Key Takeaways & Executive Findings
- •• A novel statistical damage constitutive model for gas-bearing coal is developed by integrating effective stress and the Hoek-Brown criterion, validated against triaxial compression tests under varying gas pressures. • The post-peak stress drop and softening characteristics of gas-bearing coal are accurately captured, with shape and scale parameters reflecting brittleness and strength. • As gas pressure increases from 1 to 5 MPa, both brittleness and strength decrease (shape parameter -22.18%, scale parameter -60.45%). • The proposed brittleness index, integrating energy evolution and damage parameters, effectively reveals enhanced plastic deformation sensitivity under higher gas pressures.
Abstract
Accurate assessment of coal brittleness is crucial in the design of coal seam drilling and underground coal mining operations. This study proposes a method for evaluating the brittleness of gas-bearing coal based on a statistical damage constitutive model and energy evolution mechanisms. Initially, integrating the principle of effective stress and the Hoek-Brown criterion, a statistical damage constitutive model for gas-bearing coal is established and validated through triaxial compression tests under different gas pressures to verify its accuracy and applicability. Subsequently, employing energy evolution mechanism, two energy characteristic parameters (elastic energy proportion and dissipated energy proportion) are analyzed. Based on the damage stress thresholds, the damage evolution characteristics of gas-bearing coal were explored. Finally, by integrating energy characteristic parameters with damage parameters, a novel brittleness index is proposed. The results demonstrate that the theoretical curves derived from the statistical damage constitutive model closely align with the test curves, accurately reflecting the stress−strain characteristics of gas-bearing coal and revealing the stress drop and softening characteristics of coal in the post-peak stage. The shape parameter and scale parameter represent the brittleness and macroscopic strength of the coal, respectively. As gas pressure increases from 1 to 5 MPa, the shape parameter and the scale parameter decrease by 22.18% and 60.45%, respectively, indicating a reduction in both brittleness and strength of the coal. Parameters such as maximum damage rate and peak elastic energy storage limit positively correlate with coal brittleness. The brittleness index effectively captures the brittleness characteristics and reveals a decrease in brittleness and an increase in sensitivity to plastic deformation under higher gas pressure conditions.
1. Introduction
Brittleness index is one of the fundamental properties of rocks and plays an important role in evaluating the wellbore wall rock stability and the feasibility of reservoir hydraulic fracturing technology [1, 2]. Hydraulic fracturing technology has been widely used to enhance the productivity of coal-bed gas reservoirs. However, in some cases, the hydraulic fracturing design scheme fails to achieve significant improvements in reservoir permeability. This is mainly due to inaccurate evaluation of the brittleness of the target reservoir [3, 4]. Therefore, precise rock brittleness assessment is essential for guiding drilling technology and hydraulic fracturing engineering.
The definition of rock brittleness remains ambiguous and controversial across different fields and disciplines [5]. Various researchers proposed different approaches to describe the brittleness characteristics of rocks based on laboratory or field tests. For instance, RICKMAN et al [6] and JIN et al [7] introduced the brittleness index based on mineral content, suggesting that the amount of quartz mineral is positively correlated with the rock brittleness. ALTINDAG [8], on the other hand, developed a multiplicative synthesis method that established a function of compressive and tensile strengths to describe rock brittleness from the perspective of rock strength. TARASOV et al [9] focused on the concept of post-failure response and proposed the brittleness index based on strain energy, considering rock brittleness as the ability to maintain a failure state after being subjected to stress. According to various evaluation standards, brittleness indicators can be roughly divided into six categories: mineral composition, strength parameters, stress−strain curve characteristics, hardness tests, energy theory, and impact tests or standard penetration tests.
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XUE Yi, WANG Lin-chao, LIU Yong, RANJITH P G, CAO Zheng-zheng, SHI Xu-yang, GAO Feng, KONG Hai-ling (2025). Brittleness evaluation of gas-bearing coal based on statistical damage constitution model and energy evolution mechanism. Journal of Central South University. https://doi.org/10.1007/s11771-025-5898-2
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Frequently Asked Questions
What is the focus of the study?
The study proposes a method for evaluating the brittleness of gas-bearing coal based on a statistical damage constitutive model and energy evolution mechanisms.
How does gas pressure affect coal brittleness?
As gas pressure increases from 1 to 5 MPa, both the brittleness and strength of the coal decrease, with the shape parameter reducing by 22.18% and the scale parameter by 60.45%.
What is the proposed brittleness index?
The brittleness index integrates energy characteristic parameters with damage parameters to effectively capture the brittleness characteristics of gas-bearing coal.
What experimental validation was performed?
The model was validated through triaxial compression tests under different gas pressures.
Why is brittleness evaluation important for coal mining?
Accurate brittleness evaluation is crucial for coal seam drilling, hydraulic fracturing design, and underground coal mining operations to ensure reservoir permeability improvement and stability.
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