Flue gas assisted cyclic steam stimulation (CSS) has gradually gained prominence in the development of extra-heavy oil reservoirs. The flue gas produced by fuel combustion is primarily composed of CO2, N2, and steam, and it is injected directly into extra-heavy oil reservoirs to enhance oil recovery. This method is also known as multi-thermal-fluid (MTF) huff-puff technology, which is an important CCUS technology. In this paper, the effects of different CO2 and N2 ratios on the development of extra-heavy oil reservoirs are analyzed and discussed. To begin, the viscosity and components of crude oil are obtained through physical experiment. Second, the effects of different CO2 and N2 ratios on crude oil viscosity and phase state are analyzed using the phase equilibrium model and viscosity model. Finally, numerical simulation is used to optimize the CO2 and N2 ratios. The results show that the higher the CO2 ratio, the lower the viscosity of the crude oil after MTF huff-puff. The phase diagram gradually narrows as CO2 ratios rise while N2 ratios fall. Because the primary function of CO2 is to reduce viscosity and the primary function of N2 is to improve formation energy, the optimal ratio of CO2 and N2 is 7:3.
Keywords Extra-heavy oil reservoir, Multi-thermal-fluid huff-puff, Optimal ratios of CO2 and N2, PVT simulation, Numerical simulation