METHOD AND APPARATUS FOR RUNNING TUBULARS

Abstract

A method is disclosed for running a completion string, such as casing, into a predrilled wellbore while allowing rotation of a downhole component with respect to the completion string. The downhole component may be a drill shoe or reamer shoe for example. There is also disclosed a drillable or millable drive assembly for driving the rotation of a portion of the casing assembly and other applications thereof.

Description

The present invention relates to the drilling and completion of wellbores. In particular, the present invention in one of its aspects relates to a method for running a completion string, such as casing, into a predrilled hole. In another of its aspects, the invention relates to apparatus for allowing rotation of a downhole component with respect to a completion string. In another of its aspects, the invention relates to a drillable or millable drive assembly and applications thereof.

Casing is steel pipe cemented in place during the wellbore construction in order to stabilise the wellbore. The casing string is made up of a plurality of casing sections, usually threaded together, each run from the surface to a casing point at the bottom of an open hole at a specific diameter. The wellbore is extended by drilling beyond the casing at a smaller inner diameter, and a second casing string is run from the bottom of that open hole back to the surface. The wellbore thus comprises a series of concentric casing strings, with the inner most strings extending to a greater depth in the wellbore.

When the casing is run into the wellbore, it is desirable to use casing that is a close fit to the open hole diameter. A float shoe may be used to guide the casing toward the centre of the hole and minimise the impact of the casing with rock ledges or washouts as the casing is run into the wellbore. The float shoe is typically made from a drillable material, since the float shoe must be drilled through if the well is to be deepened beyond the casing point.

When running casing using conventional float shoes, the casing string can still have a propensity to stand up on ledges and washouts. Casing reaming shoes attached to the lower end of the casing have an abrasive surface for enlarging the open hole size and assisting in deployment of the casing. During casing reaming, the entire casing string will be rotated, and the reaming shoe smoothes the run-in of the casing. However, the frictional forces of the casing string against the open bore reduce the torque transferable to the casing reaming shoe. The forces can be significant as the casing string may extend over considerable distances. In addition, rotation of the casing string in the open bore can cause wear and damage to the casing string.

Casing drilling techniques have also been developed, in which the casing is run into the hole as it is being drilled. In most cases, it would be necessary to drill beyond the casing point. One solution to this is to divert a drill string around the drill bit used for the casing drilling operation, using a whipstock provided in the casing section. Alternatively, the drill bit can be retrieved from the wellbore using a specialised fishing tool in a dedicated run. Alternatively, it may be possible to use a drillable bit that can be drilled through in a subsequent drilling operation. All of these solutions are specialised operations requiring dedicated equipment and procedures, and present significant technical challenges in some drilling environments.

Similar challenges apply to the running of completion strings such as including liners, sand control screens, slotted liners, expandable tubulars, sometimes referred to as lower completion strings or openhole completion strings.

It is one object of the invention to provide a method and apparatus for casing the running that at least mitigate one or more disadvantages of conventional casing running methods and apparatus.

It is an aim of at least one aspect of the invention to provide a method and apparatus for allowing selectable rotation of downhole equipment mounted at the bottom of the casing string.

It is a further aim of an embodiment of the invention to provide a method and apparatus for rotating a portion of a completion string by controlled pumping of circulation fluid.

It is a further aim of at least one aspect of the invention to provide a drive assembly for downhole equipment that is drillable or millable by conventional downhole drilling equipment.

According to a first aspect of the invention there is provided a method of running a casing string in a wellbore, the method comprising the steps of:

• • providing a casing string assembly in a wellbore, the casing string assembly having a first portion and a second portion; and
  • rotating a first portion of the casing string assembly relative to a second portion of the casing string assembly while running the casing string.

It should be understood that the term casing string assembly includes any casing sections along with apparatus associated with the casing sections, including a drill shoe, or reaming shoe located at the lower end of the casing string.

Preferably, the casing string assembly includes a swivel, and the first portion of the casing string assembly is a lower portion located below the swivel, and the second portion of the casing string assembly is an upper portion located above the swivel.

