ABSTRACT
A comparison of oxygen and air fireflooding is presented from the technical and economic points of view for three specific applications produced by in--situ combustion.
In order to evaluate the economic interest of using of oxygen instead of air, a technico–economic assessment was made of the supplying of oxygen. In the three cases examined, supplying pure oxygen at wellhead pressure is always at least as economical as air compression with an equivalent oxygen flowrate. It is all the more advantageous as the flowrate is high and the injection pressure is low in the range examined (10 to 30 MPa) and considering the development strategies used in this study. However, the difference between the two techniques is sufficiently small to take into consideration the implementation conditions and performances of each type of gas in the recovery process itself.
3D field–scale combustion simulations and analytical calculations were carried out to evaluate the real technical advantages and disadvantages of the enriched–air combustion process for three idealized applications. This study includes the effect of CO2 dissolution (O2 combustion) and the water injection strategy (wet combustion, following water drive) on the sweep efficiency.
The technical part of the paper leads to the following main conclusions:
For a thin heavy–oil reservoir, it has been demonstrated that it is not possible to use air in a 6.25 ha (15.4 acres) inverted five–spot because of the air injectivity limitation. Oxygen clearly has a technical advantage in this case.
On the contrary, injection of oxygen instead of air is not recommended for waterflooded or steam–stimulated shallow reservoirs.
Based on results for one inverted five–spot, it has been found that scavenging of the oxygen fill–up behind the burn–front is not detrimental to the oxygen fireflood process when followed by waterflooding. By comparison with wet oxygen combustion, this technique may lead to higher recovery factors at slightly higher technical costs.
Regarding the field developments examined, O2 wet combustion (simultaneous O2 and water injection) is definitely the most efficient process for deep heavy–oil reservoirs.
Finally, the various processes and injection strategies applied to the three cases examined were compared from an economic point of view, which considers O2/air production costs, recovery and field costs including the additional costs of oxygen over air (safety, cleaning, completion).
This study gives a complete determination of key O2/Air fireflood parameters for the field of application covered by the cases examined.
