A gas-water mixed injection CO2 hydrate storage cap rock generation system and control method based on multi-stage fracturing

Abstract

The present invention belongs to the technical field of hydrate method for the geological storage of carbon dioxide in the seabed, and proposes a gas-water mixed injection CO2 hydrate storage cap rock generation system and a control method based on multi-stage fracturing. The gas-water mixed injection CO2 hydrate storage cap rock generation system based on multi-stage fracturing includes a high-pressure water injection system, a high-pressure gas injection system, a gas-water mixing system, a fracturing control system and a seabed storage system. The innovative use of a multi-stage fracturing tool string arrangement method and a gas-water mixed injection control method combines the advantages of inclined wells and has the characteristics of horizontal wells and vertical wells. It solves the problem that large-scale safe storage cannot be achieved in the process of traditional hydrate method CO2 seabed storage, and realizes adjustable and controllable water and gas injection, which is of great significance for the safe, stable and efficient storage of CO2 and related scientific research.

Description

Technical Field

[0001] The present invention relates to the technical field of hydrate-based CO2 submarine geological storage, and in particular to a gas-water mixed injection CO2 hydrate storage cap rock generation system based on multi-stage fracturing and a control method. Background Art

[0002] At present, my country's CO2 emissions rank first in the world, and it is urgent to control the atmospheric CO2 content. CO2 capture, utilization and storage CCUS is a necessary means to reduce atmospheric CO2 emissions and alleviate climate deterioration, and CO2 geological storage is the fundamental way to achieve the reduction of CO2 content. There are two main CO2 geological storage technologies: land storage and marine storage. Common locations for land storage are abandoned oil and gas reservoirs, deep coal seams and saline water layers, but land storage requires comprehensive consideration of multiple influencing factors and has more stringent environmental requirements. In comparison, marine storage has more potential. Marine storage is the transportation of captured CO2 by pipeline or ship and injection into the deep sea. CO2 can be stored in the deep sea for up to several centuries. An effective method for marine CO2 geological storage is to store it in the form of CO2 hydrates in geological structures such as the ocean.

[0003] The marine geological environment has the characteristics of high pressure and low temperature, which can ensure the stable occurrence of CO2 hydrates. Therefore, storing CO2 in the form of hydrates in the marine strata is a stable and safe way to store CO2. However, the storage process is affected by many factors. Among them, the permeability characteristics of the formation during the storage process are an important reason that restricts the storage efficiency, storage range, and safe and stable operation. Therefore, it is particularly important to use hydraulic fracturing to increase the formation seepage channels and improve the formation seepage characteristics. Directly injecting CO2 into the formation to form a "carbon lake" is prone to leakage, which will reduce the pH value of seawater and damage the ecological environment on the seabed. Even worse, it can cause the death of a large number of deep-sea organisms, bringing serious impacts to the global ecological environment. Traditional well types used for CO2 seabed storage are mostly vertical wells or horizontal wells (such as a device and method for integrating deepwater oil and gas drilling and carbon dioxide utilization and storage disclosed in patent document CN114033322A, Li Xiaosen et al.). Vertical wells are easy to construct and the well drilling technology is relatively mature, but for formations with poor seepage characteristics, the sealing effect of this type of well is poor. If necessary, the number of wellheads needs to be increased, but the cost will increase. Although horizontal wells can achieve large-scale fracturing and sealing in the horizontal direction, this type of well is difficult to exploit, not easy to construct, and has huge economic costs.

[0004] In view of the relevant technical difficulties existing in the current hydrate method CO2 submarine storage process, the present invention provides a gas-water mixed injection CO2 hydrate storage cap rock generation system and control method based on multi-stage fracturing. Summary of the invention

[0005] In view of the above problems, the purpose of the present invention is to provide a gas-water mixed injection CO2 hydrate storage cap rock generation system and control method based on multi-stage fracturing. The system and control method are reliable in principle, simple, practical, and economical. Compared with the current traditional formation fracturing equipment and seabed CO2 storage methods, the invention has broad market application prospects.

