【作者】 胡胜勇；

【导师】 周福宝；

【作者基本信息】 中国矿业大学 ， 安全技术及工程， 2014， 博士

【摘要】 钻孔抽采是煤矿瓦斯灾害防治的主要技术措施，其中钻孔密封是决定瓦斯抽采效果的重要环节。然而瓦斯抽采过程中煤岩裂隙发育导致瓦斯抽采浓度低、衰减快，存在重大安全隐患：抽采系统内低浓度瓦斯易落入爆炸极限范围内，存在爆炸风险；孔外裂隙内高浓度氧气易引起煤体氧化自燃，可诱发瓦斯燃烧（或爆炸）。本文以提高瓦斯抽采浓度为切入点，对钻孔围岩渗流特性和粉体封堵机理进行了研究，为有效封堵孔外漏气裂隙提高瓦斯抽采浓度提供理论依据。主要成果如下：采用理论分析、模型试验和现场实测相结合的方法研究了钻孔周边裂隙区分布特性：（1）钻孔开挖形成的裂隙区范围是在巷道开挖形成的围岩裂隙区基础上沿钻孔轴向呈“先增、后减、随后稳定”的形式扩展；（2）钻孔开挖卸荷后，孔上、下方形成垮塌区，左右两侧形成破碎区，最终由圆形变成“类橄榄球”形，且其长轴与最大主应力方向垂直；（3）裂隙区内主要存在宏观和微观的直、弯折和分叉裂隙三种裂隙形态，大部分宏观裂隙的宽度和长度分别小于5mm和50mm。采用实验室试验和数值计算的方法研究了巷道及钻孔先后开挖引起的有效应力与渗透性演化特性：（1）巷道开挖使巷道径向方向依次形成完全渗流区（裂隙区）、过渡渗流区、渗流屏蔽区和原岩渗流区。钻孔开挖使“四区”以钻孔为对称轴沿钻孔方向继续扩展；（2）完全渗流区和渗流屏蔽区内煤体渗透率随有效应力增大而呈负指数递减变化，随瓦斯压力增大而降低；完全渗流区和渗流屏蔽区内的渗透率分别较原岩渗流区高2个数量级和低2倍以上；（3）对于低渗透性煤层，瓦斯抽采过程中渗流屏蔽区阻隔了本区及本区以外煤体瓦斯流入孔内，而巷道中空气却易经完全渗流区进入孔内，最终导致钻孔抽采浓度降低。应用双流体模型研究了裂隙内的气固两相流运动规律：（1）直裂隙发生堵塞前，底部颗粒沉积层以整体蠕动的方式向前运动，堵塞发生需要的时间尺度很小；（2）各种影响因素的重要程度：加料量>表观气速>弯折角度，弯折裂隙的弯折角度对堵塞过程的影响较小；（3）分叉裂隙的主管段更易堵塞，钝角分叉裂隙比锐角更易于发生堵塞。利用自主构建的粉体封堵裂隙实验平台研究了粉体封堵直、弯折和分叉裂隙的堵塞特性：（1）弯折裂隙堵塞的临界粉体质量流量与气速间呈递增的关系。直裂隙发生堵塞的临界粉体质量流量也随气速的增大而增大，依次分为三个典型阶段：中速增长阶段、低速增长阶段和高速增长阶段，比弯折裂隙多了中速增长阶段；（2）随气速增大，弯折裂隙堵塞的临界粉体固气比呈幂指数递减变化，这与已有的研究理论和实验结果相一致。但不同的是，发现存在最小固气比，当气速大于最小固气比对应的气速时，固气比反而随气速的增大呈幂指数递增变化；（3）直、分叉裂隙堵塞的临界气速与固气比间呈“S”型关系，即随气速增大，固气比呈“增、减、增”变化。由于低气速输送粉体易造成管路堵塞，基于此，直、分叉裂隙也存在最小固气比；（4）建立了直、弯折和分叉裂隙堵塞边界的经验公式，其相对误差在合理范围内，可用于工程实践。研究成果在平煤股份八矿进行了应用，结果表明：采用本文研究成果后，钻孔瓦斯抽采浓度比原先增幅一倍以上。更多还原

