Modeling of water saturation in carbonate reservoirs is a challenging task which becomes more difficult if the reservoir is highly affected by diagenesis processes. This paper addresses a robust water saturation modeling workflow that encompass multiple approaches followed to handle an onshore Abu Dhabi carbonate reservoir that have complex pore network. This covers a holistic reservoir petrophysical evaluation all the way to water saturation modeling for dynamic simulation studies.
The studied reservoir was deposited in high energy shallow, water platform environment prevailing during Early Cretaceous time. It has a large areal extent with low average porosity and moderate to low average permeability.
Considering the impact of porosity exponent (m) and water resistivity (Rw) on the water saturation evaluation, the workflow was adjusted to integrate all available log, core and well test data. By integrating the formation resistivity factor (FRF) at different net confining stress and petrography data, relationship for variable porosity exponent (m) versus porosity was developed.
The salinity variation was observed across the field (east to west) from the well test and Pickett plot. In water saturation estimation, salinity taken from one side of the field (east/west) causes either under or overestimation of water saturation by 18% on the other side of the field which has a Considerable impact on the STOIIP. To address this problem, variable formation water resistivity (Rw) was generated from log data. This was validated using Porous Plate capillary pressure data measured using samples collected from different areas of the field.
The destructive dolomitization at the bottom part of the reservoir resulted in the large transition zone and sporadic occurrence of poor lithofacies poses a great challenge in developing a saturation height function. The current case study also evaluates the best possible saturation height function which can explain the variability of water saturation across the field.