Rock-fluid interaction and phase properties of fluids in nano- and subnano-pores of shales: sorption-based studies

Abstract

Sorption-based methodologies are proposed and developed to study rock-fluid interactions and properties of the fluid-phase in organic-rich shale reservoirs. Lack of appropriate methods to study these attributes of shale reservoirs affects the efficiency and economy of the shale-based exploration and production (E&P) efforts. A macroscopic conception of fluids still guides most existing methods for studying rock-fluid interaction and fluid phase properties. However, the modified regime of surface forces in fluids confined within nanometer and sub-nanometer sized pores typical of shales render such a macroscopic treatment fundamentally inconsistent. Apart from these theoretical limitations, shales are operationally challenging for the existing methods for rock-fluid interaction studies, primarily due to their ultra-low permeability, compositional heterogeneity, and the presence of organic matter and swelling clay minerals. Therefore, I propose using sorption-based methods that are sensitive to the modified regime of the surface forces in nano- and sub-nano-pores to study rock-fluid interaction and fluid-phase properties in shales The Nitrogen adsorption method that is commonly used to study pore-structures was improvised in this thesis. In addition to nitrogen, water and hexane vapors were used to study rock-fluid interactions in organic-rich shales, which helped in quantifying the surface areas of the hydrophilic and hydrophobic pores of shales. In another study, the role of hydrophilic and hydrophobic pores in supercritical CO2 sorption was further investigated by measuring supercritical CO2 sorption isotherms for illite clay and organic-rich shale samples in dry and in water-imbibed conditions. In a separate study, ultrasonic p-wave measurements during sorption experiments allowed a determination of the phase properties of fluids confined in the nano- and sub-nanometer sized pores. BET specific surface areas (SSA) determined from the isotherms of water and hexane vapors in organic-rich shale and siltstone samples suggest that hexane vapor measures pores in both clay and in organic matter (OM) while water vapor selectively probes only clay-hosted pores. Thus, OM pores, which are not accessed by water vapor adsorption, are concluded to be hydrophobic. Nitrogen adsorption underestimates porosity in the organic-rich shales due to the kinetic-restriction faced by nitrogen in the cryogenic test conditions. The OM pores in the organic-rich shale samples retained their sorption capacity after water-imbibition. On the other hand, illite clay pores lost most of their supercritical CO2 sorption capacity in the presence of water. The diffusion of dissolved CO2 in water and its subsequent sorption in the OM pores suggests that dissolved gases can still be sorbed. As a consequence, rock-fluid interaction in nano- and sub-nanometer sized pores of shales may potentially alter the PVT properties of multi-component hydrocarbon liquids. The deformational and flow properties of confined undersaturated condensates (CUC) or the adsorbed phase of water and hexane in the nano- and sub-nanometer sized pores of various clay minerals were thus characterized and found to have liquid-like properties. However, the cation-hydrating CUC of water had unusual phase properties, as it was found to increase the overall p-wave stiffness of the clay aggregates. The sorption-based methods developed in the thesis for studying the rock-fluid interaction and fluid properties of shales are shown to be theoretically consistent and appear operationally more viable than the existing methods for rock-fluid interaction studies. Therefore, the proposed methods may have wider implications in the rock-physical and reservoir engineering studies of shales.