BAO Peng－cheng，HAN Xiao－qiang，MA Yue－qin，LU Yao＊ ，HUANG Xiao－yi ，WU Li－bin，LIU Fang－fang

（1.Technical Institute of Physics and Chemistry，Chinese Academy of Sciences，Beijing100190，China；

2.Research Institute of Experiment and Detection of Xinjiang Oilfield Company，Karamay834000，Xinjiang，China）

Among many improved oil recovery（IOR）methods，air injection technology has received a great deal of attention both in the laboratory and in the field because it has several advantages.Air injection is freely available and does not suffer from any constraint on supply.Moreover，it can be used as a secondary recovery method when water injection is not suitable，or as a tertiary recovery method to enhance oil production after water－flooding.Traditionally，air injection has been applied to heavy oil reservoirs via an in situ combustion （ISC）process［1－2］，in which a portion of the in－place oil is burned to generate steam flooding and heat to reduce oil viscosity and enhance the oil recovery.In ISC process，low－temperature oxidation（LTO）has significant influence on the success of ignition process of ISC.

LTO reaction of crude oil is very reservoir－specific.Previous reports mainly focus on LTO process of light oil［3－6］.Researches conducted on heavy oil field are mainly about the combustion of crude oil.Few researches have been conducted on the LTO process of heavy oil.Therefore，LTO process of heavy oil needs further study.In this paper，LTO research of heavy oil was conducted by using sand packed reactors.The vent gases were analyzed by agas chromatography analyzer for the content of CO，CO2，O2and CH4.Compositional analysis of the oil sample was carried out by using the SARA （saturates，aromatics，resins，and asphaltenes）analysis method［7］.Additionally，pure oil components experiments were performed to study the LTO effects on components of the oil.Pyrolysis experiments were conducted to distinguish the heat effects from oxidation effects on the oil.

1 Experimental

Table 1 Properties of the crude oil

The LTO reaction of the sample oil was conducted in the presence of sieved silicon sand.Approximately 132g sand and 25mL oil were used.When the reaction began，the temperature and pressure changes were recorded.The experiment was terminated when the temperature and pressure of tubular reactor remained constant.After being cooled to room temperature，the LTO residue was separated and analyzed.A total of 21experimental runs were conducted.The initial and operating conditions for these runs were listed in Table 2.The porosity of the packed sand was 44%for each run.

Table 2 Initial and operating conditions

The low－temperature oxidation tests were performed on the heavy oil obtained from Xinjiang Karamay oil field.The oil sample was pretreated to remove water and undesirable materials.The properties of the oil sample were shown in Table 1.The silicon sands used in high pressure reactor were sieved to 40－80 meshes.

The facility and experimental operation of pyrolysis experiments were the same as those of LTO experiments，besides that injected gas was pure N2.

A standard SARA analysis method was used for compositional analysis of oil samples before and after the LTO reaction.Details were described elsewhere［7］.Six parallel SARA separations were performed to reduce the relative error and all the solvents used in this step were analytical grade.

The LTO reactions of pure oil components（n－h(huán)exadecane，wax，phenanthrene and toluene）were also conducted.The proportion of each component was determined according to the results of the SARA analysis and GC／MS of the whole oil.For repeatability，experiments were performed three times.

Throughout the reaction，the effluent gas was analyzed at regular intervals by gas chromatography.This was performed on GC 112Aequipped with an Al2O3column and a 5Amolecular sieve column.The temperature of the column was maintained at 130℃，and the carrier gas was He with a flow rate of 30 mL／min.

2 Results and discussion

2.1 LTO process of heavy oil

Fig.1and Fig.2showed the temperature records of 150 ℃.Fig.1aand Fig.2aindicated that the oxidative reactions happened immediately as soon as air was injected into the reactor，causing the fluctuation of temperature and pressure.The temperature increased quickly in the first few minutes and then dropped to initial temperature and kept constant.The pressure of the reactor decreased slightly as time went.The results of pyrolysis experiments（Fig.1band Fig.2b）indicated that it was oxidative reaction instead of pyrolysis reaction that caused the temperature and pressure changes.The results of experiments at other temperatures showed the same trend.

Fig.1 The temperature records of the oxidative（a）and pyrolysis（b）reactor at 150℃

Fig.2 The pressure records of the oxidative（a）and pyrolysis（b）reactor at 150℃

2.2 Effects of temperature on LTO

2.2.1 Effects of temperature on the temperature increasing of the LTO process

The temperature changes（Δθmax）of the LTO were obtained.The values ofΔθmaxin different operation conditions were within the range（1－15℃）.The temperature changes were higher than those reported by previous work［8］.The results of LTO experiment at 5MPa were shown in Fig.3（a）.Two peaks appeared at 150℃and 250℃before the temperature reached 300℃.The results indicated that the reactions which happened in the LTO region were more violent at 150℃and 250℃，thus releasing more reaction heat that made the temperature rise.The results were in good agreement with the previously reported results［9］.The values ofΔθmaxrocketed and reached 15 ℃ at 400 ℃.This phenomenon indicated that the hightemperature oxidation（HTO）process might happen when the temperature reached 350℃and the violent HTO reactions accelerated the temperature rising.The values ofΔθmaxincreased as the pressure increased.The boundary of the temperature peaks between 150℃and 250℃became unclear as the pressure increased and finally incorporated as one（200℃）at 15MPa.The oxidation time associated with temperature shown in Fig.3（b）demonstrated two peaks at the same temperature ranges.The results revealed longer reaction time of the LTO reactions that happened at 150℃.

