Impact Of Cation Exchange On Polymer In-Situ Viscosity: An Experimental Investigation For A Low-Salinity Polymer Flooding Case

Cation exchange occurs when water with a different salinity as the connate brine is injected in a reservoir. During polymer flooding operations, the potential release of divalent cations by the rock can have a detrimental impact on the in-situ viscosity in the polymer bank. The objective of this work was to assess for the risk related to cations exchange in an Argentinian oilfield and to provide guidelines for the injection water design. Reservoir rock samples were first submitted to mineralogical analysis involving scanning-electron microscopy (SEM), X-Ray Diffraction (XRD) and determination of their Cation Exchange Capacities (CEC). Coreflood tests were then performed where the effluents were analyzed for their cations composition. In these experiments, two main scenarios for the composition of the low-salinity injection water (with or without softening) were investigated and the transport properties of the polymer were determined. As a more exploratory approach, polymer was also injected in a 12-meter-long slim tube filled with crushed reservoir rock, to assess if it could be exposed to released cations. The results showed that all reservoir rocks investigated had high CEC, which was consistent with their high clay contents, and that significant cations exchanges took place during low salinity water injection, although no formation damage occurred, showing the stability of the clays. During injection of the softened water, evidences of significant divalent (and monovalent) cations release from the rock were found. During injection of the unsoftened water, a marked and long-term adsorption of the injected calcium cations was observed, corresponding to a depletion in calcium of the injected water. This suggests that, quite counter-intuitively, using unsoftened water as polymer make up water could be interesting in view of economics because the cations exchanges could entail an increase of the in-situ viscosity. The coreflood test results also showed that the presence of polymer in the injected water had no impact on the cations exchanges mechanisms. The partial results from the slim tube injection test suggested, however, that the retardation of the polymer bank caused by polymer adsorption was sufficient to avoid for its viscosity to be affected by the changes in cations distribution. This study illustrates the importance of cation exchange mechanisms and their potential impact for polymer flooding. It also shows that these effects can be investigated in a representative manner at the lab and that practical guidelines for the composition of the polymer injection water can be deduced from the experiments, provided a risk for in-situ viscosity reduction is identified.