Faults as Fluid Flow Barriers and Their Role in Trapping Hydrocarbons

1
Faults as Fluid Flow Barriers and Their Role in
Trapping Hydrocarbons

• Suzanne Coogan
• Richard Nice
• Ayeni Gboyega
• Kate Carter-Walford

2
Introduction

• Fault seal mechanisms
• Influence on hydrocarbon fields
• Modelling and Flow Properties
• Case Studies

3
How Can Faults Create Barriers to Fluid Flow?

• Juxtaposition
• Cataclasis
• Cementation
• Clay Smearing

4
Juxtaposition
Juxtaposition of lithologies with different
permeabilities across the fault
5
Juxtaposition
Coarse grained material (light colour)
Analogous to a reservoir being juxtaposed against
a sealing lithology
Fine grained material (dark colour)
6
Cataclasis
Cataclastic grain-size reduction results from
abrasion during deformation smaller grains fill
pore space and reduce porosity

• Effectiveness depends on
• The hardness of the intact rock
• The magnitude of displacement
• Initial normal stress on the fault surface prior
  to movement

7
Grain Size Reduction andMica Orientation
Processes Shearing Mica Orientation Clay
formation Fluid flow
8
Cementation

• Minerals carried in solution in water under high
  pressure
• As fault opens, pressure release occurs, water
  flows through fault and minerals precipitate out
  of solution
• Crystallisation of cements in pore spaces reduces
  permeability

Cement
Reduced grain size
9
Clay Smearing
Layers of shale contained within sequence are
drawn into fault as movement along plane
progresses Impermeable layer formed along
fault depending on Shale Gouge Ratio (SGR)
ratio of sand to shale
10
Shale Gouge Ratio
This measure is useful for predicting sealing
ability of fault. 18 30 indicates high
probability of a seal The SGR is the percentage
of shale within a part of the sequence which has
moved past a point on the fault surface
11
CSP smear factor definition

• SSF lt3
• Continuous smears.
• SSF 3 -10
• 60 Continuous.
• SSF gt10
• 70 Discontinuous.

Yielding et al. (1997)
12
Shale Gouge Example
13
Shale Gouge Ratio
As SGR increases, sealing ability improves. The
clay has a small pore throat size and therefore
high capillary entry pressure With smaller gouge
ratios, brittle fracture and therefore
cataclasis dominates. Sealing is less effective
than clay smear
14
Examples of Clay Smearing
These examples show 3 faults in outcrop that
range from sand-prone  to shale-prone gouge and
an intermediate sand/shale ratio gouge.  These
faults demonstrate a spectrum of gouge
composition and of seal behaviour
15
Clay Smearing on Microscopic Scale
16
Effectiveness of Fault Sealing Mechanisms
17
Sealing Capacity of Faults

• Hd 2gh(rt-1-rp-1)/g(rw-rh)
• rt pore throat radius in the seal
• rp pore throat radius in the reservoir
• gh hydrocarbon-formation water interface
  tension
• (Oil 5-35 dynes/cm Gas 30-70
  dynes/cm).
• rw density of the formation water (1 1.2
  gm/cm3)
• rh density of the hydrocarbon phase (Oil
  0.5 1.0 gm/cm3 Gas 0.2 - 0.4 g/cm3)
• g acceleration due to gravity

18
Modelling

• Empirical methods for risking the sealing
  potential of faults have been devised in
  combination with outcrop and laboratory studies
• Estimates the sealing potential of a fault
  offsetting a particular sequence and therefore
  the entry pressure
• For a detailed model
• identify where a fault is sealed and where
  leakage may occur
• establish where significant pressure
  differences are likely to be
• supported across a fault surface and
  their magnitude
• understanding of the migration pathway and
  the column height
• Can use models to predict the flow and flow
  restrictions of hydrocarbons due to fault
  properties using a programs such as the SEMI
  migration model or TransGen
• Such modelling can only be achieved by 3-D
  analysis of the geometry and sealing
  characteristics of faults

19
Calculating fault properties

• Calculate the Shale Gauge Ratio on the fault
  surface and convert to fault seal potential
• Calculate the reservoir elevation at fault traces
  
• Construct a sequence/throw juxtaposition diagram
  form log and lithological information
• Input sequence containing shale values and
  reservoir offset
• In this example, throw is between zero and
  thickness of sequence - triangular plot
• Plot is annotated according to SGR

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What Controls Seal Effectiveness and Fluid
Transmissibility

• Hydrostatic / Capillary
• Buoyancy control
• Hydrodynamic / Capillary
• Fluid pressure gradient control
• Hydrodynamic / Open Fractures
• Network Geometry, aperture and pressure gradient
  control

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Column Heights

• Two distinct geometries have to be considered
  when estimating oil column heights
• When fault throw is less than the thickness of
  the reservoir and the reservoir is self
  juxtaposed. Column height is determined by the
  threshold pressure
• When the fault throw exceeds the thickness of the
  carrier interval, oil leakage occurs along the
  fault. Leakage can occur along fault surface if
  fault is not sealing
• Column Height is determined by the depth (Z),
  density of oil and water (po, pw) and gravity (g)

