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Measurement of short-term low dynamic interfacial tensions: Application to surfactant enhanced alkaline flooding in enhanced oil recovery
Colloids and Surfaces, 47 (1990) 245-253 Elsevier Science Publishers B.V., Amsterdam
245 -
Printed
in The Netherlands
Measurement of Short-Term Low Dynamic Interfacial Tensions: Application to Surfactant Enhanced Alkaline Flooding in Enhanced Oil Recovery KEVIN C. TAYLOR and LAURIER
L. SCHRAMM
Petroleum Recovery Institute, 3512-33rd St. N. W., Calgary, Alberta T2L 2A6 (Canada) (Received 26 May 1989; accepted
14 November
1989)
ABSTRACT The conventional spinning drop technique does not always provide a clear representation of dynamic interfacial tension behavior because it places a lower limit on the interfacial ages that can be examined. A modification of the conventional technique is described which extends the accessible range to interfacial ages as short as two seconds. By injecting an oil drop into an already spinning tube and simultaneously recording the injected drop image with a video camera, measurements can be made much more quickly than usual. This modification allows the lower limit of interfacial age measurement to be reduced from l-2 min to two seconds. Maintaining the same drop volume and spinning velocity allows results from the conventional and extended techniques to be combined into a single representation. The application of this modified technique is illustrated for crude oil/surfactant enhanced alkaline systems of interest in enhanced oil recovery research. Such systems exhibit complex dynamic interfacial tension behavior including minima which vary in both the magnitude and the interfacial age at which they occur. With the extended technique a more complete characterization was obtained than would have been possible previously.
INTRODUCTION
Occurrence of dynamic inter-facial tensions Dynamic interfacial tension phenomena occur in a number of aqueous/oil systems. In the field of enhanced oil recovery, reservoirs are frequently flooded with solutions designed to produce low interfacial tensions which facilitate oil displacement from reservoir rock. For crude oil/alkaline systems dynamic interfacial tensions have been observed repeatedly [l-4]. In some cases the interfacial tension (IFT) increased dramatically as the interface aged, which was attributed to the migration of soap products away from theinterface after their initial formation. The interest in dynamic IFT in such systems has resulted in
0166-6622/90/$03.50
0 1990 Elsevier Science Publishers
B.V.
246
several attempts to model this behavior, either physically or chemically [511 1. A somewhat more complex process, surfactant-enhanced alkaline flooding, has evolved in which synthetic surfactant is added to a crude oil/alkaline system to obtain improved oil recovery [3,12-141. These systems can also display dynamic IFT behavior. The measurement of short-term low dynamic IFT behavior in these kinds of systems is important in two ways. First, the complete dynamic behavior is needed for mathematical modelling of the process. To date, experimental data has been available only for interfacial ages greater than 1 to 2 min [ 5,111. Yet, by this age the IFT may already have dropped by three orders of magnitude from an initial value of about 30 mN m-l. The accuracy of the models could be improved if data for the earlier ages were available. Secondly, the possible effect of dynamic IFT behavior on oil recovery is of significant interest. Recent results [ 151 suggest that oil recovered through the surfactant-enhanced alkaline flooding of Berea sandstone cores correlates better with “initial” IFT, than with the equilibrium IFT. Since the “initial” values were measured l-2 min after aqueous/oil contact occurred [ 151, they are not true initial values. Thus the behavior of these systems at shorter interface ages is of interest both for potential oil recovery correlation and in terms of the mechanisms involved. To pursue these aspects, a method capable of measuring very low dynamic IFT corresponding to interface ages of between several seconds and several minutes is required. Measurement of dynamic interfacial tensions Many traditional IFT methods are not capable of dynamic measurement, or cannot measure sufficiently low values. These include the capillary rise, du Nouy ring, Wilhelmy plate and drop weight (volume) methods [ 16-191. The pendant drop, oscillating jet and spinning drop methods are more suitable and have been used to study the dynamic IFT across aqueous/oil interfaces [4,5,11,20-261. However, these latter methods have practical limits on the ranges of IFT values and interface ages that can be accessed. Where very low IFT is of interest, the spinning drop technique [31] is preferred, but while this technique can be used to study interface ages extending to several days, the shortest age accessible is about 1 to 2 min. These limitations also apply to the spinning rod tensiometer [ 281.A laminar contracting jet has been used [ 241to measure IF?