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Influence of the Main Border Faults on the 3D Hydraulic Field of the Central Upper Rhine Graben
(2019)
The Upper Rhine Graben (URG) is an active rift with a high geothermal potential. Despite being a well-studied area, the three-dimensional interaction of the main controlling factors of the thermal and hydraulic regime is still not fully understood. Therefore, we have used a data-based 3D structural model of the lithological configuration of the central URG for some conceptual numerical experiments of 3D coupled simulations of fluid and heat transport. To assess the influence of the main faults bordering the graben on the hydraulic and the deep thermal field, we carried out a sensitivity analysis on fault width and permeability. Depending on the assigned width and permeability of the main border faults, fluid velocity and temperatures are affected only in the direct proximity of the respective border faults. Hence, the hydraulic characteristics of these major faults do not significantly influence the graben-wide groundwater flow patterns. Instead, the different scenarios tested provide a consistent image of the main characteristics of fluid and heat transport as they have in common: (1) a topography-driven basin-wide fluid flow perpendicular to the rift axis from the graben shoulders to the rift center, (2) a N/NE-directed flow parallel to the rift axis in the center of the rift and, (3) a pronounced upflow of hot fluids along the rift central axis, where the streams from both sides of the rift merge. This upflow axis is predicted to occur predominantly in the center of the URG (northern and southern model area) and shifted towards the eastern boundary fault (central model area).
Understanding the interactions between the different processes that control the geothermal and fluid flow fields in sedimentary basins is crucial for exploitation of geothermal energy. Numerical models provide predictive and feasible information for a correct assessment of geothermal resources especially in areas where data acquisition is demanding. Here, we present results from numerical efforts to characterize the thermal structure and its interaction with the fluid system for the area of the North East German Basin (NEGB). The relative impact of the different (diffusive and advective) processes affecting the hydrothermal setting of the basin are investigated by means of three- dimensional numerical simulations. Lithospheric-scale numerical models are evaluated to understand the specific thermal signature of the relevant factors influencing the present-day conductive geothermal field in the NEGB. Shallow and deep structural controls on the thermal configuration of the basin are addressed and quantified. Interaction between the resulting thermal field and the active fluid system is investigated by means of three-dimensional simulations of coupled fluid flow and heat transport. Factors influencing stability and reliability of modeling predictions are discussed. The main effort is to build a physically consistent model for the basin which integrates the impacts of thermal gradients on the regional fluid regime and their coupling with the main geological units defining the basin.
The deep thermal field in sedimentary basins can be affected by convection, conduction or both resulting from the structural inventory, physical properties of geological layers and physical processes taking place therein. For geothermal energy extraction, the controlling factors of the deep thermal field need to be understood to delineate favorable drill sites and exploitation compartments. We use geologically based 3-D finite element simulations to figure out the geologic controls on the thermal field of the geothermal research site Gro Schonebeck located in the E part of the North German Basin. Its target reservoir consists of Permian Rotliegend clastics that compose the lower part of a succession of Late Carboniferous to Cenozoic sediments, subdivided into several aquifers and aquicludes. The sedimentary succession includes a layer of mobilized Upper Permian Zechstein salt which plays a special role for the thermal field due to its high thermal conductivity. Furthermore, the salt is impermeable and due to its rheology decouples the fault systems in the suprasalt units from subsalt layers. Conductive and coupled fluid and heat transport simulations are carried out to assess the relative impact of different heat transfer mechanisms on the temperature distribution. The measured temperatures in 7 wells are used for model validation and show a better fit with models considering fluid and heat transport than with a purely conductive model. Our results suggest that advective and convective heat transport are important heat transfer processes in the suprasalt sediments. In contrast, thermal conduction mainly controls the subsalt layers. With a third simulation, we investigate the influence of a major permeable and of three impermeable faults dissecting the subsalt target reservoir and compare the results to the coupled model where no faults are integrated. The permeable fault may have a local, strong impact on the thermal, pressure and velocity fields whereas the impermeable faults only cause deviations of the pressure field.
