Coal Methane Research Articles

Investigation on future perspectives of ex-situ biogenic methane generation from solid waste coal and coal washery rejects

Biogenic methane production from the coal faces difficulties in the conventional combustion process. Understanding the coal rank, structure, and the biogenic pathway is essential to introduce the innovations in the exploitation of coal reserves. Furthermore, in-depth investigation of the microbial diversity present in the coal beds is necessary to develop cost-effective and environment-friendly techniques for energy production. In the present review, we provide an overview of the availability, properties, and processing of coal for the production of biogenic methane. The survey on the microbial communities present in various coal seams and laboratory-scale conversion of ex-situ coal to methane is also provided. We have elaborated the pathway of methanogenesis and the source of microbes for the coalification process. Finally we discussed the enhancement strategies to improve the bioavailability of coal towards microbes. Recent researchers showed that a maximum of 22.9 mM/g of coal, 0.162 mM/g coal, and 0.08 mM/g coal methane has been produced from various consortiums. The bioaugment and biositmulation process has lead to 25% increase in methane production which proves the possibility of transiting technology from laboratory to large scale.

Adsorption and Diffusion of Methane in Coal Slit Pores: Insights into the Molecular Level

Coal contains multi-scale pore structures, which determine the adsorption and diffusion of methane. To further reveal the effect of the pore structure from the molecular level, we construct the different slit pore models of coal and carry out a series of molecular simulations. The results show that the increase of slit pores increases the absolute adsorption capacity and isosteric heat of adsorption. The increased slit pores can promote the self-diffusion and transport diffusion of methane molecules. This research can provide a basis for the theory of the coal–methane interaction.

ОЦІНКА СОРБЦІЙНОЇ РІВНОВАГИ МЕТАНУ ПРИ ЙОГО ГЕНЕРАЦІЇ У ВУГІЛЬНОМУ ПЛАСТІ

Purpose. Study the adsorption equilibrium of adsorbed during the generation of methane in coal massif to determine the conditions for its implementation at different depths of mining operations. Methods. The thermodynamic research methodology, numerical calculation methods, mathematical processing of research results using approximation methods. Results. The entropy change of the «adsorbed methane – coal» system shows that the initial state of adsorbed methane during its generation in a coal seams determined by the depth of the seam and the degree of filling of its pores with methane. The results of the calculation showed, that the sorption equilibrium of the “methane – coal” system in the mountain massif is energetically most probable at the degree of filling of the pores with methane, which is θ = 40%. When θ < 40 % an irreversible spontaneous process of adsorption of methane by coal occurs, but when θ > 40% – its desorption, the degree intensity of which can be estimated by the ratio obtained in the work. It has been established that at the depth of the coal seam there is only low-intensity process, and at Н > 500 m – the process is more intensive. Therefore, when methane is generated at depths of Н < 500 m, it is likely that due to weak desorption and high sorption bond energy, the pores tend to be filled with methane almost completely. When Н > 500 m they tend to sorption equilibrium, so they are filled with methane by only 40%. The rest of the volume of gas generated in the coal seam at depths greater than 500 m, as a result of an intensive desorption process, migrates into the intervening rock seam. Scientific novelty. As opposed to a priori accepting opinions about what the initial state of methane adsorbed in coal is always characterized by the complete filling of pores with gas, numerical calculations dedicated to this issue have been performed for the first time in the paper. Calculations are based on modern ideas about the generation of methane in coal with the help of the separation of methyl group and hydrogen atoms from aliphatic fringes, which combine to form methane molecules. At the same time, the dependence of the change in the entropy of the “adsorbed coal – methane” system on the depth of occurrence and the degree of filling of its pores with methane during the generation of methane in the coal seam has been established. Practical significance. The research results make it possible to obtain fundamentally new regularities of the processes of mass transfer and filtration of methane in the mountain massif at great depths. The use of new laws makes it possible to adjust the existing technologies of mine ventilation and methods of safe mining operations in emission-hazardous and highly gas-bearing coal seams, to develop fundamentally new technologies in this direction, as well as to make more accurate calculations of methane reserves in various rocks of the mountain massif.

