Propagation behavior of hydraulic fractures in shale under triaxial compression considering the influence of sandstone layers

Abstract

The shale-sandstone interbedded reservoirs are widely distributed in China and the natural gas reserves are huge, and the particularity of this type of reservoir has brought great challenges to hydraulic fracturing design. In this respect, laboratory tests modelling the hydraulic fracturing process in the horizontal well under true triaxial compression were carried out based on real rock specimens. Considering factors such as stress state, sandstone proportion, and natural fractures, the spatial morphology and influence mechanism of hydraulic fractures were studied. Besides, based on acoustic emission (AE) monitoring and real-time monitoring of water pressure, the influence of sandstone layers on the propagation behavior of hydraulic fractures was further investigated. The results show that, the fracture morphology in sandstone is relatively simple, while the fracture morphology and orientation in shale are more complex due to serious bedding interference. A large vertical stress difference is the guarantee for realizing hydraulic fractures penetrating multiple lithologic layers, and the long distance between wellbore and shale-sandstone interface reduces the probability of the hydraulic fracture reaching the sandstone layer. Whether hydraulic fractures passed through the shale-sandstone interface can be judged from the fluctuation characteristics of the water pressure curve and the AE signal. The research results are expected to provide theoretical support for the efficient exploitation of shale-sandstone interbedded reservoirs.