WO1992007881A1

WO1992007881A1 - A method for inhibiting hydrolysis of polyacrylamide

Info

Publication number: WO1992007881A1
Application number: PCT/US1991/004734
Authority: WO
Inventors: James E. Tackett, Jr.; Wayne E. Luetzelschwab
Original assignee: Marathon Oil Company
Priority date: 1990-11-05
Filing date: 1991-07-03
Publication date: 1992-05-14
Also published as: CA2086597A1; GB2262529B; AR245150A1; GB9300880D0; GB2262529A; AU8845191A; TNSN91094A1; CN1061226A

Abstract

Aqueous solutions of partially hydrolyzed polyacrylamide are storable by adjusting and maintaining solution pH from about 3.5 to about 6.8 prior to storage. Storage at a pH above 6.8 or below 3.5 results in hydrolysis. The desired solution pH is achieved by addition of appropriate mineral acids or bases or by the addition of buffer systems with pKa values of about 3.5 to about 6.8.

Description

A METHOD FOR INHIBITING HYDROLYSIS OF POLYACRYLAMIDE

BACKGROUND OF INVENTION

TECHNICAL FIELD The present invention relates to a method for storing aqueous solutions of partially hydrolyzed polyacrylamidβ without increasing the percentage of polyacrylamide hydrolysis.

DESCRIPTION OF RELATED ART

Polyacrylamide has many uses in the petroleum industry. Of particular importance is the use of polyacrylamide for matrix conformance gels during enhanced oil recovery. When used to improve matrix conformance, aqueous solutions of polyacrylamide are crosslinked to form gels within subterranean hydrocarbon-bearing formations. Inventions employing matrix conformance gels are exemplified by U.S. Patent Nos. 4,548,272, 4,561 ,502, and 4,679,625 and 4,683,949. Controlling temperature and pH during polymerization reactions influences the molecular weight and degree of hydrolysis (polymer qualities) of the polyacrylamide product. For example, U.S. Patent No. 2,813,088 discloses that hydrophobic polymers can be obtained by polymerizing monomer in an aqueous medium using redox chemistry under controlled temperature and pH conditions. U.S. Patent No. 4,335,237 discloses the use of an endothermic compound to control temperature and thereby control polymer qualities during polymerization reactions. U.S. Patent No. 3,215,680 discloses that polyacrylamide qualities can be controlled by maintaining polymerization reaction temperatures between 30-60° C. U.S. Patent No. 2,983,717 discloses the use of the polymerization reaction exotherm in combination with catalysts to control the qualities of polyacrylamide polymer. Finally, U.S. Patent No. 4,312,969 discloses that polyacrylamide qualities can be controlled by using low boiling point fluids present in the reaction mixture to control polymerization temperature and by adjustment of reaction mixture pH to 8-9. However, none of these patents address how to maintain low levels of hydrolysis within polyacrylamide during storage of polyacrylamide in aqueous solutions. Unhydrolyzed polyacrylamide has the following structure:

Polyacrylamide that has undergone partial hydrolysis, i.e., partially hydrolyzed polyacrylamide (PHPA), will have less than 100% of the amide moiety (CONH2) replaced by a carboxylate moiety (COO^*) via hydrolysis that occurs either during acrylamide monomer polymerization or after polymerization has taken place.

Partially hydrolyzed polyacrylamide will have the general structure:

with the carboxylate moiety occupying a percentage of the sites formerly occupied by the amide moiety.

Aqueous solutions of very low hydrolysis polyacrylamide (less than 0.1 mole % hydrolysis) are useful for obtaining delayed gelation during gel treatments of oil-containing formations. The gels obtain improved conformance and flow properties of fluids within formations (see commonly assigned, copending U. S. patent application to J. E. Tackett entitled "Process for Enhanced Delayed In Situ Gelation of Chromium Polyacrylamide Gels", which is being filed concurrently herewith). Generally, as the amount of hydrolysis of the partially hydrolyzed polyacrylamide increases, the gelation delay time decreases. Therefore, it is desirable to be able to store aqueous solutions of very low hydrolysis polyacrylamide without the level of hydrolysis increasing.

