Coal Mine Goaf Research Articles

Effective 3D-transient electromagnetic inversion using finite-element method with a parallel direct solver

The correct interpretation of time-domain or transient electromagnetic (TEM) data relies on effective inversion techniques. Herein, we have developed an effective 3D-TEM inversion method based on the vector finite-element method. The domain was discretized with unstructured tetrahedra to simulate complex geologic structures. We develop a second-order backward-Euler scheme, rigorously with nonuniform time-step sizes, to approximate the time derivative of the electric field. The waveform was directly simulated and considered in the forward modeling and inversion. In a 3D inversion, the most time-consuming element of the workflow is solving a large system of linear equations. To alleviate this problem, we adopt a parallel direct solver for solving a large system of equations involved in forward modeling and sensitivity calculations. We use the Gauss-Newton approach to solve the inverse problem and formulate the least-squares problem to obtain model updates. We avoided the explicit sensitivity calculation by solving the matrix-vector multiplication between the sensitivity matrix (or its transpose) and vector. The developed inversion algorithm is validated using several realistic synthetic models. Finally, we apply the method to practical TEM data with complex topography for coal mine goaf detection. We thus obtain a detailed 3D resistivity model that revealed the layering structure and some localized anomalies related to the coal mine goaf.

Disaster-Causing Mechanism of Hidden Disaster-Causing Factors of Major and Extraordinarily Serious Gas Explosion Accidents in Coal Mine Goafs

Hidden disaster-causing factors (HDCFs) in coal mines can be identified via the rerefinement and classification of disaster-causing factors (DCFs) in coal mines. The study of the disaster-causing mechanism of accidents from the perspective of HDCFs in coal mines could be helpful to analyze the accident occurrence process from a new perspective, and new ideas for accident prevention and control could then be proposed. To clarify the disaster-causing mechanism of HDCFs of major and extraordinarily serious gas explosion accidents (MESGEAs) in coal mine goafs, 32 MESGEAs in coal mine goafs in China from 2000 to 2021 were adopted as a data source. By redefining the definition, connotation and characteristics of HDCFs in coal mines, 10 HDCFs were identified. Consequently, an improved decision-making trial and evaluation laboratory (DEMATEL)-interpretive structural model (ISM)-matrix of cross impact multiplications applied to classification (MICMAC) model was used to comprehensively analyze HDCFs in 3 aspects, including the centrality and cause degrees, hierarchical structure, and driving and dependence powers, from a completely objective perspective. The results demonstrated that (1) the considered MESGEAs in coal mine goafs were caused by DCFs in the management aspect by affecting the DCFs in the 3 aspects of human factors, equipment and environment, as well as under the combined effect of DCFs internal interaction contained in itself. (2) There were 2 types of disaster-causing mechanisms of HDCFs of MESGEAs in coal mine goafs: (a) the indirect disaster-causing by HDCFs in the management aspect and (b) the random coupling disaster-causing by HDCFs in human factors, equipment and environment 3 aspects.

Heterogeneous rock mass detection in the Goaf underlying the Yingbin substation

The existence of heterogeneous rock mass in the goaf easily causes surface deformation, subsidence, and even collapse, which restricts the development of urbanization of the buildings above the goaf. Because of the saturation of the city and the shortage of power supply, the idea of building the Yingbin substation was proposed on the old coal mine goaf in Xinhua District, Pingdingshan City, Henan Province, China. To ensure the safety and stability of the foundation of the proposed substation, the comprehensive detection method was proposed, which is mainly based on transient electromagnetic detection and supplemented via drilling detection. It was used to determine the spatial position and location of heterogeneous rock mass in the goaf underlying the proposed substation. The results show that 1) after eliminating the interference to the transient electromagnetic detection, it is found that there are low-resistance anomalies near the depth of −160 m at points 1 to 2 and point 8 of the X2 line and at points 1 to 3 of the Y3 line, which are preliminarily presumed to be the influence of the heterogeneous rock mass. 2) The apparent resistivity anomalous areas of −160 and −200 m depth are highly consistent, and the location and scope of the heterogeneous rock mass were basically determined. 3) The abnormal depth range of core rate obtained by drilling is in good agreement with the abnormal range of apparent resistivity at the same location obtained by transient electromagnetic, which indicates the reliability of the detection results. 4) Combining the detection results of the two methods, the location and scope of the heterogeneous rock mass were basically determined, which is located in the western part of the proposed area. The detection results can provide a basis for the safety and stability of the proposed substation foundation. To ensure the safe and stable operation of the proposed substation foundation, effective governance measures for the heterogeneous rock mass of the goaf should be taken.

