In-situ conversion method of medium-low maturity shale oil injected with high-temperature supercritical CO 2 -nanoparticle system

Abstract

The present invention discloses a method for in-situ conversion of medium-low maturity shale oil by injecting a high-temperature supercritical CO2-nanoparticle system, belonging to the field of medium-low maturity shale oil development. This method first arranges dual horizontal wells and uses supercritical CO2 for formation fracturing to form a fracture network; subsequently, a nano-SiO2 aerogel slug is injected to block the fractures and transfer heat, promoting the pyrolysis of organic matter to generate movable hydrocarbons, and high-temperature supercritical CO2 at 400°C to 450°C is injected for displacement; finally, nano-SiO2 particles with a particle size of 10 nm to 15 nm are injected in cooperation with high-temperature supercritical CO2 at 400°C to 450°C to enhance the oil and gas extraction efficiency. The present invention effectively solves the problems of gas channeling and small swept area in the in-situ conversion by injecting high-temperature supercritical CO2. Using the method proposed by the present invention can significantly improve the exploitation efficiency of medium-low maturity shale oil, and is a key technology for promoting the green and efficient in-situ development of medium-low maturity shale oil.

Description

Technical Field

[0001] The present invention belongs to the field of medium and low maturity shale oil development. Specifically, it relates to a method for in-situ conversion of a high-temperature supercritical CO 2 -nanoparticle system. Background Art

[0002] Medium and low maturity shale oil is the main body of continental shale oil in China and is one of the most realistic conventional oil and gas replacement resources with scale. Although the resource volume of medium and low maturity oil shale in China is huge, due to the low permeability and strong heterogeneity of medium and low maturity shale oil reservoirs, a large amount of solid organic matter and retained hydrocarbons are not affected; some cracking products undergo adsorption and dissolution with organic matter and cannot be driven; the remaining cracking products cool and increase viscosity at the displacement front, reducing the transfer efficiency and even causing blockage. The above phenomena lead to problems such as "low production, small swept area, and easy blockage" in pilot tests. This results in higher development difficulty and cost than conventional oil reservoirs. In-situ upgrading development mainly involves injecting oxygen-containing gas into a locally preheated medium and low maturity shale oil reservoir and using the heat released by the oxidation reaction of the residues generated after kerogen pyrolysis to heat the medium and low maturity shale oil reservoir, thereby completing the exploitation of shale oil. In in-situ upgrading development, there are many methods for heating the reservoir, including electric heating, radio frequency heating, convective heating, etc. Among them, convective heating relies on the flow of fluid to achieve heat transfer and is the most direct, fastest heating speed, and highest displacement efficiency heating method. During convective heating, high-temperature nitrogen or air is often selected as the fluid medium. Supercritical CO 2 can dissolve and extract oil and gas. Selecting it as the fluid medium during convective heating can significantly improve the recovery rate and is an important development trend for the medium and low maturity shale oil industry. At the same time, high-temperature supercritical CO 2 in-situ upgrading development can also sequester a large amount of CO 2 greenhouse gas in the formation after development, which is conducive to implementing the environmental protection goal of carbon sequestration. Currently, due to problems such as gas channeling and small diffusion range in the high-temperature supercritical CO 2 development technology, it restricts the economicization and commercialization of this technology. Under high-temperature conditions, some common gels for solving gas channeling problems will become ineffective due to high-temperature modification and destruction, and surfactants mainly composed of organic matter will also have such problems. Therefore, solving the CO 2 channeling in the high-temperature supercritical CO 2 development technology, improving the CO 2 extraction efficiency, and increasing the swept area is of great significance.

