Processes operating during the extraction of heat from fractured rocks influence dynamically their fluid flow and heat transport characteristics. The incorporation of pressure- and ...temperature-dependent rock parameters, coupled with geomechanical deformation, is particularly important for predictive modelling of geothermal reservoirs hosted in crystalline rock masses. Changes in flow and transport parameters of fractures caused by variations in local effective stress are computed using an experimentally validated geomechanical model McDermott, C.I., Kolditz, O., 2006. Geomechanical model for fracture deformation under hydraulic, mechanical and thermal loads. Hydrogeol. J. 14, 487–498. Local effective stress changes are linked to alterations in reservoir fluid pressures, and to
in situ stress conditions, including the build-up of thermal stresses resulting from the cooling of the rock mass. These processes are simulated using a finite-element model in order to study the behaviour of the Spa Urach (southwestern Germany) potential geothermal reservoir. The model couples mechanical deformation and alteration of fracture parameters with pressure-, temperature- and salinity-dependent fluid parameter functions. The effects of potential reservoir damage on reservoir productivity are investigated to help identify optimal heat recovery schemes for the long-term economical exploitation of geothermal systems. Simulation results indicate that preferential fluid flow paths and shortcuts may develop, depending on the mechanical and thermal stress releases that occur during intense exploitation of these systems.
Islamic colored glass windows (qamarīyāt) have long been disregarded by scholarship despite their popularity with nineteenth-century travelers. Their vibrant light and bright colors have sparked the ...enthusiasm of artists, architects, and collectors who depicted, recreated, and displayed this fragile art form. By focusing on the British architect James William Wild, the eccentric traveler and art collector Karl von Urach, and the iconic American artist Louis Comfort Tiffany, this contribution will highlight the artistic and cultural significance of qamarīyāt as an expression of the intense colors of the East.
We present halogen compositions in a series of alkaline rocks from the Kaiserstuhl, Hegau and Urach areas of the Upper Rhine Graben region (South Germany). Most primitive rocks (olivine melilitites ...and olivine nephelinites) have lower Cl and Br concentrations (generally below 100μg/g and below 0.3μg/g, respectively) compared to more evolved tephrites, phonolites and related rocks (up to 7600μg/g Cl and 34μg/g Br). However, the Cl/Br ratios of the majority of the investigated samples are relatively uniform (371±120), regardless of rock type and sample locality, suggesting that partial melting, fractional crystallization, and degassing have limited effects on the fractionation of Cl from Br. The mean value of the Cl/Br ratio is similar to previous estimates for basaltic rocks representing MORB and OIB mantle signatures.
Fluorine concentrations of the primitive rocks show limited variations (900–1100μg/g) and are within the range defined by the evolved rocks (400–2100μg/g), but are much higher than previous estimates for the MORB and OIB mantle (50–135μg/g). This may indicate a relatively F-rich mantle source beneath the Rhine Graben region. In contrast to Cl/Br ratios, the F/Cl ratios vary significantly over three orders of magnitudes (from <0.1 to around 100) and decrease from primitive rocks to more evolved ones, implying that magmatic processes such as fractional crystallization and degassing strongly effect this ratio.
Positive correlations between F, Cl and Br contents in a series of tephritic rocks from Kaiserstuhl probably record degassing processes, since these samples have similar geochemical and isotopic signatures with the only difference being textural: they vary from glassy to phanerocrystalline. Simplified calculations imply that about 50% F and 90% Cl and Br were released during the crystallization process, resulting in increasing F/Cl, but relatively constant Cl/Br ratios.
•Olivine nephelinites and olivine melilitites have high F but low Cl and Br contents.•Evolved tephrites, phonolites and related rocks show variable halogen contents.•In magmatic processes, Cl is not strongly fractionated from Br but largely from F.
Permeability is a critical geologic parameter because migrating fluids play a fundamental role in heat and mass transfer. Because of their assumed low hydraulic conductivities, crystalline basement ...rocks are being considered for nuclear waste repositories. A compilation of existing data and a description and interpretation of new hydraulic test data in the crystalline basement of the 4.5-km-deep Urach 3 borehole (southwest Germany) are presented, together with an interpretation of all test campaigns for Urach 3. The tests showed that within the crystalline basement, permeability is decreasing with depth. In the Urach borehole many high-pressure tests with well-head pressures of more than 600 bar were carried out. During hydraulic tests with well-head pressures above 176 bar, permeability of the crystalline basement increased dramatically, showing the elastic reaction of the rock as a result of pressure; during tests with well-head pressures below 176 bar, there was no significant elastic reaction of the rock. The pressure-dependent change of transmissivity was described in terms of a set of power laws relating injection rate to transmissivity and fracture width. Probably due to the lack of shear stress, no significant increase in permeability remained after any high-pressure tests.
