Nanoparticles for Enhanced Oil Recovery: Influence of pH on

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Nanoparticles for Enhanced Oil Recovery: Influence of pH on Aluminum-Cross-linked Partially Hydrolyzed PolyacrylamideInvestigation by Rheology and NMR Abduljelil Sultan Kedir,*,†,‡ John Georg Seland,‡ Arne Skauge,† and Tormod Skauge† †

Centre for Integrated Petroleum Research (CIPR), Uni Research, Realfagbygget, Allégaten 41, N-5007 Bergen, Norway Department of Chemistry, University of Bergen, Realfagbygget, Allégaten 41, N-5007 Bergen, Norway

‡

S Supporting Information *

ABSTRACT: New methods are continuously being evaluated for Enhanced Oil Recovery (EOR). Nanoparticle flooding is an intriguing new approach in which one of the main applications is microscopic diversion. Linked polymer solution (LPS) is a nanoparticle system that consists of partially hydrolyzed polyacrylamide (HPAM) cross-linked by aluminum(III). The source of aluminum is an aluminum citrate (AlCit) complex, where citrate serves as a carrier ligand. The large size and flexibility of HPAM as well as the multicomponent species of AlCit present at most relevant reservoir conditions make the system highly complex and challenging to characterize. In the literature, there is a lack of systematic and consistent data describing the various chemical species involved in LPS. This work used nuclear magnetic resonance (NMR) spectroscopy and dynamic rheology to investigate the reaction between HPAM and Al3+ as a function of solution pH. The results of NMR spectra indicate that AlCit is more reactive at near neutral pH and cross-linking reactions should preferably be performed at this condition. Structural conformation and viscoelastic properties of the HPAM-Al complex appear to be dominated by the protonation−deprotonation state of the carboxylic acid groups as result of pH changes.

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INTRODUCTION Nanoparticles have received an increased interest in the oil and gas industry in recent years. Their use has been proposed for stabilization of CO2-foams,1 as nanofluids for wettability change,2,3 as nanosensors for well applications,4,5 and as flooding agents.6 We have studied nanoparticles in the form of cross-linked polyacrylamide polymers7−10 and well-defined silica particles.11 The polymer particle system has been called Linked Polymer Solution (LPS) and applied as a flooding agent in onshore fields in China.6 Field experience from onshore fields showed good results,12−14 and laboratory studies also revealed from 19 to 61% increase in recovery.10 LPS is a class of macromolecules, generated by cross-linking of high molecular weight of HPAM with aluminum citrate (AlCit) in the semidilute or dilute concentration regime at saline condition.15 The interacting species exhibit a multitude of conformations and complexes in solution and may form primarily intra- or intermolecular cross-links depending on concentration ratios, pH, salinity, and temperature16 and makes it a highly complex system to study. Hence, both laboratory studies and field implementation are hampered by the lack of consistent tools for LPS analysis and quality control. The pH change was expected to have a significant impact on LPS properties as it is a polyelectrolyte with the possibility of attaining several conformations. This study was therefore initiated to serve several purposes. The first purpose was to evaluate if conformational changes at low or high pH might improve LPS properties. Second, the sensitivity of LPS properties to changes in pH was important to characterize. Third, for core floods, field injections and back production analysis, alteration in LPS properties with pH were important © 2014 American Chemical Society

to characterize. In this work, we presented a characterization of conformational changes of LPS as a function of pH measured by NMR and dynamic rheology. Partially hydrolyzed polyacrylamide (HPAM) is one of the most extensively used polymers in the oilfields. It generates gel phases at concentration above the critical overlap concentration (C*). Metal ions such as Cr6+ and Cr3+ have been used to reinforce the polymer network to form stronger gel phases with very high viscosity.17,18 These cross-linked polymer gels have been used successfully for conformance control as blocking agents in watered out reservoir zones. The polymer gel deviate the flow of water in to less flooded areas and thus enhance oil production. Novel application of this technology has been developed by using low polymer concentrations in the semidilute regime.19 Addition of polyvalent metal ions in this concentration regime results in reduction of polymer hydrodynamic radius (intramolecular cross-linking) or reinforcement of small aggregates (intermolecular cross-linking). Mack and Smith20 used aluminum citrate to cross-link ultrahigh molecular weight HPAM (>20 MDa) at concentrations in the semidilute regime. This has been branded as colloidal dispersion gels (CDG). Colloidal dispersion gel (CDG) has been applied successfully in oilfields and improves sweep efficiency in adverse heterogeneous reservoirs by in-depth permeability control.20−23 Propagation of CDG in porous media was studied by Ranganathan et al.,24 who reported that although polymer Received: December 2, 2013 Revised: March 14, 2014 Published: March 24, 2014 2343

