Article pubs.acs.org/jced
Vapor−Liquid Equilibrium of Bitumen−Ethane Mixtures for Three Athabasca Bitumen Samples Mohammad Kariznovi, Hossein Nourozieh,# and Jalal Abedi* Department of Chemical & Petroleum Engineering, University of Calgary, Calgary, T2N 1N4, Canada ABSTRACT: Steam−solvent coinjection processes have received more attention in the past decade due to the environmental impact of the pure steam injection process. Increasingly restrictive environmental regulations including carbon tax have served to push the industry to consider coinjection processes (steam + solvent) to reduce greenhouse gas emissions. Understanding the phase behavior of solvent/ bitumen mixtures is critical for feasibility studies as well as the design and implementation of a successful coinjection process. This study presents the vapor−liquid equilibria for bitumen/ ethane mixtures and their applications for bitumen recovery processes. Experiments were conducted for temperatures up to 190 °C and pressures up to 10 MPa to simulate the conditions of in situ steam processes. The results of our vapor−liquid equilibrium experiments include solubility, viscosity, and density measurements of the saturated liquid phase, k-values, and gas oil ratio. Increasing the temperature from 50 to 150 °C resulted in a significant drop in ethane solubility in the bitumen. However, increasing the temperature from 150 to 190 °C had a negligible impact on solubility. As a result, viscosity reduction is much lower at higher temperatures. The viscosity of ethane-saturated bitumen changed linearly with pressure at three temperatures (100, 150, and 190 °C) in a semilog plot. A nonlinear trend was recorded at high pressure and 50 °C with liquid−liquid behavior characteristics. The solubility of ethane is in the same range for the three bitumen samples used in this study which indicates that this characteristic of Athabasca bitumen is not dependent on geographical location.
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INTRODUCTION Steam-assisted gravity drainage (SAGD) and cyclic steam stimulation (CSS) have been used in the Canadian Oil Sands for the last three decades. Unfortunately, these processes are very energy intensive making them economically vulnerable to high fuel prices. In addition, these processes result in high greenhouse gas emissions raising both environmental and economic concerns. Environmental restrictions have increased significantly over the past decade and are expected to become even more rigid in the near future. For instance, new Carbon Tax regulations by the Government of Alberta will put a cap on greenhouse emissions. The substantial difference in selling price and higher production costs of Canadian bitumen make it challenging to stay competitive in a low price environment. In addition, industry has already developed the highest quality reservoirs with thick pay and high oil saturation. Economically sustainable development of new, lower quality resources will require more efficient recovery techniques. Solvent-based processes or steam−solvent coinjection processes are an alternative to the current steam-based recovery processes. In these processes; the dissolution of a solvent in the bitumen provides an additional viscosity reduction compared to the steam-based process, leading to a lower steam−oil ratio (SOR). This results in decreased energy consumption, lower water treatment requirements, and reduced greenhouse gas emissions. The reduced energy requirements of coinjection processes improve current recovery processes efficiency and make it practical to produce lower quality reservoirs. The fact © XXXX American Chemical Society
that the solvent is recovered from the produced bitumen and recycled back into the reservoir makes coinjection economically attractive. Low production rate is the main concern with solvent-based processes, which can be mitigated by coinjection (steam + solvent), maximizing the advantages of both solvent and steam. Currently, the development of solvent-based and coinjection (solvent + steam) processes is hampered by limited data and modeling methodologies for bitumen/solvent systems.1 The economics of these processes depend on the efficiency of oil recovery and solvent recycling, both of which are a function of the phase behavior of the particular solvent/bitumen system. In addition, accurate phase behavior data are critical for pipeline transportation, surface upgrading, and the design of refinery processes. Despite its importance, the study of phase behavior has received little attention. There is a need to study the effect of solvents on bitumen viscosity, density, and transport mechanisms. This study aims to achieve a better understanding of the equilibrium properties of bitumen/solvent systems at conditions approaching the operating conditions of in situ thermal processes. The intention is not to develop recovery processes, but to provide the basic data and mechanistic understanding necessary for quantitative assessment of these processes. This Received: April 5, 2017 Accepted: June 7, 2017
