Cation Exchange, Surfactant Precipitation, and Adsorption in Micellar

1 Cation Exchange, Surfactant Precipitation, and Adsorption

Downloaded by PENNSYLVANIA STATE UNIV on July 10, 2013 | http://pubs.acs.org Publication Date: March 2, 1979 | doi: 10.1021/bk-1979-0091.ch001

in Micellar Flooding ROBERT D. WALKER, JR. and W. E. RAY—Department of Chemical Engineering, University of Florida, Gainesville, FL 32611 Μ. Κ. THAM—USDOE Bartlesville Energy Technology Center, Bartlesville, OK 74003 M. C. LEE—Department of Chemical Engineering, Oklahoma State University, Stillwater, OK 74074 It is commonly known that the constituents of a micellar slug may interact in several ways with both the rock and the formation fluids when injected into a reservoir, and a considerable body of literature exists (1-8). In spite of this knowledge, however, it is not yet possible to design a micellar slug for tertiary oil recovery from basic principles because of the complexity of the phenomena and inadequate understanding of the processes involved. The primary objectives of this paper are to present the results of some experiments on the structure and mineralogy of selected rock and reservoir core samples, on the interactions within surfactant solutions and between surfactant solutions and rock, and to attempt to draw from these observations some conclusions as to the phenomena and mechanisms involved-especially surfactant loss processes-as these can affect the maintenance of low interfacial tension between oil and water. In the course of attempts to determine adsorption isotherms of anionic surfactants on selected clays two other phenomena re quiring separate investigation were noted, namely, salting-out of surfactants by NaC1, and surfactant precipitation as calcium or magnesium salts by multivalent cations displaced from clays. Each of these and their significance for adsorption measurements will" be dealt with prior to discussion of surfactant adsorption. Electron Microscopy of Selected Reservoir Core Samples Scanning electron micrographs of fracture surfaces of Berea sandstone and two reservoir cores (made available to us by Dr. F.W. Smith-ARCO Research Laboratories) are shown in Figure 1. The sample identifications are as follows: 1) Berea sandstone, Amherst, Lorain County, OH, 2) Glenn Sand, Glenn Pool Field, Creek County, OK, 3) San Andres Formation, Wasson Field, Yoakum County, TX. The primary purpose of presenting these is to illus trate the geometric and mineralogical heterogeneity of typical reservoir cores, and the SEM in Figure 1 illustrate these points quite graphically. The basic sand matrix is clearly visible in 0-8412-0477-2/79/47-091-001$05.00/0 © 1978 American Chemical Society In Chemistry of Oil Recovery; Johansen, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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Figure 1. Electron micrographs of Berea sandstone and selected core samples: (a) Berea sandstone, representative fracture surface, 102.75X;(b) Berea sandstone, clay on quartz crystals, 959X;(c) Glenn sand core, representative fracture surface, 123.3X;(d) Glenn sand core, clay crystals on quartz, S938.75X;(e) San Andres core, representative fracture surface, 123.3X;(f) San Andres core, clay and dolomite crystals, 993.25 X.

In Chemistry of Oil Recovery; Johansen, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.


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the low m a g n i f i c a t i o n SEM, w h i l e those a t higher m a g n i f i c a t i o n r e v e a l the heterogeneity o f s t r u c t u r e o f c l a y s and other m i n e r a l s , and t h e i r general d i s t r i b u t i o n over the surface o f the sand r a t h e r than being concentrated i n cementation bridges between sand g r a i n s . Energy d i s p e r s i v e X-ray a n a l y s i s (EDXA) i s o f a s s i s t a n c e i n i d e n t i f y i n g the p r i n c i p a l chemical elements i n p a r t i c u l a r c r y s t a l s . This i n f o r m a t i o n , along w i t h c r y s t a l shape, enables one to i d e n t i f y reasonably w e l l the minerals l i k e l y to be contacted by a s u r f a c t a n t s l u g when i n j e c t e d i n t o a core o r a r e s e r v o i r formation. Thus, the b a s i c sand m a t r i x o f these m a t e r i a l s i s revealed w h i l e the presence o f p a r t i c u l a r c l a y m i n e r a l s , such as k a o l i n i t e , can be seen i n Berea sandstone and Glenn Sand; dol o m i t e appears to be present i n s i g n i f i c a n t amounts i n the core from the San Andres formation. These SEM, then, show c l e a r l y the t y p i c a l geometric and m i n e r a o l o g i c a l heterogeneity o f r e s e r v o i r rocks. They a l s o give one an idea of the shapes and s i z e s o f c l a y and mineral c r y s t a l s , and they suggest intimacy of contact between m i c e l l a r f l u i d and c l a y c r y s t a l s . F i n a l l y , i n some cases they suggest the p o s s i b i l i t y of s u r f a c t a n t p r e c i p i t a t i o n , e.g., dolomite i n the San Andres core sample. S a l t i n g Out And P r e c i p i t a t i o n of S u r f a c t a n t s By E l e c t r o l y t e s R e s e r v o i r b r i n e s and s u r f a c t a n t formulations normally c o n t a i n s u b s t a n t i a l concentrations of e l e c t r o l y t e s * one wt.% NaCl or greater i s t y p i c a l . They may a l s o c o n t a i n s i g n i f i c a n t conc e n t r a t i o n s o f m u l t i v a l e n t c a t i o n s but one u s u a l l y attempts to minimize these because o f the low s o l u b i l i t y o f the s u l f o n a t e s of m u l t i v a l e n t c a t i o n s . Owing to the h i g h s u r f a c t a n t concentrat i o n s normally used i n m i c e l l a r slugs ( t y p i c a l l y 5 w t . % ) , e l e c t r o l y t e e f f e c t s tend to be masked i n experiments w i t h cores. When one i s studying the e q u i l i b r i u m a d s o r p t i o n , however, they cannot be ignored; indeed, they may be the p r i n c i p a l phenomena which a r e observed. Surfactant S a l t i n g Out By Sodium C h l o r i d e . For the most part t h i s study has been confined to d e s a l t e d , d e o i l e d a l k y l b e n zene s u l f o n a t e s ( f o r procedures of d e s a l t i n g and d e o i l i n g see Ref. 9 ) . Low e q u i v a l e n t weight alkylbenzene s u l f o n a t e s are so w a t e r - s o l u b l e that they are extremely r e s i s t a n t to s a l t i n g out. However, when the a l k y l chain contains more than about 12 carbons, s a l t i n g out becomes i n c r e a s i n g l y s i g n i f i c a n t . Short chain a l cohols are commonly added to m i c e l l a r s o l u t i o n s to s t a b i l i z e them against p r e c i p i t a t i o n , but i t should be noted that these concent r a t e d s u r f a c t a n t s o l u t i o n s normally c o n t a i n about 5 wt.% s u r f a c tant and are u s u a l l y t u r b i d .



