The conventional and widely used way of distributing saturation arrays inreservoir simulation models is through porosity-weighted water saturationvalues. In this way, each grid cell has an assigned porosity and initial watersaturation. The porosity and water saturation are estimated with the help ofwell logs using established procedures. However, for producing reservoirs, logderived saturations may not represent initial saturations due to variousreasons like depletion due to production and effect of water injection etc. Oneof the ways to estimate the initial water saturation is by the use ofrelationship between depth and bulk volume of water (BVW). Such relationship, known as Saturation-Height function is used to estimate saturation values awayfrom the well locations and to calculate the hydrocarbons in placevolumetrically. This approach has been used in a multi layered carbonatereservoir of an Indian offshore field.

Layer-wise saturation height functions are developed by establishingrelationships between height above the free water level and bulk volume ofwater derived from the wells drilled in the initial phase of field development.The scatter in the BVW plot has been reduced by further classifying the datafor different porosity facies. These porosity intervals are treated as rocktypes for that layer. Since each layer has a particular range of porosity, different porosity based rock types are identified. Height above the free waterlevel versus water saturation plots are then generated for different rock typesusing the relationship developed for each geological layer.These equations wereused to assign initial water saturation in the reservoir simulation model.

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