Note: Descriptions are shown in the official language in which they were submitted.
IN SITU COMBUSTION FOLLOWING SAGD
FIELD OF THE INVENTION
[0003] The invention relates to recovery of petroleum from a petroliferous
formation, in
particular to in situ processing of a reservoir containing heavy oil and/or
bitumen.
BACKGROUND OF THE INVENTION
[0004] Production of heavy oil and bitumen from a subsurface reservoir can
be quite
challenging, especially when the viscosity of the oil at reservoir temperature
is often greater than
a million centipoise (cP). High viscosity oil cannot be pumped out of the
ground using typical
methods, but must be mined or processed in situ. Surface mining is limited to
reservoirs to a
depth of about 70 meters. Greater depths are not economical to access and most
reserves are not
accessible by mining. Since only a relatively small percentage of bitumen and
oil sand deposits
(such as the Athabasca oils sands of Alberta, Canada), are recoverable through
open-pit mining,
the majority of reservoirs require some form of in situ extraction.
[0005] In situ combustion (ISC) is an enhanced oil recovery method for both
light oil and
heavy oil reservoirs, wherein the heat to liquefy the deposits is provided by
combusting some of
the fuel in situ. ISC typically involves injection of an oxidant into a
formation, and the oil
present in the reservoir serves as fuel for combustion once ignited. Heat,
oxygen and fuel must
be readily available to sustain the reaction, but in a bitumen reservoir,
combustion can be
interrupted by immobile fuel. Therefore, combustion gas products (CO, CO2,
H2S, etc.) and
mobilized oil can be trapped in the reservoir, extinguishing the combustion
front.
[0006] Furthermore, interwell fluid communication must be established for
ISC to work.
Heating rods and other approaches can achieve this outcome, but they are not
as yet economical.
[0007] US7516789 and W00674555 describe a hydrocarbon recovery process
comprising,
among other things, injecting an oxidizing gas into a formation through an
injection well to
support in situ combustion and mobilize hydocarbons in the heavy oil;
producing fluids from a
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combustion gas production well, to direct combustion gases to the combustion
gas production
well; and recovering the mobilized hydrocarbons from the reservoir through a
hydrocarbon
production well. These publications also suggest that the method can be
applied to a reservoir
that has been depleted or partially depleted by a petroleum recovery process,
leaving a residual
oil deposit in the reservoir. US7516789 and W00674555 require a separate
combustion gas
production well, and fail to teach or suggest disposition of the oxidizing gas
injection well within
the formation relative to the overburden and/or to the hydrocarbon production
well.
[0008] US20070187094 and US20090044940 describes a method referred to as
Combustion
Assisted Gravity Drainage (CAGD), wherein oxygen is co-injected with steam
into a SAGD
steam chamber, thereby reducing the amount of steam required to produce a
barrel of oil. In
particular, US20090044940 describes a method for producing oil comprising:
providing a steam
chamber within an oil formation wherein the steam chamber defines steam
chamber walls;
injecting oxygen into the steam chamber and initiating combustion of oil at
the steam chamber
walls; allowing heated and cracked oil to drain toward a production well, and
recovering oil
through the production well. These applications fail to teach or suggest use
of in situ combustion
after a formation is developed with a steam-assisted process, and describe
recovery only at steam
chamber walls rather than from the formation as a whole, such as edges of the
formation and
residual oil within the steam chamber.
[0009] There thus exists a need to overcome fuel immobility in
petroliferous formations so
that ISC can be applied as an improved recovery and/or secondary recovery
method. Such a
method is especially needed to recover residual oil from a steam chamber and
at the boundaries
of a petroliferous foiniation after a SAGD process reaches its economic end.
SUMMARY OF THE INVENTION
[0010] Steam-assisted gravity drainage (SAGD), a leading recovery method
already in use
for heavy oil and bitumen in Canada, can be used to condition a reservoir
prior to ISC
implementation. The steam chamber generated by a SAGD wellpair provides two of
the main
requirements for ISC: oil mobility and established communication between
wells. By drilling a
horizontal well near the top of the reservoir and injecting an oxidizing
agent, such as air, oxygen,
or oxygen-enriched air, ISC recovers residual oil in the steam chamber and
provides heat to
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recover residual oil at the edges of the steam chamber and from the boundaries
of the formation.
Using ISC after a SAGD pattern has matured can recover additional reserve past
the normal
SAGD economic limit, and the compression equipment used in ISC can be used to
repressurize
the reservoir before abandonment.
