Investigation of fracture propagation characteristics caused by hydraulic fracturing in naturally fractured continental shale

Continental shale from the Ordos Basin, China, is naturally fractured and characterized by silty laminae and interbeds that is different from marine shale. To investigate the hydraulic fracture propagation in this kind of shale, hydraulic fracturing experiments were performed in cylinder continental shale samples under different stress ratio conditions, and propagation characteristics of the visible surface fractures, the main fractures as well as the main fracture surfaces were observed and discussed. The results reveal that the visible induced fractures on the sample surfaces primarily extend along natural fractures, bedding planes and interbeds, and situation of crossing beddings and interbeds can be found, which is different from fractures in marine shale that are mainly vertical to bedding planes with some branches. Besides, the main fractures were observed and measured through electronic microscope. They are wavelike and tortuosity tends to increase as stress ratio decreases. Moreover, three dimensional scanner was employed to study the main fracture surfaces, and Area Ratio and Standard Deviation were used to evaluate the roughness. With the decrease of stress ratio, partial vertical fractures can be found on the main fracture surface, and Area Ratio and Standard Deviation get higher, which indicates that the roughness of the main fracture surfaces becomes higher. Therefore, hydraulic fracture propagation characteristics of continental shale are strongly influenced by the stress and some its own properties, including natural fractures, beddings and interbeds. And the result contributes to understanding the mechanism of hydraulic fracture propagation in continental shale formation.