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Successful design of enhanced geothermal systems (EGSs) requires accurate numerical simulation of hydraulic stimulation processes in the subsurface. To ensure correct prediction, the underlying model assumptions and constitutive relationships of simulators need to be verified against experimental datasets. With the aim of generating laboratory-scale benchmark datasets, a state-of-the-art testing facility was developed, allowing for experiments under controlled conditions. Samples of size 30 cm × 30 cm × 45 cm were subjected to confining stresses while high-pressure fluid was injected into the sample through a pre-drilled borehole, where a saw-cut notch was used to initiate a penny-shaped fracture. Fracture growth and propagation was monitored by measuring pressure data and acoustic emissions detected using 32 seismic sensors. Subsequently, samples were split along the fracture plane to outline the created fracture marked by a red-dyed injection fluid. Finally, a 2D fracture contour was generated using photogrammetry. Presented datasets, accessible via a public repository, include experiments on granite and marble samples. They can be used for verifying and improving numerical codes for field stimulation designs., Measurement(s) geological fracture • pressure • fluid flow rate • acoustic emission spectrum Technology Type(s) visual inspection of rocks after fracturing • pressure transmitters (Keller P33x/1000 bar/80794) • syringe pump • acoustic sensors Factor Type(s) rocks (granite and marble) Sample Characteristic - Environment laboratory facility Machine-accessible metadata file describing the reported data: 10.6084/m9.figshare.12490064

Comparisons of fix and variable costs for converting different forms of primary into electric energy performed by national and international agencies as well as by a major German electrical utility consented in concluding: geothermal electricity produced from natural steam or permeable hot water reservoirs is among the cheapest renewable forms of electricity. However, in years to come, geothermal electricity may find itself restricted to the limited number of regions worldwide which feature these particularly favorable reservoirs. This would exclude most of the continental land mass of the earth which is hot but lacks natural steam or hot water reservoirs. This is due to the current lack of proven technology for engineering man-made geothermal reservoirs where natural ones do not exist. At the same time, proven technologies for generating wind and solar electricity are readily available. This may turn out to be a serious competitive disadvantage for geothermal energy as fossil primary energies are being replaced by renewables. Therefore, dedicated and proactive funding of geothermal technology development is required at much larger scale than previously for making geothermal electricity competitively available almost everywhere.

Reconstructing the thermal history in active volcanic complexes characterized by multiple magmatic events is challenging due to the limited knowledge of the nature and extent of the transient heat sources. Although understanding of the geometry and architecture of a magmatic system is of prime importance for accurate temperature assessments, it is still one of the most uncertain parameters in numerical models. In this work, we presented a methodology for thermal assessment in active volcanic systems, whereby field-based geological, geochemical and petrological data are integrated to define the transient heat sources of a magma plumbing system. This time-varying heat source conceptual model is applied in the Los Humeros Volcanic Complex, an active Quaternary caldera complex in the Trans Mexican Volcanic Belt, for evaluating the thermal footprint related to the major volcanic events. The site is characterized by two caldera-forming eruptions, the Los Humeros (164 000 years ago) and the Los Potreros (69 000 years ago) and numerous episodes of post-caldera bi-modal volcanism during Holocene period (8 000 – 3 000 years old). The transient nature of the heat sources is implemented as time-varying temperature boundary conditions and the complete temporal evolution for a period of 182 000 years is simulated in 13 modeling stages. The thermal impact due to the voluminous caldera-forming events and the later short-lived magma pockets of Holocene ages is simulated by emplacing heat sources in the numerical model distributed heterogeneously in space and active at different instants of time. The depth, volume and age of the magma pockets are constrained from geochemical, petrological, geochronological and thermo-barometric analysis of erupted material. The present temperature state obtained from this approach agrees well with the temperature data recorded in the geothermal wells. The thermal footprint of the individual volcanic events indicates that almost 80 % of the present-day thermal contribution results from the massive caldera-forming events. The post-caldera Holocene magma pockets had additionally increased temperatures locally by 10 % - 20 % depending on the volumes and ages of the magma pockets. The present-day thermal regime of the younger Holocene magma pockets suggests existence of super-hot resources at shallow depths in the southern part of the geothermal field, making it a potential site for future exploration activities.

