The Yield Stress Concept in Drilling Fluids

Abstract Viscosity models are needed to present input parameters for hole cleaning and other well hydraulic items. The simplest model describing the drilling and completion fluid adequately at shear rates valid for most well geometries and pump rates is the Herschel-Bulkley model, which is a three-parameter model. The model parameters are the yield stress (the flow initiation stress), a consistency index and a flow index (being the index indication the curvature of the shear stress as function of shear rate). Also, the Bingham model with yield stress and plastic viscosity has been frequently used in the drilling industry. The yield stress is established to be a sort of transient parameter dependent on the time dependencies of both the flow and the fluid. In drilling engineering, the temporal responds of the flow is much shorter than the temporal responds of the fluid, making use of simple a yield stress concept applicable. Improved methods to determine this yield stress have been assessed. In the assessment simple shear stress vs shear rate curves and oscillatory rheometry has been applied. In addition, a simplified method using a Marsh funnel to determine the yield stress has been assessed. In this method the gravity flow out of a standardised Marsh funnel continue until the yield stress is reached where the flow stops. At this point the yield stress can be determined simply by measuring the fluid level inside the funnel.