U.S. patent application number 13/021726 was filed with the patent office on 2011-07-21 for full gauge milling bottom hole assembly with optimal contact force and build rate capability.
This patent application is currently assigned to Baker Hughes Incorporated. Invention is credited to Christopher W. Guidry, Daniel R. Hart, Reena Thomas, James S. Trahan, Suhas S. Verma.
Application Number | 20110174477 13/021726 |
Document ID | / |
Family ID | 46603228 |
Filed Date | 2011-07-21 |
United States Patent
Application |
20110174477 |
Kind Code |
A1 |
Verma; Suhas S. ; et
al. |
July 21, 2011 |
Full Gauge Milling Bottom Hole Assembly with Optimal Contact Force
and Build Rate Capability
Abstract
A milling bottom hole assembly (BHA) for use in cutting a full
gauge window in a wellbore casing wall, the resultant length of the
window being greater than or equal to the whipstock ramp length. A
milling BHA is described which includes two shaft portions, a
window mill and two bearing mills. The design, which involves
strategically placed bearing mills, allows the milling BHA to stay
on the whipstock ramp for the entire casing window milling
operation and, thereafter, to optimally rapidly build angle and
move laterally away from the whipstock and casing, creating a
significantly long window which allows for easy passage of
directional drilling BHAs through the milled window.
Inventors: |
Verma; Suhas S.; (Austin,
TX) ; Trahan; James S.; (Magnolia, TX) ; Hart;
Daniel R.; (Sugar Land, TX) ; Guidry; Christopher
W.; (Spring, TX) ; Thomas; Reena; (Houston,
TX) |
Assignee: |
Baker Hughes Incorporated
Houston
TX
|
Family ID: |
46603228 |
Appl. No.: |
13/021726 |
Filed: |
February 5, 2011 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
12325184 |
Nov 29, 2008 |
|
|
|
13021726 |
|
|
|
|
60991432 |
Nov 30, 2007 |
|
|
|
Current U.S.
Class: |
166/55.7 |
Current CPC
Class: |
E21B 7/061 20130101;
E21B 29/06 20130101 |
Class at
Publication: |
166/55.7 |
International
Class: |
E21B 29/06 20060101
E21B029/06; E21B 29/00 20060101 E21B029/00 |
Claims
1. A bottom hole assembly for use in milling a window in a wellbore
casing in association with a whipstock with an angled ramp having a
whipstock ramp length, the bottom hole assembly having a bottom
hole assembly length and comprising: a shaft providing a bottom
hole assembly length; a window mill located proximate a lower end
of the shaft; a first bearing mill upon the shaft; a second bearing
mill upon the shaft; the window mill and the second bearing mill
being spaced from each other by a first distance; the first bearing
mill being spaced from the window mill at a second distance that is
from about one-fifth to one-half of the first distance; and the
first distance is from about 75% to about 90% of the bottom hole
assembly length.
2. The bottom hole assembly of claim 1 wherein the first distance
is from about 80% to about 85% of the bottom hole assembly
length.
3. The bottom hole assembly of claim 1 wherein the second distance
is about one-third of the first distance.
4. The bottom hole assembly of claim 1 wherein the first and second
bearing mills are separated from each other by a third distance
that is at least as great as the second distance.
5. The bottom hole assembly of claim 1 wherein the first distance
is greater than the whipstock ramp length.
6. The bottom hole assembly of claim 1 wherein the first bearing
mill has an arrowhead-shaped configuration.
7. The bottom hole assembly of claim 6 wherein: the angled ramp
presents a whipstock scoop angle; the first bearing mill includes a
lower portion which presents a mill blade taper angle; and the mill
blade taper angle is from 1.5 times to 3 times the whipstock scoop
angle.
8. The bottom hole assembly of claim 1 wherein the second bearing
mill presents a cross-section having a substantially flat bearing
surface.
9. The bottom hole assembly of claim 1 wherein the bottom hole
assembly length has a midpoint and wherein the first bearing mill
reaches an upper end of a whipstock during a milling operation
before the midpoint of the bottom hole assembly length reaches the
upper end of the whipstock.
10. The bottom hole assembly of claim 1 wherein the window mill
contacts the whipstock to experience a contact force which
gradually increases until the window mill reaches halfway across
the whipstock ramp and then gradually declines as the first bearing
mill passes a top of the ramp.
11. The bottom hole assembly of claim 1 wherein the shaft is formed
of first and second shaft sections that are threaded together and
wherein: the first bearing mill is carried by the first shaft
section; and the second bearing mill is carried by the second shaft
section.
12. The bottom hole assembly of claim 1 wherein: the second
distance is less than half the length of the ramp; and the first
distance is greater than half the length of the ramp.
13. A window cutting arrangement for forming a window within a
wellbore casing, the window cutting arrangement comprising: a
whipstock to be disposed within the wellbore casing, the whipstock
presenting an angled ramp and having a whipstock ramp length; a
bottom hole assembly for contacting the angled ramp and cutting a
window within the wellbore casing, the bottom hole assembly having
a bottom hole assembly length and comprising: a shaft; a window
mill proximate a lower end of the shaft and operable for cutting a
window in the wellbore casing; a first bearing mill upon the shaft;
a second bearing mill upon the shaft; the window mill and the
second bearing mill being spaced from each other by a first
distance; the first bearing mill being spaced from the window mill
at a second distance that is from about one-fifth to one-half of
the first distance; and the first distance is from about 75% to
about 90% of the bottom hole assembly length.
