High-Pressure Flow Reactor: Design and Application to Pertinent Oil

VOLUME 14, NUMBER 3

MAY/JUNE 2000

© Copyright 2000 American Chemical Society

Articles High-Pressure Flow Reactor: Design and Application to Pertinent Oil Recovery Studies R. A. Kazi,†,§ H. B. Al-Saffar,† D. Price,‡ and R. Hughes*,† Chemical Engineering Unit, University Of Salford, Maxwell Building, Manchester M5 4WT, U.K., and Department of Chemistry and Applied Chemistry, University Of Salford, Manchester M5 4WT, U.K. Received February 1, 1999

Continuing studies on combustion cell reactor experiments are expected to promote a better understanding of the problems and mechanisms involved in laboratory investigations and field applications of the in-situ combustion process for enhanced oil recovery. A new high-pressure flow reactor has been designed for analyzing the specific features of in situ combustion in consolidated core materials (real core) at elevated pressures. The purpose of this facility is to enable physical simulation of the in-situ combustion process and air injection process for oil recovery at reservoir conditions of temperature and pressure. This paper describes in detail the high-pressure flow cell reactor facility in terms of specific equipment operation and features. Furthermore, the results of some experiments are presented.

Introduction In-situ combustion and air injection (for light oil) continue to be an important oil recovery processes which could be used to increase both the amount and the rate of oil recovered from a petroleum reservoir.1,2 In-situ combustion has been developed as a technique for extraction of heavy crude. Injection of air or an air/ oxygen mixture into the oil bearing formation oxidizes the oil generating heat and producing carbon oxides, oxygenated hydrocarbons, and water. The heat gener* Corresponding author. Fax: +44(0)161-2955380. E-mail: [email protected]. † Chemical Engineering Unit. ‡ Department of Chemistry and Applied Chemistry. § Present address: Department of Petroleum and Gas Engineering, Mehram University of Engineering and Technology, Pakistan. (1) Farouq Ali, S. M.; Thomas, S. J. Can. Pet. Technol. 1996, 35, 57-63. (2) Greaves, M.; Ren, S. R.; Rathbone, R. R. Air injection technology (LTO process) for IOR from light oil reservoirs: Oxidation rate and displacement studies. Presented as SPE/DOE Improved Oil Recovery Symposium, Tusla, OK, April 19-22, 1998.

ated forms a combustion front which lowers the viscosity of the oil ahead of this front and the resultant oil is driven to the production well by the combustion gases. Air injection has been developed recently as a production process for extraction of light crudes in which the oxygen in the air reacts with the oil to produce lowtemperature oxidation compounds and flue gas. The resultant flue gas mixture is used as a gas drive for recovery of the oil at the production well. All investigators agree that in-situ combustion and more recently air injection techniques should be regarded as a potentially very effective process for producing oil. The applicability of both techniques to a wide range of reservoirs and recent advances show that these processes have unique advantages. Most of the in-situ combustion field projects have been conducted for tertiary recovery purposes. However, in some cases, combustion has been used for primary recovery process.3 However, the phenomena observed are much too com(3) Chu, C. J. Pet. Technol. 1983, 1412-1418.

10.1021/ef9900177 CCC: $19.00 © 2000 American Chemical Society Published on Web 04/27/2000

