Search Results (626)

Dynamic damage to the seepage characteristics of the near-well zone during natural gas hydrate exploitation is a key factor limiting production stability. There is still a lack of systematic understanding of the microscopic mechanisms underlying fine-particle migration and blockage in the near-well zone. In this study, a long sand-packed column was segmentally packed with clayey-silt sediments from the South China Sea and quartz sand to simulate the near-well reservoir and the packed layer, respectively. Long-term seepage processes in the near-well zone were simulated using water flow experiments at constant flow velocities. By combining pressure distribution monitoring with particle-size analysis, the spatiotemporal evolution of seepage characteristics in the near-well zone is revealed from both macroscopic and microscopic perspectives. Results indicate that under long-term displacement, the reservoir permeability near the injection end increased from 0.0149 mD to 0.0159 mD; the reservoir permeability near the packed layer exhibits the greatest decline, dropping from 0.0089 mD to 0.0065 mD. Combined with the particle-size analysis of the packer layer, the boundary between the reservoir and the packed layer is identified as the critical site for permeability damage in the near-well zone. Radial flow inversion shows that a reduction in wellbore radius leads to an increase in reservoir pressure, with the increase being greater the farther from wellbore. A decrease in the permeability of packed layer causes an increase in reservoir pressure, but the magnitude of the increase is consistent across different locations. It provides a theoretical basis for the optimized design of production wells. Full article

Connections between earth–rockfill and concrete dams are critical components of hybrid-dam seepage-control systems because material-stiffness contrasts and complex foundation conditions can create localized preferential seepage paths. Using HS Reservoir as a case study, this predictive design-stage assessment employed a full-domain three-dimensional model of the dam–foundation–abutment system and a local three-dimensional model of the cutoff-spur-wall connection. The seepage field, hydraulic gradients, and zonal seepage discharges were evaluated under the normal pool, design flood, and check flood levels, together with the responses of the connection interface and right-abutment grout curtain. Across the three baseline scenarios, the impervious core accounted for 82.2–83.6% of the total head difference at the maximum riverbed section, and the reported control-location gradients remained below the corresponding design values. At the check flood level, the modeled 178 and 179 m head contours passed above the local curtain crest at elevation 177.5 m, identifying an over-curtain seepage pathway. From the design flood level to the check flood level, right-abutment discharge increased from 259.86 to 544.49 m³/d (109.5%), while total discharge increased by 28.6%. Flow in the connection zone diverted around and beneath the cutoff spur wall, and the connection-surface gradients increased with reservoir level. These model predictions characterize the three-dimensional seepage response of the connection zone and right-abutment seepage-control system and can inform curtain-crest review, construction quality control, and post-impoundment monitoring. Full article

The lightning withstand performance of transmission lines is critically affected by grounding impulse characteristics, particularly for river-crossing towers where soil conditions are complex. This study develops a coupled seepage–electric field model to evaluate these characteristics under dynamic hydrological influences. A complex soil model is constructed integrating Bernoulli’s laminar flow equation with Richards’ equation for unsaturated seepage; long-term finite-element iterations simulate seepage dynamics, yielding distributed soil conductivity parameters that vary with river flow velocity, water depth, and impermeable layers. These parameters are then coupled with an electroquasistatic Maxwell framework to model impulse current dispersion. Validation against experimental data confirms the model’s accuracy. Results show that seepage increases moisture and lowers resistivity. Increasing flow from static to 10 m/s reduces riverbed pressure from 5.61 × 10⁴ Pa to 1.86 × 10⁴ Pa, shifting the 0 Pa isobar downward by 5.1 m, weakening seepage and raising impulse resistance. A shallower impermeable layer deflects seepage laterally, reducing nearby resistivity. Raising water depth from 5 m to 10 m increases pressure from 1.96 × 10⁴ Pa to 5.61 × 10⁴ Pa, enhancing seepage and lowering resistivity. These findings indicate that grounding design must holistically account for flow velocity, water depth, and subsurface barriers to ensure reliable lightning current dissipation and stable grid operation. Full article

