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英文原文Numeric Analysis of Hydraulic Fracturing Technique in Preventing Outburst and the Control TechniqueZhao xue-bing Wei wei Ni mao-longXuzhou mining Group Co Ltd Xu zhou,china,221006Abstract: To give full play to the technique of hydraulic fracturing in removing the danger of coal and gas outburst by overall relieving the stress, the FLAC3D software was used to analyze the stress distribution after the hydraulic fracturing, and the relation of spatiotemporal coupling of hydraulic fracturing and controlled blasting techniques was theoretically analyzed. The result shows that the technique of hydraulic fracturing should generate high pore pressure and distribute in chaos while relieving the ground stress, which results new hidden danger. The controlled blasting technique used before the hydraulic fracturing should generate crannies which lead the fracturing direction and relieve the stress overall. By analyzing an outburst example of a mine, the spatiotemporal coupling relationship of hydraulic fracturing and controlled blasting techniques directly affects the stress relieving effect. Finally, a number of proposals to avoid incidents were post out to improve the safety use of the hydraulic fracturing technique.Keywords: controlled blasting; hydraulic fracturing; spatiotemporal coupling; overall relieving the stress; coal and gas outburst, pore pressureCoal and gas outburst is affected by ground stress and gas pressure 1, and for a long time it is one of the main problem to highlight the safety production of a mine. Scholars at home and abroad have done a great deal of research works about relieving coals stress, and have played a significant role in preventing outburst 2,5. However, with the deepening of the depth of coal mining, the increasing stress reduce the permeability of coal, conventional methods cant be effectively removed the stress and gas pressure, coal and gas outburst occur from time to time in complex geological condition region 6, 7. Hydraulic fracturing technology is an effective method to preventing outburst by injecting high.pressure water into the rock and fracture the coal and rock to release stress and increase the coal.rock permeability. Because the fracturing direction is uncontrollable 8, new high pore pressure will be generated at another places while relieving the stress and distribute in chaos, which has a bad effect on safety production and restricts the extent use. To control fracturing, this paper gave a detailed analysis of the overall relieving stress mechanism of spatiotemporal coupling of controlled blasting and hydraulic fracturing techniques, and made a number of recommendations with the view to make a good use of the techniques to reduce the similar incidents and serve in the coal mine safety production.1. Hydraulic Fracturing Technique and the DeficiencyHigh pressure water can fracture and expand crannies around a hole to relief the ground stress around, but at the same time high pore pressure and addition stress concentration will formed at some other place, which was proved in China Pigmies Sheena Group. Because the fracturing direction is uncontrollable and difficult to detect at present, most of the cases the fracturing direction is unclear. The coal and mine outburst maybe occur when the coal mine get into the high pore