14. The window cutting arrangement of claim 13 wherein the first
distance is from about 80% to about 85% of the bottom hole assembly
length.
15. The window cutting arrangement of claim 13 wherein the second
distance is about one-third of the first distance.
16. The window cutting arrangement of claim 13 wherein the first
and second bearing mills are separated from each other by a third
distance that is at least as great as the second distance.
17. The window cutting arrangement of claim 13 wherein the first
distance is equal to or greater than the whipstock ramp length.
18. The window cutting arrangement of claim 13 wherein the first
bearing mill has an arrowhead-shaped configuration.
19. The window cutting arrangement of claim 13 wherein the second
bearing mill presents a cross-section having a substantially flat
bearing surface.
20. The window cutting arrangement of claim 13 wherein the bottom
hole assembly length has a midpoint and wherein the first bearing
mill reaches an upper end of the whipstock during a milling
operation before the midpoint of the bottom hole assembly length
reaches the upper end of the whipstock.
21. The window cutting arrangement of claim 13 wherein the first
distance is from about 1.0 to about 1.25 times the whipstock ramp
length.
22. The window cutting arrangement of claim 13 wherein the first
distance is from about 1.15 to about 1.20 times the whipstock ramp
length.
23. The window cutting arrangement of claim 13 wherein: the angled
ramp presents a whipstock scoop angle; the first bearing mill
includes a lower portion which presents a mill blade taper angle;
and the mill blade taper angle is from 1.5 times to 3 times the
whipstock scoop angle.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part of U.S. patent
application Ser. No. 12/325,184 filed Nov. 29, 2008 which claims
priority to provisional patent application Ser. No. 60/991,432
filed Nov. 30, 2007.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] 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.
[0004] 2. Description of the Related Art
[0005] In modern 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
[0006] 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
[0007] 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:
[0008] FIG. 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.
[0009] FIG. 1A is a side view of an exemplary arrowhead-shaped mill
used with the milling BHA shown in FIG. 1.
[0010] FIG. 1B illustrates an exemplary relationship between the
angle of the lower portion of the first bearing mill blades and the
associated whipstock scoop angle.
[0011] FIG. 2 is a side, cross-sectional view of an exemplary
wellbore containing the whipstock, and the milling BHA shown in
FIG. 1, during an initial window cutting stage.
[0012] FIG. 3 is a side, cross-sectional view of the arrangement
depicted in FIG. 2, now with the window cutting operation further
advanced.
[0013] FIG. 4 is a side, cross-sectional view of the arrangement
depicted in FIGS. 2 and 3, now with the window cutting operation
further advanced.
[0014] FIG. 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.
[0015] FIG. 6 is a graph depicting an exemplary contact force on a
window mill as the milling BHA is moved along a whipstock ramp.
[0016] FIG. 7 is a graph depicting exemplary contact forces versus
distance along a whipstock ramp.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0017] FIG. 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.
[0018] 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 FIG. 1A. 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.
[0019] 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 centerlines
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. FIG. 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). FIG. 1B also illustrates an angle "Y" 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 "Y" such
that Y=(1.5 to 3)X.
[0020] 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 flat 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.
[0021] The overall length "L" of the milling BHA 12 (the milling
BHA length) exceeds the longitudinal length "l" of the ramp 34 of
the whipstock 10 (the whipstock ramp length). The second bearing
mill 28 is preferably located at a distance "x" from the window
mill 24 that is from about 1.0 to about 1.25 times the length "l"
of the ramp 34. Most preferably, the distance "x" is about 1.15 to
about 1.20 times the length "l" 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 "x". Most preferably, the distance "d" is about one-third of
the length "x". It is further noted that the spacing ("d1") between
the first and second bearing mills 26, 28 preferably exceeds the
distance "d".
[0022] The distance "x" 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
"x" is from about 80% to about 85% of "L".
[0023] FIGS. 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. FIGS. 2-4 also depict the milling BHA 12 exiting the
primary wellbore 44 along a departure path 46 through the
surrounding earth 48.
[0024] 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 FIG. 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 FIGS. 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 FIGS. 1 and 3) of the milling BHA 12
due to the spacing of the first bearing mill 26 proximate to the
window mill 24.
[0025] During the milling operation, as illustrated by FIG. 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.
[0026] 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. FIG. 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.
[0027] As noted, the first bearing mill 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. FIG. 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.
[0028] FIG. 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 FIG. 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. FIG. 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 FIG. 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. FIG. 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.
[0029] 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.
[0030] 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. FIG. 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).
[0031] Contact forces at defined intervals are experienced by the
mills 26, 28 (which are at drift OD) 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 FIG. 7. FIG. 7
shows that, when the window mill 24 is on the ramp 34, the contact
forces (distance 1-18 in FIG. 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 FIG. 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 FIG. 7). The negative peak on the lower mill 26
contact force (distance 8 in FIG. 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 FIG. 7), which indicates
that the lower 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 FIG. 7) on
the window mill 24 helps establish the desired build rate for the
rat hole that is subsequently drilled.
[0032] 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.
* * * * *