Static Adsorption and Retention of Viscoelastic Surfactant in Porous

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Static Adsorption and Retention of Viscoelastic Surfactant in Porous Media: EOR Implication Ke-xing Li, Xueqi Jing, Song He, and Bing Wei Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01732 • Publication Date (Web): 19 Sep 2016 Downloaded from http://pubs.acs.org on October 3, 2016

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Static Adsorption and Retention of Viscoelastic Surfactant in Porous Media: EOR Implication Kexing Li,*†§ Xueqi Jing, † Song He,‡ Bing Wei*†§ †

School of Petroleum and Natural Gas Engineering, SWPU, Chengdu, 610500,China

§

State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation(SWPU),Chengdu, 610500,China

‡

Dagang Oilfield Branch PetroChina, Tianjing, 300280,China

ABSTRACT: The mass loss of surfactant during flowing through porous media is one of the major concerns to surfactant-based Enhanced Oil Recovery (EOR) techniques and also an critical issue for viscoelastic surfactant (VES), which is a novel and promising flooding agent compared against the traditional displacing chemicals. This paper comprehensively investigated the interactions between a VES and several minerals. The static adsorption tests of VES on quartz, montmorillonite and kaolinite was first carried out at 65oC. Empirical models were established to describe the VES adsorptive isothermal beahviors on the mineral surfaces. The data indicated that the VES showed L, S and LS adsorption isotherm patterns on quartz, montmorillonite, and kaolinite, respectively. The adsorption of VES were notably influenced by temperature, pH value, salinity and Ca2+. As a result of adsorption on clay surfaces, the viscosity and surface activity of the VES solution were significantly decreased. Moreover, the large hydrodynamic size of VES aggregates caused an inaccessible pore volume (IPV) during flowing through porous media. The magnitude of VES retention was considerably impacted by permeability (pore radius), VES concentration, displace velocity and salinity under reservoir conditions. This work provided some new insights into oil recovery mechanisms of viscoelastic surfactant flooding. 1. INTRODUCTION Viscoelastic surfactant (VES) is considered a promising flooding agent for enhanced oil recovery (EOR), which not only enables to reduce oil-water IFT as conventional flooding surfactant and but also correct the unfavorable mobility ratio of waterflooding.1-3 However, the mass loss of surfactant during flowing through porous media usually exerts a detrimental effect on oil recovery factor of any surfactant-based EOR methods,4 and adsorption is known to be one of the principle mechanisms causing surfactant loss in porous media. In the past decades, numerous efforts have been made to the adsorption behaviors of surfactant. For example, hydrogen bonding was known to be responsible for the adsorption of nonionic surfactant.5 The properties of hydrophobic stationary phases coated with zwitterionic surfactant were found to depend on several factors.6 The dynamic surface tension proved that

