Oil-Field Chemistry - American Chemical Society

Chapter 4

Downloaded by UNIV OF ALABAMA on September 15, 2016 | http://pubs.acs.org Publication Date: July 10, 1989 | doi: 10.1021/bk-1989-0396.ch004

Predictions of the Evolution with Time of the Viscosity of Acrylamine—Acrylic Acid Copolymer Solutions Houchang Kheradmand and Jeanne Francis Institut Charles Sadron, CRM-EAHP, CNRS-ULP, 6 rue Roussingault, 67083 Strasbourg-Cedex, France

This work deals with an attempt to p r e d i c t the evolution at long term of the thickening properties of acrylamide-acrylic acid copolymer solutions, from t h e i r hydrolysis and degradation k i n e t i c s . A MonteCarlo method i s proposed to simulate hydrolysis process. By introducing at each step of t h i s c a l c u l a t i o n the molecular weight deduced from degradation equations and using semi-empirical laws for the molecular weight and charge density dependences of the i n t r i n s i c v i s c o s i t y , we have obtained some tendencies f o r the v a r i a t i o n s of the thickening power with time, under various conditions of temperature, pH and s a l i n i t y .

One of the main problem encountered when hydrosoluble polymers are used i n chemical t e r t i a r y process of o i l recovery i s the p r e d i c t i o n of the evolution of the thickening properties of t h e i r solutions. In the case of acrylamide-acrylic acide copolymers, such a p r e d i c t i o n requires a good knowledge and understanding on the three following aspects: i ) the hydrolysis of amide groups which leads to the enhancing of the polyelectrolyte character of the polymer. Different experimental works have dealt with the dependence of this k i n e t i c s on pH ,temperature and i n i t i a l polymer composition(l-6). More recently a Monte-Carlo simulation method has been proposed i n order to predict the v a r i a t i o n of the hydrolysis degree under d i f f e r e n t conditions(7-8). i i ) the chemical degradation of the chain which can be due to various mechanims according to the pureness of the samples,the method used for i t s synthesis,the nature of the ions present i n the brine (oxidizing or reducing ions),the oxygen content of the brine and the temperature. 0097--6156/89A)396-Olll$06.00A) o 1989 American Chemical Society Borchardt and Yen; Oil-Field Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1989.


112

OIL-FIELD CHEMISTRY

i i i ) these two chemical processes leading to changes i n the polymer charge and molecular weight respectively are expected to strongly modify the solution v i s c o s i t y . Then the r e l a t i o n between v i s c o s i t y and these two parameters must be known for the given conditions of a p p l i c a t i o n . In recent works, we have studied the k i n e t i c s of both hydrolysis and degradation of a acrylamide-acrylic acid copolymer containing 17% of acrylate groups. The purpose of this paper is to give some predictions of the thickening properties evolution based upon semi-empirical v i s c o s i t y laws. Hydrolysis k i n e t i c s It i s well known that the base hydrolysis of polyacrylamide is catalyzed by OH" ions ( f i r s t order reaction) and obeys autoretarded k i n e t i c s due to the e l e c t r o s t a t i c repulsion between the anionic reagent and the polymeric substrate(3-5). In the range of slightly acid pH (3 < pH < 5), Smets and Hesbain(6) have demonstrated a mechanism of intramolecular c a t a l y s i s by undissociated neighbouring carboxylate groups analogous to that observed i n low molecular weight compounds such as phtalimic acid(10). By assuming that Hydrolysis simply results from these two mechanisms no reaction was expected i n the range of pH near n e u t r a l i t y . However, Muller(2) has shown that the modification of already p a r t i a l l y hydrolyzed polyacrylamide cannot be neglected i f one considers reaction times of several months. A more recent systematical study(7,8) of the reaction at different pH ,temperatures and i n i t i a l carboxylate contents led us to propose the simple following model , f o r 3 < pH < 9. We have considered two types of reacting monomer units: the units which have an undissociated neighbouring group which catalyses the reaction with a rate constant 1^ independent on pH (units X) - the other units (units Y) whose hydrolysis rate k i s not simply proportional to (OH") concentration (as i n the range of high pH) but varies with pH according to an empirical r u l e : a

