Journal of Advanced Oxidation Technologies. 2017; is the corresponding author by Walter De Gruyter GmbH.

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1 Journal of Advanced Oxidation Technologies. 2017; Xi an Shiyou University, Xi an, Shanxi , P.R. China, yflliu@xsyu.edu.cn, jingyinghanting@hotmail.com The ignition process has a significant influence on the success of in-situ combustion at heavy oil reservoirs. During this process, oxidation reactions between crude oil and injected air mainly occurred. In this paper, a series of oxidation experiments were performed at different reaction temperatures and air-oil ratios to investigate the heavy oil oxidation characteristics at different stages of the ignition process. The results revealed that heat release and production of CO and CO 2 were observed during the entire oxidation process, while transformation of heavy components (resins and asphaltenes) in crude oil to light components (saturates and aromatics) and production of H 2 S occurred at higher temperatures. The heavy oil oxidation in ignition process can be divided into three stages based on physical and chemical characteristics of the reaction, they are low-temperature oxidation, pseudo-high temperature oxidation, and high temperature oxidation, respectively. in-situ combustion, ignition, heavy oil, oxidation characteristic /jaots March 25, 2016; May 6, 2016; May 26, 2016 In-situ combustion (ISC) is a common thermal recovery technology which has been studied for decades and used widely around the world. ISC technique have several advantages such as wide application range, high driving efficiency, and convenient acquisition of the injection fluid, which is usually applied to heavy oil reservoirs [1, 2]. Heat, steam and CO 2 are generated by burning a portion of oil after the injection of air. As a result, the composite gases, high temperature pyrolysis reactions mainly contribute to enhance oil recovery by reducing crude oil viscosity and increasing sweep efficiency. Most of China s heavy oil reservoirs with a low initial temperature. Thus, spontaneous ignition of the reservoir cannot be achieved. The injection of large amounts of air or oxygen-enriched gas into the reservoir was required in the ignition process for a successful ISC process. Typically, heat are released accompany with the multi-stages oxidation reactions between injected oxygen and crude oil, consequently uplift reservoir temperature of the near borehole zones. Then combustion could be achieved until reservoir temperature reached the spontaneous ignition point of oxidized oil. Apparently, the oxidation characteristics of crude oil during ignition process will significantly affect the implementation of ISC technology. In previous studies, much attention have been focused on low-temperature oxidation mechanisms of crude oil. Babu and Cormack investigated the effect of low temperature oxidation on the composition of bitumen in an oxidation temperature range of C [3]. Fassihi et al. conducted a series of low temperature oxidation of four oils, compositional, viscosity and density changes of which were analyzed [4]. Chen et al. carried out the high pressure low temperature oxidation experiments in a temperature range of C to examine the exothermic behavior [5]. Pu et al. performed an isothermal oxidation experiment at a condition of 120 C and 30 MPa and a series of thermogravimetry experiments to study the oxidation behavior. Effects of formation water, reservoir core, catalyst, and different kind of crude oils on low temperature oxidation were also discussed [6, 7]. However, little studies have been devoted to characteristics and reaction rate of heavy oil oxidation at high temperature conditions. Furthermore, the research of oxidation characteristics at a continuous increasing process of temperature from low to high which typically representative the ignition process, is particularly rare. In this work, motivated by the previous attempts and based on ignition process of heavy oil ISC technology, we aim to study the oxidation characteristics of the heavy oil at different air-oil ratios and reaction temperatures. Different phenomena including exothermic behavior, components change of produced gas and crude oil of the oxidation reactions were discussed. Hopefully, more data could be provided for a better understanding of oxidation reaction through our work. Moreover, it is expected that the results of our study can be applied to provide a theoretical support for the optimization of ISC ignition process. is the corresponding author by Walter De Gruyter GmbH.

2 LIU and LIU According to the previous research, a considerable factors, including reservoir temperature, pressure, initial saturation, injected air flux, type of porous media, and clay content, will affect the oxidation reaction, such as oxidation type and oxidation kinetics. Among these factors, reservoir temperature and injected air flux were usually considered as the direct influence factors, which are obviously shown in the kinetics equations. Ultimately, we choose initial temperature and air-oil ratio as the variable parameters to study the oxidation characteristics. Three air-oil volume ratios (13:1, 34:1, 68:1) and a series of reaction temperatures (65, 90, 170, 190, 220, 240, 260 C) under different air-oil ratios were eventually selected. The crude oil selected for experiments is from Liaohe oilfieldnortheast China. The original viscosity and density of the oil are 1,325 mpa s at 25 C and g/cm 3 at standard surface conditions. The oil sample was pretreated to remove water and gas. Glass beads, with a particle size of 40 to 80 meshes, were chosen to enlarge reaction areas by imitating porous media, the average apparent density of which is g/cm 3. A schematic diagram of the experimental apparatus is shown in Figure 1. Crude oil and glass beads were mixed in proportion in a stainless-steel reaction container, the volume of which is 430 ml. To obtain initial reservoir temperature before air injection, the reactor was preheated at a certain temperature for 24 h through an isothermal oil bath tank around it. Pressured air was then charged into the reactor until the designed air-oil ratio was reached, then a spontaneous oxidation will take place in the reaction container. Figure 1: Schematic diagram of oxidation experimental apparatus: 1, air pressure booster; 2, stainless-steel container; 3, pressure gauge; 4, isothermal oil bath tank; 5, oil + glass beads; 6, temperature acquisition system; 7, pressure gauge; 8, gas sample analyzer. Temperature and pressure of the reaction system were continuously recorded through an acquisition system during the experiments. Each of the experiments was sustained for more than 7 days to make sure that the entire procedure of the multiple oxidation was achieved. After each experiment, component analysis including CO, CO 2, H 2 S of produced gas and SARA (saturates, aromatics, resins, asphaltenes) fraction analysis of oxidized oil were conducted.

