Stresses Around a Borehole

Prof. Dr. Eissa Mohamed Shokir

Common Borehole Stability Symbols

s1,s2,s3: Major, intermediate, minor stress

Sv, Sh, SH: Total earth stresses, or Sv, Shmin, SHMAX, or sv, shmin, sHMAX

sr, sq: Radial, tangential, borehole stresses

sr, sq, sv, shmin, sHMAX, etc…: Effective stresses

r, ri: Radial direction, borehole diameter

po, p(r): Initial pressure, p in radial direction

MW, pw: Mudweight, pressure in borehole

E, n: Young’s modulus, Poisson’s ratio

f, r, g: Porosity, density, unit weight

k: Permeability

These are the most common symbols we use

Terminology and Symbols Problems

Often, the terminology and symbols used are confusing and irritating

This complexity arises because: The area of stresses and rock mechanics is

somewhat complex by nature

The terminology came from a discipline other than classical petroleum engineering

There is still some inconsistency in symbology, such as Sh, Sh, Shmin, sh, all for shmin …

We will try to be consistent

Please spend the time to understand

Physical principles are the most important

Other Conundrums How do we express stresses?

As absolute stresses? As stress gradients? As equivalent density of the overburden? As equivalent mud weights?

e.g. PF = 18 ppg means 18 pounds per US Gallon is the fracture pressure at some (unspecified) depth (fracture gradient = (s3/z).

e.g. shmin gradient is 21 kPa/m (or 21 MPa/km)

e.g. The minimum stress is 2.16 density units

e.g. shmin is 66 MPa (at z = 3.14 km depth)

All of these are the same! (or could be)

Which method is used usually depends who you are talking to! (Drillers like MW…)

The Basic Symbols, 2-D Borehole

Far-field stresses are natural earth stresses and pressures, genera-ted by gravity, tectonics…

Borehole stresses are generated by creation of an opening in a natural stress field

Far-field stresses: scale: 100’s of metres

Borehole stresses scale: 20-30 ri (i.e. local- to small-scale)

Far-field stress

r q

s’r

s’q

ri

pw

shmin

sHMAX

po

Borehole stress

Important to Remember…

sq is the tangential stress, also called the hoop stress, you will see it repeatedly referred to in these terms

sq lies parallel (tangential) to the wall trace The magnitude of sq is affected by:

In situ stresses MW and cake efficiency Temperature and rock behavior

It is the most critical aspect of the stress condition around a borehole… High sq values lead to rock failure Lower sq values usually imply stability

Borehole Stability Analysis Concept

First, we need stresses around the borehole… In situ stresses are vital

Δp, ΔT, chemistry affect these stresses

Mud cake efficiency

In some cases, rock properties are also needed

Then, we must compare the maximum shear stress with the rock strength… We need to know the rock strength

We need to know if the rock has been weakened by poor mud chemistry and behavior

If matrix stress exceeds strength, we say the rock has yielded (or “failed”)

Plotting Stresses Around a Borehole

Usually, we plot sq, sr values along one or the other of the principal stress directions

Vertical

borehole

sr

sq

radius

s

pw = 0

smin

smax

Far-field stresses

Vertical borehole

smax

smin

Stresses Around a Borehole One Dimensional Case:

A borehole induces a stress concentration

Two- and three-dimensional cases are more complicated (discussion deferred)

Stress “lost” must be redistributed to the borehole flanks (i.e.: s concentration)

F (F/A =

stress) F F

Initial stress

High sq near

the borehole,

but low sr!

(F/A)

(2F/A) F

F = force, A = Area, F/A = stress

Stress Redistribution

Around the borehole, a “stress arch” is generated to redistribute earth stresses

elastic rocks have rigidity (stiffness)

“lost” s

“elastic” rocks resistribute the “lost” stress

Everyone carries an equal

load (theoretical socialism)

In reality, some carry more

load than others (higher s’q

near the borehole wall)

Far away (~5D): ~no effect

These guys may “yield”

if they are overstressed

D

Stresses “Arch” Around Borehole

The pore pressure in the hole is less than the total stresses

Thus, the excess stress must be carried by rock near the hole

If the stresses now exceed strength, the borehole wall can yield

However, “yield” is not “collapse”! A borehole with yielded rock can still be stable…

shmin

circular

opening,

pw s H

MA

X

Arching of Stresses

arches lintels

load

stress arching

Typical Borehole Instability Issues

Pack-offs

Excessive tripping and reaming time

Excessive mud losses (fracturing losses)

Stuck pipe and stuck or wedged BHAs

Loss of equipment and costly fishing trips

Sidetracks, often several in the same hole

Cannot get casing to bottom

Poor logging conditions, cleaning trips…

Poor cementing conditions, large washouts

These are all related in some way to rock failure and sloughing

Yield of Rock Around a Borehole

Borehole pressure

= pw = MW z

sHMAX

shmin

Axial borehole fractures develop

during drilling when MW is higher

than sq (surges, yield). (This is

related to ballooning as well.)

