Fracture mechanical properties of carbonate and evaporite caprocks in Sichuan Basin, China with implications for reservoir seal integrity

Abstract

Fracture development in reservoir rocks enhances the flow and transport properties of the petroleum system, yet fracture in caprocks can significantly risk reservoir seal integrity. Over geological time scales, the slow, subcritical fracture propagation can contribute considerably to the 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 suggest 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. Microstructural analysis suggests a predominantly opening mode for both the natural fractures and induced fractures during fracture mechanical testing. Our results further demonstrate that subcritical fracture growth process is also strongly related to rock composition and stress conditions. 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.