(140a) Adsorption and Phase Behavior of Mixed Alkanes in Nano Slit Graphite Pores: An iSAFT Application

The promising development in shale gas/oil makes it critical to understand fluid properties in pores of shale kerogen. Confinement in nanopores shifts the fluid phase behavior making the exploration and production predictions challenging. As the pore shape, pore size and solid composition can all affect the fluid behavior, prediction of fluid properties in shale networks is still under study by many researchers. Molecular density functional theory (DFT) is and efficient and accurate approach to model the thermodynamic properties of confined fluid.

In this work, we have applied interfacial Statistical Associating Fluid Theory (iSAFT) to model alkane (C₁-C₈) adsorption/desorption and phase behavior in graphite slit pores for pure fluids and mixtures. iSAFT accurately predicts the effect of size and shape on component partitioning and phase behavior. The pure component parameters were determined by fitting to bulk saturated liquid density and vapor pressure data in certain temperature ranges. In quantifying the fluid-solid interactions, we have introduced the potential of mean force from molecular simulation to evaluate the external potential in the non-local DFT framework. The density distribution of molecules in adsorption of methane and heptane are in good agreement with molecular simulation. The predicted shift in critical properties of confined pure methane and ethane is of similar magnitude to that from simulation. For a binary system, a methane and ethane mixture at 250 K in a 5 nm pore was chosen as a showcase. The shifted bubble point, dew point and critical pressure show good agreement with molecular simulation, with the errors mainly arisen from the mean field approximation in treating dispersion energy. We predict the competitive adsorption of multicomponent systems and discuss the dominating factor that determines the pore selectivity to the heavy components in gas or liquid adsorption. With these studies, we confirm the validity of the simplest form of iSAFT in predicting confined molecule phase behavior and show the promising application in oil and gas industry with its advantage in being more computationally efficient than molecular simulation method.