STEERABLE DRILLING WITH DOWNHOLE MOTOR

Abstract: A method is provided for creating a borehole in an earth formation using a drilling assembly comprising a drill string (5) extending into the borehole, a downhole motor (6) including a housing (7) and a drive shaft (8) for rotating a drill bit (9), which drive shaft has an inclined orientation relative to a longitudinal axis of the lower part of the drill string. The motor housing is connected to the lower part of the drill string in a manner so as to allow rotation of the motor housing about said longitudinal axis. The drilling assembly further comprises an electric motor (23) for rotation of the motor housing about said longitudinal axis and relative to the drill string. The method comprises drilling a substantially straight section of said borehole by inducing said electric motor to rotate the motor housing continuously about said longitudinal axis and relative to the drill string while the downhole motor is operated to rotate the drill bit.

Description: 635 PCI7EP95/05163 - 1 STEERABLE DRILLING WITH DOWNHOLE MOTOR The present invention relates to a method and a system for creating a borehole in an earth formation. More specifically the invention relates to drilling of boreholes whereby a drilling assembly is applied which allows steering of a drill bit to a desired subsurface target area. A typical application in which there is a need for such steering of the drill bit, is, for example, in offshore drilling whereby a plurality of deviated wellbores are drilled from an offshore platform to multiple locations of a hydrocarbon containing zone in an earth formation. Other applications in which there is a need for steerable drilling, include drilling of horizontal or highly inclined wellbores into such a hydrocarbon containing zone. EP-571045-A1 discloses a method of creating a borehole in an earth formation using a drilling assembly comprising a drill string extending into the borehole, a downhole motor including a housing and a drive shaft for rotating a drill bit, which drive shaft has an inclined orientation relative to a longitudinal axis of the lower part of the drill string, the motor housing being connected to the lower part of the drill string in a manner so as to allow rotation of the motor housing about said longitudinal axis, the drilling assembly further comprising control means to control rotation of the motor housing about said longitudinal axis and relative to the drill string. In the known method, a downhole motor having a housing with an adjustable bend is applied, whereby during straight drilling the bend is adjusted so that the bend angle is zero, and during curved drilling the bend is adjusted so that the bend angle corresponds to the desired borehole curvature. Prior to drilling a curved borehole section the motor housing is rotated relative to the drill string about a selected number of incremental angular steps so as to orient the motor housing in the desired azimuthal direction. It is an object of the invention to provide a method of creating a borehole in an earth formation whereby a downhole motor is used which allows curved drilling as well as straight drilling, without the need of a motor housing having an adjustable bend. It is a further object of the invention to provide a drilling assembly for creating a borehole in an earth formation using a downhole motor which allows curved drilling as well as straight drilling, without the need of a motor housing having an adjustable bend. The method according to the invention therefor comprises the step of drilling a substantially straight section of said borehole by inducing said control means to rotate the motor housing continuously about said longitudinal axis and relative to the drill string while the downhole motor is operated to rotate the drill bit. The continuously rotating motor housing ensures that the orientation of the inclined drive shaft of the motor continuously changes so that there is no preferential direction in which the drill bit will proceed in the formation. As a consequence thereof, the drill bit will drill a straight borehole section during such continuous rotation of the housing. The drilling assembly according to the invention comprises a drill string extending into the borehole, a downhole motor including a housing and a drive shaft for rotating a drill bit, which drive shaft is oriented inclined relative to a longitudinal axis of the lower part of the drill string, the motor housing being connected to the lower part of the drill string in a manner so as to allow rotation of the motor housing about said longitudinal axis, the drilling assembly further comprising control means to control rotation of the motor housing about said longitudinal axis and relative to the drill string, and means for drilling a substantially straight section of said borehole by inducing said control means to rotate the motor housing continuously about said longitudinal axis and relative to the drill string while the downhole motor is operated to rotate the drill bit. The invention will be described hereinafter in more detail with reference to the accompanying drawing in which Fig. 1 shows schematically a lower part of a drilling assembly as used in the method according to the invention. The drilling assembly shown in Fig. 1 extends into a borehole 1 formed in an earth formation 3. The assembly, of which only the lower part is shown in Fig. 1, includes a drill string in the form of a tubing 5 of relatively small diameter, for example a diameter of about 50 mm. Before being lowered into the borehole 1 the tubing 5 is stored in the form of a coil on a reel (not shown) located at surface. As drilling progresses the tubing 5 is unreeled from the reel and gradually lowered into the borehole 1. The tubing 5 is therefore also referred to as "coiled tubing", and drilling with such coiled tubing is also referred to as "coiled tubing drilling". A downhole drilling motor 6 having a housing 7 is located at the lower end of the drilling assembly, which motor 6 drives a drive shaft 8 provided with a drill bit 9 which cuts into the rock at the borehole bottom 11 during drilling. The downhole motor 6 forms a hydraulic motor of the Moineau type which is well-known in the art of steerable drilling. The drive shaft 8 and the drill bit 9 are arranged inclined relative to the motor housing 7, so that a longitudinal axis 13 of the lower end part of the coiled tubing 5 is oriented at angle 15 relative to a longitudinal axis 17 of the drive shaft 8. The longitudinal axes 13 and 17 have a point of intersection 19 located below the motor housing 7. The housing 7 of the downhole motor 6 is provided with a number of stabiliser blades 21 to stabilise and centralise the lower part