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BACKGROUND OF THE INVENTION
The present invention relates to profile modification agents for altering
the permeability of preselected portions of a subterranean formation. In
preferred embodiments, the present invention relates to a new and improved
composition and method for profile modification of a subterranean
hydrocarbon-containing formation to reduce water:oil ratios and improved
petroleum recovery during enhanced oil recovery operations. More
particularly, it relates to new and improved aqueous gelable compositions
exhibiting high temperature gel stability at temperatures up to about
150.degree. C. and in harsh brines and methods for using same.
The enhanced secondary recovery of oil from oil-bearing or containing
subterranean formations by fluid drive processes, wherein a fluid is
injected into the formation by one or more injection wells to drive the
oil through the formation to one or more production wells is a known
process, commonly referred to as enhanced oil recovery. Fluids used in
such processes include liquids such as water and various hydrocarbons, and
gases such as hydrocarbon gases, carbon dioxide, steam, etc. Many oil
reservoirs comprise layers or zones of porous rock which can vary in
permeability from zone to zone. In all fluid drive processes, a recognized
problem is the predilection of the drive fluid to channel along or through
the more permeable zones of the formation. This is commonly referred to as
channeling. Another problem is viscous fingering which occurs, for
example, by the over-ride of a viscous fluid by a less viscous fluid. The
more conductive zones after the oil has been largely displaced therefrom
function as "thief zones" which permit the drive fluid to channel directly
from injection to production wells. In many instances, such channeling or
fingering results in leaving substantial quantities of oil in the less
permeable zones of the formation which are bypassed. Such channeling or
fingering can occur when the mobility i.e. the quotient of the reservoir's
permeability to the drive fluid divided by the viscosity of the drive
fluid becomes large relative to the mobility of the reservoir oil.
One of the significant problems, therefore, attendant to the production of
oil and gas from subterranean hydrocarbon containing formations, is the
concomitant production of water. Such produced water can be reservoir
water occasioned by coning or a similar phenomena of the aquifier, or it
can be injection water from secondary or tertiary recovery treatments
being applied to the formation. Whatever the source, there is an upper
limit beyond which water production can no longer be tolerated and its
further entry into the producing well bore must at least be reduced if
further production of hydrocarbon resources at that location is to be
continued.
Regardless of whether the undesired water is a natural drive fluid or an
artificial drive fluid such as from secondary or tertiary recovery
projects, the problem is primarily occasioned by the predilection of the
drive fluid to preferentially seek the higher permeability zones and to
more or less bypass the lower permeability zones. The mobility of a fluid
in a permeable geological formation is the effective permeability of the
formation to that fluid divided by the viscosity of the fluid. In the
past, a conventional method for reducing the mobility of drive fluids
through permeable formations has been to increase the drive fluids
viscosity. Such an increase in viscosity is generally accomplished by
using viscous solutions of high molecular weight polymers such as
polyacrylamides, cellulose ethers, polysaccarides and the like. Such
polymeric solutions have been found effective for reducing the water:oil
ratio in the total producing well effluent and for increasing the daily
production of hydrocarbonaceous fluids.
In actual field practice, however, such mobility altering polymers elute
out of producing wells quickly and the water:oil ratios rapidly rise back
to an undesirable level necessitating retreatment of the producing
interval with the viscous polymer solutions. These viscosity increasing
polymers are relatively expensive materials and a one time treatment would
be particularly desirable.
More recently, reduction in the permeability of the pre-selected portions
of various subterranean oil bearing formations has been accomplished with
gelable solutions of polymeric materials. The formation of gels by the
cross-linking of polymers is well known in the art for this purpose. A
great deal of literature has been generated concerning the formation of
gels in situ in underground formations for the purpose of treating the
formations to better produce oil and gas from bore holes drilled into the
formations and to decrease undesired water output. It is well recognized
that polymer gels and processes incorporating same facilitate the plugging
of underground formations in desired areas e.g. by modifying the fluid
flow profile, and in particular by decreasing the relative permeability of
the most permeable portions of the formations.
