Molecular dynamic simulation and experimental study of wettability alteration by hydrolyzed polyacrylamide for enhanced oil recovery: A new finding for polymer flooding process

Abstract

Abstract Polymer flooding by hydrolyzed polyacrylamide (HPAM) is a well-known enhanced oil recovery (EOR) technique which is mainly applied to viscosify the pushing fluid and improve the mobility ratio; however, the role of this polymer on the rock wettability alteration, one of three main recovery mechanisms, has remained a topic of debate. In this study, the adsorption of hydrolyzed-polyacrylamide on the carbonate surface was investigated by molecular dynamic simulation considering a simple Young model based on the surface tensions of each phase to calculate the water contact angle and then continued by an experimental procedure after aging carbonate slices in the oil and polymer solutions. Dreiding and Universal force fields were selected for the simulation of water and HPAM boxes, respectively, to calculate non-bonding/bonding energies and surface tension for the water, polymer, and three-phase system including calcite rock. The polymer adsorbing decreased the final water contact angle to about 130 and 140° at the ambient and reservoir temperatures, respectively, to provide a moderately oil-wet state and a good agreement with the experimental results for the surface wettability alteration toward the water wetting. However, the theory and laboratory observations showed a different relation between the polymer adsorption and increasing temperature. Lennard-Jones potentials showed the polymer surface desorption at the increased kinetic energy of molecules due to the molecular distance whereas in the experiment condition the tendency of polymer adsorption on the calcite surface increased with temperature, probably because of ion exchange and thermal decarboxylation phenomenon. These findings make an opportunity to further molecular dynamic investigations of different polymers as a wettability modifier through the EOR process.