Impact of Pre-equilibration on the Assessment Methodology of

ARTICLE pubs.acs.org/EF

Impact of Pre-equilibration on the Assessment Methodology of Interfacial Tension Measured between Aqueous and Heavy Oil Phases M. Sharath Chandra,* Jose Zacharia, and Geza Horvath-Szabo DBR Technology Center, Schlumberger Canada Limited, 9450-17 Avenue, Edmonton, Alberta T6N 1M9, Canada ABSTRACT: Equilibrium interfacial tension (IFT) between toluene-diluted bitumen and a series of alkyltrimethylammonium bromide surfactant solutions was measured at various temperatures as a function of the carbon chain length of surfactants. A mixing and separating protocol to pre-equilibrate the aqueous and organic phases was introduced prior to the measurements. The influence of the equilibrated phase densities on the IFT values obtained by the pendant drop method is discussed. A possible systematic error in the assessment of the IFT values with the pendant drop and spinning drop methodologies was pointed out. In general, bitumen
aqueous phase IFTs increase with an increase in the temperature and decrease with an increase in the carbon chain length of the surfactants. During the equilibration process of the bitumen and aqueous phases, the phase densities change significantly, which has implications on the gravity drainage process.

’ INTRODUCTION The development of enhanced oil recovery (EOR) processes, including water, chemical, and steam flooding, steam and carbon dioxide (CO2) injections, and in situ combustion, has been driven by the reality that only 10
50% of original oil in place (OOIP) is recovered using conventional processes. The use of surfactants for EOR1 is not economical when the oil price is low in comparison to the cost associated with the chemicals. However, the increase in demand for oil is prompting the industry to revisit their use for EOR processes that are considered reasonably economical for today’s market. The EOR process usually includes some viscosity-reducing methodology for crude oils of higher viscosity. It is the temperature increase that is used primarily to reduce the viscosity of these crudes. For example, the somewhat lower viscosity crude oil of the “Orinoco Belt” in Venezuela is recovered with hot-water injection, while steam injection is used for the recovery of the extra-high-viscosity Athabasca bitumen in Canada. During the steam-assisted gravity drainage (SAGD) process, the mobilized bitumen is drained to the bottom of the steam chamber along with the hot water produced by condensation. In the next step, the melted bitumen is produced together with the water in the production well. Consequently, in either the hot-water injection or the SAGD process, the recovery rate is dependent upon the properties of a two-phase flow [i.e., heavy oil (HO) and water] in porous media under high-temperature conditions. One such important property is the interfacial tension (IFT) between the oil and water phases besides the viscosities of the phases for the hot-water injection process. Clearly, the value of oil
water (O/W) IFT is also important for SAGD recovery. Furthermore, the density of the bitumen and water phases is also on the list of the recovery rate-controlling parameters in the case of gravity drainage. Lately, the expanding-solvent, steam-assisted gravity drainage (ES-SAGD) process2 is also considered, which uses the beneficial viscosity reduction effects of both the temperature increase and solvent addition. Therefore, both the O/W IFT and the densities of organic and aqueous phases are needed as a r 2011 American Chemical Society

function of the temperature in the presence of hydrocarbon solvents, to assess recovery rates of the ES-SAGD process. It can also be expected that the next game changing step for the hot water or ES-SAGD process is the surfactant injection. Surfactants can either reduce the O/W IFT, alter the wettability of the reservoir rock, or both. The outcome of surfactant injection is the reduced capillary pressure, which leads to improved recovery. Although the role of capillary pressure on the recovery had been recognized quite a while ago, it is still in the focus of research interest today.3,4 During the imbibition process, lowering the IFT can transform a capillary-driven flow to gravity-driven flow.5 Most surfactant studies reported in the petroleum-industryrelated literature involved use of anionic surfactants6
9 and non-ionic surfactants.10
13 Researchers are also exploring the capabilities of Gemini surfactants14,15 in reducing the liquid/ liquid IFT. The major drawback of anionic surfactants is their adsorption onto the reservoir rock (except for silica rocks), and the benefit derived from their usage can only be realized when they are employed in the presence of alkali.16,17 Cationic surfactants were studied extensively to alter the wettability of the reservoir rock by promoting ion-pairing between adsorbed naphthenic acid and the cationic surfactants.18
20 The advantages of these studies are more prominent in carbonate reservoirs. For the reason mentioned, cationic surfactants are intensely used for imbibition studies. It has been reported that dodecyltrimethylammonium bromide and amine surfactants are most effective in enhancing a spontaneous imbibition process. However, there are few published reports on the usage of cationic surfactants for IFT reduction, except for recent studies with cationic Gemini surfactants.14 Hence, the parameter space, which already contains the temperature, oil/water ratio, and diluent concentration, should first be expanded by the surfactant concentration. Then, the oil Received: November 24, 2010 Revised: May 2, 2011 Published: May 09, 2011 2542

