Application of Xanthan Gum for Enhanced Oil Recovery

19 Application of Xanthan G u m for Enhanced O i l Recovery

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Ε. I. SANDVIK and J. M. MAERKER Exxon Production Research Co., P.O. Box 2189, Houston, TX 77001

High molecular weight water soluble polymers find applica tion in two different enhanced oil recovery processes. At present, the principal use is for an improved form of waterflooding in which polymers are used to increase the efficiency with which water can contact and displace reservoir oil. However, it is anticipated that polymer requirements for processes of this type will be overshadowed by the quantity needed to provide mobility control for future micellar-polymer projects. The latter processes have potential for producing oil that is unre coverable by polymer augmented waterflooding. In both applica tions--polymer waterflooding and micellar-polymer flooding--the function of polymer is to reduce the mobility of injected water. Mobility is defined as the flow capacity of a rock-fluid system, or the volumetric flow rate per unit area achieved with a given pressure gradient (Figure 1). It is usually expressed as effective rock permeability divided by fluid viscosity, and the common petroleum reservoir engineering units are darcies (or millidarcies) per centipoise. As will be discussed later, poly mer can reduce mobility by decreasing effective rock permeability and by increasing effective fluid viscosity. Effects of mobility and mobility ratio on the efficiency of reservoir displacements may be illustrated with Hele-Shaw models (1) which give a simplified portrayal of areal displacement effi ciency in a reservoir element. These models commonly consist of two square glass plates that are separated and sealed at the edges by a thin spacing gasket and have provisions to inject and withdraw fluid at opposite corners to simulate injection and pro duction wells. Mobilities of displacing and displaced fluids 242

In Extracellular Microbial Polysaccharides; Sandford, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.


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may be v a r i e d b y c h a n g i n g f l u i d v i s c o s i t i e s . The e f f e c t o f m o b i l i t y r a t i o on displacement e f f i c i e n c y i s i l l u s t r a t e d by t h e H e l e - S h a w m o d e l r e s u l t s d e p i c t e d i n F i g u r e 2. The m o d e l s a r e i n i t i a l l y o i l f i l l e d , and water i s i n j e c t e d i n t h e lower l e f t corner. When t h e m o b i l i t y o f d i s p l a c i n g w a t e r ( u n s h a d e d ) e q u a l s the m o b i l i t y o f d i s p l a c e d o i l (shaded), about t h r e e - f o u r t h s o f the o i l i s produced before water a r r i v e s a t t h e p r o d u c t i o n w e l l (Figure 2a). A f t e r water breakthrough, production increases i n water content w i t h continued throughput. At a favorable v i s c o s i t y r a t i o o f 0.03, a b o u t n i n e - t e n t h s o f t h e o i l i s p r o d u c e d before water breakthrough ( F i g u r e 2b). An u n f a v o r a b l e v i s c o s i t y r a t i o o f 30 c a u s e s a n o b v i o u s l y u n s t a b l e d i s p l a c e m e n t t o o c c u r , and o n l y a b o u t o n e - t h i r d o f t h e r e s i d e n t o i l i s p r o d u c e d a t w a t e r breakthrough (Figure 2c). These e x p e r i m e n t s i l l u s t r a t e t h e i m p o r t a n c e o f m a i n t a i n i n g as f a v o r a b l e a m o b i l i t y r a t i o a s p o s s i b l e d u r i n g d i s p l a c e m e n t s . The d i s p l a c e m e n t i l l u s t r a t e d i n F i g u r e 2c c a n be made t o l o o k l i k e t h e o n e i n F i g u r e 2a b y i n c r e a s i n g t h e w a t e r - p h a s e v i s c o s i t y t h i r t y f o l d , and t h i s would s u b s t a n t i a l l y improve o i l r e c o v e r y . S i m i l a r r e s u l t s c a n be e x p e c t e d i n a c t u a l r e s e r v o i r s i t u a t i o n s . C o n s e q u e n t l y , m o b i l i t y c o n t r o l polymers a r e used i n r e c o v e r y p r o c e s s e s t o r e d u c e t h e m o b i l i t y o f i n j e c t e d w a t e r and i n c r e a s e process e f f i c i e n c y . Two b a s i c t y p e s o f p o l y m e r s — x a n t h a n gums a n d p a r t i a l l y hydrolyzed p o l y a c r y l a m i d e s — c o n s t i t u t e the large majority of those c u r r e n t l y used i n enhanced r e c o v e r y . At present, polya c r y l a m i d e s s t r o n g l y dominate polymer w a t e r f l o o d i n g a p p l i c a t i o n s , w h i l e x a n t h a n gums p l a y a v e r y m i n o r r o l e . Substantial increases i n x a n t h a n gum u s e c a n be e x p e c t e d , h o w e v e r , a s m i c e l l a r - p o l y m e r p r o c e s s e s a r e f u r t h e r t e s t e d and then a p p l i e d i n l a r g e r - s c a l e applications. T h i s paper i s devoted p r i m a r i l y t o a comparison o f t h e p e r f o r m a n c e f o r t h e s e two p o l y m e r t y p e s i n l a b o r a t o r y e v a l u a t i o n s and a c t u a l r e s e r v o i r use. H o p e f u l l y , t h i s comparison w i l l serve t o p i n p o i n t s p e c i f i c a s s e t s o r l i a b i l i t i e s and p r o v i d e g u i d e l i n e s f o r needed i m p r o v e m e n t s . Mobility