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In preparing d i l u t e s o l u t i o n s of a s e r i e s of alkylbenzene s u l f o n a t e s f o r adsorption experiments, i t was observed that most of them became cloudy upon the a d d i t i o n of NaCl, and that a prec i p i t a t e of salted-out s u r f a c t a n t formed i n a considerable number. Since i t was deemed necessary to use c l e a r s u r f a c t a n t s o l u t i o n s only f o r adsorption measurements, a more d e t a i l e d study of the s a l t i n g - o u t phenomenon was undertaken, and some of these r e s u l t s are presented here. Aqueous stock s o l u t i o n s of s e l e c t e d a n i o n i c s u r f a c t a n t s were prepared; i f a l c o h o l was to be added, i t was incorporated i n the stock s u r f a c t a n t s o l u t i o n . These s u r f a c t a n t s were desalted and d e o i l e d . Aqueous NaCl s o l u t i o n s were a l s o prepared. Surfactant (or s u r f a c t a n t / a l c o h o l ) and NaCl s o l u t i o n s were mixed i n 10 ml screw-capped t e s t tubes i n the proportions necessary to give the d e s i r e d f i n a l concentration of each c o n s t i t u e n t . A f t e r thorough mixing, the t e s t tubes were set aside and observed p e r i o d i c a l l y f o r c l a r i t y and/or p r e c i p i t a t i o n . The r e s u l t s are summarized i n Table I . In general, cloudiness or p r e c i p i t a t i o n developed almost i n stantaneously i f i t occurred at a l l . The r e s u l t s shown i n Table I i n d i c a t e that sodium dodecylbenzene s u l f o n a t e (SPBS) alone of the s u r f a c t a n t s tested was s t a b l e i n s a l t s o l u t i o n s . As l i t t l e as 0.1 wt. % NaCl caused cloudiness i n s o l u t i o n s of a l l of the other s u r f a c t a n t s t e s t e d . The a d d i t i o n of 5 wt.% n-butanol prevented s a l t i n g out of sodium pentadecylbenzene s u l f o n a t e (SPBS) at both 0.1 and 1.0 wt.% NaCl, but none of the other butanols were e f f e c t i v e i n preventing s a l t i n g out of the s u r f a c t a n t . I t a l s o seems worth n o t i n g that the 5 wt.% n-butanol s o l u t i o n s of SPBS became hazy at 1.25 wt.% NaCl and a c l e a r lower phase separated at 3 wt. % NaCl l e a v i n g a hazy upper phase. I t appears, then, that s a l t i n g out of s u r f a c t a n t occurs when the s u r f a c t a n t equivalent weight exceeds 350 and the NaCl concentration exceeds 0.1 wt.%. The a d d i t i o n of short chain a l c o h o l s seems to be e f f e c t i v e only f o r SPBS (Eq. wt. = 390.5). Although the data are not included i n Table I , i t has been noted that 0.1 wt.% s o l u t i o n s of SPBS, TRS 10-410, and Aerosol OT a l s o become c l o u d l y upon the a d d i t i o n of 1.0 wt.% NaCl. Aside from the s i g n i f i c a n c e of the s a l t i n g - o u t phenomenon i t s e l f , these observations are important f o r adsorption measurements i n that i t appears that the s u r f a c t a n t concentration a c t u a l l y i n s o l u t i o n i s l e s s than 0.1 wt.% when appreciable concentrations of NaCl are present. Not only does the d i s s o l v e d s u r f a c t a n t concent r a t i o n appear to be l e s s than about 0.1 wt.% but there i s the e f f e c t on the apparent adsorption i f the s a l t e d out s u r f a c t a n t p a r t i a l l y or completely separates w i t h the c l a y or other adsorbent being s t u d i e d . Complete separation of the salted-out s u r f a c t a n t leads to l a r g e values of apparent adsorption and low e q u i l i b r i u m s u r f a c t a n t concentrations; n e g l i g i b l e separation of the s a l t e d - o u t s u r f a c t a n t leads to low adsorption and l a r g e apparent e q u i l i b r i u m s u r f a c t a n t concentrations but the a c t u a l d i s s o l v e d s u r f a c t a n t con-


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centrâtion may be q u i t e low. I t should be noted here that there i s l i t t l e or no evidence f o r m i c e l l e adsorption o f a n i o n i c s u r factants . These experiments have made c l e a r that some o f our own e q u i l i b r i u m a d s o r p t i o n data are erroneous. I t seems p o s s i b l e that some o f the data i n the l i t e r a t u r e may have been a f f e c t e d by s a l t i n g - o u t o f the s u r f a c t a n t , and some o f these may warrant réexaminât i o n .