Preferably, the first portion of the casing string assembly includes a shoe. More preferably, the shoe includes an abrasive or cutting surface. The shoe may be a casing reaming shoe.

The second portion of the casing string assembly may include a plurality of casing sections. Preferably, the second portion of the casing string assembly provides the majority of the length of the casing string assembly. More preferably, the second portion of the casing assembly provides greater than 70% of the length of the casing string assembly. More preferably greater than 80%. More preferably, at least 90%.

Preferably, the casing string is run while the second portion of the casing string assembly is substantially not rotated with respect to the wellbore.

In this manner, the invention allows for a reduction to the length of tubular that is rotated with respect to the wellbore, reducing frictional forces and wear to the casing string.

Preferably, all casing sections in the casing string assembly are provided in the second portion.

The method may include the step of driving rotation of the first portion of the casing string assembly by circulation of fluid through the casing string. The rotation of the first portion of the casing string assembly may be driven by a drive assembly, which may comprise a mud motor. In this way, pumping of fluid controls the rotation and reaming function.

The method may include the step of locating a drive assembly with respect to the casing string assembly. The drive assembly may be run in hole with the casing string assembly. Where this is the case, the drive assembly may be located in the casing string assembly at surface.

The method may include the step of retrieving the drive assembly from the casing string assembly. In this way, after the reaming operation is complete, for example when no further reaming is required due to formation conditions or because the casing string has reached its casing point, the drive assembly can be retrieved to surface.

The drive assembly may be retrieved using a wireline fishing tool. Alternatively, the drive assembly may be retrieved using drill pipe. In a further alternative, the drive assembly may be retrieved by reverse circulation of the fluid in the wellbore to pump the drive assembly upward in the well.

In a preferred embodiment, the method includes the step of locating the drive assembly with respect to the casing string when the casing string has been at least partially run into the wellbore.

In this way, initial running of the casing could be by floating the casing string into the wellbore. If difficulties are experienced while running the casing in this fashion, for example, if the casing string stands up on a rock ledge or a washout, the drive assembly can be deployed through the casing string to locate with respect to the swivel components.

The method may include the step of pumping the drive assembly through the casing string. The drive assembly may comprise a swab cup or wiper. In an alternative embodiment, the drive assembly is run on drill pipe.

The method may include the additional step of engaging first and second portions of the casing string assembly with the driving assembly. The first and second portions of the casing string assembly may be rotationally keyed with corresponding portions of the drive assembly.

The method may include the additional step of rotating the casing string during running. That is, the casing string could be run as a conventional casing reamer over distances or through formation for which casing reaming is achievable and desirable. If difficulties are encountered, for example, if the reaming shoe stands up on a ledge or washout and sufficient torque is not available to continue the casing reaming operation, or alternatively, if the frictional wear on the casing string poses a risk of damage to the string, the equipment could then perform in its swivel mode, with just a portion of the casing string being rotated.

In an alternative embodiment, the first portion includes a drill shoe or drill bit, and the method is implemented as a casing-while-drilling operation.

The method may include the additional step of preventing rotation of the first portion of the casing string assembly with respect to the second portion. Preferably, the first portion of the casing string assembly is locked with respect to the second portion.

The method may include the additional step of cementing the casing. The cementing of the casing may prevent rotation of the first portion of the casing string assembly with respect to the second portion. The first and second portions of the casing string assembly may thereby be locked by a cementing operation.

The method may include the additional step of drilling beyond the casing string assembly.

Advantageously, all components of the casing string assembly left downhole are drillable or millable by conventional drilling apparatus.

The method may include the additional step of drilling through the drive assembly. The drive assembly may be a drillable or millable downhole motor. The drive assembly may be integral with the casing string assembly.

The method may include the steps of:

• • running the drive assembly with the casing; and
  • cementing the casing with the drive assembly downhole.

The method may include the steps of running a float collar with the casing, above the drive assembly.

