Theme 6: INTEGRATION OF STRUCTURAL DATA AND RESERVOIR MODELS

1. Theme 6:INTEGRATION OF STRUCTURAL DATA AND RESERVOIR MODELS

2. Basis of fault modeling in reservoir simulations • Reservoir models of entire field (‘full-field’) or part of a field (‘sector’) • Faults considered as single plane • Modelled flow path as part of cross-cell flow calculation • Use modifiers of transmissibility between cells

3. Manzocchi et al. (2002)

4. Fault zone transmissibility Fault Rock Thickness Fault Rock Permeability Transmissibility (Perm x Fault rock thickness) Hydraulic Resistance (Fault rock thickness / Perm) Matrix Properties Cell Size

5. Transmissibility multipliersand flow modeling Only Cross-fault cells used : - No along fault flow considered. - No Threshold Capillary Pressure considered. Separate cells for faults allows along fault flow evaluation.

6. Fault zone hydraulic resistance • Flow across a fault in reservoir models follows Darcy flow: The rate for linear flow is: q = (k/L) (A/h) (f1 - f2) For a given cross-sectional area, A, across the fault and a constant pressure gradient and fluid viscosity, the flow rate is dependent on the fault zone hydraulic resistance or, (k/L), where L is the fault rock thickness.

7. L L1 L2 k1 k2 reservoir cell 2 reservoir cell 1 Transmissibility – no fault • Fault zone properties are introduced into reservoir models as transmissibility multipliers. • Average permeability for flow between adjoining cells with no fault is: k undeformed = L / [(0.5L1/ k1) + (0.5L2/ k2)] And transmissibility (T trans) is K undeformed /L No fault

8. L Lf L1 L2 k1 k2 fault reservoir cell 2 reservoir cell 1 Fault transmissibility – with fault • Average permeability for flow between adjoining cells with a fault is: k faulted = L / [0.5 (L1 - Lf) / k1] + [0.5 (L2 - Lf) / k2] + [Lf / kf] With fault

9. Transmissibility multiplier - T • Transmissibility with a fault is altered by transmissibility multiplier, T Ttrans = T (kundeformed/L) for no fault T=1 and for a completely sealing fault T=0 • The transmissibility multiplier is the ratio of the faulted permeability to the undeformed permeability that is: T = kfaulted/kundeformed This is the key relationship introduced into reservoir models.

10. Transmissibility multiplier - T • The transmissibility multiplier is: T = kfaulted/kundeformed where, k faulted = L / [0.5 (L1 - Lf) / k1] + [0.5 (L2 - Lf) / k2] + [Lf / kf] is a function of the fault permeability, kf and fault rock thickness, Lf. • The fault rock thickness is associated with the fault throw, Lf.

11. Fault rock thickness Fault rock thickness scales with fault displacement

12. Manzocchi et al. (2002)

13. Fault rock permeability vs. clay content

14. Fault Zone Flow Transmissibility depends on cell size

15. Fault Zone Flow Transmissibility depends on cell size

16. Fault Rock Prediction: Heidrun field Knai & Knipe (1998)

17. Geocellular models for reservoir modeling Fault rock thickness

18. Geocellular reservoirmodels Fault rock permeability Geologists provide reservoir engineers input along faults for modeling.

19. Threshold Pressures and Flow Modeling Constant Fault Rock Properties P > Pth Fault leaks Sealing Capacity P < Pth Fault seals No Flow. Permeability based Transmissibility not applicable, Water filled fault. Base Hydrocarbon

20. Fault zone flow effectiveness • Fault zone complexity cannot be explicitly modelled in current reservoir simulators. • Capture essential details by determining effective fault rock thickness. • Minimise fault rock thickness on all pathways - assumed to be most efficient flow path.

21. 5cm Cumulative fault rock thickness Cataclastic faults in porous sandstones build up cumulative fault rock thickness

22. COMPONENTS: Matrix Flow Modified Matrix Tortuosity Factor Fault Rock Flow 0 100% • Fault Populations • Fault Connectivity • Fault Rock Permeability • Fault Rock Distribution Fault Transmissibility Assessment

23. Fault transmissibility evaluation: workflow 1. Define number of faults crossed along critical flow paths through fault zones % by-passed ? 2. Define total thickness of fault rocks present along the critical flow path 4. Calculate the effective transmissibilities, and threshold pressures of fault zones 3. Define permeabilities and threshold pressures of the different fault rocks along pathways

24. Complex fault modelling • Study impact of 3D spatially distributed faults on flow properties of complex fault zones. • Analyse tortuosity and connectivity in terms of fault zone geometry. • Analyse spatial clustering techniques (core, outcrop & seismic scale). • Model influences of host rock and fault rock permeability ratio. • Results accessible to reservoir simulation packages - fault rock thickness, transmissibility multipliers.

25. Model attributes • Position, length, width, strike, dip, aspect ratio • Clustering technique – hierarchical • Throw:thickness and throw:length ratios BASIC ASSUMPTIONS: • Fault lengths are members of power law size-frequency distribution • Faults elliptical and planar; orientation unrestricted • Damage zone of clustered faults around major faults

26. A Two-Dimensional Illustration

27. Strike view of fault zone Collapsed Fault Rock Thickness Hierarchical Clustering Technique

28. Two-Dimensional Horizontal Slices Hierarchical Clustering Technique Fault Spacing Along 1D Traverse

29. Modelled Volume of Interest

30. Controls on Pathway Length and Fault Rock Thickness • Direct Path (low frequency): Low connectivity of fault array, low fault rock thickness. • Tortuous Path (medium frequency): Increased connectivity, long pathways, low fault rock thickness. • Direct Path (higher frequency): Effective barriers, low tortuosity pathways, significant increase in fault rock thickness.

31. High Kh/Kf Kh ~ Kf Impact of permeability ratio of host rock and fault rock • Permeability ratio controls host rock pathway lengths which can be traversed before faults are crossed. • Modelled by adding a “background” value to each cell in addition to fault rock thickness values.

32. Transmissibility Multiplier Permeability ratio  3334