Note: Descriptions are shown in the official language in which they were submitted.
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Description
Flexible drill shaft
Technical field
[0001] This invention relates to flexible drill shafts and in particular
flexible drill shafts
able to transmit a high torque during drilling of lateral boreholes.
Background art
[0002] In oil and gas wells, it is often desirable to drill holes laterally
out from a main
vertical borehole to increase the communication with the formation sun=ounding
the main borehole and the drainage area. These lateral holes are generally
known
as drainholes. They provide a passageway through the surrounding formation and
can increase the flow rate of the hydrocarbons into the well casing of the
main
borehole and improve the amount of hydrocarbon extracted.
[0003] In order to have drainholes having a significant length, the equipment
or apparatus
used to drill the drainholes generally have a flexible shaft to have the
ability to
change the direction of the apparatus from the main borehole and the lateral
borehole and to transmit the rotational power and the rate of penetration to
the
drill bit.
[0004] US 6,220,372 discloses an apparatus for drilling lateral drainholes
comprising a
flexible shaft formed from at least two helically wound coil springs. The
document discloses that the springs should be formed from wire having a
circular
cross section.
[0005] US 4,658,916 discloses a flexible drill shaft formed from at least two
helically
wound coil springs formed from wires having rectangular cross section. This
document describes that the wire used to form the coils springs has a
nectangular
cross section. However if a coiled spring is formed from rectangular cross
sectional wire, during the winding process the wire can be distorted due to
the
pressures altering the final cross section shape of the wire that is forming
the
spring, such that is no longer rectangular, as shown in figure 1.
[0006] One of the difficulties of having a flexible shaft is that while the
flexibility allows
it curve to direct the drill bit at an angle away from the main borehole to
drill a
drainhole and therefore help the drilling apparatus negotiate the bend in the
bore
hole so that it can drill in a path at an angle away from the main borehole.
The
flexibility of the shaft can reduce stability of the shaft when it is required
to drill
in a straight path and can reduce the torque that is provided to the drill
bit.
[0007] Therefore it is an object of the invention to provide a flexible shaft
with planar
contact between coils of the springs, for stably transmitting torque and axial
load
in a straight guide path or in a curved guide path, to a drill bit.
Disclosure of the invention
[0008] One aspect of the invention comprises a flexible drill shaft for
drilling lateral
boreholes comprising: a helical outer wire coil spring; and at least one inner
helical wire coil spring residing concentrically within the outer coil spring,
with
the coils of adjacent springs having an opposite pitch; wherein the wire
forming
each helical coil springs has a cross section such that when the drill shaft
is in a
straight alignment there is planar contact between the wire of adjacent coils
of the
spring. There is planar contact between the adjacent coils when a rotational
force
and an axial load are applied to the drill shaft during drilling in a straight
pathway.
Having planar contact between the coils forming each coiled spring helps
prevent
buckling of the drill shaft. The cross section of the wires is such that
planar
contact between adjacent coils is maintained even when the drill shaft is
drilling
in a straight direction.
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[0009] Preferably the cross section of the wire is prismatic. More preferably
the cross
section of the wire of the flexible drill shaft is square, rectangular or
lozenge.
These cross section shapes allows for planar contact between the adjacent
coils of
a coil spring.
[0010] The coils of coiled spring can have ridges on the contact surface of
the coils
and/or grooves on the contact surface on the coils. Having ridges and/or
grooves
increases the friction ratio between the coils of each coiled spring.
[0011] The coiled springs can be formed from at least two wires coiled in
parallel.
Having two or more wires coiled in parallel allows more torque to be
transmitted
and reduces the coiling angle.
[0012] A flexible drill shaft can be formed from coil springs having a
preload. Having a
preload between spires gives the springs a longitudinal stiffness which helps
provide the shaft with stability during drilling in a straight trajectory.
[0013] The flexible drill shaft can further comprise a torque spike filter.
This protects the
shaft from damage caused by a torque spike. The filter can be placed at any
place
along the shaft.
[0014] The flexible drill shaft can further comprise a drill bit at the lower
end of the
shaft.
[0015] A another aspect of the invention comprises a method of making a
flexible drill
shaft as described above comprising: winding wire to form a helical coil
spring;
applying a mechanical deformation to the wire as it is being wound; and using
the
helical coil spring to form a flexible di7ill shaft; wherein the mechanical
deformation applied results in the wire after being wound having a cross
section
such that when an axial load is applied to the drill shaft there is planar
contact
between adjacent coils of the spring.
[0016] Preferably the method comprises applying a preload to the wire as it is
being
wound.
[0017] The method can comprise using dimensional or thermal expansion to
tightly
assemble two or more coil springs together. This can help reduce the gap
between
coil springs of the flexible drill shaft.
Brief description of the drawings
[0018] Figure 1 shows a schematic view of the deformation of wire after
winding;
Figure 2 shows a schematic view of the wire before and after the winding
process;
Figure 3 shows a schematic view of one wire cross section for use in making a
coiled spring of the invention;
Figure 4 shows a schematic view of one of the coiled spring that can be used
for
the invention; and
Figure 5 shows the forces taking place on a coiled spring.
Mode(s) for carrying out the invention
[0019] The flexible shaft is foimed of a helically wound outer coil spring and
one or
more helically wound and smaller inner coils springs residing concentrically
therein. Each successively smaller inner coil spring has an outer diameter
substantially the same as the inner diameter of the adjacent larger coil
spring.
