Coal Research Articles

Error analysis and improvement of water displacement method in measuring gas desorption volume from coal particles

The water displacement method is widely employed in experiments to investigate the adsorption and desorption characteristics of coal gas. However, conventional desorption apparatus faces challenges in accurately quantifying gases with low desorption rates, leading to significant inaccuracies in experimental assessments of residual gas content and adsorption–desorption hysteresis. In order to solve the issue, a new desorption device was invented to carry out isothermal ultimate desorption experiments together with the conventional device under two different equilibrium pressures with coal samples of four particle sizes. The results reveal that after 100 min, the gas desorption rate gradually decreases, with the desorption percentage of the conventional device remaining relatively constant, while that of the new device continues to rise. By the end of the experiment, the conventional device measured gas residual percentages ranging from 30 % to 50 %, whereas the new device recorded percentages between 10 % and 25 %. The lower residual percentages obtained by the new device prove the effectiveness to solve the issue that the conventional devices struggle to quantify low-rate desorption gas.When applying the conventional device, as desorption progresses, the desorption rate decreases, and the pressure within the coal sample tank increases more slowly, which results in the difficulty for the gas to reach a pressure of 71.18 Pa to overcome fluidic constraints and enter into the graduated cylinder and ultimately accumulates in the soft, large-volume silicone pipeline. In contrast, the new device, designed with its gas outlet strategically positioned above the liquid level, bypasses the constraints of liquid forces. Additionally, the modest inner diameter of the rigid tube undergoes minimal deformation under experimental conditions. Differences in liquid forces and piping allow the new device to accumulate 71 mL less gas than the conventional device. The utilization of the new device in coal gas desorption experiments effectively mitigates experimental errors stemming from low desorption rates, thereby driving advancements in the investigation of coal seam gas parameters and residual gas content.

Research on the Characteristics of Seepage Failure in the Surrounding Rock (Coal) of the Goafs

During mining, the brittle fracture structure of coal makes it highly susceptible to disturbance, leading to changes in the permeability of the coal seam from non-conductive to water-conductive, which poses a significant threat to the stability and safety of coal pillars in goafs. Therefore, understanding the damage mechanisms of coal during water–rock interactions is crucial for ensuring mine safety. In this paper, based on laboratory seepage tests, the impact of hydrodynamic forces on the microstructure of fissured coal and its subsequent effect on permeability is examined. The study found that increasing confining pressure causes the “closure” of coal fissures, leading to a reduction in permeability. Additionally, during the initial stage of seepage, fine particles within the coal samples are mobilized due to seepage damage, leading to channel blockages and further reductions in permeability. However, as seepage continues, the hydraulic channels eventually open fully, resulting in a sharp increase in permeability. Furthermore, using a two-dimensional fracture seepage model, the study investigated how the scale of fractures in the water-conducting channels influences seepage behavior. A critical fracture width method was proposed to predict permeability surges, offering a new approach for analyzing the stability of coal pillars in mining areas.

A Study on Acid Dissolution Characteristics and the Permeability Enhancement of Deep Coal Rock

In order to reveal the acidification and dissolution characteristics of deep coal rock, core acidification and dissolution experiments are carried out based on low-field nuclear magnetic resonance technology to study the dissolution characteristics of different acid types when applied to coal rock, and to quantitatively evaluate the dissolution characteristics of acid solutions when applied to different-scale pore throats and the karst corrosion characteristics of primary fractures. This will help to further understand the dissolution rate and pore volume growth rate of coal powder under the action of different acid types. Improving the seepage effect of coal seams is of great significance. The results show that 15% acetic acid has the best effect with regard to karst erosion and permeability. The pore volume growth rate is 442.49%, and the permeability increases by up to 31 times. With large pores, the rapid dissolution stage of mud acid, hydrochloric acid, and mixed acid mainly occurred in the first 36 h, and the rapid dissolution stage of acetic acid and hydrofluoric acid applied to the core mainly occurred at 36–72 h. The dissolution rate of acid solution is strongly correlated with porosity and permeability, and the higher the acetic acid concentration, the larger the permeability increase.

