Non-ionic surfactant loaded on gel capsules to protect downhole tubes from produced water in acidizing oil wells

2.1 Materials and solutions

Polyoxyethylene sorbitan monostearate (T-60), sodium alginate, barium sulfate, and calcium chloride were purchased from Sigma-Aldrich. The composition (wt%) of carbon steel tubes used was 0.07% C, 1.35% Mn, 0.24% Si, 0.017% P, 0.16% Cr, 0.18% Ni, 0.005% S, 0.12% Mo, and the remainder being iron. The oil well produced water used in this study was obtained from the Egyptian Badr El-Din Petroleum Company (BAPETCO) with physical and chemical properties shown in Tables 1 and 2, respectively. The aggressive solution of acidizing produced water was prepared by dilution of analytical grade 37% HCl with filtered oil well-produced water to produce 1 mol l⁻¹ HCl.

2.2 Preparation of gel capsules

The excess concentration of corrosion inhibitor (T-60) was mixed with 0.6 g sodium alginate and 1.2 g barium sulfate as a heavy additive under stirring at 70°C for 20 min until obtaining a homogeneous viscous mixture. Then, the capsules were formed with average diameter of 0.73 cm by injecting the viscous mixture into (2% wt) calcium chloride solution as presented in Figure 2. The formed capsules containing T-60 were collected and left to dry. The average diameter of dried capsules was 0.22 cm.

Figure 2:

Prepared capsules loaded with T-60.

2.3 Weight loss measurements

Weight loss measurements were carried out according to the American Society for Testing and Materials (ASTM) standard method (ASTM International, 1999). The coupons with dimensions of 7 cm×2 cm×0.3 cm were immersed in aerated covered bottle containing acidizing produced water without and with a constant weight of gel capsules loaded with T-60, at different time intervals (3, 6, 12, 24, and 48 h) and different temperatures (298, 323, and 343 K). Then, the carbon steel tube coupons were taken out, carefully washed with distilled water, ultrasonically cleaned in acetone, dried, and then weighed again. The average loss in weight (ΔW) was calculated using the following equation (Farag, 2018):

where W₁ and W₂ are the weight values of coupons before and after immersion in the acidizing produced water, respectively. All experiments were repeated three times and average values were obtained.

2.4 Electrochemical measurements

A Volta Lab 40 (Tacussel-Radiometer PGZ301) potentiostat was used for the electrochemical measurements. The Volta Lab 250 ml corrosion cell with a conventional three-electrode system was used. The carbon steel electrode with 1 cm² embedded in the specimen holder was used as a working electrode. The platinum wire and saturated calomel electrode (SCE) were used as an auxiliary and a reference electrode, respectively. All experiments were performed under aerated and unstirred conditions. Prior to all experiments, the working electrode was immersed in the test solution for 1 h to stabilize the potential and establish a steady-state open circuit potential (OCP). The polarization curves were obtained from −900 to −200 mV (vs. SCE) with a scan rate of 0.5 mV s⁻¹ in the potentiodynamic polarization study. The extracted parameters, such as corrosion potential (E_corr), corrosion current density (i_corr), anodic Tafel slope (β_a), and cathodic Tafel slope (β_c), were analyzed using a Tacussel corrosion analysis software model (Voltamaster 4). In addition, EIS measurements were carried out in the frequency spectrum limit from 100 kHz to 0.03 Hz at OCP with an amplitude of 10 mV using alternating current sine wave. The obtained impedance data were fitted with Zsimpwin 3.6 software. All measurements were performed in triplicate under the same condition to obtain reproducibility of experiments.

2.5 Instrument

The prepared gel capsules without and with T-60 inhibitor were analyzed using Nicolet iS10 FT-IR spectrometer (Thermo Fisher Scientific, Waltham, MA, USA), with measurement ranges from 4000 to 400 cm⁻¹ with a 4 cm⁻¹ resolution. A TGA SDT Q600 V20.5 Build 15 instrument (TA Instruments, New Castle, DE, USA) with a heating rate of 283 K/min and temperature range from 293 to 1123 K, under nitrogen atmosphere, was used to estimate the thermal stability of the gel capsules loaded without and with T-60 inhibitor. For UV-visible absorption, a Cary 50 Scan spectrophotometer (Palo Alto, CA, USA) was used to detect the absorbance of the released T-60 into the acidizing produced water at different times and temperatures to calibrate the inhibitor concentration. A JEOL 5410 scanning electron microscope (Tokyo, Japan) was used for studying the surface morphology of the carbon steel surface.

