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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Estimate Elastic Properties
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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Estimate Elastic Properties
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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Estimate rock strength
Empirical relations to estimate UCS for shales
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Estimate rock strength
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
Estimate stresses (Max & Min horizontal)
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 stresses (Max & Min horizontal)
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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