An in-situ oxidation heat extraction system and method for oil shale

Abstract

The present invention discloses an in-situ oxidation heat extraction system and method for oil shale, belonging to the technical field of in-situ development and utilization of oil shale; it includes a well pattern structure and a ground power generation device; the well pattern structure includes injection wells, heat extraction wells and multiple observation wells; an electromagnetic sensor is arranged in each observation well, and the oil shale layer is fractured by hydraulic fracturing to form fractures, so that the injection wells and the heat extraction wells are connected, and nano-magnetic induction particles are added as proppants in the fracturing fluid; the observation wells are used to monitor the distribution of nano-magnetic induction particles and the magnetic field intensity; the present invention directly oxidizes and releases heat from the organic matter and fixed carbon in the oil shale in the in-situ state underground, and uses low-temperature steam as the heat-carrying fluid to extract the heat energy released by the oxidation of the oil shale for power generation, and can be used for the development of oil shale and medium- and low-maturity shale oil and gas resources.

Description

Technical Field

[0001] The present invention belongs to the technical field of in-situ development and utilization of oil shale, and relates to an in-situ oxidation heat extraction system and method for oil shale. Background Art

[0002] Oil shale is a fine-grained sedimentary rock containing solid organic matter (kerogen) and has high energy potential. Currently, the development of oil shale mainly relies on open-pit mining and surface retorting technologies. However, the existing mining modes have many drawbacks and are limited by the mining depth, mainly focusing on oil shale resources with relatively shallow burial depths, and deep oil shale resources have not been effectively utilized. To solve this problem, scholars at home and abroad have proposed various in-situ mining technologies, but so far, large-scale industrial applications have not been achieved. The main reason is that the energy density of oil shale is relatively low, and its calorific value is only about 30% of that of standard coal. In addition, most of the organic matter in oil shale exists in solid form, and the dense rock matrix mainly composed of clay minerals has strong sealing properties for its organic matter, and the permeability of the solid matrix is extremely low. Even if there is a small amount of liquid or gaseous organic matter, it is difficult to migrate and discharge. This makes it difficult for many in-situ mining methods to achieve ideal economic benefits.

[0003] During the exploitation of oil and gas, the degree of reservoir cracking and the oxidation heat release process are important factors affecting the exploitation efficiency and safety. Traditional reservoir monitoring methods mainly rely on means such as seismic exploration, electromagnetic measurement, and pressure monitoring, but these methods have certain limitations in terms of resolution, real-time monitoring ability, etc. In recent years, nano-magnetic sensing particles, as a new type of functional material, have shown great potential in the field of reservoir monitoring. Nano-magnetic sensing particles have unique physical and chemical properties, such as high specific surface area, adjustable magnetism, and excellent biocompatibility, making them show superior performance in complex geological environments. After injecting nano-magnetic sensing particles into the reservoir, they can migrate with the crack propagation and fluid flow, and under the action of an external magnetic field, the particles generate response signals. By monitoring the changes in these signals, the degree of reservoir cracking can be reflected in real time.

[0004] In addition, during the process of oil and gas exploration and production, the temperature changes within the reservoir are crucial for optimizing the production plan and enhancing the recovery rate. However, traditional temperature detection methods are often restricted by the complex downhole environment and it is difficult to accurately and real-time monitor the temperature changes in the reservoir. The development of nanotechnology provides a new solution to this problem. Due to their unique magnetic properties and high sensitivity, nano-magnetic sensing particles have become ideal temperature monitoring tools. When the temperature changes, the magnetic field intensity of the nano-magnetic sensing particles will change significantly. By monitoring the change in magnetic field intensity, the temperature changes inside the reservoir can be indirectly reflected, thereby achieving precise monitoring of temperature changes. This technology not only improves the accuracy and real-time performance of temperature monitoring but also overcomes the limitations of traditional methods in complex downhole environments. Compared with the traditional PIV particle technology, nano-magnetic sensing particles have shown significant advantages in monitoring the degree of reservoir fracture cracking and temperature changes. Nano-magnetic sensing particles are more sensitive to temperature changes and can monitor the temperature changes inside the fractures in real-time; at the same time, nano-magnetic sensing particles can be continuously and real-time monitored through magnetic field detection technology to provide a stable data stream, while PIV technology usually requires intermittent capture of flow images; due to their small size, nano-magnetic sensing particles can enter smaller fractures and pores, thus achieving a higher spatial resolution; in addition, magnetic field detection technology can penetrate a certain depth of rock to monitor the degree of cracking of deeper fractures; in high-temperature and high-pressure environments, nano-magnetic sensing particles can maintain stable performance and are suitable for long-term monitoring tasks. Therefore, nano-magnetic sensing particles have important technical advantages in reservoir monitoring and can provide new technical means for improving the efficiency and safety of oil and gas production.

[0005] In the currently published patents, the patent with publication number CN109736762A proposes a method for extracting shale oil and gas by in-situ catalytic oxidation of oil shale; the patent with publication number CN10477581A proposes a vacuum spiral tube type nitrogen heater for in-situ underground conversion of oil shale; the patent with publication number CN109444020A proposes an observation device and observation method for microscopic seepage characteristics of rock mass fractures based on 3D printing technology.

