Investigation on the mechanical and failure behaviors of deep coal-rock combination using a hybrid continuum–discontinuum numerical method

Abstract Deep coalbed methane reservoirs often contain a type of coal-rock combinations—the gangue interlayers—which can lead to significant drilling problems with wellbore instability. Understanding the mechanical behaviors of these coal-rock combinations is crucial for analyzing wellbore stability. However, their mechanical behaviors differ significantly from those of homogeneous rocks, and further research is necessary to fully understand them. Therefore, this study investigated the mechanical behaviors of deep coal-rock combinations using a hybrid continuum–discontinuum method (hCDM). Firstly, the numerical model of the coal-mudstone-coal (CMC) combination was proposed using the hCDM and calibrated with experimental data. Then, numerical simulations of uniaxial compression were conducted for CMC combinations with different interlayer inclination angles, and the influences of the height ratio, joint layer, and gangue interlayer were investigated systematically. Finally, differences between the hCDM and theoretical methods were compared and discussed. The results indicated that the numerical simulation matched the experiment on the binary coal-rock combination within 2% error. As the interlayer inclination angle increased, the uniaxial compressive strength (UCS) exhibited fluctuations around that of coal but notably lower than that of mudstone, while the Young’s modulus gradually increased within the range observed for coal and mudstone. Meanwhile, the stress localization within the CMC combination became more pronounced, and the Poisson’s effect had a significant impact on stress localization, consequently altering crack initiation and failure modes. The UCS of the CMC combination was significantly enhanced by increased height ratio, joint layer strength, and interlayer rock strength, but the strength increase was consistently less than the interlayer’s enhancement. Greater strength improvements occurred at higher inclination angles, but excessive joint layer strength did not proportionally strengthen the CMC combination. These findings provide theoretical guidelines for deep CBM drilling and exploitation.