New saturation functions for carbonate reservoirs TZs based on fundamental petrophysical properties

In the present study a core analysis program including NMR measurements were performed to get a better petrophysical characterization of carbonate reservoir transition zone in the Abu Dhabi region. The results reveal three distinct rock types with average NMR-T2 cutoffs of 292 ms, 164 ms and 63 ms for the topmost, middle and lowermost samples of the reservoir respectively. Electrical resistivity and capillary pressurewater saturation were also measured at ambient and reservoir (2500 psi and 85 °C) conditions using a DCI Pc-RI system mimicking drainage and imbibition to investigate the hysteresis and variation in saturation exponent. It was found that the cementation exponent increases from 1.9 to 2.3 under overburden conditions and decreases in a stepwise manner during reduction of overburden, but not to the initial value due to hysteresis. This implies that the electrical parameters at ambient condition would lead to underestimation of water saturation. The saturation exponents estimated during drainage and spontaneous imbibition at reservoir conditions range from 1.6 - 7.5 for the different rock types tested in this study. New saturation functions were developed for each rock type from the measured electrical parameters at reservoir conditions by using the modified Archie's and Skelt Harrison's equation for the transition zone of this carbonate reservoir. The outcomes of the present study lead to a correction of the field resistivity log data that, overestimate the water saturation and hence predictions of saturation distributions, mobility and original oil in place.