Emulsions in Enhanced Oil Recovery - American Chemical Society

7 Emulsions in Enhanced Oil Recovery Downloaded by NORTH CAROLINA STATE UNIV on December 7, 2012 | http://pubs.acs.org Publication Date: May 5, 1992 | doi: 10.1021/ba-1992-0231.ch007

Kevin C . Taylor and Blaine F. Hawkins Petroleum Recovery Institute, 3512 33rd Street N . W . , Calgary, Alberta, Canada T 2 L 2A6

Micellar-polymer flooding and alkali-surfactant-polymer (ASP) flooding are discussed in terms of emulsion behavior and interfacial properties. Oil entrapment mechanisms are reviewed, followed by the role of capillary number in oil mobilization. Principles of micellarpolymer flooding such as phase behavior, solubilization parameter, salinity requirement diagrams, and process design are used to introduce the ASP process. The improvements in “classical” alkaline flooding that have resulted in the ASP process are discussed. The ASP process is then further examined by discussion of surfactant mixing rules, phase behavior, and dynamic interfacial tension.

E M U L S I O N S A R E O F G R E A T I M P O R T A N C E i n e n h a n c e d o i l recovery ( E O R ) t e c h n i q u e s . I n some cases, e m u l s i o n s m a y be an u n w e l c o m e c o n s e q u e n c e o f the process, b u t i n o t h e r cases, the use o f emulsions is c r i t i c a l a n d f u n d a m e n t a l to the o i l recovery process. I n general, processes that r e l y o n the i n j e c t i o n o f surfactants or s u r f a c t a n t - f o r m i n g materials i n t o a reservoir r e l y heavily o n e m u l s i o n technology. M i c e l l a r - p o l y m e r flooding a n d a l k a l i - s u r f a c t a n t - p o l y m e r flooding are two examples i n w h i c h e m u l s i o n t e c h n o l o g y specific to the process has e v o l v e d to m e e t special needs. I n these processes it is necessary to u n d e r s t a n d the b e h a v i o r o f an e m u l s i o n as it is i n j e c t e d i n t o o r f o r m e d i n a reservoir, as it travels t h r o u g h that reservoir over a p e r i o d o f weeks o r m o n t h s , a n d as it flows out o f the reservoir t h r o u g h a p r o d u c i n g w e l l . T h i s chapter discusses the basics r e q u i r e d for an a p p r e c i a t i o n o f these processes. T h r o u g h o u t this chapter, m i c r o e m u l s i o n s w i l l be treated as a type o f e m u l s i o n , e v e n t h o u g h there are f u n d a m e n t a l differences b e t w e e n the t w o . M i c r o e m u l s i o n s are t h e r m o d y n a m i c a l l y stable a n d w i l l not segregate w i t h 0065-2393/92/0231-0263 $08.75/0 © 1992 American Chemical Society

In Emulsions; Schramm, L.; Advances in Chemistry; American Chemical Society: Washington, DC, 1992.

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EMULSIONS IN THE PETROLEUM INDUSTRY

Downloaded by NORTH CAROLINA STATE UNIV on December 7, 2012 | http://pubs.acs.org Publication Date: May 5, 1992 | doi: 10.1021/ba-1992-0231.ch007

t i m e . I n contrast, e m u l s i o n s w i l l e v e n t u a l l y separate w i t h t i m e a n d o w e t h e i r stability t o processes that d e l a y the a p p r o a c h t o e q u i l i b r i u m . T h e p r o g r e s s i o n f r o m m i c e l l a r - p o l y m e r flooding to a l k a l i - s u r f a c t a n t p o l y m e r ( A S P ) flooding has special significance i n this chapter. M i c e l l a r p o l y m e r flooding is t e c h n i c a l l y w e l l - d e v e l o p e d , relatively w e l l - u n d e r s t o o d , a n d has u n d e r g o n e n u m e r o u s t e c h n i c a l l y successful field trials. H o w e v e r , this process is i n h e r e n t l y expensive because o f the relatively large surfactant concentrations that m u s t b e i n j e c t e d i n t o t h e reservoir. A l k a l i - s u r f a c t a n t p o l y m e r flooding is a m u c h n e w e r technology, is m o r e c o m p l e x , a n d is n o t t e c h n i c a l l y w e l l - d e v e l o p e d . M a n y o f t h e lessons l e a r n e d f r o m m i c e l l a r p o l y m e r flooding c a n b e a p p l i e d t o t h e A S P process. A l k a l i - s u r f a c t a n t p o l y m e r flooding is i n h e r e n t l y m u c h less expensive t h a n t h e m i c e l l a r - p o l y m e r process, p r i m a r i l y because t h e surfactant c o n c e n t r a t i o n is significantly l o w e r . F i e l d trials are i n progress, a l t h o u g h m a n y o f t h e details r e m a i n c o n f i d e n t i a l . T h i s t e c h n o l o g y is at t h e stage that t h e m i c e l l a r - p o l y m e r p r o cess was i n d u r i n g the early 1970s. A s m o r e is l e a r n e d , this process m a y c o m e i n t o m u c h m o r e w i d e s p r e a d use.

Oil Entrapment and Mobilization in Porous Media O i l E n t r a p m e n t M e c h a n i s m s . E n h a n c e d o i l r e c o v e r y processes d e p e n d i n large part o n t h e e l i m i n a t i o n o r r e d u c t i o n o f c a p i l l a r y forces. C a p i l l a r y forces are t h e strongest that o c c u r u n d e r t y p i c a l reservoir c o n d i tions, a n d are most r e s p o n s i b l e for o i l e n t r a p m e n t . V i s c o u s forces, w h i c h act to displace o i l , are c o m p o s e d o f t h e a p p l i e d pressure gradient, gravity, density differences b e t w e e n phases, a n d viscosity ratio. I n a p e r m e a b l e m e d i u m , c a p i l l a r y forces result w h e n t h e pores c o n s t r a i n t h e o i l - w a t e r interface t o a h i g h degree o f c u r v a t u r e . F r o m t h e L a p l a c e e q u a t i o n , t h e c a p i l l a r y pressure P i n a c a p i l l a r y tube c a n b e d e r i v e d : c

P ,P _P c

o

w

=

2iL£2L* r

(1)

w h e r e Ρ is t h e pressure i n a fluid phase (subscripts ο a n d w for o i l o r water, respectively), σ is t h e i n t e r f a c i a l t e n s i o n , r is t h e radius o f c u r v a t u r e o f the interface, a n d Θ is t h e contact angle, w h i c h is t h e angle that t h e o i l - w a t e r interface makes w i t h the s o l i d surface, as m e a s u r e d t h r o u g h the w a t e r phase. W h e n t h e contact angle is z e r o , t h e surface is said to b e h i g h l y water-wet. I f the contact angle is 180°, t h e n o i l c o m p l e t e l y wets t h e surface a n d i t is o i l w e t . T h e p r e f e r e n c e o f the surface for o i l o r w a t e r is its w e t t a b i l i t y . I n c r u d e o i l reservoirs, w e t t a b i l i t y varies f r o m relatively o i l - w e t i n carbonate reser voirs to relatively w a t e r - w e t i n sandstone reservoirs, w i t h m a n y exceptions.


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A l t h o u g h the c a p i l l a r y t u b e is a s i m p l e r e p r e s e n t a t i o n o f fluid i n a p o r e , several v a l i d c o m m e n t s c a n b e m a d e . T h e c a p i l l a r y pressure increases as p o r e d i a m e t e r decreases, o r as i n t e r f a c i a l t e n s i o n ( I F T ) o r contact angle increases. C a p i l l a r y forces are t h e r e f o r e i n f l u e n c e d b y p o r e geometry, i n t e r facial t e n s i o n , a n d surface w e t t a b i l i t y .


W h e n an aqueous phase flows t h r o u g h a p o r o u s m e d i u m c o n t a i n i n g o i l , some o i l w i l l b e r e a d i l y d i s p l a c e d , b u t c a p i l l a r y forces w i l l act to trap o i l i n some o f the p o r e spaces. N o matter h o w m u c h aqueous phase flows t h r o u g h the m a t e r i a l , a c e r t a i n a m o u n t o f o i l , c a l l e d r e s i d u a l o i l , w i l l r e m a i n t r a p p e d . T h e r e s i d u a l o i l saturation is generally expressed as a p e r c e n t a g e o f t h e o r i g i n a l o i l i n place ( % O O I P ) , a n d c a n b e greater t h a n 4 0 % . T h i s o i l is t h e target o f m a n y e n h a n c e d o i l r e c o v e r y t e c h n i q u e s . T w o m o d e l s h e l p to e x p l a i n t h e m e c h a n i s m s b y w h i c h o i l is e n t r a p p e d i n p o r o u s m e d i a : t h e p o r e d o u b l e t a n d the s n a p - o f f m o d e l s . T h e p o r e d o u b l e t m o d e l was i n t r o d u c e d b y M o o r e a n d S l o b o d i n 1956 (1), a n d has b e e n c r i t i c a l l y r e v i e w e d b y C h a t z i s a n d D u l l i e n (2). I n this m o d e l , t w o capillaries o f d i f f e r e n t diameters are c o n n e c t e d at t h e i n l e t a n d outlet ends to create a n i d e a l i z e d m o d e l o f a p o r e structure. I n i t i a l l y , t h e d o u b l e t is f u l l o f o i l , a n d w a t e r w i t h viscosity t h e same as that o f the o i l flows i n f r o m o n e e n d ( F i g u r e l a ) . A s t h e w a t e r c o n t i n u e s to flow, t h r e e o u t c o m e s are p o s s i b l e : 1. O i l c o u l d b e c o m p l e t e l y d i s p l a c e d f r o m b o t h c a p i l l a r i e s . 2. S o m e o i l c a n r e m a i n i n t h e s m a l l e r c a p i l l a r y . 3. S o m e o i l c a n r e m a i n i n t h e larger c a p i l l a r y . F o r t h e first o u t c o m e to o c c u r , viscous a n d c a p i l l a r y forces w o u l d have to b e e q u a l , w h i c h is u n l i k e l y u n d e r realistic reservoir c o n d i t i o n s . M o r e likely, viscous forces w i l l b e s m a l l relative to c a p i l l a r y forces u n d e r n o r m a l flow c o n d i t i o n s i n a reservoir. I n this case, u n d e r w a t e r - w e t c o n d i t i o n s , c a p i l l a r y pressure is m u c h greater i n t h e c a p i l l a r y w i t h a s m a l l d i a m e t e r , so t h e w a t e r moves m o r e r a p i d l y , t r a p p i n g t h e o i l i n t h e larger c a p i l l a r y ( F i g u r e l b ) . Q u a l i t a t i v e l y , t h e m o d e l p r e d i c t s that t h e n o n w e t t i n g phase w i l l b e t r a p p e d i n large pores, w h i l e t h e w e t t i n g phase w i l l b e t r a p p e d i n s m a l l cracks a n d crevices. A s c a p i l l a r y forces are l o w e r e d , a decrease i n t r a p p i n g w i l l result i n the w a t e r - w e t case, because t h e relative v e l o c i t y o f t h e w a t e r i n t h e s m a l l c a p i l l a r y w i l l decrease relative to that i n t h e large c a p i l l a r y . T h e p o r e d o u b l e t m o d e l is a s i m p l e o n e that tends to greatly overestimate r e s i d u a l o i l satura t i o n i n p o r o u s m e d i a . H o w e v e r , it does s h o w h o w a n o n w e t t i n g phase c a n become trapped i n water-wet porous media. T h e snap-off m o d e l has b e e n d e t a i l e d w i t h e x p e r i m e n t a l w o r k b y C h a t z i s et a l . (4). I n this m o d e l , o i l i n i t i a l l y fills a series o f c o n n e c t e d p o r e


266


OU Phase (nonwetting)

^ Water Phase (wetting)


a) beginning of displacement

Water Phase (wetting) b) oil phase trapped Figure 1. Fore doublet model. (Reproduced with permission from reference 3. Copyright 1989 Prentice Hall.) b o d i e s ( F i g u r e 2). T h e surface o f each p o r e is w a t e r - w e t , h o w e v e r , a n d is c o a t e d w i t h a t h i n film o f water. C a p i l l a r y pressure varies a l o n g t h e flow p a t h , b e i n g highest at t h e p o r e throat c o n s t r i c t i o n s . A s w a t e r flows i n t o o n e e n d o f t h e p o r e series, some o i l w i l l b e d i s p l a c e d . B u t i f t h e c a p i l l a r y pressure b e c o m e s h i g h e n o u g h , t h e o i l phase w i l l snap o f f i n t o globules w i t h i n t h e pores i n t h e flow p a t h . S n a p - o f f begins to o c c u r as t h e ratio o f p o r e d i a m e t e r to p o r e throat d i a m e t e r (aspect ratio) exceeds a c r i t i c a l v a l u e . I n p o r o u s m e d i a the p o r e structure is m u c h m o r e c o m p l e x t h a n i n t h e s i m p l e s n a p - o f f m o d e l , d i s p l a y i n g a range o f p o r e sizes a n d aspect ratios. I n B e r e a sandstone, about 8 0 % o f t h e t r a p p e d n o n w e t t i n g phase occurs i n t h e snapo f f g e o m e t r y , a n d t h e r e m a i n i n g a p p r o x i m a t e l y 2 0 % occurs i n p o r e d o u b l e t s or c o m b i n a t i o n s o f the t w o (4). T h e n o n w e t t i n g r e s i d u a l o i l b e c o m e s t r a p p e d i n l a r g e r pores i n globules that c a n b e several p o r e diameters l o n g .

