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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.