Key Takeaways & Executive Findings
- •• Pore water storage characteristics significantly influence methane adsorption in coal, with effects varying by coal rank. • During water diffusion, adsorbed water penetrates micropores, with brown coal retaining more adsorbed water than anthracite. • High-rank coal (anthracite) shows a marked decrease in methane adsorption capacity (Langmuir volume reduction of 21.22 cm3/g) during both water diffusion and addition phases. • Molecular dynamics simulations reveal that hydroxyl groups enhance electrostatic interactions, increasing steric hindrance and inhibiting hydrogen bond formation, correlating with pore water content.
Abstract
This study mainly investigates the influence of pore water characteristics on the adsorption properties of coalbed methane through integrated low field nuclear magnetic resonance (LF-NMR), adsorption experiments, and molecular dynamics (MD) simulations. Pore water states in three coal ranks were characterized during progressive hydration. Multi-scale analysis revealed how pore water evolution regulates methane adsorption processes. During the diffusion-dominated stage (M2–M3), adsorbed water penetrates into the micropores. In the highly wettable brown coal (L1), the adsorbed water content reaches 2.12 g while in the anthracite (A1), it is only 0.29 g. During the active water injection stage (M4–M6), non-adsorbed water dominates in anthracite (over 85% of the total water content of 4.01 g), while adsorbed water remains dominant in lignite (over 60% of the total water content of 3.52 g). Water content plays a key role in methane adsorption in coal. During the water addition phase, the influence of methane adsorption on medium-to-low-rank coal is relatively weak, while the methane adsorption capacity of high-rank coal A1 shows a significant decrease during both the water diffusion and water addition phases, corresponding to a reduction in Langmuir volume of 21.22 cm3/g. Molecular dynamics (MD) results further show that the free energy between molecules on the surface of hydroxyl-modified coal increases, with hydroxyl groups driving electrostatic interactions between coal and water molecules. Increased steric hindrance inhibits hydrogen bond formation and reduces the rate of hydrogen bond growth. There is a significant correlation between pore water content and coal-water molecular interaction energy, which cross-scale validates the results of LF-NMR testing and MD simulations.
1. Introduction
China's sustainable energy development strategy emphasizes that the country's energy structure is characterized by ''abundant coal, limited oil, and scarce natural gas'' [1]. This structural feature ensures that coal will continue to play a dominant role as the ''ballast'' energy source in the short term [2,3]. However, as shallow coal resources are gradually depleted, the depth of coal mining in China increases by 10 to 25 m annually [4]. Deeper coal seams are characterized by higher geological stress, gas pressure, and gas content, leading to lower permeability [5,6]. This not only makes gas extraction more difficult but also heightens the risk of coal and gas outbursts, rock bursts, and other disasters, which pose a serious threat to the safety of coal mines and workers [6].
To improve gas extraction rates and utilization, researchers both domestically and internationally have developed various reservoir modification techniques to enhance coal seam permeability [7,8]. These techniques aim to expand the influence of extraction boreholes, promote gas desorption, and improve extraction efficiency. High-pressure water injection has been proven to be an effective method for enhancing coal seam permeability and has gained widespread application.
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CHEN Aikun, ZHAI Cheng, CAI Yuliang, SUN Yong, YU Xu, XU Jizhao, CONG Yuzhou, ZHENG Yangfeng, TANG Wei (2025). Influence law of pore water storage characteristics on the gas adsorption characteristics of coal. SinoTechIntel Verified Research. https://doi.org/10.1016/j.ijmst.2025.07.008
Research & Educational Purpose Only:The translations, structured abstracts, analytical annotations, and data reports provided by SinoTechIntel are intended exclusively for academic research, internal corporate R&D, and educational benchmarking. They do not constitute formal engineering, chemical safety, legal, or professional advice.
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Frequently Asked Questions
How does pore water affect methane adsorption in coal?
Pore water significantly influences methane adsorption by occupying adsorption sites and altering the coal surface chemistry. The effect varies with coal rank: in high-rank coal (anthracite), methane adsorption capacity decreases markedly during water diffusion and addition, while in low-rank coal (lignite), the effect is relatively weaker during water addition.
What methods were used in this study?
The study integrated low-field nuclear magnetic resonance (LF-NMR), adsorption experiments, and molecular dynamics (MD) simulations to characterize pore water states and methane adsorption across three coal ranks.
What are the key findings regarding water distribution in different coal ranks?
During water diffusion, adsorbed water penetrates micropores, with brown coal retaining more adsorbed water (2.12 g) than anthracite (0.29 g). During active water injection, non-adsorbed water dominates in anthracite (over 85% of total water), while adsorbed water remains dominant in lignite (over 60%).
How does molecular dynamics simulation explain the experimental results?
MD simulations show that hydroxyl groups on coal surfaces enhance electrostatic interactions with water, increasing steric hindrance and inhibiting hydrogen bond formation. This correlates with pore water content, cross-scale validating the LF-NMR and adsorption experimental results.
What is the practical significance of this research?
Understanding the influence of pore water on gas adsorption is crucial for optimizing water injection techniques for coalbed methane extraction and for assessing the risk of coal and gas outbursts in deep mines.
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