Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR RECOVERING HYDROCARBONS FROM A
SUBTERRANEAN RESERVOIR
FIELD OF THE INVENTION
[0001] The present invention relates to the formation of vertical fractures in
subterranean earth formations, and particularly for the purpose of secondary
or tertiary
recovery of hydrocarbons.
BACKGROUND OF THE INVENTION
[0002] It is a common practice to treat subterranean formations to increase
the gross
permeability or conductivity of such formations with procedures which are
identified
generally as fracturing processes. For example, it is a conventional practice
to
hydraulically fracture a well in order to produce one or more cracks or
"fractures" in
the surrounding formation by a mechanical breakdown of the formation.
Fracturing
may be carried out in wells which are completed in subterranean formations for
virtually any purpose. The usual candidates for fracturing, or other
stimulation
procedures, are production wells completed in oil or gas containing
formations.
However, injection wells used in secondary or tertiary operations, for
example, for the
injection of water or gas, may also be fractured in order to facilitate the
injection of
fluids into such subterranean formations.
[0003] Hydraulic fracturing may be propagated from vertical or horizontal
wells and
is accomplished by injecting a hydraulic fracturing fluid into the well and
imposing
sufficient pressure on the fracturing fluid to cause the formation to
breakdown with the
attendant formation of one or more fractures. The fracture or fractures formed
may be
vertical or horizontal with the former usually predominating and with the
tendency
towards vertical fracture orientation increasing with the depth of the
formation being
fractured.
[0004] Fracturing techniques and the challenges that arise with such
techniques are
well known to those skilled in the art. Fracturing in unconsolidated
formations wherein
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the earthen particles are not cemented together but are loosely associated, in
particular,
present several difficulties. For example, fracturing in such formations are
difficult to
complete due to the tendency of unconsolidated formations to collapse into the
fracture
when the pressure is removed. As well, due to the loose nature of the
formation,
earthen particles tend to be produced into the well with produced fluids.
Production of
such earthen particles can result in numerous problems including plugging the
fracture
and filling the well which can result in shortening the life of pumping
equipment.
[0005] To address these challenges, a thickened carrier fluid having a
propping agent
such as sand or other particulate material suspended therein is typically
introduced into
the fracture simultaneously with or subsequent to its formation. The propping
agent is
deposited within the fracture and serves to hold the fracture open after the
pressure is
released and the fracturing fluid withdrawn back into the well. Even with the
addition
of such propping agents, the challenges encountered with fracturing in
unconsolidated
formations often persists.
100061 United States Patent No. 6,644,407 describes a method for indirectly
fracturing an unconsolidated subterranean formation to avoid the production of
particulates from the unconsolidated formation. A vertical lined and cemented
well is
perforated to induce a horizontal fracture in the consolidated reservoir
region, creating
a region of disturbance that extends to some extent into the nearby
unconsolidated
region. Propping agent is utilized in the consolidated region to hold back
sand from the
unconsolidated region from entering the wellbore. By only directly fracturing
the
consolidated region, the unconsolidated region is disturbed to a lesser degree
thereby
reducing the production of particles. The method requires that the reservoir
has at least
one consolidated region near the unconsolidated formation of interest.
Accordingly, the
method cannot be applied to reservoirs that contain completely unconsolidated
regions
as occurs commonly in heavy oil and shallow gas reservoirs. This limitation
further
limits the possible number of horizontal fractures to the number of identified
regions of
consolidated rock. Moreover, the method is further limited to application in a
vertical
well process that can vertically traverse through consolidated and
unconsolidated
regions in a formation. As a result, the difficulties related to fracturing
unconsolidated
formations remains unaddressed in particular in horizontal well processes.
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[0007] Directional or horizontal wells, as is well known by persons of skill
in the art,
can result in higher production than a vertical well. As the reservoir rocks
which
contain hydrocarbons are usually horizontal, or sub-horizontal; a horizontal
well placed
in a production zone has more surface area in the production zone than a
vertical well,
and in this way allows for far greater exposure to a production zone than a
vertical well.
[NOM Hydraulic fracturing of a horizontal well usually occurs in a number of
stages
to create multiple fractures along the length of the horizontal well,
extending through
the production zone. The method by which the fractures are placed along the
well is
most commonly achieved by one of two methods, known as "plug and perforate
sequence" and "sliding sleeve" which can allow for more than 30 vertical
fractures to
be pumped into the horizontal section of a single well, which may be up to
10,000 feet
in length, as compared to the single fracture provided by a vertical well.
