Highly Stable Foam Stabilized by Alumina Nanoparticles for EOR

Aug 22, 2017 - Foam stabilized by particles has been applied in enhanced oil recovery (EOR). However, many difficulties in establishing a foam system ...
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Highly Stable Foam Stabilized by Alumina Nanoparticles for EOR: Effects of Sodium Cumenesulfonate and Electrolyte Concentrations Weipeng Yang, Tengfei Wang, and Zexia Fan* School of Petroleum Engineering, China University of Petroleum (East China), Qingdao 266580, Shandong, China ABSTRACT: Foam stabilized by particles has been applied in enhanced oil recovery (EOR). However, many difficulties in establishing a foam system with high stability, foamability, and resistance to harsh conditions still exist. In this study, foam stabilized by hydrophobic modified alumina nanoparticles was systematically studied. Sodium cumenesulfonate (SC) was used to modify nanoparticles and generate foam. A competitive effect between nanoparticles and SC was found. Specifically, foam stability increased with SC concentration at lower SC concentration range. Foam with high stability and relatively high foamability was obtained at an SC concentration of 20 mM, and then, foam stability sharply decreased because of the competitive effect. The presence of an electrolyte slowed down the adsorption of SC at the air−water interface by forming an ion pair between the headgroup of SC and the cation, which influences foamability and stability. Meanwhile, the electrolyte also caused detachment of particles from foam due to the decrease of hydrophobicity as a result of the decreased SC adsorption. Despite these effects, the particle-stabilized foam maintained high stability in a wide range of electrolyte concentrations. Sandpack flooding experiments showed that the oil recovery rate increased with SC concentration because of the enhanced foamability and stability, leading to a higher displacement efficiency and water flush resistance. ΔG = πR2gg/w (1 − |cos θ|)2

1. INTRODUCTION Foam has been widely used in many industries such as food,1,2 porous material,3−5 flotation,6,7 and enhanced oil recovery (EOR).8−11 Foam stability is one of the decisive factors that ensures successful implementation. Because of the thermodynamically unstable characteristics, foams are inclined to reduce surface energy via coalescence and disproportionation, which finally leads to collapse. Foam stability is more important for EOR due to the harsh conditions of high temperature and salinity, which accelerate the rupture of foams. Polymers have been used to enhance foam stability by increasing the viscosity and surface charge between interfaces to slow down drainage, coalescence, and disproportionation.12 However, the effect of a polymer is limited by temperature and salinity.13,14 Meanwhile, the coalescence and disproportionation cannot be totally halted by adding a polymer to the foam system. Currently, a large number of theoretical and experimental studies on particle-stabilized foams and emulsions have been conducted.15−19 Foam stabilized by particles shows prominent stability. Specifically, the foam can maintain stability for several weeks or even years, and the coalescence and disproportionation can be completely halted by the adsorption of particles at the air−water interface, which contributes to the extreme stability.1,20,21 Because of the larger size of particles compared with surfactants, the desorption energy of particles is larger than that of surfactants by several orders of magnitude, which indicates that particles are better foam stabilizers and less subjected to external disturbance.15 Meanwhile, the unabsorbed particles aggregate in liquid films and form a network structure that can also enhance foam stability.17 Particle wettability is one of the most important factors that determines whether the particle can be adsorbed tightly at the air−water interface.16,22 The desorption energy10,17 of particles ΔG can be expressed as © XXXX American Chemical Society

(1)

where R is the radius of the particle, γg/w is the gas−water surface tension, and θ is the contact angle of water on the solid surface. This indicates that if the particle is highly hydrophilic (θ < 30°) or highly hydrophobic (θ > 150°), the desorption energy is not large enough to guarantee a stable foam, and an adequate hydrophobicity is necessary to obtain a stable foam.15,17 Alargova et al.20 acquired foams with ultrastability that were stabilized by rod-like synthesized hydrophobic polymer particles without a surfactant. The volume of particle-stabilized foam remained unchanged for more than 20 days, and the addition of surfactant sodium dodecyl sulfate (SDS) made the foam unstable because of the adsorption of SDS on the particle surface, which made the surface hydrophilic. Gonzenbach et al.5 investigated the interaction between Al2O3 particles and short-chain amphiphiles (butyric acid and propyl gallate) and concluded that the short-chain amphiphiles can be adsorbed on opposite charged particle surfaces via electrostatic interaction, making the particle surface hydrophobic. These hydrophobic modified particles stabilize foam, with the bubble size almost unchanged for 4 days and no drainage observed. These studies show that particle-stabilized foam exhibits superior stability, and the stable foam has low foamability due to the absence of a surfactant. Binks et al.1 studied foam stabilized by in situ hydrophobized CaCO3 particles and a food-grade surfactant. The foam was stable for weeks, and the foam volume was not large. In many studies, surfactants with strong foamability (such as SDS) were used to hydrophobize particles. However, in this scenario, the foam Received: May 1, 2017 Revised: July 25, 2017 Published: August 22, 2017 A