Figure 2. Pore snap-off model. (Reproduced with permission from reference 3. Copyright 1983 Society of Petroleum Engineers.)


7.

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Capillary Number in Oil Mobilization, T h e c a p i l l a r y n u m b e r N is a d i m e n s i o n l e s s ratio o f viscous to c a p i l l a r y forces; i t p r o v i d e s a m e a sure o f h o w strongly t r a p p e d r e s i d u a l o i l is w i t h i n a g i v e n p o r o u s m e d i u m (5). V a r i o u s definitions have b e e n u s e d f o r c a p i l l a r y n u m b e r , b u t the f o l l o w i n g e q u a t i o n is c o m m o n : c

N = ^ σ

(2)


c

w h e r e υ is D a r c y v e l o c i t y , μ is the viscosity o f the d i s p l a c i n g phase, a n d σ is the i n t e r f a c i a l t e n s i o n b e t w e e n the d i s p l a c e d a n d d i s p l a c i n g phases. T h e D a r c y v e l o c i t y is expressed i n units o f distance over t i m e a n d is o b t a i n e d b y d i v i d i n g t h e flow rate i n t o a p o r o u s m e d i a b y t h e cross-sectional area t h r o u g h w h i c h flow occurs. F i g u r e 3 shows several c a p i l l a r y n u m b e r curves o b t a i n e d f r o m the literature f o r w a t e r - w e t B e r e a sandstone. T h e shape o f the c u r v e is affected b y w e t t a b i l i t y a n d p o r e size d i s t r i b u t i o n . F o r the o i l phase, m o b i l i z a t i o n o f r e s i d u a l o i l u s u a l l y begins at a c a p i l l a r y n u m b e r o f about 10" (the c r i t i c a l c a p i l l a r y n u m b e r ) , a n d c o m p l e t e o i l r e c o v e r y occurs at h i g h values o f N o f about 10~ . F o r a variety o f w a t e r - w e t sandstones, C h a t z i s a n d M o r r o w (6) f o u n d a c r i t i c a l c a p i l l a r y n u m b e r o f 10~ a n d c o m p l e t e r e c o v e r y o f o i l at N o f 10" . 5

2

c

5

3

c

T h e c a p i l l a r y n u m b e r f o r flow i n a t y p i c a l o i l r e s e r v o i r u n d e r g o i n g w a t e r - f l o o d i n g can easily be c a l c u l a t e d . A t a flow rate o f about 0.26 m/day (3 χ 10" m/s), an o i l - w a t e r i n t e r f a c i a l t e n s i o n o f 30 m N / m , a n d w a t e r viscosity 6

10"

6

10"

5

1CT

4

10

-3

10'

2

Capillary Number Figure 3. Capillary number correlations from the literature. Key: . . Gupta and Trushenski (34); - - -, Taber (5); and —-, Chatzis and Morrow (6).


268


o f 1 mPa-s, the c a p i l l a r y n u m b e r is 1 x 10" . T h i s n u m b e r is 2 orders o f m a g n i t u d e b e l o w the c r i t i c a l value r e q u i r e d to b e g i n m o b i l i z a t i o n o f r e s i d u a l o i l . T o r e c o v e r a l l o i l , the c a p i l l a r y n u m b e r must b e i n c r e a s e d b y a factor o f 10 . T h i s increase c o u l d b e d o n e b y d e c r e a s i n g the I F T to 3 x 10~ m N / m ; s u c h a decrease is possible i n some s u r f a c t a n t - o i l systems. A n increase i n viscosity b y 4 orders o f m a g n i t u d e is not feasible because o f w e l l - b o r e injectivity p r o b l e m s , a l t h o u g h an increase o f 1 o r d e r o f m a g n i t u d e c o m b i n e d w i t h a decrease i n I F T b y 3 orders o f m a g n i t u d e w o u l d suffice. A n increase o f the flow rate b y several orders o f m a g n i t u d e is i m p r a c t i c a l because the r e s u l t i n g pressure w o u l d fracture the o i l - b e a r i n g f o r m a t i o n . A l s o , i n j e c t i o n w e l l s u s u a l l y d o not have significant u n u s e d i n j e c t i o n capacity. So, to greatly increase c a p i l l a r y n u m b e r , i n t e r f a c i a l t e n s i o n can be decreased b y a large a m o u n t , a n d d i s p l a c i n g phase viscosity c a n be i n c r e a s e d m o d e r a t e l y . 7


4

3

Interfacial Parameters Important in Enhanced Oil Recovery Interfacial Tension. A s already seen, i n t e r f a c i a l t e n s i o n is o f f u n d a m e n t a l i m p o r t a n c e i n d e t e r m i n i n g the c a p i l l a r y forces a c t i n g o n t r a p p e d o i l w i t h i n p o r o u s m e d i a . Interfacial t e n s i o n arises f r o m an i m b a l a n c e i n the forces o f attraction b e t w e e n m o l e c u l e s i n the b u l k phase a n d m o l e c u l e s at the interface. It c a n be d e f i n e d as the i s o t h e r m a l reversible w o r k o f f o r m a t i o n o f u n i t area o f interface i n a system o f constant c o m p o s i t i o n (7). I f i n t e r f a c i a l t e n s i o n b e t w e e n t w o phases b e c o m e s z e r o , t h e n the t w o phases b e c o m e m i s c i b l e . T h i s result is the u l t i m a t e a i m o f m a n y types o f E O R : to m a k e o i l - w a t e r i n t e r f a c i a l t e n s i o n e q u a l to 0, so that a d i s p l a c i n g fluid c a n m i s c i b l y displace o i l t r a p p e d i n the p o r o u s m e d i u m . I n p r a c t i c e , it is d i f f i c u l t to m a k e i n t e r f a c i a l t e n s i o n a p p r o a c h 0 f o r l i q u i d s o f s u c h d i f f e r e n t characteristics as o i l a n d w a t e r . V e r y l o w values o f I F T are usually m e a s u r e d w i t h the s p i n n i n g d r o p m e t h o d m a d e p r a c t i c a l b y C a y i a s et a l . (8), because this m e t h o d is the most versatile m e t h o d for t a k i n g measurements at values o f 0.1 m N / m a n d l o w e r . C l a s s i c a l m e t h o d s s u c h as the c a p i l l a r y rise, d u N o u y ring, W i l h e l m y plate, a n d d r o p w e i g h t (and v o l u m e ) m e t h o d s are generally u s e d for i n t e r f a c i a l t e n s i o n measurements o f greater t h a n 1 m N / m . T h e s p i n n i n g d r o p m e t h o d also has the advantage o f b e i n g able to measure d y n a m i c changes i n I F T . T h e o n l y o t h e r p o t e n t i a l l y u s e f u l m e t h o d to date is laser l i g h t scattering, w h i c h is largely u n p r o v e n f o r c r u d e - o i l - w a t e r i n t e r f a c i a l t e n s i o n measure m e n t (9). I n the s p i n n i n g d r o p m e t h o d , an o i l d r o p l e t o f a p p r o x i m a t e l y 1 is p l a c e d i n a glass c a p i l l a r y (2 m m i.d.) f u l l o f aqueous s o l u t i o n . T h e glass tube is sealed a n d s p u n h o r i z o n t a l l y a r o u n d its axis at 5000 to 10,000 r p m . C e n t r i f u g a l force w i l l cause the l i g h t e r phase (usually the oil) to f o r m an


7.

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269

e l o n g a t e d d r o p i n the c e n t e r o f the glass c a p i l l a r y t u b e . T h e shape o f this d r o p is the result o f a n e q u i l i b r i u m b e t w e e n r o t a t i o n a l forces a c t i n g to elongate the d r o p a n d c o n t r a c t i o n b y i n t e r f a c i a l t e n s i o n a c t i n g to m i n i m i z e surface area. P r o v i d i n g that the d r o p is q u i t e l o n g relative to its w i d t h , i n t e r f a c i a l t e n s i o n c a n be c a l c u l a t e d w i t h V o n n e g u t ' s f o r m u l a (10):


σ

= ^ 1

(3)

w h e r e Δ ρ is the density d i f f e r e n c e b e t w e e n the t w o phases, ω is the angular v e l o c i t y , a n d R is h a l f o f the d r o p w i d t h . W h e n the d r o p is not l o n g relative to its w i d t h , a m o r e c o m p l i c a t e d treatment is r e q u i r e d (8). I n t e r f a c i a l t e n s i o n is g e n e r a l l y at a m i n i m u m w h e n the i n t e r f a c i a l c o n c e n t r a t i o n o f a d s o r b e d surfactant m a t e r i a l is at a m a x i m u m . Possibly, m a x i m u m a d s o r p t i o n at the interface occurs w h e n the surfactant is e q u a l l y s o l u b l e i n the o i l a n d aqueous phases. Shah et a l . (II) s h o w e d that the I F T b e t w e e n an o i l a n d an aqueous s u r f a c t a n t - c o n t a i n i n g phase is at a m i n i m u m w h e n the p a r t i t i o n c o e f f i c i e n t is u n i t y . T h i s c o n d i t i o n occurs w h e n the surfactant is e q u a l l y soluble i n b o t h phases.

Interfacial Viscosity. I n a c l e a n system i n w h i c h t w o p u r e l i q u i d s p r o d u c e an interface, the viscosity o f the interface s h o u l d be the same as the b u l k s o l u t i o n viscosity. H o w e v e r , surfactant o r i m p u r i t y a d s o r p t i o n at an interface c a n cause a resistance to flow to o c c u r that c a n be m e a s u r e d as the i n t e r f a c i a l shear viscosity. T h i s viscosity is d e f i n e d as the ratio b e t w e e n the shear stress a n d the shear rate i n the p l a n e o f the interface (12). M e t h o d s u s e d to m a k e these measurements i n c l u d e a viscous t r a c t i o n surface v i s c o m eter (12), d r o p l e t - d r o p l e t coalescence (13), the r o t a t i n g r i n g v i s c o m e ter (14), a n d surface laser l i g h t scattering (9). L o w i n t e r f a c i a l viscosity is desirable i n e n h a n c e d o i l r e c o v e r y o p e r a tions, so that d i s p l a c e d o i l globules may r e a d i l y coalesce i n t o an o i l bank. E m u l s i o n stability decreases as i n t e r f a c i a l viscosity decreases, a n d this c o n d i t i o n increases the ease w i t h w h i c h an o i l b a n k c a n b e f o r m e d . W a s a n et al. (15) f o u n d a qualitative c o r r e l a t i o n b e t w e e n coalescence rates a n d i n t e r facial viscosities f o r c r u d e o i l . In extreme cases, m a t e r i a l c a n adsorb at an interface to create a film. I n t e r f a c i a l film f o r m a t i o n c a n o c c u r i n c r u d e - o i l systems a n d has b e e n r e p o r t e d b y B l a i r (16), a n d b y R e i s b e r g a n d D o s c h e r (17), F i l m f o r m a t i o n is relatively c o m m o n w i t h c r u d e oils a n d c a n effectively stabilize e m u l s i o n s b y p r e v e n t i n g d r o p l e t coalescence e v e n w i t h h i g h values o f i n t e r f a c i a l t e n s i o n .