log k = log k a

2

aQ

+ pH ( C - C a r - C ( a r) ) A

fi

(1)

c

where r i s the f r a c t i o n of carboxylate groups i n the polymer, a is t h e i r i o n i z a t i o n degree and C , C , C are constants. This expression corresponds to the following experimental observations: i ) for the unhydrolyzed polyacrylamide ,only units of type Y must be considered i n the i n i t i a l step of the reaction and intramolecular c a t a l y s i s has not to be taken into account. In r e l a t i o n (1) , we have obtained a rate constant k varying as : A

B

C

fl

log k - log k a

a0

+ C pH

(2)

A

This shows that pH increase favors hydrolysis reaction of Y u n i t s . i i ) for p a r t i a l l y hydrolyzed polymers ( or polyacrylamide i n a second step), the increase of k with pH i s lowered when the charge density (a r) increases (see fig.10 of r e f . 8 ). This retardating effect i s expressed by the terms (C a r) and (C ( a r ) ) . a

2

B

c

Borchardt and Yen; Oil-Field Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

4.

KHERADMAND AND FRANCOIS

N,

Acrylamine-Acrylic Acid Solutions

At each reaction time , and for a the number of units X i s :

113

given polymerization degree

where i s the number of diades AB ( acrylamide :A; a c r y l i c acid :B). The number of Y units i s N-N . Then the modelization of the hydrolysis k i n e t i c s requires at each time the knowledge of a and N ^ . a can be calculated by w r i t i n g the d i f f e r e n t relations of d i s s o c i a t i o n e q u i l i b r i a of water,polyacid and NH (produced by the hydrolysis reaction). We have proposed to determine at each reaction step and simulate the whole kinetics by using a Monte-Carlo method .(see r e f . 8 ). In Figure 1 we compare the calculated and measured variations of r for a copolymer of i n i t i a l r — 17%. Let us remark that in these experiments, the pH has not been adjusted at a constant value and the hydrolysis process induces a change of pH i n the solution. We have taken into account this effect i n our c a l c u l a t i o n s . In fig.2, we give the predicted hydrolysis k i n e t i c s for the same polymer sample but i n the case where pH remains constant. Degradation k i n e t i c s We have previously performed a systematical study of the degradation of a acrylamide-acrylic a c i d copolymer c a l l e d sample C( of acrylate content r 17% and molecular weight M = 6*10 ) prepared by photocopolymerization by using benzyl methyl k e t a l as catalyst(11). The observed behaviors have been compared with those of a sample obtained by base hydrolysis of polyacrylamide c a l l e d sample H ( approximately same M and r — 30%), i n some p a r t i c u l a r cases. In the p r a c t i c a l a p p l i c a t i o n , the chemical s t a b i l i t y of these polymers at high temperature w i l l e s s e n t i a l l y depend on the content of oxygen and ions of t r a n s i t i o n metals i n the b r i n e . It i s the reason why we have investigated t h e i r behaviors for several months ( at l e a s t 3) ,at 80°C and under three main conditions (see f i g . 3 ) . -without oxygen and s a l t s of t r a n s i t i o n metals Under such conditions , sample C i s not degradated ,M measured by l i g h t scattering remaining constant for 6 months. A s l i g h t increase of reduced v i s c o s i t y (*7 ) w i l l be l a t e r explained by the hydrolysis of amide groups. - i n the presence of oxygen and without s a l t s of t r a n s i t i o n metals After 3 months of ageing, » j is reduced by a factor 6 while M i s 10 times lower than i n i t i a l M . We have observed that the x


3

6

w

w

red

red

/ n x

w(0)

w

degradation follows a l i n e a r expression* *:

random process

of l i n k breaking since the

12,13

In

( 1 - M

w(0)

)

- in< 1 - M

w ( t )

) - k t

(4)

has been obtained. Moreover, the degradation leads to the formation of shorter l i n e a r chains and at each time the i n t r i n s i c v i s c o s i t y (77) can be obtained from r e l a t i o n (4) and the c l a s s i c a l Mark-Houwink law: (5)


114


OIL-FIELD CHEMISTRY

Figure 2.