The complexity of micrometer-scale pore-throat structures in sandstone reservoirs strongly controls the occurrence state and mobilization degree of residual oil after water-flooding. To clarify the differences in residual oil occurrence between pure oil-zone and transition-zone reservoirs and their microscopic controlling mechanisms, sandstone samples were collected from the SII oil layer group, which belongs to the Upper Cretaceous Yaojia Formation, in the Nanqi area of the Daqing Oilfield, northern Songliao Basin, NE China, and were investigated. Mercury intrusion capillary pressure (MICP), two-dimensional nuclear magnetic resonance (2D NMR), laser scanning confocal microscopy (LSCM), micro-computed tomography (micro-CT), X-ray diffraction (XRD), wettability measurement and fractal analysis were integrated to systematically characterize the pore-throat architecture, mineral composition, seepage capacity, and residual oil occurrence of the two reservoir types. The results show that the pore-throat radius distributions are mainly unimodal. In the pure oil-zone samples, the pore-throat distribution is highly consistent with the corresponding permeability contribution curve, whereas evident deviations occur in some transition-zone samples. Large and medium pore throats exert the most significant control on seepage capacity, and the difference in fractal characteristics is mainly reflected by D1, the fractal dimension of large pore throats. The transition-zone reservoirs generally exhibit moderate to strong water-wet characteristics. Owing to the development of fine pore throats and strong capillary forces, water is prone to retention within pore-throat spaces, resulting in pronounced water-blocking and Jamin effects. After water-flooding, the pure oil-zone reservoirs exhibit lower residual oil saturation, with residual oil occurring mainly in a bound state; in contrast, the transition-zone reservoirs show higher residual oil saturation and relatively high proportions of free and semi-bound residual oil. Mineral composition further modifies pore-throat complexity and residual oil occurrence. D1 is negatively correlated with feldspar content, indicating that increased feldspar content helps improve the pore-throat structure, but positively correlated with clay mineral content, suggesting that clay minerals enhance structural complexity. In the transition-zone reservoirs, kaolinite and illite–smectite mixed-layer minerals are relatively well developed. Their velocity-sensitive and water-sensitive effects readily induce pore-throat blockage and increased flow resistance, which are important causes of residual oil enrichment and difficult oil mobilization in the transition zone. Full article

This study addresses stability degradation of cemented unclassified tailings backfill (CTB) under seepage-stress coupling in deep water-rich metal mines. CTB specimens with diverse mix proportions were prepared to explore their mechanical and permeability responses under varying seepage water pressures, and a response surface regression model was built to quantify multi-factor interactive effects on permeability. Results reveal that increased seepage pressure degrades CTB mechanical performance, while the degradation rate gradually declines. Higher cement-tailings ratios amplify the weakening effect of seepage pressure on elastic modulus. The permeability-strain evolution curve of CTB resembles its σ-ε, and the strain at peak permeability k_max always exceeds peak stress strain. The interaction between cement-tailings ratio and seepage water pressure dominates the variation in k_max and k_min. This work deepens the understanding of CTB seepage-mechanical behaviors and offers experimental references for proportion design and stability assessment of CTB in water-rich underground mines. Full article

Long-term agricultural irrigation in the loess platform region of Northwest China has raised the groundwater level and triggered numerous irrigation-induced loess landslides. The widely developed fissures in loess provide preferential pathways for irrigation water infiltration and serve as key factors that control the hydrological evolution and stability of landslides. The Jiaojiayatou landslide in the Heifangtai platform, Gansu Province, was selected as the study case. A coupled saturated-unsaturated seepage–stress numerical model incorporating fissure structures was established to systematically investigate the effects of fissure depth, location, and number on the seepage field evolution, stability, and deformation characteristics of loess landslides under long-term irrigation. The results show that fissures significantly accelerate the advance of the wetting front, enlarge the high-water-content zone, increase pore water pressure, and reduce the factor of safety. Among these parameters, the effect of fissure depth is the most significant: for fissure depths of 5 m and 10 m, the simulated average annual rise in groundwater level is 0.63 m/a and 1.21 m/a, respectively. When the fissure depth increases to 15 m, irrigation water directly recharges the groundwater, leading to landslide instability (factor of safety drops to 0.97). The displacement at the slope shoulder increases by 54% compared with that in the no-fissure case, and the displacement pattern shifts from predominantly horizontal sliding to vertical settlement. Furthermore, the closer the fissure is to the platform edge and the greater the number of fissures, the lower the stability becomes and the larger the soil displacement at the slope shoulder. Full article

