Gas Pressure Evolution Characteristics in Multiple Coal Seams during Cross-Seam Borehole Extraction at Different Angles

In order to investigate the influence of borehole angles on the gas extraction efficiency of multicoal seams in floor rock roadways, a three-dimensional geometric model was established in this paper. Based on the fluid-solid coupling model, COMSOL software was used to carry out the gas extraction process of single and multiple boreholes with different angles. These simulations enabled a comprehensive analysis of gas pressure evolution in the coal seam and an accurate determination of the effective drainage volume. The results indicate that when the angle between the borehole and the coal seam is 90°, the contour lines of gas pressure are circularly distributed with the borehole serving as the center. As the angle decreases, the contour lines of gas pressure tilt toward the direction of the borehole's inclination, with the contour lines of the coal seam becoming more inclined. The smaller the inclination angle of the borehole relative to the coal seam, the larger the contact area with the coal seam, resulting in a larger gas extraction area at any given time. As time progresses, the effective extraction area gradually expands during gas extraction, with a smaller angle leading to a faster rate of increase. After 150 days of gas extraction, the gas pressure within the areas between boreholes in the coal seam with smaller drilling spacing (<i>M</i>1) has decreased to below 0.74 MPa, whereas there remain inadequately extracted regions in the coal seam with larger drilling spacing (<i>M</i>2), with gas pressures exceeding this critical threshold. This research provides theoretical support for optimizing the spatial layout of gas extraction boreholes in multicoal seams, which aims to improve extraction efficiency, and prevent gas outbursts and explosions.