A Laboratory Observation on the Acoustic Emission Point Cloud Caused by Hydraulic Fracturing, and the Post-pressure Breakdown Hydraulic Fracturing Re-activation due to Nearby Fault

We re-investigate the laboratory acoustic emission (AE) source location from a hydraulic fracturing test. This test produces temporally well-separated AE releases exhibiting first the reverse Omori–Utsu law (ROU-AEs), and following later the normal Omori–Utsu law (NOU-AEs) behaviors. One fault was cut in the hydraulic fracturing specimen. The spatial permeability changes along the direction of minimum in-situ stress were previously measured on two series of sub-cores. However, the previously published AE source location, which is produced from a commercial software, suffers from several issues. Through data re-analysis, we discover: (1) areas of high AE source concentration in the period of ROU-AEs can support the reverse permeability-distance relationship which is discovered by the commercial software, as well as can be closely correlated with the actual hydraulic fracturing path. (2) The AE point cloud in the period of NOU-AEs after pressure breakdown has re-oriented towards the pre-existing fault, highlighting the influence of nearby structure on hydraulic fracture growth. (3) The peak of ROU-AEs is well synchronized with the borehole pressure breakdown, while the NOU-AEs are released under significantly decreased borehole pressure. (4) The evidence suggests the ROU-period is primarily associated with hydraulic fracture creation, while the NOU-AEs may be correlated with the re-activation of the hydraulic fracture under the influence of the nearby fault.