Microfluidic experiments of biological CaCO₃ precipitation in transverse mixing reactive environments

Enzymatically induced carbonate precipitation (EICP) in heterogeneous subsurface is of interest in various geoscience and environmental applications. Through fabricating microfluidic cells with different pore networks, the pore-scale formation of biological CaCO₃ was investigated experimentally by transverse mixing using time lapse high-resolution camera. The precipitation pattern over time was processed to obtain the evolution of distribution of CaCO₃ volume fraction. The impact of flow rate and pore-scale heterogeneity were quantitatively evaluated with reaction index including overall pore filling ratio, precipitation rate and precipitation efficiency. The results showed that low flow rates and strong heterogeneity in porous media are the two favorable conditions for precipitation process due to more nucleation sites. At grain scale, a statistic of three-dimensional morphologies of individual crystals was evaluated with specific surface area and degree of anisotropy. Localized precipitates in multiple pores were adopted as a representative of the whole porous media. The analysis showed that the complex pore structures are generated due to precipitates in pores, thus limiting transport of reactants and result in the permeability reduction. We furthermore confirmed different polymorphs of calcium carbonate, mainly containing vaterite and calcite. Pore-scale analysis of biological CaCO₃ in porous media significantly contributes to the understanding of advection–diffusion-reaction coupling effect, and further revealing the role of pore network on biomineralization process.