Marine gas hydrates are highly sensitive to temperature and pressure fluctuations, and deviations from in-situ conditions may cause irreversible changes in phase state, microstructure, and mechanical properties. However, conventional samplers often fail to maintain sealing and thermal stability, resulting in low sampling success rates. To address these challenges, an in-situ temperature- and pressure-preserved sampler for marine applications has been developed. The experimental results indicate that the self-developed magnetically controlled pressure-preserved controller reliably achieves autonomous triggering and self-sealing, provides an initial sealing force of 83 N, and is capable of maintaining pressures up to 40 MPa. Additionally, a custom-designed intelligent temperature control chip and high-precision sensors were integrated into the sampler. Through the design of an optimized heat transfer structure, a temperature-preserved system was developed, achieving no more than a 0.3 °C rise in temperature within 2 h. The performance evaluation and sampling operations of the sampler were conducted at the Haima Cold Seep in the South China Sea, resulting in the successful recovery of hydrate maintained under in-situ pressure of 13.8 MPa and a temperature of 6.5 °C. This advancement enables the acquisition of high-fidelity hydrate samples, providing critical support for the safe exploitation and scientific analysis of marine gas hydrate resources.
As mining depth increases, the temperature of the surrounding rock rises, drawing global attention to the potential for geothermal energy extraction from high-temperature water stored in collapsed rock masses—a prospect that offers both promise and challenges. In response, this study proposes a functional backfilling method using mining solid waste to construct a high-porosity heat extraction space. The research integrates experiments, theoretical analysis, and simulations to examine the mechanical and permeability properties of solid waste backfill materials. It further aims to elucidate how flow velocity and initial temperature influence the evolution of the temperature field and the thermal performance. Results indicate that the backfill material achieves optimal mechanical strength with a glass fiber content of 10‰ and a length of 6 mm. Furthermore, the permeability of the solid waste backfill demonstrates a quadratic relationship with both axial and confining pressure. During the recovery stage, the temperature in the heat extraction space remains lower than that of the surrounding rock, with geothermal energy being extracted via convective heat transfer between the water medium and the rock. The amount of heat extracted shows a positive correlation with the flow velocity of the water medium and a negative correlation with its initial temperature.
Biotite content critically influences rock mechanical behavior and threatens underground engineering stability. Uniaxial compression tests with acoustic emission (AE) monitoring were conducted on granite pegmatite samples having varying biotite content. Peak frequency distribution analysis, rise angle-average frequency (RA-AF) analysis, multifractal theory, and a dynamic multifractal algorithm were applied to explore the relationship between damage evolution and AE characteristics. Results indicate that increased biotite content reduces uniaxial compressive strength and elastic modulus, enhances plastic deformation, and increases the proportion of shear cracks. The segmented evolution of the dynamic multifractal parameter △ αm is biotite-dependent. Oscillations during the elastic phase signify localized shear crack initiation and propagation; their attenuation in the plastic phase reflects frictional closure along biotite cleavage planes, promoting elastic energy storage and delaying release. AE-based damage models and time-varying signals characterize rock damage progression. Stress concentrations around biotite minerals foster localized shear band formation, leading to concentrated shear failure at lower damage levels. Higher biotite content accelerates crack propagation, while smooth cleavage planes lower the fracture energy threshold, reducing strength and stiffness. These findings enhance understanding of biotite-influenced progressive rock damage and underpin stability monitoring and early-warning systems for underground engineering.
