An Integrated Assessment of Fault Behaviour during Depressurization

R.J. Knipe¹, A.I.F. Welbon², S.R. Freeman¹, S.D. Harris¹, O. Hostad², M. Hettema², T. Berre³, K.B. Lyslo², and T. Olsen^2
1 Rock Deformation Research Ltd, Leeds, England
² Statoil, ASA, Stavanger, Norway
³ Norwegian Geotechnical Institute, Oslo, Norway

The results of an integrated analysis of fault behaviour during depressurization in the Statfjord field, N. Sea incorporates specifically designed fault rock flow property and strength testing, with high resolution seismic mapping, flow simulation and stress modeling. New relationships between fault clay content and fault rock flow and strength properties were determined from core samples and the derived fault rock property relationships mapped onto the fault zones in a high resolution geo-cellular model. The model was used to assess the potential for cross-fault flow and to evaluate the distribution of faults susceptible to mechanical failure during depressurization along different stress path (ratio between the change in the horizontal stress and the change in the fluid pressure) scenarios. The susceptibility to failure was determined via a mobilization factor defined by the distance in stress space between the failure envelope and the stress resolved onto discontinuities (faults and bedding planes). An uncertainty assessment including variations in the fault properties and geometries on the mobilization values captures the sub-seismic dip variations on the large faults, the impact of small faults (with more variable dips in damage zones to the large faults) as well as the variations in flow and strength property values. Mapping the distribution of mobilization factors for key faults reveals the distribution fault patches of high fluid communication and / or higher failure susceptibility, which successfully contributed to the integrated analysis of the project planning for depressurization of the field.