Method for drilling with casing

[0001] This invention relates to a method for wellbore drilling and, in particular, a method for drilling a wellbore using casing as the drill string.

[0002] The drilling of wells, for example, for oil and gas production conventionally employs relatively small diameter strings of drill pipe to which is secured a drill bit of somewhat larger diameter. After a selected portion of the wellbore has been drilled, the well bore is usually lined with a string of tubulars known as casing. The casing (herein used to encompass any wellbore liner) has a smaller diameter than the drill bit. This conventional system which requires sequentially drilling the borehole using drill pipe with a drill bit attached thereto, pulling the drill pipe out of the borehole and running casing into the borehole is time consuming and costly. In addition, control of the well is difficult during the period that the drill pipe is being removed and the casing is being run in.

[0003] Drilling with casing is gaining popularity as a method for drilling wherein the casing is used as the drill string and drilling conduit and, after drilling, the casing remains downhole to act as the wellbore liner. A drilling assembly, including a drill bit and one or more hole enlargement tools such as, for example, an underreamer, is used which drills a borehole of sufficient diameter to accommodate the casing. The drilling assembly is deployed on the advancing end of the casing. The drill bit can be retractable and/or removable through the casing.

[0004] Casing drilling has been tested for drilling vertical, straight and deviated wellbores.

[0005] Recently, as described in copending PCT application s.n. PCT/CA99/00636, a process has been invented for drilling directional holes using casing as a drill string. According to that process a directional borehole assembly and a drilling assembly are connected to the distal end of a drill string. The directional borehole assembly includes a biasing means for applying a force to the drilling assembly to drive it laterally relative to the wellbore. The directional borehole and drilling assemblies can include drill bits with rotary steerable tools or downhole motors equipped with bent housings and/or bent subs that permit control of forces acting perpendicular to the drill string to steer the drill bit in a selected direction while drilling.

[0006] In casing drilling, the drilling assemblies can extend out a considerable distance, for example, 75 to 150 feet, in front of the end of the casing. Thus, during the drilling process the distal end of the casing does not extend fully to the bottom of the well. Whenever the drilling assemblies, including the directional assembly if any, are removed, the distal end of the casing is a considerable distance from the bottom of the borehole. However, it is desirable to have the borehole fully cased to the bottom and, therefore, once the drilling assemblies are removed often attempts are made to advance the casing to the bottom. This process has experienced considerable difficulties. In particular, while advancing the casing to the bottom it can become hung up on the sides of the wellbore and/or, depending on the integrity of the formation, it can steer off away from the drilled wellbore and into the formation. The borehole can also cave-in in the intervening period between removing the bottom hole assemblies and attempting to move the casing downhole. The problems increase with increased time delay between stopping drilling and advancing the casing.

[0007] Similar problems have been experienced in another form of casing drilling, more commonly termed liner drilling. In liner drilling, the drilling assemblies operate and advance to extend the borehole while being mounted on the end of a section of liner. The liner is connected to surface by a length of drill pipe.

[0008] A method for drilling boreholes using casing has been invented. The term casing is used herein to encompass any wellbore liner capable of supporting an operational drilling assembly. The present invention provides a method for advancing the casing to substantially the bottom of the drilled wellbore while reducing the problems previously associated with this procedure. In the method the drilling assembly is used as a guide over which the drill string is advanced toward the bottom of the borehole. This method can be used whenever it is desired to remove the drilling assembly from the wellbore such as when drilling assembly maintenance is required or when drilling is complete.

[0009] In accordance with a broad aspect of the present invention, there is provided a method for drilling a well with a well casing as at least a portion of an elongated tubular drill string and a drilling assembly retrievable from the lower distal end of the drill string without withdrawing the well casing from a wellbore being formed by the drilling assembly, the method comprising: providing the casing as at least a portion of the drill string; providing the drill string with the drilling assembly engaged at the distal end thereof; inserting the drill string and the drilling assembly into the wellbore and driving the drilling assembly to operate at the bottom of the wellbore to extend the wellbore, the wellbore being formed having a diameter greater than the diameter of the drill string; actuating the drilling assembly to disengage from the distal end of the drill string; advancing the drill string over the drilling assembly toward total depth; removing the drilling assembly out of the wellbore through the drill string without removing the drill string from the wellbore; and leaving the casing portion of the drill string in the wellbore.

