in situ stress

8/10/2019 In Situ Stress

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In situ stress &Deformation mechanismsJan Kees Blom

November 2011



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In-situ stress

•

How do we know what stresses occur in the crust?• Stress measurements

• Complex conditions due to heterogeneities

• Sometimes multiple deformation phases with different stressfields

• Crust can „freeze in‟ stress and preserve remnants over long times



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Why stress matters

• Influences stability of boreholes,

tunnels, mines, open pits, mine bursts• Influences natural and man-induced

earthquakes and faulting

• Influences reservoir / aquifercompaction and land subsidence

• Controls hydraulic fracturing for wellstimulation

• Influences preferred subsurface flowdirections

• Influences injector - producer wellpatterns and spacing



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Present-day in-situ stress

• Result of:

• Gravitational stresses (overburden)• Current tectonic stresses (plate tectonics)

• Remnant/residual stresses (from past tectonic orgravitational stress)

• Measured from outcrops, bore holes, earthquakes

• Continuous over 100(0)s of km, but local changes

• Related to plate movements and to local weaknesses (e.g.

faults, weak layers)



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Direction of stress field

• Breakouts of rock fragments in tunnel (or borehole) gives

information about the orientation of the principal stress andthe differential stress

Fossen 2010



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Local perturbations

• Shear stress is zero along free surface, so one of theprincipal stress must always be perpendicular to thatsurface

• Weak faults can also influence stress field

• Keep this in mind when measuring stress near free surface



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Measuring stress• Developed in engineering, mining and energy industries

• At surface of earth use stress relief techniques:

• Overcoring: drill hole (1), attach strain gauges in it, drill annulus around it

(2), stress release causes change in shape of first hole (3).Use elasticity theory to get stress state



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Measuring stress 2• Flat Jack : make reference grid with pins, drill slots, inject flat jacks and

repressurise slots until reference grid has been restored. Gives normalstress component only. By combining several measurements in several

orientations, get state of stress



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Measuring stress 3• At depths of up to 5 km:

• Hydraulic fracturing: magnitude• P frac_propagation ~ σ3 + To

• Pshut in = σ3 (often σHmin)

• To is rock tensile strength

•

Borehole images: direction (& approximate magnitude)

• At shallow to great depths (100‟s km):

• First motions of earthquakes (approximate direction)

• Aftershocks indicate fault orientation

• P wave first motion gives sense of shear



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Reference state of stress

• Models for idealized state of stress as if tectonic processesdo not occur

• Litho/hydrostatic reference state

• Uniaxial-strain reference state

• Constant-horizontal stress reference state



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Lithostatic / Hydrostatic stress

• Lithostatic stress is isotropic stress

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Controlled by height and density of overlying rocks• σ1 = σ2 = σ3 = ρgz

• Average density crust ~ 2.7 g/cm3 => stress gradient~26.5 MPa/km

• Lower with porous rocks

• Hydrostatic stress : gz (water: ρ = 1 g/cm3 (different foroil)), if water is intercoonected to the surface

• Fluids trapped in rocks may lead to overpressures



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Practice vs theory

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Stress measurements inNorwegian mines (a) ,worldwide and oilfields(b) plotted againsttheoretical values.

• Note too low pressures in

oilfields, indicating fluidpressures, and thusoverpressured formations

Fossen 2010



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Stress magnitude and fault style

• Upper limit determined by rock strength

• In present-day extensional setting:• σ V = σ1 = weight of overlying rocks

• σH = σ3 (and σ2) harder to obtain. Possibly by

• Hydraulic fracturing

•

Assuming ratio between σ V and σH Relaxed elastic crust: σH / σ V ~ 0.3 to 0.5

S i d



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Stress magnitude

• In present-day

compressional setting• ơ V = ơ3 = weight of

overlying rocks

• Thus stresses muchgreater: destructive

earthquakes, harder todrill

• In present-day strike-slip

• Stresses difficult toobtain...