Study on the rockburst risk of alternate exterior entry under island coal pillar

The extraction of extremely thick coal seams through slicing mining requires leaving behind several island coal pillars in the top slicing, which could lead to rockburst in the bottom slicing. This article proposed a method of alternate exterior entry (AEE) for rockburst prevention based on the stress distribution of bottom slicing. The influences of coal pillar width [Formula: see text] and stress concentration factor [Formula: see text] on stress distribution were explored using two-dimensional stress imaging. The results indicated that K value had a greater effect on stress distribution comparing with B value. Both stress relaxed and stress rate angles increased linearly with an increase in K value. In order to get the optimized location of AEE in the coal mine stress rate angle was chosen as the stress extension angle, which was further verified by field data from 1210 coal faces. Finally, numerical simulation was used to explore the optimized location of special tail entry under dip island coal pillars for rockburst prevention. It was revealed that plan first was found to be most effective due to its presence in a stress-relaxed area, thus verifying the effectiveness of the AEE method for rockburst prevention.

Research on an identification model for mine water inrush sources based on the HBA-CatBoost algorithm

Accurate and efficient identification of water inrush sources, as one of the three critical elements of mine water hazards, is crucial for mine water management. To identify the sources of mine water inrush effectively, a model named HBA-CatBoost is introduced. This model is established on the hybrid bat algorithm (HBA)-optimized category feature gradient boosting tree (CatBoost), and the shapley additive explanation (SHAP) method is employed to elucidate the model’s decision-making process. Given the prevalent occurrence of water hazards in coal seam roofs and floors in the northern Guizhou coalfield, coupled with the challenges in pinpointing water inrush sources in mines, the HBA-CatBoost model is tested at the Longfeng Coal Mine in northern Guizhou to validate its practicality. Comparative analysis with the HBA-RF, HBA-XGBoost, CatBoost, RF, and XGBoost models demonstrates that the hybrid bat algorithm significantly enhances the classification performance of the CatBoost model, resulting in improved convergence speed and classification accuracy. The HBA-CatBoost model outperforms the aforementioned models in terms of classification effectiveness, achieving accuracy, recall, precision, and F1 scores of 96.43%, 97.22%, 96.43%, and 96.61%, respectively. The SHAP method elucidates the decision mechanism of the optimal HBA-CatBoost model, highlighting the significance of sample features and bolstering the model’s credibility. These outcomes underscore the superior performance of the HBA-CatBoost model and its potential for effectively identifying water inrush sources in mines.

Research on Enhancing Gas Extraction Efficiency through CO2 Gas Fracturing in Outburst-Prone Coal Seams.

Given the background of significant Coal Mine gas disasters worldwide, the research and development of efficient gas control technologies have become critical to ensuring mining safety. CO2 gas fracturing (CO2-Frac), an innovative technology for Coal Mine gas control, has demonstrated efficiency in various mining areas across China. However, its application in outburst-prone coal seams is not yet fully understood. This study presents a field-scale CO2-Frac project conducted in an outburst Coal Mine in China, specifically the Pingshu Coal Mine, where the previously employed dense-borehole gas extraction technology failed to efficiently achieve gas extraction and outburst prevention. The CO2-Frac plan was evaluated through fracture propagation experiments utilizing both single-hole and dual-hole configurations, highlighting the advantages of the dual-hole CO2-Frac method. Subsequently, on-site gas extraction tests were conducted to further assess the efficacy of the CO2-Frac plan. The results indicate that (1) In the dual-hole CO2-Frac scheme, the fractured and pressure relief area expanded to approximately 26.82 m2, which is 220% larger than that of the single-hole scheme. (2) The dual-hole CO2-Frac significantly enhanced gas extraction effectiveness, increasing the flow rate from 0.026 to 0.216 m3/min in a 100m borehole, a 7.3-fold improvement. Additionally, the gas extraction period to reach the standard was reduced from 20 to 6 days. These findings conclusively demonstrate that the dual-hole CO2-Frac technique is an effective method for safe excavation in outburst-prone coal seams, providing both theoretical and practical validation for its application in similar geological settings.

Reasonable layout range of lower mining roadways in the close and variable interval coal seams mining: a case study.

