Reservoir Description and Performance Analysis of a Mature Miscible Flood in Rainbow Field, Canada

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Abstract
The Rainbow Keg River AA pool is a dolomitized carbonate reservoir in a mature stage of a vertical hydrocarbon miscible flood. The long history of enhanced recovery, plus the fact that the pool had not produced according to initial expectations, formed an ideal framework for an interdisciplinary re-evaluation of the pool and its depletion mechanisms. The result provided a new interpretation of the pool and allowed the development of an optimized depletion strategy based on minimizing coning and maximizing recovery in the remaining oil sandwich. Acquisition of 3-D seismic data provided a better definition of the shape and size of the reservoir and led to the drilling of four flank wells. The 3-D seismic data, information from the new wells and reinterpretation of old wells resulted in a reassessment of pool volumetrics. A new geological model suggested the presence of unswept oil, which was later substantiated when one well was recompleted above the solvent/oil contact and produced oil which had been bypassed by the solvent front. In combination with a revised material balance and a review of miseibility, the new reservoir interpretation explained past performance and demonstrated the success of the miscible flood. After the reservoir description and performance analysis were completed, the major challenge was to develop a production strategy which optimized the oil recovery from the remaining 15-metre oil bank. The main objective of the strategy was to reduce the sandwich loss. Using xylene washes, significant increases in the productivity index were achieved without large increases in gas production. A simplified coning simulation model provided a qualitative view of how the major reservoir facies affected cone development. The model plus reservoir description led to recommendations of optimum production methods (reverse coning, gas cycling and horizontal well application) for different parts of the pool.