Fracture system identification of coal reservoir and the productivity differences of CBM wells with different coal structures: A case in the Yanchuannan Block, Ordos Basin

Fracture system identification of coal reservoirs plays a vital role in evaluating favorable zone of coalbed methane (CBM) development and improving CBM well productivity. In order to identify the fracture system of the No. 2 coal reservoir, and to determine the productivity differences of CBM wells with different coal structures in the Yanchuannan (YCN) Block, Ordos Basin, China, a comprehensive analysis method from macroscopy to microscopy and from static to dynamic was provided in this work. Site observations on coal sections and observations under light microscope and scanning electron microscopy (SEM) were combined to understand the fracture characteristics of the coal seam, and the spatial geological model of fracture development was built. The selected logging data was conducted cluster analysis to identify the coal structures of coring wells and to predict the coal structures of non-coring wells. The analysis results show that the No. 2 coal seam has a high fracture development degree, and is dominated by type II coal structure (cataclastic coals). The coals with different structures show evident zonation on the plane, which is largely controlled by faults. The well-developed fracture system and high permeability zone mainly concentrate on the tectonic rolling areas and the areas with small faults. Furthermore, the output data of coal fine and the drainage curves of CBM wells were performed dynamic analysis to indirectly reflect the fracture system difference and the resulting productivity difference. The results indicate that the wells with poor-preserved coals tend to produce more coal fine, and the gas production has a negative correlation with coal fine amount. The productivity pattern of CBM wells can be divided into “high yield-long-term stable yield” and “low yield-decline quickly”. Almost all CBM wells with type I coal structure and most CBM wells with type II coal structure have the first productivity pattern, and part wells with type II coal structure and all wells with type III coal structure present the second productivity pattern.