DOI: 10.1021/acs.energyfuels.7b01248 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels generated from the surfactant/particle mixture was less stable compared with the foam stabilized only by hydrophobic particles or particles with a weak foamability surfactant. Meanwhile, to our knowledge, there are few studies that report foam with a very high stability and foamability and that is stabilized by particles and a strong foamability surfactant. It can be inferred that foam stability and foamability are dominated by different mechanisms, and there are conflicts between these mechanisms. Because of the high salinity of formation water and because electrolyte compositions vary between different formations, the effects of electrolyte concentration on foam stability and the interaction between particles and amphiphiles are major considerations. The effect of electrolyte concentration on the stability of foam stabilized solely by hydrophobic modified particles has been studied by some researchers.23−25 Kostakis et al.23 investigated the influence of NaCl concentration on foam stabilized by silica particles with 33% SiOR. The results showed that at high NaCl concentrations, the particles aggregate due to screening of electrostatic repulsion, and hydrophobicity was increased, which led to a more stable foam. Bournival and Ata25 demonstrated that an increase in salt concentration reduced the zeta potential of particles, which made it easier for them to be adsorbed at the air−water interface, and therefore, the foam stability was enhanced at high salt concentrations. However, the influence of an electrolyte on foam stability and the interaction between the particles and amphiphiles will be more complicated, and to our knowledge, few studies have been conducted on this topic. This study aims to investigate the effects of short chain amphiphile (sodium cumenesulfonate) concentration on foamability and the stability of foam stabilized by a mixture of alumina nanoparticles and the relationship between foamability and stability. In addition, the effects of electrolyte type and concentration on foamability and stability will be systematically investigated. The adsorption of SC onto alumina nanoparticle surfaces was studied using zeta potential and pyrene fluorescence measurements. Last, sandpack flooding experiments were conducted to evaluate the foam’s displacement and plugging ability in EOR.

Figure 1. Size distribution of the 1.0 wt % AlOOH nanoparticles used in the experiments.

Figure 2. Molecular structure of sodium cumenesulfonate.

4.42. The pH level of the AlOOH/SC dispersion was changed because of the addition of SC to the AlOOH dispersion. Therefore, HCl was used to adjust the pH of the AlOOH/SC dispersion to 4.42. 2.3. Zeta Potential and Particle Size Measurements. The zeta potential and particle size of AlOOH nanoparticles at different SC concentrations were measured using a Nano-ZS ZEN3600 (Malvern Instruments) at room temperature. The dispersion was ultrasonically treated for 10 min before measurement. Every sample was measured at least three times to minimize errors. 2.4. Contact Angle Measurement. After dispersions were prepared, they were centrifuged at 6000 rpm for 1 h to separate nanoparticles. Then nanoparticles were washed with deionized water to wash away the extra SC molecules. Nanoparticles were dried in an oven at a temperature of 80 °C for 5 h and pressed into a slice under 16 MPa. The contact angle of every sample was measured three times using Harke-SPCA (Harke Instruments). 2.5. Pyrene Fluorescence. Pyrene was dissolved in dichloromethane to prepare a solution with a pyrene concentration of 6 × 10−6 M. Then, 2 mL of pyrene solution was transferred into a beaker and dried to remove dichloromethane. Finally, 20 mL of AlOOH/SC mixture was added to the beaker and stirred for 12 h. An FLS-920 (Edinburgh Instruments) was used to measure the fluorescence spectra, and the excitation wavelength was 337 nm. 2.6. Preparation and Characterization of the Foam. The foam of the AlOOH/SC dispersion and SC solution was generated using the stirring method. A total of 100 mL of liquid was poured into a mixing cup and then stirred at 6000 rpm for 3 min. After the stirring process, the foam was immediately transferred to a cylinder to measure the foam volume, and the cylinder was sealed with parafilm to avoid evaporation and external disturbance. The timer started immediately when the foam was generated. The change of foam volume above the

