Patent Publication Number: US-8528644-B2

Title: Apparatus and method for milling casing in jet drilling applications for hydrocarbon production

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a National Phase application of International Application No. PCT/US2008/080630, filed Oct. 21, 2008, which claims the benefit of U.S. Provisional Application No. 60/999,723, filed Oct. 22, 2007, both of which are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to apparatus and methods for milling holes in wellbore casings of the type used for hydrocarbon production, and especially those wellbores in which coiled tubing is used to initially lower a milling device and subsequently lower a jet drilling hose to the bottom of the wellbore. In one of its aspects, the invention relates to a method and an apparatus for transferring a known amount of weight to a bit to mill a hole in a wellbore casing. In another of its aspects, the invention relates to a method and an apparatus for milling a hole in a wellbore casing in a relatively quick and cost effective manner. In another of its aspects, the invention relates to a method and an apparatus for milling a hole in a wellbore casing that is deviated. In another of its aspects, the invention relates to a method and an apparatus for milling a hole in a wellbore casing at greater depths than heretofore possible. In another of its aspects, the invention relates to a method and an apparatus for milling a hole in a wellbore casing wherein the skill of the operator in controlling the operating tools is lessened. In another of its aspects, the invention relates to a method and an apparatus for milling a hole in a wellbore casing wherein the tools are less expensive to build and operate. In another of its aspects, the invention relates to a method and an apparatus for milling multiple holes in a wellbore casing without removing the cutting tools from the wellbore. 
     2. Description of Related Art 
     Hydrocarbon wellbore casings often have lateral holes milled in them using a small diameter motor-driven “knuckle” joint drive assembly with a bit on the leading end. The motor used is often a fluid-driven motor known as a mud motor, lowered on the end of standard coiled tubing. Once the holes are milled the milling equipment is removed, and the coiled tubing is subsequently used to lower a jet drilling assembly down to where it can be pushed out through the milled holes to drill into the surrounding well formation. 
     Using a motor-driven knuckle joint drive for the milling operation entails several problems for the operator. The lowered knuckle joint drive assembly is poorly stabilized during the cutting operation, requiring additional time to cut a hole in the casing. Lowering the knuckle drive assembly requires significant skill on the part of the operator, particularly when standard size coiled tubing is used, since the operator has virtually no “feel” over the milling operations and must depend on surface gauges hundreds or thousands of feet above to determine how to control the milling operation. Some of the available torque from the motor is expended on frictional drag resulting from the joint assembly rubbing against the inside of the deflector shoe, or resulting from the coiled tubing rubbing on the inside wall of the production tubing or “work string”, making it even more difficult for the operator to know how much torque is available for the milling operation. Wellbores with increased deviation angle reduce the amount of weight that can be transferred to the bit via the knuckle drive for the milling operation. Small diameter knuckle joint assemblies cannot be used in high angle or horizontal wells; they make it difficult to know how much torque is really reaching the milling bit; and they make it difficult to know when the bit has completed milling a hole in the casing. 
     Alternatives to knuckle drive assemblies exist, but they also have drawbacks. One alternative is jointed pipe with a milling bit on the end, used in conjunction with a whipstock to drill a slot in the side of a wellbore casing. But conventional jointed pipe is time-consuming to put together and take apart on the surface, which is of particularly concern with wells drilled for hydrocarbon production because it results in high operating expense due to labor, rig rental, etc. 
     Another alternative uses coiled tubing to drive a jet nozzle using abrasive cutting fluids to cut a hole in the casing. But abrasive cutting fluid rapidly deteriorates and damages the pumping and metering equipment at the surface. 
     SUMMARY OF THE INVENTION 
     According to the invention, a bit-weighting “sub” assembly is provided at the lower end of coiled tubing during the milling operation, adjacent a mud motor. The sub transfers a known, constant weight to the bit through a rotary drive for the purpose of milling a hole in the casing in a relatively quick, controlled, cost effective manner. 
