Since 2003, the main challenge of selective matrix treatments with coiled tubing (CT) has been to manage the severe corrosion rates of CT strings with large acid volumes and long exposure times to hydrochloric acid (HCl), resulting in reduced pipe life. Generally, the average volume required per job is 300 bbl of 15% to 20% HCl, depending on the bottomhole conditions and damage mechanisms; severe material losses of carbon steel coiled tubing string grade 90,000 psi have been regularly measured above 0.20 lb/ft2.

Our local stimulation team has developed an engineering workflow that consists of several stages and can reduce corrosion rates below 0.05 lb/ft2. A laboratory database was built to perform corrosion tests under time and temperature conditions and evaluate available corrosion inhibitors. The database allows the selection of the inhibitor and concentration range for each application. To reinforce the quality control, field preparation of acids is monitored and samples are tested to compare with laboratory design. Finally, measurements of coiled tubing wall thickness give an estimated value of material weight loss.

As a result of the implementation of the workflow, average pipe life has been extended from 1,200 to 3,000 bbl of 15% HCl pumped.

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