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Abstract

Potash salts belong to the most soluble minerals and their unintended dissolution can result in a safety risk for the construction and utilisation of salt caverns and mines [1]. One
main challenge in modelling the formation of leaching zones
within potash seams is the representation of fluid-rock
interactions within regions exhibiting highly varying porosities.

Chemical reactions cannot take place if small porosities inhibit
the inflow of solution, although present solutions may be undersaturated with respect to certain minerals. These porosity
variations only occur at the dissolution front in binary systems,
such as NaCl solution and solid halite. Its progress can be
described by a mass transfer rate, depending on the concentration
of present solutions or by assuming a saturated interface between
dry rock and solution, subtracting out the diffusive mass
transport. In contrast, the dissolution of potash salt results in the
formation of a porous rock matrix, consisting of undissolved and
precipitated minerals that can further react with the surrounding
solution. Accordingly, fluid-rock interactions and largely varying
porosities also occur remote from the dissolution front. The
interchange approach [2] was developed to describe these
interactions. Coupled with a reactive transport model including
PHREEQC [3] and TRANSE [4] this approach is capable to
quantify, e.g., the leaching process of carnallite-bearing potash
seams due to natural density-driven convection. The dissolution
rate is essential for both, the timely progress and geometric shape of evolving leaching zones in the potash seam. Therefore, the
interchange approach has been adapted in the scope of the present study to consider saturation-dependent dissolution rates for each mineral. In this contribution, we discuss the feasibility and limitations of our approach to represent fluid-rock
interactions between brine and different types of potash salts at
the metre scale.