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CO2mobility control in porous media is crucial to geological sequestration of CO2and enhanced oil recovery (EOR). Although aqueous foam has been widely used to control the mobility of CO2, bubble dispersion with low gas content is formed easily because of gas–liquid separation caused by gravity differentiation, which reduces the storage volume of CO2and limits its utilization. In this work, the use of armored bubbles with silica nanoparticles for CO2mobility control in porous media was explored. Visual micromodel flooding experiments were conducted to study the pore-scale flow behaviors of bubbles. Both homogeneous and heterogeneous flooding experiments were used to evaluate the mobility control ability of bubble dispersions. The results showed that the nanoparticles were adsorbed on the interface of CO2bubbles and formed an armor layer, which made the interface more solid-like and enhanced the roughness of the bubble film. Subsequently, the effective viscosity of the armored CO2bubble dispersions increased and showed a shear-thinning property. By observing the pore-scale flow behaviors, it was found that armored bubbles showed high rigidity and strong plugging ability, thereby trapping CO2in porous media and enlarging the sweep region. A synergistic plugging effect occurred between the armored bubbles, which drove the subsequent bubbles to invade into the residual oil region. Moreover, as the armored bubbles became rough, the residual oil was activated more easily by the scraping effect, and the oil-transporting ability of bubbles was enhanced. Thus, residual oils formed after water flooding could be displaced by armored bubbles thoroughly. In homogeneous porous media, the breakthrough and flow channels of the displacing fluid were controlled by the strong plugging of armored bubbles in porous media, and the displacement efficiency was enhanced. The ultimate oil recovery of armored bubble flooding was improved by about 11.4% compared to that of bare surfactant bubble flooding. In heterogeneous porous media, the mobility of the displacing fluid in relatively high permeability sandpack was reduced more effectively by armored bubbles. Even during the subsequent water flooding stage, the armored bubbles showed relatively strong resistance to water erosion and inhibited the differentiation of outflow percentage. Therefore, the flow profile was controlled by armored bubbles, and the ultimate oil recovery was improved by about 14.3% using armored bubbles. The results confirmed the high mobility control performance of nanoparticle-armored bubbles for EOR.

The use of CO2as a fracturing fluid for reservoir stimulation to enhance oil and gas recovery in low-permeability formations is widespread. However, during the CO2injection process, the low viscosity of supercritical CO2(SC-CO2) at high temperature and pressure conditions usually causes serious fluid loss in porous media, thus restricting its efficient utilization. In this work, the dynamic filtration control properties of nanoparticle-enhanced dry SC-CO2foams in porous media were explored, and the effects of nanoparticle and surfactant concentration, foam quality, pressure drop, temperature, and permeability were systematically studied. The results showed that the SC-CO2/liquid interfacial viscoelasticity modulus and the corresponding foam viscosity were improved by the adsorption of silica nanoparticles at the SC-CO2/liquid interface. At high foam quality (⩾90%), the nanoparticles reduced the amount of coarse bubbles and prevented bubble disproportionation, both of which helped to maintain a higher viscosity in the ultradry foam. The nanoparticles also significantly enhanced the foam filtration control performance; compared to bare dioctyl sodium sulfosuccinate (AOT) foam, the total filtration coefficient of CO2was decreased by a factor of about 2.2–6.5 with an increase in SiO2nanoparticle concentration from 0.5 to 1.5 wt %. The effect of surfactant concentration on the filtration of foam with nanoparticles also correlated well with its effect on interfacial viscoelastic modulus and foam viscosity. Increasing the foam quality from 80 to 97% only increased the filtration control performance to a certain extent, as the foams became ultradry and unstable if the foam quality was too high (⩾90%); thereafter, a continuing increase in foam quality caused by CO2expansion at high pressure drop values led to low flow resistance and weakened the filtration control performance. The addition of silica nanoparticles reduced the temperature dependence of the foam filtration coefficient. The permeability and total filtration coefficient followed a power law relationship, with the addition of nanoparticles causing a decrease in the power law exponent. Return permeability tests after filtration result confirmed that nanoparticle-enhanced dry SC-CO2foams were relatively clean fluids for porous media.