Study of the thermodynamic parameters influence on the phase state of methane in coal

Study of the thermodynamic parameters influence on the phase state of methane in coal

Решение проблемы изменения климата на планете. Возможный сценарий развития газои нефтедобычи в России

A new scenario for the development of gas and oil production is proposed, based on the use of greenhouse gas sequestration projects that allow increasing the oil recovery coefficient of reservoirs depleted by flooding, ensuring the production of viscous, shale, and hard-to-recover oil, as well as starting the production of methane from its hydrates or from coal. The main idea of the article is that oil and gas production and sequestration of greenhouse gases are not antagonists. Reducing greenhouse gas emissions is possible through the use of natural gas, which is an environmentally friendly fuel. It is necessary to involve huge reserves of methane hydrate or coal methane in the development while using collectors for sequestration of CO2. To improve the economy of sequestration projects, it is proposed to abandon the simultaneous commissioning of the entire technological chain of greenhouse gas sequestration and the use (where possible) of a high degree of concentration; it is recommended to pump greenhouse gas in the form of a water-gas mixture with pumping and booster units; for sequestration, first of all, use reservoirs with hard-to-recover oil reserves.

USING THE METHOD OF PULSATING ELECTROMAGNETIC SOUNDING TO DETERMINE DISTURBED ZONES IN THE LITHOSPHERE

The study was carried out using the method of pulsed electromagnetic sounding (PEMZ) to determine the accumulation of coal methane in one of the sections of the coal basin San Juan (USA) using the Phoenix-1 station. Based on the results of studies of the area of the indicated coal basin, criteria for the separation of planar anomalies promising for gas production from coal seams located at shallow depths, as well as gas migration paths through weakened zones, were obtained. The hydrocarbon source is much deeper (1400 - 1700 m in depth). The FEMZ method allows you to localize the deep source and vertical zones along which gas migration occurs.

Design of an information-measuring system for monitoring deformation and displacement of rock massif layers based on fiber-optic sensors

This paper reports a study into designing an information-measuring system that could be used in coal mines that are dangerous in terms of the explosion of coal dust and methane gas. The results of reviewing technical advancements in the field of fiber-optic system development are given. To solve the set task, prototypes of a fiber-optic sensor of a new type and a hardware-software complex were constructed. The research aims to improve the safety of workers at coal enterprises. The result of the theoretical research has established that additional losses related to a micro bending should be taken into consideration while accounting for the effect of photoelasticity. The fundamental difference between the idea reported here and existing analogs is the development of a hardware-software complex capable of working with a single-mode optical fiber of great length with a significant noise level. The data processing unit is equipped with a television matrix and can analyze changes in the pixels of a light spot. The proposed system is quasi-distributed; it controls individual points within a rock massif. The designed hardware-software system provides high noise immunity of measuring channels when the external temperature changes. The research results helped develop an information-measuring system for monitoring the deformation and displacement of rock massif layers based on fiber-optic sensors, capable of operating in an explosive environment. The system makes it possible to control several layers located in the roof of the workings, while the fiber-optic sensor may contain two or three sensitive elements that are connected to different channels. With a sharp fluctuation in pressure and an increase in the displacement parameter, the system triggers a warning signal about the danger.

Comparative Study of Pore Structure Characteristics between Mudstone and Coal under Different Particle Size Conditions

In order to investigate the difference of pore structure characteristics between mudstone and coal under different particle size conditions, samples acquired from Henan province were smashed and screened into three different particle sizes (20–40, 80–100, and >200 mesh) to conduct the experiments, using the high-pressure mercury intrusion porosimetry (MIP) and low-temperature N2 adsorption (LT-N2A) techniques. The results demonstrated that the proportion of open pores or semi-enclosed pores increased, and the pores became preferable contacted each other for both mudstone and coal during the crushing process. These variations of pore structure characteristics in the coal were beneficial to methane storage and migration. The total specific surface areas and pore volumes all showed a tendency of increasing continually for both mudstone and coal, as the particle sizes decreased from the LT-N2A test. The mudstone and coal were non-rigid aggregates with micropores, plate-shaped pores, and slit-shaped pores developed inside. The effect of the crushing process on the pore shape for the mudstone and coal was inappreciable. Moreover, the influence of the particle sizes on the mesopore was the most significant, followed by the macropore; and on the micropore, the influence was negligible for both mudstone and coal. The crushing process only had a significant impact on the pore structure of mudstone with a particle size of less than 100 mesh, while it could still alter the pore structure of coal with a particle size of larger than 100 mesh. It is believed that this work has a significant meaning to explore the diffusion and migration rules of coal-bed methane in coal.