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DOI: 10.1021/acs.jced.7b00322 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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rates. Enhancing our understanding of the bitumen/ethane interaction is critical for optimizing the process. The phase behavior of the bitumen/ethane system and equilibrium properties are governed by complex interactions that demand further study. In addition to oil recovery processes, ethane is a potential candidate for bitumen upgrading and supercritical extraction processes.16 Ethane has favorable critical properties and behaves as a liquid-like solvent with significantly increased solubility at the supercritical condition. The main advantage of supercritical extraction is high solvent recovery wherein the solubility of the solvent changes dramatically with an adjustment of the operating conditions allowing most of the solvent to be recovered. The low critical temperature of ethane means that the extraction process can be operated at a lower temperature than the distillation process and therefore requires less energy.17 Supercritical extraction has been a well-known technique for more than a century with lots of applications in the food industry.18 However, application of supercritical extraction for heavy oil and bitumen is relatively new and a significant gap exists in the literature. While field and commercial applications using ethane require phase behavior data, studies using supercritical fluids and bitumen are limited and can be categorized into constant solvent-to-bitumen ratios and semibatch extractors. In the second category, the extractor is fed a fixed amount of bitumen or heavy oil. The solvent then flows through the extractor and withdraws the lighter components. The constant solvent−bitumen ratio studies have wider applications for modeling and thermodynamic studies of bitumen/ethane systems while the semibatch method is more practical for field applications. There is a limited number of experimental studies in the literature that focus on the phase behavior of ethane/bitumen system. Frauenfeld et al.,19 Fu et al.20 and Mehrotra and Svrcek21−24 measured the equilibrium properties for the ethane/bitumen system. These properties include ethane solubility in bitumen and saturated liquid density and viscosity. Fu et al.20 used a modified Ruska rocking cell apparatus to measure the vapor−liquid equilibrium properties of a Cold Lake bitumen/ethane mixture. They conducted experiments for temperatures up to 150 °C and pressures up to 12 MPa. The authors reported the composition of each phase at equilibrium condition. Mehrotra and Svrcek21−24 experimental data covered temperatures up to 115 °C and pressures up to 10 MPa. The authors did the measurements for four different bitumens: Athabasca, Peace River, Cold Lake, and Wabasca. Puttagunta et al.25 investigated the effect of dissolved gases including ethane on the Lloydminster heavy oil and Athabasca bitumen viscosity. They used the experimental data to develop a correlation for predicting the viscosity of the saturated mixture. They conducted the experiments for the ethane− bitumen system at pressures and temperatures up to 12 MPa and 120 °C, respectively. More recently, Frauenfeld et al.19 measured ethane solubility in Cold Lake blend oil and in Lloydminster Aberfeldy oil at temperatures of 15 and 19 °C and pressures up to 3 MPa. Lim et al.3 conducted flood experiments on a sand pack saturated with Cold Lake bitumen. They considered both subcritical and supercritical ethane and the aim was to evaluate the change in production rate moving from subcritical to the supercritical condition. Zirrahi et al.26 measured the solubility of gaseous hydrocarbon solvent including ethane in Mackay River Bitumen.
study focused on measuring and predicting the phase behavior and thermophysical properties of bitumen/ethane mixtures over a temperature range of (50 to 200 °C) and pressure range of (2 to 10 MPa).