Swelling or other geochemical filtrate

effects (strength deterioration,

cohesion loss) lead to rock yield

High shear stresses cause shear

yield, destroying cohesion

(cementation), weakening the rock

Low sq

High sq

Shear yield

Tensile yield

Shear Stresses

Shear stress is the cause of shear failure

The maximum shear stress at a point is half the difference of s1 and s3

max = (s‛1 - s‛3)/2, or (s‛q - s‛r)/2 in the figure

Vertical

borehole

sr

sq

radius

s

pw = 0

Vertical borehole

smax

smin

Assumptions:

The simplest stress calculation approach is the Linear Elastic rock behavior model

This behavior model is very instructive

It leads to (relatively) simple equations

r

i2

2

i2

2

i4

4

i2

2

i4

4

ri2

2

i4

4

i

= ( + )

2(1-

r

r) +

( - )

2(1-

4 r

r+

3r

r) 2

= ( + )

2(1 +

r

r) -

( - )

2(1 +

3r

r) 2

= -( - )

2(1 +

2 r

r-

3r

r) 2

in all cases, r r , is taken CCW from reference

ss s s s

q

ss s s s

q

s s

q

q

q

q

max min max min

max min max min

max min

cos

cos

sin

.

r

q

sr

sq

ri

Symbols used

smin

smax

Far-field stress

pw = 0 Known as the “Kirsch” Equations

Comments

Note that the equations are written in terms of effective stresses (sq, sr, s’min…), with no pore pressure in the hole

Far-field effective stresses are the earth stresses, and they have fixed directions

sq, sr can be calculated for any specific point (r, q) around the borehole, for r ri

Later, one may introduce more complexity: T, p(r), non-elastic behavior, and so on…

These require software for calculations; various commercial programs are available

Calculations with In Situ Stresses

For a vertical borehole, the least critical condition is when s’hmin = s’HMAX = s’h s’q]max in this case = 2· s’h if pw = po

However, we can still get rock yield!

However, in most cases, especially in tectonic regions and near faults… The stresses are not the same!

This means that the shear stresses are larger around the borehole after it is drilled

This means that rock yield is more likely!

Borehole stability issues are more severe

Lost circulation more critical

What is a Linear Elastic Model?

The simplest rock behavior model we use… Strains are reversible, no yield (failure) occurs

Linear relationship between stress & strain

Rock properties are the same in all directions

σ‛a

σ‛r = σ‛3

σ’a = σ‛1

εa – axial strain

σ’ –

str

ess (σ‛

1 –

σ‛3)

E = Ds/De =

Young’s modulus

Stress-strain plot

From The Elastic Model

Even in an isotropic stress field (e.g. shmin sHMAX for a vertical hole), shear stress concentration exists around the hole This can lead to rock yield. How to counteract?

We can partly counteract with mud weight E.g.: if pw = shmin = sHMAX = sh (i.e.: MW = sh/z)

If the filter cake is perfect (no Dp near hole)

In practice, this is not done: if MW = sh/z, we are at fracture pressure & drilling is slower!

Higher MW reduces the magnitude of the shear stress, which reduces the risk of rock yield, but increases LC risk, slows drlg…

From The Elastic Model

Fracture breakdown pressure is calculated to be Pbreakdown = 3σ’hmin - σ’HMAX + po In practice, this is not used for design

Fracture propagation is Ppropagation = shmin, also taken to be PF (fracture pressure) for planning of MW programs This is often taken to be MW]max

MW is usually maintained to be less than shmin

In practice, it is often possible to use some methods to “strengthen” the borehole

This allows drilling somewhat “overbalanced”, when pw > σhmin, (this must be done carefully!)

Here, we plot the tangential stress, s’q

Higher stress difference is serious! It gives rise to higher s’q values. Rupture??

