Desorption Mechanism of Asphaltenes in the Presence of Electrolyte

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Desorption mechanism of asphaltenes in the presence of electrolyte and the extended DLVO theory Yang Guang, Ting Chen, Juan Zhao, Danfeng Yu, Fang Hui Liu, Dong Xue Wang, Ming Hong Fan, Wen Juan Chen, Jian Zhang, Hui Yang, and Jinben Wang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b00866 • Publication Date (Web): 15 Jun 2015 Downloaded from http://pubs.acs.org on June 16, 2015

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Desorption mechanism of asphaltenes in the presence of electrolyte and the extended DLVO theory Guang Yang1, Ting Chen2, Juan Zhao1, Danfeng Yu2, Fanghui Liu2, Dongxue Wang2, Minghong Fan2, Wenjuan Chen1, Jian Zhang1, Hui Yang2,*, Jinben Wang2

1. CNOOC Research Institute, State Key Laboratory of Offshore Oil Exploitation, Beijing 100027, P. R. China 2. Key Laboratory of Colloid, Interface and Chemical Thermodynamics, Institute of Chemistry, the Chinese Academy of Sciences, Beijing 100190, P. R. China

ABSTRACT: Desorption of asphaltenes from silica-coated quartz crystals upon exposure to a series of saline solutions was studied through the measurements of quartz crystal microbalance with dissipation (QCM-D), atomic force microscopy (AFM), and contact angle. Interestingly, it was found that the mass loading and thickness of asphaltene film decreased during the injection of sodium chloride solution at the concentrations ranging from 1 mM to 10 mM, with the surface tending to be hydrophilic; whereas, the mass loading and film thickness increased gradually when the concentration increased from 10 mM to 1000 mM, with the surface inclined to be hydrophobic. It was also found that the electrostatic force had a great effect on this process due to the interactions between the charged interfaces of oil/water and water/solid. Besides, some additional interactions may arise under small distance at the presence of the electrolyte solution, and therefore, a direct force-measuring technique was introduced, in which the functionalized

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AFM tips felt a solid surface to model the interactions among three phases of oil, water, and solid. Based on the computed results of disjoining pressure isotherms, the theory of Derjaguin-Landau-Verwey-Overbeek (DLVO) was extended, taking into account of the participation of hydration forces which played an important role at short range. These structural forces mostly originated from the overlap of the hydrated layers under a variety of salinity concentrations, resulting in the balance of resultant interactions. 1. INTRODUCTION Waterflooding technology has been one of the most successful approaches to improve oil recovery due to the effect of salinity on reservoir rock-fluids interactions.1-4 During the last decade, a great number of laboratory core flooding tests with salinities lower than 5000 mg/L have been demonstrated to be efficient to enhance oil recovery (EOR), which has also been confirmed by field tests.5-8 The key points to reach this success of water flooding can be attributed to the research progresses of water chemistry and salinity level on oil recovery both from the experiments in the laboratory and field trial.2, 3, 9 Although tremendous effort has been made to understand the phenomenon of salinity effect, the molecular level processes that control EOR have not been completely revealed.2 Different mechanisms of the adsorption of crude oil components onto mineral surfaces include polar interactions, surface precipitation, acid/base interactions, and ion-binding interactions.10-12 Adsorption and wettability alteration in the absence of water mainly occur between polar groups (including N, O, S, or metal contents) in the crude oil and polar sites on the mineral surfaces.13 With the oil being a poor solvent for asphaltenes, asphaltene precipitation and oil-wet surface are induced after aging. Water plays an important part in both of the acid/base interactions and ion-binding interactions. The former interactions are of coulombic type, present between charged


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sites at the mineral surface and oppositely charged sites at the oil/water interface. In the case of the latter interactions, divalent and multivalent ions bind to negatively charged polar compounds siting in the interfaces. Exposed in the low salinity water, the mineral surface can be altered to be less oil-wet due to the desorption of polar components, which was explained by different mechanisms of the wettability alteration or wettability modification. 3, 7, 14-16 The theory of the salting-in mechanism is on the basis of the higher solubility below a critical ionic strength of the injected water, leading the adsorbed organic components to loosely bond to the surface and resulting in more water-wetness. In the case of the injection of low salinity water, the multicomponent ion exchange may occur in a way of replacing organic polar components or organo-metallic complexes with noncomplexed cations. Till now, a theory in the explanation of the wettability alteration process has been considered that the diffuse double layer between oil/water and water/solid interfaces expands with the decrease in salinity, increasing the electrostatic forces and promoting the release of organic materials to an extent.9 However, the mechanism of asphaltene desorption from the reservoir surface at the molecular scale has not been completely revealed when the salinity of the injected water is decreased, which will probably prevent the researchers from understanding the wettability alteration of reservoir surfaces and finding more efficient ways to EOR. The preference of the solid surface for one fluid over another phase is usually quantified by measuring the contact angle which defines the tendency of the nonwetting fluid either to adhere and spread on the surface or to remain separated from it, depending upon the balance between the pressure in the nonwetting phase and the disjoining pressure.17 Generally, the disjoining pressure plays an important role when the thickness of the wetting film confined between the


