Carbon Dioxide-in-Brine Foams at High Temperatures and Extreme

Sep 12, 2017 - CO2-enhanced oil recovery (CO2-EOR) is a common tertiary oil recovery ... zones and migrate to the top of the reservoir, overriding the...
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Article Cite This: Energy Fuels 2017, 31, 10680-10690

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Carbon Dioxide-in-Brine Foams at High Temperatures and Extreme Salinities Stabilized with Silica Nanoparticles Shehab Alzobaidi,† Mohammad Lotfollahi,‡ Ijung Kim,‡ Keith P. Johnston,*,† and David A. DiCarlo*,‡ †

McKetta Department of Chemical Engineering, University of Texas at Austin, Austin, Texas 78712-1589, United States Department of Petroleum and Geosystems Engineering, University of Texas at Austin, Austin, Texas 78712-1585, United States

‡

S Supporting Information *

ABSTRACT: The stabilization of carbon dioxide-in-water (C−W) foams with nanoparticles (NPs) becomes highly challenging as the temperature and salinity increase, particularly for divalent ions, as the nanoparticles often aggregate in the brine phase. For silica nanoparticles with a medium coverage (MC) and high coverage (HC) of organic ligands, the hydrophilic−CO2-philic balance (HCB) was found to be in the appropriate range to produce a large reduction in the C−W interfacial tension (IFT). Furthermore, the nanoparticles were colloidally stable in concentrated brine (15% total dissolved solids, TDS) up to 80 °C. With these interfacially active nanoparticles, C−W foams were stabilized with apparent foam viscosities up to 35 cP and foam textures with bubble sizes on the order of 40 μm at various gas fractional flows (foam qualities) in beadpack experiments. At the foam quality where the apparent viscosity was a maximum (transition quality) in the beadpack, we also produced CO2 foams in Boise and Berea cores versus temperature with apparent viscosities up to 26 cP at 70 °C and 15% TDS and hysteresis in the apparent viscosity versus the interstitial velocity. The reductions in the IFT and foam strength at elevated temperature were modestly larger for the HC nanoparticles than for the MC nanoparticles but were low for the low-coverage case. Given that the interfacial adsorption increased with salinity up to 15% TDS, the screening of the charge helped drive the particles from the brine phase to the interface, which was necessary to stabilize the foams.

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INTRODUCTION CO2-enhanced oil recovery (CO2-EOR) is a common tertiary oil recovery technique, but it can suffer from low sweep efficiency due to the high mobility of CO2. CO2 tends to channel through high permeability zones and migrate to the top of the reservoir, overriding the oil-rich zone. To remedy this issue, surfactant foams have been studied extensively for their ability to reduce gas mobility and provide the essential mobility control in CO2-EOR.1−5 For example, water-soluble anionic and cationic surfactants have been commonly used for CO2 foams,6,7 along with CO2-soluble nonionic8 and switchable nonionic to cationic surfactants.7,9,10 In some cases, foam propagation may be more effective for nonionic CO2-soluble surfactant, which can partition somewhat equally between the aqueous and CO2 phases,11 with either co-injection or wateralternating gas injection.10,12,13 Whereas nonionic surfactants, mostly commonly with ethylene oxide head groups, have limited thermal stability above 100 °C and often become insoluble in brine, switchable substituted amine surfactants are soluble and have been used to stabilize foams up to 120 °C.10 Surface-treated silica nanoparticles (NPs) have become of interest as an alternative for surfactants for stabilization of C− W foams14 because of their high chemical stability at high temperature, potentially low retention on minerals, and unusually strong adsorption at CO2−water interfaces.15−19 Espinosa et al. generated a supercritical CO2-in-water (C−W) foam in beadpack using 5 nm silica nanoparticles whose surfaces were treated with short-chain polyethylene glycol (PEG) for a range of salinity up to 4% NaCl and temperatures up to 95 °C.20 Aminzadeh-goharrizi et al. altered the propagation of the CO2 displacement front in Boise sandstone © 2017 American Chemical Society

