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

Key words:  heavy oil  CO₂  swelling and viscosity reduction  dissolution diffusion  huff and puff process  EOR mechanism

Abstract:

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.