Mechanistic understanding of chemical flooding in swelling porous media using a bio-based nonionic surfactant

Journal of Molecular Liquids 229 (2017) 76–88

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Mechanistic understanding of chemical flooding in swelling porous media using a bio-based nonionic surfactant Aghil Moslemizadeh a,⁎, Abbas Shirmardi Dezaki a, Seyed Reza Shadizadeh b a b

Department of Petroleum Engineering, Ahwaz Faculty of Petroleum Engineering, Petroleum University of Technology, Ahwaz 63134, Iran Department of Petroleum Engineering, Abadan Faculty of Petroleum Engineering, Petroleum University of Technology, Abadan, Iran

a r t i c l e

i n f o

Article history: Received 3 September 2016 Accepted 8 December 2016 Available online 18 December 2016 Keywords: Enhanced oil recovery (EOR) Swelling porous media Nonionic surfactant Sweep efficiency

a b s t r a c t A large quantity of world's oil reserves is stored in sandstone reservoirs where clay swelling phenomenon, owing to its negative impact on reservoir quality, imposes a challenge on applicability of surfactant flooding, a subset of chemical enhanced oil recovery (EOR). Therefore, surfactant flooding in swelling porous media should be carried out with more sensitivity. In this study, a great attempt was made to mechanistically understand the performance of a bio-based nonionic surfactant named Zizyphus Spina Christi leaf extract (ZSCLE) in EOR process from swelling porous media. This purpose was achieved through determination of linear swelling, viscosity, pore plugging index, oil recovery, zeta potential, particle size as well as FT-IR analysis. Based on the results, the capability of montmorillonite (Mt, familiar swelling clay) to plug the pores was lost upon getting exposed to ZSCLE aqueous solution, a finding which is promising for surfactant flooding in swelling porous media, especially in the cases where high brine salinity is limited. In contrast to water flooding, ZSCLE was able to do a uniform sweeping with an improvement of sweep efficiency at about 10.72%. It was concluded that in addition to interfacial tension reduction and lowering of the mobility ratio, the increasing of the pore connectivity is another key parameter which is strongly effective in oil recovery improvement using ZSCLE. The adsorbed ZSCLE on Mt particles resulted in a significant decrease in the absolute magnitude of zeta potential and an increase in particle size. From FT-IR analysis, some new picks were detected in infrared spectra of ZSCLE-modified Mt. These indications suggest the interaction between hydrophilic head of ZSCLE and oxygen atoms available on the surface of Mt. This study presents a new criterion for selecting appropriate surfactants so as to stimulate the reservoirs rich in active clays. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Oil recovery process is generally categorized into three different stages comprising primary, secondary and tertiary oil recovery [1]. Primary oil recovery refers to the stage in which oil production relies on the native energy of the reservoir. After this stage, the reservoir pressure reduces as it is insufficient to produce oil; thereby, it is necessary to implement the secondary oil recovery stage which includes applying an external energy (e.g. water flooding) into the reservoir for the purpose of pressure maintenance and oil displacement towards the production wells. Unfortunately, the first two stages of oil recovery are not quite efficient since a large quantity of oil always remains in reservoirs due to capillary forces as well as mobility issues. The residual oil of this stage is often the target of tertiary oil recovery or enhanced oil recovery (EOR). Actually, EOR is a general term for any technique used to increase oil production after the primary and the secondary oil recovery processes [2]. It is necessary that the extraction of the residual oil is quite ⁎ Corresponding author at: Ahwaz Faculty of Petroleum Engineering, Petroleum University of Technology, P. O. BOX 63134, Ahwaz, Iran. E-mail address: [email protected] (A. Moslemizadeh).

http://dx.doi.org/10.1016/j.molliq.2016.12.036 0167-7322/© 2016 Elsevier B.V. All rights reserved.

necessary to respond to the present high rate energy demand of the world. This issue necessitates the implementation of EOR techniques such as chemical flooding, gas flooding and thermal recovery [3]. The main goal of these techniques is to increase the volumetric sweep efficiency and enhance the displacement efficiency [4]. Chemical EOR processes are helpful in many reservoirs. Surfactant flooding is a chemical EOR method during which a slug of surfactant is injected into the reservoir from one or several injection wells in a special pattern [5]. In this process, surfactants decrease the existing interfacial tension (IFT) between oil and aqueous phase, leading to lower capillary forces, flowing the trapped oil bank, lower residual oil saturation and eventually higher ultimate oil recovery [6–8]. Clay swelling phenomenon which comes from the interaction of active clays with aqueous phase can strongly influence the efficacy of chemical flooding by its adverse impact on reservoir quality and ultimately oil recovery [9,10]. This phenomenon is more acute in reservoirs with smaller porosity and higher clay content [11]. Hence, it is imperative to consider the issue of formation damage due to clay swelling in chemical EOR scheme. In much simpler words, all the fluids or chemicals that are injected into the clay containing reservoirs must be checked for compatibility. One of the relevant studies

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in this area is the work of Kazempour et al. [12] who investigated the role of active clays on alkaline-surfactant-polymer (ASP) performance. They stated that the performance of ASP flooding is significantly affected by active clays such as smectite. The evaluation of fluid-rock interaction has strongly been recommended in their work. Most recently, Moslemizadeh et al. [13] assessed the performance of Mulberry leaves extract for improving oil recovery from reservoirs rich in active clays. Despite showing the high capability of active clays in oil entrapment, they concluded that clay swelling cannot be problematic in the case of this surfactant because it can potentially inhibit the swelling of active clays such as smectite. The major restrictions of synthetic surfactants are their cost and environmental concerns which limit their use. An alternative for synthetic surfactants is bio-based ones which are both environmentally friendly and cost effective. Zizyphus Spina Christi leaf extract (ZSCLE) is a bio-based surfactant which has attracted increasing attention in petroleum industry both in EOR and drilling areas [14–24]. Nevertheless, the performance of this surfactant on oil recovery from swelling porous media has not yet been investigated in literature. This article assesses for the first time the performance and mechanism of ZSCLE in surfactant flooding process in reservoirs rich in montmorillonite (Mt, familiar swelling clay). To this end, first, the tests including linear swelling, viscosity measurements and pore plugging are carried out to examine the interaction between ZSCLE aqueous solution and Mt. Second, water flooding with and without ZSCLE into the heterogeneous swelling porous media is performed to evaluate the performance of ZSCLE on oil recovery factor. As the final step, the tests comprising particle size distribution, zeta potential measurements and FT-IR analysis are conducted to clarify the interaction mechanism between ZSCLE and Mt. The results obtained from this study are presented and discussed in greater details throughout the paper. This investigation is quite instructive because it offers a new criterion for selecting appropriate surfactants in stimulation of the reservoirs rich in active clays.

