Variation in gas drainage rate from a coal seam during mining

Gas flow patterns during draining of gas from a coal seam during mining are discussed. The coal seam is treated as a dual medium with both pores and cracks. The seepage, diffusion, and desorption processes are treated using a gas flow equation that describes flow around drill holes. MATLAB is used to solve the differential equations. The permeability tracer test results from a mined coal seam are used to study the variation in gas drainage from a coal seam during mining. The results show that mining can increase the permeability of a coal seam, which then increases the gas drainage. There are inflection points in this variation over time. A close relationship between this variation and the rate of change in coal seam permeability is observed.     

构造煤及其对煤与瓦斯突出的控制作用

高空隙率、低透气性使构造煤能够保持较高的瓦斯压力 ;破碎性、“隔离”作用及“气垫”作用 ,使构造煤抵御外力作用的能力大大降低 ;构造煤变形幅度大的特性 ,为瓦斯的迅速解吸、放散和快速流动创造了条件 ;构造煤薄弱分层或“通道层”的存在 ,则为煤与瓦斯突出的初始激发和持续发展奠定了基础 ;上述因素的共同作用 ,影响和制约了煤与瓦斯突出的强度和分布 .尽管如此 ,一定厚度的构造煤的存在只是发生煤与瓦斯突出的必要条件和有利条件 ,而非充分条件 .     

长平矿“立体式”瓦斯抽采模式的探索与实践

瓦斯抽放是防治煤矿瓦斯灾害事故的根本措施,长平矿经过几年的探索实践,逐步形成了煤层瓦斯抽放、地面钻井抽放及开掘底板岩巷穿层钻孔抽放等独具特色的立体式瓦斯抽放模式,多措并举,大大降低了煤层的瓦斯含量及矿井瓦斯涌出量,为进一步提高矿井瓦斯抽放率及瓦斯治理技术奠定了坚实基础．     

煤层瓦斯抽放钻孔的二次封孔方法研究

针对煤层钻孔抽放瓦斯后煤层孔隙裂隙发育致使瓦斯浓度下降,钻孔有效抽放期缩短的技术难题,首次提出二次封孔方法.阐述了二次封孔方法的基本原理,建立了微细膨胀粉料颗粒在孔隙裂隙中运动的数学模型,并在晋城煤业集团公司寺河矿开展了6个月的现场工业性试验.结果表明:应用二次封孔方法可提高瓦斯抽放浓度25%～50%,延长钻孔的有效瓦斯抽放期3个月.     

A case study of gas drainage to low permeability coal seam

Gas drainage at low gas permeability coal seam is a main barrier affecting safety and efficient production in coal mines. Therefore, the research and application of drainage technology at low permeability coal seam is a key factor for gas control of coal mine. In order to improve the drainage effect, this paper establishes a three-dimensional solid-gas-liquid coupling numerical model, and the gas drainage amounts of different schemes are examined inside the overburden material around the goaf. The Yangquan mine area is selected for the case study, and the gas movement regularity and emission characteristics are analyzed in detail, as well as the stress and fissure variation regularity. Also examinations are the released gas movement, enrichment range and movement regularity during coal extraction. Moreover, the gas drainage technology and drainage parameters for the current coal seam are studied. After measuring the gas drainage flow in-situ, it is concluded that the technology can achieve notable drainage results, with gas drainage rate increase by 30%–40% in a low permeability coal seam.     

Effect of protective coal seam mining and gas extraction on gas transport in a coal seam

A gas–solid coupling model involving coal seam deformation,gas diffusion and seepage,gas adsorption and desorption was built to study the gas transport rule under the effect of protective coal seam mining.The research results indicate:(1) The depressurization effect changes the stress state of an overlying coal seam and causes its permeability to increase,thus gas in the protected coal seam will be desorbed and transported under the effect of a gas pressure gradient,which will cause a decrease in gas pressure.(2) Gas pressure can be further decreased by setting out gas extraction boreholes in the overlying coal seam,which can effectively reduce the coal and gas outburst risk.The research is of important engineering significance for studying the gas transport rule in protected coal seam and providing important reference for controlling coal and gas outbursts in deep mining in China.     

