Coal and oil shale in-situ gasification and high temperature fluid pyrolysis of the coordinated exploitation method

Abstract

This invention discloses a method for the co-mining of coal and oil shale through in-situ gasification and high-temperature fluid pyrolysis. Injection wells and extraction wells are respectively arranged in the coal seam and oil shale layer. After cementing and targeted perforation completion, multi-stage fracturing is performed on the coal seam to form a dense fracture network. Subsequently, retreating combustion gasification is carried out in the coal seam to produce gas, and the gas is extracted and high-temperature CO2 is separated. The high-temperature CO2 is injected into the oil shale layer through insulated pipelines, serving both as a pyrolysis medium to promote the pyrolysis of kerogen to produce oil and gas, and to achieve fracturing and permeability enhancement of the oil shale. Finally, oil and gas are extracted through oil shale extraction wells. After the coal seam is mined, the goaf is grouted and filled, and displacement fluid is injected to protect the wellbore. This invention enables in-situ co-mining of coal and oil shale, utilizing the high-temperature CO2 byproduct of coal gasification to solve the high energy consumption problem of oil shale pyrolysis, reducing resource waste and process flow, and lowering mining costs.

Description

Technical Field

[0001] This invention relates to the field of coordinated mining technology of coal and coal-bearing strata, specifically to a method for the coordinated mining of coal and oil shale through in-situ gasification and high-temperature fluid pyrolysis. Background Technology

[0002] Oil shale (also known as oil shale mother rock) is a high-ash sedimentary rock containing combustible organic matter, rich in organic matter—kerogen. It can be converted into shale oil and gas through high-temperature pyrolysis at 300-400℃ or higher. my country has abundant coal resources, with currently proven reserves of approximately 207 billion tons. Shallow coal resources (buried less than 1000 m) account for only about 20%, while deep coal resources (buried more than 2000 m) account for over 60%, approximately 120 billion tons. These deep coal resources are abundant and represent a key area for energy extraction. In my country, some oil shale resources are associated with coal resources, existing as interlayered coal seams or directly above the coal seam. Given my country's current high dependence on imported oil and gas energy and the urgent situation of coal resource scarcity, coordinated mining of oil shale and coal seams is an inevitable trend.

[0003] Both underground coal gasification and in-situ pyrolysis of oil shale are in-situ development technologies. Underground coal gasification (UCG) involves controlled combustion underground, converting deep coal into combustible gases such as methane and hydrogen through thermal and chemical reactions. Injection wells are drilled in the target reservoir, and a gasifying agent rich in oxygen and water vapor is injected into the underground coal seam, where it is ignited, causing partial combustion. However, mining underground coal resources through underground coal gasification can be prohibitively expensive. The large amounts of CO2 generated during the process, if not properly captured, utilized, and stored, could lead to leaks, contradicting my country's dual-carbon emission reduction strategy. Furthermore, the byproducts generated during underground coal gasification cannot be reused, increasing extraction costs and reducing economic efficiency. In-situ underground oil shale extraction involves first fracturing the oil shale reservoir using high-pressure fluid to increase permeability and expand the volume of high-temperature fluid in contact with the oil shale. This high-temperature fluid is then pumped in to pyrolyze the oil shale, and the resulting oil and gas resources are extracted through wells. However, this technology has drawbacks. Heating the underground oil shale reservoir to temperatures above 400°C requires enormous energy, potentially resulting in a low energy input-output ratio and high extraction costs. Furthermore, the poor thermal conductivity of oil shale reservoirs leads to uneven heat transfer and low heating efficiency, resulting in a very slow extraction process.

[0004] Therefore, how to organically combine underground coal gasification with underground in-situ pyrolysis technology of oil shale to better support the coordinated mining of oil shale and coal seams is an urgent problem to be solved. Summary of the Invention

[0005] The purpose of this invention is to propose a method for the co-extraction of coal and oil shale through in-situ gasification and high-temperature fluid pyrolysis. While carrying out underground coal gasification, the high-temperature medium such as CO2 generated is injected into the roof of the oil shale for pyrolysis to produce oil and gas resources. This method achieves the co-extraction of oil and gas resources generated from underground coal gasification and oil shale pyrolysis, thereby improving the extraction efficiency of coal resources and strategic alternative unconventional oil and gas resources.

