Note: Descriptions are shown in the official language in which they were submitted.
CA 02651527 2009-01-29
METHOD AND SYSTEM FOR ENHANCING A RECOVERY PROCESS EMPLOYING
ONE OR MORE HORIZONTAL WELLBORES
FIELD OF THE INVENTION
The present invention relates generally to a system and method for enhancing a
hydrocarbon recovery process. More particularly, the present invention relates
to a
system and method for enhancing a recovery process employing one or more
horizontal
wellbores by selectively altering vertical permeability in a reservoir.
BACKGROUND OF THE INVENTION
Oil and gas are nonrenewable natural resources relied upon heavily in the
industrialized world. The rising demand for oil and gas, combined with
declining
conventional resources, has necessitated the development of alternative
sources of oil,
as well as methods of enhancing recovery from both conventional and
alternative
resources.
The Athabasca oil sands of Alberta, Canada, contain some of the largest
deposits
of hydrocarbons in the world. Oil sands are an important alternative source of
crude oil,
such as bitumen and heavy oil, that can be extracted and processed for fuel.
Bitumen is
classified as an extra heavy oil, with an API gravity of about 10 or less,
referring to its
gravity as measured in degrees on the American Petroleum Institute (API)
Scale. Heavy
oil has an API gravity in the range of about 22.3 to about 10 . The terms
heavy oil and
bitumen are used interchangeably herein since they may be extracted using
similar
processes.
Heavy oil can be recovered from oil sands by various methods, including in-
situ oil
recovery methods. In-situ heavy oil recovery methods are typically applied
when a
deposit cannot be mined economically due to the depth of overburden. The aim
of most
in-situ heavy oil recovery processes is to reduce the viscosity of heavy oil
in the reservoir
to enable it to flow into a well and be produced therefrom. Thermal in-situ
heavy oil
recovery processes utilize heat, typically provided by steam injection, to
mobilize the
bitumen. Hydrocarbon solvents and may also be utilized. Such processes
frequently
employ one or more horizontal wellbores and typically utilize gravity drainage
as a fluid
drive mechanism. Horizontal wellbores can also be employed in conventional oil
or gas
recovery processes, including sweeping and flooding processes that utilize
displacement
as a fluid drive mechanism, and have been shown to offer significant potential
to
maximize production or injection rates.
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A key challenge with the practical application of any recovery process
utilizing one
or more horizontal wellbores, or otherwise relying on vertical movement of
flowable
material through a reservoir, is that permeability in most reservoirs is not
homogeneous.
Reservoir permeability generally refers to the capacity of formation material
to permit the
passage of fluid through it. The customary unit of measurement is the darcy
(D) or
millidarcy (mD). While the permeability of oil sand may generally be in the
range of a few
darcies to a few hundred darcies, the vertical permeability of an oil sands
reservoir is
frequently disrupted by layers of material having substantially reduced
permeability.
These layers or matrices can form vertical permeability impediments that
inhibit or
prevent the vertical flow of materials through the reservoir, relative to the
more permeable
pay zones, and thus limit production rate and resource recovery. The degree to
which a
vertical permeability impediment will hinder a recovery process will depend on
such
factors as the permeability and thickness of the impediment, the particular
recovery
process utilized and the viscosity of the hydrocarbon being recovered.
Methods of increasing vertical permeability in a reservoir, or enhancing
hydrocarbon recovery from a reservoir having vertical permeability
impediments, have
been proposed. However, each suffer from disadvantages that will be apparent
to
persons skilled in the art.
A known method of increasing reservoir permeability in general involves
hydraulic
fracturing of a formation with viscous fracturing fluid containing a proppant
material. Such
fractures are typically induced from a production or injection well. However,
attempts to
propagate fractures through compartmentalized or stratified formations have
historically
yielded unpredictable and poor results, such as insufficient fracture
extension and
inadequate propping of fractures adjacent to less permeable areas of the
formation.
U.S. 6,119,776 to Graham et al. describes a method of stimulating and
producing
a stratified reservoir by fracturing a horizontal injection well such that a
vertical fracture
extends through multiple layers of the formation and connects the injection
well to a
horizontal well below, thereby providing a passage for the flow of
hydrocarbons. The
lower well is sloped downward and intersects a vertical well that serves as a
sump and a
production well. Such a system is not designed for gravity drainage and is not
suited for
reservoirs where fracture extension occurs in a horizontal versus a vertical
orientation.
Moreover, a thick or substantially impermeable impediment, such as shale, may
not be
effectively propped open.
U.S. 2,280,851 (1942) to Ranney discloses early methods of drilling deviated
wellbores for collection of oil. The wellbore may be undulating and may cross
and re-
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cross a layer of impermeable material such that portions of the wellbore lie
in productive
stratum on either side of the impermeable layer in order to tap into both
zones. The fluids
that collect in the wellbore are vacuum pumped to the surface.
U.S. 6,708,764 to Zupanick discloses a drainage pattern of wellbores, which
may
include one or more undulating wellbores, extending from a main articulated
wellbore to
provide access to a large subterranean area from the surface. An undulating
drainage
wellbore may provide access to multiple layers of subterranean gas deposits
separated
by layers of impermeable material, such as a coal seam. The drainage pattern
connects
to a vertical wellbore having an enlarged cavity that can act as a sump.
U.S. 2007/0039729 to Watson et al. discloses a method of increasing vertical
permeability in a reservoir by drilling a series of substantially vertical
piercing wells
upwardly and on various angles from a large service tunnel or workspace
located deep
within the formation. Each vertical well pierces the impermeable layer once
from below to
provide multiple openings in the impermeable layer. The openings cover a broad
area in
the horizontal plane of the impermeable layer. The entire process is carried
out while
maintaining isolation between the service tunnel and the formation fluids.
Such a system
for increasing vertical permeability in a reservoir is costly and inefficient.
Tunneling is
rarely practiced for this reason.
There exists a need for enhancing hydrocarbon recovery in general. In
particular,
there exists a need for enhancing hydrocarbon recovery processes employing one
or
more horizontal wellbores.
SUMMARY OF THE INVENTION
It is an aim of the present invention to provide a method and system for
enhancing
a hydrocarbon recovery process employing one or more horizontal wellbores,
which
involves selectively altering permeability in the reservoir. The system and
method of the
invention are particularly useful in reservoirs having one or more vertical
permeability
impediments.
In a first aspect, there is provided a method of enhancing a hydrocarbon
recovery
process. The recovery process employs one or more substantially horizontal
wellbores.
The method comprises providing a zone of increased permeability in a
hydrocarbon
reservoir to facilitate movement of flowable materials through the reservoir
to thereby
enhance the recovery process. The zone of increased permeability is preferably
provided
in a region substantially above or below the one or more horizontal wellbores.
In some
embodiments, the zone of increased permeability extends generally in a
direction
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substantially parallel the horizontal axis of the one or more horizontal
wellbores along at
least a portion of the length thereof. In some embodiments, the zone of
increased
permeability may be provided by an accessory conduit in the reservoir. In some
embodiments, the reservoir has one or more vertical permeability impediments.
In another aspect, there is provided a method of enhancing a recovery process
employing one or more substantially horizontal wellbores in a hydrocarbon
reservoir
having at least one vertical permeability impediment. The method comprises
providing at
least one accessory conduit in a region substantially above or below at least
one said one
or more substantially horizontal wellbores. The at least one conduit extends
generally in a
direction substantially parallel to the horizontal axis of the one or more
horizontal
wellbores along at least a portion of the length thereof.
