Patent Publication Number: US-6216802-B1

Title: Gravity oriented directional drilling apparatus and method

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to the field of methods and apparatus for drilling a wellbore along a desired trajectory. More specifically, the invention relates to tools for controlling the direction of a wellbore while drilling by rotating a drill pipe from the earth&#39;s surface is in progress. 
     2. Description of the Related Art 
     Wellbores used for petroleum production are often drilled along trajectories other than vertically from the earth&#39;s surface in a process referred to as directional drilling. The main purpose of directional drilling is for the wellbore to penetrate earth formations at a subsurface location different from the surface location from which the wellbore is started. 
     Various tools are well known in the art for directionally drilling wellbores, including hydraulically powered motors which turn a drill bit by converting the flow of drilling fluid (“mud”) into rotational energy, the mud flow otherwise being used to cool the drill bit and lift drill cuttings out of the wellbore. Typical motors designed for directional drilling purposes include a housing which is bent at a preselected angle. These are known in the art as “bent-housing” motors. The power generation section of such motors is coupled to an output shaft, which ultimately turns the bit, by a flexible coupling. When this type of motor is used to adjust the trajectory of the wellbore, the entire drilling assembly, which includes drill pipe, drill collars, the motor, stabilizers and the drill bit, is slowly rotated from the earth&#39;s surface by a rotary drilling rig or similar apparatus so that the bend of the motor housing is oriented in the direction towards which the wellbore trajectory is to be adjusted. As is well known in the art, after the desired trajectory adjustment to the wellbore is finished, the bent-housing motor must be removed from the drilling assembly. This requires a time-consuming “trip out of the hole”, where the entire drilling assembly is removed from the wellbore and a different assembly, which may exclude the bent-housing motor, is inserted into the wellbore to continue drilling along the adjusted trajectory. 
     In other cases, a so-called “steerable” motor can be used both to adjust and to maintain the trajectory of the wellbore during drilling. The typical steerable motor has a bent housing as does the bent-housing motor, but the bend is much smaller in magnitude. Adjusting the trajectory of the wellbore is accomplished with a steerable motor by adjusting the orientation of the motor housing as is done for the bent-housing motor, but when the desired trajectory is achieved, the trajectory can be maintained by rotating the entire drilling assembly from the earth&#39;s surface. Rotating the housing of a steerable motor generally causes the existing trajectory of the wellbore to be maintained. 
     Limitations of mud motor-based directional drilling include limited life of the power-generation section of the typical motor, which includes a positive displacement rotor disposed inside an elastomeric-lined stator. An additional limitation is that orientation of the motor housing can often be difficult to maintain, because as the drill bit contacts the earth formations to drill them, a reactive torque is generated against the motor housing which changes the orientation. A particular limitation of directional drilling using steerable motors is that steerable motors tend to drill a “corkscrew” shaped hole where the motor housing is rotated to maintain trajectory of the wellbore. 
     A different type of steerable rotary tool for directional drilling is presented in U.S. Pat. Nos. 5,484,029 and 5,529,133 to Eddison and U.S. Pat. No. 5,617,926 to Eddison et. al, hereafter collectively referred to as Eddison. This steerable tool comprises an upper housing which connects to the drill pipe and a lower driveshaft which attaches to the drill bit. The housing and driveshaft are coupled so that rotary torque from the housing is transmitted to the shaft while allowing the rotational axis of the bit to pivot universally to a limited degree relative to the longitudinal axis of the housing. Enclosed inside the housing is an internal “eccentric weight” arranged to have relative rotation with respect to said housing. Due to the effects of gravity, the weight remains substantially stationary at the low side of the directional wellbore. The upper end of the driveshaft is coupled to the stationary weight through an eccentric bearing to maintain the bit axis pointed in only one direction as the bit is rotated. Additionally, Eddison discloses an intricate clutch system used to alter the orientation of the drill bit downhole and a measuring-while-drilling (MWD) tool for monitoring directional parameters with respect to the position of the weight. 