Reduction

As m e n t i o n e d e a r l i e r , d i l u t e p o l y m e r s o l u t i o n s w o r k i n two ways t o r e d u c e w a t e r m o b i l i t y i n p o r o u s m e d i a : 1) b y i n c r e a s i n g v i s c o s i t y a n d 2) b y d e c r e a s i n g p e r m e a b i l i t y . D i f f e r e n t p o l y m e r s depend o n t h e s e two mechanisms i n v a r y i n g d e g r e e s . However, b o t h mechanisms a r e i n f l u e n c e d b y m o l e c u l a r w e i g h t , m o l e c u l a r w e i g h t d i s t r i b u t i o n , s a l i n i t y , f l o w r a t e and p e r m e a b i l i t y . I n t h e c o n c e n t r a t i o n range u s u a l l y c o n s i d e r e d f o r enhanced o i l r e c o v e r y a p p l i c a t i o n s — 200 t o 1500 ppm — a n d i n w a t e r s a l i n i t i e s n o r m a l l y e n c o u n t e r e d i n r e s e r v o i r s , X a n t h a n gums g e n e r a l l y e x h i b i t h i g h e r v i s c o s i t y and a lower s e n s i t i v i t y o f v i s c o s i t y t o s a l i n i t y changes t h a n p a r t i a l l y h y d r o l y z e d p o l y a c r y l a m i d e s . F i g u r e 3 shows v i s c o s i t y - c o n c e n t r a t i o n b e h a v i o r f o r s e v e r a l


EXTRACELLULAR MICROBIAL POLYSACCHARIDES


PRESSURE GRADIENT,

= EFFECTIVE ROCK PERMEABILITY EFFECTIVE FLUID VISCOSITY Figure 1.

=

FLOW RATE/UNIT AREA PRESSURE GRADIENT

Definition of mobility

A-VISCOSITY RATIO OF 1.

- FAVORABLE VISCOSITY RATIO OF 0.03 (STABLE).

AC-

UNFAVORABLE VISCOSITY RATIO OF 30 (UNSTABLE).

Canadian Journal of Chemical Engineering

Figure 2.

Disphcements

in HeleShaw cosity ratios (1)

