Water-saturated Samples Research Articles

Deterioration mechanism of physical and mechanical properties of phosphate rock under seismic disturbance and saturation

Introduction: To explore the influence of seismic disturbance and saturation on the mechanical properties of phosphate rock, the paper divided phosphorus rock into natural group, water-saturated Group and “earthquake and water-saturated” group.Methods: Fatigue loading and conventional triaxial compression tests were carried out to study the physical characteristics, degeneration characteristics, strength characteristics and failure characteristics of the samples.Results: The conclusions are as follows: The average wave velocity of samples of water-saturated group and “earthquake and saturation” group is 29.7% and 52.9% lower than that in natural group, respectively, and the average peak strength under various confining pressure conditions is 32.2% and 48.3% lower than that in natural group, respectively, indicating that both seismic stress loading and saturation have deterioration effects on the samples.Discussion: Compared with natural group and watersaturated group, after simulated seismic stress loading, the stress-strain curve of “earthquake and water-saturated” group shakes obviously after the peak, and the steps appear frequently, showing obvious plastic characteristics. Due to the different deterioration degree of the samples, the samples of the natural group were in the overall state of compression during the triaxial test, while the samples of the water-saturated group were compressed before failure and expanded after failure. The sample of earthquake and water-saturated group is in the whole state of expansion. The failure characteristics of natural samples and water-saturated samples are shear or shear failure. The shear plane starts from the edge of sample end face and presents typical diagonal shear failure. The failure characteristics of “earthquake and water-saturated” group samples are heavy shear and through failure. There are several fracture zones extending inward from the end of the sample, resulting in obvious transverse expansion.

Density-Dependent Pore Water Pressure Evolution in a Simplified Cyclic Shear Test

When specimens of different sands are produced using the same preparation method and sheared under the same conditions (consolidation stress, loading, etc.), while simultaneously keeping the drainage closed, the resulting tendencies of these sands regarding the PWP build-up will be different. This research paper presents a simplified cyclic shear test, which is used to evaluate the accumulation of PWP in sands under defined specimen preparation procedure and testing conditions. In the proposed experiment, a comparison of different sands with this respect is easily achieved. The principle of this experimental method is based on the evolution of the PWP during cyclic shearing of a water-saturated sand sample. Undrained conditions during the experiment allow for the evolution of the PWP, which is quantified by the rate of the PWP build-up. The duration of a single cyclic shear test, including specimen preparation, is approximately 30 min. The evaluation of the rate of the PWP build-up for different densities resulted in an exponential dependence of the PWP build-up on the variation of the relative density. The results confirmed a higher generation of PWP in a fine sand compared to a coarse sand. A comparison with the results of undrained cyclic triaxial tests in the case of eight different sands demonstrated a good agreement between both experimental methods. The basis for the comparison was the density-dependent evolution of PWP in these methods. The presented method delivers a value (index) that quantifies the PWP build-up in sands under the defined testing conditions.

Ceramic Bricks of Increased Frost Resistance of Kushchevsky Deposit Clay Raw Material as the Building Material within Architectural Shaping Dynamics

