Abstract:

Crystalline rock formations are commonly encountered in the deep stages of ultra-deep geothermal drilling. The degradation of their mechanical properties under the coupled effects of thermal, hydraulic, chemical, and stress fields exerts a significant impact on wellbore stability, operational safety, and efficiency. This paper elaborates on the current status of ultra-deep drilling and its downhole environmental conditions, while analyzing the mechanical characteristics of crystalline rocks. On this basis, the paper further clarifies the action mechanisms of single-field factors (including thermal field, seepage field, chemical field and stress field) during ultra-deep drilling, as well as the influence pathways of more complex multi-field coupling effects on the mechanical properties of crystalline rocks. Focusing on typical crystalline rocks (granite and gneiss), this study summarizes the microstructural evolution laws and macroscopic mechanical response behaviors under thermal shock, seepage intrusion, chemical corrosion, and stress disturbance. It also generalizes the degradation characteristics of key mechanical parameters such as strength, fracture toughness, and elastic modulus, and compares the degradation mechanisms between granite and gneiss. It is explicitly clarified that in ultra-deep drilling, high temperatures can induce the initiation of microcracks inside crystalline rocks and further promote their propagation; fluid intrusion not only exacerbates the dissolution of primary minerals but also facilitates the precipitation of secondary minerals; stress redistribution resulting from drilling-induced perturbations further promotes fracture connectivity and the formation of macroscopic failure surfaces; and multi-field synergy drives the transition of rocks from a brittle to a ductile deformation mode.Existing theories have limitations in terms of multi-field coupling mechanisms, coverage of crystalline rock types, and the long-term effects of actual drilling fluids. Future research urgently needs to strengthen fully coupled thermal-hydraulic-mechanical-chemical experiments and simulations, develop a “fully coupled thermal-hydraulic-mechanical-chemical numerical model” capable of accurately describing cross-scale damage processes and a “digital twin wellbore model” integrated with real-time drilling data, and propose an “active wellbore stability control strategy” focusing on multi-field coupling mechanisms. Ultimately, within a multi-scale and multi-field coupling framework, this study aims to establish a crystalline rock wellbore stability evaluation and degradation control method suitable for deep underground environments, providing solid theoretical support for in-depth understanding of the mechanical behaviors of crystalline rocks under multi-field coupling conditions and the safe and efficient implementation of ultra-deep drilling.

Key words: deep drilling, thermal-hydraulic-mechanical-chemical coupling, crystalline rocks, mechanical degradation, borehole wall stability

摘要：

结晶岩层常见于深地钻探的深部阶段，其在热流化力多场耦合作用下产生的力学性质劣化，对井壁稳定性及作业安全与效率产生了显著影响。阐述了深地钻探现状及其井下环境条件，分析了结晶岩的力学特征。在此基础上，进一步梳理了深地钻探过程中热场、渗流场、化学场及应力场等单场因素的作用机制，以及更为复杂的多场因素耦合作用对结晶岩力学性质的影响路径。重点总结了典型结晶岩(花岗岩、片麻岩)在热冲击、渗流入侵、化学腐蚀及应力扰动作用下的微观结构演化规律与宏观力学响应行为，归纳了强度、断裂韧度、弹性模量等关键力学参数的劣化特征，对比了花岗岩与片麻岩的劣化机理。明确了深地钻探中，高温可诱发结晶岩内部产生微裂纹并促使其扩展，流体的侵入加剧矿物的溶解与次生矿物沉淀，应力重分布进一步促进裂隙贯通与宏观破坏面的形成，多场协同推动岩石由脆性向延性转变。现有理论在多场耦合机理、结晶岩岩类覆盖以及真实钻井液长期作用效应等方面存在不足。未来研究亟需加强热流化力全耦合实验与模拟，构建能够精确描述跨尺度损伤过程的“热流化力全耦合数值模型”与融合实时钻井数据的“数字孪生井筒模型”，并提出聚焦于多场耦合机理的“井壁稳定性主动控制策略”。最终在多尺度、多场耦合框架下，形成适用于深地环境的结晶岩井壁稳定评价与劣化控制方法，为多场耦合环境下结晶岩力学行为的深入认识及深地钻探安全高效实施提供坚实的理论支撑。

关键词: 深地钻探, 热流化力耦合, 结晶岩, 力学劣化, 井壁稳定

CLC Number: 

• TE21

• TU45

• P634.1