| Acoustic Emission (AE) signals are elastic body waves produced by a sudden release of acoustic energy, as a result of a localized or a distributed failure, and of redistribution of stresses (e.g. grain crushing, grain sliding, microscopic fracturing and macroscopic fracturing). Acoustic emission technology (AET) uses AE events to locate fractures in real time. This technology is of particular importance for mapping the propagation of hydraulic fractures in the subsurface and particularly important on tight reservoirs. Results give the operator an opportunity to visualize the fracture development, during hydraulic treatment, and potentially take corrective actions to control fracture growth, if necessary. For these applications, understanding the sources of AE during fracturing in rocks is of critical importance for characterizing the final fracture geometry.;In this work, controlled fracturing tests were conducted on relatively homogeneous and isotropic sandstone rock slabs to map fracture propagation, using AET. Fracturing was done by pressurizing a drilled borehole in the sample using an inflated cylindrical bladder. The experimental configuration permitted some control of the final fracture. Finite element analysis (FEA) was used to understand the stress distributions at specific times, during the fracturing process, and based on these results; the distribution of AE events was anticipated in time.;A strong correlation between the stress concentrations from FEA and localized AE was observed. Acoustic emissions were detected before, during and after the visible failure of the rock. AE localizations show that, before and after the failure, the highest density of AE events exist in the vicinity of the region where the fracture eventually develops. This indicates that an incipient fracture develops slowly, before the rapid unstable fracturing, generating a large amount of AE events during the process. The rapid fracturing process generates a considerably smaller number of AE events. Results also show a low density of localized AE events away from the fracture.;The petrographic analysis verifies the development of incipient fracturing as a precursor to fracturing and fracture detachment. Grain level damage in the form of grain crushing and sliding and submillimeter fracture branching are observed. The submillimeter fracture branching events are outside the resolution of AE localization. |
