A novel numerical simulation of CO2 immiscible flooding coupled with viscosity and starting pressure gradient modeling in ultra-low permeability reservoir

CO2 immiscible flooding is an environmentally-friendly and effective method to enhance oil recovery in ultra-low permeability reservoirs. A mathematical model of CO2 immiscible flooding was developed, considering the variation in crude oil viscosity and starting pressure gradient in ultra-low permeability reservoirs based on the non-Darcy percolation theory. The mathematical model and numerical simulator were developed in the C++ language to simulate the effects of fluid viscosity, starting pressure gradient, and other physical parameters on the distribution of the oil pressure field, oil saturation field, gas saturation field, oil viscosity field, and oil production. The results showed that the formation pressure and pressure propagation velocity in CO2 immiscible flooding were lower than the findings without considering the starting pressure gradient. The formation oil content saturation and the crude oil formation viscosity were higher after the consideration of the starting pressure gradient. The viscosity of crude oil considering the initiation pressure gradient during the formation was higher than that without this gradient, but the yield was lower than that condition. Our novel mathematical models helped the characterization of seepage resistance, revealed the influence of fluid property changes on seepage, improved the mathematical model of oil seepage in immiscible flooding processes, and guided the improvement of crude oil recovery in immiscible flooding processes.