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Spina Christi [31]. Saponins are bio-surfactants mainly produced by plants and less frequently by marine organism and insects [32]. These bio-surfactants could present in N500 plant species [33,34]. The leaves of Zizyphus Spina Christi are rich in Saponins [30]. It is important to consider that molecules of Saponins (Fig. 1) contain both hydrophobic and hydrophilic parts much the same as the synthetic surfactants. The hydrophobic part contains a triterpenoid, steroid or steroid backbone, while the hydrophilic part consists of several saccharide residues attached to the hydrophobic scaffold via glycoside bonds [35]. For the purpose of this study, leaves of the Zizyphus Spina Christi were hand-picked from the cultivated trees in Petroleum University of Technology campus, Ahwaz, southern Iran. Then, Saponin which is a biodegradable nonionic surfactant was extracted from the collected leaves by spray dryer method [36–38]. The extracted powder, Zizyphus Spina Christi leaf extract (ZSCLE), was utilized in this study. The properties of ZSCLE are given in Table 1.

2.1.2. Swelling clay Typical smectite clay, montmorillonite (Mt), with cation exchange capacity of 70.5 meq/100 g characterized by methylene blue test was utilized in this study. The raw sample was characterized by X-ray diffraction (XRD). The test analysis revealed that Mt content is about 65.5%, while other minerals are quartz 12%, cristobalite 10%, muscovite 0.5%, gypsum 0.5%, anorthite 11.5%.

2.1.3. Oil In the present study, the oil sample was taken from one of the Lavan's oil fields, Lavan Island, southern of Iran. The general properties of the utilized oil are given in Table 2.

2.2. Methods 2. Experimental section 2.1. Materials 2.1.1. Surfactant Surfactants, surface active agents, are usually organic compounds that are containing both hydrophobic (tails) and hydrophilic (heads) groups. Therefore, a surfactant molecule contains both water insoluble and water soluble components. Based on the nature of the hydrophilic head, surfactants are classified into four groups including anionic, cationic, zwitterionic, and nonionic [25]. Anionic and cationic surfactants bear a negative and a positive charge on the hydrophilic heads, respectively. On the other hand, zwitterionic surfactants have both a negative and a positive charge. Nonionic surfactants bear no apparent ionic charge; however, the hydrophilic part is soluble in water because of polar groups. These groups can be hydroxyl (OH) or polyethylene oxides (OCH2CH2)2 [26]. Nonionic surfactants have several fascinating aspects compared to other surfactants including inexpensively, compatibility with most other chemicals, better control properties [27], less toxicity, and higher biodegradation potential [28]. Therefore, more effective surfactants with much broader applicability can be obtained from nonionic surfactants alone or by combining them with other surfactants [27]. Zizyphus Spina Christi commonly known as Christ's Thorn Jujube is a deciduous tree with light-grey and very cracked bark. Tropical and subtropical regions are the most suitable areas for growing this tree [29]. It is typically found in Jordan, Iran, Iraq and Egypt [30]. In Iran, different regions, especially south regions, are dedicated for cultivating this tree. Humans have employed the leaves of this plant, which are locally known as “Sedr” and “Konar”, for washing the hair and body. It has been reported that three cyclopeptide alkaloids, four Saponin glycosides, and several avonoids can be extracted from the leaves of Zizyphus

2.2.1. Preparation of surfactant solutions In this study, homogeneous surfactant aqueous solution was prepared by mixing a certain amount of ZSCLE in deionized water using magnetic stirrer. The powder of ZSCLE needs to be slowly added to vortex of deionized water. After dissolving the whole of ZSCLE, the stirrer speed was retarded and solution was stirred for 2 h. In the cases where various concentrations of ZECLE aqueous solution are required, master ZSCLE solution was first prepared and then lower concentrations were achieved by appropriate dilution of it.

Fig. 1. General chemical structure of saponins. The notation R1-R4 represents either H or various sugure groups [16].

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Table 1 Properties of the ZSCLE [17,22]. Product

Total extracted powder of Zizyphus Spina Christi

Used part Preparation Description Color Solubility in cold water Solubility in alcohol pH value (10 wt%) CMC (wt%) Density (g/cm3) Loss on drying (LOD) at 110 °C after 6 h (%) Total asfh at 550 °C after 4 h (%) Applications

Leaves Spray drier Fine powder Light brown Soluble Soluble 5.7–5.9 3.22 0.09 1.6–2 11.7–12 Personal care, special shampoo, herbal cream and ointment, chemical EOR, drilling fluid.

2.2.2. Linear swelling tests As earlier mentioned in the context, clay swelling can strongly impact the reservoir quality and subsequently oil recovery. Here, dynamic linear swelling test is presented to show the influence of adsorbed ZSCLE on linear swelling capacity of Mt in aqueous phase. In this connection, 10 g of Mt was compressed in the form of pill using a hydraulic compactor (under a pressure of 41 MPa for 30 min). Continuing, the pills were placed in linear swelling cup assemblies (LSCA) which were located between the magnetic stirrers and linear variable differential transducers. The solutions were then poured into the LSCA. Finally, linear swelling data were recorded over time. It should be also noted that the fluids were in dynamic condition by magnetic bars located at the bottom of the LSCA. In addition, this phase of study was carried out at 28 °C and atmospheric pressure. 2.2.3. Viscosity measurements The viscosity of Mt dispersion with and without modification by special product could represent the type of particles association, a characteristic which is extremely linked to the chemical environment in which Mt particles are placed. Hence, the kind of interaction between the Mt particles and ZSCLE could be revealed by viscosity measurements. To this end, the dispersion of 15 wt% of Mt was prepared by adding dry Mt to deionized water and different concentrations of ZSCLE aqueous solution. After shaking the dispersion for 30 min, Brookfield DV-II + Viscometer was employed to measure viscosity versus shear rate. 2.2.4. Visual experiments 2.2.4.1. Experimental set-up. Nowadays, glass micro-models can be utilized for different purposes. Here, a glass micro-model was designed,

