Macrolithotype controls on natural fracture characteristics of ultra-thick lignite in Erlian Basin, China: Implication for favorable coalbed methane reservoirs

Understanding the distribution of the fracture system in the reservoir scale could significantly benefit coalbed methane (CBM) development. Coal fractures are controlled by multiple factors, among which the influence of macrolithotypes has been widely studied. However, few studies have explained their relationship from the perspective of mechanics. Furthermore, the macrolithotypes are controlled by coal facies and thus a relationship between coal facies and fracture system characteristics is likely to be built, which could largely facilitate the prediction of fracture system development in a CBM reservoir, especially for those having ultra-thick coal seams. In the current research, mechanical properties, in combination with in-situ and laboratory observations, are carried out for different coal macrolithotypes from ultra-thick lignite reservoirs to reveal their influence on coal fracture development in Erlian Basin, China. In this basin, xylitic coal and detritic coal are mainly developed, and the former has better developing natural fractures which appears to be caused by its weaker mechanical strength as identified by the mechanical tests including the compressive, shear, and tensile strength. Moreover, coal facies based on petrography and palynology tests were analyzed to clarify the spatial distribution of macrolithotypes in the reservoir. The results show that in an ideal coal-forming cycle, the coal seams occur starting from the detritic coal, to xylitic coal, and fusitic coal at last upwards in order. In a realistic sag, however, multilayer individual xylitic coal intervals are likely to exist, especially for ultra-thick lignite. In this situation, coal facies analysis could explain xylitic coal distribution, thereby predicting the fracture system development.