Mechanical behavior of anhydrite caprock and implications for CO2 sealing capacity

Maintaining caprock integrity is prerequisite for geological storage of CO2. We investigated the mechanical strength and damage behavior of anhydrite-rich caprock, which seals many potential CO2 storage sites around the world. Conventional triaxial tests were performed at temperatures of 20°C–80°C, confining pressures of 1.5–50 MPa and strain rates of ∼10−5 s−1. We determined the failure and dilatation envelopes for dry anhydrite and studied the effect upon strength and dilatation of high-pressure pore fluids (Pf = 15 MPa), namely presaturated CaSO4 solution and CO2-saturated CaSO4 solution. For dry samples, we observed an increase in strength with confining pressure and a slight weakening with temperature. Fluid penetration prior to failure resulted in a direct effective pressure effect on strength, but not on volumetric behavior. Fluid penetration during failure is too slow to influence mechanical and volumetric behavior. Overall, we found no short-term chemical effects of CO2 and pore fluid on the strength of anhydrite. Penetration of the samples by CO2-saturated pore fluid was more effective than by solution alone, most likely as a result of the lower interfacial tension of the CO2/water system. Simple analytical calculations based on the elastic flexure of a seal formation, combined with our failure and dilatation envelopes, show that, for realistic conditions, caprock integrity will not be compromised by mechanical damage. In addition, long-term chemical reactions of anhydrite with CO2 will most likely not lead to significant CO2 penetration, though more research is needed.