SPE DISTINGUISHED LECTURER SERIESis funded principally

through a grant of the

SPE FOUNDATIONThe Society gratefully acknowledges

those companies that support the programby allowing their professionals

to participate as Lecturers.

And special thanks to The American Institute of Mining, Metallurgical,and Petroleum Engineers (AIME) for their contribution to the program.

2

Wellbore Stability Issues in Shales or Hydrate Bearing Sediments

Reem Freij-AyoubCSIRO Petroleum

Australia

Dr Reem Freij-Ayoub Tel: 61864368631Reem.freij-ayoub@csiro.au, http://www.dpr.csiro.au/

CSIROPETROLEUM

3

Acknowledgements

Geomechanics team at CSIRO petroleum with > 17 Geomechanics team at CSIRO petroleum with > 17 years of industry supported research (e.g. years of industry supported research (e.g. PETRONAS, PDVSA Intevep, Woodside ..etcPETRONAS, PDVSA Intevep, Woodside ..etcmillions of savings /projectmillions of savings /projectXavier ChoiXavier ChoiChee Tan and Bailin Wu (currently Schlumberger) Chee Tan and Bailin Wu (currently Schlumberger) Mohammed AmanullahMohammed Amanullah

4

OutlineWhen do we need Geomechanics?When do we need Geomechanics?Consequences of wellbore instabilityConsequences of wellbore instabilityWellbore stability model dataWellbore stability model dataWellbore stability in shale formationsWellbore stability in shale formations–– Main processesMain processes–– Modelling resultsModelling results–– Key MessagesKey MessagesWellbore stability in hydrate bearing sedimentsWellbore stability in hydrate bearing sedimentsFurther challenges in GeomechanicsFurther challenges in Geomechanics

5

When do we need Geomechanics?Wellbore stability in difficult formations Wellbore stability in difficult formations loss of loss of

US$ 2 billions/yrUS$ 2 billions/yr

–– Shale formations Shale formations 90% of incidents 90% of incidents

–– Hydrate bearing sediments (HBS)Hydrate bearing sediments (HBS)

–– Salt formationsSalt formations

–– HPHT reservoirsHPHT reservoirs

–– Brown fieldsBrown fields

Sand production Sand production US$ billions/yrUS$ billions/yrReservoir subsidenceReservoir subsidenceFault seal analysisFault seal analysis

shalesshales

6

Consequences of wellbore instabilitySloughing or cavingSloughing or caving

Packoffs, blowouts, or mud lossesPackoffs, blowouts, or mud losses

Increased mud treatment costIncreased mud treatment cost

Stuck pipe and loss of equipmentStuck pipe and loss of equipment

Side tracks or well abandonmentSide tracks or well abandonment

7

Wellbore stability model data

Stability prediction model

In-situ stressesRegional dataLeakoff testBreakoutsFractures

Petrophysical dataPorosity

PermeabilityBulk density Salt concentration

Reflection coefficient

Chemical properties

Specific heatConductivity

Thermal properties

Reservoir pressure

Log dynamic propertiesLab tests

Correlations

Rock mechanics data

8

Processes affecting wellbore stability in shaleformations when using water based mud

shalesshales

Hydrational stress

Pore p

ress

ure

Fluid Flow & Mud Pressure

penetrationPore pressure

Mechanical Deformation(Poro - elasticity)

Swellingor

Shrinkage

Heat TransferPore pressure

Thermal stre

ss

Chemical PotentialMechanism

Pore pressure

9

Mud pressure penetration mechanism

Overbalance drilling conditions Overbalance drilling conditions meanmeanMud filtrate (viscosity, adhesion and density) will invade Mud filtrate (viscosity, adhesion and density) will invade the formationthe formationReduction of differential pressure Reduction of differential pressure leads to reduction ofleads to reduction ofthe mechanical support to the wellbore wallthe mechanical support to the wellbore wallTo eTo ensure high breakthrough pressure nsure high breakthrough pressure –– Optimize drilling mud propertiesOptimize drilling mud properties–– Consider the formation pore size distributionConsider the formation pore size distribution

shalesshales

10

Chemical potential mechanism

The chemical potential The chemical potential controls controls direction of flowdirection of flow–– The chemical potential of a fluid = F (dissolved The chemical potential of a fluid = F (dissolved

ion concentration)ion concentration)

–– Water flows from low to high salt concentration Water flows from low to high salt concentration

