Polymer-Based Scale Inhibition of Calcium Sulfate Using Phosphino-Polycarboxylic Acid: Experimental Evaluation, RSM Modeling, and Process Optimization

Calcium sulfate scale formation is a major challenge in oilfield production systems. In this study, the performance of phosphino-polycarboxylic acid (PPCA) as a polymeric scale inhibitor was evaluated using static jar tests and modeled using Response Surface Methodology (RSM). The effects of temperature (50–100 °C), inhibitor concentration (10–50 ppm), and calcium ion concentration (1000–10,000 ppm) on inhibition efficiency (IE%) were investigated through 60 experimental runs. The results showed that IE% varied from 33.7% to 95.4%, depending on operating conditions. A quadratic RSM model demonstrated excellent predictive capability (R2 = 0.9977) with a low average error of 1.4%. Among the variables, inhibitor concentration had the strongest effect, followed by calcium ion concentration and temperature. Increasing PPCA concentration significantly improved inhibition efficiency, exceeding 90% at dosages above 35 ppm, while higher temperature and calcium concentration reduced performance. Response surface analysis indicated that IE% above 94% could be achieved at 43–50 ppm inhibitor and temperatures below 63 °C. Under harsh conditions (100 °C and 10,000 ppm Ca2+), 42–50 ppm PPCA maintained IE above 90%, with a predicted maximum of 90.81%. These results confirm the effectiveness of PPCA and provide a reliable basis for optimizing scale inhibition performance.