Note: Descriptions are shown in the official language in which they were submitted.
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STEERABLE GAS TURBODRILL
RELATED APPLICATIONS
[0001] The present application claims the benefit of U.S. Provisional Patent
Application,
Serial No. 61/643,145 filed on May 4, 2012 all of which is herein incorporated
by reference
in its entirety.
BACKGROUND
1. Field of the Invention
[0002] The present invention relates to gas turbodrills for downhole drilling
operations.
2. Description of the Related Art
[0003] It is generally desirable to operate a drill motor on dry gas for
completion drilling of
water sensitive formations. However, some types of drill motors are not
suitable for this
purpose. For example, progressive cavity motors incorporate elastomeric
stators that can
rapidly degrade when operated on dry gas. Turbodrills are capable of operation
on dry gas,
but these tools stall easily when operated on gas, and the motor speed is
generally much too
high for effective drilling. Typical turbodrill motors also tend to be very
long, which limits
the steer-ability of the drill string. ln a paper entitled, "Downhole
pneumatic turbine motor:
testing and simulation results," SPE Drilling Engineering, September pp 239-
246, Lyons et
al. describe the development and testing of a gas turbine motor for drilling.
As described in
this paper, the gas turbine motor included a single stage radial-flow turbine
operating at
extremely high rotary speed (i.e., at more than 100,000 rpm) and a multi-stage
planetary
transmission to reduce the speed and increase torque to the level needed to
drive a
conventional roller cone drill bit. There are technical challenges that arise
when exiting an
open hole during completion drilling, which include:
1. Orientation of the lateral bores in vertical, inclined, or horizontal
wells;
2. The kickoff of the lateral;
3. Transport of cuttings away from the drill bit;
4. Hole stability; and
5. Trajectory control.
[0004] Coiled tubing drilling ("CTD") systems capable of sidetracking and
drilling
multiple lateral bores are available. These systems have been used extensively
in Alaska to
access compartmentalized oil reservoirs. The cost of a CTD bottom hole
assembly ("BHA")
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including measurement-while-drilling and downhole bit face orientation tools
is relatively
high, as is the cost of the surface equipment required to support this
apparatus. These
systems offer full steer-ability and tracking and are capable of drilling at
up to 500/100 ft
dogleg severity ("DLS"). [DLS is a normalized estimate of the overall
curvature of a well
path between two consecutive directional survey stations, according to the
minimum
curvature survey calculation method.] A conventional CTD system incorporates a
positive
displacement motor ("PDM") designed to operate on drilling mud. This system
develops
significant torque and requires constant trajectory measurement using
measurement while
drilling tools and steering adjustment using a downhole orienter. These
steering systems are
complex and expensive and greatly increase the length of the BHA. Wire in coil
systems can
be required for operation on dry gas since mud pulse telemetry is not feasible
when running
dry gas.
[0005] It would be desirable to develop a steerable gas turbodrill ("SGTD")
that enables
high-power, high-rotary speed drilling at a lower torque than a PDM system and
which
requires minimal steering, once the SGTD is properly oriented. This approach
would
eliminate the need for high-cost measurement while drilling and the need for
bit face
orientation systems in the bottomhole assembly. This tool should be relatively
compact and
capable of being readily steered, for example, at least through a 200 ft.
lateral arc having a
constant 120 ft. radius, i.e. a spur lateral.
[0006] It would further be desirable to employ a SGTD that uses dry nitrogen,
and which
includes a gear box, enabling operation at a high rotary speed, for efficient
power conversion,
and but achieving a lower rotational speed on the output of the gear box, than
is possible for a
gas turbine power section.
SUMMARY OF THE INVENTION
[0007] In accordance with the present invention, the problems discussed above
are solved
by a gas turbodrill that includes a drill-bit section, a bearing assembly, a
gearbox assembly, a
gimbal assembly, a high-speed gas turbine power section and a flexible tubing
string are fed
downhole at the end of a string of pipe for a spur lateral drilling
application.
