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Title:
FORCE STACKING ASSEMBLY FOR USE WITH A SUBTERRANEAN EXCAVATING SYSTEM
Document Type and Number:
WIPO Patent Application WO/2017/106479
Kind Code:
A1
Abstract:
A force stacking assembly for use with an earth boring system that includes a series of actuators (37) that each generate a force, and that are arranged to create a combined force that is cumulative of all of the actuators. The actuators (37) include members (38) that react in response to an applied stimulus, such as from an electrical current or magnetic field. The members (38) are arranged in series in a hollow housing (26), planar bulkheads (30) are transversely mounted in the housing (26). Each of the members (38) have an end axially abutting a corresponding bulkhead (30). Ends of each member (30) distal from its corresponding bulkhead (30) couple to a ram member (32), that in turn couples to a drill bit (44). Energizing the members (38) causes each to exert a force against the ram member (32), which is transferred to the bit (44).

Inventors:
FRASED SCOTT (NL)
BAMFORD BEN (GB)
Application Number:
PCT/US2016/066899
Publication Date:
June 22, 2017
Filing Date:
December 15, 2016
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
SAUDI ARABIAN OIL CO (SA)
ARAMCO SEVICES COMPANY (US)
International Classes:
E21B7/24; E21B4/06; E21B4/12; E21B28/00
Domestic Patent References:
WO2002099242A12002-12-12
WO2016043709A12016-03-24
Foreign References:
US2893692A1959-07-07
US20120241219A12012-09-27
US7740088B12010-06-22
Other References:
None
Attorney, Agent or Firm:
RHEBERGEN, Constance, Gall (P.O. Box 61389Houston, TX, US)
Download PDF:
Claims:
CLAIMS

What is claimed is:

1. A system for excavating within a wellbore comprising:

a drill string;

a housing having an end that couples to the drill string;

actuators in the housing that are selectively extendable and that each have an end coupled with the housing; and

a ram assembly having an end coupled to a drill bit, and that couples to ends of the actuators opposite from the ends of the actuators that couple with the housing, so that when the actuators are selectively extended, the drill bit selectively extends a distance from the drill string.

2. The system of Claim 1, wherein each of the actuators exerts a force onto the ram assembly when selectively extended, and wherein the actuators are arranged in series in the housing such that a sum of the forces is transmitted to the ram assembly.

3. The system of Claim 2, wherein when the actuators are selectively extended, the drill bit is axially displaced an amount substantially equal to axial elongation of one of the actuators.

4. The system of Claim 1 or Claim 2, wherein the actuators comprise members made from an activatable material that elongates in response to applied electricity.

5. The system of Claim 4, wherein the activatable material comprises a substance selected from the group consisting of a piezoelectric material, a magnetorestrictive material, and combinations thereof.

6. The system of any of Claims 1-5, wherein the drill bit comprises an outer bit having an axial bore, and an inner bit that reciprocates within the axial bore in response to the actuators being changed into an activated state.

7. The system of any of Claims 1-6, wherein the actuators are axially elongated when selectively activated.

8. The system of any of Claims 1-7, wherein the housing is hollow and bulkheads are formed within the housing at axially spaced apart locations, and wherein outer peripheries of each of the bulkheads couple with an inner surface of sidewalls of the housing.

9. The system of Claim 8, wherein the ends of the actuators that couple with the housing are in abutting contact with the bulkheads.

10. The system of any of Claims 1-9, wherein planar radial walls are provided inside of the ram assembly, and that extend in a direction transverse to an axis of the ram assembly, and wherein ends of the actuators that couple with the ram assembly abut the radial walls.

11. The system of any of Claims 1-10, wherein the ram assembly coaxially moves within the housing when the actuators are selectively extended.

12. A method of excavating within a wellbore comprising:

rotating a drill string in the wellbore that comprises drill pipe, a drill bit coupled to the drill pipe, and actuators disposed between the drill pipe and drill bit;

generating actuating forces with the actuators by selectively elongating each of the actuators a designated distance; and

imparting a summation of the actuating forces against the drill bit to urge at least a portion of the drill bit away from the drill pipe an urged distance that is substantially the same as the designated distance.

13. The method of Claim 12, wherein the actuators are elongated at a frequency that comprises a resonant frequency selected from the group consisting of a resonant frequency of the drill string and a resonant frequency of a formation that surrounds the wellbore.

