热冲击花岗岩力学响应及损伤特征显微CT试验研究

Study on mechanical properties and damage characteristics of granite under thermal shock based on CT scanning

在深部地热资源开发过程中,通常利用低温流体的冲击作用诱导高温岩石热破裂来提高储层的渗透特性。为了揭示热冲击作用下岩石的损伤破裂机理,对高温加热后的花岗岩(20℃、150℃、300℃、450℃、600℃和750℃)进行了自然冷却和水冷却处理,并对处理后的花岗岩开展了波速测试、单轴压缩试验和CT扫描试验,探讨了热冲击作用对花岗岩纵波波速、抗压强度、弹性模量等力学参数以及细观结构损伤的影响。研究结果表明:①随着热处理温度升高,花岗岩的纵波波速、抗压强度与弹性模量逐渐减小,峰值应变逐渐增加,且相比于自然冷却,水冷却作用后岩石的波速与力学性质劣化更显著。②通过CT扫描试验,获得了不同加热温度与热处理方式作用下花岗岩的孔裂隙结构空间分布特征,可以直观反映岩石细观结构的热损伤程度。当热处理温度小于等于450℃时,花岗岩扫描切片中的热致裂纹数量较少,裂隙连通性较差;超过450℃后,花岗岩内部微裂纹快速发育和扩展,并逐渐有形成裂隙网络的趋势,且水冷却对花岗岩的细观损伤致裂效果更明显。③基于三角网格离散技术,结合椭球模型重构算法和裂隙张量计算理论,对热冲击后花岗岩的三维裂隙场进行定量表征,并建立了裂隙组构张量与峰值强度的关系,进一步揭示了热冲击花岗岩细观结构对其力学性质的影响机理。

During the exploitation of deep geothermal resources,the thermal fractures of high-temperature rocks are usually induced by the impact of low-temperature fluids to improve the permeability of reservoir rocks.In order to reveal the damage and fracture mechanism of rock after thermal shock,the granites heated at high temperature(20℃,150℃,300℃,450℃,600℃and 750℃)were treated by natural cooling and water cooling respectively,and the wave velocity test,uniaxial compression test and CT scanning were carried out on the treated granites.The mechanical effect of thermal shock on P-wave velocity,compressive strength and elastic modulus of granite were also discussed.The experimental results show that with the increase of heat treatment temperature,the P-wave velocity,compressive strength,and elastic modulus of rock gradually decrease,and the peak strain gradually increases.Compared with natural cooling,the wave velocity and mechanical properties of rock deteriorate more significantly after water cooling.Based on CT scanning,the spatial distribution charac teristics of pore and fracture structure of granite under different heating temperatures and heat treatment methods were obtained,which can directly reflect the thermal damage degree of rock microstructure.When the heat treatment temperature is not higher than 450℃,the number and size of thermally induced cracks in granite scanning slices are less and the connectivity of cracks is relatively poor.When the temperature exceeds 450℃,the micro-cracks in granite develop and expand rapidly,and tend to form fracture network gradually,and the damage and cracking effect of water cooling on the microscomic-structure of granite is more obvious than that of natural cooling.In addition,based on triangular mesh discretization technique,ellipsoid model reconstruction algorithm and fracture tensor calculation theory,the three-dimensional fracture field of granite after thermal shock is quantitatively characterized,and the relationship between fracture fabric tensor and peak strength was established,which further reveals the influence mechanism of granite microscomic-structure on its mechanical properties under thermal shock.