[0008] In one embodiment of the invention, the high-speed gas turbine power
section in
the upper section of the gas turbodrill rotates a flexible shaft that extends
through a gimbal
assembly. The lower section of the turbodrill then contains the gearbox
assembly, bearing
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assembly and drill-bit section. The gimbal assembly serves as a flex joint for
the entire gas
turbodrill, which allows the drill to move at an angle away from the central
wellbore, with a
whipstock serving as a guide. In a preferred embodiment of the invention, the
power section
is located above the gearbox which is above the gimbal section and the
flexible shaft passes
through the gimbal and drives the bit.
[0009] As the gas turbodrill is lowered downhole on the end of a pipe string
and the gas
turbodrill reaches the whipstock, which has been pre-installed, the lower
turbodrill section
will change direction with the gimbal assembly providing a pivot point. As the
gas turbodrill
and drill string are lowered further into the wellbore, the flex joint bends
until it reaches a
preset bend angle limit. A highly compressed spring inside of the gimbal
assembly locks the
bend into position.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] Various aspects and attendant advantages of one or more exemplary
embodiments
and modifications thereto will become more readily appreciated as the same
becomes better
understood by reference to the following detailed description, when taken in
conjunction with
the accompanying drawings, wherein:
[0011] FIG. 1A is a is a cross-sectional view of an exemplary openhole spur
lateral drilling
configuration showing the steerable gas turbodrill configured in accordance
with the
invention at the start of drilling the lateral;
[0012] FIG. 1B is a cross-sectional view of an exemplary openhole spur lateral
drilling
configuration showing the steerable gas turbodrill in accordance with FIG. 1A
just after the
spur lateral has started;
[0013] FIG. 1C is a cross-sectional view of an exemplary openhole spur lateral
drilling
configuration showing the steerable gas turbodrill in accordance with FIGS. 1A
and 1B at the
completion of spur lateral drilling;
[0014] FIG. 2A is a cross-sectional view of the exemplary steerable gas
turbodrill pinch
point of FIGS. IA, 1B, and 1C.
[0015] FIG. 2B is a cross-sectional enlarged view of the pinch point of FIG.
2A.
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[0016] FIG. 3 is an exemplary fixed cutter bit selection chart, which is
published by
Dimatec Inc., for use in selecting a suitable cutter bit that can be driven by
the exemplary
SGTD of FIG. 1;
[0017] FIG. 4 is an exemplary graph that can be used for a gas turbodrill
circulation
analysis, in connection with the SGTD discussed herein;
[0018] FIG. 5A is a cross-sectional view of an exemplary gas turbodrill
configuration;
[0019] FIG. 5B is an enlarged cross-sectional view of the exemplary gas
turbodrill
configuration of FIG. 5A;
[0020] FIG. 5C is an enlarged cross-sectional view of a gimbal assembly of the
exemplary
steerable gas turbodrill configuration of FIG. 4A;
[0021] FIG. 5D is a side perspective view of the exemplary steerable gas
turbodrill
configuration of FIGS. 5A, 5B, and 5C;
[0022] FIG. 6A is a side-elevational view of the exemplary steerable gas
turbodrill
configuration; and
[0023] FIG. 6B is a cross-sectional view of the exemplary gas turbodrill
configuration of
FIG. 6A.
DETAILED DESCRIPTION
[0024] It is to be understood that the invention is not limited in its
application to the details
of construction and the arrangement of components set forth in the following
description or
illustrated in the drawings. The invention is capable of other embodiments and
of being
practiced or of being carried out in various ways. Also, it is to be
understood that the
phraseology and terminology used herein is for the purpose of description and
should not be
regarded as limiting. The use of "including," "comprising," or "having" and
variations
thereof herein is meant to encompass the items listed thereafter and
equivalents thereof as
well as additional items. Unless limited otherwise, the terms "connected,"
"coupled," and
"mounted," and variations thereof herein are used broadly and encompass direct
and indirect
connections, couplings, and mountings. In addition, the terms "connected" and
"coupled"
and variations thereof are not restricted to physical or mechanical
connections or couplings.
