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第 155 页英文原文Research into Comprehensive Gas Extraction Technology of Single Coal Seams with Low Permeability in the Jiaozuo Coal Mining AreaFu Jiangwei (a.b), Fu Xuehai (a), Hu Xiao (c), Chen Li (a), Ou Jianchun (d) a:School of Resource and Earth Science, China University of Mining & Technology, Xuzhou221008, Chinab:Henan Provincial Coal Seam Gas Development and Utilization Co, LTD, Zhengzhou 450016,Chinac:Department of Industry and Business Administration, Henan Engineering College,Xinzheng451191, Chinad:Faculty of Safety Engineering, China University of Mining & Technology, Xuzhou 221008, ChinaAbstract: For a low permeability single coal seam prone to gas outbursts, pre-drainage of gas is difficult and inefficient, seriously restricting the safety and efficiency of production. Radical measures of increasing gas extraction efficiency are pressure relief and infrared antireflection. We have analyzed the effect of mining conditions and the regularity of mine pressure distribution in front of the working face of a major coal mine of the Jiaozuo Industrial (Group) Co. as our test area, studied the width of the depressurization zone in slice mining and analyzed gas efficiency and fast drainage in the advanced stress relaxation zone. On that basis, we further investigated and practiced the exploitation technology of shallow drilling, fan drilling and grid shape drilling at the working face. Practice and our results show that the stress relaxation zone is the ideal region for quick and efficient extraction of gas. By means of an integrated extraction technology, the amount of gas emitted into the zone was greatly reduced, while the risk of dangerous outbursts of coal and gas was lowered markedly. This exploration provides a new way to control for gas in working faces of coal mines with low permeability and risk of gas outbursts of single coal seams in the Jiaozuo mining area.Key words: Single coal seam with low permeability; High risk gas and outburst coal seam; Stress relaxation zone; Comprehensive gas extraction; Jiaozuo coal mining area; Gas emission1 IntroductionAntireflection pressure relief is an effective method to improve gas extraction rates. Based on mining pressure relief theory, protecting the exploitation of coal seams has become an effective way of controlling gas in multi- seam mining in China. Protecting the exploitation of seams not only reacts to antireflection pressure relief of coal outbursts, but can also effectively release gas and ground stress and hence has become an effective method of prevention of coal and gas outbursts. Lacking adequate conditions for protecting the exploitation of a single seam with low permeability and a high risk of gas outbursts, pre- mining drainage of coal seams that have been worked is the only method to control regional gas controlling. However, high gas outburst seams, with low permeability and poor drilling conditions, require long pre-drainagetimes and the existence of blank tapes at working faces. Not only does this threaten the conditions for safe production, but also seriously restrict the safety and productivity of coal mines. Theoretical research and practice have shown that coal is in a state of pressure relief affected by mining activity within a specified range of the stope, whose flow- increasing effect appears as antireflection. According to flow theory the occurrence of coal seam gas, given the conditions of similar basic gas-geology, antireflection pressure relief is the most important factor controlling the amount of gas draining from drill holes. Based on this theory, we have used the example of the Jiaozuo mining area as a typically developed single low permeability and high risk gas and coal bed outburst area to study the technical foundation of gas extraction, the distribution of pressure release zones and the increasing effect of permeation at its working face, affected by mining activity. Given the theoretical basis for gas extraction in pressure release zones, we carried