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Phase behavior of the ternary system carbon dioxide + toluene + asphaltene
Fuel 137 (2014) 405–411
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Phase behavior of the ternary system carbon dioxide + toluene + asphaltene Samaneh Soroush a, Eugene J.M. Straver b, E. Susanne J. Rudolph a, Cor J. Peters c,d,⇑, Theo W. de Loos b, Pacelli L.J. Zitha a, M. Vafaie-Sefti e a
Delft University of Technology, Faculty of Civil Engineering and Geosciences, Delft, The Netherlands Delft University of Technology, Process & Energy Laboratory, Delft, The Netherlands Eindhoven University of Technology, Department of Chemical Engineering and Chemistry, Eindhoven, The Netherlands d The Petroleum Institute, Department of Chemical Engineering, Abu Dhabi, United Arab Emirates e Tarbiat Modares University, Department of Chemical Engineering, Tehran, Iran b c
a r t i c l e
i n f o
Article history: Received 4 August 2013 Received in revised form 3 May 2014 Accepted 14 May 2014 Available online 2 July 2014 Keywords: Asphaltene Carbon dioxide Toluene Phase behavior Precipitation
a b s t r a c t The effect of carbon dioxide on the phase behavior of a model oil and, in particular, on asphaltene precipitation has been studied in this work. A mixture of toluene and asphaltene with a constant asphaltene mass fraction of 0.0034 on a CO2-free basis was chosen as the model oil. The asphaltene was extracted from a crude oil. The phase transitions from liquid–vapor to liquid and from solid–liquid–vapor to liquid–vapor were determined in the temperature range from 295 K to 365 K and in a pressure range from atmospheric pressure to 7 MPa with the so-called synthetic method. Thereby, different amounts of carbon dioxide were added to the model oil. It was found that the temperature and the CO2 concentration affect the stability of the asphaltene containing model oil solution. If the CO2 concentration is higher than 10 mass%, precipitation of asphaltene occurs. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Carbon dioxide injection has been used successfully as an enhanced oil recovery (EOR) method for several decades [1]. Light to medium oil reservoirs have been developed by either miscible or near-miscible CO2 flooding [2]. The CO2 flooding method has gained popularity due to the growing concerns about global warming. The injection of CO2 in reservoirs containing asphaltenic oil may force the asphaltenes to precipitate. Asphaltenes represent the heavy fraction of crude oils which is soluble in low-molecularweight aromatics and insoluble in low-molecular-weight alkanes [3]. They usually contain heteroatoms such as N, O, and S, and metals such as V, Ni, and Fe [4]. Once asphaltenes precipitate, they tend to flocculate, and after reaching a critical size, they deposit [5]. This leads to clogging in the near-wellbore area (formation damage), the production tubing, or the surface production facilities [6–8]. A better understanding of asphaltene precipitation is crucial for the optimization of oil recovery by CO2 injection. According to Yen ⇑ Corresponding author at: The Petroleum Institute, Department of Chemical Engineering, Abu Dhabi, United Arab Emirates. Tel.: +971 26075492. E-mail addresses: [email protected], [email protected] (C.J. Peters). http://dx.doi.org/10.1016/j.fuel.2014.05.043 0016-2361/Ó 2014 Elsevier Ltd. All rights reserved.
and Chilingarian [9], CO2 dissolves in oil and decreases the asphaltene solubility in oil. Srivastava and Huang [2], used a high pressure–volume–temperature (PVT) cell to study asphaltene precipitation in presence of CO2. They found that the onset of asphaltene precipitation occurs at CO2 concentrations ranging from 40–46 mol%. Takahashi et al. [10], examined the asphaltene precipitation from a crude oil from a Middle East carbonate reservoir in presence of CO2 in a standard PVT cell. Using near-infrared light scattering to determine the onset pressure of asphaltene precipitation, they found that asphaltene precipitation starts when the CO2 content is above 50 mol%. Idem and Ibrahim [11], studied the kinetics of asphaltene precipitation induced by the mixing CO2 with three Saskatchewan crude oils varying the temperature (in the range of 300–338 K) and a constant pressure (17.2 MPa). They used a solid detection system (SDS) to investigate the onset of asphaltene precipitation. They found that the rate of asphaltene precipitation increases with asphaltene and CO2 content. Further, they showed that the rate of formation of the asphaltene obeys Arrhenius law. Verdier [12], conducted CO2 flooding with two different oils for their experiments. They found that the solubility of asphaltenes in CO2–crude oil mixtures increases with pressure and with decreasing temperature. Tavakkoli et al. [13], described asphaltene precipitation by a so-called micellization model. They
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reported experimental data for an Iranian heavy crude oil, for which major problems had occurred due to the asphaltene precipitation during CO2 injection. The amount of asphaltene precipitating increased with the CO2 amount injected. Several models have been proposed for describing the phase behavior of asphaltenic oils, including models based on Flory– Huggins theory, micellization models, or the models using advanced equations of state such as the statistical associating fluid theory (SAFT) [14–21,4,12]. None of the available models accounts satisfactorily for asphaltene precipitation due to the presence of carbon dioxide. The purpose of this study is to investigate experimentally the effect of CO2 on the phase behavior of a model asphaltenic oil, and more specifically the onset of the asphaltene precipitation. The model oil is a well-defined mixture of toluene and asphaltenes, extracted from a real crude oil. A similar model oil was previously used by Ting et al. [22] and Gonzalez et al. [23] in their studies of asphaltene precipitation in presence of CO2. The Cailletet apparatus (see De Loos et al. [24] for information on this equipment) was used to examine the phase transitions of samples having a constant overall composition, from vapor–liquid to liquid and from solid– liquid–vapor to solid–liquid at various temperatures and carbon dioxide contents. The phase transition from solid–liquid–vapor to liquid–vapor is of particular interest as it corresponds to asphaltene precipitation. The CO2 content in the toluene–asphaltene mixture was varied from 2.99 to 19.45 mass% to identify its effect on the phase behavior in the mixture. 