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Title:
DRILLING OSCILLATION SYSTEMS AND SHOCK TOOLS FOR SAME
Document Type and Number:
WIPO Patent Application WO/2018/119151
Kind Code:
A1
Abstract:
A shock tool for reciprocating a drillstring includes an outer housing. The outer housing has a central axis, a first end, a second end opposite the first end, and a passage extending axially from the first end to the second end. In addition, the shock tool includes a mandrel assembly coaxially disposed in the passage of the outer housing and configured to move axially relative to the outer housing. The mandrel assembly has a first end axially spaced from the outer housing, a second end disposed in the outer housing, and a passage extending axially from the first end of the mandrel assembly to the second end of the mandrel assembly. The mandrel assembly includes a mandrel and a first annular piston fixably coupled to the mandrel. The first annular piston is disposed at the second end of the mandrel assembly and sealingly engages the outer housing.

Inventors:
DONALD SEAN MATTHEW (US)
SCOTT ANDREW LAWRENCE (US)
YANG YONG (US)
KVASNYTSIA ROMAN (CA)
Application Number:
PCT/US2017/067734
Publication Date:
June 28, 2018
Filing Date:
December 20, 2017
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
NAT OILWELL DHT LP (US)
International Classes:
E21B7/24
Domestic Patent References:
WO2007113477A12007-10-11
Foreign References:
US20020050359A12002-05-02
US20050236190A12005-10-27
US4830122A1989-05-16
US20120247832A12012-10-04
US6279670B12001-08-28
US6508317B22003-01-21
US6439318B12002-08-27
US6431294B12002-08-13
Attorney, Agent or Firm:
FALESKI, Thaddeus J. (US)
Download PDF:
Claims:
CLAIMS

What is claimed is:

1 . A shock tool for reciprocating a drillstring, the shock tool comprising:

an outer housing having a central axis, a first end, a second end opposite the first end , and a passage extending axially from the first end to the second end;

a mandrel assembly coaxially disposed in the passage of the outer housing and configured to move axially relative to the outer housing, wherein the mandrel assembly has a first end axially spaced from the outer housing, a second end disposed in the outer housing, and a passage extending axially from the first end of the mandrel assembly to the second end of the mandrel assembly;

wherein the mandrel assembly includes a mandrel and a first annular piston fixably coupled to the mandrel, wherein the first annular piston is disposed at the second end of the mandrel assembly and sealingly engages the outer housing.

2. The shock tool of claim 1 , further comprising a second annular piston moveably mounted to the mandrel assembly, wherein the second annular piston is disposed in a first annulus radially positioned between the mandrel assembly and the outer housing, wherein the first annulus is axially positioned between the first annular piston and the first end of the outer housing;

wherein the second annular piston is configured to move axially relative to the mandrel assembly and the outer housing.

3. The shock tool of claim 2, wherein the second annular piston divides the first annulus into a first section and a second section, wherein the first section of the first annulus is axially positioned between the second annular piston and the first end of the outer housing and the second section of the first annulus is axially positioned between the second annular piston and the first annular piston;

wherein the outer housing includes a first port extending radially from the passage of the outer housing to a radially outer surface of the outer housing, wherein the first port is in fluid communication with the second section of the first annulus.

4. The shock tool of claim 3, further comprising:

an annular seal assembly positioned between the outer housing and the mandrel assembly proximal the first end of the outer housing;

a hydraulic oil chamber radially positioned between the mandrel assembly and the outer housing, wherein the hydraulic oil chamber extends axially from the annular seal assembly to the second annular piston.

5. The shock tool of claim 1 further comprising a biasing member disposed about the mandrel assembly in an annulus radially positioned between the mandrel assembly and the outer housing, wherein the biasing member is configured to generate an axial biasing force that resists axial movement of the mandrel assembly relative to the outer housing.

6. The shock tool of claim 1 , wherein the mandrel assembly includes a washpipe having a first end fixably coupled to the mandrel and a second end distal the mandrel, and wherein the first annular piston is fixably attached to the second end of the washpipe.

7. The shock tool of claim 3, further comprising a third annular piston fixably coupled to the mandrel, wherein the third annular piston is disposed in a second annulus radially positioned between the mandrel assembly and the outer housing, wherein the second annulus is axially positioned between the first annular piston and the second annular piston, and wherein the third annular piston sealingly engages the outer housing.

8. The shock tool of claim 7, wherein the third annular piston divides the second annulus into a first section and a second section, wherein the first section of the second annulus is axially positioned between the third annular piston and the second annular piston and the second section of the second annulus is axially positioned between the third annular piston and the first annular piston;

wherein the outer housing includes a second port extending radially from the passage of the outer housing to a radially outer surface of the outer housing, wherein the second port is in fluid communication with the first section of the second annulus; wherein the mandrel assembly includes a fourth port extending radially from the passage of the mandrel assembly to a radially outer surface of the mandrel assembly, wherein the fourth port is in fluid communication with the second section of the second annulus.

9. A shock tool for reciprocating a drillstring, the shock tool comprising:

an outer housing having a central axis, an upper end, a lower end, and a passage extending axially from the upper end to the lower end;

a mandrel assembly disposed in the passage of the outer housing and extending telescopically from the upper end of the outer housing, wherein the mandrel assembly is configured to move axially relative to the outer housing to axially extend and contract the shock tool, wherein the mandrel assembly includes a mandrel and a first annular piston fixably coupled to the mandrel, wherein the first annular piston sealingly engages the outer housing;

a second annular piston disposed about the mandrel assembly within the outer housing, wherein the second annular piston is axially positioned between the first annular piston and the upper end of the outer housing, wherein the second annular piston is configured to move axially relative to the mandrel assembly and the outer housing, and wherein the second annular piston sealingly engages the mandrel assembly and the outer housing.

10. The shock tool of claim 9, wherein the second annular piston is disposed in a first annulus radially positioned between the mandrel assembly and the outer housing, wherein the second annular piston divides the first annulus into an upper section and a lower section;

wherein the outer housing includes a first port extending radially from the passage of the outer housing to a radially outer surface of the outer housing, wherein the first port is in fluid communication with the lower section of the first annulus.

1 1 . The shock tool of claim 10, wherein the port extends radially from the lower section of the first annulus to the radially outer surface of the outer housing.

12. The shock tool of claim 10, further comprising: an annular seal radially positioned between the outer housing and the mandrel assembly proximal the upper end of the outer housing;

a hydraulic oil chamber radially positioned between the mandrel assembly and the outer housing, wherein the hydraulic oil chamber extends axially from the annular seal assembly to the second annular piston.

13. The shock tool of claim 9, further comprising:

an annulus positioned between the mandrel assembly and the outer housing, wherein the annulus extends axially upward from the first annular piston;

a port extending radially through the outer housing from the passage of the outer housing to a radially outer surface of the outer housing, wherein the port is in fluid communication with the annulus.

14. The shock tool of claim 10, further comprising a third annular piston fixably coupled to the mandrel, wherein the third annular piston is disposed in a second annulus radially positioned between the mandrel assembly and the outer housing, wherein the second annulus is axially positioned between the first annular piston and the second annular piston, and wherein the third annular piston sealingly engages the outer housing.

15. The shock tool of claim 14, wherein the third annular piston divides the second annulus into an upper section and a lower section, wherein the upper section of the second annulus is axially positioned between the third annular piston and the second annular piston and the lower section of the second annulus is axially positioned between the third annular piston and the first annular piston;

wherein the outer housing includes a second port extending radially from the passage of the outer housing to a radially outer surface of the outer housing, wherein the second port is in fluid communication with the first section of the second annulus;

16. The shock tool of claim 15, further comprising a third annulus radially positioned between the mandrel assembly and the outer housing, wherein the third annulus is axially positioned between the first annular piston and the third annular piston; wherein the outer housing includes a third port extending radially from the passage of the outer housing to the radially outer surface of the outer housing, wherein the third port is in fluid communication with the third annulus.

17. The shock tool of claim 16, wherein the mandrel assembly includes a fourth port extending radially from the passage of the mandrel assembly to a radially outer surface of the mandrel assembly, wherein the fourth port is in fluid communication with the lower section of the second annulus.

18. A method for cyclically extending and contracting a shock tool for a drillstring extending through a subterranean borehole, the method comprising:

(a) flowing drilling fluid down a drillstring and up an annulus positioned between the drillstring and a sidewall of the borehole;

(b) generating pressure pulses in the drilling fluid with a pressure pulse generator disposed along the drillstring;

(c) transferring the pressure pulses through the drilling mud to a first annular piston fixably coupled to a mandrel of the shock tool; and

(d) moving the mandrel axially relative to an outer housing of the shock tool in response to (c).

19. The method of claim 18, further comprising:

exposing a first end of the first annular piston to the drilling fluid flowing down the drillstring; and

exposing a second end of the first annular piston to the drilling fluid flowing up the annulus while exposing the first end of the first annular piston to the drilling fluid flowing down the drillstring.

20. The method of claim 18, further comprising:

(e) transferring the pressure pulses through the drilling mud to a second annular piston fixably coupled to the mandrel;

wherein (d) comprises moving the mandrel axially relative to the housing in response to (c) and (e).

21 . The method of claim 20, further comprising:

exposing a first end of the second annular piston to the drilling fluid flowing down the drillstring; and

exposing a second end of the second annular piston to the drilling fluid flowing up the annulus while exposing the first end of the second annular piston to the drilling fluid flowing down the drillstring.

22. The method of claim 18, further comprising:

isolating a hydraulic oil chamber in the shock tool from the pressure pulses.

23. A method for increasing an amplitude of reciprocal axial extensions and contractions of a shock tool, the method comprising:

(a) selecting the shock tool, wherein the shock tool has a central axis and an axial length, wherein the shock tool includes an outer housing, a mandrel assembly telescopically disposed within the outer housing, and a first annular piston fixably coupled to the mandrel assembly, and wherein the shock tool has a first amplitude of reciprocal axial extension and contraction at a pressure differential between a first fluid pressure in the mandrel assembly and a second fluid pressure outside the outer housing;

(b) fixably coupling a second annular piston to the mandrel assembly of the shock tool and increasing the axial length of the shock tool after (a), wherein the second annular piston is axially spaced from the first annular piston, wherein the shock tool has a second amplitude of reciprocal axial extension and contraction at the pressure differential between the first fluid pressure in the mandrel assembly and the second fluid pressure outside the outer housing after (b), wherein the second amplitude of reciprocal axial extension and contraction is greater than the first amplitude of reciprocal axial extension and contraction.

24. The method of claim 23, further comprising:

(c) fixably coupling a third annular piston to the mandrel assembly of the shock tool after (b) and further increasing the axial length of the shock tool, wherein the third annular piston is axially spaced from the first annular piston and the second annular piston, wherein the shock tool has a third amplitude of reciprocal axial extension and contraction at the pressure differential between the first fluid pressure in the mandrel assembly and the second fluid pressure outside the outer housing after (c), wherein the third amplitude of reciprocal axial extension and contraction is greater than the first amplitude of reciprocal axial extension and contraction and greater than the second amplitude of reciprocal axial extension and contraction.

