The 'Toe-to-Heal' Air Injection (THAI) process has been proposed as a possible method for heavy oil recovery. This process involves injecting oxygencontaining gas (air) into the reservoir through vertical or horizontal wells, burning some of the oil underground, and producing heated oil from a horizontal well. Laboratory studies of the process have been reported (Greaves et al., 1996, 1997).
This paper reports a numerical study of the THAI process. The study includes history matching of the laboratory experiments to give a better understanding of the process and a field scale numerical study to derive predictions of the process performance under field conditions.
With the limited time and limited laboratory information available the numerical history match of the experiments was able to show that a horizontal advancing combustion front could be achieved, producing a constant production rate with high recovery. The field scale simulations suggested an advancing front could be obtained with good vertical and aerial sweep. A constant oil production rate was also obtained. However, as the front progresses, only a short section of the production well is utilized at a time, resulting in a low oil production rate.
The idea of using in situ combustion process to recover the huge resources of heavy oil and bitumen in Western Canada has been around for many years. The combustion process involves injecting oxygen-containing gas (air) into the reservoir and burning some oil underground. As the formation is heated and oil viscosity reduced, oil can be recovered. The process makes all the sense economically and environmentally. The heavy equipment for steam generation is not needed. The amount of water need to be treated and CO2 emission to environment are greatly reduced. In addition, some degree of upgrading of the oil may also be achieved. All of these make combustion an attractive process. However, combustion has failed to achieve general success in the field.
There are several obstacles that need to be overcome in order to develop a successful in situ combustion operation. The first is to maintain high temperature combustion at the combustion front. Usually highpressure air or enriched air injection is used to ensure oxygen supply at the front. However, this may create the second obstacle, the fingering phenomena, and air override instability. These reduce the sweep efficiency and eventually may cause burnout of the production well. The third obstacle is how to effectively remove mobile oil. The last one is particularly critical for Canadian heavy oil and bitumen, where initial oil mobility is so low that the oil ahead of the combustion front can hardly be displaced. Any successful in situ combustion process has to overcome all of the obstacles.
The traditional in-situ combustion process involves vertical injection well(s) and vertical production well(s). Many field tests have been conducted in Canada. A summary of these works can be found in review papers by Moore et al.1, 2. Most of them used the traditional vertical wells.