Residual oil saturation (Sor) is defined as fraction of pore volume occupied by oil at the end of the oil displacement by a specific fluid. It signifies the ultimate recovery under a given displacement process and represents the endpoint of the relative permeability curves in reservoir simulation. The estimation of Sor is critical in understanding the behavior of the reservoirs during various recovery mechanisms and it is a very important measure used to decide the EOR process selection and feasibility for further exploitation of the reservoir.

The residual oil saturation varies depending on lithology, pore size distribution, permeability, wettability and fluid characteristics. There are several ways to estimate the Sor including core analysis methods, well log methods, and other saturation and volumetric assessment methodologies. However, none of the methods is regarded as a single best method for determining the Sor. In addition, there could be circumstances that the remaining oil saturation (ROS) is misinterpreted as Sor. The integration of various data sources is therefore critical in estimating the true residual and remaining saturations.

This paper highlight number of offshore field case studies where significant difference observed in Sorw estimation using various approaches from core and logs analysis. In these examples, SCAL data and logs in hydrocarbon column as well as swept intervals together with the wells/reservoir performances have been considered in estimating the Sor. It was observed that the production forecasting, reserve estimates, EOR mechanism are hugely affected by the Sorw estimation.

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