Surfactant P r e c i p i t a t i o n By M u l t i v a l e n t

Cations

P r e c i p i t a t i o n of a n i o n i c s u r f a c t a n t s by m u l t i v a l e n t c a t i o n s i s w e l l known and i t has been s t u d i e d i n t e n s i v e l y (10, 11, 12,13). Powney and Addison (10) found that the a d d i t i o n o f small amounts of CaCl^ to d i l u t e sodium dodecyl s u l f a t e s o l u t i o n s caused prec i p i t a t i o n o f the calcium s a l t . The a d d i t i o n o f small amounts o f n-hexanol was reported to postpone p r e c i p i t a t i o n to higher CaCl^ concentrations, and i n c r e a s i n g the s u r f a c t a n t concentration t o a value greater than the CMC was found by Pearson and Lawrence (13) to prevent p r e c i p i t a t i o n o f the calcium s a l t o f dodecyl s u l f a t e owing to f i x a t i o n o f calcium ions by the m i c e l l e s . More r e c e n t l y Smith (8), and H i l l and Lake (14) studied c a t i o n exchange as i t a f f e c t e d the behavior o f m i c e l l a r slugs i n t y p i c a l r e s e r v o i r cores. These authors found that c a t i o n exchange i n cores was q u i t e complex, but that calcium and magnesium could, f o r a l l p r a c t i c a l purposes, be t r e a t e d as a s i n g l e species. Moreover, they found that p r e - f l u s h i n g o f a core reduced s u r f a c tant l o s s e s i n most cases. H i l l and Lake found that s u r f a c t a n t adsorption i n cores was reduced by d i s s o l u t i o n o f carbonate minerals and by converting the c l a y s to t h e i r sodium form. Where s u r f a c t a n t s were used i n these experiments they were present i n r e l a t i v e l y l a r g e c o n c e n t r a t i o n . H i l l and Lake, for example, i n j e c t e d a s o l u t i o n c o n t a i n i n g 0.046 raeq/ml o f a s u r f a c t a n t mixture having an average equivalent weight of 410; thus, the s u r f a c t a n t concentration was about 1.9 wt.%. Since t h i s conc e n t r a t i o n i s f a r above the CMC f o r the s u r f a c t a n t s i n v o l v e d , m u l t i v a l e n t cations may be bound by the m i c e l l e s with the r e s u l t t h a t calcium s u l f o n a t e p r e c i p i t a t i o n does not occur. In e q u i l i b r i u m adsorption experiments, however, one must work w i t h much smaller s u r f a c t a n t concentrations and those d i l u t e s u r f a c t a n t s o l u t i o n s behave d i f f e r e n t l y than concentrated ones. This was made evident i n a dramatic way i n two d i f f e r e n t e x p e r i ments. In the f i r s t case, when a 6 i n c h column o f crushed Berea sandstone was c a t i o n exchanged w i t h 1.0 Ν NaCl, a s l u g of solution c o n t a i n i n g more than 200 ppm Ca 2 (measured by atomic absorption) and about 0.5 pore volumes wide issued from the column. The peak Ca concentration was about 1500 ppm. When a small amount of t h i s s o l u t i o n was added to an approximately one wt.% SDBS s o l u t i o n , a copious white p r e c i p i t a t e formed. I n another e x p e r i ment, a s o l u t i o n o f SDBS i n 1.0 wt.% NaCl was added to a sample o f +



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M i s s i s s i p p i montmorillonite i n the r a t i o of 10 ml of l i q u i d per gram of c l a y . A f t e r e q u i l i b r a t i o n and c e n t r i f u g a t i o n to separate the s o l i d s , i t was noted that there were two l a y e r s of s o l i d s ; the lower one was tan i n c o l o r and due to the c l a y ; the upper s o l i d l a y e r was an o f f - w h i t e c o l o r and i t was l a t e r shown to cons i s t mostly of s u r f a c t a n t p r e c i p i t a t e d as calcium and magnesium sulfonates. To evaluate the importance of m u l t i v a l e n t c a t i o n p r e c i p i t a t i o n of s u r f a c t a n t s by m u l t i v a l e n t c a t i o n s present i n the formation b r i n e or r e s u l t i n g from c a t i o n exchange and d i s s o l u t i o n of minerals such as limestone, dolomite, e t c . , a very l i m i t e d study of the calcium t o l e r a n c e of s e l e c t e d alkylbenzene s u l f o n a t e s was undertaken. Experimental Procedure. The technique used i n these e x p e r i ments was q u i t e simple: A stock s o l u t i o n of the s u r f a c t a n t was made and CaCl2 s o l u t i o n i n v a r y i n g concentrations was added, the f i n a l volume being kept constant. I f a l c o h o l was to be added, i t was incorporated i n the s u r f a c t a n t s o l u t i o n i n an amount s u f f i c i e n t to make the f i n a l concentration 3 wt.% a l c o h o l . The tubes were capped and r o t a t e d at one rpm f o r 24 hours i n a t h e r mostat set at 25°C. Experiment at 40°C and 60°C were a l s o conducted. A p o r t i o n of the l i q u i d was t r a n s f e r r e d to an absorption c e l l and the transmission was measured at a wavelength of 650 nm ( i t having been determined p r e v i o u s l y that the s o l u t i o n and the p r e c i p i t a t e were r e l a t i v e l y i n s e n s i t i v e to wavelength i n that range). The onset of p r e c i p i t a t i o n was chosen as the Ca ^ conc e n t r a t i o n at which the transmission f e l l below 98% when compared to the s u r f a c t a n t s o l u t i o n to which no CaCl2 bad been added. +

Experimental R e s u l t s With Crude S u r f a c t a n t s . In general, as the CaCl2 concentration increased no d i s c e r n a b l e change i n the transmission occurred u n t i l the onset of p r e c i p i t a t i o n ; a f t e r prec i p i t a t i o n was i n i t i a t e d , the transmission decreased r a p i d l y and p r e c i p i t a t i o n was u s u a l l y noted unless the s u r f a c t a n t concentrat i o n was very s m a l l . As the CaCl2 concentration was increased f u r t h e r , the p r e c i p i t a t e began to f l o c c u l a t e and the s o l u t i o n e v e n t u a l l y became c l e a r . The a d d i t i o n of 3 wt.% n-butanol and 2-butanol modified the behavior somewhat but d i d not prevent precipitation. The calcium tolerance of a s e l e c t e d group of crude a l k y l benzene s u l f o n a t e s i s summarized i n Table I I . These r e s u l t s , w h i l e r e l a t i v e l y imprecise, i n d i c a t e that the calcium tolerance of these s u r f a c t a n t s i n d i l u t e s o l u t i o n s i s q u i t e low. In general, the calcium tolerance decreases as the equivalent weight i n c r e a s e s , and the a d d i t i o n of 3 wt.% n-butanol or 2-butanol does not appear to improve s t a b i l i t y . F i n a l l y , the onset of p r e c i p i t a t i o n does not appear to be very s e n s i t i v e to temperature.


