Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02777862 2015-11-06
PROCESS FOR ENHANCED PRODUCTION OF HEAVY OIL USING
MICROWAVES
FIELD OF THE INVENTION
[0003] The present invention relates generally to a process for recovering
heavy
oil from a reservoir.
BACKGROUND OF THE INVENTION
[0004] Heavy oil is naturally formed oil with very high viscosity but often
contains impurities such as sulfur. While conventional light oil has
viscosities
ranging from about 0.5 centipoise (cP) to about 100 cP, heavy oil has a
viscosity that
ranges from 100 cP to over 1,000,000 cP. Heavy oil reserves are estimated to
equal
about fifteen percent of the total remaining oil resources in the world. In
the United
States alone, heavy oil resources are estimated at about 30.5 billion barrels
and heavy
oil production accounts for a substantial portion of domestic oil production.
For
example, in California alone, heavy oil production accounts for over sixty
percent of
the states total oil production. With reserves of conventional light oil
becoming more
difficult to find, improved methods of heavy oil extractions have become more
important. Unfortunately, heavy oil is typically expensive to extract and
recovery is
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CA 02777862 2012-05-22
much slower and less complete than for lighter oil reserves. Therefore, there
is a
compelling need to develop a more efficient and effective means for extracting
heavy
oil.
[0005] Viscous
oil that is too deep to be mined from the surface may be
heated with hot fluids or steam to reduce the viscosity sufficiently for
recovery by
production wells. One thermal method, known as steam assisted gravity drainage
(SAGD), provides for steam injection and oil production to be carried out
through
separate wellbores. The
optimal configuration is an injector well which is
substantially parallel to and situated above a producer well, which lies
horizontally
near the bottom of the formation. Thermal communication between the two wells
is
established and, as oil is mobilized and produced, a steam chamber or chest
develops.
Oil at the surface of the enlarging chest is constantly mobilized by contact
with steam
and drains under the influence of gravity.
[0006] There are
several patents on the improvements to SAGD operation.
U.S. Patent No. 6, 814,141 describes applying vibrational energy in a well
fracture to
improve SAGD operation. U.S. Patent No. 5,899,274 teaches addition of solvents
to
improve oil recovery. U.S. Patent No. 6,544,411 describes decreasing the
viscosity of
crude oil using ultrasonic source. U.S. Patent No. 7,091,460 claims in situ,
dielectric
heating using variable radio frequency waves.
[0007] In a
recent patent publication (U.S. Patent Publication
20070289736/US-Al, filed May 25, 2007), it is disclosed to extract
hydrocarbons
from a target formation, such as a petroleum reservoir, heavy oil, and tar
sands by
utilizing microwave energy to fracture the containment rock and for
liquification or
vitalization of the hydrocarbons.
[0008] In
another recent patent publication (US Patent Publication
20070131591/US-Al, filed December 14, 2006), it is disclosed that lighter
hydrocarbons can be produced from heavier carbon-base materials by subjecting
the
heavier materials to microwave radiations in the range of about 4 GHz to about
18
GHz. This publication also discloses extracting hydrocarbons from a reservoir
where
a probe capable of generating microwaves is inserted into the oil wells and
the
microwaves are used to crack the hydrocarbons with the cracked hydrocarbon
thus
produced being recovered at the surface.
[0009] Despite
these disclosures, it is unlikely that direct microwave cracking
or heating of hydrocarbons would be practical or efficient. It is known that
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CA 02777862 2012-05-22
-
.
microwave energy is absorbed by a polar molecule with a dipole moment and
bypasses the molecules that lack dipole moment. The absorption of the
microwave
energy by the polar molecule causes excitation of the polar molecule thereby
transforming the microwave energy into heat energy (known as the coupling
effect).
Accordingly, when a molecule with a dipole moment is exposed to microwave
energy
it gets selectively heated in the presence of non-polar molecules. Generally,
heavy
oils comprise non-polar hydrocarbon molecules; accordingly, hydrocarbons would
not
get excited in the presence of microwaves.
[00101
Additionally, while the patent publication above claims to break the
hydrocarbon molecules, the energy of microwave photons is very low relative to
the
energy required to cleave a hydrocarbon molecule. Thus, when hydrocarbons are
exposed to microwave energy, it will not affect the structure of a hydrocarbon
molecule. (See, for example, "Microwave Synthesis", CEM Publication, 2002 by
Brittany Hayes).
