Mechanism of the vapor extraction process for heavy oil and bitumen

Journal of Petroleum Science and Engineering 21 Ž1998. 43–59

Mechanism of the vapor extraction process for heavy oil and bitumen Swapan K. Das ) , Roger M. Butler Department of Chemical and Petroleum Engineering, The UniÕersity of Calgary, Calgary, AB, Canada T2N 1N4 Received 20 May 1997; revised 13 November 1997; accepted 13 November 1997

Abstract The vapor extraction process may be suitable for the recovery of huge resources available in the form of highly viscous heavy oil and bitumen. Vaporized hydrocarbon solvents are used to reduce the viscosity; the diluted oil drains by gravity. Low energy consumption, less environmental pollution, in situ upgrading, lower capital costs, etc., make the process superior to the currently used thermal processes. Experiments were carried out in a Hele–Shaw cell and in a scaled packed cell to evaluate the performance of this process and study the mechanism. The experimental results showed that in porous media, the process performs as much as an order of magnitude better than expected from typical calculation using molecular diffusivity alone. In porous media, the process takes place in a contact zone. In this zone, the high-viscosity oil contacts the solvent vapor in the fine capillaries that offer a higher interfacial area of contact. The process involves transient diffusion of solvent into the bitumen at the interface. As soon as the oil at the interface attains mobility due to viscosity reduction, it drains, exposing a new interface of bitumen having a very low concentration of solvent. Surface renewal, aided by capillary imbibition, yields a higher mass transfer rate that enhances the rate of extraction. q 1998 Elsevier Science B.V. All rights reserved. Keywords: vapor extraction; mass transfer; mechanism; heavy oil; bitumen; enhanced recovery

1. Introduction With the decline of light oil reserves, a major thrust of oil industries throughout the world is on the exploitation of the huge resources present in the form of heavy oil and bitumen. These resources are approximately six trillion barrels of oil in place throughout the world, a major part of which is present in Venezuela and Canada. Heavy oil and

)

Corresponding author. Present address: Petroleum Recovery Institute, Calgary, AB, Canada T2L 2A6.

bitumen are characterized by their high viscosities and low-degree API gravities ŽSpeight, 1991.. In some Canadian reservoirs, such as Athabasca, the oil viscosity is in the millions of mPa s at reservoir conditions. In thermal recovery processes, namely, Cyclic Steam Stimulation ŽCSS., In Situ Combustion ŽISC., Steam-Assisted Gravity Drainage ŽSAGD., etc., which are currently being applied to produce these crudes, the viscosity is reduced by heating the reservoir. The maximum recovery in the CSS process seldom exceeds 20%; usually it is followed by a steam flood. For ISC to be successful, the original oil should have sufficiently low viscosity Ž m - 1000

0920-4105r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 9 2 0 - 4 1 0 5 Ž 9 8 . 0 0 0 0 2 - 3

44

S.K. Das, R.M. Butlerr Journal of Petroleum Science and Engineering 21 (1998) 43–59

mPa s. so that it can be displaced away from the combustion zone into the cold portion of the reservoir ŽMoore et al., 1994.. Although the SAGD technique ŽButler et al., 1981; Butler and Stephens, 1981. is successful in exploiting these resources, low energy efficiency, especially in thin reservoirs, can make the process uneconomic. Apart from the energy inefficiency some other important concerns are treatment andror disposal of the effluent water and the related environmental problems, requirement of a huge source for the supply of fresh water, possibility of formation damage by clay swelling, etc. Thus, the quest for a cleaner and more efficient process continues.

2. Solvent methods for extracting heavy oil and bitumen The viscosity of heavy oil and bitumen can also be reduced by the addition of solvents. Thus, energy losses inherent in the steam processes may be avoided if a solvent is used instead of steam. Although various solvents have been studied for the miscible displacement since early fifties, the use of solvents for recovery of heavy oil and bitumen began to evolve in the seventies. Allen Ž1974. patented a ‘huff and puff’ type process in which propane or butane is injected in cycles to extract oil from a cell packed with Athabasca tar sands; in addition, Allen and Redford Ž1976. proposed the injection of a liquid solvent and a non-condensible gas at reservoir temperature and pressure. The purpose of the non-condensable gas was to prevent the formation of an impermeable bitumen bank. A combination of thermal and solvent processes are proposed in other patents ŽBrown et al., 1977; Allen, 1973; etc..; but they are overcomplex for field applications. Another solvent process ŽNenniger, 1979. used pure gases or mixtures at a pressure equal to, or slightly lower than, the saturation vapor pressure. Recommended solvents are carbon dioxide, lower alkanes, e.g., methane, ethane, propane, etc., and alkenes such as ethylene, propylene and isomeric butenes; carbon dioxide and ethane were preferred for their availability and lower cost. All of the solvent extraction processes discussed above consider injection in a vertical well and pro-

duction, from the same well or through another closely spaced vertical well. With this arrangement and the slow rate of solvent diffusion in bitumen, the production rates are usually below economic level; this is probably why these processes have not been applied in field operations. In the eighties, the horizontal wells were introduced and used in the SAGD process to enhance the production rate. Dunn et al. Ž1989. applied the principle of SAGD for both theoretical and experimental studies of bitumen recovery by gravity drainage using low-temperature soluble-gas injection. Experiments were carried out with Athabasca bitumen in a glass bead-packed cell. Carbon dioxide and ethane at pressures below, but close to, the respective dew point pressures were used as solvents. Both continuous injection–production and pressure-cycling were used. To match the experimental results using the theoretical model, it was necessary to use an effective diffusivity, two to three orders of magnitude higher than the reported molecular diffusivity. This could not be explained using correlations available for dispersion. Although absolute production rates for field conditions were not reported, they turned out to be very low. Butler and Mokrys Ž1989. studied the extraction of untreated Athabasca bitumen and Suncor coker feed bitumen with toluene Žliquid. in a line source Hele–Shaw cell. A solvent analog model of SAGD was developed to analyze the results. It was concluded that due to the orders of magnitude difference in molecular and thermal diffusivities, the solventleaching would be much slower than the thermal processes. It appeared that the use of vaporized solvent in combination with deasphalting may enhance the rate considerably.

3. The vapor extraction process The above considerations lead to the idea of the extraction of heavy oil and bitumen using vaporized hydrocarbon solvents. The concept of the Vapor Extraction ŽVAPEX. process is presented in Fig. 1, which shows a vertical cross-section of the reservoir. The vaporized solvent is injected into the reservoir using a horizontal injection well at A, and oil is produced through the horizontal production well at B, placed vertically below the injection well and

S.K. Das, R.M. Butlerr Journal of Petroleum Science and Engineering 21 (1998) 43–59

45

scaled packed model. The Hele–Shaw cell or the packed cell represents a physical model of a thin vertical section of the Vapex chamber in a reservoir in which a pair of horizontal wells is used for injection and production. The modified Butler– Mokrys model, as discussed below, was used to analyze experimental results.

4. Butler–Mokrys model for solvent extraction in Hele–Shaw cell

Fig. 1. The concept of the Vapex process: solvent vapor dissolves at the interface, diffuses through the oil, diluted and low-viscosity oil drains by gravity to the production well. At higher solvent concentration, deasphalting may take place. Deasphalted oil drains, leaving the asphaltene in the matrix.

close to the bottom of the reservoir. The solvent vapor dissolves in the bitumen at the solvent bitumen interface and diffuses into the bulk of bitumen. Solvent dilutes the oil, reduces its viscosity and the diluted oil drains along the interface to the production well driven by gravity. Several other configurations of injection and production wells are also possible. The use of vaporized, rather than liquid solvent, produces a higher driving force for gravity drainage due to the higher density difference between bitumen and solvent vapor, and ensures that the residual amount of solvent in the extracted reservoir is less than that with liquid solvents. If the pressure used is close to the vapor pressure of the hydrocarbon at the operating temperature, deasphalting takes place; this causes an additional reduction in viscosity. Although this in situ-upgraded oil is of better quality, the reduction of permeability due to asphaltene deposition in the reservoir matrix, plugging of wells, etc., and consequent hindrance to the flow of oil out of the reservoir are the major concerns. The objectives of this work were to visualize and study the mechanism of the Vapex process, assess the performance of the process and the impact of asphaltene deposition on its performance. Experiments were carried out using Hele–Shaw cells and a

Butler and Mokrys Ž1989. developed a mathematical model for the extraction of bitumen with liquid solvent in a vertical Hele–Shaw cell made with two glass plates separated by a thin brass spacer to maintain a constant distance between the plates. They assumed that oil flows along the interface in a thin diffusion boundary layer. Drainage of the undiluted bitumen was considered to be negligible. Since the solvent was completely miscible with bitumen, there was no interfacial tension in the systems studied. In the Vapex process, the solvent is being used as a vapor; therefore, the presence of surface tension cannot be ignored. As shown later, the extraction rate is enhanced in porous media by interfacial phenomena. In the Hele–Shaw cell, surface tension restricts the fluid from flowing out through the open edge due to capillary action. This should not, however, affect the drainage of fluid taking place upstream of this draining edge. Consider a small part of the vapor–bitumen interface away from the draining edge, as shown in Fig. 2. The flow of fluid in the layer is nearly parallel to the interface. Assuming pseudosteady state, any position of the interface advances at a constant unspecified velocity, U. The solvent volume fraction, cs , and the concentration-dependent properties like viscosity, density, diffusivity, etc., can be assumed to be uniform across the cell thickness. By material balance over a thin shell, the cs in the diffusion layer can be given by the one-dimensional diffusion equation: yD

d cs dj

s Ucs

Ž 1.

where j is the perpendicular distance from the interface. Concentration in the diffusion layer varies from

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46

For small spacing between the plates Žthickness. of the Hele–Shaw cell, Butler and Mokrys Ž1989. showed that the second-order differential term that arises due to the change in velocity gradient in the direction normal to the flow surface could be neglected. Then, Eq. Ž2. becomes the equivalent of Darcy’s equation. Substituting expressions for d j and n from Eqs. Ž1. and Ž2. Žneglecting the secondorder differential term., respectively, and using the proper limits of integration, Eq. Ž4. yields: Fig. 2. Butler–Mokrys model for the Vapex process: fluid flow and mass transfer take place in the boundary layer between the solvent vapor and immobile bitumen.

Qs

kg sin u U

ci

D r Ž 1 y cs . D

m

m cs

Hc

d cs s

kg sin u U

Ns

Ž 5. where:

c i , the interfacial concentration which is the solubility of the solvent vapor in bitumen, to c m , the minimum concentration of solvent required to mobilize the otherwise immobile bitumen. The thickness of the diffusion layer is j max , over which the solvent concentration changes from c i to c m . In a Hele–Shaw cell at steady state, the balance of pressure gradients in the direction of flow within the diffusion layer can be given by a simplified form of Brinkman’s equation as: D r g sin u s

Õm k

E 2n qm

Ej 2

Ž 2.

where n is the average velocity across the thickness of the cell. With this average velocity, the apparent permeability of the Hele–Shaw cell is given by ŽLamb, 1932.: ks

b2

Ns s

ci

D r D Ž 1 y cs .

m

m cs

Hc

d cs

Ž 6.

Ns is a dimensionless number consisting of the physical properties of the solvent–bitumen system and hence, at constant temperature and pressure for a particular bitumen and solvent and with a fully developed diffusion boundary layer, it should be a constant. This number accounts for the concentration dependence of D r , D and m. It is also a function of c i , the solubility of solvent in bitumen which depends upon the temperature and pressure. Fig. 3 presents a schematic of a typical Hele–Shaw cell experiment. The cross-hatched area represents

Ž 3.

12

where b is the distance of separation between two plates. There is a capillary force normal to the interface due to surface tension. Since the direction of fluid flow is parallel to the interface, this force does not appear in the force balance of Eq. Ž2.. As the diluted bitumen has a volume fraction of solvent, cs , dissolved in it, the flow rate of bitumen in terms of vertical area drained, Q, through a section of the diffusion layer is: Qs

jmax

H0

Õ Ž 1 y cs . d j

Ž 4. Fig. 3. Schematic of the vapor extraction in Hele–Shaw cell.

S.K. Das, R.M. Butlerr Journal of Petroleum Science and Engineering 21 (1998) 43–59

the amount of oil drained through the section at ordinate, y, during time, t, from the beginning. This area should be equal to the amount of bitumen drained through this section during the same time period. Hence: t

h

H0 Qd t y fD S Hy xd y 0

Ž 7.

Differentiating with respect to t and y in sequence: EQ Ey

s yfD S0

Ex Et

Ž 8.

From the geometry of the interface: Ex

U s sin u

Et

Ž 9.

Qsy

kg f D S0 Ns EQrE y

(

Ž 10 .

Ž 11 .

The flow rate out of the cell, that is Q at ordinate y s 0 will be:

(

Q s 2 kg fD S0 Ns h

In the Butler–Mokrys model, the mass balance equation in the diffusion boundary layer in the Hele–Shaw cell experiment is given by Eq. Ž1.. For porous media, the same equation can be modified as: yADp

d cs dj

s A f Ucs

Ž 13 .

where A is the cross-sectional area of the solvent– bitumen interface and Dp is the apparent diffusion coefficient of the solvent in bitumen in porous media and is related to the intrinsic diffusivity Ds by the following expression ŽDas, 1995..

Ž 14 .

substituting this into Eq. Ž13. we get:

This may be integrated to obtain the flow rate at any ordinate, y, in the diffusion layer as: Q y s 2 kg f D S0 Ns Ž h y y .

6. Modified Butler–Mokrys model for porous media

Dp s Df V

Combining Eqs. Ž5., Ž8. and Ž9., we get:

47

Ž 12 .

It should be noted that in a Hele–Shaw cell, f and D S0 are unity. The above analysis does not include the effects of deasphalting on the process.

5. Model developed by Dunn et al. Dunn et al. Ž1989. developed a theoretical model to study the results of their experiments in which ethane and CO 2 were injected at low temperature to recover bitumen by gravity drainage. An approach similar to that used by Butler et al. Ž1981. for analyzing the SAGD process was adopted. This model neglected the strong concentration dependence of the molecular diffusivity and may yield erroneous results. Also, the form of viscosity temperature correlation is used as such for the concentration dependence of viscosity, which may not be a valid assumption ŽDas and Butler, 1996..

yD

d cs dj

s f 1y V Ucs

Ž 15 .

Following the analysis presented above for Hele– Shaw cell, the expression for extraction rate in a porous media in terms of the area of vertical crosssection drained is given by:

(

Q s 2 kg f VD So HNs

Ž 16 .

The above analysis considers the flow of oil from one side of the Vapex chamber, which is symmetric against the vertical line, through the horizontal injector and producer. Thus, to get the volumetric flow rates from both sides of the Vapex chamber, Q is to be multiplied by a factor of two and by the length of the horizontal well. Hence, the production rates from the Vapex process with a horizontal well of length, L, will be:

(

q s 2 L 2 kg f VD So Ns H

Ž 17 .

7. The vapex parameter ‘a’ Eq. Ž17. can be written in the form: q s 2 La'H

Ž 18 .

where:

(

a s 2 kg f VD So Ns

Ž 19 .

S.K. Das, R.M. Butlerr Journal of Petroleum Science and Engineering 21 (1998) 43–59

48

The composite parameter, a, incorporates reservoir parameters and physical properties of the bitumen–solvent system at the desired temperature and pressure; it is named as the ‘Vapex Parameter’. For a particular reservoir and operating conditions, this parameter is constant. It is clear from the above that this parameter can be used as a quantitative measure of the relative rates at different conditions. During the experiments using a Hele–Shaw cell, the interface height falls with time. Hence, a comparison of performance on the basis of the Vapex parameters, rather than the extraction rates, is considered to be more appropriate. In Hele–Shaw cells, where D So and f are unity, the Vapex parameter is given by:

(

a s 2 kgNs

Ž 20 .

8. Relation between production rates and permeability For identical conditions of temperature and pressure for the same bitumen–solvent system, the values of Ns are equal. The production rates and the Vapex parameters of two different media under such condition can be related using Eqs. Ž17. and Ž19. as follows: q1 q2

s

L1 L2

)

k 1 f 1V 1 D So 1 k 2 f 2V 2 D So 2

;

a1 a2

s

)

k 1 f 1V 1 D So 1 k 2 f 2V 2 D So 2

Ž 21 . i.e., if all conditions other than permeability remain the same, the production rate and the Vapex parameter should be proportional to the square root of permeability.

9. The experimental work 9.1. Vapex process as obserÕed in Hele–Shaw cell In the Hele–Shaw cell experiments, a thin vertical layer of crude oil held in the cell cavity is contacted with the solvent vapor along its vertical open edge; the diluted oil drains by gravity through the bottom of this edge. The extraction process is observed on a TV terminal using a video camera. The experimental

equipment and procedure are described elsewhere ŽDas and Butler, 1994a.. Different temperatures, pressures and crudes: Lloydminster Tangleflags heavy oil and Cold Lake, Peace River and Athabasca bitumens were used. It was observed that below the saturation pressure of the solvent at a constant temperature, extraction rates were higher at higher pressures. Deasphalting occurred when operating pressure was maintained close to the vapor pressure of the solvent at the operating temperature. The precipitated asphaltenes stick to the glass wall of the cell cavity Žwidth 254 m m. forming an interesting regular pattern and do not move as the diluted oil drains below. Fig. 4 presents a photograph of the Hele– Shaw cell taken during one of the experiments with Peace River bitumen and propane as solvent. The brown zone is the extracted area of the cavity originally completely filled with the bitumen. Deposited pattern of asphaltenes and the microlayers are visible in this area. The bulk of unextracted bitumen at this point of time during the experiment is the black zone at the bottom of the photograph. Drainage of diluted oil was observed to take place near the interface of the bitumen. The underlying principle of the Vapex process is the reduction of oil viscosity by dilution Žand deasphalting. with solvents. The mechanism of the process, as understood from the Hele–Shaw cell experiments, is that the propane vapor dissolves in bitumen at the interface and diffuses into it. As the propane concentration builds beyond a critical value, asphaltenes precipitate and adhere to the glass wall in the form of fringes Žmicro layers.. Diluted, deasphalted oil, which has a much lower viscosity than the original oil, drains down. As the interface moves, propane vapor flows through the deposited asphaltene layers, contacts fresh bitumen and the process continues in a periodic fashion. Even if the propane pressure is not sufficient to build the concentration up to that required for deasphalting, oil flows down due to the viscosity reduction in the presence of solvent. There is also some flow of the oil because of the swelling resulting from dilution. 9.2. Consequences of deasphalting In the Hele–Shaw cell, asphaltenes deposit in regular patterns. Although there may not be any

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49

Fig. 4. Extraction of Peace River bitumen with propane in a Hele–Shaw cell. Deposited asphaltenes remain adhered to the glass plate of the cell forming a pattern. Deasphalted oil drains at the solvent–oil interface.

regular pattern of deposition in the reservoir, since deasphalting is a thermodynamic phenomenon, the same mechanism would occur in the bitumen reservoir also. Due to the change in vapor–liquid equilibrium in curved interfaces of the reservoir capillaries, the pressure of solvent required for precipitation to occur may change, but this effect is negligible for the usual pore sizes of reservoir matrix. Silica and clay minerals adsorb asphaltenes, probably through hydrogen bonding ŽDubey and Waxman, 1991.. Gonzalez and Middea Ž1991. confirmed the irreversibility of the adsorption of asphaltenes on the reservoir sand. Thus, in a reservoir, the deposited asphaltenes would remain adsorbed on the reservoir matrix. A certain minimum concentration of solvent is required for deasphalting to take place. This minimum concentration varies from solvent to solvent; with propane, this is greater than 20–32% by weight ŽMokrys and Butler, 1993.. As the concentration of

propane is highest at the propane bitumen interface, deasphalting tends to take place there most readily. As diluted and deasphalted oil flows downward, the interface moves away. Asphaltenes are left behind, adhering to the sand and clay surfaces. If a pair of horizontal wells is used for injection and production and oil is allowed to drain by gravity, then the oil flow mechanism will be similar to that observed in the Hele–Shaw cell, i.e., diluted and deasphalted oil will flow underneath the deposited asphaltene layer along the interface. The permeability and wettability of the asphaltene-deposited sand matrix may be adversely altered. Since that part of the reservoir is already drained, these alterations of wettability and permeability would not play a significant role in the flow of diluted oil or in the deasphalting process taking place at the new position of the interface. Although the solvent vapor has to pass through this asphaltene-deposited sand matrix, it should be noted that asphaltenes occupy less than 20% of the void

50

S.K. Das, R.M. Butlerr Journal of Petroleum Science and Engineering 21 (1998) 43–59

space even if most of the asphaltenes are precipitated from the crude. In the actual process, only partial deasphalting is achieved. Hence, propane vapor bypasses the deposited asphaltenes and contact fresh bitumen, and the deasphalting process continues. Since deasphalting causes drastic viscosity reduction, the production rate is expected to increase with asphaltene deposition. This was indeed observed in the experimental results; the extraction rate of Peace River bitumen using propane as the solvent was 35% higher with deasphalting than without asphaltene precipitation. The extent of enhancement is not as high as would be expected from the possible viscosity reduction by deasphalting; probably, the diffusion of solvent is the controlling factor. It should be noted that it is possible to drain the oil even without deasphalting.

Fig. 5. Stabilized production rates in porous media are proportional to the square root of permeabilities.

chamber. In this phase, the production rate is higher due to the counter current nature of the extraction process. Once the vapor chamber reaches the cap rock, it spreads sideways. During this sidewaysleaching phase which continues for a considerable period of the project life, the production rate is constant; this continues until the boundary of the pattern is reached when the interface of the unextracted oil begins to fall, reducing the gravity head and the production rate. In the field, this falling interface phase may be continued until the production rate drops below the economic limit of the operation. To avoid liquefaction of butane in the porous bed, the operating pressures in the experiments described in this paper were maintained lower than the vapor pressure of butane at the operating temperature. Thus, the solvent concentration in the bitumen was low, and since the deasphalting with butane is less than that with propane, the effect of deasphalting in these

9.3. Vapex process in porous media The packed bed in the physical model used is 34 cm long, 22 cm high and 3.2 cm thick. A vertical transparent wall facilitates the observation of the process in real time. The model is packed with Ottawa sand or glass bead and saturated with the desired crude. It is then extracted by injecting butane vapor at a constant pressure. Due to the pressure limitation of the model, butane was used as the solvent. The amount of produced oil is weighed from time to time and the extraction rate is obtained from these weights. The experimental set up and procedure are presented elsewhere ŽDas and Butler, 1994b.. In the Vapex process, initially, the solvent vapor rises above the injection well and forms a vapor Table 1 Effect of permeability on the Vapex parameter Crude and viscosity at 208C ŽmPa s.

Cell and propane pot temperatures Ž8C.

Cell permeability Ž m m2 .

Vapex parameter Žcm1.5rh.

ar6k Žcm0.5rh.

Peace River Ž130 000.

25.0r21.7

Peace River Ž130 000.

25.0r20.8

Lloydminster Ž10 000.

25.0r20.8

1344 5376 3441 5376 1344 5376

1.28 2.47 1.27 1.76 1.66 3.56

0.035 0.034 0.022 0.024 0.045 0.049

Extraction with propane using Hele–Shaw cell.

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51

Table 2 Comparison of results of experiments with Peace River bitumen and butane in packed cell and Hele–Shaw cell Cellrbutane temperature Ž8C.

Cell and permeability Ž m m2 .

kh Ž m m2 m.

Vapex parameter experimental Žcm1.5 rh.

