structural geology application in petroleum industry

GEOLOGIST

PRACTICAL USE AND REFERENCE

INTRODUCTION STRUTURAL GEOLOGY

GEOMECHANIC

FAULT SEAL ANALYSIS

OUTLINES

Structural Geology : Study deformation of the rocks

Deformation = Changes in rocks caused by Force

Force = Stress (s)

Changes (Place, Length, Volume, Angle) = Strain (e)

INTRODUCTION STRUTURAL GEOLOGY

Term for Stress and Strain

*) Important distinction between two quantity

STRESS (s) AND STRAIN (e)

STRESS (s)

SEDIMENTARY ROCKS DEFORMATION

Twiss and Moores, 1992

STUDY ROCK

DEFORMATION

FORCES AND VECTORS

• Force is any action which alters, or tends to alter • Newton II law of motion : F = M a • Unit force : kgm/s2 = newton (N) or dyne = gram cm/s2; N = 105 dynes

(a). Force: vector quantity with magnitude and direction

(b). Resolving by the parallelogram of forces

Modified Price and Cosgrove (1990)

Two Types of Force

• Body Forces (i.e. gravitational force)

• Contact Forces (i.e. loading)

Stress defined as force per unit area:

s = F/A

A = area, Stress units = Psi, Newton (N), Pascal (Pa) or bar (105 Pa)

(Davis and Reynolds, 1996) (Twiss and Moores, 1992)

DEFINITIONS

• Stress at a point in 2D

• Types of stress

Str

ess

(s

)

Normal stress (sN)

(+) Compressive (-) Tensile

Shear stress (sS)

(+) (-)

NOMENCLATURES

STRESS ON A PLANE AND AT A POINT

Stress Tensor Notation

s11 s12 s13

s = s21 s22 s23

s31 s32 s33

NOMENCLATURES

Stress Ellipsoid

a) Triaxial stress

b) Principal planes of

the ellipsoid

(Modified from Means, 1976)

Stress Ellipsoid

FUNDAMENTAL STRESS EQUATIONS

Principal Stress:

s1 > s2 > s3

• All stress axes are mutually

perpendicular

• Shear stress are zero in the

direction of principal stress

Stress Tensor Notation

s11 s12 s13

s = s21 s22 s23

s31 s32 s33

s12 = s21, s13 = s31, s23 = s32

sn

r

n

(p) sn (p)

s s

2

2

s s 3

2

s s 3

sn

sn ,

(p)

s

(p)s s

s s 2 3 cos

2

s s 2 3 sin

ss

2x3

(p)s s

(p)s n

s3

s

Plane P

x

s3

s1 + s3 - s1 – s3 sN = cos 2

2 2

ss = Sin 2 s1 – s3

2

Stress Equation:

Mohr Diagram 2-D

Mohr Diagram 3-D

(Twiss and Moores, 1992)

Geometry of a three-dimensional

Stress on a Mohr diagram

• Mohr diagram is a graphical representative of state of stress

• Mean stress is hydrostatic component which tends to produce dilation

• Deviatoric stress is non hydrostatic which tends to produce distortion

• Differential stress, if greater is potential for distortion

(Davis and Reynolds, 1996)

STATE OF STRESS

Relationship Between Stress and Strain

• Evaluate Using Experiment of Rock

Deformation

• Rheology of The Rocks

• Using Triaxial Deformation Apparatus

• Measuring Shortening

• Measuring Strain Rate

• Strength and Ductility

(Davis and Reynolds, 1996)

TRIAXIAL TEST

(Modified from Park, 1989)

Deformation and Material

A. Elastic strain

B. Viscous strain

C. Viscoelastic strain

D. Elastoviscous

E. Plastic strain

Hooke’s Law: e = s/E, E = Young Modulus or Elasticity

Newtonian : s = he, h = Viscosity, e = Strain-Rate

BRITTLE AND DUCTILE

DEFORMATION MECHANISM

Stress (s) Strain (e)

STRESS vs. STRAIN

SUMMARY OF DEFORMATION

FRACTURES AND FAULTS

SUBSURFACE

FRACTURES IN IMAGE LOG

GMI (2001)

