Abstract
AbstractEstimation of water saturation, Sw, in shaly sandstone is an intricate process. The surface conduction of clay minerals adds up to the electrolyte conduction in the pore spaces, thus generating high formation conductivity that overshadows the hydrocarbon effect. In each resistivity-based water saturation model, the key parameter is formation factor, F, which is typically derived from Archie’s Law. Referring to a log–log plot between formation factor and porosity, cementation factor reflects the slope of the straight line abiding Archie’s Law. In the case of shaly sandstone, derivation based on Archie’s Law in combination with Waxman–Smits equation leads to higher cementation factor, m*. In the shaly parts of the reservoir, high m* is counterbalanced by clay conductivity. Nonetheless, high m* used in clean parts increases Sw estimation. In this study, the variable cementation factor equation is introduced into the standard correlation of Sw versus Resistivity Index, RI, to develop a water saturation model with shaly sandstone parameters. Data retrieved from two fields that yielded mean arctangent absolute percentage error (MAAPE) were analysed to determine the difference between calculated and measured data within the 0.01–0.15 range for variable cementation factor method. The conventional method yielded maximum MAAPE at 0.46.
Highlights
Water saturation, Sw, refers to crucial petrophysical information for volumetric estimation
Archie expanded the equation by embedding quantity F, known as the formation resistivity factor or referred as formation factor
This result contravenes the Waxman and Smits theory on excess clay conductivity, which depicts that the cementation factor is Variable Formation Factor, F*, from variable cementation factor
Summary
Sw, refers to crucial petrophysical information for volumetric estimation. The intrinsic cementation factor, m, is determined from the slope of bi-logarithmic plot of formation factor versus porosity, which is in accordance to Archie’s Law: F=. This technique leaves cementation factor constant for the entire zone represented by the respective core samples. The shaly sandstone cementation factor, m*, which was obtained from the linear regression of F* (calculated from F multiplied by (1 + BQvRw)), generated a higher value than m This result contravenes the Waxman and Smits theory on excess clay conductivity, which depicts that the cementation factor is Variable Formation Factor, F*, from variable cementation factor. The comparison of calculated Sw values between variable m* method and conventional method for Field A is presented in Figs. 10, Sample 1, Field A 1.2
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