基于核磁共振监测的砂岩强度软化实验及微观机制研究

Experimental study on strength softening behaviors and micro-mechanisms of sandstone based on nuclear magnetic resonance

随着煤炭开采向深部不断发展,岩石所处地质条件愈发复杂,地下水作用使得围岩强度明显降低,严重影响现场人员及生产安全。为研究巷道开挖后水岩相互作用对围岩性质的影响,以万福煤矿深部巷道砂岩为研究对象,通过X射线衍射(XRD)、扫描电镜(SEM)、压汞(MIP)及核磁共振实验(NMR),对砂岩样品矿物成分、孔隙尺寸及微观结构进行监测,研究孔隙结构随含水率的动态变化规律。结合关键含水率(0%,0.8%,1.6%,2.4%,3.3%)的室内力学实验结果,探讨砂岩强度软化机制。研究表明:(1)本次砂岩核磁共振T2谱曲线呈现单峰特征,随着含水率增加,总谱面积不断增加,峰值对应弛豫时间不断右移。根据实验过程中谱面积占比出现稳定阶段及相互转化阶段,将传统吸水三阶段过程进一步分为急速吸水、均匀吸水、孔隙转化及稳定吸水4个阶段,其中,孔隙转化过程主要发生在第Ⅲ阶段(31~113 h);(2)砂岩强度随含水率增加呈非线性衰减,衰减程度受围压影响,随围压增大强度软化程度越低,其中,单轴抗压强度软化系数为0.372;(3)实验过程中黏土矿物的膨胀及石英骨架的约束是小孔闭合的原因,而石英颗粒的错动、孔隙水的流动及可溶性矿物的溶解,会导致吸水过程中孔隙变大;(4)对不同吸水阶段的软化机制进行探讨,除了结合水膜的润滑作用、黏土矿物胶结能力下降外,孔隙的扩张、连通也是导致砂岩软化的重要原因,且影响砂岩最终的破坏形态。

As coal mining goes deeper,the rocks are in more complex geological conditions. The strength of the surrounding rock is obviously reduced due to the action of groundwater,which seriously affects the personnel on site and the safety of production. In order to study the influence of water-rock interaction on surrounding rock properties after roadway excavation,the sandstone in a deep roadway of Wanfu Coal Mine was taken as the research object. The mineral composition,pore size and microstructure of sandstone samples were monitored by X-ray diffraction(XRD),scanning electron microscope(SEM),mercury intrusion(MIP) and nuclear magnetic resonance(NMR),and the dynamic variation of the pore structure with the water content was studied. According to the mechanical test results of key water contents(0%,0.8%,1.6%,2.4% and 3.5%),the strength softening mechanism of sandstone is discussed. The results show that the NMR T2 spectrum curve of sandstone presents a single peak characteristic. With increasing the water content,the total spectral area increases continuously,and the relaxation time corresponding to the peak shifts to the right. According to the stable stage and transformation stage of the spectral area proportion in the process,the traditional three-stage water absorption process can be further divided into four stages including rapid water absorption,uniform water absorption,pore transformation and stable water absorption,in which the pore transformation process mainly occurrs in the Ⅲ stage(31–113 h). The strength of sandstone decreases non-linearly with increasing the water content,and the degree of attenuation is affected by the confining pressure. The softening degree of the strength decreases with increasing the confining pressure,and the softening coefficient of the uniaxial compressive strength is 0.372. It is believed that the expansion of clay minerals and the restriction of quartz framework are the reasons for the closure of the pores,and that the dislocation of quartz particles,the flow of pore water and the dissolution of soluble minerals will lead to the enlargement of the pores during the water absorption process. The softening mechanism at different water absorption stages is discussed. It is indicated that,in addition to the lubrication of bound water film and the decrease of the cementing capacity of clay minerals,the expansion and connectivity of the pores are also important reasons for the softening of sandstone and affect the final failure morphology of sandstone.