Note: Descriptions are shown in the official language in which they were submitted.
CA 02893170 2015-05-29
THERMALLY INDUCED EXPANSION DRIVE IN HEAVY OIL RESERVOIRS
FIELD OF THE INVENTION
[0001] The invention is in the field of hydrocarbon reservoir engineering,
particularly
thermal recovery processes such as steam assisted gravity drainage (SAGD)
systems
in heavy oil reservoirs.
BACKGROUND OF THE INVENTION
[0002] Some subterranean deposits of viscous hydrocarbons can be extracted
in situ
by lowering the viscosity of the petroleum to mobilize it so that it can be
moved to, and
recovered from, a production well. Reservoirs of such deposits may be referred
to as
reservoirs of heavy hydrocarbon, heavy oil, bitumen, tar sands, or oil sands.
The in situ
processes for recovering oil from oil sands typically involve the use of
multiple wells
drilled into the reservoir, and are assisted or aided by thermal recovery
techniques,
such as injecting a heated fluid, typically steam, into the reservoir from an
injection well.
One process of this kind is steam-assisted gravity drainage (SAGD).
[0003] The SAGD process is in widespread use to recover heavy hydrocarbons
from
the Lower Cretaceous McMurray Formation, within the Athabasca Oil Sands of
northeastern Alberta, Canada. A thick sequence of marine shales and siltstones
of the
Clearwater Formation unconformably overlies the McMurray Formation in most
areas of
northeastern Alberta. In some areas, glauconitic sandstones of the Wabiskaw
member
are present at the base of the Clearwater. The Grand Rapids Formation overlies
the
Clearwater Formation, and quaternary deposits unconformably overlie the
Cretaceous
section. The pattern of hydrocarbon deposits within this geological context is
complex
and varied.
[0004] A typical SAGD process is disclosed in Canadian Patent No. 1,130,201
issued on 24 August 1982, in which the functional unit involves two wells that
are drilled
into the deposit, one for injection of steam and one for production of oil and
water.
Steam is injected via the injection well to heat the formation. The steam
condenses and
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gives up its latent heat to the formation, heating a layer of viscous
hydrocarbons. The
viscous hydrocarbons are thereby mobilized, and drain by gravity toward the
production
well with an aqueous condensate. In this way, the injected steam initially
mobilises the
in-place hydrocarbon to create a "steam chamber" in the reservoir around and
above
the horizontal injection well. The term "steam chamber" accordingly refers to
the volume
of the reservoir which is saturated with injected steam and from which
mobilized oil has
at least partially drained. Mobilized viscous hydrocarbons are typically
recovered
continuously through the production well. The conditions of steam injection
and of
hydrocarbon production may be modulated to control the growth of the steam
chamber,
to ensure that the production well remains located at the bottom of the steam
chamber
in an appropriate position to collect mobilized hydrocarbons.
[0005] In the ramp-up stage of the SAGD process, after communication has
been
established between the injection and production wells during start-up,
production
begins from the production well. Steam is continuously injected into the
injection well
(usually at constant pressure) while mobilized bitumen and water are
continuously
removed from the production well (usually at constant temperature). During
this period
the zone of communication between the wells is expanded axially along the full
well pair
length and the steam chamber grows vertically up to the top of the reservoir.
The
reservoir top may be a thick shale (overburden) or some lower permeability
facies that
cause the steam chamber to stop rising.
[0006] Heavy oil recovery techniques such as SAGD create mobile zone chambers
in a reservoir, from which at least some of the original oil-in-place has been
recovered.
However, reservoirs depleted by such processes typically contain a significant
volume
of residual hydrocarbons, often in reservoir zones that are hydraulically
segregated from
a mobile production zone, separated from the production zone for example by
lower
permeability facies such as a shale overburden. There remains a need for
methods that
may be used to recover these residual hydrocarbons.
[0007] In the context of the present application, various terms are used in
accordance with what is understood to be the ordinary meaning of those terms.
