Brine-Soluble Zwitterionic Copolymers with Tunable Adsorption on Rocks.

Injection of aqueous fluids into reservoirs as an enhanced oil recovery (EOR) tool has been of great interest in petroleum engineering. EOR using viscous polymer solutions improves the volumetric sweep efficiency. However, significant polymer adsorption on reservoir rock surfaces is one of the greatest challenges in polymer-flooding EOR. We have synthesized and characterized five zwitterionic copolymers and studied their static adsorption on limestone surfaces in seawater at high temperatures and salinities. Our results indicate that polymer adsorption directly correlates to a small percentage of functional co-monomers on the polymer backbone. One particular copolymer shows negligible static adsorption on limestone surfaces.

Introduction

Polymer flooding is widely used in chemical enhanced oil recovery (CEOR) and allows for a more efficient water flood sweep by increasing solution viscosity.[1,2] However, retention of polymers is among the biggest challenges when developing applications in this field.[3−7] Numerous studies have shown that adsorption of polymers on rock surfaces, mechanical entrapment of polymers in pore matrices, and fluid flow-induced hydrodynamic retention of polymers[4,8−10] contribute to the overall polymer loss (polymer retention) in the porous media of rocks. These factors reduce the fluid viscosity and therefore the efficacy of chemicals, requiring the injection of large quantities of chemicals to compensate for the material (viscosity) loss.[11] The widely studied polymers for EOR are polyacrylamide (PAM), partially hydrolyzed polyacrylamide (HPAM), and hydrophobically modified polyacrylamide (HMPAM), which is a type of associative polymer.[1] PAM and HPAM polymers have limited long-term stability especially in the high-temperature and high-salinity downhole environment, and HMPAM with improved long-term stability in brines has shown high polymer retention.[12]

Polyzwitterions[13] containing both anionic and cationic groups can be stable in high salinity fluids at high temperatures as a result of the “anti-polyelectrolyte effect”.[14,15] In recent years, polyzwitterions have been used in anti-biofouling coatings, drilling fluids, and EOR, which all rely on their unique surface properties.[16,17] For example, Sabhapondit et al. reported the synthesis of N,N-dimethyl-acrylamide-co-Na-2-acrylamido-2-methyl propanesulfonate, which was able to achieve 66% viscosity retention after a month at 120 °C.[18] Subsequently, Ye et al. synthesized a novel water-soluble associative polymer by copolymerization and sulfomethylation using acrylamide and N-allyl benzamide with even better salt and temperature tolerance.[19] As mentioned previously, the retention of these polymers in porous media is the collective result of adsorption, mechanical entrapment, and flow-induced hydrodynamic retention.[4,5,11] On the other hand, the retention of polyzwitterions in porous media is less studied or understood. The competition between attractive forces (e.g., van der Waals interactions, entropic contribution, or specific interactions) and electrostatic forces, which can be either repulsive or attractive, governs polymer adsorption on a charged surface.[20,21] Structural modifications on polymers could be used to control polymer adsorption. We intend to study the polymer adsorption on rock surfaces focusing on the contribution of particular functional groups on the side chains of zwitterionic polymers. Furthermore, the potential to fine-tune polymer adsorption by varying a small fraction of certain functional groups on the polymers is extremely attractive not only in EOR but also in some other applications[22] that prefer polymer adsorption on mineral surfaces.

Experimental Section

Materials

All reagent-grade materials were purchased from Sigma-Aldrich and used without further purification unless otherwise noticed. Crushed, sieved, and hot solvent (toluene and methanol)-cleaned pieces of Indiana limestone with diameters ranging from 0.5 to 1.41 mm were purchased from Core Laboratories, Houston, TX, and used as received. The chemical component of Indiana limestone is 98–99% calcite (CaCO3).

