Module L:Module L:More Rock Mechanics More Rock Mechanics

Issues in Drilling Issues in Drilling

Argentina SPE 2005 Course on Earth Stresses and Drilling Rock Mechanics

Maurice B. DusseaultUniversity of Waterloo and Geomec a.s.

““Predicting” Onset of InstabilityPredicting” Onset of Instability

Now, we have methods of estimating in situ stress conditions

Also, we have methods of measuring or estimating strength

Furthermore, we have methods of calculating stresses around a circular opening, subject to several assumptions…

Putting this together allows prediction of shearing initiation on the borehole wall

…An estimate of “breakouts initiation”

Linear Poroelastic Borehole Linear Poroelastic Borehole Model…Model… Eqn: Where:

pw]cr critical wellbore pressure, shear initiation

pi pressure just inside the borehole wall

σ1, σ3 largest, smallest ppl σ in borehole planeA = α(1-2)/(1-) ( = Poisson’s ratio)α Biot’s coefficient (1.0 for soft rocks)N friction coefficient = (1 + sin’)/(1 - sin’) UCS, Unconfined Compressive Strength,

friction angle (MC yield criterion) Δp “drawdown” = pi - po

1N

p)1N(UCSpA3]p i31crw

Discussion of ParametersDiscussion of Parameters

pw – pi is support pressure

Usually, we ignore effects of “α”, except in low porosity, stiff shales (E > 30-40 GPa)

UCS and N are equivalent to the c’, ’ of the linear MC yield criterion for shear

Poisson’s ratio for shales, 0.25 to 0.35 σ1, σ3 are computed using equations

converting 3-D stress to stresses in the plane of the borehole (90° to hole axis)

pw

pi

po

radius - r

Control Parameters in DrillingControl Parameters in Drilling Mud weight, mud rheological properties,

the geochemistry of the filtrate, cake quality, mud type (WBM, OBM, foam, etc.)

LCM content, type and gradation Tripping and connection practices:

Surging (run-in), swabbing (pull-out) pressures Drilling parameters:

ROP, bit type… Hydraulics and hole cleaning ECD (BHA characteristics, mud properties) Well trajectory, and maybe a few others

Defining Limits in Our Well PlanDefining Limits in Our Well Plan

Predicted MW for onset of unmanageable sloughing

po, onset of blowout if in a sand zone

hmin, danger of LC

Onset of ballooning in shale zones

Depth

Pressure or stress

v

v

Depth

Gradient

How are the Limits Defined?How are the Limits Defined?

Lower MW limitPressure controlRock Mechanics stability, experience, use of

correlations to predict stability line, etc.How much sloughing can we live with?Underbalanced Drilling is a good example of

RM Upper MW limit

Avoiding massive lost circulationFracture gradient, earth stresses analysisEffects on ROPThe new concept of overbalanced drilling is

an example of RM extending this envelope

Are All Limits Absolute?Are All Limits Absolute?

No, and here are examples: Drilling underbalanced? OK as long as it is

shales or lower permeability sands, and if the shales are strong (little sloughing)

Drilling overbalanced? OK for up to ~1000 psi with properly designed LCM in mud!

Drilling below sloughing line? OK if good hole cleaning, use increased MW for trips…

Pushing the envelope is typical in offshore drilling, HPHT wells… (e.g. mud cooling…)

Vigilance and RM understanding needed…

Example: Drilling Example: Drilling UnderbalancedUnderbalanced It is a Rock Mechanics issue, a pore

pressure issue, and a fluids type issue If the shale is strong enough to be self

supporting in a bore hole with a negative r

If the pore pressure is not so high that it “blows” sand and shale into the borehole

If the fluids that enter the hole are “safe”, i.e., not oil and gas in large quantities

Excellent for drilling through depleted zones, fast drilling through good shale, entering water sensitive gas-bearing strata, reservoirs that are easy to damage

Underbalanced Stress Underbalanced Stress ConditionsConditions

pw < po

r

radius

– stress

po

pw

Some tensile stress exists near the hole wall in underbalanced drilling because po > pw

High shear stress at the borehole wall

hmin = HMAX

Mud RheologyMud Rheology High gel strength can

cause mud losses on connections, trips

Increases surge and swab effects when BHA is in a small dia. Hole

Also affects ECD Mud rheology & density

can be changed for trips to sustain hole integrity

Hydraulics is a vital part of borehole stability!

