Scale Effects on the Application of Saturation-Height Functions to Reservoir Petrofacies Units

Abstract

Summary An internally self-consistent reservoir model has been constructed to investigate the effects of scale on the numerical form and application of four saturation-height functions and on the resulting predictions of reservoir fluid saturations. The treatment uses reservoir partitioning into strongly characterized, physically distinct lithounits, or petrofacies, for purposes of petrophysical interpretation, and a separate, correlative reservoir zonation for subsequent parameter mapping and volumetrics. The context is therefore that of a deterministic reservoir model. The numerical difference between apetrofacies-specific saturation-height function calibrated at the well-log scale (< 1 m) and the same type of function generated at the reservoir zonal scale (>10 m) varies with the function used and the physical character of the particular petrofacies unit. The application of saturation-height functions established separately at these two different scales has allowed the departures introduced through scale transgression to be assessed quantitatively. This has been done by comparing the predicted zonal hydrocarbon saturations generated for reservoir mapping with benchmark values of the same parameter. For this deterministic approach to reservoir description, an extended algorithm that uses bulk volume water as predictand provides the most accurate evaluation of water saturation at the reservoir zonal scale, which must be used in volumetrics applications. In contrast, an exponential form of the saturation-height function shows relatively large errors at both scales. The significance and variability of scale effects have required the formulation of a structured approach to the optimum application of saturation-height functions in deterministic reservoir description. Through this facility, it is possible to contain the uncertainty associated with the distribution of hydrocarbon saturation at the reservoir mapping and volumetrics stages.