The various equations of state and modifications proposed for use in nanoporous systems are critically examined. The fluid properties modifications in nanoporous systems produced by the effect of pore wall potentials and the limited number of molecules in nanopores are investigated for liquid-rich shale. These properties include phase behavior, interfacial properties, gas and liquid transport, storage, and composition. The existing theoretical equations are modified to predict the vapor-liquid equilibrium for shale and the unique behavior of fluids in kerogen and inorganic pores. The pore geometry, molecule size, interaction between the sorbed molecules and the nanoporous framework are included to enable accurate prediction of fluid phase behavior, critical properties, and composition. The predictions of the proposed approaches are compared to results obtained from molecular dynamic simulations.
The study of fluid properties in nanoporous systems continues to receive considerable attention because of the shift towards the development of liquid-rich shale plays. In these nanoporous shales, it is now increasingly recognized that pore-wall proximity determines to a large extent the fluid phase behavior, its interfacial properties, gas and liquid transport, storage, and composition. We focus on a comprehensive and critical evaluation of existing equations-of-states (EOS) to quantify vapor-liquid equilibrium for shales and review the need for modified EOS's in order to address the unique behavior of fluids in kerogen and inorganic pores. Several factors that need to be considered are the pore geometry, molecule sizes, interaction between the sorbed molecules and the nanoporous framework. Proper consideration of these factors will enable more accurate prediction of fluid phase behavior, its critical properties, and composition which will lead to improved forecasting and reserves estimation.
We review the limitations in the applicability of existing EOS for multicomponent, multiphase fluid description in shale nanopores. A comparison is made of the predictive capability of these equations against results obtained from pore scale molecular dynamic simulations describing fluid phase behavior in nanopores. We then review the modifications to these equations of states proposed to capture pore wall proximity effects and explore the validity of these for compositional shale gas simulation. Finally, we propose convenient modifications to existing EOS that quantifies fluid phase and compositional behavior across a wide range of pore sizes and pore pressures in adsorbing and non-adsorbing walls. The approach to modification of equations-of-state outlined in our work enables improved reservoir performance forecasting, reserves estimation, calculations of condensate dropout and additionally, enables operators to characterize the original in-situ fluid composition from the produced gas and liquid streams.