Inga Moeck

Modelling of fractured carbonate reservoirs: outline of a novel technique via a case study from the Molasse Basin, southern Bavaria, Germany

Fluid flow in low-permeable carbonate rocks depends on the density of fractures, their interconnectivity and on the formation of fault damage zones. The present-day stress field influences the aperture hence the transmissivity of fractures whereas paleostress fields are responsible for the formation of faults and fractures. In low-permeable reservoir rocks, fault zones belong to the major targets. Before drilling, an estimate for reservoir productivity of wells drilled into the damage zone of faults is therefore required. Due to limitations in available data, a characterization of such reservoirs usually relies on the use of numerical techniques. The requirements of these mathematical models encompass a full integration of the actual fault geometry, comprising the dimension of the fault damage zone and of the fault core, and the individual population with properties of fault zones in the hanging and foot wall and the host rock. The paper presents both the technical approach to develop such a model and the property definition of heterogeneous fault zones and host rock with respect to the current stress field. The case study describes a deep geothermal reservoir in the western central Molasse Basin in southern Bavaria, Germany. Results from numerical simulations indicate that the well productivity can be enhanced along compressional fault zones if the interconnectivity of fractures is lateral caused by crossing synthetic and antithetic fractures. The model allows a deeper understanding of production tests and reservoir properties of faulted rocks.

Controls on the deep thermal field: implications from 3-D numerical simulations for the geothermal research site Groß Schönebeck

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 Groß Schönebeck 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.

Fault reactivation potential as a critical factor during reservoir stimulation

Hydraulic stimulation is frequently used to enhance reservoir productivity. The aim of hydraulic stimulation is to increase the formation pressure by fluid injection to create artificial fractures that act as additional fluid pathways. But large-scale fluid injection as applied in hydrocarbon and geothermal reservoirs can also induce seismicity and fault reactivation depending on the reservoir geomechanics and stress regime. Recent case studies in stimulation of geothermal reservoirs have shown induced seismicity as an undesirable side effect which needs to be understood prior to massive fluid injection. Slip tendency analysis has been successfully used to characterize fault slip likelihood and fault slip directions in any stress regime. In our study, we applied slip tendency analysis to assess the reactivation potential of shear and dilational fractures in a deep geothermal reservoir in the North-East German Basin, based on the notion that slip on faults is controlled by the ratio of shear to normal effective stress acting on the plane of weakness. The results from slip tendency analysis are supported by the spatial distribution of recorded microseismicity, which indicates slip rather than extension along a presumed NE-striking failure plane.

Linking gas fluxes at Earth’s surface with fracture zones in an active geothermal field

The percolation of fluids is of utmost relevance for the utilization of underground resources; however, the location and occurrence of fractures are not always known, and important characteristics of faults, such as stress state and permeability, are commonly uncertain. Using a case study at the Brady’s geothermal field in Nevada (USA), we demonstrate how permeable fractures can be identified and assessed by combining fault stress models with measurements of diffuse degassing and emanations at Earth’s surface. Areas of maximum gas emissions and emanations correspond to fault segments with increased slip and dilation tendency, and represent a fingerprint of the geothermal system at depth. Thus, linking gas fluxes with fault stress models serves as a measure of the connectivity between surface and subsurface.

Joint Research Project Brine: Carbon Dioxide Storage in Eastern Brandenburg: Implications for Synergetic Geothermal Heat Recovery and Conceptualization of an Early Warning System Against Freshwater Salinization

Brine was a scientific joint-project implemented to accompany a prospective CO2 storage site in Eastern Brandenburg, Germany. In this context, we investigated if pore pressure elevation in a CO2 storage reservoir can result in shallow freshwater salinization involving the conceptual design of a geophysical early warning system. Furthermore, assessments of a potential synergetic geothermal heat recovery from the CO2 storage reservoir and hydro-mechanical integrity were carried out. The project results demonstrate that potential freshwater salinization is strongly depending on the presence and characteristics of geological weakness zones. The integrated geophysical early warning system allows for reliable monitoring of these potential leakage pathways at different spatial and time scales.

Geochemical/hydrochemical evaluation of the geothermal potential of the Lamongan volcanic field (Eastern Java, Indonesia)

Magmatic settings involving active volcanism are potential locations for economic geothermal systems due to the occurrence of high temperature and steam pressures. Indonesia, located along active plate margins, hosts more than 100 volcanoes and, therefore, belongs to the regions with the greatest geothermal potential worldwide. However, tropical conditions and steep terrain reduce the spectrum of applicable exploration methods, in particular in remote areas. In a case study from the Lamongan volcanic field in East Java, we combine field-based data on the regional structural geology, elemental and isotopic composition of thermal waters, and the mineralogical and geochemical signatures of volcanic rocks in exploring hidden geothermal systems. Results suggest infiltration of groundwater at the volcanoes and faults. After infiltration, water is heated and reacts with rocks before rising to the surface. The existence of a potential heat source is petrologically and geophysically constrained to be an active shallow mafic-magma chamber, but its occurrence is not properly reflected in the composition of the collected warmed spring waters that are predominantly meteoric in origin. In conclusion, spring temperature and hydrochemistry alone may not always correctly reflect the deep geothermal potential of an area.

The new Geothermal Energy: Science, Society, and Technology

In light of the recent worldwide move to reduce dependence on coal and nuclear energy, renewable energies have taken on significant importance on a global level. Geothermal energy offers considerable future potential for heating and cooling (near-surface geothermal energy) as well as electricity production (deep geothermal energy also known as enhanced/engineered geothermal systems [EGS]), and there still remain many open questions. To shift the topic of geothermal energy more strongly into the focus of scientific research and development, two open-access journals formed simultaneously in 2013, both focusing on the science behind geothermal energy: Geothermal Energy and Geothermal Energy Science. As such, the editors of both journals recognized that the journals covered the same scope and would be stronger by cooperating rather than competing. The editors thus agreed to merge the journals into one to combine efforts and build on each other’s strengths. To preserve the Scopus listing already achieved by Geothermal Energy, the decision was made to keep the journal name Geothermal Energy. A new subtitle to the journal—Science, Society, and Technology—differentiates it from the former, but preserves the Scopus listing. The newly merged journal’s launch date was 01 July 2017, and the transition from the former journal to the new one was smooth and unnoticeable. All articles published under Geothermal Energy are still and always will be available at the journal website. Geothermal Energy: Science, Society, and Technology now bears a new title, cover and has the support of both scientific institutions (UFZ, GFZ, KIT) and professional organizations, including the leading international organization in geothermal research, the International Geothermal Association (IGA), and the leading geothermal organization in Germany, the German Geothermal Association (BVG). We are proud to present to you this new, stronger journal and, with increasing interest in and demand for renewable technologies, have high expectations for Geothermal Energy: Science, Science, and Technology.

Geothermal Exploration – Ensuring an Optimized Utilization of Geothermal Energy in Low-enthalpy Sedimentary Settings

An adequate comprehensive understanding of the subsurface geology is a pre-requisite for a precise planning and successful operating of geothermal applications and reduces the financial risks considerably. An exploration concept is needed which is tailored for the geological setting to be evaluated and the level of exploration performed prior to the geothermal exploration. We present examples from ongoing geothermal exploration projects encompassing, for example, studies on the geological structure, including the stress field, on the hydraulic and thermal properties of geological formations, and on the temperature prognoses for target reservoirs.