Cation Valence and Concentration Modulate the Structure and Transport of Methane-Brine Two-Phase Flow in Quartz Nanopores: A Molecular Dynamics Study

Abstract Most molecular dynamics (MD) studies of gas–water two-phase flow in nanopores have treated the aqueous phase as pure water or employed Na+ as a representative cation. However, formation water in deep shale reservoirs is typically high-salinity brine containing cations of different valences, and how cation valence and concentration affect methane–brine transport remains poorly understood. In this study, we constructed a hydroxylated α-quartz (100) nanoslit pore at approximately 27% water saturation. Equilibrium and non-equilibrium MD simulations were conducted at 353 K and 40 MPa to examine methane–brine two-phase flow in NaCl, KCl, CaCl2, and MgCl2 systems at nominal cation concentrations of 1, 2, and 4 mol/L. Equilibrium results show that cations are confined within the near-wall water film, where they reduce water–water hydrogen bonding and suppress water self-diffusion, with the suppression increasing in the order KCl < NaCl < MgCl2 < CaCl2 at each cation concentration. In contrast, methane diffusion shows no systematic dependence on salt composition or cation concentration. Non-equilibrium results indicate that the effective viscosity of methane shows no systematic dependence on salt composition, whereas the apparent permeability shows an overall decrease as the cation concentration increases. Over the same concentration range, the methane mass flow rate decreases monotonically by up to 19% relative to the salt-free system, with larger reductions observed in the MgCl2 and CaCl2 systems than in the NaCl and KCl systems at each cation concentration. This transport impairment arises from water-film thickening, which narrows the effective methane channel, together with changes in methane–water interfacial dynamics, as indicated by the reduced streamwise water-film velocity. These findings highlight brine composition as an additional factor that warrants explicit consideration in nanoscale shale gas transport modeling.