Steam-Assisted Gravity Drainage (SAGD) is a commercially viable recovery method for oil sands of Athabasca, where other methods have been unsuccessful. In one variation of SAGD, a small amount of a non-condensable gas is added to the injected steam to maintain pressure in the chamber while utilizing the energy in place, reducing steam consumption and providing thermal insulation from overburden heat losses. The role of gas during steam-gas co-injection processes, in terms of its effects on chamber development, bitumen flow rates and heat losses is not fully understood and therefore is the main focus of this work.
A new analytical model for gas injection in SAGD is derived, taking into account the three-phase flow of gas, oil and water in the reservoir. The analytical theory is used to predict the fluid flow rates as well as phase mobility, relative permeability and saturation profiles in the mobile oil region. The theoretical results are replicated by fine grid numerical simulations. Methane was used as the non-condensable gas for the purpose of this study as it is the main solution gas in most reservoirs. It is, however, believed that the findings of this study are equally applicable to other non-condensable gases such as nitrogen, air, helium and others. Fine grid numerical simulations were performed to gain a visual understanding of gas distribution in a SAGD chamber and its effect on in-situ steam quality, overburden heat losses, phase saturations and fluid flow rates. The simulation results support the predictions of the mathematical theory.
The results of the analytical and numerical study reveal that methane co-injection with steam is in general unfavorable in a SAGD operation. The injected methane tends to accumulate at the steam condensation front which lowers the heat transfer rate of steam to the adjacent oil resulting in lower oil production rates and slower growth of the chamber.