Characterization and Kinetics of Light Crude Oil Combustion in the

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Energy & Fuels 2004, 18, 858-865

Characterization and Kinetics of Light Crude Oil Combustion in the Presence of Metallic Salts Mustafa Versan Kok* and Suat Bagci Department of Petroleum and Natural Gas Engineering, Middle East Technical University, 06531, Ankara, Turkey Received October 23, 2003. Revised Manuscript Received March 9, 2004

In this research, a reaction cell, thermogravimetry (TG), and differential thermal analysis (DTA) were used to characterize the light crude oil combustion and kinetics in the presence of copper(I) chloride (CuCl) and magnesium chloride (MgCl2‚6H2O). In TG-DTA experiments with magnesium chloride, three reaction regions were identified, known as distillation, low-temperature oxidation (LTO), and high-temperature oxidation (HTO). In the case of copper(I) chloride, two main transitional stages are observed with distillation and high-temperature oxidation (HTO). It was also observed that, as the mol % of magnesium chloride increased, the high-temperature oxidation peak shifted to the lower-temperature region reflecting more homogeneous composition of the solid residue. In the case of reaction cell experiments, it was observed that the molar CO2/CO ratio values of fuel combustion increased with the addition of metallic salts. A decrease in the atomic H/C ratio with an increase in temperature was observed in all experiments performed.

Introduction The application of an in-situ combustion process to light crude oil reservoirs is limited by the amount of fuel available for combustion. To artificially regulate the fuel formation and combustion reactions, metallic salts are usually considered as the catalyst because they are water soluble and cheap. Metallic ions would be distributed throughout the reservoir as a dilute salt solution. Metals have long been recognized in the chemical industry for their catalytic potential in both hydrocarbon oxidation and cracking reactions. Metals catalyze by their ability to transform the reactant molecules into a form in which they can more readily undergo reaction. Metals also accelerate oxidation indirectly by destroying the antioxidants that are naturally present in most crude oils.1,2 Metals have long been known to catalyze hydrocarbon oxidation and cracking reactions, and they have been extensively used as catalysts in chemical and petroleum refining industries. Gureyev and Sublina3 found that by adding metals such as copper, brass, iron, aluminum, tin, lead, or zinc, oxidation of the crude oil was promoted by destroying the antioxidants naturally present in the crude. Burger and Sahuquet4 studied the influence of 1.0% nickel oxide and 2000 ppm copper in the sand on the oxidation of a 27° API crude. They found that the activation energy of the low-temperature reaction was * Corresponding author. E-mail: [email protected]. (1) Donaldson, R. E.; Rice, T.; Murphy, J. R. Ind. Eng. Chem. 1961, 721. (2) Boreskov, G. K. Catalyst and Chemical Kinetics; Academic Press Inc.: New York, 1964. (3) Gureyev, A. A.; Sablena, Z. A. Scientific Research Institute of Fuel and Lubricating Materials; Pergamon Press: Elmsford, NY, 1965. (4) Burger, J. C.; Sahuquet, B. C. Soc. Pet. Eng. AIME 1972, 410422.

reduced with a corresponding increase in the amount of fuel deposited. This was evident from the occurrence of oxidation at a lower temperature with a high peak for the high-temperature reaction. Fassihi5 studied the oxidation of a 27° API crude and the effect of adding 2000 ppm copper to the sand. He found that the activation energy of the low-temperature reaction was unaffected, but higher reaction rates were observed in the low-temperature reaction as a result of an increased Arrhenius rate constant. Drici and Vossoughi6 applied DSC and TG to crude oil combustion in the presence and absence of metal oxides. Vanadium, nickel, and ferric oxides behaved similarly in enhancing the endothermic reactions. In the presence of a large surface area such as on silica, the surface reactions are predominant and unaffected by the small amount of metal oxide present. Kisler and Shallcross7 performed experiments to study the effects of various metallic salts on the oxidation kinetics of light crude oil. The experimental results showed that a plot of oxygen consumed versus temperature contained from two to five peaks, depending on the salt present. This range of behavior is substantially different from that observed for heavy oils. The crude oil oxidation reactions were classified into three broad groups depending on the ratio of carbon oxides produced to oxygen consumed. Sodium, copper, and iron enhanced the fuel combustion reactions, while lithium, magnesium, and cobalt reduced the amount of fuel available. Castanier et al.8 attempted to modify the (5) Fassihi, M. R.; Brigham, W. E.; Ramey, H. J., Jr. SPEJ 1984, 408-416. (6) Drici, O.; Vossoughi, S. SPE Reservoir Engineering 1987, 591595. (7) Kisler, J. P.; Shallcross, D. C. In Situ 1996, 20, 137-160. (8) Castanier, L. M.; Baena, C. J.; Holt, R. J.; Brigham, W. E. Proceeding of the Second Latin American Petroleum Engineering Conference, 1992, pp 199-204.

