Module B: Module B: In Situ StressesIn Situ Stresses

Earth 437

Maurice B. DusseaultUniversity of Waterloo

Common Symbols in Earth Common Symbols in Earth StressesStresses

v,hmin,HMAX: Vertical, minor and major horizontal stresses (usually v to surface)

Sv,Sh,SH: Same as above, different symbols 1,2,3: Major, intermediate, minor stress

1,2,3: Effective or matrix (solid) stress E, : Young’s modulus, Poisson’s ratio : Porosity (e.g. 0.25, or 25%) po Density, unit weight, pore pressure

k: Permeability (kv, kh…) These are the most common symbols used in

discussing stresses in the earth

Stresses in the Earth: Intro IStresses in the Earth: Intro I In situ stresses: a vital initial condition for

all geomechanics issues, not just drilling!To carry out any quantitative analysis, it is

necessary to start from the initial stress stateFor example, deep reservoir depletion can lead to

a Δp of perhaps -75 MPa, so that Δ‛v = +75 MPa. The stress change is what is important; it is

defined as Δ‛ = ‛ final - ‛ initial

This Δ‛ value is used to compute subsidence, rock behavior (shearing, collapse), and so on

In hard rocks (mining), []ij can be calculated from direct strain measurements – []ij

Stresses in the Earth: Intro IIStresses in the Earth: Intro II

In sedimentary rocks (oil and gas applications), it is far more difficultThe locations are deep, hard to get toAnd, the strains are small, hard to measureThe rocks are porous, poor strain response

So… hydraulic fracture-based methods are widely used - Minifrac™, LOT, XLOT

+Core-based methods (DSCA, vP(), …) +Geophysical logging based methods +Geological inference (burial and

tectonic histories of the basin give excellent clues)

Stress DefinitionsStress Definitions

hmin

HMAX

v

HMAX > hmin

a

rr = 3

a = 1

max planesslip

planes

TriaxialTest

Stresses

In SituStresses

11

2

2

3

1 > 2 > 3

3

PrincipalStresses

Borehole Stresses

r

r

ri

z

x

yWe usually

assume v is a principal stress

Local, Reservoir and Regional Local, Reservoir and Regional Scales Scales Regional Scale Stresses

Basin scale: 50 km to 1000 kmOften called “far-field stresses”

Reservoir Scale StressesA reservoir, or part of a reservoirScale from 500 m to several kmSalt dome region: 5-20 km affected zone

Local Scale StressesBorehole region: 1-5 mDrawdown zone (well scale) 100-1000 m

Small Scale Stresses (less than 10-20 cm)

~200 km

~4 km

~400 m

Common Stress RegimesCommon Stress Regimes

The most common stress regimes are:Relaxed, or non-tectonic (no faulting, flat-lying):

vertical stress, v, is = 1 (major stress)

Normal fault regime: v is 1

Thrust fault regime: v is 3 (least stress)

Strike-slip regime: v is 2 (intermediate stress)Listric (growth, down-to-sea or GoM) fault

regime: v changes from 1 to 3 at depth, then back to 1

Most sedimentary basins with O&G have relatively simple stress regimes

But, there are local complications, such as multiple faults, salt domes, uplift, etc.

Faults and Plate TectonicsFaults and Plate Tectonics

The Big Picture!

Regions of crustal extension

Compression region

Where Are Tectonics Important?Where Are Tectonics Important?

Near active plates (eg: California, Sumatra, Colombia), tectonics governs stresses

Near mountains, tectonic forces dominate Away from plate margins and mountains

(eg: Williston Basin, Kalimantan, GoM), other factors are important

In continental margin basins salt tectonics (domes and tongues) can be very important

In non-tectonic intracontinental basins (Michigan, Williston, Permian…), the shape and burial/erosion history are more important than tectonics

Basins: Major Examples in USABasins: Major Examples in USA

PE

RM

IAN

B

AS

INS

C

OM

PLE

X

WILLISTON BASIN

MICHIGAN BASIN

APP

ALA

CH

IAN

BA

SIN

Gulf Coast

Basin – G

oM,

passive or

relaxed b

asin

Paradox B.

Powder River B.

