# Profile Control Using Fly Ash Three-Phase Foam Assisted by Microspheres with an Adhesive Coating
## 1 Introduction  
Steam injection has been acknowledged as one of the most effective approaches for heavy-oil exploitation [1, 2, 3]. Nevertheless, due to the great density disparity, steam tends to flow in the upper reservoir (the so-called \"gravity override\"), which aggravates as the reservoir thickness increases [4, 5]. Moreover, steam viscosity is much lower than that of the reservoir fluids, resulting in severe viscous fingering [6, 7]. Steam channeling also emerges in heterogeneous reservoirs [8]. The abovementioned weaknesses lead to significant steam loss and low sweep efficiency. Foam, marked by low density, small filtration loss and strong particle-carrying ability, preferentially enter high-permeability regions, block large pores or channels, and reduce the permeability of high-permeable layers [9, 10, 11]. Foam has been widely used to assist steam flooding in heavy-oil exploitation, enabling us to decrease vapor mobility, enlarge the swept volume and enhance oil recovery [12, 13]. However, long-term foam stabilization remains challenging at high temperatures [14, 15], concerning the effectiveness of profile control treatment by foam when injecting steam into heavy-oil reservoirs. Solid particles, such as polymers and nanoparticles, have been commonly applied to enhance foam stability [16, 17]. Dispersing solid particles into the conventional foam yieldsthe so-called three-phase foam (TPF) system [18]. The resultant high dispersity of bubbles and anomalous foam structure give rise to excellent foam stability, resulting in increased flow resistance, improved plugging efficiency and an extended swept volume [16; 18]. Polymers are capable of generating a viscoelastic shell on the water-gas interface, which could mitigate liquid drainage between bubbles and hinder foam collapse, thereby improving foam stability [19; 20; 21]. Nevertheless, polymer-enhanced foam exhibits poor stability at reservoir conditions when contacting resident brines, crude oils, or minerals in porous media due to faster collapse of the thin liquid films at the gas-liquid interface [22; 23]. Moreover, polymers may lose their viscosity-enhancing properties at a high-temperature and high-salinity condition [24]. Polymer concentrations can also increase by up to 10 to 15 times of their original value in formation water, causing pore blocking and the consequent formation damage [25]. Therefore, polymers might not be an appropriate candidate as the foam stabilizer in hydrocarbon reservoirs due to their short-term effectiveness and potential damage to a formation. Hydrophilic nanoparticles can strongly adsorb onto the interface at harsh reservoir conditions and behave like surfactant molecules. The generated three-dimensional network structure allows slowing down thin liquid film drainage and improving foam stability [26; 27]. In contrast, non-adsorbed nanoparticles in the intervening thin film could separate the dispersed phases [16; 28]. It has been well documented that the synergetic effect of surfactant and certain types of nanoparticles is able to generate stronger foams and enhance foam stability against shrinkage and coalescence [29; 30; 31]. However, the aggregation of nanoparticles necessitates special chemical treatment in terms of their surface wettability, making them less cost-effective. As a result, using solid particles from widely available and commercially viable raw materials confers a distinguished economic superiority on particle-stabilized foam for enhanced oil recovery. Fly ash is a coproduct of coal-fired power plants, serving as a source for the commercial foam stabilizers [32]. Using the abundantly available fly ash to generate the fly ash three-phase foam (FATPF) can significantly reduce the expense of the sealing system and lower the discharge of fly ash that could lead to waste pollution [33]. [PERSON] et al. [34] acquired stable and dense CO\({}_{2}\) foam by adding fly ash nanoparticles (referred to \"nanoash\") combined with additives. [PERSON] et al.