Gas-bearing Coal Research Articles

Evolution of Cracks at the Edge of a Gas-Bearing Coal Seam under Stationary Mining

In this paper, we consider the conditions under which the gas enclosed in a main crack located at the edge of a coal or rock formation can produce fracture of the formation. Kinetic theory is developed for two competing physical processes: formation unloading due to rock pressure and gas filtration from the crack cavity into the surrounding rock. The first process promotes fracture, and the second leads to a decrease in the gas pressure causing the fracture. The evolution of the crack is determined by the ratio of the rates of these processes. It is found that a modified Griffith’s criterion is a necessary but not sufficient condition for fracture. For formation fracturing, it is also necessary that the ratio of the unloading rate to the filtration rate exceed a certain threshold value.

Experimental Study on Damage and Gas Migration Characteristics of Gas-Bearing Coal with Different Pore Structures under Sorption-Sudden Unloading of Methane

Coal and gas outburst is one of the most serious hazards in underground mine operations. In this paper, we explored the damage and gas seepage characteristics of gas-bearing coal samples with different pore structures under methane sorption-sudden unloading conditions using a self-developed experimental apparatus. The results show that (1) the deformation of coal samples can be divided into three stages, namely, pressure-increase-induced compression, sorption-induced expansion, and sudden-unloading-induced deformation or failure stage; (2) with the increase of the number of diffusion pores in the coal sample, the amount of gas sorption gradually increases and the expansion and failure of coal samples become more obvious; (3) in the experiment, the damage of coal samples is mainly along the axial direction and rarely along the radial direction; (4) the radial strain is always greater than the axial strain.

Experimental Study on Electric Potential Response Characteristics of Gas-Bearing Coal During Deformation and Fracturing Process

Coal mass is deformed and fractured under stress to generate electrical potential (EP) signals. The mechanical properties of coal change with the adsorption of gas. To investigate the EP response characteristics of gas-bearing coal during deformation and fracture, a test system to monitor multi-parameters of gas-bearing coal under load was designed. The results showed that abundant EP signals were generated during the loading process and the EP response corresponded well with the stress change and crack expansion, and validated this with the results from acoustic emission (AE) and high-speed photography. The higher stress level and the greater the sudden stress change led to the greater EP abnormal response. With the increase of gas pressure, the confining action and erosion effect are promoted, causing the damage evolution impacted and failure characteristics changes. As a result, the EP response is similar while the characteristics were promoted. The EP response was generated due to the charge separation caused by the friction effect etc. during the damage and deformation of the coal. Furthermore, the main factors of the EP response were different under diverse loading stages. The presence of gas promoted the EP effect. When the failure of the coal occurred, EP value rapidly rose to a maximum, which could be considered as an anomalous characteristic for monitoring the stability and revealing failure of gas-bearing coal. The research results are beneficial for further investigating the damage-evolution process of gas-bearing coal.

Mechanical properties and permeability evolution of gas-bearing coal under phased variable speed loading and unloading

It is of great significance for the analysis and prediction of coal-gas outburst disasters to understand the mechanical properties and permeability evolution of coal and rock under conditions of stope stress evolution. In this study, mechanical tests were conducted on gas-bearing coal under four stress paths, including conventional triaxial compression (CTC), phased variable speed triaxial compression (PVSTC), unloading confining pressure (UCP), and phased variable speed unloading confining pressure (PVSUCP), simultaneously measuring the permeability in mechanical tests. The mechanical properties and permeability evolutions in gas-bearing coal under four different stress paths were compared. The obtained results show that the deviatoric stress-strain curves of gas-bearing coal under four stress paths could be divided into five stages: compaction, linear elasticity, plastic deformation, stress drop, and residual stress stage. The permeability-strain curves under four stress paths could also be divided into five stages: fast drop, slow decrease, slow increase, sharp increase, and slowed growth. Compared to the CTC conditions, the peak strain and strength of coal under PVSTC conditions increased. Furthermore, the stress drop and energy release under PVSTC were more intense at the moment of instability failure. Compared to both loading paths, the coal was damaged more rapidly under unloading paths and the damage was stronger. Additionally, among both unloading paths, the time required for the failure of coal under PVSUCP was shorter than that under UCP, while the damage under PVSUCP was stronger. The strength characteristics of the gas-bearing coal under PVSTC and PVSUCP still met the Mohr–Coulomb criterion. This preliminary study has guiding significance for the understanding of the co-occurrence mechanisms of coal-gas outburst disasters.

