In-situ conversion method for injecting high-temperature supercritical CO2-nanoparticle system into medium-low-maturity shale oil

Abstract

The invention discloses an in-situ conversion method for injecting a high-temperature supercritical CO2-nanoparticle system into medium and low-maturity shale oil, and belongs to the field of medium and low-maturity shale oil developing.The method comprises the steps that firstly, double horizontal wells are arranged, and supercritical CO2 is utilized for stratum fracturing to form a fracture network; then, a nano SiO2 aerogel slug is injected to block the crack and transfer heat, organic matter is promoted to be pyrolyzed to generate movable hydrocarbon, and high-temperature supercritical CO2 at the temperature of 400-450 DEG C is injected for displacement; and finally, injecting nano SiO2 particles with the particle size of 10-15nm in cooperation with high-temperature supercritical CO2 at the temperature of 400-450 DEG C, so as to enhance the oil gas extraction efficiency. According to the method, the problems of in-situ conversion gas channeling and small sweep range during injection of high-temperature supercritical CO2 are effectively solved, the mining efficiency of the medium-low-maturity shale oil can be remarkably improved by adopting the method provided by the invention, and the method is a key technology for promoting green and efficient in-situ development of the medium-low-maturity shale oil.

Description

Technical Field

[0001] The present invention belongs to the field of medium-low maturity shale oil development, and specifically relates to an in-situ conversion method of medium-low maturity shale oil by injecting a high-temperature supercritical CO2-nanoparticle system. Background Art

[0002] Medium-low maturity shale oil is the main body of China's continental shale oil and one of the most realistic conventional oil and gas scale replacement resources. Although my country's medium-low maturity oil shale resources are huge, due to the low permeability and strong heterogeneity of medium-low maturity shale oil reservoirs, a large amount of solid organic matter and retained hydrocarbons are not affected; some cracking products are adsorbed and dissolved with organic matter and cannot be driven; the remaining cracking products cool down and increase viscosity at the displacement front, reducing the transfer efficiency and even causing blockage. The above phenomenon causes the pilot test to face problems such as "low production, small range of influence, and easy blockage". As a result, its development difficulty and development cost are higher than conventional oil reservoirs. In-situ reforming development mainly heats the medium-low maturity shale oil reservoir by injecting oxygen-containing gas into the locally preheated medium-low maturity shale oil reservoir, and uses the residues produced after kerogen pyrolysis to release heat by oxidation reaction, thereby completing the exploitation of shale oil. In in-situ reforming development, there are many ways to heat the reservoir, including electric heating, radio frequency heating, convection heating, etc. Among them, convection heating relies on the flow of fluid to achieve heat transfer. It is the most direct, fastest heating speed and highest displacement efficiency heating method. High-temperature nitrogen or air is often used as the fluid medium in the convection heating process. Supercritical CO2 can dissolve and extract oil and gas. Selecting it as the fluid medium in the convection heating process can greatly improve the recovery rate, which is an important trend in the development of medium and low-maturity shale oil industry. At the same time, high-temperature supercritical CO2 in-situ reforming and development can also seal a large amount of CO2 greenhouse gas in the formation after the development is completed, which is conducive to the implementation of the environmental protection goal of carbon sequestration. At present, the economic and commercialization of this technology are restricted by the problems of gas channeling and small diffusion range in the high-temperature supercritical CO2 injection development technology. Under high temperature conditions, some common gels that solve the gas channeling problem will fail due to high-temperature modification and destruction. Surfactants with organic matter as the main component will also have such problems. Therefore, it is of great significance to solve the CO2 channeling in the high-temperature supercritical CO2 injection development technology, improve the CO2 extraction efficiency, and increase the affected range.

[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. This method injects room temperature liquid CO2 into the injection well, so that CO2 at different well depths is in ordinary liquid, subcritical and supercritical states when it passes from the small holes in the well wall to the medium and low maturity shale oil reservoir, dissolving the shale oil in the reservoir. This method utilizes the effects of supercritical CO2 such as viscosity reduction, miscibility, and diffusion, and can dissolve and extract oil and gas to develop shale oil. However, this method uses room temperature supercritical CO2 to develop shale oil reservoirs. Although this method takes into account the lower energy consumption compared to high temperature steam displacement, the oil and gas resource development rate is lower without heating the reservoir and using room temperature supercritical CO2. This method also does not take into account the problems of gas channeling and small diffusion range when using CO2 as a displacement medium.

