Influence of Calcium on Adsorption Properties of Enhanced Oil

Chapter 11

Influence of Calcium on Adsorption Properties of Enhanced Oil Recovery Polymers

Downloaded by UNIV OF BATH on October 28, 2014 | http://pubs.acs.org Publication Date: July 10, 1989 | doi: 10.1021/bk-1989-0396.ch011

L. T. Lee, J. Lecourtier, and G. Chauveteau Institut Français du Pétrole, B.P. 311, 92506 Rueil-Malmaison, Cedex, France

The influence of calcium on the adsorption of high molecular weight EOR polymers such as flexible polyacrylamides and semi-rigid xanthans on siliceous minerals and kaolinite has been studied in the presence of different sodium concentrations. Three mechanisms explain the increase in polyacrylamide adsorption upon addition of calcium: (i) reduction in electrostatic repulsion by charge screening, (ii) specific interaction of calcium with polymer in solution, decreasig its charge and affinity for solvent, and (iii) fixation of calcium on the mineral surface, reducing surface charge and creating new adsorption sites for the polymer. The intrinsic viscosities of polyacrylamide solutions are significantly lowered in the presence of calcium, and the increase in Huggins constant at high calcium concentrations suggests attractive polymer-polymer interactions. The effects of calcium on polymer-solvent and polymer-surface interactions are dependent on polymer ionicity; a maximum intrinsic viscosity and a minimum adsorption density as a function of polymer ionicity are obtained. For xanthan, on the other hand, no influence of specific polymer-calcium interaction is detected either on solution or on adsorption properties, and the increase in adsorption due to calcium addition is mainly due to reduction in electrostatic repulsion. The maximum adsorption density of xanthan is also found to be independent of the nature of the adsorbent surface, and the value is close to that calculated for a closely-packed monolayer of aligned molecules. A controlling factor in the success of polymer flooding in enhanced oil recovery (EOR) is the level of polymer adsorption on reservoir rocks. Adsorption depletes polymer from the mobility control slug leading to delayed oil recovery and too high a level of adsorption renders the EOR process uneconomical. Although there has been extensive research in the field of polymer adsorption, a comprehensive study of adsorption of high molecular weight EOR polymers under imposed field conditions has been few (1-10). One of the commonly encountered conditions is the presence of high levels of monovalent and even multivalent ions which can interact with both polymers and solid surfaces, hence complicating further the understanding of the adsorption mechanism. In our previous studies on the 0097-6156/89/0396-0224$06.00/0 o 1989 American Chemical Society In Oil-Field Chemistry; Borchardt, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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influence of pH and monovalent ions on adsorption of polyacrylamides and xanthan (9, 10), it has been shown that adsorption of these polymers is mainly governed by a competition of attractive H-bonding and repulsive electrostatic interactions, and that monovalent ions increase adsorption by screening polymer and surface charges and thus reducing electrostatic repulsion. This study aims at determining the effects of calcium on the adsorption of polyacrylamides and xanthans on siliceous minerals and kaolinite.


