Time-Dependent Impact of CO2-Shale Interaction on CO2 Storage Potential

Abstract

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.

Keywords: Shale Rock, CO2 Storage, Gas Adsorption, Pore Characterization, Bakken Formation, fractal dimension, density functional theory