[0011] This application provides a method for recovering petroleum from a
formation,
wherein said formation is intersected by at least one wellpair consisting of a
horizontal
production well and a horizontal injection well, and wherein said formation
comprises at least
one steam chamber developed by a steam-assisted process, said method
comprising: providing
an oxidizing agent near the top of said formation; initiating in situ
combustion (ISC); and
recovering petroleum from said at least one production well.
[0012] The petroleum can comprise heavy oil and/or bitumen. The oxidizing
agent can be
selected from the group consisting of air, oxygen and oxygen-enriched air,
preferably oxygen.
The duration of the process can be, for example, 5 years of SAGD followed by 5
years of air
injection, but obviously, the time will vary with the size and conditions of
the reservoir.
[0013] The ISC can heat the formation or parts of the formation to a
temperature of, for
example, 500 C to 1000 C. The steam-assisted process can be selected from the
group consisting
of steam-assisted gravity drainage (SAGD) (with or without solvents), steam-
assisted gravity
push (SAGP), and cyclic steam stimulation (CSS). For example, the at least one
wellpair can be
a SAGD wellpair.
[0014] The oxidation agent is provided 1-10 meters below the overburden of
said formation.
The wellpairs can be at any distance relative to one another determined, for
example, by details
for the formation and economic concerns. The oxidizing agent is provided
through a horizontal
oxidant injection well. In some embodiments, an oxidant injection well is
placed at the top of a
formation between sets of SAGD wellpairs. This configuration allows the
oxidant injection wells
to supply an oxidizing agent to two steam chambers simultaneously. For
example, the formation
can comprise at least two wellpairs and at least two steam chambers, and the
oxidant injection
well can be disposed so that said oxidizing agent is supplied to said at least
two steam chambers.
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[0015] In addition to recovering more oil, the method herein also allows
repressurization of
the formation. Some governments require that a reservoir produced using a SAGD
method must
be pressurized back to virgin reservoir pressure before abandonment, so as to
avoid subsidence
and other post-production complications.
[0016] In a particular embodiment, there is provided a method for
recovering heavy oil
and/or bitumen from a formation, comprising: providing an oxidizing agent
through a horizontal
oxidant injection well disposed 1-10 meters below the overburden of said
formation, wherein
said formation is intersected by at least one steam-assisted gravity drainage
(SAGD) wellpair
consisting of a horizontal production well and a horizontal injection well,
wherein said formation
comprises at least one SAGD steam chamber, and wherein said oxidizing agent is
selected from
the group consisting of air, oxygen and oxygen-enriched air; initiating in
situ combustion (ISC);
and recovering petroleum from said at least one production well. The formation
can comprise at
least two SAGD wellpairs and at least two SAGD steam chambers, and said
oxidant injection
well is disposed so that said oxidizing agent is supplied to said at least two
SAGD steam
chambers.
[0017] In another embodiment, a method of enhanced oil recovery is provided
using at least
one horizontal production well and at least one horizontal injection well,
said production well
being at or near a bottom of a hydrocarbon reservoir, and said injection well
being above said
production well and at or near a top of said hydrocarbon reservoir. The method
can include
additional injection or production wells, arranged as is known in the art. The
first step is injecting
steam (and/or solvents and/or gases) into said injection well and recovering a
first amount of
hydrocarbon from said production well. The second step is injecting an oxidant
into said
injection well when steam assisted hydrocarbon production begins to decrease,
and initiating
combustion. Finally, the remaining hydrocarbon can be produced using the
additional heat
produced by in situ combustion.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1. Typical SAGD wellpair, wherein an injection well 2 is above
a production
well 4. The SAGD process forms a steam chamber 6 in a formation between the
overburden 8
and the underburden 10, mobilizing oil to gravity drain at the production well
4. In FIG. 1,
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reference character 14 refers to heat, reference character 16 refers to
mobilized oil and reference
character 18 refers to oil sand.
[0019] FIG. 2. Oil saturation of a formation containing a SAGD wellpair at
the pattern's
economic limit.
[0020] FIG. 3. Horizontal oxidant injection well layout for ISC after a
SAGD process.
[0021] FIG. 4. Temperature profile for ISC that shows the combustion front
moving toward
the production well.
[0022] FIG. 5. Oil saturation of a SAGD-treated formation after ISC, which
creates a bank of
oil that moves toward the producer as the combustion front progresses.
DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0023] The following abbreviations are used herein:
BOE barrel of oil equivalent
CAGD combustion assisted gravity drainage
cP centipoise
cSOR cumulative steam-oil ratio
CSS cyclic steam stimulation
CWE cold water equivalent
DSG direct steam generation
GOR gas-oil ratio
MPa megapascals
NCG non-condensable gas
SAGD steam-assisted gravity drainage
SOR steam-to-oil ratio
[0024] "Formation" as used herein refers to a geological structure,
deposit, reserve or
reservoir, which includes one or more hydrocarbon-containing layers, one or
more non-
hydrocarbon layers, an overburden and/or an underburden. The hydrocarbon
layers can contain
non-hydrocarbon material as well as hydrocarbon material. The overburden and
underburden
contain one or more different types of impermeable materials, for example
rock, shale,
mudstone, wet carbonate, or tight carbonate. A "petroliferous formation" is a
formation that
contains or yields petroleum.
Date Recue/Date Received 2020-12-04
[0025] "Petroleum deposit" refers to an assemblage of petroleum in a
geological for ination.
The petroleum deposit can comprise light and heavy crude oils and bitumen. Of
particular
interest for the method described herein are petroleum deposits that primarily
comprise heavy
petroleum, such as heavy oil and/or bitumen.
[0026] "Injection well" or "injector" refers to a well into which a fluid
is injected into a
geological formation. The injected fluid can comprise, for example, a gaseous
mixture of steam,
non-condensable gas (NCG) and/or hydrocarbon solvent. The injected fluid can
also comprise a
liquid solvent, such as a liquid hydrocarbon solvent or CS2.
[0027] "Production well" or "producer" refers to a well from which a
produced fluid is
recovered from a geological formation. The produced fluid can comprise, for
example, a
petroleum product, such as heavy oil or bitumen.
[0028] "Horizontal drilling" refers to a process of drilling and completing
a well, beginning
with a vertical or inclined linear bore, which extends from the surface to a
subsurface location in
or near a target reservoir (e.g., gas, oil), then bears off at an arc to
intersect and/or traverse the
reservoir at an entry point. Thereafter, the well continues at a horizontal or
nearly horizontal
attitude tangent to the arc, substantially or entirely remaining within the
reservoir until the
desired bottom hole location is reached. (Of course, the "bottom hole" of a
horizontal well is the
terminus of the horizontal wellbore rather than the gravitational bottom of
the vertical wellbore.)
[0029] A "horizontal well" is a well produced by horizontal drilling.
Horizontal
displacements of more than 8000 feet (2.4 km) have been achieved. The initial
linear portion of a
horizontal well, unless very short, is typically drilled using rotary drilling
techniques common to
drilling vertical wells. A short-radius well has an arc with a 3-40 foot (1-12
m) radius and a
build rate of as much as 3 per 100 feet (30 m) drilled. A medium-radius well
has an arc with a
200-1000 foot (61-305 m) radius and build rates of 8-30 per 100 feet drilled.
A long-radius
well has an arc with a 1000-2500 (305-762 m) foot radius. Most new wells are
drilled with
longer radii, while recompletions of exiting wells tend to employ medium or
short radii.
Medium-radius wells are the most productive and most widely used.
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[0030] Horizontal wells confer several benefits. Operators are often able
to develop a
reservoir with fewer horizontal wells than vertical wells, since each
horizontal well can drain a
larger rock volume about its bore than a vertical well could. One reason for
this benefit is that
most oil and gas reservoirs are more extensive in their horizontal (areal)
dimensions than in their
vertical (thickness) dimension. A horizontal well can also produce at rates
several times greater
than a vertical well, due to a higher wellbore surface area within the
producing interval.
[0031] "Multilateral well" refers to a well that is one of a plurality of
horizontal branches, or
"laterals", from a vertical wellbore. Such wells have at least two such
branches and allow access
to widely spaced reservoir compartments from the same wellbore, thus saving
the cost of drilling
multiple vertical wellbores and increasing the economy of oil and gas
extraction. For example, a
well with a fishbone configuration has a single vertical wellbore and a
plurality of non-vertical
(e.g., horizontal), deviated portion connected to the vertical wellbore and
extending into the
formation. The non-vertical portions of a fishbone-configured well can further
progress through
the reservoir at angles different from the original angle of deviation.
[0032] "Ex situ processing" refers to petroleum processing which occurs
above ground. Oil
refining is typically carried out ex situ.
