- Email: [email protected]
Drag reduction in reservoir rock surface: Hydrophobic modification by SiO2 nanofluids
Accepted Manuscript Title: Drag reduction in reservoir rock surface: Hydrophobic modification by SiO2 nanofluids Author: Yong-Li Yan Ming-Yue Cui Wei-Dong Jiang An-Le He Chong Liang PII: DOI: Reference:
S0169-4332(16)32667-8 http://dx.doi.org/doi:10.1016/j.apsusc.2016.11.209 APSUSC 34514
To appear in:
APSUSC
Received date: Revised date: Accepted date:
25-5-2016 9-11-2016 26-11-2016
Please cite this article as: Yong-Li Yan, Ming-Yue Cui, Wei-Dong Jiang, An-Le He, Chong Liang, Drag reduction in reservoir rock surface: Hydrophobic modification by SiO2 nanofluids, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.11.209 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Drag reduction in reservoir rock surface: hydrophobic modification by SiO2 nanofluids
Yong-Li Yana,*, Ming-Yue Cuib, Wei-Dong Jiangb, An-Le Heb, Chong Liangb
aCollege
of Chemistry & Chemical Engineering, Xi’an Shiyou University, Xi’an 710065, China
bLangfang
To
Branch of Research Institute of Petroleum Exploration & Development, Langfang 065007, China
whom correspondence should be addressed:
Yong-Li Yan College of Chemistry & Chemical Engineering, Xi’an Shiyou University, Xi’an 710065, P. R. China Telephone: +86-029-88382691; Fax: +86-029-88382693 Email address: [email protected]
1
Graphical abstract
The micro-nanoscale hierarchical structures at the sandstone core surface are constructed by adsorption of the modified silica nanoparticles, which leads to the effect of drag reduction to improve the low injection rate in ultra-low permeability reservoirs.
Research highlights
A micro-nanoscale hierarchical structure is formed at the reservoir rock surface;
An inversion has happened from hydrophilic into hydrophobic modified by nanofluids;
The effect of drag reduction to improve the low injection rate is realized;
The mechanism of drag reduction induced from the modified core surface was unclosed.
2
Abstract Based on the adsorption behavior of modified silica nanoparticles in the sandstone core surface, the hydrophobic surface was constructed, which consists of micro-nanoscale hierarchical structure. This modified core surface presents a property of drag reduction and meets the challenge of high injection pressure and low injection rate in low or ultra-low permeability reservoir. The modification effects on the surface of silica nanoparticles and reservoir cores, mainly concerning hydrophobicity and fine structure, were determined by measurements of contact angle and scanning electron microscopy. Experimental results indicate that after successful modification, the contact angle of silica nanoparticles varies from 19.5° to 141.7°, exhibiting remarkable hydrophobic properties. These modified hydrophobic silica nanoparticles display a good adsorption behavior at the core surface to form micro-nanobinary structure. As for the wettability of these modified core surfaces, a reversal has happened from hydrophilic into hydrophobic and its contact angle increases from 59.1° to 105.9°. The core displacement experiments show that the relative permeability for water has significantly increased by an average of 40.3% via core surface modification, with the effects of reducing injection pressure and improving injection performance of water flooding. Meanwhile, the mechanisms of drag reduction and improving water injection operation induced from the modified core surface were uncovered. The present study will establish a fundamental understanding on the drag reduction at the core surface modified by nanofluids and its applications in more industries.
Keywords silica nanoparticle; hydrophobic modification; micro-nanobinary structure; drag reduction; reservoir rock surface 3
Introduction Water flooding is one of the key technologies for improving oil recovery in oilfields. For a long time,
major problems faced by oil production operation in the low and ultra-low
permeability oilfields are the high water injection pressure and the insufficient injection rate, which seriously restrained the developments and production operations in oilfields[1-2]. There exist two approaches to reducing the high pressure in the microchannel of the reservoir. One of them is reservoir reconstruction that enlarges the pore size and the other is the modification for the microchannel surface and thus resulting in the wettability alteration of the wall. Because the microchannel of the reservoir has a complicated structure of being tortuous and cannot be unfolded. A failure of reconstruction will bring immediate severe damage for the reservoir. On the other hand, by means of the “self-cleaning” property of the super-hydrophobic interface[3-5], the construction of it on the wall of the reservoir will contribute to the distinct hydrophobicity, through which the flow resistance can be decreased and the injection rate will be increased. For this reason, surface-modification in situ has attracted much attention as a powerful technology[6,7]. The wettability of the rock surface can be altered using some chemical agents, such as alkaline, surfactants, and polymers[8-10]. For using polymers, with a molecular weight of 10 to 25 million, the critical difficulty is chemical and thermal degradation in reservoirs. Surfactant flooding processes become challenging in hard salinity and high temperatures, which raise the loss of effectiveness and possible plugging. Recently, nanoparticles are attractive agents to enhance the oil recovery at the laboratory scale[11-13]. In 2000, the flow resistance of injection water was finally reduced through the introduction of injection technology of SiO2 nanoparticles from Russia[14]. The field experiment 4
was successful, but it failed to reveal the drag reduction mechanism in detail. Since 2002, Di Qinfeng’s research group[15,16] has carried out some related studies to disclose the functional principle of hydrophobic surfaces from the perspective of mechanics. Nevertheless, a large amount of basic researches still needs to be done so as to put the drag reduction for improving water-injection technology into practice from the laboratory to the fields. Unfortunately, little research systematically has been devoted to the wettability alteration of reservoir core for water floodings. Here we report the wettability inversion of reservoir microchannel by adsorption of hydrophobic SiO2 nanoparticles so as to reduce the injection pressure and enhance the performance of water flooding. In this presentation, the wet process[17] was adopted for hydrophobic modification in SiO2 nanoparticles and the modifying effects was presented with tests on the hydrophobicity and the contact angle. To construct the micro-nano binary structure with striking hydrophobicity, the modified nanoparticles as nanofluids were adsorbed on the core surface. A series of core displacement experiments have been made to validate the existence of the slippage on the walls of microchannel deposited by nanoparticles. Finally, we identified the mechanisms of drag reduction and improving water injection operation derived from the modified core surfaces.