22
Potential Oil Column Heights
1000
Oil column heights
100
Shale smear / cementation
Cataclasites
10
Fine sandstone
1
Coarse sandstone
.1
-6
-5
-2
-3
-4
Pore throat radius (log cm)
23
Migration
Migration is the movement of hydrocarbons though
rock pores and fault networks Migration is
driven by buoyancy and resisted by capillary
pressure Leakage occurs when the entry pressure
(Pe) equals the pressure of oil and water (Po, Pw)
Po
Pw
Po
24
Flow Model Predictions

• It is possible to predict fault properties and
  attach them to flow models, eg. TransGen
  (Mansocchi et al 1999)
• Attach a transmissibility multiplier a unique
  property attached to the face of a grid block
• Transmissibility is assessed using the length of
  the block (Lg), the permeability (k) and the
  fault thickness (tf)
• Transmissibility of 0 is sealing, 1 is for
  unimpeded cross-flow

25
Applications of Modelling Fault Sealing

• Using different fault sealing properties to
  perform migration modelling
• In this example the following conditions exist
• reservoir contains known hydrocarbon
  accumulations
• different hydrocarbon-water contact levels in
  adjacent fault blocks
• a dry fault-bounded structural high exists

• All indicate that faults and fault sealing play a
  key role in hydrocarbon distribution and
  migration
• Different fault seal properties result in
  different hydrocarbon distributions and migration
  pathways

26
Larger accumulation of hydrocarbons due to
sealing capacity of bounding faults with little
excess spilling to east
All faults are open with no fault sealing
small accumulation in east and spills to the south
27
Ninian Field - Juxtaposition

• Lower Jurassic marine shale, Dunlin Group,
  overlain by Middle Jurassic Brent Group, a prime
  reservoir in the area
• The Kimmeridge Clay acts as a cap to the
  formation and is an excellent hydrocarbon source
• Several faults place the Brent Group against the
  older Dunlin Group
• In the horst block the Brent Group is faulted
  against the Kimmeridge clay

28
Moab Fault - Juxtaposition

• Due to exposure at surface lithologies can be
  defined using field and published data.
• The lithological descriptions can be used to
  model the geometry of the fault yielding
  juxtaposed seal analysis.
• Sand units in the hangingwall and footwall are
  seen sealed due to the fault.
• Areas with less displacement represent leak
  points along the fault.

29
Moab Fault - SGR

• Modeling of shale gouge ratios along the fault
  are consistent with field observations.
• Greatest SGR where displacement is greatest

• Juxtaposition predicted pathways in the north
  remain
• Those in the central region are sealed by clay
  smear

30
Entrada Sandstone Cementation

• Hydrocarbon bearing reducing fluids
• Cementation occurred post faulting and was
  coeval with but not related to hydrocarbon
  migration
• Calcite cementation occurred around faults but
  the faults were not conduits
• Cements related to the faults acted as ephemeral
  seals causing fluid pressure fluctuations
• Increased pressure due to ponding of hydrocarbons
  caused dissolution of earlier calcite deposits
• subsequent pressure release resulted in
  exsolution of gaseous CO2, forming calcit
  structures

31
Artemis Field, North Sea

• The Artemis field is much smaller scale than the
  Moab Fault region
• Accumulations of gas migrate through the
  reservoir toward high south eastern corner
• .however gas also accumulates in the footwall
  and hangingwall of faults
• No one fault has produced a seal for hydrocarbons
  but the combined result of the many faults has
  created many local trapping geometries.

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Conclusion

• Faults as Fluid Flow Barriers and Their Role in
  Trapping Hydrocarbons
• In this presentation we have briefly shown how
  faults serve as fluid flow barriers by forming
  low transmissivity membranes, and their further
  role in trapping hydrocarbons by juxtaposing
  lithologies of differing permeabilities.

33
References

• The Moab Fault, Utah, U.S.A. - A
  Three-Dimensional Approach to Fault Seal and
  Hydrocarbon Flow Pathway Modelling - S.M. Clarke,
  S.D. Burley G.D. Williams
• The 3D fault segmentation development A
  conceptual model. Implications on fault sealing
  A. BENEDICTO1, T. RIVES2 AND R. SOLIVA1- EAGE, In
  Proceedings Fault and Top Seals, Extended
  Abstracts volume, ISBN 90-73781-32-9,
  Montpellier, September 2003
• A Method for Including The Capillary Properties
  of Faults in Hydrocarbon Migration Models O
  Sylta, C Childs, S Moriya, JJ Walsh, T Manzocchi
• An Exhumed Paleo-Hydrocarbon Migration Fairway In
  a Faulted Carrier System, Entrada Sandstone of SE
  Utah, USA Garden, Guscott, Burley, Foxford,
  Walsh and Marshall
• Knipe, R.J., Jones, G., and 1998 Fisher, Q.J.
  Faulting fault sealing and fluid flow in
  hydrocarbon reservoirs An introduction. In
  Faulting Fault Sealing and Fluid Flow in
  Hydrocarbon Reservoirs, edited by Jones, G., 1998
  Fisher, Q.J andKnipe, R.J. Geological Society of
  London Special Publication 147, p 7-21