热处理砂岩微观结构及力学性质试验研究

Experimental study of sandstone microstructure and mechanical properties under high temperature

采用核磁共振、电镜扫描、X射线衍射、单轴压缩等试验手段对某砂岩试样不同温度下(25~900℃)微观结构及力学性质的变化情况进行研究。研究表明:温度对砂岩试样微观结构具有重要影响,试样总孔隙率随温度的升高而升高,在150℃以下,由于中、大孔隙的减少,试样渗透性反而降低;当温度超过600℃时,中、大孔隙快速增加,试样渗透性能大幅增加;温度升高导致砂岩试样弹性模量减小、峰值应变增大以及孔隙压密阶段变长,宏观表现为脆性降低、塑性明显增强;热处理条件下,除微观结构的变化会导致砂岩试样力学强度改变外,试样矿物成分对其力学强度也具有十分重要的影响;450℃以下,由于矿物成分变化较小,试样力学强度主要受到孔隙度增加的影响,表现为强度随温度升高而降低;450~600℃,虽然孔隙率继续增长,但由于主要矿物高岭石发生脱水以及与其他离子形成新相矿物,进而导致试样强度没有随着孔隙增加而降低,反而出现一定的增长;超过600℃后,孔隙尤其是大孔隙的急剧增加,导致强度重新开始降低。

This research investigated the relationship between temperature(25-900℃)and the microstructure and mechanical properties of sandstone through the application of nuclear magnetic resonance,electron microscopy,X-ray diffraction and uniaxial compression techniques.The result firstly suggested temperature has a significant impact on sandstone microstructure because an increase in temperature can improve the total porosity.This phenomenon is especially obvious when temperature is higher than 600℃,in which the high temperature quickly increases the number of medium and large sized pores,which in turn lead to a significant permeability increase.Meanwhile,a temperature rise may also lead to a decrease in elastic modulus,an increase in peak strain and an elongation in pore compaction stage,these can be reflected by a decrease in brittleness and an obvious enhancement in plasticity.In addition,the mechanical strength of sandstone under heat condition may also be impacted by mineral composition.Such impact was not significant when temperature was below 450℃.That is because low temperature has minor impact on mineral composition,in this way,the rock strength was mainly influenced by porosity.But when temperature raised to 450-600℃,the dehydration of kaolinite and the formation of new phase minerals increased the rock strength dramatically,this offset the effects of porosity increase,therefore increased the strength of sandstone.However,when temperature exceeds 600℃,the rock strength decreases again because of a rapid increase in large sized pores.