Adjustable stabilizer for directional drilling

A drillstring carries a stabilizer sub above the drill bit for steering or directing drilling. The stabilizer body is rotatably carried by the stabilizer sub, wherein the stabilizer body remains substantially stationary relative to the borehole as the drillstring rotates. At least one stabilizer blade is carried by the stabilizer body, the stabilizer blade being radially extendable from the stabilizer body and into engagement with the sidewall of the borehole. Each stabilizer blade is extendable and retractable from the stabilizer body independently of the others. The stabilizer blades are coupled to the stabilizer body such that the blades are capable of collapse to minimum radial extension if the stabilizer assembly becomes stuck in the borehole.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to apparatus for use in drilling 
directional boreholes. More specifically, the present invention is related 
to stabilizer assemblies carried by a drillstring for altering the 
direction of drilling from vertical. 
2. Background Information 
The earliest efforts to drill directionally for petroleum hydrocarbons 
employed mechanical whipstocks, which were used to deflect a rotating 
drillstring from vertical in a previously vertical wellbore. The chief 
drawback to the use of whipstocks is that directional control of the bit 
and drillstring is lost once the drillstring is kicked off or deflected by 
the whipstock. Additionally, whipstock operations are time-consuming and 
therefore expensive. 
Another method of directional drilling uses a bent or bendable sub with a 
downhole motor or turbine. The bent sub has a bend formed therein to 
position the drill bit a few degrees from the vertical axis of the 
remainder of the drillstring. A downhole motor is coupled between the bent 
sub and drill bit or is incorporated in the bent sub itself. The 
drillstring and downhole motor may be rotated to cause the bit to 
disintegrate formation and drill straight ahead at the same angle and 
azimuth of the existing borehole. When altering the direction of drilling 
is desirable, rotation of the drillstring is stopped and the bit is 
rotated by the drilling motor. This mode of operation is known as the 
"sliding" mode, because the drillstring is sliding rather than rotating 
with respect to the sidewall of the borehole. In the deviated portion of 
the borehole, the drillstring experiences sufficient frictional contact 
with the sidewall of the borehole to make it difficult to apply 
significant weight to the bit, resulting in reduced rates of penetration 
compared with rotary drilling. Examples of bent sub or motor directional 
drilling systems and method are disclosed in U.S. Pat. No. 5,311,953, May 
17, 1994 to Walker; U.S. Pat. No. 5,139,094, Aug. 18, 1992 to Prevedel et 
al; and U.S. Pat. No. 5,050,692, Sep. 24, 1991 to Beimgraben. 
In another directional drilling system and method, a pair of stabilizers is 
provided in the drillstring and are spaced-apart above the drill bit. The 
difference in diameter between the upper stabilizer and the near-bit 
stabilizer, whether adjustable or fixed, and the spacing between the 
stabilizers, provide lateral forces that assist in deflecting the bit from 
the vertical axis of the borehole. Such stabilizer arrangements are 
employed in both rotary drilling and downhole motor arrangements. If the 
stabilizers are adjustable and employed in surface rotation drilling, each 
stabilizer blade must extend from the stabilizer body the same distance to 
maintain symmetry and avoid eccentricity and associated rough running. If 
drilling is accomplished with a drilling motor, no such limitation is 
imposed on the upper stabilizer, above the drilling motor, because it is 
not rotated. Examples of stabilizer arrangements are found in U.S. Pat. 
No. 5,332,048, Jul. 26, 1994 to Underwood et al; U.S. Pat. No. 5,293,945, 
Mar. 15, 1994, to Rosenhauch et al.; U.S. Pat. No. 5,181,576, Jan. 26, 
1993 to Askew et al.; and U.S. Pat. No. 4,754,821, Jul. 1, 1988 to 
Swietlik. 
A variation on the adjustable stabilizer theme is to provide stabilizer 
bodies having fixed stabilizer blades, but having pistons acting between 
the drillstring or stabilizer sub and the fixed stabilizer bodies to 
introduce eccentricities between the upper and lower stabilizers and 
resulting lateral deflection forces. These arrangements require multiple 
piston actuations per revolution of the drillstring and thus present 
mechanical and reliability disadvantages. Examples of such arrangements 
can be found in U.S. Pat. No. 5,038,872, Aug. 13, 1991 to Shirley and U.S. 
