Coal Structure and Its Implications for Coalbed Methane Exploitation: A Review

Coal structure refers to the internal structural characteristics of coals including the degree of macroscopic and microscopic deformation, pore structure, and mechanical properties after various geothermal and geological stresses, which have substantial impacts on coal mining safety and coalbed methane (CBM) exploitation. The coal structure is an indicator of coal mechanical strength, which determines the petrophysical properties of CBM reservoirs and gas extraction or CBM production efficiency from coals. Coals with different structures are divided into primary coals and tectonically deformed coals (TDCs). To explore the macro- and microscopic petrophysics of TDCs, a wide range of techniques including geophysical logging, seismic inversion, amplitude variation with offset (AVO), scanning electron microscopy (SEM), nuclear magnetic resonance (NMR), and Fourier transform infrared spectroscopy (FTIR) have been employed. From a macroscopic perspective, the geological and geophysical methods are generally used to distinguish the degree of deformation of whole coal seams. Microscopic methods including SEM, NMR, and FTIR, are normally used to investigate the internal petrophysics (e.g., pore and fracture, macromolecular structure, and fluid performance) of TDCs. Herein, the macro- and micro-petrophysical properties of TDCs and their impacts on CBM exploration and exploitation are systematically reviewed and potential opportunities of future directions are recommended. Extensive studies have shown that the development zone of TDCs in the brittle series, notably cataclastic coals, has high potential for CBM exploration and exploitation, while that of TDCs in the ductile series is a dangerous region for coal mining and subject to gas outbursts. This review aims to provide background on the coal structure, summarize evaluation methods, highlight its geological drivers, and explain its significance in CBM exploitation. We will end with proposals for future research directions.