Research progress on mechanism of wellbore instability in deep fractured formations and related countermeasures
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 well-bore instability.Moreover,five key points are proposed as follows:(1)To make a breakthrough in the discrete-continuous cou-pling 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 coeffi-cient on stress transfer and macroscopic stability of the broken wellbore rock.(3)To explore the mechanism of subcritical propa-gation 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.
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