Vapex parameter: scaled-up from Hele–shaw cell results Žcm1.5rhr.

Flow rates predicted Žgmrh. Žm3 rday m.

Stabilized rates experimental Žgmrhr. Žm3rday m.

25.0r25.0 21.0r20.6 22.8r22.1 21.5r20.8 21.0r20.8

Hele–Shaw 5400 Packed model 830 Packed model 217 Packed model 43.5 Packed model 27

432.0 182.6 47.7 9.6 5.9

0.68 1.41 0.65 0.31 0.17

0.68 0.128 0.068 0.027 0.023

y 3.78 Ž0.0029. a 1.92 Ž0.0015. 0.80 Ž0.0006. 0.70 Ž0.00051.

y 41.2 Ž0.0309. 19.0 Ž0.0143. 8.2 Ž0.0061. 5.0 Ž0.0038.

a Numbers inside the parentheses in this table are the production rates expressed in m3rday for one meter length of the production well from two sides of the Vapex chamber.

experiments was not very significant; however, considerable quality improvement was observed in experiments carried out using propane as solvent, as described elsewhere ŽDas and Butler, 1995..

10. Enhancement of mass transfer in porous media For experiments with the same solvent–bitumen system, if the temperature and pressure remain the same for two different permeabilities of the medium, Vapex parameters are expected to be proportional to the square root of permeabilities as given by Eq. Ž21.. Table 1 presents a comparison of the results of three sets of experiments carried out for extraction of two crudes from different sources using propane as a solvent in Hele–Shaw cells. In each set, two cells of different permeabilities were used. Each of these experiments were carried out at 258C; the operating pressure in each set was maintained constant by fixing the temperature of the propane supply cylinder. Propane pressure was maintained much lower than the vapor pressure to avoid deasphalting in these experiments. Column 5 of Table 1 presents the ratio of the Vapex parameter to the square root of permeability. For each set of experiments, the ratio is almost constant, as expected from Eq. Ž21.. This confirms the applicability of the Butler–Mokrys model for the process. Fig. 5 presents a plot of the stabilized production rate vs. the square root of permeability for a few experiments carried out for extraction of Peace River bitumen with butane in porous media of different

permeabilities. The linear nature of the plot is in agreement with Eq. Ž21.. Vapex parameters for one of the Hele–Shaw cell experiments and that of a few experiments carried out in the physical model using different permeabilities at almost identical conditions are compared in Table 2. Column 4 of this table presents the Vapex parameters estimated from the experimental results, and column 5 shows the predicted results using Eq. Ž21., considering Hele–Shaw cell results as the basis. 1 Although the predicted Vapex parameter is 0.128 cm1.5rh in 830 m m2 permeability packing Ž1-mm glass beads., the experimental value is 1.41 cm1.5rh, indicating a more than 10-fold increase in the extraction rate. These predicted Vapex parameters are converted to the corresponding production rates available from the packed model and are presented in column 6 along with the experimental rates in column 7 of Table 2. For example, the experimental production rate for the experiment in 30–50 mesh sand Žpermeability 43.5 m m2 . is 4.1 grh in place of the predicted value of 0.4 grh. This shows that the production rates in porous media are significantly higher than expected from the scaling of the Hele–

1 Vapex parameter is 0.68 cm1.5 rh for the line source Hele–Shaw cell of permeability 5400 d, f s 1.0 and D S o s 1.0. For identical conditions of tem perature and pressure for same solvent–bitumen system, the Vapex parameter Ž a p . for a porous medium of permeability 830 m m2 , f s 0.35 and D So s 0.86 can be calculated using Eq. Ž21. as:

a p s 0.68

(

830=0.351 .3 =0.86 5400

s 0.128.

52

S.K. Das, R.M. Butlerr Journal of Petroleum Science and Engineering 21 (1998) 43–59

Shaw cell experimental results. Similar enhancement also were observed in the experiments carried out for the extraction of Lloydminster heavy oil in porous media; rates are 3–5 times faster than predicted. Reasons for this enhancement are discussed in the following sections. One interesting feature is that although the rate is enhanced in porous media, as presented in Fig. 5, experimental production rates are nearly proportional to 6k, as expected from Eq. Ž21..

11. Increased interfacial contact A part of the enhancement of production is thought to be due to the increased interfacial area for diffusion in porous media. In the Hele–Shaw cell experiments, diffusion occurs through a smooth interface having a curvature across the width of the cell. The radius of curvature is approximately half of the cell width, and the ratio of the interface area to the cross–sectional area is about pr2 s 1.57. On many occasions, the analysis of transport processes in porous media has been modelled, considering the system as a bundle of capillaries. In a bundle of capillaries, the interfacial area, assuming complete wetting and thus hemispherical interfaces, as shown

in Fig. 6a, is twice that of the cross-sectional area. Hence, even if a macroscopically smooth interface is considered for the porous media, the area available for diffusion will be approximately twice the area of cross-section of the pores. For the same cross-sectional area, the ratio of the interfacial area in the bundle of capillaries to that of the curved interface in the Hele–Shaw cell is 4rp . This does not explain the order of enhancement mentioned previously. The microscopic segment of the hypothetical macroscopically smooth interface in the sand grains may be highly extended, as illustrated in Fig. 6b. Although there is a 5–10% reduction of flux in a curved interface, increased contact area is much higher to yield a higher rate of mass transfer. The theoretical analysis is based on the assumption of a smooth interface. The interface in a packed cell is far from being smooth, even macroscopically, as can be seen, for example, in the photograph in Fig. 7 that shows the extraction of Peace River bitumen using butane in 43 m m2 permeability sand. The contact between solvent vapor and bitumen in porous media does not take place at a simple surface. Instead, as illustrated in Fig. 8, it extends as a transition zone. The oil saturation in this zone

Fig. 6. Extended interfacial contact at the interface in porous media.