FRACTURES IN SANDSTONE

OUTCROP/SURFACE

FRACTURES MECHANICS

sc = so + tan (sn)

The Coulomb Law of Failure

sc = critical shear stress

so = cohesive strength

tan = coefficient

of internal friction

sn = normal stress

(Modified from Davis and Reynolds, 1996)

Compressive Fractures

ROLE OF FLUID PRESSURE (Pf)

Effective stress (s*)

sn* = sn - Pf

sc = so + tan (sn- Pf)

If sn* = 0

sc = so + tan (sn*)

sc = so

sc = Critical stress

so = Tensile strength

of rock

(Twiss and Moores, 1992)

FAULT GEOMETRIES AND CLASSIFICATION

• Anderson’s Dynamic Fault Classification

• Separation Classification

• Slip Classification

Foot wallblock

Rotationalfaults

Hanging wallblock

F. Sinistral-reverse

Foot wallblock

G.E. Sinistral-normal

Hanging wallblock

Oblique-slipfaults

Dip-slipfaults

Dip-slipfaults

B. Thrust D. Left-lateral, or sinistralA. Normal C. Right-lateral, or dextral

Smax = s1

SInt = s2

Smin = s3

s<s2<s3

FAULT AND STRESS

ANDERSON (1951) FAULT CLASSIFICATION

1 m

0.1 m

FAULT AND FAULT ZONES

Clay Gouge

FAULT BRECCIA

FAULT SYSTEMS AND CLASSIFICATIONS

Characteristics of Faults and Fault Systems

1. Fault are dynamic structures that evolve in both space and time.

2. Faults commonly occur in linked systems.

3. Faults are not simple planar surface but may show complex

shape change in 3-D (i.e. in plan and in cross-section).

4. Deformation by brittle faulting in the upper crust must be

balanced by plastic deformation in the lower crust.

5. Faults and fault systems are fundamental in the location and

deformation of orebodies.

6. Faults are dilatant (volume increase) zones in the crust that focus

fluid flow.

TECTONIC REGIMES FOR FAULTING

• CONTRACTIONAL REGIME

• Collision Orogens

• Subduction Orogens

• Inversion Orogens

• Foreland Fold and Thrust Belts

• Accretionary Wedges

• EXTENSIONAL REGIME

• Extensional Orogens

• Rift Systems

• Passive Continental Margins

• Extensional Collapse Basins

• Delta System

• Salt Terranes

Modified from McClay (1997)

•STRIKE-SLIP REGIME

• Oceanic Transform Zones

• Intracontinental (Intra-Plate) Strike-Slip Zones

• Trench-linked Strike-Slip Zones

• MIXED MODE TECTONIC REGIMES

• Transtensional

• Transpressional

• Positive Inversion – Extension Followed by

Contraction

• Negative Inversion – Contraction Followed by

Extension

• Multiple – Oblique Slip

Reservoir Geomechanics is the integrated study of the state of stress, pore pressure and physical properties of reservoirs, natural fractures/faults, cap rocks and the formations in the overburden. • Interactions between geological conditions and engineering and production practices The state of in-situ stress Rock strength Bedding orientation Pore pressure Distribution of fractures and faults Wellbore trajectory Mud weight

(Castillo and Moos, 2001)

Definitions

GEOMECHANIC

Geomechanics ???

Drilling Engineering

• Drilling to Reduce Cost and Formation Damage • Hydrofrac Propagation • Well Placement (Azimuth and Deviation, Sidetracks) • Wellbore Stability During Drilling (mud weights, drilling direction) • Long-term Reservoir Stability (Sand Production)

Geology

• Fault Seal Integrity • Reservoir Compartmentalization • Optimizing Drainage of Fracture Reservoirs • Hydrocarbon Migration