3.1 Gel capsule characterization

The prepared gel capsules without and with T-60 inhibitor were analyzed using FT-IR spectroscopy, which is a great tool to reveal whether the T-60 inhibitor molecules were encapsulated into the alginate gel or not, as shown in Figure 3. The FT-IR absorption peak of gel capsules alone without T-60 showed a symmetric vibration peak of carboxylate (COO⁻) at 1431 cm⁻¹ (Macher et al., 2015). The weak limited peak at 2925 cm⁻¹ can be assigned to the asymmetric stretching vibration of C–H, due to the cross-linkage of alginate with calcium ions instead of sodium ions, which confirms the transformation of sodium alginate to calcium alginate after being injected in the calcium chloride solution (Taha et al., 2005). The broad peaks around stretching 3424 cm⁻¹ and bending 1615 cm⁻¹ of O–H vibrations suggest the presence of physically adsorbed water molecules in the gel capsule samples. The FT-IR spectrum of T-60 showed typical stretching bands of –C–O–C groups at 1110 cm⁻¹ and 1735 cm⁻¹ band from –C=O stretching of the ester bond from the hydrophilic portion of T-60. The asymmetric 1644 cm⁻¹ and symmetric 1462 cm⁻¹ vibrations were related to COO⁻ groups. The much broader band around 3424 cm⁻¹ likely included the contribution from the -OH groups of T-60 molecules. The FT-IR spectrum of gel capsules loaded with T-60 shows the characteristic bands of both the capsules and the T-60 inhibitor, which confirm that the T-60 inhibitor molecules have been successfully encapsulated into the gel capsules.

Figure 3:

FT-IR chart of gel capsules without and with T-60.

Figure 4 shows the TGA curves of the gel capsules loaded without and with T-60. The weight loss that started for both samples at the temperature range of 303–393 K can be attributed to the desorption of physically adsorbed moisture (Chen et al., 2019). The TGA curves illustrate that the gel capsules without T-60 began to decompose at 453 K, and their complete decomposition occurred at approximately 542 K. The capsules loaded with T-60 showed a higher decomposition temperature and began to degrade at 650 K due to the blending of gel capsules with the T-60 molecules. This confirms that the T-60 inhibitor loaded in the gel capsules is stable at high temperatures and has a long duration time.

Figure 4:

TGA curves of gel capsules loaded without and with T-60.

3.2 Release of T-60 to the acidizing produced water

The idea of the releasing experiments is to calibrate the released concentration of T-60 inhibitor molecules into the acidizing produced water solution at various times and temperatures. Figure 5 shows the releasing of the T-60 inhibitor from gel capsules into the acidizing produced water medium at different times (3, 6, 12, 24, and 48 h) and different temperatures (298, 323, and 343 K). The experiments were conducted using UV-visible measurements with related concentrations, and the data are listed in Table 3. It was found that the T-60 inhibitor molecules loaded in the gel capsules diffused faster at higher temperatures, whereas the capsules were harder to splinter at 298 K than that at 343 K. The experiments revealed that capsule swelling occurred in the acidizing produced water solution and release was induced by capsule splintering. Figure 6 shows a representative scheme of the acidizing process and the addition of inhibitor capsules for an oil well. First, the dry gel capsules became swollen because the acidizing produced water permeated into the interior channels and pores of the gel capsules. Therefore, the inhibitor molecules of T-60 were released rapidly at all temperatures in the initial time. This observation indicates that the inhibitor molecules in open pores at the gel capsule surface dissolve easily and leave quickly. Second, the interior pores and channels of the gel capsules broke due to the acidity and salinity effect of acidizing produced water and finally the capsules splintered, leading to the permeation of inhibitor molecules into solution, which increased more at high temperatures.

Figure 5:

Release curves of T-60 in acidizing produced water at different temperatures.