In the context of close-distance coal seam mining, the stress transfer mechanism following the extraction of the upper coal seam is crucial for the layout of roadways in the lower coal seam. This study utilizes ground-penetrating radar (GPR) to analyze the stability and plastic zone extent of residual coal pillars after upper seam extraction. A theoretical model for the stress distribution of multiple goaves and coal pillars was established. Based on a case study with variable inter-seam spacing, the stress distribution and transfer mechanisms of the floor were analyzed. The results indicate that during the transition from the center of the coal pillar to the goaf, stress concentration initially decreases significantly and then gradually returns to the original rock stress, forming a distinct stress gradient distribution. As the depth of the floor strata increases, the weight of the overlying strata is more evidently transferred through the coal pillars to the goaf. Finally, a reasonable layout range for the lower seam mining roadways under different inter-seam spacings is proposed, providing a reference for optimizing roadway layout in similar geological conditions.

Geochemistry in Geological CO2 Sequestration: A Comprehensive Review

The increasing level of anthropogenic CO2 in the atmosphere has made it imperative to investigate an efficient method for carbon sequestration. Geological carbon sequestration presents a viable path to mitigate greenhouse gas emissions by sequestering the captured CO2 deep underground in rock formations to store it permanently. Geochemistry, as the cornerstone of geological CO2 sequestration (GCS), plays an indispensable role. Therefore, it is not just timely but also urgent to undertake a comprehensive review of studies conducted in this area, articulate gaps and findings, and give directions for future research areas. This paper reviews geochemistry in terms of the sequestration of CO2 in geological formations, addressing mechanisms of trapping, challenges, and ways of mitigating challenges in trapping mechanisms; mineralization and methods of accelerating mineralization; and the interaction between rock, brine, and CO2 for the long-term containment and storage of CO2. Mixing CO2 with brine before or during injection, using microbes, selecting sedimentary reservoirs with reactive minerals, co-injection of carbonate anhydrase, and enhancing the surface area of reactive minerals are some of the mechanisms used to enhance mineral trapping in GCS applications. This review also addresses the potential challenges and opportunities associated with geological CO2 storage. Challenges include caprock integrity, understanding the lasting effects of storing CO2 on geological formations, developing reliable models for monitoring CO2–brine–rock interactions, CO2 impurities, and addressing public concerns about safety and environmental impacts. Conversely, opportunities in the sequestration of CO2 lie in the vast potential for storing CO2 in geological formations like depleted oil and gas reservoirs, saline aquifers, coal seams, and enhanced oil recovery (EOR) sites. Opportunities include improved geochemical trapping of CO2, optimized storage capacity, improved sealing integrity, managed wellbore leakage risk, and use of sealant materials to reduce leakage risk. Furthermore, the potential impact of advancements in geochemical research, understanding geochemical reactions, addressing the challenges, and leveraging the opportunities in GCS are crucial for achieving sustainable carbon mitigation and combating global warming effectively.

Technology and application of through layer borehole ultra-high pressure hydraulic slotting in soft coal seam to pressure relief and permeability enhancement

ABSTRACT It is a vital method that controls outburst disasters through gas drainage to reduce gas pressure. But the permeability of coal seam in China is low. Therefore, technologies that can decrease pressure and increase permeability such as ultra-high pressure hydraulic slotting (UPHS) are widely applied. Based on the technology of UPHS, the principle of coal breaking and mechanism of pressure relief and permeability enhancement (PRPE) were first analyzed. The effects of PRPE between contrast borehole and UPHS were compared through numerical simulation. Finally, combined with the field test, the parameters of UPHS were determined, and difference in gas drainage effect between contrast borehole and UPHS was investigated. It is found that the exposed area of coal increased due to macroscopic slotting formed by hydraulic slotting, which provide space for coal deformation. The stress of coal relieved the permeability enhanced, resulting in the gas drainage efficiency increasing. The simulated results showed the pressure relief area and plastic deformation caused by UPHS are significantly increased compared with contrast borehole. The average concentration and total of gas drainage in UPHS are 1.49 and 3.13 times than that of contrast borehole. And radius of gas drainage of UPHS 1.77 times than that of contrast borehole under reduce the 46% engineering quantities.