model at different vis-



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polymers i n two b r i n e s : 1% NaCl and a s y n t h e t i c r e s e r v o i r b r i n e c o n t a i n i n g calcium and magnesium. Each xanthan sample represents a d i f f e r e n t commercial source, and, although they d i f f e r s i g n i f i c a n t l y from each other i n v i s c o s i t y , each e x h i b i t s l i t t l e s a l t s e n s i t i v i t y . The polyacrylamide, on the other hand, gives s i g n i f i c a n t v i s c o s i t y d i f f e r e n c e s f o r the two b r i n e s used. Many i n v e s t i g a t o r s (2-20) have observed p e r m e a b i l i t y reduct i o n with polyacrylamide s o l u t i o n s by f l u s h i n g a polymerflooded sandstone core o r sand pack with b r i n e and comparing the f l u s h e d , b r i n e m o b i l i t y with that o f b r i n e p r i o r to polymer. The r a t i o of i n i t i a l b r i n e m o b i l i t y t o b r i n e m o b i l i t y a f t e r i n j e c t i o n of a polymer bank has been c a l l e d a r e s i d u a l r e s i s t a n c e f a c t o r , but t h i s f a c t o r i s not always a good measure of p e r m e a b i l i t y reduct i o n during polymer flow (9). Two p o s s i b l e mechanisms can be r e s p o n s i b l e f o r p e r m e a b i l i t y r e d u c t i o n : 1) adsorption of p o l y mer molecules on main-flow-channel w a l l s which reduces c r o s s s e c t i o n a l area a v a i l a b l e f o r flow, and 2) entrapment of polymer molecules i n narrow pore c o n s t r i c t i o n s which p a r t i a l l y s h u t s - o f f a p o r t i o n of the interconnected pore network. Interested readers may gain an a p p r e c i a t i o n f o r each mechanism by comparing the works o f Thomas (19) and Domingues and W i l l h i t e (20). The degree of p e r m e a b i l i t y r e d u c t i o n v a r i e s i n v e r s e l y with o r i g i n a l b r i n e p e r m e a b i l i t y (2-4_, 10, 21). This r e l a t i o n s h i p i s r e a d i l y understandable by r e c o g n i z i n g that an adsorbed polymer molecule of given s i z e w i l l cause a greater percentage r e d u c t i o n of c r o s s s e c t i o n a l area i n a small diameter pore (lower p e r m e a b i l i t y ) than i n a l a r g e r pore (higher p e r m e a b i l i t y ) . Xanthan gum s o l u t i o n s cause very l i t t l e r e d u c t i o n of permea b i l i t y i n porous media (4, 19). As a r e s u l t , m o b i l i t y c o n t r o l design f o r a secondary (polymer waterflood) or t e r t i a r y (micellar-polymer flood) o i l recovery process i s s i m p l i f i e d ( 7 ) , but the very r e a l advantage o f continued i n j e c t i o n a t a reduced m o b i l i t y i s l o s t f o r b r i n e i n j e c t e d behind a xanthan gum polymer bank. F i g u r e 4 compares r e s i s t a n c e f a c t o r s as a f u n c t i o n of throughput i n one-foot Berea sandstone cores f o r a 600-ppm p o l y acrylamide s o l u t i o n and a 750-ppm xanthan gum s o l u t i o n . Under these t e s t c o n d i t i o n s , steady-state m o b i l i t y r e d u c t i o n during polymer flow and the r e s i d u a l r e s i s t a n c e f a c t o r ( p e r m e a b i l i t y reduction) a f t e r b r i n e flow a r e l a r g e r f o r the polyacrylamide, even though i t has l e s s v i s c o s i t y . However, i t must be noted t h a t , because of the p r e v i o u s l y mentioned dependence o f permea b i l i t y r e d u c t i o n by polyacrylamides on i n i t i a l b r i n e permeabili t y , much l e s s m o b i l i t y r e d u c t i o n would be expected i f the polyacrylamide t e s t of F i g u r e 4 had been conducted i n 500-md, r a t h e r than 100-md, sandstone. In the case of xanthan gum, l i t t l e change i n m o b i l i t y r e d u c t i o n would be expected with changes i n i n i t i a l b r i n e p e r m e a b i l i t y .


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Figure 4.