【Abstract】 Methane extraction from boreholes is the main approach for prevention andcontrol of methane disasters in coal mine, and borehole sealing is an important linkwhich determines methane extraction effect. However, the growth of coal and rockcracks in the process of methane extraction leads to low concentration and fastattenuation, leaving a significant hidden danger: there is risks of explosion that lowconcentration methane in methane extraction system is easy to fall into the explosionlimit range; there is high concentration oxygen in the cracks around borehores that islikely to cause the oxidation and spontaneous combustion of coals, may inducingmethane burning. In order to improve the methane extraction concentration, this paperstudies the seepage characteristics around borehole and blockage mechanism ofpowder, which presents theoretical support for effective plugging the leakage cracksaround boreholes and the improvement of methane extraction concentration. The mainachievements are as follows:Using the combination of methods including theoretical analysis, model test andfield test to study the fracture distribution characteristics around boreholes:(1)Fracture zone induced by borehole excavation is based on the formation of thefracture zone induced by roadway excavation along borehoe axial "first increases,then decreases, then stability";(2) After borehole excavation unloading, the zonesabove and below the borehore collapse, on the left and right sides become brokenzones, and the round shape of borehoe finally comes to a "class a football" form, andits long axis and the maximum principal stress are vertical in the direction;(3) Thereare the macroscopic and microscopic straight, bending, and bifurcation slits in fracturezone, most width and length of the macroscopic slits are less than5mm and50mmrespectively.The effective stress and permeability redistributions induced by successiveroadway and borehole excavations, are studied through the methods of laboratory testand numerical calculation:(1) After roadway excavation, four zones, including thefull flow zone, the transitive flow zone, the flow shielding zone, and the in situ rockflow zone are consecutively formed along the radial direction of the roadway. Afterborehole excavation, these zones expand along the borehole direction;(2) Theeffective stress-permeability relationship in the full flow zone and the flow shieldingzone is a piecewise exponential function, and the coal permeability reduces with the increase of methane pressure. The permeability of the coal mass in the full flow zoneis two orders of magnitude higher than that of the original coal, and the permeabilityin the flow shielding zone is minimized at a value two times lower than that of theoriginal coal mass;(3) For all coal seams with low permeability when drainingmethane around boreholes, the flow shielding zone may prevent the methane in thiszone and the other zones from flowing into boreholes, whereas air in the roadwaycould easily pass the full flow zone and enter the boreholes, which eventually leads tolow methane concentration obtained from boreholes.Application of the two-fluid model to research the movement of gas-solidtwo-fluid flow in slit:(1) Before the blockage of a straight slit, at the bottom of theparticle sedimentary sequences creep travel forward in the form of integral, and theblockage time is very small;(2) The importance of the various factors affectingblockage is as follows: feeding amount> superficial gas velocity> bending angle.The bending angle of bending slit has less effect on the blocking process;(3) Themain pipe section of bifurcation slit is easy to block, and obtuse angle of bifurcationslit is easier than acute angle to block.The experiment platform for simulating the blockage of slits using powder wasestablished independently to research the blockage characteristics of straight, bendingand bifurcation slits:(1) For the bending slit, the solid mass flow rate for blockageincreases as the air velocity increases. For the straight slit, the relationship betweenthe solid mass flow rate and air velocity is also like that and it could be divided intothree typical stages in turn: medium growth stage, slow growth stage and rapid growthstage. The straight slit has stage of medium growth which the bending slit isinexistence;(2) The solid loading ratio and air velocity in the bending slit display apower function decreasing relationship. This finding is in agreement with existingtheory and experiment results. What is different, however, is that we found aminimum solid loading ratio exists. When air velocity is greater than the minimumsolid loading ratio’s corresponding velocity, the solid loading ratio and air velocityactually exhibit a power function increasing relationship;(3) The solid loading ratioand air velocity in the straight and bifurcation slits display a "S" shape relationships:with the increasing of air velocity, the solid loading ratio increases at first, thendecreases, and finally increases. Because low speed conveying of powder is easy tocause pipe blockage, straight and bifurcation slits also have the minimum solidloading ratio;(4) The formulas about the blockage condition of straight, bending and bifurcate slits are established, and the relative error of formulas is in the reasonablerange, which can be used in Engineering.The results of field experiments in No.8Mine of Pingdingshan Mines showedthat the application of the research results in this paper could increase theconcentration of methane two times as high as the original one of less than30%in theboreholes.更多还原