2. Experimental section 2.1. Materials and methods Toluene and carbon dioxide with purities of 99.00 and 99.99 mol% respectively, were obtained from Merck and HoekLoos. The chemicals were used as received from the supplier. The asphaltenes used to conduct the experiments were extracted from a heavy crude oil according to a modified IP-143 procedure (Standard for Petroleum and its Products, 1985, [25]): 40 mL of n-pentane was added to 1 g of oil and mixed before the mixture was left to settle for at least 14 h. Thereby, asphaltenes which are insoluble in pentane were separated from maltenes which are soluble in pentane. The sample was then centrifuged and the supernatant liquid was decanted. The asphaltene residue was repetitively washed until no more maltenes dissolved in the n-pentane, as indicated by a colorless supernatant. The extracted asphaltenes were characterized by thermogravimetric analysis (TGA) [26], Electron Microprobe (EMP) and scanning electron microscopy (SEM); see for more details Appendix A. The TGA analysis was done in nitrogen to avoid combustion. The TGA results showed that a substantial weight loss had occurred in asphaltene when heating up to a temperature of around 470 °C. For lower temperatures, only a small amount of the volatile components are released from the asphaltenes. According to EMP analysis the asphaltenes contained carbon, sulfur, sodium, and oxygen in their structure. 2.2. Experimental setup The experiments were carried out in a Cailletet apparatus by the synthetic method [24], whereby the phase transitions for each sample in the Cailletet tube were observed visually. The temperature of the sample was kept constant by circulating a liquid through a glass thermostat jacket surrounding the Cailletet tube. For our experiment water was used as thermostat liquid. The
thermostat liquid was kept at the desired temperature by means of a thermostat bath within ±0.02 K in the temperature range between 295 K and 365 K. A platinum resistance thermometer, located near the sample-containing part of the Cailletet tube, recorded the temperature of the thermostat liquid with an accuracy of 0.01 K. Mercury was used as the sealing and pressure-transmitting fluid between the hydraulic oil and the sample. The pressure inside the tube was determined by a dead-weight pressure balance. The maximum pressure that can be applied in this apparatus is 14 MPa. The uncertainty of the pressure measurements was ±0.0025 MPa. Further details of the equipment and the procedures are described by De Loos et al. [24]. 2.3. Sample preparation First a toluene–asphaltene solution with 0.0034 mass fraction asphaltene was prepared. The amount of asphaltene dissolved in the toluene is less than its maximum solubility. However higher concentrations of asphaltene would result in a solution too dark for the visual observation of the phase transitions in the Cailletet facility. The toluene–asphaltene solution was used as a reference for all experiments. 2.4. Experimental procedure Before starting the experiments, the Cailletet tube was first weighed. A certain amount of the toluene–asphaltene solution was then injected into the Cailletet tube. In the following step, the tube containing the toluene–asphaltene solution was weighed again to determine the exact amount of added solution. The subsequent degassing of the liquid solution and the addition of a known amount of CO2 was done in the so-called gas rack (see Gauter [27], for a more detailed description). The concentration of carbon dioxide in the overall solution was varied from 2.99 to 19.45 mass%. In order to obtain the bubble point curves, the temperature was set to a desired value and the pressure in the tube was varied using a hand screw pump until the last gas bubble disappeared. A similar procedure was applied to obtain the solid–liquid–vapor (SLV) to liquid–vapor (LV) phase transition curves (L, V, and S denote liquid, vapor, and solid phase, respectively). For measuring the onset of asphaltene precipitation due to an increase in the amount of CO2 in the liquid phase, first the pressure is increased in small steps. If the solid phase does not appear within 30 min after each pressure increase step, the pressure is then increased again. Since the asphaltene precipitation is a time-dependent process and the thermodynamic equilibrium is not reached instantaneously, a delay occurs to reach equilibrium. This process continues until the solid phase is formed. Upon formation of the solid phase and in order to estimate the onset pressure as accurate as possible, the reverse procedure is practiced. That is, the pressure is decreased in small steps each with intervals of 30 min until the solid phase disappears. The corresponding pressure is then recorded. The above procedure is repeated several times with refined pressure increase/ reduction steps such that the onset pressure is measured with enough accuracy. In all measurements, the sample was stirred inside the tube by a stainless steel ball in order to ensure a proper contact between the phases. The lower onset of asphaltene precipitation is rarely reported in literature due to practical difficulties. In fact, measuring the lower onset of asphaltene precipitation is hardly possible with near infrared techniques. In this work the lower onset of asphaltene precipitation has been measured. The obtained data can be used to evaluate the effect of CO2 injection in asphaltenic oil reservoirs where very high pressure levels are not present.