Applications of Heterogeneous Catalysts in Green Chemistry

In our day-to-day lives, the utilization of industrial products, including chemicals and fuels, is essential [...]

Study on failure mechanism of cracked coal rock and law of gas migration

China possesses abundant coal resources and has extensive potential for exploitation. Nevertheless, the coal rock exhibits low strength, and the coal seam fractures due to mining activities, leading to an increased rate of gas emission from the coal seam. This poses significant obstacles to the exploration and development of the coal seam. This paper focuses on studying the failure mechanism of fractured coal rock by conducting uniaxial and triaxial compression experiments on the coal rock found at the Wangpo coal mine site. Simultaneously, in conjunction with the findings from the field experiment, a gas migration model of the mining fracture field is constructed to elucidate the pattern of coal seam gas distribution during mining-induced disturbances. The study structure reveals that coal rock exhibits three distinct failure modes: tensile failure, shear failure, and tension-shear failure. The intricate fissure in the rock layer will intensify the unpredictability of rock collapse patterns. The compressive strength of coal rock diminishes as the confining pressure drops. The coal rock in the working face area will collapse as a result of the lack of confining pressure. In the rock strata above the mining fracture zone, the gas pressure is first higher and then significantly falls with time. After 100 days of ventilation, the low gas pressure area changes little, so to ensure the safety of the project, the ventilation time of the fully mechanized mining surface is at least 100 days. The research results will help to establish the core technology system of coal seam development and improve the competitiveness of coal seam resources in China.

Optimization of small coal pillar width and control measures in gob-side entry excavation of thick coal seams

This study introduces an innovative optimized bolting support system specifically tailored for gob-side entry excavation in thick coal seams at a coal mine in southwestern Shandong, China. Employing theoretical analysis, numerical simulation, and field measurements, the research focuses on examining the failure characteristics of surrounding rock during gob-side entry excavation. The key innovation lies in the development of a 5-meter optimal coal pillar width, ensuring balanced stress distribution and structural integrity. Additionally, a lag time of at least 46 days between gob-side entry excavation and the upper working face retreat is recommended to mitigate roof subsidence and surrounding rock deformation. The optimized bolting support system, featuring increased bolt pretension, utilization of high-strength steel strips, and reinforcement of weak points, effectively reduces deformation of the roadway surrounding rock, meeting support requirements for normal production. This novel approach successfully addresses the support challenges in thick coal seam gob-side entry excavation, enhancing mining safety and resource recovery rates.

Highwall mining productivity enhancement with energy conservation: A case study

ABSTRACT Open-cast mining in India is transitioning from closing existing mines to extracting remaining coal seams from dormant sites. Highwall machines are key to this efficient extraction process. To meet international safety and environmental standards, audits are essential. This article evaluates the power quality of the substation’s incomer, transformer, and feeder, and proposes energy-efficient changes to boost production. Tests on the main substation transformers included voltage, current, power, power factor, harmonics, and AC waveform analysis. Thermal imaging of electrical equipment was conducted to verify operating temperatures. Results were graphed for each test. The study recommends installing an Automatic Power Factor Correction (APFC) unit and lowering the tap value from 5 to 4 to improve the transformer’s power factor. It is estimated that a 4% voltage reduction will save 1% energy, while a 5% reduction will save 1.25% energy.

Compound disaster characteristics of rock burst and coal spontaneous combustion in island mining face: A case study