[0005] Patent document CN118090528A discloses an apparatus and method for screening the optimal CO2-nanoparticle composite system for displacing shale oil. The method mentions that single gas injection development has problems such as gas channeling, and nanoparticle oil displacement agents can reduce the interfacial tension between crude oil and CO2, promote miscibility, and provide reservoir plugging effect. This method explores the effects of the same CO2-nanoparticle composite system in increasing the swept range and reducing viscosity through microfluidic experiments and interfacial tension tests. However, in the development process, there is a sequence of plugging operations and displacement operations, and the optimal CO2-nanoparticle composite system under the corresponding operating conditions can be selected to improve operating efficiency. Room temperature CO2 is used in this patent document, and the high temperature and high pressure resistance characteristics of inorganic nanoparticles are not taken into account.

[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 purpose of the present invention is to propose an in-situ conversion method for low-mature shale oil injected with high-temperature supercritical CO2-nanoparticle system. The method adopts gel segment injection-high-temperature supercritical CO2-surfactant to collaboratively develop low-mature shale oil, aiming to improve the recovery rate of such resources. First, double horizontal wells are arranged and the medium-mature shale oil reservoir is volume fractured by supercritical CO2 foam method to form a fracture network; then, nano-SiO2 aerogel is injected as a plugging agent to plug the fractures and transfer heat, so that kerogen in the reservoir is pyrolyzed to generate mobile hydrocarbons; then, nano-SiO2 particles as surfactants are injected to further improve the recovery rate and complete CO2 storage. The present invention ensures the efficient implementation of CO2 circulation and oil and gas separation process by monitoring the pressure, displacement and temperature of the reflux CO2 gas.

[0008] Specifically, the in-situ conversion method of low-mature shale oil injected with high-temperature supercritical CO2-nanoparticle system proposed in the present invention comprises the following steps: 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 classification 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 and 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 pressure, displacement and temperature of the reflux CO2 gas to ensure stable circulation.

[0009] Furthermore, in the in-situ conversion method of low to medium maturity shale oil by injecting high-temperature supercritical CO2-nanoparticle system, in step 1, the vertical distance between the horizontal sections of the injection well and the production well is controlled to be between 25m and 30m, and the length of the horizontal sections of the injection well and the production well is controlled to be between 700m and 1500m according to the actual length of the low to medium maturity shale oil reservoir, so that the low to medium 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 low to medium maturity shale oil reservoir, ensuring that the horizontal sections of the injection well and the production well evenly and completely penetrate the low to medium maturity shale oil reservoir, and are horizontally developed along the direction of the principal stress.

[0010] Furthermore, in step 2, supercritical CO2 foam fracturing is used to perform volumetric fracturing on the low-middle-maturity shale oil reservoirs. The pumping pressure reaches between 30Mpa and 50Mpa, which can effectively generate a fracture network in conventional low-middle-maturity shale oil reservoirs. As the cracks are generated, the pumping pressure decreases and eventually maintains a flow pressure of 20Mpa.

[0011] Furthermore, in step 2, supercritical CO2 foam fracturing is used to perform volume fracturing on the medium- and low-maturity shale oil reservoirs. After the fracturing is completed, the well is shut down for 1.5 days to 3 days to expand the affected area.

[0012] Furthermore, 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.

[0013] Further, in step 3, the nano-SiO2 aerogel configuration method is as follows: nano-SiO2 aerogel silicon source powder with a particle size of 40nm~50nm is purchased on the market, and dispersed in a solvent, and hydrolyzed and catalyzed with an inorganic acid or 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. The solvent is selected from alcohols or ketones, including but not limited to methanol, isobutyl ketone, and acetone; the inorganic acid is hydrochloric acid or hydrofluoric acid, and the inorganic base is ammonia water. Considering that the local pressure in the development process will exceed the pump flow pressure, the brittle damage of the gel solid structure will be caused, and the gel blocking effect will be reduced. SiC whiskers with a weight percentage of 4wt% are added to the nano-SiO2 aerogel silicon source powder to improve the strength of the finished gel.

