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QUESTION: Dear Sir,

The classical Archie’s formula is not correct in shaly formations. How we can estimate the effect of this modeling error on our results? What is the possible solution if we are working in a sand zone have 20-30% shale content?

Secondly what kinds of corrections are applied to the sonic log?

ANSWER: Hi Nisar,

Thanks for putting through an interesting question.

Archie assumed that a rock is a perfect insulator and the only medium which conducts electricity is the brine present in the rock. He combined Formation Factor "F" and Resistivity Index "RI" to come up with his classical equation. The measurements were performed on clean clastic rocks so Archie is only valid in clean formations. Soon after Archie the successors noticed that the clay also conduct due to additional cations which cling to clay surface due to the substitution in the structure of Al+3 for Si+4, Fe+2 for Al+3 and Mg+2 for Al+3.

In case of shaly sands, we use two types of equations, i.e Vsh and Cation exchange equations. Shale is present in rocks in three forms, 1. Laminar 2. Structural 3. dispersed.

Laminated : inter-bedded sand and shale

Structural : shale comprises a fraction of the grains

Dispersed : occurring within the pore-system of a rock.

You can use Poupons, simandoux or poupon - Leveaux (Indonesian) equations (Vsh type) or Waxman Smits, Dual water and Juhasz equation (cation exchange equations).

You can calculate the extra conductivity imparted by clay into the rocks with the help of these equations. Once that is done, then you can calculate your Sw easily.

For sonic correction, we edit the bad sonic sections i.e cycle skipping, spikes etc

I hope this will help you in your assignment.

Regards,

Syed

---------- FOLLOW-UP ----------

QUESTION: Dear sir thank you very much for your answer.

Please can you give me the complete references of research articles of the geoscientists who use these equations (Poupons, simandoux or poupon - Leveaux (Indonesian) equations (Vsh type) or Waxman Smits, Dual water and Juhasz equation (cation exchange equations)).

Actually I have to refer these ones in my research article.

However, about sonic correction my question was for corrections applied to sonic porosity. In case of compact and consolidated rock can we use Wyellie time equation directly to compute porosity? I means without applying any corrections?

Again thanks

Below are the references of the papers you requested,

Clavier, C., Coates, G., and Dumanoir, J. 1984. Theoretical and Experimental Bases for the Dual-Water Model for Interpretation of Shaly Sands. SPE J. 24 (2): 153-168. SPE-6859-PA.

Juhasz, I., 1981, Normalized Qv – the key to shaly sand elaluation using the Waxman-Smits equation in the absence of core data, Transactions, SPLWA 22nd Annual Logging Symposium, June

Poupon, A., and Levaux, J.,1971, Evaluation of water saturation in shaly formations:

Society of Professional Well Log Analysts 12th Annual Logging Symposium Transactions

Poupon, A., Loy, M.E., and Tixier, M.P.,1954, A contribution to electric log

interpretation in shaly sands: Transactions of the American Institute of Mechanical

Engineers: v. 201

Simandoux, P., 1963, Dielectric measurements on porous media application to the

measurement of water saturations: study of the behaviour of argillaceous formations

Waxman, M. H., and Smits L. J. M., 1968, Electrical conductivities in oil-bearing sands, SPE Journal, June

We apply corrections on logs if they give anomalous reading in the the interval where we intend to interpret its properties. You need to look if the log is giving good response then there is no need to apply an correction or edition, if not then you will have to edit the log in order to acquire optimum properties. We normally don't apply environmental corrections on sonic logs.

Regards,

Syed

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