[0003] Patent document CN107420079A discloses a dual horizontal well SAGD heavy oil mining mechanism and method, which uses microwave heating magnetic control devices and microwave reflection devices for formation heating and uses steam to drive oil. This method is mainly applicable to the field of heavy oil mining. In the development of medium and low-maturity shale oil, the use of steam to drive oil may cause the hydration and expansion of clay mixed in the medium and low-maturity shale oil reservoir. On the one hand, it affects the fluid sweep range, which is not conducive to the generation and development of cracks and reduces the permeability of high-temperature steam in the reservoir. On the other hand, it is easy to cause accidents such as well wall instability, affecting development safety.

[0004] Patent document CN103835683A discloses a method for in-situ extraction of shale oil using carbon dioxide, which injects room temperature liquid CO into an injection well. 2 , so that CO 2 When the wellbore is connected to the medium- and low-maturity shale oil reservoir, it is in the ordinary liquid, subcritical and supercritical states, dissolving the shale oil in the reservoir. 2 Viscosity reduction, miscibility, diffusion and other effects can dissolve and extract oil and gas to develop shale oil. 2 For shale oil reservoir development, although this method takes into account the lower energy consumption compared to high-temperature steam displacement, it does not heat the reservoir and uses room-temperature supercritical CO 2 The development rate of oil and gas resources is relatively low. This method also does not take into account the use of CO 2 As a displacement medium, it has problems such as gas channeling and small diffusion range.

[0005] Patent document CN118090528A discloses a method for screening the optimal CO for displacing shale oil. 2 - Equipment and method for nanoparticle composite system. This method mentions the problems of gas channeling in single gas injection development, and nanoparticle oil displacement agent can reduce the interaction between crude oil and CO. 2 The interfacial tension promotes miscibility while providing reservoir plugging effect. This method uses microfluidic experiments and interfacial tension tests to explore the effects of the same CO 2 -Nano-particle composite system has two effects: increasing the swept area and reducing viscosity. However, in the development process, plugging and displacement operations have a sequence, and the optimal CO under the corresponding operating conditions can be selected respectively. 2 -Nano-particle composite system improves work efficiency. This patent document uses room temperature CO 2 , without taking into account the high temperature and high pressure resistance characteristics of inorganic nanoparticles.

[0006] In summary, in the field of medium and low-maturity shale oil development, increasing the affected area, reducing gas channeling and efficiently extracting and displacing hydrocarbons are issues that need to be urgently addressed in this field. Summary of the invention

[0007] In view of the above problems existing in the prior art, the object of the present invention is to propose a method for in-situ conversion of a high-temperature supercritical CO 2 -nanoparticle system for medium and low maturity shale oil, which uses a gel slug injection - high-temperature supercritical CO 2 -surfactant to co-develop medium and low maturity shale oil, aiming to improve the recovery rate of such resources. First, two horizontal wells are arranged and the supercritical CO 2 foam method is used to perform volume fracturing on the medium and low maturity shale oil reservoir to form a fracture network; then, nano-SiO 2 aerogel is injected as a plugging agent to plug the fractures and transfer heat, so that the kerogen in the reservoir pyrolyzes to generate mobile hydrocarbons; subsequently, nano-SiO 2 particles as a surfactant are injected to further improve the recovery rate and complete the CO 2 sequestration. The present invention monitors the pressure, displacement and temperature of the reflux CO 2 gas to ensure the efficient progress of the CO 2 circulation and the oil-gas separation process.

[0008] Specifically, the method for in-situ conversion of a high-temperature supercritical CO 2 -nanoparticle system for medium and low maturity shale oil proposed by the present invention includes the following steps:

[0009] Step 1: Determine the target development area, deploy two horizontal wells according to the conditions of the medium and low maturity shale oil reservoir, which are used as the injection well and the production well respectively, and the horizontal well section of the injection well is located above the horizontal well section of the production well;

[0010] Step 2: Use the supercritical CO 2 foam method to perform volume fracturing on the medium and low maturity shale oil reservoir to form a fracture network;

