Clean geothermal heat deep from the earth

Geothermal energy is an exciting option in comes to zero-emission heat production. The Otaniemi pilot project has in many respects explored world's first deep EGS (Engineered Geothermal System) plant of this type. The pilot project enables researching and piloting how geothermal heat could be produced in Finland and other Nordic countries. 

The process to harvest and utilise geothermal heat is quite simple. First, two production wells of  several kilometres are drilled into the ground. One of the holes pumps water down to the bedrock in order to heat it as a result of the warmth in the earth’s crust. The hot water is pumped up via the other hole, and the produced heat is captured with a heat exchanger and fed into the district heating network. 

Penetrating the hard Finnish granitic bedrock requires both a purpose-built drilling technology and some persistence. The drilling of the production wells first utilised air hammer drilling technology (often referred as DTH drilling) to reach a depth of 4.5 kilometres. After this, drilling of the first well to the hard bedrock was continued with both water hydraulic hammer drilling technology and the traditional rotary drilling method. By optimising these technologies, drilling has reached a depth of 6.4 kilometres.

Another challenging stage of the project is achieving water flow between the two boreholes. The stimulation phase, during which we  investigated how water injected in the first well flows between cracks in the fractured zone of the bedrock, was successfully completed in July 2018. The purpose of stimulation results was to pinpoint the direction to which the final stage of the second production well should be drilled, and to determine how water flows in the bedrock between the wells.  Geophones installed in the deep boreholes were used to monitor the flow of water in the bedrock. 

After  the stimulation stage, the collected data on water flow was analysed. During the analysis stage the drilling was paused and the drilling technology was optimized and further developed more cost-efficient. Drilling of the second production well was continued in fall 2019 and was completed in February 2020.

In April 2020 the Otaniemi geothermal pilot project progressed to the finalization of the production wells. First, hydraulic tests were performed to find out the need for so-called counter stimulation. That was followed by a counter stimulation phase during which water was pumped into the second production well to ensure that the water supplied to the system flows as well as possible in the rock fracture between the wells. The counter stimulation was completed in the second half of May 2020.

During the design phase of the cross-flow test scheduled for fall 2021, detailed data from 2018 and 2020 stimulations were analyzed to provide information on the water flows in the deep bedrock fractures and how it can be intensified if necessary. As a result of the analysis, it was concluded that the new cross-flow test between wells does not provide any additional information that would support to run the test and the time spent on it.

The pilot project of St1's Otaniemi geothermal heating plant explores the technical implementation options for the heating plant. The Engineered Geothermal System (EGS) technology, which was originally planned, has proved to be very challenging from a techno-economic point of view, and the current mapping phase aims to define the connection of the boreholes and above-the-ground production technologies to achieve the best possible thermal output.