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Surfact ant Salting—Out by Sodium C h l o r i d e Surfactant, SDBS TRS-10-410 SPBS A e r o s o l OT 1.0 wt.% Surfactant Equiv. wt. 348.5 418 390.5 444.5 NaCl Alcohol wt.% 5 wt.% clear clear clear None clear ted not t e s t e d not t e s t e d n-BuOH not tes ted not tes ted not t e s t e d not t e s t e d i-BuOH not tes ted not tes ted not tested not t e s t e d t-BuOH not tes ted not tes 0.1 none clear cloudy cloudy cloudy n-BuOH not tes ted clear cloudy cloudy i-BuOH not t eted s cloudy cloudy cloudy t-BuOH not t eted s cloudy cloudy cloudy 1.0 none clear cloudy cloudy,ppt. cloudy,ppt. n-BuOH not tes ted c l e a r * cloudy,ppt. cloudy,ppt. i-BuOH not t eted s cloudy cloudy,ppt. cloudy,ppt. t-BuOH not tes ted cloudy cloudy,ppt. cloudy,ppt. ^becomes cloudy a t 1 25 wt.% NaCl; phase separation occurs a t about 3 wt.% NaCl. Table I I . Calcium Tolerance of Crude Alkylbenzene Sulfonates

Surfactant SPBS 0.00625 wt.%

TRS-10-410 0.025 wt.%

Petrostep 420 0.025 wt.%

Petrostep 450 0.0125 wt.%

Petrostep 465

Precision!,

Temp. °C 24 30 40 60 24 30 40 60 24 30 40 60 24 30 40 60 24 30 40 60

+2 Ca Necessary to I n i t i a t e P r e c i p i t a t i o n , ppm No A24* lcohol 3 wt.% 2-BuOH 24n-BuOH 3 wt.%20 24* 24 20 24* 24 20 24* 24 20^ 24 20 20 24 20 20 24 20 20 24 20 20 15 2 2 15 2 2 15 2 2 15 2 2 15 2 2 12 2 2 10 2 2 5 2 2 15 2 2 15 2 2 15 2 2 15 2 2

approximately + 2 ppm Ca



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Experimental Results w i t h P u r i f i e d S u r f a c t a n t s . A s i m i l a r s e r i e s of experiments was c a r r i e d out w i t h desalted and d e o i l e d alkylbenzene s u l f o n a t e s . Aside from the apparently smaller s o l u b i l i t y of the p u r i f i e d s u r f a c t a n t s , the r e s u l t s were essent i a l l y the same as f o r the crude s u r f a c t a n t s . The calcium i o n c o n c e n t r a t i o n necessary to i n i t i a t e p r e c i p i t a t i o n from SDBS s o l u t i o n s was about 200 ppm. For p u r i f i e d SPBS i t was 10 to 20 ppm, and f o r p u r i f i e d TRS 10-410 i t was about 5 ppm. The onset of p r e c i p i t a t i o n was r e l a t i v e l y i n s e n s i t i v e to temperature i n the range 25-60°C, and l i t t l e a f f e c t e d by the a d d i t i o n of 3 wt.% of e i t h e r n-butanol or 2-butanol. I t should be noted that p r e c i p i t a t i o n occurred i n a l l cases i n s p i t e of the f a c t that the s u r f a c t a n t concentrations were w e l l above the CMC. In s h o r t , f i x a t i o n of calcium ions by m i c e l l e s d i d not prevent s u r f a c t a n t prec i p i t a t i o n i n these experiments. S i g n i f i c a n c e For E q u i l i b r i u m Adsorption Measurements. While these r e s u l t s appear to have l i t t l e relevance f o r m i c e l l a r s l u g s , they are q u i t e r e l e v a n t f o r e q u i l i b r i u m adsorption measurements. They show that p r e c i p i t a t i o n of s u r f a c t a n t s can be expected to occur i f the calcium i o n concentration exceeds the l i m i t r e q u i r e d to i n i t i a t e p r e c i p i t a t i o n (SDBS ~ 200 ppm: and TRS 10-410 < 10 ppm). Unless the adsorbent i s s t r o n g l y colored and the s u r f a c t a n t c o n c e n t r a t i o n i s s u b s t a n t i a l , i . e . , > 0.1 wt.%, one may experience d i f f i c u l t y i n d e t e c t i n g the presence of p r e c i p i t a t e d s u r f a c t a n t . I f a p p r e c i a b l e p r e c i p i t a t i o n occurs, however, i t leads to e r r o neous adsorption d a t a — a s we have noted i n s e v e r a l cases. The problem i s o b v i o u s l y more s e r i o u s w i t h the higher equivalent weight s u r f a c t a n t s (Eq. wt. > 400) and these are the s u r f a c t a n t s of greatest i n t e r e s t f o r improved o i l recovery by m i c e l l a r f l o o d ing. I t should be noted that the s u r f a c t a n t s used i n these experiments are commercial products; they are complex mixtures of many compounds and the equivalent weights represent averages. The i n fluence of the mixture i s not shown by these experiments; i t i s being i n v e s t i g a t e d now and w i l l be reported i n a l a t e r p u b l i c a t i o a The Adsorption of A n i o n i c Surfactants on Clays and Related Material The adsorption of s u r f a c t a n t s on r e s e r v o i r sands and c l a y s has been known from e a r l y s t u d i e s on s u r f a c t a n t f l o o d i n g , and from time to time the r e s u l t s of i n v e s t i g a t i o n s d e a l i n g s p e c i f i c a l l y w i t h adsorption l o s s e s have been reported (1-7, 15,16,17). These have made c l e a r that adsorption l o s s e s and s e l e c t i v e adsorpt i o n are s i g n i f i c a n t f a c t o r s i n determining the e f f i c i e n c y and economic f e a s i b i l i t y of s u r f a c t a n t f l o o d i n g f o r enhanced o i l recovery. However, the mechanism of adsorption i s not w e l l understood and s e v e r a l other aspects of the process i n d i c a t e the need f o r a d d i t i o n a l i n f o r m a t i o n and understanding. This i n v e s t i g a t i o n