[0006] The technical solution of the present invention:

[0007] A gas-water mixed injection CO2 hydrate storage cap rock generation system based on multi-stage fracturing includes a high-pressure water injection system, a high-pressure gas injection system, a gas-water mixing system, a fracturing control system and a seabed storage system.

[0008] The high-pressure water injection system includes a water storage tank 1, a first filter 2, a first valve 3, a first non-return valve 4, and a liquid centrifugal pump 5 which are sequentially connected through a water injection pipeline 6; the liquid centrifugal pump 5 is connected to the top of the air-water mixer 7.

[0009] The high-pressure gas injection system includes a gas storage tank 8, a second filter 9, a second valve 10, a second non-return valve 11, and a high-pressure gas injection pump 13 which are connected in sequence through a gas pipeline 12; the high-pressure gas injection pump 13 is connected to the lower part of the gas-water mixer 7.

[0010] The gas-water mixing system includes a gas-water mixer 7, a pressure relief port 14, a pressure relief valve 15, and a collector 16; a magnetic stirrer 17 is arranged in the gas-water mixer 7 for fully mixing CO2 gas with water; a pressure relief port 14 is arranged at the bottom of the gas-water mixer 7, which is connected to the pressure relief valve 15 and the collector 16 in sequence; the side of the gas-water mixer 7 is connected to the first delivery pump 18 through a pipeline; the first delivery pump 18 is connected to the second delivery pump 19 and the third delivery pump 20 through two branch pipelines respectively; the second delivery pump 19 and the third delivery pump 20 are connected to the seabed storage system through branch pipelines respectively.

[0011] The fracturing control system includes a fracturing fluid storage tank 21, a third filter 22, a third valve 23, a third one-way valve 24, and a fracturing fluid pump 25 which are sequentially connected through pipelines; the fracturing fluid pump 25 is connected to the first delivery pump 18;

[0012] The subsea storage system includes a fracturing tool string, a booster pump, a fracturing hole set, a one-way valve and a monitoring station 42; the first fracturing tool string 26 and the second fracturing tool string 27 are arranged at an angle φ to the horizontal plane; the fracturing fluid casing 37 is nested in the fracturing tool string, one end of which is connected to the branch pipeline through the casing valve 38, and the other end is close to the bottom of the fracturing tool string, on which a second casing injection port 40 is provided; the fracturing fluid or gas-water two-phase flow flows into the fracturing tool string through the fracturing fluid casing 37; the first casing injection port 39 and the liquid injection port 41 are arranged at the top and bottom of the fracturing tool string respectively; the second delivery pump 19 and the third delivery pump 20 are connected to the casing valve 38 and the second ... The first injection port 39; multiple booster pumps are arranged in sequence in the fracturing tool string, and each booster pump can boost the pressure of the fracturing fluid or the gas-water two-phase flow, and fracturing hole sets are arranged between each level of booster pumps; when implementing the fracturing or sealing process from top to bottom, the fracturing fluid or the gas-water two-phase flow is firstly boosted by the first-level booster pump 28, and injected into the formation through the first-level fracturing hole set 31 to achieve the fracturing of the upper part of the formation or the formation of the caprock, and then the fracturing fluid or the gas-water two-phase flow is again boosted by the second-level booster pump 29, and injected into the formation through the second-level fracturing hole set 32 ​​to achieve the fracturing of the middle part of the formation or the formation of the caprock, and finally the fracturing fluid or the gas-water two-phase flow is again boosted by the third-level booster pump 30 The three-stage fracturing hole set 33 is pressurized and injected into the formation to realize the fracturing or caprock formation process of the lower part of the formation; when the fracturing or sealing process is carried out from bottom to top, the fracturing fluid or gas-water two-phase flow is pressurized by the three-stage, two-stage and one-stage booster pumps in sequence and then injected into the formation from the fracturing hole set; a one-way valve is arranged between each stage of the fracturing hole set and the formation to prevent the backflow of sediment; an optical fiber and ultrasonic monitoring system are arranged in the ground monitoring station 42, which is connected to each working device (pump, valve, etc.) on the fracturing tool string through a submarine pipeline to monitor the fracturing tool string in real time; the fracturing fluid and gas-water two-phase flow are pressurized by the booster pumps and fracturing hole sets at each stage on the fracturing tool string and injected into the ground layer; when the casing valve 38 is closed and the first casing injection port 39 is opened, the fracturing fluid or the gas-water two-phase flow flows through the first casing injection port 39 from the upper end into the fracturing tool string, thereby realizing a fracturing process from top to bottom or a hydrate caprock formation process; when the casing valve 38 is opened and the first casing injection port 39 is closed, the fracturing fluid or the gas-water two-phase flow flows through the second casing injection port 40 from the lower end into the fracturing tool string, thereby realizing a fracturing process from bottom to top or a hydrate caprock formation process; finally, the CO2 is liquefied through the boosting pumps at various levels, and a large amount of pure liquid CO2 is injected at the liquid injection port 41 at the bottom end of the fracturing tool string to complete the storage of liquid CO2.

