松软煤层及砂岩顶板水力挠动卸压增透效果对比分析

Gas pressure relief-permeability increase effect comparative analysis about hydraulic disturbance to soft coal seam and its sandstone roof

为了考察水力挠动措施对软煤及砂岩的卸压增透效果,克服采取高压注水进行煤层瓦斯增浓提效的盲目性。采用自行设计改造的"真三轴水力挠动-瓦斯渗流实验装置"对正方体砂岩及松软煤样试件进行了水力挠动实验。根据泵注压力实时演化曲线分析掌握了2种试件注水过程中裂隙发育、延展规律:泵注过程中砂岩试件经历压裂—多次压裂—完全破裂,最终被高压水完全压裂,其内部裂隙得到充分扩张、衍生,最终相互贯通的裂隙网形成;相反,软煤样试件则经过压裂—压实—闭合,最终被压实闭缝,内部裂隙网络未能展开连通。基于挠动前后渗透率分析对比的基础上实验验证了水力挠动措施对2种试件的适用性:挠动后砂岩试件的渗透率成百倍增加,而软煤试件的渗透能力几乎丧失。与模拟实验相对应,在试验井松软煤层及顶板砂岩中分别布置钻孔进行高压水挠动,措施后采用相关指标对挠动后的瓦斯抽采效果进行了考察:砂岩挠动钻孔措施后的抽采流量、浓度及纯量大幅增加,衰减系数急剧下降;而煤层挠动钻孔相应指标则呈相反趋势。实验结果及现场工程实践均表明:水力挠动作用下砂岩内部发生脆性变形,持续高水压作用下裂隙较好地发育、延展、贯通,较发育的网络系统形成,为下部煤体瓦斯解吸、扩散和运移提供了畅通的流动通道。相反,尽管挠动初期松软煤体内部出现裂隙,但随着注水持续,煤体发生塑性变形,水与煤体形成的煤泥堵塞了天然及人造裂缝(隙),软煤逐渐被压实,瓦斯流动性弱化,渗透性降低。研究成果表明转移挠动对象松软煤体至其坚硬顶板是松软煤层卸压增透的有效途径之一,应用于不适宜采用水力挠动的低透气性突出煤层有望取得较理想的瓦斯治理效果。

In order to investigate the gas pressure relief-permeability increase effect of hydraulic disturbance to soft coal and sandstone,and overcome the blindness of adopting high-pressure water injection to increase the concentration and efficiency of coal seam gas.Hydraulic disturbance experiments on cubic sandstone and soft coal body samples are carried out with the self-designed modified"true triaxial hydraulic disturbance-gas seepage test device".The law of crack development and extension about the two cubic samples during the water injection are obtained according to the analysis about the real-time pumping pressure evolution curve of pumping pressure:undergoing fractured-multiple fractured-completely broken,finally completely fractured by high-pressure water during the pumping process,and its internal fractures fully expanded and derived,and finally forming intercon-nected fracture network.On the contrary,undergoing fractured-compacted-closed,the soft coal samples are finally compacted and closed,and the internal fracture network fails to expand and communicate.The applicability of hydraulic disturbance to the two kinds of samples is experimentally validated based on the permeability comparative analysis before and after hydraulic disturbance:the permeability of soft coal samples almost loses.In the test coal mine,the fracturing boreholes are arranged in the soft coal seam and its roof sandstone respectively for the verification corresponding to the simulation experiment,the gas drainage effect after hydraulic disturbance is investigated adopting the relative indicators:after hydraulic disturbance,the gas drainage flow rate,the concentration and purity of sandstone drilling increase greatly,and the attenuation coefficient decreases sharply,however,the corresponding indexes of soft coal seam drilling shows the opposite trend.Both experimental results and field practice show that the brittle deformation occurs within the sandstone under the action of hydraulic disturbance,the internal fracture develops,extends and penetrates well under the action of continuous high water pressure,the well-developed network system forms,providing a smooth flow channel for the gas desorption,diffusion and migration from the lower coal body.On the contrary,in the initial stage of hydraulic disturbance,there are cracks in the soft coal body,but as the water injection continues,plastic deformation occurs inside the coal body,natural and man-made cracks(gaps)are blocked by coal slime mixed with water and coal body,and the coal body is gradually compacted,gas fluidity weaken,and gas permeability decreases.The research results show that for soft coal body,transferring the hydraulic disturbance object to its hard roof is one of the effective ways to relieve pressure and increase permeability in soft coal seam,it is expected to achieve ideal gas control effect when it is applied to low permeability outburst coal seam which is unsuitable for hydraulic disturbance.