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RECENT ADVANCES IN THE STUDY OF LOW INTERFACIAL TENSIONS
RECENT ADVANCES IN THE STUDY OF LOW INTERFACIAL TENSIONS James C. Morgan, Robert S. Schechter and William H. Wade
The University of Texas at Austin I.
ABSTRACT The study of interfacial tensions in an oil/surfactant/ water system is greatly facilitated by use of the alkane model. In this, the crude oil is replaced by that member of the homologous n-alkane series which shows interfacial tension behavior identical to that of the crude oil. A complete scale of hydrocarbon properties is used to study system variables affecting the low interfacial tension state. II.
SCOPE Almost 70% of the total proven crude oil reserves will still be in the ground when production by standard techniques ceases to be economic ( 1 ) . A successful tertiary oil recovery process, to supplement the present methods of pumping under natural forces, followed by a secondary water flood, is now an urgent research objective. In the forefront of proposed methods is chemical flooding with surfactant solutions. Two distinct types of surfactant flooding are possible, those employing high and low surfactant concentrations. The Maraflood process (2-4) is an example of a high concentration process which has been shown to enhance oil recovery under field conditions. A small slug of micellar solution of surfactant, such as petroleum sulfonate, is used at about 1 2 % volume concentration. Oil displacement is initially by a miscible process in this type of system, but Healy and Reed (5-6) have shown that the single phase microemulsion of oil, water and surfactant first involved breaks down into a m u l t i phase emulsion when mixing dilutes the surfactant slug. Further displacement is then by an immiscible process. The alternative is to use a continuous low concentration of surfactant (2% or much l e s s ) , all displacement then being immiscible. A possible advantage here is that considerably less surfactant may be required overall. Possible drawbacks are that loss of surfactant from solution by adsorption onto the rock surface, or by solubilization into the oil, may more seriously affect oil recovery. Also, the method may be more sensitive to salinity and temperature changes than is miscible
101
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displacement. However, as miscible displacement inevitably degenerates by dilution to an immiscible process, understanding and control of immiscible displacement is of paramount importance. Melrose and Brandner (7) and Taber (8) have shown that successful immiscible oil displacement depends on the existence of a very low interfacial tension, γ, between the oil and the water phases. A value of about 10~3 dyne/cm or less is required to mobilize the oil. Certainly, recovery of residual oil from laboratory test cores is greatly improved for systems with ultra low interfacial tension ( 7 ) . The achievement and maintenance of low interfacial tensions during chemical flooding therefore seems essential. Practical surfactant flood systems are usually very complex: present are surfactant, oil, water, electrolyte and probably a thickening agent for viscosity control, plus cosurfactants and blocking agents to enhance or protect the main surfactant. Surfactants which combine an ability to produce a low interfacial surface tension with low cost and large scale availability are not common. Petroleum sulfonates are perhaps the main candidates. These are usually sodium salts of sulfonated crude oil. The oil is fractionated by molecular weight before sulfonation, but the surfactant produced still has a range of molecular weights which may be broad. Crude oils themselves are also very complex, and vary considerably from field to field. The salinity of formation water, and type of ions present, will change with the field, as will the temperature. This type of practical system is difficult to use in the study of variables affecting low interfacial tension. Changing the nature of one component of the system will demonstrate if, and in what manner, it affects γ, but quantitative assessments during this approach are not straightforward. In particular, achievement of a sufficiently low γ for one system does not show immediately how the system will react if a different crude oil is used. III.
THE ALKANE MODEL The model system introduced by Cayias e£ al. (9), in which a pure n-alkane replaces the crude oil, solves the above problem. Development of the alkane model began when a study of the interfacial tension of a series of pure n-alkane drops against a petroleum sulfonate saline solution revealed that only one alkane gave a really low γ. An example of this specificity is shown in Figure 1. Here a solution containing 0.2% by weight of the petroleum sulfonate Witco 10-80, plus 1%
A D V A N C E S IN LOW INTERFACIAL TENSIONS
103
INTERFACIAL TENSIONS OF 10-80 ( 0 . 2 % , I % No CI) VS: .0°.
—n-ALKANES — O - n-ALKYLBENZENES n-ALKYLCYCLOHEXANES
Id'-
Υ
«ι
ιο" 3
ιο-Ο 4
Fig. 1.
4 8 ALKYL GROUP CARBON NUMBER
12
Interfacial tensions of three homologous hydrocarbon series with 0.2 wt.% 10-80 (1.0 wt.% NaCl)
by weight sodium chloride, gives the lowest γ against heptane. When a series of 1-phenyl n-alkanes is tested, the same surfactant phase gives a minimum at heptyl benzene. Similarly, a minimum γ is found at butyl cyclohexane for this surfactant phase against a series of 1-cyclohexyl n-alkanes (Figure 1 ) . Thus, heptane, heptyl benzene and butyl-cyclohexane may be said to act as equivalent oil droplets. This equivalency is virtually independent of the nature of the surfactant phase. Comparisons such as this using different surfactant systems have been combined with a study of hydrocarbon m i x tures (10) to produce the concept of the equivalent alkane carbon number (EACN) for pure hydrocarbons and their mixtures (11). For instance, as heptyl benzene is equivalent to heptane, the EACN of heptyl benzene is 7. The EACN of butyl cyclohexane is similarly 7. Mixtures of hydrocarbons average very simply. If a surfactant phase gives a low, minimum γ against dodecane for example, it will also give a low γ against a mixture of 0.5 mole fraction decane and 0.5 mole fraction tetradecane. The mixture therefore has an EACN of 1 2 . All
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other mixtures of decane and tetradecane will give higher interfacial tensions against this surfactant. Any pure hydrocarbon, whether a member of a homologous series or not, may be assigned its individual EACN by testing in binary mixtures with an alkane. An average EACN may be calculated for any mixture of hydrocarbons, providing the EACN of each component is known. This is found from
( E A C N )
MIXTURE
= I
(
E
A
C
N
)
i i X
(
1
)
1 where X is the mole fraction of the i*-* component of the mixture. Values of EACN are not confined to integers, so that the EACN of an equal mole fraction mixture of heptane and octane is 7.5, for instance. By coincidence, all three hydrocarbon series in Figure 1 give minima almost exactly at integral carbon numbers, but in Figure 4 only two minima are at integral carbon numbers. The EACN of a crude oil cannot be calculated directly using Equation ( 1 ) , because all the hydrocarbon components of a specific crude oil have never been identified. However, applicability of Equation (1) to crude oils is inferred by its successful application to complex synthetic mixtures containing up to 29 aliphatic, alicyclic and aromatic hydrocarbons (11). Measurements of the EACN of a crude oil requires the preparation of a series of surfactant solutions giving individual minimum tensions against steadily increasing alkane carbon numbers. The crude oil is then tested against each surfactant solution, and the lowest γ found shows the EACN of the oil. By changing the surfactant series used, it may be demonstrated that the EACN of a crude oil is a property essentially characteristic of the oil, and not of the surfactant type used. EACN values varying from 6.2 to 8.6 have been reported for eight stock tank oils (11). This variation, though apparently small, is extremely important because most surfactant systems are very selective. If a surfactant gives a low tension of 10~3 dyne/cm or less with the 8.6 EACN oil, it will probably give a far higher tension of perhaps 10"^ dyne/cm with the 6.2 EACN, and vice versa. Use of the complete alkane series instead of individual crude oils has several advantages. Firstly, it is immediately evident whether any particular surfactant system is optimally adjusted for giving a low γ with any given crude oil by comparing the alkane of minimum γ with the EACN of the oil. Also, the effect of changing one system variable on the a l kane of minimum tension may be studied in detail, because a wide range of alkanes is now available. With these variable 1
i
A D V A N C E S IN LOW INTERFACIAL TENSIONS
105
rules complete, adjustment of a surfactant system to any desired crude oil EACN becomes simple. IV.
PARAMETERS AFFECTING LOW INTERFACIAL TENSIONS Variables currently identified as important in the achievement of the low interfacial tension state in a water/ oil/surfactant/electrolyte system are: the surfactant average molecular weight and molecular weight distribution; surfactant molecular structure; surfactant concentration; electrolyte concentration and type; oil phase average molecular weight and structure; system temperature; and the age of the system. This list includes variables which may be deliberately tailored to give a low-tension surfactant flood system for a particular field, and others which may be expected to vary during production from a field or from field to field. Several variables are, of course, in both groups. The effect of changing each variable on interfacial tensions is now surveyed. All surface tensions referred to as "low or "minimal" have values of 2 χ 10"^ dyne/cm or lower unless otherwise stated. The standard aqueous phase has a total surfactant concentration of 0.2% by weight and a sodium chloride concentration of 1% by weight, the oil phase being an alkane. No other components are present in the surfactant phase unless mentioned. All petroleum sulfonates and xylene sulfonates have been deoiled on a silica gel column prior to use by a method described elsewhere ( 1 2 ) . No preequilibration of the oil and aqueous phases was employed before measurement of the interfacial tensions, which are made at 27°C using the spinning drop technique ( 1 3 ) . In early papers (10,11), this technique was not standard, and results from these papers have been repeated before inclusion here if comparison with later results is to be made. In fact, differences found on repetition were usually within experimental error, which is considered to be ±0.3 of a alkane carbon number. The molecular weights of petroleum and xylene sulfonates quoted here are, strictly, equivalent weights, determined by estimation of the SO-^Na content of a known weight of surfactant. 11
V.
