log based wbs

8/10/2019 log based wbs

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Log based WBS

Vamegh Rasouli

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Build Mechanical Model of the Earth

This shows mechanical properties of different layers,

stresses magnitude & direction & Pore Pressure

A log based model is presented here, which is used

for WBS analysis

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Mechanical Model can have usefulness during different

stages of oilfield activities

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Sources of data used to build a Mechanical Model & calibrate it

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Process of building a Mechanical Model

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Clay support

Grain support

Grain/clay support characteristics

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Clay support

Grain support

QC: Gamma Ray Log

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Available logs

Density(g/cm3)Density log is

usually available

for reservoir

sec on

Density log is

extrapolated for

overburden

Cutting density

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calibration

Sonic logs

(m/s) Sonic logs (specially shear)

is vital to Geomechanics

study & strongly advised to

Available logs

compressionshear

Sonic logs usually run

across reservoir section. Ifneeded can be extrapolated

to overburden, but implies

uncertainty

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If no shear log available, it

can be estimated from

compression through

experimental relations


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Estimate Elastic Properties

Empirical relations to estimate Dynamic elastic properties (Youngs

modulus, Bulk modulus, Shear modulus & Poissons ratio) from sonic logs

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Dynamic Shear & Bulk Modulus can be estimated from dynamic Youngs

Modulus:

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dyn

dyn

EG

3 1 2

dyn

dyn

EK

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Static elastic properties are less than Dynamic values (up to about 40%)

& can be estimated through empirical relations or correlations obtained

through laboratory sample data

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Estimate Elastic PropertiesYoungs modulus

(GPa)Poissons ratio

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Estimate Elastic PropertiesYoungs modulus

(GPa)Poissons ratio

Calibration against lab

test data on cores

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Estimate rock strength

Empirical relations to estimate UCS for sandstones

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Empirical relations to estimate UCS for shales

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Empirical relations to estimate UCS for carbonates & limestones

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Estimate stresses (vertical or overburden)

Kirchs equations is used to calculate induced stresses induced at

distance r, from a borehole (radius R) in an elastic medium (P=Pw-Pp):

__

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Estimate stresses (vertical or overburden)

Vertical stress is simply estimated from density log

Sv

(MPa)

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Estimate stresses (Max & Min horizontal)

Assuming poroelastic behavoiur, horizontal stresses can be

calculated from Poissons ratio, horizontal strains, fluid pressure &

Biot factor:

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Biot factor ()

Kb & Kg are Bulk modulus of a porous drained rock & intact rock,

respectively. Biot factor is zero for an intact medium & tends towards

unity for a highly porous medium


Sh

(MPa)

In practice, firstly it is

assumed that x=y, then Sh is

estimated & calibrated against

available). If not a good match

obtained the ratio of x/y is

changed & Sh is recalculateduntil a good match is obtained.

This is an estimation of the

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It is difficult to calibrate SH,

usually it is checked against

WBS results


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Estimate horizontal stress direction

Hydraulic fractures generally

occur in the maximum principal

stress direction around the

wellbore, where the hoop stress

s ens ona

In a vertical well, the hydraulic

fracture direction indicates the

direction of the maximum

horizontal stress

Drilling induced hydraulic

fracture direction can be

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identified from the image logs

Estimate horizontal stress direction

Shear failure

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Tensile failure


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Estimate stresses (Pore Pressure)

PP can be estimated through empirical relations exist with sonic,

density, resistivity, logs

Most often, in sandstone, Eaton equation, which relates PP to

compression sonic data is used

Estimated PP log is calibrated against pressure test data obtained

from MDT

There is not a good relation exist for carbonates. Local correlations to

be used & calibrated

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Estimate stresses (Pore Pressure)

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Failure modes

Effective stresses around a borehole (borehole parallel to principal

stresses) in an elastic medium:

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Failure modes

Tensile Failure: when hoop stress becomes tensile & exceeds tensile

strength of rock. This could be due to a high wellbore pressure or a

big difference between principal stresses normal to wellbore axis

rac ure cr er on can e cons ere as w en 3=

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Failure modes

Shear Failure:

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Failure modes

Shear Failure:

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Failure modes

Shear Failure:

Mohr-Coulomb criterion

F=UCS

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,

N=(1+sin)/(1-sin)

S0: Cohesion

: friction angle

Failure modes

Shear Failure:

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Choose proper MW to avoid failure

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WBS analysis

Results are

calibrated

against

Caliper data

MW used is

insufficient atsome intervals,

causing shear

failure. Caliper

data confirms this

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FMI or other

images can

also be

used for

calibration


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Effect of wellbore deviation & Azimuth on

stability

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Effect of wellbore deviation & Azimuth on

stability

Losses

Tensile failure

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KickShear failure

log based wbs

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