Applications of Water-Soluble Polymers as Shale Stabilizers in Drilling

10 Applications of Water-Soluble Polymers as Shale Stabilizers in Drilling Fluids

Downloaded by UNIV OF MICHIGAN ANN ARBOR on June 10, 2013 | http://pubs.acs.org Publication Date: May 5, 1986 | doi: 10.1021/ba-1986-0213.ch010

R. K. Clark Production Operations Research Department, Shell Development Company, Houston, T X 77001

Water-soluble polymers are often used in oil and gas well drilling fluids as stabilizers for water-sensitive shales encountered in subsurface formations. Polymer chemistry, ionic character, degree of charge, molecular weight, and other factors play a role in deter mining the effectiveness of a polymer as a shale stabilizer. Some of the more common polymer types that have been used, with varying degrees of success, are natural gums (guar, xanthan, and flaxseed), cellulose derivatives (carboxymethyl and hydroxyethyl), starches, and high molecular weight acrylate-acrylamide copolymers. This chapter reviews laboratory test methods for evaluating polymer performance, presents results from one of the methods described, and discusses possible shale stabilizing mechanisms.

A / PPLICATIONS OF WATERS -OLUBLE POLYMERS

i n o i l a n d gas w e l l d r i l l i n g f l u i d s i n c l u d e use as v i s c o s i t y b u i l d e r s , f i l t r a t i o n c o n t r o l agents, f l o c c u l e n t s , d e f l o c c u l e n t s , a n d shale s t a b i l i z e r s . S h a l e s t a b i l i z e r s m a y b e o n e o f t h e least u n d e r s t o o d uses o f p o l y m e r s i n d r i l l i n g f l u i d s . P r o b l e m s a r i s ing f r o m borehole instability resulting f r o m adverse interactions b e t w e e n the d r i l l i n g f l u i d a n d c l a y - b e a r i n g shales a r e a m o n g t h e m o r e c o s t l y d i f ficulties that occur i n d r i l l i n g operations. M a i n t e n a n c e of a stable b o r e h o l e is o n e o f t h e m a n y f u n c t i o n s that a d r i l l i n g f l u i d m u s t f u l f i l l . S e l e c t i o n o f t h e p r o p e r p o l y m e r f o r i n c o r p o r a t i o n i n t o the s y s t e m c a n o f t e n r e d u c e o r e l i m i n a t e shale s t a b i l i t y p r o b l e m s , w h i l e a n i m p r o p e r c h o i c e c a n i n c r e a s e w e l l costs s i g n i f i c a n t l y .

Drilling Fluids: Functions and Properties T h e m o s t i m p o r t a n t f u n c t i o n s o f a d r i l l i n g f l u i d a r e to (1) r e m o v e f o r m a t i o n c u t t i n g s f r o m the b o t t o m o f t h e h o l e a n d t r a n s p o r t t h e m to the surf a c e , (2) p r o v i d e s u f f i c i e n t h y d r o s t a t i c p r e s s u r e against t h e f o r m a t i o n t o p r e v e n t i n f l u x o f f o r m a t i o n f l u i d s , (3) s t a b i l i z e d o w n h o l e f o r m a t i o n s a n d 0065-2393/86/0213-0171$06.(W0 © 1986 American Chemical Society

In Water-Soluble Polymers; Glass, J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1986.

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p r e v e n t h o l e c o l l a p s e , (4) p r e v e n t loss o f f l u i d to p e r m e a b l e f o r m a t i o n s , (5) c o o l a n d l u b r i c a t e the b i t a n d d r i l l s t r i n g , a n d (6) h e l p s u s p e n d the w e i g h t o f the d r i l l s t r i n g a n d c a s i n g ( I ) . T h e f l u i d is e x p e c t e d to p e r f o r m these a n d o t h e r f u n c t i o n s s i m u l t a n e o u s l y .