The Mao open-pit coal mine waste dump in Hequ, Shanxi, is a loose, anthropogenic mass accumulated over the original topography. Following a recent sliding and significant settlement event, this dump became the subject of intense stability concerns. Due to the high moisture sensitivity of its interlayered soil and coal gangue structure, rainfall infiltration can reduce internal effective stress, triggering slope instability. Although conventional geological surveys have mapped surface fractures, implementing precise, targeted drainage control requires characterizing the internal geometric structure and preferred seepage directions. To address this, this study integrates electrical resistivity tomography (ERT), surface nuclear magnetic resonance (SNMR), and spontaneous potential (SP) methods. Multiple ERT profiles (270–600 m long) were deployed across several benches at varying elevations, supplemented by fixed-point SNMR sounding over typical low-resistivity anomalies and dense SP grid scanning. The integrated results successfully delineate the internal architecture and seepage characteristics of the dump. Specifically, ERT imaging resolves the primary geoelectrical interface (tentatively inferred as the potential sliding surface) separating the overlying loose mass from the stable underlying strata while mapping the spatial extent of the inferred water accumulation zone (IWAZ). SNMR sounding quantitatively reveals a two-layer water-bearing structure at the specific sounding site, with a deep primary water-bearing zone at 45–80 m depth. Furthermore, SP inversions illuminate the seepage process, demonstrating that meteoric water deflects along the geoelectrical interface to converge laterally toward the central axis at approximately 42°, before transitioning into a high-angle vertical deep infiltration zone (61.7°) within the axial region. These findings suggest a potential engineering direction for remediating surficial fractures and designing subsurface drainage along this 1040 m bench axis, which would mitigate future landslide risks by reducing internal pore water pressure. Full article

Understanding the dynamic evolution of pore and fracture structure (PFS) in coal under mining disturbance is essential for safe coal extraction. In this study, coal specimens collected from the Dongqu Mine, Taiyuan, Shanxi Province, China, were subjected to stepwise cyclic confining pressure loading–unloading tests using a triaxial in situ nuclear magnetic resonance (NMR) system. Based on T₂ spectrum measurements, the real-time evolution of PFS, stress–strain response, permeability-related behavior, average pore diameter, and fractal characteristics were systematically investigated. The results show that irreversible damage developed in the coal specimens during cyclic confining pressure loading–unloading. With increasing cycle number, the load-bearing capacity gradually decreased, internal damage intensified, and pore expansion and coalescence became more pronounced. Seepage pore porosity showed an overall increasing trend, indicating a gradual enhancement of inferred permeability. Therefore, seepage pore porosity can be used as an effective indicator for evaluating permeability-related evolution in coal. During both loading and unloading stages, the relative volumes of small pores (SP), medium pores (MP), and large pores and fractures (LPF) continued to increase, whereas their average pore diameters fluctuated. This indicates that pore volume growth was controlled not only by the enlargement or shrinkage of pre-existing pores but also by new PFS generation. Fractal analysis showed that the fractal dimensions of MP, LPF, and total pores exhibited clear scale-dependent evolution, whereas the calculated SP fractal dimensions were lower than 2 and were therefore not suitable for pore-surface fractal interpretation. Among the valid pore systems, LPF exhibited the highest fractal dimension, indicating that LPF dominate the structural complexity of coal. These findings provide new insight into the fractal evolution of unloading-induced PFS damage and offer theoretical support for mitigating gas outburst and water inrush hazards during coal mining. Full article

