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Low-Temperature Oxidation of Oil Components in an Air Injection Process for Improved Oil Recovery Baolun Niu,* Shaoran Ren, Yinhua Liu, Dezhi Wang, Lingzhi Tang, and Bailian Chen
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College of Petroleum Engineering, China University of Petroleum, Qingdao, Shandong 266555, People’s Republic of China ABSTRACT: Air injection is an effective technique for improved oil recovery in light oil reservoirs. It is speculated that the main mechanism of the process is via spontaneous low-temperature oxidation (LTO) to consume oxygen and generate “flue gas” that displaces oil out of the reservoir. In this study, laboratory experiments have been conducted to study the effects of oil composition and main reservoir parameters on the kinetics of LTO, in a range of reservoir temperatures from 70 to 150 °C. Saturates, aromatics, resins, and asphaltenes (SARA) analysis and experiments using pure oil components were preformed to study the oxidation activity of different oil compounds and components. Reaction rates of typical light and heavy oil samples were also measured for comparison. Effects of temperature, pressure, water saturation, sand type, and residence time on reaction rates and products were investigated under static and dynamic conditions. The results indicate that different oil components exhibit different reaction activity under the LTO conditions. Heavy oils can be more readily oxidized than light oils at low temperatures. The data shed more light on the mechanisms of LTO reactions and can provide guidelines for reservoir selection and air injection process design.
1. INTRODUCTION Air injection is an effective method for improved oil recovery (IOR), which could increase oil recovery over 6% of the original oil in place (OOIP) from selected oil reservoirs.1,2 Traditionally, air injection has been applied to heavy oil reservoirs via an in situ combustion process,3,4 and oil recovery is enhanced mainly because of steam flooding and viscosity reduction as a result of heat generated in the oxidation reaction at high temperatures. In light oil reservoirs, however, heat generation is not necessary for IOR and air injection can be considered as a conventional gasinjection process if oxygen can be removed through spontaneous low-temperature oxidation (LTO) at the reservoir temperature, in terms of safe operation (to prevent gas and oil explosion in production wells). The LTO reaction consumes oxygen in the injected air and produces flue gas (mainly contains N2, CO2, and light hydrocarbons) to displace oil, and the thermal effect generated from the reaction can be a bonus for oil recovery. A simplified LTO process of air injection can be shown in Figure 1. Air injection techniques have been applied in many oil reservoirs worldwide. Field tests have being conducted in a few light oil reservoirs in China in recent years, including the Zhongyuan oilfield, Baise oilfield, Shanshan oilfield.2,5,6 Results from the Zhongyuan oilfield have shown significant benefits of air and airfoam injection via the LTO process. Up to now, 3 4% oil production has been achieved by air injection in the pilot region during 3 years of air injection.7 Importantly, there is no significant oxygen breakthough observed in production wells during the test. Overall, air injection can be a high-risk operation process because of safety issues associated with the high level of oxygen and complexity of reaction phenomena.8 11 Particularly, the risk of potential explosion because of oxygen breakthrough in producers is the main concern, which is particularly important for air injection LTO operations. Thus, the oxygen consumption or oxidation rate in light oil reservoirs is one of the key factors for reservoir selection and evaluation, as well as for the process design. r 2011 American Chemical Society
Many factors can affect the LTO reactions, including the oxidation activity of various types of hydrocarbons and reservoir conditions, such as temperature, pressure, water saturation, and formation rocks. In previous studies of the LTO process,12 16 compositional analysis using the saturates, aromatics, resins, and asphaltenes (SARA) method has been tried to reveal the LTO reaction mechanisms, intending to find which oil components are oxidized during the reaction by conducting SARA analysis to the oil samples before and after the reaction. However, because the SARA analysis can only reveal the relative changes among the oil components, the method could not give unique information to identify which component can be more readily subjected to oxidation at low temperatures. For instance, complex aromatic components may contain short or long branches of saturates, which may become simple saturate compounds during the reaction. Subcomponents in resins and asphaltenes may also exchange during LTO, and it is difficult to identify them correctly after the reaction. The LTO reaction kinetics is also sensitive to various reaction conditions and is very reservoir-specific. In this study, both SARA analysis of oil samples before and after the LTO reaction and experiments using pure oil compounds and components, such as n-hexadecane, wax, anthracene, asphaltenes, and typical light and heavy oil samples, are conducted for studying the LTO reaction process in a small-batch reactor (SBR). Effects of the temperature, pressure, water saturation, and sands on the oxidation rate of light oil samples were also investigated. In addition, the oxygen consumption rate and gas composition of the reaction products were studied using a long oxidation tube under dynamic (air flooding) conditions to simulate a long residence time of air in the reservoir. Received: June 17, 2011 Revised: August 25, 2011 Published: August 25, 2011 4299
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Figure 1. Schematics of the air injection LTO process.
