Modeling of Multifield Coupling and Borehole Placement for Hydraulic Flushing and Engineering Applications

To investigate the pressure-relief and permeability enhancement mechanisms of hydraulic flushing (HF), this study conducted the triaxialseepage experiments on raw coal; the permeability changes during the loading process were analyzed, and the coal permeability evolution model was derived considering the damage effect. Given the boundary conditions of gas seepage and stress fields, a multifield coupling model was developed to reflect coal seam methane (CBM) extracted by the HF borehole. Based on the actual geological conditions of Yi'an Coalmine, the three-dimensional (3D) numerical simulation was implemented to investigate the effects of coal discharge amount on the relief range, coal permeability, and effective extraction radius of the HF borehole. Moreover, the validity of the coupling model was confirmed through field tests on hydraulic flushing. Then, the interactions of various factors, including coal discharge amount, borehole spacing, initial CBM pressure, and coal permeability, were investigated using the response surface methodology (RSM). Through the results obtained, it was found that the interactions between the factors involved were very significant and caused an obvious time effect on CBM extraction. The interaction between coal discharge amount and borehole spacing increasingly affected the extraction effect of HF boreholes, whereas the interaction between initial CBM pressure and coal permeability gradually weakened. Then, the RSM regression model was established to optimize the borehole placement parameters using predicted extraction time and coal discharge amount as variables. The regression prediction data were consistent with the simulation results and mutually validated. Therefore, based on the elimination of gas outbursts and reduced construction costs, the modeling approach and the RSM model introduced in this study can be used to optimize the precise placement parameters of HF boreholes for enhancing CBM recovery within the expected extraction period.