Synthesis of the Copolymer of Poly(vinyl imidazole-co-styrene) (0.9:0.1) 1′ and Sulfozwitterionic 1

1-Vinyl imidazole (2 g, 21.25 mmol), styrene (0.2459 g, 2.36 mmol), azobisisobutyronitrile (AIBN, 0.03877 g, 0.236 mmol, 1 mol %), and DMSO (5 mL) were combined in a Schlenk tube with a stir bar. The reaction mixture was degassed using freeze–pump–thaw (3×) and sealed under a static inert atmosphere and stirred at 60 °C for 4 days. The resulting light yellow viscous solution was cooled and diluted by the addition of methanol. The mixture was precipitated into acetone (2×) and then dried overnight to produce copolymer 1′, which was a clear hard light yellow solid. 1′ was characterized by 1H NMR and 13C NMR in deuterated methanol-d4 (Figure S1). To a solution of copolymer 1′ (1 g, 9.46 mmol of imidazole moieties) in methanol was added 1, 3-propane sultone (1.386 g, 11.35 mmol, 1.2 equiv). The reaction mixture was refluxed overnight under an inert atmosphere and the resulting zwitterionic copolymer 1 precipitated out. Zwitterionic copolymer 1 was filtered out and then washed with a mixture of methanol and diethyl ether (1:1, 2×). Zwitterionic copolymer 1 was characterized by 1H NMR in deuterated brine (D2O brine) (Figure S2).

Synthesis of Poly(vinyl imidazole-co-4-aminostyrene) (0.95:0.05) 2′ and Sulfozwitterionic 2

1-Vinyl imidazole (5 g, 53.1 mmol), 4-aminostyrene (0.333 g, 2.796 mmol), azobisisobutyronitrile (AIBN, 0.092 g, 0.559 mmol, 1 mol %), and DMSO (5 mL) were combined in a Schlenk tube with a stir bar. The reaction mixture was degassed using freeze–pump–thaw (3×) and sealed under a static inert atmosphere and stirred at 60 °C for 4 days. The resulting light yellow viscous solution was cooled and diluted by the addition of methanol. The mixture was precipitated into acetone (2×) and then dried overnight to produce copolymer 2′, which was a clear hard light yellow solid. To a solution of 2′ (1.085 g, 10.6 mmol imidazole moieties) in methanol was added 1, 3-propane sultone (1.2978 g, 10.6 mmol, the equiv of 1, 3-propane sultone is exactly equal to the equiv of the imidazole moiety in copolymer 2′ to minimize the side reaction with the −NH2 group on 4-aminostyrene). The reaction mixture was refluxed overnight under an inert atmosphere and the resulting zwitterionic copolymer 2 precipitated out. Zwitterionic copolymer 2 was filtered out and then washed with a mixture of methanol and diethyl ether (1:1, 2×).

Synthesis of Poly(vinyl imidazole-co-4-trifluoromethylstyrene) (0.95:0.05) 3′ and Sulfozwitterionic 3

1-Vinyl imidazole (2 g, 21.25 mmol), 4-trifluoromethylstyrene (0.406 g, 2.36 mmol), azobisisobutyronitrile (AIBN, 0.0388 g, 0.236 mmol, 1 mol %), and DMSO (5 mL) were combined in a Schlenk tube with a stir bar. The reaction mixture was degassed using freeze–pump–thaw (3×) and sealed under a static inert atmosphere and stirred at 60 °C for 4 days. The resulting light yellow viscous solution was cooled and diluted by the addition of methanol. The mixture was precipitated into acetone (2×) and then dried overnight to produce copolymer 3′, which was a clear hard light yellow solid. To a solution of copolymer 3′ (1.2 g, 10.6 mmol imidazole moieties) in methanol was added 1, 3-propane sultone (1.555 g, 12.75 mmol, 1.2 equiv). The reaction mixture was refluxed overnight under an inert atmosphere and the resulting zwitterionic copolymer 3 precipitated out. Zwitterionic copolymer 3 was filtered out, washed with a mixture of methanol and diethyl ether (1:1, 2×), and dried under vacuum to yield a light yellow solid.

