CA1275792C - Conformance improvement in a subterranean hydrocarbon- bearing formation using a polymer gel - Google Patents
Conformance improvement in a subterranean hydrocarbon- bearing formation using a polymer gelInfo
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- CA1275792C CA1275792C CA000520374A CA520374A CA1275792C CA 1275792 C CA1275792 C CA 1275792C CA 000520374 A CA000520374 A CA 000520374A CA 520374 A CA520374 A CA 520374A CA 1275792 C CA1275792 C CA 1275792C
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- C—CHEMISTRY; METALLURGY
- C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
- C09K—MATERIALS FOR MISCELLANEOUS APPLICATIONS, NOT PROVIDED FOR ELSEWHERE
- C09K8/00—Compositions for drilling of boreholes or wells; Compositions for treating boreholes or wells, e.g. for completion or for remedial operations
- C09K8/50—Compositions for plastering borehole walls, i.e. compositions for temporary consolidation of borehole walls
- C09K8/504—Compositions based on water or polar solvents
- C09K8/506—Compositions based on water or polar solvents containing organic compounds
- C09K8/508—Compositions based on water or polar solvents containing organic compounds macromolecular compounds
- C09K8/512—Compositions based on water or polar solvents containing organic compounds macromolecular compounds containing cross-linking agents
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- Engineering & Computer Science (AREA)
- Materials Engineering (AREA)
- Organic Chemistry (AREA)
- Consolidation Of Soil By Introduction Of Solidifying Substances Into Soil (AREA)
- Addition Polymer Or Copolymer, Post-Treatments, Or Chemical Modifications (AREA)
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Abstract
CONFORMANCE IMPROVEMENT IN A SUBTERRANEAN HYDROCARBON-BEARING
FORMATION USING A POLYMER GEL
ABSTRACT
Conformance improvement is achieved in a subterranean hydrocarbon-bearing formation using a gel comprised of a high molecular weight water-soluble acrylamide polymer, a complex crosslinking agent and an aqueous solvent. The gel components are combined at the surface and injected into the desired treatment zone via a wellbore to form a continuous single-phase gel.
FORMATION USING A POLYMER GEL
ABSTRACT
Conformance improvement is achieved in a subterranean hydrocarbon-bearing formation using a gel comprised of a high molecular weight water-soluble acrylamide polymer, a complex crosslinking agent and an aqueous solvent. The gel components are combined at the surface and injected into the desired treatment zone via a wellbore to form a continuous single-phase gel.
Description
Description CONFORMANCE tMPROVEMENT IN A SUBTERRANEAN HYDROCARBON-BEARING
FORMAT10~ USI~G A POLYMER GEL
Background of the InventiQn 05 Technical Field:
The invention relates to a process for reducing the permeability in a relatively high permeability region of a subterranean hydrocarbon-bearing formation and more particularly to a process for improving areal and vertical conformance and flow profiles at or away from a production and/or injection wellbore penetrating the hydrocarbon-bearing forma-tion.
Description of Related Art:
Poor vertical conformance results from the vertical juxtaposition of relatively high permeability geologic zones to relatively low permea-bility zones within a subterranean formation. Poor areal conformance results from the presence of high permeability streaks and high permea-bility anomalies within the formation matrix, such as vertical fractures and networks of the same, which have very high permeability relative to the formation matrix. Fluids generally exhibit poor flow profiles and sweep efficiencies in subterranean formations haviny poor vertical or areal conformance. Poor conformance is particularly a problem where 57~3Z
FORMAT10~ USI~G A POLYMER GEL
Background of the InventiQn 05 Technical Field:
The invention relates to a process for reducing the permeability in a relatively high permeability region of a subterranean hydrocarbon-bearing formation and more particularly to a process for improving areal and vertical conformance and flow profiles at or away from a production and/or injection wellbore penetrating the hydrocarbon-bearing forma-tion.
Description of Related Art:
Poor vertical conformance results from the vertical juxtaposition of relatively high permeability geologic zones to relatively low permea-bility zones within a subterranean formation. Poor areal conformance results from the presence of high permeability streaks and high permea-bility anomalies within the formation matrix, such as vertical fractures and networks of the same, which have very high permeability relative to the formation matrix. Fluids generally exhibit poor flow profiles and sweep efficiencies in subterranean formations haviny poor vertical or areal conformance. Poor conformance is particularly a problem where 57~3Z
-2- Docket 850003-B
vertical heterogeneity and/or fracture networks or other structural anomalies are in fluid communication with a subterranean wellbore across which fluids are injected or produced.
A number of attempts to remedy conformance problems exist. U.S.
~5 Patents 3,7~2,476; 3,981,363, 4,018,286; and 4,039,029 to &all or Gall et al describe various processes wherein gel compositions are formed in high permeability zones of subterranean formations to reduce the per-meability therein. According to U.S. Patent 3,762,476, a polymer such as polyacrylamide is injected into a formation followed sequentially by a crosslinking agent. The sequentially injected slugs are believed to perrneate the treatment zone of the formation and gel in situ.
It is generally held that effective polymer/crosslinking agent systems necessitate sequential injection of the gel components because gel systems mixed on the surface often set up before they can effec-tively penetrate the treatment region. HowevPr, in practice, treatments such as that disclosed in U.S. Patent 3,762,476 using sequentially in~ected gel systems have proven unsatisfactory because of the inability to achieve complete Inixing and gelation in the formation. As a result, gels only form at the interface of the unmixed gel components and often in regions remote from the desired treatment region. A need exists for a gelation process capable of forming gels having a predetermined gelation rate, strength, and stability to satisfy the particular demands of a desired treatment region in a subterranean hydrocarbon-bearing formation.
~2757~2 -3- Docket 850003-B
Summary of the Invention The present invention provides a process for improving vertical and areal conformance in a subterranean hydrocarbon-bearing formation pene-trated by a production and/or injection well and for correspondingly im-05 proving flow profiles and sweep efficiencies of injected and/or produced fluids in the formation. The objectives are achieved by means of a tailor-made flowing or non-flowing polymer gel.
The gel comprises a high molecular weightl water-soluble acrylamide polymer and a crosslinking agent comprised of complex ions and/or mole-cules. The gel is prepared by forming a gelation solution above ground containing the polymer and crosslinking agent and injecting the solution into the desired treatment region via a wellbore in fluid communication therewith. The gelation solution is advantageously at least partially gelled by the time it reaches the treatment region to inhibit or pre-vent its propagation into adjoining regions where no treatment is de-sired. The final gel is a continuous single-phase gel which substan-tially reduces permeability in the treatment region.
After the gelation treatment, fluids may be injected into or pro-duced from the hydrocarbon-bearing regions of the formation in fluid communication with the wellbore. The gel is substantially incapable of flowing from the treatrnent reyion and is substantially permanent and resistant to in situ degradation.
The process provides distinct advantages over gelation processes known in the art. The practitioner of the present invention customizes ~;~75792 -4-- Docket 850003-B
or tailors a gel to a specific subterranean application by first deter-mininy the treatment demands of a desired subterranean region. Given these treatment demands, one can predetermine the gelation rate and resultant gel strength and stability which are required of a gel to meet 05 the demands. Thereafter, a gel having the required predetermined prop-erties is produced under controlled conditions at the surface by utiliz-ing observed correlations between specific controllable gelation param-eters and resultant gel properties.
Description of the Preferred Embodiments l`he present invention is described in the context of specific terms which are defined as follows. The formation consists of ~wo general regions, the "matrix" and "anomalies." An "anomaly" is a volume or ~oid space in the formation having very high permeability relative to the matrix. It is inclusive of terms such as streaks, fractures, fracture networks, vugs, solution channels, caverns, washouts, cavities, etc.
The "matrix" is substantially the remainder of the formation volume characterized as essentially homogeneous, continuous, sedimentary reser-voir material free of anomalies and often competent.
The matrix consists of horizontal "zones" of distinctive subter-ranean material of continuous geologic properties which extend in the horizontal direction. "Vertical conformance" is a measure of the degree of geologic uniformity in permeability as one moves vertically across the formation. "Areal conformance" is a measure of the degree of 127S7~Z
vertical heterogeneity and/or fracture networks or other structural anomalies are in fluid communication with a subterranean wellbore across which fluids are injected or produced.
A number of attempts to remedy conformance problems exist. U.S.
~5 Patents 3,7~2,476; 3,981,363, 4,018,286; and 4,039,029 to &all or Gall et al describe various processes wherein gel compositions are formed in high permeability zones of subterranean formations to reduce the per-meability therein. According to U.S. Patent 3,762,476, a polymer such as polyacrylamide is injected into a formation followed sequentially by a crosslinking agent. The sequentially injected slugs are believed to perrneate the treatment zone of the formation and gel in situ.
It is generally held that effective polymer/crosslinking agent systems necessitate sequential injection of the gel components because gel systems mixed on the surface often set up before they can effec-tively penetrate the treatment region. HowevPr, in practice, treatments such as that disclosed in U.S. Patent 3,762,476 using sequentially in~ected gel systems have proven unsatisfactory because of the inability to achieve complete Inixing and gelation in the formation. As a result, gels only form at the interface of the unmixed gel components and often in regions remote from the desired treatment region. A need exists for a gelation process capable of forming gels having a predetermined gelation rate, strength, and stability to satisfy the particular demands of a desired treatment region in a subterranean hydrocarbon-bearing formation.
