nanoparticles-assisted surface charge modification of the porous medium to treat colloidal particles...

Accepted Manuscript

Title: Nanoparticles-Assisted Surface Charge Modification ofthe Porous Medium to Treat Colloidal Particles MigrationInduced by Low Salinity Water Flooding

Author: Danial Arab Peyman Pourafshary

PII: S0927-7757(13)00633-XDOI: http://dx.doi.org/doi:10.1016/j.colsurfa.2013.08.022Reference: COLSUA 18593

To appear in: Colloids and Surfaces A: Physicochem. Eng. Aspects

Received date: 18-5-2013Revised date: 6-8-2013Accepted date: 7-8-2013

Please cite this article as: D. Arab, Nanoparticles-Assisted Surface Charge Modificationof the Porous Medium to Treat Colloidal Particles Migration Induced by Low SalinityWater Flooding, Colloids and Surfaces A: Physicochemical and Engineering Aspects(2013), http://dx.doi.org/10.1016/j.colsurfa.2013.08.022

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http://dx.doi.org/doi:10.1016/j.colsurfa.2013.08.022

http://dx.doi.org/10.1016/j.colsurfa.2013.08.022

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• Nanoparticle (NP) treatment of low salinity water (LSW) flooding was investigated. • NP alters the surface charge of the bed and critically mitigates fines migration. • The Effect of the nanofluids’ ionic strength on the NP performance was studied. � The repulsive energy barrier vanishes due to the effect of nanoparticle. • Severe permeability impairment induced by LSW flooding is treatable by γ-Al2O3 NP.

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Nanoparticles-Assisted Surface Charge Modification of the Porous Medium to Treat Colloidal Particles Migration Induced by Low Salinity Water Flooding

SHORT TITLE: Nanoparticles-assisted remediation of fines migration induced by low salinity water flooding

Danial Araba, Peyman Pourafsharya,1

a Institute of Petroleum Engineering, School of Chemical Engineering, University of Tehran, P.O. Box 11155/4563, Tehran, Iran

E-mail addresses:

[email protected]

[email protected]

ABSTRACT

In this experimental study, nanoparticles (NPs) treatment of low salinity water (LSW) flooding in order to mitigate the induced colloidal particles migration in the medium has been 1 Corresponding author. Tel.: +(98)21-61114712 Fax: +98 (21)-88632976 E-mail: [email protected]

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investigated. Two sets of coreflood experiments were conducted. In the first set, engineered cores were utilized as porous media. In these experiments five types of metal oxide nanoparticles, γ-Al2O3, CuO, MgO, SiO2, and ZnO, were utilized to treat the medium. In twelve flooding tests, LSW was injected into the already NP-treated core and the effluent’s particle concentration was analyzed using a turbidimeter apparatus. These experiments were conducted to find the best NP as an adsorbent of tiny particles. In the second set of the experiments, coreflood tests were performed using Berea cores to investigate the NP treatment of permeability impairment induced by LSW flooding. Quantification methods of dynamic light scattering and zeta potential analysis were done to compare different scenarios. It was found that soaking the medium with a nanofluid slug prior to LSW flooding can be a very promising remedy for the formation damage subsequently induced. The surface charge of the medium treated by γ-Al2O3 NP increases to a critically high value of 33.2 mV which in turn, results in a 70 percent reduction of fines migration compared with the blank test. In addition, the ionic strength of the nanofluid was recognized as an important parameter that affects the treatment efficiency. It was also found that when nanoparticles disperse better in solution they have greater tendencies to alter the surface properties of the medium. The calculated total surface forces quantitatively confirmed the experimental results. Furthermore, the results confirmed that the severe permeability impairment induced by LSW flooding can be dramatically remedied due to the effect of γ-Al2O3 NP.

Keywords

Colloidal particles transport; Fines migration; Low salinity water flooding; Nanotechnology-assisted enhanced oil recovery; Porous media; Zeta potential alteration

1. Introduction

Recent laboratory and field studies have revealed that oil recovery improves when conventional high-salinity seawater injection is replaced by flooding with low salinity water (LSW) with a typical salt concentration ranging from 200 to 8000 ppm [1–4]. In this process, oil is pushed by the injected water toward the producing well. The salinity of the water has a dominant effect on the ultimate oil recovery and a greater amount of oil can be produced when the salinity of the injected water is lowered [1,5]. It is generally accepted that LSW flooding alters the rock wettability toward more water-wet state, which is demonstrated by decreasing contact angles as a result of lowering the water salinity [6–9]. The main mechanism explaining the improvement in oil recovery due to the LSW flooding is cation exchange with the rock surface [6,9,10]. Low-salinity water whose ion concentration is lower than that of the rock tends to exchange cations with the rock surface and, as a result, makes the rock surface more negatively charged. Decreasing the water salinity results in more negative surface charges of the rock and, in turn, lowers electrostatic attractive forces between the rock surface and crude oil, which results in the release of a greater volume of oil [11]. Therefore, altering the wettability of the rock and

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changing the balance of forces between the rock surface and crude oil are responsible for the improved oil recovery obtained by LSW flooding.