Synthesis of Poly(vinyl imidazole-co-N-vinyl pyrrolidone) (0.9:0.1) 4′ and Sulfozwitterionic 4

1-Vinyl imidazole (2.5 g, 26.5 mmol), N-vinyl pyrrolidone (0.3 g, 2.7 mmol), azobisisobutyronitrile (AIBN, 0.048 g, 0.293 mmol, 1 mol %), and DMSO (5 mL) were combined in a Schlenk tube with a stir bar. The reaction mixture was degassed using freeze–pump–thaw (3×) and sealed under a static inert atmosphere and stirred at 60 °C for 4 days. The resulting light yellow viscous solution was cooled and diluted by the addition of methanol. The mixture was precipitated into a mixture of acetone and diethyl ether (1:1, 2×) and then dried overnight to produce copolymer 4′, which was a clear hard light yellow solid. To a solution of copolymer 4′ (1 g, 9.24 mmol imidazole moieties) in methanol was added 1, 3-propane sultone (1.354 g, 11.1 mmol, 1.2 equiv). The reaction mixture was refluxed overnight under an inert atmosphere and the resulting zwitterionic copolymer 4 precipitated out. Zwitterionic copolymer 4 was filtered out, washed with a mixture of methanol and diethyl ether (1:1, 2×), and dried under vacuum to yield a light yellow solid (yield: 91%).

Synthesis of Poly(vinyl imidazole) 5′ and Sulfozwitterionic 5

1-Vinyl imidazole (3 g, 32 mmol), azobisisobutyronitrile (AIBN, 0.0523 g, 0.32 mmol, 1 mol %), and DMSO (3 mL) were combined in a Schlenk tube with a stir bar. The reaction mixture was degassed using freeze–pump–thaw (3×) and sealed under a static inert atmosphere and stirred at 60 °C for 4 days. The resulting light yellow viscous solution was cooled and diluted by the addition of methanol. The mixture was precipitated into acetone (2×) and then dried overnight to produce polymer 5′, which was a clear hard light yellow solid. To a solution of polymer 5′ (1 g, 10.6 mmol imidazole moieties) in methanol was added 1, 3-propane sultone (1.557 g, 12.75 mmol, 1.2 equiv). The reaction mixture was refluxed overnight under an inert atmosphere and the resulting zwitterionic polymer 5 precipitated out. Zwitterionic polymer 5 was filtered out and then washed with a mixture of methanol and diethyl ether (1:1, 2×).

Thermogravimetric Analysis

Thermogravimetric analysis (TGA) measurements were carried out under a nitrogen atmosphere (Airgas, ultrahigh purity grade) using a TGA 550 from TA Instruments. The ramp speed was 20 °C min–1 from room temperature to 650 °C.

NMR Spectroscopy

1H and 13C NMR spectra for all the polymers were acquired on a Bruker Advance Spectrometer operating at 400 MHz. Chemical shifts (δ) are reported in parts per million (ppm) and referenced with D2O for 1H NMR. NMR of polymer 1′, 2′, 3′, 4′, and 5′ (Figure S1 in the Supporting Information) was performed in methanol-d4 solvent and polymer 1, 2, 3, 4, and 5 (Figure S2 in the Supporting Information) was performed in abrine using D2O as the solvent. The formula of the brine is in Table S4 in the Supporting Information.

Gel Permeation Chromatography

The molecular weight of polymer 4 was determined using a Viscotek GPCmax with a Viscotek aqueous A6000M (300 × 8.0 mm) column and an Aguard guard column. A Malvern 270 dual detector and a Viscotek VE 3580 RI detector were used to detect reflective index (RI), viscosity (IV), and low angle light (LAL) and right angle light (RAL) scattering signals. The dextran standards with Mw 1270, 5220, 11 600, 25 000, and 68 082 g/mol were purchased from Sigma-Aldrich. All the standards and polymer 4 were dissolved in 0.3 M NaCl solution, and 0.3 M NaCl solution was also used as the eluent at 0.5 mL/min. The retention volume was selected using the RAL peaks, although other signals generate the same result.

