FAULT SEAL ANALYSIS: Mapping & modelling. EARS5136 slide 1

Transcription

1 FAULT SEAL ANALYSIS: Mapping & modelling EARS5136 slide 1

2 Hydrocarbon field structure Compartments 1 km Depth ~2.5km How to produce field? EARS5136 slide 2

3 Predict flow patterns and communication Fault compartments in the Sleipner field, Norwegian North Sea Different oil-water contacts Ottesen Ellevset et al. (1998) EARS5136 slide 3

4 Seal Mapping - Complexities Horizon / fault zone resolution (thin sand problem) Lack of reflectors for mapping Stratigraphic architecture / sediment pinchout Erosional truncation Intersecting faults Sub-seismic seal elements Multiple faulting events (reactivation) and impact on seal distribution and properties EARS5136 slide 4

5 Fault Seal Workflow Define geometry of fault array Establish sub-seismic fault density and fault zone structure Assess sealing mechanisms and fault rock properties Evaluate juxtapositions and seal distributions Test models against hydrocarbon contact levels if known Map seal distributions on fault planes which might form compartment boundaries Model reservoir flow and impact of faults on drainage patterns EARS5136 slide 5

6 Allan diagrams Footwall template > < Hangingwall template EARS5136 slide 6

7 Allan diagrams Areas where sands not in contact are juxtaposition seals Migration possible by stair-stepping between hangingwall & footwall across sand-sand windows Use fault seal algorithms to predict behaviour of juxtaposed sands EARS5136 slide 7

8 Allan Diagrams: Bed-Fault Intersections EARS5136 slide 8

9 Seismic data in juxtaposition analysis Example showing modelled fault surface with stratigraphic juxtapositions EARS5136 slide 9

10 Do we assess fault juxtapositions correctly? Allan Maps Accuracy; Horizon uncertainty: +20m to -20m Fault Uncertainty: ~100m Assume single fault, not complex damage zone EARS5136 slide 10

11 Snapping horizons to faults EARS5136 slide 11

12 Impact of Seismic Data Interpretation on Resolution and Quality of Allan Diagrams EARS5136 slide 12

13 Impact of Seismic Data Interpretation on Resolution and Quality of Allan Diagrams uncertainty EARS5136 slide 13

14 Impact of branch-lines from intersecting faults uncertainty Branch-lines EARS5136 slide 14

15 Complex Fault Plane Mapping Intra-formational erosion / pinchout Fig 6-41 S N m BCU FW BCU HW F FW F HW TOP Å FW INTRA ÅRE FW INTRA ÅRE HW TOP Å HW F FW INTRA ÅRE FW TOP Å HW UPPER ÅRE / LOWER ÅRE F2 BCU FW BCU HW F HW INTRA TOP Å HW ÅRE FW INTRA ÅRE HW TOP ÅRE HW INTRA ÅRE FW TOP ÅRE HW INTRA ÅRE HW F6 Intra F Fault Juxtapositions 1 km Erosional contact Erosional Contact BCU in HW BCU in HW Upper Jurassic in HW U Jur in HW Fangst / Fangst Fangst / Fangst BCU FW BCU HW F FW F HW 1800 Depth (m) Erosion and 30m Seismic Resolution Fangst in HW / Upper Åre in FW Fangst HW / Up Are FW Fangst in HW / Lower Åre in FW Fangst HW / Lr Are FW Upper Åre / Upper Åre Upper Åre / Lower Åre TOP ÅRE FW Up Are HW / Up Are FW TOP ÅRE HW INTRA ÅRE FW Up Are / Lr Are FW INTRA ÅRE HW EARS5136 slide 15

16 Seismic horizon juxtaposition example Four seismically mapped horizons displayed on strike view of fault. EARS5136 slide 16

17 Fault throw Four seismically mapped horizons displayed on strike view of fault. EARS5136 slide 17

18 Stratigraphic juxtaposition: relative reservoir quality I Relative reservoir quality index based on a scale normalized to lithological property seals have larger numbers. The larger index juxtaposed across the fault controls the seal and is displayed. seal Reservoir quality Reservoir against reservoir EARS5136 slide 18

