A review on the effect of confinement on phase behavior in tight formations

Unconventional resources, including tight oil and gas formations and shale plays, have become a vital source of energy all over the world. The unique characteristic of these reservoirs, which has made their development challenging, is the low-to ultra-low-permeability due to the abundance of nano-scale pores. In these tiny pores and pore-throats, phase behavior and saturation pressures of the confined fluid are shifted from those of the bulk fluid within larger pores of the conventional medium-to high-permeability reservoirs. During the past few decades, many scholars attempted to compare this alteration in fluid phase behavior inside the tiny pores of tight formations to that of the bulk by studying the fundamentals behind this behavior through mathematical models, simulations, and experimental studies. Reduced pore size and pore structure, mineralogy, adsorption, and capillary condensation phenomena have been addressed in different studies as the source of this deviation in properties. Attempts to model fluid phase behavior in narrow pores started by applying the knowledge of classical thermodynamics in molecular simulations of the confined fluid, which has an inhomogeneous distribution inside the narrow pores. This was followed by modifying different types of equations of state to capture the difference in saturation pressures and temperatures of the confined fluid and that of the bulk. Experimental efforts in this area cover a wide range of non-visual approaches and precise visual approaches using the lab-on-a-chip techniques. However, conducting experiments at the nano-scale, specifically less than 100 nm, is rare due to many experimental limitations. This review provides a comprehensive summary of the theoretical and experimental studies in this area, highlights the advantages and disadvantages of each method, and indicates a lack of data at the challenging range of pore scales, less than 10 nm, for simulation validation purposes.