Laboratory hydraulic fracturing of granite: Acoustic emission observations and interpretation

Hydraulic fracturing is routinely used, but the fracturing processes that occur when rocks are hydraulically-fractured are not entirely understood and require further investigation. This study presents the acquisition, analysis and interpretation of acoustic emissions data from a series of laboratory hydraulic fracturing experiments on granite. Specimens with different orientations of two pre-cut flaws were tested under both 0 MPa and 5 MPa of vertical uniaxial stress to understand the effect of the external stress conditions. Acoustic emissions data are related to corresponding visual observations made using high-resolution and high-speed imaging. We find that in general, (1) the AE begin to occur at approximately 80% of peak pressure, (2) the focal mechanisms suggest that 55–60% of the radiation pattern could be explained by a double couple mechanism and is well-explained by simulation results, (3) hypocenter locations tended to agree well with visually observed white patching (process zone) and crack patterns, (4) spatio-temporal analysis revealed points in time at which microcrack coalescence were detected by AE, (5) that the AE could be used to make a non-unique prediction of crack initiation, and lastly (6) that exponential AE energy accumulation in the last 5–10 s prior to crack initiation can be modelled by time-to-failure methods.