3 LIU and LIU Due to the exothermal behavior of oxidation reaction, a clearly transient temperature rise was observed simultaneously when the air was injected into the reactor. The temperature rise of the reaction system at different reaction temperatures and air-oil ratios, calculated by highest temperature minus initial temperature of reaction system for each experiment, have shown in Figure 2. The values of temperature rise can be employed to reflect the magnitude of oxidation heat release. Figure 2: Temperature rise at different reaction condition. Figure 2 indicates that at different experimental conditions, temperature rise of reaction system ranges from 0 to 20 C. When the reaction temperature kept constant, temperature rises with respect to a higher air-oil ratio in the system was apparently higher when compares to the ones with a low air-oil ratio, it also means the reaction become more intense. Additionally, when the air-oil ratio was the same, temperature rise in the system increased gradually with the reaction temperature at temperatures below 240 C. Once the reaction temperature reached 260 C, there had been a drop out compare to the previous points. Table 1 shows the relative content of H 2 S in produced gas, among the process of reaction temperature continuously increasing, H 2 S has not been detected until the temperature reached 260 C, indicating that pyrolysis reaction of crude oil occurred at this temperature. According to the theories of petroleum refining chemistry [8], the pyrolysis of crude oil is an endothermic reaction, part of the heat was absorbed by oil cracking, therefore lead to a decline of temperature rise in the system at this reaction temperature. Regard the appearance of pyrolysis reaction as the beginning of high temperature oxidation, then the lower critical temperature of high temperature oxidation of the experimental oil should be between 240 and 260 C. Table 1: H 2 S content in produced gas at different reaction conditions. Air-oil ratio of 34:1 Air-oil ratio of 68:1 90 C 170 C 190 C 220 C 240 C 260 C On the other hand, according to the variation trend of temperature rise before 240 C, the oxidation reaction can be divided into two stages. There is only a small change of temperature rise in the system with the increasing temperature between 65 and 170 C. While at the range of C, temperature rise increased rapidly, it implies oxidation reaction went through an accelerated exothermic stage. For a successful implement of ISC, a larger heat release is always hoped in the ignition process. It is obvious that the two different ways including rising reservoir temperature or injected air flux will play a positive role in heat release. Data shown in Figure 2 indicated that at the same temperature, the air-oil ratio increase twice will substantially double the heat release, and the maximum heat release at a certain temperature is three times to

4 LIU and LIU the heat release at initial reservoir temperature (65 C). Both of the two techniques require an extra cost. Hence, to get a large amount of oxidation heat release with smaller economic costs, a thorough consideration combined with actual technological process of oilfields is needed. In general, CO 2 and CO are the landmark products of oil oxidation, thus qualitative judgment of reaction type and degree can be achieved by an analysis of the relative content of CO 2 and CO in produced gas. Figure 3 reflects the relative content of CO 2 and CO in produced gas at different experimental conditions. It can be observed that the amount of CO 2 was relatively higher, while the amount of CO maintained at a low level. According to the low temperature oxidation theories [9], hydrocarbon molecules attacked by oxygen atoms lead to chain scission, most of which generated the carboxyl with a high chemical stability and further be peeled to create CO 2. A small portion of which generated hydroxyl and carbonyl, however, even fewer of the produced hydroxyl and carbonyl were then decarburized to produce CO. A specific schematic diagram of low temperature oxidation is shown in Figure 4. Figure 3: (a) CO and CO 2 content in produced gas at air-oil ratio of 13:1 (b) CO and CO 2 content in produced gas at ait-oil ration of 34: 1 (c) CO and CO 2 content in produced gas at air-oil ration of 68:1. Figure 4: Mechanism of the low temperature oxidation process [5]. From Figure 3, 3(b), and 3(c), it shows that at the range of C, CO 2 content increased with the reaction temperature at the same air-oil ratio. Furthermore, at the same temperature, CO 2 content also increased as the air-oil ratio increased. The increase of temperature and air-oil ratio could promote the conversion procedure of hydroxyl and carbonyl to carboxyl at this stage, which can be responsible for the increasing content of CO 2. Since CO content was low, the relative variation can be ignored.

