Modelling of a Large Depleted and Tilted Carbonate Oil Rim Reservoir - An Integrated Case Study from UAE

Abstract

When a reservoir undergoes gas cap production without proper pressure maintenance, the underlying oil rim development at a later stage of the field development is very critical and challenging. Adding to the complexity is the tilted Free Water Level. This paper presents a study that was performed to model a large tilted oil rim reservoir that had been massively depleted by the production of the overlaying gas cap. The first part of the paper explains the development history of the oil rim and the low recovery factor that was obtained as a result of the massive gas cap depletion, and the results of previous attempts to revive dead oil wells through artificial means (ESP, booster pumps) which had limited success so far. The second part of the paper elaborates on the study which was carried out to build the concept of a tilted reservoir and later modelled it to get the proper initial water saturation distribution. The variable salinity concept was brought in to validate the proposed tilting scenario. The variable salinity was supported by produced water salinity data. The formation pressure test data also conforms to the tilting concept. The tilting in Free Water level (FWL) was also observed while analyzing capillary pressure data. Model was prepared to map the tilted FWL by krigging FWL depths at individual wells. The challenges that were encountered and overcome during the feasibility study: (1) construction of a new reservoir model incorporating a large set of static and dynamic data showing significant complexity with tilted fluid contacts, variable formation water salinity and initial fluid saturation; (2) multi-scenarios history matching with complex fluid movements and tilted Free Water Level; (3) reservoir uncertainty analysis using learnings from history matching; (4) accurate remaining oil saturation in oil rim. The originality of this study resides in the complex reservoir geology and field production history, in the integrated approach to address requirements of both oil rim and gas cap developments, and in the fact that proposed field revitalization calls for unusual static and dynamic reservoir property modelling which can conceptualize the fluid movement before and after production.