Drilling muds have four basic properties that determine the behavior of the mud as a drilling fluid – viscosity, density, gel strength and filtration. Several other properties, although of lesser importance, need to be checked, especially if problems are anticipated or encountered. These properties include sand content, pH (alkalinity or acidity) and calcium content (hard water). Although tests are available to measure each of these properties, simple field tests for viscosity and density, coupled with an understanding of drilling and the capabilities of available mud products, can satisfy the drilling needs for most applications.
Viscosity is defined as the resistance offered by a fluid (liquid or gas) to flow. The thicker a particular fluid, the higher its viscosity. Accurate measurement of the viscosity of drilling mud is dependent on a number of factors and requires special equipment. The basic factors which affect the viscosity of a mud are the viscosity of the base fluid (water); the size, shape and number of suspended particles; and the forces existing between particles, as well as between particles and the fluid.