Method for underground gasification of deep coal

Abstract

The invention relates to a deep coal underground gasification method which comprises the following steps: drilling a plurality of gasification wells in a land parcel, sequentially arranging the plurality of gasification wells into three rows from the edge of the land parcel, and fracturing a bottommost coal seam; the first row of gasification wells are injection wells, the second row of gasification wells are production wells, and the bottommost coal seam between the first row of gasification wells and the second row of gasification wells is gasified; when the gasification high-temperature area of the gasified coal seam is expanded to the front edge of the gas cooling area, the second gas gasification well is closed, the third gas gasification well serves as a production well, and the coal seam continues to be gasified; meanwhile, a fourth row of gasification wells are drilled; after the coal bed is gasified, the bottommost coal bed between the second row of gasification wells and the third row of gasification wells is gasified, and so on, the bottommost coal bed between all the rows of gasification wells is gasified; meanwhile, cement paste is injected into the first exhaust gasification well to fill the gasified goaf; and so on, filling the goaf between each row of gasification wells; sequentially gasifying and refilling each coal seam from bottom to top according to the method.

Description

Technical Field

[0001] The present invention belongs to the technical field of underground coal gasification, and particularly relates to a method for underground coal gasification in deep coal seams. Background Art

[0002] The buried depth of deep underground coal seams is below 1000 meters, and it is impossible to directly mine the coal. Underground coal gasification refers to the controlled combustion of coal buried deep underground (about 950 °C), while injecting a mixture of high-temperature steam and air. Through thermal action, a chemical reaction occurs to produce coal gas (a mixed gas of CH 4 、CO、H 2 ), and then the coal gas is output to the ground through a gas production well.

[0003] Deep underground coal seams generally have multiple coal seams up and down, the thickness of each coal seam is different, the underground stress is large, and the permeability of the coal seam is poor. At present, the research on deep coal gasification is mainly in the experimental exploration stage, and there are few practical applications. The main problem is that the gasification efficiency is low, resulting in high costs. Currently, generally one injection well corresponds to one production well for point-to-point gasification, and the gasification efficiency is low. Summary of the Invention

[0004] In view of the above problems, the present invention provides a method for underground coal gasification in deep coal seams, including the following steps:

[0005] S1: Drill a number of gasification wells distributed in a matrix in the target plot. The gasification wells are arranged in three rows in sequence starting from the edge of the target plot, and the bottom coal seam is fractured through the gasification wells;

[0006] S2: Drill at least one horizontal branch well at the corresponding bottom coal seam for each gasification well, introduce an injection nozzle pipeline into the horizontal branch well. The first row of gasification wells at the edge of the target plot are all used as injection wells, and the second row of gasification wells are all used as production wells to gasify the bottom coal seam between the first row and the second row of gasification wells;

[0007] S3: When the gasification high-temperature zone of the gasified coal seam extends to the front of the gas cooling zone, close the second row of gasification wells, use the third row of gasification wells as production wells, and continue to gasify the coal seam;

[0008] Meanwhile, drill a number of gasification wells in the fourth row, fracture the bottom coal seam again, drill at least one horizontal branch well at the corresponding bottom coal seam for the fourth row of gasification wells, and introduce an injection nozzle pipeline into the horizontal branch well;

[0009] S4: After the bottom coal seam between the first row and the second row of gasification wells is completely gasified, use the second row of gasification wells as injection wells to gasify the bottom coal seam between the second row and the third row of gasification wells, and so on, to gasify the bottom coal seam between each row of gasification wells;

[0010] Meanwhile, inject cement slurry into the first gasification well to fill the goaf after gasification; and so on, fill the goafs between each gasification well;

[0011] S5: Fracture the sub-bottom coal seam of the first gasification well. Then, use the first gasification well as the injection well and the second gasification well as the production well to gasify the sub-bottom coal seam between the first row and the second gasification well. After that, gasify the sub-bottom coal seams between each gasification well according to the methods of steps S3 - S4, and then fill the goaf;

[0012] Then, gasify and fill each coal seam from bottom to top in turn according to the method of S5.

