Modeling Alkane Partitioning and Phase Behavior in Non-Permeable and Permeable Slit Pores

In unconventional shale reservoirs, hydrocarbon adsorption and absorption both affect reservoir fluid distribution and phase behavior, which makes the prediction of fluid thermodynamic properties (pressure/volume/temperature) increasingly challenging. It has been recognized that the nano-confinement lowers the bubble point and critical point of fluid mixtures, and solvation of hydrocarbon in kerogen is also significant. For a hydrocarbon mixture, the pore fluid composition has a large deviation from extracted bulk fluid due to different surface affinities of different components. Although sorption can be described by some macroscopic models, density functional theory (DFT) can provide detailed information of the fluid structure in the pore-wall system.

In this presentation, we apply interfacial Statistical Associating Fluid Theory (iSAFT) to investigate the fluid adsorption in a non-permeable graphite pore and both adsorption and absorption in a permeable polymer pore. We show the shifted pure and binary phase diagrams in a non-permeable 3 nm pore and the pore size dependency of pure component critical properties, where the results all have a reasonable agreement with molecular simulation. The competitive adsorption of mixtures is predicted and suggests different dominating factors, enthalpic or entropic, determine the pore selectivity depending on the bulk conditions. For a permeable pore wall, we show the adsorbed density in the pore and the penetrated density in polymer wall, and meanwhile the sorption isotherms obtained from density distribution. We compare the enhanced sorption of fluid in a pore-wall system with bulk phase equilibrium solvation and discuss the phase equilibrium in different pore width and wall thickness.