The effect of the synthetic- and oil-based drilling fluid's internal water-phase composition on barite sag

One important issue for drilling operations is control of downhole pressures. When drilling with synthetic- or oil-based drilling fluids, the ability to maintain all the weighting agents in suspension is particularly difficult because these fluids are more vulnerable toward sag than water-based drilling fluids. In the current study, it will be shown that the ability to keep barite in suspension also depends on the chemical composition of the water phase. The sag tendency of a mineral-oil-based drilling fluid, a linear paraffin-based drilling fluid, and both an ester- and a linear-alpha-olefin (LAO) -based synthetic drilling fluid has been evaluated. For comparison, all fluids were formulated with equivalent water activity in the internal brine phase. The results show that for all these fluids, improved performance was observed if the traditional calcium chloride (CaCl2) salt was replaced with a selection of other salts. Formates as the internal salt gave generally better performance than if calcium chloride was used. The best stability in the tests was observed using an ammonium calcium nitrate [NH4Ca(NO3)] as the internal salt phase, which also enhances the environmental performance of the fluid. This paper describes in detail the effect that different salts in the internal brine phases in synthetic- and oil-based drilling fluids had on sag performance in a series of tests.