Surface Charge at Interfaces. T h e interface i n a c r u d e - o i l - w a ter system u s u a l l y carries a net charge, w h i c h c a n be c a u s e d b y the a d s o r p t i o n o f surface-active ions. T h e s e surfactants m a y be c a r b o x y l i c acids that


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originate f r o m the o i l o r synthetic surfactants a d d e d to the system. T h e r m a l m o t i o n o f the c o u n t e r i o n s i n the aqueous phase results i n a d i f f u s e d d o u b l e layer adjacent to the i n t e r f a c e w i t h a n excess o f c o u n t e r i o n s . T h e s e c o u n t e r i o n s screen the attraction b e t w e e n ions a d s o r b e d at the interface a n d c o u n t e r i o n s that are m o r e distant. T h i s s c r e e n i n g causes c o u n t e r i o n c o n c e n t r a t i o n to decrease e x p o n e n t i a l l y w i t h distance f r o m the interface, a n d a net charge results. I n f o r m a t i o n about the surface charge o f droplets i n an e m u l s i o n c a n b e o b t a i n e d f r o m e l e c t r o p h o r e s i s . I n this process, a s o l u t i o n c o n t a i n i n g s m a l l o i l d r o p l e t s is p l a c e d b e t w e e n o p p o s i t e l y c h a r g e d electrodes. B y m e a s u r i n g the v e l o c i t y o f a p a r t i c l e u n d e r a k n o w n field gradient a n d d i v i d i n g v e l o c i t y b y field gradient, the e l e c t r o p h o r e t i c m o b i l i t y is o b t a i n e d . I n g e n e r a l , the absolute v a l u e o f e l e c t r o p h o r e t i c m o b i l i t y is at a m a x i m u m w h e n I F T is at a m i n i m u m i n s u r f a c t a n t - c r u d e - o i l systems (9, 18) as w e l l as i n a c i d i c c r u d e o i l - s o d i u m h y d r o x i d e systems (19). T h u s , surface charge is at a m a x i m u m w h e n surface-active ions are present at m a x i m u m c o n c e n t r a t i o n at the i n t e r face. C h a n g e s i n surface charge c a n result i n changes i n the w a y that o i l d r o p l e t s react w i t h t h e i r s u r r o u n d i n g s . A negative surface charge w i l l r e d u c e the attraction o f o i l d r o p l e t s to negatively c h a r g e d s a n d surfaces, f o r i n stance. C h i a n g et a l . (18) f o u n d that o i l r e c o v e r y i n sand packs a n d B e r e a sandstone was at a m a x i m u m w h e n the absolute value o f surface charge was at a m a x i m u m . T h e y f o u n d that surface charge was at a m a x i m u m a n d i n t e r f a c i a l viscosity at a m i n i m u m b e t w e e n 3.5 w t % s o d i u m c h l o r i d e s o l u t i o n a n d Seeligson c r u d e o i l . I n s a n d packs, r e c o v e r y i m p r o v e d f r o m 6 5 % O O I P to 7 3 % O O I P w h e n i n c r e a s i n g the salinity o f the d i s p l a c i n g phase f r o m 0 to 3 . 5 % . I n B e r e a sandstone, r e c o v e r y i m p r o v e d f r o m 48 to 5 8 % O O I P . I m p r o v e d o i l r e c o v e r y c o u l d not b e a c c o u n t e d f o r b y e i t h e r the alteration o f w e t t a b i l i t y o r the s m a l l increase i n c a p i l l a r y n u m b e r .

Surface Wettability. A s m e n t i o n e d earlier, w e t t a b i l i t y affects cap i l l a r y pressure a n d thus the e n t r a p m e n t a n d d i s p l a c e m e n t o f o i l i n p o r o u s m e d i a . T h e i m p o r t a n c e o f w e t t a b i l i t y o n the d i s p l a c e m e n t o f o i l b y w a t e r o r b r i n e solutions has l o n g b e e n k n o w n . I n 1956, after several t h o u s a n d flood i n g e x p e r i m e n t s i n a v a r i e t y o f p o r o u s m e d i a , M o o r e a n d S l o b o d (I ) c a m e to the c o n c l u s i o n that w e t t a b i l i t y is the single most i m p o r t a n t factor a f f e c t i n g w a t e r - f l o o d r e c o v e r y efficiency. L a t e r (20, 21), i n t e r m e d i a t e w e t t a b i l i t y was s h o w n to be u n f a v o r a b l e f o r e n h a n c e d o i l r e c o v e r y processes, because the c a p i l l a r y n u m b e r r e q u i r e d for o i l m o b i l i z a t i o n is h i g h e r t h a n occurs w i t h w a t e r - w e t p o r o u s m e d i a , a n d less o i l is available to b e r e c o v e r e d . W a t e r flood o i l r e c o v e r y f r o m w e a k l y w a t e r - w e t cores was h i g h e r t h a n that o b t a i n e d w i t h strongly w a t e r - w e t B e r e a sandstone. Surface w e t t a b i l i t y is p a r t i c u l a r l y i m p o r t a n t i n e n h a n c e d o i l recovery, because the fluids u s e d c a n change the surface w e t t a b i l i t y d u r i n g the course o f the r e c o v e r y process.


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T h i s change i n w e t t a b i l i t y i n t u r n c a n change relative p e r m e a b i l i t y a n d flow characteristics o f b o t h o i l a n d w a t e r i n p o r o u s m e d i a .

Principles of Micellar-Polymer Flooding M i c e l l a r - p o l y m e r flooding relies o n t h e i n j e c t i o n o f a surfactant s o l u t i o n t o l o w e r i n t e r f a c i a l t e n s i o n t o u l t r a l o w levels, o n t h e o r d e r o f 10" m N / m . T h e r e s u l t i n g increase i n c a p i l l a r y n u m b e r allows t h e r e c o v e r y o f r e s i d u a l o i l f r o m p o r o u s m e d i a . T h e t e r m " m i c e l l a r " is u s e d because the concentrations o f i n j e c t e d surfactant solutions are always above t h e i r c r i t i c a l m i c e l l e c o n c e n t r a t i o n . T h a t is, t h e y are always above t h e c o n c e n t r a t i o n at w h i c h m i celles f o r m .


3

Microemulsions. T h e structure o f m i c r o e m u l s i o n systems has b e e n r e v i e w e d (22). B o t h b i c o n t i n u o u s a n d d r o p l e t - t y p e structures, a m o n g o t h ers, c a n o c c u r i n m i c r o e m u l s i o n s . T h e d r o p l e t - t y p e structure is c o n c e p t u a l l y m o r e s i m p l e a n d is a n extension o f the e m u l s i o n structure that occurs at relatively h i g h values o f I F T . I n this case, v e r y s m a l l t h e r m o d y n a m i c a l l y stable droplets o c c u r , t y p i c a l l y s m a l l e r t h a n 10 n m (7). E a c h d r o p l e t is separated f r o m the c o n t i n u o u s phase b y à m o n o l a y e r o f surfactant. B i c o n t i n u o u s m i c r o e m u l s i o n s are those i n w h i c h o i l a n d w a t e r layers i n t h e m i c r o e m u l s i o n m a y b e o n l y a f e w m o l e c u l e s t h i c k , separated b y a m o n o l a y e r o f surfactant. E a c h layer m a y e x t e n d over a m a c r o s c o p i c distance, w i t h m a n y layers m a k i n g u p t h e m i c r o e m u l s i o n . C o m p o s i t i o n s o f i n j e c t e d m i c e l l a r .fluids c a n vary greatly. T h e y i n c l u d e aqueous o r o l e i c solutions o f surfactant as w e l l as c o m p l e x mixtures c o n t a i n i n g c o m p o n e n t s s u c h as cosurfactants, cosolvents, o r stabilizers, i n a d d i t i o n to surfactant, o i l , a n d b r i n e . Regardless o f t h e c o m p o s i t i o n o f t h e i n j e c t e d fluid, o n c e i n t h e reservoir t h e fluid system consists p r i m a r i l y o f o i l , w a t e r , and surfactant. T h e phase b e h a v i o r o f the fluid system c a n b e q u i t e c o m p l e x b u t m a y b e a p p r o x i m a t e l y d e s c r i b e d b y means o f p s e u d o t e r n a r y diagrams i n w h i c h the p s e u d o c o m p o n e n t s are surfactant, b r i n e , a n d o i l ( F i g u r e 4). D e p e n d i n g o n the system b e i n g s t u d i e d , the p s e u d o c o m p o n e n t s c a n range f r o m p u r e substances to c o m p l e x mixtures. F o r e x a m p l e , the o i l m a y b e a p u r e h y d r o c a r b o n o r a c r u d e - o i l m i x t u r e . T h e surfactant c a n i n c l u d e cosurfactants a n d cosolvents, a n d t h e b r i n e m a y i n c l u d e a variety o f i o n i c constituents. T h e p s e u d o t e r n a r y d i a g r a m is separated b y a m u l t i p h a s e b o u n d a r y i n t o a single phase r e g i o n above a n d a m u l t i p h a s e r e g i o n b e l o w t h e phase b o u n d a r y .

Phase Behavior. N e l s o n a n d c o - w o r k e r s ( 2 3 - 2 5 ) a n d H e a l y et al. (26) have w r i t t e n extensively o n phase b e h a v i o r i n m i c e l l a r flooding. I n Nelson's methodology, three different phase-behavior environments occur


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surfactant

brine

oil

Figure 4. Pseudoternary diagram of oil-water-surfactant system with three compositions of interest. (Reproduced with permission from reference 40. Copyright 1977 Academic Press.) ( F i g u r e 5). T h e s e are the type I I ( - ) , the type II(+), a n d the type I I I phase environments. B o t h t h e type I I ( - ) a n d t h e type II(+) phase e n v i r o n m e n t s have a m a x i m u m o f t w o phases. I n b o t h eases, o n e phase o n l y m a y be present at h i g h surfactant c o n c e n t r a t i o n s . T y p e s I I ( - ) a n d II(+) phase e n v i r o n m e n t s are d i s t i n g u i s h e d w h e n the phases are p l o t t e d o n a p s e u d o t e r n a r y d i a g r a m . T h e tie lines i n the two-phase r e g i o n give e i t h e r a negative slope f o r type I I ( - ) b e h a v i o r o r a p o s i t i v e slope for type II(+) b e h a v i o r . W i n s o r (27) as s i g n e d m i c r o e m u l s i o n s o c c u r r i n g i n the two-phase r e g i o n o f a type I I ( - ) d i a g r a m as type I, a n d d e f i n e d t h e m as a m i c r o e m u l s i o n i n e q u i l i b r i u m w i t h excess o i l . T h e m i c r o e m u l s i o n contains m o s t l y b r i n e a n d surfactant, a n d any o i l present is s o l u b i l i z e d i n m i c e l l e s . A s s u c h i t is a n o i l - i n - w a t e r (waterexternal) e m u l s i o n . T h e p l a i t p o i n t o f the p s e u d o t e r n a r y d i a g r a m tends to be close to the o i l apex. M i c r o e m u l s i o n s i n the two-phase r e g i o n o f a type II(+) phase e n v i r o n m e n t c o r r e s p o n d t o a W i n s o r type I I e m u l s i o n . T h e m i c r o e m u l s i o n is i n e q u i l i b r i u m w i t h excess b r i n e a n d contains m o s t l y o i l a n d surfactant. T h e b r i n e is s o l u b i l i z e d i n m i c e l l e s ; thus this is a w a t e r - i n - o i l (oil-external) e m u l s i o n . I n this case, the p l a i t p o i n t o f the p s e u d o t e r n a r y d i a g r a m tends to be close to the b r i n e apex. T h e type I I I phase e n v i r o n m e n t m a y c o n t a i n a m a x i m u m o f t h r e e phases. W h e n this is the case, the e m u l s i o n present c o r r e s p o n d s to W i n s o r type I I I , i n w h i c h a m i c r o e m u l s i o n is i n e q u i l i b r i u m w i t h p u r e o i l a n d p u r e b r i n e phases. H o w e v e r , type I I ( - ) b e h a v i o r a n d type II(+) b e h a v i o r m a y also b e o b s e r v e d u n d e r c e r t a i n c o n d i t i o n s . I n p r a c t i c e , type I I ( - ) or II(+) behav i o r occurs w h e n a l l o f t h e b r i n e o r o i l c a n b e i n c o r p o r a t e d i n t o t h e m i c r o e m u l s i o n o r w h e n i n s u f f i c i e n t surfactant is present to p r o d u c e a m e a surable m i c r o e m u l s i o n .


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surfactant

Typell(-)

two phases plait point


brine

overall Oil composition

surfactant

P fS^r two phases three phases na

Type III brine

overall Oil composition

surfactant

Typell(+)

p l a l t p o i n t

brine

two phases

overall °^ composition

Figure 5. Phase-behavior environments. (Reproduced with permission from reference 24. Copyright 1978 Society of Petroleum Engineers.)

