ABSTRACT

An alternate method for SW calculation, based on direct computation of phi x Sw (designed as phi_w) from the resistivity logs, was recently elaborated, and is briefly exposed. The application of this method to shaly sands is described. Several formulas for the calculation of SW in shaly sands have been published so far. Applications of these formulas to the phi_w method is made. The differences observed between the results obtained respectively are illustrated by a number of resistivity/Porosity cross-plots constructed for a wide range of parameters, and comments are made on possible reasons for such discrepancies. A new formula is proposed, based on these observations:phi_w = (1 - C. Vcl ^ 1 - Vcl/2) x (Rw/Rt)^1/nC being a constant valid for the whole reservoir interval analyzed, and nthe saturation exponent retained. In many cased the product, C?? Vcl ^ 1 - Vcl/2happens to approximate Vcl. This alternate approach enables the analyst to calculate SW in all the shortest delays, yet keeping along with acceptable accuracy requirements whatever the clay content may be. This approach therefore is deemed of valuable help for analyses each time an immediate access to the computer is not available.

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