D r i l l i n g f l u i d s are p u m p e d d o w n the w e l l t h r o u g h a h o l l o w d r i l l s t r i n g , o u t the d r i l l b i t at the b o t t o m o f the h o l e , a n d b a c k u p to the s u r f a c e i n t h e a n n u l u s f o r m e d b y t h e b o r e h o l e w a l l a n d the r o t a t i n g d r i l l s t r i n g . T h e fluid is p a s s e d t h r o u g h a series o f v i b r a t i n g screens, h y d r o c y c l o n e s , a n d c e n t r i f u g e s o n the s u r f a c e to r e m o v e f o r m a t i o n c u t t i n g s , w h i c h a r e d i s c a r d e d . O n e o r m o r e a d d i t i v e s are t h e n u s e d to r e t u r n the s y s t e m to a d e s i r e d set o f p h y s i c a l a n d c h e m i c a l p r o p e r t i e s , after w h i c h the f l u i d is p u m p e d b a c k d o w n the w e l l to r e p e a t the c y c l e . T h e f l u i d is a d y n a m i c , c i r c u l a t i n g s y s t e m that c h a n g e s f r o m d a y to d a y i n r e s p o n s e to the d r i l l i n g c o n d i t i o n s b e i n g e x p e r i e n c e d . T h e d r i l l i n g f l u i d s y s t e m m a y b e u s e d f o r a f e w d a y s , w e e k s , o r m o n t h s a n d i n s o m e cases is r e c o n d i t i o n e d f o r use o n s u b s e q u e n t w e l l s . D r i l l i n g f l u i d s a r e m o s t f r e q u e n t l y f o r m u l a t e d w i t h w a t e r as the c o n t i n u o u s p h a s e (2). V i s c o s i t y - b u i l d i n g c l a y s o r p o l y m e r s a n d v a r i o u s o r g a n i c c h e m i c a l s are a d d e d f o r c o n t r o l l i n g the r h e o l o g i c a l a n d f i l t r a t i o n properties. Water-soluble polymers used for rheological a n d filtration control include (earboxymethyl)- a n d (hydroxyethyl)cellulose, xanthan gum, starch, a n d acrylate-acrylamide copolymers of various compositions a n d m o l e c u l a r weights. N a t u r a l l y d e r i v e d c o m p l e x p o l y m e r s such as l i g n o s u l f o n a t e s , l i g n i t e s , a n d p l a n t t a n n i n s m a y b e a d d e d to m o d i f y the r h e o l o g i c a l a n d f i l t r a t i o n b e h a v i o r o f a d d e d c l a y s . T h e c o m p o s i t i o n o f the w a t e r p h a s e r a n g e s f r o m f r e s h t o h i g h l y saline solutions that m a y c o n t a i n s u b s t a n t i a l l e v e l s o f s o l u b l e c a l c i u m a n d m a g n e s i u m i o n s . W a t e r - b a s e d f l u i d s a r e c o m m o n l y m a i n t a i n e d at a n alkaline p H w i t h s o d i u m hydroxide, c a l c i u m hydroxide, or potassium h y d r o x i d e . T h e r e s u l t i n g c o m p l e x a q u e o u s s o l u t i o n constitutes the e n v i ronment i n w h i c h c o m m e r c i a l clays a n d polymers must w o r k a n d be effective. P o w d e r e d , h i g h s p e c i f i c g r a v i t y s o l i d s s u c h as b a r i t e o r h e m a t i t e are a d d e d t o r e a c h t h e d e s i r e d f l u i d d e n s i t y , w h i c h m a y r a n g e f r o m 1 t o 2.5 g / c m . T h e d e n s i t y r e q u i r e m e n t is d i c t a t e d b y the f o r m a t i o n pressures that a r e e n c o u n t e r e d w h i l e the w e l l is b e i n g d r i l l e d a n d b y the e a r t h stresses i m p o s e d o n the f o r m a t i o n . A h i g h - d e n s i t y d r i l l i n g f l u i d m a y c o n t a i n u p to 40 v o l % s u s p e n d e d s o l i d s ( f o r m a t i o n s o l i d s , c o m m e r c i a l c l a y s , a n d w e i g h t i n g m a t e r i a l s ) . T h e s p e c i f i c c o m p o s i t i o n o f the f l u i d d e p e n d s o n the d o w n h o l e c o n d i t i o n s b e i n g e x p e r i e n c e d . T e m p e r a t u r e s e x c e e d i n g 205 ° C (400 ° F ) a n d pressures u p to 140 M P a (20,000 p s i ) are n o t u n c o m m o n i n d e e p w e l l s . W a t e r - i n - o i l e m u l s i o n ( o i l c o n t i n u o u s phase) f l u i d s are o f t e n u s e d f o r b o r e h o l e s t a b i l i t y a n d d e e p w e l l d r i l l i n g b u t are o u t s i d e the s c o p e o f this c h a p t e r . 3