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phase which in turn destabilizes the dissolved asphaltene and induces its precipitation.
3. Results and discussion 3.1. Pressure–temperature diagram
3.2. Pressure–composition diagram The bubble point curves, i.e. the LV to L and SLV to SL phase transition lines, for the samples with different overall compositions are depicted in Fig. 1. As to be expected, for all asphaltene concentrations the bubble-point pressure increases either with increasing temperature or with increasing amount of CO2 in the sample. Even though the effect of CO2 is not as significant as that of the temperature, above 10 mass% carbon dioxide causes precipitation of the asphaltene. The determined phase transition points between the solid–liquid–vapor phase and the liquid–vapor phase – which can be regarded as the onset of asphaltene precipitation – are presented in Fig. 2. At high pressures more CO2 dissolves in the liquid 10 2.99%, LV>L
9
From the pressure–temperature diagrams of the samples with different overall composition, pressure–composition diagrams at 310.1 K and 333.15 K were constructed (see Figs. 3 and 4). Fig. 3 shows the pressure–composition diagram at a temperature of 310.1 K. In this figure, the phase transitions between LV to L, SLV to SL, and SL to L are presented. Unfortunately, the phase transition from solid–liquid to liquid could not be determined experimentally. Therefore, the latter transition was added as a dashed line in Fig. 3, assuming that these phase transitions are pressure independent. The data obtained from this work were compared to literature bubble-point curves for the asphaltene-free toluene–CO2 system at 310.1 K [28] and at 333.15 K [29]. Even though the differences
4.98%, LV>L 7.51%, LV>L
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10.01%, LV>L
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P (MPa)
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5 4 3 2 1 0 290
300
310
320
330
340
350
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T (K) Fig. 1. P–T diagram displaying the LV M L and SLV M SL phase transition lines for the CO2 + toluene + asphaltene system. The curves are determined for samples with different overall composition and with variation of the CO2 mass%. The massfraction of asphaltenes in the CO2-free toluene–asphaltene solutions was 3.4 10 3. The lines are drawn to guide the eye.
Fig. 3. P–mass% CO2 diagram of the CO2 + toluene + asphaltene – system, constructed from the P–T diagrams obtained from the Cailletet experiments at 310.1 K. The CO2-free solution contains 0.0034 mass fraction of asphaltene in toluene. The diamonds display the bubble points, whereas the stars show the onset of asphaltene precipitation. The triangles show the transition from SLV M SL. The solid lines are drawn to guide the eye. The dashed line displays the expected SL M L phase transition curve.
6
310.1 K, LV>L
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15.00% CO2
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P (MPa)
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Chang et al,1995, 310.1 K
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Tochigi et al, 1998, 333.15 K
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310.1 K, SLV>SL
3 1
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2 0 0
1 0
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Mass% of CO2 270
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T (K) Fig. 2. P–T diagram displaying the SLV M LV phase transitions for the carbon dioxide + toluene + asphaltene system. The abbreviations L, V, and S are used for the liquid, vapor, and solid phase, respectively. The curves are determined for samples with different overall composition and with variation of the CO2 mass%. The CO2free toluene–asphaltene solution contains 0.0034 mass fractions of asphaltene. The lines are merely drawn to guide the eye.
Fig. 4. P–CO2 mass% diagram of the CO2 + toluene + asphaltene system constructed from the P–T diagrams obtained from the Cailletet experiments. The CO2-free solution contains 0.0034 mass fraction of asphaltene in toluene. This figure displays the bubble points and SLV M SL phase transitions for two different temperatures. The experimental data of this work are displayed by the solid triangles, the solid diamonds, the stars and the solid bullets; the first two give the phase transitions from LV M L at 310 K and 330 K, the latter two the phase transitions from SLV M SL at 310 K and 330 K obtained in this work. The empty triangles and the empty diamonds display literature data of the asphaltene-free CO2 + toluene system of Tochigi et al. [29] at 333 K and Chang et al. [28] at 310 K.
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also could be assigned to differences in the asphaltenes as these structures are ill-defined and multi-disperse. 4. Conclusions
Fig. A.1. Weight loss as a function of temperature from TGA of asphaltene under nitrogen atmosphere. The heating rate is 10 K/min. The inset of this figure shows the variation of mass% versus temperature in the range of 33–100 (mass%) with a higher resolution.
are small it can be concluded that the bubble point curve is affected by the presence of asphaltene. Of course, these differences
In this study the effect of CO2 on asphaltene precipitation from a model oil was explored. For that purpose, a mixture of toluene and asphaltene with fixed overall composition was used as a model oil. A comparison of the data from this study to literature data of an asphaltene-free toluene–CO2 system revealed that the bubblepoint curve was not that much affected by the presence of asphaltene. Additionally, the presence of CO2 affects the solubility of asphaltene in the oil considerably. Asphaltene precipitation was observed at high CO2 concentrations (above 10 mass%) whereas at low CO2 concentrations asphaltenes stayed in solution even in the investigated pressure range between atmospheric pressure and 7 MPa. This observation confirms once more that, in general, at low concentrations CO2 acts as a co-solvent and at elevated concentrations it switches into anti-solvency behavior [30,31]. The composition of the phases could not be measured due to the fact that the sample could not be brought out of the Cailletet setup for a composition measurement. However, it can be concluded from the experimental observations that upon reduction of pressure, a part of the dissolved CO2 migrates to the vapor phase which in turn results in lower CO2 content in the liquid phase. The asphaltene precipitation process is then reversed resulting in disappearance of the solid phase. The obtained experimental data confirm that high CO2 concentrations should be avoided during miscible CO2 flooding to, on the one hand, minimize the chance of asphaltene precipitation and, on the other hand, to optimize the oil recovery. To identify and to understand the underlying mechanism responsible for the asphaltene precipitation at high CO2 concentrations, further experimental phase behavior studies have to be conducted with a well-defined system using a model asphaltene of known composition and chemical structure.
