Idealized Shale Sorption Isotherm Measurements to Determine Pore Capacity, Pore Size Distribution, and Surface Area

One potential method for mitigating the impacts of anthropogenic CO2-related climate change is the sequestration of CO2 in depleted geological gas and oil formations, including shale. The accurate characterization of the heterogeneous material properties of shale, including pore capacity, surface area, pore-size distributions, and composition is needed to understand the potential storage capacities of shale formations. Powdered idealized shale samples were created to explore reduction of the complications in characterization of pore capacity that arise from the heterogeneous rock composition and pore sizes ranging over multiple orders of magnitude. The idealized shales were created by mechanically mixing incremental amounts of four essential powdered components by weight and characterized with low pressure gas adsorption/desorption isotherms. The first two components, organic carbon and phyllosilicates (such as clays, micas, and chlorite), have been shown to be the most important components for CO2 uptake in shales. Organic carbon was represented by kerogen isolated from a Silurian shale, and phyllosilicate groups were represented by powdered illite from the Green River shale formation. The remainder of the idealized shale was composed of equal parts by weight of SiO2 to represent quartz and CaCO3 to represent carbonate components. Three idealized sample groups were prepared to be approximately 10, 30, and 55% illite by weight. Each of the sample groups consisted of four samples, incrementing the percent kerogen from 1.5 to 6%. Eagle Ford, Baltic, and Barnett shale sorption measurements were used to validate the idealized sample methodology. The sorption isotherms were measured volumetrically using low pressure N-2 (77 K) and Ar (87 K) adsorbates on Quantachrome Autosorb IQ2. Both idealized and validation samples were outgassed using a standardized procedure that produced repeatable results while minimizing changes to the material properties of the shale. The idealized sample results indicated a positive linear correlation with increasing total organic carbon and pore capacity. This work is essential toward the development of predictive models weighted and scaled by the corresponding mineral compositional description of the reservoir.