New Flat-Rheology Synthetic-Based Mud for Improved Deepwater Drilling

Abstract

Abstract In deepwater drilling operations, synthetic based mud (SBM) is the preferred drilling fluid because it delivers high drilling rates and excellent wellbore stability. However, lost circulation problems caused by adverse elevation of viscosity and increase in equivalent circulation density (ECD) as a function of temperature and pressure often result in significant operational cost increase. Controlling the pressure and temperature dependence of SBM rheology thus is an important step towards achieving successful and cost-effective deepwater drilling operations. This paper highlights the development and application of an improved SBM that optimally balances the requirements for excellent rheological properties, ECD control, and improved barite suspension. Unlike conventional SBMs, the new SBM exhibits a constant ("flat") rheological profile over a wide temperature and pressure range. For instance, certain key rheological parameters, such as 6-rpm reading, yield point, and gel strengths, remain virtually unchanged when temperature and pressure are varied. This flat-rheology characteristic allows for a higher viscosity to be maintained without negatively affecting drilling rate or ECD. Moreover, cuttings carrying capacity and barite suspension properties are greatly improved. The flat-rheology profile is achieved through the usage of a re-designed package of emulsifiers, rheology modifiers and viscosifiers. The emulsifier package minimizes the impact of drill solids on the rheological properties of the new SBM. Optimal rheology modification is achieved through minimal use of organophilic clays. Moreover, a new rheology modifier also reduces the key viscosity parameters at low temperatures while raising them at high temperatures. The viscosifier is used to provide the desired enhancement in overall viscosity and suspension capacity. Extensive field data from Gulf of Mexico (GoM) wells (including drilling performance data, ECD and downhole pressure data, etc.) shows the benefits of the new flat-rheology SBM (FR-SBM) system, including lowering of ECD (resulting e.g. in lower SBM mud losses), improved cuttings evacuation from the well and prevention of barite sag problems. The operational success of FR-SBM has led to rapid acceptance in the field, allowing it to displace conventional SBM in Shell's GoM deepwater drilling operations.