Lei Wang

With the development of unconventional oil and gas resources (such as shale gas and tight oil/gas), the widespread application of multistage fracturing technology has significantly increased the difficulty of wellbore integrity maintaining. The cement sheath serves as the core barrier for preserving wellbore integrity, particularly at the first interface (cement–casing) and the second interface (cement–formation). The high temperature, high pressure, and cyclic dynamic loading imposed by multistage fracturing represent severe challenges to the integrity of cement sheath. To simulate underground conditions realistically, a high-temperature, complex stress path loading system coupled with real-time gas flow monitoring was developed. Using this system, gas leakage monitoring and displacement-controlled cyclic loading tests were conducted on cement–steel (simulating the first interface) and cement–shale (simulating the second interface) composite specimens. It focused on investigating the effects of different temperatures, cyclic stress levels, and cycle counts on the sealing performance of the cement–steel and cement–shale composites. The findings reveal that elevated temperatures significantly degrade cement properties and accelerate damage accumulation. Cyclic stress levels and cycle counts are core drivers of interface fatigue failure, exhibiting synergistic destructive effects with temperature. The first interface is more prone to seal failure due to material property differences and a relatively high stress level. This research elucidates the cumulative damage mechanism underlying interfacial seal failure. It is of significant engineering implications for enhancing well safety and development efficiency.

The rapid development of renewable energy requires the support of large-scale energy storage technologies to maintain system balance. Salt cavern compressed air energy storage (CAES) is regarded as one of the key technological pathways for large-scale energy storage. In such systems, the wellbore serves as a critical structure connecting surface facilities and the underground salt cavern, while the cement sheath—formed between the casing and formation during well cementing—acts as the primary barrier ensuring the overall sealing integrity of the wellbore. Through cyclic loading–unloading tests on cement slurries under different temperatures, this study yields the following main conclusions: (1) Increasing temperature aggravates the accumulation of fatigue damage in cement specimens. Taking cumulative plastic strain as an example, it rises from 0.45% to 0.99% as the temperature increases from 25 °C to 115 °C. (2) Elevated temperature promotes greater irreversible energy dissipation under fixed cyclic stress limits. When the temperature rises from 25 °C to 115 °C, the dissipated energy density increases from 0.0033 mJ/m³ to 0.0046 mJ/m³, and its proportion relative to the input energy also increases from 5.52% to 7.13%. (3) Temperature rise leads to notable deterioration of the internal pore structure. At 115 °C, the NMR T₂ distribution peak shifts rightward by 0.49 ms, the total pore volume increases by 150.53 mm³, and the corresponding permeability rises by 1.398 × 10⁻³ μm². (4) Elevated temperature (up to 115 °C) weakens material performance through a dual mechanism: it accelerates dehydration of the cementitious system, reducing interparticle bond strength, while also promoting plastic slip. It is recommended to optimize the cement slurry formulation (e.g., by incorporating thermal stabilizers) to enhance its long-term sealing performance under service conditions.

Fracture–cavity carbonate reservoirs account for a considerable proportion of oil and gas resources. Because of the complicated relationships between cavities, fractures and pores in these reservoirs, which are defined as cavity clusters, fracturing technology is employed to enhance their hydrocarbon productivity. However, almost all previous studies have just considered the effect of a single natural cavity or fracture on the propagation of a hydraulic fracture; therefore, the mechanism by which a hydraulic fracture interacts with a cavity cluster needs to be clarified. In this study, cavity clusters with different distributions were accurately prefabricated in synthetically made samples, and large-scale simulation equipment was employed to systematically perform fracturing experiments considering different horizontal differential stress levels. Meanwhile, the hydraulic fracture propagation behaviors were comprehensively analyzed through fracture morphology, fracturing curves, the complexity of the fracture network and acoustic emission monitoring. It was found that a natural fracture with a smaller approach angle is favorable in guiding a hydraulic fracture to a cavity. The fracturing curves were divided into the following four types: frequent fluctuations with “step-like” shapes, great fluctuations with slightly lower closure pressure, fluctuations with obviously lower closure pressure, and little fluctuations with obviously lower closure pressure. And different cavity cluster distributions play a dominant role in the complexity of generated hydraulic fracture networks. In addition, AE energy was used to judge the ease of crossing the cavity. The above findings indicated that for the actual exploration and exploitation of carbonate reservoirs, the geological exploration of different fracture–cavity structures in reservoirs would be required, and targeted fracturing engineering designs need to be carried out for different fracture–cavity carbonate reservoirs.

