Reservoir characterization is a key element of asset evaluation and optimal field development strategies. Accurate water saturation evaluation and permeability profiling are crucial parameters in determining volumetrics and productivity of the multiple, stacked carbonate reservoirs in offshore Abu Dhabi and derisking the reservoir management process. The main objective of this paper is to look at saturation and permeability interpretations by comparing the various static and dynamic logging measurements.
Archie in 1942 postulated the water saturation calculation equation for sandstones, but it has also been used in carbonates because of its simplicity. Multiple authors have understood the effect of different textural parameters on water saturation calculation and have come up with different workaround techniques. The current case study illustrates how the integration of static measurements, such as dielectric dispersion and nuclear magnetic resonance (NMR) with dynamic measurements improves our understanding of the reservoir properties (saturation and permeability), and in turn support a more accurate reservoir evaluation. Sampling and downhole fluid analysis (DFA) performed by the wireline formation tester (WFT) identifies the fluid and rock properties in the various flow units.
One key observation in this case study is how the conventional Archie-based equation has struggled to provide satisfactory and accurate water saturation results in this reservoir, whereas the dielectric dispersion measurement proves its value by providing direct water volume measurement that is validated with the fluid pressure gradient and the sampling results. The WFT sampling and DFA stations—which account for the flowing fractions and the hydrocarbon characterization—are followed by an extended build up, otherwise known as mini drill stem test (mini-DST). The radial flow regime is used to derive the horizontal/radial permeability across the tested interval. Mini-DST measurements provide a depth of investigation much deeper into the reservoir compared to standard core or log analysis, yet at a fraction of the operational times typically needed by a full drill-stem test. This integration of static (saturations and permeability) with dynamic measurements (DFA and Mini-DST) aids in providing accurate reservoir characterization and better understanding of its properties.
Multifrequency dielectric measurements add a weapon in the arsenal of the petrophysicist and can be used in conjunction with dynamic measurements from the wireline formation tester to understand the rock and fluid properties and get a better understanding of the Cretaceous reservoirs of interest in offshore Abu Dhabi.