Coalbed methane drainage model and key technology for “one surface well used in three field” in coal mining areas

In order to realize the whole life cycle of the same surface well in the process of pre-mining, in-mine cooperative drainage and residual coalbed methane recovery in the goaf, and improve the service life and gas drainage efficiency of the wellbore of the coalbed methane surface well. Relying on the stratified mining project of high gas thick coal seam in Yuecheng Mine of Jinneng Holding Group, the surface pre-pumping well of the 2301 working face of the mine was selected as the object, and the collaborative research on overlying strata movement and wellbore response was carried out, and the structural stress and drainage channel evolution characteristics of the surface well under the condition of “one surface well used in three field” were analyzed. Based on the migration theory of “horizontal three zones and vertical three belts” in the goaf, the stress section and potential failure site of the surface well at different mining stages are revealed, and the combination technology of local protection of the wellbore and the protection of the screen pipe section is proposed to form a wellbore anti-deformation system composed of safety protection screen pipe and cut seam casing. Combined with the characteristics of overlying strata movement and fracture evolution after upper stratified mining, the transformation and reuse process of surface pre-pumping wells is constructed. The engineering application results show that: The surface well is arranged at the inflection point of surface subsidence and appropriately biased to the center of the stope, which can effectively avoid the influence area of the mining core and ensure the stability of the wellbore. By hanging at the wellhead and laying the cutting casing in the fractured zone, a four-in-one efficient gas migration channel of “wellbore-cutting casing-perforating casing-overlay fracture” is constructed. After the upper layered mining, the pre-pumping well located in the “O” circle, at the anticline structure and within the oblique length of 0.2-0.4 times the working face from the return air lane can achieve “one surface well used in three field”. Applying this technology, the surface wells of Yuecheng Mine have extracted a total of 5 856.5×10⁴ m³ of coalbed methane, with an average methane concentration of 51.1%, which realizes the extension of the function of pre-mining pre-extraction wells to in-mining and post-mining extraction. Through “one well and three uses”, the maximum gas concentration in the upper corner of the 2301 working face was reduced from 0.77% to 0.45%, the minimum value was reduced from 0.22% to 0.06%, and the average concentration was reduced from 0.34% to 0.25%. The integrated technology of “one surface well used in three field” gas drainage mode and wellbore protection and channel construction has been researched and formed, which provides a technical idea for the multi-stage continuous utilization of surface wells and the coordinated development of coal-gas in high-gas mining areas.