Methane Drainage Research Articles

Tests to Ensure the Minimum Methane Concentration for Gas Engines to Limit Atmospheric Emissions

During the extraction of hard coal in Polish conditions, methane is emitted, which is referred to as ‘mine gas’. As a result of the desorption of methane, a greenhouse gas is released from coal seams. In order to reduce atmospheric emissions, methane from coal seams is captured by a methane drainage system. On the other hand, methane, which has been separated into underground mining excavations, is discharged into the atmosphere with a stream of ventilation air. For many years, Polish hard coal mines have been capturing methane to ensure the safety of the crew and the continuity of mining operations. As a greenhouse gas, methane has a significant potential, as it is more effective at absorbing and re-emitting radiation than carbon dioxide. The increase in the amount of methane in the atmosphere is a significant factor influencing global warming, however, it is not as strong as the increase in carbon dioxide. Therefore, in Polish mines, the methane–air mixture captured in the methane drainage system is not emitted to the atmosphere, but burned as fuel in systems, including cogeneration systems, to generate electricity, heat and cold. However, in order for such use to be possible, the methane–air mixture must meet appropriate quality and quantity requirements. The article presents an analysis of changes in selected parameters of the captured methane–air mixture from one of the hard coal mines in the Upper Silesian Coal Basin in Poland. The paper analyses the changes in concentration and size of the captured methane stream through the methane capturing system. The gas captured by the methane drainage system, as an energy source, can be used in cogeneration, when the methane concentration is greater than 40%. Considering the variability of CH4 concentration in the captured mixture, it was also indicated which pure methane stream must be added to the gas mixture in order for this gas to be used as a fuel for gas engines. The balance of power of produced electric energy in gas engines is presented. Possible solutions ensuring constant concentration of the captured methane–air mixture are also presented.

Investigation of a hazardous uncontrolled dumpsite in an oxbow lake of the Nitra River for pollution potential: a case study.

This paper deals with the engineering-geological investigation of uncontrolled dumpsites that are abundant in post-communist countries. The sites may be redeveloped in an optimal manner by using the applied methodology of engineering-geological investigations. The research tool is a case study dealing with hazardous uncontrolled dumpsites chemically contaminated by various substances, particularly carcinogenic chromium. The dumpsite is located in the alluvial sediments of an oxbow lake of the Nitra River in the Slovak Republic. The hazard is seen in the fact that the alluvial sediments are permeable and thus the contamination may spread easily. At the same time, it is located near a river, which makes the hazard greater. Apart from the risk of contamination, another risk is related to the methane generated by the dumpsite and thus the risk of self-ignition. In order to identify the uncontrolled dumpsite body, the research was grounded in the different physical properties of the diverse geological environments. Quasi-homogenous blocks of the dumpsite body and its alluvial surroundings were well identified by using the combined three geophysical methods, namely dipole electromagnetic profiling (DEMP), electrical resistance tomography (ERT) and spontaneous polarization (SP). In order to eliminate the risk of contamination spread, redevelopment measures for the uncontrolled dumpsite in the form of sealing walls and surface sealing foil were proposed. A system of methane drainage boreholes was proposed to eliminate the risk of self-ignition. The methodology in this case study is well applicable for other uncontrolled dumpsites, which is an important outcome of the study.

Characterization and Quantification of Mining-Induced Fractures in Overlying Strata: Implications for Coalbed Methane Drainage

Coalbed methane (CBM) production in the overlying strata of coal reservoirs is often hampered by the unknown distribution of the mining-induced fractures. Mining-induced fractures are CBM migration pathways in the fractured overlying strata, and the excavation of coal seams within a mine causes the CBM in adjacent coal seams to flow into the overlying strata. The mining-induced fracture field in the overlying strata is the best place from which this CBM is drained. Here, to better understand the distributions of vertical and horizontal fractures caused by excavation, we propose a novel approach to quantify the dimensions of vertical and horizontal fractures in fractured zones. In addition, we demonstrate that there are negligible changes in the dimensions of horizontal fractures and great changes in the dimensions of vertical fractures when there is an increase in the height of the fractured zone. We further demonstrate that mining-induced angles mainly concentrate on 0°–10°, 50°–70°, 110°–120° and 170°–180°, and larger width fractures exist in both sides and top due to the de-stressed effect and fractures in the middle of model close under mining-induced stress. The approach described here could be used to improve the accuracy of cross-measure borehole positioning and the efficiency of CBM drainage.