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plicated to permit interpretation simply on the basis of the occurrence of a heating effect. A more intricate and intrinsic picture is required. Any general review of combustion field projects should cover screening guides, reservoir performance predictions, project design, well completions, ignition methods, and operational problems. Not all the reported field projects indicated the use of laboratory experiments to help in the design and conduct of field projects. However, laboratory experimentation is advised as an integral part of the development program of a combustion project because laboratory results prove useful in many ways. Both in-situ combustion and air injection are complex processes which involve simultaneous heat and mass transfer in a multiphase environment coupled with the chemical reactions involved in crude oil combustion. While much work has been done to study the thermal and fluid dynamic aspects of the in-situ combustion process, the chemical reaction kinetics remain the least investigated aspects. Adequate kinetics data are necessary for reliable performance predictions by mathematical or numerical models because of the coupling between heat transfer, mass transfer and chemical reaction phenomena taking place during in-situ combustion. Experimental work on the combustion oil recovery process has consisted of both laboratory and field studies. Although field experiments are the ultimate test of any oil recovery process, they are costly, timeconsuming, and difficult to analyze quantitatively. A close coordination between laboratory research and field experimentation could benefit any combustion project in many ways. A laboratory combustion reactor could be used to measure the oxidation rate of oil under reservoir conditions, and numerical models incorporating this rate could then be used to calculate temperature variations with respect of time and position and thus to evaluate the possibility of successful in-situ combustion. Furthermore, experiments conducted with core samples saturated with oil could be used to study the kinetics of chemical reactions involved, and how the reaction rates are affected by the properties of the oil and core material and by any catalytic material present in the oil or the matrix. Moreover, the combustion apparatus can provide data on fuel availability, combustion front velocity, air requirement, and composition of the produced fluids.3 The majority of experiments on oil combustion have been conducted at relatively low pressures. While the in-situ combustion process has been successfully applied in the field to reservoirs with pressures below 10 bar, more recently in-situ combustion and also air injection processes are being applied to reservoirs with pressures in excess of 200 bar. Thus there is a need for the chemical reactions to be studied at the high pressures more likely to be encountered in a reservoir in-situ combustion process. This paper reports a new high pressure flow reactor specially designed for analyzing the specific features of in-situ combustion and air injection in consolidated core materials at elevated pressures up to 100 bar. Oil Oxidation Reactions In-situ combustion involves various competing reactions occurring over different temperature ranges. Low-

Kazi et al.

temperature oxidation (LTO) is believed to occur when very little or no carbon oxides are produced in the effluent gas despite the oxygen consumption being at its highest level. The extent of LTO reactions has a significant influence on subsequent fuel lay down (medium-temperature oxidation, MTO) and fuel combustion (HTO) processes. The movement or transition from LTO to HTO during an air flood is determined by the rate of temperature rise. Several workers have developed procedures for determining the oxygen consumed by each of the oxidation regimes, i.e., de-coupling the total oxygen consumption curve into LTO, MTO, and HTO regimes.4-6 Most of the literature on crude oil oxidation kinetics has been largely concerned with HTO occurring during in-situ combustion for the heavy oil recovery process. However, it was also recognized that LTO was important in this process as a precursor, or necessary step, for the formation of fuel prior to combustion.7 Unlike HTO, which produces carbon oxides and water as its primary reaction products, LTO yields mainly water and oxygenated hydrocarbons, but with light oils, LTO produces small quantities of carbon oxides.8 The resulting oxygenated oils can have significantly higher viscosity, and lower volatility than the virgin oils. For many oils and reservoir conditions, especially where the residual oil saturation post-water flood is low, the reactions may be limited to LTO. However, it is essential to be able to determine the boundary between the LTO regime and where full in-situ combustion (HTO) commenced. A screening tool is therefore required to delineate which oils are primarily candidates for LTO and which are primarily candidates for HTO, and to establish the relevant kinetics parameters for each, so that a more detailed evaluation of their asset potential may be made.6 High Pressure Flow Reactor As stated earlier, the purpose of the high-pressure combustion cell reactor facility is to enable physical simulation of the in-situ combustion and air injection processes for oil recovery at reservoir conditions of temperature and pressure. A detailed description is given elsewhere.9 The central units of the system are the reactor cell and high-pressure vessel. The other components provide the necessary gas metering and control, gas analysis, pressure and temperature regulations, and data gathering systems. Figure 1 shows a schematic diagram of the experimental setup. A 500 mL capacity autoclave reactor provided by Baskerville and Lindsay Limited, U.K., was modified to accommodate the in-house flow reactor designed with independent temperature control. The original stirrer (4) Fassihi, M. R.; Brigham, W. E.; Ramey, H. J. Soc. Pet. Eng. J. 1984, 399-407. (5) Shallcross, D. C.; de los Rios, C. F.; Castanier, L. M.; Brigham, W. E. SPE Reservoir Eng. 1991, 6, 287-294. (6) Al-Saffar, H. B.; Price, D.; Soufi, A; Hughes, R. Fuel 1998. In press. (7) Belgrave, J. D.; Moore, R. G.; Ursenbach, M. G.; Bennion, D. W. SPE Adv. Technol. Ser. 1993, 1, 98-107. (8) Kisler, J. P.; Shallcross, D. C. Tran. IchemE 1997, 72, 163. (9) Kazi, R. A. A high-pressure kinetics study of the in-situ combustion process for oil recovery. Ph.D. Thesis, University of Salford, U.K., 1995.

