Note: Descriptions are shown in the official language in which they were submitted.
CA 02521022 2004-02-16
1
METHOD AND SYSTEM FOR EXTRACTION OF RESOURCES
FROM A SUBTERRANEAN WELL BORE
This is a division of co-pending Canadian Patent
Application No. 2,457,902, which was filed on February 16,
2004.
TECHNICAL FIELD OF THE INVENTION
The present invention relates generally to recovery of
subterranean resources and more particularly to a method
and system extraction of resources from a subterranean well
bore.
BACKGROUND OF THE INVENTION
Subterranean deposits of coal, also referred to as
coal beds, contain substantial quantities of entrained
resources, such as natural gas (including methane gas or
any other naturally occurring gases). Production and use
of natural gas from coal deposits has occurred for many
years. However, substantial obstacles have frustrated more
extensive development and use of natural gas deposits in
coal beds.
SUMMARY OF THE INVENTION
Certain exemplary embodiments can provide a method for
extracting resource from a subterranean well bore,
comprising: forming a drainage well bore in the
subterranean coal bed, the drainage well bore having a
first cross-sectional diameter, a first end, and a second
end; positioning a liner in the well bore, the liner having
a wall including a plurality of apertures and a second
cross-sectional diameter that is at least ten percent
smaller than the first cross-sectional diameter; at the
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2
first end, collecting a mixture flowing from the second
end, the mixture comprising fluid, a plurality of coal
fines, and any resource from the well bore; and collapsing
the well bore around the liner after positioning the liner
in the well bore.
Certain exemplary embodiments can provide a method for
stimulating production of gas from a coal seam, comprising:
forming a drainage well bore including a substantially
horizontal section in a coal seam; inserting a liner into
the drainage well bore; and purposefully collapsing the
drainage well bore around the liner.
Certain exemplary embodiments can provide a method for
producing gas from a coal seam, comprising: forming a
drainage well bore comprising a substantially horizontal
section in a coal seam; inserting a liner into the drainage
well bore; collapsing the drainage well bore around the
liner; and wherein diameter of at least part of a drainage
well bore is sized for collapse based on characteristics of
the coal seam.
Certain exemplary embodiments can provide a method,
comprising: determining one or more characteristics of a
coal bed; determining a size of at least part of a well
bore to drill in the coal bed such that the well bore may
be collapsed by pumping fluids from the well bore to reduce
bottom hole pressure before or during production.
Certain exemplary embodiments can provide a method for
producing resources from a coal seam, comprising: forming a
substantially horizontal well bore in a coal seam;
inserting a liner into the substantially horizontal well
bore; collapsing the substantially horizontal well bore
around the liner; and forming at least one lateral in the
coal seam from the substantially horizontal well bore.
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2a
Certain exemplary embodiments can provide a method for
producing resources from a coal seam, comprising: forming a
substantially horizontal well bore in a coal seam;
inserting a liner into the substantially horizontal well
bore; collapsing the substantially horizontal well bore
around the liner; and producing fluid from the coal seam
through the liner and reinjecting at least a portion of the
fluid.
Certain exemplary embodiments can provide a method for
producing resources from a coal seam, comprising: forming a
substantially horizontal well bore in a coal seam;
inserting a liner into the substantially horizontal well
bore; collapsing the substantially horizontal well bore
around the liner; and injecting a fluid into the liner to
remove coal fines.
Certain exemplary embodiments can provide a method for
producing resources from a coal seam, comprising: forming a
substantially horizontal well bore in a coal seam;
inserting a liner into the substantially horizontal well
bore; collapsing the substantially horizontal well bore
around the liner; and wherein the substantially horizontal
well bore is drilled using low loss drilling fluid.
Some embodiments of the invention provide numerous
technical advantages. Some embodiments may benefit from
some, none, or all of these advantages. For example,
according to certain embodiments, resource production from
a well bore is improved by an efficient removal of water
and obstructive material. In particular embodiments, such
water and obstructive material may be moved without the use
of a down hole pump.
Furthermore, in certain embodiments, efficiency of gas
production may be improved in coal beds by increasing the
permeability of parts of the coal by providing controlled
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2b
collapse of a portion of the coal or other forms of stress
relief in portions of the coal. Such stress relief may be
particularly useful in low permeability, high gas content
coal beds and can stimulate production in such coal beds.
