Tailoring polymer molecular weight distribution to pore size distribution using filtration and mechanical degradation
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High molecular weight polymers are used for mobility control during oil recovery operations. The cost effectiveness of polymer solutions improves as the molecular weight of the polymer increases, but very high molecular weight polymers can be as large in solution as the throats of the pores that support transport. An envelope exists in pore size and permeability that marks the limits of transport without plugging for polymers of different sizes. The concept of a polymer’s “size” is a complicated one; polymers adopt non-spherical shapes in solution, a distribution of molecular weights is present in any given solution, and the exact relationship between a polymer’s apparent size and the size of pores as inferred from mercury injection capillary pressure is unclear. This study contends that filter plugging is the most practical measurement of the sizes of polymers in a polymer solution, and builds a quantitative theory to maximize the quality of the information extracted. Specifically, the most effective size characterization method is the filtration pore size assay, in which plugging of a polymer solution is measured on filters across a range of pore sizes. The utility of this assay is demonstrated in comparisons of polymer shearing, variations in salinity, post-hydrolysis, and filtration processing. It is shown that mechanical degradation of polymer solutions shifts the high molecular weight tail of the polymer size distribution without appreciably reshaping it, that swelling by salinity and hydrolysis has a differential effect on plugging at large and small pore sizes, and that serial filtration can produce high-quality, high-viscosity polymer solutions that cannot be produced from a single filtration. Furthermore, mechanical degradation and serial filtration can be used in combination to produce polymer solutions that can be filtered at extremely small pore sizes, down to one tenth of a micrometer in diameter, with surprising retention of viscosity that makes them suitable for chemical EOR core floods. Single phase core floods with HPAM and scleroglucan are used to assay the limits of transport without plugging, while a successful surfactant flood in a seventeen millidarcy reservoir carbonate core proves the power of the optimized tight-filtration technique.