AMENDED CLAIMS

received by the International Bureau on 28 May 2018 (28.05.2018)

In the Claims

What is claimed is:

1. A shock tool for reciprocating a drillstring, the shock tool comprising:

an outer housing having a central axis, a first end, a second end opposite the first end , and a passage extending axially from the first end to the second end;

a mandrel assembly coaxially disposed in the passage of the outer housing and configured to move axially relative to the outer housing, wherein the mandrel assembly has a first end axially spaced from the outer housing, a second end disposed in the outer housing, and a passage extending axially from the first end of the mandrel assembly to the second end of the mandrel assembly;

wherein the mandrel assembly includes a mandrel and a first annular piston fixably coupled to the mandrel, wherein the first annular piston is disposed at the second end of the mandrel assembly and sealingly engages the outer housing;

a second annular piston moveably mounted to the mandrel assembly within the outer housing.

2. The shock tool of claim 1 , wherein the second annular piston is disposed in a first annulus radially positioned between the mandrel assembly and the outer housing, wherein the first annulus is axially positioned between the first annular piston and the first end of the outer housing;

wherein the second annular piston is configured to move axially relative to the mandrel assembly and the outer housing.

3. The shock tool of claim 2, wherein the second annular piston divides the first annulus into a first section and a second section, wherein the first section of the first annulus is axially positioned between the second annular piston and the first end of the outer housing and the second section of the first annulus is axially positioned between the second annular piston and the first annular piston;

wherein the outer housing includes a first port extending radially from the passage of the outer housing to a radially outer surface of the outer housing, wherein the first port is in fluid communication with the second section of the first annulus.

4. The shock tool of claim 3, further comprising: an annular seal assembly positioned between the outer housing and the mandrel assembly proximal the first end of the outer housing;

a hydraulic oil chamber radially positioned between the mandrel assembly and the outer housing, wherein the hydraulic oil chamber extends axially from the annular seal assembly to the second annular piston.

5. The shock tool of claim 1 further comprising a biasing member disposed about the mandrel assembly in an annulus radially positioned between the mandrel assembly and the outer housing, wherein the biasing member is configured to generate an axial biasing force that resists axial movement of the mandrel assembly relative to the outer housing.

6. The shock tool of claim 1 , wherein the mandrel assembly includes a washpipe having a first end fixably coupled to the mandrel and a second end distal the mandrel, and wherein the first annular piston is fixably attached to the second end of the washpipe.

7. The shock tool of claim 3, further comprising a third annular piston fixably coupled to the mandrel, wherein the third annular piston is disposed in a second annulus radially positioned between the mandrel assembly and the outer housing, wherein the second annulus is axially positioned between the first annular piston and the second annular piston, and wherein the third annular piston sealingly engages the outer housing.

8. The shock tool of claim 7, wherein the third annular piston divides the second annulus into a first section and a second section, wherein the first section of the second annulus is axially positioned between the third annular piston and the second annular piston and the second section of the second annulus is axially positioned between the third annular piston and the first annular piston;

wherein the outer housing includes a second port extending radially from the passage of the outer housing to a radially outer surface of the outer housing, wherein the second port is in fluid communication with the first section of the second annulus; wherein the mandrel assembly includes a fourth port extending radially from the passage of the mandrel assembly to a radially outer surface of the mandrel assembly, wherein the fourth port is in fluid communication with the second section of the second annulus.

9. A shock tool for reciprocating a drillstring, the shock tool comprising:

an outer housing having a central axis, an upper end, a lower end, and a passage extending axially from the upper end to the lower end;

a mandrel assembly disposed in the passage of the outer housing and extending telescopically from the upper end of the outer housing, wherein the mandrel assembly is configured to move axially relative to the outer housing to axially extend and contract the shock tool, wherein the mandrel assembly includes a mandrel and a first annular piston fixably coupled to the mandrel, wherein the first annular piston sealingly engages the outer housing;

a second annular piston disposed about the mandrel assembly within the outer housing, wherein the second annular piston is axially positioned between the first annular piston and the upper end of the outer housing, wherein the second annular piston is configured to move axially relative to the mandrel assembly and the outer housing, and wherein the second annular piston sealingly engages the mandrel assembly and the outer housing.

10. The shock tool of claim 9, wherein the second annular piston is disposed in a first annulus radially positioned between the mandrel assembly and the outer housing, wherein the second annular piston divides the first annulus into an upper section and a lower section;

wherein the outer housing includes a first port extending radially from the passage of the outer housing to a radially outer surface of the outer housing, wherein the first port is in fluid communication with the lower section of the first annulus.

11. The shock tool of claim 10, wherein the port extends radially from the lower section of the first annulus to the radially outer surface of the outer housing.

12. The shock tool of claim 10, further comprising:

an annular seal radially positioned between the outer housing and the mandrel assembly proximal the upper end of the outer housing; a hydraulic oil chamber radially positioned between the mandrel assembly and the outer housing, wherein the hydraulic oil chamber extends axially from the annular seal assembly to the second annular piston.

13. The shock tool of claim 9, further comprising:

an annulus positioned between the mandrel assembly and the outer housing, wherein the annulus extends axially upward from the first annular piston;

a port extending radially through the outer housing from the passage of the outer housing to a radially outer surface of the outer housing, wherein the port is in fluid communication with the annulus.

14. The shock tool of claim 10, further comprising a third annular piston fixably coupled to the mandrel, wherein the third annular piston is disposed in a second annulus radially positioned between the mandrel assembly and the outer housing, wherein the second annulus is axially positioned between the first annular piston and the second annular piston, and wherein the third annular piston sealingly engages the outer housing.

15. The shock tool of claim 14, wherein the third annular piston divides the second annulus into an upper section and a lower section, wherein the upper section of the second annulus is axially positioned between the third annular piston and the second annular piston and the lower section of the second annulus is axially positioned between the third annular piston and the first annular piston;

wherein the outer housing includes a second port extending radially from the passage of the outer housing to a radially outer surface of the outer housing, wherein the second port is in fluid communication with the first section of the second annulus;

16. The shock tool of claim 15, further comprising a third annulus radially positioned between the mandrel assembly and the outer housing, wherein the third annulus is axially positioned between the first annular piston and the third annular piston;

wherein the outer housing includes a third port extending radially from the passage of the outer housing to the radially outer surface of the outer housing, wherein the third port is in fluid communication with the third annulus.

17. The shock tool of claim 16, wherein the mandrel assembly includes a fourth port extending radially from the passage of the mandrel assembly to a radially outer surface of the mandrel assembly, wherein the fourth port is in fluid communication with the lower section of the second annulus.

18. A method for cyclically extending and contracting a shock tool for a drillstring extending through a subterranean borehole, the method comprising:

(a) flowing drilling fluid down a drillstring and up an annulus positioned between the drillstring and a sidewall of the borehole;

(b) generating pressure pulses in the drilling fluid with a pressure pulse generator disposed along the drillstring;

(c) transferring the pressure pulses through the drilling mud to a first annular piston fixably coupled to a mandrel of the shock tool; and

(d) moving the mandrel axially relative to an outer housing of the shock tool in response to (c);

(e) isolating a hydraulic oil chamber in the shock tool from the pressure pulses during (b) with a second annular piston moveably coupled to the mandrel of the shock tool.

19. The method of claim 18, further comprising:

exposing a first end of the first annular piston to the drilling fluid flowing down the drillstring; and

exposing a second end of the first annular piston to the drilling fluid flowing up the annulus while exposing the first end of the first annular piston to the drilling fluid flowing down the drillstring.

20. The method of claim 18, further comprising:

(f) transferring the pressure pulses through the drilling mud to a third annular piston fixably coupled to the mandrel;

wherein (d) comprises moving the mandrel axially relative to the housing in response to (c) and (f).

21. The method of claim 20, further comprising: exposing a first end of the third annular piston to the drilling fluid flowing down the drillstring; and

exposing a second end of the third annular piston to the drilling fluid flowing up the annulus while exposing the first end of the third annular piston to the drilling fluid flowing down the drillstring.

22. (Cancelled).

23. A method for increasing an amplitude of reciprocal axial extensions and contractions of a shock tool, the method comprising:

(a) selecting the shock tool, wherein the shock tool has a central axis and an axial length, wherein the shock tool includes an outer housing, a mandrel assembly telescopically disposed within the outer housing, and a first annular piston fixably coupled to the mandrel assembly, and wherein the shock tool has a first amplitude of reciprocal axial extension and contraction at a pressure differential between a first fluid pressure in the mandrel assembly and a second fluid pressure outside the outer housing;

(b) fixably coupling a second annular piston to the mandrel assembly of the shock tool and increasing the axial length of the shock tool after (a), wherein the second annular piston is axially spaced from the first annular piston, wherein the shock tool has a second amplitude of reciprocal axial extension and contraction at the pressure differential between the first fluid pressure in the mandrel assembly and the second fluid pressure outside the outer housing after (b), wherein the second amplitude of reciprocal axial extension and contraction is greater than the first amplitude of reciprocal axial extension and contraction.

24. The method of claim 23, further comprising:

(c) fixably coupling a third annular piston to the mandrel assembly of the shock tool after (b) and further increasing the axial length of the shock tool, wherein the third annular piston is axially spaced from the first annular piston and the second annular piston, wherein the shock tool has a third amplitude of reciprocal axial extension and contraction at the pressure differential between the first fluid pressure in the mandrel assembly and the second fluid pressure outside the outer housing after (c), wherein the third amplitude of reciprocal axial extension and contraction is greater than the first amplitude of reciprocal axial extension and contraction and greater than the second amplitude of reciprocal axial extension and contraction.

Description:
DRILLING OSCILLATION SYSTEMS AND SHOCK TOOLS FOR SAME

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims benefit of U.S. provisional patent application Serial No. 62/436,955 filed December 20, 2016, and entitled "High Energy Agitator Systems," which is hereby incorporated herein by reference in its entirety. In addition, this application claims benefit of U.S. provisional patent application Serial No. 62/513,760 filed June 1 , 2017, and entitled "Drilling Oscillation Systems and Shock Tools for Same," which is also hereby incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] Not applicable.

BACKGROUND

[0003] The disclosure relates generally to downhole tools. More particularly, the disclosure relates to downhole oscillation systems for inducing axial oscillations in drill strings during drilling operations. Still more particularly, the disclosure relates to shock tools that directly and efficiently convert cyclical pressure pulses in drilling fluid into axial oscillations.

[0004] Drilling operations are performed to locate and recover hydrocarbons from subterranean reservoirs. Typically, an earth-boring drill bit is typically mounted on the lower end of a drill string and is rotated by rotating the drill string at the surface or by actuation of downhole motors or turbines, or by both methods. With weight applied to the drill string, the rotating drill bit engages the earthen formation and proceeds to form a borehole along a predetermined path toward a target zone.