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nonwetting phase and the solid surface is in the order of nanometers, and therefore, the interactions are to be considered at molecular level.18 A quantitative understanding of these interaction forces is essential for describing wetting and dewetting phenomena of immiscible liquids on solid surfaces, which can deduce a functional dependence of thin film stability on the concentration by considering intermolecular and surface interactions in the system of crude oil/brine/rock. The interactions of the solid substrate, thin wetting aqueous film and the non-wetting phase are studied using the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory.9, 19, 20

It considers the electrostatic forces, van der Waals forces and additional forces acting

together to model the resultant forces which can be obtained by the disjoining pressure isotherms at different film thickness.9, 21, 22 However, it is still a difficulty to understand the experimental results under low brine in the oil/solid system due to the participation of non-DLVO forces, such as hydration forces which are usually too complex to be analyzed in short-range and remain in a controversial issue.23,

24

Instead, if the non-DLVO forces can be quantified, the interactions

among the oil/water/solid interfaces can be interpreted and an efficient way of EOR will be proposed under appropriate conditions of ion environment. In the present work, the mass loading and film thickness of asphaltene films were measured by QCM-D; the morphology of adsorption and desorption films was measured through AFM; the hydrophobic and hydrophilic properties of the films were detected by contact angle. Moreover, we reported on a direct measurement of the electrostatic forces, van der Waals forces and structural forces between chemical modified AFM tip and substrate under low salinity solutions, and calculated such forces. Based on the DLVO theory, the present model was extended by introducing non-DLVO forces, giving a better understanding on the mechanism of the asphaltene deposition from silica surfaces.


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2. EXPERIMENTAL SECTION 2.1 Materials 2.1.1 Extraction and Solution Preparation of Asphaltenes Asphaltenes extracted from the oil sand (Long Lake, Canada) were used as the adsorbate, precipitated at a 1:40 volume ratio of excess n-hexane (AR, 99.5%). Kept in vacuum distillation, the mixture was stirred until the reflux was colorless. After cooled down to the room temperature, the flask was put in the dark for 1 h. The precipitate was collected and rinsed to make sure other ingredients (including resins) be removed. Finally, the asphaltenes were dried overnight in a vacuum drying oven. A stock solution of asphaltenes (1.0 g/L) was prepared by dissolving the sample in pure toluene (AR, 99%) and magnetic stirred for 1 h. The solution was stored in a dark and cool place. 2.1.2 AFM Tips Functionalization and Substrate Selection The AFM cantilever (NPG-10, Bruker Corporation) with a nominal spring constant of 0.06 N/m was used throughout this study. The material of tips is silicon nitride coated with 15 nm Cr on the bottom and 60 nm Au on the top, both of which are the same on the front side and back side. To create the alkyl terminated tips, the fresh tips were cleaned in ozone for 30 s and then submerged them in an ethanol (AR, 99%) solution of 1 mM 1-Octadecanethiol CH3(CH2)17SH (AR, 96%) for at least 24 h,2 after which the tips were submerged in pure ethanol solution and pure water again respectively to keep the surface clean, and finally was dried with pure nitrogen. The sectional area of the tip was determined through AFM measurement. Mica was used as the substrate in our experiments, because it has some properties similar to clay minerals in sandstones.18 Mica was in the form of slip disks of 9 mm diameter, and atomically smooth and well suited for AFM imaging.


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2.1.3 Aqueous Solutions All the Millipore Milli-Q grade water was used in all experiments. Aqueous electrolyte solutions were made by dissolving NaCl (AR, 99.5%) at the concentration of 1 mM ~ 1000 mM under the pH value of around 7. 2.2 Methods 2.2.1 Quartz Crystal Microbalance with Dissipation (QCM-D) QCM-D (Q-SENSE E4, Sweden) was used to measure the adsorption of asphaltene and the desorption of brine on silica-coated quartz crystal sensors (QSX 303). When an AC voltage is applied to the electrodes, the crystal oscillates at a specific resonance frequency that is highly sensitive to the total mass of the crystal. When a mass is added to the sensor due to material adsorption, the resonance frequency of the sensor will decrease, and then the mass uptake can be calculated by measuring the decrease of the frequency, △f, in Hz. If the adsorbed layer is thin and rigid, the Sauerbrey relation is applicable,25-27 as shown in equation (1): ∆m = −

C × ∆f n

(1)

where △m is the adsorbed mass or mass uptake in ng/cm2; n is the number of harmonic overtones of the crystal sensor, n = 1, 3, 5, 7, 9, 11; C is a constant of the crystal, C = 17.7 ng/(Hz·cm2) for a 5 MHz crystal. For the sake of simplicity, three harmonics (n=3, 5, 7) were shown in the results. The third overtone was used to determine the frequency and dissipation shifts and to calculate the corresponding mass uptake. In addition to determining the quantity of the adsorbed mass, the dissipation (D) of the QCM-D monitors reflects an energy loss due to the viscous dissipation when the adsorbed materials oscillate with the crystal. Shifts in D during adsorption provide an indication of the


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rigidity of the adsorbed layer. Dissipation is energy loss in the molecular layer on the sensor, as shown in equation (2)28:

D=

Ed 2 πE s

(2)

where Ed is the energy dissipated during one oscillation and Es is the energy stored in the oscillating system. The measurement of ∆D is based on the fact that the voltage over the crystal decays exponentially as a damped sinusoidal when the driving power of a piezoelectric oscillator is switched off.29 By switching the driving voltage on and off periodically, a series of changes of the resonant frequency and the dissipation factor can be obtained simultaneously. The profiles of ∆f and ∆D versus time are evaluated with the Q-Tools software. Prior to each experiment, the silica crystals were soaked in excess toluene (AR, 99%) for 30 min, followed by thorough rinsing with Milli-Q water, to remove any contaminations from the silica crystals surface. Then the crystals were blow-dried with nitrogen and placed in an aqueous solution of 2 wt% sodium dodecyl sulfate (AR, 99%) for 30 min. Subsequently, they were rinsed with Milli-Q water and blow-dried with nitrogen. The QCM-D chamber and connecting tubes were rinsed with 100 mL water and then with 100 mL toluene prior to each measurement. Before starting an experiment, the resistance across the crystal in absence of liquid was measured. The temperature was kept at 25 oC in all experiments. 2.2.2 Contact Angle Measurements The contact angle of a water droplet on the treated wafer surfaces from QCM-D experiments was measured through the analysis system of interface chemistry (SL200B, USA) at the temperature of 25 oC. The method of ‘Pendant Drop’ and the analysis software CAST 2.0 of interface chemistry were applied. 2.2.3 Atomic Force Microscopy (AFM)


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The surface topography of asphaltenes aggregating at the interface was measured via AFM in tapping mode (Digital Instruments, Bruker Corporation). The AFM measurements were conducted using a probe made of silicon nitride. The topography images were performed on a scan area of 10 µm × 10 µm at a speed of 4 Hz. AFM force-distance measurements were conducted through a Multimode VIII AFM with an O-ring liquid cell and in contact mode. The actual spring constants of the cantilevers were determined using the “thermal tune” method introduced by Hutter and Bechhoefer.30 Force curves were obtained by converting the cantilever deﬂections (mV) and piezeotube displacements in accordance with Hooke’s law, in which the deflection sensitivity was recalibrated once the solution was changed. The AFM fluid cell was rinsed with ethanol and dried with nitrogen prior to the experiments. The forces between the flat surface and the probe were measured as a function of the separation distance. The ramp rate for an approach/retraction cycle was adjusted to 1 Hz. At least 50 force curves were recorded at different locations on the surface for each experimental condition. After analyzed through the NanoScope Analysis software, the force curves were fitted and computed in one profile. 2.2.4 Zeta (ζ)-Potential Measurements Zeta-potential of oil/brine and solid/brine interfaces was measured through a Zetasizer (Nano-ZS, Malvern). The brine solutions were of NaCl with the concentration of 1 mM, 10 mM, 100 mM, and 1000 mM. The solutions of oil/brine were prepared at a volume ratio of 1:19 and octane represented oil phase, which were stirred by a magnetic stirring apparatus. The mica powder was in the size of 5 µm and 0.2 g/L in NaCl solution. In the system of solid/brine, the mica powder was added to the brine and stirred using a magnetic stirring apparatus to make sure that the solid powder was well-distributed. All the measurements were conducted at 25 oC and


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each zeta-potential value was obtained from the average of three measurements as shown in Supporting Information. 2.2.5 Elemental Analysis The elemental composition of the asphaltenes (C, N, O, S) was determined by multi-functional X-ray photoelectron spectrometer (XPS, ESCALab220I-XL) and, C, H, and N were determined by thermal conductivity measurements (FLASH EA1112). Asphaltenes contained a range of functional groups (see Supporting Information). The high percentage of heteroatoms in the molecules endowed the asphaltenes highly polar property entities. Oxygen and silicon, totally accounting for more than 10 % wt in the molecules, were the dominant heteroatoms; while the C/H molar ratio showed that the aromaticity of the asphaltenes was not high. 2.2.6 Dielectric Constants Measurements Dielectric constants were measured through microwave network analyzer (N5224A, Agilent) in the frequency range of 200 MHz ~ 20 GHz at the room temperature. Both of the relative permittivity (ε') and the imaginary part of the permittivity (ε'') of saline solutions, mica, octane were recorded as shown in Supporting Information. 3 THEORY AND CALCULATION 3.1 Theory Background The intermolecular force comprise of the van der Waals, electrostatic, and structural forces, as expressed in Equation (3)31-33: Πtotal = Πvan der Waals + Πelectrical + Πstructural

(3)

where Πtotal is the disjoining pressure of the specific intermolecular interactions which imitate interactive forces between the interface of water/oil and water/rock. A brief introduction of the forces and calculation procedures are presented as below.


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3.2 Van der Waals Forces The van der Waals forces play a central role in all phenomena involving intermolecular forces, which are always present and important, both at small and large separations. These forces are recognized as the strength of the attachment between solids, which is expressed as Equation (4)34: Πvan(h) =

- A(15.96 / λ + 2) 12πh 3 (1 + 5.32h / λ ) 2

(4)

where A is the Hamaker constant; λ is the London wavelength, 100 nm; h is the film thickness. The key point in calculating the van der Waals forces is to determine the Hamaker constant, as expressed by Equation (5)34: A=

3 3h ∞ ε (iv) − ε 3 (iv) ε 2 (iv) − ε 3 (iv) ε −ε ε −ε kT ( 1 3 )( 2 3 ) + ∫ ( 1 )( )dv 4 ε1 + ε 3 ε 2 + ε 3 4π v1 ε1 (iv) + ε 3 (iv) ε 2 (iv) + ε 3 (iv)