[0013] The multi-stage booster pump includes a first-stage booster pump 28, a second-stage booster pump 29 and a third-stage booster pump 30; a first-stage fracturing hole set 31 and a fourth one-way valve 34 are arranged between the first-stage booster pump 28 and the second-stage booster pump 29; a second-stage fracturing hole set 32 ​​and a fifth one-way valve 35 are arranged between the second-stage booster pump 29 and the third-stage booster pump 30; a third-stage fracturing hole set 33 and a sixth one-way valve 36 are arranged between the third-stage booster pump 30 and the second injection port 40 of the casing.

[0014] The specific arrangement of the fracturing tool string is selected according to the seabed environment:

[0015] 1) When the seabed environment is a uniform geological environment, a parallel inclined well is used: the first fracturing tool string 26 and the second fracturing tool string 27 are arranged in parallel at an angle φ to the horizontal plane;

[0016] 2) When the seabed environment is a dense and tight upper layer and loose lower layer seabed geological environment, an eight-shaped fork well is used: the first fracturing tool string 26 and the second fracturing tool string 27 are arranged in an "eight" shape at angles of -φ and φ with the horizontal plane respectively;

[0017] 3) When the seabed environment is a loose upper layer and a dense lower layer, an inverted figure eight fork well is used: the first fracturing tool string 26 and the second fracturing tool string 27 are arranged in an "inverted figure eight" shape with angles of φ and -φ with the horizontal plane respectively.

[0018] The components of the high-pressure water injection system, high-pressure gas injection system, gas-water mixing system, fracturing control system and seabed storage system are all made of corrosion-resistant materials. Monitoring devices are installed at pipeline nodes and bends to monitor the pressure, temperature and stress changes inside and outside the transmission pipeline in real time. The CO2 in the high-pressure gas injection system is dried and treated to remove impurities in advance.

[0019] A method for regulating and controlling CO2 hydrate storage caprock by gas-water mixed injection based on multi-stage fracturing, comprising the following steps:

[0020] Step 1, formation fracturing process: the fracturing fluid is transported from the fracturing fluid storage tank 21 to the first delivery pump 18 through the third filter 22, the third valve 23, the third one-way valve 24 and the fracturing fluid pump 25 in sequence. The first delivery pump 18 pumps the fracturing fluid into the seabed storage system through the second delivery pump 19 and the third delivery pump 20. The fracturing fluid reacts with a special cementing fluid and a microbial solution that promotes the formation of carbonate ions. After reaching the interior of the formation for a period of time, the fracturing fluid expands itself to form a porous structure to support the formation cracks.

