Evaluating Seal Capacity of Caprocks and Intraformational Barriers for Carbon Dioxide Containment

Richard (Ric) Daniel and John Kaldi
Cooperative Research Centre for Greenhouse Technologies, at the Australian School of Petroleum, The University of Adelaide, Adelaide, SA, Australia

The petrophysical properties of cap rocks and intraformational barriers can constrain the CO₂ containment volumes of potential geosequestration sites and requires an understanding of the seal capacity, which is the capillary pressure at which a trapped fluid commences to leak or move through a seal rock. Seal rocks are effective because of very fine pore and pore-throat sizes that result in low porosities and permeabilities, which in turn generate high capillary threshold pressures. These pressures, together with wettability and interfacial tension (IFT) properties, determine the final column height that a seal can support. A review is presented on the role of wettability and IFT in the geological storage of CO₂ and its effect on seal capacity (CO₂ column height) with respect to capillary pressure, the potential for the movement of CO₂ through the seal and the effect on reservoir storage volumes.

Mercury injection capillary pressure analysis has been used extensively in the petroleum industry to determine the effectiveness of the top seal in relation to hydrocarbon column height retention. With the burgeoning interest in geological storage of carbon dioxide (CO₂), this technology can be applied to establish the suitability of a top seal for containment of CO₂; however, the role of IFT and wettability in the CO₂-water-rock systems is not well understood. It is unclear how supercritical CO₂ (scCO₂) effects these two properties, especially as the water front becomes saturated with scCO₂ and eventually becomes miscible with the scCO₂ at reservoir conditions.

Literature-to-date shows that wettability and IFT of the CO₂-water-rock system may be more significant than in the hydrocarbon-water-rock systems and that calculated CO₂ column heights based on non-wetting assumptions could result in column heights being as much as 50% less than otherwise predicted.

AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery