A vertical well in a carbonate reservoir was damaged due to invasion of large volumes of water from an upper formation. Diagnostic work (temperature survey and chemical analysis of bailer samples) confirmed the communication between the upper formation and the reservoir. Obviously, invasion of large volumes of water can damage the well due to water blockage and the formation of viscous emulsions inside the formation. As a result of water invasion, the well ceased to flow. A workover was conducted on this well to eliminate the downhole communication, but still the well did not flow. Therefore, there was a need for an acid treatment to remove the damage and restore well productivity

The well was perforated after the workover in 112 ft perforated intervals. In addition, the carbonate reservoir is heterogeneous with several high permeability zones. For these reasons, acid placement and diversion were important. A viscoelastic-based acid system was selected to combine diversion and dissolving capabilities in one system. Moreover, the viscoelastic-based acid was foamed to enhance diversion and improve lifting of the spent acid following the treatment. Mutual solvent was added to the pre and post flushes to break the gel formed by the viscoelastic surfactant and help in removing the water blockage.

The treatment was applied using coiled tubing for better diversion in the wellbore. The treatment was very successful and resulted in major increases in oil production (more than 6,000 bbl/day). The well is still flowing at high rates nine months after the treatment. No significant change in water-cut was noted after the treatment.

This paper describes problem identification, fluid selection, and diversion. It will also discuss the first field application of foamed viscoelastic based-acid system.

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