Shearing rate

She

arin

g re

sist

ance

– mud viscosity

Yield point

Static condition

Dynamic conditions

Mud Rheology Diagram

YP

Effect of Mud Weight IncreaseEffect of Mud Weight Increase

n, normal stress

, shear stress

r a

MC failure line

c

Mohr’s circleof stresses

tanmax nc

Increasing MW (with good cake) reduces the stresses on the wall

yield

no yield

Effect of Loss of Good Filter Effect of Loss of Good Filter Cake Cake

n, normal stress

, shear stress

r a

MC failure line

c

Mohr’s circleof stresses

tanmax nc

With loss of mudcake effect, radial support disappears, shear stress increases

failure

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 60-80° cone

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

Favored holeorientation

Uncontrollable ParametersUncontrollable Parameters

Constrained trajectory (no choice as to the wellbore path)

Sequence of rock types (stratigraphy) Rock strength and other natural properties

Fractured shalesClay type in shales (swelling, coaly, fissile)Salt, etc.

Formation temperatures and pressures, plus other properties such as geochemistry

Natural earth stresses and orientations

Can You Live with Breakouts?Can You Live with Breakouts?

Yes, in most cases the breakouts are a natural consequence of high stress differences, and can be controlled

In exceptional cases, the breakouts are so bad that massive enlargement takes place

If hole advance is necessary, there are special things that can be done:Some new products, silicates, polymers that

set in the hole and can even be set and then drilled

Increase MW, even to the point of overbalanceGilsonite and graded LCM can help somewhat In desperation, set casing!

Some Diagnostic Hole Some Diagnostic Hole GeometriesGeometries

a.

drillpipe

b.

f.

e.

HMAX

hmin

Induced by high stress differences

General sloughing and washout

Keyseating

Fissility sloughing

Swelling, squeeze

Breakouts

Only breakouts are symmetric in one direction with an enlarged major axis

c.

d.

c.

Equivalent Circulating Density Equivalent Circulating Density

Viscous resistance increases the apparent mud weight at the bottom of the hole

This is a kinematic (viscosity) effect, and takes place only as the mud is circulating

ECD can lead to fracture at the bit though static pressure of mud column is below PF

As high as 2.0#/gal recorded in 4¾” hole! Real-time BHP pressure data allow it to be

measured and managed (offshore drilling)This leads to early warnings of high ECDThis leads to better control and mitigation

ECDECD

High ECD!

15 16 17 18 19 ppg

MW = 16.7 ppg (static value)

Dynamic pressure (ECD) because of friction, hole restrictions, high mud

Pressure gradient plot

De

pth

reamers and stabilizers

mud rings also increase ECD

BH

A a

nd c

olla

rs

PF (hmin)

A hydraulic fracture is induced at the base of the hole where the ECD exceeds PF (hmin). When the pumps stop, much of the mud comes back into the hole!

ECDECD

pBH = mud weight plus friction p loss High ECD values (>0.5 ppg) are related to:

High mud viscosities and gel strengths (evident on connections and trips as “breathing” of hole)

Rapid slim hole drilling leading to large cuttings loads in the drilling fluids near the bit

Limited clearance with BHA (MWD system), reamer system, extra large collars…

Sloughing of shales leading to partial mud rings or high cavings loads in the mud

Reducing ECD is the same as expanding your safe MW window for drilling!

High ECD EffectsHigh ECD Effects

High ECD!

15 16 17 18 19 ppg

poPF = hmin

MW = 16.7 ppg (static value)reamers

stabilizers

mud rings

BH

A a

nd c

olla

rs

Large mud losses at hole bottom because of fracturing

Cannot reduce the MW much because of borehole instability uphole or blowout danger on trips, connections, gas cutting…

Dynamic pressure (ECD) because of friction, hole restrictions, high mud

Gradient plot

Dep

th

Top of restrictive BHA

Reducing High ECD ValuesReducing High ECD Values

High ECD: excessive ballooning, high losses, increased risk, reducing the drilling window

