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Water color is an important optical proxy for trophic status and water quality, but its integration into regulatory assessment frameworks is still limited. This study assesses the potential of the Forel–Ule Index (FUI) derived from Sentinel-2 as a proxy indicator to support the assessment of the ecological status of reservoirs under the European Union’s Water Framework Directive (WFD). Seventeen reservoirs located in semi-arid Mediterranean climate agricultural basins in southern Portugal (Sorraia, Sado, and Guadiana) were analyed, combining 4316 FUI observations (2017–2024) with in situ water quality data and official WFD ecological status classifications. The results showed that the values on the FUI scale (which ranges from 1 to 21) fell, for the most part, between 12 and 18 and with marked spatial and seasonal contrasts, particularly between more transparent reservoirs and persistently turbid ones, probably eutrophicated reservoirs. Principal component analysis showed that the first component (PC1, 39.5% of variance) represents a trophic gradient dominated by turbidity, chemical oxygen demand and chlorophyll-a, and is positively, albeit moderately, correlated with FUI (Spearman’s ρ = 0.439, p < 0.001), while the second component, dominated by nitrogen, showed no significant association. To make the Water Framework Directive (WFD) regulations compatible with the structure of the available dataset, ecological status was dichotomized into “Satisfactory” and “Deterioration”. Binary logistic regression showed that increasing FUI values were significantly associated with a lower probability of classification as “Satisfactory” (β = −0.682, p = 0.0137; odds ratio = 0.51, 95% CI: 0.29–0.87). The model performance was moderate (balanced accuracy = 0.682; AUC = 0.754), with better identification of “Deterioration” conditions than “Satisfactory” conditions. The Mann–Whitney U test confirmed that mean FUI values differed significantly between the two ecological status groups (U = 65, p = 0.0166), with lower values associated with reservoirs meeting the “Good” threshold. Overall, FUI proved to be a low-cost and temporally flexible screening and early-warning tool, particularly useful for identifying departures from favorable ecological conditions. However, the index is not a direct substitute for the official ecological classification and is best applied in combination with physicochemical and biological metrics when assessing changes in water quality over a shorter period of time. Full article

Accurate interwell connectivity characterization is critical for fine-grained waterflood optimization and reservoir management, often requiring integrated analysis across multiple disciplines and methods. Among these, injection–production response analysis stands as the most cost-effective and widely adopted approach. However, it remains highly subjective, heavily reliant on senior engineers’ decades of accumulated experience, and prohibitively labor-intensive for large-scale oilfields with hundreds of wells. With the exponential growth of production data in modern oilfields, manual analysis has become the bottleneck restricting the timeliness of reservoir management decisions. While signal processing techniques offer a promising path to automation, general-purpose algorithms fail to incorporate fundamental reservoir fluid flow laws, resulting in insufficient accuracy for practical engineering applications. To address this gap, we propose a novel connectivity identification method that mimics expert analysis logic by focusing on large-amplitude fluctuation segments rather than full-curve matching. Using curve slope as the core metric, cosine similarity quantifies trend consistency, while Root Mean Square Error (RMSE) measures amplitude proximity. Three targeted strategies enhance accuracy: key region screening with segmented matching, outlier removal accounting for time-varying lags, and multi-index weighted fusion. Validated on synthetic and mature carbonate waterflood field cases, the method improves the identification performance over benchmark Normalized Cross-Correlation (NCC) and Capacitance-Resistance Model (CRM) methods by more than 12% in both cases. It retains the reliability of traditional response analysis while achieving full automation, and can help estimate the timing of preferential flow path formation, requiring only routine production data to provide valuable reference for timely field development decision-making. Full article

