Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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SYSTEM AND METHOD FOR OPERATING AN INFILL AND/OR A STEP-OUT WELL FOR IN
SITU BITUMEN RECOVERY
TECHNICAL FIELD
[0001] The following relates to systems and methods for operating infill
wells and/or step-
out wells for in situ bitumen recovery.
DESCRIPTION OF THE RELATED ART
[0002] Bitumen is known to be considerably viscous and does not flow like
conventional
crude oil. As such, bitumen is recovered using what are considered non-
conventional methods.
For example, bitumen reserves are typically extracted from a geographical area
using either
surface mining techniques, wherein the overburden is removed to access the
underlying pay
(e.g., ore-containing bitumen) and transported to an extraction facility; or
using in situ
techniques, wherein subsurface formations (containing the pay), e.g., oil
sands, are heated such
that the bitumen is caused to flow into one or more wells drilled into the pay
while leaving
formation rock in the reservoir in place. Both surface mining and in situ
processes produce a
bitumen containing fluid that is subsequently sent to an upgrading and
refining facility, to be
refined into one or more petroleum products such as gasoline and jet fuel.
[0003] Bitumen reserves that are too deep to feasibly permit bitumen
recovery by mining
techniques are typically accessed by drilling wellbores into the hydrocarbon
bearing formation
(i.e. the pay) and applying an in situ bitumen recovery process.
[0004] There are various in situ technologies available, such as steam
driven or in situ
combustion-based techniques. However, currently Steam Assisted Gravity
Drainage (SAGD) is
considered to be the most popular and effective in situ process. SAGD is an
enhanced oil
recovery process whereby a long horizontal steam injection well is located
above a long
horizontal production well. Injected steam forms a steam chamber above the
SAGD well pair,
heating the reservoir rock and reservoir fluids. Heated bitumen plus condensed
steam flows
down the sides of the steam chamber towards the production well. The condensed
steam and
bitumen are then lifted to surface with a downhole pump or by gas lift. SAGD
typically operates
at elevated pressures and elevated temperatures, e.g., with temperatures
exceeding 190 C.
Once at surface the bitumen and water are separated from one another in
treatment vessels
that operate at relatively high temperatures (e.g., 170 C). Bitumen is sent
to refineries, while
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,
produced water is recycled. The reservoir rock (typically unconsolidated rock,
i.e., sand) that
once contained the bitumen remains in place, and is not produced to surface.
[0005] SAGD has become an increasingly popular method for extracting
bitumen from oil
sand reservoirs that are too deep for surface mining, largely due to the high
recovery factor from
SAGD. In situ techniques such as SAGD are normally used to access deeper pay
wherein
wellbores are drilled from the surface into the subsurface hydrocarbon-bearing
formation. While
vertical wellbores can be drilled deep enough to access the oil sands, bitumen
recovery from
vertical wells has not been found to be as effective as SAGD, which utilizes
horizontally drilled
wells.
[0006] Typically, multiple SAGD well pairs are drilled from surface into
the subsurface
hydrocarbon-bearing formation to cover a particular geographical area. While
each SAGD well
pair operates to heat bitumen on each side of the steam injector well, there
often exists an area
between adjacent SAGD well pairs where bitumen cannot be easily extracted
using the SAGD
well pairs due to the way in which the steam chambers develop for each well
pair. To access
these areas between adjacent SAGD well pairs, infill wells have been used,
which are drilled
into areas between SAGD well pairs. Step-out wells, which are located adjacent
a single SAGD
well pair, e.g., at the outer edges of a SAGD operation, can also be used to
access areas
beyond the SAGD well pair steam chambers. During early production stages, the
infill wells are
heated from diffused heat from the surrounding SAGD chambers. The temperature
around the
infill wells and step-out wells therefore increase with time but in the early
stages the temperature
can be too low to have a meaningful effect on mobilizing the bitumen.
SUMMARY
[0007] In one aspect, there is provided a method for injecting solvent into
a bitumen reserve
at a well located adjacent at least one steam assisted gravity drainage (SAGD)
well pair in the
bitumen reserve, the well comprising an infill well or a step-out well. The
method comprises:
injecting a first solvent into the bitumen reserve at the well, wherein a
region of the bitumen
reserve near the well is at a first temperature; and at a later time when the
region of the
bitumen reserve is at a second temperature higher than the first temperature,
injecting a second
solvent into the bitumen reserve at the well, wherein the second solvent has a
higher boiling
point than the first solvent and the first and second solvents are operable to
reduce the viscosity
of the bitumen in the reserve.
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[0008] In another aspect, the first solvent is selected based at least in
part on the first
temperature and the second solvent is selected based at least in part on the
second
temperature.
[0009] In yet another aspect, there is provided a method for injecting
solvent into a bitumen
reserve at a well located adjacent at least one steam assisted gravity
drainage (SAGD) well pair
in the bitumen reserve, the well comprising an infill well or a step-out well.
