Carboxyl Groups as a Determinant of Scale Inhibition Efficiency in Chitosan Derivatives for Oilfield Calcite and Gypsum Scales

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Mineral scale deposition continues to cause serious operational problems in oil and gas production. Conventional synthetic inhibitors can effectively limit scale formation; however, their low biodegradability and environmental persistence remain major concerns. In this work, chitosan was chemically modified to produce three derivatives with different carboxyl contents: monocarboxylated chitosan (MCC), dicarboxylated chitosan (DCC), and polycarboxylated chitosan (PCC). The synthesis involved stepwise oxidation of chitosan hydroxyl groups in aqueous media using sodium metaperiodate, sodium chlorite, and hydrogen peroxide. The resulting derivatives were investigated as environmentally friendly alternatives to conventional chemicals used for controlling oilfield scale. FTIR and NMR spectroscopy confirmed the structural modifications and the progressive conversion of hydroxyl and aldehyde moieties into carboxylic acid groups. Zeta-potential measurements revealed an increasingly negative surface charge with increasing carboxyl content. Carboxyl-group density significantly affected both physicochemical behavior and antiscaling performance. Each additional carboxyl group provided an extra coordination site for multidentate chelation of Ca 2+ ions, strengthened electrostatic attraction to calcium-rich crystal surfaces, and enabled multi-anchor adsorption that disrupted nucleation and crystal growth. Static bottle tests and high-pressure dynamic tube-blocking tests demonstrated a clear structure-activity relationship for both gypsum and calcite scales. PCC exhibited the highest inhibition efficiency and the lowest failure inhibitor concentration. Inhibition efficiency and calcium-binding strength increased in the order MCC < DCC < PCC. All three derivatives also exhibited excellent calcium compatibility over a wide range of Ca 2+ concentrations without precipitation and retained good thermal stability. These findings establish carboxyl-group density as an important and tunable design parameter governing chelation capacity, surface adsorption, and threshold inhibition, thereby positioning carboxylated chitosan derivatives as promising sustainable alternatives to conventional antiscalants for demanding oilfield environments.