Calculated hydrolysis k i n e t i c s f o r Copolymer C at 80°C at d i f f e r e n t constant pH






OIL-FIELD CHEMISTRY

116

Under these conditions, the degradation k i n e t i c s of sample H i s similar,with a d i f f e r e n t value of k i n r e l a t i o n (4). However, the o r i g i n of the phenomenon i s d i f f e r e n t : for sample H, the degradation i s due to the decomposition of chain hydroperoxides by traces of F e . The sample C does not contain peroxides nor F e and we have explained i t s i n s t a b i l i t y by the presence of catalyst residues. The p u r i f i c a t i o n by p r e c i p i t a t i o n i n methanol of both samples allows to obtain a very good s t a b i l i t y for 6 months. - i n absence of oxygen and with s a l t s of t r a n s i t i o n metals In f i g . 3, we compare the evolutions of i y of free oxygen solutions of copolymer C with and without 5 ppm of Fe . As confirmed by l i g h t scattering measurements, the presence of ferrous ions induce a quasi instantaneous degradation of this polymer followed by a much slower reaction. The p u r i f i e d sample C i s not degradated under such conditions while degradation i s measured before and after p u r i f i c a t i o n of sample H. This results confirms the difference of degradation mechanism according to the method of preparation of the polymer. Oxidizing ions ( Cu or F e ) as well as reductants ions ( Cu or Fe ) induce the degradation of these polymers. Dependence of the v i s c o s i t y on molecular weight and charge density of the polymer I f the polymer concentration c i s lower than c , the c r i t i c a l concentration of chain overlapping , one can express the v i s c o s i t y r\ of the solution by: m

m

red


++

++

+

+++

++

V = % + % (•») c + p

2

K' „„ ( „ ) c

2

(6)

p

where rj i s the solvent v i s c o s i t y K'is the Huggins constant which varies with the thermodynamical quality of the solvent ( s a l i n i t y and temperature) For the c a l c u l a t i o n of rj of a solution of copolymer of given r ,a and M under given conditions of s a l i n i t y and temperature , one must know the v a r i a t i o n laws of (»j) and K' with these parameters. - i n t r i n s i c v i s c o s i t y (»?) In the c l a s s i c a l theories of polyelectrolytes , the chain expansion i s characterized by the e l e c t r o s t a t i c excluded volume parameter , z with: Q

w

el

z . el

i

2

M

1 / 2

/ c

w

'

(7) N

s

'

where i i s the i o n i z a t i o n degree and c the concentration of added electrolyte(14). A dependence of the i n t r i n s i c v i s c o s i t y with l/ i s then predicted while experiments show that (77) varies as l / c ' Fixmann et Al.(15) have proposed an expression which gives a better account for experimental data (( a ' - l ) varying as l / c , a' being the chain expansion). Nevertheless, i n spite of the numerous expressions proposed, they generally do not take into account the strong interactions between counter ions and polyions for the high values of the charge parameter f c

s

1 / 2

s

3

1 / 2

s

£ = e where e Boltzamm

2

/ D k T b

(8)

i s the proton charge, D the d i e l e c t r i c constant, k the constant, T the absolute temperature and b the average


