Heats of Combustion of Selected Crude Oils and Their SARA Fractions

Abstract

Abstract In situ combustion and high-pressure air injection are enhanced oil recovery (EOR) processes used to recover oil from both heavy and light oil reservoirs. These processes are quite complex and involve consideration of heat and mass transfer, phase behaviour of oil, water and gas, as well as relative permeability effects. This paper outlines a study that was conducted in order to develop a better understanding of the heats of combustion (HOC) for three different types of crude oils and their respective saturate, aromatic, resin and asphaltene (SARA) fractions. One outcome of the study indicated that saturates and aromatics have higher heating values than resins and asphaltenes, where this value in both saturates and aromatics (in any given crude oil) is close. Resins and asphaltenes also displayed heating values that were almost the same, however, were consistent in having a lower heating value than saturates and aromatics. The linear mixing rule was applied to predict the heat of combustion for the three crude oils studied. The HOCs for the maltene and asphaltene fractions were mathematically combined (per the mixing rule) to predict the actual observed HOC of the combined maltene/asphaltene crude. This rule did not hold true for all the crude oils studied, however, which suggests that the heat of combustion is not necessarily independent of the presence of other fractions. Introduction In situ combustion and high pressure air injection are technologies used for the recovery of both heavy and light crude oils. These technologies involve the creation of an oxidation front in the reservoir with subsequent propagation by air injection. Generally, air is injected in the reservoir and the oxygen contained in the air reacts with the oil through various oxidation reactions. The burning front is formed and the combustion gases produced from these reactions are available to help displace the oil. This process offers economic and technical opportunities for improved oil recovery in many reservoirs. Many thermal analysis studies on both light and heavy crude oils have been conducted and several oxidation tests for modelling the process have been performed. Verkoczy and Freitag(1) applied the relevance of various oxidation reactions to the modelling of in situ combustion in heavy oils, through three different sets of experiments. They performed thermogravimetric scans and autoclave tests on three heavy oils and their SARA fractions. They found that low temperature oxidation had significant and sometimes dramatic effects on the amount of coke formation. They also found that asphaltenes apparently underwent low temperature oxidation more rapidly than other crude fractions. K?k et al.(2) used thermogravimetric analysis under an air atmosphere at a 10 °C/min heating rate. Two oils (medium and heavy) were separated into their SARA fractions. Then a quantitative investigation was performed in order to determine the temperature intervals at which evaporation, oxidation and combustion effects operated for each fraction. Kinetic parameters of SARA fractions according to the Coat and Redfern technique were also established. K?k and Karacan(3) studied the behaviour and effect of SARA fractions of two different oils during combustion using a thermogravimetric analyzer and a differential scanning calorimeter.