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

[0014] A1: Conduct geological exploration on the target plot, drill several gasification wells in the target plot, and the gasification wells are distributed in a matrix; several gasification wells are arranged in three rows in sequence starting from the edge of the target plot. After the gasification wells are cemented and completed, fracture the bottommost coal seam;

[0015] A2: Drill at least one horizontal branch well at the corresponding bottommost coal seam of the first three rows of gasification wells, and introduce an injection nozzle pipeline into the horizontal branch well for releasing the gasifying agent; the first gasification well at the edge of the target plot is used as the injection well, and the second gasification well is used as the production well, and then gasify the bottommost coal seam between the first row and the second gasification well;

[0016] A3: When the high-temperature gasification zone of the bottommost coal seam between the first row and the second row extends to the front of the gas cooling zone, close the second gasification well, use the third gasification well as the production well, and continue to gasify the coal seam;

[0017] Meanwhile, drill several gasification wells in the fourth row, fracture the bottommost coal seam again, drill at least one horizontal branch well at the corresponding bottommost coal seam of the fourth gasification well, and introduce an injection nozzle pipeline into the horizontal branch well;

[0018] A4: After all the bottommost coal seams between the first row and the second gasification wells are gasified, use the second gasification wells as injection wells, and the third gasification wells still serve as production wells to gasify the bottommost coal seams between the second row and the third gasification wells, and so on, gasify the bottommost coal seams between each gasification well;

[0019] Meanwhile, inject cement slurry into the first gasification well to fill the goaf corresponding to the bottommost coal seam between the first row and the second gasification well to prevent the collapse of the mining pit area; and so on, fill the goafs corresponding to the bottommost coal seams between each gasification well;

[0020] A5: Fracture the sub-bottom coal seam of the first row of gasification wells, then drill a horizontal branch well corresponding to the sub-bottom coal seam, and introduce the gas injection nozzle pipeline. Then, the first row of gasification wells serves as the injection wells, and the second row of gasification wells serves as the production wells to gasify the sub-bottom coal seam between the first row and the second row of gasification wells. After that, according to the method of steps A3 - A4, gasify the sub-bottom coal seam between each row of gasification wells, and then fill the goaf;

[0021] Then, according to the method of A5, gasify and fill each coal seam from bottom to top in turn.

[0022] Through the reasonable layout of gasification wells, the present invention realizes the three-dimensional full gasification mining of the deep coal seam in the target plot in a more efficient, simpler and lower-cost way, and conducts full filling to avoid the collapse of the coal seam after mining. Specifically, a number of gasification wells are arranged uniformly in a matrix in the target plot, and the gasification wells are alternately used as injection wells and production wells, which greatly reduces the drilling cost and can evenly control the gasification of the coal seam and the filling of the goaf. The general idea of the coal seam gasification of the present invention is: in the direction from the first row of gasification wells to the last row of gasification wells, gradually push forward to gasify and burn the bottommost coal seam, then gasify the sub-bottom coal seam upward in the same way, and then gasify each coal seam layer by layer upward; the gasified coal seam becomes a goaf, and cement slurry is injected into the gasification well, and after the cement slurry solidifies, it supports the goaf to avoid collapse.

[0023] The present invention sets a number of rows of gasification wells and gasifies and burns row by row. The first row of gasification wells is the injection wells, and the second row of gasification wells is the production wells. When the high-temperature gasification area expands to the front of the gas cooling area, the second row of gasification wells is closed, and the third row of gasification wells is used as the production wells to continue gasifying the coal seam. In this way, rolling continuous gasification is carried out to achieve full combustion of the coal seam. After a part of the coal seam is burned out, cement slurry is injected into the first row of gasification wells, and the goaf area is filled in a rolling and continuous manner. Then, each coal seam above is gasified layer by layer in the same way to achieve three-dimensional full gasification of the target plot.

