Note: Descriptions are shown in the official language in which they were submitted.
F-1184-L
SOLVENT FLOODING TO RECO~ER VISCOUS OILS
This invention is directed to a method of ~ecovering oil from
viscous oil-containing subsurface formations. More particularly, this
invention is directed to an improved hydrocarbon solvent recovery
method for recovering oil from subsurface formations that are
penetrated by at le~st one injection well1 one production well, and
one interior production well which extend from the surface of the
earth and into the subsurface formation containing viscous
hydrocarbons.
In U.S. Patent 4,293,035, there is disclosed a method of
recovering viscous oil from a viscous oil-bearing subsurface formation
wherein a solvent is injected into a high mobility channel formed in
the bottom of the formation intermediate an injection well and a
production well. The solvent is injected until the ratio of produced
oil to solvent becomes unfavorable and thereafter the inJection of
solvent is terminated and gas is injected into the high mobility
channel to produce solvent and oil from the formation. In U.S. Patent
3,838,738 there is described a method for recovering viscous petroleum
from petroleum-containing formations by first establishing a fluid
communicatlon path low in the formation. A heated fluid is then
injected into the fluid communication path followed by injecting a
volatile solvent such as carbon disulfide, benzene, or toluene into
the preheated flow path and continuing injecting the heating fluid and
recovering fluids including petroleum from the production well.
In U.S. Patent 3,500,~17 there is disclosed a method for
recovering crude oil from an oil-bearing formation having a
water-saturated zone underlying the oil-saturated zone. A mixture of
an aqueous fluid which has a density greater than the density of the
crude oil and a solvent having a density less than the density of the
crude oil are injected into the water-saturated zone and oil is
produced from the formation.
U.S. Patent 4,û26~358 discloses a method for recovering heavy
oil from a subterranean hydrocarbon-bearing formation traversed by at
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least one in~ection well and one production well wherein a slug of
hydrocarbon solvent in amounts of 0.1 to about 20 percent of the
formation pore volume and having a gas dissolved therein is injected
into the formation via the injection well. Thereafter, a thermal sink
is created in the formation by in-situ combustion or by injecting
steam. The wells are then shut-in for a predetermined time to permit
the formation to undergo a soak period, after which production is
continued. Optionally, after the production period, the formation may
be water-flooded to recover additional oil from the formation.
The present invention involves a solvent flood oil-recovery
process for recovering viscous oil from a viscous oil-containing
formation that has no significant vertical perr~ility barrier in the
portion of the formation to be treated, involving at least one
injection well and at least one spaced-apart production well for
injecting a hydrocarbon solvent into a fluid communication path in the
lower portion of the formation and recovering oil from the formation.
The improvement comprises employing a third well or plurality of
wells, referred to herein as an interior production well, drilled into
the upper portion of the oil-containing formation between injection
and production wells to achieve more efficient mixing of the solvent
in the formation whereby solvent more readily migrates upward through
the formation by gravity-convection and dissolves the viscous oil
contained therein reducing its viscosity and permitting improved flow
of the less viscous oil to the surface of the earth by a fluid drive
process. A fluid communication path is established through the lower
portion of the formation between the injection well and production
well, and a hydrocarbon solvent, liquid under formation conditions,
and having a specific gravity less than the oil contained in the
formation is injected via the injection well into this path and fluids
including oil are recovered from the formation via the production well
until solvent is detected in the produced fluid. When this occurs,
production is terminated and the production well is shut-in.
Injection of solvent is continued and fluid including oil is recovered
from the formation via the interior production well which is in fluid
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F-1184-L
communication with the upper portion of the formation until the fluid
recovered con-tains a predetermined amount of solvent, preferably from
5 to 10 percent by volume. Thereafter, production via the interior
production well and injection of solvent via the injection well is
terminated. The injection well, production well, and interior
production well are shut-in to permit the formation to undergo a soak
period for a time sufficient for gravity~driven convection to
substantially mix the solvent with the oil in the formation and
thereby reduce its viscosity and increase its mobility, preferably for
a period of time between 1 to 10 days per vertical thickness in feet
of the viscous oil-containing formation. During this soak period~ the
light hydrocarbon solvent will tend to rise and the heavy oil will
move downward in a gravity-driven convection process forming a pattern
of fingers. This fingering causes a more effective contact between
the oil and solvent thus providing a greater volume of oil that is
reduced in viscosity and which can be produced more readily.
