Stability Mechanism of Nitrogen Foam in Porous Media with Silica

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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers

The Stability Mechanism of Nitrogen Foam in Porous Media with Silica Nanoparticles Modified by Cationic Surfactants Yining Wu, Sisi Fang, Kaiyi Zhang, Mingwei Zhao, Baolei Jiao, and Caili Dai Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01187 • Publication Date (Web): 11 Jun 2018 Downloaded from http://pubs.acs.org on June 12, 2018

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The Stability Mechanism of Nitrogen Foam in Porous Media with Silica Nanoparticles Modified by Cationic Surfactants

Yining Wu1, Sisi Fang1, Kaiyi Zhang1, Mingwei Zhao1*, Baolei Jiao2, Caili Dai1,3* 1. School of Petroleum Engineering, China University of Petroleum (East China), Qingdao 266580, People’s Republic of China 2. Petroleum engineering institute, northwest branch of Sinopec, Urumchi 830000, People’s Republic of China. 3. State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China), Qingdao 266580, People’s Republic of China

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ABSTRACT This work aims at studying the effect of electrostatic interactions between cationic surfactants and silica nanoparticles (NPs) on foam stability in porous media. The physio-chemical property of NPs, the gas-liquid interface properties, the foam flow characteristics together with the stability under different concentrations of surfactant and NPs were investigated and compared. It was found that the affinity of silica NPs to surface is tunable by variation of surfactant concentrations. NPs and surfactants as a whole assembling at surface substantially improve the foam stability in static and dynamic tests. These surfactant modified NPs accumulate at bubble surface and retain stable under dilution of brine, providing a barrier effectively preventing coalescence. In addition, foam stability is enhanced since the layer of NPs significantly reduces the mass transfer rate, consequently, mitigating the Ostwald ripening. Keywords: nanoparticle, surfactant, foam, anti-coarsening, adsorption

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Introduction Foam is a well-known gas-liquid two-phase system and is related to many fields such as biological engineering, food industry, chemical engineering, pharmaceutical industry, petroleum industry and so on 1-8. In recent years, foam as a displacement agent or water shutoff agent is highly favored in oil field due to the economic efficiency and environmental friendliness

8, 9

. Due to

Jamin effect, the apparent viscosity of foam in porous media is relatively high compared with continuous gas flow 9, 10. Thus, it is generally used for flooding, water shutoff and profile control 8-10. But the stability of foam is an outstanding problem for a long time. Surfactants are the most commonly used additive to form and stabilize foam at present 11, 12. However, the surfactants are easy to detach from bubble surface when foam was applied in high temperature, high-salinity subsurface reservoir or 13-15

other harsh condition

. Once the surface concentration is inferior to a certain

critical value, foam becomes unstable and turns into continuous gas flow leading to premature gas breakthrough and low sweep efficiency 16-17. In order to overcome the weakness of surfactant and expand the life-time of foam in formation, nanoparticles were proposed as interface stabilizer13,

18-24

. Some

researchers utilized the flocs formed by nanoparticles to stabilize foam 13, 25. The flocs provide a barrier between the gas bubbles and increase the solution viscosity, therefore, slow down the drainage rate of the lamellae between two bubbles. Not only that, the self-assembly of nanoparticles at the interface properties improve the foam or emulsion stability

20-26

. Binks et al.13 reported the energy E required to remove a

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nanoparticle from the interface ( E = πR2γ ( 1 ± cos θ ) 2 ) can be as large as several thousands of kT, much more than the thermal energy. Clearly, the attachment of nanoparticles to interfaces is more stable compared with surfactant

27-30

. So far two

main kinds of NPs are developed and applied. The first kind is a biopolymer based NPs made from polysaccharide or protein 31-34, the second kind is inorganic NPs like silica NPs, calcium carbonate NPs and carbon NPs

15,18-24,35

. For these NPs, the

relevant parameter is thought to be the contact angle of NP at the interface. To strongly anchor NPs at the interface, the contact angle θ of NPs should be adjusted to circa 90°

13, 36

. For this purpose, several methods, for instance, surface chemical

grafting, surface roughness regulation, salinity adjustment were investigated to modify the surface properties of bare NPs. For chemical grafting methods, some kinds of functionalized modifiers such as polymer or silane coupling agent were connected to NP surface through chemical bonds, which endow the NPs with proper surfactancy to anchor firmly at interface

37, 38

. Zanini et al.