In addition, in particular embodiments, a drainage well
bore having a f latter curvature may be used to ef f iciently
produce resources by angling the drainage well bore
downward relative to the horizontal in the coal seam.
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Other technical advantages will be readily apparent
to one skilled in the art.
BRIEF DESCRIPTION OF THE DRAWINGS
Reference is now made to the following description
taken in conjunction with the accompanying drawings,
wherein like reference numbers represent like parts, in
which
FIGURE 1 is a schematic diagram illustrating one
embodiment of a resource extraction system constructed in
accordance with one embodiment of the present invention;
FIGURE 2A is a cross sectional diagram illustrating
one embodiment of a liner and a tube in a well bore shown
in FIGURE 1;
FIGURE 2B is a cross sectional diagram illustrating
one embodiment of the liner and the tube positioned in
the well bore of FIGURE 2A after a collapse of the well
bore ; and
FIGURE 3 is a flow chart illustrating one embodiment
of a method for extraction of resources from the well
bore of FIGURE 1.
DETAILED DESCRIPTION
Embodiments of the invention are best understood by
referring to FIGURES 1 through 3 of the drawings, like
numerals being used for like and corresponding parts of
the various drawings.
FIGURE 1 is a schematic diagram illustrating one
embodiment of a well system 10. Well system 10 includes
a resource extraction system 12 positioned on a ground
surface 36 and a drainage well bore 14 that extends below
ground surface 36. Drainage well bore 14 includes an
open end 16, a substantially vertical portion 18, an
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articulated potion 20, and a drainage portion 22. Any
one of portions 18, 20, and 22 of well bore 14 may
individually constitute a well bore, and may be referred
to as a well bore herein. Drainage portion 22 of well
bore 14 includes a first end 24 and a second end 28. As
shown in FIGURE 1, first end 24 of drainage portion 22 is
accessible from a location above ground surface 36, such
as open end 16. In one embodiment, second end 28 of
drainage portion 22 may be a closed end that is not
accessible from a location above ground surface, except
through first end 24 of drainage portion 22, as shown in
FIGURE 1. As used herein, second end 28 is also referred
to as a closed end 28. Second end 28 also constitutes an
end 28 of drainage well bore 14. Drainage portion 22 of
well bore 14 may be positioned at least partly in a coal
bed 30 or any other appropriate subterranean zone that
includes resources to be extracted.
Drainage well bore 14 may be drilled using an
articulated drill string that includes a suitable down
hole motor and a drill bit. A measurement while drilling
("MWD") device may be included in articulated drill
string for controlling the orientation and direction of
the well bore drilled by the motor and the drill bit.
As shown in FIGURE 1, drainage portion 22 is
approximately horizontal. In one embodiment where ground
surface 36 is substantially horizontal, a distance 34
from ground surface 36 to end 24 is approximately equal
to a distance 38 between ground surface 36 and end 2-8.
However, portion 22 is not required to be horizontal.
For example, where well bore 14 is a down-dip or an up-
dip well bore, portion 22 may be sloped. In a down-dip
configuration, distance 38 may be greater than distance
34, which allows articulated portion 20 to be less
CA 02521022 2004-02-16
curved. This is advantageous because a less extreme
curvature at portion 20 allows the overall length of well
bore 14 to be greater, which improves efficiency of
resource production. Because a flow of fluid is
5 generated from end 28 of portion 22 to move the gas in
portion 22 to ground surface 36, production
inefficiencies conventionally associated with a down-dip
well bore is reduced. In one embodiment, drainage
portion 22 may be approximately horizontal with respect
to coal bed 30, regardless of whether coal bed 30 is
parallel to ground surface 36. In one embodiment,
portion 22 may be angled with respect to coal bed 30
rather than ground surface 36.
Production of resources, such as natural gas, may be
dependent on the level of resource content in coal bed 30
and permeability of coal bed 30. Gas is used herein as
an example resource available from a coal region, such as
coal bed 30; however, the teachings of the present
invention may be applicable to any resource available
from a subterranean zone that may be extracted using a
well bore. In general, less restricted movement of gas
within coal bed 30 allows more gas to move into well bore
14, which allows more gas to be removed from well bore
14. Thus, a coal bed having low permeability often
results in inefficient resource production because the
low number and/or low width of the cleats in coal bed 30
limit the movement of gas , into well bore 14. In
contrast, high permeability results in a more efficient
resource production because the higher number of pores
allow freer movement of gas into well bore 14.
