In fields under gas injection, appropriately modeling and understanding gas miscibility is critical to developing accurate production forecasts and managing the field development. The field under consideration in this paper is a deep-water brownfield with crestal gas re-injection and peripheral water injection to maintain reservoir pressure. Over the past 10 years of oil production from the field, associated production gas has been largely re-injected into the reservoirs and has led to significant increases in the reservoir gas saturation. Initial fluid studies performed indicated that the injected gas is miscible above a given minimum miscibility pressure (MMP). However, as the pressure is depleted in the field, this may be challenged.

Modeling of the gas miscibility in the field is done using a modified black oil dynamic simulation model that includes Todd-Longstaff miscibility formulation. However, this method may not fully capture the change in fluid behavior as the pressure depletes and cumulative gas injection into the reservoir is increased.

This study focuses on the gas phase behavior and gas miscible displacement process through a full-field compositional simulation model to evaluate the performance of the black oil model as well as to gain additional insight into the fluid behavior in this field. This study also compares the long-term production prediction between the two models where the gas production and gas injection rates are high and reservoir pressures decline near to minimum miscibility pressure. The comparison results in a similar oil recovery in the two models. The results also indicate that the black oil model with Todd-Longstaff option can effectively simulate the gas miscible process, but the compositional model is essential to gain a more in-depth understanding of the gas miscible process and displacement over time.

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