Coal Seam Gas Content Research Articles

Improvement of Gas Drainage Efficiency of Low-Permeability Coal Seams Using Ultra-High Pressure Hydraulic Cutting Technique

With the increase in the coal mining depth, the dynamic disasters of deep coal seam under the combined effects of high crustrals stress and high gas pressure seriously restrict the safety production of the coal mines. Moreover, it is very difficult to lower the gas content of the low-permeability coal seams. Even though numerous boreholes have been drilled inside the coal seam and gas drainage have been performed for a long period. High-pressure water jet cutting technology can effectively increase the exposure area of coal seams, improve the gas flow state in coal seams, so that the coal can be evenly and fully discharged in a certain range, and then the goal of rapid discharge and permeability enhancement can be achieved. After cutting the coal seam between 1m and 2m interval, a uniform pressure relief space is formed inside the boreholes to increase the permeability of the coal seam and improve the gas drainage efficiency The field test results shows that the equivalent diameter of boreholes with cutting is about 5 times that of boreholes without cutting. The gas drainage radius of boreholes with cutting is 2.8 times that of boreholes without cutting, which significantiy shortens the gas drainage time of the coal lane strip, and then attributes to relizing the rapid and safe excavation of coal seam roadway. This technique can provide reference significance for the modification of low-permeabilety coal seans in the similar condition area.

Influence of Ground Stress on Coal Seam Gas Pressure and Gas Content

Influence of Ground Stress on Coal Seam Gas Pressure and Gas Content

Application of Pre-extraction Drilling in Anti-burst of Low Permeability Coal Seam

In order to eliminate the outburst danger of coal seam with low gas permeability and consider the economic cost of anti-burst measures, the gas pre-extraction coal seam gas prevention technology in the coal roadway excavation process is studied to relieve the coal seam pressure and gas content. The pressure is reduced, and the gas content of the coal body is reduced to 8m3/t within the allowable excavation range. The maximum gas cuttings Smax of the gas is reduced to less than 6kg/m after the anti-burst effect test, which effectively eliminates the prominent danger of the coal seam. During the implementation of the regional anti-burst measures, it can assist the gas release from the gas release, reduce the gas content of the coal seam, release the gas potential of the coal body, and strictly implement the regional defense according to the requirements of the two “four-in-one” anti-burst measures. The comprehensive anti-burst measures supplemented by the sudden measures and the local anti-burst measures eliminate the prominent danger of the coal seam and ensure the safe excavation of the coal roadway.

Research on the Mining Practice of Protection Layer in the Complex Geological Conditions of Qidong Coal Mine

To study the effect of 82 coal seam unloading and outburst prevention with 71 coal seam mining,as well as the influence of the integrated application of research methods of theoretical analysis and field practice, the feasibility of the protective layer under the condition of mining research, and in the process of 71 coal seam mining, below 82 coal seam gas content, gas pressure and the effect of the deformation of coal unloading field measurement and comprehensive theoretical analysis. The research results show that the protective layer of 71 coal seam as the lower 82 coal mining is feasible in theory, in 7137 working face of the actual mining process, the lower 82 significant increase in the permeability of coal seam, the gas content is greatly reduced, unloading deformation effect is obvious, further verify the three 71 coal seam as the lower protective layer of 82 coal mining, the discharge pressure and outburst prevention with dual effectiveness, to ensure safe and efficient mining of coal seam 82.

New method for predicting coal seam gas content

ABSTRACTCoal seam gas (CSG) content is an important factor affecting the safety related to gas explosions, and it is also an indicator of natural gas resource assessment. The conventional method of establishing a network model for predicting gas content is mainly to predict the gas content of an individual location point, and achieve gas content prediction in the target area through an interpolation algorithm, which cannot accurately control the interpolation beyond the prediction point. Therefore, we selected a certain study area as an example, conducted a comprehensive analysis of CSG enrichment logging and seismic response characteristics, to determine the relative sensitive parameters, and used a support vector machine (SVM) network for sensitive parameters training based on genetic constraints. The algorithm (GA) optimizes the penalty parameters and kernel functions of the prediction model, completes the prediction of the gas content at the drill site, and uses neural network inversion to train and predict the target gas curve. The distribution of the gas content in the three-dimensional volume of the coal seam was obtained. A variety of gas content influencing factors and sensitive parameters of gas enrichment were integrated to establish a set of prediction methods for the volume gas content, which provided a theoretical basis for accurately predicting coal seam gas content. Comparison with subsequent measured data verified that this prediction method has a higher accuracy in this study area.

