Effect of Anionic Comonomers on the Hydrolytic Stability of

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Chapter 3

Effect of Anionic Comonomers on the Hydrolytic Stability of Polyacrylamides at High Temperatures in Alkaline Solution R. W. Dexter and R. G. Ryles American Cyanamid Company, Chemical Research Division, Stamford, CT 06904-0060

Hydrolysis of amide groups to carboxylate is a major cause of i n s t a b i l i t y i n acrylamide-based polymers, e s p e c i a l l y a t a l k a l i n e pH and high temperatures. The performance of o i l - r e c o v e r y polymers may be adversely affected by excessive hydrolysis, which can promote p r e c i p i t a t i o n from sea water solution. T h i s work has studied the e f f e c t s of the sodium s a l t s of a c r y l i c acid and AMPS*, 2-acrylamido-2-methylpropanesulfonic acid, as comonomers, on the rate of hydrolysis of polyacryl amides i n a l k a l i n e solution a t high temperatures. Copolymers were prepared containing from 0-53 mole % of the anionic comonomers, and hydrolyzed i n aqueous solution a t pH 8.5 a t 90°C, 108°C and 120°C. The extent of hydrolysis was measured by a conductometric method, analyzing f o r the t o t a l carboxylate content. I t was found that the rate of hydrolysis decreased as the mole r a t i o of the anionic comonomers increased, and that AMPS was more e f f e c t i v e i n preventing hydrolysis a t all of the temperatures studied.

Polymers designed f o r the enhancement of o i l recovery must remain i n s o l u t i o n throughout the predicted l i f e of the flood to provide the required v i s c o s i t y f o r o i l displacement. During polymer flooding using brines or sea water as solvents, the hydrolysis o f amide groups present i n polyacrylamides to form carboxylates l i m i t s the u s e f u l l i f e t i m e of the polymer due to the formation o f complexes with magnesium and calcium ions [2,3]. These may be p r e c i p i t a t e s or gels. The objective of t h i s work was to i n v e s t i gate copolymers of acrylamide having greater resistance to hydro l y s i s a t high temperatures, and with a lower tendency to form insoluble p r e c i p i t a t e s i n sea water. AMPS* i s a registered trademark of the L u b r i z o l Corporation 0097-6156/89/0396-0102$06.00A)

o 1989 American Chemical Society


3.

DEXTER AND RYLES

Effect of Anionic Comonomers

103

The hydrolysis of polyacrylamide and acrylamide/sodium a c r y l a t e copolymers has been extensively studied [1,2,3,5,6,7,8,-9,10], i n r e l a t i v e l y strongly a l k a l i n e conditions, above pH 12. These studies demonstrated that the hydrolysis of the amide groups i s hydroxide-catalyzed and that neighboring ionized carboxyl groups i n the polymer i n h i b i t the hydrolysis by e l e c t r o s t a t i c repulsion of the hydroxide ions. Senju e t a l . [6] showed that a t temperatures up to 100°C, there i s an apparent l i m i t to the extent of hydrolysis of polacrylamide when approximately 60% of the amide groups are hydrolyzed. The hydrolysis of acrylamide copolymers i n d i l u t e a l k a l i n e conditions f o r long periods a t high temperatures up to 120°C, as found i n harsh reservoirs, has not been extensively studied. Metzner e t a l . [1] have considered the chemical s t a b i l i t y of acrylamide copolymers with various proportions o f sodium a c r y l a t e i n sea water a t 90°C. They showed that p r e c i p i t a t i o n of the poly mers occurred when approximately 40% of the polymer was i n the carboxylate form, and a t t r i b u t e d t h i s to the formation of magnesium and calcium complexes. Nonionic polyacrylamides are unsatisfactory i n polymer floods because of t h e i r low v i s c o s i t y and high adsorption. Anionic polyacrylamides containing 20-30% a c r y l i c a c i d can be obtained with higher v i s c o s i t y and lower adsorption, and are generally quite e f f e c t i v e . However, even moderate increases i n the hydrolysis of these polymers r a i s e s the carboxylate content to a l e v e l a t which p r e c i p i t a t i o n of calcium and magnesium complexes occurs, thereby shortening the e f f e c t i v e l i f e t i m e of the polymer. (The separate and independent i n s t a b i l i t y due to chain s c i s s i o n of polyacrylamides by oxidation has been shown to be minimal i n reservoirs i n the absence of molecular oxygen [ 4 ] ) . EXPBBIMH1TAL

