Nanopore structure and surface roughness in brittle tectonically deformed coals explored by atomic force microscopy

The pore structure of coal is the key to coalbed methane (CBM) exploration and exploitation, and coal mining safety. Compared to primary coals, tectonically deformed coals (TDCs) have stronger heterogeneity and are of great importance in coal reservoir study. Here, the pore structure and surface roughness of brittle TDCs were obtained with atomic force microscopy (AFM) and NanoScope Analysis and Gwyddion software. The results show that tectonic deformation generally promotes the pore development in brittle TDCs, but the extent of tectonic impact varies greatly among different brittle TDCs. As a result, two stages in brittle TDCs were identified: weak brittle deformation stage (primary coal, cataclastic coal, schistose coal and mortar coal) and strong brittle deformation stage (mortar coal, granulitic coal and flaky coal). In the weak brittle deformation stage, tectonic stress has little impact on coals’ pore structure. The mean pore number increases slowly and the mean pore size decreases slowly. In this stage, the tectonic process mainly promotes the development of 100-200 nm macropores. In the strong brittle deformation stage, tectonic stress has a major impact on coals’ pore structure. The mean pore number increases quickly and the mean pore size decreases quickly. The tectonic process in this stage mainly promotes the development of mesopores of 10-50 nm and macropores of 50-100 nm. The two different tectonic stages demonstrate that the whole brittle TDCs do not evolve linearly with tectonic deformation. The arithmetical average height (R_a) and root-mean-square (R_q) roughness are 3.00-6.05 nm and 3.94-7.62 nm respectively. The weak brittle TDCs’ R_a and R_q fluctuate slightly, while the strong brittle TDCs’ R_a and R_q decrease fast. A mathematical model of the morphology of pores in the coal surface was established based on AFM sectional analysis. The R_a simulation of coal samples based on this model shows that pores of larger diameter are mainly responsible for surface roughness. R_a and R_q of coal samples are controlled mainly by nanopore development during tectonic deformation.