     In one embodiment, a bit-weighting sub is applied to a known type of milling assembly, for example, a deflector shoe milling assembly including (in order from the lower end up) a production tubing anchor, a deflector shoe with an orientation sub, and a rotary drive with a milling bit. In a preferred embodiment, the rotary drive includes a knuckle joint type drive assembly (hereafter “knuckle drive”). A motor including a Kelly shaft and bushing are lowered on the end of coiled tubing to couple with the knuckle drive and rotate the bit. In a one embodiment, the bit-weighting sub is mounted between the lower end of the coiled tubing and the upper end of the drive motor, and the motor can be considered part of the rotary drive since the bit-weighting sub applies its force to the milling bit through the motor. In another embodiment, the bit-weighting sub is mounted below the mud motor. 
     The bit-weighting sub comprises a spring-driven tubular support that applies a consistent amount of weight to the rotary drive. The bit-weighting sub is activated by lowering the coiled tubing to a “no-go” point in the workstring tubing, where it is stopped by complementary structure in the workstring when the weight of the coiled tubing compresses the bit-weighting sub&#39;s spring a pre-set amount. When the milling operation begins, i.e. when the rotary drive begins rotating the milling bit, the bit-weighting sub spring expands to apply a consistent amount of weight to the milling bit to advance the revolving bit through the casing. The weight of the coiled tubing is accordingly removed from the rotary drive and milling bit and the bit-weighting sub spring force controls the milling operation. 
     In another embodiment, the bit-weighting sub includes a housing, a Kelly-type non-rotating shaft, and a spring. The shaft is shaped to prevent rotation and has, for example, a square or hexagonal cross-section, or any other multi-sided shape that maintains the shaft in a linear path without rotating. Alternatively, the shaft can contain a key or keyway to prevent its rotation. Various types of springs, such as conventional coiled springs, Belleville springs, or leaf springs, can be used. 
     As a further embodiment, the bit-weighting sub can use a hydraulic lift, rather than a spring, to transfer weight to the milling bit. 
     As a preferred embodiment, the bit-weighting sub can be mounted to rotate with the motor and rotary drive when the sub is mounted below the motor. 
     Further according to the invention, a method of milling a hydrocarbon wellbore casing wherein a rotary drive with a milling bit and a motor are lowered by tubing down the wellbore to rotate the milling bit to form a hole in the casing comprises controlling the force exerted on the milling bit through the rotary drive during the milling operation. 
     In one embodiment, the act of controlling the force on the milling bit comprises removing the weight of the tubing from the rotary drive and milling bit after the motor has been landed in operative connection with the rotary drive and is ready to mill a hole in the casing. Further, a milling force is exerted on the milling bit through the rotary drive, independent of the weight of the tubing. Thereafter, the motor is operated to rotate the rotary drive and milling bit under the milling force to form a hole in the wellbore casing. 
     These and other features and advantages of the invention will become apparent from the detailed description below, in light of the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of a casing milling assembly containing the bit-weighting sub according to the invention as it is lowered into a wellbore. 
         FIG. 2  is a view of the casing milling assembly of  FIG. 1  with its rotary drive assembly landed in the deflector shoe before weight is applied to the bit-weighting sub. 
         FIG. 3  is a view of the casing milling assembly of  FIGS. 1 and 2  as weight is applied to the bit-weighting sub in the landed condition of  FIG. 2 , compressing the internal bit-weighting spring prior to the start of milling operations. 
         FIG. 4  is an exploded view of the parts of the bit-weighting sub of  FIGS. 1-3  relative to the lower end of standard coiled tubing. 
         FIG. 5A  is a side elevation view of the bit-weighting sub of  FIGS. 1-4  secured between the lower end of the coiled tubing and the upper end of the mud motor, with the sub spring uncompressed. 
         FIG. 5B  is a side elevation view similar to  FIG. 5A , but showing the coiled tubing in a lower position to weight the sub and compress the spring. 