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recovery rates of surfactant-injection-assisted SAGD and ESSAGD processes should be optimized over this parameter space. To better understand the underlying mechanisms of recovery alterations in these SAGD processes, one should measure the three major variables that would influence the recovery rates. These variables are (i) the O/W IFT, (ii) the density of the oil phase, and (iii) the density of the water phase. There is a common perception that, the higher the temperature, the better the performance of the steam or hot-water injection process because of the more pronounced viscosity reduction of crude oils. Indeed, the viscosity is a crucial parameter for the gravity drainage but so is the O/W IFT. Unfortunately, there is not too much published experimental data on the dependence of the O/W IFT as a function of the temperature. Perhaps this lack of experimental data has its roots in the understanding that the IFT between the gas and liquid phases of hydrocarbons vanishes at the critical temperature; hence, a decreasing tendency of the gas
liquid (G/L) IFT can well be expected. However, one should not overlook the fact that the O/W IFT of crude oils is also dependent upon the interfacial excesses of the surface-active components, which are abundant in HOs. If the adsorption isotherms of these polar compounds (surface-active components) are temperature-dependent, then the temperature-dependent interfacial excesses would not necessarily lead to diminishing O/W IFT with an increasing temperature. If, for instance, the O/W IFT increases with the temperature, then the beneficial effect of the decreasing viscosity (with an increasing temperature) on the recovery rates could be counteracted with the increasing IFT. There should be an optimum temperature for the SAGD and ES-SAGD processes for this latter case. Consequently, it is imperative to obtain the tendency of the IFT change with the temperature for these systems. It is important to realize as well that, in the above-discussed EOR processes, a relatively longer time is provided for the equilibration of the organic and aqueous phases, as compared to the usual time scale used in laboratories for testing IFT. Hence, the impact of the equilibration process on the IFT and, especially, the phase densities is of interest. As for the IFT, it has already been recognized that heavy crude samples would need longer equilibration with the aqueous phase than it is usually observed with light crudes.10,21 This observation has thus far been exclusively interpreted by the time-dependent interfacial properties, i.e., interfacial adsorption and rearrangement rates of adsorbed species. The possibility that the bulk properties of the oil and water phases would have an impact on the obtained values of the organic
water IFTs has never been considered in the literature according to the knowledge of the authors of the present paper. In fact, none of the papers available in the literature that publishes experimental data of IFT between aqueous phases and crude oils measured by either the spinning drop22
31 or pendant drop32,33 methodology discusses this possibility. For the assessment of the raw data obtained by these measurement techniques, some density differences of the fluids are needed to calculate the IFT. For example, eq 1 describes the pendant drop method34 used to calculate the IFT. γ¼

gΔFDe 2 H

ð1Þ

Here, g is the gravitational acceleration; γ is the IFT; ΔF is the density difference between the fluids; De is the equatorial

diameter of the drop; and H is a correction term related to the shape factor of the pendant drop. If the density of the organic and aqueous phases is changing during equilibration and this fact is not considered in the assessment of the experiment, an apparent time dependency of the IFT is going to be reported even for cases in which the IFT is constant. The same conclusion can be obtained for data measured by the spinning drop method. Therefore, it is unclear in what extent the time-dependent phase densities should have been considered to correct the thus far reported time-dependent IFTs and what are the correct IFT values. This issue can become serious especially when crude oils, for example, HOs, with a high concentration of polar compounds are equilibrated with aqueous-phase-containing surfactants, because the redistribution of polar compounds and the synthetic and natural surfactants will have an impact on not only the IFT but the densities of the phases. Beside these newly recognized needs that we discussed above, there are some traditional issues around IFT determination worth reiterating below. The traditional challenges in determining the IFT at bitumen and aqueous surfactant interfaces are the smaller density difference in this combination than in the one with light hydrocarbons and the high viscosity of bitumen at ambient temperatures. Because of these issues, work at ambient temperatures is difficult and a long time is required to obtain equilibrium IFT. Researchers have tried various approaches to deal with these challenges. To increase the density difference between bitumen and aqueous systems, either bitumen was diluted with some organic solvent9 or heavy water (D2O) was used instead of normal water (H2O)11,21 for surfactant solution preparation. The dilution technique has an additional advantage of reducing the bitumen viscosity while simultaneously increasing the density difference between the two fluids. The high viscosity of bitumen at room temperature limited the bitumen IFT measurements to high temperatures only.35
37 Obtaining equilibrium IFT is time-consuming.10,21 Sometimes, it is practically not possible to obtain equilibrium IFT, especially using drop-shape analysis techniques, because the drop may detach from the capillary tip before reaching equilibrium as a result of low IFT between the fluids. As of today, we could not obtain equilibrium IFT at a shorter time scale. The confidence levels for equilibrium IFT values obtained after long wait periods are low. It is believed in the literature that the longer equilibration times are a result of slow migration of active molecular species across the interface between the bitumen and aqueous surfactant solution.38 The small phase density differences and the low IFT values initiated a debate on the suitability of conventional IFT measurement techniques to deal with these situations. The well-accepted pendant drop technique was restricted to fluids with relatively larger density differences or to the measurement of only high IFT values, whereas the spinning drop technique11,21 was considered more suitable to measure low IFTs. Researchers also employed dynamic interfacial tensiometers to study fluids with small density differences.36 Recent advances in the image-capturing techniques are encouraging researchers to use the pendant drop technique to measure very low IFT values. In the current study, we used the rising drop method with image-capturing software to determine the IFT between diluted bitumen and various aqueous surfactant systems. The objective of this exploratory study is to demonstrate that the densities of the organic and aqueous phases can be 2543