2. METHODOLOGY 2.1. Materials. The AlOOH nanoparticles used in the experiments were VK-L01A, which is a 20 wt % aqueous dispersion. The size distribution of the AlOOH nanoparticles is shown in Figure 1. SC with a purity of 40% was purchased from Huntsman. Figure 2 shows the molecular structure of SC. Pyrene with a purity of 98% was purchased from Aladdin. Sodium chloride (99.5% purity), calcium chloride (96.0% purity), and dichloromethane (99.5% purity) were purchased from Sinopharm Chemical Reagent. Nitrogen (99.9% purity) was purchased from Tianyuan. Deionized water was used during the experiments. All experiments were carried out at room temperature (20 ± 2 °C) unless otherwise noted. 2.2. Preparation of AlOOH/SC Dispersion. The AlOOH/SC dispersion was prepared by mixing an AlOOH dispersion and SC solution. First, the concentrated AlOOH dispersion was added dropwise into deionized water under continuous magnetic stirring. Then, the SC solution was prepared at different concentrations by diluting the high concentration SC. Finally, 50 mL of AlOOH dispersion and SC solution were mixed at a ratio of 1:1 by adding the SC solution to the AlOOH dispersion dropwise under continuous magnetic stirring to avoid strong flocculation. The stirring was maintained for 12 h to achieve adsorption equilibrium. On the basis of our previous studies, the nanoparticle concentration was fixed at 1 wt %. The pH level of the 1 wt % AlOOH nanoparticle dispersion was B

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Energy & Fuels liquid phase over time was recorded to evaluate foam stability, and the relative foam volume VR can be defined as VR =

Vt V0

(2)

where V0 is the initial foam volume minus the liquid phase volume (mL) and V(t) is the foam volume above the liquid at time t (mL). Considering that the SC foam is highly unstable, the half-life was used to assess its foam stability. 2.7. Accumulation of AlOOH Nanoparticles in the Foam. The AlOOH/SC dispersion with a nanoparticle concentration of 1 wt % and different SC concentrations was used to generate foam. After the foam drainage process finished, the foam above the liquid phase was removed, and the particles in the liquid were centrifuged and washed to separate the particles. Then, the particles were dried in an oven at 85 °C, and after drying, the weight of the particles was measured. The accumulation percentage ( f) and accumulation density (ρ) can be defined as follows:

w − wr f= 0 w0

(3)

w0 − wr V0

(4)

ρ=

Figure 3. Zeta potential and contact angle of AlOOH nanoparticles for different SC concentrations.

where w0 and wr are the weights of the AlOOH nanoparticles initially and after the accumulation in foam, respectively. 2.8. Sandpack Flooding Experiments. Sandpack flooding was carried out to evaluate the plugging and displacement abilities of the foam. Sandpacks were filled with a single type of quartz sand. The permeability (2.5 ± 0.2 D) and porosity (36.1 ± 0.2%) were nearly equal for the flooding experiments. First, brine water (300 mM NaCl) was injected at a rate of 1.0 mL/min to saturate the sandpack. After the saturation of water, crude oil was injected into the sandpack at a rate of 0.5 mL/min until no water was produced. The back pressure was set to 6 mPa during the flooding experiments, and all of the experiments were conducted at 60 °C. Then, water flooding was carried out until no oil was produced, and the oil volume was recorded. The AlOOH/ SC dispersion was injected at a rate of 0.4 mL/min, and the nitrogen was at a rate of 0.6 mL/min to carry out foam flooding. The foams was injected for 1 pore volume (PV). Finally, subsequent water flooding was conducted at a rate of 1.0 mL/min until no oil was produced. Pressure and oil production were recorded in the experiment.

Figure 4. Photograph of the AlOOH/SC dispersion after 24 h. The AlOOH nanoparticle concentration is 1 wt %, and the SC concentrations (mM) are noted in the photograph.

concentration is lower than 3 mM, the AlOOH/SC dispersions were clear, which indicates that the colloid remained stable. With the increase of the SC concentration, the dispersion became turbid, and phase separation occurred, which indicated the flocculation of nanoparticles. The zeta potential is the key factor that determines the stability of a colloidal dispersion. The adsorption of SC led to a reduction of the zeta potential, and flocculation occurred when the electrostatic repulsion between nanoparticles was lower than the van der Waals attraction.18 When the SC concentration was higher than 30 mM, nanoparticles strongly flocculated, and no sign of redispersion was observed. The phenomenon is different from the interactions between a long chain surfactant and oppositely charged particles, which always results in the reversal of the zeta potential and redispersion of particles.18,29,30 The free energy of adsorption27,31 (ΔGads ° ) of an amphiphile on a charged surface can be represented as