Figure 3. Schematic diagram of the SBR at high pressure and isothermal conditions.
Figure 2. Speculated reaction mechanism and paths of the hydrocarbon oxidation process.
2. LTO OF HYDROCARBONS The mechanisms of LTO and its possible reaction paths were speculated during a feasibility study of air injection IOR for light oil reservoirs in the North Sea oilfield.9 11 Experimental results have shown that the LTO reactions, at isothermal conditions of 100 120 °C, can generate up to 10% CO2 and some CO, while the reaction rates were very low (in days and weeks). A simplified reaction model of LTO is described in Figure 2. Hydrocarbon molecules can be partly oxidized, generating hydrocarbons branched with CH2 OH (alcoholic) and CHO (aldehyde) groups. CO2 generation is via route 2 for the CHO groups to be further oxidized into compounds containing carboxyl groups, and carboxyl groups are broken off from the main hydrocarbon chains to generate CO2. CO can be generated via route 1, in which some radicals of R CO• can be first generated and then CO can be released. CO is active and can be further oxidized to CO2. The results of bond energy calculation from simple reaction paths indicate that the oxidations are an exothermic process, which may increase the reservoir temperature in the reaction zones and have an accelerating effect on the reaction.17 3. EXPERIMENTAL SECTION 3.1. Materials. A light crude oil sample from the Zhongyuan oilfield in China is selected for experiments in this study. The viscosity of the oil is of 3.96 mPa s at reservoir conditions (87 °C and 22.3 MPa).
Its viscosity and density at standard surface conditions are 46.24 mPa s and 0.886 g/cm3, respectively. The heavy oil sample tested has a surface density of 1.02 g/cm3 and viscosity of 112 000 mPa s. Four oil components, namely, n-hexadecane (purity of 97%, Alfa Aesar, Ltd.), wax (paraffin, mainly n-dodecane and n-octadecane), anthracene (purity of 98%, Wok, Ltd.), and asphaltenes, were selected for the study. Crushed reservoir cores and washed quartz sands were used to make oil sand samples. Brine water with a salinity of 20 mg/L was reconstituted on the basis of the composition of the reservoir aquifer. 3.2. SARA Analysis Method. A standard SARA analysis method was adopted for compositional analysis of oil samples before and after the LTO reaction.18 The principle of the SARA analysis is to use different solvents to dissolve different components and precipitate them out (using filter paper and an alumina column) on a weight basis. In detail, n-heptane can be used to dissolve other components in crude oil, except asphaltenes; therefore, asphaltenes can be filtered out first after the oil sample is dissolved in a certain amount of n-heptane. After asphaltenes are filtered out, the n-heptane solution, containing saturates, aromatics, and resins, will pass through an alumina oxide column, in which they will be adopted on and along the column (similar to a chromatograph in principle). Then, saturates can be dissolved and washed out using n-pentane. Similarly, aromatics can be washed out using toluene, and finally, resins will be washed out using a mixture of ethanol and toluene. The important thing to note is that the compositional determination of each oil component is on a weight basis, which means if oxygen is added to any oil molecules, the weight of the relevant component will be increased. 3.3. Static Oxidation Experiment. A SBR (100 mL in volume) is used for the LTO reaction at static conditions (no stirring; see Figure 3), which has been described in detail in previous studies.9 11 In static experiments, 60 mL of oil or oil sand mixture was filled into the reactor, leaving the rest of the space for air to be then charged to the required pressure. During the experiment, pressure and temperature profiles were recorded and, normally, a reduction of pressure was observed, which is due to oxygen consumption. 3.4. Dynamic Oxidation Experiment. A dynamic oxidation experiment (see Figure 4) was carried out in a high-pressure oxidation tube, which was similar to that used for oil displacement studies.10,19 Two oxidation tubes with different sizes were used to investigate the effect of the residence time on oxygen consumption. One was a regular sandpacked tube, 0.7 m long and 25 mm inner diameter, and the other was a 4300
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Figure 4. Experimental setup of oil dynamic oxidation.