Fig.3 The temperature changes（Δθmax）and reaction time of the LTO in different operating conditions

2.2.2 Effects of temperature on the pressure increasing of the LTO

Fig.4 The pressure changes（ΔPmax）of the LTO under different operating conditions

The results of pressure changes （ΔPmax）of LTO were shown in Fig.4.Two peaks appeared at 150℃and 300℃respectively.The boundary of the two peaks became much clearer as the pressure increased.The first peak was due to the consumption of oxygen at lower temperature and the second peak which appeared at higher temperature might be resulted from gas products such as CO2and CO.These results were in good agreement with the information obtained from the gas products of the LTO.

2.2.3 Effects of temperature on the gas products of the LTO

Carbon oxides in the vent gases were used as an indication of the presence of LTO［10］.Fig.5（a）showed the relative amount of LTO gas products and O2.It is noticeable that O2consumption changed slightly in lower temperature region and increased when the temperature reached 250℃.CO2and CO were the main LTO products that were detected.The amount of CO kept low.The amount of CO2increased quickly after 250℃and continued to increase and reached the highest value at 400℃.The generation of CO2might be partly responsible for the decrease of O2content after 250℃.CH4was not detected until the temperature reached 250℃.The amount of CH4reached the highest value at 350℃and then decreased as the temperature increased which could be resulted from the further oxidation of CH4.Similar results were obtained from 10MPa and 15MPa LTO experiments（Fig.5（b）and Fig.5（c））.

2.2.4 Effects of temperature on the oxygen consumption of the LTO

Overall，air injection had not been widely accepted as a risk－free technique for either heavy－oil or lightoil reservoirs due to the complexity of the reaction in the process and the safety problems associated with the high level of oxygen which increased the risk of potential explosion［11］.So achieving O2complete consumption was important to the operation safety of air injection process and became the main concern.Air injection process could be considered as a conventional gas－injection process，so long as the oxygen could be removed efficiently through spontaneous low－temperature oxidation.Fig.6showed the results of oxygen consumption of LTO process.As the temperature rose，oxygen consumption decreased firstly and then increased quickly after 250℃.The highest oxygen consumption value was obtained at 400℃.The whole process consumed nearly 90%of O2.Therefore，LTO reactions were responsible for the most of the oxygen consumption.Operating conditions which were suitable for LTO reactions would benefit the operation safety of the air injection process.

Fig.5 The gas products of the LTO at 5MPa（a），10MPa（b），15MPa（c）

2.3 Pure oil components analysis

Fig.6 The oxygen consumption of the LTO under different operating conditions

There was a controversy over which oil components of the SARA（saturates，aromatics，resins，and asphaltenes）were susceptible to LTO reactions.Some researches concluded that saturates were more susceptible to LTO but others showed that the aromatics and resins were the most susceptible to the LTO process［12］.More recently，a study using compositional analysis and pure oil components investigated the oxidation activity of oil components，the results of LTO reactions at 70－150℃indicated that oil compounds with long hydrocarbons（wax or paraffin）were more readily subjected to oxidation［13］.In this work，pure oil components experiments were conducted to study oxidation effect on oil components.The pure oil components included the saturate（n－h(huán)exadecane，wax）and aromatic（phenanthrene and toluene）fraction but without reins and asphaltenes.The results summarized in Table 3were the even values of three experiments.It was noticeable that the LTO process of pure oil components created resins and asphaltenes.The results for the aromatic fraction showed significant reduction for both crude oil components and pure oil components.The increasing of temperature led to the decreases of aromatics and saturates and increments in resins and asphaltenes for pure oil components.The results of crude oil components analysis showed similar trend except for the saturate and resin fraction.The results revealed that the aromatics were more easily subjected to LTO.And more complicated components would be formed as LTO process progressed.

Table 3 Pure oil and crude oil components analysis before and after LTO experiments

3 Conclusions

In this research，LTO experiments of crude oil were conducted on Karamay heavy crude oil.The data can provide guidelines for air injection in specific oil reservoir and air injection process design.The following conclusions were derived from this work：

（1）Oxidation of crude oil happened immediately as soon as air was injected into the reactor.The temperature increased quickly in the first few minutes.The pressure decreased slightly as time went.The temperature and pressure changes were caused by oxidation reaction instead of pyrolysis reaction.

（2）Temperature and pressure changes had significant effects on the temperature changes（Δθmax），pressure changes（ΔPmax），gas products and oxygen consumption.As to the studied crude oil，the appropriate temperature and pressure for LTO process were 150℃ and 10MPa respectively.CO and CO2products had important effects on the pressure changes and oxygen consumption.

（3）Pure oil components analysis results indicated that aromatics were more easily subjected to oxidation.More complicated components were formed as LTO process progressed，which would benefit the fuel formation process.

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