3. RESULTS AND DISCUSSION 3.1. Interaction between the AlOOH Nanoparticles and SC. The AlOOH nanoparticles are strongly positively charged at pH = 4.42 because of protonation of the large number of hydroxyls (Al−OH).26 The short chain amphiphile SC is negatively charged due to the sulfonic group (−SO3−). Thus, it can be adsorbed onto the positively charged nanoparticle surface via electrostatic attraction.27,28 To determine the interaction between the AlOOH nanoparticles and the oppositely charged amphiphile SC, the zeta potential of the nanoparticles was measured, as shown in Figure 3. Bare AlOOH nanoparticles are highly positively charged with a zeta potential of +55.36, which makes the colloidal particles stable in the absence of SC. The zeta potential of nanoparticles decreased rapidly in the low concentration range (SC < 10 mM), and the dispersion gradually became turbid with the increase of the SC concentration. With a further increase of the SC concentration, the zeta potential of the nanoparticles decreased slower than at low SC concentrations and nearly reached a constant value when the SC concentration was higher than 75 mM. Photographs of the AlOOH/SC dispersions for different SC concentrations after 24 h are shown in Figure 4. From the figure, we can observe that when the SC

° = ΔGelec ° + ΔGspec ° ΔGads

(5)

° = ΔGcolu ° + ΔGdip ° ΔGelec

(6)

° = ΔGchem ° + ΔGc°− c + ΔGc°− s + ΔG H° + ΔG H° 2O ... ΔGspec (7)

where ΔG°elec is the electrical interaction free energy (attraction of repulsion) between the amphiphile and the particle. ΔG°colu and ΔGdip ° are the free energy of the columbic interaction and dipole interaction, respectively. ΔGspec ° represents the specific C

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Energy & Fuels adsorption free energy, which contains all other interactions beside the electrical interaction. ΔG°chem is the free energy of covalent bonding. ΔG°c−c is the free energy of the hydrophobic interaction between the hydrocarbon chains of adsorbed amphiphiles. ΔGc−s ° is the free energy of the hydrophobic interaction between the hydrocarbon chains and particle hydrophobic sites. ΔG°H is the free energy of hydrogen bonding, and ΔG°H2O is the free energy due to solvation and hydration. It can be found in eq 5 that the adsorption free energy of an oppositely charged amphiphile consists of electrical attraction and specific attraction. For an anionic amphiphile, there is no significant difference in electrical attraction. ΔGspec ° , ΔGc−c ° , and ΔGc−s ° are the dominant factors during the adsorption of the amphiphile. The free energy of adsorption of SC on AlOOH is relatively low because the hydrocarbon chain length of SC is much shorter than that of a normal surfactant, which leads to a low ΔGspec ° . As reported before,32−34 the hydrophobic interaction between the adsorbed surfactants and monomers contributes to the rapid adsorption of surfactants, which leads to the rapid decrease and reversal of the zeta potential. Because of the weak hydrophobic interaction between the SC molecules, the saturation adsorption was reached before the surface charge neutralization, and no second adsorption layer was formed.35,36 Therefore, the zeta potential of AlOOH is positive at all SC concentrations, and the hydrophobicity of nanoparticles increased due to the hydrophilic group to surface and hydrocarbon chain to water adsorption monolayer. The particle hydrophobicity showed no reduction in the high SC concentration range because of the absence of the second adsorption layer, which adopts a hydrophilic group to water orientation. This can explain why there is no redispersion of nanoparticles at high SC concentrations. Meanwhile, the contact angle measurement shown in Figure 3 also indicates the single adsorption layer. The contact angle increased with SC concentration and reached a plateau without decline, which supported the single layer adsorption. Pyrene was used as a fluorescent probe to investigate the change of hydrophobicity of the AlOOH/SC dispersion and SC solution. Because the solubility of pyrene is very low in pure water, the intensity ratio of the first and third bands (I1/I3) of pyrene in the spectrum is very sensitive to the polarity of the environment, and the ratio decreases with polarization.37,38 The I1/I3 ratio decreases rapidly once the SC monomers form hemimicelles on the nanoparticle surface or micelles in solution that can solubilize pyrene. Figure 5 shows the value of I1/I3 versus the SC concentration. The I1/I3 ratio of the SC solution remained approximately unchanged before the rapid reduction, which indicates the formation of micelles. This means that hydrophobic areas do not exist in the SC solution when the SC concentration is lower than the critical micelle concentration (cmc). For the AlOOH/SC dispersion, the decrease of I1/I3 can be detected even in the low SC concentration range. The abrupt decrease of I1/I3 occurred when the SC concentration was between 3 mM and 5 mM, which indicated the formation of hemimicelles on the particle surface. Meanwhile, in Figure 3, we observe that the zeta potentials of AlOOH nanoparticles for the SC concentrations of 3 mM and 5 mM are +46.2 mV and +44.43 mV, respectively. If we only take the zeta potential into account, the little reduction in the zeta potential should not cause an obvious flocculation, as shown in Figure 5. If the hydrophobic interaction between nanoparticles due to the

