The effects of confinement and wettability on the phase behavior of pure and multicomponent fluids in nanoporous media

The phase behavior of fluids stored in nanopores deviates significantly from bulk behavior dueto intense molecular interactions in confined spaces. Understanding these confinement-induced effects isessential for applications such as carbon capture using nanoporous materials, gas storage, and enhancedhydrocarbon recovery in unconventional reservoirs. While most geological formations containmulticomponent fluid systems, existing studies typically focus on the phase behavior of single-componentfluids. As a result, the phase behavior of mixtures under confinement remains poorly understood. Thisknowledge gap limits the evaluation of fluid storage in nanoporous media, particularly storing fluid mixturesin materials with varying wettability. In such systems, fluid-solid interactions can lead to distinct phasebehavior in mixtures compared to pure fluids. The present study addresses this gap by investigating thethermodynamic behavior of pure carbon dioxide at 10 °C and an n-butane/n-hexane binary mixture (90mol% n-butane) at 10, 20, and 30 °C in Mobil Composition of Matter No. 41 (MCM-41) nanoporousmaterials. Adsorption and desorption isotherms were measured in hydrophilic and hydrophobic MCM-41samples with a pore size of 8 nm using a patented nano-condensation apparatus. The results show that theadsorption behavior of the fluid mixture closely resembles that of pure carbon dioxide, with higher storagecapacities in hydrophilic samples. However, desorption behavior, critical for hydrocarbon production andfluid withdrawal from storage systems, diverged significantly. Unlike carbon dioxide, desorption isothermsof the n-butane/n-hexane mixture did not follow the same trajectory as adsorption. Consequently, capillaryevaporation pressures of the mixture could not be identified from the measured isotherms, indicating adistinct confinement-induced phase transition characteristic of multicomponent systems. Additionally,selective adsorption and desorption were found to be composition-driven rather than wettability-driven.These findings underscore the complexities of fluid storage and withdrawal in nanoporous media, offeringvaluable insights into optimizing storage applications in unconventional formations.