Performance Enhancement of Vapex by Varying the Propane Injection

Apr 13, 2012 - To that end, lab-scale experiments were designed and carried out to investigate this concept. Experiments were performed with two diffe...
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Performance Enhancement of Vapex by Varying the Propane Injection Pressure with Time Hameed Muhamad, Simant R. Upreti,* Ali Lohi, and Huu Doan Department of Chemical Engineering, Ryerson University, 350 Victoria Street, Toronto, Ontario M5B 2K3, Canada ABSTRACT: Vapex or vapor extraction is an emerging green technology for heavy oil recovery. However, the oil production rates with Vapex are lower than those with the conventional recovery processes. This work aims at enhancing the oil production rates by investigating the effect of varying the injection pressure of solvent propane with time. For this purpose, experiments were designed and performed by injecting pure propane at injection pressures of 482.6, 551.6, 620.5, and 689.5 kPa and 21 °C into lab-scale physical models of heavy oil reservoirs. The physical models were packed with a porous medium and saturated with heavy oil. Three different permeabilities of the porous medium were used with heavy oils of two different viscosities and bed heights. The experiments were performed using different policies of solvent injection pressure versus time. Pressure variations were introduced by sudden release and re-injection of the solvent gas. In comparison to constant injection pressure, the pressure pulsing enhanced the oil production rate by 20−30%.

1. INTRODUCTION With the increasing worldwide demand for fossil energy sources, heavy oil and bitumen represent a significant energy supply to meet this demand. As conventional crude oil reserves are becoming consumed, the world focus is on the heavy oil and bitumen resources in Canada and Venezuela to meet the evermore increasing demands for energy and petroleum products. The importance of unconventional oil reserves (heavy oil and bitumen) has increased because of their much higher in-place volumes. The enormous heavy oil and bitumen deposits in the world are estimated to be approximately 4800 billion barrels in-place,1 of which most of them reside in Canada. The main challenge in the exploitation of heavy oil resources is an effective oil recovery process to mobilize the oil in the reservoir. Until the advent of horizontal wells, heavy oil was considered too viscous to flow and be recovered economically at the reservoir temperature. Water flooding without heat does not enhance recovery because water does not mobilize the oil. On the other hand, viscosity decreases greatly with increases in the temperature. As a result, thermal processes, such as steamassisted gravity drainage (SAGD) and cyclic steam stimulation (CSS), have been applied to some extent in heavy oil fields. However, it is questionable whether these methods are sufficient and economical in reservoirs with large heat requirements, specifically in some reservoirs with thin pay zone, low thermal conductivity, high water saturation, or bottom water aquifers.2,3 Moreover, steam generation facilities account for about 30% of the capital cost in SAGD.4 Steam production also requires a large source of water. In addition, a significant amount of surface equipment is required to produce steam and separate the produced oil−water mixture. Also, several environmental issues, such as greenhouse gas emissions and effluent water disposal, are associated with the SAGD process.5,6 Vapex was proposed by Butler and Mokrys to recover heavy oil from highly viscous reserves of heavy oil deep inside the © 2012 American Chemical Society

earth crust. In this process, a light hydrocarbon solvent or a solvent mixture is injected into an upper horizontal well inside a reservoir. The absorption of the solvent(s) in the heavy oil reduces its viscosity, thereby causing it to drain into an underlying horizontal production well from where the oil is easily pumped to the surface. The researchers found that oil recovery was even higher when pure propane gas is injected close to its dew point under reservoir conditions.7 These results revealed the suitability of Vapex for effective heavy oil and bitumen recovery from thick as well as frequently occurring thin reservoirs with much smaller energy losses than those with a conventional thermal process, such as SAGD. Especially for thin reservoirs, the conventional recovery methods, such as surface mining, CSS, SAGD, and cold heavy oil production, are not viable. The use of solvents in Vapex alleviates the energy requirements and environmental impacts that plague thermal recovery processes. For example, Vapex uses about 3% of the energy consumed by SAGD and reduces greenhouse gas emission by 80%.8 Because of these reasons, interest in Vapex for heavy oil recovery has grown considerably as a viable and environmentally friendly alternative to the currently used thermal methods. The oil production in Vapex is directly related to the transfer of solvent into the heavy oil. In the presence of solvent, the viscosity of heavy oil reduces, which, in turn, facilitates solvent penetration and mixing with the heavy oil.9 Because the primary mode of solvent transfer is concentration-dependent molecular diffusion, the oil production in Vapex builds up slowly with the solvent concentration. Thus, oil production is slow in the beginning and generally lower than that in SAGD driven by the faster mechanism of thermal diffusion.10 Nonetheless, the advantages of Vapex make it worthwhile to Received: February 3, 2012 Revised: April 6, 2012 Published: April 13, 2012 3514

