Note: Descriptions are shown in the official language in which they were submitted.
5~
MISCIBLE DISPLACEMENT OIL RECOVERY METHOD
D#73, 635-F
FIELD OF THE INVENTION
This invention pertains to a miscible flooding oil
recovery process and particularly to an improved oil recovery
process utilizing a vertical, downward moving, miscible
blanket in which the pressure required in the reservoir to
attain miscibility is reduced.
BACKGROUND OF 1~ INVENTION
Petroleum is found in subterranean formations or
reservoirs in which it has accumulated, and recovery is
initially accomplished by pumping or permitting the petroleum
to flow to the surface of the earth through wells drilled
into the subterranean formation or ~hat purpose. Petroleum
can be recovered from subterranean formations only if certain
conditions are satisfied. For example, there must be an
adequately high concentration of petroleum in the formation,
and there must be sufficient porosity and permeability or
interconnected flow channels t]hroughout the formation to
permit the flow of fluids therethrough if sufficient pressure
is applied to the fluids. Furthermore, the formation
petroleum viscosity must be sufficiently low that petroleum
will flow through the flow channels if pressure is applied
thereto. When the subterranean petroleum-containing
formation has natural energy present in the form of an
underlying active water drive, or solution gas, or a high
pressure gas cap above the petroleum-satura-ted zone, this
natural energy is utilized to recover petroleum. This phase
of oil recovery is referred to as primary recovery. When
this natural energy source is depleted, or in the instance of
those formations which do not originally contain sufficient
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natural energy to support primary recovery operations, some
form of enhanced recovery or supplemental recovery process
must be applied to the formation. Supplemental oil recovery
is sometimes referred to in the 'iterature as secondary or
tertiary recovery, although in fact it may be primary,
secondary or tertiary in sequence of employment.
Al~hough waterflooding or water injection is the
simplest and most widely used form of oil recovery, it is
only partially effective because water does not displace
petroleum efficiently. Persons skilled in the art of oil
recovery have recognized this inefficiency of waterflooding,
and it has been proposed in the literature to inject a
solvent for petroleum into the ormation to reduce the
viscosity of the naturally occurring petroleum, followed by
injecting a drive fluid, such as water or natural gas, in
order to recover a higher perc:entage of the formation
petroleum ~han is possible utilizing water or gas alone.
A particularly promising type of miscible flooding
which has been applied successfully to reservoirs having
substantial vertical thickness, is referred to as vertically
downward moving, miscible blanket flooding. This type of oil
recovery is especially suitable for use in thick reservoirs,
e.g., petroleum reservoirs having vertical thickness in
excess of 50 feet or more. In mi~cible blanket flooding, a
solvent, e.g., a material which is miscible under reservoir
conditions with formation petroleum, is injected into the
upper portion of the petroleum reservoir. After a pre-
determined volume of solvent is injected, sufficient to form
a thin layer or blanket on the top of the oil-saturated
portion of the formation, a drive fluid such as lean or dry
gas, e.g., natural gas or methane, is injected into the upper
portion of the formation in order to displace the slug or
blanket of solvent vertically downward. The idealized
version of downward miscible blanket flooding contemplates
the establishment of a discrete, relatively thin layer of
solvent which is spread completely across the top of the
petroleum formation, with the layer of solvent then being
displaced downward in a substantially piston-like manner by
the subseguently-injected dry gas. Oil pr~duction is
ordinarily taken from wells completed in and in fluid
communication with the bottom of the petroleum-containing
formation. Drive gas will displace liquid solvent and
petroleum efficiently only if it is displacing the solvent
and petroleum vertically downward, thereby employing
lS gravitational forces to stabilixe the process and avoiding
viscous ~ingering as is sometimes encountered when ga~
injection is applied to a formation in a horizontally moving
displacement process.
Although in this simplified description, the
solvent action is obtained from the intermediate (C~ - Cg)
hydrocarbon components injected for the purpose of
functioning as a solvent, and the gas is descrihed only as an
inert displacing agent, in fact a multicomponent mixture is
formed in the formation, comprising the heavy components,
e-g-, the hydrocarbon or pe-troleum naturally occurring in
the formation, the intermediate component hydrocarbon species
which is the liquid solvent injected into the formation, and
the drive gas which may be considered to be essentially pure
methane. Since methane is substantially less expensive than
the intermediate component hydrocarbon, e.g., LPG or
s~
llquified petroleum gas or other hydrocarbons which may be
injected for the purpose of functioning as a solvent or
miscible displacement agent for petroleum, it is highly
desirable to operate under conditions where miscibility is
achieved between components of the naturally-occurring
petroleum, the injected solvents, and the injected dri~e gas,
utilizing the smallest volume possible of solvent. To
accomplish this, it is fre~lently necessary to raise the
pressure existing in the formation where it is desired to
obtain a condition of miscibility.
