Geophysical signatures of fluid-rock interactions in carbonates: Laboratory experiments and a dimensionless analysis to link microstructure heterogeneities to flow and chemical reactivity

In both natural and engineered systems, fluid-rock interactions may occur due to changes in temperature, pressures or nature of the circulating fluids. Dissolution and precipitation of minerals will cause changes in the rock microstructure, which will affect the rock properties (hydraulic, electrical and elastic) and in turn affect the fluid-rock interactions themselves. I will present the methodology employed in the laboratory in order to better understand this complex coupled problem and to assess the possibility of geophysical monitoring. Experiments consist in injecting CO2-saturated fluids in various limestones and in using a combination of geochemical fluid analysis to quantify fluid-rock interactions, imaging techniques to document changes in the rock microstructure, and rock properties monitoring (permeability, electrical formation factor, and P- and S-wave velocities). These experiments provide key methods and experimental data needed to revise current rock physics models in the case of reactive systems. I will then show an approach to quantify the effects of microstructure heterogeneities on the flow and fluid chemical reactivity: it consists in computing the Péclet and Damköhler numbers, rewritten in such a way that they link parameters characterizing the pore space to the magnitude of the different transport and kinetic processes. This analysis shows that the main controlling factors are in that case permeability and reactive surface area.