Mineral scale prediction is an important tool for effective scale management in oil and gas flow assurance.
Accurate prediction of scale formation is particularly challenging at high temperatures and pressures that are encountered as the industry develops progressively deeper and overpressured reservoirs. To address the need to predict scaling at conditions ranging from ambient to extreme, a comprehensive thermodynamic model has been developed. This model has been designed to represent the solubility of scaling minerals at temperatures up to 300 °C and pressures up to at least 1,700 atm. The model is based on the previously developed Mixed-Solvent Electrolyte (MSE) thermodynamic framework and relies on a detailed treatment of speciation in the liquid phase. It represents the standard-state properties of individual species using the Helgeson-Kirkham-Flowers equation of state and it predicts the species activity coefficients by accounting for long-range electrostatic, short-range ionic, and non-ionic interactions. The model has been parameterized to reproduce the solubility of sulfate, sulfide, and carbonate scales in water and in multicomponent brines ranging from dilute to highly saline (typically up to ~6 m Cl). The model accurately represents the effects of temperature, pressure and common salt components on the solubility of the minerals. Additionally, it takes into account the effect of metastability for scales that may occur in multiple crystalline forms. Application of the model to zinc sulfide, lead sulfide and calcium sulfate scales is analyzed in detail.
Key words: Scaling, flow assurance, thermodynamics, electrolyte solutions, solubility, speciation