[0006] The technical solution adopted in this invention is: a method for the synergistic mining of coal and oil shale through in-situ gasification and high-temperature fluid pyrolysis, characterized by comprising the following steps:

[0007] Step S1: Drill a horizontal well in the coal seam as injection well I along the direction of the coal seam, and arrange a extraction well I on each side of injection well I in parallel. Drill a horizontal well in the oil shale layer as injection well II along the direction of the oil shale layer, and arrange a extraction well II on each side of injection well II in parallel.

[0008] Step S2: Cementing is performed on injection well I, injection well II, extraction well I, and extraction well II respectively. After cementing, casing is arranged in the coal seam for perforation completion configuration. Injection well I uses fixed-face perforation, and extraction well I uses half-plane fixed-face perforation. The direction of the half-plane perforation of extraction well I is towards injection well I. In the oil shale layer, casing is arranged for perforation completion configuration. Fixed-face perforation is used in the direction of injection well II, and extraction well II uses half-plane fixed-face perforation. The direction of the half-plane perforation of extraction well II is towards injection well II.

[0009] Step S3: Perform hydraulic fracturing on the coal seam, following these steps:

[0010] Step S31: Use the coal seam between the horizontal sections of the two extraction wells as the fracturing zone;

[0011] Step S32: Pump clean water into injection well I and adopt the fracturing mode of prioritizing the propagation of the main fracture. Stop the fracturing operation after the initiation pressure curve reaches the maximum peak value; at the same time, the two extraction wells I are fracturing through incremental gradient pumping and circulation.

[0012] Step S33: Injection well I is switched to decremental gradient pumping and circulation fracturing to drive the main fracture to extend until it connects with the dense fracture network formed by fracturing in extraction well I;

[0013] Step S34: After the fracturing of this section of the coal seam is completed, the remaining fracturing zone is fracturing in a backward manner until the fracturing of all coal seams is completed.

[0014] Step S4: Perform hydraulic fracturing operations on the oil shale layer, following these steps:

[0015] Step S41: Pump clean water into injection well II. First, use a large-displacement pump to perform segmented multi-cluster fracturing. Stop fracturing after the pressure reaches the peak value.

[0016] Step S42: Then set the pumping parameters to a maximum of 80% of the peak pressure and the pumping time to T min. During the pumping process, while forming the main fracture, the natural fractures are connected to form a dense fracture network.

[0017] Step S43: The two extraction wells II are fracturing by incremental gradient pumping and circulation to form fracturing fractures, which are connected to the fractures constructed in the injection well II;

[0018] Step S5: Install pressure and temperature monitors in the coal seam and oil shale layer;

[0019] Step S6: Conduct coalbed gasification gasification operations, following these steps:

[0020] Step S61: Inject gasifying agent into injection well I and launch ignition equipment. Use a backward combustion method to burn the coal seam between the horizontal section of injection well I and extraction well I through fracturing fractures.

[0021] Step S62: Injection well I continuously injects gasifying agent to maintain coal seam combustion and gas production, while extraction well I remains in a simmering state and its temperature is monitored; when the temperature of extraction well I rises to the set value, the injection of gasifying agent is stopped and combustion is terminated.

[0022] Step S63: Continue to shut down the coal seam and monitor the formation pressure. If the formation pressure no longer increases, start gas extraction operation in extraction well I.

[0023] Step S7: After the coal seam is fully combusted, the generated methane, H2 and high-temperature CO2 gases are extracted to the surface through the gasification channels formed by the hydraulic fracturing fractures by the two extraction wells I. Combustible gas and high-temperature CO2 are obtained through gas separation. The high-temperature CO2 is transported by insulated pipelines and injected into the oil shale layer from the injection well II, causing the kerogen in the oil shale layer to pyrolyze and produce oil and gas compounds. At the same time, the hydraulic fracturing structure fractures are formed to create gasification channels, which are then extracted to the surface by the two extraction wells II.