A n u m b e r o f factors c a n affect t h e phase type that is o b s e r v e d . T h e s e factors g e n e r a l l y act b y c h a n g i n g t h e p a r t i t i o n i n g o f the surfactant b e t w e e n the b r i n e a n d o i l phases. I n g e n e r a l , any change i n the s u r f a c t a n t - o i l - b r i n e system that increases t h e s o l u b i l i t y o f surfactant i n o i l relative t o b r i n e w i l l cause t h e phase e n v i r o n m e n t type t o shift f r o m I I ( - ) t o I I I t o II(+) as i n d i c a t e d i n the f o l l o w i n g s c h e m e :

i n c r e a s i n g surfactant s o l u b i l i t y i n o i l

->

N e l s o n type I I ( - ) W i n s o r type I

N e l s o n type I I I W i n s o r type I I I

N e l s o n type II(+) W i n s o r type I I

l o w e r phase

m i d d l e phase

u p p e r phase m i c r o e m u l s i o n



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I n c r e a s i n g salinity g e n e r a l l y decreases the s o l u b i l i t y o f the surfactant i n the b r i n e phase. T h i s d e c r e a s e d s o l u b i l i t y tends to shift the phase b e h a v i o r f r o m I I ( - ) to I I I to II(+). C h a n g i n g the o i l type so that the surfactant is m o r e s o l u b l e i n the o i l w i l l also shift the phase b e h a v i o r f r o m left to r i g h t . B y i n c r e a s i n g the m o l e c u l a r w e i g h t o f the h y d r o p h o b i c part o f the surfactant, s o l u b i l i t y i n the aqueous phase w i l l g e n e r a l l y decrease, a n d phase b e h a v i o r w i l l shift f r o m left to r i g h t . D e c r e a s i n g the a m o u n t o f b r a n c h i n g w i l l have the same effect. S i m i l a r l y , d e c r e a s i n g the p o l a r i t y o f the h y d r o p h i l i c g r o u p w i l l also decrease s o l u b i l i t y i n the aqueous phase. T h e effects o f changes i n surfactant s t r u c t u r e o n phase b e h a v i o r have b e e n d i s c u s s e d i n d e t a i l (28). T h e f o l l o w i n g changes w i l l also shift phase b e h a v i o r f r o m I I ( - ) to I I I to H(+): • decrease i n t e m p e r a t u r e f o r a n i o n i c surfactants • increase i n t e m p e r a t u r e f o r n o n i o n i c surfactants • increase i n a l c o h o l c o n c e n t r a t i o n (alcohols o f f e w e r t h a n f o u r carbons) • decrease i n a l c o h o l c o n c e n t r a t i o n (alcohols o f m o r e t h a n f o u r carbons) • increase i n the d i v a l e n t i o n c o n c e n t r a t i o n o f the b r i n e

Solubilization Parameter.

T h e s o l u b i l i z a t i o n p a r a m e t e r (S) is d e

fined as S = ^

or

^

(4)

w h e r e V is the o i l v o l u m e , V is the v o l u m e o f surfactant, a n d V is the v o l u m e o f w a t e r , a l l three m e a s u r e d i n the m i c r o e m u l s i o n phase. I n t e r f a c i a l t e n s i o n σ c a n b e m e a s u r e d b e t w e e n the m i c r o e m u l s i o n a n d o i l phases ( a ) or b e t w e e n the m i c r o e m u l s i o n a n d w a t e r phases ( a ) . A s e i t h e r measure o f i n t e r f a c i a l t e n s i o n decreases, the s o l u b i l i z a t i o n p a r a m e t e r increases. H e a l y a n d R e e d (29) first s h o w e d that l o w i n t e r f a c i a l t e n s i o n correlates w i t h h i g h s o l u b i l i z a t i o n p a r a m e t e r , a n d H u h (30) s h o w e d it to be t h e o r e t i c a l l y v a l i d . G l i n s m a n n (31) a n d G r a c i a a et a l . (28) have v a l i d a t e d the c o n c e p t e x p e r i 0

s

w

mo

m w

m e n t a l l y . T h i s c o r r e l a t i o n is v e r y u s e f u l because it enables the results o f p h a s e - b e h a v i o r e x p e r i m e n t s to p a r t i a l l y r e p l a c e the e x p e r i m e n t a l l y m o r e difficult measurement of interfacial tension.

Salinity Requirement Diagrams. M a x i m u m s o l u b i l i z a t i o n p a r a m e t e r occurs v e r y close to the salinity at w h i c h m a x i m u m c o r e - f l o o d o i l


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r e c o v e r y is o b t a i n e d . T h e salinity r e q u i r e m e n t o f a c h e m i c a l flooding system is d e f i n e d b y N e l s o n (32) as the salinity at w h i c h a type I I I m i c r o e m u l s i o n is at m i d p o i n t salinity. T h i s c o n d i t i o n occurs w h e n the o i l a n d b r i n e c o n c e n t r a tions i n the m i c r o e m u l s i o n m i d d l e phase are e q u a l . M a n y authors (24, 25, 32-34) have r e p o r t e d that o i l r e c o v e r y is at a m a x i m u m i n a m i c e l l a r flood w h e n the system is near m i d p o i n t salinity. A salinity r e q u i r e m e n t d i a g r a m aids i n the design a n d u n d e r s t a n d i n g o f a m i c e l l a r - p o l y m e r system. T o construct a salinity r e q u i r e m e n t d i a g r a m , 5 to 10 d i f f e r e n t b r i n e salinities are p r e p a r e d f o r at least three surfactant concentrations i n screwcap test tubes. T y p i c a l l y , surfactant c o n c e n t r a t i o n w i l l range f r o m 0 to 10 w t % , a n d salinity w i l l vary a c c o r d i n g to the reservoir o f interest. S a m p l e tubes a l l c o n t a i n an i d e n t i c a l a m o u n t o f b r i n e , u s u a l l y b e t w e e n 50 a n d 8 0 % by v o l u m e . S a m p l e tubes are m i x e d r e g u l a r l y f o r several days, t h e n a l l o w e d to e q u i l i b r a t e . T h e e q u i l i b r a t i o n process c a n take a n y w h e r e f r o m several days to several m o n t h s , d e p e n d i n g o n e m u l s i o n stability. F i g u r e 6 shows a n i d e a l i z e d salinity r e q u i r e m e n t d i a g r a m . W i t h i n the type I I I phase e n v i r o n m e n t , three phases o c c u r i n the area i n d i c a t e d , b u t two phases o c c u r i n the rest o f the type I I I r e g i o n . T y p e II(+) phase b e h a v i o r occurs above the type I I I r e g i o n , a n d the type I I ( - ) b e h a v i o r occurs b e l o w . M i d p o i n t salinity is s h o w n near the m i d d l e o f the type I I I r e g i o n . M i d p o i n t salinity generally decreases w i t h d e c r e a s i n g surfactant c o n c e n t r a t i o n . A s surfactant c o n c e n t r a t i o n decreases, the type I I I phase e n v i -

l

0

ι

2

4

6

8

Total Surfactant (volume %)

.

1

10

Figure 6. Idealized salinity requirement diagram. (Modified and reproduced with permission from reference 32. Copyright 1982 Society of Petroleum Engi neers.)



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r o n m e n t b e c o m e s n a r r o w e r . T h i s n a r r o w i n g has i m p o r t a n t i m p l i c a t i o n s w h e n c a r r y i n g out a m i c e l l a r flood i n reservoir c o r e . I f a constant salinity flood is c a r r i e d out i n w h i c h the salinity is the same i n the w a t e r - f l o o d b r i n e , the c h e m i c a l s l u g , a n d the p o l y m e r d r i v e , recovery w i l l generally be i n f e r i o r to a salinity gradient flood, even t h o u g h b o t h floods use the same o p t i m u m c h e m i c a l c o m p o s i t i o n . F o r instance, w i t h a constant salinity flood d e s i g n e d to be at m i d p o i n t salinity, surfactant c o n c e n t r a t i o n w i l l decrease i n the front a n d rear m i x i n g zones. T h e surfactant s l u g w i l l t h e n change f r o m type III b e h a v i o r to type II(+) b e h a v i o r . I f a salinity gradient is u s e d , the f o l l o w i n g may o c c u r . A h i g h salinity w a t e r - f l o o d , w i t h salinity h i g h e r than m i d p o i n t salinity, is f o l l o w e d b y a c h e m i c a l s l u g w i t h salinity at o r just b e l o w m i d p o i n t salinity. T h e c h e m i c a l s l u g is f o l l o w e d by a p o l y m e r d r i v e that is m u c h b e l o w m i d p o i n t salinity. In the front m i x i n g z o n e , type III phase b e h a v i o r q u i c k l y develops f r o m i n i t i a l type II(+) behavior. A t the rear m i x i n g z o n e , type III b e h a v i o r is m a i n t a i n e d b e f o r e type I I ( - ) b e h a v i o r occurs. In the l o w e r salinity p o l y m e r d r i v e , surfactant that h a d p a r t i t i o n e d i n t o any r e m a i n i n g o i l partitions back i n t o the l o w e r salinity aqueous phase a n d thereby keeps the system i n type III b e h a v i o r for a l o n g e r p e r i o d . T y p i c a l l y , less surfactant a d s o r p t i o n occurs w i t h a salinity gradient flood than w i t h a constant salinity flood.

Process Design. T h e various applications o f m i c e l l a r flooding can be r e p r e s e n t e d o n a p s e u d o t e r n a r y d i a g r a m ( F i g u r e 4). T h e injected slug u s e d i n aqueous surfactant flooding, i n d i c a t e d b y p o i n t 1 o n the d i a g r a m , has no a d d e d o i l i n the slug m a t e r i a l . S o m e o i l may be i n t r o d u c e d f r o m either surfactant o r p o l y m e r m a n u f a c t u r i n g processes, but i n v e r y s m a l l amounts. A n o i l - i n - w a t e r m i c r o e m u l s i o n injectant is u s e d w h e n the c o m p o s i t i o n is r e p r e s e n t e d b y p o i n t 2. T h i s type o f system is u s e d i n a c o m m e r c i a l ( M a r a f l o o d ) process. C o m p o s i t i o n s i n d i c a t e d by p o i n t 3 are t e r m e d soluble o i l (35, 36) a n d can spontaneously e m u l s i f y water w i t h o i l r e m a i n i n g as the external phase. T h i s type o f c o m p o s i t i o n forms the basis o f another c o m m e r c i a l ( U n i f l o o d ) process. Ideally, the injected m i c e l l a r solutions w i l l be m i s c i b l e w i t h the fluids that they are i n contact w i t h i n the reservoir a n d can thus m i s c i b l y displace those fluids. I n t u r n , the m i c e l l a r solutions may be m i s c i b l y d i s p l a c e d by water. H i g h e s t o i l recovery w i l l result i f the injected m i c e l l a r s o l u t i o n is m i s c i b l e w i t h the reservoir o i l . I f there are no interfaces, i n t e r f a c i a l forces that trap o i l w i l l be absent. Injection o f c o m p o s i t i o n s l y i n g above the m u l t i phase b o u n d a r y i n i t i a l l y s o l u b i l i z e s b o t h w a t e r a n d o i l a n d displaces t h e m i n a m i s c i b l e l i k e m a n n e r . H o w e v e r as i n j e c t i o n o f the m i c e l l a r s o l u t i o n p r o gresses, m i x i n g occurs w i t h the o i l a n d b r i n e at the flood front, a n d surfac tant losses o c c u r because o f a d s o r p t i o n o n the reservoir rock. T h e s e c o m p o sitional changes move the system into the m u l t i p h a s e r e g i o n . T h e a b i l i t y o f


