Nitrogen pressurization system and pressurization method for carbon dioxide fracturing

Abstract

The invention discloses a nitrogen pressurization system for carbon dioxide fracturing, which belongs to the technical field of carbon dioxide fracturing operation and comprises a waste heat utilization module, a CO2 utilization module and a liquid nitrogen gasification module. The invention further discloses a nitrogen pressurizing method for carbon dioxide fracturing, through utilization of waste heat in high-temperature flue gas and CO2 capture, CO2 liquefaction and storage are carried out by utilizing cold energy of liquid nitrogen gasification, and finally fracturing operation is carried out by utilizing high-pressure nitrogen to displace liquid CO2, so that the construction effect is effectively improved, the equipment, labor and fuel consumption cost is saved, and the method is suitable for industrial production. And the carbon dioxide fracturing operation process is more efficient, reliable, economical and environmentally friendly.

Description

Technical Field

[0001] This invention belongs to the field of carbon dioxide fracturing operation technology, specifically relating to a nitrogen pressurization system for carbon dioxide fracturing, and also to a nitrogen pressurization method for carbon dioxide fracturing. Background Technology

[0002] Carbon dioxide fracturing, which increases shale gas production while storing carbon dioxide underground, represents a green development of fossil energy and has become a new direction for future shale gas extraction and development. However, in traditional carbon dioxide fracturing operations, the pumping action of the booster pump and pipeline friction cause partial vaporization of the liquid CO2 in the surface supply pipeline, leading to the fracturing truck running dry, making it difficult to increase discharge capacity and resulting in instability and sand blockage. Furthermore, the different rates of liquid level reduction in various storage tanks can cause excessively rapid pressure drops in some pipelines, leading to CO2 dry ice formation and equipment malfunction.

[0003] When dozens of diesel-powered fracturing trucks operate at large-scale carbon dioxide fracturing well sites, they emit large amounts of high-temperature flue gas, with temperatures reaching up to 500°C and a carbon dioxide dry basis concentration greater than 10 vol%. At the same time, nitrogen displacement consumption is enormous. Using liquid nitrogen vaporization and pressurization is more in line with the needs of the field conditions, but the entire vaporization process involves a huge temperature jump from -196°C to ambient temperature. This not only wastes cold and heat energy but also contributes to the greenhouse effect and causes environmental pollution. Summary of the Invention

[0004] The primary objective of this invention is to provide a nitrogen pressurization system for carbon dioxide fracturing, which solves the technical problems of poor performance and waste of cold and hot energy in existing carbon dioxide fracturing operations.

[0005] A second objective of the present invention is to provide a nitrogen pressurization method for carbon dioxide fracturing.

[0006] The first technical solution adopted in this invention is a nitrogen pressurization system for carbon dioxide fracturing, including a waste heat utilization module, a CO2 utilization module and a liquid nitrogen vaporization module; The waste heat utilization module includes a steam waste heat boiler, which is connected to a diesel-driven fracturing truck, a steam turbine power generation unit, a CO2 capture device, and a liquid nitrogen vaporizer. The steam waste heat boiler forms a circulation loop with the steam turbine power generation unit and the liquid nitrogen vaporizer. The CO2 utilization module includes a CO2 condenser connected to a CO2 capture device and a liquid nitrogen vaporizer. The CO2 condenser is also connected to a CO2 liquid tank and a sand mixing device. The CO2 liquid tank and the sand mixing device are interconnected. The sand mixing device is connected to a diesel-powered fracturing truck. The liquid nitrogen vaporization module includes a liquid nitrogen storage tank connected to a CO2 condenser; The diesel-powered fracturing truck is also connected to the wellhead.

[0007] The first technical solution of this invention is further characterized by: The diesel-powered fracturing truck includes a first inlet, a first outlet a, and a first outlet b; The steam waste heat boiler includes a second inlet a, a second inlet b, a second inlet c, and a second outlet a, a second outlet b, and a second outlet c. Steam turbine power generation equipment includes a third inlet and a third outlet; The CO2 capture unit includes a fourth inlet and a fourth outlet; The liquid nitrogen vaporizer includes a sixth inlet a, a sixth inlet b, a sixth outlet a, and a sixth outlet b; Specifically, the first outlet a is connected to the wellhead, the first outlet b is connected to the second inlet b, the second inlet a is connected to the third outlet, and a condenser and a feed water pump a are sequentially connected between the second inlet a and the third outlet along the gas flow direction. The second inlet c is connected to the sixth outlet a, and a feed water pump b is connected between the second inlet c and the sixth outlet a. The second outlet a is connected to the third inlet, and a steam booster pump a is connected between the second outlet a and the third inlet. The second outlet b is connected to the fourth inlet, the second outlet c is connected to the sixth inlet a, and a steam booster pump b is connected between the second outlet c and the sixth inlet a.