5
1 Experimental 1.1 Chemicals SiO2 nanoparticles (primary particle size of 30±5 nm) were from Beijing DK Nano-Technology Co. Ltd. Anhydrous ethanol (A.R.) was purchased from Xi’an Chemical Reagent Factory. Dichlorodimethylsilane (C2H6Cl2Si, A.R.) was supplied by Sunuo Chemical Reagent Factory. Other reagents used were of analytical grade. 1.2 Surface modification of nano-silica (1) Modification procedure The wet process was adopted to modify surfaces of SiO2 nanoparticles. A fixed amount of fumed silica particles with primary diameter about 30 nm and surface area 160 m2g-1 was charged into the round bottom flask, stirred and heated up to 120 °C, and then it was dried at the constant temperature for 50min. This was followed by the addition of a certain amount of anhydrous ethanol to prepare SiO2 nanoparticle suspension with a mass fraction of 4.8%. Being further stirred for 10min, 15% of dichlorodimethylsilane was added gradually. Meanwhile, a certain amount of distilled water was dropped slowly. After which the reaction flask was heated up to 130 °C with a reflux of 50min. As the reaction finished, the suspension was centrifugally washed for 3 to 4 times by anhydrous ethanol and was dried again to the constant weight. (2) Hydrophobic characterization To witness the modification effects of nanoparticles, we design a experiment, in which a few of the nano-SiO2 were charged into the colormetric cylinder filling with water and kerosene. The disperse state of these nanoparticles within the oil and water phases could be observed. These
6
observations will help us to identify the hydrophilic and hydrophobic properties of these modified nanoparticles. For contact angle measurements, the SiO2 nanoparticles were compressed as a thin film. Analysis was done using instruments manufactured by Data Physics model OCA15. The values reported here were obtained as the average of three measurements of liquid droplets deposited in various regions of the film. 1.3 Wettability inversion on reservoir core surface (1) Nanofluid preparation Colloidal suspensions were made by mixing hydrophobic SiO2 nanoparticles into filtrated diesel with concentration of 1.5 mg L-1. The dispersions were stirred at the speed of 7000r/min for at least 20 min. It seems that some of particles may flocculate in the dispersions. Finally, the SiO2 nanofluid dispersions with better stability were sealed for use. (2) Adsorption test To perform the adsorption test, the core plug was sliced into small plates by DW50NC slicing machine. The sliced plates were fully saturated with formation water using a vacuum pump for 2h,
and then suspended in the SiO2 nanofluid dispersions for 48h at the constant temperature of
60 °C. Finally, these core plates covered by SiO2 nanoparticles were
dried in the oven for 12h
and reserved. (3) Wettability determination Microstructures of the original and treated cores were observed, respectively, by a field emission scanning electron microscopy (SEM, Quanta400 FEG) so as to evaluate the performance of the SiO2 nanofluids. The second technique to confirm the wettability alteration was with 7
contact angle measurements. The surfaces of the original core, the oil-immersed core and the core adsorbed with SiO2 nanoparticles were measured by OCA15 optical contact angle tester, respectively. It's operation is same like description in section 1.2. 1.4 Water flooding experiments Core flooding experiments were carried out to verify the performance of wettability alteration on water injection, under the temperature of 60 °C and pressure of about 10 MPa. The nanofluids with concentration of 1.5g/L were flooded to cores at 5 PV (pore volume). For both the original and modified cores, the primary water flooding operations were conducted and the parameters including flow rate and pressure were recorded, respectively[13].
2 Results and Discussion 2.1 Surface modification of SiO2 nanoparticles Figure 1 The surface modification on the SiO2 nanoparticles was undertaken firstly through silanization with dichlorodimethylsilane. To demonstrate the modified effects on the SiO2 nanoparticles, we have constructed a experiment concerning the dispersion status of the nano-silica particles in the oil-water phase. As shown in Figure 1, colorimetric cylinder A presents the dispersion state of SiO2 nanoparticles before modification. We can see that SiO2 nanoparticles disperse in water with uniformity and white creamy. Meanwhile, the oil phase in the upper of the colorimetric tube is transparent without any variation, which indicates the unmodified SiO2 nanoparticles displaying absolute hydrophilicity and oleophobicity. For the modified SiO2 nanoparticle, on the contrary, the water phase is absolutely transparent as observed in colorimetric cylinder B in Figure 1. Most of the nanoparticles are suspended at the water-oil interface, among which only a few disperse in the 8
oil phase. These exhibit considerable hydrophobicity and partially oleophilicity reaching the expected results for the modification of SiO2 nanoparticles. Figure 2 Contact angle measurements were performed to evaluate the performance of
the treated
nanoparticles as presented in Figure 2, respectively. For the original nanoparticles, the water drop spreads immediately and quickly as soon as it touches the surface of SiO2 nanoparticle tablet and partially diffuses to pores of the tablet shown in Figure 2A. The contact angle is measured around 19.5°, exhibiting significant hydrophilic feature. On the contrary, the contact angle of SiO2 nanoparticle tablet after modification is around 141.7°, as presented in Figure 2B, much larger than that of the former, displaying strong hydrophobicity. Combined with the results shown in Figure 1 and 2, it is clear that the modification for SiO2 nanoparticles is successful, through which the alteration from intense hydrophilicity to distinct hydrophobicity was realized.
2.2 Inversion of wettability on reservoir core surface Figure 3 The invisible reservoir core is of three-dimensional network and its structure cannot be etched manually. The surface is mainly composed of clay minerals and quartz, presenting distinct hydrophilicity. Therefore, the key of wettability alteration and drag reduction is to construct an adsorption layer of lotus-like strong or super hydrophobic nanoparticles on the wall of reservoir channel[18-20]. A series of experiments were conducted to reveal the adsorption behavior and distribution properties of nanoparticles on the porous walls. Scanning electron microscopy was used to observe the microstructure of the samples, as shown in Figure 3, in which A (top) is the SEM image of the slice surface of the original core; B (bottom) for that of adsorbed hydrophobic 9
nanoparticles. It is apparent from Figure 3A that there are many sheets of clay mineral with clear edges and corners. However, as shown in Figure 3B, the nanoparticles adsorbed surface is covered entirely with spherical nanoparticles that are tightly connected with each other, and there are many continuous papillate dots on it. The whole surface appears nano-patterned and remains the shape of the porous core, which indicates that the entire framework of the core is not changed by nanoparticles. Through the adsorption of nanoparticles, the micro-nanoscale hierarchical structure was formed at the core wall. The globular aggregate randomly ordered on the surface of the core slice, is submicron with the size of each as hundreds of nanometers. Nevertheless, nanoparticles used in this experiment are about 30 nm. Moreover, being composed of countless nanoparticles, aggregates themselves are rough structures. The surface of this binary microstructure are similar to that of the lotus-like binary microstructure, through which, the contact area of water drop and the slice surface sharply decreases, and thus presenting extreme hydrophobicity. These observations reveal that the nanoparticles could be adsorbed tightly on the surface of the porous wall, and could be distributed continuously and densely to form the micro-nano binary structure on the surface as well. These features are beneficial not only to changing the wettability of the surface but also to preventing the clay mineral from hydrating and inflating by separating water from the core. Figure 4 To gain insight into the wettability of core surface deposited by SiO2 nanofluids, the contact angles between the water drop and the surfaces of the original core, the oil immersed core and the 10
core with adsorbed SiO2 nanoparticles were measured as shown in Figure 4, respectively. We could observe from Figure 4 that the water drop spreads quickly on its surface due to high porosity of the original core. The contact angle is determined around 59.1°, presenting significant hydrophilicity (Figure 4A). For the oil-immersed core, the water drop spreads slow on the surface and its contact angle is around 89.5°, displaying neutral wettability (Figure 4B). On the contrary, the water drop spreads extremely slow on the surface of the core with adsorbed SiO2 nanoparticles (Figure 4C). Indeed, it can keep its shape for longer time. The contact angle is around 105.9° and much larger than the other two. It is obvious that the alteration of wettability of the channel wall is realized and significant hydrophobicity is presented.
