WITH OPTIMAL CONTACT FORCE AND BUILD RATE CAPABILITY

Cross-Reference to Related Applications

[0001] This application is a continuation-in-part of U.S. Patent Application Serial No. 12/325,184 filed November 29, 2008 which claims priority to provisional patent application Serial No. 60/991,432 filed November 30, 2007.

BACKGROUND OF THE INVENTION

Field of the Invention

[0002] The invention relates generally to the arrangement and design of mills on bottom hole assemblies that are used to cut windows in casing strings for the creation of lateral wellbores.

Description of the Related Art

[0003] In modem hydrocarbon production, it is common to create one or more lateral production wellbores which extend outwardly from a central, generally vertical wellbore. In order to form a lateral production wellbore, a window must be cut into the side of casing in the central wellbore. Thereafter, drilling tools are used to form an extended lateral wellbore. Traditionally, whipstocks and milling tools are used to create the window in the central wellbore casing wall. SUMMARY OF THE INVENTION

[0004] The invention provides an improved milling bottom hole assembly (BHA) for use in cutting a window in a wellbore casing wall. An exemplary milling BHA is described which includes a shaft that is made up of two shaft sections. The distal end of the shaft carries a window mill. A pair of bearing mills is carried by the shaft sections above the window mill. Preferably, each of the bearing mills is carried by a different shaft section. Placement of the bearing mills permits the milling BHA to cut a window having a greater length and quality as it allows the milling BHA to stay on the whipstock ramp for the entire milling operation and then exit the ramp and casing rapidly, such that the lateral build rate of the milling BHA away from the whipstock and its anchor is optimum and both risks of casing reentry of the milling BHA and excessive damage to the milling BHA are mitigated. The resultant milled casing exit window is superior for subsequent ingress and egress of long and stiff directional drilling BHAs. A full gauge arrowhead-shaped mill is preferably used for the lower bearing mill. A full gauge watermelon-shaped mill is preferably used for the upper bearing mill. All three mills, the window mill, the arrowhead-shaped mill and the watermelon-shaped mill, present the same full gauge diameter. BRIEF DESCRIPTION OF THE DRAWINGS

[0005] The advantages and further aspects of the invention will be readily appreciated by those of ordinary skill in the art as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference characters designate like or similar elements throughout the several figures of the drawing and wherein:

[0006] Figure 1 is a side, cross-sectional cutaway drawing of an exemplary milling BHA constructed in accordance with the present invention depicted alongside an associated exemplary whipstock. £0007] Figure 1 A is a side view of an exemplary arrowhead-shaped mill used with the milling BHA shown in Figure 1.

[0008] Figure 1B illustrates an exemplary relationship between the angle of the lower portion of the first bearing mill blades and the associated whipstock scoop angle,

[0009] Figure 2 is a side, cross-sectional view of an exemplary wellbore containing the whipstock, and the milling BHA shown in Figure 1, during an initial window cutting stage.

[0010] Figure 3 is a side, cross-sectional view of the arrangement depicted in Figure

2, now with the window cutting operation further advanced.

[0011] Figure 4 is a side, cross-sectional view of the arrangement depicted in Figures

2 and 3, now with the window cutting operation further advanced.

[0012] Figure 5 is a graph depicting the correlation of side forces on the window mill with distance of the window mill from the whipstock kick-off point

[0013] Figure 6 is a graph depicting an exemplary contact force on a window mill as the milling BHA is moved along a whipstock ramp.

[0014] Figure 7 is a graph depicting exemplary contact forces versus distance along a whipstock ramp.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] Figure 1 illustrates an exemplary whipstock 10 and a milling BHA 12, which is constructed in accordance with the present invention. The milling BHA 12 includes a threaded upper end 14 which is used for securing the milling BHA 12 to a drill string 16. The milling BHA 12 includes a shaft 17 formed of upper and lower shaft sections 18, 20, which are secured together at threaded joint 22, and a window mill 24. The window mill 24, of a type known in the art, is secured to the distal end of the milling BHA 12.