Rockies Foreland Basins, compressive stresses controlled by mountain thrust

San Joachim, a rift valley

Southern CA basin complex, strike-slip and normal faulting

Atlantic coastal plain and offshore basin complexes, passive margins

MIDCONTINENT BASINS

Thrust basins

Non-Tectonic RegimesNon-Tectonic Regimes

On stable continental plates, far from active plate boundaries. Some examples Mid-continent basins: Williston, Michigan,

Permian age basins, East Texas, Songliao Basin (richest Chinese basin), interior Russia…

On passive continental plate oceanic margins such as GoM, Kalimantan, Nova Scotia, NW Norway coast, Angola, etc.

Basin geometry, history of sedimentation, compaction, burial, erosion, diagenesis, salt dissolution features…, salt tectonics affect stress states locally & substantially

Stresses and Basin ShapeStresses and Basin Shape

Yuc

utan

Houston

New Orleans

Florida

Gulf of Mexico

USA

Mexico

Edge of continental shelf

Regional3 directions

Cross-section

shoreline

GoM example: regional stress directions are dominated by the continental slope, except locally near salt domes and a few structures such as the Mississippi canyon

listric faults

Normal Fault RegimeNormal Fault Regime

v = 1

hmin = 3

HMAX

=

The normal fault regime is also called the extensional regime. It is characteristic of shallow rocks in all non-tectonic sedimentary basins without large erosion.

The San Joachim Valley in California, the Rhine Valley between France and Germany, the Gulf of Thailand are all normal fault grabens

Horst-graben structure

extension

horstgraben

Some Classic Normal Fault AreasSome Classic Normal Fault Areas

Red Sea Around UK, Ireland

Normal Fault Zones (Pull-Apart)Normal Fault Zones (Pull-Apart)

Mid-ocean rifts East African Rift Upper zones, GoM Gulf of Thailand Upper Cook Inlet On flanks of thrust

faults, etc.

Normal Faulting RegimesNormal Faulting Regimes High angle faults at surface (60°-70° dip) This indicates that v = 1 when faulting

occurred. (But, is the fault old or active?)

Also, HMAX = 2 and hmin = 3

Characteristic of extensional strain Also, typical of non-tectonic basins Hydraulic fractures are vertical, to hmin

However, high angle surface faults may “flatten” at greater depth (as in the GoM)

Many continental margins, passive basins, regions of crustal “pull-apart” …

Strike-Slip or Wrench FaultStrike-Slip or Wrench Fault

Surfaceview

v = 2

hmin = 3

HMAX

= acuteangle

~vertical fault plane

hmin

Associated normal faults

HMAX

Block diagram

Strike-Slip Stress RegimeStrike-Slip Stress Regime

Very high angle faults (>80° usually) Indicates v = 2 (HMAX = 1, hmin = 3)

when the fault formed Characteristic of plate margins Common at depth in eroded basins Common some distance from

compression Usually, normal faults are found nearby at

the surface, away from the main fault trace, to accommodate strata movements

Hydraulic fractures vertical, to hmin

Small Window BasinsSmall Window Basins

Small “window” basins between strike-slip faults have complex stress conditions Small basin opened up

between parallel strike-slip faults. Locally, stresses can vary from normal to thrust regimes, very complex

Southern CA basin complex

San Joachim Valley Basin

Graben basin

15 km

Small basin

Thrust Fault Regime and Thrust Fault Regime and StructuresStructures

static basal sheet

overthrust sheet

brittle quartz-illite shale

hinge points

highly fractured zone

largely unfractured shale

AB C

The shale bed in zone A has gone throughone hinge point, through two in zone B, and through three hinge bends in zone C.

HMAX = 1

v = 3

overthrust sheet

compression

high-p shale

strong lateral thrust

RAMP

Thrust and Reverse FaultsThrust and Reverse Faults

Less than 45° angle on fault plane If less than 20-25°, it is almost always

called a thrust fault rather than a reverse fault

This angle is always less than 45°, usually less than 30°

Sometimes, thrust faults can take on very complex, stacked structures

Thrust Faults and MountainsThrust Faults and Mountains

AlbertaSyncline

Basinedge Canadian Shield

Williston Basin

The Western Canadian Sedimentary Basin

USA

ALTA SASK MAN

NWT Nunavut

BC

Canadian Shield

(Precambrian)

