Geomechanical modeling of faults in layered sequences · • Yielding, G., B. Freeman, and T. Needham, 1997, Quantitative Fault Seal Prediction: AAPG Bulletin, v. 81, p. 897 – 917.
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Wouter van der Zee (wvanderzee@geomi.com)
Geomechanical modeling of faults in
layered sequences
NKAM Symposium
October 16th 2009
Acknowledgements
Janos Urai
Marc Holland
Martin Brudy
Endogne Dynamik @ RWTH Aachen
DFG
GeoMechanics International
Outline
– Faults in layered sequences
– Clay smear
• Statistics
• Lateral Clay Injection
– Effect of clay on fault stability
network of deformation
bands
Mature Fault Gouge
“Clay Smear”
• loosely defined term for all processes which transform clay in the wall rock into clay in the fault
• knowing the clay content of the fault gouge is
important because the clay has a large effect on:
– transport properties (fault seal)
– strength
“Clay Smear”
Shale Gouge Ratio
• Yielding, G., 2002, Shale gouge ratio— Calibration by geohistory, in A. G. Koestler and R. Hunsdale, Hydrocarbon seal quantification: Amsterdam, Elsevier, Norwegian Petroleum Society (NPF) Special Publication 11, p. 1– 15.
• Yielding, G., B. Freeman, and T. Needham, 1997, Quantitative Fault Seal Prediction: AAPG Bulletin, v. 81, p. 897– 917.
Bretan et al. AAPG Bulletin, v. 87,
no. 3 (March 2003), pp. 397–413
Shale Gouge Ratio, Airportroad Outcrop, Miri
0
10
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0 50 100clay % in fault zone
SG
R
0
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0 5 10 15clay thickness (mm)
SG
R0
10
20
30
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100
0.1 1 10 100 1000faultzone thickness)
SG
R
y = 1.2957x
R2 = 0.9955
0
10
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30
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0 20 40 60 80
SGR
Cla
y %
Shale Gouge Ratio, Airportroad Outcrop, Miri
Lateral Clay Injection
lignite mine Hambach, Germany
Field Example of Lateral Clay Injection (I)
Field Example of Lateral Clay Injection (II)
Lateral Clay Injection Enhancedby Squeezing Blocks
Finite Element Analyses: Mesh
• Injection:no pull-apart
plasticity in
clay
• No injection:
pull-apartno plasticity Open void
Horizontal and Vertical Displacement due to Clay Injection
0
20
40
60
80
0 10 20 30 40
Friction Angle (deg)
Co
he
sio
n (
Mp
a)
INJECTION
NO INJECTION
Sensitivity Analyses
Clay Injection Criterion
• constant σv at fixed depth
• σh decreases in “pull apart”
• injection criterion: σ3 =0
σ1
σ3
τ
( )( )φ
φσ
cos2
sin11
−⋅=C
Clay Injection in Mohr Space
• σσσσ3
– reduced near pull-apart but >0 at interface
• σσσσ1
– also reduced near pull-apart
-40
-90
0
150
Mechanical Clay Injection Potential
Lateral Clay Injection
• first order conditions for clay injection are:
– kinematic:
• fault must have a releasing bend or step
– mechanical:
• clay must be weaker than the sand
• clay must flow under overburden load
• by ongoing deformation forming of squeezing block which enhance injection
Effect of clay smear on fault stability
residual friction angle vs. clay fraction
0
5
10
15
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30
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0 20 40 60 80 100
Clay fraction (%)
fric
tion
ang
le (
deg)
Beyerlee 1978
Fault Stability - introduction
pp
σn
τ
Fault Stability - introduction
pp
σn
τ
Pore pressure, Sv, SHmax,
Shmin, SHmaxAziGeomechanical model
Stress Tensor
Stress on the fault
patch
Fault Stability - introduction
Stress on the fault
patches
pp
σnτ
Fault Stability (I)
CFF
Coulomb failure function
CFF=τ−µσn
<<0: stable
Close to 0: sliding
Fault Stability (II)
PPcrit
Critical pore pressure
(absolute/EMW)
“…at which pore pressure
will the fault slip*?”
Fault Stability (III)
IPcrit
Critical injection pressure
(absolute/EMW)
“What is the difference between
the pore pressure and the
critical pore pressure*?”
Fault Stability (IV)
TAUratio
Actual shear stress/critical
shear stress
TAUratio = 1: slip*
TAUratio <1: “stable*”
Homogeneous Friction Coefficient
Friction coefficient = 0.65
Varying Friction Coefficient
Friction coefficient high = 0.65, low = 0.35
Questions ?
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