Utilizing Borehole Failure to Constrain Stress Magnitude and Rock Strength

Compressional

• Borehole Breakouts • Incipient Borehole Breakouts

Tensile

• Tensile Wall Failure • Drilling Enhanced Natural Fractures

Shear

• Slip on Pre-Existing Faults and Bedding

Borehole Breakouts

COMPRESSIVE WELLBORE BREAKOUTS

DETERMINING SHmin

CALCULATION SHmin

DRILLING INDUCED TENSILE FRACTURE

GEOMECHANICAL MODELING

FAULT, FRICTION AND STRESS

FAULT AND STRESS

Utilizing Borehole Failure to Constrain Stress Magnitude and Rock Strength

Compressional

• Borehole Breakouts • Incipient Borehole Breakouts

Tensile

• Tensile Wall Failure • Drilling Enhanced Natural Fractures

Shear

• Slip on Pre-Existing Faults and Bedding

FAULT SEAL ANALYSIS INJECTOR PRODUCER

Reservoir Target

• Poor Sweep Efficiency

• Fault Stability

Strike-slip

Fault plane

Oblique-slipDip-slip

Oblique-slipDip -slip

Heave

Horizontal component

Str ike-slip

component

Verticalcomponent

Throw

FAULT ATTRIBUTES

Foot wallblock

Rotationalfaults

Hanging wallblock

F. Sinistral-reverse

Foot wallblock

G.E. Sinistral-normal

Hanging wallblock

Oblique-slipfaults

Dip-slipfaults

Dip-slipfaults

B. Thrust D. Left-lateral, or sinistralA. Normal C. Right-lateral, or dextral

FAULT TYPES

Twiss and Moore (1992)

Geologist/Explorationist

Faulting can act as impermeable barrier to hydrocarbons and creates a trap (i.e. fault sealing and compartmentalization).

Sealing faults may also act as barrier for hydrocarbon migration.

Reservoir Engineer (PE)

A Sealing faults is not necessary an impermeable barrier but may have reduced permeability and affect fluid flow during production life of a field.

Faults and fault zones are sealing membrane in which their properties are related to capillary entry pressure of the membrane.

TWO VIEWPOINTS OF FAULT SEALING

• The effect of faulting on migration and entrapment depends on rock properties of strata juxtaposed by fault • Material within the fault zone can be act as effective barriers where reservoir facies are juxtaposed

Fault-Seal Prediction in Exploration

1 m

0.1 m

FAULT AND FAULT ZONES

Fault zone on Pre-Tertiary sandstone, Central Sumatera

Clay Gouge

FRACTURED ROCKS

CLAY SMEAR AND SLICKENSIDES

Fault-seal analysis uses • Faults act both as seals and as conduits for migration • Faults effect migration scenarios / volume estimates • Fault zone properties influence reservoir simulation

FSA benefits • Assign risk (leaking or sealing)

• Estimate potential column heights • Better understanding of fault zone properties • More cost-effective reservoir management • Increased recovery (= Budget savings)

(Badley-TT5)

Summary

Fault-seal analysis requires • Geometrically consistent structural framework • Prediction of fault-zone rock type Fault seal in Exploration is mainly dependent upon the capillary entry pressure of the fault zone (static trapping) Fault seal in Production is mainly dependent upon the low permeability of the fault zone (dynamic trapping)

(Badley-TT5)

Summary

ALLAN MAP/DIAGRAM

Seismic Horizons, Faults & Well data

Fault polygons at horizon / fault intersections

Allan Map

Faulting processes that lower a formation’s porosity, permeability and increase entry pressure:

• Juxtaposition where reservoir sands are juxtaposed against a low-permeability unit (i.e. shale) with high entry pressure • Clay Smear incorporation of clay materials into fault plane by ductile deformation, therefore generating the fault itself a high entry pressure. • Cataclasis, which is the crushing of sand grains to produce a fault gouge of finer grained material, giving the fault a high entry pressure (reduction pore throat geometry). • Diagenesis /Mineralization/Alteration, where cementation of original permeable fault plane may partially or completely remove porosity, finally creating a hydraulic seal

FAULTING AND SEALING MECHANISM

FAULT ZONE DIAGENESIS

• Fluid flow in fault zone zones results in precipitation of authigenic mineral in dilation zones and pore spaces. • Mineral precipitation decreases f, K and increase entry pressure of fault zone • Fluid moving through fault zone s react with primary wall rock composition and form secondary minerals. This alteration reaction can cause dissolution of detrital component and/or precipitation of secondary minerals. • Common mineral precipitation in fault zones:

• Carbonates: Calicite, Dolomite, Siderite • Fe-Oxides/Hydroxides: Gothite • Fe-Sulphides: Pyrite • Clay Minerals: Illite, Smectite, Chlorite, Kaolinite • Quartz

(Chevron, 1992)

STRATIGRAPHIC JUXTAPOSITION

JUXTAPOSITION

Up thrown


Sst

Sh

Sh

Sst

4

3

3

3

LITHOLOGY JUXTAPOSITION

Th

row

1 = Shale on Shale

2 = Sand on Sand

3 = Shale on Sand

4 = Sand on Shale

1

2

1

2

1

2


Juxtaposition ….. Resistance of Cross Fault and/or Along Fault Migration Caused by Structural Apposition of Relatively Permeable and impermeable Formations

SHEAR ZONE AND DEFORMATION BANDS

CLAY SMEAR, SMEAR GOUGE & CATACLASTIC

cataclastic def’m bands

shale

smear

breccia

gouge

Fault-zones in nature

• Structure & Composition of fault-zones are extremely heterogeneous.

• In terms of fault-seal prediction, it is the clay content that is the primary control on seal behavior of faults in mixed clastic sequences (though geohistory is significant).

• Require some method or algorithm to predict the fault-zone composition.

(Badley-TT5)

Myloniteicfaultrocks

Surface trace of fault

Mylonites

Cohesivecataclasites

Fau

lt zone

Cataclasticfaultrocks

Incohesivecataclasites 1-4 km.

4-10 km.

250º-350º CTemperature

FAULTING AND SEALING MECHANISM

(Sibson, 1977)

• Cataclastic gouge is common in the fault zones in sandstone • Deformation bands are mm veins of crushed local rock:

From in <1 mm throw Form at low confining pressure (<2000 psi) Reduce permeability by factor 30 – 10,000 From anastomosing complex or cataclastic shear zone Cataclastic shear zone are reservoir seals Zome width related to formation strength

(Chevron, 1992)

CLAY SMEAR, SMEAR GOUGE & CATACLASTIC

CLAY SMEAR AND SMEAR GOUGE

• Dragging of ductile clays into the fault plane during deformation creates a seal between juxtaposed sandstone • The amount of clay and sand smeared into the fault is related to the sand/shale ratio of displaced stratigraphy

Available in the public domain:

Clay Smear Potential (CSP)*

Shale Smear Factor (SSF)

Smear Gouge Ratio (SMGR)**

Shale Gouge Ratio (SGR) * Although the algorithm has been published, calibrations using the CSP algorithm have only been described in a qualitative way

** Algorithm and calibrations using the Smear Gouge Ratio have only been described in a qualitative way in proprietary reports Other algorithms are reported in the literature but no details are available

Fault Seal Algorithms

(Badley-TT5)

• Assumes that material is incorporated into the fault gouge in the same proportions (ratio) as it occurs in the wall rocks of the slipped interval • Assumes mixing of wall-rock components in any throw- window • SGR can utilize bed-by-bed input or proportion of clay or shale distributed through the sandstone units • SGR is a predictor of upscaled fault-zone composition • SGR does NOT correlate with the thickness of the shale smear or shale gouge

Shale Gouge Ratio (SGR)

(Badley-TT5)

Seismic data (>20 metres)

Predicting fault-zone composition

Outcrop data (meter scale)

Well core data (cm scale)

Throw, T

Slipped interval (T)

Vsh5, Dz5

Vsh4, Dz4

Vsh3, Dz3

Vsh2, Dz2

Vsh1, Dz1

Sand

Shale

(Badley-TT5)

A) B) C)

D) E) Smear Factor Algorithms A) Bouvier et al., 1989 B) Fulljames et al., 1996 C) Lindsay et al., 1993

Gouge Ratio Algorithms D and E

by Yielding et al., 1997

FAPS Badley Earth Sciences

Fault Seal Algorithms

Shale Gouge Ratio (SGR) Algorithm

(Badley-TT5)

A: No lateral stratigraphic variation (layer-cake or tabular):

SGR computed along A-B (dip-slip) equals SGR computed along A-C (oblique slip)

