Carbon dioxide storage potential of shales Andreas Busch a,1, *, Sascha Alles a , Yves Gensterblum a , Dirk Prinz a , David N. Dewhurst b , Mark D. Raven c , Helge Stanjek d , Bernhard M. Krooss a a RWTH Aachen University, Institute of Geology and Geochemistry of Petroleum and Coal, Lochnerstr. 4-20, D-52056 Aachen, Germany b CSIRO Petroleum, Australian Resources Research Centre, Western Australia, Australia c CSIRO Land and Water, Urrbrae, South Australia, Australia d RWTH Aachen University, Clay and Interface Mineralogy, Aachen, Germany international journal of greenhouse gas control 2 (2008) 297–308 article info Article history: Received 15 June 2007 Received in revised form 14 February 2008 Accepted 7 March 2008 Published on line 24 April 2008 Keywords: CO 2 storage Cap rocks CO 2 sorption CO 2 diffusion Sealing integrity abstract Options for the geologic storage of carbon dioxide vary from saline aquifers and depleted oil and gas reservoirs to unminable coal seams and abandoned coal mines. Important aspects include the sealing integrity of the cap rock and potential changes in this integrity, owing to the interaction with CO 2 . In this study, diffusive transport and gas sorption experiments on one well characterised shale sample (Muderong Shale, Australia) and different clay minerals were performed to obtain information on the sealing integrity and the CO 2 storage potential of these materials. All measurements were performed under reservoir conditions relevant for CO 2 storage (T = 45–50 8C; p < 20 MPa). Repeat diffusion experiments on one shale plug yielded increased effective diffusion coefficients and a decrease in the concentration of the bulk CO 2 volume in the sample. The latter is believed to be dissolved in formation water, sorbed to mineral surfaces or involved with geochemical reactions. For the Muderong Shale, bulk volume CO 2 concentrations are greater within the experimental time frame (222–389 mol/m 3 ), when compared to coal and cemented sandstone (3–4 and 8–10 mol/m 3 ), respectively. This high CO 2 storage potential could not fully be explained by CO 2 dissolution in water alone. Thus, gas sorption experiments were performed on crushed shale and various clay minerals. High CO 2 sorption capacities (e.g. up to 1 mmol/g for the Muderong Shale) show that the high CO 2 concentration is related to a combination of CO 2 dissolution in water and gas sorption on clay minerals. Additionally, changes in specific surface areas before and after the sorption experiments and variations in the CO 2 sorption and diffusion behaviour due to repetitive experiments on the identical sample were observed, possibly related to geochemical alteration of the Muderong Shale and the clay minerals. These could not be quantified however and seemed to occur only at high pressures. Results obtained in this study provide a more positive view on the sealing integrity of intact cap rock formations. Carbon dioxide that migrates from a storage reservoir into the cap rock through the pore network will be immobilised to a certain extent, hence mini- mising (slow, diffusion-driven) leakage and providing additional CO 2 storage potential. # 2008 Elsevier Ltd. All rights reserved. * Corresponding author. Tel.: +49 241 8098293; fax: +49 241 8092152. E-mail address: busch@lek.rwth-aachen.de (A. Busch). 1 Present address: Shell International Exploration and Production B.V., Kessler Park 1, 2288 GS Rijswijk, The Netherlands. E-mail: busch_a@web.de available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/ijggc 1750-5836/$ – see front matter # 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijggc.2008.03.003