out studies and applications of integrated gas extraction technology in single coal seams with low permeability. Practice has shown that pressure release zones are ideal regions for highly efficient gas exploitation, with marked effects of its utility model that suppresses the risks of emission and coal and gas outbursts.2 Strata behavior regularity of excavation working face and width observation of stress relaxation zoneBecause of the effect of coal mining activity, support conditions change. In a sequence from the bottom of the mined zone, followed by the formation of the caving-, the fracture- and bending-subsidence zones above the gob area, roof strata movements change. Roof strata stress of different coal seams in front of the working face may change along the seams to the front followed by the formation of a pressure relief zone, a stress concentration zone and an original stress zone (see Fig. 1). Because of top coal caving from a false leaf trace roof, the immediate roof caves as mining progresses and in the end the cantilevered roof will collapse, resulting in a wide range of overlying strata relief. Given the decrease of the support pressure of the coal in the pressure relief range, a stress relaxation zone will be formed in first instance. With the release of the coal stress, where original fractures open or expand. New fractures will be generated and regional permeability of coal increases rapidly. Part of the adsorbed gas is analyzed and together with the free gas this adsorbed gas migrates rapidly to the free space, showing a flow effect by relief, which is the ideal area to improve the effect of pumping gas. Within a specified range below the stress relaxation zone, due to the cyclical destruction and instability of the rock wall, coal pressure is transferred downward to form a stress concentration zone, where stress will increase, fractures and large pores will close, decreasing the permeability of the coal seam and hence, the conditions for desorption and seepage of coal gas. Below this level, coal and its rock mass have not yet been affected by mining activity. This zone bears normal stress, called the original stress zone, where the gas flow is not affected by mining.Theoretical studies suggest that the width of the stress relaxation zone is proportional to the thickness of the coal seam, while the side pressure coefficient and the depth of coal mining are inversely proportional to the friction coefficient of the interface and the tensile strength of coal.Because differences among some factors in the same mine are not large, we can analyze the variation in underground pressure and amount of gas flow from the boreholes at the working face to investigate the width of the stress relaxation zone. The Jiaozuo coalfield is a typical low permeability and high gas outburst single coal seam zone in China. The main seam is the #21 coal seam of Shanxi formation, with an average thickness of approximately 6 m, a high coal gas content (generally 2034 m3/t daf), and high pressure (0.72.42 MPa). The measured seam permeability coefficient ranges between 0.11 and 5.86 m2/(MPa2d). Pre-pumping the coal gas is the most important technology to remove the risk of an outburst at the working face. Currently, the major mining methods at Jiaozuo are top-coal mining and slice mining, with mine technologies such as inclined longwall mining and fully- mechanized mining. During our tests, we selected from the Jiaozuo Industrial (Group) Co, LTD, the working faces of the #14101 Jiulishan seam and of the #22041 Yanmazhuang seam. Given the variation in mine pressure distribution in different stress zones and gas flow patterns, we investigated the width of the advanced stress relaxation zone at both of working faces with the help of two conditions, i.e., the underground pressure at the working face and the amount of gas flowing from boreholes. From our investigation of mine pressure distribution regularity and the variation of gas flows in the various stress zones (Figs. 2 and 3), we see that the advanced