Numerical Simulation of Elastoplastic Wellbore Surrounding Stress Field Model Coupling Thermal‐Hydro‐Mechanical‐Adsorption (THMA) in Coalbed Methane Wells

ABSTRACT Existing studies on coalbed methane wellbore stability have inadequately addressed the systematic consideration of coal's elastoplastic constitutive behavior and matrix shrinkage/swelling stresses induced by gas adsorption–desorption. It perturbs the initial wellbore stress equilibrium and incurs risks of borehole collapse and formation instability. This study, based on the stress equilibrium equation of elastoplastic mechanics and combined with seepage mechanics, Langmuir theory, and thermodynamic equations, thoroughly considers the heterogeneity of circumferential stress around coal–rock gas wellbores, and finally establishes a thermal‐hydraulic‐mechanical‐adsorption coupling elastoplastic circumferential stress field model for coalbed methane wells. As verified, the model satisfies the Krisch formula. This study analyzed the influence of coal–rock adsorption and various stress fields on wellbore stability. Results show that: (1) after considering adsorption stress, in the plastic zone, the radial stress increases; the circumferential stress decreases when r > 0.10537 m. In the elastic zone, the radial stress increases while the circumferential stress decreases. (2) Furthermore, after introducing thermal stress, compared with the original model, in the plastic zone, the radial stress increases significantly; the circumferential stress decreases when r > 0.13407 m. In the elastic zone, the radial stress increases and the circumferential stress decreases. (3) Specifically, the influence of the adsorption parameter ε L is analyzed. In the plastic zone, as ε L increases, the radial stress increases accordingly; the circumferential stress decreases when r > 0.10538 m. In the elastic zone, the radial stress increases and the circumferential stress decreases. Moreover, considering different parameters has an impact on the plastic radius, wellbore circumferential stress. This study reveals the law of variation in the stress distribution state around the wellbore under multi‐field coupling effects, providing a solid theoretical foundation for the accurate determination of the safe density window of drilling fluid and the maintenance of wellbore stability in coal–rock gas well drilling engineering.