CO2 has the advantages of strong solubility, good viscosity reduction ability and low miscible pressure, which has a great potential in enhancing oil recovery (EOR). Shale has a high content of organic porous media and clay minerals, which is conducive to long-term adsorption and retention of CO2, and is one of the best CO2 storage environments. However, with the exploitation of hydraulic fracturing and CO2 huff-n-puff, the water cut of the reservoir is increasing, and the mechanism of oil, water, CO2 molecular interaction and fluid-rock molecular interaction in shale is still unclear. In previous studies, we developed a new nano-scale porous media model based on the lattice Boltzmann method (LBM). A CO2 huff-n-puff model including fracture and matrix (including macro poress, micro-pores (inorganic), nano-pores (organic) ) were designed. The multi-relaxation Shan-Chen model was used to simulate the diffusion and adsorption behavior of CO2 in the miscible state of shale. In this study, we additionally consider the water cut of different development stages of reservoir development to set the oil-water occurrence state in the matrix. Meanwhile, we comprehensively considered the effects of heterogeneous wettability caused by organic porous media and inorganic porous media in shale and the varying interfacial fluid viscosity of interfacial caused by adsorption on CO2 huff-n-puff. Through the simulation of oil-water phase separation in the matrix, we set up four oil-water occurrence states (water cut is 10 %,30 %, 50 % and 70 % respectively). Water molecules mainly aggregate in inorganic matter. By injecting CO2 into the fracture, the oil and gas density distribution and oil-water-gas three-phase distribution are analyzed. The results show that the adsorption quantity of CO2 is reduced due to the stronger adsorption capacity of water molecules on the surface of inorganic rock particles, and the quantity of CO2 entering the matrix in the fracture is reduced. The contact area between CO2 and crude oil decreases with the increase of water content, and the miscibility of oil and gas slows down. We also found that the increase of water content will promote oil recovery in inorganic matter. But it will also reduce the contact area of oil and CO2. When the water content is 0.5, the two effects are the weakest, and the oil recovery is the smallest. This paper provides theoretical guidance for the field application of CO2 huff-n-puff mining unconventional oil and gas resources and the field application of CO2 storage technology in different development stages.
Keywords CO2 huff-n-puff, Water cut, LBM, EOR, CCUS