Fig. A.2. SEM images of asphaltene particles before TGA.
Fig. A.3. SEM images of asphaltene particles after TGA under nitrogen.
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Fig. A.4. EMP results of asphaltene particles.
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Acknowledgements The authors gratefully acknowledge Dr. Karl-Heinz A.A. Wolf for the valuable discussions on the asphaltene characterization and Dr. Bianca Breure for valuable comments. Dr. Jack H.L. Voncken and Ing. Michel M. van den Brink are also greatly acknowledged for their efforts in the EMP and SEM analyses. Appendix A. Thermogravimetric analysis Thermogravimetric analysis (TGA) was used to characterize the asphaltene and determine the volatile components in its structure. For the analysis, a DSC-TGA instrument (SDT Q600, manufactured by TA Instruments) was used. It is capable of performing both differential scanning calorimetry (DSC) and TGA analysis at the same time. The asphaltene analysis was done under nitrogen atmosphere with a heating rate of 10 K/min. During TGA in nitrogen atmosphere the mass loss due to evaporation of components in the sample is determined. The mass loss percentage as a function of temperature for the asphaltenes used in this study is shown in Fig. A.1. Up to temperatures of around 500 K, there is hardly any change in the mass. Between 500 K and around 700 K, 10% of the total weight is lost. In this temperature range most likely smaller poly-aromatic hydrocarbons such as naphthalene (C10H8, consisting of two benzene rings), phenanthrene (684 K, C14H10, consisting of three benzene rings) or pyrene (677 K, C16H10, consisting of four benzene rings) evaporate. In the temperature range between 700 K and 723 K 50% of the mass is lost. At temperatures above 723 K the residue started to foam and continued up to 923 K. The negative values of weight loss and the strong scattering of the weight loss data in this temperature range indicate that material is released spontaneously, comparable to combustion. The fact that the weight loss observed at 720 K and the weight loss determined at around 950 K differs only slightly indicates that low weight gases were captured in the asphaltene structure. Due to the evaporation of some of the poly-aromatic hydrocarbons forming the asphaltenes, the asphaltene structure is weakened allowing the gases to escape. This is accompanied by strong fluctuating weights but small absolute weight losses. This behavior might indicate that also oxygen had been incorporated into the asphaltene structure which combusts when released at these high temperatures. A.1. Scanning electron microscopy The asphaltene samples were also studied by scanning electron microscopy (SEM) before and after TGA. Fig. A.2 shows the asphaltene structure before TGA. It can be seen that the asphaltene is porous. This might have been caused by the washing procedure during the asphaltene separation from the crude oil [32]. Fig. A.3 shows the image of the asphaltene particle after TGA. The image is different than the image of the particle before TGA. The small holes seen in the image of the asphaltene before TGA are probably from degasification during the separation. The open structure of the asphaltene sample after TGA are attributed to the plastic state and deformed flows in the particle due to the second spontaneous degasification [33]. This image supports the hypothesis that in the temperature range between 723 K and 923 K captured gas is released spontaneously. This spontaneous release could also include combustion of hydrocarbons with oxygen initially captured in the asphaltene. A.2. Electron microprobe An electron microprobe (EMP) is an analytical tool used to determine the chemical composition of small volumes of solid materials
in a non-destructive manner. The study of elemental analysis of the asphaltenes was performed with an EMP (JEOL 8800 M JXA Superprobe-1993) equipped with 3 WDS-spectrometers, LDE1, TAP, PET, and LIF analytical crystals. Samples were placed on a cuprum holder and analysis was performed at 20 kV of acceleration voltage. During the analysis, EMP was focused close to the edge of the asphaltene where the sample was thinner. Three different representative analyses were carried out on each sample to obtain reliable results. The analysis of asphaltenes by EMP shows that the asphaltenes used in this study contain carbon, sulfur, sodium, and oxygen; see Fig. A.4. There is some doubt if the asphaltenes contain more oxygen or sodium. In the top graph of Fig. A.4 it is given that sodium and oxygen are found at the same frequencies. However, combining the results of the TGA and the EMP, it can be concluded that the amount of oxygen is higher than of sodium so that the peak in the top graph can be assigned to oxygen. References [1] Mathiassen OM. CO2 as injection gas for enhanced oil recovery and estimation of the potential on the Norwegian continental shelf. Norwegian University of Science and Technology, Norway, MSc, 2003. [2] Srivastava KR, Huang SS. Asphaltene deposition during CO2 flooding: a laboratory assessment. In: SPE #37468, Proceedings of production operation symposium, Oklahoma City, Oklahoma, 9–11 March 1997. [3] Mansoori GA. Modeling of asphaltene and other heavy organic depositions. J Pet Sci Eng 1997;17:101–11. [4] Qin X, Wang P, Sepehrnoori K, Pope GA. Modeling asphaltene precipitation in reservoir simulation reservoir simulation is an area of reservoir engineering in which computer models are used to predict the flow of fluids (typically, oil, water, and gas) through porous media. 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Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Phase behavior of the ternary system carbon dioxide + toluene + asphaltene Samaneh Soroush a, Eugene J.M. Straver b, E. Susanne J. Rudolph a, Cor J. Peters c,d,⇑, Theo W. de Loos b, Pacelli L.J. Zitha a, M. Vafaie-Sefti e a
