Note: Descriptions are shown in the official language in which they were submitted.
1060785
FIELD OF THE INVENTION
This invention relates to a method for the recovery
of oil by a vertical miscible flood wherein a miscible slug is
formed at the crest of the oil column or the gas-oil interface,
and which slug is driven downward through the reservoir by a
drive agent wherein the oil is produced from the lower part of
the reservoir, and simultaneously the reservoir is produced
from the gas cap in a manner whereby improved coverage of the
slug is accomplished and at an increased rate thereby minimizing
dispersion and mixing of the slug into the oil column.
PRIOR ART
In the recovery of oil from a subterranean oil-bearing -
reservoir, one method that has been suggested for increasing
- oil recovery is that of miscible flooding wherein a solvent that
is miscible with the reservoir oil is injected as a slug via
an injection weIl, and thereafter a drive agent is injected to
drive the solvent slug and the oil through the reservoir toward
a production well from which the oil and solvent are recovered.
The process of miscible flooding is extremely
effective in stripping and displacing the reservoir oil from
the reservoir through which the solvent flows. This effective-
ness is derived from the fact that a two-phase system within
the reservoir and bet~een the solvent and the reservoir oil
is eliminated at the conditions of temperature and pressure
of the reservoir whereby a miscible transition zone is for~.ed
which eliminates the retentive forces of capillarity and
interfacial tension. These forces are significant factors in
reducing the recovery efficiency of oil in conventional
flooding operations where the displacing agent and the
reservoir oil exist as two phases in the reservoir.
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lO~iV785
In the usual miscible slug process, after the
solvent slug has been injected in sufficient amounts to form
the said transition zone, a drive agent is injected to drive
the solvènt slug and the reservoir oil through the reservoir.
A second transition zone is formed at the trailing edge of
the solvent slug between the solvent and the drive agent.
Miscibility may exist between the solvent and the drive agent,
dependent upon the reservoir conditions and the composition of
the solvent and the fluid used as drive agent.
In steeply-dipping reservoirs or thick reservoirs
having good vertical permeability, vertical displacement
processes are known to improve the displacement efficiency
resulting in increased recovery. Thick reservoirs may include
reef reservoirs which herein mean oil-bearing formations whose
matrix is a reef vis-a-vis a sandstone sediment or limestone
deposit.
In a vertical slug displacement method, the solvent
is injected at the crest of the oil column or at the gas-oil
interface to form a slug or a "blanket" of the solvent between
the gas cap and the oil column. Thereafter, a displacing
fluid or drive agent is injected at or near the crest of the
gas cap tc displace the formed slug or blanket and the oil
downward toward production wells that have communication with
the lower horizons of the oil-bearing reservoir.
The composition of the solvent used for the miscible
slug is generally a light hydrocarbon such as propane or LPG,
or a mixture of light hydrocarbons having from two to six
carbon atoms in the molecule, although higher molecular weight
hydrocarbons can be used under certain conditions. The
0 solvent may also include in its composition methane or a
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lean gas, that is, a gas containing methane with minimum
amounts of C2-C6 hydrocarbons. In determining the composition
- of the solvent to be used, the criterion is that the solvent
be miscible with the reservoir oil at reservoir conditions of
pressure and temperature.
The drive agent is generally a gaseous hydrocarbon
such as natural gas or methane, that is capable of forming a
miscible transition zone with the slug material. Prior art
also teaches that the drive agent may be inert gases such as
air, nitrogen or flue gases. The drive agent may also be
water, in which case miscibility does not occur at the trailing
edge of the solvent slug, in the situations where the solvent
i is a low molecular weight hydrocarbon or mixtures thereof.
;~ The success of the process is greatly dependent
~ upon maintaining the integrity and discreteness of the slug
,~ so that miscibility is retained during the flooding operation.
At the same time, in order to attain optimum economic benefits,
the size of the slug should be minimal.