Prior art gelling compostions for use in profile modification applications
generally comprise water, polymers capable of being cross-linked by
polyvalent metal cations and polyvalent metal ion crosslinker. Prior art
crosslinkable polymers have included polyacrylamides,
carboxymethylcelluloses and polysaccharides, generally of high molecular
weight in excess of one million. A commonly used system for generating
polyvalent metal ions has been to provide them in the form of chelated
metal ion complexes or as part of a redox system. The redox system will
generally comprise redox couples wherein oxidizing agent is selected from
water soluble compounds of polyvalent metals wherein the metal is present
in a valence state which is capable of being reduced to a lower polyvalent
state as exemplified by potassium permanganate, sodium permanganate,
ammonium chromate, ammonium dichromate, the alkali metal chromates, the
alkali metal dichromates and chromium trioxide. Sodium dichromate and
potassium dichromate because of low cost and ready availability are the
most widely used of the oxidizing agents. The reducing agents in the redox
couples have included sulfur containing compounds such as sodium or
potassium sulfide, sodium or potassium hydrosulfide, sodium or potassium
metabisulfite, sodium or potassium bisulfite, hydrogen sulfide, sodium or
potassium thiosulfate, thioacetamide and others, as well as non-sulfur
containing compounds, such as hydroquinone, perahydrazinobenzoic acid,
hydrazine phosphite, hydrazine dichloride and others. Illustrative prior
art profile modification compositions and methods are disclosed in U.S.
Pat. Nos. 2,718,497; 3,502,149; 3,727,689; 3,749,172; 3,762,476;
3,785,437; 3,795,276; 3,952,806; 3,964,923; 3,981,363; 4,018,286;
4,039,029; 4,040,484; 4,043,921; 4,055,502; 4,110,230; 4,120,361; and
4,498,539 to list but a few.
The crosslinkable polymers used in the past have comprised mainly high
molecular weight partially hydrolyzed polyacrylamide compounds. A serious
shortcoming of the high molecular weight polyacrylamides is that the
effective life of the gel as a profile modifier is seriously decreased by
the natural temperature of oil-bearing formations having temperature
above, for example, 60.degree. C. and the hydrolysis caused thereby. This
temperature effect is further complicated by the significant divalent ion
concentrations found in most reservoir fluids, which can cause
precipitation of the modifier. Lower molecular weight polyacrylamides
which are partially hydrolyzed to about 10 mol percent carboxylate groups
have also been used. The higher molecular weight polyacrylamides may be
used at lower polymer concentration and hence have been considered more
economical. However, the thermal stability of the higher molecular weight
polyacrylamide materials is poorer than for the lower molecular weight
polyacrylamides and the lower molecular weight materials have exhibited
the best stability properties of the materials currently in use. Gelable
compositions comprising optimum concentrations of low molecular weight
polyacrylamide and cross-linking agent perform satisfactorily up to
temperatures of about 90.degree. C. However, at higher reservoir
temperatures such as those occurring naturally in a number of locations,
such as the North Sea, for example, temperatures at or about 120.degree.
C., frequently as high as about 150.degree. C., or often higher, even up
to 200.degree. C. are commonly encountered. At these higher temperatures,
even the low molecular weight polyacrylamide gelable composition loses all
of its strength within a matter of days. Moreover, as has been mentioned,
the polyacrylamides and partially hydrolyzed polyacrylamides are
susceptible to degradation and precipitation in harsh environment
reservoirs containing divalent ions such as Ca.sup.2+, Mg.sup.2+ and
Ba.sup.2+. Effective profile modification requires the gels to retain
their strength and water diverting characteristics for a time sufficient
to accomplish the flood at higher temperatures up to at least about
120.degree. C., preferably up to about 150.degree. C., and especially
preferable to about 200.degree. C. in harsh brine environments. At the
higher temperatures shorter time periods are required.
N-sulfohydrocarbon-substituted acrylamide monomers and polymers comprising
same are known. See, for example, U.S. Pat. No. 3,547,899, which discloses
a homopolymer of poly(2-acrylamido-2-methylpropanesulfonic acid) (AMPS).
In U.S. Pat. No. 3,679,000, it is disclosed that polymers and copolymers
of N-sulfohydrocarbon-substituted acrylamide monomers are useful as
mobility control agents, i.e., as viscosifiers in polymer-flooding or
fluid drive processes.
