Note: Descriptions are shown in the official language in which they were submitted.
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F-0732 -1
SURFACTANT WATERFlOO~ING IN
A HIGH BRINE ENVIRONMENT
This invention relates to the recovery of petroleum from
subterranean oil reservoirs. In particular, it relates to improved
waterflooding operations involving the injection of a surfactant slug and
drive fluid comprising concentrated brine.
In the recovery of oil from oil-bearing reservoirs, it usually
is possible to recover only minor portions of the original oil in place
by primary recovery methods which utilize only the natural forces present
in the reservoir. A variety of supplementa:L recovery techniques has been
employed in order to increase the recovery of oil from subterranean
reservoirs. A widely used supplemental recovery technique is
waterflooding, which involves the injection of aqueous media into the
reservoir. As the water moves through the reservoir, it acts to displace
oil therein to a production well system through which the oil is
recovered.
Interfacial tension between the injected waterflooding medium
and the reservoir oil, the relative mobilities of the reservoir oil and
injected media, and the wettability characteristics of the rock surfaces
within the reservoir are factors which influence the amount of oil
recovered by waterflooding. Thus, it has been proposed to add
surfactants to the flood water in order to lower the oil-water
interfacial tension and/or alter the wettability characteristics of the
reservoir rock. Viscosifiers such as polymeric thickening agents may be
added to all or part of the injected water in order to decrease the
mobility ratio between the injected water and oil and improve the sweep
efficiency of the waterflood.
Techniques involving the injection of an aqueous solution of
brine-tolerant sulfonate surfactants have been developed for use under
controlled conditions of salinity. Processes which involve the injection
of aqueous surfactant solutions have been described in U.S. Patents
3,508,612, 3,827,497, 3,890,239, 3,977,471 and 4,018,278, for instance.
The surfactant slug may be followed by a thickened water slug which
contains a viscosifier such as a water-soluble biopolymer in a graded
concentration in order to provide a maximum viscosity greater than the
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1 ~ 7
F-0732 -2-
effective viscosity of the flowing oil-water bank and a terminal
viscosity near that of water. A driving fluid such as a field brine may
be injected with or without the thickener to carry the process to
conclusion.
~ y adding brine-tolerant surfactants to the injected fluids,
recovery of the petroleum can be enhanced. However, employing adequate
surfactant to substantially enhance the recovery of oil from the
subterranean formation by the flooding water has not been generally
economically feasible heretofore because the surfactants are adsorbed
from the surfactant solution onto the rock surfaces of the subterranean
formation. As a result of this adsorption of the surfactant, the
concentration of the surfactant in the flooding water becomes less than
that required to achieve enhanced recovery of the oil. Moreover, the
adsorption, where the surfactant is a mixture, causes a chromatographic
dispersion to separate components of the surfactant mixture on the basis
of their relative sorptivity. Frequently, this dispersion destroys the
efficacy of the surfactant mixture in lowering the interfacial tension
between the flooding water and the oil being displaced within the
formation.
Brine-tolerant surfactants are generally expensive chemical
compositions, and less expensive sacrificial agents can be employed to
prevent undue material losses. A surfactant composition has been found
which overcomes most of the shortcomings of proprietary combinations.
These combinations are not known in the prior art in waterflooding to
recover oil from subterranean reservoirs.
In accordance with the invention, there is provided an improved
method of recovering oil from a subterranean formation containing oil
having at least one injection welI and at least one production well, the
improvement comprising the steps of: (1) injecting through an injection
well and into said subterranean formation an aqueous solution containing
high salinity and a sufficient amount of a surfactant combination
comprising a water soluble ether-linked sulfonate or sulfate, a petroleum
sulfonate and a preferentially oil soluble aliphatic alcohol; (2)
injecting through said injection well subsequent to said aqueous solution
an aqueous brine drive fluid; and (3) recovering oil from a production
well. The surfactant combination, in addition to the two types of
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F-0732 -3-
sulfonates and the alcohol named, may contain a sacrificial
lignosulfonate agent. Also, alkylbenzene sulfonates may be used in place
of the petroleum sulfonates. Thus, the invention involves a
multi-component surfactant system containing cosurfactants and,
preferably, sacrificial agents.