High Pressure Flow Reactor

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Figure 1. Flow diagram of the experimental setup.

shaft and magnetic drive supplied with the original autoclave were removed. The original shaft through the top flange was then modified to accommodate highpressure multiple probe seal glands (Conax Buffalo) facilitating four probes through a lava sealant. These probes are for electrical and thermocouple connections to the reactor, which was located within the highpressure vessel as, indicated in Figure 1. A detailed sketch of the high-pressure flow reactor as fabricated is shown in Figure 2. The outside diameter of the reactor was 52.5 mm, some 8.75 mm less than the inside diameter of the autoclave, with the cell inside diameter of 30.5 mm. The reactor cell wall was drilled to accommodate five cartridge heaters of 5 mm diameter. A disk of actual reservoir rock of 30 mm in diameter and of 10 mm in length was placed inside the reactor cell. A brass O-ring of 2.5 mm thickness containing asbestos was then placed on the topside of the disk, to avoid inlet gas bypassing. This ring was then compressed by the inlet casing on tightening three nuts and bolts, which hold the reactor casing together. A 1 mm thick copper gasket was used to seal the disk cell at the two flanges. Once the disk cell was assembled, it was then attached to the gas outlet connection by a swagelock fitting to the gas outlet at the top flange of the autoclave pressure vessel, while the electrical connections were made through the connectors to the heaters. Five cartridge heaters, each of 200 W, were used to heat the cell to a maximum temperature of 625 °C, while the cell assembly was insulated from the outside by two layers of carborundum heat resistance material. Heating of the reactor was regulated by a chromel-alumel

Figure 2. Detailed diagram of high-pressure flow reactor

thermocouple attached to the outside wall of the cell and a temperature controller (Eurotherm programmer 812). A second chromel-alumel thermocouple was situated inside the cell in a position facing the sample disk to measure the sample temperature. A dome type back

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pressure regulator (DR 4MK-4 Hale Hamilton), designed for a working pressure of 415 bar, was positioned downstream from the reactor. The dome of the regulator was then connected through the main control panel to a high-pressure air cylinder, while a high-pressure Bourdon gauge was positioned close to the dome. Pressurizing the dome was manually controlled; this was accomplished by adjusting two needle valves situated at the inlet and outlet of the dome and a high-pressure gas inlet valve controlled by the main control panel board. A bursting disk was also used to ensure a safe and controlled operation. A helical coil made of stainless steel tube was fitted down stream from the reactor in order to trap any oil in the gas stream. This was done to prevent the oil being deposited in the backpressure regulator, and thus blocking the flow path of the effluent gas. After an electrical continuity check and once the system was leak free, the system was pressurized and all operations were carried out through the control panel located outside the reinforced room designed to accommodated high-pressure operations. After setting the operation parameters, i.e., heating rate, maximum temperature, flowrate, etc., the system was then operational. The cooled and dried effluent gas was then analyzed for composition by means of a gas chromatograph. A concentric 1.83 m column (Alltech CTR1-8700) with an inner column of 3.175 mm o.d. packed with a porous polymer mixture, and an outer column of 6.35 mm o.d. filled with activated molecular sieve, was used to measure oxygen, carbon dioxide, carbon monoxide, methane and nitrogen concentrations at room temperature. Helium was used as a carrier gas, while the response of the thermal conductivity detector (TCD) was recorded on a Hewlett-Packard 3390A integrator. Readings of temperature, pressure, and flowrate were taken, simultaneously. Experiments over a range of pressures from 10 to 100 bar were carried out to assess the applicability of the equipment. The results of these tested are reported here. Consolidated Formation. The use of consolidated core material in combustion experiments is desirable for purposes of better simulating reservoir conditions. Consolidated cores may also be potentially of much lower porosity and more uniform than unconsolidated material. Samples from actual reservoir rock cores (Green-brae) were used in this work. These cores were cut to disks of 30 mm in diameter and 10 mm in length. The porosity of the consolidated disk was 22.3%. Impregnation of Disks with Oil. Two different methods have been used to impregnate the core disks with oil: (1) A disk of known weight was placed in the reactor cell and 10-15 cm3 of oil was poured over the top of the disk. Air was then applied to the disk side in contact with oil and the pressure drop across the disk was observed (usually 10 cm water). After the oil had been absorbed into the disk, the disk was removed from the reactor cell. The amount of oil impregnated in the disk was determined by weighing the impregnated disk and the saturation was evaluated using the following formula:

Kazi et al.

oil saturation, % ) volume of oil in sample (cm3) volume of sample (cm3)

100 100 porosity (%)

After each impregnation procedure, the disk cell was washed with dicholoromethane before the impregnated disk was relocated in the cell for combustion experiments. This method is very time-consuming and laborious and was subsequently discounted in favor of vacuum impregnation. (2) For the vacuum impregnation process, the disk was secured in a circular plastic holder which in turn was placed in a flat bed glass filter funnel. A rubber O-ring was situated over the top face of the disk to ensure a complete vacuum. Once the disk was properly placed, a vacuum pump with an additional trap was then switched on. With the aid of a spatula, a very small amount of oil was initially spread on the center of the disk and the oil was allowed to impregnate into the pores. Following the absorption of the first layer of oil, another small amount of oil was placed again on top of the disk; this procedure was repeated for 4 or 5 times. After sufficient oil had been impregnated into the center of the disk, the rubber O-ring was removed and the oil was allowed to dry. During the impregnation procedure, a hot air stream at 30-35 °C was blown onto the disk. This procedure provided a reasonably well saturated disk within 2-3 h. To confirm the uniformity of oil distribution through the disk, a few impregnated disks were broken and the cross section examined; the disks were found to be evenly saturated. To avoid excessive oil saturation at the bottom face of the disk, the disks were placed over absorbent paper and repeatedly dried under vacuum till the bottom face was seen to be dried. This method gave an oil saturation of around 65 to 75% of the total pore volume and around 8 to 9 wt %. The disks were re-impregnated and reused up to a maximum of three times. Experimental Results Consolidated porous formations impregnated with Wolf Lake crude oil (10.9° API gravity) were studied. The physical and chemical properties of the oil are shown in Table 1. An effluent gas analysis technique was used to study the reaction kinetics of the crude oil. The method involves heating the consolidated reservoir core disk impregnated with oil, increasing the temperature at a constant rate. A constant and controlled flow rate of air, or enriched oxygen stream, is passed through the mixture during the heating process. The effluent gas was then continuously analyzed for oxygen and carbon oxides content. The experiments were conducted with a temperature ramp of 5 °C/min, from room temperature to 550 °C, and most of the experiments were carried out at a gas flux of 18 m3/h m2. The latter was selected because of its relative insensitivity to heating rate variations on the extent of fuel deposition. For comparison purposes, tests were also carried out on unconsolidated (crushed) core impregnated with oil and placed on a fine mesh plate located in the same position, as was the disk in the reactor cell. All other conditions and procedures were the same.



Table 1. Physical and Chemical Properties of Wolf Lake Crude Oil API gravity density 15 kg/m 60 kg/m elemental analysis (wt %) carbon hydrogen nitrogen oxygen sulphur H/C metals (ppm) vanadium nickel hydrocarbon type (wt %) saturates aromatics resins A resins B

10.95 992.5 995 83.9 10.5 0.5 0.6 4.3 1.5 62 178 10.8 ( 0.5 61.2 ( 2.5 20.7 ( 0.9 0.6 ( 0.1

Figure 3. Consolidated impregnated disc with Wolf Lake oil, 9.06% by weight. Experiment carried out at atmospheric pressure.

