Ind. Eng. Chem. Res. 2007, 46, 365-368
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Compositional Changes and Rheological Properties of Athabasca Bitumen under CO2/O2 Mixtures Associated with Carbon Dioxide Flooding/Sequestration Process Na Jia,* Gordon Moore, Sudarshan (Raj.) A. Mehta, Elizabeth Zalewski, and Matthew Ursenbach Department of Chemical and Petroleum Engineering, UniVersity of Calgary, 2500 UniVersity DriVe NW, Calgary, Alberta T2N 1N4, Canada
Research described in this paper was conducted in support of a more extensive study that has been ongoing at the University of Calgary to quantify the effect of the presence of carbon dioxide and oxygen mixtures in an Athabasca bitumen reservoir undergoing a carbon dioxide flooding/sequestration process. This paper concentrated on the compositional and rheological changes of Athabasca bitumen. The experiments were performed in the oscillating batch reactor using clean oil plus distilled water only. Viscosity was measured, and the compositional data were expressed in terms of the components maltenes, asphaltenes, and coke. In CO2/O2 tests, the amounts of maltenes decreased with increasing reaction temperatures as a result of conversion to asphaltenes and coke. The data had direct applicability to recovery processes involving the injection of carbon dioxide containing oxygen as an impurity in the bitumen reservoir. Introduction Carbon dioxide flooding is a well-established enhanced oil recovery process in regions where CO2 is available. Recently, the public’s concern with greenhouse gas emissions and Canada’s support of the Kyoto Agreement provide environmental incentives to allow for the application of CO2 flooding as a combined EOR/sequestration process. Capturing the CO2 from stack gas is an expensive process, and the actual cost would depend on the purity of the CO2 stream. Oxygen is an important impurity because it can modify the native oil properties through low-temperature oxidation reactions, promote corrosion in the injection and production piping, and alter the phase behavior of the reservoir fluids. The study conducted by Xu et al.1 quantified the effect of the presence of air in the unheated portions of an Athabasca reservoir and evaluated whether low-temperature oxidation reactions could be used to achieve in situ upgrading. Jia et al.2 investigated compositional changes of Athabasca bitumen under a nitrogen atmosphere. The purpose of both programs was to understand the compositional changes that might occur at temperatures ranging from those of the native reservoir to those experienced in a steam injection oil recovery process. The tests described in this paper involved contacting the Athabasca bitumen and injection fluid (CO2/O2) in a closed reactor under the same two-stage temperature setting as experiments described by Jia et al.2 The reason for choosing this particular temperature setting in this program was to compare the results for CO2/O2 tests with nitrogen tests. From the literature,3-5 it is expected that, when an O2 containing gas mixture was injected into a reservoir, liquid-phase lowtemperature oxidation (LTO) and vapor-phase bond scission reactions may take place to change the crude oil composition. Also, from the information provided by the literature,6 the injection of CO2 promoted an increase of the production of heavy hydrocarbons, which would affect the oil transportation in a pipeline. The present study examined how a CO2/O2 gas mixture was reacted in a bitumen reservoir. Measured data included the composition and emulsification characteristics of * To whom correspondence should be addressed. Tel.: +1-403-282 6058. Fax: +1-403-284 4852. E-mail:
[email protected].
the oil, the pH levels of the connate water, and the vapor-phase composition. Experiments (A) Apparatus. Two-stage oxidation experiments on bitumen under a CO2/O2 and CO2 only atmosphere were conducted. The experimental apparatus was designed and constructed in 1993 for Wichert’s study.5 It consisted of ten high-pressure, threephase batch reactors, each with an inner volume of 250 cm3. All reactors could be operated simultaneously to an upper range of 68.93 MPa and 220 °C. Each reactor cell was equipped with type-K thermocouples to continuously measure the inner vapor and liquid temperatures. Heat was supplied with flexible silicone strip heaters that were coiled around the outer surface of each cell. A thermocouple used as the input signal in the control loop was inserted between the reactor’s outer surface and the heater. Once the reactors were assembled, they were mounted into individual containment chambers and were set to oscillate for a period of time. The reactor arms could reach 30 degrees from the vertical position, and rocking speeds could be set from 0 to 11 rpm. Heat control and data acquisition were completed through National Instruments “Labview” software. A schematic design of the experimental setup is shown in Figure 1. (B) Materials. The materials used in the experiments were as follows: 1. Athabasca bitumen: It was steam-flood produced from the Underground Test Facility (UTF)-Devon mine near Fort McMurray and delivered to the laboratory in 1998. Properties of the Athabasca bitumen are shown in Table 1. 2. CO2 and CO2/O2 streams: Carbon dioxide and its mixtures with oxygen were used as a feed gas for the reactors. Either pure carbon dioxide, 99% carbon dioxide + 1 mol % oxygen, or 95% carbon dioxide + 5 mol % oxygen were used. All gases were supplied by Praxair, Inc. (C) Procedures. A measured amount of bitumen and distilled water was introduced into the stainless steel batch reactor (as previously described) and allowed to react under specific gas atmosphere, temperature, and pressure conditions. The mass of oil and distilled water were ∼50 g each. After the loading of oil and water, the cells were assembled immediately and the gas (CO2/O2 mixture or CO2 only) was injected up to a
10.1021/ie0612461 CCC: $37.00 © 2007 American Chemical Society Published on Web 12/02/2006
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Ind. Eng. Chem. Res., Vol. 46, No. 1, 2007 Table 2. Description of Reaction Runs
run no. 1 2 3 4 5 6 7 8 9 Figure 1. Schematic design of experimental apparatus.