pressure region without realization.Its very difficult to get clear of the stress distribution after hydraulic fracturing through laboratory or field test, and the repeatability is also weak, while the numeric simulation technique, as a new method, can overcome these shortcomings. Three conditions of one crack, two cracks or three cracks as show in figure 1 were simulated separately. In the figure, “c.i” means crack number i, crack 1 goes along the roadway, crack 2 and 3 goes start with the injection hole and vertical to the roadway, but the crack 3 is upper to crack 2. And the nij means the node number j in condition i, and high pressure water was injected into the injection hole. Fig. 1 Crack distribution （a） one crack （b） two cracks （c） three cracks1.1 Numeric Simulation AnalysisTo enable the calculation more close to reality, the model of 504020 m was built according to the outburst location of a mine. The coal seam is 3.5 m thick, 700 m deep, with an inclination of 22. The fluid.solid coupling mode of FLAC3D was used to analyze the stress distribution after hydraulic fracturing 9. The upper face was applied 18 MPa pressure, and norm to others was fixed. The model was shown as figure 2:Fig. 2 Numeric analysis model1.1.1 Bringing no main crack after hydraulic fracturingWhen no main crack formed after hydraulic fracturing, the high pore pressure shown in figure 3 will be generated.Fig. 3 The stress distribution when no main crack formedFigure 3 shows that although no main crack formed after hydraulic fracturing, the high pressure water had already been injected into the coal, which generated 9 MPa high pore pressure 6 m in front of the roadway, and couldnt be released for a certain period of time. The concentric contour.shaped of pore pressure was changed to Meniscus because of the excavation of the roadway. The isograms of the pore pressure in front of the roadway is much more intensive compare to other regions, indicating that the pore pressure can easily destroy the coal, and result in coal and gas outburst. So the failure use of hydraulic fracturing technique will generate high pore pressure, which will make it much more danger. So when it is difficult to fracture the coal, some other measures should be taken to ensure the successful fracturing to avoid negative effects.1.1.2 Analyzing the effect of the hydraulic fracturingThe effect will not be perfect when the crack distribution is not like pre.designed. Figure 4（a） shows the “一”.shaped distribution of pore pressure when there is only one crack like figure 1（a）, figure 4（b） shows the “T”.shaped distribution of pore pressure when there are two cracks, figure 4（c） shows the “十”.shaped distribution of pore pressure when there are three cracks like figure 1（c）. Fig. 4 Distribution of pore pressure （a） one crack （b） two cracks （c） three cracksFigure 4（a） shows the pore pressure ahead the roadway reduced heavily when the coal was fractured along the roadway, and the pore pressure, about 6 MPa, transferred to both sides of the crack. The pore pressure becomes more complex, and the gradient is also larger at the both sides of the crack near the water injection hole in front of the roadway, which makes the coal still easily be destroyed and cause outburst.When the coal was fractured along the roadway and vertical to it at a side, then there will be two cracks in front of the