[0006] However, the patent with publication number CN109736762A focuses on reducing the heating temperature and enhancing the catalytic effect, which is suitable for improving production efficiency and energy utilization rate; the patent with publication number CN10477581A uses nitrogen heating and directly heats the injected nitrogen through a downhole combustion heat exchange system, focusing on nitrogen heating and downhole monitoring, and is more focused on reducing heat loss and increasing oil recovery rate, suitable for developing deep and high-thickness oil shale with low cost and less land occupation; the patent with publication number CN109444020A is composed of a 3D printed transparent resin specimen, a servo motor controller, a flowmeter, a solenoid valve, and a control main board, etc., and is used for observing seepage characteristics under laboratory conditions. Summary of the Invention

[0007] The present invention overcomes the deficiencies of the prior art and provides an in-situ oxidation heat extraction system and method for oil shale. The present invention abandons the inherent mode of positioning the products as oil and gas in the existing oil shale development and utilization technologies, directly oxidizes the organic matter and fixed carbon in the oil shale to release heat in the in-situ state underground, and uses low-temperature steam as the heat-carrying fluid to extract the heat energy released by the oxidation of the oil shale for power generation. It can be used as a strategic reserve technology for the development of oil shale and medium- and low-maturity shale oil and gas resources.

[0008] To achieve the above object, the present invention is realized through the following technical solutions:

[0009] An in-situ oxidation heat extraction system for oil shale includes a well network structure and a ground power generation device; the well network structure includes injection wells, heat extraction wells and multiple observation wells; the depths of the injection wells and the heat extraction wells penetrate the oil shale layer, and the depth of the observation wells is determined according to the vertical distance from the wellhead of the observation well to the roof of the oil shale layer;

[0010] An oxygen injection device, a combustible gas injection device and an injection well temperature and pressure monitoring device are connected to the wellhead of the injection well; a heat extraction well temperature and pressure monitoring device is connected to the wellhead of the heat extraction well; electromagnetic sensors are arranged in each observation well, the lower part of the electromagnetic sensor is in contact with the roof of the oil shale layer, and the upper part of the electromagnetic sensor is connected to a high-sensitivity geomagnetic exploration device; the number of electromagnetic sensors arranged in each observation well ≥ 1; hydraulic fracturing is used to fracture the oil shale layer to form fractures to connect the injection well and the heat extraction well, and nano-magnetic induction particles are added as a proppant in the fracturing fluid; the observation wells are used to monitor the distribution of the nano-magnetic induction particles and the magnetic field intensity;

[0011] The ground power generation device includes a binary working fluid heat exchanger, a flash evaporator and a generator; the binary working fluid heat exchanger is connected to the flash evaporator through a superheated steam transmission pipeline, the turbine on the flash evaporator drives the generator to generate electricity, the electricity generated by the generator is transformed by a transformer and then incorporated into the national power grid, and the low-temperature and low-pressure condensate after power generation re-enters the binary working fluid heat exchanger through the post-generation steam condensate recovery pipeline. The binary working fluid heat exchanger is connected to the injection well through a low-temperature water reinjection pipeline, and the binary working fluid heat exchanger is connected to the heat extraction well through a heat extraction pipeline.

[0012] Furthermore, the layout spacing between the injection well and the heat extraction well is 50 - 100 meters.

[0013] Furthermore, a downhole igniter control device is connected to the wellhead of the injection well.

[0014] Furthermore, an oxygen injection control valve is connected to the injection port of the oxygen injection device, and a combustible gas injection control valve is connected to the injection port of the combustible gas injection device.

[0015] Furthermore, a low-temperature water injection pump and a check valve are provided on the low-temperature water reinjection pipeline; a hot water circulation pump is connected to the steam condensate recovery pipeline after power generation.

[0016] Furthermore, electromagnetic sensors are evenly distributed at different depths and lateral positions of the observation well for comprehensively monitoring the cracking conditions of the oil shale formation.

[0017] An in-situ oxidation heat extraction method for oil shale, based on the described in-situ oxidation heat extraction system for oil shale, and includes the following steps:

[0018] S1. Wellbore arrangement and connection to the oil shale formation

[0019] S1.1. Well pattern design: Design the described well pattern structure according to the geological structure of the oil shale formation;

[0020] S1.2. Wellbore construction: Using the directional drilling method, arrange injection wells, heat extraction wells and observation wells on the oil shale formation;

[0021] S1.3. Establish a fracture network in the oil shale formation by hydraulic fracturing: Add nano-magnetic induction particles as proppants to the fracturing fluid; Fracture the oil shale formation through the fracturing fluid to form fractures, so that the injection well and the heat extraction well are connected;

[0022] Before the fracturing operation, observe the internal image of the oil shale formation through a high-sensitivity geomagnetic exploration device, and find that the concentration of nano-magnetic induction particles is 0 and the image does not show any information; As the fracturing operation progresses step by step, the concentration of nano-magnetic induction particles inside the oil shale formation gradually increases, and the high-sensitivity geomagnetic exploration device begins to show a preliminary image; When at least one fracture image running through between the injection well and the heat extraction well is observed, it is judged that the fracturing operation has successfully reached the predetermined target, and the fracture has been formed and has effective connectivity;

[0023] S2. Preheating of the oil shale formation and oxygen injection

[0024] Inject a mixed gas of combustible gas and oxygen into the oil shale formation through the injection well until the temperature of the oil shale formation near the injection well reaches the ignition point of 350°C; And ignite at the bottom of the injection well to start preheating the oil shale formation; At this time, stop injecting the combustible gas and switch to injecting pure oxygen;

