It is of prime importance for a drilling fluid to have good and optimal rheology in order to achieve good hole-cleaning and have good barite sag resistance. Invert emulsion drilling fluids with organoclay as a viscosifier sometimes fail to maintain sufficient rheology during drilling due to the thermal degradation of organoclay with time and temperature. Thus, there was a need to develop a drilling fluid with optimal rheology sufficient to give good hole cleaning and barite sag resistance in high temperature and pressure conditions.

This paper describes the formulation of invert emulsion drilling fluids formulated with a synergistic rheology modifier combination comprising of two different additives viz. rheo1 and rheo2. Invert emulsion fluids formulated with either rheo1 or rheo2 did not show good and optimal rheology required for successful drilling. However, the synergistic effect of these two additives rheo1 and rheo2 resulted in fluids with low PV and enhanced yield point and low end rheology thereby increasing their capacity for better hole-cleaning and barite-sag resistance.

The effectiveness of this rheology modifier combination was shown by formulating 68pcf low density, 90pcf medium density and 120pcf high density invert emulsion drilling fluids. The 68pcf, 90pcf and 120pcf fluids were hot rolled at 250°F, 300°F and 325°F respectively. Rheology of the 120pcf invert emulsion fluid was measured across high temperature and pressure range. The paper also describes the contamination and static aging studies of 90pcf and 120pcf fluids at 300°F and 325°F respectively.

68pcf, 90pcf and 120pcf invert emulsion drilling fluids formulated with the novel rheology modifier combination of rheo1 and rheo2 showed optimal rheology and low HTHP fluid loss. Static aging studies of 90pcf and 120pcf fluids at 300°F and 325°F respectively showed that the fluids were resistant to barite sag. Contamination studies of 90pcf fluid showed that the contaminants have minimal effect on the rheology and filtration properties of the invert emulsion fluid. HTHP rheology of the 90pcf invert emulsion fluid shows consistent rheology across high temperatures and pressures. The paper thus demonstrates the superior performance of the rheology modifier combination to achieve good rheological and filtration properties.

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