The oil rim reservoirs are referred to the reservoirs with limited oil column underlain with aquifer and overlain by a gas cap. In such reservoirs, the whole oil column can be in the capillary transition zone with a complex flow dynamics. In some cases, oil might have been displaced by imbibition process over geological time during the fluid migration history following the initial primary drainage. Understanding the capillary transition zone, displaced oil zone, saturation modeling, mobile oil distribution and flow dynamics can influence the resource estimation, field development planning and ultimate recovery.

In this paper, we present an extensive review on the concept of the transition zone characterization and the current limitations and challenges in modeling static and dynamic properties in oil rim reservoirs. Various actual field data and examples are presented to better understand the flow characteristics in capillary transition zone. To perform the study, we utilize sector modeling from real oil rim fields with displaced oil zone, the actual drainage and imbibition SCAL measurements, and the historical field performance data. Modeling approaches based on sole drainage and sole imbibition for initialization, imbibition and hysteresis impact on the flow dynamics and residual oil saturation (Sor) dependent to the initial oil saturation (Soi) have been investigated.

The study show that the sole drainage based approaches could underestimate the potential of the transition zone, which can make the project less commercially attractive. This method can lead to significant errors in residual oil saturation and inevitably unrealistic history match assumptions. The resulted model could be very mis-leading in the production forecasting. It is shown that proper description of representative relative permeability and capillary pressure data and residual/initial oil saturation dependency will lead to a more realistic history matched model. This could play a major role in the field performance predictions including the value of the tertiary oil recovery, in particular, for heterogeneous reservoirs.

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