基于低场磁共振的北山花岗岩热损伤研究

Thermal damage study of Beishan granite based on low field magnetic resonance

研究岩石的热破裂和损伤机制在高放废物地质处置工程中越来越受到重视。以我国甘肃北山的高放废物处置库的花岗岩岩样为研究对象,采用低场核磁共振系统、MTS岩石力学试验机、倒置镜像光学显微镜对重点预选场址花岗岩的热稳定性开展室内试验研究。研究发现:①核磁共振T 2谱图在0~400℃没有明显变化,当温度高于500℃时,T 2谱图振幅显著增大且向右大幅移动,T 2谱面积与孔隙率在加热过程中呈现幂率关系;②峰值应力随着温度和孔隙度的增加以幂律关系降低;③通过核磁共振成像(MRI)发现,温度低于500℃时质子密度分布均匀且没有发现明显的质子密度簇,说明岩石内部结构稳定。当温度高于500℃时,晶体裂纹和边界裂纹的产生致使出现大量的高质子密度区域,并随着温度持续升高质子密度高的微小区域融合成大的连通区域;④花岗岩岩样在不同温度条件下的核磁成像像素的概率密度函数都服从对数正态分布,当温度超过500℃,概率密度函数整体向右转移;⑤通过显微结构观察,500℃时在长石晶间和长石晶粒与石英晶粒间有边界裂纹产生,并在晶体内部出现少量的穿晶裂纹,当试样温度加热到600℃时,超过了石英α/β相变点,石英颗粒产生大面积透明状穿晶裂缝,同时伴随着明显的穿晶网络。

The thermal rupture and damage mechanism of rock has received more and more attention in the high-level waste geological disposal project.The granite rock samples from the high-level waste disposal repository in Beishan,Gansu Province,China were collected in this study.The thermal stability of granite was investigated by low-field nuclear magnetic resonance,MTS rock mechanics test machine and inverted optical microscope.It was found that①NMR T 2 spectrum has no significant change from 0℃to 400℃.When the temperature is higher than 500℃,the amplitude of the T 2 spectrum increases significantly and shifts to the right.The T 2 spectral area and porosity show a trend of power-law during heating;②As the increase of temperature and porosity,the peak-stress decreases with power-law relationship;③It was found by magnetic resonance imaging(MRI)that the proton density distribution is homogeneous and no obvious proton density clusters appear below 500℃,explaining the internal structure of granite is stable.When the temperature is higher than 500℃,a large number of high proton density regions caused by the occurrence of crystal and boundary cracks,and the micro-regions with high proton density merge into a large connected region as the temperature increases continuously;④The probability density function of the MRI pixel of granite obeys the lognormal distribution under different temperatures,the probability density function moves to the right with the occurrence ofα/βphase transition;⑤According to microstructure observation,boundary cracks generate between feldspar crystals or between feldspar grains and quartz grains at 500℃,and a small amount of transgranular-cracks appear inside the crystals.The quartz particles produce large-area transparent transgranular cracks accompanied by a distinct transgranular network when the temperature exceeds theα/βphase transition point.