Disclosure of Invention
In order to solve the problems, the invention provides a method for underground gasification of deep coal, which comprises the following steps:
S1, drilling a plurality of gasification wells distributed in a matrix in a target land block, sequentially arranging the plurality of gasification wells into three rows from the edge of the target land block, and fracturing the bottommost coal seam through the gasification wells;
S2, drilling at least one horizontal branch well at the position corresponding to the bottommost coal seam, introducing an air injection nozzle pipeline into the horizontal branch well, taking a first row of gasification wells at the edge of a target land block as injection wells, taking a second row of gasification wells as extraction wells, and gasifying the bottommost coal seam between the first row and the second row of gasification wells;
s3, when the gasified high-temperature zone of the gasified coal bed is expanded to the front edge of the gas cooling zone, closing the second row of gasification wells, taking the third row of gasification wells as extraction wells, and continuously gasifying the coal bed;
Simultaneously, drilling a plurality of gasification wells in a fourth row, fracturing the bottommost coal seam, drilling at least one horizontal branch well in the position corresponding to the bottommost coal seam in the fourth row of gasification wells, and introducing a gas injection nozzle pipeline into the horizontal branch well;
S4, after the bottommost coal beds between the first row and the second row of gasification wells are gasified, the second exhaust gasification wells are used as injection wells, the bottommost coal beds between the second row and the third row of gasification wells are gasified, and the like, the bottommost coal beds between the exhaust gasification wells are gasified;
meanwhile, cement slurry is injected into the first row of gasification wells to fill the gasified goaf, and the goaf among the gasification wells is filled by analogy;
S5, fracturing a secondary bottom coal bed by a first row of gasification wells, taking the first row of gasification wells as injection wells, taking a second row of gasification wells as extraction wells, gasifying the secondary bottom coal bed between the first row of gasification wells and the second row of gasification wells, gasifying the secondary bottom coal bed between the gasification wells according to the method of the steps S3-S4, and refilling a goaf;
And then gasifying and refilling each coal bed in sequence from bottom to top according to the method of S5.
More specifically, the method for underground gasification of deep coal comprises the following steps:
A1, performing geological investigation on a target land block, drilling a plurality of gasification wells in the target land block, wherein the gasification wells are distributed in a matrix, the plurality of gasification wells are sequentially distributed into three rows from the edge of the target land block, and fracturing the bottommost coal bed after well cementation and well completion are performed on the gasification wells;
a2, drilling at least one horizontal branch well at the position corresponding to the bottommost coal seam of the first three rows of gasification wells, and introducing a gas injection nozzle pipeline into the horizontal branch well for releasing gasification agent; the first row of gasification wells at the edge of the target land block are all used as injection wells, the second row of gasification wells are all used as extraction wells, and then the bottommost coal seam between the first row and the second row of gasification wells is gasified;
a3, when the gasification high-temperature zone of the bottommost coal bed between the first row and the second row is expanded to the front edge of the gas cooling zone, closing the second row of gasification wells, taking the third row of gasification wells as extraction wells, and continuously gasifying the coal bed;
Simultaneously, drilling a plurality of gasification wells in a fourth row, fracturing the bottommost coal seam, drilling at least one horizontal branch well in the position corresponding to the bottommost coal seam in the fourth row of gasification wells, and introducing a gas injection nozzle pipeline into the horizontal branch well;
A4, after the bottommost coal beds between the first row and the second row of gasification wells are gasified, the second exhaust gasification wells are all used as injection wells, the third row of gasification wells are still extraction wells, the bottommost coal beds between the second row and the third row of gasification wells are gasified, and so on, the bottommost coal beds between the exhaust gasification wells are gasified;
Meanwhile, cement slurry is injected into the first row of gasification wells, goafs corresponding to the bottommost coal layers between the first row and the second row of gasification wells are filled, collapse of a mining pit area is avoided, and the goafs corresponding to the bottommost coal layers between the exhaust gasification wells are filled by analogy;
A5, fracturing a secondary bottom coal bed by a first row of gasification wells, drilling a horizontal branch well corresponding to the secondary bottom coal bed, introducing an air injection nozzle pipeline, taking the first row of gasification wells as injection wells, taking a second row of gasification wells as extraction wells, gasifying the secondary bottom coal bed between the first row and the second row of gasification wells, and gasifying the secondary bottom coal bed between the exhaust wells and refilling a goaf according to the method of the steps A3-A4;
And then gasifying and refilling each coal bed in sequence from bottom to top according to the method of A5.