Figure 6:

Scheme of acidizing process and addition of inhibitor capsules for an oil well.

3.3 Gravimetric study

3.3.1 Effect of concentration

The concentration effect of the released T-60 inhibitor on the corrosion rate of carbon steel in the acidizing produced water was investigated using weight loss measurements. The weight losses of steel coupons were used to calculate the corrosion rate (CR, mg cm⁻² h⁻¹), surface coverage (θ), and corrosion inhibition efficiency (η) from the following equations (Farag et al., 2015a,b):

(2) C R = Δ W S t ,

(3) θ = C R o − C R C R o ,  

(4) η = θ × 100 ,

where S is the surface area (cm²) of the steel coupon, t is the immersion time (h), and CR_o and CR are the corrosion rate values without and with inhibitor, respectively.

The values of calculated parameters are listed in Table 4, and the decay of carbon steel corrosion rate with various released T-60 concentrations is presented in Figure 7. It can be seen that the value of corrosion rate in the presence of released T-60 is much smaller compared to only the acidizing produced water (blank). Obviously, the corrosion rate decreases with increasing released T-60 inhibitor into the acidizing produced water, which may be related to the adsorption of inhibitor molecules on the carbon steel surface and formation of a stable protective film. In addition, the inhibition efficiency increases with the increase of inhibitor releasing, which shows inhibition efficiencies of 64.0% and 87.9% at released T-60 concentration after 3 and 48 h of immersion, respectively. This increase in the inhibition efficiency after 48 h of immersion of the carbon steel in the acidizing produced water confirms that the releasing rate of T-60 inhibitor increases with time, which increases the surface coverage on the carbon steel. Moreover, the inhibition effect of T-60 can be related to its high molecular weight (648.919 g/mol), which covers a large size of the steel surface, as well as the high electronic density of 10 oxygen atoms (molecular formula: C₃₅H₆₈O₁₀) in the inhibitor molecules that increase the adsorption centers on the steel surface.

Table 4:

Weight loss parameters obtained for carbon steel in acidizing produced water without and with various released T-60 at 298 K.

Release time (h)	Released conc. (mol.)	ν (mg cm−2 h−1)	θ	η (%)
0	–	1.0143	–	–
1	0.00010	0.3654	0.640	64.0
3	0.00016	0.2204	0.783	78.3
12	0.00019	0.1811	0.822	82.2
24	0.00021	0.1511	0.851	85.1
48	0.00029	0.1225	0.879	87.9

Figure 7:

Decay of corrosion rate of carbon steel in acidizing produced water solution with different T-60 released at 298 K.

3.3.2 Adsorption isotherm

Different isotherm modules were employed to study the interactions between the released concentration of T-60 from the gel capsules and the carbon steel surface coverage (θ) in the acidizing produced water. Adsorption isotherms (Frumkin, Langmuir, and Temkin) have been applied to simulate the relationship between θ obtained from gravimetric measurements and the released T-60 concentration. The adsorption isotherms can be expressed in the following equations (Gürten et al., 2015):

(5) θ 1 − θ exp ( − 2 a θ ) = K ads C   ( Frumkin isotherm ) ,  

(6) C θ = 1 K ads + C   ( Langmuir isotherm ) ,

(7) exp ( − 2 a θ ) =   K ads C   ( Temkin isotherm ) ,

where C is the inhibitor concentration, K_ads is the equilibrium constant, and a is the molecular interaction parameter. According to the values of correlation coefficient (R²), as shown in Figure 8A–C, it can be concluded that the Langmuir isotherm (Figure 8B) was the best-fit model for the adsorption of T-60 molecules on the steel surface. The correlation coefficient (R²) was found close to unity, which indicates the existence of molecular adsorption within the adsorbed layer. The slope value in the Langmuir model was less than unity and indicates the existence of interactions between the adsorbed inhibitor molecules and the metal surface. The equilibrium constant K_ads can be calculated from the intercept of Figure 8B. The higher value of K_ads (16,666.7 mol⁻¹) implies more efficient adsorption. The dimensionless separation factor, R_L (Mall et al., 2005), can be calculated by Eq. (8), indicating a highly favorable adsorption process in the case of a smaller value (0<R_L<1).

