Investigation of Spontaneous Imbibition by Using a Surfactant-Free

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Investigation of Spontaneous Imbibition by Using a Surfactant-Free Active Silica Water-Based Nanofluid for Enhanced Oil Recovery Yuyang Li, Caili Dai,* Hongda Zhou, Xinke Wang, Wenjiao Lv, and Mingwei Zhao* School of Petroleum Engineering, State Key Laboratory of Heavy Oil, China University of Petroleum (East China), Qingdao, Shandong 266580, China S Supporting Information *

ABSTRACT: Interests in using nanofluids for enhanced oil recovery (EOR) applications has been increasing. Herein, a novel surfactant-free water-based nanofluid for EOR was constructed using active silica nanoparticles. Active silica nanoparticles were synthesized via condensation of hexanedioic acid with the −OH group of silica. Water-based nanofluid was obtained by transforming carboxyl into carboxylate on the surface of active silica nanoparticles in water. The particle size of the active silica nanoparticles in water ranged from 10 to 20 nm. The interfacial activity of the nanoparticles was endowed through the shape change of the active silica nanoparticle surface groups to minimize their interface energy. The morphology and surface components of the active silica nanoparticles were characterized by transmission electron microscopy and Fourier transform infrared spectroscopy. The interfacial activity of the active silica nanoparticles was proved through interfacial tension and interfacial dilational modulus measurements. Active silica nanoparticles exhibited a stronger ability to reduce interfacial tension and enhance the interfacial film strength than silica nanoparticles. Contact angle measurements showed that this nanofluid exhibited excellent capabilities of oil displacement from a solid surface and wettability alteration. Spontaneous imbibition tests of ultralow permeability cores using different liquid phases (active silica nanofluid, silica nanofluid and brine) were conducted. Oil recovery using active silica nanofluid was higher than that of using silica nanofluid or brine. Active silica nanofluid at a low concentration could display an equal EOR efficiency with highly concentrated silica nanofluid. These results indicated the possible application of the proposed active silica water-based nanofluid in EOR. This preparation method could be used to prepare surfactant-free active nanofluids, and the surfactant-free active nanofluid shows great potential for EOR applications.

1. INTRODUCTION

Surfactants used as a dispersant or a surface modifier are often blended with nanoparticles for preparing nanofluids.26−28 Surfactant molecules interact with nanoparticles through electrostatic interaction. Surfactant adsorption onto nanoparticles has been proven by Zeta potential measurements, contact angle measurements, and so forth.29−31 When the nanoparticle surface is modified by the surfactant, the interfacial property of a biphase system can be changed by the surfactant-coated nanoparticles. The nanofluid property is related to the concentration ratio between the surfactant and the nanoparticle;5,32 however, electrostatic interaction between the surfactant and nanoparticle is unstable. Nanoparticles are easily flocculated as the concentration ratio between the surfactant and the nanoparticle changes or salt is presented.33 The flocculated nanoparticles easily cause a loss of permeability in low-permeability reservoirs. On the basis of our previous study, we have reported a kind of self-dispersing silica nanofluid for EOR in a low-permeability reservoir.34 The nanofluid improved the oil recovery of lowpermeability cores by the mechanisms of wettability alteration and structural disjoining pressure. However, there was little reduction in oil−water interfacial tension under the effect of the nanofluid. Low interfacial tension is conducive to EOR. Wasan et al. propounded a novel mechanism of structural

Nanofluids are novel fluids that have received increased attention in the oil and gas industry.1−3 Nanofluids are obtained by adding nanomaterials into liquids for endowing or improving their special properties.4 Nanomaterials have large surface area, high density of crystal lattice defects, and high surface energy due to their nanostructure. Nanofluids have special thermal, optical, electrical, rheological, and interfacial performance based on the nanomaterial features. The main characteristics of nanofluids rely on the amount, size, and components of nanomaterials; therefore, the properties of nanofluids often go beyond the properties of conventional fluids.5−9 Existing oil field production is declining, and new exploration is reduced; thus, significant EOR techniques and difficult-to-produce reservoirs are important research topics.10,11 Chemical flooding is used extensively as an effective technology for EOR. However, traditional chemical EOR technologies, such as surfactant flooding and polymer flooding, are challenges with expensive chemicals, high consumption, chemical loss, and possible reservoir damage.12−14 A low cost, highly efficient, and environmentally friendly EOR technology needs to be developed. On the basis of this, nanofluids are proposed for EOR because of their advantages. Underlying EOR mechanisms of nanofluid have been reported in the literature, mostly including wettability alteration, interfacial tension decrease, and structural disjoining pressure.15−25 © XXXX American Chemical Society