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the m i c e l l a r fluid t d r e c o v e r o i l is a l t e r e d d e p e n d i n g o n the phase e n v i r o n ment. In the two-phase r e g i o n , the type II(+) system has an o i l - r i c h m i c e l l a r phase i n e q u i l i b r i u m w i t h an excess b r i n e phase. Surfactant is f o u n d almost exclusively i n the o i l - r i c h phase, a n d the c o n c e n t r a t i o n o f surfactant i n that phase can greatly e x c e e d the c o n c e n t r a t i o n o f surfactant i n the i n j e c t e d c h e m i c a l slug. I n the type II(+) e n v i r o n m e n t , the m i c e l l a r phase r e m a i n s m i s c i b l e w i t h the o i l b u t is i m m i s c i b l e w i t h the b r i n e . O i l c o n t i n u e s to b e r e c o v e r e d b y a m i s c i b l e l i k e process. T h e opposite occurs i f the phase e n v i r o n m e n t is type I I ( - ) . T h e b r i n e - r i c h m i c e l l a r phase is i m m i s c i b l e w i t h the o i l phase, a n d o i l r e c o v e r y is b y l o w I F T i m m i s c i b l e d i s p l a c e m e n t . N e l s o n a n d P o p e (24) d e s c r i b e d these phase relationships a n d d e m o n strated that phases o b s e r v e d i n l a b o r a t o r y test tube e x p e r i m e n t s also f o r m and are t r a n s p o r t e d i n a p o r o u s m e d i u m subjected to a c h e m i c a l flood at r e s e r v o i r flow rates. C h e m i c a l floods c o n t i n u o u s l y m a i n t a i n e d i n a type II(+) phase e n v i r o n m e n t r e c o v e r e d substantially m o r e r e s i d u a l o i l t h a n those m a i n t a i n e d i n a type I I ( - ) e n v i r o n m e n t . N e l s o n a n d P o p e c o n c l u d e d that c h e m i c a l flood design s h o u l d b e s u c h as to m a i n t a i n as m u c h surfactant as possible i n the type I I I phase e n v i r o n m e n t . T h i s c o n d i t i o n c a n be a c c o m p l i s h e d b y d e s i g n i n g the m i c e l l a r fluid s u c h that the i n i t i a l phase e n v i r o n m e n t o f the i m m i s c i b l e d i s p l a c e m e n t is type II(+). A negative salinity g r a d i e n t is i m p o s e d , a n d it moves the phase e n v i r o n m e n t to type I I I a n d , eventually, to I I ( - ) . In p r a c t i c e , because o f e c o n o m i c constraints, a finite c h e m i c a l s l u g must b e i n j e c t e d a n d effectively d i s p l a c e d t h r o u g h the reservoir. T h i s step is a c c o m p l i s h e d b y u s i n g a p o l y m e r d r i v e fluid. T h e p o l y m e r d r i v e fluid, b e i n g aqueous, is i m m i s c i b l e w i t h type II(+) m i c e l l a r fluids b u t m i s c i b l e w i t h type I I ( - ) . T h u s , i f a type II(+) system w e r e to be d i s p l a c e d b y a p o l y m e r d r i v e fluid, phase t r a p p i n g o f the m i c e l l a r s o l u t i o n c o u l d o c c u r . T h e use o f salinity gradient to p r o d u c e a type I I ( - ) e n v i r o n m e n t at the t r a i l i n g edge o f the c h e m i c a l s l u g allows m i s c i b l e d i s p l a c e m e n t b y the p o l y m e r d r i v e fluid. F i g u r e 7 shows the g e n e r a l sequence o f a m i c e l l a r - p o l y m e r flood. A n i n i t i a l p r e f l u s h is sometimes u s e d to l o w e r salinity a n d d i v a l e n t i o n c o n c e n t r a t i o n i n the reservoir. T h i s p r e f l u s h is f o l l o w e d b y the surfactant slug, c o n t a i n i n g a s u r f a c t a n t - p o l y m e r m i x t u r e d e s i g n e d to p r o d u c e a m i c r o e m u l s i o n w i t h the c r u d e o i l . A p o l y m e r d r i v e f o l l o w s , a n d it prevents " f i n g e r i n g " o f the b r i n e i n t o the surfactant slug. T h e p o l y m e r d r i v e is o f t e n i n j e c t e d w i t h a c o n c e n t r a t i o n gradient; p o l y m e r c o n c e n t r a t i o n decreases as the i n j e c t i o n progresses.

Oil Bank Formation. I f a surfactant o r s u r f a c t a n t - f o r m i n g mate r i a l is i n j e c t e d i n t o a r e s e r v o i r a n d m o b i l i z e s r e s i d u a l o i l , t h e n o i l r e c o v e r y is m o r e efficient i f the m o b i l i z e d o i l droplets c a n coalesce to f o r m an o i l bank.


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flow

Brine

Polymer Drive

Surfactant Preflush Slug


Polymer Drive: 0 «100% pore volume 200 - 2500ppm polymer biocides Surfactant Slug: 5 - 20% pore volume 200 · 5000ppm polymer 1 - 20% surfactant 0 - 5% cosurfactant biocides Preflush:

0 -100% pore volume

Figure 7. Typical microemulsion injection scheme.

T h e o i l b a n k that f o r m s w i l l exist at an o i l saturation that is greater t h a n the r e s i d u a l o i l saturation. A t the f r o n t o f the bank, r e s i d u a l o i l is taken u p , w h i l e at the back, the c a p i l l a r y n u m b e r must r e m a i n h i g h to m i n i m i z e o i l e n t r a p m e n t . I n this way, the o i l b a n k grows larger a n d forms slightly a h e a d o f the injected chemicals. T h e f o r m a t i o n a n d d i s p l a c e m e n t o f the o i l b a n k d e p e n d s u p o n t h e nature o f the phases f o r m e d i n t h e p o r o u s m e d i u m a n d t h e i r relative p e r m e a b i l i t i e s , w h i c h m a y also change as a result o f changes i n w e t t a b i l i t y . D e t a i l e d d i s c u s s i o n o f these factors is b e y o n d the scope o f this c h a p t e r ; C h a p t e r 6 a n d references 3 7 a n d 3 8 address this t o p i c .

Nonideal Behavior. T h e d i s c u s s i o n o f phase b e h a v i o r u p t o this p o i n t represents the i d e a l case. A n u m b e r o f factors cause d e v i a t i o n f r o m ideality. T h e phases present m a y i n c l u d e l i q u i d crystals, gels, o r s o l i d p r e c i p itates i n a d d i t i o n t o the o i l , b r i n e , a n d m i c r o e m u l s i o n phases (39, 40). T h e h i g h viscosities o f these phases are d e t r i m e n t a l t o o i l recovery. T o c o n t r o l the f o r m a t i o n o f these phases, the p r a c t i c e has b e e n to a d d l o w - m o l e c u l a r w e i g h t alcohols to the m i c e l l a r s o l u t i o n ; these alcohols act as cosolvents o r i n some cases as cosurfactants. T h e a l c o h o l cosolvents o r cosurfactants m a y p a r t i t i o n b e t w e e n aqueous a n d o i l phases i n d i f f e r e n t p r o p o r t i o n s t h a n the p r i m a r y surfactant, a n d t h e r e f o r e , g r o u p i n g these c o m p o n e n t s i n the surfactant p s e u d o c o m p o n e n t is i n a p p r o p r i a t e . C h r o m a t o g r a p h i c separation o f the c o m p o n e n t s m a y o c c u r d u r i n g flow i n the reservoir, a n d u n w a n t e d phases m a y f o r m . I o n exchange b e t w e e n i n j e c t e d b r i n e a n d reservoir r o c k m a y result i n l o c a l l y h i g h concentrations o f d i v a l e n t ions that c a n p r e c i p i t a t e a n i o n i c


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surfactants. T o g e t h e r w i t h d i f f u s i o n a n d d i s p e r s i o n p h e n o m e n a i n the p o rous m e d i u m , p r e c i p i t a t e d surfactants c a n result i n l o c a l i m m i s c i b i l i t i e s a n d phase t r a p p i n g .


I n c o m p a t i b i l i t y o f surfactant a n d p o l y m e r that is u s e d i n the m i c e l l a r slug or chase fluid can o c c u r because p o l y m e r m a y not be i n c o r p o r a t e d r e a d i l y i n t o m i c e l l e s . U s e o f alcohols mitigates this d i f f i c u l t y .

Field Application. T h e m i c e l l a r - p o l y m e r process f o r e n h a n c e d o i l recovery has b e e n u s e d i n m a n y field trials. P e t r o l e u m sulfonates are the most c o m m o n l y u s e d surfactant (41, 42). O t h e r surfactants have b e e n u s e d , s u c h as ethoxylated a l c o h o l sulfates (43) a n d n o n i o n i c surfactants m i x e d w i t h p e t r o l e u m sulfonates (44). T h e L o u d o n field i n I l l i n o i s , o p e r a t e d b y E x x o n , is an i n t e r e s t i n g exam p l e o f m i c e l l a r - p o l y m e r flooding design (45, 46). T h e reservoir is a m o d e r ate p e r m e a b i l i t y sandstone w i t h excellent p r o p e r t i e s f o r m i c e l l a r - p o l y m e r flooding, except one: T h e salinity is v e r y h i g h , a p p r o x i m a t e l y 10.5 w t % total d i s s o l v e d solids a n d 4 0 0 0 p p m o f divalent ions. E x x o n has b e e n s t u d y i n g the m i c e l l a r - p o l y m e r process i n this field f o r m o r e than 10 years, a n d to date (mid-1991) has c o m p l e t e d t w o p i l o t projects a n d has t w o others i n progress. T h e s e q u e n c e o f the i n j e c t e d m i c r o e m u l s i o n a n d p o l y m e r d r i v e is o u t l i n e d i n F i g u r e 8. A m i c r o e m u l s i o n o f 0.3 p o r e v o l u m e s , c o n t a i n i n g a relatively l o w surfactant c o n c e n t r a t i o n , 2.3 w t % , was u s e d . T h e surfactant was a sulfate o f a p r o p o x y l a t e d ethoxylated t r i d e c y l a l c o h o l , o f the f o l l o w i n g structure: i-C

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Polymer Drive: 1.0 pore volume biopolymer, 38 cp @ 11 s" salinity 70% of resident brine 1

Microemulsion: 0.3 pore volume 2.3 wt% surfactant 2.65 wt% 250 white oil base biopolymer Flocon 4800,28 cp © 11 s" 500 -1500ppm formaldehyde (biocide) 90 ppm citric acid (iron control) 96% salinity Figure 8. Loudon micellar-polymer

flood.


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w h e r e m a n d η are e i t h e r 4 a n d 2 o r 3 a n d 4, respectively. I n p r a c t i c e , a m i x t u r e o f these t w o surfactants was u s e d , so that surfactant c o m p o s i t i o n c o u l d easily b e v a r i e d i n the field to c o r r e c t for any changes i n i n j e c t i o n b r i n e c o m p o s i t i o n . T h i s type o f surfactant is p a r t i c u l a r l y s u i t e d to h i g h salinities. I f the n u m b e r o f ethoxyl groups is i n c r e a s e d , o p t i m a l salinity increases, b u t i f the n u m b e r o f p r o p o x y l groups increases, the o p t i m a l s a l i n ity decreases. T h e m i x t u r e u s e d h a d o p t i m a l salinity at the h i g h salinity a n d hardness levels associated w i t h this reservoir. T h e synthesis o f this surfactant a n d its use i n m i c e l l a r - p o l y m e r flooding has b e e n p a t e n t e d b y E x x o n (47). I n a d d i t i o n to the surfactant, a w h i t e o i l was a c o m p o n e n t o f the m i c r o e m u l s i o n . T h i s o i l was a d d e d i n the m i n i m u m a m o u n t r e q u i r e d to s o l u b i l i z e e n o u g h xanthan p o l y m e r to p r o d u c e the target viscosity. I n the absence o f the w h i t e o i l , the p o l y m e r c o u l d not be s o l u b i l i z e d . T h e xanthan p o l y m e r itself was r e q u i r e d f o r m o b i l i t y c o n t r o l . T o p r e v e n t biodégradation o f the p o l y m e r , f o r m a l d e h y d e was a d d e d . C i t r i c a c i d was also a c o m p o n e n t o f the m i c r o e m u l s i o n , a d d e d to p r e v e n t the o x i d a t i o n o f ferrous i o n present i n the b r i n e to f e r r i c i o n . T h e p r e s e n c e o f f e r r i c i o n w o u l d l e a d to p r e c i p i t a t i o n o f i r o n c o m p o u n d s as w e l l as c r o s s - l i n k i n g o f the b i o p o l y m e r . A p a r t i c u l a r l y i n t e r e s t i n g part o f the p i l o t i n v o l v e d the t r e a t i n g o f p r o d u c e d e m u l s i o n s . O v e r the life o f the p i l o t , 9 3 % o f the i n j e c t e d surfactant was p r o d u c e d at the p r o d u c t i o n w e l l s , a n d this situation l e d to serious e m u l s i o n p r o b l e m s . H e a t i n g the e m u l s i o n to a specific, b u t u n r e p o r t e d , t e m p e r a t u r e c a u s e d the surfactant to p a r t i t i o n c o m p l e t e l y i n t o the aqueous phase a n d leave the c r u d e o i l w i t h v e r y l o w levels o f surfactant a n d b r i n e . T h e r e s u l t i n g o i l was suitable for p i p e l i n e t r a n s p o r t a t i o n . T h e c r i t i c a l separa t i o n t e m p e r a t u r e h a d to be c o n t r o l l e d to w i t h i n 1 °C. A t h i g h e r t e m p e r a tures, surfactant p a r t i t i o n e d i n t o the o i l , a n d at l o w e r t e m p e r a t u r e s , signifi cant quantities o f o i l r e m a i n e d s o l u b i l i z e d i n the b r i n e . R e c o v e r e d surfactant was e q u i v a l e n t to the i n j e c t e d surfactant i n terms o f phase b e h a v i o r , a n d h a d the p o t e n t i a l for reuse. T h e p i l o t area u s e d for this test was relatively s m a l l , 0.71 acres. H o w ever, the test was a t e c h n i c a l success, r e c o v e r i n g 6 8 % o f the w a t e r - f l o o d r e s i d u a l o i l . T h e p i l o t began i n 1982 a n d e n d e d i n N o v e m b e r 1983. S i n c e that t i m e , E x x o n has i n i t i a t e d t w o other m i c e l l a r - p o l y m e r floods i n the L o u d o n field, one a 40-acre p i l o t a n d the o t h e r an 80-acre p i l o t .

