Fractal analysis of high rank coal from southeast Qinshui basin by using gas adsorption and mercury porosimetry

In order to better understand the pore structure of high rank coal, low-temperature N-2 isotherm adsorption/desorption and mercury porosimetry experiments were conducted on 20 coal samples from the southeast Qinshui basin. Fractal analysis was used to characterize the heterogeneity of pore structure. To obtain credible fractal dimension of pores, several models for calculating the fractal dimension were used and their calculation results were compared. The results show that fractal dimensions of pore surface and structure in adsorption pores can be obtain by Frenkel-Halsey-Hill (FHH) model from type A of N-2 adsorption isotherms. The mercury intrusion data reflect the fractal information about seepage pores and partial transition pores. Because sensitivities of various models on pores with different widths are different, fractal dimensions obtained from different models based on mercury intrusion data are inconsistent. The coal samples can be divided into three types according to the crossplot of the volume percentage of seepage pore and the ratio of fixed carbon content and volatile content. For type I and II, Langmuir volume is positively correlated with the pore structure fractal dimension of transition pores and micropores. This shows that high pore structure fractal dimension of adsorption pores related to more small pores is conducive to coalbed methane (CBM) enrichment. The relationships between fractal dimension and permeability in type I and II are different, which may be related to volume and connectivity of mesopores and macropores. For type III, high ash content greatly reduces the adsorption and seepage capacity of coal.