‘ ( > 40 mN m-’ ) at interfacial ages in the range 0.01 to 1 s. However experimental errors, and the fact that the contracting jet relies on fast contractions due to the area-minimising action of the IFT, appear to limit this technique to systems of fairly high IFT. An oscillating pendant drop technique has been used [ 251to measure IFT ( > 25 mN m-l ) at interface ages between 0.25 and 5 s. Here again only relatively high interfacial tensions are accessible. Sauer et al. [26] used an electrocapillary wave
247
technique to measure IFT ( > 16 mN m- ’ ) at extremely short interface ages corresponding to surface waves of frequency 14 to 800 Hz. This technique also seems only to have been applied to high tension systems, and requires high viscosities as well. The cited studies involved measurement of either high IFT at very short interface ages or else low IFT at longer interface ages, but none are applicable to the measurement of low IFT at short interface ages ( << 1 mN m-’ for 1 to 120 s) . It was decided to extend the range of the spinning drop technique, which is already convenient for measuring low IFT, to shorter interface ages. The conventional spinning drop technique is limited to interface ages greater than about 1 to 2 min which is the time required to place an oil drop in the tube containing the aqueous phase, place the tube in the apparatus, bring the tube to a stable rate of spinning, bring the drop into view, and make the necessary drop shape measurements. In this work a modification of the procedure was used which involves inserting a drop of oil into an already spinning tube containing the aqueous phase and then immediately recording the oil drop shapes using a video camera. With careful drop placement it is possible to obtain interfacial tensions corresponding to interface ages as short as 2 s. The method was used to study the short-term dynamic IFT behavior of an oil interface against some surfactant- and surfactant/polymer-e hanced alkaline systems. EXPERIMENTAL
Materials Surfactant and polymer The surfactant used was Neodol25-38, a commercial alcohol ethoxysulfate (Pecten Chemicals), which was used as received (as a 60% active solution). The polymer used was Dow Pusher lOOOE,a commercial polyacrylamide. Oil The crude oil used was from the David Lloydminster area. It was centrifuged at 9000 g for 70 min to remove suspended solids and water (final water content about 0.5 mass%). The density of the oil was 0.922 g ml-’ and the viscosity was 144 mPa s, both at 23.O”C. Physical properties Densities The sample densities were measured by the vibrating reed principle (Anton Paar, Graz, Austria, Model DMA 46), the refractive indices were measured with a Bausch and Lomb refractometer (Model 33-45-58).
248
Interfacial tensions
Conventional dynamic IFT measurements were made using the spinning drop tensiometer (Univ. of Texas, Model 300) and the procedure of Cayias et al. [ 271. Glass sample tubes of 0.24 ml volume, 2 mm i.d., were obtained from Wilmad Glass Co. (Buena, NJ). Measurements were made at 23.5 + l.O”C. The oil drop volume used was 1.0 ~1, dispensed into a glass capillary full of aqueous solution using a 10 ~1 Hamilton syringe. The tube was then placed into the instrument, accelerated to the desired steady angular velocity and drop shape measurements made. To minimize the measurement errors, angular velocities were optimised for each sample to obtain drops with a length/diameter ratio greater than four whenever possible. The first measurements could usually be made no earlier than about 2 min of interface age. Interfacial tensions were calculated by the procedure of Cayias et al. [ 271. Where the drop length/ drop diameter ratio was greater than 4.0, only the diameter was used, following Vonnegut’s method [ 291. Drop shape measurements were calibrated by placing a stainless steel rod of accurately known diameter in one of the glass tubes
my 0 0
0
Telescope
Motor
-
Syringe
4
Strobe
light
Fig. 1. Schematic diagram of the modified spinning drop apparatus.
249
filled with the aqueous solution of interest. The standard deviation of the IFT measurement was + 5%.