[0010] In accordance with another broad aspect of the present invention, there is provided a method for advancing a casing string to a desired depth within a wellbore, the wellbore having been drilled using the casing string as at least a portion of an elongated tubular drill string and a drilling assembly connected to the drill string and retrievable from the lower distal end of the drill string without withdrawing the casing string from the wellbore being formed by the drilling assembly, the method comprising: after drilling a wellbore, releasing the drilling assembly from engagement with the drill string; advancing the drill string over the drilling assembly to a desired depth; and removing the drilling assembly from the wellbore through the drill string without removing the casing string portion of the drill string from the wellbore.

[0011] The drilling assembly includes drill bits such as, for example, a pilot bit and underreamers and can include any of: directional assemblies such as rotary steerable tools or downhole motors equipped with bent housings and/or bent subs; mud motors; measurement while drilling (MWD) instruments; and other downhole tools.

[0012] Preferably the drill string is advanced such that its distal end is substantially at the bottom of the borehole prior to removing the drilling assembly from the wellbore.

[0013] As is known, drilling assemblies are attached to the drill strings in various ways such as, for example, by locking dogs latching in recesses in the casing. A release tool is generally used to retract the locking arrangement of the drilling assembly from engagement with the drill string. The drilling assembly can be actuated to disengage from the drill string by any useful method such as, for example, using a release tool conveyed on a work string such as, but not limited to, wireline, drill pipe or coiled tubing.

[0014] In wellbore drilling it is preferable that fluid circulation be maintained even during periods in which the drill bit is not operating. Fluid circulation acts to condition the well, remove debris and prevent cave in. Therefore, in a preferred aspect of the present invention the disengagement of the drilling assembly from the drill string is carried out using a means that permits circulation during the actuation operation. Further, the chances of a cave-in increase with increased time between stopping the drilling operation and advancing the casing. Therefore, a process is preferred wherein the step of disengaging to initiate the advancement of the drill string is minimised with respect to time required. In other words, a process for releasing the drilling assembly from engagement with the drill string is preferred that minimises time taken. Since the use of a work string requires that the work string be fully removed from the casing before further joints of casing, or drill pipe in the case of liner drilling, can be added to advance the drill string, any process that uses a work string inserted through the casing requires more time than one not using an inserted work string. Thus, a process is preferred wherein the step of disengagement is accomplished without the use of a work string. These processes permit continued circulation during actuation of release of the drilling assembly from the drill string. In addition, these processes do not require a delay between release of the drilling assembly from the drill string and advancement of the drill string over the drilling assembly. The step of disengagement can include for example, releasing a fluid or gravity conveyed tool such as a ball, spear, a drop bar or a fluid-conveyed dart or actuation by a mud pulse, or electromagnetic or electrical actuation. A ball can be dropped where the wellbore is substantially vertical. Where the wellbore contains some deviations, the use of a fluid-conveyed dart is preferred. The use of a mud pulse, electrical signal or an electromagnetic signal can be used but will require the use of a drilling assembly which can recognize the signal to disengage.

[0019] Referring to FIG. 1, a wellbore 10 is shown during a drilling operation. Wellbore 10 extends between surface 12 and bottom 14 of the wellbore. A drill string 16 formed of casing extends from surface into the wellbore. A drilling assembly 18 is connected at the distal end 16′ of the drill string and extends a distance d from distal end 16′ to total depth 14. Drilling assembly 18 includes a pilot bit 19 and a plurality of underreamers 20. Also included in drilling assembly are a drill lock member 22 for engaging the drilling assembly to the drill string, a drill collar 24, a stabilizer 25, a non-magnetic drill collar 26 including an MWD survey instrument and mud pulse generator 27 and a mud motor 28 for driving the bits 19 and 20. In such an arrangement the distance d is about 100 to 120 feet. This distance can be reduced in a non-directional drilling operation and/or by use of a non-magnetic MWD.

[0020] Drilling assembly 18 is connected into drill string 16 by means of latches on drill lock member 22. As will be appreciated by a person skilled in the art, the latches are activated to retract by manipulation of member 22. Similar assemblies are available, for example in stage cementing collars, available from Davis-Lynch, Inc., Pearland, Tex.

[0021] Member 22 can be manipulated by running in a work string 30 such as drill pipe having a release tool 32 attached thereto. As is known, release tool 32 manipulates member 22 such that the latches are driven to retract out of engagement with drill string 16. Work string 30 is inserted through surface equipment 34, as is known. A pack off 36 seals the opening through which work string 30 passes.

