Relationship between nanoscale deformation of coal structure and metamorphic-deformed environments

Abstract

There is a more consanguineous relation between nano-scale deformation of coal structure and metamorphic-deformed environment. In different metamorphic-deformed environments, deformation in the coal structure can occur not only at micro-scale, but also at nano-scale, and even leads to the change of molecular structure and nano-scale pore (< 100 nm) structure. The latter is the main space absorbing coalbed methane. Through X-ray diffraction (XRD) and liquid—nitrogen absorption methods, the characteristics of macromolecular and nano-scale pore structures of coals in different metamorphic-deformed environments and deformational series of coals have been studied. By combining with high-resolution transmission electron microcopy (HRTEM), the macromolecular and nano-scale pore structures are also directly observed. These results demonstrate that the stacking L_c of the macromolecular BSU in tectonic coals increases quickly from the metamorphic-deformed environment of low rank coals to that of high rank coals. For different deformed tectonic coals, in the same metamorphic-deformed environment, the difference of L_c is obvious. These changes reflect chiefly the difference of different temperature and stress effect of nano-scale deformation in tectonic coals. The factor of temperature plays a greater role in the increase of macromolecular structure parameters L_c, the influence of stress factor is also important. With the stress strengthening, L_c shows an increasing trend, and L_aL_c shows a decreasing trend. Therefore, L_c and L_aL_c can be used as the indicator of nano-scale deformation degree of tectonic coals. With increasing temperature and pressure, especially oriented stress, the orientation of molecular structure becomes stronger, and ordering degree of C-nets and the arrangement of BSU are obviously enhanced. For the deformation of nano-scale pore structure, in the same metamorphic-deformed environment, along with the strengthening of stress, the ratio of mesopores to its total pores volume of tectonic coals reduces to a large extent, the ratio of volume of micropores and the pores whose diameters are lower than micropores increases, and sub-micropores and ultra-micropores can be found. Moreover, the ratio of specific surface area of mesopores to its total pores reduces rapidly while the amount of sub-micropores increases more quickly. The ductile structure coal has a change in pore parameters similar to that of weak brittle deformation. There are differences in the deformation and evolution of nano-scale pore structure of different kinds of tectonic coals formed in different metamorphic-deformational environments. In short, temperature and confining pressure play some role in the change of nano-scale pore structure parameters, whereas stress has important influence on the evolution of characteristic parameters in nano-scale pore structure of tectonic coals.