(5)

where ε1, ε2, ε3 are the static dielectric constants and medium 3 stands for aqueous solution; ε1(iv), ε2(iv), ε3(iv) are the electronic absorption terms; k is the Boltzmann constant, 1.381 × 10-23 J/K; T is the absolute temperature, 298.15 K; h is the plank constant, 6.626 × 10-34 J·s. 3.3 Electrostatic Forces These forces are due to the development of the charge between interacting surface in aqueous solution, as expressed by Equation (6)35:

2ψ r1ψ r 2 cosh(κh ) + ψ r21 + ψ r22 ） Πelectrical(h) = nkT（ sinh 2 (κh )

(6)

where n is the number of cations (or anions) per unit volume; k is the Boltzmann constant, 1.381 × 10-23 J/K; h is the film thickness. κ denotes the reciprocal Debye length as described by the following Equation (7)36, 37:


10

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κ -1 =

εε 0 k B T 2ρ ∞ e2

(7)

where e is the electron charge, 1.602 × 10-19 C; ρ∞ corresponds to the charge density in the bulk; ε0 is the permittivity of vacuum, 8.854 × 10-12 C2/J·m; ε denotes the relative permittivity (the dielectric constant) of the solution, 78.4; T is the absolute temperature. ψr1, ψr2 are the reduced potentials at the oil/water interface and water/solid interface based on the Debye-Hückel equation as shown in Equation (8)38: ψh ≈ ψ0 × e-κh

(8)

where again the Debye length appears as the characteristic decay length of the potential; ψ0 is the potential of the isolated surfaces; h is the film thickness. 3.4 Structural Forces When two surfaces or particles approach closer than a few nanometers, continuum theories of attractive van der Waals and repulsive double-layer forces often fail to describe their interactions. Other non-DLVO forces come into play and become dominating, being repulsive, attractive, or oscillatory at small separations. Based on a single-exponential function,39 the structural force is expressed as a double-exponential function as described by Equation (9)24:

Π structural = C1e − h / λ + C2e − h / λ 1

2

(9)

where C1 and λ1 describe the magnitude and the decay length of short-range forces for the exponential model; the parameters C2 and λ2 represent the magnitude and decay length of the long-range forces; h is the film thickness. 4. RESULTS AND DISCUSSION In each QCM-D experiment, as shown in Figure 1, two types of data were recorded simultaneously, including variations in resonance frequency and energy dissipation response.


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And the former provides information on the mass of adsorbate on the surface, while the latter provides information about the elastic property of the adsorbed film.40-43 After the injection of toluene as the baseline (region I), region II shows an immediate drop in the resonance frequency when the crystal surface is exposed to the asphaltene solution, exhibiting a rapid adsorption of asphaltenes onto the silica surface. And then it shows a saturated absorption on the silica surface, without any further shift in frequency. In region III, the surface is rinsed by toluene to remove the weakly bound asphaltenes, resulting in a slight increase in the frequency. At the same time, the corresponding dissipation shift increases first, then decreases and stabilizes around a constant value upon rinsing with toluene, which shows that a small amount of weakly bound asphaltenes is removed and the remaining asphaltenes form a rigid layer at the silica surface. The irreversibly adsorbed mass is calculated to be 425.3 ng/cm2, agreeing well with other studies.26,

27

An

interesting observation result is the distinct shifts of f and D especially at the third overtone (region IV), showing that the difference value of △f is more than 80 Hz and the ratio of △D/△f is about 0.6 × 10-6 /Hz, after the introduction of the sodium chloride solution. The whole adsorption/desorption process can be divided into two areas, being compact adsorbed layer and soft adsorbed layer, in terms of Sauerbrey relation of eq 1. It is well documented that the lower order overtones are more sensitive to the outer region of the adsorbed layer (regions far away from the solid surface).44 The characteristic changes in the dissipation of the third overtone indicate that the outer region of the film is softer than the inner region. The softness of the outer region of the film can be attributed to the intermolecular exchanges and interactions between sodium chloride molecules and asphaltene molecules, and the adsorption of the water molecules, resulting in the structural changes of the interface, as well as the characteristics of viscosity and elasticity.


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D3

Frequency (Hz)

0

40

D5

-20

D7

30

D9

-40

F9

-60

I

II

III

IV

F5

-80

20

F7

10

F3

-100 0

1000

2000

3000

Dissipation (1E6)

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0 4000

Time (s) Figure 1. Frequency and dissipation shift as a function of adsorption time in QCM-D experiments in sodium solution at the concentration of 10 mM (region I: toluene injection; region II: asphaltene injection; region III: toluene injection; region IV: NaCl injection)

In order to further understand the desorption process of the asphaltenes-coated film, the adsorbed films are exposed to sodium chloride solutions at different concentrations separately. The amount of desorption differs when different sequences of aqueous solutions flush the asphaltene surfaces, as summarized in Figure 2 and Table 1. With the increase of concentration ranging from 1 to 10 mM, the shift of frequency and the calculated mass of film decrease, while these values increase with the concentration increasing from 10 to 1000 mM, which induces that there is a turning point around the concentration of 10 mM. Furthermore, the ratio of △D/△f increases in the presence of NaCl ranging from 1 mM to 10 mM; instead, the ratio decreases at the concentrations ranging from 10 mM to 1000 mM. It provides more information about the structural changes of adsorbed layers in different systems, indicating that there may be a