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pressure vessel directly. The pressure inside the vessel is either kept constant or varied with time. The temperature controller is designed to maintain the temperature within ±0.5 °C of the set point. We used research-grade propane of 99.99% purity (MEGS Specialty Gases Inc., Montreal, Quebec, Canada) as a solvent at the laboratory ambient temperature, which varied between 21 and 22 °C in the experiments. The experimental conditions are recorded as a function of time continuously by the data acquisition system connected to a computer. Labview (version 7.1, National Instruments, Montreal, Quebec, Canada) was used for graphical user interface and online monitoring of the following inputs: (1) two pressure control valves, (2) temperatures of the pressure vessel and water bath, (3) pressure in the pressure vessel, (4) inlet flow of propane, and (5) mass of the physical model. The load cell reading was taken every minute. Physical reservoir models with an inside radius of 3 cm and two different heights of 25 and 45 cm were used to study the effect of the drainage height as well as the effect of variation of the propane injection pressure on the production rate of heavy oil. 2.1. Physical Model Preparation. Two different heavy oils were obtained from Saskatchewan Research Council (SRC), Regina, Saskatchewan, Canada, which had 14 500 and 20 000 mPa s viscosities at 21 °C. The physical reservoir models were carefully prepared to avoid any air from becoming trapped in the simulated reservoir medium of heavy oil and glass beads. The heavy oil was placed in a temperature-controlled heater. The oil was heated for at least 30 min at 70 °C for sufficient reduction in oil viscosity to promote mixing with glass beads. Samples were prepared on the basis of the weight of the beads and the weight of the heavy oil for a given model height. Glass beads of known permeability were gradually added to the heated oil in the form of thin layers, layer by layer, inside the temperaturecontrolled heater. The glass beads in the layer were allowed to settle as a result of gravity before another layer was added to the heated oil. This procedure was repeated until the heavy oil could not take in any more beads. This method of preparing the heavy oil−glass beads mixture (i.e., the simulated reservoir medium) ensured that the heavy oil was fully and homogenously saturated with glass beads without any air bubbles. The mixture thus prepared was packed into a cylindrical wire mesh outlining the physical reservoir model. With this method, we prepared the physical models of 6 cm in diameter and 25 and 45 cm in height for the experiments. Before use in an experiment, a physical model was kept in an air bath for 15 h. This step ensured that the model temperature reached the room temperature of approximately 21 °C. 2.2. Permeability Measurement. We prepared samples of a saturated mixture of heavy oil and glass beads of different permeabilities to study the permeability effect on the production rate. Different glass beads sizes (industrial names BT 3, BT 4, BT 5, and BT 6) were used. The packing material simulating a reservoir was glass beads obtained from Flex-O-Lite, Ltd. (St. Louis, MO). To measure the permeability of the porous media consisting of the heavy oil and glass beads mixture, a horizontal cylindrical physical model having a cavity size of 26 × 4 cm was used. The model setup was filled with the glass beads. The cylinder had two ports: one for the air inlet and one for discharge air with a screen placed at the two sides to avoid any glass bead passage. Two pressure gauges at both ends of the cylinder were used to measure the air pressure drop across the media when air was passed through it. The airflow rate was measured by a flow meter at the outlet. Darcy’s law for single-phase steady-state flow was used to calculate the permeability (K) of the glass beads packing as follows:11