[0025] Whether the ignition point is reached is detected by a high-sensitivity geomagnetic exploration device at the upper part of the observation well to detect the magnetic field intensity inside the oil shale formation, and judged through the attenuation curve and attenuation formula of the magnetic field intensity with temperature;

[0026] The attenuation formula is:

[0027] In the formula, is Magnetic field strength at a temperature; is the initial magnetic induction intensity at the reference temperature; is the temperature; is the characteristic temperature constant;

[0028] S3. Low-temperature water injection and heat energy collection

[0029] S3.1 Low-temperature water injection: After the oil shale layer is oxidized and heated, low-temperature water below 50°C is injected through the injection well. During the flow of water in the oil shale layer, heat exchange occurs with the heat of the oil shale layer and is converted into high-temperature steam;

[0030] S3.2 Steam collection: During the in-situ oxidation process of oil shale, the generated high-temperature steam flows through the fracture network in the oil shale layer to the heat extraction well, and the high-temperature steam is collected to the ground through the outlet of the heat extraction well; A dual-fluid heat exchanger is used for heat exchange to release the heat of the high-temperature steam to drive the generator to generate electricity and convert it into electrical energy;

[0031] The cooled steam after heat energy recovery is injected into the wellbore of the injection well through the dual-fluid heat exchanger and then injected into the underground oil shale layer again.

[0032] The beneficial effects of the present invention compared with the prior art are as follows:

[0033] 1. High-efficiency energy utilization: The present invention releases heat energy through the in-situ oxidation process and uses a low-temperature steam circulation system to convert the heat released underground into electrical energy, significantly improving the energy recovery efficiency and reducing energy waste.

[0034] 2. Low cost and environmental protection: Compared with the traditional oil and gas pyrolysis extraction technology, the present invention does not require the extraction of oil and gas, the process is simple, avoids environmental damage, and significantly reduces the development cost, with strong economic efficiency and environmental friendliness.

[0035] 3. Wide applicability: The present invention is applicable to the development of deep oil shale resources, especially has obvious advantages in areas where open-pit mining and retorting methods are difficult to apply. In addition, this technology can also be applied to the development and utilization of medium- and low-maturity shale oil and gas resources, broadening its scope of application.

[0036] 4. Low carbon dioxide emissions: The in-situ oxidation process of the present invention produces a relatively high concentration of carbon dioxide, and the emissions are concentrated, which is convenient for subsequent treatment, can effectively reduce the carbon dioxide emissions, and meets the environmental protection requirements.

[0037] 5. Precise monitoring and optimized control: By introducing nano magnetic induction particles, the present invention can monitor the cracking situation inside the reservoir and the exothermic process in the oxidation reaction in real time, optimize the heat extraction effect, and improve the production efficiency. Description of the Drawings

[0038] Figure 1 This is a connection schematic diagram of an in-situ oxidation heat extraction system for oil shale of the present invention;

[0039] Figure 2 This is an arrangement diagram of injection wells, heat extraction wells and observation wells;

[0040] Figure 3 This is a decay curve diagram of magnetic field intensity with temperature;

[0041] In the figure: 1 - oil shale layer; 2 - injection well; 3 - heat extraction well; 4 - binary working fluid heat exchanger; 5 - combustible gas injection device; 6 - oxygen injection device; 7 - downhole igniter control device; 8 - injection well temperature and pressure monitoring device; 9 - low-temperature water injection pump; 10 - hot water circulation pump; 11 - heat extraction well temperature and pressure monitoring device; 12 - flash evaporator; 13 - turbine; 14 - generator; 15 - transformer; 16 - national power grid; 17 - superheated steam transmission pipeline; 18 - recovered pipeline for steam condensate after power generation; 19 - heat extraction pipeline; 20 - check valve; 21 - combustible gas injection control valve; 22 - oxygen injection control valve; 23 - low-temperature water reinjection pipeline; 24 - downhole igniter; 25 - injection well bottom temperature and pressure sensor; 26 - heat extraction well bottom temperature and pressure sensor; 27 - observation well; 28 - high-sensitivity geomagnetic exploration equipment. Specific embodiments

[0042] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present invention more clear and understandable, the present invention will be further described in detail in combination with embodiments and drawings. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention. The technical solutions of the present invention will be described in detail below in combination with embodiments and drawings, but the protection scope is not limited by this.

[0043] Embodiment 1

[0044] See Figure 1 and Figure 2 In this embodiment, an in-situ oxidation heat extraction system for oil shale is proposed; it includes a well network structure and a ground power generation device.

[0045] The well network structure includes injection wells 2, heat production wells 3 and multiple observation wells 27. The spacing between the injection wells 2 and the heat production wells 3 should be reasonably set according to the geological conditions of the oil shale layer 1, and the spacing is usually 50-100 meters. In this embodiment, the spacing between the injection wells 2 and the heat production wells 3 is 80 meters; the depths of the injection wells 2 and the heat production wells 3 should penetrate the oil shale layer 1 to ensure that the wellbore is completely connected to the oil shale layer 1 to achieve effective heat collection and gas injection. The depth of the observation well 27 is usually determined based on the vertical distance from the wellhead of the observation well 27 to the top plate of the oil shale layer 1, and its spatial orientation should avoid being arranged linearly with other wellbores to reduce interference between wellbores and improve the heat production effect.