In commonly assigned U.S. Pat. No. 4,573,533 a mobility control reagent
comprising an aqueous composition of a polymer consisting of acrylamide
units and units of 2-acrylamido-2-methylpropanesulfonic acid or its salts,
is disclosed which is resistant to viscosity degradation in the presence
of divalent salt containing brines up to or at temperatures of about
90.degree. C. These acrylamide copolymers, however, are generally not
crosslinkable to form gels, and therefore are not suitable for extended
profile modification applications.
In commonly assigned copending application, Ser. No. 729,512, now U.S. Pat.
No. 4,746,687 there is disclosed copolymers of 1)
2-acrylamido-2-methylpropanesulfonic acid and salts thereof and 2) acrylic
acid and salts thereof as gellable compositions in conjunction with water
and polyvalent metal cross-linkers. While effective, the polymers are less
than all encompassing and, or such, the search continues for polymer
systems useful as profile modifiers.
Accordingly, it is an object of the present invention to provide a new and
improved composition and method for profile modification operations which
is effective at elevated temperatures of up to at least about 120.degree.
C. preferably up to about 150.degree. C. and especially preferably up to
about 200.degree. C., even in harsh environment reservoirs.
SUMMARY OF THE INVENTION
Unexpectedly, in view of the foregoing, it has now been discovered that
effective profile modification in high temperature reservoirs having
formation temperatures of up to about 120.degree. C., preferably up to
150.degree. C., and especially preferably up to about 200.degree. C. may
be achieved using a gelable composition comprising a polymer which
contains dialkylacrylamide units.
More particularly, the present invention provides a new and improved
gelable composition, useful for altering the fluid flow profile of a
subterranean formation, which exhibits high temperature stability in the
gelled state at temperatures up to about 200.degree. C., comprising:
(a) water;
(b) a water thickening amount of a water dispersible copolymer comprising:
(i) from about 1 to about 99 mol percent of units of the formula:
##STR1##
wherein R.sup.1 is hydrogen or a lower alkyl radical and R.sup.2 is a
lower alkyl radical; and
(ii) from about 1 to about 99 mol percent of units derived from acrylic
acid or salts thereof; and
(c) an amount of a polyvalent metal capable of cross-linking said copolymer
to form a high temperature stable gel.
The present invention also includes a second gellable composition, useful
for altering the fluid flow profile of a subterranean formation, which
exhibits high temperature stability in the gel state at temperatures up to
about 200.degree. C., comprising:
(a) water;
(b) a water thickening amount of a water-dispersible terpolymers comprising
(i) from about 5 to about 75 mol percent of units of the formula:
##STR2##
wherein R.sup.1 and R.sup.2 are as described above, (ii) from about 5 to
about 35 mole percent of acrylic acid units or salts thereof and
(iii) from about 10 to about 90 mole percent of
2-acrylamido-2-methylpropanesulfonic acid units or salts thereof and
(c) an amount of a polyvalent metal capable of cross-linking said
terpolymer to form a high temperature stable gel.
The concentration of the polymers in the gelable compositions may vary and
the amount of polymer required to form a satisfactory gel will generally
depend upon the molecular weight thereof, the number of crosslinkable
sights per molecule, crosslinker concentration and the desired gel
characteristics for a particular end use. Generally, and without
limitation, in the gelable compositions, the polymer concentration should
be at least 3 times the overlap concentration for the polymer, preferably
at least 3-5 times the overlap concentration. Generally, insufficient
overlap provides a poorer gel. Expressed differently, the concentration of
the polymer employed should be within the range referred to as the
"concentrated region of polymers in solution." Generally good results are
obtained wherein the polymer concentration of the gelable composition is
from about 0.5% to about 5.0% and preferably is from about 2.0 to about
4.0% based on the total weight of gelable composition.
As mentioned above, the concentration of the polymer required to form
satisfactory gels varies inversely with molecular weight of the copolymer.
Generally, the molecular weight of the polymer should be from about
100,000 to about 15 million, and preferably from about 500,000 to about
5.0 million expressed in terms of M.sub.w.
