Water-soluble carboxymethylchitosan as green scale inhibitor in oil wells

Carbohydrate Polymers 215 (2019) 137–142

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Carbohydrate Polymers
journal homepage: www.elsevier.com/locate/carbpol

Water-soluble carboxymethylchitosan as green scale inhibitor in oil wells
a

a

T

a

Ruza Gabriela M. de A. Macedo , Nívia do N. Marques , Luciana C.S. Paulucci ,
⁎
João Victor Moura Cunhaa, Marcos A. Villettib, Bruno B. Castroc, Rosangela de C. Balabana,
a
b
c

Laboratório de Pesquisa em Petróleo – LAPET, Instituto de Qmica, Universidade Federal do Rio Grande do Norte – UFRN, Natal, RN, Brazil
Departamento de Física, Universidade Federal de Santa Maria – UFSM, Santa Maria, RS, Brazil
Centro de Pesquisa e Desenvolvimento Leopoldo Américo Miguêz de Mello – CENPES/PETROBRAS, Rio de Janeiro, RJ, Brazil

A R T I C LE I N FO

A B S T R A C T

Keywords:
Carboxymethylchitosan
Scale inhibitor
Calcium carbonate
Oil industry

A water-soluble carboxymethylchitosan (CMC) was prepared in water/isopropanol (2/8) medium, at 10 °C, and
characterized by UV–vis, FT-IR and NMR techniques. Its performance as an environmentally friendly scale inhibitor in oil wells was evaluated under the physicochemical conditions of oil wells in northeast of Brazil, by
using SEM, visual compatibility and dynamic tube blocking test. The synthesis conditions led to a degree of
carboxymethylation of 0.45 and water-solubility in all pH range studied (1–11). CMC acted as a scale inhibitor of
CaCO3 under synthetic brine medium, presenting a minimum inhibitor concentration (MIC) of 170 ppm
(1000 psi, T = 70 °C). SEM images showed that CaCO3 crystals were deformed by CMC, which was attributed to
effective interactions of CMC through its carboxylate ions and lone pair of electrons on OH and NH2 groups with
calcium ions, preventing scale deposition.

1. Introduction
Inorganic scale formation is one of the most severe problems in
petroleum industry, leading to total or partial obstruction of equipment
and pipes, causing great damage and economic losses (Kamal, Hussein,
Mahmoud, Sultan, & Saad, 2018; Khormali, Sharifov, & Torba, 2018).
In this scenario, chemicals are often applied as scale inhibitors,
avoiding nucleation or crystal growth. One of the most applied types of
antiscaling are phosphorus-containing materials, which are highly effective, but are nutrients after discharge to sea, leading to eutrophication, besides of being able to promote calcium phosphate deposition
(Zhang, Zhang, Li, Hu, & Hannam, 2010). The growing concern on the
utilization of environmentally safe chemicals and legislation control has
led to the search for green scale inhibitors, which display nontoxicity,
non-bioaccumulation and easy biodegradation (Kumar, Naiya, &
Kumar, 2018; Liu, Xue, & Yang, 2017; Mady, Charoensumran, Ajiro, &
Kelland, 2018).
Polymers have been extensively applied as antiscaling in oil and gas

fields, because of their enhanced thermal stability and better environmental compatibility (Younes, El-Maghrabi, & Ali, 2017). In recent
years, some biopolymers and their derivatives, such as guar and xanthan gums (Elkholy, El-Taib Heakal, Rashad, & Zakaria, 2018), carboxymethylinulin (Kırboga & Öner, 2012), copolymers of β-cyclodextrins (Liu, Zou, Li, Lin, & Chen, 2016; Gu et al., 2013; Liu, Kan et al.,