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structural rearrangement of adsorbed asphaltene molecules and their aggregates at the series of concentrations of sodium chloride solutions, especially around 10 mM. The silica surface is covered by siloxane and silanol groups, and the latter is usually either as single silanol group or adjacent silanol pairs through hydrogen bonding.3, 45 As strong adsorption sites, the polar silanol sites are tend to be adsorbed by the polar parts of the asphaltene molecules, in which hydrogen bonds and van der Waals forces are considered to be the predominant interactions.45 Dissolved in toluene, asphaltenes always form molecular aggregates through intermolecular hydrogen bondings between polar groups in the asphaltene structures and π electron interactions of the aromatic rings.46 The high amount of adsorption indicates the adsorption of aggregates on the silica crystals. When they are flushed by NaCl aqueous solutions, functional groups of asphaltenes, mainly including hydroxyls and carboxylic acids, are almost ionized. The ionic strength of Na+ has a great effect on the diffuse double layer and Debye length, which is not only considered to have a relationship with electrostatic forces, but also may encounter some speciﬁc interactions (which will be discussed in the following section). 3000

△F (Hz) Mass (ng·cm-2)

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2000

1000 120 100 80 60 40 20 0 0 20 40 60 80 100 120

900

950

1000

CNaCl (mM)

Figure 2. The changes of frequency and mass with the increase of concentration of sodium solutions


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Table 1. Asphaltene desorption upon exposure to aqueous solutions △F (Hz)

△D (1E-6)

△D/-△f

Mass (ng/cm2)

Asphaltene

26.2

1.4

0.05

425.3

Water flush

106.6

41.9

0.39

1851.6

1 mM NaCl flush

85.0

39.4

0.46

1440.0

5 mM NaCl flush

79.8

40.7

0.51

1359.1

10 mM NaCl flush

65.4

41.3

0.63

1139.5

25 mM NaCl flush

76.6

42.3

0.55

1377.2

50 mM NaCl flush

84.2

39.6

0.47

1491.0

100 mM NaCl flush

105.1

44.6

0.42

1845.0

1000 mM NaCl flush

123.9

52.0

0.42

2088.7

Topographical images of the sample surfaces of bare silica substrate, exposed to a series of aqueous solutions are shown in Figure 3. Figure 3(a) shows a typical topographical feature of asphaltenes which are randomly and continuously distributed in the form of thin film, with a maximum roughness around 10 nm. When the asphaltenes-coated surface is exposed to water, there is nearly no difference between Figure 3(a) and Figure 3(b) in topographical feature. After the surface exposed to sodium chloride solutions, there are differences in morphology and height profile with the concentration ranging from 1 mM to 1000 mM (Figure 3(c) ~ Figure 3(h)), indicating the thickness of exposed films reducing from asphaltenes-coated film at least 1 nm. Worthy of note is that the modified film appears thinner as the concentration gets to the range of 10 mM (Figure 4), indicating that more asphaltene molecules are removed. As shown from the


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results of contact angle measurements that asphaltene adsorption causes bare surface to become more oil-wet (see Table 2), while NaCl solution at the concentration of 1 mM makes the surface intermediate-wet, and the surface becomes more hydrophilic after exposed to NaCl solution at the concentration around 10 mM. It indicates the transition from oil-wet to water-wet of the thin film after the deposition of asphaltenes.

(a) Asphaltene-coated surface

(b) Asphaltene-coated surface flushed by water

(c) Asphaltene-coated surface flushed by 1 mM NaCl

(d) Asphaltene-coated surface flushed by 5 mM NaCl


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(e) Asphaltene-coated surface flushed by 10 mM NaCl

(f) Asphaltene-coated surface flushed by 25 mM NaCl

(g) Asphaltene-coated surface flushed by 100 mM NaCl

(h) Asphaltene-coated surface flushed by 1000 mM NaCl Figure 3. AFM topography images and histograms of point height of the samples

11 10

Height (nm)

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9 8 7 6 5

Asphalt Water

1

5

10

25

50

100

1000

Concentration (mM)

Figure 4. The statistical analysis of height profiles


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Table 2.

Page 18 of 29

Magnitude of contact angle in different systems

Bare chip

Contact angle (ο)

27.7

1 mM NaCl

10 mM NaCl

100 mM NaCl

flush

flush

flush

73.2

68.7

80.2

Asphaltene film

79.3

Considering all the results discussed above, it can be deduced that possible processes take place during the recovery of asphaltenes via brine flooding, as exhibited in Figure 5. The bare surface of silica is mainly covered by silanol groups which can be present either as single surface silanol groups or as adjacent hydrogen bonded exposed in the air. The polar silanol sites play an important part in interacting with asphaltene molecules due to the predominant interactions of hydrogen bonds and van der Waals forces upon adsorption. Meanwhile, asphaltenes form molecular aggregates in toluene, because intermolecular hydrogen bondings and π electron interactions control the aggregation, resulting in the hydrophobicity of the surface. With the introduction of aqueous solutions of sodium chloride, functional groups of hydroxyls and carboxylic acids of asphaltenes tend to ionize. The ionic strength of Na+ may have a special effect on desorption of asphaltene molecules when the concentration is around 10 mM, leading more deposition, thinner layers, and more water-wet of the asphaltenes-coated layer than other cases. It indicates an important relationship between alsphaltene molecules and the exchanges between alsphaltene and sodium chloride molecules, due to the electrostatic forces induced by the different diffuse double layer at interface, as well as other important interactions: structural forces (as discussed below).


