Note: Descriptions are shown in the official language in which they were submitted.
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ACCELERATED START-UP USING SOLVENT INJECTION
FIELD OF THE INVENTION
The present invention relates to oil and gas production. More particularly,
the
present invention relates to the recovery of heavy oil or bitumen from, for
example an oil
sand or tar sand deposit. More particularly, the present invention relates to
a method for
starting up such operations in a gravity controlled recovery system, such as
Steam
Assisted Gravity Drainage (SAGD).
BACKGROUND OF THE INVENTION
Numerous patents, patent applications and technical papers describe means of
establishing fluid communication between proximate horizontal wells that are
located in
porous media of limited fluid mobility, such as an oil sand at original
conditions. However,
as will be explained, none of these relies on or teaches the key elements of
the present
invention to accomplish that communication.
For example, U.S. Patent No. 3, 913,672 filed October 15, 1973, by J.C. Allen
and
D. A. Redford and titled "Method for establishing communication path in
viscous
petroleum-containing formations including tar sands for oil recovery
operations" describes
a method to establish fluid communication in a viscous petroleum-containing
formation
such as an oil sand. The method comprises "forming an initial entry zone into
the
formation by means such as hydraulic fracturing and propping, or utilizing
high
permeability streaks naturally occurring within the formation, and injecting
into the
propped fracture zone or high permeability streak a solvent for the petroleum
contained in
the formation, said solvent being saturated with a gas or containing
appreciable quantities
of gas dissolved therein". In contrast to this, the present invention requires
no prior
establishment or presence of permeable zone or streak. Furthermore, the
present
invention, while using a solvent, requires no dissolution in that solvent of a
gas. Thus, the
present invention, in not requiring these well-known aids to fluid conveyance
in an oil
sand or other formation of limited mobility, is not only novel but also non-
obvious.
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The abstract of U.S. Patent No. 3,706,341 to Redford, and titled "Process For
Developing Interwell Communication In A Tar Sand", reads in part: "A hot,
competent,
permeable communications zone, connecting injection and production wells
completed in
a tar sand, is developed as follows: A cold, aqueous solution containing
sodium hydroxide
and a non-ionic surfactant is injected into a propped fracture system
connecting the wells.
... Bitumen is slowly emulsified in the solution and removed through the
fracture system.
". The present invention involves none of the chemicals, or chemical
combinations, or
chemical types described in this patent. Also, Redford utilizes a prior means
of
communication, described as a "propped fracture system connecting the wells"
whereas
the present invention relies on no such prior communication artifact.
U.S. Patent No. 4,249,604 to Frazier and titled "Recovery method for high
viscosity petroleum" states that "....the invention relates to oil recovery
methods wherein
heated aqueous fluids are injected into directionally drilled injection wells
which are drilled
radially inward towards central producing well....". The present invention
differs materially
from this patent in both process and well configuration.
U.S. Patent No. 4,301,868 filed October 15, 1979 by G. Scherubel et al and
titled
"Method using hydrocarbon foams as well stimulants" states in the abstract
that "This
invention relates to the use of silicone-induced hydrocarbon foams as well
stimulants, for
example as illustrated by their use in fracturing well formations, removal of
paraffin in
wells, removal of condensate hydrocarbons from blocked gas wells, etc". The
process of
the present invention differs materially from that of this patent and
specifically does not
use foams.
U.S. Patent No. 7,464,756 filed February 4, 2005 by I. Gates and A. Gutek and
titled "Process for in situ recovery of bitumen and heavy oil" includes as
claim 1 in its
entirety the following: "A method to recover heavy hydrocarbons from an
underground
reservoir, the underground reservoir being penetrated by an injection well and
a
production well, the method comprising the steps of: (a) injecting steam and a
heavy
hydrocarbon solvent into the injection well over time, while producing
reservoir
hydrocarbons from the production well; (b) transitioning from steam and heavy
hydrocarbon solvent injections to a lighter hydrocarbon solvent injection,
while continuing
to produce hydrocarbons from the production well; (c) continuing to inject the
lighter
hydrocarbon solvent while producing hydrocarbons from the production well; and
(d)
enhancing the solubility of solvents in steps (a) through (c) by monitoring
and adjusting
reservoir pressure". The cited patent describes a recovery process rather than
an
interwell communication technique. That aside, the process initially requires
the injection
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of steam as a concomitant of the solvent injection "over time" while producing
from the
production well. The present invention requires no concomitant injection fluid
along with
the solvent, and involves no step of concurrently producing at another well
while the
solvent is being injected. Indeed, the present invention requires injection of
only the
solvent with no injection of other fluids on either a precedent, concomitant,
or successor
basis. It should also be noted that, according to claims of the cited patent,
solvent
injection occurs only at the injection well. In the present invention, solvent
is a means of
establishing interwell communication and occurs at either an injector or a
producer or
both.