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studied both in solution and solid state, using 1H and 13C,48−50 and 27Al NMR and XRD.50−52 However, the results were inconsistent with regard to stoichiometry and stability of the complexes at equilibrium. The present work was motivated by the complexity of the LPS system and the lack of literature data describing physicochemical properties of its components. The goal of this study is to serve as a basis for the further investigation of LPS properties and aid in species determination in laboratory and field studies. The pH influence on the LPS system may be better understood by characterization of individual species. Therefore, we investigated the influence of pH on the chemistry of individual species, NaCit, AlCit, HPAM, and LPS by using 1H NMR and dynamic rheology. Representative pH ranges for petroleum and EOR application were chosen to understand how individual species are sensitive to the pH changes.

was detected at the outlet, the gel properties were lost by filtration at the inlet. A number of similar particle-like systems were developed;25−28 however, they differ in the preparation method and particle hydrodynamic radius.9 The LPS system is developed for flooding and not for indepth diversion. Building on successful field implementation in China, we initiated studies on the underlying mechanisms of oil mobilization by polymer particles.7 The purpose of the LPS system is improved microscopic sweep efficiency and reduced oil saturation by microscopic diversion.10 Through the experimental work by Spildo et al.,10 log-jamming was proposed as the main mechanism for increased oil production by polymer particles (LPS). The working hypothesis is that pore blocking is induced by particles smaller than the pore radius as a result of accumulation at pore throats. The blocking of smaller pores will lead to redistribution of the local pressure field and consequently a change in the local flow pattern. The increased oil production may come as a result of improved microscopic sweep efficiency. A number of polyvalent ions like Al3+(AlCl3),29−31 Cr3+,32−38 4+ 39 Ti , Zr4+,40 and Ca2+41 have been used as cross-linker in addition to AlCit. However, the aspiration to use AlCit is because of its high valency and favorable environmental status being a natural component of the reservoir. Citrate is used as a carrier ligand to reduce the concentration of free Al(III) in solution, which would lead to high degree of intermolecular cross-linking and to ensure a slow release of Al(III) to HPAM.42 Aluminum citrate was shown to propagate through porous media and showed similar adsorption values as HPAM.43,44 LPS has been implemented successfully at onshore fields in China6,12 and extensive core flood experiments have showed high recovery potential.7,9,10 However, only one injection pilot has been performed at offshore field.45 To adapt LPS to offshore conditions, the properties have to be optimized to meet specific requirements regarding pH, salinity and temperature. Physicochemical properties have been studied by rheology and dynamic light scattering (DLS).8,15 A major challenge in characterizing the LPS system is the presence of several species existing at equilibrium condition. LPS consists of unreacted AlCit, solvated Al3+, solvated citrate, unreacted HPAM and cross-linked HPAM-Al complex species. In addition, the cross-linked HPAM-Al complex may exist in different conformations, intramolecular cross-linked HPAM (coils), intermolecular cross-linked HPAM (gel) and a mix of both (aggregates). The physicochemical behavior of these species varies with regard to pH, salinity, temperature and other factors. In this work, we have focused on investigating how the change of pH influences the physicochemical properties of LPS. To the best of our knowledge, no study in the pH influence on a LPS system has been published, although similar systems have been investigated where other cross-linkers have used.29−31 The chemical environment of sodium citrate solution is affected by change in pH. This is shown by the work of Moore and Sillerud.46 Graaf and Heerschap47 reported the effect of cation binding and solution pH influence on chemical behavior of sodium citrate. Certainly, the chemistry of AlCit is highly complex. A multitude of species coexist in solution at any pH and one species dominate over the other at certain pH interval. Experimental studies by various methods may not establish a complete characterization because of the dynamic equilibrium between complexes and versatility of the polynuclear complexes in aqueous environment. This metal−ligand complex has been