~2757~2 -3- Docket 850003-B
Summary of the Invention The present invention provides a process for improving vertical and areal conformance in a subterranean hydrocarbon-bearing formation pene-trated by a production and/or injection well and for correspondingly im-05 proving flow profiles and sweep efficiencies of injected and/or produced fluids in the formation. The objectives are achieved by means of a tailor-made flowing or non-flowing polymer gel.
The gel comprises a high molecular weightl water-soluble acrylamide polymer and a crosslinking agent comprised of complex ions and/or mole-cules. The gel is prepared by forming a gelation solution above ground containing the polymer and crosslinking agent and injecting the solution into the desired treatment region via a wellbore in fluid communication therewith. The gelation solution is advantageously at least partially gelled by the time it reaches the treatment region to inhibit or pre-vent its propagation into adjoining regions where no treatment is de-sired. The final gel is a continuous single-phase gel which substan-tially reduces permeability in the treatment region.
After the gelation treatment, fluids may be injected into or pro-duced from the hydrocarbon-bearing regions of the formation in fluid communication with the wellbore. The gel is substantially incapable of flowing from the treatrnent reyion and is substantially permanent and resistant to in situ degradation.
The process provides distinct advantages over gelation processes known in the art. The practitioner of the present invention customizes ~;~75792 -4-- Docket 850003-B
or tailors a gel to a specific subterranean application by first deter-mininy the treatment demands of a desired subterranean region. Given these treatment demands, one can predetermine the gelation rate and resultant gel strength and stability which are required of a gel to meet 05 the demands. Thereafter, a gel having the required predetermined prop-erties is produced under controlled conditions at the surface by utiliz-ing observed correlations between specific controllable gelation param-eters and resultant gel properties.
Description of the Preferred Embodiments l`he present invention is described in the context of specific terms which are defined as follows. The formation consists of ~wo general regions, the "matrix" and "anomalies." An "anomaly" is a volume or ~oid space in the formation having very high permeability relative to the matrix. It is inclusive of terms such as streaks, fractures, fracture networks, vugs, solution channels, caverns, washouts, cavities, etc.
The "matrix" is substantially the remainder of the formation volume characterized as essentially homogeneous, continuous, sedimentary reser-voir material free of anomalies and often competent.
The matrix consists of horizontal "zones" of distinctive subter-ranean material of continuous geologic properties which extend in the horizontal direction. "Vertical conformance" is a measure of the degree of geologic uniformity in permeability as one moves vertically across the formation. "Areal conformance" is a measure of the degree of 127S7~Z
-5- Docket 850003-B
geologic uniformity in permeability as one moves horizontally across the formation. A "flow profile" qualitatively describes the uniformity of fluid flow through a subterranean formation while "sweep efficiency" is the quantitative analog of "flow profile." "Plugginy" is a substantial 05 reduction in permeability in a region of a formation.
The term "gel" as used herein is directed to a continuous three-dimensional crosslinked polymeric network having an ultra high molecular weight. The gel is qualitatiYely defined as "flowing" or "non-flowing"
based on its ability to flow under the force of gravity when unconfined on the surface at ambient atmospheric conditions. A flowing gel flows under these conditions; a non-flowing gel does not. Nonetheless, both a non-flowing gel and a flowing gel are defined herein as having suffi-cient structure so as not to propagate from the confines of the desired treatment region into a less permeable adjoining region when injected into the subterranean treatment region.
Partially gelled solutions are also referred to herein. A par-tially gelled solution is at least somewhat more viscous than an un-crosslinked polymer solution such that it is incapable of entering a less permeable region where no treatment is desired, but sufficiently fluid such that it is capable of displacement into a desired treatment zone. The crosslinking agent of the partially gelled solution has re-acted incompletely with the polymer, but is capable of continued reac-tion to completion thereafter, resulting in the desired gel.
~Z~5792 -6- Docket 850003-B
The gel composition utilized in the present invention is comprised of an acrylamide polymer and a crosslinking agent. The acrylamide poly-mer is either polyacrylamide or partially hydrolyzed polyacrylamide, depending on the number of carboxylate groups it has. An acrylamide S polymer having substantially less than 1g of the acrylamide groups in the form of carboxylate groups is termed polyacrylamide (PA); an acryl-amide polymer having at least 1% but not 100~ of the acrylamide groups in the form of carboxylate yroups is termed partially hydrolyzed poly-acrylamide (PHPA). The average molecular weight of the acrylamide polymer is in the range of about 10,000 to about 50,000,000 and prefer-ably about 100,000 to about 20,000,000, and most preferably about 200,000 to about 12,000,000.
The crosslinking agent is a complex or mixture of complexes. The term "complex" is defined herein as an ion or molecule containing two or 1~ more interassociated ionic, radical or molecular species. A complex ion as a whole has a distinct electrical charge while a complex molecule is electrically neutral.
The complex of the present invention includes at least one or more electropositive chromium III species and one or more electronegative carboxylate species. The complex may advantageously also contain one or more electronegative hydroxide and/or oxygen species. It is believed that, when two or more chromium III species are present in the complex, the oxygen or hydroxide species may help to bridge the chromium III
species. Each complex optionally contains additional species which are :~2~7~7~32 -7- Docket 850003-B
not essential to the polymer crosslinking function of the complex. For example, inorganic mono- and/or divalent ions, which function merely to balance the electrical charge of the complex, or one or more water molecules may be associated with each complex. Representative formulae 05 of such complexes include:
[Cr3(CH3C02)6(0H)2] 1;
[Cr3(0H)2(CH3C02)6]N03 ^6H2;
[Cr3(H20~2(CH3C02)6]
[cr3(H2o)2(cH3co2)6](cH3co2)3 H2o; etc-Trivalent chromium ana chromic ion are equivalent terms encompassed by the term chromium III species as used herein. The carboxylate species are advantageously derived from water-soluble salts of carbox-ylic acids, especially low molecular weight mono-basic acids. Carboxy-late species derived from salts of fornlic, acetic, propionic, and lactic acid, lower substituted derivatives thereof and mixtures thereof are especially preferred. The carboxylate species include the following water-soluble species: formate, acetate, propionate, lactate, lower substituted derivatives thereof, and mixtures thereof. The optional inorganic ions include sodium, sulfate, nitrate and chloride ions.
A host of complexes of the type described above and their method of preparation are well known in the leather tanning art. These complexes are described in Shuttleworth and Ru~ssel, Journal o~ The Society of ~;~757~;~
geologic uniformity in permeability as one moves horizontally across the formation. A "flow profile" qualitatively describes the uniformity of fluid flow through a subterranean formation while "sweep efficiency" is the quantitative analog of "flow profile." "Plugginy" is a substantial 05 reduction in permeability in a region of a formation.
The term "gel" as used herein is directed to a continuous three-dimensional crosslinked polymeric network having an ultra high molecular weight. The gel is qualitatiYely defined as "flowing" or "non-flowing"
based on its ability to flow under the force of gravity when unconfined on the surface at ambient atmospheric conditions. A flowing gel flows under these conditions; a non-flowing gel does not. Nonetheless, both a non-flowing gel and a flowing gel are defined herein as having suffi-cient structure so as not to propagate from the confines of the desired treatment region into a less permeable adjoining region when injected into the subterranean treatment region.
Partially gelled solutions are also referred to herein. A par-tially gelled solution is at least somewhat more viscous than an un-crosslinked polymer solution such that it is incapable of entering a less permeable region where no treatment is desired, but sufficiently fluid such that it is capable of displacement into a desired treatment zone. The crosslinking agent of the partially gelled solution has re-acted incompletely with the polymer, but is capable of continued reac-tion to completion thereafter, resulting in the desired gel.
~Z~5792 -6- Docket 850003-B
The gel composition utilized in the present invention is comprised of an acrylamide polymer and a crosslinking agent. The acrylamide poly-mer is either polyacrylamide or partially hydrolyzed polyacrylamide, depending on the number of carboxylate groups it has. An acrylamide S polymer having substantially less than 1g of the acrylamide groups in the form of carboxylate groups is termed polyacrylamide (PA); an acryl-amide polymer having at least 1% but not 100~ of the acrylamide groups in the form of carboxylate yroups is termed partially hydrolyzed poly-acrylamide (PHPA). The average molecular weight of the acrylamide polymer is in the range of about 10,000 to about 50,000,000 and prefer-ably about 100,000 to about 20,000,000, and most preferably about 200,000 to about 12,000,000.
The crosslinking agent is a complex or mixture of complexes. The term "complex" is defined herein as an ion or molecule containing two or 1~ more interassociated ionic, radical or molecular species. A complex ion as a whole has a distinct electrical charge while a complex molecule is electrically neutral.
The complex of the present invention includes at least one or more electropositive chromium III species and one or more electronegative carboxylate species. The complex may advantageously also contain one or more electronegative hydroxide and/or oxygen species. It is believed that, when two or more chromium III species are present in the complex, the oxygen or hydroxide species may help to bridge the chromium III
species. Each complex optionally contains additional species which are :~2~7~7~32 -7- Docket 850003-B
not essential to the polymer crosslinking function of the complex. For example, inorganic mono- and/or divalent ions, which function merely to balance the electrical charge of the complex, or one or more water molecules may be associated with each complex. Representative formulae 05 of such complexes include:
[Cr3(CH3C02)6(0H)2] 1;
[Cr3(0H)2(CH3C02)6]N03 ^6H2;
[Cr3(H20~2(CH3C02)6]
[cr3(H2o)2(cH3co2)6](cH3co2)3 H2o; etc-Trivalent chromium ana chromic ion are equivalent terms encompassed by the term chromium III species as used herein. The carboxylate species are advantageously derived from water-soluble salts of carbox-ylic acids, especially low molecular weight mono-basic acids. Carboxy-late species derived from salts of fornlic, acetic, propionic, and lactic acid, lower substituted derivatives thereof and mixtures thereof are especially preferred. The carboxylate species include the following water-soluble species: formate, acetate, propionate, lactate, lower substituted derivatives thereof, and mixtures thereof. The optional inorganic ions include sodium, sulfate, nitrate and chloride ions.
