Calcium sulfate is one of the dominant scales in the oil field. At high temperatures, hemihydrate (CaSO4·0.5H2O) and anhydrite (CaSO4) are the predominant sulfate scales formed, while gypsum (CaSO4·H2O) scale may form at low temperature during offshore production with long tie-back particularly if hydrate inhibitors are used. The prediction and prevention of calcium sulfate scales require accurate knowledge of the solubility and phase behavior of the three major phases of calcium sulfates.

The phase behaviors of the different calcium sulfate phases are related to the supersaturation state, temperature, and fugacity of water. In this study, the effect of a common hydrate inhibitor, monoethylene glycol (MEG), on calcium sulfate solubility and phase behavior was investigated. Three approaches were taken to determine the kinetics of calcium sulfate phase transition at various temperatures, ionic strengths, and MEG concentrations: (1) Dissolution of gypsum as the starting material, (2) Dissolution of anhydrite as the starting material, (3) Precipitation of calcium sulfate by mixing calcium and sulfate containing solutions. The effect of scale inhibitors on phase transition was also evaluated. This study was run in NaCl/CaSO4/MEG/H2O solutions at 0–6 molal NaCl and 0–95 wt% MEG at 4–70°C. The calcium concentrations in liquid samples were analyzed by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) and EDTA (ethylenediamine tetraacetic acid) titration, and the morphology, phase composition and water content of solid samples were examined by Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD), and Thermogravimetric Analysis (TGA). Phase transition of gypsum to anhydrate was observed in the presence of high concentrations of NaCl and MEG, regardless of the experimental approach. The transition boundary of temperature and NaCl and MEG concentration can be estimated quantitatively from solubility and fugacity of water. The inhibition of hexamethylenediamine tera (Methylene Phosphonic acid) (HDTMP) was also tested by using precipitation method to investigate the precipitation kinetics of calcium sulfate.

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