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

Abstract:

One method for mitigating the impacts of anthropogenic CO₂-related climate change is the sequestration of CO₂ in depleted gas and oil reservoirs, including shale. The accurate characterization of the heterogeneous material properties of shale, including pore volume, surface area, pore size distributions (PSDs) and composition is needed to understand the interaction of CO₂ with shale. Idealized powdered shale sorption isotherms were created by varying incremental amounts of four essential components by weight. The first two components, organic carbon and clay, 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 clay groups were represented by illite from the Green River shale formation. The rest of the idealized shale was composed of equal parts by weight of SiO₂ to represent quartz and CaCO₃ to represent carbonate components. Baltic, Eagle Ford, and Barnett shale sorption measurements were used to validate the idealized samples. The idealized and validation shale sorption isotherms were measured volumetrically using low pressure N₂ (77K) and CO₂ (273K) adsorbates on a Quantachrome Autosorb IQ₂. Gravimetric isotherms were also produced for a subset of these samples using CO₂ and CH₄adsorbates under subsurface temperature and pressure conditions using a Rubotherm magnetic suspension balance. Preliminary analyses were inconclusive in validating the idealized samples. This could be a result of conflicting reports of total organic carbon (TOC) content in each sample, a problem stemming from the heterogeneity of the samples and different techniques used for measuring TOC content. The TOC content of the validation samples (Eagle Ford and Barnett) was measured by Rock-Eval pyrolysis at Weatherford Laboratories, while the TOC content in the Baltic validation samples was determined by LECO TOC. Development of a uniform process for measuring TOC in the validation samples is underway. The validity of the idealized samples can be reevaluated upon obtaining reliable and consistent TOC measurements. Validated idealized sample data will lend itself toward the development of predictive models weighted and scaled by the percentages of these essential components in the shales’ corresponding compositional breakdown.