Prediction of Barite Sag Potential of Drilling Fluids From Rheological Measurements

Abstract

Abstract Barite sag under static and dynamic conditions has been studied in the laboratory. Static sag has been related to viscoelastic properties of several fluids. These viscoelastic properties are related to gel formation which needs to be sufficient to keep barite in suspension. This static sag was not predictable by the use of standard VG viscometers. Dynamic sag can be severe even when static sag is negligible. The understanding of dynamic sag is much more difficult than sag under static conditions. Introduction Settling of barite or other types of weighting agents may cause several problems when drilling and completing a well. While drilling these problems range from having insufficient mud density for well control to fracturing the formation when resuspending a barite bed. In addition the settling of barite may hinder the running of casing as well as cause insufficient displacement efficiency during the cementing operations. Some time ago the rate of settling of particles from a fluid was shown to occur more rapidly in an inclined tube than in a vertical tube. This phenomenon, known as Boycott settling, has its application in the sag of weighting agents in an inclined well. Jamison and Clements used an inclined tube to investigate barite sag in a static drilling fluid. Analysis of their data reveals an increased tendency towards sag with lower viscosity or lower gel strength. However, there is a significant scatter in the data. Therefore they conclude that static sag could not be reliably related to YP or PV, 10 second or 10 minute gel strengths as measured by standard API methods. Hanson et al. studied the situation where in a slow fluid flow gel structures seem to be less important for preventing sag than in the static case. This dynamic sag was evaluated in a deviated pipe where fresh drilling fluid was circulated at a slow rate. They found dynamic sag of significantly greater importance than static sag. P. 663