Abstract Drilling and completion fluids based on cesium formate brines were selected by Statoil for use in the development of the high pressure high temperature Kvitebjørn field. Cesium formate brine was selected primarily to minimize well control problems and maximize well productivity. These important benefits had been recognized by Statoil in previous HPHT drilling and completion operations over the past 5 years. The use of the same fluid system for both drilling and completion gives the additional benefits of simplified operations, reduced waste, and elimination of fluid incompatibility problems. The challenge on the Kvitebjørn field was to drill long deviated well paths through significant sequences of shales into reservoirs with pressures of up to 81 MPa (11,700 psi) and temperatures up to 155°C (311°F). So far the cesium formate brine has enabled the successful drilling, completion, and logging of 7 high angle HPHT production wells on Kvitebjørn, two completed with a cemented perforated liner and five with sand screens. Additionally, an extended-reach exploration well was drilled from the Kvitebjørn platform to the Valemon structure. The 705 m (2,313 ft) long reservoir section of this 7,380 m (24,213 ft) long well with an inclination of 69°, was successfully drilled with the same cesium formate fluid system. In all these wells the cesium formate brine system once again demonstrated clear performance benefits such as very low ECDs, moderate to high ROPs, good hole-cleaning, and excellent wellbore stability while logging. Quick, trouble-free, safe, and robust completion operations were also accomplished, and the wells that have been put on production show high production rates with low skin. Full open-hole formation evaluation of the Kvitebjørn reservoir has been carried out with LWD tools. The evaluation has been aided by the development of a novel logging interpretation solution for a LWD density tool, in which the extremely high photoelectric effect of cesium-rich filtrate plays a vital role. Using photoelectric factor and bulk density data, combined with resistivity measurements from both the LWD drill pass and the ream pass, produces a very reliable and consistent net reservoir definition. The final interpretation result matches the core porosity from different lithologies in 3 different wells. Cesium formate brine has helped Statoil to achieve a remarkable record of zero well control incidents in all 15 HPHT drilling operations and 20 HPHT completion operations in the Kvitebjørn, Kristin, and Huldra fields over a period of 5 years. Introduction The Kvitebjørn Field Development Kvitebjørn is a HPHT gas/condensate field under development in block 34/11, located in the South-Eastern part of the Tampen Spur area in the North Sea. The field, which is east of Gullfaks and North of Huldra in block 30/2, is in 190 meters of water. The Kvitebjørn field sits on a down-faulted rotated block, flanked by the Gullfaks Sør and Gullfaks fault blocks in the west and the North Viking Graben in the south, east, and northeast. The Brent group is a deltaic deposit, comprising interbedded sand, shale, and coal layers. The heterogenic Brent Group sequence has a thickness of approx. 160–190 m in the Kvitebjørn field. The reservoir lies at approximately 4,000 meters depth and is classified as high temperature (155°C / 311°F) and high pressure (81 MPa / 11,700 psi). To date seven out of the planned eleven production wells have been drilled and completed, all with an inclination varying from 20–45°. All drilling takes place from the Kvitebjørn platform's fixed rig. Drill cuttings and produced water are injected into a dedicated disposal well. Of the seven wells drilled and completed so far, five have been completed with OH sand screens and two with cemented liners. Production started after drilling the first two wells, and the remaining part of the well construction program is taking place in an environment of progressive reservoir pressure depletion.

Abstract Conventional drilling and completion fluids containing weighting solids or hydrocarbons or halide brines can create problems with hydraulics, well control, well integrity and well productivity in HPHT operations. The negative influence of conventional fluids on drilling and completion operations can be sufficiently serious to compromise safety and degrade the economics of challenging HPHT field developments. Formate brines have been developed specifically to provide improved drilling and completion fluids that are free of the troublesome components found in conventional fluids and therefore better suited to meet the needs of oil companies involved in difficult HPHT well constructions. Formate brines have been successfully used as reservoir drill-in, completion, workover and suspension fluids in more than 130 HPHT well construction operations over the past 10 years. These applications have included 100 cases in which high down-hole pressures have necessitated the use of cesium formate brines for well control purposes. Some 15 applications of cesium formate brines to date have been HPHT reservoir drill-in operations in high angle wells where operators considered that conventional fluids could create a safety risk and adversely effect project economics. We review the published information on the field performance of the cesium formate brines in HPHT applications, and conclude that the novel benefits of the technology that were first promised some 15 years ago during the early product development phase have now been fully validated. Introduction The objective of the drilling and completion process is to safely deliver high quality wells that are optimized in terms of providing shareholder value: Best well productivity at lowest drawdown Best well integrity and longest structural lifetime Lowest well construction cost Lowest environmental impact and liability exposure Best reservoir information capture The choice of drilling and completion fluid used in a well construction operation has a critical influence on the extent to which an operator can meet this objective. In particular the fluid's performance will play a significant part in determining whether or not an operator meets its key performance indicator targets in the following areas: Time to drill and complete Well control and safety incidents Well integrity Well lifetime and maintenance costs Well productivity index Waste management costs Logging capability and interpretation Environmental footprint and impact Exposure to liability (short- and long-term) The drilling fluid chosen for the upper well sections must offer a host of functionalities: Ability to maintain the integrity of weak rocks Ability to minimize fluid loss into permeable rocks Ability to provide stable well control Ability to efficiently transfer hydraulic power Ability to move cuttings to the surface Provide steel/steel and steel/rock lubricity Provide protection against all forms of corrosion Allow formation evaluation Pose little or no hazard to rig personnel Have little or no adverse effect on the environment Have little or no adverse effect on elastomers If the drilling fluid is to be used in reservoir sections without further intervention it must cause minimal change to the native permeability of the reservoir rock in the near wellbore area. The drilling fluid filtrate must also be compatible with other filtrates that might leak-off from subsequent cementing and completion operations. A completion fluid should have the same overall properties as a reservoir drilling-in fluid and, ideally, should be the same fluid minus any drilled solids.