围压及烧变温度对砂岩力学性质和渗透性的影响规律研究

Influence patterns of confining pressure and burning temperature on mechanical properties and permeability of sandstone

摘要

【目的】煤层自燃产生的高温会造成围岩热损伤，导致其结构和力学性质发生显著改变，诱发突水、顶板垮塌等地质灾害。为探究不同烧变程度及埋深条件下岩石的力学性质、渗透性及破裂特征演变规律，【方法】以经过不同高温预处理后的砂岩为研究对象，开展了恒定渗透压、不同围压条件下的三轴压缩渗流试验，并同步进行声发射监测。【结果】结果显示：砂岩的力学性质随围压增大逐渐增强，随烧变温度升高先略微增强后逐渐劣化；不同围压和烧变温度条件下渗透率均呈先下降后上升的变化趋势。【结论】结果表明：(1)三轴压缩过程中，砂岩的峰值强度和弹性模量随烧变温度升高先增大后减小，随围压升高而增大；轴向峰值应变随烧变温度升高不断增大，随围压增大也逐渐增大；闭合应力、起裂应力和屈服应力均随烧变温度升高先略微增大再逐渐下降，温度阈值在200℃附近；(2)烧变砂岩的渗透率演变模式可分为三类：“√”型、“U”型、“”型，且围压和烧变温度均可导致演变模式由“√”型逐渐变为“U”型及“”型；(3)存在渗透压条件下，声发射振铃计数在岩石破坏前均较平静，在临近峰值破坏前出现大量信号；(4)烧变温度对砂岩声发射振铃计数的变化特征影响不显著，但压密阶段的声发射信号在400~600℃间有较大提升，800℃后屈服阶段声发射信号比较剧烈；(5)砂岩的三轴破坏形态均为剪切破坏，围压和烧变温度对主裂纹的渗透性有抑制作用。

Abstract

[Objective] High temperatures generated by coal seam spontaneous combustion cause thermal damage to surrounding rock, leading to significant changes in its mechanical properties and structure and potentially inducing geological disasters such as water inrushes and roof collapses. The evolution patterns of mechanical properties, permeability, and fracture characteristics of rock under different burning temperatures and burial depths are investigated in this study. [Methods] Sandstone specimens subjected to different high-temperature pretreatments were selected as the research objects. Triaxial compression seepage tests were conducted under constant seepage pressure and varying confining pressures, with synchronous acoustic emission monitoring. [Results] The result showed that the mechanical properties of sandstone progressively strengthened with increasing confining pressure, while they initially exhibited slight enhancement and then gradual deterioration as the burning temperature rose. Under varying confining pressures and burning temperatures, the permeability followed a trend of initial reduction followed by a subsequent increase. [Conclusion] The result indicate that:(1) during triaxial compression, the peak strength and elastic modulus of sandstone initially increase and then decrease with rising burning temperature, while both parameters increase with higher confining pressure. Axial peak strain increases continuously with elevated burning temperature and also rises gradually with increasing confining pressure. Closure stress, crack initiation stress, and yield stress exhibit a slight initial increase followed by a progressive decline as burning temperature increases, with a temperature threshold observed near 200 ℃.(2) The permeability evolution patterns of burnt sandstone are classified into three types: “√”-shaped, “U”-shaped, and “”-shaped. Both confining pressure and burning temperature promote a transition from the “√”-shaped pattern to the “U”-shaped and “”-shaped patterns.(3) Under seepage pressure conditions, acoustic emission ring-down counts remain low before rock failure but exhibit a sharp surge immediately prior to peak failure.(4) Burning temperature has no significant effect on the variation characteristics of acoustic emission ring-down counts, although acoustic emission signals are notably enhanced during the compaction stage from 400 ℃ to 600 ℃, and become intense during the yielding stage after 800 ℃.(5) The triaxial failure modes of sandstone are consistent with shear failure. Both confining pressure and burning temperature exert an inhibitory effect on the permeability of dominant fractures.

关键词

砂岩 / 深部开采 / 煤层自燃 / 力学性质 / 渗透率演化 / 岩石力学 / 热-力耦合 / 影响因素

Key words

sandstone / deep mining / coal seam spontaneous combustion / mechanical properties / permeability evolution / rock mechanics / thermal mechanical coupling / influencing factors

基金资助

国家自然科学基金项目(41807233)