Materials. Monomers used i n the preparation of the copolymers were as follows: acrylamide as a 50% solution i n water, stablized with cupric ion, supplied by American Cyanamid Company; a c r y l i c a c i d supplied by BASF; and AMPS*, 2-acrylamido-2-methylpropanesulfonic acid, ( r e c r y s t a l l i z e d grade) obtained from L u b r i z o l . The sodium s a l t s of a c r y l i c a c i d and AMPS were prepared by gradual n e u t r a l i zation of the monomers with sodium hydroxide solution, maintaining a temperature of 0 to 5°C, to give a f i n a l concentration o f 50%. A l l copolymers were prepared by solution polymerization, under adiabatic conditions, giving a t l e a s t 99.9% conversions. The polymer gels were granulated and then dried a t 90 °C to a residual water content o f 10 to 12%. The a c t i v e polymer content of each sample was calculated from the i n i t i a l weight of the comonomers and the weight of the dried g e l . Hydrolysis of the polymers was determined by conductometric t i t r a t i o n to be l e s s than 0.2% of the acrylamide charge. The molecular weight of the polymers was 8-10 m i l l i o n as determined by i n t r i n s i c v i s c o s i t y measurements. The composition and concentration of polymers i n the t e s t solutions f o r hydrolysis are shown i n Table 1. The concentration of the sodium a c r y l a t e and sodium AMPS copolymers with acrylamide were calculated to provide 0.025 molar solutions of amide units, to simplify the k i n e t i c s .

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OIL-FIELD CHEMISTRY Table 1. Composition and Concentration of Polymers


Mole % Anionic Comonomer i n Copolymers

0.00 7.70 15.40 22.40 33.50 43.00 53.30

% w/w Active Polymer i n Solution Sodium acrylate Sodium AMPS Copolymers Copolymers 0.178 0.197 0.222 0.254 0.296 0.355 0.444

0.178 0.226 0.286 0.349 0.467 0.609 0.803

Solutions o f each copolymer were prepared by d i s s o l v i n g the appropriate amount o f copolymer i n deionized water and r o l l i n g the solutions f o r 16 hours.

ANIONIC ACRYLAMIDE COPOLYMERS

HYDROLYSIS EXPERIMENTS Conditions f o r hydrolysis experiments were selected t o simulate harsh r e s e r v o i r environments. Moderately a l k a l i n e (pH 8.5) solutions, high temperatures, and long reaction times up to 120 days were used. The pH o f the solutions remained a t 8.5 o r s l i g h t l y higher, ensuring that a l l anionic groups were f u l l y ionized. A portion o f each s o l u t i o n was retained f o r analysis o f carb oxylate content a t zero time. Samples o f the polymer solutions were weighed i n t o glass j a r s , the pH adjusted to 8.5 and the j a r s were sealed with t i g h t l y f i t t i n g screw caps. The j a r s were placed i n thermostatted ovens a t 90°, 108°, and 120°C. A f t e r the appro p r i a t e time, the j a r s were removed, cooled and weighed to ensure no loss o f contents, p r i o r to analysis f o r hydrolysis. The extent o f hydrolysis o f the copolymers was determined by conductometric t i t r a t i o n . The increase i n carboxylate content was determined by difference, before and a f t e r hydrolysis. (The AMPS content o f the polymers, where measured, was determined by c o l l o i d t i t r a t i o n with poly [ d i a l l y l dimethyl ammonium chloride].)

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DEXTER AND RYLES

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Effect ofAnionic Comonomers

KINETICS The rate o f hydrolysis o f acrylamide i s assumed to be equal to the rate o f formation o f carboxylate groups i n the e a r l y stages o f reaction, f o r both sodium a c r y l a t e and AMPS copolymers.

f


d[C00 ] = - d[AMD] = K [AMD] [OH] dt dt d[000~] = K[AMD] dt K = d[000~] dt 0.025

a t zero time and constant pH [AMD] = 0.025 a t zero time

P l o t s of the concentration o f carboxylate formed vs. time were drawn f o r each copolymer, and the i n i t i a l rates of hydrolysis were determined by measurement o f the slope o f the tangent t o the curve at zero time. The pseudo-unimolecular rate constant (K) i s given by:

K = i n i t i a l slope 0.025

Confirmatory values of K were obtained from p l o t s o f l o g 0.025 vs. time i n the e a r l y stages o f the reaction, 0.025-[COO ] e

although deviations from a s t r a i g h t l i n e occurred i n l a t e r stages. RESULTS The rate of hydrolysis o f acrylamide i n copolymers with sodium a c r y l a t e o r AMPS, 2-acrylamido-2-methylpropanesulfonic acid, decreased as the proportion o f the anionic comonomers was increased. T h i s e f f e c t was much more marked with AMPS than with sodium acrylate, and occurred a t 90°, 108°, and 120°C. T y p i c a l r e s u l t s a t 108°C [Figs. 1 and 2] show the increase i n carboxylate content o f acrylamide copolymers containing sodium a c r y l a t e and AMPS respectively. The calculated pseudo-unimolecular rate constants (k) f o r the hydrolysis reaction [ F i g . 3 ] , c l e a r l y show the i n h i b i t i n g e f f e c t o f AMPS, r e l a t i v e t o sodium a c r y l a t e a t a l l three temperatures. The t o t a l carboxylate content i n a range o f sodium acrylate copolymers i s shown i n F i g . 4, calculated as the sum o f the i n i t i a l carboxylate and carboxylate formed during hydrolysis. These values may be compared with the t o t a l carboxylate content i n AMPS copolymers [ F i g . 2 ] .

