MOBILITY CONTROL REAGENTS WITH SUPERIOR THERMAL STABILITY

Abstract: Homopolymers of 2-acrylamido-2-methylpropane sul­ fonic acid or its salts or copolymers thereof with N,N-­ dimethylacrylamide are used to maintain the viscosity of aqueous solutions thereof in the presence of poly­ valent salt containing-waters during the enhanced re­ covery of petroleum from subterranean petroleum-bearing deposits.

Description: BACKGROUND OF THE INVENTION U.S. Patent Nos. 3,679,000; 3,692,673; 3,858,652; 3,945,929; 3,948,783; 3,953,341; 3,953,342; 4,309,523; 4,326,970; 4,395,340; 4,404,111; 4,409,110; 4,439,334; 4,515,635; 4,555,269; and European Published Application Nos. 0094898 and 0115836 disclose the use of various N-­ sulfohydrocarbon substituted acrylamide homopolymers and copolymers for controlling the viscosity of aqueous media used in water-flooding operations for enhanced petroleum recovery. The copolymers are described as containing 5-­ 95%, by weight, of the N-sulfohydrocarbon-substituted acrylamide, preferably 50-95%, by weight, and even more preferably 70-95%, by weight. Comonomers such as acrylic monomers e.g. acrylic or methacrylic acid or salts, acrylamide, methacrylamide etc., are taught. Addition­ ally, U.S. Patent No. 4,573,533 discloses the use of analogous polymers as mobility control reagents under harsh conditions and in the absence of oxygen. While most of the polymers discussed above and other commercially used polymers such as acrylamide-­ ammonium acrylate copolymers and polysaccharides have usually proven to be effective for the control of the viscosity of aqueous media, in many instances, the vis­ cosity of the copolymers or material breaks down when the temperature of the well interior at the petroleum bearing deposit reaches above about 90°C i.e. preferably about 100°C-120°C or, alternatively, when prepared and/or used in the presence of oxygen, or both. Furthermore, the presence of polyvalent metal ions in the water already present in the wells or in that used to flood the wells during the enhanced oil recovery also tends to participate in the degradation of the copolymers and other materials which are added thereto. Such polyvalent metal ions are normally found in hard waters. It therefore would solve a long-felt need if a method could be discovered for treating wells which exhi­ bit temperatures above about 90°C i.e., those preferably ranging from about 100°C to about 120°C, during secondary oil recovery, especially in the presence of mineral water containing polyvalent salts without the substantial ac­ companying break-down of polymer viscosity. SUMMARY OF THE INVENTION It has now been found that the viscosity degrada­ tion exhibited by many of 2-acrylamido-2-methylpropane-­ sulfonic acid (AMMPS) polymers used in enhanced oil re­ covery, caused by the presence of high temperatures and/or polyvalent salt-containing water which is either present in the subterranean petroleum-bearing deposits or which is used to form the polymer solution per se which is injected into the well is not experienced by homopolymers of 2-acrylamido-2-methylpropane sulfonic acid or its salts or copolymers thereof with N,N-dimethylacrylamide during and after the injection of the water solution thereof under anaerobic conditions. DESCRIPTION OF THE INVENTION INCLUDING PREFERRED EMBODIMENTS The present invention refers to a method for re­ covering petroleum from a subterranean petroleum-bearing deposit wherein a viscous polymer solution of a homo­ polymer of AMMPS or a copolymer thereof with N,N-di­ methylacrylamide is injected into the area of said deposit through at least one input well to cause said petroleum to flow from said area for collection through at least one output well. The present improvement comprises con­ trolling and maintaining the viscosity of the homopolymer or copolymer solution by utilizing the homopolymer or copolymer in solution and in the substantial absence of oxygen, i.e., under anaerobic conditions, the temperature at the petroleum-bearing deposit being above about 90°C, preferably from about 100°C to about 120°C, and even more preferably from about 110°-120°C. As mentioned above, the viscosity of the water solution of the homopolmeric AMMPS or the copolymer of AMMPS and N ,N -dimethylacrylamide which is injected into the input well is maintained substantially constant over the period of time required to force the petroleum out of the deposits and into the output well. The presence of polyvalent salts and/or the temperature of the deposits, which usually contributes significantly to the viscosity breakdown of other polymers, do not materially affect the polymers used herein especially when the polymer is main­ tained and used under anaerobic conditions. Although the use of fresh water for the water-­ flooding