The Urach 3 research borehole in SW Germany has been drilled through a sedimentary cover sequence and reached gneisses of the Variscan crystalline basement at 1604 m below surface. An additional ...2840 m has been drilled through fractured basement rocks. The borehole has been used for hydraulic tests in the context of a ‘hot dry rock’ (HDR) project. The sedimentary cover ranges from the Carboniferous to the Middle Jurassic (Dogger) in age and comprises mostly clastic sediments in the Paleozoic and limestone and shale in the Mesozoic. Water composition data from 10 different depths include samples from all major lithological units. The total dissolved solids (TDS) increases from the surface to about 650 m where it reaches 4.1 g l−1 in Triassic limestone. In lower Triassic sandstones, TDS increases very sharply to 28.5 g l−1 and the water is saturated with pure CO2 gas. With increasing depth, TDS does not change much in the clastic sediments of the Permian and Carboniferous. The crystalline basement is marked by a very sharp increase in TDS to 55.5 g l−1 at about 1770 m depth. TDS increases within the basement to more than 78.5 g l−1 at about 3500 m depth. The data suggest that there is limited vertical chemical communication over long periods of time. The CO2 gas cap in the lower Triassic sandstones requires a gastight cover. The chemical stratification of the fluids relates to the permeability structure of the crust at the Urach site and fits well with hydraulic and thermal data from the site.
Seismic lamination in the lower crust associated with marked anisotropy has been observed at various locations. Three of these locations were investigated by specially designed experiments in the ...near vertical and in the wide-angle range, that is the Urach and the Black Forrest area, both belonging to the Moldanubian, a collapsed Variscan terrane in southern Germany, and in the Donbas Basin, a rift inside the East European (Ukrainian) craton. In these three cases, a firm relationship between lower crust seismic lamination and anisotropy is found. There are more cases of lower-crustal lamination and anisotropy, e.g. from the Basin and Range province (western US) and from central Tibet, not revealed by seismic wide-angle measurements, but by teleseismic receiver function studies with a P–S conversion at the Moho. Other cases of lamination and anisotropy are from exhumed lower crustal rocks in Calabria (southern Italy), and Val Sesia and Val Strona (Ivrea area, Northern Italy). We demonstrate that rocks in the lower continental crust, apart from differing in composition, differ from the upper mantle both in terms of seismic lamination (observed in the near-vertical range) and in the type of anisotropy. Compared to upper mantle rocks exhibiting mainly orthorhombic symmetry, the symmetry of the rocks constituting the lower crust is either axial or orthorhombic and basically a result of preferred crystallographic orientation of major minerals (biotite, muscovite, hornblende). We argue that the generation of seismic lamination and anisotropy in the lower crust is a consequence of the same tectonic process, that is, ductile deformation in a warm and low-viscosity lower crust. This process takes place preferably in areas of extension. Heterogeneous rock units are formed that are generally felsic in composition, but that contain intercalations of mafic intrusions. The latter have acted as heat sources and provide the necessary seismic impedance contrasts. The observed seismic anisotropy is attributed to lattice preferred orientation (LPO) of major minerals, in particular of mica and hornblende, but also of olivine. A transversely isotropic symmetry system, such as expected for sub-horizontal layering, is found in only half of the field studies. Azimuthal anisotropy is encountered in the rest of the cases. This indicates differences in the horizontal components of tectonic strain, which finally give rise to differences in the evolution of the rock fabric.
Earth-reservoirs are increasingly exploited today with the extraction of resources, such as heat and hydrocarbons, and the large-scale emplacement of waste, such as CO
2
sequestration. The ...characterization, site investigation, predictive modeling and long-term monitoring are dependent on the processes being investigated and modeled. In most cases complex coupled processes have to be addressed in a geologically complex rock mass system. In this paper we present a conceptual holistic framework known as geomechanical facies linking all the scales of investigation, characterization and reservoir development methods. We demonstrate this concept on the work undertaken during the design and development of the enhanced geothermal systems (EGS) systems at the forefront of European Hot-Dry-Rock (HDR) technology, Soultz-sous-Forêts (France) and Spa Urach (Germany). Soultz-sous-Forêts is situated within granitic rocks and an active tectonic graben system in the central part of the Rhine Graben. It presents conditions of lithology, temperature, stress, hydraulics and geochemistry that are very different from those at Spa Urach, located in a very dense gneiss formation in the South German crystalline complex. Spa Urach exhibits more elastic behavior and is set tectonically within an almost inactive strike-slip stress field described in more detail in Sects. “
Drill core analysis
” and “
Hydraulic stimulation at Spa Urach
”. This paper compares the exploration and field development methods used at these two sites against the back drop of the geomechanical facies concept. Issues addressed include the key parameters for flow and heat transport properties, coupled hydro-mechanical process identification, the success of the HDR reservoir as a heat exchanger and exploration techniques applicable to the different facies. Identification of the key geomechanical facies gives an indication as to which technologies will prove more efficient in the application of HDR technology. The results of this study will hopefully help in developing heat recovery schemes for the long-term economical operation of future HDR plants and EGS as well as assist in the understanding of engineered geosystems.