cores using 5 nm silica nanoparticles with a 5 nm PEG coating in 2% NaBr at room temperature.21 Aroonsri et al. tested three different silica nanoparticles, a PEG-ylated NP (3M-PEG) and two NPs with proprietary coatings (EOR-5XS and EOR-12), to generate C−W foam at 50 °C and 2800 psi in NaCl brine.15 As the salinity increased, the foam viscosity increased for all tested nanoparticles. Eventually, as the salinity reached 4 wt %, EOR5XS and EOR-12 nanoparticles aggregated and no longer provided foam stability. Yu et al. created CO2 foam using three different nanosilica particles in 2% NaCl brine at 25 °C and observed that, as the nanoparticle surface changed from hydrophilic to somewhat hydrophobic, the volume of foam generated increased.22 Lotfollahi et al. generated C−W foam using 0.5 wt % surfacetreated silica nanoparticles (EOR-5XS) in 9.6 wt % brine (8.53 wt % NaCl and 1.11 wt % CaCl2) at 70 °C in Boise sandstones.16 San et al. reported the effect of monovalent ion (NaCl) and divalent ion (CaCl2) concentrations and temperature on C−W foam generation.23 They observed that the amount of CO2 foam and foam stability increased upon increasing the NaCl concentration at room temperature from 1.0 to 10%, but a further increase in NaCl to 15% resulted in nanoparticle aggregation and core plugging, which is consistent with Aroonsri et al.’s observations. They also found that upon increasing CaCl2 concentration from 0.1 to 1% at low temperature, foam generation and stability were improved, but higher divalent ion concentrations were not reported. The Received: June 24, 2017 Revised: September 10, 2017 Published: September 12, 2017 10680

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foam stability was also reduced in their experiments as the temperature increased from 25 to 65 °C.23 Moreover, the addition of surfactants to nanoparticle amphiphiles can significantly enhance foam generation and stability.17,24−29 The surfactant can weakly bind to the surface of the nanoparticle and decrease the hydrophilicity of the nanoparticle, leading to greater interfacial adsorption. Emrani and Nasr-El-Din measured the surface tension between CO2 and AOS surfactant solution in the presence and absence of nanoparticles and observed that surface tension was reduced in the presence of nanoparticles at 150 °C and 435 psi.27 They also found that adding nanoparticles increased the half-life of CO2 foam at ambient pressure and temperature. Various researchers have studied the effect of nanoparticle size on emulsion/foam generation in porous media. Lin et al. reported that smaller nanoparticles are transported more readily to the liquid−liquid interface and, thus, enhance creation of emulsion/foam.30 Binks and Lumsdon proposed a linear relationship between particle size and emulsion droplet size.31 Kim et al. tested silica nanoparticles with four different sizes with nonionic surface ligands32 and found that smaller particles lead to more stable emulsion/foam, due to greater diffusivity and/or higher interfacial concentrations of nanoparticles. Worthen et al. found that modifying the surfaces of silica nanoparticles with glymo ligands led to colloidal stability up to 80 °C for salinities up to 10% TDS (total dissolved solids).33 Thus, it may be anticipated that the range of salinity and temperature for C−W foams may be further extended by the advanced design of the ligands on the nanoparticle surface. It would be desirable to study the interfacial behavior of nanoparticles at the CO2−water interface to more fully understand the stabilization of CO2 foams; however, very few if any examples have been reported to show that nanoparticles lower the interfacial tension between CO2 and water. The objective is to demonstrate that C−W foam can be generated in high-salinity brine (15 wt % total: 8.5% NaCl, 4.3% CaCl2, and 2.2% MgCl2) and high temperature up to 80 °C using surface-modified silica nanoparticles that also lower the CO2−water interfacial tension. The brine composition was chosen to represent high-salinity reservoirs. We compare the colloidal properties (hydrodynamic diameter and ζ-potential) in brine and the interfacial properties (surface tension and water−CO2 interfacial tension) for three different silica nanoparticles with low coverage (LC), medium coverage (MC), and high coverage (HC) as characterized by thermogravimetric analysis (TGA), but each with a nominal average silica size of 10 nm and a hydrodynamic diameter in the range of 13−15 nm at pH 8.5. With the MC and HC NPs, two seemingly conflicting goals have been met simultaneously, high colloidal stability in brine and adsorption at the water−CO2 interface. The effects of foam quality and interstitial velocity are examined for C−W foams in a beadpack and in Boise and Berea sandstones for temperatures from ambient up to 80 °C. In both cases, the strength of the foam for the NPs, as characterized by the apparent viscosity and bubble size, is shown to increase with an increase in the surface pressure (NPs adsorption) at the C−W interface as the degree of ligand coverage is increased. The ability to stabilize C−W foams with nanoparticles at high temperature and salinity and to understand the formation and stabilization mechanisms will be of benefit to practical applications of CO2-EOR.