A host of complexes of the type described above and their method of preparation are well known in the leather tanning art. These complexes are described in Shuttleworth and Ru~ssel, Journal o~ The Society of ~;~757~;~
-8- Docket 850003-B
Leather Trades' Chemists~ "The Kinetics of Chrome Tannage Par~
-United Kingdom, 1965, v. 49, p. 133-154; "Part III.," United Kingdom, 1965, v. 49, p. 251-260; "Part I~.," United Kingdom, 1965, v. 49, p.
261-268; and Von Erdman, Das Leder, "Condensation of Mononuclear Chro-05 mium (III) Salts to Polynuclear Compounds," Eduard Roether Verlag, Darmstadt, Germany, 1963~ v. 14, p. 249.
Udy, Marvin J., Chromium, Yolume 1: Chemistry of Chromium and lts Compounds, Reinhold Publishing Corp., N.Y., 1956, pp. 229-233;
and Cotton and Wilkinson, Advanced Inorganic Chemistry 3rd Ed., John Wiley & Sons, Inc., N.Y~, 1972, pp. 836-839, further describe typical complexes which may be within the scope of the present invention.
The present invention is not limited to the specific complexes and mixtures thereof described in the refer-ences, but may include others satisfying the above-stated definition.
The gel is formed by admixing an acrylamide polymer and crosslink-ing ayent at the surface ~o form an injectable gelation solution. Sur-face admixing broadly encompasses inter alia m~xing the solution in bulk at the surface prior to injection or simultaneously m;xing the solution at or near the we11head by in-line mixing means while injecting it.
Admixing is accomplished for example by dissolving the starting materials for the crosslinking agent in an appropriate aqueous solvent.
Exemplary starting materials include solid CrAc3-H20, solid Cr3Ac7-(OH)2 or a solution labeled "Chromic Acetate 50~ Solution" commercially available, for example, from McCean Chemical Co., Inc., 1250 Terminal i,, ~. . ~ , lZ~7~i'79Z
Leather Trades' Chemists~ "The Kinetics of Chrome Tannage Par~
-United Kingdom, 1965, v. 49, p. 133-154; "Part III.," United Kingdom, 1965, v. 49, p. 251-260; "Part I~.," United Kingdom, 1965, v. 49, p.
261-268; and Von Erdman, Das Leder, "Condensation of Mononuclear Chro-05 mium (III) Salts to Polynuclear Compounds," Eduard Roether Verlag, Darmstadt, Germany, 1963~ v. 14, p. 249.
Udy, Marvin J., Chromium, Yolume 1: Chemistry of Chromium and lts Compounds, Reinhold Publishing Corp., N.Y., 1956, pp. 229-233;
and Cotton and Wilkinson, Advanced Inorganic Chemistry 3rd Ed., John Wiley & Sons, Inc., N.Y~, 1972, pp. 836-839, further describe typical complexes which may be within the scope of the present invention.
The present invention is not limited to the specific complexes and mixtures thereof described in the refer-ences, but may include others satisfying the above-stated definition.
The gel is formed by admixing an acrylamide polymer and crosslink-ing ayent at the surface ~o form an injectable gelation solution. Sur-face admixing broadly encompasses inter alia m~xing the solution in bulk at the surface prior to injection or simultaneously m;xing the solution at or near the we11head by in-line mixing means while injecting it.
Admixing is accomplished for example by dissolving the starting materials for the crosslinking agent in an appropriate aqueous solvent.
Exemplary starting materials include solid CrAc3-H20, solid Cr3Ac7-(OH)2 or a solution labeled "Chromic Acetate 50~ Solution" commercially available, for example, from McCean Chemical Co., Inc., 1250 Terminal i,, ~. . ~ , lZ~7~i'79Z
-9- Docket 850003-B
Tower, Cleveland, Ohio 44113, U.S.A. The crosslinking agent solution is then mixed with an aqueous polymer solution to produce the gelation solution. Among other alternatives, the starting materials for the crosslinking agent can be dissolved directly in the aqueous polymer 05 solution ts form the gelation solution in a single step.
The aqueous solvent of the gelation solution may be fresh water or a brine having a total dissolved solids concentration up to the solu-bility limit of the solids in water. Inert fillers such as crushed or naturally fine rock material or glass beads can also be added to the gelation solution to reinforce the gel network structure.
ïhe present process enables the practitioner to customize or tailor-make a gel having a predetermined gelation rate and predetermined gel properties of strength and stability from the above-described compo-sition. The gelation rate is defined as the degree of gel formation as a function of time or, synonymously, the rate of crosslinking in the gelation solution. The degree of crosslinking may be quantified in terms of gel viscosity and/or strength. Gel strength of a non-flowing gel is defined as the coherence of the gel network or resistance to deformation under external forces. Gel strength of a flowing gel is defined as the resistance of the gel to filtration or flow. Stability is defined as either thermal or phase stability. Thermal stability is the ability of a gel to withstand temperature extremes without degrada-tion. Phase stability is the ability of a gel to resist syneresis which can detract from the gel structure and performance.
~l27S~7~Z
Tower, Cleveland, Ohio 44113, U.S.A. The crosslinking agent solution is then mixed with an aqueous polymer solution to produce the gelation solution. Among other alternatives, the starting materials for the crosslinking agent can be dissolved directly in the aqueous polymer 05 solution ts form the gelation solution in a single step.
The aqueous solvent of the gelation solution may be fresh water or a brine having a total dissolved solids concentration up to the solu-bility limit of the solids in water. Inert fillers such as crushed or naturally fine rock material or glass beads can also be added to the gelation solution to reinforce the gel network structure.
ïhe present process enables the practitioner to customize or tailor-make a gel having a predetermined gelation rate and predetermined gel properties of strength and stability from the above-described compo-sition. The gelation rate is defined as the degree of gel formation as a function of time or, synonymously, the rate of crosslinking in the gelation solution. The degree of crosslinking may be quantified in terms of gel viscosity and/or strength. Gel strength of a non-flowing gel is defined as the coherence of the gel network or resistance to deformation under external forces. Gel strength of a flowing gel is defined as the resistance of the gel to filtration or flow. Stability is defined as either thermal or phase stability. Thermal stability is the ability of a gel to withstand temperature extremes without degrada-tion. Phase stability is the ability of a gel to resist syneresis which can detract from the gel structure and performance.
~l27S~7~Z
-10- Docket 850003-B
Tailor-making or customizing a gel in the manner of the present in-vention to meet the demands of a particular treatment region is provided in part by correlating the independent gelation parameters with the dependent variables of gelation rate and resultant gel strength and 05 stability. The independent gelation parameters are the surface and in situ gelation conditions including: temperature, pH, ionic strength and specific electrolytic makeup of the solvent, polymer concentration, ratio of the weight of polymer to the combined weight of chromium III
and carboxylate species in the mixture, degree of polymer hydrolysis, and average molecular weight of the polymer.
The operable ranges of the gelation parameters are correlated with the dependent variables of gelation rate and resultant gel properties by means including qualitative bottle testing, quantitative viscosimetric analysis, packed-column flooding, and core flooding. The operable ranges of a number of gelation parameters and their correlation with the dependent variables are described below.
The lower temperature limit of the gelation solution at the surface is the freezing point of the solution and the upper limit is essentially the thermal stability limit of the polymer. The solution is generally maintained at ambient temperature or higher at the surface. The temper-ature may be adjusted by heating or cooling the aqueous solvent. In-creasing the temperature within the prescribed range increases the gela-tion rate.
~Z75 ~'9;Z
~ Docket 850003-B
The initial pH of the gelation solution is within a range of about 3 to 13 and preferably about 6 to 13. Althouyh gelation can occur at an acidic pH, lowering the initial pH of the solution below 7 does not favor gelation. The initial pH of the solution is most preferably alka-05 line, i.e., greater than 7 to about 13. Increasing the pH within the prescribed range increases the rate of gelation.
The polymer concentration in the solution is about 1000 ppm up to the solubility limit of the polymer in the solvent or the rheological constraints of the polymer solution, preferably about 1000 to about 200,000 ppm, and most preferably about 3000 to about 100,000. Increas-ing the polymer concentration increases the gelation rate and ultimate gel strength at a constant ratio of polymer to crosslinking agent.
The ionic strength of the solvent can be from that of deionized distilled water to that of a brine having an ion concentration approach-ing the solubility limit of tne brine. Increasing the ionic strength of the solution can increase the gelation rate.
The weight ratio of acrylamide polymer to chromium III and carboxy-late species comprising the mixture is about 1:1 to about 500:1, prefer-ably about 2.5:1 to about 100:1, and most preferably about 5:1 to about 40:1. Decreasing the ratio generally increases the gelation rate and up to a certain point generally increases the gel strength, especially at a constant high polymer concentration.
