A method for deep underground coal gasification

Abstract

This invention relates to a method for deep underground coal gasification, comprising: drilling a plurality of gasification wells within a block, the wells being arranged in three rows from the edge of the block to fracturing the bottommost coal seam; the first gasification well being an injection well and the second gasification well being a production well, gasifying the bottommost coal seam between the first row and the second gasification well; when the high-temperature gasification zone of the gasified coal seam extends to the front of the gas cooling zone, the second gasification well is shut down, and the third gasification well is used as a production well to continue gasifying the coal seam; simultaneously, a fourth gasification well is drilled; after the coal seam is completely gasified, the bottommost coal seam between the second row and the third gasification well is gasified again, and so on, gasifying the bottommost coal seam between each gasification well; simultaneously, cement slurry is injected into the first gasification well to fill the gasified goaf; and so on, filling the goaf between each gasification well; and then, following the above method, each coal seam is gasified and filled sequentially from bottom to top.

Description

Technical Field

[0001] This invention belongs to the field of underground coal gasification technology, specifically relating to a method for deep underground coal gasification. Background Technology

[0002] Coal seams buried deep underground are less than 1,000 meters deep, making direct extraction impossible. Underground coal gasification refers to the controlled combustion (around 950℃) of coal buried deep underground, while simultaneously injecting a mixture of high-temperature steam and air. Through thermal action, a chemical reaction occurs, producing coal gas (a mixture of CH4, CO, and H2), which is then output to the surface through a gas extraction well.

[0003] Deep coal seams typically consist of multiple layers, each with varying thicknesses, resulting in high underground stress and poor permeability. Currently, research on deep coal gasification is primarily in the experimental exploration stage, with limited practical applications. The main problem is low gasification efficiency, leading to high costs. Currently, the common practice is to use a single injection well for each production well, performing point-to-point gasification, which results in low efficiency. Summary of the Invention

[0004] To address the above problems, this invention provides a method for deep underground coal gasification, comprising the following steps:

[0005] S1: Drill several gasification wells in a matrix distribution within the target area. The gasification wells are arranged in three rows starting from the edge of the target area. The bottom coal seam is fractured through the gasification wells.

[0006] S2: Each gasification well drills at least one horizontal branch well at the corresponding bottom coal seam, and introduces gas injection nozzle pipelines into the horizontal branch wells. The first gasification well located at the edge of the target block is used as an injection well, and the second gasification well is used as a production well. The bottom coal seam between the first and second gasification wells is gasified.

[0007] S3: When the high-temperature gasification zone of the gasified coal seam expands to the front of the gas cooling zone, the second gasification well is closed, and the third gasification well is used as the production well to continue gasifying the coal seam.

[0008] At the same time, several gasification wells in the fourth row were drilled, and the bottom coal seam was then fracturing. At least one horizontal branch well was drilled in the fourth gasification well corresponding to the bottom coal seam, and gas injection nozzle pipelines were introduced into the horizontal branch wells.

[0009] S4: After the bottom coal seam between the first row and the second degassing well has been completely gasified, the second degassing well will be used as an injection well to gasify the bottom coal seam between the second row and the third degassing well, and so on, to gasify the bottom coal seam between each degassing well.

[0010] Simultaneously, cement slurry is injected into the first gasification well to fill the gasified goaf; and so on, filling the goaf between each gasification well.

[0011] S5: The first degassing well fractures the sub-bottom coal seam. Then, the first degassing well is used as an injection well and the second degassing well is used as a production well to gasify the sub-bottom coal seam between the first and second degassing wells. After that, the sub-bottom coal seam between each degassing well is gasified according to the method of steps S3-S4, and then the goaf is filled.

[0012] Then, following the method in S5, the coal seams are gasified and filled sequentially from bottom to top.

[0013] More specifically, the method for deep underground coal gasification includes:

[0014] A1: Conduct geological surveys of the target plot and drill several gasification wells within the target plot. The gasification wells are distributed in a matrix. The gasification wells are arranged in three rows from the edge of the target plot. After cementing and completion of the gasification wells, the bottom coal seam is fracturing.

[0015] A2: The first three gasification wells drill at least one horizontal branch well at the corresponding bottom coal seam, and introduce gas injection nozzle pipelines into the horizontal branch wells to release gasification agent; the first gasification wells located at the edge of the target block are all used as injection wells, and the second gasification wells are all used as production wells, and then the bottom coal seam between the first and second gasification wells is gasified.

