A method for constructing an enhanced geothermal system for abandoned oil and gas wells

Abstract

This invention discloses a method for constructing an enhanced geothermal system for abandoned oil and gas wells, characterized by the following steps: logging and testing the target abandoned oil and gas well to assess wellbore integrity, reservoir parameters, and the type, content, and spatial distribution of residual alkyl compounds; injecting an igniter and oxygen-containing gas into the reservoir, followed by the injection of high-pressure nitrogen for ignition, thereby igniting the residual alkyl compounds to establish an initial combustion zone; injecting an oxidizer into the combustible gas enrichment zone, mixing it with the combustible gas therein to form an explosive mixture and detonating it; periodically or continuously injecting an oxidizer and water vapor alternately into the reservoir through the fracture network, continuously heating the reservoir through combustion and gasification reactions; stopping the injection of oxidizer, injecting a low-temperature working fluid into the artificial geothermal reservoir space through injection wells, and subsequently extracting it from production wells for thermal energy utilization; thus, this invention achieves in-situ energy upgrading and enrichment of abandoned resources.

Description

Technical Field

[0001] This invention relates to the field of energy development technology, and in particular to a method for constructing an enhanced geothermal system for abandoned oil and gas wells. Background Technology

[0002] Globally, there are a vast number of abandoned oil and gas wells. These wellbores serve as readily available deep underground channels, providing potential entry points for geothermal resource development. However, traditional geothermal utilization models for abandoned wells face two fundamental bottlenecks: First, insufficient reservoir heat source intensity. Due to long-term oil and gas extraction leading to fluid production and pressure decline, the original heat of the reservoir is lost, forming a "heat decay zone," resulting in small temperature differences and low efficiency for direct heat extraction. Second, low reservoir permeability. Many oil and gas reservoirs are located in low-porosity, low-permeability tight reservoirs. After extraction, compaction, and pressure depletion, permeability further deteriorates, leading to poor working fluid circulation and limited heat exchange area.

[0003] However, existing technologies such as acidizing and hydraulic fracturing mainly focus on improving the flow capacity of the near-wellbore zone in the short term, but cannot actively, on a large scale and sustainably enhance the heat source intensity of the reservoir.

[0004] Therefore, there is an urgent need for an innovative method that can fundamentally solve the two major problems of "cold reservoir" and "poor permeability" at the same time, and extract geothermal resources from abandoned wells. Summary of the Invention

[0005] The technical problem solved by this invention is to overcome the two interrelated fundamental technical bottlenecks of insufficient reservoir heat source intensity and low seepage capacity in the geothermal development of traditional abandoned wells, while realizing the resource utilization and harmless use of residual alkyl compounds in the well.

[0006] To solve the above-mentioned technical problems, the present invention provides the following technical solution:

[0007] A method for constructing an enhanced geothermal system for abandoned oil and gas wells includes the following steps:

[0008] S1. Logging and testing of the target abandoned oil and gas wells to assess wellbore integrity, reservoir parameters, and the type, content, and spatial distribution of residual alkyl compounds;

[0009] S2. Inject an ignition agent and oxygen-containing gas into the reservoir, then inject high-pressure nitrogen and ignite it to ignite the residual alkyl compounds to establish an initial combustion zone; wherein, the initial combustion zone is used to thermally crack and desorb hydrocarbons in the reservoir, forming a combustible gas enrichment zone in front of the initial combustion zone.

[0010] S3. Inject an oxidant into the combustible gas enrichment zone, mix it with the combustible gas therein to form an explosive mixture and detonate it, in order to form a fracture network in the reservoir.

[0011] S4. Through the fracture network, oxidant and water vapor are periodically or continuously injected into the reservoir alternately to continuously heat the reservoir through combustion and gasification reactions, thereby raising the overall temperature of the reservoir to the target temperature to construct an artificial thermal reservoir space.

[0012] S5. Stop injecting oxidant and inject cryogenic working fluid into the artificial thermal reservoir through the injection well. The cryogenic working fluid is used for heat exchange when flowing through the fracture network and is then extracted from the production well for thermal energy utilization. The cryogenic working fluid is circulated and injected after being treated by the surface system.

[0013] Preferably, step S1 further includes injecting auxiliary fuel into the reservoir based on the evaluation results, wherein the auxiliary fuel includes propane or diesel.

