Comprehensive study on the enrichment mechanism of coalbedmethane 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 <500,500-800 and >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.