Experimental study on fracture propagation in anisotropy rock under cyclic hydraulic fracturing

•In hydraulic fracturing tests, on average, the breakdown pressure values of cyclic injection are reduced by 30% compared to monotonic injection. Therefore, the effect of the hydraulic fatigue phenomenon is clearly visible.•The fracture geometry in the samples subjected to cyclic injection is in the form of a network of multiple and intersecting fractures, while that subjected to monotonic injection is in the form of a single flat surface.•In the samples with horizontal stress difference, the sample fracturing is not affected by sample anisotropy in a way that the single fracture surface with relative curvature grows in the direction of the maximum stress.•In the cyclic injection using the damaged-controlled method (cycle duration: 16 s), the Energy caused by the injection has been concentrated around the borehole, and as a result, a complex fracture network with short crack lengths has been created; meanwhile, the cyclic injection using the ramp signals method (cycle duration: 8 s) tends to form simple cracks with higher expansion and penetration. Hydraulic fracturing is commonly used to enhance the hydraulic conductivity of geothermal, oil, and gas reservoirs, particularly those situated in enclosed and unconventional formations. Shale oil and gas reservoirs exhibit distinct characteristics in rock mechanics parameters, morphology, natural fracture geometry, and geological features. Structural anisotropy in materials generally arises from changes in grain type and size, weak layering, and variations in layering angles. This study aims to investigate the impact of two key factors, namely cyclic injection and structural anisotropy, on the hydraulic fracturing process. To achieve this goal, the phenomenon of hydraulic fatigue in rock was examined through various cyclic injection methods, and true-triaxial hydraulic fracturing tests were conducted on cubic samples composed of different materials and layers. To evaluate and analyze the fracture morphology of these samples, CT scan X-ray imaging technology was employed. The results revealed that samples subjected to cyclic injection exhibited an average breakdown pressure approximately 30% lower than those subjected to monotonic injection. Fracture geometry in the cyclic injection samples displayed a network pattern with multiple crack branches, while monotonic injection primarily resulted in a single-surface pattern. In samples with differences in horizontal stress, the anisotropy of the sample did not affect