Search Results (1,035)

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

Fractal analysis has been widely applied to characterize the complexity of pore systems in tight sandstone reservoirs. However, fractal parameters derived from different experimental techniques are frequently interpreted without sufficient consideration of their distinct physical meanings. In this study, tight sandstones from the Fuyu Formation in the Songliao Basin were investigated using cast thin sections, scanning electron microscopy, high–pressure mercury intrusion porosimetry, and nuclear magnetic resonance. The objective was to establish a comparative interpretation framework that distinguishes capillary–pressure–controlled geometric fractal dimensions from NMR–derived apparent relaxation–response fractal parameters (ARR fractal parameters). The pore–throat system is dominated by medium– to fine–sized throats, with the dominant pore–throat radii concentrated between 0.05 and 0.15 μm. Mercury–intrusion–derived fractal parameters primarily characterize the geometric heterogeneity of relatively large and fine pore–throat systems, whereas NMR–derived parameters mainly reflect the apparent relaxation response associated with pore–scale fluid occurrence rather than strict geometric complexity. In the present MICP–NMR subset, comparative regression trends indicate that the geometric fractal parameter representing fine pore throats is more responsive to median pore–throat radius and movable–fluid saturation, whereas the NMR–derived apparent relaxation–response parameter associated with longer relaxation times shows a relatively closer relationship with log–transformed permeability. These relationships should be regarded as exploratory response trends rather than universal predictive models. These findings demonstrate that the two types of fractal parameters provide complementary rather than interchangeable information for reservoir evaluation. The proposed comparative framework offers a physically constrained basis for integrating pore–throat geometry, relaxation response, fluid occurrence, and seepage properties in the characterization of tight sandstone reservoirs. Full article

Tight dolomite gas reservoirs are promising exploration targets, yet their evaluation is complicated by multiscale pore-throat heterogeneity and poor seepage connectivity. Here, high-pressure mercury intrusion (HPMI), nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), and deep-learning-assisted pore extraction were integrated to characterize the pore-throat structure and fractal features of the Middle Ordovician Majiagou Formation in the Ordos Basin. The reservoir is dominated by diagenetic-origin pores, mainly intercrystalline and intragranular dissolution pores, together with microfractures, and can be classified into three types with progressively poorer connectivity and flow capacity. Type I reservoirs contain more regular pores, larger pore-throat systems, and better storage and seepage capacity; Type II reservoirs are intermediate, whereas Type III reservoirs exhibit complex pore morphology, isolated pore networks, poor petrophysical properties, and limited gas-flow potential. The corresponding fractal dimensions are weakly correlated but complementary: D_SEM captures pore-boundary complexity, D_HPMI reflects pore-throat architecture and capillary-pressure-controlled seepage pathways, and D_NMR reflects multiscale movable-fluid distribution. Clay minerals, especially illite-rich mixed layers, further intensify pore-throat heterogeneity. Increasing fractal dimension is generally associated with higher displacement and median pressures, but poorer connectivity, porosity, permeability, movable-fluid content, and gas deliverability. These results provide a basis for the quantitative evaluation of multiscale pore systems and reservoir quality in tight dolomite gas reservoirs. Full article

Retained water associated with water-locking can restrict gas production in heterogeneous tight sandstones, and the activity and phase stability of many surfactant treatments are reduced under high-temperature, high-salinity (HTHS) conditions. In this study, a composite surfactant formulation, CSF-1, designed to lower gas–brine capillary resistance and adjust sandstone wettability under representative reservoir constraints, is reported. Its performance was evaluated using thermal aging, surface tension and contact angle measurements, geochemical compatibility tests, laser diffraction, SEM/EDS, and core flooding combined with stagewise low-field nuclear magnetic resonance (LF-NMR). CSF-1 remained macroscopically homogeneous after aging at 170 °C in 188.314 g/L hypersaline brine and retained low gas–brine surface tension when measured at 25 °C after aging. In core flooding tests, CSF-1 increased the apparent gas permeability from 0.203 to 0.388 mD relative to the SFW-saturated water-locked state, corresponding to a 91.1% improvement. One- and two-dimensional NMR measurements provided comparative relaxation domain evidence that CSF-1 promoted the removal and redistribution of relatively mobile and weakly restricted fluid signals and reduced residual signal clustering. The shortest T₂ relaxation domains were less affected. The absence of replicate core flooding and associated LF-NMR runs, together with the non-equivalent Ref-S comparison, precludes a statistically rigorous cross-agent performance ranking. These results support the laboratory water-locking mitigation potential under the tested conditions, without implying calibrated pore-size-resolved removal or field-scale confirmation. Full article

