Brine

Subscriber access provided by CMU Libraries - http://library.cmich.edu

Article

The Effect of Ionic Strength on the Interfacial Forces between Oil/Brine/Rock Interfaces: A Chemical Force Microscopy Study Jiazhong Wu, Fang Hui Liu, Gang Chen, Xu Wu, Desheng Ma, Qingjie Liu, Shijing Xu, Shizhe Huang, Ting Chen, Wei Zhang, Hui Yang, and Jinben Wang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b02614 • Publication Date (Web): 26 Dec 2015 Downloaded from http://pubs.acs.org on January 1, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Energy & Fuels is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 18

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

The Effect of Ionic Strength on the Interfacial Forces between Oil/Brine/Rock Interfaces: A Chemical Force Microscopy Study Jiazhong Wu1, Fanghui Liu2, Gang Chen2, Xu Wu3, Desheng Ma1, Qingjie Liu1, Shijing Xu1, Shizhe Huang2, Ting Chen2, Wei Zhang2, Hui Yang2,*, Jinben Wang2

1. State Key Laboratory of Enhanced Oil Recovery, Research Institute of Petroleum Exploration and Development of PetroChina, Beijing 100083, 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 3. College of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou 510006, P. R. China

ABSTRACT: The presence of thin aqueous films and their stability has a profound effect on the interactions between oil/brine/rock interfaces. In a previous report, we proposed that hydration forces, originating from the overlap of hydrated layers of different surfaces in the presence of sodium chloride, played an important role at short range. In the present work, divalent ions were introduced to the liquid films and the mechanisms in improving oil recovery from low-salinity brine and the low-salinity effect at the molecular level were revealed. Through a direct force-measuring technique of chemical force microscopy (CFM), the functionalized AFM tips felt a solid surface to mimic the oil/rock interactions in brine. It was found that not only the van der Waals and electrostatic forces had a great effect on this process due to the interactions between the charged interfaces of oil/water and water/solid, but also some important additional interactions appeared at short range under a variety of salinity concentrations or compositions. Being taken into account the important role of structural forces under small distance, the force profiles were fitted well with the theory of extended Derjaguin-Landau-Verwey-Overbeek (denoted by EDLVO) through a double-exponential or Gaussian model. Interestingly, low adhesion appeared in the presence of sodium sulfate, because hydration forces contributed to the resultant force depending on the ACS Paragon Plus Environment

1

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 18

intrinsic properties of the solvent or solute molecules; while in the presence of calcium chloride, high adhesion emerged due to the dispersion interaction between water and hydrocarbon molecules, as well as the reorientation or restructuring of water molecules with tiny breakage of H-bonds. Therefore, based on the EDLVO theory, additional forces were suggested to play an important part in short range, proposing a better understanding on the effect of divalent ions on the thin liquid films in the process of increasing oil recovery. 1.

INTRODUCTION Waterflooding is an effective approach to increase oil recovery (IOR) due to the effect of salinity on reservoir

rock/fluids interactions.1-5 As an important method that has attracted great attention in the past two decades, low-salinity waterflooding (LSW) has been further reported for IOR and low-salinity effect (LSE) has been confirmed in core plug testing by several groups.6-9 Attributed to the complex nature among brine/oil/rock interactions and conflicting observations from experimental researches, LSE seems to result from a combination of mechanisms.10 The interactions of the solid substrate, thin wetting aqueous film and non-wetting phase are studied using the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory in which electrostatic and Van der Waals forces act together to model resultant forces.2,6 Double-layer expansion (DLE), as one of such mechanisms, enlarges the electrostatic repulsion between brine/oil and brine/rock interfaces through the expansion of two electrical double layers in the presence of low-salinity brine.11 Another mechanism is focused on chemical mechanism in low salinity water which affect non-DLVO interactions on thin brine films, taking multicomponent ionic exchange (MIE) for an example that wettability alteration may occur because of MIE involving divalent cations near the clay surfaces of sandstones.12 From numerous experiments in laboratory and field trial, active ions Ca2+ and SO42- have been found to be important for wettability alteration and oil recovery process in low salinity.13-16 A glaring question that remains is whether there exists other dominating mechanisms during the injection of low-salinity brine, especially in the presence of the divalent ions which may have a significant effect on the oil/rock interactions? Fathi et al indicated that sulfate from seawater adsorbs onto the positively charged water-wet sites and lowers the positive surface charge as well as the electrostatic repulsion, resulting in the location of excess Ca2+ being close to the chalk surface.17 And then Ca2+ reacts with carboxylic groups bonding to the surface and releases some of the ACS Paragon Plus Environment