According to a second aspect of the invention, there is provided a method of running a completion string in a wellbore, the method comprising the steps of:

• • providing a completion string assembly in a wellbore, the completion string assembly having a first portion and a second portion; and
  • rotating a first portion of the completion string assembly relative to a second portion of the completion string assembly while running the completion string.

The completion string may be tubular including liners, sand control screens, slotted liners, expandable tubulars or casing. These strings may be referred to as lower completion strings or openhole completion strings. Embodiments of the second aspect of the invention are as embodiments of the first aspect, with references to casing string changed to completion string accordingly.

According to a third aspect of the invention, there is provided apparatus for connection to a tubular forming part of a completion string, the apparatus comprising an upper section adapted to be coupled to the completion string; a lower section rotatable with respect to the upper section and adapted to be coupled to equipment located below the tubular; and engaging means corresponding to engaging means provided on a drive assembly adapted to be coupled to the apparatus to allow rotation to be imparted to the lower section, wherein the apparatus is operable to be engaged or disengaged with the drive assembly in the wellbore.

Preferably, the apparatus is operable to be engaged or disengaged with the drive assembly in the wellbore to allow the drive assembly to be run into the wellbore or retrieved from the wellbore separately from the apparatus.

Preferably, the engaging means arranged to be rotationally keyed on engagement with the corresponding engaging means. Preferably, the engaging means comprises a plurality of splines.

Preferably, the apparatus includes an upper spline section, a lower spline section, and a bearing portion.

Preferably, the apparatus is adapted to have a rotating mode and a non-rotating mode. The bearing portion may be operable to rotate under load or under pressure. In one embodiment, the apparatus is provided with means for preventing rotation of the upper portion with respect to the lower portion. Preferably, the upper and lower portions of are provided with formations which are lockable by a cementing operation.

The apparatus may be adapted to be coupled to a shoe, such as a casing reamer shoe or drill shoe.

According to a fourth aspect of the invention, there is provided a drive assembly for a downhole apparatus, the drive assembly comprising a main body portion and a rotating portion and a motor for effecting rotation of the rotating portion with respect to the main body portion; wherein the drive assembly comprises engaging means for engagement of the main body portion and the rotating portion with corresponding engaging means provided on the downhole apparatus to allow rotation to be imparted to the downhole apparatus, and the drive assembly is operable to be engaged or disengaged with the downhole apparatus in the wellbore.

Preferably, the drive assembly is operable to be engaged or disengaged with the downhole apparatus in the wellbore to allow the drive assembly to be run into the wellbore or retrieved from the wellbore separately from the downhole apparatus.

Preferably, the engaging means arranged to be rotationally keyed on engagement with the corresponding engaging means. Preferably, the engaging means comprise a plurality of splines.

In one embodiment, the drive assembly comprises a fluid circulation path therethrough. Preferably, the motor is a positive displacement motor or mud motor.

The drive assembly may be adapted to be run in and/or retrieved from a completion string, such as a casing string. The drive assembly may be adapted to be received in a shoe assembly of a completion string. Preferably, the drive assembly is adapted to be received in a shoe assembly to allow rotation of the shoe to be driven by the drive assembly.

In one embodiment, the drive assembly is adapted to be pumped downhole. Preferably, the drive assembly comprises a resilient member for providing a seal with a completion string, against which the drive assembly is pumped. Preferably, the drive assembly comprises a swab cup or wiper.

The drive assembly may be provided with means for preventing fluid flow through its circulation path. Such means may be a rupture disc assembly. In this way, a sufficient increase in pressure will rupture the disc and open the circulation path, and allow the motor to be activated.

The drive assembly may comprise a wireline fishing neck.

According to a fifth aspect of the invention, there is provided an assembly comprising the apparatus of the third aspect of the invention, and the drive assembly of the fourth aspect of the invention.

