Wells – Processes – Distinct – separate injection and producing wells
Reexamination Certificate
1998-11-20
2001-02-13
Suchfield, George (Department: 3672)
Wells
Processes
Distinct, separate injection and producing wells
C166S295000, C166S300000, C405S264000, C507S207000, C507S224000, C507S903000, C523S130000
Reexamination Certificate
active
06186231
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a method of injecting gel-forming chemicals into one or more relatively high permeability regions of an underground hydrocarbon bearing formation in order to reduce the permeability of these regions. This invention may therefore be used as a method of improving areal and vertical conformance and flow profiles at or away from a production and/or injection wellbore for the purpose of increasing hydrocarbon recovery.
Poor vertical conformance and excessive water production are problems encountered in secondary oil recovery processes in heterogeneous formations with high permeability contrast. This leads to poor recovery efficiencies by bypassing large concentrations of hydrocarbons, and the uneconomical production of high water-oil ratios fluids from producing wells. Most methods to improve conformance involve the injection of a chemical solution into the formation which forms a gel in high permeability regions and thereby lowers permeability.
A number of gel systems have been described over the years for improving conformance in subterranean hydrocarbon bearing formations. One approach commonly used for conformance improvement is injection of polyacrylamide based gels into the wellbore. Polyacrylamide gels are useful over a certain range of temperatures but at higher temperatures they undergo thermal hydrolysis which leads to Ca
2+
and Mg
2+
ion intolerance and/or gel degradation. Gel systems with increased stability, especially in the presence of high temperatures and hard water, were found to be desirable.
To address the need for conformance improvement of high temperature subterranean regions, other types of gel treatment methods were developed that make use of lignosulfonates and sulfonated lignins. These compounds have frequently been proposed to be used in permeability reducing processes due to their inexpensive cost and favorable chemical structure. The sulfonate groups of lignosulfonates do not complex readily with salts and therefore lignosulfonate gels are more resistant to precipitation due to hardness and high temperature than other types of gels. The disadvantage to this system is that the gelant solutions containing lignosulfonate are only set by high formation temperatures. U.S. Pat. No. 4,074,757 describes injecting lignosulfonate and water in the absence of other gelation promoters in order to achieve high temperature plugging. To achieve gelation at lower temperatures, lignosulfonates have been mixed with an activator comprised of dichromate (i.e. chromium (VI)) and an alkali metal or alkaline earth salt. U.S. Pat. No. 3,896,827 describes a lignosulfonate gel formed by injecting a lignosulfonate solution with a mixed activator comprising a dichromate and an alkali metal or alkaline earth salt. However, the use of dichromate is not environmentally preferred due to its high toxicity and carcinogenicity. Other variations of lignosulfonate based gelants have been described. U.S. Pat. Nos. 4,257,813 and 4,275,789 teach methods using silicate activated lignosulfonate gels. U.S. Pat. No. 4,428,429 discloses a process where a lignin solution gels in the formation. A profile control process is described in U.S. Pat. No. 4,110,231 where lignosulfonate and carboxymethyl cellulose comprise the treatment system.
Lignosulfonate-acrylic acid copolymers have been proposed for use as gels for conformance improvement. U.S. Pat. No. 4,721,161 discloses the use of lignosulfonate and acrylic acid which are reacted in situ using a catalyst and low pH to copolymerize the lignosulfonate and acrylic acid and form a gel to decrease permeability around a wellbore. However, due to the fast reaction rates involved in this method, the use of this system has been found to be undesirable in most instances. The fast reaction rates prevent the use of this method where deep penetration is required, such as injection well treatments for profile modification. In addition to limiting the amount of gel that can be used to treat the formation, the fast reaction rates of this gel system also necessitate the mixing of the gel components and catalyst within the wellbore. The use of gel treatment methods requiring in situ mixing of gel components have proven unsatisfactory in the art due to the difficulty in achieving complete mixing. Incomplete mixing in situ leads to uncontrolled and non-uniform gel placement in the underground formation.
The need therefore exists for a gel treatment system for conformance improvement that is stable over a wider range of temperatures and in the presence of hard water, and that can be delivered efficiently and controllably to the underground region in need of treatment.
These and other features of the present invention are more fully set forth in the following description of illustrative embodiments of the invention.
SUMMARY OF THE INVENTION
The disclosed invention is a method for reducing the permeability of a region of an underground formation by injecting a gelant solution that forms a stable gel into the region. The gelant solution includes an aqueous solvent, about 2% to about 16% by weight of a lignosulfonate-acrylic acid graft copolymer and a sufficient concentration of a crosslinking agent to crosslink the copolymer. The lignosulfonate-acrylic acid copolymer used in the invention has an acrylic acid content of about 10% to about 50% by weight and the crosslinking agent is a complex of one or more positively charged chromium (III) species and one or more negatively charged carboxylate species. Preferably, the chromium (III) species used in the invention has a concentration of about 0.1% to about 8.0% by weight. In one preferred embodiment, the carboxylate species of the crosslinker may be selected from formate, acetate, propionate, lactate, oxalate and malonate. In another preferred embodiment the crosslinker may further comprise one or more species selected from hydroxide and oxygen.
A particular advantage to the disclosed invention is that brine may be used as the aqueous solvent of the gelant solution. Further, in one embodiment, the gelant solution of the disclosed invention may be prepared prior to injection into the subterranean region and does not require mixing in situ. In preferred embodiments the gelant solution may be prepared at the surface either in a batch process or on the fly.
Finally, the gelant solution of the invention may be tailored to the treatment needs of a high permeability region of an underground formation by a process of determining the characteristics of the high permeability region, providing a gelant solution of the disclosed invention having predetermined gel properties appropriate to the region to be treated, and injecting the gelant solution into the region.
DETAILED DESCRIPTION
The disclosed invention is carried out by injecting a gelant solution into a region of an underground formation in order to reduce its permeability. The gelant solution includes about 2% to about 16% by weight of a lignosulfonate-acrylic acid graft copolymer and a sufficient concentration of a crosslinking agent to crosslink the copolymer dissolved in an aqueous solvent. The lignosulfonate-acrylic acid graft copolymer used in the gelant solution should have an acrylic acid content of about 10% to about 50% by weight. The crosslinking agent is a complex of one or more positively charged chromium (III) species and one or more negatively charged carboxylate species. Lignosulfonate-acrylic acid graft copolymers form stable gels over a wide range of temperatures and under harsh salinity conditions when crosslinked with chromium (III) ions. These gels have the advantage of being thermally stable due to the presence of sulfonate groups on the graft copolymer. Chromium (III) is also less toxic than dichromate and therefore less environmentally hazardous when used in underground gel treatment systems.
The amount of lignosulfonate-acrylic acid graft copolymer in the gelant solution will be one factor in the amount of time required for the gelant to form a stable gel. The higher the copolymer concent
Howrey Simon Arnold & White
Reinisch Morris N.
Suchfield George
Texaco Inc.
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