SURFACTANT AVERAGE MOLECULAR WEIGHT A linear relationship has been found between the average molecular weight of a mixture of two surfactants and n ^ , the carbon number of the n-alkane with which the mixture gives a minimum interfacial tension (at constant total surfactant concentration, salinity and temperature). This holds for all but one (14) of the many binary mixtures studied to date (11, 14,15). The individual surfactants may each be of reasonably well-defined structure, such as the two Exxon alkyl orthoxylene m
n
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M O R G A N et al.
sodium sulfonates (11) (Figure 2, approximate molecular structures in Figures 3(a) and 3 ( b ) ) . Also, each component
cΕ
.
— Δ — MARTINEZ 470·**MARTINEZ
.
—Ο-MARTINEZ <—Φ-> C
3 2
+C
E
IE
380
3 8 0 + C, O-XYLENE S0 NO E
Ο-XYLENE
3
S O , ΝΑ
- Ό - 10-80 + C O-XYLENE |E
S0 NA 8
0~C
XYLENES
| 5
Ο
(Τ ζ ο ω Κ < ο ω ζ <
/
/
•
ιο-βο
p
/
tf—C
/
XYLENES
E
'
A L L AT 0.2 % TOTAL SURFACTANT, L%SALT 380
400
420
AVERAGE
Fig. 2.
440
EQUIV.
460
480
WT.
Dependence of alkane carbon number of minimum interfacial tension (n ^ ) on the average equivalent weight of a mixture of two surfactants m
n
Η H
l5 7~9" e l7 C
C
H
CH -CH-£CH -CH 3
2
(A) C O - X Y L E N E | 5
CH,
S0 NA
(C) 8TF>C S0 NA
3
CH,
|6
/ = <
3
H
3^C-C
3
R 4
H
2 9
CH -CH-(-CH -CH 3
2
S0 NA 3
(B) C
Fig. 3.
|2
O-XYLENE
S0 NA 3
Representative surfactant structures (approximate for the alkyl o-xylene sulfonates)
A D V A N C E S IN LOW INTERFACIAL TENSIONS
107
might contain a range of molecular weights and structures, as with mixtures of the two Shell petroleum sulfonates Martinez 380 and Martinez 470 (11) in Figure 2. The linear molecular w e i g h t / n ^ ^ relationship again holds when surfactants of completely different types are mixed, such as Martinez 380 with the 0-^5 o-xylene sulfonate (14) (Figure 2 ) . It may not hold for mixtures of two isomerically pure surfactants, however (14). Of practical importance is that the composition of a surfactant mixture giving the lowest γ with any given crude oil may now be calculated. If, for instance, the crude oil has an EACN of 8, and a mixture of Martinez 380 and 470 is to be used at a 0.2% by weight overall concentration in 1% by weight NaCl solution, then Figure 2 shows that a surfactant mixture of average molecular weight 440 is required. The surfactant requirements can be found using, Wt. of Martinez 470 in g/1 Wt. of Martinez 380 in g/1
=
_470 380
(440-380) 470-440
.
( K
}
No other 380/470 mixture will give this low a γ against the crude oil. Figure 2 shows that a mixture of C^2 * 1 5 l"~ fonates of average molecular weight 400 will also give a low tension with a crude oil of EACN = 8. The linear averaging rule allows prediction of for any mixture of two surfactants providing the individual value of is known for each component. For some individual surfactants, such as C^2 o-xylene sulfonate and Martinez 380, a value of cannot be measured directly because it is not within the liquid alkane range. Reliable values of may be found by extrapolation, however (14). The alkane carbon number of minimum γ for a mixture of two surfactants is then given by a n c
mm
MIXTURE
A
mm
A
Β
mm
c
su
Β
Testing of Equation (3) when expanded to deal with three or more surfactant components is not yet complete. VI.
SURFACTANT MOLECULAR WEIGHT DISTRIBUTION To treat the surfactant molecular weight distribution as an independent variable is not practicable at present. If the surfactant average molecular weight is kept constant and the range of molecular weights is changed, then the "mean structure type" is almost certain to change also, and the surfactant structure is known to be a variable to which n ± is particularly sensitive. However, the presence of a distribution of surfactant molecular weights is certainly an important factor because it introduces to the system a dependence on overall surfactant concentration, discussed later. A m
n
108
MORGAN et al.
susceptibility to aging phenomena is also introduced by the presence of a range of surfactant molecular weights and structures. For example, Cash jet al. (17) find that for 10-80 shifts gradually to lighter alkanes over a period of months. The origin of this aging process is not understood, but it is only found for complex mixtures such as petroleum sulfonates. VII.
SURFACTANT STRUCTURE There is no single line of surfactant molecular weight against i ^ ^ , although lines connecting surfactant pairs of similar structure (such as two o-xylene sulfonates, or two Martinez petroleum sulfonates) have similar slopes (Figure 2 ) . Surfactants, or surfactant mixtures, of the same average molecular weight may be far apart on the ΐΐ ± scale. For instance, 15 ° " " sulfonate and the Martinez 380-470 mixture of the same average molecular weight (420) are nine alkane units apart. Doe and Wade (16) have shown that shifts of this magnitude are due to differences in surfactant molecular structure. Shifts in n ^ are seen when a series of isomers of n-hexadecyl benzene sodium sulfonate (molecular weight 404) are studied. There are eight possible para-isomers, of which two, the 8phenyl hexadecane sulfonate, and the much less highly branched 2-phenyl isomer, are illustrated in Figures 3(c) and 3 ( d ) . Interfacial tensions of each isomer against the range of a l kanes are shown in Figure 4. These results are for a system containing 0.07% surfactant by weight and 0.3% sodium chloride. Isopentanol (2% by volume) is also added as a cosurfactant in order to increase the solubility of the 3 and 2 isomers. The 1-phenyl isomer is not soluble. In isopentanol-free solution, the trend of Figure 4 is repeated qualitatively, but not quantitatively, for isomers remaining soluble. αί
C
η
x v l e n e
m
n
Figure 4 shows that as the branching of the alkyl chain increases from the 2 isomer towards the 8 isomer, there is a steady movement of the value of to higher alkanes (at a fixed molecular w e i g h t ) . This result probably explains the differences between surfactants of less well-defined structure. For example, if surfactants of molecular weight 415 are compared in Figure 2, the o-xylene sulfonate mixture is at the highest Π β ^ . The o-xylene sulfonates have highly branched main alkyl groups (Figure 3(a)) because of their method of manufacture, and they also have two additional alkyl groups, albeit only methyls, on the benzene ring. Witco 10-80 has a lower ii ± than the xylene sulfonate mix, and therefore should have a less highly branched alkyl chain structure, on the average, though this remains to be proven. Further work on structural effects must rely heavily on the study of surfactants of absolutely known structure. The structure, and even the molecular weights of the petroleum sulfonates cannot be m
Ti
A D V A N C E S IN LOW INTERFACIAL TENSIONS
Fig. 4.
109
Interfacial tensions of n-alkanes with isomers of phenyl hexadecane sulfonate
defined well e n o u g h — t h e manufacturers quoted equivalent weights will be below the true molecular weights if disulfonates are present. This makes petroleum sulfonates largely unsuitable for study of shifts at constant molecular weight. The noted effect of branching is of great interest in selecting surfactants for a practical surfactant flood system. To achieve a low tension with a crude oil of given EACN, the choice lies between a high molecular weight, straight alkyl chain type of structure and a low molecular weight highly branched structure. The branched surfactant may have several advantages. It will be more soluble, so that it may be used in higher concentration if required, and its increased solubility will probably lead to decreased adsorption on the reservoir rock. It will also block a much greater surface area of rock than will an adsorbed, straight-chain molecule. Lower molecular weight surfactants are also far more tolerant of increasing salt concentration, as is discussed below. 1
110
M O R G A N et al.
VIII.
SURFACTANT
CONCENTRATION
The concentration of surfactant in a solution containing only one surfactant molecular species may be varied over a wide range without affecting (at constant salinity, e t c . ) . For example, 8-phenyl n-hexadecane sodium sulfonate (Figure 3(c)) has a fixed at any concentration down to its c.m.c. (16). For C - j c o-xylene sulfonate at 1% salt, stays fixed at 12.4 at all surfactant concentrations from 1% down to about 0.01% (Figure 5)· The value of at 0.01% is slightly shifted, and the minimum γ slightly raised (at 6 χ 10"3 dyne/ cm) when compared with results at higher concentrations. For
Δ
Δ
12-
C, ο - X Y L E N E 5
A
S0 Na (l%NaCI) 5
10·
8-
^
^
^
^
10-80
(l%NaCI)
6
Ο ()
0.2
0.4
SURFACTANT
Fig. 5.
