Rahmani Didar, Behnaz (Petroleum Engineering Dept., Texas A&M University, College Station, Texas, USA) | Akkutlu, I. Yucel (Petroleum Engineering Dept., Texas A&M University, College Station, Texas, USA)
Recent studies have shown that the behavior of pure fluid in confinement deviates from its bulk state. This means that the vapor and liquid saturation curves, critical temperatures, pressures and densities of fluid in confinement are different from their well-documented bulk state values. While these single-component studies have been influential in our understanding of this peculiar phenomenon, the multi-component fluid phase behavior has yet to be investigated in depth. This is an important step, because production of liquids and natural gases from organic rich nanoporous shales involves fluids with a wide range of composition. Nanopore walls have significantly varying degrees of affinity to each component (selective adsorption). Consequently there will be large gradients across the pore width in not only density and pressure but also in concentration. This work is an investigation of phase changes of single and multi-component fluids in confinement with the help of molecular simulation. In the first part of this paper, phase diagrams of three single component (pure) fluids in confinement are constructed and studied. In the next part, a different methodology is utilized to construct phase diagrams of binary and ternary mixtures with light, intermediate and heavy hydrocarbon components. The methodologies are initially explained and verified by the bulk-state fluid behavior.
This study shows that the behavior of single component fluids approaches its bulk state at a confinement of approximately 13nm width. Furthermore, the impact of confinement appears to be greater on the vapor saturation curve than on the liquid curve. In the case of multi-component fluids, the phase diagrams seem to shift more severely with the increase of the percentage of light component in the mixture. Contrast in concentrations of the light and heavy components, amplifies the confinement effect. More specifically, confined multi-component fluids with high percentage of light components such as methane and ethane are expected to exhibit more dramatic changes in phase behavior. Lastly, critical temperature and pressures of confined mixtures obtained from our molecular simulations are compared to those obtained from other mixing rules and equations, such as Peng-Robinson equation of state, which are extensively used for fluid in bulk state. The differences in values obtained show the necessity for the development of new approaches for considering hydrocarbon fluids in confinement. The observations undermine the current practice of assuming bulk fluid parameters for fluid in shale plays.