[0011]
Conventional cyclic steam stimulation involves the process of injecting a
predetermined amount of steam into wells that have been drilled or converted
for
injection purposes. These wells are then shut in to allow the steam to heat or
"soak"
the producing formation around the well. After a sufficient time has elapsed
to allow
adequate heating, the injection wells are put back in production until the
heat is
dissipated with the producing fluids. Each cycle can last from weeks to months
and
this process continues until the reservoir is depleted or it is no longer
economically
feasible to produce.
[0012]
There exists a need to combine the technology of conventional cyclic
steam stimulation with in situ upgrading to both increase the amount of oil
produced
from the reservoir and in situ upgrade the oil from the reservoir.
BRIEF SUMMARY OF THE DISCLOSURE
[0013]
A process of injecting H2O into a subterranean region through a first
wellbore of a cyclic steam stimulation operation. Microwaves are introduced
into the
region at a frequency sufficient to excite the H20 molecules and increase the
temperature of at least a portion of the H20 within the region to produce
steam. The
subterranean region is then soaked with the steam. Revaporized steam and
superheated steam are then produced by directing the microwaves to the steam
and
H2O molecules. At least a portion of the heavy oil in the region is heated by
contact
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with the revaporized steam and the superheated steam to produce heated heavy
oil.
Heated heavy oil is then produced through a second wellbore of the cyclic
steam
stimulation operation, thereby recovering heavy oil with the cyclic steam
stimulation
operation from a subterranean region. In this embodiment a portion of the H20
is
injected as steam and the steam contacts with at least a portion of the heavy
oil in the
region so as to heat the portion of the heavy oil and reduce its viscosity so
that it flows
generally towards the second wellbore. Furthermore it is important to note
that the
temperature of the superheated steam is greater than the temperature of the
revaporized steam and the steam.
[0014] In another embodiment a process is describes injecting liquid H20
into a
region through a first wellbore of a cyclic steam stimulation operation.
Microwaves
are introduced into a subterranean region at a frequency sufficient to excite
the liquid
H20 molecules and increase the temperature of at least a portion of the liquid
H20
within the region to produce steam. The subterranean region is stoked with the
steam.
Revaporized steam and superheated steam is produced by directing the
microwaves to
the steam and H20 molecules. At least a portion of the heavy oil the in region
is
heated by contact with the revaporized steam and superheated steam to produce
heated heavy oil. Heated heavy oil is then produced through a second wellbore
of the
cyclic steam stimulation operation, thereby recovering heavy oil with the
cyclic steam
stimulation operation from a subterranean region. In this embodiment a portion
of the
H20 is injected as steam and the steam contacts with at least a portion of the
heavy oil
in the region so as to heat the portion of the heavy oil and reduce its
viscosity so that
it flows generally towards the second wellbore. Furthermore it is important to
note
that the temperature of the superheated steam is greater than the temperature
of the
revaporized steam and the steam.
[0015] In yet another embodiment a process begins with injecting H20 into a
subterranean region through an injection wellbore of a cyclic steam
stimulation
operation. Microwaves are introduced into the region at a frequency sufficient
to
excite the H20 molecules and increase the temperature of at least a portion of
the H20
within the region to produce steam. The subterranean region is then soaked
with the
steam. Revaporized steam and superheated steam are then produced by directing
the
microwaves to the steam and H20 molecules. At least a portion of the heavy oil
in the
region is then heated by contact with the revaporized steam and the
superheated steam
to product heated heavy oil. Heated heavy oil is then produced through a
second
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wellbore of the cyclic steam stimulation operation, thereby recovering heavy
oil with
the cyclic steam stimulation operation from a subterranean region. In this
embodiment a portion of the H20 is injected as steam and the steam contacts
with at
least a portion of the heavy oil in the region so as to heat the portion of
the heavy oil
and reduce its viscosity so that it flows generally towards the second
wellbore.
Furthermore it is important to note that the temperature of the superheated
steam is
greater than the temperature of the revaporized steam and the steam and that
the total
time soaking the bitumen with the steam and soaking the bitumen with the
revaporized steam and the superheated steam is greater than 30 days.
