geomechanical study of wellbore stability

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GEOMECHANICAL STUDY OF WELLBORE STABILITY -VIDIT MOHAN

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Page 1: Geomechanical Study of Wellbore Stability

GEOMECHANICAL STUDY OF

WELLBORE STABILITY

-VIDIT MOHAN

Page 2: Geomechanical Study of Wellbore Stability

TOPICS

• Geomechanical Model

• Need of Geomechanical Model

• Wellbore Stability

• Developing Comprehensive Geomechanical Model

• Variation of Effective Hoop Stress

• Compressional and Tensile Failure

• Failure Criteria

• Normal Compaction Trend (NCT)

• Pore Pressure (PP) Estimation

• Observations

• Fracture Pressure (FP) Determination

• Sensitivity Analysis

• Conclusions

• References

Page 3: Geomechanical Study of Wellbore Stability

• In-situ stress orientations

• In-situ stress magnitudes

• Pore Pressure

• Rock strength and mechanical

properties

• Fracture patterns

• Geology and structure

GEOMECHANICAL MODEL

Model involving detailed knowledge of:

Page 4: Geomechanical Study of Wellbore Stability

NEED OF GEOMECHANICAL MODEL

Courtesy: Baker Hughes

Page 5: Geomechanical Study of Wellbore Stability

ADVANTAGES OF GEOMECHANICS

Reduction of drilling problems:

• Wellbore stability analysis- Reducing stuck pipe, sidetracks, washing and reaming

• Improved pore and fracture pressure prediction- Reducing kicks and lost circulation

Improving reservoir performance:

• Predicting sand production

• Predicting permeable natural fractures to optimize production

• Prediction of fault controlled hydrocarbon column heights

• Injection or depletion induced fault reactivation

• Determination of fracture propagation direction and reorientation

• Sweep efficiency

• Compaction and subsidence

Page 6: Geomechanical Study of Wellbore Stability

WELLBORE STABILITY

• Modeling anisotropic breakouts with given in-situ stress state.

• Tendency for Breakout Initiation for different stress regimes.

• Design for variations in strength.

Key is to control the width of failure zones

Page 7: Geomechanical Study of Wellbore Stability

DEVELOPING COMPREHENSIVE GEOMECHANICAL MODEL

Parameter Data

Vertical Stress, Sv(z) g0𝑧ƿ(z) dz

Minimum Horizontal Stress, Shmin

XLOT, LOT, minifrac, lost circulation,

ballooning

Maximum Horizontal Stress, SHmax Analysis of wellbore failure

Pore Pressure, Pp

Measurements (RFT, DST, etc), Log-

based, Seismic

Stress orientation Orientation of wellbore failures

Faults/Bedding Planes Wellbore Imaging

Rock StrengthLab measurements, Logs, Modelling

wellbore failures

Page 8: Geomechanical Study of Wellbore Stability

IN-SITU PRINCIPAL STRESSES

Fig.: (A) Rock formation in-situ stresses, (B) Rock formation in-situ principal

stresses for a drilled vertical well

A B

Page 9: Geomechanical Study of Wellbore Stability

VARIATION OF EFFECTIVE HOOP STRESS

SHmax = 90 MPa

SHmax orientation is N90E (East West)

Sv= 88.2 MPa

Shmin= 51.5 MPa

Pp=Pmud=31.5 MPa

Page 10: Geomechanical Study of Wellbore Stability

COMPRESSIONAL AND TENSILE WELLBORE FAILURES

Page 11: Geomechanical Study of Wellbore Stability

MOHR-COULOMB FAILURE CRITERION

Represents linear envelope obtained from plot of shear strength of material versus applied

normal stress,

τ = Б tan(Ø) + c

where τ is the shear strength, Б is the normal stress, c is the intercept of failure envelope

with the τ axis, and Ø is the slope of failure envelope.

Page 12: Geomechanical Study of Wellbore Stability

VON MISES FAILURE CRITERION

• Yielding of materials begins when second deviatoric stress invariant reaches

yield strength.

• Mathematically, the von Mises yield criterion is expressed as:

J20.5 = (1/30.5)*( б1- б3)

Бm- Po= {( б1+ 2*б3) – Po}/3

Бv= бy= (3*J2)0.5

БV2= 3*J2=3*k2

Бv2 = [ (Б11- Б22)

2 + (Б22- Б33)2 + (Б33- Б11)

2 + 6*(Б232+ Б31

2+ Б122)]/2

Page 13: Geomechanical Study of Wellbore Stability

NORMAL COMPACTION TREND (NCT)

• Straight line in log linear space fitted as a function of depth where sediments are

compacting.

