Plug in the Earth? Get heat from rocks!

When we observe the Earth from space, we see a beautiful blue planet. Since the blue of the ocean and the green of the forest are both cool colors, the Earth may seem clear and cool to us.

However, eyes always deceive us, and the actual situation may be beyond everyone's expectation. The earth is actually a small fireball that is cold on the outside and hot on the inside, with a very high heat content inside. Although it cannot be compared with a big fireball like the sun, it is estimated that the temperature at the center of the earth has reached an astonishing 6000 ℃.

Figure 1 Earth's internal structure Image source: flickr/ Argonne's Advanced Photon Source

Some scholars believe that there are three main sources of heat on Earth: the first is the heat generated by the decay of radioactive elements inside the Earth; the second is the heat converted from the gravitational potential energy when the dense matter inside the Earth sinks to the center of the Earth; and the third is the heat that has not yet been dissipated when the Earth was formed. In short, these are all the heat generated by the Earth itself.

Can this energy be used by humans? Today, I will introduce to you a "future energy" - hot dry rock.

In addition to hot springs, the rocks underground are also hot.

Although the center of the earth is hot, according to data, the average annual surface temperature is only about 15°C. This is because heat gradually dissipates from the center of the earth to the surface, and the temperature becomes lower and lower, which also causes a geothermal gradient in the strata. In areas close to the surface, the average geothermal gradient is 3°C/100m, that is, the temperature rises by 3°C for every 100m underground; conversely, the temperature drops by 3°C for every 100m closer to the surface.

After discovering the heat coming from underground, humans wanted to make use of it. They called it geothermal resources and divided it into hydrothermal geothermal resources and dry geothermal resources according to the production method.

Hydrothermal geothermal resources can be simply understood as underground hot water. Hot springs belong to hydrothermal geothermal resources, which are the product of underground hot water coming out of the ground. Some underground hot water does not come out of the ground, but is always sealed underground. In addition to hot springs, the most famous application example of hydrothermal geothermal resources in China is the Yangbajing geothermal field in Tibet. In 1977, my country built a geothermal power station here and achieved good results.

Figure 2 Hot springs use hydrothermal geothermal resources. Image source: Wikipedia

Correspondingly, dry-heat geothermal resources can be understood as the underground rock layer is very hot, but there is no water or only very little water. Generally speaking, it refers to high-temperature rock bodies buried 3 to 10 km deep in the earth's crust with a temperature higher than 180 ℃ (some scholars believe it is 150 ℃ or 200 ℃), so dry-heat geothermal resources are also called dry hot rocks.

The reason why we emphasize the depth of 3~10 km is not that it is not hotter at deeper depths, but because we cannot reach deeper places. The deepest drilling record of mankind is the Kola Superdeep Well in the Soviet Union, which took 20 years to reach 12.2 km and was extremely expensive. Therefore, we generally only focus on places with high geothermal gradients, that is, places where it is hot without digging deep, where the input-output ratio is relatively high. China's hot dry rock resources are mainly concentrated in the Yangbajing area of ​​Tibet, the Tengchong area of ​​Yunnan, and the Gonghe Basin of Qinghai.

Figure 3 Distribution map of hot dry rock resources in China Image source: Reference [13]

The resources of hot dry rocks are considerable. According to conservative estimates by the Massachusetts Institute of Technology, the reserves of exploitable hot dry rocks in the earth's crust are close to 1.3×1027 J, which can be used worldwide for about 217 million years. The hot dry rock resources at a depth of 3 to 10 km in mainland China are estimated to be about 2.52×1025 J, roughly equivalent to 860 trillion tons of standard coal. Based on China's total energy consumption of 5.24 billion tons of standard coal in 2021, if this part of hot dry rocks can achieve a 2% mining rate, it will be able to maintain China's energy supply for 3282 years.

However, since hot dry rock is such a good resource, why haven’t we mined and utilized it yet?

The energy is attractive, but it is not easy to mine

Although the prospect of hot dry rock resources is very attractive, it is extremely difficult to mine. It was not until the 1970s that a major theoretical breakthrough was made. In 1970, the Los Alamos Laboratory in the United States proposed the concept of an enhanced geothermal system (EGS). The basic principle is roughly as follows: drill two wells in the geothermal reservoir, an injection well and a production well. Inject cold water into the injection well, wait for the cold water to flow through the geothermal reservoir and be heated, and then use the production well to pump up hot water, and then you can use this hot water for heating or power generation. After use, you can also introduce this water into the underground for recycling.

Figure 4 Schematic diagram of EGS project Image source: self-made by the author

This solution sounds simple and cost-effective at first glance, which means that cold water can go to the geothermal reservoir and return with a full load of heat. However, there are actually many engineering and technical difficulties. For example, the strata where hot dry rock resources are located are often hot and hard, making drilling difficult and costly. Another example is that the hot dry rock reservoir may be dense and impermeable, so the injected cold water cannot diffuse around the drilling well to absorb heat and is difficult to flow to the production well to be pumped out.

Although we can improve the permeability of the reservoir through hydraulic fracturing technology (injecting high-pressure water to destroy the reservoir structure and form a network of fractures in it), since underground operations are invisible and intangible, it is difficult to ensure that the fractures will only develop in the direction we want after the high-pressure water is injected. In case it develops in a direction away from the production well, we will not be able to collect the injected water.