of subterranean deposits and the preparation of polymer solutions used for such purpose has been employed in the past, it is of course, from an economical stand­ point, preferred to use mineral-containing waters there­ for. The present invention encompasses the use of such fresh water or mineral-containing waters having a poly­ valent ion concentration of at least 100 ppm, preferably at least 500 ppm for preparation of the polymeric solu­ tion. The polyvalent ions usually are present in such water in the form of calcium iron and magnesium salts alone or in conjunction with sodium salts. Alternatively, of course, fresh water solutions of the polymer can be injected into wells which already contain polyvalent salt containing waters. The homopolymers useful in the instant process employ 2-acrylamido-2-methylpropanesulfonic acid (AMMPS) or a salt thereof and the copolymers employ AMMPS or a salt thereof and N,N-dimethylacrylamide. The salts of the AMMPS monomer are usually alkali metal or ammonium salts with sodium and potassium salts being preferred. The copolymers may contain a mole ratio of from about 20-99%, of the AMMPS and correspondingly, from about 1-80%, of the N,N-dimethylacrylamide. Up to about 10% of the above comonomers may be replaced by a third monomer such as acrylonitrile, methyl methacrylate, acrylic acid, vinyl chloride and the like. The polymers may be prepared by any known method such as bulk, solution, suspension in emulsion so long as the polymerization is conducted in the absence of oxygen and the resultant polymer is maintained and used under a substantially oxygen-free atmosphere. The molecular weight of the polymer should be at least about 500,000, preferably at least about 1,000,000. By the terms "in the substantial absence of oxygen" or "under anaerobic conditions" etc., as used herein, is meant that from the point where the polymer is added to the water, through the oil well pipeline and to the sub­ terranean strata, the presence of oxygen is maintained at a minimum, i.e. <1ppm. The polymer itself should also be prepared and stored under conditions conducive to oxygen elimination. The elimination of the oxygen can be accom­ plished by the use of oxygen scavengers such as sodium dithionite, sodium metabisulfite etc., by the use of an inert gas sparge such as nitrogen, argon etc., and the like. The most pertinent of the above-disclosed refer­ ences with regard to the instant copolymers is EPO pub­ lished application 0115836. This application teaches various sodium AMMPS copolymers of N,N-dimethylacryl­ amide. In Tables XXXV and XLIV, the results of testing copolymers containing a mole ratio 50-60% of N,N-di­ methylacrylamide and 40-50% of AMMPS for viscosity sta­ bility in the presence of sea water at 250°F (121°C) are presented. The tests described in the application are all conducted on solutions which were aged for only seven days. Such short duration tests do not predict long term longevity and may result in misleading conclusions. Since polymer flooding operations are normally conducted over many months or even years, water-soluble polymers which are used as mobility control reagents must be stable to reservoir conditions over the same period of time. The EPO application shows relatively poor to moderate stability over a very short time scale of aging. It is believed that these results are less positive because of the presence of oxygen during the manufacture and/or testing of the poly­ mers. The following examples are set forth for purposes of illustration only and are not to be construed as limi­ tations on the present invention except as set forth in the appended claims. All parts and percentages are by weight unless otherwise specified. In the following examples, polymer solutions are prepared by dissolution in distilled water, normally 2 hours for dry and liquid polymers and about 16 hours for gel polymers. Salt is added and the solution is then filtered through a 150 mesh stainless steel screen. The composition of the brine is as follows: 9.0% NaCl 1.0% CaCl₂ The composition of the synthetic sea water is as follows: 2.3% NaCl 0.13% Na₂SO₄ 0.53% MgCl₂ 0.11% CaCl₂ Test solutions are essentially free from dissolved oxygen and are aged at controlled elevated temperature by the procedure described in Society of Petroleum Engineers, Paper No. 12008; "Thermal Stability Testing of Mobility Control Reagents" The copolymer concentration in all examples is 2500 ppm. The viscosity of the polymers in all examples is measured by use of a Brookfield Viscometer fitted with a UL adaptor at 25°C. The Viscosity Ratio %(VR%) is defined as the Brookfield Viscosity (12 r.p.m.) after 4 days of aging divided by the initial viscosity. ANAEROBIC TEST METHOD Conditions close to the anaerobic are achieved by aging