The Weyburn oilfield is in south-east Saskatchewan, Canada (Figure 1a) and lies in the north-eastern part of the Williston Basin. It was discovered in 1954, is now operated by EnCana Resources and covers some 180 square kilometres of prairie. Medium gravity sour crude oil (25 to 34 degree API) is produced from the uppermost Midale Beds (Charles Formation), which represent a transgressive-regressive cycle and comprise a succession of upwards shoaling, shallow marine carbonate-evaporite sediments of Mississippian (Early Carboniferous) age. The Midale Vuggy unit is a succession of highly fractured and permeable vuggy, heterolithic limestones that were deposited in relatively deep water. This unit is overlain by the cryptocrystalline dolomites of the Midale Marly unit, which represent shallow water conditions (Wegelin, 1987). The Midale Marly unit contains the majority of the remaining oil reserves and is the target for the miscible CO2 flood (Figure 1b).
Since 1964, water injection has been used as a secondary oil recovery strategy and has efficiently swept incremental oil from the Midale Vuggy Beds due to the high permeability of this unit. The water flood helped the field to achieve its peak production in 1965. The field was revitalized in 1991 by the drilling of horizontal wells. The Midale Marly Beds are less permeable and CO2 injection is an effective method of extracting oil from this unit. This is because CO2 is an excellent solvent and dissolves into the oil, thereby simultaneously reducing viscosity and increasing volume. The reduced viscosity makes the oil flow more easily and the swelling of the CO2-rich oil enhances reservoir pressure. It is anticipated that this CO2-EOR operation will extend the life of the Weyburn oilfield by the production of 130 million barrels of incremental oil (Figure 1b). The injection of CO2 was started in September 2000 as Phase 1A and the initial injection rate was 5000 tonnes per day. Injection was originally in 18 patterns of nine wells, each at the west end of the oilfield. The CO2 flood has been extended in a south-easterly direction and the ultimate aim is to flood 75 patterns in phases over the next 15 years. The CO2 used is a purchased by-product of coal gasification and supplied to Weyburn through a 320 km long pipeline from the Great Plains Synfuels Plant in Beulah, North Dakota, USA that is operated by the Dakota Gasification Company (Figure 1a) (Stelter, 2001).
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