Improving Oil Recovery In Gas-flooded, Oil-wet Carbonate Reservoirs by Wettability Altering Surfactants

Abstract

Abstract Carbonate reservoirs have complex pore structure and tend to be oil-wet. The remaining oil saturation is often high after a gas flood. The goal of this work is to develop and evaluate surfactant formulations that can improve the oil recovery after a gas flood. An anionic surfactant was used to develop a low IFT and wettability altering formulation for a soft injection brine and EDTA was added to modify the formulation for a hard injection brine. These surfactant solutions changed the wettability of an oil-wet calcite plate to intermediate-wet/water-wet conditions. Core spontaneous imbibition tests and corefloods were conducted. These displacements were simulated using CMG-STAR to study in-situ viscous fingering during gas injection and improved oil recovery using surfactant formulations. The hard brine surfactant formulation recovered 54% of the original oil in 60 hours by spontaneous imbibitions compared to 12% for the soft brine surfactant formulation and no oil for the formation brine. Initial gas flood left a lot of oil unswept. EDTA assisted hard brine surfactant formulation could recover over 40% of the oil, higher than 25% for the soft brine surfactant formulation. The gas injection after the surfactant slug increased the pressure drop due to foam formation. Numerical simulation of the GSG flood showed in-situ viscous fingering during the first gas injection and efficient oil displacement during the surfactant injection and the subsequent gas injection.