Short-term interfad
tensions
The range of dynamic IFT measurements was extended using modifications to the above apparatus and procedure. First, the capillary tube was filled with only aqueous solution, placed into the instrument, and brought to a previously optimised steady angular velocity. Next 1.0 ~1 of oil was carefully injected into the spinning tube, using a 10 ~1 Hamilton syringe, so as to leave the drop in the field of view of the eyepiece. The apparatus was modified by fitting a video camera (Panasonic, Model WV-3240) to the viewing telescope and video-recording the entire experiment (see Fig. 1). Using an on-screen electronic timer, and by starting recording from before injection of the oil drop, drop images were recorded with a time reference from the initial oil/aqueous phase contact. The angular velocities were chosen from preliminary measurements so as to produce drops whose length to diameter ratio was greater than 4.0. This enabled the use of Vonnegut’s method [ 291 and the measurement of drop diameter only. Replaying the video-recordings allowed drop diameters to be measured from enlarged still frames. The image enlargement factor was determined from replicate measurements of the calibration rod, mentioned above, placed in the aqueous solutions. The standard deviation of these measurements varied somewhat with the magnitude of IFT being measured, typically 2 5% at 0.1 mN m-‘, + 10% at 0.05 mN m-l and + 20% at 0.01 mN m-l. RESULTS AND DISCUSSION
Using the conventional spinning drop technique, interfacial tensions corresponding to interface ages of from about 1 min to 17 h were obtained. Using the modified procedure it was possible to obtain interfacial tensions corresponding to interface ages from about 2 s to 3 min. Greater ages could be measured this way as well. The earliest measurements could only be made at 2 s, which seems to be the time required to form, dislodge and accelerate the drop to the angular velocity of the aqueous solution and tube. It is well-known that the dynamic IFT behavior of crude oil/alkaline systems depends on the interfacial contact areas when there is transfer of surfactant species from the oil to the aqueous phase involved [ 30,311. In the present case the same oil drop volumes and spinning velocities were used in both conventional and modified spinning drop techniques. Thus the effects of interfacial area should be the same in each case. The results from the conventional and extended IFT measurements are shown in Fig. 2 (a-e) for the crude oil in contact with aqueous solutions of constant sodium carbonate and sodium chloride concentration, for increasing concentrations of surfactant. A second set of curves corresponding to the same
250
0.01 a) 0 wtf6 Neodol
f
d) 0.1 0.001
. . . . . ..
. . . . . . ..
. . . . . ..
0.01
0.1
. . . . . . .. 1
, . ...111: 10
wtX Neodol 25-35
I . . . . . . ..
e) 0.5 I ~.,...A
253S
,.....:
wtX Neodol
I . . . . . . .. 100
I....,.: 1000
. .
25-3s . .,..v.+ 10000
Time (minutes)
Fig. 2. Dynamic interfacial tensions for crude oil in the presence of 1 wt% Na2C03, 0.5 wt% NaCl and varying surfactant concentration (Neodol25-38). Data points are from video ( X ) and conventional ( + ) methods.
crude oil/aqueous systems as above but with a constant polymer concentration are shown in Fig. 3 (a-d). It can be seen that the two time curves meet fairly closely in all of the figures so that the combination yields a reasonable representation of the dynamic IFT behavior from 2 s to 30 h. There are some curves for which the agreement is not exact. Due to the nature of the crude oil used, a certain variability is always encountered in studies of this type and the agreement is considered to be good. Several features now emerge that would not have been obvious from either the long-term or short-term measurements by themselves. It was expected that each curve would exhibit a minimum IFT. However, the magnitude of the minimum IFT and the interfacial age at which the minimum occurs can only be determined from the combined curves. A summary is given in Table 1. It can be seen from the table and figures that the interfacial ages at which the IFT
251 1
0.01
0.01 d) 0.5 0.001
. . 0.01
. . . ...
.,.,.:
. . . . . . . .. 1
0.i
10 Time
wtX Neodol
. . . ...
.
25-35
. . . . .. 1000
100
. .
._ 10000
(minutes)
Fig. 3. Dynamic interfacial tensions for crude oil in the presence of 1 wt% Na2C03, 0.5 wt% NaCl, 0.1 wt% polymer (Dow Pusher 1000E) and varying surfactant concentration (Neodol 25-38). Data pointe are from video ( x ) and conventional ( + ) methods.
