Ekofisk Field: The First 20 Years

Abstract

Summary Ekofisk field, discovered in late 1969, is now more than 20 years old. During its first 20 years, Ekofisk has undergone both gas and water injection. The initial $1.5-billion waterflood project (1983) has been expanded (1988), extended (1989) and optimized (1990). Subsidence, discovered in late 1984, wasrectified by the deck-elevation (1987) and the protective barrier (1989)projects, at protective barrier (1989) projects, at a total cost of about $1billion. Introduction Ekofisk produced 894 × 10 bbl of oil and 2.8 × 10 SCf of gas (net ofinjection) during its first 20 years. Solution-gas drive and compaction drivehave been the dominant natural production mechanisms. Oil expansion contributedduring the first few years. Water influx also has played an important role. Natural drive mechanisms have been augmented by gas and water injection. Aconcerted effort is now under way to improve the characterization of the Ekofisk reservoirs by use of an integrated interdisciplinary approach. Thisshould result in significant production improvement in the future throughbetter well/sidetrack placement and completion. placement and completion. Discovery By Fall 1969, exploration efforts in the North Sea were rapidly declining. More than 200 exploratory wells had been drilled in the hostile environment ofthe North Sea, including 32 in the Norwegian sector. None of these wells foundcommercial oil, although there were some gas discoveries in the U.K. sector.Ocean Viking spudded Well 2/4–1AX in the southern portion of the Norwegiansector on Sept. 18, 1969 (Fig. 1). On Oct. 25, the oil-bearing Ekofisk chalkreservoir was penetrated. On Dec. 31, 1969, the Ekofisk penetrated. On Dec. 31,1969, the Ekofisk discovery well tested 1,071 BOPD on a 34/64-in. choke. Thistest was limited by equipment capacity. Further testing was hampered by severeweather, but the well subsequently was tested at sustained rates exceeding10,000 BOPD. Structure. Following the drilling of the three appraisal wells, the Ekofiskstructure was still not frilly understood. The center of the structure wasthought, on the basis of seismic interpretation, to be depressed and wasavoided by the discovery and appraisal wells (Fig. 2). The dining of Well 2/4C-8 in 1974 showed that seismic results were affected by a gas cloud over thecenter of the structure and that a normal anticline existed. The anticline iselongated and covers some 12,000 acres. The major axis (north/ south) is about6 miles long, while the east/west axis extends roughly 4 miles (Fig. 3). Theformation of the structure is still disputed. Some geologists think that thestructure was created by an underlying salt dome, but because no well on thestructure has penetrated salt, compressional forces penetrated salt, compressional forces resulting in a deep flower fault could be responsible forthe anticline (Fig. 4). Formations. The producing horizons of the Ekofisk field are the Ekofisk and Tor formations. Both formations are fine-grained limestones, or chalks, composed of the skeletal debris of pelagic unicellular algae known ascoccolithophorides. Coccospheres, the calcareous exoskeletons of these algae, are composed of wheel-shaped coccoliths, which in turn are composed ofcoccolith platelets. The majority of the matrix is composed of coccolithplatelets that range in size from 1 to 2 pm. Whole coccoliths are commonlyfound, but coccospheres are rarely preserved. The Ekofisk formation is locatedat a depth of about 9,600 ft. This chalk formation is of Danian Age in the Paleocene period. The Tor formation, which underlies the period. The Torformation, which underlies the Ekofisk formation, is of Maastrichtian Age inthe Cretaceous period. The Tight Zone, where it exists, forms an impermeablebarrier between the two producing formations. The chalks exhibit high porosity(25 to 48 %) and low matrix permeability (1 to 5 md). Natural fracturesincrease the effective permeability to up to 100 md. An overall paypermeability to up to 100 md. An overall pay thickness exceeding 1,000 ft hasbeen encountered (Fig. 5). The overburden consists of about 9,300 ft of claysand shales and is overpressured below about 4,500 ft. Fluid Properties. Ekofisk initially contained undersaturated volatile oil at7,135 psia and 268 degrees F at datum (- 10,400 ft). The 36 API-gravity crudeoil had a solution GOR of 1,530 scf/bbl. The bubblepoint pressure was 5,560psia. Phased Development Phased Development The producibility of these naturallyfractured chalk reservoirs was initially in doubt. October 1991 P. 1265