Pat. No. 3,593,810, Jul. 20, 1971 to Fields. 
U.S. Pat. No. 4,947,944, Aug. 14, 1990 to Coltman et al. discloses a 
stabilizer that employs electric motors to actuate stabilizer blades 
independently in a stabilizer sub that rotates independently of the 
drillstring to which it is coupled. This permits the stabilizer blades to 
remain stationary relative to the borehole and simplifies the process 
significantly. One drawback to the Coltman device is that is does not 
appear to be collapsible in a "fail-safe" state to a reduced radial 
dimension in the event the stabilizer becomes stuck in the borehole due to 
malfunction. 
A needs exists, therefore, for a directional drilling assembly or system 
for use with an efficient rotating drillstring that permits the driller to 
control precisely the trajectory of the bit during drilling operation and 
that is capable of being withdrawn from the borehole relatively easily in 
the event of malfunction. 
SUMMARY OF THE INVENTION 
It is a general object of the present invention to provide an improved 
assembly for steering a rotating drillstring in a borehole. 
This and other objects of the present invention are accomplished by 
providing a stabilizer sub for attachment into a drillstring proximal to a 
drill bit. A stabilizer body is rotatably carried by the stabilizer sub, 
wherein the stabilizer body remains substantially stationary relative to 
the borehole as the drillstring rotates. At least one stabilizer blade is 
carried by the stabilizer body, the stabilizer blade being radially 
extendable from the stabilizer body and into engagement with the sidewall 
of the borehole. Each stabilizer blade is coupled to the stabilizer body 
or sub in such a manner that the blades can be collapsed to a reduced 
radial dimension upon malfunction or failure of the stabilizer or in the 
event it becomes stuck in the borehole. 
According to the preferred embodiment of the present invention, at least 
three stabilizer blades are spaced apart on the circumference of the 
stabilizer body. Each stabilizer blade is selectively extendable and 
retractable independently of the others. 
According to the preferred embodiment of the present invention, each 
stabilizer blade is carried in a longitudinal slot in the stabilizer body, 
the slot having an inclined bottom such that relative longitudinal 
movement between the stabilizer blade and stabilizer body causes extension 
or retraction of the stabilizer blade. A motor is coupled between each 
stabilizer blade and the stabilizer body to cause relative longitudinal 
movement there between. 
According to the preferred embodiment of the present invention, the 
stabilizer sub includes a fixed stabilizer at an end opposite the drill 
bit. A lead screw couples the motor to the stabilizer blade, wherein 
rotation of the lead screw by the motor cause the relative longitudinal 
movement.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the Figures, and specifically to FIG. 1, a longitudinal 
section view of a borehole 1 having a steering assembly disposed therein 
is depicted. Steering assembly includes a stabilizer sub 3, which is 
conventionally connected by a threaded tool joint into a conventional 
rotary drillstring (not shown). A drill bit 5, of either the fixed or 
rolling cutter variety, is secured to the lowermost end of stabilizer sub 
3. A fixed stabilizer 7 is carried by stabilizer sub 3 and spaced apart 
from bit 5. An adjustable stabilizer 9, including a plurality of 
stabilizer blades 11, is carried by stabilizer sub 3 at its lower end, 
near drill bit 5. Alternatively, upper stabilizer 7 can be an adjustable 
stabilizer, as well, further increasing the versatility of the steering 
assembly according to the present invention. 