The method
comprises: determining a first temperature of the bitumen reserve in a region
near the well;
based on the first temperature, selecting a first solvent and injecting the
first solvent into the
bitumen reserve using the well; at a later time, determining a second
temperature of the region,
where the second temperature is higher than the first temperature; based on
the second
temperature, selecting a second solvent and injecting the second solvent into
the bitumen
reserve using the well; wherein: the first solvent and the second solvent are
operable to reduce
the viscosity of bitumen in the reserve; and a first diffusion boundary of the
first solvent at the
first temperature is a greater distance from the well than a second diffusion
boundary of the
second solvent at the second temperature, such that a lesser volume of the
second solvent
penetrates the bitumen reserve than the first solvent.
[0010] An advantage of the diffusion controlled mobilization process
described herein stems
from the process beginning earlier (that is, at lower temperatures) with
relatively lighter solvents,
which can reduce the steam-to-oil ratio (SOR). By increasing the surrounding
temperature of the
infill or step-out well, progressively heavier solvents are then used. An
advantage of this lighter
to heavier progression of solvents comes from the nature of the diffusion
mechanism in lighter
versus heavier solvents. That is, at the early stages in which the temperature
of the area
surrounding the infill or step-out well is lower, the correspondingly lighter
solvents are used,
which are less costly in comparison to heavier solvents. The diffusion
boundary layer can be
significantly larger for lighter solvents, meaning that the correspondingly
larger solvent loss is
offset by the lower cost. As such, for early stages in which SAGD operators
have less control
over aspects of the process such as temperature variations along the infill or
step-out well, less
expensive solvents are being used. Moreover, heavier solvents can be injected
at higher
pressures due to steam chamber advancement and higher steam chamber pressures
and, as
such, the solvent is contained around the infill or step-out wells thus
further reducing solvent
loss. As a result, operating at a relatively lower pressure than the steam
chamber pressure can
further limit solvent loss and improve pad economics.
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BRIEF DESCRIPTION OF THE DRAWINGS
[0011] Embodiments will now be described by way of example only with
reference to the
appended drawings wherein:
[0012] FIG. 1 is a cross-sectional elevation view of a SAGD production
site;
[0013] FIG. 2 is a cross-sectional elevation view of a SAGD production site
having multiple
SAGD well-pairs;
[0014] FIG. 3 is a cross-sectional elevation view of a SAGD production site
having multiple
SAGD well-pairs, multiple single infill wells, and a step-out well;
[0015] FIG. 4 is a cross-sectional elevation view of a SAGD production site
having multiple
SAGD well-pairs, multiple infill well-pairs, and a step-out well;
[0016] FIG. 5 is a cross-sectional elevation view of adjacent SAGD well-
pairs and
illustrating heat diffusion towards a single infill well between the SAGD well-
pairs;
[0017] FIG. 6 is a cross-sectional elevation view of adjacent SAGD well-
pairs and
illustrating a temperature profile towards a single infill well between the
SAGD well-pairs;
[0018] FIG. 7 is a chart illustrating a series of vapor pressure-
temperature curves and a
series of corresponding solvent injection stages for a diffusion controlled
mobilization process;
[0019] FIG. 8 is a chart illustrating a series of mixed bitumen and solvent
viscosity-
temperature curves at the edge of an infill well and viscosity changes for
different stages of a
diffusion controlled mobilization process at varied injection pressures;
[0020] FIG. 9 is a chart illustrating a series of mixed bitumen and solvent
viscosity-
temperature curves at the edge of an infill well and viscosity changes for
different stages of a
diffusion controlled mobilization process at a constant injection pressure;
[0021] FIGS. 10(a) to 10(g) are cross-sectional elevation views of adjacent
SAGD well-
pairs during operation of a diffusion controlled mobilization process for a
single infill well
between the SAGD well pairs; and
[0022] FIG. 11 is a flow chart illustrating operations performed in a
diffusion controlled
mobilization process for extracting bitumen at an infill well.
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DETAILED DESCRIPTION
[0023] There is provided a method of operating an infill or step-out well
for in situ bitumen
recovery from a bitumen reserve, wherein the infill or step-out well is
located adjacent at least
one SAGD well pair in the bitumen reserve. A solvent can be injected at the
infill or step-out
well when a region of the bitumen reserve near the infill or step-out well is
at a first temperature.
A second solvent can be injected at the infill or step-out well when the
region of the bitumen
reserve is at a second temperature greater than the first temperature. The
second solvent has
an associated boiling point that is higher than the boiling point of the first
solvent.
[0024] In some implementations, the method includes recovering bitumen at
the infill or
step-out well. The method also can include, in other implementations,
injecting at least one
additional solvent at the infill or step-out well as the region of the bitumen
reserve near the infill
or step-out well increases in temperature beyond the second temperature. In
such
implementations, each additional solvent comprises a higher boiling point than
a previously
injected solvent.
[0025] In other implementations, the method includes determining the first
temperature and
the second temperature in order to determine the first and second solvents to
be used. For
example, at least one of the first temperature and the second temperature can
be determined
via simulation or modeling a temperature profile in the bitumen reserve
relative to the at least
one SAGD well pair. Similarly, the first temperature and the second
temperature can be
determined by obtaining a measurement from a temperature sensing device
located in or near
the region of the bitumen reserve.
[0026] Turning now to the figures, FIG. 1 illustrates an example of a SAGD
production site
at a surface location 12 in a particular geographical region. The SAGD
production site 10 is
positioned to allow one or more SAGD well-pairs 14 to be drilled from the
surface location 12
towards a bitumen reserve (i.e., the pay 24). The one or more SAGD well-pairs
14 include an
injector well 16 configured to inject steam into the pay 24, positioned above
a producer well 18
configured to recover a bitumen-containing fluid that has been mobilized by
the injected steam.