On the other hand, brine chemistry has an effect on the resultant electrostatic forces acting on in-situ fine particles [6,10]. There exists a critical salt concentration (CSC) in the water sensitive sandstones, above which particles tend to become attached to the pore walls and, as a result, no reduction in rock permeability occurs. If the ionic strength of the permeating fluid falls below the CSC, which is the case in LSW flooding projects, the tiny particles start to become dislodged and to migrate through the medium and they may finally become trapped at some pore sites [12]. This occurrence is known as formation damage and may result in an uneconomical oil production rate. Accordingly, LSW flooding should be done in a controlled formation damage regime where salinity of the water is low enough to yield improving oil recovery and at the same time high enough to prevent fines migration [13,14]. This limitation makes one choose optimum water salinity rather than the lower one; therefore, there is a narrow water salinity window where still oil recovery is improved but the fine particles remain intact [15].

Khilar and Fogler conducted several systematic experiments and found that a CSC exists only in the case of monovalent cations [16]. The CSC was found to be nonexistent for cations whose valences are greater than one. Table 1 lists the CSC values for different monovalent cations. The CSC depends on the valency of the cations, the specific characteristics of the cations, and the type of porous media in which the fine particles migrate [17]. The dependencies of the CSC on the aforementioned factors have been addressed by Kia et al. [18].

Table 1: Critical salt concentration for single salt systems

Monovalent cation type Porous media CSC (M) References

Na+ Naturally consolidated sandstone 0.070 ± 0.002 [16]

Packed bed of soil 0.250 [19]

Packed bed of glass beads 0.200 [20]

K+ Packed bed of soil 0.067 [19]

Naturally consolidated sandstone 0.044 ± 0.003 [16]

Li+ Naturally consolidated sandstone 0.068 ± 0.002 [16]

The summation of surface forces existing between fine particles and the pore surface determines whether the particles adhere to the surface or are released and migrate into the porous media. The electric double layer repulsion (EDLR) and London–van der Waals attraction (LVA) are the

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most dominant contributors among the electrostatic surface forces. The EDLR is found to be a function of the permeating fluid’s characteristics such as its ionic strength and pH. Ionic conditions of low salinity and high pH result in a reduction in zeta potential and make the total interactions that exist between a fine particle and the rock surface increasingly repulsive, and as a result fine particles detach from the surface [21–28].

As mentioned, LSW flooding has been recognized as a promising method for improving oil recovery, but it also causes indigenous particles to become dislodged and to migrate through the medium, which results in severe formation damage. This creates some uncertainty in the success of LSW flooding and extensive researches are still required. Our goal was to investigate nanoparticle (NP) treatment of suspension transport in the medium as a remedy for this problem. In such a way, one can gain the distinguished advantages of LSW flooding for improving oil recovery without encountering the subsequent induced migration of colloidal particles in the medium. In this study, two types of porous media, engineered porous material and Berea cores, were utilized in coreflood experiments. Five types of metal oxide nanoparticles, γ-Al2O3, CuO, MgO, SiO2, and ZnO, were chosen to investigate their effects on the migration of colloidal particles in porous media. The effect of nanofluids’ ionic strength on the efficiency of the treatment was investigated as well. In several core flooding experiments, the NP-treated medium was flooded by a stable suspension. The salinity of the suspension was very much lower than the CSC. The fine particle concentration of effluent samples obtained at different pore volumes was measured using a turbidimeter apparatus. Quantification methods of dynamic light scattering (DLS) and zeta potential analysis were carried out to compare different scenarios.

2. Background

Nanotechnology is a recently emerging science which has been found to provide solutions to old challenges. Due to their size, which ranges from 1 to 100 nm, nanomaterials show very enhanced properties which are of interest in petroleum production engineering. These properties of NPs include their very high specific surface area, distinguished heat conductivity and thermal properties, chemical potential to modify and alter the wettability of the reservoir rocks, surface charges, ability to alter the surface charge of the reservoir rock, and effects on the rheological properties of suspensions [29–37].

There are some indications in the literature addressing the remedial effects of NPs for fixing the in-situ tiny particles. If the summation of forces existing between a particle and the rock surface is repulsive, the fine particle may detach and migrate into the medium. Following the transport of suspended particles in the porous medium, they may plug some pore throats, impairing the permeability of the reservoir. Therefore, strengthening the attractive forces with respect to the repulsive ones by employing the effect of NPs is a promising remedy to prevent the detachment of fine particles. The presence of NPs on the surface of porous media disturbs the force balance and, as a result, the detachment of particles from the surface can be prevented [38,39]. This idea was first introduced by Huang et al., who used nanocrystals in proppant packs to fix fine

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particles [30]. Belcher et al. conducted a field case study in which the ability of NP-treated proppants to capture the migrating particles was investigated [40]. They reported that the treated proppants have the tendency to capture the migrating particles and as a result the production life of the oil well can be maximized without producing fine particles.

Habibi et al. conducted a series of experiments to investigate the effect of NPs on the migrating fine particles [39]. They utilized MgO, SiO2, and Al2O3 NPs to treat the packed beds. They mixed glass beads with the fine particles to create a porous medium containing in-situ fine particles and used distilled water (DW) injected into the NP-soaked porous medium. They reported that the MgO NPs deposited on the beads could fix indigenous particles even at injection rates above the critical rate. They also investigated the effect of MgO NPs as a clay stabilizer to prevent the water shock phenomenon. They reported that switching the injection fluid from high-salinity brine to low-salinity water caused a drastic pressure drop across the core as a result of water shock. They found that soaking the cores with MgO nanofluid for 120 min resulted in a reduction in permeability impairment due to water shock [41].

