Key technology of surface extraction for coalbed methane wells crossing goaf

In order to ensure the advanced extraction and continuous development utilization of coalbed methane(CBM) resources in coal mine areas, the extraction of coal reservoir CBM underlying goaf has become one of the important topics for coal and CBM co-mining. Taking Sihe Mine in Jincheng, Shanxi Province as an example, the stress-strain distribution law of the underlying strata in the goaf of No. 3 coal seam, the evolution law of fissures in the underlying strata in the goaf and the change of permeability of the underlying coal seam were analyzed. According to the pressure relief effect of the goaf and the desorption degree of CBM in the underlying coal seam, the extraction mechanism of CBM wells crossing goaf was revealed. Through installing an open hole packer and a reverse buckle device outside 50 m position of the second spud casing above the goaf, the reverse buckle device and the above casing are recovered after the second spud section was cemented, so that the third spud casing realized effective cementing in the whole well section. It was drilled by N2 in the second spud section process from 90 m above the goaf of No. 3 coal seam to 20 m below the floor of the goaf. A complete set of system for surface drilling of CBM crossing goaf was developed, including air compressor unit, nitrogen generator unit and booster unit, the unit parameters were optimized. Based on the fracture monitoring results of CBM wells in Sihe mining area in the past, the fracturing construction parameters were optimized, and the scale of No. 9 and No. 15 coalbed fracturing was appropriately reduced. According to the extraction mechanism and gas production characteristics of wells crossing goaf, three types of gas production were classified and their gas production laws were analyzed. The research results show that No. 9 coal seam and No. 15 coal seam are both located in the deformation zone of floor drum. After mining in the No. 3 mined-out area, the permeability of No. 9 coal seam and No. 15 coal seam increased 2.70 times and 2. 65 times respectively, and the permeability of No. 9 coal seam increased to about 10×10-15 m2, the pressure relief effect is favorable. The installation of open hole packer and reverse buckle device to optimize the well bore structure, the whole well cementing has been realized and the cementing quality has been improved for the wells crossing goaf; the second spud section passes through the goaf with N2 medium, which can ensure the safe and efficient construction of the wells passing through the goaf. The fracture monitoring results show that the total length of fracture in No. 15 coal seam is 290 m, and the strike is NE150°; the total length of fracture in No. 9 coal seam is 370 m, and the strike is NE80°, the fracture effect is good. 60% of the wells in the goaf generally produce gas in one month, 29% in two to five months and 11% in more than seven months. The three types of gas production are as follows: Single peak type I: slow increase-decrease type (mainly adsorbed gas); Single peak type II: gas production rapid increase-decrease type (mainly free gas); Double peaks type (free gas+adsorbed gas). There are obvious differences in gas production law of different types of CBM. There are obvious differences in gas production laws of different types of CBM wells. For the three types of wells crossing goaf, the wellhead casing pressure was generally kept above 0.5 MPa. After entering the gas production stabilization phase, the casing pressure of most wells was controlled to be stable within the range of 0.2-0.4 MPa, so as to ensure the continuous high production of wells crossing goaf. © 2020, Editorial Office of Journal of China Coal Society. All right reserved.