Coal Outburst Research Articles

Early warning of coal and gas outburst based on abnormal gas emission

Taking the dynamic time series data of gas emission in mining face as the research object, the early warning model of coal and gas outburst was established based on single-time gas emission information function. Based on the data of 21118 heading face before outburst of Panyi Mine in Huainan, the single-time gas emission information function diagram was drawn, named as G-line diagram in short. The result showed that during normal production period, the entity of G-line diagram was small, which was close to a stable horizontal line. And before the outburst, the G-line diagram showed an upward trend. The negative and positive entity of G-line diagram became larger. At the same time, there were many times positive lines in the process of rising. According to the different shape, colour, length and other characteristics of G-line diagram, the change trend of coal body state in front of working face can be judged. Based on that, the outburst symptoms in the incubation stage of coal and gas outburst can be identified, and the early warning of outburst can be realized. It is of great significance to ensure the safety of mine production.

Hypothesis about physical-chemical nature of gas-dynamic phenomena in coal mines

Practical experience in the development of a gas-saturated coal-rock massif, as well as the results of scientific research, point to the imperfection of the existing ideas regarding the processes in the extremely stressed coal-rock massif, mechanisms of the occurrence and behavior of gas-dynamic phenomena (GDP). The conducted analysis for the problem state regarding the nature and mechanism of sudden outbursts of coal, rocks and gas, in general, confirm that the outburst hazard was due to the complex interaction of three main natural factors (stress state, gas factor and physical-mechanical properties of rocks) with technological ones, and the formation of outburst hazardous conditions occurs in geological environments that are characterized by certain properties and conditions. The purpose of the paper is to obtain new theoretical understanding of the GDP nature with taking into account geological conditions and the impact of external factors by developing a hypothesis regarding the physicochemical nature of the GDP. It is proved that relief of stresses arisen in the molecular structure under the impact of geological factors leads to the activation of relaxation processes in the substance. The relaxation of the accumulated energy system results in structural transformations with the release of low-molecular-weight compounds. According to this pattern, accumulation of gases genetically connected with fossil organic matter and the emergence of gas-dynamic phenomena.is formed in the coal-rock massif. The key point of the hypothesis is that partial stress relief of the coal-rock massif creates in the fractured zones of the coal bed conditions for relaxation of elastic energy accumulated in the molecular structure of coal, hence, promoting the activation of structural transformations by the free-radical mechanism, which results in the release of low-molecular-weight compounds (methane, in particular). Under appropriate conditions, in the zone of relaxation of elastic deformations, the process can assume an avalanche character, namely, initiate sudden outbursts of coal and gas. According to the physicochemical model of methane release in coal under the impact of geomechanical, geological, and technogenic factors, which is proposed within the framework of this work, partial discharge creates in the coal bed conditions for relaxation of the elastic energy accumulated in the molecular structure of coal. That is, it promotes the activation of structural transformations by the free-radical mechanism, the result of which is the release of low-molecular-weight compounds (methane). Under the condition of sufficiently high fracturing of the elastic deformation zone, the process acquires an avalanche character, that is, sudden outbursts of coal and gas are developed. Keywords: coal, coal mines, gas-dynamic phenomena, outbursts of coal and gas, stress-strain state.

A Multiscale Structural Analysis of Soft and Hard Coal Deposits in Deep High‐Gas Coal Seams

In recent years, with the increases in coal mining depths, the risk of coal seam outburst occurrences has increased. Therefore, it is of major significance to study the multiscale structures of soft and hard coal deposits in order to prevent and control the coal and gas outbursts. In this research investigation, soft and hard coal multiscale structures were comprehensively examined using various laboratory methods. The results revealed the following: (1) From a macrostructural aspect, the physical and mechanical properties of the soft coal were weaker than those of the hard coal. It was found that the majority of the examined specimens were characterized by scaly structures without blocks larger than 50 mm. The hard coal was observed to be mainly massive with only a small part being clastic. Therefore, the structural characteristics were considered to be stable. (2) From a microstructural perspective, the surfaces of the soft coal specimens were observed to be rough. The pores were found to be more developed, with the edge of pores being mainly hackly. At the same time, fractures were also relatively developed, showing good connectivity. (3) From a micropore structural perspective, it was found that the BET‐specific surface areas and BJH‐specific surface areas of the soft coal specimens were higher than those of the hard coal specimens, which indicated that the gas adsorption and diffusion migration abilities of the soft coal were greater than those of the hard coal. (4) It was suggested from the study results that the ventilation and gas extraction processes should be strengthened in the mining activities of coal seams with high, soft stratification content. At the same time, the methods used for water injection modification should be enhanced in order to improve the mechanical stability of soft coal. Consequently, the instantaneous released gases will be decelerated, and the occurrences of coal and gas outburst events in mine working faces can be prevented.