39

proposed a method to adjust the

roughness of NP surface. The results demonstrate that particle surface roughness determine the particle's wettability. Particles with a certain surface roughness can effectively stabilize different types of emulsions

39

. Although chemical grafting and

surface roughness regulation these two methods can meet the requirement but some kind of organic solvent are need such as methyl alcohol, ethyl alcohol or dimethyl formamide during the reactions. The production processes are complicated and costly. In previous case, the surface activity of NPs can also be regulated by changing the solution salinity

40

. Although this method costs less, in many natural setting and

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industrial applications, the salinity is hard to change, such as in oil reservoir, which limits the application. For the sake of simplicity, researchers have proposed to mix NPs with surface-active agents to increase the surface activity of NPs. Anionic， cationic and amphoteric surfactant, even polymers were used to act with NPs 8,10,18,22,25,30,41-43

. The objective of this method is to decorate the surface of NPs in situ

through physio-chemical interactions. Liu et al. 40 utilized the electrostatic interaction between cellulose nanoparticle and functionalized polymer to increase the surfactancy of the NPs. The formation, assembly and jamming of the modified NPs at the interface form a monolayer which brings exceptional mechanical properties. Shi et al. 41

took the same approach and fixed the interface shape by the interfacial jamming of

cellulose nanocrystal surfactants. Gonzenbach et al.

44

stabilized foam by using and

short-chain amphiphiles and Al2O3 particles. It was found that the short-chain amphiphiles can be adsorbed on oppositely charged Al2O3 particles by electrostatic interaction. The short-chain amphiphiles increased the hydrophobicity of the particles, which impart the NPs surfactancy

44

. These decorated Al2O3 particles improve

substantially the foam stability. The mixture of NPs and surfactants may offer a novel technique to stabilize foam. Whereas, the mechanisms of NPs and surfactants to stabilize foam is still not well understood. The present work aims at investigating experimentally the function of the NPs adsorbed at gas-liquid interface, and the influence of NPs on foam behaviors through porous media and micro pore-throat models.

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Material and methods Materials Three

homologous

cationic

surfactants

(Fisher

Scientific)

were

used:

Decyltrimethylammonium bromide (DETAB), Dodecyltrimethylammonium bromide (DTAB) and Tetradecyl Trimethyl Ammonium bromide (TTAB). The bare silica NPs (LUDOX HS-30, Sigma-Aldrich) were purchased as an aqueous dispersion (30 wt %) with a pH of 9.9, surface area of 220m2/g, an average diameter around 13 nm and a density of 1.21 g/mL at 25℃. Hydrochloric acid (1N solution, Fisher Scientific) and sodium hydroxide (1N solution, Fisher Scientific) were used to alter the solution pH to 7. Ultra-pure water was prepared by using a reverse osmosis unit (ULUPURE, UPT-II-5T).

Physio-chemical properties measurements The surface tensions σg between nitrogen and aqueous solution were measured by a tensiometer on a Tracker apparatus (Dataphysics, Gemany). Malvern Zetasizer HS-3000 instrument was used to measure the diameter and zeta potential of NPs. Interfacial viscoelasticity was measured with a dynamic interfacial oscillatory drop tensiometer, Tracker-H, from Teclis France. The dilatational viscoelasticity and interfacial tension were recorded in real time.

Preparation of Aqueous dispersions To minimize aggregation of NPs, high concentrated surfactant dispersions was first diluted in water and stirred to ensure homogeneity. For 100 g surfactant-NPs dispersion, a certain amount of highly concentrated surfactant solution (0.1mol/L) was

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weighed in glass bottle and diluted with water to 93 g, then the solution was stirred for 5 hours. 30 wt % NPs aqueous dispersion was added to surfactant solution dropwisely to avoid the aggregation of NPs. The dispersions were stirred for ten minutes and then the pH was adjusted to 7 ± 0.2 by addition of HCl/NaOH solution. At last, a small amount of water was added to dilute the dispersion to 100g. The concentration range of surfactant was from 0 mmol/L to 2 mmol/L.

Static tests The objective to perform static foam comprehensive value (foam comprehensive value = foaming volume times foam half-life, where half-time represents the time when the foaming volume declined 50 percent) tests is to optimize the surfactant-NPs system forming three homologous cationic surfactants to generate stable foams. Foam is generated by Waring blender method. The agitator vessels were filled with nitrogen before stirring process. The aqueous dispersion used in these tests was prepared with mixtures of the three cationic surfactants (0 mmol/L to 2 mmol/L) and 0.50 wt% NPs. Once the dispersion became cloudy or precipitate was generated, the dispersion was discarded. Then 100mL dispersion was added into an agitator vessel and stirred for 10 minutes to ensure abundant NPs adsorption. Subsequently the foaming volume was measured in graduated cylinder rapidly and the foam half-life was recorded, which was used to calculate the foam comprehensive value. Ultimately, the optimal formula was obtained according to the foam comprehensive value.

