Search Results (57)

The compaction deformation and load-bearing behavior of crushed gangue in the caved zone of goafs directly affect overburden movement, fracture evolution, and stability evolution. To clarify the compaction deformation mechanism of crushed gangue under wetting conditions in goafs, confined compression tests coupled with synchronous acoustic emission (AE) monitoring were carried out. Sandstone and mudstone crushed gangue were selected as the research objects, and Talbot gradation indexes of $n$ = 0.2, 0.4, 0.6, and 0.8 were adopted. The effects of lithology, particle gradation, and moisture condition on compaction deformation, particle-structure adjustment, and AE response were systematically analyzed under dry and short-term wetting conditions. The results show that: (1) the confined compression process of crushed gangue exhibits pronounced nonlinear strain-hardening behavior and can be divided into rapid compaction, slow compaction, and stable compaction stages. Water dripping shifts the stress–strain curves toward the higher-strain side and significantly enhances the compression deformation of mudstone, indicating a stronger wetting response of mudstone than sandstone. Meanwhile, water dripping reduces the equivalent compressive stiffness of crushed gangue. (2) Particle gradation affects the compaction response by modifying the proportions of coarse and fine particles and the initial pore structure. With increasing Talbot gradation index $n$, the proportion of coarse particles increases, resulting in more pronounced skeleton collapse, localized particle breakage, and secondary filling by fine particles, and the final compression deformation generally increases. After wetting, the final strain of mudstone samples with different gradations concentrates within 0.32035–0.33439, indicating that the control of water-induced softening on mudstone compaction deformation is stronger than the gradation effect. (3) AE results indicate that the compaction process of broken rock can be divided into a flow sliding deformation stage, a fracture deformation filling stage, and a compaction elastic deformation stage, corresponding, respectively, to particle sliding and rearrangement, particle breakage and pore filling, and structural stabilization and consolidation. Water action affects damage evolution by modifying particle contact conditions, promoting fine-particle migration, and facilitating structural adjustment. Among them, mudstone exhibits a more pronounced wetting response, whereas sandstone maintains relatively higher structural stability. The findings provide a reference for analyzing overburden movement, predicting residual subsidence, and evaluating stability in water-influenced goafs. Full article

Deep shale reservoirs in the tectonically complex margin of the southern Sichuan Basin have experienced multistage deformation, resulting in strong spatial heterogeneity of the present-day geostress field. However, the influence of stress heterogeneity on multiscale pore structure evolution and reservoir quality remains poorly constrained. Here, we integrate in-situ stress measurements, overburden porosity and permeability experiments, CO₂/N₂ adsorption, high-pressure mercury intrusion, SEM-MAPS (Scanning Electron Microscopy-MAPS) pore imaging, stress well profile interpretation, and multifractal analysis to quantify the controls of present-day geostress heterogeneity on pore structure evolution in deep Longmaxi Formation shale. The results show that the present-day stress regime is characterized by a strike-slip pattern (σ_H > σ_v > σ_h), with significant variations among different structural deformation zones. Increasing structural deformation results in enhanced differential stress, increasing by 30–80% from gentle structures to tight folds and fault-affected zones, accompanied by a 60–70° rotation of the maximum principal stress orientation. Differential stress, effective stress, differential stress coefficient, and stress structure index exhibit strong negative correlations with porosity, whereas permeability decreases nonlinearly with increasing stress, indicating progressive pore-throat compression and connectivity degradation under heterogeneous stress conditions. Multifractal analysis reveals that pore-size domains exhibit different sensitivities to stress heterogeneity. The macropore fractal dimension (DN3) shows the strongest response, followed by mesopores (DN2), whereas micropores (DN1) exhibit relatively limited variations. Fault-affected zones and strongly deformed regions display higher DN3 values (>2.8), reflecting enhanced complexity of macropore and fracture networks. In contrast, gentle structural zones characterized by curvature values <0.10 km⁻¹ and distances >500 m from faults exhibit relatively low and stable fractal dimensions (<2.73), indicating more homogeneous pore structures. Increasing stress heterogeneity induces the transformation of organic matter pores from regular subcircular shapes to flattened and slit-like morphologies, accompanied by pore-size migration toward smaller scales (<15 nm) and enhanced pore heterogeneity (Df > 1.35). These findings reveal that present-day geostress heterogeneity governs shale pore fractal evolution and promotes the transition from micropore-dominated to heterogeneous macropore–fracture systems. This study provides quantitative insights into stress-controlled pore evolution and reservoir quality evaluation in deep shale reservoirs under complex tectonic settings. Full article

