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Vogt C.,RWTH Aachen | Mottaghy D.,Geophysica Beratungsgesellschaft GmbH | Wolf A.,RWTH Aachen | Rath V.,RWTH Aachen | And 3 more authors.
Geophysical Journal International | Year: 2010

Quantifying and minimizing uncertainty is vital for simulating technically and economically successful geothermal reservoirs. To this end, we apply a stochastic modelling sequence, a Monte Carlo study, based on (i) creating an ensemble of possible realizations of a reservoir model, (ii) forward simulation of fluid flow and heat transport, and (iii) constraining post-processing using observed state variables. To generate the ensemble, we use the stochastic algorithm of Sequential Gaussian Simulation and test its potential fitting rock properties, such as thermal conductivity and permeability, of a synthetic reference model and-performing a corresponding forward simulation-state variables such as temperature. The ensemble yields probability distributions of rock properties and state variables at any location inside the reservoir. In addition, we perform a constraining post-processing in order to minimize the uncertainty of the obtained distributions by conditioning the ensemble to observed state variables, in this case temperature. This constraining post-processing works particularly well on systems dominated by fluid flow. The stochastic modelling sequence is applied to a large, steady-state 3-D heat flow model of a reservoir in The Hague, Netherlands. The spatial thermal conductivity distribution is simulated stochastically based on available logging data. Errors of bottom-hole temperatures provide thresholds for the constraining technique performed afterwards. This reduce the temperature uncertainty for the proposed target location significantly from 25 to 12 K (full distribution width) in a depth of 2300 m. Assuming a Gaussian shape of the temperature distribution, the standard deviation is 1.8 K. To allow a more comprehensive approach to quantify uncertainty, we also implement the stochastic simulation of boundary conditions and demonstrate this for the basal specific heat flow in the reservoir of The Hague. As expected, this results in a larger distribution width and hence, a larger, but more realistic uncertainty estimate. However, applying the constraining post-processing the uncertainty is again reduced to the level of the post-processing without stochastic boundary simulation. Thus, constraining post-processing is a suitable tool for reducing uncertainty estimates by observed state variables. © 2010 The Authors Journal compilation © 2010 RAS.

Mottaghy D.,Geophysica Beratungsgesellschaft GmbH | Schwamborn G.,Alfred Wegener Institute for Polar and Marine Research | Rath V.,Complutense University of Madrid
Climate of the Past | Year: 2013

This study focuses on the temperature field observed in boreholes drilled as part of interdisciplinary scientific campaign targeting the El'gygytgyn Crater Lake in NE Russia. Temperature data are available from two sites: the lake borehole 5011-1 located near the center of the lake reaching 400 m depth, and the land borehole 5011-3 at the rim of the lake, with a depth of 140 m. Constraints on permafrost depth and past climate changes are derived from numerical simulation of the thermal regime associated with the lake-related talik structure. The thermal properties of the subsurface needed for these simulations are based on laboratory measurements of representative cores from the quaternary sediments and the underlying impact-affected rock, complemented by further information from geophysical logs and data from published literature.

The temperature observations in the lake borehole 5011-1 are dominated by thermal perturbations related to the drilling process, and thus only give reliable values for the lowermost value in the borehole. Undisturbed temperature data recorded over more than two years are available in the 140 m deep land-based borehole 5011-3. The analysis of these observations allows determination of not only the recent mean annual ground surface temperature, but also the ground surface temperature history, though with large uncertainties. Although the depth of this borehole is by far too insufficient for a complete reconstruction of past temperatures back to the Last Glacial Maximum, it still affects the thermal regime, and thus permafrost depth. This effect is constrained by numerical modeling: assuming that the lake borehole observations are hardly influenced by the past changes in surface air temperature, an estimate of steady-state conditions is possible, leading to a meaningful value of 14 ± 5 K for the post-glacial warming. The strong curvature of the temperature data in shallower depths around 60 m can be explained by a comparatively large amplitude of the Little Ice Age (up to 4 K), with low temperatures prevailing far into the 20th century. Other mechanisms, like varying porosity, may also have an influence on the temperature profile, however, our modeling studies imply a major contribution from recent climate changes. © Author(s) 2013.

Vogt C.,RWTH Aachen | Iwanowski-Strahser K.,University of Kiel | Marquart G.,RWTH Aachen | Arnold J.,Geophysica Beratungsgesellschaft GmbH | And 4 more authors.
Renewable Energy | Year: 2013

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%. © 2012 Elsevier Ltd.

