Mechanism Transition for Fracture Energy Dissipation of Shale Under Crack-Parallel Compression

ABSTRACT: The microcracking processes in front of the rock fracture tip could determine the macro-fracture propagation. Understanding the effect of crack-parallel stress on the microcracking processes could facilitate characterization of fracture propagation in shales and guide hydraulic fracturing. The gap test is improved by replacing elastoplastic pads with a micro servo pump for applying precise crack-parallel stresses. The fracture propagation with crack-parallel stress is captured, where microcracking processes are identified unifying digital image correlation (DIC) and acoustic emission (AE) method. Based on the crack cohesive model, the influence of crack-parallel stress on fracture energy is characterized. Experiments show that the parallel stress of 10MPa increases the fracture energy of shale by about 25%, due to more shear type microcracks forming macro-fracture. The microcracks are spread over a wider damage zone rather than concentrated in fracture processes zone (FPZ) because of the crack-parallel compression. Hence the whole damage zone instead of FPZ dominates fracture extension. This work provides fundamental understandings in propagation of hydraulic fracture in shale under reservoir conditions. 1. INTRODUCTION Hydraulic fracturing is a key technology increasing reservoir permeability by injecting high-pressure fluids to form fractures(Xing, Zhang, Luo, et al., 2019). Essentially, hydraulic fracture extends in the reservoir under the crack parallel compression. There are various standard specimens to characterize fracture resistance of rock, such as three-point-bend and compact tension, but all the standard tests almost ignore the effect of crack parallel stress. The crack parallel stress is part of triaxial stresses from confining pressure, which is parallel to the fracture propagation. Some fracture tests under confining pressure have checked the effect of triaxial stresses. The straight notched round bar was proposed to obtain mode I fracture toughness of shale influenced by confining pressure(Antinao Fuentealba et al., 2024), using linear fracture mechanics. Experiments present that fracture toughness is double when confining pressure is increased to 70 MPa. The Brazil splitting tests with confining pressure indicate that the tensile strength of shale increases by up to 58.5%(Guo et al., 2022), as well as the sandstone and limestone samples. It indirectly shows obvious effect of confining pressure on rock fracture toughness because fracture toughness is proportionate to tensile strength(Dutler et al., 2018).