         FIG. 6  is a schematic side elevation view of a casing milling assembly similar to  FIG. 3  but with an alternate position for the bit-weighting sub, mounted below the mud motor. 
         FIG. 7  is an enlarged side elevation view of a portion of the casing milling assembly of  FIG. 6  and illustrating a preferred no-go structure for the below-motor mounting arrangement of  FIG. 6 . 
         FIG. 8A  is a view similar to  FIG. 5A , but schematically illustrates a hydraulic force-exerting structure in place of a spring in the bit-weighting sub. 
         FIG. 8B  shows the hydraulic bit-weighting sub of  FIG. 8A  with the hydraulic force-exerting structure in a bit-weighting condition. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings and to  FIG. 1  in particular, a deflector shoe milling assembly  20  is mounted in a hydrocarbon wellbore  10  for milling wellbore casing  12  using a knuckle drive  22  with a bit  24  on its leading end inside a deflector shoe  21 . The deflector shoe  21  is anchored relative to casing  12  using a tubing anchor  18 . Deflector shoe  21  has an orientation sub  26  on its upper end to receive a mud motor  30  and a motor-driven Kelly drive shaft  32  that engages a Kelly bushing  28  of known type on the upper end of the knuckle drive  22 . Mud motor  30  is lowered into the wellbore from surface  14  on the end of standard coiled tubing T (visible in  FIGS. 2 and 3 ) until shaft  32  is operatively coupled to knuckle drive  22 , and then fluid pumped from the surface drives the motor to rotate the bit  24  through a rotary drive that includes the shaft  32  and knuckle drive  22  to cut a hole in casing  12 . 
     The deflector shoe milling assembly  20 , which is not part of the present invention, and which can be the type disclosed in WO 2007/067544, which is incorporated herein by reference in its entirety, is used to re-orient the milling bit for milling multiple holes in the casing  12  at the anchored depth. It will be recognized that alternative devices for orienting the knuckle drive  22  and milling bit  24  relative to casing  12  known in the art and can be used for the milling operation. Alternative devices for applying rotary power to the milling bit will also be known, for example, using a turbine drill with a speed reducer in lieu of a mud motor. Various modifications to the rotary drive can be made, for example, placing the Kelly shaft in the deflector  20  and the mating Kelly bushing above. The knuckle drive  22  can be coupled to the mud motor at the surface and lowered into the deflector, instead of residing in the deflector. The invention is believed to be suitable for use with these and other such alternatives and modifications to the structural environment in which a rotary drive is lowered on tubing to rotate a milling bit against the wellbore casing to form a hole, and should not be limited to the specific milling assembly shown in the illustrated example. 
     The milling assembly described up to this point is already known and further detail will be omitted as being unnecessary for an explanation of the invention. 
     The present invention resides in a “weight on bit” or bit-weighting sub  40  associated with the mud motor  30  on the end of the coiled tubing. In  FIG. 1 , the bit-weighting sub  40  is supported by the coiled tubing T and is positioned above the mud motor  30  and knuckle-driving Kelly shaft  32 . 
     Referring to  FIGS. 2 and 3 , coiled tubing T is shown lower in the well casing  10  to land the mud motor  30  and Kelly shaft  32  in rotary driving engagement with knuckle drive  22  in deflector shoe  20 . In the illustrated example, the knuckle drive  22  rests in the deflector shoe  21  and is disengaged from the mud motor  30 . The coupling between the Kelly shaft  32  and knuckle drive  22  can be a locking mechanism, for example, using known locking dogs. Alternately, the coupling can be made non-locking by leaving the typical spring-loaded dogs out of the assembly, disconnecting the motor  30  from the knuckle drive  22 , for example, when the motor is removed by the coiled tubing to re-orient the milling bit, or for a subsequent jet drilling operation through the newly milled hole in casing  12 . Mud motors and Kelly shaft/bushing structures and equivalents for giving rotary motion to knuckle drives are well known in the art, and further detail will be omitted. 