The molecular weight of polymer 1′, 2′, 3′ and 5′ was determined using the same GPCmax and detectors but with Guard-0478 and Malvern CLM1012 (I-MBMMW-3078) columns. The polystyrene (PS) standards with Mw 5200, 30 000, 200 000, 400 000, and 900 000 were purchased from Alfa Aesar. The PS standards and polymer 1′, 2′, 3′ and 5′ were dissolved in a mixture solvent of 20% methanol in THF. The same mixed solvent was used as the eluent at 0.5 mL/min for the PS standards and the polymer samples.

pH of the Polymer Solutions in A Brine

Polymers 1, 2, 3, 4, and 5 completely dissolve in a SW brine (SW, composition shown in Table S4) at 0.2 wt % after heating at 90 °C in a sealed vial for 30–60 min. The pH of the polymer solutions was measured using a pH meter, Orion Star A111 of Thermo Scientific, at 25 °C. The reported pH is an average after three repeats.

Density and Viscosity Measurement of the Polymer Solutions

The density (g/cm3) and the dynamic viscosity (mPa·s) of the SW and polymer solutions were determined using an Anton Paar Stabinger viscometer SVM 3001. The density and viscosity values were taken at 80–85 °C.

Dynamic Light Scattering

We used a NanoBrook Particle and Zeta potential analyzer (Brookhaven Instruments Corporation) to determine the hydrodynamic diameter (dh) of 0.2 wt % polymers in the SW brine and in NaCl or CaCl2 solutions at the same ionic strength of 0.115 M as SW. The freshly prepared polymer solutions were first heated at 90 °C in an oven for 30–60 min and then cooled to room temperature before dynamic light scattering (DLS) measurement. All the measurements used a PS cell and 659 nm of laser at 90° angle. The solution was equilibrated at 25 °C for 1 min before the measurement. The reported dh is an average of three repetitive measurements at 5 min duration for each measurement, and the size distribution was calculated using CONTIN algorithms with an auto (slope analysis) baseline normalization.

Zeta Potential of Limestone Powders

We used a NanoBrook Particle and Zeta potential analyzer (Brookhaven Instruments Corporation) to determine the zeta potential of limestone powders in 1 mM KCl solution at 25 °C. The reported zeta potential is an average of five repeats at 1 min duration.

Scanning Electron Microscopy

Scanning electron microscopy (SEM) images of rock limestone (LS) particles were collected in a JEOL JSM-6010LA in the high vacuum mode using a secondary electron detector. The LS samples were coated with 10 nm of gold in a Quorum SC7640 sputter coater prior to imaging.

Brunauer–Emmett–Teller

The Brunauer–Emmett–Teller (BET) surface area of the LS particles was measured with N2 sorption at 77 K using a Micromeritics ASAP 2020. Limestone particles were degassed under high vacuum at 150 °C for 6 h prior to the measurement. The analysis of the pore-size distributions was performed using a nonlocal density functional theory model for carbon slit pore geometry provided using ASAP 2020.

Results and Discussion

Herein, we report the synthesis and static adsorption of four copolymers that share the same zwitterionic backbone with different functional co-monomers. These polymers show excellent solubility and stability in a SW brine at ambient and elevated temperatures. Most importantly, they exhibit a wide range of adsorption on carbonate LS, depending on a small fraction of the functional co-monomers incorporated during copolymerization. Scheme shows the synthetic procedures for these random, zwitterionic copolymers. The major monomer is 1-vinylimidazole that is polymerized by free-radical methods with functional co-monomers styrene (1), 4-aminostyrene (2), 4-trifluoromethylstyrene (3), and vinyl pyrrolidone (4) to test the “substituent effect” on the polymers’ adsorption properties. For comparison, a pure 1-vinylimidazole-based polyzwitterion (5) is also synthesized. After free-radical polymerization, prezwtterionic polymers 1′, 2′, 3′, 4′, and 5′ were subjected to a postpolymerization functionalization using 1,3-propane sultone to yield their zwitterionic counterparts 1, 2, 3, 4, and 5.

