Heap, and 5 co-authors, reported in 2019 that the Muschelkalk, composed of Triassic limestones, marls, dolomites, and evaporites, forms part of the Permo-Triassic cover of sedimentary rocks that directly overlies the fractured granitic reservoir used for geothermal energy exploitation in the Upper Rhine Graben. Petrophysical data for this lithostratigraphic unit are sparse, but are of value for reservoir investigations, stimulation, and optimization strategies at existing and prospective geothermal sites throughout the Upper Rhine Graben. To this end, The authors present here a systematic microstructural, mineralogical, and petrophysical characterization of the Muschelkalk core (from the Middle to Lower Muschelkalk; from a depth of ~930 to~1001 meters) from exploration borehole EPS-1 at Soultz-sous-Forêts (France).
First, the authors assessed the microstructure and mineral content of samples from six depths that we consider represent the variability of the available core. The majority of the core is composed of fine-grained, interbedded dolomites and marls; however, anhydrite and a dolomitic sandstone bank were found in the Upper and Lower Muschelkalk core, respectively. A larger suite of samples (from 15 depths, including the six depths chosen for microstructural and mineral content analysis) were then characterized in terms of their petrophysical properties. The matrix porosity of the measured Muschelkalk samples is low, from~0.01 to~0.1, and their matrix permeability is below the resolution of our permeameter (≪10−18 m2).
P-wave velocity, thermal conductivity, thermal difusivity, specifc heat capacity per unit volume, Young’s modulus, and uniaxial compressive strength range from 2.60 to 5.37 km/s, 2.42 to 5.72 W/mK, 1.19 to 2.46 mm2/s, 1.63 to 2.46 MJ/m3 K, 9.4 to 39.5 GPa, and 55.1 to 257.6 MPa, respectively. Therefore, and despite the narrow range of porosity, the petrophysical properties of the Muschelkalk units are highly variable. The authors compared these new data with those recently acquired for the Buntsandstein unit (the Permo-Triassic unit immediately below the Muschelkalk) and thus provide an overview of the petrophysical properties of the two sedimentary units that directly overly the fractured granitic reservoir.
The authors concluded that the Muschelkalk exhibited high petrophysical variability, an important lithostratigraphic unit for geothermal energy development in the Upper Rhine Graben. Although the matrix porosity, a parameter that typically exerts a first-order control on rock physical properties, is low and falls within a narrow range (from~0.01 to~0.1), however, the equivalent permeability of the Muschelkalk may be high as a result of highly fractured zones. The measured range in petrophysical properties is a result of the textural, microstructural, and compositional variations of the rocks comprising the Muschelkalk. The range of petrophysical properties observed in the Muschelkalk exceeds that of the Buntsandstein, the unit situated below the Muschelkalk and above the fractured reservoir granite, despite the large range in matrix porosity observed within the Buntsandstein.
Although these data can be used to assist reservoir investigations, stimulation, and optimization strategies at geothermal sites within the Upper Rhine Graben, the authors concluded that caution is perhaps required when targeting the Muschelkalk as a reservoir/aquifer due to its high variability in terms of not only petrophysical properties, but also rock mass structure. For example, the equivalent permeability of the Muschelkalk unit will be too low to support flow in the absence of highly fractured zones. The authors trust that the petrophysical properties provided herein will be useful for:
1) thermo-hydro-mechanical modelling designed to better understand large-scale fluid circulation, heat flow calculations, and temperature estimations and
2) reservoir stimulation strategies and assessments of borehole stability at geothermal sites throughout the Upper Rhine Graben.
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