So, based on the reported remedial effects of NPs as an adsorbent of tiny particles, we conclude that it is possible to utilize NPs to mitigate fines migration and the subsequent formation damage induced by LSW flooding. In this paper, the capability of NPs to filter the suspended particles existing in the flowing LSW has been investigated. In addition, two types of NP dispersing fluid, DW and LSW, were utilized to investigate the effect of the nanofluids’ ionic strength on the treatment performance.

3. Materials and methods

3.1 Porous material specifications and characterization

3.1.1 Engineered porous medium

In order to mimic sandstone reservoirs, spherical glass beads with an average diameter of 420–595 microns (30–40 U.S. mesh) were utilized as the porous medium. Prior to each experiment, the porosity of the bed packed under confining pressure was determined. It was calculated by measuring the volume of the fluid needed to saturate the core of known total volume.

In order to compare the capability of different NPs to alter the surface charges of the glass beads, zeta potential analysis was carried out using Zetasizer Nano series equipment (Malvern Instruments Inc., London, UK, ZEN 3600). To determine the zeta potential of the beads in the presence of nanoparticles, glass beads were ultrasonicated in the nanofluid of interest. It should be mentioned that in the reference scenarios, i.e., first and seventh experiments (Table 6) the beads were dispersed in the base fluids, i.e., DW and LSW respectively. Following the sonication, the electrophoretic mobility measurement was performed at ambient temperature and neutral pH. Zeta potentials were calculated from the measured electrophoretic mobilities using the approximate expression of Swan and Furst for Henry’s function [42].

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3.1.2 Berea sandstone core

Berea sandstone core (7.62 cm length and 3.8 cm diameter) was utilized to investigate the NP treatment of permeability impairment induced by LSW flooding. The mineralogical composition of the utilized cores was analyzed through X-Ray Diffraction (XRD) technique and the results are given in Table 2.

Table 2: Mineralogy of Berea sandstone cores utilized in this study

Mineral Concentration (Wt. %)

Quartz 86

Kaolinite 9

Feldspar 2

Chlorite 2

Others 1

Table 3 gives the petrophysical properties of the cores. It should be mentioned that in order to prevent the release of in-situ fines and in turn the permeability impairment, high-salinity brine (0.7 M NaCl) was utilized to estimate the initial permeability of the core.

Table 3: Petrophysical properties of the Berea sandstone cores

3.2 Aqueous phase

Distilled water, produced by GFL Water Still 2001/4 (Gesellschaft für Labortechnik Corp., Burgwedel, Germany), was utilized as the base fluid for all the tests. In order to minimize the contamination of water by carbon dioxide, distilled water was produced just before usage and utilized at once for preparation of solutions. It should be mentioned that the pH of the solutions was in the range of 6.6–6.9.

3.3 Fines

The utilized fine particles were characterized using X-Ray Fluorescence (XRF) method and the results are presented in Table 4. It should be mentioned that the XRF apparatus was not

Core ID Initial permeability (md)

Porosity (%)

A 427 20.5

B 354 18.7

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calibrated for each compound; therefore, Table 4 presents relative oxides concentrations. Furthermore, the mean grain size of dry fine particles is between 1 and 10 microns and the precise particle size existing in the suspension was obtained by the dynamic light scattering (DLS) method.

Table 4: Mineralogy of fine particles based on XRF

Component LiO Na2O MgO Al2O3 SiO2 P2O5 SO3 K2O Cao TiO2 Fe2O3

Wt.% 2.020 2.073 0.873 8.236 80.843 0.141 0.155 3.087 1.528 0.118 0.925

3.4 Nanoparticles

Nanosized gamma alumina (γ-Al2O3), copper oxide (CuO), silica (SiO2), magnesium oxide (MgO), and zinc oxide (ZnO) nanoparticles were purchased from U.S. Research Nanomaterials Company, and their characteristics are shown in Table 5. The main criterion for the selection of the NPs was their role in adsorption phenomena reported in the literature as well as their specific surface area.

Table 5: Characteristics of nanoparticles

3.5 Procedure

Several core flooding tests were conducted to investigate the potential effect of NPs on migration of fines in porous media during LSW flooding. The water salinity was set at 0.03 M of NaCl, which is a typical salinity of the water utilized in LSW flooding projects. In addition, the selected salt concentration is much lower than the CSC for Na+cations, which is equal to 0.07 and 0.2 M for the naturally consolidated sandstones and engineered porous medium, respectively (Table 1). In this way, we tried to ensure that the adsorption of suspended colloidal particles on the medium surface did not occur due to the effect of salt concentration.

3.5.1 Coreflood experiments using engineered cores

Nanoparticles Morphology Specific surface area (m2/g)

γ-Al2O3 Spherical 90–160

CuO Nearly spherical ≈ 20

MgO Tetragonal > 160

SiO2 Spherical > 600

ZnO Nearly spherical 20–60

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These coreflood experiments aimed to find the best NP as an adsorbent of fine particles. To begin the experiments, the glass beads were packed into a sleeve of 11.5 cm length and 3.8 cm diameter. A prepared core was fitted into the core holder and 700 psia overburden pressure was applied to its surroundings. In order to saturate the porous medium, the packed bed was vacuumed for 2 hours. Then the porous medium was saturated with different types of nanofluids. After that, the medium was soaked with the nanofluids for 24 hours. Subsequent to the soaking time, the core was flooded by a stable suspension including LSW, cationic surfactant, and naturally occurring fine particles. During the coreflood experiments effluent samples were collected at different pore volumes and their fine particle concentration was analyzed using a turbidimeter apparatus (Hach 2100 Q). Figure 1 shows the experimental set-up. It should be mentioned that the data acquisition system was utilized to record the cumulative volume injected, pressure drop across the bed, and overburden pressure during the coreflood experiments. As shown in Fig. 1, there is a valve prior to the inlet of the core which was used to take four influent samples. After taking these influent samples, the flooding tests were started. The influent concentration was determined from the average of these samples’ concentrations. This procedure was repeated in all the tests in order to obtain the precise concentration of the influent suspension.