Principles of Alkaline Flooding A l k a l i n e flooding is an o l d c o n c e p t , first p a t e n t e d b y A t k i n s o n (48) i n 1927. H y d r o x i d e i o n i n an alkaline s o l u t i o n reacts w i t h a c i d i c c o m p o n e n t s present i n some c r u d e oils to p r o d u c e p e t r o l e u m soaps, w h i c h are generally s o d i u m salts o f carboxylic acids. T h e s e p e t r o l e u m soaps are capable o f a d s o r b i n g at the o i l - w a t e r interface a n d l o w e r i n g i n t e r f a c i a l t e n s i o n . C r u d e oils suitable


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for alkaline flooding g e n e r a l l y have a total a c i d n u m b e r ( T A N ) o f 0.1 to 2 m g o f K O H p e r g r a m o f o i l . T h e i n j e c t i o n o f alkaline solutions i n t o a reservoir c a n i m p r o v e o i l r e c o v e r y b y several m e c h a n i s m s . E m u l s i f i c a t i o n a n d e n t r a i n m e n t , e m u l s i f i c a t i o n a n d e n t r a p m e n t , a n d w e t t a b i l i t y reversal have b e e n p r o p o s e d (49, 50). C l a s s i c a l alkaline flooding uses o n l y alkaline solutions a n d has several disadvantages. O n e o f t h e most serious is that t h e p e t r o l e u m soaps are v e r y sensitive to increases i n t h e i o n i c strength o f t h e aqueous phase. I n 1975, B u r d y n et a l . (51) b e g a n to address this p r o b l e m w i t h the a d d i t i o n o f s y n thetic surfactants to t h e alkaline flooding process. I n this case, a synthetic a l k y l aryl sulfonate was a d d e d to s o d i u m h y d r o x i d e solutions to p r o v i d e l o w I F T b e h a v i o r over a b r o a d e r range o f salinity t h a n c o u l d b e o b t a i n e d w i t h alkali alone. B u r d y n also p a t e n t e d t h e use o f p o l y m e r w i t h alkali to increase the aqueous-phase viscosity. T h i s i n c r e a s e d viscosity h a d t h e effect o f i m p r o v i n g t h e sweep o f t h e aqueous phase t h r o u g h t h e reservoir. E x c e s s i v e alkali c o n s u m p t i o n also p l a g u e d t h e classical alkaline flooding process. S o d i u m h y d r o x i d e a n d s o d i u m silicate solutions react w i t h r e s e r v o i r r o c k o r p r e c i p i t a t e i n t h e presence o f d i v a l e n t cations. B u f f e r e d alkaline solutions s u c h as s o d i u m carbonate have i n c r e a s e d i n i m p o r t a n c e because o f l o w e r a l k a l i - r o c k interactions. C u r r e n t l y , the a l k a l i - s u r f a c t a n t - p o l y m e r process represents t h e state o f t h e art i n alkaline flooding. M a n y o f t h e basic concepts

of micellar-polymer

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alkaline flooding. H o w e v e r , a l k a l i n e flooding is f u n d a m e n t a l l y d i f f e r e n t b e cause a surfactant is c r e a t e d i n the reservoir f r o m t h e r e a c t i o n o f h y d r o x i d e w i t h a c i d i c c o m p o n e n t s i n c r u d e o i l . T h i s r e a c t i o n means that t h e a m o u n t o f p e t r o l e u m soap w i l l v a r y l o c a l l y as t h e w a t e r - t o - o i l ratio varies. T h e a m o u n t o f p e t r o l e u m soap has a large effect o n phase b e h a v i o r i n c r u d e - o i l - a l k a l i surfactant systems.

Surfactant Mixing Rules. T h e p e t r o l e u m soaps p r o d u c e d i n alkaline flooding have an e x t r e m e l y l o w o p t i m a l salinity. F o r instance, most a c i d i c c r u d e oils w i l l have o p t i m a l phase b e h a v i o r at a s o d i u m h y d r o x i d e c o n c e n t r a t i o n o f a p p r o x i m a t e l y 0.05 w t % i n d i s t i l l e d water. A t that c o n c e n t r a t i o n (about p H 12) essentially a l l o f t h e a c i d i c c o m p o n e n t s i n t h e o i l have r e a c t e d , a n d type I I I phase b e h a v i o r o c c u r s . A n increase i n s o d i u m h y d r o x i d e c o n c e n t r a t i o n increases t h e i o n i c strength a n d is e q u i v a l e n t to an increase i n s a l i n i t y because m o r e p e t r o l e u m soap is not p r o d u c e d . A s s a l i n i t y increases, t h e p e t r o l e u m soaps b e c o m e m u c h less s o l u b l e i n t h e aqueous phase t h a n i n t h e o i l phase, a n d a shift to o v e r - o p t i m u m o r type II(+) b e h a v i o r occurs. T h e w a t e r i n most o i l reservoirs contains significant q u a n t i ties o f d i s s o l v e d solids, r e s u l t i n g i n i n c r e a s e d I F T . I n t e r f a c i a l t e n s i o n is also i n c r e a s e d because h i g h concentrations o f alkali are r e q u i r e d to c o u n t e r t h e effect o f losses d u e to a l k a l i - r o c k interactions. A s o l u t i o n to t h e p r o b l e m has b e e n to a d d a synthetic surfactant to


282



m o d i f y t h e p r o p e r t i e s o f the p e t r o l e u m soap that is p r o d u c e d f r o m r e a c t i o n w i t h h y d r o x i d e (51, 52). T h i s process has b e e n t e r m e d s u r f a c t a n t - e n h a n c e d alkaline flooding. T h e a d d e d synthetic surfactant is c h o s e n to have a v e r y h i g h o p t i m a l salinity, a n d the r e s u l t i n g p e t r o l e u m - s o a p - s y n t h e t i c - s u r f a c t a n t m i x t u r e p r o d u c e s o p t i m a l phase b e h a v i o r at i n t e r m e d i a t e salinities. T h e m i x i n g o f a synthetic surfactant a n d a p e t r o l e u m soap c a n b e ex p l a i n e d i n terms o f surfactant m i x i n g rules p r o p o s e d b y W a d e et a l . i n 1977 (53). T h e s e rules are b a s e d o n p r e v i o u s studies (54) o f the e q u i v a l e n t alkane c a r b o n n u m b e r ( E A C N ) c o n c e p t , w h i c h show that h y d r o c a r b o n b e h a v i o r t o w a r d surfactants is a d d i t i v e a n d w e i g h t e d b y m o l e f r a c t i o n a c c o r d i n g to t h e f o r m u l a : E A C N ^ I E A C N . X , i

(5)

where E A C N is t h e E A C N o f the m i x t u r e , E A C N ; is the E A C N o f c o m p o n e n t i, a n d X is the m o l e f r a c t i o n o f c o m p o n e n t i. a v g

t

Cayias et a l . (55) a n d others (31, 56) f o u n d that this r e l a t i o n s h i p c o u l d be a p p l i e d to c r u d e oils. T h e y f o u n d that c r u d e oils i n g e n e r a l b e h a v e d e q u i v a l e n t l y to n-alkanes i n the range o f pentane to decane, e v e n w i t h greatly d i f f e r i n g o i l types (55). T h u s , the b e h a v i o r o f a c r u d e o i l t o w a r d a g i v e n surfactant c a n b e d e s c r i b e d as t h e s u m o f the b e h a v i o r o f each o f its h y d r o c a r b o n c o m p o n e n t s t o w a r d that surfactant. T h e b e h a v i o r o f m i x e d surfactant systems has b e e n d e s c r i b e d i n s i m i l a r terms (53): (H „)av = X ( ] V i mi

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w h e r e (IV ) is the alkane c a r b o n n u m b e r o f the i n t e r f a c i a l tension m i n i m u m o f surfactant i, ( N ) is the alkane c a r b o n n u m b e r m i n i m u m o f the I F T m i n i m u m o f the surfactant m i x t u r e , a n d X is t h e m o l e f r a c t i o n o f surfactant i. min

i

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M o r e s i m p l y stated, the b e h a v i o r o f a surfactant m i x t u r e t o w a r d a g i v e n o i l c a n b e d e s c r i b e d as t h e s u m o f the b e h a v i o r o f e a c h o f its c o m p o n e n t s t o w a r d that o i l . T h i s hypothesis shows that the n a t u r a l surfactant that is p r o d u c e d i n a l k a l i n e flooding c a n b e m o d i f i e d b y an a d d e d synthetic surfac tant i n a p r e d i c t a b l e w a y .

Phase Behavior. T h e use o f p h a s e - b e h a v i o r diagrams i n surfactante n h a n c e d alkaline flooding is m o r e c o m p l i c a t e d t h a n i n m i c e l l a r - p o l y m e r flooding f o r several reasons. O n e reason is that phase b e h a v i o r is v e r y sensitive to t h e w a t e r - t o - o i l ratio e m p l o y e d . F r o m surfactant m i x i n g rules, v a r y i n g the a m o u n t o f o i l present w i l l vary the a m o u n t o f p e t r o l e u m soap



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present, a n d the nature o f the m i x e d surfactant w i l l change. A n o t h e r c o m p l i c a t i o n is that stable e m u l s i o n s w i t h h i g h values o f i n t e r f a c i a l t e n s i o n are m u c h m o r e l i k e l y to o c c u r w i t h the heavier oils u s e d i n the process, a n d these s t a b i l i z e d emulsions c a n l e a d to i m p r o p e r c o n s t r u c t i o n o f phase-be h a v i o r diagrams. T h e t h i r d p r o b l e m is that total surfactant c o n c e n t r a t i o n is m u c h l o w e r t h a n is seen i n m i c e l l a r - p o l y m e r flooding. I n some cases, I F T m a y b e v e r y l o w , a n d phase b e h a v i o r can b e i n the type I I I e n v i r o n m e n t , b u t a m i d d l e phase m a y not be r e a d i l y apparent because o f the l o w surfactant concentration. F i g u r e s 9 a n d 10 show p h a s e - b e h a v i o r diagrams f o r D a v i d L l o y d m i n s t e r c r u d e o i l a n d the surfactant N e o d o l 25-3S i n the p r e s e n c e o f 1 w t % s o d i u m carbonate. P h a s e - b e h a v i o r measurements w e r e c a r r i e d out a c c o r d i n g to the m e t h o d o f N e l s o n et a l . (52). T h e D a v i d L l o y d m i n s t e r o i l field is near the A l b e r t a - S a s k a t c h e w a n b o r d e r d i r e c t l y east o f E d m o n t o n . T h e o i l has a density o f 0.922 g/mL a n d a viscosity o f 144 M P a - s at 23 °C. T h e r e g i o n o f o p t i m a l phase b e h a v i o r is s h o w n at a surfactant c o n c e n t r a t i o n o f 0.1 w t % i n F i g u r e 9. T h e r e g i o n o f o p t i m a l phase b e h a v i o r is s h a d e d . A b o v e this r e g i o n , type II{+) b e h a v i o r occurs, a n d type I I ( - ) b e h a v i o r occurs b e l o w the r e g i o n o f o p t i m a l phase b e h a v i o r . V o l u m e p e r c e n t o i l refers to the a m o u n t o f o i l present i n the p h a s e - b e h a v i o r tube u s e d . F o r a g i v e n o i l - t o - w a t e r ratio, a t r a n s i t i o n f r o m type I I ( - ) to type I I I to type II(+) occurs as salinity increases. As the a m o u n t o f o i l increases relative to the a m o u n t o f aqueous phase, the same t r e n d i n phase b e h a v i o r is seen. F i g u r e 10 shows the same system, b u t w i t h a l o w e r synthetic surfactant

Figure 9. Activity map with David Lloydminster crude oil, 0.1 wt% Neodol 253S, and 1 wt% sodium carbonate.



