The Effect of Fly Ash Nanoparticles on Foam Stability for CO₂ Flooding ^†

Abstract

Foam–CO₂ EOR Flooding is not very successful if unaccompanied by foam stabilizers such as nanoparticle fly ash (NFA). This study was conducted to determine the effect of NFA on foam stability by considering particle characteristics using the Bulk Foam Method as an additional condition. The test resulted in half-life times, which showed that in the absence of NFA, when oil was added, it was 211.5 s, and in salinity conditions, it was 232.5 s. This succeeds in improving half-life times to 226 s (with oil) and 241.5 s (with salinity) by adding NFA-Type F. For further research, conducting tests using reservoir conditions is recommended.

1. Introduction

Naturally, only 40–50% of crude oil can be extracted from reservoirs after primary and secondary oil recovery stages [1]. Enhanced oil recovery (EOR) offers a great solution to assist in the process of extracting additional crude oil, which is trapped in a reservoir [2]. One common method in EOR is CO₂ injection, which improves the miscibility of oil and is thus more easily produced. It improves the oil recovery factor by up to 25% over the primary and secondary recovery and can be achieved via CO₂ injection [3]. In addition, CO₂ flooding supports Net Zero Emission (NZE) 2050 as part of limiting the release of greenhouse gas emissions [4]. Based on the data of the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2021, the gas that most contributes is CO₂ at 79.4%, followed by methane at 11.5%, N₂O at 6.2%, and others at 3% [5]. Thus, CO₂ injection, referring to the implementation of Carbon Capture, Utilization, and Storage (CCUS) is a great solution to limit the effect of global warming and climate change [4].

The application of CO₂ injection in heterogeneous reservoirs has a high propensity of encountering problems related to unfavorable mobility due to the properties of pure CO₂, leading to an ineffective displacement process [6], as shown in Figure 1. The physical properties of CO₂, such as its lower density and viscosity compared to reservoir fluids, cause gravity override, viscous fingering, and early CO₂ breakthrough [7]. The gas tends to flow preferentially in high-permeability streaks and may bypass the reservoir fluids in low-permeability regions, resulting in poor sweep efficiency [8]. To overcome these challenges, foam has been proposed, which reduces gas mobility in high-permeability zones relative to low-permeability zones, thus achieving a uniform displacement front [9].

One of the challenges in foam applications is foam stability. Lamellae, as part of the foam, are thermodynamically unstable and easily break. Generally, the stability of foam is influenced by the surfactant as the foaming agent [10]. However, the application of a surfactant is expensive and has a high adsorption rate to the rock surface [11]. Thus, in recent years, nanoparticles have become potentially applicable, especially for conventional good-quality (<5 mD) and unconventional or low-permeability (<0.1 mD) oil reservoirs [12]. One potential nanoparticle to replace the expensive foam booster is fly ash [13]. Fly ash is a coal power plant by-product with excessive quantities, up to 500 million tonnes across the world annually, of which only 25 to 30% is re-utilized in several sectors [14]. The main compound of coal fly ash is SiO₂ [15], which is as the most effective foam-stabilizer additive [16]. In this work, the fly ash particle size was reduced to nanoparticles and characterized to improve its effectiveness in stabilizing foam. The aims of this research were to investigate the effect of nano fly ash on foam stability, considering the compounds of the nanoparticles, using static tests where salinity and the presence of oil were observed.

2. Materials and Methods

2.1. Nanoparticle Characterizations

Nanoparticles (NPs) can be characterized by their size and shape, as well as their size distribution, degree of aggregation, and surface area, which influences NP applications. X-Ray Diffraction (XRD) is one of the widely used techniques to characterize NPs. This technique provides chemical information associated with the crystal structure, such as the degree of crystallinity. For this research, XRD was used to investigate the particle compositions and crystal structures of fly ash before and after a mechanical activation process, which reduces the size of the nanoparticles. The results were determined by comparing the position and intensity of the peak with the reference pattern data, which are available from the International Centre for Diffraction Data (ICDD). However, this technique is not suitable for amorphous materials and it exceeds the XRD peak for particle sizes under 3 mm [17]. The other technique is Particle Size Analysis (PSA), which measures the distributions of particle sizes with equivalent spherical diameters (D₁₀, D₅₀, D₉₀) for both liquid and powder samples [18]. Using this method, the results are not influenced by the flow behavior and can be obtained rapidly. In addition, the particle shape cannot be investigated through PSA. For this research, there were two types of nano fly ash (NFA) that were observed, which are Type F and Type C. This classification is based on chemical compounds such as SiO₂, which is the main compound used to stabilize foam.