TABLE I Minimum interfacial tension results Concentration (mass% ) Surfactant
Polymer
0.00 0.01 0.02 0.1 0.5 0.0
0.0 0.0 0.0 0.0 0.0 0.1 0.1 0.1 0.1
0.02
0.1 0.5
Minimum IFT (mN m-l) 0.08 0.009 0.008 0.009 0.04 0.06 0.005
0.01 0.04
Time needed to reach minimum IFT bin) 0.1 3.0 0.6 1.3 1.0 60.0 20.0 4.0 2.0
minima occur are sometimes earlier and sometimes later than the 1 to 2 min previously taken as “initial” interfacial tensions. By considering a fuller range of interfacial ages a clearer representation of the interfacial behavior emerges. For example, Table 1 shows the effect of adding the polymer into the alkaline-surfactant system. The polymer raises the viscosity of the aqueous solution. Apparently, in the higher viscosity medium the diffusion of at least one of the chemical species is reduced since the minimum IFT now occurs at much longer interfacial ages. For example, in 0.02 mass% surfactant solution minimum IFT values of 0.005 to 0.008 mN m-l are reached in 0.6 min for a viscosity similar to that of water, while in the higher viscosity polymer solution (about 14 mPa s) the minimum was reached only after 20 min. The 1 to 2 min “initial” IFT values from the conventional method are, of course, not initial values at all. Neither are they always the minimum IFT values. Future work will address the question of whether it is the minimum IFT or the IFT value at some specific interfacial age that best correlates with oil recovery in a surfactant-enhanced alkaline flood. From the present work we note that in either case, and indeed to make such distinctions, a combination of the short-term and conventional IFT measurements is needed. CONCLUSIONS
Crude oil/aqueous systems can exhibit complex dynamic IFT behavior. This is especially so in the field of surfactant-enhanced alkaline flooding in enhanced oil recovery, where the aqueous phase may contain various concentrations of alkali, surfactant and polymer. Such systems exhibit IPI’ minima which not only vary in magnitude but also in the interface age at which they occur. It is shown that the conventional spinning drop technique does not always provide a clear representation of the dynamic IFT behavior because it is not capable of measurements at interfacial ages of less than 1 min. A simple extension of the technique allows this limit to be reduced from 1 to 2 min to about 2 s. By keeping the same relative oil drop sizes and geometry with respect to the aqueous solutions, results from these two techniques can be combined into a single representation. With the expanded range of interfacial ages accessible, the surfactant-enhanced alkaline systems of the present work could be characterised more completely than would have been possible previously. The modified spinning drop technique should be useful in other areas where the low IFT magnitude and moderate interfacial ages of interest are not accessible by other techniques.
253 REFERENCES
I 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
H. Atkinson, U.S. Pat. No 1,651,311 (1927). P.G. Nutting, Oil Gas J., 25 (1927) 76; 150. J. Reisberg and T.M. Doscher, Prod. Mon., November (1956) 43. F.G. McCaffery, J. Can. Pet. Technol., 15 (1976) 71. E. Rubin and C.J. Radke, Chem. Eng. Sci., 35 (1980) 1129. M.M. Sharma L.K. Jang and T.F. Yen, Preprint, SPE/DOE 4th Symp. on Enhanced Oil Recovery, Tulsa, OK, 15-18 April SPE/DOE paper 12669,1984. E.F. dezabala and C.J. Radke, SPE Res. Eng., 1 (1986) 29. E.M. Trujillo, Sot. Pet. Eng. J., 23 (1983) 645. T.S. Ramakrishnan and D.T. Wasan, Sot. Pet. Eng. J., 23 (1983) 602. M. Chan. and T.F. Yen, Can. J. Chem. Eng., 60 (1982) 305. R.P. Borwankar and D.T. Wasan, AIChE J., 32 (1986) 455. R.C. Nelson, J.B. Lawson, D.R. Thigpen and G.L. Stegemeier, SPE/DOE 4th Symp. on Enhanced Oil Recovery, Tulsea, OK, 