An Effective Acid Placement Technique to Stimulate High-Temperature Sandstone and Carbonate Formations

Viscoelastic surfactant-based acids have been used in the field for several years, and have been the subject of many laboratory and field studies. An extensive literature survey revealed that corrosion inhibitors and high-temperature dramatically weaken the surfactant structure and significantly reduce its viscosity. Therefore, these fluids are typically limited to placement in relatively low-temperature formations. This study was conducted to evaluate a new VES system that can form a gel and maintain useful viscosity at high temperature. The effects of temperature, initial HCl concentration, corrosion inhibitor concentration, surfactant concentration, and breaker type and concentration on the viscosity and corrosion rate of the new VES were investigated. Additionally, a core flood study was conducted using Indiana limestone and Berea sandstone cores to evaluate the diversion ability of VES system. Experimental results show that because of the low viscosity of the new VES system at live condition, it can be used at an initial HCl concentration of 15 wt% or higher. Even in the presence of a high concentration of corrosion inhibitor, the new VES system maintained its viscosity at 300°F. With no breaker in the system, the viscosity of the new VES system was stable for at least 3 days, and viscosity increased with surfactant concentration. Breakers based on mineral oil caused a smooth viscosity decline, while breakers based on mutual solvent caused instantaneous viscosity reduction. The pre-flush stage of mutual solvent did not prevent gel formation during acid neutralization. Mixing the new VES in 5 wt% NH4Cl brine formed a gel at surface. The new VES acid system increased the pressure drop by a factor of 18 inside Berea sandstone core and resulted in a 60.5 % permeability enhancement. Corrosion inhibitor reduces viscosity of the new VES system, while intensifier has nearly no effect. Also, increasing the concentration of either corrosion inhibitor or intensifier reduced the corrosion rate of the new VES system. Therefore, in some cases, a higher intensifier loading can be recommended to achieve viscosity goals along with corrosion protection.