FIGS. 2 and 3 are elevation and longitudinal section views, respectively, 
of adjustable stabilizer 9 of the steering assembly according to the 
present invention. A generally cylindrical stabilizer body 13 is coupled 
to the exterior of generally cylindrical stabilizer sub 3 by bearings and 
seals 15, which permit stabilizer body 13 to rotate relative to stabilizer 
sub 3 and retain lubricant in the annular gap there between. According to 
the preferred embodiment of the present invention, at least four 
stabilizer blades 11A, 11B, 11C, 11D are received in longitudinal slots 17 
in stabilizer body 13 and are retained therein by a tongue-and-groove 
arrangement. Each longitudinal slot 17 has an inclined bottom 17A, which 
defines a ramp wherein relative longitudinal movement between the 
stabilizer blades 11A-11D and ramp 17A causes radial expansion or 
retraction of stabilizer blades 11A-11D from stabilizer body 13. 
Associated with each slot 17 is a one-half horsepower electric motor 19. 
Motor 19 rotates a lead screw 21, which engages a ball nut (not shown) 
carried in each stabilizer blade 11A-11D to cause the relative 
longitudinal movement. Because each stabilizer blade 11A-11D is provided 
with its own actuator, in the form of motor 19 and lead screw 21, the 
stabilizer blades are independently extendable and retractable with 
respect to stabilizer body 13. Motors 19 preferably are stepper or servo 
motors adapted to control precisely the rotation of lead screws 21 and the 
extension of each stabilizer blade 11A-11D from stabilizer body 13. 
A microprocessor or control unit 23 is coupled to each motor 19 and carried 
in stabilizer body 13 to control the rotation of motor 19 and lead screw 
21, and thus the extension of stabilizer blades 11A-11D from stabilizer 
body 13. Microprocessor 23 contains conventional means for reading 
position data from encoders associated with each motor 19 to ascertain the 
extension of each stabilizer blade 11A-11D. 
According to the preferred embodiment of the present invention, 
microprocessor or controller 23 and motors 19 are powered by a battery 25 
carried in stabilizer body 13. Alternatively, if the drillstring (or a 
component thereof) is hardwired with a power supply, an inductive coupling 
between a plurality of coils 27 circumferentially spaced in stabilizer sub 
3 and corresponding coils in body 13 can transmit electrical power from 
the drillstring to stabilizer body 13 in a reliable fashion. 
According to the preferred embodiment of the present invention, the 
stabilizer blades are configured to collapse or retract to a reduced or 
minimum radial dimension relative to stabilizer sub 3 in the event that 
stabilizer 9 malfunctions or becomes stuck in the borehole. According to 
the preferred embodiment of the present invention, this is achieved by 
applying sufficient axial force, usually upward or uphole, to stabilizer 9 
through drillstring 1. There are several ways by which stabilizer blades 
can be made to collapse upon application of sufficient axial force. In one 
embodiment, each lead screw 21 is designed to buckle in compression (from 
an uphole force) when stabilizer 9 is subjected to axial sticking loads of 
10,000 pounds per stabilizer blade (or 40,000 pounds total). With the 
stabilizer blades thus detached from motors 19, blades 11A-11D are free to 
slide, aided by gravity and/or the removal of drillstring 3 from borehole 
1, to a position on inclined surface 17A in slot 17 corresponding to a 
reduced or minimum radial extension. 
FIG. 4 illustrates one embodiment of a stabilizer blade 11 that is designed 
to yield upon application of sufficient force. A ball nut or threaded nut 
41 is secured within stabilizer blade 11 by one or more shear pins or 
screws 43, which are designed to yield upon application of sufficient 
axial force. Lead screws 21 are coupled to blades 11 through nuts 41 and 
application of force sufficient to yield shear pins or screws 43 detaches 
blades from nuts 41, lead screws 21, and motors 19, and allows them to 
collapse within slots 17. Similarly, the threads, of a separate nut or 
formed in the blades themselves, can be designed to yield upon application 
of sufficient axial force. 
FIG. 5 illustrates another embodiment of blade 11 in which the blade 
comprises two sections or portions 45, 47, which are held together by 
shear pins or screws 49. Upon application of sufficient axial force, pins 
or screws 49 yield and portions 45, 49 separate. When outer portion 45 
becomes separated from inner portion 47, the effect is a reduction in 
diameter of stabilizer 9 sufficient to permit withdrawal of drillstring 3 
from borehole 1. 