Thereafter, a driving fluid such as water is injected into the
injection well to displace the mixture of oil and solvent toward the
production well for recovery. Production is continued until the fluid
including oil recovered from the production well contains an
unfavorable ratio of oil to driving fluid In a variation of the
process, a predetermined amount of solvent is injected into the
formation, preferably in a total amount of 0.05 to 0.30 pore volume.
During the fluid drive recovery process, the injection well and
production well may be in fluid communication with essentially the
entire oil-containing formation and the fluid communication path
b~ en the injection well and production well closed off.
/ The drawing represents a cross-sectional view of a subsurface
oil-containing formation with the improved solvent recovery method
depicted therein.
The process of the present invention may best be understood
by referring to the attached drawing which illustrates a minimum
three-well unit for employing the process of the invention, wherein a
viscous oil-containing formation that has no significant vertical
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F-1184-L -4
permeability barriers is penetrated by an injection well 12 which is
in fluid communication with the lower portion of the formation through
per~orations 14, a production well 16 which is in fluid communication
with the lower portion of the formation through perforations 18, and
an interior production well 20 which is in fluid communication with
the upper portion of the formation through perforations ~2. For both
the injection well 12 and the production well 16, the distance in
which the perforations are formed above the bottom 24 of the viscous
oil-containing formation lû designated as 26 in the drawing is no
longer than 10 percent of the total thickness 28 of the viscous
oil-containing formation.
The drawing illustrates how interior production well 20 is
drilled into the formation 10, with respect to injection well 12 and
production well 16. Interior production well 20 must be drilled into
the recovery zone within the formation 10 defined by injection well 12
and production well 16. It is not essential that intPrior production
well 20 be located on a line between injection well 12 and production
well 16, and may be offset in either direction from a straight line
arlarg~r -,t, although it is preferred that interior production well is
in alignment with wells 12 and 16. S;~ rly, it is not essential
that well 2û be located exactly midway between injection well 12 and
production well 16 although it is preferred that interior well 20 be
from 40 to S0 percent of the distance between injection well and
production well. It is also within the scope of the invention to use
a plurality of interior production wells located between injection and
production wells using the process described herein.
A fluid communication path 30 is established intermediate the
injection and production wells 12 and 16 through the lower portion of
the viscous oil-containing formation 10. Therea~ter, a hydloca}bon
solvent which is liquid at formation temperature and injection
pressure and having a specific gravity less than the specific gravity
of the oil contained in the formation 10 is injected via the injection
well 12 through the fluid communication path 30 and fluid including
oil is recovered from the formation via the production well 16.
3L ~.924~8S
F-1184-L -5-
Solvent injection via injection ~Yell 12 and recovering fluid includingoil from the formation by means of production well 16 is continued
until solvent breaks through at the production well 16. A tracer can
be incorporated in the solvent to aid in its detection in the fluid
recovered from the production ~ell 16. Once solvent is detected in
the fluid being produced from well 16, production is terminated and
well 16 is shut-in.
Thereafter, solvent is continued to be injected into the
formation lC via injection well 12 through the fluid communication
path 30 and fluid including oil is recovered from the formation via
interior production well 20 through perforations 22. The solvent
having a specific gravity less than the specific gravity of the oil
contained in the formation will tend to finger upward into the
oil-containing formation by gravity-driven convection aided by the
production of well 22 in fluid communication with upper portion of the
formation. As solvent moves upward through the formation 10 toward
perforations 22 in production well 10, it dissolves viscous oil,
forming a bank of oil and solvent as shown by the dashed lines 32 in
the drawing in which the oil content increases as the bank moves
upward from the irme~;ate vicinity of the concentrated layer of
solvent contained in the fluid communication path 30. Solvent
injection and production of oil from well 20 are continued until the
fluid recovered from well 20 contains a predetermined amount of
solvent, preferably from 5 to 10 percent by volume. Once the amount
of solvent in the fluid being produced from the formation by means of
well 20 increases to a predetermined value, production of fluids from
the formation by means of well 20 and injection of solvent into the
formation by means of well 12 is terminated.