Mobility reductions in Berea sandstone cores


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Retention

Polymer r e t e n t i o n i n porous r o c k d e l a y s polymer bank a r r i v a l a t a p r o d u c i n g w e l l and i n c r e a s e s t h e q u a n t i t y o f p o l y mer r e q u i r e d t o p r o v i d e m o b i l i t y c o n t r o l t h r o u g h o u t a n o i l r e s e r voir. C o n s e q u e n t l y , t h i s l o s s o f p o l y m e r t o t h e f o r m a t i o n must not be e x c e s s i v e . The two mechanisms m e n t i o n e d a b o v e f o r perme a b i l i t y r e d u c t i o n — c h e m i c a l a d s o r p t i o n and p h y s i c a l e n t r a p m e n t — a r e a l s o two ways i n w h i c h p o l y m e r m o l e c u l e s a r e removed f r o m s o l u t i o n when f l o w i n g t h r o u g h p o r o u s m e d i a . A l t h o u g h p o l y m e r r e t e n t i o n and p e r m e a b i l i t y r e d u c t i o n a r e d e f i n i t e l y i n t e r r e l a t e d , no o n e has y e t d e t e r m i n e d t h e s e p a r a t e c o n t r i b u t i o n s f r o m a d s o r p t i o n and p h y s i c a l entrapment f o r a g i v e n polymer-rock system. S e v e r a l w o r k e r s ( 6 , 10, 19_, 20^,) h a v e p u b l i s h e d d a t a i n d i c a t i n g t h a t p h y s i c a l e n t r a p m e n t i s t h e more i m p o r t a n t mechanism f o r p o l y a c r y l a m i d e s i n water-wet sandstone c o r e s o r sand packs. P o l y a c r y l a m i d e s c a n e a s i l y l o s e 300 t o 4 0 0 pounds p e r a c r e - f o o t i n c o n s o l i d a t e d sandstone. As w i t h p e r m e a b i l i t y r e d u c t i o n d i s c u s s e d e a r l i e r , t h i s l o s s i s a l s o a n i n v e r s e f u n c t i o n o f perme a b i l i t y ( 2 1 ) . X a n t h a n gums e x h i b i t l e s s r e t e n t i o n — o n t h e o r d e r o f 1 5 0 t o 300 pounds p e r a c r e - f o o t . F u r t h e r w o r k i s n e c e s s a r y t o a s s e s s r e l a t i v e i m p o r t a n c e o f a d s o r p t i o n and e n t r a p m e n t f o r x a n t h a n gum s o l u t i o n s . I n a c c e s s i b l e P o r e Volume I t has b e e n shown ( 1 9 , 22) t h a t m o l e c u l a r s i z e f o r p o l y m e r s o f i n t e r e s t h e r e c a n e x c e e d t h e d i a m e t e r s o f some o f t h e s m a l l e r pores i n n a t u r a l porous media (16). This i m p l i e s that a p o r t i o n of the i n t e r c o n n e c t e d pore volume i s i n a c c e s s i b l e t o polymer molecules. When p o l y m e r s o l u t i o n s f l o w t h r o u g h p o r o u s m e d i a , t h e r e s u l t i s a n a c c e l e r a t i o n o f polymer through l a r g e r pores r e l a t i v e t o s i m u l t a n e o u s l y i n j e c t e d s o l v e n t ( 2 3 ) , i n a manner r e m i n i s c e n t o f g e l permeation chromatography. T h i s e f f e c t i s i l l u s t r a t e d b y F i g u r e 5, w h i c h shows e f f l u e n t c o n c e n t r a t i o n response t o a p u l s e o f p o l y a c r y l a m i d e and a t r a c e r i n j e c t e d s i m u l t a n e o u s l y i n t o a sandstone c o r e t h a t had p r e v i o u s l y been contacted w i t h a h i g h e r polymer c o n c e n t r a t i o n t o s a t i s f y r e t e n tion. The p o l y m e r p u l s e , t h e r e f o r e , i s n o t d e l a y e d b y r e t e n t i o n and b r e a k s t h r o u g h a b o u t 2 2 % o f a p o r e v o l u m e e a r l y b e c a u s e 2 2 % of the pore space i s i n a c c e s s i b l e t o polymer molecules. I n a c c e s s i b l e p o r e v o l u m e does n o t r e q u i r e p o r e c o n s t r i c t i o n s t o o s m a l l f o r p o l y m e r m o l e c u l e s t o p a s s ; Thomas (19) h a s shown that b r i d g i n g o f adsorbed molecules i n c o n s t a n t - r a d i u s c a p i l l a r i e s w i t h diameters l e s s than f o u r times the average molecular d i a m e t e r can l e a d t o r e d i r e c t i o n o f subsequent polymer i n t o l a r g e r flow channels. P o l y m e r may a l s o b e a c c e l e r a t e d r e l a t i v e t o i t s s o l v e n t b y a mechanism t e r m e d h y d r o d y n a m i c c h r o m a t o g r a p h y (24) w h e r e b y t h e mean v e l o c i t y o f a p a r t i c l e i n f l o w i n g f l u i d i s a r e f l e c t i o n o f the pore v e l o c i t y p r o f i l e . Because o f the s i z e