5 LIU and LIU It is noteworthy that from Figure 3 and 3(c), a valley of CO 2 content was show between 190 and 240 C. From the low temperature oxidation theories, the temperature increase will without any question to promote the oxidation procedure which CO 2 was produced, and none of the previous work presented the similar phenomenon which produced CO 2 content corresponding to a higher oxidation temperature is lower than that at a lower oxidation temperature. Thus, a further discussion was required to find out the cause in others ways. The results of SARA fraction analysis for oil sample measured before and after the oxidation at a certain air-oil ratio (68:1) were listed in Table 2. It indicated that at a lower temperature region ( C), saturates content of the oxidized oil decreased slightly and aromatics content have a significant decline compare to the initial oil sample. It can be explained that hydrocarbon was oxidized into polar oxides such as aldehydes, alcohols, ketones, and hydroperoxides during the low temperature oxidation, which were insoluble in n-heptane at SARA analysis, then attributed to resins or asphaltenes in the test results. Table 2: SARA analysis of crude oil before and after oxidation. Saturates/% Aromatics/% Resins/% Asphaltenes/% Initial oil sample C C C C C As the temperature increases ( C), the content of saturates, aromatics in oxidized oil continued to rise, while resins and asphaltenes content were relatively reduced. This results revealed that at the continuous variation of temperature during the ignition process, also the variation process of low temperature oxidation to high temperature oxidation, showing a trend that heavy components transformed to light ones. It is worth noting that within this period ( C), asphaltene deposition had appeared for the first time, as shown in Figure 5. Typically, asphaltenes are partly dissolved and partly in suspension state, stabilized by resin molecules which are absorbed on asphaltene surface [10]. In general, asphaltene deposit occurred in CO 2 injection projects, and it is proved that CO 2 concentration is the most important factor on asphaltene precipitation. Evidence also shows that the asphaltenes were stable at a wide range of pressure (from 7 MPa to 13 MPa) without any CO 2 [11]. While CO 2 molecules are dissolved into oil, the relative content of resins will reduce, and the initial resin-asphaltene system is more likely to be damaged and finally lead to asphaltene deposition. Hence, the unusual decline of CO 2 mentioned above just matched the asphaltene deposition exactly, can be responsible for valley of CO 2 content (see Figure 3(b) and 3(c)).

6 LIU and LIU Figure 5: Asphaltene sediments in experiments. 1. Heavy oil oxidation in ignition process of in-situ combustion can be divided into three stages based on physical and chemical characteristics, they are low temperature oxidation, pseudo-high temperature oxidation and high temperature oxidation, respectively. 2. During the low temperature oxidation of heavy oil ( C), a small amount of heat was released and a certain amount of CO and CO 2 were produced. 3. During the pseudo-high temperature oxidation of heavy oil ( C), heat release was increased quickly compares to low-temperature oxidation, a tendency of heavy components transformed to light components was clearly observed. The rapid increase of heat release accompanied with CO 2 solution can be responsible for the deposition of partial heavy components caused by destruction of resins system. 4. During the high-temperature oxidation of heavy oil (above 260 C), pyrolysis reaction started to occur, accompanied with the production of CO, CO 2, and H 2 S especially. 5. Both of reaction temperature and air-oil ratio will affect oxidation characteristics of crude oil. As reaction temperature increase, heat release and CO 2 content of the oxidation increase, so did the air-oil ratio. 1. Moore RG, Laureshen CJ, Mehta SA, Ursenbach MG. J Can Petroleum Technol Dec 31;38(13): Nissen A, Zhu Z, Kovscek A, Castanier L, Gerritsen M. SPE Reservoir Eval Eng May 1;18(02): Babu DR, Cormack DE. Fuel Jun 1;63(6): Fassihi MR, Meyers KO, Baslie PF. SPE Reservoir Eng Nov 1;5(04): Chen Z, Wang L, Duan Q, Zhang L, Ren S. Energy Fuels Feb 12;27(2): Pu WF, Liu PG, Jia H, Zhao S, Du DJ, Wang S, et al. Petroleum Sci Technol Jul 18;33(13-14): Pu WF, Yuan CD, Jin FY, Wang L, Qian Z, Li YB, et al. Energy Fuels Jan 14;29(2): Gary JH, Handwerk GE, Kaiser MJ. Petroleum refining: technology and economics. Boca Raton, USA: CRC press; 2007 Mar5. 9. Ren SR, Greaves M, Rathbone RR. Chem Eng Res Des Jul 31;77(5): Leontaritis KJ, Mansoori GA. Asphaltene fllocculation during oil production and processing: a thermodynamic collodial model. SPE International Symposium on Oilfield Chemistry 1987 Jan 1. Society of Petroleum Engineers. 11. Novosad Z, Costain TG. Experimental and modeling studies of asphaltene equilibria for a reservoir under CO 2 injection. SPE Annual Technical Conference and Exhibition 1990 Jan 1. Society of Petroleum Engineers.