Coal Fracture Research Articles

Coal cleat network stimulation through a combination of acidizing and liquid nitrogen fracturing

Gas extraction from Coal Bed Methane has proved challenging mainly due to the low permeability of coals. This study investigated a combination of 2 stimulation techniques, Liquid Nitrogen (LN2) fracturing and acidizing, to increase pore structure and cleat network connectivity in coals. Microcomputed Tomography (μ-CT) scanning results suggested that a combination of those 2 applications, in either sequence of implementation, has substantial potential in recovering mineralized fractures and inducing new fractures; thus, leading to wide-open cleat networks. X-ray images show that if LN2 fracturing of coal is preceded by acidizing, the stimulation efficiency increases from more than double the initial value to over 700% of it, where a porosity of 12% can be achieved. Alternatively, the efficiency of acidizing in porosity enhancement can increase from 300% to 530% when a preceding application of LN2 freezing provides an open cleat network to facilitate acid access to remote areas. Acid diffusion and digital microscopy images also determined that in fractured coal, HCl concentrations of both 1% and 3% can result in desirable demineralization (13% porosity increment), whereas in the acidizing of original coals, 1% HCl might not be promising. Further, the latter experiments created a wide and connected cleat network after LN2 fracturing of efficiently acid-stimulated coal. Therefore, a combination of two enhanced recovery techniques, acidizing and LN2 fracturing, was used in this study to achieve a dramatically boosted cleat network in coals. Additionally, an analytical model was developed to predict the acid reaction rate with coal minerals; it had a good match with experimental results.

Influence of plasma breakdown on pores and fractures in bituminous coal: A new characterization method of breakdown energy

Plasma technology has a potential application prospect in the field of coal seam permeability enhancement. However, the relationship between plasma breakdown energy and the improvement effect of coal pores and fractures is lack of in-depth research. This study proposed a novel method for characterizing the plasma breakdown energy, and three testing techniques, namely, particle size distribution, mercury intrusion porosimetry (MIP) and low-field nuclear magnetic resonance (NMR), were utilized to examine the pore and fracture distribution of coal samples obtained from Jiulong Mine under varying breakdown energy conditions. The influence between breakdown energy and pores and fractures development is revealed. The results show that: (1) The degree of coal sample fragmentation increases with the increase of plasma breakdown energy. (2) Different breakdown energy induced the development of different pore distribution patterns. At lower breakdown energy, an increase in the volume of macropores dominates. As the breakdown energy increases, there is a rapid increase in the volume of mesopores, a slight increase in minipores, and the least increase in micropores, indicating that plasma treatment primarily improves pores with diameters greater than 100 nm. The test results of the three techniques were mutually verified. (3) Compared with capacitance energy and voltage, the effective breakdown energy obtained by the integral method has a more significant linear relationship with the particle size distribution, total pore volume, and average pore diameter, indicating that the effective breakdown energy is more suitable for characterizing the crushing effect of plasma. During the plasma breakdown process, the expansion stress and thermal stress formed by the effective breakdown energy exceeded the local tensile strength of coal, leading to the generation and development of pores and fractures, which in turn led to the rupture of the coal body. The larger the breakdown energy, the larger the crushing circle formed around the ion channel, the finer the coal is crushed, and the more pore fractures are developed. These nascent pores and fractures provide channels for gas transport, which is extremely favorable for coal mine gas extraction. The research conclusions provide theoretical support for the application of plasma in fracturing and permeability enhancement in coal seams.

