Note: Descriptions are shown in the official language in which they were submitted.
HYDROCARBON RESOURCE HEATING SYSTEM INCLUDING CHOKE FLUID
DISPENSER AND RELATED METHODS
Field of the Invention
[0001] The present invention relates to the field of
hydrocarbon resource recovery, and, more particularly, to
hydrocarbon resource recovery using RF heating.
Background of the Invention
[0002] Energy consumption worldwide is generally
increasing, and conventional hydrocarbon resources are being
consumed. In an attempt to meet demand, the exploitation of
unconventional resources may be desired. For example, highly
viscous hydrocarbon resources, such as heavy oils, may be
trapped in tar sands where their viscous nature does not
permit conventional oil well production. Estimates are that
trillions of barrels of oil reserves may be found in such tar
sand formations.
[0003] In some instances these tar sand deposits are
currently extracted via open-pit mining. Another approach for
in situ extraction for deeper deposits is known as Steam-
Assisted Gravity Drainage (SAGD). The heavy oil is immobile at
reservoir temperatures and therefore the oil is typically
heated to reduce its viscosity and mobilize the oil flow. In
SAGD, pairs of injector and producer wells are formed to be
laterally extending in the ground. Each pair of
injector/producer wells includes a lower producer well and an
upper injector well. The injector/production wells are
typically located in the pay zone of the subterranean
formation between an underburden layer and an overburden
layer.
[0004] The upper injector well is used to typically
inject steam, and the lower producer well collects the
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heated crude oil or bitumen that flows out of the formation,
along with any water from the condensation of injected steam.
The injected steam forms a steam chamber that expands
vertically and horizontally in the formation. The heat from
the steam reduces the viscosity of the heavy crude
oil or bitumen which allows it to flow down into the lower
producer well where it is collected and recovered. The steam
and gases rise due to their lower density so that steam is not
produced at the lower producer well and steam trap control is
used to the same effect. Gases, such as methane, carbon
dioxide, and hydrogen sulfide, for example, may tend to rise
in the steam chamber and fill the void space left by the oil
defining an insulating layer above the steam. Oil and water
flow is by gravity driven drainage, into the lower producer
well.
[0005] Operating the injection and production wells at
approximately reservoir pressure may address the instability
problems that adversely affect high-pressure steam processes.
SAGD may produce a smooth, even production that can be as high
as 70% to 80% of the original oil in place (00IP) in suitable
reservoirs. The SAGD process may be relatively sensitive to
shale streaks and other vertical barriers since, as the rock
is heated, differential thermal expansion causes fractures in
it, allowing steam and fluids to flow through. SAGD may be
twice as efficient as the older cyclic steam stimulation (CSS)
process.
[0006] Many countries in the world have large deposits of
oil sands, including the United States, Russia, and various
countries in the Middle East. Oil sands may represent as much
as two-thirds of the world's total petroleum resource, with at
least 1.7 trillion barrels in the Canadian Athabasca Oil
Sands, for example. At the present time, only Canada has a
large-scale commercial oil sands industry, though a small
amount of oil from oil sands is also produced in Venezuela.
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Because of increasing oil sands production, Canada has become
the largest single supplier of oil and products to the United
States. Oil sands now are the source of almost half of
Canada's oil production, although due to the 2008 economic
downturn work on new projects has been deferred, while
Venezuelan production has been declining in recent years. Oil
is not yet produced from oil sands on a significant level in
other countries.
[0007] U.S. Published Patent Application No. 2010/0078163
to Banerjee et al. discloses a hydrocarbon recovery process
whereby three wells are provided, namely an uppermost well
used to inject water, a middle well used to introduce
microwaves into the reservoir, and a lowermost well for
production. A microwave generator generates microwaves which
are directed into a zone above the middle well through a
series of waveguides. The frequency of the microwaves is at a
frequency substantially equivalent to the resonant frequency
of the water so that the water is heated.
[0008] Along these lines, U.S. Published Application No.
2010/0294489 to Dreher, Jr. et al. discloses using microwaves
to provide heating. An activator is injected below the surface
and is heated by the microwaves, and the activator then heats
the heavy oil in the production well. U.S. Published
Application No. 2010/0294489 to Wheeler et al. discloses a
similar approach.
