Carbon dioxide storage potential of shales

Options for the geologic storage of carbon dioxide vary from saline aquifers and depleted oil and gas reservoirs to unminable coal seams and abandoned coal mines. Important aspects include the sealing integrity of the cap rock and potential changes in this integrity, owing to the interaction with CO₂. In this study, diffusive transport and gas sorption experiments on one well characterised shale sample (Muderong Shale, Australia) and different clay minerals were performed to obtain information on the sealing integrity and the CO₂ storage potential of these materials. All measurements were performed under reservoir conditions relevant for CO₂ storage (T = 45-50 °C; p < 20 MPa). Repeat diffusion experiments on one shale plug yielded increased effective diffusion coefficients and a decrease in the concentration of the bulk CO₂ volume in the sample. The latter is believed to be dissolved in formation water, sorbed to mineral surfaces or involved with geochemical reactions. For the Muderong Shale, bulk volume CO₂ concentrations are greater within the experimental time frame (222-389 mol/m³), when compared to coal and cemented sandstone (3-4 and 8-10 mol/m³), respectively. This high CO₂ storage potential could not fully be explained by CO₂ dissolution in water alone. Thus, gas sorption experiments were performed on crushed shale and various clay minerals. High CO₂ sorption capacities (e.g. up to 1 mmol/g for the Muderong Shale) show that the high CO₂ concentration is related to a combination of CO₂ dissolution in water and gas sorption on clay minerals. Additionally, changes in specific surface areas before and after the sorption experiments and variations in the CO₂ sorption and diffusion behaviour due to repetitive experiments on the identical sample were observed, possibly related to geochemical alteration of the Muderong Shale and the clay minerals. These could not be quantified however and seemed to occur only at high pressures. Results obtained in this study provide a more positive view on the sealing integrity of intact cap rock formations. Carbon dioxide that migrates from a storage reservoir into the cap rock through the pore network will be immobilised to a certain extent, hence minimising (slow, diffusion-driven) leakage and providing additional CO₂ storage potential.