of the drilling assembly in the borehole 1. The downhole motor 6 is connected to the coiled tubing 5 via an electric motor 23 having a housing 25 and an output shaft 27, the upper end of the housing 25 being fixedly connected to the lower end of the coiled tubing 5, and the lower end of the output shaft 27 being fixedly connected to the upper end of the housing 7 of the downhole motor 6. The output shaft 27 of the electric motor 23 is rotatable about its longitudinal axis, relative to the housing 25 of the electric motor 23. When the electric motor 23 is operated the output shaft 27 rotates about its longitudinal axis, relative to the housing 25. A bore (not shown) extends through the interior of the electric motor 23, which bore provides a fluid flow path between the interior of the coiled tubing and the fluid inlet of the downhole motor 6. Thus, when drilling fluid is pumped through the coiled tubing to drive the downhole motor 6 the drilling fluid flows through the bore of the electric motor 23 to the inlet of the downhole motor 6. Electric power is provided to the electric motor 23 via a power supply line extending through the interior of a protective steel sheath 29 which is rigidly connected along the coiled tubing 5, to surface. The power supply line is connected at surface to an electric power supply (not shown) which is controlled by a suitable control system (not shown) . The electric motor 23 is furthermore provided with an orientation indicator (not shown) for providing an indication of the angular orientation of the output shaft 27, and thus also of the angular orientation of the downhole motor 6, relative to the coiled tubing 5. The orientation indicator provides a signal representing said orientation of the output shaft 27 to an operator at surface, via a suitable signal line extending through the steel sheath 29. During normal operation of the drilling assembly, a substantially straight borehole section is drilled as follows. Drilling fluid is pumped through the coiled tubing 5 and through the bore of the electric motor 23 to the fluid inlet of the downhole motor 6. Thereby the downhole motor 6 is operated to drive the drive shaft 8 and the drill bit 9. Simultaneously with operating the downhole motor 23, the control system at surface induces the power supply to provide electric power to the electric motor 23, so that the output shaft 27 of the electric motor 23 is rotated continuously at a controlled speed and thereby the housing of the downhole motor 6 is rotated at the same controlled speed. The drill bit 9 consequently rotates in the borehole 1 about both axes 13 and 17. Since the orientation of the drill bit 9 in the borehole 1 continuously changes due to the rotation of the drill bit 9 about the axis 13, there is no preferential direction for the drill bit 9 to deepen the borehole 1. Consequently the drill bit 9 drills a straight section which is substantially directed along the axis 13. It should be noted that deviations from the straight direction of the borehole section drilled in this manner, can occur if the earth formation 3 is anisotropic or inhomogeneous. However such deviations can be corrected by measuring the direction of the borehole 1 at regular intervals, for example by well-known telemetry methods, and drilling a short curved section as described hereinafter, if necessary. When a curved borehole section is to be drilled following drilling of the straight borehole section, drilling proceeds as follows. First the control system at surface induces the power supply to operate the electric motor 23 so as to rotate the output shaft 27 about a selected angle corresponding to a selected orientation of the housing 7 of the downhole motor 6 in the borehole 1, which orientation of the motor housing 7 determines the direction in which drilling of a curved borehole section proceeds. To achieve the desired orientation of the motor housing 7, the orientation indicator is induced to provide an indication of the angular orientation of the output shaft 27 to an operator at surface. After the desired orientation is reached, the electric motor 23 is stopped, and the downhole motor 6 is operated so as to rotate the drive shaft 8 and the drill bit 9 by pumping drilling fluid through the coiled tubing 5 and through the bore of the electric motor 23 to the fluid inlet of the downhole motor 6. Thus, drilling proceeds with the housing 7 of the downhole motor 6 stationary while the drill bit 9 rotates. Since the drill bit 9 is inclined relative to the longitudinal axis 13 of the lower part of the drilling assembly, the borehole 1 is deepened in the direction of inclination of the drill bit 9 so that a curved borehole section is drilled. When the borehole 1 is found to be directed along the desired course, and drilling should be continued in a straight line, the electric motor 23 is operated again so as to simultaneously rotate the motor housing 7 continuously and to operate the downhole motor 6 so as to rotate the drill bit 9. To reach a target area in the earth formation 3 the drilling operator may repeat the above procedure for alternatingly drilling straight and curved borehole sections. During drilling the coiled tubing will twist due to reaction moments acting on the coiled tubing, the degree of twist being dependent on various factors, such as the diameter and wall-thickness of the coiled tubing, the weight-on-bit, the pressure of the fluid driving the downhole motor, or the magnitude of friction forces between the coiled tubing and the borehole-wall. Instead of orienting the downhole motor by operating the electric motor in the above described manner, the motor can alternatively be oriented by varying the twist angle of the coiled tubing, for example by adjusting the weight- on-bit or by adjusting the pressure of the drilling fluid which drives the downhole motor, or by a combined operation of the electric motor and a variation of the twist angle. Instead of applying an electric motor to rotate the housing of the downhole motor which drives the drill bit, a hydraulic motor or any other suitable motor which can be operated in a controlled manner can be applied. Preferably said control means does not include means for controlling rotation of the motor housing about said longitudinal axis and relative to the drill string, which rotation is caused by reactive torque forces exerted to the motor housing due to the reactive torque action of the rotating drill bit in the borehole. In conclusion, the above described method and system using a coiled tubing, a downhole motor and a second motor to control rotation of the downhole motor housing, allows drilling of straight and curved borehole sections without the need of a drill string which is rotated at surface.

