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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.

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In-situ combustion (ISC) is an effective thermal recovery process that provides an alternative to steam injection. Air is injected into a reservoir to oxidize a small portion of the hydrocarbons present thereby generating heat and pressure that enhance recovery. ISC suffers from fewer limitations than steam injection, but is not applied widely. One factor that has limited application of ISC is the difficulty of evaluating ISC candidates in the laboratory to obtain critical information such as crudeoil oxidation kinetics, combustion front propagation, and the burning qualities of various crude-oil components.The Karamay Field (Xinjiang, China) has crude oil that is roughly 12.0°API and is a potential candidate for ISC. We have conducted a screening study to evaluate the likelihood of the success of ISC in Karamay. Questions of interest for Karamay include the fraction of the crude oil that is converted to fuel and the particular crude-oil components that become fuel for ISC. To investigate ISC properties of the oil, true boiling point fractions were collected from Karamay crude oil and the kinetics of oxidation of each fraction in porous media were measured using ramped temperature oxidation (RTO). The isoconversional approach is used to interpret the RTO results of each boiling point fraction. The approach obtains the reaction kinetics for a given rock/oil sample at identical reaction extent from multiple experiments with different heating rates.The 500+°C boiling point fraction presents reaction kinetics most similar to the whole crude oil indicating that the crude-oil components in this fraction contribute the most to fuel production. Reaction kinetics of the whole oil appeared to be favorable for successful propagation of a combustion front. This prediction was validated by conducting displacement tests in a 1 m long combustion tube using reservoir sands. The Karamay crude oil demonstrated significant in-situ upgrading as a result of ISC. The initial gravity was 11.8 °API whereas the average gravity of the produced oil was 19.3 °API. Importantly, our results add to the knowledge base of both conditions for successful ISC as well as significant upgrading.

This paper shows the close relation between the displacement effects and reservoir properties in CO2 flooding for low permeability reservoirs. Low permeability reservoirs are characterized by tight matrix, strong heterogeneity and developed fractures. With the increase of CO2 injection rate, the CO2 viscous fingering gets worse in low permeability homogeneous reservoirs. While for low permeability heterogeneous reservoirs, the increase in displacement differential pressure contributes to the enhanced oil recovery in less permeable layers. The moderate increase in effective pressure is conductive to improveing the oil recovery in matrix. The displacement effect and CO2 channeling can be improved through the adjustment of injection-production parameters in CO2 flooding. Additionally, the interaction among CO2, oil and brine can improve the water-oil viscosity ratio. The gas viscous fingering can be weakened. The temporary performance of oil-water emulsion band formed in carbonate water can plug high permeability layers, and the production capacity of low permeability layers can be enhanced. The compound gel system in the sour environment of CO2 can plug large gas-channeling paths, restrict gas channeling and enlarge the sweep efficiency.