井下碎软煤层顶板加砂分段压裂瓦斯高效抽采技术

High-efficiency gas extraction technology of staged fracturing roof with sand of underground broken and soft coal seam

碎软煤层瓦斯治理常采用的底板穿层钻孔抽采瓦斯方式存在掘进工程量大、治理周期长、钻孔揭煤段短、抽采治理效果受限等问题,顺层短孔抽采方式存在成孔性差、抽采钻孔短、抽采区域小等问题。通过统计淮北、淮南、焦作、晋城、阳泉5个典型碎软煤层矿区煤层及其顶底板围岩力学参数和地应力,可得顶板岩层弹性模量为碎软煤层的2.56~6.71倍,泊松比为煤层的0.48~0.84倍,分析认为碎软煤层顶板岩层具有高弹性模量、低泊松比特征,顶板较碎软煤层更易压裂改造。参考地面煤层气水平井顶板加砂分段压裂思路,提出了井下碎软煤层顶板加砂分段压裂瓦斯抽采思路,即在煤层顶板稳定岩层中施工定向长钻孔(与煤层距离一般小于10 m),对钻孔由里向外逐段携砂压裂,形成以定向长钻孔将岩层完全联通、煤岩层中压裂缝网将煤层充分沟通的多级缝网,通过支撑剂保障缝网处于开启状态,实现碎软煤层瓦斯顶板定向长钻孔大区域高效抽采。建立了山西新景矿煤业有限责任公司某工作面3号煤层顶板加砂压裂地质模型,采用FracproPT软件对煤层及顶板水力加砂压裂进行数值模拟,得出顶板压裂裂缝在垂直方向上主要向煤层延伸,在水平方向上压裂缝长为煤层压裂缝长的3.49倍,表明碎软煤层顶板间接压裂较煤层直接压裂效果更好。在该工作面3号煤层顶板施工2个609 m定向长钻孔进行水力加砂分段压裂瓦斯抽采工程应用试验,2个钻孔压裂影响半径为20~38 m,压裂钻孔瓦斯抽采纯量分别为1025.11,2810.60 m^(3)/d,百米瓦斯抽采纯量为同区域顺层未压裂钻孔的5.6~15.4倍,实现了碎软煤层大区域瓦斯高效抽采。

The gas extraction method of floor cross-layer drilling commonly used in gas control of broken and soft coal seam has problems such as large excavation quantity,long control period,short coal uncovering section of drilling,limited extraction control effect and so on.The gas extraction method of bedding short hole has problems such as poor drill-forming property,short extraction drilling hole,small extraction area and so on.This paper makes statistics on mechanical parameters and in-situ stress of coal seams and their roof and floor surrounding rocks in five typical broken and soft coal seam mining areas in Huaibei,Huainan,Jiaozuo,Jincheng and Yangquan.It is concluded that the elastic modulus of roof rock is 2.56-6.71 times of that of the broken and soft coal seam.The Poisson's ratio is 0.48-0.84 times of that of the coal seam.The analysis shows that roof rock of broken and soft coal seam is characterized by high elastic modulus and low Poisson's ratio.The roof is easier to be fractured than broken and soft coal seam.Referring to the idea of staged fracturing roof with sand of the horizontal borehole of surface coalbed methane,the idea of gas extraction by staged fracturing roof with sand of underground broken and soft coal seam is put forward.The directional long boreholes in stable strata of coal seam roof are constructed.The boreholes are generally less than 10 m away from the coal seam.The sand-carrying fracturing shall be carried out from the inside to the outside section by section in the boreholes.It will form a multi-stage fracture network in which the rock layer fully connects through directional long holes and the coal seam fully connects through fracture network in coal strata.The proppant is used to ensure that the fracturing network is in open state,so as to realize efficient gas extraction in a large area by the directional long hole in roof of broken and soft coal seam.The geological model of fracturing roof with sand in No.3 coal seam of a working face in Shanxi Xinjing Coal Industry Co.,Ltd.is established.The numerical simulation of hydraulic fracturing coal seam and roof with sand is carried out by using FracproPT software.The result shows that the fracturing cracks in roof mainly extend to the coal seam in vertical direction.The length of fracturing cracks in roof in horizontal direction is 3.49 times of that of fracturing cracks in coal seam.This result shows that indirectly fracturing roof of broken and soft coal seam is better than directly fracturing the coal seam.Two 609 m directional long boreholes are drilled in roof of the No.3 coal seam in the working face to carry out gas extraction engineering application test of hydraulic staged fracturing with sand.The fracturing influence radius of the two boreholes is 20-38 m.The gas extraction pure amount of the fracturing holes is 1025.11 m^(3)/d and 2810.60 m^(3)/d respectively.The gas extraction pure amount of 100 m is 5.6-15.4 times of that of bedding un-fracturing boreholes in the same area.This study realizes high-efficiency gas extraction in a large area of broken and soft coal seam.