【作者】 程庆迎；

【导师】 李增华； 黄炳香；

【作者基本信息】 中国矿业大学 ， 安全技术及工程， 2012， 博士

【摘要】 煤层水力致裂增透是低透气性煤层瓦斯抽采、突出煤层消突的有效技术途径，特别是对于单一低透煤层。本文以含瓦斯煤层水力致裂驱赶为研究对象，基于大量的实验和现场试验，并结合理论分析等研究方法对低透煤层水力致裂增透与驱赶瓦斯效应进行了较为系统的研究，具有重要理论意义和广泛工程应用前景。通过扫描电镜、压汞实验和亲水性实验等研究了煤层孔隙裂隙结构特征。煤层的内部结构由节理－割理与层理－微裂隙－孔隙四级空间结构体系组成。煤层质软、瓦斯的吸附解吸效应、天然裂缝发育等因素导致煤体水力致裂变得复杂。薛湖煤矿二2煤层裂隙比较发育，绝大多数裂隙宽度在1~10μm之间，孔隙率平均为4.73%，接触角平均51.15°，润湿性好；孔隙体积主要由微孔构成，小孔次之；表面积构成主要由微孔贡献构成。孔隙率大，骨架分维数会降低，逾渗分维数增加。煤的润湿性影响毛细管压力大小，进而影响含瓦斯煤层水力致裂增透抽采过程中水和瓦斯两相不混溶流体在孔隙裂隙系统中流动的界面压力差。采用真三轴水力致裂实验系统研究了煤体水力致裂的裂缝扩展规律。水压裂缝的形态整体呈椭圆形态扩展，水压裂缝的扩展方向平行于最大主应力方向。最大主应力与最小主应力的差值越小，对应的破裂压力越大；围压主应力差越小，水压裂缝越容易发生空间转向。钻孔径向水压裂缝的产生首先必须满足最小主应力方向为钻孔轴向方向，同时沿钻孔致裂段轴向存在局部应力集中或缺陷。增大水力致裂排量，裂缝扩展的动力效应变明显，水压裂缝越易出现分叉与转向，裂缝表面的粗糙度也增大。煤层水力致裂会形成以三维应力控制的水压主裂缝为基础，两侧是交叉贯通扩展的成组张开节理的空间裂缝网络系统。含瓦斯煤层水力致裂应采用大排量快速致裂工艺，达到产生较多且范围大的水压裂缝，利于后续排水采气等目的。在MTS试验机上采用瞬态压力脉冲法实验研究了煤体固液耦合的结构及渗透性演变规律，包括煤体全应力应变、卸围压和变渗透水压差过程的渗透性变化测试。煤体的渗透性与其内部的结构密切相关。在固液耦合不产生损伤的条件下，瞬态渗透系数整体与水力梯度成正比关系。在固液耦合作用下，渗透水压力可引起结构面的错动闭合或导致破裂碎屑集聚堵塞渗流通道。在假三轴和恒定轴压条件下，煤样卸围压过程中的变形主要为径向膨胀变形。卸围压过程中，体积应变综合反映了煤样内裂隙增生、张开等损伤。当轴压大于单轴抗压强度且小于三轴抗压强度时，煤样卸围压过程中典型的卸围压-体积应变曲线分为三个阶段，分别为弹性变形恢复、塑性变形和破坏阶段。煤体卸围压过程渗透系数的变化规律与其体积应变变化规律一致，煤样的损伤和渗透系数的变化存在卸围压阀值和卸围压拐点。高于此阀值，渗透系数快速增加；煤样的力学性质和初始应力状态决定了卸围压阀值大小。卸围压超过拐点值，煤样宏观破裂，渗透系数急剧增大。发现并研究了含瓦斯煤层水力致裂的瓦斯驱赶现象。含瓦斯煤层水力致裂在产生水压裂缝的同时，压力水向裂缝两侧渗透，孔隙水压力与孔隙裂隙内的游离瓦斯接触，引起裂缝围岩内孔隙水和瓦斯压力的变化，孔隙压力分布的不均匀会产生孔隙压力梯度。游离态瓦斯由孔隙（瓦斯）压力高的位置向孔隙（瓦斯）压力低的位置运移。瓦斯压力梯度是煤层水力致裂驱赶瓦斯的根本动力。煤矿井下高瓦斯煤层注水常会引起回风流瓦斯浓度有一定的升高，这是煤层注水驱赶瓦斯导致的结果。突出煤层掘进工作面深孔水力致裂后，后续掘进过程中煤体瓦斯含量呈现“低-高-低”的现象，验证了含瓦斯煤层水力致裂瓦斯驱赶现象的存在。水压裂缝扩展的不均匀性和瓦斯驱赶的时间效应会引起瓦斯驱赶的不均匀。在实践中针对瓦斯驱赶现象应该扬长避短。提出并实施了突出煤层深孔水力致裂驱赶与浅孔抽采消突技术。通过优化水力致裂钻孔布置、互为卸压孔和观测孔、深封孔短封孔长度等保障了突出煤层掘进头超前深孔水力致裂驱赶的安全性。实践证明，通过超前深孔水力致裂和浅孔瓦斯抽采相结合的方式，实现了突出煤层掘进头增透、弱化、瓦斯驱赶、抽采与注水湿润的有机结合，有效地提高了瓦斯抽采率和消除了突出危险性。薛湖矿突出煤层掘进节省了10个区域孔的工程量和施工时间，减少了瓦斯抽采时间，具有显著的技术经济效益。更多还原

【Abstract】 Hydraulic fracturing for increasing permeability in coal seam is an effectivetechnological approach to gas extraction in low-permeability coal seam and outburst removalin outburst coal seam, especially in single low-permeability coal seam. Based on a lot ofexperiments and filed tests, this paper takes hydraulic fracturing driving for gassy coal seamas research object and carries out a systematic study on permeability improvement andmethane driven effect of hydraulic fracturing for low permeability coal seam with the methodof theoretical analysis, laboratory and site experiments, which is provided with importanttheoretical significance and widely engineering application prospects.Study on pore and fracture structure characteristics of coal seam are conducted byscanning electron microscope, mercury penetration test and hydrophilic experiment, etc. Theinternal structure of coal seam is constituted by jiont fissure, cleat and bedding, microfractureand pore levels. Those factors including soft coal seam, gas adsorption and deabsorptioneffect and developed natural fissures make hydraulic fracturing for coal-rock mass complex.In22coal seam of Xuehu colliery, fissures are very developed and most of them are1~10μmwidth. Average porosity is4.73%, average contact angle51.15°, wettability