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of polymer molecules, t h e i r c e n t e r s a r e excluded from t h e slowest s t r e a m l i n e s c l o s e s t t o p o r e w a l l s , a n d t h e y move f a s t e r t h a n t h e average s o l v e n t flow r a t e .


Transient Flow

Behavior

S e v e r a l i n v e s t i g a t o r s ( 9 , 20, 2 5 ) , have observed t h a t , i n porous media, a s t e a d y - s t a t e e q u i l i b r i u m e x i s t s between r e t a i n e d and f l o w i n g p o l y m e r . I n t e r r u p t i o n s o r c h a n g e s i n f l o w r a t e c a n perturb t h i s steady s t a t e , r e s u l t i n g i n t r a n s i e n t s i n both r e t e n t i o n and s o l u t i o n c o n c e n t r a t i o n . Figure 6 i l l u s t r a t e s this b e h a v i o r f o r x a n t h a n gum. E f f l u e n t c o n c e n t r a t i o n a n d m o b i l i t y r e d u c t i o n ( r e s i s t a n c e f a c t o r ) a r e p l o t t e d v e r s u s pore volumes i n j e c t e d f o r a 500-ppm x a n t h a n gum s o l u t i o n w i t h 2 p e r c e n t N a C l i n a 6 - i n c h , 121-md B e r e a c o r e . A t p o s i t i o n A, f l o w was i n t e r r u p t e d f o r 16 h o u r s a n d t h e n resumed a t t h e same p r e s s u r e d r o p . This r e s u l t e d i n sharp increases i n both e f f l u e n t c o n c e n t r a t i o n and t h e d e g r e e o f m o b i l i t y r e d u c t i o n r e l a t i v e t o p r e v i o u s s t e a d y s t a t e c o n d i t i o n s . T h i s r e s u l t may be e x p l a i n e d w i t h t h e same m e c h a n i s t i c c o n s i d e r a t i o n s t r e a t e d e a r l i e r — t h a t i s , under a p o s i t i v e p r e s s u r e g r a d i e n t p o l y m e r m o l e c u l e s become p a c k e d i n t o p o r e c a v i t i e s t h a t have downstream o u t l e t s so c o n s t r i c t e d t h a t molecules cannot pass through. This contributes t o permeability reduction. C e s s a t i o n o f f l o w e l i m i n a t e s hydrodynamic drag and p e r m i t s t h e m o l e c u l e s t o assume r e l a x e d c o n f i g u r a t i o n s . M o l e c u l a r d i f f u s i o n i s then able t o reduce t h e c o n c e n t r a t i o n gra d i e n t s e x i s t i n g between c a v i t i e s w i t h r e s t r i c t e d f l o w and main channels. When f l o w i s r e s u m e d , t h e i n c r e a s e d c o n c e n t r a t i o n o f f l o w i n g polymer i n c r e a s e s v i s c o s i t y and, hence, a l s o i n c r e a s e s m o b i l i t y r e d u c t i o n . P e r m e a b i l i t y may a l s o i n c r e a s e , b u t e v i d e n t l y t h i s i s overwhelmed b y t h e a t t e n d a n t i n c r e a s e i n v i s c o s ity. S u b s e q u e n t l y , polymer t r a p p i n g r e c u r s and d e c r e a s e s t h e e f f l u e n t c o n c e n t r a t i o n below t h e i n j e c t e d value. This, i n turn, l o w e r s i n s i t u s o l u t i o n v i s c o s i t y a n d m o b i l i t y r e d u c t i o n . When a l l t r a p p i n g s i t e s a r e once a g a i n s a t u r a t e d , t h e system r e t u r n s to i t s i n i t i a l steady s t a t e . P o s i t i o n s Β a n d C i n F i g u r e 6 i n d i c a t e where p r e s s u r e d r o p a c r o s s t h e c o r e was i n c r e a s e d w i t h o u t i n t e r r u p t i n g t h e f l o w . I n these cases a d d i t i o n a l polymer i s immediately r e t a i n e d , l o w e r i n g b o t h e f f l u e n t c o n c e n t r a t i o n a n d t h e amount o f m o b i l i t y r e d u c t i o n . The m i n i m a a n d a s y m p t o t i c a p p r o a c h e s t o s t e a d y s t a t e w i t h c o n t i n u e d i n j e c t i o n occur as b e f o r e , b u t a lower e q u i l i b r i u m m o b i l i t y r e d u c t i o n r e s u l t s f o r each i n c r e a s e i n p r e s s u r e drop. This i s a t t r i b u t e d t o lower polymer s o l u t i o n v i s c o s i t i e s a t h i g h e r shear r a t e s ( p s e u d o p l a s t i c , non-Newtonian b e h a v i o r ) . Here a g a i n , t h e i n c r e m e n t a l r e d u c t i o n o f p e r m e a b i l i t y a s s o c i a t e d w i t h a d d i t i o n a l polymer r e t e n t i o n , w h i c h opposes t h e e f f e c t o f v i s c o s i t y on m o b i l i t y r e d u c t i o n , i s d o m i n a t e d b y t h e v i s c o s i t y contribution to mobility reduction. Comparing t h e b e h a v i o r o u t l i n e d above f o r x a n t h a n s o l u t i o n s