[0014] Furthermore, the in-situ conversion method of low-middle-mature shale oil by injecting high-temperature supercritical CO2-nanoparticle system mainly starts the production stage by injecting 400°C~450°C high-temperature supercritical CO2 into the low-middle-mature shale oil reservoir after the reservoir transformation. 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 the recovery operation is carried out. The production pressure is controlled at 12Mpa in this stage.

[0015] Furthermore, in the in-situ conversion method of low- to medium-maturity shale oil by injecting high-temperature supercritical CO2-nanoparticle system, in step 6, nano-SiO2 particles with a particle size of 10nm~15nm are added to liquid CO2 to prepare a dispersion, and the amount of nano-SiO2 particles added to the dispersion is 3wt%~4wt%.

[0016] Compared with the prior art, the method proposed by the present invention has the following advantages: 1. The present invention adopts the in-situ conversion method of injecting high-temperature supercritical CO2-nanoparticle system into medium- and low-maturity shale oil to develop and recover medium- and low-maturity shale oil. Supercritical CO2 is a water-free phase of fracturing fluid and will not expand with the hydration of clay in shale. At the same time, supercritical CO2 reduces the fracture pressure during the fracturing process due to its high permeability, thereby improving economic benefits.

[0017] 2. The present invention adopts the in-situ conversion method of injecting high-temperature supercritical CO2-nanoparticle system into medium-low-maturity shale oil to develop and recover medium-low-maturity shale oil. The medium-low-maturity shale oil reservoir is heated by high-temperature supercritical CO2 to promote the occurrence of kerogen pyrolysis. At the same time, supercritical CO2 can replace and extract the dissolved hydrocarbons, and can act deeply in the pores and roars, greatly improving the oil and gas yield.

[0018] 3. The present invention adopts a double horizontal well layout method, which reduces the impact of shale horizontal heterogeneity on recovery to a certain extent, increases the scope of high-temperature supercritical CO2, and uses gravity-assisted oil recovery to improve recovery rate.

[0019] 4. The present invention performs cementation treatment and dispersion treatment on nano-SiO2 particles of different particle sizes respectively.

[0020] Cementation treatment: Disperse the commercially available nano-SiO2 aerogel silicon source powder with a particle size of 40nm~50nm in a solvent, and hydrolyze and catalyze under the conditions of inorganic acid or inorganic base. The hydroxyl groups on the molecules aggregate with each other through polycondensation and aging to form a nano-porous network structure gel, and form a nano-SiO2 particle aerogel with a particle size of 40nm~50nm after drying and dehydration. The aerogel maintains a porous network structure, making it difficult for the gas inside and on both sides of the pores to flow, and has a good blocking effect, which can effectively increase the gas sweep range during the displacement process.

[0021] Dispersion treatment: Nano-SiO2 particles with a particle size of 10nm~15nm are added to liquid CO2 to form a dispersion. The nano-particles in the dispersion penetrate into the organic matter, destroy the entanglement between large molecular hydrocarbons, heavy viscous oil and asphaltene molecular chains, and play the role of surfactant to wrap the oil droplets, promote miscibility, and effectively reduce surface tension.

[0022] Nano-SiO2 particles are selected as plugging agents and surfactants, and their thermal stability, chemical stability, formation affinity and cheapness are also utilized. Conventional gels and surfactants will be inactivated at 400℃~450℃; while more efficient metal oxide nanoparticles are not only more expensive, making the development cost too high, but also pollute the formation. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 Flow chart of the in-situ conversion method for low- to medium-maturity shale oil by injecting high-temperature supercritical CO2-nanoparticle system.

[0024] Figure 2 It is a schematic diagram of the dual horizontal well development mode of the present invention.

[0025] Figure 3 This is a graph showing the relationship between harvest volume and time under different action conditions.

[0026] Figure 4 This is a relationship diagram between construction displacement, production pressure and time.