This study systematically investigates the effects of varying Ni doping levels (1.0–7.0 wt.%) on the microstructure, static magnetic properties, and high-frequency dynamic magnetic performance of Fe-5.5 wt.% Si soft magnetic composites (SMCs). Toroidal core samples were fabricated using powder metallurgy combined with silicone resin coating and high-temperature annealing. The influence of Ni doping on phase composition, morphology, saturation magnetization, coercivity, effective permeability, quality factor, total core loss and its components, and DC bias characteristics was comprehensively evaluated by XRD, SEM, EDS, hysteresis loop testing, and DC bias measurements. The results indicate that an appropriate Ni content (3.0–5.0 wt.%) promotes the formation of α-Fe(Si,Ni) solid solution and (Fe,Ni)₃Si ordered phases, optimizes grain size and structural ordering, enhances saturation magnetization, and reduces coercivity. In contrast, excessive Ni doping (7.0 wt.%) leads to Ni segregation at grain boundaries, forming strong pinning centers that significantly increase coercivity and hysteresis loss. Within the wide frequency range of 1–100 kHz, Ni doping improves the permeability retention under DC bias but reduces the initial effective permeability. Notably, the sample with 5.0 wt.% Ni exhibits the highest quality factor (Q value) across the entire frequency range, demonstrating the best overall performance. This study provides experimental evidence and theoretical guidance for developing high-saturation-resistance, low-loss soft magnetic composites for medium-to-high-frequency applications. Full article

Continental shale oil reservoirs contain multiscale pore–fracture systems with strong heterogeneity and fractal characteristics, which complicate oil mobilization during post-fracturing shut-in imbibition. In this study, shale cores from the LGS Formation (a lacustrine continental shale oil formation in China) were used to investigate capillary-force-regulated pressurized shut-in imbibition by integrating interfacial tension measurements, apparent contact angle tests, capillary pressure calculation, time-lapse nuclear magnetic resonance (NMR), NMR-based fractal characterization, visual observations, and pre-/post-imbibition permeability measurements. Two surfactant-based imbibition agents with different capillary-force regulation mechanisms were compared to represent different capillary-force regulation pathways. Agent 1 mainly modified apparent wettability, increasing the contact angle from 51.0° to 66.1°, whereas Agent 2 reduced the oil–water interfacial tension from 31.85 to 22.12 mN/m while maintaining a favorable apparent contact angle of 49.3°. Time-lapse NMR results showed that oil recovery increased with shut-in time and reached approximately 12–30% after 144 h. Agent 2 generally produced higher recovery than Agent 1, with the optimum response at 0.15 wt%. NMR-derived fractal dimensions ranged mainly from 2.32 to 2.61, confirming the multiscale heterogeneity of the LGS shale pore system. Pore-size-resolved recovery further showed that oil mobilization was dominated by pores larger than 20 nm and microfracture-related spaces, whereas pores smaller than 20 nm contributed only limited bulk recovery. This indicates an apparent nanopore mobilization threshold near 20 nm, controlled by fractal pore complexity, pore-throat connectivity, oil adsorption, capillary pressure, and molecular accessibility of imbibition agents. Visual and permeability evidence further showed that pressurized imbibition can selectively activate connected pore–fracture pathways. Post-imbibition dry-core permeability increased in all tested samples, although the enhancement was highly heterogeneous. These results demonstrate that shut-in imbibition in LGS shale is governed by coupled interfacial regulation, fractal pore heterogeneity, pore-size-dependent oil accessibility, and selective pore–fracture structural modification. Full article

