Abstract
The yield stress appears as a quantity that is frequently used to describe drilling fluid rheological properties. For the driller it is important to use a simple model to predict the pressure losses in the total fluid circulation system. A very simple model, to which good prediction models exists, is the Bingham model using the yield point (YP) and plastic viscosity (PV) as determined following API procedures. This is acceptable for the driller when looking for the drill pipe and bottom hole assembly conditions as most of the pressure losses are to be found in these components and the bit nozzles where the shear rate is high.
In the flow phenomena occurring in the annulus is different. The concept of YP and PV cannot generally be used to describe the necessary properties of the fluid. A low shear yield point (LSYP) is frequently used by drilling engineers to describe the yield stress of a Herschel-Bulkley fluid. When the LSYP is used it is possible to select measurement data from relevant shear rate regions to be able to produce estimates of frictional pressure losses and to determine the fluids equivalent circulating density (ECD) during circulation.
In oscillatory tests yield stresses are determined as the stress where the linear viscoelastic strain region ends. Also, a flow point is measured, being the stress at the amplitude where the viscoelastic loss modulus becomes equal to the elastic storage modulus. These two viscoelastic yield stress properties relate to interactions in particle suspensions, emulsions and polymer solutions. Thus, these properties are of importance when dealing with shelf life of fluid properties.
In the article the different application of the yield stress concept is outlined. It is shown that these values have a real significance for any flow phenomenon when used correctly. It is also shown that there is no reason for these different yield stress estimates to be equal. Still, none of these estimates are incorrect or correct, but reflects the physical conditions the yield stress is meant to describe.
