Multiphase flow law in wellbore during downhole dual-gradient controlled pressure drilling
Gas invasion is one of the main challenges in the process of downhole dual-gradient controlled pressure drilling on land.The study of the gas-liquid two-phase flow law in the wellbore during gas invasion has a vital importance for gas invasion detection and treatment.In this paper,based on the energy conservation equations,momentum and quality of drilling fluid,gas and hollow sphere,the new model of total transient gas-liquid two-phase flow under gas invasion conditions of downhole dual-gradient controlled pressure drilling is established by considering the variable mass flow in annulus caused by variable gradient parameters and the multi-phase flow behavior in wellbore.With this model,the gas-liquid two-phase flow law in the wellbore during gas invasion of downhole dual-gradient controlled pressure drilling was analyzed.The simulation results show that:the variable gradient parameters will not only change the volume fraction and equivalent circulation density(ECD)of the gas phrase,but also affect the evolution of gas invasion.The annular ECD distribution in downhole dual-gradient controlled pressure drilling is approximately"broken line",which can better adapt to the narrow safety density window in deep and ultra-deep layers.The distribution of annular ECD can be flexibly adjusted by combining the concentration of hollow spheres,the position of cyclone separator and the separation efficiency.The research has significant theoretical and engineering significance for solving the drilling problem of narrow safe density window formation on land by using downhole dual-gradient controlled pressure drilling.
dual-gradient controlled pressure drillinggas invasionmultiphase flow in wellboreequivalent circulating density
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