2.3 Modification effects on water flooding process Table 1 Table 2 The core displacement experiments were conducted to investigate the water flooding behavior in the porous media modified by the hydrophobic nanofluids[21,22]. Several natural core samples of sandstone were tested to measure their water-phase effective permeability. The drag reduction effects can be confirmed with a comparison of the water phase permeability between the original and modified cores . The parameters of the cores and experimental results are described in Table 1 and 2, respectively. These results suggest that, from Table 2, there is a significant improvement (with variation rate of permeability of 28.12% to 55.17%, respectively, average of 40.3%) in water phase permeability via the treatment of nanoparticle dispersion liquids, which indicates the formation of hydrophobic surface of reservoir cores after adsorption of hydrophobic SiO2 nanofluids. At the same time, the 11
wettability of core wall becomes hydrophobic or even strong hydrophobic. As the flooding water flows through core wall, it slides on the uneven surface of the micro-nano binary structure, thus, leading to the effects of drag reduction and enhancement of water injection rate duration oil recovery operations.
2.4 Mechanism
of drag reduction in reservoir channels modified by SiO2
nanofluid Figure 5 Sandstone rock, which is composed of grains with different sizes, is microchannel media deposited under the combination of consolidation and compaction throughout a long geological period, and also has large specific surface area. Since the property of minerals determines the wettability of porous walls, and the wettability of reservoir rock dominates, to a great extent, the flow and distribution of oil, water, and gas in reservoirs, the distributive characteristics, relative permeability of water and oil and flow dynamics of fluids in reservoir microchannels can be changed by modifying the wettability of porous walls[23,24]. Nanofluids were used in oilfields to enhance water injection by changing wettability of porous media[25,26], thus leading to improve relative permeability of the water-phase by adsorption of SiO2 nanoparticles on the porous surface of sandstone. With the formation of the micro-nano binary structure rooting from the adsorption of the modified nanoparticles, as shown in Figure 5, the reservoir wall exhibits remarkable hydrophobicity. The large number of unsaturated bonds on the surface of nanoparticles has excellent adsorption capacity. Entering the pore channel of the rock, the unsaturated bonds, hydrogen bonds and the molecular force interact with the hydration layer and thus adsorption takes place. 12
When the injecting water fluid undergoes laminar flow across reservoir rock surface, it is commonly assumed that a no-slip boundary condition applies, requiring the velocity of the liquid to match that of the rock surface[27-29]. The resulting velocity profile in a cross-section of a circular microchannel enclosed by a solid wall has a parabolic profile with a maximum flow rate at the mid-point between the walls as shown schematically in Figure 5a. On the surface of the modified cores, hydrophobicity of nanoparticles and the micro-nano binary structure produce a significant sliding effect[30-32]. The adsorption layer of hydrophobic nanoparticles makes the rock surface displaying perfect hydrophobic characteristics. When the water flows through the surface of the hierarchical structure, the flow will not spread as it does on the surface of the hydrophilic cores. Instead, it shrinks to the middle of the pore channel and forms an outward spherical surface. In this way, the flow can be kept away from hydrophilic rocks. Therefore, the contact area of the flow and the hydrophilic pore wall reduces and the hydraulic resistance decreases. Finally, the flow slides more quickly with the push of the external force. From the velocity profile, it can be seen that the flow rate on the surface of the nanoparticle layer is not zero. It increases evenly at each point and on the whole significantly. The flow pattern changes from a parabolic velocity profile (Figure 5a) to a plug (Figure 5b) with the water flow increase. Consequently, a significant effect of drag reduction duration water injection is witnessed.
3 Conclusions On the basis of the above experiments, we have demonstrated that with wettability transformation from extreme hydrophilicity to excellent hydrophobicity, the modified SiO2 nanoparticles can be adsorbed on the core surface successfully. They form a dense hydrophobic interface with a micro-nano binary structure. The dramatic hydrophobicity of nanoparticles and 13
the micro-nano binary structure results in a significant sliding effect, and therefore it has a drag reduction effect and an excellent improving water injection mechanism. We believed that such findings are potentially important for a better understanding on the mechanisms of drag reduction at the core surface modified with nanofluids. Looking out to the future, our report attempts to provide a new efficient technology for the improvement of displacement efficiency and the water injection operation in the low and ultra-low permeable oilfields.
Ackowledgments This project has been financially supported by the Natural Science Foundation of China (NO. 21073140) and Science & Technology Research Program of Shaanxi Province (NO. 2014JM2048).