Calculation Model and Rapid Estimation Method for Coal Seam Gas Content

Coalbed gas content is the most important parameter for forecasting and preventing the occurrence of coal and gas outburst. However, existing methods have difficulty obtaining the coalbed gas content accurately. In this study, a numerical calculation model for the rapid estimation of coal seam gas content was established based on the characteristic values of gas desorption at specific exposure times. Combined with technical verification, a new method which avoids the calculation of gas loss for the rapid estimation of gas content in the coal seam was investigated. Study results show that the balanced adsorption gas pressure and coal gas desorption characteristic coefficient (Kt) satisfy the exponential equation, and the gas content and Kt are linear equations. The correlation coefficient of the fitting equation gradually decreases as the exposure time of the coal sample increases. Using the new method to measure and calculate the gas content of coal samples at two different working faces of the Lubanshan North mine (LBS), the deviation of the calculated coal sample gas content ranged from 0.32% to 8.84%, with an average of only 4.49%. Therefore, the new method meets the needs of field engineering technology.

Coalbed methane reservoir characteristics of coal seams of south Karanpura coalfield, Jharkhand, India

Coalbed methane has emerged as a viable natural gas resource in India since 2007. The understanding of gas genesis kinetics, storage mechanisms and the geological controls is vital for exploration and successful recovery of gas in a cost-effective manner. In this respect, a multidisciplinary analytical approach including gas content, stable isotopes (δ13C1), hydrocarbons distribution, reconstruction of original organic matter, chemical properties, sorption kinetics and the role of petrographic constituents have been assessed. The volatile matter content varies from 20.2 to 32.1 wt%; indicating medium volatile to high volatile bituminous rank of coal with the maximum vitrinite reflectance (VRo%) ranging from 0.63 to 0.98%. The studied coals are vitrinite rich and contains vitrinite group macerals in the range of 41–65 vol%. Dendritic micro-fractures are confined to vitrite microlithotype, and secondary mineralization helps preserve fracture connectivity. The average gas content of coal seams is 2.06 cc/g. The increase in gas content values per 100 m is about 0.38 cc/g emphasizing escape and migration of hydrocarbons during restructuring and tectonic activities in the basin. The relationship of C3/C1 and C2/C1 ratios is indicating that the hydrocarbons in desorbed gas originated from the thermogenic process. The large concentration of methane in desorbed gas and its stable isotope signatures (δ13C1 < −40‰) also indicates the thermogenic origin of gases in coal seams. Few desorbed gas samples have been found in the mixed gas region, which may be due to the influx of fresh water carrying bacteria leading to the generation of biogenic gases and subsequent mixing with the sorbed thermogenic gases. The values of S1 (0.19–0.78 mg HC/g) and S2 (10.31–25.63 mg HC/g) indicate excellent source rock potentials. The positive correlation of original hydrogen index (HIO) with present-day hydrogen index (HIPD) and original organic content (TOCO) with present-day organic content (TOCPD) suggests uniform consistency in organic carbon conversion to hydrocarbons.The gas content compared with sorption capacity reveals the undersaturation of coal seams. The experimental results summarize that the low gas content is a critical issue. Similarly, the low, quality and quantity of CH4 may affect the exploration and recovery of methane in the long term into this basin. But, the low values of sorption time (0.12–6.65 days) signifies good diffusion characteristics that may support the recovery of gas. Finally, it is concluded that depth of occurrence, maturity and pore/cleats associated with microconstituents control the gas accumulation and transport in coal seam reservoirs of South Karanpura coalfield.