• Response of petrophysical properties to reduction of porosity due to compaction

disequilibrium.

• Basis for measuring pressure from seismic, from wireline and in basin modelling.

Page 14: Geomechanical Study of Wellbore Stability

PLOTTING NCT

Estimate the onset of overpressure

1.• Plot porosity vs. depth.

2.

• Estimate porosity assuming an exponential compaction trend.

• Ø = Øo * e^ (-c*h), where ϕ is the porosity, ϕ0 is the initial porosity & c is the coefficient of compaction

3.

• Calculate the theoretical compaction trend. Db=Dma*(1-Ø) + Dfl*Ø

• Plot this trend on the same plot as the porosity data.

Page 15: Geomechanical Study of Wellbore Stability

Db=Dma*(1-Ø) + Dfl*Ø and Ø = Øo * e-c*h

Bulk Density = Db

Density of Fluid = Dfl

Density of Matrix = Dma

h=Depth

Onset of Overpressure

Page 16: Geomechanical Study of Wellbore Stability

PLOTTING NCT

Using Sonic Transit Time data

ΔTn=ΔTm+ (ΔTml-ΔTm) exp (-cz)

where,

ΔTml=Mudline Transit Time

ΔTm=Compressional Transit Time

z=Depth

c=0.27 (Sandstone)

Onset of Overpressure

Page 17: Geomechanical Study of Wellbore Stability

PORE PRESSURE ESTIMATION

Page 18: Geomechanical Study of Wellbore Stability

EQUIVALENT DEPTH METHOD

NCT is fitted to the decrease in slowness as a

function of depth where sediments are normally

compacting.

The effective stress at depth Z is equal to

effective stress at depth A, and thus, the pore

pressure at depth Z is

Pz = Pa + (Sz–Sa).

where Pa,z and Sa,z are pore pressure and stress

at z, the depth of interest and a, the depth along

the normal compaction trend at which the

measured parameter is the same as it is at the

depth of interest.

Page 19: Geomechanical Study of Wellbore Stability

RATIO METHOD

Pore pressure is the product of the normal pressure multiplied (or divided by)

the ratio of the measured value to the normal value for the same depth.

where the subscripts n and log refer to the normal and measured

values of density, resistivity, or sonic delta-t; Pp is the actual pore

pressure, and Phyd is the normal hydrostatic pore pressure.

Can lead to unphysical situations, such as calculated pore pressures that are

higher than the overburden.

Pp=Phyd ΔTlog/ΔTn

Page 20: Geomechanical Study of Wellbore Stability

EATON METHOD

PP=S-[(S-Ph)*(ΔTlog/ΔTn)3]

S=Overburden Stress

Ph=Hydrostatic Pore Pressure

If the NCT is defined over an interval with elevated pore pressure, the method

will give the wrong (too low) pore pressure, leading to severe risks for drilling.

Page 21: Geomechanical Study of Wellbore Stability
Page 22: Geomechanical Study of Wellbore Stability

OBSERVATIONS

• Selection of appropriate normal compaction curve.

• Equivalent effective stress method should be used if most of overpressure is generated

by disequilibrium compaction.

• All these methods require that rock obeys a single, monotonic, compaction-induced

trend, and that no other effects are operating.

• Pore fluid properties can also have a significant effect on pore-pressure predictions.

• Fluid salinity consideration.

Page 23: Geomechanical Study of Wellbore Stability

FRACTURE PRESSURE DETERMINATION

Page 24: Geomechanical Study of Wellbore Stability

FRACTURE FORMATION PRESSURE

Fracture pressure is the pressure in the wellbore at which a formation will crack .

Formation will fracture when pressure in borehole exceeds the least of stresses within the

rock structure.

Normally, fractures will propagate in a direction perpendicular to the least principal

stress.

Definition and Mechanism

Page 25: Geomechanical Study of Wellbore Stability

• The minimum wellbore pressure required to extend an existing fracture was

given as the pressure needed to overcome the minimum principle stress :

•The minimum principle stress in the shallow sediments is approximately one-

third the matrix stress resulting from weight of the overburden.

•Assumed elastic behaviour.