The island mining face in coal mines often encounters stress concentration and significant air leakage, elevating the risk of rock burst and spontaneous combustion of residual coal in the goaf. This study centers on the investigation of the 9305 island mining face within a mine situated in Shandong Province, China. Leveraging the features of coal spontaneous combustion and data from microseismic and gas monitoring throughout the mining process, a novel method for calculating safe mining speeds under compound disasters is introduced. The safe mining speed of the 9305 island mining face is stratified, and a prevention and control technology integrating long distance directional drilling for impact-spontaneous combustion compound disasters is proposed. Findings suggest that to mitigate spontaneous combustion of residual coal, considering the seam's spontaneous combustion tendency, the safe mining speed should exceed 3.7 m per day. Accounting for the distribution of microseismic events during coal seam extraction, the safe mining speed is maintained below 4.8 m per day in the unprotected mining area and capped at 8 m per day in the protected zone. Through a comprehensive analysis considering coal seam spontaneous combustion tendency, impact tendency, and time to coal seam oxidation to critical temperature, the safe mining speed for the island mining face ranges from 3.7 m per day to 4.8 m per day in the unprotected area and from 5.14 m per day to 8 m per day in the protected area. The proposed long distance directional drilling 'one hole, multiple purposes' scheme enables an integrated approach encompassing pressure relief before coal seam extraction, water injection during mining, and grouting post-mining. This method effectively prevents and controls the compound disasters of rock burst and coal spontaneous combustion, presenting innovative technical solutions for the safe extraction of coal resources.

Experimental study on the influence of horizontal stress difference coefficient on coal rock fracture

Hydraulic fracturing is a pressure-relief and permeability-enhancing technique widely used in the mining of low-permeability coal seams. The horizontal stress difference coefficient is an important factor that affects the fracture propagation during the fracturing. To study the mechanism of the hydraulic fracturing process, in this paper, the initiation and propagation of cracks under horizontal stress difference coefficient are studied by using the developed true triaxial hydraulic fracturing system. The fracture surface morphology was obtained by 3D scanner. The experimental results are verified by the mathematical model. The results show that the fracture pressure and the peak values of AE count rate and b value increase and the cumulative AE count decreases with the increase of the horizontal stress difference coefficient. After the first pressure drop, the “pressure built-up” in the specimen increases and the peak value of AE count rate decreases with the increased horizontal stress difference coefficient. The 3D scanning results show that when the horizontal stress difference coefficient is larger, the hydraulic fracture propagation is straighter and the fracture surface area is smaller. The experimental results are in good agreement with the theoretical values. The experimental results provide a basis for engineering practice.

Clumped isotopes constrain thermogenic and secondary microbial methane origins in coal bed methane

Methane is an economic energy resource and potent greenhouse gas. Distinguishing secondary microbial methane from thermogenic gas is important for natural gas exploration and consideration of subsurface microbial activity in the global carbon cycle, but remains challenging. To understand controls on methane origins in natural gas systems, we investigated the methane clumped isotopologue distributions in the Qinshui Basin high-thermal maturity coal bed methane (CBM) reservoir. Here, near-equilibrium clumped isotopologues distribution (Δ13CH3D and Δ12CH2D2) inferred a temperature interval of 21.6–252.3 °C. The high-temperature thermodynamic equilibrium most likely represents original thermogenic CBM characteristics during coalification. The low-temperature equilibrium clumped isotopologue distributions suggest microbial alteration to CH4 isotopic bond ordering by increased enzymatically catalyzed isotopic exchange. The independent constraints from clumped isotopes, integrated with other geochemical and genomic evidence, confirm notable secondary microbial methane from biodegradation in the highly mature reservoir. Thus, methane clumped isotopes can be used as unequivocal tracers to distinguish secondary microbial methane from thermogenic gases and hence provide the ability to incorporate them separately into global methane budgets.

Risk analysis of coal seam floor water inrush based on GIS and combined weight TOPSIS method

Risk analysis of coal seam floor water inrush based on GIS and combined weight TOPSIS method

Analysis of dominant flow in tectonic coal during coalbed methane transport

Diffusion and seepage are the main flow forms of coal seam gas transport, and are one of the key factors in the selection of gas extraction improvement methods. Changes in the physical structure of tectonic coal make gas transport more complex during coalbed methane extraction. In this paper, we develop a multi-field coupled model of methane transport in coal seams, taking into account the effects of tectonics, and theoretically analyze the dominant flow patterns for methane extraction. Then, the evolution of gas dominated flow is analyzed for different initial pressures, initial permeabilities, and initial diffusion coefficients of tectonic and intact coal seams. The results show that the amount of daily methane seepage in tectonic coal increases with the initial pressure of the coal reservoir, but decreases with the initial diffusion coefficient of the coal reservoir. Methane seepage in tectonic coal has a longer control time than in intact coal at different initial pressures, initial permeabilities, and initial diffusion coefficients of the coal reservoir. For different coal reservoir initial pressures, coal reservoir initial permeabilities, and coal reservoir initial diffusion coefficients, the maximum seepage control time for tectonic coal is 20, 17, and 15 times longer than for intact coal, respectively. Finally, the discrepancies of methane dominant flow in tectonic coal and intact coal during methane extraction were analyzed by using the double bottleneck flow model, and methods for methane enhanced extraction in tectonic coal and intact coal were discussed. The results presented in this paper may provide a theoretical reference for the extraction of differentiated gas in coal seams.