⁎

2016), carboxymethylstarch (Wang, Li, & Yang, 2017) and starch-g-poly
(acrylic acid) (Yu, Wang, Li, & Yang, 2018) have been evaluated as
green scale inhibitors and the results obtained have been considered
very promising. The presence of hydroxyl and carboxyl groups on the
polymer structure provides chelation, dispersion and crystal distortion
effects.
In particular, chitosan, a polysaccharide mainly obtained from
deacetylation of chitin, exhibits biocompatibility, biodegradability and
hydrophilicity and its degradation products are nontoxic. Because of its
acid-limited solubility, chitosan has been chemically modified to improve its range of applications and some chitosan derivatives have
shown the ability to act as scale inhibitors (Guo et al., 2012; Liang,
Zhao, Shen, Wang, & Xu, 2004; Yang et al., 2010; Yang, Xu, Chen, &
Sui, 2012; Zhang et al., 2015; Zhao et al., 2010). These studies, however, were performed under mild conditions, very different from the
complex environment of petroleum wells.
The selection of an appropriate chemical to prevent mineral scale in
petroleum industry is a challenging task, due to the diversity of parameters that should be taken into account, such as pH, salinity, composition of the water, temperature and pressure of the well, since these
conditions combined can completely change the performance of a scale
inhibitor. The antiscaling product should have the following characteristics: solubility in the medium, stability at the operating conditions and functional groups able to interact with the fouling ions.

Corresponding author.
E-mail address: (R.d.C. Balaban).

/>Received 17 December 2018; Received in revised form 11 February 2019; Accepted 25 March 2019
Available online 25 March 2019
0144-8617/ © 2019 Elsevier Ltd. All rights reserved.

Carbohydrate Polymers 215 (2019) 137–142

R.G.M.d.A. Macedo, et al.

Besides, the concentration applied in the field should be high enough to
effectively inhibit scale deposition, but not too high to avoid incompatibility (insolubility in the aqueous medium) or waste of money
(2016b, Liu, Kan et al., 2016; Popov et al., 2019).
In a recent paper of our group (Macedo, Marques, Tonholo, &
Balaban, 2019), carboxymethylchitosan has been proven to act as an
excellent corrosion inhibitor in media of high salinity, typical of oil
wells. This performance was attributed to interactions of eCOOH,
eNH2 and eOH groups with the metal surface. Knowing that these
functional groups can also contribute to scale inhibition processes, the
main objectives of this work are to describe the preparation and
structural characterization of a water-soluble carboxymethylchitosan
and evaluate it as a novel green inhibitor of CaCO3 scale in pipelines
used in the oil well installations of the northeast region of Brazil, taking
into account the physicochemical conditions of the medium.

Grasdalen, Tokura, & Smidsrød, 1997):

⎡
DS = ⎣

( ). (I ) ⎤⎦ + ⎡⎣ ( ). (I ) ⎤⎦
1

2

1

8

2

9

(1)

IH 1

Where, I1 is the integral of the hydrogen bonded to anomeric carbon, I8
is the integral corresponding to 3- and 6-substituted OeCH2eCOOD
groups, and I9 is integral corresponding to the hydrogens of NeCH2eCOOD groups.
2.5. Conductimetric titration

2. Experimental

The DS was also determined by conductimetric titration, as presented elsewhere (Bidgoli, Zamani, & Taherzadeh, 2010). CMC (0.1 g)
was added to 100 mL of 0.05 M HCl and left under magnetic stirring
during 24 h. The solution was then titrated with 0.1 M NaOH.

2.1. Materials

2.6. UV–vis

Low-molecular weight chitosan was obtained from Polymar S.A
(Brazil). It has a viscosity-average molecular weight of 3.57 × 104 g/

mol and a degree of deacetylation (DD) of 80%, according to procedure
performed previously by our research group (dos Santos Alves, Lima
Vidal, & de Carvalho Balaban, 2009). Isopropanol, ethanol, acetic acid,
NaOH and HCl were obtained from Synth. Salts used in the preparation
of the brine, namely, NaCl, KCl, CaCl2, MgCl2, and NaHCO3 were acquired from Sigma-Aldrich. Chitosan was purified as described previously (dos Santos Alves et al., 2009), with a mass yield of 72%. All the
other reagents were used as received.