Based on a numerical model of the Northeast German Basin (NEGB), we investigate the sensitivity of the calculated thermal field as resulting from heat conduction, forced and free convection in response to consecutive horizontal and vertical mesh refinements. Our results suggest that computational findings are more sensitive to consecutive horizontal mesh refinements than to changes in the vertical resolution. In addition, the degree of mesh sensitivity depends strongly on the type of the process being investigated, whether heat conduction, forced convection or free thermal convection represents the active heat driver. In this regard, heat conduction exhibits to be relative robust to imposed changes in the spatial discretization. A systematic mesh sensitivity is observed in areas where forced convection promotes an effective role in shorten the background conductive thermal field. In contrast, free thermal convection is to be regarded as the most sensitive heat transport process as demonstrated by non-systematic changes in the temperature field with respect to imposed changes in the model resolution.
Three dimensional modelling of fractured and faulted reservoirs : framework and implementation
(2010)
Modelling of coupled physical processes in fractured and faulted media is a major challenge for the geoscience community. Due to the complexity related to the geometry of real fracture networks and fault systems, modelling studies have been mainly restricted either to two dimensional cases or to simplified orthogonal fracture systems consisting of vertical and horizontal fractures. An approach to generate three dimensional meshes for realistic fault geometries is presented. The method enables representation of faults in an arbitrary incline as two dimensional planes within a three dimensional, stratified porous matrix of a generic geometry. Based on a structural geological model, the method creates three dimensional unstructured tetrahedral meshes. These meshes can be used for finite element and finite volume numerical simulations. A simulation of a coupled fluid flow and heat transport problem for a two layered porous medium cut by two crossing faults is presented to test the reliability of the method.
The internal geological structure of the Northeast German Basin (NEGB) is affected by intense salt diapirism and by the presence of several stratified aquifer complexes of regional relevance. The shallow Quaternary to late Tertiary freshwater aquifer is separated from the underlying Mesozoic saline aquifers by an embedded Tertiary clay enriched aquitard (Rupelian Aquitard). An important feature of this aquitard is that hydraulic connections between the upper and lower aquifers do exist in areas where the Rupelian Aquitard is missing (hydrogeological windows). Three-dimensional thermohaline numerical simulations are carried out to investigate the effects of such hydrogeological windows in the Rupelian Aquitard on the resulting groundwater, temperature, and salinity distributions. Numerical results suggest that hydrogeological windows act as preferential domains of hydraulic interconnectivity between the different aquifers at depth and enable vigorous heat and mass transport which causes a mixing of warm and saline groundwater with cold and less saline groundwater within both aquifers. In areas where the Rupelian Aquitard confines the Mesozoic aquifer, dissolved solutes from major salt structures are transported laterally giving rise to plumes of variable salinity content ranging from few hundreds of meters to several tens of kilometers. Furthermore, destabilizing thermal buoyancy forces may overwhelm counteracting stabilizing salinity induced forces offside of salt domes. This may result in buoyant upward groundwater flow transporting heat and mass to shallower levels within the same Mesozoic Aquifer.
To investigate and quantify main physical heat driving processes affecting the present-day subsurface thermal field, we study a complex geological setting, the Northeast German Basin (NEGB). The internal geological structure of the NEGB is characterized by the presence of a relatively thick layer of Permian Zechstein salt (up to 5000 m), which forms many salt diapirs and pillows locally reaching nearly the surface. By means of three-dimensional numerical simulations we explore the role of heat conduction, pressure, and density driven groundwater flow as well as fluid viscosity related effects. Our results suggest that the regional temperature distribution within the basin results from interactions between regional pressure forces as driven by topographic gradients and thermal diffusion locally enhanced by thermal conductivity contrasts between the different sedimentary rocks with the highly conductive salt playing a prominent role. In contrast, buoyancy forces triggered by temperature-dependent fluid density variations are demonstrated to affect only locally the internal thermal configuration. Locations, geometry, and wavelengths of convective thermal anomalies are mainly controlled by the permeability field and thickness values of the respective geological layers.
