A numerical technique for an accurate determination of formation resistivity factor using F R -R O overlays method

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ORIGINAL PAPER

A numerical technique for an accurate determination of formation resistivity factor using FR-RO overlays method Walid M. Mabrouk & Khaled S. Soliman

Received: 29 August 2013 / Accepted: 23 December 2013 # Saudi Society for Geosciences 2014

Abstract The exactness of water saturation value for given reservoir conditions depends on the accuracy of Archie parameters a, m, and n. The terms of Archie relationship have been subjected to many laboratory investigations and even more speculation. There are many factors that affect porosity exponent m, saturation exponent n, and tortuosity factor a. Usually, assumptions are made to approximate a and m; often m is 2, while a may be 0.81 or 1.0 depending on the type of lithology. But it is very difficult to fix Archie parameters regardless of reservoir characteristics; rock wettability, formation water salinity, permeability, porosity, and fluids distribution. This work illustrates a simple numerical method to calculate a and m which depends on FR-RO overlays method which is used as hydrocarbon indicator. The method is tested using synthetic and real data to ensure its ability in determining formation factor parameters a and m. Keywords Formation resistivity factor . a and m . Tortuosity and porosity exponent Nomenclature FR Ro A Rt Sw Rcal R-Calc (0.52–1.6) ϕ

Formation resistivity factor Resistivity of fully saturated S.S. (Ω m) Tortuosity factor True resistivity (Ω m) Water saturation Calculate resistivity (Ω m) Calculated resistivity (a-m) Porosity

W. M. Mabrouk (*) : K. S. Soliman Geophysics Department, Faculty of Science, Cairo University, Giza, Egypt e-mail: [email protected]

Water resistivity (Ω m) Cementation exponent Flushed zone resistivity Synthetic porosity

Rw M Rxo ϕSyn.

Introduction The need for greater accuracy for the determination of in situ fluid saturations has become more important in recent years because of efforts to develop new technology to enhance oil recovery from old oil fields. This accurate assessment of the existing reservoir oil saturation is required to project the overall economic feasibility of the new or improved oil-recovery process. This need for greater accuracy and more data from in situ testing has led to the development of new well-logging tools (the repeat formation tester, the 64-electrode microresistivity tool, and the Phasor induction tool). The formation resistivity factor of a porous medium is a valuable concept in the area of formation evaluation. It has been denned as the ratio of the resistivity of the medium when completely saturated with a conducting fluid to the resistivity of the saturating fluid as follows: F R ¼ R0 =Rw

ð1Þ

Next, Archie cross-plotted porosity versus formation factor on log-log scales, and noted a linear trend that is equivalent to the following:

FR ¼

1 ϕm

ð2Þ

Arab J Geosci Table 1 Coefficients and exponents used to calculate formation factor (F) as a function of porosity

F=a / ϕ m

General relationship (Archie 1942) where: a=tortuosity factor m=cementation exponent

F=1

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