A numerical simulation study was made to assist in the design of a tertiary miscible flood in the previously water flooded Keg River II Pool of the Rainbow Field in Northern Alberta. Because of the high vertical relief of the pool, recovery mechanisms in the reservoir are gravity controlled. The water flood was in a upward vertical direction while the miscible flood, which was started in July 1983, is in a downward vertical direction.
The simulation study indicated that coning to the wells' production intervals of water from below the oil bank and solvent and drive gas from above the oil bank would limit the efficacy of the tertiary process' unless an appropriate balance was made of oil production from the oil column and simultaneous water production from the underlying aquifer. Further, it was found that without water injection into the aquifer subsequent to the oil bank reaching the original water-oil contact, significant oil would be permanently lost to the aquifer.
The study predicts that tertiary miscible flooding will increase the ultimate recovery from the II Pool to 16.3 million barrels of oil or 75 percent of the original oil-in-place from the 11.5 million barrels of oil or 53 percent of original oil-in-place expected under continued waterflooding.
The Rainbow Keg River II pool, located in the Rainbow Field of northwestern Alberta, was discovered in 1967. The pool is a dolomitize bioherm reef of the Keg River member of the Devonian Elk Point formation. The reef has up to 392 feet of gross oil pay (Map 1) and an areal extent of approximately 180 acres at the original oil-water interface of 4,537 feet subsea. The Keg River II pool appears to be in limited hydraulic communication with the adjacent Keg River II pool through a common aquifer.
A total of four wells were drilled into the II pool from 1967 through 1979. A pilot water injection project was commenced in July 1968 which was followed in April 1969 by a waterflood pressure maintenance with water injected into the aquifer. Cumulative oil production to the end of 1982 was 9.9 million barrels or 45.3 percent of the original oil-in-place of 21.9 million barrels. The maximum oil column remaining at that time was estimated to be 85 feet.
Due to the advanced nature of the water flood, a tertiary recovery method was necessary to maximize the recovery from the II pool. Hydrocarbon solvent flooding the pool was the most attractive tertiary oil recovery scheme. Slim tube work indicated that a solvent containing 55 mol percent ethane – plus components would be miscible with the II pool reservoir fluid with a reasonable safety margin1. It was anticipated that a new producing well would have to be drilled to replace one of the producing wells which would have to be converted to solvent injection.
Hydrocarbon miscible flooding, as a secondary process, has been successfully applied during the last fifteen years in nine Keg River reefs in Rainbow.