Hydraulic fracture initiation and propagation in deep coalbed methane reservoirs considering weak plane: CT scan testing

Hydraulic fracturing technology in coal reservoirs helps increase coalbed methane (CBM) production, and the development of hydraulic fractures (HFs) is influenced by in situ stress and original fractures. Therefore, in this study, triaxial HF tests and computed tomography (CT) were used to determine HF initiation and propagation in coal, considering weak structural planes and in situ stress. The experimental results show that weak structural planes control HF initiation and propagation in coal under a deep-stress environment. Most HFs occurred along bedding planes, followed by exogenetic fractures. The influence of the horizontal stress difference coefficient k, which is equal to the maximum horizontal stress minus the minimum horizontal stress and divided by the minimum horizontal stress, on the propagation of the HF was studied. When k increases to 1.04, the HF becomes perpendicular to the direction of the minimum principal stress. When k < 1.04, the main controlling factor of HF propagation is a weak structural plane, and when k ≥ 1.04, the main controlling factor of HF propagation is in situ stress. Based on the CT reconstruction model, the average aperture distributions of the discrete bedding fractures and exogenetic fractures of coal in this experiment were calculated to be 0.0947 mm and 0.1940 mm, respectively. This determines the different strength properties of exogenetic fractures and bedding planes, and determines the priority initiation position of HF in coal. During the HF initiation and propagation, the average energy along the exogenous fracture was lower than that along the bedding planes. On this basis, this study predicts that the main factor controlling HF propagation in a deep coal seam in the Qin-Shui Basin, China, is the weak structural plane in the coal seam.

Research on rockburst prevention systems based on the attenuation law of coal and rock vibration wave energy

During the coal and rock mass fracture process, elastic properties are released and vibration waves are radiated outward. The energy attenuation characteristics of these waves can describe the cumulative damage and elastic energy accumulation of the mass. To investigate coal and rock mass failure characteristics and energy attenuation rules during rockburst, numerical simulation and laboratory testing were utilized to study the energy transfer laws under various parameters. Six variables, including elastic modulus, Poisson’s ratio, bulk density, cohesion, internal friction angle, and void ratio, were selected to simulate the rockburst energy release process under different parameter combinations by adding surface pressure to the model. The coal and rock mass energy attenuation coefficient was obtained by fitting the node energy straight line using the least squares method. The six variables’ influence on vibration wave energy transfer was obtained using analytic hierarchy process program written in MATLAB, and a comprehensive calculation formula was proposed. Using the energy attenuation coefficient, the rock layer energy diffusion distance was calculated and compared with the roof collapse rock layer step distance, resulting in the roof rock layer cutting distance determination. By roof rock strata precutting, rockburst occurrence can be prevented, ensuring safe and efficient coal mine production.

Determination of critical energy for coal impact fracture under coupled static-dynamic loading

Deep coal is constantly subjected to high static loading, and the accompanying mining disturbances will exacerbate the energy changes in the coal specimen. This study utilized the Split Hopkinson Pressure Bar (SHPB) system to examine the critical energy required for coal impact fracture under one-dimensional (1D) and three-dimensional (3D) coupled static-dynamic loading. The results revealed that a sufficiently large impact disturbance was required to cause deformation of the coal specimen when the initial static energy storage was low. Additionally, the coal with higher initial static energy storage was more likely to fracture under minor impact. Compared to the 1D loading, the 3D loading resulted in a higher critical impact energy and velocity for intact coal to fracture. Additionally, a model was developed to demonstrate the inherent energy evolution of coal specimen breakage under coupled static-dynamic loading, based on the relationship between energy and coal specimen deformation. Reducing the initial energy storage, weakening the dynamic absorbed energy, and increasing the critical instability energy of the coal matrix were the primary recommendations for disaster prevention. And corresponding prevention and control measures are suggested. This research can provide a scientific foundation and guidance for predicting dynamic disasters in order to ensure safe and efficient mining engineering.