Table 1. Results of the SARA Analysis and Oxygen Content, Measured for Light Oil Samples before and after the LTO Reaction before reaction
after reaction
saturates (wt %)
68.69
66.61
aromatics (wt %)
17.06
11.64
resins (wt %)
13.87
17.32
0.38 0.89
4.43 1.77
asphaltenes (wt %) oxygen (wt %)
slim tube, 5 m long and 8 mm inner diameter, pre-packed using quartz sands with a porosity of 30% and a permeability of around 5 D.
4. RESULTS AND DISCUSSION 4.1. SARA Analysis. Static oxidation of 60 mL of the light oil sample was carried out using the SBR, at 20.75 MPa and 87 °C, for over 70 h. After the reaction, the compositions of the produced gas (in the reactor) was measured, in which the oxygen content was reduced to 16% (from originally 21%) and the CO2 content was increased to 1%. The results of the SARA analysis and oxygen content for the oil samples measured before and after the reaction are listed in Table 1. It is noticeable that the contents of saturates and aromatics were decreased after the reaction, while the fractions of resins and asphaltenes were raised. Similar results were observed in other studies.12,13,15 The decrease of saturates and aromatics could be partially due to the dilution effect of the increased components, such as oxygen. On the other hand, the increase of the oxygen content in the oil sample after oxidation clearly indicates that oxygen has been added into oil samples (if the effect of water was eliminated). It is possible that the weight fraction of resins and asphaltenes could increase when oxygen was reacted with them and added into their molecules. It is understood that the SARA fraction analysis can certainly reveal the changes in the oil composition, while it cannot identify which oil components are taking part in the reaction and their reactivity in the LTO reactions. Therefore, experiments using pure oil components can be a better way to determine the reactivity of individual compounds and components at different temperatures. 4.2. Experiments of Pure Oil Components. 4.2.1. Reaction Rate. The LTO reaction rates (oxygen consumption) for the four pure oil components (n-hexadecane, wax, anthracene, and asphaltenes) measured at elevated temperatures are shown in Figure 5. At a low temperature (i.e., 70 °C), the asphaltene
Figure 5. Effect of temperatures on the oxidation rate.
compound exhibited the highest reaction rate, meaning that asphaltene is more readily oxidized than the other components, but its reaction rate increased slightly up to 110 °C. It is interesting to note that the reaction rate of the wax compound (heavy saturates) increased rapidly over the temperature range of 70 100 °C. The relatively lower reaction rates of light saturate and aromatic compounds (n-hexadecane and anthracene) at low temperatures show that light oil components can be more resistible to oxidation than heavy wax and asphaltenes. It has been observed that, in a few repeated experiments, n-hexadecane exhibited a very high reaction rate around 120 °C, nearly consuming all of the oxygen in the reactor in a short period of time. The anthracene compound was relatively stable in the low-temperature range tested, but its oxidation started to increase significantly at higher temperatures over 120 °C. The experimental results obviously indicate that all of the oil components can take part in LTO reactions, while their reactivity at relatively low temperatures varies because of their different molecular structures. Anthracene consists of three aromatic rings, and it is more stable structurally compared to large alkane molecules with long and straight carbon chains. Asphaltene compounds have a complex molecular structure, which contains multiple large carbon chains and polycyclic/aromatic rings with higher aromaticity and polarity; therefore, it may speculate that some carbon atoms, such as those at the ends of carbon chains or on branched chains, can be easily attacked by oxygen or be oxidized. n-Hexadecane has a shorter carbon chain, which is relatively stable compared to wax compounds with longer and branched carbon chains. While at higher temperatures, both n-hexadecane and wax showed very a high reaction rate, indicating that, at a certain temperature, the carbon atoms on the main chains can no longer be resistible to oxygen attack with sufficiently high energy; therefore, the oxidation rate starts to increase rapidly. 4.2.2. Composition of Produced Gas. During the experiments, the reaction was completed if no further pressure reduction was observed or it was terminated because of operational reasons. To maintain the pressure in the reactor, the compositions of the produced gas were only measured after the reaction was completed over the temperature range tested. Table 2 shows the measured results of O2 and CO2 from the experiments using the four oil components. Please note that the measured O2 and CO2 contents vary significantly from different reactions with different oil components. These differences could be due to the difference in the end temperature and reaction rate of each experiment. That is, at higher 4301
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Table 2. Measured Gas Compositions after the Oxidation Reactions Using Different Oil Components in the SBR n-hexadecane
wax
reaction temperature (°C)
70 120
70 100
70 150
70 120
O2 content (vol %) CO2 content (vol %)
1.1 3.2
1.2 3.7
5.5 9.6
2.3 5.4
anthracene asphaltene
Table 3. Experimental Results of LTO Reactions at Different Pressures and Temperatures, with and without Crushed Cores O2 P
CO2
produced produced
(MPa) T (°C)
reaction rate [10, 1
5 mol of O2 h 1
gas (%)
gas (%)
(mL of oil) ]
notes
10
100
15.7
1.1
3.42
oil only
15
100
12.8
1.4
3.69
oil only
20 20
100 80
11.4 18.0
1.7 0.7
4.57 3.23
oil only oil only
20
90
16.1
1.0
3.83
oil only
20
110
3.5
3.8
9.02
oil only
20
100
11.9
4.6
5.81
oil + crushed
20
100
17.9
0.9
3.33
oil + sands,
20
90
16.1
1.0
3.83
oil + sands, Sw = 0.3
cores Sw = 0
Figure 6. Curve of the pressure of light and heavy oils under the same conditions in LTO.