2

Page 3 of 18

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

organic carboxylic material. So the wettability alteration mechanism is described as a symbiotic interaction between Ca2+, SO42-, and the adsorbed carboxylic material on the surface.18 Ion bridging was proposed by Lager et al to be another mechanism that could contribute to LSE.19 When salinity decreases and the number of cations in water decreases, cations are released from the surface which will be reestablished, and therefore, decreasing the ionic strength of cations is available for bridging and decreasing adhesion. Usually, cationic bridging is stronger for smaller ions with higher charge, so the positive effect would decrease in the order Mg2+ > Ca2+ > Na+ > K+.3, 20 However, there seems to be less reports on the non-DLVO interactions in the presence of divalent cations or anions between oil and sandstone surfaces, which is extremely important to reveal the key mechanism to LSE at the molecular level.1, 21 To increase the fundamental understanding of such processes that lead to a more efficient way to IOR, chemical force microscopy (CFM) studies with model systems have been conducted and presented in the present manuscript. As one kind of clay mineral widely distributing in ore,22 mica was chosen to be the substrate of CFM measurements. Through a tip that was functionalized by model oil, the interactions between the molecules on the tip and the mica surface under low salinity solutions were performed. In the presence of different composition and concentration of brine, the van der Waals and electrostatic forces were calculated. It was found that the resultant forces from experiments cannot fit well with the classical DLVO theory. So we accounted for the additional forces in the presence of active ions by the aid of EDLVO theory. 2.

EXPERIMENTAL SECTION 2.1 2.1.1

Materials Tip and Substrate Selection

AFM cantilever (NPG-10, Bruker Corporation) with a nominal spring constant of 0.06 N/m was employed 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. The sectional area of the tip was about 2826 nm2 determined through AFM measurement. Mica was used as substrate in our experiments, because it's properties are similar to clay minerals in sandstones. It was in the form of disks of 9 mm diameter and atomically smooth. 2.1.2

Aqueous Solutions ACS Paragon Plus Environment

3

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 18

All the Millipore Milli-Q grade water (18.2 MΩ·cm) was used in all our experiments. Aqueous electrolyte solutions were prepared by dissolving NaCl, CaCl2, and Na2SO4 (AR, 99.5%) at the concentration of 10, 100, and 1000 mM under the pH value of around 7. 2.2 2.2.1

Methods Tips Functionalization

Considering nonpolar components from crude oil to be a key item in the low salinity effect,21 we chose 1-Octadecanethiol that resembled known crude oil compounds. To create the alkyl terminated tips, the fresh tips were cleaned in plasma for 30 min and then submerged in an ethanol (AR, 99%) solution of 1 mM CH3(CH2)17SH (AR, 96%) for at least 24 h, after which the tips were rinsed in pure ethanol solution and pure water (Milli-Q, 18.2 MΩ·cm) respectively to keep the surface clean, and finally was dried with pure nitrogen.2, 7, 23

2.2.2

Atomic Force Microscopy

AFM force-distance measurements were performed using Multimode VIII AFM with an O-ring liquid cell and contact mode. Actual spring constants of cantilevers were determined through the thermal tune method introduced by Hutter and Bechhoefer.24 Force curves were obtained by converting the cantilever deﬂections (mV) and piezeotube displacements in accordance with Hooke’s law, where the deflection sensitivity was recalibrated once the solution was changed. The fluid cell was rinsed with ethanol and dried with nitrogen prior to every experiment. The forces between the probe and flat surface were measured as a function of the separation distance. The ramp rate for an approach/retraction cycle was adjusted to 1 Hz and more than 100 force curves were recorded at different locations for each experimental condition. After being analyzed through NanoScope Analysis software, force curves were fitted and computed in one profile as the tip-sample approaching profiles. Disjoining pressure isotherms were applied which manifested the surface forces. The adhesion force between tip and surface can be measured from the deflection on y axis in the use of force coordinate25, when the tip retracts from the sample as shown in Figure 1. After the peaks of force curves being searched, histograms of adhesion force are computed and fitted.

ACS Paragon Plus Environment

4

Page 5 of 18

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Approach

Force Adhesion force

Retract

Separation Distance, nm

Figure 1. Typical force vs. separation distance plot as obtained by AFM

2.2.3

Zeta (ζ)-Potential Measurements

Zeta-potential of solid/brine and oil/brine interfaces was measured via a Zetasizer (Nano-ZS, Malvern). The brine solutions were of NaCl, CaCl2, and Na2SO4 at the concentration of 10 mM, 100 mM, and 1000 mM. Octane was chose to represent oil phase.2, 11 The solutions of oil/brine were prepared at a volume ratio of 1: 19 and stirred by a magnetic stirring apparatus at the rate of 350 rpm for about 1 min.2 To measure zeta-potential of the interface between solid and brine, mica powder was added to the brines and stirred by a magnetic stirring apparatus. The powder was in the size of ~1 µm and of 0.2 g/L in NaCl, CaCl2, and Na2SO4 solution, respectively. All the measurements were performed at 25 °C and each zeta-potential value was obtained from the average of three measurements (see Table S1). 2.2.4

