Pore-scale study of transverse mixing induced CaCO₃ precipitation and permeability reduction in a model subsurface sedimentary system

A microfluidic pore structure etched into a silicon wafer was used as a two-dimensional model subsurface sedimentary system (i.e., micromodel) to study mineral precipitation and permeability reduction relevant to groundwater remediation and geological carbon sequestration. Solutions containing CaCl ₂ and Na₂CO₃ at four different saturation states (Ω = [Ca²⁺][CO₃^2-]/K _spCaCO3) were introduced through two separate inlets, and they mixed by diffusion transverse to the main flow direction along the center of the micromodel resulting in CaCO₃ precipitation. Precipitation rates increased and the total amount of precipitates decreased with increasing saturation state, and only vaterite and calcite crystals were formed (no aragonite). The relative amount of vaterite increased from 80% at the lowest saturation state (Ω_v = 2.8 for vaterite) to 95% at the highest saturation state (Ω_v = 4.5). Fluorescent tracer tests conducted before and after CaCO₃ precipitation indicate that pore spaces were occluded by CaCO₃ precipitates along the transverse mixing zone, thus substantially reducing porosity and permeability, and potentially limiting transformation from vaterite to the more stable calcite. The results suggest that mineral precipitation along plume margins can decrease both reactant mixing during groundwater remediation, and injection and storage efficiency during CO₂ sequestration.