Time-dependent impact of CO₂-shale interaction on CO₂ storage potential

The assessment and implementation of CO₂ storage projects require a critical study of CO₂-shale interaction and its potential effect on pore morphology and fractal characteristics. In this study, we investigated the temporal effect of CO₂-shale interaction on the mineralogy, pore morphology, and fractal characteristics, and its impact on long-term CO₂ storage potential. In our analyses, shale samples from upper Bakken (UB) and lower Bakken (LB) were exposed to CO₂ for 60 days. We conducted different investigations to gain insight into changes in the pore structure pre- and post-CO₂ exposure, which includes organic chemistry, mineralogy by X-ray diffraction (XRD), and gas adsorption. Fractal dimension was obtained from the gas adsorption data using the fractal Frenkel-Halsey-Hill (FHH) model. The relationship between mineral composition, pore size, total organic carbon (TOC) content, and fractal dimension was assessed. Our pre- and post-CO₂ exposure results also indicated an increase in the calcite contents of UB and LB samples, whereas the quartz content increased in UB (+87%) and decreased in LB (-21%). Further, the pre- and post-CO₂ exposure clay content analyses showed a higher decrease in UB (-75%) than LB (-7%). The changes in the mineral composition may be attributed to the dissolution of pre-CO₂ exposure minerals and precipitation of new or more minerals post-CO₂ exposure. An increase in post-CO₂ exposure fractal dimension in both UB and LB indicated that the pore morphology can become more complex, rougher, irregular, and heterogenous after CO₂ exposure. The pre- and post-CO₂ exposure analyses of our results showed that UB had a smaller pore size with a higher fractal dimension and higher adsorption capacity, whereas LB had a larger pore size with a lower fractal dimension and lower adsorption capacity. In post-CO₂ exposed UB, the increased calcite and quartz contents with a significant decrease in clay content, relatively smaller pore size and higher adsorption capacity in comparison to the LB suggests that UB can provide a greater long-term CO₂ storage potential.