Gravity drainage is an elegant concept for in-situ extraction. The basic idea is to inject a vapour and produce a liquid. However, gravity drainage has an important physical constraint that is widely misunderstood.
This paper shows that gravity drainage processes that inject blended fluids such as propane-methane or a steam-solvent tend to have an inherently unstable material balance due to the difference between dew point and bubble point compositions. This can lead to the accumulation of volatile components within the chamber and impair both heat and mass transfer.
This material balance constraint does not appear to be adequately represented in many laboratory experiments and in many reservoir simulations. Ignorance of this constraint may help to explain the lack of commercial progress beyond the original SAGD concept, even though SAGD is now 40 years old.
To mitigate the material balance instability, the dew point and bubble point compositions should be as similar as possible. This paper concludes that gravity drainage will work best with injection of a pure fluid such as pure propane.
In a gravity drainage processes such as SAGD, it is undesirable to inject hot water because the water will short circuit directly to the production well and therefore hot water represents a loss of plant capacity and efficiency. Similarly in SAGD, it is undesirable to produce live steam vapour from the production well because any steam production represents a short circuit across the reservoir and again a loss of plant capacity and energy efficiency.
The dew point and bubble point are key features of vapour-liquid phase equilibria and provide constraints that define the physical mechanism for gravity drainage extraction. The dew point is the set of conditions (temperature composition and pressure) where the first tiny amount of liquid appears within a vapour phase. A familiar example is that of fog, where the mass of liquid water is inconsequential. Similarly, the bubble point is the set of conditions (temperature, pressure composition) where the first tiny bubble of vapour is present within a liquid phase. At the bubble point, the mass of vapour is inconsequential compared to the total mass of liquid.
To minimize short circuiting, gravity drainage processes generally try to inject at conditions near the dew point temperature at the injection well and produce at conditions at or slightly below the bubble point temperature at the production well. The latter is generally called steam trap control, where the withdrawal rate in the production well is restricted to ensure that the produced fluid remains in the liquid phase. Or in more familiar terms, the liquid withdrawal rate is restricted to ensure that the production well is fully submerged in liquid and not in direct communication with the vapour chamber.
The implementation of steam trap control is fairly straightforward if the injected fluid is steam and the produced fluids are bitumen and water. However, things are more complicated if there are non-condensable gases such as methane present or solvents or solvent blends.