Underground gas storage converted from depleted oil and gas reservoirs requires reliable long-term sealing of caprocks, faults, and other geological barriers during cyclic injection and withdrawal. Conventional evaluations mainly focus on static geological parameters, whereas the effects of stress evolution during operation are often insufficiently addressed. In contrast, four-dimensional geomechanical simulations based on fluid–solid coupling can capture the dynamic evolution of geological sealing behavior under injection and withdrawal conditions. The review summarizes progress in heterogeneous geomechanical model construction, stress-field evolution under cyclic loading, dynamic sealing assessment of caprocks and faults, and determination of safe operating pressure limits. Geological sealing evaluation has evolved from a static assessment based on geological characteristics to a dynamic assessment controlled by mechanical criteria. Geostress inversion has developed from three-dimensional heterogeneous mechanical models to four-dimensional geomechanical dynamic coupling analyses that account for seepage, stress, temperature, and other factors. Safe pressure evaluation has also progressed from conventional gravity-driven storage construction to the assessment of critical pressure evolution during the safe operation stage. Existing studies indicate that heterogeneous parameter characterization, coupled flow-stress simulation, and dynamic pressure management strongly affect the reliability of sealing evaluation in reservoir-type UGS. The results further show that pressure history, stress redistribution, and creep effects should be considered together when assessing long-term storage safety. The engineering cases listed at the end of this paper verify some of the research findings. The results presented above are of great significance for the construction and safe operation of reservoir-type gas storage facilities. Full article

This study used silicate cement as the base material and nano-silica, fly ash, and bentonite as the composite modification components. Through laboratory tests, the engineering characteristics and impermeability and drying shrinkage properties of the modified slurry were systematically investigated. The results showed that the verification test mix proportion of the slurry was a water–binder ratio of 0.7, a nano-silica content of 2%, a fly ash content of 40%, and a bentonite content of 6%. This ratio of the slurry had the best comprehensive performance. Compared with pure cement slurry, the water loss rate decreased by 61.90%; the 3d, 7d, and 28d compressive strengths increased by 30.60%, 36.08%, and 20.08% respectively; the fluidity decreased by 6.38%; and the initial setting time decreased by 9.77%. The anti-seepage pressure of the verification test mix proportion slurry group reached 1.05 MPa, which was 43.84% higher than the pure cement reference group and was superior to each single addition group. Combined incorporation of nano-silica, fly ash, and bentonite remarkably improved the impermeability. The 56d drying shrinkage rate was 1257 × 10⁻⁶, which was 19.16% lower than that of the reference group. Based on X-ray diffraction (XRD) and scanning electron microscopy (SEM) tests, the microstructure was analyzed, and the hydration mechanism was discussed. The composite modification did not change the type of hydration products but significantly improved the microstructure. Nano-silica reacted with the hydration product Ca(OH)₂ in the early stage of hydration, accelerating the hydration process and promoting the interwoven coating of the hydration product on the calcium aluminosilicate crystals, thereby improving the compactness of the matrix. Fly ash participated in the pozzolanic reaction in the later stage of hydration, adhering to the secondary hydration products on the surface and gradually consuming them, further filling the pores and optimizing the interface structure. Combined with bentonite, nano-silica and fly ash jointly densified the matrix, refining the microstructure of modified samples and forming a continuous integrated hydration product network inside the grout. Full article