Scheme 1

Synthesis of Zwitterionic Polymers 1, 2, 3, 4, and 5 Based on 1-Vinylimidazole and Various Functional Co-monomers via Free-Radical Polymerization and Postpolymerization Sulfonation

1H and 13C NMR of polymers 1′, 2′, 3′, 4′, and 5′ were performed in methanol-d4 (Figure S1 in the Supporting Information). The 1H NMR spectra of the five polymers are very similar with aromatic protons of vinylimidazole and styrenic monomers on polymers overlapping in broad peaks at 7.5–6.5 ppm and aliphatic protons at 3.6, 3.2, 2.9, and 2.0 ppm. 1H NMR peaks of vinyl pyrrolidone of 4′ are overlapped with the 1H peaks of the aliphatic backbone. 13C NMR spectra of 1′, 2′, 3′, and 5′ are also very similar with broad aromatic vinylimidazole and styrenic co-monomers peaks at 136, 129, and 117 ppm (Figure S1) and broad aliphatic carbon peaks at 52 and 41 ppm. The 13C NMR spectra of 4′ show the amide peak (−NCC=O) on vinyl pyrrolidone at 176 ppm besides the aromatic vinylimidazole carbon at 136, 129, and 117 ppm. Integration of the carbonyl peak and the vinylimidazole carbon peaks suggest that the vinyl pyrrolidone on 4′ is around 11 mol % (Figure S1 in the Supporting Information). The mol % of co-monomers on 1′, 2′, and 3′ is difficult to determine using NMR data because of their broadly overlapped 1H and 13C peaks. We also performed 19F NMR of 3′ in methanol-d4 with 4-fluorobenzoic acid as an internal reference. The 19F peak of −CF3 appears at −63.91 ppm (Figure S1), confirming the formation of a copolymer.

After sulfonation, polymers 1, 2, 3, 4, and 5 are only soluble in brines and not soluble in common organic solvents. 1H NMR in D2O-containing mixed salts was used for characterizations. The downfield shift of the 1H peak of Ha on vinylimidazole from 7.34 to 8.97 ppm for each polymer is a clear indication of sulfonating the imidazole groups on the polymers (Figure S2 in the Supporting Information). The broad peaks of propane (Hf, He, and Hh) protons of sulfone groups appear at 2.50, 2.98, and 4.29 ppm on the 1H NMR spectrum and at 49.05 and 24.65 ppm on the 13C NMR spectrum (Figure S2).

We use pyrolysis-GC/MS (Py-GC/MS) to determine the molar ratios of the comonomer to vinyl imidazole (VIM) for each random copolymer 1′, 2′, 3′, and 4′. Flash thermal de-polymerization (pyrolysis) at 550 °C of the copolymers generates the molecular ions of VIM, co-monomers, and fragments of the polymer backbone, which are separated by gas chromatography and detected using a mass spectrometer. The chromatogram (total ion count vs retention/acquisition time) of each polymer after pyrolysis shows the elution of the monomers and their derivatives at specific retention times (Figure S3 in the Supporting Information). The area under each peak correlates to the concentrations of the monomers and their derivatives. The same Py-GC/MS condition is also applied to thermally depolymerize polymer 5′ to generate a baseline chromatogram of the decomposed products from the polyvinyl imidazole homopolymer (see Supporting Information). The chromatogram of each copolymer is compared with that of 5′, and the peaks of VIM and the co-monomers are identified based on the retention time and their m/z. The area ratio of the co-monomer and the vinyl imidazole peaks in the chromatogram correlates with the molar ratio of the two monomers in each copolymer, assuming that the mass loss from the further degradation of the monomers is negligible. Table presents the average molar ratio of the two monomers in each copolymer, after three repeats of Py-GC/MS (Table S1 in the Supporting Information). The mol % of vinyl pyrrolidone on 4′ determined by Py-GC/MS is consistent with that determined using the 13C NMR spectrum.