Fig. 1: Schematic view of experimental set-up

In order to investigate the effect of the type of NP dispersing fluid on the NPs’ performance, two sets of experiments were done. In the first set of experiments, NPs were uniformly dispersed in DW using an ultrasonic probe. The chosen concentration of NPs was constant and equal to 0.03

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wt% for different scenarios. In addition, in these experiments, the utilized suspension was prepared with DW. In the second set of experiments, 0.03 wt% of NPs was dispersed in LSW containing NaCl (0.03 M). In this set of experiments, the salinity of the NP dispersing fluid was chosen to be exactly the same as that of suspension to be flooded; therefore, in this way one can conclude that the only potentially remedial effects on the transport of colloidal particles in the medium resulted from NPs deposited on the surface of the beads and not from the effect of salt concentration. Furthermore, since NPs tend to agglomerate in solution, the DLS method was utilized to obtain the size of the dispersed NPs. Scanning electron microscopic (SEM) images are provided to visualize the adsorption of fine particles on the treated glass beads as well.

3.5.2 Coreflood experiments using Berea cores

These experiments aimed to investigate the capability of NP to remedy the permeability impairment induced by LSW flooding. Based on the results obtained in the experiments using engineered cores, γ-Al2O3 NP was chosen as the best adsorbent of fine particles; therefore, this NP was utilized in this set of coreflood experiments. In these experiments, the dry core was fitted into the core holder and then, was placed under vacuum while 1400 psia overburden pressure was applied to its surrounding. Afterwards, the core was saturated with LSW-based γ-Al2O3 nanofluid (0.03 wt% NP concentration). Subsequent to 24 hours soaking time, the core was flooded by LSW at a constant flow rate of 4 ml min-1 while pressure drop across the core was monitored. It should be mentioned that in the reference scenario the core was saturated with LSW prior to the coreflood experiment. Accordingly, one can conclude that the only difference between the reference test and the NP-treated scenario is the presence of γ-Al2O3 NP. Furthermore, it should be mentioned that the new intact cores whose properties are presented in Table 3 were utilized in different experiments.

4. Results and discussion

4.1 Experiments using engineered cores

4.1.1 Core flooding tests results

As mentioned, prior to the flooding tests the medium was saturated with different types of nanofluids, whereas in the reference tests, the medium was saturated with DW and LSW (0.03 M NaCl) in the first and seventh experiments respectively. The specifications of the tests performed are presented in Table 6. In each test, the influent suspension concentration is measured based on the turbidity analysis and the results are presented in nephelometric turbidity units (NTU) (Table 6).

Table 6: Specifications of the experiments

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Tests in which NPs were dispersed in DW

Experiment ID Influent concentration (NTU)

Porosity (%)

Flow rate (ml/min)

Overburden pressure (psia)

NP concentration

1st run: reference 48.85 33.85 4 700 –

2nd run: γ-Al2O3NP 59.20 38.14 4 700 0.03

3rd run: CuO NP 50.50 38.89 4 700 0.03

4th run: MgO NP 64.05 38.89 4 700 0.03

5th run: SiO2 NP 68.22 38.14 4 700 0.03

6th run: ZnO NP 46.80 39.66 4 700 0.03

Tests in which NPs were dispersed in LSW

Experiment ID Influent concentration (NTU)

Porosity (%)

Flow rate (ml/min)

Overburden pressure (psia)

NP concentration

7th run: reference 52.00 38.14 4 700 –

8th run: γ-Al2O3 NP 62.90 38.89 4 700 0.03

9th run: CuO NP 76.00 38.14 4 700 0.03

10th run: MgO NP 59.30 38.89 4 700 0.03

11th run: SiO2 NP 64.30 38.89 4 700 0.03

12th run: ZnO NP 72.10 38.14 4 700 0.03

The particle concentration profiles of effluents obtained at different values of pore volume injected (PVI) are presented in Fig. 2. It should be noted that all the values are dimensionless with respect to the pertinent influent suspension concentrations presented in Table 6.

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Fig. 2: Dimensionless form of particle concentration breakthrough profiles: (a) for the tests in which the medium was treated with NPs dispersed in DW; (b) for the tests in which the medium

was treated with NPs dispersed in LSW

The results presented in Fig. 2 indicate that the presence of NPs on the surface of the beads results in a critical reduction of the concentration of fine particles in the effluent samples compared with the non-treated media. As shown in Fig. 2a, ZnO NPs dispersed in distilled water show the best remedial effects and make the treated medium a very good adsorbent of suspended colloidal particles. As can be inferred from Fig. 2b, the particle concentration breakthrough profile obtained in the case treated by LSW-based γ-Al2O3 nanofluid is critically lower than that in the other scenarios, which indicates a very high particle-adsorption capability of the beads in this case. Comparing the results presented in Fig. 2a and b reveals that the medium treated by LSW-based ZnO nanofluid shows the least tendency to filter the suspended particles; therefore, the type of NP dispersing fluid is an important parameter governing the NPs’ performance in treating fines migration in porous media. In addition, the increasing ionic strength of the NP dispersing fluid does not have a uniform effect on the performance of various NPs. In order to investigate this effect, different scenarios are compared separately in Fig. 3.

