Development and Evaluation of EOR Polymers Suitable for Hostile Environments Part 1: Copolymers of Vinylpyrrolidone and Acrylamide

Abstract

Doe, Peter H., SPE, Phillips Petroleum Co. Phillips Petroleum Co. Moradi-Araghi, Ahmad, SPE, Phillips Petroleum Co. Phillips Petroleum Co. Shaw, James E., Phillips Petroleum Co. Phillips Petroleum Co. Stahl, G. Allan,* Phillips Petroleum Co. Phillips Petroleum Co. November 1987 Summary. This paper describes the properties of synthetic water-soluble polymers that are stable for extended periods of time in hard brines at polymers that are stable for extended periods of time in hard brines at very high temperatures. Several copolymers of vinylpyrrolidone (VP) and acrylamide (AM) were prepared and evaluated in our laboratories for EOR application in hostile environments. VP in the copolymer composition protects AM against extensive thermal hydrolysis, which otherwise will protects AM against extensive thermal hydrolysis, which otherwise will result in loss of viscosity and precipitation. A range of VP/AM copolymer compositions was found to tolerate the harsh conditions of 250F [121C] in seawater for extended periods of time and to be suitable for EOR application under these conditions. The performance of these polymers in porous media was evaluated by extensive coreflood experiments in Berea porous media was evaluated by extensive coreflood experiments in Berea sandstone at 250F [121C] with synthetic seawater. The results indicate that these copolymers can easily be injected into porous media and that they can be effective polymers for EOR application in hostile environments. Introduction In recent years, oil production has been moving toward deeper, and consequently hotter, reservoirs. This trend has produced a challenge for manufacturers of water-soluble polymers. Polyacrylamides, the most widely used products for mobility Polyacrylamides, the most widely used products for mobility control, suffer extensive thermal hydrolysis at high temperatures and, as a result, may precipitate in the presence of divalent cations. A recent study indicates a "safe" temperature limit of only 167F [75C] for typical oilfield brines. Somewhat lower limits of 154F [68C] have been observed for a brine with 3,604 ppm Ca2+ ions. Polysaccharidese.g., xanthan gums-have acquired some popularity, but are also sensitive to high temperatures and are probably not suitable for EOR application in reservoirs hotter than 200F [93C]. The use of stabilizing packages might raise this limit somewhat. Other synthetic polymers such as polyethylene oxide, polyvinyl alcohol, and cellulose derivatives suffer thermal degradation, precipitation, and substantial viscosity losses as a result of aging in seawater at elevated temperatures. An extensive investigation of the thermal stability of commercially available water-soluble polymers was conducted to determine the suitability of these products for EOR application in seawater at 250F [121C]. The objective of this study was to identify products that can tolerate this harsh combination of temperature, salinity, and hardness (Table 1) for a period of at least 3 years without a substantial loss in viscosity. Almost all but scleroglucan and polyvinylpyrrolidone (PVP) failed this test by precipitating within a few weeks. A similar conclusion was reached by Davison and Mentzer, who tested some 140 polymers for EOR application in sea-water at 194F [90C]. Although scleroglucan passed initial screening at 250F [121C], it failed to survive a passed initial screening at 250F [121C], it failed to survive a few months of aging at this temperature. PVP is not a good viscosifier, and high concentrations are needed to produce viscosities comparable to polyacrylamides. The need for 8 to 10 times higher concentrations of this polymer, coupled with a price about three times that of polyacrylamide, makes PVP unsuitable for EOR applications. polyacrylamide, makes PVP unsuitable for EOR applications. We found that copolymers of N-VP and AM have unusual thermal stability. These systems demonstrate stabilities beyond what would be expected from a simple dilution of AM with VP molecules. This paper reports the properties of poly(N-VP/co-AM) produced in our laboratories. Experimental Techniques Polymer Preparation. VP/AM copolymers used in this study were Polymer Preparation. VP/AM copolymers used in this study were prepared by polymerization as 10 to 20 wt% solutions in distilled prepared by polymerization as 10 to 20 wt% solutions in distilled water or synthetic seawater. The polymerization was initiated by free-radical initiators under an oxygen-free atmosphere. Reagent-grade commercial monomers were used in all copolymer preparations. Details regarding the polymerization process are preparations. Details regarding the polymerization process are given elsewhere. Sample Preparation and Aging. To avoid polymer degradation, all the sample preparation and aging tests were carried out under an oxygen-free atmosphere. Samples (25 mL) prepared for monitoring the extent of hydrolysis or cloud points were placed in 50-mL Pyrex TM ampules and torch-sealed under a vacuum. Procedures for Pyrex TM ampules and torch-sealed under a vacuum. Procedures for determining the cloud point and extent of hydrolysis are reported in Ref. 1. To evaluate the effects of aging on the viscosity of polymer solutions, stainless-steel pressure bombs equipped with Pyrex polymer solutions, stainless-steel pressure bombs equipped with Pyrex glass liners were charged with about 260 to 280 mL of polymer solution, covered with a watch glass, sealed with a stainless-steel cap, and pressurized to 100 psi [690 kPa] of N2. The bombs were then aged in ovens for the appropriate times. Capillary Viscometer. A homemade capillary viscometer was used to scan viscosity/temperature relationships in the range of 100 to 300F [38 to 149C] for each solution. The viscometer consists of a 20-ft [6.1-m] "preheating" coil and a 40-ft [12.2-m] stainless- steel measuring coil with an ID of 0.069 in. [0.175 cm]. Both coils are immersed in a silicone oil bath and heated with a temperature programmer set at 3.6F [2C]/min. The polymer solution is driven programmer set at 3.6F [2C]/min. The polymer solution is driven through the coil by N2 pressure. The pressure drop across the measuring coil is monitored by a differential-pressure transducer. After the solution passes through the measuring coil, it goes through the flow-control section of the apparatus, which is equipped with a valve to maintain 90-psi [620-kPa] backpressure on the system. This is enough to keep the polymer solution liquid at 300F [149C]. SPEFE