0.6
0.8
ΙΟ
CONCENTRATION ( W t . % )
Variation of alkane carbon number of minimum tension ( hni/n) ^tfo surfactant concentration at 1.0 wt.% NaCl r
w
12 °" yl sulfonate in 1% salt and between concentrations of 1% and 0.2%, is at 4 (tested against butyl benzene, equivalent to b u t a n e ) . As n |_ for C-^ o-xylene sulfonate is at 4, and n ^ for C*15 o-xylene sulfonate is at 12.4, a 1:1 molar mixture should give a minimum γ at 8.2, according to Equation ( 3 ) . In fact, njiiin for this mixture is at 8.0 for the concentration range 1% to 0.2%. This shows that the linear relationship between mixture average molecular weight and n ^ holds over a wide range of concentrations. Also, if both components in the mixture are safely above some critical concentration, for the mixture will not be dependent on the overall concentration. C
x
e n e
m:
n
m
m
n
n
A D V A N C E S IN LOW INTERFACIAL TENSIONS
111
In the low, overall concentration region, for the 1:1 molar mixture of C-^ * i 5 xylene sulfonates becomes concentration dependent. As the concentration decreases to 0.01%, H J J ^ shifts from 8 to 12.6, which is very near the 15 " y l sulfonate line. The xylene sulfonate now seems ineffective compared with the C-j^ xylene sulfonate. This may be a reflection of the relative adsorptions at the interface, the o-xylene predominating because it is still above a critical concentration required for strong adsorption, whereas the C-j^ o-xylene sulfonate is not. This idea requires that the "critical concentration" mentioned is higher for C^2 than for C-^ xylene sulfonate, and it may be relevant here that Klevens (18) has shown c.m.c.'s to vary inversely with the alkyl group chain length. The exploratory xylene sulfonate results offer an explanation of the concentration dependences found with petroleum sulfonates. The shift of with changing concentration of Witco 10-80, at 1% salt, is shown in Figure 5 (data taken from (15)). As a range of molecular weights and structures are present, the components of 10-80 will have a range of critical concentration values. The concentration dependence noted in Figure 5 may be explained if, on the average, high molecular weight species or groups of species reach their critical concentrations at lowest overall concentration. Molecules active in the interface at the lowest concentrations then have a high average molecular weight and, noting the trends in Figure 2, a high value of n ^ will be obtained. At higher overall concentrations, steadily lower molecular weight species reach their individual critical concentrations. The average molecular weight at the interface will slowly decrease so that ^ i n decreases with increasing concentration. Some species, of very low molecular weight and present only in small proportion, will need a very high overall concentration before they can reach their critical concentrations. Therefore, the 10-80 line only approaches asymptotically a concentration independent region. a n c
C
0
x
c
e n e
m
n
IX. ELECTROLYTE CONCENTRATION AND TYPE Several studies have now been reported on the effect of electrolyte concentration in systems producing low interfacial tensions (11,15,19,20), with sodium chloride being almost exclusively the electrolyte used. Wilson, Murphy and Foster (19) have found that the salinity range in which a surfactant is interfacially most active is primarily a function of surfactant molecular weight. They suggest that if the molecular weight is too low, the solubility may be too high for strong adsorption at the interface, and a low γ will not be obtained unless electrolyte is added to decrease the solubility to some
112
M O R G A N et al.
required lower level. More electrolyte will be required if the molecular weight is low. Therefore, as the molecular weight of the surfactant decreases, the optimum salinity for production of a low γ against a given crude oil should increase, which is the result found by Wilson jet a l . (19). Exactly the same conclusion is reached by Healy, Reed and Stenmark (21), and by Puerto and Gale (20) in their studies of optimum salinities for production of low γ values in microemulsion systems. Use of the alkane model in salinity studies (11,15) can produce additional information, and some recent results are given in Figure 6. Here, three alkyl o-xylene sulfonates, of different molecular weight but of the same basic structure (approximately as in Figures 3(a) and ( b ) ) , are compared at V
C
e
o-XYLENE
S0 Na
Δ
C
l 8
o-XYLENE
SOjNo
0 . 2 % TOTAL
C
l 8
o - X Y L E N E S O , Να
SURFACTANT
•
G
16·
/I
12·
8-
C +C B
J1
/
/
W
3
o - X Y L E N E S 0 N a , 1:1 MOLAR MIX3
/
---Δ~
1 I
4ι
ι
ι
0
Fig. 6.
ι
ι
ι
—
γ
AT n
—
y
AT n
ι
m j n
< 2 ΧΙΟ"
m i n
ι
>2XI0" 1
-
e
I
(dynes/cm) (dynes/cm) I
I
1 2 No CI CONCENTRATION (wt.%)
I 3
Variation of alkane carbon number of minimum tension (^in^ ith concentration of NaCl at 0.2 wt.% surfactant w
0.2% concentration. In the alkane model, the equivalent to maintaining a crude oil at a fixed minimum γ is to maintain the value of constant at the EACN of the oil. If is to be kept constant, at nonane in Figure 6, for example, the NaCl concentration requirements to give a minimum γ with ^12» ^15 * ^18 ° ~ y l sulfonates are C j ^ ^18» which is in accord with other reported results (19-21). As crude oils have different EACN's, each will require a different salinity at a fixed surfactant concentration and type, which has also been noted by Wilson, Murphy, and Foster ( 1 9 ) . a n (
x
e n e
>
>
A D V A N C E S IN LOW INTERFACIAL TENSIONS
113
One surfactant mixture is included in Figure 6, a 1:1 mole ratio of d o-xylene sulfonates at 0.2% total concentration. This falls about midway between the curves of the individual components, so that the linear relationship between surfactant molecular weight and n is shown to hold, at least approximately, over a wide range of salinity. The NaCl tolerances shown in Figure 6 are particularly interesting. The highest molecular weight surfactant, the 1 8 ° ~ y l e sulfonate, is by far the least NaCl tolerant. Although very little NaCl is required to produce a low tension, the low γ is quickly lost as the NaCl concentration is increased further. Moreover, the slope of the curve is very steep, so that only a very small NaCl concentration fluctuation would be enough to change drastically, and to lose the low γ with a given crude oil. The C-^ o-xylene sulfonate, with a lower molecular weight, requires more NaCl to produce the first low tension, but low tensions are maintained over a larger NaCl concentration range, and the curve is less steep, so that the salt tolerance may be said to be increased. The C 2 2 o-xylene sulfonate is tolerant of NaCl over a still wider concentration range, though the slope is not decreased further. The 1:1 molar mixture of C ^ and C-^ o-xylene sulfonates is approximately intermediate to the single components in behavior. a n
m i
c
x
e n
The C 9 o-xylene sulfonate is off the low end of the a l kane range at low NaCl concentrations, but at high concentrations it may be moved onto the bottom part of the alkyl benzene scale. At 3% salt a minimum is seen at toluene, which is equivalent to methane as an oil phase, so that = 1. At 4% salt, iijj^ = 3, and at 5% salt, = 3.4. All these minima are only at 1 0 " 2 dyne/cm. An interesting feature here is that precipitation of surfactant and/or NaCl occurs in all three systems, but despite this, fairly low y's are found, and the minimum is still shifting in its usual direction with increasing NaCl. This may be contrasted with the behavior of the higher molecular weight o-xylene sulfonates, which lose their low tensions completely well before the precipitation stage. For instance, C - i o o-xylene sulfonate gives a minimum tension of 1.5 χ 1 0 ~ dyne/cm at 2.5% NaCl, but 2 dynes/cm at 2.625% NaCl. It seems possible that the loss of a low γ is associated with a sudden change in the nature of the interfacial phase when the electrolyte concentration is increased beyond a certain critical value, and that the critical electrolyte concentration decreases as the surfactant molecular weight increases. The effect of changing electrolyte type has not been studied extensively. Wilson e £ al. (19) find that the chloride anion can produce a lower γ than several other 3
114
MORGAN et al.
anions. The replacement of N a by Ca"""*" by ion exchange from reservoir clays may have an effect on oil displacement, though Healy, Reed and Stenmark (21) found little effect on optimal salinity or on γ produced when a 10:1 mixture of NaCl:CaCl22H20 was used instead of NaCl alone. +
1
X.
