Optimal selection of coal seam pressure-relief gas extraction technologies: a typical case of the Panyi Coal Mine, Huainan coalfield, China

Pressure-relief gas extraction from a protected seam can be realized by surface well drilling and the use of net-like penetrating boreholes (NPB). The selection of the extraction mode is affected by the geological conditions, characteristics of the coal seam occurrence and inherent characteristics of the extraction mode. Based on a numerical simulation and field test, the Panyi Coal Mine in the Chinese Huainan coalfield is used as an example to study the effective extraction range, efficiency, cost, total amount extracted and applicable conditions of two methods. The results show that the average gas extraction concentration (93.8%) and average extraction purity (28 m(3)/min) of surface wells are higher than those (41.8% and 17 m(3)/min) of NPB by approximately 124.4% and 64.7%, respectively. The superiority of the extraction efficiency from the use of surface wells is obvious from intact wells. In addition, the unit methane control costs, methane drainage costs per cubic meter and construction period of surface drillings are less than those of the NPB. However, Surface drilling is easily restricted by poor wellbore stability and high surface environment requirements. This case study can provide valuable examples for other coal mines with similar geological conditions for the selection of pressure-relief gas treatment methods.