[0024] Optionally, in step S2, the horizontal branch well extends along the corresponding coal seam towards 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 corresponding to a gasification well, and they extend towards the adjacent gasification wells on the left and right respectively.

[0025] Further optionally, in step S2, in the part of the injection well corresponding to the bottommost coal seam, use a coiled tubing drilling rig to drill horizontally (i.e., left and right) towards the adjacent injection well to form a horizontal branch well. Lower the gas injection pipeline into the vertical straight well part of the injection well, and lower the gas injection nozzle pipeline into the horizontal branch well. The bottom of the gas injection pipeline is connected to the gas injection nozzle pipeline, and a number of nozzles are evenly distributed on the gas injection nozzle pipeline for releasing the gasifying agent; lower the gas production pipeline into the gas production well for outputting coal gas.

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

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

[0028] (1) After the bottommost coal seam between the first and second rows of gasification wells is completely gasified, all the second-row gasification wells are used as injection wells, and all the third-row gasification wells are used as production wells to gasify the bottommost coal seam between the second and third rows of gasification wells;

[0029] Meanwhile, cement slurry is injected into the first row of gasification wells to fill the gob area corresponding to the bottommost coal seam between the first and second rows of gasification wells;

[0030] (2) When the high-temperature gasification zone of the bottommost coal seam between the second and third rows extends to the front of the gas cooling zone, the third row of gasification wells is closed, and the fourth row of gasification wells is used as the production well to continue gasifying the coal seam;

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

[0032] Meanwhile, drill several gasification wells in the fifth row, then fracture the bottommost coal seam through the fifth row of gasification wells. The fifth row of gasification wells drill horizontal branch wells at the corresponding bottommost coal seam, and inject the gas injection nozzle pipeline and the gas production pipeline into the horizontal branch wells;

[0033] (3) After the bottommost coal seam between the second and third rows of gasification wells is completely gasified, the third row of gasification wells is used as the injection well, and the fourth row of gasification wells remains the production well to gasify the bottommost coal seam between the third and fourth rows of gasification wells;

[0034] Meanwhile, cement slurry is injected into the second row of gasification wells to fill the gob area corresponding to the bottommost coal seam between the second and third rows of gasification wells;

[0035] (4) When the high-temperature gasification zone of the bottommost coal seam between the third and fourth rows extends to the front of the gas cooling zone, the fourth row of gasification wells is closed, and the fifth row of gasification wells is used as the production well to continue gasifying the coal seam;

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

[0037] Meanwhile, drill several gasification wells in the sixth row, then fracture the bottommost coal seam through the sixth row of gasification wells. The sixth row of gasification wells drill horizontal branch wells at the corresponding bottommost coal seam, and inject the gas injection nozzle pipeline and the gas production pipeline into the horizontal branch wells;

[0038] Meanwhile, the first gasification well and the second gasification well are fractured at the positions corresponding to the sub-bottom coal seam. The first gasification well and the second gasification well then drill horizontal branch wells corresponding to the sub-bottom coal seam, and inject gas pipelines and gas injection nozzle pipelines are introduced. Then, the first gasification well is used as the injection well, and the second gasification well is used as the production well to gasify the sub-bottom coal seam between the first row and the second row of gasification wells;

[0039] And so on according to the above steps, gasifying and then filling each coal seam from bottom to top in turn.

