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 (kabs(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