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Low temperature oxidation (LTO) of heavy oils involves many complex reaction mechanisms that are important for the success of ignition and front propagation during in situ combustion (ISC). In this...

In situ combustion is an enhanced method to recover heavy oil. The formation and oxidation of coke are crucial to promote the combustion front. Heavy oil from China and montmorillonite, a major type of clay, were used as samples in this study. The thermogravimetric analyzer (TGA) was applied to temperature-programmed oxidation/pyrolysis experiments to study the effect of montmorillonite on the thermal conversion characteristics of heavy oil. A fixed-bed reactor was then used to obtain coke and study the effect of montmorillonite on coke properties. The characteristic temperatures of thermal conversion decreased with montmorillonite in the oxidizing atmosphere but remained unaffected in the pyrolysis atmosphere. The fuel deposition increased in both atmospheres because of montmorillonite’s strong adsorption. In the oxidizing atmosphere, the presence of montmorillonite obviously promoted the progress of coke formation and increased coke yield. The content of O was increased, and the contents of C and H were...

Pyrolysis and oxidation of heavy oil is the fundamental of in situ combustion process. Heavy oil from Xinjiang, China and its main components, saturates, aromatics and resins (SAR), were used as samples to study their pyrolysis and oxidation characteristics with a thermogravimetric analyzer (TGA) and a fixed-bed reactor. The DTG curves and the detected gas products differed between pyrolysis and oxidation of the heavy oil. Elemental analysis of pyrolysis and oxidation cokes indicates that low temperature oxidation (LTO) primarily accounted for the differences. Compared with independent pyrolysis and oxidation of saturates, aromatics and resins, they showed little interactions during co-pyrolysis, but significant interactions during co-oxidation. When saturates co-oxidized with aromatics or resins, both the beginning temperatures of LTO were increased, the DTG curve at high temperature oxidation (HTO) remained unchanged during co-oxidation with aromatics, while the DTG peaks at HTO changed from two into one during co-oxidation with resins. Co-oxidation of aromatics and resins showed similar effect at HTO. Oxygen-adding reactions of saturates, aromatics, and resins at low temperature promoted the polycondensation between intermediate products and altered the subsequent cracking reactions, which caused special characteristics during their co-oxidation.

Coke formation during the process of in-situ combustion is affected by clay minerals, among which montmorillonite shows the most significant effect. A fixed-bed reactor was used to conduct the thermal conversion experiments of heavy oil and model compounds to reveal the mechanism of montmorillonite affecting the coke formation. Under the inert atmosphere, in the presence of montmorillonite, the temperature at which aromatic C–C groups of coke appeared decreased from 500 °C to 450 °C and polymers were formed when toluene was heated, indicating that montmorillonite catalyzed aromatization and polymerization, respectively. The increase in the supporter surface area promoted the formation of coke from pyrolysis by shortening the induction of coking. Under the oxidizing atmosphere, in the presence of montmorillonite, the initial temperature of O2 consumed by heavy oil decreased from 270 °C to 250 °C, indicating that montmorillonite catalyzed oxygen-adding reaction. The temperature of COx released by acetaldehyde and acetic acid decreased, and the release amounts increased, indicating that montmorillonite catalyzed decarbonylation and decarboxylation. Polymers were formed when acetaldehyde, acetic acid, ethanol, and acetone were heated, indicating that montmorillonite catalyzed polycondensation. When the oil mass fraction in the sample was within 18%, further increase in the supporter surface area did not affect the formation of coke from oxidation. The catalysis of montmorillonite was the main mechanism affecting coke formation during the thermal conversion of heavy oil under the inert and oxidizing atmospheres.

Air injection for in situ combustion (ISC) and air injection assisted cyclic steam stimulation (AACSS) techniques have a good application prospective in the development of heavy oils, while the explosion risk in the process of air injection is of great concern and has restricted the application of the technology. So that injection of oxygen-reduced air has been proposed to eliminate and control the explosion of oil/gas mixture with air. In this study, low-temperature oxidation experiments of heavy oil samples were conducted using a small batch reactor under the pressure of 5–15 MPa and oxygen content of 5%–15% at 225 °C in order to investigate the effect of oxygen content and pressure on the reaction rate and heat release during oxidation of heavy oil with oxygen-reduced air. The variations of temperature and pressure during the oxidation reaction were measured in the experiment, and the explosion phenomena of heavy oils were observed in the air with high oxygen contents (oxygen content more than 15%). The reaction rate and the exothermic heat of the reaction were calculated based on the pressure and temperature curves and using an improved heat loss model. The experimental results showed that reducing the oxygen content (e.g. reduced oxygen less than 10%) in air can effectively prevent the explosion of oil-gas mixtures, and the reaction rate and heat release during heavy oil oxidation are linearly proportional to pressure and oxygen content in the injected air when oil is in excess. The results of this study indicate that the heat generated in oil oxidation, which is important for the ISC and AACSS processes, can be controlled by pressure and oxygen content of the injected air, which can lay a good foundation for the oxygen-reduced air injection technique, especially for its application in deep heavy oil reservoirs, in which injection of oxygen-reduced air at high pressure can offset the effect of low oxygen content on heat release.

When in situ combustion is applied to reservoirs that have been exploited by steam recovery, the influence of high water saturation on the thermal conversion of heavy oil into coke needs to be evaluated. A Xinjiang heavy oil was used as the sample. A thermogravimetric analyzer and a pressurized reactor were used to study the influence of steam on the pyrolysis and oxidation characteristics of heavy oil under atmospheric pressure (0.1 MPa) and reservoir pressure (4 MPa), respectively. Under atmospheric pressure, the presence of steam showed little effect on the pyrolysis and low temperature oxidation characteristics of heavy oil but accelerated the rate of coke oxidation. Under reservoir pressure in an inert atmosphere, steam showed little influence on the coke yield but increased the proportion of alkanes and hydrogen in gas products due to the hydrogen donation of steam. Additionally, the oxidation activity of coke produced with steam decreased. The main reason was a decrease in the number of active sites of coke oxidation according to kinetic analysis. Under reservoir pressure in an oxidizing atmosphere, the coke yield decreased significantly because of the existence of steam, but the coke oxidation activity remained unchanged. The main reason for this coke yield decrease was that steam accelerated coke oxidation and increased the oxidation consumption of the produced coke in the coking temperature range of 250–400 °C. In addition, steam promoted the cleavage of oxygen-containing functional groups in heavy oil conversion intermediates, thus inhibiting the formation of coke precursors.