temperatures, more CO2 could be produced. However, it is noticeable that compounds with similar chemical structures have similar gas-phase composition in terms of O2 and CO2 levels. The remaining O2 contents for the n-hexadecane and wax experiments are low (meaning most oxygen was consumed), while the produced CO2 was also low, indicating that oxygen was added to carbon chains in the compounds and less CO2 was released because of the weaker decarboxylation reaction applied at low temperatures. For the reactions of anthracene and asphaltene, more CO2 was produced, although less oxygen consumption was observed. This may be due to the higher temperatures applied that could promote more decarboxylation reactions to generate CO2. 4.3. Comparison of Light and Heavy Oils. The experimental results described above indicate that heavy oil components can be more easily oxidized than the light oil components at low temperature. Figure 6 shows the pressure drop curves (oxygen consumption) for heavy and light oil samples loaded in the reactor for the LTO reaction at 100 °C and 10.7 MPa. Noteworthy, the heavy oil sample exhibited a higher oxidation activity (higher oxygen consumption rate) than the light oil during the LTO process. 4.4. Influence Factors of LTO Comparison of Light and Heavy Oils. 4.4.1. Static Experiments. Effects of reservoir pressure, temperature, water saturation, and existence of sands on the oxidation rate and products were investigated using the SBR under static conditions, and the experimental results are summarized in Table 3. The experimental data show that the LTO reaction rate increases with the pressure and temperature. This is because oxidation can occur at the oxygen oil interface and in oil solution when oxygen is dissolved in oil. The increases of the oxygen partial pressure can increase oxygen dissolution in oil. The temperature is the main factor that can promote oxidation significantly, especially when the mixture is subjected to high temperatures. It is notable that there is a 3 time increase in the reaction rate for a temperature increase of only 30 °C. A modest increase in the oxidation rate is observed in the presence of crushed cores compared to the cases with quartz
Table 4. Experimental Conditions and Results for the LTO Reaction Using Different Tubes
experiment slim tube sand pack tube
oxidation
produced
tube
gas compositions
length diameter residence time (cm) (cm) (gas breakthrough, h) 500
0.8
164
70
2.5
6
O2 (%)
CO2 (%)
13
4.6
20.5
0.7
sands and in pure oil only (without sands). This can be attributed to a catalytic/surface effect. It has also been observed that the presence of water can increase the reaction rate. The reason for this is not clear; either water can promote contact between oxygen and oil, or it has a catalytic or water mediation effect, which needs further studies. 4.4.2. Experiments under Dynamic Conditions. Dynamic LTO reactions were conducted using the oxidation tube (filled with sand) facilities, as described above. The experimental condition and results are listed in Table 4. The residence time of air in the sand pack is defined as the time required for the air to pass through the oxidation tube, which is related to the air injection rate, the porosity, and the length of the sandpack. The dynamic or air flooding experiments were conducted at 60 °C and 16 MPa. The composition variations of the produced gas for both tests using the slim tube and shorter sandpack are shown in Figure 7. For the shorter sandpack tube, the oxygen content in the produced gas rapidly increased to 20.5% after gas breakthrough, meaning very little O2 has been consumed with air passing through the sandpack in a short time and at a low temperature and also very little CO2 was produced. However, for the slim tube experiment, significant oxygen has been consumed with air passing through the tube in a longer residence time and a lot of CO2 can be produced. The reaction rate remained stable over the experimental period up to 500 h. Figure 8 shows another experimental result using the slim tube under different temperatures. The experiment was conducted at 20 MPa, and the air injection rate was 0.005 mL/min. It can be observed that, at 60 °C, the oxygen content in the produced gas 4302
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Figure 7. Compositions of production gas during dynamic LTO experiments using different oxidation tubes at 60 °C and 16 MPa.