[0021] Step 2, water injection and gas injection process: ① two-phase flow process; the treated seawater is stored in the water tank 1, and it flows into the top of the gas-water mixer 7 from the water tank 1 through the first filter 2, the first valve 3, the first non-return valve 4, and the liquid centrifugal pump 5 in sequence; the gas flows into the bottom of the gas-water mixer 7 from the gas storage tank 8 through the second filter 9, the second valve 10, the second non-return valve 11, and the high-pressure gas injection pump 13 in sequence; after being mixed by the magnetic stirrer 17, a gas-water two-phase flow is formed and pumped into the main pipeline by the first delivery pump 18, and then pumped into the seabed storage system by the second delivery pump 19 and the third delivery pump 20; ② single-phase flow process, when the first valve 3 or the second valve 10 is closed, a single-phase flow process of CO2 or water is realized;

[0022] Step 3, CO2 hydrate generation and storage process: ① In the cap layer generation stage, the gas-water two-phase flow transported in the previous stage is first pressurized by the first-stage booster pump 28, and then enters the formation through the first-stage fracturing hole set 31, and flows in the formation along the seepage channel formed after fracturing; the first-stage booster pump 28 is located in the muddy surface layer of the seabed formation. After a large amount of hydrate is generated here, a "cap layer" is generated on the upper part of the entire fracturing area, and the CO2 to be stored later is "confined" under the cap layer; ② In the later storage stage, after the cap layer is formed, the gas-water two-phase flow is injected with secondary and tertiary pressurization; the secondary booster pump 2 9, after a large amount of hydrate is generated near it, it serves as a "cap layer" to further ensure the stability of the sealing; subsequently, the formed seepage channel diffuses in a large range inside the formation to further generate CO2 hydrate. This part is located in the deep seabed of the seabed, and the cap layer formed in the cap layer generation stage provides an effective closed space to ensure the sealing effect; ③ In the final sealing stage, a complete and solid hydrate cap layer has been generated on the upper part of the formation. The first valve 3 is closed to realize the pure CO2 injection process. After being pressurized by each booster pump, a large amount of pure liquid CO2 is injected at the liquid injection port 41 at the lower part of the fracturing tool string to finally complete the liquid CO2 sealing process.

[0023] When the seabed environment is unevenly distributed, the fracturing fluid casing 37, the casing valve 38, the first casing injection port 39, and the second casing injection port 40 are controlled to adjust the fracturing sequence.

[0024] The special cementing fluid is a calcium ion solution; and the microbial solution that promotes carbonate generation is a Bacillus pasteurianus solution.

[0025] The beneficial effects of the present invention are as follows: the proposed gas-water mixed injection CO2 hydrate storage caprock generation system and regulation method based on multi-stage fracturing innovatively adopts a multi-stage fracturing tool string arrangement method, so that it has the characteristics of both horizontal wells and vertical wells, and solves the problem that large-scale storage cannot be achieved in the traditional hydrate method CO2 seabed storage process. At the same time, it realizes the adjustable and controllable water injection and gas injection, which is of great significance to the safe, stable and efficient storage of CO2 and related scientific research. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a schematic diagram of the cap rock generation system for CO2 hydrate storage based on multi-stage fracturing and gas-water mixed injection in parallel inclined wells;

[0027] Figure 2 It is a caprock generation system for CO2 hydrate storage based on multi-stage fracturing and gas-water mixed injection in the Ba-shaped fork well.

[0028] Figure 3 This is a schematic diagram of the cap rock generation system for CO2 hydrate storage based on multi-stage fracturing and gas-water mixed injection in the inverted figure eight fork well.