The high ECD values can be reduced in several ways, here are a few examples:Reduce the mud weight (careful about gas cuts!)Reduce the viscosity and gel strengthAvoid sloughing above bit (increases ECD)Circulate out cavings and cuttings as neededUse less restrictive BHA, reduce ROPUse an off-center bit (lower friction losses)Redesign well plan (one less casing, larger hole)OBM probably somewhat better than WBM

North Sea ECD ExampleNorth Sea ECD Example

Serious ECD problems, but extra depth needed

Very long & restrictive BHA was being used

Drill (mud motor) to Z with 8.5” hole size

Trip out, replace bit with eccentric 9¾” bit

Ream to bottom & trip Drill to TD with the 8.5”

drill bit size Set 7” casing to TD

10¼” casing

High ECD Underream

Drill to TD

Some Other Comments on ECDSome Other Comments on ECD

If high drill chip loads from rapid ROP are contributing to ECD, reduce ROP

Lower viscosity and gel strength during drilling, but increase it a bit for trips

Break the gel strength of the mud during trips by pumping, rotating pipe as you are breaking circulation

Be careful in inclined and horizontal holes where pipe is not being rotated much, better to rotate more aggressively

Use LCM in mud to plug fractures

ECD ServicesECD Services Example of output

from BHI service MWD gauges used Gives ECD, MW,

annular pressure, connection effects…

This data can be used in a diagnostic manner during drilling to manage ECD and aid well performance

This website gives many useful formulae

http://www.tsapts.com.au/formulae_sheets.htm

Drilling and Shale FissilityDrilling and Shale Fissility

If a hole is within 20° of strong fissility… Sloughing is more

likely Shale breaks like

small brittle beams Breakouts can

develop deep into strata

In this GoM case, in the “tangent” section, the hole angle was 61°

Vertical offset hole, no problems

beddingdirection

Courtesy Stephen Willson, BP

Coping with Fissile Shale Coping with Fissile Shale SloughingSloughing If possible, stay at least 30° away from the

fissility dip direction (see sketch) Otherwise, keep your mud properties

excellent, keep circulation rate & ECD low, gilsonite and fn-gr LCM in mud may help…

Normal to bedding planes

100-120° coneKeep the drillhole within this cone to avoid severe fissility

sloughing problems

GRDENSITYCURVES

PHOTOELECTRICFACTOR CURVES

BOTTOM OFBOREHOLE

SECTION OFSHALE

BREAKOUTNote that the

majority of theshale sloughingappears to be

from the top of the borehole.

DENSITY NEUTRON IMAGE OF12500’ MD SHALE BREAK OUT

Density Neutron ImageDensity Neutron Image

From: Bruce Matsutsuyu

Drilling Through FaultsDrilling Through Faults

The fault plane region is often:Broken, sheared, weak shales and rocks It may have a high permeability It can be charged with somewhat higher po

First, expect the faults from your data:Seismic data analysisNear salt diapirs, especially shoulders

Accurate mud V(t) measurements can be of great value to good drilling

Cavings monitoring MWD (ECD, resistivity, bit torque…)

Borehole Shear DisplacementBorehole Shear Displacement

High angle faults, fractures can slip and cause pipe pinchingNear-slip earth stresses conditionHigh MW causes pw charging

Reduction in n leads to slipBHA gets stuck on trip out

Can be identified from borehole wall sonic scanner logs (profile logs)

Raising MW makes it worse! Lowering MW is better…

Also, LCM materials to plug the fault or joint plane are effective

npw

Slip of a High-Angle Fault PlaneSlip of a High-Angle Fault Plane

high pressuretransmission

slip of joint

slip of joint surface

borehole

casing bendingand pinching in

completed holes

(after Maury, 1994)

v = 1

h = 3

pipe stuck on trips

Slip Affected by Hole Slip Affected by Hole Orientation!Orientation!

OFFSET ALONG PRE-EXISTING DISCONTINUITIESOFFSET ALONG PRE-EXISTING DISCONTINUITIESFILTRATE

40

45

50

55

60

65

70

75

0 10 20 30 40 50 60 70 80 90 100

Inclination () (deg)

Eff

ec

tiv

e n

orm

al

str

es

s (

ba

r)

0

10

20

30

40

50

60

70

80

90

Azimuth:

TYPICALMUD

OVER-PRESSURE

TYPICALMUD

OVER-PRESSURE Courtesy Geomec a.s.