Dielectric Constants Measurements

Dielectric constants were measured via microwave network analyzer (N5224A, Agilent) in the frequency range of 200 MHz ~ 20 GHz at room temperature. Both of relative permittivity (ε') and imaginary part of permittivity (ε'') of saline solutions, mica, and octane were recorded as shown in Table S2~4. 3 3.1

THEORY AND CALCULATION Theory Background

The intermolecular forces comprise of the van der Waals, electrostatic, and structural forces, as shown in Equation (1)26, 27: Πtotal = Πvan der Waals + Πelectrostatic + Πstructural

(1)


5

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 18

where Πtotal is the disjoining pressure of the specific intermolecular interactions reflecting interactive forces between the interfaces of water/oil and water/rock. A brief introduction of the forces and calculation procedures are presented as following. 3.2

Van der Waals Forces

Van der Waals forces are considered to be negative and electrostatic forces are positive forces. The former forces are always present and important in all phenomena involving intermolecular forces, both at small and large separations. These forces are recognized as the strength of the attachment between two surfaces, expressed as Equation (2)28: Πvan(h) =

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

(2)

where h is the film thickness; λ is the London wavelength, 100 nm; A is the Hamaker constant, which is the key point in calculating van der Waals forces, as expressed by Equation (3)28, 29: A=

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

(3)

where ε1, ε2, and ε3 are the static dielectric constants; media 1 and 2 represent oil and solid phase, respectively; media 3 stands for aqueous solution; ε1(iv), ε2(iv), and ε3(iv) are the electronic absorption terms; k is the Boltzmann constant, 1.381 × 10-23 J/K; T is the Kelvin temperature, 298.15 K; h is the plank constant, 6.626 × 10-34 J·s.

3.3

Electrostatic Forces

These forces originate from the development of the charges between interacting surface in aqueous solution, as expressed by Equation (4)10, 30: Πelectrostatic(h) = nkT（

2ψ r1ψ r 2 cosh(κh ) + ψ r21 + ψ r22 ） sinh 2 (κh )

(4)

where n denotes the number of cations (or anions) per unit volume; k represents the Boltzmann constant, 1.381 × 10-23 J/K; h is the film thickness; κ is the reciprocal Debye length as described by the following Equation (5)31, 32:


6

Page 7 of 18

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

κ -1 =

εε 0 k B T 2ρ ∞e2

(5)

where e denotes the electron charge, 1.602 × 10-19 C; ρ∞ is the charge density in the bulk; ε0 corresponds to the permittivity of vacuum, 8.854 × 10-12 C2/J·m; ε is the relative permittivity (the dielectric constant) of the solution, 78.4; T is the room temperature. where ψr1, ψr2 are the potentials at oil/water and water/solid interfaces based on the Debye-Hückel equation31: ψh ≈ ψ0 × e-κh

(6)

where 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 the approach distance between two surfaces or particles is closer than a few nanometers, continuum theories of attractive van der Waals and electrostatic forces often fail to describe their interactions.31 In this case, other non-DLVO forces appear and dominate, being repulsive, attractive, or oscillatory at small separations. Based on a single-exponential function, the structural force is expressed as a double-exponential function as described by Equation (7)2, 33, 34:

Π structural = C1e − h / λ1 + C2e − h / λ2

(7)

where C is the coefficient; λ is the decay length; C1 and λ1 describe the parameters of short-range forces and C2 and λ2 represent the parameters of long-range forces; h is the film thickness.

4.

RESULTS AND DISCUSSION In order to illustrate a variety of brine/oil/solid interactions, the forces upon approach as a function of the

separation between a modified probe and substrate in a series brine of NaCl, CaCl2, and Na2SO4 solutions are obtained as shown in Figure 2 (a), (b), and (c), separately. Positive disjoining pressure means repulsive interaction and negative disjoining pressure means attractive interaction. In Figure (a), the attraction grows stronger as the concentration increases from 10 mM to 1000 mM, with the positive disjoining pressure being the leading force at the concentration of 10 and 100 mM and negative disjoining pressure dominating at separations below 10 nm at the concentration of around 1000 mM. It is worth noting that only attractive interactions can be ACS Paragon Plus Environment

