Effectiveness of CO2 Foamed Acid for Stimulating Low Permeability Carbonate Formations

Carbon dioxide (CO2) has so many applications in petroleum industry. In carbonate water injection wells, acid stimulation treatments are conducted as a common practice following drilling of the target zone, to remove mud-induced formation damage and improve formation permeability. Obviously, injected acids will preferentially go to the non/less damaged zones (high permeability) over the damaged zones (low permeability), which will lead to poor zonal coverage. Being more stable in high-permeability layers, CO2 foams can be used to provide effective acid diversion during acidizing treatments.

Four surfactants were used to evaluate the foam generation and rheological properties of different CO2 foamed acid systems at ambient and reservoir conditions, e.g., high-pressure, high-temperature (HPHT). Compatibility, thermal stability, bulk, and dynamic tests were conducted to evaluate the effectiveness of each acid system to generate stable and viscous foams. Dynamic foam viscosity measurements was conducted in a special foam rheology apparatus at HPHT. The foam was generated by injecting gas and surfactant containing acid solutions at different surfactant concentrations, acid concentrations, shear rates, injection qualities, and temperatures.

The results show that there is a direct correlation between surfactant concentration and foam strength. Also, the results indicate that as the acid concentration increased, the foam viscosity decreased. Foams exhibited shear thinning behavior, where foam viscosity decreased with increased shear rates. Also, the results show that the foam quality had a significant impact on foam stability and viscosity. For each surfactant-acid solution, there was a certain quality at which the highest stability and viscosity was attained. The results also report that the temperature has a negative impact on foam stability and viscosity, with the foam becoming less stable and weak with increased temperature.

This paper presents in detail the role of CO2 foamed acid on improving diversion stages in the stimulation treatments.