According to a sixth aspect of the invention, there is provided apparatus for running a casing string, the apparatus comprising an upper section adapted to be coupled to the casing string; a lower section rotatable with respect to the upper section and including a shoe located below the tubular; and engagement means for engaging a drive assembly adapted to rotate the shoe with respect to the upper section.

According to a seventh aspect of the invention there is provided a drive assembly for a downhole apparatus comprising an upper section adapted to be coupled to a tubular; a lower section rotatable with respect to the upper section; and a motor for imparting rotation to the lower section; wherein the drive assembly is drillable or millable by conventional downhole drilling equipment.

Preferably, the motor is a positive displacement motor such as a mud motor. More preferably, the motor comprises a drillable or millable rotator. The rotator may be made from a drillable alloy. The motor may comprise a stator formed from a rubber or nitrile rubber material.

The drive assembly may be provided with a fluid circulating path for actuating the motor and a fluid bypass path. The fluid bypass path may be provided with a rupture disc. In this way, the rupture disc prevents fluid flow through the bypass channel until pressure behind the disc is sufficient to break the disc.

Preferably, the lower section is connected to a shoe, such as a casing reamer shoe or a drill shoe. More preferably, the show is drillable or millable by conventional downhole drilling equipment. The shoe may include a non-return valve to prevent fluid flow into the shoe from the wellbore.

In one embodiment, the drive assembly is adapted to have a rotating mode and a non-rotating mode. A bearing portion may be operable to rotate under load or under pressure. In one embodiment, the drive assembly is provided with means for preventing rotation of the upper portion with respect to the lower portion. Preferably, the upper and lower portions of are provided with formations which are lockable by a cementing operation.

It will be appreciated that where the terms ‘up’ and ‘down’ are used in this specification, they are used in a relative sense and the invention could equally apply to deviated or horizontal wellbores, in which case the references would convert accordingly.

There will now be described by way of example only, various embodiments of the invention with reference to the following drawings, of which:

FIG. 1 is a schematic representation of a shoe assembly according to an embodiment of the invention;

FIGS. 2A and 2B are schematic representations of a drive assembly, used in accordance with the apparatus of FIG. 1;

FIGS. 3A, 3B, and 3C show the apparatus of FIGS. 1 and 2 at different stages of running;

FIG. 4 is a schematic representation of a shoe assembly in accordance with a second embodiment of the invention;

FIG. 5 is a schematic representation of a casing string comprising the shoe assembly of FIG. 4.

Referring firstly to FIG. 1, there is shown in schematic form a shoe assembly, generally depicted at 10, at its downhole end. The apparatus is shown in half section in order to reveal internal components. At an upper end 12, the apparatus 10 is connected to a casing string (not shown). The apparatus includes an upper spline section 16, which comprises longitudinally oriented internal splines 17, protruding radially inward from the inner wall of the section, and spaced circumferentially around the inner bore 34 of the apparatus. Bearing assembly 18 is coupled to the upper spline section 16, in this case via an optional spacer section 20, and provides a downhole swivel. The bearing assembly comprises upper and lower bearing portions, respectively 22a and 22b, and a bearing housing 24 functioning to keep the bearing assembly together. Upper and lower bearing portions 22a and 22b are rotated with respect to one another.

Coupled to the lower bearing portion is a lower spline section 26, which in turn is coupled to shoe 28. The lower spline section 26 comprises longitudinally oriented internal splines 27, protruding radially inward from the inner wall of the section, and spaced circumferentially around the inner bore 34 of the apparatus. The shoe 28 in this example is a casing reaming shoe, having abrasive portions 30 on its out surface. Internal to the shoe is a dual non-return valve arrangement 32 allowing one-way flow of circulating fluid from the internal bore 34 of the apparatus through the shoe 28 and out through the jets 36.

Referring now to FIGS. 2A and 2B, there is shown a drive assembly, generally depicted at 50, for use with the apparatus of FIG. 1. FIG. 2B shows a continuation of the same of the drive assembly 50, in half section to show internal components.