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Figure 6. Effluent concentration and mobility reduction profiles vs. pore volumes injected for a 500-ppm xanthan gum solution with 2% NaCl(9)




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w i t h s i m i l a r t r a n s i e n t experiments f o r polyacrylamide s o l u t i o n s (20) again shows a b a s i c d i f f e r e n c e i n porous media behavior, which may be a t t r i b u t e d to molecular conformation d i f f e r e n c e s . Resumption of polyacrylamide s o l u t i o n flow a f t e r a short i n t e r r u p t i o n r e s u l t s i n a m o b i l i t y r e d u c t i o n decrease. T h i s occurs, apparently, because the e f f e c t of increased p e r m e a b i l i t y r e s u l t ing from d i s l o d g i n g molecules that had been trapped i n pore cons t r i c t i o n s i s greater than the simultaneous v i s c o s i t y i n c r e a s e a t t r i b u t a b l e to a higher flowing c o n c e n t r a t i o n . Polyacrylamide s o l u t i o n s a l s o show higher steady-state m o b i l i t y r e d u c t i o n s f o l lowing i n c r e a s e s i n flow r a t e , but i t i s not c l e a r whether t h i s e f f e c t i s due mainly to reduced p e r m e a b i l i t i e s from higher r e t e n t i o n l e v e l s or higher e x t e n s i o n a l v i s c o s i t i e s r e s u l t i n g from the v i s c o e l a s t i c nature of polyacrylamide s o l u t i o n s (26). In cont r a s t , xanthan gum s o l u t i o n s are r e l a t i v e l y i n e l a s t i c . Mechanical

Degradation

One c r i t i c a l problem a r i s i n g from i n j e c t i o n of polymer s o l u t i o n s i n t o o i l r e s e r v o i r s i s the p o s s i b i l i t y of imposing f l u i d s t r e s s e s l a r g e enough to rupture molecules and reduce molecular weight. Because of the r a d i a l - f l o w nature of i n j e c t i o n w e l l s , f l u i d s e n t e r i n g a formation a t t y p i c a l flow r a t e s are subjected to very high f l u x e s at the sand face. These l a r g e f l u x e s and the converging-diverging nature of flow channels i n porous media cause s e c t i o n s of entangled molecules to be s t r e t c h e d very r a p i d l y , and some molecules rupture before entanglements can rearrange to r e l i e v e the s t r e s s (27). Polyacrylamide s o l u t i o n s are very s u s c e p t i b l e to t h i s mechanical degradation, while xanthan gum s o l u t i o n s are q u i t e r e s i s t a n t (7, 27, 28). F i g u r e s 7 and 8 compare shear v i s c o s i t i e s vs shear r a t e before and a f t e r high-shear flow through bead packs f o r 300-ppm s o l u t i o n s of a polyacrylamide and a xanthan gum, r e s p e c t i v e l y . The p o l y a c r y l amide s o l u t i o n shows an e i g h t f o l d v i s c o s i t y l o s s f o l l o w i n g bead pack flow, whereas the xanthan s o l u t i o n undergoes n e g l i g i b l e v i s c o s i t y l o s s a f t e r experiencing order of magnitude higher shear r a t e s i n the bead pack. I t should be pointed out that v e l o c i t y gradient i n the flow d i r e c t i o n , or s t r e t c h r a t e , appears to be a b e t t e r measure of deformation r a t e than shear r a t e f o r c o r r e l a t i o n of mechanical degradation (27). The maximum i n apparent v i s c o s i t y f o r polyacrylamide i n F i g u r e 7 i s due to i t s v i s c o e l a s t i c c h a r a c t e r , and there i s a strong c o r r e l a t i o n between v i s c o e l a s t i c s t r e s s e s and mechanical degradation (27). Injectivity

