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Ekofisk field
The Ekofisk oil field[1][2][3] is in the North Sea, south of Norway, with an estimated 6.4 billion bbl stock tank original oil in place (STOOIP). It is a large, carbonate reservoir that has two zones, Ekofisk and Tor, that are high-porosity, fractured chalks with matrix permeabilities of approximately 1 md and effective permeabilities that range from 1 to 50 md. Discovered in 1969, the Ekofisk field was found at very high pressure [7,120 psia at 10,400 ft true vertical depth subsea (TVDSS)] but with an initial bubblepoint pressure that was 1,600 psi below initial reservoir pressure. Ekofisk’s oil is 38°API, has a viscosity of approximately 0.25 cp, and has a solution gas/oil ratio (GOR) of more than 1,500 scf/STB.
Waterflooding assessment
Primary production began in June 1971 and peaked in 1976 at 350,000 barrels of oil per day (BOPD) from 30 production wells (with 8 gas reinjection wells). Fig. 1 shows the structure of the Ekofisk field and its injection- and production-well locations as of early 2003. Laboratory-test results indicated that waterflooding by water imbibition into the low-permeability chalk was favorable for the Tor formation, but the laboratory results for the Ekofisk formation were variable.[4] A waterflood pilot of the Tor formation was initiated in 1981, and favorable results were obtained by 1983. A 30-slot water-injection platform with an injection capacity of 375,000 barrels of water per day (BWPD) started up in 1987. A second waterflood pilot was run in the lower Ekofisk formation from 1985 to 1987, and its results were positive.[5] Further laboratory studies of the upper Ekofisk formation were somewhat negative, but a water-injection test into this zone and a coring of a sidetrack well 6 months later indicated that the upper Ekofisk formation also could be waterflooded successfully. By the early 1990s, all three intervals were receiving injected water.
Fig. 1 – Areal map of Ekofisk field with injection- and production-well locations (image provided by courtesy of ConocoPhillips).
Subsidence
Surface subsidence was a major issue at Ekofisk. By 1984, the seabed had subsided approximately 10 ft, prompting a major project to jack up the offshore platforms. A major field study in 1992 concluded that using water-injection pressure maintenance to arrest the reservoir pressure decline could minimize future seabed subsidence. Voidage replacement was achieved in 1993. Additional laboratory studies found that water injection had induced shear in the chalk. Shear failure and water-weakening of the rock matrix causes additional deformation of the chalk, even under conditions of constant or decreasing stress levels. Despite the use of voidage-replacement waterflooding, seabed subsidence continued until 1998, when the subsidence rates slowed dramatically because the water-weakening effect was expended and the reservoir pressure had increased.
Phase II facilities
In 1997, production began from the 50-slot production platform 2/4 X, and in 1998 full processing of the Ekofisk fluids was handled by the 2/4 J processing platform. These new "Ekofisk II" facilities replaced the aging original facilities and were designed to increase operational efficiency and to allow safe and economical production until at least 2028, the end of the current license period. The current best estimate of the ultimate recovery factor from the start of production through waterflooding is 44% of original oil in place (OOIP).
During the past 20 years of waterflooding, many operational changes have been made at Ekofisk. The changes were a logical progression that was based on laboratory studies, field pilot tests, and engineering analyses of field production and pressure data. All of this has led to a very successful waterflood project, but with a few unexpected complications that the engineers had to handle in the course of the waterflood project.
References
- ↑ Torsaeter, O. 1984. An Experimental Study of Water Imbibition in Chalk From the Ekofisk Field. Presented at the SPE Enhanced Oil Recovery Symposium, Tulsa, Oklahoma, 15-18 April 1984. SPE-12688-MS. http://dx.doi.org/10.2118/12688-MS
- ↑ Hermansen, H., Thomas, L.K., Sylte, J.E. et al. 1997. Twenty Five years of Ekofisk Reservoir Management. Presented at the Annual Technical Conference and Exhibition, San Antonio, Texas, 5–8 October. SPE-38927-MS. http://dx.doi.org/10.2118/38927-MS
- ↑ Agarwal, B., Hermansen, H., Sylte, J.E. et al. 2000. Reservoir Characterization of Ekofisk Field: A Giant, Fractured Chalk Reservoir in the Norwegian North Sea—History Match. SPE Res Eval & Eng 3 (6): 534-543. SPE-68096-PA. http://dx.doi.org/10.2118/68096-PA
- ↑ Currier, J.H. and Sindelar, S.T. 1990. Performance Analysis in an Immature Waterflood: The Kuparuk River Field. Presented at the SPE Annual Technical Conference and Exhibition, New Orleans, 23–26 September. SPE-20775-MS. http://dx.doi.org/10.2118/20775-MS
- ↑ Sylte, J.E., Hallenbeck, L.D., and Thomas, L.K. 1988. Ekofisk Formation Pilot Waterflood. Presented at the SPE Annual Technical Conference and Exhibition, Houston, 2–5 October. SPE-18276-MS. http://dx.doi.org/10.2118/18276-MS
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External links
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Ekofisk Industrial Heritage web site: http://www.kulturminne-ekofisk.no/