106

OIL-FIELD CHEMISTRY oc

ID

I 2

100 80 0.0 MOLE% A 7.7 MOLE % A 15.9 MOLE % A 24.4 MOLE % A 33.5 MOLE % A 43.0 MOLE % A


cc o z

o CO 5 o 2

10 20 TIME, D A Y S A T 1 0 8 ° C

Figure 1. Hydrolysis of amide in sodium acrylate (A) copolymers at 108 ° C, pH 8.5, 0.025 M amide.

2

> 2

ID 0.0 MOLE% S 7.70 MOLE % S 15.9 M O L E % S 24.4 MOLE % S 33.5 MOLE % S 43.0 MOLE % S 53.3 MOLE % S

CC o z

5

s 10 20 30 TIME, DAYS AT 108PC

40

Figure 2. Hydrolysis of amide in AMPS (S) copolymers at 108 ° C, pH 8.5, 0.025 M amide.

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DEXTER AND RYLES

107

Effect ofAnionic Comonomers


-4

0

10

20

30

40

50

60

Mole % anionic comonomer

Figure 3. Initial rate constants for hydrolysis of acrylamide copolymers at 90, 108, and 120 °C.

10 20 TIME, DAYS AT108C

Figure 4. Total mole percent carboxylate in sodium acrylate (A) copolymers at 108 ° C, pH 8.5, 0.025 M amide.

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OIL-FIELD CHEMISTRY


DISCUSSION The mechanism o f base catalyzed hydrolysis o f e i t h e r homopolyacrylamide o r o f copolymers o f acrylamide and a c r y l i c a c i d has been studied extensively. I t i s well known that the rate o f hydrolysis of amide groups i n such copolymers decreases s i g n i f i c a n t l y as the charge on the polymer i s increased [2,3,5,7]. This phenomenon has been mainly a t t r i b u t e d t o e l e c t r o s t a t i c e f f e c t s , repulsion between charges on the macroion and on the approaching hydroxide anion. I t i s generally believed that s p e c i f i c neighboring group e f f e c t s , i n h i b i t i o n by adjacent pendant carboxyl groups, dominate [7], However, Morowetz [5] has proposed that the t o t a l charge on the polymer does play an important r o l e . The data presented here confirms the work o f these previous authors showing that the rate o f amide group hydrolysis decreases as the l e v e l of a n i o n i c i t y i s increased. This was found to be true for both carboxylated and sulfonated copolymers. However, the rate of amide group hydrolysis i n the AMPS copolymers was found to be further i n h i b i t e d f o r any given l e v e l o f a n i o n i c i t y , e.g., a t 30 mole % a n i o n i c i t y the rate o f hydrolysis o f amide groups i n the AMPS copolymers was found to be ca. 1/2 that o f the corresponding acrylate copolymer a t a l l o f the temperatures studied. Since the t o t a l charge on these copolymers was the same and a l l groups were f u l l y ionized under these reaction conditions, t h i s difference cannot be a t t r i b u t e d to a macroion charge e f f e c t . As f a r as neighboring group i n h i b i t i o n i s concerned, sequence d i s t r i b u t i o n can play an important role, e.g. Morowetz [5] showed that p a r t i a l l y hydrolyzed polyacrylamide hydrolyzes more slowly than an a c r y l i c a c i d copolymer o f the same charge. The former has a more even d i s t r i b u t i o n o f groups leading t o a greater proportion of the l e a s t reactive BAB t r i a d (where A are the acrylamide and B are the a c r y l i c a c i d moieties). Also, Higuchi and Senju [7] have proposed that the o v e r a l l rate constant i s composed o f three d i s t i n c t rate constants corres ponding to the hydrolysis o f the three possible t r i a d configura tions, AAA, AAB, and BAB, and have found that the r e l a t i v e reac t i v i t y i s 1:0.25:0.005. Thus, the o v e r a l l rate i s determined by the r e l a t i v e proportions o f these configurations and a r e l a t i v e composite rate constant K can be derived as follows: KINETIC SCHEME FOR THE HYDROLYSIS OF ACRYLAMIDE COPOLYMERS

n—i—i—i—i—i—i—i—r ©

0

©

0

©

k,

©

©

k,

©

© k

"i—i—i—i—i—i—i—i—r ® ® ® ® ® ® © ® © A B k

1

AMIDE COMONOMER -

0.25 k

-

0.005 k

k [AAA] + 0.25 k [AAB] + 0.005 k [BAB] K =


[AAA]

