Abstract:
A downhole motor operated by circulating mud fluid in the wellbore is revealed. The motor has nested rotors and is geared to a bit drive. The motor is a dual-rotor pump that is operated as a motor with mud flow through the rotor housing on end connections. The structures of the rotor housing and the rotors can be made of the same material. An angular offset can be incorporated between the centerline of the output of the motor and the bit drive. In the preferred embodiment, the motor output is through a gear located within a bigger gear connected to the bit so as to provide a speed reducer. The gear on the bit shaft is preferably made of spaced rods to mesh with the gear on the motor output shaft. The drive between the rotors and the bit can accommodate angular offsets of a predetermined amount for directional drilling. The design is compact and can be used to drill wellbores as small as about 2½″ in diameter, or even smaller.

Description:
This is a continuation in part of application Ser. No. 08/885,337 Jun. 30, 1997 and issued as U.S. Pat. No. 5,911,284 field Jun. 15, 1999. 
    
    
     FIELD OF THE INVENTION 
     The field of this invention relates to drilling with downhole motors, and more particularly to directional drilling with a downhole motor having a particular transmission design. 
     BACKGROUND OF THE INVENTION 
     Fluid-powered motors have been in use in drilling assemblies in the past. These designs are primarily a fixed stator rotating rotor, which are powered by fluid flow based on the original principles developed by Moineau. Typical of such single-rotor, progressive cavity downhole motor designs used in drilling are U.S. Pat. Nos. 4,711,006 and 4,397,619. The stator in Moineau motors is built out of elastic material like rubber. Other designs have put single-rotor downhole power sections in several components in series, with each stage using a rotor connected to the rotor of the next stage. Typical of these designs are U.S. Pat. Nos. 4,011,917 and 4,764,094. 
     Dual-rotor devices have been used as pumps. U.S. Pat. No. 4,820,135 uses a twin-rotor device which is fluid-operated which has output shafts connected to a downhole pump, which is also of the twin-rotor type, for use in producing low-pressure formations and especially if pumping three-phase media (gas-oil-sand). In essence, the twin-rotor design provides the mechanical energy to rotate another twin-rotor downhole pump to pump formation fluids and gases to the surface. U.S. Pat. No. 4,314,615 illustrates a self-propelled drilling head used in large-bore applications where hydraulic fluid is provided to drive twin-rotor motors through supply and return lines. The motors, through a complex planetary gear system, are connected to a bit. The technology and tools shown in U.S. Pat. No. 4,314,615 are used to drill mining shafts and tunnels. 
     Despite all these prior developments, what has been lacking is a compact design suitable in drilling a typical wellbore which has the desirable features of providing sufficient torque and power to the bit to accomplish the drilling in an expeditious manner. The disadvantages of the single-rotor designs is that they required complex controls to avoid damage if the bit became stuck or if the bit was suddenly picked up while fluid was circulating and the load on the bit relieved. Impurities in the mud were also a problem for the rubber of the stator in this design. Entrained solids and gas were particularly an issue in the reliable operation of the single-rotor, Moineau-type mud motors. Temperature limitations of the Moineau-type mud motor cause unreliable operation, especially for geothermal drilling applications. The control requirements, as well as the output limitations of the single-rotor designs, have been overcome by the present invention, which provides a compact design using a downhole motor having a twin-rotor design which is geared to the bit. 
     In directional drilling in the past, universal joints have been used, as indicated in some of the above-mentioned patents, to connect the output of the single-rotor power section to the drillbit. Universal joints have also been used to accommodate an offset in the motor housing or drillstring to permit directional drilling. One of the advantageous features of the design of the present invention is to provide, in a compact bottomhole assembly, an angular bend which is accomplished through the gearing of the output of the twin rotors to the drive for the bit. The gearing can be accomplished with a speed reduction using a straight cut gear meshing with an open structure comprising of spaced rods which will give long life in the hostile mud environment. Accordingly, complex structures that use universal joints are eliminated in the present design which can optionally provide for a bend angle as required and accomplish the connection between the bit drive and the rotating rotor through a gear system involving the requisite angular offset. By adaptation of a twin-rotor design used primarily in pumping applications, a compact downhole motor has been developed which can run on the circulating mud, with fewer controls, and can be constructed to accommodate directional drilling. Additionally, vibration is eliminated, which is common in Moineau motors due to orbital movements. Therefore, measurement while drilling procedures can be achieved much more accurately and economically with the present invention. Those and other beneficial features of the present invention will become apparent to those of ordinary skill in the art by a review of the specification and the drawings. 