Borehole Stresses if shmin sHMAX

pw

2·σhmin

σ HMAX σ hmin

= 1.0) (

σhmin

σHMAX

= 1.4) (

1.6·σ hmin

3.2·σ hmin

σ HMAX σ hmin

σHMAX

Calculated from Kirsch equations,

along principal stress directions

2σhmin

σhmin

Far-field stresses, shmin, sHMAX, are: shmin – po, sHMAX – po

wellbore pressure pw assumed to be equal to po

pw

sq ~ sq ~

It gets worse in tectonic cases!

When shmin - sHMAX is large, the borehole wall in the sHMAX direction is in tension! Induced fractures can be generated during pw surges

High sHMAX - shmin Cases (Tectonic)

σ hmin

pw

σ hmin

σ HMAX

sq ~ 5σ hmin

σ hmin sq ~ 8σ

hmin

= 2.0) ( σ HMAX σ hmin

= 3.0) ( σ HMAX σ hmin

σ HMAX

*Note: here, borehole pressure, pw, is assumed = po

pw

θ rw

0

+90°

-90°

Plot of the Tangential Stresses

σHMAX

σHMAX

Refer to paper by Grandi for details

Here, σθ stresses at the wall (ri) are plotted as a function of θ

Note the symmetry

σθ(ri)

Borehole Wall Stresses (@r = ri)

Now, introduce effective stresses: e.g. symbols s for total, s for effective

Maximum stress at the borehole wall: σq]max = 3·σHMAX - σhmin – po (total stresses)

sq]max = 3·σ’HMAX - σ’hmin (effective stresses)

Minimum stress at the borehole wall:

σq]min = 3·σhmin - σHMAX - po (total stresses)

s’q]min = 3·σ’hmin - σ’HMAX (effective stresses)

For a general 3-D solution for inclined wellbores: use a software solution (big equations!)

Preliminary Comments…

Creation of a borehole: high tangential stresses (sq), low radial stresses (sr)

The larger sHMAX - shmin, the higher sq is (in the direction of shmin), the lower sq is (in the direction of sHMAX)

Radial effective stress (sr) is low near the borehole wall, zero right at the wall

pw = 0

sr

sq

radius

s

More Preliminary Comments…

If both stresses are equal (sh) and MW = po: at borehole wall: sq = 2sh, and sr = 0

If sHMAX – shmin is large, sq is increased, and sr doesn’t change too much

This greatly increases the shear stresses

These shear stresses are responsible for failure of the rock, breakouts, sloughing…

How do we control this? High effective mud weights reduce this

Mud cooling shrinks rock, reduces stresses

Avoid shale swelling, promote shale shrinkage

Mud Weight Effect (equal s case)

pw = 0.3s

sr

sq

pw = 0.8s

sr

sq

Here, we assume for simplicity that we

have “perfect” mud cake, and that the

pore pressure in the rock is zero

radius

s

radius

s

pw = 0

sr

sq Assume sHMAX = shmin = s

radius

s

Let’s Include Pore Pressures…

pw = 0.6s

sr

sq

Assume sHMAX = shmin = s

radius

s

Pore pressure - po

Positive support force = pw – po is applied in the case of a perfect mud cake:

this is a strong stabilizing force because it increases confining stress, this

will be discussed later, when we introduce rock strength

Mud

pressure -

pw

Much of what we do in mud chemistry and MW management is to try and

keep a positive support force right at the wall. This acts like a liner in a

tunnel, keeping the rock from deteriorating and reducing the shear stresses.

If it is lost by poor cake…, deterioration can be expected, especially in shale.

perfect cake

Filter Cake Efficiency

The better the filter cake, the better the support pressure on the borehole wall Support pressure = pw - pi

If there is poor filter cake, support pressure on a shale may be almost zero!

This support pressure is a true effective stress that is acting in a radial outward direction, holding rock in place!

In WBM in shales, the support pressure tends to decay with time! Soon after increase in MW – good stability After some time (days, weeks), sloughing can

start again because support p decays

Geochemical Effects

Swelling or shrinkage can occur because of geochemical effects in shales Geochemical changes lead to swelling or shrinkage!

