Spatial distribution of coalbed methane migration channel and optimization of extraction target area in abandoned mines

Objective and Methods The spatial distribution of effective migration channels and their long-term evolution characteristics not only determine the migration path of coalbed methane (CBM) left behind in abandoned mines but also serve as an important basis for the selection of CBM pumping drilling target areas and the maximization of pumping efficiency. Taking the first level and first mining panel of Chongqing Songzao Coal Mine as the research object, this research systematically investigates the distribution patterns of effective CBM migration channels in abandoned mines under inclined multi-seam conditions through an integrated approach combining theoretical analysis, numerical simulation, and physical similarity experiments. A methodology for optimizing extraction target zones is proposed. Results and conclusions (1) The maximum heights of the repetitively mined fracture zones in the K2b, K1 and K3b coal seams obtained from theoretical calculations, numerical simulations and physical similarity tests are 72.89 m, 76.4 m and 77.3 m, respectively, and the results are highly consistent. (2) The evolution of the overburden stress field shows three-stage characteristics of ".arch-shaped pressure relief-asymmetric transfer - dynamic balance.", and eventually forms a three-dimensional asymmetric structure of " roof pressure relief dome - lateral stress shell ". (3) The development of overburden fractures follows the co-evolution law of "mining-dominated-spatial differentiation-time series accumulation." In the initial stage, the overlying strata migrate rapidly under the influence of mining to form a basic fracture network. In the middle stage, the superimposition and expansion of repeated mining impacts occur. In the later stage, after more than 10 years of consolidation and recovery, a three-dimensional "O"-shaped fracture trap is finally formed, which enters a relatively stable stage, provides an effective migration channel for CBM, and forms an enrichment area under the barrier of thick and hard limestone above. (4) After long-term compaction, the fracture height decreases to 67.97 m, showing a differentiation pattern of symmetrical trapezoid and inclined trapezoid. Combined with the migration law of CBM, it is suggested that the extraction target area should be preferably located in the upper corner area within the range of 6.41-36.70 m above the floor of K3b goaf.