Steam-Assisted Gravity Drainage (SAGD) is being operated by several operators in Athabasca and Cold Lake reservoirs in Central and Northern Alberta. In this process, steam, injected into a horizontal well, flows outwards, contacts and loses its latent heat to bitumen at the edge of a depletion chamber. As a consequence, the viscosity of the bitumen falls, its mobility rises, and it flows under the action of gravity towards a horizontal production well located several meters below and parallel to the injection well. In practice, the temperature difference between the injected steam and produced fluids, called the subcool, is maintained between 15 and 30°C. Despite many pilots and commercial operations, it remains unclear what the impact of subcool on the performance and thermal efficiency of SAGD especially in reservoirs with a top gas zone. The objective of this study was to define a steam chamber operating strategy that leads to optimum oil recovery for minimum cumulative steam to oil ratio in a reservoir with a top gas zone. These findings were established from extensive simulation runs that were built from a detailed geostatistically generated static reservoir model. The strategy devised uses a high initial chamber injection rate and pressure prior to chamber contact with the top gas. Subsequent to breakthrough of the chamber into the gas cap zone, the chamber injection rates are lowered to balance pressures with the top gas and avoid or at least minimize convective heat losses of steam to the top gas zone. The results are also analyzed by examining the energetics of SAGD.

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