15-18 April, SPE/DOE paper 12672,1984. S.M. Saleem and M.J. Faber, Rev. T&z. Intevep, 6 (1986) 133. P.J. Shuler, D.L. Kuehne and R.M. Lerner, J. Pet. Technol., 41 (1989) 80. K.C. Taylor, B.F. Hawkins and M.R. Islam, J. Can. Pet. Technol., 29 (1990) 50. W.C. Reynolds, J. Chem. Sot., 119 (1921) 460. H.H. Zuidema and G.W. Waters, Ind. Eng. Chem., 13 (1941) 312. W.D. Harkins and E.C. Humphery, J. Am. Chem. Sot., 38 (1916) 228. C.A. Miller and P. Neogi, Interfacial Phenomena Equilibrium and Dynamic Effects, Marcel Dekker, New York, 1985, pp. 29-36. R.P. Borwankar and D.T. Wasan, AIChE J., 32 (1986) 467. M. Burkowsky and C. Marx, Oil Gas Eur., 3 (1977) 3. D.R. Babu, V. Hornof and G. Neale, Can. J. Chem. Eng., 62 (1984) 156. Y. Huang, P. Yang and T. Qin, Shiyou Kantan Yu Kaifa, 14 (1987) 37,81; Chem. Abst., 108 (1988) 170346h. D.C. England and J.C. Berg, AIChE J., 17 (1971) 313. A.A. Badran and E. Marschall, Rev. Sci. Instrum., 57 (1986) 259. B.B. Sauer, J. Skarlupka, M. Sano and H. Yu, Polym. Prepr. Am. Chem. Sot. Div. Polym. Chem., 28 (1987) 20. J.L. Cayias, R.S. Schechter and W.H. Wade, ACS Symp. Ser., 8 (1975) 234. P. Than, L. Preziosi, D.D. Joseph and M. Arney, J. Colloid Interface Sci., 124 (1988) 552. B. Vonnegut, Rev. Sci. Instrum., 13 (1942) 6. J.B. Brown and C.J. Radke, Chem. Eng. Sci., 35 (1980) 1458. C.J. Radke and W.H. Some&on, in B. Linville (Ed.), Proc. 3rd ERDA Symp. Enhanced Oil Rec. Improved Drill. Methods, Pet. Pub. Co., 1 (1977) B5/1.
245 -
Printed
in The Netherlands
Measurement of Short-Term Low Dynamic Interfacial Tensions: Application to Surfactant Enhanced Alkaline Flooding in Enhanced Oil Recovery KEVIN C. TAYLOR and LAURIER
L. SCHRAMM
Petroleum Recovery Institute, 3512-33rd St. N. W., Calgary, Alberta T2L 2A6 (Canada) (Received 26 May 1989; accepted
14 November
1989)
ABSTRACT The conventional spinning drop technique does not always provide a clear representation of dynamic interfacial tension behavior because it places a lower limit on the interfacial ages that can be examined. A modification of the conventional technique is described which extends the accessible range to interfacial ages as short as two seconds. By injecting an oil drop into an already spinning tube and simultaneously recording the injected drop image with a video camera, measurements can be made much more quickly than usual. This modification allows the lower limit of interfacial age measurement to be reduced from l-2 min to two seconds. Maintaining the same drop volume and spinning velocity allows results from the conventional and extended techniques to be combined into a single representation. The application of this modified technique is illustrated for crude oil/surfactant enhanced alkaline systems of interest in enhanced oil recovery research. Such systems exhibit complex dynamic interfacial tension behavior including minima which vary in both the magnitude and the interfacial age at which they occur. With the extended technique a more complete characterization was obtained than would have been possible previously.
INTRODUCTION
Occurrence of dynamic inter-facial tensions Dynamic interfacial tension phenomena occur in a number of aqueous/oil systems. In the field of enhanced oil recovery, reservoirs are frequently flooded with solutions designed to produce low interfacial tensions which facilitate oil displacement from reservoir rock. For crude oil/alkaline systems dynamic interfacial tensions have been observed repeatedly [l-4]. In some cases the interfacial tension (IFT) increased dramatically as the interface aged, which was attributed to the migration of soap products away from theinterface after their initial formation. The interest in dynamic IFT in such systems has resulted in
0166-6622/90/$03.50
0 1990 Elsevier Science Publishers
B.V.
