1TP00-09

Advanced Wellbore Stability Model

(WELLSTAB-PLUS)

Advanced Wellbore Stability Model

(WELLSTAB-PLUS)Dr. William C. Maurer

2TP00-09

DEA-139 Phase I

DEA Sponsor: MarathonDuration: 2 YearsStart Date: May 1, 2000End Date: April 30, 2002Participation Fee: $25,000/$35,000

3TP00-09

Typical Occurrences of WellboreInstability in Shales

soft, swelling shale

brittle-plastic shale

brittle shale

naturally fractured shale

strong rock unit

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Wellbore Stability Problems

8 High Torque and Drag8 Bridging and Fill8 Stuck Pipe8 Directional Control Problem8 Slow Penetration Rates8 High Mud Costs8 Cementing Failures and High

Cost8 Difficulty in Running and

Interpreting Logs

7TP00-09

Effect of Borehole Pressures

8TP00-09

PW PW

smax smax

smin smin

High Support Pressure Low Support Pressure

Effect of Mud Support Pressureon Rock Yielding

10TP00-09

Rock Failure Mechanisms

PLASTICBRITTLE

11TP00-09

Rock Yielding around WellboresLaboratory Tests

Rawlings et al, 1993

Isotropic Stresses Anisotropic Stresses

12TP00-09

Change In Near-Wellbore StressesCaused by Drilling

V (overburden)

Hmin

Hmax Hmin

Hmax

Pw (hydrostatic)

Before DrillingIn-situ stress state

After DrillingLower stress within wellbore

13TP00-09

Stress Concentration around an Open Wellbore

Pw

Po

sHmin

sHmax

szsq

sq

sr

sz

sr

s

r

14TP00-09

Strength vs StressIdentifying the Onset of Rock Yielding

Sh

ear

Str

ess

Shear Strength

r´

Effective Compressive Stress

Stable Stress State

q´

r´

Sh

ear

Str

ess

Shear Strength

r´

Effective Compressive Stress

Unstable Stress State

q´

r´

q´

MinStress

MaxStress

q´

15TP00-09

Effect of Pore Fluid Saturation

POROUS ROCKSOLID ROCK

Pf = Fluid Pressure

o=zo=z+pf

18TP00-09

Effect of Near-WellborePore Pressure Changeon Effective Stresses

Sh

ear

Str

ess

No Yield

Yield

Shear Strength

Effective Compressive Stress

sr´ sq´sr´ sq´

Po increase

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20TP00-09

MEI Wellbore Stability Model:(mechanical model, does not include chemical

effects)

8Linear elastic model (BP)8Linear elastic model

(Halliburton)8Elastoplastic Model (Exxon)8Pressure Dependent

Young’s Modulus Model(Elf)