For
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example, "petroleum" is a naturally occurring mixture consisting predominantly
of
hydrocarbons in the gaseous, liquid or solid phase. In the context of the
present
application, the words "petroleum" and "hydrocarbon" are used to refer to
mixtures of
widely varying composition. The production of petroleum from a reservoir
necessarily
involves the production of hydrocarbons, but is not limited to hydrocarbon
production.
Similarly, processes that produce hydrocarbons from a well will generally also
produce
petroleum fluids that are not hydrocarbons. In accordance with this usage, a
process for
producing petroleum or hydrocarbons is not necessarily a process that produces
exclusively petroleum or hydrocarbons, respectively. "Fluids", such as
petroleum fluids,
include both liquids and gases. Natural gas is the portion of petroleum that
exists either
in the gaseous phase or in solution in crude oil in natural underground
reservoirs, which
is gaseous at atmospheric conditions of pressure and temperature. Natural gas
may
include amounts of non-hydrocarbons. The abbreviation POIP stands for
"producible oil
in place" and in the context of the methods disclosed herein is generally
defined as the
exploitable or producible oil structurally located above the production well
elevation.
[0008] It is common practice to segregate petroleum substances of high
viscosity
and density into two categories, "heavy oil" and "bitumen". For example, some
sources
define "heavy oil" as a petroleum that has a mass density of greater than
about 900
kg/m3. Bitumen is sometimes described as that portion of petroleum that exists
in the
semi-solid or solid phase in natural deposits, with a mass density greater
than about
1000 kg/m3 and a viscosity greater than 10,000 centipoise (cP; or 10 Pa.$)
measured at
original temperature in the deposit and atmospheric pressure, on a gas-free
basis.
Although these terms are in common use, references to heavy oil and bitumen
represent categories of convenience, and there is a continuum of properties
between
heavy oil and bitumen. Accordingly, references to heavy oil and/or bitumen
herein
include the continuum of such substances, and do not imply the existence of
some fixed
and universally recognized boundary between the two substances. In particular,
the
term "heavy oil" includes within its scope all "bitumen" including
hydrocarbons that are
present in semi-solid or solid form.
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[0009] A "reservoir" is a subsurface formation containing one or more
natural
accumulations of moveable petroleum, which are generally confined by
relatively
impermeable rock. An "oil sand" or "tar sand" reservoir is generally comprised
of strata
of sand or sandstone containing petroleum. A "zone" in a reservoir is an
arbitrarily
defined volume of the reservoir, typically characterised by some distinctive
property.
Zones may exist in a reservoir within or across strata, and may extend into
adjoining
strata. In some cases, reservoirs containing zones having a preponderance of
heavy oil
are associated with zones containing a preponderance of natural gas. This
"associated
gas" is gas that is in pressure communication with the heavy oil within the
reservoir,
either directly or indirectly, for example through a connecting water zone.
[0010] "Thermal recovery" or "thermal stimulation" refers to enhanced oil
recovery
techniques that involve delivering thermal energy to a petroleum resource, for
example
to a heavy oil reservoir. There are a significant number of thermal recovery
techniques
other than SAGD, such as cyclic steam stimulation, in situ combustion, hot
water
flooding, steam flooding and electrical heating. In general, thermal energy is
provided to
reduce the viscosity of the petroleum to facilitate production. The addition
of heat may
also have geophysical effects within the reservoir, for example causing the
expansion of
reservoir fluids, which may in turn lead to increases in pore pressures. In
oil sand
reservoirs, thermal expansion of bitumen within a reservoir may for example
create pore
pressures that are high enough to produce reservoir expansion, in effect
moving sand
grains apart (R.M. Butler, The expansion of tar sands during thermal recovery,
Journal
of Canadian Petroleum Technology, 1986, volume 25, issue 5, p. 51-56). The
evolution
of temperature and heat flow within a reservoir depends upon the thermal
properties of
the reservoir materials, including volumetric heat capacity, thermal
conductivity, thermal
diffusivity and the coefficients of thermal expansion.
[0011] A "chamber" within a reservoir or formation is a region that is in
fluid/pressure
communication with a particular well or wells, such as an injection or
production well.
For example, in a SAGD process, a steam chamber is the region of the reservoir
in fluid
communication with a steam injection well, which is also the region that is
subject to
depletion, primarily by gravity drainage, into a production well.
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[0012] "Reservoir compartmentalization" is a term used to describe the
segregation
of a petroleum accumulation into a number of distinct fluid/pressure
compartments. In
general, this segregation takes place when fluid flow is prevented across
sealed
boundaries in the reservoir. These boundaries may for example be caused by a
variety
of geological and fluid dynamic factors, involving: static seals that are
completely sealed
and capable of withholding (trapping) petroleum deposits, or other fluids,
over
geological time; and dynamic seals that are low to very low permeability flow
barriers
that significantly reduce fluid cross-flow to rates that are sufficiently slow
to cause the
segregated chambers to have independent fluid pressure dynamics, although
fluids and
pressures may equilibrate across a dynamic seal over geological time-scales
(Reservoir
compartmentalization: an introduction, Jolley et al., Geological Society,
London, Special
Publications 2010, v. 347, p. 1-8). A reservoir compartment may be
hydraulically
confined, so that fluids are prevented from moving beyond the compartment by
sealed
boundaries confining the compartment.
SUMMARY OF THE INVENTION
[0013] The invention involves the production of hydrocarbons from reservoir
compartments that are initially segregated into distinct fluid/pressure
compartments,
with the compartments in thermal communication. Thermal recovery techniques
applied
to one compartment are used so as to provide thermal energy to a second,
adjoining
but distinct fluid/pressure compartment, in which the second compartment is
hydraulically confined by sealed boundaries. Production from the first
compartment is
managed in conjunction with effecting thermal communication from the first
compartment to the second compartment. Heating of the second, confined
compartment, increases fluid pressures within the second compartment. This
increase
in fluid pressure in the second compartment, which may be coupled to
production of
fluids from the first compartment, gives rise to a pressure differential that
is used to
drive hydrocarbons from the second compartment to the first compartment, so
that
hydrocarbons originating from the second compartment are produced from the
first
compartment. The hydrocarbons may for example be a heavy oil that is
originally
CA 02893170 2015-05-29
immobile in the second compartment, which is mobilized by the thermal energy
communicated from the first compartment, and driven by the pressure
differential to the
first compartment for production.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] Figure 1 is a schematic illustration of a typical SAGD well pattern,
showing
paired injector and producer well pairs, each have a heel and a toe within the
hydrocarbon rich pay zone of the formation.
[0015] Figure 2 is schematic illustration of cretaceous stratigraphy of the
Athabasca
oil sands.
[0016] Figure 3 is a schematic illustration of a compartmentalized heavy
oil
reservoir.
[0017] 'Figure 4 is a cross sectional view of an exemplary completion for an
injector
well, referring to the use of slotted liners, as for example disclosed in
Canadian Patent
Application 2,616,483 of Cenovus Energy Inc. published 29 June 2008.
[0018] Figure 5 is a cross sectional view of an exemplary completion for a
production well, in a start up configuration for circulation, illustrating an
embodiment
employing gas lift.
[0019] Figure 6 is a cross sectional view of an exemplary completion for a
production well, illustrating an embodiment employing an electric submersible
pump
(ESP) for production operations following start up. Typically, after
circulation start-up,
the 2" coiled tubing string will be removed and the well equipped with a high
temperature ESP capable of pumping fluid from the well into production
gathering
facilities.
[0020] Figure 7 is a cross sectional view of an alternative completion for
a
production well.
