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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 order to accurately understand the initiation, propagation and evolution of micro-and micro-cracks in rocks, the “arch” shale samples containing prefabricated edge cracks were uniaxially stretched under a scanning electron microscope with a tensile loading device Experimental, real-time observation of prefabricated edge crack tip micro-nano crack initiation and propagation process. After crack penetration, the in-situ observation of the distribution characteristics of the secondary micro-nano-forked bifurcation at the edge of the main fracture was continued. After the sample was taken out, the fracture morphology was observed under a scanning electron microscope. Based on the above microscopic observation, the mechanical process of micro-nano crack propagation in shale was analyzed. The results show that: (1) It is easier to successfully observe the initiation and propagation of micro-and nano-scale cracks in pre-cracked shales by using “arch” specimens containing prefabricated cracks. (2) The micro-and nano-scale cracks are usually initiated and cracked from the prefabricated crack tip. The horizontal and vertical crack growth occurs along with the longitudinal extension. Before the rapid brittle fracture, the crack propagates steadily. (3) The micro-nano-scale secondary bifurcation cracks usually start to crack and expand from weak micro-structural surfaces such as the natural micro-cracks at the edge of the main fracture, and finally crack in the edge of the main fracture, or the front edge turns and breaks with the main fault Crack confluence. (4) The main fault surface exposes the natural micropores and cleavage planes, the lamellar mineral bedding plane and the natural microcracks in the shale mineral crystals, which is beneficial to the formation of micro-cavities and micro-micro-scale pores Crack networks provide an effective microchannel for shale gas production.