Joint ground and underground gas extraction mode and its key technology for outburst coal seam group

China’s coal seams are characterized by high ground stress, low permeability, high gas pressure, and high gas content, and their gas outburst hazards are serious. Therefore, some effective gas extraction measures must be taken for coal seams. However, only ground gas extraction technology is prone to emerge extraction gaps, resulting in poor extraction effects. Moreover, it is a challenge to achieve coal seam abatement in a short period because of the large work and extended period using underground gas extraction technology only. Combining ground and underground gas extraction technologies to achieve a joint ground and underground gas outburst prevention is one of the key technologies to ensure a safe coal mining. In the coal seam group mines, different modes of joint ground and underground gas outburst prevention should be adopted according to the distance between coal seams in order to rapidly eliminate gas outburst risk in the coal seam group. Based on the advantages of combined ground and underground gas extraction technology, the connotation of time and space conversion and complementary advantages of combined ground and underground gas outburst prevention technology is clarified. Long-time and comprehensive coverage extraction of gas from ground wells achieves a coal seam gas reduction in exchange for underground operation space. Underground drilling in the underground space accurately extracts gas from the target area, shortens the gas outburst prevention time and enhances the effect of coal seam gas extraction. Because of the characteristics of close distance coal seam groups, such as significant influence of interlayer mining, this paper puts forward a typical prevention model of gas outburst in close distance coal seam group “boreholes group coverage - cooperative extraction”, which realizes advance planning, advance construction and overall prevention of gas outburst. Given the characteristics of long distance coal seams with small mining pressure relief area, the typical model of “enhanced reflection improvement and progressive extraction” for the gas outburst prevention mode of joint ground and underground technology is proposed, which realizes the strengthening pressure relief, mining follow-up and progressive key gas outburst prevention in the management of long distance coal seams. In addition, the authors have independently developed the gas outburst prevention technology of joint ground and underground technology for close distance coal seams and long distance coal seams to further improve the gas outburst prevention effectiveness of joint ground and underground technology and analyzed the application effect by using the Shaqu No.1 Mine and Zhuji West Mine as typical cases. The application results show that the amount of coal mined in the Shaqu No.1 Mine in the close distance coal seam group is about 800 thousand tonnes, and the amount of gas efficiently extracted is about 4.2 million m³. In the long distance coal seam group of the Zhujixi Mine, the underground drilling quantity is reduced by about 60%, and the gas extraction standard time is shortened by about 40%.