Observation of Hydraulic Fracture Morphology for Laboratory Experiments by Using Multiple Methods

How to monitor and identify fracture initiation and propagation is a significant work for the laboratory experiment studies of hydraulic fracturing. In this paper, four main monitoring methods are reviewed and compared through the tri-axial fracturing experiments with natural shale outcrops, including tracer labeling, acoustic emission (AE) monitoring, computerized tomography (CT) scanning and visual fracturing method. Results show that for tracer labeling, fluorescent tracer could visually monitor the hydraulic fracture distribution morphologies, but it has a poor compatibility with slippery water. For the method of AE monitoring, dynamic three dimensional (3D) hydraulic fracture propagation process can be monitored by fixing AE sensors on specimen surfaces. However, the AE signal might display inconsistency with fracture path, which is affected by shale heterogeneity, natural fractures and bedding planes. For the method of CT scanning, many internal rock features can be well recognized, including the location of primary and activated fractures, fracture scales and the interactions between hydraulic fracture and natural fractures. However, CT scanning method cannot identify some micron-and nanoscale fractures characteristics. For the method of visual fracturing, the injected low-temperature metal can extract the fractures skeleton and measure fracture morphologies precisely, but temperature have a significant impact on experiment results. Therefore, combinations of multiple monitoring methods are recommended in laboratory experiment studies. For example, the combination of AE monitoring and CT scanning can effectively identify the propagation behavior of complex fracture network.