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Abstract

Daniel, R. F., and J. G. Kaldi, 2009, Evaluating seal capacity of cap rocks and intraformational barriers for CO2 containment, in M. Grobe, J. C. Pashin, and R. L. Dodge, eds., Carbon dioxide sequestration in geological media—State of the science: AAPG Studies in Geology 59, p. 335345.

DOI:10.1306/13171247St59227

Copyright copy2009 by The American Association of Petroleum Geologists.

Evaluating Seal Capacity of Cap Rocks and Intraformational Barriers for CO2 Containment

Richard F. Daniel1, John G. Kaldi2

1Cooperative Research Center for Greenhouse Gas Technologies (CO2CRC), Australia; Present address: Australian School of Petroleum, University of Adelaide, Adelaide, Australia
2Cooperative Research Center for Greenhouse Gas Technologies (CO2CRC), Australia; Present address: Australian School of Petroleum, University of Adelaide, Adelaide, Australia

ACKNOWLEDGMENTS

The authors wish to acknowledge the support of the Cooperative Research Center for Greenhouse Gas Technologies (CO2CRC), Australia, and fruitful discussions on CO2 storage with fellow researchers Catherine Gibson-Poole, Maxwell Watson, and Jonathan Ennis-King from the CO2CRC research group. The manuscript was also enhanced by the constructive comments of Peter D'Onfro, who reviewed this chapter.

ABSTRACT

The petrophysical properties of cap rocks and intraformational barriers can constrain the CO2 containment 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 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 CO2, this technology can be applied to establish the suitability of a top seal for containment of CO2; however, the function of IFT and wettability in the CO2-water-rock systems is not well understood. How supercritical CO2 (scCO2) affects these two properties is unclear, especially as the waterfront becomes saturated with scCO2 and may eventually become miscible with the scCO2 at reservoir conditions.

To date, literature shows that the 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 nonwetting assumptions could result in column heights being as much as 50% less than otherwise predicted.

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