The drive assembly 50 comprises a main cylindrical body 52, shown predominantly in FIG. 2A, and an upper end 54, shown in FIG. 2B. The drive assembly is dimensioned to fit inside the internal bore of the apparatus 10. The upper end 54 comprises a wireline fishing neck 56, but could be provided with any other suitable retrieval means. Provided in the body 52 at the upper end 54 are inlet ports for entry of circulating fluid. In this embodiment, a rupture disc assembly 58 is provided internal to the body 52. The drive assembly 50 is also provided with a swab cup 59 made of resilient material to form a seal with the inner wall of the casing.

The drive assembly further comprises an upper spline sub 60, comprising a plurality of longitudinally oriented splines 62, extending radially from the outer surface of the upper spline sub. The splines 62 are distributed circumferentially around the upper spline sub 60, and are arranged to complement and engage with the splines 17 of the upper spline section 16.

Beneath the upper spline sub 60 is a housing 64 for a mud motor, rotationally coupled to a lower spline sub 66 via a bearing assembly 68. The spline sub 66 comprises splines 70, distributed circumferentially around the lower spline sub 66, and arranged to complement and engage with the splines 27 of the lower spline section 16.

At the lower end 72 of the drive assembly 50 there are provided a plurality of outlet ports for circulating fluid.

In use, the drive assembly 50 fits inside of the apparatus 10, such that the upper spline sub 60 is rotationally keyed with the upper spline section 16, and the lower spline sub 66 is rotationally keyed with the lower spline section 26. When fluid is circulated in the mud motor 64 by pumping from surface, the lower spline sub 66 is rotated with respect to the main body 52. The upper spline sub 60 becomes a reactive torque sub with the upper spline section 16 acting as a reactive torque section of the apparatus 10, against which the lower spline sub 66 rotates. The rotation of the sub 66 causes rotation of the casing reaming shoe 28.

In the embodiment of FIGS. 1, 2A and 2B, the apparatus is adapted to be run into the casing to the apparatus 10 in a subsequent run, when the apparatus 10 is already downhole. The running method will be described with reference to FIGS. 3A to 3C, each of which shows schematically the apparatus 10, 50 at a different stage of running in a pre-drilled bore 100.

In FIG. 3A, the apparatus 10 has been run into bore 100 on the downhole end of a casing string 110. The apparatus 10 is connected to the casing string 110 via an optional spacer section 120. The casing string is provided with a landing collar 122 for landing a cementing plug, which is a non-float collar.

The casing string and apparatus 10 has been run to a first depth A conventionally. During this stage of running, the casing string might not be rotated in the bore, or the casing string could be run as a conventional casing reamer over distances or through formation for which casing reaming is achievable and desirable. At depth A, difficulties are encountered, for example, the reaming shoe has stood up on a ledge or washout and sufficient torque is not available to continue the casing reaming operation. Alternatively, the operator may decide that frictional wear on the casing string poses a risk of damage to the string. The operator elects to run the drive assembly 50.

As shown in FIG. 3B, the drive assembly 50 into the casing. The swab cup 59 provides a seal with the casing wall and along with the resistance provided by the rupture disc assembly 58, allows the drive assembly 50 to be pumped downhole in the direction of arrow B. The drive assembly 50 and landing collar 122 are selected to allow the drive assembly to pass the collar. When the drive assembly 50 locates in the apparatus, the splines of the respective spline subs 60, 66 and spline sections 16, 26 are rotationally keyed, and the lower end 72 abuts a formation in the apparatus 10. The pump pressure breaks the rupture disc assembly 58 and fluid circulates in the drive assembly. The mud motor 64 causes the lower spline sub 66, the lower spline section 26, and shoe 28 to rotate against the portions of the apparatus above the bearing portions. Downweight on the casing string 110 allows the depth of the casing to be increased to the casing point while reaming the bore 100, as indicated by arrow C.

When the reaming operation is complete, the pump pressure is bled off and the wireline fishing neck 56 allows the drive assembly 50 to be retrieved from the casing prior to cementing.