work concentrates stress in front of the working face and exceeds the constant. Beyond 20 m, the coal seams gradually become areas of stress concentration, where roof pressure increases, coal fractures close, permeability reduces, gas flows from drilling decreases to an average gas flow of only 0.03 m3/min. Within 1020 m from the face, the coal forces the stress concentration zone into the transition zone and further into the stress relaxation zone, where permeability and gas flow from drilling gradually increase to 0.12 m3/min. Less than 10 m from the face and as the distance to the working face decreases, the effective extraction length of drilling is reduced, resulting in a declining gas flow. Because the roof lets go and collapses in this area, the pressure in the coal seam is fully released and the fractures in the coal seam increase substantially. The volume extracted by drilling is still increasing. Given our data on mining pressure at the working face and our investigation of gas flows, we consider the range of the full stress relaxation zone in front of a slice of the mining working face to be 2030 m, where a distance of 20 m or less is clearly within the range of a stress relaxation zone under normal conditions.4.2. Analysis of basic technology of gas extraction in preact stress relaxation zoneMethane drainage is not only related to drainage time, negative drainage pressure, the diameter of the drill hole, drilling depth but also to other parameters depends on the permeability of coal seams. When underground coal is not affected within an influence radius of drilling during the initial stage of drainage, coal fractures are connected, adsorbed gas is gradually desorbed and the extraction results are quite good. When the drainage time is extended, the gas flow from drilling attenuates quickly. As mining activities are carried out, the stress balance in the coal is broken; stress redistribution takes place in front of the working face to reach a newbalance. As the working face advances, the front of the coal is unloaded within a specific pressure range and coal fractures become further connected, forming a stress relaxation zone. Meanwhile, within a specified range of deep coal, a higher stress appears, holes and fractures close, forming a stress concentration zone in the area of mining activities. In this area, not only is construction difficult, but poor permeability is also a problem, given the original stress area in this deep area. In stress relaxation zones, the permeability of coal seams increase, the conversion rate of the adsorbed gas to free gas increases, a new rise in the amount of drill gas inevitably occurs, thus making the area an ideal region for drawing gas fast and efficiently. According to a study by Zhou and Lin, an application of this technology to draw gas into the stress relaxation zone, shrinkage of coal will certainly occur, giving rise to two benefits. On the one hand, coal strength will be increased; on the other hand the permeability of coal seams is further improved, thus reducing the methane pressure gradient and effectively preventing coal and gas outbursts. As the coal working face advances continuously, there will be always stress relaxed coal in certain areas of the working face. The determination of the head relief pressure area of a coal mining face is the basis for a suitable arrangement of gas drill holes and for strengthening methane drainage measures and management in a stress relaxation zone. Therefore, reducing the risk of gas emissions for the purpose of safe mining can be achieved by strengthening the management of pre-drainage holes, arranging drainage holes suitably and optimizing drainage parameters. Such achievements should be applied at the front of coal mining areas, as well as at other parts of comprehensive drainage areas.4.3. Study and application of comprehensive gas extraction at a working face4.1 Technology of gas extraction from shallow boreholesWhen drainage holes are drilled into the working face of a coal seam, gas will