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the adsorption kinetics of a star-shaped gluconamide-type cationic surfactant solution above CMC was a mixed diffusion-kinetics.7 Cationic surfactants having similar structures with CTAB present almost identical adsorptive behaviors and the magnitude of the adsorption varied linearly with the length of the hydrophobic tails.8 The adsorption characteristics of binary or ternary surfactant systems were also previously investigated. The adsorption data of three surfactants on kaolinite indicated that the surfactant structure had considerable effect on the adsorption isotherms, competitive and synergistic effects occurred in the surfactant-polyacrylamide system.9 In addition, when the molar ratio of cationic and anionic surfactants binary systems was over 1.1, the adsorption was negligible and the presence of nonionic surfactant impaired the adsorption of the ionic species.10 The adsorption of an anionic and cationic binary system reached an equal molar ratio and the combination of single- and double-head surfactants could improve the adsorption degree.11 The desorption behaviors of an individual and binary anionic surfactant systems were also studied by varying washing procedures.12 In surfactant flooding, surfactant molecules will permeate through porous media and the magnitude of surfactant adsorption strongly depends on the exposing minerals. In the case of sand, cationic surfactant suffered more adsorption than anionic surfactant.13 Moreover, the adsorption and desorption of calcium lignosulfonate on a porous dolomite exhibited a twostep pattern.14 The adsorptive behavior of surfactant on carbonate can be fitted to Freundlich isotherm model.15 The adsorption of surfactant on quartz surface was found to be influenced by NaCl concentration, Ca2+ and pH value.16 In addition, the kinetics of surfactant adsorption on clays was also influenced by oxygen species.17 When surfactant molecules confronted rock, the surfactant tails would adsorb onto rock surface, consequently, the hydrophobicity of quartz surface was reduced and the wettability was then reversed.18 Surfactant adsorption is also associated with other factors including concentration, molecular structure, solid-liquid ratio, adsorption time, mass concentration, rock type, flow state and temperature, etc.14, 19, 20 Ziegler et al. studied the static and dynamic adsorption of a nonionic surfactant on Berea core and observed that the adsorption decreased as a function of temperature at low surfactant concentration.21 Surfactant retention is believed to be one of the dominant factors determining the displacing efficiency of surfactant-based EOR methods. The mechanisms of retention in porous media includes adsorption, precipitation, partitioning into residual oil and entrapment of immiscible microemulsion phase.22 The magnitude of surfactant retention is closely related to brine salinity, pH, surfactant structure, crude oil, porous media, etc.23 In addition to the


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aforementioned factors, surfactant solubility, phase behavior and slug size are also associated with the retention behaviors of surfactant in porous media.24 As a consequence of surfactant adsorption and/or retention, the wettability of rock surface would be altered, which thus leads to variations in the fluid flow capacity of the reservoir.25,26 Differing from conventional surfactants, the large hydrodynamic size of VES aggregates usually renders the VES solution problematic in permeating resulting from adsorption and/or retention. Therefore, the primary objective of this work was to figure out the static adsorption and retention behaviors of VES in porous media, which seems have been rarely studied before. The findings derived from this work can provide some new insights into VES behaviors regarding EOR use. To accompany this research objective, the static adsorption kinetics of VES on three main mineral surfaces-quartz, montmorillonite and kaolinite, was first studied and the experimental data were fitted to some empirical adsorptive models. Afterwards, the VES retention in porous media during permeating in sandstone cores was investigated. This work is supposed to address the following queries: 1. static adsorption isotherms of VES on different minerals; 2. inaccessible pore volume (IPV) when VES permeates through porous media; and 3. the dependence of VES retention on several common factors. 2.THEORY 2.1. ADSORPTION ISOTHERM It has been documented that Langmuir and Freundlich models are two well-know empirical models describing the adsorption isotherm of surfactant.15 The most extensively used isotherms include L-curve, S-curve and LS-curve.27 L-curve is very prevalent on solid-liquid interface for dilute solutions, which can be described using Langmuir monomolecular adsorption equation. S-curve has a small slope at the beginning and also a fast increasing interval, which shows that the IFT is relatively small at low concentration and can be described by BET equation.28 While, LS-curve has two flat zones, demonstrating that the aggregates of surfactants on interface could be monomolecular and bilayer adsorptions. In terms of Langmuir model, two phase adsorption on interface are usually assumed.27 The first phase is the adsorption of molecules or ions due to electrostatic attraction and/or the interaction between Vander Waals force and the solid surface; the second phase is caused by hydrophobic interaction causing the molecules or ions form micelles. The adsorption on solid-liquid interface can be described by general isotherm Equation (1):19,28


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1 Γ ∞ k1C ( + k2C n −1 ) n Γ= 1 + k1C (1 + k2C n −1 )

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(1)

Γ and Γ ∞ can be gained by experiments; k1 and k2 are equilibrium constants. Theoretical results show that Eq (1) can be converted into L-curve, S-curve and LS-curve through adopting appropriate values of k1, k2 and n. When k2 approaches to 0 and n approaches to 1, Eq (1) can be restored back to Langmuir model representing L-curve:

Γ= n −1 When n>1, if k2C
> 1 or k1C > 1 , Eq (1) can be rewritten as (5), representing LS-curve:

1 Γ ∞ ( + k2C n −1 ) n Γ= 1 + k2C n −1

(5)

With the surfactant concentration increasing, the eventual adsorption value can be described as Eq (6), which implies that all adsorption sites are taken by micelles:

Γ = Γ∞

(6)

2.2. Retention model The diameter of water molecule is much smaller than that of pore and throat of the porous media. Therefore, theoretically, water molecules are supposed to pass through all the pore and throat space of a given core. However, in the case of other materials such as macromolecular polymers, as a result of the relatively larger hydrodynamic size, not all the space can be accessed, which thus creates a certain volume of IPV. IPV cuts the passing time of macromolecular polymers; while, the retention causes a time lag of slug breakthrough. In EOR process, the dynamic retention of VES is more concerned than the static adsorption loss.


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Conventional surfactants can access almost all the pore volume because of the small molecule size. Nevertheless, for VES, the aggregates have the similar hydrodynamic size with polymer molecules, which accordingly makes some pores inaccessible during VES flooding.29 The magnitude of the dynamic retention can be determined through plotting the effluent profile of a core flood experiment.30 According to the theory of diffuse percolation, when a solution floods a water-saturated core at a constant velocity, the point, at which the dimensionless concentration (Cr=C/Co) on the concentration profile at any time is 0.5, moves at velocity=V.31 Therefore, on the concentration profile of the outlet and at Cr=0.5, the cumulative injected PV is the dimensionless time of the solution slug front breaking through, i.e. reaching the outlet. Using the dual slug concentration profile method,32,33 the concentration profile was plotted during VES solution flowing through the porous cores and IPV = 1 − VP ,C

r

= 0.5

. In addition, on

the dual slug concentration profile, the retention porous volume (RPV) caused by retention loss is RPV = VP1,C

r

= 0.5

− VP 2,C r = 0.5 . Based on the mass balance, the surfactant loss after passing

through the core can be calculated using Eq (7):16

Γr =

V (C0 − C ) M

(7)

If IPV exists, Eq (7) can be converted into Eq (8) in the dual slug test:

Γr =

RPV ⋅VP ⋅ C0 M

(8)

The values of IPV and RPV can be gained from the dual slug concentration profile.

3. EXPERIMENTAL 3.1. Materials Water: deionized water was used throughout the work. VES: the VES used in this work is an anionic viscoelastic EOR surfactant, which was provided by Shanghai Research Institute of Petrochemical Technology. The moduli of G' and G'' of VES solution as a function of concentration is shown in Fig. 1 (65oC). The rheological results revealed the viscoelastic properties of this surfactant.


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1.E+01 1.E+00 1.E-01 1.E-02

G' & G'' (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

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1.E-03 1.E-04 1000mg/L,G' 1000mg/L,G'' 5000mg/L,G' 5000mg/L,G'' 10000mg/L,G' 10000mg/L,G''

1.E-05 1.E-06 1.E-07 1.E-08 1.E-09 0.01

0.1

Frequency (Hz)

1

10

Figure 1. Storage modulus and loss modulus of VES sample at different concentrations

Mineral grains: the materials used in the static adsorption tests are three main sandstone minerals including quartz, montmorillonite and kaolinite. The size of the grains ranged from 80 to 100mesh. All the chemicals were purchased from Kelong Co., Ltd (China) and used as received. Artificial cores: The artificial cores are homogeneous porous sandstone cores and mainly composed of quartz(>99%). The petrophysical properties of the cores were tabulated in Table 1.

Table 1. Petrophysical properties of the cores 3

Core No.

