Investigating the effects of plug viscosity on annular pressure drop and cuttings transport in a concentric annulus

摘要

The API recommended approach to model the rheology of a drilling fluid is to use the Herschel Bulkley equation. This equation breaks down as shear rate approaches zero and predicts infinite viscosity at zero shear rate limit. In an annulus, shear rate can drop to values approaching zero and the use of Herschel Bulkley equation in flow models can lead to inaccurate prediction of drilling hydraulics. This paper investigates the effects of zero shear rate viscosity, called plug viscosity, on annular pressure drop and cuttings transport predictions in a concentric annulus. Flow is modeled using the mixture model approach and is simulated using computational fluid dynamics (CFD) methodology. The rheology of the drilling fluid is characterized using a modified bi-viscosity function. The first half of this paper examines the effect of common drilling parameters on the existence of plug zones, regions with zero shear rate viscosity, whereas the second half of this paper investigates the effects of plug viscosity on annular pressure profile and cuttings transport. The results show that plug viscosity has a significant impact on annular pressure profile and cuttings transport predictions. The same set of Herschel Bulkley parameters can produce different values of annular pressure drop and volumetric cuttings concentration if plug viscosity is ignored. Therefore, to accurately predict annular pressure profile and cuttings transport, it is imperative that plug viscosity is incorporated in the rheological functions.