Ro

ckies

Athabasca Basin

EdmontonTectonic

stressBreakouts to HMAX

Alberta is the “classic” compressional (thrust fault) regime

Compressional Basin SectionCompressional Basin Section

Massive heavy oil deposits

+

++ +++

+++ ++ ++

+

+ ++

+ +

+

++

++

++

+

++

++ +

+

+

++ +

++ +

+

++ +

+

+

+

+ ++ +

+

Cretaceoussands, shales

Jurassic andolder carbonates,

sandstones, shales

Prairie Evaporites (halite)

Alberta SynclineRockies

Edmonton

Precambrian rocks

SW NE

not toscale

Schematic cross-sectionthrough Edmonton, Alberta

RegionalCretaceous

unconformity

Thrust faults

Salt solution andcollapse features

Devonian reefs

Thrust FaultsThrust Faults Low angle faults (dip of 0° to 30° usually) Indicates v = 3 (HMAX = 1) when the

fault formed (or if it is still active) Characteristic of compression regions,

associated with thrust mountain ranges Same stress condition can often be found

at shallow depths in eroded basins Usually, thrust fault “sheets” are bounded

by systems of normal and strike-slip faults Hydraulic fractures will be horizontal (in

fact, usually they propagate gently upward)

Cross Section: Stress & Cross Section: Stress & StructureStructure

Sing07.024

Mountains Distant plainsGolden Colorado Eastern ColoradoBanff Alberta Calgary Alberta

Near mountains:•Very high HMAX

•For great depth, v = 3 •Thrusts, folds…•Fractured strata•Low to modest po

Distant from mountains:•Moderate to high HMAX

•For some depth, v = 3 •Flat-lying, no faults•Strata are relatively intact•Low pressures

HMAX HMAX

Generally very high pore pressures are not found in thrust regimes and their forebasins, as rocks are somewhat fractured, pressures dissipate

forebasin

Real Thrust Faulting StructuresReal Thrust Faulting Structures

Sing07.024

High stresses near the tectonic compression front

Lower stresses far from the tectonic compression front

Folded belt in front of last thrust

Rocks are permeable because of fractures and folds, po is rarely overpressured

Folding Thrust sheets Thrust fault planes Undeformed sediments

Moderate deep overpressure may remain in the deepest part of the foreland basin (po ~ 1.3-1.4 w·z)

Thrusting Aided by High pThrusting Aided by High poo

Sing07.025

0 10 20 3030 20 10

MILES

Axis of geosyncline

Eroded

Zone of abnormal fluid pressure

Normal faultsThrust fault

Undeformed

Undeformed strata

This is a massive gravitational “landslide” (Wyoming), similar to listric faults. This could not be possible without high local pore pressures in shales, which allowed the fault block to virtually “float” along the fault plane

Listric Faulting and StressesListric Faulting and Stresses

“down-to-the-sea” faults

zone where faults coalesce(detachment or décollement zone)

sea

slip planes

steepat top

v

h

stress

depth

Stresses change with z!

Listric faults on continental marginslead to unusual stress regimes wherethe major stress changes from vertical

to near-horizontal at depthgrabens

Listric FaultsListric Faults

Characteristic of passive continental margin basins that are “open-to-the-sea” (GoM)

Look like normal faults at the surface At depth, the faults flatten to become

thrust faults Stress regimes change with depth! Often associated with overpressured

zones These faults are like massive landslides

India and Tibet ExamplesIndia and Tibet Examples

Passive basins

Active basins

Bay of Bengal Region and NorthBay of Bengal Region and North

A continental margin basin exists offshore south of Dacca and Calcutta, we will expect a relaxed stress condition, GoM features

A strong thrust basin to the north, along the Himalaya front, fractures, no oil

Strike-slip to the east (Sagaing zone) Shan-Thai Plateau is partly a zone of

extension, some N-S faults are normal Sichuan Basin, relatively undeformed,

but under strong compression Etc.

Tectonic Structure Map of

the Region

Tectonics Give Stress State Tectonics Give Stress State CluesCluesThis NASA image of Ganymede shows complex tectonism, giving

clues about the stresses and dynamics which caused the structures

Shear zones

Normal faulting

Extraterrestrial Geomechanics!

