The permeability of composite coal-rock specimens is crucial for engineering applications, such as coal seam gas extraction, CO2/H2 geological storage, and the prevention of gas-induced coal-rock disasters. This study investigated the permeability behavior of coal, rock, and composite coal-rock specimens. Single-component permeability evolution tests were conducted under true triaxial stress conditions to examine the effects of stress on the permeability of each specimen. Moreover, permeability variations among the specimens were compared under the same stress path. Differences in gas adsorption between coal and rock and their stress interactions were incorporated using the elastic-stage permeability evolution model of coal and rock. A mathematical model for the permeability of composite coal-rock specimens under true triaxial stress was developed. The permeability evolution model was further extended to describe the entire instability process of composite coal-rock along the true triaxial path. The results reveal that the model accurately captures the permeability characteristics of composite coal-rock under true triaxial stress.
To understand the permeability and energy evolution of coals under the mining-induced stress, constant and unloading confining pressure tests are conducted. The permeability and energy evolution of coals under unloading confining pressure for simulating the excavation process of coal seam are compared with a constant confining pressure condition. The results show that the peak strength and elastic modulus under unloading confining pressure are lower than that under constant confining pressure. Besides, the unloading confining pressure results in a lower normalized permeability decrement and a larger normalized permeability increment than constant confining pressure condition. Moreover, permeability is well related to elastic and dissipative energy density. The energy evolution could well characterize the damage and permeability evolution. The damage is initiated earlier under constant confining pressure tests than under unloading confining pressure.
In this study, a coupled strain-damage framework based on mesoscale elastoplastic elements is proposed to numerically investigate rock deformation and dynamic damage (from the initiation of mesoscale damage to macroscopic failure) under cyclic loading. Building on mesoscale elastic-damage mechanics, elements are upgraded to elastoplastic counterparts to capture plastic strain accumulation and strength degradation during deformation. Damage is formulated as a strain- and time-dependent field variable and solved together with the solid-mechanics field through a staggered coupling scheme. The framework is evaluated by cyclic uniaxial compression and cyclic Brazilian splitting tests. The simulations reproduce the complete deformation-failure process and key laboratory-observed features, including hysteresis, irreversible strain growth, and fatigue failure. Quantitative validation shows that key mechanical parameters are reproduced with relative errors below 5% (elastic modulus: 4.6%, UCS: 0.9%, UTS: 0.4%, and failure angle: 3.2%). The results indicate that the ultimate failure strain under cyclic loading is governed by its monotonic counterpart, and they quantify the influences of specimen heterogeneity (homogeneity index m), maximum stress ratio, and loading frequency on fatigue life. The proposed approach provides a physics-grounded and computationally tractable basis for analysing rock fatigue and failure in geoengineering settings subjected to dynamic loading.
CO2-enhanced coalbed methane recovery (CO2-ECBM) can significantly improve gas extraction efficiency while simultaneously inducing permeability reduction. However, the evolution on permeability during displacement is still indistinct. To examine the evolution of permeability and gas displacement efficiency, a suite of experiments is performed to measure the strain characteristics and gas flow rates of coal samples. A mixed gas permeability model is established, and the advantages of injecting pressure at different phases of displacement are discussed. The finding reveals that the coal exhibits higher deformability in the direction perpendicular to bedding planes. CO2 adsorption significantly reduces permeability. While the displacement process is taking place, the permeability is expressed in the increase and decrease phases, and finally, it displays a stable tendency. Under different pressures, the permeability decreases to 95.9%-98.2% of the initial permeability, respectively. The CH4 flow rate experiences decreases, increases, and sustained decline phases during displacement. A stepwise increase in gas injection pressure during different displacement stages enhances the gas displacement efficiency.
In coal mining, mixed-mode I-II fractures caused by offset cracks are commonly observed. While previous research has primarily focused on stress intensity factors (SIFs) at the crack tip during fracture initiation, the variation of SIFs during crack propagation has received limited attention. As the SIF is a singular parameter, the precise localization of the crack tip is critical for its determination. However, traditional high-speed photography and visual localization methods often introduce systematic errors in experimental measurements. Therefore, this study employs an ultrafast time-resolution method (15 ps) based on pulsed laser technology combined with an immune algorithm to eliminate such errors and calculate I-II mixed-mode SIFs. Semi-circular bend (SCB) experiments were conducted on coal specimens with offset fractures positioned at 5 mm, 10 mm, and 15 mm from the center. Experimental results demonstrate that both Mode I and Mode II SIFs at crack initiation increase with greater offset distances. Notably, a significant decrease in SIFs is observed after the onset of crack propagation. Additionally, the energy release rates at initiation were measured as 140.77 J/m2, 328.45 J/m2, and 2063.61 J/ m2, showing a clear upward trend with increasing offset distance. This paper presents a novel experimental approach for characterizing the coal fracture process and SIF evolution, providing valuable insights for laboratory testing in rock mechanics.
