Pore Characteristics and Fractal Dimension Analysis of Tectonic Coal and Primary-Structure Coal: A Case Study of Sanjia Coal Mine in Northern Guizhou

Abstract

Understanding the pore heterogeneity of tectonic coal and primary-structure coal is of great significance for predicting and preventing tectonic coal. This study adopts the low-temperature nitrogen adsorption method, mercury injection experiment, and other methods, combined with fractal theory, to quantitatively analyze the pore distribution of coal samples inside and outside the outburst cavities of the Sanjia coal mine. The experiments have shown that the contents of aliphatic functional groups and hydrogen in tectonic coal are higher than those of aromatic structural functional groups. Raw coal has more straight chains than side chains, whereas aliphatic hydrocarbon mostly has short chains, and the branching degree is high. Soft and primary-structure coals have similar elemental content and tectonic effects endow the coal with better connectivity. The pores are filled with particles and flakes, and the surfaces of tectonic coal have more pores and fissures on them. According to the experimental curve, the pores are divided into five types. The pore size of primary-structure coal is mainly type II pores, and the pore size distribution of tectonic coal is relatively wide, with the majority being class I and class II pores. The specific surface area of tectonic coal is 60.7% more than that of primary-structure coal. The box fractal dimension of coal decreases with the increase in scanning electron microscopy (SEM) magnification. The minimum fractal dimension of tectonic coal is 2.49, which is 7.8% lower than the peak of 2.70. It can be seen from the fractal dimension that the fractal dimensions of pore types II, III, and IV are rougher.

Effect of Water on Effective Pore Structures for Medium-Rank Coal: Based on the Prefreezing Nitrogen Adsorption–Desorption Experiment

“…This result is consistent with the research result of Gu et al for water-bearing shale. First, the pores of coal have mixed wetting properties that contain oxygen-containing functional groups, , hydrophilic substances such as clay minerals and brittle minerals, as well as hydrophobic substances such as organic matter and tar. Due to the different water absorption capabilities of these substances, the adsorption of water in the pores is uneven, which makes the pore structure complex in the water-containing state.…”

Effect of Tectonic Deformation on the Pore System and Methane Adsorption of Anthracite Coal

“…14,15 Subsequently, pore characterization methods, including low-temperature N 2 and CO 2 adsorption experiments, have been routinely employed to investigate the nanopore size distribution of tectonically deformed coals. 16,17 Moreover, CH 4 isothermal experiments represent a pivotal technique for assessing the CH 4 adsorption capacity of coals. 10,18 Additional investigations include the applications of Raman spectroscopy, 13 C nuclear magnetic resonance ( 13 C NMR), and HRTEM to analyze the molecular structure of tectonic coals.…”

“…In the realm of coal mining gas burst investigations, coals subjected to tectonic influences have been denoted as “tectonic coals”. − Conversely, coalbed methane geologists commonly refer to them as “tectonically deformed coals” and classify them into distinct categories based on the type and extent of deformation. ,− To understand the tectonic impact on the CH 4 adsorption capacity and pore system of coal, a collection of natural coal samples exhibiting varying degrees of deformation was assembled and compared. , Subsequently, pore characterization methods, including low-temperature N 2 and CO 2 adsorption experiments, have been routinely employed to investigate the nanopore size distribution of tectonically deformed coals. , Moreover, CH 4 isothermal experiments represent a pivotal technique for assessing the CH 4 adsorption capacity of coals. , Additional investigations include the applications of Raman spectroscopy, 13 C nuclear magnetic resonance ( 13 C NMR), and HRTEM to analyze the molecular structure of tectonic coals. ,, Yu et al compiled a comprehensive classification of tectonically deformed coals, categorizing them into three subgroups: brittle deformation (encompassing cataclastic, mortar, and granulitic coals), ductile deformation (comprising wrinkle and mylonitic coals), and shear deformation (including schistose, flake, and scaly coals). , Notably, previous studies have revealed a continuous increase in pore volume and surface area with increasing deformation levels. , However, certain studies have shown that the mesopore volumes of primary anthracite coals are not significantly lower than those of tectonic coals . Additionally, Wang et al .…”

Fractal Analysis of Coal Pore Structure Based on Computed Tomography and Fluid Intrusions

“…The pore size distribution measured using NMR was higher than that of LP-N 2 A. Wang et al [19] explored the nature of micropores and mesopores during the adsorption/desorption of bituminous coal cycle gases from a microscopic perspective using N 2 and CO 2 adsorption experiments. Lin et al [20] used SEM, low-temperature N 2 adsorption, mercury pressure experiments, and other methods to investigate the pore heterogeneity of tectonic and primary tectonic coals.…”