Advanced Wellbore Stability Model (WELLSTAB-PLUS)

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Advanced Wellbore Stability Model(WELLSTAB-PLUS)
DEA-139
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 WellboreInstability in
Shales
soft, swelling shale
brittle-plastic shale
brittle shale
naturally fractured shale
strong rock unit
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Cost of Wellbore Instability Problems
500 million/year, before 1992 G.M. Bol, SPE
24975 1992 SPE European Petroleum Conference 92
million, BP 1997 38 million, BP 1998 first
quarter J. Kijowski, BP-Amoco Downhole Talk,
Issue 80
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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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Wellbore Failure MechanismsMAURY et al., 1987
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Effect of Borehole Pressures
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Effect of Mud Support Pressure on Rock Yielding
High Support Pressure
Low Support Pressure
smax
smax
smin
PW
smin
PW
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Rock Failure
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Rock Failure Mechanisms
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Rock Yielding around WellboresLaboratory Tests
Rawlings et al, 1993
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Change In Near-Wellbore StressesCaused by
Drilling
Before Drilling In-situ stress state
After Drilling Lower stress within wellbore
?V (overburden)
Pw (hydrostatic)
?Hmax
?Hmax
?Hmin
?Hmin
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Stress Concentration around an Open Wellbore
sq
s
sz
sr
sq
sz
r
sr
Po
Pw
sHmin
sHmax
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Strength vs StressIdentifying the Onset of Rock
Yielding
?q
Shear Strength
Shear Strength
?q
Shear Stress
Shear Stress
?r
?r
Unstable Stress State
Min Stress
Max Stress
Stable Stress State
?r
?r
?q
?q
Effective Compressive Stress
Effective Compressive Stress
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Effect of Pore Fluid Saturation
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Effective Stresses Partioningof Total Stress
betweenMineral Grains and Pore Fluids
s
s
Po
s s - a Po s - effective stress s -
total stress Po - pore pressure a - Biot
Coefficient ( 1 for weak, porous rocks)
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Effective Rock Stress
?z ? o - pf
?o Overburden Stress ?z Matrix Stress pf
Pore Fluid Pressure
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Effect of Near-WellborePore Pressure Changeon
Effective Stresses
Yield
Shear Strength
No Yield
Shear Stress
Po increase
sr
sq
sr
sq
Effective Compressive Stress
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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 Youngs 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
tzq
s2
sz
sr
a
b
tqz
q
sq
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 Youngs Modulus
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Shale Borehole Stability TestsDarley, 1969
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Montmorillonite Swelling PressurePowers, 1967
80,000
5000
4000
60,000
3000
kg/cm2
SWELLING PRESSURE, psi
40,000
2000
20,000
1000
0
0
4th
3rd
2nd
1st
LAYERS OF CRYSTALLINE WATER
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Shale Water AdsorptionChenevert, 1970
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4
3
WEIGHT WATER
2
DESORPTION
1
ADSORPTION
0
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
WATER ACTIVITY - aW
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Shale Swelling TestsChenevert, 1970
0.4
0.3
Activity of Internal Phase
1.00
LINEAR SWELLING -
0.2
0.91
0.88
0.1
0.84
0.75
0
0.25
-0.1
.01
0.1
1.0
10
TIME - HOURS
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Effect of KIons on Shale SwellingBaroid, 1975
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Effect of Swelling Strains onWellbore Stability
Most Likely Scenario for SoftReactive Shales in
Low Stress Settings
Soft, Swelling Shale
Hole Closure due to Swelling Strains
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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 DrivingFlow of Water and
SoluteInto/Out of Shales
Force
Chemical Potential Gradient (Amud ¹ Ashale)
Hydraulic Gradient (Pw ¹ Po)
Flow
P
Fluid (water)
Hydraulic Diffusion (Darcys Law)
Chemical Osmosis
t3
t2
t1
r
Diffusion (Ficks Law)
Advection
Solute (ions)
Other Driving Forces Electrical Potential
Gradient
Temperature Gradient
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Osmotic Flow of Water throughIdeal
Semi-Permeable Membrane
Ideal Semipermeable Membrane - permeable
to water - impermeable to dissolved
molecules or ions
High concentration of dissolved molecules or ions
( Low Aw )
Low concentration of dissolved molecules or ions
( High Aw )
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Effect of Osmotic Flow on Near-Wellbore Pore
Pressure for a Balanced Bottomhole Pressure
Condition
Pore Pressure Decrease
Pore Pressure Increase
P
P
PW
P
PW
P
fm
fm
Osmotic flow from shale to mud
Osmotic flow from mud to shale
r
r
a
gt
a
a
lt
a
mud

shale
mud

shale
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Water Activity in Brine at Room Temperature
Water Acitivity in Salt Solution
1.0
0.9
KCl
0.8
NaCl
0.7
0.6
Water Activity
0.5
0.4
CaCl
2
0.3
0.2
0.1
0.0
0
5
10
15
20
25
30
35
40
45
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Salt Concentration, w/w
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A Critical IssueHow Efficient Are Shale
²Membranes² ?
Laboratory Measurements, Chenevert, 1998
Membrane Efficiency of Speeton Shale when Exposed
to Various Water-based Fluids
de-ionized water
de-ionized water
de-ionized water
0.78 aw CaCl2
0.4 aw CaCl2
0.4 aw KCOOH
0.78 aw Glycerol
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
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Osmotic Membrane Efficiency
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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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Wellbore Stability Design (through Mud
Weight-Inclination diagram)
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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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Effect of Concentration of Salt in Mud
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Effect of Membrane Efficiency
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Wellbore Stability Design (continued)
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Wellbore Stability Design (continued)
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Wellbore Stability Design (continued)
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Wellbore Stability Design (continued)
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Wellbore Stability Design (continued)
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Wellbore Stability Design (continued)
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Wellbore Stability Design (through Mud
Weight-Salinity diagram)
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Wellbore Stability Design (continued)
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Wellbore Stability Design (continued)
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Wellbore Stability Design (continued)
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Wellbore Stability Design (continued)
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Wellbore Stability Design (continued)
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Field Calibration
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Field Calibration (continued)
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Effect of Concentration of Salt in Mud
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Multi-Depth Data/Calculation Display
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Microsoft Word Report
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Microsoft PowerPoint Presentation
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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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Benefits

• Accelerate Technology Implementation
• Affordable Software
• Compound R D Funds
• Technical Interchange
• Unbiased Information
• Schools and Forums

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The End
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