[0005] During drilling, the drillstring may rub against the sidewall of the borehole. Frictional engagement of the drillstring and the surrounding formation can reduce the rate of penetration (ROP) of the drill bit, increase the necessary weight-on-bit (WOB), and lead to stick slip. Accordingly, various downhole tools that induce vibration and/or axial reciprocation may be included in the drillstring to reduce friction between the drillstring and the surrounding formation. One such tool is an oscillation system, which typically includes an pressure pulse generator and a shock tool. The pressure pulse generator produces pressure pulses in the drilling fluid flowing therethrough and the shock tool converts the pressure pulses in the drilling fluid into axial reciprocation. The pressure pulses created by the pressure pulse generator are cyclic in nature. The continuous stream of pressure peaks and troughs in the drilling fluid cause the shock tool to cyclically extend and retract telescopically at the pressure peak and pressure trough, respectively. A spring is usually used to induce the axial retraction during the pressure trough.

BRIEF SUMMARY OF THE DISCLOSURE

[0006] Embodiments of shock tools for reciprocating drillstrings are disclosed herein. In one embodiment, a shock tool for reciprocating a drillstring comprises an outer housing. The outer housing has a central axis, a first end, a second end opposite the first end, and a passage extending axially from the first end to the second end. In addition, the shock tool comprises a mandrel assembly coaxially disposed in the passage of the outer housing and configured to move axially relative to the outer housing. The mandrel assembly has a first end axially spaced from the outer housing, a second end disposed in the outer housing, and a passage extending axially from the first end of the mandrel assembly to the second end of the mandrel assembly. The mandrel assembly includes a mandrel and a first annular piston fixably coupled to the mandrel. The first annular piston is disposed at the second end of the mandrel assembly and sealingly engages the outer housing.

[0007] In another embodiment, a shock tool for reciprocating a drillstring comprises an outer housing having a central axis, an upper end, a lower end, and a passage extending axially from the upper end to the lower end. In addition, the shock tool comprises a mandrel assembly disposed in the passage of the outer housing and extending telescopically from the upper end of the outer housing. The mandrel assembly is configured to move axially relative to the outer housing to axially extend and contract the shock tool. The mandrel assembly includes a mandrel and a first annular piston fixably coupled to the mandrel. The first annular piston sealingly engages the outer housing. Further, the shock tool comprises a second annular piston disposed about the mandrel assembly within the outer housing. The second annular piston is axially positioned between the first annular piston and the upper end of the outer housing. The second annular piston is configured to move axially relative to the mandrel assembly and the outer housing. The second annular piston sealingly engages the mandrel assembly and the outer housing.

[0008] Embodiments of methods for cyclically extending and contracting a shock tool for a drillstring extending through a subterranean borehole are disclosed herein. In one embodiment, a method for cyclically extending and contracting a shock tool for a drillstring extending through a subterranean borehole comprises (a) flowing drilling fluid down a drillstring and up an annulus positioned between the drillstring and a sidewall of the borehole. In addition, the method comprises (b) generating pressure pulses in the drilling fluid with a pressure pulse generator disposed along the drillstring. Further, the method comprises (c) transferring the pressure pulses through the drilling mud to a first annular piston fixably coupled to a mandrel of the shock tool. Still further, the method comprises (d) moving the mandrel axially relative to a housing of the shock tool in response to (c).

[0009] Embodiments of methods for increasing an amplitude of reciprocal axial extensions and contractions of a shock tool are disclosed herein. In one embodiment, a method for increasing an amplitude of reciprocal axial extensions and contractions of a shock tool comprises (a) selecting the shock tool. The shock tool has a central axis and an axial length. The shock tool includes an outer housing, a mandrel assembly telescopically disposed within the outer housing, and a first annular piston fixably coupled to the mandrel assembly. The shock tool has a first amplitude of reciprocal axial extension and contraction at a pressure differential between a first fluid pressure in the mandrel assembly and a second fluid pressure outside the outer housing. In addition, the method comprises (b) fixably coupling a second annular piston to the mandrel assembly of the shock tool and increasing the axial length of the shock tool after (a). The second annular piston is axially spaced from the first annular piston. The shock tool has a second amplitude of reciprocal axial extension and contraction at the pressure differential between the first fluid pressure in the mandrel assembly and the second fluid pressure outside the outer housing after (b). The second amplitude of reciprocal axial extension and contraction is greater than the first amplitude of reciprocal axial extension and contraction.

[0010] Embodiments described herein comprise a combination of features and advantages intended to address various shortcomings associated with certain prior devices, systems, and methods. The foregoing has outlined rather broadly the features and technical advantages of the invention in order that the detailed description of the invention that follows may be better understood. The various characteristics described above, as well as other features, will be readily apparent to those skilled in the art upon reading the following detailed description, and by referring to the accompanying drawings. It should be appreciated by those skilled in the art that the conception and the specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] For a detailed description of the preferred embodiments of the invention, reference will now be made to the accompanying drawings in which:

[0012] Figure 1 is a schematic view of a drilling system including an embodiment of an oscillation system in accordance with the principles described herein;

[0013] Figure 2 is a side view of the shock tool of the oscillation system of Figure 1 ;

[0014] Figure 3 is a cross-sectional side view of the shock tool of Figure 2;

[0015] Figure 4 is an enlarged partial cross-sectional side view of the shock tool of

Figure 2 taken in section 4-4 Figure 3;

[0016] Figure 5 is an enlarged partial cross-sectional side view of the shock tool of Figure 2 taken in section 5-5 Figure 3;

[0017] Figure 6 is an enlarged partial cross-sectional side view of the shock tool of Figure 2 taken in section 6-6 Figure 3;

[0018] Figure 7 is a cross-sectional side view of the outer housing of the shock tool of Figure 3;

[0019] Figure 8 is a side view of the mandrel assembly of the shock tool of Figure 3;

[0020] Figure 9 is a side view of an embodiment of a shock tool;

[0021] Figure 10 is a cross-sectional side view of the shock tool of Figure 9;

[0022] Figure 1 1 is an enlarged partial cross-sectional side view of the shock tool of

Figure 9 taken in section 1 1 -1 1 of Figure 10; [0023] Figure 12 is a flowchart illustrating an embodiment of a method for increasing the reciprocal axial extension and contraction of a shock tool in accordance with principles described herein.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] The following discussion is directed to various exemplary embodiments. However, one skilled in the art will understand that the examples disclosed herein have broad application, and that the discussion of any embodiment is meant only to be exemplary of that embodiment, and not intended to suggest that the scope of the disclosure, including the claims, is limited to that embodiment.

[0025] Certain terms are used throughout the following description and claims to refer to particular features or components. As one skilled in the art will appreciate, different persons may refer to the same feature or component by different names. This document does not intend to distinguish between components or features that differ in name but not function. The drawing figures are not necessarily to scale. Certain features and components herein may be shown exaggerated in scale or in somewhat schematic form and some details of conventional elements may not be shown in interest of clarity and conciseness.

[0026] In the following discussion and in the claims, the terms "including" and "comprising" are used in an open-ended fashion, and thus should be interpreted to mean "including, but not limited to... ." Also, the term "couple" or "couples" is intended to mean either an indirect or direct connection. Thus, if a first device couples to a second device, that connection may be through a direct connection of the two devices, or through an indirect connection that is established via other devices, components, nodes, and connections. In addition, as used herein, the terms "axial" and "axially" generally mean along or parallel to a particular axis (e.g., central axis of a body or a port), while the terms "radial" and "radially" generally mean perpendicular to a particular axis. For instance, an axial distance refers to a distance measured along or parallel to the axis, and a radial distance means a distance measured perpendicular to the axis. Any reference to up or down in the description and the claims is made for purposes of clarity, with "up", "upper", "upwardly", "uphole", or "upstream" meaning toward the surface of the borehole and with "down", "lower", "downwardly", "downhole", or "downstream" meaning toward the terminal end of the borehole, regardless of the borehole orientation. As used herein, the terms "approximately," "about," "substantially," and the like mean within 10% (i.e., plus or minus 10%) of the recited value. Thus, for example, a recited angle of "about 80 degrees" refers to an angle ranging from 72 degrees to 88 degrees.

[0027] Referring now to Figure 1 , a schematic view of an embodiment of a drilling system 10 is shown. Drilling system 10 includes a derrick 1 1 having a floor 12 supporting a rotary table 14 and a drilling assembly 90 for drilling a borehole 26 from derrick 1 1 . Rotary table 14 is rotated by a prime mover such as an electric motor (not shown) at a desired rotational speed and controlled by a motor controller (not shown). In other embodiments, the rotary table (e.g., rotary table 14) may be augmented or replaced by a top drive suspended in the derrick (e.g., derrick 1 1 ) and connected to the drillstring (e.g., drillstring 20).

[0028] Drilling assembly 90 includes a drillstring 20 and a drill bit 21 coupled to the lower end of drillstring 20. Drillstring 20 is made of a plurality of pipe joints 22 connected end-to-end, and extends downward from the rotary table 14 through a pressure control device 15, such as a blowout preventer (BOP), into the borehole 26. Drill bit 21 is rotated with weight-on-bit (WOB) applied to drill the borehole 26 through the earthen formation. Drillstring 20 is coupled to a drawworks 30 via a kelly joint 21 , swivel 28, and line 29 through a pulley. During drilling operations, drawworks 30 is operated to control the WOB, which impacts the rate-of-penetration of drill bit 21 through the formation. In adition, drill bit 21 can be rotated from the surface by drillstring 20 via rotary table 14 and/or a top drive, rotated by downhole mud motor 55 disposed along drillstring 20 proximal bit 21 , or combinations thereof (e.g., rotated by both rotary table 14 via drillstring 20 and mud motor 55, rotated by a top drive and the mud motor 55, etc.). For example, rotation via downhole motor 55 may be employed to supplement the rotational power of rotary table 14, if required, and/or to effect changes in the drilling process. In either case, the rate-of-penetration (ROP) of the drill bit 21 into the borehole 26 for a given formation and a drilling assembly largely depends upon the WOB and the rotational speed of bit 21 .

[0029] During drilling operations a suitable drilling fluid 31 is pumped under pressure from a mud tank 32 through the drillstring 20 by a mud pump 34. Drilling fluid 31 passes from the mud pump 34 into the drillstring 20 via a desurger 36, fluid line 38, and the kelly joint 21 . The drilling fluid 31 pumped down drillstring 20 flows through mud motor 55 and is discharged at the borehole bottom through nozzles in face of drill bit 21 , circulates to the surface through an annulus 27 radially positioned between drillstring 20 and the sidewall of borehole 26, and then returns to mud tank 32 via a solids control system 36 and a return line 35. Solids control system 36 may include any suitable solids control equipment known in the art including, without limitation, shale shakers, centrifuges, and automated chemical additive systems. Control system 36 may include sensors and automated controls for monitoring and controlling, respectively, various operating parameters such as centrifuge rpm. It should be appreciated that much of the surface equipment for handling the drilling fluid is application specific and may vary on a case-by-case basis.