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Table 1. International Union of Pure and Applied Chemistry (IUPAC) Names of the ATAB Compounds Used in the Current Study along with Their Respective Abbreviations, Hydrophobic Chain Lengths, and Purities compound name

hydrocarbon purity abbreviation chain length (wt %)

1-octyltrimethylammonium bromide

C8TAB

8

97

1-decyltrimethylammonium bromide

C10TAB

10

98

1-dodecyltrimethylammonium bromide

C12TAB

12

99

1-tetradecyltrimethylammonium

C14TAB

14

98

C16TAB

16

99

bromide 1-hexadecyltrimethylammonium bromide

significantly altered under conditions relevant for the ES-SAGD process. We want to illustrate that this finding, beyond its trivial implications, has a significant impact on assessing the methodology of IFT data, which has not been recognized thus far. This also indicates that probably a significant portion of the published IFT data for crude oil
water systems has a systematic error. From the assessment, it will become obvious that this systematic error cannot be eliminated by increasing the equilibration time.

’ EXPERIMENTAL SECTION Materials. Bitumen (HO 7) used in the study has an American Petroleum Institute (API) gravity of about 8, total acid number (TAN) of 3.37 mg of KOH/g of oil, and ash of 0.12%. The viscosity of undiluted bitumen was 5.7  105 mPa s at 25 °C and 1.37  103 mPa s at 80 °C. The bitumen was diluted by adding 25 wt % high-performance liquid chromatography (HPLC)-grade toluene. This mixture is dubbed as diluted bitumen in this paper. At 25 °C, the density of the toluene, bitumen, and diluted bitumen was found to be 0.865, 1.012, and 0.973 g/cm3, respectively. The resultant density of the diluted bitumen is close to the expected 0.972 g/cm3, which can be calculated by linear interpolation between the corresponding densities. The overall change in the density of bitumen as a result of toluene dilution is 0.039 g/cm3. This change is relatively small compared to the original density of the bitumen. The alkyltrimethylammonium bromide (ATAB) compounds with alkyl chain lengths of 8, 10, 12, 14, and 16 were of 97.0þ% purity. They were purchased from Aldrich and used without any further purification. Table 1 lists the names of all of the compounds used in this study along with their respective abbreviations, hydrophobic chain lengths, and purities. Freshly drawn ultrapure water of 18.2 MΩ cm resistivity was used to prepare all of the surfactant solutions in the study. This water was produced with a Barnstead EASYpureRoDi system that uses reverse osmosis followed by ion exchange and ultraviolet (UV) irradiation. Thermal Stability Testing. Prior to usage, the thermal stability of the surfactant was confirmed at 100 °C for 4 h under an oxygen-free environment in a reactor constructed in-house. The thermal stability of the surfactant was tested using the following protocol: 200 mg of surfactant was dissolved in 15 g of water (final concentration ≈ 34.0 mM) under a nitrogen blanket in a glovebox. The oxygen levels in the glovebox were maintained below 50 ppm. The prepared solutions were packed in acid digestion cylinders before removal from the glovebox. The cylinders containing the sample solutions were then placed in the grooves of a customized heating block mounted on a manually controlled hot plate at room temperature. The heating block was covered with an insulating thermal jacket to prevent heat loss, and the hot plate was set at 100 °C with an appropriate ramp. The temperature was then continuously