The pH-based solubility was estimated on a UV–vis spectrometer
from Shimadzu. About 2 mg of sample was dissolved in 10 mL of 1%
HCl and the resulting solution had pH adjusted by adding 1% NaOH.
The sample was considered insoluble when the transmittance of its
solution at λ =450 nm was ≤ 85% (Chen & Park, 2003; de Abreu &
Campana-Filho, 2009; Sashiwa & Shigemasa, 1999).
2.7. Preparation of brine
Cationic and anionic species of interest were dissolved separately
within individual containers to prevent premature interaction. Thus,
KCl, MgCl2, CaCl2 and NaCl were successively added to water and the
resulting solution was named as “cationic” water; NaHCO3 and NaCl
were dissolved in water to obtain the “anionic” water. After preparation, the pH of the solutions was adjusted to 8.2. Then, they were filtered through 0.45 μm cellulose acetate membrane, under reduced
pressure. The mixing ratio of “cationic” and “anionic” water was of 1:1
to all tests, to give final brine composition showed in Table 1.

2.2. Synthesis of carboxymethylchitosan (CMC)
Carboxymethylchitosan was prepared as described previously (Chen
& Park, 2003), however, with minor modifications (Macedo et al.,
2019). In this case, 10 g of chitosan was added to a water/isopropanol
(2/8) alcoholic solution containing 13.5 g of NaOH. The dispersion was
kept under stirring for 1 h, at 10 °C. Thereafter, 15 g of monochloroacetic acid was dissolved in 20 mL of isopropanol and added to
the reaction mixture. The reaction proceeded for 4 h, at 10 °C. After this
time, 200 mL of 70% aqueous ethanol solution was added to quench the

reaction. The precipitate was washed with 70–90% ethanol and then
with anhydrous ethanol. The product obtained was dried under reduced
pressure.
In order to obtain the acidic form of CMC, 1 g of sample was suspended in 100 mL of 80% ethanol. Then, 10 mL of 37% HCl was added
and the suspension was stirred for 30 min, at room temperature. The
sample was then filtered and the solid was washed with 70–90%
ethanol and dried under vacuum. The mass yield was of 87%.

2.8. Compatibility test
Visual assessment of compatibility between CMC and self-precipitating brine was verified by means of the NACE TM0197 standard
(NACE, 2010). Therefore, 50 mL of "cationic" water was added in a
100 mL Schott flask, and 50 mL of "anionic" water was added into another 100 mL Schott flask. CMC was dissolved in the "anionic" water at
different concentrations. All bottles were then kept in oven for 1 h, at
70 °C. After this time, the "cationic" and "anionic" waters were mixed
and photographed with a Nikon 16.1 Megapixels digital camera, in
order to observe if there is formation of precipitate and/or turbidity in
the medium. The photos were recorded at 0, 1, 2 and 24 h after mixing
the waters, and the samples were kept statically under heating at 70 °C.

2.3. Infrared spectroscopy
Infrared spectroscopy was performed on a Perkin Elmer spectrometer by using an attenuated total reflectance (ATR) accessory. For
each sample, 12 scans were performed from 650 to 4000 cm−1, with a
spectral resolution of 4 cm−1.

Table 1
Composition of self-precipitating brine, which simulates
a real produced water of a well in the Northeast of
Brazil.

2.4. Nuclear magnetic ressonance

Ions

1

H NMR spectra were obtained at 70 °C on a BRUKER AVANCE
400 MHz spectrometer. Chitosan (10 mg/mL) and carboxymethylchitosan (20 mg/mL) were dissolved in D2O/HCl (1%), for
24 h, before analyses. The degree of carboxymethylation (DS = degree
of substitution) was determined by 1H NMR. For that purpose, Eq. 1 was
employed, based on the method described previously in literature
(Mourya, Inamdar, & Tiwari, 2010; Nordtveit Hjerde, Vårum,

+

Na
K+
Ca2+
Mg2+
HCO3−
Cl−

138

Concentration (mg/L)
2231
85
152
33
1000
2686

Carbohydrate Polymers 215 (2019) 137–142

R.G.M.d.A. Macedo, et al.