constructed and used to investigate the effect of ZSCLE on swelling capacity of porous media rich in Mt as well as on oil recovery of water flooding. Fig. 2 shows a sketch of the utilized experimental setup in this study. It is comprised of a glass micro-model, injection and vacuum pumps, a light source located under the micro-model, and a camera attached to computer system. To construct the micro-model, first a pattern of heterogeneous porous media was designed in Corel Draw X7 software. Then, the laser beam was implemented to engrave the designed pattern on the glass surface. The next step was to place another glass on the engraved pattern and eventually to put both of them in the furnace under temperature of 725 °C. During this time period, two glasses were firmly adhered together. Two paths for fluid input and output were also designed at the two ends of the micro-model. Table 3 displays the physical properties of the micro-model. In the experiments, the test fluids were injected into the micromodel by a precise syringe pump (Nexus 6000, Chemyx, USA). After passing the test fluid through the micro-model, the discharged fluid was stored in a waste storage tank. It is noteworthy that the discharge line is also connected to the vacuum pump. The visualization system uses a high resolution camera connected to a computer system. A backlight was also implemented to increase the quality of pictures. The pictures were captured over time and stored in the computer for further analysis. The saturation of oil within the micro-model was calculated using pixel analysis. To do this, Adobe Photoshop CS6 software was used. This method is on the basis of color contrast between the different phases. In the captured images the displaced phase (oil) is quite black and the displacing phase including water and surfactant solution (ZSCLE) are colorless and light brown, respectively. For the image processing in Adobe Photoshop CS6 the Histogram option was used in order to sharpen the contrast of colors between the different phases. The uncertainty observed in this method was b3% for all the measurements. 2.2.4.2. Pore plugging tests. This test is a newly developed method to assess the inhibition effect of a product on swelling capacity of porous media rich in Mt during chemical flooding. In this phase of study, a swelling porous media was made by injecting of Mt dispersion (10 wt%) into the micro-model. The micro-model was then put into the furnace at 150 °C for 5 h. After cooling the micro-model to room temperature, 5 pore volumes (PV) of test fluids were injected into the micro-model with an injection rate of 0.5 ml/h. It should be noted that the test fluids were deionized water and ZSCLE aqueous solution of 3 wt%. In this state, Mt found its maximum capacity to swell depending on the surrounded fluid. The next step was to measure the swelling capacity of porous media through injection of oil, a visible and inert fluid, into the micro-model at different flow rates. The flow rate was increased one after another. Definitely, Mt extremely loses its hydration and swelling potential after exposing to inhibitive fluids. Therefore, the injected oil is capable of touching and then wetting many locations in the porous media, and thus it

Table 2 General properties of the used crude oil. Value

Specifications

Value

Specifications

Specific gravity at 15.5 °C API

0.8486 35.6 9.522

Nitrogen content (wt%) H2S content (ppm) Mercaptan content (ppm)

0.07 21 278

2

6.272

Nickel content (ppm)

9.0

Kinematic viscosity at 40 °C ðmm ;sec Þ

2

3.889

Vanadium content (ppm)

b1

Water content (vol%) Salt content (P. T. B) Wax content (wt%) Sulfure content (wt%) Asphaltenes (wt%)

b0.05 3.0 4.6 1.71 1.2

Iron content (ppm) Lead content (ppm) Sodium content (ppm) Zinic content (ppm) Copper content (ppm)

b1 b1 b1 b1 b1

2

Kinematic viscosity at 10 °C ðmm ;sec Þ Kinematic viscosity at 20 °C ðmm ;sec Þ

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Fig. 2. A sketch of micro-model setup.

leads to longer breakthrough time as well as higher oil saturation in the micro-model. In contrast, when Mt comes into contact with a non-inhibitive fluid such as deionized water, it finds a high tendency to hydrate and swell, which hinders the oil from covering the whole micro-model; thereby, the flow of oil has to pass through open paths. This undoubtedly leads to a much shorter breakthrough time and lower oil saturation in the micro-model. The above explanations well indicate that the swelling capacity of Mt exposed to aqueous phase could be evaluated by calculating oil saturation within the micro-model. To this aim, the picture of micro-model was captured after a certain time period for each injection rate and then was saved on the computer. Oil saturation was then acquired using pixel analysis. The following equation provides an index representing plugging index of Mt in the micro-model. PI ¼

Vt−oil −V f−oil  100 Vt−oil

ð1Þ

where PI is the plugging index, % (0 ≤ PI ≤ 100); Vt − oil, refers to the total volume of the micro-model which can be filled by injected oil, Pixel; and Vf − oil is the final volume of oil in the micro-model after accomplishment of each injection rate, Pixel. 2.2.4.3. Oil recovery tests. Water-sensitive formation damage (i.e., clay swelling) can greatly influence the efficacy of chemical flooding. Hence, its adverse impact needs to be minimized through improving the inhibition potential of injected fluid. Here, the aim is to appraise the performance of ZSCLE as a reservoir stimulator in chemical flooding through porous media rich in Mt. Initially, the dispersion of Mt was injected into the micro-model and then dried like the procedure described for pore plugging tests. Afterwards, water injection was carried out until the micro-model was completely saturated. The next step was Table 3 Physical properties of the used micro-model. Pore diameter (μm)

Throat diameter (μm)

Etched thickness (μm)

Micro-model dimensions (mm)

Pore volume (cc)

Porosity (%)

250–400

150–200

110

37 × 102

0.473

38.2

to start oil injection into the micro-model until the condition similar to irreducible water saturation was obtained. Finally, water flooding with a rate of 0.5 PV/h was performed with and without 3 wt% ZSCLE. In order to calculate oil recovery as the percentage of original oil in place (OOIP), the picture was taken from the whole micro-model after each injected PV and then analyzed via Pixel analysis method. 2.2.5. Adsorption mechanism investigation This section describes tree different experiments in order to achieve a reliable adsorption mechanism of ZSCLE on Mt. 2.2.5.1. Zeta potential measurements. Zeta potential was measured at 28 °C by Malvern Zen 3600 Zetasizer (Malvern, U.K), according to Henry's equation [39]:

UE ¼

2εZF ðKaÞ 3μ

ð2Þ

in which, UE is the electrophoretic mobility; ε refers to the dielectric constant; Z stands for the zeta potential; F(Ka) represents the Henry's function; and μ is the viscosity. First, Mt dispersion was prepared by mixing Mt powder with deionized water via magnetic stirrer. The Mt powder should be slowly added to the sides of eddy of the deionized water. The dispersion was stirred until homogeneous dispersion was achieved. It was then kept for 24 h to reach full hydration. Afterwards, two concentrations of ZSCLE (0.15 wt% and 0.4 wt%) were added to the pre-hydrated dispersion. At the final step, all the dispersion were shaken for 24 h at 28°C to reach the equilibrium condition and consequently a certain volume of every one was taken for zeta potential measurements. To make particle visible, it was not possible to use ZSCLE at concentration N0.3 wt% owing to coloring property of ZSCLE. 2.2.5.2. Particle size distribution. The particle size distribution of Mt dispersion was measured using Malvern Zen 3600 Zetasizer (Malvern, U.K) at 28 °C. The size of Mt particles was recorded in equivalent spherical diameter (nm). Initially, Mt dispersion (0.2 wt%) was prepared by adding Mt powder to deionized water and two concentrations of ZSCLE aqueous solution (0.15 wt% and 0.4 wt%). The dispersion was

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shaken for 24 h and then a certain volume of it was taken for measuring particle size distribution. 2.2.5.3. Fourier transform infrared spectroscopy (FT-IR). To further corroborate of the adsorption mechanism of ZSCLE on Mt, the characteristic adsorption of functional group of ZSCLE, Mt, and modified Mt was studied using FT-IR analysis. Modified Mt was prepared by adding 0.5 g Mt powder to 100 ml ZSCLE aqueous solution in two concentrations (1 wt% and 3 wt%). The dispersion was shaken for 24 at 28 °C to reach equilibrium. After centrifuging at 6000 rpm for 2 h, the recovered solids were dried at 105 °C for 24 h. Finally, the solids were pulverized to fine powder. A little powder of pure ZSCLE, pristine Mt, and modified Mt was mixed with KBr salt using mortar and pestle, and then compressed into thin pellet. Eventually, the infrared spectra were recorded using ATR-IR spectrometer (Bruker Co., USA) with the wavenumber range 400–4000 cm−1 and resolution of 4 cm−1. 3. Results and discussion This section illustrates the performance of ZSCLE, a bio-based nonionic surfactant, in chemical flooding through swelling porous media using extensive experiments. Before discussing the results, it is a good opportunity to explain the Mt structure and its swelling process in aqueous phase. 3.1. Montmorillonite (Mt) and its swelling in aqueous phase Mt represents a large portion of all clays in petroleum reservoirs, especially sandstones. It has received a lot of attention owing to its impact on reservoir quality. Mt is a familiar member of dioctahedral semectite clay group and it is made of a succession of TOT layers comprised of an octahedral (O) sheet sandwiched between two tetrahedral (T) sheets [40]. Isomorphic substitution, which mainly occurs in octahedral sheet and less frequently in tetrahedral sheet, is a process whereby an ion is replaced by another ion with lower charge; for example, Al3 + by mg 2 + and Fe2 + in octahedral sheet and Si4 + by Al3 + and Fe3 + in tetrahedral sheet [41,42]. This process gives a surplus of negative ions at basal surface. This positive charge

deficiency is compensated by adsorbing charge balancing cations (Na+ , K+ Ca2 + or Mg+ 2) in the region named interlayer space, a region between the two TOT layers [40]. The type, size and charge of charge balancing cations greatly affect the magnitude of clay swelling [43]. In Mt, where charge balancing cations are mainly sodium, Mt is referred to as sodium Mt which accounts for a big challenge in the oil-field. What follows is a description of the swelling of Mt in aqueous phase. Generally, clay swelling is defined as a phenomenon during which water molecules encompass a clay crystal structure and cause an increase in the volume of particles. This phenomenon occurs under two distinct regimes: innercrystalline and osmotic. In order to illustrate Mt swelling, visualize dry Mt particles (Fig. 3). In this state, Mt layers are held together tightly via charge balancing cations and van der Waals attraction force. When Mt particles come into contact with deionized water, water molecules orient their negative dipoles towards charge balancing cations. Then, the cations will start to hydrate and arrange themselves on plan halfway between the layers, forcing TOT layers apart in a series of steps. Since this type of swelling takes place due to the hydration of cations, it is called innercrystalline swelling [43]. It has been reported that the distance between TOT layers may reach to the range of 9 Å to 20 Å [44]. On the other hand, osmotic swelling refers to osmotically penetration of water into the interlayer region when the concentration of cations in the interlayer region is higher than that of the surrounding water. Accordingly, this type of swelling which arises from the difference in concentration of cations between the interlayer region and surrounding water is called osmotic swelling [43]. It should be added that the distance between TOT layers has been seen in the range of 20 Å to 130 Å [44]. 3.2. Linear swelling tests Fig. 4 the linear swelling of Mt versus time in deionized water and different concentrations of ZSCLE aqueous solution. Swelling curves showed a similar trend with steep rise within the initial few hours when the lines overlapped. Mt in deionized water revealed a continuous swelling within the whole experiment time so that it had ultimate linear swelling (10.39 mm) much higher than other samples. However, it showed very low swelling capacity in aqueous solution of ZSCLE. Unlike

Fig. 3. Mt swelling in water.

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Linear Swelling (mm)

12

for ZSCLE because maximum IFT reduction, which is the main purpose of surfactant flooding, occurs at CMC not only for ZSCLE but also for most of surfactants [21].