Nitrogen enhanced drainage of CO_2 rich coal seams for mining

Coal seams with high CO_2 gas contents can be difficult to drain gas for outburst management. Coal has a high affinity for CO_2 with adsorption capacities typically twice that of CH_4. This paper presents an analysis of nitrogen injection into coal to enhance drainage of high CO_2 gas contents. Core flooding experiments were conducted where nitrogen was injected into coal core samples from two Australian coal mining basins with initial CO_2 gas contents and pressures that could be encountered during underground mining. Nitrogen effectively displaced the CO_2 with mass balance analysis finding there was only approximately 6%–7% of the original CO_2 gas content residual at the end of the core flood. Using a modified version of the SIMED II reservoir simulator, the core flooding experiments were history matched to determine the nitrogen and methane sorption times. It was found that a triple porosity model(a simple extension of the Warren and Root dual porosity model) was required to accurately describe the core flood observations. The estimated model properties were then used in reservoir simulation studies comparing enhanced drainage with conventional drainage with underground in seam boreholes. For the cases considered, underground in seam boreholes were found to provide shorter drainage lead times than enhanced drainage to meet a safe gas content for outburst management.     

煤体瓦斯渗透性的电场响应研究

通过试验研究了施加交变电场条件下的煤体瓦斯渗透特性，并分析了其作用机理．研究结果表明，煤体瓦斯渗透率对电场有明显的响应：施加电场后，煤体瓦斯的渗透率提高，并且随着电场作用频率和强度的增加而提高；其作用机理是：外加电场作用使煤瓦斯分子热运动加剧，吸附势阱降低，吸附量降低，活性提高，增强了瓦斯的解吸和扩散，使煤体瓦斯的有效渗透通道增大，这对于提高煤矿瓦斯排放率和煤层气的开发利用，对于防治煤矿瓦斯灾害都有重要的现实意义。     

Reservoir reconstruction technologies for coalbed methane recovery in deep and multiple seams

Multiple coal seams widely develop in the deep Chinese coal-bearing strata. Ground in situ stress and coal seam gas pressure increase continuously with the increase of the mining depth, and coal and gas outburst disasters become increasingly severe. When the coal is very deep, the gas content and pressure will elevate and thus coal seams tends to outburst-prone seams. The safety and economics of exploited firstmined coal seams are tremendously restricted. Meanwhile, the multiple seams occurrence conditions resulted in different methane pressure systems in the coal-bearing strata, which made the reservoir reconstruction of coal difficult. Given the characteristics of low saturation, low permeability, strong anisotropy and soft coal of Chinese coal seams, a single hydraulic fracturing surface well for reservoir reconstruction to pre-drain the coalbed methane(CBM) of multiple seams concurrently under the different gas pressure systems has not yet gained any breakthroughs. Based on analyses of the main features of deep CBM reservoirs in China, current gas control methods and the existing challenges in deep and multiple seams, we proposed a new technology for deep CBM reservoir reconstruction to realize simultaneous high-efficiency coal mining and gas extraction. In particular, we determined the first-mined seam according to the principles of effectiveness and economics, and used hydraulic fracturing surface well to reconstruct the first-mined seam which enlarges the selection range of the first-mined seam. During the process of mining first-mined seam, adjacent coal seams could be reconstructed under the mining effect which promoted high-efficiency pressure relief gas extraction by using spatial and comprehensive gas drainage methods(combination of underground and ground CBM extraction methods). A typical integrated reservoir reconstruction technology, ‘‘One well for triple use", was detailed introduced and successfully applied in the Luling coal mine. The application showed that the proposed technology could effectively promote coal mining safety and simultaneously high-efficiency gas extraction.     

Pore characterization of different types of coal from coal and gas outburst disaster sites using low temperature nitrogen adsorption approach

To characterize the pore features of outburst coal samples and investigate whether outburst coal has some unique features or not, one of the authors, working as the member of the State Coal Mine Safety Committee of China, sampled nine outburst coal samples(coal powder and block) from outburst disaster sites in underground coal mines in China, and then analyzed the pore and surface features of these samples using low temperature nitrogen adsorption tests. Test data show that outburst powder and block coal samples have similar properties in both pore size distribution and surface area. With increasing coal rank, the proportion of micropores increases, which results in a higher surface area. The Jiulishan samples are rich in micropores, and other tested samples contain mainly mesopores, macropores and fewer micropores. Both the unclosed hysteresis loop and force closed desorption phenomena are observed in all tested samples. The former can be attributed to the instability of the meniscus condensation in pores,interconnected pore features of coal and the potential existence of ink-bottle pores, and the latter can be attributed to the non-rigid structure of coal and the gas affinity of coal.     

