Miscible water-alternating-gas (WAG) flooding has proven to be an attractive enhanced oil recovery (EOR) method the world over. Successful WAG floods can yield 5-20% additional oil recovery over waterflood because of improved microscopic displacement efficiency by the solvent coupled with improving areal and vertical sweep efficiency through the managed-mobility nature of the WAG displacement process.

WAG floods are complex in nature since reduction of residual oil in the pore spaces depends on mass transfer. Maintaining miscibility between the injected gas and the reservoir oil over a large rock volume is challenging. This challenge is more manageable in a small-scale pilot flood or a core flood than a large field implementation. Numerical simulation efforts can provide guidance to designing an optimal flood. However, the field application will often reveal challenges that are not discovered in the pilot stage or by the full field simulation model because the geologic properties and heterogeneity of the reservoir rock is not accurately represented. Sometimes the incomplete understanding of the PVT characteristics, such as fluid interaction and miscibility, or rock properties, such as wettability and capillary pressures, can exacerbate the challenges.

Integrated surveillance of a WAG flood is the only means to determine whether the flood is working efficiently and the planned additional recovery will be delivered. A well implemented surveillance plan allows timely intervention to improve the efficiency of an underperforming WAG flood.

This paper presents a systematic approach for applying EOR surveillance tools and methods in large miscible WAG floods in the Ivishak reservoirs at the Prudhoe Bay and Eileen West End of the North Slope, Alaska. Highlights of these surveillance methods are (1) designed and implemented by a multi- disciplinary team, (2) based on proven theory and corroborated with field data, (3) requires easily obtainable and relatively inexpensive field data and analysis, and (4) applied from fault block down to zone levels. Implementation of these tools has helped to identify efficient flood patterns and areas of poor performance, which then can be modified through infill drilling, well recompletion, or WAG ratio modification to maximize EOR recovery.

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