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EXPERIMENTAL SECTION

Materials and Sample Preparation. A concentrated brine (NaCl) solution was prepared by dissolving p.a. grade salts (Fluka, Riedel-de Haën) in deionized water and filtered through 0.45 μm filters (Millipore). Stock solution was diluted to the desired concentration, 0.5 wt % (0.085 M) NaCl. Sodium Citrate. A stock solution of 3000 ppm (w/w) sodium citrate dihydrate (≥99.0, Sigma Aldrich) was prepared. The stock solution was diluted to 20 ppm (0.068 mM) in 0.5 wt % NaCl and pH was adjusted to 1.58, 3.19, 7.10, and 10.72 by addition of NH4OH and HCl. Aluminum Citrate. A stock solution of 3000 ppm (weight Al/total weight) of aluminum citrate was prepared in 0.5 wt % NaCl and adjusted to neutral pH to avoid precipitation. The solution was prepared from an aluminum citrate solution (Dr. Paul Lohmann AG) which contained 8.8 wt % Al. Aluminum concentration was measured by ICP-OES (Perkin-Elmer). This solution was diluted to 20 ppm (weight Al/total weight) (0.74 mM) and the pH adjusted by adding NH4OH and HCl to match the series: 1.71, 2.74, 3.85, 5.39, 6.53, 7.60, 9.26, and 10.15. Polymer and LPS. Partial hydrolyzed polyacrylamide (HPAM) polymer, Flopaam 3630S, (SNF Floerger) with an average molecular weight of 20 MDa and 25−30% degree of hydrolysis was used. A stock solution of 5000 ppm (w/w) was prepared by dispersing dry polymer granulate in 0.5 wt % NaCl solvent, as described by API RP 63.53 The stock polymer solution was diluted to the desired concentrations of HPAM by adding 0.5 wt % NaCl solution and stirring the solution for a minimum of 24 h using a magnetic stirrer at low speed. HPAM solutions were adjusted to three different pH values; 1.15, 7.02, and 9.84. Linked polymer solutions (LPS) were prepared by adding stock solution of aluminum citrate (pH 6.5) into HPAM solutions, all at neutral pH. The pH was subsequently adjusted to five values, 1.71, 6.63, 7.48, 8.46, and 9.84, by addition of NH4OH and HCl. LPS solutions were kept under slow stirring on a magnetic stirrer for three days before use to ensure the reaction between polymer and AlCit to reach equilibrium. The cross-linked polymer generates primarily coiled particles with hydrodynamic radius in the order of 50−100 nm,54 as measured by dynamic light scattering (DLS) and scanning electron microscopy (SEM). All samples in this study are prepared at ambient temperature. The following nomenclature was introduced to distinguish between HPAM and LPS formulations: ‘polymer concentration_cross-linker concentration_solvent concentration’, for instance 600_20_0.5 is used for a solution which consists of 600 ppm HPAM and 20 ppm Al3+ in 0.5 wt % NaCl solvent. Methodology. pH Meter. All the sample pH was measured by ISFET pH meter (Model 24309). The pH meter has a water proof nonglass probe (Model 24307) that measure with resolution of 0.01 and accuracy of ±0.01.The probe was calibrated with three (i.e., pH of 4.01, 7.00, and 10.01) buffer solutions before measurement. 2344