Article

EXPERIMENTAL SECTION

Materials. Silica nanoparticles (NPs) with three different amounts of similar ligands grafted to the surface of the nanoparticles were provided by Nissan Chemicals as 20 wt % concentration aqueous dispersions. All the concentrations (%) in this study are in wt % (w/v). The EOR-5XS-V2 particles were low coverage (LC), EOR-5XS-V3.2 were medium coverage (MC), and EOR-5XS-V4.2 were high coverage (HC). The exact type of the organic ligands grafted on the surface of the nanoparticles is proprietary. HCl (1 N solution, Fisher Scientific), NaCl (ACS grade, Fisher Scientific), MgCl2 (ACS grade, Fisher Scientific), and CaCl2·H2O (ACS grade, Amresco) were used to prepare the 15% TDS brine (8.5% NaCl, 4.3% CaCl2, and 2.2% MgCl2). Deionized (DI water) (Nanopure II, Barnstead, Dubuque, IA) was used to prepare brine solutions for all the experiments. Thermogravimetric Analysis (TGA). To remove the ungrafted ligands and any stabilizers in the NP solution, the NPs solution was washed three times with 2× v/v % DI water using Amicon 30K MWCO centrifuge filters at 6000 rpm for 10 min. Prior to TGA measurement,33 the samples were dried in an oven at 80 °C overnight to remove residual water. Dynamic Light Scattering. The hydrodynamic diameter was measured for 1% nanoparticles in DI water or in 15% TDS using a Brookhaven ZetaPALS instrument with BI-MAS configuration. The scattered light was collected at 90° using an avalanche photodiode detector. All nanoparticle size measurements were repeated six times, and the average size with standard deviation of the runs was recorded. ζ-Potential. ζ-Potential samples were prepared in DI water with background electrolytes of 10 mmol KCl at pH 5 adjusted by 1 N HCl. Brookhaven ZetaPALS was used for the measurements. The Smoluchowski model of electrophoretic mobilities of silica nanoparticles at room temperature was used to calculate the zeta potentials. All the samples were prepared at least 1 h before the measurement. Four measurements of each sample were conducted, and the average value is reported. CO2−Water Interfacial Tension and Air−Water Surface Tension Experiments. For CO2−water interfacial tension, an axisymmetric drop shape analysis of a reverse pendant droplet, composed of liquid CO2, formed on the tip of a stainless-steel capillary was used to determine the interfacial tension for 1% NPs in 15% TDS at 3000 psig and 25 °C. A Theta optical tensiometer (Biolin Scientific) was used for the measurement; the details of the apparatus are described elsewhere.2 The NP solution was presaturated with CO2 before the bubble is generated. The droplets were equilibrated for 10 min until a steady-state value of interfacial tension was reached. The recorded images were analyzed with the OneAttension software package. The same instrument was used to measure air−water (A−W) surface tension. The high-pressure chamber was replaced by a syringe and a stainless steel needle to generate a pendant drop at ambient conditions as described elsewhere.34 A pendant drop made of the desired solution was generated and the surface tension was measured with air at ambient conditions. A minimum of three runs for each sample was recorded and the average value was reported. Silica Nanoparticles Stability Tests. Silica nanoparticles LC, MC, and HC were tested for stability via visual inspection and dynamic light scattering (DLS) in 15% TDS brine (8.5% NaCl, 4.3% CaCl2, and 2.2% MgCl2) at room temperature and at elevated temperatures with or without pH adjustment. The pH was left unadjusted (pH 8.5) or adjusted to pH 4 with 1 N HCl and measured with a Mettler-Toledo FiveGo pH meter equipped with a micro tip. Nanoparticles were diluted to 1% w/v in 15% TDS brine and placed in an oil bath over 40 h. The 15% TDS brine viscosity of 1.24 cP at 22 °C, measured with a viscometer (CANNON Instrument Co., no. 50), was used to determine the hydrodynamic diameter (HDD) of the NPs from the Stokes−Einstein equation via DLS. The stable and nonturbid NP dispersions with NPs sized 1000 21.1 ± 0.2 17.6 ± 0.9

