CO₂-H₂O can damage the rock microstructure and change the tensile failure characteristics and fracture propagation mode during CO₂ fracturing in shale reservoirs. X-ray diffraction (XRD) tests, scanning electron microscope (SEM) observation, and Brazilian tests are conducted to investigate the microscopic damage and failure characteristics, and fracture propagation mode of Longmaxi and Chang-7 shale specimens after CO₂-H₂O treatment. The results show that the microscopic damage of bedding after CO₂-H₂O treatment is more significant than that of the matrix. The volume of bedding clay minerals is reduced due to dehydration, the organic matter is extracted and contracted, and the large-size microfractures (10−30 μm in length and 1−5 μm in width) are generated in the laminae distributed along the bedding. Carbonate and feldspar minerals in the matrix are dissolved and induce randomly distributed small-size microcracks (< 1 μm in length and < 0.5 μm in width). After CO₂-H₂O treatment, the tensile strength of shale decreases, and the anisotropy increases. The failure mode of shale changes from tensile failure to mixed tension-shear failure, and the shear action of specimens loaded vertically to the bedding is stronger. Fracture propagation is restricted by the bedding for specimens loaded vertically to the bedding, leading to fracture propagation along the bedding; for specimens loaded horizontally to the bedding, the bedding exerts stronger constraints on fracture propagation, resulting in fracture propagation merely within the bedding.

Massive developed discontinuities are the salient geological features of unconventional oil and gas reservoirs, and the hydraulic fractures’ capabilities of crossing the discontinuities concern the stimulation effects of hydraulic fracturing. To study the development of the fracture process zone (FPZ) when the hydraulic fracture orthogonally propagates through a discontinuity, the self-designed visual fracturing equipment was adopted to carry out hydraulic fracturing tests on sandstone plates with a prefabricated unbounded friction interface. Based on the digital image correlation method, the displacement and strain characteristics during the hydraulic fracture propagation across the orthogonal interface were monitored in real time. The test results show that the FPZ has developed across the interface before the hydraulic fracture extends across the interface. Whether the fracture can propagate through the interface is predetermined at the initial developmental stage of the FPZ and is not affected by the stress-softening process in the FPZ. Based on the Renshaw-Pollard criterion, a criterion considering the FPZ boundary was established for estimating the fracture propagation across the friction interface, and it was verified by test data and existing results. In comparison, the improved criterion considers a more accurate application scope of elastic fracture mechanics at the fracture front. The aspect ratio of the FPZ has a significant effect on the improved criterion, and the lower limit of friction coefficient required for the fracture propagation orthogonally across the interface declines as the aspect ratio of the FPZ rises under the same conditions.