Equation of state for nanopores and shale: Pore size–dependent acentric factor

Abstract

Nanopores pose a challenge to phase-behavior modeling when using the equation of state (EOS), and common methods do not capture their vapor pressure or become difficult to implement because they require additional tuning parameters. This study focuses on improving the vapor-pressure prediction of EOS in nanopores by proposing a pore size–dependent acentric factor (ACF). It determines the ACF from experimental data and implements it in EOS along with critical pressure and temperature. The proposed approach is then applied to the vapor-pressure measurements of nitrogen, argon, oxygen, methane, and ethane in pores ranging from 3.5 nm to 8.1 nm. The results show that the ACF in nanopores increases as the pore size decreases, and the degree of size dependency varies across different pure components. The results also demonstrate that the proposed approach enables EOS predictions to match the measurements with good accuracy. This study quantifies the effects of pore size on the ACF for the first time and presents simple correlations for estimating the ACF when the pore radius is smaller than 10 nm. The proposed approach simplifies the application of EOS in nanopores and unconventional hydrocarbon reservoirs.