[0040] Optionally, step S2 further includes:

[0041] (a) Plan a gasification area between the injection well and the production well in one-to-one correspondence. The gasification area is square, the width of the gasification area is the distance between the injection well and the production well, the length of the gasification area is the distance between two adjacent injection wells. The injection well is located in the middle of one long side of the gasification area, and the production well is located in the middle of the other long side of the gasification area;

[0042] (b) After fracturing the bottommost coal seam, drill a number of gasification channels at the positions of the injection well and / or the production well corresponding to the bottommost coal seam. The head of the gasification channel is connected to the injection well or the production well, and the tail faces the production well or the injection well and extends to the boundary of the gasification area. The included angles formed between adjacent gasification channels are the same, and the number of gasification channels is evenly distributed radially in the gasification area with the injection well or the production well as the center;

[0043] (c) Through the injection well or the production well, input gravel materials into each gasification channel until the gasification channel is filled.

[0044] Further optionally, a number of gasification channels corresponding to the same production well are at the same height, and a number of gasification channels corresponding to the same injection well are at the same height.

[0045] Further optionally, between the injection well and the production well in one-to-one correspondence, the gasification channels with the head connected to the production well are in the upper-middle part of the coal seam, and the gasification channels with the head connected to the injection well are in the lower-middle part of the coal seam, so as to form gasification channels in different upper and lower layers.

[0046] Further optionally, when the thickness of the coal seam to be gasified is not greater 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 not greater than 1 meter, it is a thin coal seam, and one gasification channel is set. 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. 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-middle part of the coal seam are all connected to the injection well, and the heads of the gasification channels in the upper-middle part of the coal seam are all connected to the production well. Description of the Drawings

[0049] Figure 1 It is a schematic plan view of the layout of gasification wells in the target plot;

[0050] Figure 2 It is a schematic diagram of the underground coal gasification working condition of a pair of injection wells and production wells;

[0051] Figure 3 It is a schematic sectional view of the injection well;

[0052] Figure 4 It is a schematic diagram of the injection well and the gasification channel;

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

[0054] In the drawings, 1 - gasification well, 2 - injection well, 3 - production well, 4 - injection pipeline, 5 - injection nozzle pipeline, 6 - production pipeline, 7 - gasification channel. Detailed Embodiment

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

[0056] S1: Conduct geological exploration on the target plot, drill several gasification wells 1 in the target plot, and the gasification wells are distributed in a matrix; several gasification wells are arranged in three rows in sequence starting from the edge of the target plot. After cementing and completion of the gasification wells 1, the bottom coal seam is fractured;

[0057] S2: Drill at least one horizontal branch well at the corresponding bottom coal seam of the first three rows of gasification wells, introduce an injection nozzle pipeline into the horizontal branch well for releasing the gasifying agent; the first row of gasification wells at the edge of the target plot are all used as injection wells 2, the second row of gasification wells are all used as production wells 3, and then gasify the bottom coal seam between the first row and the second row of gasification wells;

[0058] S3: When the gasification high-temperature zone of the bottommost coal seam between the first row and the second row extends to the front of the gas cooling zone, close the second gasification well, use the third gasification well as the production well, and continue to gasify the coal seam;

[0059] Meanwhile, drill several gasification wells in the fourth row, then fracture the bottommost coal seam. At least one horizontal branch well is drilled at the corresponding position of the bottommost coal seam in the fourth gasification well, and an injection nozzle pipeline is introduced into the horizontal branch well;

[0060] S4: After all the bottommost coal seams between the first row and the second gasification wells are gasified, the second gasification well becomes the injection well, gasify the bottommost coal seam between the second row and the third gasification wells, and so on, gasify the bottommost coal seams between each row of gasification wells;

[0061] Meanwhile, inject cement slurry into the first gasification well to fill the goaf corresponding to the bottommost coal seam between the first row and the second gasification wells, to avoid the collapse of the mining pit area; and so on, fill the goafs corresponding to the bottommost coal seams between each row of gasification wells;

[0062] S5: The first gasification well fractures the sub-bottommost coal seam, then drills a horizontal branch well corresponding to the sub-bottommost coal seam, and introduces an injection nozzle pipeline and a gas production pipeline. Then, the first gasification well is used as the injection well, and the second gasification well is used as the production well to gasify the sub-bottommost coal seam between the first row and the second gasification wells. After that, according to the methods of steps S3 - S4, gasify the sub-bottommost coal seams between each row of gasification wells, and then fill the goafs;

[0063] Then, according to the method of S5, gasify and fill each coal seam from bottom to top in turn.