[0029] In the figure: 1-water storage tank, 2-first filter, 3-first valve, 4-first non-return valve, 5-liquid centrifugal pump, 6-water injection pipeline, 7-gas-water mixer, 8-gas storage tank, 9-second filter, 10-second valve, 11-second non-return valve, 12-gas pipeline, 13-high-pressure gas injection pump, 14-pressure relief port, 15-pressure relief valve, 16-collector, 17-magnetic stirrer, 18-first delivery pump, 19-second delivery pump, 20-third delivery pump, 21-fracture fluid storage tank, 22-third filter, 23- The third valve, 24-the third one-way valve, 25-the fracturing fluid pump, 26-the first fracturing tool string, 27-the second fracturing tool string, 28-the first booster pump, 29-the second booster pump, 30-the third booster pump, 31-the first fracturing hole set, 32-the second fracturing hole set, 33-the third fracturing hole set, 34-the fourth one-way valve, 35-the fifth one-way valve, 36-the sixth one-way valve, 37-the fracturing fluid casing, 38-the casing valve, 39-the first injection port of the casing, 40-the second injection port of the casing, 41-the liquid injection port, 42-the monitoring station. DETAILED DESCRIPTION

[0030] The embodiments of the present invention are further described below in conjunction with the technical solutions and drawings.

[0031] Among them, the specific layout method and implementation process of the gas-water mixed injection CO2 hydrate storage cap rock generation system based on multi-stage fracturing.

[0032] 1) When the seabed environment is a uniform geological environment, a parallel inclined well is used: the first fracturing tool string 26 and the second fracturing tool string 27 are arranged in parallel at an angle of 65° to the horizontal plane according to the geological conditions;

[0033] 2) When the seabed environment is a seabed geological environment with a dense upper layer and a loose lower layer, an eight-shaped fork well is used: the first fracturing tool string 26 and the second fracturing tool string 27 are arranged in an "eight" shape at angles of -65° and 65° to the horizontal plane respectively;

[0034] 3) When the seabed environment is a loose upper layer and a dense lower layer, an inverted figure eight fork well is used: the first fracturing tool string 26 and the second fracturing tool string 27 are arranged in an "inverted figure eight" shape at angles of 65° and -65° to the horizontal plane respectively.

[0035] 4) Formation fracturing process: The fracturing fluid is transported from the fracturing fluid storage tank 21 to the first delivery pump 18 through the third filter 22, the third valve 23, the third non-return valve 24 and the fracturing fluid pump 25 in sequence. The first delivery pump 18 pumps the fracturing fluid into the main pipeline, and then the second delivery pump 19 and the third delivery pump 20 pump the fracturing fluid into the seabed storage system. The fracturing fluid uses a special cementing fluid and a microbial solution that can promote the formation of carbonate, such as a Bacillus pasteurianus solution, to react. After reaching the interior of the formation for a period of time, it expands itself to form a porous structure to support the formation fractures.

[0036] 5) Water injection and gas injection process: ① Two-phase flow process; the treated seawater is stored in the water tank 1, and it flows into the top of the gas-water mixer 7 from the water tank 1 through the first filter 2, the first valve 3, the first non-return valve 4, and the liquid centrifugal pump 5 in sequence; the gas flows into the bottom of the gas-water mixer 7 from the gas tank 8 through the second filter 9, the second valve 10, the second non-return valve 11, and the high-pressure gas injection pump 13 in sequence; after being mixed by the magnetic stirrer 17, a gas-water two-phase flow is formed and pumped into the main pipeline by the first delivery pump 18, and then pumped into the seabed storage system by the second delivery pump 19 and the third delivery pump 20; ② Single-phase flow process, when the first valve 3 or the second valve 10 is closed, a single-phase flow process of CO2 or water is realized;

[0037] 6) CO2 hydrate generation and storage process: ① In the cap layer generation stage, the gas-water two-phase flow transported in the previous stage is first pressurized by the first-stage booster pump 28, and then enters the formation through the first-stage fracturing hole set 31, and flows in the formation along the seepage channel formed after fracturing; the first-stage booster pump 28 is located in the muddy surface layer of the seabed formation. After a large amount of hydrate is generated here, a "cap layer" is generated on the upper part of the entire fracturing area, and the CO2 to be stored later is "confined" under the cap layer; ② In the later storage stage, after the cap layer is formed, the gas-water two-phase flow is injected with secondary and tertiary boosting; the secondary booster pump 29 After a large amount of hydrate is generated nearby, it serves as a "cap layer" to further ensure the stability of the sealing; subsequently, the formed seepage channel diffuses over a large range inside the formation to further generate CO2 hydrate. This part is located in the deep seabed, and the cap layer formed in the cap layer generation stage provides an effective closed space to ensure the sealing effect; ③ In the final sealing stage, a complete and solid hydrate cap layer has been generated on the upper part of the formation. The first valve 3 is closed to realize the pure CO2 injection process. After being pressurized by each booster pump, a large amount of pure liquid CO2 is injected at the liquid injection port 41 at the lower part of the fracturing tool string to finally complete the liquid CO2 storage process.

