APPARATUS AND A METHOD FOR DRILLING BOREHOLES

Abstract: An apparatus (10) for inclusion in a string having a drill bit located at one end is described. The apparatus (10) comprises a body (20) having a longitudinal axis which is substantially coaxial with the longitudinal axis of the string. A portion (30) of the body (20) has a cross section where the centre of mass of the body (20) on that cross section is offset from the longitudinal axis of the string. A method of reducing the variation of a drill bit from an intended path (1) in a borehole is also described, where the drill bit is coupled to one end of a string having a longitudinal axis. The method comprises the steps of including a body (20) in the string, where a portion of the body (20) has a cross section where the centre of mass of the body (20) on that cross section is offset from the longitudinal axis of the string. The string is inserted into a borehole, and at least the portion of the body (20) is rotated.

Description: APPARATUS AND A METHOD FOR DRILLING BOREHOLES The invention relates to apparatus and a method for use do nhole and particularly relates to apparatus for inclusion in a drill string which is used to drill a borehole, and to a method of reducing the variation of a drill bit from an intended path in a borehole. In the art of drilling boreholes, and particularly in relation to drilling boreholes for hydrocarbon exploration and production, it is well known to use drill bits located at one end of a drill string. Conventionally, there are two basic methods which can be used to impart motion to the drill bit in order to operate it. The first method is to rotate the drill string from a drilling rig located at the surface. In this method of use of the drill string, the drill string can be likened to a rotating shaft, where the drill string may be many thousands of feet long. The second method includes use of a downhole motor which is located in the drill string just above the drill bit, and which provides power to rotate the drill bit. In both of these existing methods of drilling a borehole, it is very difficult to maintain the drill bit on its intended path. For instance, if the intended path is vertically downwards, such as path 1 shown in Fig. 1, it is usual for the actual path of the drill bit to be in the form of a helix around the intended path, akin to a corkscrew path. The helix or corkscrew path can be seen in Fig. 1 as path 2 drilled by the drilling rig B. This helix or corkscrew path is caused by the drill bit and/or the drill string rotating out of balance. The helix or corkscrew path will mean that the drill bit and drill string must take a longer path than that intended. Conventional methods and apparatus for reducing this problem include running a number of stabilisers spaced apart along the string, where the stabilisers contact the just-drilled borehole with spiral fluted fins, in an attempt to keep the string straight. However, this conventional method is extremely inefficient, since the stabilisers inevitably create drag on the progress of the string into the borehole, in the form of friction between the fins and the inner surface of the borehole. Accordingly, it would be advantageous and desirable to reduce the helix or corkscrew path to a path which approaches that intended, with a reduced or obviated requirement for stabilisers, where a reduced helical or corkscrew path is shown in Fig. 1 as path 3. According to a first aspect, the present invention provides an apparatus for inclusion in a string having a drill bit located at one end, the apparatus comprising a body having a longitudinal axis which is substantially coaxial with the longitudinal axis of the string, a portion of the body having a cross-section where the centre of mass of the body on that cross- section is offset from the longitudinal axis of the string. The invention thus provides a string having an eccentric weight e.g. offset from the axis. According to a second aspect, the present invention provides a method of reducing the variation of a drill bit from an intended path in a borehole, where the drill bit is coupled to one end of a string having a longitudinal axis, the method comprising the steps of including a body in the string, where a portion of the body has a cross-section where the centre of mass of the body on that cross-section is offset from the longitudinal axis of the string; inserting the string into a borehole; and rotating at least the portion of the body. The invention has the advantage that rotation of the portion of the body about its longitudinal axis produces a vibratory action by the portion of the body. The intended path is preferably straight. Preferably, the mass of the body on that cross-section is distributed such that one half of the cross-section has a lower mass than the other half of the cross- section. Optionally, the body is a tubular member, which typically comprises a throughbore which is in communication with a throughbore of the string. Optionally, the portion of the tubular member comprises a non-uniform tubular cross-section, and preferably comprises a cavity which is typically formed along a length of the longitudinal axis of the tubular member. The invention has the advantage that the cavity can be left empty, partially filled, or wholly filled with a filling material, in order to vary the centre of mass of the tubular member. Optionally, the filling material is substantially secured within the cavity, and more preferably, the filling material is substantially secured (or its position in the cavity influenced) by a magnetic force. Preferably, the cavity is formed by removing a cover portion from the tubular member, and removing material from either, or both of, the tubular member or the cover portion. Preferably, the cover portion is replaced on the tubular member such that the cavity is formed in the space between the tubular member and the cover portion. Preferably, the cover portion includes an aperture to permit access to the cavity, and more preferably to permit the filling material to be inserted into, or removed from, the cavity. Preferably, the apparatus further comprises a closure device to permit the aperture to be obturated. Preferably, the cover portion is replaced on the tubular member by securing it to the tubular member. More preferably, the cover portion is secured to the tubular member by welding it thereto. An embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: - Fig. 1 is a schematic