Polymers The polyacrylamides are homopolymer (PAM) and copolymers (HPAM) of acrylamide and acrylate varying from 0 to 50% acrylate content as determined by potentiometric titrations. The average molecular weight for all the samples measured by low angle light scattering is about 8x10 daltons. Solutions are prepared by gentle stirring using a magnetic stirrer in de-ionized water containing the required salts and 400 ppm of NaNo as stabilizer. The xanthans (XCPS L XCPS II) are fully pyruvated samples with mean molecular weights of 1.8x10 and 4.2x10 daltons respectively (11, 12). Solutions are prepared by diluting a fermentation broth. Any possible existing microgels are removed by a filtration method described elsewhere (13), and low molecular weight impurities are eliminated by ultrafiltration. Minerals Siliceous minerals carrying surface silanols (sand and silicon carbide (SiC)) and kaolinite carrying silanols and aluminols are used in this study. The sand is from Entraigues, France. The particle size ranges from 80 to 120 um and the average specific surface area is ~ 0.1 m /g. It is washed in 1 M H C l before use. The SiC has a particle size of 18 um and a specific surface area of 0.4 m /g. The surface of SiC after heat treatment at 300°C and acid washing is oxidized and resembles that of silica (14). The kaolinite is from Charentes (France) and has undergone ion exchange with Na to obtain a homoionic Na-kaolinite. TJie particle size ranges from 0.2 to 0.8 um and the total specific surface area is 20 m /g (15 and 5 m /g for basal and lateral surface respectively (15)). Adsorption Measurements Adsorption is determined by the depletion method using a Dohrmann D C 80 carbon analyzer. The mineral is contacted with the polymer solution and agitated with a mechanical tumbler for 24 hours, a time which has been verified to be sufficient for adsorption to be complete (9). A more detailed description of experimental procedures is given elsewhere (10). A l l the data reported in this study are taken in the plateau region of the adsorption isotherm. Polymer Solution Properties The properties of polymers in solution are investigated in conjunction with adsorption since solution properties are dependent on polymer-solvent interactions

In Oil-Field Chemistry; Borchardt, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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which affect the polymer behavior at the interface. The reduced specific viscosities of HPA M (ionicity = 30%) in 2 g/1 NaCl and various concentrations of C a C l are plotted as a function of polymer concentration in Figure 1. For a flexible polyelectrolyte, an increase in salt concentration significantly reduces the intrinsic viscosity, fn], due to screening of charged groups on the polymer and thus decreasing the electrostatic persistence length. This is observed upon increasing NaCl from 2g/l (solid line) to 20g/L (dotted line) in the absence of C a C l where [n] decreases from 12700 to 4200 cm /g respectively. For HPA M in the presence of monovalent ions, the [-n] has been found to be proportional to c " , where c is the monovalent salt concentration (9, 10). In the presence of C a C l (2, 5 and 10 g/1) at 2g/l NaCl, the [T|] of HPA M decreases even more significantly than expected from ionic strength effect due to specific interactions of the divalent ions with the polymer. Such interactions of HPA M with divalent ions which have also been studied elsewhere (16) using other techniques such as conductivity, densimetry and light scattering alter not only the charge and dimension of the macromolecule but also inter-molecular interactions. The latter is characterized by the Huggins constant, k'(17), which is deduced from the slope of the reduced specific viscosity versus concentration curve. In Figure 2 are [r\] and k' values (in parentheses) of HPA M (ionicity = 30%) measured in 20g/l NaCl as a function of CaCl? concentration. In the absence of Ca , k' is slightly less than 0.4, a value which corresponds to the theoretical value for free-draining, strictly repulsive nondeformable molecules (17). Upon addition of Ca , this value increases and reaches 1 at 20g/l CaCl^, and remains at around the same value at 40g/l C a C l . Since the intrinsic viscosities do not decrease significantly in this calcium concentration range and remain at values relatively high compared to 9 conditions ([n] ~ 500 cm /g), this high value of k' indicates a tendency for inter-molecular attraction of the macromolecules. For xanthan (XCPS I), even though its degree of ionicity is higher than that of HPA M , the [T|] does not appear to be as sensitive to Ca (Figure 3). The presence of 2 and 20§Jl C a C l produces the same slight effect of reducing the [r\] from 4300 to 3400 cm /g, the limiting value at high salinity (11). This is due to the high rigidity of xanthan and thus its large structural persistence length (11, 12) which limits the change in molecular conformation and which renders relatively weak the effects of electrostatic repulsions between charged groups on the polymer chain. The k' in this case remains at 0.4 even up to 20g/l CaCl , showing the absence of attractive polymer-polymer interactions. 2

2

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2

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Polymer Adsorption Properties Polyacrylamides Adsorption on Siliceous Minerals. The adsorption of polyacrylamides on siliceous minerals in the presence of monovalent ions has been discussed previously (9, 10). While P A M adsorption is unaffected by monovalent ions since it is not governed by electrostatic factors, HPA M adsorption is increased due to reduction in electrostatic repulsion by charge screening. In the presence of divalent ions, apart from charge screening, adsorption can be modified due to additional factors arising from specific interactions of the divalent ions with the polymer and the surface. In this case, adsorption of calcium on the S i 0 surface (15, 18) not only reduces the surface charge but can also 2




LEE ET AL.