St i ti ith



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Stress variations withlayering

• Strong layers “carry” the in-situ stresses

• They act as beams in a bridge or thechassis of a car

• Stress contrast also dependant on time

• Joints develop in hard and brittle rocks

• E.g. sandstone can sustain higher

differential stress than shale => uplift orhigh fluid pressure will break sandstonebefore shale



Ductile vs brittle deformation

• Ductile material accumulates permanent strain withoutmacroscopically fracturing

• Brittle material deforms by fracturing when subjected tostress beyond rock strength (yield stress)

• Ductile deformation can be dependent upon scale ofobservation…

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• Plastic deformation ispermanent strain withoutfracture, produced bydislocation movement



Ductile vs brittle

• So we can haveductile deformationby brittle process, butnot the other way

round

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Tectonic stress



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Tectonic stress• Stresses due to tectonic forces

• Anderson‟s classification of tectonic stress and faulting:

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σ V = σ1 : normal fault regime• σ V = σ2 : strike slip fault regime

• σ V = σ3 : thrust fault regime

Global stress



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Global stresspatterns

Version 2008

For stress maps of the world, see theWorld Stress Map at:

http://dc-app3-14.gfz-potsdam.de/

or next page..

Data collected in mines, tunnels, drilling,earthquake monitoring

www-wsm.physik.uni-karlsruhe.de











22Fossen 2010



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Global plate movement directions



Plate movement history



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Plate movement history

• Hot-spot trail

Plate scale forces



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Plate-scale forces

Fossen 201



S th A i R lt



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South America Results



Strength of lithosphere

• Strength ( resistance against

shear, blue lines) increasesdownwards in brittle crust

• At depth, plastic flow occurs,following a different path

•

Flow paths are derived fromexperimental deformation ofquartzite

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Strength profile lithosphere



Strength profile lithosphere

• Different materials have different flow paths.

• A layered crust can thus give several brittle-ductiletransitions

• Note that dry rock is stronger than wet rock

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St ess o ientations & fa lt st le



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Stress orientations & fault style

• Extension: normal faults-> σ1= Vert, σ2 &, σ3 = Hor

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Strike-slip: σ2= Vert , σ1 &, σ3 = Hor• Shortening: thrusts-> σ3 = Vert, σ1 &, σ2 = Hor

Never mix geological past with present-day setting.....

Fossen 201

Recognizing



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paleo in-situ stress• Based on

• opening mode cracks / joints

(perpendicular to ơ3)

• stylolites

(perpendicular to ơ1)

• striated fault surfaces

(Striae parallel to shear tractionresolved on fault plane)



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Palaeostresses from joints

• Appalachians in

New York state•Curved foldedand thrustedmountain chain

•Joint patterns

persist over largeareas

•Stress in linewith mountainbuildingkinematics

• Arches NP, Utah



Palaeostress state from faults



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Palaeostress state from faults

Present-day to Pleistocene Mio- to Pliocene

Mercier et al., 1991

Borehole image logs



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Borehole image logs

• Acoustic or resistivity image ofthe borehole wall

• Important to distinguish• natural fractures (which

give information aboutgeological past)

• drilling-induced fractures

(which give informationabout present-day state ofstress).

• Compressive (“breakout”) andtensile borehole failure



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Borehole image logs

• Borehole wall is a cylinder

• Layering and fractures are planarfeatures that intersect the borehole

• After “unwrapping” the cylinder, planarfeatures appear as sinusoidal curves

• High sine amplitudes indicate planesnearly parallel to the borehole axis. Lowsine amplitudes indicate planes nearlyperpendicular to the borehole axis

• Low point of sine indicates dip direction

“Unwrapped” cylinder

Borehole image log example



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Borehole image log example

• Acoustic & resistivityimage logs

• Resistivity imagesusually higherresolution

• Partial borehole

coverage ofresistivity logs cancreate uncertainty ininterpretation

Geothermal well Japan

Okabe et al. 1996

Borehole image log example• Induced fractures



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Borehole image log exampledue to boreholetensile failure do notextend across entire

borehole. Visible onlyon two “pads”

• Simple straightinduced fractures ifhole is parallel to

principal stresses(left)

• Multiple en-echeloninduced fractures ifhole is oblique toprincipal stresses(right)

KTB borehole, Germany

Zoback & Peska, 1995

Stresses around boreholes



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Stresses around boreholes• Borehole is a hole filled with fluid

• Fluid cannot sustain shear stresses

• Thus there cannot be any shear stress on borehole wall

• Therefore the borehole locally perturbs the in-situ stressfield

Pre-drillingShear stress on imaginaryplane along future borehole wall

Post-drillingShear stress cannot existalong borehole wall

Stresses around borehole



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Stresses around borehole• Analytic solution to an elastic plate with a hole

• Change in orientation of principal stresses (upper diagrams)

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Change in magnitude of principal stresses (lower diagrams)

-2 -1 0 1 2

yellow-green = lower values ; b lue-violet = higher values .

