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As a new mechanism of enhancing oil recovery by low-salinity water-flooding, osmotic pressure has been widely studied in recent years. The injected water molecules enter the original formation water-drops wrapped by oil droplets under osmotic pressure, resulting in apparent expansion of crude oil. However, at present, the effect of osmotic pressure alone on crude oil droplets expansion is not obvious, and the research is only limited to qualitative research, lacking quantitative analysis. In this study, three kinds of nano silica are used as an oil droplet expansion displacement agents, and the water in oil ( W/0) emulsion is used to simulate the original oil-water distribution in the reservoir to observe the expansion effect of W/O emulsion droplets and quantitatively calculate droplets' expansion ratio. The experimental results show that both hydrophilic and hydrophobic nano silica can enhance the expansion ratio of oil droplets, and the expansion ratio by hydrophilic HS-40 nano silica is higher than that by other two kinds of nano silica because of its smaller particle size and higher absolute zeta potential. With the increase of HS-40 concentration, the surfactant molecules at the oil-water interface enter the oil droplets and accelerate the expansion of the droplets. The maximum expansion ratio reaches 3. 29 when the concentration of HS-40 is 0. 20%. With the continual increase of HS-40 concentration, nano silica particles will hinder the transfer of water molecules at the oil-water interface into the droplets. On the condition that there are large salinity differences between formation water and injection water, the existence of nano silica can accelerate the expansion of oil droplets, which provides a new way for low-salinity waterflooding. [ABSTRACT FROM AUTHOR]

Zero-dimensional carbon nanomaterials show intriguing potential in the field of unconventional oil resource development for reducing injection pressure and enhancing oil recovery. However, the complicated synthesis procedure, necessity for surfactant and limited understanding of the mechanism impede their practical applications. In this study, novel high-hydrophilic carbon dots (hh -CDs) with an average particle size of 2.54 ± 0.016 nm were facilely synthesized through an electrochemical approach by employing cost-effective petroleum coke as the carbon source, followed by treating with ozone for further oxidation. The abundant surface functional groups render hh -CDs superior hydrophilicity, dispersibility and stability. Core flooding tests show 0.20 wt% hh -CDs nanofluid delivers a prominent pressure-reducing rate of 23.81% and an enhancement in oil recovery of 26.38% without any surfactant. Analytical results of atomic force microscope (AFM) with hydrophobic probe reveal hh -CDs can adsorb on the rock surface to alter the micro-scale wettability from oil-wet to homogeneous water-wet. The hh -CDs can also efficiently reduce the core surface roughness and afford excess disjoining pressure for oil displacement, accounting for the impressive performance. This work provides a feasible oil systemic circulation for the synthesis of versatile surfactant-free carbon dots for boosting injection pressure reduction and enhancing oil recovery in tight oil reservoirs. [Display omitted] [ABSTRACT FROM AUTHOR]