Nanoscale Analysis of Shale Matrix Alteration after Supercritical CO2 Treatment: Implications for scCO2 Fracturing in Shales

Utilizing supercritical CO2 instead of water for shale gas stimulation has been proposed as a potential alternative. Understanding the scCO2–shale interaction during fracturing and its response to micromechanical as well as mineralogical properties of the rock is crucial to improve its application. To achieve this goal, dry scCO2 soaking experiments were carried out on two distinct shale samples (Eagleford and Mancos) for a 24 h duration. Multiple characterization techniques, including nanoindentation, scanning electron microscopy (SEM), energy-dispersive spectroscopy, and X-ray diffraction, were employed to understand the micromechanical and physical alterations to the two rocks caused by the short-term interaction with scCO2. The results of the nanoindentation indicate that the short-term interaction of scCO2 has a higher impact on the mechanical properties of Eagleford than the Mancos shale. This is due to the calcareous nature of the Eagleford rocks, composed of around 78% carbonate minerals. Nanoscale analysis of specific mineral-rich sites on the surface of both rocks reveals that the micromechanical response to the exposure of scCO2 is different for each mineral. The hierarchy of indentation modulus reduction is carbonate minerals, followed by clay, organic matter, and quartz. SEM images after CO2 exposure showed calcite etching along the calcite-accumulated areas, blurring of clay crystals in clay-dominated regions, and the generation and extension of the microcracks along the pre-existing fractures. Overall, the study's results suggest that dry scCO2 exposure can alter the properties of both Eagleford and Mancos shale even during short-term exposure (e.g., fracturing operation). The formation of microcracks and a reduction in strength could potentially aid scCO2 fracturing by reducing the pressure needed for rock breakdown and creating more defined microfractures along the primary fracture planes. This, together with the dissolution and etching of minerals, has the potential to enhance the rock's porosity and permeability, thereby improving the performance of scCO2 fracturing. The extent and characteristics of these modifications and their distribution within a rock will rely on heterogeneity and the arrangement of various minerals in it. Therefore, such interactions should be carefully considered along with the nature of the targeted shale formation when contemplating the use of CO2-based fracturing in shale reservoirs.