Earthquake hits Ekofisk

by Trude Meland, Norwegian Petroleum Museum

An earthquake struck the Greater Ekofisk Area on 7 May 2001, when the 852 workers felt the installations swaying and various small loose objects fell down. No personal injuries were reported.

— "Earthquake report to the Petroleum Directorate". From ConocoPhillips' Pioneer no. 11, 2002

The epicentre of the quake was about 18 kilometres north-west of the Ekofisk Complex, and it registered as 4.0 on the Richter scale.[REMOVE]Fotnote: Teknisk Ukeblad, “Jordskjelv på Ekofisk”, 9 May 2001. and Norsar Jordskjelv.no. Downloaded from https://www.jordskjelv.no/#modal.

A duration of roughly one minute was long for a tremor in this part of the world. It ranked as the second medium-sized quake (up to 4.0) on Ekofisk, after one in 1998. Phillips also registered a quake of 3.0 in 2000.

Inspections showed that the platforms had not been damaged. In consultation with the Norwegian Petroleum Directorate (NPD), Phillips therefore found that it was safe to stay on stream.

Nor were checks confined to the platforms. The wells had to be inspected for over a year to ensure that they had suffered no after-effects from the quake.

According to the Norwegian Seismic Array (Norsar) monitoring station, the incident related to oil production and to a collapse in a reservoir.

The seabed over Ekofisk had been subsiding for many years as a result of petroleum output, and water was being pumped into the reservoir to counter this.

Ahead of the quake, an error had meant that water was injected into the rock formation immediately above the oil reservoir. That led in turn to the “human-induced” tremor.[REMOVE]Fotnote: Department for geoscience in collaboration with Bergen Museum, at the University of Bergen.

The NPD came to the same conclusion. With the reservoir compressed, the strain was released when injection water leaked in error into the overlying clay and created instability. This compression resulted in microseismicity – a minor quake.[REMOVE]Fotnote: Norsar jordskjelv.no. Downloaded from https://www.jordskjelv.no/jordskjelv/om-jordskjelv/faq/generelt/kan-oljeutvinningen-i-nordsjoen-forarsake-jordskjelv-article1003-939.html.

Following this incident, new equipment was installed and tested on the seabed around Ekofisk to provide information on general seismic activity associated with subsidence.[REMOVE]Fotnote: Pioner, no 11, “Jordskjelvrappport til Oljedirektoratet”, ConocoPhillips, November 2002.