Search Results (13)

It is vital to stabilize pillar dams in underground reservoirs in coal mine goafs to protect groundwater resources and quarry safety, practice green mining, and protect the ecological environment. Considering the actual occurrence of coal pillar dams in underground reservoirs, acoustic emission (AE) mechanical tests were performed on dry, naturally absorbed, and soaked coal samples. According to the mechanical analysis, Quantitative analysis revealed that dry samples exhibited the highest mechanical parameters (peak strength: 12.3 ± 0.8 MPa; elastic modulus: 1.45 ± 0.12 GPa), followed by natural absorption (peak strength: 9.7 ± 0.6 MPa; elastic modulus: 1.02 ± 0.09 GPa), and soaked absorption showed the lowest values (peak strength: 7.2 ± 0.5 MPa; elastic modulus: 0.78 ± 0.07 GPa). The rate of mechanical deterioration increased by ~25% per 1% increase in moisture content. It was identified that the internal crack development presented a macrofracture surface initiating at the sample center and expanding radially outward, and gradually expanding to the edges by adopting AE seismic source localization and the K-means clustering algorithm. Soaked absorption was easier to produce shear cracks than natural absorption, and a higher water content increased the likelihood. The b-value of the AE damage evaluation index based on crack development was negatively correlated with the rock damage state, and the S-value was positively correlated, and both effectively characterized it. The research results can offer reference and guidance for the support design, monitoring, and warning of coal pillar dams in underground reservoirs. (The samples were tested under two moisture conditions: (1) ‘Soaked absorption’—samples fully saturated by immersion in water for 24 h, and (2) ‘Natural absorption’—samples equilibrated at 50% relative humidity and 25 °C for 7 days). Full article

Rainfall can easily cause local sliding and collapse of carbonaceous mudstone deep road cut slopes. In order to study the strength characteristics of carbonaceous mudstone under different water environments, large-scale horizontal push shear tests were conducted on carbonaceous mudstone rock masses in their natural state and after immersion in saturated water. The push shear force–displacement relationship curve and fracture surface shape characteristics of carbonaceous mudstone samples were analyzed, and the shear strength index of carbonaceous mudstone was obtained, and numerical simulations on the stability and support effect of carbonaceous mudstone slopes were conducted. The research results indicate that carbonaceous mudstone can exhibit good structural properties and typical strain softening characteristics under natural conditions. The fracture surface, shear strength, and shear deformation process of carbonaceous mudstone samples will undergo significant changes after being soaked in saturated water. The average cohesion decreases by 33% compared to the natural state, and the internal friction angle decreases by 15%. The numerical simulation results also fully verify the attenuation of mechanical properties of carbonaceous mudstone after immersion, as well as the effectiveness of prestressed anchor cables and frame beams in supporting carbonaceous mudstone slopes. The research results provide an effective method for understanding the shear performance of carbonaceous mudstone and practical guidance for evaluating the stability and reinforcement design of carbonaceous mudstone slopes. Full article

The long-term immersion of coal rock may affect its mechanical properties and failure modes, potentially impacting the stability of coal pillars. This work aims to investigate the influence of the immersion duration on the mechanical properties and fracture evolution processes of coal, employing acoustic emission detection and the digital image correlation (DIC) method. The work focuses on the weakening law of the coal pillar dam in contact with water and obtains a model of the strength deterioration after different periods of water immersion. The stress–strain curves of coal specimens with varying immersion durations are obtained. The results show that the peak absorption rate of coal samples immersed in water transpires within 24 h, with fundamental saturation being achieved at between 25 and 30 days at saturation moisture content of 1.97%. The specimen’s compressive stress after being immersed in water for 7 days is 3.34 MPa, with strain of 0.18%. The cracking stress is 15.60 MPa, with strain of 0.54%. The peak stress is recorded at 27.65 MPa, with strain of 0.92%. The complete rupture stress measures 23.37 MPa, with the maximum strain at 0.95%. During the yielding stage, the specimen has the highest strain increment of 0.38%. Short-term immersion brings about an increase in the coal sample’s plasticity, exhibiting a relatively minor softening impact of water, resulting in comparatively intact fragmentation and modest breakage. A negative exponential function relationship is observed between the compressive strength of coal and the immersion duration. The mechanical reduction relationship is utilized to analyze the failure patterns of coal pillars in underground reservoirs. With prolonged water immersion, the damage area expands to include the coal pillars and the surrounding rock of the excavation area. Full article

