Enhanced oil recovery by alkaline-surfactant-alternated-gas/CO2 flooding

The volumetric sweep efficiencies of CO2 flooding for enhanced oil recovery (EOR) are generally low due to problems of viscous fingering and gravity override. This paper attempts to study a relatively new and promising method to reduce the mobility of CO2 flooding and increase oil recovery under reservoir conditions. Referred to as alkaline-surfactant-alternated-gas/CO2 (ASAG) flooding, this method is essentially the synergic combination of chemical and immiscible CO2 flooding. In this work, chemical formulations were identified through foam stability tests based on their foaming ability coefficients. The selected formulations were further tested for their capabilities to reduce oil-water interfacial tensions (IFT) to ultra-low value. With the best performing formulations, the laboratory-scale core flooding experiments were conducted to evaluate their EOR potential. The core flooding experiments were performed with sandstone reservoir core samples from two different depths of a major depleted oil field of Upper Assam Basin, India. This study reports the successful application of a natural anionic surfactant (black liquor) as a co-surfactant and foaming agent during ASAG flooding. It was observed that higher oil recovery of 14.26% original oil in place (OOIP) was obtained bysurfactant-alternated-gas (SAG) flooding compared to 12.03% OOIP by immiscible CO2 alternated with brine (WAG) flooding. The highest residual oil recovery of 20% OOIP was obtained for ASAG flooding with the alkali, surfactant and black liquor in the chemical slug. Oil recovery performances during SAG and ASAG flooding were found to be better for core samples with lower porosity-permeability due to stronger foam formation in lower permeability cores.