Received: October 16, 2017 Revised: December 18, 2017 Published: December 19, 2017 A

DOI: 10.1021/acs.energyfuels.7b03132 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 1. Schematic illustration of the active silica nanoparticle fabrication process.

disjoining pressure for EOR.19−24 According to their structural disjoining pressure mechanism, nanofluids with lower interfacial tension can easily displace oil from a solid surface. It is necessary to develop a kind of nanofluid with interfacial activity for EOR and for understanding its EOR mechanism. The aim of this work is to exploit a surfactant-free active silica water-based nanofluid for EOR. The water-based nanofluid was obtained by transforming carboxyl into carboxylate on the surface of silica nanoparticles in water. The silica nanoparticles with interfacial activity (active silica nanoparticles) were synthesized via condensation of hexanedioic acid with the −OH group of silica. The interfacial activity of nanoparticles was endowed through the shape change of the active silica nanoparticle surface groups to minimize their interface energy. Active silica nanofluid exhibited a stronger ability to reduce interfacial tension, enhance the interfacial film strength, displace oil from a solid surface, and alter wettability than silica nanofluid. Spontaneous imbibition experiments revealed that oil recovery using active silica nanofluid was higher than that by using silica nanofluid. The proposed nanofluid provides a useful tool for EOR, and this strategy shows potential for EOR applications in ultralow permeability reservoirs.

interfacial dilational modulus measurements were performed on a drop shape analyzer with an oscillating pendant drop tensiometer (Tracker, Teclis, France). The viscosity was measured by a rotary viscometer (DV3T, Brookfield, U.S.A.). The contact angle measurement was conducted using a contact angle measuring system (Tracker, Teclis, France). The pore size distribution of the core was measured by a mercury intrusion porosimeter (AutoPore IV 9500, Micromeritics, U.S.A.). 2.2. Preparation of Nanofluid Based on Active Silica Nanoparticles. Active silica nanoparticles were synthesized via condensation of hexanedioic acid with the −OH group of silica (Figure 1). Silica sol (10 mL) and hexanedioic acid (3 g) were dispersed in 500 mL of DMF. The mixture was stirred at 110 °C. After 12 h, the mixture was cooled to room temperature. The vacuumrotary evaporation procedure method was used to concentrate the mixture. The concentrated mixture was added into 100 mL water and then separated centrifugally. The obtained white solid was washed three times with water and ethanol. After purification, the obtained white solid was dried in a vacuum at 60 °C for 24 h. Finally, 4.8 g of active silica nanoparticles was obtained. Approximately 1 g of active silica nanoparticles was dispersed into 1.0 L of brine. To transform carboxyl into carboxylate on the surface of active silica nanoparticles, a 1 mol/L NaOH solution was added into the dispersion, and the pH of the dispersion was adjusted to 10. With the help of ultrasonic dispersion at 60 °C, a clear and transparent nanofluid was obtained. 2.3. Spontaneous Imbibition Tests. Sandstone cores were dried to remove bound water in pores. Then, the cores were saturated with oil according to methodologies presented in earlier reports.34 At 60 °C, cores were immersed in oil for 24 h to avoid the high temperature influence on the oil volume. Under 60 °C, cores were placed in brine and nanofluids with different concentrations. Oil was discharged from cores. Under the action of buoyancy, the discharged oil was aggregated into a calibrated glass tube at the top of imbibition cell. The precision of the calibrated glass tube was 0.005 mL. The volume of discharged oil was recorded versus time.