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Dynamic tests These tests were performed to assess the performance of surfactant-NP dispersion to generate and stabilize foam in porous media. Sand-packs and transparent pore-throat micro-structures were used to act as porous media. The results of dynamic tests can provide the characteristics of foam from complex pore-throat scale to single pore-throat scale. Single sand-pack tests were divided into two stages: the foam injection stage and sequent reverse water flooding. The experimental diagram is shown in Fig. 1a and Fig. 1b. The sand-pack (φ 2.5cm×100cm) was filled with sand and saturated with brine, the permeability and pore volume of which were determined subsequently. The measuring procedures were as follows: gas (1 ml/min) and surfactant-NPs dispersion (1 ml/min) were co-injected into the sand-pack until the total volume reached 4 pore volume. During this period, the pressure drop of the sand-pack was recorded in real-time to evaluate the foam stability in first stage. Subsequently, brine (NaCl 30 000mg/L) was injected into the sand-pack in the opposite direction. Likewise, the pressure drop across the rock samples was measured in real time. The pressure difference in the sand-pack, and the flow characteristics in the outlet indicate the foam stability in porous media.

Microscopy investigation Microscopy investigation was carried out to directly study the bubble behaviors in micro-structures using inverted microscope (Leica DMi8 C) equipped with a Photron Fastcam SA-Z high speed camera shown in Fig.2a.

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The microfluidic devices were fabricated in a plate (180×120×8 mm) of polymethyl methacrylate (PMMA) by precisely milling and sealed with another PMMA plate (180×120×2 mm). The layout of the device is displayed in Fig. 2b. This microchannel consists of a bubble formation section, a side inlet, a snail-like microchannel, a 3-D pore-throat structure and a bubble entrapment section. N2 was introduced from the middle main channel at a flow rate of Qg and the NPs dispersion was driven into two branches perpendicular to the main channel at the same flow rate of Qw/2. These three flow streams focused at the intersection downstream to generate bubbles in the NPs dispersion. Next to the bubble formation section locates the second inlet of NPs dispersion. In our experiments, the bubble formation section was used to control the bubble size while the second inlet of continuous phase is used to regulate the bubble velocity and bubble intervals. After flowing through the second inlet of continuous phase, the formed bubbles were driven into the snake-like microchannel. The length of the snail-like microchannel (from C point to D point) is of 1450 mm to ensure an adequate time for NPs adsorption. The bubbles were introduced towards the 3-D pore-throat structures to study the bubble breakup behaviors. At the end of the microfluidic devices, there is a bubble entrapment section. This section is connected by two narrow channels. The bubbles will be trapped due to surface tension when they flow into the section while the continuous phase can flow through the narrow section. In this section, mass transfer rate between bubbles and foam stability can be studied by tracing the bubble volume. The bubble formation section, snaillike microchannel, has a uniform square cross section of 400 µm wide and 400 µm high

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(Wc = 400 µm).

Results and Discussion Foam comprehensive-value in static tests Foam comprehensive-value measurements were performed using the three homologous cationic surfactants. Foam static performances for mixtures of 0.5 wt % silica NPs with different surfactant concentrations (0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 1.0 and 2.0 mmol/L) were examined. For TTAB - NPs and DTAB – NPs, the precipitate appeared when the concentration exceeded 0.3 and 0.7 mmol/L, respectively. Notably, although there is no precipitate in the 0.7 mmol/L DTAB – NPs system, the dispersion became slightly opaque, which implies the flocs were generated. For DETAB – NPs, no precipitate was generated in the range of experimental concentrations. These phenomena imply the hydrophobicity of NPs increase with increasing the chain length of surfactants. The dispersibility of modified NPs decrease with increasing the chain length. Fig.3 shows that, based on foam comprehensive-value measurements, NPs alone could barely stabilize foam. In the presence of surfactants and NPs, the foam property has improved. Compared with other two surfactants, DTAB-NPs produces highest quality foams at moderate surfactant concentrations (0.6 and 0.7 mmol/L), and the foam transformed into something like aerogel over time. The foam volume in graduated cylinder decreased first then remained constant for 0.6 mmol/L DTAB - 0.5 wt % NPs system. Due to the appearance of flocs at 0.7 mmol/L DTAB, the optimum formula was set at 0.6 mmol/L DTAB and 0.5 wt % NPs, ultimately.