Introduction. The architectural and construction discourse, which has become widespread today, states that even though there develops the informational networking type of interaction among the participants of the socio-cultural exchange, the architecture will still get its rightful place because it is the most important component of the living environment of a community.Despite the rapid and successful development of applied areas of the Architectural Science caused by the dynamics of mass construction (or construction of single buildings and structures, but those, which are originally clearly meant to meet the mass requirements of the society, its stereotyped ideas about the look of a modern urban metropolis) and changes in construction technologies, design and engineering methods, forms and production resources, there is no clear understanding of what does the architecture implicate, not merely in the frame of scientific achievements, but also in the frame of the technical, technological and material progress of civilization.The problem set by the latest dynamics in the architectural and construction field (industry) is increasing the durability of wall building materials, mainly the ceramic bricks. The method of increasing the frost resistance of ceramic bricks made of Kushchevsky deposit clay raw material by adding a structuring additive into the clay mass, which ensures the required properties of the material’s pore structure, is presented in this scientific article.Materials and Methods. The present scientific work builds up on the results of the research on the effect of the proposed additive on the frost resistance of a ceramic crock made of Kushchevsky clay raw material by plastic molding. The results of the chemical analyses of the clay raw material of Kushchevsky deposit and the modifying additive are shown. As an additive there is used the waste (a calcium-bearing by-product) which is released during manufacture of the phosphate mineral fertilisers and has the form of spherical granules of 30–100 microns in diameter.In addition to determining the physical and mechanical characteristics of the burnt samples modified with a mineral additive, the porous characteristics of a ceramic crock were studied using the mercury porosimeter. With the help of the device, which is made on the basis of a quartz dilatometer and a freezer, the dilatometric measurements were carried out by freezing the water-saturated samples at the temperature of up to –20°C.Results. The carried-out experiments showed the increase in the frost resistance of the plastic-molded ceramic crock made of the clay rock of the Kushchevsky deposit when adding the carbonate-bearing mineral additive due to formation of the optimal porous structure. During numerous successive periods of freezing and thawing there was recorded good resistance of the modified samples to tensile stresses arising in the crock. The implementation of this method (of adding the proposed additive into the clay mass) in the brick factories is guaranteed to provide the manufacture of the ceramic building products with increased frost resistance.Discussion and Conclusions. Deformation curves of pores calibrating graduation in samples with and without a mineral additive are presented, along with the study results of the samples’ deformation during freezing.In conclusion, a matrix with the results of the climatic annual parameters of the Russian Federation is presented, confirming the need for the building materials to comply with the improved performance characteristics. The implementation of this method in ceramic brick manufacturing lines will foster the production of products with increased frost resistance.

Modelling and validation of dry and saturated velocities in carbonates from Saudi Arabia – Part II

Ultrasonic compressional (P-wave) and shear (S-wave) velocity data, obtained from a set of 35 dry and water-saturated (carbonate) core samples, were presented as a first part to this study, in a separate paper by Bakhorji and Schmitt (2022). These carbonate samples were obtained from a prominent reservoir in Saudi Arabia.In this follow-up paper, the velocity measurements, obtained from these 35 core samples, were utilized to, firstly, predict and validate four appropriate rock physics models and to, secondly, derive empirical relationships relating the P- and S-wave velocities to the porosity and calcite content of the samples (over the effective pressure range from 5 MPa to 25 MPa). The derived empirical relationships were validated against the wireline P- and S-wave velocities assuming insitu reservoir pressure conditions. The empirical models predicted the P- and S-wave velocities accurately over the reservoir interval of interest. The shear strengthening / weakening effect, or the saturation effect on the shear moduli, was also modelled and is discussed further.The comparisons of the Biot, Gassmann, Mavko-Jizba-Biot (MJ-Biot) and Mavko-Jizba-Gassmann (MJ-Gassmann) model predictions, with the measured water-saturated P- and S-wave velocities, show that the squirt mechanism was not applicable for our samples. The Gassmann model consistently over-predicts the water-saturated S-wave velocities at low pressure, but closely fits the measured velocities at high pressures, whereas Biot over-predicts the water-saturated velocities in most of the studied samples. We conclude that Biot global-flow is most likely to be the principle dispersion mechanism in these samples.

Thermophysical properties of n-alkyl-ammonium nitrate ionic liquids (n = 2,3,4) pure and water saturated for energy applications

Phase transition temperatures of three ionic liquids (ILs) with ammonium cations of different alkyl chain length and nitrate common anion were determined in this paper by differential scanning calorimetry. Thermal stability of these ILs in air and nitrogen atmosphere and the maximum operation temperatures of the compounds were also studied using a thermogravimetric analyser. Furthermore, thermophysical properties as density, viscosity, thermal conductivity, and heat capacity against temperature of these pure ionic liquids were determined and compared with the corresponding values for water saturated samples. For this purpose, different techniques were employed: for density and viscosity a rotational automated viscodensimeter; for thermal conductivity measurements a thermal conductimeter that follows the transient hot-wire method; and for heat capacity measurements a microdifferential scanning calorimeter. Different behaviours were observed depending on the analysed thermophysical properties. In case of dried samples, density and thermal conductivity decrease with alkyl chain; for saturated samples, same tendency is observed for density but is the opposite in case of thermal conductivity. Isobaric specific heat and viscosity increase with alkyl chain length in case of dried samples; for saturated samples same tendency is observed for the viscosity, whereas the tendency is the contrary in case of isobaric specific heat.

