basic well logging analysis -9 (log interpretation)
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BASICWELLLOGGINGANALYSIS
LOGINTERPRETATION
Hsieh, Bieng-Zih
Fall 2009
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ARCHIEEQUATIONSW
Water saturation (Sw) of a reservoirs uninvaded zone is calculatedby the Archie (1942) formula.
Where: Swwater saturation of the uninvaded zone (Archie method)
Rwresistivity of formation water at formation temperature
Rttrue resistivity of formation
porosity
atortuosity factor (1.0 for carbonates; 0.81 for consolidatedsandstone; 0.62 for unconsolidated sandstone)
mcementation exponent (2.0 for carbonates and consolidatedsandstone; 2.15 for unconsolidated sandstone)
nsaturation exponent (normally equal to 2.0) 2
n
t
w
mw R
RaS
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ARCHIEEQUATIONSW(CONT.)
The uninvaded zones water saturation (Sw), determinedby the Archie equation, is the most fundamentalparameter used in log evaluation.
But, merely knowing a zones water saturation (Sw) willnot provide enough information to completely evaluate azones potential productivity.
A geologist must also know whether: (1) hydrocarbons aremoveable, (2) water saturation is low enough for a water-freecompletion, (3) the zone is permeable, and (4)whether (volumetrically) there are economic, recoverablehydrocarbon reserves. 3
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ARCHIEEQUATIONSXO
Water saturation of a formations flushed zone (Sxo) is
also based on the Archie equation, but two variables are
changed:
Where:
Sxowater saturation of the flushed zone
Rmfresistivity of the mud filtrate at formationtemperature
Rxoshallow resistivity
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n
xo
mf
mxo R
RaS
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ARCHIEEQUATIONSXO (CONT.)
Water saturation of the flushed zone (Sxo) can be used as
an indicator of hydrocarbon moveability.
For example, if the value of Sxois much larger than Sw,then hydrocarbons in the flushed zone have probably
been moved or flushed out of the zone nearest the
borehole by the invading drilling fluids (Rmf).
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RATIOMETHOD
The Ratio Method identifies hydrocarbons from the
difference between water saturations in the flushed zone
(Sxo) and the uninvaded zone (Sw).
When water saturation of the uninvaded zone (Sw) is
divided by water saturation of the flushed zone (Sxo), the
following results:
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n
t
w
mw R
Ra
S
1
n
xo
mf
mxo R
RaS
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RATIOMETHODWITHOUTKNOWINGPOROSITY
When Swis divided by Sxo, the formation factor (Fa/m)is cancelled out of the equation because formation factoris used to calculate both Swand Sxo.
This can be very helpful in log analysis because, from theratio of (Rxo/Rt)/(Rmf/Rw), the geologist can determine avalue for both the moveable hydrocarbon index (Sw/Sxo)and water saturation by the Ratio Method withoutknowing porosity.
Therefore, a geologist can still derive useful formationevaluation log parameters even though porosity logs areunavailable. 7
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RATIOMETHOD-- MOVEABLEHYDROCARBONINDEX
Formulas for calculating the moveable hydrocarbon index
and water saturation by the Ratio Method are:
If the ratio Sw/Sxois equal to 1.0 or greater, then
hydrocarbons were not moved during invasion.
Whenever the ratio of Sw/Sxois less than 0.7 for
sandstones or less than 0.6 for carbonates, moveable
hydrocarbons are indicated (Schlumberger, 1972). 8
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RATIOMETHOD
To determine water saturation (Sw) by the Ratio Method,
you must know the flushed zones water saturation.
In the flushed zone of formations with moderate invasion
and average residual hydrocarbon saturation, the
following relationship is normally true:
by substituting the above equation in the relationship:
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RATIOMETHOD
Where: Swr water saturation uninvaded zone, Ratio
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RATIOMETHODQUALITYCHECK
After the geologist has calculated water saturation of the
uninvaded zone by both the Archie and Ratio methods, he
should compare the two values using the following
observations:
(1) If Sw(Archie) Sw(Ratio)
the assumption of a step-contact invasion profile is indicated
to be correct,
all values determined (Sw, Rt, Rxo, and di) are correct.
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RATIOMETHODQUALITYCHECK
(2) If Sw(Archie)Sw(Ratio)
the value for Rxo
/Rtis too low.
Rxois too low because invasion is very shallow, or Rtis too high
because invasion is very deep.
Also, a transition type invasion profile may be indicated
Sw(Archie) is considered a good value for Sw
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RATIOMETHODQUALITYCHECK
If Sw(Archie)Sw(Ratio)
the value for Rxo
/Rtis too high because of the effect of
adjacent, high resistivity beds
an annulus type invasion profile may be indicated
or SxoSw1/5
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RATIOMETHODQUALITYCHECK
In the case of Sw(Archie)Sw(Ratio),a more accurate
value for water saturation can be estimated using the
following equation (from Schlumberger, 1977):
Where:
(Sw)CORcorrected water saturation of the uninvaded zone
Swa
water saturation of the uninvaded zone (Archie Method)
Swrwater saturation of the uninvaded zone (Ratio Method)
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BULKVOLUMEWATER
The product of a formations water saturation (Sw) and its
porosity () is the bulk volume of water (BVW).
If values for bulk volume water, calculated at several
depth in a formation, are constant or very close to
constant, they indicate that the zone is homogeneous and
at irreducible water saturation (Sw irr).
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BULKVOLUMEWATER
When a zone is at irreducible water saturation, water
calculated in the uninvaded zone (Sw) will not move
because it is held on grains by capillary pressure.
Therefore, hydrocarbon production from a zone at
irreducible water saturation should be water-free(Morris
and Biggs, 1967).
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BULKVOLUMEWATER
A formation notat irreducible water saturation (Sw irr) will
exhibit wide variations in bulk volume water values.
Figure 39 illustrates three crossplots of porosity (
)versus Sw irrfor three wells from the Ordovician Red River
B-zone, Beaver Creek Field, North Dakota.
Note, that with increasing percentages of produced water,scattering of data points from a constant value of BVW
(hyperbolic lines) occurs.
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PERMEABILITYFROMLOGS
Log-derived permeability formulas are only valid for
estimatingpermeability in formations at irreducible water
saturation(Sw irr; Schlumberger, 1977).
The common method for calculating log-derived
permeability is the Wyllie and Rose (1950) formulas.
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PERMEABILITYFROMLOGS
Before these formulas can be applied, a geologist must
first determine whether or not a formation is at
irreducible water saturation.
Whether or not a formation is at irreducible water
saturation depends upon bulk volume water (BVWSw
) values.
When a formations bulk volume water values are constant, a
zone is at irreducible water saturation.
If the values are not constant, a zone is not at irreducible
water saturation .
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PERMEABILITYFROMLOGS
The Wyllie and Rose (1950) method for determiningpermeability utilizes the following two formulas:
Where:
K1/2square root of permeability (K is equal to
permeability in millidarcies) porosity
Sw irrwater saturation (Sw) of a zone at irreduciblewater saturation 21
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HOMEWORK #6 -- LOGINTERPRETATION
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HOMEWORK#6
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RxoRt
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HOMEWORK#6LOGINTERPRETATION
Depth Rxo Rt Sw Sxo Sw/Sxo Swr BVW K
7600
7610
7620
.
.
.
.
.
.
.
7840
785025
Information:
Consolidated sandstone
a = 0.81
m = 2.0
n = 2.0