Fluids incorporating carbon dioxide (CO2) microbubbles have been utilized to promote enhanced oil recovery from hydrocarbon reservoirs. The performance of such fluids in porous media is greatly affected by both the bubble size and stability. On this basis, the present study evaluated the effects of varying the concentrations of a xanthan gum (XG) polymer, a surfactant (sodium dodecyl sulfate: SDS) and sodium chloride (NaCl) on both the stability and bubble size distribution (BSD) of CO2 microbubbles. CO2 microbubble dispersions were prepared using a high-speed homogenizer in conjunction with the diffusion of gaseous CO2 through aqueous solutions. The stability of each dispersion was ascertained using a drainage test, while the BSD was determined by optical microscopy and fitted to either normal, log-normal or Weibull functions. The results showed that a Weibull distribution gave the most accurate fit for all experimental data. Increases in either the SDS or XG polymer concentration were found to decrease the microbubble size.However, these same changes increased the microbubble stability as a consequence of structural enhancement. The addition of NaCl up to a concentration of 10 g/L (10g/1000g) decreased the average bubble size by approximately 2.7%. Stability was also reduced as the NaCl concentration was increased because of the gravitational effect and coalescence.

A novel two-step preparation of a silica nanofluid (Si-NF) with potential for light oil recovery is herein presented. It was observed that a Si-NF, prepared by dispersing silica nanoparticles (Si-NPs) in poly­(vinyl alcohol) (1%) under carbon dioxide gas bubbling, is stable for 30 days at a temperature as high as 55 °C without any visible sedimentation. Unstable Si-NFs formed siliceous gels whose strength depends on Si-NP load. The spectral characterization revealed that prepared Si-NFs consisted of silanols, carbonyls, and alcohols. As part of an effort to enhance the light oil production (API 31°), 1, 0.5, and 0.1 wt % Si-NPs were injected in waterflooded Berea sandstone at 55 °C. An increase in oil as high as 7.6% was achieved for the Si-NF with the highest Si-NP load (1 wt % Si-NP). Oil production encompassed the discontinuity of Si-NFs, channel plugging, wettability, and interfacial tension alterations.

The underlying effect of preflush salinity and silica nanofluid (Si-NF) on oil production is examined. The influence of salinity on the stability of Si-NFs is studied. A series of sand-pack floodings evaluating oil production was conducted at different concentrations of preflush salinity (0 to 4 wt.%), followed by the injection of a Si-NF (0.5 wt.%) at the trail of which postflush water was injected. The effluent water and solids were collected and analyzed using X-ray fluorescence (XRF). Interfacial tension (IFT) and contact angle measurements were conducted on the Si-NF in the presence of salinity to confirm the effect. The Si-NF became unstable and formed precipitate in the presence of salinity. The sand-pack flooding showed that when the preflush salinity was increased, the displacement efficiency (ED) using the Si-NF and postflush injection was increased (ED = 44%). The XRF of the precipitated effluent revealed that the preflush salinity and Si-NF caused mineral leaching, which triggered pore clogging. The IFT value reduced from 13.3 to 8.2 mN/m, and the wettability was altered to be more strongly water-wet when the salinity increased. The primary mechanisms of oil recovery using the Si-NF after preflush salinity is attributed mainly to the clogging mechanism. This clogging helps block the high-perm area, shift the fluid flow to the oil-trapped zone, and free the oil out. Other contribution mechanisms are IFT reduction and wettability alteration.