Interpreting Post-Injection Acoustic Emission in Laboratory Hydraulic Fracturing Experiments

Bunger, A.P. (University of Pittsburgh) | Kear, J. (CSIRO Earth Science and Resource Engineering) | Dyskin, A.V. (The University of Western Australia) | Pasternak, E. (The University of Western Australia)



This paper presents data from laboratory scale hydraulic fracturing experimentation with acoustic emission (AE) monitoring. The motivation is firstly to confirm existence of the post-injection surge in the AE rate observed by other investigators. Secondly, unlike previous investigations, a re-pressurization stage is included to test whether AE is driven by the interaction of the two hydraulic fracture surfaces as they contact one another during the closure period that follows an injection-shut in cycle. Our results show that the AE rate indeed increases when the pressure is relieved after the first injection/propagation stage. However, re-opening the fracture through a second pressurization and allowing it to close again proved unsuccessful in causing a second increase in the AE rate. Instead, the AE rate is observed to decay hyperbolically with the time from the first moment of pressure relief with no impact from the second injection/closure stage. This hyperbolic decay in AE rate is in accordance with Omori’s law, that is, it is statistically similar to earthquake aftershocks. The AE in our laboratory experiments were therefore apparently not associated with closure but rather to the somewhat surprising propensity of the rock to produce AE aftershocks from the vicinity of a hydraulic fracture under zero-loading conditions during the hours to days following its creation.