Prediction of Fracture Pressure

Hubbert and Willis Equation

fff PP min

Page 26: Geomechanical Study of Wellbore Stability

Prediction of Fracture Pressure

fma

ff PP 3

f

fob

ff PP

P

3

3

2 fob

ff

PP

Hubbert and Willis Equation

Pf =Pore Pressure

σob=Overburden Pressure

Page 27: Geomechanical Study of Wellbore Stability

Prediction of Fracture Pressure

Replaced the assumption that the minimum stress was one-third the matrix stress

by

where the stress coefficient was determined empirically from field data taken in

normally pressured formations.

Not valid for deeper formation.

Matthew and Kelley Correlation

maF min

Page 28: Geomechanical Study of Wellbore Stability

Prediction of Fracture Pressure

The vertical matrix stress at normal pressure is calculated (subscript “n” is for normal

pressure)

(Sma)n = Sobn – Pfn

Di is the equivalent normal pressure depth

Matthew and Kelley Correlation

iiinma DDD 535.0465.01)(

At the depth at which the abnormal pressure presents:

535.0535.0535.0

)( ffobnmai

PDPD

Pfi = Fracture Initiation Pressure

Pfi= Smin + Pp

Pfi= [ (0.61*Di) - (0.61*Pp)] + Pp

Page 29: Geomechanical Study of Wellbore Stability

The overburden and Poison ratio vary with depth.

Prediction of Fracture Pressure

FG=[(S-P)*ϒ/D*(1-ϒ)]+ P/D

S=Overburden

D=Depth

ϒ=Poisson Ratio

Eaton Correlation

Page 30: Geomechanical Study of Wellbore Stability

Prediction of Fracture Pressure

•Stress coefficient is correlated to the bulk density of the sediments.

•Take into consideration the effect of water depth on overburden stress.

Christman Correlation

ϴ=ϴoexp(-KD)

ϴ=Porosity

K=Christman Constant

Pff= (бmin+Pp)/D

D=Depth

Page 31: Geomechanical Study of Wellbore Stability
Page 32: Geomechanical Study of Wellbore Stability

SENSITIVITY ANALYSIS

• All the methods take into consideration the pore pressure gradient.

• As the pore pressure increases, so does the fracture gradient.

• Hubbert and Willis apparently consider only the variation in pore pressure

gradient.

• Matthews and Kelly also consider the changes in rock matrix stress coefficient

and the matrix stress.

• Eaton considers variation in pore pressure gradient, overburden stress, and

Poisson’s ratio. It is probably the most accurate of the three.

• None consider the effect of water depth except Christman approach.

Page 33: Geomechanical Study of Wellbore Stability

Top Down Casing Design

Pore Pressure

Fracture Pressure

Page 34: Geomechanical Study of Wellbore Stability

Bottom Up Casing Design

Pore Pressure

Fracture Pressure

Page 35: Geomechanical Study of Wellbore Stability

Top Down

Casing Design

Page 36: Geomechanical Study of Wellbore Stability

Bottom Up

Casing Design

Page 37: Geomechanical Study of Wellbore Stability

CONCLUSIONS

• Uncertainty in pore pressure prediction analyzed by examining spread in predicted

pore pressure obtained using parameter combinations consistent with available well

data.

• Pore pressure prediction from well logs has spatial and depth limitation.

• Results of wellbore stability assessment are required to mitigate consequences of

instability.

• Individual evaluation of each well.

• Pore pressure & Fracture gradient determination Casing setting depth selection

Page 38: Geomechanical Study of Wellbore Stability

REFERENCES

• Drill Works – Halliburton User Guide

• Dr Mark D Zoback – Reservoir Geomechanics tutorials

• Petrophysics by Dr Paul Glover

• Well Engineering & Construction by Hussain Rabia

• European Association of Geoscientists & Engineers (EAGE) journals & short courses

• Bowers, G. L., 1995, Pore pressure estimation from velocity data: Accounting for overpressure

mechanisms besides undercompaction: SPE Drilling and Completion, 27488.

• Eaton, B. A., The equation for geopressure prediction from well logs: SPE, 5544.

• Rancom, R.C., A Method for Calculation Pore Pressures from Well Logs

• Papers:

http://petrowiki.org/Methods_to_determine_pore_pressure

http://petrowiki.org/Subsurface_stress_and_pore_pressure#Pore_pressure

https://www.linkedin.com/groups/What-is-Normal-Compaction-Trend

3858625.S.126429683

Page 39: Geomechanical Study of Wellbore Stability

THANK YOU!