The mechanical properties of deep volcanic reservoir rocks are complex and the brittleness characteristics are not clear. In order to study the brittleness characteristics of deep volcanic rocks, triaxial compression tests and XRD tests under different confining pressures were carried out to study the brittleness characteristics of deep volcanic rocks. Three methods were used to evaluate the brittleness of volcanic rocks from different sides: the brittleness evaluation method based on stress–strain curve characteristics (BI₁), the brittleness evaluation method based on energy dissipation (BI₂) and the brittleness evaluation method based on mineral composition (BI₃). The three methods showed that the brittleness index of volcanic rocks decreases with the increase in confining pressure, and the higher the depth of volcanic rocks, the lower the brittleness. On the basis of previous studies, an improved brittleness evaluation method based on internal friction Angle was proposed, and the obtained evaluation results were almost consistent with the results obtained by the brittleness evaluation method based on stress–strain curve characteristics (BI₁) and the brittleness evaluation method based on energy dissipation (BI₂), indicating the feasibility of the improved brittleness evaluation method. The brittleness characteristics of deep volcanic rocks are clearly defined to provide reference for the exploration and development of deep volcanic rocks reservoir.

With the progress of underground energy exploration and development, the research on deep shale was becoming increasingly important. Laboratory tests were designed and combined with theories to study the fracture characteristics of deep shale true triaxial hydraulic fracturing under different stress difference coefficients. Combined with the results of laboratory triaxial test and the evaluation method of rock brittleness based on energy change, it was shown that with the increase of confining pressure, the brittleness of shale decreased, indicating that it was difficult for deep shale to form complex fracture networks. In high-stress hydraulic fracturing tests with different stress difference coefficients, combining fractal theory and energy release rate principle, it was found that hydraulic fractures were easy to communicate with shale bedding planes under low horizontal stress difference coefficients, which can improve the complexity of hydraulic fractures ignoring the overall increase of the triaxial stress. Although the low horizontal stress difference coefficient in deep shale reservoir makes it easy for hydraulic fractures to communicate with shale strata and thus promote the formation of complex shale fracture network, deep shale reservoir fracture expansion is difficult and reservoir reconstruction scope is limited. Therefore, hydraulic fracturing tests under different viscosity and displacement were designed and multiple linear regression method was adopted to obtain the relationship between the coupling effect of fracturing fluid viscosity and displacement on the fractal dimension, which can not only improve the fracture complexity to a large extent, but also control the fracture propagation direction, provide reference for field fracturing work.

A deep volcanic reservoir has high geostress, and the rock in the reservoir exhibits strong heterogeneity. Moreover, extensive natural fractures are distributed in the reservoir, and the hydraulic fracture propagation mode is unclear. A high-stress true triaxial hydraulic fracturing test was carried out on volcanic rock to study the influence of vertical stress difference under different maximum horizontal principal stresses on the fracture propagation characteristics. We studied the fracture characteristics using image processing and fracture characterization methods. The high-stress hydraulic fracturing test showed that the vertical stress difference mainly affected the direction of fracture propagation and fracture roughness. Less vertical stress difference implied chaotic fracture propagation, complex fracture network, and rough fracture surface. Hydraulic fracture interface characteristics indicated that the peak pump pressure linearly increased with an increase in the vertical stress difference. Moreover, the overall fractal dimension and crack width decreased and increased, respectively, with the increase of vertical stress difference, which were verified from the decreased fractal dimension of cracks and increased crack width at different positions, respectively.