19 Stratigraphic juxtaposition: relative reservoir quality II Relative reservoir quality index based on a high, med or low determination The juxtaposition combination of the reservoirs on either side of the fault are color-coded as shown. EARS5136 slide 19

20 Shale Gouge Ratio Juxtaposed reservoirs on either side of the fault are color-coded for SGR as shown. EARS5136 slide 20

21 Seal Comparison seal Reservoir quality High risk windows for fault seal juxtaposition may be sealed by shale gouge mechanism. EARS5136 slide 21

22 Juxtaposition diagrams EARS5136 slide 22

23 Juxtaposition Diagrams Rapid modelling of seal distributions possible Seismic mapping input not required initially Possible to analyse reverse faults, growth faults and variable FW/HW stratigraphy, but more difficult EARS5136 slide 23

24 Communication Map EARS5136 slide 24

25 Fault Throw Distributions EARS5136 slide 25

26 Fault Rock Type Map EARS5136 slide 26

27 Seismic Throw: Hangingwall Communication EARS5136 slide 27

28 Seal mapping & vertical continuity Separate Risk for : a) Faults linked to Zechstein b) Faults not linked EARS5136 slide 28

29 Overall Seal Workflow (1) Create depth structure map (2) Map fault activity and linkage (3) Evaluate reactivation risk and top seal (4) Undertake juxtaposition / seal mapping for faults trapping unreactivated prospects (5) Evaluate impact of seismic resolution, depth conversion etc. (6) Re-integrate with larger-scale tectonic / fluid flow evolution EARS5136 slide 29

30 Putting it all together.. the reservoir model Geocellular models of reservoir rock properties..but what about the faults? Models should attempt to capture fault properties but upscaling can be difficult Porosity model Gullfaks field EARS5136 slide 30

31 Fault Throw EARS5136 slide 31

32 Fault rock thickness EARS5136 slide 32

33 Stratigraphic juxtaposition EARS5136 slide 33

34 Fault rock permeability EARS5136 slide 34

35 Sand-Sand windows EARS5136 slide 35

36 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 EARS5136 slide 36

37 Manzocchi et al. (2002) EARS5136 slide 37

38 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 EARS5136 slide 38

39 Transmissibility multipliers and 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 EARS5136 slide 39

40 Fault zone hydraulic resistance Flow across a fault in reservoir models follows Darcy flow: The rate for linear flow is: q = (k/l) (A/η) (φ 1 - φ 2 ) 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. EARS5136 slide 40

41 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 / [(0.5L 1 / k 1 ) + (0.5L 2 / k 2 )] And transmissibility (T trans ) is K undeformed /L EARS5136 slide 41

42 Fault transmissibility with fault Average permeability for flow between adjoining cells with a fault is: k faulted = L / [0.5 (L 1 -L f ) / k 1 ] + [0.5 (L 2 -L f ) / k 2 ] + [L f / k f ] EARS5136 slide 42

43 Transmissibility multiplier - T Transmissibility with a fault is altered by transmissibility multiplier, T T trans = T (k undeformed /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 = k faulted /k undeformed This is the key relationship introduced into reservoir models. EARS5136 slide 43

44 Transmissibility multiplier - T The transmissibility multiplier is: where, T = k faulted /k undeformed k faulted = L / [0.5 (L 1 -L f ) / k 1 ] + [0.5 (L 2 -L f ) / k 2 ] + [L f / k f ] is a function of the fault permeability, k f and fault rock thickness, L f. The fault rock thickness is associated with the fault throw, L f. EARS5136 slide 44

45 Fault rock thickness Fault rock thickness scales with fault displacement EARS5136 slide 45

46 Manzocchi et al. (2002) EARS5136 slide 46

47 Fault rock permeability vs. clay content EARS5136 slide 47

48 Fault Zone Flow Transmissibility depends on cell size EARS5136 slide 48

49 Fault Zone Flow Transmissibility depends on cell size EARS5136 slide 49

50 Fault Rock Prediction: Heidrun field Knai & Knipe (1998) EARS5136 slide 50