10.1021/ef0301755 CCC: $27.50 © 2004 American Chemical Society Published on Web 04/01/2004

Light Crude Oil Combustion and Metallic Salts

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Figure 1. TG-DTA curve of Beykan crude oil + limestone.

Figure 2. TG-DTA curve of Beykan crude oil + limestone + 1 mol % MgCl2‚6H2O.

Figure 3. TG-DTA curve of Beykan crude oil + limestone + 1 mol % CuCl.

fuel deposition reactions with water-soluble metallic additives. Modifications in the nature and in the amount of fuel formed were observed whenever the additive was introduced. Iron consistently increased the fuel deposition, but with light crude the additive allowed sustained

combustion under conditions where a run failed without any additive. Iron and tin increased the efficiency of the combustion, reducing the amount of oxygen produced as well as eliminating the fluctuations in gas compositions. Zinc does not seem as effective as iron and tin.

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Figure 4. Gas composition curve for Beykan + 1 mol % CuCl.

Figure 5. Gas composition curve for Beykan + 1 mol % MgCl2‚6H2O.

Experimental Section Samples. The crude oil used throughout the experiments was from Beykan-South-Eastern Turkey-region. The °API gravity of the crude oil is 31.5, and the dynamic viscosity is 11.7 cp. The premixing method has been used in preparing sand pack mixtures for the reaction cell experiments. Aqueous metallic solutions were prepared in concentrations of 1 and 2 mol % using distilled water. The metallic catalysts were magnesium chloride (MgCl2‚6H2O) and copper(I) chloride (CuCl). Aqueous solutions of two metallic salts with 1, 5, and 10 mol % were mixed with limestone and crude oil for TGDTA experiments, 1 and 2 mol % for reaction cell experiments. Mixtures of oil and solid particles were prepared in a small plastic weighing boat and mixed thoroughly. A 40-g quantity of crushed limestone, oil, and salt solution mixture was packed in a reaction kinetics cell. The limestone pack had a porosity of 38%, and oil and water saturations of 65% and 35%, respectively.

Equipment and Procedure. In the first part of the experiments, simultaneous TG-DTA experiments were carried out using a Netzsch thermal analysis system. TG has the capability of measuring the weight loss, whereas DTA has the capability of measuring the temperature difference either as a function of temperature or time in a varied but controlled atmosphere. Prior to the experiments, the TG system was calibrated with calcium oxalate monohydrate for temperature readings and silver was used in order to correct for buoyancy effects. A simultaneous TG-DTA experimental procedure involves placing sample (100 mg), setting the heating and gas (air) flow rate, then commencing the experiment. All experiments were performed at 10 °C/min. heating rate over the temperature range of 25-1200 °C and performed twice for repeatability. In the second part of the experiments, a reaction kinetic cell was used. The cell was equipped with a furnace, temperature programmer, air and nitrogen flow rate controller, digital



Figure 6. Oxygen composition curve for Beykan + 1-2 mol % CuCl.

