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Rocks are known to fail in a time‐dependent manner similar to many brittle and quasi‐brittle materials. However, observed time dependence of hydraulic fracture initiation has ubiquitously been attributed to fluid flow‐related mechanisms without consideration of the intrinsic time dependence of rock failure. Laboratory delayed hydraulic fracturing breakdown experiments on three rocks (granite, sandstone, and limestone) show sensitivity to rock properties associated with subcritical crack growth as well as fluid viscosity and ambient confining pressure applied to the specimens. A new hydraulic fracture initiation model accounting for subcritical crack growth and fluid flow in a poroelastic medium, as well as the additional energy dissipation required for fluid flow in the rough fracture tip, is then used to estimate the value of the subcritical index n based on characterization experiments. Given this characterization data, the model is shown to be capable of predicting time‐dependent hydraulic fracture initiation with variation of fluid viscosity and confining stress.

We present a distributed fiber optic sensing scheme to image 3D strain fields inside concrete blocks during laboratory-scale hydraulic fracturing. Strain fields were measured by optical fibers embedded during casting of the concrete blocks. The axial strain profile along the optical fiber was interrogated by the in-fiber Rayleigh backscattering with 1-cm spatial resolution using optical frequency domain reflectometry (OFDR). The 3D strain fields inside the cubes under various driving pressures and pumping schedules were measured and used to characterize the location, shape, and growth rate of the hydraulic fractures. The fiber optic sensor detection method presented in this paper provides scientists and engineers an unique laboratory tool to understand the hydraulic fracturing processes via internal, 3D strain measurements with the potential to ascertain mechanisms related to crack growth and its associated damage of the surrounding material as well as poromechanically-coupled mechanisms driven by fluid diffusion from the crack into the permeable matrix of concrete specimens.