Halfdan: Developing Non-Structurally Trapped Oil in North Sea Chalk

Abstract

Abstract Mærsk Olie og Gas AS has as operator for the Danish Underground Consortium (DUC) discovered and initiated the development of the Halfdan accumulation located in the southern part of the Danish North Sea. The accumulation comprises oil and gas in a low permeability chalk reservoir located outside structural closure. This paper describes how the phased, fast track development of Halfdan was planned and executed while testing a new concept for constructing an aligned sweep pattern. Tilted oil-water and gas-oil contacts across the Halfdan area are indicative of non-equilibrium conditions. The non-structural trapping in the Halfdan area is provided by a combination of up dip thinning of the hydrocarbon bearing section, late structural tilting and the slow re-adjustment caused by the low permeability. Accelerated and stepwise development was pursued to allow continuous optimisation of the development in light of the significant uncertainties associated with the fluid distribution and reservoir production characteristics. Appraisal drilling from a template and tie-back of the appraisal wells to temporary production facilities via the drilling rig and early deployed pipelines enabled production from the Halfdan 10 months after finalising the discovery well. Furthermore, a temporary water injection package installed on the drilling rig enabled evaluation of water injection before the installation of permanent facilities including determination of displacement efficiency, fracturing pressure and induced fracture orientation. Halfdan was produced through permanent facilities less than 18 months after finalising the discovery well. Introduction The Halfdan accumulation is situated in the Danish North Sea Central Graben about 250 kilometres off the West Coast of Denmark (Figure 1), and is located between the Dan and Skjold fields (Figure 2). Halfdan was discovered in December 1998 with a 30,000-ft long horizontal well drilled from the Dan field and the vertical Nana-1XP well completed in May 1999. First production from Halfdan was obtained late February 2000 and Halfdan is currently producing 50,000 stb/d from 7 wells. Halfdan is operated by M rsk Olie og Gas AS on behalf of the DUC (Danish Underground Consortium), which is a partnership between A.P. Møller, Shell Olie- og Gasudvinding Danmark B.V. (Holland) and Texaco Denmark Inc. The Halfdan area is estimated to contain some 1,500 MMstb STOIIP and covers an area of more than 12,000 acres. A small gas cap containing some 500 Bscf is present in the developmed area. The reservoir comprises unfractured, low permeability chalk (0.5 to 2.0 mD) of mainly Maastrichtian age. Oil and gas is present in Halfdan despite the lack of structural closure and both the gas-oil and oil-water contacts exhibit an apparent tilt from east to west (Figures 2 and 10). The complex trapping rendered an accurate determination of the STOIIP uncertain. Further, the low permeability and the relatively thin oil column including a significant transition zone added to the development risk. This paper describes how the phased, fast track development of Halfdan was planned and executed while testing a new concept for constructing an aligned sweep pattern. The paper focuses on the following aspects of the Halfdan development:Mechanisms for trapping of oil in the Halfdan area outside structural closure.The application of advanced saturation modelling based on horizontal wells in the assessment of hydrocarbon in place.How the uncertainty associated with the complex fluid distribution and reservoir performance is handled through a phased development.