+

109

Effect of Anionic Comonomers

3. DEXTER AND RYLES

[AAB] +

[BAB]

where k i s the rate f o r AAA hydrolysis

The r e l a t i v e proportions o f t r i a d s i s determined by the synthetic conditions chosen as described above f o r a c r y l i c a c i d copolymers o f acrylamide derived by e i t h e r d i r e c t copolymerization o r by hydrolysis. Also, the polymerization pH has a considerable e f f e c t on the r e a c t i v i t y i n acrylamide/acrylic a c i d copolymerization. Table 2 shows the t r i a d d i s t r i b u t i o n , integrated t o f u l l conversion, f o r 30 mole % anionic copolymers o f acrylamide using the r e a c t i v i t y r a t i o s taken from Ponratnam [11] f o r a c r y l i c a c i d and from McCormick [12] f o r AMPS. From these data (the a c r y l amide/acrylic a c i d copolymer prepared a t pH = 4 i s shown f o r comparison) composite r e l a t i v e rate constants K were obtained assuming equal r e a c t i v i t y f o r the a c r y l i c and AMPS based t r i a d s o f the same sequence. These data show that K f o r the sodium aery l a t e copolymer should be ca. 17% lower than f o r the sodium AMPS copolymer. Since our experimental data show a s i g n i f i c a n t reduction, ca. 50%, f o r the sodium AMPS copolymer, we can only conclude that sequence d i s t r i b u t i o n e f f e c t s on neighboring group i n h i b i t i o n cannot be the dominant c o n t r o l l i n g f a c t o r i n the hydrolysis o f these copolymers. However, the pendant group o f the AMPS monomer does possess a geminal dimethyl group which may associate more strongly with the hydrophobic polymer backbone. Such a c o n f i g u r a t i o n a l arrangement may place the negatively charged sulfonate group i n very close proximity to any neighboring amide group r e s u l t i n g i n increased repulsion o f hydroxide anion. The carboxyl groups i n a c r y l i c a c i d copolymers are bonded d i r e c t l y t o the polymer chain and are therefore, u n l i k e l y t o form associations over several bond lengths. Table 2. Triad Distributions and Composite Rate Constants (K) for 30 Mole % Anionic Acrylamide Copolymers

Monomer B Acrylic Acid Sodium Aerylate AMPS Na

Polym. r r. Triad Distribution pH AAA AAB BAB ABA ABB BBB 4.0 0.57 0.32 0.30 0.28 0.12 0.22 0.07 0.01

0.53k

8.0 0.12 0.63 0.25 0.34 0.11 0.27 0.03 0

0.47k

9.0 0.49 0 98 0.31 0.31 0.08 0.18 0.10 0.02

0.55k

R

K

LITERATURE CITED 1. 2. 3.

P. Davidson and E. Mentzer, SPE 9300, presented a t the 55th Annual Technical Conference, D a l l a s , TX, 1980. R.G. Ryles, SPE 13585, presented a t the I n t e r n a t i o n a l Symposium on Oilfield and Geothermal Chemistry, Phoenix, AZ, April, 1985. A. Moradi-Araghi and P.H. Doe, SPE 13033, presented a t the 59th Annual Technical Conference, Houston, TX, Spet. 1984.

110


4.

11.

OIL-FIELD CHEMISTRY

R.G. Ryles, SPE 12008, presented a t the 58th Annual Technical Conference, San Francisco, CA, 1983. 5. S. Sawant and H. Morowetz, Macromolecules, 17, 2427, (1984). 6. K. Nagase and K. Sakaguchi, J. Polym. Sci. (A), 3, 2475, (1965). 7. M. Higuchi and R. Senju, J. Polym. S c i . , (3), 3, 370, (1972). 8. G. Smets and A.M. Hesbain, J. Polym. S c i . , 40, 217, (1959). 9. J. Moens and G. Smets, J. Polym. S c i . , 23, 931 (1957). 10. S. Mukhopadhyay, B. Ch. Mitra, and S.R. Pailt, Indian J. Chem., 7, 903 (1963). S. Ponratnam and S.L. Kapur, Makromol. Chem., 178, 1029, (1977). 12. C.L. McCormick and G.S. Chen, J. Polym. S c i . , 20, 817, (1982). R E C E I V E D September 7, 1988

Effect of Anionic Comonomers on the Hydrolytic Stability of

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