Evaluating seal capacity of caprocks and intraformational barriers for the geosequestration of CO2

Abstract

The petrophysical properties of cap rocks and intraformational barriers can constrain the carbon dioxide (CO2) containment volumes of potential geosequestration sites. Characterisation 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 fl uid commences to leak through a seal rock. Seal rocks are effective due to very fi ne pore and pore-throat sizes that result in low porosities and permeabilities. These in turn generate high capillary threshold pressures. High threshold pressures, together with wettability and interfacial tension (IFT) properties determine the fi nal column height that a seal can hold. Wettability and IFT play an important role in the geological storage of CO2 through their effect on seal capacity (CO2 column height) with respect to capillary pressure, thereby controlling the potential for the movement of CO2 through the seal and affecting ultimate 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 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) affects these two properties, particularly as the water front becomes saturated with scCO2 and eventually becomes miscible with the scCO2 at high pressure reservoir conditions. Selected examples of top and intraformational seals from the Bowen, Otway, Gippsland and Cooper basins are discussed in light of new experimental evidence on wettability and IFT variations in the CO2-water-rock system. These variations may be more signifi cant than in hydrocarbon-water-rock systems and based on non-wetting assumptions, the calculated CO2 column heights may be signifi cantly different than previously predicted.