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DETAILED DESCRIPTION OF THE INVENTION
[0021] Various aspects of the invention may involve the drilling of SAGD
well pairs
within a reservoir 11, as illustrated in Figure 1, with each injector well 13,
19, 23, paired
with a corresponding producer well 15, 17 and 21. Each well has a completion
14, 12,
16, 18, 20 and 22 on surface 10, with a generally vertical segment leading to
the heel of
the well, which then extends along a generally horizontal segment to the toe
of the well.
In very general terms, to provide a general illustration of scale in selected
embodiments,
these well pairs may for example be drilled in keeping with the following
parameters.
There may be approximately 5 m depth separation between the injection well and
production well. The SAGD well pair may for example average approximately 800
m in
length. The lower production well profile may generally be targeted so that it
is
approximately 1 to 2 m above the SAGD base. The development of steam chambers
around each well pair may be illustrated in cross sectional views along axis
24, which is
perpendicular to the longitudinal axial dimension of the horizontal segments
of the well
pairs.
[0022] As illustrated in Figure 2, the stratigraphy of the Athabasca oil
sands varies
geographically, and in places includes oil sand deposits that are separated by
distinct
barrier layers, such as marine shales. Figure 3 is a cross sectional view
along axis 24
of Figure 1, illustrating a hydrocarbon reservoir in which a primary heavy oil
compartment 30 is hydraulically separated from a secondary heavy oil
compartment 40
by a permeability barrier 32, so that under initial reservoir conditions heavy
oil does not
flow across the permeability barrier. The secondary heavy oil compartment 40
is
hydraulically confined, for example by shale cap rock 42.
[0023] In the embodiment illustrated in Figure 3, a SAGD thermal recovery
technique is applied to the primary heavy oil compartment 30, forming steam
chamber
28 around injection well 19, to mobilize heavy oil for production through
production well
17. Thermal energy applied to the primary heavy oil compartment 30 by way of
steam
chamber 28 is communicated across permeability barrier 32 to secondary heavy
oil
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compartment 40 to heat heavy oil in the secondary heavy oil compartment 40.
Alternatively, other thermal recovery techniques can be used, including cyclic
steam
stimulation (CSS), electrical or electromagnetic heating, hybrid solvent-steam
processes
and in-situ combustion. This is carried out so as to increase fluid pressure
within the
secondary heavy oil compartment 40 by way of thermal expansion of the confined
fluids
in secondary compartment 40.
[0024] By adjusting the production and/or injection of fluids in primary
heavy oil
compartment 30 or by adjusting the thermal energy delivered to the reservoir
via an
electric or electromagnetic heating source, as well as the delivery of thermal
energy to
secondary heavy oil compartment 40, conditions may be arranged so that the
fluid
pressure in the secondary heavy oil compartment 40 rises above the fluid
pressure in
the primary heavy oil compartment 30, to create a fluid pressure differential
between the
compartments. Under these circumstances, a fluid flow path may be provided
across
the permeability barrier, for example by a well completed so as to drain
mobilized heavy
oil from secondary heavy oil compartment 40 to primary heavy oil compartment
30,
driven by the fluid pressure differential between the compartments. In this
way, fluids
may be recovered from primary heavy oil compartment 30 that include heavy oil
from
secondary heavy oil compartment 40, for example by way of SAGD production well
17.
[0025] In alternative embodiments, the relative positions of primary and
secondary
heavy oil compartments may be varied. For example, the primary compartment may
be
above or below the secondary compartment, with an intervening permeability
barrier
that is substantially horizontal. Alternatively, the compartments may be
spaced apart
horizontally, with a substantially vertical permeability barrier. In practice,
the adjoining
compartments will typically have a complex geometric relationship, with
vertical and
horizontal components of offset.
[0026] The hydraulic confinement of the secondary compartment may be by way of
a
static seal formed by a geological pattern of surrounding permeability
barriers.