According to the invention, by reasonably arranging the gasification wells, three-dimensional full gasification exploitation of the deep coal bed of the target land block is realized in a more efficient, simpler, more convenient and lower-cost mode, and full filling is performed, so that the collapse of the exploited coal bed is avoided. Specifically, a plurality of gasification wells are uniformly arranged in a matrix manner in the target land parcel, the gasification wells are used as injection wells and extraction wells alternately, the drilling cost is greatly reduced, and the gasification of the coal seam and the filling of the goaf can be uniformly controlled. The general idea of the coal seam gasification is that the coal seam at the bottommost part is gasified and burned step by step in a pushing manner according to the direction from the first row of gasification wells to the last row of gasification wells, the coal seam at the bottommost part is gasified upwards according to the same method, each coal seam is gasified layer by layer, the gasified coal seam becomes a goaf, cement slurry is injected by the gasification wells, and the goaf is supported after the cement slurry is solidified so as to avoid collapse.
The invention sets several rows of gasification wells, which burn by exhaust, the first row of gasification wells is injection well, the second row of gasification wells is extraction well, when the gasification high temperature area expands to the front of the gas cooling area, the second row of gasification wells is closed, the third row of gasification wells is used as extraction well, and the coal seam is continuously gasified, thus the continuous rolling gasification is realized, and the full burning of the coal seam is realized. After a part of coal layers are burnt, cement slurry is injected into the first row of gasification wells, and the mining area is continuously filled in a rolling mode. And gasifying all coal beds above the target land by layers in the same method to realize three-dimensional full gasification of the target land.
Optionally, in step S2, the horizontal branch wells extend along the corresponding coal seam to 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, and preferably, two horizontal branch wells are drilled in one gasification well corresponding to each coal seam and extend to the adjacent left and right gasification wells respectively.
Further optionally, in step S2, a horizontal branch well is formed by drilling a part of the injection well corresponding to the bottommost coal seam horizontally (i.e., left and right) with a coiled tubing drilling machine, an air injection pipeline is arranged below a vertical well part of the injection well, an air injection nozzle pipeline is arranged in the horizontal branch well, the bottom of the air injection pipeline is connected with the air injection nozzle pipeline, a plurality of nozzles are uniformly distributed on the air injection nozzle pipeline and used for releasing gasifying agent, and an air production pipeline is arranged in the air production well and used for outputting coal gas.
Optionally, in step S3, the front edge 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 to the extraction well, when the distance between the edge of the gasification high temperature area nearest to the extraction well and the injection well reaches 1/2-2/3 of the distance between two adjacent rows of gasification wells, the second row of gasification wells is closed, and the third row of gasification wells are used as the extraction wells to continuously gasify the coal seam.
Optionally, the steps S4 and S5 are specifically:
(1) After the bottommost coal seam between the first row and the second row of gasification wells is gasified, the second exhaust gasification wells are all used as injection wells, the third row of gasification wells are all used as extraction wells, and the bottommost coal seam between the second row and the third row of gasification wells is gasified;
Meanwhile, cement slurry is injected into the first row of gasification wells, and goafs corresponding to the bottommost coal seams between the first row and the second row of gasification wells are filled;
(2) When the gasification high-temperature zone of the bottommost coal bed between the second row and the third row is expanded to the front edge of the gas cooling zone, closing the third row of gasification wells, taking the fourth row of gasification wells as extraction wells, and continuously gasifying the coal bed;
simultaneously, waiting for the cement paste in the step (1) to solidify;
simultaneously, a plurality of gasification wells of a fifth row are drilled, the bottommost coal bed is fractured through the gasification wells of the fifth row, a horizontal branch well is drilled at the position corresponding to the bottommost coal bed in the gasification wells of the fifth row, and a gas injection nozzle pipeline and a gas production pipeline are led into the horizontal branch well;
(3) After the bottommost coal seam between the second row and the third row of gasification wells is gasified, taking the third row of gasification wells as injection wells, and taking the fourth row of gasification wells as extraction wells, and gasifying the bottommost coal seam between the third row and the fourth row of gasification wells;
Meanwhile, cement slurry is injected into the second gasification well, and goafs corresponding to the bottommost coal beds between the second row and the third row of gasification wells are filled;
(4) When the gasification high-temperature zone of the bottommost coal bed between the third row and the fourth row is expanded to the front edge of the gas cooling zone, closing the fourth row of gasification wells, taking the fifth row of gasification wells as extraction wells, and continuously gasifying the coal bed;
Simultaneously, waiting for the cement paste in the step (3) to solidify;
simultaneously, drilling a plurality of gasification wells in a sixth row, fracturing the bottommost coal seam through the gasification wells in the sixth row, drilling a horizontal branch well at the position corresponding to the bottommost coal seam in the gasification wells in the sixth row, and introducing an air injection nozzle pipeline and an air production pipeline into the horizontal branch well;
Simultaneously, the first row of gasification wells and the second row of gasification wells are used for fracturing at the positions corresponding to the secondary bottom coal seam, the first row of gasification wells and the second row of gasification wells are drilled with horizontal branch wells corresponding to the secondary bottom coal seam and are led into gas injection pipelines and gas injection nozzle pipelines, then the first row of gasification wells are used as injection wells, the second row of gasification wells are used as extraction wells, and the secondary bottom coal seam between the first row of gasification wells and the second row of gasification wells is gasified;
and according to the steps and the like, gasifying and refilling each coal bed sequentially from bottom to top.