Figure 7. Oxygen composition curve for Beykan + 1-2 mol % MgCl2‚6H2O. temperature indicator, pressure gauges, and continuous gas analysis equipment. A variable temperature program was applied to the reaction kinetics cell with an increase at a constant heating rate, to determine reaction rate data. During the experiments, initial temperature of the reaction cell was set around 20 °C, then the cell temperature was raised to 100 °C in 5 °C/min rate and retained for a half hour at 100 °C in order to reach thermal equilibrium in the reaction cell. Then the reaction cell temperature was increased at a constant rate of 1 °C/min after this point; throughout the heating of the cell, air was injected at a constant rate of 1.8 L/min. During the experiments, center temperatures in the cell and O2, CO2, CO gases were analyzed and recorded from a continuous gas analyzer. This recording procedure continued until no carbon

oxide gases were observed in the produced gas from the reaction kinetics cell. To check the reproducibility of the experimental data, two standard runs for the oils were carried out at the same experimental conditions and the results were identical.

Results and Discussion The generally accepted theory of in-situ combustion is that three competing oxidation reaction exists, known as low (LTO)-, medium (MTO)-, and high-temperature oxidation (HTO). These three classes involve quite different chemical reactions but occur across over-

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Figure 8. Atomic H/C ratio and molar CO2/CO curve for Beykan + 1 mol % CuCl.

Figure 9. Atomic H/C ratio and molar CO2/CO curve for Beykan + 1 mol % MgCl2‚6H2O.

lapping temperature ranges. Because of the chemical complexity of crude oil, lumped groups of reactions are considered rather than individual reactions. Lowtemperature oxidation (LTO) reactions involve the oxygenation of liquid-phase hydrocarbons. These are heterogeneous reactions between gas and liquid phases. The products of LTO are oxygenated hydrocarbons, which are more viscous, denser, and less volatile than the original crude oil. Significant amounts of oxygen are consumed by LTO reactions but little carbon oxide production occurs. As the crude oil is heated through intermediate temperatures, it undergoes distillation and pyrolysis. This leads to the deposition of a solid fuel on the reservoir grains and the production of light hydrocarbons in the gas phase. The oxidation of these light hydrocarbons is known as middle-temperature oxidation

(MTO). The thermal energy to drive the entire process is generated by the combustion of the solid fuel deposited by pyrolysis. These exothermic high-temperature oxidation reactions (HTO) are heterogeneous.9-11 In combustion with air, three main transitional stages are detected (Figures 1-3) in crude oil + limestone + magnesium chloride. These are the following: distillation between room temperature and about 300 °C, lowtemperature oxidation (LTO) between 300 °C and 420 °C ,and high-temperature oxidation (HTO) between 460 (9) De los Rios, C. F., Brigham, W. E.; Castanier, L. M. Report DOE/ BC/14126-4, U.S. Department of Energy: Washington, DC, 1988. (10) Cram, P. J.; Redford, D. A. J. Can. Pet. Technol. 1977, 16, 7277. (11) Mamora, D. D.; Ramey, H. J.; Brigham, W. E.; Castanier, L. M. Topical Report 91, Stanford University Petroleum Research Institute, 1993.



Figure 10. Relative reaction rate (RRR) vs 1/T curve for Beykan + 1 mol % CuCl.

Figure 11. Relative reaction rate (RRR) vs 1/T Curve for Beykan + 1 mol % MgCl2‚6H2O. Table 1. Reaction Intervals of the Samples (°C), TG/DTA sample Beykan + Lst Beykan + Lst + 1% MgCl2‚H2O Beykan + Lst + 5% MgCl2‚6H2O Beykan + Lst + 10% MgCl2‚6H2O Beykan + Lst + 1% CuCl Beykan + Lst + 5% CuCl Beykan + Lst + 10% CuCl

distillation low-temp. high-temp. region oxidation oxidation 25-300 25-300 25-300 25-300 25-300 25-300 25-300

300-420 300-410 300-410 300-410

420-605 410-580 400-575 410-570 300-535 300-455 300-450

°C and 620 °C, respectively (Table 1). These are the same stages identified in the literature.12,13 It was observed that, as the mol % of magnesium chloride

increased, the high temperature oxidation peak shifted to the lower temperature region and became smoother, reflecting a more homogeneous composition of the solid residue. In the case of crude oil + limestone + copper(I) chloride, two main transitional stages are observed as distillation and high-temperature oxidation (Table 1). It is believed that the hydrocarbons that were preferably undergoing cracking in the liquid phase are now being burned on the increasingly available solid surface. It was observed that both additives lowered the reaction intervals and peak temperatures of the high-tempera(12) Verkocy, J.; Kamal, N. J. Can. Pet. Technol. 1986, 47-51. (13) Kok, M. V. Thermochim. Acta 1993, 214, 315-324.