Figure 5. Fluorescence intensity ratio I1/I3 at different SC concentrations.

formation of hemimicelles is considered, the abrupt flocculation can be successfully explained. The continuous decrease of I1/I3 means that hemimicelles formed continuously with the increase of the SC concentration. The change of I1/I3 acted in concert with the variation of the zeta potential. 3.2. Characterization of the Foam in the Absence of an Electrolyte. AlOOH/SC dispersions containing 1 wt % nanoparticles with SC concentrations of 0 to 30 mM were used to generate foam. The foam was generated via shaking and stirring, and Figure 6 shows photographs of the foam generated

Figure 6. Photographs of foam generated via shaking for different periods of time. The SC concentrations are labeled in the photographs.

by shaking the bottle for 10 s. It can be seen that the foam was easily generated by shaking, which means good foamability of AlOOH/SC dispersions. The foam showed good stability when the SC concentration was between 5 and 20 mM. Foamability and foam stability of the foam generated by stirring are shown in Figure 7. It can be found in Figure 7a that the foamability and foam stability of SC were very weak because its short hydrocarbon chain length cannot form a compact adsorption layer at the air−water interface. The foam was generated during stirring but ruptured very quickly, leading to a very low foamability. Foamability increased when the SC concentration was higher than 15 mM, but the foam collapsed quickly at all SC concentrations. However, foamability increased significantly D

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Figure 7. (a) Foam volume of the AlOOH/SC dispersion and half-life of SC foam versus SC concentration. (b) Relative foam volume of the AlOOH/SC dispersion for different SC concentrations versus time.

Figure 8. Schematic of two different bubble coalescence processes depending on the surface particle coverage. (Γ and Γ* are the particle adsorption amount and threshold particle adsorption amount, respectively.)

that SC molecules contributed to foamability, and nanoparticles contributed to foam stability. As shown in Figure 7b, the foam stabilized with hydrophobic modified AlOOH nanoparticles with SC concentrations of 5 to 20 mM remained stable for more than 7 days. The most stable foam can be obtained when the SC concentration was 20 mmol/L, and the small decrease of foam volume was due to evaporation in the cylinder. In the SC concentration range of 0 to 20 mM, foam stability increased with SC concentration. Both good foamability and foam stability can be obtained at the SC concentration of 20 mM. The same change pattern can be found in former studies,36 and this pattern occurs because the hydrophobicity of nanoparticles increases with SC concentration.41 However, foam stability drastically decreased at the SC concentration of 30 mM, but foam stability was supposed to keep increasing. In general, in most cases, the decrease of foam stability at high surfactant concentrations was caused by the hydrophilization of particles due to the second adsorption layer

by adding 1 wt % AlOOH nanoparticles to the SC solution, and the foam volume increased steadily with SC concentration. By referring to eq 1, it can be found that the desorption energy of nanoparticles is proportional to the contact angle, and the contact angle of the nanoparticles increases with SC concentration because of the formation of an SC monolayer on the particle surface. The adsorption of hydrophobic modified nanoparticles at the air−water interface enhanced the foam stability. Meanwhile, according to a previous study, SC molecules adsorbed much quicker to the interface than to nanoparticles because of kinetic reasons.39,40 The quick rupture of the SC foam led to the small increase of foam volume with SC concentration. However, the hydrophobic modified nanoparticles attached to the newly formed air−water interface prevented the rupture of foam. Thus, the foam volume increased with SC concentration because more SC molecules attached to the air−water interface, and then, the adsorption of nanoparticles stabilized the foam. Therefore, it is concluded E