Gas Content Evaluation of Coalbed Methane Reservoir in the Fukang Area of Southern Junggar Basin, Northwest China by Multiple Geophysical Logging Methods

To study the gas potential of coalbed methane (CBM) in the Fukang area, southern Junggar Basin (SJB) of North China, different methods including multiple geophysical logging, the Kim method with proximate analysis data, and Langmuir adsorption were used to evaluate the gas content. Furthermore, the geological controls on gas content were evaluated. One hundred sixteen CBM wells with geophysical logging and 20 with field-measured gas content were adopted to assess the gas content in the Fukang area of SJB, NW China. The results show that the two geophysical logging variables (DEN and CNL) were favorable for evaluating the gas content due to the perfect correlation with the measured gas content. The gas content varies from 4.22 m3/t to 16.26 m3/t, and generally increases with increasing burial depth. The gas content in coal seams along the synclinal axis is significantly higher than that along the synclinal wing in the west zone. In the east zone, the gas content of the westward is higher than that of the eastward because of the fault coating effect by reverse fault. Generally, the gas content of the SJB is in the order of syncline &gt; surrounding reverse fault &gt; slope of syncline &gt; slope of anticline &gt; central of reverse fault, if only geological structure features are considered. The favorable areas for CBM concentration appear to be a composite gas controlling result of multiple geological factors. Two typical geological scenarios with low gas content and high gas content were revealed. In the Fukang area of SJB, the low gas content is mainly due to the normal fault and roof lithology of sandstone. The most favorable area of high gas content for CBM exploration and development is in the northeast, where reversed fault, synclinal axis, mudstone roof lithology, and burial depth coincide with high gas content.

Simulation of Ground Stress Field and Advanced Prediction of Gas Outburst Risks in the Non-Mining Area of Xinjing Mine, China

In order to predict in advance the coal and gas outburst risks in the No. 3 coal seam in the non-mining area of the Xinjing mine, the strata were divided into seven rock assemblage types based on the lithologic characteristics of strata in the research area, and eight geological profile models were constructed. The finite element methods were used to simulate the ground stress field of the No. 3 seam floor. Based on the log curve, the coal structural type of the No. 3 coal seam was identified, with the thicknesses of the coal body under different types of damage being marked off in each borehole. A damage index of the coal structure (DV) was also proposed, and the DV indexes for all boreholes were calculated. The coal and gas outburst risks in the research area were comprehensively evaluated and predicted through a superposition analysis of the spatial distribution states of three indexes, namely: ground stress, coal structure damage degree, and coal seam gas content. The results show that the equivalent stress of the No. 3 coal seam floor is usually within the range of 9–26 MPa. The high-stress zone presents a strip distribution along the northeast–southwest (NE–SW) direction. The distribution of ground stress is mainly subject to the folds and buried depth of the coal seam. The distribution range where the damage degree of the coal structure falls under Types II and III is DV ≥ 22. The gas outburst risk in the mid-southern and northeastern parts of the research area is high, whereas that in the mid-western part is low. The zones with a high and low degrees of gas outburst risks are all mainly present as strips in the NE–SW direction. The gas outburst risk in the northwest and southeast is moderate. The research results can provide guidance for gas control in non-mining areas.

Geological Factors and Reservoir Properties Affecting the Gas Content of Coal Seams in the Gujiao Area, Northwest Qinshui Basin, China

Coalbed methane (CBM) well drilling and logging data together with geological data were adopted to provide insights into controlling mechanism of gas content in major coal seams and establish gas accumulation models in the Gujiao area, Northwest Qinshui Basin, China. Gas content of targeted coals is various in the Gujiao area with their burial depth ranging from 295 to 859 m. Highly variable gas content of coals should be derived from the differences among tectonism, magmatism, hydrodynamism, and sedimentation. Gas content preserved in the Gujiao area is divided into two parts by the geological structure. Gas tends to accumulate in the groundwater stagnant zone with a total dissolved solids (TDS) value of 1300–1700 ppm due to water pressure in the Gujiao area. Reservoir properties including moisture content, minerals, and pore structure also significantly result in gas content variability. Subsequently, the gray correlation statistic method was adopted to determine the most important factors controlling gas content. Coal metamorphism and geological structure had marked control on gas content for the targeted coals. Finally, the favorable CBM exploitation areas were comprehensively evaluated in the Gujiao area. The results showed that the most favorable CBM exploitation areas were in the mid-south part of the Gujiao area (Block I).