     SUMMARY OF THE INVENTION 
     The invention is an apparatus for orienting a drilling assembly. The apparatus includes a first driveshaft coupled to the drilling assembly, a second driveshaft flexibly coupled to the first driveshaft, and an orientation collar disposed outside the first and second driveshafts so that the first and second driveshafts can rotate freely inside the collar while the collar remains rotationally fixed. The collar is substantially coaxial with the first driveshaft. The apparatus includes an adjuster for selecting a center of rotation of the second driveshaft with respect to an axis of the collar so that an axis of rotation of the second driveshaft is selected by changing the position of the center of rotation. 
     In one embodiment, the adjuster includes screws to set the position of a bearing disposed on the outside of the second driveshaft to change the position of a center of rotation of one point on the second driveshaft. In another embodiment, the adjuster includes a sleeve having a bore non-parallel with the axis of the collar, and a rotation and translation mechanism to slide and rotate the sleeve with respect to the collar. The bearing supporting the lower driveshaft is supported in the bore, so that changing the position and rotary orientation of the sleeve changes the relative angle of the second driveshaft with respect to the center line of the collar. 
     The collar in one embodiment is oriented by earth&#39;s gravity because the collar has asymmetric mass about its center line. The collar in various embodiments includes devices to resist rotation of the collar including vanes, mud discharge jets and a sprag which contacts the wellbore wall. 
     In another embodiment, the orientation of the collar is measured, and the relative angle of rotation of the second driveshaft with respect to the center line of the collar is adjusted in response to the measured orientation. In this embodiment, the collar can include various devices to resist rotation thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows one embodiment of the invention in cross-section as it is used to directionally drill a wellbore penetrating earth formations. 
     FIG. 2 shows one embodiment of an orientation collar for the invention, which is an asymmetrically weighted collar. 
     FIG. 3 shows the embodiment of FIG. 2 in cross-section. 
     FIG. 4A shows embodiments of several improvements to the orientation collar of FIG. 2, which reduce the tendency of the asymmetrically weighted collar to rotate as a result of fluid friction between the collar and a driveshaft. 
     FIG. 4B shows another embodiment of an improvement used to reduce rotation of the collar. 
     FIGS. 5A and 5B show an embodiment of a sleeve used to adjust the orientation of the drill bit with respect to asymmetrically weighted collar. 
     FIG. 6 shows an alternative embodiment of the collar. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows one embodiment of the invention as it is to be used to directionally drill a wellbore through earth formations. The wellbore is shown generally at  2  as it has been drilled through the earth formations, shown generally at  4 . The wellbore  2  can be drilled using a rotary drill bit  30  of any type well known in the art. 
     As is well known in the art, rotary power to turn the drill bit  30  can be provided by a drilling rig (not shown) or the like located on the earth&#39;s surface. The drilling rig (not shown) is typically coupled to the drill bit  30  by a drilling assembly which includes sections of threaded drill pipe, one section of which is shown at  6 . As is also well known in the art, the drill pipe  6  can include, generally at the bottom end, larger diameter, high-density sections known as “heavy-weights” or “drill collars” which increase the bottom-end weight of the drilling assembly so that earth&#39;s gravity can assist in providing axial force to the drill bit  30 . A drilling assembly which includes only drill pipe  6 , collars, the bit  30 , and centering tools known as stabilizers, shown generally at  8  and  28 , will follow a trajectory affected by gravity, the flexibility of the drilling assembly and the mechanical properties of the earth formations  4  through which the well is drilled. The rotational axis (not shown) of the drill bit  30  in such drilling assemblies is substantially coaxial with the center line  10  of the drilling assembly, not taking account of any flexibility of the drilling assembly. 
     Directional drilling systems, such as described in the Background section herein, cause the rotational axis (not shown) of the drill bit  30  to be deflected from the center line (rotational axis)  10  of the drill pipe  6  in a selected direction. This embodiment of the invention, shown generally at  32  and for convenience referred to hereafter as a “steering system”, provides an improved mechanism to place the axis of rotation of the drill bit  30  along such a selected direction. 