14. The method of Claim 12 or Claim 13, wherein selectively elongating each of the actuators a designated distance comprises directing electricity to a magnetorestrictive member disposed in an actuator that axially expands and generates one of the acutating forces.

15. The method of any of Claims 12-14, wherein the portion of the drill bit urged away from the drill pipe comprises an inner bit that is proximate an axis of the drill bit.

16. The method of any of Claims 12-15, wherein at least a portion of the drill bit comprises all of the drill bit, and when at least a portion of the drill bit is urged away from the drill pipe the urged distance, the drill bit is urged into excavating contact with a bottom of the wellbore.

17. A system for excavating within a wellbore comprising:

a bottom hole assembly that selectively couples to a drill string; actuators in the bottom hole assembly that are selectively extendable a designated distance and that each exert a force when extended;

a drill bit coupled with the bottom hole assembly; and

a means for transferring the combined forces exerted by the actuators to the drill bit, and urging the drill bit a distance away from the drill string that is substantially the same as the designated distance.

18. The system of Claim 17, wherein the actuators comprise members having material that is responsive to an application of electricity.

19. The system of Claim 18, wherein the bottom hole assembly further comprises a housing that is coupled with the drill string, and wherein members are arranged in series in the housing, and ends of each of the members are coupled with the housing.

20. The system of Claim 19, wherein the bottom hole assembly further comprises a ram assembly that couples to the drill bit, and wherein ends of the members opposite from the ends that couple with the housing couple to the ram assembly, so that when the members expand, the ram assembly is urged axially a distance that is substantially the same.

Description:
PCT PATENT APPLICATION

FORCE STACKING ASSEMBLY FOR USE WITH A SUBTERRANEAN

EXCAVATING SYSTEM

BACKGROUND

1. Field of the Disclosure

[0001] The present disclosure relates to a system for use with a borehole excavating system that employs reactive materials that selectively generate impulse forces in the excavating system.

2. Description of Prior Art

[0002] Hydrocarbon producing wellbores extend below the Earth's surface where they intersect subterranean formations in which hydrocarbons are trapped. The wellbores generally are created by drill bits that are on the end of a drill string, where typically a drive system above the opening to the wellbore rotates the drill string and bit. Cutting elements on the drill bit scrape or otherwise impact the bottom of the wellbore as the bit is rotated and excavate material from the formation thereby deepening the wellbore. Drilling fluid is typically pumped down the drill string and discharged from the drill bit into the wellbore. The drilling fluid flows back up the wellbore in an annulus between the drill string and walls of the wellbore. Cuttings produced while excavating are carried up the wellbore with the circulating drilling fluid.

[0003] During drilling, cutters or teeth formed on the cutting surfaces of the drilling bits impart forces onto the subterranean formation. The forces include shear forces generated by rotation of the drill bit with respect to the bottom of the borehole. Compressional forces are also transferred between the bit and formation, where the compressional forces are from a combination of the weight of a drill string on which the bit is attached and a column of drilling fluid flowing within an axial bore in the drill string. Except when changing bits due to wear or failure, the bit remains in contact with the formation during drilling of the wellbore.

SUMMARY OF THE INVENTION

[0004] Disclosed herein is an example of a system for excavating within a wellbore and that includes a drill string, a housing having an end that couples to the drill string, actuators in the housing that are selectively extendable and that each have an end coupled with the housing, and a ram assembly having an end coupled to a drill bit, and that couples to ends of the actuators opposite from the ends of the actuators that couple with the housing, so that when the actuators are selectively extended, the drill bit selectively extends a distance from the drill string.

[0005] In an example, each of the actuators exerts a force onto the ram assembly when selectively extended, and wherein the actuators are arranged in series in the housing such that a sum of the forces is transmitted to the ram assembly. In an example when the actuators are selectively extended, the drill bit is axially displaced an amount substantially equal to the axial elongation of a one of the actuators. The members can optionally be made from an activatable material that elongates in response to applied electricity. Examples of activatable material include piezoelectric material, a magnetorestrictive material, and combinations thereof.

