SWITZERLAND IS SEEING SIGNIFICANT USE of geothermal energy, mainly by means of probes that collect heat energy from the ground to provide heating and hot water for buildings.
However, the heat residing in the deeper layers of the earth is hardly used: if one were to drill down to 1,000m and more, one would come across an enormous heat reservoir that can be used for heating purposes, industrial processes and electricity production.
At the Bedretto rock laboratory of the Swiss Federal Institute of Technology (ETH) Zurich and with its scientific support, Geo-Energie Suisse has tested a new approach for the safe use of deep geothermal energy.
ENDLESS ENERGY
An almost inexhaustible source of energy lies directly beneath our feet: in Switzerland, drilling down to a depth of one kilometre will encounter rock at a very warm 40°C. At a depth of five kilometres, the temperature can be as high as 160°C.
Geothermal energy provides an enormous supply of heat that can also be used to produce electricity, provided the temperatures are above 100°C.
Currently, the average household in Switzerland draws four billion kilowatt hours (kWh) of geothermal energy every year; this covers 4% of the country’s heating needs. Most of the energy comes from the top layer of the earth. Geothermal probes collect the energy before heat pumps (running on electricity) raise it to the temperature level required for heat and hot water.
WATER FLOWS THROUGH JAGGED HOT ROCK
The contribution of geothermal energy to the energy supply can be increased considerably by expanding the use of geothermal probes and by using geothermal energy at medium depths (500–3,000m) and also at much deeper levels (3,000m and deeper). According to the basic variant of the net zero (ZERO) scenario of the ‘Energy Perspectives 2050+’ published by the Swiss Federal Office of Energy (SFOE) at the end of 2020, geothermal energy could contribute up to 5.5TWh to the energy supply in 2050, of which 2TWh would be supplied as electricity and 3.5TWh for the production of district heat.
The umbrella organization Geo- Energie Schweiz (Geo-energy Switzerland) even places the potential contribution to the heat supply at 17TWh; this would be about a quarter of the heat demand assumed by the ‘Energy Perspectives 2050+’ for the year 2050.
Depending on the drill depth, different methods are recommended for extracting geothermal energy. At depths of 3,000m and more, the petrothermal geothermal method is the most evident. This method, involves water injected at high pressure via a borehole into the crystalline rock. The water penetrates existing rock cracks (fractures) and widens them. The hydraulic stimulation creates so-called ‘reservoirs’ in the rock, i.e. spaces in which the circulating water can absorb heat from the jagged rock. The water, heated to 100°C or more, then returns to the earth’s surface through a second borehole. The hot steam can be used to produce heat, but above all to produce useful electricity.
This was exactly the idea behind a 5,000m-deep geothermal well created in 2005 in Basel. However, the ‘Deep Heat Mining Basel’ project had to be abandoned after several perceptible earthquakes occurred and raised fears of a major earthquake.
In 2011, municipal utilities and regional energy providers from all over Switzerland founded Geo-Energie Suisse. The company drew lessons from the Basel experience and decided to adopt a new approach. To safely extract geothermal energy, those responsible turned to the multi-stage stimulation concept familiar from the petroleum industry. “In 2012, we patented this process for Switzerland; in the late fall of 2020, we were able to demonstrate in the Bedretto Laboratory at ETH Zurich that we can use the process to reduce the earthquake risk in deep geothermal drilling to a minimum,” says Dr Peter Meier, CEO of Geo-Energie Suisse.
STIMULATION IN ISOLATED ZONES OF THE BOREHOLE
The basic idea of the new approach is as follows. In Basel, the entire lower part of the borehole was pressurised during the process of injecting of water. In the new method, the borehole is divided into several stages (zones) using rubber collars (packers). A reservoir can now be stimulated independently in each zone, with water injections staggered spatially and temporally. The method allows the simulation of micro-earthquakes in each rock section with exactly the strength necessary to create a reservoir. Potentially critical zones can thus be detected at an early stage, thereby in theory avoiding strong, uncontrolled earthquakes.