Table 1

Molar Ratio of Co-monomer to VIM via Py-GC/MS

polymer	comonomer/VIM average	standard deviation	comonomer/VIM stoichiometric
1′	0.111	0.029	0.111
2′	0.034	0.004	0.052
3′	0.082	0.002	0.111
4′	0.114	0.011	0.111
5′	0	0	0

The intent to determine the molecular weight of polyzwitterions 1, 2, 3, and 5 in 0.3 M NaCl solution using an aqueous column via gel permeation chromatography (GPC) was not successful because of strong polymer retention in the column. Only polymer 4 passed through the column and its molecular weight was determined to be 807 435 g/mol using a calibration curve of dextran in 0.3 M NaCl solution (Figure S4 and Table S2 in the Supporting Information). Alternatively, we measured the molecular weights of prezwtterionic polymers 1′, 2′, 3′, and 5′ using GPC and a polystyrene calibration curve in a mixed solvent of THF (80%) and methanol (20%) (Figure S5 in the Supporting Information). Interestingly, 4′ is not soluble in the mixed solvent. The number-average molecular weights (Mn, g/mol) of the polymers 1′, 2′, 3′, and 5′ are 483 062, 350 263, 562 385, and 397 289, respectively, which are in the same order of magnitude (Table S3 in the Supporting Information) due to the same loading of the azobisisobutyronitrile (AIBN) initiator. Mn of 4, measured after sulfonation in brine using a different calibration standard, may not be directly compared with that of 1′, 2′, 3′, and 5′.

Polymers 1, 2, 3, and 4 are associative polymers having hydrophobically modified hydrophilic polymers. Previous researches have shown that the associative polymers have complicated fluid behaviors in water and especially in brines.[12] We are particularly interested in polymer adsorption in the SW because SW is extensively used in Middle Eastern oil fields. The SW used in our experiments contains both monovalent and divalent cations and anions, including NaCl, CaCl2, MgCl2, and Na2SO4, and the ionic strength of the SW is 1.15 M (Table S4 in the Supporting Information). We tried to dissolve 0.2 wt % of each polymer, a typical polymer concentration for oil field applications, in SW. Polymers 1′, 2′, 4′, and 5′ are completely soluble in SW at room temperature upon 30 min sonication. Polymer 3′, however, has a slow dissolution kinetics and dissolves in SW after heating at 94 °C for several hours. These polymer solutions in SW are stable over a temperature range of 25–85 °C without any precipitates during heating and cooling cycles. The pH of the polymer solution is in the narrow range of 6.6–6.9 for 1, 2, 3, and 4, and the polymer 5 solution has the lowest pH of 5.7. The viscosity of the polymer 1, 2, 3, 4, and 5 at 80 °C is 0.447, 0.454, 0.436, 0.520, and 0.434 (mPa·s) respectively, which are all higher than 0.425 mPa·s of SW (Table S5 in the Supporting Information). The hydrodynamic diameter dh of the polymers in SW is measured using DLS. Figure shows the dh of 0.2 wt % polymers 1, 2, 3, 4, and 5 in SW at 25 °C. The dh of polymers 1, 3, and 5 in SW has little change over the temperature range of 25–85 °C (Figure S6 in the Supporting Information). The dh of 2 and 4 in SW has large variation between each repeated measurement at temperatures above 55 °C. The hydrodynamic diameter dh of the polymers in the SW are governed by multiple factors including molecular weight, inter- and intrachain electrostatic interactions, counter ions, Lewis acid–base coordination, and van der Waals interactions.

Figure 1

Hydrodynamic diameter of 0.2 wt % zwitterionic polymers 1, 2, 3, 4, and 5 in SW.

In order to quantify the relative adsorption of five zwitterionic polymers on the rock, the static adsorption experiments were conducted using clean Indiana Limestone (LS) particles. The zeta potential ξ of the limestone powders in a 1 mM KCl solution at 25 °C is −31.72 ± 1.55 mV, indicating positive surface charges on limestone. The ξ of the LS powders in SW is hard to measure accurately because of the high ionic strength of the brine. SEM images show that the size of LS particles is ∼1 μm or less (Figure S7 in the Supporting Information). The surface area of LS particles is 0.2975 m2/g determined by BET measurements (Figure S8 in the Supporting Information). It is worth noting that the cumulative pore volume of LS particles is ∼0.001 cm3/g (Figure S9 in the Supporting Information). This indicates that the LS particles have little pores, and the polymer adsorption should be primarily surface-based. For the static adsorption experiments, 2 g of LS particles and 5 mL of 0.2 wt % polymer solution were mixed, sealed, and heated at 80 °C for 3 days (Figure a). As a control experiment, 5 mL of 0.2 wt % blank polymer solution without LS particles was also heated at 80 °C for 3 days to quantify the polymer adsorption onto the glass surface of the vial. Total adsorption was calculated by comparing the UV–vis absorbance after heating for 3 days to a predetermined UV–vis calibration curve for each polymer at various concentrations (Figures S10–S14 in the Supporting Information). Subtracting polymer adsorption onto glass surfaces from total adsorption gave the polymer adsorption on LS surfaces (Tables S6–S10 in the Supporting Information).