[0016] A first bearing mill 26 is located on the lower shaft section 20 above the window mill 24. The first bearing mill 26 is preferably of full gauge and is preferably of an arrowhead-shaped configuration, as illustrated in Figure 1 A. The blades of the first bearing mill 26 present an enlarged, full gauge cutting diameter 25 that is located within the upper half of the length of the mill 26. As a result, the portion 27a of the first bearing mill 26 that is located above the full gauge diameter 25 quickly increases from the mill's shaft 17 diameter radially outwardly to the full gauge diameter 25. The portion 27b of the mill 26 that is located below the full gauge diameter 25 decreases gradually from the full gauge diameter to the diameter of the shaft 17. The tapered lower portion 27b facilitates easy movement and entry of the mill 26 onto a whipstock ramp and reduces chances of getting stuck. Also, the positioning of the cutting structures on the gauge section of the mill 26 allows effective cutting. The tapered lower portion 27b is designed to improve the longevity of the cutting portion of the arrowhead-shaped first bearing mill 26. In an embodiment, the milling BHA 12 with all milling sections at full gauge diameter is designed such that, as the first bearing mill 26 transitions from the primary wellbore 44 into the window 40, the contact forces between the first bearing mill 26 and the surrounding casing 42 are increased.

[0017] Also of note is that the angle of the taper on the lower portion 27b of the arrowhead-shaped first bearing mill 26 is derived from the predicted angular position between the centeriines of the first bearing mill 26 and the whipstock 10 when the first bearing mill 26 transitions from the primary wellbore 44 into the window 40. Because maximum forces are encountered at this transition point, the angle of the taper is such that the surface area on the cutting surface is optimized, damage to the mill 26's cutting structure is minimized, and cutting structure life expectancy is maximized. Figure 1B, depicts an exemplary whipstock scoop angle "X," which is the angle between the vertical axis of the whipstock 10 and the inclination of ramp 34 (i.e. , the whipstock scoop angle). Figure 1B also illustrates an angle "V which is the angle at which the blades of the lower portion 27b of the first bearing mill 26 are disposed from the vertical axis of the milling BHA 12 (i.e., the mill blade taper angle). In a currently preferred embodiment, the angle "X" is derived from angle T such that Y = (1.5 to 3)X.

[0018] A second bearing mill 28 is located on the upper shaft section 18. The second bearing mill 28 preferably presents a cross-section that is curved and oblong, thereby presenting a substantially fiat center segment 30 and arcuately curved end sections 32. The second bearing mill 28 may be of the type generally known in the industry as a "watermelon mill." In an alternate embodiment, the second bearing mill 28 presents a cross-section that is arcuately rounded, in the same manner as the first bearing mill 26. Both the first and second bearing mills 26, 28 extend radially outwardly to full gauge.

[0019] The overall length "L" of the milling BHA 12 (the milling BHA length) exceeds the longitudinal length T of the ramp 34 of the whipstock 10 (the whipstock ramp length). The second bearing mill 28 is preferably located at a distance V from the window mill 24 that is from about 1.0 to about 1.25 times the length T of the ramp 34. Most preferably, the distance "x" is about 1.15 to about 1.20 times the length T of the ramp 34. The first bearing mill 26 is preferably located at a distance "d" from the window mill 24 that is from about one-fifth to about one-half of the length V. Most preferably, the distance "d" is about one-third of the length V. It is further noted that the spacing ("d1") between the first and second bearing mills 26, 28 preferably exceeds the distance "d".

[0020] The distance V of the second bearing mill 28 from the window mill 24 is also preferably from about 75% to about 90% of the overall milling BHA length "L". More preferably, the distance V is from about 80% to about 85% of "L".

[0021] Figures 2, 3 and 4 illustrate the milling BHA 12 in operation to create a window 40 in the casing 42 surrounding a primary wellbore 44. Figures 2-4 also depict the milling BHA 12 exiting the primary wellbore 44 along a departure path 46 through the surrounding earth 48.

