Wellbore Stability for Extended-Reach Drilling in Deep Coal Seams Under Heterogeneous In Situ Stresses: A Laboratory-Calibrated Framework

Wellbore instability is a critical challenge in deep coalbed methane (CBM) development, especially for extended-reach horizontal wells subjected to pronounced horizontal in situ stress anisotropy. This study integrates uniaxial and triaxial laboratory testing of deep coal samples with an analytical Mohr–Coulomb-based model to quantify how horizontal stress contrast redistributes near-wellbore stresses and controls collapse pressure. Mechanical parameters from core experiments and log-derived stresses are embedded into the model and applied to six representative horizontal wells in the Ordos Basin. At 2000 m depth, circumferential stress perpendicular to the minimum horizontal stress direction exceeds orthogonal directions by 20 MPa (wells 1–3) and 40–50 MPa (wells 4–6). As the horizontal stress ratio n = σH/σh (where σH and σh are the maximum and minimum horizontal in situ stresses, respectively) increases from 1.07 to 1.28, the equivalent mud density required to prevent collapse rises from 1.53 to 1.77–1.81 g/cm3, representing a 15–18% increase. These results demonstrate that explicitly accounting for horizontal stress anisotropy—calibrated by uniaxial and triaxial tests—is essential for reliable collapse-pressure estimation in extended-reach wells drilled in deep coal seams, without invoking additional trajectory-optimization assumptions.