[0009] U.S. Patent No. 7,441,597 to Kasevich discloses
using a radio frequency generator to apply RF energy to a
horizontal portion of an RF well positioned above a horizontal
portion of an oil/gas producing well. The viscosity of the oil
is reduced as a result of the RF energy, which causes the oil
to drain due to gravity. The oil is recovered through the
oil/gas producing well.
[0010] Unfortunately, long production times, for example,
due to a failed start-up, to extract oil using SAGD may lead
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to significant heat loss to the adjacent soil, excessive
consumption of steam, and a high cost for recovery.
Significant water resources are also typically used to recover
oil using SAGD, which impacts the environment. Limited water
resources may also limit oil recovery. SAGD is also not an
available process in permafrost regions, for example.
[0011] Despite the existence of systems that utilize RF
energy to provide heating, such systems suffer from the
inevitable high degree of electrical near field coupling that
exists between the radiating antenna element and the
transmission line system that delivers the RF power to the
antenna, resulting in common mode current on the outside of
the transmission line. Left unchecked, this common mode
current heats unwanted areas of the formation, effectively
making the transmission line part of the radiating antenna.
One system which may be used to help overcome this problem is
disclosed in U.S. App. Serial No. 14/167,039 filed January 29,
2014, which is also assigned to the present. This reference
discloses a system for heating a hydrocarbon resource in a
subterranean formation having a wellbore extending therein
which includes a radio frequency (RF) antenna configured to be
positioned within the wellbore, an RF source, a cooling fluid
source, and a transmission line coupled between the RF antenna
and the RF source. A plurality of ring-shaped choke cores may
surround the transmission line, and a sleeve may surround the
ring-shaped choke cores and define a cooling fluid path for
the ring-shaped choke cores in fluid communication with the
cooling fluid source.
[0012] Despite the advantages of such systems, further
approaches to common mode current mitigation may be desirable
in some circumstances.
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Summary of the Invention
[0013] A system for heating a hydrocarbon resource in a
subterranean formation having a wellbore extending therein may
include a radio frequency (RF) source, a choke fluid source,
and an elongate RF antenna configured to be positioned within
the wellbore and coupled to the RF source, with the elongate
RF antenna having a proximal end and a distal end separated
from the proximal end. The system may also include a choke
fluid dispenser coupled to the choke fluid source and
positioned to selectively dispense choke fluid into adjacent
portions of the subterranean formation at the proximal end of
the RF antenna to define a common mode current choke at the
proximal end of the RF antenna.
[0014] More particularly, the choke fluid may comprise an
electrical conductivity enhancing fluid, such as water, for
example. Furthermore, the RF antenna may include a cylindrical
conductor, and the system may further include an RF
transmission line extending at least partially within the
cylindrical conductor and coupling the RF source to the RF
antenna. Furthermore, the choke fluid dispenser may be carried
by the transmission line and include an inner sleeve
surrounding the RF transmission line, a liner surrounding the
inner sleeve and defining a first annular chamber therewith,
the liner having a plurality of ports therein in fluid
communication with the choke fluid source, and an outer sleeve
surrounding the liner and defining a second annular chamber
therewith to receive choke fluid from the plurality of ports.
The outer sleeve may have a plurality of openings therein to
pass choke fluid from the annular chamber into the
subterranean formation adjacent the antenna. Moreover, the
inner sleeve may be slidably moveable with respect to the
liner, and the liner may be fixed to the outer sleeve.
[0015] The choke fluid dispenser may further include a
respective seal at opposing ends of the inner sleeve. The RF
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antenna may comprise a cylindrical conductor having a
plurality of collection openings therein to collect
hydrocarbon resources from adjacent portions of the
subterranean formation, and the choke fluid dispenser may be
positioned in spaced relation from the collection openings.
[0016] A related choke fluid dispenser, such as the one
described briefly above, and method for heating a hydrocarbon
resource in a subterranean formation having a wellbore
extending therein are also provided. The method may include
applying radio frequency (RF) power to an elongate RF antenna
positioned within the wellbore using an RF source, the
elongate RF antenna having a proximal end and a distal end
separated from the proximal end. The method may further
include selectively dispensing choke fluid from a choke fluid
source into adjacent portions of the subterranean formation
via a choke fluid dispenser positioned in the wellbore at the
proximal end of the RF antenna to define a common mode current
choke at the proximal end of the RF antenna.