Optionally, step S2 further includes:
(a) Planning a gasification zone between injection wells and extraction wells which are in one-to-one correspondence, wherein the gasification zone is square, the width of the gasification zone is the distance between the injection wells and the extraction wells, the length of the gasification zone is the distance between two adjacent injection wells, the injection wells are positioned in the middle of one long side of the gasification zone, and the extraction wells are positioned in the middle of the other long side of the gasification zone;
(b) After fracturing the bottommost coal bed, drilling a plurality of gasification channels at the positions of the injection well and/or the extraction well corresponding to the bottommost coal bed, wherein the heads of the gasification channels are communicated with the injection well or the extraction well, the tails of the gasification channels face the extraction well or the injection well and extend to the boundary of a gasification zone, the included angles formed between the adjacent gasification channels are the same, and the gasification channels are uniformly distributed in a radial manner in the gasification zone by taking the injection well or the extraction well as the center of a circle;
(c) Gravel is input into each gasification channel through the injection well or the extraction well until the gasification channels are filled.
Further optionally, the plurality of gasification channels corresponding to the same extraction well are at the same height, and the plurality of gasification channels corresponding to the same injection well are at the same height.
Further alternatively, between the injection well and the extraction well which are in one-to-one correspondence, the gasification channel of the head communicated extraction well is positioned at the middle upper part of the coal bed, and the gasification channel of the head communicated injection well is positioned at the middle lower part of the coal bed, so that the gasification channels of different layers are formed.
Further alternatively, when the thickness of the coal seam to be gasified is not more than 3 meters, the included angle between the adjacent gasification channels is about 60 degrees, and when the thickness of the coal seam to be gasified is more than 3 meters, the included angle between the adjacent gasification channels is about 30 degrees;
When the thickness of the coal bed to be gasified is greater than 1 meter, at least two layers of gasification channels are arranged, one layer of gasification channels is positioned at the middle upper part of the coal bed, and the other layer of gasification channels is positioned at the middle lower part of the coal bed.
Further optionally, when the number of layers of the gasification channels in the coal seam is greater than two, the heads of the gasification channels at the lower part in the coal seam are communicated with the injection well, and the heads of the gasification channels at the upper part in the coal seam are communicated with the extraction well.
