The results of a series of combustion tube tests conducted with a medium gravity crude oil from the Esperson Dome Field, Texas, are described in this paper. The effects of water/oxygen ratios (WO2R), ranging from 0 to 10 bbl/Mcf, on residual oil saturation in the burned zone, fuel and oxygen requirements, and rates of combustion/steam fronts advance were determined.
The results show that up to 7% residual oil saturation (based on pore volume) remains in the burned zone at the highest WO2R studied. Thus, one of the main benefits of wet combustion, viz., reduced fuel/O2 consumption, must be balanced against the reduction in displaced oil.
Normal wet combustion occurs at WO2R's up to 2.5 bbl/Mcf, and the steam zone ahead of the combustion front continues to grow with increasing WO2R. Oil production is accelerated and the oxygen/produced oil ratio decreases. A transition zone of constant fuel and oxygen requirements is observed as the combustion process changes from normal wet to super wet. This is the first time such data are reported for fireflooding of a medium gravity oil with oxygen. At high WO2R's, super wet combustion occurs, characterized by combustion at steam temperature.
An analytical heat transfer model successfully matches the experimental results. It predicts the minimum and maximum WO2R's for normal wet combustion. Below the minimum WO2R, the model suggests no significant growth of the steam zone. The model should be useful for estimating WO2R's in field projects.
In-situ combustion is a thermal enhanced oil recovery process in which heat is generated in the reservoir by burning a portion of the oil-in-place. Air combustion projects date back to the early 1950's. A recent modification of the process involves the use of oxygen at high concentrations (about 95%) instead of air. As a result, the injected gas is more reactive than air. The combustion process can therefore be extended to medium-gravity (18–30 ° API) and high-gravity (> 30 ° API) oils which would not otherwise sustain combustion. Other potential advantages of oxygen over air have been documented in the literature.1 Wet combustion is a process modification in which water is coinjected with oxygen to scavenge the heat stored in the burned zone. 2 Additional advantages of wet combustion are:
reduced fuel consumption, thereby leading to lower oxygen requirements, and
accelerated oil production because of a larger steam zone and pressure effects.
The injected water/oxygen ratio (WO2R) dictates the behavior of the wet combustion process, as shown schematically in Figure 1. 3 At low WO2R's, all of the injected water is vaporized behind the combustion front.
Heat is transported forward, thus enlarging the steam zone ahead of the combustion front. The process is called "normal wet" combustion. In the limiting case of normal wet combustion, all of the heat stored in the burned zone is recuperated. In this case, the process is referred to as "optimum wet" combustion. At higher WO2R's, the heat, stored in the burned zone is insufficient to vaporize all of the injected water and the firefront is "partially quenched" (super wet combustion).