Flat Rheology Invert Drilling Fluid (FRIDF) has been successfully used for deepwater drilling due to its outstanding performance in providing excellent hole cleaning, temperature-independent rheology profile, good barite sag control and good ECD control. The high performance is achieved by using correct combinations of emulsifier, wetting agent, rheology modifiers and supplementary viscosifiers. Because of the multiple products used, the FRIDF can be complex to engineer and manage. An improved and simplified flat rheology system has been developed to aid in the field engineering and ease of system maintenance without sacrificing fluid performance.

The newly developed flat rheology system utilizes a single emulsifier component to provide dual functions of emulsification and surface wetting. This helps to improve emulsion stability and enhance thermal stability and fluid lubricity. The system can be formulated for deepwater applications with mud weights up to 18.0 lb/gal and temperatures up to 350°F. In addition, the system uses a new rheology modifier that provides a temperature-independent rheology profile for hole cleaning, barite suspension, ECD management and lost circulation control.

Recent field trials indicated that the new system is easy to maintain and provides good fluid performance in terms of drilling rate, ECD management, lost circulation control and hole cleaning. Even when the system was contaminated with a severe saltwater flow, there were no fluid-related problems before the synthetic/water ratio was restored. The new fluid system exhibited flat rheology profiles and non-progressive gel structures. Hydraulic modeling showed excellent hole cleaning with low ECDs and breaking-circulation pressure. This resulted in a noticeable reduction of lost circulation potential in lost circulation prone areas. The performance of the new flat rheology system (NFRS) also will be compared to the current system to demonstrate some advantages of the new system.

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