[0033] "In situ processing" refers to processing which occurs within the
ground in the
reserve itself. Processes include heating, combustion, pyrolysis, steam
cracking, and the like. In
situ processing has the potential of extracting more oil from a given land
areas than ex situ
processes since they can access material at greater depths than surface mines
can. Examples of in
situ processing include SAGD and ISC.
[0034] "Steam-assisted process" refers to any method wherein heated water
or steam, used
alone or in combination with other solvents and/or gases, is injected into a
petroliferous
formation so as to produce petroleum from that formation. Solvents may include
hydrocarbon
solvents, such as methane, ethane, propane, butane, pentane, hexane,
acetylene, and propene, or
solvents containing heteroatoms, such as carbon disulfide (CS2). Other gases
may include non-
condensable gases (NCGs) such as nitrogen (N2), oxygen (02), air, CO2, CO,
hydrogen (H2), flue
gas and combustion gas. Examples of steam-assisted processes include, but are
not limited to
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steam-assisted gravity drainage (SAGD), steam-assisted gravity push (SAGP),
and cyclic steam
stimulation (CSS).
[0035] "Steam-assisted gravity drainage" or "SAGD" refers to an in situ
recovery method
which uses steam to assist in situ processing, including related or modified
processes such as
steam-assisted gravity push (SAGP), and the original SAGD method as described
by Butler in
US4314485. In general, the method requires two horizontal wells drilled into a
reservoir. The
wells are drilled vertically to different depths within the reservoir then,
using direction drilling,
the wells are extended horizontally, resulting in horizontal wells vertically
aligned to and spaced
from each other. Typically the production well is located above the base of
the reservoir but as
close as possible to its bottom, for example between 1 and 3 meters above the
base of the oil
reserve. The injection well is placed above (or nearly above) the production
well, and is supplied
steam from the surface. The steam rises, forming a steam chamber that slowly
grows toward the
reservoir top, thereby increasing reservoir temperature and reducing viscosity
of the petroleum
deposit. Gravity pulls the petroleum and condensed steam through the reservoir
into the
production well at the bottom, where the liquid is pumped to the surface. At
the surface, water
and petroleum can be separated from each other.
[0036] "In situ combustion" or "ISC" refers to a process wherein an
oxidizing agent is
introduced into a formation and a combustion reaction is initiated to consume
fossil fuel that is
present in the foimation. ISC can be controlled, for example, by metering the
volume of
oxidizing agent introduced to the formation, adjusting the pressure of the
oxidizing agent or
overall pressure of the formation, and/or producing combustion gases and/or
petroleum from the
formation. ISC can be used over the period of years to heat and pressurize a
petroliferous
formation in order to mobilize, liquefy, upgrade, and/or produce petroleum.
Time can depend on
economic limits, for example simulations were run for 5 years of SAGD followed
by 5 years of
air injection.
[0037] Oxidizing agents include, but are not limited to, oxygen, air,
oxygen-enriched air, and
the like. Oxygen is preferred because of its relatively low cost and
effectiveness in the ISC
process. ISC can be catalyzed, for example for upgrading purposes. Although
upgrading is not
ISC per se, upgrading can result from an ISC process. Temperatures in the
formation should be
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below the melting temperature of well completion to avoid well failure, for
example from 500 C
to 1000 C. The entire formation need not attain the target combustion
temperature for the process
to be effective, but different portions of the formation or hydrocarbon
reservoir can attain the
target temperature at different times.
[0038] Temperature can depend on asphaltene content of the bitumen and air
injection rates.
Air injection duration depends on air injection rate, which depends in part
upon economic factors
that have not been investigated. Speed of the front can be controlled by the
rate of oxidizing
agent injection and pressure at which it is injected. A balance between front
speed and
compression cost is determined on a reservoir-by-reservoir basis. Pressure and
volume can be
adjusted and are determined, for example, by the economic factors associated
with each
reservoir.
[0039] "Cumulative steam-oil ratio" or "cSOR" refers to the ratio of
cumulative injected
steam (expressed as cold water equivalent, CWE) to cumulative petroleum
production volume.
The thermal efficiency of SAGD is reflected in the cSOR. Typically a process
is considered
thermally efficient if its SOR is less than 3, such as 2 or lower. A cSOR of
3.0 to 3.5 is usually
the economic limit, but this limit can vary project to project and with oil
prices.