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was undertaken w i t h the p a r t i c u l a r o b j e c t i v e s o f e l u c i d a t i n g t h e mechanism and i d e n t i f y i n g the p r i n c i p a l e f f e c t s o f system para meters . Experimental: M a t e r i a l s and Procedure. The e q u i l i b r i u m ad s o r p t i o n o f sodium dodecylbenzene s u l f o n a t e (SDBS), and d e o i l e d TRS 10-410 (a commercial petroleum s u l f o n a t e w i t h an equivalent weight o f 418) on s i l i c a g e l (Davison Grade 62), and crushed Berea sandstone was measured a t 30°C a t two b r i n e concentrations (0 and 1 wt.% NaCl). A weighed amount o f adsorbent which had been d r i e d overnight at 110°C was t r a n s f e r r e d to a 15 ml screw-capped t e s t tube, 10 ml of s u r f a c t a n t s o l u t i o n c o n t a i n i n g e i t h e r no NaCl o r one wt.% NaCl were added, and an amount of deionized water s u f f i c i e n t to s a t u r ate the adsorbent (determined i n a separate experiment i n which adsorbent i s e q u i l i b r a t e d w i t h water o r b r i n e a t the adsorption temperature) added. The t e s t tube was covered w i t h p o l y v i n y l c h l o r i d e f i l m , the cap screwed on, and the tubes mounted on a drum which r o t a t e s a t one rpm at the adsorption temperature. A f t e r twenty-four hours r o t a t i o n (previous experiments had shown that e q u i l i b r i u m was reached i n 16 h r s . o r l e s s ) , the tubes were r e moved, uncapped, and c e n t r i f u g e d to give a c l e a r supernatant l i q u i d . The l i q u i d was sampled and analyzed f o r s u r f a c t a n t content by complexing the s u r f a c t a n t w i t h methylene blue i n an a c i d i f i e d sodium s u l f a t e s o l u t i o n , e x t r a c t i n g the complex i n t o chloroform, and measuring the absorbance of the chloroform so l u t i o n a t the absorption maximum of the complex. The e q u i l i b r i u m or r e s i d u a l c o n c e n t r a t i o n of s o l u t i o n was c a l c u l a t e d from a ma t e r i a l balance. The p r e c i s i o n o f the measurements appears to be ±2%, and the accuracy appears to be about ±5%. Adsorption on S i l i c a G e l . The a d s o r p t i o n isotherms of sodium dodecylbenzene s u l f o n a t e and TRS 10-410 on s i l i c a g e l a t 30°C and pH =5.8 are shown i n Figure 2 f o r zero and one wt. % NaCl. A l though the equivalent weights of these s u r f a c t a n t s d i f f e r sub s t a n t i a l l y (SDBS = 348; TRS-10-410=418) the isotherms are very s i m i l a r i n shape: there i s a concave toe, a shoulder, and a long f l a t p l a t e a u i n each case. The a d d i t i o n o f one wt.% NaCl to the s o l u t i o n r e s u l t s i n a sharp r e d u c t i o n i n the adsorption p l a t e a u (or s a t u r a t i o n l e v e l ) f o r SDBS (one wt.% NaCl causes s a l t i n g - o u t of TRS-10-410, see Table I , so no adsorption isotherm was measured f o r TRS-10-410 and one wt % NaCl). The concave shape o f the adsorption isotherms a t low s u r f a c t a n t concentrations i n d i c a t e s that the presence o f some ad sorbed s u r f a c t a n t makes e a s i e r the adsorption o f a d d i t i o n a l sur f a c t a n t , and t h i s process continues up to s a t u r a t i o n at which p o i n t the isotherm breaks over sharply to the f l a t p l a t e a u . I n the case o f SDBS the c r i t i c a l m i c e l l e concentration i s known to be 0.056 wt.% or 1.61 χ 10"3 moles/1 (18,19). Thus, the shoulder of the adsorption isotherm occurs a t or near the CMC. The con-


10

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stancy of the amount of SDBS adsorbed on s i l i c a g e l as the m i c e l l e s i z e and c o n c e n t r a t i o n increases i s evidence that m i c e l l e a d s o r p t i o n does not occur. This r e s u l t i s to be expected s i n c e both the s i l i c a s u r f a c e and the m i c e l l e s are n e g a t i v e l y charged and strong coulombic r e p u l s i o n e x i s t s between them. The isotherm for TRS-10-410 has the same shape as those f o r SDBS. However, the shoulder occurs at lower s u r f a c t a n t concentration and the adsorpt i o n p l a t e a u i s lower than the SDBS isotherms. This i s c o n s i s tent w i t h a smaller CMC f o r TRS-10-410. The CMC of TRS-10-410 and i t s NaCl c o n c e n t r a t i o n dependence are not known w i t h the p r e c i s i o n of that f o r SDBS, but the CMC i s probably somewhat l e s s than 0.5 wt.% ( 1 . 2 x l 0 ~ moles/1). From Figure 2, one can observe that the shoulder of the a d s o r p t i o n isotherm occurs near the CMC, and that there i s no evidence of m i c e l l e a d s o r p t i o n on s i l i c a g e l . The sharp r e d u c t i o n i n the a d s o r p t i o n s a t u r a t i o n l e v e l when one wt.% NaCl i s added to the s o l u t i o n seems to be a s s o c i a t e d p r i n c i p a l l y w i t h the e f f e c t of e l e c t r o l y t e on the s t r u c t u r e of the e l e c t r i c a l double l a y e r and on the i n f l u e n c e of NaCl on the CMC of the s u r f a c t a n t s . The CMC decreases as the i o n i c s t r e n g t h of the s o l u t i o n increases and t h i s has the e f f e c t of reducing the maximum s u r f a c t a n t monomer c o n c e n t r a t i o n .


