log interpretation of shaly formations part4

7/27/2019 Log Interpretation of Shaly Formations Part4

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1.4 Porosity Logs in shaly Formations

Porosity log response can be expressed generally by

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In shaly liquid filled formation assumed to havenormal compaction and no secondary porosity .

The presence of shale complicates the interpretation

of the tool response due to :1. The diverse characteristics of shale

2. The different responses of each porosity tool to theshale content

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N.B. Density porosity logs shale displays low to

moderate porosity values .

On sonic and neutron logs shale display moderate to

relatively high porosity values .

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From this figure that depends on porosity, shale

content and the fluid ,shale and matrix properties .

It shows schematically the fractions and densities of different constitutes of shaly formation .

Rearranging will result in the following form

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ΦD is the apparent porosity that is equal to true porosity in

case of clean liquid filled formations

ΦDSh is the apparent density porosity displaced by the tool

if it is placed in 100% shale formation having the same

characteristic as the shale present in the formation of

interest

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True porosity

Φ = Φa – Vsh (Φa )sh

Where

Φa is the apparent porosity displayed by the tool or calculatedfrom raw data assuming that the formation is clean and liquid

filled .

(Φa )sh is the apparent porosity displayed by the tool or calculated

in adjacent shales from raw data assuming that the formation is

clean and liquid filled .

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1.5 Effective Porosity and Shale

Content From Porosity LogsThe effective porosity of shaly formations can be

calculated from the response of any porosity tool but thisrequires the knowledge of Vsh .

Experience has shown that ,when one porosity tool is usedto calculate (Φ) , the density log provides the bestestimates .

Because (ΦD)sh is much smaller (Φ N)sh and (ΦS)sh thecorrection term [VSh(Φa)sh] is less sensitive to incorrectestimation of VSh .

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In order to get the best value of Vsh we should avoid non

representative value estimated from gamma ray log .

So we will solve 2 porosity tools responses. Then solving

the equations simultaneously will give us the values of

(Φ) and (Vsh)

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Because the sonic log depends on compaction and

secondary porosity in addition to porosity , shale andmatrix , density and neutron responses are used to set up

the simultaneous equations.

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Estimation of V sh from gamma ray log :



By solving simultaneous equations :

Comparing the results from different methods :

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log interpretation of shaly formations part4

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