A Fractal-Based Quantitative Method for the Study of Fracture Evolution of Coal under Different Confining Pressures

To study the dynamic crack evolution process of loaded coal from the perspective of fractals, we carried out in situ industrial CT scanning tests of loaded coal under different confining pressures, visualizing loaded coal fracturing. Combined with fractal theory, the temporal and spatial evolution law of coal cracks is described quantitatively. The results provide two findings: (1) from the perspective of two-dimensional images and three-dimensional space, the evolution characteristics of cracks in coal under different confining pressures were basically the same in each loading stage. During the loading stages, the cracks exhibited a change rule of a slow reduction, initiation/development, rapid increase, expansion, and penetration. (2) The fractal dimension of coal was calculated by introducing fractal theory, and its change law was in good agreement with the dynamic changes of the cracks, which can explain the influence of the confining pressure on the loaded coal. The fractal dimension showed three stages: a slight decrease, a stable increase, and then a significant increase. The larger the confining pressure, the more obvious the limiting effect. Thus, our approach provides a more accurate method for evaluating the spatial and temporal evolution of cracks in loaded coal. This study can be used to predict the instability failure of loaded coal samples.

Failure Behavior and Fracture Evolution Mechanism of Coal with Fissures around Holes.

The fracture of coal is the main channel of gas flow and an important factor affecting the stability and efficiency of gas drainage boreholes. The coal structure, the development of hole cracks, and the degree of deformation are different. It affects the strength and mechanical deformation characteristics of coal to a great extent. In order to investigate the law of fracture evolution around the borehole of fractured coal, uniaxial and triaxial compression tests of raw coal samples have been carried out. The stress field evolution characteristics of fractured coal under compression were analyzed by Particle Flow Code (PFC2D). The strength, deformation, and fracture evolution behavior of fractured coal around boreholes under different confining pressures were studied. The results show that the compressive strength and fracture morphology evolution characteristics of coal around the hole are obviously related to the confining pressure and fracture occurrence of raw coal. The borehole structure itself has an important influence on the distribution location of the shear failure zone of the fracture around the hole, and its influence degree increases with the decrease of borehole confining pressure. During the deformation of coal with cracks around the hole, the initiation, propagation, and union behavior of cracks are related to the crack angle β. The cracks with β 0 and 180° are most easily closed during compression and the cracks with β 90° have little effect on the crack propagation zone. When the crack angle β is 45°, it is most easy to sprout and expand at the end; when the coal is compressed to the ultimate strength, the increase rate of the tensile crack increases, and the polymerization and combination behavior of the crack is more obvious. The evolution cloud map of the stress field can better reflect the evolution characteristics of fracture development, expansion, and fracture in the process of coal loading. Studying the failure behavior and fracture evolution mechanism of the coal around the hole can better predict and control the gas migration and extraction effect, which is of great significance to prevent the occurrence of gas accidents.

Research on Hydraulic Fracturing Technology of Long Boreholes along Strata of High Vast Thick Coal Seam

Gas extraction is a major technique for regional gas regulation and coal and gas comining in China. Assuring effective gas extraction operations is a crucial step in ensuring the supply of energy. The effect range of extraction drilling, pressure relief degree, and standard period of gas extraction are all constrained because of the geological constraints affecting coal gas permeability and occurrence. Combined with the advantages of directional drilling and high-efficiency pumping technology and antireflection enhanced pumping technology of hydraulic fracturing, directional long-drilling hydraulic fracturing can effectively improve the efficiency of gas control and expand the scale of gas control. The present study focuses on the exploration of directional long-hole hydraulic fracturing technique in thick coal seams with high gas content using Dafosi Mine as a case study. The research findings demonstrate that hydraulic fracturing contributes to the enlargement of pore size, pore density, and pore connectivity in coal seams. In-depth research was conducted on the expansion of coal seam fractures during the hydraulic fracturing process using the RFPA3D-flow numerical simulation program. Additionally, a comprehensive analysis of stress distribution around the fractures under the flow-solid coupling condition was performed. To further improve the effectiveness of hydraulic fracturing technique, the research team optimized the fracturing tools and construction processes in the four coal seams of Dafosi Mine. The impact of segmented hydraulic fracturing in coal seam bare hole drilling was also studied. Furthermore, an investigation method specific to the coal seam bare hole segmented hydraulic fracturing effects applicable to Dafosi Mine was developed. The maximum fracture extension pressure, minimum fracture closure pressure, and fracture morphology change characteristics during drilling and fracturing were measured, and the fracturing influence radius of coal seam was determined to be 46−58 m, the gas extraction concentration after fracturing increased by 2.20–4.22 times, and the 100-m extraction flow increased by 4.93–11.03 times. It gives other mines technical assistance so they can keep advocating and utilizing the horizontal directional long-drilling stage hydraulic fracturing technique.

