Low Tension Gas Flooding with CO₂ in a Low Permeability, High Salinity Carbonate Rock

Low tension gas flooding is an innovative oil recovery technique that relies on the reduction of the oil–water interfacial tension for oil mobilization and the generation of foam for proper mobility control. When CO2 is used as the working gas, LTG offers the dual benefit of improving oil recovery as well as geological carbon storage. This first-of-its-kind investigation aims to investigate the dynamics of LTG with CO2 in an oil-wet carbonate rock under varying conditions of salinity, pressure, and surfactant concentration. Two series of core floods were conducted at 1400 and 2000 psi at 69 °C. The significance of CO2 and surfactant in oil displacement at low pressure was established through a reduction in residual oil saturation from 14 to 2%. The impact of CO2-microemulsion interaction on the LTG process was evaluated by accounting for the reduction in optimum salinity due to the in situ modification of crude oil composition with dissolved CO2, which led to 90% oil recovery after only 3 PVs of injection, with a reduction of optimum salinity from 179,000 to 150,000 ppm. Reducing the slug concentration from 0.5 to 0.2 wt % resulted in a decrease in oil recovery and weaker foam propagation. The role of CO2 in improving oil mobilization at high pressure was demonstrated by achieving comparable results to the low-pressure case at a lower surfactant concentration. These results offer valuable insight into designing optimal injection strategies for LTG flooding when CO2 is used.