[0016] A3: When the high-temperature gasification zone of the bottom coal seam between the first and second rows expands to the front of the gas cooling zone, the second gasification well is closed, and the third gasification well is used as the extraction well to continue gasifying the coal seam.

[0017] At the same time, several gasification wells in the fourth row were drilled, and the bottom coal seam was then fracturing. At least one horizontal branch well was drilled in the fourth gasification well corresponding to the bottom coal seam, and gas injection nozzle pipelines were introduced into the horizontal branch wells.

[0018] A4: After the bottom coal seam between the first row and the second degassing well is completely gasified, the second degassing well will be used as an injection well, and the third degassing well will remain a production well. The bottom coal seam between the second row and the third degassing well will be gasified, and so on, the bottom coal seam between each degassing well will be gasified.

[0019] At the same time, cement slurry is injected into the first degassing well to fill the goaf area corresponding to the bottom coal seam between the first and second degassing wells, so as to prevent the mining area from collapsing; and so on, to fill the goaf area corresponding to the bottom coal seam between each degassing well.

[0020] A5: The first degassing well is used to fracture the bottom coal seam, and then a horizontal branch well corresponding to the bottom coal seam is drilled and a gas injection nozzle pipeline is introduced. Then, the first degassing well is used as an injection well and the second degassing well is used as a production well to gasify the bottom coal seam between the first and second degassing wells. After that, the bottom coal seam between each degassing well is gasified according to the method of steps A3-A4, and then the goaf is filled.

[0021] Then, following the method in A5, the coal seams are gasified sequentially from bottom to top and then filled.

[0022] This invention achieves three-dimensional, full gasification mining of deep coal seams in a target area through a rational layout of gasification wells, enabling more efficient, simpler, and lower-cost operations, and ensuring adequate filling to prevent coal seam collapse after mining. Specifically, several gasification wells are evenly arranged in a matrix within the target area. These wells are used alternately as injection and production wells, significantly reducing drilling costs and allowing for uniform control of coal seam gasification and goaf filling. The overall concept of coal seam gasification in this invention is as follows: following the direction from the first to the last gasification well, the bottommost coal seam is progressively gasified and burned, then the next lower coal seam is gasified upwards using the same method, and so on, layer by layer upwards. The gasified coal seam becomes the goaf, where cement slurry is injected using the gasification wells. After solidification, the cement slurry supports the goaf and prevents collapse.

[0023] This invention employs several gasification wells for sequential gasification and combustion. The first gasification well serves as an injection well, and the second as a production well. When the high-temperature gasification zone extends to the forefront of the gas cooling zone, the second gasification well is closed, and the third gasification well becomes the production well, continuing the gasification of the coal seam. This continuous, rolling gasification ensures complete combustion of the coal seam. After a portion of the coal seam has burned out, cement slurry is injected into the first gasification well to continuously fill the goaf. The same method is then used to gasify the upper coal seams layer by layer, achieving three-dimensional, complete gasification of the target area.

[0024] Optionally, in step S2, the horizontal branch well extends along the corresponding coal seam to the adjacent gasification wells in the same row; the direction of the horizontal branch well of a gasification well is consistent with the arrangement direction of the gasification wells in the same row; preferably, two horizontal branch wells are drilled for each coal seam of a gasification well, extending to the adjacent left and right gasification wells respectively.

[0025] Further optionally, in step S2, in the portion of the injection well corresponding to the bottommost coal seam, a coiled tubing drilling rig is used to drill horizontally (i.e., left and right) toward the adjacent injection well to form a horizontal branch well. A gas injection pipeline is run into the vertical section of the injection well, and a gas injection nozzle pipeline is run into the horizontal branch well. The bottom of the gas injection pipeline is connected to the gas injection nozzle pipeline, and several nozzles are evenly distributed on the gas injection nozzle pipeline for releasing the gasifying agent. A gas production pipeline is run into the gas production well for outputting coal gas.

[0026] Optionally, in step S3, the leading edge of the gas cooling zone is 1 / 2 to 2 / 3 of the distance between two adjacent gasification wells; the gasification high-temperature zone gradually moves from the injection well to the production well. When the distance between the edge of the gasification high-temperature zone closest to the production well and the injection well reaches 1 / 2 to 2 / 3 of the distance between two adjacent gasification wells, the second gasification well is closed, and the third gasification well is used as the production well to continue gasifying the coal seam.

[0027] Optionally, steps S4 and S5 are as follows:

[0028] (1) After the bottom coal seam between the first row and the second gasification well is completely gasified, the second gasification well is used as an injection well and the third gasification well is used as a production well to gasify the bottom coal seam between the second row and the third gasification well.