246
several attempts to model this behavior, either physically or chemically [511 1. A somewhat more complex process, surfactant-enhanced alkaline flooding, has evolved in which synthetic surfactant is added to a crude oil/alkaline system to obtain improved oil recovery [3,12-141. These systems can also display dynamic IFT behavior. The measurement of short-term low dynamic IFT behavior in these kinds of systems is important in two ways. First, the complete dynamic behavior is needed for mathematical modelling of the process. To date, experimental data has been available only for interfacial ages greater than 1 to 2 min [ 5,111. Yet, by this age the IFT may already have dropped by three orders of magnitude from an initial value of about 30 mN m-l. The accuracy of the models could be improved if data for the earlier ages were available. Secondly, the possible effect of dynamic IFT behavior on oil recovery is of significant interest. Recent results [ 151 suggest that oil recovered through the surfactant-enhanced alkaline flooding of Berea sandstone cores correlates better with “initial” IFT, than with the equilibrium IFT. Since the “initial” values were measured l-2 min after aqueous/oil contact occurred [ 151, they are not true initial values. Thus the behavior of these systems at shorter interface ages is of interest both for potential oil recovery correlation and in terms of the mechanisms involved. To pursue these aspects, a method capable of measuring very low dynamic IFT corresponding to interface ages of between several seconds and several minutes is required. Measurement of dynamic interfacial tensions Many traditional IFT methods are not capable of dynamic measurement, or cannot measure sufficiently low values. These include the capillary rise, du Nouy ring, Wilhelmy plate and drop weight (volume) methods [ 16-191. The pendant drop, oscillating jet and spinning drop methods are more suitable and have been used to study the dynamic IFT across aqueous/oil interfaces [4,5,11,20-261. However, these latter methods have practical limits on the ranges of IFT values and interface ages that can be accessed. Where very low IFT is of interest, the spinning drop technique [31] is preferred, but while this technique can be used to study interface ages extending to several days, the shortest age accessible is about 1 to 2 min. These limitations also apply to the spinning rod tensiometer [ 281.A laminar contracting jet has been used [ 241to measure IF?‘ ( > 40 mN m-’ ) at interfacial ages in the range 0.01 to 1 s. However experimental errors, and the fact that the contracting jet relies on fast contractions due to the area-minimising action of the IFT, appear to limit this technique to systems of fairly high IFT. An oscillating pendant drop technique has been used [ 251to measure IFT ( > 25 mN m-l ) at interface ages between 0.25 and 5 s. Here again only relatively high interfacial tensions are accessible. Sauer et al. [26] used an electrocapillary wave
247
technique to measure IFT ( > 16 mN m- ’ ) at extremely short interface ages corresponding to surface waves of frequency 14 to 800 Hz. This technique also seems only to have been applied to high tension systems, and requires high viscosities as well. The cited studies involved measurement of either high IFT at very short interface ages or else low IFT at longer interface ages, but none are applicable to the measurement of low IFT at short interface ages ( << 1 mN m-’ for 1 to 120 s) . It was decided to extend the range of the spinning drop technique, which is already convenient for measuring low IFT, to shorter interface ages. The conventional spinning drop technique is limited to interface ages greater than about 1 to 2 min which is the time required to place an oil drop in the tube containing the aqueous phase, place the tube in the apparatus, bring the tube to a stable rate of spinning, bring the drop into view, and make the necessary drop shape measurements. In this work a modification of the procedure was used which involves inserting a drop of oil into an already spinning tube containing the aqueous phase and then immediately recording the oil drop shapes using a video camera. With careful drop placement it is possible to obtain interfacial tensions corresponding to interface ages as short as 2 s. The method was used to study the short-term dynamic IFT behavior of an oil interface against some surfactant- and surfactant/polymer-e hanced alkaline systems. EXPERIMENTAL
Materials Surfactant and polymer The surfactant used was Neodol25-38, a commercial alcohol ethoxysulfate (Pecten Chemicals), which was used as received (as a 60% active solution). The polymer used was Dow Pusher lOOOE,a commercial polyacrylamide. Oil The crude oil used was from the David Lloydminster area. It was centrifuged at 9000 g for 70 min to remove suspended solids and water (final water content about 0.5 mass%). The density of the oil was 0.922 g ml-’ and the viscosity was 144 mPa s, both at 23.O”C. Physical properties Densities The sample densities were measured by the vibrating reed principle (Anton Paar, Graz, Austria, Model DMA 46), the refractive indices were measured with a Bausch and Lomb refractometer (Model 33-45-58).
