Characterization of multicomponent-hydrocarbon phase behaviors in nanopores with a modified simplified local-density model

The process of gas and oil production is significantly influenced by the phase behaviors of hydrocarbons in the reservoir. Due to confinement effects, phase behaviors in nanopores deviate from those of the bulk fluid, making conventional equations of state (EOSs) inapplicable to shale gas and oil reservoirs. Various methods have been proposed to characterize the complex phase behavior and investigate the confinement effects in nanopores. In this paper, the applicable conditions of different methods are discussed, including the modified EOSs, molecular simulations, density functional theory, and the simplified local-density (SLD) method. The SLD method has advantages in terms of modeling the non-uniform fluid distributions in nanopores for multicomponent systems. However, thermodynamic inconsistencies in the multicomponent SLD derivation from prior studies limit its broader application. A modified multicomponent SLD model is developed to characterize the confined fluid phase behaviors for complex hydrocarbon systems. With the introduction of position-dependent pressure in the nanopore, the modified model overcomes the consistency problem in previous works. Nanoscale visualization experimental results from the literature are employed to validate the modified model. Phase diagrams for nanopores of varying sizes are computed for a binary mixture and two real shale oil samples containing up to eight components. Accordingly, both single- and multicomponent hydrocarbon phase behaviors in nanopores are investigated. This study provides insights into the development of shale oil reservoirs.