Adsorption induced critical shifts of confined fluids in shale nanopores

Phase equilibrium of shale gas or oil is highly disturbed due to fluid adsorption in densely-developed nanopores. Accurate description of adsorption and its induced critical shifts for confined fluids in nanopores is essential for a better understanding of the underlying mechanisms governing the storage, transport, and recovery of shale gas or oil in shale reservoirs. In this study, first, a novel method for describing fluid adsorption in nanopores was presented on a basis of adsorption thickness (delta) coupled with an initially-proposed parameter (reduced adsorption density beta). Second, the two parameters were introduced to modify the molar volume term and then the Peng-Robinson equation of state (PR-EOS) was updated. This proposed model could characterize the quantitative relationship between fluid adsorption and its induced critical shifts. Third, the literature data of critical temperature shift were collected to correlate the reduced adsorption density and dimensionless pore radius. Then, the modified PR-EOS was solved and two generalized analytical formulas were developed for calculating the shifts of critical temperature and pressure caused by fluid adsorption in nanopores. Finally, several key factors including hydrocarbon component and pore size were investigated to demonstrate that the modified PR-EOS showed a satisfactory ability to predict the shifted critical properties of confined fluids.