Containment of CO₂ in CCS: Role of Caprocks and Faults

The successful commercial scale deployment of carbon capture and storage (CCS) requires assurance of the confinement of the injected CO₂ at each potential storage site. The most critical elements of the confinement of CO2 are the caprock overlying the storage formation, and any faults or fractures which occur within the caprock.

The most significant aspect of containment is the seal potential of the caprock, defined as the seal capacity, geometry and integrity. The sealing capacity refers to the CO₂ column height that the caprock can retain before capillary forces allow the migration of the CO₂ through the caprock. Determination of capacity is achieved primarily through petrophysical analyses such as mercury injection capillary pressure (MICP) tests. For storage in depleted fields, assessments of seal capacity can be made from empirical observations of actual hydrocarbon column heights and converting these to CO₂ physical properties (density, temperature, pressure). Where these data sources are unavailable, the use of analogs (from known comparable caprocks) can be a viable alternative. The measured seal capacity from MICP data must, however, be tempered by the hydrodynamic environment above and below the seal which may modify the calculated seal capacity.

Seal geometry refers to the thickness and lateral extent of the caprock. The caprock must have sufficient lateral extent to cover whatever structural, stratigraphic or hydrodynamic storage reservoir in which the CO₂ is trapped. In addition, it must be thick enough to maintain an effective barrier despite faults through it. In other words, the seal thickness should exceed the throw of faults that cut it.

Seal geometry is evaluated through detailed stratigraphic and sedimentological analyses, wireline log data and seismic techniques, which are also required for baseline surveys prior to CO₂ injection.

Seal integrity refers to the geomechanical properties of the caprock. These properties are controlled by caprock mineralogy, regional and local stress fields as well as any stress changes induced by injection or withdrawal of water or CO₂. The modification of the stress field within a storage formation during and after injection of CO₂ can lead to reservoir and caprock mechanical failure. This failure can result in the generation of new faults and fractures, reactivation of existing faults and/or bedding parallel slip.. The key parameters determining whether faults might act as conduits or as seals are the juxtaposition relationships of rocks on either side of a fault plane, the material on the fault plane itself or the reactivation potential of the fault. The greatest likelihood of fluid migration up faults is during or immediately after reactivation. However, the mere existence of faults should not automatically prohibit geological storage of carbon dioxide. On the contrary, sealing faults commonly trap hydrocarbons and compartmentalize oil and gas reservoirs. Such sealing faults could also form suitable confining barriers at CO₂ storage sites.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California