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Energy & Fuels of the surfactant.18,20 However, as we have demonstrated before, the hydrophobicity of AlOOH kept increasing with SC concentration because the hydrophobic interaction between SC molecules is too weak to form the second adsorption layer. Thus, the desorption energy theory is not applicable in this case. Research on competitive adsorption of surfactants and particles at interfaces has been conducted by some researchers. Pichot et al.42 showed that both silica particles and surfactants can be adsorbed at the interface at a low surfactant concentration. At a high surfactant concentration, the adsorption of silica particles at the interface is restrained by the surfactant, which can be adsorbed faster than the silica particles. The fast collapse of foam at the SC concentration of 30 mM was controlled by the same mechanism. Because of the weak stability of the foam stabilized by SC, the foam stabilized by AlOOH/SC at the SC concentration of 30 Mm collapsed very quickly. Actually, even at the low SC concentrations, some foam bubbles were covered mainly by SC because the rupture of foams could be observed by the naked eye after the foam was generated, but the rupture of the foam ceased after several minutes. Combining the study of Pichot et al.42 with this study, we conclude that the foam surface was covered with AlOOH nanoparticles and SC at all SC concentrations. However, the surface area covered by nanoparticles decreased with the increase of SC concentration. Additionally, studies40,43 showed that bubbles with low surface coverage adopted coalescence to increase the coverage by particles, which enhances the stability of foam. Thus, this theory can explain the ceasing of the rupture sound in minutes. Furthermore, it can be inferred that there is a threshold surface particle coverage. If the surface particle coverage is higher than the threshold value, a stable foam can be obtained after the coalescence. If the surface particle coverage is lower than the threshold, the coalescence process will not stop, which leads to foam collapse. Figure 8 presents the relationship between the coalescence process and surface coverage. Meanwhile, the increase of SC concentration led to a larger particle aggregate size, as shown in Figure 9. Former studies25,44 reported that it is more difficult to generate foam from a dispersion without a surfactant, which means that the attachment of particles at the air−water interface is difficult.

The energy barrier between the particles and interface can be presented as F ≈ 2πR

∫h

∞

π (h) dh

(8)

where R is the particle radius, π(h) is the disjoining pressure, and h is the distance between the particles and interface. Equation 8 reveals that the energy barrier between the particles and interface increases with particle size, which makes it more difficult and requires more time for larger particles to attach to the interface.45 Thus, we can summarize the effects of a high concentration of SC on foam stability: (1) a higher concentration of SC leads to a higher surface SC coverage, which makes foam unstable, and (2) a higher concentration of SC results in a larger particle aggregate size, which further reduces surface particle coverage. These two factors result in an unstable foam at the SC concentration of 30 mM. Because surfactants (such as SDS) have higher foamability than the short chain amphiphile SC, which results in a lower surface particle coverage compared with the short chain amphiphile, the stability of foam stabilized by a particle/surfactant mixture is usually lower than that of foam stabilized by particles/short chain amphiphiles. Because the adsorption of nanoparticles on the foam surface is necessary for stabilizing foam, the accumulation of particles in foam was measured, as shown in Figure 10. It can be seen that

Figure 10. Particle accumulation amount and accumulation density versus SC concentration.

the particle accumulation amount increased with SC concentration because of the hydrophobization of particles via adsorbing SC.46 Almost all particles accumulated in foam after the SC concentration was higher than 20 mM. However, the foam was highly unstable at the SC concentration of 30 mM. This indicated that these particles only accumulated in foam but did not adsorb on the foam surface, which correlates with the discussion about the surface particle coverage. Therefore, the use of the particle accumulation amount to reflect the relationship between the foam stability and adsorption of particles has limitations because the accumulation of particles in foam does not mean adsorption. The foam’s ability to maintain a good stability in the presence of oil is an important factor which determines the success of displacement process. In this experiment, kerosene was used to test the stability of particle-stabilized foams at the nanoparticle concentration of 1 wt % and SC concentration of