[0038] The sequence of formation fracturing and sealing process is not restricted to the top-middle-bottom implementation process, and can be flexibly adjusted according to the actual fracturing effect or sealing effect. For areas where hydrates are already present on the seabed surface and caprock is easily formed, casing can be used to adjust the fracturing sequence and sealing sequence to bottom-middle-top or middle-bottom-top. At the same time, the fracturing levels of the fracturing tool string are not limited to the "three-level" arrangement, but can be one, two or more levels. Ultimately, a stable and solid sealing caprock is formed to achieve the goal of safely and massively storing CO2 on the seabed.

[0039] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by any technician familiar with the technical field within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A gas-water mixed injection CO2 hydrate storage cap rock generation system based on multi-stage fracturing, characterized in that: The gas-water mixed injection CO2 hydrate storage cap rock generation system based on multi-stage fracturing includes a high-pressure water injection system, a high-pressure gas injection system, a gas-water mixing system, a fracturing control system and a seabed storage system; The high-pressure water injection system comprises a water storage tank (1), a first filter (2), a first valve (3), a first non-return valve (4), and a liquid centrifugal pump (5) which are connected in sequence through a water injection pipeline (6); the liquid centrifugal pump (5) is connected to the top of the air-water mixer (7); The high-pressure gas injection system comprises a gas storage tank (8), a second filter (9), a second valve (10), a second non-return valve (11), and a high-pressure gas injection pump (13) which are connected in sequence through a gas pipeline (12); the high-pressure gas injection pump (13) is connected to the lower part of the gas-water mixer (7); The gas-water mixing system comprises a gas-water mixer (7), a pressure relief port (14), a pressure relief valve (15), and a collector (16); a magnetic stirrer (17) is arranged in the gas-water mixer (7) for fully mixing CO2 gas with water; a pressure relief port (14) is arranged at the bottom of the gas-water mixer (7), and is sequentially connected to the pressure relief valve (15) and the collector (16); the side of the gas-water mixer (7) is connected to a first delivery pump (18) through a pipeline; the first delivery pump (18) is respectively connected to a second delivery pump (19) and a third delivery pump (20) through two branch pipelines; the second delivery pump (19) and the third delivery pump (20) are respectively connected to a seabed storage system through branch pipelines; The fracturing control system comprises a fracturing fluid storage tank (21), a third filter (22), a third valve (23), a third check valve (24), and a fracturing fluid pump (25) which are sequentially connected through pipelines; the fracturing fluid pump (25) is connected to the first delivery pump (18); The seabed storage system comprises a fracturing tool string, a booster pump, a fracturing hole set, a one-way valve and a monitoring station (42); a first fracturing tool string (26) and a second fracturing tool string (27) are arranged at an angle φ to the horizontal plane; a fracturing fluid casing (37) is nested in the fracturing tool string, one end of which is connected to a branch pipeline through a casing valve (38), and the other end is close to the bottom of the fracturing tool string and is provided with a second casing injection port (40); fracturing fluid or gas-water two-phase flow flows into the fracturing tool string through the fracturing fluid casing (37); a first casing injection port (39) and a liquid injection port (41) are arranged at the top and bottom of the fracturing tool string respectively; a second delivery pump (19) and a third delivery pump (41) are arranged at the top and bottom of the fracturing tool string respectively; (20) are respectively connected to the first injection port (39) of the casing; multiple booster pumps are arranged in sequence in the fracturing tool string, and each booster pump can boost the pressure of the fracturing fluid or the gas-water two-phase flow, and fracturing hole sets are arranged between each booster pump; when implementing the fracturing or sealing process from top to bottom, the fracturing fluid or the gas-water two-phase flow is firstly boosted by the first-stage booster pump (28), and injected into the formation through the first-stage fracturing hole set (31), so as to realize the fracturing of the upper part of the formation or the formation of the caprock, and then the fracturing fluid or the gas-water two-phase flow is again boosted by the second-stage booster pump (29), and injected into the formation through the second-stage fracturing hole set (32), so as to realize the fracturing of the middle part of the formation or the formation of the caprock, and finally the fracturing fluid or the gas-water two-phase flow is boosted by the third-stage booster pump (30) The fracturing fluid or gas-water two-phase flow is pressurized again and injected into the formation through the three-stage fracturing hole set (33) to realize the fracturing of the lower part of the formation or the formation of caprock; when implementing the fracturing or sealing process from bottom to top, the fracturing fluid or gas-water two-phase flow is pressurized by the three-stage, two-stage, and one-stage booster pumps in sequence and then injected into the formation through the fracturing hole set; a one-way valve is arranged between each stage of the fracturing hole set and the formation to prevent sediment backflow; an optical fiber and ultrasonic monitoring system is arranged in the ground monitoring station (42), which is connected to each working device on the fracturing tool string through a submarine pipeline to monitor the fracturing tool string in real time; the fracturing fluid and gas-water two-phase flow are pressurized by the booster pumps and fracturing hole sets on the fracturing tool string at each stage and injected into the formation; When the casing valve (38) is closed and the first casing injection port (39) is opened, the fracturing fluid or the gas-water two-phase flow flows through the first casing injection port (39) from the upper end into the fracturing tool string, thereby realizing a fracturing process from top to bottom or a hydrate cap formation process; when the casing valve (38) is opened and the first casing injection port (39) is closed, the fracturing fluid or the gas-water two-phase flow flows through the second casing injection port (40) from the lower end into the fracturing tool string, thereby realizing a fracturing process from bottom to top or a hydrate cap formation process; finally, the CO2 is liquefied through the boosting of various levels of booster pumps, and a large amount of pure liquid CO2 is injected into the liquid injection port (41) at the bottom end of the fracturing tool string, thereby completing the storage of liquid CO2.

