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Abstract 
In geological storage of CO2 there are four main trapping mechanisms; structural, dissolution, mineral and residual trapping. While structural and dissolution trapping depend on the large scale structure of the reservoir, both residual and mineral trapping are strongly influenced by geometry at the pore scale. The dominant mechanism for residual capillary trapping is snap off which depends largely on the interconnectivity of the pore space and ratio of the pore throats to pore bodies. Mineral trapping depends largely on the surface area and mineralogy of the rock. Digital core analysis makes it possible to directly enumerate the detailed pore and mineral phase structure of core material in 3D. Flooding experiments enable the amount and microscopic distribution of residual fluid phases to be directly imaged in 3D. In this paper the pore scale properties associated with mineral and residual capillary trapping are directly enumerated using core material from the CO2CRC Otway CO2 storage project in Australia. Surface areas associated with varying mineral phases are quantified. 3D pore network characteristics are derived from the image data which allow the quantification of the pore network structure (e.g., interconnectivity) and the pore geometry (pore/throat size, pore shape, aspect ratio). Heterogeneity of the permeability and anisotropy are calculated directly. The amount of residual non-wetting phase of analog fluid pairs is directly measured on Otway core material after flooding at the pore scale in 3D. The microscopic distribution of the residual non-wetting phase within the pore space is quantified and individual pore occupancies defined. This information will provide a better understanding of recovery mechanisms and assist in the design and implementation of CO2 flooding processes. Comparison of individual pore occupancies based on 3D image data provides a foundation for quantitative comparison of experiment to pore scale modeling and enable testing and calibration of pore network predictions of CO2 storage. 
Introduction 
The Otway Project, an undertaking of the Australian Cooperative Research Centre for Greenhouse Gas Technologies, is located in the on-shore part of the Otway sedimentary basin in south-eastern Australia. In the first stage, between March 2008 and August 2009, 65445 tonnes of approximately 77 mole % carbon dioxide, 20 mole % methane and 3 mole % other gas components was injected into a depleted gas field. The injection well, CRC-1, was located 290 m down-dip from the existing natural gas production well, so that the CO2 migrated upwards towards the remaining methane cap. Injection was into the Waarre “C” sandstone at a depth of about 2060 m. An assessment of the long-term security of storage depends on an understanding of the form and distribution of the injected CO2. 

In geological storage of CO2 there are four main trapping mechanisms: structural, residual, dissolution and mineral trapping. While both structural trapping and dissolution trapping depend on large-scale geometry, both residual trapping and mineral trapping are strongly influenced by geometry at the pore-scale. The dominant mechanism for residual capillary trapping of non-wetting CO2 is snap-off which depends on the ratios of pore throats to pore bodies. Surface area is a key parameter in determining the rate of mineral trapping (Ennis-King, et al. 2006; Ennis-King and Paterson, 2007; Kang et al. 2009). Residual and mineral trapping are of particular interest because they represent secure forms of long-term storage (Ide et al, 2007; Qi et al. 2009; Al Mansoori et al. 2010). Recent advances in digital core analysis now means that these pore-scale properties can be studied from X-ray microtomographic images. It is the purpose of this paper to examine applicability of digital core analysis to CO2 storage problems on core samples from the CRC-1 well.