[0024] Step S8: Repeat steps S6 and S7, with the coal seam undergoing retreating combustion and the oil shale layer undergoing retreating extraction, until both the coal seam and oil shale layer are mined out.

[0025] As a further improvement of the present invention, in step S32, the two extraction wells I are fracturing by incremental gradient pumping and circulation, specifically as follows:

[0026] Based on the maximum peak value of the fracturing pressure curve of injection well I, set the circulation pumping parameters and gradient pumping pressure. The upper limit of the pumping pressure is increased step by step in stages of 20% of the maximum pressure until the upper limit of the pumping pressure is reached. Set the pumping time of each stage to t min and the interval between pumping stages to T min. Repeat the above stages N times to fully develop the microfractures in extraction well I and form a dense fracture network.

[0027] As a further improvement of the present invention, in step S33, the injection well I is switched to a decreasing gradient pumping and cyclic fracturing method, specifically: the maximum pumping pressure is set for the first time for pumping, and then each stage is gradually reduced by 20% of the maximum pressure. The pumping time of each stage is set to t min, and the interval between pumping stages is T min. The above stages are repeated N times.

[0028] As a further improvement of the present invention, after the coal seam mining is completed, the goaf left after underground coal gasification is filled by grouting.

[0029] Compared with the prior art, the present invention has the following technical advantages:

[0030] (1) This invention utilizes underground coal gasification to separate and introduce the generated high-temperature CO2 into oil shale formations. On one hand, this CO2 is used to fracture the formations, further forming a network of pressure fractures and increasing the contact area between the pyrolysis medium and the oil shale reservoir, thus enhancing the permeability of the oil shale formation. On the other hand, it enables high-temperature heating of the oil shale, serving as a pyrolysis catalyst and extractant for the kerogen pyrolysis reaction, ensuring a full reaction and increasing yield. This overcomes the problems of insufficient heat supply and low heating efficiency in underground coal gasification. Especially when the coal reservoir and oil shale reservoir are spatially distant, injecting the separated high-temperature CO2 into the oil shale formation breaks the limitation of joint mining due to the distance between the two resource reservoirs. This allows for the full utilization of byproducts generated from underground coal gasification, improving resource extraction efficiency and saving extraction costs.

[0031] (2) This invention, targeting the structural characteristics of coal and oil shale geological reservoirs, implements different pumping fracture strategies to generate multiple fractures and dense fracture networks in the coal and oil shale geological reservoirs, thereby increasing the contact area of ​​the pyrolysis medium and enabling the coal resources and oil shale layers to fully undergo pyrolysis, thus improving mining efficiency. In the coal reservoir, an alternating fracturing strategy of injection well pumping and extraction well pumping is adopted. The injection wells preferentially form the main fractures, and then the extraction wells adopt a circulating pumping strategy to form micro fractures that communicate and interweave with the main fractures, promoting the formation of a dense fracture network structure in the coal reservoir. The cyclic loading effect of multi-stage circulating pumping is used to subject the existing fracturing network to alternating loads, promoting the formation of new fractures on the existing fracture surface, and forming micro-fracture zones of different scales at the fracture tips, further increasing the complexity of the micro-fracture zones. In oil shale formations, a multi-cluster pumping fracturing strategy is adopted. Based on the geological characteristics of the multi-layered bedding structure of oil shale formations, the resulting fracturing fractures will turn and extend along the bedding planes. The use of multi-cluster pumping technology can generate a network of interwoven fractures within the oil shale formation, thereby increasing the permeability of the oil shale formation.

[0032] (3) This invention solves the problem of high energy consumption caused by heating oil shale in the co-mining technology of coal and oil shale, realizes in-situ co-mining of coal and oil shale, reduces process flow and resource waste, and lowers costs; it is not only applicable to the co-existence or superposition of coal and oil shale, but can also be extended to the co-mining of coal and shale oil at a distance, providing a new idea for the coordinated mining of coal and associated coal resources. Attached Figure Description

[0033] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0034] Figure 1 This is a flowchart of the co-mining method of in-situ gasification of coal and oil shale and high-temperature fluid pyrolysis according to the present invention.