Seepage and Damage Evolution Characteristics of Gas-Bearing Coal under Different Cyclic Loading–Unloading Stress Paths

The mechanical properties and seepage characteristics of gas-bearing coal evolve with changes in the loading pattern, which could reveal the evolution of permeability in a protected coal seam and allow gas extraction engineering work to be designed by using the effect of mining multiple protective seams. Tests on gas seepage in raw coal under three paths (stepped-cyclic, stepped-increasing-cyclic, and crossed-cyclic loading and unloading) were carried out with a seepage tester under triaxial stress conditions. The permeability was subjected to the dual influence of stress and damage accumulation. After being subjected to stress unloading and loading, the permeability of coal samples gradually decreased and the permeability did not increase before the stress exceeded the yield stage of the coal samples. The mining-enhanced permeability of the coal samples in the loading stage showed a three-phase increase with the growth of stress and the number of cycles and exhibited an N-shaped increase under the stepped-cyclic loading while it linearly increased under the other two paths in the unloading stage. With the increase of peak stress and the accumulation of damage in coal samples, the sensitivity of the permeability of coal samples to stress gradually declined. The relationship between the damage variable and the number of cycles conformed to the Boltzmann function.

Research on the Macro and Micro Evolution Law of Shear Fracture of Gas-Bearing Coal Under Different Shear Rates

Using a self-researched micro-structure shear testing device for gas-containing coal, this paper studies the characteristics of micro-cracks forming, expanding, and ultimately destroying the sample at different shear rates and then discusses the macro-failure characteristics and the evolution law of micro-cracks in the gas-containing coal at different shear rates. The results show that the shear strength decreases with an increasing shear rate, and the relation between them is a logarithmic relation; the cracking tips are in the upper and lower parts of the coal, and a fine crack appears in the sample when the shear stress is 10% of the maximum shearing stress, while the cracks fully penetrate when the shear stress is 70% of the max; finally, macroscopic damage forms when the shear stress equals the maximum amount. The angle between the direction of the lower cracks and the shear increases with an increasing shear rate; the main fracture of the gas-containing coal is around the coal particles, the fracture surface of the coal is injustice, and the fracture surface roughness becomes worse with an increase in the shear rate. With an increasing shear rate, the gas-containing coal’s cranny ratio and crack fractal dimension increases, the degree of damage increases, and the carrying capacity decreases, which leads to a lower stability and a higher tendency for coal and gas outbursts.

A Novel True Triaxial Apparatus for Testing Shear Seepage in Gas-Solid Coupling Coal

To study the effects of shear stress on the mechanical and seepage properties of gas-bearing coal in three-dimensional stress conditions, a novel true triaxial apparatus (TTA) was developed for the rigid loading of the major principal stress σ1 and the intermediate principal stress σ2, and for the flexible loading of the minor principal stress σ3. Both the upper and lateral pressure heads do not interfere with each other when loading σ1 and σ2. The control and measurement of gas flow sealed effectively in coal samples were achieved by using a gas seepage system. The TTA was used to perform a series of experiments on shear seepage in coal samples under true triaxial stress conditions. The experimental results about coal’s shear failure modes, shear stress-shear displacement curve, and permeability-shear displacement curve all showed that the TTA with its better accuracy and reliability had advantages in studying the effects of both the intermediate principal stress and shear deformation on the mechanical properties of coal samples and on the characteristics of gas seepage. Thus, it could provide a new test means for further studies on shear-induced seepage in the gas-solid coupling coal.

Investigation of the acoustic emission characteristics during deformation and failure of gas-bearing coal-rock combined bodies

With increasing mining depth, coal-gas compound dynamic disasters have become an important factor restricting mining safety. In the present study, conventional triaxial compression (CTC) tests were conducted on the gas-bearing coal, gas bearing coal-mudstone combination and gas bearing coal-sandstone combination using the RLW-500G triaxial experimental system. The gas bearing coal-sandstone combined samples were subjected to unloading tests, including unloading confining pressure (UCP) under constant axial tests and UCP-reloading axial stress (UCP-RAS) tests. In addition, the acoustic emission (AE) signals and permeabilities were measured simultaneously during the mechanical process. The experimental results indicate that the deformation of the coal-rock body is stronger under unloading conditions than in the CTC tests. Moreover, the damage of the coal-rock combination body is more severe in the UCP-RAS tests than in the UCP tests. Under lower confining pressure, the AE cumulative counts and the energy are higher for the gas-bearing coal-rock body than the gas bearing coal body. As the confining pressure increases, the AE cumulative counts and the energy are lower for the gas-bearing coal-rock body than for the gas bearing coal body. The AE cumulative counts and the energy of the three specimens under different stress paths decrease with the increase in the confining pressure or with the decrease in the gas pressure. The AE cumulative counts and energy of the gas-bearing coal-rock body are highest for the UCP-RAS test, followed by the UCP test and the CTC test. This study provides some references for understanding the mechanisms of coal-gas compound dynamic disasters and the basis for an early warning system.

