Use of PP and PS time-lapse stacks for fluid-pressure discrimination.

1. Use of PP and PS time-lapse stacks for fluid-pressure discrimination. ALEXEY STOVAS 1, MARTIN LANDRØ 1 & BØRGE ARNTSEN 2 1NTNU, Dept. of Petroleum Engineering and Applied Geophysics, Trondheim, Norway 2 Statoil R&D centre, Trondheim, Norway

2. Outline • Change in PP and PS reflectivity due to change in pressure and saturation (reflection pattern) • Pressure-saturation discrimination and uncertainties • Application on the Gullfaks synthetic data set • Conclusions • Acknowledgments

3. Method • Methodology (Stovas & Landrø, 2002a,b) • Water saturation model (Gassmann, 1951) • Pressure model (Mindlin, 1949) • Reflection coefficients (Ursin & Stovas, 2002) • The Gullfaks synthetic data set (Arntsen, 2002)

4. Reflectivity versus saturation and pressure (1) Gassmann and Hertz-Mindlin models give

5. Reflectivity versus saturation and pressure (2) Reflection coefficients versus incident angle

6. Reflectivity versus saturation and pressure (3) Stacked reflection coefficients versus opening angle (Q)

7. Basic principle (1) We establish relationship between the change in the PP and PS stack amplitudes and the change in water saturation and pressure where operator maps the input vector of the change in saturation and pressure into the output vector of the change in the stacks amplitudes

8. Mapping

9. Opening angle (ray tracing)

10. Basic principle (2) The procedure: • Compute elastic parameters • Compute PP&PS reflection coefficients • Evaluate min/max opening angle • Compute stacked PP&PS reflection coefficients • Build up the reflection patterns • Compute PP&PS calibration factors • Compute the difference PP&PS stacks • Place amplitudes into corresponding reflection patterns

11. Uncertainties in saturation&pressure from uncertainties in PP&PS stacked amplitudes a = discrimination angle

12. Weighting factors for uncertainties

13. Gullfaks synthetic data Data set includes: • 8 types of reservoir rock (Tarbert and Ness formations) overlaid by shale (Shetland formation) • 3 time-lapse models with PP and PS seismic data Saturation-pressure condition is known for Model I and has to be predicted for Model II and Model III

14. P-wave Gullfaks model Distance, m Top reservoir Oil water contact Time, ms

15. Reservoir rock physics parameters

16. P- and S-wave velocities for reservoir rock SM1

17. Isolines for reflectivity changes for the interface Shetland/SM1

18. PP&PS stacks for Gullfaks Model I

19. PP&PS stacks for Gullfaks Model II

20. PP&PS stacks for Gullfaks Model III

21. Saturation-pressure prediction for reservoir rock SM1

22. Seismic amplitudes and saturation-pressure uncertainties

23. Weighting factors for uncertainties

24. Uncertainties vs. opening angle (water saturation)

25. Uncertainties vs. opening angle (gas saturation)

26. Conclusions • Method of fluid-pressure discrimination from PP and PS stacks is developed • Method is applied on synthetic data set from Gullfaks model which consists of three time-lapse situations. The results of water saturation and pressure prediction are very close to the modelled data (2-3% error in average). • The analysis of weighting factors for uncertainties in water saturation and pressure shows that for all reservoir rocks representing Gullfaks Field the relative uncertainties in saturation are bigger than the corresponding uncertainties in pressure.

27. Acknowledgments We want to acknowledge the financial support from the EC project ENK6-CT-2000-00108, ATLASS and Statoil.