A Geo‐Structurally Based Correction Factor for Apparent Dissolution Rates in Fractured Media. Issue 15 (12th August 2022)

• Record Type: Journal Article
• Authors: Hyman, Jeffrey D.; Navarre‐Sitchler, Alexis; Andrews, Elizabeth; Sweeney, Matthew R.; Karra, Satish; Carey, J. William; Viswanathan, Hari S.
• Is Part Of: Geophysical research letters. Volume 49:Issue 15(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-08-12
• Subjects: reactive transport modeling -- fracture network -- subsurface flow and transport -- apparent dissolution rates -- geochemistry; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2022GL099513
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: Field measurements of apparent geochemical weathering reaction rates in subsurface fractured porous media are known to deviate from laboratory measurements by multiple orders of magnitude. To date, there is no geologically based explanation for this discrepancy that can be used to predict reaction rates in field systems. Proposed correction factors are typically based on ad hoc characterizations related to geochemical kinetic models. Through a series of high‐fidelity reactive transport simulations of mineral dissolution within explicit 3D discrete fracture networks, we are able to link the geo‐structural attributes with reactive transport observations. We develop a correction factor to linear transition state theory for the prediction of the apparent dissolution rate based on measurable geological properties. The modified rate law shows excellent agreement with numerical simulations, indicating that geological structure could be a primary reason for the discrepancy between laboratory and field observations of apparent dissolution rates in fractured media. Plain Language Summary: Fractures are the principal conduits for fluid flow through low permeability rock in the Earth's subsurface. In many of these systems, fluids passing through the fractures are out of equilibrium with the resident minerals, and various reactions, such as dissolution and precipitation, occur. These geochemical processes change the fracture permeability and drive fracture propagation, thereby dynamically changing flows. Field measurements of apparent geochemical weathering reaction rates are lower than laboratory measurements by multiple orders of magnitude, which makes predictions of geochemical reaction rates highly uncertain. These slow apparent dissolution rates are particularly pronounced in fracture networks where geo‐structural attributes, for example, the network connectivity and fracture geometry, determine the flow field structure and dictate transport. Through a series of high‐fidelity reactive transport simulations of mineral dissolution in fractured media, we uncovered a new link between the geo‐structural attributes of the underlying fracture network with reactive transport observations. Guided by this information, we develop a correction factor to linear transition state theory to predict the apparent dissolution rate that is based on these geological attributes. The excellent agreement of the proposed model with our numerical simulations indicates that geological structure could be one of the reasons for the commonly observed discrepancy. Key Points: Observations of apparent reaction rates in fractured media are orders of magnitude lower than those measured in laboratory conditions Reactive transport simulations are used to characterize the influence of 3D fracture network structure on apparent dissolution rates A geo‐structurally based modification to linear transition state theory for the prediction of the apparent dissolution rate is presented