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Geothermal Energy Basel, 2006

The Deep Heat Mining Project in Basel was a ground-breaking Swiss energy research project that planned to build a pilot plant for a geothermal power station using the Enhanced Geothermal System (EGS) or 'petrothermal energy generation'. This entails forcing a cold liquid, usually water, down into hot rock, where it heats up before being conveyed back up to the surface to be used to generate heat or electricity.

In the medium term, the project was intended to generate environmentally-friendly power, harnessing Swiss energy sources and lowering the country's reliance on imported electricity. A pilot plant, due to be built in Basel, was supposed to generate 6 MW of electricity and 17 MW of heat, whilst emitting hardly any CO2 and barely producing any waste. This output would have sufficed to meet the energy needs of around 10,000 households and the heating requirements of 2,700 homes. The plan was to drill to a depth of 5,000 metres to exploit the temperatures of 200°C there.

The intended site of the future geothermal power plant was the Basel Industrial Works (IWB) site in Basel-Kleinhüningen. From there, the heat could have been injected into the extensively developed municipal district heating system. During the summer months, when there is low demand for district heating, the plant would mainly have generated power.

Geothermal Energy Basel, 2006

The first geothermal project in Switzerland designed to generate electricity was launched in Basel. In December 2006, after an exploratory phase, water was due to be forced under high pressure into the crystalline basement rock over a two-week period. The aim of this procedure was to increase its permeability at a depth of between 4,000 and 5,000 m and create a geothermal reservoir where the liquid would circulate and heat up.

The geothermal injection process was tracked by a dense network of seismic monitoring stations, including six of the borehole seismometers operated by Geopower Basel AG at depths of between 300 and 2,700 m. As anticipated, these stations recorded thousands of microquakes. The SED runs a compact network of seismic stations in the Basel area, which was substantially densified during the Deep Heat Mining Project. In addition to this, the federal state of Baden-Württemberg's Earthquake Office (LED) set up a number of monitoring stations on the German side of the border with Switzerland. The SED had access to the data generated by these stations and was tasked to estimate the magnitude of the detected earthquakes.

The rate of injection (quantity of liquid over time) was gradually increased until, on the sixth day, the maximum rate was reached. Shortly after, an earthquake with a local magnitude of 2.6 occurred, whereupon the rate of injection was first decreased and then stopped altogether a few hours later. Approximately five hours after that, an earthquake with a local magnitude of 3.4 (moment magnitude 3.1) occurred. Having an intensity of V ("moderate" on the Mercalli scale), it was felt over a wide area and caused minor damage. Three more quakes with local magnitudes of higher than 3.0 ensued, the last occurring in February 2007. In all, more than 900 quakes with moment magnitudes of 0.9 or above were recorded. Consequently, the project managers stopped implementing their plan and definitively terminated the project in 2009 after a comprehensive risk assessment.

For the most part, the damage reported concerned small cracks in buildings' plasterwork. Most claims, which totalled CHF 6 million, resulted in the payment of compensation.

Using deep boreholes to inject cold water into the substratum forcibly expands existing gaps in the rock, causing movements between both sides of a fault or rift which in turn trigger a large number of small-scale earthquakes known as microquakes. These mark the path sought by the water underground, so pinpointing the quakes helps us to reach conclusions about the heat reservoir opened up by the water. Accordingly, microquakes are a key part of the stimulation process, showing the rock's rising permeability and the circulation of the required mass of water through the hot reservoir. If the stimulation is sufficient, the configuration of quakes, i.e. the position of the reservoir, is used to determine the position of the second borehole for the production well.

The expected earthquakes are usually so small that they are not felt on the surface, though they are detected and recorded by sensitive seismometers. However, in individual instances a larger earthquake can occur, which the local population experiences as a jolt or bang. The Swiss Seismological Service (SED) was responsible for independently monitoring the seismic stimulation process in Basel. Recorded quakes were constantly registered by a purpose-expanded earthquake-monitoring network, their epicentre was located by the SED and the results were published in a database and on maps on the project website. When a recorded quake had a local magnitude of 2.0 or more, the SED also informed the relevant authorities.

Learn More

Extensive information on the geothermal project in Basel on the project website (which is no longer being updated)

Earthquakes and geothermal energy

How a petrothermal project to generate geothermal energy works

Field of Research Induced Seismicity