Numerous patents involve well stimulation using a solvent in which that
solvent is
heated by some means, such as the concomitant injection of steam or prior
heating of
solvent, for example by electrical means.
By way of illustration two of the patents that include these features are:
U.S. Patent No. 5,400,430 by J. Nenniger filed January 21, 1994 and titled
"Method for injection well stimulation". The process involves electrical
heating of a solvent
prior to its injection into the reservoir. Also, as the title implies, and as
described therein,
the process is intended only for an injection well.
U.S. Patent No. 7,363,973 by J. Nenniger and E. Nenniger filed February 2,
2005
and titled "Method and apparatus for stimulating heavy oil production"
describes in the
abstract a process which "has the steps of heating a solvent, such as propane,
and then
placing the solvent into the extraction chamber at a temperature and a
pressure sufficient
for the solvent to be in a vapor state in the chamber and to condense on the
extraction
surface". Not only does the patent describe the heating of the solvent, but it
describes the
necessity for the solvent to be in a vapor state. The present invention
requires neither the
prior heating of the solvent, nor the requirement that it be in a vapor state
nor the need for
a change of state.
Other processes involving solvent include U.S. Patent No. 6,769,486 by G. Lim
et
al. filed May 30, 2002 and titled "Cyclic solvent process for in situ bitumen
and heavy oil
production". This patent involves injection of a solvent at a pressure
sufficiently high to
cause formation dilation or pore fluid compression, allowing solvent and
hydrocarbons to
mix at pore dilation conditions, and then reducing the reservoir pressure so
as to cause a
solvent gas drive. The present invention entails no dilation of the formation,
involves
injection of the solvent strictly to establish interwell communication, and
does not function
as part of a solvent gas drive.
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U.S. Patent No. 5,607,016 by Butler filed November 1993 and titled "Process
and
apparatus for the recovery of hydrocarbons from a reservoir of hydrocarbons"
describes
in the abstract the process as follows: "A method for the recovery of
hydrocarbons from a
reservoir of hydrocarbons including injecting a hydrocarbon solvent into the
reservoir
along with a displacement gas to mobilize hydrocarbons in the reservoir of
hydrocarbons;
and producing mobilized hydrocarbons from the reservoir of hydrocarbons. The
hydrocarbon solvent is injected along one or more predominantly horizontal
injection
wells in the aquifer that are spaced from the production well or wells. The
hydrocarbon
solvent is a hydrocarbon solvent selected from the group consisting of ethane,
propane
butane". The present invention differs materially from this patent. According
to the claims,
the patent involves the use of a gas to first establish a communications path,
and then
continued use of gas as a concomitant of solvent injection. The present
invention requires
no prior step, such as gas injection, to establish communication, and involves
injection
into the reservoir of the solvent alone, without any concomitant fluids. Also,
one
embodiment of the present invention involves the injection of xylene, whereas
the Butler
patent confines its specifications to a solvent from the group consisting of
ethane,
propane, butane.
SUMMARY OF THE INVENTION
This invention pertains to a method of establishing or accelerating fluid
communication between horizontal wells located in a formation of very limited
fluid
mobility, such as an oil sand reservoir. The horizontal wells may be
vertically displaced
from each other, for example as are the injector and producer in a SAGD
pattern. Or they
may be staggered with respect to each other, for example as when a horizontal
infill well
is situated in the region between two surrounding SAGD well patterns.
The method of accelerating communication according to this invention involves
injecting a modest amount of solvent, one embodiment of which is xylene, into
a subject
well. A subject well is herein defined as a well that is located in a region
of the reservoir
that has very limited fluid mobility and from which communication is to be
established with
a target well by means of solvent injection. Conversely, a target well is
herein defined as
a well with which communication is to be established by means of solvent
injection into
the subject well.
The solvent is injected via the subject well into the reservoir at sub-
fracturing
pressures. Injection of the solvent may be into any individual subject well or
into any
combination of subject wells. After some time period of soak, and without the
aid of
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previously created flow channels at a subject well, such as through fracturing
or pre-
injection of mobile fluids, and without the aid of prior or concurrent thermal
means to
establish or enhance communication at a subject well, such as steam or in situ
combustion or electrical heating, communication between a subject well and a
target well
is established, notwithstanding the modest volume of solvent injected at the
subject well.
