Note: Descriptions are shown in the official language in which they were submitted.
CA 02777942 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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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. A variety of processes are used to recover viscous
hydrocarbons,
such as heavy crude oils and bitumen, from underground deposits. There are
extensive deposits of viscous hydrocarbons throughout the globe, including
large
deposits in the Northern Alberta tar sands that are not recoverable with
traditional oil
well production technologies. A problem associated with producing hydrocarbons
from such deposits is that the hydrocarbons are too viscous to flow at
commercially
viable rates at the temperatures and pressures present in the reservoir. In
some cases,
these deposits are mined using open-pit mining techniques to extract the
hydrocarbon-
bearing material for later processing to extract the hydrocarbons.
[0006]
Alternatively, thermal techniques may be used to heat the reservoir fluids
and rock to produce the heated, mobilized hydrocarbons from wells. One such
technique for utilizing a single well for injecting heated fluids and
producing
hydrocarbons is described in U.S. Patent No. 4,116,275, which also describes
some of
the problems associated with the production of mobilized viscous hydrocarbons
from
horizontal wells.
[0007] One
thermal method of recovering viscous hydrocarbons using two
vertically spaced wells is known as steam-assisted gravity drainage (SAGD)
process.
The SAGD process is currently the only commercial process that allows for the
extraction of bitumen at depths too deep to be strip-mined. Various
embodiments of
the SAGD process are described in Canadian Patent No. 1,304,287 and
corresponding
U.S. Patent No. 4,344,485. In the SAGD process, steam is pumped through an
upper,
horizontal injection well into a viscous hydrocarbon reservoir while the
heated,
mobilized hydrocarbons are produced from a lower, parallel, horizontal
production
well vertically spaced proximate to the injection well. The injection and
production
wells are typically located close to the bottom of the hydrocarbon deposits.
[0008] The
SAGD process is believed to work as follows. The injected steam
creates a "steam chamber" in the reservoir around and above the horizontal
injection
well. As the steam chamber expands upwardly and laterally from the injection
well,
viscous hydrocarbons in the reservoir are heated and mobilized, especially at
the
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margins of the steam chamber where the steam condenses and heats a layer of
viscous
hydrocarbons by thermal conduction. The heated, mobilized hydrocarbons (and
steam condensate) drain under the effects of gravity towards the bottom of the
steam
chamber, where the production well is located. The mobilized hydrocarbons are
collected and produced from the production well. The rate of steam injection
and the
rate of hydrocarbon production may be modulated to control the growth of the
steam
chamber to ensure that the production well remains located at the bottom of
the steam
chamber and in a position to collect the mobilized hydrocarbons.
[0009] In order
to initiate a SAGD production, thermal communication must be
established between an injection and a production SAGD well pair. Initially,
the
steam injected into the injection well of the SAGD well pair will not have any
effect
on the production well until at least some thermal communication is
established
because the hydrocarbon deposits are so viscous and have little mobility.
Accordingly, a start-up phase is required for the SAGD operation. Typically,
the
start-up phase takes about three months before thermal communication is
established
between the SAGD well pair, depending on the formation lithology and the
actual
inter-well spacing.
[0010] The
traditional approach to starting-up the SAGD process is to
simultaneously operate the injection and production wells independently of one
another to circulate steam. The injection and production wells are each
completed
with a screened (porous) casing (or liner) and an internal tubing string
extending to
the end of the liner, forming an annulus between the tubing string and casing.
High
pressure steam is simultaneously injected through the tubing string of both
the
injection and production wells. Fluid is simultaneously produced from each of
the
injection and production wells through the annulus between the tubing string
and the
casing. In effect, heated fluid is independently circulated in each of the
injection and
production wells during the start-up phase, heating the hydrocarbon formation
around
each well by thermal conduction. Independent circulation of the wells is
continued
until efficient thermal communication between the wells is established. In
this way,
an increase in the fluid transmissibility through the inter-well span between
the
injection and production wells is established by conductive heating. The pre-
heating
stage typically takes about three to four months. Once
sufficient thermal
communication is established between the injection wells, the upper, injection
well is
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dedicated to steam injection and the lower production well is dedicated to
fluid
production.
[0011] 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.
[0012] 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.
[0013] 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.
[0014] Despite these disclosures, it is unlikely that direct microwave
cracking
or heating of hydrocarbons would be practical or efficient. It is known that
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.
[0015] 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
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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).
BRIEF SUMMARY OF THE DISCLOSURE
[0016] A process of injecting H20 and sulfur hexafluoride into a
subterranean
region through a first wellbore of a steam assisted gravity drainage
operation.
