Deep geothermal drilling in Kotka confirms high heat potential in southeastern Finland’s bedrock

A newly drilled medium-deep geothermal well in Kotka has confirmed that southeastern Finland’s rapakivi bedrock produces up to 30 percent more heat than the national average, reports Finnish public broadcaster Yle.

The Geological Survey of Finland (GTK) announced successful results from its 800-meter-deep test well in Kotka’s Sunila district, marking the first medium-deep geothermal drilling project in the Kymenlaakso region. Project manager Annu Martinkauppi called the findings “extremely promising,” noting the borehole was completed without complications and that bedrock temperatures matched expectations.

“The thermal conductivity of the surrounding rock was also about 10 percent higher than anticipated,” Martinkauppi said, adding that a single medium-deep well could now supply annual heating for 5–7 detached homes.

Southeastern Finland’s bedrock ideal for geothermal energy

The drilling is part of the Geoenergialoikka project, a collaboration between GTK and South-Eastern Finland University of Applied Sciences (Xamk) to assess medium-deep geothermal potential in the region’s rapakivi bedrock—an area covering most of South Karelia and nearly all of Kymenlaakso.

Teppo Arola, GTK’s lead geoenergy expert, explained that the rapakivi bedrock’s high radioactive mineral content generates exceptional heat as these materials decay. “This is Finland’s best region for geoenergy production,” Arola stated. “The rapakivi here yields about 30 percent more heat than the Finnish average and roughly double that of the country’s least productive areas.”

While medium-deep wells (600–2,000 meters) are already in use in the Helsinki region, their adoption in southeastern Finland has been slower due to occasional fault zones in the rapakivi that complicate deeper drilling. However, Rototec CEO Vesa Sinisalo, whose company executed the Sunila borehole, believes this success could spur further investment.

“If drilling proceeds this smoothly, there’s definitely potential for more wells in the region,” Sinisalo said. GTK’s pre-drilling geological surveys helped avoid complications, and the agency is now mapping additional suitable sites in Kymenlaakso.