Weight/g

L/cm

D/cm

Φ/%

V p /cm

kg (×10-3) µm2

1#

65.82

6.962

2.506

16.20

5.56

102.6

2#

65.50

7.000

2.516

16.90

5.88

103.3

3#

65.50

6.970

2.520

16.66

5.79

103.6

4#

65.78

6.934

2.508

15.60

5.34

98.7

5#

64.96

6.952

2.514

17.02

5.87

102.8

6#

65.64

6.968

2.507

18.34

6.31

101.4

7#

65.33

6.992

2.510

17.21

5.95

99.5

8#

65.48

7.021

2.508

16.92

5.87

99.2

9#

65.61

6.941

2.514

17.58

6.05

102.1

10#

65.28

6.986

2.509

16.98

5.86

99.6

11#

65.53

6.973

2.511

16.75

5.78

100.5

12#

65.30

6.952

2.513

17.10

5.89

102.1

13#

65.59

6.954

2.522

21.60

7.50

310.5

14#

65.73

6.968

2.516

29.81

10.32

935.1

3.2. Apparatus


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The UV spectrophotometer used in this work is UV-1800 (SHIMADZU, Japan). The flooding apparatus for the dual slug concentration profile tests were assembled by a constant speed pump, core holder, oven, pressure gauge and DBR pump, etc., as illustrated in Fig. 2.

Figure 2. Experimental set-up of dual slug concentration profile tests 3.3. Experimental procedures Static adsorption

Mineral pretreatments are necessary prior to the static adsorption tests.12 Quartz was first heated and then soaked in distilled water followed by immersing in hydrochloric acid (1.0%) for 10 hours. Afterwards, the sand was washed using distilled water until no chloride ions could be detected in the AgNO3 tests. Finally, the sand was dried in a thermostatic oven at 60oC. Kaolinite and montmorillonite were heated and soaked in distilled water until the electrical conductivity became constant. Thereafter, the minerals were dried in an oven at 60oC. The basic procedures of the static adsorption test are as follows: 1) The bulk VES solution was prepared using distilled water at a concentration of Co; 2) Standard correlation curve of VES solution concentration and absorbance was established using UV spectrophotometer; 3) Mineral grains were mixed with the VES solution in a sealed ampoule bottle and then vibrated; 4) The bottle was placed in a 65oC waterbath for a certain duration time; 5) The solution was centrifuged at 2000 r/min for 30 minutes; 6) Diluting the supernatant and then measuring the UV absorbance; 7) Calculating the adsorption equilibrium concentration based on the standard curve.


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Dual slug concentration profile22 To measure the VES retention and IPV in porous media, dual slug concentration profile experiments have been conducted as follows: 1) Determining pore volume (VP) and porosity of the cores; 2) Displacing water out of the core with VES solution at a constant temperature of 65oC (the first VES slug injection) and effluent was sampled every 0.2 PV. The concentration of the solution was determined using spectrophotometer until a plateau was reached; 3) Draining the deionized water out of the core until the concentration at the outlet become zero again; 4) Injecting VES solution into the core again at a constant velocity (the second VES slug injection) and effluent was sampled every 0.2 PV. The concentration of the solution was determined using spectrophotometer until the concentration leveled off.

4. RESULTS AND DISCUSSION 4.1. Kinetics of VES static adsorption Standard curve A wave length sweep was first conducted to identify the characteristic absorption peak of VES (297nm).14 The correction of UV absorbance and VES concentration was thus established, which was used to study the kinetics of the static adsorption. The relevant correlation is y = 145.91x + 2.8014 , where y is the solution concentration, and x is the value of UV absorbance.