CO₂ flooding plays a crucial role in enhancing oil recovery (EOR) in heterogeneous reservoirs; however, the reservoir’s vertical rhythmic characteristics and permeability gradients significantly influence the flooding efficiency. Unlike homogeneous reservoirs, CO₂ migration in heterogeneous reservoirs is jointly controlled by gravitational overburden and differences in flow conditions, which can easily lead to gas short-circuiting and the formation of dominant flow paths, thereby reducing the degree of crude oil mobilization. However, systematic research on the mechanisms of CO₂ flooding under different rhythm types and permeability difference remains relatively scarce. In this study, two-dimensional large-scale physical model experiments were conducted using stratified core plates with a planar size of 30 × 30 cm² and a single-layer thickness of 1 cm. The experiments were performed at 70 °C and 18 MPa, corresponding to the target reservoir conditions, with CO₂ injected from the inlet side and outlet pressure controlled by a backpressure valve. Under these conditions, CO₂ remained in the supercritical state during displacement. These experiments were designed to comparatively investigate the effects of reservoir rhythm and permeability contrast on pressure distribution, CO₂ migration patterns, and crude oil mobilization. The study elucidated the mechanisms by which reservoir heterogeneity influences the effectiveness of CO₂ flooding. The results show that the positive rhythmic unit delays upward CO₂ migration and gas breakthrough because of the low-permeability top layer, resulting in the highest ultimate oil recovery of 73.35%. In contrast, the reverse rhythmic unit promotes rapid CO₂ breakthrough through the high-permeability top layer and forms dominant flow paths, causing insufficient mobilization of the middle and bottom layers and yielding the lowest oil recovery of 51.03%. In the sandwich-type rhythmic unit (low–high–low permeability configuration), the interaction between the high-permeability middle layer and gravity override enhances mobilization in the top and middle layers, whereas oil mobilization in the bottom layer remains limited. Under interlayer conditions, increasing the permeability contrast from three-fold to five-fold strengthens preferential flow in the high-permeability layer and reduces oil recovery from 65.58% to 60.99%. Full article

Staged upward seepage can trigger particle migration and pore-structure adjustment in graded sandy soils, but the resulting transition behavior remains difficult to identify quantitatively. In this study, three representative sandy soils from a deep overburden deposit in southeastern Tibet were tested using a laboratory vertical upward seepage apparatus. Eight specimens with different nominal preparation states were subjected to stepwise increases in hydraulic head difference. Local hydraulic gradient, seepage velocity, hydraulic conductivity, and macroscopic outflow phenomena were monitored. Apparent seepage-transition hydraulic gradients were identified by combining abrupt changes in k–i curves, conductivity ratios between eligible staged records, and observed seepage responses. The clearest transition occurred in the nominal loose specimen of Soil 2, where the temperature-corrected hydraulic conductivity k₂₀ increased from 1.76 × 10⁻³ to 2.25 × 10⁻² cm s⁻¹ as i increased from 0.20 to 0.25, giving k_20,2/k_20,1 = 12.80 and i_c = 0.225. A clear transition was also identified for the nominal dense specimen of Soil 3, with k_20,2/k_20,1 = 6.61 and i_c = 0.583. Clear transitions were identified in the tested specimens only for Groups D and H, whereas the remaining specimens showed weak or phenomenon-assisted responses, local high-gradient fluctuations, or anomalous loading-path records rather than uniformly identifiable transition points. These results show that apparent transition gradients are path-dependent and should be evaluated together with loading history, seepage-velocity evolution, conductivity ratios, and macroscopic observations. Full article