10
Table 1. Initial Properties of Athabasca Bitumen viscosity (mPa‚s) density (g/cm3) initial composition (mass % of initial oil)
at 40 °C at 55 °C at 70 °C at 25 °C at 40 °C asphaltenes maltenes coke
22 000 4 725 1 460 1.00 0.99 18.30 81.58 0.12
predetermined charge pressure (0.45-3.54 MPag). The celltemperature controls were then set to the desired experiment temperature. After the preset reaction time, the gas-phase composition in each cell was analyzed using a Hewlett-Packard 5890 gas chromatograph. Free water present in the cell was collected, and its pH was measured using VWR Scientific model 8005 pH meter. Oil was then removed from the cell, and the cell was rinsed with toluene. The oil-toluene rinses were combined, and a Dean-Stark distillation was conducted to separate the water. The oil/toluene mixture was then filtered to separate any toluene insoluble materials (i.e., coke and salts). Finally, the toluene was removed by rotary evaporation followed by a mild vacuum at a temperature between 30 and 40 °C until the oil mass was constant. Asphaltenes, defined as pentane insoluble, was measured by applying filtration methods. Viscosity measurements were made on the clean oil using a Brookfield RVDV-1+ viscometer. Results A total of 10 experiments involving the two-stage temperature treatment under a CO2/O2 or CO2 only atmosphere were completed. The lower temperature phase was at 80 °C (lowtemperature soak, LTS), and the higher temperature was either 200 °C or 220 °C (elevated-temperature soak, ETS). The duration of the tests ranged from 6 to 24 days. Table 2 provides detailed conditions for the test program. As described before, the purpose of the two-stage (LTS + ETS) process was to simulate first injecting gas into a formation at low temperature and then heating the oil to a relatively high temperature. The reactors were not depressurized between the LTS and ETS stages; hence, the residual oxygen that remained in the gas phase after low-temperature soak was still available for reaction. The initial gas pressure during LTS was in the range of 0.52-3.85 MPag, and it was higher in the ETS stage at 2.49-7.82 MPag as a result of the temperature increase. Figure 2 shows typical temperature and pressure profiles
reaction description 6 days LTS at 80 °C, 7 days ETS at 200 °C 6 days LTS at 80 °C, 9 days ETS at 200 °C 6 days LTS at 80 °C, 6 days ETS at 220 °C 6 days LTS at 80 °C 6 days LTS at 80 °C, 6 days ETS at 220 °C 18 days LTS at 80 °C, 6 days ETS at 200 °C 6 days LTS at 80 °C, 12 days ETS at 200 °C 6 days ETS at 200 °C 12 days LTS at 80 °C, 6 days ETS at 200 °C 6 days LTS at 80 °C, 6 days ETS at 220 °C
O2 mol % in the feed gas
LTS initial pressurea (MPag)
ETS initial pressureb (MPag)
agitation speed (rpm)
5%
0.84
2.75
1/3
5%
0.79
2.43
1/3
5%
3.85
7.82
1/3
1% 1%
1.89 1.52
4.87
1/3 1/3
1%
0.83
2.49
1/3
1%
0.86
2.61
1/3
0% 0%
[/] 0.87
2.99 2.77
1/3 1/3
0%
0.52
2.88
1/3
a LTS initial pressure measured when liquid temperature equaled 80 °C. ETS initial pressure measured when liquid temperature equaled 200/220 °C.
b
Figure 2. Temperature and pressure profiles. Table 3. Gas Composition of Vapor Phase (mol %) before reaction
after reaction
run no.