good. Porevolume mainly consists of micro pore and then small pore, superficial area is mainlycomposed of micro pore. If the porosity rises, fractal dimensions of skeleton reduce. Fractaldimensions of percolation are close related to porosity, namely that when the porosityincreases, fractal dimensions of percolation augment. Coal wettability affects capillarypressure, and then pressure difference of two phase interface when two kinds of immisciblefluid, water and gas, flow in pore and fracture system of coal mass during increasingpermeability and extracting gas by hydraulic fracturing for gassy coal seam.Research on crack propagation law of hydraulic fracturing for coal mass is performedwith the help of true triaxial hydraulic fracturing experiment system. Hydraulic crackpropagates like elliptic form on the whole and the propagation direction parallels to themaximum principal stress direction. The less the difference value of maximum and minimumprincipal stress is, the higher the rupture pressure of corresponding sample is. The less theprincipal stress difference of confining pressure is, the easier spatial turn of hydraulic crackoccurs. Hydraulic crack generates along drilling hole radial direction must satisfy that theminimum principal stress direction is parallel axial direction of drilling hole at first and localstress concentration exists along the axial direction of drilling hole fracturing segment. When the output volume of hydraulic fracturing rises, dynamic effect of crack propagation becomesobvious, hydraulic crack is easier to bifurcate and turn, crack surface roughness also increases.Hydraulic fracturing for gassy coal seam should adopt fast fracturing technology with bigoutput volume in order to create more hydraulic cracks with big range and then be beneficialto water drainage and gas extraction, etc.Fluid-solid coupling structure and permeability evolution law of coal mass is researchedon MTS test machine with transient pressure pulse method, including overall stress and strainof coal mass, test on permeability variation during confining pressure relief and seepagewater pressure difference change. The permeability of coal mass is close related to its internalstructure. Under the condition of fluid-solid coupling without damage, transient permeabilitycoefficient is proportional to hydraulic gradient as a whole. Depend on fluid-solid coupling,seepage water pressure can cause dislocation and closure of structural plane or make rupturedebris gather and block seepage channel. Under the condition of false triaxial and constantaxial compression, the deformation of coal sample is mainly radial expansion deformation inthe process of confining pressure relief. During confining pressure relief, volume straincomprehensively reflects coal sample damage of crack growth and expansion. When axialpressure is above uniaxial compressive strength and below triaxial compressive strength, coalsample typical curve of confining pressure relief and volume strain in the process ofconfining pressure relief contains three stages: elastic deformation