2. EXPERIMENTAL SECTION 2.1. Materials and Apparatus. Silica sol (10 nm, 30 wt %) was obtained from Dezhou Jinghuo Technique Glass Co., Ltd. Hexanedioic acid (AR), n-heptane (AR) and N,N-dimethylformamide (DMF, AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. Oil with a density of 0.786 g/cm3 and a dynamic viscosity of 2.02 mPa·s at 60 °C was the mixture of crude oil and kerosene with a volume ratio of 1:19. A NaCl solution (3 wt %) was used as the reservoir brine. Sandstone cores (length 25 mm and diameter 25 mm) were obtained from Haian Oil Scientific Research Apparatus Co., Ltd. The porosity and gas permeability of sandstone cores were 14% and 0.6 mD, respectively (Table 1S of the Supporting Information, SI). The pore size distribution of the core is shown in Figure S1. The viscosity of the nanofluid is shown in Figure S2. The structure and morphology of nanoparticles were observed through a transmission electron microscope (TEM, Tecnai-G20, FEI, U.S.A.). Infrared spectroscopy was obtained from a Fourier transform infrared spectrometer (FT-IR, NEXUS, Thermo Nicolet, U.S.A.). Dynamic light scattering (DLS) and Zeta potential measurements were conducted using a laser particle size analyzer (Zetasizer Nano ZSP, Malvern, England). Interfacial tension was measured by a spinning drop interfacial tensiometer (TX500C, Kono, U.S.A.). The

3. RESULTS AND DISCUSSION 3.1. Characterization of Active Silica Nanoparticles. Figure 2 shows the TEM images of silica nanoparticles and active silica nanoparticles. As shown in Figure 2, active silica nanoparticles showed spherical-like microstructure, which is similar to silica nanoparticles. The size of the active silica nanoparticles ranged from 10 to 20 nm. The result indicated that the active silica nanoparticles were well dispersed and successfully overcame the agglomeration of nanoparticles. The surface chemistry of silica nanoparticles and active silica nanoparticles was analyzed by FT-IR (Figure 3). A peak at B

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Zeta potential of active silica nanoparticles or silica nanoparticles in water was −35 mV and −38 mV, which improved the electrostatic repulsion among nanoparticles. The result was indicative of a well-dispersed water-based nanofluid. The good dispersion laid a good foundation for further EOR application. 3.3. Interfacial Activity of Nanofluid. To assess the influence of active silica nanoparticles on oil−water interfacial properties, oil−water interfacial tensions between the silica nanofluid or active silica nanofluid and oil were measured (Figure 5). The oil−water interfacial tension decreased with Figure 2. TEM images of silica nanoparticles (a) and active silica nanoparticles (b).

Figure 5. Effect of different nanofluids with different concentrations on oil−water interfacial tension at 60 °C. Error bar = RSD (n = 5). Figure 3. FT-IR of active silica nanoparticles and silica nanoparticles.

increasing nanofluid concentration, and the relation of nanofluid concentration with oil−water interfacial tension was a logarithmic function. However, active silica nanofluid owned a greater ability to reduce interfacial tension than silica nanofluid. When the oil/water interfacial tension was 26.8 mN/m at 60 °C, 0.001 wt % active silica nanofluid and 1 wt % active silica nanofluid could reduce oil−water interfacial tension to 17.5 and 7.0 mN/m, respectively. Meanwhile, 0.001 wt % silica nanofluid and 1 wt % silica nanofluid could just reduce oil−water interfacial tension to 26.5 and 20.6 mN/ m, respectively. The results showed that hexanoate groups on the surface of active silica nanoparticle could endow silica nanoparticles with the powerful ability to reduce oil−water interfacial tension. To further explore the influence of active silica nanoparticles on oil−water interfacial properties, dilational modulus of the interfacial film between the silica nanofluid or active silica nanofluid and n-heptane were determined as a function of the time and concentration of nanofluid (Figure 6). The results showed that the interfacial dilational modulus rose with time, indicating an increase in the interfacial film strength. Compared with silica nanofluid, active silica nanofluid exhibited a greater ability to enhance interfacial film strength. One possible explanation for this result was that hexanoate groups on the surface of active silica nanoparticle were conducive to the adsorption of nanoparticles at oil−water interface and nanoparticles were self-assembled to form a stable interfacial film. On the basis of the results of interfacial tension and interfacial dilational rheology, the schematic illustration of active silica nanoparticles and silica nanoparticles at oil−water interface is shown in Figure 7. The adsorption of active silica nanoparticles at the water/oil interface could obviously reduce the interfacial energy of the system for the contribution of active silica nanoparticles to reduce interfacial tension and enhance the interfacial film strength. This may be due to the

