Did EBN set a trend? That’s to be seen, but the four-well geothermal drilling campaign the Croatian Hydrocarbon Agency recently embarked on to prove the potential of an equal number of towns has similarities to what EBN is doing in the Netherlands. Here, up to seven wells are being drilled back to back, also to prove the potential for geothermal energy production.
However, where the wells drilled in the Netherlands were chosen in underexplored areas first and foremost, the Croatian wells have been planned at sites as close to population centres as possible, and in places where district heating systems are in place already. “And, of course, areas that have the subsurface potential as well,” says Martina Tuschl, who is overseeing the project on behalf of the Croatian Hydrocarbon Agency.
Another big difference between the two projects is that the wells drilled in Croatia will be completed with the idea to convert them into either a producer or an injection well. “We can’t just afford drilling exploration wells,” says Marijan Krpan, President of the Management Board of the Croatian Hydrocarbon Agency. “The wells will be completed with a 7” slotted liner at reservoir depth, and the well heads are prepared for ESP pumps as well,” adds Martina.
The project in Croatia is exciting and must be seen as a serious attempt to de-risk four towns for the further development of their local geothermal resource. The first well is currently being drilled near the town of Velika Gorica, just south of Zagreb, the capital.
“The primary target of this well are the lithotamnic limestones of Middle Miocene age at a depth of around 2,100 m,” says Martina, “with the second target are older Miocene rocks 700 m deeper. The expected temperatures are 90° C and 140° C, respectively.
“We plan to test the wells for a duration of around seven days following completion, and depending on the results, we will either plug or temporarily abandon them in order to come back later. The other wells are planned near the towns of Zaprešić (north of Zagreb), Osijek, in the far east of the country, and Vinkovci, just south of Osijek.
It is no surprise that the locations of all four planned wells are in the northern part of the country, which is part of the Pannonian Basin. Here, the geothermal gradient is close to 50° C / km, in contrast to the southern part, where it is only 18° C / km.
Seismic surveys were acquired in all areas to narrow down the best location for the wells, supplemented with magneto-telluric surveys.
“There is potential for high-temperature geothermal in our country as well,” says Marijan Krpan, “but the investment framework has to be established for that first. Hence, with this campaign, we are looking to tap into temperatures that lend themselves for use in district heating networks.”
