Session: 11-02-01 Well Drilling Fluids & Hydraulics

Submission Number: 157289

The Yield Stress Concept in Drilling Fluids 

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 Bingham model as presented in oil field standards has little relevance for the flow in the annular sections during drilling.  Its accuracy is generally only acceptable for flow inside the drill string and through the bottom hole assembly and the drill bit.  Thus, this model is more of use for the driller who can compare the pump pressure with calculations to see if there are any drill pipe leakages or a lost drill bit orifice.  For well engineering it is of lesser use as the yield stress normally is significantly overestimated; sometimes by several hundred percents.  The yield stress applied in the Herschel-Bulkley model is as accurate as possible give the viscometer design.  Still the yield stress is a complicated feature that needs a thorough assessment.

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 Mash funnel to determine the yield stress has been applied.  In this method the gravity flow out of a standardised Mash 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.  

In the current article different aspects of the yield stress is analysed.  Oscillatory rheometry, low shear rate measurements and funnel flow are used in the analysis.

Presenting Author: Arild Saasen University of Stavanger

Presenting Author Biography: The author works for ALKAS Atlantic AS. He also hold a position as professor.