Conventionally, a well bore is drilled to reach a
coal bed that includes resources, such as natural gas.
Once a well bore is formed, a mixture of resources,
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water, and coal fines may be forced out of the coal bed
through the well bore because of the pressure difference
between the ground surface and the coal bed. After
collecting the mixture at the ground, surface, the
resource is separated from the mixture. However,
production of resources from a well bore in such a manner
may be inefficient for numerous reasons. For example,
the level of resource production may be reduced due to
the coal fines that may obstruct the well bore or a
possible collapse of the well bore. A well bore in a
coal bed having low permeability or under lower pressure
may produce a lower level of resources. Additionally, a
"down dip" well bore, which refers to an articulated well
bore having a flatter curvature and a portion that slopes
downward from the horizontal, may produce a lower level
of resources due to a higher producing bottom hole
pressure resulting from the hydrostatic pressure of the
water collecting up to the pumping point.
According to some embodiments of the present
invention, a method and a system for extracting resources
from a subterranean well bore are provided. In certain
embodiments, efficiency of gas production may be improved
in a coal beds by increasing the permeability of parts of
the coal by providing controlled collapse of a portion of
the coal or other forms of stress relief in portions of
the coal. Such stress relief may be particularly useful
in low permeability, high gas content coal beds and can
stimulate production in such coal beds. Tn particular
embodiments, a drainage well bore having a flatter
curvature may be used to efficiently produce resources.
Additional details of example embodiments of the methods
and the systems are provided below in conjunction with
FIGURES 1 through 3.
CA 02521022 2004-02-16
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Referring back to FIGURE 1, resource extraction
system 12 is provided for gas production from drainage
well bore 14. System 12 includes a liner 44, a tube 58,
a fluid injector 70 (which may inject gas, liquid, or
foam), a well head housing 68, and a separator 74. Liner
44 has a first end 48 and a second end 50. Tube 58 has
an entry end 60 and an exit end 64. Fluid injector 70 is
coupled to entry end 60 of tube 58 through outlet 68.
Housing 72 is coupled to separator 74 and is operable to
direct any material from well bore 14 into separator 74.
Separator 74 is coupled to fluid injector 70 through a
pipe 94.
Fluid injector 70 is operable to urge an injection
fluid out through outlet 68. An example of fluid
injector 70 is a pump or a compressor. Any suitable type
of injection fluid may be used in conjunction with fluid
injector 70. Examples of injection fluid may include the
following: (1) production gas, such as natural gas, (2)
water, (3) air, and (4) any combination of production
gas, water, air and/or treating foam. In particular
embodiments, production gas, water, air, or any
combination of these may be provided from an outside
source through a tube 71. In other embodiments, gas
received from well bore 14 at separator 74 may be
provided to injector 70 through tubes 90 and 94 for use
as an injection fluid. In another embodiment, water
received from well bore 14 at separator 74 may be
provided to injector 70 through tubes 75 and 94 for use
as an injection fluid. Thus, the fluid may be provided
to injector 70 from an outside source and/or separator 74
that may recirculate fluid back to injector 70.
Separator 74 is operable to separate the gas,, the
water, and the particles and Lets them be dealt with
CA 02521022 2004-02-16
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separately. Although the term "separation" is used, it
should be understood that complete separation may not
occur. For example, "separated" water may still include
a small amount of particles. Once separated, the
produced gas may be removed via outlet 90 for further
treatment (if appropriate). In one embodiment, a portion
of the produced gas may be provided to injector 70 via
tube 94 for injection back into well bore 14. The
particles, such as coal fines, may be removed for
disposal via an outlet 77 and the water may be removed
via an outlet 75. Although a single separator 74 is
shown, the gas may be separated from the water in one
apparatus and the particles may be separated from the
water in another apparatus. Furthermore, although a
separation tank is shown, one skilled in the art will
appreciate numerous different separation devices may be
used and are encompassed within the scope of the present
invention.