The injector well 16 is typically located about 4 to 6 meters above the
producer well 18, although
a shorter or longer distance is possible, as is a lateral offset. The one or
more SAGD well-pairs
14 are drilled vertically into the overburden 22 towards and into the
underlying pay 24, and as
they are drilled become oriented substantially horizontally, such that the
producer well 18 is
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above but near the formation 26 underlying the pay 24 (hereinafter the
"underlying formation
26"). The one or more SAGD well pairs 14 are operated using surface production
equipment
20. After determining the surface location 12 and production site 10, and
determining where the
one or more SAGD well-pairs 14 will be located at the production site 10
(e.g., by conducting
typical computer simulations using geological and reservoir data), the
corresponding locations
of the production site 10 are drilled, as is known in the art.
[0027] After drilling the wells 16, 18, the surface production equipment 20
is installed in one
or more production facilities for operating the one or more SAGD well pairs
14. Completing a
particular well for production can involve several steps, as is known in the
art. To access the
target pay 24, perforating is performed to create holes through the well's
casing and cement,
which can be performed before or after production tubing is installed in the
wells 16, 18.
Alternatively, the pay section of the well can be lined with a slotted liner
or other form of sand
control that is not cemented in place. The liner can utilize packers and
inflow or ICDs that divide
the injector or production wells into segments. The production tubing is then
installed using the
service rig. In addition to production tubing, the operator may install
downhole instrumentation
that can include temperature sensors, pressure sensors or fiber optic cable.
Once the tubing
has been landed, a wellhead is installed over the production casing.
[0028] Typically, multiple SAGD well pairs 14 are drilled from the surface
location 12 into
the subsurface hydrocarbon-bearing formation to recover bitumen within the
particular
geographical area. FIG. 2 illustrates multiple SAGD well pairs 14 used to
extract a targeted
region of the pay 24, the view in FIG. 2 being towards and along the ends of
the horizontal
portions of the injector and producer wells 16, 18. It will be appreciated
that three SAGD well
pairs 14 are shown for illustrative purposes only and more or fewer SAGD well
pairs 14 can be
employed in different implementations.
[0029] As illustrated in FIG. 2, during production, each SAGD well pair 14
develops a steam
chamber 30 as high pressure steam is injected into the injector well 16, as is
known in the art.
The steam chamber 30 grows vertically and horizontally in the formation
containing the pay 24,
e.g., as shown in FIG. 2. The steam injected into the injector well 16 heats
the bitumen in the
pay 24 reducing the viscosity of the bitumen which allows bitumen containing
fluid to flow along
the wall of the steam chamber 30 down towards and into the lower producer well
18 with the
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assistance of gravity. That is, the mobilized bitumen flows into the producer
well 18 along with
any water resulting from the condensation of the injected steam (i.e. bitumen
containing fluid).
[0030] While each SAGD well pair 14 operates to heat bitumen to each side
of the steam
injector well, there typically exists an "inter-well-pair region" (denoted by
numeral 32) between
adjacent SAGD well pairs 14 where bitumen cannot be easily extracted using the
SAGD well
pairs 14 due to the way in which the steam chambers 30 develop for adjacent
well pairs 14. For
example, adjacent steam chambers 30a, 30b may coalesce near the top as the
steam
chambers 30a, 30b grow horizontally, with bitumen being stranded below. The
distance
between adjacent SAGD well pairs 14 can also be affected by efforts to
economically balance
the costs associated with having more SAGD well pairs 14 against both heat
losses to the
overburden 22 when the SAGD well pairs 14 are spaced far from each other, and
the ability of
the SAGD well pairs 14 to drain the targeted pay 24 when spaced in this way.
It should be
noted that the inter-well-pair regions 32 are in contrast to interwell
regions, which typically refer
to the regions between the injector well 16 and producer well 18 of a SAGD
well pair 14.
[0031] FIG. 2 illustrates a first inter-well-pair region 32a located
between a first SAGD well
pair 14a and a second SAGD well pair 14b, and a second inter-well-pair region
32b located
between the second SAGD well pair 14b and a third SAGD well pair 14c in this
illustrative
example. To produce bitumen in these areas between adjacent SAGD well pairs
14, infill wells
40 are used, which are drilled into the inter-well-pair regions 32 between the
adjacent SAGD
well pairs 14 as illustrated in FIG. 3.
[0032] Turning now to FIG. 3, a first infill well 40a is located between
the first SAGD well
pair 14a and the second SAGD well pair 14b in the first inter-well-pair region
32a, and a second
infill well 40b is located between the second SAGD well pair 14b and the third
SAGD well pair
14c in the second inter-well-pair region 32b. Typically the infill wells 40
are positioned
approximately halfway between adjacent SAGD well pairs 14 although other
locations are
possible depending, e.g., on the physical characteristics of the inter-well-
pair regions 32.
[0033] Although the examples described below are directed towards producing
bitumen at
infill wells 40 located between adjacent SAGD well pairs 14, it can be
appreciated that the
principles described herein can equally be applied to step-out wells 44, as
illustrated in FIG. 3.