diagram of two boreholes being drilled; Fig. 2 shows an exploded side view of an apparatus for use downhole in accordance with the present invention; Fig. 3 shows an exploded perspective view of the apparatus of Fig. 2; Fig. 4 shows a partial-cut-away side view of the apparatus of Fig. 2 assembled; Fig. 5 shows a cross-sectional view across section AA of the apparatus of Fig. 4; and Fig. 6 shows a partial-cut-away perspective view of the apparatus Fig. 2, fully assembled. A downhole tool 10 is shown in Fig. 2, and which is intended for inclusion in a string (not shown), such as a drill string made up of a number of drill pipes, where the string is inserted into a borehole 3 (see Fig. 1) to be drilled by a drilling rig/platform A. The tool 10 comprises a body 20, which has a tapered screw thread pin connection 18 at one end and a tapered screw thread box connection 19 at the other end, for inclusion in a drill string in a conventional manner. The body 20 also comprises a throughbore 25, which is shown in dotted phantom lines, and which is in fluid communication with the pin 18 and box 19 connections, thus allowing fluid, such as conventional drilling fluid, to flow through the body 20. The lower half of the body 20 as shown in Fig. 2 is in the form of a standard tubular member, such as a drill pipe, and thus has a semi-circular outer circumference which is of the same diameter as the circular outer circumference of the box connection 19. The outer surface diameter of the box connection 19, and the semi-circular outer surface diameter of the lower half of the body 20, is 8 inches and the inner diameter of the throughbore 25 is 2 7 / 8 inches, which provides a side wall thickness of the lower half of the body 20 of 2 9 / 16 inches . A length, which is semi-circular in cross-section, of the upper half of the body 20 shown in Fig. 2 has been removed, although a semi-circular side wall portion 30 of the body 20 remains, thus ensuring that there is no fluid leak path from the throughbore 25 through the sidewall portion 30. The sidewall portion 30 has a thickness of h inch. A cover 22 is shown in Fig. 2 and has a semi-circular cross-section of the same outer diameter as that of the lower half of the body 20 (that is 8 inches). The sidewall thickness of the cover 22 is h inch. In use of the downhole tool 10, the cover 22 is fitted onto the upper half of the body 20 as shown in Fig. 2, such that the outer circumference of the cover 22 is in line with the outer circumference of the pin connection 19. Therefore, there is a cavity formed between the inner circumference of the cover 22 and the outer circumference of the upper half of the body 20 as shown in Fig. 2. The cross-sectional radius of the cavity 32 is l 5 / 8 inch. The cover 22 is secured to the upper half of the body 20, as shown in Figs. 4, 5 and 6, and preferably the cover 22 is welded to the body 20. Preferably, the cover 22 is formed from same the material removed from the body 20 to form the upper half of the body 20. An aperture 26 having a 1*_ inch diameter is formed through the sidewall of the cover 22, and in use, the aperture 26 is obturated by a plug 24. Hence, the centre of mass of the body 20 through the cross-section shown in Fig. 5 does not coincide with the longitudinal axis of the downhole tool 10, since the lower half of the body 20 will have a greater collective mass than the upper half of the body 20. The collective mass of the upper half of the body 20 can be increased by inserting a filling material 34 into the cavity 30 through the aperture 26. A suitable filling material is shown in Fig. 6 as a number of 1 inch diameter steel balls, collectively designated as 34. Thus, by varying the number of steel balls 34 retained within the cavity 32, the centre of mass of the body 20 can be altered. The steel balls 34 can optionally be secured within the cavity 32 by attaching a permanent magnet 36 to the outer surface of the side wall portion 30. The length, depth and width of the material removed from the upper half of the body 20 to form the side wall portion 30, the cover 22, and the resulting cavity 32 can be varied. Further, the number of one inch diameter steel balls 34 inserted into the cavity 32 can be varied, as can the location at which the downhole tool 10 is included in the string, depending on the formation through which it is required to be drilled. Accordingly, when the body 20 is rotated, within the drill string, a vibratory action will be produced, since the downhole tool 10 will be rotating out of balance. Depending on the aforementioned variables, this vibratory action will tend to counterbalance the out of balance rotation of the drill bit, and thus reduce or preferably eliminate the variation of the drill bit from its intended path. The vibratory motion produced by the body 20 can be tuned to values chosen to counteract vibration or deviation of the string from its intended path. Accordingly, by reducing the vibration imparted to the end of the drill string by the out of balance rotation of the drill bit, the spiralling or helical action of the drill bit is reduced or optimally eliminated. Further, the invention has the advantage that by providing a cavity 32 so that the centre of mass of a cross-section of the body 20 does not coincide with that of the downhole tool 10, or that of the string, the outer circumference of the body 20 has no upsets, and thus is flush with the outer circumference of the drill string. If the downhole tool 10 is included in the string between a drilling motor and the drill bit, typical rotation speeds are likely to be in the region of 500 to 600 rpm. If the rotation is provided to the drill bit by rotary drilling, that is by rotating the drill string from the drilling rig/platform, then typical rotation speeds of the drill string, and thus the body 20, are likely to be in the region of 80 to 150 rpm, but could be as high as 220 rpm. Accordingly, by use of the present invention, the requirement for conventional stabilisers in the string is reduced or optimally obviated. Modifications and improvements may be made without departing from the scope of the invention. For example, the cavity can be filled with filling material with a different density than the material forming the tool 10. Particularly, the filling material can be a denser material such as lead. Additionally, more than one cavity can be formed in the same portion of the tool 10, so as to allow increased control of variation of the string's path.

