Impact of Additives with Opposing Effects on the Rheological Properties of Bentonite Drilling Mud: Flow, Ageing, Microstructure and Preparation Method

Drilling fluids are formulated with a range of additives to achieve the desired functional properties including specific rheological properties. Its rheology, a critical property, is sensitive to the chemicals used in its formulation and the environment encountered during drilling. The flow and time-dependent behaviour of bentonite slurries were evaluated under the influence of KCl and P2O74− or MgCl2 and P3O105− at specific concentrations. The multi-charge phosphate anions weakened the gel and its ageing behaviour at low KCl or MgCl2 content. At high KCl concentration, the sequence of salt addition in the gel preparation has a significant effect on the gel strength and stability. The gel was very strong when the platelets were allowed to exfoliate before the salt addition. When the delamination was hindered, a weak and at time an unstable gel was produced as a result of low platelet concentration. The gels prepared by both methods displayed time-independent behaviour at high KCl concentration. The ageing behaviour was described well by the Leong model. The cryo-SEM images revealed different microstructures and particle interaction configurations between gels with low and high salt concentration containing 0.5 dwb% P2O74−. The phosphate-based additives adsorbed at the platelet edge and weakened the platelet-platelet attraction at the network junctions. The opposing effect of Mg2+ at 0.01M to the effect of 0.5 dwb% dwb% P3O105− on the rheological properties of the bentonite gel is weak.