As shown as FIGURE 1, in particular embodiments,
second end 50 of liner 44 is located approximately at
closed end 28 of well bore 14. End 48 of liner 44 is
approximately at opening 16 of well bore 14; however, end
48 may be anywhere along vertical portion 18 or
articulated portion 20 of well bore 14. In certain
embodiments, liner 44 may be omitted. In particular
embodiments, the wall of liner 44 may include a plurality
of apertures 54. Apertures 54 may include holes, slots,
or openings of any other shape. In particular
embodiments, the use of holes as the apertures may allow
production of more coal fines than the use of slots,
while the use of slots may provide more alignment of the
apertures with cleats in the coal than when using holes .
Although apertures in a portion of the liner 44 are
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illustrated, apertures may be included in any appropriate
portion of the length of liner 44. The size of apertures
54 may be adjusted depending on the size of coal
particles or other solids that are desired to be kept
outside of liner 44. For example, if it is determined
that a piece of coal having a diameter greater than one
inch should not be inside liner 44, then each aperture 54
may have a diameter of less than one inch. In particular
example embodiments, apertures 54 may be holes having a
diameter of between 1/16 and 1.5 inches or slots having a
width of between 1/32 and 1/2 inches (although any other
appropriate diameter or width may be used).
Tube 58 is positioned inside well bore 14. In
embodiments where liner 44 is used, tube is positioned
inside liner 44. As shown in FIGURE 1, in one
embodiment, exit end 64 is positioned approximately at
closed end 28 of well bore 14. Entry end 60 is
positioned approximately at open end 16 of well bore 14.
In one embodiment, coil tubing may be used as tube 58;
however, any suitable tubing may be used as tube 58 (for
example, jointed pipe).
In operation, a well bore, such as well bore 14, is
formed in coal bed 30. In particular embodiments, well
bore 14 is formed without forming a secondary well bore
that intersects portion 22; however, a secondary well
bore may be formed in other embodiments. Fluid injector
70 injects an injection fluid, such as water or natural
gas, into entry end 60 of tube 58, as shown by an arrow
78. The injection fluid travels through tube 58 and is
injected into closed end 28, as shown by an arrow 80.
Because end 28 is closed, a flow of injection fluid is
generated from end 28 to end 24 of portion 22 through
gaps 104 and/or 102, as shown by arrows 84. In
CA 02521022 2004-02-16
particular embodiments gap 102 may be blocked by a plug,
packer, or valve 106 (or other suitable device) to
prevent flow of fluid to the surface via gap 102 (which
may be inefficient). In other embodiments, gap 102 may
5 be removed due to the collapse of the coal against liner
44, as described in further detail below.
As the injection fluid flows through gaps 102 and
104, the injection fluid mixes with water, coal fines,
and resources, such as natural gas, that move into well
10 bore 14 from coal bed 30. Thus, the flow of injection
fluid removes water and coal fines in conjunction with
the resources. The mixture of injection fluid, water,
coal fines, and resources is collected at separator 74,
as shown by arrow 88. Then separator 74 separates the
resource from the injection fluid carrying the resource.
Although the injection fluid may be used for some time to
remove fluids from well bore 14, at some point (such as
during the mid-life or late-life of the well) a pump may
replace the use of the injection fluid to remove fluids
from the well bore 14 in certain embodiments. The "mid-
life" of the well may be the period during which well 14
transitions from high fine production to a much lower
fine production. During this period, the coal may
substantially stabilize around liner 44. In other
embodiments, a pump may be used for the entire life of
the well, although in such embodiments the particles in
the well may not be swept out (or the extent of their
removal may be diminished).
In one embodiment, the separated resource from
3.0 separator 74 is sent to fluid injector 70 through tube 94
and injected back into entry end 60 of tube 58 to
continue the flow of fluid from end 28 to ends 24 and 16.
In another embodiment, liquid, such as water, may be
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injected into end 28 using fluid injector 70 and tube 58.
Because liquid has a higher viscosity than air, liquid
may pick up any potential obstructive material, such as
coal fines in well bore 14, and remove such obstructive
material from well bore 14. In another embodiment, air
may be injected into end 28 using fluid injector 70 and
tube 58. In one embodiment, any combination of air,
water, and/or gas that are provided from an outside
source and/or recirculated from separator 74 may be
injected back into entry end 60 of tube 58.
