Structure feature and evolution mechanism of pores in different metamorphism and deformation coals

Coal pore structure plays a crucial role in the adsorption, migration and production of coalbed gas, and its feature and evolution are mainly controlled by metamorphism and deformation. In this study, the original and tectonically deformed structures of middle- and high-rank coal samples (R-o,R- max = 1.14-3.44%) were obtained through low-temperature nitrogen adsorption (LTNA), small-angle X-ray scattering (SAXS) and scanning electron microscopy (SEM) to analyze the pore structure of both open pores and closed pores. Moreover, we investigate the evolution features and mechanism of pore structure with different metamorphism and deformation degrees by comparing the pore structures of these samples with those of low-rank tectonically deformed coals (R-o,R- max = 0.88-0.96%). The differentiation and inheritance are summarized as the distinctive evolution feature of pore structure of coal with different metamorphism and deformation degree. The pore size of tectonically deformed coals decreases with increase in the degree of deformation under the action of the tectonic stresses. This phenomenon is in line with the metamorphism evolution phenomenon of the original structure that the macro-pores decrease and the micro-pores develop as the coal metamorphism degree increases. Tectonic stress has a dual effect on the transformation between closed pores and open pores of coal in the process of deformation. Closed pores increase is controlled by local uneven shrinkage of the matrix, reduction of pore throat caused by tectonic stress, and partial dislocation of open and semi-open pores. Larger closed pores may open more easily due to deformation caused by shear stress.

Automated summary

The study focuses on the characteristics and evolution mechanisms of coal pore structure, revealing that pore size decreases and the increase of closed pores are controlled by tectonic stress during metamorphism and deformation processes.