Deionized water 1 Wt% ZSCLE 3 Wt% ZSCLE 6 Wt% ZSCLE

10

81

8

3.3. Viscosity measurements

6 4 2 0 0

10

20

30

40

50

Time (h) Fig. 4. Linear swelling of Mt versus time in deionized water and different concentrations of ZSCLE aqueous solution.

deionized water, the ultimate linear swelling of Mt decreased by factor of 20.01%, 49.18%, and 57.2% in ZSCLE aqueous solution of 1 wt%, 3 wt% and 6 wt%, respectively. Therefore, the inhibition potential of ZSCLE rose with increasing its concentration; nevertheless, no significant improvement was observed after doubling ZSCLE concentration (3 wt% to 6 wt%). This confirms the conclusion of the previous study of the authors [22] who introduced ZSCLE as a shale inhibitor in water-based drilling fluids and stated that the inhibition potential of ZSCLE has no remarkable improvement above critical micelles concentration (CMC). CMC is the concentration in which monomers start to form micelles such that an increase in surfactant concentration will result in an increase in micelles concentration while the concentration of monomers remains unchanged [5]. Since CMC is 3.22 wt% for ZSCLE [22], inefficiency of ZSCLE to restrain linear swelling of Mt at 6 wt% is justifiable. The argument behind this issue is the fact that micelles diffuse more slowly throughout the solution owing to their larger size than a surfactant monomer. This makes them a less efficient charge carrier leading to less ZSCLE-Mt interaction and eventually lower inhibition potential. Overall, the results of linear swelling test indicate that monomers are more successful than micelles in inhibiting Mt swelling, representing CMC as an optimum concentration to do this role. This is a great merit

Fig. 5 shows viscosity versus shear rate for Mt dispersion and its modification by different concentrations of ZSCLE aqueous solution. The viscosity of Mt dispersion decreased extremely after modification by ZSCLE. This can be explained by the fact that upon exposing Mt particles to deionized water, TOT layers adsorb water and then hydrate and swell rapidly. In this state, applying shear rate (agitation) can easily lead to dissociation of TOT layers and eventually a high viscosity profile because more separated TOT layers cause an increased resistance to flow. Alternatively, when Mt particles come into contact with aqueous solution of ZSCLE, their ability to adsorb water reduces considerably. Indeed, ZSCLE keeps clay particles as stacked TOT layers, which in turn leads to fewer particles, less available surface area and eventually fairly low viscosity profile. Another important point to mention is the impact of ZSCLE concentration. The magnitude of viscosity decreased considerably up until concentration of 3 wt% ZSCLE, beyond which no significant reduction was observed such viscosity profiles of Mt dispersion modified by ZSCLE overlapped at concentrations of 3 wt% and 6 wt%. This conveys the message that inhibiting Mt particles is more affected by monomers than micelles. 3.4. Pore plugging tests The aim of this test is to assess the influence of ZSCLE on swelling capacity of Mt in most factual case, porous media. In order to make an interaction between Mt and aqueous phase, 5 PV deionized water and ZSCLE aqueous solution were injected into the micro-model coated by Mt. Afterwards, the capacity of Mt to swell in the porous medium was appraised through oil injection at five different rates. The injection rates which are labeled from Q1 (lowest) to Q5 (highest) increased one after another. In the micro-model exposed to water (Fig. 6), Mt swelling caused an extreme pore plugging or, in much simpler words, a significant decrease in pore connectivity. Hence, as soon as the first injection rate (Q-1) was started, the flow of oil had to divert into a path

100000000 10000000 1000000

0 Wt% ZSCLE 3 Wt% ZSCLE

Viscosity (cp)

100000

0.5 Wt% ZSCLE 6 Wt% ZSCLE

1 Wt% ZSCLE

10000 1000 100 10 1 0.1 0.01 0

20

40

60

80

100

Shear Rate (1/Sec) Fig. 5. Viscosity versus shear rate for Mt dispersion and its modification by ZSCLE.

120

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Fig. 6. Flow pattern of oil at five injection rates (Q-1 to Q-5) after injecting 5 PV water.

Fig. 7. Flow pattern of oil at five injection rates (Q-1 to Q-5) after injecting 5 PV ZSCLE aqueous solution.

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with lower swelling potential. This resulted in a shorter breakthrough time and the lower saturation of oil inside the micro-model (PI = 92.72%). Furthermore, no significant change was seen in pore connectivity after increasing injection rates such that PI was obtained as 83.22%, 83.01%, 80.84% and 74.07% for Q2 to Q5, respectively. This represents maximum capacity of Mt to hydrate and swell in deionized water. An enlarged area of the micro-model after accomplishment of the highest oil injection rate (Q5) is also shown in Fig. 6 which clearly confirms poor pore connectivity due to Mt swelling. Therefore, it can be concluded that the presence of Mt in porous media is a great threat for reservoir quality reduction; consequently, flooding fluids should be fairly inhibitive in such cases. The above procedure was repeated for the micro-model which was in contact with the ZSCLE aqueous solution (Fig. 7). In contrast to deionized water system, Mt had a very low potential to swell in ZSCLE aqueous solution, which led to an extended breakthrough time as well as high oil saturation after accomplishment of each injection rate. Owing to the high pore connectivity, the saturation of oil was observed to augment significantly by increasing the injection rate so that PI was acquired as 74.04%, 42.26%, 21.7%, 17.42% and 10.47% for Q1 to Q5, respectively. Also, an enlarged zone of the micro-model after accomplishment of the highest oil injection rate (Q5) is presented in Fig. 7. It obviously shows that the injected oil can attain every location within the micro-model due to a lack of Mt swelling. The test outcome proves that adsorbed ZSCLE onto Mt could strongly reduce the capacity of Mt to hydrate and swell in porous media. This finding is quite important for chemical flooding through reservoirs containing high amount of active clays (e.g. sandstone reservoirs) especially in cases where high brine salinity condition is limited.

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clear issue is the remarkable portion of the micro-model which is not swept by water flooding. On the other hand, ZSCLE aqueous solution swept the micro-model uniformly such that it could touch almost every location inside the micro-model (Fig. 8-b). Compared with water flooding, ZSCLE aqueous solution was also capable of sweeping a high saturation of oil available at the inlet and outlet of the micromodel. In order to assess the quality of sweep, we have enlarged a region of the micro-model before and after 1 PV flooding as it has been shown in Fig. 9. ZSCLE flooding shows a good quality of sweep as a great number of pores are free from oil. This is quite opposite in the case of water flooding where all pores approximately contain a noticeable amount of oil. The pictures were captured from the entire micro-model after each injected PV and then, using pixel analysis they were analyzed so as to quantify sweep efficiency. From Fig. 10 1 PV flooding by water and ZSCLE aqueous solution recovered respectively 47.36% and 58.08% of OOIP from the micro-model. In much simpler words, ZSCLE improved the magnitude of sweep efficiency at about 10.72%. The amount of sweep efficiency rose slightly with increasing PV for ZSCLE; however, it was nearly constant for water flooding. This might be due to the influence of contact time between the Mt and injected water, because the potential of Mt to swell and consequently to entrap oil increases over time. Capillary number is a dimensionless magnitude which represents the ratio of viscous to capillary forces. It is defined as the following formula [45]:

NC ¼

Viscous force V μ ¼ Capillary force σ  cosθ

ð3Þ

3.5. Oil recovery measurements Fig. 8 presents a picture of the entire micro-model after 1 PV flooding by water and ZSCLE aqueous solution. With a glance at Fig. 8-a, the first

where V and μ are the actual velocity and the viscosity of displacing fluid, σ the displacing phase-displaced phase IFT, and θ is the contact angle.