[0022] In operation, the drill string 16 and milling BHA 12 are rotated within the casing 42, and the milling BHA 12 is lowered within the wellbore 44 until the milling BHA 12 encounters the whipstock 10 proximate the kick-off point 43. As Figure 2 illustrates, the window mill 24 is urged against the casing 42 and begins to cut the window 40. As the milling operation continues, the window mill 24 cuts downwardly from the upper window end 50 to increase the length of the window 40 (as shown in Figures 3 and 4). At the same time, the incline of ramp 34 urges the window mill 24 laterally outside of the wellbore 44. The lower string section 20 remains substantially rigid between the window mill 24 and the first bearing mill 26. However, due to the substantial distance between the first and second bearing mills 26, 28, the portion of the lower string section 20 above the first bearing mill 26 and the portion of the upper string section 18 below the second bearing mill 28 will bend and flex. The first bearing mill 26 will cut away the upper end 50 of the window 40 during the milling operation, thereby increasing the length of the window 40. It is noted that, as the milling operation progresses, the first bearing mill 26 will reach the upper end of the whipstock 10 before or at the same time as does the mid-point (52 in Figure 1 and 3) of the milling BHA 12 due to the spacing of the first bearing mill 26 proximate to the window mill 24.

[0023] During the milling operation, as illustrated by Figure 4, the flat portion 30 of the second bearing mill 28 will contact the surrounding casing 42 and be urged to remain radially inside of the casing 42. This urging results in additional lateral forces to be imparted to the lower portion of the milling BHA 12, causing the milling BHA 12 to hold against the whipstock 10 for a longer time, thus leading to a longer window 40.

[0024] The design of the milling BHA 12 provides high constraining forces at the window mill 24 while it traverses the midsection of the ramp 34 of the whipstock 10. The use of a milling BHA 12 constructed in accordance with the present invention produces a milled window 40 having an extended length, as measured from the upper end 50 to the lower end 52. The proximity of the first bearing mill 26 to the window mill 24 creates restraining forces on the window mill 24 to urge it properly along the departure path 46 from the primary wellbore 44. Additionally, the proximity of the first bearing mill 26 to the window mill 24 helps in harnessing the efficiency of the cutters of the first bearing mill 26 for additional cutting of the upper end 50 of the window 40. This results in a longer window 40 than with many conventional techniques. Figure 3 depicts the upper end 50 of the window 40 being milled away by the first bearing mill 26. At the same time, the first bearing mill 26 is spaced at an optimum distance from the window mill 24 to avoid an early jump-off of the window mill 24 from the casing 42 near the mid-point of the whipstock ramp 34. [0025] As noted, the first bearing miii 26 preferably has an arcuate cross-section, thereby providing for point-type contact between the bearing mill 26 and the surrounding casing 42 or the whipstock 10. Point-type contact results from the fact that the surface of the curved bearing mill 26 cross-section will contact the surrounding casing 42 or whipstock 10 at a single point. Figure 3 illustrates the mill 26 contacting the casing 42 at point 54. In addition, the milling BHA 12 can pivot with respect to the surrounding casing 42 about the point 54. Binding of the milling BHA 12 as it turns while moving onto the upper end of the whipstock ramp 34 is dramatically reduced as a result of this point-type contact between the first bearing mill 26 and the casing 42. The combination of these advantages results in a longer service life for the milling BHA 12.