14
References [1] A. Muggeridge, A. Cockin, K. Webb, H. Frampton, I. Collins, T. Moulds, P. Salino, Recovery rates, enhanced oil recovery and technological limits, Phil Trans R Soc A, 2014, 372, 20120320. [2] B. Wei, Q. Li, F. Jin, H. Li, C. Wang, The potential of a novel nanofluid in enhancing oil recovery, Energy Fuels, 2016, 30, 2882-2891. [3] X.L. Wang, Q.F. Di, R.L. Zhang, C.Y. Gu, Progress in theories of super-hydrophobic surface slip effect and its application to drag reduction technology, Adv Mech, 2010, 40, 241-248. [4] F. Sofos, T. E. Karakasidis, A. Liakopoulos, Surface wettability effects on flow in rough wall nanochannels, Microfluid Nanofluid, 2012, 12: 25-31. [5] G. A. Pilkington, W. H. Briscoe, Nanofluids mediating surface forces, Adv Colloid Interface Sci, 2012, 179-182: 68-84. [6] G. R. J. Artus , S. Jung, J. Zimmermann, H. P. Gautschi , K. Marquardt , S. Seeger , Silicone nanofilaments and their application as superhydrophobic coatings, Adv Mater, 2006, 18, 2758-2762. [7] R. Crick Colin, Ivan P. Parkin, Preparation and characterisation of super-hydrophobic surfaces, Chem Eur J, 2010, 16, 3568-3588. [8] W. W. Anderson, Wettability literature survey: Part 5-The effects of wettability on relative permeability. J Petrol Technol, 1987, 39, 1453-1468. [9] W. W. Anderson, Wettability literature survey: Part 6-The effects of wettability on water flooding. J Petrol Technol, 1987, 39, 1605-1622. [10] S. Al-Anssari, A. Barifcani, S. Wang, L. Maxim, S. Iglauer, Wettability alteration of oil-wet carbonate by silica nanofluid, J Colloid Interface Sci, 2016, 461, 435-442. 15
[11] B. A. Suleimanov, F. S. Ismailov, E. F. Veliyev, Nanofluid for enhanced oil recovery, J Petrol Sci Eng, 2011, 78, 431-437. [12] B. Ju, T. Fan, Z. Li, Improving water injectivity and enhancing oil recovery by wettability control using nanopowders, J Petrol Sci Eng, 2012, 86-87, 206-216. [13] H. Ehtesabi, M. M. Ahadian, V. Taghikhani, M. H. Ghazanfari, Enhanced heavy oil recovery in sandstone cores using TiO2 nanofluids, Energy Fuels, 2014, 28, 423-430. [14] M. Aminnaji, H. Fazeli, A. Bahramian, S. Gerami, H. Ghojavand, Wettability alteration of reservoir rocks from liquid wetting to gas wetting using nanofluid, Transp Porous Med, 2015, 109, 201-216. [15] C.Y. Gu, Q.F. Di, L.Y. Shi, F. Wu, W.C. Wang, Z.B. Yu, Experimental investigation of superhydrophobic properties of the surface constructed by nanoparticles, Acta Phys Sinica-Ch Ed, 2008, 57, 3071-3076. [16] X.L. Wang, Q.F. Di, R.L. Zhang, W.P. Ding, W. Gong, Y.C. Cheng, The strong hydrophobic properties on nanoparticles adsorbed core surfaces, Acta Phys Sinica-Ch Ed, 2012, 61, 216801. [17] R. G. Chaudhuri, S. Paria, Core/shell nanoparticles: classes, properties, synthesis mechanisms, characterization, and applications, Chem Rev, 2012, 112, 2373-2433. [18] A. Karimi, Z. Fakhroueian, A. Bahramian, N. P. Khiabani, J. B. Darabad, R. Azin, S. Arya, Wettability alteration in carbonates using zirconium oxide nanofluids: EOR implications, Energy Fuels, 2012, 26, 1028-1036. [19] F.C. Wang, H.A. Wu, Enhanced oil droplet detachment from solid surfaces in charged nanoparticle suspensions, Soft Matter, 2013, 9, 7974-7980. [20] J. Giraldo, P. Benjumea, S. Lopera, F. B. Cortés, M. A. Ruiz, Wettability alteration of 16
sandstone cores by alumina-based nanofluids, Energy Fuels, 2013, 27, 3659-3665. [21] B. Ju, T. Fan, M. Ma, Enhanced oil recovery by flooding with hydrophilic nanoparticles, China Particuology, 2006, 4, 41-46. [22] L. Hendraningrat, S. Li, O. Torsæter, A coreflood investigation of nanofluid enhanced oil recovery, J Petrol Sci Eng, 2013, 111, 128-138. [23] O. Jia, P. Blair, P. R. Jonathan, Laminar drag reduction in microchannels using ultrahydrophobic surfaces, Phys Fluids, 2004, 16, 4635-4644. [24] J. D. Wang , Y. Yu , D. R. Chen , Progress in effects of superhydrophobic surface morphology, Chin Sci Bull, 2006, 51, 2097-2099. [25] S. Lu , Z. H. Yao , P. F. Hao , C. S. Fu , Drag reduction in ultrahydrophobic channels with micro-nano structured surfaces, Sci China Phys Mech Astron, 2010, 53, 1298-1305. [26] H. Zhang, A. Nikolov, D. Wasan, Enhanced oil recovery (EOR) using nanoparticle dispersions: underlying mechanism and imbibition experiments, Energy Fuels, 2014, 28, 3002-3009. [27] C. Choi, K. Westin, K. Breuer, Apparent slip flows in hydrophilic and hydrophobic microchannels, Phys Fluids, 2003, 15, 2897-2902. [28] M. Daas, C. Kang, W. P. Jepson, Quantitative analysis of drag reduction in horizontal slug flow, SPE J, 2004, 7, 337-343. [29] G. McHale, M. I. Newton, N. J. Shirtcliffe, Immersed superhydrophobic surfaces: Gas exchange, slip and drag reduction properties, Soft Matter, 2010, 6, 714-719. [30] D. T. Wasan, A. D. Nikolov, Spreading of nanofluids on solids, Nature, 2003, 423, 156-159. [31] D. C. Tretheway, C. D. Meinhart, A generating mechanism for apparent fluid slip in 17
hydrophobic microchannels, Phys Fluids, 2004, 16, 1509-1515. [32] D. Song, R. J. Daniello, J. P. Rothstein, Drag reduction using superhydrophobic sanded Teflon surfaces, Exp Fluids, 2014, 55, 1783.