248
Interfacial tensions
Conventional dynamic IFT measurements were made using the spinning drop tensiometer (Univ. of Texas, Model 300) and the procedure of Cayias et al. [ 271. Glass sample tubes of 0.24 ml volume, 2 mm i.d., were obtained from Wilmad Glass Co. (Buena, NJ). Measurements were made at 23.5 + l.O”C. The oil drop volume used was 1.0 ~1, dispensed into a glass capillary full of aqueous solution using a 10 ~1 Hamilton syringe. The tube was then placed into the instrument, accelerated to the desired steady angular velocity and drop shape measurements made. To minimize the measurement errors, angular velocities were optimised for each sample to obtain drops with a length/diameter ratio greater than four whenever possible. The first measurements could usually be made no earlier than about 2 min of interface age. Interfacial tensions were calculated by the procedure of Cayias et al. [ 271. Where the drop length/ drop diameter ratio was greater than 4.0, only the diameter was used, following Vonnegut’s method [ 291. Drop shape measurements were calibrated by placing a stainless steel rod of accurately known diameter in one of the glass tubes
my 0 0
0
Telescope
Motor
-
Syringe
4
Strobe
light
Fig. 1. Schematic diagram of the modified spinning drop apparatus.
249
filled with the aqueous solution of interest. The standard deviation of the IFT measurement was + 5%.
Short-term interfad
tensions
The range of dynamic IFT measurements was extended using modifications to the above apparatus and procedure. First, the capillary tube was filled with only aqueous solution, placed into the instrument, and brought to a previously optimised steady angular velocity. Next 1.0 ~1 of oil was carefully injected into the spinning tube, using a 10 ~1 Hamilton syringe, so as to leave the drop in the field of view of the eyepiece. The apparatus was modified by fitting a video camera (Panasonic, Model WV-3240) to the viewing telescope and video-recording the entire experiment (see Fig. 1). Using an on-screen electronic timer, and by starting recording from before injection of the oil drop, drop images were recorded with a time reference from the initial oil/aqueous phase contact. The angular velocities were chosen from preliminary measurements so as to produce drops whose length to diameter ratio was greater than 4.0. This enabled the use of Vonnegut’s method [ 291 and the measurement of drop diameter only. Replaying the video-recordings allowed drop diameters to be measured from enlarged still frames. The image enlargement factor was determined from replicate measurements of the calibration rod, mentioned above, placed in the aqueous solutions. The standard deviation of these measurements varied somewhat with the magnitude of IFT being measured, typically 2 5% at 0.1 mN m-‘, + 10% at 0.05 mN m-l and + 20% at 0.01 mN m-l. RESULTS AND DISCUSSION
Using the conventional spinning drop technique, interfacial tensions corresponding to interface ages of from about 1 min to 17 h were obtained. Using the modified procedure it was possible to obtain interfacial tensions corresponding to interface ages from about 2 s to 3 min. Greater ages could be measured this way as well. The earliest measurements could only be made at 2 s, which seems to be the time required to form, dislodge and accelerate the drop to the angular velocity of the aqueous solution and tube. It is well-known that the dynamic IFT behavior of crude oil/alkaline systems depends on the interfacial contact areas when there is transfer of surfactant species from the oil to the aqueous phase involved [ 30,311. In the present case the same oil drop volumes and spinning velocities were used in both conventional and modified spinning drop techniques. Thus the effects of interfacial area should be the same in each case. The results from the conventional and extended IFT measurements are shown in Fig. 2 (a-e) for the crude oil in contact with aqueous solutions of constant sodium carbonate and sodium chloride concentration, for increasing concentrations of surfactant. A second set of curves corresponding to the same
250
0.01 a) 0 wtf6 Neodol
f
d) 0.1 0.001
. . . . . ..
. . . . . . ..
. . . . . ..
0.01
0.1
. . . . . . .. 1
, . ...111: 10
wtX Neodol 25-35
I . . . . . . ..
e) 0.5 I ~.,...A
253S
,.....:
wtX Neodol
I . . . . . . .. 100
I....,.: 1000
. .
