Numerical simulation of velocity profile in eccentric annulus

Abstract

Annulus flow is a phenomenon which is frequently encountered in many industrial processes and has been under investigation for many decades. It is relatively simple to study concentric annulus flow. However, if the annulus is eccentric, calculations become complex due to the asymmetry of the eccentric annulus flow. During drilling operation, the space between the drill stem and the well wall is generally an eccentric annulus. Especially in highly deviated wells and horizontal wells, the drill stem will be located at the low side of the wellbore because of gravitational effects, thus, a fully eccentric annulus is formed. The velocity profile in the eccentric annulus will exert a great impact on the transportation and distribution of the cuttings, which is very important to prevent the drill string from being buried and avert sticking incidents. Based on a single-phase flow model and a solidliquid two phase mixture drift model, (using the computational fluid dynamics (CFD) software) we analyze the influence of six factors in this paper. These factors are the • flow rate, • fluid viscosity, • the type of fluid, • eccentricity, • annulus geometry size and • cuttings concentration. According to the results of numerical simulation, we can see that the velocity profile in the eccentric annulus is asymmetrical. There will be a high-speed basin in the wide gap of the annulus while the low-speed basin will lie in the narrow gap. With increasing flow rate, annulus geometry size and a decrease of the eccentricity, the area of the high-speed basin in the annulus will be increased and that of the low-speed basin will be decreased. This will improve the uniformity of the annulus flow. With increasing fluid viscosity and cuttings concentration, the velocity in high-speed basin of the annulus will be increased, at the same time, the velocity in low-speed basin will be decreased and the change of the velocity is not evident. However, the pressure loss in annulus will increase rapidly. Therefore, we have to ensure that the velocity at the low-speed basin is large enough to transport the cuttings and to avoid the debris deposition in the drilling operation. At the same time, the borehole pressure should be taken into consideration when we select the reasonable drilling fluid viscosity.