basic well logging analysis -9 (log interpretation)

7/22/2019 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

Method10


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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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(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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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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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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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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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

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Information:

Consolidated sandstone

a = 0.81

m = 2.0

n = 2.0

basic well logging analysis -9 (log interpretation)

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