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Use as Shale Stabilizers in Drilling Fluids

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Shale Composition and Properties Shales a r e f i n e - g r a i n e d s e d i m e n t a r y r o c k s that c o n t a i n v a r i o u s c l a y m i n erals, q u a r t z , f e l d s p a r , c a l c i t e , etc. T h e i n t e r a c t i o n b e t w e e n a d r i l l i n g f l u i d a n d a shale is d o m i n a t e d b y the r e s p o n s e o f the c l a y m i n e r a l s i n the shale to c o n t a c t b y the w a t e r p h a s e o f the f l u i d . T h e m o s t w a t e r s e n s i t i v e c l a y m i n e r a l s are the s m e c t i t e s ( t r a d i t i o n a l l y k n o w n as m o n t m o r i l l o n i t e ) a n d s m e c t i t e - i l l i t e m i x e d - l a y e r c l a y s . Shales c o n t a i n i n g l a r g e a m o u n t s o f i l l i t e c a n also e x h i b i t e x t r e m e w a t e r s e n s i t i v i t y . C h l o r i t e a n d k a o l i n i t e a r e c o m m o n l y f o u n d i n s u b s u r f a c e shales b u t are r a r e l y c o n s i d e r e d d e t r i m e n t a l to b o r e h o l e s t a b i l i t y a n d d r i l l i n g o p e r a t i o n s . T h e c l a y m i n e r a l s are c o l l o i d a l , h i g h s u r f a c e a r e a , l a y e r e d a l u m i n o silicates that f r e q u e n t l y o c c u r i n n a t u r e as assemblages o f t h i n p l a t e s h a p e d p a r t i c l e s s t a c k e d f a c e to f a c e l i k e p a g e s i n a b o o k (3). T h e p l a n a r surfaces c a r r y a n e g a t i v e c h a r g e a r i s i n g f r o m s u b s t i t u t i o n of l o w e r v a l e n t cations i n the clay crystal lattice. T h e edges m a y be positively or negat i v e l y c h a r g e d d e p e n d i n g o n the e n v i r o n m e n t : p o s i t i v e at l o w p H a n d n e g a t i v e at h i g h p H . C a t i o n s a s s o c i a t e d w i t h the p l a n a r surfaces m a y b e l o o s e l y h e l d a n d r e a d i l y e x c h a n g e d b y o t h e r cations o r m a y b e t i g h t l y h e l d a n d n o n e x c h a n g e a b l e . W a t e r c a n b e d r a w n i n t o the i n t e r l a y e r r e g i o n o f s o m e c l a y s b y h y d r a t i o n o f the e x c h a n g e c a t i o n s a n d the s i l i c a t e s u r f a c e . A n o s m o t i c f o r c e a r i s i n g f r o m d i f f e r e n c e s b e t w e e n the a c t i v i t y o f the i n t e r l a y e r w a t e r a n d that o f the d r i l l i n g f l u i d c a n r e s u l t i n increases i n l a y e r s e p a r a t i o n b e y o n d that o f the i n i t i a l s u r f a c e h y d r a t i o n . S u r f a c e a r e a a n d c a t i o n - e x c h a n g e c a p a c i t y are k e y p r o p e r t i e s that d e t e r m i n e t h e r e s p o n s e o f a c l a y m i n e r a l to w a t e r . T h e s a m e is t r u e o f a shale (4). Shales h i g h i n s m e c t i t e o r s m e c t i t e - i l l i t e m i x e d - l a y e r c l a y s are t y p i c a l l y y o u n g m a r i n e shales that s w e l l a n d m a y d i s p e r s e c o m p l e t e l y i n f r e s h w a t e r . T h e s e are the " g u m b o " shales c o m m o n a l o n g the G u l f C o a s t that a r e c h a r a c t e r i z e d b y h i g h s u r f a c e a r e a a n d m o i s t u r e c o n t e n t , m o d erate to h i g h cation-exchange c a p a c i t y , a n d l o w m e c h a n i c a l strength. A t the o t h e r e n d o f the scale are the h a r d , b l a c k i l l i t i c shales f o u n d f r o m T e x a s t o the R o c k i e s a n d i n m a n y d e e p w e l l s . T h e s e shales are o l d e r a n d m u c h m o r e h i g h l y c o m p a c t e d t h a n the soft g u m b o shales e v e n t h o u g h t h e y m a y b e f o u n d at o r n e a r the s u r f a c e . A l t h o u g h the t o t a l c l a y m i n e r a l c o n t e n t m a y e x c e e d 5085, the i l l i t i c shales are l o w s w e l l i n g a n d n o n d i s persive a n d are characterized b y l o w surface area, cation-exchange capacity, moisture content, a n d h i g h m e c h a n i c a l strength. I m m e r s i o n o f a l a r g e c o m p e t e n t p i e c e o f a h a r d i l l i t i c shale i n f r e s h w a t e r m a y result i n d i s a g g r e g a t i o n a l o n g f r a c t u r e p l a n e s i n t o f i r m shale s p l i n t e r s . T h e b o x lists t h e p a r a m e t e r s o f t e n m e a s u r e d o n shale s a m p l e s f o r the p u r p o s e o f c h a r a c t e r i z a t i o n a n d c l a s s i f i c a t i o n . B y c l a s s i f y i n g shales, o n e m a y b e a b l e to e s t i m a t e the d r i l l i n g f l u i d c h e m i s t r y r e q u i r e d to d r i l l a g i v e n shale p r o b l e m - f r e e (4).