In the Bedretto laboratory, the ‘isolated zones’ method was tested by placing two packers in the borehole 6.5m apart and stimulating the rock in between them. By moving the two packers in the borehole, stimulations could be performed at different locations within it.
A total of ten borehole sections were stimulated in this way during the tests in late fall 2020. The multi-stage stimulation concept proved successful: according to initial evaluations, water permeability in the compact Rotondogranite was increased by a factor of 10 to 100.
“Although we injected significantly smaller amounts of water than in Basel, the reservoirs we created have the permeability required for economic heat utilisation,” says hydrogeologist Peter Meier. The microearthquakes (microseismicity) required for the cracking had a maximum magnitude of -1.8Mw on the Richter scale. The earth thus shook about a hundred thousand times weaker than during the largest quake at the geothermal project in Basel (magnitude of 3.4). Meier concludes: “With these results, the use of petrothermal deep geothermal energy is within reach.”
ETH ZURICH’S MEASUREMENT PROGRAMME
In summer 2021, another test series is planned in the Bedretto laboratory. In a 400m-long borehole, packers will be used to create a total of ten zones, each 10–20m long, which will be stimulated individually or in various combinations. The test series will increase the volume of water injected to expand the reservoirs. As with the first test series in the fall of 2020, scientists from ETH Zurich will accompany the tests with an extensive measurement programme. However, many detailed questions still remain unanswered: do the packers seal the zones well, and do they do so over the long term? How much heat can be extracted from the artificially created reservoirs? How can flow rates be controlled? What chemical processes take place in the rock? Can the reservoirs be used as seasonal heat stores from summer to winter?
ETH Zurich has expertise in very sensitive sound measurements. Its measuring instruments register vibrations that are 10 million times smaller than a perceptible earthquake. Based on such measurements, scientists have developed a method for predicting seismicity that was successfully used in the current series of experiments. To do this, researchers measure the slightest tremors in the rock, which are considered harbingers of larger tremors. “Our method allows us to identify rock zones, in the case of stimulation, where there is a threat of a severe earthquake and therefore should not be stimulated,” says Dr Marian Hertrich, who heads the research team at ETH Zurich.
PILOT PROJECT IN HAUTE-SORNE
Following the positive results in the Bedretto rock laboratory, Geo-Energie Suisse intends to test the multi-stage stimulation concept at a test site in the US state of Utah before the end of the year. There – unlike in the Bedretto rock laboratory – the temperatures required for petrothermal geothermal energy are above 100°C.
In Switzerland, the next stage for Geo-Energie Suisse is an exploratory well at the Haute-Sorne pilot site in the canton of Jura. A pilot plant is planned there that would supply electricity to 6,000 households with a capacity of 5MW. The plant has so far met with resistance in the canton. The new results from the Bedretto rock laboratory could help to promote public acceptance of the use of heat from deep-earth regions.
1. ETH Zurich rock laboratory
ETH Zurich’s Bedretto rock laboratory is located about 20 minutes by bus from Airolo in the southern foothills of the Gotthard Pass. The laboratory is housed in a tunnel, originally excavated for the construction of the Furka Base Tunnel, which was opened in 1982. The project outlined above builds on investigations carried out in 2017 at the Grimsel rock laboratory of the National Co-operative for the Disposal of Radioactive Waste (Nagra).
The reservoir stimulation research project in the Bedretto laboratory, which will run until summer 2021, is being carried out by Geo-Energie Suisse AG (Zurich) with scientific support from ETH Zurich, financially supported by the Swiss Federal Office of Energy (SFOE). Geo-Energie Suisse is sponsored by seven energy utilities, including the municipal utilities of Basel, Bern and Zurich. The company’s main goal is to build a pilot power plant for deep geothermal energy. Such a plant is envisaged at the Haute-Sorne site (Canton of Jura).
2. Pilot and Demonstration Projects
Under its Pilot and Demonstration Programme, the SFOE promotes the development and testing of innovative technologies, solutions and approaches that make a significant contribution to energy efficiency or the use of renewable energies. Applications for financial assistance can be submitted at any time.
www.bfe.admin.ch/pilotdemonstration
