Containment of CO₂ in CCS: Role of caprocks and faults

Abstract

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 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, seal geometry and seal integrity. The seal capacity refers to the CO2 column height that the caprock can retain before capillary forces allow the migration of the CO2 into and possibly 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 can be a viable alternative. 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, its thickness should exceed the throw of any faults that cut so as to maintain an effective barrier despite faults through it. Seal geometry is evaluated through well data (stratigraphic, sedimentological and wireline log analyses) and seismic surveys, which are pre-requisites to any viable storage project. 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 properties of the fault zone itself or the reactivation potential of the fault. The greatest likelihood of fluid migration up faults is during or immediately after reactivation. Thus, the mere existence of faults does not automatically rule out a site for geological storage of carbon dioxide. On the contrary, sealing faults commonly trap hydrocarbons and compartmentalize oil and gas reservoirs and could also form suitable confining barriers at CO2 storage sites. Seal capacity, geometry and integrity interpretations must be tempered by the potential geochemical reactions between fluids and rocks and injected CO2 as well as by the hydrodynamic environment above and below the seal which may modify the calculated pressure regimes.