Key Takeaways & Executive Findings
- •• The undulation degree of rock tensile fracture surfaces is critically influenced by the initial fracture angle, with a threshold at 45° separating high and low undulation intervals. • Quantitative indices for relative undulation degree, undulation frequency, and matching degree were developed to characterize fracture surface morphology. • A direct relationship was established between fracture surface undulation changes and gas flow guidance, enabling quantitative characterization of gas flow stability, tortuosity, and uniformity. • Numerical models validated the proposed indices, confirming their utility in predicting gas flow patterns in fractured rock strata.
Abstract
Fractures in rock strata serve as flow pathways for gas flow. The undulation of fracture channels can influence the guidance of gas flow. In this context, four-point bending experiments on prefabricated fractured rocks at different angles under stable stepped loading stress. The experiment results clarified the evolutionary law that the undulation degree of the rock tensile fracture surface is separated by an initial fracture angle of 45°. The high undulation intervals were less than 45°, whereas the low undulation intervals were more than 45°. Furthermore, the relative undulation degree, undulation frequency, and matching degree of the fracture surface were quantified. The relationship between the change in fracture surface undulation and gas flow guidance was established. Based on this, the stability, tortuosity, and uniformity of the gas flow in the fracture channel were quantitatively characterized. Subsequently, numerical models of the fracture channels were constructed to validate the indices proposed in this study. The results of the study clarified the influence of different initial fracture angles on the undulation changes of fracture surfaces, and established the relationship between these changes and gas flow, which is conducive to understanding the role of internal fracture channels in rocks in guiding the gas flow process.
1. Introduction
During coal mining, the original stress state in the overlying rock strata is disrupted [1]. This leads to the collapse of the overlying rock strata in the goaf after the coal mining is completed. The immediate roof bends and subsides [2], forming a fractured rock beam structure [3], as shown in Fig. 1. Moreover, after the coal seam is mined, long-term stable stress acts on the rock strata overlying the goaf. The rock strata gradually fracture under long-term stable stress, resulting in tensile damage [4] and internal fracturing within the rock strata.
Gas residual in the goaf after mining of a coal seam or gas desorbed from coal and coal pillars will spread along the fractures of the overlying rock strata in the goaf [5,6]. Additionally, fractures in the overlying rock strata of the goaf act as channels for gas to migrate upward [7]. Consequently, both the morphology and roughness of the fracture surfaces can interfere with the gas flow process. It, in turn, affects the distribution and diffusion of gas within the overlying strata of the goaf. Additionally, due to the impact of coal mining, fractures are included in the overlying strata of the goaf. The existence of these fractures not only influences the fracturing of the rock strata but also further impacts the fracture development pattern. Hence, to clarify the distribution and expansion pattern of gas in the overlying rock strata of the goaf, it is necessary to quantify the fracture morphology of the overlying rock strata of the mining zone after fracture and its surface roughness and to elucidate the guiding effect of the fracture surface on the gas flow.
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Zihan Chen, Quanle Zou, Feixiang Lv, Qican Ran, Xiaoyan Sun, Xianwei Heng (2025). Quantitative characterization of fracture surface undulations and gas-guiding patterns in fractured rocks under steady loading. SinoTechIntel Verified Research. https://doi.org/10.1016/j.ijmst.2025.08.017
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Frequently Asked Questions
What is the significance of the 45° initial fracture angle in this study?
The study found that the undulation degree of rock tensile fracture surfaces is separated by an initial fracture angle of 45°. Specifically, fracture surfaces with initial angles less than 45° exhibit high undulation, while those greater than 45° show low undulation. This threshold is critical for predicting fracture morphology and its influence on gas flow.
How were the fracture surface undulations quantified?
The researchers quantified the relative undulation degree, undulation frequency, and matching degree of the fracture surfaces. These indices provide a comprehensive characterization of the surface morphology, enabling a quantitative link to gas flow behavior.
What is the relationship between fracture surface undulation and gas flow?
The study established that changes in fracture surface undulation directly influence gas flow guidance. Specifically, the stability, tortuosity, and uniformity of gas flow in fracture channels are quantitatively characterized based on the undulation parameters, demonstrating that higher undulation leads to more tortuous and less uniform gas flow.
How were the proposed indices validated?
Numerical models of fracture channels were constructed to validate the indices proposed in the study. The models confirmed the accuracy of the indices in predicting gas flow patterns, thereby supporting their applicability in real-world scenarios.
What are the practical applications of this research?
This research provides a framework for understanding gas flow in fractured rock strata, which is essential for predicting gas distribution and diffusion in goaf areas during coal mining. The findings can aid in designing safer mining operations and improving gas drainage strategies.
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