[0022] Once the latches are disengaged from the drill string, the work string is removed from the well. The drilling assembly remains downhole. When the work string has been completely removed from the well, further joints of casing are added at surface such that the drill string of casing is advanced into the wellbore. Since drilling assembly 18 remains in the wellbore and within the distal end of the drill string, drill string 16 is moved down over the outer surface of the drilling assembly. To advance the drill string, some reaming of the casing may be required. Reaming is accomplished by rotating the drill string. The drill string is advanced to a desired position, after which the drilling assembly is removed from the wellbore.

[0023] Referring now to FIG. 2, it shows a vertical section through another well in which the drill string 16 has been advanced down over the drilling assembly 18. While in FIG. 1 member 22 is actuated by a work string, in FIG. 2 member 22 is actuated by means of a fluid-conveyed dart 40 (shown in actuating position, partially inserted into member 22). Dart 40 is introduced by opening briefly the surface well head and is conveyed with drilling fluid circulation downhole. Dart 40 requires no attached work string and is conveyed by fluid flow into engagement with the drilling assembly. As will be appreciated, the dart can be, for example, formed at its leading edge to act against and depress shoulders on member 22 that in turn cause the disengagement of the locking means on member 22 from drill string 16.

[0024] Drill string 16 is then advanced into the well by adding additional casing joints at surface and, if necessary, rotating the casing as it is lowered into the well. As the drill string advances, the drilling assembly acts as a guide over which the drill string moves. The drilling assembly maintains the drill string on course in the borehole and reduces the chances of the casing being hung up on the borehole wall. Once the drill string is advanced to a desired depth (as shown), for example substantially to bottom 14 of the borehole, the drilling assembly can be retrieved to surface by use of a fishing tool (not shown). In so doing, underreamers 20 collapse to fit within drill string 16.

[0025] By use of dart 40, no work string is required to be used and circulation of drilling fluid can continue during the entire disengaging operation, with the exception of a short period during which the drill string is opened to introduce the dart and when joints of casing are added to the drill string.

[0026] Referring to FIG. 3, another form of casing drilling is shown. The process is commonly known as liner drilling wherein a borehole 10a is drilled using a liner 44 with a drilling assembly 18a connected thereto. As will be appreciated, a liner is casing that does not extend all the way to the surface. A string of drill pipe 46 is connected via a liner hanger 47 to the liner. The string of drill pipe extends to surface (not shown). During the drilling operation, advancement of liner 44 is made by adding at surface further joints of drill pipe to the drill pipe string 46. Liner drilling is usually conducted through an already drilled wellbore 10 that can be lined with casing 16a.

[0027] Drilling assembly 18a is engaged to the distal end of liner 44 by latches or other locking means on a lock member 22. Drilling assembly 18a can extend out beyond the end of liner 44 a distance d of between about 10 and 150 feet depending on the components included in the drilling assembly.

[0028] Lock member 22 can be manipulated to release from engagement with liner 44. In accordance with the present invention, when it is desired to remove the drilling assembly because, for example, one of the drilling assembly components requires maintenance/replacement or the drilling is complete, a release dart 40b is launched. Dart 40b is conveyed by a flow of fluid and acts on member 22 to cause the locking means of the member to release from engagement with the liner. Dart 40b can be launched from surface through drill pipe 46. Alternately, dart 40b can be installed downhole during the drilling operation such as, for example, in liner hanger 47. Dart 40b can include a bore therethrough such that the drilling fluid can pass the dart during the drilling operation. When it is desired to launch the dart, a bore sealing member such as a ball is released from surface to seat in the bore of the dart such that it is released, by fluid pressure, from its mounted position to act on the locking means of member 22.

[0029] Liner 44 can then be advanced towards borehole bottom 14 by reaming over the drilling assembly until the liner is at a desired depth. As such, drilling assembly 18a is positioned in part within liner 44. The drilling assembly can then be removed by disconnecting drill pipe string 46 from liner 44 and removing the drill pipe string from the well. A work string (not shown) is then run in through the wellbore and into liner 44 to engage drilling assembly 18a and pull it to surface.

[0030] It is to be understood that while mechanical manipulation of the drill lock has been shown in FIGS. 1 to 3, the drill lock can alternatively be selected to permit manipulation by mud pulse, electric or electromagnetic means. However these means are not conducive to illustration by drawings.

[0031] It will be apparent that many other changes may be made to the illustrative embodiments, while falling within the scope of the invention and it is intended that all such changes be covered by the claims appended hereto.