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(from Fejerskov and Lindholm, 2000)

EKOFISK SEISMIC EVENT OF MAY 7, 2001 Lars Ottem öller (BGS, UK) Hardy Hartmann Nielsen ( PhillipsConoco , Norway) Jochen Braunmiller (ETHZ, Switzerland ) Kuvvet Atakan , Jens Havkov (University of Bergen, Norway). (from Fejerskov and Lindholm, 2000).

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(from Fejerskov and Lindholm, 2000)

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  1. EKOFISK SEISMIC EVENT OF MAY 7, 2001 Lars Ottemöller (BGS, UK)Hardy Hartmann Nielsen (PhillipsConoco, Norway)Jochen Braunmiller (ETHZ, Switzerland) Kuvvet Atakan, Jens Havkov (University of Bergen, Norway)

  2. (from Fejerskov and Lindholm, 2000)

  3. Date 07.05.2001; Time 09:43:34Loc. 56.565 N / 3.182; RMS=1.1; Mb=4.4; MS=4.6

  4. Strike: 356Dip: 85Rake: -92 Mw: 5.1

  5. Natural: Earthquakes caused by purely tectonic stresses • Triggered: Earthquakes caused by tectonic stresses that are initiated by human activity • Induced: Earthquakes caused by stresses that can be traced directly to human activity

  6. Basic mechanisms • Local fluid injection decreases effective normal stress and induces seismic cracks M<3 within the reservoir • Fluid withdrawal causes pore pressure to decrease within reservoir, stresses are transferred to surrounding region where M<5 occur above or below reservoir • Isostatic compensation after load removal through hydrocarbon recovery can cause M>6 at larger distance (after Grasso, 1992)

  7. Ekofisk facts • Started production in 1971 • Chalk reservoir at depth of ~3 km • Production now is ~340000 barrel oil and ~11*106 m3 gas per day • Total of more than 8 m subsidence • Water flooding program started early 1990s, led to increase in pore pressure and reduced subsidence • Drill cuttings re-injection into overburden started 1996 after extensive risk and cost evaluation • Induced micro-earthquakes on reservoir level, about 100 per day • Causal relation between seismicity and production not resolved

  8. Bathymetric Difference Map 1999-2001

  9. (From Segall, 1989)

  10. Conclusions • Event induced • Located in the overburden • Cause is unintended water-injection into overburden • Event was sudden compaction of overburden

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