The influence of gas and humidity on the mineralogy of various salt compositions – implications for natural and technical caverns

Bettina Strauch,Anja M Schleicher,Martin Zimmer,Axel Zirkler,Stefan Höntzsch

doi:10.5194/adgeo-45-227-2018

Abstract

Abstract. Storage caverns are increasingly located in heterogeneous salt deposits and filled with various fluids. The knowledge of phase behaviour in heterogeneous systems of salt, liquid and gas and the requirements for reliable analytical techniques is, therefore, of growing interest. A method that allows for the continuous monitoring of mineral compositions at distinct humidity and gas content using XRD measurements is presented here. Various saliniferous mineral compositions have been investigated in pure CO2, N2 or CH4 atmospheres with varying humidity in a closed chamber. All mineral compositions experience dissolution and/or mineral conversion reaction accompanied by volume loss. Dissolution-recrystallization reactions of complex mineral assemblages involving halite, sylvite, kieserite, carnallite and kainite were observed using this method. For carnallite-rich mineral assemblages, the mineral conversion from carnallite to sylvite was observed when humidity exceeded 50 % RH. In the presence of CO2, acidification of the aqueous phase occurs which enhances the dissolution rate and reaction kinetics.

Highlights

Undisturbed salt formations are practically impermeable to fluids up to lithostatic pressure (e.g. Durup, 1994; Berest et al, 2001)
Various saliniferous mineral compositions have been investigated in pure CO2, N2 or CH4 atmospheres with varying humidity in a closed chamber
Detailed knowledge on the interaction between mineral dissolutionrecrystallization processes and porosity around cavern areas is, crucial in order to estimate the propagation potential of salt solution processes. This is likewise relevant for mining activities and for technical cavern usage where mineralogical heterogeneities occur. We investigate these processes using X-ray diffraction (XRD) analyses on a variety of saliniferous mineral compositions in a humidity chamber with defined gas composition

Summary

Introduction

Undisturbed salt formations are practically impermeable to fluids up to lithostatic pressure (e.g. Durup, 1994; Berest et al, 2001). Undisturbed salt formations are practically impermeable to fluids up to lithostatic pressure Due to that specific characteristic together with the high solubility of saliniferous rocks, the construction of technical caverns in bedded or domal salt structures is nowadays common practice in order to create tight storage space for different commodities. Storage caverns are of cylindrical shape in homogeneous rock salt deposits. A large number of cavern structures show strong variation in their contour which is primarily caused by variable salt mineralogy of the host rock. Salt formations may contain zones with lowsoluble impurities (e.g. anhydrite, dolomite) as well as intercalated layers with minerals of higher solubility (e.g. potash beds). The formation of a “spongy” salt rock mass along the cavity walls where fluid infiltrates into the saliniferous host rock is described in the literature (Heekeren et al, 2009)

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Conclusion