The morphology and surface-chemistry of gas-wetting nanoparticles

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Thermodynamics, Transport, and Fluid Mechanics

The morphology and surface-chemistry of gas-wetting nanoparticles and its effect on the liquid menisci in porous media Jiafeng Jin, Yanling Wang, Tuan Anh Huu Nguyen, Baojun Bai, Wande Ding, and Mutai Bao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05525 • Publication Date (Web): 21 Mar 2019 Downloaded from http://pubs.acs.org on March 21, 2019

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Industrial & Engineering Chemistry Research

The morphology and surface-chemistry of gas-wetting nanoparticles and its effect on the liquid menisci in porous media

Jiafeng Jin1, Yanling Wang 2*, Tuan A.H. Nguyen 3, Baojun Bai 4, Wande Ding 1, Mutai Bao 1*, 1

Key Laboratory of Marine Chemistry Theory and Technology (Ocean University of China), Ministry of Education, 238 Song-Ling Road, Qingdao 266100, PR China

2Petroleum

Engineering College, China University of Petroleum (East China), Qingdao Shandong, P.R. China

3 4

School of Chemical Engineering, The University of Queensland, QLD 4072, Australia

Department of Geological Science and Engineering, Missouri University of Science and Technology, 1400 N Bishop Avenue, Rolla, Missouri, USA

*Corresponding author E-mail: [email protected] [email protected] The first three authors contributed equally to this paper.

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

Abstract The transformation of the liquid menisci at pore throats is of great importance for mitigating the liquid-blocking effect of condensate reservoirs. Here, we reported a super gas-wetting peanut-like nanoparticle which can facilitate the liquid menisci to transform from concaveshape to convex-shape by coating a super gas-wetting adsorption with high surface roughness. The morphology and surface chemistry of gas-wetting nanoparticles were investigated by SEM, AFM, and XPS analysis. The mechanism of surface modification was further explored by TEM, the adsorption layer coated on the nanoparticle surface can be recognized as monolayer absorption. Gas-wetting model is recommended as the combination of the Wenzel model and Cassie-Baxter model, which is in close agreement with the results of AFM and Contact-angle measurement. Core flooding visualization was performed to identify the effect of gas-wetting alteration on the transformation of liquid menisci in porous media. Results showed that the addition of gas-wetting nanoparticles could decrease the liquid saturations by inducing the transformation of liquid menisci in the pore throat. Additionally, a unique “Amoeba effect” and miscibility effect can synergistically improve the mobility of the oil phase, further enhance the oil recovery. Keywords: Super gas-wetting; Peanut-like nanoparticle; Surface roughness; Convex liquid menisci; Liquid saturation


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1. Introduction The accumulation of condensate easily causes an annular blocking area near the wellbore region during the isothermal depletion production, resulting in a sharp reduction in the flow efficiency, further curtailing the gas deliverability 1, 2. Gas-wetting alteration derives from the classical theory based on the wettability alteration3, 4, which has proved to be a feasible and cost-effective stimulation to alleviate the liquid blockage of the condensate reservoir. The performance of gas-wetting alteration mainly depends on the agents, condensate, the geometry of pore throat, and the liquid menisci, among which both of gas-wetting agents and the flow of liquid menisci play vital roles in mitigating the liquid-blocking effect. The extensive efforts have been devoted to the synthesis and characterization of gaswetting agents, fluorosurfactants and fluoropolymers dominate

5-7

. Fluorosurfactant typically

exhibits a more pronounced performance than fluoropolymer. However, the present fluorochemicals can only achieve intermediate gas-wetting due to the inability to fabricate multiscale adsorption layer. Surface modification has been a feasible approach to alter the surface chemistry of nanoparticles, by which multiscale adsorption layer with desired functional groups for different applications can be prepared 8. Fluorosurfactant molecules can be introduced onto nano-silica particles to prepare spherical gas-wetting nanoparticles, the contact angles of liquid droplets could sharply climb to more than 120° after gas-wetting alteration 9. Ponnapati et al.

