EOR mechanisms of CO2 huff and puff process for heavy oil recovery

A heavy oil sample from Dagang Oilfield was used to study the EOR mechanisms of CO2 huff and puff process using a PVT device and a sand-pack displacement set-up. The properties and interactions of the CO2-crude oil system were measured under high pressure and temperature conditions, along with the reservoir numerical simulation carried out for the history matching of the pilot testing of the CO2 huff and puff operation in a horizontal well. The results show that, under reservoir conditions, the interfacial tension between CO2 and the heavy oil sample is 2.9 mN/m, which is immiscible. The solubility of CO2 in the crude oil is 45 m3/t, with the expansion coefficient of 1.17, in which the viscosity reduction rate can reach to 99.18%.The dissolution rate of CO2 in the heavy oil by diffusion at static condition is only 0.005 9 mL/mL/min, but it can reach to 0.291 2 mL/mL/min under condition of dynamic miscibility. Supercritical CO2 can effectively extract C5-C14 alkane components from the crude oil, which makes the content of aromatics, resin and asphaltene in the remaining oil increase by 10.14%, and a slight increase of the oil viscosity. In the field pilot testing, an excellent performance of CO2 huff and puff process has been achieved in a horizontal well, in which the CO2-oil exchange ratio was of 1.21, larger than that measured in the laboratory. The EOR mechanisms of the CO2 huff and puff process mainly include oil swelling and viscosity reduction, dissolved gas drive and inhibition of bottom water coning in the field operation, while the bottom water can have a positive effect on increasing CO2 sweep volume and supplementing formation energy. © 2021, Editorial Office of Journal of China University of Petroleum(Edition of Natural Science). All right reserved.