Early attempts at in situ combustion involved what is referred to as the forward dry combustion process. The crude oil was ignited downhole, and then a stream of air or oxygen-enriched air was injected in the well where the combustion was originated. The flame front was then propagated through the reservoir. Large portions of heat energy were lost to the overburden and underburden with this process. To reduce the heat losses, a reverse combustion process was designed. In reverse combustion, the oil is ignited as in forward combustion but the air stream is injected in a different well. The air is then “pushed” through the flame front as the flame front moves in the opposite direction. The process was found to work in the laboratory, but when it was tried in the field on a pilot scale, it was never successful. It was found that the flame would be shut off because there was no oxygen supply and that, where the oxygen was being injected, the oil would self-ignite. The whole process would then revert to a forward combustion process.
When the reverse combustion process failed, a new technique called the forward wet combustion process was introduced. This process begins as a forward dry combustion does, but once the flame front has been established, the oxygen stream is replaced by water. As the water comes in contact with the hot zone left by the combustion front, it flashes to steam, using energy that otherwise would have been wasted. The steam moves through the reservoir and aids the displacement of oil. The wet combustion process has become the primary method of conducting combustion projects.
Not all crude oils are amenable to the combustion process. For the combustion process to function properly, the crude oil has to have enough heavy components to serve as the source of fuel for the combustion. Usually this requires an oil of low API gravity. As the heavy components in the oil are combusted, lighter components as well as flue gases are formed. These gases are produced with the oil and raise the effective API gravity of the produced oil.
The number of in situ combustion projects has decreased since the 1980s. Environmental and other operational problems have proved to be excessively burdensome to some operators.26–27
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