Kinetic Modelling of Thermal Cracking and Low Temperature Oxidation Reactions

Abstract

Abstract Two kinetic models were developed to describe thermal cracking and low temperature oxidation (LTO) reactions of Athabasca bitumen for the in situ combustion process. These unified kinetic models can describe the compositional changes in the Athabasca bitumen under thermal cracking or LTO. Bitumen composition was expressed in terms of the pseudo-components maltenes, asphaltenes, and coke. Oxidation reactions occurring in the low temperature range (less than 300 °C) are complex with two primary oxidation reaction modes: oxygen addition and bond scission. For Athabasca bitumen, oxygen addition reactions are generally dominant in the low temperature range (less than 300 °C) however bond scission or carbon oxide forming reactions occur to some extent at reaction temperatures greater than approximately 150 °C. The variation in the oxygen uptake rate with time at a given temperature is accounted for through rate equation describing the change in composition of the oil and the rate of oxygen uptake at the temperature of interest for each of the pseudo-components. The oxidation model for the low temperature range that was developed in this work accounts for both oxygen addition and bond scission modes of reaction. It is capable of predicting the effect of temperature, pressure, oxygen concentration, and time on bitumen composition and oxygen uptake rates. Introduction For nearly 90 years, in situ combustion (ISC) has been employed to improve recovery from oil reservoirs. Until now, this technology has not been widely used because of the mixed history of success of the field implementation. The failures have resulted from many reasons, however a key point is that the transition of experimental data to commercial pilot stages has not been achieved. This stems from the fact that the fundamental reaction mechanisms of the in situ combustion process have not been understood completely. It is well known that the combustion front advance and air (or fuel) requirement of ISC are determined by the kinetics of the reactions occuring in the vicinity of the burning front. Three major reactions have been reported:thermal cracking;liquid phase low temperature oxidation (LTO); and,high temperature oxidation (HTO) of an immobile hydrocarbon residue. This paper will focus on the first two reactions because they are the two principal reactions associated with fuel deposition during the in situ combustion process. Thermal cracking reactions are traditionally referred to as the fuel deposition reactions for in situ combustion. The carboncarbon bonds of the heavier hydrocarbon components are broken to form low carbon number hydrocarbon molecules, plus an immobile fraction which is referred to as coke. In order to better understand thermal cracking reaction mechanisms and correctly predict product concentration, it is necessary to understand the thermal cracking reaction within the timeframe of an in situ combustion process. A significant amount of data on the thermal cracking of Canadian heavy oils has been published(1–3) and compositional models have been developed to describe the change in the oil composition as a function of temperature and time.