flow under the effect of negative pressure in the drainage area and methane from the coal wall parallel to this flow changes to a radial flow and reduces gas emission at the working face. When the gas is extracted, the coal undergoes shrinkage, causing pressure relief to some extent which helps to reduce the risk of coal and gas outbursts. The mode of arrangement of shallow drainage boreholes has some advantages, requiring only small efforts in technical ingenuity, easy construction and high efficiency. For complex structures, soft coal development and long working faces, coal and gas outbursts can be prevented by rational control of the density and depth of shallow boreholes, and reduce gas emissions preventing accidents due to methane overrun.In the #15021 working face of Jiulishan of the Jiaozuo Industrial (Group) Co., Ltd., the coal seam gas content is 30 m3/t, the coal seam is 5.3 m thick, on top of which there is soft a coal layer 0.31.0 m thick. Prior to drilling 523 pre-drainage holes, the amount of gas drained within 100 m of the working face was 0.0023 m3/min and a large amount of gas was released into the initial mining area, of which the largest concentration of methane, after roasting, reached 1.42%. In order to reduce gas emissions during mining, drainage drilling proceeded along the outward direction of the working face, to depths of 1215 m, the holes were spaced 1.5 m apart. Immediately after their construction, each drainage hole continuously pumped gas, lasting at least 4 h. Compared to pre-drainage face holes, the methane scalar of a single hole used a shallow hole drainage method with a range of 0.020.1 m3/min. The average amount of gas drained per 100 m was0.68 m3/min hm, which improved efficiency 296 times. Data obtained from continuous monitoring of the shallow drainage holes (Fig. 4) show that over a specified time period, the drainage gas attenuation coefficient decreased to 0.00020.0021/d, indicating a significant improvement in coal seam permeability in the stress relaxation zone. Coal bed methane extraction from shallow holes or advanced emissions is effective measures to reduce gas emission during the mining at working faces.3 Technology of gas extraction from fan boreholesIn the Weicun Coal Mine of the Jiaozuo Industrial (Group) Co.,Ltd., the measured coal-bedgas content of the #14101 working face is 18.46 m3/t r. A top grade mechanized mining technique of a long-arm strike way is applied, with a fully subsident method for coal roof management. Before the extraction of the working face, pre-drainage of coal bed gas at the working face is carried out through parallel borehole bedding in the upper and lower ducts at the working face. Given a low extraction rate of the working face and in order to reduce gas extraction at the working face effectively and guarantee safe production and simultaneously constructing drainage boreholes along the strike of cutting holes, one drilling field was allocated per 100 m along the slope of the transport roadway, 20 m deep. Drainage boreholes are fanned out inside (see Fig. 5), 5060 m long. The #1 drilling field is 85 m removed from the cutting hole and boreholes in groups of four are laid out in the drilling field. Group a consists of 12 boreholes at the left working slope; Group b consists of 9 boreholes at the right front; Group c consists of 13 boreholes at the right working slope and Group d consists of 14 boreholes at the right working slope. During our experiment, we traced the amount of gas flow for each of the four groups of boreholes.Drilling the #1 field was carried out on time and the boreholes were linked. The total amount of gas drained per group of boreholes ranged from 0.34770.671 m3/min to 0.0290.072 m3/min per single borehole per 100 m, which is not a clear change compared with pre-drainage conditions. In order to evaluate the process of coal extraction at working faces with fan boreholes, we drew a trend map of the amount of gas extracted per group a boreholes as the working face advances (see Fig. 6). In Fig. 