(i) Consolidated Core. Figure 3 shows the oxygen consumed, effluent gas composition, and temperature profiles as a function of time for a typical nonisothermal experiment conducted at atmospheric pressure on the consolidated formation impregnated with 9.06 by weight of Wolf Lake oil. Usually effluent gas analysis data should show two oxygen consumption peaks at low and high temperatures, i.e., LTO and HTO. The results in Figure 3 show unusual behavior in that no LTO reactions are observed as identified by the absence of the oxygen consumption peak at lower temperatures. On the other hand, LTO reaction were much more extensive in experiments involving the unconsolidated formation (green-brae core impregnated with Wolf Lake oil) under the same operating conditions (Figure 4). It is believed that the increased bed thickness (by 50%) and looser packing in the unconsolidated formation compared with that of the consolidated formation gave better accessibility to the oil which favored the occurrence of LTO reactions. LTO peaks were observed in all the experiments conducted using unconsolidated formations and also on consolidated formations, which had been only partially impregnated with oil, Figure 5. The appearance of LTO in this partially filled disk is attributed to gradual oil displacement over the dry portion of the formation, thereby increasing the accessibility of oxygen to the residual oil and favoring LTO.

Figure 4. Unconsolidated formation impregnated with Wolf Lake oil, 10% by weight. Experiment carried out at atmospheric pressure.

Figure 5. Consolidated disc impregnated to 30% at top in thin layer with Wolf Lake oil, 4% by weight. Experiment carried out at atmospheric pressure. Table 2. Pressures and Oxygen Concentrations Used in the Experimental Study total pressure (bar)

O2 concentration (%)

1.0 35.5 69.9 97.5 1.0 35.5 69.9 97.5 1.0 35.5 69.9 97.5 1.0 35.5 69.9 97.5

10 10 10 10 21.0 21.0 21.0 21.0 30 30 30 30 50 50 50 50

(ii) Total Pressure and Partial Pressure. The effect of pressure on the oxidation kinetics of consolidated core samples was also during this study. The effect of pressure can be divided into two separate categories; total pressure and partial pressure of oxygen. A range of total pressures from 1 to 100 bar and oxygen concentrations from 10% to 50% were investigated; details are presented in Table 2. Figure 6 shows oxygen consumption versus temperature curves at different pressures. It was anticipated that the higher pressures would suppress the amount of distillation occurring at lower temperatures, and

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Figure 6. Oxygen consumption at various pressures.

Figure 7. Coke burned as a fuel at different total pressures.

hence, more fuel would remain for oxidation resulting in a higher rate of oxygen uptake in the LTO region. However, increasing the pressure shows no appreciable effect on oxygen consumption in the LTO region, since a small peak only was observed at a pressure of 97.5 bar. Similar results were observed for various oxygen concentrations at low and medium temperatures. Similarly, variation in total pressure had little effect on CO2 production. However, CO production was reduced from 7% at atmospheric pressure to 1.5% at 97.5 bar, probably due to the increase in combustion efficiency with total pressure. Figure 7 shows the amount of carbon burned as fuel at different total pressures. This was estimated by multiplying the area under the sum of the carbon oxides curves by the corresponding gas flow rate. For 10%, 21%, and 30% oxygen concentration and for pressures up to 70 bar, there was an increasing trend in the amount of carbon burned. Normally, an increase in the total pressure increases the amount of coke formed and accordingly increases oxygen consumption.10-12 However, on further pressure increase, the amount of coke burned showed a decreasing trend. Furthermore, at 50% oxygen concentration, the reduction in the amount of coke burned was observed earlier than that with other (10) Moore, R. G.; Bennion, D. W.; Belgrave, J. D.; Gle, D. N.; Ursenbach, M. G. J. Pet. Technol. 1990, 916-923. (11) Abu Khamsin, S. A.; Brigham, W. E.; Ramy, H. J. Soc. Pet. Eng. Res. Eng. 1988, 1308-1316. (12) Showalter, W. E. Soc. Pet. Eng. J. 1963, 53-58.

Kazi et al.