     The principal components of this embodiment of the steering system  32  include an orientation collar, shown at  16  in FIG.  1 . The purpose of the orientation collar  16  is to provide a rotationally fixed reference against which to set an axis of rotation of the drill bit  30 , as will be further explained. In this embodiment, the orientation collar  16  is an asymmetrically weighted collar (“AWC”), which includes means of bearings  12 ,  18  and  20  to enable free rotation within the orientation collar  16  of an upper driveshaft  14  and a lower driveshaft  24 . As will be further explained, in this embodiment the orientation collar  16  is asymmetric in mass radially or circumferentially about its axis (that is, it is rotationally unbalanced) so that one side of the orientation collar  16  will tend to rest downwardly, that is, in the direction of gravity. The asymmetry of the mass of the orientation collar  16  in this embodiment provides one element of the steering system  32  which is substantially rotationally fixed during drilling. Components which are rotationally coupled to the collar  16 , which will be further explained, will thus be oriented with respect to earth&#39;s gravity. 
     Rotary torque can be transmitted from the drilling rig (not shown) at the earth&#39;s surface directly to the bit  30  through the steering system  32 . The upper driveshaft  14  is coupled at one end to the drill pipe  6 , optionally through upper stabilizer  8 . The upper driveshaft  14  can be flexibly coupled to the lower driveshaft  24  by means of a universal joint, flexible coupling, constant velocity joint or any similar flexible rotary connection, shown generally at  22 , which enables transmission of rotary torque across a change in direction of the axis of rotation. The upper driveshaft  14  rotates substantially collinearly with the drill pipe  6  immediately connected thereto because it is held in position relative to the collar  16  by the upper bearing  12  and center bearing  18 , both of which are positioned substantially collinearly with the collar  16 . The lower driveshaft  24  can be coupled through lower stabilizer  28  to the bit  30 , through a mud motor (not shown) or any other drilling tools. Selection of and location of stabilizers  8 ,  28  in the drilling assembly depends on the intended trajectory of the wellbore as is known in the art, and such placements of stabilizers and drilling tools are not intended to limit the invention. 
     In the invention, the orientation of the axis of rotation of the lower driveshaft  24  with respect to the center line  10  of the orientation collar  16  is generally changed by changing the position of the center of the lower bearing  20  with respect to the center line  10  of the orientation collar  16 , as will be further explained. The orientation of the axis of rotation of the lower driveshaft  24  will thus be determined by the relative position of the lower bearing  20  with respect to the center line  10  of the orientation collar  16 . 
     In one embodiment of the invention, the position of the lower bearing  20  can be changed by means of adjusting screws, shown generally at  26 . The adjusting screws  26  are preferably located in the lower part of the orientation collar  16 , whereby an outer case of the collar  16  serves as a fixture against which to adjust the position of the lower bearing  20 . The means used to adjust the position of the lower bearing  20  with respect to the center of the collar  16  shown in FIG. 1 is only one example of possible adjustment means. The means actually used to adjust the position of the lower bearing  20  with respect to the center of the collar  16  is a matter of convenience for the system designer and is not meant to limit this aspect of the invention. 
