Journal
JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH
Volume 115, Issue -, Pages -Publisher
AMER GEOPHYSICAL UNION
DOI: 10.1029/2009JB006954
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- Shell International Exploration and Production
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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 degrees C-80 degrees C, confining pressures of 1.5-50 MPa and strain rates of similar to 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 (P-f = 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.
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