In some instances, the subject well and the target well may both be situated
in a
region of immobile bitumen. One embodiment of this instance involves a SAGD
well pair
that has not yet been placed in operation. In this embodiment, the subject
well might be
either the injector, or the producer, or both.
In other instances, the subject well may be situated in an extensive region of
immobile bitumen whose distal boundary is defined, in part or in whole, by an
adjacent
region of mobile bitumen. One embodiment of this instance involves an infill
horizontal
well that is located within the unheated region between adjacent operating
SAGD
horizontal well pairs. In this embodiment, the infill well is the subject well
and the target
wells are those located within a SAGD steam chamber or heated zone, which zone
is
separated from the subject well by a region containing bitumen of very limited
mobility.
In the case of a new SAGD well pair, the communication thus achieved is
sufficient to then permit inception of a gravity-dominated process, such as
SAGD, or other
gravity-dominated processes such as those involving the use of solvents as
part of a
thermal or non-thermal technique, without the need for additional procedures
or
processes whose purpose is to further facilitate or expedite inter-well
communication,
such as steam circulation in the wells.
In the case of an infill well located within a region of very limited fluid
mobility that
is situated between SAGD well pairs, communication is thus achieved without
the need
for additional procedures or processes and is sufficient to permit the infill
well to
participate in the recovery process by acting as a supplementary means of
production.
In one aspect, the present invention provides a method wherein a selected
amount of solvent is injected at sub-fracturing conditions (pressure or rate
or both) and
ambient temperature into a first horizontal well located in an immobile
formation or
reservoir, so as to accelerate fluid communication in the formation or
reservoir between
the first horizontal well and a second horizontal well with which the first
horizontal well is
not in fluid communication.
In an embodiment of the present invention the solvent is xylene, benzene,
toluene, phenol or mixtures or derivatives thereof. In an embodiment of the
present
invention the solvent is substantially xylene.
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In an embodiment of the invention, the solvent is injected into either the
first
horizontal well or the second horizontal well, or into both the first
horizontal well and the
second horizontal well.
In an embodiment of the invention, the immobile formation or reservoir is an
oil
sand reservoir at original conditions. In an embodiment of the invention, the
original
conditions include a bitumen or heavy oil density of about 12 API or heavier.
In an
embodiment of the invention, the original conditions include a bitumen or
heavy oil
density of about 15 API or heavier.
In an embodiment of the invention, the selected amount of solvent is a volume
less than ten times the volume of the first horizontal well completion.
In an embodiment of the invention, a further selected amount of solvent is
injected
at sub-fracturing conditions and ambient temperature into the second
horizontal well. In
an embodiment of the invention, the further selected amount of solvent is a
volume less
than ten times the volume of the second horizontal well completion. In an
embodiment of
the invention, the total of the selected amount of solvent and the further
selected amount
of solvent is a volume less than ten times the volume of the total of the
first horizontal well
completion and the second horizontal well completion.
In an embodiment of the invention, the solvent is displaced into the immobile
formation or reservoir using a displacing fluid sufficient to displace
substantially all of the
selected volume of solvent into the formation or reservoir. In an embodiment
of the
invention, the displacing fluid is a non-condensing, non-oxidizing gas. In an
embodiment
of the invention, the displacing fluid is substantially nitrogen.
In an embodiment of the invention, the injected solvent is allowed to remain
resident in the immobile formation or reservoir for a selected period of time
before a
subsequent recovery process is implemented. In an embodiment of the invention,
the
solvent is displaced into the immobile formation or reservoir using a
displacing fluid
sufficient to displace substantially all of the selected volume of solvent
into the formation
or reservoir after the selected period of time before the subsequent recovery
process is
implemented.
In an embodiment of the invention, a subsequent recovery process is
implemented following application of the method to establish inter-well
communication. In
an embodiment of the invention, the subsequent recovery process is SAGD or
other
gravity-dominated recovery processes that involved the use of solvents, either
thermally
or non-thermally.
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In an embodiment of the invention, the first horizontal well is an infill well
completed in an immobile hydrocarbon region located between surrounding well
patterns
of a gravity-dominated recovery process.
In an embodiment of the invention, the immobile hydrocarbon region is in a
bypassed region, remaining between adjacent mobilized hydrocarbon regions of a
gravity-dominated recovery process.
In an embodiment of the invention, fluid communication is established between
the first horizontal well and an adjacent mobilized hydrocarbon region of a
gravity-
dominated recovery process.
In an embodiment of the invention, fluid communication is established between
the first horizontal well and an adjacent common mobilized hydrocarbon region
of a
gravity-dominated recovery process.