Evolution characteristics of reservoir parameters during coalbed methane drainage via in-seam horizontal boreholes

In-seam horizontal boreholes are used to collect coalbed methane (CBM). Physical simulations of CBM drainage by in-seam horizontal boreholes are conducted. The evolution of gas pressure and permeability ratio indicate a rapid decline in a short time. As time goes by, the permeability ratio recovers, and the gas pressure declines slowly. In the borehole area, initially, the decline rates of the gas pressure and permeability ratio are positively related to the distance from the borehole, and the shape of the distribution of the gas pressures and permeability ratio is similar to that of multiple rings. In the non-borehole area, the decline rates of the gas pressures and permeability ratio is almost the same at different points. When the borehole spacing is optimum, the interaction effect accelerates the drainage of CBM. The permeability is initially controlled by effective stress and is mainly affected by matrix shrinkage after a period of time.

Forecasting Methane Emissions from Hard Coal Mines Including the Methane Drainage Process

With regard to underground mining, methane is a gas that, on the one hand, poses a threat to the exploitation process and, on the other hand, creates an opportunity for economic development. As a result of coal exploitation, large amounts of coal enter the natural environment mainly through ventilation systems. Since methane is a greenhouse gas, its emission has a significant impact on global warming. Nevertheless, methane is also a high-energy gas that can be utilized as a very valuable energy resource. These different properties of methane prompted an analysis of both the current and the future states of methane emissions from coal seams, taking into account the possibilities of its use. For this reason, the following article presents the results of the study of methane emissions from Polish hard coal mines between 1993–2018 and their forecast until 2025. In order to predict methane emissions, research methodology was developed based on artificial neural networks and selected statistical methods. The multi-layer perceptron (MLP) network was used to make a prognostic model. The aim of the study was to develop a method to predict methane emissions and determine trends in terms of the amount of methane that may enter the natural environment in the coming years and the amount that can be used as a result of the methane drainage process. The methodology developed with the use of neural networks, the conducted research, and the findings constitute a new approach in the scope of both analysis and prediction of methane emissions from hard coal mines. The results obtained confirm that this methodology works well in mining practice and can also be successfully used in other industries to forecast greenhouse gas and other substance emissions.

Permeability of coal associated with cleat and bedding structure: measurement and modeling

ABSTRACT The permeability of coal shows anisotropic features due to its cleat and bedding structures. This work measured methane permeability of three cylindrical coal specimens, which represents the three orthogonal axes of the coal seam about bedding plane and cleat (face cleat and butt cleat). Twenty-seven permeability measurement tests were conducted under the different confining pressures and axial compression (4, 6, and 8 MPa) at different gas pressures (0.5, 1, and 1.5 MPa). Results clearly revealed the anisotropy in permeability of the tested coal specimens under test conditions. The specimen parallel to the bedding plane in face cleat direction shows the highest permeability while the specimen perpendicular to the bedding plane shows the lowest permeability. Within gas pressure range from 0.5 to 1.5 MPa, the lower the gas pressure the higher the measured coal permeability. Spatial permeability distribution around coal mine methane drainage borehole is obtained through a three-dimensional permeability model. The spatial permeability around drainage borehole parallel to butt cleat direction is much higher than that parallel to face cleat direction. In-mine methane drainage boreholes parallel to bedding plane in the butt cleat direction are recommended to use in engineering application.