To mitigate global warming induced by excessive carbon dioxide emissions, carbon dioxide displacement technology for carbon utilization and storage has attracted growing attention. In this paper, laboratory displacement experiments combined with Nuclear Magnetic Resonance (NMR), constant-rate mercury intrusion, and X-ray diffraction (XRD) tests are adopted to investigate the residual storage characteristics of carbon dioxide in tight sandstone reservoirs. This study evaluates the effect of depletion pressure on carbon dioxide storage efficiency. Combined with the pore-throat fractal dimension (D_f) obtained from constant-rate mercury intrusion and the capillary tortuosity fractal dimension (D_T) calculated via models, the relevant controlling mechanisms are illustrated. The results show that the residual storage efficiency can exceed 62.00% when the depletion pressure is higher than the supercritical pressure of carbon dioxide. Storage efficiency exhibits a significant correlation with the average pore-throat ratio, indicating that pore-throat matching characteristics play a vital role in carbon dioxide retention. Both the pore-throat fractal dimension D_f and capillary tortuosity fractal dimension D_T are positively correlated with the average pore-throat ratio and negatively correlated with the pore-throat radius, reflecting the impacts of pore structure complexity and fluid channel tortuosity on carbon dioxide migration and storage processes. X-ray diffraction test results further verify that quartz and clay minerals indirectly affect carbon dioxide storage performance by altering the preservation status and connectivity of pore throats. The innovation of this study lies in establishing a coupled analysis system integrating pore-throat heterogeneity, fluid channel complexity, and carbon dioxide phase evolution, which reveals the residual storage mechanism of carbon dioxide in tight sandstone reservoirs. Relevant research findings provide new insights for studies on carbon dioxide storage laws at the pore scale, and offer theoretical support for optimizing geological carbon dioxide storage schemes in tight reservoirs. Full article

The transformation of abandoned open-pit mines into pumped-storage power station reservoirs offers an effective way to optimize China’s energy structure and supports China’s dual-carbon objectives by providing large-scale flexible storage. However, the steep slopes and complex geological conditions formed by mine excavation pose severe challenges to the stability of reservoir slopes. This paper focuses on a pumped-storage power station project converted from an abandoned mine pit in Luanping, Hebei Province. Specifically addressing high and steep slopes containing weak faults, it proposes a stability analysis method for the excavation stage that considers the influence of faults. The analysis assumes dry conditions, excluding reservoir-induced pore water pressure and seepage forces. Adopting the Janbu method and the M-P method within the limit equilibrium framework, safety analyses were conducted on multiple typical analysis sections of the abandoned mine pit before and after excavation. Analytical results indicate that the minimum safety factors derived from the Janbu method pre- and post-excavation are 1.33 and 2.13, respectively, whereas those obtained via the M-P method are 1.40 and 2.47. Given its consistently lower and thus more rigorous estimations, the Janbu method is recommended for conservative engineering design and risk assessment in similar geological settings. It is found that when the fault outcrop daylighting on the slope face constitutes a substantial spatial proportion, the potential sliding surface of the slope will extend from the top of the weak fault along an arc to the bottom of the stepped excavation. This indicates that the position of the fault influences both the sliding mode and the safety performance of the slope. The research findings of this paper could provide technical support for the slope engineering design, excavation and construction of pumped-storage power station reservoirs converted from abandoned mine pits. Full article

Ultra-low-permeability tight oil reservoirs, including the Chang 6 and Chang 8 formations in the Changqing Oilfield and the Fuyu reservoir in the peripheral Daqing Oilfield, are characterized by poor reservoir properties and limited waterflooding efficiency. Conventional areal waterflooding either fails to establish effective displacement or results in rapid local water breakthrough, leading to rapid production decline and a recovery degree of less than 10%. Large-scale refracturing combined with modification of the water injection strategy has, therefore, become an important approach for improving single-well productivity. However, long-term injection–production imbalance may cause substantial formation-energy depletion and an increase in horizontal stress contrast, which are unfavorable for the development of complex fracture networks during refracturing. To investigate the mechanism and optimize the design of pre-fracturing energy enhancement, rock-mechanics experiments were first conducted on cores from the Fuyu reservoir in the Daqing Oilfield. The resulting pore pressure and stress responses were interpreted based on poroelastic coupling and the effective-stress principle. A three-dimensional coupled reservoir–geomechanical model was subsequently established for a Chang 6 tight oil block in the Changqing Oilfield using formation-specific geological, petrophysical, geomechanical, and production data, and field performance was further used for validation. The laboratory results show that pre-fracturing water injection increases pore pressure, reduces effective confining stress, and decreases the horizontal principal-stress difference, thereby promoting a transition in rock failure from isolated shear fractures toward intersecting fracture patterns. The laboratory-derived mechanical trends were transferred to the Chang 6 model primarily at the mechanistic level, while quantitative parameters were recalibrated using reservoir-specific data. By establishing the relationship between the energy enhancement ratio, defined as the ratio of injected fluid volume to cumulative produced fluid volume, and formation pressure recovery, and further considering sensitivity, economic feasibility, and operational constraints, the optimal energy enhancement ratio was determined to be 0.8–1.0. These results clarify the geomechanical mechanism and key design parameters of pre-fracturing energy enhancement and provide practical guidance for refracturing design in ultra-low-permeability tight oil reservoirs. Full article

