FAULT SEAL ANALYSIS: Mapping & modelling EARS5136 slide 1
Hydrocarbon field structure Compartments 1 km Depth ~2.5km How to produce field? EARS5136 slide 2
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
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
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
Allan diagrams Footwall template > < Hangingwall template EARS5136 slide 6
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
Allan Diagrams: Bed-Fault Intersections EARS5136 slide 8
Seismic data in juxtaposition analysis Example showing modelled fault surface with stratigraphic juxtapositions EARS5136 slide 9
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
Snapping horizons to faults EARS5136 slide 11
Impact of Seismic Data Interpretation on Resolution and Quality of Allan Diagrams EARS5136 slide 12
Impact of Seismic Data Interpretation on Resolution and Quality of Allan Diagrams uncertainty EARS5136 slide 13
Impact of branch-lines from intersecting faults uncertainty Branch-lines EARS5136 slide 14
Complex Fault Plane Mapping Intra-formational erosion / pinchout Fig 6-41 S N 1200 1340m 1400 1600 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
Seismic horizon juxtaposition example Four seismically mapped horizons displayed on strike view of fault. EARS5136 slide 16
Fault throw Four seismically mapped horizons displayed on strike view of fault. EARS5136 slide 17
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
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
Shale Gouge Ratio Juxtaposed reservoirs on either side of the fault are color-coded for SGR as shown. EARS5136 slide 20
Seal Comparison seal Reservoir quality High risk windows for fault seal juxtaposition may be sealed by shale gouge mechanism. EARS5136 slide 21
Juxtaposition diagrams EARS5136 slide 22
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
Communication Map EARS5136 slide 24
Fault Throw Distributions EARS5136 slide 25
Fault Rock Type Map EARS5136 slide 26
Seismic Throw: Hangingwall Communication EARS5136 slide 27
Seal mapping & vertical continuity Separate Risk for : a) Faults linked to Zechstein b) Faults not linked EARS5136 slide 28
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
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
Fault Throw EARS5136 slide 31
Fault rock thickness EARS5136 slide 32
Stratigraphic juxtaposition EARS5136 slide 33
Fault rock permeability EARS5136 slide 34
Sand-Sand windows EARS5136 slide 35
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
Manzocchi et al. (2002) EARS5136 slide 37
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
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
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
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
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
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
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
Fault rock thickness Fault rock thickness scales with fault displacement EARS5136 slide 45
Manzocchi et al. (2002) EARS5136 slide 46
Fault rock permeability vs. clay content EARS5136 slide 47
Fault Zone Flow Transmissibility depends on cell size EARS5136 slide 48
Fault Zone Flow Transmissibility depends on cell size EARS5136 slide 49
Fault Rock Prediction: Heidrun field Knai & Knipe (1998) EARS5136 slide 50