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Page 19 of 29

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Asphaltenes in toluene

NaCl aqueous solution

Figure 5. Schematics of asphaltenes displacement by sodium chloride aqueous solution on a silica surface

In order to qualify the interaction forces among oil/water/solid surfaces, the interactions upon approach and retraction as a function of the separation between a modified probe and substrate in a series of NaCl solutions are obtained as shown in Figure 6 (a) and (b), separately. In Figure 6 (a), positive disjoining pressure which means repulsive interaction is the leading force in the systems with NaCl concentration of 1 ~ 100 mM, while negative disjoining pressure which means attractive interaction is observed at separations below 10 nm when the concentration is around 1000 mM. Interestingly, the repulsion grows stronger as the concentration increases from 1 mM to 10 mM. Taking into account the resultant of the intermolecular and surface forces during the approaching process, different kinds of interactions are calculated respectively. Van der Waals forces often play a central role in all phenomena involving intermolecular forces, being present and important both at small and large separations.47 Ended up with the expression for the nonretarded Hamaker constant based on the Lifshitz theory, the total energy of van der Waals interactions are calculated, including Keesom, Debye dipolar and London energy contributions. There is not a big difference in Hamaker constants with the increase of salt concentration, about 3×10-21 J at 298.15 K, where the static dielectric constant of aqueous


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solution is much bigger than those of the other media and the Hamaker constants are around 3/4 kT, mostly from the contributions of Keesom and Debye dipolar interactions. So van der Waals forces in a solvent medium are much reduced from their values in free space, forming a similar trend with the increase of film thickness at the series of concentrations. Generally, the Debye length (1/κ) describes the ionic atmosphere near a charged surface, the magnitude of which solely depends on the properties of the solution and not on any property of the surface such as the charge or potential, named as the diffuse electric double-layer. The Debye length decreases (1/κ = 9.62 nm, 3.04 nm, 0.96 nm, 0.30 nm at the concentrations of 1 mM, 10 mM, 100 mM, 1000 mM, respectively) when the concentration increases from 1 mM to 1000 mM. And therefore, the higher concentration results in shorter Debye length and effects on the less repulsion between the oil phase and solid phase. Figure 6 (b) shows that repulsive interactions can be observed in the cases of 1 and 10 mM NaCl, and the adhesion force increases as the concentration increases from 100 to 1000 mM, and noticeably, the most positive value of disjoining pressure observed in the short range is in the system of around 10 mM NaCl between surfaces. The main factor inducing the tip and the solid surface to come into adhesive contact in a minimum is usually due to the lowering of their surface charge or potential, brought out by increasing the screening of the double-layer repulsion with the increase of the salt concentration.38 Therefore, the minimum adhesive force appears at the concentration of 10 mM, leading to an unusual phenomenon, which indicates that two surfaces may hardly attract one another due to the “Jump out” behavior.


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Page 21 of 29

5

3x10

5

2x10

5

1x10

5

1 mM 10 mM 100 mM 1000 mM

Disjoining Pressure (Pa)


4x10

0

4x10

5

3x10

5

2x10

5

1x10

5

10

20

30

1 mM 10 mM 100 mM 1000 mM

0 -1x10

0

5

0

10

20

30

Film Thickness (nm)

Film Thickness (nm)

(a)

(b)

Figure 6. The approach (a) and retraction (b) of disjoining pressure isotherms for sodium chloride solutions at different concentrations

To illustrate the turning point appearing in this measurement, the disjoining pressure is calculated according to the classical DLVO theory, as shown in Figure 7. The calculated profiles do not fit with the experimental profiles, mainly attributed to the fact that when two surfaces approach close with each other, continuum theories of attractive van der Waals and repulsive double-layer forces sometimes fail to describe their interaction. It can be deduced that non-DLVO forces may come into play and be much stronger than either of the two DLVO forces at small separations.

0.0

DLVO Interaction (Pa)

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5

-5.0x10

1 mM 10 mM 100 mM 1000 mM

6

-1.0x10

0

2

4

6

8

10

Film Thickness (nm)

Figure 7. Calculated profiles of DLVO interactions verses film thickness in the presence of NaCl aqueous solutions


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A term describing non-DLVO forces is introduced, as expressed in Equation (9) using two decay lengths to get a better fit of the experimental results. After computed on the basis of the EDLVO theory, the calculated profiles of structural force match well with the experimental data in Figure 8 (a). All the structure interactions behave repulsive in short range, revealing the existence of a strong hydration force associated with the presence of a structured layer of water molecules at solid surface. With the increase of concentration from 1 mM to 10 mM, the magnitude of hydration force increases; by comparison, when the concentration rises to 1000 mM, it decreases gradually. It can be seen from Figure 8 (b) that the comparison between force profiles treated with the DLVO and EDLVO models, respectively. As DLVO model predicts that a large attractive force would be present at a distance of less than 10 nm, which would have originated from the balance of the attachment and the diffuse layer overlap of the surfaces. Under this condition, van der Waals and double-layer forces act together in DLVO theory exhibiting similar profiles as shown in other papers, which exhibits the interaction potentials that can occur between two similarly charged surface or colloidal particles in a 1:1 electrolyte solution.38 Obviously, the existence of this attractive force is inconsistent with the experimental results in this paper. Instead, short-ranged deep repulsions emerge in the presence of the electrolyte and in fact the non-DLVO forces exist. From the results, the EDLVO model successfully predicts the force distance and the behavior observed in the experimental disjoining pressure-film thickness curves, revealing that other interactions such as hydration forces which should also be considered in the presence of electrolytes (other fitting profiles at the concentrations of 1 mM, 100 mM and 1000 mM are shown in Figure 1 of Supporting Information).