[0006] In one embodiment, the bit is made up of an outer bit having an axial bore, and an inner bit that reciprocates within the axial bore in response to the actuators being changed into the activated state. The actuators can be axially elongated when selectively activated. Optionally, the housing can hollow with bulkheads formed in the housing at axially spaced apart locations, and wherein outer peripheries of each of the bulkheads couple with an inner surface of sidewalls of the housing. In this example, the ends of the actuators that couple with the housing are in abutting contact with the bulkheads. In an embodiment, planar radial walls are provided inside of ram assembly, and that extend in a direction transverse to an axis of the ram assembly, and wherein ends of the actuators that couple with the ram assembly abut the radial walls. In an alternative, the ram member coaxially moves within the housing when the actuators are selectively extended.

[0007] Also disclosed herein is a method of excavating within a wellbore and that includes rotating a drill string in the wellbore that includes drill pipe, a drill bit coupled to the drill pipe, and actuators disposed between the drill pipe and drill bit, generating actuating forces with the actuators by selectively elongating each of the actuators a designated distance, and imparting a summation of the actuating forces against the drill bit to urge at least a portion of the drill bit away from the drill pipe an urged distance that is substantially the same as the designated distance.

[0008] The actuators can be elongated at a resonant frequency, such as a resonant frequency of the drill string, or a resonant frequency of a formation that surrounds the wellbore. Selectively elongating each of the actuators a designated distance can involve directing electricity to a magnetorestrictive member disposed in the actuator that axially expands and generates a one of the axial forces. The portion of the drill bit urged away from the drill pipe can be an inner bit that is proximate an axis of the drill bit. In one embodiment, at least a portion of the drill bit is all of the drill bit, and when at least a portion of the drill bit is urged away from the drill pipe the urged distance, the drill bit is urged into excavating contact with a bottom of the wellbore.

[0009] Another example of a system for excavating within a wellbore is described herein and that includes a bottom hole assembly that selectively couples to a drill string, actuators in the bottom hole assembly that are selectively extendable a designated distance and that each exert a force when extended, a drill bit coupled with the bottom hole assembly, and a means for transferring the combined forces exerted by the actuators to the drill bit, and urging the drill bit a distance away from the drill string that is substantially the same as the designated distance. The actuators can include members made up of material that is responsive to an application of electricity. The bottom hole assembly can further include a housing that is coupled with the drill string, and wherein members are arranged in series in the housing, and ends of each of the members are coupled with the housing. In one alternate embodiment, the bottom hole assembly includes a ram assembly that couples to the drill bit, and wherein ends of the members opposite from the ends that couple with the housing couple to the ram assembly, so that when the members expand, the ram assembly is urged axially a distance that is substantially the same.

BRIEF DESCRIPTION OF DRAWINGS

[0010] Some of the features and benefits of the present disclosure having been stated, others will become apparent as the description proceeds when taken in conjunction with the accompanying drawings, in which:

[0011] Figure 1 is a sectional view of an example of a drilling system having actuators for delivering an axial force to a drill bit.

[0012] Figure 2 is a sectional view of an alternate example of the drilling system of Figure 1.

[0013] Figure 3 is an axial view of an example of the drilling system taken along lines 3-3 of Figure 1.

[0014] Figures 4A and 4B show an example of the drilling system of Figure 1 respectively in a retracted and an extended configuration.

[0015] Figures 5A and 5B show an example of the drilling system of Figure 2 respectively in a retracted and an extended configuration.

[0016] Embodiments described here are not intended to limit the present disclosure to those embodiments. On the contrary, the present disclosure is intended to cover all alternatives, modifications, and equivalents, as may be included within the spirit and scope of what is described.

DETAILED DESCRIPTION OF THE INVENTION

[0017] The method and system of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings in which embodiments are shown. The method and system of the present disclosure may be in many different forms and should not be construed as limited to the illustrated embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey its scope to those skilled in the art. Like numbers refer to like elements throughout. In an embodiment, usage of the term "about" includes +/- 5% of the cited magnitude. In an embodiment, usage of the term "substantially" includes +/- 5% of the cited magnitude.

[0018] It is to be further understood that the scope of the present disclosure is not limited to the exact details of construction, operation, exact materials, or embodiments shown and described, as modifications and equivalents will be apparent to one skilled in the art. In the drawings and specification, there have been disclosed illustrative embodiments and, although specific terms are employed, they are used in a generic and descriptive sense only and not for the purpose of limitation.

