Patent Application
20080035344
The gelling system of the present invention comprises a delayed co-gel formed from an acidic aqueous solution of acid-soluble polyacrylamide and an at least partially neutralized acid aluminum salt, with additional gelling control by a Lewis base and optional polyvalent anionic gel modifiers. The crosslinking reactions of this new system are controllable and robust. The pre-gel fluid before it sets has an adjustable rheology for pumping and placement. When set, this system provides a three-dimensional interconnected polymer molecular structure interspersed in and crosslinked by a network of an inorganic gel structure.
As used herein, “polymer” may be used to refer to homopolymers, copolymers, interpolymers, terpolymers, etc. Likewise, a copolymer may refer to a polymer comprising at least two monomers, optionally with other monomers, and may be a random, alternating, block or graft copolymer.
As used herein, when a polymer is referred to as comprising a monomer, the monomer is present in the polymer in the polymerized form of the monomer or in the derivative form the monomer. A “cationic polymer” is a polymer with positively charged ionic sites spaced along the polymer; an “anionic polymer” is one with negatively charged ionic sites; and an “amphoteric polymer” is one with both positively and negatively charged ionic groups. The ionic sites can be either integral or pendant with respect to the polymer backbone. The terms “cationic polymer” and “anionic polymer” each encompass amphoteric polymers, without regard to whether one charge predominates over the other, or whether the polymer is zwitterionic.
The polyacrylamide (PAM) suitable in the present invention is generally a water-soluble polymer or copolymer of acrylamide that is soluble in acidic solutions, e.g. at a pH less than 6, from 3 to 5 in one embodiment, and especially from 4 to 4.5. The polyacrylamide or other polymer should also be stable in acidic solution with partially neutralized acid aluminum salts such as aluminum hydroxyl chloride (AHC), i.e. it should not prematurely crosslink before the desired set time.
The polyacrylamide may be anionic, cationic, or amphoteric. Anionic polyacrylamide or APAM can be made by copolymerizing an acidic comonomer such as acrylic acid or methacrylic acid with the acrylamide, or more commonly by hydrolysis or other derivatization of a portion of the amide groups. For example, polyacrylamide can be partially hydrolyzed at the formation conditions. Although not intending to be bound by theory, it is thought that the anionic groups can facilitate cross-linking by providing reactive sites for the polyvalent cationic aluminum species that are present upon activation. In an embodiment, only a very small amount of anionic groups are required for the desired cross-linking density, e.g. a degree of hydrolysis of just 0.01 to 0.1 (100 to 1000 ppm mole basis) may be sufficient in some embodiments. In one embodiment, this degree of hydrolysis is achieved fairly quickly at the elevated temperature conditions of subterranean formations, i.e. before the inorganic (aluminum) gel sets.
Cationic polyacrylamide or CPAM is a copolymer of acrylamide and a cationic monomer in an amount to introduce the desired charge density into the copolymer, although it is also known to form a derivative of polyacrylamide via modification of the amide groups to introduce the pendent cationic moieties into the polymer, and such cationic polyacrylamides are also suitably employed in the invention. Although not fully understood and not wishing to be bound by theory, the presence of the cationic comonomer appears to impart acid solubility to the polymer and to play a role in helping to stabilize the system to facilitate control of the delayed gel formation.
The cationic comonomer typically introduces pendant quaternary amines into the copolymer, such as, for example, 4-vinylpyridine or a beta-acryloxyalkylenetrialkylammonium salt of the formula CH2=C(R1)[CONH—R2N+(R3)3]Z− wherein Z is an anion (such as chloride, bromide, iodide, hydroxide, carbonate, phosphate, nitrate, etc.); R1 and R3 are the same or different and are independently selected from hydrogen and C1 to C10 alkyls, and R2 is C1 to C10 alkylene, e.g. methacryloxyethylenetrimethylammonium chloride or acryloxyethylenetrimethylammonium hydroxide monomers. In various embodiments, the R1, R2 and R3 moieties in the beta-acryloxyalkylenetrialkylammonium salt may each independently have from 1 to 5 carbon atoms, from 1 to 3 carbon atoms, 1 or 2 carbon atoms, or especially 1 carbon atom.