     As shown in  FIGS. 2 and 3 , bit-weighting sub  40  is connected between the lower end of coiled tubing T and the upper end of mud motor  30 . Sub  40  includes a sliding, non-rotating hex Kelly shaft  54  connected in fixed manner to the upper end of the mud motor  30 , for example, with a threaded connection or set screws or pins, and a bit-weighting spring  46  between the mud motor  30  and the coiled tubing T. Kelly shaft  54  is mounted to slide up and down a limited distance within the bit-weighting sub&#39;s housing  56 .  FIG. 2  shows the spring  46  in an uncompressed state, just as the mud motor  30  and knuckle-driving Kelly  32  land in a landing profile of orientation sub  26  to couple with knuckle drive  22  in deflector shoe  20 .  FIG. 3  shows sub spring  46  compressed as weight from the coiled tubing T is set down on the bit-weighting sub; i.e., as the coiled tubing is lowered further from the position in  FIG. 2 , until a no-go projection  60  on the lower end of the bit-weighting sub&#39;s housing  56  abuts a no-go profile  16   a  in tubing  16 , positively stopping the upper end of sub  40  (and thus the coiled tubing T) from being lowered any further. The no-go profile  16   a  can be a ring or a series of circumferentially spaced projections welded or otherwise fastened to the interior surface of the production tubing  16  before the production tubing is lowered into the well casing  10 . 
     Once the mud motor  30  is landed and bit-weighting sub spring  46  is compressed against the stationary knuckle drive  22  as shown in  FIG. 3 , fluid can be pumped down the coiled tubing to drive (rotate) the mud motor  30  in known manner to begin rotating the bit  24  on the end of knuckle drive  22 . It may be preferred to slightly lift the coiled tubing T from this position before starting motor  30 , for example, a few inches, and then lower it back down to begin milling a hole through casing  12 . 
     FIGS.  4  and  5 A- 5 B illustrate the bit-weighting sub  40  in more detail. In the illustrated embodiment, bit-weighting sub  40  includes an upper cap  42  secured to the lower end of coiled tubing T with a connection such as a threaded connection. Cap  42  has a shoulder or stop  42   a  that rests on the upper end of upper housing  56  and is secured thereto through screws  42   b  or by a threaded connection (not shown). A spacer ring  44  can be used to adjust the amount of compression applied to spring  46 . Spring  46  fits axially over a centralizer sub  48 , inside housing  56 , with the lower end of spring  46  seated on a ring  50 . Ring  50  has a seal  52 , for example, an O-ring, in sliding contact with the inner wall of the housing  56 . Centralizer sub  48  includes a lower Kelly shaft portion  54 , in the illustrated embodiment a hex Kelly, although any multi-sided or keyed shape or structure that can permit shaft  50  to slide longitudinally but prevent it from rotating with respect to housing  56  can be used. Upper housing  56  mounts a lower housing  58  that has a hexagonal (or multi-sided or similar) interior shape to receive hex Kelly  48  with an axially-sliding but non-rotating fit. The lower housing  58  includes a no-go radial projection  60  that is configured to abut a corresponding no-go internal projection or abutment  16   a  in tubing  16 , as best shown in  FIG. 3 . The radial projection  60  can be annular or circumferentially spaced individual pieces. Likewise, the no-go internal projection or abutment  16   a  can be annular or circumferentially spaced individual pieces. 
     While the illustrated embodiment in  FIGS. 1 through 5  show bit-weighting sub  40  mounted between the coiled tubing T and mud motor  30  (above the motor), it is also possible to mount bit-weighting sub  40  between the mud motor and knuckle drive  22  (below the motor) as shown in  FIG. 6 . Whereas above-motor bit-weighting sub  40  in  FIGS. 1-5  is non-rotational, it is preferred that below-motor sub  40  in  FIG. 6  is attached to rotate with the lower end or drive shaft of motor  30 . It is also possible to mount sub  40  below the motor so that a rotational drive element passes through sub  40  without rotating the bit-weighting sub itself, but it has been found that rotating sub  40  with the motor improves the milling operation. 
