Experimental Study on Transport of Ultra-Dispersed Catalyst Particles

Aug 31, 2010 - Energy Fuels 2010, 24, 4980–4988 . DOI:10.1021/ ... Experimental Study on Transport of Ultra-Dispersed Catalyst ... Received April 24...
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Energy Fuels 2010, 24, 4980–4988 Published on Web 08/31/2010

: DOI:10.1021/ef100518r

Experimental Study on Transport of Ultra-Dispersed Catalyst Particles in Porous Media Amir Zamani, Brij Maini,* and Pedro Pereira-Almao Department of Chemical and Petroleum Engineering, University of Calgary 2500 University Drive NW, Calgary, Alberta, Canada, T2N 1N4 Received April 24, 2010. Revised Manuscript Received August 20, 2010

In situ upgrading of heavy oil by catalytic hydrogenation using submicrometer sized dispersed catalysts during thermal recovery is a promising new idea to achieve an environmentally sustainable method for unlocking heavy oil and bitumen resources. This requires placement of the ultradispersed catalyst particles deep into the formation where it can accelerate the high-temperature upgrading reactions. The objective of this work was to investigate the feasibility of transporting such ultradispersed catalyst particles through porous rock formations. This paper presents the results of experiments carried out to systematically examine the propagation of ultradispersed catalyst suspensions in sand packs. These experiments involved the injection of submicrometer-sized catalyst particles suspended in oil into a sand pack and analysis of the produced fluid samples and the sand bed. The results show that it is possible to propagate the ultradispersed catalyst suspension through sand beds. However, a fraction of the catalyst particles are retained by the sand (around 14 to 18%), and much higher retention occurs in the entrance region of the bed. Particles appear to be deposited on sand surfaces by an attachment mechanism deep inside the bed, but larger particles appear to be strained by mechanical trapping near the inlet face. The deposition of particles was found to be almost irreversible in the sense that the deposited particles could not be remobilized by reverse flow of the suspending medium.

with thermal recovery methods by using the reservoir as a high-temperature reactor.3-6 It requires placement of the catalyst deep into the oil-sand formation by propagating a nanosized catalyst suspension through the sand. One of the obvious concerns with the aforementioned idea is the feasibility of transportation of nanodispersed catalysts to the desired depth inside the reservoir without causing damage to the formation. Propagation of ultradispersed catalysts suspended in an oil medium over long distances in the reservoir is essential for placing the catalyst in contact with heated oil during the thermal recovery processes. Currently, very little information is available on the flow behavior of such suspensions through porous media. Much of what is known comes from studies of formation damage due to migration of clays and other fines or deep bed filtration of aqueous suspensions in subsurface environments.7-9 However, these earlier results cannot be easily applied to ultradispersed catalysts suspended in oil except in a qualitative sense. Flow of suspended nanoparticles through porous media has not been studied deeply, and there is a serious lack of experimental data in this area. Consequently, the proposed transport of a colloidal dispersion of nanoparticles suspended in oil through a porous medium is fraught with uncertainties and unknown behavior. The purpose of this study was to systematically examine the propagation of ultradispersed catalyst suspensions in porous