The step-out well 44 is similar in configuration in this example to the infill
wells 40a, 40b but is
located adjacent a single SAGD well pair 14 as opposed to being located
between two SAGD
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well pairs 14. For example, a step out well 44 can be positioned at the outer
edges of the
geographical region of the pay 24 being produced that is beyond either of the
outermost SAGD
well pairs 14. In the implementation shown in FIG. 3, the step-out well 44 is
located at a
distance d2 from the third SAGD well pair 14c, which is less than a distance
dl between the infill
well 40b and the third SAGD well pair 14c. The distance d2 is chosen to be
less than dl in this
example since only the third SAGD well pair 14c contributes diffused heat in
the area of the
step-out well 44 with correspondingly less heat contribution than that
contributed to an infill well
40, which is situated between two SAGD well pairs 14.
[0034] To increase early-time bitumen production at the infill wells 40a,
40b, solvent is
injected into the infill wells 40a, 40b according to a diffusion controlled
mobilization process.
The solvent injection at the infill wells 40a, 40b can occur without steam
having been injected
into the reserve through the infill wells 40a, 40b, such that a bitumen
containing fluid can be
produced from a region of the bitumen reserve in the absence of steam
injection into the region
of the bitumen reserve, e.g., during such an early-time bitumen production
phase. It can be
appreciated that the solvent can be heated similar to the NSolvTM process,
with the heated
solvent being injected. The solvent is injected in a liquid state and
penetrates the pay 24
surrounding the infill wells 40a, 40b before vaporizing at deeper zones,
thereby creating a
region of solvent 42a, 42b around each infill well 40a, 40b. Each region of
solvent 42 has a
diffusion boundary 43 at the periphery of the region 42. The diffusion
boundary 43 represents
substantially the outermost boundary of the region of solvent 42, within which
diffusion of the
solvent occurs. The diffusion boundary 43 can be used to determine a relative
volume that
would be consumed, for different solvents being injected (as described in
greater detail below).
Therefore, the larger the region of solvent 42 defined by the diffusion
boundary 43, the larger
the volume of solvent that penetrates the pay 24, and the higher volume of
solvent that would
be consumed. As such, injected solvents having a closer diffusion boundary 43
and smaller
region of solvent 42 consume less injected solvent. In the example shown in
FIG. 3, the first
infill well 40a includes a first region of solvent 42a with a corresponding
diffusion boundary 43a
at the periphery of the first region of solvent 42a. Similarly, the second
infill well 40b includes a
second region of solvent 42b with a corresponding diffusion boundary 43b at
the periphery of
the second region of solvent 42b. Moreover, in this example, the step-out well
44 can also be
operated according to the diffusion controlled mobilization process described
herein, and
includes a region of solvent 46 with a corresponding diffusion boundary 45.
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[0035] The implementation shown in FIG. 3 illustrates a single infill well
40 and single step-
out well 44 configuration in which a cyclic solvent injection process is used.
For a cyclic solvent
injection process, solvent is injected for a first period of time to create
the region of solvent 42,
46, followed by a second period of time during which bitumen is mobilized and
produced at the
same infill well 40 and step-out well 44 respectively.
[0036] As illustrated in FIG. 4, it can be appreciated that an infill well
pair 50 can also be
used in other implementations. Shown in FIG. 4 are a first infill well pair
50a, having a first
solvent injector well 52a and a first infill producer well 54a; and a second
infill well pair 50b,
having a second solvent injector well 52b and a second infill producer well
54b. With an infill
well pair 50, the solvent is injected into the injector wells 52a, 52b, and
bitumen is produced at
the producer wells 54a, 54b. The infill well pair 50 also develops a region of
solvent 53 from the
injection of solvent into the pay 24, with a corresponding diffusion boundary
55 at the periphery
of the region of solvent 53. In the example shown in FIG. 4, the first infill
well pair 50a creates a
first region of solvent 53a having a first diffusion boundary 55a, and the
second infill well pair
50b creates a second region of solvent 53b having a second diffusion boundary
55b. Similar to
the configuration shown in FIG. 4, a step-out well pair 56 can also be
employed adjacent the
third SAGD well pair 14c. The step-out well pair 56 includes a solvent
injector well 58 and a
production well 60. The step-out well pair 56 creates a region of solvent 57
having a diffusion
boundary 59.
[0037] As discussed above, adjacent SAGD well pairs 14 contribute to an
increasing
temperature around an infill well 40 (or infill well pair 50) and a step-out
well 44 (or step-out well
pair 56). However, during the early stages of SAGD well production, the
temperature
surrounding the infill wells 40 (or infill well pairs 50) and step-out wells
44 (or step-out well pairs
56) is relatively low and therefore only mobilizes the surrounding bitumen to
some extent. As
illustrated in the upper view A of FIG.5, heat diffusion 70 caused by
development of the steam
chambers 30a, 30b in adjacent SAGD well pairs 14a, 14b exhibits a temperature
profile 74 as
shown in view B of FIG. 5 wherein the temperature near the SAGD steam chambers
30a, 30b
tapers towards the infill well 40. It can be appreciated that a similar
tapering effect would be
experienced with respect to a step-out well 44 (or step-out well pair 56) with
heat diffusion 70
being contributed from a single SAGD well pair 14.