The mol percent ratio of (i) N,N.sup.1 -dialkylacrylamide units to said
(ii) units in the polymers may vary depending on the final gel properties
required or desired. In this connection, the gel strength depends upon the
quantity of chelatable i.e., carboxyl groups present contributed by the
(ii) units, the strength of the metal chelate bond, molecular weight of
the polymer, and salinity of the solution to be more fully described
hereinafter. Polymer stability depends on the quantity of
N-sulfohydrocarbon-substituted acrylamide units present.
In preferred embodiments, the copolymer and terpolymer are comprised of (i)
N,N.sup.1 -dimethylacrylamide units and (ii) sodium acrylate units. In
addition, in preferred embodiments, the copolymers will comprise from
about 5-95 mol percent of (i) units and 5 to 95 mol percent of (ii) units,
and copolymers comprising from about 30 to about 70 mol percent of (i)
units and 30 to 70 mol percent of (ii) units are especially preferred for
long term gel stability at temperatures up to about 150.degree. and
preferably up to about 200.degree. C.
Preferred concentrations of monomers in the terpolymers are from about
10-50 mol percent of the (i) units, from about 10-30 mol percent of the
(ii) units and from about 20-80 mol percent of the (iii) units. Even more
preferred concentrations comprise from about 30-50 mol percent of the (i)
units, from about 15-25 mol percent of the (ii) units and from about 40-60
mol percent of the (iii) units.
In all concentrations of the components of the copolymers and terpolymers
expressed herein, the total concentration is, of course, 100 mol percent.
The polyvalent metal ions for use as component (c) may comprise any
polyvalent metal ions capable of crosslinking the copolymer component (b)
to form a high temperature stable gel. Illustrative examples of some of
these polyvalent metal ions include Fe.sup.2+, Fe.sup.3+, Al.sup.3+,
Ti.sup.4+, Zn.sup.2+, Sn.sup.4+, Cr.sup.3+, Ce.sup.4+, Zr.sup.4+,
Ba.sup.2+, Ca.sup.2+, Mg.sup.2+ and Mn.sup.4+ to name but a few.
Generally, the polyvalent metal ions may be added as is, but in many in
end use applications, the polyvalent metal ions may be added in the form
of a redox couple, or in the form of a chelated complex, each being
capable of generating the polyvalent metal ion crosslinking agents in
situ, to provide flexibility with respect to onset of gelation times and
placement of the gel at desired locations within the formation. Rate of
reaction in each case will be determined by the difference in the redox
potential or by the strength of the complex, respectively. The
crosslinking agents and crosslinking methods will be more particularly
described hereinafter.
The present invention provides alternate embodiments of the gelable
compositions. For example, instead of aqueous gelable compositions, the
compositions of this invention may be provided in the form of solutions,
inverted emulsions or as dry products. Moreover, the compositions may be
provided in the form of a wet or dry two package gelation systems.
In an alternate aspect of the present invention, there is provided a new
and improved method for altering the fluid flow profile of a
petroleum-bearing, underground formation penetrated by at least one
wellbore to provide improved production of oil from the formation, said
method comprising:
(a) injecting into said formation the gelable compositions of the present
invention defined above; and
(b) permitting gelation of the polymer to proceed until substantially
complete. In accordance with an alternate method of the present invention,
the gelable polymer and the crosslinking polyvalent metal ion may be added
separately in alternating slugs, to mix the polymer and crosslinking agent
within the formation to generate the high temperature stable gels in situ.
Using either embodiment of the present method, the gelable compositions
will preferentially travel to the more permeable zones within the
formation and gel, rendering these zones less permeable to fluid flow.
Modifying the fluid flow profile of the formation in this manner provides
a substantial decrease in the volume ratio of water:oil produced from the
formation at the production wells, thereby improving the overall oil
recovery and economics of the improving the overall oil recovery and
economics of the operation. Moreover, the compositions and methods of the
present invention permits effective profile modification in high
temperature harsh brine reservoirs heretofore unattainable with prior art
compositions and methods.
The compositions and methods of the present invention are also useful in
sealing underground formations to be used as waste containment sites,
whereby after treatment, an underground formation may be rendered
impermeable to the flow of water materials away from the site and
impermeable to the flow of water into or through the site.
Other objects and advantages provided by the present invention will become
apparent from the following detailed description of the invention and
illustrative working Examples.
