CO₂-Foam Rheology: Effect of Surfactant Concentration, Shear Rate and Injection Quality

Injecting carbon dioxide (CO₂) into oil reservoirs has the potential to enhance oil recovery (EOR) and sequester CO₂ underground. While CO₂ injection has been successful, a major challenge facing this technique is enhancing volumetric sweep efficiency. Some of the factors that contribute to this challenge are the low density and viscosity of injected gas relative to reservoir fluids. The use of foam is one of the most promising techniques to increase the CO₂ apparent viscosity, thereby improving volumetric sweep efficiency.

Increasing the CO₂ viscosity by using surfactants has the potential to mitigate some of the challenges associated with CO₂ injection projects. The objective of this work is to investigate the effect of various surfactants on CO₂-foam viscosity. Four surfactants were used to evaluate the foam generation and rheological properties of CO₂-foams at high pressure and temperature. Dynamic foam viscosity measurements were conducted in a special foam rheology apparatus with supercritical (sc-CO₂) under high pressure (3200 psi), temperature (210°F) and salinity conditions. The foam was generated by injecting sc-CO₂ and surfactant at different concentrations (0.20, 0.50 and 1.00 wt %), shear rates (10-600 s^-1) and qualities (70, 85 and 90%).

The results indicate that all surfactants were able to generate good quality foam at high pressure and temperature. The foam viscosity increases with surfactant concentration. All foams exhibited shear thinning behavior, with foam viscosity decreasing with increasing shear rates. In general, the highest viscosity for each surfactant was reported at the lowest shear rates. The results also showed that the foam quality has an impact on foam viscosity for some cases. The highest foam viscosities for surfactant 2 were achieved at 85% quality. However, the quality had no significant impact on foam viscosity for surfactant 3.