Berlin: geothermal energy from the shell limestone?
Unused reservoir: 500 meters below the urban area of Berlin lies a layer of rock that could possibly provide the metropolis with geothermal energy. Because the shell limestone is very porous and contains around 32 degrees warm deep water. A research project is now investigating whether this water can be pumped and how much thermal energy can be gained from it. Its big advantage: old natural gas storage facilities offer easy access to the rock.
Geothermal energy uses the heat from the subsurface to generate heat - and this is also possible in Germany. Because we also have hot deep water or even warm rock layers in which water pumped down can be heated. With the help of geothermal probes or heat pumps, even relatively small temperature differences to the surface can be used for heat generation.
Porous limestone layers at a depth of 500 meters
Such a heat reservoir could be located under the urban area of Berlin. Because at a depth of 500 meters under the Grunewald in southwest Berlin there is a layer of prehistoric shell limestone that could offer good conditions for heat generation. This carbonate rock has a temperature of around 32 degrees and comprises two layers of particularly porous foam limestone, each 15 meters thick.
This rock, with its innumerable pores, was formed around 240 million years ago at the bottom of a shallow sea. At that time, lime was deposited on grains of sand, shell fragments and other crumbs. Over the course of millions of years, most of the once enclosed particles weathered and water-filled cavities formed. This is how the spongy but hard layer of the foamy lime was created. Because of its many pores and the warm water stored in it, this rock could in principle be suitable as a heat supplier.
Old gas wells as access
But whether this works and is worthwhile depends crucially on how much water there is in the carbonate rock of the foamy lime and what quantities can flow through the naturally formed fissures below. Because only if the warm water can also be pumped upwards can it be used directly for heat recovery.
"To find out, you would normally have to drill a hole right into the limestone," explains Guido Blöcher from the GFZ German Research Center for Geosciences in Potsdam. However, such a bore is complex and expensive.
But the researchers are lucky: under the foam lime layer lies an old natural gas storage facility, which used to store gas reserves for the city's fluctuating needs. Numerous drill holes from this use have been preserved, which also cross the shell limestone layer on the way to the 1,000-meter-deep gas storage facility. The plant, which was shut down in 2017, offers scientists a unique opportunity to explore the potential heat source.
The project examines the suitability of the lime
This is exactly what is now happening as part of the ATES iQ project. First of all, the researchers insert a measuring cable into the borehole, which measures, among other things, the temperature along the entire length of the borehole. This shows how this changes when water is pumped up or when surface water is pressed. "In this way, we can see where permeable rock layers are along the borehole," explains GFZ researcher Jan Hennings.
In another test, the chemical composition of the deep water is determined. To do this, the scientists bring water samples from the shell limestone to the surface. They examine the substances it contains to find out where the water comes from and whether it could be used. Because the amount of dissolved salts reveals, among other things, how strong the corrosion will be during later pumping.
Pushed up and pushed down
In order to estimate the amount of water flowing out of the cracks in the limestone, the scientists will pump nitrogen under pressure into the borehole. As a result, deep water shoots up and the researchers can measure how many cubic meters of it rise per hour. In a supplementary test, the water in the borehole is pushed down around a hundred meters using nitrogen.
The injected nitrogen is then released very quickly via a valve and the water rises up again in the bore. “From this increase, we can calculate the amount of water that can later be pumped in one hour,” explains Blöcher. Using these quantities, he and his colleagues can then estimate whether the use of geothermal energy there is profitable.
Should that be the case, the foam lime could in future not only supply the metropolis of Berlin, but perhaps also the surrounding area with heat. Because the porous shell limestone layer continues beyond the city limits of Berlin.