[0014] Preferably, in step S2, the igniter includes triethylaluminum or triethylborane; the oxygen-containing gas is oxygen-enriched air or pure oxygen.

[0015] Preferably, in step S3, the injected oxidant is pure oxygen or hydrogen peroxide solution; the ignition method is thermal auto-ignition or active ignition via an electric spark plug.

[0016] Preferably, in step S4, the injected oxidant is air or oxygen-enriched air, and the cycle and ratio of injected oxidant and water vapor are adjusted to control the expansion of the combustion / gasification chamber and the reservoir heating process.

[0017] Preferably, in step S5, the cryogenic working fluid is supercritical carbon dioxide.

[0018] Preferably, in step S5, the injection well and the production well form a U-shaped well structure.

[0019] Preferably, before step S2, the method further includes:

[0020] The fracturing tool with a built-in ignition electrode is lowered to the predetermined location in the reservoir via a coiled tubing and connected to the surface injection equipment and ignition device.

[0021] Preferably, in steps S2, S3 and S4, the igniter, oxygen-containing gas, oxidant and water vapor are all pumped to a predetermined downhole location through the coiled tubing.

[0022] Preferred options also include:

[0023] Real-time data collection of temperature, pressure, micro-vibration, and gas composition allows for dynamic optimization and control of oxidant injection amount and timing, combustion and explosion timing, and geothermal fluid circulation parameters.

[0024] The beneficial effects of this invention are:

[0025] First, the enhanced geothermal system retrofit method for abandoned oil and gas wells provided by this invention establishes an initial combustion zone by injecting an igniter and oxygen-containing gas into the reservoir to ignite residual hydrocarbons. The high temperature of the gas then causes the heavy hydrocarbons to crack and adsorbed hydrocarbons to desorb, forming an in-situ flammable gas enrichment zone. This invention solves the technical problem that residual hydrocarbons in abandoned wells are traditionally difficult to utilize effectively, and their chemical energy is not incorporated into the geothermal development system. Furthermore, this invention efficiently converts residual alkyl compounds, which were originally considered a burden or pollution source, into directly usable fuels and "raw materials" for subsequent high-intensity retrofitting, achieving in-situ energy upgrading and enrichment of abandoned resources and providing a low-cost endogenous energy source for the entire process.

[0026] Secondly, the enhanced geothermal system stimulation method for abandoned oil and gas wells provided by this invention injects an oxidant into the combustible gas enrichment zone to form an explosive mixture and detonates it, using the resulting shock wave to create a complex fracture network in the reservoir. This invention solves the technical problems of extremely low permeability in tight reservoirs, limited fracture creation range of conventional hydraulic fracturing, and high costs. Furthermore, by utilizing in-situ prepared combustible gas as the explosion source, this invention achieves highly efficient volumetric stimulation with multi-branched, three-dimensional structures, far exceeding the capabilities of conventional hydraulic fracturing. This significantly improves the reservoir's conductivity and heat exchange surface area, while the heat released by the explosion further contributes to reservoir warming, achieving a synergistic effect of "fracture creation" and "heat enhancement."

[0027] Third, the enhanced geothermal system modification method for abandoned oil and gas wells provided by this invention periodically and alternately injects oxidant and water vapor into the existing fracture network, continuously "roasting" the reservoir through combustion and water-gas reactions. This invention solves the technical problems of insufficient original geothermal temperature in abandoned well reservoirs, lack of sustainable high-intensity heat sources, and how to store heat on a large scale in underground rock masses. Furthermore, this invention transforms a one-time energy release into long-term, stable, and large-scale heating of the reservoir rock, successfully constructing a high-temperature (e.g., above 300°C) "artificial thermal reservoir space," fundamentally transforming "cold reservoirs" into "thermal reservoirs," providing a stable and sufficient heat source guarantee for long-term geothermal extraction. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the wellbore structure and reservoir in the enhanced geothermal system modification method for abandoned oil and gas wells provided by the present invention;

[0029] Figure 2 This is a schematic diagram of the process flow and equipment in the enhanced geothermal system retrofit method for abandoned oil and gas wells provided by the present invention;

[0030] Figure 3 This is a schematic diagram of the injection of igniter and pure oxygen in the enhanced geothermal system modification method for abandoned oil and gas wells provided by the present invention.