[0019] Shown in a side sectional view in Figure 1 is one example of a drilling system 10 for use in forming a wellbore 12. In this example wellbore 12 intersects formation 14, and a wellbore wall 15 is defined at the intersection of wellbore 12 and formation 14. A drill string 16 is shown projecting into wellbore 12 and which is rotated by a rotary table 18 on surface. Sections of drill pipe 20 may be added on top of drill string 16 with use of a derrick 22 shown mounted over an opening of wellbore 12. Optionally, a top drive (not shown) may be mounted to derrick 22 and used for rotating drill string 16 in lieu of rotary table 18. A bottom hole assembly ("BHA") 24 is shown coupled to drill string 16. BHA 24 is made up of an elongated housing 26 that is hollow and whose outer periphery is made up of sidewalls 27 that extend along a length of the housing 26 and curve around an axis Αχ of BHA 24. The outer surface of sidewalls 27 resembles a cylindrical shape. Inside of housing 26 are elongate compartments 28i -n that are formed in series. The compartments 28i -n are defined between planar bulkheads 30i_ n that project radially between the sidewalls 27 of housing 26 at axially spaced apart locations. A ram assembly 32 is shown coaxially disposed within housing 26, and which has sidewalls 33 that define an outer lateral periphery of the ram assembly 32. Sidewalls 33 of the ram assembly 32 are curved around the axis Αχ of the bottom hole assembly 24 and extend generally parallel with sidewalls 27 of housing 26. Similar to the bulkheads 30i_ n in housing 26, are planar radial walls 34i, 34 2 that extend radially between the sidewalls of the ram assembly 32 at axially spaced apart locations to form compartments 36i_n within ram assembly 32.

[0020] BHA 24 further includes actuators 37i_ n that selectively apply a cumulative force against the housing 26, and an opposing force against ram assembly 32. More specifically, actuators 37i_ n of Figure 1 are made up of reactive members 38i_ n , that in the illustrated embodiment are disposed in housing 26. Further illustrated is that each of the reactive members 38i_ n have an end that is coupled with the housing 26 via contact with an associated bulkhead 30i_ n . Examples of the reactive members 38i_ n include things that change in size or shape. Embodiments exist where the change in size or shape is in response to applied energy, such as electricity or magnetism; or introducing a fluid to the actuators 37i_ n , such as hydraulic or pneumatic. Changes in size include becoming longer, shorter, wider, thinner, or combinations thereof. Example constituents of the reactive members 38i_ n include electro- active materials, magnetostrictive materials, magneto- active materials, lead-zirconate- titanate, lead-magnesium-niobate, terfenol-D, galfenol, and combinations thereof. An opposing end of each of the reactive members 38i_ n couples with the ram assembly 32 via resilient members 40i_ n where each of the resilient members 40i_ n are in contact with the ram assembly 32. In the example of Figure 1, resilient member 40i abuts a drill chuck 42 shown formed on a lower end of ram assembly 32. As will be described in more detail below, ram assembly 32 and drill chuck 42 are recriprocatable with respect to the housing 26 and drill pipe 20 portion of the drill string 16. In the illustrated example, resilient member 4(½ mounts on radial wall 34 resilient member 40 3 mounts on radial wall 34 2 , and resilient member 40 n mounts on radial wall 34 n . Examples of the resilient members 40i -n include springs, Belleville washers, elastomeric members, combinations thereof, and the like. In an alternate embodiment, resilient members 40i -n are not included so that the ends of the reactive members 38i -n directly contact the ram assembly 32.

