Not So Deep Heat

Shallow geothermal energy has been gaining traction as a clean energy source that avoids some of the challenges felt by other renewables. In geophysical terms, it’s just below the surface – and that makes it practically on the doorstep for many urban areas across the world.

The traditional use for geothermal energy has been to generate electrical power, and that generally calls for very high-temperature fluids to deliver the temperature differences necessary. Unfortunately, such applications are extremely geographically constrained; they only tend to operate in areas where there are volcanoes or very high/low temperatures, eg New Zealand, Indonesia and Iceland.

Shallow geothermal can be reached with a much smaller and less expensive drilling rig than high temperature geothermal needs. It’s also less cumbersome and requires less complex and costly technology.

Jeremy O’Brien: Global Director – Energy, Seequent

But what geothermal also affords is the ability to just provide heat. That can be useful at much lower temperatures, and this low-temperature geothermal resource can be found in many more locations, without drilling so deeply. While this approach may not have the capacity to generate large amounts of electricity, it can play an important role in displacing the need to create electricity from another source.

“The greatest value of shallow geothermal energy is that it replaces baseload energy,” says Jeremy O’Brien, Seequent’s Energy Business Manager. “Around 40% of all the energy used in Europe is for heating and cooling, so if you were able to get half of that 40% from drilling some holes in the ground, that is an enormous benefit.”

Geothermal also offers a particular advantage over other renewable sources. It’s always there. “If the sun’s not shining or the wind’s not blowing it still works. It’s 24/7 clean energy and it’s not going away.”

Baseload is a key target for emissions controls
Much of the baseload energy our society relies on has come in the past from coal or gas generation, and those are the sources currently being curtailed by CO2 agreements. (In 2020 the world’s use of coal fired electricity is on track for its biggest annual fall ever recorded, after four-decades of almost uninterrupted growth.)

While solar and wind energy also have a critical role to play in reducing our CO2 emissions, they are usually not focused on purely replacing baseload. Shallow geothermal doesn’t need a battery to store the energy it creates. It just sits there in the ground, waiting to be tapped. The footprint of a geothermal plant will also typically be far smaller than that of a solar power array or wind farm as all the heavy lifting is going on underground.

All of this means that shallow geothermal – especially heat pump applications – has the ability to be highly ‘local’. (A good example being the trend for supermarkets to extract heat from below their own stores and use heat exchangers to offset their refrigeration power.) Or it can be city-wide. Copenhagen is one of a number of European cities exploring the potential of shallow geothermal to support its district heating for residents.

“Often you’re only looking for temperatures in the range of 50 to 80 degrees Celsius,” says Jeremy O’Brien, “but in many cases that’s all you need for baseload replacement.” Neither is the idea actually that new. “Many people don’t realise that Paris has had geothermal heating since the 1970s…”

Down in the not so depths
So how deep is shallow? “I suppose in our language we would say anything less than 1000 metres, whereas in a normal geothermal sense, the average depth of a well would be 2000 metres.

“What’s important is that everything within those 1000 metres of the surface can be reached with a much smaller and less expensive drilling rig than high temperature geothermal needs. It’s also less cumbersome and requires less complex and costly technology.”

And in many instances, simple heat pumps can be effective at far less than that. London’s Tate Modern gallery opted for a geothermal solution that goes just five metres down to a bed of river gravel. It uses the borehole to satisfy part of the building’s heating demands in the winter, and cooling needs in the summer, keeping the invaluable collection of Picassos, Dalis, Rothkos and more at a comfy (and internationally required) 18 to 25 degrees.

Again in the UK, researchers are exploring how the country’s legacy of abandoned coal mines could be used to create a second life of heat generation – this time using the slightly raised temperatures (around 30 degrees) of the miles of empty voids sitting there within the earth.

How shallow geothermal potential reveals itself
A key to making shallow geothermal work is locating the particular formations and stratigraphic units that have good temperature fluids in them, and which can be employed effectively. Surveys might include seismic, gravity, magnetic telluric data, but it can be invaluable to incorporate that with what’s already known about the location, points out Jeremy O’Brien.

“Are there old oil and gas exploration wells or perhaps old groundwater wells where the data can be integrated with the geophysics? What are the flow rates from the existing wells? What does that tell you about the areas that might be most interesting to explore? Where are the highest temperatures and what’s the geology?”

Good detective work can come in many forms. For example, a Google Earth tour of Almeria in south-east Spain reveals a glittering landscape of greenhouses covering almost the entire peninsula. It’s the largest collection in Europe. When looking for areas of high geothermal potential, researchers reasoned that the farmers might perhaps know something they didn’t

“Just inland you can see the fault lines running through the topography, and the farmers were drilling for water up in these hills. The fault channels fluids deep down and back up again in a very efficient manner so they get hot relative to benign groundwater. It was no good for the plants as it had picked up too many salts, but for geothermal energy it was ideal….”

CASE STUDY – Assessing shallow geothermal potential in urban areas; Catalonia, Spain

European urban areas are decarbonising, and the energy market is shifting towards renewables. The popularity of shallow geothermal energy is growing. Spain’s Catalonia is one region exploring the possibilities, and is a case study for the MUSE project (Managing Urban Shallow-geothermal Energy). The urban area of Girona was chosen as the first pilot area – not without its challenges.

It’s the thermal properties of the subsoil that determine how much energy can be extracted via heat exchangers, and around Girona the geological and hydrogeological properties of each stratigraphic unit are complex and vary significantly. A detailed 3D model was needed, and Leapfrog software was used to prepare that model from all the available data, which was substantial.

All in all, around 1400 drillholes, 4 geological maps scale 1:25000, 5 geological maps scale 1:5000, 2 hydrogeological maps scale 1:25000 and a wealth of geophysical data were used to prepare a detailed geological model of an area 10km wide, 9km long and 300m deep. A total model volume of 29km3. Average ground water temperatures were also monitored.

In order to interpret and present subsoil geology in the best way possible, it was necessary to build a base model defining the Paleogene-Palaeozoic, Neogene and Quaternary periods. In fact 31 geological units were modelled in total, between Ordovician and Quaternary. The focus was on determining the depth and spatial expansion of Girona’s La Selva sedimentary basin (the basin’s sedimentary fill is likely to be the principle medium through which to utilise geothermal resources). The research is ongoing and will ultimately contribute to the GeoEnergy SGE Project that will provide a platform to assess geothermal potential at a regional and local scale and help Girona decide which areas are the most feasible for installing open and closed-loop heating systems.

Using Leapfrog to chase the heat
Increasingly governments are looking to screen and sift all the data they’ve gathered in areas, like Almeria, where they suspect there could be useable heat gradients due to faulting or pressured sedimentary aquifers, etc.

“I think Leapfrog can make a real contribution to that,” considers Jeremy O’Brien. “It’s a very intuitive tool for combining all those different data sets together in one place. You can build a picture of the subsurface that will identify potential spots that might not have been intuitive otherwise.

“If you’re starting a project, you might have an old geological cross-section that covers a massive region and you need to integrate all that data into one place. Leapfrog can speed that up enormously, quickly building a picture and generating visualisations. In comparison it might take you days to explicitly draw temperature contours by hand or by using another package.

“Screening a lot of data quickly makes it a powerful tool in identifying areas with potential for shallow geothermal. Then, once the drilling campaign is planned, the fresh data can be input to update the model and show what’s happening in the subsurface. Because Leapfrog connects with other simulation software you can also look at the flow of water in the ground or changes in temperature. The software can guide teams on where drilling should occur, and then go on to help understand and manage the resource with time.

“It means that Leapfrog can support a project from beginning to end, throughout the value chain.”