monitored and recorded every 30 min. After the samples were treated for 4 h at 100 °C, the hot plate was turned off and the change in the temperature was continuously monitored. When the heating block reached the “finger-touch” temperature, the cylinders were removed from the heating block. Sample solutions of specific concentrations were then prepared to determine and compare the critical micelle concentration (cmc) of the surfactant before and after thermal treatment at 100 °C. The cmc of a given surfactant was determined by performing surface tension measurements on a series of concentrations and determining the break point of the resulted function the following way: The surface tension data was plotted against the concentration on a log scale, and a breaking point in the surface tension versus log concentration function was identified by independently fitting lines to the experimental points in both the high- and low-concentration regions while omitting the values next to the approximate break point. An insignificant change in the cmc was detected, which indicated thermal stability of the surfactant in the temperature range used in this study. The same procedure was adopted for all of the surfactants studied. In the IFT experiments for diluted bitumen and aqueous phases, the concentrations of the surfactants were 3 times higher than the respective surfactant cmc at 20 °C. Pre-equilibration. The following procedure was adopted to equilibrate the diluted bitumen and surfactant solutions prior to IFT and density measurements: The organic and aqueous phases were mixed in 1:4 volume ratios in a separating funnel and left at 80 °C for 24 h with intermittent shaking. The experiment was performed in a closed system to minimize the evaporation losses. The 1:4 volume ratio was chosen because it is similar to the expected oil/water ratio during the SAGD production process. After the 24 h treatment, both the aqueous and organic phases were separated at 80 °C. A 1.0 mm thick rag layer was observed between the oil and water phases. The volume of this layer was about 5.6% of the water phase. These separated phases were then cooled to the respective experimental temperatures for further studies. Density Measurements. The densities of the separated aqueous and organic phases were measured at 24, 60, and 80 °C using an Anton Paar densitometer that consisted of a DMA HPM cell and a mPDS 2000, version 3, evaluation unit. The procedure for the density measurements was as follows: a two-point calibration of the sensor, above and below the expected density of the fluid in study, was performed under the specified temperature conditions. The sample density was measured immediately after these calibrations. In practice, we made three measurements under identical conditions at every temperature: two measurements with the calibrating fluids and one measurement with the sample. n-Decane with a density of 0.7301 g/cm3 and heavy water with a density of 1.1056 g/cm3 were used as calibrating fluids. Each of these measurements was performed at least 2 times to confirm the repeatability of the data. In our common laboratory practice, the repeatability of the density measurements was better than 0.0001 g/cm3. Because this repeatability involves the repeated calibration of the instrument with an n-alkane and heavy water, deviation of the measured density value from the actual value would be less than the error of repeatability. Densities at 40 and 70 °C were obtained by fitting the polynomial formula (eq 2) to the experimental data and interpolating. FðTÞ ¼ A þ BT þ CT 2

ð2Þ

IFT Measurements. All of the IFT measurements were performed at five different temperatures: 24, 40, 60, 70, and 80 °C. Surface tension measurements required to determine the cmc of the individual surfactants to confirm the thermal stability as well as to determine the required surfactant concentrations for this investigation were performed using the conventional pendant drop technique. IFT measurements between the aqueous and organic phases were carried out using the rising drop (inverted pendant drop) technique. Figure 1 shows the 2544

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Table 2. cmc Values of Various ATAB Surfactants Determined at 20 °C Using the Pendant Drop Method compound C8TAB C10TAB C12TAB C14TAB C16TAB

Figure 1. Schematic of the IFT apparatus used in the current study: (1) CCD camera, (2) cell body, (3) light source, (4) vibration-free table, (5) PC with image-capturing software, and (6) sample syringe.

Figure 2. Real-time data logging of IFT for diluted bitumen and water phases at 24 °C. The values on the y axis are not the final IFT numbers. The final IFT was obtained by further analysis of the drop shape at a given time. schematic of the apparatus used for IFT measurements in the study. Diluted bitumen was used as the organic phase, and either pure water or surfactant solution (before and after equilibration) was used as the aqueous bulk phase in the cell. The temperature of the cell was controlled by a circulating water bath. The absolute temperature in the cell was monitored independently of the water bath temperature using a thermocouple dipped into the cell. The temperature of the cell was controlled within (0.2 °C. With a screw-type syringe, a drop of the diluted bitumen was made at the tip of the capillary in the aqueous bulk phase. The maximum drop size was prepared for all of the experiments. The drop was illuminated with a light source, and its projected image shape was captured using a charge-coupled device (CCD) camera, which was directly connected to a dedicated personal computer (PC). Image-capturing software allowed for the capture of both still images and video streams of the drop shape. These data can be retrieved and analyzed later at any given time window. The sensitivity of the instrument and the data analysis process is < (0.1 mN. Figure 2 shows the change of the IFT in real time starting from the formation of the drop. It also demonstrates that, once a stable drop is formed around 3000 s, the change in IFT is negligible. The overall approximate change in the smoothed IFT from 3000 to 7500 s is 0.03 mN/m. Please also note that these data are not the final IFT values that we reported, because the software was initially provided with hypothetical density values only, which made it possible to quickly record the relative change in the IFT. Hence, the scale on the y axis is only for a comparison purpose. Once the drop is stabilized, the drop shape was re-analyzed at a given time, the measured densities of the phases were entered into the software, and the final IFT value was calculated. For water and diluted bitumen phases at 24 °C (Figure 2), the drop shape was analyzed at 4500 s.

cmc (mM, at 20 °C) 219.7 54.14 16.85

literature values 140.0a 68.0a 16.4a

3.66

3.5b

3.41

c

0.820

a

14.0b c

0.824

0.95b

b

From ref 42 (refractive index methodology). From ref 40 (surface tension methodology). c From ref 41 (refractive index methodology).