One of the difficul~ies that has been realized in
the miscible slug process is the disintegration of the slug
with consequent loss of miscibility. In U.S.P. 3,845,821 there
is taught that the undesirable mixing may be minimized thereby
maintaining the integrity of the slug by establishing the slug
"in-situ" at the crest of the oil column or at the gas-oil
interface by the separate and simultaneous injection of a
stream of the light constituents and a stream of the heavy
constituents comprising the miscible slug. The slug material
is then followed by the injection of the drive agent to dis-
place the slug and the reservoir oil downward through the
reservoir.
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:
In the present invention, the problems of establish-
ing the slug or blanket that relate to the time required to lay
down the slug as a blanket are overcome by establishing a
pressure drop at the horizontal layer at the top of the oil
column and at the same time producing fluids from the formed
gas cap so as to minimize the dilution effect of the solvent
slug. The idealized version of downward miscible blanket
flooding contemplates the formation of a discrete, relatively
thin layer of solvent which has spread completely across
10 the top of the oil column from which oil recovery is sought,
with the miscible slug or blanket being displaced downward
in substantially piston-like manner by the subsequently
injected drive agent. Oil production normally will be from a
well or wells completed in the bottom of the oil-bearing
reservoir. Initially only oil will be recovered, and after a
substantial amount of time has elapsed a mixture of the
previously injected solvent slug and oil will be recovered from
the reservoir. Since the upper portion of the oil column has
a reduced viscosity as a result of the presence of the miscible
20 blanket therein, much more efficient displacement of oil from
the reservoir is achieved than would be possible utilizing
lean gas alone.
If the miscible blanket fails to spread over all of
the top of the oil co~umn or oil saturated zone, only a portion
of the reservoir will be subjected to miscible blanket flooding,
and the portion not covered by the spreading miscible blanket
will be subjected only to downward displacement by lean gas.
Gas displacement is relativeIy inefficient, so a portion of the
reservoir over which the miscible blanket has not spread will
30 experience much lower recovery efficiency than is achieved in
the portion of the reservoir which has been covered by the ?
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lO~V785
miscible blanket. Accordingly, the anticipated high recovery
efficiency of vertically downward miscible blanket flooding
is achieved only if the injected solvent blanket spreads at
a sufficiently high rate that it covers the top of the oil
column completely. Thus, in U.S.P. 3,850,243 there is taught
the improvement of spreading the slug of solvent material
- more rapidly by using the conventional solvent to which has
been added a high density solvent such as carbon disulfide
or certain halogenated hydrocarbons so as to more nearly
match the density of the solvent to a value slightly less
than the density of the reservoir oil. In a related patent,
U.S.P. 3,878,892 there is taught the use of a high density
solvent which is injected separately and simultaneously with
the conventional solvent.
In view of the foregoing discussion, it can be
appreciated that the total oil recovery efficiency will be
reduced dramatically in applications of vertical downward
moving miscible blanket flooding if complete spreading of the
injected solvent blanket does not occur because of, for
example, slow spreading rate. Accordingly, there is a
substantial need for a method for improving the spreading rate
of an injected solvent blanket over the top of the oil-
saturated zone of the reservoir being subjected to miscible
blanket flooding.
Still another problem is sometimes encountered in
miscible blanket flooding. In application of this technique
in formations containing appreciable quantities of asphaltic
or bituminous materials, complete miscibility between the
injectea solvent and the reservoir oil may not be achieved.
This is particularly true since the nature of the solvent is
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106i~)785
frequently influenced by the types of solvent materials
available in the area. Mixtures of Cl to C6 aliphatic hydro-
carbons, for example, sometimes are utilized as the miscible
blanket, and saturated hydrocarbons such as these are not
suitable solvents for asphaltic materials. Accordingly, there
is also a substantial, unfulfilled need for an improved
miscible blanket flooding technique which will achieve
efficient recovery of high asphalt-content oil.
In the establishment of the solvent blanket on top
lQ of an oil column, gravity alone has been utilized for spreading
the blanket on the oil column. Where the injection time is so
- long, dispersion of the blanket occurs before the slug reaches
the periphery of the oil column trap.