North Sea Stress TrajectoriesNorth Sea Stress Trajectories

From the World Stress Map Project

In the central part, complex of grabens and wrench faults

Many “blocks”, each with a stress pattern

Farther north, the Continental Shelf is “open to the sea”

Breakouts, LOT, HF tests…

North America (World Stress Map)North America (World Stress Map)

(available online at www.world-stress-map.org)

Stress Map of EuropeStress Map of Europe

Many solutions for earthquake focal mechanisms in southern Europe give the dense stress coverage

In the hard-rock areas – strain relief methods

In quiescent basins, data from breakouts, hydraulic fracturing, LOT

Geological History!!Geological History!!

This basin opened, filled, was compressed (thrusts and folds), uplifted and eroded

Later, it subsided with new sediment fill The different lithologies compacted

differently, leading to normal faults

gravelsclays and silts

20 – 100 km

3-10 km

Thrust condition

Relaxed stresses

Normal faults

Folds and closed structures

Conclusions on Tectonics and Conclusions on Tectonics and FaultsFaults The tectonic condition and the nature and

orientation of faults give important clues:The principal stress directionsThe relative magnitude of the stressesWhether stresses are intense or relaxed

To be confident of the stress conditions, the faults must be shown to be “active”

Geological history can be complex, giving different stress fields at different depth

The first task in a new area is to study the stresses and tectonic features

Burial and Diagenetic HistoryBurial and Diagenetic History

What controls stresses during burial? How do stresses change with diagenesis? What happens during uplift and erosion? Do all rocks behave the same? What happens if pore pressures change? When there is tectonic loading or

unloading, how are stress changes partitioned in strata?

Hydrocarbon generation effect? Etc, etc… (it gets complicated…)

Stresses at DepthStresses at Depth

σv from density logs, σhmin, σHMAX from various methods (geological estimation, HF…)

We often use the “K” coefficient.

Ratio of least horizontal effective stress to the vertical effective stress (in situ)

<1 – vertical fracturing >1 – “horizontal” fracturing

v

minh

ov

ominh

p

pK

Friction Angle Control of Friction Angle Control of StressesStresses If soft sediment is in a state of plastic yield

during sedimentation and burial: K’]min = (1 - sin )/(1 + sin ) (soft seds)

is the Mohr-Coulomb friction angle For loose sand: = 30°, thus K’]min = 0.33 For shales, much lower friction angles = 10°: this gives K’ = 0.70 We observe that horizontal stresses in soft

shale are much higher than in sands during burial, until sediments are indurated

Upper GoM, Gulf of Thailand…

Burial Stresses, Friction ControlBurial Stresses, Friction Control

Ka = 0.33

Ka = 0.70

Ka = 0.33

Ka = 1.0

po

Note: = - po

(Terzaghi’s law)

UC sand

shale

salt

sandstone E

0.5E

0.75E

salt is viscoplastic

These values are the limits, not actual values in situ

E = stiffness

Frictional Control of StressesFrictional Control of Stresses

In fact, the strata we encounter are rarely purely frictional materials

They also have cohesion The frictional stress control “model” is

only intended to give the theoretical lower bound of hmin for high porosity strata

If rocks are strongly cemented, it is possible to have stresses lower than this

In exceptional cases, open fractures!Shallow, above flanks of salt domes In mountainous areas

Stresses In and Around SaltStresses In and Around Salt

Salt is a very special material:Highly solubleLow density (2.16 g/cm3 or 18 ppg equivalent)Viscoplastic, so all stresses are the same

Drilling long sections of salt is a challenge Drilling near salt structures such as

diapirs and sand tongues is challenging Therefore, a special Module on salt

drilling is included, and not treated here…

See Module G for a full discussion…

Deep Salt Diapir ExampleDeep Salt Diapir Example

Gas Pull Down

Mid-Miocene regional pressure boundary

Top BalderTop Chalk

Intra Hod/Salt

Sands and Shales, Sands and Shales, hh vs. Depth vs. Depth

0 0.5 1.0

hsand

~ 0.45

stress, units of density (/z)

depth

h

shalehyd

rost

at

= g

wz

mud

clay

mud-stone

shale

v, vertical stress

Ko - effective stress ratio

h/v h/v

depth

sands &sandstones

clays &shales

This modelapplies only to

upper 2000 m of soft sediments and

no tectonics!