[0035] Figure 2 This is a schematic diagram of fracturing a coal reservoir injection well in an embodiment of the present invention.

[0036] Figure 3 This is a scene diagram of the synergistic mining of coal and oil shale through in-situ gasification and high-temperature fluid pyrolysis.

[0037] Explanation of reference numerals in the attached diagram: 1-Injection well I; 2-Drainage well I; 3-Injection well II; 4-Drainage well II; 5-Gas separation unit; 6-Coal seam; 7-Oil shale layer; 8-Main fracture; 9-Branch fracture. Detailed Implementation

[0038] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the described embodiments of the present invention are within the scope of protection of the present invention.

[0039] Please see Figure 1 , Figure 2 and Figure 3 The present invention discloses a method for the synergistic mining of coal and oil shale through in-situ gasification and high-temperature fluid pyrolysis, comprising the following steps:

[0040] Step S1: Following the strike of coal seam 6, drill a horizontal well as injection well I1 within coal seam 6. Within the plane of injection well I1, arrange a parallel extraction well I2 on each side. Following the strike of oil shale layer 7, drill a horizontal well as injection well II3 within oil shale layer 7. Within the plane of injection well II3, arrange a parallel extraction well II4 on each side. Injection well I1, injection well II3, extraction well I2, and extraction well II4 all include both vertical and horizontal sections. Figure 3 As shown, the extraction well I2 is connected to one end of the gas separation device 5, and the other end of the gas separation device 5 is connected to the injection well II3.

[0041] Step S2: Cementing is performed on injection well I1, injection well II3, extraction well I2, and extraction well II4. After cementing, perforation completion configuration is carried out in coal seam 6. Injection well I1 uses fixed-face perforation, while the two extraction wells I1 on either side use semi-planar fixed-face perforation. The semi-planar perforations in extraction well I2 face injection well I1. A similar configuration is also found in injection well II3 within oil shale layer 7. Specifically, casing is installed in injection well I1, with a ring of perforations (fixed-face perforations) arranged circumferentially at the front section. In extraction well I2, the casing has perforations along the semi-circumference of its front section, facing injection well I1. This prepares for subsequent coal seam fracturing and gas collection from coal combustion. In oil shale layer 7, the casing in injection well II3 and extraction well II4 has more perforations along each length, i.e., multiple clusters.

[0042] Step S3: Perform hydraulic fracturing operation on coal seam 6, following these steps:

[0043] Step S31: Use the coal seam between the horizontal sections of the two extraction wells as the fracturing zone.

[0044] Step S32: Pump clean water into injection well I1, using a fracturing mode that prioritizes the propagation of the main fracture. Fracturing operations are stopped once the initiation pressure curve reaches its maximum peak. The pumping fracturing stage is monitored using acoustic emission technology, and its fracturing effect is displayed in real-time via microseismic signals. When a large main fracture 8 is formed, a large number of microseismic signals are generated, and the pumping strategy is adjusted in real-time based on the feedback from the microseismic signals. Simultaneously, the two extraction wells I1 are initiated using circulating water injection and gradient pumping methods, respectively. Specifically, the two extraction wells I2 are initiated using circulating water injection and gradient pumping methods, respectively: Based on the maximum peak value of the initiation pressure curve of injection well I1, circulating pumping parameters and gradient pumping pressures are set. The upper limit of the pumping pressure is increased incrementally in stages of 20% of the maximum pressure until the upper limit of the pumping pressure is reached. The pumping time for each stage is set to t min (minutes), and the interval between pumping stages is T min. This process is repeated N times to ensure that the microfractures 9 in extraction well I2 fully develop, forming a dense fracture network. In this embodiment, both T and t are set to 30.

