Study of surfactant alternating gas injection (SAG) in gas-flooded oil-wet, low permeability carbonate rocks

Many low permeability carbonate reservoirs are gasflooded but leave behind a large amount of residual oil. Surfactant solutions can be injected with gas to improve recovery. Surfactants and gas have the potential to form foam that can improve both oil displacement efficiency and sweep efficiency in oil reservoirs. However, in many carbonate reservoirs, foams need to overcome two adverse conditions: oil-wettability and low permeability. This study investigates the performance of 2 surfactant formulations in gas-flooded low permeability, oil-wet carbonate cores: one that combine wettability alteration, low interfacial tension (IFT), and coarse foaming and the other that is only foaming. Phase behavior experiments were performed to determine the optimal salinity for low oil-water IFT. Contact angle and imbibition experiments were performed on initially oil-wet media to identify surfactants for wettability altering capabilities. Coinjection coreflood experiments of surfactant solution and gas (with no crude oil) were performed at a constant injection rate with varying gas/water ratio and at varying injection rates to determine the critical foam parameters. Finally, oil displacement experiments were performed in low permeability limestone cores using the promising surfactant formulations to study the effect on incremental oil recovery after gas EOR injection process. In addition, dynamic adsorption tests were performed to estimate the adsorption of surfactants in limestone rocks. Mobility reduction factors obtained in coinjection experiments were low (less than 10 in most cases) in tight carbonate cores. The foaming was very coarse or weak, if any. There was a significant hysteresis effect at higher foam injection velocity; the foaming behavior in low permeability carbonate rocks were very weak and very different compared to those in high permeability rocks. Wettability alteration and low IFT can be achieved with anionic surfactants that generate coarse foam and help in fluid diversion. Adsorption of the anionic surfactants studied was between 0.04 and 0.11 mg/gm of rock in presence of alkali and EDTA in tight carbonate formations. Oil displacement experiments in oil-wet carbonate cores revealed that tertiary oil-recovery with injection of wettability-altering, low IFT surfactant can recover a significant amount of oil (about 28-70% ROIP) over the secondary gas flood. The pressure drops in the presence of oil suggested rapid foam coalescence and unstable foam in oil-wet carbonate cores. Good foaming surfactant that does not change wettability or achieve low IFT recovered less oil. Low IFT and wettability alteration in synergy with coarse foaming improves incremental oil recovery significantly and is crucial for higher oil recovery in oil-wet porous media.