Improvement of Thick Coal Seam Gas Drainage Efficiency Using Highly Pressurized Multidischarge CO2 Gas Fracturing Technique.

Thick coal seam fracture stimulations were conducted to enhance pre-gas drainage efficiency through the use of a highly pressurized multidischarge carbon dioxide gas fracturing technique. This method also offers potential as a strategy for carbon dioxide sequestration, aiding in the reduction of atmospheric carbon dioxide levels and thereby contributing to the fight against climate change. This paper discusses findings from both field experiments and numerical simulations. Data from the field show that the multidischarge fracturing approach significantly improves permeability in thick coal seams, thereby boosting gas drainage effectiveness. Additionally, the impact of increasing the number of fracturing devices is more pronounced at distances of 2.5 or 7.5 m from the borehole but becomes more complex at 12.5 m or further. The numerical simulations reveal that this technique primarily enhances coal seam gas drainage by improving the seam permeability and establishing a gas pressure gradient within the seam. It is noted that the radius of failure around the borehole wall expands with higher discharge pressures, while the radius of effective drainage narrows as the gap between discharge heads increases. Moreover, adding more discharge sets significantly influences the deformation and permeability of the coal seam within a 5 m radius of the fracturing borehole, but the influence is not obvious after 10 m from the fracturing borehole.

Experimental investigation on visualization and quantitative characterization of filling and plugging performances of fractures in coal

Underground coalbed methane extraction is an important way for preventing coal mine gas disasters and reducing greenhouse gas emission, but air leakage around extraction boreholes is a critical factor that limits efficient coalbed methane extraction. Dynamically filling and plugging fractures around extraction boreholes can prevent air leakage. Due to the invisibility and complexity of underground geological formations, observing the process and effectiveness of plugging is extremely challenging. To visualize and quantitatively characterize the filling and plugging performances of fractures in coal. First, experiments are conducted to fill the coal damaged by the uniaxial and the triaxial loading-induced using a novel fracture filling experimental system under the constraint conditions. Then the multidimensional (point, surface, volume) plugging performances of fractures in coal before and after filling is visualized by the method of the X-ray computed tomography imaging. Finally, the quantitative characterization of filling and plugging performances of fractures in coal is conducted by testing changes in weight, fracture volume, and permeability before and after filling process. The experimental results show that after filling, the width and quantity of fractures in the coal significantly decreases, and the damaged coal is repaired. The filling ratio of fractures in coal fractured by uniaxial loading is 72%, and the fracture filling ratio in coal under triaxial loading is 68.1%. Under the influence of seepage and diffusion of plugging materials, the fractures are continuously filled, leading to a continuous decrease in the permeability of the fractured coal. The significant reduction in permeability of the fractured coal body is evident. The filling and plugging performances of fractures in coal is effective. The findings in this paper provide meaningful references and guidance for determining the filling and plugging parameters in underground gas extraction engineering practices.