     With respect to the example shown in FIG. 1, while the adjuster for setting the position of the lower bearing  20  is fixed, in another aspect of the invention, an adjuster which can be operated while the steering system  32  is in the wellbore  2  can also be used. For example, a sliding sleeve adjuster  34  having an internal bore  29  which is not parallel to the center line  10  of the orientation collar  16  can be used to adjust the center position of the lower bearing  20  with respect to the center line  10  of the collar  16 , as shown in FIGS. 5A and 5B. Using the sliding sleeve  34  having such a non-parallel internal bore  29 , the angular displacement of the bit  30  can be adjusted by axially displacing the sleeve  34  relative to the orientation collar  16  until the desired magnitude of the angle of lower shaft  24  with respect to the center line  10  of the orientation collar  16  is selected. This is illustrated in FIGS. 5A and 5B, wherein the sleeve  34  can be translated from a first position  44 A (FIG.  5 A), to a second position  44 B (FIG.  5 B), thereby adjusting the magnitude of the angle of the lower shaft  24  with respect to the center line  10  of the collar  16 . Rotating the sleeve  34  relative to the collar  16  provides the desired azimuthal orientation of the bit  30  with respect to the orientation of the collar  16 . Mechanisms for translating and rotating the sliding sleeve  34  with respect to the collar  16  are known in the art. One example of such a translation and rotation mechanism is shown in FIGS. 5A and 5B, wherein the sliding sleeve  34  uses worm gears  36  driven by motors M 1 B, M 2 B for translation, and spur gears  38  driven by motors M 1 A, M 2 A in meshing engagement with gear teeth  40  on the collar  16  for rotation. Alternatively, hydraulic actuation or other means may be used. The actual translation and rotation means used with the sliding sleeve  34  are a matter of convenience for the system designer and are not meant to limit this invention. 
     Additionally, the shape of the bore  29  of the sliding sleeve need not be a straight as shown in FIGS. 5A and 5B. Alternatively an increasing-helix radius helical bore (not shown) can be used wherein the sleeve  34  may simply be axially displaced to adjust both the angle magnitude and azimuthal orientation of the lower shaft  24 , and consequently the drill bit  30 , with respect to the center line ( 10  in FIG. 1) of the collar  16 . 
     Referring once again to FIG. 1, alternatively, an hydraulic cylinder type adjuster (not shown) could be disposed between the outer race of the lower bearing  20  and the collar  16  to adjust the center position of the lower bearing  20  with respect to the center line  10  of the collar  16 . 
     The hydraulic cylinder-based adjuster, as well as the sleeve adjuster shown in FIGS. 5A and 5B can be configured, using control circuits well known in the art, to be responsive to measurements from a measurement-while-drilling (MWD) system (not shown) forming part of the drilling assembly, or to be responsive to drilling mud pressure-based command signals sent from the earth&#39;s surface. Such remotely operable adjusters make possible both wellbore trajectory adjustments during drilling, and trajectory maintenance settings where the center of rotation of the lower bearing  20  is set to be axially parallel with the center line  10  of the orientation collar  16 , so that the extant trajectory of the wellbore  2  will be maintained. 
     The orientation collar  16  and components running through it are shown in more detail in FIGS. 2 and 3. In FIG. 2, the collar  16  can include the previously mentioned case  16 A which in this embodiment can be a steel pipe or the like preferably being cylindrically shaped and having an outside diameter comparable to that of the drill pipe ( 6  in FIG. 1) connected to the upper driveshaft  14 . For example, if the portion of the drill pipe ( 6  in FIG. 1) connected to the upper driveshaft is a 6.75 inch (171.45 mm) O. D. “heavy weight” or “drill collar”, then the case  16 A preferably has the same 6.75 inch (171.45 mm) outside diameter to maintain overall stability of the drilling assembly. It should be understood, however, that the shape of and the outside diameter of the case  16 A is a matter of convenience for the system designer and is not meant to limit the invention. The upper driveshaft  14 , as well as the lower driveshaft  24  preferably include a centrally located passage or bore  14 A through which the drilling mud can flow. 
     The inner diameter of the case  16 A, although its actual dimension is not critical to the invention, should preferably be selected to provide a space  14 B for the bearings  12 ,  18 ,  20  between the inner diameter of the case  16 A and the outer diameter of the driveshafts  14 ,  24 . The inner diameter of the case  16 A should also be as small as is practical, as should be the outside diameter of the driveshafts  14 ,  24 , to enable the mass of the collar  16  to be as large, and as asymmetric about the axis of rotation as possible, consistent with the need for of adequate bending stiffness of the driveshafts  14 ,  24  and of the overall drilling assembly, and consistent with the driveshafts  14 ,  24  having the capacity to transmit adequate rotary torque to the bit ( 30  in FIG. 1) without breaking. 