~ 0.25

1.0 1.5 2.0 2.5

Stress plot (density) Stress plot (ratio of h / v) 2.0 = 16.7 ppg

( = Poisson’s ratio)

Porosity-Depth RelationshipsPorosity-Depth Relationships

mud

clay

mud-stone

shale

0 0.25 0.50 0.75 1.0

clay & shale,“normal” line

sands &sandstones

effect ofoverpressures

on porosity

depth

porosity

4-8 km

The specific details ofthese relationships area function of basin age,diagenesis, heat flow ...

Diagenesis and RocksDiagenesis and Rocks

Mechanical compaction, most important in shales, drives the particles closer together

Pressure solution, important in sands but not shales, lowers porosity substantially

Cementation, usually SiO2 or CaCO3, bonds grains together, reducing porosity as well, most important in sands (sandstones)

Very deep, clay minerals change, leading to fracturing & stress changes (shales only)

Diagenesis rate and intensity is ƒ[T, (ie: depth), t, chemistry…]

Stress and DiagenesisStress and Diagenesis

line

of

v =

h, o

r K o =

1

yiel

d,

= 3

0°v

burial

diagenesis

Sand burial in a non-tectonic environment results in h < v because of friction. Diagenesis seems to reduce this stress difference slowly over long periods of time.

h

is the friction angle for sand

Stress & Diagenesis, no Stress & Diagenesis, no TectonicsTectonics If there has been no tectonic activity, h is

less than v

In sands, the ratio Ko (defined as the ratio of horizontal to vertical stress, h/v), can be as low as 0.3, usually 0.4 – 0.6

Shales have a low angle of friction, usually Ko is 0.6 – 0.8, even as high as 0.95 in muds

Thus, the fracture gradient is higher in mud or shale in non-tectonic areas (GoM)

Deep burial and diagenesis tend to reduce the stress differences

Diagenesis and StrengthDiagenesis and Strength

shear stress -

chemical cementation

densification(more interlock)

originalsediment

diageneticstrengthincrease

3

diagenesiseffects on the

strength

cohesion

a

rr = 3

a = 1 max planes

slipplanes

TriaxialTest

Stresses

normalstress - n

Principal stresses – see inset

strength increase

Diagenesis, Strength, StiffnessDiagenesis, Strength, Stiffness

Mechanical compaction in the early burial stages increases the friction angle,

Chemical effects increase cohesion, c, often cementing particles together

The sediments also become stiffer (higher Young’s modulus)

In general, rocks become stronger, ƒ() However: deep, intense shale diagenesis

often generates shrinkage fractures; they become weaker, hmin and HMAX , k, even though they are lower porosity

Erosion and StressesErosion and Stresses

K o =

1.0

line

(i.e

.: 1

= 3

)

yiel

d,

= 3

0°

’v

’h

buria

l

diagenesis

Elastic behavior governs unloading because the rock is stiff and strong; lateral stresses increase naturally

eros

ion

v

h

Effect of ErosionEffect of Erosion Once a sediment is buried and diagene-

tically indurated, it behaves elastically Direct erosion without tectonic loading

leads to the so-called “Poisson effect”:

Thus, erosion naturally leads toward the shallow condition K’o > 1.0 (except for salt, which behaves as a viscous fluid)

vh '1

'

Rocks, Stresses in an Eroded Rocks, Stresses in an Eroded BasinBasin

Erosion has created a “skin” near the surface where HMAX = 1, and v = 3

Deeper, a strike-slip regime condition HMAX = 3, v = 2 is found

I.e. fracture gradient increases with depth

Rocks are stronger, stiffer

High pore pressures (po > 1.3 v) are rare in eroded basins

, po

Z

v

hv = 3

v = 2

Assuming that both horizontal stresses are equal

thrust stress state

strike-slip stress state

Eroded BasinEroded Basin

The “Poisson effect” during unloading generates a region at shallow depth where horizontal stresses are larger than vertical

Also, the rocks are strong Drilling underbalanced is becoming

common in such regions because of rock strength

Pore pressures in such regions are rarely in the overpressure domain

For large overpressures, there is a special module

Conclusions on ErosionConclusions on Erosion Generation of high Ko values requires

plastic deformation (incl. diagenesis) This happens naturally in sedimentary

basins, even without tectonics, during the burial phase, however, after induration…

The unloading phase is largely elastic It is unlikely that glaciation has a large

effect on previously eroded basins Any eroded basin will have a skin that is

“overstressed”, with v = 3 to some depth

Changes in StressesChanges in Stresses

Opening a hole (drilling) changes stresses massively r is now 3 at a local scale

In drilling, heating or cooling the wall can change the stresses, affecting stability

High mud pressures can cause joints to slip Stresses changed by injection or production

E.g.: Depletion of the reservoir decreases the fracture gradient (loss of horizontal stress)

Hot or cold water injection changes stresses Pressurization or depletion can result in

shearing of casing, faulting, minor seismicity

In Situ Stresses: SummaryIn Situ Stresses: Summary

Stresses are needed for casing programs, borehole stability analysis, etc. etc.

