Mechanical Properties Of Coal Research Articles

Characterization and mechanism of multi-scale pore changes in scCO2-water injection into different porosity coal specimen

The process of supercritical CO2 (scCO2) injection into the coal body induces inhomogeneous changes in pore, which has a significant effect mechanical changes of the geological storage of CO2 coal seams. In this study, the effect of scCO2-water intrusion on coal samples with different porosity was investigated by NMR equipment and the change in the mechanical properties were tested by uniaxial compression and acoustic emission techniques after treatment. Results showed that during the treatment of scCO2-water, the pores of different scales showed an exponential change trend of y = a·xb, with the macropore of low-porosity coal samples continuously increasing and the macropore of high-porosity coal samples continuously decreasing. The pore changes after treatment mainly occurred in the micropore range, and the changes of low-porosity coal samples were more than 6%, while the changes of high-porosity coal samples were less than 2.3%. The changes in the fractal dimension of low-porosity coal samples exceeded those observed in high-porosity. The geochemical impact, effective stress of scCO2-water mixed fluid, and the adsorption and expansion mechanism of matrix macromolecules are influenced by varying porosity. This study helps to understand the internal multi-scale pore coupling mechanism and the effect on the coal mechanical properties after CO2 injection.

Numerical simulation study on the evolution law of mechanical properties of different metamorphic coals after heat treatment.

In order to study the effect of temperature on the structure and mechanical properties of coal with different metamorphic degree. Three coal samples with varying degrees of metamorphism were chosen for analysis. The discrete element software PFC2D is used to simulate the heat treatment and compression of coal. The findings indicate that during the heating process, low-order coal exhibits noticeable thermal cracks at an early stage, while thermal crack development in middle-order coal is concentrated in the later stages. In contrast, high-order coal demonstrates a more stable macroscopic structure. The strength and stiffness of low rank coal show the lowest value and decrease significantly within 135°C. However, the strength and stiffness of medium rank coal decrease significantly after 135°C. The changes of mechanical properties and damage modes of coal caused by thermal damage are often ignored, which may lead to the deviation of design and research results from the actual situation. Therefore, this study is of great significance to the prevention and control of coal mine disasters.

Anisotropic Damage Mechanism of Coal Seam Water Injection with Multiphase Coupling.

After coal seam water injection, coal mechanical properties will change with brittleness weakening and plasticity enhancement. Aiming at the problem of coal damage caused by the coal seam water injection process, based on nonlinear pore elasticity theory and continuum damage theory, a nonlinear pore elastic damage model considering anisotropic characteristics is proposed to calculate and analyze the gas-liquid-solid multiphase coupling effect with the fully coupled finite element method during the coal seam water injection process. The research results indicate that the wetting radius of calculated results by the model agrees well with the in situ test results, and the relative errors are less than 10%. Water saturation and induced damage of the coal body in the parallel bedding direction are greater than that in the vertical bedding direction during the coal seam water injection process, which exhibits significant anisotropic characteristics. With the increasing water injection time, the induced damage of the coal body also increases near the water injection hole. Considering the inherent permeability arising with damage, it has a significant impact on both water saturation and induced damage, which also indicates that there is a strong interaction between water saturation and induced damage. The theoretical model reveals the coal damage mechanism of gas-liquid-solid multiphase coupling after coal seam water injection and provides a theoretical prediction of coal containing water characteristics in engineering practice.