The membrane can be nonThe membrane can be non--ideal or leakyideal or leaky

shalesshales

Shales have fine pores and (Shales have fine pores and (--) charges, they act as a semi) charges, they act as a semi--permeable membranepermeable membraneIdeal semiIdeal semi--permeable membranes permeable membranes permitpermit flow of water but flow of water but not salt ions (osmosis)not salt ions (osmosis)

11

The drilling mud and formation differ in temperature The drilling mud and formation differ in temperature –– Geothermal gradient: the drilling mud is cooling Geothermal gradient: the drilling mud is cooling

the bottom and heating the top of the wellborethe bottom and heating the top of the wellboreHeating the formation Heating the formation thermal expansion of thermal expansion of formation & pore fluid:formation & pore fluid: thermal stresses develop. thermal stresses develop. Pore fluid & formation have different coefficients of Pore fluid & formation have different coefficients of thermal expansion thermal expansion pore pressure increases.pore pressure increases.Hydraulic and thermal diffusivities are different Hydraulic and thermal diffusivities are different leading toleading to pressure buildpressure build--up.up.Other effects on drilling mud characteristicsOther effects on drilling mud characteristics……..

Heat transport mechanismshalesshales

12

Swelling mechanism

Reactive shales + nonReactive shales + non--inhibitive muds inhibitive muds pore pore pressure increase pressure increase && adsorption of wateradsorption of water

This leads to hydrational strain=swelling or This leads to hydrational strain=swelling or hydrational stresses & a possible shear failurehydrational stresses & a possible shear failure

Water is reactive with shales, low viscosity, high Water is reactive with shales, low viscosity, high wetting characteristics i.e., nonwetting characteristics i.e., non--inhibitiveinhibitive

Inhibitive water based mud with low wetting Inhibitive water based mud with low wetting characteristics reduces the risk of wellbore instabilitycharacteristics reduces the risk of wellbore instability

shalesshales

13

Modelling results: mud chemical composition

shalesshales

Drilling mud has 20% lower salt concentration

Mud weight 45 MPa

Drilling mud has 20% higher salt concentration

Mud weight 45 MPa

25

30

35

40

45

50

55

60

65

70

1 1.5 2 2.5 3

Normalized Radial Distance From Wellbore Centre

Pore

Pre

ssur

e (M

Pa) 0.0008 hr

0.1 hr0.266 hr17.5 hr

23

24

25

26

27

28

29

30

31

1 1.5 2 2.5 3

Normalized Radial Distance From Wellbore Centre

Pore

Pre

ssur

e (M

Pa)

0.0008 hr0.1 hr0.266 hr17.5 hr

14

2025303540455055606570

1 1.2 1.4 1.6 1.8 2

Normalized Radial Distance From Wellbore Centre

Pore

Pre

ssur

e (M

Pa)

L at 0.0008 hrL at 0.1 hrL at 0.266 hrL at 17.5 hrH at 0.0008 hrH at 0.1 hrH at 0.266 hrH at 17.5 hr

Comparison between high and low salt concentration mud

Mud weight 45 MPa

15

Modelling results: mud temperature

Hot mud Cold mud

Temperature (ºc) contours

Pressure (MPa) contours

shalesshales

30

28.6929.4

31.3

3030.65

50

2037

50

2037

wellbore wall

Temperature Temperature

Pore Pressure Pore Pressure

16

Modelling results: mud temperature

Pore Pressure evolutionPressure change due to 30º (heating or cooling) > 2 MPa

shalesshales

Hot mud

Cold mud

17

Modelling results: swelling shales

Swelling is significant when non-inhibitive mud is used with highly stiff reactive shales

shalesshales

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

0 0.5 1 1.5 2 2.5

Time (hrs)

Safe

ty F

acto

r in

Shea

r

Soft shale &inhibitive mudStiff shale &inhibitive mudSoft shale &noninhibitive mudStiff shale &noninhibitive mud

18

Careful design of water base drilling fluids is needed to control PP and SF evolution

