Caprocks are essential to the safe sequestration of CO2. Clayey rocks (clays, claystones, shales, mudrocks, siltstones) represent a major constituent of sedimentary basin fill and act as potential flow barriers and seals for subsurface fluid transport. Thus the investigation of caprock is usually an understanding of clayey rocks. Preliminary tests were conducted on the Pierre Shale from North Dakota's Pembina Gorge. Test results indicate that shales have a much lower Young's modulus compared with many other types of rock, such as limestones. In a basin scale, one may deduce that the clayey layer will bear much more deformation than its neighboring formations under the same tectonic activity. Dry shale shows a much higher peak axial strength than oil-wet shale; however, their residual strengths are comparable, which may indicate that for shale, the Mohr- Coulomb failure envelope constructed based on the residual strength may be even more reliable. Shale strength may also be decreased by exposure to CO2. However, serious deterioration was observed on the sample under CO2 flow after water flow. Both steady state and transient methods were used to determinate the permeability of rock. The rock matrix's permeability is found in the order of 10nD. The permeability is sensitive to confining pressure, the presence of fractures and flow history.
Disposal of CO2 into deep aquifers or depleted oil reservoirs is increasingly being studied as a strategy for limiting the anthropogenic CO2 emissions (USDOE, 2002; Nelms et al., 2004; Fischer et al., 2005). In order to proceed with a large-scale carbon storage project, a risk assessment is likely to be required, with leakage estimation at its core (Celia et al., 2009). Leakage through caprocks may occur as (1) rapid (“catastrophic”) leakage due to seal-breaching or damage of well casing (corrosion of pipes and cements); (2) slow leakage governed by capillary sealing efficiency and relative permeability (after capillary break-through pressure is exceeded); (3) diffusive loss of dissolved gas through saline water or hydrocarbon-saturated pore space (Zoback and Zinke, 2002; Rutqvist et al., 2007; Al-Basali et al., 2005; Chiquet et al., 2005; Busch et al, 2008; Shafeen et al., 2004; Krooss et al., 1988). Clayey rocks (clays, claystones, shales, mudrocks, siltstones) represent a major constituent of sedimentary basin fill and act as potential flow barriers and seals for subsurface fluid transport (Hildenbrand et al., 2003). Thus, very often, the caprocks are composed by the clayey rocks. A combination of diffusion experiments and conventional gas sorption tests on the Muderong Shale from Western Australia has provided evidence for significant CO2 storage capacity in clayey sequences (Busch et al., 2008). However, limited by the poor accessibility due to low permeability, this retention capacity can only be considered as an additional beneficial feature of the clayey caprocks overlying potential CO2 storage sites. Long term reactive transport modeling of CO2 into the Nordland Shale caprock at Sleipner shows that the exact mineralogical composition of the plagioclase fraction in the caprock plays a crucial role (Gaus et al., 2002 & 2005).