Brief Description of the Drawings
[0017] FIG. 1 is a schematic diagram, partially in section,
of a system for heating a hydrocarbon resource in accordance
with an example embodiment including a choke fluid dispenser.
[0018] FIG. 2 is a side view of the downhole antenna
portion of the system of FIG. 1 illustrating a region of
desiccation adjacent the RF antenna.
[0019] FIGS. 3(a)-3(f) are a series of time-lapsed
simulated cross-sectional views of the desiccation region of
FIG. 2 demonstrating changes to the desiccation region over a
time period of operation of the RF antenna.
[0020] FIGS. 4(a)-4(c) are side and cross-sectional views
of the choke fluid dispenser of the system of FIG. 1
illustrating example choke fluid dispensing portions thereof.
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[0021] FIGS. 5(a)-5(c) are side and cross-sectional views
of the choke fluid dispenser of the system of FIG. 1
illustrating example end attachment and sealing configurations
thereof.
[0022] FIG. 6 is a side view, partially in section, of the
choke fluid dispenser of the system of FIG. 1 as carried
around the transmission line to allow relatively movement to
accommodate thermal expansion.
[0023] FIG. 7 is a perspective sectional view of the choke
fluid dispenser and RF transmission line of the system of FIG.
1 illustrating the various components and annuli therein.
Detailed Description of the Embodiments
[0024] The present invention will now be described more
fully hereinafter with reference to the accompanying drawings,
in which embodiments of the invention are shown. This
invention may, however, be embodied in many different forms
and should not be construed as limited to the embodiments set
forth herein. Rather, these embodiments are provided so that
this disclosure will be thorough and complete, and will fully
convey the scope of the invention to those skilled in the art.
Like numbers refer to like elements throughout.
[0025] Referring initially to FIG. 1, a system 30 for
heating a hydrocarbon resource 31 (e.g., oil sands, etc.) in a
subterranean formation 32 having a wellbore therein is first
described. In the illustrated example, the wellbore is a
laterally extending wellbore, although the system 30 may be
used with vertical or other wellbores in different
configurations. The system 30 further includes a radio
frequency (RF) source 34 for an RF antenna or transducer 35
that is positioned in the wellbore adjacent the hydrocarbon
resource 31. The RF source 34 is illustratively positioned
above the subterranean formation 32, and may be an RF power
generator, for example. In an exemplary implementation, the
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laterally extending wellbore may extend several hundred meters
(or more) within the subterranean formation 32. Moreover, a
typical laterally extending wellbore may have a diameter of
about fourteen inches or less, although larger wellbores may
be used in some implementations. Although not shown, in some
embodiments a second or producing wellbore may be used below
the wellbore, such as would be found in a SAGD implementation,
for collection of petroleum, bitumen, etc., released from the
subterranean formation 32 through heating.
[0026] Referring additionally to FIG. 7, a coaxial
transmission line 38 extends within the wellbore 33 between
the RF source 34 and the RF antenna 35. The transmission line
38 includes an inner conductor 36 and an outer conductor 37.
In some embodiments, one or more radial support members (not
shown) may be positioned between the inner and outer
conductors. The radial support members may have openings
therein which may be used to route tubes 40 for fluid, gas
flow, etc. For example, the space between the inner conductor
36 and the outer conductor 37 may be filled with an insulating
gas, such as nitrogen, if desired. Moreover, the tubes 40 may
also be used to deliver fluids such as a solvent to be
dispensed in the pay zone where the hydrocarbon resource 31 is
located, for example.
[0027] A drill tubular 42 (e.g., a metal pipe) surrounds
the outer conductor 37 and may be supported by spacers (not
shown). A space between the outer conductor 37 and the drill
tubular 42 defines a passageway 43 which may be used for
returning reservoir fluid (e.g., bitumen) back to the surface,
for example, to a well head 51, if desired. In such a
configuration, proximal and/or distal slotted liner portions
53, 56 of the antenna 35 would include a plurality of
collection openings 80 therein to collect hydrocarbon
resources 31 from adjacent portions of the subterranean
formation 32, and the choke fluid dispenser 60 may be
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positioned in spaced relation (i.e., up hole) from the
collection openings as shown, such as adjacent the heel of the
antenna 35.