1. Introduction In the past decade, the continuing decline of conventional crude oil resources has motivated researchers to investigate various new methods for exploitation of vast heavy oil resources all around the world. Heavy oil deposits are known to be one of the abundant sources of crude oil in the world, especially in Canada, Venezuela, and the United States. However, only a small fraction of these recourses is recoverable using the current in situ and surface mining processes. These processes are usually carried out with substantial environmental cost, which limits their largescale deployment.1,2 In situ upgrading of heavy oil is an innovative idea that promises more environmentally friendly technology and is attracting considerable attention. This idea is based on the possibility of integrating the catalytic hydrogenation reaction *To whom correspondence should be addressed. Tel.: þ1 (403) 2208777. Fax:þ1 (403) 282-3945. E-mail: [email protected]. (1) Alboudwarej, H.; Felix, J.; Taylor, S.; Badry, R.; Bremner, C.; Brough, B. et al. Highlighting Heavy Oil. Oilfield Rev. 2006, 18 (2). (2) Energy Market Assessment, Canada’s Oil Sands: Opportunities and Challenges to 2015: An Update. National Energy Board of Canada: Canada, June 2006. http://www.neb-one.gc.ca/clf-nsi/rnrgynfmtn/ nrgyrprt/lsnd/lsnd-eng.html (accessed on April 2010). (3) Weissman, J. G.; Kessler, R. V. Downhole heavy crude oil hydroprocessing. Appl. Catal., A 1996, 140, 1–16. (4) Weissman, J. G.; Kessler, R. V.; Sawicki, R. A.; Belgrave, J. D. M.; Laureshen, C. J.; Mehta, S. A.; et al. Down-hole catalytic upgrading of heavy crude oil. Energy Fuel 1996, 10, 883–889. (5) Weissman, J. G. Review of processes for downhole catalytic upgrading of heavy crude oil. Fuel Process. Technol. 1997, 50, 199–213. (6) Moore, R. G.; Laureshen, C. J.; Mehta, S. A.; Ursenbach, M. G.; Belgrave, J. D. M.; Weissman, J. G.; et al. A downhole catalytic upgrading process for heavy oil using in situ combustion. J. Can. Petrol. Technol. 1999, 38 (13), 1–8. r 2010 American Chemical Society

(7) Tien, C.; Payatakes, A. C. Advances in deep bed filtration. AIChE J. 1979, 25, 737. (8) Herzig, J. P.; Leclerc, D. M.; Le Goff, P. Flow of suspensions through porous media: application to deep filtration. Ind. Eng. Chem. 1970, 62 (5), 8. (9) Zamani, A.; Maini, B. Flow of Dispersed Particles through Porous Media - Deep Bed Filtration. J. Pet. Sci. Eng. 2009, 69, 71–88.

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differential pressure transmitters (3051S1CD, Rosemount) are connected to the sand pack such that the first five measure the pressure difference along the sand pack in five consecutive length intervals. The length segments, starting from the top, are 0.5, 4, 4.5, 4.5, and 4 in. The last pressure transmitter measures the pressure at the bottom end of the sand-pack. The continuous phase for the preparation of the ultradispersed catalyst suspension was heavy vacuum gas oil (HVGO), and the same metal oxide type catalyst was used in all experiments. An inductively coupled plasma (ICP) instrument (IRIS Intrepid II XDL, Thermo-Instruments) was used for measuring the catalyst concentration in suspension samples. A dynamic light scattering instrument (Zetasizer Nano, Malvern) was used for measuring the size of catalysts dispersed in the medium. 2.2. Experimental Procedure. After checking the integrity of the system, the packed sand pack holder is installed in its place in the experimental rig and overburden pressure is applied and monitored by the pressure gauge on top of the sand pack holder. The sand pack holder is flushed with CO2 and then evacuated for 24 h. Then, it is saturated with the desired fluid for porosity measurement. Transfer vessels are filled with the oil. The oil is in fact the continuous phase which is used for preparing the suspension of nanocatalysts, i.e., heavy vacuum gas oil (HVGO). The HVGO is flowed through the sand pack for several pore volumes to stabilize the system. The effective permeability or absolute permeability is measured by measuring the pressure drop at constant flow rates. The system is then ready for the suspension injection. Once the suspension of ultradispersed catalysts is ready, the concentration of suspended particles and particle size distribution are measured. One cylinder of transfer vessels is filled with HVGO and the other one with ultradispersed catalysts suspension. HVGO is flowed through the sand pack to stabilize the flow conditions. At any desired time, the fluid can be switched from particulate-free HVGO to the ultradispersed catalyst suspension. During the experiment, produced fluid is collected, and the collection time for each sample is recorded. The following parameters are monitored and recorded during the test by a LabVIEW program: pump injection flow rate, pump pressure, total injected volume, differential pressures measured by each pressure transmitter, temperature, and elapsed time for each measurement. The size distribution of particles and the particle concentration in each produced sample are measured subsequently. After finishing the flow experiment, sand samples along the sand pack are extracted and analyzed for the amount of particles inside the sand pack using the inductively coupled plasma instrument. The particle concentration in the sand comprises two parts: (1) particles present in the suspension that are present in the pore space (accumulated catalyst) and (2) particles deposited on the sand surfaces (retained catalyst). 2.3. Suspension Preparation. The catalyst suspensions used in these tests were prepared using the microemulsion technique. A water-in-oil microemulsion containing a precursor salt in the water phase was prepared and then heated to a high temperature in a flow system that ensured rapid heating to a high temperature. The heating resulted in quick decomposition of the precursor salt and the complete vaporization