(a) (b)

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Fig. 3: Comparison of different scenarios: (a) reference test; (b) test with ZnO NP; (c) test with MgO NP; (d) test with SiO2 NP; (e) test with CuO NP; (f) test with γ-Al2O3 NP

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(a) (b)

(c) (d)

(e) (f)

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In order to estimate the repeatability of the experiments, coreflood tests investigating the effect of different NPs on migrating fines, i.e., all the tests except first and seventh experiments were repeated three times and the results presented in Fig. 3 are the average of particle concentration breakthrough profiles obtained in the three repetitions. It should be mentioned that the average error was between 2 and 4%, which proves the repeatability of the experiments. It is clear from Fig. 3a that the suspension’s ionic strength was chosen wisely because around 80% of the suspended particles existing in the influent suspension were detected in the effluent samples. Therefore, the observed adsorption of fine particles in the NP-treated scenarios resulted solely from the NPs’ effects. Figure 3b indicates that the beads treated with DW-based ZnO nanofluid are a very good particle adsorbent and, as a result, the concentration breakthrough profile is approximately 50% of the influent suspension concentration, whereas beads treated with LSW-based ZnO nanofluid showed the least tendency to adsorb the migrating particles, which, as will be discussed later, resulted from the weak ability of the nanofluid to alter the surface charge of the beads in this case. Figure 3d, e, and f reveal that increasing the ionic strength of the nanofluid increases the remedial effect of SiO2, CuO, and γ-Al2O3NPs. These improvements can be explained by the effect of nanofluid ionic strength on the NPs’ capability to change the beads’ surface charge, which will be addressed later.

In order to investigate the dispersion characteristics of NPs in solution and the effect of the ionic strength of the NP dispersing fluid on this property of NPs, the size of NP aggregates was analyzed using the DLS method and the results are presented in Table 7.

Table 7: Size of nanoparticles used in this study at neutral pH

Nanoparticles Size according to manufacturer (nm)

Size of NPs dispersed in DW according to

DLS (nm)

Size of NPs dispersed in LSW according to

DLS (nm)

γ-Al2O3 10–20 113 107

CuO 40 879 503

MgO 63 2830 2940

SiO2 48 587 415

ZnO 10–30 920 1130

According to the results presented in Table 7, increasing the nanofluids’ ionic strength affects the NP dispersion characteristic. As mentioned previously, the advanced properties of NPs result from their very small sizes; therefore, if they can be dispersed appropriately in solution to maintain their nano-scale dimensions, their enhanced properties are attainable. As can be inferred from Table 7, increasing the ionic strength of the dispersing fluid results in a better dispersion of

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CuO and SiO2 NPs in solution. Therefore, the improvements in the adsorption capability of the beads treated with CuO and SiO2 NPs dispersed in LSW can be attributed to the better dispersion characteristics of these NPs in LSW compared to DW.

The size of gamma alumina NPs in low salinity water is 107 nm, which is an indication of their critically good dispersion characteristic and, as a result, γ-Al2O3 NPs deposited on the beads can alter the beads’ surface characteristics appropriately. Accordingly, treated beads in this scenario tend to adsorb more than 70% of the particles existing in the flowing suspension (Fig. 3f). In order to visualize the adsorbent role of the treated beads, SEM images are presented in Fig. 4.

Fig. 4: SEM images: (a) fresh glass beads; (b) particles attached on the surface of non-treated beads; (c) adsorbed particles on the surface of the beads treated with LSW-based γ-Al2O3

nanofluid; (d) closer view of the adsorbed particles on the treated beads

(a) (b)

(c) (d)

Coating of γ-Al2O3 nanoparticles deposited on the glass beads’ surfaces

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The fresh non-treated glass beads are shown in Fig. 4a. Figure 4b visualizes the particles’ attachment on the surface of the non-treated beads obtained after the coreflood experiment in the reference scenario. Furthermore, adsorbed fines on the surface of the γ-Al2O3 NP-treated beads are shown in Fig. 4c. By comparing Fig. 4b and c, one can conclude that the NP-treated beads show a stronger tendency to adsorb the fines compared with the non-treated glass beads. The presence of NPs on the surface of the beads alters the surface charge of the beads and facilitates the adsorption of fines on the surface. Furthermore, the presence of NPs on the beads’ surface provides more collector sites on which the suspended particles have the chance to attach. A closer view of the adsorbed fine particles on the surface of the treated beads is shown in Fig. 4d.

4.1.2 Surface forces

In order to investigate the effect of NPs on the migration of colloidal particles induced by LSW flooding, we invoke surface forces theory. As mentioned, NPs deposited on the surface of the medium alter the surface charge of the medium and, as a result, disturb the force balance in the system. Therefore, the calculated total surface forces can reflect the effect of the NPs on the suspension transport in the medium and provide a descriptive tool to quantify the interactions between a fine particle and the rock surface. The resultant force acting on a suspended fine particle approaching a glass bead is the sum of the electric double layer repulsion, London–van der Waals attraction, Born repulsion, and hydrodynamic forces, which are quantified by Khilar and Fogler [17,43]. Therefore, an equation for the total energy of interaction is obtained by algebraically adding all the contributions and is presented in Eq. 1.

where V is the potential as a function of the separation distance between a fine particle and the pore surface (h) and the meanings of the subscripts are as follows: T: total; EDLR: electric double layer repulsion; LVA: London–van der Waals attraction; BR: Born repulsion; and HR: hydrodynamic potential. Furthermore, negative and positive signs represent the attractive and repulsive energies, respectively. The dimensionless form of the total energy of interaction is shown in Eq. 2 [17].