Using alternative sources of methane for energy as a method of environmental protection and improvement of economy condition, on example of Poland

The article analyses the types of waste generated in economy and the possibilities of their management, on the example of Poland. The main focus was on using selected types of waste for methane production. The impact of the decomposing mass of organic waste on the production of greenhouse gases was examined and various types of biogas plants operating in Poland were presented (biogas plants at landfills, biogas plants at sewage treatment plants, agricultural biogas plants). The article also presents the benefits for the economy from the use of methane from methane drainage. Unfortunately, in Poland, methane resources are used to a small extent. In the case of mines, landfills or sewage treatment plants it is related to high costs of constructing methane capture installations, which are not offset by revenues from sales of gas and green certificates. Although much more favourable blue certificates have been introduced for agricultural biogas plants, the obstacle standing in the way of their progress is the lack of local spatial development plans for the places designated for their construction

Взрывы и выбросы метана: квантовая теория метаноносности, выбросоопасности и дегазации угольных пластов

Взрывы и выбросы метана: квантовая теория метаноносности, выбросоопасности и дегазации угольных пластов

Comparative Analysis of Methane Concentration Near the Junction of the Longwall and Top Road

Abstract Mining of longwalls ventilated by the „U” method is willingly applied in Polish coal-mines due to low costs of workings maintenance, low costs of ventilation and a lower fire threat because of a limited flow of air through goafs. However, such a way of ventilation may pose an increased risk of methane explosion. For this reason, the “U” ventilation has been limited in longwalls with methane risk. The mining regulations in force provide that ventilation methane-bearing capacity, i.e. the intensity of methane flow into the ventilation air cannot exceed 20 m3 CH4/min. The regulations also provide that in the event the absolute methane-bearing capacity, i.e. a sum of methane released to the ventilation air and captured by the methane drainage system is higher than 25 m3 CH4/min and the “U” method of ventilation is applied, the effectiveness of methane drainage should be minimum 50% in relation to the forecast absolute methane-bearing capacity. To streamline the process of ventilation near the junction of the longwall and the gallery carrying off the used air, auxiliary ventilation means are applied, such as a ventilation partition, a ventube – which supplies air without methane or with a low concentration of methane, injectors etc. Application of these means is limited by the cross-section of the heading carrying off the air from the longwall. Deformations of the ventilating roadway, which is usually located in the one-sided vicinity of goafs, may prevent the use of a ventilation partition, which has a negative influence on the conditions of ventilating the junction of the longwall and ventilating roadway. The author of the article also refers to such conditions, presenting average values and maximum concentrations of methane concentrations recorded with four methane concentration sensors, located in the vicinity of the junction of the longwall and ventilating roadway.

Study on Characteristics of Airflow Spatial Distribution in Abandoned Mine Gob and Its Application in Methane Drainage

At present, there are many known sources of methane resources in abandoned mine gobs. Therefore, investigations into the characteristics of airflow spatial distributions in abandoned mine gobs will be helpful in future methane drainage processes. In this study, a physical simulation experiment was first adopted to simulate the formation processes of abandoned gobs based on the geological conditions of the abandoned Yongan Mine in China. Gas injection experiments were then conducted to study the gas concentration distributions and variations in a simulated gob based on the measured gas concentrations at certain sampling points. The results indicate that the abandoned gob comprises a gas enrichment zone (GEZ), gas diversion zone (GDZ), and gas isolation zone (GIZ) from the bottom to the top in the incline edges of the abandoned gob, respectively. However, only the GEZ and GIZ exist in the incline center of the abandoned gob. The gas concentration in the GEZ is observed to be the highest, although the concentration is also found to be relatively high in the GDZ. Moreover, a shielding is found to exist between the boundaries of the GEZ and GDZ, which results in a dramatic decline in concentration in the latter due to its extremely low permeability. The study results were verified by using the methane drainage data from the surface vertical wells in the abandoned Yongan Mine. The field application indicated that the methane extraction flow rate could potentially increase by 1.5 times when the well bottom traversed the GDZ and entered the GEZ, compared to the flow rate when the well bottom was located in the GDZ.