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53

Fig. 7. Solvent bitumen interface in porous media. Extraction of Peace River bitumen using butane in 43 darcy sand; shown half of the Vapex chamber after 73 h.

changes from the original oil saturation to the residual oil saturation. The solvent chamber extends up to the bulk of the bitumen through the capillary network and contacts oil drops trapped in the capillary pores. Diluted oil flows in slow streams in the direction perpendicular to the flow of solvent vapor. This cross-current flow provides an extended contact of solvent and oil. Performance of the Vapex process is directly related to the amount of solvent dissolving into the bitumen. Microscopically, the process involves transient diffusion of solvent into bitumen at the interface. In this unsteady process, a sharp concentration profile develops near the interface. As the concentration of solvent near the interface becomes sufficient, the oil becomes mobile and starts flowing, exposing a new interface of bitumen having a very low concentration of solvent. Hence, solvent does not have

Fig. 8. Cross-flow of solvent vapor and oil in the contact zone enhances mass transfer in porous media.

S.K. Das, R.M. Butlerr Journal of Petroleum Science and Engineering 21 (1998) 43–59

54

to penetrate a long distance into the bitumen, i.e., diffusion at the surface is important rather than diffusion in the bulk of bitumen. This explains the enhancement of rate due to the increased contact area. If we consider that the area for fluid flow, A f , in the porous media is different from the interfacial area for diffusion, A d , Eq. Ž13. can be rewritten as: yA d Dp

d cs dj

s A f Ucs

Ž 22 .

This can be rearranged as: yDeff

d cs dj

s Ucs

Ž 23 .

where effective diffusivity Deff is higher than Dp , the apparent diffusion coefficient by a factor of A drA f . The similarity between Eq. Ž23. and Eq. Ž1. shows that the Modified Butler Mokrys model would be applicable using a higher value of the diffusivity to account for the enhancement. Thus, although the extraction rate is enhanced in porous media, Eq. Ž21. may still be valid for this situation, as observed from the experimental results. The specific surface area of the porous media may also play a role in the diffusion of solvent in the residual bitumen in the chamber. The effect, however, is not significant, as indicated by the consistency of results in Fig. 5 for Peace River bitumen. Although the specific surface area increases sharply as the sand grains become finer, the production rates were almost proportional to the square root of permeabilities, as expected from the theory.

12. Increased effective diffusivity in porous media The effective diffusivities of carbon dioxide in bitumenrOttawa sand have been observed to be twice those expected from diffusivity in pure bitumen ŽSchmidt, 1989.. A similar enhancement in effective diffusivity may also be true for the diffusion of butane or propane in bitumen in the presence of porous media.

13. Increased solubility A decrease in vapor pressure at the curved vapor–liquid interface in fine capillaries of porous media will cause the condensation of the solvent vapor for pressures less than the vapor pressure at a flat interface. This enhances the solubility of the vaporized solute in the liquid medium. A small part of the enhancement observed in porous media may be related to this increase in solubility of the gaseous solvent at the interface. This effect may be negligible considering the pore dimension in the high-permeability packed-beds used.

14. Enhancement due to capillary imbibition and surface renewal In our experiments in Hele–Shaw cells with asphaltene deposition, it was observed occasionally that the diluted oil of low viscosity rises up against gravity due to the capillary action in the reduced spacing in the asphaltene deposited region. The same phenomenon in which diluted oil is removed from the interface due to capillary imbibition may also occur in the fine capillaries in porous media. Imbibition of diluted oil into the solvent vapor region can occur because the vapor is non-wetting. The bitumen at the interface, prior to mixing with sufficient solvent, is highly viscous, and although it has a higher surface tension than the diluted oil, it takes a very long time to move under the action of capillary pressure. As soon as the oil becomes diluted at the solvent–bitumen-bulk interface, viscosity and surface tension both decrease and the oil swells. As discussed in Appendix A, the effect of viscosity reduction surpasses the effect of the reduction of surface tension on the rate of capillary imbibition. The capillary imbibition is aided by the positive spreading coefficient of diluted oil over water in a water-wet sand as presented in Appendix B. Thus, the diluted oil is drawn away from the interface by the adjacent capillary pore in the extracted sand matrix. The bitumen surface is renewed and the concentration of solvent in bitumen at the interface drops to a lower value, resulting in a higher transient diffusion flux, as discussed below. This procedure repeats in cycles.

S.K. Das, R.M. Butlerr Journal of Petroleum Science and Engineering 21 (1998) 43–59

55

In Hele–Shaw cell experiments, the capillary effect is as small as the plate separation is in hundred microns compared to a porous media where the pore radius may be a few microns. Thus, the role of the imbibition process mentioned above and the corresponding effect of surface renewal is not prominent in the drainage of oil in Hele–Shaw cell. The diluted oil in Hele–Shaw cell cannot flow through the open space separated from the main bulk of oil and has to flow along the interface, and probably, the presence of a diffusion boundary layer is always maintained.

15. Transient mass flux at the interface If we consider the diffusion of a solvent in a semi-infinite stationary slab of bitumen, the concentration profiles gradually change with time, as shown in Fig. 9. In essence, the concentration gradient at the interface decreases with time. The expression for the transient mass flux of solvent at the interface is given by: js

rs M

(

D

pt

k

Ž cs y c o . s

't

Ž 24 .

The graphical presentation of Eq. Ž24. for an arbitrary value of k Žas defined by Eq. Ž24.., is shown in Fig. 10. It is clear that as the concentration profile builds up with time, the mass flux reduces. In the pseudo-steady state analysis applied to the Vapex

Fig. 10. Transient mass flux at the interface of a semi-infinite slab. The mass flux decreases with time.

process, it is assumed that the points on the interface move at a constant rate, and that the concentration profile assumes a pseudo-steady shape in the diffusion layer. This profile is in between the ultimate steady state linear profile and the initial transient profile. Thus, the pseudo-steady state model represents a lower rate of mass transfer than the actual transient process. The pseudo-steady state may be true for the system in Hele–Shaw cell where the capillary imbibition effect is negligible. However, in a porous medium where capillary imbibition can remove the diluted oil, the interface is renewed periodically and the initial transient concentration profiles prevail at the interface, leading to a higher rate of mass transfer. Even with this periodic renewal, the interface will advance at a constant rate, macroscopically resembling the pseudo-steady state model. Hence, the overall pseudo-steady state analysis will still be valid, however, necessitating a higher value of the diffusion coefficient to justify the higher mass transfer rate. If we consider D t to be the average time period of the surface renewal, then the amount of mass transferred per unit area in this time span can be obtained by integrating Eq. Ž24. over this time period and is given by: Q cum SR s

Fig. 9. Transient concentration profiles inside a semi-infinite slab. The concentration gradient decreases with time.

rs M

(

DD t

p

Ž cs y c o . .