Results at 5% well radius inside the formation

shalesshales

Drilling fluid pressure = 45 MPaFormation Pressure =30 MPa

Time=0.0025 hour Time=0.1 hour

Drilling Fluid Pressure Safety Factor Pressure Safety Factor

Water 33.37 1.36 42.39 1.07

Low filtrate invasion 31.25 1.42 38.05 1.19

Lower salt concentration than the formation

36.55 1.28 54.42 0.76

Higher salt concentration than the formation

29.80 1.48 25 1.60

Hot water 38.33 1.25 43.71 1.02

Cold water 28.41 1.48 41.07 1.11

Cold-low filtrate invasion-higher salt concentration than formation

23.76 1.62 23.2 1.72

Key Messages

19

DrillersDrillers’’ wellbore stability toolwellbore stability tool

Mud weight vs. well orientation

In-situ stress bounds

shalesshales

High pressure High pressure triaxial celltriaxial cell

NN

σσHH

σσhh

0.5

0.55

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2

N

σh/ σ

v Low

er b

ound

N

Upp

er b

ound

N Hydraulic fracture limit(standard leak-off test)

Permissible horizontal stresses

Lower bound σh/σv

Normal fault condition

20

What are hydrates and why are hydrate bearing sediments (HBS)

important ?

Hydrate stability domain

Hydrates on sea bed (USGS)

Hydrates are• Source of energy• Way to transport natural gas• Geohazard• Climate change• Drilling hazard• Problem in flow assurance

21

Hydrate stability domain

DepressurizationDepressurizationThermal stimulationThermal stimulationChemicals affect hydrate stability:Chemicals affect hydrate stability:–– Thermodynamic inhibitors Thermodynamic inhibitors

(salts, alcohols, g(salts, alcohols, glycols) can lycols) can inhibit hydrate formation in the inhibit hydrate formation in the mudmud

–– Kinetic additives (eg lecithin + Kinetic additives (eg lecithin + poly Npoly N--vinyl pyrrolidonevinyl pyrrolidone ((PVP)) PVP)) stabilize hydrates in the stabilize hydrates in the formation formation

00

250250

500500

750750

10001000

12501250

Pres

sure

(Psi

a)Pr

essu

re (P

sia)

1010 2020 3030 4040 5050 6060 7070 8080 9090 100100

Temperature (Temperature (°°F)F)

With inhibitorWith inhibitorNo inhibitorNo inhibitor

Chemical inhibitionChemical inhibition

ThermalThermal

DeDe--pressurizationpressurization

HydratesHydratesunstableunstable

HydratesHydratesstablestable

http://www.ench.ucalgary.ca/~hydrates/kinetics.html#E

22

Constrained modulus of HBS

Pre-dissociation (with hydrates stable) constrained modulus dependence on

hydrate saturation

Post-dissociation (after hydrates dissociated) constrained modulus dependence on hydrate saturation

Results of confined compression tests on natural gas hydrate bearing silica sands (In collaboration with Heriot-Watt University)

0

1

2

3

4

5

0 10 20 30 40Hydrate Saturation (%)

Pre-

Dis

soci

atio

n C

onst

rain

ed M

odul

us(G

Pa)

0

0.2

0.4

0.6

0.8

1

0 10 20 30 40Hydrate Saturation (%)

Post

-Dis

soci

atio

n C

onst

rain

ed M

odul

us(G

Pa)

23

HBS well control problems

Gas hydrate-related drilling problems (Adapted from Maurer Engineering, Inc.)

Hydrates are sensitive to the drilling mud:Hydrates are sensitive to the drilling mud:–– PressurePressure–– TemperatureTemperature–– Chemical compositionChemical composition

Hydrate destabilization Hydrate destabilization –– Drilling mud contamination with gasDrilling mud contamination with gas

Reduction of mud densityReduction of mud densityChange of mud rheologyChange of mud rheology

–– Decrease of formation strength & loss of Decrease of formation strength & loss of mud support mud support wellbore instabilitywellbore instability

24

HBS and casing integrity

During hydration of casing cement During circulating of hotter drilling mud

Casing collapse

Gas hydrate-related casing problems (Adapted from Maurer Engineering, Inc.)

Productionfacilities

Free-gas

HydrateCollapsedcasing

Casedborehole

Production of hot hydrocarbons

Circulation of hot fluid

Cased Borehole

Free Gas

25

Wellbore stabilization strategies

Optimal design of the drilling mud to enhance hydrate Optimal design of the drilling mud to enhance hydrate stability (use kinetic additives e.g.: lecithin, etc.)stability (use kinetic additives e.g.: lecithin, etc.)

