The main drainage strategy in Brage-Statfjord reservoir is pressure support by water injection to produce light oil. Continuous water injection is effective in maintaining the reservoir pressure and improving the volumetric sweep efficiency. However, waterflooding can leave behind large volumes of un-swept oil, mainly in the roof/attic of the reservoir. This attic oil can be targeted by gas injection with its properties of low density but gas injection alone cannot provide sufficient pressure support and high gas mobility can lead to quick gas breakthrough leaving the oil behind.

WAG (Water Alternating Gas) injection combines the macroscopic sweep efficiency and pressure maintenance of the waterflooding operation with displacement and microscopic sweep efficiency of gas injection to enhance the oil recovery by targeting the attic oil. This study looked at different WAG injection scenarios by varying the gas-water cycles and gas injection rates. The additional recovery due to WAG was normalized to the cumulative gas injection volumes as WAG efficiency. Role of hysteresis and gas trapping in 3-phase flow was also investigated with the help of numerical simulations. The impact of varying the parameters controlling the 3-phase flow such as Land's parameter, Secondary drainage reduction factor, critical gas saturation and critical oil saturation in gas phase was then included in a thorough uncertainty analysis using Latin hypercube sampling.

Simulation studies showed that the WAG response can be observed several years after injection of the first gas cycle. 3-phase hysteresis plays a significant role in enhancing the recovery due to gas trapping and reduced gas phase mobility. Results in this paper confirm that WAG has the potential to significantly reduce the attic oil saturation and provide an alternative to in-fill drilling, which both can result in higher ultimate recovery factors as compared to the conventional waterflooding.

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