Identification of fracture zones at the drill sample recovery in the rock mass

The purpose of the work was to develop a new method for identifying fracture zones in a coal-rock mass. The main task of the study was to identnify the fracture zones in the studied sector of the mine field and give them a characteristic. The method is based on the collection and processing of information on geological sections for the investigated section of the mine field. Selection of fracture zones is made by core. For the selected stratigraphic interval, the dimensionless fracture factor is calculated for each well, on the basis of which a predictive fracture map of the investigated area is constructed by the interpolation method. The author proposes a new method for identifying of fracture zones in the coal-rock mass, which was compared with the previously approved one. The given research data showed a high convergence of the results. Application of the method will make it possible to promptly and with a certain degree of probability predict fracture zones in a coal-rock massif, all other things being equal. The use of this method will make it possible to predict fracture zones that may be associated with the release of methane into mine workings, spontaneous combustion and a number of other phenomena, which, in turn, can adversely affect the increase of gas content in mining workings, manifestations of rock and coal outbursts during mining the coal bed. Thus, the use of this method will allow to improve safety of work. Keywords: fracturing, well, rock mass, interval, gas release.

Deflection Laws of Gas Drainage Boreholes in Interbedded Soft and Hard Seams: A Case Study at Xinzheng Coal Mine, China

Coal and gas outbursts can lead to serious disasters in coal mines. The drilling of boreholes to predrain the gas is an effective measure for preventing such accidents. However, due to the complexity of the geological situation, the drilling trajectory often deviates from the design trajectory, resulting in poor gas extraction. To solve the problem of gas drainage borehole deflection, an analytic hierarchy process (AHP) model is established based on geological factors, technical factors, and human factors. The AHP model is used to rank the weights of various influencing factors, and the analysis is combined with a drilling model and engineering examples. Finally, the results show that soft and hard interlayers are the most important factors affecting the deflection of the borehole. The rock drilling model is mainly affected by the formation forces. The regularity of the change in the azimuth angle during drilling is not obvious when the angle of the encountered layer is less than some critical value. When the borehole is skewed downward, the deflection angle ranges from 0 to 4°, and the deflection of the borehole occurs mainly at the interface of the rock layers. When the angle of the encountered layer is greater than the critical value, the borehole is skewed upward, with a deflection angle of 0–6°, and the deflection occurs at the rock interface. The trajectory curve obtained by theoretical predictions from field data is found to be consistent with that of an actual project.

Experimental Study on Intensity and Energy Evolution of Deep Coal and Gas Outburst

Experimental Study on Intensity and Energy Evolution of Deep Coal and Gas Outburst