[0021] A drill bit 44 is shown mounted to drill chuck 42 on an end of drill chuck 42 that is opposite from its connection to ram assembly 32. Drill bit 44 is equipped with cutters 46 on its cutting face for excavating wellbore 12. Further shown in Figure 1, is a controller 48 which connects to a communication means 49 for communicating signals and/or electrical power to the reactive members 38i -n . In one example of operation, reactive members 38i -n respond to applied electrical energy (such as that provided from controller 48 via communication means 49) by elongating, which imparts a force against the housing 26, and another force against ram assembly 32 that is in a direction opposite to the force applied to the housing 26. Embodiments exist where controller 48 includes a power supply (not shown) from which electricity is selectively provided to reactive members 38i -n . In an alternate embodiment, a dedicated power supply 50 is shown with an output line connecting to communication means 49 and through which electricity is routed downhole. An interface 51 between the controller 48 and power supply 50 provides communication from controller 48 to power supply 50 for providing electricity to communication means 49. It should be pointed out that ram assembly 32 is axially movable with respect to housing 26, so that the oppositely directed forces applied by the reactive members 38i -n to the housing 26 and ram assembly 32 causes ram assembly 32 to move axially with respect to housing 26. In one example, the applied forces of the reactive members 38i -n axially urges the ram assembly 32, thereby axially moving drill chuck 42 and drill bit 44 in a direction away from drill string 16 and towards the bottom of the wellbore 12. Further, the axial movement of the drill bit 44 is with respect to the rest of the drill string 16, increases the force exerted by the drill bit 44 against the bottom of wellbore 12 to above that of the weight on bit.

[0022] Thus selectively generating forces against ram assembly 32 with reactive members 38i_n can generate a reciprocating motion of bit 44 against the bottom of wellbore 12, wherein the resultant force is greater than the standard weight on bit that takes place during a normal drilling operation. An advantage of the strategic combination of the reactive members 38i_ n within housing 26 and ram assembly 32 creates a resultant force on the ram assembly 32, and thus drill bit 44, which is cumulative of the forces generated by each of the reactive members 38i_ n . Moreover, the axial displacement of the ram assembly 32 with respect to the rest of the drill string 16 is about that of an axial extension of a single one of the reactive members 38i -n rather than a sum of all of their elongations. In one example, controller 48 energizes actuators 37i_ n at designated intervals of time, and at designated durations of time, so that the frequency at which the bit 44 strikes the bottom of the wellbore 12 is at a designated frequency. Examples of designated frequencies are a resonant frequency of the drilling system 10, a resonant frequency of the rock making up the formation 14, or a combination thereof. Resonance is a phenomenon seen by some cyclical systems, whereby energy from one cycle is stored by the system and used in the next cycle. In one example of the drilling system 10 described herein, recycling of energy between cycles allows for a greater impact force of the percussive elements than could be achieved for a non-resonant percussive system using the same energy input. It is well within the capabilities of one skilled in the art to operate controller 48 so that the actuators 37i_ n are energized at the designated time intervals and durations so the bit 44 strikes the bottom of the wellbore 12 at the designated frequency.

[0023] The high frequency vibration imparted against the formation 14 creates a series of impacts that cause compressive failure of the formation 14 under load, which is in addition to the shear failure caused by rotating the bit 44 while in contact with the formation 14. Tuning the frequency of vibration of the drilling system 10 to a resonance mode increases drilling efficiency above that of operating at a range of different frequencies, or by rotating the drill string 16 alone. An advantage of the arrangement shown is that although the actuators 37i_ n are arranged in series, the resulting force is as though the actuators 37i_ n were in parallel, that is, the resulting force is substantially equal to the sum of force exerted by each of the actuators 37i_ n . Moreover, in an example the axial displacement of the bit 44, due to the cumulative axial displacement of the actuators 37i_ n , is substantially the same as if the actuators 37i_ n are in parallel. In an embodiment, the Young's modulus of the rock making up the formation 14 can be inferred from the frequency of vibration of the BHA 24, as the stiffness of the rock will have an effect on the resonant frequency of the system 10.

[0024] The velocity of the mass m of the bottom hole assembly 24 changes by Δν during impacts of the oscillator of period x, due to the contact harmonic force F = P d sin(jii/x) which is governed bv Eauation 1. for the changing momentum of the system.

mAv = I P d sin (-^) dt = P d (— j Equation 1.

o

In one example, uie uniaxial compressive strength of a rock is defined as the value of the peak stress sustained by a rock specimen subjected to failure by uniaxial compression. It is the maximum load supported by the specimen during the test divided by the effective contact area subjected to the compression. Thus the compressive strength of the rock;

Us = Pd/A e , Equation 2;

where A e is the effective area, which in an example is assumed to be about 5% of the area of the hole drilled.

mAv = (by substituting Eqn. 2 into Eqn. 1) Equation 3.

