The effects of lateral variations in rock composition and texture on anhydrite caprock integrity of CO2 storage systems

• Lateral changes in caprock microstructure may lead to local variations in strength
• We investigated the behaviour of two texturally different Zechstein anhydrites
• A significant difference in mechanical behaviour was observed between the two
• No chemo-mechanical effect of CO2-saturated fluid on strength was measured

We investigated the effect of rock texture and composition on the mechanical strength and volumetric behaviour of anhydrite-rich caprock. Conventional triaxial experiments were performed at 80 °C, confining pressures of 1.5–35 MPa and strain rates of ~10−5 s−1, both dry and in the presence of fluids. We determined the failure envelope, and the effect of fluids upon it, for anhydrite displaying an equigranular texture consisting of euhedral grains. We observed a general pattern of increasing peak compressive strength with increasing confining pressure, consistent with a previous study on anhydrite from the same formation, which showed an irregular texture consisting of interlocking, acicular grains embedded in a fine-grained matrix. The peak strength of euhedral anhydrite was found to be 15–65% lower than that of acicular anhydrite. No chemical effects were observed for the fluids tested. Hydraulic fracturing experiments showed the tensile strength to be approximately 4 MPa for euhedral and 6 MPa for acicular anhydrite. We combined the failure envelopes with simple, analytical calculations on the effect of cooling on in-situ stress, showing that tensile caprock failure due to cooling would be unlikely for both types of anhydrite, assuming a hypothetical reservoir at a depth of 3800 m. However, it should be noted that tensile failure may pose an issue for shallower reservoirs or weaker caprock formations.