Detailed Description
The embodiment provides a method for underground gasification of deep coal, as shown in fig. 1-5, comprising the following steps:
S1, geological investigation is carried out on a target land block, a plurality of gasification wells 1 are drilled in the target land block, and are distributed in a matrix, wherein the plurality of gasification wells are sequentially distributed into three rows from the edge of the target land block, and after well cementation and well completion are carried out on the gasification wells 1, the bottommost coal bed is fractured;
S2, drilling at least one horizontal branch well at the position corresponding to the bottommost coal seam in the first three rows of gasification wells, and introducing a gas injection nozzle pipeline into the horizontal branch well for releasing gasification agent; the first row of gasification wells at the edge of the target land block are all used as injection wells 2, the second row of gasification wells are all used as extraction wells 3, and then the bottommost coal seam between the first row and the second row of gasification wells is gasified;
s3, when the gasification high-temperature zone of the bottommost coal bed between the first row and the second row is expanded to the front edge of the gas cooling zone, closing the second row of gasification wells, taking the third row of gasification wells as extraction wells, and continuously gasifying the coal bed;
Simultaneously, drilling a plurality of gasification wells in a fourth row, fracturing the bottommost coal seam, drilling at least one horizontal branch well in the position corresponding to the bottommost coal seam in the fourth row of gasification wells, and introducing a gas injection nozzle pipeline into the horizontal branch well;
s4, gasifying the bottommost coal bed between the second row and the third row of gasification wells after the bottommost coal bed between the first row and the second row of gasification wells is gasified, wherein the second row of gasification wells are injection wells, gasifying the bottommost coal bed between the second row and the third row of gasification wells, and so on;
Meanwhile, cement slurry is injected into the first row of gasification wells, goafs corresponding to the bottommost coal layers between the first row and the second row of gasification wells are filled, collapse of a mining pit area is avoided, and the goafs corresponding to the bottommost coal layers between the exhaust gasification wells are filled by analogy;
S5, fracturing a secondary bottom coal bed by a first row of gasification wells, drilling a horizontal branch well corresponding to the secondary bottom coal bed, introducing an air injection nozzle pipeline and an air production pipeline, taking the first row of gasification wells as injection wells, taking a second row of gasification wells as extraction wells, gasifying the secondary bottom coal bed between the first row of gasification wells and the second row of gasification wells, gasifying the secondary bottom coal bed between the exhaust gasification wells according to the method of the steps S3-S4, and refilling a goaf;
And then gasifying and refilling each coal bed in sequence from bottom to top according to the method of S5.
The geological survey in step S1 is a conventional survey, the content of which includes, but is not limited to, the formation of the coal seam, the number of coal layers, the thickness of the coal seam, the pressure and temperature of the coal seam, the water content, the sulfur content, the range of the target land mass according to the geological survey result, and the drilling scheme.
Optionally, in step S1, the gasification wells are vertical wells, and can penetrate through each underground coal seam from top to bottom, and the distance between adjacent gasification wells is determined according to the actual situation of the target land parcel, and the distance is generally 300-500 meters;
The gasification well is subjected to conventional well cementation and well completion operations, specifically, a sleeve is arranged in the gasification well, the sleeve can resist high temperature of 1000-1200 ℃, meanwhile, hydrogen, sulfur and carbon dioxide are resistant, well cementation cement is also resistant to high temperature, cement returns to a well head, after well drilling and well completion pressure testing are qualified, hydraulic fracturing is carried out on the bottommost coal bed, the coal bed is reformed, conventional fracturing fluid and propping agent are injected into the coal bed, large-area fracture cracks are formed, the coal bed combustion and gas flow are facilitated, and after the fracturing operation is completed, water and fracturing fluid of the bottommost coal bed are pumped out after a gas injection nozzle pipeline is led into the horizontal branch well.
Optionally, in step S2, the horizontal branch wells extend along the corresponding coal seam to 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, and preferably, two horizontal branch wells are drilled in one gasification well corresponding to each coal seam and extend to the adjacent left and right gasification wells respectively.
Further optionally, in step S2, a coiled tubing drilling machine is used to drill horizontally (i.e. leftwards and rightwards) the part of the injection well corresponding to the bottommost coal seam to form a horizontal branch well, an air injection pipeline 4 is arranged below the vertical well part of the injection well, an air injection nozzle pipeline 5 is led into the horizontal branch well, the bottom of the air injection pipeline is connected with the air injection nozzle pipeline, a plurality of nozzles are uniformly distributed on the air injection nozzle pipeline and used for releasing gasifying agent, and an air production pipeline 6 is arranged in the air production well and used for outputting coal gas.
The first row of gasification wells is internally provided with gas injection nozzle pipelines, and the inner parts of all the gasification wells in the rear row are internally provided with gas injection nozzle pipelines and simultaneously are also downwards provided with gas production pipelines.
And (3) injecting a conventional gasifying agent into an air injection pipeline of the injection well, igniting the coal bed, and keeping the temperature at about 900-950 ℃. And (3) forming a combustion area with a certain area near the coal bed of the injection well, then injecting mixed gas of air and high-temperature steam, and comprehensively determining the injection air quantity according to the pressure and the temperature of the coal bed, the gas composition of the gas production well and the yield.
The gasification high-temperature area is a high-temperature area where coal is combusted, and gas (namely CH4、CO、H2) formed by the reaction of water vapor and coal flows to the gas production well through the carbonization area and is produced through the gas production well.
Optionally, in step S3, the front edge 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 to the extraction well, when the distance between the edge of the gasification high temperature area nearest to the extraction well and the injection well reaches 1/2-2/3 of the distance between two adjacent rows of gasification wells, the second row of gasification wells is closed, and the third row of gasification wells are used as the extraction wells to continuously gasify the coal seam.