European Geosciences Union General Assembly 2017, EGU Division Energy, Resources & Environment (ERE) / Edited by Sonja Martens, Christopher Juhlin, Suzanne Hangx, Michael Kühn European Geosciences Union General Assembly 2017, ERE, Vienna, Austria, 23 Apr 2017 - 28 Apr 2017; Amsterdam [u.a.] : Elsevier, Energy Procedia, 125, 318-326 (2017). doi:10.1016/j.egypro.2017.08.206, Published by Elsevier, Amsterdam [u.a.]

We analyze the likelihood of success for heat production strategies in a sandstone reservoir in the north-eastern German basin in a depth of about 2 km by simulating both double and single well configurations. For this test case study we use an exploited oil and gas field. We combine seismic interpretation, numerical modeling, and stochastic estimation of rock properties to predict the transient temperature and pressure variations and their uncertainties in a geothermal reservoir. We demonstrate the essential necessity in geothermal reservoir modeling to account for heterogeneity of rock properties. We use 3D seismic data and stratigraphy data from about 100 wells at 1500 m – 2500 m depth for setting up a 3D stratigraphic model. Rock properties are assigned to this model by a Monte Carlo approach using Sequential Gaussian Simulation. Using 3D inversion of temperature data obtained in the wells we estimate a specific heat flow of 77.7 mW m −2 ± 1.2 mW m −2 at 6 km depth, in agreement with a temperature of 87.1 °C ± 1.8 K in the Rhaetian sandstone target layer at a depth of ∼2 km. For different types of potential geothermal well installations inside the Rhaetian sandstone layer the probability of success is just 1.6%.

Granitoid intrusions are the primary heat source of many deep geothermal reservoirs in Tuscany. The depth and shape of these plutons, characterised in this study by a prominent seismic reflector (the K horizon), may vary significantly within the spatial scale of interest. In an exploration field, simulations reveal the mechanisms by which such a heat source influences temperature distribution. A simple analysis quantifies the sensitivity of potentially measurable indicators (i.e. vertical temperature profiles and surface heat flow) to variations in depth, temperature, and shape of the heat source within given ranges of uncertainty., Geothermal Energy, 4, ISSN:2195-9706

Hydrothermal systems are characterised by complex interactions between heat transfer, fluid flow, deformation, species transport and chemical reactions. Numerical models can provide quantitatively constrained information in regions where acquisition of new data is difficult or expensive thus providing a means for reducing risks, costs, and effort during targeting, production, and management of resources linked to hydrothermal systems. Here we show how numerical simulations of hydrothermal processes can be used to better understand coupled reactive transport in modern geothermal systems and in ancient hydrothermal ore deposits. We give examples based on the Enhanced Geothermal System at Soultz-sous-Forets in France, hydrothermal mineralisation at Mount Isa in Australia, and the geothermal resource at Hamburg-Allermohe in Germany.

The heat of the Earth derives from internal and external sources. A heat balance shows that most of the heat provided by external sources is re-emitted by long-wavelength heat radiation and that the dominant internal sources are original heat and heat generated by decay of unstable radioactive isotopes. Understanding of the thermal regime of the Earth requires appreciation of properties and mechanisms for heat generation, storage, and transport. Both experimental and indirect methods are available for inferring the corresponding rock properties. Heat conduction is the dominant transport process in the Earth’s crust, except for settings where appreciable fluid flow provides a mechanism for heat advection. For most crustal and mantle rocks, heat radiation becomes significant only at temperatures above 1200°C.