Uniaxial compressive strength (UCS) and SEM study of liquid nitrogen for waterless hydraulic fracturing in coalbed methane reservoirs of Karaganda Basin in Kazakhstan

Four different cryo-fracturing liquid nitrogen (LN2) treatment processes are compared for stimulation of coal. Namely, peak compression strength, acoustic emission, and energy absorption tests, plus scanning electron microscope (SEM) observations for fracture evolution were conducted for freezing time (FT), freezing-thawing cycle (FTC), negative freezing-thawing cycle (NFTC) and positive freezing-thawing cycle (PFTC) LN2 exposure processes for both dried and water-saturated specimens. The positive and negative freezing-thawing cycle experimental processes are novel variations on multi-contact cryogenic stimulation reported for the first time. In all freezing-thawing processes, water-saturated experiments exhibited lower Uniaxial Compression Strength (UCS) peaks compared to dry coal samples, presumably due to the additional frost forces degrading the overall sample integrity. For all LN2 exposure processes using dry coal specimens, the compaction line decreased and the elastic line decreased in acoustic emission (AE) tests, concurrent with a reduction in compressive strength, as LN2 treatment time increased. This trend reversed in water-saturated samples, highlighting the importance of sample water content on fracturing potential. Energy absorption analyses clearly showed that the strength of coal samples decreased with increasing the freezing time and freezing-thawing cycles, as the LN2 treatment produced more cracks and damage to coal. Furthermore, SEM analysis and image processing confirmed that all LN2 treatment processes resulted in the generation of fractures evident in fracture trace statistic changes on the surface of coal samples. FTC and PFTC processes showed the greatest promise as stimulation methods due to the extent of the network of fractures created. Engineering results were augmented with the inclusion of the geological setting, important for the transportability of these outcomes to coals of similar origin.

Laboratory, numerical and field assessment of the effectiveness of cyclic geomechanical treatment on a tournaisian carbonate reservoir

Results are discussed for evaluation of effectiveness of the cyclic geomechanical treatment (CGT) on a Tournaisian carbonate reservoir. Analysis of laboratory experiments performed according to a special program to assess permeability changes for Tournaisian samples under cyclic changes in pore pressure is presented. The main conclusion is the positive selectivity of the CGT: an increase in permeability is observed for samples saturated with hydrocarbons (kerosene) with connate water, and maximal effect is related to the tightest samples. For water-saturated samples, the permeability decreases after the CGT. Thus, the CGT improves the drainage conditions for tight oil-saturated intervals. It is also confirmed that the CGT reduces the fracturing pressure in carbonate reservoirs. Using flow simulations on detailed sector models taking into account the results of laboratory experiments, a possible increase in well productivity index after CGT with different amplitudes of pressure variation was estimated. Results of a pilot CGT study on a well operating a Tournaisian carbonate reservoir are presented, including the interpretation of production logging and well testing. The increase in the well productivity index is estimated at 44-49 % for liquid and at 21-26 % for oil, with a more uniform inflow profile after the treatment. The results of the field experiment confirm the conclusions about the mechanisms and features of the CGT obtained from laboratory studies and flow simulations.