This ΔV changes the tangential stresses (Δσ’θ)

Swelling always leads to problems: Rock yield from high hoop stresses

Deterioration of cohesion from chemistry changes and small volume changes

Squeezing of borehole, mudrings, poor mud…

Shrinkage can also reduce strength because any ΔV helps degrade grain-to-grain cohesion

Modest shrinkage or no shrinkage are best

What is a Washout? When shale yields (high sq), it weakens and

tends to fragment

If filter cake is poor, sr is low (no support for the shale fragments) sloughing

Washouts develop all around the borehole, roughly symmetric (made worse by fissility)

gage

gage = ri

shmin

sHMAX

Stresses

“flow”

around

borehole

breakouts

Washouts,

no strong

orientation

yielded shale

Borehole Wall Features & Failure

Axial fractures (high MW) are not rock failure and deterioration

Breakouts are evidence of rock shear failure

Large washouts as well, leading to problems…

Natural fractures are not usually a problem, except if they are high-angle and can slip

This case is more common than thought

0 90 180 270 360

washout

breakouts

axial fractures

Natural fracture traces

Sandstone Mudcake, Dp Support

borehole

p(r), steady-state,

no mud-cake

mudcake

limited solids invasion depth

Dp across mudcake

p(r) with mudcake

pressure

po

pw

distance (r)

sandstone

Excellent support MW

sHMAX

shm

in

Filter Cake in Sandstones

Filter cake is made of clays, polymers, etc.

Very low permeability

Sand k is much larger than cake k…

Allowing the pressure difference to give a direct support stress

Therefore: sands almost never slough, but:

Differential sticking is an issue in sandstones

pw

The positive support pressure in a

sandstone is usually close to pw – po

because permeability is high

po

Damaged rock held

in place by +ve

mud support

Filter cake

Shale Mudcake, Dp Support

borehole

p(r), steady-state, @ t = ∞

now, no more mud-cake effect!

mudcake?

p(r) initially, @ t = 0. This is an

excellent support condition

pressure

po

pw

distance (r)

shale

MW

shale

Because no mudcake can form on a shale, slow

pressure penetration takes place, and the support

pressure effect is slowly destroyed

This is a time-dependent process

sHMAX

shm

in

Filter Cake in Shales

Intact shale k is much lower than cake k…

A true filter cake cannot form on the borehole wall

Initially, support is good

But, with t, it decays…

Rock yields = microfissures

pw penetrates more fully into the damaged region

Dp support is lost leading to sloughing and breakouts…

A time-dependent process!

pw

The support pressure in

shale is a function of time

po

Damaged rock is not

held in place by mud

pressure and high k

Support lost with time

Cake Efficiency Management

Using OBM in intact shale gives excellent efficiency, good Dp support, reducing the shear stresses in the borehole wall

In fractured shale, OBM often ineffective: Filtrate penetrates the small fractures

No Dp across wall can be sustained (no cake)

These shales easily slough on trips, connections

When using WBM Gilsonite, dispersed glycol, fn.-gr. solids can

help plug small induced microfissures

This helps maintain good Dp across the wall

But! Geochemical effects can take place.

Gilsonite: natural asphalt

Glycol is any of a class of organic compounds belonging to the alcohol family; in the molecule of a glycol, two hydroxyl (OH) groups are attached to different carbon atoms. The term is often applied to the simplest member of the class, ethylene glycol.

What Happens with Hot Mud?

The rock in the borehole wall is heated

Thermal expansion takes place

This “attracts” stress to the expanding zone around the well

The peak stress rises right at the borehole wall, and yield and sloughing is likely

For cooling, the rock shrinks; this allows the stress concentration to be displaced away from the borehole, helping stability

Cooling occurs at and above the bit

Heating occurs farther uphole

Heating and Cooling in the Hole

depth

T

casing

geothermal temperature

bit

cooling

heating

mud temperature

shoe

+T

-T

mud down pipe

mud up annulus

cooling in tanks

BHA

drill

pipe

open

hole

Heating occurs uphole, cooling

downhole. The heating effect can

be large, exceptionally 30-35°C in

long open-hole sections in areas

with high T gradients.

Heating is most serious at the last

shoe. The shale expands, and this

increases sq, often promoting

failure and sloughing.

At the bit, cooling, shrinkage, both

of which enhance stability.

Commercial software exists to draw

these curves

Thermal Stresses Around Boreholes

Heat transfer: conductive or convective Conductive: low permeability rock – shale, salt

Convective: high permeability rocks – sandstone

The stress distributions are different for these cases, and conduction is much slower

Heating increases σθ, and shear failure is more likely (= sloughing)

Cooling reduces hoop stresses, and short axial fracturing is more likely

In general, the effects of axial fracturing on stability are not substantial

Horizontal vs. Vertical Wellbore?