- In addition to this another difficulty lies in the
dilution effect by the mixing of the slug with the residual
oil that exists in the gas column. As the solvent in the slug
invades the gas cap in a horizontal manner it displaces the
gas phase preferentially to the oil because of the high
relative permeability and the low viscosity of the gas.
The efficiency of the displacement varies directly
with the pressure. In order to maintain the pressure at a
high level the gas is injected into the crest of the reservoir
via other wells while the solvent is injected at the top of
the oil column. The volume of gas cap invaded is high due to
the high vertical build-up of the slug around the injection
well that is necessary to impose adequate driving potential to
invade horizontally the entire top of the oil column. The
time of arrival of the slug at the periphery is delayed and a
larger volume of gas cap is invaded which adds to the dilution
of the slug by the residual oil.
106078S
In order to overcome these difficulties and to
minimize the amount of slug required I have found that simulta-
neous injection of the solvent slug into the top of the oil
column together with production of the gas cap near the top of
the oil column will improve the laying down of the blanket of
solvent and maintain the integrity of the slug.
BRIEF DESCRIPTION OF THE FIGURE
The accompanying FIGURE is an illustrative embodiment
of the invention showing a solvent injection well and two
production wells.
DESCRIPTION OF THE INVENTION
This invention may be applied to steeply-dipping
- reservoirs or thick reservoirs having good vertical permeability.
In the practice of this invention, the reservoir is penetrated
by at least one crestal injection well into the upper horizon
of the oil saturation zone and at least two production wells
one of which penetrates the lower horizon of the oil saturation
zone of the reservoir and the second of which penetrates the
upper horizon of the oil saturation zone, and which well bears
a spaced horizontal relation to the solvent injection well.
Thus the present invention provides a method for the
recovery of oil from a subterranean oil-bearing reservoir having
an oil saturation zone, a gas-oil transition zone, and a gas
saturation zone wherein a slug or blanket of solvent miscible
with said oil is driven downwardly through said reservoir by a
drive agent, thereby displacing said oil downwardly through said
reservoir, comprising the steps of:
(a) providing a first injection means extending into said
reservoir adjacent said gas-oil transition zone for injection
of said solvent into said reservoir,
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1060785
(b) providing a second injection means extending into
` said reservoir adjacent said gas saturation zone for injecting
said drive agent into said reservoir,
(c) providing a first production means in spaced relation
: to said first injection means and adjacent said gas-oil transi-
tion zone,
(d) providing a second production means extending into
said reservoir and adjacent the lower horizon of said oil
saturation zone,
(e) injecting via said first injection means a solvent
in amounts sufficient to establish said blanket of said solvent
in the vicinity of said gas-oil transition zone,
(f) simultaneous therewith producing fluids via said first
production means, from said gas-oil transition zone thereby
: creating a pressure gradient at said gas-oil transition zone
during the establishment of said solvent blanket at said gas-oil
transition zone,
(g) injecting via said second injection means a drive
agent thereby displacing said solvent blanket and said reservoir
oil downwardly through said reservoir,
(h~ producing said oil via said second production means.
In a preferred embodiment the above method is provided ?
wherein said first and second injection means comprise a well
completed by setting a casing to at least said gas-oil transi-
tion zone said casing being perforated in two intervals thereby
forming a set of first perforations to a depth adjacent said
gas-oil transition zone and a set of second perforations adjacent
the upper portion of said gas saturation zone, running a tubing
into said casing to a depth of said gas-oil transition zone and
setting a packer in the annulus formed by said casing and said
tubing and intermediate between said two sets of perforations.
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1060785
In another embodiment the present invention provides
a vertical downward miscible flobding technique for recovering
oil from a subterranean oil-bearing reservoir, said reservoir
having an oil saturation zone and a gas saturation zone wherein
a blanket of solvent miscible with said oil is driven downwardly
through said reservoir by a drive agent thereby displacing said
oil downwardly through said reservoir comprising the steps of:
(a) providing a first injection well penetrating said
reservoir to the top of said oil saturation zone,
(b) providing a second injection well extending to the
top of said gas saturation zone,
(c) providing a first production well extending to the
top of said oil saturation zone,
(d) providing a second production well extending to the
bottom of said oil saturation zone,
(e) injecting said solvent via said first injection well
and simultaneously producing fluid via said first production
well, to establish said blanket of miscible solvent,
(f) injecting said drive agent via said second injection
well and producing said oil via said second production well.
