Storing heat in basement rocks

“Having a look at our lineament map,” says Jon Engström from the Geological Survey of Finland, “that is the first thing you need to do when you’re planning a subsurface project in our country.” Jon is a structural ge­ologist who has extensive experience in mapping of structural deforma­tion in Finland’s subsurface, mostly gained through the site selection pro­cess for the nearly completed reposi­tory for nuclear waste.

The lineament map provides a good indication of the bedrock blocks in southern Finland, which is primarily composed of granites that were subsequently metamorphosed and deformed to varying degrees. As such, the lineaments dissect the country into a patchwork of small­er blocks. In turn, it is these smaller blocks that form the basis for most subsurface projects because the de­gree of fracturing and faulting is usually less than in the lineaments themselves. This reduces the risk of geomechanical instability, reactiva­tion or groundwater flow pathways.

It is groundwater flow within the bedrock that is especially critical to the Varanto project in the town of Vantaa, southern Finland, where the world’s largest thermal energy storage project is currently under construc­tion. It will be a seasonal storage fa­cility that consists of a series of three huge man-made caverns that togeth­er will store 1,1 million m3 of water at a temperature of around 140° C. In total, the energy stored this way amounts to 90 GWh, enough to heat a medium-sized town for as long as a year. The caverns will be 20 m wide, 300 m long, and 40 m high and are situated at around 100 m below the surface.

“We know that the rocks them­selves are impermeable,” says Jon, “but we also know that fracture zones are the most risky when it comes to potential fluid escape. That’s why a rigorous mapping exercise is critical; not only through mapping the region­al lineaments, but also by more de­tailed work carried out on site.” That’s where cores from boreholes come in, which were drilled at the project site. “You always need that type of data to gain a better understanding of the local geology,” explains Jon, “also because the types of basement rocks vary and with that does the density of fractures.”

“When you look at just granite, the fractures tend to be a set of two conjugates at a defined angle, but when the rocks are heavily metamor­phosed, as is the case with the succes­sions we are dealing with at the site, the fractures tend to follow the foli­ation and therefore it is more mixed up,” Jon says. “That is the main rea­son as to why the cores are so critical to the overall assessment of deforma­tion zones and fracture density.”

The database covering a nationwide overview of interpreted lineaments can be found through the website of the Geological Survey of Finland through this link.