# Profile Control Using Fly Ash Three-Phase Foam Assisted by Microspheres with an Adhesive Coating
## 3 Results and Discussion
### Foam Microstructure  
Foam is a thermodynamically unstable system, and its stability is controlled by the process of liquid film thinning and bubble coalescence. During liquid film thinning, bubbles push against each other, while the total surface area is changeless. In comparison, bubble coalescence results in a larger bubble, leading to a decrease in the total surface area. The coalescence rate of bubbles is determined by the speed of liquid film thinning and rupture. The smaller is the bubble volume, the stronger is the pressure-resistant ability, and the harder is the liquid film rupture.  
The fluid loss in a foam system arises from the mutual squeezing of bubbles and the effect of gravity. Bubble squeezing originates from the surface pressure of the so-called \"Plateau border\". As seen in Figure 4a, the conventional foam exhibits an unstable hexagonal structure that may lead to fast bubble fluid loss and rupture. However, in comparison, the bubbles of the three-phase foam exist in the liquid phase independently, showing high dispersity and a smaller size ranging from 10 to 100 \(\upmu\)m if compared to the conventional foam (Figure 4b). Figure 4b demonstrates typical microstructure of aqueous foam stabilized by solid particles. Analogous figures have been reported in the literature [18; 32; 40]. The smaller bubble size of the three-phase foam might be attributed to the irreversible adsorption and aggregation of fly ash particles at the liquid-gas interface of the foam and plateau borders. The adsorbed particles can reduce the direct contact between the fluids to mitigate liquid drainage and decrease the rate of film rupture and bubble coarsening [32], thereby improve foam stability. Moreover, although the presence of fly ash particles cannot significantly affect the interfacial tension [27; 40], the adsorbed solid particles can notably increase the dilatational interfacial viscoelasticity that characterizes bubbles' ability to resist external disturbances. At high viscoelasticity, the liquid film shows a solid-like behavior that is favorable to restrain the coalescence and rupture of bubbles [40]. Therefore, due to the protection of adsorbed particles, the tiny bubbles can survive longer than the conventional foam. The coagulation of MCL is anticipated to help the three-phase foam achieve better profile control, enhancing the plugging performance. Furthermore, the starch gel, serving as a suspending agent, increases liquid viscosity due to its reticulated texture and contributes positively to the mechanical strength of the liquid film surrounding a bubble.  
Figure 4: Images of the conventional foam system and the three-phase foam system: (**a**) conventional foam (adopted from [49]); (**b**) three-phase foam (magnified by 20 times).  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Consolidation Agent Concentration, wt\%** & **Settling Time, min** \\ \hline
0.5 & 1210 \\
5 & 2180 \\
10 & 2360 \\
20 & Not observed \\
30 & Not observed \\ \hline \hline \end{tabular}
\end{table}
Table 5: Three-phase foam properties.