Dynamic Behaviors of CO2 Enhanced Shale Oil Flow in Nanopores by Molecular Simulation

Summary With the development of unconventional oil and gas, shale oil has become a significant focus for exploration and development. The mineral composition of shale is notably complex, and the mechanisms underlying carbon dioxide (CO2)-enhanced shale oil flow remain insufficiently understood. While many studies have addressed adsorption in shale oil and gas, research into the dynamic flow of CO2 and shale oil within pore spaces is limited. To investigate the mobility behavior of CO2 and shale oil in nanopores from a microscopic perspective, a dynamic flow model for CO2-enhanced shale oil flow, considering wall adsorption effects, was established by using the Non-Equilibrium Molecular Dynamics (NEMD) method. This model simulated CO2-enhanced shale oil flow within organic nanopores under reservoir conditions and analysed the effects of pore size, temperature, and injection pressure. The results show that shale oil forms four adsorption layers in 4-nm graphene pores, with a density of 2.25 g/cm3 in the first adsorption layer and 0.63 g/cm3 in the free zone, closely aligning with the standard shale oil density of 0.66 g/cm3 at 343 K and 25 MPa, thereby validating the accuracy of the model. The peak density of the first adsorption layer is 3.55 times that of the free zone, highlighting shale oil’s strong adsorption capacity at the pore wall. The study reveals that the diffusion coefficients of CO2 within the pores are 1.05, 1.14, and 1.41 times higher than those of pentane, octane, and dodecane, respectively. Additionally, the diffusion coefficient of shale oil increased by 10.3 times when the pore size increased from 2 to 5 nm, and by 3.9 times when the temperature rose from 303 to 383 K. Injection pressure also led to a 1.5 times increase in diffusion coefficients. Thus, in shale oil development, adjusting pore size, temperature, and injection pressure can enhance production, although excessive injection pressure may result in CO2 gas channeling, negatively impacting CO2-enhanced shale oil flow. This study offers a microscopic exploration of CO2-enhanced shale oil flow mechanisms and provides a theoretical foundation for efficient shale oil development.