Effect of pH and scale inhibitor concentration on phosphonate–carbonate interaction

In this paper, we present results from five corefloods (RC1 to RC5) from the Jurassic Portlandian limestone (ф ~ 19.80% and k = 606 mD) using 5000 ppm, 10,000 ppm, 25,000 ppm and 27,000 ppm of partly neutralized Diethylenetriamine pentamethylenephosphoric acid (DETPMP) at pH 4 and 2. The purpose of this study was to study the effect of inhibitor concentration and pH on the inhibitor adsorption and on the evolution of the inhibitor and cation (calcium and magnesium) return concentrations. These corefloods were performed using long cores (12 in.), which were treated with just 0.5 pore volume (PV) of inhibitor. Another purpose was to study the transport and inhibitor/carbonate rock interactions when less than 1 PV of inhibitor solution is injected. This allows for consumption of the inhibitor during propagation and return, rather than saturating the core with many PV to full adsorptive capacity of the inhibitor/rock system. This study showed that the higher the concentration of SI and lower the pH, the more calcium dissolution is observed (from the [Ca2+] effluents). In all treatments there is a decrease in the [Mg2+] effluent corresponding directly to the increase in calcium. The effluent cation results in the long corefloods which strongly support the view that both magnesium and calcium are binding quite strongly to the DETPMP scale inhibitor. These observations lead us to a number of conclusions on the factors that must be included in a full carbonate model. In particular, our experimental results, along with some simple modeling, greatly clarify the role of both calcium and magnesium in the mechanism of the scale inhibitor retention in carbonate systems.