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Mathematical Algorithms

8 Dr Martin Chenervert (Un. Texas)

8 Dr. Fersheed Mody (Baroid)

8 Jay Simpson (OGS)

8 Dr. Manohar Lal (Amoco)

8 Dr. Ching Yew (Un. Texas)

22TP00-09

Stress State on Deviated Wellbore

s3

s2sz srtqzb

q

tzq

sq

a

s1

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24TP00-09

(BP)Linear Elastic Model

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26TP00-09

(Halliburton) Linear Elastic Model

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28TP00-09

(Exxon)Elastoplastic Model

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(Elf)Pressure Dependent

Young’s Modulus

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35TP00-09

Shale Borehole Stability TestsDarley, 1969

DIESELDISTILLED WATER

36TP00-09

Montmorillonite Swelling PressurePowers, 1967

80,000

60,000

40,000

20,000

04th 3rd 2nd 1st

5000

4000

3000

2000

1000

0

SW

ELL

ING

PR

ES

SU

RE

, p

si

kg

/cm

2

LAYERS OF CRYSTALLINE WATER

37TP00-09

Shale Water AdsorptionChenevert, 1970

0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00

5

4

3

2

1

0

WEIG

HT %

WATER

WATER ACTIVITY - aW

DESORPTION

ADSORPTION

38TP00-09

Shale Swelling TestsChenevert, 1970

TIME - HOURS

LIN

EA

R S

WE

LLIN

G -

%

.01 0.1 1.0 10

0.4

0.3

0.2

0.1

0

-0.1

1.00

0.910.880.840.75

0.25

Activity of Internal Phase

39TP00-09

Effect of K+Ions on Shale SwellingBaroid, 1975

Ca ++

K+

K+

K+

Na+

Cs+

Na+

Ca++

Li+

K+

Rb+

Cs+

Na+

Mg++

Na+

10A°

Na+

--

-- -

--

-

-

--

- --

-

41TP00-09

North Sea Speeton Shale SpecimenExposed at Zero DP to Drilling Fluid

Drilling Fluid: Ionic Water-Base (CaCl2 Brine)Activity = 0.78

42TP00-09

North Sea Speeton Shale SpecimenExposed at Zero DP to Drilling Fluid

Drilling Fluid: Oil-Base Emulsion (Oil with CaCl2 Brine)Activity = 0.78

43TP00-09

North Sea Speeton Shale Specimen

Exposed at Zero DP to Drilling Fluid

Drilling Fluid: Non-Ionic Water-Base (Methyl Glucoside in Fresh Water)Activity = 0.78

44TP00-09

Principle Mechanisms DrivingFlow of Water and Solute

Into/Out of Shales

Force

Flow

Fluid(water)

Solute(ions)

Hydraulic Gradient (Pw ¹ Po)Chemical Potential

Gradient (Amud ¹ Ashale)

HydraulicDiffusion

(Darcy´s Law)

Advection Diffusion(Fick´s Law)

ChemicalOsmosis

H2O

H2O H2O

H2Ot1

t2

t3

P

r

Other Driving Forces: Electrical Potential Gradient Temperature Gradient

H2O H2O

H2O H2O

H2OH2O

H2O

+ -

-

-

++

+

-

45TP00-09

Osmotic Flow of Water throughIdeal Semi-Permeable Membrane

Ideal Semipermeable Membrane - permeable to water - impermeable to dissolved molecules or ions

Water flow directionHigh concentrationof dissolved molecules

or ions ( = Low Aw )

Low concentrationof dissolved molecules

or ions ( = High Aw )

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50TP00-09

Limitations of Existing Models

8Do not handle shale hydration

8Very complex8 Input data not available8Limited field verification8Cannot field calibrate

51TP00-09

Mathematical Algorithms

8 Dr Martin Chenervert (Un. Texas)

8 Dr. Fersheed Mody (Baroid)

8 Jay Simpson (OGS)

8 Dr. Manohar Lal (Amoco)

8 Dr. Ching Yew (Un. Texas)

52TP00-09

Mechanical/Chemical Property Input

53TP00-09

Help Information as Clicking Question Mark

54TP00-09

Pore Pressure Input/Predict

55TP00-09

Pore Pressure Predictionvia Interval Transit Time Log Data

56TP00-09

In-Situ Stresses Input/Predict

57TP00-09

Correlation to DetermineHorizontal Stresses

58TP00-09

Output Windows

59TP00-09

Safe Mud Weight vs Well Inclination

61TP00-09

Safe Mud Weight Distribution by Azimuth

62TP00-09

Near-Wellbore Stresses Distribution

63TP00-09

Mohr Diagram

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Wellbore Stress Distribution

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Propagation of Swelling Pressure

68TP00-09

Too large inclination

Wellbore Stability Design (continued)

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Wellbore Stability Design (continued)

Decrease inclination

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Wellbore Stability Design (continued)

Too high mud weight

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Wellbore Stability Design (continued)

Decrease mud weight

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Not enough salinity

Wellbore Stability Design (continued)

73TP00-09

Increase salinity

Wellbore Stability Design (continued)

74TP00-09

Wellbore Stability Design(through Mud Weight-Salinity diagram)

Too low mud weight

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Wellbore Stability Design (continued)

Increase mud weight

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Wellbore Stability Design (continued)

Not enough salinity

77TP00-09

Increase salinity

Wellbore Stability Design (continued)

78TP00-09

Wellbore Stability Design (continued)

Low Value Membrane Efficiency

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Wellbore Stability Design (continued)

High Value Membrane Efficiency

80TP00-09

Field Calibration

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Field Calibration (continued)

86TP00-09

Project Tasks

8 Distribute Wellbore Stability Model (WELLSTAB)

8 Develop Enhanced Model (WELLSTAB-PLUS)

8 Add time dependent feature to model

8 Hold workshops8 Conduct field verification tests8 Write technical reports

87TP00-09

Field Verification Goals

8Determine model accuracy8 Improve mathematical

algorithms8Field calibrate model8Make models more user-

friendly8Convert wellbore stability

from an art into a science

89TP00-09

90TP00-09