Abstract: Wellbore instability in fractured formations of deep and ultra-deep wells can easily induce accidents such as drill pipe sticking and borehole fillings, thus threatening the safe and efficient drilling. Based on research progress and existing problems, the paper puts forward a research idea of stratigraphy-wellbore cross-scale modeling and static/dynamic simulation in case of wellbore instability. Moreover, five key points are proposed as follows:(1)To make a breakthrough in the discrete-continuous coupling modeling and numerical simulation methods, with the aim of achieving stratigraphy-wellbore cross-scale modeling and fine description of fractured rock mass around the well. (2)To clarify the changing law of rock friction coefficient under the coupling effect of high stress and temperature and drilling fluid in deep layers, and further reveal the influence mechanism of friction coefficient on stress transfer and macroscopic stability of the broken wellbore rock. (3)To explore the mechanism of subcritical propagation in rock cracks, with the aim of clarifying the mechanical mechanism of bond failure at crack tip under molecular scale, and revealing the time lag effect of wellbore instability. (4)To carry out the dynamic evolution simulation of wellbore instability in terms of borehole trajectory extension and local dynamic load impact, and further reveal the influence mechanism of engineering factors on the static/dynamic instability of wellbore. (5)To upgrade the large-scale physical model experimental equipment and methods for wellbore instability, and further explore pressure-bearing evolution mechanism and rocks falling/collapse laws for fractured wellbore by use of force chain theory. Finally, the paper proposes a wellbore instability risk assessment framework based on formation parameters, engineering statics and dynamic parameters, providing support for the prediction, prevention and control of wellbore instability in fractured formations.

Key words: fractured formation, wellbore instability, cross-scale modeling, discrete-continuous method, subcritical crack propagation

摘要： 深 部破碎地层井壁失稳极易诱发卡钻和井眼填埋等事故,威胁安全高效钻井。基于研究进展和存在问题总结阐述了井壁失稳的地层—井眼跨尺度建模与静/动力学模拟研究思路,并提出后续研究着力点:1攻关离散—连续耦合建模与数值模拟方法,实现地层—井眼跨尺度建模和井周岩体破碎特征精细表征;2明确深部高应力高温与钻井液耦合作用下岩块摩擦系数变化规律,揭示其对破碎井壁应力传递和稳定性影响机制;3开展岩石裂缝亚临界扩展机理探索,在分子尺度上阐明岩石裂缝尖端胶结破坏与亚临界扩展力学机制,揭示井壁失稳时间滞后效应;4从轨迹延伸和动载荷冲击等方面开展失稳动态演化模拟,揭示钻井工程因素对井壁静/动力学失稳的影响机制;5升级井壁失稳大尺寸物理模拟实验设备与方法,结合力链理论探索破碎井壁承压演化机制和掉块/垮塌规律。最后,提出基于地层参数、工程静力学和动力学参数的井壁失稳风险评价框架思路,支撑破碎地层井壁失稳预测与防控。

关键词: 破碎地层, 井壁失稳, 跨尺度建模, 离散—连续, 亚临界扩展