On methods of categorizing the outburst-hazardous of coal seams

Abstract. Having analyzed the existing methods for categorizing the outburst hazard of coal seams the author worked out the proposals on improving the methodology for categorizing the outburst hazard of coal seams during their mining. The research methodology includes analysis of existing methods for categorizing of prone-to-outburst coal seams depending on the depth and speed of mining operations driving. It is established that the regulatory document on categorization of the prone-to-outburst coal seams needs to be updated in order to take into account possible manifestation of not only sudden outbursts of coal and gas, but also other known types of gas-dynamic phenomena and to specify the main criterion features and indicators, which should be determined quite simply and without significant time costs. Categorization of several coal seams was assessed: in some cases it depends on the grade of the seam (volatile content), but not always on the indicator M. An obvious defect of the obtained dependencies was found in the regulatory document (perhaps the reason is the lack of necessary data on the study of natural gas content and other properties and conditions for the manifestation of sudden outbursts of coal and gas). Thus, with indicators М = 24,5‑26,2 and М = 23,7‑27,6, critical value of natural gas content and the depth of prediction are the same. The authors of the regulatory document give no explanation to this fact as well as how and why, when determining the indicator for different dependencies, the same criteria of outburst hazard are obtained. The comprehensive studies on assessing a gas-dynamic state of potential prone-to-outburst zones in the threatened formations reveals that with greater depth of mining operations these zones should be timely transferred to the category of prone-to-outburst ones before the first outburst happens. In the paper, methods for referring the coal seams to the outburst hazard categories are discussed. It is concluded that the normative document needs to be revised in terms of categorization of prone-to-outburst coal seams with taking into account a more complete classification of all types of gas-dynamic phenomena. Keywords: categorization, outburst hazard, coal seam, sudden outbursts, normative document.

Moisture Content on Methane Desorption Characteristics in Coal and Its Effect on Outburst Prediction

Facts have proved that coal and gas outbursts require rapid desorption in a short period of time. Due to the limitation of experimental conditions, the methane desorption characteristics in the first few seconds (0–60 s) in coals with different moisture contents have not been fully studied at present. In this article, the initial desorption characteristics of methane in coals with different moisture contents were investigated using a self‐developed experimental setup. In order to collect enough methane pressure data for analysis and calculation, a self‐developed real‐time data acquisition system with a time interval of about 10 ms was used in the experiment to calculate the initial gas desorption amount and the initial gas velocity diffusion index (ΔP). Experiments show that coal with low water content and methane outburst is more dangerous than coal with high water content and outburst; and the degree of outburst of coal and methane decreases exponentially with the increase of moisture content.

Effect of Low Gas Pressure on the Physical Properties of Outburst Coal during Uniaxial Compression

Effect of Low Gas Pressure on the Physical Properties of Outburst Coal during Uniaxial Compression

Research and Application of Hydraulic Punching Pressure Relief Antireflection Mechanism in Deep “Three‐Soft” Outburst Coal Seam

To solve the problems of gas predrainage in deep seams with “three softs” and low‐air permeability, hydraulic punching pressure relief antireflection technology is proposed on the basis of the research background of gas predrainage technology in Lugou Mine to alleviate technical problems, such as low gas drainage efficiency, in this mine. Through the analysis of the mechanism of hydraulic punching and coal breaking, combined with FLAC3D software, a hydraulic punching pressure relief antireflection model is established. Then, the fracture radii of coal rock are simulated and calculated. The results show that, under hydraulic punching with a water pressure of 10 MPa and coal outputs of 3 m3, 6 m3, 9 m3, and 12 m3, the fracture radii of coal and rock are 3.4 m, 4.8 m, 5.5 m, and 5.9 m, respectively. Using the software to fit the relationship between coal output V and hydraulic punching fracture radius R under the same water pressure, R = 2.32479 V0.3839 is obtained. The field test is carried out in the bottom drainage roadway of 32141 in Lugou Mine. The application effect is as follows: the gas concentration of hydraulic punching with a coal output of 3 m3 is twice that of ordinary drilling, and the coal output of hydraulic punching with a coal output of 6 m3 is four times that of ordinary drilling. The extraction concentration is four times that of ordinary drilling, and the extraction concentration of hydraulic punching with a coal output of 9 m3 is 6.4 times that of ordinary drilling. Combining the results of the numerical simulation and taking into account the actual construction situation on site, the coal output of water jetting from the borehole is 9 m3, and the fracture radius is 5.5 m. This outcome means that the effective half radius is 5.5 m, and the borehole spacing is 7.7 m. These values are the construction parameters for large‐scale applications. This proposal provides effective technology and equipment for gas drainage in the deep three‐soft coal seam. Consequently, it has promotion and reference significance for gas drainage in coal seam of the same geological type.