The prominent multi-scale pore heterogeneity widely developed in deep-buried coal reservoirs, which seriously restricts accurate reservoir characterization and precise resource evaluation for deep coalbed methane exploitation. Taking deep-buried coal reservoirs of the Benxi Formation in the Ordos Basin as the research target, this study integrates data from low-temperature CO₂/N₂ adsorption and high-pressure mercury intrusion experiments. Segmented monofractal quantification and multi-fractal singularity analysis are further adopted. Fractal differentiation characteristics, scale effects of pores at different scales, and the synergistic control mechanism of multiple geological factors were indicated. Eight segmented single-fractal dimensions (D1~D8) are defined. The results indicate an obvious scale-dependent zonal distribution of pore heterogeneity in deep-buried coal. Micropores smaller than 1.2 nm possess the largest fractal dimension and the strongest heterogeneity. They act as the primary adsorption and storage space for coalbed methane. Mesopores ranging from 1.1 nm to 8 nm have the lowest fractal dimension with the most homogeneous structure, serving as major gas migration pathways. The heterogeneity of macropores gradually increases with pore size. A continuous full-scale pore size distribution curve is reconstructed. Coal reservoirs exhibit a typical bimodal pore structure dominated by micropores and macropores, with the micropore-dominated storage and macropore-dominated seepage. Key multi-fractal indicators including Δα, Δf and the Hurst index are used for quantitative comparison. Micropores display strong aggregation and weak interpore connectivity. Mesopores own superior connectivity, while their heterogeneity differs greatly between individual samples. Macropores feature moderate aggregation and connectivity. Coalification degree, organic macerals, clay minerals and industrial parameters are associated with the formation and differentiation of pore heterogeneity. Each factor differentially regulates the fractal evolution of pores across various scales. Full article

To reveal the heave deformation characteristics and influencing factors of high-speed railway subgrade–culvert transition section (SCTS) on expansive soil foundations under water immersion, this study takes the Xi’an–Ankang High-Speed Railway as the engineering background. An in situ vertical swelling pressure test was conducted, and the equivalent thermal expansion coefficient was back-calculated based on the temperature–moisture equivalence theory. A three-dimensional numerical model was then established. The effects of immersion zone width, swelling potential, and immersion location were analyzed. The results show that, as the immersion center gradually deviates from the culvert center, the maximum subgrade heave deformation exhibits a non-monotonic trend of first decreasing and then increasing. When the immersion zone is locally distributed beneath the culvert, differential heave deformation of the SCTS is likely to occur. Gray relational analysis indicates that the maximum heave deformation is highly associated with both immersion zone width and swelling potential, while the width of heave area is more closely associated with immersion zone width. In engineering practice, the immersion range of the foundation near the SCTS should be carefully controlled, and drainage, anti-seepage, waterproofing, and soil improvement measures should be adopted to reduce the risk of differential heave deformation. Full article

Under complex terrains and extreme environmental conditions such as high wind speeds, prestressed rock anchor foundations for onshore wind turbines are subjected to multiple coupled adverse effects during long-term service, including cyclic wind loading, temperature variation, groundwater intrusion, and rock mass weathering. These factors significantly affect structural performance and service life through corrosion and material degradation processes, while conventional design methods mainly focus on ultimate bearing capacity and lack a systematic consideration of corrosion-induced deterioration mechanisms and long-term performance evolution. Without changing the theoretical framework of current design codes, this study introduces a durability-oriented design concept and explicitly incorporates corrosion effects and material degradation into the analytical system of prestressed rock anchor foundations. First, from the perspective of anchor force evolution, a time-dependent analysis method for long-term prestress loss is established, considering the coupled effects of steel corrosion, material relaxation, and cyclic loading. Second, for the mechanical behavior of group anchor systems, a shear capacity model is proposed that accounts for rock mass strength degradation and grout–rock interface deterioration. Meanwhile, the coupling relationship between foundation void development and groundwater seepage is analyzed, revealing its critical role in the corrosion evolution process. On this basis, a coordinated design method for foundation dimensions and prestress parameters is developed to satisfy both load-bearing capacity and durability requirements. Finite element analysis is further conducted to verify the stress and deformation characteristics of the foundation–rock–anchor system under nonlinear conditions. Engineering case studies demonstrate that the proposed method not only meets bearing capacity requirements, but also effectively suppresses void development, reduces corrosion risk, delays structural performance degradation, and improves long-term service reliability. The findings provide a theoretical basis and engineering reference for the durability design and lifecycle performance optimization of prestressed rock anchor foundations for onshore wind power structures in extreme environments. Furthermore, the study underscores the critical role of advanced anti-corrosion coatings and surface protection systems in mitigating the coupled corrosion-degradation mechanisms, aligning with the scope of this Special Issue on corrosion protection and durability of infrastructure in harsh environments. Full article