6, about 60 m away from the cutting hole, the effective drainage length decreases as the working face becomes shorter, while the total amount of gas drained continued to increase. About 60 m from the cutting hole, the bottom of the boreholes of group a generally became uncovered, with the attended effect of sharply shortening their length and a gradual decline in the total amount of gas drained. However, the boreholes were affected by an advanced pressure release zone, with a clear increase in the amount of gas drainage per 100 m. Owing to mining activity in the pressure release range, the permeability of the coal rock clearly increased. The amount of gas flow from the fan boreholes has increased 522 times in this pressure release zone. To make the most of the antireflection effect of the pressure release zone, as the working face constantly advances, drilling fields should be designed in a timely fashion in airflow and drilling roadways and fan boreholes should be allocated suitably based on the size of the pressure release zone. In other words, high-density and wide-aperture boreholes should be laid out following the cutting hole. This should improve the rate of gas drainage quickly and decrease the amount of gas drainage at the working face.4 Technology of gas extraction from grid shaped boreholesGrid-drainage of working faces, middle roadways joining upper and lower ducts is laid at a specified distances from working faces. In roadways, strike drainage boreholes are allocated along working faces. The 5.5 m thick coal seam thickness at the #22041 working face in the Yanmazhuang Coal Mine has a measured permeability coefficient of 4.15 m2/(MPa2d). Within a distance of 197 m, 258 boreholes are laid out in the transport roadway of the working face, with average space between them of 0.75 m, of which 252 are laid out in the return airway. The lengthof the cutting holes and traverse are 97 m, each of them with 35 boreholes laid out, with an average space between boreholes of 2 m and an average aperture of 94 mm. Gas extraction started when the dip boreholes in the upper duct of the #22041 working face were finished. Owing to better permeability of coal seam in this working face, the original amount of gas extracted is greater than 0.02 m3/min, with an attenuation coefficient of boreholes gas extraction of 0.0072/d (see Fig. 7a). In order to investigate the change in the a mount of gas extracted in gridding as the working face advances during the stoping period at the working face, successive ion tracking was carried out in grid shaped boreholes. The average amount of gas extracted from these boreholes is shown in Fig. 7b, where the amount of gas drainage per 100 m at the #22041 working face is seen to increase gradually within a distance of 100 m from the working face. The amount of gas extracted per 100 m has increased by more than 3 times. Especially within 40 m of the working face, gas extraction has improved markedly, implying that the region is within range of antireflection pressure relief.If we only take the mechanism of borehole extraction into account because of the space across the gridding boreholes, a mutually affecting zone and a sufficiently affecting zone are formed in the destructive region of the boreholes, which can improve fracture connectivity between boreholes and the surrounding coal rock. This is of benefit for enlarging the range of gas release of the pressure-relief coal rock and improves the speed of gas desorption, as well as the rate of gas extraction. Considering the effect of gas extraction on the #22041 working face in the Yanmazhuang Coal Mine, Within a range of 80 m from the working face, the amount of gas flows from the boreholes clearly increased, which means that the rate of coal bed gas extraction can be quickly and effectively improved before stopping of the working face, because of the effect of pressure-relief caused by mining activity. Given the results of our tests, a suitable middle roadway should be located 100200 m from the cutting hole. Within