OA10429A

OA10429A - Steerable drilling with downhole motor

Info

Publication number: OA10429A
Application number: OA70030A
Authority: OA
Inventors: Alban Michel Faure
Original assignee: Shell Int Research
Priority date: 1994-12-21
Filing date: 1997-06-20
Publication date: 2002-03-21
Also published as: AU692040B2; NO972878L; NO310036B1; CA2207923C; BR9510493A; EG20620A; DE69509557T2; CO4480786A1; WO1996019635A1; MY115387A; CN1171142A; RU2149248C1; CA2207923A1; SA95160479B1; TNSN95131A1; AR004469A1; EP0799363A1; MX9704585A; DE69509557D1; AU4435796A

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

010429 - î -

STEERABLE DRILLING WITH DOWNHOLE MOTOR

The présent invention relates to a method and aSystem for creating a borehole in an earth formation.

More specifically the invention relates to drilling ofboreholes whereby a drilling assembly is applied which 5 allows steering of a drill bit to a desired subsurface target area. A typical application in which there is aneed for such steering of the drill bit, is, for example,in offshore drilling whereby a plurality of deviatedwellbores are drilled from an offshore platform to 10 multiple locations of a hydrocarbon containing zone in an earth formation. Other applications in which there is aneed for steerable drilling, include drilling ofhorizontal or highly inclined wellbores into such ahydrocarbon containing zone. 15 EP-571045-A1 discloses a method of creating a borehole in an earth formation using a drilling assemblycomprising a drill string extending into the borehole, adownhole motor including a housing and a drive shaft forrotating a drill bit, which drive shaft has an inclined 20 orientation relative to a longitudinal axis of the lower part of the drill string, the motor housing beingconnected to the lower part of the drill string in amanner so as to allow rotation of the motor housing aboutsaid longitudinal axis, the drilling assembly further 25 ' comprising control means to control rotation of the motorhousing about said longitudinal axis and relative to thedrill string. In the known method, a downhole motorhaving a housing with an adjustable bend is applied,whereby during straight drilling the bend is adjusted so 30 that the bend angle is zéro, and during curved drilling the bend is adjusted so that the bend angle corresponds 2 010429 to the desired borehole curvature. Prior to drilling acurved borehole section the motor housing is rotatedrelative to the drill string about a selected number ofincrémental angular steps so as to orient the motor 5 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 adownhole motor is used which allows curved drilling aswell as straight drilling, without the need of a motor 10 housing having an adjustable bend.

It is a further object of the invention to provide a drilling assembly for creating a borehole in an earthformation using a downhole motor which allows curveddrilling as well as straight drilling, without the need 15 of a motor housing having an adjustable bend.

The method according to the invention therefor comprises the step of drilling a substantially straightsection of said borehole by inducing said control meansto rotate the motor housing continuously about said 20 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 theorientation of the inclined drive shaft of the motorcontinuously changes so that there is no preferential 25 direction in which the drill bit will proceed in the formation. As a conséquence thereof, the drill bit willdrill a straight borehole section during such continuousrotation of the housing.

The drilling assembly according to the invention 30 comprises a drill string extending into the borehole, a downhole motor including a housing and a drive shaft forrotating a drill bit, which drive shaft is orientedinclined relative to a longitudinal axis of the lowerpart of the drill string, the motor housing being 35 connected to the lower part of the drill string in a - 3 - 010429 manner so as to allow rotation of the motor housing aboutsaid longitudinal axis, the drilling assembly furthercomprising control means to control rotation of the motorhousing about said longitudinal axis and relative to the 5 drill string, and means for drilling a substantially straight section of said borehole by inducing saidcontrol means to rotate the motor housing continuouslyabout said longitudinal axis and relative to the drillstring while the downhole motor is operated to rotate the 10 drill bit.

The invention will be described hereinafter in moredetail with reference to the accompanying drawing inwhich Fig. 1 shows schematically a lower part of adrilling assembly as used in the method according to the 15 invention.

The drilling assembly shown in Fig. 1 extends into aborehole 1 formed in an earth formation 3. The assembly,of which only the lower part is shown in Fig. 1, includesa drill string in the form of a tubing 5 of relatively 20 small diameter, for example a diameter of about 50 mm.

Before being lowered into the borehole 1 the tubing 5 isstored in the form of a coil on a reel (not shown)located at surface. As drilling progresses the tubing 5is unreeled from the reel and gradually lowered into the 25 borehole 1. The tubing 5 is therefore also referred to as "coiled tubing", and drilling with such coiled tubing isalso referred to as "coiled tubing drilling". A downholedrilling motor 6 having a housing 7 is located at thelower end of the drilling assembly, which motor 6 drives 30 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 theMoineau type which is well-known in the art of steerabledrilling. The drive shaft 8 and the drill bit 9 are 35 arranged inclined relative to the motor housing 7, so 4 010429 that a longitudinal axis 13 of the lower end part of thecoiled tubing 5 is oriented at angle 15 relative to alongitudinal axis 17 of the drive shaft 8. Thelongitudinal axes 13 and 17 hâve a point of intersection 5 19 located below the motor housing 7. The housing 7 of the downhole motor 6 is provided with a number ofstabiliser blades 21 to stabilise and centralise thelower part of the drilling assembly in the borehole 1.