Delft University of Technology, Faculty of Civil Engineering and Geosciences, Delft, The Netherlands Delft University of Technology, Process & Energy Laboratory, Delft, The Netherlands Eindhoven University of Technology, Department of Chemical Engineering and Chemistry, Eindhoven, The Netherlands d The Petroleum Institute, Department of Chemical Engineering, Abu Dhabi, United Arab Emirates e Tarbiat Modares University, Department of Chemical Engineering, Tehran, Iran b c
a r t i c l e
i n f o
Article history: Received 4 August 2013 Received in revised form 3 May 2014 Accepted 14 May 2014 Available online 2 July 2014 Keywords: Asphaltene Carbon dioxide Toluene Phase behavior Precipitation
a b s t r a c t The effect of carbon dioxide on the phase behavior of a model oil and, in particular, on asphaltene precipitation has been studied in this work. A mixture of toluene and asphaltene with a constant asphaltene mass fraction of 0.0034 on a CO2-free basis was chosen as the model oil. The asphaltene was extracted from a crude oil. The phase transitions from liquid–vapor to liquid and from solid–liquid–vapor to liquid–vapor were determined in the temperature range from 295 K to 365 K and in a pressure range from atmospheric pressure to 7 MPa with the so-called synthetic method. Thereby, different amounts of carbon dioxide were added to the model oil. It was found that the temperature and the CO2 concentration affect the stability of the asphaltene containing model oil solution. If the CO2 concentration is higher than 10 mass%, precipitation of asphaltene occurs. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Carbon dioxide injection has been used successfully as an enhanced oil recovery (EOR) method for several decades [1]. Light to medium oil reservoirs have been developed by either miscible or near-miscible CO2 flooding [2]. The CO2 flooding method has gained popularity due to the growing concerns about global warming. The injection of CO2 in reservoirs containing asphaltenic oil may force the asphaltenes to precipitate. Asphaltenes represent the heavy fraction of crude oils which is soluble in low-molecularweight aromatics and insoluble in low-molecular-weight alkanes [3]. They usually contain heteroatoms such as N, O, and S, and metals such as V, Ni, and Fe [4]. Once asphaltenes precipitate, they tend to flocculate, and after reaching a critical size, they deposit [5]. This leads to clogging in the near-wellbore area (formation damage), the production tubing, or the surface production facilities [6–8]. A better understanding of asphaltene precipitation is crucial for the optimization of oil recovery by CO2 injection. According to Yen ⇑ Corresponding author at: The Petroleum Institute, Department of Chemical Engineering, Abu Dhabi, United Arab Emirates. Tel.: +971 26075492. E-mail addresses: [email protected], [email protected] (C.J. Peters). http://dx.doi.org/10.1016/j.fuel.2014.05.043 0016-2361/Ó 2014 Elsevier Ltd. All rights reserved.
and Chilingarian [9], CO2 dissolves in oil and decreases the asphaltene solubility in oil. Srivastava and Huang [2], used a high pressure–volume–temperature (PVT) cell to study asphaltene precipitation in presence of CO2. They found that the onset of asphaltene precipitation occurs at CO2 concentrations ranging from 40–46 mol%. Takahashi et al. [10], examined the asphaltene precipitation from a crude oil from a Middle East carbonate reservoir in presence of CO2 in a standard PVT cell. Using near-infrared light scattering to determine the onset pressure of asphaltene precipitation, they found that asphaltene precipitation starts when the CO2 content is above 50 mol%. Idem and Ibrahim [11], studied the kinetics of asphaltene precipitation induced by the mixing CO2 with three Saskatchewan crude oils varying the temperature (in the range of 300–338 K) and a constant pressure (17.2 MPa). They used a solid detection system (SDS) to investigate the onset of asphaltene precipitation. They found that the rate of asphaltene precipitation increases with asphaltene and CO2 content. Further, they showed that the rate of formation of the asphaltene obeys Arrhenius law. Verdier [12], conducted CO2 flooding with two different oils for their experiments. They found that the solubility of asphaltenes in CO2–crude oil mixtures increases with pressure and with decreasing temperature. Tavakkoli et al. [13], described asphaltene precipitation by a so-called micellization model. They
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reported experimental data for an Iranian heavy crude oil, for which major problems had occurred due to the asphaltene precipitation during CO2 injection. The amount of asphaltene precipitating increased with the CO2 amount injected. Several models have been proposed for describing the phase behavior of asphaltenic oils, including models based on Flory– Huggins theory, micellization models, or the models using advanced equations of state such as the statistical associating fluid theory (SAFT) [14–21,4,12]. None of the available models accounts satisfactorily for asphaltene precipitation due to the presence of carbon dioxide. The purpose of this study is to investigate experimentally the effect of CO2 on the phase behavior of a model asphaltenic oil, and more specifically the onset of the asphaltene precipitation. The model oil is a well-defined mixture of toluene and asphaltenes, extracted from a real crude oil. A similar model oil was previously used by Ting et al. [22] and Gonzalez et al. [23] in their studies of asphaltene precipitation in presence of CO2. The Cailletet apparatus (see De Loos et al. [24] for information on this equipment) was used to examine the phase transitions of samples having a constant overall composition, from vapor–liquid to liquid and from solid– liquid–vapor to solid–liquid at various temperatures and carbon dioxide contents. The phase transition from solid–liquid–vapor to liquid–vapor is of particular interest as it corresponds to asphaltene precipitation. The CO2 content in the toluene–asphaltene mixture was varied from 2.99 to 19.45 mass% to identify its effect on the phase behavior in the mixture. 