The evolution of nanopores and surface roughness in naturally matured coals in South China: An atomic force microscopy and image processing study

The coals in South China has not been well investigated because the coal mines are small in volume compared with the well-known large-scale coal accumulations in North China, thus the nature of such coals is of significance to study. The nanopore characteristics and surface roughness of 14 coal samples that have experienced low degrees of tectonic deformation as a naturally matured coal series in South China were addressed. Atomic force microscopy (AFM) and image processing were applied in a study of relationships between nanopore development, surface roughness, and thermal maturity (indicated by vitrinite reflectance; Ro = 0.65%-4.43%) in the naturally matured coal series. The results show that areal porosities of the coal samples are 0.78%-9.33%, with mean pore sizes of 7.9-27.9 nm. There were three main stages of pore evolution related to thermal maturity, with Ro values of <1.3%, 1.3%-3.7%, and >3.7%. Pore number, areal porosity, and mean pore size follow a cyclical downward/upward trend across the three stages. Arithmetical mean (Ra) and root-mean-square (Rq) surface roughness values are 1.69-17.74 nm and 2.24-28.81 nm, respectively. Ra and Rq values of coal samples with relatively low thermal maturity (Ro < 1.3%) are controlled mainly by coal maceral and mineral composition, whereas values for coals with relatively high thermal maturity (Ro > 1.3%) are controlled by nanopore development. Thermal maturity is an important factor that influences the nanopore and surface roughness characteristics of naturally matured coals in South China.