DETAILED DESCRIPTION OF THE INVENTION
In accordance with the present invention effective profile modification of
high temperature and harsh brine environment reservoirs is provided with a
gelable water-dispersible copolymer comprising:
(i) from about 1 to 99 mol percent of the N,N.sup.1 -dialkylacrylamide
units, and
(ii) from about 1 to about 99 mol percent of the acrylic acid units or
salts thereof.
More particularly, the (i) units of the copolymer are derived from an
N,N.sup.1 -dialkylacrylamide monomer having the formula set forth above.
These monomeric units are represented by the above formula in which
R.sup.1 is hydrogen or a lower (C.sub.1 -C.sub.4) alkyl radical and
R.sup.2 is a lower (C.sub.1 -C.sub.4) alkyl radical. Preferred units are
those derived from N, N.sup.1 -dimethylacrylamide.
The copolymers also comprise from about 1 to about 99 mol percent of units
derived from acrylic acid or salts thereof. Illustrative salts are the
alkali metal or ammonium salts, to name but a few.
Those units represented by components (i) and (ii) of the terpolymers is
useful herein are the same as those described above for the copolymers.
The unit represented by component (iii) is that derived from
2-acrylamido-2-methylpropylsulfonic acid and its salts. Alkali metal or
ammonium salts are exemplary with sodium or potassium salts being
preferred.
The crosslinkable, water dispersible polymers for use in the compositions
and methods of the present invention may be prepared in bulk, solution,
suspension or emulsion. Since the polymers should be at least
water-dispersible, if not water-soluble, it is convenient to prepare them
in aqueous solution. Another method is to prepare an aqueous solution of
the monomers and to suspend this solution in a water-immiscible solvent
such as an aliphatic or aromatic hydrocarbon or halogenated hydrocarbon.
Generally, the sulfonic acid monomer and the acrylic acid monomer are
converted to their alkali metal or ammonium salts prior to polymerization
by means of a suitable alkaline reagent. When the polymerization is
effected in suspension, ordinary suspension agents known to those skilled
in the art are used.
The polymerization may be promoted by typical initiators used in aqueous
systems, especially peroxides, persulfates and the like. It has been found
that the alkali metal salts, especially the sodium salts, of
2-acrylamido-2-methylpropane sulfonic acid may be polymerized in the
absence of an initiator. moreover. polymerization may be initiated by the
various Azo-type initiators or actinic radiation, e.g. ultraviolet or
electron beam, sources and methods may be used.
It is sometimes advantageous to carry out the polymerization in the
presence of a small amount of chain transfer agent, to provide polymer
products having more uniform molecular weights, i.e. a narrower molecular
weight distribution range. Suitable chain transfer agents are well known
and will suggest themselves to those skilled in this art.
As has been mentioned above, the gelable compositions of the present
invention are for use as profile modification agents and in this
application the polymers should be prepared so as to provide polymers
having a molecular weight, M.sub.w, of from about 100,000 to about 15
million, preferably from about 500,000 to about 5 million. The polymer
should be at last 3 and preferably at least 3-5 times the overlap
concentration of the polymers. Polymers concentrations for polymers of
molecular weight within the range set forth above, will generally be from
about 0.5% to about 5.0%, and preferably from about 2.0% to 4.0% by weight
of the gelable composition.
As has been mentioned above, the acrylic acid or salt containing (ii) units
are responsible for the formation of the crosslinked, gel network, and the
N,N.sup.1 -dialkylacrylamide or (i) units are responsible for high
temperature and harsh environment stability of the network. In the
terpolymers, the 2-acrylamido-2-methylpropane-sulfonic acid and salt units
also contribute to the high temperature and harsh environment stability.
The gelable composition of the present invention also comprise, as
component (c), an amount of polyvalent metal capable of crosslinking the
polymers to form a high temperature stable gel. The polyvalent metals may
be added as an aqueous solution of their respective water-soluble salts,
or as chelates, however, they are preferably added in the form of a redox
couple.
More particularly, the crosslinking agents, component (c), are preferably
added in the form of a redox couple wherein the redox couple comprises:
(i) at least one oxidizing agent comprising a water soluble compound of a
polyvalent metal wherein the metal is present in its highest valence state
and is capable of being reduced to a lower polyvalent valence state and
being in a form normally unavailable for reaction until contacted with a
reducing agent; and
(ii) a reducing agent effective to reduce the higher valence metal in
oxidizing agent (i) to a lower polyvalent valence state.