The safe and efficient development of geothermal energy is a key driver of the energy revolution and environmental governance in this century. To understand the effect of water driving pressure on drilling safety and hydraulic fracturing efficiency during the development of geothermal energy under varying reservoir temperatures, dynamic compression tests were conducted on granite samples subjected to thermal treatment (25, 100, 200, 300, 400 and 600 °C) and subsequent forced water absorption (0, 4, 8, 12 MPa) using a split Hopkinson pressure bar system. The results indicate that a higher water driving pressure exacerbates the deterioration of dynamic compressive strength with increasing temperature, while it enhances the rate dependence of dynamic compressive strength, except at 600 °C. The dynamic increase factor (DIF) of dynamic compressive strength vs. strain rate is determined by both temperature and water driving pressure. A prediction model for the deterioration of dynamic compressive strength considering reservoir temperature and water driving pressure is proposed for geothermal reservoirs. While the splitting failure of samples remains unchanged, crack density increases with increasing temperature and water driving pressure, exhibiting multiscale failure cracks parallel to the loading direction. The structure effective strength model, the wing-crack propagation model, the effect of pore water pressure on dynamic stress intensity factor, and the dynamic response of forced absorbed water can collectively reveal the response mechanisms of dynamic strength. Based on the experimental findings, implications for safe and productive geothermal energy development are discussed, with particular attention to the effect of drilling fluid leakage on wellbore stability and the impact of residual fracturing fluid after backflow on repeated fracturing. This study has important reference value for understanding dynamic wellbore stability under drilling disturbance loads and for the design of repeated dynamic hydraulic fracturing schemes in geothermal energy development.
It is of great significance to study the failure mode of mining roadways for safe coal mining. The unconventional asymmetric failure (UAF) phenomenon was discovered in the 9106 ventilation roadway of Wangzhuang coal mine in Shanxi Province. The main manifestation is that the deformation of the roadway on the coal side is much greater than that on the coal pillar side. A comprehensive study was conducted on on-site detection, theoretical analysis, laboratory tests and numerical simulation of the UAF phenomenon. On-site detection shows that the deformation of the coal sidewall can reach 50–80 cm, and the failure zone depth can reach 3 m. The deformation and fracture depth on the coal pillar side are much smaller than those on the coal side. A calculation model for the principal stress of surrounding rock when the axial direction of the roadway is inconsistent with the in-situ stress field was established. The distribution of the failure zone on both sides of the roadway has been defined by the combined mining induced stress. The true triaxial test studied the mechanical mechanism of rock mass fracture and crack propagation on both sides of the roadway. The research results indicate that the axial direction, stress field distribution, and mining induced stress field distribution of the roadway jointly affect the asymmetric failure mode of the roadway. The angle between the axis direction of the roadway and the maximum horizontal stress field leads to uneven distribution of the principal stress field on both sides. The differential distribution of mining induced stress exacerbates the asymmetric distribution of principal stress in the surrounding rock. The uneven stress distribution on both sides of the roadway is the main cause of UAF formation. The research results can provide mechanical explanations and theoretical support for the control of surrounding rock in roadways with similar failure characteristics.
Weak structural planes commonly exist in underground engineering. These planes make anchor structures more prone to failure, threatening rock stability, threatening the safety and stability of underground engineering. Optical-Thermal-Acoustic (OTA) monitoring was applied during uniaxial compression tests on cross-layer anchored rock masses. The study revealed the mechanical properties, failure characteristics, and energy evolution of rock masses with different anchoring methods and bedding angles. Key findings: anchoring suppresses transverse deformation and tensile crack propagation, increasing elastic modulus and bearing capacity; anchored rock shows more intense acoustic emission but smaller infrared temperature changes; the structural plane angle controls the direction of crack extension and the evolution of the strain characteristics, and the rock is prone to instantaneous slip failure of the structural surface at 45°-75°, and the lower strength with significant IR change characteristics. Distinct OTA characteristics during rupture validate the method’s reliability for rockburst early warning and intensity assessment. Moreover, based on the failure characteristics of cross-layer anchored rock masses, a shear failure criterion for anchored structural planes is established. This criterion enables prediction of rock mass failure modes, analysis of bolt support resistance, reference for support design/construction in underground engineering within complex strata.