>1000 24.7 ± 0.13 20.6 ± 1.1

>1000 34.2 ± 0.8 27.4 ± 2

−19.2 ± 2.2 −12.3 ± 0.1 −9.3 ± 0.4

18.8 ± 0.6 13.6 ± 0.5 12.7 ± 0.9

66 83 100

Unless indicated otherwise, the aqueous phase was 15% TDS. bHDD was measured by DLS. cIn 10 mM KCl. dFor 1% NPs, at 3000 psig, 25 °C, and 15% TDS. eHC NPs organic fraction was considered as 100% coverage.

a

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Figure 3. TEM images of silica NPs with different degrees of coverage of organic ligands: (from left to right) low coverage (LC), medium coverage (MC), and high coverage (HC) nanoparticles.

Figure 4. Images of nanoparticle stability tests: (A) silica nanoparticles in 15% TDS, at 25 °C and pH 4, and (B) silica nanoparticles in 15% TDS at pH 4 after 3 days at T = 50 °C; note the clouded sample of LC NPs.

indicates an increase in the ligand concentration consistent with the ζ-potential results above. The effect of salt was investigated to determine how it influences the interfacial adsorption of the NPs at the air−water and CO2−water interfaces. Water−air surface tension measurements at ambient pressure are shown at pH 8.5 in Figure 5 and

Furthermore, the hydrogen bonding of water to the ligands decreases with increasing temperature, whereby the stronger ligand−ligand interparticle interactions lead to increased aggregation. However, the steric stabilization was still sufficient to maintain sub-50-nm particles that will be shown below to stabilize C−W foams. Effect of NPs and Added Salt on CO 2 −Water Interfacial Tension and Air−Water Surface Tension. The CO2−water interfacial tension (IFT) is presented in Table 2 for 15% TDS at 3000 psia and 25 °C and a nanoparticle concentration of 1%. The IFT was 25 mN/m for the control experiment without added nanoparticles (Table 3). Table 3. Salinity Effect on C−W Interfacial Tension (IFT) with or without Added HC NPs system

IFT (mN/m)

CO2−brine (15% TDS) no NPs (control) CO2−DI water +1% HC NPs CO2−brine (15% TDS) + 1% HC NPs

25.0 ± 1.8 16.9 ± 0.3 12.7 ± 0.9

Figure 5. Surface tension of 1% NPs with varying salinity at ambient conditions. The pH was not adjusted (pH 8.5) for these measurements.

The surface pressures (difference in IFT with and without NPs, π = γ − γo) were 6.2, 11.4, and 12.3 mN/m for LC, MC, and HC, respectively. These large π values indicate very significant adsorption of the NPs at the interface. From previous studies on NPs at the oil−water and air−water interfaces, the π values are very small for bare silica particles. Thus, the ligands on the surface of the modified NPs lowered the hydrophilicity and increased the interfacial adsorption. Furthermore, the increase in the surface pressure (and adsorption) for LC to MC to HC

at pH 3 in Figure S1 of the Supporting Information (SI). The effect of pH was small. Up to 10% TDS, the π values were small, indicating relatively low NP adsorption. However, at 15% TDS, the π increased for each type of NP and increased more for MC and HC relative to LC. Interestingly, these π values were much smaller than those observed at the CO2−water 10684

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Figure 6. Micrographs of the initial foam texture with Dsm under each image (top) and the corresponding apparent foam viscosities versus foam quality of 1% MC in 15% TDS, 1% LC in 15% TDS, and 1% HC in 15% TDS nanoparticles in a 22 darcy glass beadpack at shear rate of 2200 s−1, 25 °C, and 3000 psig (bottom).

The adsorption energy (ΔE) of a spherical particle is a function of the loss of CO2−water interfacial tension, which is replaced by CO2−NP and water−NP hemispherical interfaces. Given that the contact angle of the NP was unknown, a value of 90° was assumed, as is typically done.40,41 For a spherical particle with an air/water/NP contact angle of 90° and a measured surface pressure (γo − γ)