2. The system for generating CO2 hydrate storage caprock by gas-water mixed injection based on multi-stage fracturing according to claim 1 is characterized in that: The multi-stage booster pump comprises a primary booster pump (28), a secondary booster pump (29) and a tertiary booster pump (30); the multi-stage fracturing hole set is arranged as a primary fracturing hole set (31), a secondary fracturing hole set (32) and a tertiary fracturing hole set (33).

3. The CO2 hydrate storage cap rock generation system based on multi-stage fracturing and gas-water mixing injection according to claim 1 or 2, characterized in that: The specific arrangement of the fracturing tool string is selected according to the seabed environment: 1) When the seabed environment is a uniform geological environment, a parallel inclined well is used: the first fracturing tool string (26) and the second fracturing tool string (27) are arranged in parallel at an angle φ to the horizontal plane; 2) When the seabed environment is a seabed geological environment with a dense upper layer and a loose lower layer, an eight-shaped fork well is used: the first fracturing tool string (26) and the second fracturing tool string (27) are arranged in an "eight" shape at angles of -φ and φ with the horizontal plane respectively; 3) When the seabed environment is a seabed geological environment with loose upper layers and dense lower layers, an inverted figure eight fork well is used: the first fracturing tool string (26) and the second fracturing tool string (27) are arranged in an "inverted figure eight" shape at angles of φ and -φ with the horizontal plane respectively.