FIGS. 6A-6D are cross section views of borehole 1 and stabilizer body 13 
and blades 11A-11D, taken along section line 6--6 of FIG. 1, depicting 
various configurations of stabilizer blades 11A-11D having varying effects 
on the trajectory of drill bit 5. For convenience, upper stabilizer blade 
is labeled 11A, right stabilizer blade is labeled 11B, bottom stabilizer 
blade is labeled 11C, and left stabilizer blade is labeled 11D. 
In FIG. 6A, stabilizer assembly 9 is configured to drop angle, or reduce 
the amount of deviation or deflection from vertical. In this 
configuration, upper stabilizer blade 11A is extended beyond stabilizer 
body 13 and into contact or engagement with the sidewall of borehole 1, 
while bottom stabilizer blade 11C is fully or near fully retracted. 
According to the preferred embodiment of the present invention, opposing 
stabilizer blades 11A, 11C are extendable to a diameter larger than the 
gage of the bit 5 or borehole 1. Of course, opposing stabilizer blades 
11A, 11C are never simultaneously fully extended to avoid sticking in 
borehole 1. The same applies for opposing stabilizer blades 11B, 11D, 
which, in the drop angle configuration, are extended to an intermediate or 
retracted degree less than the gage of bit 5 and borehole 1. 
In FIG. 6B stabilizer 9 is depicted in a configuration to build angle, or 
increase the amount of deviation or deflection from vertical in borehole 
1. In this configuration, bottom stabilizer blade 11C is fully or near 
fully extended and upper stabilizer blade 11A is fully or near fully 
retracted. Again, right and left stabilizer blades 11B, 11D are extended 
to an intermediate or retracted degree less than the gage of bit 5 and 
borehole 1. 
FIG. 6C illustrates stabilizer 9 in a configuration for turning bit 5 to 
the left in which right stabilizer 11B is extended and left stabilizer 
blade 11D is retracted, permitting changes in the azimuth of bit 5. Upper 
and lower stabilizer blades 11A, 11C are extended to an intermediate or 
retracted degree less than the gage of bit 5 and borehole 1 to hold angle. 
Similarly, FIG. 6D depicts stabilizer 9 in a configuration to turn bit 5 
left in which right stabilizer blade 11D is extended and right stabilizer 
blade 11B is near fully retracted, while upper and lower stabilizer blades 
11A, 11C are extended to an intermediate or retracted degree to hold 
angle. 
While FIGS. 6A-6D depict only four of the configurations of stabilizer 9 of 
the steering assembly according to the present invention, because each 
stabilizer blade 11A-11D is extendable independently of the others, a 
virtually infinite variety of stabilizer configurations and thus bit 
trajectories are possible. Of course, the virtually infinite adjustability 
of stabilizer 9 is made possible by coupling stabilizer body 13 for 
rotation to stabilizer sub 3, wherein it remains substantially stationary 
relative to borehole 1 as the drillstring rotates. This permits the 
differential or asymmetric extension of stabilizer blades 11A-11D, which, 
in turn, permits the wide range of trajectories achieved by the various 
configurations of stabilizer 9. 
Of course, stabilizer body 13 cannot be expected to remain entirely 
stationary with respect to the sidewall of the borehole. Friction between 
the inner diameter of stabilizer body 13 and the outer diameter of 
stabilizer sub 3 is less than that between stabilizer blades 11A-11D and 
the sidewall of the borehole such that stabilizer body 13 makes 
approximately one revolution for each 100 to 500 feet drilled. As this 
slow rotation occurs, upper stabilizer 11A will tend to move toward the 
orientation of right stabilizer 11B and the same is true of stabilizer 
blades 11C and 11D. As the orientation of stabilizer blades 11A-11D 
changes with respect to the sidewall of borehole 1, corrections must be 
made to maintain the trajectory of bit 5 on the desired course. 
A three-axis accelerometer having each accelerometer aligned on orthogonal 
axes is carried by stabilizer body 13 and coupled to microprocessor 23 to 
permit measurement of the inclination angle of stabilizer body 13 and the 
rotational orientation of stabilizer body 13 and blades 11A-11D. 