10

synthesized soluble polymer-functionalized nano-silica

particles with the assistance of surface modification, which can effectively improve the mobilization of residual oil without plugging the core. Based on the synergy between nanoparticles and surfactants, nano-silica particles are increasingly being used as a carrier for wettability alteration and stabilizing fluid in Enhanced oil recovery (EOR) 11, 12. Shape-controlled synthesis has been a versatile approach to prepare nanoparticles with varying morphologies, the considerable endeavor has been dedicated to synthesizing inorganic materials with three-dimensional nano-structures

13

. Lee et al.

14

synthesized

peanut-like hematite particles by the gel-sol method, which is of good stability and can yield 3 to 5 times body interactions than the spherical particle. Sasaki et al. 15 analyzed the growth mechanism of the peanut-like particle by computer simulation, found that the growth of peanut-like particles is controlled by surface nucleation and the subsequent growth of crystallites on both ends of secondary particle. Pietak et al.

16

found that there was a close

correlation between the surface wettability and surface morphology. However, few research



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involved in the peanut-like nano-silica particles, especially for the surface modification of peanut-like nano-silica particle. Kovscek et al.

17

firstly conducted pore-level flooding in an axisymmetric model, a

delicate interaction between pore shape and liquid menisci can predict the wettability of porous rocks. Simons et al.

18

investigated the mechanisms of the liquid menisci

transformation and developed an approach to predict adhesion forces and liquid menisci distribution. The imbibition dynamics of liquid menisci in pore-scale model depends on the orientation, continuity, and distribution of the pore throats

19, 20

. Li and Wardlaw 21 indicated

that under quasistatic conditions, the liquid menisci can detach off the pore throat when the pore-to-throat effective diameter ratio is about 1.5. Danesh et al.

22

confirmed that liquid

saturation can be one of the key parameters to characterize the mobility of fluids in the porous media. Considering the effect of wettability on the fluid fluidity, the transformation of liquid menisci in the pore-throat would be an appropriate measure to predict the rock wettability. Nevertheless, limited research takes into account the effect the transformation of liquid menisci on the fluid fluidity. In this article, we presented a facile approach to fabricate gas-wetting peanut-like nanoparticles, contact angle measurement was used to quantitatively characterize the wettability change in rock surface after gas-wetting alteration. An improved wetting model for gas-wetting has been developed, which provides an insight into the relationship between gas-wetting and the surface morphology. The morphology and surface chemistry of gaswetting were analyzed by Scanning Electron Microscope (SEM), Atomic Force Microscopy (AFM), and X-ray photoelectron spectrometer (XPS). Furthermore, the mechanism of the liquid menisci transformation in the presence of gas-wetting nanoparticles was investigated by the visualization flooding. This research provides theoretical and practical support for enhancing gas deliverability in gas reservoirs.

2. Experimental Methodology The nonionic fluorosurfactant (FG40, CP) was purchased from Harbin Chemical Reagent Co., Ltd; tetraethoxysilane (TEOS, AR), ammonium hydroxide (NH3·H2O, CP), hexadecane (AR) and ethanol (AR), purchased from Sinopharm Chemicals (China). CH4 was supplied by Shandong Gas Chemicals (99 %, China). Rock samples, 2.5×0.5 cm, supplied by the Shengli Oilfield company (China). Quartz chamber, 5×5×5 cm, purchased from Shandong Glass company (China).


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2.1 Fabrication of gas-wetting nanoparticles Nano-silica particles with peanut-like structure (PNP) were synthesized with TEOS and NH3·H2O, which was functionally modified by FG40 to prepare gas-wetting peanut-like nano-silica particles (FG40-PNP). The procedures are as follow: 0.5 % FG40 solution was added into TEOS solution at a temperature of 40 ℃, 2 mL NH3·H2O (0.4 M) was pipetted into the mixture in 2 h, then the mixture was dispersed at a temperature of 60 ℃ for 6 h. After dried at a temperature of 80 ℃ for 4 h, the peanut-like gas-wetting nanoparticles were prepared, as can be seen from Fig. 1. The principle of synthesizing peanut-like gas-wetting nanoparticles is to control the temperature, FG40, and the molar ratio between TEOS and NH3·H2O

