Effect of Engineered Water and Scale Inhibitor Concentration on the Adsorption Performance of Inhibitors on Multi-Walled Carbon Nanotubes and Crushed Sandstone

Engineered waterflooding is an effective process for enhanced oil recovery, but it can lead to mineral scale deposition due to the incompatibility of brines. Previous studies on scale mitigation have mainly focused on high-temperature baryte and anhydrite scales, neglecting gypsum precipitation and inhibitors adsorption at low temperatures. Additionally, earlier research has used simple brines that do not reflect the actual injection and formation water in reservoirs. In this study, the impact of temperature, various brine mixtures, and thermodynamic databases on saturation ratio and scale precipitation is explored using PHREEQC (pH-REdox-EQuilibrium-C program). The study reveals that the copresence of calcium and magnesium ions improves the gypsum inhibition efficiency of scale inhibitors (SIs) at low concentrations to a maximum of 79%. However, this effect is reversed or neutral at higher SI concentrations. The study also shows that the presence of monovalent ions reduces the adsorption of SIs by multi-walled carbon nanotubes (MWCNTs). Removing sodium ions from seawater while leaving calcium and magnesium ions intact increases MWCNTs' adsorption capability to 93%. This is because monovalent cations attach to the adsorbent surface and block the active sites, whereas divalent cations act as a bridge between MWCNT and SIs. The study establishes that the behaviour of SIs regarding adsorption on MWCNT and crushed sandstone depends on various factors, including molecule size, calcium toleration of the SIs, point of zero charge, and solution pH. Understanding these factors can improve the effectiveness of SIs, reduce chemical costs, and prolong the life of oil wells.