The optimal use of enhanced oil recovery polymers under hostile conditions
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The purpose of this work is to frame the main issues one must face in the design of a mobility control process using polyacrylamide and related acrylic polymers under hostile conditions. Proper preliminary lab evaluation techniques, chemical degradation and related calcium tolerance issues, thermal degradation, and economic optimization based upon injectivity are discussed. Emphasis is placed on stability under alkaline conditions, the use of sodium dithionite to prevent thermal degradation, and the beneficial use of in-situ hydrolysis to increase injectivity. Filtration properties are a focus of screening experiments, and though it often takes several days to achieve acceptable filter ratios in the lab, experience from two field observations indicate that even high molecular weight polymers have filtration ratios on the order of 1.2 or less before they are injected, so preparation procedures that do not result in this may not yield results that scale to the field. Chemical stability issues with acrylamide polymers are addressed in two parts, the first describing the kinetics of hydrolysis under neutral and alkaline conditions and the second estimating the calcium tolerance of aged polymers using industrial and lab produced analogues. Under alkaline conditions, hydrolysis is very rapid, even at low temperatures. Though aged copolymers of acrylamide (AM) and 2-acrylamide 2-methyl propane sulfonate (AMPS) exhibit similar calcium tolerances to similarly aged polyacrylamide (PAM), viscosity loss is much higher for the latter as this limit is approached. Thermal, or "oxidative" degradation, is examined using Pourbaix diagrams for iron to understand the commonly reported relationships between pH, Eh, and stability. The beneficial effects of sodium carbonate and sodium dithionite on polymer solutions as well as some inconsistencies in the literature point towards a catalytic role played by ppb level amounts of iron in oxidative degradation mechanisms. It is put forward that addition of sodium dithionite is a conservative approach to all acrylic-backboned polymer floods, and practical issues related to this are discussed. A simple analytical model is developed to take a brief look at economic optimization of polymer viscosity, and this is used to demonstrate the benefits of in-situ hydrolysis in alkaline or high-temperature floods.