Figure 1. Schematic of the experimental setup.

media. To this end, a well instrumented core flooding rig was developed for experimentation with a flow of ultradispersed particles through a sand pack under simulated reservoir conditions. Particle (catalyst) size and concentration in the inlet and outlet streams and the pressure drop along the sand pack at different locations were recorded. The concentration of retained particles along the sand pack was measured at the end of each experiment. 2. Materials and Methods 2.1. Experimental Setup. The schematic of the experimental rig is shown in Figure 1. Two stainless steel transfer vessels with freely floating pistons were used for injecting the suspensions. The pistons were driven hydraulically by injecting water with a positive displacement pump (Teledyne Isco, E100DM, dual-pump with electric valve continuous flow systems). An annular type sand pack holder, with the capability of applying overburden pressure, was designed and fabricated using the design described by Maini and Nicola.10 It consists of a housing forming an open-ended longitudinal bore and a radially deformable and expandable sleeve, like lead pipe, which is extended through the bore in such a way that an annular space between housing sidewall and sleeve is formed. The sand with an average diameter of 197.41 μm is packed inside the annulus, and open ends are sealed with annular end-caps. Two ports are formed at each end of the annulus so that fluids (suspension) can be injected into the sand pack at one end and discharged from the other end. There are five pressure ports along the housing for communicating the annulus with pressure probes. One thermocouple is placed in the middle of the sand pack holder body for measuring the temperature inside the porous medium. A pressurizing liquid, e.g., water, can be injected into the lead sleeve through one of the overburden ports to radially expand the lead sleeve and compress the sand pack against the housing sidewall. A confining pressure of 900 psi was used in all experiments. The production line, at the bottom of the sand pack, is connected to a back pressure regulator at 500 psi. Samples are collected downstream of the back pressure regulator. Six (10) Maini, B. B.; Nicola, F. C. United States Patent No. 5719327, 1998.

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Table 1. Experiments Specificationsa parameters 3

pore volume, PV (cm ) initial porosity, φ0 (%) irreducible water saturation, Swir (%) absolute permeability, K (darcy) effective permeability of HVGO, Keo (darcy) Viscosity of HVGO, μHVGO (cp) @ 20 °C viscosity of suspension, μSusp (cp) @ 20 °C suspension particle diameter, dp (nm) suspension particle concentration, C (mg/L) a

1st run

2nd run

3rd run

4th run

442.08 38.28 8.39 10.45 8.09 380.7 301.9

390.75 33.84 7.68 7.46 5.26 784.8 785.6 212.2 134.78

407.75 35.31

406.75 35.22

7.32

6.77

380.7 394.0 541.8 145.61

380.7 394.0 554.7 149.80

Bed length, L (cm) = 45.72; bed volume, BV (cm3) = 1154.73; flow rate, q (mL/min) = 2.

