Repair of irregular caverns using gas blanket holds broad application prospects, yet implementation methods and effectiveness require elucidation. This study classifies irregular caverns into three types and proposes a novel repair mode. The formation mechanisms and repair laws of irregular caverns are investigated through model experiments and numerical simulation. Typical irregular caverns include edge protrusion, narrow-elongated, and partially soluble types. Controlling gas-brine interface lifting manner and avoiding salt layers with faults or fractures can prevent irregular cavern formation. The repair priorities are protrusions and flat roofs, overall cavern expansion, and under-dissolved sides for different cavern types. Enlarged volume and regularized shape act inversely on hydrogen storage stability performance. During repair of edge protrusion and partially soluble cavern, the gas-brine interface concentrates at deformed sections with small adjustment distances, whereas narrow-elongated cavern features large repair ranges with more uniform interface positioning. This study provides insights for irregular cavern repair and utilization.

Utilizing underground spaces of abandoned coal mines for compressed air energy storage (CAES) systems represents a feasible and promising option, yet its feasibility is highly dependent on the system's thermodynamic performance, sealing integrity, and coordination with the power grid. Taking the Datong No.1 Coal Mine in Chongqing as a case study, this study develops a thermodynamic model of CAES suitable for coal mine roadways and performs a comprehensive parameter sensitivity analysis. A quantitative evaluation framework for mine-based CAES is innovatively proposed, and the air tightness performance of storage roadways is investigated under various combinations of air storage pressure and lining permeability. Furthermore, a multi-objective coordinated dispatch model integrating mine-based CAES with the power grid is established, and an improved particle swarm optimization (PSO) algorithm is employed to solve the model. The thermodynamic analysis results show that system efficiency is improved by approximately 10.2% when the compressor isentropic efficiency is increased from 0.80 to 0.94. A more significant improvement of 14.3% is achieved by raising the heat-exchanger efficiency from 0.80 to 0.94. However, a fivefold increase in energy storage density(ESD) achieved by elevating the upper storage pressure limit from 6 to 15 MPa is accompanied by an 8% efficiency loss, revealing an inverse relationship between efficiency and ESD. After CAES retrofitting, the available roadway volume of the Datong No.1 Coal Mine reaches 458,000 m3, corresponding to a total storage capacity of 1374 MWh. Numerical simulations indicate that for the long-term operation of mine-based roadway CAES, the permeability of the sealing layer must be maintained below 1 × 10−20 m2. Grid-coordination results indicate that integrating roadway CAES reduces total electricity cost by 2.25% relative to the non-CAES case, yields peak-shaving/valley-filling revenue of $41,547, and increases renewable penetration from 51.4% to 54.1%, accompanied by absorbable capacity of installed renewable energy increases of 11.7%. Overall, the proposed assessment and dispatch framework jointly quantifies engineering feasibility, including capacity and leakage constraints, and system level value, thereby providing a transferable basis for deploying mine roadway CAES to support higher renewable energy integration.

Salt rock is widely recognized as an ideal host medium for underground energy storage and radioactive-waste disposal. Nevertheless, the presence of fractures within salt cavern gas storage can markedly compromise the integrity of the surrounding rock mass. Focusing on the mechanisms of fracture evolution and mechanical response in salt rock, this study conducted uniaxial compressive strength (UCS) tests on cubic salt specimens containing prefabricated flaws at various inclinations. Acoustic Emission (AE) monitoring and Digital Image Correlation (DIC) were integrated to capture the damage development and crack evolution in real time. The results show that: (1) Flaw inclination exerts a significant effect on both the UCS and the crack-propagation path of salt rock, exhibiting a clear and reproducible angle effect. (2) DIC effectively captures orientation-dependent crack trajectories, revealing the geometric complexity of surface crack evolution during failure. (3) According to the RA-AF criterion, the tensile-event fraction ranks 90° > 0° > 45° > 30° > 60°, consistent with the DIC observations and energy-index analysis. (4) The fractal dimension D increases over time, indicating progressive network densification and multiscale damage amplification. (5) The spatiotemporal distribution of AE events exhibits a damage-localization trend consistent with the evolution of the DIC principal strain field, confirming the complementary capabilities of the two monitoring techniques. The results provide a solid experimental basis for the quantitative evaluation of inclination effects and for the stability assessment of salt cavern storage, offering practical guidance for cavern design and long-term performance appraisal. 