Behavior

A major problem a s s o c i a t e d with use of xanthan gum f o r m o b i l i t y c o n t r o l a p p l i c a t i o n s has been poor i n j e c t i v i t y behavior. This problem may be i l l u s t r a t e d by i n j e c t i v i t y t e s t s (28) conducted i n the Coalingua F i e l d , C a l i f o r n i a . During i n j e c t i o n of



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Figure 7.

Viscosity vs. shear rate for 600-ppm polyacrylamide in 300-ppm NaCl brine (7)



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w a t e r a n d x a n t h a n gum s o l u t i o n s , f l o w r a t e s a n d p r e s s u r e s w e r e monitored t o determine i n j e c t i v i t i e s o f thevarious f l u i d s . I n j e c t i o n was t h r o u g h c a s i n g p e r f o r a t e d w i t h 4 s h o t s - p e r - f o o t i n t o e i t h e r o n e o r two s a n d s a t d e p t h s o f 1600 t o 1800 f e e t s u b surface. S o f t e n e d f r e s h w a t e r was u s e d f o r p o l y m e r h y d r a t i o n t o y i e l d a 6000-ppm c o n c e n t r a t e a n d f o r f u r t h e r d i l u t i o n t o 300 ppm p r i o r t o c a r t r i d g e f i l t r a t i o n and i n j e c t i o n . F i g u r e 9 shows i n j e c t i v i t y a s a f u n c t i o n o f i n j e c t e d volume. D u r i n g i n j e c t i o n o f x a n t h a n gum ( p o i n t s " L " a n d "P", F i g u r e 9 ) , s u b s t a n t i a l i n j e c t i v i t y d e c r e a s e s w e r e o b s e r v e d . Upon r e t u r n i n g t o b r i n e i n j e c t i o n ( p o i n t s M and "Q", F i g u r e 9 ) , i n j e c t i v i t y l e v e l s r e m a i n e d s i g n i f i c a n t l y below pre-polymer v a l u e s . T h i s i n d i c a t e d n e a r w e l l b o r e p l u g g i n g , w h i c h c o u l d b e removed b y two w e l l c l e a n u p procedures: e i t h e r a s i m p l e b a c k wash o r a t r e a t m e n t d e s i g n e d t o remove b a c t e r i a a n d o t h e r d e b r i s a s i n d i c a t e d b y p o i n t s N a n d R i n F i g u r e 9. C l e a r l y t h e r e a r e c o n d i t i o n s o f u s e w h e r e i n j e c t i o n o f x a n t h a n gum c a n r e s u l t i n u n d e s i r a b l e p l u g g i n g a n d injectivity loss. I n j e c t i v i t y p r o b l e m s w i t h x a n t h a n may be a t t r i b u t e d t o t h r e e p r i m a r y f a c t o r s : w a t e r q u a l i t y , polymer c o m p o s i t i o n and injection well configuration. I n g e n e r a l , water q u a l i t y r e q u i r e ments a r e more s t r i n g e n t f o r p o l y m e r t h a n f o r p l a i n w a t e r i n j e c tion. I n t h e C o a l i n g u a i n j e c t i o n t e s t , i t was c o n c l u d e d t h a t f i n e l y d i v i d e d s o l i d s i n t h e i n j e c t i o n s o u r c e w a t e r were b e i n g f l o c c u l a t e d by polymer and c o n t r i b u t e d t o p l u g g i n g problems ( 2 8 ) . In a d d i t i o n , any species present i n t h e i n j e c t i o n water that can c r o s s l i n k x a n t h a n , s u c h a s f e r r i c i r o n o r b o r a t e i o n , s h o u l d be avoided. Composition o f t h e polymer