Fig. 8. Photos of the micro-model for oil recovery tests: a) after one PV water flooding, b) after one PV ZSCLE flooding.

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Fig. 9. Enlarged region of micro-model after one PV flooding by: a) water which shows low sweep quality, b) ZSCLE aqueous solution which shows high sweep quality.

Here, the capillary number was calculated in order to compare the effect of capillary forces on residual oil saturation in water flooding with and without ZSCLE. To this end, the velocity of displacing fluids was calculated based on the injection rate and the micro-model specifications. The parameters including viscosity of ZSCLE aqueous solution, oil-water IFT and oil-ZSCLE IFT were acquired from the literature. Since our micro-model is coated by Mt, the contact angle was measured according to the procedure described by Wu [46]. In this connection, several microscope glass slides (2.2 cm × 2.2 cm) were coated by Mt dispersion of 10 wt% and then allowed to dry for 24 h. The glass slides were placed in environment chamber holder of Theta Optical tensiometer equipped to firewall digital camera and four syringe steel needle. Continuing, water droplets with and without ZSCLE were deposited on glass slides surrounded by oil. The images of free drop at equilibrium were then captured and saved on the computer. Finally, contact angle was obtained via numerically solving the Young–Laplace equation for sessile drops and obtaining the Laplacian curve which best matches the drop profile. Capillary number and all the data used for its determination are summarized in Table 5. It is well-known that a significant increase in the magnitude of the capillary number is required in any EOR process in order to reduce the residual oil saturation [47]. Based on Eq. (1), the capillary number can be increased either by reducing the IFT or altering the contact angle. In contrast to the water flooding, the magnitude of capillary number was observed to have a sharp increase when flooding by ZSCLE aqueous solution such that the ratio of the capillary

70

Recovery (%)

60 50 40 30

ZSCLE flooding

Water flooding

20 10 0 0

2

4

6

8

10

PV Fig. 10. Oil recovery versus PV injected of water and ZSCLE aqueous solution.

12

number of the ZSCLE flooding to the water flooding was obtained at 20.22. This indicates that the capillary force is more dominant in the case of the water flooding than the ZSCLE flooding. In much simpler words, the ZSCLE is capable of decreasing the magnitude of residual oil saturation significantly. It is noteworthy that clay swelling can occur in the micro-model undergoing water injection which results in reducing the magnitude of porosity and eventually increasing the displacing fluid velocity. As a result, capillary number may increase to some extent. This phenomenon cannot be occurred in the case of ZSCLE flooding owing to the capability of ZSCLE in inhibiting Mt swelling. It should be added that clay swelling can lead to significant oil entrapment especially in reservoirs with low porosity. It can be concluded that the ability of ZSCLE in improving the sweep efficiency is probably attributed to three main parameters: the first one is related to the formation of micro-emulsion between the oil and aqueous phase by lowering IFT; the second one is reducing the mobility ratio between the oil and aqueous phase by increasing the viscosity of injected fluid (e.g. 7.32 cp for 4 wt% ZSCLE and 12.75 cp for 8 wt%) [17]; and the third one is restraining Mt swelling which was seen in the current study. The aforementioned discussion implies that ZSCLE is a multi-functional surfactant as it reduces IFT, increases viscosity and inhibits Mt swelling. All these features help to improve the magnitude of oil recovery. 3.6. Zeta potential measurements Zeta potential is an important electrokinetic property of clay particles. It could be implemented as an index measurement representing the stability of water film surrounding clay particles. A high negative zeta potential reflects the great stability of water film surrounding the clay particles and the high degree of clay swelling. Since zeta potential could represent charges at the interface of Mt particles and aqueous solution of ZSCLE, it is quite helpful in identifying the adsorption mechanism of ZSCLE onto Mt. In order to gauge the effect of ZSCLE on surface charge of Mt, certain amount of Mt was added to deionized water and kept for one day. In this state, Mt particles are fully hydrated and their TOT layers can easily dissociate by partial agitation. Since isomorphic substitution in octahedral and less frequently in tetrahedral sheets causes a net negative charge on Mt surfaces, more separated TOT layers are expected to show high negative zeta potential similar to what is shown in Fig. 11 (−29.9 mv). This

Zeta Potential (mv)

A. Moslemizadeh et al. / Journal of Molecular Liquids 229 (2017) 76–88

85

12

adsorbed ZSCLE on Mt particles at quite low concentrations can protect Mt particles from dissociation.

17

3.8. FT-IR analysis

22

27

32 0

0.1

0.2

0.3

0.4

ZSCLE Concentration wt% Fig. 11. Zeta potential of Mt particles modified by different concentrations of ZSCLE.

indicates that the formation of a stable and thick water film surrounding Mt particles results in greater repulsion forces; thereby, dispersion of Mt in deionized water is fully stable owing to high hydration and swelling potential. Surprisingly, when low concentrations of ZSCLE was added to hydrated Mt dispersion, the absolute amount of zeta potential was seen to have a sharp decrease (i.e., shifted from negative to positive). This is due to the fact that ZSCLE could interact with TOT layers and probably coat them. Therefore, it can be concluded that a stable water film cannot be formed surrounding the Mt layers, which points to the low degree of hydration and swelling.

