Abstract: Understanding wellbore instability mechanisms in coalbed methane (CBM) horizontal wells under complex stress conditions is crucial for optimizing CBM reservoir drilling. Using a self-developed true triaxial device to simulate surrounding rock deformation, this study examines how differential horizontal stresses and vertical stress variations affect deformation and failure in CBM horizontal wellbores, and obtains the following results: 1) Wellbore deformation can be divided into three stages: slow shrinkage deformation stage, stable shrinkage deformation stage, and accelerated shrinkage until closure stage. 2) In the latter two stages of wellbore deformation, the deformation of the specimen in three directions is mainly controlled by wellbore deformation and failure. 3) As the maximum or minimum horizontal principal stress increases, the slope of the vertical principal stress-deformation curve during the stable shrinkage deformation stage of the wellbore significantly increases, and the strain in all three directions per unit stress increment decreases; simultaneously, the cross-sectional area at wellbore failure increases, with a corresponding increase in critical vertical principal stress and a decrease in vertical strain. 4) When the two horizontal principal stresses increase simultaneously, the aforementioned trends become more pronounced, which is more conducive to maintaining wellbore stability. The findings of this study can provide guidance for the drilling design of CBM horizontal wells.

Key words: coalbed methane, horizontal in-situ stresses difference, wellbore deformation, wellbore stability, physical model