Deep coalbed methane (CBM) reservoirs have the characteristics of low permeability, low porosity, and low water saturation, which easily experience wellbore instability due to drilling fluid, severely affecting drilling safety. Based on the physical property analysis of coal samples, the wellbore instability mechanism of the deep CBM reservoir was investigated by multiple methods. It was found that the wellbore instability is mainly caused by drilling fluid intrusion and the interaction between drilling fluid and coal formation; the fracture pressure of coal after immersion decreased from 27.4 MPa to 25.0 MPa because of the imbibition of drilling fluid. A novel nano-plugging agent with a size of 460 nm was prepared that can cement coal particles to form disc-shaped briquettes with a tensile strength of 2.27 MPa. Based on that, an effective anti-collapse drilling fluid for deep coal rock reservoirs was constructed, the invasion depth of the optimized drilling fluid was only 6 mm. The CT result shows that the number of fractures and pores in coal rock significantly reduced after treatment with the wellbore-stabilizing drilling fluid; nano-plugging anti-collapse agent in drilling fluid can form a dense layer on the coal surface, and then the hydration swelling of clay in the wellbore region can be effectively suppressed. Finally, the drilling fluid in this work can achieve the purpose of sealing and wettability alternation to prevent the collapse of the wellbore in the deep coal reservoir. Full article

Underground reservoir technology in coal mines enables the effective storage and utilization of water resources disturbed by mining activities. Owing to the effects of mining operations and water extraction/injection activities, the water head in underground reservoirs fluctuates dynamically. The total bearing capacity of a coal pillar dam is significantly reduced due to the combined effects of overlying rock stress, dynamic and static water pressures, and mining-induced stresses, which are critical for ensuring the safe operation of underground reservoirs. Based on the correlation between different water head heights and the corresponding water pressures on the coal pillar dam, a custom-made coal rock pressure water immersion test device was used to saturate the coal samples under various water pressure conditions. The mechanical deformation and failure characteristics of the samples and fracture propagation patterns under different water pressure conditions were studied using uniaxial compression, acoustic emission (AE), and three-dimensional X-ray microimaging. The results indicated that, compared with the dry state, the peak strain of the water-immersed coal samples increased to varying degrees with increasing water pressure. Additionally, the average porosity and the number of pores with diameters in the range of 0 to 150 μm significantly increased in water-immersed coal samples. Under the combined influence of water immersion pressure and uniaxial stress, loading the water-saturated coal samples to the fracture damage threshold significantly intensified deformation, failure, and fracture propagation within the samples, and the failure mode changed from tension to a composite tensile–shear failure. Full article

When water damage occurs in a mine, variations in the immersion levels of tunnels at different burial depths can be observed. There is a significant relationship between the stability of the surrounding rock and the depth of immersion. Therefore, studying the deformation and damage characteristics of sandstone with circular holes at varying immersion depths, along with their acoustic properties, plays a crucial role in maintaining the stability of water-rich roadways. The TAW-2000 press and static strain system were utilized to investigate the mechanical properties, crack evolution, and deformation field distribution of sandstone with circular holes at varying immersion depths. Additionally, this study analyzed the impact of immersion depth on the characteristic parameters of acoustic emission. The results indicate that immersion depth is negatively correlated with the compressive strength and modulus of elasticity of sandstone; as immersion depth increases, the duration of the compression and yield phases of the rock samples also increases, while the duration of the elastic phase remains relatively unaffected. Furthermore, greater immersion depths correspond to a decrease in the total number of cracks, although the proportion of tensile cracks increases, making the formation of secondary cracks less likely. The frequency of acoustic emission events (transient elastic waves generated by the formation, extension, or closure of tiny cracks within the rock) shows a closely correlated dynamic with the stress–time curve of the rock sample. The acoustic emission ringing counts generated by rock samples under submerged water conditions tend to stabilize with a slight increase before signs of rupture appear. Additionally, the cumulative total energy of acoustic emissions from the rock samples decreases as the water level rises. These research findings provide significant reference value for addressing issues related to water immersion and the extent of water saturation in roadways within rock engineering. Full article