[0029] At the same time, cement slurry was injected into the first degassing well to fill the goaf area corresponding to the bottom coal seam between the first and second degassing wells;

[0030] (2) When the gasification high-temperature zone of the bottom coal seam between the second and third rows expands to the front edge of the gas cooling zone, the third gasification well is closed and the fourth gasification well is used as the extraction well to continue gasifying the coal seam.

[0031] Meanwhile, wait for the cement slurry from step (1) to cure;

[0032] At the same time, several gasification wells in the fifth row are drilled, and the bottom coal seam is then fracturing through the fifth gasification well. Horizontal branch wells are drilled at the corresponding bottom coal seam of the fifth gasification well, and gas injection nozzle pipelines and gas production pipelines are introduced into the horizontal branch wells.

[0033] (3) After the bottom coal seam between the second row and the third gasification well is completely gasified, the third gasification well is used as the injection well, and the fourth gasification well is still the production well, and the bottom coal seam between the third row and the fourth gasification well is gasified.

[0034] At the same time, cement slurry was injected into the second degassing well to fill the goaf area corresponding to the bottom coal seam between the second and third degassing wells;

[0035] (4) When the gasification high-temperature zone of the bottom coal seam between the third and fourth rows expands to the front of the gas cooling zone, the fourth gasification well is closed and the fifth gasification well is used as the extraction well to continue gasifying the coal seam.

[0036] Meanwhile, wait for the cement slurry from step (3) to cure;

[0037] At the same time, several gasification wells in the sixth row are drilled, and the bottom coal seam is then fracturing through the sixth gasification well. Horizontal branch wells are drilled at the corresponding bottom coal seam of the sixth gasification well, and gas injection nozzle pipelines and gas production pipelines are introduced into the horizontal branch wells.

[0038] Simultaneously, the first and second gasification wells are fracturing at the corresponding sub-bottom coal seams. The first and second gasification wells are then drilled to form horizontal branch wells for the corresponding sub-bottom coal seams, and gas injection pipelines and gas injection nozzle pipelines are introduced. Then, the first gasification well is used as an injection well and the second gasification well is used as a production well to gasify the sub-bottom coal seam between the first and second gasification wells.

[0039] Following the steps described above, the coal seams are gasified and then filled sequentially from bottom to top.

[0040] Optionally, step S2 may also include:

[0041] (a) A gasification zone is planned between the corresponding injection well and production well. The gasification zone is square. The width of the gasification zone is the distance between the injection well and the production well. The length of the gasification zone is the distance between two adjacent injection wells. The injection well is located in the middle of one long side of the gasification zone, and the production well is located in the middle of the other long side of the gasification zone.

[0042] (b) After fracturing the bottom coal seam, several gasification channels are drilled at the injection well and / or production well corresponding to the bottom coal seam. The head of the gasification channel is connected to the injection well or production well, and the tail is directed toward the production well or injection well and extends to the boundary of the gasification zone. The included angle between adjacent gasification channels is the same. Several gasification channels are evenly distributed radially in the gasification zone with the injection well or production well as the center.

[0043] (c) Gravel material is fed into each gasification channel through injection wells or production wells until the gasification channel is filled.

[0044] Alternatively, several gasification channels corresponding to the same production well may be at the same height, and several gasification channels corresponding to the same injection well may be at the same height.

[0045] Alternatively, between the corresponding injection well and production well, the gasification channel connecting the head to the production well is located in the upper part of the coal seam, and the gasification channel connecting the head to the injection well is located in the lower part of the coal seam, thus forming gasification channels in different layers.

[0046] Alternatively, when the thickness of the coal seam to be gasified is no more than 3 meters, the included angle between adjacent gasification channels is about 60°; when the thickness of the coal seam to be gasified is greater than 3 meters, the included angle between adjacent gasification channels is about 30°.

[0047] When the thickness of the coal seam to be gasified is no more than 1 meter, it is considered a thin coal seam. One gasification channel is set up, and the head of the gasification channel can be connected to either the injection well or the production well. When the thickness of the coal seam to be gasified is greater than 1 meter, at least two gasification channels are set up, one gasification channel is located in the upper middle part of the coal seam, and the other gasification channel is located in the lower middle part of the coal seam.