[0028] However, it should be noted that the illustrated
configuration need not be used for production in all
embodiments, and that the passageway 43 could be used for
other purposes, such as to supply other fluids (e.g., cooling
fluid, etc.), or remain unused. Further details regarding
exemplary transmission line 38 support and interconnect
structures which may be used in the configurations provided
herein may be found in co-pending application nos. 13/525,877
filed June 18, 2012, and 13/756,756 filed February 1, 2013,
both of which are assigned to the present Applicant.
[0029] A surface casing 51 and an intermediate casing 52
may be positioned within the wellbore as shown. In the
illustrated example the RF antenna 35 is coupled with the
intermediate casing 52, and the RF antenna illustratively
includes a proximal slotted liner portion 53, a center
isolator 55 (i.e., a dielectric) coupled to the proximal
slotted liner portion, and a distal slotted liner portion 56
coupled to the center isolator opposite the proximal slotted
liner portion. The proximal slotted liner portion 53 and
distal slotted liner portion 56 are cylindrical conductors
(e.g., metal) in the illustrated example, and the RF
transmission line 38 extends at least partially within the
proximal slotted liner portion and couples the RF source 34 to
the RF antenna 35. By way of example, an electromagnetic
heating (EMH) tool head 58 may be carried by the drill tubular
42 to plug the transmission line 38 into the antenna 35 when
the transmission line is inserted into the wellbore. In the
illustrated example, the EMH tool head 58 includes a guide
string attachment 59, although other EMH or antenna attachment
arrangements may be used in different embodiments.
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[0030] The RF source 34 may be used to differentially drive
the RF antenna 35. That is, the RF antenna 35 may have a
balanced design that may be driven from an unbalanced drive
signal. Typical frequency range operation for a subterranean
heating application may be in a range of about 100 kHz to 10
MHz, and at a power level of several megawatts, for example.
However, it will be appreciated that other configurations and
operating values may be used in different embodiments. The
transmission line 38 and tubular 42 may be implemented as a
plurality of separate segments which are successively coupled
together and pushed or fed down the wellbore.
[0031] The system 30 further illustratively includes a
choke fluid dispenser 60 coupled to the transmission line 38
adjacent the RF antenna 35 within the wellbore. The RF antenna
35 may be installed in the well first, followed by the
transmission line (and choke assembly 60) which is plugged
into the antenna via the EMH tool head 59, thus connecting the
transmission line to the antenna. Further details on an
exemplary antenna structure which may be used with the
embodiments provided herein is set forth in co-pending
application no. 14/076,501 filed November 11, 2013, which is
also assigned to the present Applicant. However, it should be
noted that in some embodiments the RF antenna assembly may be
connected to the transmission line at the wellhead and both
fed into the wellbore at the same time, as will be appreciated
by those skilled in the art.
[0032] Generally speaking, the choke fluid dispenser 60 is
used for common mode suppression of currents that result from
feeding the RF antenna 35. More particularly, the choke fluid
dispenser 60 may be used to confine much of the current to the
RF antenna 35, rather than allowing it to travel back up the
outer conductor 37 of the transmission line, for example, to
thereby help maintain volumetric heating in the desired
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location while enabling efficient, and electromagnetic
interference (EMI) compliant operation.
[0033] By way of background, because the wellbore diameter
is constrained, the radiating antenna 35 and transmission line
38 are typically collinearly arranged. However, this results
in significant near field coupling between the antenna 35 and
outer conductor 37 of the transmission line 38. This strong
coupling manifests itself in current being induced onto the
transmission line 38, and if this current is not suppressed,
the transmission line effectively becomes an extension of the
radiating antenna 35, heating undesired areas of the
geological formation 32. The choke fluid dispenser 60, which
in the illustrated example is carried on the drill tubular 42,
advantageously performs the function of attenuating the
induced current on the transmission line 38, effectively
confining the radiating current to the antenna 35 proper,
where it performs useful heating.