Stacked coalbed methane (CBM) reservoirs can increase the drainage thickness of a single well, but commingled production is influenced by stress-sensitive permeability, gas desorption, water drainage, and interlayer heterogeneity. This study presents a three-segment reservoir model for well GP-2 in the Tucheng block. Pore-fracture compressibility was estimated from overburden low-field nuclear magnetic resonance measurements and compared with stress-dependent permeability obtained by the pulse-decay method. The resulting coefficient was used in the dynamic permeability relationship and held fixed during history matching. The model was calibrated against gas- and water-production data from the first 240 d. One-factor simulations were then run over a common 3000 d calculation window to compare the relative responses to geological, adsorption, and stimulation parameters. In the GP-2 base model, average gas rate increased with equivalent coal thickness, gas content, Langmuir pressure, stimulated area, and stimulated-region permeability; inverse responses were obtained for cleat-fracture porosity, proportional three-layer initial permeability, initial reservoir pressure, and Langmuir volume. Adsorption time and interlayer spacing had comparatively small effects. These trends are specific to the selected model and parameter ranges and should not be interpreted as validated long-term forecasts or established causal relationships. This study demonstrates a practical way to carry a laboratory-derived stress-sensitivity parameter into a multilayer field model. Full article

The residual magnetic field of the magnetic shielding system is a key factor limiting the bias stability of high-precision atomic gyroscopes. Due to the temperature dependence of hysteresis in soft magnetic materials, variations in ambient temperature can cause drift in the residual magnetic field inside the shielding cavity, thereby introducing measurement errors. Existing studies mostly rely on time-consuming finite element methods (FEM), which struggle to efficiently characterize the temperature–magnetic coupling effect. To address this issue, this paper develops a theoretical model for a fast solution. First, a static magnetic field analytical model for the multilayer cylindrical magnetic shielding system is established. Second, nonlinear magnetization theory is introduced to correct the calculation errors caused by the nonlinear variation in material permeability under weak fields. On this basis, an improved Jiles-Atherton (J-A) model incorporating a temperature correction factor is constructed to accurately characterize the magnetic field distribution inside the shielding system at different temperatures. The results demonstrate that the proposed analytical model can independently and rapidly predict the residual magnetic field distribution at different temperatures, without requiring any calibration or fitting based on FEM simulations. After accounting for hysteresis nonlinearity, the deviation of the shielding factor at the center point between the analytical model and FEM simulations is approximately 5%. The static residual magnetic field at the center point exhibits a negative correlation with temperature variation. Within the actual operating temperature range of the atomic gyroscope from −40 °C to 60 °C, the measured results agree with the model predictions regarding the temperature-dependent trend of the radial residual magnetic field. The relative deviation of the radial residual magnetic field ranges from 2.78% to 7.69%, and that of the axial residual magnetic field ranges from 7.94% to 14.47%, thereby verifying the accuracy of the theoretical model. This model effectively predicts the residual magnetic field drift law of the magnetic shielding system under varying temperature conditions and can provide theoretical support for the analysis and active compensation of thermally induced magnetic errors in atomic gyroscopes. Full article

The quantitative characterization of microscopic pore structure has long been a challenge in reservoir evaluation for igneous reservoirs, owing to their pronounced heterogeneity and complex pore geometry. In this study, thin-section casting, X-ray diffraction, high-pressure mercury intrusion, nuclear magnetic resonance, and fractal theory were employed to investigate the reservoir-space types, pore-structure characteristics, and fractal features of the igneous rocks both quantitatively and qualitatively. The relationships among reservoir petrophysical properties, pore structure, movable-fluid saturation, and fractal dimension were examined. The results indicate that the reservoirs in the study area are characterized by medium-to-low porosity and medium-to-low permeability, with mean values of 6.57% and 2.06 mD, respectively; the storage performance of andesite was found to exceed that of tuff. Based on the morphology of the mercury intrusion curves and the petrophysical parameters, the reservoirs were classified into three categories. From Class I to Class III, the displacement pressure increased progressively, the movable-fluid saturation declined from 9.65% to 8.54%, and the heterogeneity was markedly enhanced. The fractal analysis revealed that the reservoirs exhibit distinct piecewise fractal behavior with a well-defined inflection point, allowing two fractal intervals to be distinguished: large pore-throats (D₁) and small pore-throats (D₂). The mean total fractal dimension was 2.8996, and the large pore-throat fractal dimension (mean = 2.9607) exceeded that of the small pore-throats (mean = 2.3863), indicating that large pore-throats serve not only as the principal contributor to reservoir space but also as the dominant control on heterogeneity. Correlation analysis demonstrated that D₁ is significantly negatively correlated with both porosity and permeability, making it a key indicator for evaluating reservoir flow capacity, whereas D₂ is positively correlated with petrophysical properties, reflecting the role of fine throats in improving the connectivity of isolated pores. Notably, the large-pore-throat fractal dimension (D₁) of these igneous reservoirs generally exceeds that of tight sandstone, whereas the small-pore-throat fractal dimension (D₂) is positively correlated with petrophysical properties rather than negatively, in contrast to sandstone reservoirs; this indicates that the pore-structure behavior of igneous reservoirs is distinct from that of conventional clastic reservoirs. This study offers a new perspective on the quantitative characterization of pore structure in igneous reservoirs and provides a scientific basis for reservoir evaluation and exploration-and-development efforts in the study area. Full article

