Modeling phase behavior of nano-confined fluids in shale reservoirs with a modified Soave-Redlich-Kwong equation of state

Abstract

• A m-SRK EOS for modeling the phase behavior of nano-confined fluids is proposed. • A more accurate empirical formula for critical temperature shift is developed. • Phase transformation of nano-confined fluid is revealed by nano-confined index. • There is a critical pore radius around 2 nm that phase behavior deviates sharply. The conventional cubic equation of state (EOS), as a robust and efficient method for phase behavior prediction of bulk fluids, is no longer applicable for nano-confined fluids because of the fluid phase behavior deviation induced by the confinement effect in shale reservoirs. In this work, a modified Soave-Redlich-Kwong (m-SRK) EOS is proposed for modeling the fluid phase behavior in nanopores. Three parameters, including the fluid distribution coefficient, relative effective molecular volume coefficient, and nano-confined index, are defined to revise the molar volume term in the SRK EOS. Then, the empirical formulas of adsorption layer thickness and critical temperature shift are established by correlating the data points collected from the literature to solve the m-SRK EOS. Finally, two analytical formulas are derived for calculating critical property shifts. Computational accuracy of critical temperature shift in this work is remarkably improved for the dimensionless pore radius more than 0.2 compared with analytic models proposed by other scholars. Besides, the nano-confined index can comprehensively reflect the phase transformation law of nano-confined fluids. Modifying the molar volume term in EOSs is crucial for the phase behavior prediction when the pore radius is less than 100 nm, especially for the pores less than 10 nm. There is a critical pore radius around 2 nm that the molar volume of nano-confined fluids changes sharply. When the pore radius equal 1 nm, the descend range of the bubble point pressure increases sharply as the temperature increases compared with that of bulk fluids.