1100 cm−1 was presented due to the stretching vibration absorption of Si−O. An absorption peak at 3330 cm−1 was ascribed to the stretching vibration of −OH, and it demonstrated that hydroxyl groups were present on the surface of silica nanoparticles. Peaks at 1440 and 2935 cm−1 were ascribed to the group of −CH2 and −C−H, respectively. A peak at 1660 cm−1 was ascribed to the CO group. The results showed that hexanedioic acid was grafted on the surface of the silica nanoparticles. 3.2. Dispersion of Active Silica Nanoparticles. As shown in Figure 4, a clear and transparent water-based

Figure 4. Size distribution of active silica nanoparticles in water.

dispersion was obtained. The size distribution of the active silica nanoparticles in water was measured by DLS. The particle size of active silica nanoparticles in water ranged from 10 to 20 nm, which was similar to the particle size of silica nanoparticles in water. There was no nanoparticles aggregation in the preparation process of active silica nanoparticles. The C

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Figure 6. Interfacial dilational modulus for interfaces between silica nanofluid or active silica nanofluid and n-heptane at 60 °C. Frequency 0.1 Hz.

Figure 8. Impact of different liquid phases (1 wt % active silica nanofluid, 1 wt % silica nanofluid and brine) for the oil contact angle on the oil-wet glass substrate at 60 °C.

Figure 7. Schematic illustration of active silica nanoparticles (left) and silica (right) nanoparticles at oil−water interface.

be obviously influenced by 1 wt % active silica nanofluid. When the oil droplet captured by the oil-wet glass substrate was immersed in 1 wt % active silica nanofluid, the oil contact angle on the treated glass substrate was changed from 35° to 142°. Accordingly, the oil contact angle in 1 wt % silica nanofluid was just changed from 35° to 72° and the oil contact angle in brine was barely changed. According to the results, the proposed water-based nanofluid exhibited an excellent capability of oil displacement from a solid surface. To further research influences of active silica nanofluid concentration on oil displacement, the oil droplet captured by the oil-wet glass substrate was immersed into active silica nanofluids with different concentrations (1 wt %, 0.5 wt %, 0.1 wt %, 0.05 wt %, 0.01 wt %, 0.005 wt %, and 0.001 wt %). Figure 9 shows that the oil contact angle change was increased

shape change of the active silica nanoparticle surface groups to minimize their interface energy.35 As shown in Figure 7, hexanoate groups on the surfaces of active silica nanoparticles were stretched in the aqueous phase and curved in the oil phase. In the aqueous phase, hydrophilic carboxylate groups were exposed, and hydropholic carbon chains were hidden. In the oil phase, hydropholic carbon chains were exposed, and hydrophilic carboxylate groups were hidden. The suitable water−oil amphipathy of active silica nanoparticles enhanced the adsorption of active silica nanoparticles at the oil−water interface and active silica nanoparticles formed a stable interfacial film. Correspondingly, silica nanoparticles are arranged in the side of aqueous phase at oil−water interface owning to their single hydrophilic property. The adsorption equilibrium of silica nanoparticles at oil−water interface was easy to break. The strength of the interfacial film formed by active silica nanoparticles was stronger than that by silica nanoparticles. The excellent interfacial activity of active silica nanoparticles was beneficial for their application in EOR. 3.4. Oil Displacement from a Solid Surface. To verify oil displacement from a solid surface using different liquid phases (1 wt % active silica nanofluid, 1 wt % silica nanofluid and brine), an oil droplet was captured on an oil-wet glass substrate treated with paraffin. The oil droplet captured by the oil-wet glass substrate was immersed into different liquid phases. The oil contact angle on the treated glass substrate with time was recorded by a contact angle measurement. Figure 8 shows the impact of different liquid phase for the oil contact angle on the treated glass substrate. The results showed that the oil contact angle on the glass substrate could