explore different ways to enhance and maximize the oil production rate. Although Vapex has a number of benefits over other thermalbased enhanced oil recovery (EOR) processes, its field implementation is hindered in the need for higher oil production rates. The oil production in Vapex primarily depends upon the solvent mass transfer into the heavy oil phase, which is a combined effect of solvent diffusion, interface renewal, solvent mixing, contact area, and capillary imbibitions. It is the optimization of the associated process parameters that can enhance production in Vapex. Of these parameters, the solvent injection pressure is the one that lends itself to easy manipulation to control the process. In this paper, the enhancement of oil production was investigated by varying the propane injection pressure with time in Vapex experiments. To that end, lab-scale experiments were designed and carried out to investigate this concept. Experiments were performed with two different heavy oils, three different permeabilties, and two physical heights. In these experiments, propane was injected at different pressures below the dew point pressure. Sharp pressure changes (pressure blips) were introduced by sudden release and re-injection of propane.

2. EXPERIMENTAL SECTION The experimental setup mainly consists of a pressure vessel controlled by two proportional control valves (model PV101-10 V, Omega Engineering, Inc., Canada). Figure 1 shows a schematic diagram of the

Figure 1. Schematic diagram of the experimental setup. experimental setup. It comprises a cylindrical pressure vessel of 80 cm in height and 15 cm in inside diameter tubing sealed at both the bottom and top of the pressure vessel. To collect the produced oil, a small carbon steel funnel at the vessel bottom is used. The funnel is connected via a one-way valve to a collection tube to measure the produced oil. The pressure vessel is placed inside a water bath. The pressure vessel is equipped with a load cell and monitoring instruments for the temperature and pressure inside the vessel and the temperature of the water bath. The load cell is used to record the weight of the physical model, which decreases with time as the oil drains out and becomes produced. Propane is supplied via a mass flow meter to the top of the pressurized vessel system, where a pressure transducer measured the system pressure. Two proportional control valves are placed at the upstream propane gas feed line, and the other is attached to the

K=

Pu 1 2μair L P2ΔP

(1)

where P1 and P2 are the pressures at the inlet and outlet of the cylinder, respectively, u2 is the velocity at the outlet, Pm is the mean pressure, ΔP is the pressure difference, μair is the air viscosity at the experimental temperature, and L is the length of the media. 3515

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The permeability of the packing material was also estimated from the particle size diameter using the model by Carman-Kozeny12

K CK =

tested. The porosities of the media used in these experiments were close to 38%. Figure 2 presents the comparison of the cumulative live oil produced versus time for the physical model of 25 cm height

ϕ3Dp2 180(1 − ϕ3)

(2)

where K is the permeability and φ and Dp are the porosity of the medium and the diameter of the particle, respectively. Table 1 shows

Table 1. Permeability of the Glass Beads glass bead type BT BT BT BT

3 4 5 6

average diameter (mm)

estimated KCK (darcy)

experimental K (darcy)

porosity

0.717 0.506 0.334 0.229

427 204 87 40

439.2 220.3 97.4 44.4

0.385 0.38 0.378 0.376

Figure 2. Cumulative live oil production versus time at different permeabilities (model height, 25 cm; heavy oil viscosity, 14 500 mPa s; and pressure, 689.5 kPa).

values of estimated sample permeability as well as the measured permeability measurement for four different permeabilties. 2.3. Experimental Details and Procedure. Before starting each experiment, the load cell was calibrated over the full scale and the pressure vessel was tested for any potential leak. The leak test was conducted by pressurizing the cylindrical pressure vessel with air and leaving it for 24 h. After no pressure drop was confirmed, the top flange of the pressure vessel was opened and the cylindrical model with a saturated heavy oil and glass beads mixture was attached to the load cell (see Figure 1). After sealing the vessel, the leak test was performed again for a short period of time to ensure proper sealing of the vessel. Air was purged from the pressure vessel by applying vacuum close to −15 mmHg using a vacuum pump. To ensure complete displacement of any residual air, the vessel was flushed with propane for 10 min and vacuumed again. Propane was injected into the vessel at a constant pressure of 689.5 kPa (100 psig) corresponding to a temperature of 21 °C. A constant temperature during the experiment was maintained by a water bath (200 cm in height and 150 cm in diameter) made of poly(vinyl chloride). After the water bath was filled to the height of the vessel, water was heated to the temperature of the surroundings. This was performed by circulating water underneath the tank through a heat exchanger. Once steady temperature was attained, the physical model was located inside the pressure vessel and the Vapex oil extraction process was started. The injection pressure was controlled through two control valves installed in the setup. As propane came in contact with the exposed surface of the physical model, it diffused into the heavy oil−glass beads mixture. The presence of hydrocarbons, such as propane, is known to significantly reduce the heavy oil viscosity. This phenomenon makes the heavy oil mobile and drain under the action of gravity. It was observed that, after some time, the heavy oil started to drain out of the physical model and accumulate in the funnel placed at the bottom of the pressure vessel. The production of the live oil was then continued as a result of exposure of the new oil-filled pores to the solvent gas, resulting from boundary layer drainage, and the process continued as a result of gravity drainage, until the production was stopped. The load cell recorded the mass of the physical model every minute as the production continued. At the end of the experiment, the propane supply was shut off. The pressure vessel was vented and flushed with air.

