Wettability Alteration of the Quartz Surface in the Presence of Metal

Article pubs.acs.org/EF

Wettability Alteration of the Quartz Surface in the Presence of Metal Cations Ziyuan Qi,*,†,‡ Yefei Wang,† Hong He,† Dandan Li,† and Xiaoli Xu† †

School of Petroleum Engineering, China University of Petroleum (East China), Qingdao 266580, People’s Republic of China Department of Applied Mathematics, Research School of Physics and Engineering, Australian National University, Canberra, Australian Capital Territory 0200, Australia

‡

ABSTRACT: Experimental research was conducted to clarify the wettability alteration mechanism in the presence of metal cations. Wettability was studied by measuring the contact angle on a quartz−crude oil−water system. The quartz surface is aged in an asphaltene/toluene solution to study the effect of aging time, asphaltene concentration, water film on the quartz surface, and metal cations in the water film on wettability alteration of the quartz surface. The quartz surface property is characterized with ζ-potential measurements and infrared (IR) spectroscopy to clarify the wettability alteration mechanism. Results show that quartz surface becomes more oil-wet with the increase of the asphaltene concentration. The quartz with a pure water film shows less oil-wetting after soaking compared to the dry quartz surface, while the presence of metal cations in the water film can improve oil wettability of the surface. ζ-Potential measurements show that Ca2+ and Mg2+ compress a diffused double layer of quartz powder more effectively than Na+, which makes asphaltene adsorption easier because of the reduced electrostatic repulsion. IR spectra analysis indicates that asphaltene molecules can interact with hydroxyls on the quartz surface by polar interactions and asphaltene can be absorbed on the quartz surface with ion binding of hydrated divalent metal cations.

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INTRODUCTION Asphaltenes usually include a wide range of chemical species with high molecular weight, aromaticity, and polarity.1−4 The complex structure of asphaltene generally involves a large graphite-like plate formed by connected aromatic rings with several alkyl chains attached to the plate.3 The major elements constituting asphaltene are therefore carbon and hydrogen. Heteroatoms, such as nitrogen, sulfur, and oxygen, also appear in the aromatic ring or the alkyl chains.4,5 Because of its strong polarity, when contacting the rock surface, asphaltene would very likely alter the surface wettability to more oil-wetting. Wettability alteration of the reservoir surface has great influence on the flow of oil and formation in the reservoir and the ultimate oil recovery. Many researchers believed that the adsorption and precipitation of asphaltene on the reservoir rock is the major reason for wettability alteration and formation damage.6−11 Many factors can affect the adsorption and precipitation of asphaltene on the rock surface.12 According to Buckley et al., during the wettability alteration process of the rock surface, there are four kinds of interactions among the system of oil, brine, and reservoir rock, which are polar interactions when there is no water film between the oil and rock surface, surface precipitation of asphaltenes, acid/base interactions, and ion binding interactions.13 There is always brine in the reservoir whether because of the natural aquifer around the reservoir or during the water flooding to increase oil recovery. The brine salinity and pH are very important factors in determining wettability.14 As a result, extensive research was conducted on the system of brine, crude oil, and rock, and it was proven that the brine composition of injected water can alter the wettability of the oil reservoir during water flooding to improve oil recovery.15−17 Therefore, injection of modified water with a © 2013 American Chemical Society

suitable salinity and brine composition, including low-salinity water flooding and seawater flooding, has been applied as an enhanced oil recovery (EOR) method, which aimed at wettability alteration by proper design of the brine composition.18,19 Wettability has been studied on mineral surfaces, such as quartz, mica, calcite, or kaolinite.20−23 The advantage of using single mineral flat surfaces is to study the factors that can impact the wettability in a more controllable way without the additional complexity caused by pore geometry of porous media.14 Quartz is typically used as the model substrate of sandstone. However, it can be observed that even a smooth quartz plate has a relatively rough surface because of the existence of sharp-edged particles that can be observed under scanning electron microscopy (SEM).24 The roughness of the quartz surface may influence the measurement of the contact angle, which means that it needs some more time to deal with wettability hysteresis. In the present paper, we used quartz plates as the model substrate of sandstone, and the plates were aged with different methods to gain an oil-wet state. Then, we used a pendent drop method to measure the contact angle of the crude oil on the quartz surface in water and used infrared (IR) spectroscopy to analyze the surface chemistry changes before and after asphaltene treatment. Several metal cations, including Ca2+, Mg2+, and Na+, were taken into consideration during the aging process. The aim of this paper is to figure out asphaltene adsorption and precipitation rules on quartz plates and clarify the mechanism of asphaltene adsorption in the presence of Received: September 24, 2013 Revised: November 20, 2013 Published: November 22, 2013 7354

