The behavior of confined fluids in shale reservoirs is significantly different from that of bulk fluids due to the confinement of nanopores. The unique physicochemical properties of confined fluids invalidate the current available equations of state (EOS) used in such reservoirs. In this study, first, the molar volume term of the Peng-Robinson equation of state (PR-EOS) is modified to describe the phase behavior of fluid under confinement. Second, a new dimensionless parameter β is proposed to describe fluid adsorption in nanopores and its empirical correlation is given. Finally, based on the modified PR-EOS, a generalized analytical formula is proposed to calculate critical temperature and pressure shifts by considering fluid adsorption in the nanopores. The modified PR-EOS, new adsorption correlation, and generalized formulations have been validated to be accurate by comparing with the experimental and molecular simulation results. The modified PR-EOS can provide a consistent fluid phase behavior description over the macro-to nano-scale pore sizes.

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