Thereafter, all three wells, 12, 16 and 20, are shut-in to
permit the formation to undergo a soak period for a time sufficient
for gravity-driven convection to further substantially mix the solvent
with the oil in the formation and thereby reduce its viscosity and
increase its mobility, preferably for a period of time between 1 to 10
days per vertical thickness in feet of the viscous oil-containing
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F -1184-L -6-
formation 10. It will be recognized by those skilled in the art of
oil recovery that during this shut-in or soak period minor amounts of
injection or production fluid, such as for the purpose of testing, may
be done without significant detrimental effects on the process.
During the soak period, the liquid hydrocarbon solvent being lighter
than khe oil contained in the viscous oil-containing formation, that
is, having a specific gravity less than the specific gravity of the
oil under formation conditions, will tend to flow by gravity-driven
convection upward into the oil-containing formation and the heavy oil
will flow by gravity-driven convection downward to obtain more
efficient mixing between the oil and solvent. Furthermore, because of
viscous instabilities, the rising hydrocarbon solvent and descending
heavy oil form a pattern of fingers. This fingering is important in
that the distribution, size and extension of these fingers will
provide intimate contact between the solvent and the heavy oil
allowing the two to mix more effectively9 thus forming a greater
volume o~ oil that is reduced in viscosity and which can be procluced
more readily. This relatively low viscosity mixture can be much more
effectively displaced from the reservoir than the original heavy oil
by flooding the formation with water or other su~table fluid, as is
well known to those skilled in the art of petroleum engineering~
Once the soak period has been completed, a driving fluid is
injected into the fluid communication path 30 via the injection well
12 and fluid including oil and hydrocarbon solvent is recovered from
the production well 16. It is preferred that additional perforations
may be added to the injection well 12 or the production well 16, or
both if desired, to establish fluid communic:ation between both wells
and the full thickness of the oil-containing formation and thus obtain
a more uniform displacement of the mobilized reservoir oils by the
driving fluid. In still another embodiment 7 the portion of the
injection well 12 and production well 16 in fluid communication with
the fluid communication path 30 in the lower portion of the formation
10 is sealed-off by cementing or other means, and perforations or
other fluid communication means are established between both wells and
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F-1184-L 7
the remaining portion of the oil-containing formation. The driving
fluid displaces both the oil and hydrocarbon solvent from the viscous
oil-containing formation into the production well 15 and production is
continued until the recovered ~luid contains an unfavorable ratio of
oil to driving fluid. The driving fluid for use in the invention may
be gaseous or liquid. For example, gases include light aliphatic
hydrocarbons having from one to four carbon atoms; carbon dioxide and
nitrogen may be used for the process of the invention. Aqueous fluids
are particularly preferred driving fluids in the process of the
invention. Water, brine and thickened a~lleous fluids are all suitable
aqueous fluids for the purpose of the invention.
The fluid communication path 30 through the lower portion of
the viscous oil-containing subsurface formation 10 may be formed by
injecting a brine having a specific gravity greater than the specific
gravity of the oil in the subsurface formation via the injection well
12 and through the perforations therein and into the oil-containing
subsurface formation until the brine breaks through at the production
well 16. In some formations, brine may underlie the oil contained in
the subsurface formation 10. In such cases, an oil-water contact will
exist in the ~ormation 10 and no additional brine need be injected
into the formation to establish the fluid communication path ~û. In
such cases, it will be considered that the fluid communication path 30
is established by determining the existence of the oil-water contact
and the brine-filled portion of the formation that underlies the
oil-containing formation.
In the case of an oil-containing formation that is underlain
by brine and thus has an oil-water contact, the injection well is
perforated and the perforation interval is provided adjacent the
underlying water. Desirably, the upper perforations of this
perforation interval are located about adjacent the oil-water contact
though it could extend soT,e~.,at above this oil-water contact without
seriously affecting the eTfficiency of the present recovery method.
