Multifractal analysis of Hg pore size distributions of tectonically deformed coals

Abstract

To get deep insight into the variability and heterogeneity of pore size distributions (PSDs) in different tectonically deformed coals (TDCs) (mean maximum vitrinite reflectance Ro,max ranging from 1.720% to 1.857%) collected from Hancheng Mine, Weibei coalfield, the multifractal analysis using generalized dimensions Dq was employed to study PSDs of TDCs based on mercury intrusion porosimetry. The mercury intrusion data under pressure over 10MPa was corrected in combination with gas adsorption data. The results show that, for the test coals, tectonic deformation (TD) mainly increases seepage-porosity while maceral composition (MC) is the main control on adsorption-porosity. The variations of Dq versus q show that the PSDs of TDCs exhibit multifractal behavior, however, differ in their multifractality suggested by the extracted parameters from PSDs including information dimension D1, the Hurst exponent H, the width of the right side D0–D10 and the left side D−10–D0 of Dq spectra. TD and MC have great influence on the variability and heterogeneity in the inner distribution of seepage-porosity and adsorption-porosity respectively, as demonstrated by the change of D1, H and D0–D10 for moment q>0 and D−10–D0 for moment q<0. Both D1 and H increase with TD but D0–D10 shows a reverse trend. Consequently, TD leads to narrower distribution with higher fluctuation, lower pore connectivity and greater complexity in the distribution of seepage-pores, which may be due to the presence and uneven distribution of microfracture and inter-granular pores formed through deformation process. Increasing vitrinite and decreasing inertinite contribute to a heterogeneous distribution of adsorption-porosity, as observed by the high correlation between D–10–D0 and MC. The aforementioned results confirm that the multifractal approach is useful to characterize the internal heterogeneity of PSDs and to detailedly distinguish between PSDs of TDCs.