Influence of magma intrusion on coal geochemical characteristics: a case study of Tiefa Daxing coal mine

Magma intrusion has an important influence on the physical and mechanical properties of coal and rock. In the area of magma intrusion, disasters such as gas outburst are prone to occur. Revealing its invasion law will be conducive to disaster management and energy development. For this purpose, changes in industrial analysis components of coal, mineral composition, major oxides, trace elements, and rare earth elements of coal under the thermal metamorphism of magma intrusion were analyzed. It is found that the moisture and volatile matter contents of the thermally affected coals in the mining face are generally lower than that of normal coals, while moisture and volatile matter contents are reduced towards to the magma intrusion contact. For example, the moisture and volatile matter of coal sample M01 decreased by 64.6% and 38.6% respectively compared with coal sample M05. During magma intrusion, some minerals remain on the surface of the coal body, resulting in changes in the mineral composition of the coal body. The decrease in carbon atom net spacing, the increase in crystallite aggregation and ductility, and aromaticity in thermally affected coals have a positive impact on the improvement of coal metamorphism. Due to the influences of magmatic intrusion, the variation rules of major oxides in coal are different, and the closer to the magmatic intrusion zone, the easier the major oxides are to be depleted. However, magma intrusion will not lead to the loss of all major oxides in thermally affected coals, such as content of CaO is 54.8%, which is higher than that of coal not affected by magmatic hydrothermal fluid. Most of the trace elements in the thermally affected coals of the No. 9 coal seam are depleted. The contents of rare earth elements are low on the whole coalbasis, with an average of 29.48 μg/g, and the distribution pattern towards to magmatic intrusion shows a wide and gentle “V” curve with left high and right low, showing the characteristics of enrichment of light rare earth elements.

Nanomechanical behavior of coal with heterogeneous minerals and pores using nanoindentation.

The coal's mechanical properties have a significant influence on mining safety and the mine environment. Preparing a standard sample and conducting repeat mechanical testing are challenging because the coal is primarily soft, fragmented, and rich in developed fractures. This study used nanoindentation technology, combined with X-ray diffraction, small-angle X-ray, a high magnification microscope, and mechanical parameter scale-up analysis, to study the micromechanical of three coals being dominated by heterogeneous components and pores. The results show that load-displacement curves with different maximum loads (50 mN, 100 mN, and 200 mN) all appear the pop-in events, and coal heterogeneity affects the frequency of their occurrence. As the maximum load is increased, pop-in event of DSC appears once each, YW increases from zero to three times and HM decreases from four to two times. The heterogeneity of pore structure has little effect on residual displacement, which is mainly affected by hard minerals, and hard minerals reduce the law that residual displacement increases with the increase in maximum load. The micromechanical parameters of soft coals are mainly affected by large pores, while hard coals are mainly affected by hard minerals. The coal's heterogeneity does not affect the linear relationship between hardness and elastic modulus, but stronger heterogeneity will weaken the linear relationship between fracture toughness and elastic modulus. Compared to the mechanical parameters after scale-up, the values obtained based on nanoindentation are less than 15.588% larger, and the increase in the heterogeneity and hard minerals can make the predicted parameters more accurate. The nanoindentation technique can not only provide an efficient and accurate method for studying the mechanical properties of heterogeneous coal at the nanoscale, an important guide for large-scale coal.

Evaluating the impact of calcite and heterogeneity on the mechanical behavior of coal: A numerical study with grain-based finite-discrete element method

Understanding the mechanical properties and fracture behaviors of coal is essential for the safety of underground mining engineering. This research aims to evaluate the impact of calcite and heterogeneity on the mechanical properties of coal. Neper, a software package for polycrystal generation and meshing, is used to create numerical models with a non-uniform grain size distribution. A random parameter assignment method is proposed to simulate the mechanical properties and failure behavior of coal using the combined finite-discrete element method (FDEM). A comprehensive parametric investigation of the properties of triangular and joint elements is conducted, and calibration procedures are suggested for numerical modeling. The model was calibrated to experimental results and then used to understand the impact of calcite grains on the mechanical behavior of coal. After calibration, a triaxial compression was performed on coal sampleswith and withoutcalcite grains at differentconfining pressures. It is found that the presence of calcite in a coal sample increases Young's modulus and decreases the peak strength of coal. This research uses Voronoi tessellation technology to present a Weibull statistical grain-based model (GBM) to investigate the microstructural heterogeneity induced by grain morphologies. The deformation response, strength characteristics and cracking behavior are investigated by varying the heterogeneity of numerical models under compressive tests with different confining pressures. The simulation results of triaxial compression indicate that the compressive strength increases linearly as the heterogeneity m value increases. The consistency of the numerical results with the experimental observationssuggests that the GBM approachcan be used to better understand the mechanics of brittle rock failure at the micro-structure scale.