     The case  16 A in this embodiment includes therein a high specific gravity section, shown generally at  16 B. The high specific gravity section  16 B is shown as subtending about half the total circumference of the case  16 A, but it should be understood that the amount of the circumference subtended by the high specific gravity section  16 B is a matter of convenience for the system designer and is not meant to limit the invention. The actual shape of the high specific gravity section  16 B is also a matter of convenience, and the generally cylindrical-section shape shown in FIG. 2 should not be construed as a limitation on the invention. A cross-section of the collar  16 , including the case  16 A, the high specific gravity section  16 B and a corresponding low specific gravity section  16 C, is shown in FIG.  3 . The high specific gravity section  16 B can be formed, for example, by filling the part of the case  16 A with very dense materials such as lead, depleted uranium or the like. The low specific gravity section  16 C may be merely enclosed air space, but preferably includes filling that portion of the case  16 A with a low density, relatively incompressible material, such as oil or aluminum for example, so that the case  16 A will resist crushing under hydrostatic pressure in the passage  14 A and in the wellbore ( 4  in FIG.  1 ). The high specific gravity section  16 B will tend to rest in the direction of gravity, providing a rotationally fixed reference against which to set the position of the lower bearing  20  with respect to the center of the collar  16 . As previously explained, setting the position of the center of the lower bearing  20  at a known location from the center of the orientation collar  16  provides an axis of rotation for the lower driveshaft  24  which is different from the axis of rotation of the upper driveshaft  14  and which is oriented in a known, selected direction with respect to the known rotational reference, i. e. earth&#39;s gravity. 
     Additional features which can reduce the tendency of the orientation collar  16  to be rotated by fluid friction between the driveshafts ( 14 ,  24  in FIG. 1) and the collar  16  are shown in FIG.  4 A. In one such improvement, the low specific gravity section  16 C, where filled with a solid such as aluminum, for example, can include spiral passages  17  therethrough which can be hydraulically connected to the passage ( 14 B in FIG.  2 ). Fluid inertia of the mud flowing in the spiral passages  17  can reduce the tendency of the orientation collar  16  to rotate away from its gravitational orientation. 
     Another such improvement can include helically spaced-apart vanes or fins  19  disposed on the exterior of the case  16 A so that fluid flow up the annulus ( 2  in FIG. 1) will tend to stabilize the rotational position of the collar  16 . 
     Still another improvement can include jets  21  formed through the collar  16  which interconnect the passage ( 14 B in FIG. 2) and the annulus ( 4  in FIG. 1) and have a discharge direction such that drilling mud discharged through the jets  21  will create a thrust tending to oppose fluid-friction induced rotation of the collar  16  in the direction of rotation  23  of the drill pipe ( 6  in FIG.  1 ). 
     Still another example of an improvement to the case  16 A used to resist rotation of the case  16 A while drilling is shown in FIG.  4 B. The case  16 A includes in the heavy weight section  16 B a sprag  19  which can extend by gravity so that friction teeth  21  disposed on the outside of the sprag  19  can contact the wall of the wellbore. Lateral movement of the sprag  19  can be limited by pins  23  loaded by springs  25  to mesh in mating slots  27  in the sprag  19 , the slots  27  being shaped to enable the sprag  19  to move laterally inward, but also to limit lateral outward movement of the sprag  19  from the case  16 A. The sprag  19  shown in this example is actuated by gravity, but it should be clear to those skilled in the art that powered forms of actuation for the sprag, such as hydraulic cylinders, solenoids, springs or the like can also be used to extend the sprag  19  laterally from the case  16 A. 
