Microemulsion-enhanced displacement of oil in porous media containing carbonate cements

Subsurface porous formations containing nonaqueous phase liquids (NAPLs) such as crude oils are often targets for surfactant flooding during enhanced oil recovery (EOR) or aquifer remediation processes designed to mobilize and solubilize oil. Recent studies suggested that putting surfactants into micro-emulsified state prior to injection might improve their performance. Most of these studies proved that microemulsion (ME) efficiency depends on test conditions and the proper selection of their chemical formulations and brine chemistry. However, the impact of rock characteristics on the complex fluid-rock interactions is still unclear, especially in heterogeneous rocks. The goal of this fundamental study was to examine the effect of MEs on oil displacement in three different aged rocks (Berea, Edwards, and Tensleep) and identify the test conditions in which MEs outperform surfactants. The effectiveness of surfactants and MEs was evaluated at two concentrations from different sets of spontaneous imbibition tests and petrographic analyses. Several mechanisms such as reduction of interfacial tension (IFT), oil emulsification, and wettability alteration were responsible for the improved recovery. Wettability alteration of aged cores by surfactants and MEs led to an early oil removal by spontaneous imbibition while emulsification increased the ultimate amount produced. Both additives lowered the IFT from 12 to less than 1 mN/m. However, high ME concentration was able to decrease the size of oil droplets by one order of magnitude, significantly enhancing oil mobilization in all three rocks. The solubilization capability of MEs was superior in Tensleep due to their unique ability to penetrate dolomite cements and alter their wettability.