Saturated aqueous NaCl solution and pure water in Na-montmorillonite clay at thermodynamic conditions of hydraulic fracturing: Thermodynamics, structure and diffusion from molecular simulations

We present a Grand Canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulation study of an aqueous NaCl solution in Na-montmorillonite (Na-MMT) clay pores, including the thermodynamic, structural and dynamical properties. We represent the clay pores by Na-MMT slits of an interlayer separation ranging from about 10 to 32 Å, covering clay pores from dry clay to clay pores with a bulk-like water layer in the middle of the pore. We consider the clay in equilibrium with the saturated salt solution at a typical shale gas reservoir condition of a temperature of 365 K and a pressure of 275 bar. The equilibrium amount of the adsorbed ions and water molecules obtained from the GCMC simulations are employed in MD to simulate the disjoining pressure and in turn the swelling free energy curve which determines the number of stable states in the clay, their composition, and the corresponding equilibrium interlayer separations. In addition, using MD simulations, we evaluate the hydration energy, atomic and charge density profiles, orientation distribution, hydrogen-bond network, and in-plane self-diffusivity of the water and ions to provide insight into the microscopic behaviour of the aqueous NaCl solution in the interlayer galleries of the hydrophilic Na-MMT pores. We compare the simulation results for the confined aqueous NaCl solutions with those obtained for the Na-MMT in equilibrium with a pure water reservoir at the same temperature and pressure.