4. KHERADMAND AND FRANCOIS


117

distance between two charged groups along the chain, varying as 1/ar. The Manning theory(16,17) predicts a c r i t i c a l value f above which coun-erions are condensated on poly ion. In the case of monovalent ions , £ = 1 and for £ > £ , the chain expansion predicted by the c l a s s s i c a l theories i s overestimated i f the i o n i z a t i o n i i s not corrected by a f i x a t i o n term. Koblansky et A l have shown(18) that ion condensation occurs for values of £ much lower than £ i n the case of p a r t i a l l y hydrolyzed polyacrylamide. They have found for Na that the a c t i v i t y c o e f f i c i e n t 7 does not vary with £ as predicted by the Manning theory (16,17) but according to a empirical law: c

c


+

7 - 0.96 - 0.42 £

1 / 2

(9)

i n the absence of simple e l e c t r o l y t e s . Kowblansky et A l (19) have also measured the i n t r i n s i c v i s c o s i t y of hydrolyzed polyacrylamide of a given molecular weight as a function of r for a — 1 and their results can be f i t t e d by the expression: (ij) = (r ) ?

Q

+ k * £ * 7 * M s

(10)

w

where (r)) i s the i n t r i n s i c v i s c o s i t y of the uncharged polymer Q

^>0(25) " and k

g

9

'

3

*

1

"

3

*

M

w°"

7

5

1

(^V )

a

t

2 5

°

C

( 1 9 >

U

< >

i s a c o e f f i c i e n t which depends on the s a l i n i t y : k

c

0

s

- 4.7*10"7c

1/2 s

- 1.33*10"*

(12)

i s the molar concentration of monovalent s a l t ( NaCl) Such empirical expressions have been also v e r i f i e d by Kulkarni et Al.(21) and Kheradmand(7) but they have been established for room temperature. In fact, we have to know the temperature dependence of the two terms of r e l a t i o n (10): -For the f i r s t non e l e c t r o s t a t i c term, such a dependence can be calculated from the c l a s s i c a l Flory theory and the value of the theta temperature of unhydrolyzed polyacrylamide ( Q = 265°K (22)) g

O7 , an increase of v i s c o s i t y can be expected but the form of the curves changes with pH: at high pH , the i n i t i a l hydrolysis rate i s higher than at pH 7 and this leads to a higher i n i t i a l increase of v i s c o s i t y . But i n the second step of the hydrolysis, the k i n e t i c s i s strongly autoretarded at ph>9 and the slow v a r i a t i o n of r corresponds to a s l i g h t increase of v i s c o s i t y while at pH 7 , complete hydrolysis can be reached and a higher v i s c o s i t y is expected for a long aging time. For pH 2. 5*10" M / l and this l i m i t i s lower for higher i n i t i a l values of r . I f CaCl < 5 10" M / l , no p r e c i p i t a t i o n w i l l occur even i f r reachs the value 1. For an intermediate concentration of CaCl , for instance 10~ M / l , phase separation can be expected after 20 months and 3.5 months at 60°C and 80°C respectively. -when degradation occurs i ) i n the presence of oxygen In this case, we must take into account the v a r i a t i o n of M through r e l a t i o n (4). An example of results i s given i n Figure 8. It is obvious that the degradation is the main phenomenon and a high loss of v i s c o s i t y can be predicted. Since hydrolysis has an increasing effect on the v i s c o s i t y , i t was i n t e r e s t i n g to determine the M v a r i a t i o n which could lead to a constant value of v i s c o s i t y (see Figure 5b). We can observe that M must remain higher than 3.7*10 i i ) i n absence of oxygen and with ions of t r a n s i t i o n metals In this case, the very fast degradation induces the high i n i t i a l loss of v i s c o s i t y as already measured and predictions for a long time do not present interest. Conclusion In this work, we propose a method based on different combined semiempirical laws to predict the evolution of the v i s c o s i t y of a solution of acrylamide - a c r y l i c acid copolymer. In f a c t , i t appears


w

2+

2

2

3

2

2

w

w

6



4.