roadway and formed a “T”.shape, the pore pressure distribution was shown as figure 4（b）. The figure shows that the pore pressure adjacent to the fractured side was reduced sharply, while the stress still kept as high as about 5 MPa adjacent to the non.fractured side with large gradient. The distribution of the pore pressure is also complex, and the danger of outburst also exists. It may also result in outburst when the roadway excavates into this region without realization.When the coal was fractured along the roadway and vertical to it at both sides, and then formed “十”.shaped crack distribution shown as figure 4（c）. The pore pressure was sharply reduced near the crack in a large region, and the high pore pressure of about 2.5 MPa, distributes far away from the roadway, and had little affects on the roadway when excavating. So it was safe.Some pore pressure of some grids were sampled and stored during the model run and were shown as figure 5. Fig. 5 History of pore pressure （a） node number 3 at different condition （b） different node at condition 2Figure 5（a） shows the pore pressure of node number 3 in different condition, the curve of no crack is on the top, the curve of n13 and n23 was overlapping in the middle, the curve of n33 was at the bottom. So when no crack was formed in the process of fracture, the pore pressure of node 3 will keep higher, the high pressure may broke the coal, but when there was one crack was formed the pressure will drop highly. The crack 2 has less effect on the node 3, so the curve of n13 and n23 is overlapping. But the pore pressure of node 3 was clearly affected by crack 3, so the curve of n33 is lower than the curve of n23. Figure 5（b） shows the pore pressure history of different node in condition 2 when there were two cracks. From the figure we know the pressure of n22 is lower than n24, and the n21 is lower than n23, which suggests that the pore pressure of node at the crack side drop much lower than the others.From the analysis above, we know that the technique of hydraulic fracturing can not only relief the stress, but also generate high pore pressure for a certain time at both sides of the crack when there are no other cracks formed nearby. The high gradient pore pressure will easily destroy the coal and cause outburst. When there are enough cracks and distribute in order, the high pore pressure and the gradient can be sharply reduced and protect the roadway.2. Example analysisThe coal seam is 700 m underground with inclination of 21and thickness of 3.7 m, the gas pressure is 1.6MPa and gas content is 18 m3/t. The perch roadway was used in the roof to protect the excavating of coal mine roadway. The distance between the two roadways is about 25 m in plan and 4 m in vertical. The injection hole was drilled from the perch roadway to the coal mine roadway, and the distance between adjacent holes is about 30 m. Water of 30 MPa pressure was injected into the holes to fracture the coal seam. The coal and gas outburst happened when the coal mine roadway had already got through three injection hole, and just 6 m to the forth injection hole. The high pressure was injected 12 days before the outburst accident. The measures and cavity after outburst were shown as figure 6: Fig 6 Methods and the cavity after outburst （a） injection hole （b） front view of the cavity （c） top view of the cavityFigure 