       FIG. 7  illustrates an alternate no-go structure  70  especially useful for the below-motor mounting of  FIG. 6 . No-go structure  70  includes an oversize tubular adapter  72  that is threadably mounted between two sections of the workstring tubing  16 , a no-go sleeve  74  with adjustment grooves  74   a , and a sub  76  threaded to the upper end of motor  30  and threaded to a lower end of the coiled tubing T. The no-go sub  76  has an outer diameter that is adapted to abut the upper end of the sleeve  74  to positively stop motor  30  (and thus the coiled tubing T used to lower the motor) against the upper end of sleeve  74 . The tubular adapter  72  is threaded to the adjacent sections of the workstring tubing  16  at the well head prior to lowering the workstring  16  into the well bore  10 . In the illustrated embodiment, setscrews are inserted through holes  72   a  to project into grooves  74   a  on sleeve  74  when the sleeve is in the desired position. It will be understood that while no-go structure  70  is preferred when the bit-weighting sub  40  is mounted below motor  30 , the no-go structure  16   a  and  42  shown in  FIGS. 1-5  could also be adapted to a below-motor mounting of sub  40 . Further, the no-go structure  70  in  FIG. 7  can be adapted to an above-motor mounting of the bit-weighting sub, replacing the no-go profile  16   a  in workstring tubing  16 . 
     Still referring to  FIGS. 6 and 7 , coiled tubing T lowers motor  30  down through adapter  72  and no-go sleeve  74  into operative connection with the knuckle drive  22  in deflector shoe  21 . The spring in bit-weighting sub  40  below motor  30  is compressed until no-go cap  76  on the upper end of motor  30  and on the lower end of the coiled tubing T stops against the upper end of no-go sleeve  74 , at which point the weight of the coiled tubing is taken off sub  40  and knuckle drive  22 . The bit-weighting sub spring alone will then apply pressure to the milling bit  24  for the milling operation. 
     As mentioned previously, it is possible to replace the known, controllable spring force of the bit-weighting sub spring  46  with a hydraulic force-exerting structure operated with the fluid pumped down coiled tubing T, or with an independent fluid supply delivered downhole. Such hydraulic force-exerting structure is illustrated at H in  FIGS. 8A and 8B , with  FIG. 8B  schematically representing the hydraulic structure H in a bit-weighting condition. Once the motor  30  is landed in operative connection with knuckle drive  22 , hydraulic fluid could be forced downhole to operate the hydraulic force-exerting structure H in the bit-weighting sub to apply milling pressure to the bit in a manner similar to the illustrated spring in  FIGS. 5A and 5B . 
     While the invention has been illustrated in use with a rotary drive lowered and operated through standard coiled tubing, it will be understood that the invention could also be used with other types of tubing such as jointed tubing. 
     The invention provides an apparatus and a method to drill one or more holes in a wellbore casing quicker than is possible with prior apparatus. The invention reduces the skill required by operators to drill a hole in a wellbore casing with minimal problems. Further, the invention provides a preset amount of force to be constantly applied to the bit as it is milling a hole in the casing. Further, the casing can be milled in deviated or horizontal wells. Still further, the casing can be milled in flowing wells and further can be milled in the casing at depths which are greater than currently possible with prior apparatus. Further, the required torque on the motor is reduced because the milling assembly does not have to support the weight of the coiled tubing. The invention provides for holes to be milled in casing using standard size coiled tubing units. 
     It will finally be understood that the disclosed embodiments are representative of presently preferred forms of the invention, but are intended to be illustrative rather than definitive of the invention. Reasonable variation and modification are possible within the scope of the foregoing disclosure and drawings without departing from the spirit of the invention.