Simulation investigation of N2-injection enhanced gas drainage: Model development and identification of critical parameters

The performance of pre-gas drainage in low permeable seams especially in CO2 abundant coal seams is not satisfactory. Due to the high affinity of coal to CO2, much lead time is needed for gas drainage boreholes to reduce the coal seam gas content below the threshold limit value (TLV), thus meeting the demand of the scheduled mining activities. In this study, simulations of N2 injection enhanced gas drainage were carried out for CO2-rich coal seams. A binary gas migration model was developed to illustrate gas migration in coal seam. The simulation results matched well with the laboratory test results, indicating that this model could be used to investigate the mechanism of N2 flushing coal seam gas. The parameters affecting the flushing performance were then studied using this model, including coal seam permeability, gas diffusion coefficient, N2 injection pressure andin-situ gas content. These results suggest that coal seam permeability and CO2 diffusion coefficient are the key parameters having significant impacts on the drainage performance whilst N2 diffusion coefficient only has negligible effect. A critical parameter theory was developed to describe the correlation between these parameters (especially coal seam permeability and the diffusion coefficient of coal seam gas). Specifically, in low permeable coal seams, permeability was the critical parameter and the effects of other parameters were insignificant. Similarly, if the diffusion coefficient of coal seam gas was very low, it could become a critical parameter.

Evaluation of gas production from multiple coal seams: A simulation study and economics

Gas production from multiple coal seams has become common practice in many coal basins around the world. Although gas production rates are typically enhanced, the economic viability of such practice is not well studied. In order to investigate the technical and economic feasibility of multiple coal seams production, reservoir simulation integrated with economics modelling was performed to study the effect of important reservoir properties of the secondary coal seam on production and economic performance using both vertical and horizontal wells. The results demonstrated that multiple seam gas production of using both vertical and horizontal wells have competitive advantage over single layer production under most scenarios. Gas content and permeability of the secondary coal seam are the most important reservoir properties that have impact on the economic feasibility of multiple seam gas production. The comparison of vertical well and horizontal well performance showed that horizontal well is more economically attractive for both single well and gas field. Moreover, wellhead price is the most sensitive to the economic performance, followed by operating costs and government subsidy. Although the results of reservoir simulation combined with economic analysis are subject to assumptions, multiple seam gas production is more likely to maintain profitability compared with single layer production.

Study on Permeability Improvement Technology by Injecting Air into the Gas Drainage Borehole in Low‐Permeability Coal Seam

Gas extraction cycle is too long in low‐permeability coal seam. In order to solve the problem, the basic principle about gas drainage drilling for gas injection technology is studied to increase permeability. And the mathematical model is established. Gas is injected into the low‐permeability coal seam by numerical simulation. The results indicate that the best condition is a negative pressure drainage at 26 kPa and a gas injection pressure at 0.6 MPa in the vertical direction and in the horizontal direction of the injection hole. In Shanxi Daping Coal Mine 3113 working face, the field test is implemented. As a result, the test is successful. During the 14 d gas injection constantly, gas content of coal seam is reduced from 12.33 m3/t to 7.12 m3/t, greatly reducing the risk of coal and gas outburst elimination time required.