The downhole motor 6 is connected to the coiled 10 tubing 5 via an electric motor 23 having a housing 25 and an output shaft 27, the upper end of the housing 25 beingfixedly connected to the lower end of the coiled tubing5, and the lower end of the output shaft 27 being fixedlyconnected to the upper end of the housing 7 of the 15 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. Whenthe electric motor 23 is operated the output shaft 27rotâtes about its longitudinal axis, relative to the 20 housing 25. A bore (not shown) extends through the interior of the electric motor 23, which bore provides afluid flow path between the interior of the coiled tubingand the fluid inlet of the downhole motor 6. Thus, whendrilling fluid is pumped through the coiled tubing to 25 drive the downhole motor 6 the drilling fluid flows through the bore of the electric motor 23 to the inlet ofthe downhole motor 6. Electric power is provided to theelectric motor 23 via a power supply line extendingthrough the interior of a protective Steel sheath 29 30 which is rigidly connected along the coiled tubing 5, to surface. The power supply line is connected at surface toan electric power supply (not shown) which is controlledby a suitable control System (not shown). The electricmotor 23 is furthermore provided with an orientation 35 indicator (not shown) for providing an indication of the 010429 10 15 20 25 30 35 angular orientation of the output shaft 27, and thus alsoof the angular orientation of the downhole motor 6,relative to the coiled tubing 5. The orientationindicator provides a signal representing said orientationof the output shaft 27 to an operator at surface, via asuitable signal line extending through the Steel sheath29.

During normal operation of the drilling assembly, asubstantially straight borehole section is drilled asfollows. Drilling fluid is pumped through the coiledtubing 5 and through the bore of the electric motor 23 tothe fluid inlet of the downhole motor 6. Thereby thedownhole motor 6 is operated to drive the drive shaft 8and the drill bit 9. Simultaneously with operating thedownhole motor 23, the control System at surface inducesthe power supply to provide electric power to theelectric motor 23, so that the output shaft 27 of theelectric motor 23 is rotated continuously at a controlledspeed and thereby the housing of the downhole motor 6 isrotated at the same controlled speed. The drill bit 9consequently rotâtes in the borehole 1 about both axes 13and 17. Since the orientation of the drill bit 9 in theborehole 1 continuously changes due to the rotation ofthe drill bit 9 about the axis 13, there is nopreferential direction for the drill bit 9 to deepen theborehole 1. Consequently the drill bit 9 drills astraight section which is substantially directed alongthe axis 13. It should be noted that déviations from thestraight direction of the borehole section drilled inthis manner, can occur if the earth formation 3 isanisotropic or inhomogeneous. However such déviations canbe corrected by measuring the direction of the borehole 1at regular intervals, for example by well-known telemetrymethods, and drilling a short curved section as described hereinafter, if necessary. 010429

When a curved borehole section is to be drilledfollowing drilling of the straight borehole section,drilling proceeds as follows. First the control System atsurface induces the power supply to operate the electric 5 motor 23 so as to rotate the output shaft 27 about a selected angle corresponding to a selected orientation ofthe housing 7 of the downhole motor 6 in the borehole 1,which orientation of the motor housing 7 détermines thedirection in which drilling of a curved borehole section 10 proceeds. To achieve the desired orientation of the motor housing 7, the orientation indicator is induced toprovide an indication of the angular orientation of theoutput shaft 27 to an operator at surface. After thedesired orientation is reached, the electric motor 23 is 15 stopped, and the downhole motor 6 is operated so as to rotate the drive shaft 8 and the drill bit 9 by pumpingdrilling fluid through the coiled tubing 5 and throughthe bore of the electric motor 23 to the fluid inlet ofthe downhole motor 6. Thus, drilling proceeds with the 20 housing 7 of the downhole motor 6 stationary while the drill bit 9 rotâtes. Since the drill bit 9 is inclinedrelative to the longitudinal axis 13 of the lower part ofthe drilling assembly, the borehole 1 is deepened in thedirection of inclination of the drill bit 9 so that a 25 curved borehole section is drilled.