Additionally the
injection wellbore and the production wellbore are from 3 meters to 7 meters
apart
and the injection wellbore is located higher than the production wellbore.
[0015al In a further embodiment of the present invention there is provided a
process comprising: (a) injecting H20 into a subterranean region through a
first
wellbore of a cyclic steam stimulation operation; (b) introducing microwaves
into the
region at a frequency sufficient to excite the H20 molecules and increase the
temperature of at least a portion of the 1120 within the region to produce
steam; (c)
soaking the subterranean region with the steam; (d) producing revaporized
steam and
superheated steam by directing the microwaves to the steam and H20 molecules;
(e)
heating heavy oil in at least a portion of the subterranean region by contact
with the
revaporized steam and superheated steam to produce heated heavy oil; (f)
producing
the heated heavy oil through a second wellbore of the cyclic steam stimulation
operation; thereby recovering heavy oil with the cyclic steam stimulation
operation
from the subterranean region; wherein a portion of the H20 is injected as
steam and
the steam contacts heavy oil in at least a portion of the subterranean region
so as to
heat the portion of the heavy oil and reduce its viscosity so that it flows
generally
towards the second wellbore; and wherein the temperature of the superheated
steam is
greater than the temperature of the revaporized steam and the steam.
[0015b] In yet a further embodiments of the present invention there is
provided a
process comprising: (a) injecting liquid H20 into a subterranean region
through a first
wellbore of a cyclic steam stimulation operation; (b) introducing microwaves
into the
subterranean region at a frequency sufficient to excite the liquid H20
molecules and
increase the temperature of at least a portion of the liquid H20 within the
subterranean
region to produce steam; (c) soaking the subterranean region with the steam;
(d)
CA 02777862 2015-11-06
producing revaporized steam and superheated steam by directing the microwaves
to
the steam and H20 molecules; (e) heating heavy oil in at least a portion of
the
subterranean region by contact with the revaporized steam and superheated
steam to
produce heated heavy oil; and (f) producing the heated heavy oil through a
second
wellbore of the cyclic steam stimulation operation; thereby recovering heated
heavy
oil with the cyclic steam stimulation operation from the subterranean region;
wherein
a portion of the liquid H20 is injected as steam and the steam contacts heavy
oil in at
least a portion of the subterranean region so as to heat a portion of the
heavy oil and
reduce its viscosity so that it flows generally towards the second wellbore;
and
wherein the temperature of the superheated steam is greater than the
temperature of
the revaporized steam and the steam.
[0015e] In another embodiment of the present invention there is provided a
process
comprising: (a) injecting H20 into a subterranean region through an injection
wellbore of a cyclic steam stimulation operation; (b) introducing microwaves
into the
subterranean region at a frequency sufficient to excite the H20 molecules and
increase
the temperature of at least a portion of the H20 within the subterranean
region to
produce steam; (c) soaking the subterranean region with the steam; (d)
producing
revaporized steam and superheated steam by directing the microwaves to the
steam
and H20 molecules; (e) heating bitumen to below 3000 cp in at least a portion
of the
subterranean region by contact with the revaporized steam and superheated
steam to
produce a heated heavy oil and an imposed pressure differential between the
injection
wellbore and a production wellbore; and (f) producing the heated heavy oil
through
the production wellbore of the cyclic steam stimulation operation; thereby
recovering
heavy oil with the cyclic steam stimulation operation from the subterranean
region;
wherein the injection wellbore and the production wellbore are from 3 meters
to 7
meters apart and the injection wellbore is located higher than the production
wellbore;
wherein the H20 is injected as steam and the steam contacts heavy oil in at
least a
portion of the subterranean region so as to heat the portion of the heavy oil
and reduce
its viscosity so that it flows generally towards the production wellbore; and
wherein
the temperature of the superheated steam is greater than the revaporized steam
and the
steam temperature and wherein the total time soaking the bitumen with the
steam and
soaking the bitumen with the revaporized steam and the superheated steam is
greater
than 30 days.
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CA 02777862 2015-11-06
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] A more complete understanding of the present invention and
benefits
thereof may be acquired by referring to the following description taken in
conjunction
with the accompanying drawings in which:
[0017] Figure 1 is a schematic diagram illustrating a heavy oil
heating process,
wherein wave guides are used to introduce the microwaves to the reservoir.