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Parameters Measured for Shale Characterization Geologic age and depositional environment Mineralogy by X-ray diffraction (clay content and composition) Bulk and grain density Cation-exchange capacity Surface area Moisture content Composition of interlayer water Water absorption-swelling properties Dispersibility Mechanical properties (compressive and tensile strength)

Drilling Fluid-Shale Interactions T h e w a t e r phase of the d r i l l i n g f l u i d c a n d r a m a t i c a l l y alter the p r o p e r ties o f e x p o s e d s u b s u r f a c e shales. T h e e f f e c t ranges f r o m s w e l l i n g a n d d i s p e r s i o n o f soft shales to m e c h a n i c a l s p a l l i n g o f h a r d shales, a l l o f w h i c h c a n s e v e r e l y r e d u c e d r i l l i n g e f f i c i e n c y . T h e c o m p o s i t i o n o f the w a t e r p h a s e p l a y s a n i m p o r t a n t r o l e i n d e t e r m i n i n g the r e s p o n s e . F r e s h w a t e r has the m o s t p r o n o u n c e d e f f e c t : m a x i m u m a l t e r a t i o n o f the shale p r o p e r t i e s results. I n c r e a s i n g the s a l i n i t y r e d u c e s the h y d r a t i o n a n d s w e l l i n g r e s u l t i n g f r o m w a t e r a b s o r p t i o n b y t h e shale. B e c a u s e the m e c h a n i c a l s t r e n g t h o f the shale is i n v e r s e l y p r o p o r t i o n a l to the m o i s t u r e c o n t e n t , r e d u c i n g w a t e r a b s o r p t i o n w i l l r e t a r d d e g r a d a t i o n o f the m e c h a n i c a l p r o p e r t i e s a n d p r o m o t e shale s t a b i l i t y . S o d i u m c h l o r i d e , p o t a s s i u m c h l o r i d e , g y p s u m , a n d l i m e are o f t e n u s e d to r e d u c e the r a t e a n d m a g n i t u d e o f w a t e r a b s o r p t i o n b y shales. B o t h laboratory a n d f i e l d experiences indicate a preference for potass i u m o v e r s o d i u m f o r shale s t a b i l i t y (4, 5 ) . C a l c i u m ions d e r i v e d p r i m a r i l y f r o m g y p s u m a n d l i m e a r e also p r e f e r r e d o v e r s o d i u m i o n s , b u t are less c o m p a t i b l e w i t h the o t h e r f l u i d a d d i t i v e s . T y p i c a l c o n c e n t r a t i o n r a n g e s f o u n d f o r the v a r i o u s c a t i o n s w h e n u s e d f o r shale s t a b i l i t y are 100-1000 m g / L f o r c a l c i u m , 2000-100,000 m g / L f o r p o t a s s i u m , a n d 12,000-125,000 m g / L f o r s o d i u m .

