Abstract

Low salinity polymer (LSP), in which the salinity of the make-up water is significantly lower than of the formation water, is an attractive alternative to conventional polymer flooding because of the reduced amount of polymer required. Viscosity loss due to cation exchange with the rock surface however is a significant risk. In this paper, the influence of partially hydrolyzed polyacrylamide (HPAM) polymer on the rock-fluid interactions has been studied by analyzing produced fluids of several core flooding experiments. It was found that all produced fluids containing polymer show elevated levels of divalent cations (mainly Ca2+ and Mg2+) compared to the produced fluids of slugs of brine injected without polymer. This was observed for both high (formation water) and low salinity polymer solutions. The divalent cation levels typically remain elevated for at least several pore volumes of polymer injection. Additionally, produced water from a field on polymer flood was found to contain elevated levels of divalent cations as well.

These results confirm that polymer affects the rock-brine equilibrium and tends to attract divalent cations, which are provided by the rock surface. In our experimental results, the concentration of divalent cations could increase by as much as a factor of 5 compared to the equilibrated brine. The expected viscosity loss varied from no loss (even for LSP) to a loss of ~30%. The extent of cation exchange and viscosity loss strongly depends on the rock-fluid system, in which the charge ratio (CR) of positive charges on the divalent cations in the solution to negative charges on the polymer as well as the divalent to monovalent cation mass-action-ratio (MAR) appear to be important and predicting parameters.

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