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MgCl2 solutions of various concentrations were used as the brines. Zetasizer Nano-ZS (Malvern Instruments) was used to measure the electrophoretic mobility in brine of 0.1 wt % suspensions of ground quartz powder, and then the mobility was converted to ζ potential using the Smoluchowski equation. Quartz suspensions were left for 24 h to reach equilibrium. Each measurement consisted of 25 runs, and each sample was run 3 times to eliminate errors. The natural pH of brines of quartz is in the small range of 5.9−7.15, which is close to the pH value of brines without particles; thus, there is no need to adjust the pH before the measurement.

metal cations, which would shed light on the EOR process involving wettability alteration.

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EXPERIMENTAL SECTION

Materical Used. Quartz plates used in the present study have the dimensions of 25 × 25 × 2 mm. The asphaltene was obtained using the n-heptene precipitation method from Lungu Oilfield in China. The average molecular weight of the asphaltene is 5600, and weight percentages of H, C, N, S, and O are 7.46, 82.54, 1.39, 4.85, and 3.76%, respectively. Toluene, quartz powder, NaCl, MgCl2·6H2O, and CaCl2·2H2O are all analytical-grade. Deionized water was used for preparation of salt solutions. Crude oil used for contact angle measurement was obtained from Shengtuo Oilfield, China. The density of the oil was 0.8617 g/cm3. The viscosity of the oil at room temperature was 61.5 mPa s. The acid number of the oil was 1.423 mg of KOH/g. Wettability Test. The wettability of the quartz plate was evaluated by contact angle. The schematic diagram of the apparatus for measuring the contact angle is shown in Figure 1. The slides were first

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RESULTS AND DISCUSSION Factors Influencing the Contact Angle. The quartz slides were aged in different concentrations of asphaltene in toluene for a variation of time, and then contact angles were measured on these aged substrates. Figure 2 shows the results

Figure 1. Schematic diagram of the apparatus for measuring the contact angle. Figure 2. Effects of the aging time on contact angles of different aging solutions.

soaked in chromic acid for 24 h and then washed by deionized water. As during the adsorption and precipitation of crude oil onto the rock surface, there is usually brine present in the oil reservoir, and to simulate the process, the quartz plates were treated in three different ways: dried, soaked in deionized water for 24 h, and soaked in salt solutions for 24 h. The last two ways were mentioned as wet aging. The wet slides taken out from water or salt solutions were then put into centrifugation at 3000 rpm for 10 min to remove the excess water and then immediately transferred into toluene solution of asphaltene to avoid drying out. The dry plates were also put into toluene solution for aging for different times. Then, the slides were dried in a desiccator, and the contact angle measurements were performed at room temperature using a pendant drop method on the treated substrate with a contact angle goniometer. The measuring cell, which was also made of quartz, was filled with pure water, and the substrate was mounted on two stubs. After the system was submerged, an oil droplet of ∼1 μL was injected out upward onto the substrate from a stainlesssteel hooked syringe needle. Photos were taken by a digital camera. The contact angle between the oil−water interface and rock surface was then measured from the photo of the droplet. Five parallel oil droplets were placed on the substrate to obtain the average contact angle. Because of the roughness of the quartz surface, the oil droplets were left for 40 min to reach equilibrium before the final photo of drop shape was taken. IR Spectra. Before IR spectroscopy scanning, the quartz powder went through the same aging process as the plates. After aging in asphaltene−toluene solution, the powder was vacuum-filtered and dried in a 75 °C oven for 12 h. Then, IR spectra of quartz and asphaltene samples were recorded between 650 and 4000 cm−1 on an IR spectrophotometer, Nicolet 6700. ζ Potential. ζ potential, which exists at the shear plane of the particles, has a major effect on surface properties.25 NaCl, CaCl2, and

of the measurement. The contact angle increases sharply initially for all concentrations of asphaltene, but for toluene, the effect is not as dramatic. For the slides aged in toluene, the contact angles are below 50°, indicating a water-wet state. While for all of the asphaltene−toluene solutions, the contact angles are in the range of 75−105°, and a higher asphaltene concentration produces a higher contact angle, although there is no big difference when the asphaltene concentration is higher than 0.005%. Also, with the increase of the asphaltene concentration, less time is needed for the contact angle to reach its maximum value. The equilibrium process took up to 3 days, indicating the long and complex interactions between asphaltene molecules and the quartz surface. Researchers have found that adsorption isotherms for asphaltene dispersed in toluene showed multilayer or stepwise adsorption, which has been related to the aggregates formed in toluene solution.26 Even in the much diluted toluene solutions, asphaltene molecules may still form aggregates or colloids. The possible explanation of asphaltene adsorption is sketched in Figure 3. For a low asphaltene concentration, the asphaltene molecules have low coverage on the surface. The adsorbed molecules are isolated and may individually desorb and be replaced by other asphaltene molecules or solvent molecules. While for a higher asphaltene concentration, it seems that the desorption process is diminished, which is because of the reorganization of asphaltene molecules at the 7355