Optimal selection of coal seam pressure-relief gas extraction technologies: a typical case of the Panyi Coal Mine, Huainan coalfield, China

ABSTRACT Pressure-relief gas extraction from a protected seam can be realized by surface well drilling and the use of net-like penetrating boreholes (NPB). The selection of the extraction mode is affected by the geological conditions, characteristics of the coal seam occurrence and inherent characteristics of the extraction mode. Based on a numerical simulation and field test, the Panyi Coal Mine in the Chinese Huainan coalfield is used as an example to study the effective extraction range, efficiency, cost, total amount extracted and applicable conditions of two methods. The results show that the average gas extraction concentration (93.8%) and average extraction purity (28 m3/min) of surface wells are higher than those (41.8% and 17 m3/min) of NPB by approximately 124.4% and 64.7%, respectively. The superiority of the extraction efficiency from the use of surface wells is obvious from intact wells. In addition, the unit methane control costs, methane drainage costs per cubic meter and construction period of surface drillings are less than those of the NPB. However, Surface drilling is easily restricted by poor wellbore stability and high surface environment requirements. This case study can provide valuable examples for other coal mines with similar geological conditions for the selection of pressure-relief gas treatment methods.

A novel anti-air-leakage method and an organic polymer material for improving methane drainage performance

Coal mine methane (CMM) drainage is an important method to curb the occurrence of gas disasters and to reduce the amount of greenhouse gas emission, and to make full use of the methane resource. High-grade sealing boreholes is the key factor to achieve efficient drainage of methane. In order to improve the efficiency of CMM drainage as well as to seal in-seam horizontal boreholes and fractures around these boreholes effectively, a dynamic anti-air-leakage method called the dynamic fractures filling technology was presented, and an organic polymer material called the PM sealing material matching with this technology was developed in this paper. Firstly, the destruction of coal body around boreholes because of a roadway and a borehole excavation was analyzed from the view of stresses, and the range of fractures zone caused by roadway excavation was determined, then the sealing technology and sealing principle of the novel technology were analyzed systematically. Moreover, the development process of the PM sealing material was introduced, and the influence of preparation parameters on material properties was investigated. Besides, taking the coal seam in the 39,021 Transportation Lane in the Zhongmacun Mine(China) as the test site of this study, the attenuation law of the methane concentration in drainage boreholes under different sealing depths was investigated via adopting the conventional sealing technology. Finally, the methane-drainage effect of the novel sealing technology and the conventional technology were compared. The results indicate that: (1) Both a roadway excavation and a borehole excavation will cause the redistribution of stresses of the original coal mass, and the coal structure around a drainage borehole has been destroyed in different degrees due to stresses overlapping effect. Moreover, the dynamic developing fractures zone is the main channel of air-leakage during drainage process. (2) The novel technology uses the conventional sealing system and the dynamic sealing system to seal drainage boreholes and dynamic development of fractures around them. (3) When the water-to-material ratio is 7:1 and the treatment temperature is 180°C for 30min, the PM material has the best performance. (4) Comparing with the conventional sealing technology, the novel technology has the obvious characteristics of short sealing depth, long efficient continuous drainage time and high drainage amount of methane. and its advantages of methane-drainage effects and economic benefits are remarkable.

Methane occurrence, emissions and hazards in the Upper Silesian Coal Basin, Poland

The paper presents the variability of methane emissions into mining excavations and the atmosphere in the Upper Silesian Coal Basin (USCB) (Poland) against the background of natural and mining factors. Emissions of methane from exploited coal seams have become a serious problem in the USCB due to the growing methane hazard in coal mines and emissions of methane into the atmosphere. In the mid-nineties 753 million m3 of methane was emitted annually from the USCB mines. Despite a significant drop in coal production over the next 20 years, methane emissions have not decreased sharply; on the contrary, in recent years they have begun to grow, and, in 2016, reached the level of 933 million m3 per year. This represents an increase of 180 million m3 per year since the mid-nineties. One of the important reasons for this phenomenon is the constantly increasing depth of coal exploitation, which in many mines now exceeds 1000 m; this is the depth corresponding to the deep methane zone, where the volume of accumulated methane in the coal seams is particularly high. Factors influencing the volume of methane emissions in relation to mine workings can be divided roughly into two groups: natural (geological) and anthropogenic (mining-related). Natural factors include methane content in coal seams, gas pressure, the presence of free gas in fault zones, related fissures and porous sandstone, the migration of methane through faults and fissures, and the presence of a continuous and impermeable Miocene overburden. Mining factors include the depth of exploitation and the concentration of coal production as expressed in terms of the length, height, and advance of walls. The interdependence of these factors means that, despite the decline in coal production, methane emissions, both total and specific, are increasing. This problem cannot be neglected, especially since it may grow worse in future. One measure to prevent the growth of methane emissions and the associated hazard may be the intensification of mine methane drainage and the economic use of captured mining gas, which is already done in the USCB.