Filling characteristics of the micropores in coals with different metamorphic degrees

To explore the occurrence state of coalbed methane (CBM) in coals of different metamorphic degrees and accurately predict CBM, we studied the micromechanism of coal and gas outbursts in a bid to avoid or reduce personnel injuries or property losses in such underground accidents. Low-temperature N2 adsorption experiments were conducted on three coal samples of different metamorphism degrees. We explored the critical filling characteristics of the packed N2 molecules in the coal. The Dubinin–Astakhov equation in the micropore filling theory and the density functional theory method were applied to obtain the critical filling pressure and critical filling aperture range for the N2 molecules to complete the micropore filling in the coal. The results showed that in the low-temperature nitrogen adsorption experiment, the pore structure of lignite was uniform, and the adsorption of N2 molecules was better. The ratio of nitrogen molecules in the adsorbed form was higher than those found in bituminous coal and anthracite. The initial filling of the micropores in coal started when the relative pressure was P/P0 ≈ 10−6–10−4 and completed when P/P0 ≈ 10−2. The order of completion was anthracite, bituminous coal, and lignite. The slit-type channel was more conducive to microhole filling than the ink bottle channel. The critical filling pore size of the coal samples with different levels of metamorphism was in the range of 1.7–2.2 nm. Finally, the critical filling pore size was divided into a primary filling stage with the range being 0.36–1.7 nm and a secondary filling stage with the range being 1.7–2.2 nm.

Газопроницаемость угольных массивов

Conditions and special features of methane release from the gas-bearing coal massifs into preparatory workings or preliminary degassing wells are given in the paper. The results are presented related to studying gas permeability of the coal seams and indicators of their gas recovery into the seam workings and wells at the depths of 270–560 m from the earth surface at the gas pressures of 1.39–4.60 MPa, and the coal methane content of 14.0–21.4 m3/t. Studies were conducted on the workings extended from 60 to 1200 m during their sinking in the conditions of 17 shallow mines on 28 coal seams, 15 of which were worked out with the use of preliminary degassing. It is established that the gas permeability of the Kuzbass coal seams varies within 0.005–0.045 milli Darcy. The intensity of methane release from the wells ranges from 2.7 to 20.1 m3/day with a coal bed capacity of 1.5–3.7 m, initial methane release into pre-degassing wells of 0.24–0.48 m3/(m2 per day), the rate of methane release reduction in time of 0.008–0.012 days–1. The relationship is noted between the quantitative indicators of gas recovery of the coal massifs in degassing wells and in reservoir workings in the dredging field. Linear dependence in time of the inverse magnitude of the intensity of methane release into the wells of preliminary degassing of the coal seam and reservoir workings at the excavation site (K-factor) is revealed. Its numerical values for 7 Kuzbass coal seams were: 0.014–0.062 m2/m3 when methane was released into preparatory workings and 0.021–0.042 m2/m3 — into pre-degassing wells. Close numerical values of the K-factors allow to use them together in predicting the parameters of methane extraction in the projects of preliminary degassing of coal-bearing formations by the wells drilled in the plane of the coal-gas massif.

Research on coalbed methane microscopic seepage characteristics of medium and high-rank coal

ABSTRACT In the present study, the microscopic seepage characteristics of coalbed methane (CBM) in medium and high-rank coal are investigated. Moreover, the influence of the coal pore structure on the CBM migration is studied. To this end, X-ray μCT scanning technology is utilized to construct the microscopic pore structure model of medium and high-rank coal. Then, the established model is transformed into a vectorized CAD solid model. Numerical simulation under different working conditions is carried out. The obtained results show that the pore system of coal is in a connected state. However, not all pores can penetrate the entire seepage direction. It is found that pore connectivity is the most important factor determining the CBM seepage performance. As coal metamorphism increases, the corresponding seepage pore connectivity decreases, pore throat radius decreases, seepage channels in coal seams decrease and the coal permeability decreases. Meanwhile, permeability anisotropy and the pore pressure of the coal have different changing laws along different seepage paths. It should be indicated that there is a dominant seepage direction. As the degree of coal metamorphism increases, the permeability in all directions gradually decreases.