Figure 3. IFT measurements performed on various concentrations of the C16TAB at 20 °C before and after thermal treatment at 100 °C for 4 h.

Ash Test. The ash test was performed on the original (non-diluted) bitumen 3 times according to the American Society for Testing and Materials (ASTM) D482 method. The ash content of the bitumen was found to be 0.12, 0.12, and 0.13 wt % in repeated experiments.

’ RESULTS AND DISCUSSION Thermal Stability of the Additive. One of the biggest challenges with the use of surfactants in the petroleum industry is their thermal stability at higher temperatures and, preferably, at reservoir conditions. Because we could not find published data on the thermal stability of aqueous surfactant solutions that we used in this study, we screened all of the ATAB surfactants for their thermal stability or chemical degradation at 100 °C, which was 20 °C higher than the maximum experimental temperature. This test served more than confirming the thermal stability of surfactants. This test would verify whether the about 2
3 wt % impurities present in the surfactant samples could be thermally degraded in such an extent that would significantly alter the IFT values. The strong impact of a tiny amount of impurities on the IFT behavior of surfactant solutions is a well-documented fact. Because one of the major outcomes of the presented study is IFT values belonging to different temperatures, we wanted to be sure that the variation of IFT with the temperature was not an artifact introduced by the degraded impurities of the additive. Because the IFT values of surfactants around their cmc’s are prone to impurities, we determined the cmc’s from IFT measurements prior to and after the thermal treatments of the surfactant solutions. These test results indicate that the cmc’s of surfactants listed in Table 2 are the same at 100 °C for about 4 h. Considering the exponential nature of the Arrhenius equation, 2545

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Figure 4. cmc of the ATAB surfactants with varying hydrophobic chain length measured at 20 °C. Specific surfactants corresponding to a hydrophobic chain length are 8, C8TAB; 10, C10TAB; 12, C12TAB; 14, C14TAB; and 16, C16TAB.

the 4 h stability test at 100 °C would ensure a longer term stability at the lower treatment temperatures used in this study. Figure 3 shows the IFT values for C16TAB measured at 20 °C before and after the thermal treatment at 100 °C. There is no significant change in the IFT. We could not find any significant change either in the cmc’s determined with the methodology described in the Thermal Stability Testing. This indicates that thermal degradation of the used additive, if there were any, would affect neither the cmc nor the IFT values. We noticed similar trends for the rest of the surfactants with different chain lengths in the series. The temperature range of this investigation, which was up to 80 °C, as well as the thermal testing at 100 °C, was selected in such a way to cover the lower half of the temperature range of the hot-water injection process for HO recovery. To study the higher than 100 °C temperature range, high-pressure separation and IFT measuring capabilities would be necessary. Although the cmc values for most of the ATAB surfactants are published in the literature,39
41 they have some deviation from each other because of the different purity levels of the samples used for these studies. The purity of the surfactants that we used in the current study is higher or comparable to the ones reported in the literature.39
41 The cmc of the surfactant can be higher or lower than that of the pure surfactant depending upon the surface activity of the impurity. From the perspective of industrial applications, the purity of the surfactants used in this study is on the high end. Lower grade surfactants are used at field operations. Nevertheless, we decided to use highly purified surfactants to eliminate the impact of impurities on the surfactant action and explore its temperature dependence. The numerical values of the cmc’s and the IFTs could be slightly different if these experiments are repeated with extremely purified surfactants; however, the observed tendencies would remain the same. Because we decided to work with aqueous solutions having a concentration 3 times the cmc of a given surfactant, we measured the cmc of each of the surfactants used in this study at 20 °C. Table 2 lists the values obtained. Although the actual cmc values are slightly different from what we have noticed in the literature, the trend in the change of cmc with the increasing hydrophobic carbon chain length is very similar. Figure 4 shows the trend in cmc values with the increasing surfactant chain length. The increase in the hydrophobic chain length of the surfactant

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Figure 5. Density of the water and diluted bitumen at various temperatures before and after equilibration of both the fluids at 80 °C for 24 h. The lines are for the guide of the eye only.

