Numerical simulation of boreholes for gas extraction and effective range of gas extraction in soft coal seams

Abstract

Gas disasters are a major factor influencing safe production in mines: Gas extraction can reduce the gas content in coal seams, providing a guarantee of safer production. The parameters for gas extraction are the primary factors influencing the effectiveness thereof. Aiming at the creep properties of soft coal, a fluid-solid coupling mathematical model considering creep properties of coal was established based on dynamic evolution equation for permeability considering the effects of matrix shrinkage and effective stress. Additionally, by utilizing COMSOL Multiphysics software, the gas extractions from a single borehole and multiple boreholes were calculated. Moreover, the parameters for gas extraction were optimized and applied and tested in field conditions. The result showed the reduction in gas pressure around the boreholes was larger than that from a single borehole when conducting gas extraction from multiple boreholes. The borehole spacing when extracting gas in coal seams by drilling multiple boreholes should be more than twice that of the effective drainage radius. The optimal borehole spacing ranged from 3.2 to 4.2 m for gas extraction lasting 180 d. Numerical simulation was carried out to ascertain the distribution of stress on coal around a roadway. The result revealed that the damage radius of the roadway was 11.8 m, and a reasonable hole-sealing depth was 12 m. On condition that the borehole spacing during gas extraction from multiple boreholes was 4 m, the reasonable pre-extraction time was 180 days taking the gas pressure being reduced to <0.74 MPa as a critical point. Furthermore, the gas content, the amount of extracted gas, etc, in a working face after the parameters for gas extraction were optimized were measured. The result suggested that the effect of gas extraction after optimizing parameters conformed to industry standards.