Research on coal crushing behavior under supercritical CO2 transient high-pressure fracturing

CO2 fracturing enhances gas permeability by creating a complex network of fractures in coal seams. Crushing serves as the foundation of fracture development, and studying the coal crushing behavior under supercritical CO2 transient high-pressure fracturing is of great significance to understand fracture evolution and guide precise fracturing. This study utilized a self-built experimental platform for supercritical CO2 transient high-pressure fracturing to obtain fractured coals at different energy levels. Based on multifractal theory, the relationship between fracturing energy and crushing behavior was studied by laser particle analysis and scanning electron microscopy. The results showed that the volume average particle size (D4,3), and the particle sizes at which the cumulative particle size distribution reached 50% (D50), and 90% (D90) decreased significantly with the increase of fracturing energy. The converted diameter and the crushing work ratio exhibited an exponential relationship with the fracturing energy. The larger the fracturing energy, the smaller the values of the multifractal generalized dimension spectrum parameters (D0, D1, and ΔD(q)), the larger the values of the multifractal singular spectrum width (Δα (q)), and the denser and more even the particle size distribution of fractured coal. The multifractal parameters and shape coefficients exhibited phased change during the fracturing process. The coal particles progressed from an initial stage with numerous microcracks and fissures to a honeycomb structures with small pores. After the disintegration of the honeycomb structures, agglomerated ultrafine particles were formed. The findings provide a theoretical reference for optimizing the selection of fracturing parameters.

Study on key grouting blocking parameters of gas drainage boreholes in soft coal seams

The construction of gas extraction boreholes in soft coal seams is prone to collapse and deformation, and grouting reinforcement is one of the main methods to solve the problem of efficient sealing. However, the reasonable selection of key grouting parameters still needs further research. In response to the problem of selecting grouting sealing parameters for gas drainage drilling in soft coal seams, based on the “concentric ring” reinforcement sealing technology obtained in previous research, the key parameters and sealing technology of the “concentric ring” reinforcement were studied through theoretical calculation and numerical simulation experiments. The slurry diffusion morphology and range under different grouting pressures and grouting time slurry viscosity were obtained. Finally, in order to explore the application effect of key grouting parameters, on-site industrial tests were conducted in a soft and high gas coal seam. The research results indicate that the optimal grouting pressure for the “retaining wall rock hole ring” should not be less than 3 MPa, the reasonable grouting time should be 10–15 min, and the water material ratio of the grouting material should be greater than 1:1; The use of new reinforcement sealing and grouting technology can ensure long-term good extraction effect. Compared to the testing of drilling sealing effect using ordinary cloth bags with two plugs and one injection, The adoption of new reinforcement sealing technology can effectively prevent the deformation and collapse of the borehole before sealing, and due to two rounds of grouting and pre reinforcement of fractured coal, the sealing effect of the borehole is also relatively good. The research results have important theoretical value for guiding the drilling and sealing grouting engineering of gas extraction in soft coal seams.

Investigation on damage-permeability model of dual-porosity coal under thermal-mechanical coupling effect

Understanding the permeability characteristics of coal under mining disturbance is crucial for ensuring coal mine safety and efficient gas extraction. Coal is a dual porosity medium consisting of matrix and fracture, and there are some differences in mechanics and permeability between matrix pores and fracture. However, research on the damage and gas seepage characteristics of dual-porosity coal is limited. In this paper, a coupled damage-permeability model considering matrix pores and fractures in coal is established, incorporating the influences of effective stress, gas adsorption, thermal expansion, and thermal cracking. And the reliability of the newly developed model was verified using experimental data of temperature and pressure variations under different stress boundary conditions. The results indicate that during the elastic stage, the total permeability and fracture permeability of coal exhibit a trend of decreasing and then increasing with pore pressure, while matrix permeability gradually increases. In the full stress-strain stage, the overall damage-permeability and fracture damage-permeability of coal exhibit an “S"-shaped variation trend with axial strain, while matrix damage-permeability shows a slow increasing trend. To accurately characterize the evolution of gas seepage in coal, contributions from matrix pores and fractures must be comprehensively considered, and the effects of gas adsorption, effective stress, thermal cracking, and thermal expansion on permeability cannot be ignored. Furthermore, through parameter sensitivity analysis, it is found that during the elastic stage, coal permeability decreases with an increase in the adsorption sensitivity coefficient and thermal expansion coefficient. In the full stress-strain process, coal permeability increases with an increase in the damage-permeability coefficient. The results of this study are of great significance for predicting the evolution of coal permeability in the working face ahead under mining disturbance.