[0046] The injection well 2 is used to inject oxygen, combustible gas and low-temperature water to stimulate the in-situ oxidation reaction of the oil shale layer; the wellhead of the injection well 2 is connected with an oxygen injection device 6, a combustible gas injection device 5, a downhole igniter control device 7 and an injection well temperature and pressure monitoring device 8; the injection port of the oxygen injection device 6 is connected with an oxygen injection control valve 22, and the injection port of the combustible gas injection device 5 is connected with a combustible gas injection control valve 21; the heat production well 3 is used to recover the heat generated by the in-situ oxidation process of the oil shale; the wellhead of the heat production well 3 is connected with a heat production well temperature and pressure monitoring device 11; the observation well 27 is used to monitor the distribution of nano-magnetic particles and the magnetic field strength, so as to reflect the cracking of the oil shale layer 1 and the heat release of the oxidation reaction. The injection well 2 and the heat production well 3 are connected through hydraulic fracturing technology. A downhole igniter 24 and a temperature and pressure sensor 25 at the bottom of the injection well 2 are arranged at the position where the bottom of the heat production well 3 is connected to the injection well 2, and a temperature and pressure sensor 26 at the bottom of the heat production well 3 is arranged at the position where the bottom of the heat production well 3 is connected to the injection well 2; an electromagnetic sensor is arranged in each observation well 27, the bottom of the electromagnetic sensor is in contact with the top plate of the oil shale layer 1, and the top of the electromagnetic sensor is connected to a high-sensitivity geomagnetic exploration device 28. The number of electromagnetic sensors arranged in each observation well 27 is ≥1; the electromagnetic sensors should be evenly distributed at different depths and lateral positions of the observation well 27 to ensure that the cracking of the oil shale layer 1 can be fully monitored; at the same time, the arrangement of the electromagnetic sensors should avoid areas that are prone to interference, such as the well wall, to ensure the accuracy and sensitivity of the signal.

[0047] The ground power generation device includes a dual-medium heat exchanger 4, a flash evaporator 12 and a generator 14. The flash evaporator 12 is arranged on the superheated water vapor delivery pipeline 17 connected to the dual-medium heat exchanger 4. The turbine 13 on the flash evaporator 12 drives the generator 14 to generate electricity. The electricity generated by the generator 14 is transformed by the transformer 15 and then connected to the national power grid 16. After the electricity is generated, the low-temperature and low-pressure condensed water re-enters the dual-medium heat exchanger 4 through the steam condensed water recovery pipeline 18 after power generation, and continues to receive the heat energy released by the oxidation of the oil shale layer 1.

[0048] The dual - working - fluid heat exchanger 4 is connected to the injection well 2 through the low - temperature water reinjection pipeline 23. A low - temperature water injection pump 9 and a check valve 20 are provided on the low - temperature water reinjection pipeline 23. The dual - working - fluid heat exchanger 4 is connected to the heat extraction well 3 through the heat extraction pipeline 19. A hot water circulation pump 10 is connected to the post - power - generation steam condensate recovery pipeline 18.

[0049] The described dual - working - fluid heat exchanger 4 is an existing device that uses the dual - working - fluid heat exchange technology to transfer the heat in the superheated steam to the working fluid. Through the high - efficiency heat transfer characteristics of the dual - working - fluid, the thermal energy conversion process is optimized. The design of the heat exchanger needs to ensure that the materials have high temperature resistance and corrosion resistance to adapt to the high - temperature and high - pressure working environment. The superheated steam is depressurized by the flash evaporator 12, causing part of the steam to be converted into low - temperature and low - pressure steam, and the released heat is used to further heat the working fluid. The design of the flash evaporator 12 requires precise control of the depressurization rate to ensure maximum heat recovery and energy conversion efficiency. The generator 14, as the final electric - energy conversion unit, should be designed with a capacity matching the output capacity of the dual - working - fluid heat exchanger 4 to maximize the electric - energy output. The working efficiency of the generator 14 is closely related to the quality of the steam, so it needs to be optimized according to specific system parameters during selection.

[0050] The temperature and pressure sensors at various locations can monitor the working conditions in the wellbore in real - time. Based on the real - time monitoring data, the injection amounts of oxygen and water can be automatically adjusted according to the monitoring data to ensure the stable operation and efficient heat extraction of the oil shale oxidation process. Specifically, when the temperature or pressure in the wellbore is abnormal, the system will automatically adjust the injection amounts of oxygen and water to ensure that the oxidation process proceeds under optimal working conditions.

[0051] Embodiment 2

[0052] This embodiment proposes an in - situ oxidation heat extraction method for oil shale, based on the in - situ oxidation heat extraction system described in Embodiment 1, and includes the following specific steps:

[0053] S1. Connect the wellbore to the oil shale layer 1

[0054] S1.1 Well - pattern design: According to the geological structure of the oil shale layer 1, design the well - pattern structure described.

[0055] S1.2 Wellbore construction: Using directional drilling technology, arrange the injection well 2, the heat extraction well 3, and the observation well 27 on the oil shale layer 1. The arrangement of the injection well 2 and the heat extraction well 3 needs to ensure that the wellbore penetrates the oil shale layer 1 and is accurately positioned.