[0027] In the figure: 1-injection well; 2-production well; 3-injection and production process pipe; 4-high-pressure pump injection system; 5-fracture network. DETAILED DESCRIPTION

[0028] In order to make the purpose, features and advantages of the present invention more obvious and easy to understand, the following is a detailed description of the present invention in combination with the embodiments of the present invention. Figure 1 , Figure 2 , Figure 3 and Figure 4 The technical solution in the present invention is clearly and completely described. Obviously, the present invention is not limited by the following embodiments, and the specific implementation method can be determined according to the technical solution of the present invention and the actual situation. In order to avoid confusing the essence of the present invention, well-known methods, processes, procedures, components and circuits are not described in detail.

[0029] Example 1: The in-situ conversion method of low-mature shale oil by injecting high-temperature supercritical CO2-nanoparticle system comprises the following steps: The medium-low maturity shale oil reservoir with a depth of 800m~1000m is selected as the development target area. Two horizontal wells are arranged according to the conditions of the medium-low maturity shale oil reservoir, namely injection well 1 and production well 2. The vertical wellbore of production well 2 is developed to 3m~5m below 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 at 25m~30m. The length of the horizontal well section is controlled at 700m~1500m according to the actual length of the medium-low maturity shale oil reservoir. 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 follows Figure 2 shown.

[0030] (2) Circulating well-washing fluid is introduced to wash the injection well 1 and the production well 2, and the injection-production process pipe 3 is lowered into the vertical well sections of the injection well 1 and the production well 2 respectively.

[0031] (3) Liquid CO2 is transported to the surface wellhead by a high-pressure pumping system 4, which is connected to pipelines, heating equipment and pressurizing equipment to ensure that CO2 is in a supercritical state during the injection and fracturing construction process. Subsequently, supercritical CO2 foam fracturing is used in the injection well 1 to perform volume fracturing on the low-maturity shale oil reservoir, thereby forming a fracture network 5 in the low-maturity shale oil reservoir. The supercritical CO2 foam fracturing method belongs to the prior art. It should be noted that in the present invention, the supercritical CO2 start-up displacement is set to 20 t / d. After 5 days of injection, the well is shut down for 25 days. During this period, the production pressure increases during the injection period, and the production pressure is stable during the shut-down period. This is repeated 6 times until the production pressure reaches 20 MPa. The supercritical CO2 foam fracturing method is used to perform volume fracturing on the low-maturity shale oil reservoir. After the fracturing is completed, the well is shut down for 1.5 days to 3 days to expand the affected area.

[0032] (4) Injecting nano-SiO2 aerogel plugs. The nano-SiO2 aerogel configuration process is as follows: nano-SiO2 aerogel silicon source powder with a particle size of 40nm~50nm is purchased from the market and dispersed in a solvent. The gel is hydrolyzed and catalyzed using an inorganic acid or an inorganic base. The solvent is selected from alcohols or ketones, including but not limited to methanol, isobutyl ketone, and acetone; the inorganic acid is hydrochloric acid or hydrofluoric acid, and the inorganic base is ammonia water. The obtained gel is preliminarily dried to remove solvent and catalyst impurities to obtain nano-SiO2 aerogel. Considering that the local pressure during the development process will exceed the pump flow pressure, the brittle damage of the gel solid structure will be caused, reducing the gel blocking effect. SiC whiskers with a weight percentage of 4wt% are added to the nano-SiO2 aerogel silicon source powder to improve the strength of the finished gel. The obtained nano-SiO2 aerogel is mixed into high-temperature supercritical CO2 at 400℃~450℃ to prepare a nano-SiO2 aerogel plug. The nano-SiO2 aerogel is allowed to fully and evenly enter the reservoir fracture network 5, and CO2 is further circulated to dry and age the nano-SiO2 aerogel, thereby completing the plugging operation.

[0033] (5) After the reservoir transformation is completed, supercritical CO2 heated to 400℃~450℃ is introduced into the medium-mature shale oil reservoir. The kerogen is pyrolyzed at high temperature to produce movable oil and gas. The mixture of oil, gas and CO2 is sent to the ground through production well 2 to complete the recovery, and is separated by a ground classification device. The separated CO2 is then reheated to complete the CO2 cycle.