[0048] Further optionally, when the number of gasification channels in the coal seam is greater than two, the heads of the gasification channels in the lower part of the coal seam are all connected to the injection well, and the heads of the gasification channels in the upper part of the coal seam are all connected to the production well. Attached Figure Description

[0049] Figure 1 A schematic diagram of the gasification well layout within the target site;

[0050] Figure 2 A schematic diagram of underground coalbed methane processing conditions for a pair of injection and production wells;

[0051] Figure 3 This is a schematic diagram of the cross-section of a gas injection well;

[0052] Figure 4 A schematic diagram of the injection well and the gasification channel;

[0053] Figure 5 This is a schematic diagram of the gasification channels for the injection well and the production well.

[0054] In the attached diagram, 1-gasification well, 2-injection well, 3-production well, 4-gas injection pipeline, 5-gas injection nozzle pipeline, 6-production pipeline, and 7-gasification channel. Detailed Implementation

[0055] This embodiment provides a method for deep underground coal gasification, such as... Figures 1-5 As shown, the following steps are included:

[0056] S1: Conduct geological surveys of the target plot and drill several gasification wells 1 within the target plot. The gasification wells are distributed in a matrix. The gasification wells are arranged in three rows from the edge of the target plot. After cementing and completion of the gasification wells 1, the bottom coal seam is fracturing.

[0057] S2: The first three gasification wells drill at least one horizontal branch well at the corresponding bottom coal seam, and introduce gas injection nozzle pipelines into the horizontal branch wells to release gasification agent; the first gasification wells located at the edge of the target block are all used as injection wells 2, and the second gasification wells are all used as production wells 3, and then the bottom coal seam between the first and second gasification wells is gasified.

[0058] S3: When the gasification high-temperature zone of the bottom coal seam between the first and second rows expands to the front of the gas cooling zone, the second gasification well is closed, and the third gasification well is used as the production well to continue gasifying the coal seam.

[0059] At the same time, several gasification wells in the fourth row were drilled, and the bottom coal seam was then fracturing. At least one horizontal branch well was drilled in the fourth gasification well corresponding to the bottom coal seam, and gas injection nozzle pipelines were introduced into the horizontal branch wells.

[0060] S4: After the bottom coal seam between the first row and the second degassing well has been completely gasified, the second degassing well becomes the injection well to gasify the bottom coal seam between the second row and the third degassing well, and so on, to gasify the bottom coal seam between each degassing well.

[0061] At the same time, cement slurry is injected into the first degassing well to fill the goaf area corresponding to the bottom coal seam between the first and second degassing wells, so as to prevent the mining area from collapsing; and so on, to fill the goaf area corresponding to the bottom coal seam between each degassing well.

[0062] S5: The first degassing well is used to fracture the bottom coal seam, and then a horizontal branch well corresponding to the bottom coal seam is drilled. Gas injection nozzles and gas production lines are introduced. Then, the first degassing well is used as an injection well and the second degassing well is used as a production well to gasify the bottom coal seam between the first and second degassing wells. After that, the bottom coal seam between each degassing well is gasified according to the method of steps S3-S4, and then the goaf is filled.

[0063] Then, following the method in S5, the coal seams are gasified and filled sequentially from bottom to top.

[0064] The geological exploration in step S1 is a conventional exploration, and the exploration content includes, but is not limited to, coal seam structure, number of coal seams, coal seam thickness, coal seam pressure and temperature, water content, and sulfur content; based on the geological exploration results, the scope of the target block and the drilling plan are determined.

[0065] Optionally, in step S1, the gasification well is a vertical well that can penetrate all underground coal seams from top to bottom. The spacing between adjacent gasification wells depends on the actual situation of the target plot, and is generally 300-500 meters apart.

[0066] The gasification well undergoes conventional cementing and completion operations. Specifically, a casing is installed inside the gasification well. The casing is resistant to high temperatures of 1000-1200℃ and is also resistant to corrosion from hydrogen, sulfur, and carbon dioxide. The cementing cement is also heat-resistant and is returned to the wellhead. After drilling, completion, and pressure testing are passed, hydraulic fracturing is performed on the bottom coal seam to modify it. Conventional fracturing fluid and proppant are injected into the coal seam to create large-area fractures, which are conducive to coal combustion and gas flow. After the fracturing operation is completed, gas injection nozzles are introduced into the horizontal branch well, and then the water and fracturing fluid in the bottom coal seam are pumped out.

[0067] Optionally, in step S2, the horizontal branch well extends along the corresponding coal seam to the adjacent gasification wells in the same row; the direction of the horizontal branch well of a gasification well is consistent with the arrangement direction of the gasification wells in the same row; preferably, two horizontal branch wells are drilled for each coal seam of a gasification well, extending to the adjacent left and right gasification wells respectively.