Cyclical water vapor sorption-induced structural alterations of mine roof shale

Understanding the nanoscale structural changes of shale in an unsaturated gas-water-shale system can shed light on the underlying mechanisms on water retention-induced shale deterioration. The simplified hypothesis here is that pore-scale shale structure can be passively altered by pore deformation/failure under cyclical water vapor treatments and can be described by a partially saturated effective stress law. Cyclical water vapor sorption tests were conducted on dried and water-saturated shale powders prepared by a hand-crushing (larger particle size) method and a cryogenic ball-milling (smaller particle size) method. Techniques including XRD, FESEM-EDS, and nanoindentation were used to provide evidence that the nano-porous shale is highly heterogeneous in nano-to micro-scale. The sorption curves of both samples with initial water-saturation treatments are higher than the raw samples, indicating enhanced sorption capacities in the treated samples. The degree of hysteresis among eight cycles (five cycles on raw sample and three cycles for treated sample) for both samples exhibit significant differences at low relative humidities, namely lower than ∼0.6. Very slight differences were observed among eight cycles when the relative humidity was higher than 0.6. An obvious decrease in the effective diffusion coefficients is observed in the treated samples compared to the raw samples, which is attributed to the physical alterations induced by the initial drying of water-saturated sample. In situ SANS data for shale at elevated relative humidity conditions suggest that the decrease in the scattering contrast between rock solid matrix and pores can be attributed to water capillary condensation. This occurs mainly in pores with radii smaller than ∼8.3 nm but larger than ∼1.25 nm and at a relative humidity of ∼59%. The capillary condensation continuously takes place in larger pores as the relative humidity increases to ∼87%. The results can ultimately provide the data for analyzing the interactions within an unsaturated air-water-shale system.

Effect of moisture on microwave ignition of bituminous coal.

The interaction between water and microwave is of vital importance to reveal the microwave ignition mechanism of water-bearing coal. This study used two group of bituminous coal after drying and water saturation treatment, for experimental testing and contrastive analysis. During the experiment, permeability of coal samples was obtained based on nuclear magnetic resonance(NMR) test, then different power of microwaves were applied to coal samples, and the occurrence of hot spots within coal samples was regarded as a sign of microwave ignition. Microwave ignition of water-saturated coal is mainly affected by microwave power and coal permeability. The pore water in low permeability coal is conducive to microwave ignition, while the pore water in high permeability coal will prolong the ignition time. There is a permeability threshold, above which the average ignition time of water-saturated coal samples is longer than that of dry coal samples, but below which the opposite is true. These insights can be used to evaluate the safety of microwave technology when applied to coal engineering.

Experimental simulations of hydrogen migration through potential storage rocks

In the framework of future decarbonization of the energy industry, the safe and effective storage of hydrogen is an important approach to add to a climate-friendly energy system. Until the development of sufficiently large electrical storage systems, the storage of hydrogen in the order of GWh to TWh is envisaged in salt caverns or porous geological formations with a gas-tight covering of salt or claystone. In order to calculate gas losses from these H2 storage facilities, the H2 diffusivity of the storage and cap rocks must be known. To determine the hydrogen diffusion rates in these rocks, an experimental set-up was designed, constructed and tested. The set-up comprises two gas chambers, separated by the rock sample under investigation with an exposed area of approximately 7 cm2. The driving force for gas migration through the rock sample from the hydrogen-containing feed gas chamber to the hydrogen-free permeate chamber is the chemical potential (concentration) gradient. With respect to hydrogen migration behaviour, hydrogen breakthrough times and hydrogen diffusion coefficients were determined for dry and wet Bentheimer sandstone, Werra rock salt and Opalinus clay samples. Breakthrough times varied between less than 1 h and 843 h. Based on concentration changes at the permeate side, hydrogen diffusion coefficients were derived ranging from 10−9 to 10−8 m2/s. The differences between the materials and the effect that wetted or water-saturated samples have higher hydrogen retention due to closed pores and microcracks were clearly shown. The experimental set-up proves to be a suitable approach to determine site-specific rock characteristics such as hydrogen diffusion coefficients and breakthrough times for natural geomaterials.

