Constructing an Offshore Reformable Corrosion-Resistant-Alloy Multilateral Junction Well (2006)

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 96343, "Constructing and Completing the World's First Offshore Reformable CRA Multilateral Junction Well," by I. Kjosnes, S.O. Syrstad, R.A.J. Flormoelen, SPE, and H. Fordedal, Statoil, and K. Nygardsvoll, SPE, C. Hogg, SPE, and D. Westgard, SPE, Baker Oil Tools, prepared for 2005 Offshore Europe, Aberdeen, 6-9 September. A reformable corrosion-resistant-alloy (CRA) multilateral system was installed in a well in the Norwegian sector of the North Sea. The multilateral completion allowed access to more reservoir targets than would have been possible with a traditional horizontal well. Introduction The Heidrun field, discovered in 1985, is offshore mid-Norway in the Haltenbanken area 120 km south of the Arctic Circle. The field contains oil with an overlying gas cap and has a reservoir pressure near hydrostatic pressure. The reservoir consists of several geological formations with varying reservoir quality and is structurally complex. The drainage strategy consists of flooding and pressure support by use of gas injection into the gas cap and water injection below. The field was developed with a tension-leg platform in 350 m of water over a 56-slot drilling template. Two subsea production templates and three subsea water-injection templates also were installed. The field has 33 production and 11 injection wells. Since production began in 1995, approximately 1.0×108 m3 of oil has been produced. Heidrun A-39 Well Design In 2002, the decision was made to develop and use multilateral-well technology on the Heidrun field. In particular, one candidate, Well A-39, was studied in depth with reservoir simulations while quantifying key risks with expected incremental cost. A multilateral design for Well A-39 was developed on the basis of drainage strategy in the area. The key benefit was the ability to reach a second reservoir compartment (fault block) at a low cost. With this capability, production from this second zone could be accelerated and project economics maximized. Several alternatives were considered for Well A-39, including traditionally completed single-wellbore configurations along with many multilateral designs. The key economic objective when comparing these various designs was the net present value of the well covering the entire-life-cycle perspective. Designing the Multilateral System In considering the potential methods to address the challenges, reformable multilateral junctions were seen as the enabling technology. Already used in multilateral installations, existing reformable junctions have been run as part of the intermediate-casing assembly to provide a hydraulically isolated junction assembly downhole. In addition to the hydraulic-isolation benefits, the reformable approach allows larger-sized casing legs to be run downhole without a corresponding increase in the intermediate- and surface-casing size requirements. However, these reformable multilateral systems had not yet been developed in the sizes and with CRA materials that were required by Statoil for their multilateral development and also were not well suited for gravel-pack installations.