CO₂ diffusion in shale oil based on molecular simulation and pore network model

Field practices show that CO2 huff and puff can effectively promote the exploitation of shale resources, and CO2 diffusion is noteworthy as a crucial mechanism of enhanced oil recovery (EOR). Understanding the confinement effect on CO2 diffusion is imperative to accurately characterize the fluid transport in shale nanopores and further upscaling. We integrate molecular simulation and pore network model (PNM) to study CO2 diffusion behavior at the nanoscale and pore scale. We first explored CO2 diffusion into heptane in kerogen and calcite pores using molecular dynamics, and analyzed the impacts of pore size, molar fraction, temperature, and pore pressure on the confined Fick diffusion. Based on the molecular simulation results, a model characterizing the ratio of the confined diffusion coefficient to that of the bulk phase is constructed for further upscaling. Based on the PNM containing organic and inorganic pores, we established a model to simulate the CO2 diffusion at the pore scale by introducing the characterization model at the nanoscale into the convective diffusion equation. Considering the microscale effects and the influence of CO2 concentration on diffusion is beneficial to improve the simulation accuracy at the pore scale. The estimated CO2 diffusion coefficients are in good agreement with the experimental results. This work provides better insight into CO2 diffusion behavior in shale oil, which can afford effective support for industrial exploitation and the breakthrough of the technical bottleneck of shale oil production.