resulted in the decrease of the cmc. However, the values obtained for the cmc are quite different compared to the reported values, indicating that purity of the compound is different. Pre-equilibration To Approach Equilibrium for Density and IFT Determinations. In an ideal world, the aqueous and organic phases are equilibrated long enough to reach the equilibrium state at the selected temperature as well as at the selected volume ratios of the oil and water phases. Then, the phases are separated at the same temperature as the equilibration temperature. In the next step, the densities of the phases are measured at the equilibration temperature; the previously separated phases are recombined in the IFT cell, where the IFT is measured at the same temperature again. As long as this procedure is strictly applied and the specific surface of the discontinuous phase is not extremely large in the IFT cell (cf. emulsions), the obtained IFT should not have any dependency on the volume ratios of the liquid phases in the IFT cell. In reality, we cannot always follow this procedure. For instance, when the pre-equilibration and IFT measurement could not be performed by the above-described procedure, the dependency of IFT on the phase ratios could be reported.38 The time necessary to establish full equilibrium between the aqueous and highviscosity HO phases could be extremely long at ambient temperature. Hence, we did not attempt to equilibrate these phases at lower temperatures. Instead, we pre-equilibrated the aqueous and organic phases at 80 °C, because we can expect shorter equilibration time at higher temperatures, and we intended to approach equilibrium in the IFT cell at 80, 70, 60, 40, and 24 °C using the organic and aqueous phases, which were pre-equilibrated. Obviously, we did not follow the above-discussed “ideal procedure”, but we argue that the used procedure is reasonable for the HO industry considering the experimental constrains. In addition, when we want to determine IFT at 80 °C, we have a very good chance to obtain close to equilibrium values not only for the IFT but for the phase densities as well. Figure 5 presents the results of the density measurements performed on diluted bitumen (organic phase) and water (aqueous phase, without surfactant), at various temperatures using the organic and aqueous phases, which were either not equilibrated or pre-equilibrated at 80 °C. The most interesting results were found after equilibrating both the fluids at 80 °C; the densities of both the aqueous and organic phases decreased significantly. It is important to realize that this density difference exceeds the precision of the density-measuring apparatus with at least 2 orders of 2546

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Figure 6. Values of IFT between equilibrated water and diluted bitumen measured at various temperatures while using densities of equilibrated and non-equilibrated phases. The plot highlights the change in IFT trends as a function of the temperature using two different densities.

magnitude. Starting from 80 °C, the density of the diluted bitumen as well as that of the water gradually increased with the decrease in the temperature. This trend is expected for most of the solutions because the intermolecular interactions weaken with the increase in the temperature. IFT versus Phase Densities. The significant change in the densities of the aqueous and organic phases at 80 °C has some remarkable implications. When HO and aqueous phases are equilibrated, two processes are going on simultaneously. The first is a mass transport through the interface, which alters the bulk properties of phases; the second is a mass transport toward the interface, which alters the interfacial properties. During these equilibration processes, the IFT does not depend upon the phase densities at any given time. However, if the IFT measuring methodology uses the phase densities as an input parameter, then the actual phase densities belonging to the given equilibration time should be used to obtain the correct IFT. It has been already mentioned in the Introduction that this issue has been disregarded thus far. Let us look at the error that arises with this issue. At 80 °C and in the absence of surfactant, the diluted bitumen
aqueous phase IFT is 33.40 mN/m when equilibrated densities are used for the calculation, while this IFT becomes 17.83 mN/m when non-equilibrated densities are used for the calculation. This is about 87% difference. The results here indicate that simply equilibrating the phases for a longer duration will not yield reliable IFT values for methodologies, which assume the knowledge of phase densities, unless we use the right densities. Figure 6 presents the difference in IFT values and trends using equilibrated and non-equilibrated density values while measuring IFT between pre-equilibrated (at 80 °C) diluted bitumen and water at various temperatures. The maximum differences between the two data sets are observed at ambient as well as high temperatures. When we used individual phase densities at various temperatures, the IFT between diluted bitumen and water reaches a maximum at 60 °C. At higher temperatures, the IFT decreases with an increase in the temperature. Similar trends were reported in the literature.10,15,21 However, when equilibrated densities were used for the IFT measurements, an increasing (inverse) trend as a function of the temperature was noted. The data presented in Figure 7 show the change in densities of the organic and aqueous phases after equilibration at 80 °C in the

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Figure 7. Densities of aqueous surfactant (viz., C8TAB, C10TAB, C12TAB, C14TAB, and C16TAB) solutions and diluted bitumen at various temperatures after equilibrating both the corresponding aqueous and organic phases at 80 °C for 24 h. The dotted lines across the points show polynomial fitting using eq 2. Here, the water and diluted bitumen densities prior to equilibration were also presented for comparison. The solid line is just a guide for the eye.