Optimization of Numerical Simulation Algorithm for Spontaneous Combustion in Goaf via a Compression Storage and Solution Method of Coefficient Matrix

In coal mine engineering, numerical software is used to analyze the behavior of coal rock damage and fluid migration. The order of the coefficient matrix used in numerical calculations is increasing, and this increases the computation steps in obtaining the coefficient matrix solution. The storage and solution of the coefficient matrix are key factors influencing the efficiency of the numerical software. Therefore, to save storage space and reduce the computation steps, the coefficient matrix must be effectively compressed and stored. In this work, the structural characteristics of different coefficient matrices are analyzed in detail, and we find that for different computational regions, as long as the nodes are numbered according to certain rules, the corresponding coefficient matrices will have similar structural characteristics. The nonzero elements are symmetrically distributed in the diagonal band, and all the elements on both sides outside the band are zero. Based on this, the coefficient matrix is compressed by a pivoting scheme, and the compressed matrix is directly eliminated by dislocation Gaussian elimination. Thus, a compressed storage method that integrates the compression and solution of the coefficient matrix is established. The compressed storage and calculation module is incorporated into our self-developed simulation software COMBUSS-3D to simulate the evolution of the temperature field in the goaf of Luling Coal Mine. Compared with the conventional method, the compressed storage module can significantly improve the computing rate of the simulation, by approximately 80%.

Discussion on Research Status and Development Trend of Fire Prevention and Extinguishing Technology in Coal Mine Goaf

Discussion on Research Status and Development Trend of Fire Prevention and Extinguishing Technology in Coal Mine Goaf

Numerical Investigation of Safety Strategy for Gas Disaster Prevention in Successive Panels Using Upper Protective Layer Mining: A Case Study

Mine gas disasters are a major safety concern in underground coal mining. Protective layer mining is widely used in gas disaster control, but there are limited theoretical and experimental results that can provide guidance for site-specific mining circumstances. Taking the Xinji No. 1 mine as an example, gas disaster treatments were conducted in a new panel with overlying goaf located 85 m above the coal mine and adjacent goaf located at 30 m intervals. This study involved a comprehensive investigation, which included four steps: the selection of the first mining face, gas control and prevention, tracking and investigation, and effect analysis and assessment. The safety strategy focused on gas control planning in new mining areas or panels. The distribution and evolution characteristics of the stress, the gas permeability coefficient and the deformation volume within the protected layer were determined by numerical simulation. The coal deformation, gas emission and extraction effect were analyzed by field observation. The deformation and gas permeability of the coal seam were consistent with the stress evolution, for which the maximum compressional and expansional deformation of 6-1 coal were 18‰ and 28‰, respectively. Gas disaster control and prevention treatment of the mining face produced a significant protective effect on the underlying No. 6-1 coal seam. This work is beneficial for the planning of gas control in successive panels.

Characteristics and main factors of foam flow in broken rock mass in coal mine goaf.

