Restoring the injectivity of water disposal wells using a viscoelastic surfactant-based acid

Application of matrix stimulation treatments in horizontal wells with openhole completions is a challenging task. Acid diversion is one of the key factors that determine the success of these treatments. This is especially true when the target zone has several high permeability streaks. Proper acid placement in carbonate formations can be achieved by increasing the viscosity of the acid in-situ. This can be done by using polymer/cross-linker or viscoelastic surfactant-based acid systems. The latter system was selected in the present study because of the tight nature of the reservoir and the presence of hydrogen sulfide in the disposal water. Both factors do not favor the use of polymer-based acids, especially when a metal cross-linker is used. This is the first time a surfactant-based acid was used to matrix acidize water disposal wells worldwide.A multi-stage acid treatment that utilized regular 20 wt.% HCl and surfactant-based acids was successfully conducted in a highly inclined water disposal well. The subject well was damaged due to invasion of particulate matters (iron sulfide and calcium carbonate) and accumulation of oil in the critical near wellbore area. The acid treatment was successful where the skin factor decreased from + 22 before the treatment to nearly zero after the treatment. The injectivity index of this well has increased from 19.7 to 270.2 bbls/psi/day. The well maintained its injectivity even 9 months after the treatment. The surfactant-based acid system decreased the injection pressure by 35%. The reduction will allow pumping the viscoelastic-based acid. The surfactant-based acid reduced the CT surface weight by nearly 12.5%. This will allow placing the CT deeper in wells with extended reach. Finally, no operational problems were encountered during field application and this acid system is recommended, especially for sour environments.