The Effect of Pore Size on Shale Gas Recovery with CO2 Sequestration: Insight into Molecular Mechanisms

The extremely low permeability of the nanopore causes only 5%-15% content of the original shale gas to be extracted, and carbon sequestration with enhanced gas recovery (CS-EGR) has been a potentially feasible win-win solution. In this study, the adsorption isotherms, density distributions, CO2/CH4 adsorption selectivity, adsorption heat, and interaction energies of CH4 and CO2 in the montmorillonite nanopores are calculated and discussed in detail using a series of grand canonical Monte Carlo (GCMG) simulations considering the influence of pore size, temperature, and pressure. The recovery of CH4 in montmorillonite nanopores with various size under different injection pressures is also investigated. The results indicate that pore size has significant influence on the adsorption of CH4 and CO2. Under the same conditions, the adsorption capacity of CO2 is obviously stronger than that of CH4 because the interaction energy and the adsorption heat of CO2 are both relatively larger. CO2 and CH4 have different adsorption sites. CO2 molecules preferentially accumulate near the Na+ cations. In contrast, CH4 molecules are preferentially adsorb in the hollow site of the six-membered oxygen ring in the silicon tetrahedron. Due to the high energy barrier and low diffusion coefficient, CH4 in smaller pores is hard to displace and even cannot be extracted in pores with basal spacing corresponding 12 angstrom even when the pressure increases to 25 MPa. Supercritical CO2 can displace CH4 more quickly and is preferable for CO2 sequestration. We hope this study will be beneficial for better understanding of the microscopic states of gas molecules in shale and provide guidance for CS-EGR.