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In situ combustion (ISC) process has drawn more and more attention in the development of heavy oil reservoirs as a result of its high recovery efficiency. Although numerous studies have been reported that oil properties exhibit significant changes during the combustion process, the reaction mechanisms and evolution of oil components are still not well understood. In this work, the compounds of produced oils collected from a three-dimensional simulated production model (container) at different duration times after combustion being initiated and the original oil were characterized at the molecular level using gas chromatography (GC), gas chromatography–mass spectrometry (GC–MS), and high-field Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Both aromatic and acidic components were analyzed. The aromatic components showed relatively more stable characteristics than those of acidic components, and no obvious changes in aromatic compound distributions were observed by the positive ion atmospheric pressure photoionization (APPI) FT-ICR MS analysis. Small aliphatic acids were detected in the ISC oils, which were responsible for the high total acid numbers (TANs). The acidic O x (x = 1–3) compounds, which have major contributions to the increase in TAN, were generated in greater abundances compared to that of the original crude oil. The carbon number distributions of the O1 and O2 classes in the produced oils significantly shifted to a lower carbon number region, with the dominant distribution from 15–40 at the initial state to 10–30 at the longest duration time. The double bond equivalent (DBE) values decreased during the combustion process. The generated acidic O1 components with DBE values less than 4 were also found in negative ion electrospray ionization (ESI) analysis, indicating the oxidation of hydrocarbons to alcohols.

A pilot test of in-situ combustion (ISC) was carried out in Jiang oil field, Junggar basin, China, and a favorable result was obtained. In this work, we systematically studied the changes of crude oil properties during the combustion process. Crude oils were characterized by means of rheology test, SARA (saturates, aromatics, resins, and asphaltenes) fractionation and analyses, CHNO elemental analyses, and acid number (AN) measurements. Furthermore, analyses of FTIR and GC-MS on the resins were carried out to investigate the functional groups and polar compounds. Moreover, influence of particular inclined formations and sampling wells’ locations are also considered to interpret the effects of ISC process in the field. During the fireflood process, the crude oil’s viscosity reduced significantly and the reduction varied according to different sampling wells with different dip angles and distances. The crude oil was greatly upgraded based on SARA fractions analyses. The content of saturates varied among those wells, and a higher value happened and was accompanied by the decrease of aromatics content during the early stage of ISC. Non-hydrocarbons content increased within the period of 4 years of investigation. It was found that the greater the asphaltenes content is, the higher will be the oil recovery (OR) obtained. The AN of oil increased remarkably during the ISC process. To some extent, the CHNO contents and H/C and O/C ratios of the oil samples could reflect the degree of oil modification; however, these values had not been found to correlate with the production performance. Polar compounds in the resins fraction such as carboxylic acids, ketones, and alcohols are detected, and the polar compounds that contribute to the increase of AN values of oils could be mostly from short-chain carboxylic acids, alkylphenols, and long-chain fatty acids.

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The in situ combustion (ISC) process has drawn a lot of attention in the field of heavy oils. However, in the case of a light crude oil reservoir, in which low-temperature oxidation (LTO) is dominant, it is still less well-understood, especially for its reaction mechanism. In this paper, ramped temperature oxidation (RTO) experiments with different temperature intervals are used to investigate the oxidation reaction behaviors on various distillation pseudo-components from Dagang light crude oil. Both RTO and isothermal experiments are conducted on the whole crude oil and the sand mixture to obtain the LTO kinetic behaviors. The results indicate that oxygen addition reaction of the crude oil occurs to a great extent in the low-temperature region of 120−200 °C. Because the LTO reaction incorporates an oxygen atom into petroleum molecules rather than forming high-temperature oxidation (HTO) products (i.e., CO 2 , CO, and H 2 O), CO 2 production is minor during the LTO process. The acid number of the crude oil increases with an increasing reaction time and temperature during the LTO as a result of the formation of organic acids. Two pseudo-component distillates were subjected to major oxygen additions as evidenced by oxygen uptake and increases of the acid numbers of oxidation products. The apparent activation energy (E a ) of the crude oil that derived from the results of RTO tests (at different temperature ranges) accompanied by the isoconversional method present the E a values varying from 160 to 350 kJ/mol as the temperature changes from 205 to 230 °C. The E a value obtained through the isothermal experiment shows a decreasing trend from 200 to 33 kJ/mol as the temperature increases from 148 to 235 °C.