The degree of hydrolysis is about 0 to 60% and preferably about 0 to 30%. Within the preferred range, increasing the degree of hydrolysis 12757~;~
-1~- Docket 850003-B
increases the gelation rate. Increasing the molecular weight of the polymer increases the gel strength.
It is apparent from these correlations that one can produce yels across a very broad range of gelation rates and gel properties as a 05 function of the gelation conditions. Thus, to effect an optimum gela-tion treatment according to the present process, the practitioner pre-determines the gelation rate and properties of the resultant gel which mee~ the treatment demands of the given region and thereafter produces the gel having these predetermined characteristics. The treatment demands include the in situ gelation conditions such as temperature, connate water properties, and permeability of the region as well as the post treatment con~itions such as injection and production pressures.
Analytical methods known to one skilled in the art are used to determine the treatment demands. The treatment demands provide criteria to pre-determine the gelation rate and resultant gel properties in the manner desoribed above and continuing hereafter.
The gelation rate is advantageously sufficiently slow to enable preparation of the gelation solution at the surface, injection of the solution as a uniform slug into the wellbore, and displacement of the entire solution into the desired treatment zone. Too rapid a gelation rate produces excessive gelation of the solution at the surface which results in a solution that may be difficult, if not impossible, to inject into the wellbore or formation due to its rheological prop-erties. At the same time, the gelation rate must be sufficiently rapid ~2'7S792 -13- Docket 850003-~
to enable completion of the reaction within a reasonable period of time so that the well may be returned to iniection or production after treat~
ment.
When treating anomalies, at least partial gelation of the solution, 05 if not complete gelation for some flowing gels, is advantageous before the solution reaches the face bounding the matrix and the anomaly to prevent the solution from penetrating the matrix as well as the anom-aly. Substantial penetration of the matrix by the solution and the ensuing permeability reduction of the matrix are counterproductive to the plugging of anomalies. The values of the independent variables in the process are carefully selected to achieve a gelation rate meeting these criteria.
The volume of solution iniected into the formation is a function of the volume and location of the desired treatment region and the degree of penetration into the treatment region by the solution. One skilled in the art can determine the required volume of gel for a given treat-ment region. Placement of the gelation solution in the treatment region may be facilitated by zone isolation means such as packers and the like.
The injection rate is a function of the gelation rate and opera-tional constraints of injection pressure and pumping limits. The re-quired injection rate is fixed such that all of the solution can be practically injected into the treatment zone before it becomes unpump-able. The gelation tirne of the gel ranges from near instantaneous for ~Z~57~Z
-14- Docket 850003-B
flowing gels up to 48 hours or longer for both flowing and non-flowing gels. Longer gelation times are limited by practical considerations of lost production when injection and production wells are shut in.
The process is applicable to conformance treatment of formations 05 under most conditions and is specific to treating regions within theformation which are in fluid communication with an injection or produc-tion well. The flowing gel is especially applicable to the treatrnent of anomalies such as streaks of relatively high permeability, fractures or fracture networks in direct communication via the anomaly with an injec-tion well but not also in direct communication via the anomaly with a production well. The ultimate gel is termed a flowing gel as defined herein because it would flow if unconfined on the surface. However, the flowing gel is sufficiently crosslinked to remain in place under injec-tion conditions in the anomaly when confined thereby. Thus, the flowing gel is capable of effectively pluyging the anomaly.
The flowing gel is not generally suitable for treatment of anom-alies in direct communication via the anomaly with production wells because flowing gels do not have sufficient strength to withstand the drawdown pressure ~uring production and may flow back into the well-bore. For treatment of anomalies in direct communication with produc-tion wells, non-flowing rigid gels having sufficient strength to with-stand the production drawdown pressure are preferred. It is preferred that substantially none of the gel flows back into the wellbore when oil is produced after the conformance treatment.
~2757~
-lS- Docket 850003-B
In some specialized cases, the degree of partial gelation of the injected solution can be maintained sufficiently low to enable the solu-tion to enter a selected high permeability zone of the matrix and crosslink to completion in situ as either a non-flowing gel or a flowing 05 gel. Both flowing and non-flowing gels can be used for treatment ofhigh permeability zones of the matrix because in general neither wi11 flow from the treatment zone upon complete gelation, a necessary condi-tion for the present invention. However, non-flowing gels are preferred for treatment of high permeability zones in direct communication with production wells because of the~r increased strength.
PA is preferred for the formulation of non-flowing gels because it has a slower gelation rate than PHPA which enab1es one to inject it into the treatment reglon before it sets up. PHPA is preferred for the for-mulation of flowing gels because In many cases flowing gels can be injected into a treatment region e~en when reacted to completion.
Gels having a predetermined gelation rate and resultant gel prop-erties to meet the treatment demands of a given region are produced by adjusting and setting the surface gelation conditions as they correlate to the gelation rate and gel properties. Accordingly the gels are pro-duced in a manner which renders them insensitive to most extreme forma-tion conditions. The gels can be applied to the treatment of many dif-ferent geological structures including high permeability zones within the formation matrix and anomalies external to the matrix such as frac-tures and other cavities. The gels can be stable at formation tempera-tures as high as 115C and at any formation pH contemplated. The gels .~ . . . .
~2'757~2 -16- Docket 850003-B
are relatively insensitive to the stratigraphy of the rock and can be employed in carbonate and sandstone strata and unconsolidated or consolidated strata having varying mineralogy. Once the gels are in place, it is extremely difficult to displace the gels by physical or 05 chemical means other than total destruction of the crosslinked network.
The gels may be reversible on contact with hydrogen peroxide or sodium hypochlorite, but are substantially insoluble in the formation fluids.
The following examples demonstrate the practice and utility of the present invention but are not to be construed as limiting the scope thereof-1,020,000 liters of a predetermined aqueous gelation solution com-prising PHPA and a crosslinking agent made up of chromium III and ace-tate ions are prepared. The gelation solution is prepared by mixing a "Chromic Acetate 50% Solution" diluted to 15% by weight with a PHPA
solution containing 3040 kg of PHPA. Mixing is conducted in an in-jection line using a simple in-line mixer. The solvent of the gelation solution is a field water having a high H2S content.
The resulting gelation solution has a PHPA to chromium III and ace-tate ion ratio of 10:1 and a PHPA concentration of 3000 ppm. The solu-tion is injected into an injection well in the Wyorning Big Horn Basin at a rate of 9900 liters/hr into a 37.2 meter perforation interval. The treatment zone is a highly fractured Pennyslvanian age sandstone forma-tion.
~7S7~
-17- Docket 850003-B
Samples of the gelation solution are taken from the wellhead during injection. The samples are unfilterable through 8 micron filter paper at 345 kPa differential pressure. The viscosities of the samples are similar to those of comparable gels formulated under laboratory condi-05 tions as described in the preceding examples.
The injection wellhead pressure prior to treatment is on vacuum during injection of a 500 ppm PHPA solution at a rate of 9900 liters/
hr. The injection wellhead pressure increases to 1380 kPa for injection of the 500 ppm PHPA solution at 9900 liters/hr immediately after termi-nation of the present treatment. Two days after termination of the flowing gel treatment, the wellhead pressure is at 1240 kPa at the same injection conditions and remains at that pressure for six months. Dur-ing this period, no chromium III ions are detected at any of the offset producing wells.
In comparison, a neiyhboring injection well in the same formation is treated with a PHPA gel formed from a chromium VI/redox crosslinking agent 501 ution and the same field water of the previous gel. The chro-mium VI reduces very rapidly in the inJection water resulting in poor mixing of the crosslinking agent with the PHPA and poor gel formation.
4,800,000 liters of the resultiny solution are injected into the injec-tion well being treated. No significant injectivity reduction is appar-ent from the treatment.
EXA~IPLE 2 An injection well in a second formation of the Wyoming Big Horn Basin characterized as carbonate strata with an extensive fracture 1~'7~7~Z
-18- Docket 850003-B
network having very open fractures is treated with a predetermined PHPA
gel crosslinked with chromium III and acetate ions. 950,000 liters of the gelation solution containing a total of 12,200 kg of PHPA and having PHPA concentrations ranging from 3000 to 7000 ppm are injected into the 05injection well having tubulars perforated into 9 meters of the forma-tion.
Prior to treatmentJ the injection wellhead pressure is on a strong vacuum when injecting a 500 ppm PHPA solution at 9,900 liters/hr.
After the flowing gel treatment, the injection wellhead pressure in-10creases to 690 kPa at the same injection conditions. Incremental oil production at two of the offset production wells in the 20-acre five-spot spacing is determined to Pxceed 132 liters/hr of oil after the gel treatment.
15A production well in a formation of the Wyoming Big Horn Basin, which is characterized as carbonate strata at a temperature of 58C and which is fractured and possibly overacidized, produces 106 liters/hr of oil and 29,000 liters/hr of water over a perforation interval of about 3.4 meters. The well is uneconomic and is shut in due to excessive 20water production. The produced water has a high H2S content and is expensive to dispose.
31,8U0 liters of a predetermined polyacrylalnide/chrolnium III and acetate gelation solution are pumped into the production well at a rate of 19,000 liters/hr. Fresh water is the solvent of the gelation solu-25tion. The polyacrylamide concentration of the gelation solution is 2%
-19- Docket 850003-B
and the polyacrylamide has an average molecular weight of 11 million.
The weight ratio of polyacrylamide to chromium III and acetate ions in the gelation solution is 20:1. The ~elation solution sets up in the formation as a non-flowing gel. Thereafter, the well is returned to 05 production. The resulting oil production is increased to an economiclevel of over 660 liters/hr while the water production is reduced to less than 13,200 liters/hr.
