Technological ways to improve the efficiency of petrothermal power plants and increase their contribution to electricity generation

The contribution of renewable energy sources to the total electricity production in the world is steadily increasing. At the same time, the share of geothermal energy in the overall balance remains unjustifiably insufficient. The prospects for obtaining heat from the earth are limited by a number of technical and technological difficulties, which especially applies to that part of geothermal energy, which is called petrothermal. The article proposes an approach to improve the efficiency of a petrothermal power plant while maintaining the general concept of environmental friendliness.


The current state of alternative energy
The main direction of alternative energy is the search and use of alternative (non-traditional) energy sources that can replace traditional energy sources that operate on oil, extracted natural gas and coal, which, when burned, emit carbon dioxide into the atmosphere, which contributes to the growth of the greenhouse effect and global warming.Prospects for the use of renewable energy sources are associated with their environmental friendliness, low operating costs and the expected fuel shortage in traditional energy [1].
Thus, according to the International Energy Agency (IEA), to achieve zero total carbon dioxide emissions by 2050 in order to prevent warming on Earth by more than 1.5 degrees Celsius, two-thirds of all energy and 90% of the electricity on the planet will be produced by green energy.By 2030, the development of green energy will create 14 million new jobs [2].
Investments in renewable energy sources are growing at a faster pace.Thus, according to a UN report, in 2008 around the world $140 billion was invested in projects related to alternative energy, while $110 billion was invested in coal and oil production.In 2023, it is expected that a total of more than $1,700 billion will be invested in such projects (Figure 1), almost double the amount of investment in fossil fuels [3].According to BP, in 2019, the share of alternative renewable energy sources (excluding hydropower) was 10.8% in global electricity generation, overtaking nuclear energy for the first time ever in this indicator [4].As of 2020, the total global installed renewable energy capacity (excluding hydropower) was 1,668 GW.Including the total global installed capacity of solar energy reaches 760 GW; wind power -743 GW; bioenergy -145 GW; geothermal energy -14.1 GW.
Figure 2 shows the distribution of shares in % of various sources in world electricity production in 2019 [5].
At the same time, for example, in the first half of 2020 in Germany, alternative energy sources generated a record 52% of electricity.The wind took first place among the sources of electricity, generating 30.6% of electricity, and the sun gave 11.4% [6].
Here it is important to note that geothermal resources are divided into two types: hydrothermal and petrothermal.Hydrothermal energy is aimed at extracting heat from groundwater of natural origin.Petrothermal -to extract heat directly from the rocks themselves, the temperature of which is the higher, the deeper they are located.Currently, hydrothermal technology is the most common, since it is much easier to implement.However, the creation of a hydrothermal system is possible only where suitable geothermal waters are available, for example, in volcanic zones.Therefore, only about 1% of all usable geothermal resources on Earth are hydrothermal, while the remaining 99% are petrothermal.This allows us to create petrothermal systems almost anywhere on Earth.
The paradox lies in the fact that, unlike other renewable energy sources, a source based on the use of petrothermal (deep) heat of the Earth is stable over time and does not depend on climatic and territorial factors.Unlike hydrothermal energy, which can only be used in some places on our planet, petrothermal energy is everywhere, literally under our feet.At the same time, heat reserves are practically inexhaustible, and are renewable in the full sense of the word due to the processes occurring in the Earth's core.
The utilization of petrotermal heat depends very much on the temperature of the rocks.The degree of increase in rock temperature with increasing depth is characterized by a geothermal gradient: on average, it is 0.02 °C/m [7].With such a gradient, the temperature of the earth's crust reaches 100 °C at a depth of 5 km.
Geothermal circulation systems (GCSs) are used to extract petrothermal energy.This system includes an underground reservoir, an injection well, a production well and a surface complex containing equipment that ensures the operation of the system.The scheme of such a GCS is shown in Figure 3. observation well; 10 -rocks The principle of operation is that the coolant is supplied to the collector through an injection well.Flowing through the collector, the coolant takes heat and is extracted through the production well.The resulting heat can be used for heating or electricity generation.After that, the spent coolant is again fed into the injection well.To improve efficiency and reduce the depth of drilling wells, the so-called binary geothermal power plants are being developed.Their main difference is that hot geothermal water and a second, additional liquid with a lower boiling point than water are passed through a heat exchanger.The heat from the geothermal water evaporates a second fluid, the vapors of which drive turbines.Temperate waters are the most abundant geothermal resource, so it is expected that most geothermal power plants of the future will operate on this principle.
In any case, the efficiency of the operation of geothermal, and petrothermal in particular, TPPs largely depends on the ability to pump a sufficient volume of coolant from the injection well to the production well.A central and as yet unanswered question is whether hot-dry-rock reservoirs can be engineered to meet the economic design criteria or whether the earth's natural properties limit the range of working conditions to lese than the design goals.In other words, can reservoir impedance be low enough and reservoir volume large enough to allow the high fluid-flow rates and long reservoir life necessary for commercial production of hot-dry-rock energy?