The production well is perforated and the perforation interval is
provided adjacent the oil-water contact such that the lower
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F~ 4-L 8
perforation of this interval is located slightly above the oil-water
contact. Again, in the case of a formation underlain by brine, as it
was with the formation that was not underlain by brine, the
perforation intervals that are provided in the injection and
production wells desirably are no longer than about 10 percent of the
total thickness of the viscous oil-containing ~ormation. It is
preferred that prior to initiation of the driving fluid phase in order
to ensure that the optimum displacement occurs in the formation, that
fluid communication be established in a relative uniform fashion over
the entire thickness of the oil-saturated interval adjacent the
injection well or production well~ or both if desired. Therefore,
perforations should be formed above the perforations originally formed
in the lower portion of both wells adjacent the oil-water contact.
Also, it may be desirable to close-off the perforations in the lower
portion of both wells that are in fluid communication with the
oil-water contact by cementing or any other suitable means.
Perforations should then be formed above the perforations closed-off
so as to establish relative uniform fluid communication between bo~h
wells 12 and 16 and the remaining vertical thickness of the
oil-containing formation.
In the case of a formation underlain by brine which forms a
fluid communication path between the injection and production wells,
the hydrocarbon solvent is selected to have a specific gravity less
than that of the brine in addition to being less than that of the oil
contained in the viscous oil-containing formation. The hydrocarbon
solvent is injected via the injection well into this brine-filled
communication path and the process is continued in the same manner
described above for the formation which is not underlain by water.
The hydrocarbon solvents used in the process must have a
specific gravity less than that of the oil and less than that of the
brine-injected or naturally present in the formation under formation
conditions. It is also highly desirable that the hydrocarbon solvent
remain liquid under the temperature and pressure conditions that exist
in the subsurface viscous oil-containing formation and not cause
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solids such as asphaltenes to precipitate from the oil in amoun-ts
sufficient to seriously plug the pores of the formation. The
preferred hydrocarbon solvent is a light crude oil. Other examples of
suitable hydrocarbon solvents include light oil condensates having an
API gravity greater than 3û API degrees and partially refined tar
which is generally known as syncrude. It may be desirable to include
in the hydrocarbon solvent up to about 10 percent of aromatic material
such as aromatic refinery stock to make the solvent compatible with
the oil contained in the formations and to prevent the deposition of
solid or gelatinous materials such as asphaltenes therefrom.
The viscous oil-containing subsurface formation to be treated
by the present hydrocarbon solvent recovery method must be one that
has no significant vertical perme~hil;ty barriers in that portion of
the formation to be treated. This allows the solvent that is injected
into the fluid communication path formed between the injection well
and production well to flow upward into the formation by convection
forces, thus solllh;li7ing the oil and forming a solvent-oil mixture of
reduced viscosity that can be more readily produced by a fluid drive
process.
Another variation of the above-described process involves
injecting a predetermined total amount of solvent into the formation
after there has been a breakthrough of solvent at production well 16
and while interior production well 20 is opened for production. In
this embodiment, after the initial step of injecting solvent into the
formation lû until solvent is detected at production well 16 and the
production well is shut-in, injection of solvent is continued and
fluids recovered from interior production well 20 until a total
quantity of solvent, liquid under formation conditions, has been
injected into the formation. The total amount of solvent injected
into the formation is preferably within the range of 0.05 to 0.30 pore
volume. Thereafter, all three wells~ 12, 16 and 20, are shut-in and
the steps of a soak period followed by a fluid drive process for
recovering the mob;~i7ed oil from the formation are conducted
according to the process previously described.
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F-l 184-L -10-
Thus, the present invention teaches how significantly more
viscous oil may be recovered from an oil formation by injecting a
solvent in-to an established fluid communication path in the lower
portion of a formation between an injection well and a production well
and employing an interior production well located between injection
and production wells in fluid communication with the upper portion of
the formation to increase the efficiency of the migration of the
solvent into the viscoùs oil contained in the formation by
gravity-convection thereby decreasing the viscosity of -the oil and
making it more mobile for subsequent recovery by a fluid drive process.
The method according to the invention is not restricted to
the use in the field in which only three wells penetrate the
formation. Any other nùmber of wells, which may be arranged according
to any pattern, may be ~ppl;ed in using the present method as
illustrated in U.S. Patent 3,927,716. By the term "pore volume" as
used herein, is meant that volume of the portion of the formation
underlying the well pattern employed, as described in greater detail
in U.S. Patent 3,927,716.
u
