Laboratory Testing on Hydraulic Fracturing of Barre Granite: Studying Visual Observations, Acoustic Emissions and Radiated Energy related to Fracture Mechanisms

Hydraulic fracturing is frequently used to increase the permeability of rock formations. This can be done by creating new fractures as usually done for hydrocarbon extraction or extending and opening fractures as usually done in Enhanced Geothermal Systems (EGS). In both cases, only indirect information such as in-situ stress, pumping records and microseisms are recorded. However, understanding the exact processes of hydraulic stimulation remains very challenging as some of the fundamental aspects such as the fracturing processes and induced/triggered seismicity are still unknown. Unique equipment developed by the MIT CEE Rock Mechanics Group allows one to visually observe hydraulic fracturing while recording Acoustic Emissions (AE) simultaneously. Important information is obtained by applying different fluid pressure in individual pre-cut flaws with distinct flow rates and external stresses. This paper focuses on the main processes inducing hydraulic fracturing, and particularly hydro-shearing, by relating the AE data to visual observations, and investigating different experimental concepts in terms of focal mechanisms and normalized radiated seismic energy. In this work we show that (1) hydro-shearing may be induced by applying high external loads i.e. induce global shear failure; (2) local failure around the flaw tips is expected to occur in tensile mode; (3) the AE hypocenter location matches well with the visual crack observations; (4) some differences of focal mechanisms based on AE data and visual observations exist; (5) cyclic pressurization results in high normalized radiated seismic energy.