Diagnostics for Fault Slip Diagnostics for Fault Slip Problems Problems In tectonic areas, near salt diapirs… On trips, BHA gets stuck at one point Easy to drop pipe, hard to raise it Borehole scanner shows strange

shapes: not the same as keyseating or breakouts

Start of keyseat Serious keyseat Evidence of fault plane slip

drill pipe

Curing Fault Plane Slip ProblemsCuring Fault Plane Slip Problems Usually occurs up-hole in normal faulting

regimes that are highly faulted, jointed, as MW is increased to control po downhole

May occur suddenly near the bit when a fault is encountered

Back-ream through the tight zone High pw contributes to the slip of the

plane, thus reduce your MW if possible Condition the mud to block or retard the

flow of mud pressure into the slip plane:Gilsonite, designed LCM in the mud

Use an avoidance trajectory for the well

Mud Volume MeasurementsMud Volume Measurements

Extremely useful, but, accurate V/t needed

Case A: fracture/fault encountered, quickly blocked, now analyze data for k and aperture!

Case B: fractured rock not healed by LCM

Other cases have their own typical response curves (ballooning, slow kick…)

Diagnostics!

20

15

10

5

0

5 6 7 8 9

Losses - gpm

Time - minHole deepening rateFiltration

fluid loss

20

15

10

5

0

5 6 7 8 9

Losses - gpm

Time - minHole deepening rateFiltration

fluid loss

A

B

A Precise Mud Volume A Precise Mud Volume InstallationInstallation

(taken from SPE 38177 - Agip well)

Outlet mud line Precision flow meter

Actual Field Example of AnalysisActual Field Example of Analysis

2890

2910

2930

2950

2970

2990

3010

3030

3050

0 0.5 1

Hydraulic Aperture (mm)

De

pth

(m

)

2890

2910

2930

2950

2970

2990

3010

3030

3050

0 20 40 60

Average permeability (D/m)

This information proved extremely valuable for reservoir engineers in this case, as a gas reservoir was found

Courtesy Geomec a.s

Losses Identify Fractured ZonesLosses Identify Fractured Zones

-20

-10

0

10

20

30

40

50

60

70

4101.5 4101.7 4101.9 4102.1 4102.3 4102.5 4102.7Depth (m)

Qlo

ss (

L/m

in)

22 liters

19 liters

35 liters

25 liters

Mu

d L

os

s R

ate

– li

tre

s/m

in

Depth - m

Likely, each event involved filling a single fracture

Problems in Coal DrillingProblems in Coal Drilling

OBM are worse than WBM in CoalFiltrate penetrates easily (oil wettability)

Coal fractures open easily if pw > po

Coal is extremely compressible Difficult to build a filter cake on the wall

Fissure apertures open with surges Sloughing on trips, connections, large

washouts, … Packing off of cuttings and sloughed

Coal around the pipe, even during trips

Drilling in CoalDrilling in Coal

r

stresses around wellbore

fracture-dominated coal

Deep pore pressure penetration because of coal fractures

Massive sloughing

Mud rings and pack-off caused by slugs of cavings and cuttings

Drilling Fractured Coal SafelyDrilling Fractured Coal Safely

Keep jetting velocities low while drilling through the coal (avoid washouts)

Keep MW modest to avoid fractures opening and coal pressuring, low ECDs while the BHA is opposite the coal seams

Drill with graded LCM in the mud to plug the fractures and build a cake zone

Avoid swabbing and surging on trips See Appendix to Module H for some results

on drilling overbalanced with LCM

A Case History of Salt Diapir A Case History of Salt Diapir Drilling in the North SeaDrilling in the North Sea

North Sea Case, Shallow DepthNorth Sea Case, Shallow DepthWell A

Shallow Gas

Gas Pull Down

1a20

00 m

Courtesy Geomec a.s.