[0009] Horizontal wells, unlike vertical wells, typically traverse through a
homogenous region or zone, oftentimes an unconsolidated formation comprising
low-
permeability formations such as shales containing oil and/or gas and having no
bottom
water. Accordingly, while technological advancements in vertical well
fracturing have
been made, these advancements may not be applicable to the unique challenges
presented by horizontal well processes.
[0010] This background information is provided for the purpose of making known
information believed by the applicant to be of possible relevance to the
present
invention. No admission is necessarily intended, nor should be construed, that
any of
the preceding information constitutes prior art against the present invention.
SUMMARY OF THE INVENTION
[0011] An object of the present invention is to provide a method for
recovering
hydrocarbons from a subterranean reservoir. In accordance with one aspect of
the
invention, there is provided a method for recovering hydrocarbons from a
subterranean
reservoir, said reservoir being contained between a pair of barrier rocks
respectively
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above and below said reservoir, said method comprising: drilling a horizontal
well in a
first barrier rock of said pair, said first barrier rock adjacent to said
reservoir to form a
first interface, wherein said horizontal well is positioned to horizontally
traverse
through said first barrier rock; establishing at least one fracture propagated
from said
horizontal well, said at least one fracture vertically extending from said
first barrier
rock into said reservoir towards a second barrier rock adjacent to said
reservoir
opposite to said first barrier rock, said second barrier rock forming a second
interface
with said adjacent reservoir, wherein said fracture terminates at said second
interface;
and producing hydrocarbons recovered from said horizontal well.
[0012] In accordance with another aspect of the invention, there is provided a
method
for recovering hydrocarbons from a subterranean reservoir, said reservoir
being
contained between a cap rock and a basement rock, said method comprising:
drilling a
horizontal well in said basement rock, said basement rock in a region adjacent
to a
bottom portion of said reservoir having an interface, and wherein said
horizontal well is
positioned to horizontally traverse through said basement rock below said
interface;
establishing at least one fracture propagated from said horizontal well, said
at least one
fracture vertically extending upwards from said basement rock into said
reservoir and
terminating at said cap rock adjacent to a top portion of said reservoir
opposite to said
basement rock; and producing hydrocarbons recovered from said horizontal well.
[0013] In accordance with another aspect of the invention, there is provided a
method
for recovering hydrocarbons from a subterranean reservoir, said reservoir
being
contained between a cap rock and a basement rock and having an unconsolidated
region therein, said method comprising: drilling a horizontal well in said cap
rock, said
cap rock adjacent to a top portion of said reservoir having an interface, and
wherein
said horizontal well is positioned to horizontally traverse through said cap
rock above
said interface; establishing at least one fracture propagated from said
horizontal well,
said at least one fracture vertically extending downwards from said cap rock
into said
reservoir and terminating at said basement rock; and producing hydrocarbons
recovered
from said horizontal well.
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BRIEF DESCRIPTION OF THE DRAWINGS
[0014] These and other features of the invention will become more apparent in
the
following detailed description in which reference is made to the appended
drawings.
[0015] Figure 1 is a schematic diagram of a typical prior art fracturing
method in
vertical well processes;
[0016] Figure 2 is a schematic diagram of a typical prior art fracturing
method in
horizontal well processes;
[0017] Figure 3 is a schematic diagram of a prior art method for indirectly
fracturing
an unconsolidated subterranean formation in vertical well processes;
[0018] Figure 4 is a schematic diagram of a fractured horizontal well for
recovering
hydrocarbons from a subterranean formation, according to embodiments of the
present
invention; and
[0019] Figure 5 is a schematic diagram of a fractured horizontal well for
recovering
hydrocarbons from a subterranean formation, according to embodiments of the
present
invention.
DETAILED DESCRIPTION OF THE INVENTION
Definitions
[0020] Unless defined otherwise, all technical and scientific terms used
herein have
the same meaning as commonly understood by one of ordinary skill in the art to
which
this invention belongs.
[0021] As is known by persons skilled in the art, there are essentially two
basic types
of geologic formations, consolidated and unconsolidated formations. As used
herein,
the term "consolidated formation" refers to a homogeneous layer composed of
solid
rock or cemented earthen material. The term "unconsolidated formation" refers
to
loose, unsorted earthen materials, or particles such as clay, silt, sand,
gravel, or stones
and, as used herein, encompasses partially consolidated formations.
