Stability and ageing behaviour and the formulation of potassium-based drilling muds

Abstract The formulation of drilling muds is currently driven by a trial-and-error process. This study aims to provide generic knowledge that helps to develop a systematic approach towards mud formulation producing muds that achieve most, if not all of their specific functionalities. The effect of KCl on the stability, viscosity and ageing behaviour of composite bentonite–barite–pyrophosphate muds was evaluated first. The ageing behaviour represented by a yield stress increasing with time was most pronounced for the 0.1 M KCl mud. At 0.2 M KCl and higher, the muds became unstable forming a two-phase suspension. The zeta potential was very small and essentially pH independent for both the 0.01 M and the 0.1 M KCl muds. Leong and Nguyen–Boger models described the ageing behaviour well, although the Leong model was found to perform better. In stage two, the effect of polyanionic carboxymethylcellulose (CMC) fluid loss prevention additives on the bentonite–barite–pyrophosphate–0.1 M KCl mud was evaluated. High molecular weight (Mw) CMC produced a thickening effect increasing both the viscosity and yield stress of the mud. Low Mw CMC (commercially known as PAC) only increased the mud yield stress marginally. All of these composite muds displayed a very similar ageing behaviour that is well described by the Leong model. The time scale-structural recovery increased with KCl and high Mw CMC concentration. All muds displayed shear thinning plastic behaviour.