Evaluation of fracture system for coal marcolithotypes in the Hancheng Block, eastern margin of the Ordos Basin, China

Fracture system (micro- and hydraulic fracture) of coal marcolithotypes, namely the bright (B), semi-bright (SB), semi-dull (SD), and dull (D) coals has a great influence on the coalbed methane (CBM) seepage in microfractures as well as gas production in hydraulic fractures. To evaluate the fracture system, four sets of marcolithotype samples from the Hancheng Block in the eastern margin of the Ordos Basin, were measured with multiple experimental techniques, including the scanning electron microscopy (SEM), optical microscopy (OM), nuclear magnetic resonance (NMR), and X-ray computed tomography (X-CT). Meanwhile, the hydraulic fracture (e.g. the fracture length, width, and orientation) was also monitored by microseism. The results indicate that, by the OM and SEM, the B and SB coals usually have higher microfracture density accompanied by the better connectivity than that of the SD and D coals. But the latter are dominated in the Hancheng Block and some of microfractures were filled by the layered calcite and pyrite. Meanwhile, the microfracture fractal dimension of the B and SB coals are larger than the SD and D coals. By reconstructions of microfracture system by the NMR and X-CT, it is further verified that the development of microfracture decreases from the B to D coal, which results the hydraulic fracturing is necessary for enhancing CBM recovery in the Hancheng Block. The monitoring of hydraulic fracture shows that the orientation of the major fracture is general NE, the length is > 100 m, and the height is usually larger than the coal thickness which indicates the hydraulic fracture extension is unrestricted by coal macrolithotypes and the formation mechanism is very different from the natural microfracture. These above observations aid in understanding the fracture system heterogeneity of the coal macrolithotypes on the refinement efficiency of the CBM exploration and development (e.g. reservoir optimization and reasonable fracturing).