Extension of SAFT equation of state for fluids confined in nano-pores of sedimentary rocks using molecular dynamic simulation

SA Goltapeh and R Miri and P Aagaard and H Hellevang, JOURNAL OF MOLECULAR LIQUIDS, 348, 118045 (2022).

DOI: 10.1016/j.molliq.2021.118045

Confined fluids in nano-mineral pores show notably different thermodynamic behavior compared with those of the same fluid in bulk. We have extended an equation of state (EoS) inspired by statistical association fluid theory (SAFT) to incorporate the fluid-mineral effect into the pure fluid properties confined in the nanopore. The established EoS is coupled with a Helmholtz free energy contribution which is a function of confinement characteristics: the potential of fluid-mineral interaction, bulk-like density and CF. The novel fitting parameter CF relies on the local properties of confined fluid and measures the density fraction within the layering and bulk-like zones, i.e., CF = rho(layering zone)/rho(bulk-like zone). The first-principle molecular dynamics (MD) simulation was utilized to estimate the confinement parameters. At T = 300 K, 350 K, 400 K, and 450 K, the NVT ensemble was applied to simulate slit-pore calcite with a continuous and homogeneous surface in contact with water, with pore widths ranging from 1.5 nm to 5 nm. After running each NVT ensemble for 6 ns, the energy and density of confined water were analyzed, and a deviation was discovered with different behavior from what had previously been reported. Coupling CF parameter with proposed EoS improves the absolute relative errors (ARE) at low temperatures up about 50%, leading to a good agreement between modified SAFT EoS and MD results. The methodology described here may be extended to include a mixture of associating fluids such as those found in oil composition and implemented in all SAFT versions. (C) 2021 The Authors. Published by Elsevier B.V.

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