of emulsified water.11-14 Initially, a commercial surfactant was used in the microemulsion preparation. In later runs, the microemulsion was prepared using only the natural surfactants present in the oil. 3. Results and Discussion Four different experiments were carried out in this study with the specifications shown in Table 1. In the first and second experiments, fresh water was used as the connate water, and then an oil flood was performed using heavy vacuum gas oil (HVGO). In the third and fourth experiments, the porous medium was saturated with HVGO during the initial imbibition process without the presence of connate water in the system. The fourth experiment included the reverse injection of particulate-free HVGO through the porous medium at the same flow rate as the forward suspension injection. 3.1. First Run. The suspension used in the first run was prepared using a commercial surfactant in the microemulsion-based technique. It was believed that the commercial surfactant would be completely decomposed in the preparation process and there would be no trace of surfactant left in the final prepared suspension. However, during the injection of the suspension through the sand pack at 0.67 pore volumes injected (PVI), the produced dark HVGO changed to a milky colored emulsion unexpectedly. The emulsion production continued until 3.22 PVI, after which it changed back to a dark colored suspension to the end of the experiment. Figure 2 shows some of the produced samples. The emulsion production clearly shows the presence of the surfactant in the suspension and removal of the connate water from the grain surfaces by the surfactant. It became apparent that the surfactant was not completely removed during the decomposition stage of the suspension preparation. This was confirmed by a simple test. Two drops of water was added to about 10 mL of the original suspension (i.e., catalyst suspension before injection through the system) in a vial, and the vial was mildly shaken by hand. Figure 3 shows the produced sample after shaking.

Figure 2. Emulsion production during injection of the catalyst suspension in the first run.

Figure 3. Emulsion produced after adding two droplets of water to the first run catalyst suspension.

Emulsification occurred by simply adding water and mild shaking. Large particles that settled on the bottom face of the vial show the presence of micrometer-sized particles inside the suspension and probable agglomeration and sedimentation of agglomerated particles after emulsification. From these observations, one can expect several phenomena to occur during the flow of such catalyst suspensions: (1) emulsification inside the sand pack, (2) straining of macroparticles at the entrance face, (3) filtration of larger particles, and (4) probable filtration of small particles. Figure 4 shows the effluent particle size versus PVI. The DLS measured average particle diameter in the original suspension as 16.21 nm. However, this average diameter may not be totally reliable since the DLS was tuned to measure submicrometer particles suspended in HVGO. The subsequent observation of sedimentation of larger particles to the bottom of the storage

(11) Thompson, J.; Vasquez, A.; Hill, J. M.; Pereira-Almao, P. The synthesis and evaluation of up-scalable molybdenum based ultra dispersed catalysts: effect of temperature on particle size. Catal. Lett. 2008, 123, 16–23. (12) Pereira-Almao, P. R.; Ali-Marcano, V.; Lopez-Linares, F.; Vasquez, A. Ultradispersed catalyst compositions and methods of preparation. Patent WO 2007/059621 A1, 2007. (13) Vasquez, A. Synthesis, characterization and model reactivity of ultradispersed catalysts for hydroprocessing. M.Sc. Thesis, Department of Chemical and Petroleum Engineering, University of Calgary, Calgary, Alberta, Canada, Feb 2007. (14) Galarraga, C. E.; Pereira-Almao, P. Hydrocracking of Athabasca Bitumen Using Submicronic Multimetallic Catalysts at Near InReservoir Conditions. Energy Fuels 2010, 24, 2383–2389.

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Figure 4. Effluent particle size versus PVI (first run).

