Chapter 3 - Equipment and Procedures for Evaluating Drilling Fluid Performance

Abstract

Most drilling fluids are complex mixtures of interacting components, and their properties change markedly with changes in temperature, shear rate, and shear history. Since the properties of muds depend so much on shear history and on temperature, it is of the utmost importance that muds to be tested in the laboratory first be subjected to conditions similar to those prevailing in the drilling well. Density, or mud weight, is determined by weighing a precise volume of mud and dividing the weight by the volume. The mud balance provides the most convenient way of obtaining a precise volume. The number obtained depends partly on the effective viscosity at the rate of shear prevailing in the orifice, and partly on the rate of gelation. The assembly is lowered to a prescribed mark in a cup of mud, and the outer cylinder rotated at a constant speed. The viscous drag of the mud turns the bob until balanced by the torque in the spring. There is general agreement that the components of drilling fluids should be evaluated in terms of performance under conditions as similar as possible to the conditions of use.