23

. The addition of NH3·H2O could trigger the formation of nucleuses at a lower

temperature, the neighboring nucleuses would grow together due to van der Waals force. The nucleuses cease to aggregate once the repulsive force between the nucleuses is greater than van der Waals force. Fluorosurfactant can accelerate the growth of nucleuses by linking several nucleuses together to grow into a peanut-like nanoparticle. The surface morphology and surface roughness of core surfaces before and after gaswetting alteration were characterized by the Scanning Electron Microscope (SEM, FEI, USA) and In-situ Atomic Force Microscopy (AFM, Veeco, USA); the surface morphology of nanosilica particle was analyzed by the Transmission Electron Microscope (TEM, JEM-2100, Hitachi, Japan). The surface elemental composition of core surface after treatment was measured by X-ray photoelectron spectrometer (XPS, Thermofisher Ltd., USA). The crystalline structure of core surfaces was determined by X-ray diffraction (XRD, Rigaku D Ltd., Japan).

Fig. 1. Sketch of preparation of gas-wetting peanut-like nanoparticles.

2.2 Characterization of core wettability Contact angle measurement has been widely used to evaluate the core wettability because of its accuracy and simplicity, the principle of this method is to measure the contact angle of



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fluid on a horizontal solid surface. To simulate the wetting state of liquid under the reservoir conditions, the contact angle measurements were performed in a gas-liquid-rock system, gas phase was methane 9. The procedures are as follow: (1) core samples were polished by sandpaper and then aged in a certain concentration of gas-wetting alteration solution for 24 h; (2) the treated core sample was placed in a sealed quartz chamber which was full of methane, then the JY-82 contact angle meter was used to measure the contact angles of liquid droplets, as shown in Fig. 2. Vaseline was used to keep the sealing performance of the quartz chamber. The water phase and oil phase used in this experiment are brine and hexadecane, respectively. Brine was provided by Shengli Oilfield.

Fig. 2. Sketch of contact angle measurement in CH4-liquid-rock system.

2.3 Gas-wetting model A multiscale structure model used to reflect the wetting regime of rock surface after gaswetting nanoparticles treatment, as shown in Fig.3. Liquid (water or oil) can wet the top region of the cavity once the gravity of the liquid is greater than the capillary force. However, the bottom and edge regions remain non-wettable, then the wetting model is composed of the Wenzel model and the Cassie-Baxter model. The surface roughness can be described by a second-order model with multiscale structures as follow: = cos θ f SL R f cos θ 0 − (1 − f SL )

= f SL

1 N    2 k 

n−2

 2h  1 +   ln −1 

 4N +1  Rf = 2   k 

(1) (2)

n −1

(3)

where R f represents the roughness factor, f SL corresponding to the fraction of the liquid droplet on the solid surface. θ 0 is the equilibrium contact angle of a liquid on the solid


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surface, N means the number of smaller-scale pillars, n is the order number of roughness, h denotes the thickness of the liquid membrane, l is the height of the pillar, and k is a constant. Substituting Eqs. 2 and 3 into Eq. 1 to obtain the contact angle as follow: N cos θ   = k 

n−2

 2h   4 N + 1  1 +    ln −1   k 

n −1

 1  N n−2  2h   cos θ 0 − 1 −   1 +     2  K   ln −1  

(4)

Fig. 3 illustrates the relationship between the surface morphology of solid and its wettability. Initial reservoir is generally water-wet or oil-wet, that means the contact angles of liquids on reservoir rock are much less than 90°, as shown in Fig. 3(a). For the multiscale rough surface shown in Fig. 3(c), k = 7 , N = 3 , the contact angle on the second-order surface ( n = 2 ) can be described as:  2h  13 1 cos θ =1 +   (1 + l1   7 γ LV 

1 Π ( h ) dh) +  − 1 ho 2

∫

∞

(5)

Assuming l1 =500 nm, then l2 ≈ 71 nm, l3 ≈ 10 nm, 71 < h < 500 nm. For the third-order surface ( n = 3 ), the contact angle can be described as follow: 2 1  3   2 h   3  θ   1 +    (1 + cos= γ LV l2   7   7 

1 Π + h dh ) ( )  −1 ∫ho 2  ∞

(6)

where 10 < h < 71 nm, the contact angle θ can be calculated by the Eqs. 5 and 6.