[0008] The CO2 condenser includes a fifth inlet a, a fifth inlet b, a fifth outlet a, and a fifth outlet b; The CO2 liquid tank includes a seventh inlet a, a seventh inlet b, a seventh outlet a, and a seventh outlet b; The sand mixing device includes an eighth inlet, an eighth outlet a, and an eighth outlet b; Among them, the fifth inlet a is connected to the fourth outlet, and a compressor is connected between the fifth inlet a and the fourth outlet. The fifth outlet a is connected to the seventh inlet b, the fifth outlet b is connected to the sixth inlet b, and a liquid nitrogen booster pump b is connected between the fifth outlet b and the sixth inlet b. The seventh inlet a is connected to the sixth outlet b, and a pressure regulating valve group is connected between the seventh inlet a and the sixth outlet b. Both the seventh outlet b and the eighth outlet b are connected to the compressor. The eighth inlet is connected to the seventh outlet a, and the eighth outlet a is connected to the first inlet.

[0009] The liquid nitrogen storage tank includes a ninth outlet, which is connected to the fifth inlet b. A liquid nitrogen booster pump a is connected between the ninth outlet and the fifth inlet b.

[0010] The second technical solution adopted in this invention is a nitrogen pressurization method for carbon dioxide fracturing, which uses the aforementioned nitrogen pressurization system for carbon dioxide fracturing and specifically includes the following steps: S1, the high-temperature flue gas generated by the diesel-powered fracturing truck enters the steam waste heat boiler and exchanges heat with the boiler water to form medium-low temperature flue gas, which heats the boiler water to boiling and generates steam. S2, after steam is output from the waste heat boiler for heat exchange, it re-enters the waste heat boiler for continuous heat exchange. S3 captures and purifies CO2 in medium and low temperature flue gas, then combines it with CO2 in the venting gas and pressurizes it for heat exchange to form liquid CO2. The liquid CO2 flows into a CO2 liquid tank for storage. S4, the cryogenic liquid nitrogen in the liquid nitrogen storage tank undergoes heat exchange to form sub-cryo liquid nitrogen. The cryogenic liquid nitrogen absorbs heat and vaporizes. The high-pressure nitrogen gas after vaporization displaces the liquid CO2 in the CO2 tank. The outflowing liquid CO2 enters the sand mixing device and mixes with the proppant to form fracturing fluid. S5, the fracturing fluid is drawn into the wellhead by a diesel-powered fracturing truck, pressurized, and then pumped into the wellhead for carbon dioxide fracturing operations.

[0011] The second technical solution of the present invention is further characterized by: S2 is specifically as follows: Steam is divided into two paths. One path of steam is pressurized by a booster pump and enters the steam turbine power generation equipment to generate electricity. The steam-water mixture coming out of the steam turbine power generation equipment is condensed into cold water after passing through a condenser. Then, the cold water is pumped back into the steam waste heat boiler by feedwater pump a for circulating heat exchange. The other path of steam is pumped into the liquid nitrogen vaporizer by steam booster pump b to exchange heat with liquid nitrogen and condense into water. Then, the cold water is pumped back into the steam waste heat boiler for circulating heat exchange.

[0012] S3 specifically refers to: medium and low temperature flue gas entering the CO2 capture device for gaseous CO2 capture and purification; the purified gaseous CO2, CO2 liquid tank and vented CO2 from the sand mixing device are combined, and then pressurized by the compressor and enter the CO2 condenser to exchange heat with liquid nitrogen; the resulting liquid CO2 flows into the CO2 liquid tank for storage.

[0013] S4 specifically refers to: the cryogenic liquid nitrogen in the liquid nitrogen storage tank is pumped into the CO2 condenser by liquid nitrogen booster pump a to exchange heat with gaseous CO2 to form sub-cryogenic liquid nitrogen, which is then pressurized by liquid nitrogen booster pump b and enters the liquid nitrogen vaporizer for heat absorption and vaporization. The high-pressure nitrogen gas after vaporization continuously and stably displaces the liquid CO2 in the CO2 tank after passing through the pressure regulating valve group. The outflowing liquid CO2 enters the sand mixing device and mixes with the proppant to form fracturing fluid.