Numerical Analysis and Prediction of Coal Mine Methane Drainage Based on Gas–Solid Coupling Model

Numerical Analysis and Prediction of Coal Mine Methane Drainage Based on Gas–Solid Coupling Model

Damage‐induced permeability model of coal and its application to gas predrainage in combination of soft coal and hard coal

AbstractCoal permeability is a key parameter that affects the drainage of coal bed methane (CBM). Owing to the lower original permeability of coal seams in China, more attention was paid to the study of damage‐induced permeability laws. However, the theory of damage‐induced permeability still needs to be improved. In this paper, the permeability evolution experiments during the complete stress–strain process of coal were carried out firstly. The results are shown that with the increase in axial strain, the coal permeability decreases slowly at first, then increases rapidly, and finally increases slowly or even remains unchanged, and the peak stress point can be regarded as the extreme point of permeability. Then, the equivalent plastic strain was used to describe the damage of coal. Based on the above results, a new damage‐induced permeability model was developed, and the new model can match the experimental data of permeability very well. In addition, solid–gas coupling models of coal were developed based on the new permeability model, and the numerical results show that there is a region of sudden increase in permeability around the borehole where plastic failure occurs, which indicates that the damage‐induced permeability model is reasonable. Finally, the results of numerical simulation of gas predrainage in combination of soft coal and hard coal under different cases were used to optimize the borehole layout of gas predrainage in the field.

Reasonable minimum sealing length of in-seam boreholes for coal mine methane drainage: a case study

ABSTRACT Coal mine methane (CMM) drainage is an effective method to curb the occurrence of coal and gas outburst and gas explosion, to reduce the amount of greenhouse gas emission, and to make full use of the clean energy(methane). Sealing in-seam horizontal boreholes plays a vital role in CMM drainage. Reasonable sealing length is an important factor to achieve high extraction efficiency and reduce the cost of construction. To determine the minimum reasonable sealing length of drainage boreholes, the radius of the fracture zone was obtained based on the ideal elastoplastic model, and this radius was taken as the minimum sealing length. The influencing factors of the minimum sealing length were analyzed and engineering verification test was taken in Zhongmacun mine in Henan province, China. The research results show that the minimum reasonable sealing length should be longer for a larger radius of the coal roadway and the coal seam with a deeper buried depth, while the minimum reasonable sealing length can be shortened with the increase of uniaxial compressive strength and the supporting stress of the coal roadway. The 11.07 m of sealing length calculated by theories was basically consistent with the 12 m of sealing length verified by the method of drilling cuttings. The reasonableness of sealing length (12 m) was verified by the test results of methane concentration and flux within 40 days. All those outcomes can provide references for other mines to determine the minimum reasonable sealing length of in-seam boreholes for CMM drainage.

Method of in-seam gas drainage borehole testing

The paper describes the method of in-seam gas dynamic logging in long drill holes. The method uses a robotic tool based on the straddle packer assembly. The choice of a measurement scheme depends on the time planned for testing, required accuracy and quality of borehole walls. By the testing data, the intervals with low gas influx can be identified for further hydraulic fracturing to stimulate methane drainage, or it is possible to undertake prompt precautions in outburst-hazardous areas.

