Facilitating The Utilization Of Deep Geothermal Energy In Denmark By Regional Derisking Using Hydrocarbon Data

Abstract

Recent reports from the IPCC calls on swift actions to change energy consumption from fossil fuels to green sustainable energy systems such as geothermal energy. Initial mapping in the early 80-ties suggested that the Danish onshore subsurface contains large geothermal resources. However, at present only a limited fraction of these resources are utilized in three existing geothermal power plants in Thisted, Margretheholm and Sønderborg (see location on figure) where warm formation water is pumped to the surface from a production well and, after heat extraction, returned to the subsurface in an injection well. To further stimulate the exploitation of the geothermal resource and thus the transformation to a more sustainable energy mix in Denmark, a number of public financed research projects has been carried out for the last 40 years, focussing on the implementation of deep geothermal energy for district heating and thereby replacing fossil fuel, especially coal and oil. A second goal was to motivate heat-demanding industries to consider geothermal energy and energy storage in their production process. In Denmark, successful geothermal exploitation in the deep subsurface requires the presence of thick and laterally coherent sandstone reservoirs with high porosity and permeability, which can ensure effective and long-term extraction and re- injection of formation water. A thick and coherent reservoir that is not hydraulically compartmentalized by faults, lateral lithological changes (e.g. grain size) or diagenetic features implies that a large volume of warm water may be accessible, and that production and injection wells can be placed at appropriate distances from each other while remaining hydraulically connected. The temperature gradient of typical 25–30°C/km in the Danish subsurface implies that at depths shallower than 800 m the temperature is generally not sufficiently high to be economically profitable for a district heating plant, whereas at depths greater than 3000 m, diagenetic alterations related to high pressure–temperature conditions reduce the porosity and permeability of the reservoir sandstones. Thus, as a rule of thumb, the potential geothermal reservoirs must occur within the 800–3000 m depth interval. An important step forwards in facilitating the utilization of deep geothermal energy was a major mapping campaign in which all existing seismic reflection data and petrophysical data (core and well-log data) acquired during former hydrocarbon and geothermal exploration activities were thoroughly evaluated, integrated and interpreted. This work formed the basis for the construction of 3D geological and temperature models and a number of thematic maps on deep geothermal energy. Thereby an overview of the structural-stratigraphical evolution from the Late Permian through Late Cretaceous of the Danish onshore subsurface as well as the thickness and lateral extent of the lithostratigraphic units known to contain geothermal reservoir sandstones was obtained. The maps are accessible from a user-friendly WebGIS portal as is a number of seismic cross-sections and an interactive 3D tool that exemplify the structural distribution of the onshore subsurface units (http://DybGeotermi.GEUS.dk). The portal provides an overview of the amount and quality of existing geodata, the geological composition of the subsurface, and interpreted thematic products such as depth and thickness maps of potential geothermal reservoirs. An important thematic map shows onshore and near-offshore areas in Denmark where the geological conditions are potentially suitable for extraction of deep geothermal energy. The maps are to be considered as indicative and meant for regional use. This is because the available geological data in many areas only provide a general picture of the subsurface, especially where data are sparse and/or of poor quality. Neither are the maps final - new well and seismic data, new interpretation tools and refined geological interpretation may modify the thematic maps. However, the maps provide good indications of where deep geothermal exploration is possible based on the geological prerequisites. The use of the portal may thus ensure that new geothermal exploration is directed towards those areas that are the most promising based on current knowledge. The various geological map themes may also form an important basis for an initial analysis of the geothermal potential at a specific site, where the construction of a geothermal plant is considered. In this first step, the maps give an overview of the potential geothermal reservoir intervals (such as the depths and thickness of lithostratigraphic units) that may be relevant at the local site, and information regarding the type, amount and quality of existing geological data, and thus potential need for acquisition of supplementing data. A more comprehensive estimate of the geothermal potential in a specific area must be based on detailed analysis of the local data and incorporation into local geological models. This may then form the input for constructing numerical reservoir models, used for simulating potential flow rates, production capacity and thermal lifecycle. Important output from these simulations can be used for assessment of the timespan before cooled water from injection wells reach the production wells and thus to optimize location, design and configuration of wells prior to drilling. The many public financed research projects during especially the last decade has considerably increased our knowledge of the Danish subsurface, has confirmed the presence of its huge geothermal resource and indicated where the geological conditions are most suitable for the extraction of deep geothermal energy. The investments in research have proven its importance, and the industry is now taking more interest in geothermal exploration and sees it as a promising business case into which it is willing to invest and take the risks associated with the drilling of expensive exploration wells.

AAPG Datapages/Search and Discovery Article #90346 ©2019 AAPG European Region, 3rd Hydrocarbon Geothermal Cross Over Technology Workshop, Geneva, Switzerland, April 9-10, 2019