absence and presence of ATAB surfactants in the aqueous phase. The presence of surfactants in the aqueous phase has not contributed to any significant alteration of the density of that phase after equilibration. The aqueous phase densities follow the same trend as a function of the temperature in the presence and absence of surfactants in the system. However, the density of the diluted bitumen in the presence of surfactants has decreased nonlinearly with the increase in the temperature. These results were discussed in detail in a later section. One of the major limitations of the current study is that the pre-equilibration was performed at only one temperature (80 °C). However, these results at 80 °C clearly demonstrate the importance of the density of equilibrated phases for accurate IFT measurements. IFT between Diluted Bitumen and Aqueous Surfactant Phases. Figure 8 presents the IFT measurements between bitumen and aqueous phase with and without surfactants at various temperatures. The surfactants present have various hydrophobic chain lengths. The data presented here show that the presence of the surfactant in the aqueous phase significantly reduced the IFT between bitumen and aqueous phase, irrespective of the surfactant type. At 80 °C, the temperature at which the two phases were equilibrated, the IFT reduced a minimum of 6.5 times with C8TAB and a maximum of 22 times with C14TAB and C16TAB. This result clearly confirms that surfactants have a beneficial effect in reducing the IFT between oil and water. In the majority of the cases, the IFT between the organic and aqueous phases has shown an increasing trend with the increase in the temperature, irrespective of the presence of surfactants. In the absence of surfactants, the IFT has linearly increased with the increase in the temperature. In contrast, in the presence of surfactants, there is a significant change in IFT from 24 to 40 °C and in some cases to 60 °C; for the later, there is a little change with the increase in the temperature. In the case of C10TAB and C16TAB, after reaching a maximum at 60 °C, the IFT gradually decreased. In the presence of C8TAB and C14TAB, there is a slight increase even after 60 °C. However, in the case of C12TAB, there is a significant change in IFT with the increase in the temperature throughout the tested temperature range. The surfactant chain length has shown considerable influence on the IFT. In general, with an increase in the surfactant chain 2547

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Figure 8. IFT between ATABs with different chain lengths and diluted bitumen at various temperatures. The samples are pre-equilibrated at 80 °C for 24 h and separated at 80 °C prior to the measurements. The surfactant concentration in the water phase was 3 time the cmc of the corresponding surfactant. The used oil/water volume ratio was 1:4.

length, the IFT gradually decreased as follows: C8TAB > C10TAB > C12TAB > C16TAB > C14TAB. This trend indicates that the IFT gradually decreased with the increase in the hydrophobic chain length from C8TAB to C14TAB. However, increasing the chain length beyond C14 resulted in an increase in IFT (viz., C16TAB). Although these trends are valid at most of the lower temperatures, at higher temperatures (70 and 80 °C), the surfactants with higher chain lengths behaved similarly (C14TAB and C16TAB) as well as the surfactants with lower chain lengths (C8TAB, C10TAB, and C12TAB). These results indicate that, by tuning one component in the surfactant molecular structure, we can decrease the IFT to a certain extent. However, to achieve ultra-low IFT values, we need to either modify the molecular structure significantly or employ formulation, i.e., the addition of co-surfactants or salt. Interpreting the Changes in the Phase Densities. It is out of the scope of this work to interpret the mechanism behind the changes in the organic and aqueous phase densities as a result of equilibration. Nevertheless, we volunteer to provide some hints for further studies. The decrease in the density of the aqueous phase can be attributed to the dissolution of toluene as well as the polar fractions of the bitumen in the aqueous phase, which is evident from the discoloration of the aqueous phase. We want to underline though that the toluene cannot be the sole source of this density change because of its limited solubility in water. Therefore, an additional contribution to the density change should come from the polar compounds of bitumen. Using a simplistic approach, one might consider that dissolution of these lower density fractions in water reduces the density of the resultant aqueous phase. However, when we consider the process more closely, we should realize that it is the partial molar volume of the partitioning component and its concentration dependence that have an ultimate effect on the phase densities. Because neither the chemistry nor the molar volume functions of the partitioning fraction is known today, the interpretation of this phenomena would probably need quite significant resources. Nevertheless, we can provide a simplified explanation for the changes of phase densities below. In principle, the densities of both the heavy and light phases could decrease when a class of molecules with intermediate density compared to the individual phase densities is moved from the lighter to heavier phase. Because, in this case, molecules with densities higher than the