The effects of porous medium parameters on electroseismic conversion

The generation of electroseismic conversion in fluid-filled porous media is closely related to reservoir parameters, however, its correlation and the influence of reservoir parameters on electroseismic conversion need to be clarified, and there is only a little analysis of measured electroseismic signals. In our study, we use experimental and theoretical methods to investigate the dependency of the reservoir parameters on the electroseismic coupling. A set of experimental devices was built to conduct electroseismic measurements on water-saturated sandstone samples. The electroseismic theory based on the capillary model is used to comprehensively analyze the experimental results and explore the effects of reservoir parameters (porosity, permeability, fluid concentration) on electroseismic conversion. The combined experimental and theoretical analysis show that the electroseismic signal increases with porosity, but its relationship with permeability is complicated. In the lower permeability range, the electroseismic conversion firstly increases with the increase of permeability, while continued increases of permeability lead to a decrease in electroseismic conversion in the high permeability range. The dependency of the electroseismic conversion on pore fluid concentration also presents two trends in different concentration ranges. It decreases rapidly with the increase of fluid concentration in the low concentration range. When the intensity of electroseismic conversion reaches the minimum, it remains almost unchanged even if the fluid concentration continues to increase. The results of the influence of reservoir parameters on electroseismic conversion indicate that electroseismic conversion is as sensitive to the parameters of fluid-filled porous media as seismoelectric conversion. The electroseismic conversion signal is more stable than seismoelectric signal, which means that compared with the difficult detection of seismoelectric signals, the detectability of electroseismic signals is higher. It may be easier to use electroseismic signals to reveal the petrophysical properties of reservoirs.

Selection Effect of Liquid Nitrogen Freeze-Thaw Cycles on Full Pore Size Distribution of Different Rank Coals.

In China, the capacity to produce coalbed methane and extract underground gas is restricted by the prevalence of low-permeability coal seams. Liquid nitrogen fracturing is a new low temperature-high-pressure anhydrous fracturing technology that uses low temperature and high frost heave forces to increase coal permeability. To better understand the liquid nitrogen fracturing effect on coal, we conduct the liquid nitrogen freeze-thaw cycle (LNCFT) experiments on different rank coals from Qinghai, Shanxi, and Shaanxi provinces. We combined the low-pressure nitrogen and carbon dioxide adsorption experiment with the non-local density functional theory model and mercury injection porosimetry with compressibility corrections to examine the full pore size distributions of untreated and water-saturated samples before and after LNCFT. The results found that LNCFT can effectively increase the pore volume (PV) and specific surface area of the water-saturated coal sample. Compared with the raw coal, the increased ratio of the full pore size PV is 70.41-100.17%. However, the scale-selective transformation effect on pores during liquid nitrogen fracturing is noticeable. Under the same conditions, LNCFT can significantly increase the pore volume of micropores (>2 nm) and macropores (>50 nm), and the increase ratios are 24.40-44.16 and 103.55-327.93%, respectively. The PV of mesopores (2-50 nm) shows a slightly increasing trend with the increase in metamorphic degree, and the increase ratio is between 8.7 and 56%. Comparing the full pore size distribution curves before and after LNCFT, it is found that the alteration of high-volatile bituminous coal (BLT coal) and anthracite (SH coal) has more significance in the range of less than 2 and 50-20,000 nm, while middle-volatile bituminous coal (YJL coal) varies between 50 and 2000 nm. Meanwhile, the ratio of micropore and mesopore PV to the total decreased gradually before and after LNCFT, while the proportion of macropores increased, indicating that small-scale pores would intersect and connect to form larger-scale pores during the fracturing. The combined effects of temperature gradient, water-ice phase transition, and heat transmission rate are the key factors that determine the impact of LNCFT on pore size distribution. Our results provide new information for enhancing the permeability of low-permeability coal seams of different ranks.