Carbonated water flooding can enhance oil recovery from low-permeability sandstone reservoirs while supporting CO₂ geological sequestration; however, the coupled effects of carbonated water–rock interactions on pore-scale fluid redistribution remain unclear. This study used online nuclear magnetic resonance (NMR), magnetic resonance imaging (MRI), and mineralogical analysis to evaluate wettability-related water redistribution, mineralogical alteration, and oil mobilization in low-permeability sandstone cores exposed to carbonated water for 0, 5, 10, and 15 days, followed by immiscible CO₂ flooding. With increasing exposure duration, NMR-derived water saturation increased from 0.490 to 0.571, indicating an apparent increase in pore-scale water affinity under the same saturation protocol. XRD results showed carbonate and clay/zeolite-related mineral alteration, including calcite falling below the detection or quantification limit and marked decreases in chlorite and laumontite, which were associated with modified pore-wall properties and improved water-phase access. During subsequent immiscible CO₂ flooding, oil was preferentially mobilized from well-connected migration pores, while carbonated water treatment enhanced oil recovery from capillary-controlled percolation pores. The overall recovery factor increased by 2.8 percentage points, reaching 53.8% after 15 days of treatment. These results indicate that carbonated water improves CO₂ flooding performance through coupled mineral alteration, pore-connectivity modification, wettability-related water redistribution, and multi-scale oil mobilization. The study provides NMR-based pore-scale evidence for interpreting carbonated water-assisted CO₂ utilization and enhanced oil recovery. Full article

The development of a magnetic composite based on a silicone matrix containing carbonyl iron (CI), combined with the Frequency Selective Surface (FSS) for radiation-attenuating material (RAM) applications in the X-band (8.2–12.4 GHz), is presented in this work. Four FSS geometries were investigated: square, circular, triangular, and hexagonal. The electromagnetic properties, namely relative electrical permittivity and magnetic permeability, were characterized using a vector network analyzer employing both waveguide and free-space measurement techniques. The attenuation performance was evaluated through reflection loss (RL) measurements and numerically simulated using FEKO software. The stability of the attenuation performance was also assessed for different wave incidence angles (0° to 45°), demonstrating a robust average peak attenuation of −32.1 dB at 11.18 GHz, with optimal resonance values reaching as low as −60.34 dB at an incidence angle of 30°, in good agreement with the simulation results. The results indicate that the capacitive and inductive behavior associated with FSS geometries plays a key role in tailoring the electromagnetic response, demonstrating the effectiveness of FSS-based magnetic composites for controlled attenuation performance. Full article

China is rich in high-sulfur natural gas resources. During reservoir development, reservoir temperature and pressure reduction induces the precipitation of elemental sulfur. Subsurface sulfur deposition seriously affects the recovery and the stable production of high-sulfur gas reservoirs. This study utilized multiple experimental techniques, including CT scanning, scanning electron microscopy, energy spectrum analysis, and nuclear magnetic resonance. The experiments were conducted under different water saturation levels and pressure differences. The results showed that the permeability of the rock samples decreased after sulfur deposition. The permeability reduction varied from 0.004 mD to 8.852 mD, with a relative change of 10.2% to 29.8%. Meanwhile, sample porosity also declined, and the porosity damage ranged from 1.5% to 11.9%. Scanning electron microscopy showed that sulfur presented a membrane adsorption morphology on the surface of skeleton particles, with spherical particles protruding from the membrane. Rock samples with poorer physical properties showed lamellar superposition sulfur deposition. Sulfur deposition damage became more severe with increasing pressure difference and weakened as water saturation increased. Beyond a water saturation of 40.6%, further increases no longer reduce sulfur deposition damage. Full article