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[0038] The temperature at the location of the infill wells 40 (or infill
well pairs 50) and step-
out wells 44 (or step-out well pairs 56) is a function of several parameters,
such as the
progressing velocity at the edge of the steam chamber 30, steam injection
temperature, initial
reservoir temperature, distance from steam chamber 30 (d1 in FIG. 3), and
steam injection
pressure. Assuming that conduction is the only heat mechanism in a SAGD
reservoir,
according to Butler (Butler, R.M., "Thermal Recovery of Oil and Bitumen",
Englewood Cliffs,
New Jersey, 1997) the temperature at any location in front of the edge of the
steam chamber 30
can be given by the following expression:
(
T ¨Tr Ux r U pr pr
_________________ = exp _____________ = exp x 1
[0039]
Tst r ¨T ();
KThermal
[0040] where, T is the temperature at a location in front of the edge of
the steam chamber,
Tr is the initial reservoir temperature, Tst is the steam injection
temperature, is the distance
from the edge of the steam chamber, Ux is the moving velocity of the steam
chamber interface,
K is the reservoir thermal conductivity, cpr is the reservoir heat capacity,
pr is the reservoir
density, and ICThermal is the thermal diffusivity of the bitumen. It can be
appreciated that while
Equation 1 can be used for estimations of temperature beyond the steam chamber
edge,
temperatures at or near infill wells 40 (or infill well pairs 50) and step-out
wells 44 (or step-out
well pairs 56) can also be evaluated using monitoring and/or measurement
techniques, e.g., by
having thermocouples, fiber optic cabling, or other temperature sensing
devices incorporated
along the length of the infill wells 40 (or infill well pairs 50) and step-out
wells 44 (or step-out
well pairs 56), e.g., inside the liner casing.
[0041] The introduction of solvents into the infill well 40, step-out well
46, infill injection
well 52, or solvent injection well 58 enables production to occur at the
infill wells 40 (or infill well
pairs 50) and step-out wells 44 (or step-out well pairs 56) at an earlier
stage compared to
production techniques where solvent injection is not employed. In one
embodiment, the solvent
is injected at a lower pressure than the steam injection pressure in the
adjacent SAGD steam
chambers 30. Injecting the solvent at a lower pressure than the steam
injection pressure can
contribute to enhancing the containment of the injected solvent around the
infill well 40, since
the pressure difference between the solvent injection pressure and the steam
injection pressure
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hinders or otherwise mitigates the migration of the injected solvent within
the formation. In other
words, the pressure gradient created in the region between the steam chamber
and the injection
point of the solvent can advantageously inhibit the solvent from migrating to
other portions of the
formation, thus enabling a more efficient recovery of the solvent and the
mobilized bitumen
containing fluids. For example, in the embodiment shown in FIG. 6, view A, the
solvent injection
pressure Pso is less than the steam pressure Pst in the first and second steam
chambers 30a,
30b. Similar principles apply to an infill well pair 50 shown in FIG. 6, view
B.
[0042] It has been recognized that the presently described diffusion
controlled mobilization
process can utilize a set of multiple solvents having different "weights"
(i.e. being relatively
lighter and relatively heavier), each of the solvents having a corresponding
boiling point. An
example of such a set of solvents is a set of alkane-based solvents. The
diffusion controlled
mobilization process can contribute to earlier production at infill and step-
out wells 40, 44, while
minimizing solvent loss and the costs associated with the solvents, by
progressing from lighter
to heavier solvents as the temperature surrounding the infill and step-out
wells 40, 44 rises.
The diffusion controlled process can achieve this by utilizing solvents from
the set according to
current (or estimated to be current) temperatures near the infill well 40 (or
infill well pair 50) and
step-out well 44 (or step-out well pair 56). For example, a relatively lighter
alkane such as
propane (i.e. relatively lower carbon solvent) can be injected into the infill
well 40 at an early
stage, and at subsequent temperature thresholds, progressively heavier alkanes
such as
butane, etc. (i.e. progressively higher carbon solvent) can be injected
according to the
increasing temperature around the infill well 40. In general, less solvent is
required at higher
temperatures, since heavier solvents used at higher temperatures intrinsically
have relatively
small diffusion boundaries.
[0043] An example of a set of solvents having multiple levels and
associated boiling points
is a set of alkanes ranging from a relatively lighter alkane such as methane
to a relatively
heavier alkane such as heptane, e.g., a set of solvents including methane
(CH4), ethane (C2116),
propane (C3H8), butane (C4H10), pentane (C51-112), hexane (C6H14), and heptane
(C7I-116), etc.
Other examples of sets of solvents can be selected from n (normal) and iso-
alkanes according
to the boiling points of such solvents. Similarly, other sets of solvents can
be chosen from the
following solvents, based on the boiling points and, in some cases the costs,
of the respective
solvents, for example: naphtha, toluene, xylene, benzene, diesel, natural gas,
etc.