Figure 8. Effect of oxygen partial pressure on oxygen consumption. Experiment carried out at total pressure of 35.5 bar.

oxygen concentrations. The decline in coke burned could be attributed to CO2 being absorbed, causing the oil to swell and become more mobile, and consequently, resulting in a decrease in fuel deposition. Dugdale13 carried out a study on the minimum miscibility pressure of CO2 in oil. He noted that the minimum miscibility was inversely related to the total amount of C5 through C30 hydrocarbons present in the crude. It was concluded that aromatics lower the minimum miscibility pressure. Wolf Lake oil used in this study contains 61.2% aromatics, which may cause an increased swelling of the oil and increase its mobility. Another reason for the decline of the amount of coke burned is the low injection gas flux (due to high pressure the gas residence time is increased). The oxidation reaction is mass transfer controlled and hence fluxes dependent. It has been suggested that the reduction in gas interstitial velocity at higher pressures would be accompanied by a decrease in global oxygen uptake rate.10 A decrease in coke loading with increased pressure and oxygen enrichment for heavy and light oils were also observed by others.14,15 Figure 8 shows the effect of oxygen partial pressure on oxygen consumption at a total pressure of 35.5 bar. Here the rate of maximum oxygen consumption is seen to increase with oxygen partial pressure. However, as oxygen concentration increased from 10% to 50%, the peak temperature decreased from about 500 to 370 °C. This indicates that the increase in oxygen concentration caused an early combustion of the hydrocarbons. The exothermicity available from this reaction increases with oxygen partial pressure before the boost due to coke combustion which starts around 480 °C. At a total pressure of 97.5 bar, the rate of oxygen consumption decreased with the increase in the oxygen partial pressure, Figure 9. The increase in total pressure diminishes the influence of oxygen concentration. One possible explanation is that oversaturation of oxygen species in the oil has decreased the global oxygen uptake rate. This effect could be attributed to the dominant (13) Dugdale, P. J. Comparison of recovery and economics for oxygen and air fire flooding in Canadian heavy oil areas. Presented at the SPE/DOE 5th Symposium, Tulsa, OK, April 20-23, 1986; pp 529567 (Paper 14921). (14) Shahani, G. H.; Hansel, G. J. Soc. Pet. Eng. Res. Eng. 1987, 583-590. (15) Hansel, G. J.; Benning, M. A.; Farbacher, J. M. J. Pet. Technol. 1984, 1139-1144.



ratios are shown in Figure 10. The lower the value of H/C, the more coke-like is the deposited fuel. A variation of (0.5 in the H/C ratio, compared to that of the parent oil (H/C)1.5) was observed in 10%, 21%, and 30% oxygen atmospheres. However, in 50% oxygen concentration at pressures above 70 bar, a significant increase in H/C ratio was observed. This increase occurs in the same region for which there was a decrease in the amount of carbon burned. It is believed that a bituminous type material was being oxidized in the HTO region for these particular experiments. Conclusions

Figure 9. Effect of oxygen partial pressure on oxygen consumption. Experiment carried out at a total pressure of 97.5 bar.

Figure 10. Hydrogen-carbon ratio (H/C) at different total pressures.

effect of total pressure. At high total pressures, both the amount of coke burnt and oxygen uptake were reduced. The nature of fuel being laid down is normally indicated by the hydrogen/carbon ratio. The effects of both total pressure and oxygen partial pressure on H/C

A new experimental set up has been designed for analyzing the specific features of in-situ combustion in consolidated core materials (actual rock core samples) at pressures up to 100 bar. The purpose of the highpressure flow cell reactor facility is to enable physical simulation of the in-situ combustion and the air injection processes for oil recovery at reservoir conditions of temperature and pressure. The main conclusions drawn from the study are as follows: (1) consolidated cores gave reduced LTO and fuel lay down in comparison with unconsolidated matrix; (2) up to 35.5 bar, oxygen consumption rate increased with oxygen partial pressure. At higher oxygen partial pressures, combustion of hydrocarbon material began at temperatures of about 370 °C, i.e., below that at which coke combustion initiates; (3) above 35.5 bar, the oxygen uptake and the amount of coke burnt decreased; (4) the oxygen partial pressure and total pressure did not influence the low and medium-temperature oxidation reactions. However, the high-temperature oxidation reaction was dependent on the total pressure. Recently, the flow cell reactor has been modified and successfully tested with pressures up to 400 bar. These high-pressure tests will be the subject of forthcoming studies. Acknowledgment. Dr. R. Kazi expresses his appreciation to the Ministry of Education, Government of Pakistan, for its financial support. EF9900177

High-Pressure Flow Reactor: Design and Application to Pertinent Oil

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