K. Bär

Thermo-Triax: An Apparatus for Testing Petrophysical Properties of Rocks Under Simulated Geothermal Reservoir Conditions

A Thermo-Triax apparatus has been developed to facilitate research on petrophysical properties of rock samples under simulated geothermal reservoir conditions. The apparatus consists of control systems for vertical stress and horizontal confining pressure, a pair of independent pore pressure controllers for applying different upstream, and downstream pore pressures at bottom and top of rock specimens, an external heater and a data logging system. Permeability of rocks is measured using steady state and transient flow methods. The thermal expansion of metallic parts in the triaxial cell and the error introduced into the readings of the extensometers at high temperatures are calibrated via experiments on an aluminum specimen with known coefficient of thermal expansion. The possibilities of studying the effect of stress and temperature on permeability and compressibilities of porous rocks with the Thermo-Triax apparatus are presented with first data. The change of pore volume during the non-isothermal process between adjacent temperature levels as well as along the measurement of permeability at leveled temperatures is interpreted and calibrated. The thermal expansion of mineral grains during heating is verified with the data of pore volume change and the magnitude of thermal expansion of mineral grains is estimated and compared with reported values. The permeability measurements along different heating paths can be used to verify the temperature dependency of stress-dependent rock properties.

3D-Modellierung der tiefengeothermischen Potenziale von Hessen – Eingangsdaten und Potenzialausweisung [3D modelling of the deep geothermal potential of the Federal State of Hesse (Germany) – input data and identifi cation of potential.]

Within the scope of the research project “3D-modelling of the deep geothermal potentials of Hesse” the deep geothermal potential of the Federal State of Hesse was quantifi ed and assessed in a qualitative analysis. The quantifi cation of the heat stored under ground and the qualitative analysis was done for different geothermal systems. These are hydrothermal and petrothermal systems, as well as fault zones and deep borehole heat exchangers. For the assessment of the deep geothermal potential, knowledge of the geological structure and the geothermal properties of the potential reservoir rocks are indispensable. Therefore, a 3D model of the deep geothermal potential of the Federal State of Hesse (Germany) has been developed (cf. Arndt et al. 2011). Systematic measurements of thermophysical and hydraulic rock properties such as thermal conductivity, heat capacity and permeability of relevant geologic formations have been combined with in-situ temperature measurements, hydrothermal upwelling zones, characteristics of geological faults and were added to the 3D geological structural model. Using a multiple criteria approach, the various rock and reservoir properties were assessed incorporating their relevance for the different geothermal systems to allow the qualitative analysis. Therefore, threshold values for each parameter were defi ned specifying whether the potential is very high, high, medium, low or very low. This method was tested for the one-dimensional case (virtual drilling) and the two-dimensional case (geological-geothermical cross-sections) before being applied to the 3D model. The resulting geothermal model, which incorporates the quantifi cation and the qualitative analysis, is an important tool, which can be used at an early stage of the planning phase for the design of geothermal power plants. Furthermore, it allows quantifi cation of the deep geothermal potential and is intended to be an instrument for public information. Schlusselworter: Geothermie, geothermisches Potenzial, thermophysikalische Gesteinseigenschaften, hydraulische Kennwerte, geologisches 3D-Modell, Oberrheingraben, Hessen, Deutschland, GOCAD

Geostatistical Interpolation of Subsurface Properties by Combining Measurements and Models

Subsurface temperature is the key parameter in geothermal exploration. An accurate estimation of the reservoir temperature is of high importance and usually done either by interpolation of borehole temperature measurement data or numerical modeling. However, temperature measurements at depths which are of interest for deep geothermal applications (usually deeper than 2 km) are generally sparse. A pure interpolation of such sparse data always involves large uncertainties and usually neglects knowledge of the 3D reservoir geometry or the rock and reservoir properties governing the heat transport. Classical numerical modeling approaches at regional scale usually only include conductive heat transport and do not reflect thermal anomalies along faults created by convective transport. These thermal anomalies however are usually the target of geothermal exploitation. Kriging with trend does allow including secondary data to improve the interpolation of the primary one. Using this approach temperature measurements of depths larger than 1,000 m of the federal state of Hessen/Germany have been interpolated in 3D. A 3D numerical conductive temperature model was used as secondary information. This way the interpolation result reflects thermal anomalies detected by direct temperature measurements as well as the geological structure. This results in a considerable quality increase of the subsurface temperature estimation.