Unmanned Aerial Vehicle-borne Ground-Penetrating Radar (UAV-GPR) has been used for ice thickness monitoring in lakes and rivers due to its non-contact measurement, high resolution, and operational flexibility. Existing algorithms can extract ice layer boundaries by tracking continuous bottom reflections in GPR images. However, they fail when the radar signal lacks a clear bottom reflection—a common condition in ice layers containing unfrozen water—and manual interpretation remains time-consuming. To address this limitation, this paper builds a freshwater ice GPR dataset covering both fully frozen and unfrozen water-bearing zones, and proposes a method for ice thickness identification based on the DeepLabv3+ neural network. The model performs pixel-level binary classification, labeling each pixel as ice layer or background, and generates a segmentation mask that constrains the subsequent thickness calculation to valid ice regions only. Field validation against drilling measurements demonstrates that the model achieves Intersection over Union (IoU) of 97.12% and an F1-score of 98.54% for ice layer identification, with a relative error in ice thickness measurement below 3% based on five borehole measurements. Field tests in two reservoirs across Tibet and Jilin, China, demonstrate that the proposed method can accurately characterize the distribution and thickness of the ice layer while effectively eliminating the interference of unfrozen water zones. The results demonstrate that the proposed method can provide automated, accurate ice thickness estimates for UAV-GPR surveys of freshwater ice. Full article

High arch dams are structurally efficient hydraulic structures for demanding loading conditions, including extreme loading, and are widely recognized for the transfer mechanism of such loads acting on them. The distinctive performance of these thin concrete structures during their service life is often intriguing and worth investigating. Unlike in the case of concrete gravity dams, in arch dams—especially, thin arch dams—the impact of temperature loads assumes significance due to the geometry and load-transfer mechanism. In this paper, an existing high double-curvature thin concrete arch dam experiencing fluctuations in reservoir levels is analyzed regarding the combined effect of thermal loads and the deflections and stresses caused. Thermal loads arising from continuous exposure of intrados and extrados faces contribute to critical loading scenarios. The FEM-based simulations assisted with field monitoring data and were used to study the structural response under the influence of temperature in steady-state conditions. The study found that the increase in body temperature is a cause of undesirable tensile stresses in the upper parts of the dam body, close to 2.0 MPa, which may cause the development of horizontal cracks. Small areas of upstream heel portion also develop higher tensile stresses due to temperature loads. The dam in its 50-year service life showed apparently aberrant behavior in deflections. The seasonal temperature variation—an increase—can be a cause of the atypical response of the dam. The anomalous nature of the behavior cannot be considered unusual, but the study also found that mitigation measures are effective, suggesting that continuous monitoring is required for sustained healthy functioning. Full article

