From rigidity to flexibility: Understanding ethane adsorption and diffusion in shale under moist and saline conditions

Understanding gas adsorption and transport in shale nanopores is crucial for accurately quantifying gas in place (GIP) and optimizing extraction, as shale gas becomes a key energy resource. Despite its significance, studies on ethane adsorption as the second major component of gas composition in mixed organic-inorganic pore systems remain limited. This work employs hybrid grand canonical Monte Carlo/molecular dynamics (GCMC/MD) simulations to explore ethane adsorption in the presence of water and salinity, emphasizing the structural flexibility of kerogen-illite pores, alongside MD simulations to study diffusion. Results reveal that water reduces the influence of hydrophilic surfaces on gas adsorption, significantly altering adsorption dynamics. While kerogen flexibility decreases adsorbed gas density due to increased surface roughness, flexible illite structures enhance the representation of adsorption behavior by allowing the free orientation of hydroxyl (-OH) groups. Although this flexibility affects Na+ and Cl− ions adsorption dynamics, it does not significantly influence ethane adsorption capacity. Self-diffusion coefficients are influenced by pore width and pressure, increasing with larger pores but exhibiting reduced sensitivity to slit aperture at pressures above 20 MPa. These findings highlight the interplay between environmental conditions and matrix properties in shale, offering guidance for resource evaluation and gas extraction optimization.