EVALUATION OF POTENTIAL GEOLOGIC CARBON DIOXIDE SEQUESTRATION SITES: TRAP VALIDATION CONCEPTS FROM THE PETROLEUM INDUSTRY

Recent and proposed experiments designed to assess the feasibility of geologic sequestration of carbon dioxide are primarily concerned with determining if injected carbon dioxide will remain in place or migrate to an undesirable location. Application of routine trap and seal concepts and analyses used in the petroleum industry would substantially improve the feasibility study chance of success with less ambiguous outcomes.

Characteristics of successful hydrocarbon traps that are applicable to subsurface carbon dioxide storage include presence of a porous and permeable reservoir, a geometric trapping configuration of the reservoir, and a robust seal envelope (base, lateral, and top seal) that can maintain a buoyant fluid column over geologically significant periods of time. Identification of a geologic trap, which may comprise both stratigraphic and structural trapping elements, is the first step in the investigation. The second step is to analyze the geologic seal around a trap. Elements of seal to be considered include seal mechanical strength, seal capillary entry pressure, and fault leak. The mechanical capacity of a seal is compromised when the absolute pressure of the non-aqueous phase on the seal unit exceeds either its opening mode tensile strength or the minimum stress. In contrast, capillary seal failure occurs when the buoyant pressure of the non-aqueous phase exceeds the capillary entry pressure of the seal, a function of seal pore throat size, interfacial tension, and wetting properties. Fault seal for long time periods is primarily a matter of what rock type is juxtaposed with the reservoir rock across a fault.

The best forward prediction of carbon dioxide storage capacity over the long-term will result from full integration of fluid and bed seal properties, fracture gradients, and structural and stratigraphic models. Such a prediction requires simultaneous evaluation of all elements that control fill capacity. Confidence may be improved in petroliferous basins that have been “leak-tested” by hydrocarbons, or where other traps along a leak pathway will capture carbon dioxide escaped from the injection site. Implementation of this integrated approach will help identify safe long-term carbon dioxide storage sites and assist in the management of any commissioned site.