In certain aspects, this invention provides for a
solvent injection well that penetrates the top several feet of
the oil column. In reservoirs that do not initially contain
a gas cap, the solvent injection well or wells are completed
in or near the crest penetrating several feet into the oil
column or oil-bearing reservoir adjacent the overburden stratum.
In reservoirs that contain a gas cap. either present initially
or formed by production and gravity drainage, the solvent
injection well or wells penetrate the reservoir
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106(~785
at least to the depth of the gas-oil transition region. This
region is a transition region between the upper gas saturation
zone and the lower oil saturation zone in which the fluid
saturation changes from one of predominantly gas saturation to
one of oil satura~ion. This transition region is often referred
to as the gas-oil interface.
In one embodiment, the injection well is completed
so as to provide for the injection of the slug material and
the drive agent by suitable completion techniques known in
the art.
A preferred method of operation of the invention is
illustrated in the accompanying FIGURE which depicts the
situation where the reservoir has an oil saturation zone (1)
containing liquid hydrocarbons, a gas saturation zone (3) and
is overlain by overburden (4~. There is also shown a gas-oil
transition zone (2), which may be referred to as the gas-oil
interface and which is the horizon where the solvent blanket
will be established. A dual-completed injection well (5~-is
depicted that traverses the gas saturation zone (3) of the
oil-bearing reservoir and is completed to the depth of the
gas-oil interface (2), above the oil-saturation zone (1).
A primary casing (6), traversing the gas saturation zone (3)
to at least the uppermost region of the oil saturation zone
is cemented in place and is perforated in two intervals as
shown by perforations (7) and (8), thereby forming a first
and second set of perforations. Thereafter, a tubing (9) is
inserted into the casing to a depth adjacent the perforations
(8~. A packer (10~ is then set in the annulus formed by the
tubing and the casing, positioned intermediate between the
0 two sets of perforations, that is above perforations (8), and
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lQ60785
below perforations t71. The lower end o~ the tubing (9~ is
open to provide commun1cation with the formation via the
perforations (8).
While the FIGURE shows the use of a single well
with dual completion, two injection wells may be used, one
penetrating at least to the gas-oil transition zone and the
second penetrating the gas cap so as to provide means for the
injection of the solvent slug and the drive agent separately.
These wells would be completed by conventional means well-known
in the art.
Referring again to the accompanying FIGURE a
production well (ll~ is provided that traverses the reservoir
to at least the bottom of the oil saturation zone (l). The
well is completed with casing (12), perforations (13),
packer (14) and tubing means (15). Communication with the
oil saturation zone (l) via the perforations (13) is provided
whereby the oil is produced from the reservoir. A pump, not
shown, may be locatea at the bottom of the tubing string.
A second production well (16) is also provided that
penetrates to the horizon of the gas-oil transition zone or
the gas-oil interface and the well is completed with casing
(17~, which is provided with perforations (18) for communica-
tion with the gas-oil transition zone (2). The well may be
completed in the conventional manner with packer tl9) and
tu~ing string means ~20) havirg communication with the gas-oil
transition zone.
It may be desirable in some instances to utilize
a dual-completed production well. The well would be completed
in a manner similar to that described for the dual-completed
injection well, with the weIl being perforated at the bottom
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1060785
of the oil saturation zone and at the gas-oil transition zone,
and with a packer set in the annulus.
While it is taught in the prior art that the rate of
spreading is determined by the viscosity of the injected
solvent and the difference in density between the solvent and
the gas in the gas saturation zone, in the present invention
the additional factor of pressure drop is considered. Thus
the production well ~16~, bears a spaced relationship to
injection well (5) that can be determined by the desired
pressure drop across the horizon at which depth both wells
have been completed. For example, at reservoir conditions
of 3000 psi for a desired pressure drop of 100 psi the second
production well would be about 2000 feet from the injection
well.
