Collapse Pressure Distribution along a Deep Horizontal Coalbed Methane Well in the Daning-Jixian Block

Abstract

Collapse-pressure estimation is essential for managing wellbore instability in coalbed methane (CBM) drilling, where weak, stress-sensitive coal formations present major operational risks. This study develops an analytical model to evaluate collapse pressure using the Mohr-Coulomb (MC) and Mogi-Coulomb (Mg-C) failure criteria. Because collapse pressure cannot be solved explicitly, the governing nonlinear function is evaluated implicitly using the Bisection Method, ensuring robust numerical convergence. The workflow integrates in-situ stress transformation, wellbore stress redistribution, principal-stress computation, and effective-stress evaluation for three azimuths and six segments along the horizontal interval. Model verification against published benchmark data confirms the correct implementation of the stress relationships and failure formulations. Application to the deep horizontal CBM well JS6-5P03 in the Daning-Jixian Block shows pronounced variation in collapse pressure with both azimuth and segment position. The 270° azimuth exhibits the highest collapse-pressure demand, whereas the 315° azimuth provides the most favorable stability conditions. The Mg-C criterion consistently predicts lower and more realistic collapse pressures than the MC criterion due to its incorporation of the intermediate principal stress however, localized deviations occur in stress-sensitive intervals (Seg-3 and Seg-5). Conversion of collapse pressures to equivalent mud density (EMD) indicates that the minimum safe mud weight for the entire interval is 1.392 g/cm3 at 315° (Seg-3) below which collapse risk increases significantly. These results underscore the importance of segment-wise analysis and true-triaxial strength modeling for designing stable horizontal well sections in deep CBM reservoirs.