[0031] Figure 4 This is a preliminary combustion schematic diagram of the enhanced geothermal system modification method for abandoned oil and gas wells provided by the present invention;

[0032] Figure 5 This is a schematic diagram of the desorption and mixing to form a gas cloud in the enhanced geothermal system modification method for abandoned oil and gas wells provided by the present invention;

[0033] Figure 6 This is a schematic diagram of pure oxygen injection in the enhanced geothermal system modification method for abandoned oil and gas wells provided by the present invention;

[0034] Figure 7 This is a schematic diagram of in-situ combustion and explosion fracture creation in the enhanced geothermal system modification method for abandoned oil and gas wells provided by the present invention.

[0035] Figure 8 This is a schematic diagram of in-situ roasting in the enhanced geothermal system modification method for abandoned oil and gas wells provided by the present invention;

[0036] Figure 9 This is a schematic diagram of the injection-production cycle heat extraction in the enhanced geothermal system modification method for abandoned oil and gas wells provided by the present invention.

[0037] In the diagram: 1. U-shaped well; 2. Reservoir; 3. Heavy hydrocarbons; 4. Light hydrocarbons; 5. Surface injection equipment; 6. Coiled tubing; 7. Fracturing tool; 8. Ignition electrode; 9. Ignition agent and oxygen-containing gas mixture; 10. Combustion zone; 11. Combustible gas enrichment zone; 12. Methane; 13. Hydrogen; 14. Carbon monoxide; 15. Pure oxygen; 16. Fracture; 17. Artificial thermal reservoir; 18. Supercritical CO2; 19. Generator; 20. Steam turbine; 21. Condenser; 22. User. Detailed Implementation

[0038] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0039] This invention addresses the fundamental, interrelated technical bottlenecks of insufficient heat source intensity and low permeability in reservoir 2 during traditional abandoned well geothermal development. It provides a method for constructing an enhanced geothermal system for abandoned oil and gas wells, specifically including the following steps:

[0040] S100, System Diagnostics and Fuel Assessment and Preparation Phase

[0041] This phase forms the foundation of the entire renovation project, aiming to comprehensively understand the current status of the abandoned target well and provide a precise basis for subsequent targeted renovation decisions. The specific implementation is as follows:

[0042] First, systematic logging and engineering testing are conducted on the selected target abandoned oil and gas wells. This includes, but is not limited to: assessing the integrity of the wellbore using logging methods such as acoustic, resistivity, and nuclear magnetic resonance to ensure its ability to withstand subsequent high-pressure operations; measuring the pressure and temperature profile of the current reservoir 2 to understand its degree of "thermal decay"; and qualitatively and quantitatively assessing the type, content, and spatial distribution characteristics of alkyl compounds remaining in reservoir 2 around the wellbore through specialized logging techniques (such as spectral analysis and pulsed neutron logging) and core analysis (if applicable). These alkyl compounds include adsorbed alkyl compounds adsorbed on the surface of rock particles, as well as free alkyl compounds present in micropores and fractures 16.

[0043] Secondly, based on the above assessment results, fuel system design and operational preparation are carried out. If the assessment determines that the total amount or effective calorific value of the residual hydrocarbons in situ is insufficient to support a stable and continuous initial combustion process, auxiliary fuel needs to be injected into the target reservoir 2 through the wellbore for adjustment. Commonly used auxiliary fuels include propane and diesel, which are used to optimize the combustion characteristics of the mixed fuel and ensure successful ignition and stable propagation of the combustion front.

[0044] Next, the downhole workover string is prepared. Using a workover rig or coiled tubing 6 workover rig, a dedicated downhole fracturing tool 7 is connected to the bottom of the coiled tubing 6 and lowered to the predetermined activation depth in reservoir 2. This fracturing tool 7 integrates a high-performance ignition electrode 8. The corresponding surface injection equipment 5 (including a high-pressure pump set, fluid storage and mixing device, control cabinet, etc.) and fracturing wellhead equipment are then installed. The ignition electrode 8 is reliably connected to the surface ignition control device via a cable pre-installed inside the coiled tubing 6. Finally, a rigorous pressure test is conducted on the entire injection pipeline (from the surface equipment to the downhole tool) to ensure that all pipelines, joints, and wellhead equipment are properly sealed under high pressure, eliminating the risk of leaks and laying the foundation for the safe injection of flammable and explosive media.

