Wells in the Gullfaks field, situated in the Norwegian sector of the North Sea, are characterized by long, highly deviated producing intervals. Cased hole completions with 177.8 mm production liners are dominant, but since 1994 several wells have been completed as open hole in one or two intervals. The Gullfaks field is producing from a reservoir that consists of a multitude of high permeability loosely consolidated sandstone formations. Formation permeability varies from 50 md to as much as 10 darcies, while reservoir pressure ranges from 220 bar to 320 bar. Sand production from the loosely consolidated formations is prevented with traditional gravel-packs. Internal gravel-packs in the Gullfaks field are performed with under-balanced snubbing, thereby eliminating the need to kill the well at any stage of the operation. Leakoff in the different formation sands is well defined by log permeability and reservoir pressure; high choke pressure during the under-balanced operation aids in controlling the pre-determined return rate throughout the job. This paper describes an approach where gravel-pack treatments pumped in the Gullfaks field have been designed and evaluated using a pseudo three-dimensional gravel placement simulator. With reservoir pressure given as a range rather than a specific value, a sensitivity study yielded the best estimate for reservoir pressure. Following verification of reservoir parameters and simulator capabilities, new jobs were optimally designed using the simulator so that potential pitfalls in traditional designs could be avoided. Designs were modified to achieve a successful annular pack, also permitting a priori knowledge of safe operational limits. This further allows better control over fluid selection, rate determination, tool position and other parameters that are critical to achieving a successful outcome of the treatment. Post-job evaluation of gravel-pack treatments using this approach confirms the validity of the designs. Three wells in the Gullfaks field operated by Statoil, serve as the basis for the case studies presented in this paper. Recommendations are made on how to extend the technique to similar fields. P. 149
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