How petrophysical heterogeneity controls the saturation calculations in carbonates, the Barremian-Aptian of the central Persian Gulf

Water saturation calculations are among the most important aspects of reservoir characterization. Despite this importance, the effects of these calculations on the resulted oil in place remain unclear. Extreme heterogeneity in most of the carbonate reservoirs strongly affects the coefficients of the Archie equation and resulted water saturation. Therefore, a practical method to evaluate and manage the heterogeneity of such reservoirs, greatly improves the reliability of the final results. In this study, porosity, permeability, thin sections data, well logs and saturation tests of Barremian-Aptian carbonate Dariyan Formation in the central Persian Gulf have been used. Both core and reservoir scale techniques including flow zone indicator, Winland R-35, electrofacies and stratigraphically modified Lorenz plot were used to evaluate the heterogeneity. Archie coefficients were arranged in different rock types. Results show that flow zone indicator is the most accurate technique to manage the heterogeneity of the reservoir with higher R-2 value between porosity and permeability. The cementation factor was measured for each unit and the amounts of water saturation was calculated using deterministic analysis. Water saturation also calculated with the constant coefficients, which are routinely used in formation evaluation. The average S-w amount for constant Archie coefficients method was determined as 0.62, whereas it is 0.73, 0.80, 0.70, and 0.77 in flow zone indicator method, Winland R-35 technique, stratigraphically modified Lorenz plot and electrofacies determination, respectively. The results demonstrate that calculating water saturation with constant Archie coefficients leads to slightly lower water saturation values, resulting in higher oil saturations and overestimating the oil in place.

Automated summary

Water saturation calculations are crucial in reservoir characterization, but their effects on oil in place remain unclear. Heterogeneity in carbonate reservoirs impacts Archie equation coefficients and resulted water saturation. Managing this heterogeneity with flow zone indicator method results in higher water saturation values compared to constant Archie coefficients method, leading to more accurate evaluation of oil in place.