Liquid/solid ratio and balance time The ratio of liquid-to-solid (mL/g), 10:1, 20:1, 30:1, 40:1, 50:1 and 60:1, were created in 100 ml of VES solution with the bulk concentration of 500 mg/L. The mixtures were placed into a 65oC waterbath for 20 hours. Clarifying the difference of solution concentration before and after adsorption is the key principle to quantify the adsorption. The magnitude of adsorption as a function of liquid-to-solid ratio was plotted in Fig. 3. Figure 3 indicates that that the magnitude of VES adsorption on the minerals almost leveled off when the liquid-to-solid ratio beyond 40:1, which was consequently set to the ratio for the following tests. Figure 4 is present to show the influence of time on VES adsorption at the ratio of 40:1 and 65oC. It was observed that the absorption equilibrium time of VES on quartz and kaolinite was approximately 15 hours and approximately 20 hours for montmorillonite. On the basis of the curves in Fig. 4, a duration of 20 hours was selected for the adsorption time in the following tests.


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Adsorption（mg/g）

5 4 3 2 Quartz Kaolinite Montmorillonite

1 0 0

10

20

30 40 50 Liquid/solid ratio（mL/g）

60

70

Figure 3. VES adsorption on minerals as a function of liquid-to-solid ratio 5


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4 3 2 Quartz Kaolinite Montmorillonite

1 0 0

5

10

15 Time（h）

20

25

30

Figure 4. VES adsorption on minerals as a function of time 4.2. VES adsorption isotherm on sandstone minerals Adsorption isotherms of VES on the surfaces of three minerals were obtained as depicted in Fig. 5. The isotherms were fitted to the Eqs. (2), (3) ... (6), and the results were tabulated in Table 2. Based on the data in Fig. 5, it can be seen that the absorption behaviors of VES on quartz, montmorillonite and kaolinite exhibit L-curve, S-curve and LS-curve features, respectively. L-curve pattern reveals the monolayer absorption characteristics of VES on quartz. VES

initially absorbs on the active parts of quartz in the form of monomolecule, after which the adsorption rises with the increase in VES concentration as indicated in the curve. After the active parts are saturated by VES, the adsorption is prone to decrease and later levels off. In the case of VES absorption on montmorillonite, a S-curve was observed, suggesting that at low mass concentration the absorption of VES is characterized by monolayer, which is


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usually very low. Nevertheless, with the concentration increasing, the monolayer adsorption switches to multilayer adsorption and the magnitude of adsorption keeps increasing until the active parts are taken, after which the adsorption isotherm becomes stable again. The adsorption of VES on kaolinite was found to well fit to a LS-curve, which has a twostage process with apparent multilayer adsorption.9 At low concentration, the adsorption is dominant monolayer and the adsorption density is proportional to the charge density on the mineral surface. As VES concentration increases, the adsorption turns into micelle adsorption and the adsorption density is proportional to the density of monolayer adsorption. 50 Quartz test Montmorillonite test Kaolinite test Quartz fitting Montmorillonite fitting Kaolinite fitting

40

Adsorportion (mg/g)

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

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30

20

10

0 0

1000

2000

3000

4000

5000

6000

VES concentration (mg/L)

Figure 5. Adsorption isotherms of VES on minerals (65oC) Table 2. Parameters of the absorption measurements Mineral type

Г∞

Equilibrium concentration

(mg/g)

(mg/L)

Curve type

Fitting

R2

Quartz

8.47

800

L

Eq. (9)

0.923

Montmorillonite

16.72

1200

S

Eq. (10)

0.996

Kaolinite

47.03

5000

LS

Eq. (11)

0.980

According to Eq. (1), the adsorption isotherm equations of VES on quartz, montmorillonite and kaolinite are as follows.

Γ=

145.65C 1 + 13.35C

(9)

Γ=

18.48 ×108 ⋅ C 7.344 1 + 1.1049 ×108 C 7.344

(10)


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Γ=

7.63 + 2.421×104 C 5.166 1 + 514.8C 5.166

(11)

Static adsorption The kinetics of VES static adsorption is associated with many factors and some key factors including temperature, pH, inorganic salts, solid surface and solvent, were thus investigated in this section.