Figure 2

(a) Experimental procedures for static adsorption using zwitterionic polymers 1, 2, 3, 4, and 5; (b) static adsorption of zwitterionic polymers 1, 2, 3, 4, and 5 using LS particles in SW at 80° C for 3 days; and (c) schematic representation of interaction between zwitterionic polymers with different co-monomers and LS surfaces.

Figure b shows the quantified static adsorption for zwitterionic polymers 1, 2, 3, 4, and 5 as milligrams of the polymer per gram of LS (mg/g) and as mg of polymer mpper square meter of the rock surface area (mg/m2). The relative adsorption of these zwitterionic polymers follows the order 2 > 4 > 1 > 5 ≫ 3. We also estimate the number density (Np) of the adsorbed polymers per unit rock surface area using mp (mg/m2) and the hydrodynamic diameter dh. The number density (Np) of the adsorbed polymers per unit rock surface area is calculated according to the following equationwhere ρp is the density of each polymer, assuming the ρp of each polymer is the same and is an arbitrary number of 1 mg/m3. The Np for the polymers follows the order 1 > 5 > 4 > 2 ≫ 3 (Figure S15 in the Supporting Information). It is not surprised that 1 and 5 have the highest Np because of their smaller hydrodynamic radius than that of 2 and 4. However, the approach to estimate Np, assuming that the polymer is hard, sphere-like nanoparticles and the hydrodynamic radius does not change upon adsorption on the rock surface, is not quite accurate because the polymers could go through conformation change, thus size changes upon binding to the surfaces.

As the LS surface in SW is predominately adsorbed with positive ions,[23−25] the interactions between the polymers and the LS surfaces include electrostatic attraction through the anionic groups of the zwitterions and the dipole interactions through the co-monomers. Polymers 2 and 4 with strong nucleophilic co-monomers have the highest adsorption to LS surfaces.[26,27] Polymer 1 with styrene as a co-monomer shows 60% less adsorption despite that mol % of styrene is twice as high as that of 4-aminostyrene in the polymer backbone. Pure polyzwitterion 5 exhibits moderately lower rock adsorption compared to that of polymer 1, indicating that styrene moieties may interact with carbonate rock surfaces via potential cation−π interaction.[28] Polymer 3 with 4-trifluoromethylstyrene as a co-monomer has negligible adsorption to LS, probably as a result of the strong repulsion force of −CF3 to most surfaces.[29] The negative adsorption of 3 most likely results from the minor variation of the polymer concentration in SW because of water diffusion (but not polymer) into pores of LS particles. It is worth noting that statistically only one in 10 or 20 repeating units is the functional co-monomer; therefore, it is striking that incorporating only 5–10 mol % of the functional co-monomers into the polymer backbone could greatly change the interaction between the zwitterionic polymers and the carbonate surface (Figure c). To the best of our knowledge, this is the first polymer that shows nearly zero static adsorption on limestone in SW. Other polymers such as the family of polyacrylamides typically show static adsorption of a few hundreds of mg/m2.[20]

Conclusions

In summary, this paper describes a simple, robust zwitterionic polymer system that exhibits tunable static adsorption to carbonate surfaces. Polymer adsorption can be tuned using different functional co-monomers at low molar percentages during free-radical polymerization. The research presented here represents an attractive and useful approach for the petroleum industry to target different applications from tags for mud logging to tracers for reservoir mapping with tailored polymer adsorption. In addition, the design concept presented here may be extended to other polymer and surfactant systems imparting them with tunable affinities toward mineral surfaces.