资源描述
英文原文Investigations of water inrushes from aquifers under coal seams（Jincai Zhang）Abstract：In many coal mines, limestone-confined aquifers underlie coal seams. During coal extraction from these mines, water inrushes occur frequently with disastrous consequences. This paper introduces the hydrogeological conditions of the coal mines and the potential water inrush disasters from aquifers under coal seams. It then presents the water inrush mechanism. The main factorswhich control water inrushes include strata pressure, mining size, geologic structures and the water pressure in the underlying aquifer. Analysis shows that reduction of confinement due to mining is the major cause of the water-conducting failure in the floor strata. The depth of the failure zone is strongly dependent on the mining width. This paper also presents field observation results of the water-conducting failure in the floor strata, and applies the finite element method coupled with stress-dependent permeability to analyze hydraulic conductivity enhancement due to coal extraction. Finally, theoretical and empirical methods to predict water inrushes are given, and technical measures for improving mine design and safety for coal extraction over aquifers are presented. These measures include fault and fracture grouting and mining method modification such as changing long-wall to short-wall Mining.Keywords: Water inrush; Coal mining; Confined aquifer; Strata failure; Stress and displacement; Hydraulic conductivity; Permeability1 Introduction China continues to rely on coal for about 75 percent of its energy. Therefore, coal production is of crucial importance for Chinas economy and development. However, mining operations in China are threatened by various kinds of groundwater during coal extractions. The most serious of the three main types of possible water disasters affecting the safe operation of coal mines 1 is water inrushes from the Ordovician limestone under the permo-Carboniferous coal seams in Northern China. The Ordovician limestone is a confined karst aquifer containing an abundant supply of water and with a very high water pressure. Furthermore, the strata between coal seams and the aquifer are relatively thin, varying in thickness from 30 to 60 m. Due to these characteristics of the aquifer, plus mining-induced strata failure and inherent geological structures (such as water-conducting faults, fractures) high-pressure groundwater can break through seam floors and burst into mining workings. Therefore, water inrushes from the aquifer occur frequently, and coal mines often suffer from serious water disasters during coal extractions. Water inrush incidents have shown that the maximum water inflow in a coal mine has reached as much as 2053 m3/min 2, which submerged the mine in a very short time. According to incomplete official statistics, about 285 of 600 key coal mines in China are threatened by water inrushes during coal mining 2. The total coal reserves threatened by bodies of water are estimated at 25 billion tons. For example, in Northern China, the yearly coal production from the Permo-Carboniferous coal-bearing formations is more than 200 million tons. However, the coal extraction has been threatened by frequent water inrushes from the Ordovician aquifer. In this region, the lower level seams which have more than half of the total coal reserves are much more difficult to mine due to this threat of water inrushes (Table 1).From 1950 to 1990, a total of 222 serious water inrush incidents took place in China causing collieries to be submerged by water intrusions from the confined karst aquifers. More recently, total water inrushes at nationalized key coal mines occur about 125 times annually resulting in an economic loss of 1.5 billion Yuan (about 180 million US dollars). In addition, local coal mines run by provinces, counties, and private business have a larger annual economic loss induced by water