Abstract

A noticeable increase in a carbonate reservoir permeability (relative to that of core) and calibration of the reservoir mobile water saturation resulted from its black model history match to 56-year primary production history.

The First Eocene carbonate reservoir of Wafra field contains a large amount of heavy oil (13-19 °API). Primary recovery since 1956 has been approximately 4 percent of the net OOIP. Steamflood has been piloted in the field since 2005. Full field steamflood performance forecasting relies heavily on reservoir simulation, which requires proper calibration of models. Long history of the field primary production is a valuable source of information for such a calibration.

Based on core data, the reservoir is not considered fractured and was modeled as a single porosity system. The model initial average horizontal permeability was about 200 md with a maximum of about 1000 md. The 258-million-cell full field geostatistical model was upscaled to a 3.6-million-cell simulation model with the reservoir heterogeneity preserved. Oil viscosity depth variation was incorporated.

The model was matched satisfactorily. Well productivity indices were increased to enable each of more than 300 wells to produce at reported liquid rates. Permeability was globally increased. An aquifer was included to match reservoir pressure. Resulting average horizontal permeability did not exceed 2-3 Darcy. Recent MDT tests support this permeability level.

Irreducible water saturation (Swir) was modified to match water production. First approximation of Swir was based on core data. Then Swir was multiplied by a factor. The resulting amount of mobile water seems modest within the producing interval and rapidly growing towards the OWC. This mobile water may become a steam thief zone.

The calibrated distributions of model permeability and mobile water will be used in steamflood forecasting.

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