A series of laboratory floods are conducted to evaluate the perfor-mance of non-flowing rigid polyacrylamide gels of the present invention formed from a chromium III and acetate ion crosslinking agent for im-proved conformance treatment of an oil production well. A sandpack con-taining 40 to 60 mesh sand and having a permeability of about lO darcies simulates the high-conductivity field zones re~uiring improved conform-ance treatment.
The gelation solution is injected into a wellbore in the center of the sandpack. After treatment, water is injected into the outside of the sandpack and produced from the center wellbore. Pressure measure-ments for two rings around the center wellbore allow monitoring of fluid mobility and permeability throughout the tests. The floods are conducted at reservoir conditions of 55C using field produced water and crude. The gel is comprised of a 20,000 ppm polyacrylamide solution.
The polyacrylamide is substantially unhydrolyzed and has a molecular weight of about 5,000,000. The weight ratio of polyacrylamide to chromium III and acetate ions is about 10:1.
'S792 -20- Docket 850003-B
Table 1 below shows the typical results for a non-flowing rigid gel treatment in a 100% water saturated sandpack. The gel reduces the per-meability of the sandpack from 15,000 md to 0.25 md, which is a factor of 60,000. Permeability after treatment increases only sliyhtly with 05 increasing throughput and increasing pressure.
i~75~2 -21- Docke~ 850003-B
Ring #1 Ring #2 Treatment Sequence ~0-0.074PV) (0-0.297 PV) Initial Brine Permeability, md 13,000 15,100 .
05 Inject 0.19 PV Gel at 192 cc/hr .
Reciprocal Relative Mobility, cp 1,500 2,100 Inject 0.03 PV Brine at 192 cc/hr Reciprocal Relative Mobility, cp 14 16 21.7-hr Shut-in Prior to Backflow Inject 0.12 PV Brine at 6 cc/hr Reciprocal Relative Mobility, cp 35,000 124,000 Permeability Reduction 69,000 242,000 Permeability, md 0.19 0.062 Produced Chromium Conc., ppm 3.9-2.9 Iniect 0.79 PV Brine at 12 cc/hr Reciprocal Relative Mobility, cp 63,000 82,000 Permeability R~duction 124,000 161,000 Permeability, md 0.104 0.094 Produced Chromium Conc., ppm 3.1-1.6 Inject 0.18 PV Brine at 24 cc/hr Reciprocal Relative Mobility, cp 41,000 49,000 Permeability Reduction 80,000 96,000 Permeability, md 0.16 0.16 Produced Chromium Conc., ppm 1.8-1.1 Iniect 0.50 PY Brine at 48 cc/hr Reciprocal Relative Mobility, cp 26,000 33,000 Permeability Reduction 52,000 65,000 Permeability, md 0.25 O.Z3 Produced Chromium Conc., ppm 1.3-0.6 Viscosity of Brine at 55C = 0.51 cp ~275~9~
-22- Docket 850003-B
The test is repeated using a much smaller gel treatment volume of 0.08 PV rather than the 0.19 PY shown in Table 1. The permeability reductions are nearly identical to those shown in Table 1.
A second serles of tests are conducted to observe the effects of OS oil saturation. The sandpack is first flooded with a Wyoming crude from a Big Horn Basin Field and then flooded with 2 PV of produced water.
The sandpack is then treated with the gel and resulting permeability reduction to oil and water i8 recorded. The results are shown in Table 2 be1Ow. Post-treatment water permeability is from about 1 to 2 md com-pared to 0.25 md for the oil-free flood shown in Table 1. As in Table 1, the permeability increases only slightly with increasing throughput and pressure. Furthermore, the treatment appears to be equally effec-tive using a treatment volume which is half the volume cf that shown in Table 2.
79~
-23- Docket 850003-B
Ring #1 Ring #2 Treatment Sequence (0-0.074PV) (0-0.297 PV) _v Initial Brine Permeability, md 8,700 8,800 05 Inject 1.27 PY Crude Oil at 192 cc/hr Oil Saturation, PV 0.774 Mobility, md/cp 24 15 Reciprocal Relative Mobility, cp370 580 Inject 2.06 PV_Brine at 192 cc/hr (Backflowl Oil Saturation, PV 0.298 Mobility, md/cp 630 1,300 Reciprocal Relative Mobility, cp14 7.0 Inject 0.164 PY Gel at 192 cc/hr ~ . ~ = = . _ Oil Saturation, PV 0.298 Mobility, md/cp 8.6 6.8 Reciprocal Relative Mobility, cp1,000 1,300 Inject 0.030 PV Brine at 192 cc/hr Oil Saturation, PV 0.298 23.8-hr Shut-in Prior to Brine 3ackflow Inject 0.38 PV Brine at 6 cc/hr Oil Saturation, PV 0.169 Mobility, md/cp 0.19 0.09 Reciprocal Relat1ve Mobility, cp47,000 93,000 Mobility Reduction >3,400 >13,000 Water Relative Permeability, md<0.09 <0.05 Produced Chromium Conc., ppm 10.9-5.1 Inject 0.21 PV Brine at 12 cc/hr Oil Saturation, PV 0.168 Mobility, md/cp 0.33 0.21 Reciprocal Relative Mobility, cp27,000 43,000 Mobility Reduction >1,900 >6,100 Water Relative Permeability, md<0.17 <0.10 Produced Chromium Conc., ppm 5.19-2.65 ;~.2~s7g;~
-24- Docket 850003-B
TABLE 2 (cont) Ring #1Ring #2 Treatment Sequence10-0.074PV) (0-0.297 PY) Inject 0.12 PV Brine at 48 cc/hr 05 Oil Saturation, PV 0.167 Mobility, md/cp 1.1 0.61 Reciprocal Relative Mobility, cp7,800 14,000 Mobility Reduction >560 >2,100 Water Relative Permeability, md<0.57 <0.31 Produced Chromium Conc., ppm 2.94-1.38 Inject 0.24 PV Brine at 192 cc/hr__ Oil Saturationl PV 0.165 Mobility, md/cp 3.4 1.6 Reciprocal Relative Mobility, cp2,600 5,600 Mobility Reduction >190 >800 Water Relative Permeability, md<1.7 <0.80 Produced Chr~mium Conc., ppm 1.77-0.70 Inject 0.23 PV Brine at 384 cc/hr _ _ Oil Saturation, PV 0.164 Mobility, md/cp 3.4 2.0 Reciprocal Relative Mobility, cp2,600 4,500 Mobility Reduction >190 >640 Water Relative Permeability, md<1.7 <1.0 Produced Chromium Conc., ppm 1.15-0.87 Viscosity of Brine at 55C = 0.51 cp Viscosity of Crude Oil at 55C = 18.5 cp The initial waterflood described in Table 2 recovers 61~5'~o of the original oil in place. Oil saturation is reduced from 0.774 to 0.298 PV. The waterflood is resumed after the non-flowiny rigid gel treatment and initial production of oil contains substantially no water. Post-treatment waterflooding produces an additional 0.134 PV of oil which is 17.3h of the oil in place.
~Z~5792 -25- Docket 850003-B
The initial large increase in oil cut by waterflooding results from the gel treatment. As indicated by Tables 1 and 2, the gel nearly elim-inates all water relative permeability in the vicinity of the production well but reduces oil relative permeability to a much lesser extent.
05 Therefore, oil is more readily produced relative to injection water.
The injected water is not produced until after tne mobile oil saturation is produced.
While the foregoing preferred embodiments of the invention have been described and shown, it is understood that the alternatives and modifications, such as those suggested and others, may be made thereto and follow in the scope of the invention.
Tailor-making or customizing a gel in the manner of the present in-vention to meet the demands of a particular treatment region is provided in part by correlating the independent gelation parameters with the dependent variables of gelation rate and resultant gel strength and 05 stability. The independent gelation parameters are the surface and in situ gelation conditions including: temperature, pH, ionic strength and specific electrolytic makeup of the solvent, polymer concentration, ratio of the weight of polymer to the combined weight of chromium III
and carboxylate species in the mixture, degree of polymer hydrolysis, and average molecular weight of the polymer.
The operable ranges of the gelation parameters are correlated with the dependent variables of gelation rate and resultant gel properties by means including qualitative bottle testing, quantitative viscosimetric analysis, packed-column flooding, and core flooding. The operable ranges of a number of gelation parameters and their correlation with the dependent variables are described below.
The lower temperature limit of the gelation solution at the surface is the freezing point of the solution and the upper limit is essentially the thermal stability limit of the polymer. The solution is generally maintained at ambient temperature or higher at the surface. The temper-ature may be adjusted by heating or cooling the aqueous solvent. In-creasing the temperature within the prescribed range increases the gela-tion rate.
~Z75 ~'9;Z
~ Docket 850003-B
The initial pH of the gelation solution is within a range of about 3 to 13 and preferably about 6 to 13. Althouyh gelation can occur at an acidic pH, lowering the initial pH of the solution below 7 does not favor gelation. The initial pH of the solution is most preferably alka-05 line, i.e., greater than 7 to about 13. Increasing the pH within the prescribed range increases the rate of gelation.
The polymer concentration in the solution is about 1000 ppm up to the solubility limit of the polymer in the solvent or the rheological constraints of the polymer solution, preferably about 1000 to about 200,000 ppm, and most preferably about 3000 to about 100,000. Increas-ing the polymer concentration increases the gelation rate and ultimate gel strength at a constant ratio of polymer to crosslinking agent.
