One important issue for drilling operations is control of downhole pressures. When drilling with synthetic or oil-baseddrilling fluids, the ability to maintain all the weighting agentsin suspension is particularly difficult since these fluids aremore vulnerable for sag than water-based drilling fluids.In the current study, it has been shown that the ability tokeep barite in suspension also depends on the chemicalcomposition of the water phase. The sag tendency of amineral oil-based drilling fluid, a linear paraffin-based drillingfluid and both an ester and a LAO-based synthetic drillingfluid has been evaluated. For comparison, all fluids wereformulated with equivalent water activity in the internal brinephase. The results show that for all these fluids, improvedperformance was observed if the traditional calcium chloridesalt was replaced with a selection of other salts. Formates asthe internal salt, gave generally better performance than ifcalcium chloride was used. The best performance with respectto sag stability in the tests was observed using an ammoniumcalcium nitrate as the internal salt phase.The paper describes in detail the effect that different saltsin the internal brine phases in synthetic and oil-based drillingfluids was found to have on sag performance in a seriesof tests.
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