B: Stratigraphy with lateral variation (channelised):

SGR computed along A-B (dip-slip) not equal to SGR computed along A-C (oblique slip)

Fault Slip & Shale Gouge Ratio (SGR)

A

B

Critical factor is slip across the stratigraphy

A

B

C

A

B

C

(Badley-TT5)

SMEAR GOUGE ANALYSIS

• Techniques for predicting trapping potential • Sealing behavior of fault controlled by sand/shale within fault gouge zone • Smear Gouge Ratio (SGR) is a quantitative estimate of this ratio • Fault with high SGR tend to leak

Fault-seal algorithms • Current fault-seal algorithms involve only a limited

number of variables (e.g. stratigraphy, throw, shale content).

• Algorithms are NOT independent

• Algorithms are essentially mechanical (measure amount of ‘mixing’ or ‘smearing’ of wall rock into the fault zone).

• The currently available algorithms do not incorporate any chemical effects such diagenetic overprinting, quartz re-precipitation, etc. Variable calibration is required to account for this.

(Badley-TT5)

SUMMARY

Others Data

- Reservoir

Properties Map

- Initial

Pressure/RFT

-Fluids Contact

(GOC,OWC)

- Bubble map

- Stress Data

Shmax,

Sv,

Shmin

FSA WORKS FLOW

3D-SEISMIC

- Mapped horizon

- Interpreted fault

Well Log Data

- Stratigraphic correlations

- Markers picks

- GR Curves

- Calculated V-Shale

Survey Data

X, Y, Z

TWT, TVD

FAPS Juxtaposition/

Allan Map

Throw Map

Fault Seal Analysis

Sealing Capacity

Slip and Dilation

Tendency RESEVOIR

CHARCTERIZATION

LITHOLOGY JUXTAPOSITION

VE : 3.50

Shaly sandstone

Shale

Sandstone

Downthrown

Up

thro

wn

Lithology Juxtapositon Map

Looking West

S N

Upthrown

Downthrown

0 100 m

ALLAN MAP/DIAGRAM

Fault Seal Capacity

SGR (%)

FAULT SEAL SAND ON SAND

VE : 3.50

Gouge ratio

Looking West

S N

Upthrown

Downthrown

Gouge Ratio

Fre

quency (

n)

0 30 40 60 50 70 80 90 100 20

200

400

600

800

1000

1200

1400

1600

1800

2000

0 100 m

FAULT ZONE PERMEABILITY

VE : 3.50

Fault Zone Permeability

(10 3 milidarcy)

Looking West

S N

Upthrown

Downthrown

0 100 m

K = 10 (-5 SGR)

OWC

OWC

Fault, Reservoir, OWC

• Where SGR is low (<15-20%), cataclastic gouge can support only minimal pressure differences (e.g. up to c.1 bar, or a few tens of meters of hydrocarbon column). Clay smears are discontinuous.

• As SGR increases from 20 to 50%, phyllosilicate-framework fault rock can support increasingly large pressures (e.g. 1-40 bars)

• The SGR scale appears to saturate around 50% when clay smears are well-developed. These can support geological pressure differences of many tens of bars, equivalent to columns of hundreds of meters.

• At depths < 3km, fault-zone composition (as predicted by SGR) is the dominant control on seal capacity. At depths > 3km, burial depth has a second-order effect especially at the cataclastic end (low SGRs) of the fault-rock spectrum. Cataclastic rocks buried to depths > 4km are capable of supporting large across-fault pressure differences.

Fault-Seal Calibration

Badley-TT5

EVALUATE SEALING POTENTIAL

• OWC AND GWC

• RFT (Across pressure difference)

• TOW (TEMPERATURE OBSERVATION WELL)

• BUBLE MAP (WELL HEAD TEMPERATURE)

• WELL HISTORY (FLUID BUDGET)

300

400

500

600

700

W

#1

50 100 150

psi

E

#2

50 100 150

psi

RFT

100.00

200.00

300.00

400.00

500.00

600.00

700.00

800.00

0 50 100 150 200 250 300

psi

Dep

th (

ft)

Well#1

Well#2

• Across Pressure Difference • Reservoir Communication • Fault Seal Characteristic

0

10

20

30

40

50

60

70

80

90

100

100 110 120 130 140 150

DEPTH (ms)

GO

UG

E R

AT

IO

Series1

SEAL

LEAK

FLUID DELTA PRESSURE ON FAULT

VE : 3.50

Looking West

S N

Upthrown

Downthrown

Delta Pressure (psi)

0 100 m 0 100 m

SEAL CAPACITY

SEAL ?

