Surface wettability alteration of shales exposed to CO₂: Implication for long-term integrity of geological storage sites

Surface wettability is a key factor controlling the CO₂ seal capacity and defines the CO₂ storage potential. Limited studies have addressed the shale/water wettability behavior during CO₂ injection, thus considerable attention is needed to understand this concept. In this paper, an ample number of supercritical CO₂ exposure experiments were conducted to evaluate the alteration of shale/water contact angles. Different types of shales with various mineralogy from Eagle Ford, Wolfcamp, and Mancos fields, were exposed to SCCO₂ at different durations, pressures, and temperatures. Shale mineralogy and surface were analyzed using X-ray diffraction and scanning electron microscope. The results indicated a strong relationship between mineral composition and the alteration in shale/water wettability. Clay-rich shales displayed a possible turn in wetting behavior to CO₂-wet with extending the SCCO₂ treatment time and increasing the treatment pressure, caused by SCCO₂ dissolution of clay and carbonate minerals. While the wettability of high-quartz contents shales remained strongly hydrophilic after various SCCO₂ treatment conditions. Increasing the temperature accelerated the CO₂/shale interactions, and a minor effect was observed on the shale hydrophilicity. Increasing the cohesive energy density of CO₂ promotes a favorable CO₂ wetting environment, which reduced the hydrophilicity of the surface and reduces the surface energy. In conclusion, shales with high quartz contents exhibit strong water wetting behavior after SCCO₂ treatment, which leads to better sealing capacity, more efficient integrity of geological storage sites, and higher potential for CO₂ containment.