[0064] The geological exploration in step S1 is a conventional exploration, and the exploration content includes but is not limited to the structural form of the coal seam, the number of coal seams, the thickness of the coal seams, the pressure and temperature of the coal seams, the water content, and the sulfur content; according to the geological exploration results, determine the scope of the target plot and the drilling plan.

[0065] Optionally, in step S1, the gasification well is a vertical straight well, which can penetrate each underground coal seam from top to bottom. The spacing between adjacent gasification wells is determined according to the actual situation of the target plot, generally with a spacing of 300 - 500 meters;

[0066] Conventional well cementing and well completion operations are carried out for the gasification wells. Specifically, a casing is lowered into the gasification well. The casing can withstand high temperatures of 1000 - 1200 °C and is also resistant to the corrosion of hydrogen, sulfur, and carbon dioxide. The well cementing cement is also heat-resistant, and the cement returns to the wellhead; after the drilling and well completion pressure test are qualified, the bottommost coal seam is hydraulically fractured to transform the coal seam, and conventional fracturing fluid and proppant are injected into the coal seam to form large-area fissures and cracks, which is beneficial to coal seam combustion and gas flow; after the fracturing operation is completed, after the injection nozzle pipeline is introduced into the horizontal branch well, the water and fracturing fluid in the bottommost coal seam are drained.

[0067] Optionally, in step S2, the horizontal branch well extends along the corresponding coal seam towards the adjacent gasification wells in the same row; the direction of the horizontal branch well of one 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 corresponding to one gasification well, and they extend towards the adjacent gasification wells on the left and right respectively.

[0068] Further optionally, in step S2, in the part 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) towards the adjacent injection well to form a horizontal branch well. An injection gas pipeline 4 is lowered into the vertical straight well part of the injection well, and an injection gas nozzle pipeline 5 is introduced into the horizontal branch well. The bottom of the injection gas pipeline is connected to the injection gas nozzle pipeline, and a number of nozzles are evenly distributed on the injection gas nozzle pipeline for releasing the gasifying agent; a gas production pipeline 6 is lowered into the gas production well for outputting the coal gas.

[0069] An injection gas nozzle pipeline is introduced into the first row of gasification wells, and injection gas nozzle pipelines are introduced into all the subsequent rows of gasification wells, and at the same time, gas production pipelines are also lowered.

[0070] A conventional gasifying agent is injected into the injection gas pipeline of the injection well to ignite the coal seam, and the temperature is maintained at about 900 - 950 °C. A combustion area of a certain area is formed near the coal seam of the injection well, and then a mixed gas of air and high-temperature steam is injected. The injection amount of air is determined comprehensively based on the coal seam pressure, temperature, gas composition, and production of the gas production well.

[0071] The gasification high-temperature area is the high-temperature area where coal burns, and the coal gas (i.e., CH 4 , CO, H 2 ) formed by the reaction of steam and coal flows through the dry distillation area towards the gas production well and is then produced through the gas production well.

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

[0073] The gas cooling area is roughly the dry distillation area. In this area, the coal is heated and may undergo chemical reactions but does not burn. The coal gas produced after coal combustion will cool down during the process of passing through the gas cooling area, which reduces the gas pressure of the production well and is beneficial to the production of coal gas. Therefore, a part of the gas cooling area is always reserved between the production well and the injection well.

