Ekofisk oil field

The Ekofisk field is a north–south trending anticline, with a central graben, forming a structure that is {{convert|49|km2|abbr=on}} in area, with {{convert|244|m|abbr=on}} of vertical closure and a hydrocarbon column {{convert|305|m|abbr=on}} long, formed by Permian Zechstein salt movement in the form of salt pillows.Van den Bark, E., and Thomas, O.D., 1980, Ekofisk: First of the Giant Oil Fields in Western Europe, in Giant Oil and Gas Fields of the Decade: 1968-1978, AAPG Memoir 30, Tulsa, American Association of Petroleum Geologists, p. 211 The production zones at a depth of about 3 km include the Paleocene Ekofisk Formation and the Upper Cretaceous Tor Formation, both Chalk Group rocks containing porosities of 30-40%.Van den Bark, E., and Thomas, O.D., 1980, Ekofisk: First of the Giant Oil Fields in Western Europe, in Giant Oil and Gas Fields of the Decade: 1968-1978, AAPG Memoir 30, Tulsa, American Association of Petroleum Geologists, pp. 195, 208 and 200 "The reservoir rock...is a true chalk-a fine-grained limestone composed of the skeletal remains of pelagic unicellular golden-brown algae or coccolithophores".Van den Bark, E., and Thomas, O.D., 1980, Ekofisk: First of the Giant Oil Fields in Western Europe, in Giant Oil and Gas Fields of the Decade: 1968-1978, AAPG Memoir 30, Tulsa, American Association of Petroleum Geologists, p. 200 The source rocks are the Upper Jurassic Kimmeridgian shales.Van den Bark, E., and Thomas, O.D., 1980, Ekofisk: First of the Giant Oil Fields in Western Europe, in Giant Oil and Gas Fields of the Decade: 1968-1978, AAPG Memoir 30, Tulsa, American Association of Petroleum Geologists, pp. 211-213

The structure was discovered using seismic reflection data in the 1960s, although the initial interpretations were distorted due to zones of high gas saturation in the overlying Cenozoic rocks causing low seismic velocities. However, the high porosity of the reservoir rock does cause an increase in seismic amplitude which can be used with an isopach map to determine net pay.

In the mid-1980s the Ekofisk field as a whole and the platforms in particular were found to be suffering from an unexpected degree of subsidence. Detailed geological investigation showed that it was the result of delayed compactional diagenesis of the Chalk Formation reservoir rocks. As hydrocarbons were produced the pore pressure declined and the effective stress increased, leading to subsidence.Sulak, R. M. & Danielsen, J. (1989), Reservoir aspects of Ekofisk subsidence Journal of Petroleum Technology, Society of Petroleum Engineers, 41, 709-716 Water injection was initiated to repressurize the reservoir, but due to the lower compaction strength of water-saturated chalk compared with oil-saturated chalk the seafloor continued subsiding and displacements of several metres were recorded. It was calculated that the total subsidence would almost be {{convert|6|m|abbr=on}} at the end of the concession of Phillips Petroleum, too much to keep the platforms secure.

The Norwegian government pressed Phillips to take action and the French company Technip was ordered to find a solution. As 5 of 7 platforms were interconnected, they had to be jacked-up by about {{convert|6|m|abbr=on}} at the same time. The eventual solution suggested was to extend the steel tubular legs of the platforms. Subsequently, large flanges were welded to these legs and when all flanges were welded and the legs cut, five platforms would be lifted simultaneously in one operation then extension pipes would be mounted in between the flanges. After bolting all flanges the platforms would be safe again.

The four days lifting was completed on 17 August 1987, at 11:30 p.m. thanks to 108 hydraulic cylinders synchronised with a network of 14 NUM 760FCNCs.{{cite journal

|publisher = NUM

|url = http://docs.niaag.net/pdfs/ekofisk_fr.pdf

|title = Special Ekofisk

|date = October 1987

|pages = 1–2

|access-date = 2011-01-26

|url-status = dead

|archive-url = https://web.archive.org/web/20110723221156/http://docs.niaag.net/pdfs/ekofisk_fr.pdf

|archive-date = 23 July 2011}}

The position tolerance of the cylinders with each other (+/- {{convert|3|mm|abbr=on}} for a {{convert|6|m|abbr=on}} extension per platform and +/- {{convert|100|mm|abbr=on}} between platforms) was to be kept for 38 hours. During the welding of the flanges to the legs, these hydraulic cylinders took over the entire load. A couple of days before this great jack-up, the hotel platform was lifted, as it was not interconnected with the others. The total lifting capacity of all these cylinders was approximately {{convert|40000|t}} and was published in the Guinness World Records as being the largest jack-up.

In April 1977, an oil well blowout occurred at the Ekofisk Bravo platform, due to an incorrectly installed downhole safety valve.[http://home.versatel.nl/the_sims/rig/ekofiskb.htm Oil Rig Disasters – Ekofisk Bravo Platform] {{webarchive|url=https://web.archive.org/web/20101203234723/http://home.versatel.nl/the_sims/rig/ekofiskb.htm |date=3 December 2010 }} At an estimated {{convert|80000|-|126000|oilbbl}} total, it was the largest blowout in the North Sea. Red Adair and his crew assisted with capping the blowout.{{Cite web |url=http://www.redadair.com/hisstory.html |title=Red Adair, Oilwell Firefighter, American Hero – His Story... |access-date=15 May 2010 |archive-url=https://web.archive.org/web/20080717184126/http://www.redadair.com/hisstory.html |archive-date=17 July 2008 |url-status=dead }}

File:Ekofisk.png|Annual oil production from Ekofisk.

File:Ekofisk complex.jpg|The Ekofisk complex in 2010.

File:Norsk oljeproduksjon pr aar pr felt.png|Petroleum production of Norway by year and oil field

• {{ Cite book | author = Kvendseth, Stig S. | year = 1988 | title = Giant discovery - A history of Ekofisk through the first 20 years | isbn = 8299177111 | publisher = Phillips Petroleum Company Norway, Public Affairs | url = http://urn.nb.no/URN:NBN:no-nb_digibok_2019090248002 }}

{{Portal|Norway|Energy}}

• List of oil and gas fields of the North Sea
• Commercial offshore diving in the North Sea
• List of oil spills
• Alexander L. Kielland (platform) 1980 disaster

{{Reflist}}

• [https://web.archive.org/web/20130926053642/http://www.conocophillips.no/EN/Norwegian%20shelf/Ekofisk/EkoEko/Pages/index.aspx Ekofisk]
• [https://web.archive.org/web/20110719221911/http://www.aftenbladet.no/energi/sokkelkart/article521528.ece?english=true Ekofisk in Interactive Energy Map]
• [https://web.archive.org/web/20050222010210/http://www.npd.no/engelsk/cwi/pbl/en/field/all/43506.htm Ekofisk field data]
• [http://www.ekofisk.industriminne.no/ Ekofisk Industrial Heritage]
• [https://www.youtube.com/watch?v=siUpnltiTc8 Jack-up mini documentary]

{{Resources in Norway}}

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