Physical heterogeneity control on mineral dissolution rates: from pore to continuum scale over geologic time

Abstract

Mineral dissolution rates are often determined by laboratory experiments performed in well-mixed conditions for a relatively short time. However, 1) geologic systems are highly heterogeneous that rarely exhibit a well-mixed condition, 2) geologic time scales cannot be reproduced in laboratories, and 3) the hydrologic accessibility of the reactive phases within the pore structure is usually not considered in experiments or continuum scale numerical simulations. These inherent differences lead to the 3~7 orders of magnitude discrepancy between field- and laboratory- measured reaction rates, which prevents direct application of laboratory measured rates to numerical simulations. The objective of this dissertation is to investigate the effect of 1) heterogeneous permeability distribution, 2) time dependent evolution of reactive surface area, and 3) pore geometry on chemical weathering rates. Reactive transport simulations conducted on random permeability fields highlight the importance of variance in permeability distribution and Péclet number in controlling the reduction of reaction rates from the laboratory measured reaction rate. In long-term simulations, highly heterogeneous domains show additional reduction in reaction rates as the remaining surface area of immobile zones over-normalizes the reaction product concentration. For the pore scale investigation, reactive microfluidic devices using silicate minerals, anorthite and albite, were fabricated with a femtosecond laser and HF etching techniques. Fluid flows that are perpendicular to the applied pressure gradient develop in a fabricated microdevice while immobile zones develop in numerical simulations, which indicate reactive microdevices can constrain numerical conditions to better represent the chemical reactions in natural pore system. Overall, the results suggest that physical heterogeneity of natural porous media could reconcile some of the large discrepancy between laboratory- and field- measured chemical weathering rates.