Spatial and Temporal Evolution of Leaching Zones within Potash Seams Reproduced by Reactive Transport Simulations

Abstract

Leaching zones within potash seams generally represent a significant risk to subsurface mining operations and the construction of technical caverns in salt rocks, but their temporal and spatial formation has been investigated only rudimentarily to date. To the knowledge of the authors, current reactive transport simulation implementations are not capable to address hydraulic-chemical interactions within potash salt. For this reason, a reactive transport model has been developed and complemented by an innovative approach to calculate the interchange of minerals and solution at the water-rock interface. Using this model, a scenario analysis was carried out based on a carnallite-bearing potash seam. The results show that the evolution of leaching zones depends on the mineral composition and dissolution rate of the original salt rock, and that the formation can be classified by the dimensionless parameters of Péclet (Pe) and Damköhler (Da). For Pe > 2 and Da > 1, a funnel-shaped leaching zone is formed, otherwise the dissolution front is planar. Additionally, Da > 1 results in the formation of a sylvinitic zone and a flow barrier. Most scenarios represent hybrid forms of these cases. The simulated shapes and mineralogies are confirmed by literature data and can be used to assess the hazard potential.

How Insoluble Inclusions and Intersecting Layers Affect the Leaching Process within Potash Seams

“…In the second calculation step, PHREEQC is used to update fluid densities and mineral saturations following transport. After that, the "interchange" step is performed, which describes the dissolution of minerals from (nearly) dry into permeated cells and allows the dissolution front to progress [6]. The terms "dry" and "permeated" are used in view of the fluid saturation in the pore space, whereas the term "saturated" refers to the chemical equilibration of the solution with respect to salt minerals.…”

“…The updated concentrations and fluid properties, such as density and viscosity, are then transferred back to TRANSE to calculate flow and transport in the next simulation time step (Figure 1). More details on this procedure are presented in Steding et al [6]. The interchange of minerals and solution has been extended in the present study and will be described in detail in the following subsection.…”

“…Steding et al [6] developed a reactive transport model complemented by the "interchange approach" to reproduce the complex interplay between chemical reactions and density-driven transport of species within potash seams. The authors showed that leaching zones can be classified into four different groups based on the Péclet (Pe) and Damköhler (Da) numbers, which indicate if a system is dominated by reactions (Da < 1) or transport (Da > 1), and if species transport is dominated by diffusion (Pe < 2) or advection (Pe > 2).…”

“…For this purpose, the reactive transport model presented by Steding et al [6] was extended by mineral-specific, saturation-dependent dissolution rates, and a scenario analysis under density-driven flow conditions was performed. Thereby, six generic mineral distributions were investigated, with insoluble areas ranging from several small inclusions to one continuous layer.…”

A Method to Increase the Leaching Progress of Salt Caverns with the Use of the Hydro-Jet Technique

“…Computer simulations allow the design of the leaching programme prior to the start of the leaching process and its correction while it is running [12,16,17]. In a more advanced stage of leaching a dissolution rate may be controlled by the modelling and analysis of fluid flow velocity and its density changes, [18].…”