Understanding heat transport in sedimentary basins requires an assessment of the regional 3D heat distribution and of the main physical mechanisms responsible for the transport of heat. We review results from different 3D numerical simulations of heat transport based on 3D basin models of the Central European Basin System (CEBS). Therefore we compare differently detailed 3D structural models of the area, previously published individually, to assess the influence of (1) different configurations of the deeper lithosphere, (2) the mechanism of heat transport considered and (3) large faults dissecting the sedimentary succession on the resulting thermal field and groundwater flow. Based on this comparison we propose a modelling strategy linking the regional and lithosphere-scale to the sub-basin and basin-fill scale and appropriately considering the effective heat transport processes. We find that conduction as the dominant mechanism of heat transport in sedimentary basins is controlled by the distribution of thermal conductivities, compositional and thickness variations of both the conductive and radiogenic crystalline crust as well as the insulating sediments and by variations in the depth to the thermal lithosphere-asthenosphere boundary. Variations of these factors cause thermal anomalies of specific wavelength and must be accounted for in regional thermal studies. In addition advective heat transport also exerts control on the thermal field on the regional scale. In contrast, convective heat transport and heat transport along faults is only locally important and needs to be considered for exploration on the reservoir scale. The general applicability of the proposed workflow makes it of interest for a broad range of application in geosciences including oil and gas exploration, geothermal utilization or carbon capture and sequestration issues. (C) 2014 Elsevier Ltd. All rights reserved.
Based on newly available data of both, the structural setting and thermal properties, we compare 3D thermal models for the area of Brandenburg, located in the Northeast German Basin, to assess the sensitivity of our model results. The structural complexity of the basin fill is given by the configuration of the Zechstein salt with salt diapirs and salt pillows. This special configuration is very relevant for the thermal calculations because salt has a distinctly higher thermal conductivity than other sediments. We calculate the temperature using a FEMethod to solve the steady state heat conduction equation in 3D. Based on this approach, we evaluate the sensitivity of the steady-state conductive thermal field with respect to different lithospheric configurations and to the assigned thermal properties. We compare three different thermal models: (a) a crustal-scale model including a homogeneous crust, (b) a new lithosphere-scale model including a differentiated crust and (c) a crustal-scale model with a stepwise variation of measured thermal properties. The comparison with measured temperatures from different structural locations of the basin shows a good fit to the temperature predictions for the first two models, whereas the third model is distinctly colder. This indicates that effective thermal conductivities may be different from values determined by measurements on rock samples. The results suggest that conduction is the main heat transport mechanism in the Brandenburg area.
The 3D thermal field across the Alpine orogen and its forelands and the relation to seismicity
(2020)
Temperature exerts a first order control on rock strength, principally via thermally activated creep deformation and on the distribution at depth of the brittle-ductile transition zone. The latter can be regarded as the lower bound to the seismogenic zone, thereby controlling the spatial distribution of seismicity within a lithospheric plate. As such, models of the crustal thermal field are important to understand the localisation of seismicity. Here we relate results from 3D simulations of the steady state thermal field of the Alpine orogen and its forelands to the distribution of seismicity in this seismically active area of Central Europe. The model takes into account how the crustal heterogeneity of the region effects thermal properties and is validated with a dataset of wellbore temperatures. We find that the Adriatic crust appears more mafic, through its radiogenic heat values (1.30E-06 W/m3) and maximum temperature of seismicity (600 degrees C), than the European crust (1.3-2.6E-06 W/m3 and 450 degrees C). We also show that at depths of < 10 km the thermal field is largely controlled by sedimentary blanketing or topographic effects, whilst the deeper temperature field is primarily controlled by the LAB topology and the distribution and parameterization of radiogenic heat sources within the upper crust.