Distribution and occurrence of pore water and retained oil in nanopores of marine-terrestrial transitional shales during oil generation and expulsion: Implications from a thermal simulation experiment on shale plug samples

The distribution and occurrence of pore water and retained oil in shale nanopores is a key scientific issue confronting the evaluation, exploration and development of shale hydrocarbon resources. At present, however, relevant researches are scarce, especially on marine-terrestrial transitional (MTT) shales. In this study, a thermal simulation experiment was performed on a set of water-saturated shale plug samples prepared from a low-maturity MTT shale. The pore water and retained oil contents of the artificially matured samples were quantified, and the pore structures under moist, dried, and solvent extracted conditions were investigated. The results show that the pore water content of the shale decreases with Ro increasing from 0.75% to 0.97% and then increases with Ro further increasing to 1.53%, which exhibits a negative correlation with the retained oil content during the oil generation and expulsion. The pore water occurs mainly in filling, adsorption and water-cluster states in micropores (<2 nm), small non-micropores (2–10 nm), and large non-micropores (>10 nm), respectively. The retained oil is mainly in a filling or dissolved state in the micropores and small non-micropores and in an oil-cluster state in the large non-micropores. The pore water and retained oil in shale micropores are stored in the inorganic matter (IM) and organic matter (OM) micropores, respectively. During oil generation and expulsion, the IM micropores are fully saturated by the pore water that is barely expelled, and most of the pore water is stored in non-micropores, especially in large non-micropores. With increasing maturity, the distribution of retained oil in nanopores changes obviously. The retained oil in non-micropores tends to be stored in the large non-micropores at low-maturity stages, occurs uniformly in both the small and large non-micropores at mature stages, and is dominantly reserved in the small non-micropores and micropores at high-maturity stages. These results have significances for understanding the accumulation mechanism of shale oil and gas resources under geological conditions and evaluating their resource potential.

Study of the response characteristics of the water–force coupling action of hard coal bodies in steeply inclined coal seams

Study of the pre-blast weakening of hard-top coal water injection is especially important to solve problems related to the low recovery rate of coal resources and frequent dynamic disasters due to the low degree of fragmentation of hard-top coal during high-stage fully mechanized top-coal caving in steeply inclined coal seams. With the application of rock mechanical tests and numerical simulations, this study carries out mechanical property testing on natural and water-saturated coal samples, investigates the effect of moisture on coal sample mechanical properties in meso-scale, and quantifies the degradation of coal samples under moisture due to mollification. It also reveals the interaction between water and acoustic emission signals based on the statistics of acoustic emission count and energy. A numerical model is established to analyze coal sample internal stress distribution features before and after water injection in macro-scale. Moreover, the detailed mitigating measures for top-coal water injection for engineering practice are designed. The results demonstrate that the mechanical properties of coal samples are significantly affected by moisture-induced degradation. The failure and collapse degrees of water-saturated samples are generally larger than those of natural samples. When the water content is higher, the acoustic emission count and energy of the coal sample are smaller—presenting a negative correlation. The internal stress of coal samples before and after water injection differs significantly. When subjected to water, the top-coal stress releases and transfers, and the peak value is significantly reduced. This study has verified in macro- and meso-scales that top coal can be fully weakened under water–force coupling. The findings of this study are of practical significance for safe and efficient mining and provide a reference for presplit weakening of hard-top coal during horizontal sublevel fully mechanized top-coal caving in steeply inclined coal seams.

Energy Evolution Characteristics of Water-Saturated and Dry Anisotropic Coal under True Triaxial Stresses

During deep underground coal mining, water-injection-related engineering methods are generally carried out to reduce the hazards of coal dynamic disasters. The energy evolution characteristics of coal can better describe the deformation and failure processes, as it is more consistent with the in situ behavior of underground mining-induced coal. In this study, experimental efforts have been paid to the energy evolution characteristics of water-saturated and dry anisotropic coal under true triaxial stresses. The effects of water saturation, intermediate stress, and anisotropic weak planes of coal on the true triaxial energy evolution were systematically evaluated. The results show that the overall energy is weakened due to the water adsorption for water-saturated coal samples. The water-weakening effect on the overall energy of water-saturated coal is more pronounced when perpendicular to the bedding plane direction than in the other two cleat directions. The accumulation elastic energy anisotropy index of dry and water-saturated coal samples is higher than 100.00%. Both accumulation and residual elastic energy of dry and water-saturated coal samples show an increasing-then-decreasing trend with intermediate stress increase. The results obtained in this study help understand the in situ behavior of coal during deep underground mining and control coal dynamic disasters.