1020 cm−1. The introduction of carboxymethyl groups on the polysaccharide backbone was evidenced by a peak at 1720 cm−1 on CMC
spectrum, which can be attributed to the symmetrical stretch of C]O
from COOH group. Also, CeO stretching of the CH2COOH group gave
rise to a band at 1240 cm−1 (Bidgoli et al., 2010; Chen & Park, 2003; Do
Nascimento Marques, Curti, Da Silva Maia, & Balaban, 2013; Doshi,
Repo, Heiskanen, Sirviö, & Sillanpää, 2017; Ge & Luo, 2005).
The 1H NMR spectra of chitosan and carboxymethylchitosan are
shown in Fig. 2. Both spectra display a chemical shift at 1.92 ppm,
which corresponds to CH3 of the acetamide groups of chitosan. The
signals at 3.04 and 3.61 ppm refer to the methine protons (CH) of C2
from acetylglucosamine and glucosamine repeat units, respectively.
The chemical shifts from 3.7 to 3.8 ppm, correspond to the protons of
the C3, C4, C5 and C6 carbon atoms. The hydrogen bonded to anomeric
C1 gives rise to the signals of 4.4–4.8 ppm. The structural modification
was confirmed by the appearance of signals corresponding to the hydrogens of the 3- and 6-substituted carboxymethyl group at
4.1–4.3 ppm. The peak attributed on literature to N-carboxymethyl
substitution (3.2 ppm) did not appear, which indicates that the O-carboxymethylchitosan was produced (Chen & Park, 2003; Zheng, Han,
Yang, & Liu, 2011).
The small change on the experimental procedure in relation to the
one reported in literature (Chen & Park, 2003), keeping temperature at
10 °C and increasing the water content, led to solubility of CMC in all
pH range studied and mass yield of 87%. Meanwhile, chitosan had
100% transmittance at pH < 6 and 85% of transmittance at pH > 6
(Table 2). At acid medium, amino groups become protonated; while at

basic medium, carboxylic acid deprotonates and form carboxylate
groups; and, at neutral medium, both amino and carboxylic acid groups
are under ionic forms, leading to solubility of CMC in all pH range.
A carboxymethylation degree of 0.45 was found by 1H NMR, similar
to the one obtained by conductimetric titration, 0.50. Chen and Park
(2003) showed that DS between 0.4 and 0.6 lead to water-soluble
carboxymethylchitosans. Besides, they demonstrated that the conditions of 2/8 water/isopropanol ratio and 50 °C promoted higher DS
(1.1) and mass yield (99.8%), but the products were insoluble at the
acid region (good amount of N-carboxymethylation). At the same time,
decreasing temperature to 10 °C and decreasing the water/isopropanol
ratio to 1/8 promoted a decrease on DS (0.3), decreasing substitution at
C2, C3 and C6 positions, leading to a low mass yield (12.8%), but resulting on a water-soluble CMC. This last condition resulted on a greater
amount of NH2 groups unreacted, which are responsible for solubility of

2.9. Dynamic inhibition efficiency
The minimum effective concentration of CMC as scale inhibitor of
CaCO3 under dynamic conditions was obtained on a Scaled Solution
Ltda's Dinamic Scale equipment, based on the NACE TM31105 standard
(NACE, 2005). The procedure evaluates the performance of the scale
inhibitor under pressure and temperature equivalent to the actual
conditions of oil wells, by using the dynamic tube block test. In this
work, the inhibitor was considered efficient when its pressure differential did not exceed 1 psi in the minimum of 3 times the blank run (no
inhibitor added) or 60 min (whichever is greater). "Cationic" and "anionic" waters were injected through pumps (pumps 1 and 2, respectively) into a capillary of 0.8 mm in diameter and 1 m in length. A third
pump injected into the same capillary a CMC solution solubilized in the
"anionic" water, and from it, together with the pump 2, the desired
concentration of CMC was dosed by the software of equipment. Each
run was automatically stopped if the pressure differential reached 5 psi.
The total flow for the three pumps was maintained at 10 mL/min, under
the pressure of 1000 psi and the temperature of 70 °C.
2.10. Scanning electron microscopy