The above-described method and apparatus allows casing reaming without rotating the entire casing string, and in a manner in which the only components left downhole may be drilled through if the wellbore is to be drilled beyond the casing point. In addition, the method and apparatus provides for elective deployment and/or retrieval of the driving assembly.

To assist in drilling out the apparatus, an alternative, non-illustrated embodiment of the invention provides a bearing assembly that has rotating and non-rotating states. For example, the bearing assembly may include a mechanism that is configured such that it only allows rotation when under a load or under pressure. In an alternative embodiment, portions of the apparatus attached to the upper and lower bearing portions are profiled or ribbed such that after the cement job has been carried out, relative rotation of the components is prevented or mitigated.

In an alternative, non-illustrated embodiment, the drive assembly is run in with the casing string 110, and not pumped down in a separate operation. In this embodiment, the drive assembly is substantially identical to that of FIGS. 2A and 2B, but the swab cup 59 and rupture disc assembly 58 will not be required and may be omitted. The drive assembly is located with respect to the spline sections at surface, and run into the bore together with the casing. If reaming is required, circulating fluid is pumped down from surface and the mud motor effects rotation of the shoe to ream the hole. Subsequent to the reaming operation the drive assembly can be retrieved from the casing, although it could be left downhole. However, it is preferable in most casing running applications to avoid leaving equipment downhole that is difficult to drill out, should the depth of the bore be increased beyond the casing point.

In a further embodiment of the invention the driving assembly is run on a wireline subsequent to running of the casing section.

In a further, non-illustrated embodiment of the invention, the landing collar 122 is modified to provide a profile that maximises its contact area for landing a cementing plug, but allows passage of the driving assembly. The landing collar could for example comprise portions of increased inner diameter to allow passage of the splines of the driving assembly. In a further alternative embodiment, the reduced inner diameter of the upper spline section functions as a landing collar for a cementing plug.

Referring now to FIG. 4, there is shown in schematic form a shoe and motor assembly in accordance with an alternative embodiment of the invention, generally depicted at 200. The apparatus is shown in half section in order to reveal internal components. At an upper end 212, the apparatus 200 is connected to a casing string (not shown). The apparatus includes an upper section 216 and a lower section 226 via a bearing assembly 218 to provide a downhole swivel. The bearing assembly 218 comprises upper and lower bearing portions, respectively 222a and 222b, and a bearing housing 224 functioning to keep the bearing assembly together. Upper and lower bearing portions 222a and 222b are rotatable with respect to one another.

The lower portion 226 is connected to shoe, in this case a standard reamer shoe 228 having tungsten carbide reaming portions 230 and stabilising portions 234. The shoe 228 includes a non-return valve 232 to prevent backflow of circulation fluid.

The upper section 216 houses a drillable or millable motor 240. The motor 240 includes a stator 242 coupled to, and static in relation to, the upper portion 216. The stator is made from a rubber or nitrile rubber material, having the properties of being elastically deformable, but resistant to wellbore and circulation fluids. The motor also comprises a rotator 244, concentric with the stator 242, and formed from a drillable or millable alloy. The rotator 244 comprises a main body portion 246 and a lower body portion 248, coupled to the lower portion 226 of the assembly by a threaded connection. Fluid ports 250 are provided in the lower body portion 248. As is understood in the art, the stator and rotator are profiled on their inner and outer surfaces respectively, such that fluid pumped through the motor causes rotation of the rotator with respect to the stator. Such rotation is transferred to the lower portion of the assembly and the shoe.

The assembly is also provided with a bypass bore 252 and a rupture disc 254. Should the motor block or jam, the pressure build up behind the motor will cause the disc to rupture and open the bore 252 for passage of fluid.

The components of the drive assembly are selected to be readily drillable or millable using conventional drilling equipment. Similarly, the components of the show 228 are readily drillable or millable. This allows the apparatus to be left downhole, even in applications where subsequent drill-out, for example to extend the wellbore beyond the casing point, is required. This arrangement also allows the drive assembly to be integral with the shoe assembly.