[0011] Step 3: After the fracture creation is completed, inject a nano-SiO 2 aerogel slug, and continuously circulate high-temperature supercritical CO 2 at 400°C to 450°C so that the nano-SiO 2 aerogel fully and evenly enters the fracture network of the medium and low maturity shale oil reservoir to complete the plugging operation;

[0012] Step 4: After the reservoir transformation is completed, introduce high-temperature supercritical CO 2 heated to 400°C to 450°C into the medium and low maturity shale oil reservoir. Under the action of nano-SiO 2 aerogel, high-temperature supercritical CO 2 at 400°C to 450°C cannot gas channel in the fractures, so it enters the pore throats of the kerogen and transfers heat, and the kerogen pyrolyzes. During pyrolysis, the medium and low maturity shale oil reservoir is heated and mobile hydrocarbons are decomposed;

[0013] Step 5: The movable hydrocarbons in the medium- and low-maturity shale oil reservoirs are heated to 400°C~450°C high-temperature supercritical CO 2 The displacement effect is discharged into the horizontal section of the production well, and the oil and gas are mixed with CO through the production well. 2 The mixture is sent to the ground for collection and separated by ground classification equipment. 2 Reheating completes CO 2 cycle;

[0014] Step 6: When the oil and gas production reaches a stable level, add nano-SiO 2 Particles dispersed in liquid CO 2 The dispersion is added into a high temperature supercritical CO at 400℃~450℃ 2 High temperature supercritical CO 2 Slug and inject into injection well;

[0015] Step 7: Repeat steps 5 to 6; establish a monitoring station on the ground and lower a telemetry cable into the injection and production process pipe in the production well to monitor the return CO 2 Gas pressure, displacement and temperature ensure stable circulation.

[0016] Furthermore, the medium-low maturity shale oil is injected with high temperature supercritical CO 2 - In-situ conversion method of nanoparticle system, in step 1, the vertical distance between the horizontal sections of the injection well and the production well is controlled between 25m and 30m, and the length of the horizontal sections of the injection well and the production well is controlled between 700m and 1500m according to the actual length of the medium-low maturity shale oil reservoir, so that the medium-low maturity shale oil reservoir is effectively penetrated, and the vertical wellbore of the production well is developed to 3m to 5m from the bottom of the medium-low maturity shale oil reservoir, ensuring that the horizontal sections of the injection well and the production well evenly and completely penetrate the medium-low maturity shale oil reservoir, and horizontal development is carried out along the direction of the principal stress.

[0017] Further, in step 2, supercritical CO 2 The foam fracturing method is used to perform volume fracturing on medium- and low-maturity shale oil reservoirs. When the pumping pressure reaches between 30Mpa and 50Mpa, a fracture network can be effectively generated in conventional medium- and low-maturity shale oil reservoirs. As the fractures are generated, the pumping pressure decreases and eventually maintains a flow pressure of 20Mpa.

[0018] Further, in step 2, supercritical CO 2 The foam fracturing method is used to perform volume fracturing on medium- and low-maturity shale oil reservoirs. After fracturing, the well is shut down for 1.5 to 3 days to expand the affected area.

[0019] Furthermore, in step 2, supercritical CO 2The foam fracturing method is used for volume fracturing of medium and low maturity shale oil reservoirs, and supercritical CO 2 The starting displacement is set at 20 t / d, injected for 5 d, and shut-in for 25 d; repeated 6 times to make the production pressure reach 20 Mpa.

[0020] Further, in step 3, nano-SiO 2 Aerogel configuration method: Purchase nano-SiO 2 aerogel silicon source powder with a particle size of 40 nm - 50 nm on the market, disperse it in a solvent, and use inorganic acid or base for hydrolysis catalysis to obtain a gel. The obtained gel is preliminarily dried to remove the solvent and catalyst impurities to prepare nano-SiO 2 aerogel. The solvent is selected from alcohols or ketones, including but not limited to methanol, isobutyl ketone, and acetone; the inorganic acid is selected from hydrochloric acid or hydrofluoric acid, and the inorganic base is selected from ammonia water. Considering that the local pressure will exceed the pump-in flow pressure during the development process, resulting in brittle damage to the solid structure of the gel and reducing the plugging effect of the gel. Add SiC whiskers with a weight percentage of 4 wt% to the nano-SiO 2 aerogel silicon source powder to improve the strength of the finished gel.