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density of the oil phase are removed from the oil and molecules with densities lower than the density of the water phase are introduced in the water. As an alternative explanation, one can also consider the accumulation of heavy surface-active molecules at the oil
water interface in the rag layer. When the amount of these molecules is considerable, as compared to the mass of the phases, their impact on the phase densities should be taken into consideration as well. The decrease in the density of the diluted bitumen after equilibration is also quite significant at 80 °C. Close examination of the experimentation process, components in the individual phases, and the data reveals that this density difference in the organic phase cannot be linked to the loss of inorganic components, viz., fine sands, or organic solids to the aqueous phase, because of the low ash content, cf. 0.12 wt %. It is important to note that, in the current study, we were always working above the Krafft temperatures of the surfactants,43 except for chain length 16, for which the Krafft temperature is about 25 °C. Similarly, in the absence of surfactants, the density of the equilibrated bitumen decreased linearly with the increase in the temperature. However, the density versus temperature function of the organic phase (diluted bitumen) changed remarkably when it was equilibrated with the aqueous surfactant solution (at 80 °C) prior to the temperature-dependent measurement. The density of these pre-equilibrated diluted bitumen samples, which were first separated from the organic phase at 80 °C and then brought to the desired temperature, can be described as a nonlinear function of the temperature. This behavior was found to be not much affected by the hydrophobic chain length of the studied surfactant family. This nonlinear change in density as a function of the temperature indicates that a portion of the surfactants traveled across the interface into the organic phase during the equilibration process, and their presence altered the temperature dependency of the thermal expansion coefficient of the organic phase. This phenomenon of surfactant migration across the oil
aqueous phase interface during the equilibration process is well-known in the literature.44,45 The presence of surfactants in the organic phase can alter the intermolecular interactions at elevated temperatures. At room temperature, the density of the organic phase was very similar in the presence and absence of the surfactant. The maximum deviation from the diluted bitumen (equilibrated with water) was noted at 60 °C. This could be because of the diminishing effect of the temperature on the intermolecular interactions at elevated temperatures, viz., >60 °C. This density alteration is probably related to the temperature-dependent association behavior of surfactants, which form reverse micelles in the organic phase. The reverse micelle formation of cetyltrimethylammonium bromide is known in organic media.46
48 An alternative hypothesis for explaining this odd behavior is the interaction between surfactants and the various polar components of bitumen, which can alter the phase behavior. For instance, it was found that the interaction of Lewis acids with Lewis bases in crude oils could significantly alter the viscosity of the HOs.49 If we consider that the interaction between the used cationic surfactants and the natural acids of the bitumen (cf., TAN = 3.37) belong to the Lewis acid
base interactions too and, furthermore, if we take into account that the phase viscosity is a bulk property of the phases, it should not come as a surprise that the phase densities can also be influenced by these intermolecular interactions. In addition to this, we should consider that intermolecular interactions affect the phase behavior and the phase ratios influence mean bulk properties.50 2548

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Energy & Fuels We know, as well, that asphaltenes associate, which alters their apparent molar volume.51 This association can be affected by surfactants; i.e., surfactant reverse micelles can solubilize asphaltenes, and this proves the temperature dependence. All of this influences the partial molar volumes of components and, consequently, the density of the organic phase. With all of the above being said, we want to emphasize again that the more detailed investigation of all of the above listed hypothesizes was out of the scope of this study. Implications of the Findings on the SAGD and ES-SAGD Processes. The altered phase densities have a strong impact on the numerical values of the O/W IFTs, which are needed for the assessment of the thermal recovery processes based on hot-water or steam injection. Because the densities of aqueous and HO phases can be significantly altered during equilibration, it is advisible to use these equilibrated density values to assess the driving force of the gravity drainage process. There should be an optimal temperature range for the hot-water or steam injection recovery processes, because the decreasing tendency of viscosities can be counterbalanced by the increasing tendency of O/W IFTs. Therefore, the SAGD and ES-SAGD processes might have an optimum temperature where the maximum drainage rate can be achieved.

’ CONCLUSION On the basis of the current investigation of measurements of equilibrium IFT between diluted bitumen and aqueous phase with and without surfactants, we drew the following conclusions: (1) Densities of the organic and aqueous phases, which were separated after equilibration, differ significantly from the original phase densities. (2) The calculated equilibrium IFTs of crude oil/water systems can be erroneous when the original densities of oil and water phases are introduced into the formulas. In addition to extend the equilibration time of the interface during IFT determination, it is also necessary to determine the pre-equilibrated phase densities. (3) The presence of C8TAB, C10TAB, C12TAB, C14TAB, and C16TAB surfactants in the aqueous phase decreases the bitumen
water IFT. For a given temperature between 24 and 80 °C, it is the addition of C14TAB that results in the smallest IFT. (4) Bitumen
water IFT increases with the temperature within the 24
80 °C range in either the absence or presence of ATAB surfactants, irrespective of the chain length of the surfactants. (5) The SAGD and ES-SAGD processes could have an optimum temperature where the maximum drainage rate can be achieved. ’ AUTHOR INFORMATION Corresponding Author

*Telephone: 1-780-577-1926. Fax: 1-780-450-1668. E-mail: [email protected] and/or sharath.mahavadi@gmail. com.

’ ACKNOWLEDGMENT We thank DBR Technology Center, Schlumberger Canada Limited, for providing the infrastructure to perform the IFT measurements and bitumen samples for the studies. We also thank Drs. Shawn Taylor and John Ratulowski for their support to the project. The ash tests were performed at Maxxam Analytics. M. Sharath Chandra thank the generous support provided by the management of the Alberta Ingenuity Fund.

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