Comprehensive study on the enrichment mechanism of coalbed methane in high rank coal reservoirs

The relative enrichment of high-rank coal coalbed methane (CBM) is one of the important natural endowments of China's CBM resources. China's CBM industry has made a world-class contribution on the breakthrough of large-scale commercial exploitation of high-rank coal CBM. The high-rank coal CBM production base, represented by the Qinshui Basin, is the main source of CBM production in China. Based on the practical data and a lot of works in the exploration and development of high-rank coal CBM in the Qinshui Basin, a systematic and in-depth research on the enrichment mechanism of high-rank coal CBM is carried out. The research works show that:① regional magmatic thermometamorphism is the basic geological background of high-rank coal CBM enrichment. The high-rank coal CBM enrichment areas in China consist of the southern Qinshui Basin(Jincheng), the southeast of the Ordos Basin (Hancheng-Southern Yanchuan), the north of the Qinshui Basin (Shouyang-Yangquan) and the Northern Guizhou-the Southern Sichuan (Zhijin-Junlian). The coal reservoirs in these areas are all affected by regional magmatic thermometamorphism. ② The enrichment laws of high-rank coal CBM are the results of the coupling configuration of gas content and permeability under different buried depths and structural conditions. In the Qinshui Basin, the high-rank coal CBM enrichment mode in syncline, ramp of fold limb and relatively high structural part correspond to coal seams at the depth of 800 m respectively. ③ Depth, structure (tectonic stress field) and coal-body structures, which can determine the permeability of high-rank coals, are the key controlling geological factors for the CBM enrichment in the high-rank coal reservoirs. The permeability of coals exponentially decreases with depth. The coals characterized by a high permeability are distributed in the structure position where the tensile fractures develop and is consistent with the direction of the current maximum principal stress. The cataclastic coal has a highest permeability compared to undeformed coal, granulated coal and mylonitized coal. ④ The control of these key geological factors on coal permeability by means of in-situ stress, fracture and strain failure is the fundamental mechanism of the CBM enrichment in the high-rank coal reservoirs. The transformation of fracture in the coal reservoirs during a strain failure process under the action of the geostress field is the intrinsic reason for the permeability change of high-rank coals. © 2022, Editorial Office of Journal of China Coal Society. All right reserved.