The CPAM may have a cationicity, defined as the molar fraction of cationic comonomer in the CPAM, from 0.001 to 0.999. In a first embodiment, the CPAM has a cationicity less than 0.05 (corresponding to a fraction of acrylamide and other nonionic and/or anionic monomer units of more than 0.95), and in a second embodiment greater than 0.05 (corresponding to a fraction of acrylamide and other nonionic monomer units of less than 0.95). In the first embodiment, the cationicity may range from 0.001 to 0.05, from 0.005 to 0.05, from 0.01 to 0.05, from 0.02 to 0.05, or from 0.025 to 0.05, and/or the fraction of acrylamide units can range from 0.95 to 0.995, from 0.95 to 0.99, from 0.95 to 0.98, or from 0.95 to 0.975; and in the second, cationicity may range from 0.05 to 0.99, with a lower limit of at least 0.05, 0.1, 0.25, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 or 0.95, and/or the fraction of acrylamide units can range from 0.01 to 0.95, with an upper limit of no more than 0.95, 0.9, 0.75, 0.6, 0.5, 0.4, 0.3, 0.2, 0.1 or 0.05.
The acid-soluble PAM can have a molecular weight from 100,000 or less to 20 million or more, depending on the pre-set rheology and gel characteristics desired. As used herein, molecular weight refers to weight average molecular weight unless otherwise specified. In general, a higher molecular weight leads to a more rigid gel, whereas lower molecular weights yield solutions of lower viscosity that may be more easily pumped. Higher molecular weight PAM is generally used at a lower concentration than the lower molecular weight PAM. In various other embodiments, the PAM has a molecular weight with a lower limit that is at least 250,000, at least 500,000, at least 750,000, at least 1 million, at least 2 million, at least 3 million, at least 4 million, at least 5 million, at least 6 million, at least 7.5 million, at least 8 million, at least 9 million, at least 10 million or at least 15 million; and with an upper limit that is less than 15 million, less than 10 million, less than 7.5 million, less than 5 million, less than 3 million, less than 2 million, less than 1 million, less than 750,000, less than 500,000, or less than 300,000; or a within a range from any lower limit value to any higher upper limit value.
The PAM can be employed in the pre-gel solution in an amount effective to form a gel of the desired morphology, and also to provide a pre-gel of the desired rheological properties. In a general embodiment the PAM comprises from 0.05 to 50 weight percent of the pre-gel solution, and in various other embodiments the PAM can have a lower limit of at least 0.1, 0.15, 0.2, 0.25, 0.3, 0.4, 0.5, 1, 1.5, 2, 2.5, 3, 4, 5, 8 or 10 percent by weight, an upper limit of not more than 30, 25, 20, 15, 10, 8, 5, 4, 3, 2.5, 2, 1.5, or 1 percent by weight, or a range from any lower limit to any higher upper limit, e.g. from 1 to 10, from 1 to 5, or from 0.2 to 2.5 weight percent. The PAM can be added to or mixed with the other components of the pre-gel solution in the form of a solid, aqueous solution or water-in-oil emulsion. For high molecular weight PAM especially, the PAM is commercially available in emulsion form.
Other polymers that can be used in addition to or instead of the PAM include alkyl substituted or unsubstituted polyacrylic acid, polymethacrylic acid, polyacrylamide, hydrolyzed polyacrylonitrile, polyvinylpyrrolidone, a combination thereof, or the like. As representative examples there may be mentioned poly(methylene-bis-acrylamide); poly(N,N′-dimethyl acrylamide); poly (2-hydroxyethylmethacrylate), poly(2-hydroxyethylacrylate), glycolated polyacrylamide, polyacrylamide copolymer, and the like. For convenience reference is made herein to PAM as a non-limiting example.
The at least partially neutralized acid aluminum salt has the general formula Aln(OH)mXp wherein X is a mineral or organic ion or a mixture thereof with average valence q, wherein pq+m=3n and a ratio m/3n is between 0.3 and 0.833 or 5/6. In an embodiment, the ratio m/3m is between 0.3 and 0.8. As mineral ions there may be mentioned chloride, bromide, iodide, phosphate, nitrate, sulfate, and the like; and as organic ions, formate, acetate, propionate, butyrate, tartrate, citrate, lactate, oxalate, succinate, and the like. Anionic groups in the PAM can also serve as the organic ion X in the acid aluminum salt in one embodiment. The partially neutralized aluminum salt may conveniently be aluminum hydroxyl chloride (AHC), also called aluminum hydroxychloride or dialuminum chloride pentahydroxide (CAS 12042-91-0), e.g. Al2(OH)5Cl·2.5(H2O).