[0068] Further optionally, in step S2, in the part of the injection well corresponding to the bottom coal seam, a coiled tubing drilling rig is used to drill horizontally (i.e. left and right) to the adjacent injection well to form a horizontal branch well. A gas injection pipeline 4 is run into the vertical section of the injection well, and a gas injection nozzle pipeline 5 is introduced into the horizontal branch well. The bottom of the gas injection pipeline is connected to the gas injection nozzle pipeline, and several nozzles are evenly distributed on the gas injection nozzle pipeline for releasing gasifying agent. A gas production pipeline 6 is run into the gas production well for outputting coal gas.

[0069] The first gasification well is equipped with a gas injection nozzle pipeline, and subsequent gasification wells are also equipped with gas injection nozzle pipelines, along with gas production pipelines.

[0070] Conventional gasifying agent is injected into the gas injection pipeline of the injection well to ignite the coal seam, maintaining the temperature at around 900-950℃. A combustion zone of a certain area is formed near the coal seam in the injection well. Then, a mixture of air and high-temperature water vapor is injected. The amount of air injected is determined comprehensively based on the coal seam pressure, temperature, gas composition of the gas well, and production rate.

[0071] The gasification high-temperature zone is the high-temperature zone for coal combustion. The coal gas (CH4, CO, H2) formed by the reaction of water vapor and coal flows to the gas production well through the dry distillation zone and is then extracted through the gas production well.

[0072] Optionally, in step S3, the leading edge of the gas cooling zone is 1 / 2 to 2 / 3 of the distance between two adjacent gasification wells; the gasification high-temperature zone gradually moves from the injection well to the production well. When the distance between the edge of the gasification high-temperature zone closest to the production well and the injection well reaches 1 / 2 to 2 / 3 of the distance between two adjacent gasification wells, the second gasification well is closed, and the third gasification well is used as the production well to continue gasifying the coal seam.

[0073] The gas cooling zone is roughly equivalent to the dry distillation zone. In this zone, the coal is heated and may undergo a chemical reaction, but it does not burn. The coal gas produced after combustion cools down as it passes through the gas cooling zone, reducing the gas pressure in the extraction well and facilitating gas extraction. Therefore, a gas cooling zone is always maintained between the extraction well and the injection well.

[0074] Optionally, steps S4 and S5 are as follows:

[0075] (1) After the bottom coal seam between the first row and the second gasification well is completely gasified, the second gasification well is used as the injection well and the third gasification well is used as the production well to gasify the bottom coal seam between the second row and the third gasification well.

[0076] At the same time, cement slurry was injected into the first degassing well to fill the goaf area corresponding to the bottom coal seam between the first and second degassing wells;

[0077] (2) When the gasification high-temperature zone of the bottom coal seam between the second and third rows expands to the front edge of the gas cooling zone, the third gasification well is closed and the fourth gasification well is used as the extraction well to continue gasifying the coal seam.

[0078] Meanwhile, wait for the cement slurry from step (1) to cure;

[0079] At the same time, several gasification wells in the fifth row are drilled, and the bottom coal seam is then fracturing through the fifth gasification well. Horizontal branch wells are drilled at the corresponding bottom coal seam of the fifth gasification well, and gas injection nozzle pipelines and gas production pipelines are introduced into the horizontal branch wells.

[0080] (3) After the bottom coal seam between the second row and the third gasification well is completely gasified, the third gasification well is used as an injection well, and the fourth gasification well is still used as a production well to gasify the bottom coal seam between the third row and the fourth gasification well.

[0081] At the same time, cement slurry was injected into the second degassing well to fill the goaf area corresponding to the bottom coal seam between the second and third degassing wells;

[0082] (4) When the gasification high-temperature zone of the bottom coal seam between the third and fourth rows expands to the front of the gas cooling zone, the fourth gasification well is closed and the fifth gasification well is used as the extraction well to continue gasifying the coal seam.

[0083] Meanwhile, wait for the cement slurry from step (3) to cure;

[0084] At the same time, several gasification wells in the sixth row are drilled, and the bottom coal seam is then fracturing through the sixth gasification well. Horizontal branch wells are drilled at the corresponding bottom coal seam of the sixth gasification well, and gas injection nozzle pipelines and gas production pipelines are introduced into the horizontal branch wells.