[0018] Figure 2 is a schematic diagram illustrating a heavy oil
heating process
wherein the microwaves are introduced into the reservoir using a microwave
generator located within the reservoir.
[0019] Figure 3 describes an embodiment of cyclic steam stimulation,
wherein a
catalyst is placed as a liner alongside the well.
[0020] Figure 4 describes an embodiment of cyclic steam stimulation,
wherein a
catalyst is placed as particles in the formation.
DETAILED DESCRIPTION
[0021] Turning now to the detailed description of the preferred
arrangement or
arrangements of the present invention, it should be understood that the
inventive
features and concepts may be manifested in other arrangements and that the
scope of
the invention is not limited to the embodiments described or illustrated. The
scope of
the invention is intended only to be limited by the scope of the claims that
follow.
=
[0022] In this description, the term water is used to refer to H20 in
a liquid state
and the term steam is used to refer to H20 in a gaseous state.
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[0023] Turning
now to Figure 1, wellbores 14, 15 and 16 are illustrated.
Wellbore 14 extends from the surface 10 into a lower portion of subterranean
region
12. Wellbore 16 extends from the surface 10 into subterranean region 12 and
generally will be higher than wellbore 14. Wellbore 16 will be used to inject
H20 and
it is preferred that it is located higher than wellbore 14 so that when the
injected H20
heats the heavy oil, the heavy oil will flow generally towards wellbore 14,
which is
used to extract the heavy oil from the reservoir. In one embodiment a portion
of the
H20 is injected as steam and the steam contacts with at least a portion of the
heavy oil
in the region so as to heat the portion of the heavy oil and reduce its
viscosity so that
it flows generally towards the second wellbore. Wellbore 15 is used to
introduce
microwaves to the reservoir and it is preferred that wellbore 15 be located
intermittent
to wellbores 14 and 15; although, other arrangements are possible.
[0024] In
operation, steam generated in boiler 11 is provided into the reservoir
12 through upper wellbore leg 16. The steam heats the heavy oil within zone 17
of
the oil-bearing portion 13 of reservoir 12 causing it to become less viscous
and,
hence, increase its mobility. The heated heavy oil flows downward by gravity
and is
produced through wellbore leg 14. While Figure 1 illustrates a single wellbore
for
injection and a single wellbore for extraction, other configurations are
within the
scope of the invention, for example, there can be two or more separate
wellbores to
provide steam injection and two or more separate wellbores for production.
Similarly,
multiple wellbores can be used for microwave introduction to the reservoir, as
further
discussed below.
[0025] Steam is
used to soak the subterranean region. After soaking the
subterranean region with steam, revaporized steam and superheated steam are
produced by directing the microwaves to the steam and water molecules. In this
embodiment the temperature of the superheated steam is greater than the
temperature
of the revaporized steam and the steam. Essentially, superheated steam is
steam
which is at a higher temperature than conventional steam or revaporized steam.
[0026] In one
embodiment the temperature of the steam can range from 220 C to
250 C. The temperature of the revaporized steam can range from 220 C to 250 C,
even upwards of 250 C when the superheated steam is accounted for. The
increased
temperature of the superheated steam allows for the superheated steam to heat
the
bitumen to a temperature higher than was previously possible by steam alone.
In one
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embodiment the temperature of the superheated steam would be sufficient to
catalytically crack the oil in the reservoir.
[0027] In one
embodiment a catalyst can be used in the process and can be
present either as particles within the reservoir or as a liner on the wall of
the well.
The addition of catalysts can decrease the viscosity and increase the API
gravity of
the oil produced as compared to traditional cyclic steam stimulation. Types of
catalyst that can be utilized include metal sulfides, metal carbides and other
refractory
type metal compounds. Examples of metal sulfides include MoS2, WS2, CoMoS,
NiMoS and other commonly known by one skilled in the art. Examples of metal
carbides include MoC, WS and others commonly known by one skilled in the art.
Examples of refractory type metal compounds include metal phosphides, borides
and
others commonly known by one skilled in the art.
[0028] Hydrogen
gas can also be added to the injected steam, the revaporized
steam and/or the superheated steam either downhole or on the surface to
stabilize the
hydrocarbons so that it is easily transportable. In one embodiment it is
preferred that
it is added at a partial pressure from 600 to 800 psi or even 50 to 1,200 psi.
