Feasibility Study of Gas-EOR Using CO2 and N2 Mixture in a Heavy Oil Reservoir: Experiments and Pilot Tests

Nanpu Onshore Block of Jidong Oil Field, China has conducted cyclic CO₂ injections since 2010, and about 109.5×10⁴ t of crude oil has been recovered until 2018. However, problems such as CO₂ corrosions, gas channeling, etc. were also observed during CO₂-EOR operations. CO₂ and N₂ mixture is then considered as an alternative gas-EOR technique for the oil field. A feasibility study of CO₂ and N₂ mixture is first evaluated in the laboratory. CO₂-N₂-oil interactions are investigated with different CO₂/N₂ molar ratio (1:0, 4:1, 7:3, 1:1, 0:1) using a PVT cell, and the high pressure properties including saturation pressure, volume factor, viscosity, etc. are measured. Then, a series of gas injection experiments using pure CO₂, pure N₂, and their mixtures are designed with outcrop cores.

PVT analysis shows that saturation pressure, density and viscosity decrease as CO₂/N₂ molar ratio increases, while volume factor increases as CO₂/N₂ molar ratio increases. A better gas-oil interactions can be obtained with a higher CO₂/N₂ molar ratio. Gas injection experiments results shows that after four cycles of gas injection with an total volume of 0.2PV, pure CO₂ and pure N₂ can achieve an oil recovery of 19.06% and 8.08%, respectively, while the mixture with CO₂/N₂ molar ratio of 4:1 can achieve an oil recovery of 17.31%. CO₂ in the mixture can drastically swell the formation oil and reduce the oil viscosity, while N₂ in the mixture can effectively supply the formation energy. With the integrated EOR mechanisms of CO₂ and N₂, gas injection using the mixture can obtain an remarkable oil recovery. An pilot test was conducted in the end of 2018, and an oil production of 275 t was obtained after 300 t of CO₂ and 40000 m³ of N₂ (CO₂/N₂ molar ratio is 4:1) were injected into the formation.