Microstructure and pore space evolution of a geological media dictates its fluid flow properties and controls its reactivity. To successfully predict reaction and transport, a quantitative understanding of rock-fluid interactions and accompanying changes in microstructure and porosity is needed. Such a predictive understanding would benefit several energy-related applications, e.g., geothermal energy production, carbon sequestration, conventional/unconventional oil and gas production, and nuclear waste management. Dissolution-reprecipitation reactions are replacement reactions, in which a parent phase is dissolved, and a product phase is precipitated without morphology changes. Studies have shown that microstructure influences replacement reaction rates. Here, we present an experimental study of volume-reducing dissolution and reprecipitation of limestone (CaCO3) by siderite (FeCO3) at 200 °C. The microstructure of the starting limestones was varied: Carrara (impermeable marble, dominated by grain boundaries), Carthage (low permeability/porosity marble dominated by nanoporosity) and Texas Cream (high porosity/permeability limestone). By combining X-ray and neutron small and ultra-small angle scattering, we quantified the porosity changes of the limestones spatially and temporally. The porosity characterization was combined with chemical imaging techniques via scanning and transmission electron microscopy. Reaction rims of siderite replacing calcite were identified. The replacement rate is controlled by pre-existing porosity/permeability and accelerated by newly formed porosity at the reaction interface and within the precipitated siderite. Full replacement was observed within 2 to 59 days for the low and high permeability/porosity limestones, with the impermeable limestone not being fully replaced within 120 days. In the impermeable limestone, the FeCl2 solution permeated the whole sample within 14 days through grain boundaries, and dolomite impurities acted as preferential nucleation sites due to lower lattice strain. After nucleation, the growing siderite exerted crystallization pressure to form micrometer-wide cracks by widening the grain boundaries. This process accelerated the fluid percolation and recrystallization of the solid. In conclusion, porosity is the main control on the replacement rate. Additionally, in the cases of low porosity, other microstructural effects, e.g., grain boundaries, accelerate fluid transport and replacement.

Dissolution and reprecipitation in the CaCO3 – FeCO3 system – influence of grain boundaries and porosity

ACS Spring 2022

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