Experimentation on hydraulic fracturing of Barre granite: from hydrofracturing to hydroshearing

Hydraulic fracturing is frequently used in the oil and gas industry to increase the permeability of rocks. Its application for Enhanced Geothermal Systems still faces many challenges such as the occurrence of induced seismicity and the difficulty to collect and analyze field data. Therefore, laboratory testing plays a significant role in the understanding of crack development in a naturally fractured environment. A test setup was developed in order to apply fluid pressure inside of individual flaws in a prismatic rock specimen containing a pre-cut flaw pair. The pressurization device is equipped with a front window allowing to record high-speed and high-resolution images throughout the experiment to observe each step of the crack initiation and propagation. The experiments are conducted under uniaxial or biaxial external stress on various flaw pair geometries. This research aims to understand better the interaction of a pressurized flaw with a non-pressurized one, by analyzing the crack processes in Barre granite. In addition, experimental procedures are developed to induce the mechanisms of hydrofracturing and hydroshearing. For this purpose, an analytical investigation is conducted to determine the evolution stress state around a pressurized opening and the type of failure induced by external stress and pressurization evolution. This research showed that visible cracks open mostly in tension at grain boundaries and that micro-cracked zones develop in the process zone leading to shear failure. Moreover, the fluid pressure inducing hydraulic fractures depends on the flaw pair geometry and the external stress. Uniaxial experiments showed that an increase of flaw inclination leads to various crack patterns evolving from tensile to shear failure. As expected from the analytical investigation, biaxial external stress induced en echelon crack structures leading hydroshearing. The results of this research will allow to further investigate the characteristics of hydrofracturing and hydroshearing mechanisms in rocks.