The partially neutralized acid aluminum salt is readily soluble in mildly acidic media, e.g. a pH below 3 or 4, and by itself forms a gel or precipitate when the pH is raised above 4.5 or 5, for example. In an embodiment, raising the pH also allows the acid aluminum salt to function as a latent cross-linking agent for the PAM, and/or to release aluminum(III) or other species that cross-link the PAM.
In a general embodiment, the partially neutralized acid aluminum salt comprises from 0.5 to 20 weight percent of the pre-gel solution. As used herein, the determination of the weight percentage of the partially neutralized acid aluminum salt in the pre-gel solution excludes any PAM present as X ions from the weight of the aluminum salt, but includes the PAM in the total weight of the solution. In various embodiments, the partially neutralized acid aluminum salt has a lower concentration limit in the pre-gel solution of at least 0.5, 1, 2, 2.5, 3, 4, or 5 weight percent, an upper concentration limit of not more than 20, 15, 10, 8, 5, or 3 weight percent, or a concentration range from any lower limit to any higher upper limit, e.g. from 1 to 10, from 3 to 10, or from 4 to 8 weight percent.
In another embodiment, the weight ratio of PAM to the partially neutralized acid aluminum salt ranges from 1:10 to 10:1, preferably from 3:8 to 8:3. In one embodiment, the total amount of PAM and partially neutralized acid aluminum salt used ranges from about 25 to about 600 pounds per thousand gallons (ppt) of aqueous fluid, preferably from about 250 to about 500 ppt.
The delayed activating agent in the pre-gel solution is generally any Lewis base source, i.e. pH adjusting agents that hydrolyze or decompose thermally to release a base or consume an acid, such as, for example: quaternary ammonium salts; urea and substituted ureas including N,N′-dimethyl urea, N,N-dimethyl urea, N,N′-diethyl urea, N,N-diethyl urea, N,N′-ethyl methyl urea, and N,N-ethyl methyl urea, especially N,N′-dimethyl urea; coordination compounds such as cupric ammonium sulfate for example; salts of a strong base and a weak acid; combinations thereof, and the like.
Lewis bases have a lone pair of electrons that can coordinate with an electron-deficient species, such as a proton. In the presence of water, Lewis bases can abstract a proton from water and produce hydroxyl ions. Lewis bases are thus sometimes also referred to as hydroxyl donors, even though such reference is technically incorrect and it is understood that the hydroxyl is obtained from water.
As specific representative examples there can be mentioned urea, N,N′-dialkyl urea such as N,N′-dimethyl urea, hexamethylene tetramine, cyanates, semicarbazide, thiourea, dithiobiurea, dithiobiuret, triazole, thiadiazole, thiosemicarbazide or the like, which may be alkyl-substituted or unsubstituted, or any combination of these. Cationic groups in the PAM, if present, can also facilitate activation in one embodiment of the invention. In a general embodiment, the activator comprises from 0.5 to 20 weight percent of the pre-gel solution, and in various other embodiments has a lower concentration limit of at least 0.5, 1, 3 or 5 weight percent; an upper concentration of no more than 20, 15, 10, 8 or 5 weight percent; or a concentration range from any lower limit to any higher upper limit, e.g. 3 to 8 percent by weight of the pre-gel solution. In one embodiment, the weight ratio of AHC to activator is generally from about 1:6 to about 2:1.
The gel modifier which is present in the pre-gel solution in one embodiment may be a polyvalent anion such as, for example, tartrate, citrate, sulfate, lactate, oxalate, succinate, or the like. The gel modifiers can affect either or both of the gel formation reaction rate and the morphology of the resulting gel. Unlike the prior art partially hydrolyzed polyacrylamide gelling systems in which they function as gel retardants, citrates and tartrates are active accelerators in the present co-gel system, as are oxalates, sulfates and lactates to a lesser extent.