σv = 0.9 psi/ft, σh = 0.6 psi/ft, p = 0.4 psi/ft

In non-tectonic systems (shmin ~

sHMAX) vertical holes are subjected

to lower shear stresses; they are

generally more stable than

horizontal holes

sq = 1.3 psi/ft, sides

Horizontal Hole

Vertical Hole

sq = 0.1 psi/ft,

top, bottom

sv = 0.5 psi/ft

sh = 0.2 psi/ft

sh = 0.2 psi/ft

Stress State 0.5

0.2

0.2

0.2

sq = 0.4 psi/ft

Tectonic Stress Conditions

Vertical effective stress = 0.5 psi/ft Min. horizontal effective stress = 0.3 psi/ft Max. horizontal effective stress = 1.0 psi/ft

Vertical well

0.1

2.7

0.1

2.7

This orientation is the

best one for this case,

showing the importance

of knowing the in situ

stresses

1.2

0.4 0.4

1.2

sv = 0.5 psi/ft

shmin = 0.3 psi/ft

sHMAX = 1.0 psi/ft

2.5

0.5

Horizontal well aligned with

minimum stress, shmin

0.5

2.5 Horizontal well aligned with

minimum stress, sHMAX

Which aligned of Horizontal

well is the best

sq =

sq =

sq

TABLE 1

Maximum Stress Minimum Stress (σθ]min)

No. Hole

Configuration Gradient ( psi/ft)

Magnitude ( psi)

Gradient ( psi/ft)

Magnitude ( psi)

1 Vertical 2.7 13,500 -0.1 -500

2 Parallel to minimum

horizontal stress 2.5 12,500 0.5 2,500

3 Parallel to maximum

horizontal stress 1.2 6,000 0.45 2,000

Stress at borehole wall (σ’θ) in a tectonically active area (Compressive stresses are +ve; Tensile stresses are -ve)

Depth of investigation is 5,000 ft

(σθ]MAX)

3-Dimensional Borehole Stresses

Effective stresses:

s1 = s1 - po s2 = s2 - po s3 = s3 - po

z

x

y

F Y

s1

s2

s3

po

F, Y are dip and dip direction (wrt x) of the borehole axis x, y, z are coordinates oriented parallel to s1, s2, s3

s1, s2, s3 are the principal total stress magnitudes po is the pore pressure

Borehole radial,

axial & tangential

stresses, sr, sa, sq

Almost always, principle stresses can be

taken as and to the earth’s surface

What About the Axial Stress??

Axial stress, sa, acts parallel to the hole wall, to sr, sq

Usually ignored in borehole stability

However, if sa is very large compared to sr & sq, it can also cause yield

More sophisticated analysis req’d

Almost always, using the hole angle and azimuth, we do the following: Determine maximum and minimum

stresses in the plane of the hole

Carry out a 2-D stability analysis

sr, sa, sq

The Best Well Orientation

In a relaxed (non-tectonic) basin, sv > shmin ~ sHMAX, vertical wells are the most stable

In a tectonic basin, an estimate of the stresses is essential; for example: If sHMAX > sv > shmin, we still have to know the

specific values to decide the best trajectory

If sHMAX = 0.7, sv = 0.5, shmin = 0.4 psi/ft, a horizontal well parallel to sHMAX is the best

If sHMAX = 0.7, sv = 0.6, shmin = 0.4 psi/ft, a well parallel to shmin is likely the best

Careful Rock Mechanics analysis is best

+0ther factors: fissility, fractures…

Stresses and Drilling

sv >> sHMAX > shmin

shmin

sHMAX

sv

sHMAX >> sv > shmin

sHMAX ~ sv

>> shmin

sv

shmin

sHMAX

sv

shmin

sHMAX

To increase hole stability, the best orientation is that which minimizes the principal stress difference normal to the axis

60-90° cone

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

Favored hole orientation

“Showing” the Best Trajectory

This is a polar plot of “ease of drilling”

Related to magnitude of shear stress on wall

This is based in situ stress knowledge

In this example, a horizontal well, W to E, seems to be “easiest”

A horizontal well N to S is the worst (all other factors being equal)

shmin

sHMAX

sv

Typical Troublesome Hole (GoM)

8.00

9.00

10.00

11.00

12.00

13.00

14.00

15.00

16.00

3000’ 4000’ 5000’ 6000’ 7000’ 8000’ 9000’

PP

Sh

Sv

Planned Casing

Actual Casing

Drill MW

MW to Keep Hole Open

Increase MW to

get out of hole

Pore pressure MWmin Lade shmin

sv

Planned Csg Actual Csg

Drill MW

MW to keep

hole open

4960 Stuck Pipe: no

rotation, no circulation

Hole tight with pumps off

Losing 300 bbl.hr (ballooning?)