Optimization of coal seam pressure fracturing fluid system in the eastern edge of the Ordos Basin based on IoT sensors

In the eastern edge of the Ordos Basin in China, coalbed methane geological resources are very abundant, and efficient extraction of coalbed methane has become an important demand for energy development strategies and coal safety production. Therefore, this article uses IoT detection technology to optimize the coal seam pressure fracturing fluid system in this area, in order to improve its efficiency and reliability. Through on-site experiments, the relationship between different concentrations of fracturing fluid systems and coal seam permeability was studied. Divide the architecture of the Internet of Things to achieve more efficient information transmission and application. Based on the actual situation of the coal industry, we have constructed a system architecture for the Internet of Things in coal mines. Based on in-depth research on the occurrence characteristics of coalbed methane in the eastern edge of the Ordos Basin and the characteristics of underground fracturing fluid technology in coal mines, we have obtained the viscosity, rheological properties, viscoelasticity of underground fracturing fluid in coal mines, as well as specific requirements for the action of coalbed methane. On this basis, an interaction model between coal seam fracturing proppant and coal reservoir was established, and its mechanical response process was analyzed using numerical simulation methods. Selecting the optimal fracturing fluid ratio can significantly improve the surface wettability and strength of coal samples, while reducing the content of clay minerals and increasing the porosity of coal samples.

Study on the Complex Electrical Response Characteristics of Loaded Coal and the Control Mechanism of the Main Fracture System Structure.

The structure of coal seam fractures is the main physical property of coalbed methane reservoir evaluation, and the complex resistivity method is a potential geophysical evaluation method for coal seam fractures. In this study, cylindrical coal samples with axial directions perpendicular to the bedding, face cleat, and butt cleat were prepared. The complex electrical parameters of the loaded specimens were tested with test frequencies ranging from 1 Hz to 10 kHz. The complex electrical response characteristics of the loaded coal are summarized, and the control mechanism of the main fracture system structure is analyzed. The results indicated that (1) as the loading pressure increased, the resistance R and the absolute values of reactance X(|X|) gradually decreased, especially in the frequency band where R slowly decreased and the characteristic frequency of X, the decreased amplitude was more significant, and the cutoff frequency of R and the characteristic frequency of X all gradually increased. (2) The complex electrical properties of coal show obvious anisotropic characteristics. Both R and |X| decreased sequentially according to the direction perpendicular to the bedding, face cleat, and butt cleat; the cutoff frequency of R and the characteristic frequency of X all increased sequentially. (3) The dispersion phenomenon of the complex electrical properties of coal is attributed to the induced polarization; the elevated loading stress enhances the polarization effects of the molecular-induced moments of the coal skeleton, and the anisotropic difference of the complex electrical properties is due to the difficulty in the degree of transport of charged particles induced by structural differences of the main fracture system. (4) The resistance R3 and capacitance Xc were selected as the complex electrically sensitive parameters of the loaded coal orthogonal fracture structures. A logarithmic inversion model reflecting the main fracture system structure of coal was constructed. This provides a certain theoretical basis for efficient electrical exploration of coal reservoir fracture structures.

Experimental study of precursory features of CO2 blasting-induced coal rock fracture based on grayscale and texture analysis.