7

Energy & Fuels

observed in the presence of CaCl2 (Figure (b)) while repulsive interactions in the presence of Na2SO4 (Figure (c)) in short range. Attractions between the two surfaces of oil/water and water/solid produce negative contributions to disjoining pressure that cause the film to collapse and film thickness to decrease, while repulsions contribute to the stability of thin aqueous films. Another evidence of DLE and chemical mechanisms in low-salinity brine comes from Lee et al's work through small angle neutron scattering, proposing that the film thickness as a function of brine composition and salinity decreases in the order: CaCl2 > NaCl > Na2SO4 and 100 mM > 10 mM.35 Whereas, so far, researches have not been likely to provide molecular level explanations of

NaCl-10 mM NaCl-100 mM NaCl-1000 mM

5

2x10

5

1x10

0 5

-1x10

0

5

10

15

20

Disjoining Pressure (Pa)

5

3x10


the low salinity phenomenon which will be discussed in detail in the following. Disjoining Pressure (Pa)

5

3x10

CaCl2-10mM CaCl2-100mM CaCl2-1000mM

5

2x10

5

1x10

0 5

-1x10

0

5

10

15

20

5

3x10

(a)

Na2SO4-10mM Na2SO4-100mM Na2SO4-1000mM

5

2x10

5

1x10

0 5

-1x10

Film Thickness (nm)

Film Thickness (nm)

0

5

10

15

20

Film Thickness (nm)

(b)

(c)

Figure 2. Approach of disjoining pressure isotherms for (a) sodium chloride, (b) calcium chloride, and (c) sodium sulfate solutions at different concentrations 80

50

NaCl-10mM

40

100

CaCl2-10mM

NaCl-100mM

20 0 NaCl-1000mM

40

0 30

Percentage (%)

0 40

CaCl2-100mM

15 0 20

CaCl2-1000mM

10

20 0 0

100

200

Force (pN)

0

(a)

0 100

Na2SO4-100mM

50 0 100

Na2SO4-10000mM

50

0

300

Na2SO4-10mM

50

25

Percentage (%)

Percentage (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 18

2000

4000

Force (pN)

0

6000

0

(b)

1000

Force (pN)

2000

(c)

Figure 3. Histograms of adhesion forces in the presence of (a) sodium chloride, (b) calcium chloride, and (c) sodium sulfate solutions at different concentrations

Figure 3 (a), (b), and (c) shows the histograms of adhesion forces in the presence of NaCl, CaCl2, and Na2SO4 solutions. When the substrate is submerged in NaCl (Figure (a)), the adhesion force increases with the increase ACS Paragon Plus Environment

8

Page 9 of 18

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

of concentration, exhibiting that the peak adhesion increases from ~20 to ~190 pN (Table 1). Interestingly, after being submerged in CaCl2, the adhesion shows the biggest value in the three salts, increasing from ~470 to ~4360 pN, while it exhibits the smallest value in the presence of Na2SO4, being around 6~9 pN. Adhesion is related with the intermolecular forces, conducting between interfacial atoms or molecules of two surfaces in contact, and related with the mechanical properties after energy being consumed through elastic or viscoelastic deformations. With a view to the approach of disjoining pressure isotherms and adhesion forces, to explore the interaction mechanisms fitting with the experiments is important, especially in the presence of divalent ions. Table 1. Average adhesion in the presence of sodium chloride, calcium chloride, and sodium sulfate at the concentrations of 10, 100, and 1000 mM Peak adhesion (pN) Brines

At the concentration of



10 mM

100 mM

1000 mM

NaCl

20.4 ± 2

121.6 ± 4

192.8 ± 2

CaCl2

472.6 ± 8

574.3 ± 33

4360.3 ± 250

Na2SO4

9.1 ± 0.6

6.7 ± 0.5

6.2 ± 0.4

Taking into account the specific intermolecular and surface forces in the approaching process, van der Waals and electrostatic interactions are calculated, respectively. Owing to the effect of polarization between the surfaces on the molecular and the atomic scale, van der Waals forces often play a central part both at small and large separations.29 The forces are described by the expression for nonretarded Hamaker constant based on the Lifshitz theory, including Keesom, Debye dipolar, and London energy contributions.2,

6

In the result of

calculation, Hamaker constant is around 3×10-21 J at 298.15 K without a big difference among these brine solutions, where the static dielectric constant of aqueous solution is much bigger than those of oil and solid media and the Hamaker constants are about 3/4 kT, resulting from the contributions of orientation and induction components. Therefore, in a solvent medium, there is a similar trend of van der Waals forces which are much reduced from their values in free space with the increase of film thickness in the three salts. Electrostatic forces are on account of the development of charges between an interacting surface and liquid bulk. In order to ACS Paragon Plus Environment

