Coal reservoir characteristics and coalbed methane resource assessment in Huainan and Huaibei coalfields, Southern North China

The Huaibei and Huainan coalfields in southern North China contain abundant coalbed methane (CBM) resources (1.1 × 1012 m3). Net accumulated thicknesses of all minable coal seams are 18–32 m in Huainan and 8–18 m in Huaibei coalfields. Coal rank in the Huainan coalfield (high volatile A and B bituminous, Ro 0.65–0.85%) is generally controlled by geothermal metamorphism. In comparison, coal rank in the Huaibei coalfield varies from high volatile A bituminous to semi anthracites (0.8 to 2.8% Ro) and is controlled by the influence of the Cretaceous Yanshanian magmatic metamorphism. In both coalfields, coal reservoirs have low permeability values of 0.0013–0.224 mD and porosity of 0.9–10.9%. Pores in coals are dominated by those with diameter < 100 nm, which are favorable for gas adsorption but unfavorable for gas permeability. Pore morphology is mainly represented by half-open tubular pores with poor interconnectivity. Coals in the Huaibei coalfield have higher pore volume than those in the Huainan coalfield. Microfractures in coal reservoirs are commonly less than 5 µm wide and 300 µm long, and have varying frequencies of 17–563 per 9 cm2. In both coalfields, the abundance of microfractures in coals is related to the coal structures; commonly more abundant in sheared coals (> 200 per 9 cm2) than in the normally structured coals (< 200 per 9 cm2). The in-place gas content is generally 8–16 m3/t in Huaibei and 10–30 m3/t in Huainan coalfields. These data yield an estimated in-place CBM resource of 375.247 × 109 m3 for the Huaibei and 722.677 × 109 m3 for the Huainan coalfields. Based on the CBM resource investigation and reservoir evaluation, the most prospective target areas for CBM production are evaluated, which include the Nanping, Taoyuan mines of the southern Suxian deep coal district, and Zhuxianzhuang and Luling mines in eastern Suxian area of the Huaibei coalfield, and the area around the Panji-2 mine in Panji-Xieqiao coal mining districts in the Huainan coalfield.