Key Tools and Dynamic Variation of Wellbore Pressure in Dual-gradient Drilling

In order to solve the problem of low separation efficiency of separator in hollow sphere injected dual-gradient drilling, the dynamic variation of wellbore pressure was studied. A filter separator capable of efficiently separating hollow spheres was designed and verified by numerical simulation and laboratory test, and the highest separation efficiency can reach 98.5%. The mathematical model of fluctuating pressure generated by the separation of hollow sphere into the annulus was built; combined with the model and considering the dynamic variation of the volume fraction of hollow sphere, the drilling fluid displacement, the separator position and the rate of penetration, the mathematical model of dynamic variation of wellbore pressure was further built. Based on drilling data, case calculation and influential factor analysis were carried out. The research results show that the wellbore pressure distribution has an obvious inflection point at the position of the separator, while the distribution of drilling fluid density in the annulus has an abrupt change; by means of dynamically adjusting the key parameters such as the volume fraction of the hollow sphere and the position of the separator, the density of unweighted drilling fluid in the annulus, the length of the liquid column and the bottom pressure while drilling can be flexibly adjusted, and thus realize the real-time prediction of the bottomhole pressure while drilling. The research results provide theoretical basis and technical support for safe drilling under narrow pressure window conditions.