9

Energy & Fuels

describe the diffuse electric double-layer, Debye length is introduced to depict the ionic atmosphere near a charged surface, the magnitude of which only depends on the properties of solution and not on any property of the surface such as the charge or potential. The Debye length decreases with the increase of concentration from 10 to 1000 mM and decreases with the increase of ionic strength from NaCl to CaCl2 or Na2SO4, showing in Table S1. Hence, higher ionic strength results in shorter Debye length and effects on less repulsion between oil/brine and solid/brine surfaces. To illustrate the contribution of DLVO interaction in these measurements, the disjoining pressure is calculated according to the classical DLVO theory in combination of van der Waals and electrostatic forces, as shown in Figure 4. The corresponding Debye length decreases with the increase of salt concentration, and the divalent ions (Ca2+ or SO42-) have more significant effect than that of the monovalent ions (Na+ or Cl-) on reducing the Debye length (see Table S1), resulting in less repulsion between functional tips and mica surfaces. Although the electrostatic forces are different in these systems, yet van der Waals forces seem to determine the total interaction calculated by the aid of the DLVO theory. Clearly, the calculated curves are not fitted with the experimental ones, mainly attributed to the fact that continuum theories of attractive van der Waals and repulsive double-layer forces may fail to describe the interactions between two surfaces at a close distance with each other. And therefore, non-DLVO forces may come into play and be much stronger than either of the two forces in such cases.

DLVO Interaction (Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 18

0 NaCl-10 mM NaCl-100 mM NaCl-1000 mM CaCl2-10 mM CaCl2-100 mM CaCl2-1000 mM Na2SO4-10 mM Na2SO4-100 mM Na2SO4-1000 mM

6

-1x10

0

5

10

15

20

Film Thickness (nm) Figure 4. Calculated profiles of DLVO interactions verses film thickness in the presence of sodium chloride, calcium chloride, and

sodium sulfate

aqueous solutions at different concentrations


10

Page 11 of 18

Furthermore, non-DLVO forces are introduced and revealed, as expressed in Equation (7) using two decay lengths to get a better fit of the experimental results. The calculated curves of structural force fit well with the experimental data in the presence of NaCl on the basis of the EDLVO theory (Figure 5 (a)). Different from this result, the calculated profiles with double-exponential function cannot match the structural interactions in CaCl2 system because of the attraction appearing in the short distance (Figure (b)). There is uncertainty about the values of four "constants" (Equation (7)) and even whether there is actually such a "universal" force-law for the additional interactions.6, 27 After being tried dozens of fitting methods, the Gaussian function is introduced as shown in Equation (8)36 and the fitting results are shown in Figure (c):

Πstructural = C1e−( h −b1 )

2

/ λ12

+ C2e−( h −b2 )

2

/ λ2 2

+ ...

(8)

where C1 and λ1 describe the coefficient and decay length of short-range forces; the parameters C2 and λ2 represent the coefficient and decay length of long-range forces; b1 and b2 are real constants; h is the film thickness. Structural Interaction (Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

5

5x10

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 0

2

4

6

8

10

Film Thickness (nm)

(a)


11


5

4x10

5

3x10

5

2x10

5

1x10

0 5

0

2

4

6

8

10

Film Thickness (nm) 5

5x10


5

4x10

5

3x10

5

2x10

5

1x10

0 2

4

6

8

10

12

Film Thickness (nm)

5

5x10


5

4x10

5

3x10

5

2x10

5

1x10

0 5

-1x10

0

2

6

8

10

(c)

5

5x10


5

4x10

5

3x10

5

2x10

5

1x10

0 5

-1x10

14

4

Film Thickness (nm)

(b) Structural Interaction (Pa)

-1x10

Page 12 of 18

Structural Interaction (Pa)

5

5x10


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Energy & Fuels

2

(d)

4

6

8

10

12

Film Thickness (nm)

14

(e)

Figure 5. Structural forces and the profiles fitted with double-exponential function in NaCl (a). Structural forces and the profiles fitted with double-exponential function (b) and Gaussian function (c) in CaCl2. Structural forces and the profiles fitted with double-exponential function (d) and Gaussian function (e) in Na2SO4.

Via such function, the calculated profiles of structural force match well with the experimental data in the presence of calcium chloride. In the case of sodium sulfate, the calculated results are in agreement with Gaussian function yet not very well with double-exponential function as shown in Figure (d) and (e), respectively, due to the additional force and the formation of a thick film. The thickness seems to reach a maximum in Na2SO4 solutions, resulting in the stability of the brine film and the largest repulsion of all the three salts. Figure 6 shows the comparison between force profiles treated with the DLVO and EDLVO models in the presence of CaCl2 and Na2SO4, separately. The predictions in the presence of NaCl was shown in our previous work.2 DLVO model predicts a large attraction present at a distance of several nanometers, which originates from the balance of attachment and diffuse layer overlap of the surfaces. Obviously, the attractive force is inconsistent with the experimental results. Instead, the total forces of short-ranged weak attractions and complete repulsions emerge in the presence of CaCl2 and Na2SO4, respectively, and therefore, it can be deduced that different non-DLVO forces are present in these systems. The EDLVO model successfully predicts the ACS Paragon Plus Environment

