MeProRisk-II: Optimization strategies and risk analysis for deep geothermal reservoirs
Within the second phase of the scientific network project MeProRisk-II, three different exploration ventures are being evaluated with respect to their geothermal energy potential by using previously developed methods (MeProRisk-I). The project is a research cooperation of five German universities (RWTH Aachen, FU Berlin, Christian-Albrecht-University Kiel, Bergakademie Freiberg, and Schiller-University Jena) and a private company.
Geophysical methods allow sophisticated exploration of the subsurface and the generation of realistic geometrical models. A region in Campania (Italy) has been identified as a possible geothermal reservoir. Simulations of the subsurface specific heat flow, optimized by means of inversion, are performed on a discretized model, on one hand for obtaining realistic temperature estimates, on the other hand for guiding further exploration.
As the availability of fossil energy sources is limited, the development of sustainable alternatives is a key to simultaneously satisfying an increasing demand for energy and reducing greenhouse-gas emissions. One such alternative, capable of providing base-load power, is geothermal energy. The quantification of uncertainties in the evaluation and exploration of geothermal reservoirs is highly relevant in this field. One tool to assess uncertainty is numerical simulation of the reservoir.
A site in Italy, “Guardia dei Lombardi”, is considered a medium-enthalpy reservoir and is currently being explored in MeProRisk-II. Based on about 20 seismic profiles and exploration wells, a geometrical model of the subsurface was established by members of the VIGOR Group (a partner in the MeProRisk-II project). The geometrical model was translated into a numerical model whose crucial parameters (thermal conductivity, porosity, permeability etc.) were measured in our laboratory on core material. To calibrate the temperature field in this simulation model, an inversion is performed based on (1) a deterministic Bayesian approach for determining a spatially constant specific heat flow from below and (2) a stochastic (Monte Carlo) approach in which this basal specific heat flow may vary with position. Temperature measurements within available exploration boreholes serve as data points. Once the basal specific heat flow is established a couples thermal and hydraulic flow simulation allows identifying areas of high geothermal energy potential.
A second study site is the Perth Basin (Australia) in which the geothermal potential is evaluated for the Perth Metropolitan Area (PMA). This region is characterized by an extended reservoir layer (Yarragadee Aquifer) up to 2 km thick with may support a large scale natural flow system depending on regional groundwater supply, geological structure and permeability heterogeneity.
Comprising the metropolitan region of Perth, a structural model is set up using the modeling software “3D GeoModeller” and data of numerous artesian and petroleum wells. The model covers an area of about 5000 km² up to a depth of 4.5 km and focuses on an adequate representation of the Yarragadee Aquifer. We construct a numerical model for fluid flow and heat transport in the Yarragadee Aquifer and consider an ensemble of stochastically varying spatial heterogeneities of porosity and permeability.
Our studies show that large-scale down-flows develop at places where the Yarragadee Aquifer is in contact with the overlaying shallow aquifers. This occurs independently of the particular heterogeneous permeability distribution which does control, however, the natural flow system on a smaller scale. For the Perth Metropolitan Area these results suggest locally reduced temperatures compared to a purely conductive thermal regime.
These kind of numerical reservoir models are required for further simulations which will comprise various geothermal doublet or multiplet designs. Finally, these simulations will enable to assess the reservoir’s economic potential.
This project is funded by the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU) for a time period of three years.Copyright: © RWTH Aachen Copyright: © RWTH Aachen
Figure 2: Perth Basin geothermal potential simulation: a) geographic location of the Perth Basin and the city of Perth in the geological context; b) geological structural model used for numerical simulations; c) (left) one Monte Carlo member of stochastic permeability variations, (right) resulting quasi-steady state temperature field.
Dr. Gabriele Marquart, Jan Niederau M.Sc., Dr. Anozie Ebigbo