Figure 9. Change of the AlOOH particle diameter versus SC concentration. F

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carried out at the SC concentration of 30 mM. The effects of CaCl2 concentration on the foamability and foam stability of the AlOOH/SC dispersion at the SC concentration of 30 mM are shown in Figure 12a. For SC, foamability decreased drastically with the increase of the CaCl2 concentration, and no foam could be generated when the CaCl2 concentration was higher than 10 mM. Meanwhile, the foamability of the AlOOH/SC dispersion also decreased at a low CaCl 2 concentration but then increased at a higher CaCl 2 concentration. The decrease in the foamability of the AlOOH/SC dispersion and SC solution at a low CaCl2 concentration was attributed to the formation of the Ca2+-SC headgroup binding and SC aggregates, which was detrimental to foamability and foam stability.47,48 Foamability is strongly affected by foam viscosity, because the formation of a new air− water interface needs to overcome viscous resistance, then foams with a higher viscosity usually have a lower volume.49 Therefore, the reduction in foam viscosity caused by the increase in CaCl2 leads to increased foamability. As previously stated, the competitive adsorption between the nanoparticles and SC resulted in an unstable foam at the SC concentration of 30 mM. Because of the strong binding effect of Ca2+ on the foamability and adsorption rate of SC due to the formation of an ion pair,50 foam stability increased rapidly when the CaCl2 concentration was higher than 5 mM. Meanwhile, the surface particle coverage increased with an increase in CaCl 2 concentration and surpassed the threshold surface coverage. Figure 12b shows the effect of NaCl concentration on the foamability and foam stability of the AlOOH/SC dispersion at the SC concentration of 30 Mm. As we can see, unlike CaCl2, the foamability of SC was not significantly affected by the NaCl concentration because of the much weaker binding ability of the monovalent ion compared to the divalent ion.51 Thus, there is no significant effect of NaCl concentration on the foamability of the AlOOH/SC dispersion. Furthermore, the minimum NaCl concentration when the rapid increase in foam stability occurred was higher than that of CaCl2. This was attributed to the weaker interaction between Na+ and the headgroup of SC. A higher concentration of NaCl was needed to slow down the adsorption rate of the SC molecule. At the NaCl concentration of 100 mM, there was still a clear decrease in foam volume after

20 mM, and foams with an SDS concentration of 10 mM were used to make a comparison. Kerosene was slightly added to the top of foams, and the initial time was recorded. Figure 11 shows

Figure 11. Changes of relative foam volume as a function of time in the presence of oil.

the change of relative foam volume over time. It can be seen that foams stabilized by SDS showed poor stability when contacted with oil, and all the foams disappeared within 4 h. Whereas foams stabilized by nanoparticles showed much better stability, about 20% of foams remained after 24 h. There are few studies about the mechanism of how particles enhance the foam stability in the presence of oil. The higher stability may attributed to the formation of particle network in foams, which slowed down the spread of oil. Moreover, the high desorption energy of the particle made it more difficult for oil to penetrate into the air−water interface, which led to foam rupture. 3.3. Characterization of Foam in the Presence of an Electrolyte. Considering that the composition of water from different reservoirs varies, CaCl2 and NaCl were added to the AlOOH/SC dispersion to study the effects of an electrolyte on foam properties. To make the phenomenon more obvious, characterization of the influence of the electrolyte on foam was

Figure 12. Effects of electrolyte concentration on foamability and foam stability. G

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charged AlOOH particles was much weaker. Thus, the particle hydrophobicity decreased less significantly, and only a few particles detached. The effects of an electrolyte on the change in particle hydrophobicity are presented in Figure 15. In summary, the presence of electrolyte influences foam properties mainly in two ways: (1) the binding effect slows down the adsorption of SC on the air−water interface, and (2) the electrolyte reduces the adsorption amount of SC on the nanoparticle surface, making it less hydrophobic. To confirm the effect of an electrolyte on foam, the CaCl2 concentration was fixed at 100 mM, whereas the SC concentration was changed. If these conclusions are right, foam stability will decrease with the increase of SC concentration, which makes the adsorption of SC on the interface faster, and foamability will also decreases because the increased particle hydrophobicity leads to a higher foam viscosity, which limits foamability. As shown in Figure 16, the foam volume gradually decreased with the increase in SC concentration, and foam stability decreased rapidly when the SC concentration was higher than 75 mM. The decrease of foam volume was attributed to the increased foam viscosity, which may be the result of increased particle hydrophobicity. Even if there is Ca2+ in the dispersion, more SC was adsorbed on AlOOH nanoparticles with the increase in SC concentration, which increased the particle hydrophobicity. The foam remained stable until the SC concentration was higher than 75 mM, which indicated a reduction in surface particle coverage. The increase in SC concentration led to more free SC monomers that adsorbed fast to the interface, which reduced the surface particle coverage.50 3.4. Sandpack Flooding Experiments. The differential pressure changes during the flooding process is shown in Figure 17. The differential pressure increased when water was injected, owing to the production of oil. Then the differential pressure reduced and stayed constant because the production of oil was ceased. The differential pressure increased rapidly when foams were injected, and foams with the SC concentration of 20 mM had the better blocking ability because they are more stable and viscous. When the subsequent water flooding was conducted, the differential pressure reduced because foams were produced and ruptured. But even after the injection of 4 PV water, the differential pressure of foam with an SC concentration of 20 mM was still higher than that of water flooding, which indicated a better foam stability. The oil recovery of sandpack flooding is shown in Figure 18. The oil recovery of water flooding was similar because of the nearly identical permeability of the sandpack.53 During foam flooding, the oil recovery increased with SC concentration because the foamability and foam stability were enhanced. Therefore, foams would not rupture easily, causing the crossflow of gas, which led to higher displacement and sweep efficiencies.54 Meanwhile, the increase in particle adsorption on the foam surface also enhanced the foam displacement efficiency by rendering foam with a higher ability to resist deformation.54 The difference between the oil recovery of subsequent water flooding was more significant because the foam with a higher SC concentration showed better stability and had better plugging ability. It can be seen in Figure 10 that only 36.49% and 73.23% of nanoparticles were accumulated in the foam, and foam with a lower particle accumulation amount more easily ruptures, especially under harsh conditions.55 Foam with higher stability and viscosity showed better resistance to water flushing, so foams migrated into an untouched area and