[0026] Figure 5 depicts the side forces imparted to the window mill 24 as it is moved along the whipstock ramp 34 from the kick-off point 43. It can be seen by reference to Figure 5 that the side forces imparted to the window mill 24 by the whipstock 10 are kept within a reasonable range throughout the milling operation. Figure 5 is a chart wherein the amount of side force (in kip-force, or klbf) imparted to the window mill (bit) 24 is represented by curve 60. As can be seen, the side forces are within an acceptable limit and are higher at locations along the whipstock ramp 34 where the window mill 24 has maximum chances of early jump-offs. In Figure 5, areas where the curve 60 presents a positive side force (1, 2, 3, 4, etc.) indicate that the window mill 24 is being urged against the ramp 34 of the whipstock 10. Conversely, areas where the curve 60 depicts negative side force (-1, -2, -3, etc.) indicate that the window mill 24 is being diverted away from the ramp 34 of the whipstock 10. Figure 5 indicates that the milling BHA 12 causes the window mill 24 to be continually urged against the ramp 34 until point 62, which generally coincides with the point at which the window mill 24 has moved entirely outside of the casing 42. As a result of this continuous positive side force, the possibility of the window mill 24 tending to undesirably "jump off of the ramp 34 during initial phases of window cutting is minimized. More specifically, when the gauge O.D. of the window mill 24 clears the casing 42, because of which the casing 42 no longer provides a restraining force urging the window mill 24 against the ramp 34, side forces are maximized to compensate for the lost casing-induced restraining force. A thorough finite element analysis of the proposed design predicts the trajectory of the lateral bore hole created in the surrounding earth formation 48 after the window mill 24 has moved past the ramp 34 (i.e., beyond point 62 of curve 60). This analysis shows that the window mill 24 and hence the milling BHA 12 will tend to desirably hold or build an angle that is more normal to the casing 42 than with other milling BHA designs, which tend to drop angle. This improved trajectory is desirable for the subsequent completion of a lateral wellbore using a drilling assembly.

[0027] It can be seen that the milling BHA 12 and the whipstock 10 collectively provide a window cutting arrangement that is operable to form a window in surrounding wellbore casing. It should also be understood that the invention provides an improved method for forming a window within wellbore casing.

[0028] In order to achieve a high build rate, the lower mill 26, which follows the window mill 24, will experience a contact force/restoring force that is in a direction towards the whipstock 10 at the time after the window mill 24 has exited the casing 42. Also, generally the magnitude of the contact force on the lower mill 26 should be equal to or greater than the maximum contact force experienced by the window mill 24. Figure 6 illustrates the contact force upon an exemplary window mill 24 as the milling BHA 12 advances along the ramp 34. The contact force of the window mill 24 against the ramp 34 increases gradually (portion 64) as the window mill 24 enters the whipstock ramp 34. The contact force is substantially constant during portion 66 as the window mill 24 advances to the middle of the ramp 34. Finally, as the window mill 24 exits the ramp 34, the contact force falls gradually (portion 68).

10029] Contact forces at defined intervals are experienced by the mills 26, 28 (which are at drift OO) when they contact the casing 42 as they pass through the deviated well profile. The contact force plots are generated for the window mill 24, lower mill 26 and the upper mill 28. For comparison purposes, these respective contact forces are superimposed on the same plot in Figure 7. Figure 7 shows that, when the window mill 24 is on the ramp 34, the contact forces (distance 1-18 in Figure 7) are positive, which indicates that the window mill 24 is pressing against the whipstock 10. At the same time, the lower mill 26 contact forces are negative, indicating that it is pressing against the casing 42. Projected distance of the positive window mill contact force curve on the x-axis is directly proportional to the length of the window that will be milled. In the case illustrated by Figure 7, the positive force distance is 19 feet. Once the contact force becomes negative, this indicates that the window mill 24 has exited the ramp 34 (distance 18-23 in Figure 7). The negative peak on the lower mill 26 contact force (distance 8 in Figure 7) is seen when the lower mill 26 is just about to enter the whipstock 10. The negative direction also indicates that the lower mill 26 is pressing against the casing 42. It will be appreciated by one of skill in the art that the window mill 24 experiences a contract force that gradually increases until the window mill 24 reaches approximately halfway across the whipstock ramp 34 and then gradually declines as the first bearing mill passes the upper end of the ramp 34. Once the window mill 24 exits the ramp 34, the lower mill 26 experiences positive contact forces (distance 21 in Figure 7), which indicates that the tower mill 26 is now pressing against the ramp 34. A higher magnitude of the positive contact force on the lower mill 26 compared to the negative contact force (distance 21 in Figure 7) on the window mill 24 helps establish the desired build rate for the rat hole that is subsequently drilled.

[0030] The foregoing description is directed to particular embodiments of the present invention for the purpose of illustration and explanation. It will be apparent, however, to one skilled in the art that many modifications and changes to the embodiment set forth above are possible without departing from the scope and the spirit of the invention.