284


Figure 10. Activity map with David Lloydminster crude oil, 0.02 wt% Neodol 25-3S, and 1 wt% sodium carbonate. c o n c e n t r a t i o n (0.02 w t % ) . T h e type I I I p h a s e - b e h a v i o r r e g i o n is s h i f t e d to l o w e r salinities a n d l o w e r o i l - t o - w a t e r ratios. T h i s shift is a d i r e c t result o f changes i n the p e t r o l e u m - s o a p - s y n t h e t i c - s u r f a c t a n t ratio as w a t e r - t o - o i l r a tio varies. N e l s o n et a l . (52) stated that these p h a s e - b e h a v i o r diagrams c a n be u s e d i n an e q u i v a l e n t fashion to the salinity r e q u i r e m e n t diagrams u s e d for m i c e l l a r - p o l y m e r flooding (32). H e c l a i m e d that a s u r f a c t a n t - e n h a n c e d alkaline flood s h o u l d be d e s i g n e d so that the flood begins at the o p t i m u m o v e r - o p t i m u m phase b o u n d a r y . T h e r e s i d u a l o i l saturation is u s e d to deter m i n e the w a t e r - t o - o i l ratio i n the d i a g r a m . T h i s d e t e r m i n a t i o n assumes that e q u i l i b r a t i o n is r a p i d a n d does not address the p o s s i b i l i t y o f p e t r o l e u m soaps b e i n g extracted a n d c o n c e n t r a t e d i n the flood front. A salinity gradient is a p p l i e d w h e n the alkaline agent is r e m o v e d f r o m the d r i v e fluid. N e l s o n ' s results have b e e n v e r y p r o m i s i n g i n p u b l i s h e d laboratory c o r e - f l o o d e x p e r i ments.

Dynamic Interfacial Tension. C r u d e - o i l - a l k a l i systems are u n usual i n that t h e y exhibit d y n a m i c i n t e r f a c i a l t e n s i o n ( F i g u r e 11). A s o l u t i o n o f 0.05 w t % s o d i u m h y d r o x i d e i n contact w i t h D a v i d L l o y d m i n s t e r c r u d e o i l i n i t i a l l y p r o d u c e s u l t r a l o w values o f I F T . A m i n i m u m value is r e a c h e d , after w h i c h I F T increases w i t h t i m e b y nearly 3 orders o f m a g n i t u d e , m e a s u r e d i n the s p i n n i n g d r o p tensiometer. T a y l o r et al. (57) s h o w e d that d y n a m i c i n t e r facial t e n s i o n c a n also o c c u r i n c r u d e - o i l - a l k a l i - s u r f a c t a n t systems. F i g u r e 11 shows i n t e r f a c i a l t e n s i o n versus t i m e f o r a s o l u t i o n c o n t a i n i n g 1 w t % s o d i u m carbonate, a n d the same s o l u t i o n c o n t a i n i n g 0.02 w t % o f N e o d o l 25-



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Time (minutes) Figure 11. Interfacial tension versus time, David Lloydminster crude oil. Key: — 0.05 wt% NaOH; . . 0.02 wt% Neodol 25-3S and 1 wt% Na C0 ; and , J wt% Na C0 . 2

2

3

3

3S, an ethoxylated a l c o h o l sulfate. T h e a d d i t i o n o f t h e synthetic surfactant greatly reduces t h e m i n i m u m I F T value o b t a i n e d . D y n a m i c I F T arises f r o m t h e r e a c t i o n o f a c i d i c c o m p o n e n t s i n t h e c r u d e o i l to f o r m p e t r o l e u m soaps. R e a c t i o n o f a c i d i c surface-active materials i n the c r u d e o i l w i t h s o d i u m h y d r o x i d e i n t h e aqueous phase is a s s u m e d to o c c u r r a p i d l y at the interface, b u t d e s o r p t i o n o f these species is taken to b e slower. T h i s slower d e s o r p t i o n leads to a m a x i m u m i n t h e c o n c e n t r a t i o n o f surface-active species at the interface at some p o i n t i n t i m e a n d h e n c e a n i n t e r f a c i a l t e n s i o n m i n i m u m . S u b s e q u e n t l y , I F T increases as e q u i l i b r i u m is a p p r o a c h e d (58). I n t h e s p i n n i n g d r o p apparatus, t h e w a t e r - t o - o i l ratio is a p p r o x i m a t e l y 200 to 1. I n a reservoir, the w a t e r - t o - o i l ratio w o u l d b e m u c h l o w e r . C h a n g e s i n this w a t e r - t o - o i l ratio are e x p e c t e d to affect t h e relative rates o f d e s o r p t i o n o f surfactants f r o m t h e o i l - w a t e r i n t e r f a c e . T h e significance o f t h e d y n a m i c I F T m i n i m u m f o r r e s e r v o i r situations has b e e n d i s c u s s e d b y R u b i n and R a d k e (58) a n d d e Z a b a l a a n d R a d k e (59). T h e y suggested that the I F T m i n i m u m f o r a c i d i c c r u d e oils is i n d i c a t i v e o f the lowest achievable reservoir e q u i l i b r i u m v a l u e . T a y l o r et a l . (57) s h o w e d a c o r r e l a t i o n b e t w e e n m i n i m u m I F T a n d c o r e - f l o o d r e c o v e r y e f f i c i e n c y i n s u r f a c t a n t - e n h a n c e d alkaline flooding. F i g u r e 12 shows a m o d i f i c a t i o n o f t h e c a p i l l a r y n u m b e r c u r v e i n t r o d u c e d i n F i g u r e 3 . C a p i l l a r y n u m b e r s f r o m c o r e - f l o o d data are p l o t t e d by u s i n g b o t h I F T m i n i m a a n d e q u i l i b r i u m values. C o r e floods w e r e c a r r i e d out i n l i n e a r B e r e a sandstone cores. A g r e e m e n t b e t w e e n p u b l i s h e d c a p i l l a r y n u m b e r correlations is v e r y p o o r w h e n u s i n g e q u i l i b r i u m I F T values, as


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Figure 12. Capillary number correlation with dynamic interfacial tension. Key:. . ., Gupta and Trushenski (34); - - -, Taber (5); — , Chatzis and Morrow (6); •, minimum IFT; and O equilibrium IFT. f

m e a s u r e d w i t h the s p i n n i n g d r o p i n s t r u m e n t , b u t it is v e r y g o o d w h e n u s i n g m i n i m u m I F T values.

Field Application. F i e l d trials o f classical alkaline flooding have b e e n d i s a p p o i n t i n g . M a y e r et a l . (60) i n d i c a t e d that o n l y 2 o f 12 projects h a d significant i n c r e m e n t a l o i l recovery: N o r t h W a r d E s t e s a n d W h i t t i e r w i t h 6 8 a n d 5 - 7 % p o r e v o l u m e , respectively. E s t i m a t e d r e c o v e r y f r o m the W i l m i n g t o n field was 1 4 % w i t h a classical alkaline flooding m e t h o d (61). H o w ever, post-project e v a l u a t i o n o f that field i n d i c a t e d n o i m p r o v e m e n t o v e r w a t e r - f l o o d i n g (62). A n u m b e r o f l a b o r a t o r y studies o f the a p p l i c a t i o n o f the a l k a l i - s u r f a c t a n t - p o l y m e r flooding to various reservoir systems have b e e n r e p o r t e d (6367), b u t field a p p l i c a t i o n o f this t e c h n o l o g y has b e e n l i m i t e d . S e v e r a l field pilots are i n progress or have b e e n c o m p l e t e d , b u t o n l y one has b e e n e v a l u ated to date i n the t e c h n i c a l l i t e r a t u r e (68). T h i s project is i n the W e s t K i e h l field i n W y o m i n g o p e r a t e d b y T e r r a R e s o u r c e s I n c . T h e W e s t K i e h l field is a m e d i u m p e r m e a b i l i t y ( 3 5 0 - m D ; m D is m i l l i d a r c i e s ) sandstone. T h e reservoir b r i n e contains 45,500 p p m o f total d i s s o l v e d solids, w i t h about 450 p p m o f d i v a l e n t ions. T h e 24° A P I c r u d e o i l has viscosity o f 19 mPa-s at a reservoir t e m p e r a t u r e o f 4 9 °C. ( A P I gravity is d e f i n e d i n the Glossary.) T h e c h e m i c a l s l u g u s e d i n the project was p r e p a r e d i n a fresh, r e l a t i v e l y soft b r i n e (800 p p m o f total d i s s o l v e d solids, 18 p p m hardness). O n the basis o f i n t e r f a c i a l t e n s i o n measurements (phase-behavior tests w e r e not re p o r t e d ) , a s o l u t i o n o f 0.8 w t % N a C 0 a n d 0.1 w t % Petrostep B 1 0 0 , a 2

3


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p e t r o l e u m sulfonate, was selected. T o this was a d d e d 0.1 w t % o f a p o l y a c r y l a m i d e p o l y m e r to p r o v i d e t h e necessary m o b i l i t y c o n t r o l . T h e project d e s i g n c a l l e d f o r i n j e c t i o n o f 0.25 p o r e v o l u m e o f t h e a l k a l i - s u r f a c t a n t - p o l y m e r s o l u t i o n f o l l o w e d b y a s i m i l a r v o l u m e o f p o l y m e r s o l u t i o n a c t i n g as a m o b i l i t y control buffer.


E a r l y results i n t h e W e s t K i e h l field are v e r y e n c o u r a g i n g . It is a n t i c i p a t e d that the use o f the A S P process w i l l increase o i l r e c o v e r y b y an a d d i t i o n a l 1 5 % over w a t e r - f l o o d .

Other Applications Emulsion Injection for Recovery of Heavy Oil. O i l - i n - w a t e r e m u l s i o n s m a y b e u s e f u l as sweep i m p r o v e m e n t agents i n h e a v y - o i l reser voirs. T o i m p r o v e the m o b i l i t y ratio o c c u r r i n g w i t h h i g h - v i s c o s i t y oils, M c A u l i f f e (69) a n d S c h m i d t et a l . (70) p r o p o s e d t h e use o f stable o i l - i n w a t e r e m u l s i o n s . T h e s e authors c o n d u c t e d laboratory experiments w i t h e m u l s i o n s p r e p a r e d b y r e a c t i o n o f s o d i u m h y d r o x i d e w i t h a synthetic a c i d i c o i l . T h e t h e o r e t i c a l b a c k g r o u n d f o r e m u l s i o n b l o c k i n g has b e e n discussed i n C h a p t e r 6, a n d i t f o r m s the basis f o r o n e o f several m e c h a n i s m s o f caustic flooding (71). T h e s e e m u l s i o n s m a y f o r m spontaneously d u r i n g o i l r e c o v e r y processes (72), b u t c a n just as easily b e p r e p a r e d a n d i n j e c t e d as e n h a n c e d o i l r e c o v e r y fluids. F i o r i a n d F a r o u q A l i (73) p r o p o s e d t h e e m u l s i o n flooding o f h e a v y - o i l reservoirs as a secondary r e c o v e r y t e c h n i q u e . T h i s process is o f interest f o r Saskatchewan h e a v y - o i l reservoirs, w h e r e p r i m a r y r e c o v e r y is t y p i c a l l y 2 8%. W a t e r - f l o o d i n g i n these fields p r o d u c e s o n l y an a d d i t i o n a l 2 - 5 % o f t h e o r i g i n a l o i l i n p l a c e because o f t h e h i g h l y viscous nature o f the o i l . I n laboratory e x p e r i m e n t s , a w a t e r - i n - o i l e m u l s i o n o f the p r o d u c e d o i l is c r e ated b y u s i n g a s o d i u m h y d r o x i d e s o l u t i o n . T h e viscous e m u l s i o n f o r m e d is i n j e c t e d i n t o t h e reservoir. Its h i g h viscosity p r o v i d e s a m o r e favorable m o b i l i t y ratio a n d results i n i m p r o v e d sweep o f the reservoir. I m p o r t a n t parameters i n c l u d e e m u l s i o n stability a n d c o n t r o l o f e m u l s i o n viscosity. D e c k e r a n d F l o c k (74) investigated t h e a p p l i c a t i o n o f e m u l s i o n injec t i o n f o r s t e a m - f l o o d i n g processes. I n laboratory m o d e l s , e m u l s i o n s c o n t a i n i n g 5 v o l % c r u d e o i l w e r e effective i n b l o c k i n g channels c r e a t e d b y steam i n j e c t i o n d u r i n g subsequent steam-injection cycles. O i l d r o p l e t s i n t h e e m u l sion w e r e p r e d o m i n a n t l y i n t h e 1 - 2 - μ π ι range, b u t d r o p l e t s as large as 10 μπι w e r e o b s e r v e d .