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Figure 12. Arrhenius Plot for Beykan crude oil with 2 mol % CuCl. Table 2. Peak Temperatures of the Samples (°C), TG/DTA sample Beykan + Lst Beykan + Lst + 1% MgCl2‚6H2O Beykan + Lst + 5% MgCl2‚6H2O Beykan + Lst + 10% MgCl2‚6H2O Beykan + Lst + 1% CuCl Beykan + Lst + 5% CuCl Beykan + Lst + 10% CuCl

peak temp. peak temp. (LTO region) (HTO region) 355 370 365 370

520 470 450 440 420 380 370

ture oxidation region. As the mol % of the additives increased, the peak temperature of the high-temperature zone decreased drastically (Table 2). It seems that the heat released during the high-temperature oxidation increases. This can be attributed to the increase in coke deposition. As expected, magnesium chloride and copper chloride caused a significant shift from a high temperature oxidation to a lower temperature region. There were also some differences among the TG-DTA curves of the two additives, however, which probably reflected their selectivity characteristics. In reaction kinetic cell experiments, two apparent peaks existed for consumed oxygen, carbon dioxide, and carbon monoxide at different temperatures and pressures for the runs with no additive. The first peak represents the low-temperature oxidation (LTO), while the second peak represents the fuel combustion or hightemperature oxidation (HTO). Between these two successive peaks, there is an interval, which shows the fuel deposition (FD) as the temperature is increased. During this reaction, the crude oil is coked and deposited on the solid matrix as fuel during an in-situ combustion process. At low temperatures, some oxygen is consumed to produce carbon oxides as carbon dioxide and carbon monoxide, which are lower than the oxygen consumed indicating some oxygen is consumed in other reactions. At high temperatures, nearly all of the oxygen is

consumed to produce carbon dioxide and carbon monoxide, which indicates complete combustion. These two peaks are observed in the standard runs, with no additives, at pressures of 25 psia and 50 psia. One oxidation peak is observed in crude oil runs with 2.0 mol % copper chloride additive. This indicates that the combustion reactions occur faster. In addition, the copper chloride additive shows one peak trend in all concentrations. The oxidation reactions in limestone medium occur more rapidly, so only one peak occurs with copper chloride additive (Figures 4, 5). On the other hand, in the case of copper chloride used with light oil at sandstone medium two large peaks are observed.7 Consumed Oxygen. Throughout the reaction cell experiments, the air injection rate was held constant. As a result of the reaction of oxygen from air with the crude oil, the rate of produced gases varied. Because of the change in gas composition and volume on reaction, it is necessary to convert the injection into the produced gas flow rate.

∆ γ ) [ γi(1 - CO2 - CO) - γo]/(1 - CO2 - CO) The above equation is used throughout the gas analysis to calculate the oxygen consumed. γi and γo are the oxygen concentrations in injected and producing gas, respectively. CO2 and CO are the concentrations in the produced gas as measured by continuous gas analyzer. In high-temperature oxidation, the amount of oxygen consumed is comparable with the amount of produced carbon oxides. The oxidation reaction peaks with metallic salts at both concentrations occur at lower temperatures compared to the standard runs. 2.0 mol % of additives lowered the peak temperatures more than the 1.0% concentrations because at 2.0 mol % of additive runs, reactions occurred faster. Copper chloride shows one peak with the 1.0 and 2.0 mol % experiments. 2.0 mol % of additives lowered the peak temperatures more