Polymers for Shale Stability A d d e d electrolyte m a y not p r o v i d e adequate stability, a n d a p o l y m e r i c m a t e r i a l is o f t e n n e e d e d . A v a r i e t y o f w a t e r - s o l u b l e p o l y m e r s h a v e b e e n t o u t e d as s h a l e s t a b i l i z e r s f o r use i n d r i l l i n g f l u i d s . T h e p o l y m e r m a y b e u s e d w i t h o n e o f the c a t i o n s t a b i l i z e r s as m e n t i o n e d a b o v e o r it m a y b e


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u s e d a l o n e , as it o f t e n p e r f o r m s o t h e r f u n c t i o n s i n the f l u i d as w e l l . S e l e c t i o n o f the p o l y m e r m u s t b e n o t o n l y o n the basis o f its shale s t a b i l i z i n g a b i l i t y , b u t also o n the b a s i s o f its c o m p a t i b i l i t y w i t h the o t h e r d r i l l i n g f l u i d c o m p o n e n t s a n d w i t h the d o w n h o l e d r i l l i n g c o n d i t i o n s , p a r t i c u larly temperature. P o l y m e r s u s e d as shale s t a b i l i z e r s i n f i e l d a p p l i c a t i o n s i n c l u d e n a t u ral, m i c r o b i a l , a n d d e r i v a t i z e d polysaccharides a n d synthetic h i g h m o l e c u l a r w e i g h t a c r y l i c c o p o l y m e r s . P r o m i n e n t a m o n g the p o l y s a c c h a r i d e s are ( c a r b o x y m e t h y l ) c e l l u l o s e (6), ( h y d r o x y e t h y l ) c e l l u l o s e (7), s t a r c h (8), a n d g u m s s u c h as x a n t h a n (5), f l a x s e e d (9), a n d g u a r . A c r y l a t e - a c r y l a m i d e c o p o l y m e r s are used i n d r i l l i n g fluids for m a n y purposes i n c l u d i n g shale s t a b i l i z e r s (10, I I ) . T h e s p e c i f i c a p p l i c a t i o n d e p e n d s o n the m o l e c u l a r w e i g h t a n d the d e g r e e a n d m a g n i t u d e o f c h a r g e o n the p o l y m e r . T h e s e a n d o t h e r p o l y m e r s n o t c o m m o n l y f o u n d i n the o i l f i e l d w i l l b e d i s c u s s e d i n t e r m s o f l a b o r a t o r y p e r f o r m a n c e as shale s t a b i l i z e r s i n the f o l l o w i n g sections.

