Fluid phase behavior under shale nanoconfinement effect: Advances and challenges from molecular simulation

The phase behavior of fluids under nanoconfinement in shale reservoirs holds significant theoretical implications for development and utilization of underground energy, yet remains elusive due to the complex interplay of fluid-rock interactions at the nanoscale. Molecular simulation has emerged as an indispensable tool to decipher these microscopic processes. While the existing literature has explored various aspects of confined fluid phase behavior by molecular simulation, a systematic review is still lacking that focuses specifically on advances and challenges in thermodynamic shift and phase transition behavior. This review comprehensively examines the evolution of molecular models and simulation methodologies tailored for shale nanoconfined environments. It then critically analyzes the key factors driving thermodynamic shifts and elucidates the underlying microscopic mechanisms governing phase behavior of confined fluids. Furthermore, the review synthesizes recent advances in understanding complex phase transitions under confinement, with a focus on capillary condensation, hydrocarbon retrograde condensation, and miscibility behavior. Persistent challenges, such as the accurate prediction of thermodynamic properties and the characterization of phase transition behavior, are rigorously evaluated, with potential pathways forward proposed. By linking molecular-scale insights to reservoir-scale applications, this work aims to provide foundational guidance for the utilization and development of shale nano subsurface energy and to outline future research directions in phase behavior of confined fluids.