     SUMMARY OF THE INVENTION 
     A downhole motor operated by circulating mud fluid in the wellbore is revealed. The motor has nested rotors and is geared to a bit drive. The motor is a dual-rotor pump that is operated as a motor with mud flow through the rotor housing on end connections. The structures of the rotor housing and the rotors can be made of the same material. An angular offset can be incorporated between the centerline of the output of the motor and the bit drive. In the preferred embodiment, the motor output is through a gear located within a bigger gear connected to the bit so as to provide a speed reducer. The gear on the bit shaft is preferably made of spaced rods to mesh with the gear on the motor output shaft. The drive between the rotors and the bit can accommodate angular offsets of a predetermined amount for directional drilling. The design is compact and can be used to drill wellbores as small as about 2½″ in diameter, or even smaller. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is the sectional elevational view of the twin rotors component of the downhole assembly. 
     FIG. 2 is a continuation of the section view of FIG. 1, showing the bit drive and the bottom end of the rotor, as well as the drive in between. 
     FIG. 3 is a section along lines  3 — 3  of FIG.  1 . 
     FIG. 4 is a section along  4 — 4  of FIG.  1 . 
     FIG. 5 is a section along  5 — 5  of FIG.  2 . 
     FIG. 6 is an alternative embodiment to FIG. 2, showing an angular displacement in the drive between the motor and the bit. 
     FIG. 7 is a sectional view of the transmission of the preferred embodiment. 
     FIG. 8 is a section along lines  8 — 8  of FIG.  7 . 
     FIG. 9 is similar to FIG. 7 but with an offset for directional drilling. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention is illustrated in FIGS. 1 and 2. A top sub  10  is connected to the drilling string (not shown) at thread  12 . Top sub  10  has an inlet path  14  which is in fluid communication with metallic twin rotors  16  and  18 . Metallic rotors can be precision machined and are more durable than Moineau pumps which are more difficult to manufacture and have one non-metallic component that can be subject to excessive wear. The rotors  16  and  18 , although preferably metallic, can be made of other materials which have similar mechanical properties. Rotors  16  and  18  are supported in bushings  20  and  22 , and the bushings  20  and  22  are in turn held in position by an upper bushing plate  24 . Rotors  16  and  18  can be axially supported off of shoulder  46  without radial bearing such as bushings  42  and  44 ,  20  and  22 . In this case, the body  32  provides radial support. As shown in FIG. 3, which is section  3 — 3  of FIG. 1, the bushing plate  24  has openings  26  and  28  which provide fluid communication from inlet  14  into cavity  30  formed by body  32 , which is connected to top sub  10  at thread  34 . The rotors  16  and  18  are disposed in cavity  30  and are in nested arrangement, as shown in FIG.  1 . Accordingly, the inlets  26  and  28  are axial so as to reduce the overall profile of the assembly for drilling of smaller wellbores. Looking further down at the top of FIG. 2, the rotor  16  has an output shaft  36 . Shaft  40  is the extension of rotor  18 . Both shafts  36  and  40  extend, respectively, through bushings  42  and  44 , which are supported by a shoulder  46  on body  32 . 
     Gear  38  is meshed to gear  48  mounted to the drive shaft assembly  50 . Referring to FIG. 5, cavity  30  has end exit ports  52  and  54  which allow the mud pumped from the surface through inlet  14  and openings  26  and  28  to pass through the chamber  30 , which in turn causes rotation of rotors  16  and  18 , and ultimately the fluid exits openings  52  and  54  into passage  56  of the drive shaft assembly  50 . A bit (not shown) is connected at thread  58 . The drive shaft assembly  50  comprises gear sub  60  which, as previously described, has gear  48  mounted internally. A body  62  engages to body  32  at thread  64 . A bushing  66  is inserted into the top end of the body  62  before it is made up at thread  64 . Bushing  66  is a radial bearing which facilitates the rotation of the drive shaft assembly  50 . Thrust transmitted to the drive shaft assembly  50  is taken up in thrust bearing assembly  68 . Thrust bearing assembly  68  is supported in part by bottom sub  70  connected to body  62  at thread  72 . 
