Establishment of the equivalent structural model for the tectonic coal and some implications for the methane migration

The establishment of the equivalent structural model (ESM) is the foundation to simplify the structures of the coal matrix and fractures for the study of methane migration. The ESM of the intact coal has been proposed and widely used by many scholars, but the ESM of the tectonic coal has not been found in the literature. The application of the ESM of the intact coal to the tectonic coal is not reasonable for the study of the methane migration properties, so the establishment of the ESM for the tectonic coal is necessary and meaningful. In this study, a tectonic coal specimen was remodeled from collected coal powders, and methane permeability tests were conducted. Then the ESM of the tectonic coal was established by analyzing the fracture structure. The results show that the tectonic coal can be idealized as a model containing a cubic matrix with square section fractures located at the twelve edges of the matrix. Because of the variation of the ESM, the permeability and the fracture porosity has a square relation for the tectonic coal, rather than the cubic relation for the intact coal. The matrix shape factor of the tectonic coal has also been proposed, which has a value of 480b(f)/7a(3)(m) and is much smaller than that of the intact coal.