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

[0075] (1) After the gasification of the bottommost coal seam between the first row and the second row of gasification wells is completed, the second row of gasification wells serves as injection wells, and the third row of gasification wells serves as production wells to gasify the bottommost coal seam between the second row and the third row of gasification wells;

[0076] Meanwhile, inject cement slurry into the first row of gasification wells to fill the goaf corresponding to the bottommost coal seam between the first row and the second row of gasification wells;

[0077] (2) When the high-temperature gasification zone of the bottommost coal seam between the second row and the third row extends to the front of the gas cooling zone, close the third row of gasification wells, use the fourth row of gasification wells as production wells, and continue to gasify the coal seam;

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

[0079] Meanwhile, drill several gasification wells in the fifth row, then fracture the bottommost coal seam through the fifth row of gasification wells. The fifth row of gasification wells drill horizontal branch wells at the corresponding bottommost coal seam, and introduce injection nozzle pipelines and production gas pipelines into the horizontal branch wells;

[0080] (3) After the gasification of the bottommost coal seam between the second row and the third row of gasification wells is completed, the third row of gasification wells serves as injection wells, and the fourth row of gasification wells still serves as production wells to gasify the bottommost coal seam between the third row and the fourth row of gasification wells;

[0081] Meanwhile, inject cement slurry into the second row of gasification wells to fill the goaf corresponding to the bottommost coal seam between the second row and the third row of gasification wells;

[0082] (4) When the high-temperature gasification zone of the bottommost coal seam between the third row and the fourth row extends to the front of the gas cooling zone, close the fourth row of gasification wells, use the fifth row of gasification wells as production wells, and continue to gasify the coal seam;

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

[0084] Meanwhile, drill several gasification wells in the sixth row, then fracture the bottommost coal seam through the sixth row of gasification wells. The sixth row of gasification wells drill horizontal branch wells at the corresponding bottommost coal seam, and introduce injection nozzle pipelines and production gas pipelines into the horizontal branch wells;

[0085] Meanwhile, fracture the first row and the second row of gasification wells at the position corresponding to the second bottommost coal seam, then drill horizontal branch wells corresponding to the second bottommost coal seam in the first row and the second row of gasification wells, and introduce injection pipelines and injection nozzle pipelines. Then, the first row of gasification wells serves as injection wells, and the second row of gasification wells serves as production wells to gasify the second bottommost coal seam between the first row and the second row of gasification wells;

[0086] By analogy according to the above steps, gasify and fill each coal seam from bottom to top in turn.

[0087] Further optionally, in step (1), since the second exhaust gasification wells are all used as injection wells, the amount of filling mortar needs to be controlled to prevent the mortar from flowing to the position of the second exhaust gasification wells and blocking the injection pipelines; when injecting filling mortar into each row of gasification wells, the amount needs to be controlled to prevent the mortar from flowing to the next row of gasification wells. The cement slurry is a conventional concrete cement slurry.

[0088] Further optionally, the sub-bottom coal seam is the coal seam above the bottommost coal seam.

[0089] According to the above method, the present invention cyclically and rollingly conducts gasification combustion and filling of the lower coal seam and gasification combustion and filling of the upper coal seam, makes full use of each gasification well, and the gasification wells respectively play the roles of injection wells, production wells, and injection of cement slurry, ensuring full combustion of the coal seam, then filling in a progressive manner in sequence, effectively supporting the coal seam, and avoiding the collapse of the mined-out coal seam. The process is reasonable, the steps are closely connected, and the efficiency is relatively high.

[0090] Deep underground coal seams have multiple sets of coal seams (i.e., multiple upper and lower coal seams), with high pressure and low permeability. Compared with the currently disclosed coal seam gasification technologies, the present invention has the following characteristics: (1) effectively controlling the gasification of large areas and multiple sets of coal seams in the plane and space through drilling, with low investment costs; (2) transforming the coal seam by fracturing the coal seam and establishing gasification channels, greatly increasing the gasification surface area and high gasification efficiency; (3) the rotation of injection and production wells in the plane and the backfilling of cement slurry in the mined-out area improve the coal seam recovery rate and avoid formation collapse; (4) being able to conduct gasification mining on deep coal seams with various structural forms; (5) being able to achieve simultaneous gasification of multiple wells and multiple layers, achieving a large-scale and high-efficiency production process.