The oxidizing agents for use herein are water-soluble compounds of
polyvalent metals wherein the metal is present in a valence state which is
capable of being reduced to a lower valence state. Examples of such
compounds include potassium permanganate, sodium permanganate, ammonium
chromate, ammonium dichromate, the alkali metal chromates, the alkali
metal dichromates, and chromium trioxide. Sodium dichromate and potassium
dichromate, are the presently preferred metal-containing compounds for use
in the practice of the invention. The hexavalent chromium in these
chromium compounds is reduced in situ to trivalent chromium by suitable
reducing agents as discussed hereinafter. In the permanganate compounds
the manganese is reduced from +7 valence to +4 valence as in MnO.sub.2.
The amount of metal-containing compounds used in the practice of the
invention will be an optimum amount, i.e., a small but finite amount which
is more than incidental impurities, but which is effective or sufficient
to cause subsequent gelation when the metal in the polyvalent metal
compound is reduced to a lower valence state. The lower limit of the
concentration of the starting metal-containing compound will depend upon
several factors including the particular polymer used, the concentration
of polymer in the water to be gelled, the water which is used, and the
type of gel product desired. For similar reasons, the upper limit on the
concentration of the starting metal-containing compound also cannot always
be precisely defined. As a general guide, the amount of the starting
polyvalent metal-containing compound used in prepariang aqueous gels in
accordance with the invention will be in the range of from 0.05 to 30,
preferably 0.5 to 30, weight percent of the amount of the total polymer
used. Those skilled in the art can determine the amount of starting
polyvalent metal-containing compound to be used by simple experiments
carried out in the light of this disclosure. For example, when brines such
as are commonly available in producing oil fields are used as the water in
preparing gels in accordance with the invention, less of the starting
polyvalent metal-containing compounds is required than when distilled
water is used. Gelation rates are frequently faster when using brines.
Such oil field brines commonly contain varying amounts of sodium chloride,
calcium chloride, magnesium chloride, etc. Sodium chloride is usually
present in the greatest concentration. The word "water" is used
generically herein and in the claims, unless otherwise specified, to
include such brines, fresh water, and other aqueous media which can be
gelled in accordance with the invention.
The reducing agents which can be used herein include sulfur-containing
compounds such as sodium sulfite, sodium hydrosulfite, sodium
metabisulfite, potassium sulfite, sodium bisulfite, potassium
metabisulfite, sodium sulfide, sodium thiosulfate, ferrous sulfate,
thioacetamide, sodium thiourea and others, and nonsulfur-containing
compounds such as hydroquinone, ferrous chloride, p-hydrazinobenzoic acid,
hydrazine phosphite, hydrazine dichloride, and others. Some of the above
reducing agents act more quickly than others. More particularly, rate of
reduction depends on the reducing agent selected, pH, and temperature. For
example, sodium thiosulfate usually reacts slowly in the absence of heat,
e.g., requiring heating to about 50.degree. C. The presently most
preferred reducing agents are sodium thiosulfate or thiourea. An
especially preferred reducing agent for use in the gelable compositions of
the present invention is thiourea.
The amount of reducing agent to be used in the practice of the invention
will be a sensible amount, i.e., a small but finite amount which is more
than incidental impurities, but which is effective or sufficient to reduce
at least a portion of the higher valence metal in the starting polyvalent
metalcontaining compound to a lower valence state. Thus, the amount of
reducing agent to be used depends, to some extent at least, upon the
amount of the starting polyvalent metal-containing compound which is used.
In many instances, it will be preferred to use an excess of reducing agent
to compensate for dissolved oxygen in the water, exposure to air during
preparation of the gels, and possible contact with other oxidizing
substances such as might be encountered in field operations. As a general
guide, the amount of reducing agent used will generally be at least 150,
preferably at least about 200, percent of the stoichiometric amount
required to reduce the metal in the starting polyvalent to said lower
valence state, e.g., +6 Cr to +3 Cr. Those skilled in the art can
determine the amount of reducing agent to be used by simple experiments
carried out in the light of this disclosure.
