Log‐based Mechanical Earth Modeling for Wellbore Stability

R R TiwariCEWELL, ONGC, Baroda

Cost of Wellbore Instability (1998)

It is a huge capital loss

• Lost circulation• Stuck pipe• Additional casing strings• Side tracks • Abandonment

Upshots of Wellbore Instability

Porepressure

CollapsePressure

Fracturepressure

Mud Weight Window

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.

• 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.

• In permeable sections it can bemeasured directly with MDT

• In shale sections it is predicted on thebasis of shale compaction theory

Pore Pressure

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

Pore Pressure Prediction

• Eaton’s sonic method:

• Eaton’s resistivity method:

Fracture Gradient

• Sets the higher side of safe mud weightwindow

• Can be approximated to minimum horizontalstress

Where ʋ is the Poisson’s ratio

• The existing near wellbore stresses arecompared with rock strength to predictfailure

• If the stress field concentration aroundwellbore exceeds rock strength, yield mayoccur

Stability Analysis

Case studies

Well: ANDW‐1

ANDW‐1

ANDW‐1Predrill Predictions

Well: ANDW‐5

Kuth‐9

Kuth‐42

Santh‐211

ANDW‐1 Breakout

Breakout azimuth from 4‐arm caliper 

Well: ANDW‐1

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.