Prediction model and risk assessment of dynamic gas emission during drilling in faulted coal seams

Influenced by multiple coupling physical fields, the disparities in gas migration characteristics between coal seams and faults may be the crucial factor triggering outbursts. This study established the initial permeability distribution of faulted coal seams using Caine's model and employed a transverse isotropic permeability model to characterize the directional of gas flow within the fault. A thermo-hydro-mechanical coupled mathematical model is proposed for simulating gas emission during drilling of faulted coal seams and validated using field data. Additionally, variations in fault azimuth, dip, width, and core width were analyzed to investigate their effects on gas emission volume during drilling. Moreover, the magnitude of the interaction of the factors was analyzed using response surface methodology to identify the fault morphology with the largest gas emission volume during drilling. Finally, a multivariate nonlinear regression model was established to describe the relationship between gas emission volume and fault core width. The results showed that the gas emission rate gradually rises as the borehole length grows but surges abruptly by approximately 95% as the borehole intersects the fault zone. Increases in fault azimuth and width enhance gas emission from the borehole, while increases in fault dip and core width inhibit it. With the fault azimuth, dip, width, and core width of 40 degrees, 80 degrees, 15 m, and 1 m, respectively, the gas emission volume during drilling peaked at 105.462 m(3).This study is helpful for better predict and evaluate the outburst risk of faulted coal seams using dynamic gas emission during drilling.