A Review of the Study of Fluid Phase Behavior at the Confined Scale of Shale Reservoirs: A Theoretical and Experimental Perspective

The fluids within the pores of shale reservoirs are influenced by nanoscale confinement effects, causing the phase behavior and flow characteristics of the oil and gas systems to deviate from bulk properties. Consequently, conventional phase theory and experimental methodologies are inadequate for studying such confined fluids. The phase characteristics and underlying microscopic mechanisms of oil and gas within confined spaces remain unclear, posing a substantial barrier to the efficient and rational development of shale reservoirs. This paper reviews current theoretical and experimental research on fluid phase behavior under nanoscale confinement in shale reservoirs. It provides a comprehensive discussion of four categories of research techniques, including mathematical models, numerical simulations, indirect measurement experiments, and direct observation experiments, detailing their principles, primary applications, and advantages and limitations. Furthermore, it compares the relationships and differences among these techniques and offers an outlook on the future development of research into the shale fluid phase behavior. Analyses indicate that various research methods can be employed to investigate the phase behavior of fluids at the confined scale of shale reservoirs. However, due to the influence of factors such as research techniques, target materials, and experimental conditions, there is no consensus on the critical pore size responsible for confinement effects, the shift in the critical properties, and the variations in bubble and dew points. The current research adopts the research idea of "theoretical exploration through mathematical modeling, mechanism revelation via numerical simulation, regularity reflection through indirect measurement experiments, and phenomenon demonstration through direct observation experiments" and forms a relatively complete research system of the confined fluid phase behavior of shale reservoir. However, the system still has some limitations and challenges, necessitating further optimization and refinement in future research.