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after centrifugation, the substrates were soaked in 0.05% asphaltene toluene solution for 5 days. Then, substrates were dried, and contact angle measurements were performed. Figure 5 shows the contact angles of different substrates treated with

Figure 3. Adsorption of asphaltene molecules in low and high concentrations: (a) low asphaltene concentration and (b) high asphaltene concentration.

interface with lateral interactions between adsorbed asphaltene molecules.27 The better steric stabilization of asphaltene would prevent any further desorption.28 Water and Ions. The substrates that were dried and soaked in deionized water or 0.05 mol L−1 salt solutions (NaCl, CaCl2, and MgCl2) were soaked in 0.05% asphaltene toluene solutions for various times. Then, the contact angle was measured. Figure 4 shows the results. For all of the treatments, contact angles

Figure 5. Effects of brine concentrations on contact angles.

various brines. As shown in Figure 5, the contact angle increases fast with the increase of the salt concentration and becomes relatively stable when the concentration is higher than 0.01 mol L−1 . For substrates soaked in three salt solutions, the contact angles are higher than that soaked in deionized water, while the contact angle is the highest for the substrate soaked in CaCl2 and lowest for NaCl when the salt concentration is higher than 0.01 mol L−1. The higher contact angle of substrate soaked in divalent ion solutions indicates different action among the asphatene molecule, divalent ion, and quartz surface.30 The electrostatic potential of quartz−brine was measured using brine of 0.1 wt % suspension of ground quartz, and the results are shown in Figure 6. From Figure 6, it can be concluded that for all solutions, when the concentration increases, the absolute value of ζ potential decreases, indicating that negativity of the quartz surface decreases, which means that negatively charged asphaltene molecules can bind with a

Figure 4. Effects of the aging time on contact angles in the presence of different ions.

increase from ∼10° to ∼100°, and after at least 3 days, the contact angles tend to be stable. The dry substrates reach a maximum value faster than the substrates with water film before aging, and the contact angle is also 5° larger than that of the latter, indicating that the water film could hinder the adsorption of asphaltene in a limited way.29 However, for those substrates soaked in salt solutions before asphaltene aging, the contact angles are generally higher than the substrates soaked in pure water, indicating that salt solutions may enhance the adsorption of asphaltene onto the quartz surface. Quartz substrates were soaked in NaCl, CaCl2, and MgCl2 solutions of various concentrations separately for 24 h, and then

Figure 6. ζ potential of quartz powder in different salt solutions. 7356

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C−H stretching vibration of the saturate (2920 and 2850 cm−1) and the C−H symmetric deformation of the saturate (1460 and 1376 cm−1). The existence of long-chain alkyl groups, (CH2)n, with n > 4, in saturates can be identified at the 758 cm−1 peak. The presence of CC stretching vibration causes the weak bands at 1590−1640 cm−1. The quartz powder was aged with 0.05% asphaltene toluene solution using dry and wet aging. Figure 8 shows the IR spectra

negative quartz surface easier because of the diminished electrostatic repulsion. In other words, there would be more asphaltene molecules adsorbed on the quartz surface, and with regard to Figure 5, the contact angles increase with the increase of the brine concentration. For MgCl2 and CaCl2 solutions, the absolute value of ζ potential is generally smaller than that of NaCl solution at the same concentration, As a result, the contact angles of quartz plates soaked in MgCl2 and CaCl2 solutions are higher than that in NaCl solution. More experiments concerning metal cations were performed using an IR spectrophotometer. The interactions between the quartz surface and asphaltene toluene solution under different conditions was studied. IR spectroscopy can detect the changes of functional groups during the adsorption of asphaltene, and thus, the adsorption mechanism can be inferred. The major functional groups in asphaltene and quartz are presented in Tables 1 and 2, respectively. Table 1. Band Assignments of Asphaltene adsorption band (cm−1) 3700−3100 2850 and 2920 1590−1640 1460−1376 758

functional group stretching vibration of the phenol O−H bonds stretching vibration of the alkyl C−H bonds (−CH3 and −CH2) stretching vibration of the CC bonds of conjugated and aromatic compounds bending vibration of the alkyl C−H bonds bending vibration of C−H of substituted benzene

Figure 8. IR spectra of quartz with wet and dry aging.