Claims: CLAIMS ; - 1. Apparatus (10) for inclusion in a string having a drill bit located at one end, the apparatus (10) comprising a body (20) having a longitudinal axis which is substantially coaxial with the longitudinal axis of the string, a portion (30) of the body (20) having a cross-section where the centre of mass of the body (20) on that cross-section is offset from the longitudinal axis of the string. 2. Apparatus (10) according to claim 1, wherein the mass of the body (20) on that cross-section is distributed such that one half of the cross-section has a lesser mass than the other half of the cross-section. 3. Apparatus (10) according to either of claims 1 or 2, wherein the body (20) is a tubular member (20). 4. Apparatus (10) according to claim 3, wherein the tubular member (20) comprises a throughbore (25) which is in communication with a throughbore of the string. 5. Apparatus (10) according to either of claims 3 or 4, wherein the portion (30) of the tubular member (20) comprises a non-uniform tubular cross-section. 6. Apparatus (10) according to any of claims 3 to 5, wherein the tubular member (20) further comprises a cavity (32) . 7. Apparatus (10) according to claim 6, wherein the cavity (32) is formed along a length of the longitudinal axis of the tubular member (20). 8. Apparatus (10) according to either of claims 6 or 7, wherein a filling material (34) is inserted into the cavity (32) such that the centre of mass of the tubular member (20) is varied. 9. Apparatus (10) according to claim 8, wherein movement of the filling material (34) within the cavity (32) is inhibited. 10. Apparatus (10) according to claim 9, wherein movement of the filling material (34) in the cavity (32) is inhibited by a magnetic force. 11. Apparatus (10) according to any of claims 6 to 10, wherein the cavity (32) is formed by removing a cover portion (22) from the tubular member (20), and removing material from at least one of the tubular member (20) and the cover portion (22). 12. Apparatus (10) according to claim 11, wherein the cover portion (22) is replaced on the tubular member (20) such that the cavity (32) is formed in the space between the tubular member (20) and the cover portion (22). 13. Apparatus (10) according to either of claims 11 or 12, wherein the cover portion (22) includes an aperture (26) to permit access to the cavity (32). 14. Apparatus (10) according to claim 13, wherein the aperture (26) permits the filling material (34) to be inserted into, or removed from, the cavity (32). 15. Apparatus (10) according to either of claims 13 or 14, wherein the apparatus (10) further comprises a closure device (24) to permit the aperture (26) to be obturated. 16. Apparatus (10) according to any of claims 12 to 15, wherein the cover portion (22) can be replaced on the tubular member (20) and secured to the tubular member (20). 17. Apparatus (10) according to claim 16, wherein the cover portion (22) is secured to the tubular member (20) by welding it thereto. 18. A method of reducing the variation of a drill bit from an intended path (1) in a borehole, where the drill bit is coupled to one end of a string having a longitudinal axis, the method comprising the steps of including a body (20) in the string, where a portion (30) of the body (20) has a cross-section where the centre of mass of the body (20) on that cross-section is offset from the longitudinal axis of the string; inserting the string into a borehole; and rotating at least the portion (30) of the body (20). 19. A method according to claim 18, wherein rotation of the portion (30) of the body (20) about its longitudinal axis produces a vibratory motion by the portion (30) of the body (20). 20. A method according to claim 19, wherein the vibratory motion is tuned to valves chosen to counteract vibration or deviation of the string from its intended path. 21. A method according to claim 20, wherein the vibratory motion is tuned by altering the distribution and/or location and/or amount of mass in the portion (30) of the body (20). 22. A method according to any one of claims 18 to 21, wherein the mass of the body (20) on that cross-section is distributed such that one half of the cross-section has a lesser mass than the other half of the cross- section. 23. A method according to any of claims 18 to 22, wherein the body (20) is a tubular member (20) and the portion (30) of the tubular member (20) comprises a cavity (32) which is formed along a length of the longitudinal axis of the tubular member (20). 24. A method according to claim 23, wherein a filling material (34) is inserted into the cavity (32) such that the centre of mass of the tubular member (20) is varied.

Inventors: OWENS DEWEY E, OWENS, DEWEY, E.
Applicants: OWENS DEWEY E, SIBILLE PATRICK NEIL, OWENS, DEWEY, E., SIBILLE, PATRICK, NEIL
Application year: 1998
Main classification: E21B7
IPC codes: E21B7, E21B17