[0014] The beneficial effects of this invention are: This invention utilizes the cold and heat energy resources within the system to pressurize liquid nitrogen vaporization for carbon dioxide fracturing operations. This invention enables the utilization of waste heat in the high-temperature flue gas and CO2 capture during the carbon dioxide fracturing process. At the same time, it utilizes the cold energy of liquid nitrogen vaporization for CO2 liquefaction and storage. Finally, it uses high-pressure nitrogen to displace the liquid CO2 for fracturing operations, making the carbon dioxide fracturing process more efficient, reliable, and environmentally friendly. Attached Figure Description

[0015] Figure 1This is a schematic diagram of the nitrogen pressurization system for carbon dioxide fracturing according to the present invention; In the diagram, 1. Diesel-powered fracturing truck, 2. Steam waste heat boiler, 3. Steam turbine generator, 4. CO2 capture device, 5. CO2 condenser, 6. Liquid nitrogen vaporizer, 7. CO2 tank, 8. Sand mixing device, 9. Liquid nitrogen storage tank, 10. Wellhead, 11. Steam booster pump a, 12. Condenser, 13. Feed water pump a, 14. Compressor, 15. Liquid nitrogen booster pump a, 16. Liquid nitrogen booster pump b, 17. Steam booster pump b, 18. Feed water pump b, 19. Pressure regulating valve assembly. Detailed Implementation

[0016] The present invention will be further explained below with reference to the accompanying drawings and specific embodiments.

[0017] like Figure 1 As shown, Y-1 represents the high-temperature flue gas discharged from the engine of the diesel-powered fracturing truck 1; Y-2 represents the medium- and low-temperature flue gas after heat exchange in the steam waste heat boiler 2; Y-3 represents the superheated steam going to the steam turbine power generation unit 3 to generate electricity; Y-4 represents the condensed return water from the steam turbine power generation unit 3; Y-5 represents the superheated steam going to the liquid nitrogen vaporizer 6 for heat exchange; Y-6 represents the condensed return water from the liquid nitrogen vaporizer 6 after heat exchange; and C-1 represents the CO2 captured in the medium- and low-temperature flue gas. C-2 represents liquid CO2 formed after pressurization and cryogenicity; C-3 represents liquid CO2 displaced by nitrogen pressurization; C-4 represents proppant-carrying fluid after mixing with proppant; C-5 represents fracturing fluid after being pressurized by diesel-powered fracturing truck 1; C-6 represents gaseous CO2 vented from CO2 tank 7 and proppant mixing device 8; N-1 represents cryogenic liquid nitrogen pumped out from liquid nitrogen storage tank 9; N-2 represents sub-cryo ...

[0018] Example 1 The nitrogen pressurization system for carbon dioxide fracturing disclosed in this invention includes a waste heat utilization module, a CO2 utilization module, and a liquid nitrogen vaporization module. The waste heat utilization module includes a steam waste heat boiler 2, which is connected to a diesel-driven fracturing truck 1, a steam turbine generator 3, a CO2 capture device 4, and a liquid nitrogen vaporizer 6. A circulation loop is formed between the steam waste heat boiler 2, the steam turbine generator 3, and the liquid nitrogen vaporizer 6. The diesel-driven fracturing truck 1 is a unit type, capable of centralized or grouped exhaust gas collection. The steam waste heat boiler 2 can be used individually or in parallel with two or more units, used for steam power generation and liquid nitrogen vaporization heat exchange, respectively. The CO2 capture device 4 employs a chemical absorption method and includes components such as pretreatment, an absorption tower, a rich liquid treatment system, a desorption tower, and a product gas treatment system. The CO2 utilization module includes a CO2 condenser 5 connected to a CO2 capture device 4 and a liquid nitrogen vaporizer 6. The CO2 condenser 5 is also connected to a CO2 liquid tank 7 and a sand mixing device 8. The CO2 liquid tank 7 and the sand mixing device 8 are interconnected. The sand mixing device 8 is connected to a diesel-powered fracturing truck 1. The liquid nitrogen vaporizer 6 is a steam-type water bath vaporizer. The CO2 liquefaction process adopts a pressurized cooling method. The gaseous CO2 is pressurized to above 4.5 MPa by a compressor and cooled to 5~10℃ by heat exchange with liquid nitrogen, which can then be condensed and liquefied.

