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

Abstract

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. INTRODUCTION Gravity drainage of heavy oils is of considerable interest to the oil industry. Because heavy oils are very viscous and, thus, almost immobile, a recovery mechanism is required that lowers the viscosity of the material to the point where it can flow easily to a production well. Conventional thermal processes, such as cyclic steam injection and steam assisted gravity drainage (SAGD) are based on thermal viscosity reduction1. Cyclic steam injection incorporates a drive enhancement from thermal expansion. On the other hand, SAGD is based on horizontal wells and maximizing the use of gravity forces2. In the ideal SAGD process, a growing steam chamber forms around the horizontal injector and steam flows continuously to the perimeter of the chamber where it condenses and heats the surrounding oil. Effective initial heating of the cold oil is important for the formation of the steam chamber in gravity drainage processes3. Heat is transferred by conduction, convection, and by the latent heat of steam. The heated oil drains to a horizontal production well located at the base of the reservoir just below the injection well. Butler et al. 4 derived Eq. (1) assuming that the steam pressure is constant in the steam chamber, only steam flows in the steam chamber, oil saturation is residual, and heat transfer ahead of the steam chamber to cold oil is only by conduction. One physical analogy of the above process is that of a reservoir where an electric heating element is placed horizontally above a parallel horizontal producing well.