Ž 25 .

On the other hand, if the pseudo-steady state concentration profile resembles that of the transient

S.K. Das, R.M. Butlerr Journal of Petroleum Science and Engineering 21 (1998) 43–59

56

profile at time, t, then for the same time period, D t, the amount of mass transferred per unit area, calculated by this approach, will be: Qcum PS s jt D t s

rs M

(

D

pt

Ž cs y co . D t.

Ž 26 .

The ratio of the amount of mass transferred predicted by these two approaches is: Qcum SR Qcum PS

s

(

t DT

Ž 27 .

Thus, the ratio of t and D t dictates the ratio of the actual mass transfer rate to that predicted by the pseudo-steady state approach. In the experiments in porous media, the production rates obtained were 3–5 times faster with Lloydminster oil and 10 times faster with Peace River bitumen than that expected from the corresponding Hele–Shaw cell experiments. In the case of lower viscosity Lloydminster oil, the oil moves faster in the diffusion layer in the Hele–Shaw cell. Thus, the concentration profile is less developed and the surface renewal rate is 3 times faster. However, in the experiments with Peace River bitumen, the flow in the diffusion boundary layer is slower with a more developed concentration profile, indicating a lower mass transfer rate in the Hele–Shaw cell experiments. Hence, the surface renewal rate in this case is about 10 times higher.

16. Enhancement during the rising of solvent chamber In some of the experiments using packed model where initially the vapor chamber rises Ždepending on the injector–producer location., the average rate during the chamber rise period was higher than that during the sideways spreading phase. The results for the rising and spreading phases were compared in terms of the Vapex parameters ŽDas, 1995.. Vapex parameters for rising phase were 2–3 times higher than those for the spreading phase of chamber growth. These can be explained if we consider the process to be taking place in a periodic manner; diffusion of solvent, followed by draining of diluted liquid, exposing a fresh bitumen interface. Since the interface

is mostly vertical in the rising chamber, the diluted liquid drains relatively faster. A very high mass transfer rate can be achieved through this process, as explained above. Moreover, in the rising chamber, the counter current contact of the solvent and oil causes better mixing of solvent and the diluted oil. This helps in faster removal of the diluted oil from the interface.

17. Conclusions Ž1. The Vapex Process can be thought to take place in the following steps: Ža. dissolution of solvent vapor in the bitumen at the vapor–bitumen interface; Žb. diffusion of solvent molecules in solution through the bitumen bulk; Žc. reduction of bitumen viscosity as the solvent concentration increases; Žd. above a critical concentration, deasphalting occurs, reducing the viscosity; Že. due to the effect of steps c and d and the difference in density between the liquid and vapor, diluted Žand deasphalted. bitumen drains down to the production well due to gravity; this drainage is also aided by capillary imbibition. Ž2. Deasphalting takes place if the pressure of the injected solvent vapor is close to, or higher than, its saturated vapor pressure at reservoir temperature. Deasphalting does not reduce the flow of oil; instead, flow rates are enhanced due to the sharp reduction of viscosity by deasphalting. Asphaltenes are left behind, resulting in the upgrading of the oil. Ž3. Production rate is proportional to the square root of the permeability of Hele–Shaw cells, as expected from the Butler–Mokrys model for Solvent-Assisted Gravity Drainage. In the porous media, the stabilized production rate is proportional to the square root of the permeability, as anticipated from the theory. The extraction rate, however, is 10 times higher for extraction of Peace River bitumen in porous media than was predicted from experiments using the Hele–Shaw cells. Ž4. This enhancement may be attributed to the following major factors: Ža. extended interfacial area of contact in porous media; Žb. transient mass transfer at the interface aided by capillary imbibition and surface renewal;

S.K. Das, R.M. Butlerr Journal of Petroleum Science and Engineering 21 (1998) 43–59

Žc. film drainage due to positive spreading coefficient. Ž5. Although there is enhancement in extraction rates in the porous media, rates are still proportional to the square roots of permeability. This may be explained from the mechanism of the process, as discussed above, and a modified Butler–Mokrys model could still be applied using a higher effective diffusivity. Ž6. Compared to the stabilized gravity drainage rate, the extraction rate is higher in the rising phase of the chamber growth due to the counter current nature of the extraction process.

P q Q cum PS Qcum SR Q r S t U Õ x y

57

pressure ŽPa. volumetric flow rate Žm3rs. cumulative mass transferred per unit area assuming pseudo-steady state cumulative mass transferred per unit area with surface renewal drainage rate in terms of the area of vertical cross-section Žm2rs. capillary radius Žm. spreading coefficient ŽNrm. time Žs. velocity of the solvent bitumen front Žmrs. velocity of the interface Žmrs. horizontal position of the interface Žm. height at any point on the interface Žm.

18. Nomenclatures Subscripts

g Dr D So Dt x m u s f V a A b c D DAO g h H j jt k l L Ns

surface tension of the Gibbs monolayer ŽNrm. density difference between solvent and bitumen Žkgrm3 . change in oil saturation time period Žs. parameter defined by Eq. Ž24 viscosity ŽmPa s. inclination of the interface with horizontal interfacial tension ŽNrm. porosity cementation factor Vapex parameter Žm1.5rs. area Žm2 . distance between two plates in a Hele– Shaw cell Žm. volume fraction of solvent in bitumen mutual diffusivity of solvent in bitumen at cs Žm2rs. deasphalted oil acceleration due to gravity Žmrs 2 . height of interface at the no-flow boundary Žm. reservoir height Žm. mass flux Žkgrm2 s. transient mass flux at time, t Žkgrm2 s. permeability Ž m m2 . length Žm. length of horizontal well Žm. dimensionless number defined by Eq. Ž6.

c d eff f i m max o p s y 1,2 w

capillary for diffusion effective for fluid flow at the interface minimum required for mobility maximum oil porous media solvent at height, y medium 1 and 2 water

Acknowledgements This work was carried out under a research contract funded jointly by CANMET, Amoco Canada, Esso Resources Canada, PanCanadian Petroleum, Sceptre Resources, Shell Canada, Texaco and Wascana Energy. The authors are grateful for this financial support.

