Geothermal resources are divided into two types: hydrothermal and petrothermal. Unlike hydrothermal energy which has already found wide application in the energy balance of many countries such as the Philippines, Iceland or Kenya, petrothermal energy is still at the stage of scientific research and pilot projects. But it is already clear that the petrothermal energy reserves of the Earth are practically inexhaustible. Therefore, if we succeed in finding the cost-effective and simple ways to use petrothermal sources, this type of energy is highly likely to become the most promising against the background of the global green agenda.
Petrothermal resources (or the use of the Earth’s deep heat) are part of thermal energy contained in practically impervious dry hot rocks located at the depths of 3-10 km where their temperature reaches 250-350°C.
Almost all the projects for the petrothermal energy use assume that cold water enters an underground reservoir of hot dry rock, gets heated, exits to the surface, through the production wells, being already very hot or in the form of steam, and enters a power plant from which this water returns back again to the injection well, thereby creating a closed circulation system.
The project of the Los Alamos National Laboratory in the USA was the first project of this kind for development of petrothermal energy. It was called HDR (Hot Dry Rock) and implemented at Fenton Hill in the Valles Caldera in New Mexico 1973 to 1996. It used hydraulic fracturing for creation of an artificial reservoir consisting of vertical fractures in a monolithic rock. Similar hydraulic fracturing is also used in oil production but water flow rates in geothermal wells must be tens of times higher than in oil production.
The project immediately revealed several problems in creation of such plants. It turned out that the standard fracturing method did not produce enough fractures to achieve the desired permeability and good heat transfer. Therefore, subsequently, scientists took the path of creating vast reservoirs with multiple fractures and natural rock defects. Such projects are called Enhanced Geothermal Systems (EGS) – the advanced geothermal systems.
In total, about twenty pilot systems have been implemented for the moment in the USA, Japan, Great Britain, France, Germany and Australia, the systems which confirmed technical feasibility of extracting deep heat from the depths of up to 5.1 km.
These studies helped to determine the minimum of the necessary requirements for establishment of such plants. So, the depth of wells should exceed 3 km; the generated power – over 3 – 10 MW; the temperature in the underground reservoir – over 250°C; the water flow rate – 50 – 100 kg/s; the distance between the wells – 0.5 – 2 km; the underground reservoir volume 0.1 – 0.3 cubic km, and the plant service life – over 25 years.
These projects have also revealed a number of serious technical problems in the use of petrothermal energy: high cost of drilling which can reach 60% of the total project costs, uncertainty and short well life, temperature drops over the time, low heat recovery efficiency which is still 1 – 5% of the reserve, the need for large volumes of water and its losses in the system, rapid corrosion of equipment and others.
At the same time, these projects have demonstrated significant advantages of petroenergy in comparison with other energy sources. Such power plants are able to operate continuously and be independent from the season or the weather. Petrostations can be installed almost everywhere in the world, including the places of energy consumption, without significant expenses for its storage. They do not require large areas, they are closed cycle systems, and they do not emit greenhouse gases.
Analysis of petrothermal resources and their potential use in the U.S. showed that the depths of up to 10 km contain 130,000 times the U.S annual energy consumption. Even with a low recovery efficiency of 1.5%, it will last for 2,000 years, and for the advanced technologies – for 20,000 years, even without considering its renewal. Moreover, preliminary calculations show that by 2030 it is possible to achieve the levelised cost of electricity (LCOE) of such projects up to 6 cents per kWh, i.e. this indicator is one of the lowest for the power industry.
Such assessments led to launching of a special program for the petrothermal energy development – GeoVision: Harnessing the Heat Beneath Our Feet in the USA in 2019. According to this program, electricity production by petroleum plants can reach 60 GW already by 2050. This would represent 3.7% of the U.S. total installed capacity in 2050, and as for production, 8.5% of all U.S. electricity generation. In terms of direct heat use, petroenergy could reach 320 GW by 2050 instead of the current 0.1 GW.
The usage of geothermal and petrothermal energy is covered in more details in the report “10 Breakthrough Ideas in Energy for the Next 10 Years“. The 3rd edition of the report will be presented in May.