Selection of casing cement with low hydration heatSelection of casing cement with low hydration heat

Use predictive modelingUse predictive modeling–– Analyze stability of the wellbore formation using coupled Analyze stability of the wellbore formation using coupled

mechanicalmechanical--thermalthermal--chemical/kineticchemical/kinetic

–– Assess conductor integrity when circulating hot fluid Assess conductor integrity when circulating hot fluid through the hydrates zone. through the hydrates zone.

26

Relative Porosity Profiles

0.4

0.5

0.6

0.7

0.8

0.9

1

1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2Normalized Distance Inside Formation

Nor

mal

ized

Po

rosi

ty

after excavationat 0.14 hrsat 0.28 hrsat 1.39 hrsat 5.56 hrs

Wellbore stability in HBS: heating the wellbore walls

Hydrates Dissociation Profiles

020406080

100

1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2

Normalized Distance Inside Formation

% H

ydra

tes

Dis

ssol

ved

after excavation

at 0.14 hrs

at 0.28 hrs

at 1.39 hrs

at 5.56 hrs

Cohesion: reduced in near wellbore region by hydrate melting

MinMin Max=initialMax=initial

Porosity: increased in near wellbore region by hydrate melting

MaxMax Min=initialMin=initial

27

Key messages: stability of wellbores in HBS

Shear failurefailure zone

Encountering hydrates in the sediments while drilling a Encountering hydrates in the sediments while drilling a wellbore can increase the size of the yield zone by 32% wellbore can increase the size of the yield zone by 32% The importance of employing techniques to stabilize HBS The importance of employing techniques to stabilize HBS in deep water drilling. in deep water drilling.

Failure zoneFailure zone

Radius of Plasticity-Failure Zone

1

1.5

2

2.5

3

3.5

4

0 0.5 1 1.5 2

Time (hrs)

Rad

ius

of P

last

icity

N

orm

alis

ed b

y W

ellb

ore

Rad

ius

Hot permeable HBS

Hot permeable sediments

Hot impermeable sediments

Hot impermeable HBS

28

Casing Integrity in HBS: 20in API casing

Progress of shear failure inside the strong cement and tension failure inside the HBS as the cement and the formation heat up 8.33 hrs after heat application to the casing. Isotropic stress field, K0=0.5

Strong cement Strong cement Thermal conductivityThermal conductivity 0.660.66 WmWm--11KK--11

Modulus of elasticityModulus of elasticity 55.16 GPa55.16 GPaPoissonPoisson’’s ratios ratio 0.40.4cohesioncohesion 11.4 MPa11.4 MPaFriction angleFriction angle 1010ººTensile strengthTensile strength 2.6 MPa2.6 MPaWeak cementWeak cementModulus of elasticityModulus of elasticity 0.4 GPa0.4 GPaPoissonPoisson’’s ratios ratio 0.20.2CohesionCohesion 1.2 MPa.1.2 MPa.Friction angleFriction angle 1010ººTensile strengthTensile strength 0.379 MPa0.379 MPa

Strong cement

tension

shear

29

Results for 20in API casing

Shear failure of all the weak cement and further part of the HBS and a band of mixed mode: tension and shear failure develops at the boundary of the cement 15minutes after heat application to the casing. No change is observed at 8.33 hours. Isotropic stress field, K0=0.5.

Weak cement

shear

Mixed mode

Casing Safety Factor for HBS and NHBS with Weak Cement-Isotropic Stress Field-Various Stress Ratios

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

0 0.5 1 1.5 2 2.5 3

Time (Days)

Cas

ing

Safe

ty F

acto

r

WS_H_K=0.5WS_H_K=1.0WS_H_K=2.0WS_NH_K=0.5WS_NH_K=1.0WS_NH_K=2.0

Casing safety factors for a wellbore drilled in NHBS or HBS and cased with 0.635in casing using weak cement. The strength degradation resulting from hydrate dissociation in the HBS upon heating reduce the SF for all stress ratios considered. Note that cases with lower stress ratios K0 have higher SF than cases with higher K0.

30

Further challenges and future trends in Geomechanics

narrow or nonexistent mud narrow or nonexistent mud windowwindow

sand & water production and sand & water production and management issuesmanagement issuesformation strengthening issuesformation strengthening issues

cuttings instead of coringcuttings instead of coring

one trip wellone trip well

HPHT reservoirsHPHT reservoirs

Brown fields and tail productionBrown fields and tail production

Rock properties measurementsRock properties measurements

Deep water drillingDeep water drilling