Matrix Acidization. E m u l s i o n t e c h n o l o g y has b e e n a p p l i e d to the a c i d treatment o f reservoir m a t e r i a l i n t h e r e g i o n near t h e w e l l b o r e . T h e p o r e structure o f the r e g i o n o f the reservoir near the w e l l b o r e m a y some times b e c o m e p l u g g e d e i t h e r b y particulates f r o m d r i l l i n g fluids o r b y p r e -


288



c i p i t a t i o n deposits caused b y pressure o r t e m p e r a t u r e changes d u r i n g p r o d u c t i o n . T h e s e p o r e - p l u g g i n g materials r e d u c e p e r m e a b i l i t y i n the r e g i o n near the w e l l b o r e a n d h e n c e r e d u c e w e l l p r o d u c t i v i t y . A c i d s t i m u l a t i o n is u s e d r o u t i n e l y to increase w e l l p r o d u c t i v i t y b y r e m o v i n g these u n w a n t e d deposits. W e l l s i n f o r m a t i o n s w i t h n a t u r a l l y o c c u r r i n g l o w p e r m e a b i l i t y c a n also be s t i m u l a t e d b y u s i n g the same process, b u t a p p l i e d to the o r i g i n a l r o c k matrix. T h i s process is r e f e r r e d to as matrix s t i m u l a t i o n . T h i s matrix a c i d i z a t i o n process consists o f i n j e c t i n g h y d r o c h l o r i c a c i d (for limestones) or a h y d r o c h l o r i c a c i d - h y d r o f l u o r i c a c i d m i x t u r e (for sand stones) i n t o the f o r m a t i o n p o r e space. T h e a c i d reacts w i t h a n d dissolves p o r t i o n s o f the o r i g i n a l r o c k matrix a n d thus increases p e r m e a b i l i t y . T h e d e p t h that the a c i d penetrates i n t o the f o r m a t i o n is one o f the factors that d e t e r m i n e s the effectiveness o f the treatment. F o r carbonate reservoirs or carbonate c e m e n t s i n p a r t i c u l a r , a c i d c o n s u m p t i o n occurs v e r y r a p i d l y at elevated f o r m a t i o n temperatures a c c o r d i n g to the e q u a t i o n : 2H + M C 0 +

3

^

M

2

+

+ C0

2

+ H 0 2

(7)

w h e r e M is c a l c i u m o r m a g n e s i u m . T h e rate o f d i s s o l u t i o n is l i m i t e d b y mass transfer, s u c h that it d e p e n d s o n the rate at w h i c h a c i d diffuses to the surface o f the m a t e r i a l b e i n g d i s s o l v e d . T h e rate o f mass transfer a c c o m p a n y i n g flow t h r o u g h the r o c k matrix is h i g h , so a c i d is c o n s u m e d v e r y q u i c k l y . T h u s d e e p p e n e t r a t i o n o f the a c i d is d i f f i c u l t to achieve, a n d significant p r o d u c t i v i t y i m p r o v e m e n t cannot always be attained. D i s s o l u t i o n o f the r o c k matrix does not o c c u r i n a u n i f o r m r a d i a l m a n ner. B e c a u s e o f p e r m e a b i l i t y contrasts i n the reservoir, d o m i n a n t flow c h a n nels c a l l e d w o r m holes c a n d e v e l o p a n d extend i n t o the f o r m a t i o n i n a r a n d o m f a s h i o n . T h e p r o d u c t i v i t y increase o f any w e l l is t h e n g o v e r n e d b y w o r m - h o l e d i r e c t i o n a n d distance. T h e l o n g e r the w o r m h o l e , the b e t t e r w i l l b e the result. L e a k - o f f o r loss o f a c i d t h r o u g h the walls o f w o r m holes o f t e n results i n w o r m holes b e i n g too short to p r o v i d e significant p r o d u c t i v i t y increase. T h e r e f o r e , effective s t i m u l a t i o n o f t e n r e q u i r e s r e t a r d a t i o n o f the m i n e r a l d i s s o l u t i o n rate. T h e use o f m i c r o e m u l s i o n s is one m e t h o d to a c c o m p l i s h this r e t a r d a t i o n . T h e h y d r o c h l o r i c a c i d is i n j e c t e d as an w a t e r - i n - o i l m i c r o e m u l s i o n . T h e d i f f u s i o n rate o f the d i s p e r s e d aqueous a c i d to the r o c k surface is slower t h a n m o l e c u l a r d i f f u s i o n o f a c i d f r o m a totally aqueous system. T h u s the rate o f l i m e s t o n e d i s s o l u t i o n is r e t a r d e d w i t h the m i c r o e m u l s i o n system. T h e degree o f r e t a r d a t i o n is d e p e n d e n t o n the m i c e l l a r structure o f the system. H o e f n e r a n d F o g l e r (75) d e s c r i b e d one s u c h m i c r o e m u l s i o n system c o n t a i n i n g c e t y l p y r i d i n i u m c h l o r i d e a n d b u t a n o l as the surfactantcosurfactant i n a 35:65 w e i g h t ratio. I n this system, d o d e c a n e was u s e d as the


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o i l phase, a n d h y d r o c h l o r i c a c i d as t h e aqueous phase. T o m a i n t a i n h y d r o c h l o r i c a c i d as t h e d i s p e r s e d phase, i t was necessary to use a c o m p o s i t i o n that sits near t h e o i l apex o f t h e p s e u d o t e r n a r y d i a g r a m . T h e d i f f u s i v i t y o f the a c i d f r o m t h e m i c r o e m u l s i o n system was 2 orders o f m a g n i t u d e less t h a n f r o m the aqueous a c i d alone.


Summary C r u d e o i l b e c o m e s t r a p p e d i n p o r o u s m e d i a as a result o f c a p i l l a r y forces. T h e r e d u c t i o n o f these forces is r e q u i r e d f o r t h e r e c o v e r y o f r e s i d u a l o i l , a n d this is t h e basis o f e n h a n c e d o i l recovery. I n practice c a p i l l a r y forces are reduced primarily by lowering interfacial tension between o i l and water phases, a l t h o u g h i n c r e a s i n g t h e viscosity o f t h e w a t e r is also i m p o r t a n t . L o w e r i n g i n t e r f a c i a l t e n s i o n leads to t h e f o r m a t i o n o f e m u l s i o n s a n d m i c r o e m u l s i o n s , w h i c h are o f great i m p o r t a n c e i n e n h a n c e d o i l r e c o v e r y techniques. M i c e l l a r - p o l y m e r flooding a n d a l k a l i - s u r f a c t a n t - p o l y m e r flooding b o t h rely o n t h e i n j e c t i o n i n t o a c r u d e - o i l reservoir o f surfactants o r surfactantf o r m i n g materials. E m u l s i o n s may b e i n j e c t e d i n t o t h e reservoir, o r they m a y be f o r m e d i n t h e reservoir, b u t t h e i r p r o p e r t i e s w i l l change as they t r a v e l t h r o u g h t h e reservoir to eventually flow f r o m a p r o d u c i n g w e l l after weeks o r months. M i c e l l a r - p o l y m e r flooding is a t e c h n i c a l l y w e l l - d e v e l o p e d process. Phase c o m p o s i t i o n a l aspects o f m i c r o e m u l s i o n d e s i g n are relatively w e l l u n d e r s t o o d , a n d several t e c h n i c a l l y successful field trials have b e e n c a r r i e d out. M i c e l l a r - p o l y m e r floods c a n b e d e s i g n e d a n d c a r r i e d o u t w i t h a g o o d c h a n c e o f success. H o w e v e r , t h e process is too expensive. T h i s h i g h cost is d u e p r i m a r i l y to t h e h i g h concentrations o f synthetic surfactants r e q u i r e d . T h e p r o b l e m is f u r t h e r c o m p o u n d e d because these synthetic surfactants are m a d e f r o m p e t r o c h e m i c a l s , a fact that ties t h e i r p r i c e to t h e p r i c e o f c r u d e oil. A l k a l i - s u r f a c t a n t - p o l y m e r ( A S P ) flooding shows p r o m i s e to b e c o m e e c o n o m i c a l l y m o r e attractive t h a n m i c e l l a r - p o l y m e r flooding. T h e process is i n h e r e n t l y less expensive because t h e c o n c e n t r a t i o n o f synthetic surfactant is significantly l o w e r . H o w e v e r , t h e A S P process is m o r e c o m p l e x a n d t e c h n i c a l l y less d e v e l o p e d than m i c e l l a r - p o l y m e r flooding. I n a d d i t i o n , t h e dis a p p o i n t i n g history o f field trials o f classical alkaline flooding has left m a n y researchers s k e p t i c a l o f the process i n g e n e r a l . B u t w i t h t h e use o f h i g h o p t i m a l salinity surfactants to l o w e r i n t e r f a c i a l t e n s i o n at realistic reservoir salinities, t h e use o f b u f f e r e d alkali to r e d u c e a l k a l i - r o c k interactions, a n d the a d d i t i o n o f p o l y m e r s to the system to increase d i s p l a c i n g phase viscosity, m a n y o f t h e p r o b l e m s associated w i t h classical alkaline flooding have b e e n addressed. A r e a s that r e q u i r e f u r t h e r investigation i n c l u d e t h e effect o f


290


d y n a m i c i n t e r f a c i a l t e n s i o n i n t h e reservoir, c o n c i l i a t i o n o f i n t e r f a c i a l t e n sion a n d phase-behavior measurements, ASP

a n d computer simulation o f the

process.


List of Symbols and Abbreviations ASP

alkali-surfactant-polymer

ΕAC Ν

e q u i v a l e n t alkane c a r b o n n u m b e r

EACN

a v g

Ε A C Ν o f a mixture o f components

EACN

4

E A C N of component i

IFT

interfacial tension capillary number

N

c

(N

m i n

)

a v g

alkane c a r b o n n u m b e r o f t h e I F T m i n i m u m o f a surfactant m i x ture

(N )i

alkane c a r b o n n u m b e r o f t h e I F T m i n i m u m o f surfactant i

%OOIP

percent o f original o i l i n place

min

P

c

capillary pressure

P

Q

p r e s s u r e i n o i l phase

P

w

p r e s s u r e i n w a t e r phase

r

radius o f c u r v a t u r e o f the i n t e r f a c e

R

half of drop width

S

solubilization parameter

TAN

total a c i d n u m b e r o f a c r u d e o i l ( m g K O H p e r g r a m o f o i l )

ν

D a r c y flow v e l o c i t y

V

Q

o i l v o l u m e i n m i c r o e m u l s i o n phase

V

s

surfactant v o l u m e i n m i c r o e m u l s i o n phase

V

w

w a t e r v o l u m e i n m i c r o e m u l s i o n phase

Xi

mole fraction o f component i

Greek θ

contact angle

μ

viscosity

ρ

density

σ

interfacial tension

σ

Γ η ο

i n t e r f a c i a l t e n s i o n b e t w e e n t h e m i c r o e m u l s i o n a n d o i l phases

a

m w

i n t e r f a c i a l t e n s i o n b e t w e e n t h e m i c r o e m u l s i o n a n d w a t e r phases

ω

angular v e l o c i t y

References 1. Moore, T . F . ; Slobod, R. L . Prod. Mon. 1956, 20, 20-30. 2. Chatzis, I.; Dullien, F . A . L. J. Colloid Interface Sci. 1983, 91(1), 199-222.



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36. 37. 38. 39.


40. 41.

42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56.

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E M U L S I O N S I N T H E P E T R O L E U M INDUSTRY

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R E C E I V E D for review December 18, 1990. A C C E P T E D revised manuscript M a y 6, 1991.


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