2. Experimental section 2.1. Materials and methods Toluene and carbon dioxide with purities of 99.00 and 99.99 mol% respectively, were obtained from Merck and HoekLoos. The chemicals were used as received from the supplier. The asphaltenes used to conduct the experiments were extracted from a heavy crude oil according to a modified IP-143 procedure (Standard for Petroleum and its Products, 1985, [25]): 40 mL of n-pentane was added to 1 g of oil and mixed before the mixture was left to settle for at least 14 h. Thereby, asphaltenes which are insoluble in pentane were separated from maltenes which are soluble in pentane. The sample was then centrifuged and the supernatant liquid was decanted. The asphaltene residue was repetitively washed until no more maltenes dissolved in the n-pentane, as indicated by a colorless supernatant. The extracted asphaltenes were characterized by thermogravimetric analysis (TGA) [26], Electron Microprobe (EMP) and scanning electron microscopy (SEM); see for more details Appendix A. The TGA analysis was done in nitrogen to avoid combustion. The TGA results showed that a substantial weight loss had occurred in asphaltene when heating up to a temperature of around 470 °C. For lower temperatures, only a small amount of the volatile components are released from the asphaltenes. According to EMP analysis the asphaltenes contained carbon, sulfur, sodium, and oxygen in their structure. 2.2. Experimental setup The experiments were carried out in a Cailletet apparatus by the synthetic method [24], whereby the phase transitions for each sample in the Cailletet tube were observed visually. The temperature of the sample was kept constant by circulating a liquid through a glass thermostat jacket surrounding the Cailletet tube. For our experiment water was used as thermostat liquid. The
thermostat liquid was kept at the desired temperature by means of a thermostat bath within ±0.02 K in the temperature range between 295 K and 365 K. A platinum resistance thermometer, located near the sample-containing part of the Cailletet tube, recorded the temperature of the thermostat liquid with an accuracy of 0.01 K. Mercury was used as the sealing and pressure-transmitting fluid between the hydraulic oil and the sample. The pressure inside the tube was determined by a dead-weight pressure balance. The maximum pressure that can be applied in this apparatus is 14 MPa. The uncertainty of the pressure measurements was ±0.0025 MPa. Further details of the equipment and the procedures are described by De Loos et al. [24]. 2.3. Sample preparation First a toluene–asphaltene solution with 0.0034 mass fraction asphaltene was prepared. The amount of asphaltene dissolved in the toluene is less than its maximum solubility. However higher concentrations of asphaltene would result in a solution too dark for the visual observation of the phase transitions in the Cailletet facility. The toluene–asphaltene solution was used as a reference for all experiments. 2.4. Experimental procedure Before starting the experiments, the Cailletet tube was first weighed. A certain amount of the toluene–asphaltene solution was then injected into the Cailletet tube. In the following step, the tube containing the toluene–asphaltene solution was weighed again to determine the exact amount of added solution. The subsequent degassing of the liquid solution and the addition of a known amount of CO2 was done in the so-called gas rack (see Gauter [27], for a more detailed description). The concentration of carbon dioxide in the overall solution was varied from 2.99 to 19.45 mass%. In order to obtain the bubble point curves, the temperature was set to a desired value and the pressure in the tube was varied using a hand screw pump until the last gas bubble disappeared. A similar procedure was applied to obtain the solid–liquid–vapor (SLV) to liquid–vapor (LV) phase transition curves (L, V, and S denote liquid, vapor, and solid phase, respectively). For measuring the onset of asphaltene precipitation due to an increase in the amount of CO2 in the liquid phase, first the pressure is increased in small steps. If the solid phase does not appear within 30 min after each pressure increase step, the pressure is then increased again. Since the asphaltene precipitation is a time-dependent process and the thermodynamic equilibrium is not reached instantaneously, a delay occurs to reach equilibrium. This process continues until the solid phase is formed. Upon formation of the solid phase and in order to estimate the onset pressure as accurate as possible, the reverse procedure is practiced. That is, the pressure is decreased in small steps each with intervals of 30 min until the solid phase disappears. The corresponding pressure is then recorded. The above procedure is repeated several times with refined pressure increase/ reduction steps such that the onset pressure is measured with enough accuracy. In all measurements, the sample was stirred inside the tube by a stainless steel ball in order to ensure a proper contact between the phases. The lower onset of asphaltene precipitation is rarely reported in literature due to practical difficulties. In fact, measuring the lower onset of asphaltene precipitation is hardly possible with near infrared techniques. In this work the lower onset of asphaltene precipitation has been measured. The obtained data can be used to evaluate the effect of CO2 injection in asphaltenic oil reservoirs where very high pressure levels are not present.