This study examines the evolving patterns and zoning characteristics of gas migration and storage zones during coal seam mining, taking the 215 fully mechanized longwall face at Huangling No. 2 Coal Mine as the engineering background. By integrating theoretical analysis, physical similarity simulation experiments, and field measurements, the research systematically explores the zonal linkage evolution mechanism of mining-induced depressurization gas migration and storage zones, together with the associated depressurization gas extraction technology. A flow regime determination equation, driven by the fracture expansion coefficient and permeability, is established on the basis of the fluid Reynolds number criterion. According to differences in gas flow states and medium morphology, the mining-induced fracture field is divided into five distinct zones: a high-permeability zone dominated by turbulent transport, a medium-to-high permeability zone with transitional flow as the secondary dominant region, a low-permeability zone featuring linear laminar flow with micro-permeability, an extremely low-permeability zone characterized by linear laminar flow in a locked state, and a zone of abrupt permeability change associated with gas enrichment. The dynamic evolution of depressurization gas migration and storage zones and their regional linkage mechanisms are clarified. On the basis of these findings, a dynamic targeted layout strategy for high-level boreholes is proposed that is consistent with the spatiotemporal evolution of the overburden permeability field. Field engineering practice shows that the optimized high-level borehole layout maintains the overall gas extraction rate at the drilling site stably above 70%, with a peak value of 93.7%, thereby ensuring safe and efficient mining of the working face. Full article

To investigate overburden movement and three-zone development under far-field strong dynamic disturbance induced by instability of typical thick and hard overburden in western mining areas, a large-scale two-dimensional physical similarity simulation was conducted using the 11N0201 working face of Maiduoshan Coal Mine as the engineering background. Four test scenarios were designed: a baseline condition, dynamic loading, pressure-relief boreholes, and coupled disturbance. The results show that dynamic loading shortened the first weighting interval of the overburden by 45.5%, while the thicknesses of the caving zone and fracture zone increased to 15 cm and 42 cm, respectively, representing increases of 36.4% and 20.6% relative to the baseline condition. At the fully mined stage, fracture connectivity increased to 45%. A fracture intersection angle of <50°, connectivity of >40%, and abrupt aperture variation can be regarded as empirical semi-quantitative precursor indicators of a dynamic instability tendency in thick and hard overburden. By introducing prefabricated weak planes, roof pre-splitting guided the directional development of fractures and caving. Under coupled disturbance, the thickness of the fracture zone was reduced by 42.9% compared with that under dynamic disturbance alone, and the amplitude of displacement fluctuation decreased by 33.3%. These changes promoted a transition in overburden movement from an “unordered dislocation” state to a controllable state of “dynamic-disturbance-induced, directionally regulated stability”. These findings provide an experimental basis for early warning and prevention of overburden instability under far-field strong dynamic disturbance in western mining areas with thick and hard overburden. Full article

To address delayed roof fracture, severe stress concentration, and strong strata pressure under thick–hard roof conditions, this study investigated the 1014 mining face of Yushuquan Coal Mine. A staged thick-plate model incorporating boundary-condition degradation was established based on Mindlin–Reissner thick-plate theory to analyze the deformation and stress redistribution characteristics of the thick–hard roof during mining. The evolution mechanism of the stress-concentration shell was systematically studied through theoretical analysis, physical simulation, numerical simulation, and field application. The results show that, with mining advancement, the boundary constraints of the thick–hard roof gradually evolve from four-sided clamped support to four-sided simply supported conditions. Meanwhile, the high-stress zone migrates from the goaf boundary toward the central suspended roof region. The stress-concentration shell undergoes a dynamic process of formation, expansion, failure, and reconstruction, and its instability is the main driving mechanism of large-scale roof caving. The plastic zone expands upward in an inverted funnel shape, while acoustic emission signals increase significantly before roof instability and exhibit strong precursor characteristics. Based on the evolution characteristics of the stress-concentration shell, a three-stage coordinated blasting technology was proposed to regulate the overburden load-bearing structure. Field application shows that this method effectively reduces suspended roof distance, caving block size, surrounding rock deformation, and hydraulic support pressure, thereby improving roof stability and mining safety. The results provide theoretical and engineering references for stability control of thick–hard roofs under similar mining conditions. Full article