Numerical Simulation and Theoretical Study of the Effect of Fracturing Strata on Multi-Stage Fracturing in the Roof of an Outburst Coal Seam

ABSTRACT Multi-stage fracturing is one of the key techniques that use boreholes instead of tunnels and can effectively transform a coalbed methane (CBM) reservoir and increase its permeability. However, due to the lack of systematic and in-depth research on the use of multi-stage fracturing for gas drainage, the development of multi-stage fracturing is slow, and there are many problems, which to some extent restrict the application of multi-stage fracturing in low permeable outburst coal seams. In this study, the No. 21 coal seam in the Zhongmacun Coal Mine was taken as the research object, and the lithologies of the roof and floor of the No. 21 coal seam and the in-situ stress were tested. Then, three types of fracturing strata were numerically simulated to study the effect of fracturing strata on the fracture initiation, propagation and morphological characteristics induced by multi-stage fracturing in the roof of an outburst coal seam. The influence of the brittleness index of fractured strata on the formation of a fracture network was analyzed theoretically. The results indicate that the sequence in decreasing order of the fracturing range and failure mode is the medium sandstone model, the sandy mudstone model, and the mudstone model. It is useful to form a fracture network when the fractured strata in the roof of the outburst coal seam is a medium sandstone or siltstone. By comparing the brittleness index of the roof and floor lithologies of the No.21 coal seam in the Zhongmacun Coal Mine, the proper strata for the multi-stage fracturing in the roof of the outburst coal seam was selected. The results of this study provide a theoretical basis and data support for selecting proper strata for multi-stage fracturing in the roof of an outburst coal seam.

Analysis of pressure relief and outburst prevention effect and mining depth effect of floor roadway in single serious outburst coal seam

In order to solve the serious problem of single serious outburst coal seam gas disaster, based on the gas geological conditions of Jiangxi Fengcheng Qujiang company, theoretical analysis, numerical simulation, field test and other comprehensive research methods were used to analyze the partition fracture law of roadway surrounding rock under different mining depth conditions. The results show that: under the conditions of high ground stress, high temperature and other environmental conditions, the radius of pressure relief loose circle of surrounding rock of roadway is significantly increased, and it is significantly larger than that of shallow part. In the deep area of - 800m elevation, the distance between floor roadway and coal seam is optimized to be 12 ~ 15m. The field test results show that the original permeability coefficient of overlying coal seam of floor roadway increases by 55.74 times, and the pressure relief effect is good.

Study on Plastic Zone Distribution Characteristic of Coal and Rock Mass in Excavation from Crosscut Coal

Coal and gas outburst is a serious disaster in the mining, and crosscut coal is the most prone to cause the coal and gas outburst. In order to study the mechanism of outburst, this paper studied the stress distribution characteristics in front of the working face in the process of rock roadway driving before implementing crosscut coal firstly. On this basis, the hypothesis of stress distribution on the “strong and soft face” of coal-rock mass was proposed. And by using the hypothesis, the mechanical equation for the parameter changes of the plastic zone in front of the working face in crosscut coal was established, and the width and morphological characteristics of the plastic zone in crosscut coal were analyzed. The results show that during the process, as the working face moves forward, the shape distribution of the plastic zone in front of the working face changes from the “semielliptical” to “semibutterfly,” And based on the distribution characteristics of plastic zone in working face in crosscut coal, modification proposals for outburst-prevention measures were proposed and provided technical support for the outburst-prevention measures and key parameters setting to prevent the coal outburst in crosscut coal.

Transition processes in coal field development in the light of intelligent control

Relevance of the work. The problem of sudden outbursts of coal and gas has been studied for more than a century and a half using various approaches to study this process. In order to prevent accidents and conduct safe mining operations. Today this problem remains relevant in the territory of the Russian Federation. In connection with the changed external and internal challenges of the development of coal mining at Russian mining enterprises, it is proposed to consider the problem in the light of two aspects, one of which is the geographical location of subsoil use objects and intelligent control. Objective. The work is devoted to the study of the development of transient processes at a coal mining enterprise from the standpoint of intellectual management. The task of the work is to create in the first approximation an industrial knowledge graph. Method and methodology of the work. The study is based on a problem-oriented approach to solving the issue of safe mining, namely, to thoroughly study the problem of sudden emissions of coal and gas before the release occurs. To solve the existing problem, it is proposed to create an industrial graph of knowledge, taking into account the peculiarities of geomechanical processes occurring in the mountain range. In the created industrial knowledge graph, the main classes are shown that contribute to the presentation and structuring of knowledge about the causes of sudden coal and gas outbursts for a particular coal seam being put into production. Results. Because of the study, based on the construction of an industrial knowledge graph, we can conclude that it is necessary to create a classification with geomechanically specific phenomena and their features. Study of normative and reference information and identification of semantic patterns in them. All these studies are complicated by the large volumes of data, their fragmentation, and in some cases the unreliability and inconsistency of information. When obtaining a semantically non-contradictory industrial graph of knowledge, knowledge can be “useful” for constructing a mathematical model for predicting sudden coal and gas emissions.