[0030] While drilling, one or more portions of drillstring 20 may contact and slide along the sidewall of borehole 26. To reduce friction between drillstring 20 and the sidewall of borehole 26, in this embodiment, an oscillation system 100 is provided along drillstring 20 proximal motor 55 and bit 21 . Oscillation system 100 includes a pressure pulse generator 1 10 coupled to motor 55 and a shock tool 120 coupled to pulse generator 1 10. Pulse generator 1 10 generates cyclical pressure pulses in the drilling fluid flowing down drillstring 20 and shock tool 120 cyclically and axially extends and retracts as will be described in more detail below. With bit 21 disposed on the hole bottom, the axial extension and retraction of shock tool 120 induces axial reciprocation in the portion of drillstring above oscillation system 100, which reduces friction between drillstring 20 and the sidewall of borehole.

[0031] In general, pulse generator 1 10 and mud motor 55 can be any pressure pulse generator and mud motor, respectively, known in the art. For example, as is known in the art, pulse generator 1 10 can be a valve operated to cyclically open and close as a rotor of mud motor 55 rotates within a stator of mud motor 55. When the valve opens, the pressure of the drilling mud upstream of pulse generator 1 10 decreases, and when the valve closes, the pressure of the drilling mud upstream of pulse generator 1 10 increases. Examples of such valves are disclosed in US Patent Nos. 6,279,670, 6,508,317, 6,439,318, and 6,431 ,294, each of which is incorporated herein by reference in its entirety for all purposes.

[0032] Referring now to Figures 2 and 3, shock tool 120 of oscillation system 100 is shown. Shock tool 120 has a first or uphole end 120a, a second or downhole end 120b opposite end 120a, and a central or longitudinal axis 125. As shown in Figure 1 , uphole end 120a is coupled to the portion of drillstring 20 disposed above oscillation system 100 and downhole end 120b is coupled to pulse generator 1 10. Tool 120 has a length L 120 measured axially from end 120a to end 120b. As will be described in more detail below, shock tool 120 cyclically axially extends and retracts in response to the pressure pulses in the drilling fluid generated by pulse generator 1 10 during drilling operations. During extension of tool 120, ends 120a, 120b move axially away from each other and length L 120 increases, and during contraction of tool 120, ends 120a, 120b move axially toward each other and length L 120 decreases. Thus, shock tool 120 may be described as having an "extended" position with ends 120a, 120b axially spaced apart to the greatest extent (i.e., when length L 120 is at a maximum) and a retracted position with ends 120a, 120b axially spaced apart to the smallest extent (i.e., when length L 120 is at a minimum).

[0033] Referring still to Figures 2 and 3, in this embodiment, shock tool 120 includes an outer housing 130, a mandrel assembly 150 telescopically disposed within outer housing 130, a biasing member 180 disposed about mandrel assembly 150 within outer housing 130, and an annular floating piston 190 disposed about mandrel assembly 150 within outer housing 130. Thus, biasing member 180 and floating piston 190 are radially positioned between mandrel assembly 150 and outer housing 130. Mandrel assembly 150 and outer housing 130 are tubular members, each having a central or longitudinal axis 155, 135, respectively, coaxially aligned with axis 125 of shock tool 120. Mandrel assembly 150 can move axially relative to outer housing 130 to enable the cyclical axial extension and retraction of shock tool 120. Biasing member 180 axially biases mandrel assembly 150 and shock tool 120 to a "neutral" position between the extended position and the retracted position. As will be described in more detail below, floating piston 190 is free to move axially along mandrel assembly 150 and defines a barrier to isolate biasing member 180 from drilling fluids.

[0034] Referring now to Figures 4-7, outer housing 130 has a first or uphole end 130a, a second or downhole end 130b opposite end 130a, a radially outer surface 131 extending axially between ends 130a, 130b, and a radially inner surface 132 extending axially between ends 130a, 130b. Uphole end 130a is axially positioned below uphole end 120a of shock tool 120. However, downhole end 130b is coincident with, and hence defines downhole end 120b of shock tool 120. [0035] Inner surface 132 defines a central throughbore or passage 133 extending axially through housing 130 (i.e., from uphole end 130a to downhole end 130b). Outer surface 131 is disposed at a radius that is uniform or constant moving axially between ends 130a, 130b. Thus, outer surface 131 is generally cylindrical between ends 130a, 130b. Inner surface 132 is disposed at a radius that varies moving axially between ends 130a, 130b.

[0036] In this embodiment, outer housing 130 is formed with a plurality of tubular members connected end-to-end with mating threaded connections (e.g., box and pin connections). Some of the tubular members forming outer housing 130 define annular shoulders along inner surface 132. In particular, moving axially from uphole end 130a to downhole end 130b, inner surface 132 includes a frustoconical uphole facing annular shoulder 132a, an uphole facing annular shoulder 132b, a downward facing planar annular shoulder 132c, an uphole facing planar annular shoulder 132d, and a downward facing planar annular shoulder 132e. In addition, inner surface 132 includes a plurality of circumferentially-spaced parallel internal splines 134 axially positioned between shoulders 132a, 132b. As will be described in more detail below, splines 134 slidingly engage mating external splines on mandrel assembly 150, thereby allowing mandrel assembly 150 to move axially relative to outer housing 130 but preventing mandrel assembly 150 from rotating about axis 125 relative to outer housing 130. Each spline 134 extends axially between a first or uphole end 134a and a second or downhole end 134b. The uphole ends 134a of splines 134 define a plurality of circumferentially-spaced uphole facing frustoconical shoulders 134c extending radially into passage 133, and the downhole ends 134b of splines 134 define a plurality of circumferentially-spaced downhole facing planar shoulders 134d extending radially into passage 133.

[0037] Referring still to Figures 4-7, inner surface 132 also includes a cylindrical surface 136a extending axially from end 130a to shoulder 132a, a cylindrical surface 136b extending axially between shoulders 132a, 134c, a cylindrical surface 136c extending axially between shoulders 134d, 132b, a cylindrical surface 136d extending axially between shoulders 132b, 132c, a cylindrical surface 136e extending axially between shoulders 132c, 132d, a cylindrical surface 136f axially positioned between shoulders 132d, 132e, and a cylindrical surface 136g extending axially from shoulder 132e. [0038] Along each cylindrical surface 136a, 136b, 136c, 136d, 136e, 136f, 136g the radius of inner surface 132 is constant and uniform, however, since shoulders 132a, 132b, 132c, 132d, 132e, 134c, 134d extend radially, the radius of inner surface 132 along different cylindrical surfaces 136a, 136b, 136c, 136d, 136e, 136f, 136g may vary. As best shown in Figures 4-6, and as will be described in more detail below, cylindrical surfaces 136a, 136d, 136f, 136g slidingly engage mandrel assembly 150, whereas cylindrical surfaces 136b, 136c, 136e are radially spaced from mandrel assembly 150.

[0039] In this embodiment, a plurality of axially spaced annular seal assemblies 137a are disposed along cylindrical surface 136a and radially positioned between mandrel assembly 150 and outer housing 130. Seal assemblies 137a form annular seals between mandrel assembly 150 and outer housing 130, thereby preventing fluids from flowing axially between cylindrical surface 136a and mandrel assembly 150. Thus, seal assemblies 137a prevent fluids from inside housing 130 from flowing upwardly between mandrel assembly 150 and end 130a into annulus 27 during drilling operations, and prevent fluids in annulus 27 from flowing between mandrel assembly 150 and end 130a into housing 130. In addition, in this embodiment, a plurality of axially spaced annular seal assemblies 137b are disposed along cylindrical surface 136f and radially positioned between outer housing 130 and mandrel assembly 150. Seal assemblies 137b form annular seals between mandrel assembly 150 and outer housing 130, thereby preventing fluids from flowing axially between cylindrical surface 136f and mandrel assembly 150.

[0040] As best shown in Figures 2 and 6, outer housing 130 includes a first plurality of circumferentially-spaced ports 138 extending radially from outer surface 131 to inner surface 132, and a second plurality of circumferentially-spaced ports 139 extending radially from outer surface 131 to inner surface 132. In particular, ports 138 extend radially from outer surface 131 to cylindrical surface 136e, and ports 139 extend radially from outer surface 131 to cylindrical surface 136g. Ports 138 are disposed at the same axial position along outer housing 130 and are uniformly angularly spaced about axis 135. Similarly, ports 139 are disposed at the same axial position along outer housing 130 and are uniformly angularly spaced about axis 135. However, ports 138 are axially spaced above ports 139. As will be described in more detail below, ports 138, 139 allow fluid communication between the annulus 27 outside shock tool 120 and through passage 133 of outer housing 130.

[0041] Referring now to Figures 4-6 and 8, mandrel assembly 150 has a first or uphole end 150a, a second or downhole end 150b opposite end 150a, a radially outer surface 151 extending axially between ends 150a, 150b, and a radially inner surface 152 extending axially between ends 150a, 150b. Uphole end 150a is coincident with, and hence defines uphole end 120a of shock tool 120. In addition, uphole end 150a is axially positioned above uphole end 130a of outer housing 130. Downhole end 150b is disposed without outer housing 130 and axially positioned above downhole end 130b. Inner surface 152 defines a central throughbore or passage 153 extending axially through mandrel assembly 150 (i.e., from uphole end 150a to downhole end 150b). Inner surface 152 is disposed at a radius that is uniform or constant moving axially between ends 150a, 150b. Thus, inner surface 152 is generally cylindrical between ends 150a, 150b. Outer surface 151 is disposed at a radius that varies moving axially between ends 150a, 150b.

[0042] In this embodiment, mandrel assembly 150 includes a mandrel 160, a tubular member or washpipe 170 coupled to mandrel 160, and an annular static piston 175 coupled to washpipe 170. Mandrel 160, washpipe 170, and piston 175 are connected end-to-end and are coaxially aligned with axis 155.

[0043] Referring still to Figures 4-6 and 8, mandrel 160 has a first or uphole end 160a, a second or downhole end 160b opposite end 160a, a radially outer surface 161 extending axially between ends 160a, 160b, and a radially inner surface 162 extending axially between ends 160a, 160b. Uphole end 160a is coincident with, and hence defines uphole end 150a of mandrel assembly 150. Inner surface 162 is a cylindrical surface defining a central throughbore or passage 163 extending axially through mandrel 160. Inner surface 162 and passage 163 define a portion of inner surface 152 and passage 153 of mandrel assembly 150.

[0044] Moving axially from uphole end 160a, outer surface 161 includes a cylindrical surface 164a, extending from end 160a, a concave downhole facing annular shoulder 164b, a cylindrical surface 164c extending from shoulder 164b, a plurality circumferentially-spaced parallel external splines 166, and a cylindrical surface 164d axially positioned between splines 166 and downhole end 160b. A portion of outer surface 161 extending from downhole end 160b includes external threads that threadably engage mating internal threads of washpipe 170.