Although significant progress has been made in micromechanical characterization and upscaling of homogeneous materials, systematic investigations into deposition-controlled micro–macro rheological relationships in heterogeneous sedimentary soft rocks remain limited, particularly concerning time-dependent viscous parameter upscaling. This study investigates six typical fluvial and lacustrine microfacies from the Ordos Basin, China, including riverbed lag, natural levee, floodplain lake, point bar, sheet sand, and shallow lake mud. Mineral composition and microstructure are characterized, and nanoindentation creep tests quantify viscoelastic properties. A micro–macro upscaling method that transforms the time-domain Burger model into the frequency domain and utilizes three traditional homogenization schemes: dilute approximation, Mori-Tanaka, and self-consistent methods, for comparative estimation of macroscopic rheological parameters is proposed. Microstructural analysis demonstrates distinct fabric patterns controlled by depositional energy. Floodplain lake and sheet sand microfacies show superior rheological stability due to dense quartz skeletons, whereas riverbed lag and shallow lake mud perform poorly, caused by skeleton relaxation and clay-dominated slip, respectively. The point bar microfacies exhibits a “rigid-soft hybrid†behavior, with high long-term stability but reduced transient stability. Comparatively, the frequency-domain upscaling framework developed in this study, incorporating the Mori-Tanaka scheme, demonstrates satisfactory agreement with experimental data, validating its capability to predict macroscopic viscoelastic properties from microstructural features.
Hydrate-based gas separation offers a promising approach for coalbed methane recovery, reaching energy conservation and emissions reduction. This study innovatively applied high-gravity technology to enhance hydrate formation in separating 25%CH4/67%N2/8% O2 for achieving rapid and efficient methane recovery. Systematic investigations were conducted at 283.2 K and 3.0 MPa with tetrahydrofuran at a molar concentration of 5.56% and L-tryptophan at a mass concentration of 0.5% additives, first evaluating liquid flow rate effects (0- 20 mL/min) on mixed hydrate kinetic performance and separation efficiency, followed by rotating speed optimization (0-1200 r·min-1) under the optimal liquid flow rate. The high-gravity system amplified the gas- liquid contact area by ~1155 times through cascaded liquid supply and secondary shear effects, methane molecules entered the hydrate phase rapidly under the highest driving force with the significantly intensified mass transfer. Optimal conditions (20 mL/min, 600 r·minˆ’1) yielded an exceptional initial hydrate growth rate of 58.59 mmol/(mol·h) and methane recovery of 50.76%, about 71.33 and 0.58 times higher than the static system, respectively. Gas chromatography and Raman spectrometer analyses revealed superior methane enrichment in hydrate phase at 90% gas uptake completion, with a concurrent 41.17% reduction in process duration. These findings demonstrate the efficacy of high-gravity-enhanced hydrate technology for coalbed methane separation, offering valuable insights for optimizing clean energy utilization.
Salt deposits in China predominantly originate from lake deposits, characterized by thin salt beds interspersed with numerous interlayers, collectively termed bedded salt formations. Historically, the solution mining practices have adopted the layered solution mining approach, inspired by coal mining techniques. However, this approach fails to account for the unique challenges of salt solution mining. Practical implementation is inefficient, costs escalate post-construction, and cavern geometry is constrained by salt beds thickness. Additionally, resource loss in abandoned beds and stability risks in adjacent mining zones remain unresolved. This study investigates mining scheme selection for low-grade salt deposits in Huai’an Salt Basin, introducing a continuous solution mining method that traverses multiple interlayers. Through comprehensive analysis of plastic deformation in caverns and surrounding rock, volume shrinkage rates, and economic costs comparing continuous and layered solution mining approaches, the results demonstrate that: (1) In the layered solution mining with horizontal interconnected wells scheme, plastic deformation zones propagate unevenly, posing interlayer connectivity risks. Concurrently, roof subsidence and floor heave destabilize the structure; (2) the continuous solution mining with horizontal interconnected wells scheme reduces plastic deformation zones to 3.4% of cavern volume, with volumetric shrinkage below 17%, markedly improving stability; (3) Economically, the continuous solution mining scheme generates caverns 2.43 times larger than the layered solution mining, slashing unit volume costs to 41.1% while enhancing resource recovery and long-term viability. The continuous method demonstrates distinct economic advantages and achieves higher resource utilization efficiency in solution mining compared to layered mining. Furthermore, its superior cavern stability presents strong potential for large-scale implementation.
