Dynamic Imaging of Reaction at Reservoir Conditions Considering the Impact of Chemical and Physical Pore-Scale Heterogeneities in Carbonates

Injecting carbon dioxide into carbonate aquifers for the purpose of sequestration causes reactions to take place between the CO₂, in situ brine, and the rock formation, which leads to partial dissolution of the rock minerals. The manner in which this dissolution materialises is dependent upon the chemical and physical pore-scale heterogeneities inherent in the host rock. Understanding dissolution dynamics is important because it affects porosity/permeability properties, and hence carbon storage capacity and long-term storage security.

We develop a combined experimental and modelling approach to study how pore-scale dissolution occurs in different mineralogical settings represented by varying chemical and physical heterogeneity. We use X-ray Microtomography to image pore-scale in situ dissolution of carbonate rocks flooded with supercritical CO₂ saturated brine at reservoir conditions. From image analysis, minerals are segmented into individual phases and the dissolution of each mineral is quantified. A Navier-Stokes flow solver was applied directly on the sequence of 3D images to calculate flow fields and the corresponding permeability changes incurred by the dissolution.

We study different mineralogical settings ranging from single to mixed mineralogy consisting of calcite and dolomite - to vary physical heterogeneity (initial pore structures and associated velocity fields) and chemical heterogeneity (intrinsic reaction rates). We looked at (a) composite core with a controlled spatial distribution of calcite and dolomite and (b) reservoir samples with random spatial distribution of the two minerals. It is found that varying heterogeneity can have a different impact on the effective reaction rates. However, mass transfer limitations cause the pore-scale effective reaction rates in all cases to be at least an order of magnitude lower than those measured in a batch experiment. We find that the nature of dissolution can be time-dependent. These observations have important implications on the coupled reactive transport behaviour that is normally found in natural carbonate aquifers where the effective reaction rates at the field-scale can differ by orders of magnitude.