Comparative Analysis of the Gas Extraction Effects With Different Numbers and Spacings of Boreholes Drilled Along a Coal Seam

ABSTRACT

The number and spacing of boreholes drilled along a coal seam are important parameters of the borehole layout. Despite the previous extensive research by many experts, quantitative and visual comparisons of the gas extraction effects with different borehole spacings and numbers are rare. Here, the effective gas extraction range, delimited by the 0.74 MPa isobaric surface and lines, was simulated. The 0.74 MPa isobaric surface around multiple sets of boreholes at 60 days of gas extraction is wavy. At 50 days, the isobaric lines around a single set of boreholes are approximately circular, whereas those around multiple sets of boreholes are approximately elliptical. When five sets of boreholes are used, the short semiaxis of the elliptical contour around the middle borehole is approximately 59% greater than the effective extraction radius of a single borehole. Among the spacings investigated, the effective extraction volume V5 peaks at a borehole spacing of 5 m at 90 days; but, the isobaric lines at the top and bottom of the working face are concave inwards, that is, certain gas pressures exceed 0.74 MPa, thereby increasing the likelihood of gas emissions. Thus, at 90 days, the efficiency of gas extraction in this studied working face is higher as the borehole spacing is 4 m. This approach, which takes into account the superimposed effect of boreholes drilled along a coal seam, the radius of gas extraction, isolines, isobaric surface and the effective extraction volume, offers theoretical guidance for the arrangement of boreholes.