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Log‐based Mechanical Earth Modeling for Wellbore Stability
R R TiwariCEWELL, ONGC, Baroda
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Cost of Wellbore Instability (1998)
It is a huge capital loss
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• Lost circulation• Stuck pipe• Additional casing strings• Side tracks • Abandonment
Upshots of Wellbore Instability
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Porepressure
CollapsePressure
Fracturepressure
Mud Weight Window
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Parameters of Importance
• Earth stresses (vertical & horizontal)• Pore pressure• Fracture gradient• Rock strength & failure criterion
All of these can be estimated from logs
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Earth Stresses
These stresses are takenas principal stresses.This may not always bethe case, however.
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• Vertical stress can be estimated from density log• Minimum horizontal stress can be estimatedfrom MDT minifrac or equated to fracturegradient
• Maximum horizontal stress can be estimatedeither from minifrac or sonic scanner
• Direction of horizontal stresses can be firmed upeither with a 4‐arm caliper or sonic scanner
Earth Stresses
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Near Well‐bore Stresses
• Mud pressure may not exactly match the stress which the removed rock exerted• This gives rise to stress imbalance or stress redistribution near wellbore wall• The new stresses are:Hoop or tangential or circumferential stressRadial stressAxial stress
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Stress Concentration Near Wellbore
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Near Well‐bore Stresses
In this model porepressure and wellborepressure are equal. Hoopstress reaches 7K psi inthe direction of minimumhorizontal stress.
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• In permeable sections it can bemeasured directly with MDT
• In shale sections it is predicted on thebasis of shale compaction theory
Pore Pressure
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2. Extrapolate normal trend
line
1. Establish “Normal” Trend Line in good “clean” shale
Transition
Porosity should decrease with depth
in normally pressured shales
3. Determine the magnitude
of the deviation
Pore Pressure Prediction
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Pore Pressure Prediction
• Eaton’s sonic method:
• Eaton’s resistivity method:
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Fracture Gradient
• Sets the higher side of safe mud weightwindow
• Can be approximated to minimum horizontalstress
Where ʋ is the Poisson’s ratio
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• The existing near wellbore stresses arecompared with rock strength to predictfailure
• If the stress field concentration aroundwellbore exceeds rock strength, yield mayoccur
Stability Analysis
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Case studies
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Well: ANDW‐1
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ANDW‐1
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ANDW‐1Predrill Predictions
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Well: ANDW‐5
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Kuth‐9
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Kuth‐42
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Santh‐211
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ANDW‐1 Breakout
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Breakout azimuth from 4‐arm caliper
Well: ANDW‐1
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Conclusions• Mechanical Earth Models can be built for ourfields with the help of logs to have a bettercontrol during drilling. This will result in reducedNPT.
• However the models have further scope ofimprovement if more data in terms of LOT,Minifrac, MDT, FMI (4‐arm caliper) andlaboratory report on rock strength, are added.