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BACKGROUND Ethane is one of the constituents of natural gas produced in gas reservoirs. The boiling point, critical temperature, and pressure of ethane are −88.6 °C, 32.27 °C, and 4.88 MPa, respectively.2 The critical temperature of ethane is close to ambient temperature which makes it a potential candidate for petrochemical supercritical extraction processes. Ethane can also be used for conventional oil recovery in miscible and immiscible displacement processes. However, its application for heavy oil and bitumen recovery has been limited to laboratory experiments, and no field-scale projects have been reported. Lim et al.3 completed a bench-scale sand pack flood test using Cold Lake bitumen and considered both subcritical and supercritical ethane. Bitumen production rate increased by an average of 25% for the supercritical ethane case, almost twice the rate of subcritical ethane in the first cycle. In addition, the supercritical ethane experiments achieved 25% higher solvent recovery when the apparatus was depressurized at the end of the experiment. The experimental data indicated that supercritical ethane performs better than subcritical ethane for in situ bitumen recovery. Ethane is a potential candidate for coinjection as a gaseous additive to steam-based bitumen recovery processes. Use of a gaseous additive for steam-based recovery processes is not a new idea; experimental and modeling studies showed that a gaseous solvent can improve the performance of steam-based processes such as SAGD when coinjected with steam. Field results by Sperry4 and modeling and simulation results by Weinstein5 support the application of a noncondensable additive to steam in thermal processes. Meldau et al.6 did field trials in the Paris Valley field which confirmed the applicability of a gas additive in steam-based processes. Experimental studies by Pursley7 confirm the field and simulation results. Redford and McKay8 conducted threedimensional (3-D) physical model experiments of the coinjection of a wide range of solvents including gaseous solvents (methane, ethane, propane, and butane), pure liquid hydrocarbon (pentane), and hydrocarbon mixtures (natural gasoline and naphtha) with steam. Their results show increased oil recovery rate with the coinjection of the additives with steam. Butler9 proposed a process to improve SAGD performance, in which noncondensable gases are coinjected with steam. He called it “steam and gas push” (SAGP). The beneficial effect of noncondensable gas additives was confirmed by physical model experiments and explained by the formation of gas insulation at the chamber edge resulting in reduced heat loss.9−11 The impact of noncondensable gases on SAGD performance was investigated by several other authors.12−15 Both methane and ethane are noncondensable gases which can be considered for SAGP. However, ethane has higher solubility in bitumen than methane making it a superior candidate. During the SAGP process, ethane is in vapor form in the steam chamber at a given steam temperature and pressure. It moves to the edge of the chamber and forms an insulating layer at the edge of the chamber which reduces heat loss. Ethane can diffuse and dissolve in the bitumen resulting in viscosity reduction and increase the production and oil recovery B
DOI: 10.1021/acs.jced.7b00322 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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They also measured the density and viscosity of the saturated liquid phase. The ethane measurements were conducted at three temperatures (100, 150, and 190 °C) and three pressures (1, 2, and 4 MPa). The experimental data in the literature is limited to a temperature range of 22 to 114 °C and the pressures up to 10 MPa. Few data points have been reported for high temperatures (150 and 190 °C). In summary, ethane can be considered for the recovery and processing of heavy oil and bitumen, as a gas additive to steambased recovery processes, or in supercritical extraction processes for surface upgrading. The phase behavior and thermodynamic properties of bitumen/ethane mixtures are extremely important to the design and optimization of these processes. In this manuscript, the phase behavior of three Athabasca bitumen samples with ethane was studied experimentally. Ethane solubility and saturated liquid phase density and viscosity were measured for temperatures up to 190 °C and at pressures up to 10 MPa. Bitumen characterization and equations of state modeling for bitumen/solvent systems including bitumen/ethane were presented in detail in our previous publication.27 The experimental data presented in this manuscript are for a binary mixture of a bitumen/ethane system. However, the data can be used to develop a model for a ternary mixture of a water/ethane/bitumen system. This ternary system has application for hybrid (steam + solvent) processes. Nighswander et al.28 incorporated the vapor−liquid data for a binary mixture of carbon dioxide/water and carbon dioxide/bitumen to predicate the ternary carbon dioxide−water−bitumen system. The modeling approach was explained in detail by Nighswander et al.28 The same approach can be used to predicate the binary data for ethane/bitumen and ethane/water mixture and predicate the ternary system of ethane−water− bitumen. In addition, the bitumen/ethane binary data can be used to tune equations of state for the modeling study of solvent and hybrid processes.