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Rheology. Rheological measurements were performed on an Anton Paar Physica MCR 300 stress-controlled rheometer. Measurements were performed at 22 °C using double gap geometry (DG 26.7). Samples were allowed to temperature equilibrate for a minimum of 10 min before starting the measurements. Amplitude sweep was performed using a frequency of 10 Hz. Strain values between 0.01 and 3000% were applied. NMR Spectroscopy. Liquid state 1H NMR measurements were performed on a Bruker AVANCE DRX 600 NMR spectrometer (University of Bergen, Norway) or a Bruker AVANCE III 900 NMR spectrometer (University of Lille 1, France) spectrometer. A 5 mm triple resonance (1H, 13C, and 15N) inverse cryogenic probe-head with z-gradient coils and cold 1H and 13C preamplification was used for the DRX 600. A 5 mm triple resonance inverse cryogenic TCI probe head with z-gradient coils (5 mm CPTCI 1H-13C/15N/D Z-GRD Z44910/0022) was also used for the III 900. The samples were prepared with 10% D2O (99,9%,Aldrich) in 5 mm 528-PP-7 NMR tubes (Sigma Aldrich). Shimming and tuning of the samples were performed automatically. Bruker standard pulse sequences were used throughout this study. In the 600 NMR spectrometer, typical acquisition parameters used for one-dimensional 1H NMR measurements were 90° pulse, 32 number of scans, 12 kHz spectral width, and 16K number of data points. The intense water signal was suppressed using excitation sculpting. Gradient selective two-dimensional COSY experiments were acquired with 2048 data points in the direct dimension and 512 in the indirect dimension with spectral widths determined from the corresponding one-dimensional experiments. In the 900 MHz spectrometer, one-dimensional 1H NMR spectra were acquired with 90° pulse, 32 number of scans, and 16K number of data points. TSP (sodium 3-(trimethylsilyl) tetradeuteriopropionate) was used as chemical shift reference. TSP chemical shift is pH dependent and we have used the equation proposed by Tynkkynen et al.55 for pH ≤ 8 to calibrate the spectra. All spectra were measured at 22 °C. All the spectra were processed using Bruker Topspin 3.2.

(δcit) of citrate decreases when the pH of solution is increased,46 as consequence of shielding effect due to deprotonation. Graaf and Heerschap47 also have investigated the effect of cations and pH change on the proton chemical shifts and the spin−spin coupling constant of citrate. The results showed an exponential decay with respect to increasing the pH of solution, in agreement with the result demonstrated in the Figure 3. Effect of pH on the Aluminum Citrate. One- and twodimensional NMR experiments were performed to generate a “map” of the pH dependency of AlCit and search for characteristic 1H NMR signal(s) that may aid conformational determination of species resulting from the reaction of Al(III) with HPAM. Eight solutions of 20 ppm of Al in 0.5 wt % NaCl were prepared and the pH adjusted to the desired value to achieve the pH values: 1.71, 2.74, 3.85, 5.39, 6.53, 7.60, 9.26, and 10.15. 1D and 2D COSY 1H NMR experiments were performed for each sample. 1D 1H NMR spectra are presented in Figure 4. Attribution of the spectra is very challenging as consequence of resonance overlap and formation of second-order coupling patterns. Furthermore, molecular exchange between aluminumbound and unbound citrate (intermolecular ligand exchange) and within bound citrate (intramolecular ligand exchange) results in line broadening. Speciation of AlCit complexes as a function of pH has been studied extensively.57−61 Ö hman showed that primarily mononuclear AlCit complexes are formed in dilute solutions, that is, citrate concentrations in the order of 10−6 M, whereas polynuclear complexes are the dominating species for semidilute solutions with concentrations about 10−3 M.57 The concentration of aluminum and citrate used in the present work is 0.7 and 1.1 mM, respectively, suggesting that polynuclear complexes are the dominating complexes formed for AlCit at these conditions. Bodor et al.48 and Matzapetakis et al.50 used concentrated solutions to characterize the three dominating species determined from the previous studies; [Al(Cit)2]3−, [Al3(Cit)3(OH)(H2O)]4−, and [Al3(H−1Cit)3(OH)4]7− using both X-ray and NMR data. In this study, we observed four dominant complex species within the selected pH range. These are discussed in the following lines. At pH 1.71, two broad AB doublets in the 1H NMR spectrum indicates that two chemical shift environments exist for CH2 groups of free citrate (H3Cit) and aluminum exist in aqueous state. Figure 2 and Figure 4 shows the line width of NaCit (pH = 1.58) and AlCit (pH = 1.71) is 1.9 and 2.6 Hz as well as the coupling constant (JAB) is 16.16 and 16.41 Hz, respectively. Thus, compared to NaCit, the AB quartet of AlCit is broad and weakly coupled as result of strong dipole−dipole interaction between Al3+ (aq) and citrate (H3Cit). Increasing the pH to 2.74, the spectrum signal becomes two AB quartets with one overlap signal of equivalent methylene group, and this is further confirmed by 2D COSY results (Figure S-1, see Supporting Information). This phenomena occurs because of simultaneous exchange between protonation and deprotonation of the terminal carboxylate groups around the aluminum ion.62 If the rate of the fluxional rearrangement is fast enough, only one AB quartet signal of the nonequivalent methylene groups could be observed. At pH 3.85, a complicated pattern resembling the trinuclear complex specie is observed. This is in agreement with previous observations.48,49,57 In fact, a trinuclear complex was observed even below this pH.51,61 Around this pH a bis-ligand complex,