Sustainable reservoir-system management requires coordinated operation to balance economic benefits, sediment regulation, and ecological requirements while maintaining equitable and durable cooperation among participating reservoirs. Focusing on the Wanjiazhai, Sanmenxia, and Xiaolangdi reservoirs in the middle and lower Yellow River Basin, this study develops an integrated framework that links multi-objective reservoir operation, coordination-oriented scheme selection, and compensation allocation under representative dry, normal, and wet hydrological conditions. NSGA-II is used to identify Pareto trade-offs among sediment transport, electricity production, and ecological water-deficit control, while the coupling-coordination degree model is applied to select schemes with balanced overall performance. CRITIC-TOPSIS is then used to allocate compensation by integrating static engineering attributes with operation-induced dynamic responses. The results show that the maximum coupling-coordination degrees reach 0.81, 0.88, and 0.90 under dry, normal, and wet conditions, respectively. Compared with actual operation, the recommended schemes increase total electricity production while reflecting hydrologically dependent trade-offs in sediment transport and ecological water-deficit control. Xiaolangdi Reservoir consistently receives the largest compensation share, followed by Wanjiazhai Reservoir and Sanmenxia Reservoir, and this ranking remains consistent across alternative allocation methods. By linking operational trade-offs with reservoir-specific contributions and compensation priorities, the framework supports more adaptive and equitable joint operation and provides a quantitative decision-support approach for improving the long-term environmental, economic, and institutional sustainability of reservoir-system management in the Yellow River Basin. Full article

Emergency treatment of algal blooms in lake and reservoir source waters requires coagulants that remain effective under changing raw water conditions. This study compared polyaluminum chloride (PAC) and chitosan (CTS) in raw water collected from Yangku Reservoir during wet and dry seasons. Jar tests evaluated algal density, algal biomass, and nitrogen and phosphorus fractions across a 3~15 mg/L reagent-mass dosage range. Treatment performance differed between seasons. Mean algal density and soluble reactive phosphorus (SRP) removals in the wet season were 63.12% and 68.51%, respectively, and an apparent 70.05% decrease in measured NH₄⁺-N concentration was also observed. Because the fate of dissolved inorganic nitrogen was not resolved, the NH₄⁺-N decrease should not be interpreted as direct coagulative removal. Dry season water had higher algal density and a higher SRP/TP ratio. PAC maintained relatively stable algal biomass removal across seasons and showed stronger phosphorus removal, whereas CTS was more sensitive to seasonal changes in the raw water matrix. These findings support season-specific preliminary screening of coagulants while highlighting the need for residual-Al, pilot-scale, and process-mechanism validation before full-scale application. Full article

Seasonal energy storage effectively addresses seasonal electricity supply–demand imbalances. This study proposes a cross-seasonal compressed air energy storage system using a depleted gas reservoir as the storage reservoir. A six-stage compression–expansion system with intercooling and reheating is designed. Also, the heat of compression is recovered for district heating, and the expansion of cold energy is for cooling supply, thereby avoiding cross-seasonal heat storage costs. For the YD1 depleted gas reservoir case, the system achieves a round-trip efficiency of 57.85% and an exergy efficiency of 70.9%. The total energy utilization ratio, which represents the combined utilization of the electricity, heating, and cooling outputs relative to the corresponding energy input, reaches 166.6%. Incorporating revenue from the heating and cooling sales, the dynamic payback period is 2.38 years, and the investment recovery ratio reaches 4.40. Parametric analysis indicates that thermodynamic performance improves with the increase in discharge pressure and decrease in discharge power. Economic performance improves with longer daily operating hours and plant lifetime. This study demonstrates that depleted gas reservoirs combined with well-designed surface combined cooling, heating, and power systems offer a new research and development direction for large-scale, long-duration seasonal storage, facilitating renewable energy integration and grid stability. Full article