12

Page 13 of 18

relationship between disjoining pressure and film thickness observed in the experimental profiles, indicating that other interactions such as hydrophobic effect or hydration force should also be considered. The hydrophobic effect relates to the low solubility of hydrophobic solutes (the tiny nonpolar oil droplets) in water, which will drive the nonpolar groups to adhere water phase to minimize their contact with water molecules.37 In addition to electrostatic and van der Waals forces, a short-range repulsive force, known as hydration force, plays an important role, because divalent anions adsorb to the tip and sample surfaces and bind few layers of water.32 5

5x10



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

CaCl2-10mM Exp data DLVO EDLVO

5

4x10

5

3x10

5

2x10

5

1x10

0 5

-1x10

0

10

20

30

Film Thickness (nm)

5x10

5

4x10

5

3x10

5

2x10

5

1x10

5

Na2SO4-10mM Exp data DLVO EDLVO

0 -1x10

5

0

(a)

10

20

Film Thickness (nm)

30

(b)

Figure 6. Disjoining pressure profiles and their predictions by DLVO and EDLVO models in (a) calcium chloride and (b) sodium sulfate solutions of 10mM

Furthermore, most of the fitting parameters of EDLVO model in a series of NaCl, CaCl2, and Na2SO4 solutions have been shown in Table 2, with other parameters supported in Table S5, such as the parameters of b3 and b4, et al. Higher values of λ1 and λ2 associate with larger structural interactions in NaCl solution, where the appearance of hydration forces mainly connects with the presence of an adsorbed layer of hydrated cations at solid surface, as well as the polarization effects of the surface dipoles (as shown in Figure 7(a)). In the presence of CaCl2 or Na2SO4, the values of b seems to make a great contribution on the structural profiles, being negative when the repulsion is the dominant force and positive when the attraction dominates the force. In the case of CaCl2, hydrophobic effect plays an important part in solute-solvent interactions, in which the strong inclination of water molecules to form H-bonds with each other influences their interactions with nonpolar molecules that are incapable of forming H-bonds. In addition, divalent cations adsorbs on mica surface and bridge with octodecane molecules surrounded as a hydration shell (Figure (b)). It should be noted that there is a maximum adhesion of nonpolar groups contacting with water molecules, due to the role of water molecules ACS Paragon Plus Environment

13

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 18

between nonpolar groups might be larger than that of the direct van der Waals interaction between these groups. While in Na2SO4, hydration force is responsible for the maximum repulsion in short-range between the modified tip and mica surfaces, originating from unique water structure in the vicinity of surfaces and the disruption of H-bonds between polar and nonpolar molecules (Figure (c)). As a result, it shows a different aggregation behavior at different saline systems, leading to a different rearrangement of polar/nonpolar environment and inducing a different interaction between alkyl terminated tips and mica surfaces. Table 2. Brine

Fitting parameters of EDLVO model with double-exponential or Gaussian function

Concentration (mM)

C1 (Pa)

λ1 (nm)

b1

C2 (Pa)

λ2 (nm)

b2

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

-

10

7.66×105

0.47

0.32

5.09×105

1.81

0.54

100

9.13×105

0.41

0.29

2.95×105

1.52

2.07

1000

2.41×106

0.58

2.38

1.40×105

0.64

0.31

10

6.92×105

1.03

-0.22

6.86×105

5.61

-0.72

100

7.09×105

1.07

-0.21

7.33×105

6.65

-1.72

1000

1.07×106

1.46

-0.76

5.58×105

4.70

0.52

NaCl

CaCl2

Na2SO4

(a)

(b)

(c)

Figure 7. Schematics of the departure of functional tip from mica surface in the presence of sodium chloride, calcium chloride, and sodium sulfate aqueous solutions (Red ball represents cation; light blue ball represents anion; blue ball represent water molecule)

5.

