Analysis of acoustic emission events induced during stress unloading of a hydraulic fractured Longmaxi shale sample

Abstract

In shale-gas exploitation, hydraulic fracturing technology is essential to successfully create complex fracture networks in formations. To investigate the dynamic fracturing process, we performed a hydraulic fracturing experiment on a Longmaxi shale sample in the laboratory. After hydrofracturing, hundreds of induced acoustic emission (AE) events were recorded during the stress unloading procedure. At the same time, the high-pressure water stored in the sample fracture network was also discharged. The fracture network inside the fractured sample was studied using medical X-ray computed tomography (CT) scanning technology. Then, the locations and characteristics of the shale failure were analyzed by localizing and inversing the source mechanism of the AE events. Furthermore, the inversion results were verified by comparing with the scanned internal cracks images. According to the source mechanism inversion result, the induced AE events mainly stemmed from the dilatation or compression of fractures and the shear slip of shale along the fracture plane. Finally, the influence of formation attenuation error (underestimated or overestimated attenuation value) on source mechanism inversion results were quantitatively analyzed. It was confirmed that the shear component of the AE event was more sensitive than the volume component to medium attenuation change, especially when the attenuation value was overestimated.