The reuse of post-mining infrastructure for pumped-storage hydropower may reduce new underground construction while supporting the repurposing of decommissioned mines. This study presents a site-specific preliminary technical and pumping-energy assessment of an underground pumped-storage system using Budryk Shaft II as the lower reservoir. The assessment integrated shaft geometry, hydraulic conditions, turbine–generator selection, pressure-pipeline configuration, structural adaptation, hydraulic isolation, and staged water return. A working water volume of 12,000 m³ was adopted. The proposed generation unit comprises a vertical Pelton turbine operating at a gross design head of 900 m, a rated net head of 837.81 m, and a discharge of 0.71 m³/s. The rated turbine output is 5287 kW, and the turbine is coupled to a 6.3 kV synchronous generator. The three-stage pumping calculation yielded energy demands of 7.037, 19.600, and 37.371 MWh, giving a total of 64.008 MWh. These values are calculation-based estimates derived from the listed nominal pump capacities and powers using a simplified proportional power–flow assumption. At the rated turbine output, the calculated generation time of 4.695 h corresponds to 24.822 MWh of mechanical energy at the turbine shaft. Because verified generator-efficiency data are unavailable, the generated electrical energy and electrical round-trip efficiency cannot be determined exactly. The ratio of turbine-shaft energy to the calculated pumping-energy demand gives an upper-bound energy-return indicator of approximately 38.8%. Further work must verify pump operating points, generator performance, hydraulic transients, structural integrity, shaft sealing, auxiliary-energy demand, and the complete hydraulic connection to the upper reservoir. Full article

Enhanced oil recovery (EOR) is a crucial technology for improving crude oil recovery; it extracts residual oil from reservoirs through chemical, physical, or biological methods, thereby further increasing recovery rates after secondary recovery. Biosurfactants, particularly lipopeptides, have become a research focus in the field of EOR due to their excellent properties. However, existing studies have mainly concentrated on their production and characterization, while systematic investigation into their deactivation mechanisms and stability limits remains lacking at the molecular level. This study integrates density functional theory (DFT), ab initio molecular dynamics (AIMD), and classical molecular dynamics (MD) simulations to systematically reveal the hydrolysis mechanisms and stability boundaries of lipopeptide model molecules under high-temperature and high-salinity reservoir conditions from a multiscale perspective. DFT calculations show significant differences in the energy barriers among different hydrolysis sites in lipopeptide molecules, with side-chain structure being a key factor influencing amide bond hydrolysis. Metal ions present in reservoir environments (Na⁺, K⁺, Ca²⁺, Mg²⁺), particularly divalent ones (Ca²⁺, Mg²⁺), can act as catalysts to reduce the hydrolysis energy barrier. Electronic structure analysis reveals that the catalytic effect originates from the polarization of the carbonyl oxygen by metal ions, weakening the covalent character of the C=O bond. AIMD simulations reveal that only Ca²⁺ can specifically activate the hydrolysis of lipopeptide molecules at certain distances (critical distance), while other cations (e.g., Mg²⁺, K⁺, Na⁺) do not exhibit similar catalytic activity. MD simulations further demonstrate that Ca²⁺ ion concentration and temperature are the dominant factors influencing Ca²⁺ permeation toward hydrolysis sites (limit distance), with other ions having a weaker effect. By systematically simulating lipopeptide behavior under varying temperature and ion concentration conditions, a catalytic hydrolysis criterion based on the effective distance of Ca²⁺ interaction (i.e., limit distance ≤ critical distance) is established through multiscale simulation, and the performance boundaries of its temperature and salt tolerance are preliminarily defined. This study provides a theoretical basis and quantitative design guidance for the applicability of lipopeptide-based biosurfactants in high-temperature and high-salinity reservoirs. Full article