Greenhouse gas emissions from shallow uncovered coal seams

This study discusses a method of quantifying emissions from surface coal mining that has been trialled in Australia. The method is based on direct measurement of surface emissions from uncovered coal seams in mine pits, concurrent measurement of residual gas content of blasted coal in mine pits, and measurement of pre-mining gas content of the same seam from cores retrieved from exploration boreholes drilled away from active mining. The results from one of the mines studied are presented in this paper. In this mine, the pre-mining gas content of the target seam was measured using cores from an exploration borehole away from active mining. Gas content varied from 0.7 to 0.8 m³/t and gas composition varied from 16% to 21% CH₄ (84–79% CO₂). In-pit measurements included seam surface emissions and residual gas content of blasted and ripped coal. Residual gas content varied from 0.09 to 0.15 m³/t, less than twofold across the mine pit. Composition of the residual gas was in general 90% CO₂ and 10% CH₄, with slight variation between samples. Coal seam surface emissions varied from 1.03 to 7.50 mL of CO₂-e per minute and per square meter of the coal seam surface, a sevenfold variation across the mine pit.     

水力压裂煤储层卸压增透技术的适用性分析

水力压裂煤储层技术在不同矿区应用过程中受不同煤体破坏类型和围压条件的影响,其卸压增透效果差异性较大.为了明确该项技术的井下适用条件,优化其实施工艺,切实提高煤层瓦斯抽采率,通过分析和总结河南省不同矿区实际煤储层的水力压裂试验数据资料发现,水力压裂增透技术对Ⅰ,Ⅱ破坏类型煤体的增透效果比较明显,而在Ⅲ,Ⅳ,Ⅴ破坏类型的松软煤层中适用性则具有一定的局限性,具有压裂范围小、裂缝闭合快、增透效果不明显等特点.所得结论对于选择合理的水力压裂储层并进一步优化完善高效预抽本煤层瓦斯技术具有一定的指导意义.     

Regional gas drainage techniques in Chinese coal mines

China’s rapid economic development has increased the demand for coal. These results in Chinese coal mines being extended to deeper levels. The eastern Chinese, more economical developed, regions have a long history of coal mining and many coal mines have now started deep mining at a depth from 800 to 1500 m. This increase in mining depth, geostresses, pressures, and gas content of the coal seam complicates geologic construction conditions. Lower permeability and softer coal contribute to increasing numbers of coal and gas outburst, and gas explosion, disasters. A search on effective methods of preventing gas disasters has been provided funds from the Chinese government since 1998. The National Engineering Research Center of Coal Gas Control and the Huainan and Huaibei Mining Group have conducted theoretical and experimental research on a regional gas extraction technology. The results included two important findings. First, grouped coal seams allow adoption of a method where a first, key protective layer is mined to protect upper and lower coal seams by increasing permeability from 400 to 3000 times. Desorption of gas and gas extraction in the protected coal seam of up to 60%, or more, may be achieved in this way. Second, a single seam may be protected by using a dense network of extraction boreholes consisting of cross and along-bed holes. Combined with this is increased use of water that increases extraction of coal seam gas by up to 50%. Engineering practice showed that regional gas drainage technology eliminates regional coal and gas outburst and also enables mining under low gas conditions. These research results have been adopted into the national safety codes of production technology. This paper systematically introduces the principles of the technology, the engineering methods and techniques, and the parameters of regional gas drainage. Engineering applications are discussed.     

利用采空区瓦斯流动规律进行瓦斯抽放的探讨

通过对采空区抽放瓦斯试验进行分析,找出了试验失败的原因．在对采空区瓦斯流动规律分析研究的基础上,提出了采空区瓦斯抽放的设想,为采空区瓦斯抽放提出了新的理论依据．研究认为,进行采空区瓦斯抽放不仅有利于大气环境保护和安全生产,而且还可以创造出可观的经济效益．     

利用采空区瓦斯流动规律进行瓦斯抽放的探讨

通过对采空区抽放瓦斯试验进行分析 ,找出了试验失败的原因 .在对采空区瓦斯流动规律分析研究的基础上 ,提出了采空区瓦斯抽放的设想 ,为采空区瓦斯抽放提出了新的理论依据 .研究认为 ,进行采空区瓦斯抽放不仅有利于大气环境保护和安全生产 ,而且还可以创造出可观的经济效益 .     

Pressure relief, gas drainage and deformation effects on an overlying coal seam induced by drilling an extra-thin protective coal seam

Numerical simulations and field tests were used to investigate the changes in ground stress and deformation of, and gas flow from, a protected coal seam under which an extra-thin coal seam was drilled. The geological conditions were: 0.5 meter min-ing height, 18.5 meter coal seam spacing and a hard limestone/fine sandstone inter-stratum. For these conditions we conclude: 1) the overlying coal-rock mass bends and sinks without the appearance of a caving zone, and 2) the protected coal seam is in the bending zone and undergoes expansion deformation in the stress-relaxed area. The deformation was 12 mm and the relative defor-mation was 0.15%. As mining proceeds, deformation in the protected layer begins as compression, then becomes a rapid expansion and, finally, reaches a stable value. A large number of bed separation crannies are created in the stress-relaxed area and the perme-ability coefficient of the coal seam was increased 403 fold. Grid penetration boreholes were evenly drilled toward the protected coal seam to affect pressure relief and gas drainage. This made the gas pressure decrease from 0.75 to 0.15 Mpa, the gas content de-crease from 13 to 4.66 m3/t and the gas drainage reach 64%.     