The ionic strength of the solvent can be from that of deionized distilled water to that of a brine having an ion concentration approach-ing the solubility limit of tne brine. Increasing the ionic strength of the solution can increase the gelation rate.
The weight ratio of acrylamide polymer to chromium III and carboxy-late species comprising the mixture is about 1:1 to about 500:1, prefer-ably about 2.5:1 to about 100:1, and most preferably about 5:1 to about 40:1. Decreasing the ratio generally increases the gelation rate and up to a certain point generally increases the gel strength, especially at a constant high polymer concentration.
The degree of hydrolysis is about 0 to 60% and preferably about 0 to 30%. Within the preferred range, increasing the degree of hydrolysis 12757~;~
-1~- Docket 850003-B
increases the gelation rate. Increasing the molecular weight of the polymer increases the gel strength.
It is apparent from these correlations that one can produce yels across a very broad range of gelation rates and gel properties as a 05 function of the gelation conditions. Thus, to effect an optimum gela-tion treatment according to the present process, the practitioner pre-determines the gelation rate and properties of the resultant gel which mee~ the treatment demands of the given region and thereafter produces the gel having these predetermined characteristics. The treatment demands include the in situ gelation conditions such as temperature, connate water properties, and permeability of the region as well as the post treatment con~itions such as injection and production pressures.
Analytical methods known to one skilled in the art are used to determine the treatment demands. The treatment demands provide criteria to pre-determine the gelation rate and resultant gel properties in the manner desoribed above and continuing hereafter.
The gelation rate is advantageously sufficiently slow to enable preparation of the gelation solution at the surface, injection of the solution as a uniform slug into the wellbore, and displacement of the entire solution into the desired treatment zone. Too rapid a gelation rate produces excessive gelation of the solution at the surface which results in a solution that may be difficult, if not impossible, to inject into the wellbore or formation due to its rheological prop-erties. At the same time, the gelation rate must be sufficiently rapid ~2'7S792 -13- Docket 850003-~
to enable completion of the reaction within a reasonable period of time so that the well may be returned to iniection or production after treat~
ment.
When treating anomalies, at least partial gelation of the solution, 05 if not complete gelation for some flowing gels, is advantageous before the solution reaches the face bounding the matrix and the anomaly to prevent the solution from penetrating the matrix as well as the anom-aly. Substantial penetration of the matrix by the solution and the ensuing permeability reduction of the matrix are counterproductive to the plugging of anomalies. The values of the independent variables in the process are carefully selected to achieve a gelation rate meeting these criteria.
The volume of solution iniected into the formation is a function of the volume and location of the desired treatment region and the degree of penetration into the treatment region by the solution. One skilled in the art can determine the required volume of gel for a given treat-ment region. Placement of the gelation solution in the treatment region may be facilitated by zone isolation means such as packers and the like.
The injection rate is a function of the gelation rate and opera-tional constraints of injection pressure and pumping limits. The re-quired injection rate is fixed such that all of the solution can be practically injected into the treatment zone before it becomes unpump-able. The gelation tirne of the gel ranges from near instantaneous for ~Z~57~Z
-14- Docket 850003-B
flowing gels up to 48 hours or longer for both flowing and non-flowing gels. Longer gelation times are limited by practical considerations of lost production when injection and production wells are shut in.
The process is applicable to conformance treatment of formations 05 under most conditions and is specific to treating regions within theformation which are in fluid communication with an injection or produc-tion well. The flowing gel is especially applicable to the treatrnent of anomalies such as streaks of relatively high permeability, fractures or fracture networks in direct communication via the anomaly with an injec-tion well but not also in direct communication via the anomaly with a production well. The ultimate gel is termed a flowing gel as defined herein because it would flow if unconfined on the surface. However, the flowing gel is sufficiently crosslinked to remain in place under injec-tion conditions in the anomaly when confined thereby. Thus, the flowing gel is capable of effectively pluyging the anomaly.
The flowing gel is not generally suitable for treatment of anom-alies in direct communication via the anomaly with production wells because flowing gels do not have sufficient strength to withstand the drawdown pressure ~uring production and may flow back into the well-bore. For treatment of anomalies in direct communication with produc-tion wells, non-flowing rigid gels having sufficient strength to with-stand the production drawdown pressure are preferred. It is preferred that substantially none of the gel flows back into the wellbore when oil is produced after the conformance treatment.
~2757~
-lS- Docket 850003-B
In some specialized cases, the degree of partial gelation of the injected solution can be maintained sufficiently low to enable the solu-tion to enter a selected high permeability zone of the matrix and crosslink to completion in situ as either a non-flowing gel or a flowing 05 gel. Both flowing and non-flowing gels can be used for treatment ofhigh permeability zones of the matrix because in general neither wi11 flow from the treatment zone upon complete gelation, a necessary condi-tion for the present invention. However, non-flowing gels are preferred for treatment of high permeability zones in direct communication with production wells because of the~r increased strength.
PA is preferred for the formulation of non-flowing gels because it has a slower gelation rate than PHPA which enab1es one to inject it into the treatment reglon before it sets up. PHPA is preferred for the for-mulation of flowing gels because In many cases flowing gels can be injected into a treatment region e~en when reacted to completion.
Gels having a predetermined gelation rate and resultant gel prop-erties to meet the treatment demands of a given region are produced by adjusting and setting the surface gelation conditions as they correlate to the gelation rate and gel properties. Accordingly the gels are pro-duced in a manner which renders them insensitive to most extreme forma-tion conditions. The gels can be applied to the treatment of many dif-ferent geological structures including high permeability zones within the formation matrix and anomalies external to the matrix such as frac-tures and other cavities. The gels can be stable at formation tempera-tures as high as 115C and at any formation pH contemplated. The gels .~ . . . .
~2'757~2 -16- Docket 850003-B
are relatively insensitive to the stratigraphy of the rock and can be employed in carbonate and sandstone strata and unconsolidated or consolidated strata having varying mineralogy. Once the gels are in place, it is extremely difficult to displace the gels by physical or 05 chemical means other than total destruction of the crosslinked network.
The gels may be reversible on contact with hydrogen peroxide or sodium hypochlorite, but are substantially insoluble in the formation fluids.
The following examples demonstrate the practice and utility of the present invention but are not to be construed as limiting the scope thereof-1,020,000 liters of a predetermined aqueous gelation solution com-prising PHPA and a crosslinking agent made up of chromium III and ace-tate ions are prepared. The gelation solution is prepared by mixing a "Chromic Acetate 50% Solution" diluted to 15% by weight with a PHPA
solution containing 3040 kg of PHPA. Mixing is conducted in an in-jection line using a simple in-line mixer. The solvent of the gelation solution is a field water having a high H2S content.
The resulting gelation solution has a PHPA to chromium III and ace-tate ion ratio of 10:1 and a PHPA concentration of 3000 ppm. The solu-tion is injected into an injection well in the Wyorning Big Horn Basin at a rate of 9900 liters/hr into a 37.2 meter perforation interval. The treatment zone is a highly fractured Pennyslvanian age sandstone forma-tion.
~7S7~
-17- Docket 850003-B
Samples of the gelation solution are taken from the wellhead during injection. The samples are unfilterable through 8 micron filter paper at 345 kPa differential pressure. The viscosities of the samples are similar to those of comparable gels formulated under laboratory condi-05 tions as described in the preceding examples.
The injection wellhead pressure prior to treatment is on vacuum during injection of a 500 ppm PHPA solution at a rate of 9900 liters/
hr. The injection wellhead pressure increases to 1380 kPa for injection of the 500 ppm PHPA solution at 9900 liters/hr immediately after termi-nation of the present treatment. Two days after termination of the flowing gel treatment, the wellhead pressure is at 1240 kPa at the same injection conditions and remains at that pressure for six months. Dur-ing this period, no chromium III ions are detected at any of the offset producing wells.
In comparison, a neiyhboring injection well in the same formation is treated with a PHPA gel formed from a chromium VI/redox crosslinking agent 501 ution and the same field water of the previous gel. The chro-mium VI reduces very rapidly in the inJection water resulting in poor mixing of the crosslinking agent with the PHPA and poor gel formation.
4,800,000 liters of the resultiny solution are injected into the injec-tion well being treated. No significant injectivity reduction is appar-ent from the treatment.
EXA~IPLE 2 An injection well in a second formation of the Wyoming Big Horn Basin characterized as carbonate strata with an extensive fracture 1~'7~7~Z
-18- Docket 850003-B
network having very open fractures is treated with a predetermined PHPA
gel crosslinked with chromium III and acetate ions. 950,000 liters of the gelation solution containing a total of 12,200 kg of PHPA and having PHPA concentrations ranging from 3000 to 7000 ppm are injected into the 05injection well having tubulars perforated into 9 meters of the forma-tion.
Prior to treatmentJ the injection wellhead pressure is on a strong vacuum when injecting a 500 ppm PHPA solution at 9,900 liters/hr.
After the flowing gel treatment, the injection wellhead pressure in-10creases to 690 kPa at the same injection conditions. Incremental oil production at two of the offset production wells in the 20-acre five-spot spacing is determined to Pxceed 132 liters/hr of oil after the gel treatment.
15A production well in a formation of the Wyoming Big Horn Basin, which is characterized as carbonate strata at a temperature of 58C and which is fractured and possibly overacidized, produces 106 liters/hr of oil and 29,000 liters/hr of water over a perforation interval of about 3.4 meters. The well is uneconomic and is shut in due to excessive 20water production. The produced water has a high H2S content and is expensive to dispose.