5 shows the cavity located about 6 m in front of the roadway and at the top side with the size of 3.6 m high, 7.3 m wide and 12 m long. The shape of the cavity was just like the distribution of the high pore pressure shown in figure 4（a） or 4（b）. So it could be concluded that the outburst accident clearly get related with the hydraulic fracturing, the complex stress distribution caused by the uncontrollable fracturing might be the main reason.3. Crack Orienting Technology ResearchHydraulic fracturing can play a role in eliminating the outburst danger by relieving the stress, but it also would generate complex high pore pressure and maybe result in outburst when the fracturing direction is uncontrollable. So the key technique of the hydraulic fracturing is to control the fracturing direction. The cracks mainly go along the weak planes; however, the direction of weak plane is unknown, which makes the fracturing direction unknown too. Sometime the weak plane is not in good distribution to relieve the stress overall, so manmade weak planes are needed to control the fracturing direction.Shock wave generated by explosive disseminates in the coal and rock to extrude it heavily, which should break the coal and rock, and forms an excess broken ring in the vicinity of the blasting hole and a cranny ring beside the excess broken ring. The shock wave energy density will decay when transporting far away and can only cause lower damage to the rock. When the shock wave energy is not sufficient enough to break the rock, and then change to stress wave, and spread like elastic wave. Although the wave cant break the rock, the rock mass have the trend to move along the wave all the same, which is bound to generate tensile stress and broke the rock mass for the tensile strength is far less than the compressive strength 10.Because the cracks distribute around the blasting hole equally, no marked weak planes were generated to control the fracturing direction. If there are some holes around the blasting hole to provide free space for coals moving and can reflect the shock wave to generate tensile stress near the holes and break the rock mass to form some weak planes which control the fracturing direction, which technique is named controlled blasting 1113. When the distance between control holes and the blasting hole is in 10 m, the stress near the control holes increased 46%66% 14, which will directly play an important role in breaking the rock mass. When the controlled blasting technique was brought forward, many scholars have made a further study. The field experiments suggest that the controlled blasting can be sure to make weak planes to improve the gas concentration in drainage. But if the distance between control hole and blasting hole is too far, then there will be too less shock wave energy spread to the control hole to break the rock mass, and less weak planes formed after explosion, just like loose explosion only. Although loose explosion can break the rock around the blasting hole, it can also generate stress concentration around the loose rock, and the phenomenon will be more evident in soft coal seam 15.Visibility, controlled blasting should generate weak planes if the holes have a proper distribution, and can be used before hydraulic fracturing to control the direction, this is the pumping integration of blasting before