[0045] This invention transforms existing geothermal system manufacturing from "blindly modifying" to "precisely implementing measures." Through precise diagnosis of the wellbore, reservoir 2, and "fuel," not only are operational risks (such as wellbore leakage) avoided, but more importantly, it provides indispensable input parameters for the customized design of subsequent critical steps such as combustion and explosion, ensuring the targeted nature, safety, and predictability of the final results of the entire technical process.

[0046] S200, in-situ combustion and formation of combustible gas enrichment zone stage

[0047] This stage of the invention aims to utilize downhole fuel to actively ignite and establish a high-temperature heat source, while simultaneously creating conditions for the subsequent combustion and explosion. The specific implementation is as follows:

[0048] After the downhole tools are in place and the pipeline is pressure tested, the surface injection equipment 5 is activated. First, a pre-prepared mixture of ignition agent and oxygen-containing gas 9 is pumped into the downhole tools, which are now positioned at the target location in reservoir 2, through the coiled tubing 6. Commonly used ignition agents include triethylaluminum or triethylborane, which are self-igniting upon contact with air or oxygen-containing gas, providing a reliable high-temperature ignition source. The oxygen-containing gas can be oxygen-enriched air (oxygen concentration higher than 21%) or pure oxygen 15, the purpose of which is to provide sufficient oxidizer for initial combustion, ensuring successful ignition and rapidly increasing the local temperature.

[0049] After injecting the predetermined amount of igniter and oxygen-containing gas, the pumping medium is immediately switched to high-pressure nitrogen to perform a "piping and displacement" operation. The purpose of this is to quickly and completely push the flammable mixture remaining in the coiled tubing 6 and downhole tool channels into the pores and fractures 16 of reservoir 2, allowing it to fully contact and mix with the surrounding residual alkyl compounds, thus constructing a homogeneous combustible system in the target area. Simultaneously, the high-pressure nitrogen "piping and displacement" also avoids the risk of premature reaction of the igniter within the tubing string.

[0050] After replacement is completed, the ignition electrode 8 inside the downhole tool is activated by the surface ignition device to generate a high-temperature electric spark, igniting the pre-mixed combustibles in reservoir 2. After successful ignition, combustion spreads rapidly, establishing an initial high-temperature combustion zone 10 in reservoir 2. The temperature of this combustion zone 10 can be controlled by adjusting the injected oxygen concentration and fuel distribution, typically reaching 600°C to 900°C.

[0051] The high temperature generated in combustion zone 10 has two key effects on reservoir 2: First, thermal cracking. Heavy hydrocarbons 3 (such as long-chain alkanes and asphaltenes) in front of and around combustion zone 10 undergo chain-breaking reactions at high temperatures, cracking into lighter hydrocarbons 4 (such as methane 12 and ethylene) and gases such as hydrogen 13 and carbon monoxide 14. Second, thermal desorption. The high temperature significantly reduces the adsorption force between hydrocarbon molecules and the surface of rock particles, causing a large amount of hydrocarbons (especially natural gas) that were originally in an adsorbed state to desorb from the rock surface and transform into free gases.

[0052] Driven by the combustion front and pressure, these newly generated and desorbed light combustible gases (mainly including methane 12, hydrogen 13, carbon monoxide 14, and a small amount of olefins) migrate and accumulate in the low-temperature region in front of the combustion zone 10, gradually forming a considerable "combustible gas enrichment zone 11" with a high gas concentration. This combustible gas enrichment zone 11 is the core link connecting combustion and explosion.

[0053] In this way, the present invention activates residual hydrocarbons from a "static occurrence" state to a "dynamic reaction" state. Initial combustion not only directly contributes the first wave of heat, but more importantly, through thermal action (cracking and desorption), it efficiently transforms and enriches heavy, adsorbed hydrocarbons in reservoir 2 that are not easily combusted or utilized directly into a "fuel package" (i.e., combustible gas enrichment zone 11) mainly composed of highly reactive, easily combustible light gases. This provides a unique in-situ material basis for subsequent high-efficiency, low-cost explosive modification of reservoir 2.