31,8U0 liters of a predetermined polyacrylalnide/chrolnium III and acetate gelation solution are pumped into the production well at a rate of 19,000 liters/hr. Fresh water is the solvent of the gelation solu-25tion. The polyacrylamide concentration of the gelation solution is 2%
-19- Docket 850003-B
and the polyacrylamide has an average molecular weight of 11 million.
The weight ratio of polyacrylamide to chromium III and acetate ions in the gelation solution is 20:1. The ~elation solution sets up in the formation as a non-flowing gel. Thereafter, the well is returned to 05 production. The resulting oil production is increased to an economiclevel of over 660 liters/hr while the water production is reduced to less than 13,200 liters/hr.
A series of laboratory floods are conducted to evaluate the perfor-mance of non-flowing rigid polyacrylamide gels of the present invention formed from a chromium III and acetate ion crosslinking agent for im-proved conformance treatment of an oil production well. A sandpack con-taining 40 to 60 mesh sand and having a permeability of about lO darcies simulates the high-conductivity field zones re~uiring improved conform-ance treatment.
The gelation solution is injected into a wellbore in the center of the sandpack. After treatment, water is injected into the outside of the sandpack and produced from the center wellbore. Pressure measure-ments for two rings around the center wellbore allow monitoring of fluid mobility and permeability throughout the tests. The floods are conducted at reservoir conditions of 55C using field produced water and crude. The gel is comprised of a 20,000 ppm polyacrylamide solution.
The polyacrylamide is substantially unhydrolyzed and has a molecular weight of about 5,000,000. The weight ratio of polyacrylamide to chromium III and acetate ions is about 10:1.
'S792 -20- Docket 850003-B
Table 1 below shows the typical results for a non-flowing rigid gel treatment in a 100% water saturated sandpack. The gel reduces the per-meability of the sandpack from 15,000 md to 0.25 md, which is a factor of 60,000. Permeability after treatment increases only sliyhtly with 05 increasing throughput and increasing pressure.
i~75~2 -21- Docke~ 850003-B
Ring #1 Ring #2 Treatment Sequence ~0-0.074PV) (0-0.297 PV) Initial Brine Permeability, md 13,000 15,100 .
05 Inject 0.19 PV Gel at 192 cc/hr .
Reciprocal Relative Mobility, cp 1,500 2,100 Inject 0.03 PV Brine at 192 cc/hr Reciprocal Relative Mobility, cp 14 16 21.7-hr Shut-in Prior to Backflow Inject 0.12 PV Brine at 6 cc/hr Reciprocal Relative Mobility, cp 35,000 124,000 Permeability Reduction 69,000 242,000 Permeability, md 0.19 0.062 Produced Chromium Conc., ppm 3.9-2.9 Iniect 0.79 PV Brine at 12 cc/hr Reciprocal Relative Mobility, cp 63,000 82,000 Permeability R~duction 124,000 161,000 Permeability, md 0.104 0.094 Produced Chromium Conc., ppm 3.1-1.6 Inject 0.18 PV Brine at 24 cc/hr Reciprocal Relative Mobility, cp 41,000 49,000 Permeability Reduction 80,000 96,000 Permeability, md 0.16 0.16 Produced Chromium Conc., ppm 1.8-1.1 Iniect 0.50 PY Brine at 48 cc/hr Reciprocal Relative Mobility, cp 26,000 33,000 Permeability Reduction 52,000 65,000 Permeability, md 0.25 O.Z3 Produced Chromium Conc., ppm 1.3-0.6 Viscosity of Brine at 55C = 0.51 cp ~275~9~
-22- Docket 850003-B
The test is repeated using a much smaller gel treatment volume of 0.08 PV rather than the 0.19 PY shown in Table 1. The permeability reductions are nearly identical to those shown in Table 1.
A second serles of tests are conducted to observe the effects of OS oil saturation. The sandpack is first flooded with a Wyoming crude from a Big Horn Basin Field and then flooded with 2 PV of produced water.
The sandpack is then treated with the gel and resulting permeability reduction to oil and water i8 recorded. The results are shown in Table 2 be1Ow. Post-treatment water permeability is from about 1 to 2 md com-pared to 0.25 md for the oil-free flood shown in Table 1. As in Table 1, the permeability increases only slightly with increasing throughput and pressure. Furthermore, the treatment appears to be equally effec-tive using a treatment volume which is half the volume cf that shown in Table 2.
79~
-23- Docket 850003-B
Ring #1 Ring #2 Treatment Sequence (0-0.074PV) (0-0.297 PV) _v Initial Brine Permeability, md 8,700 8,800 05 Inject 1.27 PY Crude Oil at 192 cc/hr Oil Saturation, PV 0.774 Mobility, md/cp 24 15 Reciprocal Relative Mobility, cp370 580 Inject 2.06 PV_Brine at 192 cc/hr (Backflowl Oil Saturation, PV 0.298 Mobility, md/cp 630 1,300 Reciprocal Relative Mobility, cp14 7.0 Inject 0.164 PY Gel at 192 cc/hr ~ . ~ = = . _ Oil Saturation, PV 0.298 Mobility, md/cp 8.6 6.8 Reciprocal Relative Mobility, cp1,000 1,300 Inject 0.030 PV Brine at 192 cc/hr Oil Saturation, PV 0.298 23.8-hr Shut-in Prior to Brine 3ackflow Inject 0.38 PV Brine at 6 cc/hr Oil Saturation, PV 0.169 Mobility, md/cp 0.19 0.09 Reciprocal Relat1ve Mobility, cp47,000 93,000 Mobility Reduction >3,400 >13,000 Water Relative Permeability, md<0.09 <0.05 Produced Chromium Conc., ppm 10.9-5.1 Inject 0.21 PV Brine at 12 cc/hr Oil Saturation, PV 0.168 Mobility, md/cp 0.33 0.21 Reciprocal Relative Mobility, cp27,000 43,000 Mobility Reduction >1,900 >6,100 Water Relative Permeability, md<0.17 <0.10 Produced Chromium Conc., ppm 5.19-2.65 ;~.2~s7g;~
-24- Docket 850003-B
TABLE 2 (cont) Ring #1Ring #2 Treatment Sequence10-0.074PV) (0-0.297 PY) Inject 0.12 PV Brine at 48 cc/hr 05 Oil Saturation, PV 0.167 Mobility, md/cp 1.1 0.61 Reciprocal Relative Mobility, cp7,800 14,000 Mobility Reduction >560 >2,100 Water Relative Permeability, md<0.57 <0.31 Produced Chromium Conc., ppm 2.94-1.38 Inject 0.24 PV Brine at 192 cc/hr__ Oil Saturationl PV 0.165 Mobility, md/cp 3.4 1.6 Reciprocal Relative Mobility, cp2,600 5,600 Mobility Reduction >190 >800 Water Relative Permeability, md<1.7 <0.80 Produced Chr~mium Conc., ppm 1.77-0.70 Inject 0.23 PV Brine at 384 cc/hr _ _ Oil Saturation, PV 0.164 Mobility, md/cp 3.4 2.0 Reciprocal Relative Mobility, cp2,600 4,500 Mobility Reduction >190 >640 Water Relative Permeability, md<1.7 <1.0 Produced Chromium Conc., ppm 1.15-0.87 Viscosity of Brine at 55C = 0.51 cp Viscosity of Crude Oil at 55C = 18.5 cp The initial waterflood described in Table 2 recovers 61~5'~o of the original oil in place. Oil saturation is reduced from 0.774 to 0.298 PV. The waterflood is resumed after the non-flowiny rigid gel treatment and initial production of oil contains substantially no water. Post-treatment waterflooding produces an additional 0.134 PV of oil which is 17.3h of the oil in place.
~Z~5792 -25- Docket 850003-B
The initial large increase in oil cut by waterflooding results from the gel treatment. As indicated by Tables 1 and 2, the gel nearly elim-inates all water relative permeability in the vicinity of the production well but reduces oil relative permeability to a much lesser extent.
05 Therefore, oil is more readily produced relative to injection water.
The injected water is not produced until after tne mobile oil saturation is produced.
While the foregoing preferred embodiments of the invention have been described and shown, it is understood that the alternatives and modifications, such as those suggested and others, may be made thereto and follow in the scope of the invention.
Claims (29)
1. A process for substantially plugging a high permeability region in a hydrocarbon-bearing formation below an earthen surface, said formation penetrated by a wellbore in fluid communication with said region, the process comprising the steps of:
a) predetermining the gelation rate, gel strength and gel stability required of a gel to meet the treatment demands of the formation at the region;
b) providing a gelation solution at said earthen surface capable of forming a gel having the required predetermined gelation rate, gel strength and gel stability, said gelation solution comprising a water-soluble acrylamide polymer and an agent capable of crosslinking said polymer comprising a complex or mixture of complexes containing at least one electropositive chromium III species and at least one electronegative carboxylate species, said gelation solution having an initial pH within a range of from about 3 to 13;
c) injecting said gelation solution into said region via said wellbore; and d) forming said gel from said gelation solution having the required predetermined gelation rate, gel strength and gel stability to substantially plug said region, said gel being substantially permanent and resistant to in situ degradation.
a) predetermining the gelation rate, gel strength and gel stability required of a gel to meet the treatment demands of the formation at the region;
b) providing a gelation solution at said earthen surface capable of forming a gel having the required predetermined gelation rate, gel strength and gel stability, said gelation solution comprising a water-soluble acrylamide polymer and an agent capable of crosslinking said polymer comprising a complex or mixture of complexes containing at least one electropositive chromium III species and at least one electronegative carboxylate species, said gelation solution having an initial pH within a range of from about 3 to 13;
c) injecting said gelation solution into said region via said wellbore; and d) forming said gel from said gelation solution having the required predetermined gelation rate, gel strength and gel stability to substantially plug said region, said gel being substantially permanent and resistant to in situ degradation.
