Role of Polymer-Based Nanofluids on Asphaltene Adsorption during Carbon Dioxide (CO₂) Injection

As part as efforts to prevent or to delay asphaltene adsorption during carbon dioxide (CO₂) oil recovery, the role of polymer-based nanofluid on asphaltene kinetics and adsorption mechanisms was studied. The model crude oil was an asphaltene solution obtained after mixing of 1 g of asphaltenes (extracted usingn-heptane) in 100 g of pure toluene. In the absence of sorbent (Berea sandstone), injecting CO₂decreased the concentration of asphaltene in the equilibrium at the rate of −0.01 mg·g^-1·MPa^-1until 5 MPa, beyond which the decrease was −0.04 mg·g^-1·MPa^-1. The model oil in contact with sandstone wetted with formation water (0.5 wt %NaCl) prompted an asphaltene adsorption, which increased with CO₂solubility and was as high as 58 μg/g-rock. Under the same conditions, the adsorption was 3-fold lower when the wetting fluid was a nanofluid (0.05 g of alumina oxide nanoparticles into 100 g of aqueous poly(vinyl alcohol), PVOH). The measurements of interfacial tension (IFT) between the asphaltene solution and the investigated wetting fluids at different CO₂injections, showed that IFT reduction depends on CO₂solubility and the nature of wetting fluids. IFT reduction was more pronounced for water compared to nanofluid or PVOH alone. A Ward-Tordai short time approximation was employed to estimate the kinetics of asphaltene at the interface with the water, the PVOH, and the nanofluid. The estimated diffusion coefficients of asphaltenes were found within the range of ∼10^-12m²/s for water-wet sandstone and ∼10^-18m²/s for PVOH-wet and nanofluid-wet sandstones, suggesting that adsorption of asphaltenes was not diffusion-controlled. Rather, it pertained to ability of the wetting fluid to provide a steric hindrance. The combined analysis of the Langmuir adsorption isotherm and the solid-liquid equilibrium model revealed that CO₂altered rather the affinity of the wetting fluid toward the asphaltenes in suspension, which defined in turn the amount of asphaltene adsorbed. A series of dynamic adsorption replicating CO₂oil recovery showed that nanofluid injected ahead of CO₂could desorb up to 35% of adsorbed asphaltene owing to a steric hindrance and the preferential affinity of alumina nanoparticles with the asphaltenes irrespective of CO₂injection pressure.