[0085] Simultaneously, the first and second gasification wells are fracturing at the corresponding sub-bottom coal seams. The first and second gasification wells are then drilled to form horizontal branch wells for the corresponding sub-bottom coal seams, and gas injection pipelines and gas injection nozzle pipelines are introduced. Then, the first gasification well is used as an injection well and the second gasification well is used as a production well to gasify the sub-bottom coal seam between the first and second gasification wells.

[0086] Following the steps described above, the coal seams are gasified and then filled sequentially from bottom to top.

[0087] Further optionally, in step (1), since the second exhaust wells are all used as injection wells, the amount of filling mortar should be controlled to prevent the mortar from flowing to the location of the second exhaust well and blocking the air injection pipeline; subsequently, when injecting filling mortar into each exhaust well, the amount should be controlled to prevent the mortar from flowing to the next exhaust well. The cement slurry is conventional concrete cement slurry.

[0088] Optionally, the second bottom coal seam is the coal seam above the bottom coal seam.

[0089] According to the above method, the gasification and filling of the lower coal seam and the upper coal seam are carried out in a cyclic rolling manner. Each gasification well is fully utilized, and the gasification wells respectively play the roles of injection well, production well and cement slurry injection well to ensure that the coal seam is fully burned. Then, the filling is carried out in a progressive manner to effectively support the coal seam and avoid the collapse of the mined coal seam. The process is reasonable, the steps are closely connected, and the efficiency is high.

[0090] Deep underground coal seams have multiple sets of coal seams (i.e., multiple layers of coal seams), high pressure, and low permeability. Compared with the currently disclosed coal seam gasification technology, the present invention has the following characteristics: (1) Through drilling, gasification of large areas and multiple sets of coal seams can be effectively controlled in both plane and space, with low investment costs; (2) Coal seam fracturing and the establishment of gasification channels can greatly improve the gasification surface area and achieve high gasification efficiency; (3) The rotation of injection and production wells in the plane and the backfilling of cement slurry in the goaf can improve the recovery rate of coal seams and avoid stratum collapse; (4) Gasification mining can be carried out on deep coal seams with various structural forms; (5) Simultaneous gasification of multiple wells and multiple layers can be realized to achieve large-scale and efficient production processes.

[0091] Optionally, step S2 may also include:

[0092] (a) A gasification zone is planned between the corresponding injection well and production well. The gasification zone is square. The width of the gasification zone is the distance between the injection well and the production well. The length of the gasification zone is the distance between two adjacent injection wells. The injection well is located in the middle of one long side of the gasification zone, and the production well is located in the middle of the other long side of the gasification zone.

[0093] (b) After fracturing the bottom coal seam, a number of gasification channels 6 are drilled at the injection well and / or production well corresponding to the bottom coal seam. The head of the gasification channel is connected to the injection well or production well, and the tail is directed toward the production well or injection well and extends to the boundary of the gasification zone. The included angle between adjacent gasification channels is the same. The number of gasification channels are evenly distributed radially in the gasification zone with the injection well or production well as the center.

[0094] (c) Gravel material is fed into each gasification channel through injection wells or production wells until the gasification channel is filled.

[0095] After step (c), a horizontal branch well is drilled in the injection well and the gas injection nozzle line is introduced. Then, the water and fracturing fluid of the bottom coal seam are discharged, and the coal seam is ignited between the first and second gasification wells.

[0096] Alternatively, several gasification channels corresponding to the same production well may be at the same height, and several gasification channels corresponding to the same injection well may be at the same height.

[0097] The length of the vaporization channel is determined by the angle between the vaporization channels and the size of the vaporization zone; the head of the vaporization channel is located in the middle of one long side of the vaporization zone, and the tail extends to the other long side, or to the two wide sides of the vaporization zone.

[0098] Alternatively, between the corresponding injection well and production well, the gasification channel connecting the head to the production well is located in the upper part of the coal seam, and the gasification channel connecting the head to the injection well is located in the lower part of the coal seam, thus forming gasification channels in different layers.

[0099] In steps S3-S5, for each coal seam and the coal seam between two gasification wells, the above-mentioned gasification channels are drilled for the corresponding injection wells and / or production wells. The tails of the gasification channels corresponding to adjacent production wells in the same row can be connected, and the tails of the gasification channels corresponding to adjacent injection wells in the same row can be connected.

[0100] To gasify the coal seam between the two gasification wells, it is necessary to switch the injection well and the production well once in between. Therefore, after fracturing the coal seam, the gasification channels corresponding to the adjacent three gasification wells are drilled and gravel (particle size about 1-5mm) is injected. Then, the horizontal branch wells of the first batch of gasification wells are drilled, and the gasification operation is carried out.