Effect of Temperature VES solution (5000mg/L) was mixed with quartz, montmorillonite and kaolinite, respectively, at the liquid-to-solid ratio of 40:1. The mixtures were then placed in a thermostatic waterbath for 20 hours. The VES adsorption as a function of temperature was plotted as shown in Fig. 6. It was found that the magnitude of VES adsorption on the minerals slightly decreased within the temperature ranging from 60-80oC. This fact is probably due to promoted activity of VES molecules with temperature. 50


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40 Quartz Kaolinite Montmorillonite

30 20 10 0 55

60

65

70 75 80 Temperature（◦C）

85

90

Figure 6. VES adsorption as a function temperature Effect of pH value Figure 7 plots the magnitude of VES adsorption as a function of pH value. In the case of quartz, we found that the influence of pH value on VES adsorption was almost negligible, whereas a more significant increase was observed for VES adsorption on clay minerals with pH value increasing. This result can be attributed to the content of H+ and OH- in the solution. High pH value results in decrease of H+ adsorption and thus leads the magnitude of VES adsorption to increase. Moreover, the repulsive forces of OH- and VES also causes the adsorption to increase on the solid-liquid interface. The adsorptive features of VES proved the major role of positive edge face of kaolinite in adsorption.9


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60


50 40 Quartz Kaolinite Montmorillonite

30 20 10 0 2

4

6

8

10

12

pH

Figure 7. VES adsorption as a function pH value (65oC) Effect of solution salinity-Na+ It has been known that salinity is one of key factors impacting the adsorption behaviors of anionic surfactants.13 Figure 8 plots the magnitude of VES adsorption as a function of Na+ concentration. As seen, the adsorption of VES on three minerals generally increases with Na+ concentration and the most remarkable variation was observed in the case of quartz. The increasing Na+ would cause a decrease in the electrostatic repulsion between VES molecules on quartz, which subsequently compacts molecular arrangement and creates additional sites VES molecules. 70 60 Adsorption（mg/g）

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50 40 30 Quartz

20

Kaolinite

10

Montmorillonite

0 0

5000

10000

15000

20000

25000

30000

35000

Total salinity（mg/L）

Figure 8. VES adsorption as a function of salinity Effect of Solution salinity-Ca2+ Figure 9 is present to show the influence of Ca2+ concentration on VES adsorption. Within the evaluated concentration range, the static adsorption of VES on the minerals all rapidly


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increase. The charge of cation Ca2+ squeezes the VES micelles from macromolecular to micromolecular aggregates and thus causes the volume of micelles to reduce, which accordingly enhances the density of the adsorbed VES on mineral surfaces. The effect of Ca2+ is quite consistent with that of Na+. 60 50 adsorption（mg/g）

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Quartz

40

Kaolinite Montmorillonite

30 20 10 0 0

100

200

300 400 Ca2+（mg/L）

500

600

700

Figure 9. VES adsorption as a function of Ca2+concentration 4.4. Effect of adsorption on the physical properties of VES solution Solution Viscosity The viscosity alteration of VES solution after adsorption as a function of concentrations is illustrated in Fig. 10. We observed that a very slight viscosity decrease of VES solution occurred on quartz indicating the low adsorption magnitude. On the contrary, for the other two minerals as depicted in Fig. 10, the viscosity of VES solution was significantly decreased, which corresponds to high adsorption magnitude. This fact proves that the mineral of clay should be incorporated in the screen criteria of VES flooding and reservoirs having high clay might be not appropriate for VES flooding.