TEMPERATURE The effect of temperature is important because oil field temperatures range from 50°C upwards, whereas most research data is gathered at ambient laboratory temperatures. Healy, Reed and Stenmark (21) have, shown that increasing the temperature increases the optimal salinity for production of a low interfacial tension with a microemulsion, at fixed surfactant molecular weight, concentration and structure. Alkane scans with a number of surfactants have shown a shift in to lighter alkanes as the temperature is increased. Shifts measured so far include A n = -0.08/°C for Martinez 470 over the range 27°C to 50°C, and the same for C-^ o-xylene sulfonate over the range 27°C to 70°C. However, a smaller shift of -.04/°C has been noted for a mixture of C-j^ o-xylene sulfonate with 6-phenyl n-dodecane sodium sulfonate, so a universal temperature coefficient seems unlikely. Although it is possible that the effect of changing temperature with a crude oil/ surfactant system will not be the same as with an alkane/surfactant system because the crude oil contains a wide range of molecular weights, no temperature dependence of the EACN of any crude oil has yet been observed. For example, the EACN of Horseshoe Gallup has been found to be 8.2 both at 28°C and at 50°C. This needs to be confirmed before inferences are taken from the observed shifts of alkane with temperature. It is worth noting, however, that increasing the temperature, and moving downwards, may be counteracted by increasing the NaCl concentration, which moves upwards (Figure 6 ) . This gives rise to the result of Healy jet al. (21) mentioned above, that in order to maintain a fixed oil phase at a low γ while the temperature is increased, the salt concentration must be increased also. Of course, high salinity is not a necessity for a low γ at high temperatures. For instance, a surfactant with a molecular weight slightly too high to give a low γ against a particular oil at ambient temperature will give a low γ with that oil at a higher temperature. Any of the trends noted in the concluding table may be utilized in this way, and all of them may be made quantitative using the alkane model. m i n
XI.
OIL PHASE STRUCTURE AND MOLECULAR WEIGHT With the pure alkane series as the oil phase, the alkane carbon number of minimum γ (n . ) varies linearly with the mm J
A D V A N C E S IN LOW INTERFACIAL TENSIONS
115
surfactant molecular weight. As is directly proportional to the alkane molecular weight, then a fixed ratio of surfactant molecular weight to oil phase molecular weight is required to give a low interfacial tension. The ratio will change, of course, if the surfactant structure or concentration, salinity or oil phase structure type is changed. Unlike the surfactants, which show a marked dependence of interfacial tension behavior on the degree of branching of the alkyl chain, the oil phase seems unaffected by this type of isomerization. For example, iso-octane behaves exactly like octane (11). The behavior of hydrocarbon mixtures therefore depends only on the average molecular weight and the ratio of aromatic:aliphatic:alicyclic hydrocarbons present. These basic factors determine the EACN of a crude oil. XII.
EFFECT OF COSURFACTANT (COSOLVENT) A number of the low tension systems discussed here contain a cosolvent, usually an alcohol (20,21). This helps to solubilize the surfactant at high concentrations. Other workers do not use a cosolvent, because it further increases the complexity of the system (11,19). None of the alkane model systems contain a cosolvent, except in the pure isomer studies of Doe and Wade (16), where isopentanol is used as a solubility aid. Generally, when alcohol is added to a system, the alkane of minimum γ will change. Addition of a higher molecular weight alcohol, such as isopentanol, will produce a system with a higher than addition of the same volume of a lower molecular weight alcohol, such as methanol. The trends noted in the concluding table are unaffected qualitatively by the addition of a l c o h o l — f o r example, n ^ ^ will shift upwards if the salinity is increased whether alcohol is present or not. The size of the shifts observed are changed by the addition of alcohol, however. Puerto and Gale (20) report that the optimal salinity for producing a low γ is dependent on the alcohol type. If the alcohol is highly oil soluble, such as n-pentanol, then the optimum salinity is lower than with a highly water soluble alcohol such as methanol, n-butanol being intermediate in effect. This would seem to be because methanol increases the solubility of a given molecular weight surfactant most, which means more NaCl is required to decrease the solubility back down to a level where strong adsorption occurs at the interface. XIII.
CONCLUSIONS Understanding of the system variables involved has now reached the point where choosing a surfactant phase formulation to give a low interfacial tension against any particular
116
M O R G A N et al.
crude oil is possible. If the alkane model is used, trial and error testing is minimal. Many different surfactants may be used with each oil, the extreme types being of high molecular weight with unbranched alkyl chain structures, or of lower molecular weight, with highly branched alkyl groups. Of these, the least sensitive to slight fluctuations in system variables, and therefore the most practicable, seems to be the highly branched, low molecular weight type. The alkane model allows study of well-defined systems, with considerable scope for varying system parameters. The effect of each system variable on the carbon number of the alkane of minimum γ ( n ^ ) is reported in the text and the results are summarized qualitatively in the following table. m
n
TABLE I Effect on η . min Increases None Decreases
Variable Increased Surfactant M o l . Wt.
/
Branching of Surf, alkyl structure
/
/
Cone, of pure surfactant
/
Cone, of complex surf, mixture Electrolyte System
/
concentration
/
temperature
Age of system with pure surfactant Age of system with complex surf.
/
XIV.
ACKNOWLEDGEMENTS The authors wish to express their appreciation of the continued interest and support of the National Science Foundation and the Robert A. Welch Foundation.
XV. 1.
LITERATURE CITED Docher, Τ. M., Wise, F. Α., "Enhanced Crude Oil Recovery P o t e n t i a l — A n Estimate, Paper SPE 5800, presented at the SPE Symposium on Improved Oil Recovery, March 2224, 1976. Gogarty, W. B., Tosch, W . C , J. Pet. Tech. 20, 1407 (1968). Gogarty, W. B., Kinney, W. L., Kirk, W. B., J. Pet. Tech. 22, 1577 (1970). 11
2. 3.
A D V A N C E S IN LOW INTERFACIAL TENSIONS 4. 5. 6.
7. 8. 9.
10.
11.
12. 13. 14.
15.
16.
17.
117
Danielson, Η. Η., Paynter, W. T., Milton, H. W., J. Pet. Tech. 28, 129 (1976). Healy, R. Ν., Reed, R. L., Soc. Pet. Eng. J. 14, 491 (1974). Healy, R. N., Reed, R. L., "Immiscible Microemulsion Flooding," Paper SPE 5817, Presented at the SPE Symposium on Improved Oil Recovery, March 22-24, 1976. Melrose, J. C , Brandner, C F., J. of Canadian Petr. Tech. 54-62 (Oct. - Dec. 1 9 7 4 ) . Taber, J. J., Soc. Pet. Eng. J, 9 (No. 1 ) , 3 (1969). Cayias, J. L., Schechter, R. S., Wade, W. H., "The Utilization of Petroleum Sulfonates for Producing Low Interfacial Tensions Between Hydrocarbons and Water," J. Coll. Int. Sci. (1976). Cash, R. L., Cayias, J. L., Fournier, G., MacAllister, D. J., Schares, T., Schechter, R. S. and Wade, W. Η., "The Application of Low Interfacial Tension Scaling Rules to Binary Hydrocarbon Mixtures," J. Coll. Int. Sci. (1976). Cash, R. L., Cayias, J. L., Fournier, G., Jacobson, J. Κ., Schares, T., Schechter, R. S. and Wade, Η. Η., "Modeling Crude Oils for Low Interfacial Tension," Paper SPE 5813, Presented at the SPE Symposium on Improved Oil Recovery, March 22-24, 1976. American Society of Testing Materials, Phil., Penn., "ASTM Standards," D-2548-69, p. 656 (1969). Cayias, J. L., Schechter, R. S. and Wade, W. H., ACS Symposium Series No. 8, p. 235 (1975). Jacobson, J. K. , Morgan, J. C , Schechter, R. S. and Wade, W. Η., "Low Interfacial Tensions Involving Mixtures of Surfactants," Paper SPE 6002, presented at the SPE-AIME 51st Annual Meeting, New Orleans, La., Oct. 3-6, "l976. Cash, R. L., Cayias, J. L., Fournier, G., Jacobson, K. J., LeGear, C. Α., Schares, T., Schechter, R. S. and Wade, W. H., "Low Interfacial Tension Variables," Proceedings of the American Oil Chemists' Society, Short Course at Hershey, Pa., June, 1975. Doe, P. H. and Wade, W. H., "Alkyl Benzene Sulfonates for Producing Low Interfacial Tensions Between Hydrocarbons and Water," J. Colloid Interface Sci. (1977). Cash, R. L., Cayias, J. L., Hayes, Μ., MacAllister, D. J., Schares, T. and Wade, W. Η., "Surfactant Aging: A Possible Detriment to Tertiary Oil Recovery," J. Pet. Tech. 985 (Sept. 1 9 7 6 ) .
118 18. 19.
20.
21.
M O R G A N et al. Klevens, Η. B., J. Phys. Colloid Chem. (J. Phys. Chem.) 52, 130 (1948). Wilson, P. Μ., Murphy, L. C. and Foster, W. R., "The Effects of Sulfonate Molecular Weight and Salt Concentration on the Interfacial Tension of Oil-BrineSurfactant Systems," Paper SPE 5812, Presented at the SPE Symposium on Improved Oil Recovery, March 22-24, 1976. Puerto, M. C. and Gale, W. W., "Estimation of Optimal Salinity and Solubilization Parameters for Alkyl Orthoxylene Sulfonate Mixtures," Paper SPE 5814, ibid. Healy, R. Ν., Reed, R. L., and Stenmark, D . G., "Multiphase Microemulsion Systems," Paper SPE 5565, Presented at 50th Annual Fall Meeting of the SPE, Dallas, Texas (1975).