Enhancing the effectiveness of GCS
The collector is a permeable zone in the rock through which the coolant flows.It must have a developed heat exchange surface to ensure efficient heat transfer from the rock by the coolant.It must also have sufficient permeability to allow the coolant to circulate.The collector can be of both natural and artificial origin.Natural formations include porous formations and zones of natural fracturing.Therefore one of the most important targets is to find cost-effective ways to increase the natural permeability of the reservoir and enhance the fluid flow potential of the system.
Most frequently an artificial reservoir is created in impermeable or low-permeable rocks by hydraulic fracturing of the massif.A working fluid is supplied to the injection well under high pressure.As a result, cracks appear and expand in the massif, through which the coolant can circulate.But stimulating the collectors of geothermal systems by hydraulic fracturing can trigger earthquakes.Maximum seismic activity can reach 3.0-3.7 units on the Richter scale [8].Similar earthquakes have already been observed in Switzerland, Germany and other countries.In addition the hydraulic fracturing is a complicated procedure, costly itself and requiring a heavy infrastructure involved.Which makes it sometimes not possible to use it (especially in remote regions), and increases the overall costs of creation of GCS and makes it economically inefficient.
To solve both of mentioned problems it is proposed to use relatively inexpensive technologies of pressure pulse impact on rocks and a reservoir, which make it possible to create "secondary fracturing" of rocks and increase their permeability.An example is the innovative technology of plasma-pulse treatment (PPT), which has been tested in oil and gas production, and creates secondary permeability in the reservoir and connects the system of natural fractures (cleavage), thereby increasing the permeability of the reservoir over a large area.The PPT technology itself as well as the references to case studies providing enhancing of oil production with application of PPT are provided in [9,10].The experience of applying the plasma-pulse technology at oil producing and injection wells has shown a multiple increase in the production/injectivity of treated wells.Most importantly, the effect after well treatment was observed for a long time.One of typical examples of the result of treatment of an injection well is shown in Figure 4. Passive microseismic studies held on the coal seams and described in [10] showed promising results in creating micro factures into the rocks within a relatively wide distance from the treatment point.Figure 5 shows a map of microseismic energy released during the treatment of one well.The map is built in the projection onto the horizontal plane (X -West-East; Y -North-South).The color indicates the density of the stored energy of microseismic events.The coordinate axes in meters, the values on the axes indicate distance in relation to wellhead (white dot).

Figure 5. Distribution of microseismic events and energy emission density during PPT
Figure 5 shows that the highest energy density of microseismic events observed under seismic antenna near the bottomhole of the well.Area of microseismic activity has elliptical shape, approximate linear dimensions of the microseismic activity area is 1,500 meters from north to south and 1,250 meters in the direction from west to east.
Based on recorded data an analytical discrete fracture model was created for estimating and predicting of the secondary filtration and reservoir properties.The results of this modeling are provided in Figure 6 as a model of technogenic fracturing of reservoir area.

Figure 6. Map of the secondary fracturing
In general, it is possible with sufficient certainty to state that the observation of the kinematic and dynamic characteristics of the area of microseismic activity in the area of installation of the observation system indicates an increase of permeability of the layers at a considerable distance on the area, which is equal to more than 1 sq.km.
At the final stage of the research, using a software module Dual-Porosity Modeling (IrapRMS), a three-dimensional cube of values of the secondary porosity and permeability resulting from Plasma Pulse Treatment has been calculated for all 4 wells treated in the same area (Figure 7) [11].As a result, all 4 wells were linked by a system of filtration channels, and in the crater formed there was an active desorption of methane and water from the coal beds, some part of which turned into the free state.Borehole annulus pressure of the wells increased, and the bleed water was saturated with the gas.
One of the most important features of technology presented is its high ecological friendliness.This type of technologies is based on electrical discharge in the liquid medium, it does not provide any waste material unlike hydraulic fracturing or chemical treatments.On top of all PPT technology has shown high cost-effectiveness when treating oil and gas wells.

Conclusions
The main advantages of petrothermal energy are the practical inexhaustibility and ubiquitous availability of petrothermal resources.In addition, its advantages include non-waste, environmental safety and relatively low labor intensity of creation and operation.
The disadvantages include the low energy potential of rocks at depths of up to 3 km and often insufficient permeability of rocks.Drilling and equipping wells in such rocks by traditional methods is very expensive and makes petrothermal stations uncompetitive.
The plasma pulse technology described in the paper could be a way to increase the effectiveness of building and utilizing the petrothermal power stations.The positive experience of this technology application in oil and gas industry could be estimated and processed for the designing new advanced geothermal circulation systems.The experimental studies provided in the paper demonstrate the potential of using such technology to increase the permeability of the natural rocks and in term to enhance the heat production from geothermal reservoirs.

Figure 1 .
Figure 1.Global energy investment in clean energy and fossil fuels, 2015-2023According to BP, in 2019, the share of alternative renewable energy sources (excluding hydropower) was 10.8% in global electricity generation, overtaking nuclear energy for the first time ever in this indicator[4].As of 2020, the total global installed renewable energy capacity (excluding hydropower) was 1,668 GW.Including the total global installed capacity of solar energy reaches 760 GW; wind power -743 GW; bioenergy -145 GW; geothermal energy -14.1 GW.Figure2shows the distribution of shares in % of various sources in world electricity production in 2019[5].

Figure 3 .
Figure 3. Scheme of petrothermal GCS: 1 -tank; 2 -pump; 3 -heat exchanger; 4 -turbine compartment; 5 -production wells; 6 -injection well; 7 -heat supply line; 8 -sedimentary rocks; 9 -observation well; 10 -rocks The principle of operation is that the coolant is supplied to the collector through an injection well.Flowing through the collector, the coolant takes heat and is extracted through the production well.The resulting heat can be used for heating or electricity generation.After that, the spent coolant is again fed into the injection well.To improve efficiency and reduce the depth of drilling wells, the so-called binary geothermal power plants are being developed.Their main difference is that hot geothermal water and a second, additional liquid with a lower boiling point than water are passed through a heat exchanger.The heat from the geothermal water evaporates a second fluid, the vapors of which drive turbines.Temperate

Figure 4 .
Figure 4. Result of injection well treatment.Vakhskoye oil field, Russia

Figure 7 .
Figure 7. Scheme of secondary permeability, which arose as a result of Plasma Pulse treatment of 4 wells.3-D image and formations map