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Structural Interaction (Pa)

5

5x10

Exp data at 1 mM Calculated Exp data at 10 mM Calculated Exp data at 100 mM Calculated Exp data at 1000 mM Calculated

5

4x10

5

3x10

5

2x10

5

1x10

0 2

4

6

8

10

Film Thickness (nm)

(a)

5

4x10


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Energy & Fuels

Exp data DLVO EDLVO

5

3x10

5

2x10

5

1x10

0 5

-1x10

0

10

20

30

Film Thickness (nm)

(b) Figure 8. Structural forces of experimental data and fitted data (a) at different concentrations; disjoining pressure profiles and their predictions by DLVO and EDLVO models (b) at 10mM

For reasons of further clarity, the fitted parameters of EDLVO model at the series of NaCl concentrations have been calculated as shown in Table 3. The values of C1 and C2 are positive at the series of concentrations of NaCl at short-range distance, which shows the hydration force mostly as the structural force and usually depends on the hydration of the surfaces. By using colloid probe atomic force microscopy technique, Yu et al. determined the hydrophobic and charge-patch interactions between surfaces in the presence of ultra-highly charged amphiphilic


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Energy & Fuels

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Page 24 of 29

polyelectrolyte through a double-exponential function, with C1 = 8 J/m2, C2 = -1.9 J/m2, D1 = 0.1 nm, and D2 = 5 nm.22 Horn et al. conducted direct force measurements by the use of surface force apparatus and measured structural forces fitted to a double-exponential function with C1 = 140 mJ/m2, C2 = 5.4 mJ/m2, D1 = 0.057 nm, and D2 = 0.57 nm.48 As shown in Table 3, all the decay lengths of λ1 are related with the diameter of the water molecule. Higher values of λ1 and λ2 associate with higher pressure of structural interactions at the concentration of 10 mM, where the appearance of hydration forces is mainly connected with the presence of an adsorbed layer of hydrated cations at solid surface, as well as the polarization effects of the surface dipoles. This mutual repulsive force in aqueous solution is even greater than the electrostatic double-layer force at short range. As a result, the saline system shows a different aggregation behavior at the concentration around 10 mM, leading to a different rearrangement of polar/nonpolar environment, inducing a different interaction between surfaces. Table 3.

The fitted parameters of EDLVO model

Concentration of C1 (Pa)

λ1 (nm)

C2 (Pa)

λ2 (nm)

1

3.89×107

0.14

7.27×105

0.87

10

7.78×106

0.25

4.20×105

3.24

100

5.13×105

0.19

1.52×107

1.89

1000

8.86×107

0.10

2.87×106

0.35

NaCl solution (mM)

5. CONCLUSIONS This study reports the desorption mechanism of asphaltenes from silica surfaces during different injection sequences of sodium chloride aqueous solutions, by the aid of QCM-D, AFM


24

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Energy & Fuels

and contact angle methods. Upon adsorption process, the asphaltenes formed a rigid layer at the surface, mainly due to hydrogen bonds and polar interaction between silanol groups and the polar parts of the asphaltenes. Desorption occurs when the asphaltene-coated surfaces exposed to saline aqueous solutions, mostly because that the charge density of both of the surfaces of water/solid and oil/water promoted and the electrostatic repulsions increased. Interestingly, mass loading and film thickness of asphaltene film decreased during the injection of saline solution with the concentration ranging from 1 mM to 10 mM, with the surface tending to be hydrophilic; while as the concentration increased from 10 mM to 1000 mM, mass loading and film thickness increased gradually, with the surface tending to be hydrophobic. Furthermore, it was not only related with the Debye length and electrostatic forces, but also associated with the structural forces. So a direct method of AFM with a chemically modified probe was introduced to have a further research on the intermolecular and surface forces in the presence of electrolytes at different concentrations. From the results of retraction profiles, adhesion force decreased when the concentration of saline increased from 1 mM to 10 mM, while increased when the concentration increased further. Computed from the approach profiles, the DLVO theory was extended to take into account the total interactions, including the van der Waals forces, electrostatic forces, and structural forces. The calculated profiles matched well with experimental curves and deduced the coefficient and characteristic decay length in different systems. As the main structural forces, the magnitude of hydration forces depended on the salt concentration, due to the interface structure composed by water molecules around nonpolar or polar solute molecules and surface. On the account of the great hydration force at 10 mM, the resultant force tended to be more repulsive, which was the driving force of asphaltenes deposition from solid surface and the wettability alteration of the surface.


