The field discussed in this paper is a highly unconsolidated sandstone reservoir with medium oil viscosities and a large bottom aquifer. Earlier simulations predicted poor primary recovery and a water flooding scenario showed early water breakthrough owing to viscous fingering. As a result, oil recovery from water flooding was severely curtailed because of unfavourable mobility ratio resulting in a low ultimate recovery factor. Therefore, enhanced oil recovery (EOR) was thought of as a possible recourse to increase recovery of this field.
Preliminary polymer flooding study carried out on the basis of the initial geological model helped in identifying some of the key design aspects impacting polymer flooding outcomes. In order to remedy these aspects, the revised geological model focused primarily on reducing uncertainties concerning property modelling and facies distribution. Screening dynamic simulations were performed to construct a recovery distribution function. The P50 case was taken forward for a more in-depth analysis of the current polymer flooding scenarios.
However, polymer flooding a thin heavy oil reservoir with a large bottom aquifer is a highly challenging scenario. Polymer losses to the aquifer are to be expected, even more so because the vertical permeability in the reservoir formation is extremely high. Although simulation results show acceleration in oil production due to improved displacement efficiency resulting from polymer slugs, the polymer losses are quite substantial and probably would render such a process uneconomic. In addition, significant amounts of oil are pushed into the aquifer as a result of injecting viscous water phase into the oil. In an attempt to mitigate these losses, a hybrid scenario was studied, in which hot water and polymer flooding is performed simultaneously. It is shown that under certain operating conditions oil losses can be eliminated, and the absolute amount of polymer losses can be reduced substantially.