The frequency domain characteristics of acoustic emission can reflect issues such as rock structure and stress conditions that are difficult to analyze in time domain parameters. Studying the influence of immersion time on the mechanical properties and acoustic emission frequency domain characteristics of muddy mineral rocks is of great significance for comprehensively analyzing rock changes under water–rock coupling conditions. In this study, uniaxial compression tests and acoustic emission tests were conducted on sandstones containing montmorillonite under dry, saturated, and different immersion time conditions, with a focus on analyzing the effect of immersion time on the dominant frequency of rock acoustic emission. The results indicated that immersion time had varying degrees of influence on compressive strength, the distribution characteristics of dominant acoustic emission frequencies, the frequency range of dominant frequencies, and precursor information of instability failure for sandstones. After initial saturation, the strength of the rock sample decreased from 53.52 MPa in the dry state to 49.51 MPa, and it stabilized after 30 days of immersion. Both dry and initially saturated rock samples exhibited three dominant frequency bands. After different immersion days, a dominant frequency band appeared between 95 kHz and 110 kHz. After 5 days of immersion, the dominant frequency band near 0 kHz gradually disappeared. After 60 days of immersion, the dominant frequency band between 35 kHz and 40 kHz gradually disappeared, and with increasing immersion time, the dominant frequency of the acoustic emission signals increased. During the loading process of dry rock samples, the dominant frequency of acoustic emission signals was mainly concentrated between 0 kHz and 310 kHz, while after saturation, the dominant frequencies were all below 180 kHz. The most significant feature before the rupture of dry rock samples was the frequent occurrence of high frequencies and sudden changes in dominant frequencies. Before rupture, the characteristics of precursor events for initially saturated and immersed samples for 5, 10, and 30 days were the appearance and rapid increase in sudden changes in dominant frequencies, as well as an enlargement of the frequency range of dominant frequencies. After 60 days of immersion, the precursor characteristics of rock sample rupture gradually disappeared, and sudden changes in dominant frequencies frequently occurred at various stages of sample loading, making it difficult to accurately predict the rupture of specimens based on these sudden changes. Full article

The Wudongde reservoir region exhibits a notable prevalence of landslides within the red-bed reservoir stratum. The red bed is a clastic sedimentary rock layer dominated by red continental deposits. It is mainly composed of sandstone, mudstone, and siltstone. The lithology is diverse and uneven. In this study, we delve into the impact of mineral dissolution on the development of red-bed landslides in the reservoir area by utilizing the Xiaochatou landslide as a representative case study. Considering the inherent susceptibility of red-bed formations to erosion, collapse, and softening when exposed to water, an investigation was conducted to examine the consequences of mineral dissolution on landslides occurring in these areas. We conducted a mineral analysis and an identification of rock samples from the Xiaochatou landslide site, revealing alternating layers of sandstone and mudstone. Sandstone and conglomerate specimens were immersed in deionized water, and advanced techniques such as scanning electron microscopy (SEM), ion chromatography (IC), and inductively coupled plasma (ICP) analysis were used to examine the effects of water immersion. We also employed the hydrogeochemical simulation software PHREEQC to understand the dissolution mechanism of gypsum during soaking. Our findings reveal that sandstone and conglomerates harbor a notable quantity of gypsum, which readily dissolves in water. Prolonged immersion leads to erosion cavities within the sandstone, thereby augmenting its permeability. The concentration of SO₄²⁻ ions in the soaking solution emerges as the highest, followed by Ca²⁺ and Na⁺. The notable significance is the dissolution of gypsum, whose intricate mechanism is contingent upon diverse environmental conditions. Variations in ion concentration profoundly influence the saturation index (SI) value, with the pH value playing a crucial role in shifting the reaction equilibrium. Regarding the deformation mode of the landslide, it manifests as a combination of sliding compression and tension cracking. The fracture surface of the landslide assumes a step-like configuration. As the deformation progresses, the mudstone layer takes control over the sliding process, causing the sandstone to develop internal narrow-top and wide-bottom cracks, which propagate upward until the stability of the slope rock mass is compromised, resulting in its rupture. In this manuscript, we delve into the dissolution traits of red-bed soft rock in the Wudongde reservoir area, using a landslide case as a reference. We simulate this rock’s dissolution under environmental water influences, examining its interaction with diverse water types through rigorous experiments and simulations. This study’s importance lies in its potential to shed light on the crucial engineering characteristics of red-bed soft rock. Full article

