Not only to the sky, but also to the earth! How much do you know about the expectations from deep inside the earth?

Going up to the sky, going down to the sea, and going into the earth are the three great feats of human exploration of nature. Today, deep space, deep sea, and deep earth have become major scientific topics vigorously promoted by all mankind, and have become household names and topics of discussion. Especially in deep space, with the announcement of the Mars exploration plan and the continuous successful launch of the "Shenzhou" series of spacecraft, all kinds of information and pictures have flooded in. From 90-year-olds to three-year-old children, everyone is willing to talk about deep space as a topic of interest. The deep sea field is also not willing to be lonely. Whether it is Jiaolong or Fendou, China's success in manned diving to a depth of 10,000 meters has attracted eager attention from all corners of the world on China's road to the ocean. The only thing that may be neglected is the deep earth. Perhaps because of its complexity, when asked about the relevant content of the deep earth, most people only have a vague understanding.

From the perspective of earth cognition, everything below the surface can be called deep earth, but the meaning of deep earth is different in different fields. In terms of human living space, 500 meters below the surface is considered deep earth; from the perspective of solid mineral resources, 1000 to 5000 meters below the surface can be considered deep earth; and in terms of oil and gas extraction, the deep earth is approximately 8000 meters below the surface to 10,000 meters underground; the deep earth discussed in geophysical science covers a wider range, from 5000 meters below the surface to the center of the earth can be considered deep earth. The deep earth contains most of the resources and energy, which is the material and energy basis for the survival of all things. The deep earth exploration mission is also called deep earth exploration. Scientists have summarized eight major research directions, and here I will share three of them with you: one is the expansion of human living space; the second is the development and utilization of solid minerals; the third is the exploration and development of oil and natural gas.

Expansion of human living space

The development and utilization of underground space is a cutting-edge topic in the world, and the whole world is studying it. At present, the utilization rate of underground space in the world is about 30%, while that in my country is only 17%. It can be seen that the utilization potential of underground space is huge, but there is also a long way to go. According to scientists' predictions, the global population is expected to reach 15 billion by 2150, and the existing arable land will reach its limit of support. At that time, people on earth will have two choices: to go to planets outside the earth or to the underground space inside the earth. Underground space itself is the safest and most reliable place to go. The total area of ​​a city is not just the ground area. The total utilization space of a city is the ground area multiplied by 40% of the development depth. As early as 1991, the Tokyo Declaration adopted by the International Academic Conference on Urban Underground Space emphasized that "the 21st century is the century for human development and utilization of underground space", and predicted that one-third of people will develop, utilize and live in underground space.

Scientists predict that the underground space accessible to human activities in the future is divided into five levels: 0 to 50 meters underground is underground rail transit, pipe network system and refuge facilities; 50 to 100 meters underground is underground home city; 100 to 500 meters underground is underground agriculture, underground medicine and underground ecosystem and strategic resource reserves; 500 to 2000 meters underground is underground energy circulation zone, underground pumped energy storage, compressed air power station, underground thermal energy and other storage and utilization; 2000 meters underground is deep ground science laboratory and deep solid resource liquefaction mining. Active development of human deep ground living space should be scientific, comprehensive, ecological and deep. It is necessary to unify the layout, scientifically and rationally plan the integration of ground and underground, and build an ecological underground city. Carry out special underground space detection and utilization (500 to 2000 meters), develop underground space detection and evaluation technology, establish an integrated technical method system, standard and safety management system for detection, evaluation and monitoring, and lead and cultivate the development and utilization of underground space through demonstration projects, which will be a new economic growth point. Deep ground life is no longer science fiction, but will become a part of human life in the future.

Deep mineral resource exploration

As exploitable resources in the shallow layer gradually decrease and become exhausted, advancing into deep-earth resources has become an inevitable strategy for many countries around the world. The development and utilization of deep-earth resources is an important part of my country's resource security. Carrying out deep earth exploration is not only necessary to solve major basic theoretical problems in geology, but also a major demand for the country to ensure energy and resource security and expand the space for economic and social development. Theoretically, the available mineralization space inside the earth is distributed from the surface to 10,000 meters underground. At present, the world's advanced exploration and mining depth has reached 2,500 to 4,000 meters, while most of my country's depths are less than 500 meters. Advancing into the deep earth is a strategic scientific and technological issue that we must solve.