Other aspects and features of the present invention will become apparent to
those
ordinarily skilled in the art upon review of the following description of
specific
embodiments of the invention in conjunction with the accompanying figures.
BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the present invention will now be described, by way of example
only, with reference to the attached Figures, wherein:
Fig. 1 illustrates the method of the present invention applied to a SAGD well
pair;
Fig. 2 illustrates a section 2-2 of Fig. 1;
Fig. 3 illustrates the method of the present invention applied to an infill
well
completed between adjacent SAGD well pairs; and
Fig. 4 illustrates the method of the present invention applied to an infill
well
completed adjacent a SAGD well pair.
DETAILED DESCRIPTION
Referring to Figs. 1 and 2, a typical SAGD recovery system 10 is shown, having
an injection well 20 for injecting steam and a production well 30 for
producing fluids
completed in a immobile formation or reservoir 40. A portion of the injection
well 20 is
open to the immobile formation or reservoir 40 via an horizontal injection
well completion
50. Similarly, a portion of the production well 30 is open to the immobile
formation or
reservoir 40 via a horizontal production well completion 60. These horizontal
well
completions are typically perforations, slotted liner, screens, or a
combination thereof
known to one skilled in the art.
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In order to establish fluid communication between the injection well 20 and
the
production well 30 in an accelerated manner, a selected amount of solvent is
injected at
sub-fracturing conditions (pressure or rate or both) and ambient temperature
into either
the injection well 20 or the production well 30 or both the injection well 20
and the
production well 30.
The selected amount of solvent may be a relatively small volume, for example
less than about ten times the volume of the horizontal well completion. As an
example, if
the horizontal production well completion 60 includes a 7 inch (approximately
0.180m)
nominal diameter slotted liner, 800 m long, the volume of the horizontal
production well
completion 60 is approximately 20 cubic metres. In such a case, a volume of
less than
about 200 cubic metres may be selected.
Referring to Fig. 3, after a period of time of operation of the SAGD recovery
system 10, a mobilized region 70a develops around the injection well 20a and
the
production well 30a (and likewise a mobilized region 70b develops around the
injection
well 20b and the production well 30b etc.). However, a portion of the immobile
formation
or reservoir 40ab remains between the adjacent well pairs. In the case where
the
mobilized region 70a and the mobilized region 70b merge or coalesce to form a
common
mobilized region 70ab, the portion of the immobile formation or reservoir 40ab
remaining
may be referred to as a bypassed region 40ab.
An infill well 80 is completed in the immobile formation or reservoir 40ab. A
portion of the infill well 80 is open to the immobile formation or reservoir
40ab via an
horizontal infill well completion 90. In order to establish communication
between the infill
well 80 and the mobilized region 70a or the mobilized region 70b or both, or
the common
mobilized region 70ab (and thus to the production well 30a or the production
well 30b or
both), a selected amount of solvent is injected at sub-fracturing conditions
(pressure and
flow rate or both) and ambient temperature into the infill well. The selected
amount or
solvent may be less than about ten times the volume of the horizontal infill
well
completion 90.
Referring to Fig. 4, after a period of time of operation of the SAGD recovery
system 10, a mobilized region 70 develops around the injection well 20 and the
production well 30. However, a portion of the immobile formation or reservoir
40 remains
adjacent the well pair.
An infill well 80 is completed in the immobile formation or reservoir 40. A
portion
of the infill well 80 is open to the immobile formation or reservoir 40 via a
horizontal infill
well completion 90. In order to establish communication between the infill
well 80 and the
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mobilized region 70 (and thus to the production well 30), a selected amount of
solvent is
injected at sub-fracturing conditions (pressure and flow rate or both) and
ambient
temperature into the infill well. The selected amount or solvent may be less
than about
ten times the volume of the horizontal infill well completion 90.
As a means of displacing the solvent into the reservoir at the subject well
while
minimizing the quantity of solvent used, a chasing fluid may be used, although
this is not
essential. If a chasing fluid is used, it does not enter the reservoir and
therefore does not
participate in the process of accelerating fluid communication. Typically the
chasing or
displacing fluid is a non-condensing, non-oxidizing gas, such as nitrogen,
sufficient to
displace most of the solvent into the formation.
The present invention relates to a method of establishing communication
between
a horizontal well that is situated in a hydrocarbon deposit of very limited
mobility, such as
an oil sand or heavy oil at original temperature, and another well or region.
As such, the
present invention provides a means of accelerating start-up of a follow-up
gravity-
dominated recovery process such as SAGD. In another aspect the present
invention
provides a means of establishing communication between a horizontal well that
is within
and surrounded by a region of immobile bitumen and a mobilized region that is
located at
some distance from said horizontal well, such as one or more operating SAGD
well
patterns.