[0091] Optionally, step S2 further includes:

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

[0093] (b) After fracturing the bottommost coal seam, drilling a number of gasification channels 6 at the positions of the injection well and / or production well corresponding to the bottommost coal seam. The head of the gasification channel is connected to the injection well or the production well, the tail faces the production well or the injection well and extends to the boundary of the gasification area, the included angle formed between adjacent gasification channels is the same, and the number of gasification channels is evenly distributed radially in the gasification area with the injection well or the production well as the center;

[0094] (c) Inputting gravel materials into each gasification channel through the injection well or the production well until the gasification channels are filled.

[0095] After step (c), a horizontal branch well is drilled in the injection well, and an air injection nozzle pipeline is introduced. Then, the water and fracturing fluid at the bottommost coal seam are discharged, and then the coal seam, the bottommost coal seam between the first row and the second row of gasification wells, is ignited.

[0096] Further optionally, 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;

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

[0098] Further optionally, between the injection well and the production well in one-to-one correspondence, the gasification channel with the head connected to the production well is in the upper-middle part of the coal seam, and the gasification channel with the head connected to the injection well is in the lower-middle part of the coal seam, so as to form gasification channels in different upper and lower layers.

[0099] In steps S3 - S5, for each coal seam and the coal seam between every two rows of gasification wells, the above-mentioned gasification channels are drilled corresponding to the injection well and / or the production well. 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 two rows of gasification wells, it is necessary to switch the injection well and the production well once in the middle. Therefore, after fracturing the coal seam, the gasification channels corresponding to three adjacent rows of gasification wells are drilled, and gravel materials (with a particle size of about 1 - 5 mm) are injected. Then, the horizontal branch well of the first batch of gasification wells is drilled, and then the gasification operation is carried out.

[0101] Based on the basic idea of traditional fracturing, the present invention further constructs gasification channels in the coal seam, greatly increasing the gasification surface area. Specifically, the present invention uses the method of drilling gasification channels in the coal seam after fracturing to construct and dredge the "blood vessel" channels of the coal seam. Gravel materials are filled in the gasification channels. There are many voids or pores in the gasification channels, which can effectively increase the gasification area and improve the diffusion rate of the gasification agent and the coal gas. The inner diameter of the gasification channel and the volume of the gravel materials are determined according to the specific actual situation of the coal seam.

[0102] Further optionally, when the thickness of the coal seam to be gasified is not greater 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 not more than 1 meter, it is a thin coal seam. One gasification channel is set, 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. 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-middle part of the coal seam are all connected to the injection well, and the heads of the gasification channels in the upper-middle part of the coal seam are all connected to the production well.

Claims

1. A method for deep underground coal gasification, characterized in that: The following steps are involved: S1: Starting from the edge of the target plot, drill several gasification wells 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 an injection well, and the second gasification well is used as a production well to gasify 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 edge of the gas cooling zone, the second gasification well is closed and the third gasification well is used as a production well to continue gasifying the coal seam; At the same time, several gasification wells in the fourth row are drilled, and then the corresponding bottom coal seam is fractured, and horizontal branch wells are drilled at the bottom coal seam corresponding to the fourth gasification well; S4: After the bottom coal seam between the first and second gasification wells is completely gasified, the second gasification well is used as an injection well to gasify the bottom coal seam between the second and third gasification wells, and so on, to gasify the bottom coal seam between each gasification well; At the same time, cement slurry is injected into the first gasification well to fill the gasified goaf; and so on, the goafs between the gasification wells are filled; S5: Fracturing the sub-bottom coal seam, using the first gasification well as an injection well and the second gasification well as a production well to gasify the sub-bottom coal seam between the first and second gasification wells; then gasifying the sub-bottom coal seam between the gasification wells and filling the goaf; According to the above method, each coal seam is gasified and filled in sequence from bottom to top.