We estimate the maximum geothermal potential in Germany available for exploitation by operated engineered geothermal systems (EGS). To this end, we assume that (a) capabilities for creating sufficient permeability in engineered deep heat exchange systems will become available in the future and (b) it will become possible to implement multiple wells in the reservoir for extending the rock volume accessible by water circulation for increasing the heat yield. While these assumptions may be challenged as far too optimistic, they allow for testing the potential of EGS, given the required properties, in countries lacking natural steam reservoirs. With this aim, we model numerically the thermal and electric energies which may be delivered by such systems by solving coupled partial differential equations governing fluid flow and heat transport in a porous medium. Thus, our model does not represent the engineered fractures in their proper physical dimension but rather distributes their flow volume in a small region of enhanced permeability around them. By varying parameters in the subsurface, such as flow rates and well separations, we analyze the long-term performance of this engineered reservoir. For estimating the maximum achievable potential for EGS in Germany, we assume the most optimistic conditions, realizing that these are unlikely to prevail. Considering the available crystalline landmass and accounting for the competing land uses, we evaluate the overall EGS potential and compare it with that of other renewables used in Germany. Under most optimistic assumptions, the land surface available for emplacing EGS would support a maximum of 13,450 EGS plants each comprising 18 wells and delivering an average electric power of 35.3 MW e . When operated at full capacity, these systems collectively may supply 4155 TWh of electric energy in 1 year which would be roughly seven times the electric energy produced in Germany in the year 2011. Thus, our study suggests that major scientific, engineering, and financial efforts are justified for developing the drilling and stimulation technologies required for creating the permeabilities required for successful EGS. Then, EGS will have great potential for contributing towards national power production in a future powered by sustainable, decentralized energy systems., Geothermal Energy, 3 (1), ISSN:2195-9706

European Geosciences Union General Assembly 2015 - Division Energy, Resources and Environment, EGU 2015 / Edited by Maria Ask, Suzanne Hangx, Viktor Bruckman and Michael Kühn European Geosciences Union General Assembly 2015, EGU 2015, Vienna, Austria, 12 Apr 2015 - 17 Apr 2015; Amsterdam [u.a.] : Elsevier, Energy procedia 76, 321-330 (2015). doi:10.1016/j.egypro.2015.07.869, Published by Elsevier, Amsterdam [u.a.]

Fluid flow in reservoirs can be identified by its characteristic signatures in temperature logs. The method presented here is based on the great sensitivity of the subsurface temperature field with respect to heat advected by fluid flow. This requires that all other temperature perturbations are eliminated. These may be caused, for instance, by heterogeneous thermal properties or paleoclimate transients, both of which may distort an otherwise linear temperature log. We propose a simple geothermal technique for quantifying regional strata-bound flow based on a detailed analysis of borehole temperatures. Depending on the spatial data distribution and the geothermal situation, this method yields results of varying quality. In a case study with data from southern Germany, the method yields a lower bound for the flow magnitude. Another case study from northern Germany yields the magnitude of the regional flow owing to a larger data set and information of the direction of flow from additional independent information. Under optimum conditions, this method also yields an estimate of the direction of regional strata-bound groundwater flow as well as the uncertainty in magnitude and direction. Finally, the full potential of the method is illustrated using a synthetic data set.

We present new heat flow values and other geothermal data in the upper crystalline crust in the immediate vicinity of the 12.4-km deep Kola super-deep borehole, NW Russia. Our results show a systematic vertical increase in geothermal gradient and heat flow density as deep as we could measure (1.6 km). Our results confirm earlier results on vertical heat flow trends of in the uppermost part of the Kola super-deep hole, and imply that the thermal regime is not in steady-state conductive conditions. In an area of 3-km × 5-km measurements were performed in 1–2-km deep boreholes surrounding the Kola super-deep hole and on core samples from these holes. Temperature logs are available from 36 holes. Core data exists from 23 boreholes with a total length of 11.5 km at a vertical resolution of 10 m. We carried out a very detailed study on thermal conductivity with regard to anisotropy, inhomogeneity and temperature dependence. Tensor components of thermal conductivity were determined on 1375 core samples from 21 boreholes in 3400 measurements. Additionally, we measured specific heat capacity, heat generation rate, density, porosity, and permeability on selected subsets of core samples. Heat flow from 19 boreholes varies between 31 and 45 mW m−2 with an average value of 38 mW m−2. In most boreholes the vertical heat flow profiles show a considerable variation with depth. This is consistent with observations in the upper part of the Kola super-deep borehole. We conclude that this variation is not caused by technical operations but reflects a natural process. It is considered to be due to a combination of advective, structural and paleoclimatic effects. Preliminary 3-D numerical modeling of heat and flow in the study area provides an indication of relative contributions of each of these factors: advective heat transfer turns out to have a major influence on the vertical variation of heat flow, although transient changes in surface temperature may also cause a significant variation. Heterogeneity of the rocks in the study area is less important.