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[0025] Air drilling systems have advantages for borehole completion
applications because
this technique leaves a dry, open borehole that requires no additional
cleanout and avoids
water contact with the formation.
[0026] An exemplary embodiment of an openhole spur lateral drilling
configuration for a
steerable gas turbodrill ("SGTD") is shown in FIGS. 1A, 1B, and 1C. A wellbore
10 is
shown with a liner 26 supporting a whipstock 30 using liner hanger. The liner
26 and
whipstock 30 may be lowered into the well and oriented using a rotary drill
rig or workover
rig with rotary capability which is not shown but are well known to those
skilled in the arts of
drilling, well completion and well intervention. A drillstring 24 extends from
above ground
into the wellbore 10 through a liner 26 that also extends from above ground
and into the
wellbore. The liner 26 guides the drillstring 24 through a medial portion of
the wellbore 10.
The drillstring 24 and SGTD 28 are fed into the wellbore with the drill rig or
workover rig
using standard methods of handling jointed tubing. Alternatively the
drillstring may be a
continuous length of tubing that is fed into the well with a coiled tubing
well service unit also
well known in the art. The SGTD 28 is coupled to the drillstring 24 before
insertion into the
wellbore 10. Prior to the insertion of the SGTD 28, a whipstock 30 is run into
the wellbore
using liner 26. Alternatively, the whipstock 30 can be run-in separately from
liner 26 and
placed with an openhole packer, not shown. The whipstock 30 serves to guide
the SGTD 28
and drillstring 24 at a desired angle to thereby allow access to oil and gas
bearing formations
that are not directly downhole from the initial wellbore 10. Once it is
inserted, the
whipstock 30 can be directionally aligned such that the whipstock 30 will
guide the SGTD 28
in a specific radial direction downhole. For example a wireline azimuth
measurement tool can
be lowered into a wireline orientation shoe just above the whipstock 30 and
the line can be
rotated from surface to the desired azimuth of the whipstock 30. The SGTD 28
has a gimbal
joint 32 that allows the SGTD 28 to bend and change direction as the SGTD 28
is guided by
the whipstock 30. The gimbal joints 32 must allow the SGTD 28 to move through
a pinch
point 34 when the whipstock 30 begins changing the direction of the SGTD drill
bit 36.
[0027] An embodiment of the pinch point 34 in FIGS. 1A-1C is shown in greater
detail in
FIGS. 2A and 2B. The whipstock 30 incorporates a whipstock ramp 38 that guides
the
SGTD 28 on a lateral spur into the formation. Stabilizer vanes 40 are located
on the
underside of the whipstock ramp 38 to hold the whipstock 30 in a position in
the borehole
adjacent the area intended for spur lateral drilling. The stabilizer vanes 40
are sized to slide
readily into the open borehole and to allow easy rotation for azimuthal
orientation. The
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vanes 40 prevent lateral motion of the whipstock 30, for example, in excess of
half of an inch
or a distance significant enough to prevent the drill bit 36 from kicking off
from the borehole
and into the formation. In an embodiment the whipstock 30 can further have a
flat spring 44
connected to a ramp 46 that pushes sideways against the SGTD as it passes
through the
whipstock 30. The ramp 46 has gradually ramped surfaces that allow the drill
bit 36 to travel
up or down in the borehole and slides past the ramp 46. The spring force of
the flat spring 44
is chosen to overcome the frictional bending resistance of the gimbal joint
32. In addition,
the spring 44 has sufficient travel to allow the drill bit 36 to pass without
excessively
dragging on inward facing surfaces of the ramp 46. Although a flat spring 44
is shown,
springs of other types could be substituted to achieve a similar effect.
[0028] This system can be designed for operation off a drilling or workover
rig, which can
includes the following steps:
1. Run a whipstock into the well using a liner.
2. Orient the whipstock using a wireline and hang in the slips, or with a
casing
hanger.