In field application of the above process,
considerable difficulty has often been encountered in
obtaining sufficient increase in formation pressure to
achieve the desired condition of miscibility. This sometimes
results from inherent low pressure in the reservoir, or the
presence of a high gas saturation of the oil zone, the
presence of a substant.ial size gas cap, or the presence of a
low pressure aquifer below the petroleum-saturated interval
which is of much greater size ~han the petroleum formation.
When the6e conditions are encountered, injection of
substantial quantities of gas, even at maximum injection
rates, raising the pressure of the reservoir sufficiently -to
achieve miscibility between oil and the injected miscible
fluids can seldom be accomplished. Accordingly, there is a
substantial problem in applying the miscible oil recovery
method described above to subterranean petroleum-containiny
formations because of the in~bility to raise the formation
pxessure substantially to attain miscibili~y with the
injected miscible displacing fluids.
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DESCRIPTION OF THE PRIOR ART
U. S. Patent, 3,850,243 describes a vertically
downward moving, miscible blanket oil recovery method in
which a dense solvent is co-mixed with the injected solvent
to increase ~he spreading rate of the solvent bank across the
oil column.
U. S. Patent 3,993,555 discloses a method for
separating bitumen from tar sand material by contacting the
material with a solvent at a temperature below the freezing
point of water in order to accomplish separation of
hydrocarbon and sand minerals without forming a froth with
water present in the tar sand material.
U. S. Patent 4,003,432 issued January 18, 1977, de-
scribes a method of separating bitumen from tar sand
formations employing a solvent having a freezing point
substantially lower than the freezing point of water, said
solvent being cooled to a temperature substantially below the
reezing point of water prior to injecting into the
formation, in order to displace hydrocarbons without
displacing water into the formation.
U. S. Patent 3,924,682 issued December 9, 1975,
describes a method of cooling a subterranean formation by
inje~ting cold water to permit use of a temperature-sensitive
surfactant in a high temperature formation.
U. S. Patent 4,050,513 issued September 27, 1977,
describes a method of injecting cold water to cool a
subterranean formation prior to injecting a solution of a
temperature-sensitive hydrophilic polymer.
SUMMARY OF THE INVENTION
_ .
Disclosed is an improved miscible oil recovery
process, especially a gas driven, vertically downward moving
miscible blanket oil recovery method in which the pressure
reguired to attain miscibility between the injected solvent
and the formation petroleum is reduced by reducing the
temperature in the portion of the formation contacted by the
injected solvent. The amount of hydrocarbon required to be
utilized în achi~ving miscibility may also be reduced ~y this
method. The temperature reduction may be accomplished by
cooling the solvent to a temperature substantially less than
the reservoir temperature prior to injecting it into the
formation, or by cooling the drive gas prior to injecting it,
or by both means.
BRIEF DFSCRIPTION OF THE DRAWINGS
Figure 1 illustrates a cross sectional view of a
formation in which a vertically downward miscible blanket
flooding oil recovery process employing the improvement of
-this invention is being applied.
Fi~ure 2 illustrates a ternary diagram for a system
coMprising methane-butane-decane showing phase envelopes at
2000 pounds per sguare inch for three different
temperatures, 280F, 160F and 40F.
DESCRIPTION_OF THE PREFERRED EMBODIMENTS
Basically, my invention is concerned with an
improvement in miscible 1Oodi~g oil recovery technolo~y.