In another aspect, there is provided a method of recovering hydrocarbons from
a
reservoir having at least one vertical permeability impediment. The method
comprises
providing a production well having a substantially horizontal portion for
collection of
production material comprising hydrocarbons and providing an injection well
having a
substantially horizontal portion for injection of injection material. The
method further
comprises providing at least one accessory conduit in the reservoir in a
region
substantially above or below the production well and/or the injection well and
extending
generally in a direction parallel to the production well and/or the injection
well along at
least a portion of its length. The conduit has one or more portions that
extend through the
vertical permeability impediment to facilitate movement of the production
material and/or
the injection material through the reservoir. The method further comprises
recovering the
hydrocarbons from the production material.
In another aspect, there is provided method of increasing permeability of at
least
one vertical permeability impediment in a reservoir to enhance a hydrocarbon
recovery
process employing one or more horizontal wellbores. The method comprises
providing at
least one accessory conduit in the reservoir in a region substantially above
or below the
one or more horizontal wellbores. The conduit has one or more portions that
extend
through the at least one vertical permeability impediment to facilitate
movement of
flowable material therethrough to enhance the recovery process.
In another aspect, there is provided a method of enhancing a hydrocarbon
recovery process employing one or more horizontal wellbores in a reservoir
having at
least one vertical permeability impediment. The method comprises increasing
the
effective permeability of the at least one vertical permeability impediment in
a region
substantially above or below the one or more horizontal wellbores to
facilitate the
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movement of flowable materials through the at least one vertical permeability
impediment
to enhance the recovery process.
In another aspect, there is provided a method of enhancing recovery process
performance in a hydrocarbon reservoir having at least one vertical
permeability
impediment. The method comprises providing a first wellbore having a
substantially
horizontal portion for collection of hydrocarbons. The method further
comprises providing
at least one accessory conduit extending through the reservoir in a region
substantially
above or below the first wellbore. The accessory conduit has one or more
portions that
extend through the vertical permeability impediment to facilitate movement of
flowable
materials therethrough to enhance recovery.
In another aspect, there is provided an in situ heavy oil recovery process for
recovering hydrocarbons from a reservoir having one or more vertical
permeability
impediments. The method comprised providing one or more horizontal wellbores
in the
reservoir, including a production wellbore, and increasing the effective
permeability of the
one or more vertical permeability impediments above the production wellbore in
a region
extending along at least a portion of the length of the production wellbore to
facilitate the
movement of flowable materials through the one or more vertical permeability
impediments and toward the production wellbore.
In yet another aspect, there is provided a system for recovering hydrocarbons
from a reservoir. The system comprises one or more substantially horizontal
wellbores,
including a production wellbore, and at least one accessory conduit located
substantially
above or below the one or more substantially horizontal wellbores for
enhancing
movement of flowable materials through the reservoir to thereby enhance
recovery. In
some embodiments, the reservoir has at least one vertical permeability
impediment and
the accessory conduit has one or more portions that extend through the at
least one
vertical permeability impediment to facilitate movement of flowable materials
therethrough.
In some embodiments, the recovery process is a conventional oil recovery
process or a gas recovery process. In some embodiments, the recovery process
is a
heavy oil recovery process. In some embodiments, the heavy oil recovery
process is
steam assisted gravity drainage (SAGD), cyclic steam stimulation (CSS), steam
flooding
or a derivative thereof.
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
embodiments of the
invention in conjunction with the accompanying figures. The scope of the
invention is not
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limited to the exemplary embodiments described herein. Furthermore, the
figures are for
illustrative purposes and may not be exact representations.
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 is an end view of a typical SAGD system in a reservoir;
FIG. 2 is an end view of a typical SAGD system in a partitioned reservoir
having
two vertical permeability impediments;
FIG. 3 is an end view showing multiple SAGD well pairs in a partitioned
reservoir
having two vertical permeability impediments;
FIG. 4 is an end view of a SAGD system in a partitioned reservoir having two
vertical permeability impediments, wherein a zone of increased permeability is
created in
a region substantially above the SAGD well pair that extends through the
vertical
permeability impediments to facilitate movement of flowable material through
the
reservoir to enhance the recovery process, in accordance with an embodiment of
the
invention;
FIG. 5 is a side (a), top (b) and end (c) view of a partitioned reservoir
showing an
undulating accessory conduit that intersects two vertical permeability
impediments to
thereby create a zone of increased permeability to facilitate movement of
flowable
material through the reservoir to enhance the recovery process, in accordance
with an
embodiment of the invention;
FIG. 6 is a side (a), top (b) and end (c) view of a partitioned reservoir
showing a
pair of laterally offset undulating accessory conduits that intersect two
vertical
permeability impediments, in accordance with an embodiment of the invention;
FIG. 7 is a side (a), top (b) and end (c) view of a partitioned reservoir
showing a
pair of stacked undulating accessory conduits each intersecting a vertical
permeability
impediment, in accordance with an embodiment of the invention;
FIG. 8 is a side (a), top (b) and end (c) view of a partitioned reservoir
showing a
main lateral accessory conduit having lateral offshoots that intersect two
vertical
permeability impediments, in accordance with an embodiment of the invention;
FIG. 9 is a side (a), top (b) and end (c) view a partitioned reservoir showing
two
lateral accessory conduits each having perforations extending therefrom that
puncture a
vertical permeability impediment, in accordance with an embodiment of the
invention;
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FIG. 10 is a side (a), top (b) and end (c) view of a partitioned reservoir
showing a
main lateral accessory conduit with multiple propped fractures extending
therefrom that
penetrate two vertical permeability impediments, in accordance with an
embodiment of
the invention;
FIG. 11 illustrates a reservoir having two vertical permeability impediments,
wherein a zone of increased permeability is created that extends through the
vertical
permeability impediments in a region substantially below the production well
to improve
primary natural gas, primary conventional oil, or water-drive conventional oil
production
from the reservoir, in accordance with an embodiment of the invention;
FIG. 12 illustrates a reservoir having two vertical permeability impediments,
wherein a zone of increased permeability is created that extends through the
vertical
permeability impediments in a region substantially above the production well
to improve
primary oil or top-gas driven production from the reservoir, in accordance
with an
embodiment of the invention;
FIG. 13 illustrates a gas flooding process in a reservoir having two vertical
permeability impediments, wherein zones of increased permeability are created
that
extend through the vertical permeability impediments in regions substantially
below the
injection well and substantially above the production well to facilitate the
movement of
flowable material through the reservoir to improve recovery, in accordance
with an
embodiment of the invention; and
FIG. 14 illustrates a liquid flooding process in a reservoir having two
vertical
permeability impediments, wherein zones of increased permeability are created
that
extend through the vertical permeability impediments in regions substantially
above the
injection well and substantially below the production well to facilitate the
movement of
flowable material through the reservoir to improve recovery, in accordance
with an
embodiment of the invention;
FIG. 15 shows the 2D results of a reservoir simulation modeling a SAGD
operation in a continuous reservoir with Athabasca type properties and no
vertical
permeability impediments, wherein the temperature and gas saturation in the
reservoir
after 1000 and 2000 days of steam injection are shown;
FIG. 16 shows the 2D results of a reservoir simulation modeling a SAGD
operation in a reservoir with Athabasca type properties and two vertical
permeability
barriers, wherein a zone of increased permeability is provided above the SAGD
well pair
and extending through the vertical permeability barriers, in accordance with
an
embodiment of the invention;
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FIG. 17 shows graphical results of the reservoir simulations shown in Figs. 15
and 16, comparing oil and water volumes between a SAGD operation in a
reservoir with
no vertical permeability impediments and a SAGD operation carried out in
accordance
with an embodiment of the invention in a reservoir having two vertical
permeability
barriers;
FIG. 18 shows the 3D results of reservoir simulations modeling a CSS operation
in a partitioned reservoir with Athabasca Cold Lake type properties and one
vertical
permeability barrier, comparing gas saturation in a typical split pay
reservoir with a split
pay reservoir having a zone of increased permeability extending through the
vertical
permeability barrier above each of two adjacent horizontal CSS wells, in
accordance with
an embodiment of the invention;
FIG. 19 shows graphical results of the reservoir simulations shown in FIG. 18,
comparing oil and water volumes between a CSS well in a split pay reservoir
having a
zone of increased permeability that extends through a vertical permeability
barrier in
accordance with an embodiment of the invention, with a CSS well in a split pay
reservoir
with no zone of increased permeability.