3. Run the SGTD into the well until it reaches the whipstock. The SGTD will
bend at the pinch point on the whipstock. Pressure applied downwardly on the
SGTD by running it further into the borehole will push the turbodrill against
the whipstock and cause the SGTD gimbal joints to bend. Additionally, in an
embodiment including the pinch point, the spring and ramp of the pinch point
will push against the gimbal joint of the of the steerable gas turbodrill as
it
travels past the pitch point thereby providing an additional force to bend the
SGTD gimbal joint. Eventually the gimbal joint will lock at a substantially
maximum angle of bend based on the configuration and internal structure of
the gimbal joint.
4. Drill a lateral at minimum weight on bit (WOB).
Cuttings are transported out of the well through the liner.
[0029] Air Compressor and Surface Equipment Pressure Capacity: For a well at
which the
SGTD will initially be employed, a current available air compressor capacity
is 1200 psig (8
MPa) @2500 scfm (70 scmm). The maximum pressure, consistent with safe
operation on air,
is 2000 psig (14 MPa). These specifications are not intended to in any way be
limiting on the
use or functionality of the SGTD.
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[0030] Exemplary Bit Design: The exemplary embodiment of the high-speed SGTD
operates with minimal torque at high speed. The SGTD may be operated with a
variety of
fixed cutter or roller come bits. In a preferred embodiment of the invention
surface set
diamond bits are used. Those skilled in the art will recognize that the
maximum bit speed is
limited by thermal wear of the diamonds. The reactive torque from a surface
set diamond bit
operating at maximum rotary speed is related to the WOB, W, bit diameter, Db,
and friction, ,u
(about 0.4 for rock drilling) according to the following equation:
(1)
3 2ff
where S is the drilling strength ¨ assumed to equal the confined compressive
strength of the
rock, A is the surface area of the bit, and is the depth of cut per
revolution. The torque will
increase with rate of penetration.
[0031] An important requirement for the SGTD is to maintain well trajectory
without any
additional steering input once the drill has exited the primary wellbore.
Conventional PDM
motors operating conventional fixed cutter bits generate enough torque to
cause the drillstring
to twist or wind up several revolutions so that it is not possible to predict
the orientation of
the SGTD bend while drilling. The present SGTD invention is designed to limit
the drilling
torque and therefore limit the windup angle to an acceptable error level. For
example if the
maximum windup can be limited to less than 45 degrees, the well azimuth can be
predicted to
within this angle. If the drilling torque is known, the windup can be
predicted and accounted
for when planning the well.
[0032] The windup of an example SGTD BHA and drillstring makeup is provided
below in
Table 2. The estimated torque while drilling with a 2-7/8" surface set diamond
bit at
about 500 lbf WOB in the Marcellus shale (15,000 psi CS) is 35 ft-lbf. The
analysis is shown
for 3-1/2 or 2-7/8 heavy wall drill pipe. Using the larger diameter pipe cuts
the windup in
half and will provide more accurate azimuthal control, however the 2-7/8"
drillstring may be
required to accommodate return circulation. In these examples the total windup
is 22 to 53
degrees. This amount of windup may be acceptable or compensated for by
rotating the
drillstring to the right by the windup angle once the lateral is spudded, or
by orienting the
whipstock to the right by the same amount.
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Table 2. Drilling Parameters and windup angle estimates.
WOB 500 Lbf
Motor Speed 640 Rpm
ROP 50 ft/hr
Shale Compressive 15000 Psi
Strength
Reactive Torque 35 ft/lbf
Drillstring 3-1/2" 12.95 #/ft 2-7/8" 8.7 4/ft
Length 6000 Ft
OD 3.5 2.88 Inch
ID 2.75 2.259 Inch
windup 22 48 Degrees
SGTD Whip 1-1/2" Type CS 2.9#/ft
Length 200 Ft
OD 1.9 Inch
ID 1.53 Inch
windup 5 Degrees
Total Windup 27 53 Degrees
[0033] Exemplary Steerable Gas Turbodrill: FIGS. 4A, 4B, 4C, and 4D show an
exemplary embodiment of a steerable gas turbodrill ("SGTD") 40 for a spur
lateral drilling
application as shown in FIG. 1. The gas turbodrill 40 includes a high-speed
gas turbine
power section 42 and a two stage planetary gearbox assembly 44 in the upper
section. The
two-stage gearbox assembly 44 reduces the speed of turbine power section 42
output by a
factor of 12:1 and has an output shaft 46 that extends into a clamp coupling
assembly 48.
