In this part of the series I will take a look at the risks of harvesting geothermal energy. Let me begin with stating that so far there has been no large accident caused by harvesting geothermal energy. On the other hand, there is no such thing as totally safe ways to harvest energy.
We all know the dangers of nuclear power plants, oil and coal destroys the biosphere and accidents in mining and drilling operations have been many. Hydroelectric dams have failed and drowned people, wind turbines have catastrophically failed and killed people and large scale solar arrays changes the heat pattern around them killing vegetation.
So then, what are the dangers of geothermal energy? Well, one big risk is actually economical. It is risky business. First you need to spend a lot of money before you even know if you have an economically viable power plant. But, that is not what this series of articles is about, it is after all only money and jobs involved in that risk. Not the lives of people.
Below I will try to give as unbiased information on the risks as possible, both on real and fictional dangers of harvesting geothermal energy.
Geothermal energy causes eruptions
The idea is that drilling into active or dormant volcanoes would cause an eruption to happen. In one scenario the magma would start to squirt out if the drill accidentally went into a magma reservoir and a new volcanic vent would be born.
An eruption occurs as the pressure in the magma reservoir goes beyond the threshold of what the rock containing the magma can withstand. If that happens there is nothing that can stop the eruption. And if the pressure is not high enough there will just simple be no eruption. If a borehole enters a magma reservoir the magma would indeed trickle into the borehole where it quickly would lose temperature and solidify, forming a perfect plug.
This has actually happened in Iceland during the International Deep Drilling Project, exactly as I described it. The only result was that a few volcanologists did somersaults while playing lip-banjo out of joy since they received a set of nice samples of magma directly out of a magma reservoir. Otherwise nothing happened.
Another proposed idea is that the harvesting of energy would cool down the lid of a magma reservoir so that it would be able to withstand a higher magma reservoir pressure. This in turn would cause a larger than natural eruption to occur at a later stage. This one is intriguing and slightly plausible actually, but as we will see it is not likely to happen.
To get a good answer we only need to look at the last eruption out of Bárdarbunga volcano. A magma reservoir is 3-dimensional, at Bárdarbunga the lid of the volcano was situated below 700 meters of ice functioning as a thermal sink lowering the temperature over time on a scale that any geothermal energy harvesting would never be able to do.
The ice there often quenches proto-eruptions from going into large eruptions and slowly the pressure in the chamber rose. In the end the pressure went beyond the capacity of the surrounding rock to hold and the side ruptured and a very long dyke formed and the eruption at Holuhraun followed.
At Bárdarbunga nature did the cooling and hardening on a scale that is quite honestly well beyond what even a bunch of the world’s largest geothermal power plants could ever do. From this we can learn that a power plant situated on top of magma reservoir would minisculy harden the roof of the reservoir, and minisculy increase the risk of a flank eruption.
If we turn to physics instead for an explanation we will see that any eruption is caused by an increase of energy (thermal) and that what a geothermal power plant does is that it leaches out that thermal energy on a minute scale.
Now it would be simple to see the geothermal power plant as a safety valve delaying or hindering an eruption. But the amount of energy in even a small volcano is so high that we are talking about fractions of a second of delay at best. But, at the same time the physics way of looking at things clearly proves that it does not raise the risk of an eruption happening.
Geothermal energy harms nature
It is true that geothermal energy can be harmful for nature. At the same time it is the safest way of harvesting energy for nature. This does not sound like a compatible set of statements does it?
Well, you are either drilling into a very hot water body or you are pumping down water into very hot volcanic rock. The water body would be filled with minerals and gasses, and if you pump water down it would also dissolve the same minerals and gasses. And a lot of those minerals and gasses are poisonous. So, yes a lot of harm could come out of this.
Both the drilling into a natural water reservoir and the pumping down of water gives polluted water as a result. If you use an open system and let that water come out into the local biosphere you basically have a toxic spill.
The solution is to use a closed loop system and pump the same water down into the natural reservoir, or into an injection borehole. Basically you are sending the minerals and gasses back where they came from. You are not adding anything new to the rock down below, you are just returning what has come up.
Another good thing with repumping the water is that the risk of land subsidence disappears. This is especially true if you are drilling into a natural hot water reservoir.
Has there ever been a large spill of toxic water from a geothermal power plant that uses closed loop technology? No, there has not been one. But, there has been a spill of cooling water on a large scale. This was turned into a tourist attraction by the Icelanders, who happily continued to pump hot cooling water into a manmade lake creating the world’s nicest industrial accident.
To be honest, at any other place the Blue Lagoon would have been a disaster, but in barren Iceland it was a boon for the tourist industry. In the end the industry learned a lot from that mishap.
Borehole blow out
This is exactly what it sounds like. In rare instances there has been sudden increases in steam pressure that has suddenly caused the borehole to blow out. This can be caused by sudden fracturing of bedrock so that over-pressurized steam ruptures the top of the borehole where it connects to pipes leading to the turbine.
The same thing happens in nature. You can have a gently steaming hydrothermal well that suddenly explodes as the underlying water comes into contact with extra hot material such as magma.
The danger here is not the general public, but it is indeed a work environment hazard.
Geothermal energy causes earthquakes
Absolutely true however you look at it, but there are actually advantages to some of these earthquakes.
First rule of geothermal power harvesting, do not drill into an active faultline. I will get back to this.
If you are harvesting dry steam out of a natural reservoir of water you must pump the residual water back into the reservoir, otherwise you will cause subsidence. The subsidence would cause damages to houses and roads. Subsidence would also cause earthquakes in the area. On the other hand, pumping back water will also cause earthquakes. These earthquakes would be small, generally it would be felt as a vibration and not as distinct earthquakes. Maximum size earthquakes would be at around M3 in this context.
If you are going to harvest from dry bedrock you first need to fraction the bedrock at the desired depth to make it more porous for the injected water. This would cause a series of earthquakes ranging up towards a maximum of M3.5 and when you repressurize the well with water you would get tremor or discrete earthquakes at a maximum of M3.
Neither of these earthquakes would be dangerous, but they would be a nuisance to the local residents. It is therefore essential to inform them, and that you do this during daytime to not hamper normal sleep patterns. Also, and this is important, the engineers can at any time shut down the injection of water if they feel that the injection is causing undue amounts of earthquakes for the local population or lower the amount of water injected per second.
Now back to the first rule of geothermal power harvesting. It is very hard to avoid doing this close to, or in a faultline, since almost all volcanoes exist at faultlines. In Iceland geothermal energy is harvested at faultlines or even triple-junction faultlines. This practice causes tectonic earthquake swarms as pressurized water enters the faultlines.
This can’t be good can it? No and yes is the answer to that. Yes you risk having series of earthquakes from the faultline as the pressurized water lubricates the faultline. Because it is true that the water would cause the rock on both side of the faultline to slide easier and this causes more earthquakes. On the other hand, all of these earthquakes releases energy that otherwise would have been dispersed in one large earthquake.
Yes, water-injection into a faultline would cause hundreds to tens of thousands of earthquakes ranging from M1 to M5. But on the other hand neither of those would cause widespread destruction. And it would lower the energy level of the next big natural earthquake, or even hindering a big natural earthquake to occur at all.
One thing is though clear, no large earthquake (M6+) has ever been caused by water injection. The reason for this is that the water injection is taking place locally and a large earthquake happens as a long distance of the faultline rips open.
In the end one thing is clear about this last issue, more research needs to be done in the future. Only science can give us the answers needed, and at the moment scientific research seems to be in favor of lubricating faultlines even though the subject is highly controversial.