interface with 15% TDS. In a control experiment without any added salt, the C−W IFT was 16.9 (Table 3), corresponding to a π of 8.1 mN/m. The much larger π of the C−W interface indicates that the solvation of the ligands by CO2 helped attract the NPs to that interface relative to the A−W interface. Additionally, the lower pH caused by buffering from CO2 lowered the magnitude of the charge on the NPs, decreasing the hydrophilicity, which may contribute to the adsorption; however, the pH effect was very small at the A−W interface. An X-ray photoelectron spectroscopy study of silica nanoparticles at the vapor−water interface indicated that the interfacial activity of the nanoparticle is a function of the surface charge density. This study concluded that the increase of negative charge on the nanoparticle results in a reduction of the interfacial activity of the nanoparticle,37 although this effect was not seen in our study at high salinity shown in Figure S1 (SI). Even though the interfacial activity of the NPs at the C−W interface was already large (γo = 25 mN/m),38 the π increased substantially to 12.3 mN/m for 15% TDS. The added salt screened the SiO− charges on the NPs and weakened the hydration of the ligands on the surface.39 Both of these effects decrease the hydrophilicity of the NPs and favored migration to the interface, as observed at the A−W and C−W interfaces. Furthermore, the increases in π with added salt were similar at the A−W and C−W interfaces, suggesting that this behavior was influenced more by the added salts (Figure 5) than pH, consistent with the lack of a significant pH effect in Figure S1 (SI) at the A−W interface.

ΔE =

(γo − γ )πa 2 η

(2)

where a is the particle radius, η is the two-dimensional packing fraction, and γo is ∼25 mN/m. For simplicity, we assume a close-packed interface where η = 0.91. For a spherical coated silica particle with radius of ∼5 nm adsorbed at an air−(15% TDS) brine interface and a surface pressure of 12.3 mN/m, the ΔE is on the order −102 kT. The essentially irreversible adsorption of a NP at a fluid−fluid interface may be contrasted with the much more dynamic adsorption and desorption of free surfactants, where ΔE values are on the order of 1 kT.42 Given the high surface pressures for the LC, MC, and HC NPs at the CO2−brine interface and the high adsorption energy, they have the potential to stabilize C−W foams. In contrast, foams are not formed with bare silica particles with low surface pressures.25 Beadpack Foam Generation Experiments. The stable NPs dispersions in 15% TDS were used to generate foam in a 22 darcy beadpack. The foam apparent viscosity (μapp) was studied versus foam quality (ϕCO2). As shown in Figure 6, the 10685

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Figure 7. Micrographs of the initial foam texture with Dsm under each image (top) and the corresponding apparent foam viscosities versus foam quality of 1% MC in 15% TDS and 1% HC in 15% TDS nanoparticles in a 22 darcy glass beadpack at shear rate of 2200 s−1 and pressure of 3000 psig (bottom).

optical micrographs for initial foam texture show a slightly larger foam bubble size for foam generated with LC NPs compared to MC or HC NPs, especially for the highest ϕCO2 values. Similarly, μapp at 25 °C for LC NPs was the lowest at all foam qualities (ϕCO2), with the highest μapp of 28.6 cP at 0.85 ϕCO2. The μapp at 25 °C increased with increasing the grafting density on the surface of the nanoparticles from LC to HC. The transition μapp values of MC and HC NPs were 34.1 and 36.5 cP at 0.85 ϕCO2, respectively, only slightly higher than for LC. The lower μapp for LC may be attributed to the lower surface pressure given the higher charge and hydrophilicity. They were repelled to a greater extent from the interface due to the stronger NP−NP and NP−interface electrostatic repulsion given the high negative charge on the surface.43 The higher surface pressure (and interfacial adsorption) in high-salinity brine of MC NPs and HC NPs compared to LC NPs facilitated generation and stabilization of C−W foam (Table 2). The minimum pressure gradient to overcome the capillary pressure to generate the lamellae in the pores decreases with the decrease in γ. Another reason for higher μapp made with higher grafting density NPs can be attributed to the relatively smaller HDD in brine.32 The stable suspensions of MC NPs and HC NPs in 15% solution at 73 or 80 °C were also utilized for C−W foam experiments in the beadpack. As shown in Figures 7 and 8, C− W foam with HC NP solutions had a higher μapp than those with MC NP solutions at high temperature. At 73 and 80 °C, the micrographs showed much larger initial bubble sizes for MC C−W foams compared to HC foams, especially at 0.9 ϕCO2. At 80 °C, the transition μapp was reduced slightly to 19 cP

Figure 8. Micrographs of initial foam texture, with Dsm under each image (top) and the corresponding apparent foam viscosities versus foam quality of 1% MC and 1% HC nanoparticles solution in 15% TDS generated in a 22 darcy beadpack at a shear rate of 2200 s−1 and pressure of 3000 psig (bottom). The temperature is indicated on the figure.