inrushes.The main coalfields threatened by the Ordovician aquifer are Jiaozhuo in Henan Province, Fengfeng,Handan and Xingtai in Hebei Province, Zibo and Feicheng in Shandong Province, and Hancheng and Chenghe in Shaanxi Province. If the problems of safe mining over the aquifer cannot be solved efficiently,some coal mines in the mentioned coalfields will be faced with gradual reduction of production or even abandonment of the mines. In general, there are two different ways of solving the problems of mining over confined aquifers. One is to drain the aquifer before mining operation, and the other is to mine without drainage. Geological investigation and mining practice have unveiled that there are many environmental problems induced by water drainage from limestone aquifers, such as the Ordovician limestone aquifer in Northern China and the Maokuo limestone aquifer in Southern China. Fissures in these karstified limestones are well developed and interconnected within the aquifer such that when water is drained from one particular region, it has an extensive influence on the whole aquifer. For example, dewatering in the Ordovician limestone aquifer was conducted in Wangfen colliery, Hebei Province. The pumping flowrate was 96m3/min; however, the drawdown in the central well was only 2.8 m, and the radius of the cone of depression extended to 10km which caused the loss of many drinking water wells. This resulted in a shortage of water supply for 100 000 people. Therefore, water drainage is infeasible. The only solution for coal extraction over the limestone aquifers is to mine with technical measures and without drainage.In order to do so, it is of vital importance to study strata failure characteristics and hydraulic conductivity changes due to mining and thereafter find a way to predict and prevent water inrushes. Various researches has been conducted in this area 215; however, the mechanism of water inrushes is still not well understood.2 Determination of the water-conducting failure zone in the seam floor2.1 In situ borehole observation of the water-conducting failure Coal extraction causes strata deformation and failure which may enhance hydraulic conductivity in the surrounding strata. Therefore, it is desirable to accurately determine pre- and post-mining hydraulic conductivities in the overburden and underlying strata of the coal seam. To measure the conductivity in the underlying strata, boreholes are drilled pre-mining in underground roadways for observation. In each borehole, water injection and a number of well logging techniques (such as electric resistivity, ultrasonic wave, acoustic emission, hole televiewer, etc.) are used to determine rock strength, borehole fissure, and changes in hydraulic conductivity. Fig. 1 gives a schematic diagram of a water injection instrument 16. The key technique during measurements is to control the injection pressure. The pressure should not be high enough to create new fractures in the strata, since the experiment is conducted to determine the changes in hydraulic conductivity induced by mining. Therefore, the injection pressure should not exceed the least principal stress of the surrounding strata. Fig. 2 shows the observed borehole locations of and layout in Xingtai coal mine. In this area, in situ stresses were as follows: . The roadway in Fig. 2 was located 36m below the mining face and four boreholes were drilled at different angles. The water injection instrument described in Fig. 1 was applied to measure the flowrate of water injection preand post-mining, using an injection pressure of 0.350.5MPa. The water injection along each borehole was conducted by pumping water into the instrument, and then into the borehole. The measurements were taken in each hole at different sections throughout the borehole and at different times.Fig. 1. The instrumentation for water injection observation in a boreholeFig. 