Hydraulic modeling was used to simulate the effect of fluid rheology (both high- and low-shear-rate rheology) on the equivalent circulating density (ECD) at critical points in the well. The modeling results provided a guide for the design of nonaqueous fluid properties necessary to achieve the required ultralow ECD values. Using a combination of a rheology modifier and a new low-end fluid rheology enhancer, ultralow ECD nonaqueous fluids were successfully developed to meet the fluid rheology and barire sag requirement. The low-end fluid rheology enhancer provided a desired gel structure that helps minimize barite sag, which is often the concern for low-rheology nonaqueous fluids.

The hydraulic modeling results showed that, although both high- and low-shear-rate fluid rheology must be very low to achieve ultralow ECD values, the reduction of high-shear-rate rheology has a larger effect on the reduction of ECD values. For a given ECD value, a fluid with a lower high-shear-rate rheology and higher low-shear-rate rheology is preferred because the higher low-shear-rate rheology is due to a much-desired gel structure that helps reduce barite sag during static aging. Hole cleaning is also expected to improve with higher low-shear-rate rheology.

Based on this information, a 14.0-lbm/gal clay-free nonaqueous drilling fluid was successfully formulated to achieve the required ECD without the use of micronized barite. As described, the fluid uses the combination of two rheology modifiers, one of which functions as a gel strength enhancer that provides the much-needed gel structure to minimize barite sag.

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