     Attached to gear sub  60  at thread  74  is output shaft  76 . In essence, the bottom sub  70  holds the thrust bearing assembly  68  in position and under compression while the assembled drive shaft assembly  50  is supported from body  32  at thread  64 . A lower bushing  80  acts as a radial bearing and is retained between the beveled washer  82 , which is in turn supported off of shoulder  84  on output shaft  76  and the inner race of the thrust bearing  68 . 
     As previously stated, flow through the rotor section past rotors  16  and  18  ultimately enters passage  56  where it ultimately goes into the bit (not shown) and into the wellbore to assist in the removal of cuttings during the drilling operation. 
     FIG. 6 is an alternative embodiment to the lower end design shown in FIG.  2 . The components are essentially the same, except that the body  32 ′ now has an offset angle between the longitudinal axis of the rotors  16  or  18  shown schematically as  86  and the longitudinal axis of the drive shaft assembly  50 ′ which is shown schematically as  88 . To compensate for the offset angle formed between the longitudinal axes  86  and  88 , the gear  38 ′ meshes with the gear  48 ′ at the desired angle offset between longitudinal axes  86  and  88 . Gears  38 ′ and  48 ′ are preferably of the internal crossed-axis helical gear type which permit such offset angles. In the preferred embodiment, the offset angle for directional drilling is between less than 1° to 10°. However, greater or smaller angles of offset can be designed without departing from the spirit of the invention. In this design, the angular offset is predetermined when the assembly is constructed so that it can be put together in the manner illustrated in FIG. 6 with a predetermined angle built into housing  32 ′. Those skilled in the art will appreciate that a reconfiguration of the gears  38 ′ and  48 ′ can allow different angles of deviation to be used between longitudinal axes  86  and  88 . Accordingly, the assembly could potentially be constructed with a mechanism in the body  32 ′ to allow a reconfiguration of the entire assembly for a deviation angle which could be functional with a gear set  38 ′ and  48 ′. Thus, there exists a potential for variability in the offset angle between axes  86  and  88  by providing a joint in the body  32 ′ which can assume different angles and a gear set compatible with the angle selected. 
     One of the advantages of the system of the present invention is that the circulating mud with any entrained solids or trapped gases can be used as the driving force for rotating the bit with the drive shaft assembly  50 . The connections within the body  32  to the rotors  16  and  18  are in axial alignment with the remainder of the assembly to give it a low profile. The nesting of gears  38  and  48  allows for a speed reduction which is determined by the needs of the particular installation. However, the nesting arrangement further reduces the profile of the entire assembly to facilitate drilling small wellbores. As opposed to some of the previous designs described above, the present invention does not require a clean circulating system of hydraulic fluid delivered by inlet and outlet lines to a hydraulic motor. Instead, a dual-rotor pump has been adapted as a motor and provided with end connections so that circulating fluid rotates the twin rotors  16  and  18  and power take-off is directly from one of those rotors to the drive shaft assembly  50 . A speed reduction is possible, as is a change in the angle of the drive shaft assembly  50  as compared to the upper section housing the rotors  16  and  18 . This facilitates directional drilling with the apparatus. As contrasted to prior installations involving a single-rotor progressive-cavity-type, Moineau fluid-powered motor, the complex controls of such prior designs are not necessary in this design. Vibrations are eliminated which are common in Moineau motors due to orbital movements. Fortunately, the body  32  and the rotors  16  and  18  can be manufactured from the same material which will allow a self adjustment of thermal expansion or contraction of these parts downhole. The drive shaft assembly  50  is adequately supported and permitted to easily rotate with respect to body  32 . Thrust loads are absorbed back through body  32  through thrust bearing assembly  68 . Universal joint drives are eliminated in favor of a direct drive, taking power output from, for example, rotor  16  into gear  38  which, through a speed reduction nesting arrangement, engages gear  48  of the drive shaft assembly  50 . 