18
Figure 1. Comparison of the dispersion status of the nano-silica particles in the oil-water phases between without (A) and with (B) surface modification
19
Figure 2. Photographs showing the contact angles at the nano-slica surfaces, A-without and B-with surface modification
20
Figure 3. SEM images of core surfaces, A-naked and B-covered by hydrophobic nano-silica particles
21
Figure 4. Photographs displaying the contact angles at the core surfaces, A-original; B-oil immersed; C-adsorbed with modified nanoparticles
22
Figure 5. Schematic diagram of drag reduction mechanisms in reservoir microchannel by hydrophobic surface modification. (a) a original reservoir with no-slip and high frictional drag at the walls, and (b) a modified reservoir with a distinct hydrophobic interface providing a slip and low frictional drag effects
23
Tables: Table 1 Physical parameters of reservoir cores Rock number
Length /mm
Diameter /mm
Porosity /%
1 2 3
50.4 50.8 55.5
24.9 24.4 24.8
11.33 15.49 17.49
24
Table 2 Variation of permeability of modified reservoir cores Rock number 1 2 3
Permeability /10-3μm2 Before modification
After modification
Variation rate permeability /%
0.32 10.87 10.73
0.41 14.96 16.65
28.12 37.63 55.17
25
of
S0169-4332(16)32667-8 http://dx.doi.org/doi:10.1016/j.apsusc.2016.11.209 APSUSC 34514
To appear in:
APSUSC
Received date: Revised date: Accepted date:
25-5-2016 9-11-2016 26-11-2016
Please cite this article as: Yong-Li Yan, Ming-Yue Cui, Wei-Dong Jiang, An-Le He, Chong Liang, Drag reduction in reservoir rock surface: Hydrophobic modification by SiO2 nanofluids, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.11.209 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Drag reduction in reservoir rock surface: hydrophobic modification by SiO2 nanofluids
Yong-Li Yana,*, Ming-Yue Cuib, Wei-Dong Jiangb, An-Le Heb, Chong Liangb
aCollege
of Chemistry & Chemical Engineering, Xi’an Shiyou University, Xi’an 710065, China
bLangfang
To
Branch of Research Institute of Petroleum Exploration & Development, Langfang 065007, China
whom correspondence should be addressed:
Yong-Li Yan College of Chemistry & Chemical Engineering, Xi’an Shiyou University, Xi’an 710065, P. R. China Telephone: +86-029-88382691; Fax: +86-029-88382693 Email address: [email protected]
1
Graphical abstract
The micro-nanoscale hierarchical structures at the sandstone core surface are constructed by adsorption of the modified silica nanoparticles, which leads to the effect of drag reduction to improve the low injection rate in ultra-low permeability reservoirs.
Research highlights
A micro-nanoscale hierarchical structure is formed at the reservoir rock surface;
An inversion has happened from hydrophilic into hydrophobic modified by nanofluids;
The effect of drag reduction to improve the low injection rate is realized;
The mechanism of drag reduction induced from the modified core surface was unclosed.
2
Abstract Based on the adsorption behavior of modified silica nanoparticles in the sandstone core surface, the hydrophobic surface was constructed, which consists of micro-nanoscale hierarchical structure. This modified core surface presents a property of drag reduction and meets the challenge of high injection pressure and low injection rate in low or ultra-low permeability reservoir. The modification effects on the surface of silica nanoparticles and reservoir cores, mainly concerning hydrophobicity and fine structure, were determined by measurements of contact angle and scanning electron microscopy. Experimental results indicate that after successful modification, the contact angle of silica nanoparticles varies from 19.5° to 141.7°, exhibiting remarkable hydrophobic properties. These modified hydrophobic silica nanoparticles display a good adsorption behavior at the core surface to form micro-nanobinary structure. As for the wettability of these modified core surfaces, a reversal has happened from hydrophilic into hydrophobic and its contact angle increases from 59.1° to 105.9°. The core displacement experiments show that the relative permeability for water has significantly increased by an average of 40.3% via core surface modification, with the effects of reducing injection pressure and improving injection performance of water flooding. Meanwhile, the mechanisms of drag reduction and improving water injection operation induced from the modified core surface were uncovered. The present study will establish a fundamental understanding on the drag reduction at the core surface modified by nanofluids and its applications in more industries.
Keywords silica nanoparticle; hydrophobic modification; micro-nanobinary structure; drag reduction; reservoir rock surface 3
Introduction Water flooding is one of the key technologies for improving oil recovery in oilfields. For a long time,
major problems faced by oil production operation in the low and ultra-low
permeability oilfields are the high water injection pressure and the insufficient injection rate, which seriously restrained the developments and production operations in oilfields[1-2]. There exist two approaches to reducing the high pressure in the microchannel of the reservoir. One of them is reservoir reconstruction that enlarges the pore size and the other is the modification for the microchannel surface and thus resulting in the wettability alteration of the wall. Because the microchannel of the reservoir has a complicated structure of being tortuous and cannot be unfolded. A failure of reconstruction will bring immediate severe damage for the reservoir. On the other hand, by means of the “self-cleaning” property of the super-hydrophobic interface[3-5], the construction of it on the wall of the reservoir will contribute to the distinct hydrophobicity, through which the flow resistance can be decreased and the injection rate will be increased. For this reason, surface-modification in situ has attracted much attention as a powerful technology[6,7]. The wettability of the rock surface can be altered using some chemical agents, such as alkaline, surfactants, and polymers[8-10]. For using polymers, with a molecular weight of 10 to 25 million, the critical difficulty is chemical and thermal degradation in reservoirs. Surfactant flooding processes become challenging in hard salinity and high temperatures, which raise the loss of effectiveness and possible plugging. Recently, nanoparticles are attractive agents to enhance the oil recovery at the laboratory scale[11-13]. In 2000, the flow resistance of injection water was finally reduced through the introduction of injection technology of SiO2 nanoparticles from Russia[14]. The field experiment 4
was successful, but it failed to reveal the drag reduction mechanism in detail. Since 2002, Di Qinfeng’s research group[15,16] has carried out some related studies to disclose the functional principle of hydrophobic surfaces from the perspective of mechanics. Nevertheless, a large amount of basic researches still needs to be done so as to put the drag reduction for improving water-injection technology into practice from the laboratory to the fields. Unfortunately, little research systematically has been devoted to the wettability alteration of reservoir core for water floodings. Here we report the wettability inversion of reservoir microchannel by adsorption of hydrophobic SiO2 nanoparticles so as to reduce the injection pressure and enhance the performance of water flooding. In this presentation, the wet process[17] was adopted for hydrophobic modification in SiO2 nanoparticles and the modifying effects was presented with tests on the hydrophobicity and the contact angle. To construct the micro-nano binary structure with striking hydrophobicity, the modified nanoparticles as nanofluids were adsorbed on the core surface. A series of core displacement experiments have been made to validate the existence of the slippage on the walls of microchannel deposited by nanoparticles. Finally, we identified the mechanisms of drag reduction and improving water injection operation derived from the modified core surfaces.
