Phase transition and pore criticality in a natural nanoporous medium: adsorption-desorption and calorimetry experimentation and equation-of-state representation.

Adsorption-desorption and differential scanning calorimetry (DSC) experiments have been extensively used to investigate the phase behavior of pure fluids and fluid mixtures in synthetic adsorbents but their application to natural nanoporous media remains relatively unexplored. In this study, these complementary methods are extended for the first time to examine the phase behavior and criticality of propane confined in a natural nanoporous medium, Mowry Shale. Capillary evaporation conditions are obtained from both methods and agree closely, providing confidence in the measured transition behavior. In contrast to observations in synthetic nanoporous media, capillary evaporation still occurs above the bulk critical point and therefore the pore critical point (PCP) exceeds the bulk critical point. The three-line approach is applied to determine, for the first time, the PCP of propane confined in shale. Since the hysteresis end point cannot be determined in the present work, its potential impact on determining the exact location of PCP remains uncertain. These findings demonstrate both the applicability and the additional complexities of extending conventional confined phase behavior and criticality methods from well-defined synthetic nanopores to natural porous media. Using the experimentally determined PCP, the Peng-Robinson equation of state (EOS) is applied to the confined fluid system and accurately reproduces the whole capillary evaporation curve.