Figure 8:

(A) Frumkin, (B) Langmuir, and (C) Temkin adsorption plots of carbon steel in acidizing produced water solution containing different T-60 released at 298 K.

(8) R L = 1 1 + K ads C .  

The calculated R_L for all concentrations of T-60 was found in the range 0.38–0.17, confirming a highly efficient adsorption (Dkhireche et al., 2010; Moschona et al., 2018). Meanwhile, the relationship between the equilibrium constant K_ads and the standard free energy of adsorption ΔGadso can be derived as follows (Farag & Ali, 2015):

(9) K ads = 1 55.5 exp ( − Δ G ads o R T ) ,

where T is the absolute temperature in K. The negative value of ΔGadso indicates that the adsorption of the T-60 molecules on the carbon steel surface is a spontaneous process (Rabizadeh & Asl, 2019). Additionally, according to the previous study, a value of ΔGadso of around −20 kJ/mol or lower suggests a physisorption mechanism (electrostatic attraction between the charged particles and the charged metal), whereas a value of ΔGadso around −40 kJ/mol or higher indicates chemisorption behavior (sharing electrons from adsorption centers of the inhibitor molecules to the carbon steel surface). The value of ΔGadso for the released T-60 inhibitor was found to be −34.6 kJ/mol. Therefore, it can be concluded that the adsorption of the released T-60 inhibitor on the carbon steel surface is a combined physisorption/chemisorption process.

3.3.3 Effect of temperature

The effect of temperature on the inhibition behavior of the released T-60 on carbon steel tube corrosion was also investigated based on gravimetric measurements. Table 5 presents the corrosion rates and the inhibition efficiencies for carbon steel in the acidizing produced water without and with various released T-60 concentrations at different temperatures of 298, 323, and 343 K. It is clear that the corrosion rate values increased with increasing temperature not only for the inhibited sample but also for the blank (acidizing produced water) sample. The increase in temperature usually accelerates the corrosion reaction, resulting in a higher dissolution rate of the carbon steel in the aggressive acidizing produced water solution. The increase in the corrosion rate with the temperature in the presence of the released T-60 inhibitor may be related to the partial desorption of inhibitor molecules at the metal surface (Cao et al., 2014). However, the released T-60 inhibitor exhibits a smaller value of CR compared with the blank uninhibited solution at the same temperature, which may be related to the strong adsorption of the inhibitor molecules on the metal surface. The apparent activation energy (E_a) for carbon steel dissolution in the acidizing produced water solution without and with released T-60 inhibitor can be calculated according to the Arrhenius equation [Eq. (10)] (Rugmini Ammal et al., 2018). The change in enthalpy and entropy of activation values (ΔH_a, ΔS_a) was calculated from the transition state theory equation [Eq. (11)]:

Table 5:

Weight loss parameters obtained for carbon steel in acidizing produced water without and with 0.00029 mol l⁻¹ of T-60 at different temperatures.

Medium	Temperature (K)	ν (mg cm−2 h−1)	ηg (%)
Blank	298	1.0143	‒
	323	3.1362	‒
	343	4.4345	‒
T-60	298	0.1225	87.9
	323	0.8663	72.4
	343	1.3864	68.7

(10) ln ( C R ) = − E a R T + ln ( A ) ,  

(11) ln ( C R T ) = − Δ H a R T + [ ln ( R N A h ) + ( Δ S a R ) ] ,

where T is absolute temperature and A is the pre-exponential factor. The E_a values are summarized in Table 6, which can be obtained from the straight-line slope of the relationship between ln(CR) vs. 1/T, as depicted in Figure 9. It was found that the calculated value of E_a in the presence of the released T-60 inhibitor was 47 kJ/mol, which was larger than the value of 28 kJ/mol in the absence of the inhibitor. In the previous study, the unchanged or lowered value of E_a compared to the blank uninhibited solution indicates strong adsorption of the inhibitor on the metal surface. This conclusion matches well with that inferred from the Langmuir adsorption isotherm that the released T-60 inhibitor predominantly chemically adsorbed on the carbon steel surface besides being physically absorbed. Further, using Eq. (11), plots of ln(CR/T) vs. 1/T gave straight lines as presented in Figure 10. The change values of the activation enthalpy (ΔH_a) and the activation entropy (ΔS_a) can be calculated through the slope (−ΔH_a/R) and the intercept [ln (R/Nh) + ΔS_a/R] of the ln(CR/T) vs. 1/T curve, respectively. The calculated values of ΔH_a and ΔS_a are listed in Table 6. The positive sign of the activation enthalpy (ΔH_a) reflects the endothermic nature of the carbon steel dissolution process. The negative values of ΔS_a indicate that the formation of the activated complex in the rate-determining step represents an association rather than a dissociation step, signifying that a decrease in disorder takes place during the transition from reactants to the activated complex (Fouda et al., 2014).