drilling and fracturing techniques （DBFP）. But if the holes have a bad distribution, there will be no weak planes formed and cant control the fracturing direction, whats more, many times people have no realization, which seems more dangerous. For different coal mine have different condition, the experience of ones cant be directly used in another. The field experiment of testing the proper distance between blasting hole and control hole was recommend to do before the technique was used to make sure to form the weak planes. The hydraulic fracturing should be used after the weak planes formed, and the injection hole should be designed according to the distribution of the weak planes. The water injection hole should be located at the cross of the weak plane or near the blasting hole. The blasting hole or control hole might be the water injection hole if possible. This is the space relationship of the two techniques used together. When the technology was used, the holes can be distributed as follow:Fig 7 Drilling distribution of DBFPNote: 1.blasting hole, 2.control hole, 3.injection holeBeside the space relationship, the time relationship is also very important. After the controlled blasting the cracks formed will be closed slowly because of the ground stress, and the ability to control the fracturing direction will be lost. This is the time relationship, so the time between the two techniques should not be too long. It can be sure that the spatiotemporal coupling is very important when the two techniques used together, which can directly affect the stress relief effect, so it should be pay more attention.4. Conclusion and SuggestionsFrom the analysis above, some conclusions can be summarized as below:1） Hydraulic fracturing technique can relief the stress of the coal and rock, but it also generates high pore pressure at the same time. When the fracturing direction is uncontrollable, the pore pressure will be very complex with the high risk of outburst.2） Controlled blasting used before hydraulic fracturing can generate weak planes, which can control the fracturing direction. But if the holes have bad distribution and cant generate weak planes, the control ability will also disappear.3） The spatiotemporal coupling of controlled blasting and hydraulic fracturing is very important. The cracks will closed slowly after blasting, and the time between the two techniques should not be too long.Some advices are listed as below:1） The mechanism of the hydraulic fracturing should be further studied; proper methods should be taken to control the fracturing direction.2） When controlled blasting technique was used to control the fracturing direction, field experiment should be made to ascertain the distribution of the holes.3） Further study the spatiotemporal coupling of controlled blasting and hydraulic fracturing to avoid failure to control the fracturing direction.Reference1 MA Pi.liang, FAN Qi.wei. China CMM drainage monographJ.China Coal, 2004,30（2）:5-8 （in Chinese）.2 XIAN Xue.fu, GU Min, LI Xiao.hong, JIANG De.yi. Excitation and occurrence conditions for coal and gas outburst J. Rock and Soil Mechanics, 2009, 30（3）:577-581 （in Chinese）.3 PAN Yue, ZHANG Yong, WANG Zhi.qiang. 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Journal of Fuxin college of Mining. 1995, 14（3）:16-21（in Chinese）.