A Rheological Approach to Differentiating Muds by Gel Structure

Drilling muds have long been known to develop weak bonds between particles and emulsion droplets in suspension, commonly known as forming a gel structure. The usual measurement for this is obtained through a flow initiation on a standard oilfield viscometer, with the gel strength recorded as the peak in this transient test. This has been a useful test but is limited in several ways, including the sensitivity of the equipment and limitations in the test itself. Hidden behind these limitations are basic differences in the gel structures of differing mud systems, where changes in the emulsion package, base oils, organophilic clays, solids loading, and other viscosifiers affect the development of structure within the fluid. The possibility of exploring differences in gel structure exists in the form of advanced rheometry. Presented in this paper is a comparison of traditional gel strength measurements with various rheometric techniques. The ability of each method to reveal complexities in the gel structure will be evaluated in a selection of downhole fluids.