O2
CO2
O2
CO2
N2a
others
1 2 3 4 5 6 7 8 9 10
0.052 0.052 0.052 0.010 0.010 0.010 0.010 0.000 0.000 0.000
0.948 0.948 0.948 0.990 0.990 0.990 0.990 1.000 1.000 1.000
1.92E-3 2.56E-3 3.78E-3 1.68E-3 1.36E-3 1.37E-3 1.29E-3 3.90E-4 4.40E-4 2.90E-4
0.981 0.973 0.962 0.983 0.988 0.986 0.992 0.996 0.996 0.997
0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
a Nitrogen concentrations were reported to ensure the system was airfree.
throughout the experiments. Higher LTS initial pressures provide enough O2 to promote the low-temperature oxidation reactions, which results in the increased viscosity. Higher ETS initial pressures usually occur after higher LTS initial pressures at the same temperature settings. When oxygen was present in the system after the LTS period, higher ETS initial pressures would enhance oxidation reactions because the reaction rates are dependent on oxygen partial pressures.3,4 (A) Gas Composition Analysis. Table 3 shows the vaporphase compositions for the Athabasca bitumen tests as measured before and after the two-stage thermal soaks. For the tests involving 1% and 5% O2, reduction in oxygen concentrations indicated that oxygen was consumed and took part in the oxidation reactions. For the tests of pure CO2 injection, a small
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Figure 3. Comparison of asphaltenes yields of Athabasca bitumen after reaction.
Figure 6. Comparison of pH values of Athabasca bitumen after reaction. Table 4. Viscosity of the Modified Oils (mPa‚s)
Figure 4. Comparison of coke yields of Athabasca bitumen after reaction.
Figure 5. Comparison of maltenes yields of Athabasca bitumen after reaction.
amount of O2 appeared in the vapor-phase compositions after reactions. This was an unexpected result, but it was very common in the tests involving recombined Athabasca bitumen. (B) Liquid-Phase Composition Analysis. The compositions of the liquid phases withdrawn from the reactors upon completion of the experiments (modified oils) were analyzed. The reported components were maltenes, asphaltenes, and coke expressed in g/(100 g of original oil). Figures 3, 4, and 5 compare the asphaltenes, coke, and maltenes yields of Athabasca bitumen in the case of CO2/O2 injection with those of the case of N2 injection,2 respectively. Figure 3 illustrates that asphaltenes yields in the CO2/O2 tests were higher than those in the nitrogen tests. The asphaltenes contents in the modified oils were also higher than that in the initial oil. The values for asphaltenes in the modified oils were (19.46-21.99 g)/(100 g) as opposed to (18.30 g)/(100 g) in the initial oil. It demonstrated that O2 was diffused into the oil and reacted with hydrocarbons through oxidation reactions. It is shown in Figure 4 that more coke was formed for CO2/ O2 when the ETS temperature was 220 °C. The coke contents of the modified oils increased as well, from (0.12 g)/(100 g) in
run no.
40 °C
55 °C
70 °C
1 2 3 4 5 6 7 8 9 10 initial
46 300 44 650 73 200 38 700 42 700 38 050 32 400 29 500 31 850 24 450 22 000
8 700 8 500 13 250 8 300 8 650 7 600 6 550 6 350 6 650 5 350 4 725
2 500 2 400 3 350 2 150 2 500 2 700 2 050 1 850 2 000 1 650 1 460
initial oil to (0.33-0.42 g)/(100 g) in modified oils. For the pure CO2 or N2 injection, it appears that, even though the reactors were evacuated for 5-10 min prior to charging with CO2 or N2, a small amount of O2 remained in the system. This might cause the increase in the reported yields of coke in the presence of pure CO2 or N2 over those of the original oil. Figure 5 shows that maltenes yields in the nitrogen tests were greater than those under the CO2/O2 environment. In the current program, maltenes yields were calculated as (100 - asphaltenes and coke yields). Thus, from Figure 5, it is seen that the sums of coke and asphaltenes yields in the nitrogen tests were less than those in the CO2/O2 tests. Also, maltenes contents of the modified oils decreased from (81.58 g)/(100 g) in initial oil to (77.59-80.21 g)/(100 g) in the modified oils. Run No. 3 in the CO2/O2 tests generated the least maltenes (highest sum of asphaltenes and coke) in comparison with the other nine runs. The reason for this is that Run No. 3 was conducted under the highest reaction pressure. It was also shown that the increased oxygen concentration in the feed gas resulted in an increase in the asphaltenes and coke yields. (C) pH Values of Free Water. Figure 6 shows that the pH values generated in the nitrogen tests were higher than those in the CO2/O2 tests. It can be seen that the pH values decreased significantly from that of 7.02 for distilled water to 4.32-5.54 for different postexperiment waters after CO2/O2 or CO2 only tests. That was caused by carbon dioxide dissolution into the water. This may cause corrosion problems, and thus, inhibition procedures may be necessary for corrosion control. (D) Viscosity of the Modified Oil. Table 4 presents the viscosity of Athabasca bitumen after reaction as measured at three different temperatures: 40, 55, and 70 °C. It is seen that the viscosity of the reacted oil increased greatly as compared to that of the initial oil. The viscosity ranges in the modified oil were 24 450-73 200 mPa‚s at 40 °C, 5 350-13 250 mPa‚s at 55 °C, and 1 650-3 350 mPa‚s at 70 °C. The highest viscosity was observed in the oil with experiment No. 3, where the CO2/ O2 feed gas containing 5% of oxygen was injected (6 days LTS at 80 °C and LTS initial pressure 3.85 MPag, followed by 6
