The coupled model of wellbore temperature and pressure for variable gradient drilling in deep water based on dynamic mass flow

Abstract Variable gradient drilling in deep water is a new drilling method of using separators to separate hollow glass spheres (Referred to as HGS, it is the glass microbeads with low density less than 1.0 kg/m3, and the worse thermal conductivity) from the drill string directly into the annulus, which can form multiple gradients of drilling fluid density in annulus. This method can better deal with the problem of narrow pressure window in deep water drilling. At present, there are few studies on the wellbore temperature and pressure distribution of this technology, and the separation efficiency of the existing separator is too low to achieve variable gradient. The purpose of this paper is to develop a new separator to improve the separation efficiency, and to study the coupled wellbore temperature and pressure field of variable gradient drilling based on dynamic mass flow. Firstly, indoor simulated experiments were used to study the separation efficiency of the newly developed filter separator, and based on the separation efficiency, wellbore flow theory and the first law of thermodynamics, the dynamic mass flow at the location of the separator were fully considered. Considering the influence of wellbore temperature and pressure on thermophysical parameters, a coupled model of wellbore temperature and pressure was established based on dynamic mass flow. Then the model was discretized and solved by finite difference method and cyclic iterative algorithm. Further, this model was verified based on the existing models and drilling site data. Finally, the effects of key parameters in variable gradient drilling on the annulus temperature, pressure and fluid density were studied. The results show that the filter separator has high feasibility, and the separation efficiency can reach 95%-98.5%. Compared with conventional drilling, distribution curve of the temperature, pressure and the mixture density in annulus of the filter separator section has obvious inflection points in variable gradient drilling, and the number of inflection points is equal to the number of separators. What’s more, the number and location of separators, the HGS volume fraction, pump flow rate, and the HGS density have significant effects on the annulus temperature and pressure. This research can significantly improve the feasibility of variable gradient drilling in deep water and provide a theoretical reference for further research in this direction.