Coal permeability evolution during damage process under different mining layouts

ABSTRACT Coal permeability is akey component that affects gas migration. Considering seepage behavior of gas during mining is critical for improving the production profile of coal-bed methane (CBM) wells, and preventing coal and gas outburst. Aseries of seepage tests have been carried out to simulate the coal permeability evolution law during mining-induced actions. Coal damage has been assessed by using atri-axial servo seepage testing system. The results show that the coal permeability under PCM, Top-coal Caving Mining (TCM) and NM are all kept constant before the deviatoric stress reaches the peak strength, then increases mildly and finally increases rapidly. The effective stress decreases during confining pressure relief, and the permeability increases exponentially with the decrease of effective stress, which is quite different from the traditional triaxial compression test. According to continuum damage mechanics, adamage constitutive model of coal was proposed to characterize coal mechanical characteristics by mining-induced activities. Furthermore, according to the matchstick model, anew coal permeability model is proposed, which could accurately reflect seepage behavior under different mining layouts.

Особенности скважинной дегазации выбросоопасных угольных пластов

The conditions and features are given concerning the degassing of outburst-prone coal seams by wells drilled from the underground workings in the plane of the mined seam. Technological schemes for the location of wells in the mining areas during the preparation and development of the coal seams prone to sudden outbursts of coal and gas were substantiated taking into account the coal strength determined using the method of M.M. Protodyakonov. The scheme of coal seams degassing by wells oriented to the working face line is recommended with a coal strength factor f ≥ 1.3, the degassing scheme with crossed wells — at f < 1.3. The wells are drilled parallel to the working face line and oriented to it. The schemes are implemented in the shallow mines of Karaganda (Kazakhstan), Ukraine and China. Degassing of coal seams not unloaded from the rock pressure by crossing wells ensured an increase in methane production from 1 ton of coal by 0.4–4.8 m3 in the conditions of seams with different coal strength. The highest indicators were achieved in the powerful outburst-prone seams with a coal strength factor of f = 0.3–0.8 in the conditions of the mines of Karaganda Basin, where the volumes of methane extraction by crossing wells were 5.0–5.6 m3 per ton of the degassed coal reserves in the working areas. In the conditions of the outburst-prone shallow seams of Ukraine, the volumes of methane extraction from 1 ton of the degassed coal reserves on the medium density seams amounted to 2.5–4.4 m3 with a coal strength factor of f = 0.6–1.1. 2-2.3 m3 of methane was removed from 1 ton of coal at the Dziulishan mine (China) at a strongly crushed coal layer II1 with a strength factor f = 0.3–0.6. The use of degassing schemes for gas-bearing and outburst-prone coal seams ensures an increase in the productivity of working faces and, as a result, high economic indicators.

Study on Multi - index Nonlinear Quantitative and Computable Normalization Model of Coal and Gas Outburst

Study on Multi - index Nonlinear Quantitative and Computable Normalization Model of Coal and Gas Outburst

Synergetic nature of oil and gas reservoir behavior and its influence on the efficiency of opening and increase well productivity

