This study addresses the important role of initial gasto-oil ratio (GOR) in steam assisted gravity drainage (SAGD) operations. A numerical model using CMG's STARS was validated through history matching of laboratory experiments conducted at the Alberta Research Council. The impacts of initial GOR on process performance were then studied using field scale numerical simulations. The results indicate that high initial GOR may have beneficial effects, namely, reduction of oil viscosity, and improvement of the oil-tosteam ratio (OSR). A detrimental impact, however, is also shown as the gas impedes the rate of steam chamber growth, and hence reduces oil production rates. Further analysis indicated that this is because of a "dynamic vacuum" effect due to steam condensation at the front of the steam chamber. This dynamic vacuum effect dominates the diffusion process and creates a gas-rich zone at the front of the steam chamber, thereby resisting further growth of the steam chamber and slowing oil production. The same effect occurs when noncondensable gas was co-injected with steam in either live oil or dead oil reservoirs.

This study addresses the importance of initial GOR in SAGD heavy oil recovery operations. By history matching two laboratory scale experiments, one with dead oil and the other with live oil, we corroborated the theoretical and numerical prediction that gas would accumulate at the front of a steam chamber. This gas accumulation could slow down oil production as well as heat loss to the overburden. It is suggested that monitoring gas production during SAGD field operations may be critical for investigation of impacts of gas, and for developing strategies for performance improvement.