ABSTRACT

Rock types in reservoir simulators are usually defined to describe different geological facies of a similar dynamic behaviour. A capillary pressure versus water saturation curve and a set of rel-perm curves will be allocated to each rock type. For both types of curves the irreducible water saturation will be used to define the curves' end-points. Reservoirs are usually composed of a wide variety of rock types varying both vertically and laterally. While it is difficult to predict in advance how such a complex distribution should be modelled, the common approach is to group the individual sets of lab capillary data by rock types, combine rock types when required for calculation constraints and eventually compare the computed water saturations with the by-layer water saturation values derived from open hole log data Rather than group rock types by permeability ranges, porosity ranges or geological facies, the basic idea of the proposed method is to obtain rock type maps, consistent with the dynamic definition of a rock type in a reservoir simulator, by mapping irreducible water saturation for each reservoir layer. Logs of Irreducible Water Saturation, similar to Water saturation logs regardless of the height above the contact, are mathematically derived from log and core data. Correlations of the shape and the displacement pressure of the associated capillary pressure curves at the reservoir model layer scale with permeability, porosity and clay volume enable the mapping of the rock types for each reservoir layer. Finally rock types maps with their associated capillary curves will be used to calculate water saturation by grid block with respect to their height above the contact. The proposed method has the advantage of being able to define and correlate the different rock types in a reservoir regardless of the height above the contact. The paper details one field case where this method was successfully applied from the log to the reservoir scale.

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