A Study of Steam-Assisted Gravity Drainage Performance in the Presence of Noncondensable Gases

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Abstract Traditionally, the addition of a non-condensable gas to steam is known to have a beneficial effect on heavy-oil production when conventional vertical wells are used. Little information and experimental evidence exists regarding the effect of the addition of such gases in the steam assisted gravity drainage (SAGD) process. The limited literature suggests that the addition of small amounts of such gases (e.g., carbon dioxide) may improve oil recovery. The gas accumulates at the top of the reservoir and provides a thermal and pressure insulation effect that in turn limits the rate of front spreading at the corners of the steam chamber. In order to investigate these phenomena, SAGD experiments with and without carbon dioxide injection were conducted in a physical model. It is packed with crushed limestone premixed with a 12.4° API heavy-oil. Temperature, pressure and production data as well as the asphaltene content of the produced oil were monitored continuously during the experiments. It was observed that for small well separations as the amount of carbon dioxide increased, the steam condensation temperature and the steam-oil ratio decreased. The heavy oil became less mobile in the steam chamber due to lower temperatures. Thus, the heating period was prolonged and the cumulative oil recovery as well as the recovery rate decreased. In this instance, the produced oil contained a relatively high concentration of asphaltenes. This supports the observation of poorer oil recovery as the fraction of carbon dioxide injected increased. A large asphaltene fraction in mobile oil serves to maintain high viscosity. On the other hand little or no change in oil recovery and oil recovery rate was observed for larger well separations regardless of the fraction of carbon dioxide in the injection gas. Similar behavior was observed when n-butane was injected along with steam instead of carbon dioxide. The impact of initial gas saturation (carbon dioxide or n-butane) was also investigated. It was observed that cumulative oil recovery, rate of oil recovery, and steam-oil ratio decreased independent of well separation compared to a reservoir with no initial non-condensable gas.