FAULT SEAL ANALYSIS: Mapping &...

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FAULT SEAL ANALYSIS:Mapping & modelling

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Hydrocarbon field structure

Compartments

How to produce field ?

1 km

Depth ~2.5km

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Predict flow patterns and communication

Fault compartments in the Sleipner field, Norwegian North Sea

Different oil-water contacts

Ottesen Ellevset et al. (1998)

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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

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Fault Seal WorkflowDefine geometry of fault array

Test models against hydrocarbon contact levels if known

Assess sealing mechanisms and fault rock properties

Evaluate juxtapositions and seal distributions

Map seal distributions on fault planes which might form compartment boundaries

Establish sub-seismic fault density and fault zone structure

Model reservoir flow and impact of faults on drainage patternsModel reservoir flow and impact of faults on drainage patterns

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Allan diagramsFootwall template >

< Hangingwalltemplate

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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

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Allan Diagrams: Bed-Fault Intersections

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Seismic data in juxtaposition analysis

• Example showing modelled fault surface with stratigraphicjuxtapositions

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Do we assess fault juxtapositions correctly?

• Allan Maps Accuracy; – Horizon uncertainty: +20m to -20m– Fault Uncertainty: ~100m

• Assume single fault, not complex damage zone

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‘Snapping’horizons to faults

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Impact of Seismic Data Interpretation on Resolution and Quality of Allan Diagrams

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Impact of Seismic Data Interpretation on Resolution and Quality of Allan Diagrams

uncertainty

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Impact of branch-lines from intersecting faults

uncertainty

Branch-lines

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Complex Fault Plane Mapping

1200

1400

1600

1800

1 kmIntra F Fault Juxtapositions

S

Depth (m)

N

Fig 6-41

1340m

UPPER ÅRE / LOWER ÅRE

F HW

BCU FW

BCU HW

BCU FW

BCU HW

F FW

F FW

INTRA ÅRE FW

F2

F6

TOP Å FW

F HW

TOP Å HW

TOP Å HW

TOP

Å HW

TOP ÅRE HW

TOP ÅRE HW

INTRA ÅRE FW

INTRA ÅRE FW

INTRA ÅRE HW

INTRA

ÅREFW

IN

TRA

ÅRE

HW

INTRA ÅRE HW

INTRA ÅRE FW

Upper Åre / Lower Åre

Upper Åre / Upper Åre

Fangst in HW / Lower Åre in FW

Fangst in HW / Upper Åre in FW

Fangst / Fangst

Upper Jurassic in HW

BCU in HW

Erosional Contact

INTRA ÅRE HW

TOP ÅRE HW

TOP ÅRE FW

F HW

F FW

BCU HW

BCU FW

• Intra-formational erosion / pinchout

BCU in HW

U Jur in HW

Fangst / Fangst

Fangst HW / Up Are FW

Fangst HW / Lr Are FW

Up Are HW / Up Are FW

Up Are / Lr Are FW

Erosional contact

Erosion and30m Seismic Resolution

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Seismic horizon juxtaposition

Four seismically mapped horizons displayed on strike view of fault.

example

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Fault throw

Four seismically mapped horizons displayed on strike view of fault.

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Stratigraphic juxtaposition: relative reservoir quality I

Reservoir quality

seal

Reservoir against reservoir

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.

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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.

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Shale Gouge Ratio

Juxtaposed reservoirs on either side of the fault are color-coded for SGR as shown.

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Seal Comparison

Reservoir qualityseal

High risk windows for fault seal juxtaposition may be sealed by shale gouge mechanism.

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Juxtaposition diagrams

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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

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Communication Map

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Fault Throw Distributions

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Fault Rock Type Map

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Seismic Throw: Hangingwall Communication

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Seal mapping& vertical continuity

Separate Risk for :

a) Faults linked to Zechsteinb) Faults not linked

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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

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Putting it all together …..the reservoir model

Porosity modelGullfaks field

Geocellular models of reservoir rock properties…..but what about the faults?

Models should attempt to capture fault properties but upscaling can be difficult

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Fault Throw

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Fault rock thickness

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Stratigraphic juxtaposition

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Fault rock permeability

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Sand-Sand windows

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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

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Manzocchi et al. (2002)

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Fault zone transmissibility

Fault Rock Thickness

Fault Rock Permeability

Transmissibility(Perm x Fault rock thickness)

Hydraulic Resistance(Fault rock thickness / Perm)

Matrix PropertiesCell Size

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Only Cross-fault cells used :- No along fault flow

considered- No Threshold Capillary Pressure considered

Separate cells for faultsallows along fault flow evaluation

Transmissibility multipliersand flow modeling

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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) (φ1 - φ2)

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.

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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

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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]

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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.

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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.

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Fault rock thicknessFault rock thickness scales with fault displacement

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Manzocchi et al. (2002)

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Fault rock permeability vs. clay content

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Fault Zone Flow

Transmissibility depends on cell size

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Fault Zone Flow

Transmissibility depends on cell size

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Fault Rock Prediction: Heidrun field

Knai & Knipe (1998)