Microfluidic experiments to rationalize coupled mineral dissolution and precipitation with gas exsolution

Coupled mineral dissolution and precipitation with gas exsolution are relevant subsurface processes occurring during CO 2 sequestration, hydrogen storage, and radioactive waste disposal. While gas exsolution during mineral dissolution has been studied, its interaction with concurrent mineral precipitation remains unclear. Here, we use a microfluidic reactor with real-time optical and 3D Raman imaging to study witherite dissolution in sulfate-rich acidic solutions, leading to barite precipitation and CO 2 exsolution. “Cauliflower-like structures” are observed, in which barite encrusts gas bubbles, forming mineral-coated structures, a phenomenon that can be explained by the electric double layer of gas bubbles that causes local increase in saturation with respect to barite, favoring precipitation. Raman imaging reveals water droplets − i.e., cloud-like dispersions − trapped inside the mineral-encrusted bubbles. In addition to the cauliflower-like structures that trap gas bubbles, we identify conditions under which the system transitions to a multiphase flow regime, i.e., gas transport along with the liquid flow. Geochemical modeling shows that such processes are heavily coupled with the exsolution of CO 2 and controlled by the acidity. The cauliflower-like structures only occur when the precipitation rate is faster than dissolution. The preferential precipitation as cauliflower-like structure is caused by competition between the rate of gas production from witherite dissolution and the barite growth rate. These cauliflower-like structures can reduce further mineral dissolution, potentially slowing down the corrosion of waste canisters, but also impeding CO 2 storage and hydrogen recovery by clogging pore spaces.