[0101] This invention, building upon the basic principles of traditional fracturing, further constructs gasification channels within the coal seam, significantly increasing the gasification surface area. Specifically, this invention employs a method of drilling gasification channels into the coal seam after fracturing, creating unblocking "blood vessels" within the coal seam. These channels are then filled with gravel, resulting in numerous voids or pores that effectively increase the gasification area and enhance the diffusion rate of the gasifying agent and coal gas. The inner diameter of the gasification channels and the volume of the gravel are determined based on the specific conditions of the coal seam.

[0102] Alternatively, when the thickness of the coal seam to be gasified is no more than 3 meters, the included angle between adjacent gasification channels is about 60°; when the thickness of the coal seam to be gasified is greater than 3 meters, the included angle between adjacent gasification channels is about 30°.

[0103] When the thickness of the coal seam to be gasified is no more than 1 meter, it is considered a thin coal seam. One gasification channel is set up, and the head of the gasification channel can be connected to either the injection well or the production well. When the thickness of the coal seam to be gasified is greater than 1 meter, at least two gasification channels are set up, one gasification channel is located in the upper middle part of the coal seam, and the other gasification channel is located in the lower middle part of the coal seam.

[0104] Further optionally, when the number of gasification channels in the coal seam is greater than two, the heads of the gasification channels in the lower part of the coal seam are all connected to the injection well, and the heads of the gasification channels in the upper part of the coal seam are all connected to the production well.

Claims

1. A method for deep underground coal gasification, characterized in that, Includes the following steps: S1: Starting from the edge of the target block, drill several gasification wells and arrange them in three rows to fracture the bottom coal seam. S2: Drill a horizontal branch well at the bottom coal seam corresponding to the gasification well; the first gasification well is used as the injection well, and the second gasification well is used as the production well, gasifying the bottom coal seam between the first and second gasification wells; S3: When the gasification high-temperature zone of the bottom coal seam reaches the front of the gas cooling zone, close the second gasification well and use the third gasification well as the production well to continue gasifying the coal seam. At the same time, drill several gasification wells in the fourth row, then fracture the bottom coal seam accordingly, and drill horizontal branch wells at the bottom coal seam corresponding to the fourth gasification well. S4: After the bottom coal seam between the first row and the second degassing well is completely gasified, the second degassing well is used as an injection well to gasify the bottom coal seam between the second row and the third degassing well, and so on, to gasify the bottom coal seam between each degassing well. Simultaneously, cement slurry is injected into the first gasification well to fill the gasified goaf; and so on, filling the goaf between each gasification well. S5: Fracturing the sub-bottom coal seam, using the first degassing well as the injection well and the second degassing well as the production well, gasifying the sub-bottom coal seam between the first and second degassing wells; then gasifying the sub-bottom coal seam between each degassing well, and then filling the goaf. Following the above method, the coal seams are gasified sequentially from bottom to top and then filled. In step S3, the leading edge of the gas cooling zone is 1 / 2 to 2 / 3 of the distance between two adjacent gasification wells; the gasification high-temperature zone gradually moves from the injection well to the production well. When the distance between the edge of the gasification high-temperature zone closest to the production well and the injection well reaches 1 / 2 to 2 / 3 of the distance between two adjacent gasification wells, the second gasification well is closed and the third gasification well is used as the production well.

2. The method for deep underground coal gasification according to claim 1, characterized in that, In step S2, the horizontal branch well extends along the corresponding coal seam to the adjacent gasification wells in the same row; the direction of the horizontal branch well of a gasification well is consistent with the arrangement direction of the gasification wells in the same row. For each coal seam, two horizontal branch wells are drilled to extend to the two adjacent gasification wells on the left and right.

3. The method for deep underground coal gasification according to claim 2, characterized in that, In step S2, in the part of the injection well corresponding to the bottom coal seam, a coiled tubing drilling rig is used to drill horizontally to the adjacent injection well to form a horizontal branch well. A gas injection pipeline is run into the vertical section of the injection well, and a gas injection nozzle pipeline is introduced into the horizontal branch well. The bottom of the gas injection pipeline is connected to the gas injection nozzle pipeline, and several nozzles are evenly distributed on the gas injection nozzle pipeline for releasing the gasifying agent. A gas production pipeline is run into the gas production well for outputting coal gas.