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[0022] The terms "barrier rock" or "barrier zone" are used interchangeably
herein to
refer to strongly consolidated rock formations that resist fracturing when
fractures are
formed in formations below or above the barrier rock or barrier zone. For this
reason,
as is known by those of skill in the art, most fracturing operations are
conducted in
subterranean formations between or beneath a barrier rock/zone. Barrier rock,
as used
herein, can further be distinguished as "cap rock" and "basement rock" to
refer to
barrier rock located above and below a target reservoir, respectively.
[0023] As used herein, the term "about" refers to an approximately +/-10%
variation
from a given value. It is to be understood that such a variation is always
included in any
given value provided herein, whether or not it is specifically referred to.
[0024] Insufficient permeability and/or reservoir pressure are typically the
factors
inhibiting the flow of natural gas and oil from subterranean reservoirs into a
well.
Hydraulic fracturing is used to increase or restore the rate at which fluids,
such as
petroleum and natural gas can be recovered from these subterranean reservoirs.
By
creating fractures from a well drilled into a reservoir rock formation, a
conductive path
is provided that connects a larger volume of the reservoir to the well and,
thereby,
releases formerly inaccessible hydrocarbons for extraction to further increase
efficiency
of hydrocarbon production from the well.
[0025] Fractures may be propagated from vertical or horizontal wells. As shown
in
Fig. 1, a vertical well 5 process vertically penetrates a subterranean
formation to extract
hydrocarbons from a target reservoir 20. Access to a target reservoir 20 in
such vertical
well processes is limited to the thickness of the target reservoir 20. While
hydraulic
fracturing can be used to improve the efficiency of hydrocarbon extraction,
the amount
of fracturing is also limited to the single region in the target reservoir 20
accessed by
the vertical well 5. Accordingly, the ability of fracturing to improve the
efficiency of
hydrocarbon recovery, in such prior art systems, is limited.
[0026] By utilizing horizontal wells where the terminal drillholc is completed
as a
"lateral" that intersects a target reservoir parallel to its plane of more
extensive
dimension, a greater region of the target reservoir is exposed and made
accessible to the
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well than would be the case with a vertical well that penetrates the reservoir
perpendicular to its plane of more extensive dimension. The efficiency of such
horizontal wells 10, as shown in Fig. 2, is further improved by allowing for
multiple
vertical fractures 50 to be created from the horizontal section 15 of the well
into the
target reservoir 20.
[0027] Both vertical 5 and horizontal 10 well processes, as shown in Figs. 1
and 2,
respectively, typically intersect the target reservoir 20. The target
reservoir 20 may be
either a consolidated or unconsolidated formation. When the target reservoir
20 is an
unconsolidated formation, hydraulic fracturing can result in a number of
challenges to
the efficiency of hydrocarbon recovery. For example, fracturing in such
formations is
difficult to complete due to the tendency of unconsolidated formations to
collapse into
the fracture when the pressure is removed. As well, due to the loose nature of
the
formation, earthen particles tend to be produced into the well with produced
fluids.
Production of such earthen particles can result in numerous problems including
plugging the fracture and filling the well which can result in shortening the
life of
pumping equipment.
[0028] Indirectly fracturing unconsolidated formations can avoid such
challenges. As
shown in Fig. 3, by initiating a horizontal fracture 55 into a consolidated
formation 35,
nearby unconsolidated formations 25 may be indirectly fractured. Specifically,
the
initial fracture 55 horizontally extends from the consolidated formation 35 in
an oval
manner into nearby unconsolidated formations 25. The scope of the extended
oval
fracture 60 expands into nearby unconsolidated formations 25, thereby,
indirectly
fracturing such unconsolidated formations 25 while avoiding the difficulties
arising
from directly fluidizing the particulate matter in the unconsolidated
formation 25. The
unconsolidated formation 25 is, therefore, disturbed to a lesser degree and
the
production of earthen particles is reduced.
[0029] Such indirect fracturing methods can be used to improve the
efficiencies of
hydraulic fracturing into unconsolidated formations, however, its requirement
for at
least one consolidated region near the unconsolidated formation of interest
limits its
application. Specifically, such methods cannot be applied to reservoirs that
contain
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completely unconsolidated formations as occurs commonly in heavy oil and
shallow
gas reservoirs. As such, the method is limited to application in vertical well
processes
that can vertically traverse through both consolidated and unconsolidated
formations in
a subterranean reservoir. The extent of indirectly fracturing into
unconsolidated
formations is further limited by the impedence caused by the high parting
pressure of
the consolidated formation. As shown in Fig. 3, once initiated the fracture 60
extends
into the unconsolidated formation 20 to a limited degree thus the extent of
fracturing
into the unconsolidated formations 25 is limited.