[0019] The liquid nitrogen vaporization module includes a liquid nitrogen storage tank 9 connected to the CO2 condenser 5; The diesel-powered fracturing truck 1 is also connected to the wellhead 10.

[0020] This invention maintains a supercritical state within the CO2 tank, preventing liquid vaporization and resource waste. It utilizes pressure differentials to ensure a continuous and stable supply pressure and flow rate for the fracturing truck and pipelines, preventing pump runaway and pressure drops leading to dry ice formation, thus effectively improving operational efficiency. It can effectively replace traditional CO2 booster devices, saving equipment and labor costs. It enables the entire CCUS process of CO2 capture, liquefaction, storage, and fracturing, resulting in significant greenhouse gas emission reduction benefits and a more economical CO2 recycling method. By utilizing the cold and hot resources of liquid nitrogen and high-temperature flue gas in a tiered manner, it solves the enormous energy load associated with equipment electricity consumption, CO2 liquefaction, and liquid nitrogen vaporization, effectively reducing equipment investment and fuel consumption costs, resulting in significant economic benefits.

[0021] Example 2 Based on Example 1, the diesel-powered fracturing truck 1 of the present invention includes a first inlet, a first outlet a, and a first outlet b; the steam waste heat boiler 2 includes a second inlet a, a second inlet b, a second inlet c, and a second outlet a, a second outlet b, and a second outlet c; the steam turbine power generation equipment 3 includes a third inlet and a third outlet; the CO2 capture device 4 includes a fourth inlet and a fourth outlet; and the liquid nitrogen vaporizer 6 includes a sixth inlet a, a sixth inlet b, and a sixth outlet a and a sixth outlet b. The first outlet a is connected to the wellhead 10, the first outlet b is connected to the second inlet b, the second inlet a is connected to the third outlet, and the second inlet a and the third outlet are connected in sequence along the gas flow direction by the condenser 12 and the feed water pump a13. The steam-water mixture from the steam turbine generator 3 is condensed into cold water after passing through the condenser 12, and then the feed water pump a13 pumps the cold water back into the steam waste heat boiler 2 for circulating heat exchange. The second inlet c is connected to the sixth outlet a. A feed water pump b18 is connected between the second inlet c and the sixth outlet a. The feed water pump b18 pumps the cold water back into the steam waste heat boiler 2 for circulating heat exchange. The second outlet a is connected to the third inlet. A steam booster pump a11 is connected between the second outlet a and the third inlet. The second outlet b is connected to the fourth inlet. The second outlet c is connected to the sixth inlet a. A steam booster pump b17 is connected between the second outlet c and the sixth inlet a. Steam is pumped into the liquid nitrogen vaporizer 6 by the steam booster pump b17 and condenses into water after exchanging heat with the liquid nitrogen. The cold water is then pumped back into the steam waste heat boiler 2 by the feed water pump b18 for circulating heat exchange.

[0022] Furthermore, the CO2 condenser 5 includes a fifth inlet a, a fifth inlet b, a fifth outlet a, and a fifth outlet b; CO2 liquid tank 7 includes a seventh inlet a, a seventh inlet b, and a seventh outlet a and a seventh outlet b; The sand mixing device 8 includes an eighth inlet, an eighth outlet a, and an eighth outlet b; The fifth inlet a is connected to the fourth outlet, and a compressor 14 is connected between the fifth inlet a and the fourth outlet. After being pressurized by the compressor 14, the gas enters the CO2 condenser 5 to exchange heat with liquid nitrogen. The fifth outlet a is connected to the seventh inlet b, and the fifth outlet b is connected to the sixth inlet b. A liquid nitrogen booster pump b16 is connected between the fifth outlet b and the sixth inlet b. After being pressurized by the liquid nitrogen booster pump b16, the liquid nitrogen enters the liquid nitrogen vaporizer 6 for heat absorption and vaporization. The seventh inlet a is connected to the sixth outlet b, and a pressure regulating valve group 19 is connected between the seventh inlet a and the sixth outlet b. Both the seventh outlet b and the eighth outlet b are connected to the compressor 14. The gaseous CO2 can combine with the CO2 liquid tank 7 and the vented CO2 from the sand mixing device 8. After being pressurized by the compressor 14, it enters the CO2 condenser 5 to exchange heat with liquid nitrogen. The eighth inlet is connected to the seventh outlet a, and the eighth outlet a is connected to the first inlet. The vaporized high-pressure nitrogen gas continuously and stably displaces the liquid CO2 in the CO2 liquid tank 7 after passing through the pressure regulating valve group 19. High-pressure nitrogen is injected into CO2 tank 7 by liquid nitrogen booster pump a15 to maintain the pressure inside CO2 tank 7, so that the carbon dioxide inside CO2 tank 7 is kept in a supercritical state, which can prevent the liquid carbon dioxide from vaporizing; at the same time, the pressure difference is used to displace the liquid CO2 in CO2 tank 7, so that the diesel-driven fracturing truck 1 can obtain a continuous and stable liquid supply, thereby increasing the construction displacement and effectively improving the construction effect.

