The Greater Burgan field has been producing since 1946 from a series of highly permeable Cretaceous reservoirs. Recently, a series of more complex reservoirs has been reassessed using advanced logging and wireline formation tester (WFT) technologies.
The techniques employed in the reassessment include fluid-quality (viscosity and presence of tar) mapping using nuclear magnetic resonance (NMR) log data and shallow invasion measurements using multifrequency, multispacing dielectric data. In addition, the dielectric logs provide a direct measurement of the Archie m exponent in water zones. Improvements in formation evaluation achieved by integrating these results with conventional logs included better differentiation of moveable from residual hydrocarbon, identification of variations in formation water salinity, and maps of oil-quality variation versus depth and across the field.
These techniques were applied to three case studies. In the first case study, formation evaluation was conducted in an Upper Cretaceous carbonate formation of unknown water salinity. The combination of dielectric logs and NMR enabled identification of water-bearing and residual oil zones where formation water salinity could be determined. The analysis revealed increasing water salinity with depth. Dielectric logging also provided a direct evaluation of Archie's m exponent in the water zones, in the absence of special core analysis. The NMR highlights variations in oil quality from one well to another.
In the second case study, a viscous oil layer located in the middle of a water zone in a Middle Cretaceous reservoir was evaluated. Moveable oil was identified by radial oil saturation variation close to the borehole detected by the dielectric log measurement. The interpretation was verified by the first oil sample ever recovered in this layer.
In the final case study, in a Lower Cretaceous reservoir, dielectric measurements provided accurate estimates of residual oil saturation required for planning enhanced oil recovery projects.
The results obtained from the application of the multifrequency dielectric dispersion and diffusion-NMR as confirmed with WFT sampling bring new insight to the evaluation of challenging formations within the Burgan field.