A thermodynamic approach is used as the basis for a computer model to simulate the coprecipitation of mixed scales (e.g., multi-cation sulfates and carbonates), which occur during enhanced oil recovery processes. Traditionally, coprecipitation is studied using partitioning coefficients (fractionation, distribution coefficients), which are empirical parameters that are not applicable to processes involving complex fluid chemistry and involving multiple component solids (mixed scales). The rigorous approach undertaken, permits coupling coprecipitation equations with the calculation of aqueous speciation, which permits modeling of complex fluid compositions and their influences on the coprecipitation. The model also uses regular solid solution models to calculate the activities of solid components, which predicts the increased scale tendency of the host phase (e.g., barite) due to the incorporation of minor components (e.g., Sr), and the inadvertent coprecipitation of Naturally Occurring Radioactive Materials (NORMs, e.g., Ra) with the scales.
As an example, computer simulations using this model have reproduced, very closely, the BaRaSO4 coprecipitation experiments published by Doerner and Hoskins. These simulations also demonstrate the empirical nature of partitioning coefficients and their limited applicability. The partitioning coefficients are not constant, but, rather, are a function of chloride concentrations. Partitioning coefficients are shown to vary from 1.8 in dilute solution, to 2.1 in 0.4 and 2.7 in 1.1 C1- molal solutions due to Ra2+ and Cl-complex formation. These results demonstrate the need for and advantage of using a computer model, in which a thermodynamic approach is employed, to simulate scale formation and the coprecipitation of NORMs in oil-field studies.
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