When the borehole 1 is found to be directed along the desired course, and drilling should be continued in astraight line, the electric motor 23 is operated again soas to simultaneously rotate the motor housing 7 30 continuously and to operate the downhole motor 6 so as to rotate the drill bit 9. To reach a target area in theearth formation 3 the drilling operator may repeat theabove procedure for alternatingly drilling straight andcurved borehole sections. 010429

During drilling the coiled tubing will twist due toreaction moments acting on the coiled tubing, the degreeof twist being dépendent on various factors, such as thediameter and wall-thickness of the coiled tubing, the 5 weight-on-bit, the pressure of the fluid driving the downhole motor, or the magnitude of friction forcesbetween the coiled tubing and the borehole-wall. Insteadof orienting the downhole motor by operating the electricmotor in the above described manner, the motor can 10 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 fluidwhich drives the downhole motor, or by a combinedoperation of the electric motor and a variation of the 15 twist angle.

Instead of applying an electric motor to rotate thehousing of the downhole motor which drives the drill bit,a hydraulic motor or any other suitable motor which canbe operated in a controlled manner can be applied. 20 Preferably said control means does not include means for controlling rotation of the motor housing about saidlongitudinal axis and relative to the drill string, whichrotation is caused by reactive torque forces exerted tothe motor housing due to the reactive torque action of 25 the rotating drill bit in the borehole.

In conclusion, the above described method and System using a coiled tubing, a downhole motor and a secondmotor to control rotation of the downhole motor housing,allows drilling of straight and curved borehole sections 30 without the need of a drill string which is rotated at surface.

Claims

8 010429 AMENDED CLAIMS

1. A method of creating a borehole in an earth formationusing a drilling assembly comprising a drill stringextending into the borehole, a downhole motor including ahousing and a drive shaft for rotating a drill bit, which 5 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 ofthe drill string in a manner so as to allow rotation ofthe motor housing about said longitudinal axis, the 10 drilling assembly further comprising control means to control rotation of the motor housing about saidlongitudinal axis and relative to the drill string, themethod comprising drilling a substantially straightsection of said borehole by inducing said control means 15 to rotate the motor housing continuously about said longitudinal axis and relative to the drill string whilethe downhole motor is operated to rotate the drill bit,wherein the step of drilling said substantially straightborehole section is alternated with a further step of 20 drilling a curved section of said borehole by inducing said control means to prevent rotation of the motorhousing about said longitudinal axis and relative to thedrill string while the downhole motor is operated torotate the drill bit, characterized in that prior to the 25 step of drilling the curved section said control means is induced to rotate the motor housing about an anglecorresponding to a selected orientation of the drill bitrelative to the borehole.
2. The method of claim 1, wherein said control means 30 does not include means for controlling rotation of the motor housing about said longitudinal axis and relative 010429 to the drill string, which rotation is caused by reactivetorque forces exerted to the motor housing due to thereactive torque action of the rotating drill bit in theborehole.
3. The method of claim 1 or 2, wherein said control means includes one of the group of a hydraulic motor andan electric motor.
4. The method of any of daims 1-3, said drill stringforming a coiled tubing which is reeled onto a reel, 10 wherein the coiled tubing is unreeled from the reel and lowered into the borehole as drilling of the boreholeproceeds.
5. A dri-lling assembly for creating a borehole in anearth formation, comprising a drill string extending into 15 the borehole, a downhole motor including a housing and a drive shaft for rotating a drill bit, which drive shaftis oriented inclined relative to a longitudinal axis ofthe lower part of the drill string, the motor housingbeing connected to the lower part of the drill string in 20 a manner so as to allow rotation of the motor housing about said longitudinal axis, the drilling assemblyfurther comprising control means to control rotation ofthe motor housing about said longitudinal axis andrelative to the drill string, means for drilling a 25 substantially straight section of said borehole by inducing said control means to rotate the motor housingcontinuously about said longitudinal axis and relative tothe drill string while the downhole motor is operated torotate the drill bit, and means for alternating the step 30 of drilling said substantially straight borehole section with a step of drilling a curved section of said boreholeby inducing said control means to prevent rotation of themotor housing about said longitudinal axis and relativeto the drill string while the downhole motor is operated 35 to rotate the drill bit, characterized in that means are 010429 10 provided for, prior to drilling the curved section,inducing said control means to rotate the motor housingabout an angle corresponding to a selected orientation ofthe drill bit relative to the borehole.
6. The drilling assembly of claim 5, wherein said control means does not include means for controllingrotation of the motor housing about said longitudinalaxis and relative to the drill string, which rotation iscaused by reactive torque forces exerted to the motor 10 housing due to the reactive torque action of the rotating drill bit in the borehole.
7. The drilling assembly of claim 5 or 6, wherein saidcontrol means includes one of the group of a hydraulicmotor and an electric motor.
8. The drilling assembly of any of daims 5-7, wherein said drill string forms a coiled tubing which is reeledonto a reel, the assembly further comprising means forunreeling the coiled tubing from the reel and loweringthe coiled tubing into the borehole as drilling of the 20 borehole proceeds. MCS13/TS6034PC