The article continues the author’s series of works on the application of synergetic theory to understand and describe the processes and phenomena that accompany the search and development of mineral resources. The research results show that the synergetic theory is universal and allows to understand and describe a wide range of phenomena and processes that occur in geological exploration and mining: dynamic phenomena in mines (outbursts of rocks and coal), spontaneous сombustion of сoal in coal mines, emergency outbursts of oil and gas from exploration wells, destruction of high-pressure pipelines, geodynamic phenomena in quaternary deposits, sedimentary rocks, and in the geological structures of the crystalline basement. One of the signs of processes and phenomena that have a synergistic nature is the nonlinear nature of the system’s response to weak perturbations. Minor disturbances slightly change the structure of geological material, in some cases change the mechanical and physical properties of formation fiuids. This creates the conditions for disturbing the unstable equilibrium of the system at the bifurcation points and its transition to another equilibrium state with other thermodynamic parameters that characterize the new state of the system. Such transitions are characterized by kinetic and phase transitions. In particular, in the reservoir rock, in coal, micro- and macrocrack systems are formed and developed. The shape of dissipative structures is described using fractal theory. The duration of self-organization of dissipative structures and the transition of the system to a new equilibrium state depends on the type of dissipative structures, the properties of the environment in which they are formed, and the speed of physicochemical processes.
 The article considers the behavior of the system “well – rock formation” as an open thermodynamic system. The internal energy of the formation is compared with the energy of external explosive action from the well. Specific examples show that the behavior of such a system is subject to the laws of synergetics and as a result of weak action on the formation from the well there are significant nonlinear effects of productivity growth due to the internal energy of the formation. This energy is generated by reservoir and rock pressures.

Geometric Model for Physical Simulation System of Coal and Gas Outburst

The physical simulation system is an important means to study the mechanism of coal and gas outburst. The small- and medium-sized coal and gas outbursts account for the largest proportion in China. So, the paper focuses on small- and medium-sized coal and gas outbursts based on key physical rules and their statistical characteristics. According to the geometric parameters statistical data of more than 100 caverns, the proportion of outburst caverns whose depth is less than 5 m is 80%, and whose depth is more than 6 m is 13%. The ratio of depth and cavern outlet’s diameter is 2.58–7.31. There are statistical relationship of 2 times between coal’s and cavern’s volume, and between gas emission volume per ton and gas content in seam. Then, the cavern of coal and gas outburst is simplified as an ellipsoid, and the shape of cavern outlet is simplified as a circle. The cavern’s and the prototype’s geometrical parameters of small and medium coal and gas outbursts were deduced. The depth of cavern is 5 m, width is 4 m, and diameter of cavern outlet is 0.68–1.94 m. The depth of coal and outburst prototype is 12.3 m, width is 6.5 m, and diameter of cavern outlet is 0.68 m. In order to guarantee the movement similarity, according to the relationships among statistical data of outburst coal particle’s size, cavern outlet’s size, and height of tunnel, the minimum cavern outlet’s size was calculated. Select 10 times the coal particle’s size (5 mm) as the smallest size. Then, maximum geometric similarity ratio of coal and gas outburst was deduced, and it is 13.6. The minimum geometrical parameters of coal and outburst similar model were obtained. Its depth is 0.9 m, width is 0.48 m, and diameter of cavern outlet is 50 mm. According to the results, the physical simulation system was developed. The results provide support to carry out physical simulation experiment of coal and gas outburst.

An experimental study of the mechanism of coal and gas outbursts in the tectonic regions

To reveal the mechanism of coal and gas outbursts in tectonic regions, a gas-driven multi-physics coal and gas outbursts experimental apparatus was developed. Based on the experimental results, the damage characteristics and the mechanism of outbursts of tectonic raw coal under different gas pressures and stress conditions are quantitatively investigated. The experimental results show that the damage to coal samples increases with the increase in gas pressure. When the gas pressure is 2.0 MPa, the damage to the coal sample tends to decrease with increases in the confining and axial stress. When the gas pressure exceeds the critical value of 2.2 MPa, a strong outburst phenomenon occurs, in which the pulverized coal is ejected and the outbursts cavern is blocked. However, when the gas pressure is 2.4 MPa, the coal samples are completely broken down after the tests, and the outbursts intensity becomes larger as the axial stress or the confining stress increases. In addition, the evolution of gas pressure and the degree of fragmentation of coal samples are also discussed quantitatively in the process of coal and gas outbursts. The present study of the mechanism of coal and gas outbursts in the tectonic regions is helpful for the assessment and forecasting of potential outbursts.