17 ½” x 20 ” 17 ½” x 20 ” 16 ” Liner 16 ” Liner 13 3/8 ” 13 3/8 ” 14 ¾” x 17 ½” 14 ¾” x 17 ½”

Pack-off

Str

ess, p

ressu

re in

pp

g

Depth in feet

The Plan … The Reality

Hole planned from offset wells (sv, shmin, log correlations to strength data, po…)

Jagged line is a prediction of MW to sustain reasonable borehole stability

Brown line: chosen MW program from stability calculation (using “Lade” criterion)

Red line was the actual mud weight needed to cope with a series of problems

The casings were set higher than expected and an extra string was eventually needed

Typical Troublesome Hole (GoM)

8.00

9.00

10.00

11.00

12.00

13.00

14.00

15.00

16.00

3000’ 4000’ 5000’ 6000’ 7000’ 8000’ 9000’

PP

Sh

Sv

Planned Casing

Actual Casing

Drill MW

MW to Keep Hole Open

Increase MW to

get out of hole

Pore pressure MWmin Lade shmin

sv

Planned Csg Actual Csg

Drill MW

MW to keep

hole open

4960 Stuck Pipe: no

rotation, no circulation

Hole tight with pumps off

Losing 300 bbl.hr (ballooning?)

17 ½” x 20 ” 17 ½” x 20 ” 16 ” Liner 16 ” Liner 13 3/8 ” 13 3/8 ” 14 ¾” x 17 ½” 14 ¾” x 17 ½”

Pack-off

Str

ess, p

ressu

re in

pp

g

Depth in feet

How do We Sustain Stability?

MW control (up or down)

Mud properties control (reduce ECD)

Trip and connection policy (speed, surge…)

Inhibitive WBM: minimize chemical effects

OBM: eliminate chemical effects

Air or foam UB drilling (shallow, strong rx)

Use fn-gr LCM, gilsonite in fractured shale

Cool the drilling mud to reduce sq, reducing the chances of rock failure

When all else fails, sidetrack, set casing

Well Design and Cost Optimization

High risks are mainly related to low MW, rapid drilling, increased well blowout risks… Low cost if successful.

Low risks are mainly associated with slow drilling and high MW, but drillings time is long… Generally costly…

In between, there is a level of acceptable risks with a lower cost factor

Well Design Costs

Ac

tual

(Lik

ely

) W

ell

Co

sts

High Risk Low Risk

Borehole Cost Optimization

Affected by drilling speed, casing string costs, cleaning problems, cost of drilling mud, risks, trip problems…

Optimizing this in “real time” is the challenging task of the Drilling Engineer

Mud Weight

Str

ess to S

trength

ratio

1.0

0.8

0.6

Flu

id influx

She

ar

failu

re

slo

ugh

ing

Safe Lost

circulation

“Ballo

onin

g”

The shape of the

cost curve changes,

depending on the

stresses and where

we are in the hole!

Borehole Stability and Hydraulics

Borehole management is not only stresses, rock strength, MW and mud properties!

It is also dependent on hydraulics: Pumping strategy and cleaning capabilities

Gel strength, viscosity, mud density

BHA design, ECD, even tripping policy

How do We Predict RM Stability?

We need to know the rock stresses in situ Vertical, horizontal usually, sv, shmin

Pore pressures (especially overpressure cases)

We need to know the rock strength Lab testing of core

Correlations to geophysical log data bases

Testing of drill chips (penetrometers, sonic…)

Then, we make predictions of stability MW

This is an indicator only! Careful monitoring on the active well

Improvement of our “calibrations”, ECD…

Lessons Learned

Stress concentrations arise naturally when a hole is drilled

The tangential stress sq is critical Affected by stress, tectonics, rock behavior…

Borehole cake and mud support are critical

We can calculate stresses, but rock parameters are (E, n, Y, Co, To…) needed

We can reduce the effects of high sq

MW, lower T, better cake, OBM…

We can use log data and correlations to predict the MW for stability