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Page 26 of 29

AUTHOR INFORMATION Corresponding Author *Address: Institute of Chemistry, the Chinese Academy of Sciences, No.2, 1st North Street, Zhongguancun, Beijing P. R. China, 100190. Tel：+8610-62523395 Fax: +8610-62523395. E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors are thankful for the Second Batch of Open Projects Funding from the State Key Laboratory of Offshore Oil Exploitation (CCL2013RCPS0244GNN) for supporting this research. REFERENCES 1. Bernard, G. G. Enhanced oil recovery in reservoirs contg. divalent metal cations|using aq. slug contg. alkaline material and amino:poly:carboxylic acid chelating agent. US4359093-A. 2. Hassenkam, T.; Pedersen, C. S.; Dalby, K.; Austad, T.; Stipp, S. L. S., Pore scale observation of low salinity effects on outcrop and oil reservoir sandstone. Colloids and Surfaces A: Physicochemical and Engineering Aspects 2011, 390, (1-3), 179-188. 3. Farooq, U.; Sjoblom, J.; Oye, G., Desorption of asphaltenes from silica-coated quartz crystal surfaces in low saline aqueous solutions. Journal of Dispersion Science and Technology 2011, 32, (10), 1388-1395. 4. Xie, Q.; Liu, Y.; Wu, J.; Liu, Q., Ions tuning water flooding experiments and interpretation by thermodynamics of wettability. Journal of Petroleum Science and Engineering 2014, 124, 350-358. 5. Tang, G. Q.; Morrow, N. R., Influence of brine composition and fines migration on crude oil/brine/rock interactions and oil recovery. Journal of Petroleum Science and Engineering 1999, 24, (2-4), 99-111. 6. Fischer, H.; Morrow, N. R., Scaling of oil recovery by spontaneous imbibition for wide variation in aqueous phase viscosity with glycerol as the viscosifying agent. Journal of Petroleum Science and Engineering 2006, 52, (1-4), 35-53. 7. Lager, A.; Webb, K. J.; Black, C. J. J.; Singleton, M.; Sorbie, K. S., Low salinity oil recovery - An experimental investigation. Petrophysics 2008, 49, (1), 28-35. 8. Matthiesen, J.; Bovet, N.; Hilner, E.; Andersson, M. P.; Schmidt, D. A.; Webb, K. J.; Dalby, K. N.; Hassenkam, T.; Crouch, J.; Collins, I. R.; Stipp, S. L. S., How naturally adsorbed material on minerals affects low salinity enhanced oil recovery. Energy & Fuels 2014, 28, (8), 4849-4858. 9. Chandrasekhar, B. D., N. R., Application of DLVO theory to characterize spreading in crude oil-brine-rock systems. SPE 89425 2004, 17-21. 10. Buckley, J. S. Evaluation of reservoir wettability and its effect on oil recovery. Energy 1990, 1-8. 11. Bryant, E. M.; Bowman, R. S.; Buckley, J. S., Wetting alteration of mica surfaces with polyethoxylated amine surfactants. Journal of Petroleum Science and Engineering 2006, 52, (1-4), 244-252. 12. Morrow, N., Evaluation of reservoir wettability and its effect on oil recovery - Preface. Journal of Petroleum Science and Engineering 1998, 20, (3-4), 107-107. 13. Nalwaya, V.; Tangtayakom, V.; Piumsomboon, P.; Fogler, S., Studies on asphaltenes through analysis of polar fractions. Industrial & Engineering Chemistry Research 1999, 38, (3), 964-972. 14. Berg, S.; Cense, A. W.; Jansen, E.; Bakker, K., Direct experimental evidence of wettability modification by low salinity. Petrophysics 2010, 51, (5), 314-322. 15. RezaeiDoust, A.; Puntervold, T.; Strand, S.; Austad, T., Smart water as wettability modifier in carbonate and sandstone: A discussion of similarities/differences in the chemical mechanisms. Energy & Fuels 2009, 23, (9), 4479-4485.


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42. Rodahl, M.; Kasemo, B., On the measurement of thin liquid overlayers with the quartz-crystal microbalance. Sensors and Actuators a-Physical 1996, 54, (1-3), 448-456. 43. Rodahl, M.; Hook, F.; Fredriksson, C.; Keller, C. A.; Krozer, A.; Brzezinski, P.; Voinova, M.; Kasemo, B., Simultaneous frequency and dissipation factor QCM measurements of biomolecular adsorption and cell adhesion. Faraday Discussions 1997, 107, 229-246. 44. Poitras, C.; Tufenkji, N., A QCM-D-based biosensor for E. coli O157:H7 highlighting the relevance of the dissipation slope as a transduction signal. Biosensors & Bioelectronics 2009, 24, (7), 2137-2142. 45. Abudu, A.; Goual, L., Adsorption of crude oil on surfaces using quartz crystal microbalance with dissipation (QCM-D) under flow conditions. Energy & Fuels 2008, 23, (3), 1237-1248. 46. Wang, S.; Liu, J.; Zhang, L.; Masliyah, J.; Xu, Z., Interaction Forces between Asphaltene Surfaces in Organic Solvents. Langmuir 2009, 26, (1), 183-190. 47. Israelachvili, J. N., Chapter 13 - Van der Waals forces between particles and surfaces. Intermolecular and Surface Forces (Third Edition) 2011, 253-289. 48. Yoon, R. H.; Vivek, S., Effects of short-chain alcohols and pyridine on the hydration forces between silica surfaces. Journal of Colloid and Interface Science 1998, 204, (1), 179-186.


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TOC Graphic

Extended DLVO Non DLVO Forces


29

Desorption Mechanism of Asphaltenes in the Presence of Electrolyte

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