this range, gridding extraction techniques can take full advantage of the effect of pressure-relief of mining activities in improving gas extraction efficiency.b Comparison and analysis of effect of gas extraction in a stress relaxation zoneAccording to a large amount of practice and preliminary theoretical analysis, the technology of shallow drilling has a feature that for the most part is effective for drilling within the stress relaxation zone, where the fracture of coal body develops and the gas permeability coefficient increases sharply. Hence, given extraction under negative pressure, the flow direction of coal- gas changed from a unidirectional into a radial flow in the boreholes, which is very favorable for improving the rate of gas extraction and reducing coal wall gas emission. This had a significant effect on gas extraction. As can be seen from Fig. 1, along the advancing direction of the coalface, the stress concentration zone is below pressure release zone. Based on a theory of coal and gas outbursts, this is the predominant zone for coal and gas outbursts to occur. Through shallow drilling, the amount of gas and gas pressure are low within the range of controlled drilling and the stress concentration zone will migrate to the inside of the coal seam in order to eliminate or lower the danger of outbursts. Shallow drilling is a very efficient way in the high gas and outburst mine of the Jiaozuo Industrial (Group) Co., Ltd. Gas flows in measured in hectometer boreholes are more than thirty times greater than those in forepumping boreholes, where the gas density of the return air from the coal face is reduced to less than 0.5%. In general, the phenomenon of methane overrun is eliminated and the extremely high risk of outbursts is reduced from the previous 17% to less than 0.6%. Given the condition of only limited space, we do not provide a detailed introduction. Detailed test results about the technology of gas pressure relief extraction from fan boreholes and grid shaped pressure relief drilling will be provided in the near future. Effects of the amount of gas emitted during extractionDuring extraction with grid shaped boreholes at the #22041 working face, we measured gas density and the amount of air from the upper air duct during normal production and calculated the rate of gas emission (see Fig. 8). As can be seen from Fig. 8, both gas density and the amount of air at the upper air duct change at different rates within the range of grid shaped drilling. At the bottom of the trend borehole, about 100 m from the working face, gas density and gas emission are low for some time.Further along the advancing coal face, both gas density and the continuous outflow from the stress relaxation zone show a gentle incline. When the trend borehole is around 25 m from the coal face, i.e., the place where the coal face reaches 175 m, the trend for both variables, i.e., gas density and the amount of gas emitted rise again as the result of abandoning and stopping the cross headings borehole in the stress relaxation zone. During the experiment, we also investigated the gas emission from the Weicun Mine which uses fan drilling at the #14101 working face (see Fig. 9). It is seen from this figure that in the area of stopping footage between 450 and 690 m, absolute amount of gushing gas ranges from 8 to 10 m3/min. As the stopping process proceeds, the frequently high gas density occurring after blas ting, will seriously constrains safe production at the coal face. After using the drainage technology in the stress relaxation zone from the fan boreholes, the absolute amount of gushing gas decreased to 46 m3/min, a decrease of 4050%. The phenomenon of extremely high gas density after blasting has essentially been eliminated with the help of this technology. Effect of test index of coal and gas outburst hazard during extractionThe application of pressure relief and gas extraction technology can effectively reduce the absolute amount of gas emission and eliminate the hazard of coal and gas outbursts during