Multifield Coupling Effects on Coal-Rock Mechanical Response and Wellbore Stability

Coal-rock samples from the Taiyuan Formation in the Ordos Basin exhibit pronounced structural heterogeneity and multifactor sensitivity. These features frequently cause wellbore instability (including collapse, spalling, and lost circulation), which severely restricts drilling efficiency and safety. This study integrates macro- and microstructural characterization, mechanical experiments, and numerical simulations to systematically investigate the individual effects of cleat heterogeneity, dynamic loading, and drilling-fluid hydration-seepage coupling on coal-rock mechanical behavior and near-wellbore stress evolution, as well as their multifactor synergistic mechanisms. Individually, cleat heterogeneity governs mechanical anisotropy and spatially dependent failure patterns. Loading amplitude and frequency control stress accumulation and damage evolution. Hydration-seepage coupling progressively degrades rock strength and permeability, promoting progressive failure. Under multifactor synergy, cleat heterogeneity causes instability zones to be spatially discrete. Hydration-induced strength reduction and permeability degradation provide the mechanical conditions for failure zone expansion, while fluctuating pressure triggers fatigue damage and drives fracture coalescence. The combined action of these factors causes the discrete instability zones to coalesce and expand, ultimately leading to large-scale wellbore collapse. This synergistic chain is identified as the primary driver of complex wellbore instability in the Taiyuan Formation. The findings provide key insights for wellbore instability prevention and engineering design optimization.