Calcareous freshwater streams near Bad Urach, southwest Germany, were studied to determine the environmental limits to cyanobacterial calcification. Daily variations in temperature, pH, calcium ...concentration, and alkalinity were measured at seasonal intervals from September 1993 to January 1995 in two small woodland streams with lengths of 1.0 and 1.5 km. The principal cause of supersaturation in these fast-flowing streams is inorganic carbon dioxide outgassing from resurging groundwater, locally enhanced by turbulence at waterfalls and cascades. Photosynthetic uptake of carbon dioxide and temperature effects are negligible. Nonetheless, organic substrates, particularly cyanobacteria-dominated microbial mats and biofilm, significantly localize precipitation by providing suitable nucleation sites. Precipitation rates on artificial substrates, up to 2.2 mm/yr on limestone, correlate with high Saturation Index values. Copper substrates inhibited microbial colonization and received negligible encrustation. Tufa formation and external calcium carbonate encrustation of cyanobacteria are conspicuous where the annual WATEQ4F Saturation Index exceeds 0.8, and are slightly below 0.7. Calcium carbonate impregnation of cyanobacterial sheaths has not been observed. We infer that in these fast-flowing streams cyanobacteria utilize CO
2 in photosynthesis whereas elsewhere, in sluggish freshwater, cyanobacteria utilize HCO
3
−, leading to sheath impregnation by calcium carbonate even where Saturation Index is only 0.2–0.3. Thus, photosynthetic influence on cyanobacterial calcification appears to be negligible in fast-flowing CO
2-rich streams and cyanobacterial calcification takes the form of external encrustation. In slow-flowing CO
2-poor streams and lakes cyanobacteria preferentially utilize bicarbonate and sheath impregnation can result. Modern tropical marine carbonate shelves have saturation indices commonly in the range 0.62–0.82 and cyanobacterial calcification is negligible. Extrapolation suggests that cyanobacterial calcification could occur in present-day seas if Saturation Index exceeded 0.8. This level, corresponding with heavy calcification in the freshwater streams studied, may have been widespread in marine environments in the geological past at times when calcified cyanobacteria and environmentally influenced non-skeletal calcium carbonate precipitates were abundant.
In order to investigate the laminated lower crust beneath Urach (SW Germany), we analysed (1) the anisotropy and reflectivity of seismic shear waves, and (2) the composition of xenoliths samples. We ...conclude that the directional dependence of shear waves of the lower crust is effectively transversely isotropic with a maximum velocity difference of 3 to 6%. Both seismic signature and petrological information are compared with reference profiles deduced from the exposed lower crustal sections of Ivrea and Calabria (Italy). The comparison, based on synthetic seismograms, shows that the observed combination of shear wave anisotropy and reflectivity pattern corresponds to alternating layers containing a high amount of metapelites, such as found in the Calabria profile and in the Val Strona subsection of Ivrea. The main portion of the whole rock anisotropy is caused by the lattice-preferred orientation of anisotropic minerals (so-called intrinsic anisotropy). The effect of alternating rock layering (so-called layering anisotropy) contributes only a minor portion to the whole rock anisotropy. Layered mafic intrusions, such as found in the Val Sesia subsection of Ivrea, are nearly isotropic. Both seismic anisotropy and xenoliths indicate that mafic layers do not play a major role for the laminated lower crust at Urach. They may be restricted to a thin layer at the crust–mantle boundary.
Combined Os–Pb–Sr–Nd isotopic compositions of melilitite samples from SW Germany provide new constraints on the origin of melts derived by low degrees of mantle melting in the course of ...Tertiary–Quaternary volcanism in Central Europe. The first Os isotopic compositions reported here show an unusually wide range in initial
187Os/
188Os isotopic ratios of 0.119 to 0.225. The low ratios overlap those in the lithospheric mantle and the highly radiogenic ratios exceed those in ocean island basalts. Pb and Sr isotopic compositions show small variations (
206Pb/
204Pb=19.19–19.57;
87Sr/
86Sr=0.7034–0.7039) whereas Nd isotopes (
ε
Nd 3.4–4.2) indicate melting of overall uniform sources. A positive correlation between initial
187Os/
188Os and
207Pb/
204Pb isotopic ratios is interpreted as a mixing trend between melts with low and lithospheric mantle-like
187Os/
188Os isotopic ratios and an old crustal component. Thus, we interpret the low
187Os/
188Os isotopic ratios in some of the samples as primary mantle source-derived signatures and propose that the melilitites originated from the lithospheric mantle. Independent evidence for an lithospheric mantle source is provided by earlier reported trace element data indicating melting of metasomatized sources with residual K-bearing phases whose stability is confined to the lithospheric mantle rather than thermal plume. A contribution from deeper mantle sources is not required to explain the data; however, the lack of Os data from other more typical basalts of the Central European Volcanic Province (CEVP) precludes an evaluation of the extent of plume involvement. Remarkable characteristics of the melilitites are their high Re/Os and U/Pb ratios. It follows that overprinting of the lithospheric mantle by melts derived by low degrees of melting of metasomatized sources will eventually produce domains with evolved Os and Pb isotopic compositions as required as source material for basalts of high
238U/
204Pb (HIMU) affinity.
PDF
