Porosity, particle velocity, and diffusion coefficient evolution due to kinetic mineral dissolution - application of iCP (interface COMSOL PHREEQC) for reactive transport modelling

Changes in porosity, pore water velocity, and diffusion coefficient due to physical and geochemical process are of great importance in reactive transport modelling. Here we present first results and planned tasks resulting from our activities in the ongoing EU-project BioMore [1] that focuses on leaching carbonaceous copper shale ore. Processes such as mineral dissolution and precipitation, clay mineral swelling and squeezing and mineral structure deformation can have significant effect on porosity, permeability and diffusion coefficient. Mineral precipitation decreases the porosity inhibiting solute transport in porous media. On the other hand mineral dissolution increases the porosity and may provide preferential pathways for fluids and dissolved solutes. Accordingly, combining methods for predicting dynamic matrix-material properties in the proposed solute transport models is useful. A finite element based reactive transport model, iCP [2], was developed to simulate kinetic dissolution of calcite and porosity evolution under high acidic condition. An acidic solution with a pH of 1.5 is injected into a 0.25 m long, 0.05 m wide axial fracture flanked by porous media containing 1wt.% calcite. The processes considered in the model are advective-dispersive transport in the fracture calculated by COMSOL Multiphysics (COMSOL, 2015) and kinetically controlled calcite dissolution and precipitation in the porous media simulated by means of PHREEQC [3]. Calcite dissolution is monitored by means of pH and Ca+2 concentrations in the pore fluid and fracture solution. The porosity evolution is calculated by considering the mineral volume fraction change and updated over throughout the time steps for each grid cell of the porous media. Induced tortuosity and effective diffusion coefficient are calculated and updated based on Archie’s low and Millington and Quirk equations respectively.
In cooperation with coworkers and partners further tasks will consider more realistic fracture structure geometry, quantified advective distributions obtained from GeoPET imaging [4] and a more complex geochemical porous matrix.
Overall aims of the project are realistic prognosis calculations for acid consumption, copper release and fluid flux rates as a function of time, aperture width and calcite content.

References
[1]https://ec.europa.eu/growth/tools-databases/eip-raw-materials/en/content/biomore-alternative-mining-concept-raw-materials-commitment
[2] Nardi, A.; Idiart, A. ; Trinchero, P. ; Manuel de Vries, L. ; Molinero, J. (2014): Interface COMSOL-PHREEQC (iCP), an efficient numerical framework for the solution of coupled multiphysics and geochemistry. Computers & Geosciences 69, 10-21.
[3] Parkhurst, L.D. ; Appelo, C.A.J. (1999): User's Guide to PHREEQC (Version 2)-A Computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations. U.S. Geological Survey, Denver, Colorado, p. 326.
[4] Kulenkampff, J.; Gründig, M.; Korn, N.; Zakhnini, A.; Barth, T.; Lippmann-Pipke, J. (2014): Application of high-resolution positron-emission-tomography for quantitative spatiotemporal process monitoring in dense