The so-called induction period, the time delay between the initial exposure to oxygen of an oil or oil fraction and the start of rapid oxidation, was examined experimentally for the saturates fraction separated from a Lloydminster heavy oil. The observed kinetics could be explained by assuming that the saturates contained a small amount of naturally occurring oxidation inhibitors, which repressed the oxidation rates by rapidly consuming an essential intermediate in the reaction chain, but which were also gradually consumed in the process. This observation explains some of the complexity that has been seen in the oxidation rates that control combustion front development during in-situ combustion, and provides some added direction in the development of a comprehensive reaction model for this process.
For enhanced oil recovery of heavy oil by in-situ combustion to be viable, it is essential that a high-temperature combustion front be established and maintained(1). Unfortunately, the creation of a combustion front can be thwarted (or sometimes aided) by the reactions that occur at temperatures below those needed for combustion. This problem has stimulated interest among some oil producers for a prediction method that can define the conditions for stable field operations.
Low-temperature oxidation (LTO) is chief among the troublesome reactions. During the initial ignition period of an in-situ combustion project, the temperature must pass through the low-temperature oxidation (LTO) region, and therefore the success of an ignition is strongly influenced by this group of reactions. Similarly, when a combustion front encounters conditions at which factors such as a low air flux or heat losses reduce the temperature so that high-temperature oxidation is weakened, the LTO rates may dictate whether or not a combustion front remains stable.
LTO has been found to be complex, and efforts to model it(2,3) have needed much more than a single reaction with Arrhenius rate parameters. In addition, the behaviour of the saturates fraction of oils has been shown(3) to be starkly different from that of the other SARA fractions.
One of the major differences between the oxidation of saturates and the other aromatic-type fractions is that the saturates, when separated from the other fractions, exhibit a prolonged period of apparent inactivity after which LTO becomes much more rapid. In the literature on the chemistry of air injection, this is normally called an "induction period".
There are two commonly accepted explanations for an induction period. One explanation is that it reflects the time for some intermediate compound in an essential chain of reactions to build up to the concentration needed for it to affect oxidation rates significantly. The other explanation is that a period of low reaction rate will exist until certain compounds that act as scavengers of an essential free-radical intermediate are consumed by their action.
The difference can be important. If the first explanation is true, then a new induction period will occur whenever oxygen supply is interrupted and the essential intermediate becomes consumed as it continues along its reaction path.
