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Case Study

 

Reservoir Compaction and Subsidence - Ekofisk Field

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Background

  • Located offshore in the North Sea, the Ekofisk field comprises Cretaceous chalks of the Tor and Ekofisk formations, with porosities ranging up to 45%.

  • The reservoir was initially overpressure with reservoir pressures of 7,000 psi (48.3 MPa) at a top reservoir depth of 2,900 m.

  • After 10 Years of production, the Ekofisk platform complex and tank were observed to be subsiding 0.5 m/yr.

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Technical Challenge

  • At 2.9 km deep, Ekofisk was the deepest compacting reservoir at that time,

  • Large depletion of ~500 psi/yr,

  • High porosity chalks up to 45% - with elastic & plastic behaviour expected over the effective stress range anticipated during production,

  • Natural fractures in reservoir and overburden,

  • Wells sheared – bedding plane slip,

  • Complex compaction behaviour,

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Solution

  • Detailed analysis of the reservoir compaction using well controlled formation rock mechanics testing which was implemented in a 3D FEM which resulted in a prediction of sea floor subsidence > 6m.

  • Matrix compaction and natural fracture shear-related compaction analyses performed

  • 12 platform complex – legs cut, and 6m long leg extensions inserted. Seafloor Ekofisk storage tank and processing complex given wave protection,

  • Led to a change of field development from depletion to pressure maintenance.

Ekofisk Top Structure.png
Chalk Critical State.png
Ekofisk Tank.png
Ekofisk Tanks.png

Business Impact

  • Estimated in today’s money, equivalent of >$2 billion,

  • Loss of reputation and credibility of operator,

  • In addition, field has been subsequently re-developed.

Publications

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  1. Addis, M.A. 1989.  The behaviour and modelling of weak rocks.  Proc. Int. Rock Mechanics Symp.: Rock at Great Depth, Pau, September 1989, V. Maury and D. Fourmaintraux (eds.), Vol. 2, pp.899-905.

  2. Addis, M.A. and Jones, M.E. 1989.  Mechanical behaviour and strain rate dependence of high porosity chalk.  Proc. Int. Chalk Symp., Brighton, September 1989; pp.239-244.

  3. Barton, N., Makurat, A., Harvik, L., Vik, G., Bandis, S., Christianson, M. and Addis, M.A. 1988. The Disconinuum approach to compaction and Subsidence Modelling as Applied to Ekofisk. In.: Proceedings of the International Conference on Behaviour of Offshore Structures, Trondheim, Norway, June 1988, pp.129-142.

  4. Jones, M.E., Leddra, M.J. and Addis, M.A. 1987.  Reservoir compaction and surface subsidence due to hydrocarbon extraction. Offshore Technology Report, OTH 87 276.  Department of Energy, HMSO, 1987.

  5. Addis, M.A. 1987.  Material metastability in weakly cemented sedimentary rocks.  Memoirs of the Geological Society of China, Vol. 9, pp.495-512.

  6. Addis, M.A. 1987. Mechanisms of Sediment Compaction Responsible for Oil Field Subsidence. Ph.D. Thesis, University of London.

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Related Publications (earliest publications only)

  1. Teufel, L.W., Rhett, D.W. & Farrell, H.E. 1991. Effect of reservoir depletion and pore pressure drawdown on in situ stress and deformation in the Ekofisk Field, North Sea. Paper ARMA-91-063, presented at the 32nd US Symposium on Rock Mechanics (USRMS), 10–12 July 1991, Norman, OK, USA.

  2. Johnson, J.P., Rhett, D.W., and Siemers, W.T. 1989. Rock Mechanics of the Ekofisk Reservoir in the Evaluation of Subsidence. SPE 17854. Journal of Petroleum Technology, July 1989, pp.717-722.

  3. Menghini, M.L. 1989. Compaction Monitoring in the Ekofisk Area Chalk Fields. SPE 17853. Journal of Petroleum Technology, July 1989, pp.735-739.

  4. Sulak, R.M. and J. Danielsen, J. 1989. Reservoir Aspects of Ekofisk Subsidence. SPE 17852. Journal of Petroleum Technology, July 1989, pp.709-716. 

  5. Jones, M.E., Leddra, M.J., Goldsmith, A., Berget, O.P. and Tappel, I. 1989. The Geotechnical Characteristics of Weak North Sea Reservoir Rods. Chapter 9, North Sea Oil and Gas Reservoirs—II:  edited by A.T. Buller, E. Berg, O Hjelmeland, J Kleppe, O Torsaeter, and J.O. Aasen, The Norwegian Institute of Technology, Graham & Trotman. pp.201-211.

  6. Jones, M.E., Leddra, M.J. and Potts, D. 1989. Ground Motions due to hydrocarbon production from the chalk. International Chalk Symposium September . Preprint No. 50, Thomas Telford, London, pp.341-347.

  7. Potts, D.M., Jones, M.E. and Berget, O.P. 1988. Subsidence above the Ekofisk Oil Reservoirs. Proceedings of the International Conference on Behaviour of Offshore Structures, Trondheim,nNorway, June 1988, pp.113-128.

  8. Johnson, J.P. and Rhett, D.W. 1987. Compaction behaviour Ekofisk Chalk as a function of stress, SPE 15872. SPE European Petroleum Conference, London, 20-22 October 1987

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