The gas cooling area is approximately a carbonization area, coal in the area is heated, chemical reaction possibly occurs, but the coal is not combusted, and gas generated after coal combustion can be cooled in the process of passing through the gas cooling area, so that the gas pressure of a production well is reduced, and the production of the gas is facilitated. Therefore, a part of gas cooling area is always reserved between the extraction well and the injection well.
Optionally, the steps S4 and S5 are specifically:
(1) After the bottommost coal seam between the first row and the second row of gasification wells is gasified, the second row of gasification wells are used as injection wells, the third row of gasification wells are extraction wells, and the bottommost coal seam between the second row and the third row of gasification wells is gasified;
Meanwhile, cement slurry is injected into the first row of gasification wells, and goafs corresponding to the bottommost coal seams between the first row and the second row of gasification wells are filled;
(2) When the gasification high-temperature zone of the bottommost coal bed between the second row and the third row is expanded to the front edge of the gas cooling zone, closing the third row of gasification wells, taking the fourth row of gasification wells as extraction wells, and continuously gasifying the coal bed;
simultaneously, waiting for the cement paste in the step (1) to solidify;
simultaneously, a plurality of gasification wells of a fifth row are drilled, the bottommost coal bed is fractured through the gasification wells of the fifth row, a horizontal branch well is drilled at the position corresponding to the bottommost coal bed in the gasification wells of the fifth row, and a gas injection nozzle pipeline and a gas production pipeline are led into the horizontal branch well;
(3) After the bottommost coal seam between the second row and the third row of gasification wells is gasified, the third row of gasification wells are used as injection wells, the fourth row of gasification wells are also extraction wells, and the bottommost coal seam between the third row and the fourth row of gasification wells is gasified;
Meanwhile, cement slurry is injected into the second gasification well, and goafs corresponding to the bottommost coal beds between the second row and the third row of gasification wells are filled;
(4) When the gasification high-temperature zone of the bottommost coal bed between the third row and the fourth row is expanded to the front edge of the gas cooling zone, closing the fourth row of gasification wells, taking the fifth row of gasification wells as extraction wells, and continuously gasifying the coal bed;
Simultaneously, waiting for the cement paste in the step (3) to solidify;
simultaneously, drilling a plurality of gasification wells in a sixth row, fracturing the bottommost coal seam through the gasification wells in the sixth row, drilling a horizontal branch well at the position corresponding to the bottommost coal seam in the gasification wells in the sixth row, and introducing an air injection nozzle pipeline and an air production pipeline into the horizontal branch well;
Simultaneously, the first row of gasification wells and the second row of gasification wells are used for fracturing at the positions corresponding to the secondary bottom coal seam, the first row of gasification wells and the second row of gasification wells are drilled with horizontal branch wells corresponding to the secondary bottom coal seam and are led into gas injection pipelines and gas injection nozzle pipelines, then the first row of gasification wells are used as injection wells, the second row of gasification wells are used as extraction wells, and the secondary bottom coal seam between the first row of gasification wells and the second row of gasification wells is gasified;
and according to the steps and the like, gasifying and refilling each coal bed sequentially from bottom to top.
Further optionally, in step (1), since the second gasification wells are all injection wells, the amount of filling mortar is controlled to avoid blocking the gas injection pipeline due to the mortar flowing to the second gasification well, and then when filling mortar into each gasification well, the amount of filling mortar needs to be controlled to avoid the mortar flowing to the next gasification well. The cement paste is a conventional concrete cement paste.
Further optionally, the secondary bottom coal seam is a coal seam above the bottommost coal seam.
According to the method, the gasification combustion and filling of the lower coal bed and the gasification combustion and filling of the upper coal bed are circularly carried out, the gasification wells are fully utilized, the gasification wells respectively play roles of the injection well, the extraction well and the injection cement slurry, the coal bed is ensured to be fully combusted, the coal bed is sequentially and progressively filled, the collapse of the mined-out coal bed is effectively supported, the technological process is reasonable, the steps are compact, and the efficiency is higher.