Microwaves are introduced into the region at a frequency sufficient to excite
the 1120
and sulfur hexafluoride molecules and increase the temperature of at least a
portion of
the 1120 and sulfur hexafluoride within the region to produce heated 1120 and
heated
sulfur hexafluoride. At least a portion of the heavy oil in the region is
heated by
contact with the heated H20 and the heated sulfur hexafluoride to produce
heated
heavy oil. Heated heavy oil is produced through a second wellbore of the steam
assisted gravity drainage operation, thereby recovering heavy oil with the
steam
assisted gravity drainage operation from the subterranean region. In this
embodiment
a portion of the 1120 is injected as steam and the steam contact 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.
[0017] In an alternate embodiment a process is taught of injecting liquid
1120 and
sulfur hexafluoride into a subterranean region through a first wellbore of a
steam
assisted gravity drainage operation. Microwaves are introduced into the
subterranean
region at a frequency sufficient to excite the liquid H20 and sulfur
hexafluoride
molecules and increase the temperature of at least a portion of the liquid H20
and
sulfur hexafluoride within the region to produce heated gaseous H20 and heated
sulfur hexafluoride. At least a portion of the heavy oil is heated in the
region by
contact with the heated gaseous 1120 and heated sulfur hexafluoride to produce
heated
heavy oil. Heated heavy oil is produced through a second wellbore of the steam
assisted gravity drainage operation, thereby recovering heavy oil with the
steam
assisted gravity drainage operation from the subterranean region. In this
embodiment
a portion of the liquid 1120 is injected as steam and the steam contact 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.
[0018] In yet another embodiment a process begins by injecting 1120 and
sulfur
hexafluoride into a subterranean region through an injection wellbore of a
steam
CA 02777942 2015-11-06
assisted gravity drainage operation. Microwaves are introduced in the region
at a
frequency sufficient to excite the H20 molecules and sulfur hexafluoride
molecules
and increase the temperature of at least a portion of the H20 and sulfur
hexafluoride
within the region to produce heated H20 and heated sulfur hexafluoride. At
least a
portion of the bitumen is heated to below 3000cp in the region by contact with
the
heated H20 and heated sulfur hexafluoride to produce a heated heavy oil and an
imposed pressure differential between the injection wellbore and a production
wellbore. Heated heavy oil is produced through the production wellbore of the
steam
assisted gravity drainage operation, thereby recovering heavy oil with the
steam
assisted gravity drainage operation from the subterranean region. In this
embodiment
a portion of the H20 is injected as steam and the steam contact 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
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.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] A more complete understanding of the present invention and benefits
thereof may be acquired by referring to the follow description taken in
conjunction
with the accompanying drawings in which:
[0020] Figure 1 is a schematic diagram illustrating a heavy oil heating
process,
wherein wave guides are used to introduce the microwaves to the reservoir.
[0021] 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.
DETAILED DESCRIPTION
[0022] 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.
[0023] 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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[0024] 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
sulfur hexafluoride and it is preferred that it is located higher than
wellbore 14 so that
when the injected H20 and heated sulfur hexafluoride 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.
[0025] This
method can also be used with a variety of enhanced oil recovery
systems. Examples of enhanced oil recovery systems include: steam assisted
gravity
drainage, steam drive, cyclic steam stimulation or combinations thereof.
[0026] In one
embodiment the sulfur hexafluoride can be injected into the region
in either liquid, gas, or even subcritical or supercritical fluid. Since
sulfur
hexafluoride is at least one hundred times more soluble in hydrocarbons when
compared to water it is able to reduce the amount of water injected region
over
conventional steam assisted gravity drainage operations. In one embodiment the
method can reduce the amount of water used by 5%, 10%, 20%, 30%, 40%, 50%,
60%, 70%, 80% even 90% of what is typically used during conventional steam
assisted gravity drainage operations.
[0027] In
another embodiment the method is capable of operating at temperatures
much less than conventional steam assisted gravity drainage operations. In one
embodiment the hydrocarbon region only needs to be heated to a temperature of
200 C before sufficient heat transfer has occurred to the hydrocarbon fluid to
promote
the flow of the heavy oil.
[0028] In
operation, steam generated in boiler 11 is provided into the reservoir 12
through upper wellbore leg 16. The steam and heated sulfur hexafluoride 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
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,
,
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.
[0029]
Generally, the wellbore for steam and sulfur hexafluoride 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 and sulfur hexafluoride 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 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
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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
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 or
sulfur
hexafluoride within the reservoir so that the microwaves excite the water
molecules
and/or sulfur hexafluoride 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
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-
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 and sulfur hexafluoride is introduced
directly
into wellbore 16 through pipe 28 and valve 30. Wellbore 16 then introduces
water
and sulfur hexafluoride 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.
[0034] 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.
[0035] 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 02777942 2015-11-06
[00361 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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