Table 6:

Thermodynamic parameters obtained for carbon steel in acidizing produced water without and with 0.00029 mol l⁻¹ of T-60.

Medium	E a	ΔH	ΔS
Blank	28	26	−237
T-60	47	44	−112

Figure 9:

(A) Arrhenius and (B) alternative Arrhenius plots for the corrosion rate of steel in acidizing produced water solution in the absence and presence of released T-60.

Figure 10:

OCP curves of carbon steel in acidizing produced water solution without and with various released T-60 at 298 K.

3.4 OCP and potentiodynamic polarization testing

The time-dependent variation of the OCP of carbon steel immersed in acidizing produced water solution without and with different concentrations of released T-60 inhibitor is presented in Figure 10. It was observed that the OCP values for all uninhibited and inhibited solutions started with decreasing as a result of the dissolution of the oxide layer formed at the carbon steel sample surface (working electrodes) due to exposure to air after polishing. They potential gradually become stable with time. In the presence of released T-60, the OCP was found to shift to a more positive potential compared to that in its absence, suggesting the formation of a protecting layer due to adsorption of the inhibitor molecules on the metal surface. However, the shift in OCP curves between the absence and presence of the inhibitor was <85 mV (30–45 mV), which indicates that the investigated inhibitor is a mixed-type inhibitor as reported by Riggs and Nathan (1973).

The potentiodynamic polarization curves obtained for carbon steel after 1 h of immersion in the acidizing produced water solution without and with the released T-60 are shown in Figure 11. The values of corrosion potential (E_corr), corrosion current density (i_corr), inhibition efficiency (η), and cathodic (βc) and anodic (βa) Tafel slopes are listed in Table 7. Based on the corrosion current density values, the values of inhibition efficiency (η) were calculated as per the following relationship (Migahed et al., 2012):

Figure 11:

Tafel plots of carbon steel in acidizing produced water solution without and with various released T-60 at 298 K.

where i_o and i are the corrosion current densities in the absence and presence of the released T-60 inhibitor. It is clear that the release of T-60 inhibitor molecules to the acidic solution of the produced water causes a remarkable decrease in corrosion current density (i_corr) at all released inhibitor concentrations compared to the uninhibited blank solution. Also, the presence of T-60 in acidizing produced water solution affects the cathodic and anodic Tafel slopes. The differences observed for the values of cathodic and anodic Tafel slopes suggest that the inhibition corrosion mechanism takes place at the active metal surface due to the adsorption of the protective layer of T-60 inhibitor on the carbon steel surface. Moreover, the corrosion potential was shifted to positive values for inhibited solutions, indicating that the carbon steel became passive by forming a protective layer from the T-60 inhibitor on the carbon steel surface capable of providing corrosion protection. However, the displacement in the corrosion potential between the absence and presence of the inhibitor is not enough to decide the anodic type inhibitor; therefore, the inhibitor behaves as a mixed-type inhibitor, which agrees with the OCP measurements (Farag & Noor El-Din, 2012).