[0021] Further, the in-situ conversion method of the medium and low maturity shale oil injection high-temperature supercritical CO 2 -nanoparticle system mainly involves injecting high-temperature supercritical CO at 400 °C - 450 °C into the medium and low maturity shale oil reservoir after reservoir transformation. 2 At the beginning of the production stage, as the CO 2 displaces the oil and gas out of the ground, the production pressure decreases. Control the injection of supercritical CO 2 for 10 d, shut-in for 20 d, and carry out production operations. The production pressure in this stage is controlled at 12 Mpa.

[0022] Further, in the in-situ conversion method of the medium and low maturity shale oil injection high-temperature supercritical CO 2 -nanoparticle system, in step 6, add nano-SiO with a particle size of 10 nm - 15 nm 2 particles to liquid CO 2 to prepare a dispersion liquid. The addition amount of nano-SiO 2 particles in the dispersion liquid is 3 wt% - 4 wt%.

[0023] Compared with the prior art, the advantages of the method proposed by the present invention are:

[0024] 1. The present invention adopts the in-situ conversion method of the medium and low maturity shale oil injection high-temperature supercritical CO 2 -nanoparticle system to develop and recover medium and low maturity shale oil. Supercritical CO 2 As the anhydrous phase of the fracturing fluid, it will not expand due to the hydration of clay in the shale. At the same time, supercritical CO2 Due to its high permeability, it reduces the fracture initiation pressure during the fracturing process and improves economic efficiency.

[0025] 2. The present invention adopts an in-situ conversion method of a medium-low maturity shale oil injection high-temperature supercritical CO 2 -nanoparticle system to develop and recover medium-low maturity shale oil. By using high-temperature supercritical CO 2 to heat the medium-low maturity shale oil reservoir, it promotes the occurrence of kerogen pyrolysis. At the same time, supercritical CO 2 can displace and extract adsorbed and dissolved hydrocarbons and can act deep into the pore throats, greatly increasing the oil and gas production rate.

[0026] 3. The present invention adopts a well pattern arrangement of double horizontal wells, which reduces the influence of heterogeneity on the horizontal plane of the shale on recovery to a certain extent and increases the sweep range of high-temperature supercritical CO 2 . At the same time, gravity-assisted oil displacement is used to improve the recovery rate.

[0027] 4. The present invention respectively performs cementation treatment and dispersion treatment on nano-SiO 2 particles of different particle sizes.

[0028] Cementation treatment: Disperse the commercially available nano-SiO 2 aerogel silicon source powder with a particle size of 40 nm to 50 nm in a solvent. Under the hydrolysis catalysis of inorganic acid or inorganic base conditions, the hydroxyl groups on the molecules aggregate with each other through polycondensation and aging to form a nano-porous network structure gel, and after drying and dehydration, nano-SiO 2 particle aerogel with a particle size of 40 nm to 50 nm is formed. The aerogel maintains the porous network structure, making it difficult for the gas inside and on both sides of the pores to flow through, having a good plugging effect, and being able to effectively increase the gas sweep range during the displacement process.

[0029] Dispersion treatment: Add nano-SiO 2 particles with a particle size of 10 nm to 15 nm to liquid CO 2 to prepare a dispersion liquid. The nano-particles in the dispersion liquid penetrate into the organic matter, break the entanglement between the molecular chains of macromolecular hydrocarbons, heavy viscous oils, and asphaltenes, and play the role of a surfactant to wrap the oil droplets, promoting miscibility and effectively reducing the surface tension.

