The structural characteristics of nanopores are known to significantly affect the wetting effect in coal seam water injection. Currently, the three-dimensional characterization of nanopores in coal relies mainly on digital images, whereas poor image resolution and segmentation methods pose significant challenges. Therefore, using coal samples from Wudong Coal Mine in China as an example, cryo-focused ion beam scanning electron microscopy (cryo-FIB-SEM) and deep learning segmentation methods were implemented to accurately characterize the nanopores and water distribution. In the obtained pore structure, the number of isolated pores was higher than that of connected pores, while the volume of connected pores was significantly larger than that of isolated pores, comprising the key path and storage space for external water to enter the coal body. The water content of isolated pores mainly depends on the permeability of the coal matrix. The connectivity of single pores can be characterized by the coordination number, whose increase leads to the number of pores exponentially decreasing. The connectivity of pore clusters depends on the number of internal branches. The number of branches in the pore cluster increases exponentially with the increasing total length, total volume and average radius of the cluster, and the connectivity is correspondingly enhanced. The increase in pore size enhances the shape factor, surface area and connectivity of pores while reducing tortuosity, which in turn facilitates coal wetting. The accurate characterization of coal nanopores in this study helps to scientifically evaluate the effect of coal seam water injection, highlighting the importance of increased pore size and improved pore connectivity for enhanced water injection effectiveness.
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Open Access
Original Article
Issue
As the main energy source in China, coal has an important strategic position in the country’s economic development. China’s coal-based energy structure model will not change in the short term, but coal mine accidents occur frequently. Mine fire is a major disaster that threatens safe coal mine production, which incurs huge economic losses and threatens the personal safety of miners and coal mine production. However, coal spontaneous combustion primarily causes mine fires. The occurrence mechanism of coal spontaneous combustion is highly complex and is affected by the inherent combustion characteristics of coal and external environmental factors. Therefore, it is necessary to comprehensively analyze various phenomena and characteristics in the process of coal spontaneous combustion and apply quantitative indicators to express to predict the risk and ensure the safe production of the mine.
Oxygen concentration is an important factor influencing coal oxidation reaction. In this paper, the temperature-programmed experimental system is used to conduct the spontaneous combustion experiment of coal samples with three oxygen concentrations of 20.9%, 10%, and 7% regarding the heat dissipation, oxidation, and asphyxiation zones in the goaf. The gas products at different temperature stages are collected, the effects of temperature and oxygen concentration on the concentration of gas products are analyzed, and the index gases are classified and evaluated in detail.
The results show the following: (1) The cross-point temperature (CPT) decreases with the increase in the oxygen concentration. The cross-point temperature under dry-air conditions is 144.6 ℃. The error between the CPT value calculated by the BM empirical relationship and the experimental data is only 6.08%. The higher the oxygen concentration, the smaller the critical temperature, and the higher the gas concentration. (2) The content of oxidation products increases with the oxygen concentration. When heating commences, minimal CO and CH4 gases are generated, but with the increase in the temperature, C2H4 and C2H6 gases appear gradually. The oxidation reaction of coal samples is primarily affected by temperature and oxygen concentration. At a certain temperature, the higher the oxygen concentration, the more intense the coal-oxygen composite reaction, and more gas products appear. (3) For different oxygen concentration environments, the regularity of generating CO gas is highly significant, which can be used as an indicator gas to evaluate the spontaneous combustion of coal. In dry air, the temperature of C2H4 is earlier than that of C3H6, implying that the existence of C2H4 indicates that the coal oxidation process has entered an accelerated stage. The emergence of C3H6 gas marks the transformation of the coal oxidation into strong oxidation. Recently, the programmed temperature rise experiment has become an important research field.
Through indoor experiments, students can deeply understand the mechanism of coal spontaneous combustion and the process of establishing the prediction index system and introduce the experiment into the classroom. This practical teaching approach not only helps students to apply theoretical knowledge to practical problems but also broadens their horizons and improves their interest in scientific research, achieving better teaching results.
Open Access
Perspective
Issue
Fluids flow within microporous and nanoporous rocks involves several industrial processes such as enhanced oil recovery, geological CO2 sequestration, and hydraulic fracturing. However, the pore structure of subsurface rocks is complex, and fluid flow is influenced by strong fluid-fluid and fluid-solid interactions, including wettability, interfacial tension, and slip effects. Characterizing this flow processes is costly and challenging through experimental techniques. At meanwhile, pore-scale simulations have been widely employed to investigate complex flow behaviors within microporous and nanoporous media. This work investigates the applications of pore-scale simulation methods for characterizing flow processes in porous rocks considering microscale and nanoscale effects. Two mainstream simulation methods, pore network modeling and direct numerical simulation, are introduced. Their application scenarios encompass immiscible flow, as well as miscible and near-miscible flow involving CO2 enhanced recovery. Additionally, some explorations of single-phase and multiphase flow processes within nanoporous media are described. Finally, future development of pore-scale simulations is discussed, with a focus on complex transport phenomena involving diffusion, reactions, and dissolution.
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