Characteristics of coal fractures and the influence of coal facies on coalbed methane productivity in the South Yanchuan Block, China

Micro-fractures and macro-fractures are abundant in coal reservoirs, and the formation of such fractures depends on the coal-bearing environment. In addition to the development of fractures, the gas content and coalbed methane productivity are also controlled by the coal-bearing environment. Research on micro-fracture characteristics is conducted through microscopic observation, and research on macro-fracture characteristics is conducted by investigating coal cores. The method of quantitative statistics is adopted for the measure of the submaceral, and the coal facies parameters are calculated. The coal structure is explained using logging data. Then, the coalbed methane productivity controlled by the coal facies is examined. The results show that Type D micro-fractures are well developed, followed by Type C micro-fractures. The micro-fractures are primarily tensional fractures, implying that the formation of micro-fractures is controlled by external stress. Micro-fractures pass through the macropores and effectively link with other micro-fractures, whereas macro-fractures are primarily developed in clarain bands and cataclastic texture coal. The gas content increases with the gelatification index (GI) and ratio of vitrinite to intertinite (V/I) in the coal reservoir, which are favourable coal facies geological conditions for coalbed methane production. However, whereas the production of the coalbed methane wells is in its initial stage, the methane produced is mostly free and strongly desorbed gas. The coal structure primarily contributes to the permeability of the coal reservoir. The development of Type II coal can increase the production of methane, whereas the development of Type III coal has the opposite effect.