[0029]
Generally, the wellbore for steam injection, wellbore 16, will be
substantially parallel to and situated above the wellbore for production,
wellbore 14,
which is located horizontally near the bottom of the formation. Pairs of steam
injection wellbores and production wellbores will generally be close together
and
located at a suitable distance to create an effective steam chamber and yet
minimizing
the preheating time. Typically, the pairs of injection and production
wellbores will be
from about 3 meters to 7 meters apart and preferably there will be about 5
meters of
vertical separation between the injector and producer wellbores. In other
embodiments it is possible for the injection and production wellbores be
anywhere
from 1, 3, 5, 7, 12, 15, 20 even 25 meters of horizontal separation apart.
Additionally,
in other embodiments it is possible for the injection and production wellbores
be
anywhere from 1, 3, 5, 7, 12, 15, 20 even 25 meters of vertical separation
apart. In
this type of SAGD operation, the zone 17 is preheated by steam circulation
until the
reservoir temperature between the injector and producer wellbore is at a
temperature
sufficient to drop the viscosity of the heavy oil so that it has sufficient
mobility to
flow to and be extracted through wellbore 14. Generally, the heavy oil will
need to be
heated sufficiently to reduce its viscosity to below 3000 cP; however, lower
viscosities are better for oil extraction and, thus, it is preferable that the
viscosity be
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=
below 1500 cP and more preferably below 1000 cP. Preheating zone 17 involves
circulating steam inside a liner using a tubing string to the toe of the
wellbore. Both
the injector and producer would be so equipped. Steam circulation through
wellbores
14 and 16 will occur over a period of time, typically about 3 months. During
the
steam circulation, heat is conducted through the liner wall into the reservoir
near the
liner. At some point before the circulation period ends, the temperature
midway
between the injector and producer will reach a temperature wherein the bitumen
will
become movable typically around 3000 cP or less or from about 80 to 100 C.
Once
this occurs, the steam circulation rate for wellbore 14 will be gradually
reduced while
the steam rate for the injector wellbore 16 will be maintained or increased.
This
imposes a pressure gradient from high, for the area around wellbore 16, to
low, for the
area around wellbore 14. With the oil viscosity low enough to move and the
imposed
pressure differential between the injection and production wellbores, steam
(usually
condensed to hot water) starts to flow from the injector into the producer. As
the
steam rate is continued to be adjusted downward in wellbore 14 and upward in
wellbore 16, the system arrives at steam assisted gravity drainage operation
with no
steam injection through wellbore 14 and all the steam injection through
wellbore 16.
Once hydraulic communication is established between the pair of injector and
producer wellbores, steam injection in the upper well and liquid production
from the
lower well can proceed. Due to gravity effects, the steam vapor tends to rise
and
develop a steam chamber at the top section 19 of zone 17. The process is
operated so
that the liquid/vapor interface is maintained between the injector and
producer
wellbores to form a steam trap which prevents live steam from being produced
through the lower wellbore.
[0030]
During operation, steam will come into contact with the heavy oil in
zone 17 and, thus, heat the heavy oil and increase its mobility by lessening
its
viscosity. Heated heavy oil will tend to flow downward by gravity and collect
around
wellbore 14. Heated heavy oil is produced through wellbore 14 as it collects.
Steam
contacting the heavy oil will lose heat and tend to condense into water. The
water
will also tend to flow downward toward wellbore 14. In past SAGD operations,
this
water would also be produced through wellbore 14. Such produced water would
need
to be treated to reduce impurities before being reheated in the boiler for
subsequent
injection. As the process continues operation, zone 17 will expand with heavy
oil
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production occurring from a larger portion of oil-bearing portion 13 of
subterranean
formation 12.
[0031] Turning again to Figure 1, the current invention provides for
microwave generator 18 to generate microwaves which are directed underground
and
into zone 17 of the reservoir through a series of wave guides 20. The diameter
of the
wave guides will preferably be more than 3 inches in order to ensure good
transmission of the microwaves. Within the reservoir, the microwaves will be
at a
frequency substantially equivalent to the resonant frequency of the water
within the
reservoir so that the microwaves excite the water molecules causing them to
heat up.