To protect the environment and reduce the occurrence of coal mine fire, foam injection in goafs is an effective measure for preventing and extinguishing mine fires. The flow characteristics of foams injected into goafs have a significant impact on the prevention and extinguishment of such fires. To study the flow characteristics of foam injected into a goaf, we first independently constructed a set of experimental platforms for the visualization of goafs. Next, we performed physical experiments on foam injection using similarity theory. Flow characteristics were simulated under different foam concentrations, flow rates, and goaf porosities. The exponential function was found to provide a good fit to the trajectory of the foam's stacking edge in the goaf. According to the foam injection volume, the trend of the fitting equation parameter a could be divided into two stages. The first stage was the rapidly decreasing stage, and the second stage was the stable stage. It was inferred that the stacking height and diffusion radius of the foam under different conditions were related to the speed of liquid film drainage. The results of this study can provide a valuable reference for the use of fire prevention and extinguishment technology in the goaf.

Underground Hydro-Pumped Energy Storage Using Coal Mine Goafs: System Performance Analysis and a Case Study for China

In response to the Paris climate agreement, the Chinese government has taken actions to improve the energy structure by reducing the share of coal-fired thermal power and increasing the use of clean energy. However, due to the extreme shortage of large-scale energy storage facilities, the utilization efficiency of wind and solar power remains low. This paper proposes to use abandoned coal mine goafs serving as large-scale pumped hydro storage (PHS) reservoir. In this paper, suitability of coal mine goafs as PHS underground reservoirs was analyzed with respects to the storage capacity, usable capacity, and ventilation between goaf and outside. The storage capacity is 1.97 × 106 m3 for a typical mining area with an extent of 3 × 5 km2 and a coal seam thickness of 6 m. A typical goaf-PHS system with the energy type αw=0.74 has a performance of 82.8% in the case of annual operation, able to regulate solar-wind energy with an average value of 275 kW. The performance of the proposed goaf-PHS system was analyzed based on the reservoir estimation and meteorological information from a typical region in China. It has been found that using abandoned coal mine goafs to develop PHS plants is technically feasible in wind and solar-rich northwestern and southwestern China.

Influence of airflow movement on methane migration in coal mine goafs with spontaneous coal combustion

Spontaneous coal combustion was proved to cause methane accumulation and methane explosions in coal mine goafs. To avoid possible methane explosions, the empirical engineering measure, ventilation dilution, is proposed in coal mines though its disaster prevention mechanism has not been well understood. Through experimental and numerical simulations, the superposition effect of the air leakage and coal combustion-induced chimney effect was studied to reveal disaster prevention effect of ventilation dilution. Research results show that the high temperature area of coal combustion is steady and can provide continuous buoyancy force to form upward airflow even under ventilation dilution; the drifting methane accumulation is observed under the superposition effect of air leakage and upward airflow; ventilation dilution can weaken and even eliminate methane accumulation by overcoming chimney effect, but an increase in coal combustion temperature will enhance the upward airflow of chimney effect to cause methane accumulation again. The competitive relationship between the coal combustion-induced chimney effect and air leakage provides a new insight to study methane migration for the disaster formation and prevention mechanism.

Effects of Confined Space Flow Fields on Explosion and Hazard Analysis

ABSTRACT In this paper, FLUENT was used to simulate the effect of inlet velocity on the flow field of a spherical closed device, a gas explosion under different gas flow field conditions in a spherical explosive device, and the gas explosion incident in the Babao coal mine goaf. Results suggest that when gas is not evenly mixed, high local pressure occurs and the reaction is unstable. Time taken for the reaction to reach a maximum pressure under uniform mixing is generally shorter than that under non-uniform mixing, and maximum temperature and pressures are typically higher than those under the condition of uniform mixing. Residual CH4 after the simulated explosion reaction under non-uniform mixing increases with an increase in CH4 concentration. Higher ignition energy levels also result in the maximum explosion pressure being closer to the constant volume explosion pressure. Higher initial temperature results in a smaller maximum explosion pressure, and higher initial pressure results in higher maximum explosion pressure and temperature. Findings from the Baboa coal mine indicate that greater air leakage speeds resulted in a larger air leakage area in the goaf; the area of the low concentration gas distribution area was proportional to the air leakage speed. As air leakage leads to a decrease in gas concentration and an uneven distribution of mixed gas in the goaf, providing the precondition for combustion and explosion, it also expanded the intensity and propagation range of the gas explosion, potentially accounting for casualties recorded in the Babao coal mine goaf incident.