Interlayer interference mechanism of multi-seam drainage in a CBM well:An example from Zhucang syncline

Based on the characteristics of the strong volatility of physical property in vertical direction, high gas content, high resource abundance and large exploitation potentiality of coal reservoir in Bide-Santang basin of Zhina coal field, we study the generation mechanism of interlayer interference, propagation rules of reservoir pressure drop and influencing factors of gas productivity in CBM multi-seam drainage in the paper. On the basis of the actual production data of X-2 well of Zhucang syncline in Bide-Santang basin,by simulating the gas production process of a CBM well under the condition of multiple seam with COMET3 numerical simulation software, we analyze the influencing factors of gas productivity during the process of multi-seam drainage, and illuminate the interlayer interference mechanism of multiseam drainage. The results show that permeability, reservoir pressure gradient, critical desorption pressure and fluid supply capacity of stratum have great influence on gas productivity of multi-seam drainage while coal thickness has little influence on it. Permeability, reservoir pressure gradient and fluid supply capacity of stratum affect the propagation velocity of reservoir pressure drop and thereby affect the final gas productivity. Moreover, the influence of critical desorption pressure on gas productivity of multiseam drainage is reflected in the gas breakthrough time and effective desorption area.     

A rapid and accurate direct measurement method of underground coal seam gas content based on dynamic diffusion theory

Coal seam gas content is frequently measured in quantity during underground coal mining operation and coalbed methane (CBM) exploration as a significant basic parameter. Due to the calculation error of lost gas and residual gas in the direct method, the efficiency and accuracy of the current methods are not inadequate to the large area multi-point measurement of coal seam gas content. This paper firstly deduces a simplified theoretical dynamic model for calculating lost gas based on gas dynamic diffusion theory. Secondly, the effects of various factors on gas dynamic diffusion from coal particle are experimentally studied. And sampling procedure of representative coal particle is improved. Thirdly, a new estimation method of residual gas content based on excess adsorption and competitive adsorption theory is proposed. The results showed that the maximum error of calculating the losing gas content by using the new simplified model is only 4%. Considering the influence of particle size on gas diffusion law, the particle size of the collected coal sample is below 0.25 mm, which improves the measurement speed and reflects the safety representativeness of the sample. The determination time of gas content reduced from 36 to 3 h/piece. Moreover, the absolute error is 0.15–0.50 m³/t, and the relative error is within 5%. A new engineering method for determining the coal seam gas content is developed according to the above research.     

Principle and engineering application of pressure relief gas drainage in low permeability outburst coal seam

With the increase in mining depth, the danger of coal and gas outbursts increases. In order to drain coal gas effectively and to eliminate the risk of coal and gas outbursts, we used a specific number of penetration boreholes for draining of pressure relief gas. Based on the principle of overlying strata movement, deformation and pressure relief, a good effect of gas drainage was obtained. The practice in the Panyi coal mine has shown that, after mining the Cllcoal seam as the protective layer, the relative expansion deformation value of the protected layer C13 reached 2.63%, The permeability coefficient increased 2880 times, the gas drainage rate of the C13 coal seam increased to more than 60%, the amount of gas was reduced from 13.0 to 5.2 m3/t and the gas pressure declined from 4.4 to 0.4 MPa, which caused the danger the outbursts in the coal seams to be eliminated. The result was that we achieved a safe and highly efficient mining operation of the C 13 coal seam.     

潘三矿立井揭煤瓦斯抽采模拟研究

瓦斯抽采对于立井揭突出煤层起到重要的作用,准确的确定钻孔瓦斯有效抽采半径和合理的在待抽煤层中布置抽采钻场对煤层消突具有关键性作用。基于多孔介质中流体流动达西定律理论,采用COMSOL Multiphysics软件对该煤层瓦斯抽采进行了模拟。模拟结果表明,此煤层的瓦斯有效抽采半径为3 m,随着抽采时间的增加,煤层瓦斯压力逐渐的降低,但降低的速率会逐渐的减小。瓦斯抽采30天后,其残余的瓦斯压力为0．18 MPa ,这与现场实测的最大残余瓦斯压力0．2 MPa相接近,这说明了模型的可信性,其模拟结果可为瓦斯抽采设计提供参考。