The use of redox couples provides additional flexibility in handling, and
positioning of the gelable composition so that rigid gel formation can be
effected in the desired locations, e.g. the high permeability zones, of
the oil-bearing formation. This is primarily because the time between
mixing and the onset of gelation is generally proportional to the redox
potential of the redox couple selected. Therefore, by carefully selecting
the oxidizing agent and reducing agent comprising the redox couple, the
user can regulate the time involved prior to gel formation, such that it
can be placed at any pre-determined location by regulation of the fluid
flow rate of the carrier of delivery fluids.
The gelable compositions of the present invention may be employed as
profile modification agents in accordance with a number of contemplated
methods. For example, either the polyvalent metal compound or the reducing
agent, if used, can be first added to a solution of the polymer in water
or other aqueous medium, or the metal containing compound and the reducing
agent can be added simultaneously to a solution or an aqueous medium
containing the polymers. Generally speaking, where convenient, the
preferred method is to first dis- perse the polymer in the water or other
aqueous medium. The reducing agent is then added to the dispersion with
stirring. The metal-containing compound is then added to the solution or
aqueous medium containing the polymer and the reducing agent, with
stirring. The newly formed lower valence metal ions, for example, +3
chromium obtained from +6 chromium, effect rapid crosslinking of the
polymers and gelation of the solution or aqueous medium containing same.
One presently preferred method of preparing the aqueous gels is to prepare
the gel while the components thereof are being pumped into the well. This
method comprises preparing a base solution of the polymer, adding to this
base solution (a) a polyvalent metal compound such as sodium dichromate or
(b) a reducing agent such as sodium thiosulfate or thiourea pumping the
base solution down the well, and during pumping adding to said base
solution the other of the reagents (a) and (b) which was not previously
added thereto. It is also within the scope of the invention to incorporate
all the components of the aqueous gel into a stream of water while it is
being pumped, e.g., into a well. For example, polymer can be added first
to the flowing stream of water and the other components added subsequently
in any suitable order. Turbulent flow conditions in the pipe will provide
proper mixing.
It is also within the scope of the invention to prepare a dry mixture of
the polymer, the metal-containing compound, and the reducing agent, in
proper proportions, and then add this dry mixture to the proper amount of
water.
An advantage of the invention is that ordinary ambient temperatures and
other conditions can be used in practically all instances in preparing the
aqueous gels of the invention or aqueous medium containing same.
Aqueous gels in accordance with the invention can be prepared having a wide
range of viscosities or firmness ranging from low viscosity or highly
mobile gels having a relatively low viscosity up to firm or rigid gels
which are nonmobile. The choice of gel viscosity or concentration will
depend upon the use to be made of the gel. The actual viscosity and/or gel
strength of the gel will depend upon the type and concentration of
polymer, the type and amount of starting polyvalent metal compound used
and the type and amount of reducing agent used.
As stated above, the gelable compositions and gels produced therewith are
particularly useful as profile modification agents in enhanced oil
recovery operations in high temperature and/or harsh environment
reservoirs. The gelable compositions of this invention are useful for
decreasing the permeability of selected portions of underground formations
prior to or during secondary or tertiary recovery operations and also for
water shut off treatments in producing wells. For example, in an enhanced
oil recovery operation, a conventional waterflood or gas drive is
performed in the conventional manner until the drive fluid breaks through
into the production well in excessive amounts. The gelable compositions of
the present invention are then pumped down the injection well and into the
formation in any suitable manner and in any suitable amount, and for any
desired length of time sufficient to obtain the desired in-depth
penetration, gel formation and consequent permeability reduction in the
high permeability zones of the formation. Usually, an in-depth penetration
of from 10 to 1,000, preferably 25 to 900, feet from the injection well
will be sufficient. However, this can vary from formation to formation,
and penetrations outside said ranges can be used. For example, there can
be injected into the formation via the injection well from about 0.001 to
about 0.5 pore volumes of gelable composition in accordance with the
invention, or by injecting a slug of about 200 to 5,000 barrels of gelable
composition into the well and then into the formation. Injection in one of
the above manners will provide a flood front adjacent the oil to be
produced. If desired, an ordinary brine or water can then be employed to
drive the gelable composition to the desired location of the f | | |