Respective cross sectional diameters 98 and 100 of
liner 44 and tube 58 are such that gaps 102 and 104 are
formed. As shown in FIGURE 1, the difference between
diameter 40 and diameter 98 results in a formation of gap
102. The difference between diameter 98 and diameter 100
results in a formation of gap 104. The larger the gap,
the more stress relief (and depth of penetration of the
stress relief) that is provided in the coal. The size of
gaps 102 and 104 may be controlled by adjusting diameters
40, 98, and 100. For example, portion 22 of well bore 14
may be formed so that diameter 44 is substantially larger
than diameter 98 of liner 44. However, a smaller
diameter 40 may be used where diameter 98 of liner 44 is
smaller: Analogously, diameters 98 and 100 may be
selected depending on the size of gap 104 that is
desired. In one embodiment, diameter 98 is less than 4.5
inches; however, diameter 98 may be any suitable length.
In one embodiment, diameter 100 is less than 2.5 inches;
however, diameter 100 may be any suitable length.
Diameter 98 may have any appropriate proportion with
respect to diameter 40 to allow the desired amount of
collapse. In particular embodiments, diameter 98 is less
than approximately ninety percent of diameter 40.
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However, in other embodiments, diameter 98 may be very
close to diameter 40 such that the coal is allowed to
slightly expand against the liner (to relief stress) but
does. not disintegrate. Such an expansion of the coal
shall be included in the meaning of the term "collapse"
or it variants.
Diameter 40 of portion 22 may be selected depending
on the particular characteristics of coal beds 30. For
example, where coal bed 30 has low permeability, diameter
40 of portion 22 may be larger for better resource
production. Where coal bed 30 has high permeability,
diameter 40 may be smaller. In particular embodiments,
diameter 40 of portion 22 may be sufficiently large to
allow portion 22 to collapse around liner 44. In one
embodiment, diameter 40 of well bore 14 may be greater
than six inches. In another embodiment, diameter 40 may
be between approximately five to eight inches. In
another embodiment, diameter 40 may be greater than 10
inches.
A collapse of well bore 14 around liner 44 may be
advantageous in some embodiments because such a collapse
increases the permeability of the portion of coal bed 30
immediately around liner 44, which allows more gas to
move into portion 22 and thus improves the ef f iciency of
resource production. This increase in permeability is
due, at least in part, to the stress relief in the coal
due to the collapse. The effects of this stress relief
may extend many feet from well bore 14 (for example, in
certain embodiments, up to fifty feet).
Furthermore, since the well bore 14 is allowed to
collapse, the well bore 14 may be drilled in an
"overbalanced" condition to prevent collapse during
drilling without adversely affecting the flow capacity of
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well bore 14. Although overbalanced drilling does force
drilling fluids (such as drilling mud) and fines into the
coal bed during drilling (which in some cases can reduce
subsequent production from the coal), the "cake" formed
around the wall of well bore 14 by the drilling fluid and
fines deposited on the wall may be formed in a manner
that is advantageous. More specifically, a thin cake may
be formed by using a low-loss drilling fluid that
minimizes fluid loss into the coal formation (for
example, an invasion of drilling fluid and/or fines less
than six inches into the coal seam may be preferable).
Furthermore, the drilling may be performed and a type
drilling fluid may be used such that the cake builds up
quickly and remains intact during drilling. This may
have the added advantage of supporting the coal to
prevent its collapse before and while liner 44 is
inserted.
In one embodiment, liner 44 is positioned in portion
22 without providing any support to prevent a collapse of
portion 22, which increases the probability of well bore
collapse. In such an embodiment, the probability of well
bore collapse may be increased by drilling a well bore
having a larger diameter than liner 44 and lowering the
bottom hole pressure. Thus the coal may be collapsed
onto the liner 44 by lowering the bottom hole pressure
below a threshold at which the coal collapses. For
example, the drilling fluid may be left in well bore 14
while liner 44 is inserted (to help prevent collapse),
and then the drilling fluid (and possibly other fluids
from the coal) may be pumped or gas lifted to the surface
to instigate a collapse of the coal. The collapse may
occur before or after production begins. The bottom hole
pressure may be reduced either quickly or slowly,
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depending, among other things, on the type of coal and
whether the coal is to be collapsed or only expanded
against liner 44.
In other embodiments, collapse of well bore 14 may
instigated using any suitable methods, such as a
transmission of shock waves to coal bed 30 using a
seismic device or a controlled explosion. Allowing a
collapse of or collapsing well bore 14 may be beneficial
in situations where coal bed 30 has low permeability;
however, coal bed 30 having other levels of permeability
may also benefit from the collapse of portion 22.