Appendix A. Rate of imbibition in a horizontal capillary If we consider the imbibition taking place in a horizontal capillary tube due to the driving force

S.K. Das, R.M. Butlerr Journal of Petroleum Science and Engineering 21 (1998) 43–59

58

capillary pressure, Pc , and the fluid flow obeys the Poiseuille flow, the velocity of imbibition in the horizontal capillary with a liquid column length, l, can be given by:

ns

dl

Pc

s

8ml

dt

r2

Ž A.1 .

In a horizontal capillary, the interface can move as long as the capillary extends, provided the liquid stream does not get disconnected from the bulk of liquid. Rearranging Eq. ŽA.1.: ld l s

Pc r 2 8m

dt

Ž A.2 .

Integrating, we get an expression for the time required for the imbibition interface to move a distance, l: ts

4m l 2 Pc r 2

Ž A.3 .

Substituting for expression of Pc and considering complete wetting Žcontact angle s 08.: ts

both viscosity and surface tension, however, the decrease in viscosity is much more prominent than the reduction in surface tension. Fig. 11 presents a plot of the ratio of viscosity to surface tension of the Peace River bitumen–butane mixture. It shows that the net effect of increasing solvent concentration is to decrease the time required for the movement of the interface over a certain horizontal distance. Without any solvent, the high viscosity original bitumen will take a very long time to move through the capillary, although the capillary pressure will be higher due to higher surface tension. With solvent, however, this time decreases by orders of magnitude. For example, in a capillary of 10 m m radius, the vapor–liquid interface of a solution of Peace River bitumen containing 30% Žby weight. of butane will move about 17.3 cm in 1 h compared to 2.1 mm for original bitumen.

2ml2

Ž A.4 . sr It is clear from this expression that viscosity and surface tension have a counteracting effect for capillary imbibition. Addition of solvent to the oil reduces

Appendix B. Spreading of diluted oil over water The spreading coefficient for spreading of film of liquid 1 over a film of liquid 2 is defined as: S1r2 s g 2 y g 1 y s 21

where g 1 and g 2 are the surface tension of the Gibbs monolayer of liquid 1 and 2, respectively, saturated with liquid 2 and 1. From thermodynamic analysis, it can be proved that S1r2 is positive if spreading is accompanied by a decrease in free energy, i.e., spontaneous. A similar expression for the spreading of diluted oil over water can be given as follows: So r w s sw y so y swo

Fig. 11. Ratio of viscosity to surface tension for bitumen–solvent mixture. The viscosity decreases at a faster rate than the surface tension with increase in solvent concentration. This increases the rate of capillary imbibition.

Ž A.5 .

Ž A.6 .

Considering the fact that water and oil are mutually insoluble, g values are replaced by the surface tensions of water and diluted oil. Interfacial tension of water and pure hydrocarbon liquids are in the range of 51 mPa m ŽAdamson, 1976.. However, experimental data show that the interfacial tensions between water and heavy oil or bitumen are considerably lower than this value ŽRobinson, 1984.. Interfacial tension ŽIFT. of heavy oil from the Lloydminster Sparky pool and formation water Ž73 218 ppm solids of which 67 400 ppm is NaCl and pH 7.56. is 20.4 mPa m where the surface tension of the oil is 31.7

S.K. Das, R.M. Butlerr Journal of Petroleum Science and Engineering 21 (1998) 43–59

mPa m. Similarly, surface tension of Athabasca bitumen and IFT, with respect to formation water, was measured to be 35.3 mPa m and 18.4 mPa m, respectively. These values, when substituted in Eq. ŽA.5., assuming surface tension of water to be 71 mPa m, indicates that for these systems, the spreading coefficients are always positive and the heavy oil or bitumen would spontaneously spread over water films. The dilution of the oil, however, decreases the surface tension, and since the solvent does not dissolve in water, the interfacial tension should not be affected. Surface tension of bitumen and heavy oil are in the range of 35 mPa m; however, surface tension of diluted bitumen may be close to 10 mPa m. This results in a higher positive value of the spreading coefficient, enhancing the spontaneity of the spreading process. Lower viscosity of the diluted oil also makes the spreading process faster and the diluted oil drains at a higher rate by film drainage. References Adamson, A.W., 1976. Physical Chemistry of Surfaces. Wiley, Chichester, pp. 103–11. Allen, J.C., 1973. Method for recovering viscous oils by solvent extraction. Canadian Patent No. 1008361, Texaco Development, April 12, 1977, US Application No. 391434, August 24. Allen, J.C., 1974. Gaseous solvent heavy oil recovery. Canadian Patent No. 1027851, Texaco Development, March 14, 1978, US Application No. 446874, February 28. Allen, J.C., Redford, A.D., 1976. Combination solvent–noncondensible gas injection method for recovering petroleum from viscous petroleum-containing formations including tar sand deposits. United States Patent No. 4109720, Texaco, New York, August 29, 1978, US Application No. 740281, November 9. Brown, A., Wu, C.H., Konopnicki, D.T., 1977. Combined multiple solvent and thermal heavy oil recovery. United States Patent No. 4004636, Texaco, New York, January 25. Butler, R.M., Mokrys, I.J., 1989. Solvent analog model of steamassisted gravity drainage. Aostra J. Res. 5 Ž1., 17–32. Butler, R.M., Stephens, D.J., 1981. The gravity drainage of steam-heated heavy oil to parallel horizontal wells. J. Can. Pet. Technol. ŽApril–June., 90–96.

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