Each coil spring is wound in the opposite direction to that of the adjacent
coil
springs, the outer coil spring sets the winding reference direction, and is
wound in
the same direction as the direction that the drill bit will rotate. The
springs are
held rigid in relation to each of the other coil springs at the drill shafts
upper and
lower ends. The springs are close wound with axially adjacent coils of each
coil
spring in contact with each other. During assembly dimensional or thermal
expansion can be used to remove any gaps between adjacent coil springs, so
that
the coil springs are assembled tightly together.
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[0020] Each of the springs is formed of wound wire, having a geometric cross
section.
However the winding process can cause the initial cross section of the wire to
deform as it is being coiled to form a spring. Therefore the wire forming the
spring has a different final cross section, once formed into the spring than
what it
initially starts off with before it is coiled.
[0021] For example, as shown in figure 1, a wire with an initial square cross
section is
deformed into a wire having a trapezium cross section due to the compression
forces placed on the wire as it is being formed into the coil spring. A coil
spring
that is formed from wire having a trapezium cross section along its length, as
in
figure 1, does not have planar contact between adjacent coils of the spring.
[0022] To fotm a coiled spring with planar contact between adjacent coils the
initial cross
section of the wire before winding has to be corrected in order to take into
account
the deformation that occurs during the winding process. With reference to
figure
2, a wire having an initial trapezium cross section when used to form a coiled
spring the final shape of the cross section of the wire once formed into the
spring
is a square due to the compression forces placed on the wire during the
winding
process. The angle, [i, is the angle by which the cross section of the wire is
corrected by to compensate for the deformation that the wire will under go
during
the winding process to form the spring. The final square cross section allows
for
planar connect between the adjacent coils. With planar contact between the
coils
of the spring, the flexible drill shaft is able to have improved transmittal
of torque
and axial load to an out put device such as a drill bit, when the drill shaft
is in a
curved and straight form.
[0023] The angle correction to the initial wire used for forming the spring
can be done by
either applying an opposite mechanical deformation to the wire during the
winding process, or by directly wire drawing with a cross section that when
altered due to the compression forces applied during the winding process will
result in a#"inal cross section that allows planar connect between coils.
[0024] Having planar contact between the coils improves the performance of the
flexible
shaft when drilling in a straight path. Planar contact between the adjacent
coils
gives the shaft a better anti- buckling effect and ensures rigidity of the
shaft, when
placed under an axial load, such as during drilling.
[0025] Planar contact between the surfaces of coils increases the friction
this reduces the
shrinking of the diameter of the spring under torque. Reducing the shrinking
of
the diameter of the spring under torque can also be helped by having a spring
with
a lozenge cross section. When the wire has a lozenge section as indicated in
figure
3, there will be planar connect between adjacent coils. Wire having a lozenge
cross section to be used to form a coiled spring will have an angle a, where
angle
a is equal to the corrected angle [i, to take into account deformation that
occurs
due winding process, and an additional angle such that the wire maintains a
lozenge cross section after the winding process.
[0026] When the spring coils forming the flexible drill sliaft are subjected
to torque this
can cause either a decrease in the diameter with a corresponding increase in
length
or an increase in diameter with a corresponding decrease in length. Having
alternate winding of the springs forming the shaft, when the springs are
placed
under torque one spring expands in diameter and its adjacent spring decreases
in
diameter thereby creating a strong, stable and flexible shaft.
[0027] The springs used to form the flexible shaft can be preloaded springs. A
preload is
applied during the winding process of the spring. This gives the drill shaft a
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lengthwise stiffness allowing the shaft to have stability when drilling in a
straight
direction.
[0028] With teferenee to figure 4, to increase the friction ratio of the
contact between
adjacent coils 1, ridges 2 and/or grooves 3 are added to the wire forming the
coils.
Processes for forming the ridges and grooves on the wire can include
mechanical
deformation of the forined wire or directly wiredrawing, however other methods
may be used. Increasing the friction ratio between adjacent coils, will reduce
the
slippage that can occur between the adjacent surfaces, and therefore improve
the
stability of the drill shaft.
[0029] In operation, a drilling apparatus comprising the flexible drill shaft
according to
the invention with a drill bit attached to the lower end of the shaft is
lowered
down into the wellbore. To drill drainholes, the flexible shaft is guided so
as to
turn and direct the drill bit to drill laterally into the side of the main
borehole. The
flexibility of the shaft enables the shaft to tum in a short radius. A rotary
motor
enables rotation of the shaft and the drill bit. Rotation is imparted by a
motor and
axial load is transmitted through to the flexible drill shaft causing the
drill bit to
rotate and drilling to proceed. A rotating torque and an axial force are
applied to
the drill bit through the flexible drill shaft.
[0030] With reference to Figure 5, with the flexible drill shaft having planar
contact
between adjacent coils of the spring, the applied torque and axial force is
more
effective in the drilling process. The torque transmitted to the drill bit is
improved.
The transmissible torque is the sum of the rotational torque 4 and clutch
effect
which is due to the combination of the axial force 5 down the length of the
shaft
and the friction ratio between the contact surfaces of adjacent coils.
[0031] Further changes can be made without departing from the scope of the
invention.