Table 3. Activation Energy Values (kJ/mol) sample Beykan + Lst Beykan + Lst + 1% MgCl2‚6H2O Beykan + Lst + 2% MgCl2‚6H2O Beykan + Lst + 1% CuCl Beykan + Lst + 2% CuCl

pressure, psi

activation energy

25 50 25 50 25 50 25 50 25 50

107.9 113.8 89.9 108.9 42.2 74.9 92.2 100.5 98.9 104.2

For the determination of activation energies from relative reaction data, experimental points were fit to a straight line and the points that show extensive departure were discarded. Then the straight line of the same slope was drawn on the Arrhenius plot as shown in Figure 12 for high-temperature oxidation region. Thus the true intercept at each pressure was computed using these straight lines. 1.0 mol % concentrations of copper chloride and magnesium chloride additives decreased the activation energy of Beykan crude oil (Table 3). Conclusions

than the 1.0% concentrations. The 1.0 mol % of magnesium chloride consumes more oxygen and the peak temperature is lower than 2.0 mol % (Figures 6, 7). Molar CO2/CO Ratio. To investigate the fuel combustion reactions, the molar CO2/CO ratios were calculated for each run. When oxygen in air reacted with crude oil, the carbon dioxide and carbon monoxide are produced as primary products. Furthermore, CO2 composition of the exit gases increased as the reaction kinetic cell temperatures increased (Figures 8, 9). Atomic H/C Ratio. The atomic H/C ratio of the reacting fuel was calculated for each combustion reaction. The atomic H/C ratio of the fuel consumed, (H/C), was calculated from the analysis of produced gases for each run as a function of temperature. These calculations were based on the assumption that all the oxygen not observed in the exit gas was reacted to form water. The differences in H/C indicate that the nature of the fuel formed during the control run is changed from the fuel during the runs with additive (Figures 8, 9). The nature of the fuel formed is dependent on the composition of each crude oil, and each crude oil was affected differently by different metallic additives.8 Modeling of Reactions. A kinetic model developed by Weijdema14 and adapted to reaction kinetics studies by Fassihi et al.5 was used for analysis of nonisothermal runs of this study. In this model, the temperature is linearly increased with time, and by proper graphing of the variables, a semilog straight line should result. Relative reaction rate was calculated and is graphed. At lower temperature (increasing values of 1/T), a departure from the straight line is observed. At high temperatures the amount of carbon oxides formed closely matches the amount of oxygen consumed, but at medium temperature the oxygen consumed is greater than the carbon oxides formed. At low temperatures, oxygen is consumed with no carbon oxides formation (Figures 10, 11).

In this research, simultaneous TG-DTA and reaction cell experiments were carried out and the following conclusions were derived. • In TG-DTA experiments, it was observed that, as the mol % of magnesium chloride increased, the hightemperature oxidation peak shifted to the lower temperature region and became smoother, reflecting a more homogeneous composition of the solid residue. • In TG-DTG experiments, it was also observed that both additives lowered the reaction intervals and peak temperatures of the high-temperature oxidation region. As the mol % of the additives increased, the peak temperature of the high-temperature zone decreased drastically. • CO2 composition of the exit gases increased as the reaction kinetics cell temperatures increased for each run. The molar CO2/CO ratio values of fuel combustion increased when additives were added. A decrease in the atomic H/C ratio with an increase in temperature was observed for all runs. • All the additives lower the peak temperature when they are compared to a standard run of crude oil. The trends of the curves are actually the same. Copper chloride shows one peak with the 1.0 and 2.0 mol % runs for both crude oils. • All the concentrations of metallic additives decreased the activation energy of Beykan crude oil, but 1.0 and 2.0 mol % have no significant difference. Only 2.0 mol % magnesium chloride run has a lower value than 1.0 mol %. • The differences of effects of additives on crude oils may depend on their compositions. However, the copper chloride additive shows only one peak for oxidation reaction and low activation energy with both crude oils in limestone medium.

(14) Weijdema, J. Report from Koninklijke/Shell, E&P Laboratorium, Rijswijk, 1968.

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Characterization and Kinetics of Light Crude Oil Combustion in the

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