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RESULTS AND DISCUSSION Effect of pH on Sodium Citrate. Citrate is used as a carrier ligand for Al3+ ion and has been shown to give a slow release of Al3+ to cross-link with HPAM in LPS. In order to achieve predominantly polymer coils, solvated Al3+ should be available at low concentration to favor intramolecular crosslinking of polymer.8,9 Sodium citrate prepared in 0.5 wt % NaCl was used as reference to investigate the influence of pH change on the chemical behavior of citrate ion. Figure 1 shows the

Figure 1. Chemical structure of trisodium citrate in solution with strongly couple methylene protons, indicated as Ha and Hb.

chemical structure of sodium citrate. The trisodium citrate sample was probed by 1H NMR and the spectrum exhibits an AB-type pattern of strongly coupled methylene proton resonances as shown in Figure 2. The two methylene groups are chemically equivalent and result in a single resonance.46 The resonance is shifted upfield as pH is increased. This is due to deprotonation of the carboxylate groups. The pKa values for citric acid are 3.14, 4.77, and 6.39 at 20 °C.56 From the result illustrated in Figure 3, both the spin−spin coupling constant (JAB) of methylene protons of citrate and chemical shift of the midpoint of methylene protons resonance 2345

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Figure 2. 1H NMR (600 MHz) spectra of trisodium citrate, showing AB-type pattern of strongly coupled methylene proton resonances. (pH increases from bottom to top accordingly: 1.58, 3.19, 7.10 and 10.72).

overlap prevents conventional peak integration to assist in spectrum assignment. We have therefore used 2D COSY spectra to aid characterization (Figure S-2, see Supporting Information). Matzapetakis et al.50 used X-ray crystallography to show a highly nonsymmetric CH2 structure, known as asymmetric trinuclear species “As”, ([Al3(H−1Cit)3(OH)]4−‑) after a structure suggested by Ö hman.57 In addition to the complicated signals, other signals appeared downfield at around 3.0 ppm. These separate signals were reported by Matzapetakis et al.50 as intermediate 1:1 complex species formed at low concentrations. However, the COSY spectra (Figure S-2, see Supporting Information) performed here revealed that these separate citrate signals are observed after a month and longer. This indicates that the separate signals are not only intermediate complex species polymerizing to trinuclear complexes, but are stable at low concentration at equilibrium conditions. The COSY spectrum shows that one of six CH2 signals does not show any coupling pattern, whereas the other five CH2 signals have different chemical environments resulting in five pairs of overlapping AB type doublets signals. The noncoupled methylene protons may be explained by local symmetry or fast dynamics.49 However, the same sample was investigated at high magnetic field NMR (900 MHz), can be seen in Figure 5b. This 1 H NMR spectrum showed a splitting of the singlet in to AB quartet and the six asymmetric CH2 group’s signals were observed (equivalent to 24 signals). This has not been reported

Figure 3. Chemical shift of the midpoint of citrate resonance (δcit) and AB spin−spin coupling constant (JAB) of citrate as a function of pH.

