Multifractal Analysis in Characterizing Adsorption Pore Heterogeneity of Middle- and High-Rank Coal Reservoirs

Abstract

Nanopore heterogeneity has a significant effect on adsorption, desorption, and diffusion processes of coalbed methane. The adsorption pore size distribution heterogeneity was calculated by combining N 2 with CO 2 adsorption data, and factors affecting multifractal and single-fractal dimensions were studied. The results indicate that pore size distribution of micropores (with pore diameters smaller than 2 nm) and meso–macro-pores (with pore diameters between 2 and 100 nm) in coal samples exhibit typical multifractal behavior. The overall heterogeneity of micropores in high-rank coal samples is higher than that in the middle-rank coal samples. The low-probability measure areas control the overall heterogeneity of pores with diameters of 0.40–1.50 nm. The high-probability measure area heterogeneity and spectral width ratio have a higher linear correlation with coal rank and pore structure parameters than those of low-probability measure areas. Heterogeneity of high-probability measure areas and overall pore size distribution are controlled by pores with diameters of 0.72–0.94 nm. Multifractal parameters of meso–macro-pores have no clear relationship with coal rank. The pore volume of 2–10 nm diameter shows a good linear correlation with heterogeneity of low-probability measure areas, and pores of this diameter range are the key interval that affected pore size distribution heterogeneity. The single-fractal dimension obtained using the Frenkel–Halsey–Hill (FHH) model shows a positive linear correlation with heterogeneity of the low-probability measure areas. It indicates that this parameter can effectively characterize the pore size distribution heterogeneity of low-probability measure areas in meso–macro-pores.

Pore Distribution and Variation Rules of the Coal Sample with CO2 Adsorption at Different Pressures Based on Small-Angle X-ray Scattering

“…The above microsize results are consistent with the fracturing technologies relying on carbon dioxide to enhance CBM recovery in the macrosize. It may also indicate that the CO 2 adsorption in a long time at different pressures can lead the adsorption pore (diameter of <100 nm) to expand to diffusion holes (diameter of >100 nm) more easily. − …”

Nanopore Size Distribution Heterogeneity of Organic-Rich Shale Reservoirs Using Multifractal Analysis and Its Influence on Porosity–Permeability Variation

“…Therefore, the corresponding value of ε differs. 26 In the case of scale variation, the water distribution probability in the interval [A, B] satisfies…”

“…D 0 −D 10 and D −10 −D 0 are the amplitudes of the right and left branches of D q , which represent the heterogeneity of the high and low probability measure areas, respectively. For the detailed derivation process, please refer to Zhang et al 26 Figure 1. Characteristics of generalized multifractal curves using LPN 2 GA tests.…”

Prediction of gas pressure in thin coal seams in the Qinglong Coal Mine in Guizhou Province, China

“…The geological structure conditions of mining areas considerably influence the present in-situ stress, and the depressions formed by fault structures are high-lying stress areas, that are conducive to gas preservation (Wang et al 2018;Li et al 2020c;Lu et al 2021). Given the differences in the associated structures and stress fields above and below the neutral layer, the large fold structures have varying control effects on gas occurrence (Li et al 2017;Lu et al 2018a;Meng et al 2019;Zhang et al 2020a).…”