-2

-1

0

1

2

Sigma 11

-2 -1 0 1 2

-2

-1

0

1

2

Cir cular hole in plate

-2 -1 0 1 2

yellow-green = lower valu es; blue-violet = hig her values .

-2

-1

0

1

2

Sigma 11-2 -1 0 1 2

-2

-1

0

1

2


Stress perturbation and failure



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• Perturbed stresses can

become greater than tensileor compressive rock strength

• If so, get tensile boreholefailure or borehole breakout

-2 -1 0 1 2

yellow-green = lower values ; blue-viole t = higher values.

-2

-1

0

1

2

Sigma 11

-2 -1 0 1 2

-2

-1

0

1

2





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• Plot of stress variation along the borehole wall

• Where stresses overcome tensile or compressive strength,

tensile and breakout occur respectively

-2 -1 0 1 2

yellow-green = lower values ; b lue-violet = higher values .

-2

-1

0

1

2

Sigma 11

Moos & Zoback, 1990

tensile

failure

breakout

failure

rock compressive

strength

rock tensile

strength

Volcanoes



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Volcanoes

• Mechanically, there is somesimilarity between an activevolcanic pipe and a borehole

• Both are holes filled with some typeof fluid

• Therefore stress perturbationaround a volcano is in someaspects similar to that predicted fora borehole

• Volcanic dikes that form duringvolcanic activity grow perpendicularto ơ3

• Therefore the pattern and directionof these dikes provides a directrecord of the (palaeo)stress stateduring times of volcanic activity

Dikes in isotropic stress field



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• Both horizontal principal stressesare similar in magnitude

• Thus radial pattern of dikesexpected (see earlier slide onisotropically loaded borehole)



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Dikes in anisotropic stress field

Stresses around other “holes”



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• Mining tunnels are holes filled by air

• Pit slopes are rock exposed to air

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In all these cases, the rock is shear-stress free at the rock - air interface• Therefore stress is locally perturbed near rock-air interface

• Perturbation depends on geometry of air - rock interface

• Sharp angles give much greater stress concentration than the curvedgeometries

• This is why there are no sharp corners in deep mine corridors, and why

road tunnels are mostly circular• This is why airplane windows and submarine windows are circular and not

rectangular

Increasing

stability

The Comet storyFirst jet airliner De Havilland Comet



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First jet airliner De Havilland Comet(1947)

First version had square windows

Comet flew higher and faster than earlierplanes

After initial success, series of mysteriouscrashes

Failure of hull at corners of windows dueto stress concentration leading to

metal fatigueLater versions had round windows

Unfortunately for British industry, the Americans had taken over.....



Stress perturbation around faults• Very local



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• Very localchanges arevisible as

changes inbreakoutdirection inimage logs

• On crustal scale,stresses changedirection too, e.g.along San Andreas fault



Local stress perturbation on fault



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• Irregularities on fault plane can create stressconcentration when the fault moves

Local compression

Stress perturbation around entire fault

F l di d i h



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• Faults die out, and at tip thedisplacement is zero

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The change in slip near the tip causesextension and compressive quadrants

Com p

Comp

Exten

Exten

Waste injection& earthquakes



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& earthquakes• First time “proof” that humans can

induce earthquakes in mid 1960s

• Injection of waste water inunderground near Denver

• Seismometers detected increasedfrequency of small earthquakes

• During injection shut-down, earthquake

frequency reduced, but picked up againafter injectors were brought on stream.

• What do you think is the cause: faultingor crack opening?

Man-induced seismicity (1)



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• Injection pressures re-open / create fractures

• gas injection fractures formation if injection pressure

exceeds minimum total stress plus rock fracturetoughness

• If rock already fractured, T ~ 0

n eff=

t- P

fl Initial DepletedGas

injection

T

failure envelope

Man-induced seismicity (2)d l ti i d ti d l t l i



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Depleted

Initial

n eff

C

failure envelope

reservoir shrinkage

• depletion induces compaction and lateral reservoirshrinkage

• this leads to unequal changes in the principal stresses

• If faults recently slipped, C~ 0

Earthquakes



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• The majority of earthquakes occur along plate boundaries

• Most earthquakes are related to fault systems, a few to

vulcanoes• Earthquakes occur in “seismic cycles” in which stress is

gradually built up over many years but violently released in afew seconds

Surface damage



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• At surface, generally there exists a fault trace. This fault isoften accompanied by wide opening-mode cracks (why?).

