Well Log & Rock Core Porosity

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soman703
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Well Log & Rock Core Porosity

Post by soman703 »

Hello everyone,
I have a question please.
How I can modify or arrange between well log porosity and core porosity.
I have one printed well log data which held neutoron porosity, density porosity and gamma ray and 3 point core porosity on interested zone.

But, well logging porosity is very different compared to core porosity.

Actually, core porosity is less than well logging one.

How can I analyze these differences?

Also is ok to calculate using NPHI/DPHI Method?
Total porosity of well logging data using NPHI/DPHI (for example two-third method ... ) is accurate? or not?

And, what is the major different between NPHI(Sandstone matrix) and NPHI(Limestone or Dolomite matrix)

if my interesting zone comprise mainly of sandstone and a bit of limestone, in this case I have to use
NPHI(sandstome matrix) and DPHI(sandstone matrix) or not.
Please guide me.

Soman
Leszek
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Re: Well Log & Rock Core Porosity

Post by Leszek »

If core porosity was done with DEAN-STARK method your log porosity will only match the data in clean zones. In shaly zones Core porosity will be always somewhere between total and effective porosity. It is because drying clays during Dean-Stark lab process - there is always some Clay Bound Water that stays in the sample. When formation is clean there is no shale so there is no CBW.

Using DPHI and NPHI is absolutely fine. Total porosity is an average of NPHI and DPHI (NPHI+DPHI)/2, and effective porosity is an average of shale corrected NPHI and shale corrected DPHI. It is called Complex Lithology Method. As for the matrix scale - in this method it doesn't matter. But if you want to be completely fine and dandy adjust scales and keep them consistent.
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mrgmm
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Re: Well Log & Rock Core Porosity

Post by mrgmm »

Leszek wrote:If core porosity was done with DEAN-STARK method your log porosity will only match the data in clean zones. In shaly zones Core porosity will be always somewhere between total and effective porosity. It is because drying clays during Dean-Stark lab process - there is always some Clay Bound Water that stays in the sample. When formation is clean there is no shale so there is no CBW.

Using DPHI and NPHI is absolutely fine. Total porosity is an average of NPHI and DPHI (NPHI+DPHI)/2, and effective porosity is an average of shale corrected NPHI and shale corrected DPHI. It is called Complex Lithology Method. As for the matrix scale - in this method it doesn't matter. But if you want to be completely fine and dandy adjust scales and keep them consistent.
Leszek,

It is new information for me. thank you :)
In the lab, the measured is effective porosity. right and from log something between effective and total

correct me plz
Leszek
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Re: Well Log & Rock Core Porosity

Post by Leszek »

That shall clarify everything:
When shale volume is close to zero then total porosity overalys effective porosity (see attached - lower section) and core porosity match the log porosity.
If shale volume is high then total porosity is higher than effective porosity due to clay bound water closed in clays (see attached - upper section) and core porosity is between total and effective porosity due to incomplete dried clays.
Your example - if you have calculated porosity properly - is showing exatly that case - Dean - Stark method. So generally in the clean zone you shall push calculated effective porosity to match core data, because core porosity in this case is effective porosity. If shale volume is high, your log calculated porosty will be somewher between total and effective porosity - do not try to match calculated log to the core data because answer would be wrong.

tldr summary :)
Dean - Stark lab method for core porosity: core porosity is effective porosity in clean zones, in shaly zones it is not.

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FANARCO
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Re: Well Log & Rock Core Porosity

Post by FANARCO »

Very impressive and illustrated answer.

Thanks Leszek,

Is this applied on sandstone and carbonate..
Leszek
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Re: Well Log & Rock Core Porosity

Post by Leszek »

Well, yes and no. Dean-Stark is mainly for sandstone reservoirs but of course operators don't care - Dean-Stark is the most favored technique since fluid saturations, permeability and porosity are determined on the same sample. And it is cheap. So depite the fact that it is mainly for sandy reservoirs, I've seen it in the Eagle Ford (about 60% of calcite), Wolfcamp (up to about 25% of calcite or even thick limestone layers) etc. So as always it is tricky and we need to follow the rule: "know your data".
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FANARCO
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Re: Well Log & Rock Core Porosity

Post by FANARCO »

Leszek wrote:Well, yes and no. Dean-Stark is mainly for sandstone reservoirs but of course operators don't care - Dean-Stark is the most favored technique since fluid saturations, permeability and porosity are determined on the same sample. And it is cheap. So depite the fact that it is mainly for sandy reservoirs, I've seen it in the Eagle Ford (about 60% of calcite), Wolfcamp (up to about 25% of calcite or even thick limestone layers) etc. So as always it is tricky and we need to follow the rule: "know your data".

:) :) :)
soman703
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Re: Well Log & Rock Core Porosity

Post by soman703 »

Thankyou senior forum users.

Beautifully explained :D :D :D
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