[0056] According to the thickness and scope of the oil shale layer 1, the wellbore depths of the injection well 2 and the heat extraction well 3 are usually controlled within 300 - 1000 meters, while the depth of the observation well 27 is usually determined according to the depth from the wellhead to the roof of the oil shale layer 1. The accuracy of the wellbore depths should be ensured, and the drilling process should be tracked through a real-time monitoring system to avoid deviations.

[0057] During the above construction process, strict quality control measures should be implemented to ensure that the wellbore diameter, the smoothness of the wellbore wall, and the stability of the wellbore wall meet the design requirements, and to avoid damage to the wellbore or fluid leakage caused by unstable wellbore walls.

[0058] After the wellbore construction is completed, each wellbore should be tested to verify the fluid transmission performance, pressure response, and temperature distribution of the wellbore, ensure the normal function of each wellbore, conduct a water injection or gas injection test on the injection well 2 to test the injectability and injection efficiency of the oil shale layer 1; conduct temperature and flow rate tests on the heat extraction well 3 to ensure effective heat extraction; conduct pressure and temperature monitoring on the observation well 27 to grasp the state of the oil shale layer 1 in real time.

[0059] Then arrange the corresponding sensors and connect various devices and equipment.

[0060] S1.3. Hydraulic fracturing: Use hydraulic fracturing technology to establish a fracture network in the oil shale layer 1. Through the action of the fracturing fluid, the oil shale layer 1 is fractured to form fractures to ensure the connectivity between the injection well 2 and the heat extraction well 3. The formation of the fracture network helps the fluid to flow in the oil shale layer 1 and enhances the permeability of the oil shale layer 1. Add nano-magnetic particles as a proppant to the fracturing fluid, which can not only enhance the stability of the fractures and keep the fractures open, but also improve the hydrodynamic performance during the fracturing process. Nano-magnetic particles are existing materials, such as magnetite (Fe3O4) nanoparticles or iron oxide (such as Fe2O3)-based nanoparticles can be selected.

[0061] Before the above-mentioned hydraulic fracturing, a detailed geological exploration of the oil shale layer 1 must be carried out to evaluate the lithology, porosity, permeability, and fracture distribution of the oil shale layer 1. The selection of the fracturing fluid should be based on the characteristics of the oil shale layer 1. The fracturing fluid should have sufficient viscosity and fluidity to facilitate the injection of the liquid into the oil shale layer 1 and form fractures. Specifically, if the permeability of the oil shale layer 1 is low, a fracturing fluid with a higher viscosity needs to be selected; if there are certain natural fractures in the oil shale layer 1, a fracturing fluid with a lower viscosity can be selected.

[0062] During hydraulic fracturing, the pressure and flow rate of the injected liquid should be precisely controlled according to the specific conditions of the oil shale layer 1. Excessive injection pressure may cause the wellbore wall to rupture, while too low pressure may not effectively open the fractures. In the initial stage, the injection pressure should be gradually increased until the predetermined fracture opening pressure is reached. At the same time, when controlling the injection pressure, the stress state of the oil shale layer 1 needs to be concerned to avoid damage to the oil shale layer 1 caused by over-fracturing.

[0063] During the injection process of the fracturing fluid, it should be ensured that the fracturing fluid can be evenly distributed throughout the entire oil shale layer 1 to make the fracture network show good connectivity. Through multi-stage injection and multiple fracturing, the fracture network can be gradually expanded to ensure sufficient connectivity between the injection well 2 and the heat extraction well 3.

[0064] The addition of the described nano-magnetic induction particles enables the proppant to form a finer network structure in the fractures, thereby optimizing the connectivity of the fractures and improving the permeability of the oil shale layer 1. In addition, the nano-magnetic induction particles can also provide real-time fracture change data through magnetic field monitoring technology, which helps to accurately evaluate the fracturing effect subsequently.

[0065] Before the fracturing operation, the internal image of the oil shale layer 1 was observed through the high-sensitivity geomagnetic exploration equipment 28, and it was found that the concentration of nano-magnetic induction particles was 0 and the image did not show any information. As the fracturing operation progresses gradually, the concentration of nano-magnetic induction particles inside the oil shale layer 1 gradually increases, and the high-sensitivity geomagnetic exploration equipment 28 begins to show a preliminary image. When at least one fracture image passing through between the injection well 2 and the heat extraction well 3 is observed, it can be judged that the fracturing operation has successfully reached the predetermined goal, and the fractures have been formed and have effective connectivity.

[0066] S2. Preheating of the oil shale layer 1 and oxygen injection

[0067] Inject a mixture of combustible gas and oxygen into the oil shale layer 1 through the injection well 2 until the temperature of the oil shale layer 1 near the injection well 2 reaches the ignition point of 350°C; and ignite it at the bottom of the injection well 2 to start preheating the oil shale layer 1. At this time, stop injecting the combustible gas and switch to injecting pure oxygen.

[0068] Whether the ignition point is reached is detected by observing the magnetic field intensity inside the oil shale layer 1 through the high-sensitivity geomagnetic exploration equipment 28 at the upper part of the observation well 27, and by Figure 3 judging according to the shown magnetic field intensity decay curve and decay formula with temperature;

[0069] The decay formula is:

[0070] In the formula, is the magnetic field intensity at temperature; is the initial magnetic induction intensity ( when it is at the reference temperature); is the temperature; is a characteristic temperature constant that reflects the rate of magnetic field strength decay.