Above a Deep Diapir, North SeaAbove a Deep Diapir, North Sea

Normal faulting observed well above the top of the diapir, these will likely be zones of substantial mud losses (low hmin)

Beds are distorted, likely shearing has occurred along the bedding planes (weaker)

Seismic data show strong “gas pull-down effect”, lower seismic velocities because of free gas in the overlying shales and high po

Free gas zones are noted in the strata, and these will increase gas cuts

(Gas “pull-down” refers to the effect of free gas on seismic stratigraphy)

Deeper, Around the DiapirDeeper, Around the Diapir

Gas Pull Down

Top BalderTop Chalk

Intra Hod/Salt

Well A1b

Mid-Miocene regional pressure boundary

2000 m

3000 m

Courtesy Geomec a.s.

This region avoided

MWD RESISTIVITY LOG SIGNATURE (OBM)MWD RESISTIVITY LOG SIGNATURE (OBM)

Well A

0.1

1

10

100

2540 2560 2580 2600 2620 2640 2660

Depth (m MD-RKB)

MW

D R

esis

tivi

ty (

Oh

m.m

)

SESP

SEDP

Well A

0.1

1

10

100

2540 2560 2580 2600 2620 2640 2660

Depth (m MD-RKB)

MW

D R

esis

tivi

ty (

Oh

m.m

)

SESP

SEDPInvaded ZoneInvaded Zone

Time-lapse and different spacing resistivity logging data identified fractured zone clearly

Courtesy Geomec a.s.

INVADED ZONE Symmetry(O-B)INVADED ZONE Symmetry(O-B)

Well A

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

2540 2560 2580 2600 2620 2640 2660

Depth (m MD-RKB)

Rat

io S

ED

P/S

ES

P (

Oh

m.m

)

Well A

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

2540 2560 2580 2600 2620 2640 2660

Depth (m MD-RKB)

Rat

io S

ED

P/S

ES

P (

Oh

m.m

)

Courtesy Geomec a.s.

What Was Done to Improve What Was Done to Improve Drlg?Drlg? A trajectory was chosen to avoid the worst

of the crestal faulting and gas pressuresShales also intersected at ~ 90to fissility

Mud losses were carefully monitored with depth in the critical zones, then analyzed

Designed LCM in the mud allowed a bit of overbalance in a critical region

Of course, gas cuts, shale chip geometry, total cutting volumes, etc., and many other things were monitored in “real-time”

Statfjord Case: North SeaStatfjord Case: North Sea

0

1

2

3

4

5

6

Mu

d P

ress

ure

min

us

s

tre

ss in

Me

ga

Pas

cal

s

B-06B B-23AT2 B-39A B-39BT2

Well

STATFJORD

These wells were drilled with overbalance: a MW above the lowest estimated hmin in the zone

Courtesy Geomec a.s.

OVERBALANCED!-800 psi

Fracturing pressure can be increased by several 100 psi by graded LCM, analysis

Young’s modulus (E) is the control parameter

Induced fractures or even natural fractures encountered open up almost immediately to their final width:This aperture controls LCM design

The plugging happens rapidly with right LCM The effect is enhanced with high viscosity

mud and slower ROP Design tools are available for this

ConclusionsConclusions

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 during trips

• monitor cavings and mud losses for warning of fractured zones

Courtesy Stephen Willson, BP

Executing this Difficult WellExecuting this Difficult Well

Background gas controlled by ROP, not MW Monitoring greatly reduced “wiper trips” Continuous ECD and mud volume

monitoring to avoid destabilization (+”charged” faults)

Chip analysis to identify fractured shales Strength profile modified “on-the-fly” using

ISONIC MWD + behavior + prognosis Ballooning analysis refined hmin data Hole condition from CRD scan on trips Weighted pills placed for trips Mud properties well maintained (ECD…)

Trajectory Variations ExampleTrajectory Variations Example

Erskine HPHT field Deviated holes need

MWD, better control, the dashed line path was abandoned

Instead, reach was established above HTHP zone, then the well turned vertical

No MWD used, hole cleaning was better, lower ECD, etc…

Also, low flow rates, low surge-swab, etc…

S-profile trajectory

A vertical trajectory in the HTHP zone proved to be cheaper and faster, rather than steering an inclined well trajectory

Reach section

5000 m

Top of HTHP zone

Real-Time Wellbore StabilityReal-Time Wellbore Stability

For deep, difficult, costly holes only Quality prognosis is needed – po(z), hmin(z) Diagnostic tools used:

Real-time pressures (ECD management)Caliper and resistivity data, D-exponent dataBorehole imagery (on trips)Accurate mud loss gauges & ballooning

analysisCuttings volumes and visual classification

Prevention and and remediation options:Mud properties and special chemicalsHydraulics, drilling parameters, reamers…Special cures… (pills, LCM,,,)

Tests on the Rig Floor on ChipsTests on the Rig Floor on Chips

Performed on “intact” cuttings Brinnell hardness is related to strength The dielectric properties can be related

to the shale geochemical sensitivity Sonic travel time can be related to

strength and stiffness empirically You can use dispersion tests in water of

different salinities to assess swelling Even some others can be used These can be taken regularly and plotted

as a log versus depth (very useful)

Mud Cooling to Increase Mud Cooling to Increase Borehole Stability in ShalesBorehole Stability in Shales

Heating and Cooling in the HoleHeating and Cooling in the Hole

depth

T

casing

geothermaltemperature

bit

cooling

heating

mudtemperature

shoe

+T

-T

muddownpipe

mud upannulus

coolingin tanks

BHA

drillpipe

openhole

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 , often promoting failure and sloughing.

At the bit, cooling, shrinkage, both of which enhance stability.

Commercial software exists to draw these curves

T Effects in the BoreholeT Effects in the Borehole

Mud goes down the drillpipe fast: ~5 to 10 faster than it returns up the annulus

It picks up heat from rising mud in annulus At the bit, still 10°-40°C cooler than rock in

HT wells with long open-hole sections Rising uphole, the mud picks up heat from

formation, and heats rapidly till the cross-over point (T diff. Is as large as 30°-40°C)

Then, it cools all the way to the surface It gets to the tanks hot, and loses some

heat, but usually goes back in quite warm

A Simple Quantitative Example…A Simple Quantitative Example…

Change in at the wall is given by: ]ri ~ (T··E)/(1-)

E = Young’s modulus = 1 to 5106 psi = Thermal expan. coef. = 10-1510-6/°C = Poisson’s ratio = 0.30 – 0.35 T = Temperature change Reasonable values are: E = 3106 psi, =

12 10-6/°C, = 0.35, T = +25°C This increases at the wall by ~1400 psi! Not good for shale stability!

Heat Also Reduces Strength a Heat Also Reduces Strength a BitBit

0

20

40

60

80

0 0.5 1 1.5 2 2.5 3 3.5

Strain (%)

Dev

iato

ric

stre

ss (

MP

a) Temperature = 20°C

Temperature = 60°C3 = 2.5 MPa

Mancos shale

About 10% strength loss for this T, so this is a secondary effect

More Temperature EffectsMore Temperature Effects

+T reduces strength, increases stress +T also makes adsorbed water more mobile Absorbed water layer thickness is reduced Either water is expelled, or stresses must

change because the pore pressure changes In either case, additional V takes place, in

addition to thermoelastic effects Furthermore, reaction rates change w. T Boy! Does this make modeling difficult!

Cooling the Mud Reduces +Cooling the Mud Reduces +TT

depth

T

cooling

+T

-T

mud upannulus

BHA

Cooling mud

The mud is cooled at surface through heat exchangers and sea water. As much as -30°C to -40°C is feasible in some cases.

Now, the amount of heating at the shoe is very small, only a few degrees.

Also, the shale remains stronger by virtue of the cooling.

There are other benefits as well…

Benefits of Mud CoolingBenefits of Mud Cooling

Increases shale stability throughout hole! Low temperature reduces the rate of

negative geochemical reactions between the mud filtrate and the shale

Generally, mud properties are far easier to maintain with cooler mud, lower cost

Tanks are less hot (in some areas, mud can exit the hole almost boiling!)

BHA is “protected” from high T Use it when appropriate!

Lessons LearnedLessons Learned

Stability in drilling involves many factorsRock mechanics information, cavings and

cuttings information, rig site tests…Hydraulics managementLithostratigraphic knowledgeMWD in difficult offshore cases (ECD)Temperature managementMW and rheology management

The key is rock mechanics behavior, as stability is mainly a stress issue

But… All factors must be considered together in difficult wells