Evaluating Seal Capacity of Cap Rocks and Intraformational Barriers for CO 2Containment

Abstract

The petrophysical properties of cap rocks and intraformational barriers can constrain the CO 2containment volumes of potential geosequestration sites. Characterization of regional seals and intraformational barriers requires an understanding of the seal capacity of the cap rock or barrier. Seal capacity is the capillary pressure (or column height) 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 high threshold pressures, together with wettability and interfacial tension (IFT) properties, determine the final column height that a seal can hold. A review is presented on the function of wettability and IFT in the geological storage of CO 2and its effect on seal capacity (CO 2column height) with respect to capillary pressure, the potential for the movement of CO 2through 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 CO 2, this technology can be applied to establish the suitability of a top seal for containment of CO 2; however, the function of IFT and wettability in the CO 2-water-rock systems is not well understood. How supercritical CO 2(scCO 2) affects these two properties is unclear, especially as the waterfront becomes saturated with scCO 2and may eventually become miscible with the scCO 2at reservoir conditions.

To date, literature shows that the wettability and IFT of the CO 2-water-rock system may be more significant than in the hydrocarbon-water-rock systems and that calculated CO 2column heights based on nonwetting assumptions could result in column heights being as much as 50% less than otherwise predicted.