5
1 Experimental 1.1 Chemicals SiO2 nanoparticles (primary particle size of 30±5 nm) were from Beijing DK Nano-Technology Co. Ltd. Anhydrous ethanol (A.R.) was purchased from Xi’an Chemical Reagent Factory. Dichlorodimethylsilane (C2H6Cl2Si, A.R.) was supplied by Sunuo Chemical Reagent Factory. Other reagents used were of analytical grade. 1.2 Surface modification of nano-silica (1) Modification procedure The wet process was adopted to modify surfaces of SiO2 nanoparticles. A fixed amount of fumed silica particles with primary diameter about 30 nm and surface area 160 m2g-1 was charged into the round bottom flask, stirred and heated up to 120 °C, and then it was dried at the constant temperature for 50min. This was followed by the addition of a certain amount of anhydrous ethanol to prepare SiO2 nanoparticle suspension with a mass fraction of 4.8%. Being further stirred for 10min, 15% of dichlorodimethylsilane was added gradually. Meanwhile, a certain amount of distilled water was dropped slowly. After which the reaction flask was heated up to 130 °C with a reflux of 50min. As the reaction finished, the suspension was centrifugally washed for 3 to 4 times by anhydrous ethanol and was dried again to the constant weight. (2) Hydrophobic characterization To witness the modification effects of nanoparticles, we design a experiment, in which a few of the nano-SiO2 were charged into the colormetric cylinder filling with water and kerosene. The disperse state of these nanoparticles within the oil and water phases could be observed. These
6
observations will help us to identify the hydrophilic and hydrophobic properties of these modified nanoparticles. For contact angle measurements, the SiO2 nanoparticles were compressed as a thin film. Analysis was done using instruments manufactured by Data Physics model OCA15. The values reported here were obtained as the average of three measurements of liquid droplets deposited in various regions of the film. 1.3 Wettability inversion on reservoir core surface (1) Nanofluid preparation Colloidal suspensions were made by mixing hydrophobic SiO2 nanoparticles into filtrated diesel with concentration of 1.5 mg L-1. The dispersions were stirred at the speed of 7000r/min for at least 20 min. It seems that some of particles may flocculate in the dispersions. Finally, the SiO2 nanofluid dispersions with better stability were sealed for use. (2) Adsorption test To perform the adsorption test, the core plug was sliced into small plates by DW50NC slicing machine. The sliced plates were fully saturated with formation water using a vacuum pump for 2h,
and then suspended in the SiO2 nanofluid dispersions for 48h at the constant temperature of
60 °C. Finally, these core plates covered by SiO2 nanoparticles were
dried in the oven for 12h
and reserved. (3) Wettability determination Microstructures of the original and treated cores were observed, respectively, by a field emission scanning electron microscopy (SEM, Quanta400 FEG) so as to evaluate the performance of the SiO2 nanofluids. The second technique to confirm the wettability alteration was with 7
contact angle measurements. The surfaces of the original core, the oil-immersed core and the core adsorbed with SiO2 nanoparticles were measured by OCA15 optical contact angle tester, respectively. It's operation is same like description in section 1.2. 1.4 Water flooding experiments Core flooding experiments were carried out to verify the performance of wettability alteration on water injection, under the temperature of 60 °C and pressure of about 10 MPa. The nanofluids with concentration of 1.5g/L were flooded to cores at 5 PV (pore volume). For both the original and modified cores, the primary water flooding operations were conducted and the parameters including flow rate and pressure were recorded, respectively[13].
2 Results and Discussion 2.1 Surface modification of SiO2 nanoparticles Figure 1 The surface modification on the SiO2 nanoparticles was undertaken firstly through silanization with dichlorodimethylsilane. To demonstrate the modified effects on the SiO2 nanoparticles, we have constructed a experiment concerning the dispersion status of the nano-silica particles in the oil-water phase. As shown in Figure 1, colorimetric cylinder A presents the dispersion state of SiO2 nanoparticles before modification. We can see that SiO2 nanoparticles disperse in water with uniformity and white creamy. Meanwhile, the oil phase in the upper of the colorimetric tube is transparent without any variation, which indicates the unmodified SiO2 nanoparticles displaying absolute hydrophilicity and oleophobicity. For the modified SiO2 nanoparticle, on the contrary, the water phase is absolutely transparent as observed in colorimetric cylinder B in Figure 1. Most of the nanoparticles are suspended at the water-oil interface, among which only a few disperse in the 8
oil phase. These exhibit considerable hydrophobicity and partially oleophilicity reaching the expected results for the modification of SiO2 nanoparticles. Figure 2 Contact angle measurements were performed to evaluate the performance of
the treated
nanoparticles as presented in Figure 2, respectively. For the original nanoparticles, the water drop spreads immediately and quickly as soon as it touches the surface of SiO2 nanoparticle tablet and partially diffuses to pores of the tablet shown in Figure 2A. The contact angle is measured around 19.5°, exhibiting significant hydrophilic feature. On the contrary, the contact angle of SiO2 nanoparticle tablet after modification is around 141.7°, as presented in Figure 2B, much larger than that of the former, displaying strong hydrophobicity. Combined with the results shown in Figure 1 and 2, it is clear that the modification for SiO2 nanoparticles is successful, through which the alteration from intense hydrophilicity to distinct hydrophobicity was realized.
2.2 Inversion of wettability on reservoir core surface Figure 3 The invisible reservoir core is of three-dimensional network and its structure cannot be etched manually. The surface is mainly composed of clay minerals and quartz, presenting distinct hydrophilicity. Therefore, the key of wettability alteration and drag reduction is to construct an adsorption layer of lotus-like strong or super hydrophobic nanoparticles on the wall of reservoir channel[18-20]. A series of experiments were conducted to reveal the adsorption behavior and distribution properties of nanoparticles on the porous walls. Scanning electron microscopy was used to observe the microstructure of the samples, as shown in Figure 3, in which A (top) is the SEM image of the slice surface of the original core; B (bottom) for that of adsorbed hydrophobic 9
nanoparticles. It is apparent from Figure 3A that there are many sheets of clay mineral with clear edges and corners. However, as shown in Figure 3B, the nanoparticles adsorbed surface is covered entirely with spherical nanoparticles that are tightly connected with each other, and there are many continuous papillate dots on it. The whole surface appears nano-patterned and remains the shape of the porous core, which indicates that the entire framework of the core is not changed by nanoparticles. Through the adsorption of nanoparticles, the micro-nanoscale hierarchical structure was formed at the core wall. The globular aggregate randomly ordered on the surface of the core slice, is submicron with the size of each as hundreds of nanometers. Nevertheless, nanoparticles used in this experiment are about 30 nm. Moreover, being composed of countless nanoparticles, aggregates themselves are rough structures. The surface of this binary microstructure are similar to that of the lotus-like binary microstructure, through which, the contact area of water drop and the slice surface sharply decreases, and thus presenting extreme hydrophobicity. These observations reveal that the nanoparticles could be adsorbed tightly on the surface of the porous wall, and could be distributed continuously and densely to form the micro-nano binary structure on the surface as well. These features are beneficial not only to changing the wettability of the surface but also to preventing the clay mineral from hydrating and inflating by separating water from the core. Figure 4 To gain insight into the wettability of core surface deposited by SiO2 nanofluids, the contact angles between the water drop and the surfaces of the original core, the oil immersed core and the 10
core with adsorbed SiO2 nanoparticles were measured as shown in Figure 4, respectively. We could observe from Figure 4 that the water drop spreads quickly on its surface due to high porosity of the original core. The contact angle is determined around 59.1°, presenting significant hydrophilicity (Figure 4A). For the oil-immersed core, the water drop spreads slow on the surface and its contact angle is around 89.5°, displaying neutral wettability (Figure 4B). On the contrary, the water drop spreads extremely slow on the surface of the core with adsorbed SiO2 nanoparticles (Figure 4C). Indeed, it can keep its shape for longer time. The contact angle is around 105.9° and much larger than the other two. It is obvious that the alteration of wettability of the channel wall is realized and significant hydrophobicity is presented.