Atmospheric effects substantially influence remote-sensing reflectance retrieval in optically complex inland waters. This study evaluated seven atmospheric correction approaches (QUAC, FLAASH, Sen2Cor, LaSRC, 6S, C2RCC, and ACOLITE) for Sentinel-2 MSI and Landsat-8/9 OLI imagery over the Danjiangkou and Luhun reservoirs. The evaluation used 67 quality-controlled, temporally matched in situ spectral observations and satellite matchups. Performance was quantified using the squared Pearson correlation coefficient (r²), root mean square error (RMSE), and average unsigned relative error (AURE). Laboratory-measured chlorophyll-a (Chl-a) concentrations were used to develop sensor-specific retrieval models and to examine how atmospheric-correction differences propagated into Chl-a estimates and spatial patterns. Because residual aerosol and sun-glint effects may remain after atmospheric correction, an exploratory SWIR-based adjustment was evaluated for the C2RCC visible-band outputs. In the pooled-band analysis, C2RCC yielded the most favorable balance of the evaluated metrics for both sensor datasets. However, performance varied among bands, and the Landsat-8/9 B5 output showed very weak covariation with the in situ measurements. Within the model-development dataset, Sen2Cor achieved the highest Sentinel-2 r² (0.762), whereas C2RCC achieved the lowest Sentinel-2 RMSE (2.30 mg/m³). C2RCC achieved both the highest Landsat-8/9 r² (0.689) and the lowest RMSE (3.18 mg/m³). Independent temporal validation used 14 Luhun observations from 2024. Sen2Cor yielded the lowest Sentinel-2 RMSE and AURE (1.658 mg/m³ and 29.28%). For Landsat-8/9 OLI, C2RCC yielded the highest r² (0.536), lowest RMSE (3.468 mg/m³), and lowest AURE (44.08%). Relative errors increased in weak-signal near-infrared bands, underscoring the need for band-specific interpretation. The SWIR-based adjustment improved both RMSE and AURE for Sentinel-2 MSI but did not provide a consistent improvement for Landsat-8/9 OLI. An exploratory comparison of quality-screened imagery from 2016 to 2025 showed broadly similar reservoir-scale Chl-a patterns in C2RCC-derived products from the two sensors. These results provide reservoir-specific evidence for atmospheric-correction selection and Chl-a retrieval under the sampled conditions. Full article

Conventional Rate Transient Analysis (RTA) models, based on homogeneous fracture assumptions, are inadequate for characterizing flow in complex fracture networks of heterogeneous unconventional reservoirs. This study develops a fractal-based RTA (FD-RTA) workflow integrating lithofacies analysis, microseismic fracture interpretation, and post-fracturing production data from the Yingxiongling shale oil field in the Q’aidam Basin. The workflow is applied to eight horizontal wells completed in layered and laminated dolomites. Results show that the two lithofacies exhibit distinct fractal flow behaviors. Layered dolomite tends to develop preferential flow pathways, characterized by rapid initial depletion followed by declining supply capacity, with the half-flow dimension (δ) decreasing from 0.299 to 0.074 during production. Laminated dolomite displays stronger fracture-matrix interaction and sustained production performance, with δ increasing from 0.469 to 0.678 as multi-scale fractures are progressively activated. The FD-RTA workflow effectively links fracture complexity with production behavior, providing a dynamic characterization tool for evaluating hydraulic fracturing effectiveness in shale oil reservoirs. Full article

Deep coalbed methane (CBM) reservoirs exhibit ultralow permeability, high in situ stress, and pronounced stress sensitivity. The resulting feedback between reservoir deliverability, wellbore liquid transport, and surface choking cannot be represented reliably by isolated reservoir or wellbore calculations. This study develops an integrated reservoir–wellbore–choke coupling model for deep CBM wells. The reservoir submodel adopts a dual-porosity, single-permeability formulation for matrix-to-natural-fracture mass transfer, incorporates hydraulic fractures through non-neighboring connections, and accounts for Langmuir adsorption/desorption and effective-stress-dependent permeability. Gas–liquid flow in the tubing or annulus is calculated with the Beggs–Brill correlation, whereas critical and subcritical flow through the wellhead choke is evaluated with the Sachdeva mechanistic model. Bottom-hole flowing pressure (BHP) serves as the coupling variable in a partitioned sequential-iterative scheme. For each time step, the reservoir model predicts gas and water rates at a prescribed BHP; these rates are passed to the choke and wellbore models, whose returned BHP updates the reservoir boundary until convergence. Newton iterations solve the reservoir equations, and the critical liquid-carrying rate identifies the end of stable natural flow and the onset of liquid-loading risk. Application to Well H1 yielded agreement scores of 80.99%, 79.93%, 94.17%, and 90.48% for the gas rate, water rate, BHP, and wellhead tubing pressure, respectively, with an overall mean of 86.39%. Gas content governed the mid- to late-time deliverability, while tubing and choke sizes controlled the trade-off between friction loss, drawdown, and liquid unloading. A 2–3/8 in tubing string combined with a 12 mm choke provided the most balanced performance. The model supports life-cycle production forecasting and integrated completion and production optimization for deep CBM wells. Full article