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.

Wouter van der Zee ([email protected])

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

20

30

40

50

60

70

80

90

100

0 50 100clay % in fault zone

SG

R

0

10

20

30

40

50

60

70

80

90

100

0 5 10 15clay thickness (mm)

SG

R0

10

20

30

40

50

60

70

80

90

100

0.1 1 10 100 1000faultzone thickness)

SG

R

y = 1.2957x

R2 = 0.9955

0

10

20

30

40

50

60

70

80

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

20

25

30

35

40

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 ?