CO2 blasting has been identified as a potent method for enhancing the permeability of coal seams and improving gas drainage efficiency. This study is focused on elucidating the deformation and fracture mechanisms of coal and rock during CO2 blasting and on identifying the precursor characteristics of these processes. To this end, a CO2 blasting-induced coal rock fracture pressure model and a gas pressure distribution model were developed. The research utilized a self-developed CO2 blasting test platform along with a non-contact full-strain field measurement analysis system. Briquette samples were subjected to CO2 blasting tests under controlled experimental conditions, which included an axial pressure of 1.0 MPa and variable gas pressures of 0.5, 1.0, and 1.5 MPa. This methodology enabled the capture of the principal strain field on the surface of the samples. The Gray Level Co-occurrence Matrix (GLCM) was employed to extract and analyze the grayscale and texture features of the strain cloud maps, facilitating a quantitative assessment of their evolution. The aim was to pinpoint the precursor characteristics associated with coal rock cracking and crack propagation. The results revealed that: (1) During the cracking and subsequent propagation of samples, the strain field's grayscale histogram underwent a transformation from a "broad and low" to a "narrow and high" configuration, with a consistent increase in peak frequency. Specifically, at 3 ms, a primary crack was observed in the sample, evidenced by a grayscale peak frequency of 0.0846. By 9 ms, as the crack propagated, the grayscale peak frequency escalated to 0.1626. (2) The texture feature parameters experienced their initial abrupt change at 3ms. Correlation with the gas pressure distribution model indicated that this was the crack initiation moment in the sample. (3) A secondary abrupt shift in the texture feature parameters occurred at 9ms, in conjunction with experimental phenomena, was identified as the crack propagation phase. Monitoring the grayscale and texture features of the principal strain field on the coal rock surface proved effective in recognizing the precursor characteristics of crack initiation and propagation. This research has the potential to reduce blasting costs in coal mines, optimize blasting effects, and provided theoretical guidance for enhancing gas extraction efficiency from deep and low permeability coal seams.

Temperature response of coal fracture induced by three-dimensional stress field

The release of thermal phenomena during coal fracture is well-established. However, current understanding of the temperature variations induced by stress fields in coal remains limited. A triaxial test was employed to explore the internal temperature response to stress and cracks in coal by incorporating moment tensor inversion for the crack distribution analysis. An equation describing the stress-induced temperature variations was derived and validated using thermodynamic parameters. The impact of coal fractures on temperature was assessed by comparing the values at different locations. The results show a strong, positive linear correlation between stress and internal temperature, indicating an increase in temperature with rising stress. The calculated temperature variations closely aligned with the observations in crack-free areas. Temperature differentiation occurred in regions with concentrated shear cracks. Significant shear damage led to a sudden increase owing to frictional and thermoelastic heating. These findings are significant for predicting spontaneous coal combustion.

A viscoelastic–plastic constitutive model incorporating creep initiation stress for surrounding rock in deep roadways and its practical application

AbstractWith the increasing depth of engineering in rock masses, the issue of significant rheological deformation becomes notably prominent. To accurately characterize the deformation and failure behavior of the surrounding rock near the goaf in deep small coal pillars, we have developed a viscoelastic–plastic constitutive model that takes into account the initial stress associated with creep. This model builds upon an existing viscoelastic–plastic constitutive framework and is implemented numerically using C++ in the FLAC3D. The Oedometer test method is used to check the creep components and the calculated parameter (η/G) ensure convergence of numerical operations. We conducted both pre‐improvement and post‐improvement tests of the constitutive model in a simple numerical analysis of roadways. The enhanced model demonstrates improved accuracy in deviational stress calculations and proves suitable for modeling deep, soft roadways. Furthermore, we applied the improved model to simulate actual working conditions at Zhaozhuang Mine, where mining operations near the working face induce a fracture zone of approximately 3 m in the small coal pillar located at the goaf's edge. The new model closely aligns with the observed results, further validating its suitability for tunneling along the goaf. Tunneling and mining activities result in significant deformation at various locations, including the shoulder socket of the working face, the top angle of the coal pillar, and the bottom angle of the floor. The crushing zone at the working face extends up to 4 m. Rheological effects transform the triangular elastic core, from its original high‐pressure elastic state to a fracture and plastic zone in the deep coal. Stress in deep coal is transmitted along the triangular elastic core. Numerical simulation results closely match the excavation profile of the tunneling, providing valuable insights into the impact of mining activities. This study can provide reference for deformation of similar geological conditions and engineering situation.