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Energy & Fuels for MC NPs and 23.5 cP for HC NPs. C−W foams could not be formed with LC NPs, given that they aggregated in the aqueous brine phase and thus were not available to stabilize the C−W interface. Thus, the greater steric stabilization with the greater ligand coverage, as reflected by the ζ-potential and surface pressure, for MC and HC was key for C−W foam formation and stabilization. The ligand design was optimized to be sufficiently hydrophilic for dispersibility in concentrated brine, while not being too hydrophilic for adsorption at the C− W interface. For bulk foam and assuming constant aqueous phase viscosity and shear rate, the apparent viscosity is a function of ϕCO2, the radius of the foam bubble, and the interfacial tension (eq 3)44,45 μapp =

−1/2 ⎛ μ γR ̇ ⎞ τ0 + 32(ϕCO − 0.73)μe ⎜ e ⎟ 2 γ̇ ⎝ γ ⎠

(3)

where τ0 is the yield stress, γ̇ is the shear rate, γ is the interfacial tension, R is the radius of a spherical bubble, μe is the viscosity of the aqueous phase, and ϕCO2 is the foam quality. Increasing ϕCO2 above 0.73 and reducing γ can significantly increase μapp. In Figures 6−8, μapp increased with increasing foam quality up to ∼0.85 and then decreased. As ϕCO2 increases to more than 0.7 (above the volume fraction of bulk close-packed spheres) (eq 3), it is harder for bubbles to pass through pores in porous media, resulting in higher μapp.44 However, when the quality approaches unity, as seen at ϕCO2 = 0.9, the capillary pressure (Pc) increases to larger values, as shown with eq 446 Pc = γ /[R(1 − ϕCO )0.5 ] 2

Figure 9. Micrographs for foam stability with Dsm under each image (top) and the analysis of the Sauter mean diameter over time generated by 1% LC in 15% TDS, 1% MC in 15% TDS, and 1% HC in 15% TDS brine at 0.75 foam quality, 25 °C, and 3000 psig (bottom).

with increasing time. On the basis of the slopes, the C−W foam generated with HC NPs was 4 times more stable than the foam generated with MC NPs. Furthermore, the foam generated with MC NPs was 1.5 times more stable than the foam generated with LC NPs. This effect can be related in part to the decrease in the interfacial tension from LC to HC (Table 2) that reduces Pc, as shown in eq 3. As Pc decreases, the drainage velocity of the lamellae decreases, as described by eq 52

(4)

where γ is the interfacial tension and R is the foam film radius (equivalent to a spherical bubble radius of the same volume). As Pc increases, it is harder to form curved lamellae in the pore throats, and the lamellae drainage rate increases as described below, leading to coalescence. Also, the rate of Ostwald ripening for C−W foams increases as the lamellae become thinner at higher ϕCO2.2,3 The modestly lower μapp at elevated temperatures versus lower temperature can be explained in part by the initial bubble size (eq 3). For example, in the micrographs of MC NPs, the initial bubble size at 25 °C and 0.9 ϕCO2 was 46 μm and increased to 63 and 71 μm when the temperature increased to 70 and 80 °C, respectively. Part of this increase is due to the greater capillary pressure at higher temperature (eq 4). Another reason could be coarsening from more rapid diffusion of CO2 from small bubbles to larger bubbles as the temperature increases, as the bubbles were smaller than the pores and thus may disappear within tens of seconds.47 Given the excellent stabilization of the lamellae and bubbles by the NPs, μapp remained quite high, even at high temperature. Bulk Foam Texture Stability. The bulk foam generated at 25 °C and 0.75 ϕCO2 with 1% NP solutions in the 22 darcy beadpack was directed to the microscope view cell in Figure 1. As shown in Figures 7 and 8, the bubble sizes were very similar for MC and HC foams at most qualities except 0.9. At this highest quality, where the spacings between bubbles are the smallest, the bubbles were larger for the MC foams, which is due in part to the lower interfacial activity. As shown in Figure 9, the Sauter mean diameter (Dsm) for all the systems increased

