Outcrop Analogue vs. Reservoir Data: Characteristics and Controlling Factors of Physical Properties of the Upper Jurassic Geothermal Carbonate Reservoirs of the Molasse Basin, Germany

摘要

In the early stages of reservoir exploration, the characterization of the reservoir is mainly accomplished by evaluating drilling data and seismic surveys. Especially in carbonate reservoirs the distinction of different facies zones is very challenging. For reservoir predictions density, porosity, permeability, thermal conductivity/diffusivity, and specific heat capacity have to be quantified as precisely as possible. Outcrop analogue studies enable the determination and correlation of facies related thermo-physical and petrophysical parameters. As these parameters show facies related trends, applying a thermofacies classification on the carbonate formations is helpful to understand the heterogeneities and to identify production zones. In combination with drilling data from a 1,600 m deep research drilling and a 4,850 m (total vertical depth, measured depth: 6,020 m) deep geothermal well (bottom hole temperature of around 170°C) the reservoir property prediction can be validated and consequently the exploration becomes more precise. The outcrops of the Swabian and Franconian Alb represent the target formations of Upper Jurassic carbonate reservoirs in the adjacent Molasse Basin. The hydraulic conductivity of these carbonate formations is mainly controlled by tectonic elements and karstification. The type and grade of karstification is also facies related. The rock permeability has only a minor effect on the reservoir's sustainability except for some grain- and dolostones with higher porosities and permeabilities. The overall rock permeability ranges from 10~(-18) m~2 to 10~(-14) m~2 (0.001 - 10 mD). A high variation of thermo-physical parameters is recognized within facies zones. Mud- and wackestones show typical thermal conductivities of around 2 Wm~(-1)K~(-1). Mudstones have lower thermal conductivities than wackestones due to their clay content. The permeability range of mud and wackestones is about the same. Reef structures show the highest values of thermal conductivity (up to 4.8 Wm~(-1)K~(-1)), due to secondary silicified sponge layers and dolomitization processes. Also in the dolomitized areas higher permeabilities can be observed. Most parameters are determined on oven dried samples. These values have to be corrected via transfer models for water saturated and according reservoir temperature and pressure conditions. To validate these calculated parameters a Thermo-Triaxial-Cell simulating the temperature and pressure conditions of the reservoir is used. Under reservoir conditions a decrease of 2-3 magnitudes in permeability is observed due to the thermal expansion of the rock matrix. From laboratory tests and analyzed drilling data can be concluded that in tight carbonates the matrix permeability is temperature controlled. The thermo-physical matrix parameters are density controlled. Density increases typically with depth and especially with dolomite content, therefore thermal conductivity increases but also decreases with increasing temperature, which is the dominant factor. Specific heat capacity increases with depth and temperature in a range from 790 to 1230 Jkg~(-1)K~(-1). In general the facies related characterization and prediction of reservoir properties proves to be a powerful tool for the exploration and operation of geothermal reservoirs.