Optimally, the microwaves will be introduced at or near the liquid vapor
interface so
that condensed steam is reheated from its water state back into steam further
supplying the steam chamber. In some embodiments the microwave frequency will
be not greater than 3000 megahertz and/or at a resonant frequency of water.
Based on
the resonant frequency of water, the optimum frequency will be 2450 megahertz;
however, power requirements and other factors may dictate that another
frequency is
more economical. Additionally, salt and other impurities may enhance the
coupling
effect (production of heat by resonance of a polar or conductive molecule with
microwave energy); thus, the presence of salt is desirable.
[0032] Turning now to Fig. 2, a further embodiment of the invention is
illustrated wherein, instead of using wave guides, power is supplied through
electrical
wire 22 to microwave generating probe 24. The electrical power can be supplied
to
wire 22 by any standard means such as generator 26.
[0033] In still another embodiment of the invention, also illustrated in
Fig. 2,
no steam boiler is used. Instead water is introduced directly into wellbore 16
through
pipe 28 and valve 30. Wellbore 16 then introduces water into the reservoir
instead of
steam and the entire steam production would be accomplished through use of the
microwave generators. This embodiment of the invention has the added advantage
of
avoiding costly water treatment that is necessary when using a boiler to
generate
steam because, as discussed above, salt and other impurities can aid in heat
generation. In a preferred embodiment, the water introduced into the reservoir
would
have a salt content greater than the natural salt content of the reservoir,
which is
typically about 5,000 to 7,000 ppm. Accordingly, it is preferred that the
introduced
water has a salt content greater than 10,000 ppm. For enhanced heat generation
30,000 to 50,000 ppm is more preferred.
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CA 02777862 2015-11-06
[0034] Figure 3 describes an embodiment of cyclic steam stimulation,
wherein a
catalyst is placed as a liner alongside the well. In this embodiment steam 102
is
injected into a well 104. The steam 102 heats the bitumen 106 in the
formation. When
the required temperature is achieved the injection of steam 102 into the well
104 is
ceased. The bitumen 106 is soaked with the steam 102 for a period of time. MW
and/or RF radiation is then directed into the well from a MW/RF antenna 1 108.
In
this embodiment the catalyst 110 is placed as a liner alongside the well 104.
The MW
and/or RF radiation is capable of heating the steam 102 into superheated steam
and
revaporized steam, which has a higher temperature than of the steam. The
bitumen
106 is then further heated with this superheated steam and any steam that has
revaporized. Hydrocarbons 112 are then produced from the well 104.
[0035] Figure 4 describes an embodiment of cyclic steam stimulation,
wherein a
catalyst is placed as particles in the formation. In this embodiment steam 102
is
injected into a well 104. The steam 102 heats the bitumen 106 in the
formation. When
the required temperature is achieved the injection of steam 102 into the well
104 is
ceased. The bitumen 106 is soaked with the steam 102 for a period of time. MW
and/or RF radiation is then directed into the well 104 from a MW/RF antenna
108. In
this embodiment the catalyst 110 are dispersed throughout the formation. The
MW
and/or RF radiation is capable of heating the steam 102 into superheated steam
and
revaporized steam, which has a higher temperature than of the steam. The
bitumen 6
is then further heated with this superheated steam and any steam that has
revaporized.
Hydrocarbons 112 are then produced from the well 104.
[0036] Microwave generators useful in the invention would be ones suitable
for
generating microwaves in the desired frequency ranges recited above. Microwave
generators and wave guide systems adaptable to the invention are sold by Cober
Muegge LLC, Richardson Electronics and CPI International Inc.
[0037] Steam to oil ratio is an important factor in SAGD operations and
typically
the amount of water required will be 2 to 3 times the oil production. Higher
steam to
oil production ratios require higher water and natural gas costs. The present
invention
reduces water and natural gas requirements and reduces some of the water
handling
involving recycling, cooling, and cleaning up the water.
CA 02777862 2015-11-06
[0038] Although the
systems and processes described herein have been described
in detail, it should be understood that various changes, substitutions, and
alterations
can be made without departing from the scope of the invention as defined by
the
following claims. Those skilled in the art may be able to study the preferred
embodiments and identify other ways to practice the invention that are not
exactly as
described herein. The scope of the claims should not be limited by the
preferred
embodiments set forth in the description, but should be given the broadest
interpretation consistent with the description as a whole.
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