The influence of rock weathering caused by freezing–thawing on stone cultural relics cannot be ignored. For immovable stone cultural relics, different parts under different environmental conditions will be under different freeze–thaw actions and suffer different degrees of damage. In this paper, three typical freeze–thaw cycle tests of sandstone are designed, namely immersion test, capillary action test, and periodic saturation test. The macroscopic and microscopic morphologies of rock samples under different freeze–thaw cycles were analyzed. Weathering indicators such as porosity, water content, wave velocity, and surface hardness were tested, as well as uniaxial compressive strength. The variation law of weathering index and uniaxial compressive strength under different freezing–thawing cycles was obtained, and the quantitative relationship between each index parameter was further analyzed. The results show that under different freezing–thawing conditions, the apparent morphology of rock samples is different, and the trend of weathering indexes is similar, but the rate of change is different. The water content of rock has a great influence on the test results of wave velocity but has little influence on the surface hardness. The function relationship between weathering index and compressive strength under different freezing–thawing modes is similar, but the fitting parameters are different. Finally, the strength and wave velocity damage factors were used to quantitatively evaluate the degree of rock weathering. The results show that the immersion freeze–thaw damage is the highest, the periodic saturated freeze–thaw damage is the second highest, and the capillary freeze–thaw damage is the least highest. This is consistent with the field observation results. The conclusion of this paper can provide reference for the detection of stone cultural relics and provides a scientific basis for the anti-weathering protection of stone cultural relics. Full article

The void of the cracked rock mass of the goaf is the main water storage space of underground reservoirs, which is in a time-space dynamic evolution process. Before the formation of the underground reservoir, the water storage space was primarily affected by disturbances. After the safe operation of the coal mine underground reservoir, the water level of the mine rises and falls repeatedly and the water storage space is affected by the water-rock interaction. To study the void evolution law of a cracked rock mass under mining disturbance and the compaction and void deformation characteristics of caving gangue under the effect of the water-rock interaction, a simulation test of a coal mine underground reservoir is conducted. Furthermore, the rupture motion law and movement deformation characteristics of the overburden during coal mining are analyzed. The digital image method and fractal theory are introduced to describe the fractal characteristics of the rock mass void of the caving zone, fracture zone, and entire goaf during the mining process. Five prototype gangue samples with different immersion times are prepared with the same grain size grading as the similar model caving gangue. The influence of the immersion times on the compaction characteristics and evolution law of the void rate of the gangue are also studied. Based on the parameter fitting method, the stress–strain relationship equation of the gangue sample and void rate-stress relationship equation of the cylindrical gangue sample, considering the influence of the immersion times, are constructed. The results show that the overburden of the model is of a “two zone” structure and the entire structure moves and sinks asymmetrically in a “∩” shape. As the advancing distance of the working face increased, the fractal dimensions of the rock mass void of the caving zone and entire goaf increased logarithmically, and the fractal dimension of the rock mass void of the fracture zone first increased rapidly (60–80 cm) and then decreased linearly (80–200 cm). As the immersion time increased, the saturated moisture content and density of the gangue samples increased logarithmically and exponentially, respectively. Under the same stress, the strain of the gangue sample increased gradually, and the void rate decreased gradually (except for the initial loading). Full article