Enrichment conditions and resource potential of coal-rock gas in Ordos Basin, NW China

To reveal the enrichment conditions and resource potential of coal-rock gas in the Ordos Basin, this paper presents a systematic research on the sedimentary environment, distribution, physical properties, reservoir characteristics, gas-bearing characteristics and gas accumulation play of deep coals. The results show that thick coals are widely distributed in the Carboniferous–Permian of the Ordos Basin. The main coal seams Carboniferous 5# and Permian 8# in the Carboniferous–Permian have strong hydrocarbon generation capacity and high thermal evolution degree, which provide abundant materials for the formation of coal-rock gas. Deep coal reservoirs have good physical properties, especially porosity and permeability. Coal seams Carboniferous 5# and Permian 8# exhibit the average porosity of 4.1% and 6.4%, and the average permeability of 8.7×10−3 μm2 and 15.7×10−3 μm2, respectively. Cleats and fissures are developed in the coals, and together with the micropores, constitute the main storage space. With the increase of evolution degree, the micropore volume tends to increase. The development degree of cleats and fissures has a great impact on permeability. The coal reservoirs and their industrial compositions exhibit significantly heterogeneous distribution in the vertical direction. The bright coal seam, which is in the middle and upper section, less affected by ash filling compared with the lower section, and contains well-developed pores and fissures, is a high-quality reservoir interval. The deep coals present good gas-bearing characteristics in Ordos Basin, with the gas content of 7.5–20.0 m3/t, and the proportion of free gas (greater than 10%, mostly 11.0%–55.1%) in coal-rock gas significantly higher than that in shallow coals. The enrichment degree of free gas in deep coals is controlled by the number of macropores and microfractures. The coal rock pressure testing shows that the coal-limestone and coal-mudstone combinations for gas accumulation have good sealing capacity, and the mudstone/limestone (roof)-coal-mudstone (floor) combination generally indicates high coal-rock gas values. The coal-rock gas resources in the Ordos Basin were preliminarily estimated by the volume method to be 22.38×1012 m3, and the main coal-rock gas prospects in the Ordos Basin were defined. In the central-east of the Ordos Basin, Wushenqi, Hengshan–Suide, Yan'an, Zichang, and Yichuan are coal-rock gas prospects for the coal seam #8 of the Benxi Formation, and Linxian West, Mizhi, Yichuan–Huangling, Yulin, and Wushenqi–Hengshan are coal-rock gas prospects for the coal seam #5 of the Shanxi Formation, which are expected to become new areas for increased gas reserves and production.

Numerical analysis of water injection in coal seams under mining influence

This study investigates the diffusion characteristics and optimization of borehole water injection in the 3# coal seam of Licun Coal Mine under mining influence. The distribution of stress and permeability ahead of working face using FLAC3D combined COMSOL Multiphysics. The influence of distances between the borehole along with the diameters of the boreholes on the stress distribution around the borehole is studied. Additionally, the diffusion characteristics of coal seam water injection are analyzed under various parameters. The study found that borehole proximity to the working face significantly enhances stress relief and permeability, while borehole diameter has a minor impact. Simulation results from COMSOL Multiphysics indicated that increasing injection pressure significantly improves seepage velocity and wetting range, though borehole diameter effects remain negligible. Optimal water injection parameters were identified, suggesting that the most effective water injection occurs at a borehole 3 m from the working face, with a diameter of 0.11 m and an injection pressure of 10 MPa, achieving saturation within 48 h. This research contributes to the theoretical and practical understanding of optimizing borehole water injection in complex geological settings and thick coal seams.