Permeability evolution of deep-buried coal based on NMR analysis: CO2 adsorption and water content effects

Permeability evolution of deep-buried coal plays a crucial role in CO2 geological storage and coalbed methane (CBM) recovery. In this study, permeability and nuclear magnetic resonance (NMR) tests were conducted on coal specimens under triaxial conditions. Based on the evolution of relaxation time (T2), the effects of CO2 adsorption and water content on permeability, pore structure, and mechanical properties of coal were discussed. Experimental results show that CO2 injection causes adsorption-induced swelling, inhibits flow in the early stage, causes the “expansion” and “complexity” of pore structure, and generates new channels for fluid transport. The presence of water in coal inhibits the flow process, resulting in an overall reduction of permeability by 2 orders of magnitude. Softening occurs in coal specimens, as evidenced by plasticity and low brittle failure and an average strength loss of 29.3 %. Additionally, a complex CO2–H2O coupling effect in coal is observed, causing the dissolution of certain minerals and organic matter, which further amplifies the “weakening effect” of water. Finally, permeability evolution characteristics of coal affected by CO2 and H2O from multiple perspectives were discussed in consideration of NMR fractal analysis. This work provides valuable insights into deep CO2 geological sequestration and CBM recovery.

Infrared Response and Mechanical Properties of Coal with Load-Bearing Damage from a Hole Defect

Infrared Response and Mechanical Properties of Coal with Load-Bearing Damage from a Hole Defect

A Laboratory-Scale Numerical Investigation of the Effect of Confinement Conditions on the Mechanical Responses of Coal under Various Saturation Conditions

Deep coal seams are generally preferred for CO2 sequestration, during which the saturation fluids and high-stress condition involved can significantly alter the mechanical attributes of coal. To understand the effect of stress conditions on the mechanical properties of coal during CO2 sequestration, a finite element model was developed and subsequently validated using experimental data. The results indicate that coal strength increases from 10.35% for a 5 MPa CO2-saturated sample to 114.54% for an 8 MPa CO2 + water-saturated sample as the confining pressure rises from 0 to 30 MPa, due to reduced porosity. However, this effect diminishes with higher confining pressures as dilation decreases. The critical confining pressure determined in this study is approximately 20 MPa, at which all samples exhibit similar failure strength (around 48.50 MPa). Moreover, the strengthening effect caused by applied stress is especially pronounced in CO2-saturated samples, particularly in those saturated with super-critical CO2 and CO2 + water. This suggests that the reduction in coal strength resulting from the adsorption of saturation fluids can be counterbalanced by the strength gain resulting from applied stress. The aforementioned results highlight the effectiveness of injecting high-pressure super-critical CO2 into deep coal seams for carbon sequestration purposes.

Effect of water on the damage and energy dissipation feature of coal under uniaxial cyclic loading–unloading condition