The University of Texas at Austin I.
ABSTRACT The study of interfacial tensions in an oil/surfactant/ water system is greatly facilitated by use of the alkane model. In this, the crude oil is replaced by that member of the homologous n-alkane series which shows interfacial tension behavior identical to that of the crude oil. A complete scale of hydrocarbon properties is used to study system variables affecting the low interfacial tension state. II.
SCOPE Almost 70% of the total proven crude oil reserves will still be in the ground when production by standard techniques ceases to be economic ( 1 ) . A successful tertiary oil recovery process, to supplement the present methods of pumping under natural forces, followed by a secondary water flood, is now an urgent research objective. In the forefront of proposed methods is chemical flooding with surfactant solutions. Two distinct types of surfactant flooding are possible, those employing high and low surfactant concentrations. The Maraflood process (2-4) is an example of a high concentration process which has been shown to enhance oil recovery under field conditions. A small slug of micellar solution of surfactant, such as petroleum sulfonate, is used at about 1 2 % volume concentration. Oil displacement is initially by a miscible process in this type of system, but Healy and Reed (5-6) have shown that the single phase microemulsion of oil, water and surfactant first involved breaks down into a m u l t i phase emulsion when mixing dilutes the surfactant slug. Further displacement is then by an immiscible process. The alternative is to use a continuous low concentration of surfactant (2% or much l e s s ) , all displacement then being immiscible. A possible advantage here is that considerably less surfactant may be required overall. Possible drawbacks are that loss of surfactant from solution by adsorption onto the rock surface, or by solubilization into the oil, may more seriously affect oil recovery. Also, the method may be more sensitive to salinity and temperature changes than is miscible
101
102
M O R G A N et al.
displacement. However, as miscible displacement inevitably degenerates by dilution to an immiscible process, understanding and control of immiscible displacement is of paramount importance. Melrose and Brandner (7) and Taber (8) have shown that successful immiscible oil displacement depends on the existence of a very low interfacial tension, γ, between the oil and the water phases. A value of about 10~3 dyne/cm or less is required to mobilize the oil. Certainly, recovery of residual oil from laboratory test cores is greatly improved for systems with ultra low interfacial tension ( 7 ) . The achievement and maintenance of low interfacial tensions during chemical flooding therefore seems essential. Practical surfactant flood systems are usually very complex: present are surfactant, oil, water, electrolyte and probably a thickening agent for viscosity control, plus cosurfactants and blocking agents to enhance or protect the main surfactant. Surfactants which combine an ability to produce a low interfacial surface tension with low cost and large scale availability are not common. Petroleum sulfonates are perhaps the main candidates. These are usually sodium salts of sulfonated crude oil. The oil is fractionated by molecular weight before sulfonation, but the surfactant produced still has a range of molecular weights which may be broad. Crude oils themselves are also very complex, and vary considerably from field to field. The salinity of formation water, and type of ions present, will change with the field, as will the temperature. This type of practical system is difficult to use in the study of variables affecting low interfacial tension. Changing the nature of one component of the system will demonstrate if, and in what manner, it affects γ, but quantitative assessments during this approach are not straightforward. In particular, achievement of a sufficiently low γ for one system does not show immediately how the system will react if a different crude oil is used. III.
THE ALKANE MODEL The model system introduced by Cayias e£ al. (9), in which a pure n-alkane replaces the crude oil, solves the above problem. Development of the alkane model began when a study of the interfacial tension of a series of pure n-alkane drops against a petroleum sulfonate saline solution revealed that only one alkane gave a really low γ. An example of this specificity is shown in Figure 1. Here a solution containing 0.2% by weight of the petroleum sulfonate Witco 10-80, plus 1%
A D V A N C E S IN LOW INTERFACIAL TENSIONS
103
INTERFACIAL TENSIONS OF 10-80 ( 0 . 2 % , I % No CI) VS: .0°.
—n-ALKANES — O - n-ALKYLBENZENES n-ALKYLCYCLOHEXANES
Id'-
Υ
«ι
ιο" 3
ιο-Ο 4
Fig. 1.
4 8 ALKYL GROUP CARBON NUMBER
12
Interfacial tensions of three homologous hydrocarbon series with 0.2 wt.% 10-80 (1.0 wt.% NaCl)
by weight sodium chloride, gives the lowest γ against heptane. When a series of 1-phenyl n-alkanes is tested, the same surfactant phase gives a minimum at heptyl benzene. Similarly, a minimum γ is found at butyl cyclohexane for this surfactant phase against a series of 1-cyclohexyl n-alkanes (Figure 1 ) . Thus, heptane, heptyl benzene and butyl-cyclohexane may be said to act as equivalent oil droplets. This equivalency is virtually independent of the nature of the surfactant phase. Comparisons such as this using different surfactant systems have been combined with a study of hydrocarbon m i x tures (10) to produce the concept of the equivalent alkane carbon number (EACN) for pure hydrocarbons and their mixtures (11). For instance, as heptyl benzene is equivalent to heptane, the EACN of heptyl benzene is 7. The EACN of butyl cyclohexane is similarly 7. Mixtures of hydrocarbons average very simply. If a surfactant phase gives a low, minimum γ against dodecane for example, it will also give a low γ against a mixture of 0.5 mole fraction decane and 0.5 mole fraction tetradecane. The mixture therefore has an EACN of 1 2 . All
104
M O R G A N et al.
other mixtures of decane and tetradecane will give higher interfacial tensions against this surfactant. Any pure hydrocarbon, whether a member of a homologous series or not, may be assigned its individual EACN by testing in binary mixtures with an alkane. An average EACN may be calculated for any mixture of hydrocarbons, providing the EACN of each component is known. This is found from
( E A C N )
MIXTURE
= I
(
E
A
C
N
)
i i X
(
1
)
1 where X is the mole fraction of the i*-* component of the mixture. Values of EACN are not confined to integers, so that the EACN of an equal mole fraction mixture of heptane and octane is 7.5, for instance. By coincidence, all three hydrocarbon series in Figure 1 give minima almost exactly at integral carbon numbers, but in Figure 4 only two minima are at integral carbon numbers. The EACN of a crude oil cannot be calculated directly using Equation ( 1 ) , because all the hydrocarbon components of a specific crude oil have never been identified. However, applicability of Equation (1) to crude oils is inferred by its successful application to complex synthetic mixtures containing up to 29 aliphatic, alicyclic and aromatic hydrocarbons (11). Measurements of the EACN of a crude oil requires the preparation of a series of surfactant solutions giving individual minimum tensions against steadily increasing alkane carbon numbers. The crude oil is then tested against each surfactant solution, and the lowest γ found shows the EACN of the oil. By changing the surfactant series used, it may be demonstrated that the EACN of a crude oil is a property essentially characteristic of the oil, and not of the surfactant type used. EACN values varying from 6.2 to 8.6 have been reported for eight stock tank oils (11). This variation, though apparently small, is extremely important because most surfactant systems are very selective. If a surfactant gives a low tension of 10~3 dyne/cm or less with the 8.6 EACN oil, it will probably give a far higher tension of perhaps 10"^ dyne/cm with the 6.2 EACN, and vice versa. Use of the complete alkane series instead of individual crude oils has several advantages. Firstly, it is immediately evident whether any particular surfactant system is optimally adjusted for giving a low γ with any given crude oil by comparing the alkane of minimum γ with the EACN of the oil. Also, the effect of changing one system variable on the a l kane of minimum tension may be studied in detail, because a wide range of alkanes is now available. With these variable 1
i
A D V A N C E S IN LOW INTERFACIAL TENSIONS
105
rules complete, adjustment of a surfactant system to any desired crude oil EACN becomes simple. IV.
PARAMETERS AFFECTING LOW INTERFACIAL TENSIONS Variables currently identified as important in the achievement of the low interfacial tension state in a water/ oil/surfactant/electrolyte system are: the surfactant average molecular weight and molecular weight distribution; surfactant molecular structure; surfactant concentration; electrolyte concentration and type; oil phase average molecular weight and structure; system temperature; and the age of the system. This list includes variables which may be deliberately tailored to give a low-tension surfactant flood system for a particular field, and others which may be expected to vary during production from a field or from field to field. Several variables are, of course, in both groups. The effect of changing each variable on interfacial tensions is now surveyed. All surface tensions referred to as "low or "minimal" have values of 2 χ 10"^ dyne/cm or lower unless otherwise stated. The standard aqueous phase has a total surfactant concentration of 0.2% by weight and a sodium chloride concentration of 1% by weight, the oil phase being an alkane. No other components are present in the surfactant phase unless mentioned. All petroleum sulfonates and xylene sulfonates have been deoiled on a silica gel column prior to use by a method described elsewhere ( 1 2 ) . No preequilibration of the oil and aqueous phases was employed before measurement of the interfacial tensions, which are made at 27°C using the spinning drop technique ( 1 3 ) . In early papers (10,11), this technique was not standard, and results from these papers have been repeated before inclusion here if comparison with later results is to be made. In fact, differences found on repetition were usually within experimental error, which is considered to be ±0.3 of a alkane carbon number. The molecular weights of petroleum and xylene sulfonates quoted here are, strictly, equivalent weights, determined by estimation of the SO-^Na content of a known weight of surfactant. 11
V.