Sorption characteristics in coal and shale: A review for enhanced methane recovery

The exploration and exploitation of hydrocarbon resources within coal and shale reservoirs is an engineering challenge. Well-developed internal micro-pore structures, complex sorption mechanism as well as numerous influencing factors affecting the gas flow are generally not well-accounted in the commercial life-cycle of shale gas and coalbed methane wells. Although large number of studies have been conducted to propose improved sorption models and study the influencing factors on adsorption and desorption characteristics of methane and CO2 in coal and shale reservoirs, a systematic review of such studies for efficient understanding of the accumulated literature is missing, especially with a focus towards coal and shale reservoirs. In that context, this study presents a review of sorption characteristics of methane in coal and shale. Firstly, theoretical mechanisms for methane sorption are introduced, followed by description of sorption models. Further, three factors influencing the sorption of gas in coal and shale are described: total organic carbon and clays, pore structures, and reservoir conditions. Finally, the preferential sorption characteristics of hydrocarbons and carbon dioxide are described, and the methods to promote methane desorption for enhanced recovery are discussed, which include technologies such as gas injection, microwave heating, and hydraulic fracturing. Cited as: Qin, X., Harpreet, S., Cai, J. Sorption characteristics in coal and shale: A review for enhanced methane recovery. Capillarity, 2022, 5(1): 1-11. https://doi.org/10.46690/capi.2022.01.01

Experimental study on the adverse effect of gel fracturing fluid on gas sorption behavior for Illinois coal

Hydraulic fracturing is an effective technology for coal reservoir stimulation. After fracturing operation and flowback, a fraction of fracturing fluid will be essentially remained in the formation which ultimately damages the flowability of the formation. In this study, we quantified the gel-based fracturing fluid induced damages on gas sorption for Illinois coal in US. We conducted the high-pressure methane and CO2 sorption experiments to investigate the sorption damage due to the gel residue. The infrared spectroscopy tests were used to analyze the evolution of the functional group of the coal during fracturing fluid treatment. The results show that there is no significant chemical reaction between the fracturing fluid and coal, and the damage of sorption is attributed to the physical blockage and interactions. As the concentration of fracturing fluid increases, the density of residues on the coal surface increases and the adhesion film becomes progressively denser. The adhesion film on coal can apparently reduce the number of adsorption sites for gas and lead to a decrease of gas sorption capacity. In addition, the gel residue can decrease the interconnectivity of pore structure of coal which can also limit the sorption capacity by isolating the gas from the potential sorption sites. For the low concentration of fracturing fluid, the Langmuir volume was reduced to less than one-half of that of raw coal. After the fracturing fluid invades, the desorption hysteresis of methane and CO2 in coal was found to be amplified. The impact on the methane desorption hysteresis is significantly higher than CO2 does. The reason for the increasing of hysteresis may be that the adsorption swelling caused by the residue adhered on the pore edge, or the pore blockage caused by the residue invasion under high gas pressure. The results of this study quantitatively confirm the fracturing fluid induced gas sorption damage on coal and provide a baseline assessment for coal fracturing fluid formulation and technology.

Gas desorption characteristics and related mechanism analysis under the action of superheated steam and pressurized water based on an experimental study