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[0044] In the present example, the solvent is selected based on the
temperature and
pressure at which the solvent is in the liquid state. As previously described,
the solvent is
injected in the liquid state, e.g., from a truck at surface 12. It is expected
that near the end of the
injection stage for a particular solvent, due to an increase in temperature
surrounding the infill
wells 40, the solvent will be in a gaseous state where the temperature is
higher than the
condensation temperature of the solvent under a given pressure. As such, the
liquid solvent
initially diffuses into the formation at a larger flux due a larger
concentration that is exposed to
the bitumen. Once the solvent is vaporized, the flux of solvent diffusion
decreases due to less
solvent being available at the interface with the bitumen. The mobilized
bitumen, due to solvent
dissolution in the surrounding bitumen is produced in a mixture of liquid
solvent and bitumen.
[0045] Accordingly, as the temperature surrounding the infill well 40 is
increased, heavier
carbon solvents are generally used, since the condensation temperature of such
solvents tend
to be higher when compared to that of lighter solvents under a given pressure.
As will be
appreciated, the use of heavier solvents at higher temperatures generally
ensures that a liquid
solvent is exposed to the bitumen, thereby creating a liquid-liquid interface
between the solvent
and the bitumen which enables larger diffusion and dissolution resulting in
larger production of
bitumen from the formation. In other words, lighter carbon solvents are
generally used during
the early stages of SAGD production when temperatures surrounding the infill
well 40 are lower
and more solvent is required to mobilize the bitumen. As such, where the
diffusion boundary
43, 55 is further from the infill well 40 during use of the lower carbon
solvents, and thus more
solvent is used, losses are attributed to the less costly solvents, which can
improve pad
economics.
[0046] For constant boundary conditions, the diffusion boundary ( 6Diff )
can be defined as
8Diff = V4Dt , where 6Diff is the diffusion length, boundary or front
(referred to herein as
"diffusion boundary"), and D is a diffusion coefficient or diffusivity in
dimensions of [length2
time-1], for example m2/sec . The diffusion boundary provides a measure of how
far the
concentration has propagated in the x-direction by diffusion in time t (Bird,
R.B., Stewart, W.E.,
Lightfoot, E.N., "Transport Phenomena", John Wiley & Sons, 1976).
[0047] The Wilke-Chang equation (Wilke C.R., Chang, P., "Correlation Of
Diffusion
Coefficients in Dilute Solutions", A.I.Ch.E. Journal, 1: 264-270, 1955) can be
used for estimating
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CA 02875846 2014-12-22
=
the diffusivity of nonelectrolytes (i.e., hydrocarbon solvent) in an
infinitely dilute solution (i.e.,
bitumen):
Vc (T+273.15)
[0048] D=7.4 x10¨ ________________
=
[1Bitu v Solv
[0049] where D is diffusion coefficient (cm2/sec), 4:13 is association
factor of bitumen,
MBitu is molecular weight of bitumen (i.e., 500-550), JtBjtu is viscosity of
the bitumen (cP) and
Vsov is molal volume of solvent at normal boiling point (cc/g.mole). Based on
current available
data, the temperature dependence of the diffusion coefficient can be assumed
to be linear.
Linear correlation is proposed in other correlations such as Stokes-Einstein
(Einstein, Albert,
Ann. Phys. 17, 549, 1905 and Miller C.C., "The Stokes-Einstein Law for
Diffusion in Solution",
Proceedings of the Royal Society of London. Series A, Containing Papers of a
Mathematical
and Physical Character, Vol. 106, No. 740, pp. 724-749, 1924) or close to
linear in other
correlations such as in Sitaraman (Sitaraman R., Ibrahim S. H., Kuloor N. R.
"A Generalized
Equation for Diffusion in Liquids" J. Chem. Eng. Data, 1963, 8 (2), pp 198-201
doi:10.1021/je60017a017). The formulae suggested above for calculating the
diffusion
coefficient are expected to hold true for low-viscosity liquids but to
introduce error for a high-
viscosity solvent. However, the solute (i.e., bitumen) the solvents use in the
present diffusion
controlled mobilization process are light and less than 10 cP viscosity.
[0050] The Wilke-Chang equation shows that by increasing the temperature
surrounding
the infill well, the diffusion increases linearly. It is noted that since the
diffusion is increasing
inversely by viscosity of the bitumen ( mu ), the diffusion reduces for
higher temperatures.
[0051] An increase in diffusion means that more solvent is diffusing and
8Diff (diffusion
length) is increasing by heating up the area surrounding the infill wells 40.
This can be
interpreted as solvent loss with no return. In the present diffusion
controlled mobilization
process, by increasing the temperature in the area surrounding the infill
wells, a heavier solvent
can be used. The advantage of using a heavier solvent is to reduce diffusion
after an increase
in temperature at the infill well surroundings. One can examine this using
Wilke-Chang
equation by substituting greater molal volume (Vsov ). This limits solvent
loss at larger
temperatures. In the present diffusion controlled mobilization process, rather
than using a
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CA 02875846 2014-12-22
heavier solvent at the lower temperatures, the use of the progressively
heavier solvents
beginning with a lighter solvent enables the mobilization of a larger volume
of bitumen at any
time, since in early-stages the process allows lighter components to mobilize
the bitumen further
away from the infill wells. The lighter solvents mobilize the deeper bitumen
zones and, at a later
time, the heat reaches closer to the infill well surroundings at which time a
heavier solvent can
be used to mobilize bitumen closer to the infill wells 40. It can be
appreciated that the infill wells
40 are generally located at the furthest distance from the steam chambers 30.