2.3 Modification effects on water flooding process Table 1 Table 2 The core displacement experiments were conducted to investigate the water flooding behavior in the porous media modified by the hydrophobic nanofluids[21,22]. Several natural core samples of sandstone were tested to measure their water-phase effective permeability. The drag reduction effects can be confirmed with a comparison of the water phase permeability between the original and modified cores . The parameters of the cores and experimental results are described in Table 1 and 2, respectively. These results suggest that, from Table 2, there is a significant improvement (with variation rate of permeability of 28.12% to 55.17%, respectively, average of 40.3%) in water phase permeability via the treatment of nanoparticle dispersion liquids, which indicates the formation of hydrophobic surface of reservoir cores after adsorption of hydrophobic SiO2 nanofluids. At the same time, the 11
wettability of core wall becomes hydrophobic or even strong hydrophobic. As the flooding water flows through core wall, it slides on the uneven surface of the micro-nano binary structure, thus, leading to the effects of drag reduction and enhancement of water injection rate duration oil recovery operations.
2.4 Mechanism
of drag reduction in reservoir channels modified by SiO2
nanofluid Figure 5 Sandstone rock, which is composed of grains with different sizes, is microchannel media deposited under the combination of consolidation and compaction throughout a long geological period, and also has large specific surface area. Since the property of minerals determines the wettability of porous walls, and the wettability of reservoir rock dominates, to a great extent, the flow and distribution of oil, water, and gas in reservoirs, the distributive characteristics, relative permeability of water and oil and flow dynamics of fluids in reservoir microchannels can be changed by modifying the wettability of porous walls[23,24]. Nanofluids were used in oilfields to enhance water injection by changing wettability of porous media[25,26], thus leading to improve relative permeability of the water-phase by adsorption of SiO2 nanoparticles on the porous surface of sandstone. With the formation of the micro-nano binary structure rooting from the adsorption of the modified nanoparticles, as shown in Figure 5, the reservoir wall exhibits remarkable hydrophobicity. The large number of unsaturated bonds on the surface of nanoparticles has excellent adsorption capacity. Entering the pore channel of the rock, the unsaturated bonds, hydrogen bonds and the molecular force interact with the hydration layer and thus adsorption takes place. 12
When the injecting water fluid undergoes laminar flow across reservoir rock surface, it is commonly assumed that a no-slip boundary condition applies, requiring the velocity of the liquid to match that of the rock surface[27-29]. The resulting velocity profile in a cross-section of a circular microchannel enclosed by a solid wall has a parabolic profile with a maximum flow rate at the mid-point between the walls as shown schematically in Figure 5a. On the surface of the modified cores, hydrophobicity of nanoparticles and the micro-nano binary structure produce a significant sliding effect[30-32]. The adsorption layer of hydrophobic nanoparticles makes the rock surface displaying perfect hydrophobic characteristics. When the water flows through the surface of the hierarchical structure, the flow will not spread as it does on the surface of the hydrophilic cores. Instead, it shrinks to the middle of the pore channel and forms an outward spherical surface. In this way, the flow can be kept away from hydrophilic rocks. Therefore, the contact area of the flow and the hydrophilic pore wall reduces and the hydraulic resistance decreases. Finally, the flow slides more quickly with the push of the external force. From the velocity profile, it can be seen that the flow rate on the surface of the nanoparticle layer is not zero. It increases evenly at each point and on the whole significantly. The flow pattern changes from a parabolic velocity profile (Figure 5a) to a plug (Figure 5b) with the water flow increase. Consequently, a significant effect of drag reduction duration water injection is witnessed.
3 Conclusions On the basis of the above experiments, we have demonstrated that with wettability transformation from extreme hydrophilicity to excellent hydrophobicity, the modified SiO2 nanoparticles can be adsorbed on the core surface successfully. They form a dense hydrophobic interface with a micro-nano binary structure. The dramatic hydrophobicity of nanoparticles and 13
the micro-nano binary structure results in a significant sliding effect, and therefore it has a drag reduction effect and an excellent improving water injection mechanism. We believed that such findings are potentially important for a better understanding on the mechanisms of drag reduction at the core surface modified with nanofluids. Looking out to the future, our report attempts to provide a new efficient technology for the improvement of displacement efficiency and the water injection operation in the low and ultra-low permeable oilfields.
Ackowledgments This project has been financially supported by the Natural Science Foundation of China (NO. 21073140) and Science & Technology Research Program of Shaanxi Province (NO. 2014JM2048).
