Caprock Integrity of Carbonate and Evaporite

In geologic time scales, the slow, subcritical fracture propagation can contribute considerably to the fault and fracture system development, especially under reactive fluid environments during petroleum migration and accumulation. Using the double torsion technique, we measure the subcritical fracture growth properties of carbonate and anhydrite caprock core samples in Sichuan Basin, China, under different environments of ambient air, de-ionized water, and formation water. Experimental results show that the aqueous fluids significantly promote subcritical fracture growth: (1) Compared to ambient air, the presence of aqueous fluids dramatically reduces both the fracture toughness (KIC) and subcritical fracture growth index (SCI) of the caprocks; (2) The KIC reduction is more prominent for the anhydrite (50%) caprock than the carbonate (30%) caprock, which indicates that mineral solubility is responsible for the fluid-weakening effect; (3) A systematic change of SCI with varying fluid conditions is observed: SCI is the largest in air, smallest in formation water, and intermediate in de-ionized water. These observations provide insights into the mechanisms controlling fracture growth in carbonate and evaporite caprocks, which could extend to brittle failure and fluid-rock interactions occurring during the seismic cycle in the upper crust.