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80 Viscosity (mPa·s）

70

Initial viscosity

60

Quartz

50

Kaolinite Montmorillonite

40 30 20 10 0 0

1000

2000 3000 4000 VES concentration （mg/L）

5000

6000

Figure 10. Viscosity of VES solution before and after adsorption (65oC) Oil-Water interfacial tension The O/W IFT variation of VES solution before and after static adsorption was measured with the results showing in Fig. 11. After the occurrence of the adsorption, the IFT of VES solution was further dropped, whereas in the case of montmorillonite and kaolinite the IFT values shifted to upper field. The mechanism of VES reducing IFT after adsorbing on quartz is closely related to the adsorption and molecular interface properties on oil-water interface. There may be an optimal concentration for VES decreasing the oil-water interfacial tension. In this work, the concentration of VES solution experienced a slight decrease after adsorbing on quartz, which might thus generate this optimal concentration for IFT reduction. 10

IFT（ mN/m）

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

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Initial IFT

1

Quartz Kaolinite Montmorillonite

0.1

0.01

0.001 0

1000

2000

3000

4000

5000

6000

VES concentration（mg/L）

Figure 11. O/W IFT as a function of concentration (65 oC) 4.5. Dynamic retention and IPV of VES in porous media


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Effect of permeability In this section, VES was forced to permeate through three sandstone cores having different permeability (pore radius) in order to figure out the effect of permeability on VES retention. The concentration profiles (C/C0) of the outlet are shown in Fig. 12 and the magnitude of retention was summarized in Table 3.

Figure 12. Dual slug concentration profiles of different permeability cores (a) 102.6×10-3µm2; (b) 310.5×10-3µm2; (c) 935.1×10-3µm2 Table 3. VES retention in porous media Core No.

kg -3

Pore radius 2

(×10 ) µm

IPV

RPV

(µm)

Retention (µg/g)

1#

102.6

2.22

0.25

0.28

70.96

13#

310.5

3.44

0.20

0.44

150.93

14#

935.1

5.06

0.12

0.56

263.82

A VES slug was pumped into 3 sandstone cores under the same experimental. Figure 12 shows the details of the dual slug tests. From the profiles, we can directly read the value of Vp corresponding to the point of C/Co=0.5 from two dimensionless concentration profile curves as highlighted in Fig. 12, after which the values of IPV and RPV can be determined. Taking Fig. 16(a) as an example, the point of C/Co=0.5 (y axis) corresponds to the Vp values of 0.75 and 1.03 according to the C/Co-Vp curves. Therefore, the IPV equals to 1-0.75=0.25 and RPV equals to 1.03-0.75=0.28. The influences of VES concentration, velocity, salinity and temperature on IPV and RPV were quantified following this method. In hydrophilic porous media, the exclusion effects of pore wall and pore influence the structure of the surfactant adsorbates.19 As shown in Fig. 12 and Table 3, the increasing permeability leads the value of IPV to decrease and VES retention to increase. This result is very understandable and could be interpreted by the average radius of pore and throat. VES aggregates would sweep more pore volume and confront more rock surface, which accordingly causes the magnitude of retention to increase. VES concentration


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Using the same method, the VES retention as a function of concentration was determined and listed in Table 4. It was found that high concentration of VES results in decrease in IPV, which subsequently leads to greater retention. It can be also understood by the permeability effect as shown in Table 3. When the hydrodynamic size of the VES solution is smaller than the pore size, VES can flow through the cores remaining an associated formation. However, if the hydrodynamic size of VES aggregates is larger than that of pore size, the associated aggregates would be torn into micro micelles under shear rate and the size might be even smaller than that of the 1000 mg/L VES solution. Table Core No.

4. VES Retention at different concentrations

Concentration

IPV

RPV

(mg/L)

Retention (µg/g)

5#

1000

0.18

0.29

26.21

3#

3000

0.12

0.32

84.84

6#

5000

0.07

0.47

225.73

Displacement velocity The VES solution with the concentration of 3000 mg/L was forced to flow through the sandstone cores at different velocities. The results of the dual slug displacement were summarized in Table 5. The increasing displacement velocity results in the decrease of the IPV and increase of retention. It is known that the shear rate in porous media is proportional to velocity. The size of VES aggregates would be decreased due to the impact of shear rate, which thus creates more sand surface for VES . Table 5. Retention at different velocities Core No.

