A MICROFLUIDIC STUDY OF THE EFFECTS OF INCOMPLETE MIXING ON CALCITE DISSOLUTION

The coupled processes of fluid flow and geochemistry alter the dissolution of carbonate rocks which has critical importance in carbon cycling, petroleum engineering, and the development of natural geologic formations, particularly karst. Inclusion of these geochemical reactions in reactive transport models is typically accomplished through effective reaction rates derived from well-mixed experiments. However, recent studies of various mineral systems demonstrated that dissolution rates under fluid flow conditions can diverge significantly from effective rates due to incomplete mixing, highlighting a limitation of these conventional Darcy-scale models. Additionally, while there is a significant amount of work studying the impacts of carbonate dissolution in static conditions or through numerical modeling, there are comparatively fewer studies which examine this mineral dissolution at the pore scale while considering flow and mixing. The primary objective of this work, therefore, is to better understand the coupling between flow and geochemistry during the dissolution of carbonate minerals at the pore scale.

Here, we present the results of a microfluidic study of calcite dissolution, in which calcite is used as a reactive substrate within a microfluidic device representing the intersection of two straight fractures. Calcite dissolution is induced by either the introduction of an acidic HCl solution or the introduction of an undersaturated solution. Mixing conditions are controlled by either pre-mixing solutions prior to infusion, simulating a well-mixed condition, or by directly infusing both solutions simultaneously which will create an incompletely mixed condition. Variations in the flow rate, and therefore, the Reynold’s number, will influence the extent of incomplete mixing, and we report observations on the change in calcite morphology in response to solution (i.e. calcite undersaturation) and flow conditions. The results of these initial experiments lay the foundation for a broader understanding of the coupling between flow and geochemistry during mineral dissolution, particularly where mixing processes can drive dissolution, such as during mixing corrosion, where the mixing of carbonate saturated solutions creates an undersaturated solution, renewing mineral dissolution.