井壁干热花岗岩卸荷-遇水冷却双重损伤本构模型研究

Hot dry rock (HDR) geothermal energy is a clean energy with enormous value and potential use, and its exploitation and utilization has great strategic significance for the adjustment of energy structure and energy security of China. How to ensure the stability of borehole surrounding rocks in the drilling process and the long-term operation is one of the most important issues to be resolved in HDR exploitation, while one of its involved scientific problems is the mechanical behavior of HDR after water cooling during drilling process. This project will take the hot dry granite borehole cores in Gonghe Basis as research subject, perform micro-mesoscopic and macroscopic experiments under the condition of high temperature with water-cooling, reveal the damage mechanism of water-cooling on hot dry granite, and establish a damage evolution equation. Through triaxial unloading and acoustic emission testing on high-temperature with water-cooling samples, the damage effect and mechanism of drilling unloading on the mechanical behavior of hot dry granite. On these bases, the effects and mechanism of double damage of unloading and water cooling on hot dry granite material properties will be studied, and then an thermo-elasto-plastic damage constitutive model considering the double damage of unloading and water-cooling for borehole hot dry granite will be established. Based on Abaqus, the constitutive model will be developed and used for numerical simulation of deformation and stability research of borehole surrounding rocks during HDR drilling in the Gonghe Basin. The research results of the project will provide theoretical bases of quantitative calculation and numerical simulation for high-temperature geotechnical problems involved in the projects such as HDR exploitation.

干热岩地热是极具应用价值与利用潜力的清洁能源，其开发利用对我国能源结构调整、保障能源安全具有重大战略意义。如何保证井壁在钻井过程及长期运行中的稳定性是干热岩开发急需解决的课题之一，其所涉及的关键科学问题之一是钻井过程中干热岩遇水冷却后的力学行为。项目以共和盆地钻获干热花岗岩岩心为研究对象，开展高温遇水冷却微细观与宏观物理力学试验，揭示高温遇水冷却对干热花岗岩的损伤机理，建立损伤演化方程。通过对高温遇水冷却岩样开展三轴卸荷试验与声发射试验，研究钻井卸荷对干热花岗岩力学行为的损伤作用与机理。在此基础上，研究卸荷-遇水冷却对干热花岗岩物性的双重损伤作用与机理，建立适于井壁干热花岗岩的考虑卸荷-遇水冷却双重损伤的热弹塑性损伤本构模型。基于Abaqus开发此模型程序模块，数值模拟研究共和盆地干热岩钻井过程中井壁围岩变形与稳定性。项目成果将为处理干热岩等高温岩土问题提供数值模拟与定量分析的理论基础。

干热岩型地热是一种储量巨大、绿色可再生的新型地热资源，应用前景广泛。其开发利用主要是采用增强型地热系统，由于干热岩热储埋深大、温度高，故存在钻井和储层改造等方面的困难。首先要弄清干热岩自身的性质。本课题根据干热岩钻采过程，对花岗岩实施了三种热处置：高温、高温遇水冷却和高温遇水冷却循环作用，通过开展微观和宏观物理力学试验，揭示这三种方式对花岗岩的损伤规律与损伤机理，损伤作用随温度的增加而增大，随循环次数的增多而增大，主要是由于矿物差异热膨胀和热冲击造成岩石微裂隙发育、扩展而形成的。发现400℃为花岗岩热损伤的阈值，低于400℃内所研花岗岩力学指标损伤小于20%，高于400℃后花岗岩力学指标随温度升高大幅降低，1000℃时花岗岩单轴抗压强度和弹性模量约降为原岩的20%。且宏观物理力学参数（如密度、波速、单轴抗压强度和弹性模量）随温度的损伤规律可近似用线性表征。通过对高温遇水冷却后岩样开展三轴加载和卸荷试验，发现高温遇水冷却和卸荷都会使得干热岩力学性质降低，两者会叠加影响，揭示了卸荷-遇水冷却对干热岩的双重损伤作用与机理，建立了适于井壁干热花岗岩的考虑卸荷-遇水冷却双重损伤作用的强度准则和损伤本构模型。基于ABAQUS二次开发功能，开发此模型程序模块，数值模拟研究了地应力场、原岩温度场、钻进速度对干热岩钻井过程中井壁围岩变形的影响，发现在钻进过程中井壁围岩由于钻井泥浆作用而导致温度降低的影响范围很小，且400℃以内花岗岩力学性质受温度变化劣化较小，故对于原岩温度小于等于400℃时花岗岩干热岩储层钻井过程中井壁围岩稳定性较好。项目成果可为处理干热岩等高温岩土问题如干热岩钻井设计与评估提供数值模拟与定量分析的理论基础。