[0071] The mixing ratio of the injected combustible gas and oxygen needs to be precisely adjusted according to the properties of the oil shale layer 1, temperature, and heat extraction requirements to ensure that the combustible gas can burn stably and achieve the expected temperature increase effect. During this process, it is necessary to continuously monitor the gas injection pressure and flow rate in the well to avoid uneven gas injection or too rapid temperature rise of the oil shale layer 1 due to too large or too small flow rate. When the temperature of the oil shale layer 1 reaches 350 °C, stop injecting the combustible gas and switch to injecting pure oxygen. At this time, it is necessary to precisely control the injection flow rate and pressure of pure oxygen to ensure that the temperature of the oil shale layer 1 further increases and remains stable. During the gas injection process, it is necessary to conduct real-time temperature monitoring through the downhole temperature and pressure sensor 25 at the bottom of the injection well to ensure uniform temperature distribution in the oil shale layer 1 and avoid local overheating or temperature fluctuations. If the temperature is too high or too low, the gas injection parameters should be adjusted in a timely manner to ensure the safety and effectiveness of the operation.

[0072] During the entire preheating process of the oil shale layer 1, strict safety monitoring measures need to be equipped, especially in key links such as gas injection, ignition, and temperature monitoring. The injected mixed gas and pure oxygen have certain flammability and explosion risks, so it is necessary to strengthen gas leakage monitoring and fire prevention measures to ensure that the operating environment meets safety requirements and prevent the occurrence of fire or explosion accidents.

[0073] Monitor the decay of the magnetic field strength inside the oil shale layer 1 through the high-sensitivity geomagnetic exploration equipment 28 above the observation well 27, and detect the decay degree of the magnetic field strength generated by the nano-magnetic induction particles inside the oil shale layer 1 with temperature change. The relationship between the magnetic field strength and the temperature of the oil shale layer 1 is calculated and analyzed in real time according to the preset magnetic field strength decay curve and decay formula to precisely monitor the temperature change of the oil shale layer 1.

[0074] Inject pure oxygen into the oil shale layer 1 through the injection well 2. The oxygen reacts with the organic matter and fixed carbon in the oil shale layer 1 to release a large amount of heat energy. The oxidation reaction gradually spreads around the wellbore, forming a high-temperature porous rock mass structure, thus providing ideal conditions for subsequent heat energy collection. The injection flow rate and pressure of pure oxygen need to be precisely adjusted to ensure that the oxygen can fully react with the organic matter in the oil shale layer 1 while avoiding damage to the structure of the oil shale layer 1 or excessive oxidation.

[0075] During the pure oxygen injection process, the injection pressure and flow rate need to be precisely controlled. Excessive injection pressure may cause the oil shale layer 1 to rupture, while too low pressure may not be able to drive the reactants of oxygen and the oil shale layer 1 into sufficient contact; the uniform distribution of pure oxygen is also crucial to ensure the stable progress of the oxidation reaction. Multiple injection wells 2 should be designed according to the distribution of the oil shale layer 1 to ensure that oxygen can effectively spread to the entire area of the oil shale layer 1.

[0076] During the pure oxygen injection process, it is necessary to monitor the temperature of the oil shale layer 1 and the progress of the oxidation reaction in real time through the high-precision geomagnetic exploration equipment 28 above the observation well 27. Too high or too low temperature will affect the efficiency of the oxidation reaction, so it is necessary to ensure that the temperature is always maintained within the optimal range.

[0077] During the entire pure oxygen injection process, safety monitoring needs to be strengthened, especially the monitoring of gas leakage and temperature. Therefore, gas leakage detection equipment needs to be set up to prevent accidents; at the same time, the heat generated by the oxidation reaction needs to be kept within a safe range to avoid damage to the structure of the oil shale layer 1 caused by too high temperature.

[0078] By adjusting the injection rate and pressure of pure oxygen, ensure the stability of the oxidation reaction, prevent overheating in local areas or insufficient oxygen supply, so as to ensure the uniform release of heat in the oil shale layer 1 and maximize the energy utilization efficiency in the oil shale layer 1.

[0079] S3. Low-temperature water injection and heat energy collection

[0080] S3.1 Low-temperature water injection: After the oil shale layer 1 is oxidized and heated, inject low-temperature water (the temperature is usually controlled below 50°C). During the flow of water in the oil shale layer 1, it effectively exchanges heat with the heat of the oil shale layer 1 and is converted into high-temperature steam.

[0081] In the above process, it is required to precisely control the injection temperature of the low-temperature water to ensure that the water temperature does not exceed the set maximum temperature so as not to affect the thermal stability of the oil shale layer 1; the flow rate and rate of the injected water should be dynamically adjusted according to the temperature, permeability and oxidation process of the oil shale layer 1 to ensure that the water can flow evenly through the oil shale layer 1 and maximize the heat conversion efficiency.

[0082] The injected low-temperature water should fully exchange heat with the heat in the oil shale layer 1, so that the water gradually heats up and is converted into high-temperature water vapor during the flow; the uniformity of the water flow and the stability of the oil shale layer 1 are crucial. It is necessary to ensure that the water does not get blocked during the flow in the oil shale layer 1, and the pressure and temperature changes are within a safe range; in addition, the purity of the water must also meet certain standards to avoid the negative impact of impurities in the water on the permeability of the oil shale layer 1 and the heat extraction process.