Figure 5 Schematic diagram of hydraulic fracturing technology, which injects high-pressure aqueous solution into the ground and uses high pressure to create cracks in the rock formation. Image source: Wikipedia

In 1973, the United States launched the Fenton Hill hot dry rock test project. Although the project ultimately failed due to drilling equipment defects and huge engineering costs, it also confirmed the feasibility of EGS technology and played a vital role in promoting hot dry rock geothermal exploitation. Since then, countries around the world have successively carried out a series of EGS engineering attempts. Among them, the EGS system jointly developed by France, Britain, Germany and Soultz is currently a relatively successful EGS demonstration project, which achieved hot dry rock geothermal power generation in 2008.

After the success of EGS technology in Europe, countries such as the United States, South Korea and China have also accelerated research in this area. In April 2015, the U.S. Department of Energy began implementing the "plug the earth" hot dry rock "Frontier Geothermal Energy Observatory Research Program" (FORGE), planning to increase the total installed capacity of EGS power generation to 100,000 megawatts by 2050, equivalent to four Three Gorges Dams.

South Korea launched its first EGS project attempt, Pohang EGS, in 2016, but a magnitude 5.5 earthquake occurred near the project site in 2017. Some studies believe that the earthquake may have been induced by the project's underground water injection, so the project was forced to stop.

Figure 6 A corner of the 2017 Pohang earthquake in South Korea, one of the largest earthquakes in the country in recent years, is suspected to be related to EGS. Image source: Wikipedia

In May 2017, Chinese scientists drilled a high-temperature rock mass with a temperature of 236°C at a depth of 3,705 meters in the GR1 geothermal well in the eastern part of the Gonghe Basin in Qinghai. This was the first time that the shallowest and highest-temperature hot dry rock mass was discovered in China. In January 2022, the country's first experimental hot dry rock power generation was successfully connected to the grid in the Gonghe Basin, achieving a major historic breakthrough.

Figure 7 Hot dry rock fracturing and directional drilling site in Gonghe Basin Image source: China Geological Survey

In addition to the traditional EGS method, some scholars have also found a new way. One of them is worth mentioning, which is the gravity heat pipe technology innovatively proposed by Chinese scholars. The principle is very clever: insert a pipe with excellent thermal conductivity into the hot dry rock formation, and then the heat pipe will automatically conduct the heat up. However, due to the long distance, the heat extraction efficiency may not be good if only the heat pipe is used. For this reason, we can inject ammonia water into the pipe. Ammonia water is easy to vaporize after being heated, and it can easily bring up the heat in the form of steam, further improving the heat extraction rate.

In January 2022, China Science reported on the 4,200-meter-long gravity heat pipe heat extraction test device, the longest in China, developed by the China Geological Survey and the Guangzhou Institute of Energy Conversion of the Chinese Academy of Sciences. A three-month on-site heat extraction test in Xiongan New Area showed that the short-term heat extraction power of a single well can reach 1.3 megawatts, the average heat extraction power is 800 kilowatts, and long-term stable operation can support a heating area of ​​more than 20,000 square meters. This technological breakthrough is also a major contribution of Chinese scientific and technological workers to the world's geothermal resource exploitation.

Figure 8 Schematic diagram of gravity heat pipe technology Image source: self-made by the author

How to solve the hidden dangers of hot dry rock mining?

Some people are concerned about the problems that hot dry rock mining may cause.

One is to induce earthquakes, such as the Korean earthquake mentioned above. However, in fact, the earthquake problem here is mainly caused by hydraulic fracturing. Other work using hydraulic fracturing technology, such as shale gas extraction, may also cause similar problems. Studies have shown that hydraulic fracturing has limited impact on earthquake activities above magnitude 3. At the same time, micro-earthquakes induced by hydraulic fracturing may help release accumulated ground stress or energy and reduce the risk of major earthquakes. In addition, some scholars believe that earthquakes caused by hydraulic fracturing can be controlled.

Second, it affects the life of the earth. Geothermal resources are part of the earth's own heat. We took away geothermal resources on our own initiative. Some people worry that it would be killing the goose that lays the golden eggs. It should be said that the huge heat inside the earth is indeed a symbol that the earth is still "alive". It is the power source of various geological activities on the earth, such as volcanoes and earthquakes. When it is consumed one day, the earth may cool down like the moon and become a wasteland without vitality. But everyone does not need to worry too much about this, because the little heat obtained by humans is really a drop in the bucket compared to the entire earth. And even if humans do not take it away, the earth will release its vigorous energy through volcanoes or earthquakes.

Third, it consumes a lot of water and affects the ecological environment. In the process of EGS reservoir transformation, tens of thousands of cubic meters of water resources are usually consumed. For example, the fracturing fluid consumption of the Soultz project mentioned above exceeds 100,000 cubic meters. my country's hot dry rock resources are mainly distributed in arid areas such as Qinghai and Tibet where water resources are scarce and the ecology is fragile. Problems such as water waste and ecological damage caused by hydraulic fracturing cannot be ignored. In this regard, some scholars have proposed the use of supercritical CO2 as a fracturing fluid, which can save water resources and help carbon neutrality. It is currently a research hotspot, and I hope it can be applied on a large scale in practice as soon as possible.

Conclusion

In general, although many countries have formulated grand plans to exploit hot dry rock resources, they are generally still in the stage of small-scale experimental exploration. Wang Guiling, a researcher at the Chinese Academy of Geological Sciences, said frankly: "(Hot dry rock mining technology) has not made particularly significant progress in 50 years." Because China started late, its technology accumulation is relatively weak.

But there is no doubt that hot dry rock is a future energy source with great potential. It has huge reserves and is green and pollution-free. Once a major breakthrough is achieved in mining technology, it will greatly benefit human society.

References

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Author information:

Author Name

Xiao Yidong

unit

Institute of Geology and Geophysics, Chinese Academy of Sciences