The tradition of using barite to increase the weight of drilling fluids dates back to the early-1920's and, while it has been of great benefit to the oil industry over the past 90 years, it has also caused some chronic and persistent well construction problems along the way. These problems, which are very familiar to drillers, include well control difficulties, stuck pipe incidents and formation damage.

The oil industry has known since the 1970's that replacing barite with suitable non-damaging solutes in reservoir drill-in fluids is an effective way of reducing formation damage, simplifying operations and eliminating the need for expensive formation damage by-pass operations. The development of brine-based drill-in fluids opened up the opportunity to connect more effectively with hydrocarbon reserves by allowing the construction of long high-angle reservoir sections completed in open hole. Despite the advantages on offer, the industry was unable to exploit this novel technology in deep HPHT gas field developments until the mid-to late-1990's when drill-in fluids based on potassium and cesium formate brine became available in commercial volumes.

Cesium formate brine was first used as a reservoir drilling fluid in the Huldra gas/condensate field in the North Sea in January 2001, and has now been used to drill a total of 29 deep HPHT gas wells. The information presented and reviewed in this paper confirms that the use of potassium and cesium formates as the sole weighting agents in reservoir drill-in fluids has enabled operators to enjoy the full economic benefits of creating low-skin open-hole completions in deep high-angle HPHT gas wells. The review also concludes that the use of these heavy formate brines as drill-in fluids over the past 10 years has facilitated the safe and efficient development of deep HPHT gas reserves by:

  • Virtually eliminating well control and stuck pipe incidents

  • Enabling the drilling of long high-angle HPHT wells with narrow drilling windows

  • Typically reducing offshore HPHT well completion times by 30 days or more

  • Improving the definition and visualization of the reservoirs

  • Eliminating the need for clean-ups, stimulation treatments or any other form of post-drilling well intervention to remove formation damage caused by the drilling fluid

This has all been made possible by the operators’ acceptance and adoption of the award-winning Chemical Leasing (ChL) and fluid management programmes that form the basis of their contracts with the sole producer of cesium formate brine. The use of the ChL model has played an important role in reducing the unnecessary consumption of what is a very rare and valuable chemical resource

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