[0040] "Steam chamber", "vapor chamber" or "steam vapor chamber" refers to
the pocket or
chamber of gas and vapor formed in a geological formation by a steam-assisted
process. In other
words, the steam chamber is the volume of the reservoir, which is saturated
with injected steam
and from which mobilized oil has at least partially drained. As the steam
chamber expands
upwardly and laterally from the injection well, viscous hydrocarbons in the
reservoir are heated
and mobilized, especially at the margins of the steam chamber where the steam
condenses and
heats a layer of viscous hydrocarbons by thermal conduction. The mobilized
hydrocarbon and
aqueous condensate drain under gravity toward the bottom of the steam chamber,
where a
production well can be located.
[0041] A steam chamber can be in fluid communication with one or more
injection wells, for
example, two injection wells. During initiation of a SAGD process,
overpressurized conditions
can be imposed to accelerate steam chamber development, followed by prolonged
underpressurization to reduce the steam-to-gas ratio. Maintaining reservoir
pressure while
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heating advantageously minimizes water inflow to the heated zone and to the
wellbore. When
petroleum is continuously recovered and the cSOR is under 4, a steam chamber
has likely
formed. A cSOR of less than 4 implies that heat from the injected steam
reaches the petroleum at
the edges of the chamber and that the mobilized bitumen is flowing under
gravity to the
production well.
[0042] "Recovery" refers to extraction of petroleum from a petroleum deposit
or hydrocarbon-
containing layer within a geologic formation.
[0043] The present invention is exemplified with respect to in situ
combustion after SAGD
in a bitumen-containing formation. However, this method is exemplary only, and
the invention
can be broadly applied to any petroliferous formation, wherein the petroleum
was mobilized
prior to ISC or wherein a combustion process is used in connection with a
steam-assisted
process. The following examples are intended to be illustrative only, and not
unduly limit the
scope of the appended claims.
EXAMPLE 1: IN SITU COMBUSTION AFTER SAGD
[0044] In a typical oil saturation of a SAGD well pattern at the end of its
economic life,
about 10% of the original oil still remains in the steam chamber, with very
high saturation on the
edges of the SAGD pattern (FIG. 2). Please note that in this figure and in
others like it (FIGS 3,
4, & 5), the producer and injector and/or their labels are superimposed on one
another. In FIG. 3,
reference character 2 refers to the injector, reference character 4 refers to
the producer and
reference characters 3A, 3B refer to horizontal oxidant injection wells (the
horizontal oxidant
injection wells are labelled as Air Injector and Air Injector - 1 in FIGS. 4
and 5).
[0045] The residual oil can be removed from the steam chamber by in situ
combustion
following the SAGD process. Oxidant injection wells are placed at the top of
the reservoir at the
edge of the SAGD pattern or, if the reservoir contains multiple SAGD
wellpairs, between SAGD
wellpairs. This placement allows the oxidant injection wells to supply an
oxidizing agent to two
steam chambers simultaneously.
Date Recue/Date Received 2020-12-04
[0046] After combustion initiates, the combustion front proceeds from the
oxidant injectors
to the SAGD wellpair, having maximum temperatures of about 500 C (FIG 4).
Temperature of
the combustion front depends on asphaltene content of the bitumen. Dark grey
indicates
propagation of the combustion front toward the production well at the bottom
of the formation.
As the combustion front advances, it creates a bank of oil that is swept
toward the production
well. At this same time step, oil saturation increases ahead of the combustion
front as the residual
oil is removed from the rock matrix (FIG. 5). Heat from the combustion front
also reduces the
viscosity of the unproduced oil on the edges of the SAGD pattern, allowing it
to flow toward to
the production well.
[0047] Simulations were run using properties of a typical Athabasca-type
reservoir with
bitumen viscosities representative of that area, for example, Surmont 1. A
process using ISC
after SAGD can increase recovery factors by about 10% and reduce cumulative
steam oil ratios
by about 15%.
[0048] The use of the word "a" or "an" when used in conjunction with the
term "comprising"
in the claims or the specification means one or more than one, unless the
context dictates
otherwise.
[0049] The term "about" means the stated value plus or minus the margin of
error of
measurement or plus or minus 10% if no method of measurement is indicated.
[0050] The use of the term "or" in the claims is used to mean "and/or"
unless explicitly
indicated to refer to alternatives only or if the alternatives are mutually
exclusive.
[0051] The terms "comprise", "have", "include" and "contain" (and their
variants) are open-
ended linking verbs and allow the addition of other elements when used in a
claim.
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Date Recue/Date Received 2020-12-04
