Three-Phase Equilibrium of Hydrocarbon Mixtures in Nanopores: A Volume-Temperature Flash Calculation Approach

The thermodynamic properties and phase behaviors of nanoconfined fluids are crucial for accurately simulating fluid transport in shale nanopores. However, the commonly used cubic equations of state (EOS) with critical parameter shifts fail to account for fluid/pore wall interactions, leading to inaccuracies in predicting fluid properties within nanopores. More precise ab initio methodologies like density functional theory and molecular simulation demand extensive computational resources, limiting their practical application in large-scale reservoir simulations. Therefore, we aim to develop an efficient and accurate method based on a modified EOS to calculate the phase behaviors of fluids confined in nanopores. A fast and robust VT-flash calculation algorithm is proposed to determine the three-phase behavior of fluids in nanopores using a modified EOS that accounts for fluid-solid interactions. The algorithm is based on directly minimizing the tangent plane distance or Helmholtz free energy using the Trust-Region modified Newton iteration method. The developed model is applied to investigate the fluid phase behavior and thermodynamic properties of four systems with increasing complexity. The results show that the Trust-Region modified Newton method is both fast and robust in VT-flash calculations, with the converging for all initial guesses considered in this study. In a single-component adsorption system, fluid–solid interactions can cause the sorbed phase density to be substantially higher than that of the bulk phase. This leads to a general spreading pressure for the sorbed phase that is much greater than that of the bulk phase. Consequently, the bulk phase pressure is not suitable as a direct input parameter for calculating fluid phase behavior and thermodynamic properties using PT-flash calculations. The calculated phase envelopes illustrate that three-phase regions tend to appear in single-component, binary-component, and more complex mixture systems in a low-temperature region. The bubble-point-like, dew-point-like, and near-pure component initial mole fraction must be considered to perform three-phase stability analysis accurately. This study provides a three-phase equilibrium calculation of fluid confined within nanopores, involving the sorbed phase, liquid phase, and vapor phase. The thermodynamic and transport properties of fluids in nanopores can be accurately and efficiently calculated, offering the potential to integrate with compositional simulation software for field-scale reservoir simulation.