with different permeabilities (427, 204, 87, and 40 darcy). It is observed that both the cumulative oil produced and the live oil production rate decreased with the model permeability. The overall oil recovery among these experiments ranged between 88 and 95% of original oil in place (OOIP). Higher permeability models resulted in higher percent OOIP recovery and production rates. Figure 3 presents the relationship between the live oil production rate and model permeability. The production

Figure 3. Variation of the production rate with model permeability (model height, 25 cm; heavy oil viscosity, 14 500 mPa s; and pressure, 689.5 kPa).

increases with permeability. The data points are fitted by a power function. ṁ = 0.0594K 0.5061

(3)

According to the above equation, the oil production rate (g/min) in the Vapex process is a square root function of the model permeability (darcy). This result is in close agreement to what is reported in the literature.13 3.2. Effect of the Pressure on the Live Oil Production Rate. To evaluate the injection pressure as one of the optimizing parameters for Vapex process enhancement, we performed a number of experiments with different injection pressure strategies as follows: (1) injecting of the propane at different but constant injection pressures and (2) introducing temporal variations in the injection pressure. The experimental results showed that the temporal variation in injection pressure enhanced oil production and improved the

3. RESULTS 3.1. Live Oil Production Rates. The effect of model permeability on oil production rates and recoveries was evaluated for 14 500 mPa s viscosity dead oil. A number of experiments were performed using the 25 cm height model packed with a homogeneous permeability medium. Varied medium permeabilities of 427, 204, 87, and 40 darcy were 3516

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process efficiency compared to constant injection pressure. The details of the results are presented next. 3.3. Effect of the Constant Injection Pressure on Live Oil Production. In the first step, we examined the effect of the constant injection pressure of propane on live oil production close to the dew point. Four experiments were carried out at different constant injection pressures of 482.6, 551.6, 620.5, and 689.5 kPa. All of these experiments were performed using the 25 cm high physical model of 14 500 mPa s heavy oil with a 204 darcy permeability. The experimental temperature was 21 °C, at which the dew point pressure of propane was 751.5 kPa. In each experiment, propane was injected at the given constant pressure. The produced oil was collected and weighed. Figure 4

Figure 5. Cumulative live oil production versus time at long pulse pressure (model height, 25 cm; heavy oil viscosity, 14 500 mPa s; and medium permeability, 204 darcy).

Figure 6. Cumulative live oil production versus time at short pulse pressure (model height, 25 cm; heavy oil viscosity, 14 500 mPa s; and medium permeability, 204 darcy).

Figure 4. Cumulative oil production versus time at different constant injection pressures (model height, 25 cm; heavy oil viscosity, 14 500 mPa s; and medium permeability, 204 darcy).

