High rock temperatures and complex engineering environments are the most significant problems in enhancing the rate of penetration (ROP) and drilling tool efficiency for hot dry rocks (HDR). Studying the failure characteristics of different thermal-treated granites by PDC cutters during percussive drilling has profound significance in enhancing the compatibility between drilling teeth and rock during HDR mining and optimizing the drilling toolʼs efficiency in breaking rocks. Through drillability tests and SHPB tests, the present study explored the relationship between drillability and the dynamic behavior of granites. In addition, granites with different heating temperatures and cooling methods were subjected to dynamic rock-breaking tests by the SHPB system installed with the PDC cutter. Furthermore, the impact crater was quantified by 3D contour scanning technology, and the fractal theory was utilized to analyze the fragments. Then, the equations for the dynamic magnification factor of intrusion, fractal dimension, and specific rock-breaking energy of granite upon impact loading are discussed. According to the results, the impact crater size is closely associated with the heating temperature of the rock. Heating and rapid cooling treatments will affect the thermal shock damage and strength of rock, and then change the penetration depth, damage volume, and area of impact craters generated from the PDC cutter. The granite at 600 ℃ exhibits about 45% smaller fragment size than that at room temperature, while the damage degree of the liquid nitrogen-cooled specimen will be greater. According to the variation of dynamic magnification factor, fractal dimension, and specific rock-breaking energy of rock with thermal shock damage, it can be found that with the temperature increasing, the problems drilling tools need to solve gradually shift from increasing ROP to improving cleaning efficiency. The experimental results are expectedly to lay the ground for high compatibility between drilling tools and formation in HDR.

A microannulus (MA) is the primary reason for sustained casing pressure in multi-stage fractured-shale gas wells. However, the effect of the casing eccentricity on the long horizontal section has not been considered. In this study, a full-scale integrity tester for cement sheaths is adopted to measure the cumulative plastic deformation. Numerical models are applied to evaluate the development of the cumulative plastic deformation and quantify the MA width considering casing centralization and eccentricity in the context of multiple loading and unloading cycles. Subsequently, the influences of the eccentricity distance and angle, cement-sheath mechanical variables, and different well depths on the cumulative sheath plastic deformation and sheath MA development are explored. The research results demonstrate that casing eccentricity significantly increases the cumulative sheath plastic deformation compared with that of the casing-centered condition. Consequently, the risk of sealing integrity failure increases. The accumulated plastic deformation increases when the eccentricity distance increases. In contrast, the initial plastic deformation increases as the eccentricity angle increases. However, the cumulative plastic deformation decreases after a specific loading and unloading cycle count. Affected by the coupled influence of the internal casing pressure and fracturing stages, the width of the MA in the horizontal section increased from the toe to the heel, and the casing eccentricity significantly increased the MA width at each stage, thus increasing the risk of gas channeling. Finally, an engineering case is considered to study the influence of casing eccentricity. The results show that cement slurries that form low and high elastic moduli can be applied to form a cement sheath when the fracturing stage is lower or higher than a specific value, respectively. The results of this study offer theoretical references and engineering support for the integrity control of cement sheath sealing.

Currently, accelerated aging tests are widely used to study the aging process of polyethylene pipelines. However, this approach can only simulate one or several main influencing factors in the natural environment, which are often quite different from the actual environment of the buried pipelines. In this study, five types of PE80 buried pipelines in service for 9–18 years were taken as the research object, while new PE80 pipelines were taken as the reference group. The aging process and mechanism of polyethylene buried pipelines were studied through mechanical and chemical property tests and microstructural analysis. The results showed that the pipeline exhibited cross-linking as the main aging mechanism after being in service for 0–18 years. The aging degree and law of the inner and outer surface of the pipeline were compared, and the observed mechanism of both surfaces was explained. After 18 years in service, the elongation at the break of the pipe decreased by 16.2%, and the toughness of the matrix in the main collapse area of the tensile sample was the fundamental reason responsible for changes in the mechanical properties. Finally, after 18 years in service, the oxidation induction time of the pipeline was 25.7 min, which was 28.5% higher than the national standard value. There were no potential safety hazards during continuous long-term service. The results of this paper provide reference data and theoretical guidance for the aging process study of buried polyethylene pipelines.

Axial and torsional impact drilling technology is used to improve the drilling efficiency of hard rock formation in the deep underground. Still, the corresponding theory is not mature, and the re are few correlative research reports on the rock-breaking mechanism of axial and torsional coupled impact drilling tools. Considering the influence of the impact hammer geometry and movement on the dynamic load parameters (i.e., wavelength, amplitude, frequency, and phase difference), a numerical model that includes a hard formation and single polycrystalline diamond compact cutter was established. The Riedel-Hiermaier-Thoma model, which considers the dynamic damage and strength behavior of rocks, was adopted to analyze the rock damage under axial and torsional impact loads. The numerical simulation results were verified by the experimental results. It was found that compared with conventional drilling, the penetration depths of axial, torsional, and axial-torsional coupled impact drilling increased by 31.3%, 5.6%, and 34.7%, respectively. Increasing the wavelength and amplitude of the axial impact stress wave improved the penetration depth. When the bit rotation speed remained unchanged, increasing the frequency in the axial and circumferential directions had little effect on the penetration depth. However, as the frequency increased, the cutting surface became increasingly smooth, which reduced the occurrence of bit vibration. When the phase difference between the axial and circumferential stress waves was 25%, the penetration depth significantly increased. In addition, the bit vibration problem can be effectively reduced. Finally, the adjustment of engineering and tool structure parameters is proposed to optimize the efficiency of the axial-torsional coupled impact drilling tool.