Evaluation of flow anisotropy within a simulated shear fracture under stress conditions from shallow

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Evaluation of flow anisotropy within a simulated
shear fracture under stress conditions from
shallow to deep reservoirs
Evaluation of flow anisotropy within a simulated
shear fracture under stress conditions from
shallow to deep reservoirs
K. Nemoto, N. Watanabe, N. Hirano, and N.
TsuchiyaGraduate School of Environmental
Studies, Tohoku University
GRC 2007 Annual Meeting, Sparks, NV,
07/Sep.29-Oct.1, Oct.3, 1110-
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Introduction
Rock fractures Major flow paths in fractured
reservoirs (e.g.) Permeability for granite
Matrix 10-18 m2 ltlt Fracture 10-12
m2_at_Pc 100MPa
??
Flow properties in fractures (permeability, flow
field) Fundamental information that should be
considered for fluid migration in reservoirs

• Performance assessments of reservoirs
• Mechanisms of fluid-induced seismicity
• Material transport

Importance of anisotropic flow within single
rough fracture (Directional deviation in flow
properties) Yeo et al., 1998 Esaki et al.,
1999 Mitani et al., 2002, Auradou et al., 2006
Depth lt 500 m _at_overburden press.
sn 010 MPa
Need to evaluate for deeper ( higher stress)
conditions
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Contact area measurements (Nemoto et al., 2005)
Overburden press. _at_3-4 km depth
Tensile fracture (50 x 50mm)
Press. sensitive sheet (0.115mm thickness)
Shear offset, d 0, 1, 3 mm
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Normal stress / shear offset dependencies of the
HIA

• An increase with normal stress
• Convergence with normal stress (to 90 _at_no
  offset)
• A decrease with increasing offset (lt30 _at_3mm
  offset)

Increasing offset
Ratio of the HIA to apparent fracture area
(Nemoto et al., 2005)
Normal stress, sn MPa
gt Used for generating numerical aperture
distributions
HIA (Hydraulically Ineffective Area) Area where
aperture is less than 0.115 mm (sheet thickness)
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Procedure for generating aperture distribution

• Measure fracture surfaces (150 x 100 mm, tensile,
  Iidate granite)
• Apply shear offset, d (1, 3 mm)
• Extract aperture area (96.25 x 96.25 mm)
• Generate numerical aperture calculate of
  contact area
• Compensate the overestimate of contact area due
  to the sheet thickness

d 1, 3 mm
Offset direction
Anisotropy evaluation
From the contact area measurements (Nemoto et
al., 2005)
Flow fields
Flow simulation
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Generated aperture distributions
Anisotropic aperture distribution (contact area
orientated in the direction perpendicular to the
shear) ? Same as the contact area (HIA)
distribution detected Suggests - validity
of our technique - a general
characteristic in sheared (offset) fracture
Contact ratio analysis was carried out to verify
the anisotropic distribution
(black contact area)
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Contact ratio analysis - Definition -
Contact ratio, R cf. Mitani et al., 2002
Aperture distribution (385 x 385)
Measured line
ncont(x) Summation of points of contact area
on a measured line at x iN Summation of
points on a measured line (385)i Num of
element (1N)
Indicator of aperture connectivity on measured
lines
...
...
Contact ratio, R
R(x)
0
1
...
...
Aperture connectivity
x
No connection (close)
1 2 3 ... i ... N
Good (open)
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Contact ratio analysis - Results -
Shear offset
Anisotropy in the aperture distributions can be
observed
(b) Perpendicular()
e.g.) d 1 mm, sn 1090 MPa
(a) Parallel (//)
a) Parallel (//)
b) Perpendicular ()
sn
sn
sn

• Directional difference in R ? Anisotropic
  aperture distribution
• Low R (well-connected aperture) ? Possible major
  flow paths
• Low R outstands for high sn ? Aperture
  connection remains
• (In the case of the direction)

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Flow simulation

• 2D D/SC simulator Watanabe et al., 2005, GRC
  Trans.
• Solve Reynolds eq. by finite difference method

Governing eq.
Boundary conditions
0
Perpendicular ()
eij Local aperture Pij Local fluid
pressure µ Viscosity of fluid (1.0 x 10-3
Pas pure water _at_20oC) i, j Num. of elements
Parallel (//)
Conditions

• Constant pressure differential 1 MPa (In 1
  MPa, Out 0)
• No inflow outflow from the side walls
• Aperture _at_contact area 0.001mm
• Incompressible fluid
• Flow field in the steady state

1 MPa
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Results from the flow simulation
(a) (Shear) // (Flow)
Shear
Flow
More tortuous flow paths than the flow
perpendicular to the shear
(b) (Shear) (Flow)
Shear
-
Flow
Common features - Convergence of major paths
with sn ? - Multiple paths with d ?
(Normalized using the max. flow rate in each
distribution)
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Anisotropy in fracture permeability
Comparison of fracture permeability
3mm
k// lt k(1 order of magnitude)
Shear
Parallel
Increase in sn
Direction perpendicular () to the shear
1mm
Shear
(k k//)
Less tortuous flow paths were observed
Permeability anisotropy ? Flow path
tortuosity (? Aperture connectivity)
Perpendicular
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Stress dependence of the permeability anisotropy
Permeability ratio, K ( k /k// )
Indicator of permeability (k) anisotropy
( k/ k// )
Shear offset
1 mm
3 mm

• K ? with sn ? Increase in permeability
  anisotropy with depth
• K ? with d ? Larger offset ? Fracture opened
  (higher perm.)

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Interpretation for fluid migration in fractured
reservoirs

• Permeability anisotropy as an important factor
  controlling fluid migration in fractured reservoir

Dimensional extension of fractured media model
(2D ? 3D) ? Anisotropy in fracture permeability
should be considered
2D
Discrete fracture network (DFN) model (e.g.)
Watanabe and Takahashi (1995) Watanabe et al.
(2000) Min et al. (2004)
Permeable direction due to the flow anisotropy
Extension
??
?
Major paths

• Importance of permeability anisotropy in deeper
  reservoirs

Permeability anisotropy of a fracture

• presents at sn 90MPa.
• shows sn (depth) dependency.

Fluid migration in deeper reservoirs (gt 90 MPa
overburden press. gt3-4 km depth)? Possibility
to be affected by the enhanced flow anisotropy
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Conclusions

• Evaluation of flow anisotropy in shear offset
  fracture on the basis of the contact area
  measurements flow simulation shows

• Anisotropic flow distributions (path tortuosity)
• Permeability (k) anisotropy (k// lt k by an order
  of mag. in max.)
• sn dependence in the k anisotropy
• As an interpretation,
• ? Importance of the permeability anisotropy on
  fluid migration in 3D fractured reservoir at
  the depths up to 3-4 km (90MPa) and more

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Thank you for your attention

Questions? Comments?
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sn dependence of fracture permeability
Shear offset
Perpendicular
3 mm
1 mm
Parallel
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Results of the contact area measurements
Contact area (HIA)
Open area (aperture)
(Different sample for each offset condition)
Shear offset direction
Shear offset, d mm
HIA Hydraulically Ineffective Area
Normal stress, sn MPa
(Nemoto et al., 2005)
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