[0030] Select nano-SiO 2 particles as the plugging agent and surfactant, and at the same time utilize the thermal stability, chemical stability, formation affinity, and low cost of nano-SiO 2 particles. Conventional gels and surfactants will become inactivated under the development stability of 400 °C to 450 °C; while more efficient metal oxide nanoparticles not only have a higher price, resulting in too high development costs, but also pollute the formation. Description of the Drawings

[0031] Figure 1 In-situ conversion method flowchart for injecting high-temperature supercritical CO 2 -nanoparticle system into medium-low maturity shale oil.

[0032] Figure 2 Schematic diagram of the dual horizontal well development mode of the present invention.

[0033] Figure 3 Graph showing the relationship between recovery and time under different operating conditions.

[0034] Figure 4 Graph showing the relationship between construction displacement, production pressure and time.

[0035] In the figure: 1 - injection well; 2 - production well; 3 - injection-production technology pipe; 4 - high-pressure pump injection system; 5 - fracture network. Detailed implementation mode

[0036] To make the objectives, features and advantages of the present invention more obvious and understandable, the following combines the Figure 1 , Figure 2 , Figure 3 and Figure 4 in the embodiments of the present invention to clearly and completely describe the technical solutions in the present invention. Obviously, the present invention is not limited by the following embodiments, and the specific implementation modes can be determined according to the technical solutions of the present invention and the actual situation. To avoid confusing the essence of the present invention, well-known methods, processes, procedures, components and circuits are not described in detail.

[0037] Example 1:

[0038] In-situ conversion method for injecting high-temperature supercritical CO 2 -nanoparticle system into medium-low maturity shale oil, including the following steps:

[0039] Select a medium-low maturity shale oil reservoir with a depth of 800 m to 1000 m as the development target area, and arrange two horizontal wells according to the conditions of the medium-low maturity shale oil reservoir, namely injection well 1 and production well 2. Develop the vertical wellbore of production well 2 to 3 m to 5 m at the bottom of the medium-low maturity shale oil reservoir, and the horizontal well section of injection well 1 is located above the horizontal well section of production well 2. The vertical distance between the horizontal well sections of the two horizontal wells is controlled within 25 m to 30 m, and the length of the horizontal well section is controlled within 700 m to 1500 m according to the actual length of the medium-low maturity shale oil reservoir, and horizontal development is carried out along the main stress direction so that the medium-low maturity shale oil reservoir is effectively penetrated. The specific structure is as Figure 2 shown.

[0040] (2) Inject circulating well-washing fluid into injection well 1 and production well 2 for well washing, and lower injection-production technology pipes 3 into the vertical well sections of injection well 1 and production well 2 respectively.

[0041] (3) Use the high-pressure pumping system 4 to transport liquid CO 2 to the surface wellhead. The high-pressure pumping system 4 is connected to pipelines, heating equipment, and pressurizing equipment to ensure that CO 2 is in a supercritical state during the injection and fracturing construction process. Subsequently, use supercritical CO 2 foam fracturing method to perform volume fracturing on the medium-low maturity shale oil reservoir, forming a fracture network 5 in the medium-low maturity shale oil reservoir. The supercritical CO 2 foam fracturing method belongs to the prior art. It should be noted that in the present invention, the starting displacement of supercritical CO 2 is set to 20 t / d. After injecting for 5 days, shut in the well for 25 days. During the injection period, the production pressure increases, and during the shut-in period, the production pressure is stable. Repeat 6 times to make the production pressure reach 20 Mpa. Use supercritical CO 2 foam fracturing method to perform volume fracturing on the medium-low maturity shale oil reservoir. After fracturing, shut in the well for 1.5 d to 3 d to expand the sweep range.

