Effects of CO2 and surfactants on the interface characteristics and imbibition process in low-permeability heavy oil reservoirs

Due to the poor physical properties and low flowing capability of crude oil in low-permeability heavy oil reservoirs, fracturing is generally required for effective development. Imbibition is considered an important development method for low-permeability and tight reservoirs after fracturing. In this paper, the imbibition efficiency of different concentrations of surfactant solutions and different CO2 pressures in low-permeability heavy oil reservoirs were studied, the oil-water interfacial tension (IFT) and contact angle (CA) of the imbibition process were tested, and the mechanism of improved oil recovery (IOR) was obtained by the modified NB−1. In addition, the effects of permeability and oil viscosity changes on imbibition efficiency were also investigated. The results show that the imbibition recovery of heavy oil increases with increasing surfactant solution concentration, and surfactants can reduce IFT and alter wettability. The oil recovery can be enhanced by 10.87% with the addition of surfactants. At the beginning of imbibition, the NB−1 value is negative, and the imbibition process is cocurrent imbibition driven by gravity. Then, the NB−1 value soon becomes positive, and the capillary force becomes the driving force of the imbibition process. As the CO2 pressure increases, the imbibition recovery increases. When the system pressure is 8 MPa, the imbibition recovery reaches 29.8%. CO2 can reduce the oil viscosity and IFT as well as strengthen the wettability alteration effect through dissolution and diffusion, thus enhancing the flowing capability of heavy oil. The imbibition recovery increases with increasing core permeability (50–200 mD), but the increase gradually decreases. In addition, imbibition recovery decreases with increasing oil viscosity, and the key factor for imbibition in low-permeability heavy oil reservoirs is to improve the flowing capability of the heavy oil.