V=−

dh f h3 = f 2 ΔPfilm dt 3μe R

(5)

where hf is the thickness of the thin film and ΔPfilm is the film pressure [=2(Pc − Πd), wherein Πd is the disjoining pressure]. Another reason for the greater foam stability of HC NPs is the higher number of small solid nanoparticles adsorbed to the C− W interface leading to slower gas diffusion (and consequently coarsening) through the foam lamella. As the drainage velocity decreases, the films remain thicker and foam coalescence and Oswald ripening slow down, leading to an increase in foam stability. Coreflood Foam Generation Experiments. The coreflood foam generation experiments with MC and HC NPs were carried out in Berea and Boise sandstone cores at varying temperatures (25, 60, and 70 °C) and varying initial pH values (pH 3, 6, and 6.5). A lower initial pH value was used, as the pH will be reduced to pH 4 when CO2 saturates the NP solution. Foam quality was 0.75 in all the coreflood experiments. More experimental details are given in Table 1. EXP 1 used MC NPs at 60 °C and an initial pH of 6.5 for the injected NP solution in a Berea core, which was the lowest permeability tested (281 mdarcy). In the absence of NPs, the apparent viscosity of the simultaneous flow of water and CO2 was measured to be μapp = 0.8 cP at all the flow rates. In the presence of MC NPs, the apparent viscosity increased from 2 to 12.5 cP, when the flow rate was increased from 2 to 6 mL/min (corresponding to interstitial velocities from 108 to 324 ft/day) 10687

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Energy & Fuels as shown in Figure 10 (points 1−3). When the flow rate was then lowered, the apparent viscosity increased to 16.5 cP (point

Figure 12. Apparent viscosity of CO2 coreflood foam as a function of interstitial velocity at a constant foam quality of 0.75 for 1% MC NPs in 15% TDS solution in 2200 mdarcy Boise sandstone at 2800 psia and 70 °C. The injected NP solution was at pH 6 (EXP 3).

Figure 10. Apparent viscosity of CO2 coreflood foam as a function of interstitial velocity at a constant foam quality of 0.75 for 1% MC NPs in 15% TDS solution in 281 mdarcy Berea sandstone at 3000 psia and 60 °C. The injected NP solution was at pH 6.5 (EXP 1).

1-R). This hysteretic behavior is discussed by Lotfollahi et al.16 and is consistent with a strong foam being made at higher velocities; once this strong foam is created, it is robust and is retained at lower flow velocities. EXPs 2 tested MC NPs at an initial pH of 3, reduced with 1 N HCl, in higher permeability Boise cores at 25 °C (Figure 11).

Figure 13. Apparent viscosity of CO2 coreflood foam as a function of interstitial velocity at a constant foam quality of 0.75 for 1% HC NPs in 15% TDS solution in 1500 mdarcy Boise sandstone at 2800 psia and 70 °C. The injected NP solution was at pH 6 (EXP 4).

HC NPs at similar interstitial velocity was 14.3 cP. The foam hysteresis effect was also observed for HC NPs when the interstitial velocity was reduced from 172 to 28 ft/day. The μapp of HC NPs in this study, with a maximum of 26 cP, was higher than the previously reported μapp of different types of NPs but a similar size at lower salinities of 9.6% TDS, where the maximum reached 19 cP.32 Upon comparing EXP 3 and EXP 4, with a similar range of permeability, we conclude that increasing the amount of grafted organic ligands on the silica NP surface led to a modestly lower Pc, due to the lower γ; hence, a higher μapp in the coreflood experiments is seen for the detailed reasons given above in the beadpack experiments. The lowering of Pc with HC versus MC foams is even more important in the cores relative to the beadpack given the lower interstitial velocities and smaller pores in the cores,48 which make it more challenging to overcome the minimum pressure gradient for lamellae mobilization.

Figure 11. Apparent viscosity of CO2 coreflood foam as a function of interstitial velocity at a constant foam quality of 0.75 for 1% MC NPs in 15% TDS solution in 4500 mdarcy Boise sandstone at 2800 psia and 25 °C. The injected NP solution was at pH 3 (EXP 2).