DETAILED DESCRIPTION
Generally, the present invention provides a method and system for enhancing a
hydrocarbon recovery process employing one or more substantially horizontal
wellbores.
In accordance with embodiments of the invention, the recovery process is
enhanced by
selectively altering the vertical permeability of the reservoir. It will
become apparent to
those skilled in the art that embodiments of the present invention may be
applied to many
different hydrocarbon reservoirs and recovery processes. A hydrocarbon
reservoir
includes one or more hydrocarbon-containing layers or zones and may also
contain one
or more permeability impediments. Although it is contemplated that the system
and
method of the invention can be utilized to enhance recovery process
performance in
substantially continuous reservoirs, embodiments of the invention are
particularly
advantageous in reservoirs having one or more vertical permeability
impediments.
Vertical permeability impediments are well known the oil and gas industry and
present significant challenges to the complete recovery of hydrocarbons,
particularly in
recovery processes employing one or more substantially horizontal wellbores. A
substantially horizontal wellbore is a wellbore that is substantially
horizontal along at least
a portion of its length. It is understood in the industry that a substantially
horizontal
wellbore may include deviations from horizontal along its length but that the
horizontal
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portions of such wellbores have an axis in a generally horizontal plane as
opposed to a
vertical wellbore. Examples of recovery processes that may employ one or more
substantially horizontal wellbores include, but are not limited to, steam-
assisted gravity
drainage (SAGD), cyclic steam stimulation (CSS), steam flooding and various
derivatives
thereof, as well as conventional oil and gas recovery processes, including
various
flooding and sweeping processes.
A vertical permeability impediment, as used herein, refers to a portion of a
reservoir that has substantially reduced permeability compared to the primary
pay zone(s)
of the reservoir, such that vertical movement of flowable material through the
reservoir is
hindered or prevented. Flowable materials comprise fluids and/or gasses,
including
mobile hydrocarbons. Skilled persons will appreciate that the degree to which
a particular
vertical permeability impediment will impede the flow of mobile hydrocarbons
in a
recovery process will depend on such factors as the permeability and thickness
of the
impediment, the reservoir characteristics, the viscosity of the particular
hydrocarbon being
recovered, and the particular recovery process utilized. These and other
factors should
be considered in carrying out the methods of the invention.
A vertical permeability impediment may form a continuous, semi-continuous or
discontinuous layer or layers in the reservoir and may be composed of one or
more
materials. There are many types of vertical permeability impediments that may
be
encountered. Common examples include baffles and barriers. A baffle in a heavy
oil
reservoir may, for example, have a permeability that is at least one order of
magnitude
less than the reservoir matrix permeability, thereby hindering vertical flow
in a recovery
process. A flow barrier, often referred to as a tight streak in the industry,
in a heavy oil
reservoir may, for example, have a permeability that is several orders of
magnitude less
than the reservoir matrix permeability, thereby substantially preventing
vertical flow. Tight
streaks are often composed of continuous or near continuous layers of shale or
mudstone
and often follow the bedding planes. They may be a few centimeters to several
meters
thick with extensive areal extent. Some thick tight streaks may be about 1 m
to 3m thick,
and some particularly thick tight streaks may be about 4m to 7m, or even
higher. The can
pose significant impediments to vertical flow in a reservoir.
As an illustrative example, consider a heavy oil reservoir having a matrix
permeability of about 1000 mD. A baffle may have a permeability in the range
of about
100 mD to about 0.1 mD. A baffle having a relatively low permeability of about
10 mD or
less would represent a significant vertical flow impediment in such a
reservoir. A flow
barrier may have a permeability of less than about 0.1 mD, or no effective
permeability.
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Vertical permeability impediments having low to no effective permeability can
essentially
partition a reservoir into split pay zones. In such cases, hydrocarbon
recovery will
primarily occur from the pay zone in which a production well is actually
located. It is
understood that lighter oils and gases can move more easily through a given
space than
heavier oils, thus whether a particular impediment will act as a barrier or
impediment will
depend, in part, on the hydrocarbon encountering the impediment.
In accordance the invention, a recovery process employing one or more
substantially horizontal wellbores is enhanced by selectively altering
permeability in the
reservoir. Vertical permeability is selectively altered by creating a zone of
increased
permeability at a targeted location or locations in the reservoir. The zone of
increased
permeability may be created in a region substantially above or below a
horizontal
wellbore employed in a recovery process. The zone of increased permeability
may
facilitate the migration of mobile hydrocarbons through a reservoir toward a
production
well to thereby enhance recovery. The zone of increased permeability may also
facilitate
the movement of injected materials, such as steam, gas, water, solvents, or
polymers,
through the reservoir to affect the mobilization or displacement of
hydrocarbons in the
reservoir. Flowable materials may be drawn into the zone of increased
permeability by
gravity drainage, pressure differential, or other factors.
In certain embodiments, the zone of increased permeability is created by
providing one or more accessory conduits in the reservoir to improve or
facilitate the
movement of flowable material through the reservoir to thereby enhance
recovery. In a
reservoir having a vertical permeability impediment, the accessory conduit has
one or
more portions that extend through the vertical permeability impediment to
improve the
movement of flowable material therethrough. The conduit may be targeted to
select areas
of relatively low permeability in the reservoir, thereby creating a higher
permeability
region(s) to facilitate the movement of flowable materials. The accessory
conduit may
facilitate the movement of flowable materials from one location to another in
a reservoir,
or may permit flowable materials to move through a permeability impediment in
the
reservoir to enhance a recovery process. By selectively facilitating the
vertical movement
of materials in the reservoir, lateral movement of flowable materials through
the reservoir
may also be enhanced.
In certain embodiments, a method of enhancing a recovery process employing
one or more substantially horizontal wellbores in a reservoir having at least
one vertical
permeability impediment is provided. The method comprises providing at least
one
accessory conduit in the reservoir thereby creating a zone of increased
permeability. The
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at least one accessory conduit has a horizontal component, although it may
also have
vertical and lateral components as well. By horizontal component, it is meant
that it
extends in a generally horizontal direction in the reservoir along a portion
of its length. By
generally horizontal direction, it is meant that the conduit travels a
horizontal distance in
the reservoir, although the conduit may deviate from horizontal along its path
and may
also have segments or extensions thereof that deviate from horizontal.
Preferably, the
conduit extends in a direction substantially parallel to a horizontal wellbore
used in the
recovery process. The conduit may have one or more portions that extend
through a
vertical permeability impediment to facilitate movement of flowable materials
therethrough
during the recovery process.
The accessory conduit may be vertically spaced from the one or more horizontal
wellbores. In some embodiments, the conduit lies substantially above or below
the
horizontal segment of one or more horizontal wellbores in the reservoir.
Depending on the
recovery process, the one or more horizontal wellbores may include a
production well, an
injection well, or both. In some embodiments, a single substantially
horizontal well may
serve as both an injection well and a production well. A production well
refers to a well in
which production materials collect. The production materials may be pumped to
the
surface from the production well or they may flow into another well for
production.