Microprocessor 23 is programmed to correct for changes in orientation of 
stabilizer sub 13 automatically, or can, through MWD apparatus, 
communicate this information to the surface for appropriate response. If 
MWD apparatus is employed, an AM radio transceiver (not shown) is carried 
by stabilizer body 13 to provide two-way radio communication between 
microprocessor 23 and the telemetry section of the MWD apparatus, which in 
turn may be in communication with the surface through one of several 
conventional telemetry or hardwire techniques. 
Similarly, it is frequently advantageous to purposefully alter the 
configuration of stabilizer 9 to correct for unanticipated changes in bit 
trajectory due to unexpected changes in the formation material, the 
drilling characteristics of bit 5, and the like. Thus, the appropriate 
configuration for stabilizer 9 is determined at the surface is 
pre-programmed into microprocessor 23 or an MWD apparatus in the 
drillstring that is in communication with microprocessor 23. Motors 19, 
lead screws 21, and stabilizer blades 11A-11D then are adjusted 
appropriately for the desired trajectory or trajectory correction. 
FIG. 7 is a flowchart depicting the control sequence and operation of the 
steering assembly according to the present invention. With reference to 
FIGS. 1-5, the operation of the steering assembly according to the present 
invention will be described. First, a bit is made up into a drillstring to 
drill an interval of vertical borehole to the kick-off or deflection point 
at which it is desired to commence directional drilling. If the kick-off 
point is sufficiently shallow so as not to deplete the life of the drill 
bit prior to or shortly after kick-off, the vertical drillstring can 
include stabilizer sub 3, along with fixed and adjustable stabilizers 7, 
9. In the vertical section of the borehole, stabilizer blades 11A-11D are 
fully retracted or positioned at an extension less than the gage of bit 5 
and borehole 1, wherein stabilizers 7, 9 simply function as centralizers. 
At the kick-off point, stabilizer 9 and stabilizer blades 11A-11D are set 
in the configuration adapted for the kick-off trajectory, as reflected at 
step 101 of FIG. 5. The controlled misalignment caused by spaced-apart 
stabilizers 7, 9 causes deflection of stabilizer sub 3 and bit 5 from the 
vertical axis of borehole 1, and directional drilling is commenced. 
As reflected at step 103 of FIG. 5, stabilizer body 13 is monitored by 
microprocessor 23 alone or together with MWD apparatus, which may be in 
communication with the surface, for rotation relative to borehole 1. If 
rotation of stabilizer body 13 is detected, this information is 
communicated to or through microprocessor 23, which takes corrective 
action to readjust the configuration of stabilizer blades 11A-11D to 
compensate for rotation of stabilizer body 13 in borehole 1. 
If no rotation of stabilizer body 13 is detected, at step 105 in FIG. 5, it 
is determined whether a change of trajectory is desired. Such a change in 
trajectory is programmed in microprocessor 23 and triggered by 
measurements from the accelerometers carried by stabilizer body 13, or by 
survey data from an MWD apparatus that indicates a change in trajectory is 
appropriate, or may be communicated to microprocessor 23 via telemetry 
from the surface when there is a surface-detected or monitored indication 
that a change in trajectory is warranted. 
As reflected by the flowchart of FIG. 5, if neither rotation of stabilizer 
body 13 is detected nor is a trajectory charge or correction warranted, 
microprocessor 23 continues to monitor both conditions for appropriate 
response in the event of the occurrence of either condition. 
The present invention provides a number of advantages over prior-art 
steering assemblies and systems. A principal advantage is that the 
steering system is adapted for use with efficient surface-rotation 
drilling techniques and their associated high rates of penetration. The 
steering assembly according to the present invention does not require 
complex hydraulic and mechanical systems to effect deflection of the bit 
or changes in its trajectory during drilling operation. 
The invention has been described with reference to a preferred embodiment 
thereof. It is thus not limited, but is susceptible to variation and 
modification without departure from the scope and spirit of the invention.