Note: Descriptions are shown in the official language in which they were submitted.
CA 02207923 1997-06-16
WO 96/19635 PCTlEP95/05163
- 1 -
STEERABLE DRILLING WITH DOWNHOLE MOTOR
The present invention relates to a method and a
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
a 30 that the bend angle is zero, and during curved drilling
the bend is adjusted so that the bend angle corresponds
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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.
Embodiments of the invention 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.
Further embodiments of the invention 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.
In one aspect of the present invention, there is
provided 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 a 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 meahod 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
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rotate the drill bit, 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, 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.
In a second aspect of the present invention, there
is provided 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, wh_~ch drive shaft is
oriented inclined relative to a longitud~_nal axis of a 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, means
for drilling a substantially straight section of said
borehole by inducing said control means t.o 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, and 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, wherein means are provided
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63293-3734
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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.
A method according to an embodiment of the
invention 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.
A drilling assembly according to an embodiment of
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
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WO 96/19635 PCT/EP95/05163
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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
Y
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
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WO 96!19635 PCT/EP95I05163
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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
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WO 96/19635 , PCTIEP95/05163
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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 d
i
ti
ev
a
ons 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.
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WO 96119636 PCT/EP95/05163
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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.
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WO 96/19635 PCT/EP95/05163
During drilling the coiled tubing will twist due to
b
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.