Gas Content and Structure of Coal in Donets Basin

The integrated study of coal is performed using methods of optical and electron paramagnetic resonances, infrared spectroscopy and Raman spectroscopy. It is found that gas content of coal is governed by the rank of coal and its material constitution as well as by the coal bed structure at the macro-, micro- and nano-levels which interwork. It is discovered that the size and amount of bubbles, of paleo-origin presumably, and the content of coalbed methane are related. The influence of each scale level of the coal bed structure on the gas content is revealed. The authors illustrate benefits of the integrated study of gasbearing coal with the physical methods that complement one the other at different levels of coal structure.

Fluid–Solid Coupling Characteristics of Gas-Bearing Coal Subjected to Hydraulic Slotting: An Experimental Investigation

Chinese coal seams are characterized by high gas content and low permeability. The permeability of coal seams should be improved to achieve maximum extraction of coalbed methane. This study explore...

Multi-scale Fractured Coal Gas–Solid Coupling Model and Its Applications in Engineering Projects

With coal mining entering the geological environment of “high stress, rich gas, strong adsorption and low permeability,” the difficulty of joint coal and gas extraction clearly augments, the risk of solid–gas coupling dynamic disasters greatly increases, and the underlying mechanisms become more complex. In this paper, based on the characteristics of coal’s multi-scale structure and spatiotemporal variation, the multi-scale fractured coal gas–solid coupling model (MSFM) was built. In this model, the interaction between coal matrix and its fractures and the mechanical characteristics of gas-bearing coal were considered, as well as their coupling relationship. By MATLAB software, the stress–damage–seepage numerical computation programs were developed, which were applied into Comsol Multiphysics to simulate gas flow caused by coal mining. The simulation results showed the spatial variability of coal elastic modulus and cross-flow behaviors of coal seam gas, which were superior to the results of traditional gas–solid coupling model. And the numerical results obtained from MSFM were closer to the measured results in field, while the computation results of traditional model were slightly higher than the measured results. Furthermore, the MSFM in a large scale was verified by field engineering project.

Mechanical properties and permeability evolution in gas-bearing coal–rock combination body under triaxial conditions

With the development of deep mining in recent years, coal and gas compound dynamic disasters become increasingly serious. In this study, uniaxial and triaxial compression tests were conducted on gas-bearing coals, coal–sandstone combined bodies and coal–mudstone combined bodies and the permeabilities in the triaxial tests were measured simultaneously. The mechanical behavior and seepage characteristics of coals and coal–rock combination bodies under triaxial conditions were compared in details. The results show that the peak strength among three samples is: coal–sandstone combined body > coal–mudstone combined body > coal. If other conditions were held constant, the strength and the elastic modulus of all specimens show that tendency increases with the increment of the confining pressure or with the decrease in the gas pressure. The strength characteristics of all three specimens met the Mohr–Coulomb criterion, and the residual strength has an increasing trend with the increase in confining pressure. The permeability evolutions of gas-bearing coals and coal–rock combination bodies which are determined by the crack propagation in the coals and rocks are not exactly the same. This preliminary study is intended to deepen our understandings of the mechanisms of coal–gas compound dynamic disasters and provide theoretical bases for their predictions.

Effects of gas adsorption on mechanical properties and erosion mechanism of coal

In order to study coal deformation and fractured mechanism under pore gas action, we chose gases N2 and CH4 as pore fluids, and carried out triaxial compression test of gas-bearing coal. To further explore the effects of gas erosion on coal deformation and failure, we tested the deformation and rapture process of coal during gas adsorption equilibrium under fixed axial load and confining pressure. The results showed that gas adsorption decreases coal's solid surface energy and lowers coal's compressive strength, leading to larger plastic deformation in coal and final rupture instability. From the physicochemical angle, we quantified the adsorbed gas erosion effects of coal based on both Griffith crack extension theory and Mohr–Columb strength criterion, explored the mechanical characteristics and erosion damage process of gas containing coals and put forwarded their mathematical models. These models can describe the experimental phenomena fairly.