Chen T.,RWTH Aachen | Clauser C.,RWTH Aachen | Marquart G.,RWTH Aachen | Willbrand K.,RWTH Aachen | And 2 more authors.
Advances in Water Resources | Year: 2015

We present a method to determine equivalent permeability of fractured porous media. Inspired by the previous flow-based upscaling methods, we use a multi-boundary integration approach to compute flow rates within fractures. We apply a recently developed multi-point flux approximation Finite Volume method for discrete fracture model simulation. The method is verified by upscaling an arbitrarily oriented fracture which is crossing a Cartesian grid. We demonstrate the method by applying it to a long fracture, a fracture network and the fracture network with different matrix permeabilities. The equivalent permeability tensors of a long fracture crossing Cartesian grids are symmetric, and have identical values. The application to the fracture network case with increasing matrix permeabilities shows that the matrix permeability influences more the diagonal terms of the equivalent permeability tensor than the off-diagonal terms, but the off-diagonal terms remain important to correctly assess the flow field. © 2015 Elsevier Ltd.

Mottaghy D.,Geophysica Beratungsgesellschaft GmbH | Pechnig R.,Geophysica Beratungsgesellschaft GmbH | Vogt C.,RWTH Aachen
Geothermics | Year: 2011

The proposed Den Haag Zuidwest district heating system of the city of The Hague consists of a deep doublet in a Jurassic sandstone layer that is designed for a production temperature of 75°C and a reinjection temperature of 40°C at a flow rate of 150m 3h -1. The prediction of reservoir temperature and production behavior is crucial for success of the proposed geothermal doublet. This work presents the results of a study of the important geothermal and geohydrological issues for the doublet design. In the first phase of the study, the influences of the three-dimensional (3D) structures of anticlines and synclines on the temperature field were examined. A comprehensive petrophysical investigation was performed to build a large scale 3D-model of the reservoir. Several bottomhole temperatures (BHTs), as well as petrophysical logs were used to calibrate the model using thermal conductivity measurements on 50 samples from boreholes in different lithological units in the study area. Profiles and cross sections extracted from the calculated temperature field were used to study the temperature in the surrounding areas of the planned doublet. In the second phase of the project, a detailed 3D numerical reservoir model was set up, with the aim of predicting the evolution of the producer and injector temperatures, and the extent of the cooled area around the injector. The temperature model from the first phase provided the boundary conditions for the reservoir model. Hydraulic parameters for the target horizons, such as porosity and permeability, were taken from data available from the nearby exploration wells. The simulation results are encouraging as no significant thermal breakthrough is predicted. For the originally planned location of the producer, the extracted water temperature is predicted to be around 79°C, with an almost negligible cooling in the first 50 years of production. When the producer is located shallower parts of the reservoir, the yield water temperatures is lower, starting at ≈76°C and decreasing to ≈74°C after 50 years of operation. This comparatively larger decrease in temperature with time is caused by the structural feature of the reservoir, namely a higher dip causes the cooler water to easily move downward. In view of the poor reservoir data, the reservoir simulation model is constructed to allow iterative updates using data assimilation during planned drilling, testing, and production phases. Measurements during an 8h pumping test carried out in late 2010 suggest that a flow rate of 150m 3h -1 is achievable. Fluid temperatures of 76.5°C were measured, which is very close to the predicted value. © 2011 Elsevier Ltd.

Kurten S.,RWTH Aachen | Mottaghy D.,Geophysica Beratungsgesellschaft GmbH | Ziegler M.,RWTH Aachen
Acta Geotechnica | Year: 2015

In this paper, a thermal resistance model for an energy wall using the example of thermo-active seal panels is presented. In the developed model, the resistances of the pipes as well as the resistance of the structure itself are considered. The resistance model is transferred to a 2D finite difference model, which itself is implemented into the general 3D subsurface heat and flow transport code SHEMAT-Suite. This coupling of a semi-analytical model with a numerical code avoids a complete discretisation of the model domain and thus enables fast computing times. This new approach has been verified by pure finite element simulations and by laboratory tests. © 2014, Springer-Verlag Berlin Heidelberg.

Mottaghy D.,Geophysica Beratungsgesellschaft GmbH | Dijkshoorn L.,RWTH Aachen
Renewable Energy | Year: 2012