[0023] Example 3 Based on Example 2, the liquid nitrogen storage tank 9 of the present invention includes a ninth outlet, which is connected to the fifth inlet b. A liquid nitrogen booster pump a15 is connected between the ninth outlet and the fifth inlet b. The cryogenic liquid nitrogen in the liquid nitrogen storage tank 9 is pumped into the CO2 condenser 5 by the liquid nitrogen booster pump a15 to exchange heat with the gaseous CO2 and then flows out to form sub-cryogenic liquid nitrogen. The sub-cryogenic liquid nitrogen is pressurized by the liquid nitrogen booster pump b16 and enters the liquid nitrogen vaporizer 6 for heat absorption and vaporization. The high-pressure nitrogen gas after vaporization continuously and stably displaces the liquid CO2 in the CO2 liquid tank 7 after passing through the pressure regulating valve group 19.

[0024] This invention also discloses a nitrogen pressurization method for carbon dioxide fracturing, which employs the aforementioned nitrogen pressurization system for carbon dioxide fracturing and specifically includes the following steps: S1, the high-temperature flue gas generated by the diesel-driven fracturing truck 1 enters the steam waste heat boiler 2 and exchanges heat with the boiler water to form medium and low temperature flue gas, which heats the boiler water to boiling and generates steam. S2, after steam is output from the waste heat boiler 2 for heat exchange, it re-enters the waste heat boiler 2 for continuous heat exchange; S3 captures and purifies CO2 in medium and low temperature flue gas, then combines it with CO2 in the venting gas and pressurizes it for heat exchange to form liquid CO2. The liquid CO2 flows into CO2 liquid tank 7 for storage. S4, the cryogenic liquid nitrogen in the liquid nitrogen storage tank 9 undergoes heat exchange to form sub-cryo liquid nitrogen. The cryogenic liquid nitrogen absorbs heat and vaporizes. The vaporized high-pressure nitrogen gas displaces the liquid CO2 in the CO2 liquid tank 7. The outflowing liquid CO2 enters the sand mixing device 8 and mixes with the proppant to form fracturing fluid. S5, the fracturing fluid is drawn into the wellhead 1 by the diesel-powered fracturing truck 1, pressurized, and then pumped into the wellhead 10 for carbon dioxide fracturing operation.

[0025] Specifically, the high-temperature flue gas Y-1 generated during the operation of the diesel-powered fracturing truck 1 is collected and combined through the induced draft duct and then enters the steam waste heat boiler 2 to exchange heat with the boiler water, heating the water to boiling and generating steam. The steam is output in two paths. One path, steam Y-3, is pressurized by the steam booster pump a11 and then enters the steam turbine generator 3 to generate electricity, which can be used by other electrical equipment in the system. The steam-water mixture from the steam turbine generator 3 is condensed into cold water by the condenser 12, and then pumped back into the steam waste heat boiler 2 by the feedwater pump a13 for circulating heat exchange. The other path, steam Y-5, is pumped into the liquid nitrogen vaporizer 6 by the steam booster pump b17 to exchange heat with liquid nitrogen and then condenses into water, which is then pumped back into the steam waste heat boiler 2 by the feedwater pump b18 for circulating heat exchange.