Production materials comprise hydrocarbons and may also comprise other
flowable
materials such as condensed steam, connate water and gas, depending on the
application and the reservoir. Hydrocarbons recovered may include conventional
oil,
natural gas, mobilized bitumen or heavy oil, or combinations thereof.
In certain embodiments, a flowable injection material is injected into the
reservoir
to aid in mobilizing hydrocarbons through the reservoir. Means of mobilizing
hydrocarbons may include, for example, reducing the viscosity of heavy oil or
bitumen
permitting it to flow by gravity drainage through the reservoir, or providing
a drive
meQhanism to direct the flow of mobile hydrocarbons through a reservoir, or
other means.
In certain embodiments, injection materials, such as water, gas, steam,
solvent, polymers
or combinations thereof, may be administered via a substantially horizontal
injection well.
In certain embodiments, the accessory conduit is located in a region
substantially
above an injection well, particularly when applied in a process that relies on
gravity
drainage. The conduit need not be directly above the well and may optionally
be laterally
offset therefrom. However, in most cases, it is advantageous to position the
conduit in
substantially the same vertical plane as the injection well. The conduit
creates a zone of
increased permeability above the injection well that may facilitate the
movement of
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flowable injection materials across a vertical permeability impediment. In an
in situ heavy
oil recovery process, the accessory conduit may facilitate the movement of
flowable
injection material into one or more upper regions of the reservoir, and may
further
facilitate the flow of production materials from the upper regions of the
reservoir toward
the production well below. Enhanced movement of flowable materials through a
vertical
permeability impediment permits fluid communication between previously
separated
productive formations and thereby enhances the recovery process. Fluid
communication
refers to the ability of flowable materials, such as fluid, gas, or oil, to
move between
different locations in a reservoir.
The location and profile of the accessory conduit can be optimized by the
person
skilled in the art having an understanding of the invention described herein.
Factors to be
considered include, for example, the hydrocarbon recovery process to be
utilized, the
formation characteristics, the location and characteristics of any vertical
permeability
impediments in the reservoir, and the location of other wellbores in the
system, including
any production or injection wells. The term accessory conduit indicates that
it is not a
primary injection or production wellbore in the recovery process.
In some embodiments, the recovery process employs a single horizontal
production wellbore, such as in primary production of natural gas or
conventional oil. In
other embodiments, the recovery process utilizes a single horizontal wellbore
as both an
injection well and a production well, such as in cyclic steam stimulation
(CSS) and various
deri~vatives thereof. In other embodiments, the recovery process utilizes
separate injection
and production wells, such as in steam-assisted gravity drainage (SAGD), and
various
derivatives thereof, or in flooding and sweeping processes that involve
displacement of
hydrocarbons in a reservoir.
The one or more accessory conduits may be drilled from the surface.
Alternatively, in some embodiments, the one or more accessory conduits may be
drilled
laterally from another wellbore. For example, the accessory conduit may be
drilled
laterally from an upper hole section of a deviated injection or production
well. In some
cases, drilling a lateral wellbore has a significant economic advantage over
drilling a new
hole from the surface. The accessory conduit may be drilled while drilling a
new
production or injection wellbore. Alternatively, an accessory wellbore may be
retrofit to a
preexisting wellbore. In some embodiments, an accessory wellbore is retrofit
to a
wellbore in a preexisting recovery process to further stimulate recovery from
the reservoir.
In some retrofit scenarios, where the preexisting holes have already been
completed, it
may be preferable to drill a conduit from the surface.
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Embodiments of the invention will be now described in more detail in reference
to
exemplary oil recovery processes. Reference to the drawings will also be
provided. It
should be understood that the scope of the invention is not limited to the
particular
embodiments described or those exemplified in the drawings.
Application to Heavy Oil Recovery Processes
Various non-limiting features and embodiments of the invention will now be
described in reference to an exemplary in situ heavy oil recovery process.
Extraction of bitumen and heavy oil from oil sand reservoirs presents unique
challenges due high viscosity, particularly if a heavy oil deposit is buried
deep within a
reservoir and cannot be surface mined due to the depth of overburden. In
general, the
aim of an in-situ heavy oil recovery process is to reduce the viscosity of
heavy oil in a
reservoir to enable it to flow into a well and be produced therefrom. Such
recovery
processes frequently utilize gravity drainage as an important fluid drive
mechanism.
Thermal in-situ oil recovery processes utilize heat, typically provided by
injection of steam
into a reservoir, to reduce the viscosity of the trapped oil and render it
mobile.
Hydrocarbon solvents or other solubilizing means may also be utilized.
Steam-assisted gravity drainage (SAGD) is a gravity-driven thermal in-situ oil
recovery process invented by Roger Butler et al. (see, for example, Butler
R.M., Thermal
Recovery of Oil and Bitumen, GravDrain Inc., Calgary, Alberta, Canada, 1997).
SAGD is
used commercially to recover heavy oil or bitumen from reservoirs,
particularly in the
Athabasca region where the in-situ oil viscosity is very high, typically on
the order of
1 million centipoises.
FIG. 1 illustrates an end view of a typical SAGD system and process of the
prior
art,in which two substantially horizontal wellbores are positioned in spaced-
apart vertical
relationship to one another in a hydrocarbon reservoir (10). A first
substantially horizontal
wellbore positioned near the bottom of the hydrocarbon reservoir serves as a
production
well (12). A second substantially horizontal well is positioned above the
production well
(12), in thermal communication therewith, and serves as an injection well
(14). The wells
are typically drilled as two separate wells from the surface using deviated
drilling
procedures well know to those skilled in the art. The upper hole sections of
the wellbores
may extend hundreds to thousands of feet into a reservoir. A steel casing is
typically
cemented into place to stabilize the upper hole sections of the wellbores. A
screened or
slotted liner typically lines the substantially horizontal portions of the
injection well (14)
and production well (12) to provide stability to the wellbores while
maintaining fluid
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CA 02651527 2009-01-29
communication between the wellbore and the formation. Other completion tools
and
procedures may also be utilized. The drilling and completion of wellbores for
an in situ oil
recovery process is very time-consuming and expensive.
Once the system is in place, steam is injected into the reservoir via the
injection
well (14), typically from the surface. As the steam penetrates into the
permeable matrix of
the formation, a heated steam chamber (16) develops. The size and temperature
of the
steam chamber (16) increases with continued steam injection into the
formation. As the
heavy oil within the steam chamber (16) is heated, its viscosity is lowered
and it becomes
mobile. The mobile oil drains downward through the heated formation material
and along
the cooler edges (18) of the developing steam chamber (16) toward the
production well
(12) below. The mobile hydrocarbons collect in the production well (12),
generally along
with condensed steam and other production materials. The production materials
collected
in the production well (12) may be pumped to the surface for separation and
processing.
The approximate drainage height of the reservoir is represented by h.
Development of the steam chamber and flow of mobilized bitumen is limited, in
part, by
the permeability of the formation.
Butler et al. developed an Equation to estimate the flow rate of bitumen for a
reservoir configuration as shown in FIG. 1:
q = 2 L 1.3 k;; csv~S,, h
m v; (1)
In Equation (1), q is the bitumen production rate, L is the horizontal well
length, k is the
reservoir effective permeability, g is the acceleration due to gravity, a is
the thermal
diffusivity of the reservoir, cp is the reservoir porosity, OSo is the change
in oil saturation,
h is the drainage height as labeled in FIG. 1, m is parameter dependant upon
viscosity-
temperature properties of crude oil and vs is the kinematic viscosity of crude
at steam
temperature. This Equation has proven to be a reasonable approximation of
bitumen flow
rates in the field for reservoirs with sands that can be characterized as
continuous clean
sands. For these types of sands it is valid to assume constant permeability of
the
formation, as in Equation 1.