2. Observing borehole layout for water injection measurements in Xingtai coal mine, Hebei Province.Fig. 3. Flowrate of water injection along a borehole (Hole 1 in Fig. 2) pre- and post-mining in Xingtai coal mine, Hebei Province.Fig. 3 gives the measured pre-mining and post-mining flowrate of water injection in Hole 1 (refer to Fig. 2). Note that in this context pre-mining corresponds to a state before the mining face passes the borehole, and post-mining means after the mining face passes the borehole. It can be seen that before the mining face passed Hole 1 (in pre-mining, the mining face was 32m away from the borehole), the injection rate was zero from 53 to 68m in the inclined borehole. This means that the strata in this area were impermeable. However, when the mining face passed the borehole, the injection rate (refer to Fig. 3 for post-mining at 19 and 63 m) increased dramatically, and the strata in some areas changed from being impermeable to permeable. Since the borehole wall collapsed by mining when the mining face passed 63m from the borehole, water injection data could not be obtained after 60m from the borehole opening. The borehole collapse post-mining illustrates that the borehole was seriously damaged, and that rocks around the borehole failed due to mining.Fig. 4 plots the increments of water injection rates after mining, which were obtained by subtracting the pre-mining injection rates from those of the post-mining.These increments represent injection rates caused by permeability changes induced by coal extraction.It can be seen from Fig. 4 that along the inclined borehole from 43 to 72m (the borehole end), the injection rate increased compared to the pre-mining (in situ) state. Therefore, the strata in this area were fissured by mining, and this area is defined as the water-conducting failure zone. Using the same method to analyze the observed data from all boreholes, the mining-induced water-conducting failure zone can be obtained. This failure zone is of critical importance for mine design and water inrush prevention for mining over aquifers.Fig. 4 Flowrate increment of water injection along a borehole (Hole 1 in Fig. 2) after mining in Xingtai coal mine, Hebei Province. Fig. 5 displays the changes of the injection rates with the distance of mining advance for two different depths beneath the coal seam in Wangfen coal mine, Hebei Province. It clearly shows that in the mined area the injection rate increases significantly compared to the unmined area. It also can be seen that due to coal extraction, the water-conducting capacity increases in the floor strata, and this water-conducting capacity decreases as the distance from the coal seam to the floor strata increases. This means that the closer the strata are to the extracted seam, the higher the permeability in the seam floor. It is also noticeable that the injection rate decreases inside the abutment. This decrease is due to the fact that stress concentration and high abutment pressure occur in this area causing the fractures to be closed.Fig. 5 Flowrate of water injection versus mining distance for two different depths beneath coal seam (the negative distance represents pre-mining state and the positive means post-mining state).Fig. 6 Observing borehole layout and observation section of the water-conducting failure zone in the underlying strata for slightly inclined coal seam in Fengfeng coal mines, Hebei Province.Fig. 7 Observing borehole layout and observation section of the water-conducting failure zone in the underlying strata for inclined coal seam in Huainan coal mines, Anhui Province. Along both strike and dip directions, failure zones increase from upstream to downstream. Field observations have shown that characteristics of failures in the floor strata are considerably different for different inclinations of the extracted seams. For flat or slightly inclined seams (inclination angle, 25), the profile of the water-conducting failure zone