In a general embodiment, the gel modifier, when present, comprises from 0.1 to 20 weight percent of the pre-gel solution. In various embodiments, the gel modifier has a lower concentration limit in the pre-gel solution of at least 0.1, 0.5, 1, 2, 2.5, 3, 4, or 5 weight percent, an upper concentration limit of not more than 20, 15, 10, 8, 5, 3, or 1 weight percent, or a concentration range from any lower limit to any higher upper limit, e.g. from 1 to 10, from 3 to 10, or from 4 to 8 weight percent.
The delayed gelling system can also include other metal compounds that can facilitate crosslinking of the polymer and/or serve as a cationic counterion to the gel modifier. These metal compounds can include any divalent or trivalent transition metal with unfilled d-orbitals, e.g. titanium, iron, nickel, copper, zinc, manganese, or the like. The system can additionally or alternatively include: one or more organoaluminum compounds, e.g. aluminum acetate, aluminum formate, aluminum acetylacetonate, aluminum lactate, aluminum tributoxide, or the like; and/or one or more inorganic aluminum compounds, e.g. hydrated aluminum ammonium sulfate, aluminum potassium sulfate, aluminum metaphosphate, aluminum nitrate, aluminum perchlorate, or the like.
The pre-gel solution may also contain various other additives used in water shutoff gel systems such as surfactants, thermal stabilizers, breakers, breaker aids, antifoaming agents, pH buffers, scale inhibitors, water control agents, and cleanup additives, and the like.
The pre-gel solution can be prepared by blending or mixing the components and water together in any particular order using conventional blending and mixing equipment and methods. The pH of the blend components should be maintained so as to avoid precipitation or premature gel formation, especially avoiding localized pH increases when high pH or neutral pH components are added. The blending and storage temperature is also maintained below the activation temperature to ensure that the gel or precipitate is not formed prematurely before the solution is placed in the appropriate location where the gel is desired.
The pre-gel solution is generally prepared shortly before it is used, and then heated to a sufficient temperature to elevate the pH so that gelation is activated. In water- or gas-shutoff applications, also known as conformance control, the pre-gel is prepared to have the rheology required for injection into the reservoir, taking into account the temperature, permeability and fluid content of the formation. The pre-gel is also prepared to give an appropriate set time upon injection into the formation, and the desired gel characteristics. For example, the set time will normally be longer than the time it takes to finish injecting the amount of pre-gel solution for the particular application. The injection of the pre-gel is otherwise similar to familiar shutoff methods known in the art.
Examples 1-4 below illustrate the manner in which the set time and pre-gel viscosity can be determined as a function of PAM concentration, AHC concentration and activator concentration at a particular downhole condition, for a particular polyacrylamide polymer. Co-gel systems were prepared by mixing a commercially available cationic polyacrylamide emulsion (20-50 wt % CPAM, unknown cationicity/molecular weight, in an aqueous emulsion of heavy aliphatic petroleum naphtha, ethylene glycol, ammonium chloride, glycol ether and surfactants) (“CPAM#1 emulsion”), solid dialuminum chloride pentahydroxide (CAS 12042-91-0) (“DCP”), solid urea (“U”), and/or solid N,N′-dimethylurea (“DMU”), in water in no particular order in the desired proportions.
The time to set at 127° C. was observed for a delayed gelling system based on 9.3 weight percent DCP and 5 weight percent CPAM#1 emulsion with DMU at various concentrations from 3 to 8 weight percent. The results seen in
The time to set at 127° C. was observed for a delayed gelling system based on 9.3 weight percent DCP and 1.3 weight percent DMU with CPAM#1 emulsion at various concentrations from 1 to 5 weight percent. The results seen in
The time to set at 127° C. was observed for a delayed gelling system based on 1.3 weight percent DMU and 5 weight percent of the CPAM#1 emulsion with DCP at various concentrations from 4 to 9 weight percent. The results illustrated in
The viscosity at 77° C. and a shear rate of 170 reciprocal seconds was measured for freshly prepared delayed gelling systems based on 9.3 weight percent DCP and 1.3 weight percent DMU for various concentrations from 1 to 5 weight percent of the CPAM#1 emulsion. As seen in
Although the invention is described in terms of shutoff applications, it is to be understood that it is applicable to any treatment in which a delayed gelling system is used. For example, the gelling system could also be used for fluid loss control from a wellbore, or to plug a formation regardless of the purpose for doing so.
Each of the patents, publications and other references mentioned herein are hereby incorporated herein by reference in their entirety for the purpose of US patent practice and other jurisdictions where permitted.