Recent scientific research shows that the earth's solid mineral resources can be buried more than 40,000 meters deep. Nearly 70% of my country's proven main solid resources are distributed below 2,000 meters, and 70% of iron ore, copper ore, bauxite and other metal mineral resources are also buried below 2,000 meters. Some scholars believe that if the exploration depth of solid minerals in my country reaches 2,000 meters, the proven resource reserves can be doubled on the existing basis. In the future, a large number of mines will enter the deep mining stage, and the problem of deep mining can no longer be avoided. At present, the depth of human well mining in my country is mainly 500 to 1,000 meters. If a deep earth exploration strategy is implemented, it is necessary to highlight the strategic scientific and technological positioning. We must keep a close eye on the world's scientific and technological frontiers, comprehensively sort out the research results and trends of deep earth exploration at home and abroad; we must strengthen basic theoretical research and form a number of high-level theoretical innovation results; strengthen the research and development of advanced technologies and equipment, improve deep exploration capabilities, effectively expand the second prospecting space and underground development space, and provide a "Chinese model" for human understanding and utilization of the earth.

Deep oil and gas exploration and production

my country's deep and ultra-deep oil and gas resources amount to 67.1 billion tons of oil equivalent, accounting for 34% of the country's total oil and gas resources. Deep and ultra-deep layers have become the main battlefield for major oil and gas discoveries in my country. Key technologies for deep oil and gas exploration and production include: (1) deep oil and gas resource evaluation and development, including deep shale oil and shale gas resource potential, enrichment area evaluation and development; (2) ultra-deep oil and gas accumulation conditions, resource potential and favorable area evaluation; (3) large-scale oil and gas basin basement structure detection, basin evolution and its control on oil and gas enrichment; (4) 10,000-meter drilling key technology.

In recent years, 60% of the world's new oil and gas reserves come from deep strata, and the exploration potential is huge. Taking the Tarim Basin as an example, the oil and natural gas resources buried at a depth of 6,000 to 10,000 meters account for 83.2% and 63.9% of the total respectively, and the total amount of ultra-deep oil and gas resources accounts for 19% of the world. In the Shunbei Oil and Gas Field in the Tarim Basin, 41 oil and gas wells with a vertical depth of more than 8,000 meters have been drilled, and 4 billion-ton oil and gas areas have been implemented. The oil reservoirs are characterized by ultra-deep, high temperature, and high pressure. The average burial depth of the reservoir exceeds 7,300 meters, and the maximum depth of the directional well reaches 9,300 meters, breaking the deepest record in Asia. This area is one of the deepest commercially developed oil and gas fields on land in the world. The successful exploration and development of this deepest oil and gas field in the world has a strong guiding significance for the exploration of deep-earth mineral resources in my country.

The deep natural gas projects in the Sichuan Basin include two areas: deep marine carbonate conventional natural gas and deep shale gas. Deep marine carbonate conventional natural gas mainly includes Puguang Gas Field, Yuanba Gas Field, and West Sichuan Gas Field. Puguang Gas Field currently has 40 wells with a depth of more than 6,000 meters, Yuanba Gas Field has 57 wells with a depth of more than 7,000 meters, and West Sichuan Gas Field has 18 wells with a depth of more than 6,000 meters. The deep shale gas resources in the Sichuan Basin are 6.3 trillion cubic meters, which is an important area for shale gas reserves and production in the future. The burial depth of shale gas put into commercial development in this area is currently the highest in my country and even in the world.

As far as the development and utilization of resources in oil and gas basins are concerned, the deep standards in the east and west of China are not exactly the same. For example, the Xingang 1 well drilled in the Bohai Bay Basin in 2011 is 6,716 meters deep, the Songke 2 well drilled in the Songliao Basin in 2014 is 7,018 meters deep, and the Lishen 1 well drilled in the Ordos Basin in 2018 is 6,535 meters deep. These deep and ultra-deep wells provide rich first-hand information for the exploration and development of oil and gas resources in their basins. The latest news on October 19, 2022 reported that another major exploration breakthrough was made in the Qiongdongnan Basin in the southeastern waters of Hainan Island, and the first deepwater (water depth of more than 1,500 meters) and deep (well depth of more than 5,000 meters) gas field in my country, Baodao 21-1, was discovered, with proven geological reserves of more than 50 billion cubic meters, which is a powerful progress in accelerating deep-sea and deep-earth exploration. Although the deep and ultra-deep layers in different oil and gas basins are not exactly the same, moving towards a depth of 10,000 meters is a goal that is just around the corner.

Conclusion

It is not easy to go up to the sky, go down to the sea, and even harder to go underground. Deep earth exploration still depends on the continuous progress of various technologies, and there is indeed a long way to go. The academic community generally believes that the 19th century was the century of bridges, the 20th century was the century of high-rise buildings, and the 21st century is the century of developing and utilizing underground space. The deep earth contains endless treasures and mysteries. The deep earth - the deep ground is waiting for human beings to explore, discover, develop, and live there.

Author: Zheng Xiujuan, China University of Petroleum (Beijing)