At 25 °C, μapp reached a plateau at 106 ft/day with μapp of 11 cP. Hysteretic behavior was observed upon the reduction in the injection velocity in this experiment. In contrast, μapp was much higher in the beadpack for MC at all temperatures, as the higher interstitial velocity and shear rate in the beadpack with larger pores overcame the capillary pressure for foam generation by a greater extent than in the Boise cores. EXP 3 was the same as EXP 2 except that the initial pH of the nanoparticle solution was pH 6 and the temperature was 70 °C (Figure 12). After mixing with CO2, the pH will drop to ∼4, given the large buffering capacity of the dissolved CO2 in brine. In this experiment, the foam apparent viscosity plateau was not achieved over the range of interstitial velocities tested. EXP 4 tested HC NPs under similar conditions as EXP 3 at 70 °C (Figure 13). HC NPs created a stronger foam compared to MC NPs, consistent with beadpack experiments, although the difference was greater in the Boise sandstone. The highest μapp for MC NPs was 7.7 cP at 173 ft/day, whereas the μapp for

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CONCLUSIONS Three types of organic-modified NPs were tested in this study, each with a different amount of ligand grafted to the surface, as shown by DLS, ζ-potential, and TGA. The HC NPs had the highest colloidal stability in 15% TDS, given the strongest steric stabilization resulting from the largest amount of organic ligands on their surface. C−W foam with μapp up to 26 cP at high temperatures up to 70 °C and high salinity of 15% TDS in 1500 mdarcy Boise sandstone was generated by high coverage (HC) modified silica NPs, despite the high divalent cation 10688

DOI: 10.1021/acs.energyfuels.7b01814 Energy Fuels 2017, 31, 10680−10690

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Energy & Fuels concentration. The hysteretic behavior in μapp versus interstitial velocity plots had the desired characteristic for practical application of EOR.16 These NPs produced the lowest interfacial tension of 12.7 mN/m (highest surface pressure and surfactant adsorption), given the lower magnitude of the charge according to the ζ-potential. Thus, it was possible to achieve simultaneously sufficient hydrophilicity for NP stabilization in concentrated brine, as well as CO2-philicity for migration from the aqueous phase to the interface to form stable C−W foams even at high temperatures. In contrast, LC NPs aggregated in brine and C−W foam could not be formed at high temperature given the insufficient steric stabilization, as reflected in the ζ-potential and TGA. For all of the NPs, increasing the salinity of the aqueous solutions made the NPs more interfacially active by screening the charges and reducing hydrophilicity. The foams were only modestly weaker for MC NPs relative to HC NPs in the beadpack but were significantly weaker in the sandstone cores, where the permeabilities were smaller and thus the minimum pressure gradients were larger. Reducing the IFT for HC relative to MC and LC NPs (1) lowered the capillary pressure to enhance lamellae generation in the pore throats, (2) slowed down thinning of the lamellae by reducing the drainage rate of bulk foam, and (3) reduced the rate of Ostwald ripening of the bulk foam, as shown with microscopy. The MC and HC NPs C−W foams, with high μapp and stability at high temperature and high salinity through proper design of the surface ligands, have properties that are of practical significance for conformance control in enhanced oil recovery applications.

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Worthen for useful discussion, Vu Tran and Chang Da for helping with surface tension measurement, and Chola Dandamudi for assistance with TEM images.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b01814. Surface tension of MC and HC NPs at ambient conditions with and without pH adjustment, repeatability of foam experiments with two different batches of 1% MC NPs, foam viscosities at a given foam quality of 1% HC and 1% washed HC, and viscosity measurements of DI water and 15% TDS brine at 22 °C and ambient pressure (PDF)

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ABBREVIATIONS USED C−W = carbon dioxide−water NPs = nanoparticles IFT = interfacial tension HDD = hydrodynamic diameter TDS = total dissolved salts

AUTHOR INFORMATION

Corresponding Authors

*K.P.J. e-mail: [email protected]. *D.A.DiC. e-mail: [email protected]. ORCID

Keith P. Johnston: 0000-0002-0915-1337 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported as part of the Center for Subsurface Energy Security an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0001114. K.P.J. acknowledges additional support from the Welch Foundation (KPJ, F-1319) on aspects involving nanoparticle surface chemistry. We thank Nissan Chemical America Corp. for donating silica nanoparticles for this study. The authors also thank Andrew 10689

DOI: 10.1021/acs.energyfuels.7b01814 Energy Fuels 2017, 31, 10680−10690

Article

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DOI: 10.1021/acs.energyfuels.7b01814 Energy Fuels 2017, 31, 10680−10690