It is very interesting that the oil production with short pressure blips virtually never drops below the oil production in the base experiment (Figure 6). This finding is in contrast with the experiment that uses long pressure blips (Figure 5). The oil production with long pressure blips is initially found to be lower than the base production for about one-third of the experimental run time. This experimental fact indicates that the duration of the short pressure blip is optimal in that it sufficiently stimulates the oil recovery process without adversely affecting the instantaneous oil production. This phenomenon is particularly noticeable in the first half of the experiment when the oil recovery is beginning to grow. Because of the above fact, we used short pressure blips in the rest of the experiments to examine the effect of different model heights and permeabilities on the oil production rate. In all of these experiments, the oil production never went below the base oil production. 3.4.2. Experiments with Short Pressure Blips and Different Model Heights. To assess the effect of the model height on oil production using short pressure blips, we carried out an additional experiment using a physical model of 45 cm height, heavy oil of 14 500 mPa s viscosity, and medium of 204 darcy permeability. Figure 7 presents the results of this run and compares the trend of the cumulative produced oil to that in the base experiment using a constant injection pressure of 689.5 kPa. It was found that the short pressure blips produced 25% more oil than that in the base experiment. Both Figures 6 and 7 show that short pressure blips in the propane injection pressure significantly enhanced the oil production. The 45 cm model produced 404 g of oil compared to 220 g of oil from the 25 cm model for the first 300 min of both experiments. The results also reveal that the increase in the oil production was more pronounced for

shows the cumulative live oil produced at the four injection pressures. As indicated in the figure, injecting propane close to its dew point pressure results in the highest oil production rate and recovery. A drop in propane pressure below the dew point pressure reduces the oil production rate as well as the overall oil recovery. 3.4. Effect of Variation in the Propane Injection Pressure. In this study, the effect of temporal variations in injection pressure on oil production rates was examined during the Vapex process. Experiments were performed with two model heights (25 and 45 cm), three different permeabilties (204, 87, and 40 darcy), and two different initial dead oil viscosities (14 500 and 20 000 mPa s). Below are the details of these results. 3.4.1. Long and Short Blips in the Injection Pressure. To study the effect of temporal variation time in the injection pressure, we performed the following two experiments: (1) experiment with long pressure blips [in this experiment, the propane injection pressure was instantly reduced several times from 689.5 to 275.8 kPa, kept at 275.8 kPa for about 13 min (“the blip interval”), and raised back to 689.5 kPa] and (2) experiment with short pressure blips (this experiment was similar to the previous experiment but with the blip interval of about 3 min). Figures 5 and 6 compare the cumulative produced oil versus time for the above experiments with the “base experiment” performed at the constant injection pressure of 689.5 kPa. As seen from the figures, the experiment performed with temporal variation in the injection pressure produced more oil compared to the base experiment. While 200 g of cumulative oil was produced with long pressure blips, the experiment with short pressure blips produced 22 g or about 10% more oil. 3517

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production for all three model permeabilities. Moreover, the effect becomes more pronounced with the decrease in the permeability. 3.4.4. Experiments with Short Pressure Blips and Different Dead Oil Viscosities. The solvent mass transfer to the oil phase primarily depends upon the initial dead oil viscosity. The lower viscosity oil is expected to uptake more solvent compared to the higher viscosity oil and, thus, result in a higher oil production and rate. Therefore, it is important to examine the effect of pressure variation on the dead oil viscosity. For this purpose, we performed experiments with permeability of 204 darcy, model height of 25 cm, and two oil viscosities of 14 500 and 20 000 mPa s. Figure 10 presents the result of the experiment with 20 000 mPa s viscosity and compares the oil production to that

Figure 7. Cumulative oil production versus time at pulse injection pressure (model height, 45 cm; heavy oil viscosity, 14 500 mPa s; and medium permeability, 204 darcy).

the model with the larger height (45 cm). The short pressure blips in the case of the 45 cm model resulted in more than 5% unit of recovery of OOIP compared to the 25 cm model. 3.4.3. Experiments with Short Pressure Blips and Different Model Permeabilities. To examine the effect of model permeability on oil production using short pressure blips, we carried out two experiments with medium permeabilities of 87 and 40 darcy, model height of 25 cm, and oil viscosity of 14 500 mPa s. Figures 8 and 9 present the results of these experiments. Figure 10. Cumulative oil production versus time at pulse injection pressure (model height, 25 cm; heavy oil viscosity, 20 000 mPa s; and medium permeability, 204 darcy).

in the base experiment with the constant injection pressure of 689.5 kPa. It is found that the short pressure blips produce 23% more oil compared to constant injection pressure. A comparison of Figure 10 to Figure 6 (for 14 500 mPa s viscosity) shows that the short pressure blips in propane injection pressure generate more oil (relative to the base oil production) when the oil viscosity is higher.

