A critical review of the CO2 huff 'n' puff process for enhanced heavy oil recovery

Heavy oil resources have become increasingly important in recent years due to a reduction in light oil production and an increase in energy consumption. A large number of heavy oil reserves are found all over the world, and traditional production methods, such as solution gas drive, water flooding, etc., cannot gain a high heavy oil recovery factor, because of the high viscosity of the heavy oil. Although the thermal method has proven efficient and economical to produce heavy oil, it cannot be applied in deep reservoirs or reservoirs with thin pay zones due to the huge heat loss in these reservoirs. Thus, in order to enhance heavy oil production, several CO2 injection processes are applied in heavy oil reservoirs. Among them, the CO2 huff 'n' puff method has proven the most applicable. In this research, the CO2 huff 'n' puff process is reviewed in detail. Among the mechanisms of the CO2 huff 'n' puff process in enhancing heavy oil production, the formation of foamy oil, viscosity reduction, and oil swelling are the most important ones, so that effect of foamy oil in the production stage is studied, and the viscosity reduction ratio with CO2 injection and oil swelling factors at different temperatures and pressures are summarized. In addition, the diffusion coefficient, which indicates the mass transfer rate and amount of CO2 dissolved into heavy oil through the two-phase interface of CO2 and heavy oil, is analyzed in various heavy oil reservoirs at different temperatures and pressures. Experimental studies on the CO2 huff 'n' puff process indicate that the process applied in the heavy oil reservoir is successful and can be carried out with an oil viscosity up to 28,646 mPa center dot s and a reservoir permeability up to 24,200 mD. In pilot tests in the field, economical CO2 huff 'n' puff processes have been applied in the heavy oil reservoirs with an oil gravity as low as 4 degrees API, a reservoir depth as high as 1985 m, and a pay zone as low as 12.2 m. Specifically, CO2 utilization can be as low as 4.2 Mscf/Stb. Numerical simulation studies can gain very good simulation results on both experimental and pilot tests studies. However, mathematical models have seldom been published on CO2 huff 'n' puff processes in heavy oil reservoirs.