Fig. 3. Sketch of the rough surface with a multiscale structure: (a) liquid-wetting surface, (b) gaswetting surface, (c) the second-order surface, and (d) the third-order surface. The wetting regime on the rock surface is the combination of the Wenzel model and Cassie-Baxter model.

2.4 Visualization flooding



Visualization flooding was utilized to determine the influence of gas-wetting alteration on the liquid menisci transformation in the porous media. The visualization equipment consists of a glass micromodel (MP-3, 5×5 cm, China), the real-time image analyzing system (Leica, DM-1000, Germany), precision pump (Longer, LSP01-1A, China), and thermostatic controller (Jintan, HJ-6A, China). Firstly, the micromodel was saturated with brine for 10 h, then followed by a displacement using CH4 or oil until the residual saturation of brine was achieved

24

; secondly, 0.5% FG40-PNP solution was pumped into the micromodel by the

precision pump at a certain flow rate; (3) gas displacement was conducted again after aging for 24 h. The real-time image analyzing system was used to record the movement of fluids in the micromodel, the camera equipped on which can capture the images with varying scales 25. The diameter of pore throat ranges from 1 to 2 mm.

3. Results and Discussion 3.1 SEM and AFM Fig. 4 shows the SEM and AFM images of surface morphology of core samples. In this study, the surface roughness was measured at 3 different locations on each core sample, and the final surface roughness was obtained by averaging these three values. Fig. 4(a) shows the surface morphology of untreated core. Figs. 4(b) and (c) correspond to the surface morphologies of cores after treated by FG40 and FG40-PNP, respectively. For the sample treated by FG40, fluorosurfactant molecules can form gas-wetting adsorption on the surface due to van der Waals force. The adsorption formed by FG40-PNP is of higher surface roughness than that formed by FG40. This is verified by the results of AFM, as shown in Figs. 4(e) and (f). The surface roughness of untreated core is approximately 38.93 nm, which climbs to 42.10 and 71.09 nm after treated by FG40 and FG40-PNP, respectively. The surface roughness of the sample treated by FG40-PNP is nearly double that of the untreated sample, which can be one of the major reasons behind super gas-wetting alteration. Figs. 4(g), (h), and (i) illustrate the vertical distance of the peaks increases from 9.919 to 31.684 and 149.110 nm after treated by FG40 and FG40-PNP, respectively. The sharp change in the vertical distance evidences the increase in the surface roughness of the core surface after treatment. Fig. 4(c) demonstrates that there are numerous gas-wetting nanoparticles which can assembly a substantial cavities

26, 27

. The liquid can wet the cavity top once its own

gravity is greater than the capillary force, leaving the bottom and edge regions non-wettable, as can be seen from Fig. 4(i).


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Fig. 4. Surface morphology and surface roughness of core surfaces before and after gas-wetting alteration: (a) untreated surface, (b) surface treated by FG40, and (c) surface treated by FG40-PNP; (d), (e), and (f) represent the surface roughness of (a), (b), and (c), which increase from 38.93 nm to 71.09 nm after gas-wetting alteration; (g), (h), and (i) illustrate the vertical distance of the peaks, corresponding to (a), (b), and (c), respectively. Table 1 Theoretical contact angles of liquid as a function of the surface roughness.

Surface roughness/ nm

Theoretical contact angle/ °

Wetting model

38.9

75

Wenzel

42.1

96

Cassie-Baxter

71.9

142

Wenzel & Cassie-Baxter

Table 1 shows the theoretical contact angles of liquid as a function of the surface roughness. It can be found that the theoretical contact angles calculated by Eq. 6 have a linear relation with the surface roughness, hence this wetting model can be used to predict the surface wettability when it comes to super gas-wetting. Furthermore, we suggest that the wetting regime on the gas-wetting surface may be a complex combination of the Wenzel model and Cassie-Baxter model.



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3.2 TEM Transmission Electron Microscope (TEM) was used to characterize the morphology of nanoparticles before and after surface modification. The interaction of nanoparticles mainly depends on a balance between van der Waals attraction and the electrostatic repulsion

28

.