As a kind of unconventional natural gas, efficient and clean energy, CBM (Coalbed methane) exhibits great potential in adjusting energy structure and formulating energy development strategy. Desorption of methane from coal is essential for CBM production. And the desorption characteristics of methane in coal under different conditions and how to improve the desorption capacity have always been a hot issue of interest to researchers. So, research in this aspect becomes urgent. In this study, a series of desorption experiments of coal samples with various particle sizes under different adsorption pressures were carried out and the influence mechanism of pressurized water and superheated steam on the desorption was analyzed. The results were as follows: 1) The gas desorption capacity is more influenced by the adsorption pressure than the particle size, the particle size mainly affects gas release rate of the desorption process. It can be found that the lower the adsorption pressure, the more obvious the enhancement effect of superheated steam on desorption. 2) The inhibition of desorption by water injection can be attributed to four inhibition mechanisms caused by water, i.e. Water blocking effect, Wetting effect, Capillary effect, and Jamin effect. 3) Superheated steam can provide the energy required for the desorption of methane, intensify the thermal movement of molecules, and can also carry and displace methane, which together increases the desorption capacity and effective diffusion coefficient of gas in coal. These research results can provide a theoretical basis for hydraulic measures and heat injection techniques related to on-site CBM extraction.

Study on the Permeability Evolution Model of Mining-Disturbed Coal

Coal permeability plays an important role in the simultaneous exploitation of coal and coal-bed methane (CBM). The stress of mining-disturbed coal changes significantly during coal mining activities, causing damage and destruction of the coal mass, ultimately resulting in a sharp increase in permeability. Conventional triaxial compression and permeability tests were conducted on a triaxial creep-seepage-adsorption and desorption experimental device to investigate the permeability evolution of mining-disturbed coal. The permeability evolution models considering the influence of the stress state and stress path on the fracture propagation characteristics were established based on the permeability difference in the deformation stages of the coal mass. The stress-strain curve of the coal was divided into an elastic stage, yield stage, and plastic flow stage. As the axial stress increased, the permeability decreased and then increased, and the curve’s inflection point corresponded to the yield point. The permeability models exhibited a good agreement with the experimental data and accurately reflected the overall trends of the test results. The results of this study provide a theoretical basis for coal mine disaster prevention and the simultaneous exploitation of coal and CBM.

Desorption Effects and Laws of Multiscale Gas-Bearing Coal with Different Degrees of Metamorphism.

Understanding gas desorption effects and laws of coal mass under different conditions is essential for the effective exploration of gas emission in underground coal mines, prediction and prevention of coal and gas outburst, accurate detection of gas [coal methane (CBM)] content in coal seams, and prediction of CBM productivity. Using a self-developed test platform, we simulated gas adsorption and desorption and performed physical simulation tests. Based on these tests, we investigated the differences in the total amount of gas desorbed, desorption rate, and initial amount of gas desorbed by long-flame coal, coking coal, meager-lean coal, and anthracite on different scales under different gas pressures. Two methods are used for compensating gas loss, namely, the method and the power function method, as stipulated in the current Standards for Determination of Gas Content in Coal Seams in China. By combining these two methods, we analyzed the applicability of these two compensation methods in coal on different scales with varying degrees of metamorphism under gas pressures. The results demonstrated that (1) under the same gas adsorption pressure, the cumulative total amount of gas desorbed per unit mass within 90 min for the four kinds of coal samples increases with the degree of metamorphism. Changes in the cumulative amount of gas desorbed per unit mass and the desorption rate with the degree of metamorphism vary with stages. Notably, a higher adsorption pressure leads to a more obvious stage change. (2) Under the same gas adsorption pressure, the cumulative total amount of gas desorbed per unit mass and the desorption rate of coal with the same degree of metamorphism are inversely proportional to the size of the coal sample. This indicates significant scale effects. The larger the degree of metamorphism and gas adsorption pressure, the more significant are the scale effects of gas desorption. (3) For coal with the same degree of metamorphism, the higher gas adsorption pressure leads to a larger cumulative total amount of gas desorbed and a higher desorption rate throughout the desorption process and a larger proportion of the cumulative amount of gas desorbed in the initial stage. The smaller the size of the coal sample, the more obvious the pressure effects of gas desorption are. (4) For coal samples with the same degree of metamorphism, when the gas content in coal seams is kept constant, the larger the size of the coal sample, the smaller the actual gas loss is. Moreover, a higher gas content in coal seams results in a greater gas loss and a larger calculation error for gas loss. Compared with the method, the power function method reveals a smaller deviation between the calculated gas loss and the actual gas loss, which is found to be more accurate. A larger size coal sample results in higher accuracy in the calculated gas loss.