The infrared spectra of Mt, ZSCLE, and modified Mt are shown in Fig. 13. For Mt, the major adsorption peaks were respectively [50]: stretching of structural hydroxyl groups (3622 cm−1), broad stretching band of water (3429 cm−1), deformation band of water (1643 cm−1), band of Si\\O stretching (1035 cm− 1),band of Si\\O\\Si bending (1089 cm−1), deformation band of Al\\Al\\OH vibration (916 cm−1), Si\\O stretching band of silica (796 cm−1), coupled out-of-plan vibration band of Al\\O and Si\\O (624 cm− 1), deformation band of Al\\O\\Si (524 cm− 1), and deformation band of Si\\O\\Si (466 cm−1). Pure ZSCLE showed a characteristic infrared absorbance of the hydroxyl group (\\OH) at 3400 cm− 1, C\\H at 2922 cm−1, C_O at 1740 cm−1, C _C at 1618 cm−1, and C\\O\\C at 1074 cm−1, which are in the range of recorded peaks for saponins in different medicinal plants [51]. Therefore, FT-IR spectrum of ZSCLE confirms the presence of saponins in ZSCLE. Comparing the spectra of Mt and modified Mt samples, we observed some changes which proved the adsorption of ZSCLE on Mt. In modified Mt samples, a peak appeared at 2925 cm− 1. In addition, a strong weakening of shoulder peak at 1089 cm−1 was seen; correspond to Si\\O\\Si bending vibration. This indicates that the adsorption of ZSCLE on Mt may occur through hydrogen bonding between the hydroxyl group (hydrophilic part) of saponin and oxygen atoms available on siloxane (Si\\O\\Si linkage) surface of Mt.

3.7. Particle size distribution

3.9. Probable adsorption mechanism

In a non-inhibitive medium such as fresh water, Mt particles tend to swell and delaminate, leading to high number of particles and thereby reducing the average particle size. Therefore, measuring particle size distribution of Mt in ZSCLE aqueous solution could serve as a good indication demonstrating the impact of ZSCLE on Mt particles delamination. Delamination of clay layers is a process during which clay particle size reduces and the specific surface area increases [48]. The degree of delamination strongly depends on the swelling potential of clay minerals in special aqueous solution [49]. High delamination potential is a sign of weak inhibition medium which leads to a decrease in particle size. Fig. 12 exhibits the average particle size of Mt versus ZSCLE concentration. In this phase of study, dry Mt was added to deionized and ZSCLE aqueous solution. In the case of deionized water, Mt particles adsorbed water easily and delaminated so that it had the lowest average particle size (672.2 nm). In contrast to deionized water, Mt yielded lager particles such that the average particle size reached 5235 nm at a low concentration of 0.4 wt%. As a conclusion, the test result exhibits that

The aim of this section will be to identify the justifiable mechanism for adsorption of ZSCLE onto Mt. The results from the current study proved that the adsorbed ZSCLE by Mt during surfactant flooding could effectively suppress the hydration and swelling potential of Mt. Considering all of the adsorption mechanisms suggested for different products, capable of inhibiting clay swelling, the indications acquired from this study as well as the chemical structure of ZSCLE, we may present the following probable adsorption mechanism: Fig. 14 represents adsorption process of ZSCLE on Mt. Isomorphic substitution in tetrahedral and less frequently in octahedral sheets leads to net charge deficiency in Mt having an overall negative charge on the individual layers. The molecules of ZSCLE (saponin) consist of hydrophilic and hydrophobic parts (Fig. 1). It is generally believed that the hydrophilic part of ZSCLE will be adsorbed onto the Mt surface, while the hydrophobic part will be oriented towards the aqueous phase. This adsorption could be due to the hydrogen bonding between the hydroxyl groups of hydrophilic part of ZSCLE molecules and available

Transmittance 0.4

0.5

3400

2800

1600

2200

1000

466

0.3

624

0.2

796

0.1

1035 1089

4000

0

1074

0

ZSCLE Mt-ZSCLE (3 wt%) Mt-ZSCLE (1 wt%) Mt

1643

1000

3429

2000

1618 1740

3000

2925

4000

3400

5000

2929

6000

3622

Average Particle Size (nm)

7000

400

Wavenumber

ZSCLE Consentration wt% Fig. 12. Particle size distribution of Mt in different concentrations of ZSCLE aqueous solution.

Fig. 13. FT-IR spectra of Mt, ZSCLE and modified-Mt by ZSCLE. The percentages in parentheses represent the concentrations of ZSCLE aqueous solution with which Mt were treated.

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Fig. 14. Orientation of ZSCLE molecules on Mt surface.

oxygen atoms on siloxane surface of Mt. In other words, ZSCLE molecules will compete with water molecules over the adsorption onto the Mt surfaces. If the quantity of ZSCLE molecules turns out to be sufficient, a film of ZSCLE will be laid down on the Mt surface which results in less water adsorption and subsequently suppression of the hydration and swelling. The aforementioned facts could be considered as the main mechanism for the adsorption of ZSCLE on Mt. 3.10. ZSCLE versus common surfactants Having a robust and economical surfactant flooding is strongly connected to various issues which mainly include cost, surfactant loss, IFT reduction, oil recovery, and environmental impact. Here, an attempt is made to compare these issues in the case of ZSCLE and well-known surfactants in petroleum industry. 3.10.1. Cost The first and perhaps most important parameter in surfactant flooding process is economic possibility. It relies on parameters including oil prices, international economics, surfactant cost, and its availability. ZSCLE which was extracted from the leaves of Zizyphus Spina Christi can be easily found in Middle East countries such as Iran and Egypt with a low cost of US$ 1.0–1.5 per kilogram of leaf [16]. The price of common surfactants in petroleum industry has been reported as US$ 3–5 per kilogram [52]. This conveys the message that using natural surfactants at high concentration is still economical; hence, low cost could be an appropriate feature for ZSCLE. 3.10.2. Surfactant loss Loss of surfactant due to adsorption on reservoir rock surface is another important issue which can reduce the technical and economic efficiency of surfactant flooding process. So far, a lot of studies have been carried out on adsorption behavior of ZSCLE onto different reservoir rocks. As pointed out earlier, Mt, which represents a large portion of

Table 4 ZSCLE versus two nonionic surfactants: surfactant loss comparison. Surfactant Injected solution (% above CMC)

Rock

Test Surfactant loss Reference condition (mg/g-rock)