By the end of 1995 direct thermal use of geothermal energy in Germany amounted to an installed thermal power of roughly 323 MWt. Of this sum, 38.7 MW, are generated in 22 major centralized installations. Small, decentralized earth-coupled heat pumps and groundwater heat pumps are estimated to contribute an additional 95 and 190 MW, respectively. By the year 2000 an increase in total installed geothermal power of about 144 MWt is expected: 115 MWt from major central and 29 MWt from small, decentralized installations. This would bring direct thermal use in Germany close to an installed thermal power of 467 MWt. At present no electric power is produced from geothermal resources in Germany, whose annual final energy consumption now amounts to about 9000 PJ. This is equivalent to a total consumed power of 285,000 MW. Almost 60% of this energy is required as heat. The technical potential for direct thermal use of geothermal energy in Germany is estimated to be 1622 PJ yr−1 for hydrothermal applications and 960 PJ yr−1 for earth-coupled and groundwater heat pumps; this is equivalent to a thermal power generation of 51,400 and 30,420 MW, respectively. The total, 81,820 MWt, corresponds to about 29% of the country's annual final energy consumption, or roughly 49% of its demand for heat. However, at present only about 4% of the existing technical potential for direct thermal use of geothermal energy meets the demand for heat. If the vast potential of geothermal energy for direct thermal use was utilized to substitute fossil fuels, roughly 110 million tons less of CO2 would be released to the atmosphere annually, equivalent to about 12% of Germany's CO2 output in 1994.

Porosity and permeability of reservoirs are key parameters for an economical use of hydrogeothermal energy. Both may be significantly reduced by precipitation of minerals. The Allermohe borehole near Hamburg (Germany) represents a case in which precipitation of anhydrite resulted in a failed reservoir development. We introduce a new numerical approach for better understanding of this, considering both the spatial variations of supersaturation on the pore scale and the pore space structure. Besides pore size this requires consideration of the effects of surface charge density on the fluid supersaturation in porous media and of the role of fluid flow velocity.

We present a novel approach for storing CO2 not only by hydrodynamic trapping within a reservoir but for converting it into the geochemically more stable form of calcite. The combination of CO2 storage and geothermal energy production is generally feasible because candidate sites are available, anhydrite is transformable into calcite and alkalinity can be provided by fly ashes or even in-situ. However, under the current circumstances the technology is not economical because the storable amount at depth is significantly less than one million tons over the life time of a geothermal heating plant.

SUMMARY The drill hole SG-3, 12261 m deep in the Pechenga-Zapolyarny area, Kola Peninsula, Russia, is currently the deepest drill hole in the world. Geothermal measurements in the hole reveal a considerable variation (30-68 mW m-') with depth in the vertical component of heat-flow density (HFD). We simulate heat and fluid flow in the bedrock structure of the Kola deep-hole site. Various potential sources for the observed HFD variation are discussed, with special emphasis on advective heat transfer, palaeoclimatic ground surface-temperature changes and refraction of heat flow due to thermal conductivity contrasts. A 2-D finite-difference (FD) porous-medium model of the Kola structure, constructed from all available data on lithology, hydrogeology , topography, thermal conductivity and heatproduction rate in the deep-drilling area, is the basis of all forward-model calculations. A conductive, steady-state simulation indicates that heat production and refraction create a variation of about 15 mW mp2 in the uppermost 15 km, but are insufficient to reproduce the measured HFD-depth curve in the uppermost 2-4 km. However, if topography-driven groundwater flow is considered in the model, the measured HFD variation is easily explained. The most sensitive parameters in fitting the model results to the observed HFD-depth curve are the permeability of the top 4 km ( 10-'4-10-'s m2) and the (constant) HFD applied at the base of the model at 15 km depth (40-50 mWmp2). The palaeoclimatic effect for the Kola structure was calculated with a conductive transient simulation. A simplified ground surface-temperature history (GTH) of the Kola area was simulated by varying the surface temperatures of the model during different intervals of the simulation. Our results indicate that the measured variation in the vertical HFD cannot be explained by the palaeoclimatic effect alone, because its amplitude decreases rapidly from about 16mW m-' near the surface to less than 2 mW m-2 at depths in excess of 1.5 km.