Therefore, as
time progresses, and the steam chambers 30 increase in size, the area
surrounding the infill
wells 40 should be mobilized to a greater extent, which is desirable. The
diffusion controlled
mobilization process described herein provides the capability of controlling
diffusion boundaries
at any stage during production.
[0052] It can be appreciated that the relative volume of solvent that
penetrates the pay 24
can also be modeled according to dispersion of the solvent, which is a
combination of diffusion
and convection and has a linear relationship with diffusion. That is, the
relative volume of each
type of solvent can also be modeled by way of a dispersion boundary. Such a
dispersion
boundary can be estimated using a constant multiplier applied to the above-
described diffusion
coefficient D, as would be understood by those skilled in the art.
[0053] As shown in FIG. 7, the sequence of solvent injection stages in the
illustrated
implementation follows vapor pressure-temperature curves. At each stage, a
particular solvent
is selected according to the temperature (e.g., to select a heavier solvent as
the temperature
increases). FIG. 7 illustrates that for the solvent injection process, the
solvent is injected as a
liquid, e.g., from a truck at surface 22, and as a result of an increasing
temperature, the solvent
vaporizes for part of each stage. Vaporized solvent has less solubility due to
having less
concentration of the solvent being exposed to the bitumen, which decreases the
efficiency of the
solvent. The decrease in efficiency is shown in FIGS. 8 and 9. At the time of
experiencing the
decrease in efficiency, a heavier solvent should then be used to increase the
efficiency and
continue to mobilize bitumen to a lower viscosity as can be seen in FIGS. 8
and 9.
[0054] An advantage of the diffusion controlled mobilization process
described herein
stems from the process beginning earlier (that is, at lower temperatures) with
relatively lighter
solvents such as C3 and C4. By increasing the surrounding temperature of the
infill well 40,
progressively heavier solvents are then used, e.g., C6 and C7. The advantage
of this lighter to
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heavier progression of solvents comes from the nature of the diffusion
mechanism in lighter
versus heavier solvents. That is, at the early stages in which the temperature
of the area
surrounding the infill well 40 is lower, the correspondingly lighter solvents
are used such as C3
and C4, which are less expensive in comparison to heavier solvents such as C6
and C7. The
diffusion boundary layer 43 can be significantly larger for lighter solvents,
meaning that the
correspondingly larger solvent loss is offset by the lower cost. As such, for
early stages in
which SAGD operators have less control over aspects of the process such as
temperature
variations along the infill well 40, less expensive solvents are being used.
However, as the pay
24 surrounding the infill well 40 continues to be heated, heavier solvents are
used. The closer
diffusion boundary 43 inherent in the heavier components means that less of
the more
expensive solvent is lost. For example, solvents such as C6 and C7 typically
have a diffusion
boundary 43 of a few centimeters when compared to lighter solvents where the
diffusion
boundary 43 can be up to 10 meters. Heavier solvents are injected at higher
pressures due to
steam chamber advancement and higher steam chamber pressures and, as such, the
solvent is
contained around the infill wells 40 thus further reducing solvent loss. As a
result, operating at a
relatively lower pressure than the steam chamber pressure can further limit
solvent loss.
[0055] As shown in FIG 8 a heavier solvent used at a higher temperature
decreases
viscosity which is not attainable with lighter solvents. The temperature at
the infill wells 40 is
increasing simultaneously with an increase in pressure. In early stages of
production, the lighter
components can be injected with a lower pressure and for larger components,
the pressure can
be increased to assist with containment of the lighter solvents. It may be
noted that in
reservoirs with higher water mobility, pressure may rise more quickly in which
case the injection
pressure should be kept constant during the process. FIG 9 illustrates the
diffusion controlled
mobilization process for constant injection, which boosts the production by
decreasing viscosity,
but can result in solvent loss at the early stages.
[0056] FIGS. 10(a) to 10(g) illustrate an example implementation of the
diffusion controlled
mobilization process in which a set of solvents is cycled as an expected or
detected
temperature near an infill well 40 is increased during SAGD production.
[0057] FIG. 10(a) illustrates the beginning of a SAGD operation in which
the first SAGD
well pair 14a and the second SAGD well pair 14b have been drilled but have not
yet begun
production. In FIG. 10(b), the first and second steam chambers 30a, 30b begin
to develop after
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start-up of the SAGD well pairs 14a, 14b during which the respective inter-
well regions between
the SAGD injector and production wells 16, 18 are heated. In addition to
developing the steam
chambers 30a, 30b, the steam injected at the injector wells 16a, 16b
contributes to heat
diffusion 70 into the pay 24 beyond the steam chambers 14a, 14b and towards
the inter-well-
pair region 32a. An infill well 40 can be drilled at a particular stage of
production for the SAGD
well pairs 14a, 14b, e.g., as shown in FIG. 10(c).
[0058] By monitoring, simulating, measuring, or estimating the temperature
profile between
adjacent SAGD well pairs 14a, 14b; after detecting a first temperature
threshold A as illustrated
in FIG. 10(d), solvent injection commences at the infill well 40. Since the
temperature near the
infill well 40 at the early stages is relatively low, a lighter solvent such
as propane (C3 in FIG.