[0030] The method according to embodiments of the present invention relates to
the
recovery of hydrocarbons from a subterranean reservoir. In particular, the
method
relates to horizontal well processes for maximizing hydrocarbon recovery by
permitting
access to a greater region of a target reservoir and further permitting
multiple vertical
fractures to be established from a single (horizontal) well into the reservoir
of interest.
[0031] According to embodiments of the present invention, the horizontal well
is
drilled into barrier rock defining the target reservoir. Fracturing is
initiated in the
barrier rock and extends into the target reservoir to create open-flow
pathways through
the reservoir to the horizontal well, facilitating the recovery of
hydrocarbons produced
through the well. The direction and the extent of the fracturing, according to
such
embodiments, can be controlled by the vertical depth positioning of the well
relative to
the interface between the barrier rock and the target reservoir. In this
way,
embodiments of the present invention permit fracturing to be targeted to the
reservoir
of interest, thereby, maximizing fracturing into the target reservoir and
minimizing
fracturing into areas of non-interest.
[0032] According to some embodiments, the method of the present invention is
adaptable to existing fracturing technologies commonly used in horizontal well
processes such as, for example, the "plug and perforate sequence" and "sliding
sleeve"
technologies well known to those of skill in the art. As such, embodiments of
the
present invention can be applied to fracturing completely unconsolidated or
partially
unconsolidated rock formations. Moreover, the challenges encountered in
fracturing in
such formations are addressed by embodiments of the present invention. For
example,
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embodiments can include the use of propping agents to assist in maintaining
the open-
flow pathway from an unconsolidated reservoir to the horizontal well and
further
prevent the flow of earthen particles into the well.
[0033] Referring now to Figs. 4 and 5, the method of the present invention
comprises
drilling a horizontal well 10 to recover hydrocarbons from a target reservoir
20 in a
subterranean formation. In some embodiments, the target reservoir 20 is in a
consolidated formation. In other embodiments, the target reservoir 20 is in an
unconsolidated formation. The target reservoir 20 comprising hydrocarbons,
will
typically be bounded by barrier rock 30, 40 above and below the target
reservoir 20
namely cap rock 30 and basement rock 40, respectively.
Drilling the Horizontal Well into Barrier Rock
[0034] The horizontal
well 10 is drilled such that the horizontal section 15 is
positioned in the barrier rock 30, 40 and runs parallel to the plane of the
target reservoir
20. In one embodiment, as shown in Fig. 4, the horizontal section 15 is
drilled into the
basement rock 40. In other embodiments, as shown in Fig. 5, horizontal section
15 can
be drilled into the cap rock 30. The horizontal section 15 may be open-hole or
completed with cement and a liner according to techniques known in the art.
[0035] Once the horizontal well 15 is in place in the barrier rock 30, 40, one
or more
fractures 50 are propagated from the horizontal section of the well 15. The
horizontal
section 15 may be fractured or multi-fractured by any of the techniques well
known to
those skilled in the art. Non-limiting examples include, the open-hole
StackFracTM
sliding sleeve process practiced by Packers Plus 1 m, and the cemented liner
sequential
isolate-and-perforate process practiced by HalihurtonTM. The fracturing fluid
may be
water-based, but in certain embodiments, particularly for oil recovery, the
fracturing
fluid may contain a solvent such as light hydrocarbons or CO2. Once the
fractures 50
are established, primary production of hydrocarbon fluids may begin.
Production
techniques well known to those skilled in the art may be used to produce
hydrocarbons
from the fractured well. In some embodiments, a cyclic injection-production
fluid
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recovery process may be undertaken. In other embodiments, production of
hydrocarbon
fluids may be undertaken using a fluid-drive enhanced hydrocarbon recovery
process.
[0036] Irrespective of the fracturing technique used, the fracture or
fractures 50 will
initially vertically extend from the horizontal section 15 in both the upwards
and
downwards direction. The extent to which a fracture 50 can penetrate and
extend
through a formation, however, will be determined by the parting pressure of
the
particular formation. Accordingly, the high parting pressure of barrier rock
30, 40 will
impede penetration of the fracture 50 into the barrier rock 30, 40. In this
way,
extension of the fracture 50 into barrier rock 30, 40 is limited. In contrast,
a target
reservoir 20, whether a consolidated or unconsolidated formation, will have a
lower
parting pressure than the barrier rock 30, 40 which will permit the fracture
50 to more
easily extend into the target reservoir 20. This parting pressure differential
between the
barrier rock 30, 40 and target reservoir 20 is utilized in embodiments of the
present
invention to control the direction of fracturing into the target reservoir 20.