A method is provided for recoverinq oil from a subterranean
formation containing oil and having at least one injection well and at
least one production well. The improved technique may include injecting
through an injection well and into the subterranean formation an aqueous
surfactant solution containing high saline content, i.e., not less than
about 3% TDS, and sufficient surfactant to effect an interfacial tension
between said aqueous saline surfactant solution and said oil of less than
about 0.1 dyne per centimeter. This can be used advantageously with a
process in which immediately following the aqueous saline surfactant
solution, or subsequently delayed, an aqueous brine drive fluid having
initial saline concentration of salt at least 75% of the surfactant
salinity concentration is injected. Sufficient additional drive fluid is
injected for recovering oil from a production well.
In one aspect of the present invention, there is provided a new
and improved waterflooding process employing a surfactant solution in
highly saline brine comprising an alcohol and an ether-linked surfactant
which exhibit contrasting oil-water solubility preferences. In carrying
out the invention, at least a portion of the fluid introduced into the
oil reservoir via a suitable injection system is an aqueous liquid
containing a preferentially oil-soluble alcohol of limited water
solubility and a preferentially water-soluble anionic surfactant
comprising a hydrocarbyl ether-linked sulfonate or sulfate wherein the
hydrocarbyl group provides a lipophilic surfactant base and wherein the
ether linkage is provided by an alkoxy linkage having a ratio of carbon
atoms to oxygen atoms within the range of 2 to 3. In a preferred
embodiment of the invention, the preferentially oil-soluble alcohol is an
aliphatic alcohol containing from 5 to 7 carbon atoms. Preferred
ether-linked surfactants for use in highly saline solutions are
sulfonated polyethoxylated aliphatic alcohols having 2 to 4 ethylene
oxide units and in which the aliphatic group providing the lipophilic
base contains from 16 to 18 carbon atoms.
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F-0732 ~4~
The ether linkage of the anionic ether-linked sulfates or
sulfonates employed in carrying out the present invention with
highly-saline water preferably is provided by an alkoxylated group
derived from ethylene oxide or propylene oxide or mixture of ethylene
oxide and propylene oxide. The number of alkoxy groups in the ether
linkage will vary depending upon such factors as the character of the
lipophilic surfactant base and the salinity of the aqueous surfactant
solution, but normally the ether linkage will contain from 1 to 20
alkylene oxide units.
Ether-linl<ed sulfonates exhibit better thermal stability than
sulfate derivatives and a preferred ether-linked sulfonate for use in
carrying out the present invention is characterized by the formula:
13 (1)
RlO-(cnH2nO)x R2 3
wherein Rl is a lipophilic base provided by a C8 to C22
aliphatic group or an aliphatic substituted aryl group containing from 5
to 24 aliphatic carbon atoms with at least one aliphatic substituent
containing at least 5 carbon atoms,
n is 2 or 3,
x is a number within the range of 1 to 20,
R2 is a Cl to C4 alkane group,
R3 is a hydrogen, a hydroxy group or a methyl group, and
M is an alkali metal or nitrogenous base.
The sulfate useful herein has the formula:
(2)
RlU-(cnH2no)x 503M
wherein Rl, n, x and M have the same meaning.
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F-0732 -5-
In this description, parts by weight and metric units areemployed, unless otherwise stated. Although significant variations in
surfactant compositions are set forth, common components include the
following:
The ether-linked surfactant used was a reaction product of
propane sultone and a sodated commercial ethoxylated Cg alkyl phenol
known as "Igepal C0430" (GAF Corp.). This surfactant, identified in the
following Table as "Cu^ 430 PS", has the following structural formula:
C9Hl9 ~ O(CH2CH20)n CH2CH2CH2 SO~Na
where n averages 4. **
The standard petroleum sulfonate used herein is "TRS-40" (Witco
Chem. Corp.), and the amounts given herein refer to active sulfonate
material in the commercial product, which also contains oil and water
along with the 40-43% active material. This petroleum sul~onate has an
average equivalent weight of about 342. The cosolvent aliphatic alcohol
preferred herein is n-hexanol. Lignosulfonate is employed optionally and
is identified as "ERA-5" ~m*erican Can Co.). The "PS 420" is a petroleum
sulfonate with a molecular weight of about 420.
