Crystallization of minerals in porous media is an important process governing the evolution of many subsurface systems. This process leads to alteration of permeability and other physical characteristics of the rock matrix that can have significant effects on subsurface solute transport. Crystallization in porous media leads to phenomena that are rarely observed in batch experiments like (a) oscillatory zoning, and (b) pore-size dependency on nucleation. Here, we developed a “lab-on-a-chip” approach (i.e., micronized experiments with in situ Raman spectroscopy and imaging) combined with pore scale reactive transport modelling to unravel and resolve current controversies on these processes. (a) The formation of oscillatory zoned crystals of (Ba,Sr)SO4: Our micromodel consists of two parallel supply channels interconnected by microfluidic reaction chambers. A Na2SO4 solution and a mixed solution of BaCl2 and SrCl2 were injected into the supply channels. The co-precipitation of (Ba,Sr)SO4 was fostered by the counter-diffusion of solutes in the microfluidic reaction chamber. The compositions of the solid solutions were determined by in situ Raman spectroscopy. We used reactive transport modelling with newly implemented theoretical approaches such as the supersaturation function and classical nucleation theory (CNT) extended to solid solutions to (i) predict the composition of the nucleating phases, and (ii) determine the driving forces for the oscillatory zoning. Our investigation shows that the composition of the nucleating phases can be approximated by using CNT, and that the oscillatory zoning results from a combination of limited diffusional transport of solutes and kinetically controlled precipitation reactions. (b) Pore size-dependency on nucleation: Our micromodel consists of two parallel supply channels interconnected by a pore network consisting of circular pores of various diameters and heights interlinked by capillaries with a cross-sectional area of 1µm2. The nucleation of BaSO4 was triggered and monitored. A preferential localization of precipitates and a shorter induction time were observed in the larger pores. However, the dynamics of nucleation were comparatively accelerated in the capillaries. Currently we (i) investigate the nature (amorphous versus crystalline) of the precipitates in the pore network and (ii) explore the theoretical approaches that can explain these phenomena.

Crystallization in porous media: A lab-on-a-chip approach and reactive transport modelling diagnostics

ACS Fall 2021

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