Surfactant enhanced imbibition in carbonate reservoirs: Effect of IFT reduction and surfactant partitioning

A majority of carbonate reservoirs are preferably oil-wet or intermediate wet. Spontaneous imbibition represents an important mechanism to enhance oil production. Targeting an oil-wet carbonate reservoir with high temperature and high salinity, several surfactant formulations were investigated to improve oil production by imbibition, focusing on the capacity in interfacial tension (IFT) reduction and surfactant partitioning in water and oil phases in addition to dominant wettability alteration effect. Oil production potentials using different surfactants were evaluated by spontaneous imbibition in Amott cells at 95 ​°C. A base case imbibition test was conducted using a high salinity water with IFT value of 15.2 ​mN/m and an oil-wet carbonate core, and the oil production was 6.4% of oil originally saturated in core. Reducing the oil-water IFT to 10° ​mN/m magnitude achieved 4% incremental oil production beyond base case. When the IFT was further reduced to 10⁻¹ ​mN/m magnitude, the incremental oil production reached 10%. For the case of water-wet core and at the same IFT range, the incremental oil production was 13%. When the IFT was reduced to 10⁻² ​mN/m, the incremental oil production was up to 17% for the tests using oil-wet cores, and up to 23% for the water-wet counterpart. When further reducing the IFT to ultra-low magnitude of 10⁻³ ​mN/m, the oil production was close to that using the imbibition agents with IFT of 10⁻² ​mN/m magnitude. For surfactants with no obvious wettability alteration capacity, those surfactants with higher molecular fraction in oil phase and higher adsorption on rock surfaces could remove more oil from the pore wall, and yielded higher imbibition efficiency. This work systematically investigated the major effects of surfactant on oil mobilization during imbibition process, which can help for better understanding of chemical imbibition in carbonates. The findings will shed light on the selection of high performance imbibition agents for carbonate reservoirs.