Yan Xi

Based on measured data from a shale gas well, this study develops a wellbore temperature cycle model and a temperature–pressure coupled finite element model to evaluate cement sheath stress during multi-stage fracturing. Dynamic temperature and pressure boundaries are applied to calculate radial and tangential stresses, while cumulative mechanical degradation and failure modes are assessed using the modified Mohr–Coulomb criterion. The results show that cement sheath temperature changes significantly, and stresses vary periodically with fracturing stages. The injection period is the most critical stage for cement sheath failure. Lower casing pressure and reduced fracturing fluid displacement can improve stress distribution and reduce damage. Higher initial fluid temperature increases radial stress but decreases tangential stress, while shallower horizontal well depth weakens temperature–pressure coupling. Optimizing these parameters can mitigate cement sheath damage, enhance structural integrity, and ensure safe fracturing operations.

Hydraulic fracturing is critical for unconventional oil and gas development, yet perforation-induced initial damage impairs the integrity of the casing–cement sheath–formation assembly, causing fracturing fluid channeling and reduced stimulation efficiency. A stress-seepage coupling numerical model was established to simulate interface fracture initiation, propagation, and sealing failure, quantifying axial and circumferential channeling evolution at the cement–formation interface. Key parameters (casing eccentricity, cement elastic modulus, injection rate, and minimum horizontal in situ stress) were systematically analyzed. Results show fluid preferentially migrates through perforation-weakened zones, with channeling initiating via axial debonding, then circumferential propagation, and finally dominant axial extension. Casing eccentricity exacerbates channeling, while higher cement elastic modulus or in situ stress mitigates it significantly; injection rate affects channeling length but not fracture initiation/propagation pressures. This study provides theoretical and practical guidance for fracturing channeling risk control.

The cement plug-casing interface is critical for long-term wellbore integrity in well abandonment to prevent fluid channeling. However, traditional cement easily debonds under long-term in situ stress and fluid exposure, causing seal failure and safety risks. To address this issue and overcome the limitations of conventional cement, a three-dimensional finite element model was established based on stress-seepage coupling theory. A systematic comparative analysis of the interface debonding mechanisms for three materials—cement, resin, and alloy—and their different combination sequences was conducted. The entire process of interface damage was quantified. The effects of material combination, formation elastic modulus, and injection rate on sealing performance were analyzed. Results show that the stiffness gradient dominates the failure mode, and the “cement–resin–alloy” configuration best suppresses damage propagation, reducing failure height by about 30%. Additionally, interface integrity is sensitive to formation constraints and operational parameters: the interface failure height decreases as the formation elastic modulus increases, and increases as the injection rate rises. The findings of this study can provide a theoretical basis and engineering reference for the optimal design of composite plugging barriers in demanding operational conditions, such as those encountered in carbon sequestration wells.

With the development of deep geothermal resources, including hot dry rock, the issues of low rock-breaking efficiency and wellbore instability encountered when drilling into high-temperature granite reservoirs have become increasingly prominent. The study aims to elucidate the physical degradation and fracture failure mechanisms of granite exposed to high temperatures and thermal shock. The mineral composition and microstructure of granite were analyzed by X-ray diffraction (XRD) combined with field emission scanning electron microscopy (FE-SEM). Systematic experiments were conducted to investigate the thermal damage mechanisms and mechanical properties of thermal-treated (25 °C to 600 °C) granite under different cooling conditions (natural cooling, water cooling, LN₂ cooling). The experimental results show that the physical parameters of granite exhibit significant path dependence on temperature and cooling rate. When the temperature exceeds 400 °C, the rock undergoes pronounced nonlinear volumetric expansion and a sharp increase in porosity, with P-wave velocity decaying exponentially as the temperature rises. Mechanical tests reveal that high temperature considerably weakens the rock tensile strength. For granite at 600 °C, the maximum reduction in strength reaches 80.79%, and faster cooling leads to greater strength degradation. Additionally, 3D morphology analysis indicates that the section roughness of granite increases exponentially with temperature, where the arithmetic mean height S_a more comprehensively reflects the overall characteristics of surface morphology and demonstrates the strongest ability for characterizing strength. These findings provide a theoretical basis for the efficient volumetric fracturing and rapid drilling technologies applicable to hot dry rock.

Summary: When drilling in high-strength, abrasive rock formations, rotary percussion drilling technology has garnered widespread attention for its efficient rock-breaking capabilities. However, existing investigations into the compatibility of polycrystalline diamond compact (PDC) cutters with rugged rock formations are still insufficient. Accordingly, based on rock dynamics theory, a numerical model for rock cutting by single-, dual-, and triple-cutter combinations composed of conventional cutters and special-shaped cutters, under dynamic-static load coupling, was established. The influence of laws of cutter type, rake angle, and cutter spacing with reference to depth of penetration and rock fragmentation output was assessed; the variation laws of rock-breaking effects under combinations of different cutter types and numbers were analyzed, and the multicutter rock-breaking mechanism under percussion drilling conditions was revealed. Research outcomes indicate that the axe cutter outperforms the planar cutter remarkably in terms of rock-breaking efficiency—its average penetration depth reaches 6.81 mm, which is 49.48% higher than that of the planar cutter (3.44 mm), and the corresponding cutting area (216.87 mm2) is also 54.0% larger than that of the planar cutter (140.81 mm2); among different cutter shape combinations, compared with the single-cutter setup, the dual-cutter structure outperforms in rock-breaking efficiency, and the combined planar-axe cutter arrangement attains the optimal effect; concerning the rake angle adjustment of the multicutter system, the optimal rake angle range for the front cutter is 15–20°, and the rear cutter achieves the best secondary fragmentation effect when its rake angle is 10–15°; in terms of the spacing setting in the multicutter system, the lateral spacing exerts a minor influence on the penetration depth, but the cutting area increases significantly with the increase in lateral spacing (with a total increment of 50.83 mm2); the effect of longitudinal spacing on penetration depth is deemed insignificant, but due to the reduced overlap of cutting zones, the cutting area gradually decreases as the longitudinal spacing increases (from 211 mm2 to 179 mm2). The findings provide valuable guidance for optimizing cutter geometry, designing cutter arrangements on drill bits, and enhancing drilling efficiency in deep formations.