In a reservoir having a vertical permeability impediment, a developing steam
chamber will expand vertically in the reservoir until the impediment is
encountered, which
impediment will inhibit or prevent further vertical expansion of the steam
chamber. An
exemplary case is illustrated in FIG. 2, wherein two continuous tight streaks
(20a
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CA 02651527 2009-01-29
and 20b) partition a reservoir (22) vertically into thirds. As steam is
injected into the
injection well (24), the steam chamber (26) can only rise to the height of the
first tight
streak (20a) and then it begins to spread laterally. Without wishing to be
bound by any
particular theory, it is anticipated that approximately one third of the oil
in the reservoir will
be produced via the production well (28), compared to a similar reservoir with
no tight
streaks, as shown in FIG. 1. Applying Equation 1, assuming all reservoir
properties
remain the same except the drainage height, h, which is now h/3, the bitumen
production
rate would now be q/-~fl
Referring to FIG. 3, one method to recover the heavy oil trapped in the pay
zones
above the tight streaks (30a and 30b) would be to drill an injection well (31)
and a
production well (32) in each productive zone. Although this is possible, two
major
disadvantages of drilling complete well pairs in each zone are cost and time.
Applying
Equation 1, and assuming all three wells pairs produce simultaneously, then
the total oil
production rate would theoretically be 3 times qd-~/ 3, or r-3 q. The total
oil production rate
increases but in proportion to the square root of the well count.
In accordance with an aspect of the invention, a significant increase in well
pair
performance and hydrocarbon recovery can be achieved by creating a zone of
increased
permeability in a region substantially above the SAGD well pair in a
reservoir. The zone
of increased permeability may be immediately above the SAGD well pair or may
be
laterally offset therefrom, so long as the location of the conduit facilitates
the recovery
process. In many cases, it is advantageous to locate the conduit immediately
above or
below a horizontal wellbore for both performance and cost reasons. If gravity
drainage is
utilized as a fluid drive mechanism, such as for heavy oil recovery, the zone
of increased
permeability is ideally provided in a region substantially above (i.e. in
substantially the
same vertical plane) the production well to facilitate drainage of production
material
towiard the production well. Where a separate injection well is also employed,
the conduit
may be positioned above the injection well.
In the embodiment shown in FIG. 4, a zone of increased permeability (40) is
created in the reservoir (42) substantially above a SAGD well pair (44, 45).
The zone of
increased permeability (40) is created by providing an accessory conduit (48)
in the
reservoir. In this embodiment, the conduit (48) has portions extending through
two vertical
permeability impediments (49a and 49b) thereby increasing fluid communication
between
the pay zones. In the embodiment shown, the vertical permeability impediments
(49a and
49b) are above the SAGD well pair (44, 45). However, it is also possible that
one or more
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= vertical permeability impediments may be located between the production well
and the
injection well, in which case a conduit could be positioned between the
production and
injection wells to facilitate fluid communication therebetween. A suitable
location and
profile for the one or more accessory conduits can be determined by the
skilled person. In
subsequent sections, various exemplary conduit profiles will be described.
The removal of reservoir material to create an accessory conduit (48) above
the
SAGD well pair (44, 45) effectively creates a 'chimney zone' of increased
permeability to
facilitate the movement of flowable materials through the reservoir. For
instance, the
chimney zone allows the steam to rise upwards (white arrow) through the tight
streaks
toward the top of the reservoir and, also allows the hot bitumen to drain
downward (black
arrows) to the production well (44). Fluids drawn into the accessory conduit
(48) will
percolate through the permeable formation material in a gravity-assisted
manner toward
the production well (44). In the embodiment shown, the accessory conduit (48)
permits
fluid communication between three pay zones of a reservoir formerly separated
by
substantially impermeable tight streaks.
The accessory conduit preferably extends in a generally horizontal direction,
along
at least a portion of its length, substantially parallel a horizontal wellbore
utilized in the
recovery process. In other words, the conduit preferably has a portion that
extends
substantially above or below, including laterally offset from, a horizontal
wellbore in the
reservoir, such as an injection or production well, to facilitate movement of
flowable
materials through the reservoir. The accessory conduit is preferably
substantially
separated from the horizontal wellbore by permeable formation material.
It was surprisingly discovered that, by altering the permeability of a
relatively
narrow vertical region above a well pair in a partitioned reservoir, recovery
performance
comparable to, or even possibly exceeding, that of a reservoir without
vertical
permeability impediments can be achieved.
Following the reasoning provided above, assuming that the chimney zone is
highly effective in allowing steam to rise and oil to drain, then the oil
production rate, qn ,
for a reservoir with n equal height layers can be written as:
Qra = 1Jrnq1 (2),
where q, is the flow rate for a case with no tight streaks and only one layer
such that the
rate is equivalent in this particular case to Equation (1). Stated simply, as
the number of
layers or tight streaks increases, the potential deliverability of the
reservoir increases.
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CA 02651527 2009-01-29
Convergent flow in the reservoir chimney zone and other factors may impact the
full
potential for reservoir deliverability. Nonetheless, this is a remarkable and
surprising
finding since tight streaks are generally considered to have a negative impact
on average
reservoir permeability and therefore reservoir productivity in a recovery
process.
If the chimney zone was not present and production occurred from only one
layer
of the reservoir, the production rate, q, would be
q = 41/"v' ^n (3)
which, as expected, is a reduction compared to the case with no tight streaks.
Thus,
embodiments of the present invention can significantly enhance recovery
process
performance in a reservoir having one or more vertical permeability
impediments. The
method and system of the invention provide an effective new means of
overcoming a
common problem in the industry.
Although SAGD is exemplified above, a skilled person will appreciate that the
method and system of the present invention can be applied to any suitable
recovery
process utilizing one or more horizontal wellbores. The skilled person will
further
appreciate that there are many different methods that can be utilized to
create the one or
more accessory conduits to achieve an increase in vertical permeability in the
reservoir to
thereby enhance recovery process performance. Since the accessory conduit is
neither
an injection well nor a production well per se, the conduit does not have to
meet the
requirements (structural, completion, location, etc) of an injection or a
production well,
which offers significant freedom in designing the accessory conduit profile in
order to
optimize recovery process performance for a particular reservoir. There is
also more
freedom in selection of drilling equipment and tools. Using methods known to
those
skilled in the art, one or more accessory conduits may be drilled form the
surface or may
be drilled from an upper hole section of an injection or production well. If
drilled as a
lateral, the lateral conduit may be sealed from the injection or production
well from which
it was drilled. Although it may not be necessary to complete the conduits with
any
mechanical devices, the conduits may be completed if desired, utilizing known
tools and
methods. For instance, various types of slotted liners or sand screens could
optionally be
placed in the conduits to ensure sand does not collapse into and plug the
holes.
Alternatively, the conduits could be packed with high permeability gravel or
'frac sand'. If
desired, the diameter of the conduit in the reservoir may be increased by
downhole tools,
such as reamers, high pressure water jetting bits, section mills with
expandable cutting
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CA 02651527 2009-01-29
arms, or the like. The latter two techniques would be particularly effective
in soft shallow
formations at increasing the size and extent of the chimney zone.
As will be appreciated by the skilled person, once having an understanding of
the
invention, there are a limitless number of different profiles that can be used
in providing
the one or more accessory conduits. The accessory conduit can have any desired
profile
that facilitates the movement of flowable materials through the reservoir to
enhance
recovery process performance. A suitable well profile for a particular
application may be
selected by a person skilled in the art.