[0054] S300, in-situ combustion and explosion cracking stage

[0055] This stage of the invention aims to utilize the "fuel pack" prepared in situ in the previous step to efficiently modify the tight reservoir 2 through controlled explosion, forming a complex fracture network 16. The specific implementation is as follows:

[0056] A strong oxidizer is injected at high speed into the combustible gas enrichment zone 11 via coiled tubing 6. According to the design, the oxidizer can be pure oxygen 15 or a high-concentration hydrogen peroxide solution. The injection rate is 0.1~0.5m. 3 / min, to ensure that the injected oxidant can quickly mix with the combustible gases (methane 12, hydrogen 13, carbon monoxide 14, etc.) in the combustible gas enrichment zone 11, forming a uniformly mixed gas mixture with a concentration within the explosion limit range in a short time.

[0057] After the mixing process is complete, the mixture is ignited using one of two methods. Method 1: Utilizing the residual heat from the incomplete combustion in zone 10, the mixed gas reaches its auto-ignition point, triggering a deflagration, i.e., "thermal auto-ignition". Method 2: Through a separate electric spark plug system built into the downhole tool, triggered by a surface command, an electric spark is actively generated to ignite the mixture, i.e., "active detonation". Both methods can achieve reliable detonation.

[0058] The detonation is instantaneous, releasing enormous energy. The explosion center generates extremely high peak pressures (tens to hundreds of megapascals) and high-speed gas shock waves (jet velocities reaching supersonic speeds). This powerful force propagates rapidly from the explosion point outwards through the surrounding reservoir 2 rock. Under the combined action of the pressure wave and the high-speed jet, the reservoir 2 rock is forcibly fractured, expanded, and sheared, forming a complex network of numerous multi-branched, radial, and interconnected fractures. The extension radius, complexity, and conductivity of these fractures typically far exceed those of conventional hydraulic fracturing fractures that expand slowly under liquid pressure.

[0059] Therefore, this invention proposes a novel reservoir 2 stimulation model that utilizes in-situ combustion products as the explosion source. Compared to traditional high-energy gas fracturing, which requires the delivery of solid explosives or pre-prepared combustible gases from the surface, this significantly simplifies the operation process and reduces costs and safety risks. In this model, the explosion energy is used to create fractures, while the heat generated further contributes to the warming of reservoir 2. Furthermore, the complex three-dimensional fracture network formed by the explosion not only greatly improves the absolute permeability of reservoir 2, providing a high-speed channel for subsequent fluid injection and heat exchange, but also creates a huge rock surface area, laying a crucial geometric foundation for subsequent working fluid heat exchange.

[0060] S400, in-situ roasting stage of reservoir 2

[0061] This stage of the invention aims to utilize the existing fracture network 16 as channels and reaction spaces to conduct large-scale, continuous heating of reservoir 2, transforming it into a high-temperature artificial thermal reservoir 17. The specific implementation is as follows:

[0062] After the combustion-induced fracture is completed, a well-connected network of fractures 16 exists in reservoir 2. At this point, through coiled tubing 6 (or an injection string that can be changed as needed), oxidant (e.g., oxygen-enriched air with an oxygen volume fraction exceeding 30%) and water vapor, mixed in specific proportions, are alternately injected into the network of fractures 16 in a periodic or continuous manner.

[0063] The injected oxidant diffuses through the fracture 16 system, contacting and undergoing combustion reactions with residual hydrocarbons (including incompletely burned heavy components, coke, etc.) on the fracture 16 wall and in the deeper matrix, continuously releasing heat. Simultaneously, the injected high-temperature water vapor reacts with the incandescent carbonaceous residues (coke) or hydrocarbons on the high-temperature fracture 16 wall (temperature typically above 400°C) in a water-gas reaction. Through a strong endothermic reaction, it absorbs some of the concentrated heat released by combustion and converts it into the chemical energy of syngas (such as CO and H2).