Figure 8. Cumulative oil production versus time at pulse injection pressure (model height, 25 cm; heavy oil viscosity, 14 500 mPa s; and medium permeability, 87 darcy).

4. DISCUSSION A key requirement in Vapex is the injection of the solvent very close to its dew point pressure at reservoir conditions. This requirement allows the solvent to be a dense vapor upon injection that has higher solubility in heavy oils compared to the solvent injected far from the dew point pressure with low-density vapor. A higher solubility results in a lower oil viscosity of the diluted oil that can drain quicker, leading to enhanced oil recovery. This fact was evident by our initial experiments. The rest of the experiments demonstrated, more importantly, that the variation in solvent injection pressure with time can play an important role in enhancing the oil production in Vapex. Among different injection schemes, constant pressure, long blips, and short blips, the last scheme had the most pronounced effect on the oil production and rate. It was observed that the short blips in the solvent injection pressure enhanced oil production, never letting it fall below the base oil production even at the pressure blip. This finding suggests that the short pressure blip optimally stimulates the oil production. The positive effect of pressure variation on the oil production and rate may be ascribed to the following:

Figure 9. Cumulative oil production versus time at pulse injection pressure (model height, 25 cm; heavy oil viscosity, 14 500 mPa s; and medium permeability, 40 darcy).

About 25 and 35% more oil was produced using short pressure blips in comparison to the base experiment with the constant propane injection pressure of 689.5 kPa. A comparison of Figures 8 and 9 to Figure 6 shows that injection pressure variation has a significant effect on oil 3518

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In an experiment with the constant injection pressure, the contact area between the oil and solvent is the surface area of stabilized solvent channels, which preserve most of their configuration throughout the Vapex process. Because the solvent injection pressure is constant, there are no major upsets. Some asphaltenes may precipitate, which is carried away in the oil draining out. A variation in the injection pressure, especially the temporal variation, generates sort of a “seismic” effect within the model. Because of this effect, the solvent not only travels through the uniformly developed channels but also forms new channels within the model. Consequently, there is more area where the solvent can contact the oil and dilute it to flowable viscosities. Thus, the oil production improves when the solvent injection pressure is varied. Moreover, as mentioned earlier for Vapex, solvent mass transfer in the heavy oil phase depends upon the initial oil viscosity. When the pressure drops during a temporal variation in injection pressure, the solvent dissolved in the oil tries to escape. However, because the pressure is not reduced entirely to the gas-phase pressure, there is always a sufficient amount of solvent dissolved in the oil that keeps the oil viscosity in a low range. Upon increasing the pressure back to the dew point pressure and replenishing solvent supply, the solvent mass transfer to the oil is much faster because the solvent now has to dilute lower viscosity oil compared to, initially, much higher viscosity oil. As a result, the oil production increases when the solvent injection pressure is varied. The experimental results of this study also show that the temporal variation in injection pressure is more beneficial for lower permeability models and higher viscosity oils. This result is relevant to heavy oil field reservoirs, which have low permeabilties (4−10 darcy) and high viscosities in millions of centipoises. Further experimental work is definitely required to study the effect of the temporal variation in injection pressure for field-type permeabilities and bitumen-type viscosities. Especially, optimized solvent injection pressure policies can play a vital role for the field implementation of Vapex.

The effect of short and long blips in solvent injection pressure with time with different petrophysical properties was examined by performing a number of experiments with two model heights (25 and 45 cm), three different permeabilties (204, 87, and 40 darcy), and two different initial dead oil viscosities (14 500 and 20 000 mPa s). This study showed that short pressure blips in the propane injection pressure produced more oil than the base case of constant solvent injection pressure. We also found that the short pressure blips are more beneficial for lower permeability models and higher viscosity oils.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: 416-979-5000, ext. 6344. Fax: 416-979-5083. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support of the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Ontario Graduate Scholarship (OGS) Program. Sincere appreciation goes to M. Imran, Research Engineer, Saskatchewan Research Council, Regina, Saskatchewan, Canada.