The preferred form o miscible displacement to which the
process of my invention may be applied is a gas driven,
vertically downward moving miscible blanket oil recovery
process. The vertically downward-moving gas driven miscible
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blanket oil recovery process may best be understood by
referring to Figure 1, in whicn Eormation 1 is penetrated by
a solvent injection well 2 which is in fluid communication
with the upper portion of formation 1, and by gas injection
well 3 which is also in fluid communicaton with the upper
portion of formation 1, and by production well 4 which is in
fluid communication of the bottom of oil formation 1. In
conventional practice, a solvent, usually an intermediate C 2
to C 9 and preferably C 3 to C 5 hydrocarbon, ordinarily a
mi~ture of hydrocarbons within this range, such as
commercially available mixtures including LPG or liquified
petroleum gas, is injected into the upper portion of the
formation via well 2. The liquid solvent spreads across the
upper portion of the formation, since the solvent density is
less than the density of the pe-troleum present in ~he
formation. Injection of solvent .is not on a continuous basis
during the course of miscible displacement oil recovery
process. Rather, a predetermined volume of solvent is
injected into the formation, allowed in some instances to
remain for a period of time sufficient to ensure that it has
spread across the desired aerial extent of the formation, and
thereafter gas is injected into the top of ~he formation and
no solvent is injected. In actual field conditions, it might
be preferred to inject solvents into the formation via both
wells 2 and 3 in the attached Figure 1, until the
predetermined amount of solvent has been injected.
Thereafter solvent injection would be discontinued in both
wells, and dry gas would be injected in-to the formation via
wells 2 and 3 simultaneously to serve as -the drive fluid. In
fact, some overlap of gas injection and solvent injection
s~
might be encountered, and so the conditions illustrated in
Figure l represents an intermediate time near the completion
of solvent injection and the beginning of drive gas
injection.
The blanket of solvent 5 spreads across the
formation and is displaced downward by drive fluid occupying
the interval of formation 6 above the solvent blanket. Since
hydrocar~on gases are commonly employed in the drive fluid,
some mixture of hydrocarbon gas in the intermediate
hydrocarbon fluid forming solvent bank 5 will occur.
Ideally, conditions should be such that miscibility exists
between all of the components injected into the formation and
the fluids naturally present in the formation. A ternary
diagram such as that shown in Figure 2 is routinely employed
by persons skilled in the related art of oil recovery for
determining the conditions under which single phase liquid
miscibility is obtained between the injected fluids and the
hydrocarbons present in the fluid. A simplification is
sometimes employed such as that illustrated in Figure 2, in
which the drive gas is shown as substantially pure methane,
the solvent as substantially pure butane and decane is
utiliz~d to represent the naturally occurring hydrocarbons in
the formation. In fact, natural gas is usually employed as
the drive fluid and while it is predominantly methane, small
concentrations of higher molecular weight paraffinic
hydrocarbons are also usually present. Similarily, the
solvent employed is generally a mixture of hydrocarbons in
the range from C 2 to C 5, and in fact there is some advantage
in employing such a mixture. Obviously, petroleum present in
the formation will represent a large range of molecular
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species o~ varying molecular weight. It has been found,
however, that the simplified ternaxy diagram can be used to
accurately predict performance in the field of the more
complicated fluids injected and present in the reservoirs.
Turning to Figure 2, there can be seen the phase
envelope of the ternary mixture of methane, butane and decane
at 2000 pounds per square inch at three different
temperatures. Curve 7 illustrates the phase envelope at
approximately 160F, which was the temperature of a
particular reservoir being studied for miscible flooding
conditions. Curve 8 illustrates how raising the temperature
to 280F would shift the phase envelope. Curve 9 illustrates
the change in the phase envelope resulting from reducing the
temperature to about 40E'. The effectiveness of changing the
temperature is illustrated by considering an equilibrium
condition designated as point 10 on the diagram. This
represents a mixture comprising approximately 50% methane,
appro~imately 34% butane and ~out 17% decane. Point 10
represents a conditions which would exist in a reservoir into
which fluids were injected suficient to attai~ this relative
proportion of the components described above. At the
temperature of Curve 7, point 10 is in a two phase, gas and
oil region. Under this condition, efficient miscible
displacement could not be attained. By lowering the
temperature sufficient to move the phase envelope from Curve
7 to Curve 9, point 10 is clearly made to fall in the single
liguid phase region where efficient miscible displacement is
obtained.
Another improvement in the operation is achieved by
decreasing the temperature. By drawing tangents to the
247
curve, it is possible to identify the minimal solvent
component of a solvent gas mixture at which single liguid
phase conditions can be attained after the mixture is
injected into the reservoir. It can be seen that by reducing
the temperature of the portion of the formation in which a
miscible condition is to be attained, the amount of the
intermediate hydrocarbon, e.g., the solvent in an oil
recovery method and butane in a ternary diagram, is reduced
from about 48% to about 38%, a significant reduction in the
amount of solvent required to attain miscibility. This would
cause very significant reduction in the cost of operating a
miscible displacement process in the field.
