It is known that ultra-high temperature drilling fluids are still quite a challenge to drilling engineering. When bottomhole temperatures exceed 200°C, the design and formulation of drilling fluids can present a host of problems. Drilling fluids destabilize under such extreme conditions, possibly causing wellbore instability, well control problems, and ultimately loss of the well. Drilling buried hill hydrocarbon reservoir in offshore oilfield around Bohai Sea Bay Basin in China has often resulted in failure to reach desired objectives. Bottomhole temperatures up to 220°C, however, the pressures requiring drilling fluid weights just to 1.04 to 1.10 g/cm3 equivalent place severe limitations on the performance of drilling fluids and often contribute in failure to reach the desired drilling objectives. Polymers as viscosifying agent that control rheological properties and fluid loss create major challenges for drilling fluid performance and stability when temperatures over 200°C are reached. Additionally, the gelation of drilling fluids containing low solids content and high fluid loss can become difficult to control under ultra-high temperature conditions. Both of these issues are compounded when the ultra-high temperature is coupled with a requirement for low drilling fluids density. The wildcat well NP3-81 and NP3-82 were drilled in this offshore oilfield around Bohai Sea Bay Basin on January 2014. TD of 6066 meters and 6037 meters were attained, respectively. Both of the bottomhole static temperatures were exceed 220°C and the pore pressure was 1.04 to 1.10 g/cm3 equivalent. Logs were run to bottom without incident and drilling fluids related problems and the primary drilling objectives achieved. The success is attributed to the new technology, fit-for-purpose drilling fluids and rigorous pre-well planning. The paper describes the laboratory development and field application of a new technology of environmentally acceptable and seawater discharged drilling fluids designed for ultra-high temperature environments, highlighting the viscosifying agent which has excellent ultra-high temperature resistance and salt resistance viscosifying property, thermal stability and thermo sensitive characteristics. The apparent viscosity retention rate of novel viscosifier after aging at 220°C after 16 h were 90.81% and 95.95% respectively and the EC50 value was 15.529×104 mg/L in both fresh water based drilling fluid and salt-water based drilling fluid. Field application data shows that this new polymer viscosifying agent could play a critical role even though at harsh conditions such as ultra-high temperature and low density drilling fluids system which decreasing daily performance maintenance problems at the expense of the large consumption of fluid additive including conventional sulfonate copolymer.

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