It is an object of this invention to provide a method for storing aqueous polyacrylamide solutions for a relatively long period of time without increasing the level of polymer hydrolysis. SUMMARY OF THE INVENTION

In accordance with the present invention, aqueous solutions of polyacrylamide are adjusted to a pH of about 3.5 to about 6.8 prior to storage to inhibit autohydroiysis.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings, which are incorporated in and form a part of the specification, illustrate the embodiments of the present invention and, together with the description, serve to explain the principles of the invention. In the drawings: Figure 1 is a graph showing the functional relationship between hydrolysis over time at 180°F (82°C); and

Figure 2 is a graph showing the portion of Figure 1 between 0 and 16 hours in expanded scale which details the functional relationship between hydrolysis and pH over time at 180°F (82°C).

DETAILED DESCRIPTION OF THE INVENTION

The process of the present invention is conducted by adjusting the pH of an aqueous polyacrylamide solution to a pH of about 3.5 to about 6.8, preferably from about 3.5 to about 6 and most preferably from about 3.5 to about 5 prior to storage. The level of polyacrylamide hydrolysis is less than about 5 mole %, preferably less than about 1 mole % and most preferably less than about 0.1 mole %. The polyacrylamide is present in solution from about 2% to about 20%, preferably from about 5% to about 12% and most preferably from about 6% to about 10% by weight. Solutions should be stored with polyacrylamide concentration above 2% because solutions containing less than about 2% by weight of polyacrylamide tend to hydrolyze faster than solutions containing higher polyacrylamide concentrations. Mineral acids such as hydrochloric, sulfuric or nitric acids or bases such as sodium hydroxide or aniline are used to adjust the pH of the polyacrylamide solution. Buffers are ideally used to adjust and maintain polyacrylamide solution pH. Buffers with pKa values of about 3.5 to about 6.8 are preferred, such as monocarboxylates, hydrogen phosphates, and polyamines such as triethylene tetraamine, tetraethylene pentamine and hexamethylene tetraamine or mixtures thereof. The end use of the polymer should be considered in buffer selection. Some dicarboxylate or tricarboxylic buffers such as malonic, oxalic and citric acids should be avoided if the polymer will eventually be used with chromium (III) gelation agents because the closely spaced dicarboxylates and tricarboxylates will chelate the gelation agents and thereby effectively prevent gelation. Acetate buffers can also be used advantageously as is suggested by a commonly assigned, copending U. S. patent application to J. E. Tackett, entitled "Process for Enhanced Delayed In Situ Gelation of Chromium Polyacrylamide Gels", which was filed concurrently herewith.

The water that makes up the bulk of the solution can be fresh water or brine containing a total dissolved solids concentration up to the solubility limit of the solids in water. The practice of this invention is best illustrated by specific examples.

For this reason the following examples are presented; however, they are not to be construed as limiting the scope of the invention.

EXAMPLE 1

A 6% by weight aqueous poiyacryiamide solution containing less than 0.1 mole % polyacrylamide hydrolysis is stored at a room temperature of approximately 68°F (20°C) at a pH of 4.8 for 2 years. After storage, the hydrolysis levels are measured and the hydrolysis level is less than 0.1 mole

% and the solution pH is 5.2.

EXAMPLE 2 A 6% by weight aqueous polyacrylamide solution containing less than

0.1 mole % hydrolysis and having a pH of 6.8 is broken into three samples. Sample 1 is stored at room temperature (68°F/20°C), sample 2 is stored in a refrigerator at between 32°F (0°C) to 50°F (10°C) and sample 3 is stored in a freezer at less than 32°F (0°C). All three samples are stored for 1 year. The hydrolysis levels of sample 1 increases to 0.4 mole % and pH increases to 7.6. The hydrolysis levels of samples 2 and 3 remain less than 0.1 mole % and the pH remains at 6.8.

EXAMPLE 3 A 6% by weight aqueous polyacrylamide solution containing less than 0.1 mole % hydrolysis and having a pH of 6.8 is heated at 180°F (82°C) while pH and percent hydrolysis are monitored. These results are shown in Figures 1 and 2. These data suggest that while the solution pH is below 7 the carboxyl content remains low. However, after about 40 minutes, the pH and the percent hydrolysis increases. This incubation period is probably due to the slight buffering capacity of the amide groups. Once it is exceeded, the pH increases significantly and so does the percent hydrolysis. EXAMPLE 4 A 6% by weight aqueous polyacrylamide solution containing less than 0.1 mole % hydrolysis is buffered with ammonium phosphate to a pH of 5.9. The buffered solution is heated to 230°F (110°C) for 170 hours. No significant increase of hydrolysis content is noted.