[0064] In this way, the present invention produces a crucial effect through the alternation or synergy of "combustion exothermic" and "water-gas reaction endothermic" processes: it avoids excessive heat concentration near the wellbore, which could lead to rock melting and blockage of fracture 16. Instead, it transfers and stores heat energy more gently and effectively to the deeper parts of reservoir 2 and the matrix. Furthermore, by controlling the cycle, ratio, and total amount of injected oxidant and water vapor, the generated "combustion / gasification chamber" can be guided to steadily expand into the interior of reservoir 2, while simultaneously conducting "in-situ roasting" of the extensive fracture 16 walls and surrounding rock mass for several weeks or even months.

[0065] The "in-situ roasting" process is equivalent to a large-scale, long-term low-temperature "kiln firing" of the entire target reservoir 2 section. The ultimate goal is to gradually and stably raise the overall average temperature of the rocks and internal fluids of reservoir 2 to a higher target temperature, such as above 180°C (180°C is the dry hot rock temperature limit specified in the national energy industry standard "NB / T 10097-2018 Geothermal Energy Terminology"), thereby constructing a "artificial thermal reservoir 17 space" with a huge spatial scale and a relatively uniform temperature field.

[0066] Therefore, this invention organically combines one-time combustion-explosion fracture creation with continuous thermal energy storage. Utilizing an alternating "gas injection-steam injection" strategy and leveraging the endothermic properties of the water-gas reaction, it achieves efficient and uniform distribution and storage of heat from a local point source to the entire reservoir 2 volume. This not only solves the problem of insufficient heat source in traditional geothermal reservoir 2, but also the stable high-temperature rock mass formed by "roasting" itself is a huge heat reservoir. Its heat capacity and sustainable extraction time far exceed those of systems relying solely on original geothermal temperature or short-term heating, thereby completing the transformation and construction of the geothermal system.

[0067] S500, Geothermal Circulation Extraction Stage

[0068] This stage of the invention marks the switch from the "Reservoir 2 modification mode" to the "Geothermal Production mode," thereby enabling the modified geothermal system to extract thermal energy from the existing artificial thermal reservoir 17 in a long-term and stable manner. The specific implementation is as follows:

[0069] Once the average temperature of the artificial thermal reservoir 17 has reached the preset extraction target (e.g., 300°C) and stabilized, as confirmed by temperature monitoring systems deployed in observation or production wells, the injection of oxidant into reservoir 2 can be stopped. The underground combustion / gasification reaction will gradually extinguish itself due to the loss of oxidant.

[0070] At this point, underground exists a vast porous geothermal reservoir composed of high-temperature rock (which has been "roasted" and heated) and a complex network of fractures. The coiled tubing and related downhole tools used for the stimulation phase are removed, and the wellhead equipment is converted to geothermal production mode. Typically, a "U-shaped well" or an "injection-production well" approach is used for this mode.

[0071] A cryogenic working fluid (preferably supercritical carbon dioxide in this embodiment) is continuously injected underground from an injection well via a high-pressure injection pump on the surface. Supercritical CO2 18 enters the target thermal reservoir 2 through a complex three-dimensional network of fractures 16 formed by combustion and roasting. These fractures 16 provide an extremely large specific surface area (fracture walls). As the cryogenic, high-pressure supercritical CO2 18 flows through the fracture channels, it undergoes direct and sufficient convective heat exchange with the high-temperature fracture wall rock (300-500°C), violently absorbing sensible heat from the rock and rapidly increasing its own temperature to near the reservoir 2 temperature, while simultaneously maintaining a high pressure level.

[0072] After absorbing heat, the high-temperature, high-pressure supercritical carbon dioxide fluid flows along the fracture network 16 to another well (production well) connected to it, driven by the pressure difference between the injection and production ends. The high-temperature, high-pressure CO2 is then concentrated and extracted to the surface by the production well.

[0073] Subsequently, the high-temperature fluid extracted from the geothermal system enters the ground thermal energy utilization system. The main pathway is as follows: the high-temperature, high-pressure CO2 first enters the turbine 20 (or expander), where it rapidly expands and performs work, driving the generator 19 to generate electricity, converting thermal and pressure energy into electrical energy. The CO2 discharged from the turbine 20, with its temperature and pressure significantly reduced, enters the condenser 21 to be cooled, re-liquefied, or reaches a high-density supercritical state. The liquid or high-density CO2 collects in a storage tank, is then pressurized by a high-pressure injection pump, and after passing through a regenerator (which recovers and utilizes the waste heat from the fluid after power generation to preheat the CO2 to be injected, improving system efficiency), it is reinjected into the injection well, forming a completely closed loop. In addition, some of the thermal energy can also be used for direct heating; for example, the waste heat from power generation or the directly extracted hot fluid can be used to heat users 22 through a heat exchanger.