Figure 6. Predictions of v i s c o s i t y evolution f o r Copolymer C without degradation at 80°C and d i f f e r e n t pH; ( — c =0N; ( ) c = 0.1N ;( ) c - 0.5N


121

)

122

OIL-FIELD CHEMISTRY

(CP)


30

TIME (MONTHS)

10

30

25

20

15

Figure 7. Predictions of v i s c o s i t y evolution for copolymer C at pH 7, 0.1 NAC1 at different temperatures

p I

I^(CP)

6-

0

2

6

M * 10"

B

1

2

3

4

5

6

TIME (MONTHS)

3

4

TIME (MONTHS)

Figure 8. a:Predictions of v i s c o s i t y evolution for Copolymer C with degradation i n the presence of oxygen at 80°C and pH7 ; ( ) c. = 0.05N ; ( ) c 0.1N b : calculated decrease of M able to compensate hydrolysis effect g

w


4.



that i n absence of degradation, the behaviors must be rather independant on the o r i g i n of the polymer and only depend on i t s i n i t i a l properties and on the pH , s a l i n i t y and temperature of the brine. When degradation occurs, it is generally due to impurities, then the k i n e t i c s is not universal and must be the object of a s p e c i f i c study for each sample.


This work has benefited from grants from Compagnie Francaise des Petroles. The authors are indebted for f r u i t f u l discussions to Doctor V.Plazanet.

Literature Cited 1. Muller G.; Fenyo J.C.; Selegny E.J. J.Appl. Polym. Sci. 1980, 25 627 2. Muller G. Polymer Bulletin 1981, 5 , 31 3. Higuchi M.; Senju R. Polymer 1972, 3, 370 4. Sawant S.; Morawetz H. Macromolecules,1984,17, 2427 5. Truong N.D.;,Galin J.C.; François J.; Pham Q.T. Polymer, 1986, 27, 459 6. Smets G.; Hesbain A.M., J.Polym.Sci. 1959, 23, 217 7. Kheradmand H. thesis L.Pasteur University Strasbourg 1987 8. Kheradmand H.; François J.; Plazanet V. Polymer 1988, 29, 860 9. Kheradmand H.; François J.; Plazanet V. J.Appl.Polym.Sci under press 10. Bender M.L.; Chow Y.L.; Chloupek F. J.Amer.Chem.Soc. 1958, 80, 5380 11. Boutin J.; Contat S. French Patent N 249 217 issued to Rhone Poulenc Ind. 12. Jellinek H.H.G.; Degradation of Vinyl Polymers Academic press, New-york 1955 13. Vink H. Makromol.Chem. 1963, 67, 105 14. Yamakawa H. Modern theory of Polymer Solutions Harper and Row New-York 1971 15. Fixman M.; Solnick J. Macromolecules, 1978, 11, 863 16. Manning G. J.Chem.Phys. 1969, 51, 924 17. Manning G. Acc.Chem.Res. 1979, 12, 442 18. Kowblansky M.; Zema P. Macromolecules, 1981, 14, 166 19. Kowblansky M.; Zema P. Macromolecules, 1981, 14, 1451 20. François J.; Sarazin D.; Schwartz T.; Weill G.Polymer, 1979 20, 969 21. Kulkarni R.A.; Gundiah S. Makromol.Chem., 1984, 185, 957 22. Kanda A.; Sarazin D.; Duval M.; François J . Polymer, 26. 406 23. Ikegami A.; Imai N. J.Polym.Sci., 1962, 56, 133 24. Truong N.D.; Galin J.C.; François J.; Pham Q.T. Polymer Communications. 1984, 25, 208 25. Truong D.N.; FRançois J. in Solid-Liquid Interactions in Porous Media. Ed.Technip, Paris , 1982, p.251 26. Medjadhdi G.; Sarazin D.; François J . Unpublished results 27. Muller G.; Kohler N. in 2nd European Symposium on Enhanced Oil Recovery. Ed. Technip, Paris, 1982 p. 87 RECEIVED

February 2, 1989


123

Oil-Field Chemistry - American Chemical Society

Recommend Documents