Non-condensable gases (CH4, CO2, and H2S) are present and play an important role in the thermal efficiency of SAGD. However, the role of these gases is not well understood and it is regarded by some people as beneficial, yet detrimental by others. The characterization of the gas flow in SAGD is crucial to predict its effect on the process performance.
One mechanism involved in the flow of gas is the viscous drag due to the falling liquids in the SAGD chamber. In this work, the production of gas in SAGD was studied by deriving flow equations that describe the viscous drag in a gas-water-oil system. Two geometries have been studied; these are flow in a capillary tube and flow of a descending film on a plate.
The three-phase flow analysis has been extended to a macroscopic level in order to predict the amount of gas produced due to the viscous drag of the falling phases. The assumption that the reservoir behaves as a bundle of capillaries with a pore size distribution was made.
Results from this analytical model indicate that some of the gas in the steam chamber flows downwards and therefore could be produced by viscous drag of the falling liquids.
The necessity and interest to fully understand the effect and behavior of non condensable gases (NCG) in SAGD has increased lately due to their important impact on the efficiency of this process, namely, steam consumption and oil production rates. These NCG's are naturally present in the reservoir generally as solution gas and they can also be added intentionally. Thimm [1] discussed on the source of these gases.
An additional amount of gas (mostly CO2 and some H 2S) can also be generated as the reservoir is heated [2, 3).
A number of simulation and laboratory scale studies have been carried out by several researches [2, 4, 5, 6, 7] to describe the role that these gases play in SAGD. However, as of today there is not a consensus or a unified understanding of the role these gases play in the process [8]. There are also some discrepancies between the results obtained in simulation studies and lab scale or field observations. The verdict about the advantages or detrimental impacts that non condensable gases may have on SAGD is not simple as they depend on reservoir and operating conditions [4, 8].
The intentional addition of non condensable gases to SAGD has as well created some controversy in regard to the benefits or disadvantages this may cause in the bitumen recovery [2, 7, 9].
Butler, for instance, seemed to defend the intentional addition of NCG's in SAGD. He claimed that the disastrous effect of these gases on the steam chamber growth and the oil production rate were noticed mostly in numerical simulations but not so much in the field or scaled model experiments.