     Referring to FIGS.  7 - 9 , an alternative and preferred embodiment of the transmission for the present invention is illustrated. FIG. 7 shows rotors  100  and  102  in a nested relationship, with gear  104  extending from rotor  100 . The output can also be taken off of rotor  102  without departing from the spirit of the invention. Axial loads from the rotors  100  and  102  are absorbed by the housing  106 . FIG. 7 schematically illustrates a support plate  108  through which extends shaft  110  which connects the nested rotors  100  and  102  to the gear  104 . As shown in FIG. 8, gear  104  has a plurality of straight cut teeth  112  which define valleys  114 . Referring to FIG. 7, the bit shaft  116  is supported in the housing  106  with regard to thrust and radial loading as previously described. Accordingly, a bushing  118  acts as a radial bearing, while a thrust bearing similar to thrust bearing  68  shown in FIG. 2 absorbs thrust loads to isolate the transmission of the present invention from loads imposed due to the drilling operation. Extending from the bit shaft is a plurality of spaced rods  120  defining what functions as a meshing gear. The valleys  114  straddle the rods  120  as the rotors  100  and  102  rotate the gear  104 , causing the speed reduction to take place because the diameter of the circle defined by rods  120  is larger than gear  104 , and gear  104  is nested within rods  120 . As shown in FIGS.  7 - 8 , the rods  120  are elongated members whose circular configuration defines an inner diameter region. In FIG. 8, the gear  104  is depicted on the first side of the inner diameter region. Longitudinal axis of gear  104  does not travel from the first side of the inner diameter region to the opposing side of the inner diameter region when gear  104  is rotated 360°. The desired speed reduction can be a function of the number of teeth  112  on gear  104 , and the corresponding spaces  122  between the rods  120 . Although the rods  120  are shown to be extending from the upper end of the bit shaft having a free end  124 , the free ends  124  can be connected to each other with a ring which would extend above gear  104 . Those skilled in the art will appreciate that the rods  120  will have to be lengthened from the depiction in FIG. 7 to accommodate a ring to connect their tops or free ends  124 . While straight cut teeth  112  are shown on gear  104  and rods  120  on the bit shaft  116 , those skilled in the art will appreciate that a reversal is possible so that a series of rods extend from shaft  110  and mesh with a series of straight cut teeth which would extend from the bit shaft  116 . 
     FIG. 9 shows the design of FIG.  7  and how it can accommodate an angular offset between longitudinal axes  126  and  128 . One of the immediate advantages that can now be appreciated by those skilled in the art is that the circulating mud which drives the nested rotors  100  and  102  can more easily pass through the transmission illustrated in FIGS. 7 or  9 . Flow can occur around the bit shaft  116 , past the bushing  118 , and down to a thrust bearing such as  68  below. A passage is generally available through the thrust bearing out of the housing  106 , as shown in FIG.  2 . Thus, some of the circulating mud will pass through passage  130 , through the bit nozzles while, due to the large open areas between the rods  120 , represented-by spaces  122 , flow will also proceed down annular passage  132 , past the bushing  118 , and down through the thrust bearings below and out of the housing  106 . The use of a gear made of a plurality of rods  120  with spaces  122  therebetween to engage a gear  104  allows for greater durability of the transmission. The large clearances reduce the erosive effects of entrained solids or the flowing fluid such as the circulating mud due to the open spaces  122  which do not materially increase the fluid velocity. Spaces  122  further promote flow down the annular passage  132  for proper lubrication of bushings  118  and thrust bearings below. Significant offsets, as described above, for directional drilling can also be employed in the make-up of the housing  106  to provide the desired skew between axes  126  and  128 . Angles of offset as much as about 10° can be accommodated. An external joint which includes O-rings, as illustrated in some prior designs of transmissions, such as in German patent 41 13986 A1 can be with the present design. Alternatively, the housing can be joined in a manner where a range of skew angles between the bit shaft and downhole motor can be accommodated. With the preferred transmission illustrated in FIGS.  7 - 9 , the compact design is retained, allowing small boreholes to be drilled while significantly increasing the reliability of the assembly to increase run time between servicing of the entire drilling assembly from the downhole motor to the bit. 
     The foregoing disclosure and description of the invention are illustrative and explanatory thereof, and various changes in the size, shape and materials, as well as in the details of the illustrated construction, may be made without departing from the spirit of the invention.