Behavior and Effect of SARA Fractions of Oil During Combustion
Authors: Kok M.V. , Karacan C.O. 
Published: December 2007

In-situ combustion technology is considered to be an efficient one not only for heavy oil reserves but also for depleted light and medium oil bearing reservoirs. Unfortunately, the lack of better understanding of the process variables in terms of the conversion of oil during combustion and reservoir characteristics, as well as the costs, limits the more effective application of this technology. In this study, SARA fractions of two (medium and heavy) Turkish crude oils were separated by column chromatographic techniques and combustion experiments were carried out on whole oils and fractions by thermogravimetric analyser (TG/DTG) and differential scanning calorimeter (DSC) under air atmosphere at 10 C/min. heating rate. TG and DSC data were analysed for the determination of weight loss parameters, and for heat data of individual fractions (which have to be known for in-situ combustion technology utilisation) and to determine the temperature domains for individual reaction for each fraction. Investigation of SARA fraction for combustion enables us to show quantitatively the temperature intervals where evaporation, oxidation and combustion effects operate for each fraction data shows that each fraction has specific and sometimes overlapping temperature domains for these reactions. Experiments show that the most similar fractions in terms of their behaviour are aromatics and resins. Asphaltenes and saturates are two different extremes. For instance, asphaltenes go into combustion directly with almost no low temperature oxidation, whereas saturates are the earliest (at low temperature) oxidised compound. It has a low temperature combustion region which, in a way, triggers combustion of whole oil. If we think that during pyrolysis and combustion, large amount of saturates is produced, which means, it is role ahead of combustion front will affect the stability of process. By using the data and findings of this study one can take advantage of studying the kinetics of SARA fractions instead of complex whole oil for modelling of the overall process accurately to predict combustion.

Introduction

In situ combustion is a process of recovering oil thermally, whereby oil is ignited underground, creating a combustion front that is propogated through the reservoir by continuous air injection. Success of such a process depends mainly on the crude oil properties and rock properties as well as operational conditions. Till today many researches have been conducted on different phases of that process, mainly on the fluid and rock interactions during combustion of fluid phase. Vossoughi et al conducted research and concluded that the addition of clay to porous media significantly affected the combustion of crude oil. Bae investigated the thermo-oxidative behaviour and fuel forming properties of various crude oils. The results indicated that oils can be classified according to their oxidation characteristics. Vossoughi has used TG/DTG and DSC techniques to study the effect of clay and surface area on the combustion of selected oil samples. The results indicate that there was a significant reduction in the activation energy of the combustion reaction regardless of the chemical composition of additives. Vossoughi and Bartlett have developed a kinetic model of the in-situ combustion process from thermogravimetry (TG/DTG) and differential scanning calorimeter (DSC). They used the kinetic model to predict fuel deposition and combustion rate in a combustion tube. Kok characterised the combustion properties of two heavy crude oils by DSC and TG/DTG. On combustion with air, three different reaction regions were identified, known as low temperature oxidation, fuel deposition and high temperature oxidation. Heat values and reaction parameters of crude oils are also obtained from DSC-TG/DTG experiments. Ciajolo and Barbella used thermogravimetric techniques to investigate the pyrolysis and oxidation of some heavy fuel oils and their separate paraffinic, aromatic, polar and asphaltene fractions.

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