Much of western Canada's conventional crude oil occurs in vertically continuous reefal carbonate structures. A common strategy has been to support oil production through downward vertical gas displacement. The gravity-stable displacement yields excellent conformance and high oil recoveries, with typical residual levels of 20% pore volume (PV). Once the oil zone has been depleted, leaving only a sandwich loss, the pools enter a blowdown phase to produce the gas cap from the top of structure. During the blowdown phase, if there is an underlying aquifer, the oil sandwich is displaced upward into the previously gas-displaced oil zone, trapping gas. Owing to the presence of the trapped-gas saturation, the remaining oil saturation in this zone, is reduced to near miscible levels (10 to 15% PV) as it is displaced by the underlying water, mobilizing incremental oil equal to 5 to 10% PV. When an aquifer is not present, bottomwater injection can be applied to ensure displacement through the entire gas-displaced oil zone. The successive displacement process (SDP), as this tertiary waterftood concept has been named, has been confirmed with full-diameter reservoir-condition core tests on carbonate cores in the laboratory. Observation from the initial stages of a full-field SDP application in Imperial Oil's Bonnie Glen reservoir after approximately 4 years of operation provides further encouragement, with performance indicating a reduction in the oil residual of 5 to 6% PV.
Many of the prolific reservoirs in the Western Canada basin are high-relief pinnacle reef structures, supported by large platform aquifer reefs. The majority of these reservoirs, discovered in the 1950's and 1960's, are depleted by use of vertical water- and/or gasfloods. Fig. 1a shows the initial saturations and the typically low connate-water saturations in the gas and oil zones in a gasflooded pinnacle reef. Oil is produced, thinning the oil bank until the economic limit of oil production is reached, as shown in Fig. lb. Oil recovery has been excellent, with recoveries from the gas-displaced pools being as high as 73% of the original oil in place (OOIP). Residual oil saturations (ROS's) in the gas-swept oil zones are typically 20% PV.