Figure 9. Impact of active silica nanofluid concentration for the oil contact angle on the oil-wet glass substrate at 60 °C.

with the nanofluid concentration increase. During the initial stage, the contact angle change was obvious, and the contact angle change rate was increased with the nanofluid concentration increase. After the initial rapid change, the contact angle change tended to slow. Finally, the oil contact angle on the treated glass substrate did not change greatly. In brief, nanofluids with high concentration or low oil−water interfacial tension can easily displace oil from a solid surface. According to the results of oil displacement from the solid surface and Wasan’s theory of structural disjoining pressure, the underlying oil displacement mechanism is shown in Figure 10. Active silica nanoparticles were adsorbed at the oil−water interface. Due to electrostatic repulsion, a wedge film in the D

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Figure 10. Schematic illustration of oil displacement from a solid surface by active silica nanofluid. Figure 12. Impact of different liquid phases (1 wt % active silica nanofluid, 1 wt % silica nanofluid, and brine) for oil recovery of cores at 60 °C.

oil/nanofluid/solid three-phase contact region tended to be formed. The wedge film was continuously extended forward owing to electrostatic repulsion and Brownian motion among the nanoparticles. Finally oil was displaced from the solid surface. In the process of oil displacement, oil displacement efficiency can be improved increasing the concentration of active silica nanofluid, which can improve the adsorption of active silica nanoparticles at the water/oil interface, strength of interfacial film, and reduction of interfacial tension. 3.5. Wettability Alteration. The wettability alteration of an oil-wet glass substrate induced by different liquid phases (1 wt % active silica nanofluid, 1 wt % silica nanofluid and brine) was explored by measuring the contact angle of the oil/water/ solid. Figure 11 a shows that oil was captured on the oil-wet

Figure 13. Impact of the concentration of nanofluids for oil recovery in cores at 60 °C. Figure 11. Oil on the oil-wet glass substrates induced by brine (a), silica nanofluid (b), and active silica nanofluid (c) at 60 °C.

%, oil recovery could reach 38.8%. Oil recovery of 0.001 wt % nanofluid could be as high as 23.9%, and oil recovery was improved by 10.1% compared with that of brine (13.8%). In addition, the speed of oil recovery using a nanofluid was increased with the increasing of nanofluid concentration. Compared with brine, the speed of oil recovery was improved using a nanofluid. After spontaneous imbibition tests, 38.8%, 36.4%, 32.6%, 29.5%, 27.6%, 26.4%, and 23.9% of oil were extracted in cores using nanofluids with different concentrations (1 wt %, 0.5 wt %, 0.1 wt %, 0.05 wt %, 0.01 wt %, 0.005 wt %, and 0.001 wt %, respectively). The results showed that there were no flocculated nanoparticles in the effective concentration range of active silica nanofluid, and the proposed water-based active silica nanofluid had wide application opportunities in the EOR of ultralow permeability reservoirs. 3.7. EOR Mechanism of Nanofluid. Low interfacial tension, oil displacement efficiency, and wettability alteration are conducive to EOR. The interfacial activity, oil displacement efficiency, and oil recovery were improved with the increasing of active silica nanofluid concentration. On the basis of the results of interfacial activity, oil displacement, and wettability alteration, the underlying EOR mechanism of the nanofluid is proposed (Figure 14). We suggest that the underlying mechanism of the nanofluid for EOR consists of the following three aspects. First, the adsorption of active silica nanoparticles at the water/oil interface could obviously reduce the interfacial energy of the system for the contribution of active silica nanoparticles to reduce interfacial tension. The adhesive work

