Technology of across-goaf drainage of coalbed methane from a lower coal seam group and its primary application: Taking Sihe mine field as an example

After many years of coal mining, a large area of goaf had been formed in the No.3 coal seam of Sihe mine field in Jincheng mining area, the pressure relief in a large area increased the permeability of the lower coal seams(seams No.9 and No.15), however, due to the limitation of goaf barrier and surface pre-extraction technology of coalbed methane, the coalbed methane of the lower coal seam group had not been effectively extracted, in order to ensure the safe production of the coal mine and the release of production capacity, based on the characteristics of goaf, a new technology of drilling, completion and fracturing in goaf was developed, focusing on the analysis of construction parameters and production capacity in the later stage, and the application effect of coalbed methane extraction technology cross the goaf in the lower coal seam group was evaluated. The results show that when the surface drilling is used to develop coalbed methane resources in the goaf, well location optimization and wellbore structure optimization should be carried out first, which can effectively ensure the success rate of drilling. The nitrogen replacement casing drilling technology and low pressure leakage grouting reinforcement technology can not only effectively reduce the risk of spontaneous combustion and even explosion of coalbed methane in the goaf, but also ensure the cementing quality of goaf section. The fracturing parameters of coal seam under goaf were optimized and the fracturing technology of coalbed methane wells with different well locations was designed to effectively extend the fracture length and avoid fracturing accidents such as fracturing through. Fine drainage control measures can effectively expand the drainage radius and improve the single well productivity. The drainage practices of more than 100 CBM wells across goaf show that the maximum gas production of a single well reached 8 832 m³/d, and the daily average gas production was up to 2 694 m³, which verifies the feasibility of the across-goal CBM extraction technology of the lower coal seam group and can be popularized and applied.