ZSCLE ZSCLE ENP95 ENP150

Sandstone Sandstone Sandstone Sandstone

Dynamic Dynamic Dynamic Dynamic

19.4 138.8 30 30

3.32 6.88 13.15 8.47

[14] [14] [53] [53]

clays containing sandstones, has attracted increasing attention owing to its adverse impact on reservoir quality. Table 4 compares ZSCLE against two nonionic surfactants at quite similar conditions. It is clear that the adsorption of ZSCLE is much lower than ENP95 and ENP150 even at higher concentrations of injected fluid. Furthermore, the results of the present study showed that adsorbed ZSCLE decreases the swelling potential of Mt and subsequently prevent reservoir quality reduction. This characteristic is a great merit for ZSCLE because it has not been reported in the case of industrial surfactants. Very recently, Moslemizadeh et al. [54] assessed the swelling inhibitive effect of CTAB adsorption on Mt in aqueous phase. According to their work, CTAB, a celebrated cationic surfactant in EOR applications, is featured by an outstanding performance in inhibiting Mt swelling though at concentration much higher than CMC, 1 wt% as an optimum concentration. Since CMC is the optimum concentration for most of surfactants as well as CTAB in chemical EOR, it can be inferred that CTAB is free from the inhibitive characteristic in chemical flooding. Thus, despite having a low adsorption rate, the swelling inhibitive characteristic of ZSCLE is to ZSCLE which makes it an appropriate surfactant for chemical EOR.

3.10.3. IFT reduction It is widely accepted that the main purpose of surfactants in chemical EOR is to reduce IFT between oil and aqueous phase. Given the literature, we know that the minimum magnitude of IFT between Kerosene oil and water are 9 mN/m, 14.5 mN/m and 17.5 mN/m for ZSCLE, alkyl poly glycosides, and alkyl sulfates, respectively [21]. Thus, the ability of ZSCLE to lower IFT between oil and aqueous phase is similar to common surfactants, raising its applicability in chemical EOR.

Table 5 The capillary number and all the data used to its determination. Parameters

Value

Reference

σoil −ZSCLE(N/m) σoil −water (N/m) μZSCLE(Pa ⋅s) μwater(Pa ⋅s) VðmsÞ θzscle flooding(degree) θwater flooding(degree) NC−zscle flooding NC−water flooding

0.0127 0.0325 0.00567 0.00100 4.2254 × 10−5 51.41 31.82 3.0939 × 10−5 1.5300 × 10−6 20.22

[17] [17] [17] Present study Present study Present study Present study Present study Present study Present study

N C−ZSCLE flooding N C−water flooding

A. Moslemizadeh et al. / Journal of Molecular Liquids 229 (2017) 76–88

3.10.4. Oil recovery The final aim of every EOR technique is to increase residual oil recovery. Ahmadi and Shadizadeh [17] investigated the effect of ZSCLE on oil recovery from carbonate reservoirs using core displacement experiments. Based on their results, ZSCLE was able to improve oil recovery of water flooding at about 13.68% and 25.63% for concentrations of 4 wt% and 8 wt%, respectively. In addition, the present study proved the high performance of ZSCLE in enhancing oil recovery from swelling porous media. It can be concluded that ZSCLE is a suitable candidate for chemical EOR.

Variables UE μ ε Z F(Ka) PI Vt−oil Vf−oil

3.10.5. Environmental impact ZSCLE is a biodegradable surfactant which has traditionally been used for washing the hair and body. Hence, environmentally it has superiority over industrial surfactants, such as Alkyl poly glycosides, Alkyl Sulfates, and CTAB. 4. Conclusions The purpose of this research was to mechanistically understand the performance of ZSCLE as a bio-based nonionic surfactant in EOR process from swelling porous media. The following findings were acquired: Linear swelling tests showed that the adsorption of ZSCLE significantly decreases the swelling potential of Mt. Based on viscosity measurements, the viscosity of Mt dispersion considerably decreased after being modified by ZSCLE. Unlike deionized water, pore plugging tests indicated that ZSCLE flooding through swelling porous media can strongly decrease the ability of Mt to plug the pores and eventually to entrap oil. The aforementioned tests revealed that Mt lost completely its capability to hydrate and swell in aqueous solution of ZSCLE, a finding which is promising for surfactant flooding in reservoirs rich in active clays especially in some cases where high salinity condition is restricted. Based on oil recovery determinations, ZSCLE can improve sweep efficiency of water flooding from a swelling porous medium at about 10.72% of the OOIP. Apart from the reduction of oil-aqueous phase IFT and mobility ratio reduction, restraining clay swelling too has a remarkable role in this improvement. Adsorption of ZSCLE on Mt decreased the absolute value of zeta potential, representing instability of water film surrounding Mt particles. In contrast to deionized water, Mt yielded larger particle size in aqueous solution of ZSCLE, indicating the lower degree of particles delamination. Based on FT-IR analysis, some picks were recognized in the infrared spectra of ZSCLE-modified Mt that indicate the interaction of ZSCLE with Mt. In view of the evidence of this study and those available in the literature, it is concluded that the adsorption of ZSCLE on Mt might occur through hydrogen bonding between the hydrophilic head of ZSCLE and oxygen atoms available on the surface of Mt. All the aforementioned results suggest that, in addition to high performance, low cost and environmentally friendly characteristics, ZSCLE has an excellent inhibitive property which is ideal for surfactant flooding through reservoirs more susceptible to formation damage due to clay swelling. Nomenclature Acronyms ZSCLE EOR ASP IFT Mt XRD CMC LSCA FT-IR OOIP

Zizyphus Spina Christi leaf extract enhanced oil recovery alkaline-surfactant-polymer interfacial reduction montmorillonite X-ray diffraction critical micelle concentration linear swelling cup assemblies fourier transform infrared spectroscopy original oil in place

87

V μ σ θ

electrophoretic mobility viscosity Dielecteric constant Stands for the zeta potential Henry's function plugging index, % total volume of micro-model which can fill by injected oil, Pixel final volume of oil in micro-model after accomplishment of each injection rate, Pixel actual velocity of displacing phase, m/s viscosity of displacing phase, Pa.s displacing phase-displaced phase IFT, N/m contact angle, degree

Acknowledgments The authors thank the Petroleum University of Technology (PUT) for providing laboratory support throughout this research.

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