     The preceding embodiments of the orientation collar  16  rely on earth&#39;s gravity to orient the collar  16 . As previously explained, the orientation of the collar  16  is used as a fixed reference against which to set the position of the bearing supporting the lower driveshaft ( 20  in FIG.  1 ). By setting the position of the lower bearing  20  with respect to the collar  16 , the magnitude and direction of the angle of the second driveshaft can be set with respect to the center line of the collar  16 . In the present embodiment of the collar  16 , the collar  16  need not include asymmetric mass but can have its relative orientation determined by means other than earth&#39;s gravity. Referring now to FIG. 6, the present embodiment of the orientation collar will be explained. The orientation collar in this embodiment is shown generally at  116  and is formed generally in the shape of a cylinder. A first driveshaft  114 , which is similar to the first driveshaft in the other embodiments of the invention, rotates inside the collar  116  on bearings  112 ,  118 . The first driveshaft, as previously explained, is rotated by a drilling rig, mud motor or similar rotary power source. The orientation collar  116  in this embodiment can be symmetric in mass distribution, to enable the collar  116  to be freely rotated irrespective of its orientation with respect to gravity. As in the previous embodiments of the invention, the first driveshaft  114  is flexibly coupled to a second driveshaft  124  through a flexible coupling. The second driveshaft  124  can be supported by a lower bearing (not shown in FIG. 6) disposed in an adjuster mechanism (not shown in FIG. 6) similar to those described in the previous embodiments (such as  20  in FIG.  1 ). In the present embodiment, the adjuster mechanism (not shown) works in substantially the same manner as in the previous embodiments shown in FIGS. 5A and 5B, and for clarity of the description will not be repeated here. 
     In the present embodiment, the first driveshaft  114  can include therein slots or perforations  130  which enable passage of pressurized drilling mud  128  pumped from the surface to flow out of the first driveshaft  114  and pressurize the annular space between the collar  116  and the first driveshaft  114 , axially between bearings  112  and  118 . The pressurized mud is selectively vented to an annular space between the collar  116  and a wellbore (not shown) through discharge jets  132  and  136 . When the pressurized mud is discharged through jets  132 , the collar  116  will tend to rotate in the direction opposite to the mud flow therethrough. Jets  136  are positioned to cause the opposite rotation of the collar  116  when mud is vented therethrough. By selective venting of the pressurized mud through the jets  132 ,  136 , the collar  116  can be rotated to to a selected rotary orientation, and the selected rotary orientation can be maintained. 
     The control of pressurized mud venting through the jets  132 ,  136  can be performed by selectively operable valves  134 A,  134 B, respectively. The valves  134 A,  134 B operate in response to a directional sensor  130 . The directional sensor  130  can be a magnetometer, accelerometers, gyroscope or any other device which makes measurements corresponding to the orientation of the sensor with respect to a fixed reference, such as magnetic north, geographic north or earth&#39;s gravity, for example. The output of the sensor  130  is used to selectively operate the valves  134 A,  134 B to maintain the selected rotary orientation of the collar  116 . Other types of mechanisms for rotating the collar  116  to maintain rotary orientation can be used in place of the jets  132 ,  136 , for example tractor pads or the like. 
     Still other embodiments of the orientation collar need not include the jets  132 ,  136  shown in FIG. 6 or other device to select the orientation of the collar  116 . Instead, the collar  116  may include the anti-rotation devices shown in FIGS. 4A and 4B to maintain the collar  116  in a rotationally fixed position. Where jets or other devices to select a rotary orientation are not used, it is preferable to include a sensor, such as  130  in FIG. 6, to measure the extant orientation of the collar  116 . The orientation and angle magnitude applied to the lower driveshaft  124  by the adjuster (such as  34  in FIGS. 5A and 5B) can be set in response to the measured orientation of the collar  116 , to provide a selected change in direction of drilling the wellbore. Note that in this embodiment, it is not necessary to set the orientation of the collar, it is only necessary to determine the orientation and to set the orientation and angle magnitude of the adjuster (such as  34  in FIG. 5A) in response to the determined orientation. 
     It will be apparent to those skilled in the art that the foregoing description is only intended to illustrate examples of the invention, and that those skilled in the art will be able to devise other embodiments of the invention which do not depart from the spirit of the invention as disclosed in the embodiments described herein. Accordingly, the scope of the invention shall be limited only by the attached claims.