A New Technique to Determine the Equivalent Viscosity of Drilling Fluids Under High Temperatures and Pressures
Authors: Sinha B.K. 
Published: December 2007

Abstract
Knowledge concerning the behavior of drilling fluids under wellbore conditions is very desirable, and experimental results have shown that the extent to which the flow properties of drilling fluids are affected by high temperatures and pressures cannot be predicted by standard API pressures cannot be predicted by standard API tests. A Fann consistometer (Model 5S-TDL) is modified to obtain the experimental data reported in this study. Data obtained from the Fann viscometer Model 50 at elevated temperatures have been included to supplement the information derived from the modified Fann consistometer. Newtonian fluids of known viscosities are used in calibrating the modified consistometer. The technique followed here keeps the sample temperature constant and allows the pressure to vary at each desired temperature level. The equivalent viscosities of both laboratory-prepared and field muds of different densities have been obtained at temperatures up to 500 deg F and pressures up to 20,000 psi. The objective of this study is to show that the modified consistometer can give much more information concerning the flow behavior of muds under wellbore conditions than that derived in the past. It can show the pressure and temperature conditions under which the tendency to thicken begins, the gradual thickening, and also the conditions at which the mud completely gels and loses its fluidity. The study shows that both temperature and pressure affect the equivalent viscosity of invert pressure affect the equivalent viscosity of invert emulsion muds. The effect of pressure is very pronounced at low temperatures. Compared to the pronounced at low temperatures. Compared to the invert emulsion muds, the equivalent viscosity of water base muds is not affected to the same extent by temperature and pressure. Temperature is the dominating variable tin case of water base muds. However, the effect of pressure on the equivalent viscosity of water base muds seems to depend on composition and temperature of the system.

Introduction
Kennedy and Crawford designed and patented the consistometer to test the setting time of cement slurries. This consistometer as manufactured and later improved by Fann. Chisholm et al. adapted the first Fann consistometer for evaluating drilling fluids under wellbore conditions in 1961; their study was later continued by Cox and Pfleger. Weintritt and Hughes used a similar consistometer with a different recording device. They measured the relative viscosity of drilling fluids in seconds and pointed out the usefulness of this data when used with viscometric and fluid loss data. They applied the term "relative viscosity" to the time required for the bob to complete movement in one direction: it is not a ratio of two viscosities. In spite of its wide usage, no standard testing procedure has been established by the industry to procedure has been established by the industry to obtain correlative data. A consistometer similar to the ones mentioned above, has been further modified and used in this study along the Fann viscometer Model 50.