中文译文水力压裂消突数值模拟及可控压裂技术研究赵雪兵 魏威 倪茂龙徐州矿务集团 徐州，中国，221006摘要：为充分发挥水力压裂技术对煤岩体的整体卸压消突作用，采用FLAC3D软件分析了水力压裂后的应力分布，并从理论上分析水力压裂和控制爆破的时空协同关系，指出水力压裂对煤岩体卸压的同时也产生空隙压力集中，由于压裂方向不可控，应力分布混乱，成为新的安全隐患。在水力压裂之前使用控制爆破技术先生成导向裂隙或弱面，控制压裂方向，均匀压裂煤岩体，达到整体卸压目的。结合某矿突出实例分析得知，控制爆破和水力压裂的时空协同关系和钻孔的布置情况直接影响技术使用效果，分析结果和实际情况比较吻合。最后提出若干建议，以避免类似事故发生，提高水力压裂技术推广使用的安全性。关键词：控制爆破；水力压裂；时空协同；整体卸压；煤与瓦斯突出煤与瓦斯突出（以下简称“突出”）是地应力和瓦斯压力综合作用的结果1，长期以来一直是困扰突出矿井安全生产的主要问题之一，国内外学者围绕对煤岩体卸压消突做了大量的研究工作，起到了显著防突作用25。但随着煤矿开采深度的加深，地应力逐渐增大，煤体透气性降低，常规方法不能有效卸除地应力而抽放瓦斯，在地质条件复杂地区突出仍然时有发生6,7。水力压裂技术通过向岩体注入高压水，压裂煤岩体并释放地应力，增大煤岩体渗透率，是有效的防突方法。但是，由于压裂方向不可控8，且在卸压的同时也形成新的空隙压力集中，影响煤矿的安全生产，成为制约该技术推广使用的主要障碍。为使得压裂方向可控，本文结合某矿突出实例，细致分析控制爆破和水力压裂技术协同卸压消突机理，并提出若干建议，以期对技术的正确使用提供指导，减少类似事故发生，服务于煤矿安全生产。1. 水力压裂技术及其缺陷水力压裂技术通过钻孔向煤岩体内注入高压水，压裂并扩展钻孔周围空隙，形成裂缝，在裂缝附近形成卸压区，达到消突的目的。但是水力压裂在对煤岩体卸压的同时，也会在未压裂区域形成空隙压力，这在平顶山十矿得到了验证。此外由于压裂方向不易控制，又难以探测，目前大多情况下不能明确其压裂方向，不能确定卸压区和空隙压力集中区域位置，当煤矿开采误入空隙压力集中区域时就可能导致事故的发生，给安全生产带来极大的隐患，阻碍了该技术的推广使用。因此，研究水力压裂方向，及其应力分布很重要。通过实验室或现场试验摸清楚压裂之后卸压区和空隙压力集中区域的分布情况极其困难，重复性也不强，而数值模拟技术能够克服这些缺点，为研究水力压裂技术提供新的方法。如图1所示，分别研究了有一条裂缝、两条裂缝和三条裂缝的情况。图中ci表示第i条裂缝，第一条裂缝沿着巷道的走向，第二条和第三条裂缝从注水孔起，并沿着垂直于巷道方向延伸，其中第三条裂缝在第二条裂缝的上侧。nij表示第i中情况下第j个节点，高压水从注水孔注入。图1 水力压裂延伸方向 （a）一条裂缝 （b）两条裂缝 （c）三条裂缝1.1 数值模拟分析为使计算贴近实际，根据某矿突出地点煤层实际赋存状况建立模型，模型尺寸为504020 m，煤层厚度3.5 m，倾角为22，埋深大致为700 m，采用FLAC3D完全流固耦合分析模式9，分析注入高压水和压裂之后空隙压力的分布情况，模型上部采用压力边界，其余面采用滚支边界，计算模型如图2所示：图2 数值计算模型图1.1.1无主裂缝时空隙压力分布水力压裂没有形成主裂缝时将引起空隙压力集中，空隙压力分布如图3所示。图3 无主裂缝生成时空隙压力分布图由图3可见，在水力压裂失败之后，不但没有产生裂缝，高压水被注入到煤体后，使得煤体内存在很大的空隙压力，并在一段时间之内将无法释放，当巷道掘进到注水孔前方6 m左右时，在巷道前方形成高达9MPa左右空隙压力。巷道开挖前，空隙压力等值线将呈现同心圆状分布；巷道开挖之后由于其卸压作用，使得空隙压力等值线呈现“半月”形状。巷道的迎头空隙压力等值线相对其他区域要密集，这表明该处的空隙压力梯度较大，煤岩体更容易被破坏，一旦操作不当就可能失稳而诱导突出。可见，当水力压裂失败后，较没有采取水力压裂技术的危险性更大，因此要确保高压水能成功压裂煤岩，当煤岩体太硬不易被压裂时，要人为采取措施，预成裂缝，确保成功压裂，避免负面效果。1.1.2 水力压裂效果分系水力压裂方向没有按照预先设计的方向延伸时将导致压裂效果不理想，这里主要分析压裂一条主裂缝，两条主裂缝和三条主裂缝的情况。图4（a）是只有一条裂缝的空隙压力分布平面图，形成“一”形卸压区，图4（b）是压裂两条裂缝的空隙压力分布图，形成“T”形卸压区。图4（c）给出了压裂三条裂缝的空隙压力分布图，形成“十”形卸压区，此种情况卸压比较充分。 图4 空隙压力分布图 （a） 一条裂缝 （b） 两条裂缝 （c） 三条裂缝由图4（a）可见，沿巷道径向被压裂之后，巷道迎头方向的空隙压力减小，并向裂缝两侧转移，产生了高达6MPa左右的空隙压力，应力分布变得复杂，在压裂缝槽两侧靠近注水孔附近的空隙压力梯度也比较大，使得该处的煤岩体容易发生破坏而突出，巷道掘进时的危险性依然很大，容易发生冲击作用而引起失稳，发生突出。当沿巷道径向和巷道下帮垂直于巷道径向被压裂时，压裂缝槽呈现“T”形分布，空隙压力分布如图4（b）所示。可见，压裂侧的空隙压力急剧降低，突出危险性也将大大降低，但在未压裂侧却仍然保存着高达5 MPa左右的空隙压力，应力等值线较密，梯度较大，空隙压力分布同样不均匀，突出危险性依然存在，当不能探测到空隙压力区域，并且空隙压力没有及时卸除，在误入该区域时，将可能会诱导突出。当沿巷道径向和巷道上下帮垂直于巷道径向被压裂时，压裂缝槽呈现“十”形分布，空隙压力分布如图4（c）所示。可见，在压裂缝槽的附近呈现大范围的卸压，在空隙压力集中区域远离巷道，其值只有2.5MPa左右，对巷道掘进已经不能起到冲击作用，在巷道前方及两侧近处，以卸压区为主，这就确保了巷道的安全。在数值计算的过程中记录了部分节点的数值变化情况，图5所示：图5 空隙压力变化（a）不同情况下3号节点 （b）第2中情况下不同节点从图5（a）可见，3号节点在不同情况下的变化曲线不同，没有裂缝时的曲线在最上方，有一条裂缝和两条裂缝时的曲线相互重合，而当有三条裂缝时，曲线在最下边。如果在压裂的过程中没有裂缝形成，3号节点的空隙压力保持很高，这种高的空隙压力可能会破坏煤岩体，为突出创造条件。但是，当有一条裂缝产生时3号节点的压力将急剧降低，第二条裂缝对节点3的影响不明显，因此第一种和第二种情况下的3号节点曲线基本上重合。但是3号节点的曲线却受到第三条裂缝的影响，因此当有三条裂缝存在的时候3号节点的历史曲线在最下方，其空隙压力最小。图5（b）给出了有两条曲线时各个节点的历史曲线，从图中可见2号节点曲线低于4号节点，1号节点低于3号节点，这表明有裂缝压裂侧空隙压力降低的更快，这也表明只有压裂才能产生卸压，否则将产生应力集中。通过以上分析可知，水力压力技术虽然能卸除地应力，但是也将形成新空隙压力集中，在压裂缝槽两侧形成很高的空隙压力梯度，容易破坏煤岩体而突出，这也是某矿突出的主导因素。当压裂的缝槽足够多，且呈现理想的分布时，水力压裂能在卸除地应力的同时，使得空隙压力大大降低，并且使其远离掘进巷道，空隙压力梯度也大大降低。2. 实例分析某矿煤层厚度3.7 m，倾角21，瓦斯压力1.6 MPa，瓦斯含量18 m3/t，垂深700 m左右，巷道掘进采用炮掘，利用高位巷道掩护煤层巷道掘进，高位巷和煤巷平距25 m左右，垂距4 m左右，从高位巷道向煤巷打4个钻孔，每孔间距30 m，采用30 MPa的高压水实施水力压裂，巷道已穿过前3个压裂钻孔控制区域，第四个压裂钻孔超前突出地点6 m左右，压裂时间超前突出12天。其措施及突出空洞示意图如图5所示： 图5 措施及突出空洞示意图 （a） 水力压裂钻孔 （b） 空洞正视图 （c） 空洞俯视图可见突出空洞分布在巷道掘进头前方上帮，空洞垂直高度3.6 m，宽7.3 m，长12 m，呈口袋形状。突出发生在水力压裂前方6 m左右，空洞的分布情况和图4（a）或4（b）的空隙压力集中区域分布类似，可能为此次突出的主要原因：由于压裂方向不可控制而导致空隙压力集中分布混乱，在巷道掘进过程中由放炮扰动而发生突出。3. 裂缝导向技术研究水力压裂技