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days ETS at 220 °C and ETS initial pressure 7.82 MPag). It was most possible due to the highest operating pressure involved in this experiment. Experiment Nos. 8, 9, and 10, in which the pure carbon dioxide was injected, displayed higher oil viscosities than that in the original oil but lower than those of the modified oils in the tests involving oxygen. The important observation in the Athabasca bitumen/CO2/O2 tests was that very viscous oils may undergo detrimental changes during a carbon dioxide sequestration process. Decreased mobility would make bitumen more difficult to displace if the dissolved gases were allowed to liberate. It should be mentioned that, when the oil samples were subject to Dean-Stark distillation and rotary evaporation as described, the loss of even trace amounts of the more-volatile hydrocarbons would have resulted in an increase in oil viscosity, while the remaining small amount of solvent with oil resulted in the reductions of viscosity. Conclusions A study of the effect of CO2/O2 gas mixtures on Athabasca bitumen during a two-stage temperature program was presented in this paper. The results demonstrated the influence of the gas mixtures composition on compositional changes and rheological properties of the Athabasca bitumen. The following conclusions were drawn: 1. The viscosity of the reacted oil increased greatly upon CO2/ O2 injection. This effect was also evident, although to a lower degree, for the CO2 only experiments. 2. The asphaltenes yields for the modified oils were higher than that for the initial oil, and more asphaltenes were generated under the CO2/O2 atmosphere compared with N2 injection. 3. The coke content in the reacted oil (after CO2/O2 injection) was increased by 3-4 times as compared to the initial bitumen. 4. The sum of coke and asphaltenes content in the reacted oil with CO2/O2 injection was greater than that of N2 injection, while maltenes content was in the reverse tendency. 5. There was an obvious correlation between oxygen concentrations in the feed gas and the sum of the asphaltenes and coke content in the reacted oil; the higher the oxygen concentration is, the higher are the increases in asphaltenes and coke yields.
6. Pure carbon dioxide injection resulted in higher asphaltenes formation than that of the original oil. 7. pH values of the collected free water show that the original neutral pH water became corrosive. There was no definite influence of oxygen concentration on pH values of the postreaction water, which suggests that the pH alterations were due to carbon dioxide dissolution in the water. Acknowledgment The financial support of AERI through COURSE program, the Department of Chemical and Petroleum Engineering at University of Calgary, and NSERC is gratefully acknowledged. Literature Cited (1) Xu, H. H.; Okazawa, N. E.; Moore, R. G.; Mehta, S. A.; Laureshen, C. J.; Ursenbach, M. G.; Mallory, D. G. In-Situ Upgrading of Heavy Oil. J. Can. Pet. Technol. 2001, 40 (8), 45-53. (2) Jia, N.; Moore, R. G.; Mehta, S. A.; Fraassen, K. V.; Ursenbach, M. G.; Zalewski, E. Compositional Changes of Athabasca Bitumen in the Presence of Oxygen under Low Temperature Conditions. J. Can. Pet. Technol. 2005, 44 (9), 51-57. (3) Adegbesan, K. O.; Moore, R. G.; Donnelly, J. K. Low-Temperature Oxidation Kinetic Parameters for In Situ Combustion Numerical Simulation. Presented at The 58th Annual Technical Conference and Exhibition, San Francisco, CA, Oct 5-8, 1983; Paper SPE 12004. (4) Millour, J. P.; Moore, R. G.; Bennion, D. W.; Ursenbach, M. G.; Gie, D. N. An Expanded Compositional Model for Low Temperature Oxidation of Athabasca Bitumen. J. Can. Pet. Technol. 1987, May-June, 24-32. (5) Wichert, G. C.; Okazawa, N. E.; Moore, R. G.; Belgrave, J. D. M. In-Situ Upgrading of Heavy Oils by Low-Temperature Oxidation in the Presence of Caustic Additives. Presented at The International Heavy Oil Symposium, Calgary, Alberta, Canada, June 19-21, 1995; Paper SPE 30299. (6) Srivastava, R. K.; Huang, S. S. Asphaltene Deposition During CO2 Flooding: A Laboratory Assessment. Presented at The SPE Production Operations Symposium, Oklahoma City, OK, Mar 9-11, 1997; Paper SPE 37468.
ReceiVed for reView September 25, 2006 ReVised manuscript receiVed November 14, 2006 Accepted November 21, 2006 IE0612461