Pore pressures are a vital aspect as well Often stress directions and relative

magnitudes can be estimated from the tectonic and burial history of the sedimentary basin, or its structure

The most recent fault patterns often reflect the stress regime

Features such as salt domes, etc., invariably indicate locally altered stresses

Lessons LearnedLessons Learned

Stresses in the earth arise from:Gravity effects (rock bulk density – v)Tectonic effects (compression, salt tectonics…)

Recent faults indicate stress patterns Basin shape is a stress pattern indicator Geology and history allow us to estimate

relative stress magnitudes and orientations Stress is involved in many basin processes

Basin fabric, diagenesis, overpressure, oil migration, gas and oil valving…

RecommendationsRecommendations

Offshore or inshore, it pays to have some stress information for drilling, hydraulic fracturing, reservoir modeling…

The first step is to use geological history to build a regional stress model

The pore pressure conditions should be inferred as well, (also offset well data…)

Then, examine reservoir & local scale factors Faults, salt features, reefs and drapes,

hydrothermalism, and other featuresThese may “perturb” regional stresses

Extra MaterialsExtra Materials

Breakouts and Natural StressesBreakouts and Natural Stresses

HMAX

hmin

principalstresses,1 > 3

Vertical borehole

breakoutsdamage,ravelling

high

Breakouts are evidence of stress anisotropy and are caused by shear rupture of the borehole wall

However, care must be taken in assessing breakouts, as other factors can “interfere”

Use only vertical wells (10) to get good stress orientations

Some “Confusing” EffectsSome “Confusing” Effects

HMAX

hmin

jointedlaminated

shale

slabbing,ravelling

HMAX

ravelling

materialanisotropy

1

3

ravelling

bedding planes

Material anisotropy means the mechanical properties are different in different directions, as in a fissile shale

3

Borehole Features: Wall ScanBorehole Features: Wall Scan

HMAX

axialfractures

Stress directions

0 90 180 270 360

largewashout

hmin

higher angleof intersection(joint plane)

low intersection angle (bedding?)

breakouts

Geometry of joint plane intersection

Sinusoidal fracture traces

“en-echelon” axial fractures

if hole is slightly inclined

Reconstructed breakout data from Schlumberger borehole scanner logs

Axial fractures and breakouts are stress direction indicators. If the stress difference is large, breakouts are also larger (deeper and wider).

brea

kout

sno

bre

akou

ts

axia

l fra

ctur

es

Directions: Breakouts, Directions: Breakouts, FracturesFractures

Borehole wall tensile fractures

Small breakouts (90° to tensile fractures)

Natural fracture

plane

Modest breakouts, no tensile fractures

Natural fracture

plane

hmin is at 40°Az in this example

This is a LWD log trace taken during a trip, so resolution is poor

Use of Breakouts, Axial Use of Breakouts, Axial FracturesFractures For orientations, use only wells that are

vertical +/- 10° (rarely more inclined) Establish quality control on your data

(length, symmetry across hole, quality…) Grade your data (“A” “B”, “C” quality…) Breakouts: HMAX is at 90° to breakout axis

Borehole wall axial fractures: HMAX is parallel to the fractures axis

Combine with geology, v calculations from density log data, LOT data, HF data…

Build a stress map for your region & use it

More about Breakouts, FracturesMore about Breakouts, Fractures Don’t confuse breakouts with hole

enlargement (breakouts are symmetrical, and the minor axis ~ hole gauge size)

Don’t confuse breakouts with sloughing in a fissile shale when the hole dip is close to the dip of the shale fissility

Joints and planar features trace sinusoidal patterns on the borehole wall; induced axial fractures do not

4-arm dipmeter data must show ~symmetry, consistency, reasonable length, etc. (QC)

