Assessing the adsorption properties of mudrocks for CO2 sequestration

摘要

The inherent complexity of microporous rocks together with their relatively low adsorption capacity as compared to commercial adsorbents represent a new scientific and technical challenge in the study of adsorption at supercritical conditions. In this paper, CO2 adsorption isotherms are analyzed that have been measured on various microporous samples, thus including natural adsorbents, such as shales and clay minerals, and engineered materials, such as zeolites. The analysis focuses on the estimation of the density of adsorbed CO2, a parameter that is key to quantify the storage capacity of a given material. It is shown that the latter is significantly overestimated when the volume of the adsorbed phase is neglected, this being unfortunately a common practice in the assessment of the storage potential of geologic formations. At the same time, estimates of the density (or volume) of the adsorbed fluid are largely uncertain with values ranging between the critical and the solid density of the adsorbed even when the same experimental conditions are considered. More data at representative conditions are needed to assess the real contribution of adsorption to the storage capacity of shales. Simultaneously, experimental protocols need to be refined to quantify high-pressure adsorption in microporous solids, thus including the characterization of structural parameters, such as the porosity and the (inaccessible) skeletal volume.