Imaging small geological structure in coal mines based on reflected acoustic logging of underground cross-seam boreholes

Accurate identification of small geological structures is the key and difficult point in coal mine geological survey. The reflected acoustic logging technology has the advantages of large detection range and high imaging resolution, and can realize the accurate investigation of small structural interfaces. Therefore, the idea of identifying small geological structures in coal mines by using cross-seam boreholes reflected acoustic logging is proposed. By placing acoustic logging tools in underground cross-seam boreholes, waveforms generated at coal-rock interfaces around the borehole are collected. The reflection wave information is then inverted to obtain interface images of small structures. First, COMSOL Multiphysics software is utilized to build finite element models for reflected acoustic logging in coal-bearing formations containing coal seam fractures, small normal faults, and coal seam thinning zones, respectively. Then forward modeling is conducted to analyze the spatiotemporal variation patterns of the full waveform signals and velocity snapshots. Finally, the reflected acoustic logging data is inverted to realize the offset imaging of small structures in coal mines. The forward modeling results show that: the arrival times of reflected waves varying with the source longitudinal coordinates exhibit good consistency with the interface morphology of borehole vicinity small structures. The inversion results show that: when fractures exist in the coal seam, the imaged dip angle of the fracture is 54.14°, with an error rate of 2.07% compared to the analytical model, and the imaged fracture length is 15.32 m, with an error rate of 4.57%. When small faults exist in the coal seam, the imaged dip angle of the fault plane is 70.63°, with an error rate of 1.41% compared to the analytical model, and the imaged fault plane length is 13.75 m, with an error rate of 3.03%. When thinning zones exist in the coal seam, the imaged dip angles of the upper and lower interfaces of the thinning zone are 44.61° and 57.80°, with error rates of 0.31% and 0.40% respectively compared to the analytical model. The imaged lengths of the upper and lower interfaces are 9.32 m and 18.73 m respectively, with error rates of 0.32% and 1.08% respectively. Acoustic reflection logging technology can effectively invert the locations and morphological characteristics of small structures in coal mines. This study can provide fundamental theoretical support for the application of acoustic reflection logging technology in cross-seam boreholes for identification of small structures in coal seams.

The evolution process of fractures and their modification effects on the liquid–solid interface during liquid nitrogen cyclic freeze–thaw of coal and shale

Liquid nitrogen cyclic freezing-thawing fracturing (LNCFT) is an innovative method for the waterless fracturing of coal and rock, harnessing the effects of low temperature, phase change, and expansion. However, the mechanisms behind micro-fracture propagation, evolution, and surface modification during LNCFT remain elusive. This study concentrates on anthracite coal and roof shale in the Jiaozuo mining area. It conducts LNCFT experiments under various saturated water conditions and employs a stereoscopic fluorescence microscope and an optical contact angle measuring instrument for observations, analyzing the surface modification effects resulting from the propagation and evolution of fractures on solid surfaces. The study unveils the following findings: (1) LNCFT effectively creates a network of micro-fractures. The quantity and length of fractures significantly increase with the number of fracturing cycles, and the effect is more pronounced with higher water content, especially in coal compared to shale. Fracture evolution falls into three categories: extension, intersecting, and newly formed independent fractures, with the scale of extension and intersecting of the original fractures determining the modification effect. (2) The LNCFT process exhibits distinct stages and selectivity. The strength of early-stage modifications (before eight cycles) directly influences the overall modification effect. (3) Throughout the LNCFT process, the solid–liquid surface contact angle first increases and then decreases with the number of fracturing cycles. This pattern can be analyzed using the Wenzel model. Liquid nitrogen fracturing increases the surface roughness of coal and shale, leading to a transition from hydrophobicity to hydrophilicity on their surfaces. The research findings of this paper provide theoretical guidance for the modification of low-permeability reservoirs.