Pore-Scale Study on Shale Oil-CO₂-Water Miscibility, Competitive Adsorption, and Multiphase Flow Behaviors

Due to the fracturing fluid imbibition and primary water, oil-water two-phase fluids generally exist in shale nanoporous media. The effects of water phase on shale oil recovery and geological carbon sequestration via CO₂ huff-n-puff is non-negligible. Meanwhile, oil-CO₂ miscibility after CO₂ huff-n-puff also has an important effect on oil-water two-phase flow behaviors. In this work, by considering the oil-CO₂ competitive adsorption behaviors and the effects of oil-CO₂ miscibility on water wettability, an improved multicomponent and multiphase lattice Boltzmann method is proposed to study the effects of water phase on CO₂ huff-n-puff. Additionally, the effects of oil-CO₂ miscibility on oil-water flow behaviors and relative permeability are also discussed. The results show that due to Jamin's effect of water droplets in oil-wetting pores and the capillary resistance of bridge-like water phase in water-wetting pores, CO₂ can hardly diffuse into the oil phase, causing a large amount of remaining oil. As water saturation increases, Jamin's effect and the capillary resistance become more pronounced, and the CO₂ storage mass gradually decreases. Then, based on the results from molecular dynamics simulations, the influences of oil-CO₂ miscibility on oil-water relative permeability in calcite nanoporous media are studied, and as the oil mass percentage in the oil-CO₂ miscible system decreases, the oil/water relative permeability decreases/increases. The improved lattice Boltzmann model can be readily extended to quantitatively calculate geological CO₂ storage mass considering water saturation and calculate the accurate oil-water relative permeability based on the real 3D digital core.