Sealing rock characteristics under the influence of CO₂

To ensure the long-term safety of geological CO₂ storage sites, such as saline aquifers or depleted gas- and oilfields, the overburden must be able to effectively retain in place the CO₂ (either gaseous, supercritical or in dissolved state). In this study the caprock sealing efficiency and potential petrophysical and mineralogical changes of caprock integrity due to CO₂ exposure are being investigated. Analysis techniques include XRD for mineralogy and N₂-BET for specific surface determination, but also high-pressure CO₂ sorption and fluid flow experiments to study the sorption (retardation) and transport/capillary sealing characteristics of argillaceous caprocks. As this study is of generic nature, argillaceous samples and one marl-/limestone have been selected from different locations, covering the scope from poorly consolidated clays to highly compacted shale/siltstones. The first results indicate that, except for the very heterogeneous marl-/limestone (water permeability values of approximately 10^-18 m²), all samples have very good to excellent sealing properties. Absolute water permeability values are in the nDarcy (10 ^-9 Darcy) to sub-nDarcy range (k_abs(water) = 10 ^-21 m²) and the capillary breakthrough experiments indicate that the clay-rich samples can retain the supercritical CO₂ phase up to capillary pressures of at least 10 MPa. Even though samples are acting as effective capillary seals up to 10 MPa, a very small CO₂ flux could be detected, which is interpreted to be due to CO₂ diffusion through the rock sample. Sorption measurements indicate the maximum CO₂ sorption capacity to vary between 0.25 and 0.63 mmol/g. This is significant and therefore sorption in thick argillaceous caprock layers may provide an important sink for CO₂ leaking from underlying storage reservoirs.