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phase which in turn destabilizes the dissolved asphaltene and induces its precipitation.
3. Results and discussion 3.1. Pressure–temperature diagram
3.2. Pressure–composition diagram The bubble point curves, i.e. the LV to L and SLV to SL phase transition lines, for the samples with different overall compositions are depicted in Fig. 1. As to be expected, for all asphaltene concentrations the bubble-point pressure increases either with increasing temperature or with increasing amount of CO2 in the sample. Even though the effect of CO2 is not as significant as that of the temperature, above 10 mass% carbon dioxide causes precipitation of the asphaltene. The determined phase transition points between the solid–liquid–vapor phase and the liquid–vapor phase – which can be regarded as the onset of asphaltene precipitation – are presented in Fig. 2. At high pressures more CO2 dissolves in the liquid 10 2.99%, LV>L
9
From the pressure–temperature diagrams of the samples with different overall composition, pressure–composition diagrams at 310.1 K and 333.15 K were constructed (see Figs. 3 and 4). Fig. 3 shows the pressure–composition diagram at a temperature of 310.1 K. In this figure, the phase transitions between LV to L, SLV to SL, and SL to L are presented. Unfortunately, the phase transition from solid–liquid to liquid could not be determined experimentally. Therefore, the latter transition was added as a dashed line in Fig. 3, assuming that these phase transitions are pressure independent. The data obtained from this work were compared to literature bubble-point curves for the asphaltene-free toluene–CO2 system at 310.1 K [28] and at 333.15 K [29]. Even though the differences
4.98%, LV>L 7.51%, LV>L
8
10.01%, LV>L
7
15.00%, SLV>SL 17.50%, SLV>SL
P (MPa)
6
19.45%, SLV>SL
5 4 3 2 1 0 290
300
310
320
330
340
350
360
370
T (K) Fig. 1. P–T diagram displaying the LV M L and SLV M SL phase transition lines for the CO2 + toluene + asphaltene system. The curves are determined for samples with different overall composition and with variation of the CO2 mass%. The massfraction of asphaltenes in the CO2-free toluene–asphaltene solutions was 3.4 10 3. The lines are drawn to guide the eye.
Fig. 3. P–mass% CO2 diagram of the CO2 + toluene + asphaltene – system, constructed from the P–T diagrams obtained from the Cailletet experiments at 310.1 K. The CO2-free solution contains 0.0034 mass fraction of asphaltene in toluene. The diamonds display the bubble points, whereas the stars show the onset of asphaltene precipitation. The triangles show the transition from SLV M SL. The solid lines are drawn to guide the eye. The dashed line displays the expected SL M L phase transition curve.
6
310.1 K, LV>L
7
15.00% CO2
5
17.50% CO2
4
333.15 K, LV>L
6
P (MPa)
P (MPa)
5
19.45% CO2
4
Chang et al,1995, 310.1 K
3
Tochigi et al, 1998, 333.15 K
2
310.1 K, SLV>SL
3 1
333.15 K, SLV>SL
2 0 0
1 0
5
10
15
20
25
Mass% of CO2 270
290
310
330
350
370
390
T (K) Fig. 2. P–T diagram displaying the SLV M LV phase transitions for the carbon dioxide + toluene + asphaltene system. The abbreviations L, V, and S are used for the liquid, vapor, and solid phase, respectively. The curves are determined for samples with different overall composition and with variation of the CO2 mass%. The CO2free toluene–asphaltene solution contains 0.0034 mass fractions of asphaltene. The lines are merely drawn to guide the eye.
Fig. 4. P–CO2 mass% diagram of the CO2 + toluene + asphaltene system constructed from the P–T diagrams obtained from the Cailletet experiments. The CO2-free solution contains 0.0034 mass fraction of asphaltene in toluene. This figure displays the bubble points and SLV M SL phase transitions for two different temperatures. The experimental data of this work are displayed by the solid triangles, the solid diamonds, the stars and the solid bullets; the first two give the phase transitions from LV M L at 310 K and 330 K, the latter two the phase transitions from SLV M SL at 310 K and 330 K obtained in this work. The empty triangles and the empty diamonds display literature data of the asphaltene-free CO2 + toluene system of Tochigi et al. [29] at 333 K and Chang et al. [28] at 310 K.
