Prediction of Mineral Scaling in Oil and Gas Production Using a Comprehensive Thermodynamic Model

A comprehensive methodology has been developed for predicting mineral scaling in oil and gas production environments. The methodology is based on a previously developed model for mixed-solvent electrolyte systems (MSE). The model 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 on the basis of contributions that reflect long-range electrostatic, short-range ionic, and non-ionic interactions. The model has been designed to calculate phase equilibria in multicomponent systems containing an aqueous phase, multiple solid phases, a gas phase, and a second liquid (typically hydrocarbon-dominated) phase. With this formulation, the model is capable of predicting the formation of scales not only in aqueous systems but also in environments that contain nonaqueous additives such as methanol or mono-, di-, and triethylene glycols. The performance of the model has been analyzed for iron sulfide, iron carbonate, calcium carbonate, calcium sulfate, barium sulfate and other scales as a function of temperature, brine composition, partial pressures of various gases and the presence of methanol and glycols. Additionally, the effects of metastability have been taken into account for the scales that may occur in various crystalline forms.