Exemplary methods of providing an accessory conduit include, but are not
limited
to, drilling one or more conduits from the surface, drilling one or more
lateral conduits
from an existing wellbore, drilling multiple lateral conduits, drilling one or
more conduits
with lateral offshoots, drilling one or more conduits and perforating the main
conduit,
drilling one or more conduits and fracturing the main conduit, and variations,
modifications, extensions and combinations thereof. Certain exemplary
embodiments will
be described further below.
In some embodiments, the accessory conduit may be drilled substantially in the
horizontal plane of a vertical permeability impediment to carve a
substantially horizontal
channel through the impediment to permit vertical movement of flowable
materials
therethrough. It is understood that the vertical permeability impediment will
follow dips
and other natural deviations from the horizontal plane but it will nonetheless
extend in a
generally horizontal plane. The accessory conduit may follow a substantially
straight path
through the substantially horizontal plane of the vertical permeability
impediment or it may
follow a deviated path, such as a horizontally undulating path.
Rather than creating an accessory conduit entirely in the plane of the
vertical
permeability impediment, it may more practical to provide a conduit that
extends in a
generally horizontal direction while having vertical and/or lateral components
such that
portions of the conduit, including extensions thereof, extend through the
impediment at
multiple locations along its length.
The embodiment shown in FIG. 5 is a side (a), top (b) and end (c) view of an
undulating accessory conduit (50) drilled laterally from an upper hole section
of an
injection well (51) to enhance vertical permeability above a well pair (51,
52). The
accessory conduit (50) intersects two tight streaks (53, 54) in the reservoir
(55). The
conduit extends in a generally horizontal direction above the well pair (51,
52) and has
both horizontal and vertical components. A key advantage of a lateral conduit
design is
that it can be drilled at relatively low incremental cost when drilled in
conjunction with the
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CA 02651527 2009-01-29
steam injection well (51). The lateral hole may be drilled before or after the
casing is set
for the injection well (51). It is important to note that even a small
diameter conduit has an
effective permeability orders of magnitude greater than the original reservoir
sand, since
sand is removed from the reservoir to create it. Furthermore, even by
providing a wellbore
profile that penetrates the tight streaks (53, 54) at intervals rather than
continuously, such
as a sinusoidal type profile, the desired increase in effective reservoir
permeability can
still be achieved, and fluid communication between the different zones of the
reservoir will
be increased. The intervals may be regular or irregular, and may vary from a
few meters
to tens to hundreds of meters. A sinusoidal wellbore profile has been drilled
in the past as
a multi-penetration pilot (MPP) hole for the purposes of reservoir evaluation
(see Lee,
Extending rotary steerable capabilities to locate thin, complex sands.
SPE/IADC Drilling
Conference, Netherlands, 2005, SPE/IADC 92152).
FIG. 6 shows another embodiment, where multiple undulating conduits (60, 61)
are drilled laterally from an injection well (62) in a reservoir (63) having
two tight streaks
(65, 66). Such a configuration may be advantageous if more frequent reservoir
penetrations are required than can be delivered with a single conduit. Note
that the
conduits can be laterally offset from the well pair (62, 64) and still be
effective.
FIG. 7 shows an alternative configuration where two stacked undulating lateral
conduits (70, 71) are used to target two separate tight streaks (72, 73) in
the reservoir
(74) above a well pair (75, 76). In this configuration, it will generally be
possible to
penetrate the tight streaks (72, 73) many more times than with a single
conduit that
penetrates both tight streaks, as shown in FIG. 4. The geologic
interpretation, separation
of the vertical permeability impediments and drilling capabilities should all
be considered
by the skilled person in determining the preferred approach.
As an alternative to having more than one main lateral, the embodiment of FIG.
8
shows a multi-lateral conduit (80) drilled in a reservoir (81) having two
tight streaks
(82, 83). The multi-lateral conduit (80) comprises one main horizontal lateral
drilled
substantially parallel to the SAGD well pair (84, 85) and multiple short
deviated laterals
extending through the tight streaks (82, 83) from the main lateral. The short
laterals could
also be vertical or any other suitable orientation, length or shape. The short
laterals are
extensions of the conduit and provide the flow pathways through the tight
streaks. These
short laterals would preferably be placed in a region substantially above the
SAGD wells
(84, 85), i.e. above the SAGD well pair in substantially the same vertical
plane, since this
is the region where they will typically be most effective in a heavy oil
recovery process
utilizing steam or solvent injection and relying on gravity drainage.
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CA 02651527 2009-01-29
Although a more costly approach, chimney zones above the injection well could
be created by drilling a series of vertical wells from the surface that
penetrate the tight
streaks. These wells could also facilitate access to upper reservoir zones for
reservoir
surveillance purposes and improve the drainage of upper zone bitumen by
supplemental
injection of steam, gas, solvents and/or polymers. Supplemental production
from vertical
wells is also possible by producing mobilized bitumen outside the vicinity of
the SAGD
horizontal well pair, for instance, due to heat conduction from a zone below
the tight
streak.
Additional techniques, such as fracturing or perforating, may be used in
conjunction with the concepts described above to increase the vertical
permeability
through the tight steaks. FIG. 9 shows an embodiment having two lateral
accessory
conduits (90, 91) drilled in close proximity to two tight streaks (92, 93) in
a reservoir (94).
The conduits are located above the well pair (95, 96). Short vertical holes
are generated
from the main laterals using perforating technologies. Perforations do not
typically
penetrate more than about a meter or, using special tools, a few meters, into
a reservoir,
so this configuration would ideally place the main lateral in close proximity
to the tight
streak. Perforations or similar high permeability tunnels emanating from the
main
accessory conduit could be generated using a variety of techniques such as
explosive
charges, lance perforating technologies, side wall coring or water jets. An
advantage of
this approach is that potentially many more closely spaced holes could be
placed in a
plane above the SAGD wells to increase the vertical permeability above the
wells.
Techniques have been developed for generating multiple fractures along
horizontal wells. Relatively small, closely placed fractures could be used in
accordance
with the invention to increase the effective permeability above a well pair.
FIG. 10 again
shows a reservoir (100) with two tight streaks (101, 102). In this embodiment,
a lateral
accessory conduit (103) is shown with spaced fractures (106) extending
therefrom in a
vertical plane oriented perpendicular to the SAGD wells (104, 105). The
fractures may be
packed with high permeability proppant to ensure effective permeability after
fracturing. A
similar approach could be utilized with fractures oriented in a vertical plane
parallel to the
SAGD wells. However, such horizontal fractures would typically need to be
employed
from a vertical wellbore.
In some situations, it may be desirable to increase the diameter of an
accessory
conduit beyond that achievable by normal drilling procedures. For instance, in
situations
where a tight streak is very thick or a large diameter accessory conduit is
desired. As an
example, high water pressure jetting bits with variable or adjustable jetting
angle could be
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CA 02651527 2009-01-29
used to progressively expand the diameter of the precondition hole while
circulating out
the cuttings. Such jets can also be used to create perforation-like tunnels
that emanate
from a main accessory conduit, similar to those shown in FIG. 9.
Alternatively, an
accessory conduit could be a tunnel or cavern created through the plane of the
tight
streak, for example, by sequentially collapsing and cleaning out formation
rock.
Acids, such as hydrofluoric acids, are commonly employed to dissolve some
types
of rock, and hydrochloric acids are very effective in carbonates. Depending on
the mineral
composition of the tight streak or other vertical permeability impediment,
chemical or acid
treatments can be used to create high permeability flow paths through the
impediments.
The acid could be administered via the accessory conduit and would penetrate
the
surrounding rock to create tunnels or paths through the vertical permeability
impediment.
