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.