Full wall scans are easier to interpret

Log Data as Stress IndicatorsLog Data as Stress Indicators

borehole,pw

Formation, po

Anisotropically invaded zone, & stress microfissured region, giving anisotropic permeability

Pad region

a

bc

dHMAX

hmin

Conductivity response

borehole

4-arm dipmeter responseSome other log properties may give stress orientation information

Mud Weight Window PrognosisMud Weight Window Prognosis1.1 1.3 1.5 1.7 1.9 2.1 2.3

prognosisfor hmin

XLOT hmin value

shoe

depth

density, g/cm3

v

prognosisfor po

overpressuretransition zone

area indicatespossible MW

MW=1.92

shoe location for next casing string

Previouscasingstring

Convert todensity unitsby dividing

pressures andstresses by

vertical depth

strong overpressure zone

This is a conservative approach: often, we can stretch the lower limit a bit, carefully

Mud Weight PrognosesMud Weight Prognoses

Data garnered from many sources, offset wells, geology, XLOT, MWD…

The interval defined is the secure window:MW below pfrac (= hmin) at the shoe

MW above pmax, usually at or near the bit

However, the window can be pushed a bit!Drilling MW + ECD can be a bit above pfrac

If shales are strong and no high k sands, drilling may take place slightly below the pore pressure

Tricks such as high weight pills on trips Underbalanced drilling in strong shales

Pushing the Envelope in Drilling!Pushing the Envelope in Drilling!1.1 1.3 1.5 1.7 1.9 2.1 2.3

prognosisfor hmin

XLOT hmin value

shoe

depth

density, g/cm3

v

prognosisfor po

overpressuretransition zone

area indicatespossible MW

MW=1.92

deeper shoe for casing string!

Previouscasingstring

Convert todensity unitsby dividing

pressures andstresses by

vertical depth

strong overpressure zone

Using high weight trip pills and careful monitoring, the lower limit can be extended

(2.0 = 16.7 ppg)

Pore Pressure GenerationPore Pressure Generation

yiel

d,

= 3

0°

v

h

Pore pressure generation reduces effective stresses, but not in a path parallel to hydrostatic because of Poisson effect. This can lead to rock shear if po is high

faulting

Poissoneffect

Stre

ss p

ath

for +

p

h = v(/1-)

/(1-

line

of

v =

h, o

r K o =

1

Diagenesis, Burial, ErosionDiagenesis, Burial, Erosion

h - MPa

v =

h (

K o =

1)

diagenesis

sedi

men

tatio

n,

~ 3

0o

actu

al p

ath?

0 5 10 15 20 25

erosion, ~ 0.2

-2000 m-

-500 m-

h =17 MPav = 7 MPa

25

20

15

10

5

0

v - MPa

This stress pathexplains the presence of

high horizontalstresses nearthe surface

Burial to 2000 m, erosion to 500 m

Stress path

A simple calculation of the probable effect of erosion of 1500 m of rocks on the stresses. We assumed initial stress state (red star), took a reasonable Poisson’s ratio for erosion (0.2), and made the calculation. (Assume that po is always 8.33 ppg)

Fluid Flow in ShalesFluid Flow in Shales

Shales are almost always the source rocks; sands, limestones the reservoirs

Most shales are water-wet, except for the rich kerogenous shales

Gas or oil as the non-wetting phase flows only with gradients of > 500

Two-phase permeability of shales = zero Thus, HC flow must be through fissures,

fractures, or other macrodiscontinuities Shales are originally intact, so O&G

expulsion is likely through induced fractures

Stress and Petroleum MigrationStress and Petroleum Migration How are microfissures generated? When 3 normal to fissures is < po

po is the pressure in the shale pores

3 is the lateral confining stress at the scale of the microfissure

This can occur by various geological and tectonic processes:Processes which increase po

Processes which decrease 3

These are linked to shrinkage of shale, loading…

Reduction in Reduction in 33

Tectonic unloading reduces 3 (hmin)

Any shrinkage in the material reduces 3

-Clay compaction, thermal shrinkage (uplift) -Loading of anisotropic shales -Smectite to mixed-layer to illite changes

When 3 drops below po, fissures can open stably, and remain open

Fissures can be dominantly vertical (usual case), or also horizontal

Now, fluids can easily flow through the open cracks, migrate to traps & accumulate