Alternatively, hydraulic confinement may be caused or enhanced by the
imposition of
dynamic fluid flow barriers that are not naturally present, such as synthetic
permeability
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barriers formed by pressurization of a hydraulically adjoining overlying gas
zone, or
underlying or overlying water zones, or laterally adjacent gas or water zones.
Alternatively, immobile bitumen may form part of a permeability barrier around
the
secondary compartment, zones of fluid injection may form a production seal or
natural
static or dynamic seals may form the production seal.
[0027] Once a fluid flow path is provided for fluids to exit the secondary
compartment, the drive mechanism for that flow may consist principally of
expansion of
pressurized liquids from the secondary compartment as it travels towards the
pressure
sink afforded by the lower pressure primary compartment. Alternatively, the
drive
mechanism may involve a solution gas drive due to expansion of gases, or may
involve
combinations of these alternative mechanisms. The basic drive mechanism
provided by
fluid expansion within the confined compartment may also be enhanced by the
addition
of other drive mechanisms, such as cyclic steam stimulation, hot water flood,
or steam
flood, combinations thereof or in-situ combustion, hybrid steam-solvent
processes,
electric and electromagnetic heating.
[0028] The fluid flow path for hydrocarbons from the secondary compartment to
the
primary compartment may for example be by way of a conduit, such as a well,
introduced to guide the flow of mobilized hydrocarbons. In the event that
formation
geology in the secondary compartment is characterized by poor effective
vertical
permeability (e.g., clasts, shale lenses, IHS), vertical or inclined wells may
for example
be employed to drain fluids horizontally (i.e., in the preferred direction of
flow) from the
secondary compartment into the wellbore that provides a fluid flow path across
the
permeability barrier to the primary compartment.
[0029] Alternatively, temperature and/or pressure can be controlled such as
to
induce stresses that can create a high degree of deformation of the rock that
comprises
the permeability barrier and eventually can yield a failure of the
permeability barrier
creating a fluid flow path for hydrocarbons from the secondary compartment to
the
primary compartment.
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[0030] In selected embodiments, permeability barriers are generally
comprised of
shale. Failure of the permeability barrier can happen because the changes in
stresses
caused by the pressure and the temperature change in the reservoir
compartments. It
can also happen because of the existence of thermal pore pressure inside the
shale,
which is caused by the heating up of the water inside the shale. Water expands
more
than the shale matrix upon heating and it cannot flow out because of the low
permeability shale ¨ creating high pore pressure inside the shale.
[0031] In select embodiments, thermal energy can also be imparted to the
permeability barrier from an adjacent well using a variety of methods such as
directing a
high temperature fluid directly to the permeability barrier until the
temperature of the
barrier increases sufficiently to induce fracturing of the shale layer.
[0032] Alternatively, injection of a fluid from an injection well to create
a localized
pressure increase in the permeability barrier sufficiently high as to create a
hydraulic
fracturing of the permeability barrier.
[0033] Alternative aspects of the invention involve completing wells in
various
configurations. Exemplary completions for injector, producer on gas lift,
producer on
electric submersible pump (ESP) and simulated producer are shown in Figures 4,
5, 6
and 7 respectively.
[0034] In accordance with various aspects of the invention, detailed
computational
simulations of reservoir behaviour may be carried out. The thermal properties
of the
reservoir may for example be characterized using two rock types. Rock type one
may
for example represent clean sand of the McMurray formation in Alberta, Canada.