[0045] Splines 166 are axially positioned between cylindrical surfaces 164c, 164d. Each spline 166 extends axially between a first or uphole end 166a and a second or downhole end 166b. In this embodiment, each spline 166 includes two segments separated by a cylindrical surface that receives a lock ring 167, which functions as a shouldering mechanism to limit the upward travel of mandrel 160 relative to housing 130. In particular, as best shown in Figure 4, mandrel 160 can move axially upward relative to housing 130 until lock ring 167 axially engages shoulders 134d at lower ends 134b of splines 134, thereby preventing further axial upward movement of mandrel 160 relative to housing 130. Limiting the upward travel of the mandrel 160 relative to housing 130 reduces the likelihood of overstressing biasing member 180. In this embodiment, the upward travel of mandrel 160 relative to housing 130 is limited to about 1 .0 in.

[0046] Referring again to Figures 4-6 and 8, the downhole ends 166b of splines 166 define a plurality of circumferentially-spaced downhole facing planar shoulders 166d. Splines 166 of mandrel 160 slidingly engage mating splines 134 of outer housing 130, thereby allowing mandrel assembly 150 to move axially relative to outer housing 130 but preventing mandrel assembly 150 from rotating about axis 125 relative to outer housing 130. Thus, engagement of mating splines 134, 166 enables the transfer of rotation torque between mandrel assembly 150 and outer housing 130 during drilling operations.

[0047] Washpipe 170 has a first or uphole end 170a, a second or downhole end 170b opposite end 170a, a radially outer surface 171 extending axially between ends 170a, 170b, and a radially inner surface 172 extending axially between ends 170a, 170b. Inner surface 172 is a cylindrical surface defining a central throughbore or passage 173 extending axially through washpipe 170. Inner surface 172 and passage 173 define a portion of inner surface 152 and passage 153 of mandrel assembly 150. A portion of inner surface 172 extending axially from uphole end 170a includes internal threads that threadably engage the mating external threads provided at downhole end 160b of mandrel 160, thereby fixably securing mandrel 160 and washpipe 170 end-to-end. With end 160b of mandrel 160 threaded into uphole end 170a of washpipe 170, end 170a defines an annular uphole facing planar shoulder 154 along outer surface 151 .

[0048] Moving axially from uphole end 170a, outer surface 171 includes a cylindrical surface 174a extending from end 170a, a downhole facing planar annular shoulder 174b, and a cylindrical surface 174c extending from shoulder 174b. A portion of outer surface 171 at downhole end 170b includes external threads that threadably engage mating internal threads of piston 175.

[0049] As best shown in Figures 6 and 8, annular piston 175 is disposed about downhole end 170b of washpipe 170 and extends axially therefrom. Piston 175 has a first or uphole end 175a, a second or downhole end 175b opposite end 175a, a radially outer surface 176 extending axially between ends 175a, 175b, and a radially inner surface 177 extending axially between ends 175a, 175b. Inner surface 177 defines a central throughbore or passage 178 extending axially through piston 175. Inner surface 177 and passage 178 define a portion of inner surface 152 and passage 153 of mandrel assembly 150. A portion of inner surface 177 extending axially from upper end 175a includes internal threads that threadably engage the mating external threads provided at downhole end 170b of washpipe 170, thereby fixably securing annular piston 175 to downhole end 170b of washpipe 170.

[0050] Outer surface 176 includes a cylindrical surface 179a. A plurality of axially spaced annular seal assemblies 179b are disposed along cylindrical surface 179a and radially positioned between piston 175 and outer housing 130. Seal assemblies 179b form annular seals between piston 175 and outer housing 130, thereby preventing fluids from flowing axially between cylindrical surfaces 136g, 179a of outer housing 130 and piston 175, respectively. As will be described in more detail below, seal assemblies 179b maintain separation of relatively low pressure drilling fluid in fluid communication with annulus 27 via ports 139 and relatively high pressure drilling fluid flowing down drillstring 20 and through mandrel assembly 150.

[0051] Referring now to Figures 4-6, mandrel assembly 150 is disposed within outer housing 130 with mating splines 134, 166 intermeshed and uphole ends 150a, 160a positioned above end 130a of housing 130. In addition, cylindrical surfaces 136a, 164c slidingly engage with annular seal assemblies 137a sealingly engaging surface 164c of mandrel 160; cylindrical surfaces 136f, 174c slidingly engage with annular seal assemblies 137b sealingly engaging surface 174c of washpipe 170; and cylindrical surfaces 136g, 179a slidingly engage with annular seal assemblies 179b sealingly engaging surface 136g of outer housing 130.

[0052] Cylindrical surfaces 136d, 174a are radially adjacent one another, however, seals are not provided between surfaces 136d, 174a. Thus, although surfaces 136d, 174a may slidingly engage, fluid can flow therebetween. Although annular seal assemblies 179b are provided between surfaces 136f, 174c in this embodiment, in other embodiments, seals are not provided between surfaces 136f, 174c, and thus, fluids can flow therebetween.

[0053] Cylindrical surface 136c of outer housing 130 is radially opposed to the lower portions of external splines 166 of mandrel 160 but radially spaced therefrom. An annular sleeve 140 is positioned about the lower portions of external splines 166 and axially abuts shoulders 134d defined by the downhole ends 134b of internal splines 134. In particular, sleeve 140 has a first or uphole end 140a engaging shoulders 134d, a second or downhole end 140b proximal shoulders 166d defined by the downhole ends 166b of external splines 160, a radially outer cylindrical surface 141 slidingly engaging cylindrical surface 136c, and a radially inner cylindrical surface 142 slidingly engaging splines 166. As will be described in more detail below, downhole end 140b defines an annular downhole facing planar shoulder 143 within housing 130.

[0054] Referring still to Figures 4-6, cylindrical surfaces 136c, 164d of outer housing 130 and mandrel 160, respectively, are radially opposed and radially spaced apart; cylindrical surfaces 136e, 174c of outer housing 130 and washpipe 170, respectively, are radially opposed and radially spaced apart; and cylindrical surfaces 136g, 174d of outer housing 130 and washpipe 170, respectively, are radially opposed and radially spaced apart. As a result, shock tool 120 includes a first annular space or annulus 145, a second annular space or annulus 146 axially positioned below annulus 145, and a third annular space or annulus 147 axially positioned below annulus 146. Annulus 145 is radially positioned between surfaces 136c, 164d and extends axially from the axially lower of shoulder 143 of sleeve 140 and shoulders 166d of splines 166 to the axially upper of shoulder 132b of housing 130 and shoulder 154 of mandrel assembly 150 (depending on the relatively axial positions of mandrel assembly 150 and outer housing 130). Annulus 146 is radially position between surfaces 136e, 174c and extends axially from shoulder 132c of housing 130 to shoulder 132d of housing 130. Annulus 147 is radially positioned between surfaces 136g, 174d and extends axially from shoulder 132e of housing 130 to uphole end 175a of piston 175. Ports 139 extend radially from annulus 147, and thus, provide fluid communication between annulus 147 and annulus 27.

[0055] Referring now to Figures 4 and 5, biasing member 180 is disposed about mandrel assembly 150 and positioned in annulus 145. Biasing member 180 has a first or uphole end 180a proximal shoulders 143, 166d and a second or downhole end 180b proximal shoulder 132b, 154. Biasing member 180 has a central axis coaxially aligned with axes 125, 135, 155. In this embodiment, biasing member 180 is a stack of Belleville springs.

[0056] Biasing member 180 is axially compressed within annulus 145 with its uphole end 180a axially bearing against the lowermost of shoulder 143 of sleeve 140 and shoulders 166d of splines 166, and its downhole end 180b axially bearing against the uppermost of shoulder 132b of housing 130 and shoulder 154 defined by upper end 170a of washpipe 170. More specifically, during the cyclical axial extension and retraction of shock tool 120, mandrel assembly 150 moves axially uphole and downhole relative to outer housing 130. As mandrel assembly 150 moves axially uphole relative to outer housing 130, biasing member 180 is axially compressed between shoulders 154, 143 as shoulder 154 lifts end 180b off shoulder 132b and shoulders 166d moves axially upward and away from shoulder 143 and end 180a. As a result, the axial length of biasing member 180 measured axially between ends 180a, 180b decreases and biasing member 180 exerts an axial force urging shoulders 154, 143 axially apart (i.e., urges shoulder 154 axially downward toward shoulder 132b and urges shoulder 143 axially upward toward shoulders 166d). As mandrel assembly 150 moves axially downhole relative to outer housing 130, biasing member 180 is axially compressed between shoulders 166d, 132b as shoulders 166d push end 180a downward and shoulder 154 moves axially downward and away from shoulder 132b and end 180b. As a result, the axial length of biasing member 180 measured axially between ends 180a, 180b decreases and biasing member 180 exerts an axial force urging shoulders 166d, 132b axially apart (i.e. , urges shoulders 166d axially upward toward shoulder 143 and urges shoulder 132b axially downward toward shoulder 154). Thus, when shock tool 120 axially extends or contracts, biasing member 180 biases shock tool 120 and mandrel assembly 150 to a "neutral" position with shoulders 132b, 154 disposed at the same axial position engaging end 180b of biasing member 180, and shoulders 143, 166d disposed at the same axial position engaging end 180a of biasing member 180. In this embodiment, biasing member 180 is preloaded (i.e., in compression) with tool 120 in the neutral positon such that biasing member 180 provides a restoring force urging tool 120 to the neutral position upon any axial extension or retraction of tool 120 (i.e., upon any relative axial movement between mandrel assembly 150 and outer housing 130).

[0057] Referring now to Figure 5, annular piston 190 is disposed about mandrel assembly 150 and positioned in annulus 146. Accordingly, piston 190 divides annulus 146 into a first or uphole section 146a extending axially from shoulder 132c to piston 190 and a second or downhole section 146b extending axially from piston

190 to shoulder 132d. Piston 190 has a first or uphole end 190a, a second or downhole end 190b opposite end 190a, a radially outer surface 191 extending axially between ends 190a, 190b, and a radially inner surface 192 extending axially between ends 190a, 190b. Piston 190 has a central axis coaxially aligned with axes 125, 135, 155.

[0058] Inner surface 192 is a cylindrical surface defining a central throughbore or passage 193 extending axially through piston 190. Washpipe 170 extends though passage 193 with cylindrical surfaces 174c, 192 slidingly engaging. Outer surface

191 is a cylindrical surface that slidingly engages cylindrical surface 136e of outer housing 130.

[0059] An annular seal assembly 196a is disposed along outer cylindrical surface 191 and radially positioned between piston 190 and outer housing 130, and an annular seal assembly 196b is disposed along inner cylindrical surface 192 and radially positioned between piston 190 and washpipe 170. Seal assembly 196a forms an annular seal between piston 190 and outer housing 130, thereby preventing fluids from flowing axially between cylindrical surfaces 191 , 136e. Seal assembly 196b forms an annular seal between piston 190 and mandrel assembly 150, thereby preventing fluids from flossing axially between cylindrical surfaces 174c, 192.