[0045] Step S33: Injection well I1 is switched to a decreasing gradient pumping and circulating fracturing method to drive the main fracture 8 to extend and form branch fractures 9, until they connect with the dense fracture network formed by fracturing in extraction well I2. Specifically, the maximum pumping pressure is set for the first pumping operation, and thereafter, the pressure is gradually decreased by 20% of the maximum pressure in each stage. The pumping time for each stage is set to t min, and the interval between pumping stages is T min. This stage is repeated N times. Acoustic emission technology is also used for monitoring in this stage. When a low-energy, high-density signal from the source is detected by microseismic monitoring, the coal seam is subjected to fatigue damage, generating multi-scale microfractures. On the other hand, natural fractures are activated, increasing the complexity of the fractures. During this process, proppant needs to be injected into the generated fractures to prevent fracture closure. All of the above operations require microseismic monitoring to monitor the extension range of the main fracture and fracture network in the reservoir in real time.

[0046] Step S34: After the fracturing of this section of the coal seam is completed, the remaining fracturing zone is fracturing in a backward manner until all operations are completed.

[0047] Step S4: Perform hydraulic fracturing operations on the oil shale layer, following these steps:

[0048] Step S41: Pump clean water into injection well II3. First, use a large-displacement pump to perform segmented multi-cluster fracturing. Stop fracturing after the pressure reaches the peak value.

[0049] Step S42: Then set the pumping parameters to a maximum of 80% of the peak pressure and the pumping time to T min. During the pumping process, while forming the main fracture, the natural fractures are connected to form a dense fracture network.

[0050] Step S43: The two extraction wells II4 are fracturing using incremental gradient pumping and circulation to form fractures, which connect with the fractures in the injection well II. The fracturing method of extraction well II4 is similar to that of extraction well I2, and will not be described in detail here.

[0051] Step S5: Install pressure and temperature monitors in coal seam 6 and oil shale layer 7. Specifically, install a coal seam pressure monitor and a coal seam temperature sensor in coal seam 6, and install an oil shale layer pressure sensor and an oil shale layer temperature sensor in the oil shale layer to monitor the pressure and temperature of the coal seam and oil shale layer.

[0052] Step S6: Conduct coalbed gasification gasification operations, following these steps:

[0053] Step S61: Inject gasifying agent into injection well I1 and deploy ignition equipment. A reverse combustion method is used to burn the coal seam between the horizontal section of injection well I1 and extraction well I2 through fracturing. The gasifying agent is a mixture of oxygen and water vapor.

[0054] Step S62: The gasifying agent is continuously injected into the injection well I1 to maintain coal seam combustion and gas production, while the extraction well I2 is kept in a simmering state and its temperature is monitored; when the temperature of the extraction well I2 rises to the set value, the injection of the gasifying agent is stopped and combustion is terminated.

[0055] Step S63: Continue to shut down the coal seam and monitor the formation pressure. If the formation pressure no longer increases, start gas extraction operation in extraction well I. The coal seam forms multiple zones during the entire underground coal gasification process: the gasification zone, the reduction zone, and the pyrolysis zone. The oxidation zone produces oxygen, which reacts with the coal to generate CO2 and high temperatures, providing a continuous source of heat for the entire process. The reduction zone follows the oxidation zone; the oxygen-deficient, high-temperature environment causes carbon dioxide and water vapor to react with the hot coal, generating combustible gases CO and H2. The area farther from the oxidation zone is the pyrolysis drying zone, where the coal seam is heated to high temperatures, releasing volatiles and undergoing drying. The syngas produced by underground coal gasification mainly consists of CO, H2, CH4, and CO2, and is then extracted to the surface for mining through extraction well I2.