SURFACTANT AVERAGE MOLECULAR WEIGHT A linear relationship has been found between the average molecular weight of a mixture of two surfactants and n ^ , the carbon number of the n-alkane with which the mixture gives a minimum interfacial tension (at constant total surfactant concentration, salinity and temperature). This holds for all but one (14) of the many binary mixtures studied to date (11, 14,15). The individual surfactants may each be of reasonably well-defined structure, such as the two Exxon alkyl orthoxylene m
n
106
M O R G A N et al.
sodium sulfonates (11) (Figure 2, approximate molecular structures in Figures 3(a) and 3 ( b ) ) . Also, each component
cΕ
.
— Δ — MARTINEZ 470·**MARTINEZ
.
—Ο-MARTINEZ <—Φ-> C
3 2
+C
E
IE
380
3 8 0 + C, O-XYLENE S0 NO E
Ο-XYLENE
3
S O , ΝΑ
- Ό - 10-80 + C O-XYLENE |E
S0 NA 8
0~C
XYLENES
| 5
Ο
(Τ ζ ο ω Κ < ο ω ζ <
/
/
•
ιο-βο
p
/
tf—C
/
XYLENES
E
'
A L L AT 0.2 % TOTAL SURFACTANT, L%SALT 380
400
420
AVERAGE
Fig. 2.
440
EQUIV.
460
480
WT.
Dependence of alkane carbon number of minimum interfacial tension (n ^ ) on the average equivalent weight of a mixture of two surfactants m
n
Η H
l5 7~9" e l7 C
C
H
CH -CH-£CH -CH 3
2
(A) C O - X Y L E N E | 5
CH,
S0 NA
(C) 8TF>C S0 NA
3
CH,
|6
/ = <
3
H
3^C-C
3
R 4
H
2 9
CH -CH-(-CH -CH 3
2
S0 NA 3
(B) C
Fig. 3.
|2
O-XYLENE
S0 NA 3
Representative surfactant structures (approximate for the alkyl o-xylene sulfonates)
A D V A N C E S IN LOW INTERFACIAL TENSIONS
107
might contain a range of molecular weights and structures, as with mixtures of the two Shell petroleum sulfonates Martinez 380 and Martinez 470 (11) in Figure 2. The linear molecular w e i g h t / n ^ ^ relationship again holds when surfactants of completely different types are mixed, such as Martinez 380 with the 0-^5 o-xylene sulfonate (14) (Figure 2 ) . It may not hold for mixtures of two isomerically pure surfactants, however (14). Of practical importance is that the composition of a surfactant mixture giving the lowest γ with any given crude oil may now be calculated. If, for instance, the crude oil has an EACN of 8, and a mixture of Martinez 380 and 470 is to be used at a 0.2% by weight overall concentration in 1% by weight NaCl solution, then Figure 2 shows that a surfactant mixture of average molecular weight 440 is required. The surfactant requirements can be found using, Wt. of Martinez 470 in g/1 Wt. of Martinez 380 in g/1
=
_470 380
(440-380) 470-440
.
( K
}
No other 380/470 mixture will give this low a γ against the crude oil. Figure 2 shows that a mixture of C^2 * 1 5 l"~ fonates of average molecular weight 400 will also give a low tension with a crude oil of EACN = 8. The linear averaging rule allows prediction of for any mixture of two surfactants providing the individual value of is known for each component. For some individual surfactants, such as C^2 o-xylene sulfonate and Martinez 380, a value of cannot be measured directly because it is not within the liquid alkane range. Reliable values of may be found by extrapolation, however (14). The alkane carbon number of minimum γ for a mixture of two surfactants is then given by a n c
mm
MIXTURE
A
mm
A
Β
mm
c
su
Β
Testing of Equation (3) when expanded to deal with three or more surfactant components is not yet complete. VI.
SURFACTANT MOLECULAR WEIGHT DISTRIBUTION To treat the surfactant molecular weight distribution as an independent variable is not practicable at present. If the surfactant average molecular weight is kept constant and the range of molecular weights is changed, then the "mean structure type" is almost certain to change also, and the surfactant structure is known to be a variable to which n ± is particularly sensitive. However, the presence of a distribution of surfactant molecular weights is certainly an important factor because it introduces to the system a dependence on overall surfactant concentration, discussed later. A m
n
108
MORGAN et al.
susceptibility to aging phenomena is also introduced by the presence of a range of surfactant molecular weights and structures. For example, Cash jet al. (17) find that for 10-80 shifts gradually to lighter alkanes over a period of months. The origin of this aging process is not understood, but it is only found for complex mixtures such as petroleum sulfonates. VII.
SURFACTANT STRUCTURE There is no single line of surfactant molecular weight against i ^ ^ , although lines connecting surfactant pairs of similar structure (such as two o-xylene sulfonates, or two Martinez petroleum sulfonates) have similar slopes (Figure 2 ) . Surfactants, or surfactant mixtures, of the same average molecular weight may be far apart on the ΐΐ ± scale. For instance, 15 ° " " sulfonate and the Martinez 380-470 mixture of the same average molecular weight (420) are nine alkane units apart. Doe and Wade (16) have shown that shifts of this magnitude are due to differences in surfactant molecular structure. Shifts in n ^ are seen when a series of isomers of n-hexadecyl benzene sodium sulfonate (molecular weight 404) are studied. There are eight possible para-isomers, of which two, the 8phenyl hexadecane sulfonate, and the much less highly branched 2-phenyl isomer, are illustrated in Figures 3(c) and 3 ( d ) . Interfacial tensions of each isomer against the range of a l kanes are shown in Figure 4. These results are for a system containing 0.07% surfactant by weight and 0.3% sodium chloride. Isopentanol (2% by volume) is also added as a cosurfactant in order to increase the solubility of the 3 and 2 isomers. The 1-phenyl isomer is not soluble. In isopentanol-free solution, the trend of Figure 4 is repeated qualitatively, but not quantitatively, for isomers remaining soluble. αί
C
η
x v l e n e
m
n
Figure 4 shows that as the branching of the alkyl chain increases from the 2 isomer towards the 8 isomer, there is a steady movement of the value of to higher alkanes (at a fixed molecular w e i g h t ) . This result probably explains the differences between surfactants of less well-defined structure. For example, if surfactants of molecular weight 415 are compared in Figure 2, the o-xylene sulfonate mixture is at the highest Π β ^ . The o-xylene sulfonates have highly branched main alkyl groups (Figure 3(a)) because of their method of manufacture, and they also have two additional alkyl groups, albeit only methyls, on the benzene ring. Witco 10-80 has a lower ii ± than the xylene sulfonate mix, and therefore should have a less highly branched alkyl chain structure, on the average, though this remains to be proven. Further work on structural effects must rely heavily on the study of surfactants of absolutely known structure. The structure, and even the molecular weights of the petroleum sulfonates cannot be m
Ti
A D V A N C E S IN LOW INTERFACIAL TENSIONS
Fig. 4.
109
Interfacial tensions of n-alkanes with isomers of phenyl hexadecane sulfonate
defined well e n o u g h — t h e manufacturers quoted equivalent weights will be below the true molecular weights if disulfonates are present. This makes petroleum sulfonates largely unsuitable for study of shifts at constant molecular weight. The noted effect of branching is of great interest in selecting surfactants for a practical surfactant flood system. To achieve a low tension with a crude oil of given EACN, the choice lies between a high molecular weight, straight alkyl chain type of structure and a low molecular weight highly branched structure. The branched surfactant may have several advantages. It will be more soluble, so that it may be used in higher concentration if required, and its increased solubility will probably lead to decreased adsorption on the reservoir rock. It will also block a much greater surface area of rock than will an adsorbed, straight-chain molecule. Lower molecular weight surfactants are also far more tolerant of increasing salt concentration, as is discussed below. 1
110
M O R G A N et al.
VIII.
SURFACTANT
CONCENTRATION
The concentration of surfactant in a solution containing only one surfactant molecular species may be varied over a wide range without affecting (at constant salinity, e t c . ) . For example, 8-phenyl n-hexadecane sodium sulfonate (Figure 3(c)) has a fixed at any concentration down to its c.m.c. (16). For C - j c o-xylene sulfonate at 1% salt, stays fixed at 12.4 at all surfactant concentrations from 1% down to about 0.01% (Figure 5)· The value of at 0.01% is slightly shifted, and the minimum γ slightly raised (at 6 χ 10"3 dyne/ cm) when compared with results at higher concentrations. For
Δ
Δ
12-
C, ο - X Y L E N E 5
A
S0 Na (l%NaCI) 5
10·
8-
^
^
^
^
10-80
(l%NaCI)
6
Ο ()
0.2
0.4
SURFACTANT
Fig. 5.