2.2. Foam Stability Test

Bulk foam is a method used in this research to test foam stability. This method was conducted with an ambient temperature and pressure using a graduated cylinder to investigate the effect of adding nano fly ash (NFA) on the stability of foam. Alpha Olefin Sulfonate, as an anionic surfactant, which is applied as a foaming agent, was dissolved in brine at a constant concentration of 0.7 wt%. Additional variables, which were the type of fly ash, light oil, and salinity (NaCl), were also observed. The constant concentration of each variable was 1 wt% for NFA, 5 wt% for light oil, and 2 wt% for NaCl.

The foam stability test was conducted using a 250 mL glass graduated cylinder and several other components, as shown in Figure 1. An aerator as porous media, to generate uniform bubble foam, was connected to a CO₂ gas tank through a PVC pipe. The flow rate of the CO₂ gas was set constant at 0.2 L/min using a flow meter. CO₂ gas was injected for 1 min, and then, the foam started to generate. After that, the column height of the foam was recorded. At that point, the time at which the foam had decayed to half the initial volume was recorded as the half-life times.

3. Results

3.1. X-Ray Diffraction (XRD)

The particle compositions of Type-C and Type-F NFA were investigated via X-Ray Diffraction or XRD, as shown in

Table 1. Comparing compounds of fly ash before and after the mechanical activation process.

Compounds	Type-C		Type-F
	Fly Ash	Nano Fly Ash	Fly Ash	Nano Fly Ash
SiO2	50%	9.2%	93.80%	59.70%
3CaO·Al2O3	46.1%	-	4.20%	-
Fe	3.9%	-	-	40.30%
Al2O3	-	29.4%	-	-
Ca3Al2O6	-	43.7%	-	-
Fe2O3	-	17.7%	2%	-

. For Type-C NFA, before the mechanical activation process, the highest mineral content was Quartz or SiO₂ at 50%. After reducing the size of the nanoparticles, the content changed and had additional compounds. The Quartz content decreased to 9.2%, and the other compound with the highest content was Calcium Cyclo-Hexaaluminate (Ca₃A_l2O₆) at 43.7%. The other result of XRD was Type-F NFA, which was similar to Type C, in that before the mechanical activation process, the highest content was Quartz or SiO₂ at 93.8%. Then, the compounds changed after the mechanical activation process, of which Quartz (SiO₂) was 59.7%.

3.2. Particle Size Analysis

The result of the Particle Size Analysis (PSA) for this research was not only the size of the particles but also the Polydispersity Index (PI), which analyzes the particle size distribution, as shown in

Table 2. Particle Size Analysis result.

Parameter	Fly Ash		Nano Fly Ash
	Type-C	Type-F	Type-C	Type-F
Particle Size	1923.7	4279.2	409.8	640.8
Polydispersity Index	0.661	0.74	0.293	0.304

. Before the mechanical activation process, the particle size of Type-C NFA was 1923.7 nm or 1.9 µm, and Type F was 4279.2 nm or 4.3 µm. Then, after that process, the size of the particles was successfully decreased, for which Type-C NFA was 409.8 nm and Type F was 640.8 nm.

3.3. Bulk Foam Test

After investigating the nano fly ash (NFA) characteristics, the bulk foam test was conducted, for which the result is shown in Figure 2 and listed in

Table 3. Foam stability test results.

Label	Compounds	Half-Life Time (s)	Foam Height (mL)
A	AOS	247	232
B1	AOS + NFA Type-F	252.5	230
B2	AOS + NFA Type-C	227	240
C1	AOS + Oil	212.5	238
C2	AOS + Oil + NFA Type-F	226	219
D1	AOS + NaCl	232.5	234
D2	AOS + NaCl + NFA Type-F	241.5	222

. The first experiment was to screen how effective the two types of NFAs were in stabilizing the foam. Sample B1 and B2 interpreted the application of Type-F and Type-C NFA, respectively. From the half-life times, the conclusion was that Type-F NFA has a longer time to decay, rather than Type C. This means that the foam was more stable using Type F as a stabilizer. Afterwards, this research applied additional conditions, which were used to investigate the effect of adding oil and salinity. The result showed that the stability of foam was influenced by oil and salinity, which was interpreted by Sample A, C1, and D1. Thus, Type-F NFA, as the most effective stabilizer foam, was applied. It successfully increased the half-life times, which is shown by comparing Sample C1 and C2 as the effect of adding oil, while Sample D1 and D2 was the effect of increasing the salinity.