Deep mining is imperative, and the consequent coal and gas outburst disasters triggered during coal uncovering are becoming increasingly severe. Therefore, this study investigated the mechanical mechanisms of outburst instability from three dimensions: experiment, numerical simulation, and field application. Based on physical simulation tests with different outburst pore diameter, it was found that the gas pressure relief rate, gas emission volume, and outburst dynamic phenomena increase with outburst pore diameter. The migration patterns of the gas-solid two-phase flow evolved over time approximately into suspension flow, plug flow, dune flow, and stratified flow. The dominant influence of gas-driven tensile failure was amplified by uncovering coal area. The employment of the “fluid-solid-damage” coupling model revealed that coal damage, gas emission volume, deflection angle of outburst hole, roof displacement, maximum horizontal tensile stress, the horizontal tensile stress zone, the peak seepage force, and the damage zone all increased with uncovering coal areas. At the gas pressure of 0.74 MPa, when the uncovering coal areas were 3.189, 4.754 and 6.225 m, the total gas emission volumes were 4.72×10-4, 16.83×10-4, and 17.67 m2/s, deflection angles of outburst hole were 150.79 °, 152.89° and 158.66°, the maximum roof displacements were 0.044, 0.046, and 0.325 m, and the peak seepage force were 0.85, 1.27, and 1.46 MPa/m, respectively. The regions of coal failure calculated by tensile failure criterion largely coincided with those calculated by the mixed failure criterion, far greater than those calculated by the shear failure criterion. As the increase of uncovering coal area, tensile weights of 80.72%, 89.78%, and 93.01%, respectively. Comparisons with field outburst cases showed that both gas emission volume and outburst hole deflection angle reflected the tensile failure of coal. The mechanical instability process of outbursts under the influence of uncovering coal area and gas pressure was analyzed, developing the progressive cyclical method of coal uncovering, which provided a novel approach for the achievement of safe coal mining.
To investigate groundwater influence on stability and rockburst mechanism of deep hard-rock rectangular tunnels, water-immersed treatment and uniaxial compressive acoustic emission (AE) experiments were conducted on rectangular tunnel specimens. Energy dissipation characteristics, AE evolution characteristics and damage evolution characteristics of rectangular tunnels were analysed under water-immersed condition. Under water-immersed condition, tunnel specimens were quite sensitive to water. Average peak stress and average peak strain energy exhibited negative exponential decay with water-immersed time. Among them, after 12 d of water immersion, average peak stress of specimens decreased by 28%. Average total strain energy decreased by 70%. Average elastic strain energy decreased by 71% and average dissipated strain energy decreased by 68%. After 62 d of water immersion, average peak stress of specimens decreased by 34%. Average total strain energy decreased by 78%. Average elastic strain energy decreased by 79% and average dissipated strain energy decreased by 75%. Water weakened bonding among mineral particles. Moreover, it undermined load-bearing capacity and diminished energy-storage properties. Under high stress, massive releasable elastic strain energy stored in natural specimens within pre-peak stage may abruptly release after peak stress. This caused rapid crack development and connection in specimens. During accumulation and release of elastic strain energy, initial failure typically occurred at sidewalls. This failure location was not affected by water. Compared with natural specimens, Specimens immersed in water for 62 d had the lowest peak values of cumulative amplitude, cumulative AE energy and cumulative AE count. After 62 d of water immersion, peak values of cumulative amplitude, cumulative AE energy and cumulative AE count of specimens decreased by 84%, 97% and 99%. Compared with AE damage model, fitting degree of energy damage model was higher. For natural specimens, fitting degree of energy damage model was 0.96. For specimens immersed in water for 12 d, fitting degree of energy damage model was 0.96. For specimens immersed in water for 62 d, fitting degree of energy damage model was 0.72. Therefore, an energy damage model had more remarkable applicability and reliability. By establishing dynamic mapping relationship between energy and damage in the model, accuracy of rockburst early warning has been significantly improved. This provided scientific basis for support structure design of rectangular tunnels and regulation of high strain energy.