A
second rock type representing an relatively impermeably strata, such as shale,
may be
used to simulate a permeability barrier. Exemplary properties of the two such
rock types
may for example be defined with the following properties:
Rocktype 1 (Sand)
Porosity Reference Pressure = 100 kPa
Compressibility = le-6 1/kPa
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Volumetric Heat Capacity 2.39e6 J/(m3*C)
Rock Thermal Conductivity = 196,820 J/(m*day*C)
Water Thermal Conductivity = 552,960 J/(m*day*C)
Oil Thermal Conductivity = 0
Gas Thermal Conductivity = 0
Rocktype 2 (Shale Overburden & Underburden)
Porosity Reference Pressure = 100 kPa
Compressibility = 1e6 1/kPa
Volumetric Heat Capacity 2.39e6 J/(m3*C)
Rock Thermal Conductivity = 146,880 J/(m*day*C)
Water Thermal Conductivity = 0
Oil Thermal Conductivity = 0
Gas Thermal Conductivity = 0
[0035] In an exemplary embodiment of the processes of the invention,
carried out in
the McMurray and Wabiskaw formations, typical values of the relevant formation
thicknesses are as follows: McMurray Formation in which SAGD is being
conducted 38
m; impermeable mudstone immediately overlying the McMurray 6 m; affected
Wabiskaw zone immediately overlying the mudstone 7 m. In this embodiment, the
ascent within the McMurray Formation of the SAGD steam chamber was confirmed
with
temperature profiles. It was also confirmed with 4D (Time Lapse) Seismic data.
Progressive heating of the overlying Wabiskaw was clearly evident in the 4D
seismic
data, over time: year 1 - No seismic anomalies evident in Wabiskaw; year 2 -
anomalies
appear, indicating some heating of Wabiskaw; year 3 ¨ anomalies signal
continued
heating of Wabiskaw.
[0036] Because of the Wabiskaw zone's geological confinement, the pressure
within
the Wabiskaw compartment increased markedly as it was heated conductively from
below. This thermally induced over-pressuring of the Wabiskaw was first
identified in
year 3, while attempting to drill a steam chamber core. Whereas the normal
formation
pressure at this depth and in this area is approximately 2000 kPa, the
pressure
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measured via drill stem test was approximately 6500 kPa. To utilize this
pressure
increase, a production well was drilled into the Wabiskaw, producing
significant
quantities of oil on a sustained basis, gradually reducing the reservoir
pressure in the
Wabiskaw.
[0037] These results indicate that conductive heating of bitumen in the
confined
Wabiskaw zone, with heat arriving from the underlying SAGD steam chamber in
the
McMurray formation, induced an increase in reservoir pressure within the
Wabiskaw
from -2000 kPa to -6500 kPa. Field data confirm that the mudstone separating
the
underlying McMurray Formation from the overlying Wabiskaw zone is competent,
allowing no hydraulic communication between the two zones. If the two zones
were
hydraulically communicating, the pressure in the Wabiskaw would equilibrate at
a value
closer to that of the McMurray Formation (e.g., -2000 kPa). Instead, the
Wabiskaw
reached a pressure of -6500 kPa, illustrating that high pressure may be
induced by
conductive heating of the Wabiskaw due to its geological confinement.
[0038] Although various embodiments of the invention are disclosed herein,
many
adaptations and modifications may be made within the scope of the invention in
accordance with the common general knowledge of those skilled in this art. For
example, any one or more of the injection, production or vent wells may be
adapted
from well segments that have served or serve a different purpose, so that the
well
segment may be re-purposed to carry out aspects of the invention, including
for
example the use of multilateral wells as injection, production and/or vent
wells. Such
modifications include the substitution of known equivalents for any aspect of
the
invention in order to achieve the same result in substantially the same way.
Numeric
ranges are inclusive of the numbers defining the range. The word "comprising"
is used
herein as an open-ended term, substantially equivalent to the phrase
"including, but not
limited to", and the word "comprises" has a corresponding meaning. As used
herein, the
singular forms "a", "an" and "the" include plural referents unless the context
clearly
dictates otherwise. Thus, for example, reference to "a thing" includes more
than one
such thing. Citation of references herein is not an admission that such
references are
prior art to the present invention. Any priority document(s) and all
publications, including
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but not limited to patents and patent applications, cited in this
specification are
incorporated herein by reference as if each individual publication were
specifically and
individually indicated to be incorporated by reference herein and as though
fully set forth
herein. The invention includes all embodiments and variations substantially as
hereinbefore described and with reference to the examples and drawings.
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