1.1. INTRODUCTION
Geological storage of carbon dioxide (CO2) has been mooted as a greenhouse gas mitigation strategy for over 20 years. The practical mechanics of such a strategy have been tested out at small scale at sites such as the Otway Basin in Australia [Sharma et al., 2009] and Frio in Texas [Doughty et al., 2008] and during industrial‐scale projects, for example, at Sleipner [Arts et al., 2008] and In Salah [Ringrose et al., 2013]. Many years of effort have been put into defining the critical parameters for potential CO2 storage sites [e.g., IPCC, 2005], and these include depth, storage capacity of the site, injectivity of the reservoir, and the containment integrity of the structure into which the CO2 is injected. Containment integrity is usually thought of in similar terms as traps and seals in petroleum systems, and similar technologies can be used to evaluate the properties of the fault and/or top seals that provide the trapping mechanisms for keeping injected CO2 in the deep subsurface. Fault seals usually result from the incorporation of material into the fault zone during fault movement, and this can comprise smearing out of ductile clay‐rich units, abrasion of harder shales, cataclasis of rigid grains, and syn‐/post‐kinematic cementation of the fault rock products [e.g., Lindsay et al., 1993; Yielding et al., 1997; Fisher and Knipe, 1998; Dewhurst et al., 2005]. Top seals are usually characterized in terms of their thickness (especially in relation to fault throw), areal extent, seal capacity (pore‐scale capillary properties), and seal integrity (mechanical properties). There are multiple techniques for assessing the potential sealing capacity of faults [e.g., Watts, 1987; Lindsay et al., 1993; Yielding et al., 1997], and these will not be discussed further here. This paper will concentrate on methods that can be used to characterize caprocks in the laboratory and the relationship between these measurement techniques and the properties noted above. In this contribution, we will concentrate on shale‐rich caprocks but acknowledge that other rocks such as anhydrites [e.g., Hangx et al., 2010] are being evaluated as caprocks for CO2 storage sites. However, it should be emphasized that any seal evaluation for a storage site or petroleum prospect should be fully integrated across both fault and top seals and for a wide range of scales.
Shale caprock properties are dependent on a number of factors, including depositional environment and resultant lithology, electrochemical conditions at deposition, mineralogy, the presence of organic matter, compaction, and diagenetic alteration. All of these processes have a significant impact on porosity and permeability, as well as the mechanical, capillary, and petrophysical properties of shales [e.g., Bennett et al., 1991a,b; Vernik and Liu, 1997; Dewhurst et al., 1998, 1999a, 1999b; Clennell et al., 2006]. A number of these properties are also controlled by human intervention during and after the coring process, such as stress relief microfracture development and drying out and desiccation of recovered core, and care must be taken for certain properties that adequate sample preservation is undertaken [e.g., Schmitt et al., 1994; Dewhurst et al., 2012; Ewy, 2015]. This study will therefore review possible preservation methods and discuss multiple mechanical and petrophysical characterization techniques that can be used to either directly measure or estimate relevant properties required for shale caprocks.