Fractal Characteristics and Acoustic Emission during the Failure Process of Argillaceous Siltstone with Different Moisture Contents

Compressive strength and tensile strength are the critical parameters to determine rock performance, which can reflect the rock’s resistance to deformation and damage. Brazilian indirect tensile and uniaxial compression tests were carried out on rocks under different water immersion conditions to study acoustic emission (AE) characteristics and crack propagation during rock fracture. The test results show that water has a deteriorating effect on the argillaceous siltstone, with significant attenuation of both compressive and tensile strengths. With the increase in moisture content, the number of AE events decreased, the cumulative AE ringing count showed a gradual rise in steps, and the proportion of AE peak frequencies in the range of 0-200 kHz gradually increased. Natural rock samples are more brittle than water-saturated rock samples. The higher frequency of AE events and the higher energy released during the destruction of natural rock samples reflect in the clustering of high-energy AE source locus near the main rupture surface. The AE source locus is 1-2 energy levels higher in the vicinity of the fracture surface than in the water-saturated rock samples. The rock samples exhibited random packing of mineral particles and contained many clay minerals by SEM analysis of fracture microstructure. Water dissolves mineral particles and cementitious materials, producing microcracks with propagation potential. Analysis of the AE time sequence based on fractal theory reveals that fractal dimension value varies with the increase of moisture content. The variation of the fractal dimension D values for the water-saturated rock samples ranges from 0.4 to 0.65. This indicates that the number of microcracks is higher and their propagation more complex when the moisture content of the specimen is higher. The failure characteristics of the rock samples in the above research can provide a reference for monitoring rock mass stability under tunnel water inflow.

Analytical Damage Model for Predicting Coal Failure Stresses by Utilizing Acoustic Emission

Overburden collapse and water inrush in mines are primarily caused by rock fractures. Mining safety can be enhanced by monitoring and identifying early signs of coal failure in the mines. This article collected acoustic emission data synchronously throughout a series of uniaxial compression (UC) experiments on natural and water-saturated coal. The influence mechanisms of water, mechanical properties, and acoustic emission signals on the stress–strain curve and the SEM results of water-saturated and dry samples are investigated. As a result, the mechanical properties of coal are not only weakened by water saturation, such as elastic modulus, strain, stress, and compressive strength but also reduced acoustic emissions. In comparison with saturated coal, natural coal has a uniaxial stress of 13.55 MPa and an elastic modulus of 1.245 GPa, while saturated coal has a stress of 8.21 MPa and an elastic modulus of 0.813 GPa. Intergranular fractures are more likely to occur in coal with a high water content, whereas transgranular fractures are less likely to occur in coal with a high water content. An innovative and unique statistical model of coal damage under uniaxial loading has been developed by analyzing the acoustic emission data. Since this technique takes into account the compaction stage, models based on this technique were found to be superior to those based on lognormal or Weibull distributions. A correlation coefficient of greater than 0.956 exists between the piecewise constitutive model and the experimental curve. Statistical damage constitutive models for coal are compatible with this model. Additionally, the model can precisely forecast the stress associated with both natural and saturated coal and can be useful in the prevention of rock-coal disasters in water conditions.