[0042] (4) Inject a nano-SiO 2 aerogel slug. The preparation process of the nano-SiO 2 aerogel is as follows. Purchase nano-SiO 2 aerogel silicon source powder with a particle size of 40 nm to 50 nm on the market, and disperse it in a solvent. Use inorganic acid or inorganic base for hydrolysis catalysis to obtain a gel. The solvent is selected from alcohols or ketones, including but not limited to methanol, isobutyl ketone, and acetone; the inorganic acid is selected from hydrochloric acid or hydrofluoric acid, and the inorganic base is selected from ammonia water. The obtained gel is preliminarily dried to remove the solvent and catalyst impurities, and the nano-SiO 2 aerogel is prepared. Considering that the local pressure during the development process will exceed the pumped-in flow pressure, resulting in brittle damage to the gel solid structure and reducing the gel plugging effect. Add SiC whiskers with a weight percentage of 4 wt% to the nano-SiO 2 aerogel silicon source powder to improve the strength of the finished gel. Mix the obtained nano-SiO 2 aerogel into high-temperature supercritical CO 2 at 400 °C to 450 °C to make a nano-SiO 2 aerogel slug. Make the nano-SiO 2 aerogel fully and evenly enter the reservoir fracture network 5, and further circulate CO 2 to make the nano-SiO 2 aerogel dry and age to complete the plugging operation.

[0043] (5) After completing the reservoir transformation, the supercritical CO 2Inject it into the medium-low maturity shale oil reservoir. Kerogen produces mobile oil and gas under high-temperature pyrolysis. Send the oil, gas and CO 2 mixture to the ground to complete the recovery, and separate it through the ground classification device. After separation, the CO 2 is reheated to complete the CO 2 cycle.

[0044] Specifically, inject high-temperature supercritical CO at 400°C - 450°C into the medium-low maturity shale oil reservoir after reservoir reconstruction. 2 , and then start the production stage. As the CO 2 displaces the oil and gas out of the ground, the production pressure is reduced to control the injection of supercritical CO 2 for 10 days, shut in the well for 20 days, and carry out the recovery operation. Control the production pressure at 12 Mpa in this stage, which can not only ensure to reach the efficient miscible pv value, but also avoid wasting supercritical CO 2 . The displacement curve of this process is as shown in Figure 4 .

[0045] When the oil and gas production reaches the part of node b as shown in Figure 3 , disperse nano-SiO 2 particles with a particle size of 10 nm - 15 nm in liquid CO 2 to prepare a dispersion liquid, and add this dispersion liquid to high-temperature supercritical CO at 400°C - 450°C 2 to prepare a high-temperature supercritical CO 2 slug with surfactant added and inject it into injection well 1. The particle size of nano-SiO 2 particles is 10 nm - 15 nm, and the addition amount is 3 wt% - 4 wt% to ensure sufficient viscosity reduction and miscibility. Under the action of the surfactant of nano-SiO 2 particles, the recovery rate is further improved.

[0046] (7) Establish a detection and treatment station on the ground, equipped with a circulation separation facility, a heating and pressurization reinjection device. While carrying out the CO 2 circulation displacement, establish a monitoring station on the ground, and lower a telemetry cable into the injection-production process pipe 3 located in the production well to monitor the pressure, displacement and temperature of the reflux CO 2 gas. Ensure the construction safety, and carry out the CO 2 sequestration treatment after the development is completed. In particular, the displacement monitoring device and the flow collector in the telemetry cable should be processed with corrosion-resistant and high-temperature-resistant materials.