Figure 6. Effluent particle size versus PVI (second run).

increases manifolds over 4.6 PVI. Notwithstanding the hump in other pressure drops (which is most likely related to the emulsification), one can see that there is not a large difference between the final pressure drop (at 4.6 PVI) and the pressure drop at the beginning of the experiment in other segments. Clearly, there was more deposition in the inlet part of the sand pack compared to other parts. The polydispersity of the prepared suspension in this experiment does not allow us to say whether a real nanosized catalyst suspension would cause severe formation damage. Bridging of big particles at the pore throats can also increase the removal of smaller size particles; nevertheless, the particle size profile shows the continued production of nanocatalyst particles in the effluent. 3.2. Second Run. The suspension used in this run was prepared without using any added surfactant. A more uniform submicrometer-sized catalyst suspension was produced. As Table 1 shows, the average size of the particle (catalyst) diameter was 212.2 nm, and the suspension concentration before injection through the system was 134.78 ppm (mg/L). After saturating the sand pack with deionized water and measuring the porosity, the test was started the same way as in the first run with the injection of HVGO for several pore volumes followed by switching to catalyst suspension injection at 2 mL/min. Figure 6 shows the produced particles’ sizes versus PVI. It is evident from the graph that nanoparticles have propagated through the porous medium. A comparison between the produced samples’ particle sizes and the size of the original suspension shows a reduction in particle size after passage through the sand pack. Figure 7 presents the concentration profile of the produced samples versus PVI. The dashed line shows the original suspension concentration. As can be seen, the concentration of produced samples increases rapidly around 1 PVI, and then it remains almost constant after 2.5 PVI to the end of the process. The difference between the produced samples’ concentration and the inlet concentration at the end of the experiment shows a continuous filtration of particles. Figure 8 shows pressure drops in different segments of the sand pack versus PVI. As we can see from the figure, unlike the first run, the pressure drop in the first half inch of the sand pack (DP1) stays almost constant, suggesting little or no straining of particles in the entrance section of the sand pack. There is a small increase in DP2 and some variation in DP3, DP4, and DP5. However, it is evident that the

Figure 5. Pressure drop along the sand pack versus PVI (first run).

container suggests that much larger particles were present in the suspension. Figure 4 shows that particles up to 160 nm traveled through the sand pack and were produced at the outlet. It appears that during the emulsification and emulsion production (from 0.68 to 3.22 PVinj), produced particles were bigger than those produced later (i.e., after 3.22 PVI). It suggests that aggregation of particles is more likely to occur during emulsification. It is likely that the emulsification started immediately when the suspension was injected, but it took time to travel the length of the sand pack. In the first experiment, we measured the concentration of produced samples by using an oil-based ICP technique that involved the injection of a highly diluted suspension into the instrument. It was later found to be not fully reliable, and a new aqueous-based technique was developed for subsequent tests. As was mentioned before, the pressure drop along the sand pack was measured across five different length segments. Figure 5 illustrates the pressure drop across each interval from the top part (DP1) to the bottom part (DP5) of the sand pack. The most prominent profile in this figure is for DP1, which depicts the pressure drop in the first half inch of the sand pack, in other words, the entrance part of the sand pack. As can be expected from the presence of large particles inside the suspension, DP1 clearly shows the straining of big particles at the inlet face of the sand pack. Surface blockage is dominant in this experiment as the inlet pressure drop 4983