actual mining. According to the requirements for outburst prevention at the Jiaozuo Industrial (Group) Co., Ltd. for every 2.5 m advance of the working face towards the coal bed strike, one row of 42 mm diameter drill testing holes should be arranged 1 m from the coal seam roof. The depth of these holes is 4.5 m and the distance between holes 5 m. The initial speed of gas emission q istaken as the index of inspection for outburst hazards. During our tests, the effects on each working face were observed and analyzed (Fig. 10). As seen from Fig. 10a, beyond the effective exhaust area of strike drilling on pressure relief and gas extraction at the working face, the value of q is relatively large and unstable. Otherwise, the situation is the opposite. At the same time, Fig. 10bd shows that, due to fan gas extraction boreholes, control of the top part of the #14101 working face is poor in this Weicun Coal Mine. Although the inspection index q clearly decreases within the control area, its average value at the top part of the working face is still larger than the value at the middle and lower parts; therefore, it shows that a grid shaped arrangement of gas extraction boreholes is helpful in drilling and control, while fan boreholes may still retain some uncontrollable areas due to its restriction in drilling.c Conclusions Radical measures of increasing gas extraction efficiency are pressure relief and infrared antireflection. We investigated a technology featuring efficient gas extraction in a stress relaxation zone of a single and low permeability and high risk gas and outburst coal seam without protective mining conditions, using mining activity processes of discharging pressure and flow effect. The technology is of practical importance for improving gas extraction rates, reducing the amount of gas emission, eliminating the risk of coal and gas outbursts and realizing safety in highly efficient mine exploitation. The difference of regular mine pressure distribution and variation in gas flows is a marked characteristic of stress distribution in different stress zones and in crack changes. Weused a method of investigating underground pressure at a working face and the amount of gas flows from boreholes in order to evaluate the suitable width of an advanced stress relaxation zone. Various practical extraction technologies employed at major coal mine of the Jiaozuo Industrial (Group) Co., Ltd. show that in discharging a pressure range, distressed gas extraction with either shallow drilling, fan drilling or grid shape drilling at a working face has a positive effect on gas extraction and improves the rate of gas extraction. The experiment shows that the position of the crosshead is the key to optimizing extraction of distressed gas with grid shaped drilling. The azimuth, density and length of fan-shaped drilling have considerable effect on distressed gas extraction. Small sized projects, easy construction and high efficiency are also technical features in shallow drilling of distressed gas extraction, which is not only a beneficial supplement of the former two, but also the most direct and effective means of preventing local coal and gas outbursts. Pre-drainage of gas is a necessary measure to realize safe and highly efficient production in a single and high risk gas and outburst coal seam. Applying this integrated extraction technology is the key point to increase gas extraction efficiency. Making the most of the effect of pressure relief and infrared antireflection in mining activities, especially for low permeability coal seams, the various extraction technologies described for distressed gas extraction with shallow drilling, fan drilling and grid shape drilling at working faces can be used. These gas extraction technologies should be carried out according mining slices and in stages, which should increase gas pre-drainage efficiency. This method effectively solves the problem of extremely high gas density in high risk coal and gas seams and insures safety at working faces in mines.