glass substrate. The wettability of the glass substrate was altered to water-wet or nearly neutral after treatment with active silica nanofluid or silica nanofluid. Active silica nanofluid exhibited a greater ability to alter wettability than silica nanofluid. We think that the paraffin absorbed onto glass substrate was fully or partly displaced by active silica nanofluid or silica nanofluid. The experimental results indicated that the proposed water-based nanofluid successfully changed the oilwet surface to a water-wet surface. A water-wet surface helps improve the capillary driving force, and the spontaneous imbibition of the core is enhanced.36−38 3.6. Spontaneous Imbibition Tests. To evaluate the potential of EOR using the proposed water-based active silica nanofluid, spontaneous imbibition tests of ultralow permeability cores using 1 wt % active silica nanofluid, 1 wt % silica nanofluid, and brine were conducted. As shown in Figure 12, the results demonstrated that the EOR became stronger from 1 wt % active silica nanofluid, 1 wt % silica nanofluid to brine. Oil recovery of cores using 1 wt % active silica nanofluid, 1 wt % silica nanofluid, and brine were 38.8%, 27.3% and 13.8%, respectively. In addition, the speed of oil recovery using 1 wt % active silica nanofluid was higher than 1 wt % silica nanofluid and brine. Figure 13 shows the influence of active silica nanofluid concentration for EOR. Oil recovery using nanofluid had an increasing tendency with the increasing of nanofluid concentration. When the nanofluid concentration was 1 wt E

DOI: 10.1021/acs.energyfuels.7b03132 Energy Fuels XXXX, XXX, XXX−XXX

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b03132. Pore size distribution of core and viscosity of nanofluid at different concentrations (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel: (86) 532-86981183. Fax: (86) 532-86981161. E-mail: [email protected] (C.D.). *Tel: (86) 532-86981183. Fax: (86) 532-86981161. E-mail: [email protected] (M.Z.).

Figure 14. Underlying EOR mechanism of the nanofluid.

ORCID

Mingwei Zhao: 0000-0002-9671-8206

between the oil and solid surface is reduced for low interfacial tension. Due to electrostatic repulsion and Brownian motion among the nanoparticles, a force pointing from the aqueous phase to the oil phase was generated. Oil can be easily displaced from the solid surface owing to the force and the reduction of adhesive work. Second, the interfacial tension and wettability alteration are interrelated.39 The adsorption of active silica nanoparticles at the water/oil interface can obviously improve the interfacial film strength and is conducive to wettability alteration. The interfacial film formed by active silica nanoparticles prevents oil from adhering to the rock surface again in the moving process of oil. Third, the sandstone surface wettability is changed from oilwet to water-wet. The influence of active silica nanofluids on the sandstone surface wettability is stronger than that on interfacial tension. On the basis of the formula of capillarity (Pc = 2σcos θ/r), the capillary force as a driving force grows stronger. Compared with the reduction of interfacial tension, wettability alteration causes the direction reverse of the capillary force which plays a more important role in spontaneous imbibition. Meanwhile, the adhesive work between oil and solid surface is reduced. The enhancement of the hydrophilicity of rock and reduction in adhesive work could promote spontaneous imbibition and oil displacement efficiency.

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Key Basic Research Program (No. 2015CB250904), the National Science Fund (U1663206, 51425406), the Chang Jiang Scholars Program (T2014152), Climb Taishan Scholar Program in Shandong Province (tspd20161004), and the Fundamental Research Funds for the Central Universities (15CX06028A).

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4. CONCLUSIONS This paper described a new surfactant-free water-based nanofluid using active silica nanoparticles as the active component for the EOR of ultralow permeability cores. Active silica nanoparticles were obtained via condensation of hexanedioic acid with the −OH group of silica. The prepared nanofluid utilized the properties of several components as follows: (1) active silica nanoparticles can be readily dispersed in water and present an excellent interfacial activity without surfactant; (2) the active silica nanofluid shows good oil displacement and wettability alteration properties; and (3) active silica nanofluid at low concentration can display an equal EOR efficiency with highly concentrated conventional silica nanofluid. Spontaneous imbibition experiments revealed that the presence of active surface groups can efficiently enhance oil recovery of ultralow permeability core. This preparation method could be widely applied to acquire surfactant-free active nanofluids, and this strategy shows great potential for EOR applications in ultralow permeability reservoirs. F

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DOI: 10.1021/acs.energyfuels.7b03132 Energy Fuels XXXX, XXX, XXX−XXX