Table 1. Molecular Weight of Bitumen Samples bitumen
MW (g/g-mol)
JACOS MacKay River Surmont
530.8 ± 4.4 512.5 ± 6.9 539.2 ± 7.9
Nourozieh et al.29 The SARA compositional analysis of three bitumen samples is presented in Table 2. Table 2. SARA Analysis for Bitumen Samples
a
fraction
JACOSa
MacKay River
Surmont
saturates aromatics resins asphaltenes
18.72 35.66 28.27 17.35
11.76 57.00 21.61 9.62
12.26 40.08 36.53 11.13
n-Pentane as precipitant.
ASTM D716931 was the method used for compositional analysis and to obtain carbon number distributions for three bitumen samples. This method provides the carbon distribution up to C100. The ASTM D716931 method uses high-temperature gas chromatography to determine the elution of the components and their boiling point distribution at temperatures up to 720 °C. At this temperature, the component n-C100 is eluted. The measurement procedure is explained in more detail by Kariznovi et al.32 The boiling point distributions for three bitumen samples are given in Table 3. Table 3. Compositional Analysis of Three Bitumen Samples
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EXPERIMENT Materials. Praxair supplied the ethane with a purity of 99.9 mol %. SHARP research consortium member companies provided the bitumen samples from their steam-assistedgravity-drainage (SAGD) operations in Fort McMurray, Alberta, Canada. Samples were received from a Japan Canada Oil Sands Limited (JACOS) SAGD operation in the Hangingstone area, Fort McMurray, Alberta, Canada; a Suncor SAGD project at MacKay River; and a ConocoPhillips SAGD operation (Surmont project) southeast of Fort McMurray. The bitumen samples were obtained from the production units and processed for sand and water removal. The water was removed from the bitumen by distillation, and the sand was removed by centrifuge. An Anton Paar density-measuring cell was used to measure sample density. The density values at 20 °C and atmospheric pressure are 1010 kg/m3 for Surmont; 1009 kg/m3 for JACOS, and 1006 kg/m3 for Mackay River Bitumen. The cryoscopy method was used to measure the molecular weight of the bitumen samples. This method is dependent on freezing-point depression. More details about molecular weight measurements can be found in Nourozieh et al.29 Table 1 summarizes the molecular weight measurements for the three bitumen samples. A modified version of ASTM D2007 was used to conduct SARA (saturates/aromatics/resins/asphaltenes) analysis.30 The procedure for SARA analysis was explained in more detail by
a
weight % off
MacKay River
JACOS
Surmont
IBP 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80
184 272 313 344 372 399 424 448 473 499 526 556 588 619 648 682 701
199 288 329 363 394 423 450 477 505 536 570 603 636 663 694 713 a
200 283 322 353 381 408 432 456 481 508 537 569 601 633 660 692 711
Deasphaltened bitumen.
Apparatus and Procedure. The details of the experimental apparatus and its validation are available in the literature. The reader is referred to these studies for further details.33−36
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RESULTS AND DISCUSSION Surmont Bitumen/Ethane Systems. Vapor−liquid measurements were conducted to determine the effect of pressure and temperature on the phase equilibrium properties of bitumen/ethane mixtures. The experiments covered 20 different conditions, combinations of four temperatures (50, 100, C
DOI: 10.1021/acs.jced.7b00322 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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150, and 190 °C), and five pressures (1, 2, 4, 6, and 8 MPa). The details of each experiment, such as the amount of bitumen and ethane charged into the equilibrium cell are presented in Nourozieh.35 The results of the vapor−liquid experiments for Surmont bitumen/ethane systems are summarized in Table 4.