3.5 EIS measurements

EIS was performed to investigate the corrosion inhibition behavior of the carbon steel in the acidizing produced water solution in the absence and presence of different released concentrations of T-60, as shown in Figure 12. Nyquist and Bode plots are shown in Figure 12A and B, respectively. The Nyquist plots (Figure 12A) exhibit a semicircle that can be attributed to the charge transfer that takes place at the carbon steel electrode-solution interface, and this process controls the corrosion reaction. It is evident from these plots that the impedance response of the carbon steel in uninhibited acidizing produced water solution is significantly changed after releasing T-60 to the solution. It was found that the semicircle diameters increased with increasing released inhibitor concentrations, and hence increased the charge transfer resistance (R_ct). The values of R_ct can be calculated by the following equation (Farag & Hegazy, 2013):

Figure 12:

(A) Nyquist and (B) Bode plots of carbon steel in acidizing produced water solution without and with various released T-60 at 298 K.

where Z_{r(At low f)} and Z_{r(At high f)} are the low- and high-frequency impedance, respectively. Meanwhile, the values of R_ct can be used to obtain the double-layer capacitance (C_dl) on the working electrode surface at the frequency f_max (at which the imaginary component of the impedance is maximal), through the following equation (Farag & Hegazy, 2013):

Moreover, some Nyquist plots are not perfect semicircles with their centers below the real axis, and this can be ascribed to surface inhomogeneity and roughness of the solid electrode (El Bakri et al., 2019). Therefore, the ideal capacity of the double-layer capacitance (C_dl) was substituted by a constant phase element (CPE) in the equivalent circuit model. The Nyquist impedance plots were analyzed by fitting the experimental data to a simple circuit model [R(QR)], which includes the solution resistance (R_s), the charge transfer resistance (R_ct), and the constant phase element (Q), and the values are listed in Table 8. The impedance of CPE can be expressed as follows:

where Y_o is the magnitude of Z_CPE, j is the imaginary root, ω is the angular frequency, and n is the exponential term. The inhibition efficiency obtained from fitting impedance data was calculated using the following relationship (Farag et al., 2015a,b):

where R_ct and Rcto are charge transfer resistance values in the presence and absence of the T-60 inhibitor, respectively. Obviously, the value of R_ct (Table 7) in the presence of the T-60 inhibitor is larger than that of the blank acidizing produced water solution, suggesting that the dissolution of carbon steel is suppressed in the presence of this inhibitor. The increase in charge transfer resistance values is attributed to the formation of an insulating protective film at the metal-solution interface. It is also noticeable that the value of layer capacitance decreased after the addition of the T-60 inhibitor, which can result from a decrease in the local dielectric constant and/or an increase in the thickness of the electrical double layer, suggesting again the formation of a protective layer by the inhibitor at the metal surface (Migahed et al., 2009). Therefore, the changes in charge transfer resistance and layer capacitance were caused by the steady replacement of water molecules by the adsorption of inhibitor on the carbon steel surface.

Meanwhile, it is apparent from Bode plots that the release of T-60 to the aggressive solution results in an increase in the impedance modulus, which further increases with increasing released inhibitor concentrations (Figure 12B). This can be attributed to the formation of a protective film of the released T-60 on the carbon steel surface, which inhibits the dissolution of iron. As shown in Figure 12B, there is only a single time constant for the corrosion process at the metal-solution interface in the phase-angle plots. The increase in the peak heights with increasing concentrations of T-60 implies a more capacitive response at the metal-solution interface owing to the adsorption of inhibitor molecules (Tang et al., 2013).

3.6 SEM analysis

To investigate the variation on the carbon steel surface images for the polished carbon steel before and after 48 h of immersion in the acidizing produced water solution without and with released inhibitor, the SEM technique was used. It can be observed from Figure 13A that the SEM image of the polished carbon steel surface is relatively smooth with some polished scratches before immersion in the acidizing produced water solution. In the case of immersed carbon steel in uninhibited blank acidizing produced water solution (Figure 13B), the carbon steel is highly corroded with an uneven surface due to the effect of acidity and salinity of the aggressive solution (Mobin et al., 2018). The inhibiting effect of the released T-60 inhibitor to the acidizing produced water is obvious in Figure 13C, where significant improvements of carbon steel, with a smoother surface compared to those in uninhibited blank solution, is shown. This observation clearly proves that the released T-60 inhibitor exhibits good inhibition effect on the corrosion of the carbon steel surface (Cui et al., 2018).

Figure 13:

SEM images of (A) freshly polished carbon steel surface, (B) after 48 h immersion in acidizing produced water without inhibitor, and (C) with released T-60 inhibitor.