Earth logo

Plugging in the Earth? Asking for heat from rocks

The Earth is blue

By Deena ThaddeusPublished about a year ago 7 min read
Like
Plugging in the Earth? Asking for heat from rocks
Photo by Guillaume de Germain on Unsplash

Looking at the Earth from space, we see a beautiful blue planet. Since the blue of the ocean and the green of the forest are cool colors, such an Earth may look refreshing and cool to us.

Earth's internal structure

But the eyes are always deceiving, and the actual situation may be unexpected. The Earth is a small fireball that is cold on the outside and hot on the inside, containing a high level of heat inside. Although it cannot be compared to a large fireball like the sun, it is estimated that the temperature at the center of the earth also reaches a staggering 6000 °C.

Some scholars believe that there are three main sources of the Earth's heat: the heat generated by the decay of radioactive elements inside the Earth, the heat converted from the gravitational potential energy of dense material inside the Earth as it sinks to the Earth's core, and the heat that has not yet been dissipated when the Earth was formed. In a word, these are the heat generated by the earth itself.

Can this energy be used by human beings? Today, we will introduce you to a "future energy" - dry heat rock.

Although the core of the earth is hot, according to the data, the annual average temperature of the surface is only about 15 ℃. This is due to the gradual dissipation of heat from the core to the surface, which results in lower and lower temperatures and a geothermal gradient in the earth's strata. In the area near the surface, the average geothermal gradient is 3 °C/100 m, which means that for every 100 m deeper into the ground, the temperature increases by 3 °C; conversely, for every 100 m closer to the surface, the temperature decreases by 3 °C.

After discovering this heat from underground, human beings want to utilize them and call them geothermal resources, and they are divided into hydrothermal geothermal resources and dry thermal geothermal resources according to the output method.

Hydrothermal geothermal resources can be simply understood as underground hot water. Hot springs are hydrothermal geothermal resources, which are the product of underground hot water emerging from the surface. There is also some underground hot water that does not emerge from the surface but remains sealed underground. In addition to hot springs, a famous example of the application of hydrothermal geothermal resources in China is the Yapping geothermal field in Tibet, where a geothermal power station was built in 1977 and has achieved good benefits.

Correspondingly, dry heat type geothermal resources can be understood as the absence of water or the presence of very little water although the underground rock layer is very hot. Generally, it refers to the high-temperature rock buried in the earth's crust at a depth of 3~10 km, and the temperature is higher than 180 ℃ (some scholars think it is 150 ℃ or 200 ℃), so the dry heat geothermal resources are also called dry heat rocks.

The reason why we emphasize the depth of 3~10 km is not that the deeper ones are not hot, but because the deeper ones are out of our reach. The deepest well drilled by man is the Soviet Kola ultra-deep well, which took 20 years to reach 12.2 km and cost a lot of money. Therefore, we generally only focus on those places with high geothermal gradients, i.e., where it can be very hot without digging deeper, and where the input-output ratio is higher. The dry thermal rock resources in China are mainly concentrated in the Slingback area in Tibet, the Chongqing area in Yunnan, and the Republican Basin in Qinghai.

The amount of dry heat rock resources is quite substantial. According to a conservative estimate by MIT, the recoverable dry heat rock reserves in the earth's crust are close to 1.3×1027 J, which can be used globally for about 217 million years. The dry heat rock resources in mainland China at a depth of 3-10 km are estimated to be about 2.52×1025 J, which is 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 dry heat rock can be mined at a rate of 2%, it will be able to sustain China's energy supply for 3282 years.

However, if the dry thermal rock is such a good resource, why hasn't it been exploited?

Attractive energy source, not easy to extract

In 1970, Los Alamo Laboratory proposed the concept of an Enhanced geothermal system, the basic principle of which is roughly like this: two wells are drilled into the geothermal reservoir, an injection well and a production well. injection wells and production wells. Cold water is injected into the injection well, and after the cold water is heated by the geothermal reservoir, the hot water is pumped up from the production well, and then the hot water can be used for heating or power generation, and the water can be recycled underground after use.

At first glance, this solution sounds easy and cost-effective, which means that the cold water can go to the geothermal reservoir and return with a full load of heat, but there are many practical engineering problems. For example, the strata in which dry heat rock resources are located are often hot and hard, making drilling difficult and costly; for example, dry heat rock reservoirs may be dense and impermeable, making it difficult for the cold water injected to diffuse and absorb heat around the drilling well and flow to the production well to be pumped out.

Although we can improve the permeability of the reservoir by hydraulic fracturing technology (injecting high-pressure water to destroy the reservoir structure and form a fracture network in it), it is difficult to guarantee that the fracture will develop only in the direction we want after the high-pressure water is injected because the underground operation is invisible and inaccessible, and in case it develops in the direction far from the production well, the water we injected will not be collected.

There are concerns about the problems that dry heat rock mining may cause. One is the induction of earthquakes, such as the Korean earthquake mentioned earlier. However, the fact is that the earthquake problem here is mainly caused by hydraulic fracturing, and other efforts that use hydraulic fracturing techniques such as shale gas extraction may also trigger similar problems. And it has been pointed out that hydraulic fracturing has a limited impact on seismic activity above magnitude 3, while fracking-induced micro-earthquakes may help to release accumulated ground stress or energy and reduce the risk of large earthquakes. In addition, some scholars believe that earthquakes caused by hydraulic fracturing can be controlled.

Secondly, it affects the life span of the earth. Geothermal resources are part of the Earth's heat, and some people are concerned that taking them away at our initiative would be a way to kill them. It should be said that the huge heat inside the Earth is indeed a symbol of the Earth is still "alive", it is the source of power for a variety of geological activities on Earth such as volcanoes and earthquakes, and when it is exhausted one day, the Earth may be cooled to the moon, turning into a wasteland without life and vitality. But we do not have to worry too much about this, because the little heat that humans get is a drop in the bucket compared to the whole planet. And even if humans do not take it away, the earth also through volcanoes or earthquakes to release its exuberant energy.

Third, water consumption is high and it affects the ecological environment. Tens of thousands of cubic meters of water are usually consumed in the process of ERGS reservoir modification, such as in the aforementioned Soulful project fracturing fluid usage is more than 100,000 cubic meters. The problems of water waste and ecological damage caused by hydraulic fracturing cannot be ignored, as dry heat rock resources in China are mainly distributed in arid areas such as Qinghai and Tibet where water resources are scarce and ecologically fragile. In this regard, some scholars have proposed the method of using supercritical COS as a fracturing fluid to save water resources and help carbon neutrality. It is currently a hot research topic, and hopefully, it can be applied to practice on a large scale soon.

Nature
Like

About the Creator

Deena Thaddeus

The best way to find out if you can trust somebody is to trust them

Reader insights

Be the first to share your insights about this piece.

How does it work?

Add your insights

Comments

There are no comments for this story

Be the first to respond and start the conversation.

Sign in to comment

    Find us on social media

    Miscellaneous links

    • Explore
    • Contact
    • Privacy Policy
    • Terms of Use
    • Support

    © 2024 Creatd, Inc. All Rights Reserved.