[Al(Cit)2]3− is reported by Bodor and co-workers at molar concentration of 0.25 M Al and 1.0 M Cit.49 However, we did not observe this complex, probably because of the low concentration as well as low ratio of Cit-to-Al used here (0.7 mM Al and 1.1 mM Cit). Between pH 5.39 and 7.60, new complicated overlapped signals appear, originating from trinuclear complexes.48,49,56This implies that trinuclear species become stable and dominant at this equilibrium condition. When the metal ion is surrounded by three citrate ligands, one can expect to observe six asymmetric CH2 groups, which is equivalent to 24 signals. The spectrum contains second order couplings and

Figure 4. 1D 1H NMR (600 MHz) spectra of aluminum citrate as function of pH (pH increases from bottom to top accordingly: 1.71, 2.74, 3.85, 5.39, 6.53, 7.60, 9.26, and 10.15). 2346

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Figure 5. 1H NMR spectrum of 20 ppm AlCit (pH, 7.01) at two different magnetic field strengths: (a) 600 MHz, and (b) 900 MHz.

Figure 6. 1H NMR (600 MHz) aligned spectra of Aluminum Citrate (AlCit) and Sodium Citrate (NaCit) to trace the free citrate signal from AlCit spectrum based on free citrate spectrum.

in previous studies. Bodor et al.,49 showed that although the 1H NMR spectrum showed a singlet, the corresponding 13C NMR spectrum showed a splitting pattern indicating that the two methylene protons were nonequivalent. This is confirmed by the high field 1H spectrum showed in Figure 5 (blue box). The symmetric trinuclear species, Sy, giving rise to an upfield shifted AB quartet, is not observed or present in very low concentration at this pH range. When the pH is 9.26 and 10.15, the symmetric trinuclear complex ([Al3Cit3(OH)4]7−), Sy, appeared as reported by Bodor et al.49 The structure is stable, highly symmetric and resembles the signals of free citrate, except that the peaks of Sy are broad. This is probably due to fluxionality of the molecule through intramolecular exchange.48 At very low pH the coupling constant (JAB) of NaCit and AlCit is 16.16 and 16.41 Hz, respectively. However, the corresponding values decrease to 15.23 and 15.28 Hz at high pH, illustrated in Figure 3 and Figure 4. This coupling constant change showed that there is difference in the separation between AB quartet signals as function of pH. This is due to the magnitude Larmor frequency of AB system (NaCit and AlCit) under low pH is greater than the corresponding solution at high pH. Thus, NaCit and AB quartet of trinuclear symmetric AlCit is strongly coupled in strong basic solution compared to the corresponding solution in strongly acidic solution. This illustrates how pH influences the chemical environment of these two systems. In Figure 6, the chemical shift of free citrate and/or Sy of AlCit has been attributed based on comparison with NaCit signals around the same pH. The NMR data presented lead us to propose 5 species present at the given concentrations of aluminum and citrate

dissolved in 0.5 wt % NaCl solution at room temperature, see Scheme 1. Generally, Cit3− ligands are slowly replaced by OH− Scheme 1. Major Species of Aluminum and Citrate Present in Solution under Given Conditions and Their Corresponding pH Range

groups as the pH of solution increase and the complex formation in AlCit solution changes from AlCit, to [Al(Cit)2]3, [Al3Cit3(OH)(H2O)]4−, [Al3Cit3(OH)4]7−, and ultimately to [Al(OH)4]− at a value higher than pH > 10, as shown by equilibrium chemical equations. Effect of pH on the Physicochemical Property of HPAM and LPS. The pH influence on the physicochemical behavior of HPAM and LPS is also investigated. The LPS spectrum is assigned and presented with interpretation in Figure 7. This is in agreement with PAM gel studied by Rajamohanan et al.,63 using 1H MAS NMR and 2D NMR (NOESY and ROSEY). A pH dependent NMR experiment was performed for both HPAM and LPS samples and the spectra are aligned and displayed in Figure 8 and Figure 9, respectively. A notable change occurred for the amide and citrate signals as pH increased. At very low pH, (pH 1.15), the HPAM spectrum shows distorted resonance peaks of backbone protons and one of the amide proton resonance peaks is split into a triplet of doublet due to protonation. When the solution pH increases to neutral range (pH= 7.02), the backbone proton resonance 2347

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Figure 7. 1H NMR (600 MHz) spectrum of LPS (600_20_0.5), at pH 6.99.

