To address the issues of traditional gels in high-temperature reservoir leakage plugging, such as injection–retention imbalance, poor high-temperature stability, and insufficient thixotropy, this study developed a thixotropic polymer gel system via molecular design and component optimization, aiming to achieve excellent thixotropy, high strength, and wide temperature adaptability (80–140 °C) while clarifying its gelation mechanism. First, the optimal polymer was selected by comparing the high-temperature stability and crosslinking activity of AM/AMPS copolymer (J-2), low-molecular-weight acrylamide polymers (J-3, J-4), and AM/AMPS/NVP terpolymer (J-1). Then, the phenolic crosslinking system was optimized: hexamethylenetetramine (HMTA) was chosen for controlled aldehyde release (avoiding poor stability/dehydration) and catechol for high crosslinking efficiency (enhancing strength via dense crosslinking sites). Urea–formaldehyde resin (UF) was introduced to form a “polymer-resin double network,” improving high-temperature compression resistance and long-term stability. Cyclic shear rheological tests showed the gel system had a larger hysteresis area than the polymer solution, indicating excellent thixotropy before gelation. It gelled completely at 80–140 °C (gelation time shortened with temperature). At 120 °C, its viscosity was 7500 mPa·s, storage modulus (G′) 51 Pa, and loss modulus (G″) 6 Pa, demonstrating good shear thixotropy. The final system (1% J-1, 0.3% catechol, 0.6% HMTA, 15% UF) is suitable for high-temperature reservoir leakage plugging.

The stability of deep marine shale wellbores is influenced by both bedding anisotropy and drilling fluid intrusion. Existing models fail to adequately account for the coupled effects of intrusion depth and strength degradation. This study, targeting Longmaxi Formation shale, established a collapse pressure prediction model incorporating drilling fluid intrusion depth through direct shear tests and nuclear magnetic resonance (NMR) techniques. Experimental results indicate that shear strength reaches its minimum at β = 45°, decreasing by approximately 60% compared to β = 0° or 90°. Intrusion causes exponential decay in bedding plane strength, with the cohesion degradation coefficient λ_c = 0.158 mm⁻¹ significantly exceeding the internal friction angle degradation coefficient λ_φ = 0.089 mm⁻¹. Sensitivity analysis indicates that bedding angle and invasion depth rank third (±3%) and fourth (±1.5%), respectively, in influencing collapse pressure. Field validation confirmed excellent model prediction accuracy (R² = 0.956; RMSE = 0.55 MPa; MAPE = 1.05%), with all errors below 4%. This model accurately predicts the time-varying characteristics of collapse pressure, providing a theoretical basis for optimizing the design of drilling fluid density.