Deep geothermal development faces a number of challenges that still need tobe overcome, according to Ryan Law.
The development of geothermal energy as a source of baseload renewable power and renewable heat is enjoying something of a renaissance.
On the power side, global output is expected to double to approximately 20GW over the next 10 years. In Europe, particularly Northern Europe, the increase in demand for renewable heat sources has spurred the development of combined heat and power (CHP) projects, often in urban areas.
However, for geothermal energy to produce similar levels of power to wind and solar energy, deeper resources will have to be developed in areas of the world that have not previously been considered as geothermal hot spots.
Routine and reliable development of such deep geothermal resources will be key to the expansion of the geothermal industry, potentially unlocking vast amounts of renewable energy. But the development of such resources is not without technical, environmental and political challenges.
It is common knowledge that the temperature of the Earth increases with depth wherever you are located. Therefore, theoretically, geothermal heat can be exploited at any location. Minimum target temperatures for geothermal power generation are currently greater than 100°C and more than 70°C for direct heating use.
Given these targets, the key questions for any developer are whether the drilling depth to the resource is commercially viable and the nature of the rock and fluid at that depth.
“For geothermal energy to produce similar levels of power to wind and solar energy, deeper resources will have to be developed”
The rate of increase in temperature with depth, or geothermal gradient, is influenced by a number of factors: thickness of the Earth’s crust, proximity to volcanic zones and presence of heat producing radiogenic granites. On average, the geothermal gradient of the Earth is approximately 25°C per kilometre depth.
Traditionally, geothermal developers have focused on areas where the geothermal gradient is high and where superheated fluids are already present in the rock. For example, the zone of Larderello in Italy, which is one of the largest geothermal electricity fields in the World, has a high geothermal gradient (more than 100°C per kilometre depth) and geothermal fluids present beneath the surface.
This means that drilling depths are relatively shallow and power can be produced from the recirculation of the existing superheated geothermal fluids.
Although these systems are by no means trouble free, the development process is well understood and has been refined over the last 100 years of development.
For successful deep geothermal development, the technical challenges are much greater. As the term suggests, deep geothermal exploitation is about targeting resources located at depths that previously would not have been considered economically viable for geothermal power production.
In the case of the project Geothermal Engineering is developing at the United Downs site in Cornwall, the target depth is 4.5km. Further to this, deep geothermal resources are often “dry”, that is to say that there are no insitu natural fluids.
There are three basic types of geothermal resource (see figure below): hydrothermal, where hot geothermal fluids are present close to the surface; hot sedimentary aquifers (HSAs) where deeper, hot sedimentary basins are exploited; and the holy grail of deep geothermal power, “hot dry rock” or EGS, which will be discussed later.
In some ways, the path currently being taken by the geothermal industry is similar to the development of the oil and gas industry. First, shallow, easy to access resources were targeted, followed gradually by the extraction of deeper and technically more complex resources.
The difference for the geothermal industry is that the potential economic returns have never been as high as the oil and gas industry. However, as electricity prices continue to increase along with concerns about energy security, deeper geothermal resources are gradually starting to become more attractive.
Certainly we are now at the stage in Northern Europe where HSAs are routinely exploited for heat and low temperature power generation.
This is well evidenced in Paris, Munich and - the only deep geothermal heat system in the UK - Southampton. But the overall potential of hot sedimentary aquifers is still limited by the local geology. For deep geothermal energy to really make an impact globally, the hot dry rock resource must also be successfully and routinely developed.
There will clearly always be technical challenges associated with drilling to depths of 4.5km, particularly in a hard rock-like granite.
Nonetheless, rigs continue to improve and with the growth of geothermal development in urban areas, rig manufacturers are raising their game. Deep drilling rigs ith
smaller footprints and lower noise levels are being produced to fit both the needs of the geothermal developer as well as the oil and gas industry.
“With the growth of geothermal development in urban areas, rig manufacturers are raising their game”
The biggest technical challenge by far for deep geothermal developers is how to turn a “hot dry rock” resource into a commercially viable geothermal reservoir. If this can routinely be achieved, the potential resource is vast.
Rock at depths of 4km and greater is more often than not “tight” (the permeability of the rock is very low) and very little natural fluid is present, so it is very difficult to flow water through the rock in sufficient quantities to generate commercially viable levels of power.
Many developers are now working on the process of “engineering” or “enhancing” the rock at depth to enable greater quantities of water to flow through the rock, absorb heat from the rock and generate power.
This has given rise to the recent adoption of the terms Engineered Geothermal System or Enhanced Geothermal System (EGS). The process of engineering the rock at depth to develop a reservoir involves the injection of cold water into the rock under high pressure.
The process is not entirely dissimilar to the fracking used in the shale gas industry, only without the potential risks associated with gas leakage and normally without the chemical additives. As the water is injected, the rock slips by small amounts, opening up zones that allow water to pass through and create pathways.
In itself, this sort of “stimulation” is not a new process, indeed the UK was one of the first to do this in Cornwall as part of the Hot Dry Rock project in the 1980s.
However, new techniques of interpreting the way in which the rock slips and the three dimensional modelling based on these movements are helping developers to understand the growth of these “artificial” reservoirs and how best to drill wells to target them.
The process of stimulation is still evolving and the industry expects that over time, successful, routine methods of developing a reservoir will become standard.
One of the environmental challenges associated with this sort of reservoir development is the management and monitoring of micro-seismicity.
As with any type of deep injection or abstraction process, whether it is oil, gas or geothermal, thousands of microseismic events are created. The vast majority of these events will never be felt at the surface. However, if an event is felt and not correctly managed, projects can be shut down or will, at the very least, be subject to significant delays.
Given the number of geothermal projects that have been developed versus the number of seismic events that have actually been felt at the surface, the industry should have very little to worry about.
But as developers target urban locations in areas where the heat produced from the power plant can be distributed, there is clearly more of a risk that micro-seismic events could become a public nuisance.
As discussed above, the monitoring and modelling of micro-seismic events is key to understanding how a reservoir is developing and there therefore has to be a balance between resource exploitation and the impact upon the local community.
“It is important to talk to the local community about this activity once drilling has started”
If managed correctly, there is no reason why these techniques should be problematical for the local community.
Indeed, past experience has shown that projects which educate the local community and involve them in the project are far less likely to encounter problems if a seismic event that is large enough to be felt does occur.
Certainly, in Cornwall, where Geothermal Engineering is developing its first deep geothermal project in the UK, the likelihood of a felt event occurring is extremely low.
However, it is still important to talk to the local community about this activity, particularly once drilling has started.
In summary, the future for deep geothermal development currently looks bright. There is significant potential for expansion of HSAs in Europe to generate heat and low temperature power.
But if the industry is to generate power at levels associated with conventional fossil fuel sources, successful development of Hot Dry Rock resources must occur.
- Ryan Law is chairman of the Deep Geothermal Sector Group and managing director of Geothermal Engineering