Closed-loop shallow geothermal boreholes drilled up to a depth of around 200 m are quite common in Estonia. But recently, the Geological Survey decided to embark on a project to tap into deeper geothermal resources by drilling into basement rocks buried beneath an interval of sedimentary strata in the northern part of the country.
The goal of the project is to produce the energy “stored” in basement rocks that are overlain by a succession of siliciclastic sediments. This is based on the concept that a 150 to 300 m thick succession of isolating sedimentary rocks forms a barrier for heat to be effectively transferred to surface, leading to relatively high temperatures in the topmost part of the igneous succession.
In southern Finland, a geological analogue to northern Estonia, this concept has already been successfully applied and it is expected that the thermogeological properties are comparable in Estonia, if not superior. For instance, geothermal gradients of up to 35° C / km have been measured in in basement rocks in northern Estonia, compared to 18° C / km in other places. This has resulted in a 5° C temperature anomaly in the higher gradient areas. Further drilling is now needed to better map these positive anomalies.
Challenges
While the geo-energy potential in northern Estonia is promising, it comes with its own set of challenges. Drilling boreholes through the unconsolidated sedimentary rocks to reach the crystalline basement increases costs. However, the economic feasibility demonstrated by similar projects in southern Finland provides a strong incentive to overcome these challenges.
The initial idea was to drill at least 600-700 m boreholes, but the length limitations of U-tube borehole heat exchangers restricted the project to 500 m boreholes. However, another way of tapping into deeper strata is now being considered without the use of a U-tube pipe. In this case, a single tube will be lowered in the borehole, with water being circulated downward through the open hole and subsequently produced back up through the tube. The heat exchanger is then used to transfer the energy from the top of the borehole to the low temperature district heating network.
The risk of these open hole circuits is the presence of a fracture system in the basement rocks, with a loss of fluid as a result. That is why only water is used as a circulation fluid, and work is being done at the same time to find ways to close off the fractures.