4. The method for deep underground coal gasification according to claim 1, characterized in that, Steps S4 and S5 are as follows: (1) After the bottom coal seam between the first row and the second gasification well has been completely gasified, the bottom coal seam between the second row and the third gasification well is gasified. At the same time, cement slurry was injected into the first degassing well to fill the goaf area corresponding to the bottom coal seam between the first and second degassing wells; (2) When the gasification high-temperature zone of the bottom coal seam between the second and third rows expands to the front of the gas cooling zone, the third gasification well is closed and the fourth gasification well is used as the extraction well to continue gasifying the coal seam. Meanwhile, wait for the cement slurry from step (1) to cure; At the same time, several gasification wells in the fifth row are drilled, and the bottom coal seam is then fracturing through the fifth gasification well. Horizontal branch wells are drilled at the corresponding bottom coal seam of the fifth gasification well, and gas injection nozzle pipelines and gas production pipelines are introduced into the horizontal branch wells. (3) After the bottom coal seam between the second row and the third gasification well has been completely gasified, the third gasification well is used as an injection well to gasify the bottom coal seam between the third row and the fourth gasification well. At the same time, cement slurry was injected into the second degassing well to fill the goaf area corresponding to the bottom coal seam between the second and third degassing wells; (4) When the gasification high-temperature zone of the bottom coal seam between the third and fourth rows expands to the front of the gas cooling zone, the fourth gasification well is closed and the fifth gasification well is used as the extraction well to continue gasifying the coal seam. Meanwhile, wait for the cement slurry from step (3) to cure; At the same time, several gasification wells in the sixth row are drilled, and the bottom coal seam is then fracturing through the sixth gasification well. Horizontal branch wells are drilled at the corresponding bottom coal seam of the sixth gasification well, and gas injection nozzle pipelines and gas production pipelines are introduced into the horizontal branch wells. Simultaneously, the first and second gasification wells are fracturing at the corresponding sub-bottom coal seams. The first and second gasification wells are then drilled to form horizontal branch wells for the corresponding sub-bottom coal seams, and gas injection pipelines and gas injection nozzle pipelines are introduced. Then, the first gasification well is used as an injection well and the second gasification well is used as a production well to gasify the sub-bottom coal seam between the first and second gasification wells. Following the steps described above, the coal seams are gasified and then filled sequentially from bottom to top.

5. The method for deep underground coal gasification according to claim 1, characterized in that, Step S2 also includes: (a) A gasification zone is planned between the injection well and the production well in a one-to-one correspondence. The gasification zone is square. The width of the gasification zone is the distance between the injection well and the production well. The length of the gasification zone is the distance between two adjacent injection wells. The injection well is located in the middle of one long side of the gasification zone, and the production well is located in the middle of the other long side of the gasification zone. (b) After fracturing the bottom coal seam, several gasification channels are drilled at the injection well and / or production well corresponding to the bottom coal seam. The head of the gasification channel is connected to the injection well or production well, and the tail is directed toward the production well or injection well and extends to the boundary of the gasification zone. The included angle between adjacent gasification channels is the same. Several gasification channels are evenly distributed radially in the gasification zone with the injection well or production well as the center. (c) Gravel material is fed into each gasification channel through injection wells or production wells until the gasification channel is filled.

6. The method for deep underground coal gasification according to claim 5, characterized in that, Several gasification channels corresponding to the same production well are at the same height, and several gasification channels corresponding to the same injection well are at the same height.

7. The method for deep underground coal gasification according to claim 6, characterized in that, Between the corresponding injection well and production well, the gasification channel connecting the head to the production well is located in the upper part of the coal seam, and the gasification channel connecting the head to the injection well is located in the lower part of the coal seam, thus forming gasification channels in different layers.

8. The method for deep underground coal gasification according to claim 7, characterized in that, When the thickness of the coal seam to be gasified is no more than 3 meters, the included angle between adjacent gasification channels is 60°; when the thickness of the coal seam to be gasified is greater than 3 meters, the included angle between adjacent gasification channels is 30°.

9. The method for deep underground coal gasification according to claim 7, characterized in that, When the thickness of the coal seam to be gasified is no more than 1 meter, one gasification channel is set up, and the head of the gasification channel can be connected to either the injection well or the production well; when the thickness of the coal seam to be gasified is greater than 1 meter, at least two gasification channels are set up, one gasification channel is located in the upper middle part of the coal seam, and the other gasification channel is located in the lower middle part of the coal seam. When there are more than two layers of gasification channels in a coal seam, the heads of the gasification channels in the lower part of the coal seam are connected to the injection well, and the heads of the gasification channels in the upper part of the coal seam are connected to the production well.

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