of dry- and wet-aged quartz. In comparison to the IR spectra of asphaltene in Figure 7, the Si−O stretching vibration (801 cm−1) and Si−O bending vibration (695 cm−1) of dry-aged quartz moved to the direction of lower wavenumber, 735 and 660 cm−1, respectively, indicating a polar interaction between asphaltene and hydroxyl on the silicone surface. However, there is no apparent difference between the IR spectra of asphaltene and that of wet-aged quartz, which means that the asphaltene molecules have little reaction with the surface of quartz when there is water film present. Also, in Figure 8, there are stretching vibrations of the alkyl C−H bonds on both spectra, while there is no such feature on clean quartz, indicating that there is asphaltene adsorption on the quartz surface during both dry and wet aging. Before the wet aging, quartz powder was soaked in 0.05% NaCl, CaCl2, and MgCl2 solutions for 24 h, and then after the aging and drying, the IR scan was performed. The results were shown in Figure 9. We can clearly see that there is a distinct stretching vibration of the phenol O−H bonds (3358 and 3411 cm−1) for CaCl2 and MgCl2 solutions, while there is little feature here for NaCl solution. As the quartz powder was dried thoroughly before the IR analysis, we can infer that the asphaltene molecules bond with hydrated divalent ions (Ca2+ and Mg2+) and then adsorb onto the quartz surface, as illustrated in Figure 10; however, it is hard for hydrated Na+ to form this ion binding. For MgCl2 solution, the Si−O stretching vibration (801 cm−1) and the Si−O bending vibration (695 cm−1) are almost identical with the untreated clean quartz, which means that, in the presence of Mg2+, asphaltene can hardly react with hydroxyl of the quartz surface. For CaCl2 solution, the magnitude of these two vibrations declined significantly, while for NaCl, the vibration at 801 cm−1 remains the same yet the vibration at 695 cm−1 has reduced intensity. The changes of position and intensity of the two peaks indicate that polar interaction barely exists when the water film contains Mg2+, while for Ca2+and Na+, it seems that the interaction

Table 2. Band Assignments of Quartz adsorption band (cm−1)

functional group

3700−3100 1103 801 695

stretching vibration of the phenol O−H bonds Si−O asymmetrical stretching vibration Si−O symmetrical stretching vibration Si−O symmetrical bending vibrations

Figure 7 shows the IR spectroscopy of asphaltene and untreated clean quartz powder. The IR spectra of pure quartz shows absorption peaks at 696 and 800 cm−1, which are the bending and stretching vibrations for Si−O bonds. The main functional groups detected on IR spectra of asphaltene are the

Figure 7. IR spectra of pure quartz and asphaltene. 7357

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bending and stretching vibrations for Si−O bonds. The main functional groups identified on IR spectra of asphaltene include C−H stretching vibration, C−H symmetric deformation, longchain alkyl groups, and CC stretching vibration. The changes of position and intensity of IR absorbance peaks indicate that asphaltene molecules can interact with hydroxyls on the quartz surface by polar interactions and asphaltene adsorption on the quartz surface can be enhanced because of the ion binding of hydrated metal cations.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-18254276316. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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Figure 9. IR spectra of quartz of wet aging with different cationic ions.

ACKNOWLEDGMENTS The authors thank the financial support from the China Postdoctoral Science Foundation(20100471576), Program for Changjiang Scholars and Innovative Research Team in University (IRT1294) and the China Scholarship Council (CSC). The College of Chemical Engineering of the China University of Petroleum is thanked for the access of the IR spectrophotometer.

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REFERENCES

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Figure 10. Schematic diagram of the ion binding effect in the presence of divalent ions.

between hydroxyl on quartz and asphaltene plays some role on asphaltene adsorption when there is water film present.

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CONCLUSION After the treatment of asphaltene in toluene solution, the wettability of the quartz surface changed from water-wet to oilwet because of the adsorption and precipitation of asphaltene onto the surface. The soaking time, concentration of asphaltene, water film on the quartz surface, and metal cations in the water film have great effect on wettability changes. The contact angle of quartz plates becomes steady after up to 3 days of aging in asphaltene/toluene solution. When the asphaltene concentration in toluene is over 0.005%, there is no big difference on the contact angle of quartz. The dry-aged quartz surface is more oil-wet than the surface soaked in pure water before aging, while the presence of metal cations in water film, especially for divalent ions, favors the adsorption of asphaltene and increases the oil-wetting property of the quartz surface. ζPotential measurements suggest that metal cations can compress the diffused double layer of quartz particles, which could decrease electrostatic repulsion and enhance asphaltene adsorption. IR spectra of pure quartz show absorption peaks of 7358

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