Welcome back after the summer break to all you VIPS enthusiasts out there. We hope you guys had a great summer! We will now continue our stories from early career researchers and our established researcher interviews. So stay tuned! Our first blog post after the break is coming in with a BANG!…or rather a SHRUSHSHSHSHRUUSH, which is the sound you can hear when overpressured steam is coming out of a vent 🙂 Enjoy this field story from Anne Pluymakers (TU Delft).
Anne Pluymakers – TU Delft
Volcanoes are hot! Geology and energy go hand in hand, and volcanoes in particular can be interesting from more than one perspective. For us humans, volcanic heat, or geothermal energy, can be used as a climate-friendly energy source. Geothermal energy is in many countries a prime target to reach climate goals. In volcano-poor countries this is a bit more tricky than in fiery countries such as Iceland and Italy. This blog is about how outcrops of intrusions can be used to teach BSc, MSc and PhD students about geothermal energy. Prof. David Bruhn (TU Delft, the Netherlands, and GFZ, Germany) organizes a yearly field trip to Italy, specifically to the Larderello area and Elba. This is done with the invaluable aid of Prof. Domenico Liotta (University of Bari Aldo Moro, Italy). The setting is the Italian Apennines, which, due to the extensional setting, are characterized by a high heat flux. The heat flux in most regions is 40 mW/mm2, whereas in Tuscany it is 150 mW/mm2. This high heat flux means that it is a prime area for geothermal energy. This blog is about the holy trinity of a usable geothermal system: the heat source, the fluid, and the fluid pathways.
Fumaroles and geothermal power plants (with crosscutting pipelines) dominate the view in the Larderello area (gif by Anne Pluymakers)
To better understand the technological background we first explore what a currently active geothermal system looks like. For this we go to the Larderello area, a beautiful region where geothermal energy is visible both through the local presence of fumaroles, as well as by the omnipresent pipelines and cooling towers. The pipelines are sometimes colored green to be less conspicuous. One could find them ugly, but given that they provide 3% of Italy’s energy plus employment throughout the region, they serve a great purpose! In this area abundant heat is present, which is tapped by in total 300 extraction wells and 34 power plants (run by Enel Greenpower). There are another 100 re-injector wells, to ensure the extracted steam and water are re-injected into the aquifers at depth. An example: the geothermal powerplant we visited is fed by 20 boreholes, spread over 7 km2.
Discussions on top of the fumaroles, where steam escapes the active geothermal system through fracture networks
Left: Tourist well: 200°C steam, at 2 bars, and 10 tonnes of steam per hour. Right: An active drill site in the Larderello area (gif and photo by Anne Pluymakers)
The two main reservoirs are (1) a shallow reservoir, with 200°C and 2 atmosphere pressure, and (2) a deep (2 km) reservoir, with the same temperature but then at high pressure, 7 atmosphere (and higher). The heat is carried by steam. The pressures aren’t very high, but the flow rates are enormous. This is demonstrated by the ‘tourist well’, that is now not in use any more. It contains 200°C steam, at just 2 bars – an average biketire carries between 2 and 5 bars. However, this tourist well produces 10 tonnes of steam per hour – so standing on the 85 decibel line it clearly shows the power of the earth! In this region the power plants function on higher pressure, which renders this specific well useless. However, if so desired, also at this location there would be possible ways to convert the heat one could obtain into energy. What is economically viable is an interplay between offer and demand, and as such is location-dependent. The steam is contaminated with fluids and other gases, which need to be separated before the steam can power the turbine and generator. Precipitation of salts in the water can lead to one of the main production issues associated with geothermal wells: scaling.
Scaling can be a big issue in geothermal pipelines (photos by Anne Pluymakers)
Fun fact: the first use of geothermal waters in the region was to produce boron salts. Production of boric acid and its sodium salt, borax, started as early as 1818, though mass production initiated in 1827. Francesco Larderel built a covered hot pond to gather natural steam, needed to feed the boric water evaporation boilers. By the start of the 20th century, the Larderello region produced electricity for local use.