4. Discussion

4.1. Nanoparticle Characteristizations

The mechanical activation process of fly ash was successful in reducing the particle size of nanoparticles, which was shown in the result of the particle characterization. Using X-Ray Diffraction (XRD), compounds of fly ash changed along with the particle size decreasing. This indicates that the crystal structure was changed to become amorphous. The other compounds that occurred were influenced by many factors. The possibility of the factors was due to new crystal structure formation after the mechanical activation process [19] or there were other contaminants during the process.

After the mechanical activation process, fly ash turned into nanoparticles, which were 409.8 nm for Type C and 640.8 nm for Type F. Particle Size Analysis (PSA) resulted in other parameters besides the particles size, which including the Polydispersity Index (PI). The result of the PI for both nano fly ash (NFA) types was below 0.5, which were 0.293 (Type-C) and 0.304 (Type-F). This indicates that the NFA size was quite uniform as the particle size distribution was tight because the PI value was less than 0.5 [20].

4.2. Nano Fly Ash Effect on Foam Stability

Type-F nano fly ash (NFA) successfully became a foam stabilizer, which improved the half-life time from 247 to 252.5 s. This was caused by rich silica compounds that are able to adsorb into the gas–liquid interface and form foam structures that are more stable, as shown in the schematic in Figure 3. However, the application of NFA at excessive concentrations can cause aggregation and precipitation in the foam film interface that causes damage to foam stability [21]. Besides the NFA concentrations, its type also influences foam stability due to NFA compounds. The result showed that Type-C NFA decreased the half-life time to 227 s. The content of Ca2+ in the Type-C NFA may cause precipitation during the flooding process, which will decrease the concentration of surfactant on the lamella [22]. In addition, due to the high content of CaO, Type-C NPs have cementitious characteristics, which may occur as coagulation if it contacts water [23]. Thus, for a further procedure, this research continued to use Type-F fly ash, which has a high content of SiO₂, which may increase the foam strength and interface film viscosity to improve foam stability.

4.2.1. Foam Stability in Light Oil Presence

The presence of light oil influenced the foam stability, which decreased the half-life time from 247 to 212.5 s. This occurred due to many factors. Oil emulsion droplets, which have smaller particles rather than the lamella, may break the gas–liquid interface. This is because the oil droplets spread out on the lamella surface and then force the liquid to discharge from the film to plateau borders, which causes a thinner film and breakage of the foam. If the oil droplets do not spread out, they will coalesce creating a lens shape on the gas–liquid interface, which breaks the bubble film if it gets into the lamella [24]. By adding Type-F NFA as nanoparticles, it adsorbed on the gas–liquid interface, which charged the foam surface. The bubbles, which have the same charge, repel each other, thus creating the lamellae thickener [25]. Thus, the foam film was more stable when Type-F NFA was applied, which improved the half-life time from 212.5 to 226 s.

4.2.2. Foam Stability in Salinity Conditions

Foam is influenced by salinity, as shown in Figure 4. In this research, salinity was increased by adding 2 wt% NaCl to investigate the foam stability, which resulted in a decrease in the half-life time from 247 to 232.5 s. In the absence of Type-F NFA, salinity reduced the foam stability because of the destruction of electrostatic forces between lamellae. This occurred because the disjoining pressure is reduced, which results in decreasing foam stability [26]. Additionally, when Type-F NFA was added, the foam stability was not improved significantly, and it was not as stable as in the absence of salinity. It only improved from 232.5 to 241.5 s. The presence of high salinity in nanoparticle applications may increase the Van der Waals forces and decrease repulsive electrostatic forces of nanoparticles. Thus, it decreases the zeta potential, which accelerates the aggregation rate of nanoparticles [27].

5. Conclusions

This study conducted tests for applying fly ash as nanoparticles to stabilize foam. Through a laboratory experiment, it was concluded that nano fly ash is suitable for foam stabilizers for both additional conditions of the presence of oil and salinity.

• 1 wt% Type-F nano fly ash (NFA) stabilized the foam, which was observed by increasing the half-life time, which has been shown in Sample B1, C2, and D2.
• Type-C NFA was not suitable as a foam stabilizer due to the cementitious characteristics because it contains high CaO, which decreased the half-life time of the foam.
• The presence of light oil caused foam film rupture, which could be prevented by adding Type-F NFA as a foam stabilizer.
• Adding 2 wt% NaCl to increase the salinity decreased the zeta potential value, which made the foam film more unstable, and adding Type-F NFA did not result in a significant improvement in foam stability due to aggregation rate acceleration.

The existing NFA analysis was conducted using raw materials of fly ash that contained several compounds, which disrupted the process of silica stabilization in the foam. Therefore, the mechanism by which the main compound of NFA, silica, stabilizes foam is not fully understood. Also, it is crucial to apply the reservoir conditions when investigating how silica can withstand harsh conditions. Thus, further in-depth analysis is required in the future.