Characteristics of middle-high-rank coal reservoirs and prospects for CBM exploration and development in western Guizhou, China

Western Guizhou is a favorable region for coalbed methane (CBM) exploration and development in Guizhou Province. Affected by complex geological conditions, the heterogeneity of middle-high rank coal reservoirs is strong, and the geological and engineering control effects on enrichment and high productivity of CBM are significantly different. This paper comparatively analyzes the characteristics of middle- and high-rank coal reservoirs in the western Guizhou region, revealing the productivity characteristics and gas/water production rules of typical middle-high rank CBM high-yield wells. Moreover, it establishes a geological-engineering control model for the enrichment and high productivity of CBM in middle-high-rank coal seams in the western Guizhou region and analyzes the exploration and development potential and direction of middle-high-rank CBM in the region. The results are as follows. First, the middle-rank coal in the western Guizhou region is represented by the Liupanshui coalfield, where cleats and fractures are well developed, the content of macropores and mesopores is relatively high, the permeability is good, and the adsorption capacity is relatively weak. The high-rank coal is represented by the Zhina coalfield, where micropores are well developed, the permeability is greatly affected by burial depth, and the adsorption capacity is strong. Second, the difference in the characteristics of middle-rank and high-rank coal reservoirs in the western Guizhou region is mainly reflected in the pore-permeability conditions and adsorption capacity. The porosity and permeability of middle-rank coal samples are both greater than those of high-rank coal samples, and the development of secondary microfractures is the main reason for the relatively high permeability of middle-rank coal, while the well-developed micropores in high-rank coal provide more space for CBM preservation. Third, the CBM high-yield wells in middle-rank coal seams in the western Guizhou region have the characteristics of “relatively high water production and high gas production,” while those of high-rank coal seams have the characteristics of “low water production and high gas production” or “high water production and low gas production.” The gas content and permeability of the coal reservoirs are the common constraints on whether CBM in middle-high rank coal seams can be highly productive under complex geological conditions. Fourth, the enrichment and high productivity of CBM in middle-high rank coal seams under complex geological conditions is the result of the synergistic matching of depth structure hydrology and adaptive fracturing production techniques. The CBM enrichment model, reservoir reformability, and gas/water production characteristics under geological and engineering synergistic control are unique and complex. Fifth, the middle-high rank coal seams in the western Guizhou region exhibit the basic resource characteristics of high gas content and high CBM resource abundance. Carrying out a precise evaluation of the CBM dual sweet spot system and reservoir volume stimulation are important guarantees for the efficient exploration and development of CBM in the western Guizhou region.