Key Takeaways & Executive Findings
- •• Liquid nitrogen cold soaking significantly enhances coal fracture networks, increasing fracture spectrum peak proportions by up to 79.939% in lignite. • The combination of micro-CT, VRA-UNet, and fractal analysis provides a robust method for quantifying fracture evolution in coal. • Fractal dimensions of coal fractures increase after liquid nitrogen treatment, indicating higher complexity and connectivity. • The findings support the application of liquid nitrogen fracturing for improving coalbed methane extraction efficiency.
Abstract
The development of coalbed methane in China is constrained by complex geological conditions characterized by low permeability, low saturation, low reservoir pressure, and high adsorption ("three lows and one high"), posing significant challenges to its efficient development. The liquid nitrogen-induced fracturing and permeability enhancement technology can effectively promote the expansion and connection of macroscopic and microscopic fractures, thereby improving the permeability of coal seams. In this study, industrial micro-CT scanning technology, the VRA-UNet method, and fractal dimension calculation methods are employed to conduct an in-depth analysis of the action mechanism of liquid nitrogen cold soaking on the fracture structure of coal bodies with different metamorphism degrees. The results indicate that liquid nitrogen cold soaking promotes the generation, expansion, and connection of new fractures inside coal bodies to form fracture networks. Via Matlab programming and VG Studio MAX image analysis software, fracture extraction and calculation are performed on CT-scanned coal samples; it is statistically found that the quantitative fracture indices of coal increase after liquid nitrogen cold soaking. Compared with the fracture spectrum peak proportions of raw coal samples, the fracture spectrum peak proportions of anthracite, bituminous coal, and lignite increase by 8.375%, 12.680%, and 79.939%, respectively after liquid nitrogen cold soaking. By combining the VRA-UNet method for coal fracture identification, the box-counting method is used to calculate that the fractal dimension of coal fractures after liquid nitrogen cold soaking is larger than that of raw coal samples. The research findings of this paper will provide theoretical and technical support for the efficient development of coalbed methane and the improvement of coal seam gas extraction rates.
1. Introduction
Coalbed methane, as a crucial unconventional natural gas resource, holds multiple strategic implications for optimizing China's energy structure, safeguarding energy security, and facilitating the low-carbon transformation. In terms of energy attributes, coalbed methane merely generates a scant amount of carbon dioxide and water upon combustion, and its pollutant emissions are merely 1/800 of those from coal, thus being a bona fide clean energy source [1, 2]. In the context of the "dual carbon" targets, expediting the development and utilization of coalbed methane can not only alleviate the pressure on the supply of traditional fossil energy but also significantly mitigate the risk of gas accidents in coal mines [3, 4], achieving the coordinated development of resource exploitation and safe production.
Nevertheless, the development of coalbed methane in China confronts substantial geological challenges. Statistical data indicate that approximately 75% of China's coal seams have a permeability of less than 1 mD and generally exhibit the typical characteristics of "three lows and one high" (low pressure, low permeability, low saturation, and high adsorption) [5 −8]. Such exceptional geological conditions give rise to the obstruction of the desorption-diffusion-percolation process of coalbed methane, resulting in generally low single-well production. Taking the Qinshui Basin as an instance, despite its abundant resources, the average daily production per single well amounts to merely 800−1500 m3, which is far lower than the level of over 3000 m3 in the San Juan Basin of the United States [9, 10]. This current low-production status not only impacts economic benefits but also restricts the process of large-scale development.
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LIU Shu-min, WANG Shuai-lin, LI Xue-long, SUN Hai-tao, WAN Ni, ZHANG Dong-ming, WANG Deng-ke (2026). Micro-CT characterization and fractal study on the fracture structure of coal under the liquid nitrogen cold soaking. Journal of Central South University. https://doi.org/10.1007/s11771-026-6272-8
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Frequently Asked Questions
What is the main objective of this study?
The study aims to investigate the effect of liquid nitrogen cold soaking on the fracture structure of coal with different metamorphism degrees using micro-CT and fractal analysis, to support coalbed methane development.
How does liquid nitrogen cold soaking affect coal fractures?
Liquid nitrogen cold soaking promotes the generation, expansion, and connection of new fractures, increasing fracture network complexity and fractal dimensions, thereby enhancing coal permeability.
What methods were used in this research?
The research employed industrial micro-CT scanning, VRA-UNet for fracture identification, and box-counting method for fractal dimension calculation, along with Matlab and VG Studio MAX for image analysis.
What are the key findings regarding different coal types?
After liquid nitrogen cold soaking, the fracture spectrum peak proportions increased by 8.375% for anthracite, 12.680% for bituminous coal, and 79.939% for lignite, indicating varying responses based on metamorphism.
What is the significance of this study for coalbed methane extraction?
The findings provide theoretical and technical support for enhancing coal seam permeability and improving gas extraction rates, which is crucial for efficient coalbed methane development in China.
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