3

Adsorption on Berea Sandstone. Berea sandstone was reported by Malmberg and Smith (20) to c o n s i s t of approximately 91 wt.% sand and 9 wt. % c l a y . The a d s o r p t i o n measurements reported here are f o r the crushed sandstone but i t should be noted that essent i a l l y a l l of the a d s o r p t i o n occurred on the c l a y f r a c t i o n . In a separate experiment the c l a y f r a c t i o n was separated from the sand and the a d s o r p t i o n of SDBS measured on both f r a c t i o n s . No adsorpt i o n on the sand could be detected w h i l e strong a d s o r p t i o n on the c l a y was found. Moreover, the a d s o r p t i o n on the c l a y agreed very w e l l w i t h that found on the o r i g i n a l crushed sandstone when converted to a common b a s i s . The e q u i l i b r i u m a d s o r p t i o n isotherms of SDBS and TRS-10-410 are shown i n F i g u r e 3 f o r zero and one wt.% NaCl. These isotherms are s t r i k i n g l y d i f f e r e n t i n shape from those obtained w i t h s i l i c a gel though there i s some s i m i l a r i t y at low r e s i d u a l s u r f a c t a n t c o n c e n t r a t i o n s . In the f i r s t p l a c e , a maximum i n the a d s o r p t i o n isotherm i s observed when the adsorbent i s c l a y . Secondly, the a d d i t i o n of NaCl r e s u l t s i n a s i g n i f i c a n t increase i n the amount of s u r f a c t a n t adsorbed i n c o n t r a s t to the decrease observed when NaCl was added to systems w i t h s i l i c a g e l as the adsorbent. Adsorption maxima have been observed by s e v e r a l i n v e s t i g a t o r s and v a r i o u s hypotheses have been advanced to account f o r them (21»12,23,24). The more recent i n v e s t i g a t i o n s a t t r i b u t e adsorpt i o n maxima to m i c e l l e e x c l u s i o n , but t h i s may be too s i m p l i s t i c an e x p l a n a t i o n s i n c e the c o n c e n t r a t i o n of s u r f a c t a n t monomers does not appear to be g r e a t l y diminished by the presence of m i c e l l e s and the amount of s u r f a c t a n t adsorbed at high r e s i d u a l


Micellar

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WALKER E T A L .

Figure 3.

Adsorption of de-oiled sodium alkylbenzene sulfonates at 30°C


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s u r f a c t a n t concentrations may, i n some cases, be as l i t t l e 20% of that at the adsorption maxima.

as

The Mechanism of A n i o n i c S u r f a c t a n t Adsorption on Clay and

Silica

Since a l l of the c o n s t i t u e n t s ( s u r f a c t a n t anions and m i c e l l e s , as w e l l as s i l i c a and c l a y s ) are n e g a t i v e l y charged a t the pH of the experiments (pH = 6 ) , one must consider why a d s o r p t i o n of the s u r f a c t a n t should occur a t a l l when e l e c t r i c a l r e p u l s i o n between the s o l i d s u r f a c e and the s u r f a c t a n t e x i s t s . A p o s s i b l e explanat i o n can be found i n the hydrophobic character of the hydrocarbon " t a i l " of the s u r f a c t a n t anion and i n the tendency f o r van der Waals a t t r a c t i o n between the hydrocarbon t a i l s to promote adsorpt i o n . However, when one c a l c u l a t e s the a d s o r p t i o n d e n s i t y and s u r f a c e area per molecule, i t seems very u n l i k e l y t h a t these kinds of forces c o u l d account f o r a d s o r p t i o n of these s u r f a c t a n t s on s i l i c a g e l . For example, the minimum surface areas per adsorbed s u r f a c t a n t anion c a l c u l a t e d from the s a t u r a t i o n l e v e l s of F i g u r e 1 are: SDBS, 4200A (no NaCl) and 7000A (1 wt.% NaCl). TRS-10-410, 31,000A (no NaCl). I t seems u n l i k e l y that i n t e r a c t i o n between adsorbsed s u r f a c t a n t anions could occur; yet the shape of the toe of the a d s o r p t i o n isotherm suggests adsorbate i n t e r a c t i o n . A more p l a u s i b l e e x p l a n a t i o n f o r a n i o n i c s u r f a c t a n t adsorpt i o n on s i l i c a g e l i s found i n the presence of about 0.2 wt.% alumina i n the s i l i c a g e l . The alumina pz-c i s about pH=8.0, so i t would be p o s i t i v e l y charged at the pH of the experiments. I f the alumina i s uniformly d i s t r i b u t e d through the s i l i c a , a l l of the a d s o r p t i o n could be accounted f o r provided a close-packed monol a y e r of s u r f a c t a n t i s formed on the alumina. T h i s circumstance would a l s o be c o n s i s t e n t w i t h the shape of the toe of the isotherm. Gaudin and Fuerstenau (25) advanced the idea of hemimicelle f o r mation (two-dimensional m i c e l l e s on a surface) to account f o r s i m i l a r observations i n f l o t a t i o n processes. In the case of sand i n sandstones, the surface area i s normally small and the surface charge d e n s i t y i s l a r g e so t h a t n e g l i g i b l e a n i o n i c s u r f a c t a n t a d s o r p t i o n i s to be expected. The s i t u a t i o n i s q u i t e d i f f e r e n t with the c l a y s commonly present i n sandsontes. The s p e c i f i c s u r f a c e area i s l a r g e to very l a r g e depending on the m i n e r a l o g i c a l d i s t r i b u t i o n , and the surface charge d e n s i t y i s l e s s than f o r s i l i c a because the point of zero charge (pzc) c l a y s i s higher than that of s i l i c a . Moreover, p o s i t i v e charges e x i s t at c r y s t a l edges and i m p e r f e c t i o n s i n c l a y s , and these can serve as primary a d s o r p t i o n s i t e s f o r a n i o n i c s u r f a c tants . There i s l i t t l e or no evidence of m i c e l l e adsorption i n these systems, and, indeed, i t i s not to be expected owing to the coulombic r e p u l s i o n between the n e g a t i v e l y charged s i l i c a or c l a y and the m i c e l l e s . S u r f a c t a n t anion a d s o r p t i o n at p o s i t i v e ( c a t i o n i c ) s i t e s followed by f u r t h e r s u r f a c t a n t a d s o r p t i o n induced by l a t e r a l a t t r a c t i o n between the hydrocarbon t a i l s to form 2



