Study on Breakage Characteristics and Anti-Breakage Theory for Pressure-Relieved CBM Drainage by Surface Wells in Coalbed Groups Covered by Super-Thick Pedosphere

Abstract

During the mining of highly gassy and low-permeability coalbed groups, massive pressure-relieved desorbed coal bed methane (CBM) is stored in the fracture zone and in the gob under the impact of mining activities. Surface wells can cross the fracture zone and the gob to drain a large amount of highly concentrated pressure-relieved CBM. CBM drainage by surface wells is an effective technology for disaster control. However, it is difficult to drill or maintain surface wells in the mining disturbance zone and overlying strata in the gob. The keyisto prevent the surface wells in the mining disturbance zone from being broken and to keep the whole surface wells unblocked. This study adopted a variety of research methods, including similar-material simulation in laboratory, numerical simulation, onsite monitoring and industrial tests. Specifically, the strata structure of surface wells for pressure-relieved CBM drainage was explored, and the characteristics of roof strata movement and stress redistribution under coalbed group mining were analyzed. Besides, positions where the surface wells are prone to breakage were found. Furthermore, based on the mechanical analysis and breakage characteristics of surface wells, the anti-breakage principle of “resistance and dodge” and the surface well completion and protection method of “upper stop and lower leak” were proposed. The proposed theory and method were then applied to pressure-relieved CBM drainage surface wells in a coalbed group covered by the super-thick pedosphere in the Huainan Coal Field. The application results indicate that the proposed theory and method succeeded in solving the breakage of surface wells and realizing the smooth transport of CBM product. The surface wells can completely resist the mining disturbance, achieving an average single-well pure CBM production of 2.71 million m3, an average service period of 482 d, and a maximum pure CBM production of 4.32 million m3.