To clarify the migration and structural evolution of mining-induced overburden following grouting reconstruction of the Fourth Aquifer, the inner section of Panel 1022-2 in Wugou Coal Mine was taken as the engineering background. The evolution law of overburden movement and the development characteristics of the caving zone were systematically investigated via theoretical analysis, similar-material simulation, and numerical simulation. In addition, the maximum caving-zone height of Panel 1022-2 was calculated based on the measured caving-to-mining ratio of the adjacent Panel 1010-1. The results show that following grouting reconstruction of the Fourth Aquifer, the water inflow and permeability coefficient decreased significantly, the mining-induced water-body grade was classified as Grade III, and the required coal pillar type was converted from a waterproof safety coal (rock) pillar to an anti-collapse safety coal (rock) pillar. The bedrock failure morphology evolved sequentially from a symmetrical trapezoid to a stepped shape and finally to an asymmetrical saddle shape, with a maximum caving-zone height of 19.0 m, whereas the Fourth Aquifer evolved from fracture initiation and bed separation to asymmetrical overall subsidence. Overburden migration is jointly controlled by bedrock thickness and the mechanical properties of the unconsolidated layer, presenting a distinct three-stage evolution pattern. As the size of the reserved safety coal (rock) pillar decreases, the overburden failure mode changes from overall plastic failure under relatively thick bedrock, to semi-block failure with longitudinal fractures penetrating to the base of the Fourth Aquifer and transverse fractures and interlayer separation initiating inside the aquifer, and finally to intensified failure under thin-bedrock conditions. Based on field analogy with Panel 1010-1, the maximum caving-zone height of Panel 1022-2 was calculated to be 19.73 m, which is in good agreement with the numerical and similar-material simulation results, verifying the reliability of the three-stage overburden evolution law and the caving-zone height evaluation. Full article

Large-scale surface subsidence induced by extra-thick seam longwall top-coal caving (LTCC) is strongly amplified by thick unconsolidated overburden, posing serious serviceability risks to overlying linear infrastructure. Taking the S103 Provincial Highway above Panel 6118 in Inner Mongolia, China, as the engineering background, this study integrates theoretical analysis, numerical simulation, and in situ monitoring to investigate the subsidence-control mechanism of directional presplitting roof cutting. The results show that roof cutting mitigates surface subsidence by reconstructing the overburden structural system and weakening the stress-transfer chain, thereby transforming key-stratum deformation from integral bending to segmented block movement and narrowing the subsidence-affected zone. An equivalent mining-depth model for subsidence-boundary convergence is proposed to characterize the inward migration of the subsidence-basin boundary under thick unconsolidated cover, and a segmented probability-integral model is developed to explain the kink-like high-gradient feature in the post-cut subsidence profile. Parametric simulations of roof-cutting positions (p = 0, 2, 4, …, 32 m) show that the most effective mitigation occurs in the range p = 4–12 m; using minimum–maximum highway subsidence together with profile flattening as the optimization criteria, the representative optimum is identified at p ≈ 10 m, for which the maximum highway subsidence is approximately 57 mm, about 76% lower than that in the non-cutting case. The results further indicate that, although roof cutting significantly reduces subsidence and deformation gradients, fissure localization and possible discontinuous deformation near the pre-split weak plane still require careful field monitoring. Full article

This study integrates physical similarity experiments with numerical simulations to examine how overburden lithology influences roof caving behavior and stress field evolution at a longwall mining face. The results demonstrate that overburden strength significantly governs the timing, extent, and periodicity of roof caving, while also strongly affecting the evolution of mining-induced stress. As lithological strength increases, both damage and displacement within the overburden strata decrease. High-strength roofs exhibit larger caving step distances and longer stress accumulation periods. In contrast, low-strength roofs enter the plastic deformation stage earlier, leading to shorter caving step distances, more frequent caving events, and a wider caving range. During coal seam extraction, roof deformation is accompanied by stress concentration and release, which are processes that are closely associated with dynamic disasters. Due to their higher elastic modulus and compressive strength, high-strength rock strata can accumulate greater elastic strain energy prior to failure. Once instability occurs, the rapid release of stored energy leads to intense stress redistribution and dynamic loading. As lithological strength increases, the stress concentration shell evolves from an arch-shaped structure to a flatter configuration. This transition results in higher internal stress levels and stronger stress concentration, thereby increasing the risk of dynamic disasters such as impact instability. Therefore, maintaining the stability of the stress concentration shell and preventing its migration into deeper strata are essential for ensuring surrounding rock stability and safe mining operations. Full article

