(260c) Modeling Alkane Partitioning and Phase Behavior on Graphite Pores: A Discussion on Dispersion Free Energy Formalism

In reservoir simulations, thermodynamic properties of fluid such as pressure /volume /temperature, bubble/dew point and critical point are required to achieve a good estimation of reservoir production performance. Lots of studies have been done in unconventional shale showing the fluid phase behavior is shifted from what is predicted by a bulk equation of state. This is due to the significant adsorption of fluids in the organic rich shale. It has been recognized that the solid composition, pore size and geometry can all affect the fluid phase boundaries, which are shown to be much narrower than for bulk phase behavior. Therefore, it is highly important to establish a phase diagram for confined fluids in a way that is both accurate and efficient. Molecular density functional theory is preferred in solving this problem since it relates intermolecular forces to fluid structure and phase behavior similar to molecular simulation.

We model the mixed alkane fluid adsorption on graphite slit pores with interfacial Statistical Associating Fluid Theory (iSAFT) and study the phase behavior of pure and mixed alkanes in different pore sizes. The excess free energy is a summation of intermolecular repulsion and attraction contributions where the repulsion is accounted for by hard sphere volume exclusion and chain connectivity that is consistent with the SAFT equation of state for a bulk fluid. However, the contribution due to long range attraction is more challenging since the pair correlation function is unavailable for inhomogeneous fluids. There are two types of approximation mainly used by people, which are the mean field approximation (MFA) and a weighted density approximation (WDA).In this study, we compare the performance (accuracy and efficiency) of three schemes, which are MFA, WDA and a combination of these two, in predicting the fluid structure and adsorption isotherms. While MFA is the simplest and most widely used method, the accuracy of bulk fluid property prediction is limited, which makes the confined fluid property also shifted systematically. Thus we explore the performance of different types of WDA or a combination of MFA and WDA in approximating the local dispersion free energy and predicting fluid phase behavior in a wide temperature, pressure, pore size and species range. The accuracy of different methods will be evaluated by comparing to simulation data and computational time of each method will also be shown.