Magnetic actuation is a fundamental enabling technology for micro/nanorobotics and biomedical manipulation. However, the trade-off between magnetic field gradient, usable workspace, and efficient heat dissipation often conflicts and constrains its performance. Here, we present an enhanced electromagnetic manipulation system (EEMS) based on a compact, high-efficiency magnetic circuit and an optimized six-electromagnet configuration. By integrating high-permeability structural components and employing finite-element-based optimization, the system achieves a spherical workspace of 106 mm in diameter while maintaining strong and spatially controllable magnetic fields. Experimental results demonstrate magnetic flux densities up to 300 mT and a magnetic field gradient up to 9.5 T/m within the workspace, with a central magnetic field gradient of approximately 2 T/m under continuous operation at 3 A. Thermal simulations and measurements confirm safe operation below human body temperature without active cooling. Magnetic manipulation experiments in viscous environments further validate precise motion control and force balancing, highlighting the system’s potential for advanced magnetic manipulation and intelligent microrobotic applications. Full article

Iron powder cores are widely used in cost-sensitive low- to medium-frequency applications because of their high saturation magnetisation, low cost and favourable formability. However, the low electrical resistivity of iron powders favours continuous conductive pathways between adjacent particles, leading to high-frequency eddy-current loss and heat accumulation. To combine electrical insulation, interfacial stability, magnetic-property retention and thermal diffusion in a single coating, a synergistic insulation/thermal-conduction coating based on thiol-functionalised boron nitride was designed for iron-based magnetic powder cores. Hexagonal boron nitride was surface-modified through ultrasonic activation followed by grafting with a mercaptosilane coupling agent, forming covalent linkages on the boron nitride surface. The resulting functionalised nanosheets were deposited onto water-atomised iron powders through interfacial interactions between nitrogen- and sulfur-containing functional groups and the iron surface. A coating content of 5 wt.% produced a relatively continuous and uniform interfacial layer with limited agglomeration, enabling the magnetic powder cores to combine interparticle insulation, loss reduction, magnetic-property retention and thermal transport. The optimised core exhibited a volume resistivity of 58.7 Ω·m and a total core loss of 81.2 kW/m³ at 10 mT and 100 kHz, corresponding to a 20.8% reduction relative to the pure iron core. The sample retained a saturation magnetisation of 201.4 emu/g and an effective permeability of 67.5 at 100 kHz, while achieving a thermal conductivity of 55.2 W/(m·K) and a thermal impedance of 0.215 K·m²/W. Loss-separation analysis indicates that the continuous insulating layer restricts interparticle induced-current pathways and suppresses high-frequency eddy-current loss, while the two-dimensional boron nitride framework promotes internal thermal diffusion. Full article

The present study investigates heat transfer characteristics in the thermally developing region of a porous tube under the local thermal nonequilibrium (LTNE) model. The influence of magnetohydrodynamic (MHD) flow on an electrically conducting fluid flowing through a porous medium under a transverse magnetic field is examined. Under the LTNE framework, two separate energy equations are employed to describe the temperature fields of the fluid and solid phases. The coupled governing equations are solved numerically using the Keller-box method. The results indicate that increasing the interphase heat transfer parameter strengthens thermal coupling between the fluid and solid phases, thereby reducing temperature differences and promoting local thermal equilibrium. In contrast, an increase in the Prandtl number reduces thermal diffusion, leading to larger temperature gradients and greater disparity between the two phases. Furthermore, the magnetic field suppresses both the velocity and temperature distributions through the Lorentz force. An increase in permeability reduces the velocity profiles due to the combined effects of the MHD and Prandtl numbers while increasing the temperature profiles. Increasing the interphase heat transfer rate drives the system from the LTNE to the LTE phase. The study confirms that LTNE effects play a significant role in thermal transfer processes in porous media and are relevant for various industrial heat transfer applications. Full article