Characteristics of Airflow Migration in Goafs under the Roof-Cutting and Pressure-Releasing Mode and the Traditional Longwall Mining Mode.

The airflow exchange between a mining face and a coal mine goaf can cause gas transfinite and spontaneous coal combustion disasters, threatening coal mining. Studying the characteristics of airflow movement in a goaf forms the basis to prevent airflow exchange for coal mining safety. Different from the traditional longwall mining mode, the roof-cutting and pressure-releasing mining mode shows new roof collapse characteristics and a ventilation system, which lead to obvious changes in the characteristics of airflow movements in coal mine goafs. To study the differences in airflow movement characteristics and the airflow disturbance influence area in a coal mine goaf between these two mining modes, the airflow movements in different goafs are compared using a numerical simulation method based on the measured parameters of the 1201 mining face in the Halagou Coal Mine, China. The results show that the airflow disturbance area in the goaf under the traditional longwall mining mode is a “η” type. Along the inclination direction of the mining face, two main exchange areas for the airflow are located in the 0–5 and 15–45 m sections, respectively. The airflow disturbance area in the goaf under the roof-cutting and pressure-releasing mining mode is a “hump” type, and there are six main exchange areas in the goaf under the roof-cutting and pressure-releasing mining mode. Along the inclination direction of the mining face, three exchange areas are located in the 0–25, 255–305, and 305–320 m sections, respectively. Along the strike direction, three exchange areas are located in the 5–25, 25–35, and 35–65 m sections, respectively. Based on the research results, sealing measures are taken to slow and eliminate airflow exchange in the goaf under the roof-cutting and pressure-releasing mining mode, and this provides theoretical guidance for safe coal mining.

Analysis and Practice of Detection Methods for Goafs in Complex Coal Mines

The structure of the coal mine goaf is complex and its stability is easily affected by many factors. The use of reasonable detection methods to obtain basic data on the development characteristics of the overburden cavities, cracks and separations in the goaf is very important for the development and construction of the goaf. Meaning. The article summarizes and analyzes the currently commonly used goaf detection methods, and compares their applicability and advantages and disadvantages. Finally, the combination of engineering examples verifies the use of drilling core, drilling TV camera technology, and geophysical logging. The method provides a certain reference for the design and construction of the goaf detection project for the reliability of the goaf detection.

Study on Multi-field Evolution and Influencing Factors of Coal Spontaneous Combustion in Goaf

ABSTRACT The coupled disaster of coal spontaneous combustion and gas explosion in the goaf threatens coal mine safety production. In order to further study the mechanism of coupled disasters and provide theoretical basis for disaster prevention and control, the multi-field evolutions and the key influencing factors of coal spontaneous combustion within methane gas condition of coal mine goaf had been investigated based on the special designed hot experimental platform and numerical simulations. Results show that the leakages of ventilation air into the goaf is positively related to the intensity of coal spontaneous combustion, but that is negatively related to the ranges of methane accumulations in the goaf. During the early stage of coal spontaneous combustion, the distributions of oxygen and methane in the goaf is less affected by the temperatures. When the temperature of coal spontaneous combustion exceeds 200°C, the oxygen-poor area, methane accumulation areas and a banded high-temperature distributions area in the non-spontaneous combustion areas would be clearly formed in the goaf. The changes of gas density in the goaf indicates that coal spontaneous combustion temperature in the goaf is another important factor influencing the change of gas density and gas accumulations, in addition to the ventilation air leakages.

