Chemical-Reaction Mechanisms That Govern Oxidation Rates During In-Situ Combustion and High-Pressure Air Injection

Abstract

Summary The lack of an accurate reaction model for petroleum-oxidation rates is a serious hindrance to the simulation of oil-recovery processes that involve air injection. However, the chemical literature on hydrocarbon oxidation contains many examples of possible reaction mechanisms that could serve as guides. These mechanisms were screened to identify generally accepted reaction paths that could help reveal how oxidation occurs in petroleum reservoirs. It was found that there are at least eight groups of fundamental reactions that can seriously affect oxidation rates of crude oils or their pyrolysis products. These eight reactions are as follows: two that lead to hydroperoxide formation; “branching” by hydroperoxides; two reactions governing the negative temperature coefficient (NTC) region; oxidation inhibition; at least one rate-controlling reaction at very high temperatures; and the combustion of coke that is produced by pyrolysis. Each of these groups exerts an influence within a separate, identifiable range of conditions. These reactions, and the conditions under which they become important, are outlined in this paper. Various oxidation behaviors that were reported for both light and heavy crude oils were then compared and aligned with the eight identified reactions. The result was a framework for selecting pseudoreactions that can facilitate the prediction of the oxidation kinetics under a wide range of oilfield conditions. Some of these pseudoreactions involve the direct representation of free radicals or other chemical intermediates, which is a departure from conventional practice for in-situ-combustion simulation. The new reaction framework is expected to serve as a reliable guide to the construction of predictive reaction models and, consequently, improved simulation of both in-situ-combustion and high-pressure-air-injection (HPAI) processes.