[0060] Referring again to Figures 4 and 5, as previously described, seal assemblies 137a seal between mandrel assembly 150 and outer housing 130 at uphole end 130a, and seal assemblies 196a, 196b and piston 190 seal between mandrel assembly 150 and outer housing 130 axially below splines 134, 166 and biasing member 180. To facilitate relatively low friction, smooth relative movement between mandrel assembly 150 and outer housing and to isolate splines 134, 166 and biasing member 180 from drilling fluid, splines 134, 166 and biasing member 180 are bathed in hydraulic oil. In particular, the annuli and passages radially positioned between mandrel assembly 150 and outer housing 130 and extending axially between seal assemblies 137a and seal assemblies 196a, 196b define a hydraulic oil chamber 148 filled with hydraulic oil. Thus, uphole section 146a of annulus 146, annulus 145, the passages between annuli 146, 145 (e.g., between cylindrical surfaces 136d, 174a), and the passages between splines 134, 166 are included in chamber 148, in fluid communication with each other, and are filled with hydraulic oil.

[0061] Floating piston 190 is free to move axially within annulus 146 along washpipe 170 in response to pressure differentials between portions 146a, 146b of annulus 146. Thus, floating piston 190 allows shock tool 120 to accommodate expansion and contraction of the hydraulic oil in chamber 148 due to changes in downhole pressures and temperatures without over pressurizing seal assemblies 137a, 196a, 196b. In this embodiment, hydraulic oil chamber 148 is pressure balanced with the relatively low pressure of drilling fluid in the annulus 27 outside shock tool 120. More specifically, lower portion 146b of annulus 146 is in fluid communication with annulus 27 via ports 138, and thus, is at the same pressure as drilling fluid in annulus 27 proximal ports 138. Thus, piston 190 will move axially in annulus 146 until the pressure of the hydraulic oil in chamber 148 is the same as the pressure of the drilling fluid in annulus 27 proximal port 138. As a result, seal assemblies 137a, 196a, 196b do not need to maintain a seal across a pressure differential - seal assemblies 137a form seals between hydraulic chamber 148 and annulus 27 proximal end 130a, which are at the same pressure (i.e. the pressure of annulus 27), and seal assemblies 196a, 196b form seals between hydraulic chamber 148 and portion 146a of annulus 146, which are at the same pressure (i.e., the pressure of annulus 27).

[0062] Referring briefly to Figure 1 , during drilling operations, drilling fluid (or mud) is pumped from the surface down drillstring 20. The drilling fluid flows through oscillation system 100 to bit 21 , and then out the face of bit 21 into the open borehole 26. The drilling fluid exiting bit 21 flows back to the surface via the annulus 27 between the drillstring 20 and borehole sidewalk In general, at any given depth in borehole 26, the drilling fluid pumped down the drillstring 20 is at a higher pressure than the drilling fluid in annulus 27, which enables the continuous circulation of drilling fluid. The drilling fluid flowing through mud motor 55 actuates pulse generator 1 10, which generates cyclical pressure pulses in the drilling fluid. The pressure pulses generated by pulse generator 1 10 are transmitted through the drilling fluid upstream into shock tool 120.

[0063] Referring now to Figure 6, downhole end 175b of piston 175 faces and directly contacts drilling fluid flowing through passage 153 of mandrel assembly 150, while uphole end 175a of piston 175 faces and directly contacts drilling fluid in annulus 147. Seal assemblies 179b prevent fluid communication between the drilling fluid in annulus 147 and the drilling fluid flowing through passage 153. The drilling fluid in each annulus 146, 147 is in fluid communication with annulus 27 via ports 138, 139, respectively, in outer housing 130. Thus, the drilling fluid within each annulus 146, 147 is at the same pressure as the drilling fluid in annulus 27 proximal ports 138, 139, respectively. Since, at a given depth, the drilling fluid flowing down drillstring 20 has a higher pressure than the drilling fluid flowing through annulus 27, there is a pressure differential across piston 175 - end 175b faces relatively high pressure drilling fluid (drillstring pressure) whereas end 175a faces relatively low pressure drilling fluid (annulus pressure).

[0064] The pressure differential across piston 175 generates an axial upward force on piston 175, which is transferred to mandrel assembly 150 (piston 175, washpipe 170, and mandrel 160 are fixably attached together end-to-end). During steady state drilling operations where changes in the pressure of drilling fluid in passage 153, annulus 27, section 146b, and annulus 147 are gradual (i.e. , there are no pressure pulses generated by pulse generator 1 10), the biasing force generated by biasing member 180 acts to balance and counteract the axially upward force on piston 175 generated by the pressure differential to maintain shock tool 120 at or near its neutral position. However, under dynamic conditions, such as when pressure pulses generated by pulse generator 1 10 act on downhole end 175b, the cyclical increases and decreases in the pressure differentials across piston 175 generate abrupt increases and decreases in the axial forces applied to piston 175. The biasing member 180 generates a biasing force that resists the axial movement of piston 175, however, it takes a moment for the biasing force to increase to a degree sufficient to restore shock tool 120 and mandrel assembly 150 to the neutral position. As a result, the pressure pulses generated by pulse generator 1 10 axially reciprocate piston 175 (and the remainder of mandrel assembly 150 fixably coupled to piston 175) relative to outer housing 130, thereby reciprocally axially extending and contracting shock tool 120. As piston 175 moves axially relative to outer housing 130, drilling fluid is free to flow between annulus 27 and annulus 147 via ports 139 to maintain the pressure in 147 the same as the pressure in annulus 27.

[0065] Many conventional shock tools do not include a piston fixably coupled to the mandrel, and instead, the pressure pulses generated by a pressure pulse generator are transferred to the mandrel through a floating piston and the hydraulic oil in the hydraulic oil chamber. In particular, the pressure pulses generate a pressure differential across the floating piston, the floating piston moves axially in response to the pressure differential, movement of the floating piston generates a pressure wave that moves upward through the hydraulic oil in the hydraulic oil chamber and acts on an uphole portion of the mandrel to move the mandrel axially relative to the outer housing. Thus, such conventional shock tools may be described as operating by indirect actuation of the mandrel. In contrast, embodiments of shock tools described herein (e.g., shock tool 120) that operate via direct actuation of the mandrel assembly - the pressure pulses from the pulse generator (e.g., pulse generator 1 10) act directly on the static piston (e.g., piston 175) fixably coupled to the mandrel (e.g., mandrel 160). Without being limited by this or any particular theory, direct actuation offers the potential for improved actuation efficiency and responsiveness as compared to indirect actuation. In particular, during the transfer of the pressure pulses through the floating piston and hydraulic oil to the mandrel in indirect actuation, energy may be lost to friction, heat, etc.

[0066] In many conventional shock tools, the seals isolating the hydraulic oil chamber from drilling fluid (e.g., the seals between the outer housing and the mandrel and the seals of the floating piston) are exposed to the relatively high pressure drilling fluid flowing down the drillstring and the pressure pulses generated by the pulse generator. In addition, such seals must withstand the pressure differentials that actuate the mandrel (the pressure pulses are transferred to the mandrel via the floating piston and hydraulic oil chamber). In contrast, embodiments of shock tools described herein isolate the floating piston, the hydraulic oil chamber, and the seals defining the hydraulic oil chamber are isolated from the relatively high pressure drilling fluid flowing down the drillstring and the pressure pulses generated by the pulse generator. Specifically, in embodiments described herein, the floating piston, the hydraulic oil chamber, and the seals separating the hydraulic oil chamber from drilling fluid are pressure balanced to the annulus of the borehole. For example, in the embodiment of shock tool 120 described above, the pressure pulses do not act on floating piston 190 and associated seal assemblies 196a, 196b, and further, the pressure pulses do not act on seal assemblies 137a. Thus, floating piston 190, seal assemblies 196a, 196b, and seal assemblies 137a are not exposed to the abrupt increases and decreases in the pressure generated by pulse generator 1 10. Rather, floating piston 190, seal assemblies 196a, 196b, and seal assemblies 137a are only exposed to the relatively low pressure of drilling fluid in annulus 27 and the hydraulic oil in chamber 148, which as described above is at the same relatively low pressure as the drilling fluid in annulus 27. In this manner, static piston 175 isolates floating piston 190, seal assemblies 196a, 196b, 137a, and hydraulic fluid chamber 148 from the pressure pulses generated by pulse generator 1 10.

[0067] Referring now to Figures 9 and 10, another embodiment of a shock tool 220 is shown. Shock tool 220 can be used in oscillation system 100 in place of shock tool 120 previously described. Shock tool 220 is substantially the same as shock tool 120 with the exception that shock tool 220 includes a plurality of static pistons fixably coupled to the mandrel and directly actuated by the pressure pulses generated by pulse generator 1 10. This functionality offers the potential to enhance the total energy transferred to the mandrel assembly by each pressure pulse. This may be particularly beneficial in drilling operations where available drilling fluid pressure pumping capacity from rig pumping systems is limited. As will be described in more detail below, in this embodiment of tool 220, the total piston area (A) to be operated on by the drilling fluid pressure differential (P) is increased via inclusion of multiple static pistons, thereby increasing the net force (F) applied to the mandrel according to the relationship F = P x A.

[0068] Shock tool 220 has a first or uphole end 220a, a second or downhole end 220b opposite end 220a, and a central or longitudinal axis 225. Tool 220 has a length L220 measured axially from end 220a to end 220b. Similar to shock tool 120, shock tool 220 cyclically axially extends and retracts in response to the pressure pulses in the drilling fluid generated by pulse generator 1 10 during drilling operations. Thus, shock tool 220 may also be described as having an "extended" position with ends 220a, 220b axially spaced apart to the greatest extent (i.e., when length L220 is at a maximum) and a retracted position with ends 220a, 220b axially spaced apart to the smallest extent (i.e., when length L220 is at a minimum).

[0069] Referring still to Figures 9 and 10, shock tool 220 includes an outer housing 230, a mandrel assembly 250 telescopically disposed within outer housing 230, a biasing member 180 disposed about mandrel assembly 150 within outer housing 230, and an annular floating piston 190 disposed about mandrel assembly 150 within outer housing 230. Thus, biasing member 180 and floating piston 190 are radially positioned between mandrel assembly 250 and outer housing 230. Biasing member 180 and floating piston 190 are each as previously described.

[0070] Mandrel assembly 250 and outer housing 230 are tubular members, each having a central or longitudinal axis 255, 235, respectively, coaxially aligned with axis 225 of shock tool 120. Mandrel assembly 250 can move axially relative to outer housing 230 to enable the cyclical axial extension and retraction of shock tool 220. Biasing member 180 axially biases shock tool 220 to the "neutral" position between the extended position and the retracted position.

