Low-Temperature-Oxidation Reaction Kinetics and Effects on the In-Situ Combustion Process
Authors: Dabbous Mahmoud K. , Fulton Paul F. 
Published: December 2007

Abstract
The kinetics of low-temperature oxidation (LTO) of crude oils in porous media was studied. Isothermal integral reactor data were analyzed to obtain rate equations for the over-all rate of the partial oxidation reactions at temperatures below partial oxidation reactions at temperatures below 500 deg. F. The reaction order with respect to oxygen was found to be between 0.5 and 1.0. The order of the reaction was dependent upon the crude but independent of the properties of the porous medium. The activation energy of the reaction was insensitive to the type of crude or porous medium and is in the neighborhood of 31,000 Btu/lb mol. LTO reactions were found to be in the kinitics-influenced region. The measured reaction rates for a 19.9 deg. API and a 27.1 deg. API crude indicated higher oxidation rates under similar reaction conditions for the higher API gravity crude. Light crudes appear to be m ore susceptible to partial oxidation at low temperatures because of the react ed oxidation reactions rather than by carbon oxidation. Other information includes the fraction of reacted oxygen utilized in carbon atom oxidation by the LTO reaction and the molar ratio of CO2 and CO produced in the low-temperature region. Effect of partial oxidation of the crude on the in-situ combustion process was studied by experimentally simulating the zones preceding the combustion front where temperatures and injection rates of linear reservoir model were programmed with time according to a predesigned schedule. Oxidation of the crude at temperatures below 400 deg.F had significant effects on the behavior of the crude-oil/water system in the porous medium at elevated temperatures and on the fuel available for combustion. A substantial decline in the recoverable oil from the evaporation and cracking zones, an increase in fuel deposition, and drastic changes in fuel characteristics and coked sand properties were obtained when the crude was subjected to LTO during the simulation process.

Introduction
The application of thermal energy to petroleum reservoirs as a means of increasing crude oil recovery has been given a great deal of attention. In underground combustion, thermal energy is induced by the partial burning of the crude oil in situ. The production of heat by the exothermic oxidation reactions of the hydrocarbons constitutes a unique feature of the in-situ combustion process. The chemical reactions and the accompanying heat released create a new temperature profile and cause drastic redistribution in the reservoir fluid saturations. With oxygen available in the transient zones of variable temperature and hydrocarbon saturations, several oxidation reactions of differing nature can take place during an underground combustion process. Because of the complex composition of process. Because of the complex composition of crudes and the great number of reaction products that can be produced, it is convenient to classify the hydrocarbon oxidation reactions ascombustion reactions that take place in the high-temperature combustion zone (above 600 deg. F) with CO2, CO, and H2O as the principal reaction products andpartial oxidation or low-temperature products andpartial oxidation or low-temperature (LTO) reactions that occur in zones where the temperature is lower than 600 deg. F.
Several partial oxidation reactions are known to take place, producing primarily water and oxygenated producing primarily water and oxygenated hydrocarbons such as carboxylic acid aldehydes, ketones, alcohols, and hydroperoxides. High-temperature combustion reactions are desirable because they generate most of the heat required for the in-situ combustion process. Partial oxidation reactions, on the other hand, are in most cases undesirable because of their adverse effect on the viscosity and distillation characteristics of the crude.
SPEJ
P. 253