2. The process of Claim 1 wherein said complex or mixture of com-plexes further contains at least one electronegative species selected from the group consisting of oxygen species, hydroxide species, and mixtures thereof.
3. The process of Claim 2 wherein said complex further contains a species selected from the group consisting of inorganic monovalent ions, inorganic divalent ions, water molecules, and mixtures thereof.
4. The process of Claim 1 wherein said wellbore is an oil pro-duction well.
5. The process of Claim 4 wherein said gel is a non-flowing gel.
6. The process of Claim 5 wherein said acrylamide polymer is sub-stantially unhydrolyzed polyacrylamide.
7. The process of Claim 5 wherein said non-flowing gel substan-tially remains in said region after complete gelation at a production drawdown pressure.
8. The process of Claim 5 wherein said gelation solution is gelled to completion in said region.
9. The process of Claim 1 wherein said wellbore is an injection well.
10. The process of Claim 9 wherein said gel is a flowing gel.
11. The process of Claim 10 wherein said acrylamide polymer is par-tially hydrolyzed polyacrylamide.
12. The process of Claim 10 wherein said ge1ation solution is gelled to substantial completion at the surface prior to injecting into said wellbore.
13. The process of Claim 10 wherein said flowing gel substantially remains in said region after complete gelation.
14. The process of Claim 1 wherein said region is a fracture or fracture network.
15. The process of Claim 1 wherein said carboxylate species is selected from the group consisting of formate, acetate, propionate, lactate, lower substituted derivatives thereof, and mixtures thereof.
16. The process of Claim 1 wherein the weight ratio of said polymer to said chromium III and carboxylate species in said gelation solution is about 1:1 to about 500:1.
17. The process of Claim l wherein the initial pH of said gelation solution is greater than about 7 to about 13.
18. A crosslinked polymer gel prepared by mixing water, a water-soluble acrylamide polymer, and an agent capable of crosslinking said polymer comprising a complex or mixture of complexes containing at least one electropositive chromium III species and at least one electronega-tive carboxylate species and crosslinking said mixture to completion.
19. A process for substantially plugging a high permeability region in a hydrocarbon-bearing formation below an earthen surface, said formation penetrated by a wellbore in fluid communication with said region, the process comprising the steps of:
a) preparing a gelation solution at the surface comprising a water-soluble acrylamide polymer and a complex capable of crosslinking said polymer, said complex containing at least one electropositive chromium III species and at least one electronegative carboxylate species, said gelation solution having an initial pH within a range of from about 3 to 13;
b) injecting said gelation solution into said wellbore; and c) displacing said gelation solution into said region to form a gel which substantially plugs said region, said gel being substantially permanent and resistant to in situ degradation.
a) preparing a gelation solution at the surface comprising a water-soluble acrylamide polymer and a complex capable of crosslinking said polymer, said complex containing at least one electropositive chromium III species and at least one electronegative carboxylate species, said gelation solution having an initial pH within a range of from about 3 to 13;
b) injecting said gelation solution into said wellbore; and c) displacing said gelation solution into said region to form a gel which substantially plugs said region, said gel being substantially permanent and resistant to in situ degradation.
20. The process of Claim 19 further comprising at least partially gelling said gelation solution before said solution contacts said region.
21. The process of Claim 19 wherein said complex further contains at least one electronegative species selected from the group consisting of oxygen species, hydroxide species and mixtures thereof.
22. The process of Claim 19 wherein said complex further contains a species selected from the group consisting of inorganic monovalent ions, inorganic divalent ions, water molecules, and mixtures thereof.
23. The process of Claim 19 wherein said wellbore is an oil pro-duction well.
24. The process of Claim 19 wherein said wellbore is an injection well.
25. The process of Claim 19 wherein said region is a fracture or fracture network.
26. The process of Claim 19 wherein said carboxylate species is selected from the group consisting of formate, acetate, propionate, lactate, lower substituted derivatives thereof, and mixtures thereof.
27. The process of Claim 19 wherein said region is a formation matrix.
28. The process of Claim 27 wherein said gel is a flowing gel.
29. The process of Claim 28 wherein said acrylamide polymer is partially hydrolyzed polyacrylamide.
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US80741685A | 1985-12-10 | 1985-12-10 | |
| US807,416 | 1985-12-10 | ||
| US06/822,709 US4683949A (en) | 1985-12-10 | 1986-01-27 | Conformance improvement in a subterranean hydrocarbon-bearing formation using a polymer gel |
| US822,709 | 1986-01-27 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1275792C true CA1275792C (en) | 1990-11-06 |
Family
ID=27123013
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000520374A Expired - Lifetime CA1275792C (en) | 1985-12-10 | 1986-10-14 | Conformance improvement in a subterranean hydrocarbon- bearing formation using a polymer gel |
Country Status (8)
| Country | Link |
|---|---|
| US (1) | US4683949A (en) |
| CN (1) | CN1005729B (en) |
| AR (1) | AR243639A1 (en) |
| BR (1) | BR8605791A (en) |
| CA (1) | CA1275792C (en) |
| GB (1) | GB2183692B (en) |
| NL (1) | NL189775C (en) |
| NO (1) | NO175791C (en) |
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| US7677313B2 (en) | 2004-09-15 | 2010-03-16 | Alberta Science And Research Authority | Method for controlling water influx into wellbores by blocking high-permeability channels |
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| FR3092329B1 (en) | 2019-02-01 | 2021-01-29 | S N F Sa | Process for modifying the water permeability of an underground formation |
| US11434718B2 (en) | 2020-06-26 | 2022-09-06 | Saudi Arabian Oil Company | Method for coring that allows the preservation of in-situ soluble salt cements within subterranean rocks |
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| US3114651A (en) * | 1960-07-22 | 1963-12-17 | American Cyanamid Co | Water insolubilization of acrylamido polymers with a salt of trivalent chromium |
| US3383307A (en) * | 1964-07-30 | 1968-05-14 | Ashland Oil Inc | Gelling agents, gels and methods for forming gels |
| US3554287A (en) * | 1966-11-07 | 1971-01-12 | Dow Chemical Co | Gelable composition, resulting gelled polymer composition and use thereof |
| US3658129A (en) * | 1970-09-01 | 1972-04-25 | Phillips Petroleum Co | In situ gelation of polymers during waterflooding |
| US3762476A (en) * | 1972-01-03 | 1973-10-02 | Phillips Petroleum Co | Subterranean formation permeability correction |
| US3833061A (en) * | 1972-12-27 | 1974-09-03 | Phillips Petroleum Co | Method for selectively reducing brine permeability in a subterranean oil-wet formation |
| US3926258A (en) * | 1972-12-27 | 1975-12-16 | Phillips Petroleum Co | Method for reducing formation permeability with gelled polymer solution having delayed gel time |
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| US4069869A (en) * | 1977-02-11 | 1978-01-24 | Union Oil Company Of California | Plugging high permeability zones of reservoirs having heterogeneous permeability |
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| US4291069A (en) * | 1977-07-20 | 1981-09-22 | The Dow Chemical Company | Rendering porous structures impermeable by treatment with gellable amine polymers |
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-
1986
- 1986-01-27 US US06/822,709 patent/US4683949A/en not_active Expired - Lifetime
- 1986-10-14 CA CA000520374A patent/CA1275792C/en not_active Expired - Lifetime
- 1986-10-23 GB GB8625445A patent/GB2183692B/en not_active Expired
- 1986-11-26 BR BR8605791A patent/BR8605791A/en not_active IP Right Cessation
- 1986-12-08 CN CN86108199.4A patent/CN1005729B/en not_active Expired
- 1986-12-08 NL NLAANVRAGE8603121,A patent/NL189775C/en not_active IP Right Cessation
- 1986-12-09 NO NO864944A patent/NO175791C/en not_active IP Right Cessation
- 1986-12-10 AR AR86306137A patent/AR243639A1/en active
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US7677313B2 (en) | 2004-09-15 | 2010-03-16 | Alberta Science And Research Authority | Method for controlling water influx into wellbores by blocking high-permeability channels |
Also Published As
| Publication number | Publication date |
|---|---|
| NO864944D0 (en) | 1986-12-09 |
| NL189775C (en) | 1993-07-16 |
| NL189775B (en) | 1993-02-16 |
| NO175791B (en) | 1994-08-29 |
| GB2183692B (en) | 1989-10-18 |
| GB2183692A (en) | 1987-06-10 |
| CN86108199A (en) | 1987-08-12 |
| CN1005729B (en) | 1989-11-08 |
| BR8605791A (en) | 1987-08-25 |
| NL8603121A (en) | 1987-07-01 |
| NO864944L (en) | 1987-06-11 |
| NO175791C (en) | 1994-12-07 |
| AR243639A1 (en) | 1993-08-31 |
| US4683949A (en) | 1987-08-04 |
| GB8625445D0 (en) | 1986-11-26 |
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