Velocity

IPV

RPV

3

(cm /min)

Retention (µg/g)

2#

0.3

0.27

0.29

78.08

3#

0.5

0.12

0.32

84.84

4#

1.0

0.08

0.53

129.10

Salinity The data of IPV and retention at different salinities are given in Table 6. Generally, the magnitude of IPV decreases with the increase in solution salinity. It has been recognized that the presence of cations promotes the adsorption of VES aggregates as a result of the reduced aggregate size and also improved flowability in porous media, which accordingly lead IPV to decrease and retention to increase.22 This results are in agreement with the influences of permeability and concentration.


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Table 6. IPV and retention at different salinities Salinity

Core No.

IPV

RPV

(mg/L)

Retention (µg/g)

3#

1000

0.12

0.32

84.84

7#

5000

0.11

0.32

87.45

8#

15000

0.10

0.36

96.75

9#

25000

0.07

0.48

132.87

Temperature The influence of temperature in IPV and retention of VES was also examined as listed in Fig. 7. It is easy to see that the magnitude of the retention in the sandstone cores slightly decreases as a function of temperature, which can be attributed to the static adsorption behavior of VES at high temperature. In terms of IPV as shown in Table 7, it was observed that the magnitude of IPV is proportional to temperature. Table 7. IPV and retention at different temperatures Core No.

Temperature

IPV

RPV

o

( C)

Retention (µg/g)

10#

45

0.10

0.49

132.0

11#

65

0.18

0.41

108.5

12#

75

0.26

0.30

81.23

5. CONCLUSIONS This work comprehensively investigated the static adsorption and retention behavior of a viscoelastic surfactant (VES). Based on the experimental data, some main conclusions can be stated as follows. 1. VES exhibits monolayer adsorption characteristics (L pattern) on quartz surface and micelle adsorption characteristics (S pattern and LS pattern) on clay minerals. 2. The noticeable adsorption of VES on clay minerals imposes a detrimental effect on the physical properties of VES solution, suggesting that the reservoirs having clay minerals might be inappropriate for VES flooding. VES experienced relatively less adsorption on quartz surface, which is about 10~20mg/g under our experimental conditions. 3. IPV was indeed observed during VES flowing through porous media due to the large aggregates size. Under our experimental conditions, the magnitude of IPV ranges from 0.07 to 0.27. 4. The magnitude of VES retention in porous media was found to be closely related to core permeability (pore radius), VES concentration, displacement velocity and salinity. The data of


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RPV suggest that the slug loss induced by retention is significant when VES flows through porous media. 5. The establishment of empirical models that correlates the static adsorption and retention with the flow behaviors of VES in porous media particularly under reservoir conditions is in progress in our group.

AUTHOR INFORMATION Corresponding Authors *Tel.: +86-28-83032040. E-mail: [email protected] (Kexing Li); *Email: [email protected] (Bing Wei) Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The authors wish to recognize financial supported received from Open Fund (PLN1513) of State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (SWPU) and The Key Subject Construction Project Science Foundation for Young Teachers (P011). JOECO, Zhiqing Shen, SINOPEC, Shanghai Research Institute of Petrochemical Technology are also acknowledged for providing the samples.

NOMENCLATURE L=Length, cm D=Diameter, cm ϕ =Porosity Kg=Air permeability, ×10-3µm2 Co=Concentration before adsorption or injected into core, mg/L C=Concentration after adsorption or flow core outlet, mg/L V=Solution volume, L m=Adsorption mass, g Γ =Adsorption, mg/g Γ ∞ =Maximum adsorption, mg/g

k1=Balance coefficient k2=Balance coefficient R2=Correlation coefficient n=Index V p =Core porous volume, cm

3


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M=Core mass, g Γ r =Retention in the core, µg/g

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Static Adsorption and Retention of Viscoelastic Surfactant in Porous

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