Scale model of the first “hot pond under cover” for boric acid extraction, where the boilers were powered by steam (display in the Geothermic Museum of Larderello, photos by Anne Pluymakers)
Geological map of Elba island (from Rocchi, S., Westerman, D. S., Dini, A., & Farina, F. (2010). Intrusive sheets and sheeted intrusions at Elba Island, Italy. Geosphere, 6(3), 225–236. https://doi.org/10.1130/GES00551.1)
After this demonstration of a working geothermal system at the earth surface, the excursion moves on to the geological analogue. Analogue field studies are a common way of using field geology to imagine what the subsurface could look like in the targeted region. Therefore we move to the island of Elba, just off the coast of Tuscany. The equivalent heat source is the Monte Capanne pluton, on the west of the island. The first evidence of geothermal activity constitutes of precursor dike and sill intrusion into an ophiolite sequence. Large scale melting happened in the Miocene, at a depth of 6 to 7 km. To generate these volumes, water would have had to be present – this geological evidence then represents the ‘mother’ of the heat flux, and where the first geothermal fluids on Elba have come from. In the outcrops of the Monte Capanne pluton xenolites and large feldspar crystals are visible. The feldspar crystals indicate large convection patterns in what must have been viscous, cooling magma. After cooling, this pluton was fractured, and meter-sized mafic intrusions with fine-grained crystals share orientations with these fractures. Analysis of fluid inclusions in the outcrop indicate that the fluids are all relatively local. That is not surprising, since upon cooling, felsic magmas release large amounts of water, and also the surrounding meta-sediments (i.e. former ocean floor) were full of water.
Two different outcrops along the Monte Capanne pluton, with different crystal composition. The feldspars in the right hand picture have centimetre size. (photos by Anne Pluymakers)
Where there is a magma, there are interesting percolating fluids, with all kinds of rare and precious minerals. As can already be seen in the geological map, the evidence of fluids is obvious throughout the island, in the form of several mines, especially on the east side. In the east the people have mined for iron for decades, specifically for hematite, limonite, pyrite, magnetite and ilvaite. Of these, ilvaite is a mineral that is quite specific to Elba. On the west, closer to the pluton, we can find tourmaline, beryllium, orthoclase and quartz.
Mine on Elba island. The elongated shape follows the fault path (photo by Anne Pluymakers)
From this mine spectacular crystals have been recovered (from www.infoelba.com)
Many of these mines follow distinct pathways: those of fractures and faults, demonstrating also in this fossil system the importance of fractures and faults as fluid pathways (i.e. compare the photo of the small scale fracture network on Elba to the photo of the fracture networks associated with the fumaroles). Elba hosts a famous shear zone: the Zuccale fault. This outcrop contains many textbook structures, from brittle to ductile deformation.
Brittle (left) and ductile (right) deformation at the Zuccale fault outcrop (photo by Anne Pluymakers)
The less weathered mineral veins provide clear evidence of past fluid pathways. A key message for geothermal energy recovery that can be obtained from outcrops like these is that permeability is directional and local. The fracture zone itself is asymmetric along the shear zone. The shear zone is impermeable across fault, but not necessarily as impermeable along the fault. As an engineer responsible to determine the best location to drill a well (approximately one million euro per km!), where would you go?
Anne Pluymakers is a VENI post-doctoral researcher at TU Delft, the Netherlands (firstname.lastname@example.org). She is an expert in experimental rock deformation and associated microstructural analysis, with a focus on fluid-rock interactions. Her research can be placed in a geo-energy context, with a specific focus on geothermal energy and CO2 storage. Anne’s current project investigates the effect of pore fluid chemistry on the failure behaviour of carbonates. Find her papers via Anne Pluymakers (TUDelft) or Google Scholar. In addition, Anne is part of the editing team of the EGU Tectonics and Structural Geology blog, and, together with @AukeBarnhoorn, she tweets about the TU Delft rock deformation laboratory activities under @RockDefTud.
Are you a VIPS enthusiast and have a great field or laboratory story that happened during the summer? Share it with us! We are always on the hunt for stories that show the experiences of other early career researchers working on, under, around and above volcanoes. Email us at: email@example.com