The long-term mechanical stability of surrounding rock in compressed air energy storage (CAES) salt caverns is crucial for the safe and efficient operation of underground energy systems. In this study, the deformation behavior of salt rock was investigated through a combined approach that involved long-term laboratory creep–fatigue tests and engineering-scale numerical simulations. Mechanical experiments were carried out at various cyclic stress levels and loading rates to replicate the creep–fatigue loading conditions that are encountered during CAES operations. The results indicate that both the magnitude and the frequency of cyclic loading significantly influence the time-dependent deformation of salt rock: higher stress levels accelerate damage, whereas lower loading rates lead to increased plastic strain. On the basis of the geological conditions of a planned CAES facility, numerical simulations were conducted using FLAC3D. The model incorporates the Norton creep law to simulate the evolution of the surrounding rock with different numbers of operational cycles and gas pressures. The operating pressure and frequency significantly affect the deformation and plastic zone distribution of the surrounding rock in salt cavern reservoirs. Higher operational frequencies and minimum gas pressures result in reduced deformation and improved cavern stability. For the first time, a direct qualitative analysis was conducted to compare the laboratory experiments and numerical simulation results. Comparative analysis reveals that the experimental and simulation results are generally consistent. These findings offer new insights into the mechanical response of salt cavern-surrounding rock and establish a foundation for predicting the long-term performance of CAES systems.

With mines reaching into greater depth, problems with violent failures become more prevalent, such as rockbursts. To assess the potential for such occurrences, understanding energy dynamics is crucial. This study delves into the impact of excavation shape, aspect ratios, inclined angles and in-situ stress on dissipated plastic energy and released kinetic energy through numerical simulations. Detailed analyses of plastic zone around rectangular and elliptical excavations, varying in aspect ratios, unveiled the distinct patterns of tensile and shear plastic zone. Quantitative assessments of dissipated plastic energy and released kinetic energy highlighted contrasting behaviors between rectangular and elliptical excavations across various aspect ratios. Although the relationships between these energies and excavation parameters are found to be intricate, it becomes evident that: with an increase in the aspect ratio, both shear and tensile plastic zones exhibit a growing trend, and their distribution shows a high degree of consistency with regions of energy concentration. When the aspect ratio is below 1.5, elliptical excavations release slightly less kinetic energy than rectangular ones. Beyond 1.5, this relationship reverses. Moreover, upon exploring inclined angles, critical angles were identified, delineating points where the influence of aspect ratio nearly diminishes. Under the given in-situ stress conditions, approximately 50° and 20° serve as the critical angles for dissipated plastic energy and released kinetic energy, respectively. Changes in the degree of in-situ stress anisotropy have a limited impact on the overall energy trends but significantly alter the critical values of aspect ratio and inclination angle. When the in-situ stress ratio matches the excavation aspect ratio, both forms of energy reach their minimum values. These findings illuminate the complex interplay between excavation geometry and energy dissipation, offering invaluable insights for designing effective excavations and devising strategies to mitigate failures.

Hydraulic fracturing in low-permeability coal seams faces significant challenges, including poor controllability of fracture propagation and difficulty in forming complex fracture networks. Existing studies lack a systematic understanding of fracture evolution under full-cycle dynamic loading, limiting their applicability to field operations. To address this, a large-scale true triaxial physical simulation system combined with real-time acoustic emission (AE) monitoring was employed to investigate hydraulic fracturing under dynamic conditions, including stepwise pressurization and pump shut-in/re-initiation. Fracture evolution was quantitatively characterized through AE analysis and source localization. Results show that during stepwise pressurization, AE spectra exhibit a broad distribution, indicating distributed microcrack nucleation that provides preconditioning damage for subsequent fracture growth. During the fracture propagation stage, AE activity intensifies, with cumulative energy reaching 2900 mV·ms, and tensile failure dominates. Fractures preferentially propagate along the maximum principal stress direction, forming branched, tree-like structures. During pump shut-in and re-initiation, AE activity decreases and the frequency range narrows; however, newly formed fractures are concentrated within azimuth ranges of 240°–255° and 315°–345°, indicating both reactivation of dormant fractures and the formation of new propagation paths. These findings demonstrate that stress redistribution during shut-in and re-initiation governs fracture reactivation, including initiation along original and non-original paths as well as borehole-induced deviation. This mechanism plays a key role in controlling fracture propagation and enhancing fracture network complexity. The study provides new insights into fracture evolution under dynamic loading and offers experimental support for optimizing controllable hydraulic fracturing in low-permeability coal seams.