Standardized laboratory oil displacement tests are performed
with regard to crude oil employing elongated flow tubes. In each
standard run, the tube is packed with unconsolidated Berea sand and then
saturated with saline water. The crude oil is then flooded into the tube
until the effluent is water free. The total amount of water displaced
from the tube during this operation is measured to determine initial oil
saturation. Each tube is subjected to a simulated waterflood by
injecting a brine until the effluent is free of oil. The amount of oil
produced during this operation is measured in order to determine the
residual oil saturation after waterflood of the tube. A simulated
surfactant waterflood is then carried out by injecting an aqueous
surfactant slug followed by injection of a driving fluid until the
effluent from the tube is free of oil. A constant flow rate is
maintained equivalent to an advance rate of about 2 meters per day (6.6
ft./day)~ The amount of oil recovered during this operation is measured
* Trademark
** Trademark
*** Trademark
~31
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F-0732 -6-
in order to determine recovery efficiency. Unless otherwise noted, waterused in the initial water saturation step, the simulated waterflood, the
surfactant slug, and the drive fluid is a mixed brine containing 16.6
weight percent total dissolved solids, dominantly NaCl with 1.2% Ca~Mg.
Surfactant slugs were typically driven by the appropriate brine
containing 0.1 wt. ~'Kelzan', which solution has a viscosity of about 45
centipoises (cp) at 76F, the temperature employed for tests unless
otherwise stated. The polymer used is the anionic polysaccharide B-1459
produced by fermentation of glucose with the bacterium Xanth~monas
campestris (NRRL ~-1459 USDA) which is available from the Kelco Company
under the trademark "Kelzan".
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F-0732 -9-
Water solubility of directly linked anionic surfactants, such as
petroleum sulfonates, is ordinarily adversely affected by increasing
salinity with the result that surfactant precipitation occurs at moderate
monovalent salt concentrations and in the presence of even smaller
amounts of divalent metal salts. The ether-linked anionic surfactants
tolerate much higher salinities since the water soluble ether moiety is
affected by dissolved salts to a lesser degree than the anionic
hydrophilic group. The sulfonate derivatives are much more stable in
high temperature environments. Therefore, the use of the sulfonate
derivatives is preferred in carrying out the present invention,
particularly where the temperature of the reservoir to be flooded is
about 50C or above.
The sulfonate or sulfate anionic group may be linked to any
suitable hydrocarbon group which provides a lipophilic base of the
surfactant. Thus, the lipophilic base of the anionic ether-linked
sulfates or sulfonates employed in the present invention may be provided
by aliphatic groups or aliphatic substituted aryl groups. Where the
lipophilic base is provided by an aliphatic substituted aryl group, the
aryl component may be mononuclear (benzene) or dinuclear (naphthalene)
and contain one or more aliphatic substituents. Preferably the aryl
component will be mononuclear in view of the practical consideration of
economy and product availability. The aryl group is substituted with one
or more aliphatic groups, at least one of which has 5 or more carbon
atoms with the total number of aliphatic carbon atoms being within the
range of 5-24. Where the lipophilic base is provided by an aliphatic
radical, it should contain from 8 to 22 carbon atoms. The aliphatic
groups or aliphatic substitutents may be unsaturated and/or contain
branched chains or may take the form of normal alkyl radicals. Where M
is an alkali metal ion, it may be sodium or potassium. Various
nitrogeneous bases, including ammonium or quaternary amines, may be
employed. Representative alkylammonium ions include methylammonium,
ethylammonium, and normal or isopropylammonium ions, and examples of
alkanolammonium ions include monoethanolammonium and triethanolammonium
ions.
