Sealing rock characteristics under the influence of CO2

Abstract

Abstract To ensure the long-term safety of geological CO 2 storage sites, such as saline aquifers or depleted gas- and oilfields, the overburden must be able to effectively retain in place the CO 2 (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 CO 2 exposure are being investigated. Analysis techniques include XRD for mineralogy and N 2 -BET for specific surface determination, but also high-pressure CO 2 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 m 2 ), 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 m 2 ) and the capillary breakthrough experiments indicate that the clay-rich samples can retain the supercritical CO 2 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 CO 2 flux could be detected, which is interpreted to be due to CO 2 diffusion through the rock sample. Sorption measurements indicate the maximum CO 2 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 CO 2 leaking from underlying storage reservoirs.