Here, KB is the Boltzmann constant and is equal to 1.38 × 10-23JK-1 [17] and T is temperature, which is equal to 297 K for the conditions of the flooding tests.

Typically, fine particles are of the order of 0.1–10 microns in size whereas the pore throats on which the migrating fine particles may be attached have diameters of approximately 30–40 microns; therefore, any curvature effect of the pore in comparison with the fine particle can be neglected and pore surface can be considered as a flat plate [17, 44]. In this study the size of fine particles is in the range of 1–10 μm and the pore throats of the engineered porous medium have

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diameters of approximately 65–92 μm. Therefore, the interactions between a suspended particle and the pore surface were modeled to have a sphere–plate geometry, as considered by previous investigators [44–46].

Hydrodynamic potential can become comparable with the colloidal potentials only at very high fluid superficial velocities above 1000 cm hr-1 [44,47]. Since the superficial velocities in this study were 21.1 cm hr-1, which is much lower than the aforementioned value, the hydrodynamic potential is neglected. Therefore, the major forces controlling the interactions between a particle and the pore surface which should be determined in this study are EDLR, LVA, and Born repulsion potentials. 4.1.2.1 Derjaguin–Landau–Verwey–Overbeek (DLVO) theory

The combination of EDLR, LVA, and Born repulsion is known as the DLVO potentials, which indicate the total electrostatic forces between a particle and the rock surface [17,48,49,50]. DLVO theory has been utilized as a quantitative tool to investigate the interactions of NPs with each other, justifying their aggregation in solution, or with the other particles present in the medium (like migrating fine particles, nano-asphaltene, ions, etc.), and also with the rock surface in adsorption and wettability alteration phenomena [29,49,50].

As a suspended particle approaches the pore surface, the diffusive layer surrounding the particle overlaps with the diffusive layer surrounding the pore surface. Thus, due to the overlapping of the diffusive double layers of like-charge surfaces, a repulsive force will form between the two surfaces. The EDLR energy of interaction is obtained by first solving the Poisson-Boltzmann equation with appropriate boundary conditions, in order to determine the potential profile from an infinite to a small distance of separation. Equation 3 is obtained for the EDLR energy of interaction for a constant potential boundary condition [17].

Here, ϵ0 is the electric constant (permittivity of free space), De is the dielectric constant, ap is the fine particle radius, h is the separation distance between a fine particle and the pore surface, and ѱ01 and ѱ02 are the surface potentials of particles and collectors-grains, respectively [17,48]. The surface potentials can be replaced by the measured values of the zeta potentials. The zeta potential of the fine particles in DW and LSW is equal to – 26.5 and – 23 mV, respectively. In this study, the mean radius of fine particles, ap, suspending in DW and LSW is equal to 1.07 and 1.34 microns respectively, which is obtained from DLS analysis of the suspension. The dielectric constant for water, De, is 78.0 and the permittivity of free space (vacuum), ϵ0, is equal to 8.854 × 10-12 C-2J-1m-1 [17,48,51]. The inverse Debye length k is solely a function of the properties of the solution and not of any property of the surface such as its charge or potential. For a monovalent electrolyte at 25 °C, the inverse Debye length of aqueous solution is given in Eq. 4 [51].

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Here, M is the molarity of the monovalent electrolyte and the inverse Debye length obtained is expressed in nm-1. Furthermore, in totally pure water at pH 7 the inverse Debye length is equal to (9.6 × 10-7)-1 m-1[17].

The London–van der Waals energy of interaction is a long range attractive potential which acts over distances as large as 10 nm. Unlike the electric double layer repulsion, this attractive potential is independent of the ionic strength of the aqueous solution [43]. The LVA energy of interaction for the sphere–plate geometry is given in Eq. 5 [17].

where

Here, A132 is Hamaker’s constant between a pore surface and a fine particle separated by an aqueous medium. Typically, Hamaker’s constant is approximately equal to 10-19 J, considering the interactions between two bodies separated by a vacuum [43,51]. In Eq. 5, it is assumed that the interaction between a suspended particle and the pore surface is not influenced by the presence of other neighboring particles and the surrounding fluid separating the particle from the pore wall. This assumption is not valid and the presence of both the other surrounding particles and the intervening fluid must be taken into account. To consider these effects, Hamaker’s constant should be modified based on Lifshitz theory [43]. Israelachvili presented an approximate (but sufficiently accurate) expression for the modified Hamaker’s constant which is a function of the existing materials’ characteristics such as their dielectric constants and the refractive indices [51]. Based on Israelachvili’s expression, Hamaker’s constant is equal to 10-

21J, which is reduced by about two orders of magnitude in the calculations relating to the release of fine particles from the surface of sandstone rocks [43]. In this study, the value A132=10-21J is of sufficient accuracy for our comparative investigation of different scenarios in which a similar suspension was utilized to flood the core.

