Comparison of Fracture and Permeability between Tectonically Deformed Coal and Briquette Coal using X-ray Computer Tomography and the Lattice Boltzmann Method

Coal and gas outburst mainly occurs in tectonically deformed coal (TDC). TDC has low strength and is difficult to core; thus, it is often replaced by briquette coal (BC) for research. However, whether the fracture and permeability in BC are consistent with TDC remains to be verified. This study used X-ray computer tomography to characterize the fracture of TDC and BC and further explored the difference of permeability between them through the triaxial permeability experiment and lattice Boltzmann method (LBM) simulation. The results showed that TDC has strong heterogeneity and developed mostly complex fracture systems with large apertures. The fractures are primarily flat with disorderly directions, slightly poor connectivity, but strong connection strength. However, BC is formed by the accumulation of particles and exhibits micro-fractures with good homogeneity, which are mostly tube and slit with good connectivity but weak connective strength. During the stress loading process, the permeability of TDC showed a three-stage (i.e., sharp → rapid → steady) declining trend , while the BC had a two-stage (i.e., sharp → rapid) decline. The fracture compressibility of TDC decreased first and then increased with the increase in stress, exhibiting mostly plastic deformation, while the fracture compressibility of BC is far less than that of TDC, exhibiting mostly elastic deformation. Moreover, the average permeability damage and irreversible permeability damage rates of TDC were 0.0133 mD (1 mD = 1 millidarcy = 9.869233 −16 m 2 ) and 52.56%, respectively, which were higher than those of BC. Therefore, a new digital rock physics method based on X-ray CT and LBM technology is proposed to study the fracture and permeability.