Geothermal energy can be harnessed for the use of its heat directly (direct use) or for electricity generation. Geothermal energy is thermal energy which is generated and stored within the Earth, as we described in our previous chapter on What is Geothermal Energy?
In the context of how one derives this energy we define four categories of geothermal resources:
Shallow geothermal heat-pump, or geo-exchange resources
- Hydrothermal resources
- Enhanced/ engineered geothermal systems
- Unconventional or advanced geothermal systems
- The type of resource determines how we can extract the thermal energy from the ground for extraction for energy utilisation on the surface.
1. Hydrothermal Resources
Hydrothermal, refers to heated water resources, which can be found in hydrothermal resources that are naturally occuring. They are created by groundwater and favourable rock characteristics, such as open fractures or fissures, that allow the flow of fluids between.
With the high(er) temperatures of the rocks, the fluids are heated and can be derived either as hot water or steam, if the temperature is high enough. So the fluid, or steam carry the heat which can then be used on the surface for either heat applications, or for electricity generation.
These hydrothermal resources range in temperature from a few degrees above the ambient conditions on the surface to temperatures beyond 350 degrees Celsius (or 660 Fahrenheit).
Hydrothermal resources can be found in volcanic settings (such as in Indonesia), in sedimentary settings (such as the German Molasse Basin) and hot wet rocks (e.g. fractured granite with water resources).
2. Unconventional Resources
Unconventional resources, such as either enhanced or engineered geothermal systems, or new so-called advanced geothermal systems, are approaching geothermal resources (heat) that lack the necessary fluids or rock characteristics that would allow for heat extraction.
Unconventional resources are geothermal energy that can be found in hot dry rocks for example – essentially hot baserock settings, where there is heat, yet no water flows that could be extracted as a carrier of the heat.
So, one would require some element of heat exchange to extract the heat from the subsurface for energy utilisation above the surface, either for heat or power generation.
i) Enhanced/ engineered geothermal systems (EGS)
The approach in enhanced geothermal systems (EGS) is to create permeability between rocks and adding fluids that would allow them to be sufficiently heated in a sustainable system (and thereby artificially create hydrothermal 'reservoirs').
This allows one to derive geothermal heat from beneath the surface to be used on the surface. As one creates these reservoirs artificially, one often also calls them engineered geothermal systems.
Another term used often is Hard Dry Rock (HDR). In the past the term was used a lot in the Australian context, whereby 'hard' described the non-permeability of the rock and 'dry' described the fact that there were no fluids that would have allowed the extraction of heat in the traditional or conventional way.
With traditional hydrothermal resources often tapping higher temperatures in accessible depths, (e.g. along the tectonic plates) EGS systems allow the tapping of geothermal energy beyond these areas essentially around the globe.
Geothermal energy can be found around the globe, yet the level of depth, the temperature and the availability of fluids have determined development so far.
The challenge with EGS technology is the cost involved in targeting sufficient temperatures in depths that are often much higher, and the economics of creating a sustainable system that would allow utilisation for the long-term.
Drilling costs, stimulation of rocks to make them permeable, the pumping of water both down into the artificial 'reservoir' and its pumping to the surface requires in itself a lot of energy. The temperatures are also often not as high, hence the technology to generate electricity is also more complex and more expensive.
The element of 'stimulation' is also often seen as critical as the pressure applied can create small earthquakes on the surface that – particularly in urban areas – create concerns of the public and other stakeholders.
While EGS systems are often described as separate systems, EGS technology can also be applied in conventional geothermal settings.
Stimulation can help make hydrothermal reservoirs more productive by increasing permeability and thereby increase output on the surface.
(ii) Advanced Geothermal Systems
The concept of advanced geothermal systems (AGS) has been coined by various groups targeting an approach that would remove the resource risk in geothermal development, namely the need to find sufficient temperature, find sufficient fluids, or in the case of EGS create sufficient permeability.
AGS does so by extracting thermal energy utilising a closed-loop system. It achieves this by circulating a working fluid through a long wellbore that conducts heat from the rock surrounding the well.
These concepts have been discussed in science for a long time, either as a single well solution, or with new efforts gaining a lot of attention in a closed loop system, not quite dissimilar to shallow heat exchange systems.
These closed-loop systems are built on wells drilled that connect with each other allowing a heat exchange-type set up beneath the surface – at deeper levels than traditional geo heat-exchange systems.
These systems would allow the extraction of geothermal heat virtually everywhere on the planet. Pilot projects have proven the concept of these closed-loop systems. The planned development of commercial scale systems will show how the economics will make these systems a valid and competitive option for both power generation and direct use.
There is yet another new approach to utilise geothermal energy which is the utilisation of so-called supercritical geothermal systems. If they would fall under the category of 'Advanced geothermal systems' is likely a bit questionable, yet it is also an advanced approach to tapping geothermal energy.
The concept of tapping supercritical geothermal systems is tapping super high temperatures of 400 degrees Celsius (750 degrees Fahrenheit).
The idea is that the energy content is so much higher that it would make the energy output per well much higher than in traditional systems.
This reservoir fluid is assumed to be in a 'supercritical' state, as fluid, water and pressure are so much higher.
With temperatures in excess of 370 degrees Celsius and a pressure of 220 bars and more, these wells could be much more powerful.
The Iceland Deep Drilling Project (IDDP) is likely the most prominent project looking at exploring how one can tap this powerful energy. While a traditional geothermal well in Iceland produces maybe around 5 MW, a well with supercritical conditions could potentially produce ten times that amount.
The management of such high temperature resources, high pressure and often the composition of the brine fluid are presenting a challenge. A lot of research remains to see how an approach to expand geothermal energy extraction from supercritical fluids will have to be seen.
As a renewable clean energy, geothermal energy is the inevitable direction of new energy development, and the Vigor team keenly captures the development trend of the energy industry, and we always believe that only by continuously exploring the future in a scientific and orderly manner can we always maintain the leading position in the industry. We also look forward to in-depth communication and cooperation with you on the development of the energy industry.
For more information, you can write to our mailbox info@vigorpetroleum.com & marketing@vigordrilling.com