Changes in morphology of CaCO3 as a function of added CMC was
observed with a Hitachi Tabletop scanning electron microscope. For
this analysis, CMC was solubilized in the “anionic” water, at different
concentrations, and the crystals were obtained by mixing the respective
“cationic” and “anionic” waters. After mixing, the system was allowed
to stand for 24 h in an oven, at 70 °C. After this time, the system was
cooled to room temperature. Then, the brines were filtered under vacuum, through 0.45 μm Millipore cellulose acetate membrane. The
crystals retained on the membrane were collected for analysis.
3. Results and discussion
3.1. Characterization of carboxymethylchitosan
Fig. 1 displays the infrared spectra of chitosan and carboxymethylchitosan. Both present bands corresponding to overlapped OeH
and NeH stretching at about 3300 cm−1, CeH stretching at about 2900
and 2860 cm−1, peaks at approximately 1650 and 1550 cm−1, corresponding to absorptions of C]O stretching (primary amide) and N-H
bending, respectively, besides of stretching of CeOeC bonds at about

Fig. 1. Infrared spectra of (a) chitosan and (b) carboxymethylchitosan.
139


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R.G.M.d.A. Macedo, et al.

Fig. 2. 1H-NMR spectra of chitosan (a) and carboxymethylchitosan (b), both in D2O/HCl (1:1 v/v), at 70 °C.

eCOOH groups have exhibited higher inhibition efficiency. Shorter
chains have greater mobility and more easily enters the crystal lattices,
disturbing the normal growth of the crystal, while acid groups act on
distortion of the crystal lattice via chelating and complexing effects
with scale ions. Polar functional groups able to bind to scale ions also

contribute to higher performance of the inhibitor (Amjad & Koutsoukos,
2014; Elkholy et al., 2018; Wang et al., 2017). Then, the choice of a low
molecular weight chitosan would give better mobility and increased
performance of CMC as scale inhibitor. Taking into account the intended oilfield application, the synthesis conditions were designed to
obtain carboxymethylchitosan with the highest degree of O-carboxymethyl substitution, which would give, at the same time, high yield,
water-solubility in wide pH range and good amount of polar and chelating groups.

Table 2
Solubility data (2 g / L) as a function of pH, obtained by UV–vis, at 25 °C.
pH

Chitosan
Transmittance (%)*

CMC

1
3
5
7
9
11

100
100
100
85
85
85

100
100
100
100
100
100

* Transmittance = 100%, indicates solubility; transmittance = 85%, indicates insolubility.

CMC at acid media. In this paper, when the amount of water was increased to 20%, it probably promoted greater attack of the base on the
polysaccharide chains, leading to higher active sites for carboxymethylation, whereas, at the same time, keeping a low temperature
(10 °C) reduced the velocity of carboxymethylation reaction. Thus, we
found that the conditions applied in this study were a good alternative
to obtain appropriate DS, CMC solubility from acid to basic medium
and suitable mass yield.
Literature has also shown that the performance of a polymer as scale
inhibitor depends on its structural parameters, such as molar mass and
degree of substitution. Lower molar mass and higher amounts of

3.2. Carboxymethylchitosan as scale inhibitor
3.2.1. Compatibility test
Compatibility tests evaluate the solubility of the inhibitor in the
presence of the cation and its ability to avoid scale deposition. Table 3
shows the results of the compatibility test between CMC and self-precipitating water, performed at 70 °C. CMC is compatible in the
50–250 ppm range, immediately after preparation. However, the formation of precipitate in the bottom of the vials was observed after 1 h of
assay at the concentrations of 50 and 100 ppm. At concentrations of 500

Table 3
Compatibility test between CMC and self-precipitating brines for formation of calcium carbonate, at 70 °C.
Time (h)

0
1
2
24

Blank
ppt
ppt
ppt
ppt

*

50 ppm

100 ppm

150 ppm

250 ppm

500 ppm

1000 ppm

limpid
ppt
ppt
ppt

limpid
ppt
ppt
ppt

limpid
limpid
limpid
limpid

limpid
limpid
limpid
limpid

turbid
turbid
turbid
turbid

turbid
turbid
turbid
turbid

* ppt = precipitate.
140


Carbohydrate Polymers 215 (2019) 137–142

R.G.M.d.A. Macedo, et al.

Fig. 3. Efficiency of dynamic CaCO3 precipitation inhibition at 1000 psi and 70 °C.