[0037] For example, as shown in Fig. 4, fractures 50 are propagated from the
horizontal well 15 and initially may extend vertically in both directions
through the
basement rock 40, in which the well 15 is situated. As the fracture 50 extends
upwards
through the basement rock 40 and reaches the target reservoir 20, the lower
parting
pressure of the target reservoir 20 takes over such that further fracture 50
extension will
be only upwards into the target reservoir 20 which provides a path of less
resistance. As
a result, the fractures 50 will not penetrate to the bottom of the basement
rock 40 and
will be directed into the target reservoir 20.
[0038] In some embodiments, particularly in unconsolidated formations, a
propping
agent as known to those skilled in the art can be used to maintain the open-
flow
pathway of the fracture 50 from the target reservoir 20 to the horizontal well
15. In
such embodiments, the propping agent can prevent collapse of the fracture 50
when the
pressure is removed and propping agent within the basement rock 40 can further
serve
to prevent the flow of fine particles from a target reservoir 20, i.e.,
unconsolidated
formation, into the horizontal well 15. Suitable propping agents are well
known to
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persons skilled in the art and may include, for example, sand, resin products,
ceramics,
small steel balls, ground walnut hulls, and resin-coated inorganic
particulates.
[0039] Referring to Fig. 5, certain embodiments of the present invention may
be
adapted for gas recovery, for example in some embodiments the present
invention may
be adapted for methane recovery from coal-beds. As shown in Fig. 5, the
horizontal
well 15 may be emplaced in the cap rock 30, with the fractures 50 extending
downward
into the target reservoir 20 comprising the gas zone. As discussed above, in
reference to
Fig. 4, the direction that the fractures 50 extend from the well 15 can be
controlled by
the parting pressure differential between the cap rock 30 and the target
reservoir 20.
Vertical Depth Positioning of the Horizontal Well
[0040] As described, the parting pressure differential between the barrier
rock 30, 40
and the target reservoir 20 allows fracturing to be directed to the target
reservoir 20.
The horizontal section 15 of the well 10 should be positioned within the
barrier rock 30,
40 nearer to the interface 70, 80 with the target reservoir 20 so that the
fractures 50 will
propagate easily into the target reservoir 20 without breaking through the
barrier rock
30, 40. To illustrate, for example, in a barrier rock 30, 40 that is 50 meters
thick in
which the horizontal section 15 of the well 10 is positioned 5-meters from the
interface
70, 80 of the barrier rock with the target reservoir, a fracture 50 need only
propagate 5-
meters vertically before reaching the lower parting pressure of the target
reservoir 20.
Once reaching the target reservoir 20, the injection pressure of the
fracturing fluid will
be relieved on account of the plastic nature of the target reservoir 20
allowing the
fracture to continue extending into the target reservoir 20 in preference to
the barrier
rock 30, 40.
[0041] According to some embodiments, the horizontal section 15 of the well 10
is
positioned in the barrier rock 30, 40 at a vertical depth from the interface
70, 80 that
will permit a fracture 50 to propagate primarily into the target reservoir 20.
In some
embodiments, the vertical depth of the horizontal section 15 of the well 10 is
between
about 1 to about 10 meters from the interface 70, 80 into the barrier rock 30,
40. In
other embodiments, the vertical depth of the horizontal section 15 of the well
10 is
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about 8 meters from the interface 70, 80 into the barrier rock 30, 40. In
other
embodiments, the vertical depth of the horizontal section 15 of the well 10 is
about 6
meters from the interface 70, 80 into the barrier rock 30, 40. In other
embodiments, the
vertical depth of the horizontal section 15 of the well 10 is about 5 meters
from the
interface 70, 80 into the barrier rock 30, 40. In other embodiments, the
vertical depth
of the horizontal section 15 of the well 10 is about 4 meters from the
interface 70, 80
into the barrier rock 30, 40. In still other embodiments, the vertical depth
of the
horizontal section 15 of the well 10 is about 2 meters from the interface 70,
80 into the
barrier rock 30, 40.
[0042] In further embodiments, the vertical depth of the horizontal section 15
of the
well 10 is between about 1% and 10% the thickness of the barrier rock 30, 40.
[0043] It will be understood by persons of skill in the art that the direction
and extent
of fractures 50 may be adjusted by varying the vertical depth of the
horizontal section
15 of the well 10 within the barrier rock 30, 40.
[0044] The scope of the claims should not be limited by the preferred
embodiments set
forth in the foregoing examples, but should be given the broadest
interpretation
consistent with a reading of the specification as a whole.
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