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HPAM

(k' = 0.39)

2 0 g / l NaCl


pH = 7

(k = 1.05) Jk'=1.01)

1000

20

25 CaCl (g/l) 2

Figure2. Effect of calcium on intrinsic viscosity and Huggins constant of HPAM.

XCPS I 8000

5 g / l NaCl pH = 6 . 5 , T = 3 0 ° C

(k' = 0.4)

(k'=0.4)

100

300 200 POLYMER CONCENTRATION (ppm)

400

Figure 3. Effect of calcium on zero shear rate reduced specific viscosity of XCPS.


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activate polymer adsorption on otherwise non-adsorbent sites (dissociated silanols). ITie effects of calcium on P A M and H P A M (ionicity = 30%) adsorption at pH 7 in 20g/l NaCl on sand and SiC are shown in Figures 4 and 5 respectively. Although the adsorption of P A M is not governed by electrostatic factors and the [n] remains unchanged with Ca (see Figure 8) the presence of Ca nevertheless provokes a slight but definite increase in its adsorption. This is attributed to the specific interaction of tte nonionic polymer with the adsorbed Ca . Indeed, weak interaction of Ca with P A M has been detected by U V spectroscopy (19). The enhancement in adsorption of P A M by Ca is also observed for SiC (Figure 5). Since oxidized SiC surface is similar to S i 0 surface with a different charge density, Ca can be expected to adsorb also on the SiC surface. The adsorption of H P A M on sand (Figure 4) is not detected below a threshold value of Ca due to strong electrostatic repulsion between the polyelectrolyte and the highly charged negative surface. This threshold value, which was also observed in the case of monovalent ions (9), represents the point where the critical adsorption energy is overcome, and once this value is surpassed, adsorption increases sharply. This form of adsorption behavior is in line with predictions of theories on polyelectrolyte adsorption (20). Mechanistically, calcium increases H P A M adsorption by (i) screening of polymer and surface charges thus reducing electrostatic repulsion, (ii) specific interaction with the polymer in solution decreasing the polymer charge and intrinsic viscosity as shown from the results above, and changing also the polymer affinity for solvent, and, (iii) fixation on the mineral surface serving as a bridge between the negative surface site and polymer. From 1 to 8g/l CaCl , adsorption increases due to the reasons stated above. Between 8 and 15g/l CaCl , adsorption reaches a first plateau where the adsorption density is only about half the value obtained for nonionic P A M ; this shows that there is a residual repulsion between the free H P A M and the partially polymer covered sand surface. With further increase in CaCU beyond 15g/l however, adsorption increases until it reaches a second plateau which coincides with the maximum adsorption level for P A M . Interestingly, the region beyond 15g/l C a C l corresponds to the region where the k' increases significantly (see Figure 2). Hence, the additional force of adsorption beyond the first plateau can be related to the occurrence of slight attractive polymer-polymer interactions. An alternative interpretation of the two plateaus observed in the adsorption curve is the existence of two different types of adsorption sites with different reactivities towards the polymer. Unlike the sand used above, the adsorption of H P A M on SiC at 20g/l NaCl is significant even in the absence of Ca (Figure 5). This is mainly due to the lower charge density of SiC, hence the weaker electrostatic repulsion. The higher affinity of H P A M for SiC may also explain the attainment of maximum adsorption at lower Ca level, and may also be the reason that the higher interaction of H P A M with can induce an adsorption level higher than that of P A M . +