4. The system for generating CO2 hydrate storage caprock by gas-water mixed injection based on multi-stage fracturing according to claim 3 is characterized in that: The components of the high-pressure water injection system, high-pressure gas injection system, gas-water mixing system, fracturing control system and seabed storage system are all made of corrosion-resistant materials. Monitoring devices are installed at pipeline nodes and bends to monitor the pressure, temperature and stress changes inside and outside the transmission pipeline in real time. The CO2 in the high-pressure gas injection system is dried and treated to remove impurities in advance.

5. A method for regulating and controlling CO2 hydrate storage in caprock by gas-water mixing injection based on multi-stage fracturing, characterized in that: The steps include: Step 1, formation fracturing process: the fracturing fluid is transported from the fracturing fluid storage tank (21) to the first delivery pump (18) in sequence through the third filter (22), the third valve (23), the third non-return valve (24) and the fracturing fluid pump (25); the first delivery pump (18) pumps the fracturing fluid into the seabed storage system through the second delivery pump (19) and the third delivery pump (20); the fracturing fluid reacts with a special cementing fluid and a microbial solution that promotes the formation of carbonate ions, and after a period of time in the formation, it expands itself to form a porous structure for supporting the formation cracks; Step 2, water injection and gas injection process: ① two-phase flow process; the treated seawater is stored in the water storage tank (1), and it flows into the top of the gas-water mixer (7) from the water storage tank (1) through the first filter (2), the first valve (3), the first non-return valve (4), and the liquid centrifugal pump (5); the gas flows into the bottom of the gas-water mixer (7) from the gas storage tank (8) through the second filter (9), the second valve (10), the second non-return valve (11), and the high-pressure gas injection pump (13); after mixing by the magnetic stirrer (17), the gas-water two-phase flow is formed and pumped into the main pipeline by the first delivery pump (18), and then pumped into the seabed storage system by the second delivery pump (19) and the third delivery pump (20); ② single-phase flow process, when the first valve (3) or the second valve (10) is closed, the single-phase flow process of CO2 or water is realized; Step 3, CO2 hydrate generation and storage process: ① In the cap layer generation stage, the gas-water two-phase flow transported in the previous stage is first pressurized by the first-stage booster pump (28), and then enters the formation through the first-stage fracturing hole set (31), and flows in the formation along the seepage channel formed after fracturing; the first-stage booster pump (28) is located in the muddy surface layer of the seabed formation, and after a large amount of hydrate is generated here, a "cap layer" is generated on the upper part of the entire fracturing area, and the CO2 to be stored later is "confined" under the cap layer; ② In the later storage stage, after the cap layer is formed, the gas-water two-phase flow is injected with secondary and tertiary boosting; the secondary booster pump ( 29) After a large amount of hydrate is generated near the formation, it serves as a "cap layer" to further ensure the stability of the storage; subsequently, the formed seepage channel diffuses in a large range inside the formation to further generate CO2 hydrate. This part is located in the deep seabed of the seabed, and the cap layer formed in the cap layer generation stage provides an effective closed space to ensure the storage effect; ③ In the final storage stage, a complete and solid hydrate cap layer has been generated on the upper part of the formation. The first valve (3) is closed to realize the pure CO2 injection process. After the booster pumps are pressurized, a large amount of pure liquid CO2 is injected at the liquid injection port (41) at the bottom of the fracturing tool string, and the liquid CO2 storage process is finally completed.

6. The method for controlling CO2 hydrate storage in caprocks by gas-water mixed injection based on multi-stage fracturing according to claim 5, characterized in that: When the seabed environment is unevenly distributed, the fracturing fluid casing (37), the casing valve (38), the first casing injection port (39), and the second casing injection port (40) are controlled to adjust the fracturing sequence.

7. The method for controlling CO2 hydrate storage in caprocks by gas-water mixing injection based on multi-stage fracturing according to claim 5, characterized in that: The special cementing fluid is a calcium ion solution; and the microbial solution that promotes carbonate generation is a Bacillus pasteurianus solution.

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