0.6
0.8
ΙΟ
CONCENTRATION ( W t . % )
Variation of alkane carbon number of minimum tension ( hni/n) ^tfo surfactant concentration at 1.0 wt.% NaCl r
w
12 °" yl sulfonate in 1% salt and between concentrations of 1% and 0.2%, is at 4 (tested against butyl benzene, equivalent to b u t a n e ) . As n |_ for C-^ o-xylene sulfonate is at 4, and n ^ for C*15 o-xylene sulfonate is at 12.4, a 1:1 molar mixture should give a minimum γ at 8.2, according to Equation ( 3 ) . In fact, njiiin for this mixture is at 8.0 for the concentration range 1% to 0.2%. This shows that the linear relationship between mixture average molecular weight and n ^ holds over a wide range of concentrations. Also, if both components in the mixture are safely above some critical concentration, for the mixture will not be dependent on the overall concentration. C
x
e n e
m:
n
m
m
n
n
A D V A N C E S IN LOW INTERFACIAL TENSIONS
111
In the low, overall concentration region, for the 1:1 molar mixture of C-^ * i 5 xylene sulfonates becomes concentration dependent. As the concentration decreases to 0.01%, H J J ^ shifts from 8 to 12.6, which is very near the 15 " y l sulfonate line. The xylene sulfonate now seems ineffective compared with the C-j^ xylene sulfonate. This may be a reflection of the relative adsorptions at the interface, the o-xylene predominating because it is still above a critical concentration required for strong adsorption, whereas the C-j^ o-xylene sulfonate is not. This idea requires that the "critical concentration" mentioned is higher for C^2 than for C-^ xylene sulfonate, and it may be relevant here that Klevens (18) has shown c.m.c.'s to vary inversely with the alkyl group chain length. The exploratory xylene sulfonate results offer an explanation of the concentration dependences found with petroleum sulfonates. The shift of with changing concentration of Witco 10-80, at 1% salt, is shown in Figure 5 (data taken from (15)). As a range of molecular weights and structures are present, the components of 10-80 will have a range of critical concentration values. The concentration dependence noted in Figure 5 may be explained if, on the average, high molecular weight species or groups of species reach their critical concentrations at lowest overall concentration. Molecules active in the interface at the lowest concentrations then have a high average molecular weight and, noting the trends in Figure 2, a high value of n ^ will be obtained. At higher overall concentrations, steadily lower molecular weight species reach their individual critical concentrations. The average molecular weight at the interface will slowly decrease so that ^ i n decreases with increasing concentration. Some species, of very low molecular weight and present only in small proportion, will need a very high overall concentration before they can reach their critical concentrations. Therefore, the 10-80 line only approaches asymptotically a concentration independent region. a n c
C
0
x
c
e n e
m
n
IX. ELECTROLYTE CONCENTRATION AND TYPE Several studies have now been reported on the effect of electrolyte concentration in systems producing low interfacial tensions (11,15,19,20), with sodium chloride being almost exclusively the electrolyte used. Wilson, Murphy and Foster (19) have found that the salinity range in which a surfactant is interfacially most active is primarily a function of surfactant molecular weight. They suggest that if the molecular weight is too low, the solubility may be too high for strong adsorption at the interface, and a low γ will not be obtained unless electrolyte is added to decrease the solubility to some
112
M O R G A N et al.
required lower level. More electrolyte will be required if the molecular weight is low. Therefore, as the molecular weight of the surfactant decreases, the optimum salinity for production of a low γ against a given crude oil should increase, which is the result found by Wilson jet a l . (19). Exactly the same conclusion is reached by Healy, Reed and Stenmark (21), and by Puerto and Gale (20) in their studies of optimum salinities for production of low γ values in microemulsion systems. Use of the alkane model in salinity studies (11,15) can produce additional information, and some recent results are given in Figure 6. Here, three alkyl o-xylene sulfonates, of different molecular weight but of the same basic structure (approximately as in Figures 3(a) and ( b ) ) , are compared at V
C
e
o-XYLENE
S0 Na
Δ
C
l 8
o-XYLENE
SOjNo
0 . 2 % TOTAL
C
l 8
o - X Y L E N E S O , Να
SURFACTANT
•
G
16·
/I
12·
8-
C +C B
J1
/
/
W
3
o - X Y L E N E S 0 N a , 1:1 MOLAR MIX3
/
---Δ~
1 I
4ι
ι
ι
0
Fig. 6.
ι
ι
ι
—
γ
AT n
—
y
AT n
ι
m j n
< 2 ΧΙΟ"
m i n
ι
>2XI0" 1
-
e
I
(dynes/cm) (dynes/cm) I
I
1 2 No CI CONCENTRATION (wt.%)
I 3
Variation of alkane carbon number of minimum tension (^in^ ith concentration of NaCl at 0.2 wt.% surfactant w
0.2% concentration. In the alkane model, the equivalent to maintaining a crude oil at a fixed minimum γ is to maintain the value of constant at the EACN of the oil. If is to be kept constant, at nonane in Figure 6, for example, the NaCl concentration requirements to give a minimum γ with ^12» ^15 * ^18 ° ~ y l sulfonates are C j ^ ^18» which is in accord with other reported results (19-21). As crude oils have different EACN's, each will require a different salinity at a fixed surfactant concentration and type, which has also been noted by Wilson, Murphy, and Foster ( 1 9 ) . a n (
x
e n e
>
>
A D V A N C E S IN LOW INTERFACIAL TENSIONS
113
One surfactant mixture is included in Figure 6, a 1:1 mole ratio of d o-xylene sulfonates at 0.2% total concentration. This falls about midway between the curves of the individual components, so that the linear relationship between surfactant molecular weight and n is shown to hold, at least approximately, over a wide range of salinity. The NaCl tolerances shown in Figure 6 are particularly interesting. The highest molecular weight surfactant, the 1 8 ° ~ y l e sulfonate, is by far the least NaCl tolerant. Although very little NaCl is required to produce a low tension, the low γ is quickly lost as the NaCl concentration is increased further. Moreover, the slope of the curve is very steep, so that only a very small NaCl concentration fluctuation would be enough to change drastically, and to lose the low γ with a given crude oil. The C-^ o-xylene sulfonate, with a lower molecular weight, requires more NaCl to produce the first low tension, but low tensions are maintained over a larger NaCl concentration range, and the curve is less steep, so that the salt tolerance may be said to be increased. The C 2 2 o-xylene sulfonate is tolerant of NaCl over a still wider concentration range, though the slope is not decreased further. The 1:1 molar mixture of C ^ and C-^ o-xylene sulfonates is approximately intermediate to the single components in behavior. a n
m i
c
x
e n
The C 9 o-xylene sulfonate is off the low end of the a l kane range at low NaCl concentrations, but at high concentrations it may be moved onto the bottom part of the alkyl benzene scale. At 3% salt a minimum is seen at toluene, which is equivalent to methane as an oil phase, so that = 1. At 4% salt, iijj^ = 3, and at 5% salt, = 3.4. All these minima are only at 1 0 " 2 dyne/cm. An interesting feature here is that precipitation of surfactant and/or NaCl occurs in all three systems, but despite this, fairly low y's are found, and the minimum is still shifting in its usual direction with increasing NaCl. This may be contrasted with the behavior of the higher molecular weight o-xylene sulfonates, which lose their low tensions completely well before the precipitation stage. For instance, C - i o o-xylene sulfonate gives a minimum tension of 1.5 χ 1 0 ~ dyne/cm at 2.5% NaCl, but 2 dynes/cm at 2.625% NaCl. It seems possible that the loss of a low γ is associated with a sudden change in the nature of the interfacial phase when the electrolyte concentration is increased beyond a certain critical value, and that the critical electrolyte concentration decreases as the surfactant molecular weight increases. The effect of changing electrolyte type has not been studied extensively. Wilson e £ al. (19) find that the chloride anion can produce a lower γ than several other 3
114
MORGAN et al.
anions. The replacement of N a by Ca"""*" by ion exchange from reservoir clays may have an effect on oil displacement, though Healy, Reed and Stenmark (21) found little effect on optimal salinity or on γ produced when a 10:1 mixture of NaCl:CaCl22H20 was used instead of NaCl alone. +
1
X.