Untreated nanoparticles tend to aggregate to achieve the minimum energy due to van der Waals attraction, which exhibited a partly amorphous aggregation, as shown in Fig. 5(a). However, the hydroxyl or carboxyl groups of fluorosurfactant molecule could react with the active groups on the nanoparticle surface after modification, leading to an increase in the -CF groups on the nanoparticle surface. Fig. 5(c) demonstrates the dispersibility of nanoparticles can be significantly improved after surface modification, reflecting the high efficient fluorocoating. In Fig. 5(d), the thickness of adsorption layer is estimated to be approximately 5 nm, which is consistent with the length of fluorosurfactant molecule (5-7 nm), so the adsorption of fluorosurfactant on nanoparticles surface can be recognized as the monolayer absorption. More interestingly, the neighboring nanoparticles can form a honeycomb-like structure to conserve the stability 29, as illustrated in Fig. 5(e). There exists a steric hindrance generated by -CF groups between the neighboring particles, which could transfer the interaction from the attractive force to the repulsive force 30.

Fig. 5. The surface morphology of nano-silica particles before and after modification: (a) untreated nano-silica particles; (c) FG40-PNP; (b) and (d) represent the magnified images of (a) and (c), respectively; the adsorption layer of FG40 on the nanoparticle surface can be observed in (d); (e) schematic of the interaction between the neighboring particles.


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3.3 XPS analysis The XPS characterization was performed to analyze the elemental composition and chemical state of the core surface after treated by FG40 and FG40-PNP. Fig. 6(a) shows the peaks that are attributed to the elements of F, O, C, and Si. The high-resolution XPS spectra of FG40-PNP can be seen in Figs. 6(c) to (f). The peak of F at the binding energy of 689.4 eV was detected on the treated core surface, which consists of three peaks at 689.5 eV (-CF2CF2-), 688.4 eV (-CF2-CH2-), evidenced the presence of FG40 on the core surface

31

. The

atomic relative content of F on the core surface climbed from 0.00 % to 1.15% and 15.87 % after treated by FG40 and FG40-PNP, respectively. The fluorine content on the sample treated by FG40-PNP is obviously higher than that treated by FG40, which could be attributed to the high specific surface area of nanoparticles. The characteristic peaks at 284.8 eV, 286.4 eV, and 288.7 eV in the C 1s spectra can be attributed to the C-C group, C-O group, and O-C=O group, respectively. The formation of O-C=O group provides the evidence for the interaction between the hydroxyl groups of the FG40 and the carboxylic group on the nanoparticle surface

32

. The peak of O 1s locates at the binding energy of 532.3 eV. The

characteristic peaks of Si 2p locate at 103.5 eV (Si-C), 102.3 eV (Si-O-C) and 101.8 eV (SiO2), among which the formation of Si-O-C group is linked to the bonding interaction between the FG40 molecules and nanoparticles, as shown in Fig. 6(f). Fig. 6(b) indicates the XRD spectra of the rock surface treated by FG40-PNP, a typical peak corresponding to the amorphous phase of the nano-silica particles can be clearly observed 33.



Fig. 6. XPS and XRD spectra of rock surface: (a) XPS spectra of core surfaces, (b) XRD patterns of core surfaces, (c) represents the high-resolution spectra of C 1s, (d) represents the high-resolution spectra of O 1s, (e) represents the high-resolution spectra of F 1s, and (f) the high-resolution spectra of Si 2p.

3.4 Evaluation of gas-wetting nanoparticles Contact angles measurement was used to evaluate the wettability change on the core surface after gas-wetting alteration. Fig. 7 plots the contact angles of liquid droplets as a function of solution concentration. The contact angles of the water phase and oil phase increased from 23° and 0° to more than 90° after gas-wetting alteration, and FG40-PNP possesses a better capacity of gas-wetting alteration compared with that of FG40. This is because the equivalent area of core surface treated by FG40-PNP can absorb more