[0034] Specifically, by injecting 400℃~450℃ high-temperature supercritical CO2 into the medium- and low-maturity shale oil reservoir after reservoir transformation, the production stage begins. As CO2 displaces 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 the recovery operation is carried out. The production pressure is controlled at 12Mpa in this stage, which can not only ensure the high efficiency of the mixed phase pv value, but also avoid the waste of supercritical CO2. The displacement curve of this process is shown in Figure 4 .

[0035] (6) When oil and gas production reaches Figure 3 At node b, nano-SiO2 particles with a particle size of 10nm-15nm are dispersed in liquid CO2 to prepare a dispersion, and the dispersion is added to high-temperature supercritical CO2 at 400℃-450℃ to prepare a high-temperature supercritical CO2 plug with added surfactant and injected into injection well 1. The particle size of the nano-SiO2 particles is 10nm-15nm, and the addition amount is 3wt%-4wt%, so as to ensure that the mixed phase is fully reduced in viscosity under the action of the surfactant of the nano-SiO2 particles, and further improve the recovery factor.

[0036] (7) Establish a detection and processing station on the ground, equipped with circulation separation facilities and heating and pressurizing re-injection devices. While circulating and displacing CO2, establish a monitoring station on the ground and lower a telemetry cable into the injection and production process pipe 3 located in the production well to monitor the pressure, displacement and temperature of the reflux CO2 gas. Ensure construction safety and seal and dispose of CO2 after development. In particular, the displacement monitoring device and collector in the telemetry cable should be processed with corrosion-resistant and high-temperature resistant materials.

[0037] Figure 2 The schematic diagram of the dual horizontal well development mode of the present invention is shown, Ⅰ represents the effective area of ​​the 400℃~450℃ high temperature supercritical CO2 segment plug with the addition of surfactant, and the nano-SiO2 particles with a particle size of 10nm~15nm are used as surfactants; Ⅱ represents the effective area of ​​the 400℃~450℃ high temperature supercritical CO2; Ⅲ represents the effective area of ​​the SiO2 nano-aerogel segment plug; Figure 3 The relationship between the recovery volume and time under different action conditions is shown. a represents the reduction of the affected area due to gas channeling; b represents the addition of a surfactant to the dispersion of nano-SiO2 particles with a particle size of 10nm~15nm; Figure 4 The relationship between the construction displacement, production pressure and time is shown, and α represents the beginning of the recovery stage.

[0038] Comparative Example 1: The method is the same as that described in Example 1, except that step (6) is not performed, that is, a dispersion of nano-SiO2 particles with a particle size of 10 nm to 15 nm dispersed in liquid CO2 is not added as a surfactant.

[0039] Comparative Example 2: The method is the same as that described in Example 1, except that step (4) is not performed, that is, the supercritical CO2 plug mixed with nano-SiO2 aerogel is not added for plugging.

[0040] Comparative Example 3: The method is the same as that described in Example 1, except that in step (5), high temperature supercritical CO2 at 400°C to 450°C is not used for displacement, but nitrogen is used for displacement.

[0041] Compared with Comparative Example 1, Example 1 adds nanoparticle surfactants in the later stage of flooding, thereby improving the CO2 miscibility, enhancing the supercritical CO2 flooding effect, and increasing the degree of oil and gas resource recovery.

[0042] Compared with comparative example 2, in example 1, aerogel was added to seal the reservoir before flooding, and supercritical CO2 entered the pore roar channel to reduce gas channeling in the main fracture. The supercritical CO2 sweep range was increased, and the recovery efficiency was higher throughout the development stage.

[0043] Compared with Comparative Example 3, Example 1 selected supercritical CO2 as the displacement gas. Compared with nitrogen, it not only has a lower mixed phase pressure and a more significant viscosity reduction effect, but is also more capable of extracting and adsorbing dissolved hydrocarbons in organic matter, with significant results.

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 classification 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 low-mature shale oil reservoirs, and the well is shut down for 1.5 days to 3 days after fracturing.

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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