The gearbox assembly output shaft 46 connects to a flexible shaft 50 in the
clamp coupling
assembly 48. FIG. 4C shows an enlarged view of the gimbal assembly 54. When
the SGTD
40 is drilling and begins to bend in the borehole, such as when coming into
contact with a
whipstock, the flexible shaft 50 bends through an arc within the gimbal
assembly 54. The
flexible shaft 50 extends through a gimbal joint 56 that enables the tool to
bend through a
fixed angle of up to five degrees. The precise angle and distance from the
bend to the bit
determines the radius of curvature of the spur lateral. The gimbal joint has a
ball and socket.
The application of internal pressure plus the force of one or more heavy
springs 58, such as
Belleville washer springs acting on a lock ring with a spherical seat that
presses against the
ball. Friction between the lock ring and ball and between the ball and socket
holds the
gimbal in the bent position, thereby allowing drilling of a fixed radius arc
or spur lateral. The
friction force is chosen to allow the gimbal joint to bend when subjected to
side loads inside
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the whipstock. The flexible shaft 50 couples to a bottom drill assembly 60
through a flow
coupling assembly 62 that extends into a bearing assembly section 64. The
bottom drill
assembly 60 rotates a drill bit 64.
[0034] An alternate configuration for this tool is shown in FIGS. 5A and 5B.
The gas
turbodrill 150 includes a high-speed gas turbine 152 and a clamp coupling
assembly 154 in
the upper section, with a gearbox 156 and a bearing assembly 158 located in
the lower
section of the gas turbodrill 150. A flexible shaft 160 connects to the
turbine output in the
clamp coupling assembly 154 and extends through a gimbal 162 that enables the
tool to bend
through an angle of up to about 5 degrees. The application of internal
pressure plus the force
applied by one or more springs in the gimbal assembly lock the gimbal in place
once drilling
commences. Drill bit assembly 164 couples to the gearbox output 156 and
rotates a drill
bit 166. Example turbine specifications are listed below, in Table 4. A
circulating model of
the turbine in a wellbore is provided in the graph shown in FIG. 3. Most of
the pressure
differential through the motor is developed through the bit nozzles. This
approach reduces
the turbine speed to a manageable level. The gearbox is a conventional two-
stage planetary
design. The output torque of the motor at maximum power is half the stall
torque and this is
the recommended operating condition. Operation at near the peak power will
require WOB
control to within 100 lbf. Over weighting the bit will cause it to stall,
while underweighting
will not enable it to drill. These characteristics are common to turbodrills,
but the relatively
light weight of the JTD discussed herein is unique to this tool.
Table 4. Gas Turbodrill Turbine specifications
Diameter 2-3/8"
Length 5.9 ft
Turbine stages 20
Gas Flow rate 2000 scfm
Turbine Pressure differential 260 psi( 1.8 MPa)
Turbine Runaway speed 15000 rpm
Two-stage planetary gear reduction 12:1
Motor stall torque 70 ft-lbf
Operating speed 625 rpm
Drilling weight on bit range 300 ¨ 600 lbf
Operating torque 35 ft-lbf
Maximum Bend 5 degrees
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[0035] Although the concepts disclosed herein have been described in
connection with the
preferred form of practicing them and modifications thereto, those of ordinary
skill in the art
will understand that many other modifications can be made thereto.
Accordingly, it is not
intended that the scope of these concepts in any way be limited by the above
description.