Effective stress of gas-bearing coal and its dual pore damage constitutive model

The dual pore structure of coal and occurence of gas adsorption make gas-bearing coal different from other non-adsorptive porous materials, and therefore interactions between coal and gas cannot be accurately described by Terzaghi effective stress. Considering adsorbed-gas-induced swelling stress and erosion, and pore/fracture-induced damage and failure to coal skeleton, the new effective stress equation for three-phase medium composed of free gas, adsorbed gas and coal skeleton is established. It can be used to reveal the effects of the spatial and temporal distribution and evolution of coal pores/fractures under different stress conditions on coal's mechanical deformation and damage characteristics, and to quantify the erosion effects of adsorbed gas on coal from a physicochemical point of view. Based on the basic principles of irreversible thermodynamics, a dual pore elastic-brittle-plastic damage constitutive model of gas-bearing coal is established. The model is further verified through the full stress–strain experiments and the adsorption-induced swelling experiments of gas-bearing coal under different axial pressures, and compared with the elasticoplasticity model established based on Terzaghi effective stress. The results show that the dual pore damage constitutive model could better describe the gas–coal interaction mechanism, and solve the fluid–solid coupling problems in gas and coal engineering practices.

Damage and deformation control equation for gas-bearing coal and its numerical calculation method

The dual pore structure of coal and occurrence of gas adsorption make gas-bearing coal mechanical properties different from other non-adsorptive porous materials. Considering adsorbed-gas-induced swelling stress and erosion, and pore/fracture-induced damage and failure to coal skeleton, the new effective stress equation, damage deformation control equation and constitutive model for gas-bearing coal is established. Based on principle of statistics and fractal theory, we combined the macro- and micro-structure characteristics of coal, and established the control equation of fracture field distribution. According to fracture mechanics and mesoscopic damage mechanics, we also established the expansion and damage evolution control equation of mesoscopic cracks. Furthermore, we revealed the relationship between meso-scale crack extension damage and macro mechanical characteristics, set up the macro-mesoscopic numerical model and calculation method of fractured rock mechanic constitutive, and developed the stress–strain numerical calculation program of gas-bearing coal using Comsol Multiphysics partial differential equation module and MatLab software. The model and methods were further validated in field tests. The results showed that our equations could better describe the deformation and damage process of gas-bearing coal and solve the fluid–solid coupling problems in gas and coal engineering practices.

Theoretical Analysis and Validation of Factors Affecting the Permeability on Coal Containing Gas

Based on definition of porosity, permeability model of gas-bearing coal was build which took porosity gas pressure, gas adsorption expansion, temperature and crustal stress into account, and the factors affecting the permeability of gas-bearing coal has been analyzed theoretically. The results show that permeability decreased exponentially with crustal stress increasing, permeability decreased linearly with temperature increasing and permeability decreased first and then increased with gas pressure increasing; crustal stress had the strongest effect than gas pressure and temperature, decreasing crustal stress was the most effective way to increase permeability and to improve extraction; the results verified by experiments data show that the smaller error and the higher prediction accuracy can meet production practical requirement, which could provide some guidance to gas extraction.

Modeling of Air-Gas and Dynamic Processes in Driving Development Workings in the Gas-Bearing Coal Seams

The models for air-gas processes of different hierarchical level are considered in designing and driving development workings in the coal seams. The procedure is proposed for model adaptation according to the on-line data, which makes it possible to estimate the state of medium and working capacity of measuring equipment.