Increase in pIncrease in poo

HC generation increases pore pressures Retarded compaction (overpressure) Smectite diagenesis releases H2O

Thermal pressuring increase po

Accumulation of a thick gas zone in a reservoir with good vertical closure

Tectonic loading (?) may increase po

Other processes (e.g. gypsum dewatering)

These processes give the driving forces for fluid flow and HC migration

clayparticle

clayparticle

gas or oil bubble

r

0.1-0.5m

water

oil orgas

p = /2r = interfacial tension r = radius of curvature p = po - pw

pw

po

Darcy flow requires huge p

Can Oil or Gas Flow in Shales?Can Oil or Gas Flow in Shales?

Conclusion: oil or gas cannot flow through intact shale!

Typical pore size in shale

Can Water Flow Easily in Shale?Can Water Flow Easily in Shale?

Free water, partly on ionsBound water

Bound water Much of the water in shales is not free to move easily:Adsorbed on the clay fragmentsHydrated onto cationsPore throats are “blocked” by adsorbed water

Na+

H2O molecules

Cations

Hydratedcations

Conclusion: oil or gas cannot flow through intact shale!

Fluid Generation & FracturingFluid Generation & Fracturing

semi-solidorganics, po < h < v

po = h < v,fractures

develop andgrow

fluids areexpelled through

the fracturenetwork,

po declines

shale

kerogen

micro-fissure

v

oil and gas

generation of hydrocarbon fluids

fluidflow

vT, p,

increase

high T, p,

3-20 mm

Flow in shales mustbe through fractures!

Effects of Increasing pEffects of Increasing poo

Fluid migration is a function of stress as well as pressure!

hydraulic fracturewhen po > h

gas cap, low density

oil

stress

depthhpo

gas capeffect

fault slips when > (n - po)·tan,

i.e. when > n·tan

stresses along A-AA

Aoil, density= 0.75-0.85

n = n - po

n

This is called “valving” of gas

Valving of Reservoirs & StressesValving of Reservoirs & Stresses

Gas is generated and accumulated, increasing the gas cap height

Because gas is light, it has a different gradient of pressure (0.1 instead of 0.8 for oil and ~1.04 to 1.10 for brine)

This causes po to increase at the crest

When po exceeds h, hydraulic fracture! This is like a valve that opens episodically

over geological time, releasing pressure Responsible for shallow gas in some areas

Tectonic StressingTectonic Stressing

UC sand

shale

salt

sandstone

stresslithotype stiffness

limestone

E

0.5E

0.75E

1.5E

loading

unloading

salt isviscoplastic

mud

depthassumedinitialh

stresses areisotropic

Tectonic StressingTectonic Stressing

From its “virgin state”, unloading (rifting) or loading (compression) can occur

This has a relatively larger effect in the stiffer rocks (small strains = large h)

Stress contrasts can be generated among the beds (except salt)

This was proven by measurements in the Appalachian Basin thrust zones in 1980s

This is easily simulated by numerical models, but real data need measurements

Typical Stress ConditionsTypical Stress Conditions

stress (or pressure)

depth

vertical stress. v

horizontal stress. h

pore pressure, po

4 kmmild

overpressure

b. Western Alberta, 100 km from Rockies

stress (or pressure)

depth

stronglyoverpressuredregion at depth

vertical stress, v

horizontal stress, h

pore pressure, po

4 km

a. Gulf Coast of USA

Relaxed continental margin Tectonically stressed rocks

A Well Plan, North SeaA Well Plan, North Sea• classical mud weight window is too

narrow; cannot avoid instability

• low mud weight breakouts

• high mud weight destabilized fractured zones & losses

• breakout problems are controllable by good hole cleaning; fracture zones are uncontrollable

Strategy:

• keep mud weight low

• manage breakouts with good hole cleaning before increasing mud weight

• monitor cavings and mud losses for warning of fractured zones

Stresses and DrillingStresses and Drilling

v >> HMAX > hmin

hmin

HMAX

v

HMAX >> > hmin

HMAX ~ v >> hmin

v

hmin

HMAX

v

hmin

HMAX

To increase hole stability, thebest orientation is that whichminimizes the principal stressdifference normal to the axis 60-90° cone

Drill within a 60°cone (±30°) from the mostfavored direction

Favored holeorientation