Times were tough for Phillips Petroleum Co. in 1969. Despite the recent discovery of the giant Groningen gas field in The Netherlands and some promising seismic surveys offshore, 11 drilling attempts in the North Sea had resulted in as many dry holes, according to “Phillips: The first 66 years.” Plans for one more well were scuttled, and the North Sea drilling team was told to sublease the rig.

But nobody wanted it; other companies were cutting back as well. So the Ocean Viking got the green light to drill one last well before giving up on the North Sea altogether. Its first attempt had favorable indications of oil but encountered a high-pressure zone. The rig was moved about 915 m (3,000 ft) southeast, and after more than a month of drilling, the bit encountered a chalk zone saturated with oil. IP was 1,071 bbl/d.

With the Ekofisk discovery, “What Phillips had at last was the first commercial oil well in the North Sea and the first giant oil field in Western Europe,” the book noted. The discovery led to the North Sea being one of the most prolific producing regions in the world, and it put Norway on the map as an energy superpower.

Field facts

According to SubseaIQ’s website, the Ekofisk Field, located in blocks 2/4 and 2/7 about 322 km (200 miles) southwest of Stavanger, sits in 75 m (246 ft) of water. The reservoir, located between 2,896 m and 3,249 m (9,500 ft and 10,660 ft), is early Paleocene and late Cretaceous in age. It is a central graben anticline that covers almost 50 sq km (19 sq miles) in area.

Initially four wells produced to tankers, but in 1973 a concrete tank was installed. Since then, 300 wells located in eight fields—Cod, Ekofisk, West Ekofisk, Tor, Albuskjell, Eldfisk, Edda and Embla—have been drilled. These wells were tied to 16 platforms known as the Ekofisk Center. The center is also a hub to other fields in the area and has now expanded to 29 platforms, transporting oil to a terminal in Teesside and gas to a terminal in Emden.

In the 1980s the field and platforms were discovered to be subsiding, a result of compactional diagenesis of the chalk formation, according to “Reservoir Aspects of Ekofisk Subsidence,” a paper presented at the 1988 Offshore Technology Conference. Water saturation was initiated, but the problem continued. Technip was hired to jack up the subsiding platforms by about 6 m (20 ft). This was accomplished by extending the tubular legs of the five affected platforms by cutting the legs, lifting the five platforms simultaneously and mounting extension pipes. The lift of about 40,000 mt was listed in the Guiness Book of World Records as being the largest jack up ever.

Water injection might not have helped with the subsidence issue, but it has been used to increase the production rate. Ekofisk 2/4 was put onstream in December 1987. Injection capacity has been increased several times since then.

In 1994 the Ekofisk license period was extended to 2018, and a plan for development and operation of Ekofisk II was adopted, according to the ConocoPhillips website. A wellhead platform was installed in 1996, and a new processing and transportation platform was installed the following year. Ekofisk II came onstream in 1998.

Ekofisk South, which is expected to contribute to a significant increase in oil production, has 36 well slots, 35 of which are for production and the last for cuttings reinjection. The wells will be drilled by a jackup next to the platform.

According to ConocoPhillips Norway’s website, the platform is able to carry out multiple tasks at the same time, including drilling, production, maintenance and intervention. The platform is remotely operated from the central control room on the 2/4 J platform but also can be monitored from the onshore operations center in Tanager.

Ekofisk 2/4 L, officially inaugurated in 2014, provides accommodations for the crew and is the largest accommodation platform in the North Sea. It provides the regional telecommunications hub for the Ekofisk Center as well as coordinating air and sea traffic in the area. It has two helicopter hangars with a capacity of 50 daily air movements and is the most important helicopter landing area in the Greater Ekofisk area.

Reservoir monitoring
Ekofisk is one of a growing number of fields in the North Sea to be equipped with permanent reservoir monitoring (PRM) equipment. According to an article titled “Ekofisk life-of-field seismic: operations and 4-D processing,” written by authors from ConocoPhillips Norge, CGG and Total E&P Norge AS and published in the February 2014 issue of The Leading Edge, the field hosts the world’s largest optical PRM system.

The system consists of almost 4,000 seabed multicomponent sensors along 200 km (124 miles) of mostly trenched fiber-optic cables. The system covers about 60 sq km (23 sq miles) of the field. The system acquires data through a topside recording unit and by using a source from a supply vessel.

Ekofisk is no stranger to 4-D seismic. The first monitor survey was shot in 1999 to repeat a streamer survey shot a decade earlier. This survey indicated significant compaction in the reservoir. Additional streamer surveys were acquired in 2003, 2006 and 2008, according to the article.

A value-of-information study performed in 2005 determined that PRM would be a better strategy than repeat streamer surveys. The permanently installed sensors provide greater repeatability, a key element in 4-D surveys. The supply vessel also is equipped with a modern steering system, which enables repeatability in source positioning.

The PRM data are now routinely used for optimizing well locations and trajectories, prioritizing well interventions, diagnosing mechanical issues and updating the reservoir model, the article noted. They are also useful in surveying the impact of injection, production, compaction and subsidence on the overburden.