David Bruhn

We investigated the deformational behavior of fine-grained calcite- anhydrite aggregates at conditions where the end-member phases are of similar strength and deform by dislocation creep and/or difømsion creep  '  various---' pUlamdunw  m,mby .,,, ,o,,o,,, ..... : pressing CaCO3 and CaSO4 powders into dense aggregates. Grain sizes are 5 and 8 gm for two different anhydrites (An) and

Dehydration of serpentinite has been proposed as an important factor for subduction related processes, such as intermediate-depth earthquakes caused by dehydration embrittlement, weakening of the mantle and arc magmatism. Better understanding of the distribution of water in the subducting slab and of the effect of dehydration on the physical properties of rocks is thus of fundamental importance for our understanding of subduction related processes. We have therefore performed laboratory experiments to determine the petrophysical signature of serpentinite with changing temperature and pressure beyond the dehydration reaction of serpentine to talc, forsterite and water. Seismic velocities and electrical conductivities were measured on various well-characterized serpentinites. In contrast to previous studies, our experiments were performed at undrained conditions, such that water produced by dehydration did not escape from the system. Electrical conductivity of water saturated samples ...

Fully quantitative textures have been measured from experimentally deformed two-phase aggregates of calcite and anhydrite. Two sets of deformation experiments were performed. Firstly, to investigate the development of microstructure and texture with composition, samples with varying volume proportions of calcite and anhydrite were deformed to 20% strain at identical conditions. Secondly, to study the texture development of the two materials

The Geothermal Technology Program at GFZ identify suitable geological structures and horizons for extracting energy and develop new methods to increase the productivity of deep geothermal reservoirs. In this context, the former gas exploration well Groß Schönebeck EGrSk3/90 (50 km northeast of Berlin) was reopened and deepened to the 4309 m depth to serve as a geothermal in situ laboratory for testing stimulation concepts. The objective of these stimulation operations was to create secondary flow paths and to improve the inflow performance of the well.In addition, Groß Schönebeck is the only test site of the EU-project I-GET (Integrated Geophysical Exploration Technologies for deep fractured geothermal systems), where new, cost-effective and reliable geothermal exploration techniques are developed to increase the success rate of drilling.Based on the data of deep neighbouring wells and the industry seismic measurements, a model was generated to visualise the geological setting and t...

The presence of fluids and melts in crustal rocks above subduction zones has often been attributed to the dehydration of subducting oceanic crust and hydrated mantle rocks. Dehydration processes have also been suggested to cause intermediate-depth earthquakes in the subduction zone. The most common mineral containing structurally bound water in the mantle is serpentine. By incorporating water into their crystal structure, serpentine minerals can store up to 13 wt% of water. This water is released when serpentines break down at elevated temperatures and pressures. In this study, we measured the electrical resistivity of dehydrating serpentinites and its evolution with time in laboratory experiments. Electrical properties of rocks respond highly sensitively to transiently available fluids, which are also detectable over very short time spans. The electrical resistivity measurements were performed at elevated pressure and temperature in a gas confining-medium pressure vessel at pressur...

ABSTRACT GEISER was a European project on understanding and mitigation of induced seismicty in geothermal operations. The project involved several European research institutions as well as industry and was funded by the European Commission within FP7. GEISER addressed a better understanding of the key parameters that control induced seismicity in response to an injection. Data from several events of induced seismicity were collected and analysed. Mechanical models were developed to understand the processes leading to induced seismicity and were combined with probabilistic seismic hazard assessment approaches to propose a new, physics based probabilistic forewarning system. This system requires the determination of a maximum acceptable seismic magnitude and its accepted probability of occurrence. The reliability of the dynamic model is based on the availability of rock physics and seismic data, with models updated from real-time monitoring. In addition to this new approach, a number of recommendations and guidelines for licensing authorities, developers and operators of geothermal projects are proposed.

ABSTRACT Energy from deep geothermal resources plays an increasing role in many European countries in their efforts to increase the proportion of renewables in their energy portfolio. Deep geothermal heat and electric power have a high load factor, are sustainable and environmentally friendly. However, the safe, sustainable, and economic development of deep geothermal resources, also in less favourable regions, faces a number of issues requiring substantial research efforts: (1) The probability of finding an unknown geothermal reservoir has to be improved. (2) Drilling methods have to be better adapted and developed to the specific needs of geothermal development. (3) The assessment of the geothermal potential should provide more reliable and clear guidelines for the development. (4) Stimulation methods for enhanced geothermal systems (EGS) have to be refined to increase the success rate and reduce the risk associated with induced seismicity. (5) Operation and maintenance in aggressive geothermal environments require specific solutions for corrosion and scaling problems. (6) Last but not least, emerging activities to harness energy from supercritical reservoirs would make significant progress with qualified input from research. In particular, sedimentary basins like e.g. the North German and Polish Basin, the Pannonian Basin, the Po Valley, the Bavarian Molasse Basin or the Upper Rhine Graben have a high geothermal potential, even if geothermal gradients are moderate. We will highlight projects that aim at optimizing exploration, characterization, and modeling prior to drilling and at a better understanding of physical, hydraulic and chemical processes during operation of a geothermal power plant. This includes geophysical, geological and geochemical investigations regarding potential geothermal reservoirs in sedimentary basins, as well as modelling of geothermally relevant reservoir parameters that influence the potential performance and long-term behavior of a future geothermal power plant. In our overview we will also highlight contributions of EGU2012-sessions ERE1.6 (Geothermal energy from deep sedimentary basins - exploration, exploitation, characterization and modeling) and ERE1.7 (Development of deep geothermal resources).

ABSTRACT To describe the poroelastic behaviour of sandstones, two factors have to be considered: the grain structure and the pore volume included. Changes in these two factors through diagenetic processes, tectonic loading or other forces lead to different results. Often external stresses induce a compaction of the rock and, therefore, a reduction of pore volume and an increased fluid pressure. Under undrained conditions, the largest pore pressure response can be observed. Besides the Biot coefficient, the Skempton coefficient ( B) is one of the most important variables of elastic rock deformation, as it describes the pore pressure change related to the acting stresses. This study shows three ways of determining the Skempton coefficient and gives evidence of its pressure dependence. First, the undrained poroelastic response of a Bentheimer sandstone sample to confining pressure change was measured. Second, a thin-section micrograph was transferred into a finite-element model, including a discretization of the grain structure and the pore space. Finally, the Skempton coefficient of a linear elastic hollow sphere was calculated to prove the laboratory experiment and the numerical simulation.