[0047] Figure 2 shows the schematic diagram of the dual horizontal well development mode of the present invention. Ⅰ represents the effective area of the 400°C - 450°C high-temperature supercritical CO 2 slug with surfactant added, and nano-SiO 2The particles serve as surfactants; II represents the high-temperature supercritical CO at 400°C to 450°C 2 Effective region; III represents SiO 2 Effective region of the SiO nanofoam slug Figure 3 Shows the relationship diagram between the recovery and time under different operating conditions. a represents the reduction in the swept area caused by gas channeling; b represents adding the SiO nanoparticle dispersion surfactant with a particle size of 10 nm to 15 nm 2 Dispersion of particles in liquid CO Figure 4 Shows the relationship diagram between the construction displacement, production pressure, and time. α represents the start of the recovery stage

[0048] Comparative Example 1:

[0049] The same method as in Example 1, except that step (6) is not carried out, that is, the SiO nanoparticle dispersion with a particle size of 10 nm to 15 nm is not added 2 Dispersion of particles in liquid CO 2 is used as a surfactant

[0050] Comparative Example 2:

[0051] The same method as in Example 1, except that step (4) is not carried out, that is, the supercritical CO slug mixed with SiO nanofoam is not added for plugging operation 2 supercritical CO 2 for plugging

[0052] Comparative Example 3:

[0053] The same method as in Example 1, except that in step (5), instead of using high-temperature supercritical CO at 400°C to 450°C 2 for displacement, nitrogen is used for displacement

[0054] Compared with Comparative Example 1, in Example 1, due to the addition of nanoparticle surfactants in the later stage of displacement and production, the CO 2 miscibility is improved, enhancing the supercritical CO 2 oil displacement effect and increasing the recovery degree of oil and gas resources

[0055] Compared with Comparative Example 2, in Example 1, due to the addition of nanofoam to plug the reservoir before displacement and production, supercritical CO 2 acts in the pore throats, reducing gas channeling in the main fractures. Increasing the supercritical CO 2 swept area, and having a higher recovery efficiency throughout the development stage

[0056] Compared with Comparative Example 3, in Example 1, supercritical CO 2 is selected as the displacement gas. Compared with nitrogen, it not only has a lower miscibility pressure and more significant viscosity reduction effect, but also can extract adsorbed and dissolved hydrocarbons in organic matter, with remarkable effects

Claims

1. In-situ conversion method of low-middle-mature shale oil by injecting high-temperature supercritical CO2-nanoparticle system, characterized in that: The steps include: Step 1: Determine the target development area and deploy two horizontal wells according to the medium- and low-maturity shale oil reservoir conditions, which serve as injection wells and production wells respectively, and the horizontal section of the injection well is located above the horizontal section of the production well; Step 2: Use supercritical CO2 foam method to perform volume fracturing on low-mature shale oil reservoirs to form a fracture network; Step 3: After the fracture is created, inject the nano-SiO2 aerogel plug, and continuously circulate the high-temperature supercritical CO2 at 400℃~450℃ to allow the nano-SiO2 aerogel to fully and evenly enter the fracture network of the medium- and low-maturity shale oil reservoir to complete the plugging operation; Step 4: After the reservoir transformation is completed, high-temperature supercritical CO2 heated to 400℃~450℃ is introduced into the medium-low maturity shale oil reservoir. Under the action of nano-SiO2 aerogel, the 400℃~450℃ high-temperature supercritical CO2 cannot gasify in the cracks, so it enters the pores in the kerogen and transfers heat, causing the kerogen to pyrolyze. The pyrolysis heats the medium-low maturity shale oil reservoir and decomposes the movable hydrocarbons. Step 5: The movable hydrocarbons in the medium- and low-maturity shale oil reservoirs are discharged into the horizontal well section of the production well under the action of high-temperature supercritical CO2 displacement at 400℃~450℃. The mixture of oil, gas and CO2 is sent to the ground through the production well to complete the recovery, and is separated by the ground separation device. The separated CO2 is reheated to complete the CO2 cycle; Step 6: When the oil and gas production reaches stability, nano-SiO2 particles with a particle size of 10nm-15nm are dispersed in liquid CO2 to prepare a dispersion, and the dispersion is added into high-temperature supercritical CO2 at 400℃-450℃ to prepare a high-temperature supercritical CO2 plug with added surfactant and inject it into the injection well; Step 7: Repeat steps 5 to 6 and monitor the pressure, displacement and temperature of the reflux CO2 gas to ensure that the cycle is stable.