Quasi-Static and Dynamic Tensile Behavior of Water-Bearing Sandstone Subjected to Microwave Irradiation

Microwave irradiation on rocks before excavation is an effective method to reduce equipment wear and energy consumption during mechanical cutting. Rock mass excavation is usually carried out in a water-rich environment and exposed to dynamic loads, thus understanding the coupled effects of water content and loading rate on the mechanical behavior of rocks under microwave radiation is essential. In this study, sandstone samples with five levels of water content (from oven-dried to water-saturated) were exposed to microwave irradiation at a power of 700 W for 10 min. Brazilian disc tests were conducted on sandstone samples after microwave radiation under both quasi-static and dynamic loading conditions. Test results revealed that, with the increase of the initial water content, the microwave heating capacity of the rock is significantly improved. The surface temperature of the saturated samples is approximately 1.38 times higher than that of the dry ones. Moreover, weight, P-wave velocity, quasi-static and dynamic tensile strength of the rock decrease, while porosity and damage factor exhibit a similar growth law. Before microwave irradiation, the average value of the P-wave velocity and the quasi-static tensile strength of sandstone were about 2521.3 m·s−1 and 4.65 MPa. However, after microwave treatment, when the initial water content was 2%, 3%, 4% and 5.4%, the P-wave velocity decreased by 6.1%, 9.8%, 16.4% and 30.2%, while that quasi-static tensile strength reduced by 9.2%, 16.7%, 30.6% and 48.9%, respectively. For water-saturated samples under microwave irradiation, the porosity increases from 13.02% to 18.12% (showing an increase of 39.2%), and the damage value rises to 0.51. In addition, the dynamic tensile strength shows a significant loading rate dependence, and as the initial water content increases, also the dynamic increase factor (DIF) increases. At a given loading rate, the energy dissipation decreases with the increase of the initial water content, which indicates that the presence of water cause more significant damage to the rock when subjected to microwave radiation. Scanning electron microscopy (SEM) results indicate that the internal damage of the rock after microwave radiation is dominated by intergranular cracks, and crack density increases with increasing initial water content of the samples. The underlying damage mechanisms of microwave radiation on water-bearing sandstone were interpreted with the theory of pore water pressure and structural thermal stresses.

Investigation of the Selectivity of the Water Shutoff Technology

High water-cut oil production is one of the major issues in the petroleum industry. The present study investigates different profile control solutions, with an emphasis on selective methods and materials that mostly decrease the permeability of water-saturated reservoir areas. To achieve the selective water flow blockage in fractured porous media, the sodium silicate-based gel-forming composition was developed. The test procedure was created to assess selective and strength characteristics of the presented composition. According to the results of this procedure, adding polyatomic alcohols to the mentioned composition enhances its hydrophilic behavior in water-saturated rocks (work of adhesion increases from 117 to 129 mJ/m2) and reduces the hydrophobic behavior in oil-saturated rocks (work of adhesion drops from 110.3 to 77.4 mJ/m2). The selectivity of the composition performance is validated by its higher wettability of water-saturated reservoir rocks compared with oil-saturated; thus, the composition creates a more stable water shutoff barrier when entering the water zone in a formation. As a result of core flooding experiments in natural, fractured, porous core samples, the efficiency of the water blocking capacity of the composition was proved. In addition, these tests showed the selectivity of the composition because the permeability decrease in water-saturated core samples was higher than in oil-saturated ones. The experimental value of the selectivity coefficient was 152.14.

Measurement of uniaxial compression mechanical properties of thermally dried coal samples by energy and fragmentation characteristic analyses

This study aimed to determine the effects of different drying temperatures on the mechanical properties of coal samples under axial compression. Processed, screened, and water-saturated coal samples were subjected to a blast drying treatment at 25, 50, 75, 100, and 125 °C for 4 h. Uniaxial compression mechanical experiments were then performed and the fragments were collected and counted after destruction of the coal samples. Based on the above experimental results, the intrinsic nature of the changes in the mechanical properties of uniaxial compression of coal samples after thermal drying is revealed by the energy evolution and the characteristics of fragment distribution. The results show that with an increase in drying temperature, the energy storage capacity of the coal sample tended to first decrease and then increase, and the energy release capacity gradually increased after the peak. The EESERR index increased exponentially with increasing drying temperature after the uniaxial compression of the coal sample reached the damaged state. In addition, as the drying temperature increased, the brittleness of the coal sample was enhanced, the intensity of uniaxial compression damage increased significantly, the fragments tended to be distributed in smaller sizes, and the degree of fragment chaos increased.