On the Concept and Challenges of Water Saturation Determination and Modeling in Carbonate Reservoirs
Authors: Masoudi Rahim , Abdul Halim Muhammad Hafidz , Karkooti Hooman , Othman Mohamad 
Published: May 2016

Water saturation in the reservoir is commonly determined by interpretation of the electrical resistivity measurements using the Archie's law. The relationship of water saturation and resistivity index is usually characterized by a constant and unique saturation exponent (i.e. n) for whole reservoir. However, our study in line with various researches in open literature have shown the dependency of the n exponent to different parameters including the saturation history, wettability, brine salinity and pore size/type/geometry. This could be among the source of error and challenges in reconciling the core and log derived water saturation especially in carbonate reservoirs which may lead to unreliable initial volumetric assessment and subsequent prediction scenarios.

A comprehensive investigation has been conducted in this study using actual carbonate field data on reliable determination of water saturation taking into account the dependency of n exponent to saturation history. In most studied cases, by applying the fix constant n exponent for the whole reservoir, the log derived water saturations are underestimating the initial water saturation (i.e., hence overestimate the hydrocarbon in-place) in the reservoir and in particular above the capillary transition zone when compared to the capillary derived water saturation. Resistivity index found to be not always in a linear relationship to water saturation (from the log-log scale plot) and hence constant saturation exponent may not be valid to be considered. A methodology has been proposed to develop a saturation dependent n exponent for carbonates where it becomes more important in lower water saturation interval above the capillary transition zone.

The proposed methodology and the derived correlation in this work for reliable Sw modeling has been trained and tested in with various real field data with especial focus on applicability in carbonate reservoirs. The correlation is developed based on a comprehensive petrophysical and SCAL database from majority of the carbonate fields in Sarawak Basin, Malaysia. The dynamic histories of production, pressure and saturation changes of some of the fields have also been used to verify the reliability of the developed methodology and correlation. The results show remarkable improvement in water saturation determination and modeling which can make the study to be a good reference on the subject.

Sw Determination: Concept and Challenges

Archies empirical equation relates the resistivity reading from logs to formation water saturation through Resistivitu Index and n exponent parameters. Plotting Resistivity Index, RI (measured from SCAL) vs. water saturation results in a line with slope of n, the saturation exponent. However, most of the points are scattered and usually one slope cannot be drawn for whole range of water saturation. Conventionally, a value of about 2 would be used as an average saturation exponent for the whole reservoir regardless of the level of heterogeneity and various rock typing. Field performance data has shown that this approach could have a huge impact on the calculated volumes, hydrocarbon recovery and hence project economics.