72 h. Figure 13 shows the photograph of the AlOOH/SC foam at the SC concentration of 30 mM after 72 h. As we can see, the

Figure 13. Photograph of AlOOH/SC foam at the SC concentration of 30 mM and at different NaCl concentrations after 72 h.

foam texture at the NaCl concentration of 100 mM was coarser than the foam texture at the NaCl concentration of 200 mM. This confirmed the conclusion that foam bubbles adopted coalescence to increase surface particle coverage. Because the surface particle coverage was lower at the NaCl concentration of 100 mM, more coalescence occurred, which led to a coarser texture and an obvious decrease in foam volume. A reduction in the particle accumulation amount was observed with the increase in electrolyte concentration, as shown in Figure 14. The particle accumulation amount rapidly

Figure 14. Influence of electrolyte concentration on the particle accumulation amount.

decreased with the increase in CaCl2, and only 58.94% of particles accumulated in foam at the CaCl2 concentration of 200 mM. For NaCl, the detachment of particles was less significant compared with CaCl2, and more than 90% of particles accumulated in foam at the NaCl concentration of 600 mM. Because the interaction between Ca2+ and negatively charged SC molecules is strong, Ca2+ competes with positively charged AlOOH particles to absorb SC molecules, which causes a reduction in the SC adsorption amount.52 Meanwhile, the increase in the Cl− concentration reduced the zeta potential of particles, which made the interaction between the particles and the SC molecule weaker. These two effects worked together to reduce the adsorption of SC on particles. The reduction of the adsorption amount of SC led to decreased particle hydrophobicity, which caused the retention of particles in the liquid phase. The competitive effect between Na+ and the positively H

DOI: 10.1021/acs.energyfuels.7b01248 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 15. Schematic representation of the effects of an electrolyte on the change of particle hydrophobicity.

Figure 16. Effect of the SC concentration on foamability and foam stability at the CaCl2 concentration of 100 mM.

Figure 18. Oil recovery as a function of the injected volume.

blocked the water channel when the water was injected, and then the oil was swept by foams and subsequent water. If the foams are not stable enough, these areas cannot be swept, and the oil recovery will be lower.

4. CONCLUSION Foam stabilized by AlOOH nanoparticle synergy with SC and the effects of electrolyte concentration were studied in this paper. The AlOOH nanoparticles were hydrophobic and modified by adsorbing SC onto the surface and then attached to the foam surface, enhancing foam stability by slowing down coalescence, disproportionation, and drainage. In the AlOOH/ SC dispersion, foamability was mainly contributed by SC, and foam stability was dominated by the adsorption of nanoparticles. However, the competitive adsorption between SC and nanoparticles was significant at high SC concentrations. Specifically, the faster adsorption of SC at the air−water interface inhibited the adsorption of nanoparticles, which led to the sharp decrease in foam stability. The presence of an electrolyte, especially CaCl2, greatly affected the adsorption of SC on nanoparticles by binding with the headgroup of SC,

Figure 17. Differential pressure changes as a function of the injected volume.

I

DOI: 10.1021/acs.energyfuels.7b01248 Energy Fuels XXXX, XXX, XXX−XXX

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which influences foamability and foam stability. The effect of NaCl on foamability and foam stability was less significant, and fewer particles were detached from foam compared with CaCl2. Despite the detachment of particles, foam stabilized by AlOOH nanoparticles showed high stability in a wide electrolyte concentration range, which guaranteed good performance under reservoir conditions. Sandpack flooding experiments showed that foam stabilized by AlOOH nanoparticles at a SC concentration of 20 mM effectively improved the displacement and sweep efficiencies. The oil recovery was positively correlated with foamability and stability.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Weipeng Yang: 0000-0002-9920-581X Notes

The authors declare no competing financial interest.

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DOI: 10.1021/acs.energyfuels.7b01248 Energy Fuels XXXX, XXX, XXX−XXX