is broad in section with extended lobes, and the maximum failure depth occurs beneath the headgate and tailgate, respectively, shown in Fig. 6. For inclined seams (2560), the failure zone propagates downwards in an asymmetric manner in the dip direction, as shown in Fig. 7. The extent of the failure zone increases gradually from updip to downdip, and the maximum failure depth appears in the floor strata beneath the area around the lower gate. For steeply inclined seams (6090), the failure zones in the floor strata are opposite to the inclined seams, i.e., the maximum failure depth appears in the strata beneath the area around the upper gate .2.2Empirical prediction of the depth of the water-conducting failure zone According to in situ observations, a number of parameters affect the development and depth of the water-conducting failure zone. Mining width of the working face and uniaxial compressive strength of the strata are the most important of all parameters. An empirical formula for predicting the depth of the water-conducting failure zone was developed from field test results in long-wall and short-wall mining faces. The formula is expressed as (refer to Fig. 8)（1） where h1 is the depth of the water-conducting failure zone starting from the immediate floor of the seam (m) and Lx is the mining width of the mining face (m). Note that the observed data were obtained from coal mines in Northern China, with mining depths ranging from 103 to 560 m, and uniaxial compressive strengths from 20 to40MPa.For mining above aquifers, it is desirable to avoid water inrushes and the extra expense of strata dewatering. This can be achieved only when aquifers are located a certain distance outside the water-conducting failure zone. If an aquifer which is confined, very permeable and with abundant water lies within the failure zone, water with high pressure will rush into the mining area, and may cause a disastrous consequence.Fig. 8 Observed maximum depth of the water-conducting failure zone in the seam floor strata for different mining widths in China.3 ConclusionsIn situ measurements and physical modeling have shown that stress increases and abutment pressure is induced in the seam floor just before the mining face is reached. This causes compressive deformation and a decrease in the water injection rate. After mining, stress decreases and stress relaxation and confinement reduction are induced, causing expansion deformation and a water injection rate increase in the floor. Essentially, as mining advances, development of stress in the floor strata includes three stages: stress increase pre-mining, stress decrease post-mining, and a gradual recovery to the original stress. Corresponding to the stress redistribution, displacement in the floor strata shows compression before mining, expansion after mining, and gradual recovery to the original state. During the floor expansion and the stress relaxation stage, the strata are more prone to creating tensile fractures. In the area of transition between floor compression and expansion located beneath the area around the coal wall of mining face, the strata are likely to create shear fractures. Therefore, the floor failure zone is the largest in the strata right beneath the area around the coal wall, where water inrush is most likely to take place.Statistical data have shown that most water inrushes from the underlying aquifers were related to faults.Therefore, it is of crucial importance to detect and map geological structures in detail before mining. Also, since the mechanism of water inrushes from faults has not been fully understood, further study, including in situ monitoring of faults, needs to be undertaken. Since more than 60% of water inrushes were ascribed in some way to faults and other geologic structures, necessary measures are needed to address faults and inherent fractures in the floor before mining operations begin. For large