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Adsorption on Kaolinite. For kaolinite, the polymer adsorption density is strongly dependent on the solidVliquid ratio, S/L, of the clay suspension. As S/L increases, adsorption decreases. This S/L dependence cannot be due totally to autocoagulation of the clay particles since this dependence is observed even in the absence of Ca at pH 7 and at low ionic strength where auto-coagulation as measured by the Bingham yield stress is relatively weak (21). Furthermore, complete dispersion of the particles in solvent by ultra-sonication before addition of +


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SAND 800

20g/l NaCl pH = 7

700

^600 >CO 500

z UJ

Q


Q.

O 300 CO Q < 200

100

10

15

20

25 CaCI (g/l)

30

35

40

45

2

Figure 4. Influence of calcium on adsorption of P A M and H P A M on sand.

Figure 5. Influence of calcium on adsorption of P A M and H P A M on SiC.


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polymer does not eliminate such dependence. The dependence of polymer adsorption on S/L is therefore attributed mainly to the flocculation of clay particles by the adsorbed polymers, a process which is favored at high S/L due to reduced inter-particle distance, and which renders some of the adsorption surface inaccessible. In the presence of calcium however, the effect of coagulation by Ca is expected to play an important role. Therefore, in order to obtain a representative level of adsorption density on clay particles, the adsorption of polymers on kaolinite throughout this study is conducted as a function of S/L and the results extrapolated to S/L=0. In the absence of Ca , it has been claimed that adsorption of P A M on kaolinite takes place only on the lateral surface while the basal surface is nonadsorbent (22). However, even though adsorption may take place predominantly on the edge surface due to the presence of the more reactive aluminols, this does not exclude some adsorption on the basal surface. In fact, some recent studies which are in progress have indicated the adsorption of P A M on the kaolinite basal surface (23). For H P A M at low salinities, however, adsorption takes place only on the more reactive aluminols of the edge surface (23). The adsorption of P A M and H P A M on kaolinite (Figure 6) is increased in the presence of Ca due to the reasons stated above for sand and SiC. The fixation of Ca on kaolinite is verified experimentally and the results plotted in the same figure.


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Effect of Monovalent to Divalent Ratio. The above mentioned interactions of Ca with the polyelectrolyte and with the negatively charged surface are electrostatic in nature, which means that an increase in ionic strength by monovalent ions should decrease such interactions. This is shown for the case of SiC at 2 and 20g/l NaCl at pH 7 (Figure 7). In the absence of Ca , adsorption of H P A M is lower at 2g/l than at 20 g/1 NaCl, but in the presence of Ca , the higher interactions of the divalent ion with the negative surface and polyelectrolyte at lower NaCl concentration result in a Wgher adsorption. Moreover, the adsorption level at high calcium content (750 ug/m ) is higher than the maximum level obtained in the presence of NaCl alone (500 ug/m ) (10). This clearly demonstrates that specific interactions of calcium with polymer and surface are significant factors in enhancing H P A M adsorption. Effect of Polymer Ionicity. The influence of polymer ionicity on solution and adsorption properties is investigated for polyacrylamides varying from 0-50% ionicity in the absence and presence of calcium. The intrinsic viscosities of the polyacrylgmides as a function of polymer ionicity in 2 g/1 NaCl and at various levels of Ca are shown in Figure 8. In NaCl solution, [T]] increases continuously with polymer charge due to tht increase in electrostatic persistence length of the polymer. In the presence of Ca , the [n] is reduced for every polymer but at low ionicity and low Ca content, the increase in [n] with charge is maintained. At higher linear charge density however, a stronger interaction of the polymer with Ca significantly reduces the polymer charge and its solubility, resulting in a larger decrease in fn] and even precipitation for hydrolysis degrees greater than 30% when more than 2g/l C a C l is added. The [n] thus exhibits a maximum as a function of ionicity. The influence of Ca on the variations of adsorption with the degree of ionicity of H P A M on sand at pH 7 in 2 g/1 NaCl is given in Figure 9. Without calcium, adsorption decreases with polymer charge because of the increase in +

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Na - Kaolinite


2 0 g / l NaCl

CaCI (g/l) 2

Figure 6. Adsorption of calcium and its influence on adsorption of P A M and H P A M on Na-kaolinite.

