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--- et_geothermal [05/07/2023 10:14] – mike_gss
+++ et_geothermal [05/07/2023 15:06] (current) – mike_gss
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 ==== Geothermal Energy and Biostratigraphy - examples from Denmark ====
-{{:geothermal01.jpg?nolink |}}Geothermal energy is a renewable energy source generated from hot, saline water which occurs naturally in geothermal reservoirs. Geothermal energy is an infinite energy source, is not dependent on the weather (unlike solar, wind and wave power) and has a low carbon footprint.
+{{:geothermal01.jpg?nolink |}}Geothermal energy is a renewable energy source that originates from radioactive decay and residual heat within the Earth, heat that slowly moves out towards the Earth’s crust and into space. Geothermal energy is an infinite energy source, is not dependent on the weather (unlike solar, wind and wave power) and has a low carbon footprint. In Denmark, the energy can be utilized as hot saline water from geothermal sandstone reservoirs. In the Danish subsurface, the temperature rises ca 25ºC for each km depth, dependent on the thermal conductivity of the rock type and location. Sandstone and chalk have relatively high thermal conductivity allowing heat to pass through, and clay has low thermal conductivity and holds heat in. [image: https://www.geoviden.dk/dyb-geotermi-i-danmark/]
-The heat within the Earth mainly originates from radioactive decay and residual heat, and slowly moves out towards the Earth’s crust and into space. A general rule of thumb is that for each km depth, temperature rises by ca. 25ºC, dependent on the thermal conductivity of the rock type and location. Sandstone and chalk have relatively high thermal conductivity allowing heat to pass through, and clay has low thermal conductivity and holds heat in. [image: https://www.geoviden.dk/dyb-geotermi-i-danmark/]
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 {{:geothermal08.jpg?nolink&300 |}}The Danish subsurface contains so much warm water that in principle it could provide up to a half of Denmark’s annual heating, but due to earlier dependence on other energy sources, the geothermal energy industry, on a wide scale, is still relatively new. Locally, however, hot water from geothermal energy is used for district heating. Warm water from an underground reservoir is pumped up to a geothermal plant at the surface, where the heat is extracted using a heat exchanger and transferred to the district heating network. At the same time cooled reservoir water is sent back into the ground, where it is warmed up again. (See 3D cube model; image - https://dybgeotermi.geus.dk/en)
-The optimal reservoir rock is sandstone. Sandstone thickness is essential as the reservoir needs to hold enough water. It has to be at depths between 800-3000 m where water temperature is between 35-90ºC (see map below).  Water has to be of a certain temperature in order that heat can be extracted. If the sandstone is too deep, its porosity falls, and reservoir quality becomes too poor.
+In Denmark, the optimal geothermal reservoir rock is sandstone. Sandstone thickness is essential as the reservoir needs to hold enough water. As a rule of thumb, the sandstone reservoir has to be at depths between 800-3000 m where water temperature is between 35-90ºC (see map below). Water has to be of a certain minimum temperature to ensure that enough heat can be extracted. If the sandstone is too deeply buried in the subsurface, its porosity may be too low, and consequently its reservoir quality too poor.
 {{:geothermal07.jpg?nolink |}}The map (left) shows the Danish subsurface and where sandstone reservoirs with the potential for geothermal energy extraction can be found. The localities of 3 geothermal facilities: Thisted, Margretheholm and Sønderborg, are shown. (https://eng.geus.dk/products-services-facilities/data-and-maps/deep-geothermal-energy-portal)
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-The Margretheholm Geothermal Plant near Copenhagen (see photo above; image - Peter Warna-Moors, GEUS) began functioning in 2005. It uses water from the Bunter Sandstone Formation which is 2.6 km deep where water is ca. 73ºC. The overburden includes the Upper Cretaceous chalks (see seismic image below) but the nature of these is poorly understood. A biostratigraphic breakdown of the chalk of the Margretheholm-1 geothermal well was carried out using calcareous nannofossils (see nannofossil biostrat image below - after Kristensen et al., 2017 and fossil photos after Sheldon, 2008). It was the first time a detailed combined bio-, litho- and log stratigraphic study had been conducted on a deep well penetrating all of the Upper Cretaceous section onshore Denmark, providing important correlation with other local and regional boreholes (see correlation image below - after Kristensen et al., 2017), increasing understanding of the regional geology, which is crucial for current and future geothermal energy development.
+The Margretheholm Geothermal Plant near Copenhagen (see photo above; image - Peter Warna-Moors, GEUS) was built in 2005. It uses water from the Bunter Sandstone Formation which is 2.6 km deep where water is ca. 73ºC. The overburden includes the Upper Cretaceous chalks (see seismic image below) but the nature of these is poorly understood. A biostratigraphic breakdown of the chalk of the Margretheholm-1 geothermal well was carried out using calcareous nannofossils (see nannofossil biostrat image below - after Kristensen et al., 2017 and fossil photos after Sheldon, 2008). It was the first time a detailed combined bio-, litho- and log stratigraphic study had been conducted on a deep well penetrating all of the Upper Cretaceous section onshore Denmark, providing important correlation with other local and regional boreholes (see correlation image below - after Kristensen et al., 2017), increasing understanding of the regional geology, which is crucial for current and future geothermal energy development.
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