Use of tortuosity for discriminating electro-facies to interpret the electrical parameters of carbonate reservoir rocks

Abstract

The electrical parameters of 80 various carbonate rock plugs from a Saudi Arabian reservoir were measured. The results were first analyzed without considering the lithology, and then subsequently lithology was considered. Tortuosity distribution and correlations with other petrophysical parameters were used in differentiating samples of similar electrical properties, called “electro-facies”. Tortuosity distribution indicated two populations of data points one of which corresponded to the oolitic and skeletal-intraclastic lithofacies, referred to as “granular electro-facies”; and the other to the muddy and the dolomitic lithofacies, named “dolomitic-muddy electro-facies”. Three methods were applied to interpret the cementation factor: (1) the Archie; (2) the best fit; and (3) the cementation factor as a function of porosity. Error analysis showed that the Archie cementation factor was not an appropriate method for the studied reservoir rocks. The best fit and the cementation factor as a function of porosity methods yielded more reasonable results and they are recommended over the Archie method. A comparison of the water saturations obtained using the electrical parameters of the overall samples with the water saturations obtained using the parameters of each electro-facies showed significant errors especially in low water saturation zones. This indicates the importance of litho- or electro-facies discrimination in heterogeneous reservoirs.