Time- And pressure-dependent source mechanisms in hydraulic fracturing tests in granite considering fluid diffusivity

The purpose of hydraulic fracturing (HF) is to enhance the permeability of the reservoir by fracturing the rock. While the hydraulically-fractured rocks are porous solids, they are typically considered an impermeable medium where the injected fluid pressurizes and leads to the propagation of fractures. Fluid diffusivity through the matrix is, therefore, frequently ignored, and its effect on the fracturing processes neglected. This paper addresses the effect of fluid diffusivity on the fracturing of a crystalline rock (granite) by evaluating the time-variation of the source mechanisms of the acoustic emission (AE) events produced during laboratory tests. By analyzing the moment tensors of the recorded AE events, these were classified as tensile, compressive, shear and mix-mode. The experiments consisted of injecting hydraulic oil into two pre-fabricated flaws, using injection rates of 20 or 2ml/min that were kept constant during each experiment. This fluid diffused into the rock matrix and progressively pressurized the flaws until cracks developed. Results show that the injection rates are inversely proportional to the number of AE events recorded, as well as to the growth of fluid fronts (fluid diffusion). However, the contribution of each type of AE source mechanisms is relatively similar for both injection rates.