AbstractTo gain insight into the reduction mechanism of water on the dynamic destabilization failure character of coal under mining disturbance conditions, the effect of water on the damage and energy dissipation feature of coal was investigated via cyclic uniaxial compressive loading–unloading test, acoustic emission (AE) monitoring technology, and theoretical analysis. It manifests that the softening effect of water on the mechanical properties of coal is still remarkable, lower uniaxial compressive strength and elastic modulus, while greater axial strain and higher Poisson's ratio are observed under the cyclic loading–unloading condition. Water‐saturated specimens dissipate more energy during the cyclic loading–unloading process, which reduces the proportion of input energy accumulated in the coal mass, thus lowering the burst proneness of coal. A greater amount of damage generates in air‐dried specimens than that in water‐saturated specimens at the first cyclic loading–unloading cycle, while lower damage increments are observed in the following cycles. This may be the reason for the minimal energy dissipation of air‐dried specimens in the following cycles. The elastic modulus behaves logarithmically versus the increasing cyclic loading–unloading cycle counts. The energy dissipated due to the damage evolution and plastic deformation of coal specimens positively correlates with the energy released by AE activities, the AE energy in air‐dried specimens has a greater concentration around the peak axial stress and a stronger correlation with the dissipated energy. The cumulative AE energy and energy dissipation density are linearly correlated during the cyclic loading–unloading process, which indicates a possibility in estimating the damage evolution and the dynamic failure risk of coal via cumulative AE energy, under the cyclic loading–unloading condition.

Prevention and Control of Coal and Gas Outburst by Directional Hydraulic Fracturing through Seams and Its Application.

The regionalized pressure relief, permeability enhancement, and outburst prevention of "three high and one low" coal seams with high gas, high ground stress, high outburst risk, and low permeability have become important problems to be solved urgently in realizing the sustainable development of coal mines. In this study, a combination of theoretical research, RFPA2D-Flow numerical simulation, and field test was used to study the initiation mechanism and propagation law of directional hydraulic fracturing fractures through the seam. The results show that fracture initiation depends on the axial and radial horizontal stress of the fracture hole, the physical and mechanical properties of coal and rock, and the inside of the weakest layer. Single-hole hydraulic fracturing can achieve a pressure relief radius of 7-8 m, but there is a stress concentration zone outside, which is not conducive to regional pressure relief and permeability enhancement. Directional hydraulic fracturing with multiple holes produces an approximate cylindrical compression and crushing ring and a penetrating fracture surface along the center line of the pressure crack hole and the directional hole, which better eliminates the phenomenon of stress concentration in nondirectional hydraulic fracturing. The technology was applied to the 2238 auxiliary roadway of Chengzhuang Mine of Jinmei Group, and the field implementation results showed that field implementation results showed that directional hydraulic fracturing through the seam reduced the gas content in the coal seam to a great extent, and the coal seam gas content was reduced by about 42.3%, indicating that the technology can effectively reduce the risk of coal and gas outbursts.

Alterations in coal mechanical properties and permeability influenced by liquid CO2 phase change fracturing

Liquid CO2 phase change fracturing (LCPCF) is a promising approach for stimulating coal reservoirs and enhancing gas drainage efficiency of low-permeability coal seams. In this work, low pressure CO2/N2 adsorption, CH4 permeability measurements and UCS tests are combined to quantitatively analyze the changes in coal mechanical properties and permeability before and after LCPCF treatment. The results show that the axial peak strain is increased and ductile failure takes place in treated specimens after 1.26 L of liquid CO2 was used to conduct physical blasting. The UCS and elastic modulus after LCPCF are reduced by 10.82–25.78% and 12.92–28.57%, respectively. Muti-scale pore structures within coal are altered to varying degrees. After LCPCF treatment, mesopore volume and macropore volume are increased by 11.33–41.57% and 33.82–67.5%, respectively, while micropores are slightly affected. Due to the adsorption swelling and Klinkenberg effect, CH4 permeability of all the specimens is reduced with the increase of injection pressure. Comparative analysis of gas permeability has been made between the raw coal and treated specimens. Coal permeability is enhanced by 14.48-29.43% and 10.54-24.82% after LCPCF treatment at the injection pressure of 0.5 MPa and 1.4 MPa, respectively. During the LCPCF process, shock waves and stress waves trigger tensile damage to coal mass, and a large number of new macropores/fractures are generated within coal. This provides more accessible paths for gas flow and coal permeability are greatly promoted. Crack propagation is accelerated by the released high-pressure CO2, and both coal porosity and pore connectivity are further enhanced to facilitate gas seepage in coal seams.