as c o m m e r c i a l l y s u p p l i e d i s another major cause o f xanthan i n j e c t i v i t y problems. Again u s i n g t h e C o a l i n g u a t e s t s a s a n e x a m p l e , i t was c o n c l u d e d ( 2 8 ) t h a t t h e xanthan c o n t a i n e d about 11 weight p e r c e n t c e l l u l a r debris. T h i s c e l l u l a r m a t e r i a l and u n h y d r a t e d polymer " g e l s " w e r e b e l i e v e d r e s p o n s i b l e f o r most o f t h e p l u g g i n g . S e v e r a l p u b l i c a t i o n s and p a t e n t s d e a l w i t h xanthan i n j e c t i v i t y problems and methods f o r improvement. T h e s e i n c l u d e t e c h n i q u e s t o f l o c c u l a t e c e l l u l a r d e b r i s onto c l a y o r other s o l i d s that enable e a s i e r removal by f i l t r a t i o n o r s e d i m e n t a t i o n (29, 3 0 ) , p r o c e d u r e s f o r d i a t o m a c e o u s e a r t h f i l t r a t i o n ( 2 8 , 2 9 , 3 0 , 31) a n d methods f o r d i s s o l v i n g p r o t i e n a c e o u s d e b r i s t h r o u g h a l k a l i n e (32) o r e n z y m a t i c (33) a c t i o n . W h i l e a l l o f t h e s e c l a r i f i c a t i o n p r o c e d u r e s p r o b a b l y do i m p r o v e p o l y m e r - s o l u t i o n p r o p e r t i e s , some q u e s t i o n e x i s t s a s t o t h e v a l i d i t y o f procedures used t o e v a l u a t e t h e degree o f i m p r o v e m e n t . B o t h o f t h e t e s t s commonly e m p l o y e d — M i l l i p o r e f i l t e r t e s t s and r o c k i n j e c t i o n t e s t s — a r e capable o f r a n k i n g polymer s o l u t i o n s on a r e l a t i v e b a s i s , b u t t e s t s r e p o r t e d i n t h e l i t e r a t u r e do n o t b e g i n t o a p p r o a c h t h e i n j e c t i o n l e v e l s e x p e r i enced i n a c t u a l w e l l s . Since the p l u g g i n g u s u a l l y observed w i t h x a n t h a n gum s o l u t i o n s i s a n e a r - s u r f a c e o r s a n d f a c e phenomenon, fl

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i t i s r e a s o n a b l e t o s c a l e i n j e c t i o n on t h e b a s i s o f Volume i n j e c t e d p e r u n i t a r e a o f s a n d f a c e , as i s commonly done i n f i l t r a t i o n s t u d i e s . T h i s means t h a t t h e t y p e o f i n j e c t i o n w e l l c o m p l e t i o n i s an i m p o r t a n t f a c t o r , s i n c e c o m p l e t i o n t y p e w i l l d e t e r m i n e sand e x p o s u r e . The f o l l o w i n g t a b l e shows i n j e c t i o n v a l u e s f o r s e v e r a l w e l l c o m p l e t i o n s commonly e m p l o y e d . Injection v a l u e s c a n r a n g e a b o u t t h r e e o r d e r s o f m a g n i t u d e d e p e n d i n g on t h e SCALED INJECTION VALUES FOR VARIOUS WELL COMPLETIONS ASSUMING 10 BBL/DAY-FT INJECTION 2 W e l l Completion 4 Collapsed Perforations/Ft 2 Perforations/Ft 6" Open H o l e 18" Underrearned and G r a v e l P a c k e d "Typical" Laboratory

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Application of Xanthan Gum for Enhanced Oil Recovery

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