Oxygen-Controlled Dynamic Evaluation of CaCO₃ Scale Inhibitors in Iron-Containing Brines

Scale inhibitors are widely used to control inorganic deposition in oil and gas production, but their laboratory evaluation is often performed under aerated conditions that may not adequately represent iron-containing brines. In this study, a modified Tube Block Test methodology was developed for dynamic scale-inhibition testing under controlled dissolved-oxygen conditions. Brines were prepared and transferred under a controlled atmosphere, and dissolved O₂ concentrations ranging from 8500 to <10 μg/L were evaluated at 65 and 120 °C in tests lasting up to 6 h. A commercially available phosphonate-based inhibitor was used as an industry-qualified benchmark. The results indicated that dissolved oxygen strongly affected precipitation behavior and apparent inhibitor performance. At 65 °C under aerated conditions, the presence of iron substantially accelerated capillary blockage, whereas negligible differential-pressure variation was observed in the corresponding iron-free control. At 120 °C, rapid blockage also occurred in the iron-free control, consistent with an increased contribution from CaCO₃ deposition. Reducing dissolved O₂ to <10 μg/L delayed blockage in iron-containing brines and improved the apparent performance of the commercial inhibitor. Under oxygen-controlled conditions, the inhibitor at 250 mg/L maintained the differential-pressure increase below 1 psi throughout the 6 h test. These findings show that dissolved-oxygen control is essential for distinguishing iron-related interference from carbonate-scaling behavior during dynamic inhibitor qualification. The proposed methodology provides a reproducible approach for extended-duration evaluation of scale inhibitors in iron-containing brines under field-relevant operating conditions.