[0074] This invention achieves the final energy recovery of all prior modifications and investments. Utilizing the constructed high-temperature, high-permeability artificial geothermal reservoir 17, combined with the circulation of highly efficient working fluids such as supercritical CO2 18, high-quality geothermal energy can be extracted in a long-term, stable, and efficient manner for power generation or heating. The entire geothermal extraction process is conducted in a closed system with no fluid loss, making it environmentally friendly, and the recycling of the working fluid also reduces operating costs.

[0075] S600, Intelligent Monitoring and Optimization of the Entire System

[0076] This step is not independent of the steps described above, but rather preferably runs throughout the entire process from S200 to S500. The specific implementation is as follows:

[0077] Before and during the operation, a distributed monitoring network is deployed both downhole (using fiber optic sensing systems, electronic pressure and thermometers, etc.) and on the surface (using microseismic monitoring stations, gas composition analyzers, etc.). Key parameters are collected in real time and continuously, including but not limited to: temperature and pressure at different depths downhole, microseismic events caused by the generation and propagation of fracture 16, and the composition of injected and produced fluids (such as the content of O2, CO2, CO, CH4, and H2).

[0078] This massive amount of data is transmitted to the central control room, where it is processed and analyzed by built-in artificial intelligence algorithms (such as machine learning models and optimization control algorithms). Based on real-time data and model predictions, the system can dynamically optimize subsequent operating parameters. For example: in the combustion stage (S200), the injection rate of subsequent oxygen-containing gas is optimized based on the temperature front advance speed and gas products; in the combustion and explosion preparation stage (S300), the volume and composition of the combustible gas enrichment zone 11 are estimated, and the oxidant injection amount is calculated to ensure that the mixed gas is at the optimal deflagration concentration; in the roasting stage (S400), the alternation cycle and ratio of gas injection and steam injection are dynamically adjusted based on the temperature field evolution; in the extraction stage (S500), the injection pump speed and generator 19 loads are optimized based on the temperature and pressure of the produced fluid.

[0079] The beneficial effect of this step is that it upgrades the entire complex transformation and mining process from experience-based operation to data-driven intelligent closed-loop control. This greatly improves the safety (such as avoiding runaway combustion or excessive pressure), efficiency (optimizing the energy input-output ratio), and the controllability and stability of the final result. It is an important manifestation of the advanced and modern technical solution of this invention.

[0080] A specific example of the overall processing flow is given, using an abandoned natural gas well located in a tight sandstone layer as an example, to illustrate the implementation process of the present invention:

[0081] 1. Diagnosis: Well logging revealed that the wellbore was intact, reservoir 2 had low pressure and a temperature of 80°C (far below the original 150°C). Assessment showed that there was residual adsorbed methane 12, combustible gas enrichment zone 11, and a small amount of condensate oil.

[0082] 2. Combustion: Lower the coiled tubing 6 tool to the production layer. Inject triethylaluminum igniter and oxygen-enriched air, displace it, and ignite. Initial combustion zone 10 (approximately 750°C) is successfully established. The high temperature causes a large amount of adsorbed methane 12 to desorb, forming a "combustible gas enrichment zone 11" rich in CH4 and H2 in front.

[0083] 3. Combustion and Explosion: Pure oxygen 15 is injected at high speed into the combustible gas enrichment zone 11 through the continuous oil pipe 6, and the mixture is ignited by an electric spark plug. The explosion creates a network of multi-branched fractures 16 with a radius of about 50 meters in the dense sandstone.

[0084] 4. Calcination: Through the fracture network 16, air is injected alternately for 3 days and water vapor for 1 day per week for 8 weeks. The water-gas reaction drives the heat to diffuse deeper, eventually heating the entire reservoir 2 within a radius of about 40 meters to 320°C, forming an artificial thermal reservoir 17.

