TP00-09 1

Advanced Wellbore Stability Model

(WELLSTAB-PLUS)

Dr. William C. Maurer

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DEA-139 Phase I

DEA Sponsor: Marathon Duration: 2 Years Start Date: May 1, 2000 End Date: April 30, 2002 Participation Fee: $25,000/$35,000

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Typical Occurrences of Wellbore Instability in Shales

soft, swelling shale

brittle-plastic shale

brittle shale

naturally fractured shale

strong rock unit

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Wellbore Stability Problems

High Torque and Drag

Bridging and Fill

Stuck Pipe

Directional Control Problem

Slow Penetration Rates

High Mud Costs

Cementing Failures and High Cost

Difficulty in Running and Interpreting Logs

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Effect of Borehole Pressures

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PW PW

smax smax

smin smin

High Support Pressure Low Support Pressure

Effect of Mud Support Pressure on Rock Yielding

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Rock Failure Mechanisms

PLASTIC BRITTLE

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Rock Yielding around Wellbores Laboratory Tests

Rawlings et al, 1993

Isotropic Stresses Anisotropic Stresses

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Change In Near-Wellbore Stresses Caused by Drilling

sV (overburden)

sHmin

sHmax sHmin

sHmax

Pw (hydrostatic)

Before Drilling In-situ stress state

After Drilling Lower stress within wellbore

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Stress Concentration around an Open Wellbore

Pw

Po

sHmin

sHmax

sz sq

sq

sr

sz

sr

s

r

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Strength vs Stress Identifying the Onset of Rock Yielding

Shear

Str

ess

sr´

Effective Compressive Stress

Stable Stress State

sq´

sr´

Shear

Str

ess

sr´

Effective Compressive Stress

Unstable Stress State

sq´

sr´

sq´

Min Stress

Max Stress

sq´

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Effect of Pore Fluid Saturation

POROUS ROCK SOLID ROCK

Pf = Fluid Pressure

so=sz so=sz+pf

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Effect of Near-Wellbore Pore Pressure Change on Effective Stresses

Sh

ear

Str

ess

No Yield

Yield

Effective Compressive Stress

sr´ sq´ sr´ sq´

Po increase

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

Linear elastic model (BP)

Linear elastic model (Halliburton)

Elastoplastic Model (Exxon)

Pressure Dependent Young’s Modulus Model(Elf)

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Mathematical Algorithms

Dr Martin Chenervert (Un. Texas)

Dr. Fersheed Mody (Baroid)

Jay Simpson (OGS)

Dr. Manohar Lal (Amoco)

Dr. Ching Yew (Un. Texas)

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Stress State on Deviated Wellbore

s3

s2 sz sr

tqz b q

tzq

sq

a

s1

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(BP) Linear Elastic Model

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(Halliburton) Linear Elastic Model

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(Exxon) Elastoplastic Model

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(Elf) Pressure Dependent

Young’s Modulus

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Shale Borehole Stability Tests Darley, 1969

DIESEL DISTILLED WATER

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Montmorillonite Swelling Pressure Powers, 1967

80,000

60,000

40,000

20,000

0 4th 3rd 2nd 1st

5000

4000

3000

2000

1000

0

SW

ELLIN

G P

RESSU

RE, psi

kg/c

m2

LAYERS OF CRYSTALLINE WATER

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Shale Water Adsorption Chenevert, 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

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Shale Swelling Tests Chenevert, 1970

TIME - HOURS

LIN

EAR S

WELLIN

G -

%

.01 0.1 1.0 10

0.4

0.3

0.2

0.1

0

-0.1

1.00

0.91 0.88 0.84 0.75

0.25

Activity of Internal Phase

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Effect of K+Ions on Shale Swelling Baroid, 1975

Ca ++

K+

K+

K+

Na+

Cs+

Na+

Ca++

Li+

K+

Rb+

Cs+

Na+

Mg++

Na+

10A°

Na+

- -

- -

-

- -

-

-

-

- - -

-

-

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North Sea Speeton Shale Specimen Exposed at Zero DP to Drilling Fluid

Drilling Fluid:

Ionic Water-Base

(CaCl2 Brine)

Activity = 0.78

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North Sea Speeton Shale Specimen Exposed at Zero DP to Drilling Fluid

Drilling Fluid:

Oil-Base Emulsion

(Oil with CaCl2 Brine)

Activity = 0.78

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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

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Principle Mechanisms Driving Flow of Water and Solute

Into/Out of Shales

Force

Flow

Fluid (water)

Solute (ions)

Hydraulic Gradient (Pw Po) Chemical Potential

Gradient (Amud Ashale)

Hydraulic Diffusion

(Darcy´s Law)

Advection

Diffusion

(Fick´s Law)

Chemical Osmosis

H2O

H2O H2O

H2O t1

t2 t3

P

r

Other Driving Forces: Electrical Potential Gradient Temperature Gradient

H2O H2O

H2O H2O

H2O H2O

H2O

+ -

-

-

+

+

+

-

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Osmotic Flow of Water through Ideal Semi-Permeable Membrane

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

Water flow direction High concentration

of dissolved molecules or ions ( = Low Aw )

Low concentration of dissolved molecules

or ions ( = High Aw )

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Limitations of Existing Models

Do not handle shale hydration

Very complex

Input data not available

Limited field verification

Cannot field calibrate

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Mathematical Algorithms

Dr Martin Chenervert (Un. Texas)

Dr. Fersheed Mody (Baroid)

Jay Simpson (OGS)

Dr. Manohar Lal (Amoco)

Dr. Ching Yew (Un. Texas)

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Mechanical/Chemical Property Input

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Help Information as Clicking Question Mark

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Pore Pressure Input/Predict

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Pore Pressure Prediction via Interval Transit Time Log Data

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In-Situ Stresses Input/Predict

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Correlation to Determine Horizontal Stresses

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Output Windows

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Safe Mud Weight vs Well Inclination

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Safe Mud Weight Distribution by Azimuth

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Near-Wellbore Stresses Distribution

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Mohr Diagram

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Wellbore Stress Distribution

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Propagation of Swelling Pressure

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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)

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Increase salinity

Wellbore Stability Design (continued)

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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

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Increase salinity

Wellbore Stability Design (continued)

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Wellbore Stability Design (continued)

Low Value Membrane Efficiency

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Wellbore Stability Design (continued)

High Value Membrane Efficiency

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Field Calibration

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Field Calibration (continued)

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Project Tasks

Distribute Wellbore Stability Model (WELLSTAB)

Develop Enhanced Model (WELLSTAB-PLUS)

Add time dependent feature to model

Hold workshops

Conduct field verification tests

Write technical reports

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Field Verification Goals

Determine model accuracy

Improve mathematical algorithms

Field calibrate model

Make models more user-friendly

Convert wellbore stability from an art into a science

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