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References [1] A. Muggeridge, A. Cockin, K. Webb, H. Frampton, I. Collins, T. Moulds, P. Salino, Recovery rates, enhanced oil recovery and technological limits, Phil Trans R Soc A, 2014, 372, 20120320. [2] B. Wei, Q. Li, F. Jin, H. Li, C. Wang, The potential of a novel nanofluid in enhancing oil recovery, Energy Fuels, 2016, 30, 2882-2891. [3] X.L. Wang, Q.F. Di, R.L. Zhang, C.Y. Gu, Progress in theories of super-hydrophobic surface slip effect and its application to drag reduction technology, Adv Mech, 2010, 40, 241-248. [4] F. Sofos, T. E. Karakasidis, A. Liakopoulos, Surface wettability effects on flow in rough wall nanochannels, Microfluid Nanofluid, 2012, 12: 25-31. [5] G. A. Pilkington, W. H. Briscoe, Nanofluids mediating surface forces, Adv Colloid Interface Sci, 2012, 179-182: 68-84. [6] G. R. J. Artus , S. Jung, J. Zimmermann, H. P. Gautschi , K. Marquardt , S. Seeger , Silicone nanofilaments and their application as superhydrophobic coatings, Adv Mater, 2006, 18, 2758-2762. [7] R. Crick Colin, Ivan P. Parkin, Preparation and characterisation of super-hydrophobic surfaces, Chem Eur J, 2010, 16, 3568-3588. [8] W. W. Anderson, Wettability literature survey: Part 5-The effects of wettability on relative permeability. J Petrol Technol, 1987, 39, 1453-1468. [9] W. W. Anderson, Wettability literature survey: Part 6-The effects of wettability on water flooding. J Petrol Technol, 1987, 39, 1605-1622. [10] S. Al-Anssari, A. Barifcani, S. Wang, L. Maxim, S. Iglauer, Wettability alteration of oil-wet carbonate by silica nanofluid, J Colloid Interface Sci, 2016, 461, 435-442. 15
[11] B. A. Suleimanov, F. S. Ismailov, E. F. Veliyev, Nanofluid for enhanced oil recovery, J Petrol Sci Eng, 2011, 78, 431-437. [12] B. Ju, T. Fan, Z. Li, Improving water injectivity and enhancing oil recovery by wettability control using nanopowders, J Petrol Sci Eng, 2012, 86-87, 206-216. [13] H. Ehtesabi, M. M. Ahadian, V. Taghikhani, M. H. Ghazanfari, Enhanced heavy oil recovery in sandstone cores using TiO2 nanofluids, Energy Fuels, 2014, 28, 423-430. [14] M. Aminnaji, H. Fazeli, A. Bahramian, S. Gerami, H. Ghojavand, Wettability alteration of reservoir rocks from liquid wetting to gas wetting using nanofluid, Transp Porous Med, 2015, 109, 201-216. [15] C.Y. Gu, Q.F. Di, L.Y. Shi, F. Wu, W.C. Wang, Z.B. Yu, Experimental investigation of superhydrophobic properties of the surface constructed by nanoparticles, Acta Phys Sinica-Ch Ed, 2008, 57, 3071-3076. [16] X.L. Wang, Q.F. Di, R.L. Zhang, W.P. Ding, W. Gong, Y.C. Cheng, The strong hydrophobic properties on nanoparticles adsorbed core surfaces, Acta Phys Sinica-Ch Ed, 2012, 61, 216801. [17] R. G. Chaudhuri, S. Paria, Core/shell nanoparticles: classes, properties, synthesis mechanisms, characterization, and applications, Chem Rev, 2012, 112, 2373-2433. [18] A. Karimi, Z. Fakhroueian, A. Bahramian, N. P. Khiabani, J. B. Darabad, R. Azin, S. Arya, Wettability alteration in carbonates using zirconium oxide nanofluids: EOR implications, Energy Fuels, 2012, 26, 1028-1036. [19] F.C. Wang, H.A. Wu, Enhanced oil droplet detachment from solid surfaces in charged nanoparticle suspensions, Soft Matter, 2013, 9, 7974-7980. [20] J. Giraldo, P. Benjumea, S. Lopera, F. B. Cortés, M. A. Ruiz, Wettability alteration of 16
sandstone cores by alumina-based nanofluids, Energy Fuels, 2013, 27, 3659-3665. [21] B. Ju, T. Fan, M. Ma, Enhanced oil recovery by flooding with hydrophilic nanoparticles, China Particuology, 2006, 4, 41-46. [22] L. Hendraningrat, S. Li, O. Torsæter, A coreflood investigation of nanofluid enhanced oil recovery, J Petrol Sci Eng, 2013, 111, 128-138. [23] O. Jia, P. Blair, P. R. Jonathan, Laminar drag reduction in microchannels using ultrahydrophobic surfaces, Phys Fluids, 2004, 16, 4635-4644. [24] J. D. Wang , Y. Yu , D. R. Chen , Progress in effects of superhydrophobic surface morphology, Chin Sci Bull, 2006, 51, 2097-2099. [25] S. Lu , Z. H. Yao , P. F. Hao , C. S. Fu , Drag reduction in ultrahydrophobic channels with micro-nano structured surfaces, Sci China Phys Mech Astron, 2010, 53, 1298-1305. [26] H. Zhang, A. Nikolov, D. Wasan, Enhanced oil recovery (EOR) using nanoparticle dispersions: underlying mechanism and imbibition experiments, Energy Fuels, 2014, 28, 3002-3009. [27] C. Choi, K. Westin, K. Breuer, Apparent slip flows in hydrophilic and hydrophobic microchannels, Phys Fluids, 2003, 15, 2897-2902. [28] M. Daas, C. Kang, W. P. Jepson, Quantitative analysis of drag reduction in horizontal slug flow, SPE J, 2004, 7, 337-343. [29] G. McHale, M. I. Newton, N. J. Shirtcliffe, Immersed superhydrophobic surfaces: Gas exchange, slip and drag reduction properties, Soft Matter, 2010, 6, 714-719. [30] D. T. Wasan, A. D. Nikolov, Spreading of nanofluids on solids, Nature, 2003, 423, 156-159. [31] D. C. Tretheway, C. D. Meinhart, A generating mechanism for apparent fluid slip in 17
hydrophobic microchannels, Phys Fluids, 2004, 16, 1509-1515. [32] D. Song, R. J. Daniello, J. P. Rothstein, Drag reduction using superhydrophobic sanded Teflon surfaces, Exp Fluids, 2014, 55, 1783.
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Figure 1. Comparison of the dispersion status of the nano-silica particles in the oil-water phases between without (A) and with (B) surface modification
19
Figure 2. Photographs showing the contact angles at the nano-slica surfaces, A-without and B-with surface modification
20
Figure 3. SEM images of core surfaces, A-naked and B-covered by hydrophobic nano-silica particles
21
Figure 4. Photographs displaying the contact angles at the core surfaces, A-original; B-oil immersed; C-adsorbed with modified nanoparticles
22
Figure 5. Schematic diagram of drag reduction mechanisms in reservoir microchannel by hydrophobic surface modification. (a) a original reservoir with no-slip and high frictional drag at the walls, and (b) a modified reservoir with a distinct hydrophobic interface providing a slip and low frictional drag effects
23
Tables: Table 1 Physical parameters of reservoir cores Rock number
Length /mm
Diameter /mm
Porosity /%
1 2 3
50.4 50.8 55.5
24.9 24.4 24.8
11.33 15.49 17.49
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Table 2 Variation of permeability of modified reservoir cores Rock number 1 2 3
Permeability /10-3μm2 Before modification
After modification
Variation rate permeability /%
0.32 10.87 10.73
0.41 14.96 16.65
28.12 37.63 55.17
25
of