Laboratory Test Methods for Shale Stabilization A l a r g e n u m b e r o f m e t h o d s h a v e b e e n u s e d to assess the p e r f o r m a n c e o f d r i l l i n g f l u i d a d d i t i v e s as shale s t a b i l i z e r s . U n f o r t u n a t e l y , n o s t a n d a r d m e t h o d is w i d e l y a c c e p t e d i n the i n d u s t r y . T h e d i v e r s i t y o f test m e t h o d s is p a r t l y d u e to the i n a b i l i t y to a c c u r a t e l y s i m u l a t e the d o w n h o l e e n v i r o n m e n t i n the l a b o r a t o r y . E v a l u a t i o n o f d r i l l i n g f l u i d s a n d a d d i t i v e s is also c o m p l i c a t e d b y the d i f f i c u l t y i n o b t a i n i n g s u i t a b l e shale s a m p l e s i n a l a r g e e n o u g h q u a n t i t y f o r t e s t i n g . A l t h o u g h shale is e n c o u n t e r e d i n v i r t u a l l y e v e r y w e l l , the state o f the shale c u t t i n g s c o l l e c t e d at the s u r f a c e u s u a l l y r e n d e r s t h e m u n s u i t a b l e f o r t e s t i n g . C o r e s are r a r e l y t a k e n i n shale e x c e p t b y m i s t a k e o r o c c a s i o n a l l y w h e n shale p r o b l e m s are s e v e r e . S u r f a c e shale o u t c r o p s are b a d l y w e a t h e r e d a n d m a y n o t b e r e p r e s e n t a t i v e o f s u b s u r f a c e f o r m a t i o n s . I n s p i t e o f the d i f f i c u l t i e s , s u f f i c i e n t shale c a n o f t e n b e o b t a i n e d to a l l o w s t a b i l i t y tests to b e c o n d u c t e d , a l t h o u g h s o m e a l t e r a t i o n f r o m i n situ p r o p e r t i e s is p r o b a b l e . L a b o r a t o r y t e s t i n g o f s h a l e - f l u i d i n t e r a c t i o n s i n c l u d e s b o t h static and d y n a m i c methods. Static methods include i m m e r s i o n - s o a k i n g , w a t e r a b s o r p t i o n (15), a n d s w e l l i n g ( l i n e a r v o l u m e t r i c ) (5,12,14) t e c h n i q u e s . S t a t i c tests assess c h a n g e s i n the p h y s i c a l size o r s t r u c t u r e o f the shale d u r i n g c o n t a c t w i t h the test f l u i d . S w e l l i n g tests u s i n g strain gauges h a v e b e e n d e s c r i b e d f o r l o w - (5), m e d i u m - (12), a n d h i g h - p r e s s u r e (13, 14) c o n d i t i o n s . F l u i d a b s o r p t i o n tests c a n b e u s e d to m e a s u r e the r a t e a n d m a g n i t u d e o f w a t e r u p t a k e u n d e r a v a r i e t y o f c o n d i t i o n s (15). Salt a n d c a t i o n e f f e c t s are r e a d i l y e v a l u a t e d i n static tests, b u t s t a b i l i z i n g p o l y m e r s m a y h a v e l i t t l e e f f e c t o n the s w e l l i n g p r o p e r t i e s o f shales, p a r t i c u l a r l y w h e n tested u n d e r pressure (12).



176


D y n a m i c m e t h o d s i n c l u d e i m m e r s i o n - s t i r r i n g (16), i m m e r s i o n t u m b l i n g (5), c a p i l l a r y s u c t i o n t i m e (15), t r i a x i a l f l o w (17), a n d m i c r o b i t d r i l l i n g m a c h i n e (18) t e c h n i q u e s . A g i t a t i o n ( s t i r r i n g o r t u m b l i n g ) o f shale c u t t i n g s i n a test f l u i d f o l l o w e d b y s i e v i n g is u s e d to d e t e r m i n e the p a r t i c l e s i z e a n d i n t e g r i t y o f the s a m p l e after c o n t a c t (5,9). T h e d i s p e r s i b i l i t y o f the shale c a n b e assessed b y the c a p i l l a r y s u c t i o n test (15). T h e p r e s e n c e o f p o l y m e r s c a n g i v e m i s l e a d i n g results i n t h e c a p i l l a r y s u c t i o n test as the m o v e m e n t o f f i l t r a t e t h r o u g h the f i l t e r m e d i a is i n f l u e n c e d b y s o l u t i o n v i s c o s i t y . W e n z e l u s e d a n a p p a r a t u s that stirs the test f l u i d o v e r a f i x e d s h a l e s a m p l e t o d e t e r m i n e the a b i l i t y o f p o l y m e r s a n d salts to p r e v e n t s o f t e n i n g a n d c r a c k i n g o f shales (16). S t i r r i n g - a n d t u m b l i n g t y p e tests a p p e a r to b e u s e f u l f o r d e t e r r n i n i n g the p e r f o r m a n c e o f p o l y m e r s as shale s t a b i l i z e r s , a l t h o u g h the test c o n d i t i o n s d o n o t s i m u l a t e the downhole environment. M o r e s o p h i s t i c a t e d d y n a m i c tests f l o w d r i l l i n g f l u i d t h r o u g h shale s p e c i m e n s w i t h (17) o r w i t h o u t (18) the shale u n d e r stress. S u c h tests assess the a b i l i t y o f the f l u i d c h e m i s t r y to m a i n t a i n the i n t e g r i t y o f the shale u n d e r c o n d i t i o n s that m o r e n e a r l y a p p r o x i m a t e the d o w n h o l e c o n d i t i o n s . S h a l e s a m p l e s u s e d i n the f l o w tests a r e u s u a l l y f o r m e d i n the l a b o r a t o r y i n a c o m p a c t i o n c e l l as n a t u r a l s a m p l e s o f s u f f i c i e n t s i z e a n d i n t e g r i t y a r e r a r e l y a v a i l a b l e f o r m a c h i n i n g to the d i m e n s i o n s n e e d e d i n the test. T h e t r i a x i a l shale tester d e s c r i b e d b y D a r l e y (17) r e q u i r e s a 5 - c m - d i a m e t e r b y 2 . 5 - c m - l o n g s a m p l e w i t h a 0 . 6 4 - c m h o l e i n the c e n t e r t h r o u g h w h i c h the test f l u i d is p u m p e d . T h e test s p e c i m e n s u s e d b y S i m p s o n (18) a r e e v e n l a r g e r . T h e s h a l e s t a b i l i z i n g a b i l i t y o f p o l y m e r s is m o r e p r o p e r l y e v a l u a t e d i n f l o w tests t h a n i n the static o r t u m b l i n g - s t i r r i n g tests, a l t h o u g h q u e s t i o n s arise o v e r the e f f e c t o f r e c o n s t i t u t i n g the shale s a m p l e s . F l o w tests r e q u i r e r a t h e r c o m p l i c a t e d e q u i p m e n t , w h i c h is c u r r e n t l y o f l i m i t e d a v a i l a b i l i t y .