We present an effective finite difference formulation for implementing and modeling multiple borehole heat exchangers (BHE) in the general 3-D coupled heat and flow transport code SHEMAT. The BHE with arbitrary length can be either coaxial or double U-shaped. It is particularly suitable for modeling deep BHEs which contain varying pipe diameters and materials. Usually, in numerical simulations, a fine discretization of the BHE assemblage is required, due to the large geometric aspect ratios involved. This yields large models and long simulation times. The approach avoids this problem by considering heat transport between fluid and the soil through pipes and grout via thermal resistances. Therefore, the simulation time can be significantly reduced. The coupling with SHEMAT is realized by introducing an effective heat generation. Due to this connection, it is possible to consider heterogeneous geological models, as well as the influence of groundwater flow. This is particularly interesting when studying the long term behavior of a single BHE or a BHE field. Heating and cooling loads can enter the model with an arbitrary interval, e.g. from hourly to monthly values. When dealing with large BHE fields, computing times can be further significantly reduced by focusing on the temperature field around the BHEs, without explicitly modeling inlet and outlet temperatures. This allows to determine the possible migration of cold and warm plumes due to groundwater flow, which is of particular importance in urban areas with a high BHE installation density.The model is validated against the existing BHE modeling codes EWS and EED. A comparison with monitoring data from a deep BHE in Switzerland shows a good agreement. Synthetic examples demonstrate the field of application of this model. © 2012 Elsevier Ltd.

The use of geothermal energy with thermo-active geothermal systems has been constantly increasing over the last years. There are examples for Energy-Piles, Energy-Diaphragm-Walls or Energy-Slabs documented. The chair of Geotechnical Engineering (RWTH Aachen University) has developed thermo-active seal panels, which are based on the principle of thermo-active structures made of concrete. The seal panels were tested in large-scale laboratory tests showing very satisfying results. The realistic description of the heat transfer between soil and structural element is paramount for the design of a geothermal plant. Regarding rotationally symmetric systems there are several modeling tools. In contrast, there are no comparable models for plane elements documented. Therefore, the chair of Geotechnical Engineering, in cooperation with Geophysica Beratungsgesellschaft mbH, developed a mathematical model, which includes all important parameters. The different heat transfer processes are summed-up to a thermal resistance model, which is implemented in a Finite Difference Code (Shemat-Suite). © 2013 Ernst & Sohn Verlag für Architektur und technische Wissenschaften GmbH & Co. KG, Berlin.

Dijkshoorn L.,RWTH Aachen | Speer S.,RWTH Aachen | Pechnig R.,Geophysica Beratungsgesellschaft GmbH
International Journal of Geophysics | Year: 2013

This study aims at evaluating the feasibility of an installation for space heating and cooling the building of the university in the center of the city Aachen, Germany, with a 2500 m deep coaxial borehole heat exchanger (BHE). Direct heating the building in winter requires temperatures of 40°C. In summer, cooling the university building uses a climatic control adsorption unit, which requires a temperature of minimum 55°C. The drilled rocks of the 2500 m deep borehole have extremely low permeabilities and porosities less than 1%. Their thermal conductivity varies between 2.2 W/(m·K) and 8.9 W/(m·K). The high values are related to the quartzite sandstones. The maximum temperature in the borehole is 85°C at 2500 m depth, which corresponds to a mean specific heat flow of 85 mW/m2-90 mW/m 2. Results indicate that for a short period, the borehole may deliver the required temperature. But after a 20-year period of operation, temperatures are too low to drive the adsorption unit for cooling. In winter, however, the borehole heat exchanger may still supply the building with sufficient heat, with temperatures varying between 25 and 55°C and a circulation flow rate of 10 m3/h at maximum. © 2013 Lydia Dijkshoorn et al.

Jorand R.,RWTH Aachen | Clauser C.,RWTH Aachen | Marquart G.,RWTH Aachen | Pechnig R.,Geophysica Beratungsgesellschaft GmbH
Geothermics | Year: 2015

We present a comprehensive statistical analysis of petrophysical properties of rocks of the northeastern Rhenish Massif and the Lower Rhine Embayment in Germany. Properties measured comprise thermal conductivity, specific heat capacity density, porosity, hydraulic permeability, and compressional wave velocity. This robust and statistically reliable data is generally useful for numerical modeling of heat transport processes and helps, in particular, reducing the risk of failure in projects of geothermal energy use. We measured the thermophysical properties of rocks from two geological settings: (1) predominantly little consolidated Tertiary rocks forming the young sedimentary cover of the Lower Rhine Embayment in the condition they arrived in the laboratory; (2) well consolidated Paleozoic rocks from the northeastern Rhenish Massif in both dry and saturated condition. We tested a total of 476 samples from different lithologies in both settings in a comprehensive laboratory program consisting of mineralogical analyses and various petro- and thermophysical measurements at ambient and elevated p-T-conditions. This yields relations between composition and thermophysical properties of different sedimentary rock types and allows distinguishing between effects due to rock matrix and structure. The results are used to prove petrophysical rock models and allow predicting thermal properties of distinct rock types for greater depth. The results show that the thermophysical properties of Paleozoic rocks are mainly controlled by their mineralogical compositions, while thermophysical characteristics of Tertiary rocks are the result of a superposition of properties of their mineral content and the water-filled pore volume. © 2014 Elsevier Ltd.

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