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A dissolution model that accounts for coverage of mineral surfaces by precipitation in core floods

Pedersen, Janne, Jettestuen, Espen, Madland, Merete V., Hildebrand-Habel, Tania, Korsnes, Reidar I., Vinningland, Jan Ludvig, Hiorth, Aksel
Advances in water resources 2016 v.87 pp. 68-79
calcite, chalk, crystals, equations, floods, magnesite, magnesium chloride, models, surface area, water resources
In this paper, we propose a model for evolution of reactive surface area of minerals due to surface coverage by precipitating minerals. The model is used to interpret results from an experiment where a chalk core was flooded with MgCl2 for 1072 days, giving rise to calcite dissolution and magnesite precipitation. The model successfully describes both the long-term behavior of the measured effluent concentrations and the more or less homogeneous distribution of magnesite found in the core after 1072 days. The model also predicts that precipitating magnesite minerals form as larger crystals or aggregates of smaller size crystals, and not as thin flakes or as a monomolecular layer. Using rate constants obtained from literature gave numerical effluent concentrations that diverged from observed values only after a few days of flooding. To match the simulations to the experimental data after approximately 1 year of flooding, a rate constant that is four orders of magnitude lower than reported by powder experiments had to be used. We argue that a static rate constant is not sufficient to describe a chalk core flooding experiment lasting for nearly 3 years. The model is a necessary extension of standard rate equations in order to describe long term core flooding experiments where there is a large degree of textural alteration.