CO2-oil diffusion, adsorption and miscible flow in nanoporous media from pore-scale perspectives

Enhancing shale/tight oil and sequestering CO2 by CO2 huff-n-puff have gained extensive attentions in recent years. However, the fundamental understanding of CO2-oil diffusion, competitive adsorption and miscible flow behaviors are mainly limited to the single pore scale based on molecular dynamics (MD) simulations, and that in nanoporous media are still ambiguous. In this work, we propose a novel multiple relaxation time lattice Boltzmann method (MRT-LBM) to simulate CO2-oil diffusion, adsorption and miscible flow in pore-scale nanoporous media, which is successfully verified by the Fick’s second law and MD simulations. The proposed model can obtain various fluid-surface molecular interaction forces (adsorption capacities) of CO2/oil single phase and CO2-oil miscible phase, and quantify the attraction and repulsion from the pore surface to CO2 and oil. Then, the effects of CO2 and oil phase heterogeneous adsorption capacity on CO2 diffusion, storage and oil extraction are quantitatively analyzed. Additionally, based on the MD simulations, we also study the adsorption and flow behaviors of CO2-nC8 miscible adsorption and flow behaviors in calcite nanoporous media. The results show that CO2 can diffuse into the oil phase in porous media more readily from the regions that are relatively attractive to CO2 and repulsive to oil. In porous media with heterogeneous adsorption capacity, CO2 first contacts with the CO2-friendly surface which is attractive to CO2 during diffusion, beneficial to CO2 storage. With an increasing nC8 mass percentage (OMP), the proportion of adsorbed CO2 increases, and that of free CO2 decreases. That indicates that the CO2 diffusing into the oil phase is firstly adsorbed on the surface. When CO2 adsorption reaches saturation, the density of CO2 in the bulk phase begins to increase. We also observed a critical OMP equaling 81% below which CO2 can enhance nC8 flow while having a negligible enhanced impact on nC8 flow beyond 81%. Additionally, with an increasing porous media size, the proportion of adsorbed CO2 decreases, and the critical OMP increases.