Technology and Performance Analysis of Preventing Surface Subsidence by Sealing and Backfilling the Goaf with Urban Waste

In order to solve the current two major environmental problems of urban waste pollution and surface subsidence above the coal mine goaf, the technology of sealing and backfilling the goaf with urban waste is proposed. This technology takes "waste disposal" and "goaf backfilling" as the theme. Aiming at the processing of loose waste, the waste drying and briquetting integrated machine is designed, the entire processing technology ensures the strength of the loose waste materials and no filtrate. For the processing of solid aggregate waste, it is crushed into recycled concrete raw materials for goaf backfilling. Aiming at the goaf backfilling, the goaf backfilling coal mining method is designed, the backfilling hydraulic support is modified, and the backfilling mechanism is added to realize the integration of “mining-backfilling” in the working face. The backfilling technology embodies the key issues of the sealing of the backfilling space and the strength of the backfilling body with "three times of sealing" and "three times of strengthening". Using urban waste as the backfilling materials for the goal area, the waste pollution and the surface subsidence are turned into “two benefits”, and the construction of waste-free cities and green mines is promoted.

Physical test of fracture development in the overburden strata above the goaf and diffusion process of permeable grout slurry

The massive mechanized and irrational coal mining has generated many goafs (i.e., mined-out areas) in China. To manage these goafs effectively, grouting technique has been commonly used to fill the fractures in the overburden strata. However, little is known about the development and distribution of fractures in the overburden strata above the goaf. The lab-scale simulation of fracture development, as well as the grouting process, can provide valuable information for managing a real goaf by the grouting technique. Based on the mechanical properties of rocks and soils in the goaf of Kangjiagou coal mine in Shanxi Province, an experimental system was designed to simulate the development of fractures in the overburden strata above the goaf and study the diffusion behaviors of the grout slurry under different conditions (such as grout volume, grouting pressure, porosity, and water-solid ratio). The physical model test showed that more fractures were found in the fractured and caved zones, with fracture porosity ranging from 15.05 to 40.79%. The grouting experiment showed that, when the water-solid ratio was smaller than 1.15 and the grouting pressure was lower than 0.3 MPa, the changes in diffusion radius were stable during the grouting process. To maintain the stable changes in diffusion radius and reach a high filling ratio, the grouting pressure should be controlled in the range of 0.2–0.4 MPa at the porosity of 24.8% and 0.5 MPa at the porosity of 35.1%. The findings derived from this study are widely applicable under real engineering contexts.

Coupling Relation between the Location of Cross-Cut Negative Pressure and Injecting Nitrogen into Coal Mine Goaf.

Injecting nitrogen into goaf has been widely adopted for preventing fire hazards in coal mines. In this paper, the coupling relation between different locations of negative pressure of cross-cut drainage and nitrogen injection was investigated. The minefield data collection was carried out by an in situ beam tube system on the intake airway and return airway of the mine goaf. The validated Computational Fluid Dynamics (CFD) model that was secondarily modified by on-site collected data was applied for further research. It is demonstrated that the area of the spontaneous combustion zone generally shows a sharp decline first, then tends to stabilize, and finally has a slight drop and rise with the increasing nitrogen injection time. It is obvious that the location of the negative pressure of cross-cut exerts a significant influence on the optimal nitrogen injection location and time. When the cross-cut is located in the center of the air leakage zone, spontaneous combustion zone, and asphyxiation zone of goaf, the optimal nitrogen injection location and time correspond to the P2 (25 m, 1200 min), P3 (30 m, 120 min), and P4 (35m, 1800 min), respectively. According to the simulation result, the specific relation between the optimal nitrogen injection point N(x) and the distance from the working distance of the cross-cut (x) by Newton interpolation polynomial analysis was figured out and verified that N(x) = 24.70808 + 0.293356x – 0.001436x2. It is hoped that the result can provide scientific guidance for coal mine fire prevention and control with nitrogen injection.