Relationship between pore structure and mechanical properties of bituminous coal under sub-critical and super-critical CO2 treatment

Geological sequestration of CO2 in unminable coal seams is one of the effective ways to reduce greenhouse gas emissions and increase coalbed methane production. However, changes in the pore structure after CO2 injection may affect the mechanical properties of the coal reservoir, posing a challenge to the long-term safety of CO2 sequestration. Therefore, a proper understanding of the relationship between pore structure and mechanical properties of coal under CO2 treatment is essential. In this paper, Low-field nuclear magnetic resonance tests were carried out on bituminous coal after sub/super-critical CO2 (Sub-CO2/Sc-CO2) treatment, and uniaxial compression tests were used to analyze the mechanical properties and energy evolution of coal samples. The results demonstrate that the CO2 treatment increased the porosity and proportion of macropores, and the effect of Sc-CO2 treatment on the pore structure was more significant. Compared to the Sub-CO2 treated sample, the uniaxial compressive strength and elastic modulus decreased more in the Sc-CO2 treated sample, which may be related to the extraction capacity of Sc-CO2. Analysis of the energy evolution shows that more energy was converted into dissipated energy in the loading process of Sc-CO2 treated coal samples, which was used to form, expand and connect fractures. Moreover, the correlation between the rate of change in porosity and the rate of change in uniaxial compressive strength was 0.9579, indicating that changes in pore structure were the main cause of the change in the mechanical properties of coal. A conceptual model was proposed to explain the mechanism of the effect of the pore structure on the mechanical properties, and pressure selection during CO2 sequestration was discussed. Therefore, the results of this study are expected to provide references for the long-term safety assessment and injection pressure selection of CO2 sequestration.

Effect of Liquid Nitrogen Cooling and Heating on Mechanical Properties and Acoustic Emission Characteristics of Coal

The instability of rock fractures has been extensively studied in domestic and international research. This paper investigates two coal types from the Jiaozuo area, which were subjected to liquid nitrogen cyclic treatment using Brazil’s indirect tensile testing (BITT). The study analyzes the characteristics of acoustic emission (AE) and crack formation during the coal cracking process and evaluates the impact of freeze-thaw treatment with liquid nitrogen cycles on the mechanical properties of coals. The results demonstrate that liquid nitrogen has a significant impact on the formation and development of coal fractures under the Brazil splitting test conditions. As the number of cyclic treatments increases, the maximum load-bearing capacity of coal decreases, indicating a gradual reduction in the effective stress on coal samples. Furthermore, the acoustic emission activity intensity of coal mass during phased loading increases as the number of freeze-thaw cycles of liquid nitrogen increases. Finally, during phased loading of coal specimens, the primary factor is the germination, extension, and failure of tensile cracks, while the secondary factor is the development, extension, and failure of shear cracks. Overall, this study provides valuable insights into the behavior of coals under freeze-thaw cycles of liquid nitrogen treatment and highlights the importance of understanding the characteristics of coal fracture instability.

Mechanical strength and porosity changes of bituminous coal induced by supercritical CO2 interactions: Influence of saturation pressure