The Born repulsive potential will form when the suspended particles approach the point of contact and, as a result, the electron clouds overlap. This short range potential, like LVA potential, is independent of the salt concentration of the permeating fluid. This potential has a dominant effect when the distance of separation is smaller than 1 nm and it can be neglected for larger distances compared with the EDLR and LVA potentials. The Born repulsive potential is given in Eq. 7 for the sphere–plate system [17].

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Here, σ is the atomic collision diameter in Lennard-Jones potential and is equal to 5 Å [17,48].

4.1.3 Calculation of DLVO potentials

In order to calculate the DLVO forces existing between a suspended particle and the medium surface, the zeta potential of the treated beads in different scenarios was measured by the DLS method (Table 8). The zeta potential is the potential difference between the so-called slipping plane, formed around a charged surface, and a point at a distance far within the bulk fluid [52,53]. Zeta potential is of high importance for explaining the surface charges, ion adsorption, and electrostatic interactions between colloidal charged particles. It is also a key parameter for investigating the transport of colloidal particles in porous media and can be used to quantify the electrostatic interactions of tiny particles with the medium surface [54,55].

Table 8: Zeta potential of the beads in different scenarios

Tests in which NPs are dispersed in DW

Case γ-Al2O3 CuO MgO SiO2 ZnO Reference

Zeta potential of glass beads(mV) + 0.82 – 7.45 0.11 – 13.60 + 1.57 – 44.00

Tests in which NPs are dispersed in LSW

Case γ-Al2O3 CuO MgO SiO2 ZnO Reference

Zeta potential of glass beads(mV) 33.20 –4.50 –5.70 –11.20 –14.20 –27.60

It is noteworthy that in the reference tests the surface potential of the beads in the presence of LSW is –27.6 mV, which is more positive compared with the potential of the beads in the presence of DW, which is equal to –44 mV. It should be mentioned that the increase in the zeta potential of silica surfaces due to the salinity effect is reported in the literature as well [56–58]. In this case, although the presence of NaCl (0.03 M) results in an increase in the zeta potential of the beads, this change in surface potential is not sufficient to affect the particles’ interactions with the surface of the beads; therefore, as Fig. 3a indicates, the beads showed only a slight capability to adsorb the suspended particles. This poor capability of LSW to alter the surface charge of the rock is the main reason for the induced migration of fines during LSW flooding projects.

As can be inferred from Table 8, the presence of NPs on the surface of the beads alters the zeta potential of the beads. Figure 5 schematically depicts the nanoparticle-assisted surface charge alteration of the medium surface. The greater the capability of the NPs to make the surface

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charge of the beads more positive, the greater the tendency for the beads to adsorb the suspended colloidal particles (Fig. 5b). By comparing the results presented in Table 8, it can be concluded that increasing the ionic strength of the nanofluids critically improves the γ-Al2O3 NPs’ ability to alter the beads’ surface potential. The surface potential of the beads in the presence of LSW is –27.6 mV, which is changed to a critically high value of 33.2 mV in the presence of LSW-based γ-Al2O3 nanofluid. In addition, increasing the ionic strength enhances the capability of CuO and SiO2 NPs to alter the zeta potential of the beads. In these cases, although the zeta potential of the beads has negative values, the increasing ionic strength of the nanofluids results in the surface charge of the beads becoming less negative compared with that found in the corresponding tests done with DW. Accordingly, improvements in the adsorbent roles of the beads treated by CuO and SiO2 NPs dispersed in LSW (Fig. 3d and e) are observed in the flooding tests. Therefore, in tests with γ-Al2O3, CuO, and SiO2 NPs, increasing the ionic strength of the NP dispersing fluid results in an increase in the NPs’ capability to make the surface charges of the beads positive (the surface charges of the beads become more positive).

Fig. 5: Schematic depiction of the beads’ surface charge and its effect on migrating fines: (a) negatively charged non-treated bead; (b) NP-treated bead

As presented in Table 8, the zeta potential of the beads treated by DW-based ZnO nanofluid is 1.57 mV, whereas the potential of the beads treated by LSW-based ZnO nanofluid is equal to –14.2 mV. Therefore, the remedial effect of ZnO NPs on the transport of colloidal particles completely vanishes when they are dispersed in LSW and, as a result, the particle concentration breakthrough profile observed in this case is higher than that of the corresponding ones treated by DW-based ZnO nanofluid (Fig. 3b). This detrimental effect of increasing ionic strength of nanofluids on the capability of NPs to alter the surface charge of the beads is observed in the test with MgO NPs as well (Table 8). As Fig. 3c indicates, the increasing ionic strength of MgO nanofluid results in lower tendencies of the treated beads to adsorb the migrating fine particles.

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In addition, according to the results presented in Table 7, increasing the ionic strength of the NP dispersing fluid has a detrimental effect on the dispersion properties of MgO and ZnO NPs. As mentioned previously, when NPs disperse better in a solution, their distinguished characteristics are more attainable. Therefore, the less remedial effects of ZnO and MgO NPs dispersed in LSW on the transport of particles in the medium can be attributed to the weakening of their dispersion characteristics with increases in the ionic strength of the NP dispersing fluid.

In order to quantify the effect of NPs on the transport of the suspended particles in the medium, the DLVO potentials existing between a particle and the surface were calculated for different scenarios (Fig. 6). The more positive the surface potential of the beads, the less EDLR exists between a suspended particle and the surface and, as a result, there is a greater adsorption of suspended particles onto the surface. In this way the remedial effect of NPs can be reflected by the EDLR energy of interaction and, in turn, by the DLVO potentials.

