Fluid flow properties in geologic media are determined by the rock’s microstructure and pore space evolution. A quantitative understanding of fluid-mineral interactions and resulting changes in microstructure and porosity is needed for successful extrapolation of lab-scale data to the field scale. This is crucial in various energy-related areas, as geothermal energy production, nuclear waste management, carbon sequestration, conventional and unconventional oil and gas, but has not been fully understood. Previous studies indicate changes in reaction rates due to changes in microstructure. In addition to porosity, other microstructural features such as grain boundaries have been recognized as potential fast pathways for fluid exchange within rocks. We present a systematic experimental study of volume-reducing limestone dissolution and reprecipitation reactions in the model systems calcite-dolomite (CaCO3 - CaMg(CO3)2 and calcite-siderite (CaCO3-FeCO3). By varying the porosity and permeability of the parent rock, we tested the effect on the replacement reaction. Through the combination of X-ray and neutron small angle scattering (SAXS & SANS), we quantified changes in the porosity as a function of depth within the sample and time. Porosity characterization was coupled with chemical mapping by scanning electron microscopy with energy-dispersive X-ray spectroscopy and time-of-flight secondary ion mass spectrometry. Our results demonstrate that the microstructure primarily controls replacement speed and the size regime of porosity development. Abundance of features that enhance the reactive surface area, such as porosity, grain boundaries or twinning within minerals, leads to a faster replacement via these pathways. We identified grain boundaries as reactive pathways via transmission electron microscopy analyses of a cross-section perpendicular to the grain boundary. Our results demonstrate the importance of the initial microstructure for the replacement reaction speed and mechanism. We will discuss possibilities of combining replacement experiments with pore-scale reactive transport modelling.

The influence of microstructure on mineral dissolution-reprecipitation: The model systems of CaCO3–CaMg(CO3)2 and CaCO3–FeCO3

ACS Spring Meeting 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.