Geothermal energy is a clean, globally available and nearly inexhaustible energy resource. However, the necessary deep drill holes demand financial investments on the order of a few tens of millions of Euros, without guarantee that the required flow rates and temperature of the produced fluids allow an economical use. This exploration risk often turns into a major show-stopper for a commercial use of geothermal energy. Improved methods for estimating the distribution of subsurface hydraulic and thermal properties will result in a reduced exploration risk. Therefore, reliable estimates of these properties are mandatory for the economic and technical planning and operation of a geothermal installation. Not in the least, this comprises quantifying the uncertainty of these estimates at an early stage of reservoir exploration and continuous updates and modifications as reservoir development progresses.

Development of geothermal energy and basin-scale simulations of fluid and heat flow both suffer from uncertain physical rock properties at depth. Therefore, building better prognostic models are required. We analysed hydraulic and thermal properties of the major rock types in the Molasse Basin in Southern Germany. On about 400 samples thermal conductivity, density, porosity, and sonic velocity were measured. Here, we propose a three-step procedure with increasing complexity for analysis of the data set: First, univariate descriptive statistics provides a general understanding of the data structure, possibly still with large uncertainty. Examples show that the remaining uncertainty can be as high as 0.8 W/(m K) or as low as 0.1 W/(m K). This depends on the possibility to subdivide the geologic units into data sets that are also petrophysically similar. Then, based on all measurements, cross-plot and quick-look methods are used to gain more insight into petrophysical relationships and to refine the analysis. Because these measures usually imply an exactly determined system they do not provide strict error bounds. The final, most complex step comprises a full inversion of select subsets of the data comprising both laboratory and borehole measurements. The example presented shows the possibility to refine the used mixing laws for Petrophysical properties and the estimation of mineral properties. These can be estimated to an accuracy of 0.3 W/(m K). The predictive errors for the measurements are 0.07 W/(m K), 70 m/s, and 8 kg/m^3 for thermal conductivity, sonic velocity, and bulk density, respectively., Comment: 13 pages, 10 figures, International Journal of Earth Sciences

This special issue of Journal of Geophysics and Engineering is dedicated to a collection of papers which resulted from an international workshop held in Aachen, Germany, on 4-7 October 2004, called 'New and Classical Applications of Heat Flow Studies'. This was the third in a series of topical geothermal workshops arranged by the Geothermal Working Group of the German Geophysical Society (DGG) and was organized by the Institute of Applied Geophysics at RWTH Aachen University under the auspices of the International Heat Flow Commission of the International Association of Seismology and Physics (IASPEI). The meeting was attended by some 60 scientists from 14 countries and three continents. Financial assistance, granted by DGG and IASPEI, allowed us to provide partial support for a total of eight students, young scientists and eminent researchers from eastern Europe and overseas. The convenors of the meeting were Christoph Clauser (Aachen), Thomas Kohl (Zurich) and Makoto Taniguchi (Kyoto). The main local organizers were Volker Rath (scientific programme) and Ute Kreutz (accommodation and financial affairs). The topics addressed in more than 50 oral and poster presentations indicated that today intriguing new applications of heat flow studies have emerged, complementing the classical topics of heat flow mapping and the tectonic implications of heat flow. In classical applications, for instance, thermal signatures of water flow or downward diffusion of variations in the Earth's mean temperature are considered as noise which needs to be corrected prior to further use of the data. In contrast, in several new applications it is exactly the information contained in these signatures which has been extracted and interpreted. For instance, over the past two decades, work on the most prominent of these new applications has been devoted to inverting the variation of the Earth's past mean ground surface temperature (GST). As of today, GST provided by the geothermal method has become widely recognized as a valuable supplement to established climate proxies. In contrast to these, GST is directly linked to climate-related temperature variations. It is based in a straightforward manner on heat transport and thermal physics and, in contrast to climate proxies, does not require transfer functions for relating temperature-sensitive phenomena to temperature.The second new application makes use of the signature of heat advection to identify and quantify flow in the deep subsurface. Based on high-quality data of sufficient number and distribution, the method's detection limit is far below that of any other technique. These are but two examples where, under a new perspective, noise has turned into signal. Last but not least, the potential of geothermal energy to supply low-pollution, low-carbon-dioxide electric power and heat has been recognized and exploited for more than 100 years, primarily in tectonically active regions with natural steam reservoirs. However, the ever-increasing price of fossil fuels and their exhaust of greenhouse gases have turned geothermal energy into an attractive option, even in geothermally less favourable regions. In this more demanding situation, understanding different heat transport processes requires more refined techniques for data correction, parameter identification and numerical simulation. This special issue comprises a selection of 11 papers which emerged from this meeting. Corresponding to the scope of the meeting, four papers address paleoclimatic topics, four papers deal with topics related to subsurface flow, and five papers discuss various aspects related directly or indirectly to geothermal energy. Thus, this special issue reflects exciting trends in current geothermal research. It illustrates that today geothermal research comprises new and challenging fields, with applications in environmental sciences, reservoir engineering, and climate and energy research.