FIGURE 2A is a cross sectional diagram illustrating
one embodiment of liner 44 and tube 58 in well bore 14 at
a location and orientation indicated by a reference
number 108 in FIGURE 1. As shown in FIGURE 2A, injection
fluid from fluid injector 70 flows in the direction
indicated by arrow 80 (pointing towards the viewer).
Because end 28 is closed, injection fluid is returned
back to end 24 in a direction indicated by arrows 84
(pointing away from the viewer) through gaps 102 and/or
104. The flow of injection fluid in the direction
indicated by arrow 84 creates a mixture of injection
fluid, gas (resources), water, and coal fines that move
into well bore 14 (as indicated by arrows 110). The
mixture moves to separator 74 through opening 16.
FIGURE 2B is a cross sectional view of liner 44 and
tube 58 in a collapsed well bore 14 at a location and
orientation indicated by a reference number 108 in FIGURE
1. As shown in FIGURE 2B, in one embodiment, well bore
14 is allowed to close gap 102 by collapsing around liner
44 to increase the permeability of coal bed 30
immediately around liner 44 by relieving stress in the
coal. Further, permeability may be increased through
CA 02521022 2004-02-16
matrix shrinkage that occurs during the degassing of high
gas content coals in coal bed 30. Thus, more gas moves
from coal bed 30 into the space defined by liner 44
through apertures 54 of liner 44. Gas is then removed
5 from well bore 14 using flow of fluid in the direction
indicated by arrow 84 through gap 104. In one embodiment
where liquid or other injection fluid having a viscosity
level higher than that of natural gas or air is
periodically injected into closed end 28 through tube 58,
l0 any coal fines 124 that may not have been removed before
may be removed by the flow of injection liquid in
direction 84.
FIGURE 3 is a flow chart illustrating one embodiment
of a method 150 for removal of resources from well bore
15 14. Some or all acts associated with method 150 may be
performed using system 12. Method 150 starts at step
154. At step 158, drainage well bore 14 having a
drainage portion 22 is formed in coal bed 30. At step
160, liner 44 is positioned in well bore 22. In
particular embodiments, step 160 may be omitted. At step
164, tube 58 is positioned in well bore 14. In
embodiments where liner 44 is used, tube 58 is positioned
within liner 44.
In embodiments where liner 44 is position in well
bore 22 at step 160, well bore 22 may be allowed to
collapse around liner 44 at step 168. In one embodiment,
the collapse of well bore 22 may be instigated using any
suitable method, such as a seismic device or a controlled
explosion. At step 170, a flow of injection fluid is
generated from end 28 to end 24. In one embodiment, the
flow may be generated by injecting injection fluid into
closed end 28 of well bore 22 through tube 58; however,
any other suitable methods may be used. The injection
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fluid may be any suitable gas or liquid. At step 174, a
mixture that includes the injection fluid, resource, and
water and/or coal fines is collected at the open end. At
step 178, the mixture is separated into different
components. In one embodiment, at step 180, a portion of
the separated resource and/or water is injected back into
closed end 28 of well bore 22 through tube 58.
Alternatively, at step 180, injection fluid from an
outside source may be injected back into closed end 28 of
well bore 22 through tube 58 to continue the fluid flow.
Steps 170 and/or 180 may be continuously performed to
continue the fluid flow in well bore 22. Step 180 may be
omitted in some embodiments. Method 150 stops at step
190.
In one embodiment, the injection fluid used to
generate a flow of fluid may be natural gas or air. In
one embodiment, the injection fluid may be liquid, such
as water. Using liquid may be advantageous in some
embodiments because liquid may be a better medium for
coal fine removal.
Although embodiments of the present invention are
only illustrated as being used in well bore 14, such
embodiments may also be used in one or more lateral well
bores drilled of well bore 14 or any other surface well
bore. For example, one or more lateral well bores may
extend horizontally from well bore 14 and a liner may be
inserted through well bore 14 and into one or more of
these lateral well bores. The method described above may
then be performed relative to such lateral well bores.
For example, multiple lateral well bores may be
successively cleaned out using such a method.
Although some embodiments of the present invention
have been described in detail, various changes and
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modifications may be suggested to one skilled in the art.
It is intended that the present invention encompass such
changes and modifications as falling within the scope of
the appended claims.