This invention relies on the use of a solvent, one embodiment of which is
xylene,
to mobilize the otherwise immobile bitumen in the immediate vicinity of the
horizontal
wellbore or wellbores. A designated volume, or slug, of solvent is injected at
a well and
displaced through its horizontal section, or portions thereof, into the
reservoir.
The solvent or combination of solvents may be selected from a wide variety of
compounds. In one embodiment the solvent comprises an organic solvent. In one
embodiment the organic solvent comprises an aromatic organic solvent. In one
embodiment the aromatic organic solvent comprises one or more of benzene,
toluene,
phenol, xylene or their derivatives or mixtures. In one embodiment the solvent
is
substantially xylene.
An aspect of the invention, and one which incorporates both novel and non-
obvious approaches, relates to the manner in which the solvent is injected
into the
reservoir so as to establish communication between the wells.
To this end, firstly, the solvent is injected at sub-fracturing pressures, and
no
means are used to fracture, dilate or otherwise alter the formation so that a
preferential
path or channel is created in the reservoir. Secondly, the solvent is injected
at ambient
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conditions, is not heated, and no other heating medium, such as steam, and no
other
displacing fluid such as a gas, accompany the solvent, precede the solvent or
follow the
solvent into the reservoir as part of the process of establishing inter-well
communication.
An option in applying the present invention is that, after the solvent is
displaced
into the reservoir, it may be allowed to reside, or soak, within the reservoir
for a period of
time.
Through the course of establishing inter-well communication via the subject
well,
there is no need for any extraneous approach to preparing the formation at
said well,
such as fracturing, or formation heating, or injection of a precedent fluid,
such as a gas, to
provide or facilitate the creation of a pre-existing pathway for the solvent,
or indeed
injection of concomitant or succeeding fluids.
A second novel and non-obvious aspect of this invention is the absence of need
for any means to augment mobility by heating the solvent, or by relying on
changes of
phase through either temperature or pressure or both.
A third novel and non-obvious aspect of this invention is the relatively small
volume of solvent slug used.
Solvent is generally very expensive. Therefore, while it may always be
possible to
establish inter-well communication using a sufficiently large volume of
solvent, there will
be some net solvent losses to the reservoir, the cost of which will easily
negate any
advantage provided by acceleration of recovery process start-up. In the
present invention,
a volume of solvent is used which is small relative to the available pore
volume that must
be traversed in order to establish interwell communication but which
nevertheless
achieves this communication.
The successful establishment of inter-well communication in the manner
consistent with this invention in a virgin oil sand using small unheated slugs
of xylene has
been verified experimentally at new SAGD wells. While many of the treatments
that were
carried out involved xylene slugs placed or injected at both the injector
(injection well,
being the upper well in a SAGD well pair) and the producer (production well,
being the
lower well in a SAGD well pair) at least one successful treatment included
placing or
injecting the solvent into only the producer. As a further check on the
efficacy of the
method, the xylene injection treatment obviated the need, during follow-up
SAGD
operations, for alternative means of establishing interwell communication such
as steam
circulation in either the injector or producer, or both.
Simulations of the xylene injection process provided a scientific basis for
our
salient field results. Simulation runs showed that the path of the injected
xylene outward
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from the subject well is not isotropic. Instead, because of the density
difference between
the xylene and the bitumen, or between the xylene-bitumen blends that evolve
as the
xylene migrates within the reservoir, and the bitumen, or between the xylene
and the
formation water, or between the xylene-bitumen blends and the formation water,
there is
a preferential upward movement of the xylene or xylene-bitumen blend front.
Thus
whereas, if the flow geometry were radial, certain limited volumes of xylene
would fail to
traverse the distance between producer and injector, and might thereby fail to
establish
inter-well communication, the situation in the reservoir created by virtue of
this invention,
with its resulting flow anisotropy, enables that same xylene volume to
establish inter-well
communication.
In one embodiment the invention provides a method of initiating or
accelerating
the establishment of fluid communication between horizontal wells located in a
formation
of very limited fluid mobility, such as an oil sand at original temperature or
an infill well
completed in an immobile hydrocarbon region located between surrounding well
patterns
in which a gravity-dominated recovery process is or has been operated.
In the preceding description, for purposes of explanation, numerous details
are set
forth in order to provide a thorough understanding of the embodiments of the
invention.
However, it will be apparent to one skilled in the art that these specific
details are not
required in order to practice the invention.
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