This method would have particular application in carbonate reservoirs or in
reservoirs with
carbonate tight streaks.
Utilizing methods known in the art, data can be acquired and analyzed from new
or existing wells in the reservoir to determine the mechanical properties of
the reservoir
and the location of any vertical permeability impediments. The data and
information
obtained may then be utilized in carrying out the methods of the invention. In
one
embodiment, the production well is the first well drilled in the system and is
utilized to
acquire reservoir data to determine the location of vertical permeability
impediments in
the reservoir before drilling an injection well or an accessory conduit.
The accessory conduit includes any lateral offshoots, perforations, fractures
or the
like extending therefrom to increase vertical permeability through a vertical
permeability
impediment. Although the accessory conduit may have vertical components such
as
undulations, lateral offshoots, perforations, fractures or the like, it is
said to extend in a
generally horizontal direction since the end of the conduit will be located a
horizontal
distance from the beginning of the conduit although possibly in a different
horizontal
plane.
Application to Conventional Oil and Gas Recovery Processes
Embodiments of the invention may be applied in conventional light oil or
natural
gas reservoirs as well to enhance horizontal well utilization. Horizontal
wells offer
significant potential to maximize production or injection rates. They can also
reduce the
surface environmental disturbance, or "footprint", of onshore operations and
minimize well
accommodation space of offshore platforms. However, as is well known to those
in the
industry, horizontal well applications are limited by vertical permeability
quality and
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CA 02651527 2009-01-29
compartmentalization. Operators sometimes try to improve recovery by drilling
high angle
production or injection wells, or 'S' wells, in order to penetrate multiple
stratigraphic flow
units. The method and system may be applied in continuous reservoirs, as well
as
complex stratified reservoirs and reservoirs segmented by one or more vertical
permeability impediments.
In accordance with embodiments of the invention, the effective vertical
permeability of a conventional oil or gas reservoir may be selectively altered
to facilitate
the movement of flowable material through the reservoir to enhance recovery.
In some
embodiments, the effective reservoir permeability is increased by providing at
least one
accessory conduit in the reservoir. In some embodiments, the reservoir may
comprise
one or more vertical permeability impediments. The accessory conduit may be
positioned
substantially above, below, lateral or distal to a production well or
injection well in the oil
or gas reservoir, depending on the particular application and reservoir
characteristics.
Figures 11 to 14 show particular exemplary embodiments where an altered
permeability zone is created substantially within the vertical plane of a
horizontal
production or injection well in the reservoir. Additional zones of increased
permeability
can be created at other locations in the reservoir, if desired. The altered
permeability
zone is beneficial to improve reservoir access and sweep of injection fluids.
The reservoir
engineer has the option to control the extent of the altered permeability zone
and
placement of the horizontal wells to best fit the specific reservoir geology
and depletion
plari. Furthermore, in designing the means of achieving the altered
permeability zone, the
design may include a mechanism to close off flow and restore the vertical
permeability
impediment. Early breakthrough of an injected fluid to the producer is an
example of
where it would be desirable to reverse the altered permeability zone. This
could be
achieved, for example, by cement squeeze of the preconditioning drill holes
used to
create the altered permeability zone in the described embodiments.
FIG. 11 shows an embodiment wherein a zone of increased permeability is
created in a region substantially below a production well (140) in a reservoir
verticai
permeability impediments (141, 142). The zone of increased permeability is
created by
providing an accessory conduit (143) that extends through the two vertical
permeability
impediments (141, 142) to facilitate flow through the reservoir (black arrows)
toward the
production well (140). Such embodiment may be utilized, for example, for
improving
primary natural gas production, primary conventional oil production, or water-
drive
conventional oil production. Note that by providing a conduit (143) to
facilitate vertical flow
toward the production well (140), lateral flow in the reservoir is also
enhanced. As with the
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CA 02651527 2009-01-29
embodiments described above for heavy oil applications, various conduit
profiles could be
used.
FIG. 12 shows an embodiment wherein a zone of increased permeability is
created in a region substantially above a production well (150) in a
reservoir. The zone of
increased permeability is created by providing an accessory conduit (153)
having portions
that extend through two vertical permeability impediments (151, 152) in the
reservoir to
facilitate flow through the reservoir (black arrows) toward the production
well (150). Such
embodiment ma y be used, for example, for improving primary conventional oil
production
or top-gas driven production. Various conduit profiles could be used.
FIG. 13 shows a system for gas flooding, in accordance with an embodiment of
the invention, wherein zones of increased permeability are created below a gas
injection
well (160) and above a production well (161) in the reservoir. The zones of
increased
permeability are created by providing a first accessory conduit (164) below
the injection
well (160) and a second accessory conduit (165) above the production well
(161), each
having portions that extend through two vertical permeability impediments
(162, 163) in
the reservoir to facilitate the flow of materials through the reservoir toward
the production
well (161). Various conduit profiles could be used.
FIG. 14 shows a system for liquid flooding, in accordance with an embodiment
of
the invention, wherein zones of increased permeability are created above a
liquid
injection well (170) and below a production well (171) in the reservoir. The
zones of
increased permeability are created by providing a first accessory conduit
(174) above the
injection well (170) and a second accessory conduit (175) below the production
well
(171), each having portions that extend through two vertical permeability
impediments
(172, 173) in the reservoir to facilitate the movement of flowable materials
through the
reservoir. Various conduit profiles could be used.
In many of the embodiments described herein, a reservoir having two vertical
permeability impediments is exemplified. This is merely for consistency and
ease in of
comparison. It is understood that the invention may be applied in a
substantially
continuous reservoir or a reservoir having one or multiple vertical
permeability
impediments. A skilled person, having an understanding of the invention as
described
herein, will be able to extend and apply the concepts of the invention to
various reservoirs
and various recovery processes. The methods and systems of the invention are
particularly useful for enhancing a recovery process utilizing one or more
horizontal
wellbores in a reservoir having one or more vertical permeability impediments.
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CA 02651527 2009-01-29
Examples of in-situ heavy oil recovery processes to which embodiments of the
present invention may be applied include, but are not limited to, steam-
assisted gravity
drainage (SAGD), cyclic steam stimulation (CSS), steam flooding and various
derivatives
thereof, such as solvent-assisted SAGD (SA-SAGD or ES-SAGD), steam and gas
push
(SAGP), combined vapor and steam extraction (SAVEX), liquid addition to steam
enhancing recovery (LASER), vapor extraction (VAPEX), constant steam drainage
(CSD), and cyclic solvent process (CSP), as well as various flooding processes
often
utilizing polymers to enhance displacement. Examples of in-situ conventional
hydrocarbon recovery processes to which embodiments of the present invention
may be
applied include, but are not limited to, primary production of light oil or
natural gas, was
well as flooding or sweeping processes, employing one or more horizontal
wellbores,
often utilizing polymers to enhance displacement. These recovery processes can
employ
one or more horizontal wellbores.
Example 1. Heavy Oil Reservoir Simulations Using SAGD
The time and cost involved in conducting field testing is often prohibitive in
the oil
and gas industry. Therefore, reservoir simulations are commonly relied upon to
test new
systems and processes before or while field testing is performed. In the
present
examples, testing was conducted using proprietary reservoir simulation
software, which
utilizes numerical modeling to mimic fluid flow through porous reservoir
media. Reservoirs
were modeled using average properties for an Athabasca oil sands deposit.
FIG. 15 shows the 2D results of a reservoir simulation modeling a typical SAGD
process in a continuous oil sands reservoir with Athabasca type properties and
no vertical
permeability impediments. Temperature and gas saturation in the reservoir are
shown for
1000 days and 2000 days of steam injection. The injection and production wells
are
located near the bottom left corner of each block but are not shown. The
results illustrate
the development of the steam chamber (labeled "Hot") over time during the SAGD
process, with corresponding increase in gas saturation where mobilized bitumen
has
drained from the heated area into the production well.