recovery stage, plasticdeformation stage and failure stage. The variation rule of permeability coefficient accordswith coal sample volume strain. Coal sample damage and permeability coefficient variationhas confining pressure relief threshold and inflection point in exist. Above this threshold,permeability coefficient rises rapidly. Mechanical properties and initial stress state of coalsample determine the value of confining pressure relief threshold. When confining pressurerelief exceeds the inflection point, coal sample gets macro rupture and permeabilitycoefficient increases sharply.Gas driving phenomenon of hydraulic fracturing for gassy coal seam is found. Whenhydraulic fracturing for gassy coal seam creates hydraulic crack, pressure water permeatesboth sides of crack at the same time. Pore water pressure contacts free gas inside pore andfissure, leading to the variation of gas pressure and pore water in crack surrounding rock.Uneven distribution of pore water can generate pore pressure gradient. Free gas migrates fromthe location of high pore (gas) pressure to that of low pore (gas) pressure. Gas pressuregradient is the fundamental power for gas driving of hydraulic fracturing for gassy coal seam.Water injection into gassy coal seam underground mining often makes gas density in returncurrent increase to a certain extent, which is induced by gas driving under coal seam injection. After deep hole hydraulic fracturing in driving face of outburst coal seam is carried out, gascontent in coal mass presents the phenomenon of “low-high-low”, verifying the presence ofgas driving phenomenon of hydraulic fracturing for gassy coal seam. The inhomogeneity ofhydraulic crack propagation and time effect of gas driving can lead to the inhomogeneity ofgas driving. In practice, fostering strengths and circumventing weaknesses of gas drivingphenomenon is in demand.The technology of increasing permeability and removing outburst by hydraulicfracturing driving in deep hole and gas extraction in shallow hole of outburst coal seam isproposed and put into effect. By means of optimizing drilling hole arrangement of hydraulicfracturing, alternating pressure relief hole and observation hole, reasonably determining thedeep hole sealing position and short hole sealing length, the safety of hydraulic fracturingdriving in leading deep hole of tunneling place of outburst coal seam is guaranteed. Asexperience proves, in combination with hydraulic fracturing in deep hole and gas extractionin shallow hole, the technology of removing outburst by hydraulic fracturing driving inleading deep hole of tunneling place of outburst coal seam and gas extraction in shallow holerealizes the dynamic integration of permeability increasing, weakening, gas driving,extraction and water injection humidification, effectively improves gas extraction efficiencyand eliminates outburst danger. Tunneling driving in outburst coal seam for Xuehu collierysaves work amount and time of10regional holes, and reduces time of gas extraction, andobtains prominent technical and economic benefits.更多还原