Table 5. Experimental Vapour-Liquid Equilibrium Properties for JACOS Bitumen/Ethane Mixtures at T = (50, 100 and 150) °C and P = (2, 4, 6, and 8 MPa)a
Table 4. Experimental Vapour−Liquid Equilibrium Properties for Surmont Bitumen/Ethane Systems at T = (50, 100, 150, and 190) °C and P = (1, 2, 4, 6, and 8 MPa)a T (°C)
P (MPa)
102ws
ρs (kg/m3)
μs (mPa·s)
51.3 50.7 50.8 50.9 50.9 100.5 100.3 100.6 100.7 100.6 149.9 149.7 149.3 149.9 149.9 189.4 189.4 189.7 189.9 189.4 50.9
1.103 2.103 4.109 6.101 8.090 1.013 1.958 3.978 6.025 8.076 1.200 1.999 4.040 6.025 8.204 1.455 2.096 4.019 6.039 8.073 8.073 8.107 1.138 1.241 1.179 2.185 2.006
1.76 3.64 7.75 12.2 14.2 0.91 1.90 3.71 6.15 8.45 0.67 1.33 2.65 4.33 5.55 0.72 1.18 2.10 3.18 4.40 14.2 14.1 0.65
974.7 957.0 916.5 874.2 856.7 949.6 940.4 919.1 897.2 876.0 921.6 915.9 900.9 885.9 869.3 900.8 897.1 884.8 872.1 859.1 854.1 859.2 921.5 921.5 921.9 896.5 897.6
1822 652 118 35.8 26.7 138 102 49.2 25.6 16.2 27.4 23.4 16.2 11.0 8.14 10.6 9.68 7.69 6.13 5.07 24.2 29.1 28.7 26.7 26.7 9.68
149.7
189.4
0.69 1.19 1.17
T (°C)
P (MPa)
102ws
ρs (kg/m3)
μs (mPa·s)
50.6 50.7 49.3 50.3 100.3 100.3 100.2 100.2 149.7 150.0 149.9 149.7 50.5
2.082 4.123 6.060 8.080 2.013 4.072 6.074 8.080 2.227 4.067 6.167 8.169 2.082 2.082 4.123 4.123 8.100 8.059 4.081 4.067 4.067 6.163 6.170
4.16 9.00 13.4 14.5 2.16 4.91 6.25 8.50 1.68 2.86 4.79 5.79 4.66 3.66 8.57 9.41 14.5 14.4 4.64 4.47 5.61 5.06 4.53
954.9 912.9 869.2 851.7 937.4 914.7 893.8 872.0 911.3 897.7 879.7 865.9 954.5 955.3 912.7 913.0 849.4 854.0 914.5 915.0 914.5 878.9 880.5
566 94.4 29.6 21.0 81.4 39.5 21.9 13.7 18.6 13.7 9.62 7.19 573 559 95.2 93.6 ---21.0 39.7 39.2 39.7 9.78 9.45
100.3
149.9
a Notation: ρs, saturated liquid density; μs, saturated liquid viscosity; ws, weight fraction of ethane in saturated liquid phase.
conducted at five pressures (1, 2, 4, 6, and 8 MPa) and four temperatures (50, 100, 150, and 190 °C) (Table 6). The repeatability of the generated data was examined by repeating one experiment in each isotherm (Table 6). The solubility measurements were precise within ±0.2 weight fraction of ethane, and the saturated liquid densities were within ±2 kg/ m3. The maximum deviation for the saturated liquid viscosities was 5%. As Tables 4 to 6 indicate, the density and viscosity of the saturated liquid phase was reduced by dissolving ethane in bitumen. Ethane solubility increased with increasing pressure at constant temperature. The slope of the decreasing trend for saturated liquid density and viscosity depends on the temperature. The curve has a lower slope at lower equilibrium temperature which indicates much higher solubility at low temperatures. Although the density and viscosity of gas-free bitumen increase with pressure at constant temperature, the dissolution of ethane in bitumen compensated for this increase and reversed the impact of pressure. The three bitumen samples followed the same trends for solubility, saturated liquid viscosity, and density with pressure and temperature. Figures 1 to 3 illustrate the experimental results for the bitumen/ethane systems. The solubility measurements are shown in Figure 1, and Figures 2 and 3 demonstrate the density and viscosity of ethane-saturated bitumen as a function of equilibrium pressure. In Figure 1, the x-axis illustrates the equilibrium pressure and the y-axis indicates the measured ethane solubilities in bitumen. Different colors are used to depict the isotherms corresponding to each temperature and the different bitumens are presented by different symbols. At constant temperature, the solubility of ethane in bitumen increases with pressure and it decreases with temperature at a
