Laboratory investigation of cyclic gas injection using CO2/N2 mixture to enhance heavy oil recovery in a pressure-depleted reservoir

CO2 has strong abilities of oil swelling and viscosity reduction for heavy oil extraction, N2 is a preferable gas media for pressure maintenance, and their mixture is then considered as an alternative method for enhanced oil recovery (EOR) in a pressure-depleted heavy oil reservoir. PVT analysis and cyclic gas injection experiments are conducted to reveal EOR mechanisms for CO2/N2 mixture, and five different CO2/N2 molar ratios (1:0 (pure CO2), 4:1, 7:3, 1:1, and 0:1 (pure N2)) are designed in this paper. PVT analysis shows that the changes of oil properties including saturation pressure, volume factor, and viscosity are highly related to CO2/N2 ratio of the mixture, and a higher CO2/N2 ratio can always lead to better gas/oil interactions. When CO2/N2 ratio is higher than 7:3, the viscosity can reduce to less than 60% of the initial oil viscosity. Gas injection experimental results show that 4:1 mixture can achieve an oil recovery of 17.31%, which is close to the recovery obtained by pure CO2. For the pressure-depleted oil reservoir, simultaneous CO2/N2 injection or N2 followed by CO2 injection is suitable injection mode for the mixture. With such a design of CO2/N2 mixture, an optimized EOR effect can be achieved with gas/oil interactions dominated by CO2 coupled with energy supplement provided by N2. A pilot test of CO2/N2 mixture injection is successfully conducted in a test well, and an oil increment of 337 t is obtained after 40,000 m3 of N2 injection followed by 80,000 m3 of CO2 injection.