10(e)) can be selected and used. While the diffusion boundary 43 for C3 is
further from the infill
well 40 and a larger region of solvent 42 develops (with a correspondingly
larger amount of C3
solvent used), the cost associated with C3 is lower than heavier, higher
carbon solvents, which
are required at higher temperatures. Therefore, the amount of solvent used,
and any losses
associated with use of the C3 solvent would be offset by the lower cost.
[0059] As the temperature surrounding the infill well 40 continues to
increase during SAGD
production at the adjacent SAGD well pairs 14a, 14b, a heavier solvent such as
butane (C4 in
FIG. 10(f)) is selected. The diffusion boundary 43 is closer to the infill
well for the heavier
solvent resulting in a relatively smaller region of solvent 42 and
correspondingly less solvent
being used in this stage.
[0060] Therefore, less of the more costly solvent is used, and losses are
reduced when
compared to a prior stage which uses a lighter solvent. Moreover, since the
progressively
heavier solvents are progressively more contained around the infill well 40,
the ability to recover
and recycle the solvents is also increased as the heavier and more costly
solvents are used.
[0061] FIG. 10(g) illustrates yet another stage, in this example using
hexane (C6 in FIG.
10(g)) with a relatively closer diffusion boundary 43 thus further minimizing
losses of costlier
solvents as the temperature continues to increase.
[0062] As illustrated in FIGS. 10(b) through 10(g), the steam chambers 30a,
30b continue
to enlarge while the SAGD well pairs 14a, 14b are in production. Since
mobilization of the
bitumen in the pay 24 surrounding the infill well 40 can begin earlier by
using solvents that are
effective at lower temperatures, the bitumen surrounding the infill wells is
mobilized more
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quickly and less steam is used, resulting in an improved steam-to-oil ratio
(SOR), and providing
an additional economic benefit. Furthermore, pay 24 can be produced in the
inter-well-pair
regions 32 prior to the steam chambers 30a, 30b coalescing, which can also
contribute to an
improved SOR by reducing the amount of time required to operate the SAGD well
pairs 14.
[0063] Turning now to FIG. 11, a flow chart is provided illustrating an
example of the
diffusion controlled mobilization process described herein. In this example,
optional step 102
can be performed in a planning process which includes determining when to
drill one or more
single infill well(s) 40 (or infill well pairs 50) and/or one or more step-out
wells 44 (or step-out
well pairs 56). It will be appreciated that the infill and step-out wells 40,
44 can also be drilled
according to a production schedule and/or can exist at the time of commencing
SAGD
operations. I will also be appreciated that the following principles equally
apply to infill well pairs
50 and step-out wells and well pairs 56, 56. For ease of illustration, the
following example
refers to operation of a single infill well 40.
[0064] A start-up temperature at which to begin drilling the infill well 40
can also be used to
determine when to begin drilling the infill well 40, at which time the infill
well 40 is drilled when
the start-up temperature is estimated or measured at step 102. It can be
appreciated that step
102 is optional in that in at least some implementations, the process
described herein can be
applied to already drilled infill wells 40 and step-out wells 46. Similarly,
infill and step-out wells
40, 46 in at least some implementations are drilled according to other factors
such as the
availability of equipment and other scheduling constraints.
[0065] At step 104, the temperature near the infill well 40 is determined,
e.g., via
measurements using temperature sensors, simulations, mathematical modeling or
predictions,
etc. In early production, a lower threshold may be used to trigger the
injection of the solvent at
the infill well 40. At step 106 a solvent is selected according to the
determined temperature and
solvent is injected at the infill well 40 at a lower pressure than the steam
pressure in the steam
chambers 30a, 30b of the adjacent SAGD well pairs 14a, 14b at step 108.
[0066] Bitumen is then extracted at the infill well 40 at step 110 and, if
applicable, solvent is
recovered at step 112, which can be reused in step 108 during the stage
associated with the
selected solvent. That is, the recovered solvent can be used while the same
type of solvent is
being injected at step 108. The process repeats by determining a next
temperature at which to
begin using a higher carbon solvent.
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[0067] For simplicity and clarity of illustration, where considered
appropriate, reference
numerals may be repeated among the figures to indicate corresponding or
analogous elements.
In addition, numerous specific details are set forth in order to provide a
thorough understanding
of the examples described herein. However, it will be understood by those of
ordinary skill in the
art that the examples described herein may be practiced without these specific
details. In other
instances, well-known methods, procedures and components have not been
described in detail
so as not to obscure the examples described herein. Also, the description is
not to be
considered as limiting the scope of the examples described herein.
[0068] It will be appreciated that the examples and corresponding diagrams
used herein are
for illustrative purposes only. Different configurations and terminology can
be used without
departing from the principles expressed herein. For instance, components and
modules can be
added, deleted, modified, or arranged with differing connections without
departing from these
principles.
[0069] The steps or operations in the flow charts and diagrams described
herein are just for
example. There may be many variations to these steps or operations without
departing from the
principles discussed above. For instance, the steps may be performed in a
differing order, or
steps may be added, deleted, or modified.
[0070] Although the above principles have been described with reference to
certain specific
examples, various modifications thereof will be apparent to those skilled in
the art as outlined in
the appended claims.
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