Reconsolidated salt, formed from crushed halite under compaction, is a promising buffer and sealing material for deep geological repositories of high-level radioactive waste (HLW) because of its low permeability and self-healing properties. This study investigated the gas permeability behavior of reconsolidated salt with varying porosities under different confining pressures and inlet gas pressures using nitrogen gas. Based on nuclear magnetic resonance (NMR) technology, the pore structure of reconsolidated salt specimens with different porosities was tested and imaged. The experimental results demonstrate that gas permeability decreases with increasing gas and confining pressures, with gas pressure having a more pronounced effect. The observed permeability‒pressure relationship is attributed primarily to the Klinkenberg effect, with gas slippage along pore walls enhancing the measured permeability under low-pressure conditions. Using the Klinkenberg correction, the absolute permeability values of reconsolidated salt were derived, reaching as low as 10−19 m2 in low-porosity samples. These values are significantly lower than the apparent gas permeability, indicating excellent sealing performance comparable to or superior to that of bentonite. A logarithmic relationship between the absolute permeability and confining pressure was established, providing a quantitative basis for permeability prediction under repository stress conditions. NMR imaging results indicate that with decreasing porosity, the connectivity between pores also gradually diminishes. Additionally, the slip factor was found to increase with increasing confining pressure, underscoring the evolving influence of pore geometry on gas transport mechanisms. Permeability of reconsolidated granular salt decreases with porosity following a power-law relationship, and the healing supports its sealing effectiveness. This study provides essential data and theoretical insights for evaluating the long-term sealing performance of reconsolidated salt in salt-based HLW repositories.

This study investigates the fatigue behavior of mudstone under cyclic triaxial loading conditions, with a focus on the effects of stress amplitude and confining pressure relevant to underground Compressed Air Energy Storage (CAES) operations. Graded triaxial intermittent fatigue tests (GTIF) were conducted to simulate the stress path of intermittent loading. The results show that the fatigue life of mudstone exhibits a non-monotonic dependence on confining pressure: the longest life occurs at 3 MPa, while higher confining pressures (6–12 MPa) cause a pronounced reduction followed by a plateau, indicating a threshold-like confinement effect. The elastic modulus decreases with increasing stress amplitude but, at a given stress ratio, increases with confining pressure, reflecting a trade-off between stress-induced damage and confinement-enhanced stiffness. Residual strain behavior was found to depend on the stress limit interval pattern: upper limit intervals led to reduced residual strain post-rest, whereas lower limit intervals caused strain reversal due to internal stress relaxation. These trends reveal that the loading history significantly influences deformation recovery and potential damage evolution. The damping ratio decreased with increasing confining pressure but increased with stress amplitude, implying that while confining pressure suppresses internal defect activity, higher stress promotes energy dissipation through microcracking. Dissipated energy showed distinct patterns under different loading modes: it increased with stress ratio but decreased with cycle number, highlighting the fatigue softening effect. Interestingly, confining pressure enhanced energy dissipation in upper-limit loading but reduced it under lower-limit conditions. Overall, these results clarify how confining pressure, stress amplitude, and loading intervals jointly govern fatigue life, residual strain, and energy dissipation in mudstone interlayers, and identify measurable damage indicators that can support safer mechanical design and long-term performance assessment of CAES in layered geological formations.

Traditional roadside backfill bodies made of high-water-content material (TRBBs) are commonly adopted in gob-side entry retaining (GER). However, their brittle response often results in crack propagation, loss of load-carrying capacity, and instability under roof rotation and mining-induced stress. This study investigates the fracture-resistance mechanism, damage evolution, and field performance of a fiber-reinforced roadside backfill body (FRBB). A fracture-mechanics-based model was developed to quantify the toughening effects of fiber bridging and fiber pull-out, and Mode I fracture toughness was measured using semi-circular bending tests. The optimum polypropylene fiber dosage was 0.3%, at which fracture toughness increased from 0.130 to 0.210 MPa·m0.5, a 61.5% enhancement relative to the plain matrix. A 3DEC model incorporating strain-softening behavior, staged material-parameter updating, and joint energy-based damage indicators was used to simulate the response of the surrounding rock and FRBB deterioration. The simulations show that stress concentration and damage mainly occur 30–80 m behind the working face, where fiber reinforcement retards crack interaction and improves damage tolerance. Parametric analyses indicate that greater GER width and longer key block B reduce stability, whereas a thicker immediate roof and larger FRBB width improve structural performance. The proposed analytic hierarchy process (AHP)–matter–element extension model showed that the GER stability grade improved from IV in the TRBB case to III in the FRBB case. Field monitoring showed roof-to-floor and rib-to-rib convergence values of 490 mm and 370 mm, respectively, with only slight FRBB surface microcracking. These findings provide a mechanical basis for designing GER systems with FRBB.