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also could be assigned to differences in the asphaltenes as these structures are ill-defined and multi-disperse. 4. Conclusions
Fig. A.1. Weight loss as a function of temperature from TGA of asphaltene under nitrogen atmosphere. The heating rate is 10 K/min. The inset of this figure shows the variation of mass% versus temperature in the range of 33–100 (mass%) with a higher resolution.
are small it can be concluded that the bubble point curve is affected by the presence of asphaltene. Of course, these differences
In this study the effect of CO2 on asphaltene precipitation from a model oil was explored. For that purpose, a mixture of toluene and asphaltene with fixed overall composition was used as a model oil. A comparison of the data from this study to literature data of an asphaltene-free toluene–CO2 system revealed that the bubblepoint curve was not that much affected by the presence of asphaltene. Additionally, the presence of CO2 affects the solubility of asphaltene in the oil considerably. Asphaltene precipitation was observed at high CO2 concentrations (above 10 mass%) whereas at low CO2 concentrations asphaltenes stayed in solution even in the investigated pressure range between atmospheric pressure and 7 MPa. This observation confirms once more that, in general, at low concentrations CO2 acts as a co-solvent and at elevated concentrations it switches into anti-solvency behavior [30,31]. The composition of the phases could not be measured due to the fact that the sample could not be brought out of the Cailletet setup for a composition measurement. However, it can be concluded from the experimental observations that upon reduction of pressure, a part of the dissolved CO2 migrates to the vapor phase which in turn results in lower CO2 content in the liquid phase. The asphaltene precipitation process is then reversed resulting in disappearance of the solid phase. The obtained experimental data confirm that high CO2 concentrations should be avoided during miscible CO2 flooding to, on the one hand, minimize the chance of asphaltene precipitation and, on the other hand, to optimize the oil recovery. To identify and to understand the underlying mechanism responsible for the asphaltene precipitation at high CO2 concentrations, further experimental phase behavior studies have to be conducted with a well-defined system using a model asphaltene of known composition and chemical structure.
Fig. A.2. SEM images of asphaltene particles before TGA.
Fig. A.3. SEM images of asphaltene particles after TGA under nitrogen.
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Fig. A.4. EMP results of asphaltene particles.
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Acknowledgements The authors gratefully acknowledge Dr. Karl-Heinz A.A. Wolf for the valuable discussions on the asphaltene characterization and Dr. Bianca Breure for valuable comments. Dr. Jack H.L. Voncken and Ing. Michel M. van den Brink are also greatly acknowledged for their efforts in the EMP and SEM analyses. Appendix A. Thermogravimetric analysis Thermogravimetric analysis (TGA) was used to characterize the asphaltene and determine the volatile components in its structure. For the analysis, a DSC-TGA instrument (SDT Q600, manufactured by TA Instruments) was used. It is capable of performing both differential scanning calorimetry (DSC) and TGA analysis at the same time. The asphaltene analysis was done under nitrogen atmosphere with a heating rate of 10 K/min. During TGA in nitrogen atmosphere the mass loss due to evaporation of components in the sample is determined. The mass loss percentage as a function of temperature for the asphaltenes used in this study is shown in Fig. A.1. Up to temperatures of around 500 K, there is hardly any change in the mass. Between 500 K and around 700 K, 10% of the total weight is lost. In this temperature range most likely smaller poly-aromatic hydrocarbons such as naphthalene (C10H8, consisting of two benzene rings), phenanthrene (684 K, C14H10, consisting of three benzene rings) or pyrene (677 K, C16H10, consisting of four benzene rings) evaporate. In the temperature range between 700 K and 723 K 50% of the mass is lost. At temperatures above 723 K the residue started to foam and continued up to 923 K. The negative values of weight loss and the strong scattering of the weight loss data in this temperature range indicate that material is released spontaneously, comparable to combustion. The fact that the weight loss observed at 720 K and the weight loss determined at around 950 K differs only slightly indicates that low weight gases were captured in the asphaltene structure. Due to the evaporation of some of the poly-aromatic hydrocarbons forming the asphaltenes, the asphaltene structure is weakened allowing the gases to escape. This is accompanied by strong fluctuating weights but small absolute weight losses. This behavior might indicate that also oxygen had been incorporated into the asphaltene structure which combusts when released at these high temperatures. A.1. Scanning electron microscopy The asphaltene samples were also studied by scanning electron microscopy (SEM) before and after TGA. Fig. A.2 shows the asphaltene structure before TGA. It can be seen that the asphaltene is porous. This might have been caused by the washing procedure during the asphaltene separation from the crude oil [32]. Fig. A.3 shows the image of the asphaltene particle after TGA. The image is different than the image of the particle before TGA. The small holes seen in the image of the asphaltene before TGA are probably from degasification during the separation. The open structure of the asphaltene sample after TGA are attributed to the plastic state and deformed flows in the particle due to the second spontaneous degasification [33]. This image supports the hypothesis that in the temperature range between 723 K and 923 K captured gas is released spontaneously. This spontaneous release could also include combustion of hydrocarbons with oxygen initially captured in the asphaltene. A.2. Electron microprobe An electron microprobe (EMP) is an analytical tool used to determine the chemical composition of small volumes of solid materials
in a non-destructive manner. The study of elemental analysis of the asphaltenes was performed with an EMP (JEOL 8800 M JXA Superprobe-1993) equipped with 3 WDS-spectrometers, LDE1, TAP, PET, and LIF analytical crystals. Samples were placed on a cuprum holder and analysis was performed at 20 kV of acceleration voltage. During the analysis, EMP was focused close to the edge of the asphaltene where the sample was thinner. Three different representative analyses were carried out on each sample to obtain reliable results. The analysis of asphaltenes by EMP shows that the asphaltenes used in this study contain carbon, sulfur, sodium, and oxygen; see Fig. A.4. There is some doubt if the asphaltenes contain more oxygen or sodium. In the top graph of Fig. A.4 it is given that sodium and oxygen are found at the same frequencies. 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