CONCLUSIONS ACS Paragon Plus Environment

14

Page 15 of 18

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

In this paper, molecular level explanations of the low salinity effect were provided during different injection sequences of sodium chloride, calcium chloride, and sodium sulfate aqueous solutions, by the aid of chemical force microscopy (CFM). The adhesion forces between a chemically modified probe and solid surface were found to be sensitive to salinity and composition, showing a decreasing trend with the decrease of concentration and close to inadhesion in the presence of divalent anions. The disjoining pressure showed a maximum attraction in calcium chloride and minimum in sodium sulfate, which was not only associated with Debye length and electrostatic forces, yet also associated with structural forces. On the basis of EDLVO theory, the van der Waals, electrostatic, and non-DLVO interactions were all taken into account. Furthermore, the contribution of structural forces to the resultants was calculated and the fitted parameters were deduced by a double-exponential decay model or a Gaussian model, indicating that a complex interaction emerged between brine/oil/rock interfaces. Hydration forces dominated in the short range in the presence of sodium sulfate, owing to the interface structure composed by water molecules around nonpolar or polar solute molecules and surface. In the case of calcium chloride, hydrophobic effect played an important part in solute-solvent interactions and formed a complex rearrangement of polar/nonpolar environment. Overall, the explanation and theoretic model proposed in the present work will lead to a deep insight into the mechanisms of improving oil recovery through low-salinity waterflooding in the molecular level.

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 Projects of New Method Researches on Improving Recovery Efficiency of Low Permeability, Ultra-Low Permeability of Oil Reservoirs, belonging to the Department of Science and Technology of China National Petroleum Co., LTD (2014B-1201 and RIPED-2015-JS-177) for supporting this research. ACS Paragon Plus Environment

15

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 18

REFERENCES 1. Myint, P. C.; Firoozabadi, A., Thin liquid films in improved oil recovery from low-salinity brine. Current Opinion in Colloid & Interface Science 2015, 20, (2), 105-114. 2. Yang, G.; Chen, T.; Zhao, J.; Yu, D.; Liu, F.; Wang, D.; Fan, M.; Chen, W.; Zhang, J.; Yang, H.; Wang, J., Desorption Mechanism of Asphaltenes in the Presence of Electrolyte and the Extended Derjaguin–Landau–Verwey–Overbeek Theory. Energy & Fuels 2015, 29, (7), 4272-4280. 3. 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. 4. Kumar, K.; Dao, E. K.; Mohanty, K. K., Atomic force microscopy study of wettability alteration by surfactants. Spe Journal 2008, 13, (2), 137-145. 5. Aman, Z. M.; Haber, A.; Ling, N. N. A.; Thornton, A.; Johns, M. L.; May, E. F., Effect of Brine Salinity on the Stability of Hydrate-in-Oil Dispersions and Water-in-Oil Emulsions. Energy & Fuels 2015, 29 (12), 7948-7955. 6. Chandrasekhar, B. D., N. R., Application of DLVO Theory to Characterize Spreading in Crude oil-Brine-Rock Systems. SPE 2004, 89425. 7. Hassenkam, T.; Mitchell, A. C.; Pedersen, C. S.; Skovbjerg, L. L.; Bovet, N.; Stipp, S. L. S., The low salinity effect observed on sandstone model surfaces. Colloids and Surfaces A: Physicochemical and Engineering Aspects 2012, 403, 79-86. 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. Gandomkar, A.; Rahimpour, M. R., Investigation of Low-Salinity Waterflooding in Secondary and Tertiary Enhanced Oil Recovery in Limestone Reservoirs. Energy & Fuels 2015, 29 (12), 7781-7792. 10. Nasralla, R.; Nasr-El-Din, H.; Nasralla, R., Double-Layer Expansion: Is It a Primary Mechanism of Improved Oil Recovery by Low-Salinity Waterflooding? Spe Reservoir Evaluation & Engineering 2014, 17, (1), 49-59. 11. Hilner, E.; Andersson, M. P.; Hassenkam, T.; Matthiesen, J.; Salino, P. A.; Stipp, S. L., The effect of ionic strength on oil adhesion in sandstone-the search for the low salinity mechanism. Scientific Reports 2015, 5, 9933. 12. Sheng, J. J., Critical review of low-salinity waterflooding. Journal of Petroleum Science and Engineering 2014, 120, 216-224. 13. Brady, P. V.; Krumhansl, J. L., A surface complexation model of oil–brine–sandstone interfaces at 100°C: Low salinity waterflooding. Journal of Petroleum Science and Engineering 2012, 81, 171-176. 14. 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. 15. Berg, S.; Cense, A. W.; Jansen, E.; Bakker, K., Direct experimental evidence of wettability modification BY LOW SALINITY. Petrophysics 2010, 51, (5), 314-322. 16. Moeini, F.; Hemmati-Sarapardeh, A.; Ghazanfari, M. H.; Masihi, M.; Ayatollahi, S., Toward mechanistic understanding of heavy crude oil/brine interfacial tension: The roles of salinity, temperature and pressure. Fluid Phase Equilibria 2014, 375, (31), 191-200. 17. Fathi, S. J.; Austad, T.; Strand, S., Water-Based Enhanced Oil Recovery (EOR) by “Smart Water”: Optimal Ionic Composition for EOR in Carbonates. Energy & Fuels 2011, 25, (11), 5173-5179. 18. Strand, S.; Austad, T.; Puntervold, T.; Høgnesen, E. J.; Olsen, M.; Barstad, S. M. F., “Smart Water” for Oil Recovery from Fractured Limestone: A Preliminary Study. Energy & Fuels 2008, 22, (5), 3126-3133. 19. Lager, A.; Webb, K. J.; Black, C. J. J.; Singleton, M.; Sorbie, K. S., Low Salinity Oil Recovery - An Experimental Investigation1. Petrophysics 2008, 49, (1), 28-35. 20. Arnarson, T. S.; Keil, R. G., Mechanisms of pore water organic matter adsorption to montmorillonite. Mar Chem. Marine Chemistry 2000, 71, (3), 309-320. 21. Skovbjerg, L. L.; Hassenkam, T.; Makovicky, E.; Hem, C. P.; Yang, M.; Bovet, N.; Stipp, S. L. S., Nano ACS Paragon Plus Environment