[0025] Ai d \ π 2 " ' 44 performs a harmonic motion between impacts, in this example the maximum velocity of the drill bit is V m =Aco, where A is the amplitude of the vibration and is its oscillation frequency in rad/s. Assuming further that the impact occurs when the drill bit 44 has maximum velocity V m and that the drill bit 44 stops during the impact, then Accordingly in this example, the vibrating mass is expressed as:

0.05 2 ¾T

Equation 4.

[0026] The period of the impact, x, in the above expression can be determined by many factors including the material properties of the formation 14 and the bottom hole assembly 24, other factors include the frequency of impacts. In one example of operation, x is estimated to be about 1.0 percent of the period of oscillation, that is, x = 0.01/ . By substituting x into Equation 4 a lower bound estimation of the resonant frequency that can aj se f or ^ j m p acls i s given by Equation 5 as follows.

Equation 5.

[0027] In an example, Equation 5 provides a lower bound estimate for the stable frequency of the oscillator. The use of a frequency too much greater than this lower bound frequency can generate a crack propagation zone in the formation 14 that is in front of the drill bit 44 during operation, which could lead to compromise borehole stability and reduced borehole quality. Moreover, if the oscillation frequency is too large then accelerated tool wear and failure may occur. A scaling/safety factor, S/, with appropriate value less than 1.0 can be applied to the frequency as a precautionary measure.

[0028] The dynamic force, P rf , applied to the oscillation system can be calculated by rearranging Equation 2 and can be expressed as follows:

V d = A e Vs = 7i/4(D e ¾) Equation 6;

where in this example D e is an effective diameter associated with effective area (A e ) of the rotary drill bit 44 which is the diameter, D, of the drill bit 44 scaled according to the fraction of the drill bit 44 which contacts the material being drilled. Thus in this example, the effective diameter, D e , can be defined as:

D e = Equation 7;

where Sc is a scaling factor corresponding to the fraction of the drill bit 44 which contacts the material being drilled. For example, estimating that only 5% of the drill bit surface is in contact with the material being drilled, . An appropriate value of scaling/safety factor can be introduced to the dynamic force, , according to the material being drilled so as to ensure that the crack propagation zone does not extend too far from the drill bit 44, and consequently compromising borehole stability and reducing the borehole quality.

[0029] Another factor to consider is that the resonant frequency changes when drilling through different rock types. The compressive strength can be related to an optimal frequency range. It was therefore considered that the lower frequency range can be in relation to changing rock properties, looking at the right hand side of Equation 5 and introducing a factor, Sf. (Z> 2 U 3 /8Q0QnAm)) < f < S f (D 2 U s /8QQQnAm) Equation 8.

[0030] Referring now to Figure 2, shown in a side sectional view is an alternate example of a drilling system 10A used in forming a wellbore 12A in a formation 14A. In this example, the drilling system 10A includes many of the same elements of the drilling system 10 of Figure 1 , that is, a drill string 16A in the wellbore 12 A, a rotary table 18 A, drill pipe 20 A, a derrick 22A, a BHA 24A having a housing 26A, and sidewalls 27A on the housing 26A. Further making up the BHA 24A are compartments 28Ai_ n in the housing 26A, and bulkheads 30Ai_ n at opposing axial ends of the compartments 28Ai_ n . A generally cylindrically shaped ram assembly 32A is coaxially disposed in the housing 26A having axial sidewalls 33A and radial walls 34Ai_ n that are transversely mounted within sidewalls 33A. Axially between the radial walls 34Ai_ n are compartments 36Ai_ n in which actuators 37Ai_ n are provided and that include reactive members 38A ! _ n . Resilient members 40A ! _ n provided in the compartments 36A ! _ n exert a biasing force against reactive members 38Ai_ n .

[0031] A difference between the embodiments of Figures 1 and 2 concerns the bit 44A. As shown, bit 44A is made up of a main bit 52A having an axial bore 54A extending therethrough. An inner bit 56A is included with the main bit 52 A that reciprocates within bore 54 A. Here, the inner bit 56A has an upstream end that attaches to a lower end of ram assembly 32A via a connecting rod 58A. Thus, in this example, actuating the reactive members 38Ai, 38A 2 , . . ., 38A n generates a resultant force in ram assembly 32 A which transfers only to inner bit 56A to reciprocate it within the main bit 52 A. Further, main bit 52A is shown mounted to a lower end of housing 26A.