An integrated GIS-based approach for geohazards risk assessment in coal mines

From the viewpoint of safety in underground coal mining, the most suitable mining panel is the one with minimum geological structures, the right machinery, and equipment selection, trained employee, and proficient stope management. Since the ground parameters are uncontrollable and inherent uncertainties exist, a high percent of risk will usually accompany the underground coal mining activities. The main purpose of this study is to present a geological–geotechnical risk assessment model for identification of high risk-prone areas in underground coal mines using an integrated GIS-geostatistics system. Tabas as the first mechanized and largest underground coal mine in Iran was selected as a case study in this study. Gas content of coal seam, Coal Mine Roof Rating (CMRR), initial in situ stress state, fault throw, and orientation were selected as hazard/risk factors. For estimating the amount of coal seam gas content, CMRR and initial in situ stress in unsampled areas and providing the prediction maps, geostatistics module in ArcGIS was used. Rock engineering system–interaction matrix method was used for attribute weight assignment. Next, the attribute layers were weighted, rated, and overlaid to create a final map of geohazards risk. The analysis results of final risk map indicate that about 45% of under study area is prone to high to very high geohazards risk. Comparison of the results with experiences obtained during the early part of the mine and mined-out panels showed generally good agreement with promising ideas. This highlights the potential application of the GIS-based approach for hazards detection and geohazards risk assessment in underground coal mines.

ТЕXНИЧЕСКИЕ ОСНОВЫ СОЗДАНИЯ ИННОВАЦИОННОГО УСТРОЙСТВА И ОПЕРАТИВНОГО МЕТОДА ОПРЕДЕЛЕНИЯ ГАЗОНОСНОСТИ УГОЛЬНОГО ПЛАСТА

ТЕXНИЧЕСКИЕ ОСНОВЫ СОЗДАНИЯ ИННОВАЦИОННОГО УСТРОЙСТВА И ОПЕРАТИВНОГО МЕТОДА ОПРЕДЕЛЕНИЯ ГАЗОНОСНОСТИ УГОЛЬНОГО ПЛАСТА

Geological controls on variable gas concentrations: A case study of the northern Gujiao Block, northwestern Qinshui Basin, China

A systematic investigation was performed on the variation of gas content and its geological controls based on the diverse geological data and the experimental results of 72 coal samples from 24 coalbed methane (CBM) wells in the northern Gujiao Block, NW Qinshui Basin, China. Coal rank varies from low-volatile bituminous coal to medium-volatile bituminous coal, with maximum vitrinite reflectance (Ro,max) ranging from 1.21 to 1.92%. CBM in the northern Gujiao Block is mainly produced from the relatively thick coal seams (∼1.0 m-7.0 m), which are distributed in the Upper Carboniferous Taiyuan formation and Lower Permian Shanxi formation. The total gas content of coal seams ranges from 3.83 to 16.82 m3/t, with increasing gas content from marginal to central areas. Highly variable gas content reflects the comprehensive effect of various geological factors and coal reservoir properties. The results of the study show that the structural types, roof lithology, burial depth and hydrodynamic conditions are key factors controlling the gas content, whereas coal thickness and coal properties have no or little correlation with the gas content. In the northern Gujiao Block, three geological features controlling gas content are identified: 1) hydrodynamic trapping of gas in the graben structure with an adequate burial depth and impermeable roof, 2) gas loss by hydrodynamic flushing and normal faults in the syncline structure, and 3) gas migration by intensive movement of groundwater in the shallow burial area. The first mechanism is considered to cause the high gas content of central study area, whereas the other two mechanisms can explain the low gas content in the north-western and south-eastern regions.

Research on mechanism of quantity discharge of firedamp from coal drift of headwork surface reflect coal and gas outburst

The flow of gas in the coal bed is an extremely complex process, and the quantity of gas emission is mainly affected by both gas pressure in coal seam and permeability coefficient of coal seam. The influence factors of quantity discharge of firedamp from coal drift of headwork surface are analyzed, and the characteristics of discharge of firedamp from coal drift of headwork surface are closely related to the factors such as geological structure, gas content, gas pressure and permeability of coal seam. Not only the mathematical and physical model of one-way fluxion of discharge of firedamp from coal drift of headwork surface is established, but also the calculation formula of the quantity discharge of firedamp from coal drift is obtained by applying of variable separation method. According to the formula, the stagnation movement, which is among coal unloading pressure zone of stress, the stress concentration zone and original stress zone stress stagnation and the firedamp package of phenomenon, is analyzed in the coal and gas outburst mine. While the gas permeability of coal seam correspondingly changes variation, the quantity gas discharge also has the corresponding dynamic change. The results show that when the coal body in front of coal drift is in a non equilibrium state in the interaction among the horizontal crustal stress by gravity, gas pressure and mechanical properties of the coal, there is a risk of coal and gas outburst.