[0056] Step S7: After the coal seam is fully combusted, the generated methane, H2, and high-temperature CO2 gases are extracted to the surface through the gasification channels formed by the hydraulic fracturing fractures via two extraction wells I2. Combustible gas and high-temperature CO2 are obtained through gas separation. The high-temperature CO2 is transported via insulated pipelines and injected into the oil shale through injection well II3, causing the kerogen in the oil shale to pyrolyze and produce oil and gas compounds. Simultaneously, hydraulic fracturing fractures form gasification channels, which are then extracted to the surface by the two extraction wells II4. Gas separation is performed using a gas separation device 5. The specific process is as follows: After the CO2-containing mixed gas enters the pressure swing adsorption (PSA) unit's adsorption tower, the adsorption capacity of the adsorbent in the adsorption bed varies with pressure. During pressurization, CO2 in the feed gas is adsorbed, while components that are not easily adsorbed are discharged from the adsorption bed outlet. During depressurization, the adsorbed CO2 and residual impurities are desorbed, and the adsorbent is regenerated by evacuation. The concentrated CO2 is then piped into the oil shale layer. The gas separation technology and equipment used here are existing technologies. For reference, you can refer to the separation equipment produced by Hangzhou Tianli Air Separation Equipment Manufacturing Co., Ltd. and Kaifeng Saipu Air Separation Group Co., Ltd., which will not be elaborated here.

[0057] Step S8: Repeat steps S6 and S7, with the coal seam undergoing retreating combustion and the oil shale layer 7 undergoing retreating extraction, until both the coal seam 6 and the oil shale layer 7 are mined out.

[0058] After the coal seam is mined out, the goaf left after underground coal gasification is grouted and filled with displacement fluid to prevent the wellbore from solidifying.

[0059] In this embodiment, a coal seam pressure monitor is used to monitor whether the underground coal gasification reaction is complete. If the pressure gradually rises to a peak and remains relatively stable while the temperature ranges from 450℃ to 1200℃, it indicates that the reaction is in progress. If oxygen is continuously injected from the injection well and the pressure is difficult to rise further, it indicates that the combustion reaction has ended, and the injection well stops injecting oxygen. A temperature monitor is installed in the oil shale to monitor the temperature changes of the oil shale layer. When the temperature monitored by the temperature monitor reaches 300-400℃, it indicates that the kerogen in the oil shale has begun to pyrolyze. Temperature monitors are installed in the surface pipeline, mainly to monitor the temperature of CO2 in the insulated pipeline. If the CO2 is above 450℃, it does not need to be heated and can be directly transported from injection well II3 to the oil shale for pyrolysis. If the temperature is below 300℃, it needs to be returned to the gas separation device 5 through the CO2 transport pipeline, heated to the required temperature, and then transported to the oil shale for pyrolysis.

[0060] This invention addresses two key aspects. First, it separates the CO2 produced by underground coal gasification and introduces it into oil shale to pyrolyze the kerogen, generating oil and gas. This significantly reduces the cost and increases the efficiency of oil shale extraction. Second, the invention employs circulating pumping and gradient fracturing in the coal seam, creating a dense network of fractures. This facilitates the efficient transport of gas from coal combustion to the two extraction wells (I2), improving gas collection efficiency. The high-temperature CO2 extracted from the coal seam is then separated and introduced into the oil shale. This high-pressure fracturing of the oil shale further creates a dense network of fractures, while simultaneously reacting with the kerogen to produce oil and gas.

[0061] The embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited thereto. Various changes that can be made within the scope of knowledge possessed by those skilled in the art without departing from the spirit of the present invention are all within the protection scope of the claims of the present invention.