To mitigate the threat posed by accumulated gob water to underlying coal seams during multi-seam mining, this study investigates the mechanism of water inrush induced by repeated mining and its control through roof cutting pressure relief. The 31110 panel of the Holowan Coal Mine is taken as an engineering case, where the 3⁻¹ coal seam is threatened by gob water from the overlying 2⁻² coal seam. The mechanisms of interlayer rock mass damage accumulation, fracture interconnection, and water-conducting channel formation were systematically analyzed using a combination of theoretical analysis, numerical simulation, and field tests. The results indicate that the superimposed mining-induced failure zones of the upper and lower coal seams significantly exceed the interlayer spacing of 46.5 m. This condition promotes through-going damage of the interlayer strata and facilitates the downward migration of gob water. Without roof cutting, the main roof fractures toward the solid coal side of the 31110 auxiliary headgate, resulting in full connectivity of the overburden plastic zones and the formation of a continuous water-conducting channel. Roof cutting pressure relief, achieved by pre-inducing artificial weak planes, effectively guides roof fracturing toward the gob side, alleviates stress concentration on the solid coal side, and suppresses the expansion of interlayer damage. When the roof cutting height exceeds 35 m, plastic connectivity between the water-resisting coal pillar and the underlying mining-induced damage zone is interrupted, preserving the integrity of the key aquiclude. Field application of directional hydraulic fracturing roof cutting confirms the formation of continuous weakened fracture planes and controlled roof caving along the designed trajectory. The overburden caving angle increases from 70° to approximately 90°, effectively blocking water-conducting pathways and eliminating the risk of gob water inrush. These findings not only deepen the understanding of water inrush mechanisms under repeated mining disturbances but also establish a proactive fracture-regulation framework for gob water hazard control, providing broadly applicable design criteria and technical references for safe and efficient multi-seam mining in water-threatened coalfields. Full article

With the increase in mining intensity and scale, the damage to groundwater resources and surface ecology caused by coal mining has become the main problem facing coal development. Coal mining can cause a redistribution of stress field and stress concentration in local areas of overlying rock, resulting in varying degrees of movement and damage to the overlying rock. Quantitative analysis of the degree of migration and damage in different areas of overlying rock and zoning control is crucial for achieving loss reduction and green mining. In this paper, the overburden damage is divided into regions according to the different causes of formation, regional characteristics of severity, and other factors, and the specific calculation method is given. UDEC7.0 numerical simulation software is used to simulate the overlying rock damage, and the best mining parameters are provided through the area changes in different zones. The research conclusions are as follows: according to the different damage states of overburden rock, the damage of overburden rock can be divided into four parts: I, caving fracture zone, II, fracture development zone, III, sliding failure zone, and IV, slight failure zone. In the four zones, the damage in zones II and IV is relatively light. During the mining process, attention should be given to controlling the development of Zone I to prevent it from abnormally enlarging; for Zone II, hydraulic fracturing can be used when there is a thick, hard key layer that poses a water inrush risk; for Zone III, the focus should be on preventing surface step fractures caused by it. For example, when a thick, hard key layer is present in Zone II, hydraulic fracturing can be applied to avoid large area hanging roofs and severe rock pressure. When the mining height is low, it mainly affects the proportion of regions I and III. With the increase in mining height, the main affected region becomes the II region. The larger the mining height is, the larger the proportion of the II region. With the increase in propulsion speed, the impact range on the surface increases, but the area with severe damage is relatively reduced. With the increase in mining width, the proportion of relatively seriously damaged areas increased. On-site measurements have shown that when the speeds of 120,401 and 22,207 working faces are slow, the rock layer pressure shows a dense state, the overburden fracture is more fully developed, and the area proportion of I and II zones is increased, which reflects the phenomenon of dense surface fracture development on the surface. When the advancing speed is large, the area proportions of zones III and IV increase, and the damage scope decreases. The on-site testing verified the conclusions drawn from theoretical analysis and numerical simulation, which can guide other mines under similar conditions to achieve safe and green production. Full article