Flu

id d

elt

a-p

res

su

re (

ps

i)

Gouge ratio

0

20

40

60

80

100

20 40 60 80 100

FAULT SEAL SAND ON SAND

VE : 3.50

Gouge ratio

Looking West

S N

Upthrown

Downthrown

SGR threshold = 25%

0 100 m0 100 m

Conclusions Geometrically consistent interpretation (Framework

Model) is required for detailed fault-seal analysis Shale Gouge Ratio is a robust method for predicting the

gross distribution of fault-rock type In an Exploration context, fault-seal evaluation can be used

to quantify seal risk in drilled and un-drilled fault-bounded prospects

Method to estimate potential column height

In a Production context, higher values of SGR indicate lower fault-zone permeabilities, and hence resistance to across-fault flow

Better understanding and prediction of fault behaviour over production time-scales

Derive geologically-realistic transmissibility multipliers for input into reservoir simulations

• Shale Gouge Ratio is a scalable indicator of fault permeability that can be used to calculate transmissibility multipliers. • The SGR-permeability relationship can be re-scaled to systematically vary transmissibility multipliers as part of history matching

Transmissibility Multipliers (T)

Reservoir simulators usually incorporate fault properties implicitly as

transmissibility multipliers - the ratio by which the slab of fault-zone

material degrades the transmissibility between juxtaposed cells.

The multiplier depends on the size and permeability of the juxtaposed

cells as well as the thickness and permeability of the fault zone.

The transmissibility multiplier is model-dependent

Fault properties in reservoir simulation:

transmissibility multipliers

k1 k2

L

t, kfz

A

t = Fault-zone thickness

Kfz = Fault-zone permeability

K1 K2 = Cell permeability

L = Distance between cell centers

A = Area of connection between cells

T=［１＋tfz ］ (2/kfz-1/k1-1/k2)

(L1/k1+L2/k2)

-1

Transmissibility multipliers based on

predicted fault-rock distributions

Cell properties Fault throw

Fault-zone

thickness

SGR,

Geohistory

Fault-zone

permeability

Transmissibility multipliers

Thr

ow

Fault displacement

Fau

lt t

hic

kn

ess

SGR

perm

eab

ilit

y (

mD

)

Transmissibility multiplier (T)

TransijNF uses

Length (Li Lj)

Permeability (Ki, Kj)

Fault thickness (tf)

Fault displacement

Fault permeability (Kf)

TransijF uses (tf) and (Kf)

Fault

permeability

from SGR

SGR calculated

from stochastic

model and fault

displacement

Fault thickness calculated from

fault displacement

Non-faulted transmissibility TransijNF

calculated from geometric connectivity across faults

(derived from displacement)

Transmissibility multipliers (T)

T= TransijF/TransijNF

Shale Gouge Ratio

(SGR)

Xyz Field TransGen analysis

TransGen view showing

net-to-gross of cells

TransGen view showing

SGR on faults

High SGR

Low SGR Shale-prone cells

Sand-prone cells

Low SGR in

regions of sand-

prone cells

High SGR in regions of

shale-prone cells

TransGen view showing transmissibility

multipliers (T) on faults

High T

Low T

Low T in regions

of high SGR

High T in regions

of low SGR 1999 HISTORY MATCH

ECLIPSE DEFAULT

INTER-UNIT SEALING

SGR CALCULATION

PRODUCTION DATA

Cu

mu

lati

ve W

ate

r P

rod

ucti

on

Time

Water Production Vs Time

= dZ

dZVs SGR

Fault displacement

Fa

ult

th

ickn

ess

pe

rmeab

ilit

y (

mD

)

SGR

The xyz Field ECLIPSE model achieved a close history match

when the SGR methodology was used to calculate transmissibility

multipliers for faults.