Next generation models of carbonate mineral growth and dissolution

Abstract

The long-term success of carbon sequestration lies in part on the ability to trap carbon dioxide as a carbonate mineral phase. As such, the ability to predict the extent of carbonate mineral precipitation over the lifetime of a proposed geologic sequestration site will be necessary. In this review, different methods of predicting the growth of carbonate minerals, particularly calcite, and their disadvantages and advantages are summarized. Starting from a simple description of the solution saturation state, more advanced affi nity-based models are described that comprise the status quo. In these, the reaction rate is measured by the difference in concentration from an equilibrium value or the Gibbs Free Energy of reaction. It is shown that these models fail to capture some important aspects of carbonate mineral growth rates. Next-generation models in development are those that refl ect the processes that occur on a mineral surface while it is growing, not just the concentration of dissolved species. While incomplete, these process-based models are already addressing some long-standing questions in geochemistry and are enhancing the accuracy and robustness of the predictive ability for calcite precipitation. Lastly, the importance of the step density, analogous to the reactive site density in a natural sample, is shown. The factors that may infl uence the step density are described and the potentially complex relationship between step density and solution conditions is presented. While still in development, these models suggest that many of the historical problems in quantitative prediction of mineral growth and dissolution reactions can be resolved.

Geochemical reaction mechanism discovery from molecular simulation

“…[1][2][3][4] There is reason to believe, however, that these are of limited usefulness in their current form, evidenced by the often-quoted two orders of magnitude discrepancy between mineral reaction rates measured in the field versus laboratory settings. [5,6] More than just a matter of matter of rate, these geochemical predictions can sometimes result in qualitative errors, e.g.…”

“…rate model predictions that a mineral ought to precipitate when no reaction is observed or may be dissolving. [2] There are numerous possibilities and ideas that have been discussed that could cause these errors. A short list includes: (1) discrepancies between total surface area of porous media and the connected, wetted, fluid-accessible surface area, [5] (2) secondary mineral formation reducing reactive surface area, [5,7] (3) pore-size-dependent effects, [8][9][10] (4) unknown reactive site densities of the mineral surfaces, [11,12] (5) impurities poisoning the reactions [5,13] and (6) issues with the appropriate functional form of the kinetic expressions.…”

“…A short list includes: (1) discrepancies between total surface area of porous media and the connected, wetted, fluid-accessible surface area, [5] (2) secondary mineral formation reducing reactive surface area, [5,7] (3) pore-size-dependent effects, [8][9][10] (4) unknown reactive site densities of the mineral surfaces, [11,12] (5) impurities poisoning the reactions [5,13] and (6) issues with the appropriate functional form of the kinetic expressions. [2,5] So how can molecular simulation help improve our understanding? All but (1) in the list above involve understanding the mechanisms of mineral growth and dissolution.…”

“…However, the atomic-scale mechanisms of mineral growth and dissolution are, for the most part, poorly constrained because mineral growth and dissolution consists of potentially dozens of reactions with variable rates and equilibrium points. [2] To name a few sources of variation: a rock sample usually contains multiple minerals with varying distributions of morphology, each mineral grain may contain multiple crystal faces with distinct structure and reactivity, each crystal face may contain multiple surface sites with varying density and reactivity because of surface roughness and multiple possible reactions for each site. Historically, most observations of mineral growth and dissolution rates have been made by measuring the composition of a solution that has reacted with a mineral.…”

“…One can readily envision a future where a macroscopic kinetic model is built to be consistent with rate-limiting atomic-scale reactions and incorporates the evolution of surface morphology with time and solution composition. Some of these 'processbased' models already exist, [2,11,14] and have found application in field settings. [15,16] Others have used molecular simulation to parameterise larger-scale reactive transport models to examine contaminants and the origin of the differences between the atomic-and field-scales.…”