Soil permeability decreases with reduced saturation, making desaturation an effective strategy for seepage control. Air injection has emerged as a promising technique to induce desaturation in engineering applications that require rapid seepage prevention. Although this method has attracted considerable attention, its specific effects on soil saturation and permeability remain insufficiently understood. In this study, a modified conventional permeameter is used to examine the influence of air injection on the degree of saturation and permeability coefficient of sandy soil; simultaneously, the variation in air injection pressure during the gas injection process was monitored, and the influence of overburden pressure on the initial gas injection value was investigated. The findings reveal the following: (1) When other factors are the same, the increase in the air injection flow rate decreases the degree of saturation of sandy soil, and the air injection rate is 40 mL/min, which results in the degree of Fujian sand to achieve a maximum reduction to about 0.750; the increase in the relative density decreases the degree of saturation of sandy soil. (2) The decrease in the degree of sandy soil decreases the permeability coefficient of sandy soil; the desaturation effect of the air injection method varies for different sand samples, and the air injection method can reduce the permeability coefficient of Fujian sand by about 60% at most. (3) The change trend of air injection pressure is related to the gas migration process. Overburden pressure has a negligible influence on the initial value of air injection pressure; the initial pressure value of the air injection method is mainly related to hydrostatic pressure and is affected by the pore structure of the soil. Full article

Large-mining-height technology has been increasingly applied in thick seam mining to enhance productivity and resource recovery. However, it also intensifies strata pressure and complicates surrounding rock control, leading to greater overburden movement, stronger roof weighting, and severe coal wall spalling. Taking the 12306 working face of the Wangjialing Mine as a case, this study employs physical similarity experiments and UDEC numerical simulations to investigate the coupled mechanism of overburden migration and coal wall instability. Results show that abutment stress induces non-uniform deformation, while strata pressure changes directly govern spalling depth. Moreover, coal wall instability is strongly affected by multiple factors: greater burial depth intensifies crack propagation, larger mining height expands failure depth, larger mining step size extends the stress-affected zone, larger dip angle shifts failure upward, and lower support resistance weakens control capacity. These findings clarify the disaster mechanism of deep large-mining-height faces and provide theoretical and engineering guidance for optimizing support design and enhancing coal wall stability. Full article

To optimize the location optimization of the coalbed methane (CBM) extraction target area in abandoned mines, based on the background of the Songzao mining area in Chongqing, theoretical analysis and numerical simulation research methods were comprehensively used to systematically evaluate the potential of residual CBM resources in the goaf of the Songzao mining area. The stress-fracture evolution law and permeability enhancement characteristics of overlying strata under repeated mining of inclined multi-coal seams were deeply revealed, and the location optimization of the residual CBM extraction borehole target area was carried out. The results show that the amount of CBM resources in Songzao Coal Mine is 5.248 × 10⁷ m³, accounting for 26.57% of the total resources, which is suitable for the extraction of CBM left in goaf. The maximum height of the overburden fracture zone caused by repeated mining of K2b, K1, and K3b coal seams in Songzao Coal Mine is 72.3 m, which is basically consistent with the results of the numerical simulation (69.76 m). The fracture development of overlying strata is in the distribution form of a symmetrical trapezoid and inclined asymmetrical trapezoid, and its development height increases with an increase in coal seam mining times, and finally forms a three-dimensional ‘O’-ring fracture area, which provides a channel and enrichment area for the effective migration of CBM. The significant permeability-increasing zone of overburden rock is stable in the range of 10~40 m above the roof of the K3b coal seam and is nearly trapezoidal. According to the calculation of the height prediction model of the fracture zone in the abandoned goaf, the fracture height of the long-term compaction of the Songzao Coal Mine is reduced to 63.74 m. Based on the stress-fracture evolution characteristics of the overburden rock, combined with the permeability-increasing characteristics of the overburden rock and the migration law of the remaining CBM, it is determined that the preferred position of the remaining CBM extraction target area of the Songzao Coal Mine should be in the upper corner of the fracture development area within the range of 10~32.47 m above the K36 coal seam. Full article