1.

WALKER E T A L .

Micelhr

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hemimicelles appears to be c o n s i s t e n t w i t h most o f the adsorpt i o n data a v a i l a b l e . There i s , however, the question of the o r i e n t a t i o n of the adsorbed s u r f a c t a n t anions. I f the primary adsorption i s a t p o s i t i v e l y charged s i t e s , the hydrophobic hydrocarbon t a i l o f the s u r f a c t a n t anion would be presented to the s o l u t i o n . This would tend to produce f l o c c u l a t i o n and to i n crease the contact angle a t the clay-water i n t e r f a c e . N e i t h e r o f these occurs, a t l e a s t i n our experience. No f l o c c u l a t i o n o f c l a y has been observed when a n i o n i c s u r f a c t a n t s are adsorbed; i n deed, s e p a r a t i o n of c l a y w i t h adsorbed a n i o n i c s u r f a c t a n t s are adsorbed* indeed, s e p a r a t i o n o f c l a y w i t h adsorbed a n i o n i c s u r f a c t a n t s has been somewhat more d i f f i c u l t than when no s u r f a c t a n t was present. Moreover, a few measurements o f the contact angle between water and c l a y w i t h adsorbed a n i o n i c s u r f a c t a n t have demonstrated that the c l a y remains very wettable by water. Cont r a r i w i s e , the adsorption o f a c a t i o n i c s u r f a c t a n t l e d to very high contact angles, i n d i c a t i n g the development of a hydrophobic water-clay i n t e r f a c e . Berg (26) has found evidence that dimers a r e the p r i n c i p a l species i n s o l u t i o n below the CMC, a t l e a s t when the hydrocarbon chain contains 12 o r more carbons. He has proposed that the dimers form by o r i e n t i n g two hydrocarbon t a i l s approximately p a r a l l e l w i t h a p o l a r head group at each end o f the dimer. The adsorption of dimers having the c o n f i g u r a t i o n suggested by Berg appears to be c o n s i s t e n t w i t h a l l of the observations made i n t h i s l a b o r a t o r y . Moreover, the decrease i n CMC w i t h e i t h e r increase i n equivalent weight or w i t h increase i n e l e c t r o l y t e concentration i s g e n e r a l l y c o n s i s t e n t w i t h the s h i f t s o f the shoulder of the a d s o r p t i o n isotherm. Conclusion T y p i c a l r e s e r v o i r rocks are q u i t e complex p h y s i c a l l y as w e l l as raineralogically. The surface o f sand grains a v a i l a b l e f o r i n t e r a c t i o n s w i t h i n j e c t i o n f l u i d s i s a s i g n i f i c a n t f r a c t i o n of the t o t a l surface area; however, the r e a c t i v i t y o f sand w i t h a n i o n i c s u r f a c t a n t s i s much l e s s than that between c l a y s and s u r f a c t a n t s , so i n t e r a c t i o n w i t h c l a y s tends to dominate behavior. S a l t i n g - o u t o f alkylbenzene s u l f o n a t e s from r e l a t i v e l y d i l u t e s o l u t i o n s by NaCl has been found to be s u b s t a n t i a l i f the a l k y l chain contains more than about 12 carbons. S i m i l a r l y , the m u l t i v a l e n t c a t i o n tolerance o f alkylbenzene s u l f o n a t e s i n d i l u t e s o l u t i o n s has been found to be s n a i l and s t r o n g l y dependent on equivalent weight. I n general, i f the equivalent weight exceeds 350 ( a l k y l chain more than 12 carbons), the calcium tolerance appears to be extremely s m a l l . The a d d i t i o n o f s h o r t - c h a i n a l c o h o l s appears to be of l i m i t e d b e n e f i t i n preventing e i t h e r s a l t ing-out by NaCl or m u l t i v a l e n t c a t i o n p r e c i p i t a t i o n . A n i o n i c s u r f a c t a n t s appear to adsorb on s i l i c a s u r f a c e s , which are n e g a t i v e l y charged above a pH o f about 2.0, only when


