Abstract
Minimizing exploration risk in deep geothermics is of great economic importance. Especially, knowledge about temperature and permeability of the reservoir is essential. We test the potential of an outcrop analogue study to minimize uncertainties in prediction of the rock properties of a fractured reservoir in the Upper Rhine Graben. Our results show that although mineralogical composition, clay content, grain size, and fabric type are basically comparable, porosity and quartz cementation are not. Young’s modulus, as observed in the outcrop closest to the reservoir is about twice as high (~ 64 GPa) as observed in the reservoir (~ 34 GPa). Most importantly, however, the parameters that describe the fracture system, which are essential to predict reservoir permeability, differ significantly. While the outcrops are dominated by perpendicular fracture sets (striking NE–SW and NW–SE), two different conjugate fracture sets (striking NW–SE and N–S) occur in the reservoir. Fracture apertures, as reported from the FMI, are one order of magnitude wider than in the outcrop. We conclude that our outcrop analogue study fails to predict important properties of the reservoir (such as permeability and porosity). This must be in part because of the tectonically complex setting of the reservoir. We propose that analogue studies are important, but they must be treated with care when attempting to predict the controlling parameters of a fractured reservoir.
Highlights
The most essential requirements for a geothermal reservoir are sufficient temperature and permeability (e.g. Jung et al 2002; Paschen et al 2003; Schulz 2011; Schulz et al 2009)
It is inherent that the quality of predictions made on reservoir properties largely depends on the distance between exploration wells in the reservoir, and on the heterogeneity of the latter (e.g. Müller et al 2010; Fitch et al 2015). This holds true for fluvial sedimentary rock sequences that are characterized by frequent changes of rock properties, both laterally and vertically (Morad et al 2010). This is the case for the Lower Triassic (Buntsandstein) in the Upper Rhine Graben (URG), where lithostratigraphic correlations are further complicated by a complex system of fault block tectonics (Boigk and Schöneich 1970; Sauer et al 1982; Villemin et al 1986)
Lithology The comparison of outcrop and well data shows that rock properties that determine lithology, such as mineralogical composition and grain size, are in good agreement, despite some uncertainties in stratigraphic correlation
Summary
The most essential requirements for a geothermal reservoir are sufficient temperature and permeability (e.g. Jung et al 2002; Paschen et al 2003; Schulz 2011; Schulz et al 2009). Estimating the potential of a fractured hydrogeothermal reservoir is a major problem This is because of the limited amount of data available to estimate the permeability in the subsurface and to predict achievable flow rates for geothermal power exploitation (at least 20 kg s−1 MW−1, Franco and Villani 2009). It is inherent that the quality of predictions made on reservoir properties largely depends on the distance between exploration wells in the reservoir, and on the heterogeneity of the latter (e.g. Müller et al 2010; Fitch et al 2015) This holds true for fluvial sedimentary rock sequences that are characterized by frequent changes of rock properties, both laterally and vertically (Morad et al 2010). This is the case for the Lower Triassic (Buntsandstein) in the Upper Rhine Graben (URG), where lithostratigraphic correlations are further complicated by a complex system of fault block tectonics (Boigk and Schöneich 1970; Sauer et al 1982; Villemin et al 1986)
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