Microscale Effects of Polymer on Wettability Alteration in Carbonates

Abstract

Summary Polymer enhances the volumetric sweep efficiency through the increased viscosity of injection water and subsequently results in enhanced oil recovery. Most of the reported experimental studies focused on only evaluating polymer viscosifying characteristics and their associated significance for achieving adequate mobility control in porous media. The microscale effects of polymer on wettability alteration in carbonates are rarely studied. In this experimental investigation, the wettability of carbonates in the presence of polymer was measured using contact angle tests. In addition, the adhesion force between carbonate and crude oil droplets in polymer solutions was determined using a custom-designed integrated thin-film drainage apparatus equipped with a bimorph sensor. The liberation kinetics of crude oil from carbonate surfaces were also measured by an optical microscope-based liberation cell to understand the wettability alteration effects on oil recovery. All the experiments, except the adhesion force, which was measured at room temperature due to the restriction of bimorph sensor, were conducted at both ambient and elevated temperatures (70°C) using a sulfonated polyacrylamide polymer (SPAM) (at 500 and 700 ppm) in high-salinity injection water. Deionized (DI) water was used as a baseline to provide a representative comparison with the high-salinity brine. The contact angles of crude oil droplets on a carbonate surface were highest in DI water and decreased in brine. The addition of polymer decreased the contact angle further, with higher concentrations of polymer resulting in a lower contact angle. The adhesion force between crude oil and carbonate showed good agreement with contact angle data, and the oil adhesion was smallest on the carbonate surface in the presence of polymer. The crude oil liberation from the carbonate surface by flooding with brine and polymer was found to be more efficient at elevated temperature than at ambient temperature, consistent with lower contact angles measured in these aqueous solutions at high temperature. The equilibrium oil liberation degree with polymer solutions increased by more than two times when the temperature was increased from 23 to 70°C. The higher liberation degree obtained with polymer solutions also correlated well with the lowest adhesion force measured between crude oil and carbonate in the presence of polymer. These consistent results obtained from different experimental techniques indicated that the oil recovery improvements observed with polymer in dynamic liberation tests are not only related to the increase in water viscosity but are also due to favorable changes in wettability as inferred from both contact angle and adhesion force measurements. This experimental study, for the first time, characterized the microscale effects of polymer on wettability alteration and crude oil liberation in carbonates. The favorable effect of polymer on wettability alteration in carbonates revealed from this study has not been reported in the literature, and it can become a novel addition to the existing knowledge.