[0034] More particularly, a choke fluid that is
conductivity enhancing liquid, such as saline or fresh water,
is delivered (e.g., in a continuous or repetitive fashion)
from the choke fluid source 50 to the choke fluid dispenser 60
via a supply line 61 at the heel or proximal end of the
antenna 35 and is allowed to infuse into the reservoir 32.
This maintains a relatively high electrical conductivity up
hole from the antenna 35 and "pins" the electric field to this
location. While the RF heating may steam water at this
location in some instances, this may be overcome by the
continuing supply of choke fluid which helps block the advance
of the RF fields beyond the location of the choke fluid
dispenser 60. Considered alternatively, the choke fluid
dispenser 60 effectively converts the reservoir 32 into a
dissipative broadband choke.
[0035] The foregoing will be further understood with
reference to FIGS. 2 and 3(a)-3(f), in which a desiccation
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region or front 65 forms where the RF heating from the antenna
35 dries or desiccates the formation. The series of time-lapse
simulations in FIGS. 3(a)-3(f) illustrates how this
desiccation region 65 grows over the course of operation of
the RF antenna 35 over weeks and months. In the illustrated
example, the simulation in FIG. 3(a) corresponds to the start
of the RF heating, while the simulation in FIG. 3(f)
represents the desiccation region 65 approximately two months
later. Power dissipation at the choke fluid dispenser 60
location (here the heel of the antenna 35) is minimal while
the tip of the antenna has direct electrical contact with the
reservoir (i.e., it is not desiccated and the formation 32 has
wet contact with the tip of the antenna). Yet, as operation of
the antenna 35 continues and the desiccation region 65 grows
over time, this increases the resistivity of the formation 32
adjacent the antenna 35, which causes common mode current to
begin to couple to the outer conductor 37 and flow back up the
transmission line 38. However, continued use of the choke
fluid dispenser 60 over time as the RF antenna 35 is operated
advantageously keeps the desiccation region 65 from advancing
back up hole past the heel of the antenna 35.
[0036] Referring additionally to FIGS. 4(a)-7, an example
implementation of the choke fluid dispenser 60 is now
described. In the illustrated example, the choke fluid
dispenser 60 is carried by the drill tubular 42/transmission
line 38 assembly and includes an inner sleeve 70 surrounding
the drill tubular 42, a liner 71 surrounding the inner sleeve
and defining a first annular chamber 72 therewith. The liner
71 has a plurality of ports 73 therein in fluid communication
with the choke fluid source 50, as seen in FIG. 4(c).
Furthermore, an outer sleeve 74 surrounds the liner 71 and
defines a second annular chamber 75 therewith to receive choke
fluid from the plurality of ports 73. The outer sleeve 71 has
a plurality of openings 76 therein (see FIG. 4(c)) to pass
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choke fluid from the annular chamber 75 into the subterranean
formation 32 adjacent the antenna 35, as described above. In
some embodiments, a sand control screen(s) 79 (e.g., a
Facsrite screen) may optionally be used to keep sand from
entering the first annular chamber 72, as seen in FIG. 4(c).
In the illustrated embodiment, the screen 79 is positioned
within the ports 73, but they may be located elsewhere in
different embodiments. Moreover, other industry standard sand
control approaches or configurations may also be used in
different embodiments, as will be appreciated by those skilled
in the art.
[0037] Moreover, to accommodate for thermal expansion, the
inner sleeve 70 may be slidably moveable with respect to the
liner 71, and the liner may be fixed to the outer sleeve 74,
as perhaps best seen in FIG. 6. Thus, as the drill tubular
42/transmission line 38 assembly and liner 70 move along the
wellbore based upon thermal expansion (as indicated by the
two-headed arrow in FIG. 6), the first annular chamber 72 will
always be in alignment with the ports 73, so that the choke
fluid will continue to flow into the second annular chamber 75
despite the relative movement of the inner sleeve 70 with
respect to the liner 71.