TEMPERATURE The effect of temperature is important because oil field temperatures range from 50°C upwards, whereas most research data is gathered at ambient laboratory temperatures. Healy, Reed and Stenmark (21) have, shown that increasing the temperature increases the optimal salinity for production of a low interfacial tension with a microemulsion, at fixed surfactant molecular weight, concentration and structure. Alkane scans with a number of surfactants have shown a shift in to lighter alkanes as the temperature is increased. Shifts measured so far include A n = -0.08/°C for Martinez 470 over the range 27°C to 50°C, and the same for C-^ o-xylene sulfonate over the range 27°C to 70°C. However, a smaller shift of -.04/°C has been noted for a mixture of C-j^ o-xylene sulfonate with 6-phenyl n-dodecane sodium sulfonate, so a universal temperature coefficient seems unlikely. Although it is possible that the effect of changing temperature with a crude oil/ surfactant system will not be the same as with an alkane/surfactant system because the crude oil contains a wide range of molecular weights, no temperature dependence of the EACN of any crude oil has yet been observed. For example, the EACN of Horseshoe Gallup has been found to be 8.2 both at 28°C and at 50°C. This needs to be confirmed before inferences are taken from the observed shifts of alkane with temperature. It is worth noting, however, that increasing the temperature, and moving downwards, may be counteracted by increasing the NaCl concentration, which moves upwards (Figure 6 ) . This gives rise to the result of Healy jet al. (21) mentioned above, that in order to maintain a fixed oil phase at a low γ while the temperature is increased, the salt concentration must be increased also. Of course, high salinity is not a necessity for a low γ at high temperatures. For instance, a surfactant with a molecular weight slightly too high to give a low γ against a particular oil at ambient temperature will give a low γ with that oil at a higher temperature. Any of the trends noted in the concluding table may be utilized in this way, and all of them may be made quantitative using the alkane model. m i n
XI.
OIL PHASE STRUCTURE AND MOLECULAR WEIGHT With the pure alkane series as the oil phase, the alkane carbon number of minimum γ (n . ) varies linearly with the mm J
A D V A N C E S IN LOW INTERFACIAL TENSIONS
115
surfactant molecular weight. As is directly proportional to the alkane molecular weight, then a fixed ratio of surfactant molecular weight to oil phase molecular weight is required to give a low interfacial tension. The ratio will change, of course, if the surfactant structure or concentration, salinity or oil phase structure type is changed. Unlike the surfactants, which show a marked dependence of interfacial tension behavior on the degree of branching of the alkyl chain, the oil phase seems unaffected by this type of isomerization. For example, iso-octane behaves exactly like octane (11). The behavior of hydrocarbon mixtures therefore depends only on the average molecular weight and the ratio of aromatic:aliphatic:alicyclic hydrocarbons present. These basic factors determine the EACN of a crude oil. XII.
EFFECT OF COSURFACTANT (COSOLVENT) A number of the low tension systems discussed here contain a cosolvent, usually an alcohol (20,21). This helps to solubilize the surfactant at high concentrations. Other workers do not use a cosolvent, because it further increases the complexity of the system (11,19). None of the alkane model systems contain a cosolvent, except in the pure isomer studies of Doe and Wade (16), where isopentanol is used as a solubility aid. Generally, when alcohol is added to a system, the alkane of minimum γ will change. Addition of a higher molecular weight alcohol, such as isopentanol, will produce a system with a higher than addition of the same volume of a lower molecular weight alcohol, such as methanol. The trends noted in the concluding table are unaffected qualitatively by the addition of a l c o h o l — f o r example, n ^ ^ will shift upwards if the salinity is increased whether alcohol is present or not. The size of the shifts observed are changed by the addition of alcohol, however. Puerto and Gale (20) report that the optimal salinity for producing a low γ is dependent on the alcohol type. If the alcohol is highly oil soluble, such as n-pentanol, then the optimum salinity is lower than with a highly water soluble alcohol such as methanol, n-butanol being intermediate in effect. This would seem to be because methanol increases the solubility of a given molecular weight surfactant most, which means more NaCl is required to decrease the solubility back down to a level where strong adsorption occurs at the interface. XIII.
CONCLUSIONS Understanding of the system variables involved has now reached the point where choosing a surfactant phase formulation to give a low interfacial tension against any particular
116
M O R G A N et al.
crude oil is possible. If the alkane model is used, trial and error testing is minimal. Many different surfactants may be used with each oil, the extreme types being of high molecular weight with unbranched alkyl chain structures, or of lower molecular weight, with highly branched alkyl groups. Of these, the least sensitive to slight fluctuations in system variables, and therefore the most practicable, seems to be the highly branched, low molecular weight type. The alkane model allows study of well-defined systems, with considerable scope for varying system parameters. The effect of each system variable on the carbon number of the alkane of minimum γ ( n ^ ) is reported in the text and the results are summarized qualitatively in the following table. m
n
TABLE I Effect on η . min Increases None Decreases
Variable Increased Surfactant M o l . Wt.
/
Branching of Surf, alkyl structure
/
/
Cone, of pure surfactant
/
Cone, of complex surf, mixture Electrolyte System
/
concentration
/
temperature
Age of system with pure surfactant Age of system with complex surf.
/
XIV.
ACKNOWLEDGEMENTS The authors wish to express their appreciation of the continued interest and support of the National Science Foundation and the Robert A. Welch Foundation.
XV. 1.
LITERATURE CITED Docher, Τ. M., Wise, F. Α., "Enhanced Crude Oil Recovery P o t e n t i a l — A n Estimate, Paper SPE 5800, presented at the SPE Symposium on Improved Oil Recovery, March 2224, 1976. Gogarty, W. B., Tosch, W . C , J. Pet. Tech. 20, 1407 (1968). Gogarty, W. B., Kinney, W. L., Kirk, W. B., J. Pet. Tech. 22, 1577 (1970). 11
2. 3.
A D V A N C E S IN LOW INTERFACIAL TENSIONS 4. 5. 6.
7. 8. 9.
10.
11.
12. 13. 14.
15.
16.
17.
117
Danielson, Η. Η., Paynter, W. T., Milton, H. W., J. Pet. Tech. 28, 129 (1976). Healy, R. Ν., Reed, R. L., Soc. Pet. Eng. J. 14, 491 (1974). Healy, R. N., Reed, R. L., "Immiscible Microemulsion Flooding," Paper SPE 5817, Presented at the SPE Symposium on Improved Oil Recovery, March 22-24, 1976. Melrose, J. C , Brandner, C F., J. of Canadian Petr. Tech. 54-62 (Oct. - Dec. 1 9 7 4 ) . Taber, J. J., Soc. Pet. Eng. J, 9 (No. 1 ) , 3 (1969). Cayias, J. L., Schechter, R. S., Wade, W. H., "The Utilization of Petroleum Sulfonates for Producing Low Interfacial Tensions Between Hydrocarbons and Water," J. Coll. Int. Sci. (1976). Cash, R. L., Cayias, J. L., Fournier, G., MacAllister, D. J., Schares, T., Schechter, R. S. and Wade, W. Η., "The Application of Low Interfacial Tension Scaling Rules to Binary Hydrocarbon Mixtures," J. Coll. Int. Sci. (1976). Cash, R. L., Cayias, J. L., Fournier, G., Jacobson, J. Κ., Schares, T., Schechter, R. S. and Wade, Η. Η., "Modeling Crude Oils for Low Interfacial Tension," Paper SPE 5813, Presented at the SPE Symposium on Improved Oil Recovery, March 22-24, 1976. American Society of Testing Materials, Phil., Penn., "ASTM Standards," D-2548-69, p. 656 (1969). Cayias, J. L., Schechter, R. S. and Wade, W. H., ACS Symposium Series No. 8, p. 235 (1975). Jacobson, J. K. , Morgan, J. C , Schechter, R. S. and Wade, W. Η., "Low Interfacial Tensions Involving Mixtures of Surfactants," Paper SPE 6002, presented at the SPE-AIME 51st Annual Meeting, New Orleans, La., Oct. 3-6, "l976. Cash, R. L., Cayias, J. L., Fournier, G., Jacobson, K. J., LeGear, C. Α., Schares, T., Schechter, R. S. and Wade, W. H., "Low Interfacial Tension Variables," Proceedings of the American Oil Chemists' Society, Short Course at Hershey, Pa., June, 1975. Doe, P. H. and Wade, W. H., "Alkyl Benzene Sulfonates for Producing Low Interfacial Tensions Between Hydrocarbons and Water," J. Colloid Interface Sci. (1977). Cash, R. L., Cayias, J. L., Hayes, Μ., MacAllister, D. J., Schares, T. and Wade, W. Η., "Surfactant Aging: A Possible Detriment to Tertiary Oil Recovery," J. Pet. Tech. 985 (Sept. 1 9 7 6 ) .
118 18. 19.
20.
21.
M O R G A N et al. Klevens, Η. B., J. Phys. Colloid Chem. (J. Phys. Chem.) 52, 130 (1948). Wilson, P. Μ., Murphy, L. C. and Foster, W. R., "The Effects of Sulfonate Molecular Weight and Salt Concentration on the Interfacial Tension of Oil-BrineSurfactant Systems," Paper SPE 5812, Presented at the SPE Symposium on Improved Oil Recovery, March 22-24, 1976. Puerto, M. C. and Gale, W. W., "Estimation of Optimal Salinity and Solubilization Parameters for Alkyl Orthoxylene Sulfonate Mixtures," Paper SPE 5814, ibid. Healy, R. Ν., Reed, R. L., and Stenmark, D . G., "Multiphase Microemulsion Systems," Paper SPE 5565, Presented at 50th Annual Fall Meeting of the SPE, Dallas, Texas (1975).