solutions in a sealed glass ampoule. Several am­ poules are attached to a manifold for loading with test solution at a single time. Each solution is purged with nitrogen (<25ppm oxygen) for at least one hour before the ampoules are filled. A gas reservoir is attached to the manifold and vacant ampoules are successfully evacuated to 0.1 mmHg and then filled with nitrogen three times. With the whole assembly under a positive nitrogen atmosphere, each ampoule is one half filled with test solution. The reservoir is evacuated to 0.1mm Hg and isolated from the vacuum source. All gases dissolved in the test solutions removed following a nine cycle procedure. Ampoules are then flame-sealed and aged at the designated temperature. EXAMPLES 1-7 Following the test method described above; various copolymer solutions, in brine at 90°C & 120°C, are tested for viscosity maintenance. As can be seen from Table 1, below, the copolymers of AMMPS and N,N-dimethylacrylamide (DMA) retain their viscosity over a very long period of time, i.e., up to 161 days. EXAMPLES 8 & 9 The procedure of Examples 1-7 is repeated except that the test results are conducted at 150°C. The results are set forth in Table II, below. EXAMPLE 10 (comparative) The procedure of Examples 1-7 is again followed except that the copolymer is composed of 70 mole% AMMPS and 30 mole% acrylamide. After 0, 17 and 56 days the VR% is 100, 57, 0 (i.e., that of water), respectively. EXAMPLES 11-16 (comparative) Again following the procedure of Examples 1-7 except that the AMMPS is replaced by sodium acrylate (SA), the results set forth in Table III, below, are achieved. EXAMPLE 17 The copolymer of Example 1 is again tested in accordance therewith except that the polymer concentration is 1750 ppm. The VR% at 0, 7, 14 and 120 days at 90°C is 100, 97, 99 and 94, respectively. A commercially available polyacrylamide polymer is tested under the same conditions. The VR% at 90°C for 0, 14 and 28 days is 100, 46 and 41, respectively, at 2000 ppm. EXAMPLES 18 & 19 (Comparative) Xanthan at 1000 ppm and at 70°C shows a breakdown of viscosity after 0, 13 and 32 days at 60 r.p.m. of 100, 34 and 16, respectively. An ionic polyacrylamide at 2000 p.p.m. shows a breakdown of viscosity after 0, 14 and 28 days of 100, 57 and 28, respectively, at 60 r.p.m. and after 0 and 14 days of 100 and 55, respectively, at 30 r.p.m. EXAMPLE 20 Following the test method described above-various polymer solutions, in harsh water* at 90°C, are tested for viscosity maintenance. As can be seen from Table IV, below, solutions of copolymers, containing 50, 70 and 90 mole percent of AMMPS, the remaining being acrylamide (AM), exhibit a break-down in viscosity after 56-294 days at 100°C-120°C whereas AMMPS homopolymer retains viscosity at 120°C. Comparisons with Xanthan and anionic polyacryl­ amide (PAM) are also shown. *water contains 9% NaCl and 1% CaCl₂. EXAMPLE 27 The thermal stability of homopolymeric AMMPS in harsh reservoir brines is shown in Table V, below.

Claims: 1. In a method for recovering petroleum from a subterranean petroleum-bearing deposit which comprises injecting an aqueous polymer solution into the area of said deposit through at least one input well to cause said petroleum to flow from said area for collection through at least one output well, the improvement which comprises maintaining and controlling the viscosity of said solu­ tion by utilizing, in the substantial absence of oxygen, a water-soluble homopolymer of 2-acrylamido-2-methylpro­ panesulfonic acid or a salt thereof or a copolymer of about 20-99%, by weight, of 2-acrylamido-2-methpropanesulfonic acid or a salt thereof and about 1-80%, by weight of N,N-­ dimethylacrylamide the temperature at the petroleum-­ bearing deposit being at least 100°C. 2. A method according to Claim 1 wherein said aqueous solution contains a polyvalent salt. 3. A method according to Claim 2 wherein said polyvalent salt is present at a concentration of at least about 100ppm. 4. A method according to Claim 2 wherein said polyvalent salt is a calcium iron or magnesium salt or mixture thereof. 5. A method according to Claim 1 wherein the temperature at the petroleum-bearing deposit ranges from about 100°C to about 120°C. 6. A method according to Claim 1 wherein the temperature at the petroleum-bearing deposit ranges from about 110°C to about 120°C. 7. A method according to Claim 1 wherein the polymer is a homopolymer of 2-acrylamido-2-methylpropane­ sulfonic acid or a salt thereof. 8. A method according to Claim 1 wherein the polymer is a copolymer of 2-acrylamido-2-methylpropane­ sulfonic acid or a salt thereof and N,N-dimethylacryl­ amide.

Inventors: RYLES, RODERICK GLYN
Applicants: AMERICAN CYANAMID COMPANY
Application year: 1987
Main classification: E21B43
IPC codes: C09K8