[0032] Because housing 26A is not axially motivated by actuators 37Ai_ n , main bit 52A does not axially reciprocate in response to operation of actuators 37Ai_ n and thus generally maintains its axial distance from the lower end of drill string 16A. Instead, main bit 52A is limited to rotation within wellbore 12A, much like a standard drill bit. Further, cutters 60A, 62 A are shown respectively formed on the downhole ends of inner bit of 56A and outer or main bit 52A. In bits that rotate about their axes, the radial speed of the bit, and thus the cutters on the bit, becomes lower with proximity to the bit axis. Meaning the region of a bit proximate its axis is less effective for rotational drilling that regions of the bit distal from the bit axis. An advantage of focusing the axial vibration of the effective bit area towards its inner radius is that when the cutters 60A on the inner bit 56A are out of contact with the formation 14 (due to reciprocation of the inner bit 56A), the amount of cutting force per bit surface area lost is less than that if an outer portion of the bit 44A is moved away from the formation 14. As such, adding the axial vibration and forces on the ensuing rock enhances the operational functionality of the bit 44A of Figure 2. Examples exist where cutters 60A, 62A are formed from composites, such as poly-crystalline diamond.

[0033] Figure 3 is an axial sectional view of an example of the BHA 24 taken along lines 3-3 of Figure 1. In this example, a coil 64 is shown between ram assembly 32 and reactive member 381. As is known, selectively energizing the coil 64 with electricity generates an electrical field that as explained above axially elongates the reactive member 381. Electricity for energizing the coil 64 can be from surface, such as from controller 48 or power supply 50 (Figure 1), from a battery (not shown) included with the bottom hole assembly 24, or from a downhole generator (not shown) that converts fluid flow to electricity. As shown reactive member 38i coaxially inserts into a sleeve 66 that can provide protection/isolation for the reactive member 38i. Further illustrated are supports 68 that extend radially between the ram assembly 32 and housing 26. Annular spaces 70 are defined in the circumferential spaces between adjacent supports 68 and the radial spaces between the ram assembly 32 and housing 26. In an example of operation, drilling fluid flows downhole within the annular spaces 70, and back uphole within an annulus 72 between the outer surface of the housing 26 and walls of the wellbore 12.

[0034] Figures 4A and 4B provide in a side sectional view an example of how the drill bit 44 of the drilling system 10 reciprocatingly contacts the bottom 74 of the wellbore 12, thereby creating fractures in the formation 14. Referring specifically to Figure 4A, here the drill string 16 of the drilling system 10 is disposed in the wellbore 12 in a retracted mode so that the bit 44 is spaced away from a bottom 74 of the wellbore 12. In the retracted mode, the members 38i -n are in an unelongated state. In an example where members 38i -n are magnetostrictive material, the members 38i -n are not energized and electricity from controller 48 or power supply 50 is not being transmitted to the members 38i -n . Referring now to Figure 4B, the members 38i -n are depicted in an elongated state. In an embodiment where the members 38i_ n are made from magnetostrictive material, the elongation can be due to applied electricity, such as from controller 48A or power source 50. In the elongated state of Figure 4B, the members 381, 382, 383, and 38 n , have elongated over their lengths shown in Figure 4A by the respective distances Di, D2, D3, and D n .

[0035] Further illustrated is that the bit 44 has moved a distance DBIT in the wellbore 12. As described above, the movement of the bit 44 is in response to movement of the members 38i -n via the coupling between the members 38i -n and ram assembly 32 (Figure 1). Additionally, in one example, the distances Di, D2, D3, and D n (that can be referred to as designated distances) all have substantially the same value. Further in this example, distance DBIT has a value that is substantially the same as the value of any one of distances Di, D2, D3, and D n . Accordingly, in this example, the novel configuration of the housing 26 and ram assembly 32 results in the distance DBIT not being a sum of the individual distances Di, D2, D3, and D n .