EXAMPLE 5 A 2% by weight aqueous polyacrylamide solution containing less than 0.1 mole % hydrolysis and having a pH of 7.1 is heated for 20 hours at 175°F (79.4°C). The solution pH increases to 8.3 and hydrolysis increases to 1.2 mole %.

EXAMPLE 6

A 2% by weight aqueous polyacrylamide solution containing 0.3 mole % hydrolysis, is buffered to a pH of 5.9 using ammonium phosphate. The solution is then heated at 175°F (79.4°C) for 40 hours. The solution pH remains at 5.9 and the solution hydrolysis increases to 0.6 mole %.

EXAMPLE 7

A 2% by weight aqueous polyacrylamide solution containing 0.3 mole % hydrolysis is buffered to a pH of 5.4 using ammonium phosphate. The solution is then heated at 162°F (72°C) for 20 hours. The solution pH remains at 5.4 and the solution hydrolysis remains at 0.3 mole %.

EXAMPLE 8 A 2% by weight aqueous polyacrylamide solution containing 0.3 mole % hydrolysis is buffered to a pH of 6.5 using ammonium phosphate. The solution is then heated at 110^βF (43^βC) for 170 hours. The solution pH remains at 6.5 and the solution hydrolysis remains at 0.3 mole %.

Based on the above Examples, an aqueous polyacrylamide solution can be stored without a significant increase in hydrolysis at temperatures below 175°F (79.4°C) if the solution pH is adjusted to about 3.5 to about 6.8 prior to storage and thereafter maintained within this pH range. While the results set forth in the Examples illustrate that aqueous solutions of partially hydrolyzed polyacrylamide are^' storable for 2 years, aqueous solutions of partially hydrolyzed polyacrylamide may be stored for longer periods of time in accordance with the process of the present invention. This process allows for an aqueous polyacrylamide solution to be stored and transported in a form that is ready for use, for example the aqueous polyacrylamide solution may be used in a gelation process, without an increase in polymer hydrolysis that decreases the solution's gelation utility.

While the foregoing embodiments of the invention have been described as shown, it is understood that alternatives and modifications, such as those suggested and others, may be made thereto and fall within the scope of the invention.

Claims

_ _

We claim: 1. A method to inhibit hydrolysis of partially hydrolyzed polyacrylamide in an aqueous solution which comprises; adjusting the pH of said solution from about 3.5 to about 6.8. 2. The method of claim 1 wherein the hydrolysis level of said aqueous partially hydrolyzed polyacrylamide solution is less than about 5 mole %. 3. The method of claim 1 wherein the hydrolysis level of said aqueous partially hydrolyzed polyacrylamide solution is less than about 1 mole %. 4. The method of claim 1 wherein the hydrolysis level of the said aqueous partially hydrolyzed polyacrylamide solution is less than about 0.1 mole %. 5. The method of claim 1 wherein said aqueous partially hydrolyzed polyacrylamide solution is stored up to about 2 years. 6. The method of claim 1 wherein the pH of said aqueous solution is adjusted to about 3.5 to about 6.8. 7. The method of claim 1 wherein the pH of said aqueous solution is adjusted to about 3.5 to about 6. 8. The method of claim 1 wherein the pH of said aqueous solution is adjusted to about 3.5 to about 5. 9. The method of claim 1 wherein said pH is adjusted by adding a mineral acid to said solution. 10. The method of claim 1 wherein said pH is adjusted by adding a base to said solution. 11. The method of claim 1 wherein said pH is adjusted by adding a buffer to said solution. 12. The method of claim 11 wherein said buffer has a pKa value of about 3.5 to about 6.8. 13. The process of claim 12 wherein said buffer is selected from the group comprising a monocarboxylate, a hydrogen phosphate, a polyamine or mixtures thereof. 14. The process of claim 13 wherein .said monocarboxylate is formate, acetate, propionate, lactate salt, or mixtures thereof. 15. The process of claim 13 wherein said polyamine is triethylene tetraamine, tetraethylene pentamine, hexamethylene tetraamine or mixtures thereof. 16. The method of claim 1 wherein the concentration of said partially hydrolyzed polyacrylamide in water is about 2% to about 20% by weight. 17. The method of claim 16 wherein the concentration of said partially hydrolyzed polyacrylamide is from about 5% to about 12% by weight. 18. The method of claim 17 wherein the concentration of said partially hydrolyzed polyacrylamide is from about 6% to about 10% by weight. 19. All inventions described herein.