In Germany the exploitation of hydro-geothermal systems becomes more and more important. Especially the (Upper Jurassic) Malm Aquifer beneath the South German Molasse Basin is predominated for a hydro-geothermal utilization. The research project PrognosPermae aims to develop a workflow to decrease the exploration risk of geothermal drillings. Several tests has been executed on analogue samples of Jurassic limestone and dolo-mite. The results are used to verify the numerical modelling.

The focus of this paper lies on using laboratory tests on increasing specimen volumes as an approach for upscaling the geomechanical properties. The carbonates were characterized by density, porosity, water-permeability, uniaxial compressive and tensile strength, cohesion and friction angle, and ultrasonic wave velocities. All these experiments were executed on both dry and saturated samples. This work aims to identify the influence of specimen size and porosity on strength and ultrasonic velocity. The results show a marginal size effect for the tested diameter range but a significant influence of the total porosity.


For nearly all engineering purposes the investigation of the scale effect is important, though it requires more effort, time and costs. The majority of researchers such as Hudson and Harrison (2005), Franklin and Dusseault (1989), Pratt et al. (1972), Bieniawski (1968) and others suggest a reduction in strength with increasing specimen size. Among others Thuro et al. (2001) and Hawkins (1998) reported a different relation. As there is still no consistent knowledge about this relation, it should be developed for every geological setting. In addition, this approach might help to upscale geomechanical parameters for numerical modelling.

Porosity is an additional important factor for hydrogeothermal purposes. Most research-ers agreed that with increasing porosity the strength (Chang et al., 2006; Palchik and Hatzor, 2004) or ultrasonic wave velocities (Fabricius et al., 2007; Eberli et al., 2003; Anselmetti and Eberli, 1993) decrease.

2.1 Specimen materials

From the intrpretation of thin sections of cuttings, one limestone and one dolomite were selected as analogue outcrop material of the reservoir rocks, Both were sampled from quaries between Ingolstadt and Regensburg in South-East Germany. These carbonates can be described as follows:

Limestone (so called ‘Kelheimer’ limestone): dense, homogeneous, crypto- to micro-crystalline with occasional calcite and quartz crystals, white to light brown. Upper Jurassic Kimmmeeridgium.

Dolomite (‘Wachenzeller’ dolomite): visually dense to slightly porous, homogeneous, crypto- to fine-crystalline, grey to beige brown, Upper Jurassic Kimmeridgium- Tighonium.

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