中文译文焦作矿区低透气性单煤层综合瓦斯抽放技术研究付江伟（a,b），付学海（a），胡骁（c），陈立（a），欧建春（d）a：中国矿业大学，资源与地球学院，中国，徐州 221008b：河南省煤层气开发与利用有限责任公司，中国，郑州 450016c：河南工程学院，工商管理系，中国，新郑 451191d：中国矿业大学，安全工程学院，中国，徐州 221008 摘要：低透气性单煤层容易发生瓦斯突出，瓦斯预抽困难且效率不高，严重制约了生产安 全和效率。提高瓦斯抽放效率的基本方法有卸压释放和采动增透。我们分析了作为测试区 的焦作工业集团有限责任公司的一个主要煤矿的开采条件和回采工作面前部矿压分布规 律，得到了分层开采中应力降低区的宽度和超前应力降低区的瓦斯可以高效快速抽放的结 论。在此基础上，我们进一步研究和实践了工作面浅部钻孔技术、扇形钻孔技术和网形钻 孔技术的开发。实践以及我们的结果都显示，应力降低区是高效快速抽放瓦斯的理想区域。 综合提取技术的利用，使得溢出到工作面的瓦斯量大大减少，与此同时，煤与瓦斯突出危 险也显著降低。这次的考察研究为焦作矿区的低透气性煤层回采工作面的瓦斯控制和单煤 层瓦斯突出危险的控制提供了一个新的方法。 关键字：低透气性单一煤层；高瓦斯有突出危险煤层；应力降低区；综合抽放技术；焦作 矿区；瓦斯涌出1 简介增透卸压是提高瓦斯抽放率的一种很有效的方法。基于开采减压理论，煤层保护开采 已经成为我国多煤层开采的瓦斯治理的有效途径，煤层保护开采不仅影响煤层突出的增透 卸压，还可以有效释放瓦斯和地压，因此，已经成为防治煤与瓦斯突出的有效方法。对于 缺乏足够保护条件的低透气性单煤层和有高瓦斯突出危险的煤层的开采，煤层采前抽放是 控制区域瓦斯治理的唯一方法。然而，低透气性和钻孔条件差的高瓦斯突出煤层，工作面 采前抽放的时间较长并且存在空白区，这不仅威胁安全生产条件，而且还严重的限制了煤 矿的安全和生产力。理论研究和实践表明，受特定范围内的开采活动的影响，煤层处于应 力降低状态，继而产生的影响就是瓦斯流量的增加。根据煤层瓦斯流动理论，鉴于相似的 基本瓦斯地质条件，增透卸压是控制钻孔瓦斯排放量的重要因素。基于这种理论，我们以 焦作矿区的低透气性高瓦斯突出危险和煤层突出的单煤层作为示例来研究受采动影响的 工作面瓦斯抽采技术基础、卸压区域分布和透气性影响的增加。鉴于卸压区瓦斯抽采的理 论基础，我们提出低透气性单煤层的瓦斯抽放技术的研究与应用。实际证明，卸压区是瓦 斯高效抽采的理想区域，还对抑制瓦斯外泄和煤与瓦斯突出危险有显著的作用。2 工作面回采的矿压显现规律和应力降低区的观测宽度由于开采活动的影响和受力环境的变化，顶板岩层的运动变化使回采后的采空区向上 依次形成垮落带，裂隙带和弯曲下沉带。工作面前部煤层受顶板岩层压力不同，沿回采面 向前依次形成应力降低区、应力集中区和原始应力区（见图 1）。由于伪顶的冒落、直接顶 随工作面推进的垮落以及最终老顶的突然垮落，导致一系列上覆岩层卸压。由于应力降低范围内的煤层支撑压力的减少，应力降低区将初步形成。随着煤层应力释放，此区域的原 生裂隙形成或扩大，新裂隙逐渐形成，区域煤层的透气性性迅速增加。一部分原本处于吸 附状态的瓦斯将会脱离吸附并和自由瓦斯一同流向可用空间，呈现出“卸压流动效应”， 这是提高瓦斯抽采的最理想的区域。在应力降低区以外的一定范围内，由于顶板周期来压 的影响，煤层压力前移形成应力集中区，区域内应力增加，裂隙和空洞闭合，煤层空隙性 降低，因此，瓦斯抽放难度加大，此区域以外，煤层及其围岩没有受到开采活动的影响， 此区域应力不变，被称为原岩应力区，这里瓦斯流动未受开采影响。理论研究表明，应力降低区的宽度和煤层的厚度成正比，而侧压力系数和煤层采深与 接触面摩擦系数和煤的抗拉强度成反比。由于同一矿井中的一些因素之间的差异不大，我 们可以通过分析地应力和工作面钻孔中瓦斯流量的变化来研究应力降低区的宽度。焦作煤 田是我国的一个典型的低透气性和高瓦斯突出的单煤层区，主采煤层是山西组的#21 煤， 平均厚度是 6m，瓦斯含量高（一般 2034 m3/t daf），地压大（0.72.42 MPa）。测量得到 的煤层透气性系数范围是 0.115.86 m2/(MPa2d)。瓦斯的采前预抽是消除工作面瓦斯突出 危险的重要技术，目前，焦作的主要采煤方法是放顶煤开采和分层开采，也采用诸如倾斜 长壁开采和综合机械化开采。测试期间，我们选择了焦作工业集团有限责任公司下属的九 里山矿#14101 工作面和演马庄矿#22041 工作面。考虑到不同应力区和瓦斯流动方式下的 应力变化的不同，为了研究超前应力降低区的宽度，我们分析了这两个工作面的以下两个 条件，即，工作面地应力和钻孔内的瓦斯流量，从调查的矿山压力分布规律和不同的应力 区瓦斯流量的不同（图 2 和图 3），我们可以看到工作面前部的应力集中区超过一个常量； 20m 开外，煤层逐渐成为应力集中区，顶板压力增大，每层裂隙闭合，透气性系数降低， 钻孔内的瓦斯流量降低到平均流量仅为 0.03 m3/min；工作面前部 1020m 的范围内，煤层 应力逐渐由应力集中区到应力过渡区再到应力降低区，应力降低区内，透气性系数和钻孔 内的瓦斯流量逐渐增加到 0.12 m3/min；距工作面 10m 以内，随着距离工作面的距离降低， 钻孔的有效长度降低，从而导致瓦斯流量的下降；由于本区域采用完全垮落发处理顶板， 煤层压力完全释放，煤层内的裂隙大幅增加。钻孔瓦斯抽采量依旧增大，结合工作面压力 和瓦斯流量变化数据，我们认为应力完全降低区的范围在单煤层开采时工作面前部 2030m，显然在正常情况下，距离 20m 或者更少的位置范围内都是应力降低区。图 1 工作面前部应力分布图3 超前应力降低区瓦斯抽采基础技术分析瓦斯抽放不仅和抽放时间、抽放阻力、钻孔直径和钻孔深度有关，也和取决于煤层透 气性系数的其他参数有关，在抽采初始阶段，地下煤层不受钻孔半径的影响，此时，煤层 裂隙联通，吸附状态的瓦斯逐渐解放，这时的抽采效果是非常好的；随着抽放时间的延长， 钻孔内瓦斯逐渐变得稀少。开采活动开始后，原本的应力平衡状态被破坏，工作面前部的 应力重（a）九里山矿#14101 工作面（b）演马庄矿#22041 工作面 图 2 巷道顶底板变形量图分布以达到新的平衡，随着工作面的推进，前部煤层在一定应力范围内卸载，煤层内的裂隙进一步连通，形成了一个应力降低区。与此同时，深部煤层的一定范围内，高应力出现， 空隙闭合，在开采活动区域内形成了一个应力集中区。在这个区域，相较于原岩应力区， 不仅施工困难，较小的透气性系数也是一个很大的问题；在应力降低区，煤层透气性性增加，吸附状态气体转化率增加，钻孔内瓦斯流量必然会有新的增加，从而使得该区域成为 瓦斯高效快速抽放的理想区域；据周和林的研究，这项技术的应用将会驱使瓦斯流向应力 降低区，还会使煤层收缩，一举两得。一方面，煤质得到提高，另一方面，煤层的透气性 性会进一步提高，从而减少瓦斯的压力梯度和有效防治煤与瓦斯突出；随着工作面的不断 向前推进，在工作面的特定区域就一直会有处于盈利降低状态的煤层。在煤层工作面应力 降低区区域的测定是合理的钻孔布置和强力的抽放方法和管理的基础，因此，通过加强预 抽钻孔的管理、合理布置抽采钻孔和优化抽采参数可以实现以减少瓦斯外泄为目的的安全 开采，前面这些成果应该利用到工作面前部以及其它综合抽采区。 （a）九里山矿#14101 工作面（b）演马庄矿#22041 工作面 图 3 预抽瓦斯钻孔瓦斯流量图4 工作面瓦斯综合抽采的研究与应用4.1.瓦斯浅层钻孔抽放工艺当钻孔钻到煤层工作面时，抽放区瓦斯将在负压作用下流动，瓦斯流向也将由平行于 煤壁变为垂直于煤壁，瓦斯溢出量也将降低。瓦斯抽放完后，煤体将收缩，也会引起应力释放，从而减少煤与瓦斯突出危险。浅层抽放钻孔的安排模式有一些优势，仅需要技术创 新上的一点小努力，施工简单，抽放高效。对于结构复杂、软煤发育和长壁工作面，合理 的布置钻孔的分布和深度可以防治煤与瓦斯突出，减少瓦斯溢出可以防止瓦斯外泄造成的 事故。在焦作工业集团有限责任公司下属的九里山矿#15021 工作面，煤层瓦斯含量是 30 m3/t，煤厚 5.3m，上覆有软煤 0.31.0m。先开凿的 523 个预抽采钻孔中，工作面百米瓦斯 抽采量为 0.0023 m3/min，大部分瓦斯流向首采区，其中，最大的瓦斯浓度可达到 1.42%。 为了减少开采期间的瓦斯溢出，钻孔由工作面向外依次布置，孔深 1215m，孔间距 1.5m， 施工完成后，每个钻孔连续抽放瓦斯，持续至少 4 个小时，相较于工作面预抽钻孔，利用浅层钻孔抽放方法的每个钻孔的瓦斯标量的波动范围是 0.020.1 m3/min，平均百米瓦斯抽放量是 0.68 m3/min，效率提高了 296 倍，浅层钻孔连续检测数据（图 4）显示，在特定的 时间段内，抽采瓦斯的衰减系数下降到 0.00020.0021/d，表明应力降低区的煤层透气性大 大改善，工作面开采期间煤层浅层瓦斯抽采或超前释放瓦斯是防治瓦斯外泄的有效措施。 图 4 九里山矿#15021 工作面浅部钻孔瓦斯流量4.2. 瓦斯扇形钻孔抽放工艺在焦作工业集团有限责任公司下属的魏村矿的#14101 工作面，测得的煤层瓦斯含量是 18.46 m3/t，采用综合机械化采煤技术，利用完全垮落法处理顶板。工作面瓦斯抽采前，采 用连接高抽巷和低抽巷的平行钻孔进行瓦斯采前预抽，考虑到工作面的抽采率较低，为了 有效提高工作面瓦斯抽采率和确保安全生产，我们从开切眼沿着运输平巷每隔 100 米布置 抽采钻孔，深度为 20m，钻孔成扇形由外向里散开（见图 5），长度为 5060 米，#1 钻孔 区域距开切眼 85m，区域内钻孔四个一