页岩微纳观断裂的原位观测

为了准确理解岩石微纳观裂纹起裂、扩展及演化规律,在带有拉伸加载装置的扫描电子显微镜下,对含有预制边缘裂纹的"拱形"页岩试样进行单轴拉伸试验,实时观测预制边缘裂纹尖端部位的微纳观裂纹起裂及扩展过程。在裂纹贯通后,继续原位观测主断裂裂纹边缘次级微纳观分叉裂纹的展布特征。取出样品后,在扫描电子显微镜下观测断口形貌,基于上述显微观测分析了页岩微纳观裂纹扩展的力学过程。研究表明:(1)采用含有预制裂纹的"拱形"试样更易于成功观测到页岩预制裂纹尖端微纳观裂纹的起裂及稳定扩展过程。(2)微纳观裂纹一般从预制裂纹尖端部位萌生、起裂,裂纹的横向伸展与纵向延伸相伴发生,在快速的脆性断裂前,经历短暂的裂纹稳定扩展过程。(3)微纳观次级分叉裂纹一般从主断裂裂纹边缘的天然微裂纹等弱结构面处起裂和扩展,最终在主断裂裂纹边缘区域止裂,或者前缘发生转向并与主断裂裂纹汇合。(4)主断裂面揭露出页岩矿物晶体内部的天然微孔洞和解理面、层片状矿物的层理面以及天然微裂纹,这有利于在微纳观尺度上形成微孔洞和微裂纹网络,为页岩气开采提供有效的微观通道。

In-situ Observation on the Micro-nano Cracks of Shale

For better understanding the law of micro-nano crack initiation,growth and evolution in rock,uniaxial tension tests of arch shale specimen with precrack were carried out under Scanning Electron Microscope (SEM) with loading devices.During tensile loading,the real-time process of micro-nano crack initiation and growth at the precrack tips were observed under the SEM and photos were also taken.After crack coalescence,observations on the secondary branch cracks in micro-nano scale nearby the main fracture crack were preceded in situ.After fetching out the specimen,fracture morphology of each specimen was observed under SEM.Then the mechanical process of the micro-nano crack growth was.We learn from the observations that,(1) the use of the pre-cracked arch shale specimen contributes to the successful observations on the process of initiation and stable growth of micro-nano cracks;(2) micro-nano cracks generally initiate from the precrack tips due to stress concentration and then propagate in the way of both lateral extension and longitudinal extension.The crack will experience a short process of stable crack growth before rapid brittle fracturing;(3) the secondary branch cracks in micro-nano scale generally initiate from the weak structural discontinuities such as the natural micro crack nearby the main fracture crack.And the branch cracks arrest at the zone around the main fracture crack or turn to converge on the main fracture crack;(4) microvoids,cleavage planes,bedding planes and other natural micro cracks could be exposed by tension fracturing,which helps formatting micro crack networks and providing efficient micro channels for shale gas exploitation.