[0026] After the high-temperature flue gas is heated by the steam waste heat boiler 2, it forms medium-low temperature flue gas Y-2, which enters the CO2 capture device 4 for gaseous CO2 capture and purification. The extracted gaseous CO2 C-1 can be combined with the CO2 liquid tank 7 and the vented gas CO2 C-6 from the sand mixing device 8. After being pressurized by the compressor 14, it enters the CO2 condenser 5 to exchange heat with liquid nitrogen. The liquid CO2 C-2 formed by condensation under pressurized low temperature environment flows into the CO2 liquid tank 7 for storage.

[0027] The cryogenic liquid nitrogen N-1 in the liquid nitrogen storage tank 9 is pumped into the CO2 condenser 5 by the liquid nitrogen booster pump a15. After exchanging heat with the gaseous CO2, it flows out to form the sub-cryogenic liquid nitrogen N-2. The sub-cryogenic liquid nitrogen N-2 is pressurized by the liquid nitrogen booster pump b16 and enters the liquid nitrogen vaporizer 6 for heat absorption and vaporization. The high-pressure nitrogen gas N-3 after vaporization continuously and stably displaces the liquid CO2 in the CO2 tank 7 after passing through the pressure regulating valve group 19. The outflowing liquid CO2 C-3 enters the sand mixing device 8 and mixes with the proppant. The resulting fracturing fluid C-4 is pressurized by the fracturing truck 1 and pumped into the wellhead 10 for carbon dioxide fracturing operations.

Claims

1. A nitrogen pressurization system for carbon dioxide fracturing, characterized in that, It includes a waste heat utilization module, a CO2 utilization module, and a liquid nitrogen vaporization module; The waste heat utilization module includes a steam waste heat boiler (2), which is connected to a diesel-driven fracturing truck (1), a steam turbine power generation device (3), a CO2 capture device (4) and a liquid nitrogen vaporizer (6). The steam waste heat boiler (2) forms a circulation loop with the steam turbine power generation device (3) and the liquid nitrogen vaporizer (6). The CO2 utilization module includes a CO2 condenser (5) connected to a CO2 capture device (4) and a liquid nitrogen vaporizer (6). The CO2 condenser (5) is also connected to a CO2 liquid tank (7) and a sand mixing device (8). The CO2 liquid tank (7) and the sand mixing device (8) are connected to each other. The sand mixing device (8) is connected to a diesel-powered fracturing truck (1). The liquid nitrogen vaporization module includes a liquid nitrogen storage tank (9) connected to a CO2 condenser (5); The diesel-powered fracturing truck (1) is also connected to a wellhead (10).

2. The nitrogen pressurization system for carbon dioxide fracturing according to claim 1, characterized in that: The diesel-powered fracturing truck (1) includes a first inlet, a first outlet a, and a first outlet b; The steam waste heat boiler (2) includes a second inlet a, a second inlet b, a second inlet c, and a second outlet a, a second outlet b, and a second outlet c; The steam turbine power generation equipment (3) includes a third inlet and a third outlet; The CO2 capture device (4) includes a fourth inlet and a fourth outlet; The liquid nitrogen vaporizer (6) includes a sixth inlet a, a sixth inlet b, a sixth outlet a, and a sixth outlet b; Among them, the first outlet a is connected to the wellhead (10), the first outlet b is connected to the second inlet b, the second inlet a is connected to the third outlet, and the condenser (12) and the feed water pump a (13) are connected sequentially between the second inlet a and the third outlet along the gas flow direction. The second inlet c is connected to the sixth outlet a, and the feed water pump b (18) is connected between the second inlet c and the sixth outlet a. The second outlet a is connected to the third inlet, and the steam booster pump a (11) is connected between the second outlet a and the third inlet. The second outlet b is connected to the fourth inlet, the second outlet c is connected to the sixth inlet a, and the steam booster pump b (17) is connected between the second outlet c and the sixth inlet a.

3. The nitrogen pressurization system for carbon dioxide fracturing according to claim 2, characterized in that: The CO2 condenser (5) includes a fifth inlet a, a fifth inlet b, a fifth outlet a, and a fifth outlet b; The CO2 liquid tank (7) includes a seventh inlet a, a seventh inlet b, a seventh outlet a, and a seventh outlet b; The sand mixing device (8) includes an eighth inlet, an eighth outlet a, and an eighth outlet b; Among them, the fifth inlet a is connected to the fourth outlet, and a compressor (14) is connected between the fifth inlet a and the fourth outlet. The fifth outlet a is connected to the seventh inlet b, the fifth outlet b is connected to the sixth inlet b, and a liquid nitrogen booster pump b (16) is connected between the fifth outlet b and the sixth inlet b. The seventh inlet a is connected to the sixth outlet b, and a pressure regulating valve group (19) is connected between the seventh inlet a and the sixth outlet b. The seventh outlet b and the eighth outlet b are both connected to the compressor (14). The eighth inlet is connected to the seventh outlet a, and the eighth outlet a is connected to the first inlet.

