INTRODUCTION
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 various solids including calcium sulfate, barium sulfate, calcium carbonate and magnesium carbonate as a function of temperature, brine composition, pressure and the presence of methanol and glycols. Additionally, the effects of metastability have been taken into account for scales that may occur in various crystalline forms.
The formation of mineral scales is a common and expensive problem in the oil and gas industry. Accumulation of scales can lead to a reduction in well productivity, fouling of equipment, damage to pumps, and concealment of corrosion. The generation of multicomponent mineral scale deposits is controlled by multiple factors including the composition of reservoir water, temperature and pressure conditions, presence of sour gases, solids introduced during drilling, corrosion of the production string, etc. Therefore, there is a strong motivation for developing accurate modeling tools that can predict the likelihood of scale formation, the chemical identity of precipitating solids and the degree of supersaturation as a function of production conditions.
The main difficulty in modeling scaling lies in the behavior of multicomponent mixtures. While the solubility and precipitation characteristics of individual solids are typically known, the presence of other components can results in significantly altered and sometimes counterintuitive behavior. The scaling behavior is also complicated by the metastability of some solid phases. Also, the presence of nonelectrolytes such as methanol and glycols can greatly alter the precipitation behavior of salts when such components are introduced as hydrate inhibitors in the transportation of produced hydrocarbons and water. To address these problems, a tremendous amount of experimental data has been published in the literature, thus creating a rich database that can be used to develop thermodynamic models.
The objective of this study is to develop a model that can accurately predict the formation of mineral scales in multicomponent aqueous systems in the absence and presence of methanol and glycols. For this purpose, we apply a previously developed thermodynamic framework1-3 to develop a detailed treatment of the solution chemistry and phase behavior of Na - K - Mg - Ca - Ba - Fe - Cl - SO4 - CO3 - S - H2O - methanol - glycol systems.
