Assessment of caprock systems will be highly site-specific and rely on a multi-disciplinary approach, utilising a combination of seismic surveys, exploration wells, wireline log data, stratigraphic and sedimentological analysis, well tests and laboratory scale testing of caprock samples.
The study has presented a qualitative methodology for assessment of seal potential at the basin scale. The seal capacity of a caprock system may be defined as the capacity, geometry and integrity of the caprock. Seal capacity refers to the maximum CO2 column height that can be retained in the underlying reservoir, before pressure exerted by buoyancy exceeds capillary entry pressure, thus allowing CO2 to migrate through the caprock. Seal geometry refers to the thickness and lateral extent of the caprock. Seal integrity refers to caprock geomechanical properties, in the context of ambient stress fields that may be modified by CO2 injection and any associated abstraction of reservoir fluids.
Key knowledge gaps identified for further research include: wettability and interfacial tension effects on supercritical CO2-water-rock systems: hydrodynamic effects of large scale injection in deep saline formations; effects of faults on caprocks in predictive modelling. There is also a case for the compilation of a comprehensive database on caprock systems, including mineralogical and petrophysical properties, to provide analogue data in storage site assessment. A compendium of caprock properties at existing CO2 storage sites would also prove useful.