Supercritical carbon dioxide (SC-CO₂) fracturing has been used in developing low permeability and water-sensitive reservoirs in recent years, which is expected to become a new generation of unconventional reservoir fracturing fluid. However, the water-rock interaction characteristics of various lithology shales under SC-CO₂ circumstance and its influence on fracturing effect still need to be investigated. Two kinds of shale samples from C7 and S1 formations of the Ordos Basin were treated by SC-CO₂ with formation water. The aims of the research are to determine the processes taking place in shale reservoir when considering minerals components transformation, porosity/permeability variation, and micro pore-structure change during the SC-CO₂ fracturing. Static and dynamic SC-CO₂ immersed experiments were conducted and the scanning of electron microscopy (SEM) and X-ray diffraction (XRD) was employed to analyze the surface morphology and newly formed minerals. Helium porosimeter, the ultralow permeability meter, and the CT scanner are employed to record the alternation of physical parameters during SC-CO₂ dynamic injection. The experimental results show that the C7 samples are rich of chlorite and easily reacting with SC-CO₂ saturated formation water to form new minerals, but the S1 samples are insensitive to aqueous SC-CO₂. The minimum value of permeability and porosity of the C7 cores appear at 24h in the long-interval experiment, but in the short-interval dynamic experiment, the minimum values move ahead to 12h. The optimal flowback time for the C7 reservoir is before 12 h or after 24 h. The high-pressure SC-CO₂ flooding pushes the new forming minerals particles to migrate to the outlet side and block the pore throat. For the S1 core results, the porosity and permeability change little in both short and long interval experiments. There is no strict flow-back time requirement for S1 reservoir during SC-CO₂ fracturing. This study is significance for the efficient application of SC-CO₂ in the exploitation of shale oil reservoirs. Full article

The Duku Highway in the study area is located in the high-altitude mountainous region of Xinjiang, China, and it is affected by various environmental factors during construction, among which the influence of freeze–thaw cycles cannot be ignored. In order to study the effect of freeze–thaw cycles on the strength of tuff surrounding rock at high-altitude mountainous areas, uniaxial compressive strength and shear wave velocity tests with different numbers of freeze–thaw cycles were conducted on dry and saturated rock samples from the tunnel entrance of the Duku Highway. The test results showed that the tuff specimens condensed a thin layer of ice on the surface with raised freezing points during the freezing stage, but the thickness of the thin ice and the density of the freezing points did not change with the increase of the number of freeze–thaw cycles. Analysis of the test data showed that the wave velocity, uniaxial compressive strength, breaking strain, modulus of elasticity, and Poisson’s ratio of the rock decreased as the number of freeze–thaw cycles increased. We believe that the freeze–thaw cycles caused the deterioration of the rock strength. The reason for this phenomenon is that we believe that the freeze–thaw cycling action changed the rock internally and affected its density, which, in turn, caused the attenuation of strength, etc. However, there is a limit to the effect of freeze–thaw cycling on the strength of the surrounding rock. After exceeding the limit of the effect of freeze–thaw cycling, the strength parameters of the surrounding rock will no longer change with the increase of the number of freeze–thaw cycles. The results of this study can provide a theoretical basis for the prevention and control of the stability of tuff surroundings at high-altitude mountain tunnel openings. Full article

This research focuses on the analysis of the influence of two secondary salt weathering processes on the durability of rocks exposed to marine environments: chemical dissolution of rock forming minerals and differential thermal expansion between halite and the hosting rock. These processes are scarcely treated in research compared to salt crystallisation. The methodology followed in this paper includes both in situ rock weathering monitoring and laboratory simulations. Four different calcite-bearing rocks (a marble, a microcrystalline limestone and two different calcarenites) were exposed during a year to a marine semiarid environment. Exposed samples show grain detachment, crystal edge corrosion, halite efflorescences and microfissuring. Crystal edge corrosion was also observed after the laboratory simulation during a brine immersion test. Calcite chemical dissolution causes a negligible porosity increase in all the studied rocks, but a significant modification of their pore size distribution. Laboratory simulations also demonstrate the deterioration of salt-saturated rocks during thermal cycles in climatic cabinet. Sharp differences between the linear thermal expansion of both a pure halite crystal and the different studied rocks justify the registered weight loss during the thermal cycles. The feedback between the chemical dissolution and differential thermal expansion, and the salt crystallisation of halite, contribute actively to the rock decay in marine environments. Full article