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Although the retention near the entrance of the sand pack was more pronounced compared to the other parts; Figure 8 showed that, unlike the first run, the pressure drop did not increase in the entrance section. It means there was no particle straining and pore throat blockage at the sand face. Table 2 shows the mass balance for different experiments. As we see from the second run analysis, part of the injected catalyst remained in the sand pack, either as deposited particles on the sand (which we call catalyst retention) or in the form of suspended particles in the fluid that remains in the medium at the end of the test (which we refer to as accumulation). The amount of catalyst present in the pores as a suspension (accumulation) was estimated from the volume of suspension in the sand pack and the average concentration of catalyst in the suspension. The average concentration was taken as the average of injected concentration and the effluent concentration at the end of the test. The difference in mass of injected and produced catalyst shows that 31.65% of the injected catalyst remained in the sand pack. Out of this, 13.7% of the injected catalyst was retained as deposits on the surfaces. The analysis of sand samples extracted from the sand pack after completion of the test showed that 32.16% of the injected catalyst remained in the sand pack, including 14.21% as deposited on the surfaces. As we can see, two different mass balances are in close agreement with each other. 3.3. Third Run. In oil-wet reservoirs, the connate water on the sand surface would not exist. To examine the influence of connate water on the filtration behavior, the next test was conducted without any connate water in the sand pack. A new sand pack was directly saturated with the heavy vacuum gas oil. The same overburden pressure (900 psi) was applied on the sand, and absolute permeability was measured by flowing HVGO through the porous media at several constant flow rates and measuring the pressure drop along the sand pack. As listed in Table 1, the concentration of the catalysts suspension was 145.61 ppm (mg/L) and the average size of catalyst particles was 541.8 nm. Figure 10 shows the average particles’ sizes in the effluent samples as a function of pore volumes injected. It is seen that the average particle size in the effluent is similar to that in the injected suspension, and particles as large as 500 nm pass through the sand pack in the absence of connate water. In the second run (with connate water), produced samples had a smaller average size compared to that in the injected suspension. Figure 11 shows the catalyst concentration in the effluent samples. The concentration increases rapidly to around 100 ppm at 1.5 PVI. The concentration profile reaches a plateau after 3 PVI and stays around 125 ppm to the end of the experiment. Similar to the second run, some filtration of catalyst particles continues to the end, yielding an almost constant effluent concentration close to (but somewhat smaller than) the inlet concentration at the end of the experiment. It would be interesting to see what will happen after many more pore volumes of continued injection; however, this was not undertaken in this work due to difficulties in preparing a much larger volume of suspension with the currently used preparation method. Figure 12 shows pressure drop profiles in different length segments of the sand pack. DP1 increases slowly from 2.2 to 4.3 psi over 5.2 PVI. This translates to a 50% reduction in permeability of the front section. There is a small initial increase in pressure drops related to the change of the fluid

Figure 7. Particle concentration of produced samples versus PVI (second run).

Figure 8. Pressure drop along the sand pack versus PVI (second run).

Figure 9. Specific deposit profile along the sand pack (second run).

propagation of the nanodispersed catalyst has not caused significant permeability damage in the sand pack. Figure 9 shows the specific deposit (i.e., amount of particles retained in the porous medium per unit bed volume) versus sand pack length, measured after finishing the flow experiment. As can be seen from the figure, much higher particle retention occurred near the entrance segment of the sand pack. 4984

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Figure 10. Effluent particle size versus PVI (third run).

Figure 12. Pressure drop along the sand pack versus PVI (third run).

Figure 11. Particle concentration of produced samples versus PVI (third run).

Figure 13. Specific deposit profile along the sand pack (third run).

from HVGO to the more viscous catalyst suspension. After 1 PVI, all pressure drop profiles except DP1 stay almost constant. Minor changes are attributed to changes in room temperature. So, except for the sizable increase in the pressure drop in the entrance part of the sand pack, there is no significant pressure drop increase along the sand pack length during the flow of the catalyst suspension. In other words, the particles that were able to pass through the entrance section were able to flow through the rest of the sand pack without causing any permeability damage. This strongly suggests that the permeability damage in the front section of the sand pack was caused by large agglomerated particles and the truly nanodispersed particles do not cause any damage.

After extracting samples from one side of the sand pack and measuring the amount of retained particles, the sample extraction in the opposite radial side of the sand pack was performed to check for the consistency of the results. As Figure 13 shows, repeating the sampling for half of the sand pack confirmed the repeatability of the extraction method and the sample analysis. Figure 13 shows that particles continue to be filtered out deep inside the porous medium, but higher retention occurs in the entrance region of the sand pack, which is partly due to the straining of large particles. As Table 2 shows, very close agreement was obtained in the mass balance for the third run. The total retention of catalyst particles in the third run was higher than that in the 4985

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Figure 14. Effluent particle size versus PVI during forward suspension and reverse HVGO injection (fourth run).