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fluorosurfactant molecules compared with that treated by FG40, leading to an increase in the contact angle. After treated by as-prepared nano-silica particles, the contact angles of brine and hexadecane on the core surface were 34° and 0°, respectively. Compared with the contact angles of water (23°) and oil (0°) on an untreated core surface, there is no obvious change in the rock wettability, so untreated nano-silica particles weren’t used in the following experiments due to its inability to wettability alteration. Generally, rock wettability is considered as liquid-wetting when the contact angle of a liquid droplet on the solid surface is less than 90°; if the contact angle of liquid phase ranges from 90 to 120°, then the wettability is intermediate gas-wetting; it’s gas-wetting when the contact angle ranges from 120-140°; the core wettability can be considered as super gas-wetting if the contact angles of liquid droplets on core surface are greater than 140° 34. Moreover, the surface free energy decreased from 70.15 to 3.27 mN/m after treated by FG40-PNP. Therefore, the wettability of the core surface treated by FG40-PNP can be regarded as super gas-wetting.

Fig. 7. The contact angles of water and oil on core surface before and after gas-wetting alteration

3.5 The liquid saturation after gas-wetting alteration The liquid saturation is one of the key parameters to characterize the flow efficiency of fluids in porous media, pore-scale gas-flooding was conducted to quantitatively study the impact of gas-wetting alteration on the liquid saturation of gas well 35, as shown in Fig. 8. Fig. 8(a) shows the water saturation as a function of time. In the primary gas-flooding, water saturation in micromodel sharply dropped to approximately 35.22 %, which further decreased by approximately 15 % after treatment with gas-wetting nanoparticles. Oil saturation shared a



similar experience. In Fig. 8(b), oil saturation decreased by approximately 12 % after gaswetting alteration, the decrease in oil saturation is slightly less than that in water saturation, this phenomenon can be partly attributed to the high viscosity of the oil. However, there might be a miscibility effect between gas (methane) and oil during the displacement, which could enhance the mobility of oil by lowering the oil viscosity.

Fig. 8. The liquid saturation in the micromodel as a function of time: (a) water saturation and (b) oil saturation.

3.6 The transformation of liquid menisci After the primary gas-flooding, plenty of liquid menisci prevent the flow path for gas by trapping at the pore throat. Fig. 9 shows the transient movement of liquid menisci located at the gas-wetting pore throat. Fig. 9(a) shows that the convex meniscus Ⅰ moves toward meniscus Ⅱ under the displacing force. Being different from the concave meniscus, the protrusion end of the convex meniscus could enhance the coalescence opportunity. A clear transformation process between the convex meniscus Ⅰ and meniscus Ⅱ can be observed in Figs. 9(b) and (c), during which a larger meniscus Ⅲ with greater fluidity generates. The momentum generated by the meniscus Ⅰ plays a decisive role in improving the flow of menisci during the transformation process, so it would be feasible to improve the fluidity of liquid menisci by transformation

36, 37

. For gas-wetting pore throat, liquid menisci would be

subjected to the resistance force generated by Jamin effect when it tried to pass through a pore throat, and then experienced the driving force after passing through, thus the meniscus Ⅲ is much more easily to detach off the pore throat under the displacing force. It’s noticeable that the geometry of the curved solid surface could also contribute to the transformation of convex meniscus 38.


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Fig. 9. The transformation of water menisci: (a) the convex menisciⅠ and Ⅱ, (b) to (c) show the coalescence process, and (d) represent the convex meniscusⅢ generated by the menisciⅠ and Ⅱ.

The flow of oil phase would have been a little different from that of water phase since the miscibility effect between methane and oil might occur at a certain temperature and pressure 39

. Figs. 10(a) and (b) show that the viscosity of oil can be reduced by generating numerous

foams with better fluidity. Fig. 10 (c) indicates a larger foam has successfully passed through the pore throat by deforming into foam Ⅰ and foam Ⅱ, this phenomenon is being named as “Amoeba effect”. Meanwhile, foams Ⅰ and Ⅱ could accelerate the detachment of oil membrane on the pore wall under the Jamin effect 40, leading to potentially high oil recovery. Fig. 10 (d) demonstrates the flow mechanism of foam in a gas-wetting pore throat

Fig. 10. Amoeba effect in the pore throat: (a) the larger foam is being squeezed by the adjacent foams, (b) the critical state of deformation, (c) the smaller foams passing through the pore throat,



and (d) represents Jamin effect in the gas-wetting pore throat. The Jamin effect in gas-wetting porous media is the opposite of that in liquid-wetting porous media.