Reduction in temperature in the portion of the
formation in which the condition of mis~ibility is sought to
be achieved can be accomplished by injecting a fluid into
that portion of the formation at a temperature substantially
less than formation tempexature. It is not necessary to
inject sufficient cold fluid into the formation to cool
substantially the entire formation from which recovery must
ultimately be sought. Miscibility is a dynamic condition and
the zone where the miscibility condition exists, moves
downward throu~h the forma-tion during the course of
conducting the miscible displacement oil recovery method. In
the application illustrated in Figure l, the zone where
miscibility should be achieved is in the region of the
miscible blanket or layer of solvent illustrated in zone 5 of
Figure l. If the solvent and the gas injected wi~h the
solvent are both chilled prior to injecting into the
formation, -~he temperature of the zone in which the solvent
contacts the formation is reduced and injection of cold gas
--10--
maintains the temperature in the desired portion of the
formation at a temperature substantially lower than that
existing in the bottom portion of formation 1 during the
course of the injection sequence, until the miscible blanket
5 had moved to a point near the bottom of formation 1.
In this particular application, it is stxongly
preferred that the temperature of any fluid injected into the
formation be maintained at a level above the freezing point
of water, in order to avoid freezing water present in the
formation, which could cause localized loss of permeability
in that zone of the formation which would interfer with
efficient movement of solvent thereby. The fluid temperature
should be in the range of from 35F to 100F and preferably
from 40F to about 70F.
Another preferred method for accomplishing
temperature reduction in the portion of the formation where
miscibility is sought to be achi.e~ed, comprises compressing
the drive gas to a pressure substantially greater than would
ordinarily be employed for gas injection, and cooling the
high pressure gas, preferably by ambient air temperature
cooled heat exchangers. The gas is then injected into the
formation at relatively low temperature, i.e., substantially
lower than that which exists after the gas compression stage.
The perforations in the gas injection well 3 would then be
made relatively small, so a substantial pressure drop exists
across these perforations. The perforations should be chosen
so the pressure drop across the perforations is at least 300
psi and preferably at least 700 psi. This ensures that
substantially all of the gas expansion occurring during the
injection phase occurs at perforations 13 of well 3.
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Accordingly, the drive gas will in this embodiment serve as a
refrigerant as well as a drive gas, and the portion of the
formation in the general vicinity of perforation 13 will be
cooled significantly.
~XAMPLE
For the purpose of illustrating the amount o
energy re~uired to produce the degree of cooling required and
the degree of improvement attained by the process of this
invention, the following calculated example is offered.
For the purpose of simplicity, a small segment of
the reservoir where miscible displacement is sought to be
achieved according to the process of this invention, is
examined. A reservoir having a porosity of 15% and an oil
saturation of 80% is considered for the purpose of
determining the amount of energy reguired to cool the portion
of the reservoir containing one barrel of oil from 160F to
40F. The rock volume in this instance is:
(1 bbl oil) ~5.6) 3
V = ~ , - = 46.7 ft
(1.5xlO ) (8xlO
The amount of energy reguired to cool this portion
of the formation, including both the formation rock and the
oil contained in the pore spaces thereo is calculated as
follows:
H = (46.7~ (36) (120) = 201,744 BTU
The foregoing illustxates that it is necessary to
expend 201,7~4 BTUIs to produce sufficient cooliny to reduce
the temperature of a segment of reservoir containing one
barrel of oil, together with the associated reservoir rock,
from 160F to 40F. This is a reasonable number, and it is
substantially less than the amount of energy expended in some
thermal oil recovery methods in connection with thermal
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recovery of viscous oil, wherein it is not unusual to expend
over one million BTU's per barrel of oil produced. The
improvement in the miscible process resuiting from cooling
the fluids to 40 has been discussed above.
The foregoing illustrates how a miscible
displacement process is improved substantially by cooling one
or more of the fluids being injected into the formation
during the course of conducting the miscible process so as to
reduce the temperature of the portion of the formation where
miscibility is to be achieved, in order to educe the pressure
re~uired to achieve a condition of miscibility between the
injected fluids and the petroleum naturally occurring in the
formation, or to reduce the amount of solvent reguired to be
injected to achieve miscibility, or both.
While my invention has been described in terms of a
number of illustrative embodiments, it is not so limited
since many variations thereof will be apparent to persons
skilled in the art of oil recovery without departing from the
true scope of my invention. It is my intention and desire
; ~0 that my invention be limited only by the limitations and
restrictions appearing in the claims appended immediately
hereinafter below.
