Research status and prospects of secondary enrichment and accumulation of residual coalbed methane resources in abandoned mines

In promoting the achievement of the carbon peak and carbon neutrality goals in China, the development of residual coalbed methane resources in abandoned mines is of important practical significance. However, the development of the residual coalbed methane is characterized by its secondary dynamic reservoir formation after disturbed under original geological conditions. Therefore, the related literature at home and abroad in recent years is summarized as follows. (1) The disturbed area of the overburden and floor in coal seam mining can be divided into the gas conduction fracture zone, pressure relief zone and non-desorption zone. The influence range of coal seam overburden mining by the long arm method can reach about 100 m, and the influence range of floor mining can reach about 50 m. (2) The residual coalbed methane resources in the mining disturbed area are mainly in the free state, adsorbed state and dissolved state, and the proportion of free gas increases compared with the in-situ reservoir. According to the distribution law of the mining disturbed stress field, fracture field, seepage field, the storage space of residual coalbed methane can be divided into the three-dimensional pressure relief zone, one-dimensional pressure relief zone and original location area. The top of the three-dimensional pressure relief zone of the overburden is the potential coalbed methane enrichment area. (3) The evaluation methods of residual coalbed methane resources in abandoned mines mainly include the monthly decline curve method, source superposition method and indirect deduction method. The latter two methods have high applicability in China. Finally, it is pointed out that accurately delineating the enrichment space and mining fracture field of residual coalbed methane, revealing its occurrence characteristics and dynamic migration and accumulation process, and establishing its dynamic evaluation model are the geological theoretical basis for the successful development of residual coalbed methane resources in abandoned mines.