In operation the solvent is injected via the
tubing (9) of injection well (5) and perforations (8) into
the gas-oil transition zone. Simultaneously with the injection
of solvent, well (16) is produced in a manner to maintain a
pressure drop across the horizon to increase the spreading
rate of the solvent, thereby establishing the solvent blanket
more rapidly. Once the solvent blanket has been established
across the oil saturation column, well ~16) may be shut in.
The well (16) may later, under certain reservoir conditions, be
utilized as an injection well for the drive fluid as the
solvent blanket is displaced downward.
In the application of this invention, the reservoir
may be repressured, if required to attain miscibility by the
injection of other fluids to establish at least saturated
reservoir conditions prior to or during the injection of the
solvent slug. Fluids that may be used for repressuring include
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1060785
methane, natural gas, carbon dioxide, nitrogen, air, water
and mixtures thereof.
It is within the scope of the practice of this
invention to include miscible floods that are termed "instant
miscible" floods, and "conditional miscible" floods. In the
former type, miscibility occurs on contact of the injected
solvent fluid with the reservoir oil. In the latter type,
miscibility is attained within the reservoir either by the
vaporizing of the lighter constituents of the oil into the
solvent fluid or by the absorption of the heavier constituents
of the solvent fluid into the oil. The composition of the
solvent for the type of miscible flood desired may be determined
by laboratory tests such as slim tube tests which involve
techniques well-known in the art.
The composition of the solvent slug may be a light
hydrocarbon such as propane or LPG, or a mixture of light
hydrocarbons having from two to six carbon atoms in the
molecule, although higher molecular weight hydrocarbons can
be used under certain conditions. The solvent may also include
in its composition methane whereby the solvent is a lean gas,
that is, a gas containing methane with minimum amounts of
C2 to C6 hydrocarbons said lean gas being miscible with the
reservoir oil at reservoir conditions of temperature and
pressure.
The solvent is injected in amounts sufficient to
form a slug or blanket at the top of the oil column or at
the gas-oil transition zone.
After the solvent blanket has been established,
the drive agent is injected to displace the blan~et downward
through the reservoir, thereby displacing the oil ahead of
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`` 106V785
the solvent toward the production wells from which the
oil is recovered.
The drive agent or driving fluid employed may be
any gaseous material that is gaseous at reservoir conditions.
Additionally, the drive agen~ may be miscible with the
solvent slug. The preferred drive agent is a relatively
inexpensive gas, such as a gas containing substantially
methane or natural gas or flue gas, or a gas from a gas-
processing facility. Other gases that may be employed include
ethane, carbon dioxide, nitrogen, air and mixtures thereof.
The drive agent is injected in an amount sufficient to
displace the solvent slug or blanket through the reservoir
thereby displacing the reservoir oil ahead of it and also
- recovering the solvent. The drive agent is injected at a
rate so that the preferred rate of movement is from about
0.3 to 10.0 feet per day.
In summary, in accordance with the practice of this
invention a vertical miscible flood is carried out in the
following manner. There is introduced into the reservoir at
the top of the oil column or oil saturation zone, a solvent
' slug or blanket that is miscible with the oil, and separately
and simultaneously production occurs from the same horizon so
as to increase the spreading rate of the slug. Thereafter,
or simultaneously therewith, a drive agent or drive fluid is
injected to displace the solvent and the reservoir oil down-
ward toward a production well completed in the lower horizon
of the oil saturation zone from which the reservoir oil is
produced. The previously injected blanket or solvent will
retain its discreteness, spread over the entire oil column,
and be continually displaced downward through the oil column
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by the gas being injected above. After the solvent blanket
is produced with residual reservoir oil dissolved in it, the
: solvent may be recovered for use in another field project.
Eventually when gas production begins, the production wells
may be recompleted as gas production wells and gas production
will be taken from the formation until pressure is depleted.
The gas may be reused in another field or sold as fuel.
Although my invention has been described in terms
of several embodiments, variations thereon will be apparent
to persons skilled in the arts without departing from the
spirit and scope of the invention. -~
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