Laboratory Performance of Polymers a s Shale Stabilizers T h e p e r f o r m a n c e o f p o l y m e r s as shale s t a b i l i z e r s d i s c u s s e d i n this sect i o n is b a s e d o n t h e results o f f l o w tests i n the t r i a x i a l shale tester d e s c r i b e d b y D a r l e y (17). M a n y d i f f e r e n t p o l y m e r s h a v e b e e n e v a l u a t e d b y u s i n g d i f f e r e n t t y p e s o f shales a n d test c o n d i t i o n s . D a t a o n o n l y t w o shales a r e g i v e n h e r e b e c a u s e p o l y m e r p e r f o r m a n c e is n o t i n f l u e n c e d g r e a t l y b y shale c o m p o s i t i o n . Shales c h o s e n f o r this s t u d y a r e A t o k a , a h a r d , i l l i t i c s h a l e f r o m O k l a h o m a ; a n d P i e r r e , a r e l a t i v e l y soft shale c o n t a i n i n g h i g h l e v e l s o f s w e l l i n g c l a y s . T h e c l a y m i n e r a l c o m p o s i t i o n o f the shales is g i v e n i n T a b l e s I a n d I I . T a b l e I lists t r i a x i a l test d a t a u s i n g A t o k a shale a n d a n u m b e r o f c o m m e r c i a l p o l y m e r s , m a n y i n use as d r i l l i n g f l u i d a d d i t i v e s . T a b l e I I g i v e s s i m i l a r d a t a f o r P i e r r e shale. P o t a s s i u m c h l o r i d e w a s u s e d i n m o s t


10.

177


CLARK

Table I . Triaxial Test Results w i t h A t o k a Shale

Polymer Polysaccharides Potato starch


Hydroxypropyl-substituted guar Xanthan gum (Carboxymethyl)cellulose Acrylate-acrylamide copolymers 0-10$ acrylate 30% acrylate 30% acrylate* fl

b

d

10% acrylate AMPS Other synthetic polymers Polyethylene oxide M e t h y l v i n y l ethermaleic anhydride L o w viscosity M e d i u m viscosity H i g h viscosity /

g

Polymer Concn (g/L)

KCl Concn (g/L)

Time to Failure (min)

Erosion (%)

0.0 15.0 50.0 0.7 1.5 1.5 2.9 1.4

30 30 30 30 30 30 0 30

5 110 muss 1380+ 21 34 38

21 9

Applications of Water-Soluble Polymers as Shale Stabilizers in Drilling

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