[0036] Further illustrated in Figure 4B are arrows that respectively represent forces F38i, F382, F383, and F38 4 generated by the members 38i -n when being actuated/elongated. Another arrow represents force FBIT which is the force being transmitted to drill bit 44 from elongation of the members 38i -n , and which is substantially equal to a summation of forces F38i, F382, F383, and F38 4 . As indicated above, ends of the members 38i -n couple with the housing 26, and opposing ends of the members 38i -n couple with the ram assembly 32. Thus the ram assembly 32, the attached drill chuck 42, and drill bit 44, are moved away from the housing 26 and drill pipe 20 by elongating the members 38i -n . Strategically coupling the members 38i -n with the ram assembly 32 via the radial walls 34i_ n and housing 26 via the bulkheads 30i -n allows for reciprocation of the drill bit 44 a distance substantially the same as the elongation of individual members 38i -n while also exerting a cumulative force onto drill bit 44 so that its reciprocating force FBIT is substantially the same as the sum of forces F38i, F382, F383, and F38 4 . An advantage of reciprocating the drill bit 44, while also rotating the drill bit 44, is that when the drill bit 44 is reciprocatingly thrust against the bottom 74 of the wellbore 12, fractures 76 are formed in the formation 14 adjacent the bottom 74 of the wellbore 12. The fractures 76 can reduce inherent stresses in the formation 14, which increases the amount of rock removed with each rotation of the drill bit 44, that in turn increases rate of penetration of the drilling operation.

[0037] Figures 5A and 5B show in a side sectional view an example of reciprocating motion of the drill bit 44 A of Figure 2. In the example of Figure 5 A the drill string 16A is in the retracted configuration with the members 38Ai_ n in an unelongated state. Further, the inner bit 56A is spaced upward from the bottom 74A of the wellbore 12A with its cutters 60A out of contact with the bottom 74A, while the main bit 52A is at the bottom 74A of the wellbore 12A and its cutters 62A in rotating contact with the bottom 74A. In an example where members 38Ai_ n include magnetostrictive material, the members 38Ai_ n are not energized and electricity from controller 48A or power supply 50A is not being transmitted to the members 38A ! _ n .

[0038] In the example of Figure 5B, the members 38Ai_ n are depicted in an elongated state. In an embodiment where the members 38Ai_ n are made from magnetostrictive material, the elongation can be due to applied electricity, such as from controller 48A or power supply 50A. In the elongated state the members 38Ai, 38A2, 38A3, and 38A n , have lengthened over that of their lengths in Figure 5 A by the respective distances D IA, D2A, D3A, and D N A. Further illustrated is that the inner bit 56A has moved a distance DBITA with respect to the main bit 52A. In this example the main bit 52A is coupled with the housing 26A by a threaded connection 78A, and unlike the inner bit 56A, the main bit 52A does not reciprocate with movement of the ram assembly 32A. As described above, the movement of the inner bit 56A is in response to movement of the members 38Ai_ n via the coupling between the members 38Ai_ n and ram assembly 32A (Figure 2).

[0039] Additionally, in one example, the distances DIA, I¾A, D3A, and D^ (that can be referred to as designated distances) all have substantially the same value. Further in this example, distance DBITA has a value that is substantially the same as the value of any one of distances DIA, D2A, D3A, and D N A. An advantage to reciprocating a portion of the cutting surface of the bit 44A proximate the axis Αχ is that the portions of the cutting surface proximate the axis Αχ have a reduced excavating effectiveness than those portions of the cutting surface distal from the axis Αχ. The bit 44A therefore can remain substantially effective in excavating even when the inner bit 56A is spaced away from the bottom 74A (Figure 5A). Moreover, the main bit 52A is shown creating fractures 76A in the formation 14A adjacent the bottom 74A, which can improve the excavating efficiency of the bit 44A as a whole.

[0040] In embodiments where the actuators 37i_ n , 37Ai_ n , do not include the members 38i_ n , 38Ai_ n , the distances DBIT, DBITA will be substantially the same as elongation of one of the individual actuators 37i_ n , 38Ai_ n rather than a sum of their distances. Similarly, the corresponding forces F B rr, F B ITA on the bits 44, 44A will be substantially the same as the sum of forces from the extended actuators 37i_ n , 37Ai_ n when the actuators 37i_ n , 37Ai_ n do not include the members 38i_ n , 38A ! _ n .

[0041] The embodiments described above are well adapted to carry out the objects and attain the ends and advantages mentioned, as well as others inherent. While a presently preferred embodiment has been given for purposes of disclosure, numerous changes exist in the details of procedures for accomplishing the desired results. These and other similar modifications will readily suggest themselves to those skilled in the art, and are intended to be encompassed within the spirit of the embodiments disclosed herein and the scope of the appended claims.