Distribution Characteristics of C-N-S Microorganism Genes in Different Hydraulic Zones of High-Rank Coal Reservoirs in Southern Qinshui Basin.

Microbial decomposition of carbon and biogenic methane in coal is one of the most important issues in CBM exploration. Using metagenomic technologies, the microbial C–N–S functional genes in different hydraulic zones of high-rank coal reservoirs were systematically studied, demonstrating the high sensitivity of this ecosystem to hydrodynamic conditions. The results show that the hydrodynamic strength of coal reservoir #3 in the Shizhuangnan block gradually weakened from east to west, forming a transitional feature from a runoff area to a stagnant area. Compared with runoff areas, stagnant areas have higher reservoir pressure, gas content, and ion concentrations. The relative abundance of genes associated with C, N, and S cycling increased from the runoff area to the stagnant area, including cellulose-degrading genes (e.g., cellulose 1,4-beta-cellobiosidase), methane metabolism genes (e.g., mcr, fwd, mtd, mer, and mtr), N-cycling genes (e.g., nifDKH, amoB, narGHI, napAB, nirK, norC, and nosZ), and S-cycling genes (e.g., dsrAB, sir, cysN, sat, aprAB, and PAPSS). This indicates that the stagnant zone had a more active microbial C–N–S cycle. The machine learning model shows that these significantly different genes could be used as effective indices to distinguish runoff and stagnant areas. Carbon and hydrogen isotopes indicate that methane in the study area was thermally generated. Methanogens compete with anaerobic heterotrophic bacteria to metabolize limited substrates, resulting in a low abundance of methanogens. In addition, the existence of methane-oxidizing bacteria suggests that biogenic methane was consumed by methanotrophic bacteria, which is the main reason why biogenic methane in the study area was not effectively preserved. In addition, weakened hydrodynamic conditions increased genes involved in nutrient cycling, including organic matter decomposition, methanogenesis, denitrification, and sulfate reduction, which contributed to the increase in CO2 and consumption of sulfate and nitrate from runoff areas to stagnant areas.

Calculation of the coal seam natural gas content

A research and methodological approach to calculating coal seam natural gas content and coal methane spatial resource evaluation is introduced in the paper. The research is based on the geological survey data about inclosing massif stratigraphy and natural properties of coals. Stresses that act in a coal seam are also taken into account here. The results allow substantiating safety-on-gas-factor technological parameters of coal mining processes beforehand.

Thermodynamic characteristics of methane adsorption about coking coal molecular with different sulfur components: Considering the influence of moisture contents

Sulfur groups are very susceptible to adsorption onto coking coal. In this study, three major organic sulfides of coking coal were extracted and identified by experiments. Then, coal molecules containing sulfide groups were respectively constructed via simulation. The adsorption isotherm curves of methane under different moisture contents were created. The experimental results show that thiophanthren, methyl-dibenzothiophene and dimethyl-naphthothiophene are the main organic sulfides found in coking coal. The simulation results indicate that the presence of moisture will cause the methane adsorption capacity to decrease sharply, which is mainly affected by the H/C ratio. With increasing of moisture content, the adsorption capacity of water molecules to coal molecules is stronger than that of methane molecules. The heat of methane adsorption, meanwhile, has no obvious relationship with moisture contents, but is mainly affected by S/C ratio. The adsorption heat of coal with an appreciable moisture content is less than that of coal without moisture. When the moisture content reaches a certain limit value (4%), its influence on the heat of methane adsorption is very small. The order of the heat of methane adsorption is dense medium component (DMC) > heavy component (HC) ＞ loose medium component (LMC). Water molecules easily combine with the chemical bonds on the surface of coal molecules and the hydrophilic functional groups inside. The heat of adsorption is mainly influenced by differences in the degree of coalification, but not by the various components extracted. When water is present, the adsorption capacity of methane is reduced to some extent, causing the adsorption system to methane coal molecules to reach equilibrium and release less heat. The results of this study were offer a quantitative understanding of the adsorption and thermodynamic properties of coal molecules with complex structures after extraction.

Variabilities in Hard Coal Production and Methane Emission in the Myslowice–Wesola Mine

Variabilities in Hard Coal Production and Methane Emission in the Myslowice–Wesola Mine