Experimental evaluation of viscoelastic surfactant acid diversion for carbonate reservoirs: Parameters and performance analysis

Formation damage minimization and removal are essential to field productivity enhancement and production target achievement. While formation damage is anticipated during drilling, an effective treatment fluid becomes important to bypass mud damage and enhance well productivity. Most of the fluids used in acid stimulation of carbonate wells are consumed in the high permeable zones - leaving the low permeability and damaged zones not effectively stimulated. Viscoelastic surfactants (VES) have been introduced in the industry to overcome this challenge and offer effective uniform treatment over long horizontal well intervals to treat damaged and low permeability zones. The viscoelastic surfactant molecules form rod-like micelles in the presence of salt at specific pH condition, which results in significant viscosity increase. Therefore, when added to acid systems, the VES helps in increasing the viscosity of the solution based on the state of the acid-carbonate rock reaction. Customizing the treatment to the field's needs is a key factor to the success of the stimulation treatment. The treatment design includes the carrying fluid type and VES concentration. These parameters need to be optimized to achieve a uniform distribution of the treatment and effective acid diversion. This paper studies the diversion ability of VES acid system in comparison to straight acid systems. Also, it evaluates the impact of permeability, salinity, and VES concentration on the diversion efficiency of the VES acid system. For the first time in the industry, the diversion efficiency was shown through return permeability results to be heavily dependent on salinity of the mixing water, VES concentration, and permeability contrast. Increasing the salinity and VES concentration enhanced the diversion efficiency significantly, while increasing the permeability contrast reduced the diversion efficiency. Core flooding equipment capable of flowing two parallel core samples was used to study the VES acid system diversion efficiency at various permeability contrasts. Carbonate core samples with permeabilities in the range of 0.7-56.25 mD were used in the study. The VES acid system carrying fluid salinity effect was investigated by comparing the diversion obtained by distilled water, field water and seawater at different VES concentrations. Two VES concentrations were used to evaluate its impact on diversion, 3.75% and 7.5%. These parameters were studied at 1,000 psi for pore pressure, 2,000 psi for confining pressure, and 150°F for temperature.