Fig. 6: Dimensionless form of DLVO potentials as a function of the particle–surface separation distance: (a) for the tests in which NPs were dispersed in DW; (b) for the tests in which NPs

were dispersed in LSW

(b)

No energy barrier for the medium treated by γ-Al2O3 NPs dispersed in

LSW

(a)

Energy barrier

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As Fig. 6 shows, at very small distances of separation the resultant force is strongly repulsive, which is due to the Born repulsive energy of interaction. The Born energy of interaction decays rapidly at distances of separation beyond the collision diameter [17]. Therefore, at larger distances, the EDLR and LVA potentials have dominant contributions in the total interactions between a particle and the surface. At these distances of separation, for the reference tests, the energy barrier formed by the EDLR potential causes the particles to become dislodged from the surface.

As mentioned previously, the NP-assisted surface charge modification of the beads results in a critical reduction of EDLR potential. Accordingly, no repulsive energy barrier exists for the scenarios treated by NPs and, as a result, the total interactions will lead to particle attachment. For the NP-treated cases, the resultant force is much more attractive, which facilitates the adsorption of suspended particles on the medium surface. This is especially observed in the case treated by LSW-based γ-Al2O3 nanofluid. The differences among the NP-treated cases resulted from the different capabilities of NPs to alter the zeta potential of the beads. The calculated total energy of interactions justifies the results of the core flooding tests which are presented in Fig. 2.

4.2 Experiments using Berea cores

As discussed in the previous section, NPs can alter the surface characteristics of the engineered porous media which leads to remediation of fine particles migration. This effect was very noticeable when the glass beads were treated by Gamma alumina NP, which was confirmed by the calculation of DLVO potentials (Fig. 6). In this set of experiments, in order to investigate the NP-treatment of fines migration in a more realistic case, the coreflood experiments were conducted using Berea cores. The utilized Berea cores contain 9 wt% of Kaolinite, the most recognized migrating species among clays (Table 2). On the other hand, the cores contain no swelling clays, especially Montmorillonite. Therefore, the Berea cores can be appropriately utilized to study clay migration. Furthermore, γ-Al2O3 NP was utilized to treat the core prior to LSW flooding, as indicated in section 3.5.2. Figure 7 gives the permeability variation with respect to the initial permeability (K/Ki) obtained at different values of PVI.

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Fig. 7: Permeability change during LSW flooding

As shown in Fig. 7, a drastic decline in permeability was observed in the reference test. This severe permeability impairment resulted from the pore throat blockage induced by LSW injection. The indigenous clays depart during LSW flooding and migrate through the bed and finally may get trapped at some pore constrictions. After one PVI, it seems that the stabilized condition is met and the migrating clays plug a significant number of pore sites; therefore, permeability decline is very dramatic during these periods of time. This phenomenon is widely addressed by previous investigators as well [16,24,41].

The severe permeability impairment observed in the reference case is completely remedied due to the effect of γ-Al2O3 nanoparticle (Fig. 7). In this scenario, the presence of γ-Al2O3 NP on the bed surface prevents clays detachment and the subsequent pore throat blockage during LSW flooding. Accordingly, one can conclude that NP-treatment of fines migration which was comprehensively addressed in the previous sections can be utilized to treat the permeability decline induced by LSW flooding. Therefore, injection of a nanofluid slug into the reservoir prior to LSW flooding can serve as a promising remedy to counteract the induced migration of colloidal particles in porous media. In this way, it is possible to flood the NP-treated medium with LSW, whose salinity is much lower than the CSC, and not encounter the severe formation damage.

5. Conclusion

In water flooding projects, lowering of the water salinity is of great interest in order to maximize oil recovery. However by lowering the ionic strength of the flooded water, the migration of colloidal particles in the medium with subsequent formation damage is an emerging challenge. Accordingly, LSW flooding should be conducted in a controlled formation damage mode where

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one should choose an optimum intermediate salinity rather than the lower one. In this experimental work, NP-assisted surface-charge modification of the porous media in order to mitigate the formation damage induced by LSW flooding was investigated. In two sets of experiments, engineered porous media and Berea cores were utilized to investigate the effects of five types of metal oxide NPs on treating LSW flooding. On the basis of the conducted measurements, the followings conclusions can be inferred:

1- The presence of nanoparticles on the beads’ surfaces alters the zeta potential of the beads, which is the main parameter affecting the interactions of a fine particle with pore surface.

2- Gamma alumina NP dispersed in LSW was selected as the best adsorbent of suspended particles existing in the flowing suspension, which was confirmed by SEM images.

3- The calculated DLVO potentials existing between a fine particle and the pore surface confirmed the adsorbent role of nanoparticle-treated medium.

4- The ionic strength of NP dispersing fluid was recognized as a governing parameter that critically affects the treatment efficiency.

5- It was found that when NPs disperse better in solution they have greater tendencies to alter the surface properties of the beads.

6- Permeability impairment induced by LSW injection into Berea core was dramatically treated due to the effect of γ-Al2O3 NP. Therefore, soaking the porous medium with a nanofluid slug prior to the LSW flooding can be a very promising remedy for the formation damage subsequently induced. In such a way it is possible to obtain the advantages of LSW flooding without encountering the induced migration of tiny particles. This technique can be of particular interest in field application where improving oil recovery is desired, yet induced formation damage should be avoided.

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