Wellbore Instability of Coal Seams under Different Lateral Stress Conditions

The stability of the wellbore in multibranch horizontal wells is a critical issue that significantly impacts gas production and service life. To gain a more comprehensive understanding of the effects of stress state on wellbore stability in multibranch horizontal wells in coal seams, a series of biaxial compression tests were conducted on coal specimens with wellholes under varying lateral stress states. Digital image correlation technology was utilized to monitor the deformation and crack propagation throughout the testing process. Furthermore, theoretical analysis was employed to investigate the influence of lateral stress states on the coal wellbore instability. The findings revealed that initial failure of the coal specimens predominantly occurred near the wellbore and primary fractures at different σ h values. As Δσ decreased, the self-stabilization time for the wellbore integrity increased. When σ h is greater than σ v , the upper and lower parts of the wellbore are more susceptible to deformation and collapse. Conversely, when σ v is greater than σ h , the left and right parts of the wellbore were more prone to collapse. Both the biaxial stress difference and the coefficient biaxial stress difference (Cov) have significant impacts on the wellbore stability in coal seams. Therefore, selecting wells with lower values for Δσ and Cov can effectively enhance the stability of multibranch wells in coal seams, improve the lifespan of coalbed methane wells, and increase coalbed methane production. The research results are of great significance in guiding the design improvements and increasing gas production for the multibranch horizontal wells in Zhi’na and similar coalfields.