4. The nitrogen pressurization system for carbon dioxide fracturing according to claim 3, characterized in that: The liquid nitrogen storage tank (9) includes a ninth outlet, which is connected to the fifth inlet b, and a liquid nitrogen booster pump a (15) is connected between the ninth outlet and the fifth inlet b.

5. A nitrogen pressurization method for carbon dioxide fracturing, employing the nitrogen pressurization system for carbon dioxide fracturing as described in any one of claims 1-4, characterized in that, Specifically, the following steps are included: S1, the high-temperature flue gas generated by the diesel-driven fracturing truck (1) enters the steam waste heat boiler (2) to exchange heat with the boiler water to form medium and low temperature flue gas, which heats the boiler water to boiling and generates steam. S2, after steam is output from the waste heat boiler (2) for heat exchange, it re-enters the waste heat boiler (2) for continuous heat exchange; S3 captures and purifies CO2 in medium and low temperature flue gas, then combines it with CO2 in the venting gas and pressurizes it for heat exchange to form liquid CO2. The liquid CO2 flows into the CO2 liquid tank (7) for storage. S4, the cryogenic liquid nitrogen in the liquid nitrogen storage tank (9) is heat exchanged to form sub-cryo liquid nitrogen. The cryogenic liquid nitrogen absorbs heat and vaporizes. The high-pressure nitrogen gas after vaporization drives the liquid CO2 in the CO2 liquid tank (7). The outflowing liquid CO2 enters the sand mixing device (8) and mixes with the proppant to form fracturing fluid. S5, the fracturing fluid is drawn into the wellhead (10) by the diesel-powered fracturing truck (1), pressurized, and then pumped into the wellhead (10) for carbon dioxide fracturing operation.

6. The nitrogen pressurization method for carbon dioxide fracturing according to claim 5, characterized in that, Specifically, S2 is as follows: Steam is divided into two paths. One path of steam is pressurized by a booster pump (11) and enters the steam turbine power generation equipment (3) to generate electricity. The steam-water mixture coming out of the steam turbine power generation equipment (3) is condensed by a condenser (12) to form cold water. Then, the cold water is pumped back into the steam waste heat boiler (2) by the feed water pump a (13) for circulating heat exchange. The other path of steam is pumped into the liquid nitrogen vaporizer (6) by a steam booster pump b (17) to exchange heat with liquid nitrogen and condense into water. Then, the cold water is pumped back into the steam waste heat boiler (2) by the feed water pump b (18) for circulating heat exchange.

7. The nitrogen pressurization method for carbon dioxide fracturing according to claim 5, characterized in that, Specifically, S3 is as follows: low-temperature flue gas enters the CO2 capture device (4) for gaseous CO2 capture and purification. The purified gaseous CO2, CO2 liquid tank (7), and the vented CO2 from the sand mixing device (8) are combined and then pressurized by the compressor (14) before entering the CO2 condenser (5) to exchange heat with liquid nitrogen. The resulting liquid CO2 flows into the CO2 liquid tank (7) for storage.

8. The nitrogen pressurization method for carbon dioxide fracturing according to claim 5, characterized in that, Specifically, S4 is as follows: the cryogenic liquid nitrogen in the liquid nitrogen storage tank (9) is pumped into the CO2 condenser (5) by the liquid nitrogen booster pump a (15) to exchange heat with the gaseous CO2 to form sub-cryogenic liquid nitrogen. After being pressurized by the liquid nitrogen booster pump b (16), it enters the liquid nitrogen vaporizer (6) for heat absorption and vaporization. After vaporization, the high-pressure nitrogen gas continuously and stably displaces the liquid CO2 in the CO2 liquid tank (7) after passing through the pressure regulating valve group (19). The outflowing liquid CO2 enters the sand mixing device (8) and mixes with the proppant to form fracturing fluid.

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