Sequestration of CO2 in the geological formations has great potential to reduce greenhouse gas emissions, and the induced mechanical stability of coal reservoirs is deeply concerned with the long-term safety of CO2 geological storage. After injection, it is likely that CO2 is stored in the form of supercritical state in coal reservoirs. To date, our current knowledge is still lacking in the underlying interaction mechanisms between coal and supercritical CO2 (ScCO2) fluids under the influence of saturation pressure. In this work, ScCO2 saturation was performed on bituminous coal at the pressures from 7.5 to 25 MPa for saturation time up to 10 days. Mercury intrusion porosimetry (MIP), X-ray diffraction (XRD) and uniaxial compression measurements were conducted to evaluate the mechanical strength and microstructure changes of bituminous coal. The results indicate that coal pore structures are greatly changed by ScCO2 interactions, including the increase of pore volume (84.8–173.2%) and surface area (42.1–85.8%). The larger hysteresis effect is found in saturated specimens, illustrating that ScCO2 interactions enhance the pore roughness of coal. The carbonates in coal are sensitive to ScCO2 fluids, which are extensively dissolved during interactions. ScCO2 saturation pressure has significant impact on the mechanical properties of coal. Rapid reduction of mechanical strength (up to 35.6%) was identified when the pressure was increased from 7.5 to 15 MPa. Subsequently, limited further reduction (<9%) occurred as the saturation pressure is further increased up to 25 MPa. Mineral dissolution and porosity changes account for the mechanical degradation, which weakens the cementation degree and interaction force of coal matrix.

Experimental and numerical study of coal mechanical properties during coalification jumps

Experimental and numerical study of coal mechanical properties during coalification jumps

Cracking and improved wettability of coal through liquid CO2 cyclic cold soaking for dust prevention

To solve the problem of water injection in low–porosity and low–permeability coal seams, a new idea of liquid CO2 cyclic cold soaking to improve the water absorption and wetting efficiency of coal is proposed. The evolution of the pore structure and mechanical properties of coal were studied using low–field nuclear magnetic resonance and uniaxial compression tests. The wettability and water absorption of coal before and after the experiment were determined using contact angle and water absorption tests. The results show that as the number of cycles increases, the effective porosity of coal increases significantly, the mechanical strength is weakened, and the compressive strength and elastic modulus decrease. The wettability and water absorption of coal samples were significantly improved by liquid CO2 treatment, and the changes in porosity, mechanical strength, and water absorption wettability of coal samples in the saturated group were more obvious compared to dry coal. This paper lays a theoretical and experimental foundation for the development of a new technology for coal seam water injection dust prevention.

Evolution of the strength characteristics of briquette and raw coal containing fluid

The mechanical properties of coal containing fluid are an important factor affecting the safe mining of soft coal seams. In particular, for class III–V coal, coal and gas outbursts and other dynamic phenomena are prone to occur due to the influences of gas pressure and groundwater, which seriously threaten the safety and lives of field workers. However, briquette samples are usually used in place of raw coal in laboratory tests conducted on class III–V coal samples. Whether the research conclusions for briquette and raw coal are consistent and whether briquette coal can replace raw coal in research on strength characteristics need to be further verified. In this paper, the evolution of the strength characteristics of fluid-bearing briquette coal and raw coal is studied. The strength characteristics, instability failure characteristics, and acoustic emission characteristics of raw coal and briquette coal under uniaxial and triaxial compression are analyzed in detail. In addition, the influence of the water content and pore pressure on the strength characteristics of class III–V raw coal and briquette coal is further studied. The results show that the failure characteristic of raw coal is overall brittle failure, mainly axial splitting failure, whereas that of briquette is overall ductile failure, mainly cone-shaped continuous spalling. The strength parameters of the raw coal and briquette coal improve under confining pressure, but the internal difference in the raw coal is significantly reduced. The cohesion of the raw coal sample initially increases and then decreases with increasing water content, and the internal friction angle increases with increasing water content. In addition, it is verified that the strength, cohesion, elastic modulus, and deformation modulus of the briquette decrease with increasing pore pressure under different pore pressures, but the strength difference of the class III–V coal decreases under increasing pore pressure. Based on the abovementioned results, the strength parameters of a coal body are estimated using the Hoek–Brown (H–B) criterion. Based on a comparison of the strength parameters of the coal sample and coal body, the estimated strength parameters of the coal body are closer to the actual values on site.

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.

Effect of Water on Mechanical Properties of Coal, Charge Induction, and Rockburst Hazard Prevention

Effect of Water on Mechanical Properties of Coal, Charge Induction, and Rockburst Hazard Prevention