Fig. 11 shows the flow behavior of oil menisci in the gas-wetting micromodel. The red circles in Fig. 11(a) show the convex oil menisci locating at gas-wetting pore throat, which becomes more easily to detach off the pore wall. Fig. 11(b) represents the detachment process of convex oil menisci on the pore wall. Oil menisci could slowly transform into slender shapes under the displacing force, leading to a decrease in oil volume on the pore wall. Even for the incompletely gas-wetting alteration region, oil saturation could reach a low level after a long-term gas-flooding process.

Fig. 11. The transformation of oil menisci: (a) the convex oil menisci, (b) the detachment of oil menisci.

3.7 The mechanism of gas-wetting alteration in different systems Generally, two conditions are indispensable to achieve super gas-wetting, one is to increase the surface roughness, the other is to decrease surface free energy 41. According to the results of AFM characterization and contact angle measurement, the adsorption layer formed by gaswetting peanut-like nanoparticles have the characteristics of high surface roughness and low surface free energy. Fig. 12 (a) demonstrates that liquid can only wet the top region of the adsorption because of the gravity, thus leaving the bottom region non-wettable, the gaswetting model can be considered as the combination of Wenzel model and Cassie-Baxter model

42

. Liquid with high surface tension cannot wet a gas-wetting surface formed by

peanut-like nanoparticles, exhibiting an increase in the contact angle, as shown in Fig. 12(a). Fig. 12(b) presents the liquid menisci transformation in a liquid-liquid-rock system. Before gas-wetting alteration, the concave menisci tends to wet the surface with high energy to lower its energy

43

, the Jamin effect acts as the resistance force during the movement of liquid

menisci. Nevertheless, liquid menisci could transform from concave-shape to convex-shape after treated by gas-wetting nanoparticles, thus Jamin effect serves as the driving force, which could be one of the main reasons for the improvement in the mobility of liquid menisci.


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Fig. 12. The mechanism of gas-wetting peanut-like nanoparticle in the different systems: (a) depicts the mechanism of gas-wetting nanoparticles in a gas-liquid-rock system, a multi-adsorption layer formed by gas-wetting nanoparticles can increase the surface roughness and lower the surface free energy of rock surface; (b) In a liquid-liquid-rock system, the liquid menisci transformation in the pore throat before and after gas-wetting alteration, the liquid droplets detach more easily off the gaswetting pore throat under the interaction of displacing phase.

4. Conclusions Based on the above investigations, the following conclusions are drawn: (1) peanut-like nano-silica particles can be functionally modified by FG40 to fabricate gas-wetting peanutlike nanoparticles (FG40-PNP); (2) 0.5 wt% FG40-PNP exhibited a fascinating performance in super gas-wetting alteration, the surface roughness of core treated by FG40-PNP is almost 2 times higher than that of untreated core; (3) gas-wetting model is recommended as the combination of the Wenzel model and Cassie-Baxter model, which can be used to predict the surface wettability when it comes to super gas-wetting; (4) the liquid saturation decreased by approximately 15% after treated by FG40-PNP, which could be attributed to the transformation of the liquid menisci in the gas-wetting porous media. The miscibility effect and Amoeba effect also played vital roles in eliminating the liquid-blocking effect. This work not only develops an excellent gas-wetting agent to address the problem of liquid blockage, but also provides perspectives for the mechanism of super gas-wetting alteration and the design of super gas-wetting surface.

Acknowledgments The authors gratefully appreciate the supports provided by the Fundamental Research Funds for the Central Universities of China (No. 201822009), Program for Innovative Research Team in Ocean University of China (No. IRT1289), and the National Key Research



and Development Program of China (No. 2016YFC1402301); This is MCTL Contribution No.***.

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379x285mm (300 x 300 DPI)


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The morphology and surface-chemistry of gas-wetting nanoparticles

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