SiC/HPAM

CaCI (g/l) 2

Figure 7. Effect of NaCl on adsorption of HPAM on SiC in the presence of calcium. (Reproduced with permission from ref. 26. Copyright 1988 Institut Francais du Petrole.)


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Figure 8. Intrinsic viscosity of HPAM versus ionicity in the presence of calcium. (Reproduced with permission from ref. 26. Copyright 1988 Institut Francais du Petrole.)


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electrostatic repulsion between the polymer and the surface. The addition of Ca increases the adsorption of each polymer, but the decrease in adsorption with polymer charge is preserved until the degree of ionicity exceeds 30%, at which point the high interaction of polyelectrolyte with divalent ion reduces the solubility of the polymer, and increases its affinity for the surface. Hence, the adsorption of H P A M with ionicity exhibits a minimum. For SiC, on which adsorption is higher, this phenomenon is more pronounced (Figure 10) and a slight adsorption increase is detected even in the absence of Ca . This is attributed to a decrease in solubility of the H P A M at high acrylate content (24). Such a minimum is enhanced in the presence of calcium due to the lower solubility of calcium acrylate. As a consequence, the minimum is shifted to a lower value of ionicity. In the case of kaolinite (Figure 11) where polymer adsorbs strongly and predominantly on the edge surface (see above discussion), the reduction in adsorption with ionicity shows that the overall adsorption is nevertheless governed by the net charge of the clay. Interestingly, an increased adsorption at high ionicity is not observed. A possible explanation is that the nature of interaction of polymer and clay surface is of higher affinity than that of polymer-siliceous surface due to the presence of more reactive aluminols (22). As such, the effect of change in polymer activity due to change in ionicity is suppressed by the more prominent polymer surface interaction.


+

Xanthans Adsorption on Siliceous Minerals. A l l adsorption studies of xanthan (XCPS) in the presence of calcium are conducted at pH 6.5 to avoid precipitation which has been reported at pH>7 for xanthan solutions containing calcium (25). The adsorption results of both xanthan samples on sand at pH 6.5 in 20g/l NaCl are shown in Figure 12. The presence of calcium is seen to increase the adsorption of both xanthan samples and a maximum in adsorption is reached at high calcium concentrations. These variations are very similar to those observed for XCPS adsorption in the presence of NaCl (26). Xanthan adsorption is governed by a competition of electrostatic repulsions and attractive H-bonding forces between polymer and surface. For XCPS II, at very low ionic strength, electrostatic repulsion dominates and thus adsorption is detected only after a threshold value of calcium. The maximum adsorption level is attained at a higher Ca concentration, and the maximum adsorption of XCPS II exceeds that of XCPS I. This may be an effect of differences in polymer molecular weight and/or structure which can be detected on sand surface due to its heterogeneity in adsorption site density which permits only partial surface coverage by XCPS I. For SiC, which has a more homogeneous adsorption site density, such a difference in maximum adsorption densities for the two samples is insignificant (Figure 13). In comparison to H P A M where Ca increases adsorption by both charge screening and specific interaction with polymer and surface, XCPS adsorption seems to be increased by Ca by charge screening only. This is deduced from solution and adsorption properties of the xanthan. Firstly, in solution, even though not much information on the degree of calcium-xanthan interaction can be derived from intrinsic viscosity data due to the structural rigidity of XCPS, studies on XCPS solution stability in the presence of calcium have shown that at pH

Influence of Calcium on Adsorption Properties of Enhanced Oil

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