[0083] S3.2, Vapor Collection: During the in-situ oxidation of oil shale, the generated high-temperature water vapor flows through the fracture network in the oil shale layer 1 towards the heat extraction well 3, and the high-temperature water vapor is collected to the ground through the outlet of the heat extraction well 3. To ensure the efficiency and stability of high-temperature water vapor collection, it is necessary to monitor the temperature and pressure of the high-temperature water vapor in real time.

[0084] During the flow of high-temperature water vapor, through the real-time data monitoring system, precisely control the temperature, pressure, and flow rate of the high-temperature water vapor to ensure that the high-temperature water vapor always remains in the best state to achieve the maximum benefit of heat energy collection.

[0085] S3.3, Heat Energy Recovery: Through the high-temperature water vapor generated during the in-situ oxidation of oil shale, a binary fluid heat exchanger 4 is used for heat exchange to release the heat of the high-temperature water vapor to drive the generator 14 to generate electricity and convert it into electrical energy. This system realizes the recycling of high-temperature water vapor.

[0086] During the heat extraction process of in-situ oxidation of oil shale, the cooled vapor after heat energy recovery is injected into the wellbore of the injection well 2 through the binary fluid heat exchanger 4 and then injected back into the underground oil shale layer 1 to utilize the heat released by the underground rock formation. This process can effectively reduce energy waste and further improve the utilization efficiency of heat energy.

[0087] S4, Wellbore Rotation and Repeated Heat Extraction

[0088] When the temperature of the high-temperature water vapor at the outlet of the heat extraction well 3 drops to a predetermined threshold, stop the current injection and heat extraction operations and perform the wellbore rotation operation. Specifically, convert the original injection well 2 into the heat extraction well 3, convert the original heat extraction well 3 into the injection well 2, and keep the observation well 27 unchanged, so as to continue the oxidation reaction and heat energy collection to ensure the continuity of heat energy collection and the stability of system operation. After the rotation operation, repeat the above process of exothermic oxidation and heat extraction by injecting low-temperature water until the organic matter and fixed carbon in the oil shale layer 1 are completely oxidized and all available heat energy is recovered.

[0089] S5, Completion of Heat Extraction and Wellbore Sealing

[0090] When all the organic matter and fixed carbon in the oil shale layer 1 are oxidized and all the recoverable heat energy in the oil shale layer 1 is fully recovered, stop the injection operations of oxygen and water, and then seal the injection well 2, the heat extraction well 3, and the observation well 27 in accordance with safety regulations to prevent wellbore collapse or pollutant leakage and ensure the safe sealing of the oil shale layer 1.

[0091] Specifically, by monitoring parameters such as the oxygen concentration, temperature change, and gas composition in the oil shale layer 1 in real time, it is confirmed that the oxidation of organic matter and fixed carbon has reached the predetermined standard, ensuring that the recoverable heat energy in the oil shale layer 1 is fully recovered. After the oxidation reaction is completed, the injection of oxygen and water is immediately stopped. The stop of gas injection and water injection should be precisely controlled to avoid the injection of excessive gas or moisture, preventing unnecessary energy loss and environmental impacts. After the gas injection and water injection are stopped, the pressure and temperature of the oil shale layer 1 may change. Therefore, it is necessary to monitor the changes in the pressure and temperature of the oil shale layer 1 in real time and take appropriate adjustment measures to ensure the stability of the oil shale layer 1.

[0092] The above content is a further detailed description of the present invention in combination with specific preferred embodiments. It cannot be determined that the specific embodiments of the present invention are limited thereto. For those of ordinary skill in the technical field to which the present invention pertains, without departing from the premise of the present invention, several simple deductions or substitutions can still be made, which should all be regarded as belonging to the patent protection scope determined by the claims submitted for the present invention.

Claims

1. An oil shale in-situ oxidation heat recovery system, characterized in that: It comprises a well network structure and a ground power generation device; the well network structure comprises an injection well (2), a heat production well (3) and a plurality of observation wells (27); the depths of the injection well (2) and the heat production well (3) penetrate the oil shale layer (1), and the depth of the observation well (27) is determined according to the vertical distance from the wellhead of the observation well (27) to the top plate of the oil shale layer (1); The wellhead of the injection well (2) is connected to an oxygen injection device (6), a combustible gas injection device (5) and an injection well temperature and pressure monitoring device (8); the wellhead of the heat production well (3) is connected to a heat production well temperature and pressure monitoring device (11); an electromagnetic sensor is arranged in each observation well (27), the lower part of the electromagnetic sensor is in contact with the top plate of the oil shale layer (1), and the upper part of the electromagnetic sensor is connected to a high-sensitivity geomagnetic exploration device (28); the number of electromagnetic sensors arranged in each observation well (27) is ≥1; the oil shale layer (1) is hydraulically fractured to form a crack, so that the injection well (2) and the heat production well (3) are connected, and nano-magnetic particles are added to the fracturing fluid as a proppant; the observation well (27) is used to monitor the distribution of the nano-magnetic particles and the magnetic field strength; The ground power generation device comprises a dual-medium heat exchanger (4), a flash evaporator (12) and a generator (14); the dual-medium heat exchanger (4) is connected to the flash evaporator (12) via a superheated steam delivery pipeline (17); the turbine (13) on the flash evaporator (12) drives the generator (14) to generate electricity; the electricity generated by the generator (14) is transformed by a transformer (15) and then connected to the national power grid (16); low-temperature and low-pressure condensed water after power generation re-enters the dual-medium heat exchanger (4) via a post-power generation steam condensed water recovery pipeline (18); the dual-medium heat exchanger (4) is connected to the injection well (2) via a low-temperature water reinjection pipeline (23); and the dual-medium heat exchanger (4) is connected to the heat recovery well (3) via a heat recovery pipeline (19); After the oil shale layer (1) is heated by oxidation, low-temperature water below 50° C. is injected through the injection well (2). The water exchanges heat with the oil shale layer (1) during its flow in the oil shale layer (1) and is converted into high-temperature water vapor. The generated high-temperature water vapor flows to the heat production well (3) through the fracture network in the oil shale layer (1), and the high-temperature water vapor is collected to the ground through the outlet of the heat production well (3). A dual-medium heat exchanger (4) is used for heat exchange to release the heat of the high-temperature water vapor to drive a generator (14) to generate electricity and convert it into electrical energy. The cooled steam after heat energy recovery is injected into the wellbore of the injection well (2) through the dual-medium heat exchanger (4) and is injected into the underground oil shale layer (1) again.