Well-slot method for degassing coal beds in which the rock pressure has not been discharged

A method developed at the Mining Institute (Siberian Branch of the Russian Academy of Sciences) for directional fluid fracturing (DFF) of rock masses can be used to create an extended slot in the rock mass in a single operation. This is done by introducing a specially designed mechanism into the well at the required depth; this mechanism cuts an initiating slot of the required size and shape into the rock at the wall of the well. Through a sealing device, a fluid is fed under pressure to the initiating slot; the fluid may be any substance that will flow, including water. When a critical pressure of the fluid is reached, the frontal part of the initiating slot is moved from place; and, as it grows in the assigned direction, it is converted to an extended, large-area slot. Information on the essence of the DFF method and results of tests in coal and hardrock mines have been presented in scientific reports and a number of articles published in the period from 1982 to 1996, mainly in the present journal (Fiziko-Tekhnicheskie Problemy Razrabotki Poleznykh Iskopaemykh). A slot that is being created in a rock mass can be given various properties. In the particular case with which we are concerned, it must have the property of a highly porous collector that is capable, in the ftrst place, of draining the surrounding mass and, in the second place, of transporting the gas through the slot/collector to the well and then to the gas line. If the creation of an extended slot in various rock masses is performed repeatedly, then the subsequent work must resolve the problem of efficient filling of the slot with solid granular material. Apart from the concepts themselves, in our case we can use practical experience in hydraulic fracturing operations in coal beds and oil-bearing formations. Here it is appropriate to note the main difference between the method of conventional hydraulic fracturing [1] and the DFF method [2]. In the first case, the liquid is propagated only along natural exogenic and endogenic cracks, which are for the most part large and are randomly oriented in space; therefore, we can hardly consider that area degassing of a coal bed through wells at a distance from each other will be accomplished uniformly. This is illustrated graphically by circle charts that have been plotted for the natural fracturing with the imposition of cracks formed by hydraulic fracturing, and also by sketches made repeatedly of the actual zones of hydraulic fracturing of beds after they have been exposed by underground mining operations [1]. In the second case, when it is proposed to use the method of directional fluid fracturing [2], there is created in a single operation just one extended slot that is located in space in such a place and such an orientation that the process of degassing a gasbearing coal bed will be accomplished uniformly. This requirement is satisfied to the greatest degree by the location of a large-area slot within the seam and along its bedding. A qualitative evaluation of the degassing capacity of a slot can be obtained from an analysis of the simplest Darcy formula

Segmented Vitrinite Reflectance Profile from the Deep Seam Project, Piceance Creek Basin, Colorado--Evidence of Previous High Pore Pressure: ABSTRACT

The mean vitrinite reflectance profile of the REI Deep Seam 32-2 well in the southern Piceance Creek basin has at least three linear segments: a shallow segment extending from the surface to about 3000 ft; an intermediate segment from 3000 to 4900 ft; and a deep segment from 4900 to 5799 ft (total depth). The shallow segment is in Tertiary rocks and is offset from the next-deeper segment. The intermediate segment is steep (low gradient) and intersects the deep, less-steep (high-gradient) segment at a vitrinite reflectance of about 0.80%. By analogy with the model of the origin of abnormal pressures proposed by Law and Dickinson, the gas-bearing subnormal-pressured coal and adjacent strata in the Upper Cretaceous Mesaverde Group have evolved from an earlier overpressured phase. This suggests that during the overpressured phase (stage III of Law and Dickinson), the expulsion of relatively hot pore fluids from the top of abnormally high-pressured rocks into the overlying normal-pressured rocks raised the levels of organic maturation and steepened the vitrinite reflectance profile in the normal-pressured rocks. The bend in the profile at a vitrinite reflectance of about 0.80% represents the top of the earlier overpressured phase. Vitrinite reflectance values of this magnitude aremore » commonly associated with the top of overpressuring in the adjacent Greater Green River basin of Wyoming, Colorado, and Utah.« less

Resource Evaluation of Gas-Bearing Coal Beds: ABSTRACT

Bituminous and subbituminous coal deposits in the United States are a significant fossil fuel resource. Furthermore, a large methane gas resource is trapped in these coal beds and associated strata. Distribution and resource assessment of these potential gas reserves in 380,000 sq mi (988,000 sq km) of coal-bearing strata is currently under investigation. Diagnostic techniques include regional and local geologic studies, mud log evaluations, geophysical wireline logging with associated digital interpretation in both open and cased boreholes, conventional and sidewall core analyses, drill-stem and production testing, and coal bed stimulation techniques. Integration and analysis of the resultant data provide valuable information as to coal bed thickness, coal rank, ash and moisture content, coal permeability, face and butt-cleat orientation, gas content, pressure, and flow potential. Overburden characteristics and the elastic rock properties of the floor and roof rocks, such as Young's modulus, bulk modulus, shear modulus and Poisson's ratio, are determined from logging techniques and provide information important to mine design, mining operation, stimulation of gas-bearing coal beds, and in-situ coal gasification projects. End_of_Article - Last_Page 774------------