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REFERENCES

(1) Saniere, A.; Hénaut, I.; Argillier, J. F. Pipeline transportation of heavy oils, a strategic, economic and technological challenge. Oil Gas Sci. Technol. 2004, 59 (5), 455−466. (2) Jiang, Q. Recovery of heavy oil and bitumen using vapex process from homogeneous and heterogeneous reservoirs. Ph.D. Thesis, Department of Chemical and Petroleum Engineering, University of Calgary, Calgary, Alberta, Canada, 1997. (3) Yazdani, J. A.; Maini, B. B. Effect of drainage height and grain size on the convective dispersion in the Vapex process: Experimental study. Proceedings of the Society of Petroleum Engineers (SPE)/Department of Energy (DOE) Symposium on Improved Oil Recovery; Tulsa, OK, April 17−21, 2004; SPE Paper 89409. (4) Das, S. K. Vapex: An effective process for the recovery of heavy oil and bitumen. SPE J. 1998, 232−237. (5) Singhal, A. K.; Das, S. K., Leggitt, S. M.; Kasraie, M.; Ito, Y. Screening of reservoirs for exploitation by application of steam assisted gravity drainage/VAPEX processes. Proceedings of the Society of Petroleum Engineers (SPE) International Conference on Horizontal Well Technology; Calgary, Alberta, Canada, Nov 18−20, 1996. (6) Talbi, K.; Maini, B. B. Evaluation of CO2-based VAPEX process for the recovery of bitumen from tar sand reservoirs. Proceedings of the Society of Petroleum Engineers (SPE) International Improved Oil Recovery Conference in Asia Pacific; Kuala Lumpur, Malaysia, Oct 20−21, 2003; SPE Paper 84868 (7) Butler, R. M.; Mokrys, I. J. A new process (Vapex) for recovering heavy oils using hot water and hydrocarbon vapor. J. Can. Pet. Technol. 1991, 30, 97−106. (8) Upreti, S. R.; Lohi, A.; Kapadia, R. A.; El-Haj, R. Vapor extraction of heavy oil and bitumen: A review. Energy Fuels 2007, 21, 1562−1574. (9) Das, S. K.; Butler, R. M. Mechanism of the vapour extraction process for heavy oil and bitumen. J. Pet. Sci. Eng. 1998, 21 (1), 43−59. (10) Gupta, S.; Gittins, S.; Picherack, P. Insight into some key issues with solvent aided process. J. Can. Pet. Technol. 2003, 43 (2), 54−61. (11) Dullien, F. A. L. Porous Media, Fluid Transport and Structure, 2nd ed.; Academic Press: New York, 1992.

5. CONCLUSION In this experimental study, the effect of model permeability on oil production rates for a Vapex process was evaluated for 14 500 mPa s viscosity dead oil. A cylindrical physical model of 25 cm in height was used. The experiments were performed with four different permeabilities of 427, 204, 87, and 40 darcy and an approximate porosity of 38%. The live oil production rate was found to be the square root function of the model permeability, which is in strong agreement with the literature. To evaluate the injection pressure as one of the parameters to enhance Vapex oil production, the number of experiments was performed with different solvent injection pressure strategies. The sensitivity of injection pressure close to the dew point pressure of the injected propane solvent at the injection conditions was studied by performing four experiments at different constant injection pressures of 482.6, 551.6, 620.5, and 689.5 kPa. All of these experiments were performed at 21 °C with a 25 cm cylindrical model packed with 204 darcy permeability media and saturated with 14 500 mPa s heavy oil. Propane injected close to the dew point pressure (at injection temperature) resulted in the highest oil recovery and oil production rate. 3519

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(12) Abukhalifeh, H.; Upreti, S. R.; Lohi, A. Permeability effect on the concentration-dependent propane dispersion coefficient in Vapex. Int. J. Oil, Gas Coal Technol. 2011, 4, 64−78. (13) Oduntan, A. R.; Chatzis, I.; Smith, J.; Lohi, A. Heavy oil recovery using the VAPEX process: Scale-up issues. Proceedings of the Canadian Petroleum Society’s International Petroleum Conference; Calgary, Alberta, Canada, June 12−14, 2001; Paper 2001-127.

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