Low tension gas flooding for secondary oil recovery in low-permeability, high-salinity reservoirs

Abstract

Low Tension Gas (LTG) flooding has been established as a successful tertiary oil recovery method for low-permeability carbonate reservoirs with high salinity and hard formation brine (~200,000 ppm and hardness 19,000 ppm). LTG flooding recovers oil using two integral mechanisms: ultra-low interfacial tension (IFT) between oil and water, and mobility control using in-situ foam generated by injected gas. In the present work, the scope of applicability of LTG flooding has been extended to secondary oil recovery under the same reservoir conditions. Secondary recovery using LTG flooding has been compared to conventional secondary recovery methods such as waterflooding. Oil recovery was observed to increase by 16% OOIP (Original Oil in Place) as compared to waterflooding, even in case of micellar flooding without gas. On introducing mobility control during LTG flooding in the form of injected gas, the secondary oil recovery was observed to increase steadily up to 81% OOIP. Co-injecting gas and surfactant also exhibited lower pressure drop than waterflood, thus underlining the importance and efficiency of mobility control using foam in secondary recovery. Gas injection strategy was improved in terms of injected foam quality and onset of gas injection. When gas was injected only during drive, ultimate oil recovery was reduced to 70% OOIP. Chemical injection strategy was also modified to test the impact of different in-situ salinity profiles on oil recovery.