Figure 8. 1H NMR (600 MHz) spectra of the HPAM (600_0_0.5) at different pH values: (a) 1.15, (b) 7.02, and (c) 9.84.

Figure 9. 1H NMR (600 MHz) spectra of the LPS (600_20_0.5) at different pH values: (a) 1.71, (b) 6.63, (c)7.48, (d) 8.46, and (e) 9.84.

indicate stable conformation, while broad amide proton peaks are observed. This is different from that observed at very low pH. However, when the pH of the HPAM solution exceeds the neutral range (pH 9.84), a marked loss of the amide proton signals was observed. A similar behavior was observed for LPS at very low pH (pH = 1.71). An exception was that one of the split amide proton signals became one sharp peak. Probably the presence of Al3+ ion is shielding the amide protons from protonation in the acidic media. In addition, the free citrate signal shows only AB quartet, which is similar to that observed in the case of the AlCit solution. When the pH increases to neutral range (i.e., pH 6.63, 7.48, and 8.46), the spectrum shows similar signals as for HPAM, in addition to a multitude of peaks originating from unreacted AlCit and free citrate. Further increasing the pH of LPS (pH = 9.84), a similar phenomenon occurs on the amide protons signal as it was observed in the corresponding HPAM solution along with symmetric trinuclear AlCit and free citrate signals. The loss of

the amide proton signals in both HPAM and LPS at high pH could be explained by hydrolysis of the amide groups. The amide groups are susceptible to chemical reaction at high pH, which undergo hydrolysis as shown by disappearance of its signals in both HPAM and LPS spectrum. The hydrolysis reaction is nonreversible reaction where the amide group is replaced by a hydroxyl group. Consequently, the polymer side chain becomes a carboxylic acid group, which alters the polymer properties. The viscoelastic behavior of HPAM and LPS was also investigated for selected pH values using a stress controlled rheometer. In this work, all samples behaved as viscoelastic liquids because G″ > G′ at any strain or frequency measurement. The rheological parameters of HPAM and LPS showed strong pH dependence. At very low pH, HPAM (pH = 1.15) and LPS (pH = 1.71), the viscoelasticity is changed with a remarkable loss of the storage modulus (G′), and marginal change of the loss modulus (G″) (see Figure 10 and 11). The main reason is that electrostatic repulsion between the negative 2348

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ical property of LPS. In addition to electrostatic interaction, hydrogen bonds also play an important role in the internal structure of polymer solution.64 The influence of pH can be explained by a gradual deprotonation of carboxylic acid groups as the pH of the solution increases, along with hydrolysis of amide groups at very high pH, which create increased repulsion between negatively charged carboxylic groups. This repulsion result in less flexibility in the polymer backbone chain. This will destabilize hydrogen bonds within or between polymer chains. Thus, based on the observed experimental results, we draw the following hypothetical schematic structure to show pH influence on the chemistry of HPAM (Scheme 2). Figure 10. Dynamic moduli as a function of strain amplitude for HPAM (600_0_0.5) solutions with different pH.

Scheme 2. Schematic Diagram of Chemical Structure of HPAM Solution at Different pH Range

Figure 11. Dynamic moduli as a function of strain amplitude for LPS (600_20_0.5) solutions with different pH.