16

Page 17 of 18

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

sized clay detected on chalk particle surfaces. Geochimica et Cosmochimica Acta 2012, 99, 57-70. 22. Jiang, H.; Xie, Z.; Liu, G. R.; Ya-Wen, Y. U.; Zhang, D., Interaction forces between muscovite and silica surfaces in electrolyte solutions measured with AFM. Transactions of Nonferrous Metals Society of China 2013, 23, (6), 1783–1788. 23. Takano, H.; Kenseth, J. R.; Wong, S. S.; O'Brien, J. C.; Porter, M. D., Chemical and biochemical analysis using scanning force microscopy. Chemical Reviews 1999, 99, (10), 2845-2890. 24. Hutter, J. L.; Bechhoefer, J., Calibration of atomic-force microscope tips. Review of Scientific Instruments 1993, 64, (7), 1868-1873. 25. Dotto, M. E. R.; Camargo, S. S.; Ziglio, C. M., Study of Force Curves on Paraffin Deposits Using Atomic Force Microscopy. Energy Fuels 2007, 21, (3), 1296-1300. 26. Israelachvili, J., Interfacial forces. Journal of Vacuum Science & Technology a-Vacuum Surfaces and Films 1992, 10, (5), 2961-2971. 27. Yoon, R. H.; Flinn, D. H.; Rabinovich, Y. I., Hydrophobic interactions between dissimilar surfaces. Journal of Colloid and Interface Science 1997, 185, (2), 363-370. 28. Gregory, J., Approximate expressions for retarded van der waals interaction. Journal of Colloid and Interface Science 1981, 83, (1), 138-145. 29. Israelachvili, J. N., Chapter 13 - Van der Waals Forces between Particles and Surfaces. In Intermolecular and Surface Forces (Third Edition), Israelachvili, J. N., Ed. Academic Press: San Diego, 2011; pp 253-289. 30. Gregory, J., Interaction of unequal double layers at constant charge. Journal of Colloid and Interface Science 1975, 51, (1), 44-51. 31. Israelachvili, J. N., Chapter 14 - Electrostatic Forces between Surfaces in Liquids. In Intermolecular and Surface Forces (Third Edition), Israelachvili, J. N., Ed. Academic Press: San Diego, 2011; pp 291-340. 32. Butt, H. J., Measuring electrostatic, van der Waals, and hydration forces in electrolyte solutions with an atomic force microscope. Biophysical Journal 1991, 60, (6), 1438-1444. 33. Derjagui.Bv; Churaev, N. V., Structural component of disjoining pressure. Journal of Colloid and Interface Science 1974, 49, (2), 249-255. 34. Yu, D.; Yang, H.; Wang, H.; Cui, Y.; Yang, G.; Zhang, J.; Wang, J., Interactions between colloidal particles in the presence of an ultrahighly charged amphiphilic polyelectrolyte. Langmuir 2014, 30, (48), 14512-21. 35. Lee, S. Y.; Webb, K. J.; Collins, I.; Lager, A.; Clarke, S.; apos; Sullivan, M.; Routh, A.; Wang, X., Low Salinity Oil Recovery: Increasing Understanding of the Underlying Mechanisms. In SPE Improved Oil Recovery Symposium, Society of Petroleum Engineers: Tulsa, Oklahoma, USA, 2010. 36. Guo, H., A Simple Algorithm for Fitting a Gaussian Function. IEEE Signal Processing Magazine 2011, 28, (5), 134-137. 37. Meyer, E. E.; Rosenberg, K. J.; Jacob, I., Recent progress in understanding hydrophobic interactions. Proceedings of the National Academy of Sciences 2006, 103, (43), 15739-15746.


17

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 18

Graphic CaCl2

Na2SO4


18

Brine

Recommend Documents