Claims

1. A method for the synergistic mining of coal and oil shale through in-situ gasification and high-temperature fluid pyrolysis, characterized in that, Includes the following steps: Step S1: Drill a horizontal well in the coal seam as injection well I along the direction of the coal seam, and arrange a extraction well I on each side of injection well I in parallel. Drill a horizontal well in the oil shale layer as injection well II along the direction of the oil shale layer, and arrange a extraction well II on each side of injection well II in parallel. Step S2: Cementing is performed on injection well I, injection well II, extraction well I, and extraction well II respectively. After cementing, casing is arranged in the coal seam for perforation completion configuration. Injection well I uses fixed-face perforation, and extraction well I uses half-plane fixed-face perforation. The direction of the half-plane perforation of extraction well I is towards injection well I. In the oil shale layer, casing is arranged for perforation completion configuration. Fixed-face perforation is used in the direction of injection well II, and extraction well II uses half-plane fixed-face perforation. The direction of the half-plane perforation of extraction well II is towards injection well II. Step S3: Perform hydraulic fracturing on the coal seam, following these steps: Step S31: Use the coal seam between the horizontal sections of the two extraction wells as the fracturing zone; Step S32: Pump clean water into injection well I and adopt the fracturing mode of prioritizing the propagation of the main fracture. Stop the fracturing operation after the initiation pressure curve reaches the maximum peak value; at the same time, the two extraction wells I are fracturing through incremental gradient pumping and circulation. Step S33: Injection well I is switched to decremental gradient pumping and circulation fracturing to drive the main fracture to extend until it connects with the dense fracture network formed by fracturing in extraction well I; Step S34: After the fracturing of this section of the coal seam is completed, the remaining fracturing zone is fracturing in a backward manner until the fracturing of all coal seams is completed. Step S4: Perform hydraulic fracturing operations on the oil shale layer, following these steps: Step S41: Pump clean water into injection well II. First, use a large-displacement pump to perform segmented multi-cluster fracturing. Stop fracturing after the pressure reaches the peak value. Step S42: Then set the pumping parameters to a maximum of 80% of the peak pressure and the pumping time to T min. During the pumping process, while forming the main fracture, the natural fractures are connected to form a dense fracture network. Step S43: The two extraction wells II are fracturing by incremental gradient pumping and circulation to form fracturing fractures, which are connected to the fractures constructed in the injection well II; Step S5: Install pressure and temperature monitors in the coal seam and oil shale layer; Step S6: Conduct coalbed gasification gasification operations, following these steps: Step S61: Inject gasifying agent into injection well I and launch ignition equipment. Use a backward combustion method to burn the coal seam between the horizontal section of injection well I and extraction well I through fracturing fractures. Step S62: Injection well I continuously injects gasifying agent to maintain coal seam combustion and gas production, while extraction well I remains in a simmering state and its temperature is monitored; when the temperature of extraction well I rises to the set value, the injection of gasifying agent is stopped and combustion is terminated. Step S63: Continue to shut down the coal seam and monitor the formation pressure. If the formation pressure no longer increases, start gas extraction operation in extraction well I. Step S7: After the coal seam is fully combusted, the generated methane, H2 and high-temperature CO2 gases are extracted to the surface through the gasification channels formed by the hydraulic fracturing fractures by the two extraction wells I. Combustible gas and high-temperature CO2 are obtained through gas separation. The high-temperature CO2 is transported by insulated pipelines and injected into the oil shale layer from the injection well II, causing the kerogen in the oil shale layer to pyrolyze and produce oil and gas compounds. At the same time, the hydraulic fracturing structure fractures are formed to create gasification channels, which are then extracted to the surface by the two extraction wells II. Step S8: Repeat steps S6 and S7, with the coal seam undergoing retreating combustion and the oil shale layer undergoing retreating extraction, until both the coal seam and oil shale layer are mined out. In step S32, the two extraction wells I are fracturing using incremental gradient pumping and circulation methods, specifically: Based on the maximum peak value of the fracturing pressure curve of injection well I, set the circulation pumping parameters and gradient pumping pressure. The upper limit of the pumping pressure is increased step by step in stages of 20% of the maximum pressure until the upper limit of the pumping pressure is reached. Set the pumping time of each stage to t min and the interval between pumping stages to T min. Repeat the above stages N times to fully develop the microfractures in extraction well I and form a dense fracture network. In step S33, the injection well I is switched to a decreasing gradient pumping and cyclic fracturing method. Specifically, the maximum pumping pressure is set for the first pumping operation, and then each stage is gradually reduced by 20% of the maximum pressure. The pumping time for each stage is set to t min, and the interval between pumping stages is T min. The above stages are repeated N times.

2. The coal and oil shale in situ gasification and high temperature fluid pyrolysis synergetic recovery method as claimed in claim 1, wherein, After the coal seam is mined out, the goaf left after underground coal gasification is filled by grouting.

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