Displacements up to ~ 10 m occur

Fault growth

Some faults (e g in



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• Some faults (e.g. inunconsolidated rocks) grow bygradual sliding. These are called

”a-seismic” faults • Other faults grow by repetition of

thousands of earthquake cycles.This is called “seismic” growth.

• Fault growth history, and “recurrence interval” of majorearthquakes can be studied byanalysing topography anddrainage patterns (e.g. offsetrivers, see arrows)

Deformation adjacent to fault



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• Main offset occurs during mainshock along “master” fault.

Aftershocks let wall rock re-adjust.

• Wall rock also deforms duringmain shock. This can bemeasured with theodolites or

GPS. This strain field can be usedto determine the fault geometryand slip at depth. Commonassumption is that fault is a crackin an elastic crust.

Earthquake distribution along fault



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• Total strain varies smoothly along fault. “Gaps” i.e.areasof limited earthquake activity either exhibit creep (e.g.

Parkfield gap along San Andreas), or are likely to be areaof future earthquakes

Deformation mechanisms



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• Three deformation mechanismscan be active to accommodatestrain:

– Fracturing, cataclastic flow

and frictional sliding – Diffusion

– Crystal Plasticity



Fracturing



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• Breccia in core, fracturing (right),

• mylonite, cataclastic flow (belowright),

• slickensides, frictional sliding(below)

www.geolab.unc.edu

Diffusional mass transfer



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• Volume and grain boundarymigration, through grains and alongedges

• very slow

•

pressure solution: add water, muchfaster

• temperature dependent

• examples: stylolites (top), suturedgrain boundaries (bottom), pressureshadows

w w w . g e ol a b . un c . e

d u

Crystal defects



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• Two types:

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Point defects• Vacancies or impurities

• Movement of vacancies is called diffusion (below)

• Line defects

• Also called dislocations

• Their movements is lead to crystal placticity

Diffusion



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• Volume diffusion

• Vacancies move throughgrain

• Nabarro-Herring creep

• Grain-boundary migration

• vacancies move alonggrain boundaries

• Coble creep

• Slow, cm‟s per Ma‟s

Fossen 201

Pressure solution

Add t t i d



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• Add water to previous processes, speeds up:

• pressure solution

• Diffusion along film of fluid on grain boundaries

• Controlled by chemistry and stress

• Material can precipitate close by (below) or far away

• Sandstone deformed by pressure solution (right)



Crystal plasticitydislocations



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• Dislocations can move through the grains

• complex patterns can lock up: strain hardening• undulose extinction

• recrystallization eliminates or reorganisesdislocations

• high temperature deformation as dislocations move

easier

Twinning or kinking of crystals



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Fossen 2010

Crystal plasticity



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•

Quartz with subgrains andundulose extinction (top) due todislocations in the lattice

• Plagioclase with twins due tokinking of the lattice (bottom)

www.geolab.unc.edu

Deformation processes



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• Which process occurswhen is mainly dependentupon:

• the material

• temperature

• stress

• presence of water

Davis & Reynolds 1996



Summary



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• In-situ stress, important for engineering purposes and productioncharacteristics

• Borehole image logs/deformation give info on in-situ stress

• Faults, stylolites, joint give info on paleo stress

• Boreholes, volcanoes, faults all influence stress patterns

• Stress is released with earthquakes

• Earthquakes can be (man-)induced

• Three types of deformation processes:

– Fracturing, cataclastic flow and frictional sliding

– Diffusional mass transfer

– Crystal Plasticity

Literature• Fossen (2010): Structural Geology

• Chapter 5: 5.1-5.7



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• Chapter 6: 6.8-6.9• Chapter 9: 9.1-9.2•

Chapter 10: 10.1-10.6• Davis & Reynolds (1996): Structural Geology of Rocks and Regions ,

2nd Edition,• Ramsey & Huber (1983): The Techniques of Modern Structural

Geology . Volume 1: Strain Analysis• Twiss & Moores (1992): Structural Geology

• websites: http://serc.carleton.edu/quantskills/methods/quantlit/stressandstrain.html

• Univ. of Wisconsin: http://www.uwgb.edu/dutchs/structge/stress.htm

• Next week: Faults and Fractures• Chapter 7•

Chapter 8

in situ stress

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