Figure 8. Composition of produced gas by the slim tube at different temperatures.
increased to 16% after gas breakthrough at the 97th hour and the CO2 content was stabilized at 1% up to the 220th hour. When the temperature was increased to 75 °C, the flowing oxygen content decreased gradually and stabilized at around 7% over 200 h and the CO2 level increased to near 3%. When the temperature was raised further to 90 100 °C, the oxygen content in the flue gas gradually reduced to less than 1% and up to 9% CO2 had been produced, indicating that a nearly complete oxidation and effective oxygen consumption can occur with air (or oxygen) passing the slim tube slowly at relatively higher temperatures. The experimental results at air-flooding conditions are in line with those obtained in the static experiments in terms of the reaction rate and gas products. These results have a significant impact on the reservoir operations. During the air injection process, if the residence time of air in oil formation is sufficiently long and/or the reservoir temperature is sufficiently high, a complete or nearly complete oxygen consumption can be achieved, which is required for preventing explosion in production wells. Therefore, a disadvantage of a low LTO reaction rate at low reservoir temperatures can be overcome if long distances between injection and production wells are taken into consideration. In addition,
the exothermic effect of the LTO reaction can accelerate the reaction rate to increase oxygen consumption and to improve safe operation, as well as to enhance oil production. Another concern of the air injection LTO process is possible oil contamination with oxygen (oxidized oil). This can be explained with the arguments below. During the air injection process, the oil produced, prior to gas breakthrough, is unreacted virgin oil. Behind the gas-flooding front (mainly N2 and CO2), oxygen will react mainly with the residual oil (oil that cannot be produced) and will be consumed; thus, the effect of the oxidation reaction on the produced oil would be minor. Moreover, for highly heterogeneous reservoirs with high permeability or highly permeable channels, air-foam injection can be applied to eliminate the problems of gas channeling and increase the sweeping efficiency of gas injection. The air-foam injection technique has shown good results in a field pilot project of the Zhongyuan oilfield, China.2
5. CONCLUSION LTO can occur spontaneously at typical reservoir temperatures after air is injected into light oil reservoirs during IOR 4303
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Energy & Fuels processes, in which oxygen is consumed and CO2 is produced as partial reaction products. In this study, to reveal the reactivity of different oil samples in terms of LTO reactions, different oil compounds and components and experimental methods were used and the following conclusions can be drawn: (1) The results of LTO reactions at 70 150 °C using four pure oil components indicate that oil compounds with long hydrocarbons (e.g., wax or paraffin and asphaltenes) are more readily subjected to oxidation at relatively low temperatures than light oil compounds. This was also verified by the experimental results using light and heavy oils in comparison, in which the reaction rate (in terms of oxygen consumption) of the heavy oil was obviously greater than that of the light oil. Light saturates and aromatics (i.e., n-hexadecane and anthracene) can be oxidized only at relatively higher temperatures. The higher the temperature, the greater the reaction rate and the more CO2 can be produced. (2) Sensitivity studies of LTO reactions show that temperature, pressure, and additions of water and reservoir sands can significantly increase the LTO reaction rate. The experimental results of air flooding using a slim tube show that oxygen can be completely consumed if the residence time of injected air in the reservoir is sufficiently long, although the reaction rate of LTO can be quite low. This may imply that, for highly heterogeneous rock formations, mobility control techniques can be employed to decrease air channeling and elongate the residence time to ensure effective oxygen consumption. (3) This study also adds clear insight to the speculated LTO mechanisms and reaction paths. Oxidation of hydrocarbon molecules is initiated via generating different radicals and/or oxygenic groups (i.e., hydroxyl, aldehyde, and carboxyl groups) on the basis of the hydrocarbon chains. Production of CO2 can be via a decarboxylation process, which is preferred to occur at relatively higher temperatures than the oxidation.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT The authors thank SINOPEC for the financial support of this research and allowing for this paper to be published. The assistance of Wei Wang, Fan Zhang, and Leibing Li in experimental work is also greatly appreciated.
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