Geological storage of carbon dioxide (CO₂) in deep-sea formations represents a pivotal strategy for mitigating atmospheric CO₂ levels, where storage security and efficacy are fundamentally governed by the pore-scale seepage behavior of CO₂. However, the microscopic displacement mechanisms of CO₂–water two-phase flow under the characteristic high-pressure, low-temperature conditions of the deep sea remain inadequately understood. This study employed a self-developed high-pressure microfluidic experimental platform (0–30 MPa, 4–50 °C) to systematically investigate the CO₂ displacement process in porous media. The effects of injection rate (0.001–5 mL/min) and system pressure (1, 5, and 10 MPa) on displacement patterns, front stability, and final saturation were quantified. The results demonstrate that injection rate is the primary controller of displacement stability: high rates (≥0.1 mL/min) induce viscous fingering and lower final saturation, whereas low rates (≤0.05 mL/min) promote stable, piston-like displacement. Crucially, elevated pressure exerts a profound stabilizing effect, effectively suppressing fingering instabilities and enhancing final gas saturation (up to 0.544 at 10 MPa). This work elucidates the synergistic regulatory mechanism between injection rate and confining pressure, providing essential pore-scale experimental evidence for optimizing injection parameters to achieve efficient and secure CO₂ storage in deep-sea reservoirs. Full article

China hosts abundant coal-rock gas (CRG) resources, which have become a critical unconventional natural gas contributor to national reserve expansion and production increment. Supercritical carbon dioxide (ScCO₂) fracturing is recognized as a green and efficient stimulation technology, exhibiting great potential for high-efficiency CRG exploitation. To clarify the effects and intrinsic mechanisms of ScCO₂ treatment on coal fracture initiation, propagation, and CRG recovery enhancement, true triaxial fracturing and CO₂-CH₄ displacement experiments were performed in combination with multiple microscopic characterization methods, including X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and Scanning electron microscopy (SEM). The multi-scale experimental investigation systematically revealed the fracture development mechanism, permeability variation characteristics, and microstructural evolution of coal reservoirs under ScCO₂ interactions. The results indicate that ScCO₂ fracturing significantly lowers the coal fracture initiation threshold compared with conventional hydraulic fracturing, with the breakdown pressure reduced by 26.2% and the initiation time shortened by 37.5%. Such advantages facilitate coal fracture activation and the development of complex fracture networks. Long-term ScCO₂ soaking induces the dissolution of inorganic minerals (e.g., calcite, plagioclase, and clay minerals) and the extraction of inherent organic matter within coal matrices. The coupled hydro-chemical reactions reconstruct the coal pore structure, enlarge pore throats, and improve reservoir permeability, achieving a maximum permeability enhancement of approximately 1.6 times. Meanwhile, ScCO₂ displacement yields a prominent CRG recovery performance, with an ultimate gas recovery factor up to 93.85%. The CRG enhancement mechanism of ScCO₂ fracturing is comprehensively attributed to three core coupled effects. First, ScCO₂ dynamic fracturing generates intricate fracture networks, which greatly optimize reservoir seepage channels and flow space. Second, the ScCO₂–formation water–coal interaction modifies coal physical properties via mineral dissolution and organic matter extraction, thereby improving reservoir permeability. Third, the preferential adsorption of CO₂ over CH₄ triggers effective competitive adsorption and gas displacement, further promoting adsorbed methane desorption and elevating CRG recovery efficiency. This study provides a solid theoretical foundation for the field application of ScCO₂ fracturing technology and offers valuable insights into the green, efficient, and sustainable development of deep coal-rock gas resources. Full article