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Foam stability in porous media Foam stability in porous media is distinguished from the stability in graduated cylinder. The foam generation process and subsequent flow behavior may be affected by confined space of pore-throat structure. Therefore, the optimum formula needs to be tested in porous media. Here, the foam stability in sand-packs were tested in foam injection stage (Fig.4a) and subsequent reversed water flooding stage (Fig.4b), respectively. Previous studies demonstrated fine and stable foam results in high flow resistance in porous media due to Jamin effect. Larger pressure drop means more stable and finer foam in sand-pack. Thus，in our experiments, the pressure drop was chosen as relevant parameter to evaluate foam stability. The permeability of sand-packs for DTAB–NPs foam system and DTAB foam system was 1.97µm2 and 2.05µm2, respectively. As shown in Fig.4a, the pressure difference of DTAB-NPs foam flow in sand-pack increased more quickly compared with surfactant foam flow. By the time the injected volume reached circa 1.5 pore volume (PV) of the sand-pack, the fine foam was drained out from the outlet for DTAB-NPs system. In contrast, discontinuous gas flow was jetted out from the outlet for surfactant system. In addition, as shown in Fig. 4a the gap of the injection pressure in two systems widened with the increase of injected volume. These results demonstrate that foam generated in DTAB-NPs system, are more stable than that in the surfactant system in injection stage. After first stage, reversed water flooding was conducted to test the stability or the plugging capacity of the two foam systems. Fig. 4b displays

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the water flooding process. At first, the pressure drop of the two systems both increased gradually with the injection of simulating water (total salinity 30,000 mg/L). DTAB-NPs system reached almost twice the maximum pressure drop as surfactant system in the water flooding stage. After reaching the maximum value, the pressure drop started to descend. However, the pressure drop in the sand-pack of DTAB-NPs foam decreased more slowly. It means the plugging capacity of DTAB-NPs foam was always stable than surfactant foam during the experiment. The abovementioned results indicate the adding of NPs in surfactant solution plays an important role to improve the foam stability.

Mechanism of foam stabilization by DTAB-NPs To understand the mechanism of foam stabilization by DTAB-NPs, the first thing to figure out is the existence form of DTAB and NPs in aqueous phase. Only in this way, can the essence of the synergistic effect of DTAB and NPs be grasped. Table. 1 shows that the absolute value of zeta potential of bare NPs was around 44.61 mV. The absolute value continued to decline with the increase DTAB concentration and dropped to around 27.66 mV when 0.6 mmol/L DTAB was added. The charge neutralization between cationic surfactant and negative charged NPs, implies the adsorption of DTAB on nanoparticle’s surface. To test this further, the diameter of the NPs with different DTAB concentrations was measured. As shown in Table.1, the diameter of NPs gets much larger with the increase of DTAB concentration. When the DTAB concentration is relatively low, the increase of the diameter is comparable to the length of surfactant which is of circa 1nm. We therefore, speculated that the

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absorption of DTAB on the nanoparticle surface was monolayer absorption. With the continuous increase of DTAB concentration the average diameter of nanoparticles increased even doubled. This may result from the aggregate of NPs due to the increase of hydrophobicity caused by more DTAB adsorption on NP surface. Furthermore, the surface tension measurement provided evidence supporting the adsorption of DTAB on NPs. Fig.5 shows that the capability of bare NPs to reduce surface tension is very limited. DTAB can reduce the surface tension to 42 mN/m. After adding NPs to DTAB, the surface tension even increased to about 51 mN/m, instead of further decreasing. The results demonstrate free surfactant in the solution will decrease due to the adsorption, which lead to the increase of surface tension. At the same time, this experiment also implied that the reduction of surface tension is not the main mechanism of foam stabilization by DTAB-NPs system. Based on the zeta potential, diameter measurements and surface tension measurements, the schematic diagram showing the interaction between DTAB and NPs is presented in Fig.6. The surface concentration of DTAB increases with the increase of bulk concentration. It can be deduced from the Fig.6 that the hydrophobicity of NPs increases with the adsorption of DTAB. Thus, the number of surfactants adsorbed on NPs may determines the affinity of NPs to the interface. The NPs decorated by surfactants may act as surface active agent. Several studies reported the adsorption of NPs with surface activity is thought of as approximately irreversible process13, 36. Compared with surfactant, the size and the mass of NP are relatively larger, thus, the diffusion coefficient of NPs is smaller than surfactant. It is

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different to attain new equilibrium for NPs adsorbed on the surface during frequent fluctuation. Consequently, the viscoelastic modulus of the surface stabilized by NPs is comparatively high. In turn, the surface viscoelastic can be used to evaluate the surface activity of NPs. The dilational surface viscoelastic modulus ɛ for surfactant alone and for NPs in the presence and absence of DTAB were measured. The surface viscoelastic modulus as a function of the DTAB concentration is shown in Fig. 7. The reorganization of DTAB and NPs at the interface plays a key role in changing the properties of interface. The results confirm that the surfactant alone exhibited a very small viscoelastic value (