循环热冲击花岗岩微观裂隙表征与渗透特性演化规律  

Microscopic fracture characterization and permeability evolutionary patterns of granites under cyclic thermal shock

作　　者：何将福[1,2] 任成程 何坤[2] 余启航 李欣儒 邓旭 HE Jiangfu;REN Chengcheng;HE Kun;YU Qihang;LI Xinru;DENG Xu(State Key Laboratory of Coal Mine Disaster Dynamics and Control,Chongqing University,Chongqing 400044,China;School of Resources and Safety Engineering,Chongqing University,Chongqing 400044,China)

机构地区：[1]重庆大学煤矿灾害动力学与控制国家重点实验室,重庆400044 [2]重庆大学资源与安全学院,重庆400044

出　　处：《煤田地质与勘探》2024年第12期131-142,共12页Coal Geology & Exploration

基　　金：重庆市自然科学基金面上项目(Z20201294)。

摘　　要：【目的】为揭示增强型地热系统(EGS)热储开发过程中,钻井液、循环取热介质等低温流体与高温花岗岩储层循环热冲击致裂微观损伤特性与渗透率演化规律。【方法】开展了不同冷却方式、循环热冲击次数、冷却温度等条件下高温花岗岩致裂特征实验研究,得到了循环注水冷却和自然冷却状态下25~700℃花岗岩的纵波波速、孔隙体积分数、分形维数等微观结构特征参数,并基于CT扫描三维重构技术与Avizo-COMSOL交互式联合建模技术,构建了花岗岩微观孔隙渗透率演化特征模型,揭示了孔隙流体流动过程中渗流场、压力场和速度场的流线分布规律,并计算了X、Y、Z方向的绝对渗透率。【结果和结论】结果表明:(1)热处理温度与循环热冲击次数均与纵波波速呈负相关,循环次数越大,波速下降越明显,岩石损伤越严重,水冷却组波速下降速率整体大于空气自然冷却组。(2)当花岗岩温度t≤300℃时,CT扫描切片中的微裂纹数量较少,连通性较差;当温度t≥400℃时,花岗岩内部微裂纹及孤立孔隙迅速发育,并逐渐形成裂隙连通网络,且水冷却对花岗岩内部损伤致裂效果更显著。(3)由于微观孔隙结构的非均质性导致X、Y、Z三个方向渗流计算结果存在差异,沿流动方向孔隙压力减小、流量变大,在裂缝通道较窄的地方流量突然增加,但在一些复杂的孔隙结构中,不可避免地导致流体停滞或回流。研究结果揭示了循环热冲击对高温花岗岩损伤机制与渗透率演化规律,为热刺激法干热岩储层改造提供了可靠参数。[Objective]This study aims to reveal the microscopic damage characteristics and permeability evolutionary patterns of high-temperature granite reservoirs fractured under cyclic thermal shock produced by low-temperature fluids like drilling fluids and circulating heat recovery media in the exploitation process of the enhanced geothermal systems(EGSs).[Methods]Using experiments on the fracturing characteristics of high-temperature granites under the conditions of varying cooling methods,numbers of thermal shock cycles,and cooling temperatures,this study determined microstructural characteristic parameters such as compressional wave(P-wave)velocity,pore volumetric fraction,and fractal dimension of granites across a temperature range of 25 to 700℃ under cooling via cyclic water injection and natural cooling.Using the CT scan-based 3D reconstruction technology and the interactive joint modeling technology based on Avizo-COMSOL,this study built a model describing the evolutionary characteristics of microscopic pore permeabil-ity.Employing this model,this study revealed the streamline distribution patterns of the seepage,pressure,and velocity fields during pore fluid flow and calculated the absolute permeability in the X,Y,and Z directions.[Results and Conclu-sions]Key findings are as follows:(1)The heat treatment temperature and the number of thermal shock cycles were neg-atively correlated with the P-wave velocity.Specifically,more thermal shock cycles corresponded to a more significant decrease in the P-wave velocity and more severe rock damage.Furthermore,the water cooling led to a more significant overall reduction in the P-wave velocity than air cooling.(2)In the case of granite temperature t≤300℃,CT scan slices revealed a small number of microcracks in granites,indicating poor connectivity.In contrast,at t≥400℃,microcracks and isolated pores occurred rapidly within granites,gradually forming an interconnected fracture network.Moreover,water cooling caused more significant internal damage-induced fractur

关 键 词：高温花岗岩 循环热冲击 CT扫描三维重构 裂隙标准 Avizo-COMSOL 增强型地热系统(EGS) 

分 类 号：TU45[建筑科学—岩土工程]