To assist in drilling out the apparatus, an alternative, non-illustrated embodiment of the invention provides a bearing assembly that has rotating and non-rotating states, as described with reference to FIG. 1. For example, the bearing assembly may include a mechanism that is configured such that it only allows rotation when under a load or under pressure. In an alternative embodiment, portions of the apparatus attached to the upper and lower bearing portions are profiled or ribbed such that after the cement job has been carried out, relative rotation of the components is prevented or mitigated.

FIG. 5 shows schematically a representation of a casing string 308 comprising the shoe assembly of FIG. 4 in a bore 300. In this arrangement, the drive assembly 240 is integral with the casing string assembly on run in, and is coupled at its upper end to a casing section 302. Further casing section 304 is provided with a float collar 306 for catching a cementing plug and providing a non-return valve. The use of the float collar is made possible because the drive assembly does not require separate deployment or retrieval.

Various modifications and improvements could be made to the above-described embodiments of the invention within the scope of the invention herein defined.

Claims

1. A method of running a casing string in a wellbore, the method comprising the steps of:

(a) providing a casing string assembly in a wellbore, the casing string assembly having a first portion and a second portion; and

(b) rotating a first portion of the casing string assembly relative to a second portion of the casing string assembly while running the casing string.

2. The apparatus as claimed in claims 49 wherein the apparatus includes a swivel, and the lower section is located below the swivel, and the upper section is located above the swivel.

3. (canceled)

4. The apparatus as claimed in claim 49 wherein the shoe includes an abrasive or cutting surface.

5. The apparatus as claimed in claim 49 wherein the shoe is a casing reaming shoe.

6. (canceled)

7. The method as claimed in claim 1 wherein the casing string is run while the second portion of the casing string assembly is substantially not rotated with respect to the wellbore.

8. The method as claimed in claim 1 including the step of driving rotation of the first portion of the casing string assembly by circulation of fluid through the casing string.

9. The method as claimed in claim 11 wherein the rotation of the first portion of the casing string assembly is driven by the drive assembly, comprising a mud motor.

10. (canceled)

11. The method as claimed in claim 1 including the steps of locating a drive assembly with respect to the casing string assembly and running the drive assembly in hole with the casing string assembly.

12. The method as claimed in claim 11 further including the step of retrieving the drive assembly from the casing string assembly.

13-22. (canceled)

23. The method as claimed in claim 1 including the additional step of preventing rotation of the first portion of the casing string assembly with respect to the second portion.

24. (canceled)

25. The method as claimed in claim 1 including the additional step of drilling beyond the casing string assembly.

26. The method as claimed in claim 11 including the step of drilling through the drive assembly.

27. (canceled)

28. The method as claimed in claim 11 including:

(a) running the drive assembly with the casing; and

(b) cementing the casing with the drive assembly downhole.

29. The method as claimed in claim 11 including the step of running a float collar with the casing, above the drive assembly.

30-48. (canceled)

49. Apparatus for running a casing string, the apparatus comprising an upper section adapted to be coupled to the casing string; a lower section rotatable with respect to the upper section and including a shoe located below the tubular; and engagement means for engaging a drive assembly adapted to rotate the shoe with respect to the upper section.

50. A drive assembly for a downhole apparatus comprising an upper section adapted to be coupled to a tubular; a lower section rotatable with respect to the upper section; and a motor for imparting rotation to the lower section; wherein the drive assembly is drillable or millable by conventional downhole drilling equipment.

51. (canceled)

52. The drive assembly as claimed in claim 50 wherein the motor comprises a drillable or millable rotator.

53. (canceled)

54. The drive assembly as claimed in claim 50 wherein the lower section is connected to a shoe.

55. A drive assembly as claimed in claim 54 wherein the shoe includes a non-return valve to prevent fluid flow into the shoe from the wellbore.

56. A drive assembly as claimed in claim 50 adapted to have a rotating mode and a non-rotating mode.