charges on repeat units of the HPAM is reduced due to protonation. Subsequently, long-range interactions, that is mainly electrostatic repulsion, between polymer random coils or within single polymer coils are weakened. This results in a transition of polymer conformation from extended coil to collapsed state and thereby, the polymer rheological behavior tends toward a Newtonian liquid. Increasing the pH of HPAM (pH = 6.83) and LPS (pH = 6.63, 7.48, and 8.46) leads to increase in both G′ and G″ as the solution gains more elastic and viscous properties, respectively. This viscoelasticity increase occurs because of deprotonation of carboxylic acid groups that impose electrostatic repulsion as pH increases. Further increasing the pH of HPAM (pH = 9.84) and LPS (pH = 9.56), result in a slight increase in G′, while G″ remains constant. At this pH both HPAM and LPS undergo hydrolysis of the amide groups, which are confirmed by NMR experiments (see Figures 8 and 9). Increase in elastic modulus (G′) might be explained by increase the polymer degree of hydrolysis as result of hydrolysis of amide group to carboxyl groups. Thus, the presence of more charged groups increases the electrostatic repulsion in the solution, which results in increase of viscoelasticity. In general, deprotonation of carboxylic acid side chain groups lead to increased electrostatic repulsion and makes the polymer molecular chain more outstretched. The changes at or near neutral pH condition resembles those observed at high pH except that the elasticity is increased because of the contribution of hydrolysis of amide groups in addition to deprotonation of acid groups that develops more electrostatic repulsion in to the system. According to the results shown, side chain interactions are the governing factor for the influence of pH on physicochem-

On Scheme 2, at very low pH, the amide protons are protonated to NH3+ as a result of excess protons and may form resonance stabilized structures. This leads to reduced electrostatic repulsion as well as hydrogen bonding, resulting in a loss of elasticity and tendency toward Newtonian flow behavior. However, once the pH reaches to neutral conditions, the polymer experience electrostatic repulsion and increased hydrogen bonding because of deprotonation of acid groups. This stabilizes the interaction between polymer chains. Hence, the system develops more elasticity and thereby, increases in viscoelasticity. When the pH exceeds the neutral range, electrostatic repulsion is increased as a result of further deprotonation of acid groups and hydrolysis of amide groups to carboxylic acid groups. This condition creates more elasticity in the polymer solution that further increases the viscoelasticity.

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CONCLUSION The influence of pH on the physicochemical properties of NaCit, AlCit, HPAM, and LPS was investigated. The combination of NMR and dynamic rheological tests were used to map these changes and provides a template for LPS analysis and quality control that may be applied for injection and back-production of LPS in the oil field. It also aids analysis of laboratory core flood experiments and give guidance for LPS preparation. Sodium citrate shows a simple spectrum with strong coupling between symmetric methylene protons. As pH increases, methylene resonances shift upfield but do not change shape or multiplicity showing that no structural change takes place. The influence of pH on the aluminum citrate is much more complicated, and it is dominated by formation of 2349

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multinuclear and multiligand complex species. The trinuclear asymmetric complex, [Al3(H−1Cit)3(OH)]4−, seems to dominate at neutral pH, at around 10−3 M concentration of citrate and aluminum. The HPAM and LPS physicochemical properties are affected by change in pH. Increasing the pH leads to deprotonation of carboxylic acid side chains of HPAM that induce electrostatic repulsion between charged groups. Hence, both HPAM and LPS gain more elasticity and thereby increase viscoelasticity. At high pH, hydrolysis leads to significant loss of amide groups that increases elasticity by increased electrostatic repulsion. Physicochemical properties of LPS were not observed to change significantly for typical oil reservoir pH range, that is, 5−7. LPS preparation and injection should be performed within this pH range. When back-produced LPS is analyzed and pH is measured to be outside this range, NMR and dynamic rheology may be used to determine the state of the polymer.

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ASSOCIATED CONTENT

S Supporting Information *

Figure S-1 and Figure S-2, the COSY spectra of AlCit at pH 2.74 and 7.60, respectively. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the Norwegian Research Council (NFR) for financial support. We also acknowledge Professor Sylvain Cristol for allowed me to work in his laboratory and Marc Bria for the aid of 900 MHz NMR measurements, and also the CNRS laboratory, University of Lille 1, France.

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ABBREVIATIONS EOR = enhanced oil recovery HPAM = hydrolyzed polyacrylamide LPS = linked polymer solution AlCit = aluminum citrate NaCit = sodium citrate CDG = colloidal dispersion gel ppm = parts per million (mg/kg) γ = gamma, strain G′ = storage modulus G″ = loss modulus

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