Background

In terms of geological storage of CO2, caprocks are layers of low permeability rock that overlay the storage formation, ensuring that buoyant dense or vapour-phase CO2 does not leak into overlying strata and towards sensitive environmental receptors. Storage security, especially in early stages after the start of injection, is largely influenced by the caprock integrity.

Caprocks can be in the form of a laterally extensive thick single seal layers across the entire
formation, or part of a multilayered system, where permeable layers are interbedded with low
permeability layers. Multilayered systems may be more risky because small faults, which might escape seismic detection, can create a connected leakage pathway through fault offset and inter-connected permeable intervals. Low permeability intervals may consist of shales, salts such as anhydrites, coals, salts or other rock-types that normally prevent upward migration of CO2 at pressures below either fracturing pressure or capillary entry pressures.

Although the caprock may act as a seal, the lower boundary of the caprock will be in contact with CO2 saturated pore water or pore fluid consisting of pure CO2. Chemical interaction between the pore fluid and the caprock may change the material properties of the caprock. Therefore knowledge of the composition of the seal rock as well as the formation waters is important to gauge the geochemical properties and the minerals formed after CO2 injection. There may be coupling between CO2 and geochemical reactions that could be self-enhancing (with permeability increases due to dissolution) or self-limiting (with permeability decreases due to precipitation).

The sealing integrity of the caprock formations may be affected by faults, fractures and microfractures, which may exist already, or be caused or enhanced by the pressure changes upon injection of CO2. The magnitude of the pressure on the caprock can depend on the permeability, the location of the injector and whether the system is open or closed to displacement of brine. Understanding the geomechanical properties of the caprock, the change in stress state with CO2 injection, and identification and understanding of changes in material properties as a result of interaction with CO2 rich fluids are essential for the overall evaluation of its sealing capacity.

CO2CRC of Australia was commissioned by IEAGHG in March 2010 to provide a comprehensive review of caprock systems for CO2 storage, in terms of required properties for storage integrity and predictive modelling of performance.

Conclusions

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 analyses, 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 potential 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 DSF; effects of faults on caprock performance; and coupling of flow, geochemical and geomechanical effects 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.
 

Recommendations

IEAGHG should consider a follow up study on caprock systems, with the modelling network providing a suitable forum for the discussion of knowledge gaps identified. The modelling network can also be used to discuss the possible compilation of a caprocks database, one of the knowledge gaps identified.

A future review of the topic would be particularly useful as data is generated by future large scale demonstration projects.