ASP flooding in tight carbonate rocks

Alkaline-Surfactant-Polymer (ASP) floods can mobilize oil left behind by waterfloods by lowering the interfacial tension. However, conducting such floods in tight carbonate rocks presents several challenges, e.g., polymer transport in low permeability carbonates, presence of divalent ions, geochemical interactions with chemicals and rock surface, pore-scale heterogeneity, and oil-/mixed-wettability. This paper addresses the first three challenges. A systematic study of polymer transport in low permeability carbonate cores was performed. Shearing of high molecular weight polymers and successive filtration treatment were performed to improve polymer size distribution. Single phase polymer transport experiments were performed in low permeability carbonate rocks, and optimum pretreatment method was developed. Surfactant phase behavior and aqueous stability experiments were performed to develop ultralow tension ASP and SP formulations with a reservoir crude oil. ASP and SP corefloods were performed in oil-wet low permeability limestone rocks. The oil recovery, pressure drop, effluent ionic composition, effluent viscosity and effluent surfactant concentrations were measured. PHREEQC simulations were performed to understand geochemical reactions during ASP floods. The polymer treatment procedure was able to successfully transport polymer through low permeability Edwards Yellow limestone cores, but not through Texas Cream limestone due to differences in their pore structures. It was found that polymer hydrodynamic radius must be smaller than the pore throat radii of the rock for successful polymer transport. Therefore, mercury porosimetry data of the porous medium is critical for polymer selection, especially in case of low permeability rocks. Tertiary ASP floods resulted in improving the cumulative oil recovery to 77-87% OOIP, whereas SP flood improved the cumulative oil recovery to 85% in an outcrop core. Surfactant retentions during ASP and SP corefloods were found to be 0.4 mg/g-rock and 0.3 mg/g-rock, respectively, in outcrop cores.