25-3s . .,..v.+ 10000
Time (minutes)
Fig. 2. Dynamic interfacial tensions for crude oil in the presence of 1 wt% Na2C03, 0.5 wt% NaCl and varying surfactant concentration (Neodol25-38). Data points are from video ( X ) and conventional ( + ) methods.
crude oil/aqueous systems as above but with a constant polymer concentration are shown in Fig. 3 (a-d). It can be seen that the two time curves meet fairly closely in all of the figures so that the combination yields a reasonable representation of the dynamic IFT behavior from 2 s to 30 h. There are some curves for which the agreement is not exact. Due to the nature of the crude oil used, a certain variability is always encountered in studies of this type and the agreement is considered to be good. Several features now emerge that would not have been obvious from either the long-term or short-term measurements by themselves. It was expected that each curve would exhibit a minimum IFT. However, the magnitude of the minimum IFT and the interfacial age at which the minimum occurs can only be determined from the combined curves. A summary is given in Table 1. It can be seen from the table and figures that the interfacial ages at which the IFT
251 1
0.01
0.01 d) 0.5 0.001
. . 0.01
. . . ...
.,.,.:
. . . . . . . .. 1
0.i
10 Time
wtX Neodol
. . . ...
.
25-35
. . . . .. 1000
100
. .
._ 10000
(minutes)
Fig. 3. Dynamic interfacial tensions for crude oil in the presence of 1 wt% Na2C03, 0.5 wt% NaCl, 0.1 wt% polymer (Dow Pusher 1000E) and varying surfactant concentration (Neodol 25-38). Data pointe are from video ( x ) and conventional ( + ) methods.
TABLE I Minimum interfacial tension results Concentration (mass% ) Surfactant
Polymer
0.00 0.01 0.02 0.1 0.5 0.0
0.0 0.0 0.0 0.0 0.0 0.1 0.1 0.1 0.1
0.02
0.1 0.5
Minimum IFT (mN m-l) 0.08 0.009 0.008 0.009 0.04 0.06 0.005
0.01 0.04
Time needed to reach minimum IFT bin) 0.1 3.0 0.6 1.3 1.0 60.0 20.0 4.0 2.0
minima occur are sometimes earlier and sometimes later than the 1 to 2 min previously taken as “initial” interfacial tensions. By considering a fuller range of interfacial ages a clearer representation of the interfacial behavior emerges. For example, Table 1 shows the effect of adding the polymer into the alkaline-surfactant system. The polymer raises the viscosity of the aqueous solution. Apparently, in the higher viscosity medium the diffusion of at least one of the chemical species is reduced since the minimum IFT now occurs at much longer interfacial ages. For example, in 0.02 mass% surfactant solution minimum IFT values of 0.005 to 0.008 mN m-l are reached in 0.6 min for a viscosity similar to that of water, while in the higher viscosity polymer solution (about 14 mPa s) the minimum was reached only after 20 min. The 1 to 2 min “initial” IFT values from the conventional method are, of course, not initial values at all. Neither are they always the minimum IFT values. Future work will address the question of whether it is the minimum IFT or the IFT value at some specific interfacial age that best correlates with oil recovery in a surfactant-enhanced alkaline flood. From the present work we note that in either case, and indeed to make such distinctions, a combination of the short-term and conventional IFT measurements is needed. CONCLUSIONS
Crude oil/aqueous systems can exhibit complex dynamic IFT behavior. This is especially so in the field of surfactant-enhanced alkaline flooding in enhanced oil recovery, where the aqueous phase may contain various concentrations of alkali, surfactant and polymer. Such systems exhibit IPI’ minima which not only vary in magnitude but also in the interface age at which they occur. It is shown that the conventional spinning drop technique does not always provide a clear representation of the dynamic IFT behavior because it is not capable of measurements at interfacial ages of less than 1 min. A simple extension of the technique allows this limit to be reduced from 1 to 2 min to about 2 s. By keeping the same relative oil drop sizes and geometry with respect to the aqueous solutions, results from these two techniques can be combined into a single representation. With the expanded range of interfacial ages accessible, the surfactant-enhanced alkaline systems of the present work could be characterised more completely than would have been possible previously. The modified spinning drop technique should be useful in other areas where the low IFT magnitude and moderate interfacial ages of interest are not accessible by other techniques.
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