Figure 15. Particle concentration of produced samples versus PVI during forward suspension and reverse HVGO injection (fourth run).

second run. This appears to be due to the absence of a wetting layer of water on the surfaces of sand grains, which can affect the attachment of catalyst particles to sand surfaces. 3.4. Fourth Run. The objective of the fourth run was to test whether or not the retained catalyst particles could be flushed out from the sand pack by the injection of pure HVGO in the reversed direction of flow. The logic behind this was that the large agglomerated particles captured by a straining mechanism would be easily flushed out by the reversed flow, while the particles attached to pore walls will be less likely to be mobilized. Thus, this test was expected to reveal whether or not the large retention of particles in the entrance region of the sand pack was mostly due to the straining capture of agglomerated particles. Three liters of the same prepared catalyst suspension that was used in the third run was kept in a freezer to minimize the chance of particle agglomeration during storage. After taking the sample out of the freezer and keeping it at room temperature, the average particle size was measured by DLS. The average size was close to that of the third run (Table 1). A new sand pack with the same packing conditions as in the third run was prepared, and then the test was run with the same injection rate and period of time as the third run (Table 1). After finishing the forward injection of the suspension, we closed the inlet and outlet valves and reconfigured the flow lines to prepare for the injection of pure HVGO in the reversed direction of flow. The heavy vacuum gas oil without any suspended particles was injected in the reverse direction through the sand pack, and the effluent samples were collected at different times. Sample collection was more frequent in the beginning. Figure 14 shows the particle size of produced samples during the fourth experiment. The average particle size of the produced samples during direct injection of the catalyst suspension is initially smaller than the original suspension particle size (554.7 nm) but later becomes similar to the injected size. This confirms the third run observation that submicrometer-sized particles can propagate through the sand. The average size of produced samples during reverse injection of the clean HVGO was smaller than the originally injected size but increased from less than 300 nm to about 440 nm. It suggests that the larger particles were preferentially retained. After one pore volume of reversed injection, the DLS was not able to detect particles in the produced samples because of the greatly reduced particle concentration in the effluent.

Figure 15 presents the concentration of produced samples during forward suspension injection and reverse clean HVGO injection. During the forward injection, particle filtration decreases quickly, and the effluent concentration reaches a plateau like in the second and third experiments. At the beginning of the reversed HVGO injection, the concentration of produced samples jumps very quickly to 180 ppm and then declines to 113 ppm at 0.84 PVI. The concentration decrease becomes sharper around 1 PV injection in the reversed direction and reaches 6.88 ppm around 1.6 PVI. The production of particles during the reverse injection of HVGO continued to the end of the experiment, the concentration decreasing slowly to 4 ppm. It is clear that during the reversed injection of clean HVGO, the particles suspended in the pore fluid along with some dislodged particles have been produced. The production of a higher than injected concentration during the initial period of reversed injection can be explained only by the dislodging of some retained particles. However, the rapid decline in the concentration of produced samples during reverse HVGO injection and continued production of a small concentration show very slow mobilization of retained particles from the sand pack. This can be confirmed by comparing the change in the retention profiles after the reverse injection of clean HVGO (end of fourth run) and the direct injection of catalyst suspension (end of third run) in the material balance discussion. Regardless of what happens during the reverse injection, one should expect to see comparable concentration profiles between the third experiment and the forward injection part of the fourth experiment. Figure 16 shows a nearly perfect match between the two aforementioned profiles. Very similar filtration behavior is seen in both experiments as they had similar original suspensions, flow conditions, and sand pack properties. This confirms the repeatability of the methodologies used in the experimentation, sampling, ICP calibration, and sample analysis. Figure 17a illustrates the pressure drop profiles along different segments of the sand pack during the forward injection of catalyst suspension. During this run, the room temperature was almost constant at 21.1 °C. Like the previous runs, an initial increase in the pressure drop up to 1 PVI is related to the change of the fluid from HVGO to a more viscous catalyst suspension. The pressure drop in all segments except the entrance region remained constant 4986

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Figure 16. Comparison between the concentration profiles of the third experiment and forward suspension injection part of the fourth experiment.