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

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 bottommost coal seam, a continuous tubing drilling rig is used to horizontally drill toward the adjacent injection well to form a horizontal branch well, a gas injection pipeline is lowered into the vertical well portion of the injection well, 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 a number of nozzles are evenly distributed on the gas injection nozzle pipeline for releasing the gasification agent; a gas production pipeline is lowered into the gas production well for outputting coal gas.

4. The method for deep underground coal gasification according to claim 1, characterized in that: In step S3, the front edge of the gas cooling zone is 1 / 2-2 / 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 / 2-2 / 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.

5. The method for deep underground coal gasification according to claim 1, characterized in that: Steps S4 and S5 are specifically as follows: (1) After the bottom coal seam between the first and second gasification wells is completely gasified, the bottom coal seam between the second and third gasification wells is gasified; At the same time, cement slurry is injected into the first gasification well to fill the goaf corresponding to the bottommost coal seam between the first and second gasification 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 a production well to continue gasifying the coal seam; Meanwhile, waiting for the cement slurry in step (1) to solidify; At the same time, several gasification wells in the fifth row are drilled, and the bottom coal seam is fractured 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 and third gasification wells is completely gasified, the third gasification well is used as an injection well to gasify the bottom coal seam between the third and fourth gasification wells; At the same time, cement slurry is injected into the second gasification well to fill the goaf corresponding to the bottommost coal seam between the second and third gasification 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 a production well to continue gasifying the coal seam; Meanwhile, waiting for the cement slurry in step (3) to solidify; At the same time, several gasification wells in the sixth row are drilled, and the bottom coal seam is fractured 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; At the same time, the first gasification well and the second gasification well are fracturing at the position corresponding to the sub-bottom coal seam, and the first gasification well and the second gasification well are then drilled with horizontal branch wells corresponding to the sub-bottom coal seam, and gas injection pipelines and gas injection nozzle pipelines are introduced, and 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; The above steps are followed by analogy to gasify and fill each coal seam from bottom to top.

6. The method for deep underground coal gasification according to claim 1, characterized in that: Step S2 also includes: (a) planning a gasification zone between injection wells and production wells corresponding to each other, 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, a plurality of gasification channels are drilled at the position of the injection well and / or the production well corresponding to the bottom coal seam, the head of the gasification channel is connected to the injection well or the production well, the tail of the gasification channel faces the production well or the injection well and extends to the boundary of the gasification zone, the angles formed between adjacent gasification channels are the same, and the plurality of gasification channels are evenly distributed in the gasification zone in a radial shape with the injection well or the production well as the center of the circle; (c) Gravel is introduced into each gasification channel through an injection well or a production well until the gasification channel is filled.

7. The method for deep underground coal gasification according to claim 6, 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.

8. The method for deep underground coal gasification according to claim 7, characterized in that: Between the one-to-one corresponding injection wells and production wells, the gasification channel whose head is connected to the production well is located in the upper middle part of the coal seam, and the gasification channel whose head is connected to the injection well is located in the lower middle part of the coal seam, thus forming gasification channels of different upper and lower layers.

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

10. The method for deep underground coal gasification according to claim 8, characterized in that: When the thickness of the coal seam to be gasified is no more than 1 meter, a layer of gasification channel is set, and the head of the gasification channel can be connected to the injection well or the production well; when the thickness of the coal seam to be gasified is more than 1 meter, at least two layers of gasification channels are set, one layer of gasification channel is located in the middle and upper part of the coal seam, and the other layer of gasification channel is located in the middle and lower part of the coal seam; When the number of gasification channels in the coal seam is greater than two, the heads of the gasification channels in the lower middle part of the coal seam are connected to the injection well, and the heads of the gasification channels in the upper middle part of the coal seam are connected to the production well.

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