[0071] Outer housing 230 is substantially the same as outer housing 230 previously described with the exception that outer housing 230 includes an additional sub at its lower end that defines additional shoulders and cylindrical surfaces along the inner surface and an additional set of radial ports. Thus, outer housing 230 has a first or uphole end 230a, a second or downhole end 230b opposite end 230a, a radially outer surface 231 extending axially between ends 230a, 230b, and a radially inner surface 232 extending axially between ends 230a, 230b. Inner surface 232 defines a central throughbore or passage 233 extending axially through housing 230 (i.e. , from uphole end 230a to downhole end 230b).

[0072] Referring now to Figure 1 1 , an enlarged view of the lower portion of shock tool 220 is shown. It should be appreciated that the portion of shock tool 220 disposed above the lower portion shown in Figure 1 1 is the same as shock tool 120 previously described. Inner surface 232 is the same as inner surface 132 previously described with the exception that inner surface 232 includes an uphole facing planar annular shoulder 132f disposed axially below cylindrical surface 136g, a downward facing planar annular shoulder 132g disposed axially below shoulder 132f, a cylindrical surface 136h axially positioned between shoulders 132f, 132g, and a cylindrical surface 136i extending axially downward from shoulder 132g. In addition, in this embodiment, a plurality of axially spaced annular seal assemblies 237b are disposed along cylindrical surface 136h and radially positioned between outer housing 230 and mandrel assembly 250. Seal assemblies 237b form annular seals between mandrel assembly 250 and outer housing 230, thereby preventing fluids from flowing axially between cylindrical surface 136h and mandrel assembly 250. As will be described in more detail below, seal assemblies 237b maintain separation of relatively low pressure drilling fluid in fluid communication with annulus 27 and relatively high pressure drilling fluid flowing down drillstring 20 and through mandrel assembly 250.

[0073] Outer housing 230 includes ports 138, 139 as previously described. However, in this embodiment, outer housing 230 also includes a third plurality of circumferentially-spaced ports 238 extending radially from outer surface 231 to inner surface 232. Ports 238 are axially positioned below ports 138, 139 and extend radially from outer surface 231 to cylindrical surface 236i. Ports 238 are disposed at the same axial position along outer housing 230 and are uniformly angularly spaced about axis 235. Similar to ports 138, 139, ports 238 allow fluid communication between the annulus 27 outside shock tool 220 and through passage 233 of outer housing 230.

[0074] Referring again to Figures 10 and 1 1 , mandrel assembly 250 is substantially the same as mandrel assembly 150 previously described with the exception that mandrel assembly 250 includes an additional washpipe at its lower end that defines an additional static piston and includes a set of drilling fluid ports. Thus, mandrel assembly 250 has a first or uphole end 250a, a second or downhole end 250b opposite end 250a, a radially outer surface 251 extending axially between ends 250a, 250b, and a radially inner surface 252 extending axially between ends 250a, 250b. Inner surface 252 defines a central throughbore or passage 253 extending axially through mandrel assembly 250 (i.e., from uphole end 250a to downhole end 250b). [0075] Mandrel assembly 250 includes a mandrel 160, a tubular member or washpipe 170 coupled to mandrel 160, and an annular static piston 175, each as previously described. However, in this embodiment, mandrel assembly 250 includes a second tubular member or washpipe 270 axially positioned between washpipe 170 and piston 175. Mandrel 160, washpipe 170, washpipe 270, and piston 175 are connected end-to-end and are coaxially aligned with axis 255.

[0076] As best shown in Figure 1 1 , washpipe 270 has a first or uphole end 270a, a second or downhole end 270b opposite end 270a, a radially outer surface 271 extending axially between ends 270a, 270b, and a radially inner surface 272 extending axially between ends 270a, 270b. Inner surface 272 is a cylindrical surface defining a central throughbore or passage 273 extending axially through washpipe 270. Inner surface 272 and passage 273 define a portion of inner surface 252 and passage 253 of mandrel assembly 250. A portion of inner surface 272 extending axially from uphole end 270a includes internal threads that threadably engage the mating external threads provided at downhole end 170b of washpipe 170, thereby fixably securing washpipes 170, 270 end-to-end. With end 170b of washpipe 170 threaded into uphole end 270a of washpipe 270, end 270a defines an annular uphole facing planar shoulder 254 along outer surface 251 .

[0077] Referring still to Figure 1 1 , moving axially from uphole end 270a, outer surface 271 includes a cylindrical surface 274a extending from end 270a, a downhole facing planar annular shoulder 274b, and a cylindrical surface 274c extending from shoulder 274b. A portion of outer surface 271 at downhole end 270b includes external threads that threadably engage mating internal threads at uphole end 170a of washpipe 170. In this embodiment, washpipe 270 includes a plurality of circumferentially-spaced ports 276 extending radially from outer surface 271 to inner surface 272. In particular, ports 276 extend radially from outer surface 271 to cylindrical surface 274c. Ports 276 are disposed at the same axial position along washpipe 270 and are uniformly angularly spaced about axis 255.

[0078] The uphole portion of washpipe 270 has an enlarged outer radius that defines or functions as an annular static piston 275 fixably coupled to mandrel 160. Pistons 175, 275 move axially together with the remainder of mandrel assembly 250. Cylindrical surface 274a defining the radially outer surface of piston 275 slidingly engages cylindrical surface 136g of outer housing 230. A plurality of axially spaced annular seal assemblies 279b are disposed along cylindrical surface 274a and radially positioned between piston 275 and outer housing 230. Seal assemblies 279b form annular seals between piston 275 and outer housing 230, thereby preventing fluids from flowing axially between cylindrical surfaces 236g, 274a of outer housing 230 and piston 275, respectively. As will be described in more detail below, seal assemblies 279b maintain separation of relatively low pressure drilling fluid in fluid communication with annulus 27 via ports 138, 139 and relatively high pressure drilling fluid flowing down drillstring 20 and through mandrel assembly 150. Although piston 275 is integral with washpipe 270 in this embodiment, in other embodiments, the piston 275 may be a distinct and separate annular static piston that is fixably coupled to mandrel assembly 250 along washpipe 270 or uphole of washpipe 270.

[0079] Annular piston 175 is disposed about downhole end 270b of washpipe 270 and extends axially therefrom. In particular, piston 175 is threaded onto downhole end 270b, thereby fixably attaching piston 175 to downhole end 270b. Seal assemblies 179b of piston 175 form annular seals between piston 175 and outer housing 230, thereby preventing fluids from flowing axially between cylindrical surfaces 136i, 179a of outer housing 230 and piston 175, respectively. Seal assemblies 179b maintain separation of relatively low pressure drilling fluid in fluid communication with annulus 27 via ports 238 and relatively high pressure drilling fluid flowing down drillstring 20 and through mandrel assembly 250.

[0080] Referring still to Figure 1 1 , mandrel assembly 250 is disposed within outer housing 230 with mating splines 134, 166 intermeshed and uphole end 250a positioned above end 230a of housing 230. In addition, cylindrical surfaces 136a, 164c slidingly engage with annular seal assemblies 137a sealingly engaging surface 164c of mandrel 160; cylindrical surfaces 136f, 174c slidingly engage with annular seal assemblies 137b sealingly engaging surface 174c of washpipe 170; cylindrical surfaces 136g, 274a slidingly engage with annular seal assemblies 279b sealingly engaging surface 136g of outer housing 230; cylindrical surfaces 136h, 274c slidingly engage with annular seal assemblies 237b sealingly engaging surface 274c of washpipe 270; and cylindrical surfaces 136i, 179a slidingly engage with annular seal assemblies 179b sealingly engaging surface 136i of outer housing 230. As previously described, cylindrical surfaces 136d, 174a are radially adjacent one another, however, seals are not provided between surfaces 136d, 174a. Thus, although surfaces 136d, 174a may slidingly engage, fluid can flow therebetween.

[0081] Shock tool 220 includes first annulus 145 that contains biasing member 180, second annulus 146 that contains floating piston 190, and hydraulic oil chamber 148 extending between seal assemblies 137a proximal uphole end 230a and seal assemblies 196a, 196b of floating piston 190. Annuli 145, 146, biasing member 180, piston 190, and hydraulic oil chamber 148 are each as previously described. In addition, shock tool 220 includes third annulus 147 axially positioned below annulus 146. However, in this embodiment, third annulus 147 extends axially between shoulder 132g and piston 175 and is in fluid communication with ports 238. Still further, in this embodiment, a fourth annulus 148 is provided between outer housing 230 and mandrel assembly 250 and extends axially between shoulders 132e, 132f. Piston 275 is disposed in annulus 148 and divides annulus 148 into a first or uphole section 148a and a second or downhole section 148b. Section 148a extends axially from shoulder 132e to piston 275 and section 148b extends axially from shoulder 132f to piston 275. Ports 139 extend to section 148a, thereby placing section 148a in fluid communication with annulus 27 and the relatively low pressure drilling fluid flowing therethrough. Section 148b is in fluid communication with ports 276 in washpipe 270, thereby placing section 148b in fluid communication with passage 253 and the relatively high pressure drilling fluid flowing therethrough. In this embodiment, section 148b is isolated from the relatively low pressure drilling fluid in annulus 27, section 148a, and annulus 147 via seal assemblies 279b, 237b.

[0082] Referring now to Figures 10 and 1 1 , shock tool 220 operates in a similar manner as shock tool 120 previously described with the exception that shock tool 220 includes two static pistons 175, 275 fixably coupled to mandrel 160, each piston 175, 275 being directly actuated by pressure pulses generated by the pulse generator (e.g., pulse generator 1 10). In particular, downhole end 175b of piston 175 faces and directly contacts the relatively high pressure drilling fluid flowing through passage 253, while uphole end 175a of piston 175 faces and directly contacts the relatively low pressure drilling fluid in annulus 147. In addition, shoulder 274b defining the downhole end of piston 275 faces and directly contacts the relatively high pressure drilling fluid flowing through passage 253 via ports 276 in washpipe 270, while shoulder 254 defining the uphole end of piston 275 faces and directly contacts the relatively low pressure drilling fluid in section 148a. Thus, there is a pressure differential across both pistons 175, 275 fixably coupled to mandrel 160. The pressure differentials across piston 175, 275 generate axial upward forces on pistons 175, 275, which is transferred to mandrel assembly 250 (pistons 175, 275, washpipes 170, 270, and mandrel 160 are fixably attached together end-to- end). During steady state drilling operations where changes in the pressure of drilling fluid in passage 253, annulus 27, section 146b, section 148a, and annulus 147 are gradual (i.e., there are no pressure pulses generated by pulse generator 1 10), the biasing force generated by biasing member 180 acts to balance and counteract the axially upward forces on pistons 175, 275 to maintain shock tool 220 at or near its neutral position. However, under dynamic conditions, such as when pressure pulses generated by pulse generator (e.g., pulse generator 1 10) act on piston 175 and piston 275 (via ports 276 and section 148b of annulus 148), the cyclical increases and decreases in the pressure differentials across pistons 175, 275 generate abrupt increases and decreases in the axial forces applied to pistons 175, 275. The biasing member 180 generates a biasing force that resists the axial movement of pistons 175, 275, however, it takes a moment for the biasing force to increase to a degree sufficient to restore shock tool 220 and mandrel assembly 250 to the neutral position. As a result, the pressure pulses generated by the pulse generator axially reciprocate pistons 175, 275 (and the remainder of mandrel assembly 250 fixably coupled to pistons 175, 275) relative to outer housing 230, thereby reciprocally axially extending and contracting shock tool 220. As pistons 175, 275 move axially relative to outer housing 230, drilling fluid is free to flow between annulus 27 and annulus 147 via ports 238, drilling fluid is free to flow between annulus 27 and section 148, and drilling fluid is free to flow between passage 253 and section 148b via ports 276.

