Amongst the main phenomena that are put forward to explain the evolution of pore network structures when aqueous solutions flow through a porous medium, the dissolution/precipitation reactions of minerals and the transport of particles play a major role. These two phenomena are generally considered separately. Using recently published experimental data on the calcium carbonate chemical system, we show that their coupling effects may become significant in some specific situations. A first type of data includes results from batch experiments describing the precipitation of amorphous calcium carbonate precursors before the more crystalline calcite mineral can form. A second type of data describe the formation of calcite during the co-injection of two aqueous solutions (one of CaCl2 and one of Na2CO3) in a microfluidic channel. To interpret consistently these observations, a new kinetic model was developed based on a transition-state-theory (TST)/surface-complexation-model (SCM) coupling. It aims to describe the successive precipitation of amorphous calcium carbonate (ACC) and calcite, taking account of their mutual influence [3]: ACC precipitates according to the standard TST and creates surface complexation sites from which calcite can form and create new surface complexation sites. When the kinetics of calcite precipitation are fast enough, the consumption of dissolved matter leads to the re-dissolution of ACC. This model is both compared to batch and flow-through experiments, showing a global good accordance with the observations. However, some discrepancies regarding the distribution of the deposited matter along the microfluidic channel highlight the need for further developments to allow the description of the transport and deposition of particles able to grow or shrink along their course. This work suggests that the combination of simple experimental microfluidic devices and reactive transport modeling could be a promising integrated methodology to identify the basic processes and study the dynamics of geochemical reactivity at the pore scale and, ultimately, to improve our ability to predict the reactivity of fluids in underground reservoirs.

Geochemical modeling of calcium carbonate formation involving a two-step mechanism

ACS Fall 2021

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