Notation: ρs, saturated liquid density; μs, saturated liquid viscosity; ws, weight fraction of ethane in saturated liquid phase. a
The oven maintains the temperature within ±0.1 °C. The repeatability of the generated data was examined by repeating one experiment at each temperature (50, 150, and 190 °C) (Table 4). The measured solubilities for the two experiments were in good agreement and the saturated phase densities were precise to less than 0.5 kg/m3. A deviation of less than 5% was obtained for saturated liquid viscosities measurements. JACOS Bitumen/Ethane Mixtures. Vapor−liquid equilibrium experiments similar to those for Surmont bitumen were conducted for JACOS bitumen. Experiments were conducted at three temperatures and four different pressures. The results of the vapor−liquid experiments for JACOS bitumen/ethane systems are summarized in Table 5. As with Surmont bitumen/ ethane mixtures, the repeatability of the generated data was examined by repeating one experiment at each temperature of 50, 100, and 150 °C (Table 5). As the table shows, the solubilities were within ±0.5 wt % of ethane at 50 °C. This may be due to high solubility values at low temperatures. The saturated phase densities are precise to less than 0.5 kg/m3. The deviation for the saturated liquid viscosities is less than 5%. MacKay River Bitumen/Ethane Mixtures. Vapor−liquid experiments for MacKay River bitumen/ethane mixtures were D
DOI: 10.1021/acs.jced.7b00322 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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Table 6. Experimental Vapour−Liquid Equilibrium Properties for Mackay River Bitumen/Ethane Mixtures at T = (50, 100, 150 and 190) °C and P = (1, 2, 4, 6, and 8 MPa)a T (°C)
P (MPa)
102ws
ρs (kg/m3)
μs (mPa·s)
51.0 53.3 51.0 50.7 53.7 100.3 99.9 100.4 100.7 101.0 149.5 149.9 149.8 150.6 149.6 190.5 189.4 189.8 190.1 190.1 50.7
1.069 1.951 4.012 6.001 8.066 1.079 2.054 4.109 6.101 8.100 1.138 1.979 4.082 6.032 8.052 1.399 2.065 4.067 6.046 8.052 5.991 6.012 2.068 2.075 2.020 4.054 4.078 4.116 2.096 2.034
1.79 3.14 7.36 11.9 14.0 1.06 2.10 3.99 6.19 8.18 0.75 1.40 2.65 4.07 5.62 0.82 1.22 2.45 3.58 4.64 11.8 12.0 2.44 1.93 1.93 2.30 2.88 2.77 1.24 1.20
967.9 951.6 910.3 868.5 846.8 942.7 932.8 911.2 889.8 868.8 915.7 909.8 894.8 879.1 863.8 894.5 890.7 877.4 864.9 851.5 868.0 868.9 933.1 931.6 933.7 897.7 893.7 892.9 890.7 890.6
1225 404 86.3 28.1 17.1 112 80.4 38.1 21.3 13.0 22.2 20.4 13.6 9.68 7.31 9.10 8.42 6.66 5.43 4.44 27.2 28.9 79.0 77.3 85.1 14.0 13.5 13.1 8.50 8.34
99.9
149.8
189.4
Figure 2. Viscosity measurements of ethane-saturated bitumen for different bitumen samples from Athabasca Field as a function of temperature at different pressures; black symbols, 50 °C; blue symbols, 100 °C; green symbols, 150 °C; red symbols, 190 °C; ●, JACOS bitumen; ○, Surmont bitumen; +, MacKay River bitumen; ×, Mehrotra and Svrcek.21
Notation: ρs, saturated liquid density; μs, saturated liquid viscosity; ws, weight fraction of ethane in saturated liquid phase. a
Figure 3. Density measurements of ethane-saturated bitumen for different bitumen samples from Athabasca field as a function of temperature at different pressures; black symbols, 50 °C; blue symbols, 100 °C; green symbols, 150 °C; red symbols, 190 °C; ●, JACOS bitumen; ○, Surmont bitumen; +, MacKay River bitumen; ×, Mehrotra and Svrcek.21
considered in this study (50 °C) the vapor−liquid equilibrium exists in the studied pressure range (