Fig. 4. SEM images obtained after compatibility test between self-precipitating brines for CaCO3 formation, at 70 °C. (a) Calcium carbonate in the absence of CMC; (b
and c) Calcium carbonate in the presence of 170 ppm CMC.

where there is a constant pressure differential from the beginning to the
end of the run (at 170 and 250 ppm). Thus, it would be reasonable to
define 170 ppm as the minimal inhibitor concentration (MIC), which is
also in the range of CMC concentration for good compatibility with the
brines (Table 3).

and 1000 ppm, the solutions presented turbidity during the entire test,
suggesting incompatibility between CMC and self-precipitating waters
for CaCO3 formation, probably due to CMC crosslinking via Ca2+ ions
bridges. However, when the concentration is 150 and 250 ppm, the
precipitate ceases to exist and the solution is clear throughout the assay,
suggesting compatibility and possible action as an inhibitor in the
formation of calcium carbonate crystals at this range of polymer concentration.

3.2.3. Scanning electron microscopy
SEM images (Fig. 4) refer to CaCO3 precipitates collected after the
compatibility test. In Fig. 4(a), a well-defined crystalline structure in
the form of needles and cubes can be observed, comparable to the ones
found in literature at this temperature range and which were ascribed
to aragonite and calcite, respectively (Yang et al., 2010; Zhao et al.,
2010). Fig. 4(b and c) displays differences in the structure of the
crystals by adding 170 ppm of CMC (minimal inhibition concentration

determined by the dynamic efficiency test). In some points, it is possible
to observe clusters (circle), whereas, at others, the edges of the cubes
are fully deformed (detailed in Fig. 4c). Therefore, it is suggested that
under these conditions, the CMC causes a morphological deformation in
the crystal structure, which difficult their organization and, consequently, minimizes scaling by CaCO3. At the pH of the brine, both
amino and hydroxyl groups can bind to Ca2+ through their lone pair of
electrons, besides of strong electrostatic interactions between Ca+2 ions
and the COO− groups. CMC can then occupy the growing points of the
initially formed crystals and hinder the arrival of further scaling ions.
The crystal morphology is then changed, leading to deformation, preventing crystal growth. CMC can also interact with calcium ions present
in the media via COO−, NH2 and OH groups, preventing the Ca2+

3.2.2. Dynamic scale inhibition efficiency
The efficiency of a scale inhibitor under dynamic conditions can be
determined by monitoring the pressure differential during the encounter of incompatible waters in the presence of the inhibitor. Fig. 3
shows the results obtained by the self-precipitating brine for the formation of CaCO3 in the presence of different concentrations of CMC, at
70 °C and 1000 psi. It can be seen that during the blank run, the pressure differential exceeds 1 psi after 15 min of testing, due to precipitation of CaCO3, that decreases the diameter of the capillary. When
50 ppm of CMC is added, the precipitation time is shifted to approximately 50 min, which indicates the existence of a certain performance
of the CMC against the precipitation of CaCO3. When concentration of
CMC is increased, the inhibitor become more efficient. At a polymer
concentration of 100 ppm, the pressure differential is less than 1 psi
after 1 h of testing, but at the end of the test a slight increase in pressure
differential was observed, which may be related to the precipitation of
CaCO3 crystals inside the test tube. As the concentration of the polymer
increases, it is verified that this slope is minimized, reaching the point
141


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4. Conclusions
The synthesis of the carboxymethylchitosan was successful, being in
agreement with the data presented in the literature. Its solubility occurred throughout the pH range studied (1–11), probably due to the
synthesis parameters adopted, which gave a degree of substitution of
0.45. CMC was compatible with the CaCO3 self-precipitating brine
(150–250 ppm) and the capillary flow test showed that this modified
polysaccharide is efficient in controlling the precipitation of CaCO3,
presenting a minimum inhibition concentration of 170 ppm, under the
conditions of pressure, temperature, pH and salinity oil wells in the
northeast of Brazil. SEM images showed that CMC modifies the morphology of calcium carbonate crystals in the brine. The data suggest a
mechanism of interaction of calcium with the polymer chains and/or
adsorption of the CMC on the initially formed CaCO3 crystals, favouring
the deformation and preventing crystals growth.
Acknowledgement
This study was nanced in part by the Coordenaỗóo de
Aperfeiỗoamento de Pessoal de Nível Superior - Brazil (CAPES) Finance Code 001.
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