FIG. 16 shows the 2D results of a reservoir simulation modeling a SAGD process
in a partitioned reservoir with Athabasca type properties and two vertical
permeability
impediments. The SAGD well pair is located near the bottom left corner of each
block but
is not shown. The impediments were designed as substantially impermeable tight
streaks,
allowing conductive heat transfer only, that split the pay zone roughly into
thirds.
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CA 02651527 2009-01-29
In accordance with an embodiment of the invention, a zone of increased
permeability was created above the SAGD well pair to facilitate the rise of
steam and the
drainage of heavy oil through the reservoir. The altered permeability zone,
labeled as
such in the figure, modeled a conduit having a vertical height sufficient to
extend through
the tight streaks and permit vertical communication between the pay zones
within a
narrow region above the well pair. The numerical permeability selected for the
conduit
was of sufficient magnitude to approximate the effective permeability of a 0.6
m x 0.6 m
simulation grid block penetrated by a wellbore. Sensitivity runs showed that
the process
was insensitive to the absolute value of the altered permeability zone in the
simulator,
provided it was sufficiently large relative to the matrix sand permeability.
Any effective
increase in permeability of the barriers in the region above the SAGD well
pair would be
expected to enhance the process.
The results show temperature and gas saturation in the reservoir after 1000
days,
1500 days and 2000 days of steam injection. It can be seen that, by creating
the altered
permeability zone above the SAGD well pair, the steam effectively penetrated
the upper
zones of the reservoir and all three zones became active SAGD zones after a
period of
time. The heated bitumen effectively drained through the reservoir from the
upper zones
and reached the production well in the lower zone, as evidenced by the
increase in gas
saturation. In the absence of the altered permeability zone, only the oil in
the bottom pay
zone would be recovered resulting in approximately 1/3 recovery. Thus,
applying the
method of the invention significantly enhanced a SAGD recovery process in a
partitioned
reservoir.
Many vertical permeability impediments will permit conductive heat transfer
from
one pay zone to an adjacent upper pay zone. Without fluid communication
between the
pay zones, this transferred heat is normally characterized as overburden heat
loss, which
is considered a detriment to SAGD thermal efficiency. In accordance with the
invention
however, this heat transfer actually benefits the operation since the zone of
altered
permeability permits fluid communication between the pay zones. Heat from the
lower
pay zone can advantageously be transferred to the upper pay zone to aid in
mobilizing
heavy oil in the upper pay zone, which mobilized oil can then drain downward
through the
reservoir toward the production well via the altered permeability zone. This
voidage also
causes a formation pressure drop and further promotes the injected steam to
rise and
contact cold bitumen in the upper pay zones. The coupled effect of an
effective but
relatively confined altered permeability zone with conductive preheating
showed
surprisingly good performance in the split pay reservoir simulation.
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FIG. 17 shows the graphical results of a comparison of water and oil volumes
for
the two reservoir simulations described above. The dashed lines represent
water and oil
volumes for a typical SAGD operation in a reservoir with no tight streaks. The
solid lines
represent water and oil volumes for an enhanced SAGD operation carried out in
a
reservoir having two tight streaks wherein a zone of increased permeability is
provided
above the SAGD well pair. The oil rates were similar for the two cases for the
first 3 years
and the final cumulative oil volumes recovered were comparable. This is a
significant
finding since recovery would generally be much lower in the reservoir with
tight streaks.
Water rates between the two models were very similar, which is indicative of
the steam
rates as well.
Further reservoir simulations demonstrated practical application of the method
of
the invention in complex heavy oil reservoirs and reservoirs having top gas or
bottom
water.
Although the simulations performed focused on a SAGD process, using 2D and
3D grids, conclusions may reasonably be extrapolated to many applications and
processes utilizing horizontal wellbores. The 2D and 3D simulations performed
suggested
that increasing the height, width and effective permeability of the altered
permeability
zone is directionally advantageous. However, for practical purposes, reservoir
deliverability may be a limiting factor as given, for example, by Equation (1)
or
subsequent variations. It is also advantageous in most cases to position the
altered
permeability zone in a region substantially above the SAGD well pair (i.e.
above the well
pair along substantially the same vertical axis) and to continue the zone in a
direction
extending substantially parallel to the SAGD well pair along at least a
portion of the
horizontal length of the SAGD well pair. An altered permeability zone having
sufficient
vertical height (i.e. a chimney zone) would promote horizontal flow toward the
conduit as
well as vertical flow through the barriers toward the production well.
While various means of optimizing performance results are described above, it
is
important to note that performance results do not have to be optimal for the
process to be
highly effective. Even a narrow diameter conduit that intersects a vertical
permeability
impediment only at intervals along its length will enhance a SAGD recovery
process
significantly compared to a typical SAGD operation in such a reservoir.
Reservoir
characteristics, cost and practicality of creating the altered permeability
zone should be
considered in designing and implementing the altered permeability zone for a
particular
application. The characteristics of the vertical permeability impediment(s)
should also be
considered, including thickness and permeability.
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Example 2. Heavy Oil Reservoir Simulation of Split Pay Reservoir Using CSS
A numerical simulation study was conducted to evaluate applicability of the
invention to cyclic steam stimulation (CSS) with horizontal wellbores. CSS is
typically a
single well recovery process where the same well receives a fixed volume of
steam and is
then produced following injection in a cyclic manner. Horizontal wells are
increasingly
being used to reduce the cost of field development. The horizontal wells are
typically
located at the base of the effective pay zone and therefore oil recovery is
vulnerable to
vertical permeability impediments.
A simulation model based on typical properties of the Cold Lake Clearwater
formation was constructed. Referring to FIG. 18, the simulation was designed
to mimic
two adjacent horizontal wells (white) completed with spaced limited entry
perforations
(LEP), as discussed in SPE Paper 50429, "Targeted Steam Injection Using
Horizontal
Wells with Limited Entry Perforations", T. Boone, D. Youck & S. Sun, 1998. The
location
of the LEP points for the two adjacent horizontal wells are at the vertical
corners of the 3D
block in the plane of the horizontal wells (white), as is evident by the gas
saturation
patterns. Two adjacent wells were used to reflect the staggered steam
scheduling
commonly implemented in a CSS operation.
Using typical cycle steam volumes and schedule the split pay reservoir was
subjected to CSS with and without an altered permeability zone. Note the
altered
permeability zone spans the entire length of the short sides of the block
model comprising
a zone through the permeability impediment above and parallel with the
horizontal wells.
Simulation results show that the altered permeability zone allowed access to
the upper
pay zone and subsequently an increase in oil rate and recovery was achieved
compared
to the split pay reservoir without an altered permeability zone, as seen in
FIG. 19.
FIG. 19 shows the graphical results of a comparison of water and oil volumes
for
the CSS reservoir simulations described above. The dashed lines represent
water and oil
volumes for a typical CSS operation in a split pay reservoir. The solid lines
represent
water and oil volumes for an enhanced CSS operation in a split pay reservoir,
carried out
in accordance with an embodiment of the invention, wherein a zone of increased
permeability was provided above the CSS well, which penetrated a tight streak.
Cumulative oil recovery was significantly increased in the presence of the
altered
permeability zone, without a significant increase in water volume.
The above-described embodiments of the invention are intended to be examples
only. Alterations, modifications and variations can be effected to the
particular
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CA 02651527 2009-01-29
embodiments by those of skill in the art without departing from the scope of
the invention,
which is defined solely by the claims appended hereto.
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