[0085] 5. Extraction: Gas injection ceases, switching to geothermal mode. Supercritical CO2 at 15 MPa and 35°C is injected from the original well (converted into an injection well), while high-temperature CO2 at 300°C and 18 MPa is extracted from another nearby connected old well (converted into a production well), driving 19 ORC generator sets to generate electricity. The cooled CO2 after power generation is compressed and reinjected, achieving a cycle.

[0086] 6. Monitoring: Temperature and pressure are monitored throughout the process via downhole fiber optics, and the propagation of fracture 16 is monitored via surface micro-seismic monitoring. Data is used to optimize injection parameters at each stage in real time.

[0087] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for constructing an enhanced geothermal system for abandoned oil and gas wells, characterized in that, Includes the following steps: S1. Logging and testing of the target abandoned oil and gas wells to assess wellbore integrity, reservoir (2) parameters, and the type, content and spatial distribution of residual alkyl compounds; S2. Ignite the reservoir (2) with igniter and oxygen-containing gas, and then inject high-pressure nitrogen to ignite the residual alkyl compound to establish an initial combustion zone (10); wherein the initial combustion zone (10) is used to thermally crack and desorb the hydrocarbons in the reservoir (2) to form a combustible gas enrichment zone (11) in front of the initial combustion zone (10). S3. Inject an oxidant into the combustible gas enrichment zone (11), mix it with the combustible gas therein to form an explosive mixture and detonate it to form a fracture (16) network in the reservoir (2); S4. Through the fracture (16) network, oxidant and water vapor are periodically or continuously injected into the reservoir (2) to continuously heat the reservoir (2) through combustion and gasification reactions, so as to raise the overall temperature of the reservoir (2) to the target temperature to construct an artificial thermal reservoir (17) space. S5. Stop injecting oxidant and inject cryogenic working fluid into the artificial thermal reservoir (17) space through the injection well. The cryogenic working fluid is used to exchange heat when flowing through the fracture (16) network and is then extracted from the production well for thermal energy utilization. The cryogenic working fluid is circulated and injected after being processed by the surface system.

2. The method for constructing an enhanced geothermal system for abandoned oil and gas wells according to claim 1, characterized in that, Step S1 also includes injecting auxiliary fuel into the reservoir (2) based on the evaluation results, the auxiliary fuel including propane or diesel.

3. The method for constructing an enhanced geothermal system for abandoned oil and gas wells according to claim 1, characterized in that, In step S2, the ignition agent includes triethylaluminum or triethylborane; the oxygen-containing gas is oxygen-enriched air or pure oxygen (15).

4. The method for constructing an enhanced geothermal system for abandoned oil and gas wells according to claim 1, characterized in that, In step S3, the injected oxidant is pure oxygen (15) or hydrogen peroxide solution; the ignition method is thermal auto-ignition or active ignition via an electric spark plug.

5. The method for constructing an enhanced geothermal system for abandoned oil and gas wells according to claim 1, characterized in that, In step S4, the injected oxidant is air or oxygen-enriched air, and the cycle and ratio of injected oxidant and water vapor are adjusted to control the expansion of the combustion / gasification chamber and the heating process of the reservoir (2).

6. The method for constructing an enhanced geothermal system for abandoned oil and gas wells according to claim 1, characterized in that, In step S5, the cryogenic working fluid is supercritical carbon dioxide.

7. The method for constructing an enhanced geothermal system for abandoned oil and gas wells according to claim 1, characterized in that, In step S5, the injection well and the production well form a U-shaped well (1) structure.

8. The method for constructing an enhanced geothermal system for abandoned oil and gas wells according to claim 1, characterized in that, Before step S2, the following is also included: The fracturing tool (7) with a built-in ignition electrode (8) is lowered to the predetermined position in the reservoir (2) through the coiled tubing (6) and connected to the surface injection equipment (5) and the ignition device.

9. The method for constructing an enhanced geothermal system for abandoned oil and gas wells according to claim 8, characterized in that, In steps S2, S3 and S4, the igniter, oxygen-containing gas, oxidant and water vapor are all pumped to a predetermined downhole location through the coiled tubing (6).

10. The method for constructing an enhanced geothermal system for abandoned oil and gas wells according to claim 1, characterized in that, Also includes: Real-time data collection of temperature, pressure, micro-vibration, and gas composition allows for dynamic optimization and control of oxidant injection amount and timing, combustion and explosion timing, and geothermal fluid circulation parameters.

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