faults, water-proofing barriers need to be left. For small faults and fractures, grouting can seal them and reduce the possibility of water inrushes. For weak aquifers existing in the seam floor, grouting cannot only change the weak aquifer into an impermeable layer but also increase the strength of the floor strata, which can reduce mining-induced failures.References1 Zhang J, Zhang Y, Liu T. Rock mass permeability and coal mine water inrush. Beijing: Geological Publication House; 1997 in Chinese.2 Zibo Mining Bureau. Data analyses and application of coal seam water inrushes in Zibo coalfield. Zibo Coal Sci Tech 1979; in Chinese.3Huainan Mining Bureau, Xian Branch of China Coal Research Institute. Hydrogeological conditions and control methods of the karst aquifer under # A coal seam in Huainan coalfield. Internal research report, 1983 in Chinese.4Fengfeng Mining Bureau, Shangdong University of Science and Technology. In situ measurement of the floor strata failure in the No. 2 coal mine of Fengfeng coalfield. Internal research report, 1985 in Chinese.5 Wang, Z. Preliminary study of water inrushes from the floor of coal mining faces. Coal Geology and Exploration 1983; (5) in Chinese.6 Wang, Y. Conditions and prevention of water inrushes from underlying confined aquifers of coal seams. Coal Sci Tech 1985;(1) in Chinese.中文译文煤层下含水层突水机理研究摘要：在许多煤矿，煤层下都存在石灰岩承压含水层。这些煤矿进行煤炭开采时，突水频频发生，造成灾难性的后果。本文介绍了煤矿的水文地质条件和煤层下含水层潜在的突水灾害。然后给出了突水机理。导致突水的主要因素包括地层压力，开采规模，地质结构和下含水层水的压力。分析表明，采矿使用限制减少是底板岩层水导电失败的主要原因。失效区的深度主要决定于开采宽度。本文还介绍了底板岩层中水承压失效的现场观测结果，并应用有限元法和应力渗透性的依从质来分析由于煤炭开采使渗透系数提高的原因。最后，给出了预测突水的理论和实证方法，并提出了含水层上煤炭开采矿井设计和安全的改进技术措施。这些措施包括断层和裂缝灌浆及采矿方法的改进，如改长壁开采为短壁开采。关键词：突水；煤炭开采；承压含水层；岩层破坏；应力和位移；水力传导系数; 渗透率1 引言中国能源的约百分之七十五将继续依赖煤炭。因此，煤炭生产对中国经济的发展至关重要。然而，在煤炭开采过程中，中国采矿生产受到地下水的各种威胁。影响煤矿生产安全运行的可能水灾害三种主要类型中最严重的是中国北部石炭二叠系煤层下奥陶系石灰岩突水。奥陶系石灰岩有一个高水压并有丰富的水源供给的密闭岩溶含水层。此外，煤层和含水层之间的地层相对较薄，厚度在30米至60米范围内变化。由于含水层的这些特点，再加上采矿诱发岩层破坏和固有的地质构造（例如水导电断层，裂缝）高压地下水可以通过缝地板和冲入开采生产区。因此，含水层突水频繁，煤矿常常在煤炭开采过程中遭受严重水害。突水事件表明，一个煤矿的最大涌水量高达2053m3/min时，能够在很短的时间内淹没这一煤矿。据不完全的官方统计，中国600个重点煤矿中约有285在煤炭开采过程中受到突水的威胁。受到水体威胁的煤炭总储量估计在25亿吨。例如，在中国北部，从石炭二叠系含煤地层中每年生产煤炭超过200万吨。在这个区域，超过煤炭总储量一半由于这种突水（表1）的威胁开采要困难得多。从1950年到1990年，在中国发生222个严重的突水事故，密闭岩溶含水层的突水导致煤矿被淹没。最近，国有重点煤矿每年共发生突水约125次，造成经济损失15亿元（约1.8亿美元）。此外，省，县，民营企业经营的地方煤矿每年由于突水导致的经济损失更大。受奥陶系含水层威胁的主要煤田是河南焦作，河北峰峰、邯郸、邢台，山东淄博、肥城，和陕西韩城、澄合。如果关于含水层的安全开采问题不能有效解决，在上述煤田的一些煤矿将面临着产量减少，甚至关闭煤矿的问题。一般来说，解决煤矿开采承压含水层问题有两种不同的方法。其一是开采前进行含水层排水，另一种是不带排水渠的开采。地质调查和矿业实践已经表明，有许多因石灰岩含水层排水引起的环境问题，如中国北部奥陶系灰岩含水层排水，和在中国南部的茅口石灰岩含水层排水。石灰岩岩溶裂隙和含水层之间的这种相互联系，使得当从一个特定的区域排水时，它对整个含水层有广泛的影响。例如，在河北省王坟煤矿对奥陶系灰岩含水层进行了排水。该泵流量为96 m3/min，但是，在中央井地下水位的降低只有2.8米，该漏斗降落半径扩展到10公里，造成了许多饮用水井的损失。这导致100000人供水短缺。因此，排水是不可行的。石灰岩含水层之上的煤炭开采的唯一的解决办法是利用技术措施，不进行排水进行开采。为了做到这一点，研究岩层破坏特性及由于采矿水力传导的变化是至关重要的，然后找到一个方法来预测和防止突水。虽然已进行过这方面的各种研究，但对突的机制仍然没有得到很好的认识。表1 在中国的煤矿受奥陶系含水层威胁的煤炭储量2 煤层底板下水承压失效区的测定2.1 钻孔原位观测水承压失效煤炭开采引起的地层变形和破坏可能提高周围地层的渗透系数。因此，最好是准确确定开采前后在煤层上覆和下伏地层的渗透系数。为了测量下伏地层中的渗透系数，钻孔都在井下巷道开采前打钻以便观察。在每一个钻孔，大量的注水和测井技术（如电阻率，超声波，声发射，孔成像等）用于确定岩石强度，钻孔裂隙和水渗透系数的变化。图1给出了一个注水仪器原理图。在测量过程中的关键技术是控制注入压力。进行实验以确定开采引起的渗透系数的变化，压力不应该高到足以制造新的地层裂缝。因此，注入压力不应超过周围地层的最小主应力。图1 钻孔注水观测仪器图2 河北省邢台煤矿注水测量钻孔布局图2显示了邢台煤矿钻孔位置布局。在这一区域，应力如下：。图2中的巷道位于采煤工作面的下方36米，并且四个钻孔分别钻在了不同的角度。图1描述的注水仪器利用0.35MPa到0.5MPa注射压力来测量开采前后水注入的流量。沿每个钻孔注水由抽进仪器的水来控制，然后流入钻孔。该测量结果是由不同地区和不同时间的每个钻孔内得出的。图3给出了测量开采前后注入孔1的水流量（参考图2）。请注意，在这种情况下开采前对应一个采煤工作面通过钻孔之前的状态，开采后则对应采煤工作面通过钻孔后的状态。可以看出，通过开采工作面通过孔1之前（开采前，采煤工作面离钻孔32米远），倾斜的钻孔从53至68米的范围内注入率是零。这意味着，在这区域的地层防渗。然而，当工作面通过钻孔，注入率（采后在19至63米参考图3）急剧增加，而且在某些地区被不透水地层向透水性转变。由于采煤工作面通过钻孔63米时井壁坍塌，注水数据无法从60米之后的钻孔获得。采后的井壁坍塌说明，由于采矿钻孔被严重破坏，而且钻孔周围的岩石也被损坏。图3 河北省邢台煤矿开采前后沿钻孔（图2中的孔1）的注水流量图4 河北省邢台煤矿开采后沿钻孔（图2中的孔1）增加的注水流量图4给出了开采后注水率增量，这是由从采后注入率减去的采前注入率得到的。这些增量表明由煤炭开采引起的渗透系数的变化导致注入率的改变。从图4可以看出沿倾斜钻孔由43至72米（钻孔结束），与开采前（原位）状态相比注射率上升。因此，在这地区的地层是由开采破坏的，而这个地区也被定义为水承压失效区。使用相同的方法来分析所有钻孔的观测资料，可以得到由采矿引起的水承压失效区。这个失效区域对于矿井设计和预防含水层之上的矿井突水至关重要。图5显示河北省王坟煤矿煤层下面两种不同深度下，注入率随开采距离的变化。这清楚地表明，与未开采的区域相比，在采空区注入速度明显增加了。这也可以看出，由于煤炭开采，底板岩层中的水的承压能力增加，并且当煤层与底板岩层之间的距离增大时水的承压能
展开阅读全文