Application of the Debye-Hückel theory for chemical reaction modeling of geothermal brines does not yield sufficiently accurate results. Thus, for the development of a new chemical reaction module for the numerical simulation model SHEMAT (Clauser and Villinger, 1990), the Pitzer formalism (Pitzer, 1991) is used to calculate aqueous speciation and mineral solubilities. It is based on an extended code of PHRQPITZ (Plummer et al., 1988). Using temperature dependent Pitzer coefficients, the system Na- K-Mg-Ca-Ba-Sr-Si-H-Cl-SO4-OH-(HCO3-CO3-CO2)-H2O can be modeled with sufficient accuracy for temperatures from 0° to 150°C. The incorporated carbonic acid system (set in parentheses in the list above) is valid for temperatures from 0 to 90°C, only. Flow, heat transfer, species transport, and geochemical reactions are mutually coupled for modeling reactive flow. Changes in porosity and permeability influence the flow and transport properties of the reservoir. These changes are taken into account by a relation derived from a fractal model of the pore space structure (Pape et al., 1999). A conceptual case study of the injection behavior of a geothermal installation focuses on the immediate vicinity of the well. The injection of cold water has a great influence firstly on the hydraulic conductivity of the aquifer indicated by continuous head pressure increase at the well and secondly on the equilibria between the minerals of the formation and the geothermal fluid. Reservoir changes are studied for the two cases of temperature dependence of solubility, prograde (i.e. barite) and retrograde (i.e. anhydrite). Dissolution of anhydrite induced by cooling down increases the permeability of the formation in a growing region around the borehole and precipitation at the temperature front decreases it. During the initial period of reinjection considered in this study, the negative effect on the injectivity by the colder water is partially compensated by the dissolution of anhydrite. Precipitation of barite around the borehole does not alter the permeability of the formation significantly because the volume of relocated mineral is too small.

This paper presents a workflow for resource characterization and assessment of exploration geothermal fields with minimum data. Our approach utilizes stochastic methods to estimate the temperature distribution at potential target depths by focusing on the impact of uncertain input parameters such as thermal conductivity and porosity. We first perform stochastic forward simulations to determine the initial steady-state thermal field and subsequently quantify the uncertainty via a Monte Carlo approach known as Sequential Gaussian Simulation (SGSim). Next, we analyze the in-field likelihood of success for Enhanced Geothermal Systems by simulating hypothetical energy production scenarios based on existing geothermal installations. This approach is applied to the case study of a Hot Dry Rock geothermal field with two exploration wells, located in Acoculco, Mexico. Data scarcity in this field necessitates the use of stochastic methods for plausible prediction of reservoir temperature used to determine the accessible thermal power. Once reliable temperature estimates are obtained at potential target depths, we simulate production scenarios by assuming a prior successful stimulation process in the existing wells. In addition to providing preliminary estimates of thermal power for different injection/production rates, stimulated volumes and created permeability, we present the long-term impact of production on the temperature and pressure fields.