2. The oil shale in-situ oxidation heat recovery system according to claim 1, characterized in that: The injection well (2) and the heat production well (3) are arranged at a distance of 50-100 meters.

3. The oil shale in-situ oxidation heat recovery system according to claim 1, characterized in that: A downhole igniter control device (7) is connected to the wellhead of the injection well (2).

4. The oil shale in-situ oxidation heat recovery system according to claim 1, characterized in that: The injection port of the oxygen injection device (6) is connected to an oxygen injection control valve (22), and the injection port of the combustible gas injection device (5) is connected to a combustible gas injection control valve (21).

5. The oil shale in-situ oxidation heat recovery system according to claim 1, characterized in that: A low-temperature water injection pump (9) and a one-way valve (20) are provided on the low-temperature water reinjection pipeline (23); and a hot water circulation pump (10) is connected to the steam condensate water recovery pipeline (18) after power generation.

6. The oil shale in-situ oxidation heat recovery system according to claim 1, characterized in that: The electromagnetic sensors are evenly distributed at different depths and lateral positions of the observation well (27) for comprehensively monitoring the cracking conditions of the oil shale layer (1).

7. An oil shale in-situ oxidation heat recovery method, characterized in that: An oil shale in-situ oxidation heat recovery system based on any one of claims 1 to 6, comprising the following steps: S1. Wellbore arrangement connected to oil shale layer (1) S1.1, well pattern design: design the well pattern structure according to the geological structure of the oil shale layer (1); S1.2, wellbore construction: using directional drilling method, an injection well (2), a heat extraction well (3) and an observation well (27) are arranged on the oil shale layer (1); S1.3, using hydraulic fracturing to establish a fracture network in the oil shale layer (1): adding nano magnetic particles as proppants to the fracturing fluid; fracturing the oil shale layer (1) with the fracturing fluid to form fractures, so that the injection well (2) and the heat production well (3) are connected; Before the fracturing operation, the image inside the oil shale layer (1) is observed by using a high-sensitivity geomagnetic exploration device (28), and it is found that the concentration of nano-magnetic particles is 0, and the image does not show any information; as the fracturing operation gradually advances, the concentration of nano-magnetic particles inside the oil shale layer (1) gradually increases, and the high-sensitivity geomagnetic exploration device (28) begins to show a preliminary image; when at least one crack image running through the injection well (2) and the heat production well (3) is observed, it is determined that the fracturing operation has successfully achieved the predetermined goal, and the crack has been formed and has effective connectivity; S2. Oil shale layer (1) Preheating and oxygen injection A mixture of combustible gas and oxygen is injected into the oil shale layer (1) through an injection well (2) until the temperature of the oil shale layer (1) near the injection well (2) reaches an ignition point of 350°C; and the mixture is ignited at the bottom of the injection well (2) to start preheating the oil shale layer (1); at this time, the injection of combustible gas is stopped and replaced with injection of pure oxygen; Whether the ignition point has been reached is determined by detecting the magnetic field strength inside the oil shale layer (1) through a high-sensitivity geomagnetic exploration device (28) at the top of the observation well (27), and judging by the attenuation curve of the magnetic field strength with temperature and the attenuation formula; The attenuation formula is: In the formula, yes Magnetic field strength at temperature; yes is the initial magnetic induction intensity at the reference temperature; is the temperature; is the characteristic temperature constant; S3. Low temperature water injection and thermal energy collection S3.1, low-temperature water injection: after the oil shale layer (1) is heated by oxidation, low-temperature water below 50° C. is injected through the injection well (2). During the flow of water in the oil shale layer (1), the water exchanges heat with the oil shale layer (1) and is converted into high-temperature water vapor; S3.2, steam collection: during the in-situ oxidation process of oil shale, the generated high-temperature water vapor flows to the heat production well (3) through the fracture network in the oil shale layer (1), and the high-temperature water vapor is collected to the ground through the outlet of the heat production well (3); a dual-medium heat exchanger (4) is used for heat exchange, and the heat of the high-temperature water vapor is released to drive the generator (14) to generate electricity, thereby converting it into electrical energy; The cooled steam after heat energy recovery is injected into the wellbore of the injection well (2) through the dual-medium heat exchanger (4), and then injected into the underground oil shale layer (1) again.

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