Effect of compressive stresses on permeability of coal with fracture filled with lightweight proppant

The paper presents laboratory testing results on effect of hydraulic fracturing on permeability of coal with and without wedging of created fractures by proppant. The tests were carried out with hollow aluminosilicate microspheres ASPM-500 without and with additional polymeric thermoreactive coating, specimens of dense long flame (grade D) coal with permeability of less than 3·10−3 μm2 and fractured fat coking coal (grade ZH) with permeability over 60·10−3 μm2. It is found that creation of fractures with and without propping considerably improves gas permeability of dense coal with weakly developed natural jointing. Created fractures are less effective in highly jointed highly permeable coal. It is experimentally proved that filling of created fractures with small opening of 0.4 mm with lightweight proppant under uniform compression of 1–5 MPa improves permeability of dense coal by 7–19.5 times. In jointed coal, application of lightweight proppant under the same conditions improves permeability less considerably—by 1.2–2.6 times. It is revealed that efficiency gas drainage in coal by hydraulic fractures without propping lowers as compressive stresses decrease. Reduction in permeability of coal with propped fracture under growing compression is probably connected with redistribution and compaction of proppant particles in the fracture. The obtained results show that application of lightweight propping materials is a promising way of advance in the technology of hydraulic fracturing for stimulation of coalbed methane drainage. The long-range research, in the authors’ opinion, should be focused on the development of lightweight proppants with improved strength characteristics.

Aging characteristic of effective distance on injecting N2 to displace CH4 in anisotropic coal seam

ABSTRACTGas injection is an effective way to enhance methane drainage in a low permeability coal seam, and the reasonable distance between injection holes and drainage holes depends on effective influence distance of injection gas in a coal seam. For anisotropic permeability of coal seam, the variation of the gas-flow rate from a test hole in the anisotropic coal seam is measured as nitrogen is injected into the injection hole near the test hole. The effective distance regularity is revealed by nitrogen injection in an anisotropic coal seam, and the model of nitrogen injection is established to numerically analyze the aging characteristic of effective influence distance in the anisotropic coal seam. The results show that the permeability in coal seam reduced exponentially with the increase in gas pressure, and it is larger in the direction of parallel to bedding than that in the direction of perpendicular to bedding. The effective distance with nitrogen injection increased exponentially in the direction of parallel/perpendicular to bedding, and the effective distance in the direction of parallel to bedding is bigger than that in the direction of perpendicular to bedding. The optimum distances between injection hole and test hole, with 0.5 MPa of injection gas pressure and 8.36 h of injection gas time, are 2.05 m in parallel to bedding direction, and 1.53 m in perpendicular to bedding direction. This study will have important theoretical and practical significance for a reasonable layout of injection-drainage holes during injecting gas to enhance methane drainage.

Time Characteristics of the Influence Radius by Injecting N2 to Displace Coalbed Methane: A Case Study

Injecting N2 to displace methane is an effective way to enhance coalbed methane drainage, and the influence radius of this process is an important factor in borehole arrangement. To determine reasonable spacing between injection boreholes and discharge boreholes, experimental and theoretical studies were carried out. The change of rule for the influence radius was determined based on the flow rate changes at the discharge boreholes when injecting gas into a coal seam in the field. Based on gas seepage-diffusion theory, a model for injecting N2 to displace coalbed methane was established. Through numerical simulation, the time characteristics of the influence radius were analyzed. The results show the following: Under different gas injection pressure conditions, the influence radius increases exponentially as injection time increases, but the rate of increase of the influence radius decreases gradually. For the same injection time, the higher the injection pressure, the wider the influence radius will be. After obtaining field results, regression analysis was applied to analyze the numerical results of gas injection at different pressures, and then, the quantitative relationship between the injection influence radius r, the injection time t, and the injection pressure p was found. According to the results calculated using this formula at an injection pressure of 0.5 MPa, the optimum spacing between boreholes was determined to be 1.5 m at the Shigang Coal Mine. The analysis of reasonable spacing between injection and extraction boreholes at different injection pressures shows that the reasonable spacing between boreholes was linearly correlated with gas injection pressures. This study has important theoretical and practical significance for the spacing between boreholes in a reasonable arrangement when injecting N2 to displace methane.