This paper describes a staged acid corrosion inhibition concept for application in matrix acidizing treatments. The approach optimizes the performance of acid corrosion inhibitor (ACI) formulations used to protect coiled tubing (CT) and downhole casing during acid injection and flowback. The concept relies on polymerizable ACI chemistry.
Matrix acidizing formulations are currently designed with a fixed ACI dosage throughout acid injection. The staged approach uses a higher ACI dosage (first stage) to establish a persistent inhibitor film followed by a lower ACI dosage (second stage) to maintain that film. The persistence of the first stage inhibitor film relies on polymerizable ACI chemistry. Staged performance was evaluated by weight loss and electrochemical testing on CT and casing steels in 15% hydrochloric acid at temperatures in the range 60 to 121°C. Rotating cylinder electrodes (RCEs) were used to compare performance under laminar and realistic turbulent flow conditions. An electrochemical flow cell (EFC) was used to quantify performance when the ACI dosage was changed during flow.
The ACI dosage used in the second stage can be at least one order of magnitude lower than that in the first stage. Staged treatments can be designed for enhanced corrosion protection using a lower total quantity of inhibitor relative to conventional treatments. Optimum first and second stage ACI dosages depend on temperature and metal type. For CT in 15% hydrochloric acid, typical stage 1 and stage 2 dosages are 0.2 and 0.01 %, respectively, at temperatures ranging 77 to 99°C; these dosages increase to 1.0 and 0.05–0.1 % at 121°C. RCE testing showed robust performance under realistic dynamic flow conditions, and EFC testing showed good performance when the ACI dosage was changed during flow.
The staged ACI concept achieves enhanced corrosion protection of CT and wellbore casing using a lower total quantity of inhibitor relative to conventional treatments. The next step is to evaluate the performance of staged ACI treatments in field experiments.