Preferably the ether linkage is provided by one or more ethylene
oxide groups because of the increased water solubility imparted to the
molecule. Thus, in a preferred form of surfactant characterized by
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F-0732 -10-
formula (1), n is 2 and x is a number within the range of 1 to 6. In thecase where Rl is an aliphatic group, it preferably contains from 12 to
20 carbon atoms. In the case where Rl is an aliphatic substituted aryl
group, the aliphatic substituents preferably contain from 9 to 18 carbon
atoms. In this instance, as noted previous:Ly, it is desirable that a
mononuclear aryl radical such as benzene, toluene, or xylene be
employed. As indicated by the formula, the alkylene yroup connecting the
sulfonate group with the ether linkage is provided by a C1 to C4
alkylene group which may be unsubstituted or which may be substituted by
a hydroxy group or a methyl group. Preferably, however, the alkylene
linkage is provided by an ethylene or propy:Lene group which is
unsubstituted or substituted by a hydroxy group. That is, R2 contains
2 to 3 carbon atoms and R3 is a hydrogen atom or hydroxy group. It is
known that many surfactant waterflooding processes are specifically
designed for the reservoirs to which they are applied. This specificity
depends upon a number of factors including the composition of the
reservoir oil, the ionic character of the reservoir water and the water
used in formulating the injected surfactant solution, and the reservoir
temperature. The HLB of the surfactant which is most effective in the
recovery of oil from a particular reservoir depends to some extent upon
the ionic strength of the injected water and the connate water within the
reservoir. Generally as the salinity increases, due to the presence of
monovalent salts such as sodium chloride or divalent salts such as
calcium chloride or magnesium chloride, the HLB at which the most
efficient oil recovery is achieved likewise increases.
The present invention may be carried out utilizing injection and
production systems as defined by any suitable arrangement of wells. One
well arrangement commonly used in waterflooding operations and suitable
for use in carrying out the present invention is an integrated five-spot
pattern of the type illustrated in U. S. Patent No. 3,927,716 to Burdyn
et al. The term "pore volume" as used herein is defined by that volume
of the portion of the formation underlying the well pattern employed, as
described in greater detail in the Burdyn et al patent.
The present invention may be carried out in conjunction with the
use of a thickening agent added for mobility control purposes. The
thickening agent may be added to the aqueous solution of alcohol and
1 ~688A~
r -0732
surfactant or it may be injected in a separate mobility control slug.
Where a separate mobility control slug is employed, it normally will be
injected immediately after the slug containing the surfactant. The
thickening agent may be added in concentrations so as to provide a graded
viscosity at the trailing edge of the mobility control slug. Use of
graded viscosities at both the leading and trailing edges of the mobilty
control slug is disclosed in U. S. Patent No. ~,018,281 to Chang.
Alternatively, the thickening agent concentration may be relatively
constant throughout. Normally, the viscosity of at least a portion of
the mobility control slug should be at least as great as the effective
maximum viscosity of the oil/water bank it displaces or typically it will
be within the range of about 1 to 9 times the viscosity of the reservoir
oil. Various thickening agents which may be employed for mobility
control purposes are well known to those skilled in the art and include
such polymers as the biopolymer "Kelzan~, previously identified, and the
various partially hydrolyzed polyacrylamides available from the Cow
Chemical Company under the trademark "Pusher" chemicals.
The ether-linked surfactant, petroleum or alkylbenzene sulfonate
and alcohol may be present in the aqueous liquid in any suitable
concentration depending upon the characteristics of the particular
reservoir involved and such factors as surfactant consumption, e.g. by
adsorption, and dispersion of the surfactant into the reservoir water.
The total surfactant concentration may range from 0.01 to 15 weight
percent although in most applications the total surfactant will be
employed in a concentration within the ran3e of 0.1 to 5 weight percent.
The alcohol concentration will, of course, depend upon the concentrations
of the surfactant, cosurfactant and the desired ratio of surfactant to
cosolvent. If a sacrificial agent, such as a lignosulfonate is employed,
its concentration in the surfactant slug may be within the range of frorn
about 1% to about lOYo by weight. The aqueous surfactant slug may be
injected in amounts ranging from 0.02 to 3.0 pore volumes with the larger
pore volume amounts being used with the lower surfactant concentratisns.
Usually it will be ~esired to inject the alcohol surfactant solution in
an amount within the range of 0.05 to 2.0 pore volumes.
.: :s
