Theme 6: INTEGRATION OF STRUCTURAL DATA AND RESERVOIR MODELS

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Theme 6INTEGRATION OF STRUCTURAL DATA AND
RESERVOIR MODELS
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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 Properties Cell Size
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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.
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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/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.

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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

No fault
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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

With fault
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Transmissibility multiplier - T

• Transmissibility with a fault is altered by
  transmissibility multiplier, T
• Ttrans T (kundeformed/L)
• for no fault T1 and for a completely sealing
  fault T0
• 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 thickness
Fault 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)
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Fault rock thickness
Geocellular models for reservoir modeling
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Fault rock permeability
Geocellular reservoir models
Geologists provide reservoir engineers input
along faults for modeling.
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Threshold Pressures and Flow Modeling
Constant Fault Rock Properties
P gt Pth Fault leaks
Sealing Capacity
P lt Pth Fault seals No Flow. Permeability based Tr
ansmissibility not applicable, Water filled fault.
Base Hydrocarbon
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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.

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Cumulative fault rock thickness
Cataclastic faults in porous sandstones build up
cumulative fault rock thickness
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Fault Transmissibility Assessment
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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
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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.

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Model attributes

• Position, length, width, strike, dip, aspect
  ratio
• Clustering technique hierarchical
• Throwthickness and throwlength 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

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A Two-Dimensional Illustration
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Collapsed Fault Rock Thickness

• Hierarchical Clustering Technique

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Two-Dimensional Horizontal Slices

• Hierarchical Clustering Technique

Fault Spacing Along 1D Traverse
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Modelled Volume of Interest
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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.

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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.

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Transmissibility Multiplier

• Permeability ratio ? 3334