CHEMISTRY OF OIL RECOVERY


14

significant amounts of positively charged impurities are present. Micelle adsorption on silica is not observed, probably owing to the strong coulombic repulsion between the negatively charged silica surface and the negatively charged micelles. The addition of NaCl sharply reduces the adsorption saturation level, partially as a result of depression of the CMC of the surfactant at higher ionic strengths but possibly also due to the influence of electrolyte of the structure of the electrical double layer at the silica/solution interface. Hemimicelle formation is observed regardless of the salt concentration. Adsorption of anionic surfactants on crushed Berea sandstone occurs on the clay only and adsorption maxima are observed. The addition of one wt.% NaCl to the surfactant solution results in greatly increased adsorption but in no significant change in the shape of the adsorption isotherm. The point of zero charge of the reservoir minerals, their physical structure, the surfactant equivalent weight and structure, and the structure of the electrical double layer at the solid/solution interface appear to be major factors determining the mechanism of adsorption and potential surfactant losses in surfactant flooding. Acknowledgement The authors wish to express their sincere appreciation to Energy Research and Development Administration (Grant No. EY-77S-05-5341), National Science Foundation-RANN (Grant No. AER 7513813) and to the industrial consortium of 21 major oil and chemical companies for their support of the research presented in this paper. This work is presently supported by USDOE Contract EW-78S-19-008. Abstract Scanning electron micrographs of fracture surfaces of Berea sandstone and representative reservoir cores reveal that the rock surface is quite heterogeneous as to geometry and mineralogy. Clay and other minor constituents cover much of the sand rather than being concentrated in cementation bridges, thus insuring intimate contact with the micellar fluid. Alkylbenzene sulfonates in dilute aqueous solution are very susceptible to salting-out by NaCl, especially if the alkyl chain contains more than 12 carbons. Cation exchange capacity measurements show that the CEC of Berea sandstone can be attributed to its clay fraction and that injection of 1N NaCl into crushed Berea sandstone generates a calcium ion wave about one pore volume wide with a peak concentration near 1500 ppm. Measurements of the calcium tolerance of several sulfonates indicate that precipitation of sulfonate begins at low calcium concentrations (200 ppm to < 10 ppm depending on sulfonate equivalent weight), and that the onset of precipitation is


1. WALKER ET AL.

Micellar Flooding

15


not materially affected by added butanol or by sulfonate concen tration. Negligible surfactant adsorption is found on sand, but it is appreciable on silica gel. The isotherm on silica gel has a concave toe and becomes constant near the CMC at a level which decreases with increasing NaCl concentration. Sulfonate adsorp tion on clay is quite different: adsorption maxima are observed and adsorption is increased by NaCl. Adsorbate association (hemimicelle formation) is observed on both silica gel and clay, and some evidence for dimer adsorption is found. Both salting-out and multivalent cation precipitation complicate adsorption measure ments. Literature Cited 1. 2. 3. 4. 5. 6. 7.

8. 9.

10. 11. 12. 13. 14. 15. 16.

17.

Gale, W.W., and Sandvik, E.I., Soc. Pet. Eng. J. (1973) 13, 191. H i l l , H.J., Reisberg, J., and Stegemeier, G . L . , J. Pet. Tech. (1973) 25, 186. Trushenski, S . P . , Dauben, D . L . , and Parrish, D . R . , Soc. Pet. Eng. J. (1974) 14, 633. Somasundaran, P . , Healy, T.W., and Fuerstenau, D.W., J. Phys. Chem. (1964) 68, 3562. Bae, J.H., and Petrick, C . B . , Soc. Pet. Eng. J. (1977) 17, 353 Hurd, B . G . , Soc. Pet. Eng. Symposium on Improved O i l Recovery (March 22-24, 1976) Tulsa, OK, SPE Paper No. 5818. Melrose, J.C., Johnson, W.F., George, R . A . , and Groenveld, H., Soc. Pet. Eng. 51st Annual Meeting (1976), SPE Paper No. 6049. Smith, F.W., Soc. Pet. Eng. International Symposium on Oil -field and Geothermal Chemistry (1977), SPE Paper No. 6598. ASTM-.D2548-69 (Reapproved 1974). "Analysis of Oil-Soluble Petroleum Sodium Sulfonates by Liquid Chromatography", 1976 Annual Book of ASTM Standards, Part 24, Am. Soc. Test. Matls., Philadelphia, PA. Powney, J., and Addison, C . C . , Trans. Farad. Soc. (1937) 33, 1253. Shinoda, Κ., and Ito, K . , J. Phys. Chem. (1961) 65, 1499. C o c k i l l , J . M . , and Goodman, J.F., Trans. Farad. Soc. (1962), 58, 206. Pearson, J.T., and Lawrence, A . S . C . , Trans. Farad. Soc. (1967) 63, 488. H i l l , H . J . , and Lake, L.W., Soc. Pet. Eng. F a l l Meeting (1977), SPE Paper No. 6770. Hower, W.F., Clays and Clay Minerals, (1970) 18, 97. Somasundaran, P . , and Hanna, H . S . , "Improved O i l Recovery by Surfactant and Polymer Flooding," p.205, D.O. Shah and R.S. Schechter, Eds., Academic Press, New York (1977). Hanna, H.S. Goyal, Α . , and Somasundaran, P . , Paper No. 239, presented at the VIIth International Congress on Surface


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Active Substances, Moscow, September 1976. 18. Ludlum, D.B., J. Phys. Chem. (1956) 60, 1240. 19. Tadros, T.F., J. Coll. and Interfac. Sci. (1974) 46, 328. 20. Malmberg, E.W., and Smith, L . , "Improved Oil Recovery by Sur factant and Polymer Flooding", p. 275, D.O. Shah and R.S. Schechter, Eds., Academic Press, New York, (1977). 21. Meader, A.L., and Fries, B.A., Ind. Eng. Chem. (1952) 44 1536. 22. Eyring, H., and Fava, Α., J. Phys. Chem., (1956) 60, 890. 23. Hsiao, L . , and Dunning, H.N., J. Phys. Chem. (1955) 59, 362. 24. Mukerjee, P., and Anavil, Α., "Adsorption at Interfaces," p. 107, (1975), K. Mittal, Ed., Am. Chem. Soc., Washington, D.C. 25. Gaudin, A.M., and Fuerstenau, D.W., (1955) Trans. AIME, 202, 958. 26. Berg, R.L., (1977) USERDA Bartlesville Energy Research Center, Report BERC/TPR-77/3. RECEIVED

November 2, 1978.


Cation Exchange, Surfactant Precipitation, and Adsorption in Micellar

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