[0038] The choke fluid may enter the first annular chamber
72 via a connection tube 81, as seen in FIGS. 5(b) and 6. A
relatively small diameter tube (e.g., 3/4") may be used as the
fluid line 61 to feed choke fluid from the choke fluid source
50 at the wellhead to the connection tube 81. The choke fluid
dispenser may further include a respective seal 77 (e.g., a
chevron seal(s)) and seal nut 78 at opposing ends of the inner
sleeve 70, as seen in FIGS. 5(a)-(c). However, other suitable
connection or sealing arrangements may be used in different
embodiments, as will be appreciated by those skilled in the
art. Thus, during operation of the example configuration,
choke fluid is pumped into the system, it fills the first
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annular chamber 72 between the inner sleeve 70 and the liner
71 between the chevron seals 77, the fluid then moves through
the screens 79 in the ports 73 and into the second annular
chamber 75, and is jetted out into the formation 32 via the
holes 76.
[0039] Choke fluid dispersion into the formation 32 may be
controlled by leaving a desired spacing between the choke
fluid dispenser 60 and any collection openings 80 used for
collecting reservoir fluids, as noted above. This offset helps
to define a desired effective area where choke fluid can
permeate without being prematurely drawn back into the
openings 80. This, in turn, helps to ensure that the choke
fluid provides the desired choke functionality, before it is
re-absorbed and "produced" with other reservoir fluids. An
example choke fluid flow or dispensing rate may be between 0.1
and 10 gallons of continuous fluid flow per minute for a
typical RF heating application, although other flow rates (and
intermittent fluid flow) may be used in some applications. In
a simulated example with a 1.4 gallon per minute flow, a total
power dissipation for a 400m antenna configuration was 400
'kilowatts for an antenna power of 4 kilowatts per meter of
antenna).
[0040] By way of comparison, a magnetic choke (such as
described in the above-noted U.S. App. Serial No. 14/167,039)
may in some implementations utilize a relatively large annular
volume to function with desired impedance, which in turn may
drive larger than standard drilling and liner sizes and
increase drilling costs. The choke fluid dispenser 60 may be
relatively compact in terms of length (e.g., it may be less
than about 10m in some applications), while remaining
compatible with standard size pipe diameters. More
particularly, drilling and completion costs typically vary
with the square of the diameter, and thus keeping the
diameters as small as possible may result in significant
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installation savings. Another potential benefit of the
relatively compact size of the choke fluid dispenser 60 is
that this may allow for sufficient envelope to package a
transmission line 38 with enough flow area to allow the
extension to longer or deeper implementation lengths.
[0041] Another contrast between the choke fluid dispenser
60 and a magnetic choke is that of efficiency, in that the
choke fluid dispenser may provide for somewhat higher
efficiency operation in terms of how much input RF energy is
lost during operation of the antenna 35. The enhanced
efficiency may also result in decreased operational costs, as
will be appreciated by those skilled in the art. Moreover,
magnetic chokes may generate significant heat and accordingly
require cooling via a cooling fluid circulation system, for
example, which is not the case with the choke fluid dispenser
60. The choke fluid dispenser 60 may not only provide broad
band choke performance over desired operating frequency ranges
similar to an magnetic choke, but it may also represent a
savings in terms of the number and complexity of components,
and thus a potential for additional cost savings. As a result,
the choke fluid dispenser 60 may be particularly useful in
"early" start-up wells used to enhance production flow at the
beginning of the recovery process, while magnetic chokes may
be more appropriate for longer term recovery wells where
enhanced tunability features may be desired over time.
However, either type of configuration may be used in
relatively short or long-term wells, and in some instances
both a magnetic choke assembly and a choke fluid dispenser may
be used in the same well, if desired.
[0042] A related method for heating the hydrocarbon
resource 31 in the subterranean formation 32 is also provided.
The method may include applying RF power to the elongate RF
antenna 35 positioned within the wellbore using the RF source
34. The method may further include selectively dispensing
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choke fluid from the choke fluid source 50 into adjacent
portions of the subterranean formation 32 via the choke fluid
dispenser 60 positioned in the wellbore at the proximal end of
the RF antenna 35 to define a common mode current choke at the
proximal end of the RF antenna, as discussed further above.
[0043] Many modifications and other embodiments of the
invention will come to the mind of one skilled in the art
having the benefit of the teachings presented in the foregoing
descriptions and the associated drawings. Therefore, it is
understood that the invention is not to be limited to the
specific embodiments disclosed, and that modifications and
embodiments are intended to be included within the scope of
the appended claims.
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