Micro-structural evolution and their effects on physical properties in different types of tectonically deformed coals

Abstract

The macromolecular structure of tectonically deformed coals (TDC) may be determined by the deformation mechanisms of coal. Alterations of the macromolecular structure change the pore structure of TDC and thereby impact physical properties such as porosity and permeability. This study focuses on structure and properties of TDC from the Huaibei and Huainan coal mining areas of southern North China. Relationships between the macromolecular structure and the pore structure of TDC were analyzed using techniques such as X-ray diffraction, high-resolution transmission electron microcopy, and the low-temperature nitrogen adsorption. The results indicated that the directional stress condition can cause the arrangement of basic structural units (BSU) more serious and closer. And, the orientation is stronger in ductile deformed coal than in brittle deformed coal. Tectonic deformation directly influences the macromolecular structure of coal and consequently results in dynamic metamorphism. Because the size of BSU in brittle deformed coal increases more slowly than in ductile deformed coal, frictional heating and stress-chemistry of shearing areas might play a more important role, locally altering coal structure under stress, in brittle deformed coal. Strain energy is more significant in increasing the ductile deformation of coal. Furthermore, mesopores account for larger percentage of the nano-scale pore volume in brittle deformed coals, while mesopores volume in ductile deformed coal diminishes rapidly along with an increase in the proportion of micropores and sub-micropores. This research also approved that the deformations of macromolecular structures change nano-scale pore structures, which are very important for gas adsorption and pervasion space for gas. Therefore, the exploration and development potential of coal bed methane is promising for reservoirs that are subjected to a certain degree of brittle deformation (such as schistose structure coal, mortar structure coal and cataclastic structure coal). It also holds promise for TDC resulting from wrinkle structure coal of low ductile deformation and later superimposed by brittle deformation. Other kinds of TDC suffering from strong brittle-ductile and ductile deformation, such as scale structure coal and mylonitic structure coal, are difficult problems to resolve.