2. The in-situ conversion method of low-middle-mature shale oil by injecting high-temperature supercritical CO2-nanoparticle system according to claim 1 is characterized in that: In step 1, the vertical distance between the horizontal sections of the injection well and the production well is controlled between 25m and 30m; the length of the horizontal sections of the injection well and the production well is controlled between 700m and 1500m according to the actual length of the medium-low maturity shale oil reservoir; the vertical wellbore of the production well is developed to 3m to 5m from the bottom of the medium-low maturity shale oil reservoir.

3. The in-situ conversion method of low-middle-mature shale oil by injecting high-temperature supercritical CO2-nanoparticle system according to claim 1 is characterized in that: In step 2, supercritical CO2 foam fracturing is used to perform volume fracturing on the medium- and low-maturity shale oil reservoirs, and the pumping pressure reaches between 30Mpa and 50Mpa. As cracks are generated, the pumping pressure decreases and eventually maintains a flow pressure of 20Mpa.

4. The in-situ conversion method of low-middle-mature shale oil by injecting high-temperature supercritical CO2-nanoparticle system according to claim 1 is characterized in that: In step 2, supercritical CO2 foam fracturing is used to perform volume fracturing on the medium- and low-maturity shale oil reservoirs, and the well is shut down for 1.5 to 3 days after the fracturing is completed.

5. The in-situ conversion method of low-middle-mature shale oil by injecting high-temperature supercritical CO2-nanoparticle system according to claim 1 is characterized in that: In step 2, supercritical CO2 foam fracturing method is used to perform volume fracturing on the medium- and low-maturity shale oil reservoirs. The supercritical CO2 startup displacement is set to 20t / d, injection is performed for 5d, and well shut-in is performed for 25d; this is repeated 6 times to achieve a production pressure of 20Mpa.

6. The in-situ conversion method of low-middle-mature shale oil by injecting high-temperature supercritical CO2-nanoparticle system according to claim 1 is characterized in that: In step 3, the nano-SiO2 aerogel preparation method is as follows: commercially available nano-SiO2 aerogel silicon source powder with a particle size of 40nm~50nm is dispersed in a solvent, and hydrolyzed and catalyzed by an inorganic acid or an inorganic base to obtain a gel, and the obtained gel is preliminarily dried to remove the solvent and catalyst impurities to obtain a nano-SiO2 aerogel with a particle size of 40nm~50nm; wherein SiC whiskers with a weight percentage of 4wt% are added to the nano-SiO2 aerogel silicon source powder.

7. The in-situ conversion method of low-middle-mature shale oil by injecting high-temperature supercritical CO2-nanoparticle system according to claim 6 is characterized in that: The solvent is selected from methanol, isobutyl ketone or acetone; the inorganic acid is selected from hydrochloric acid or hydrofluoric acid; and the inorganic base is selected from ammonia water.

8. The in-situ conversion method of low-middle-mature shale oil by injecting high-temperature supercritical CO2-nanoparticle system according to claim 1 is characterized in that: By injecting 400℃~450℃ high-temperature supercritical CO2 into the medium- and low-maturity shale oil reservoirs after reservoir transformation, the production stage begins. As CO2 drives the oil and gas out of the ground, the production pressure decreases. The supercritical CO2 injection is controlled for 10 days, the well is shut down for 20 days, and recovery operations are carried out. The production pressure is controlled at 12Mpa during this stage.

9. The in-situ conversion method of low-middle-mature shale oil by injecting high-temperature supercritical CO2-nanoparticle system according to claim 1 is characterized in that: In step 6, nano-SiO2 particles with a particle size of 10 nm to 15 nm are added to liquid CO2 to prepare a dispersion, and the added amount of the nano-SiO2 particles in the dispersion is 3 wt% to 4 wt%.

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