Hydraulic fracturing plays a crucial role in the development of unconventional energy resources. However, traditional hydraulic fracturing faces many challenges, such as excessive breakdown pressure, potential geological risk of fault slip, and pollution of the environment by acid fracturing fluids, etc. How to achieve efficient reservoir reformation on the premise of reducing the risk of inducing natural disasters is a key scientific topic, and the new fracturing methods were needed urgently to deal with these challenges. Recently, variable-load fracturing, as a form of fracturing operation that applies periodic or random variable loads (such as pressure, fluid, temperature, and high-energy gas, etc.) acting on formation rocks through physical or chemical means, has gradually attracted widespread attention. Variable-load fracturing emphasizes the multi-field coupling effect between factors such as fluid, temperature, high-energy gas, etc. and the porous medium of rocks during the fracturing process, which is fundamentally different from the mechanical fatigue failure of metals and has a broader conceptual scope. This paper systematically reviews the latest advancements in variable-load fracturing experiments and comprehensively summarizes the findings from four key perspectives: research subjects, influencing factors, research contents, and analytical methods. The variable-load actions are categorized into six primary types: cyclic load, increasing load, stepped load, pulse load, impact load, and combined load. Particular attention is given to the specific effects of variable-load on rocks both before and after fracture, summarizing four characteristics of variable-load fracturing: reduction in rock fracture pressure, decrease in rock fracture energy, alteration in rock permeability, and generation of complex fracture patterns. Furthermore, this paper delves into the underlying mechanisms of variable-load fracturing, examining aspects such as rock fatigue fracture, stress corrosion, and fracture propagation in deflagration fracturing. Finally, the future research directions on variable-load fracturing characteristics were emphasized. It is believed that the influence of rock structure characteristics on the permeability enhancement effect and complex fracture morphology should be clarified. Keeping up with the research trends in deep/ultra-deep reservoir development, the applicability and potential changes of variable-load fracturing under complex and extreme conditions should be considered thoroughly to broaden the research and development space for variable-load fracturing. The action characteristics and internal mechanisms of variable-load fracturing should be comprehensively analyzed from the perspectives of multiple fields, multiple scales, and multiple time periods. This paper aims to provide a comprehensive basic understanding of variable-load fracturing and offers ideas and directions for future research and development.

Due to the lithology difference between layers and pronounced vertical heterogeneity, the main fractures are difficult to extend vertically in thin interbedding tight sandstone reservoirs. Increasing the fracture extension rate can effectively promote the fracture pass-through-bedding extension. A three-point bending fracture experiment was carried out using prefabricated cement-sandstone specimens to study the effect of extension rate on fracture extension path. The digital image correlation method monitored the fracture process zone (FPZ) development characteristics when the fracture extends to the bedding. A prediction model of fracture extension path considering rate effect is proposed based on fracture dynamics theory. The results indicate that at low fracture extension rates, the fracture geometry exhibits a tortuous pattern, accompanied by a short and wide FPZ. Conversely, at high extension rates, the fracture geometry becomes smooth, with a long and narrow FPZ. FPZ is discrete and has the characteristics of mutual attraction. The fracture expands from low-strength rock to high-strength rock. At low extension rates, a high-strain zone (FPZ) forms in advance at the bedding plane, facilitating the extension of fractures along the bedding, causing the fractures to extend along the bedding upon reaching these interfaces. In contrast, this phenomenon is not observed during high-rate fracture extension. A positive correlation exists between the tensile strength of the rock and the average tensile strength of pass-through the element. Fractures with low extension rates preferentially propagate along micro-defects, leading to a reduction in the tensile strength of the rock, Conversely, fractures with high extension rates preferentially propagate along self-similar directions, causing an increase in the tensile strength of the rock due to the tearing of numerous high-strength elements. As the angle between the fracture and bedding increases, the fracture's ability to pass-through-bedding. The effect of the angle is maximized when it reaches 30° between the fracture and bedding, gradually diminishing beyond this threshold. These research findings hold significant implications for optimizing hydraulic fracturing parameters, enhancing fracture height, and boosting oil and gas production in thin interbedded tight sandstone reservoirs.

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.