Claims: C L A I M S 1. A method of creating a borehole in an earth formation using a drilling assembly comprising a drill string extending into the borehole, a downhole motor including a housing and a drive shaft for rotating a drill bit, which drive shaft has an inclined orientation relative to a longitudinal axis of the lower part of the drill string, the motor housing being connected to the lower part of the drill string in a manner so as to allow rotation of the motor housing about said longitudinal axis, the drilling assembly further comprising control means to control rotation of the motor housing about said longitudinal axis and relative to the drill string, the method comprising drilling a substantially straight section of said borehole by inducing said control means to rotate the motor housing continuously about said longitudinal axis and relative to the drill string while the downhole motor is operated to rotate the drill bit. 2. The method of claim 1, wherein the step of drilling said substantially straight borehole section is alternated with a further step of drilling a curved section of said borehole by inducing said control means to prevent rotation of the motor housing about said longitudinal axis and relative to the drill string while the downhole motor is operated to rotate the drill bit. 3. The method of claim 2, wherein prior to the step of drilling the curved section said control means is induced to rotate the motor housing about an angle corresponding to a selected orientation of the drill bit relative to the borehole. 4. The method of any of claims 1-3, wherein said control means does not include means for controlling rotation of the motor housing about said longitudinal axis and relative to the drill string, which rotation is caused by reactive torque forces exerted to the motor housing due to the reactive torque action of the rotating drill bit in the borehole. 5. The method of any of claims 1-4, wherein said control means includes one of the group of a hydraulic motor and an electric motor. 6. The method of any of claims 1-5, said drill string forming a coiled tubing which is reeled onto a reel, wherein the coiled tubing is unreeled from the reel and lowered into the borehole as drilling of the borehole proceeds. 7. A drilling assembly for creating a borehole in an earth formation, comprising a drill string extending into the borehole, a downhole motor including a housing and a drive shaft for rotating a drill bit, which drive shaft is oriented inclined relative to a longitudinal axis of the lower part of the drill string, the motor housing being connected to the lower part of the drill string in a manner so as to allow rotation of the motor housing about said longitudinal axis, the drilling assembly further comprising control means to control rotation of the motor housing about said longitudinal axis and relative to the drill string, and means for drilling a substantially straight section of said borehole by inducing said control means to rotate the motor housing continuously about said longitudinal axis and relative to the drill string while the downhole motor is operated to rotate the drill bit. 8. The drilling assembly of claim 7, means for alternating the step of drilling said substantially straight borehole section with a step of drilling a curved section of said borehole by inducing said control means to prevent rotation of the motor housing about said longitudinal axis and relative to the drill string while the downhole motor is operated to rotate the drill bit. 9. The drilling assembly of claim 8, comprising means for, prior to drilling the curved section, inducing said control means to rotate the motor housing about an angle corresponding to a selected orientation of the drill bit relative to the borehole. 10. The drilling assembly of any claims 7-9, wherein said control means does not include means for controlling rotation of the motor housing about said longitudinal axis and relative to the drill string, which rotation is caused by reactive torque forces exerted to the motor housing due to the reactive torque action of the rotating drill bit in the borehole. 11. The drilling assembly of any of claims 7-10, wherein said control means includes one of the group of a hydraulic motor and an electric motor. 12. The drilling assembly of any of claims 7-11, wherein said drill string forms a coiled tubing which is reeled onto a reel, the assembly further comprising means for unreeling the coiled tubing from the reel and lowering the coiled tubing into the borehole as drilling of the borehole proceeds. 13. The method substantially as described hereinbefore, with reference to the drawing. 14. The drilling assembly substantially as described hereinbefore, with reference to the drawing.

Inventors: FAURE ALBAN MICHEL, FAURE, ALBAN, MICHEL
Applicants: SHELL CANADA LTD, SHELL INT RESEARCH, SHELL CANADA LIMITED, SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1995
Main classification: E21B7
IPC codes: E21B7