Reservoir Management of Valhall Field, Norway
Authors: York S.D. , Pong C.P. , Joslin T.H. 
Published: December 2007

Summary
This paper presents the historical development and the evolution ofreservoir simulation models for the Valhall field. Reservoir simulators wereused as management tools to determine possible effects of high rockcompressibility, fracturing, and possible effects of high rock compressibility, fracturing, and fracture permeability decline on primary recovery. Theseevaluations identified additional development opportunities, resulting inhigher ultimate recoveries.

Introduction
This paper describes the field development and the evolution of reservoirsimulation tools used for the Valhall field in the Norwegian sector of the North Sea. This reservoir consists of a high-porosity chalk with naturalfractures in the crestal area of the field. As in other chalk reservoirs in thearea, the drive mechanism is a combination of fluid expansion and rockcompaction. According to laboratory test results, this high-porosity rockundergoes both elastic deformation and pore collapse (plastic deformation) withdeclining reservoir pressure. The rock compressibility resulting from porecollapse is a function of reservoir pressure and porosity and ranges from about 10 × 10 psi to more than 100 × 10 psi for chalk with 50% psi to more than 100 × 10 psi for chalk with 50% porosity. Simulation studies of field performanceindicate that porosity. Simulation studies of field performance indicate thatrock compaction will account for more than 80 million STB of oil recovery, about 25% of the predicted ultimate field recovery for current development. The Valhall field is an overpressured, undersaturated Upper Cretaceous chalkreservoir located about 180 miles offshore southern Norway (Fig. 1) in a waterdepth of 225 ft. Nearby producing fields include Ekofisk, Eldfisk, Tor, and Hod. The producing fields include Ekofisk, Eldfisk, Tor, and Hod. The Valhallfield was discovered in 1975 with the drilling of Well 2/8-6. Subsequently, five appraisal wells were drilled before installation of the three-platform,24-slot complex in mid-1981. Initial production from the field began in Oct.1982, and by early 1990, production from the field began in Oct. 1982, and byearly 1990, production was about 85,000 BOPD from 24 producers. Amoco Norwayproduction was about 85,000 BOPD from 24 producers. Amoco Norway Oil Co.operates the field on behalf of partners, including Amerada Hess Norge A/S, Enterprise Norge Ltd., and Elf Aquitaine Norge A/S.

Reservoir Description

Geology.
The trap at Valhall is an asymmetric anticline trending northwest-southeast, with a steeply dipping westerly limb. The structure is aresult of basin inversion along the Lindesnes fault during late Cretaceoustime. The Tor formation (Upper Cretaceous), the youngest chalk present at Valhall and the main pay interval, is present at 7,880 ft subsea (SS) at thestructural crest. The total area present at 7,880 ft subsea (SS) at thestructural crest. The total area under closure on the Valhall structure isabout 60,000 acres, with the last closing contour at 8,860 ft SS. However, thetotal productive limit of the field is only about 7,000 acres, with the mostproductive area comprising about 2,000 acres in the crest of the structure. Fig. 2 shows a field map with depth to top of chalk contours. A clearly definedoil/water contact (OWC) in the field has not been established because watersaturations increase gradually with depth, while porosities and permeabilitiesdecrease gradually. This results in varying capillary characteristics acrossthe field. To estimate the movable oil in place (OIP), a water saturation of80% was used as a cutoff to determine the HCPV. This water saturation is foundat about 8,525 ft SS for the Tor formation, with rock porosities of about 25%to 30%, and at about 8,300 ft SS for the Lower Hod formation, with rockporosities in the 30% to 35% range.
Rock Properties. The two producing horizons at Valhall field are the Maastrichtian Age Tor formation and the Coniacian-Turonian Age Hod formationchalks. Fig. 3 is a computed log example of a typical crestal well with theproducing layers indicated. Table 1 provides a list of pertinent reservoir, rock, and fluid-property provides a list of pertinent reservoir, rock, andfluid-property data. About 60 % of the OIP and 85 % of the production are fromthe Tor formation; the majority of the remaining production comes from the Lower Hod Unit. Most of the Tor production comes from the crestal area of thefield, where permeabilities appear to be enhanced by natural fracturing. Because of its dominance, the Tor formation will be the focus of this paper. Tor thickness varies from 0 to 164 ft across the field area, and porositiesapproaching 50% are common on the crest of the structure but decrease to around30% on the flanks. Porosity is preserved in the Tor formation as a result ofthe combined effect of preserved in the Tor formation as a result of thecombined effect of reservoir overpressuring, early oil migration, and the lowmagnesium calcite composition of the Tor.