[0083] Embodiments of shock tool 220 offer many of the same potential advantages as shock tool 120 previously described. For example, shock tool 220 is operated via direct actuation of the mandrel assembly 250 - the pressure pulses from the pulse generator (e.g., pulse generator 1 10) act directly on static pistons 175, 275 fixably coupled to mandrel 160. Such direct actuation offers the potential for improved actuation efficiency and responsiveness as compared to indirect actuation (i.e., actuation through a floating piston and hydraulic oil). As another example, in shock tool 220, floating piston 190, hydraulic oil chamber 148, and seal assemblies 137a, 196a, 196b defining the hydraulic oil chamber 148 are isolated from the relatively high pressure drilling fluid flowing down the drillstring and the pressure pulses generated by the pulse generator. Specifically, floating piston 190, the hydraulic oil chamber 148, and seal assemblies 137a, 196a, 196b defining the hydraulic oil chamber 148 are pressure balanced to the annulus 27 of the borehole 26. Thus, floating piston 190, seal assemblies 137a, 196a, 196b, and hydraulic oil chamber 148 are not exposed to the abrupt increases and decreases in the pressure generated by the pulse generator.

[0084] It should also be appreciated that embodiments described herein that include two static pistons that are directly actuated by pressure pulses (e.g., shock tool 220) offer the potential for additional benefits. In particular, such embodiments enhance the net axial force applied to the mandrel assembly (e.g., mandrel assembly 250) as the pressure differentials resulting from differences in the pressure of the drilling fluid pumped down the drillstring, the pressure of drilling fluid in the borehole annulus, and the pressure pulses are applied to both pistons, effectively multiplying the total axial force applied to the mandrel assembly. This may be particularly beneficial when axial reciprocation of the shock tool and drillstring are desired, but the pressure differential is insufficient to actuate a single piston. Although the embodiment of shock tool 120 shown in Figures 2 and 3 includes on static piston 175 disposed along mandrel assembly 150, and the embodiment of shock tool 220 shown in Figures 9 and 10 includes two static pistons 175, 275 disposed along the mandrel assembly 250, in general, any suitable number of static pistons (e.g., static pistons 175, 275) may be disposed along the mandrel assembly (e.g., mandrel assembly 150, 250) to achieve the desired axial force applied to the mandrel assembly by pressure pulses generated by a pulse generator (e.g., pulse generator 1 10). For example, in some embodiments, three, four, or more static pistons may be provided along the mandrel assembly to enhance the net axial force applied to the mandrel assembly.

[0085] As previously described, in many conventional shock tools, pressure pulses generate a pressure differential across a floating piston. The pressure differential acts over the surface area of the piston exposed to the pressure differential to generate a net axial force on the piston. The floating piston moves axially in response to the axial force, the axial movement of the floating piston generates a pressure wave that moves upward through hydraulic oil in a hydraulic oil chamber and acts on an uphole portion of the mandrel to move the mandrel axially relative to the outer housing, thereby inducing the reciprocal axial extension and contraction of the shock tool. The amplitude of the axial reciprocation of the shock tool is a function of the axial force applied to floating piston - the greater the axial force applied to the piston, the greater the amplitude of the axial reciprocation of the shock tool. As noted above, the axial force applied to the floating piston is a function of the pressure differential across the floating piston and the surface areas of the piston exposed to the pressure differential. Thus, the axial force applied to the floating piston, and hence the amplitude of the reciprocal axial extension and contraction of the shock tool, can be increased by increasing the pressure differential across the floating piston and/or increasing the surface areas of the floating piston exposed to the pressure differential.

[0086] Increasing the pressure of the drilling fluid pumped from the surface down the drillstring and through the pulse generator can increase the amplitude of the pressure pulses generated by the pulse generator. Unfortunately, this may not be possible due to upper limits in the drilling fluid pumping capacity of the rig at the surface. Increasing the diameter of the floating piston can increase the surface areas of the floating piston acted on by the pressure differential. Unfortunately, this may not be possible as diameter of the borehole limits the maximum diameter of the shock tool, which in turn limits the maximum diameter of the floating piston.

[0087] In scenarios where there is no ability to increase the pressure of the drilling fluid being pumped down the drillstring through the pulse generator and no ability to increase the diameter of the shock tool (to increase the diameter of the floating piston), it may not be possible to enhance or increase the amplitude of the reciprocal axial extension and contraction of the shock tool. However, embodiments described herein offer the potential to increase the amplitude of the reciprocal axial extension and contraction of a shock tool without increasing the pressure of the drilling fluid being pumped down the drillstring and without increasing the diameter of the shock tool. More specifically, by adding static pistons that are directly actuated by pressure pulses (e.g., moving from shock tool 120 to shock tool 220), the net axial force applied to the mandrel (e.g., mandrel 160) at a given pressure differential across the pistons is increased. [0088] Referring now to Figure 12, an embodiment of a method 300 for increasing the amplitude of the reciprocal axial extension and contraction of a shock tool is shown. In this embodiment, the amplitude of the reciprocal axial extension and contraction of the shock tool is increased by increasing the axial force applied to a mandrel of a shock tool by providing one or more additional annular static pistons fixably coupled to the mandrel assembly of the shock tool. Thus, in this embodiment, the amplitude of the reciprocal axial extension and contraction of the shock tool is increased without increasing the diameter of the shock tool and without the need to increase the pressure of drilling fluid being pumped down the drillstring.

[0089] Beginning in block 301 , a shock tool is selected. Selection of the shock tool may depend on a variety of factors including, without limitation, the drilling conditions and parameters such as the capacity of the mud pumps, the pressure and flow rate of drilling mud during drilling operations, the size (e.g., diameter of the borehole), the pressure pulses generated by a pulse generator (e.g., pulse generator 1 10) disposed along the drill string, and the geometry of the borehole. For example, the diameter of the borehole may dictate the maximum outer diameter of the shock tool. It should be appreciated that the drilling conditions and parameters can be actual conditions and parameters if drilling operations have already begun or anticipated drilling conditions and parameters if drilling operations have not yet begun or are temporarily ceased.

[0090] In embodiments described herein, the shock tool selected in block 301 is similar to shock tool 120 previously described. In particular, the selected shock tool includes has a central axis and ends that define the length L of the shock tool. In addition, the shock tool includes an outer housing (e.g. , outer housing 130), a mandrel assembly telescopically disposed within the outer housing (e.g., mandrel assembly 150), a biasing member (e.g., biasing member 180) disposed about the mandrel assembly within the outer housing, and annular floating piston (e.g., floating piston 190) disposed about the mandrel assembly within the outer housing 130. In addition, the mandrel assembly includes a mandrel (e.g. , mandrel 160) and a first annular static piston (e.g., piston 175) fixably coupled to the mandrel (e.g., with washpipe 170). Due to the axial movement of the mandrel assembly relative to the outer housing during cyclical axial extension and retraction of the shock tool, the length L of the shock tool varies between a maximum with its ends axially spaced apart to the greatest extent and a minimum with its ends axially spaced apart to the smallest extent.

[0091] Moving now to block 302, an amplitude of reciprocal axial extensions and contractions of the selected shock tool at a given pressure differential is determined. The given pressure differential is the actual or anticipated pressure differential acting across the first static piston of the shock tool during the generation of pressure pulses by a pulse generator (e.g., pulse generator 1 10). For clarity and further explanation, the amplitude of reciprocal axial extensions and contractions of the selected shock tool at the given pressure differential determined in block 302 may also be referred to herein as the "actual" amplitude. In embodiments described herein, the pressure differential is the difference between the fluid pressure of a pressure pulse within the mandrel assembly and the fluid pressure outside the housing (based on actual drilling conditions or anticipated drilling conditions). The given pressure differential defines the pressure differential acting across the first static piston of the shock tool, which results in the application of an axial force to the first static piston and the mandrel assembly as previously described. In general, the actual amplitude is equal to the difference between the maximum length of the shock tool and the minimum length of the shock tool at the given pressure differential and can be calculated using techniques known in the art.

[0092] Depending on the drilling conditions and parameters (actual or anticipated), it may be desirable to increase the actual amplitude at the given pressure differential (e.g., in response to the pressure pulses generated by pulse generator 1 10). For example, in drilling a lateral section of a borehole, it may be desirable to increase the actual amplitude to reduce friction between the drillstring and the borehole sidewalk Thus, in block 303, a desired amplitude of reciprocal axial extensions and contractions of the selected shock tool is determined. For purposes of clarity and further explanation, the desired amplitude of reciprocal axial extensions and contractions of the selected shock tool determined in block 303 may also be referred to herein as the "desired" amplitude. Then, in block 304, the desired amplitude from block 303 is compared to the actual amplitude from block 302. If the desired amplitude is less than the actual amplitude, then it is not necessary to increase the amplitude of reciprocal axial extensions and contractions of the selected shock tool. However, if the desired amplitude is greater than the actual amplitude, then the amplitude of reciprocal axial extensions and contractions of the selected shock tool is increased in block 305. In embodiments described herein, the amplitude of reciprocal axial extensions and contractions of the selected shock tool is increased in block 305 by lengthening the selected shock tool, and more specifically, by fixably coupling one or more additional annular static pistons to the mandrel assembly as previously described with respect to shock tool 220 (as compared to shock tool 120). More specifically, the first annular static piston (e.g., piston 175) and each additional annular static piston (e.g., piston 275) coupled to the mandrel assembly experiences substantially the same pressure differential - the pressure differential between the fluid pressure of pressure pulses generated by the pulse generator within the mandrel assembly and the pressure of drilling fluid flowing along the outside of the outer housing, thereby enhancing the net axial force applied to the mandrel assembly.

[0093] While preferred embodiments have been shown and described, modifications thereof can be made by one skilled in the art without departing from the scope or teachings herein. The embodiments described herein are exemplary only and are not limiting. Many variations and modifications of the systems, apparatus, and processes described herein are possible and are within the scope of the disclosure. For example, the relative dimensions of various parts, the materials from which the various parts are made, and other parameters can be varied. Accordingly, the scope of protection is not limited to the embodiments described herein, but is only limited by the claims that follow, the scope of which shall include all equivalents of the subject matter of the claims. Unless expressly stated otherwise, the steps in a method claim may be performed in any order. The recitation of identifiers such as (a), (b), (c) or (1 ), (2), (3) before steps in a method claim are not intended to and do not specify a particular order to the steps, but rather are used to simplify subsequent reference to such steps.