Figure 18. Specific deposit profile along the sand pack (fourth run).

injection compared to the end of the forward injection. This decrease occurred very quickly and confirms that the increase in the pressure drop during the forward injection is due to the plugging of pore throats by the straining capture of larger agglomerated particles near the sand face. Figure 18 shows the specific deposit profile along the sand pack for both the third and fourth runs. As we can see from the figure, the fourth run profile matches the third run except in the first 6 cm at the entrance. A comparison between two profiles demonstrates that one-third of the particles retained in the inlet section and a small portion of the particles in the downstream sections of the sand pack were flushed out by reverse HVGO injection. However, particles retained deep inside the sand pack have not been affected by the reverse HVGO injection. This suggests that the retention of nanoparticles during the propagation of ultradispersed catalyst suspension through a porous media is mainly attributed to the attachment of particles on the grain surfaces, which is similar to an adsorption process. The inlet part of the sand pack can have both surface adsorption and mechanical trapping. Consequently, the deposition mechanism involved in the retention of nanodispersed catalysts in porous media is not readily reversible. However, since a small concentration of the catalyst continued to be produced in the effluent during the reverse injection, even after five pore volumes of injection, the surface adsorption is not totally permanent. Table 2 shows a close agreement between the two methods used for calculating the retained catalyst amount at the end of the reverse injection of the fourth run. A comparison between the percentages of retained particles in the fourth run and the third run indicates that only around 2% of the retained particles were recovered by flowing the HVGO in the reverse direction. In other words, the retention is practically irreversible.

Figure 17. Pressure drop along the sand pack versus PVI during (a) forward injection of the catalyst suspension and (b) reverse injection of clean vacuum gas oil (fourth run).

4. Conclusions

during the forward injection. The entrance region pressure drop (DP1) increased by approximately 50%. Figure 17b shows that during reverse injection of clean VGO through the sand pack the pressure drop in different sections of the sand pack does not change dramatically. The change during the first pore volume injection in DP2, 3, 4, and 5 is again related to the change of fluid viscosity from a suspension to the less viscous HVGO. The pressure drop in the entrance region was much lower during the reversed

Residual commercial surfactant left over in the catalysts suspension caused the emulsification of connate water during flow of the suspension through the sand pack, which could have introduced extraneous effects in that test. The emulsification of connate water caused by the surfactant in the ultradispersed catalyst suspension caused a pressure drop increase, which is unrelated to the deposition of particles. Such an emulsion-related pressure drop increase disappeared when the emulsion had been displaced out of the sand pack. 4987

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Surface straining and mechanical trapping at the pore throats resulting in a dramatically increased pressure drop near the inlet face of the sand pack were dominant in the flow of a polydispersed suspension with micrometer-sized particles. Nanodispersed catalysts were able to propagate through the sand, but larger (micrometer size) agglomerated particles were filtered out and caused permeability damage. The deposition (filtration) rate of particles decreased with the time, but the steady-state effluent concentration remained 10% to 20% lower than the injected concentration. The retention of nanoparticles (at low concentration) had a negligible effect on the pressure drop and caused no permeability damage.

In the absence of connate water, the retained particles were more uniformly distributed along the length of the core. Reverse flow removed a very small amount of retained particles, mostly from the inlet portion of the sand pack. Retention of the nanodispersed catalysts inside the sand pack is mainly attributed to adsorption and is not reversible. Acknowledgment. The authors thank the Alberta Ingenuity Centre for In Situ Energy and the sponsor companies for financial support and permission to publish this paper. Special thanks to Dr. Carlos Scott and Carmen Galarraga for preparation of ultradispersed catalysts suspension and Dr. Nashaat Nassar for helping in ICP analysis. The great support of the Schulich School of Engineering Machine Shop at the University of Calgary in developing the experimental rig is gratefully acknowledged.

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