CO2 gas fracturing: A novel reservoir stimulation technology in low permeability gassy coal seams

Coal mine methane (CMM) is one of the major safety hazards in China’s gassy coal mines. To prevent the gas related mining hazards, effective gas drainage is a pivot technology to reduce the gas content in coal seams. For low permeability coal seams, permeability enhancement is one of the top challenges for enhancing gas drainage efficiency. In the study, a novel methane drainage technology for coal permeability enhancement, namely CO2 gas fracturing (CO2-Frac), has been proposed and investigated in Luan coal mine field. CO2-Frac is achieved by multiple stage directional CO2 gas fracturing with a newly renovated CARDOX system. Through optimization of the CO2-Frac boreholes parallel to face cleats, a complex and high permeability fracture network was generated in CO2-Frac borehole and hence the improvement of methane gas drainage effectiveness. Mine-scale tests of CO2-Frac have been conducted in 5 coal mines in Luan mine field from December 2011 to December 2015. Methane concentrations and flow rates from both CO2-Frac boreholes and adjacent boreholes without CO2-Frac were continuously monitored for evaluating the stimulation efficiency. It was observed that permeability and methane drainage effectiveness in the CO2-Frac boreholes was significantly improved: (1) gas permeability increases up to 20–50 folds; (2) extracted methane concentration increases ∼10 folds ranging from 50 to 90%, compared the concentration of 5–30% in normal boreholes without CO2-Frac; (3) methane rate in a single CO2-Frac hole varies from 140 to 280m3/d with an increase of 10–20 folds, compared the rate of 14–20m3/d in normal boreholes; (4) methane content of the coal seam in drainage area reduced from original 11.7m3/t to 6.5m3/t after 210days of drainage; (5) upgraded methane from CO2-Frac boreholes can be better utilized in air ventilation methane (AVM) power plant. CO2-Frac is confirmed as the best technology to drainage methane in this low permeability coal seams in Luan mine field, and it is not only beneficial on coal mine gas control and safety management but the CMM economic utilization by upgrading drained methane gas concentration.

GAS CONTENT APPRAISAL OF SHALLOW COAL SEAMS IN THE SOUTH PALEMBANG BASIN OF SOUTH SUMATRA

The gas content of coal seams in the South Palembang basin has been assessed using well log data and core analysis. The seams are stratigraphically exposed at shallow levels ( 39 wt. %, and fixed carbons of >43 wt. %. The amount of gas within individual coal layer varies from 4.1-5.3 m3 /t, but the gas content tends to increase with deeper burial. The sum of the estimated gas-in-place is approximately 3,019x106 m 3 . In addition, the onset of gas generation within these near surface coals may occur due to biogenic controls during the coal-forming processes, as a result of sedimentation of the overburden in Pliocene time prior to the subsequent basin inversion commencing in the Plio-Pleistocene.

Numerical investigations of methane flow characteristics on a longwall face Part II: Parametric studies

To further understand methane flow characteristics on a longwall face, this study focuses on the investigation of methane flow behaviour under six different operating and mining conditions, namely, the impact of ventilation in terms of both flow rate and the ventilation scheme, coal seam gas content, adjacent gas bearing strata, drum sprays and the maingate (MG) curtain. Modelling results obtained from the parametric studies were compared with the corresponding base model results, through which the impact of single parameter on the methane distribution characteristics along the longwall face was investigated. Modelling results indicate that: 1) flow rate has major impact on the methane concentration around the shearer and at the inner tailgate (TG) corner while the impact is minor at the upper TG corner; 2) methane emission from adjacent gas bearing strata significantly affects the methane distribution at the immediate goaf and at the upper TG corner of the face; 3) the use of a TG cut through behind the face is very effective in diluting the methane accumulated at the upper TG corner; 4) methane levels in the vicinity of drums and downstream is reduced greatly with the operation of drum sprays.