In situ measurement of water accumulation in overlying goaf of coal mine-a transient electromagnetic-based study

Water accumulation detection in mining areas is the primary technical means for preventing mine water disasters. To study the distribution of water accumulation in the goafs of the Xingnong coal mine in Jixi, the present paper systematically analysed the water accumulation regularities in the goafs based on the applied and theoretical studies using transient electromagnetic (TEM) methods for the detection of goaf water accumulation characteristics. Because of the high sensitivity of transient electromagnets to low resistance and water, the water body distribution in the goafs is determined by the exhibition of low resistance by the water bodies. Furthermore, the water-bearing anomalies are located depending on the conductivity difference among media. The analysis of the in situ measurements for the Xingwang coal mine revealed four water-rich goaf anomalous zones within the detection area, of which one was located in the shallower part and three were located in the deeper part. The water-retention capacity of the fault zones was analysed, with more reliable degree of control attained by analysing a fault with a large throw. A broad area of highly distinct low-resistance anomalies appeared in the vicinity of the fault. The suspicious water-containing anomalies were verified through field drilling experiment, proving the reliability of TEM in detecting the water distribution in the coal mine goafs.

Permeability and inertial resistance coefficient correction model of broken rocks in coal mine goaf

Goafs are composed of large, irregularly shaped broken rocks. This means in order to calculate the permeability and inertial resistance coefficient of a goaf, the Ergun equation must be modified. The coupling relationships between coefficients A and B, and particle size and porosity are established based on the existing simulation research. Coefficients A and B both have a quadratic polynomial relationship with porosity. The quadratic polynomial's constant term ratio has a good power function relationship with particle size, while the linear term ratio and quadratic term ratio both have good exponential function relationships with particle size. By introducing the shape factor of permeability and inertial resistance coefficient, which are calculated by dividing the experimental value with the theoretical value, the correction model is established for the permeability and inertial resistance coefficient of the goaf. The permeability and inertial resistance coefficient shape factor have good power function relationships with particle size.

Discharge and ignition characteristics from indentation fracture of coal mine roof

Electric effects are generated during the deformation and fracture of coal mine roof sandstones. Previous research has mostly focused on the mechanism of electric effect generation and the electromagnetic radiation released. However, few studies have investigated the hazard characteristics of the high-energy electrons released. This study explores the discharge and ignition characteristics of roof indentation fracture to identify the real ignition source of a gas explosion in a coal mine goaf. Theoretical and experimental analyses conclude that the charge of the coal mine roof sandstone will gradually escape from the surface during the loading process. The charge release process of the rock before fracture is related to the loading rate, and an instantaneous high voltage will be formed during fracture of the rock. The compressive strength of the rock plays a key role in this type of instantaneous discharge process. An electric spark can be generated at the moment of rock fracture under loading. The high-energy electrons released by rock deformation and rupture can ionise the air. Moreover, the electric spark can persist for more than 20 ms, which considerably exceeds the induction period for gas ignition. When the quartz content of the roof rock is high and reaches a certain compressive strength, stress mutations can cause the roof to undergo instantaneous rupture and generate electric sparks. The premixed gas air gathered in the area is thus broken down and ionised, igniting the gas and producing a gas explosion.

Research and Exploration of Water Storage Law in Goaf of Merger and Reorganization Coal Mine Based on Computer

With the application of modern technology represented by computer, in recent years, the probability of coal mine safety production accidents has been declining. However, the current coal mine water disasters still occur, and a considerable proportion of accidents are caused by water inrush from goaf. The destructive characteristics of coal mine goaf water make it necessary to carry out the relevant research on the occurrence law of coalmine goaf water. Based on this, this paper first analyses the characteristics of water disaster in the goaf of merger and reorganization coal mine, then studies the computer-based investigation and exploration mode of water accumulation in the goaf of merger and reorganization coal mine, and finally puts forward the prevention and control strategy of water accumulation in the goaf of merger and reorganization coal mine based on computer.