The deep underground coal bed is provided with a plurality of sets of coal beds (namely an upper multi-layer coal bed and a lower multi-layer coal bed), and has high pressure and low permeability. Compared with the coal seam gasification technology disclosed by the invention, the method has the characteristics of (1) effectively controlling the gasification of large-area and multiple sets of coal seams on a plane and in space through drilling, having low investment cost, (2) fracturing the coal seams and establishing gasification channels to reform the coal seams, greatly improving gasification surface area and high gasification efficiency, (3) injecting and mining on the plane, rotating the mining well and backfilling goaf cement paste, improving the recovery ratio of the coal seams and avoiding formation collapse, (4) carrying out gasification exploitation on deep coal seams with various structural forms, and (5) realizing simultaneous gasification of multiple wells and multiple layers and achieving a large-scale and efficient production process.
Optionally, step S2 further includes:
(a) Planning a gasification zone between injection wells and extraction wells which are in one-to-one correspondence, wherein the gasification zone is square, the width of the gasification zone is the distance between the injection wells and the extraction wells, the length of the gasification zone is the distance between two adjacent injection wells, the injection wells are positioned in the middle of one long side of the gasification zone, and the extraction wells are positioned in the middle of the other long side of the gasification zone;
(b) After fracturing the bottommost coal bed, drilling a plurality of gasification channels 6 at positions of the injection well and/or the extraction well corresponding to the bottommost coal bed, wherein the heads of the gasification channels are communicated with the injection well or the extraction well, the tails of the gasification channels face the extraction well or the injection well and extend to the boundary of a gasification zone, the included angles formed between the adjacent gasification channels are the same, and the gasification channels are uniformly distributed in a radial manner in the gasification zone by taking the injection well or the extraction well as the center of a circle;
(c) Gravel is input into each gasification channel through the injection well or the extraction well until the gasification channels are filled.
And (c) drilling a horizontal branch well in the injection well, introducing a gas injection nozzle pipeline, discharging water and fracturing fluid of the bottommost coal bed, and igniting the coal bed, wherein the bottommost coal bed is arranged between the first row and the second row of gasification wells.
Further optionally, the plurality of gasification channels corresponding to the same extraction well are at the same height, and the plurality of gasification channels corresponding to the same injection well are at the same height;
The length of the gasification channels is determined by the included angle between the gasification channels and the size of the gasification zone, the head of the gasification channels is positioned in the middle of one long side of the gasification zone, and the tail of the gasification channels extends to the other long side or to two wide sides of the gasification zone.
Further alternatively, between the injection well and the extraction well which are in one-to-one correspondence, the gasification channel of the head communicated extraction well is positioned at the middle upper part of the coal bed, and the gasification channel of the head communicated injection well is positioned at the middle lower part of the coal bed, so that the gasification channels of different layers are formed.
In the step S3-S5, the gasification channels are drilled corresponding to the injection wells and/or the extraction wells for each coal seam and each coal seam between every two rows of gasification wells, the tail parts of the gasification channels corresponding to the adjacent extraction wells in the same row can be communicated, and the tail parts of the gasification channels corresponding to the adjacent injection wells in the same row can be communicated.
To gasify the coal seam between two rows of gasifying wells, one-time injection well and one-time extraction well are needed to be switched in the middle, so after the coal seam is broken, gasifying channels corresponding to the three adjacent rows of gasifying wells are all well drilled, gravel (with the grain size of about 1-5 mm) is injected, then horizontal branch wells of the first row of gasifying wells are drilled, and then gasification operation is carried out.
According to the invention, on the basis of the basic way of traditional fracturing, the gasification channel is further constructed in the coal bed, so that the gasification surface area is greatly increased. Specifically, the invention adopts a method of drilling the gasification channel in the coal seam after fracturing to construct a 'blood vessel' channel for dredging the coal seam, gravel is filled in the gasification channel, and the gasification channel is provided with more gaps or pores, so that the gasification area can be effectively increased, and the diffusion rate of gasifying agent and coal gas can be improved. The inner diameter of the gasification channel and the material volume of the gravel material are determined according to the concrete practical condition of the coal bed.
Further alternatively, when the thickness of the coal seam to be gasified is not more than 3 meters, the included angle between the adjacent gasification channels is about 60 degrees, and when the thickness of the coal seam to be gasified is more than 3 meters, the included angle between the adjacent gasification channels is about 30 degrees;
When the thickness of the coal bed to be gasified is greater than 1 meter, at least two layers of gasification channels are arranged, one layer of gasification channels is positioned at the middle upper part of the coal bed, and the other layer of gasification channels is positioned at the middle lower part of the coal bed.
Further optionally, when the number of layers of the gasification channels in the coal seam is greater than two, the heads of the gasification channels at the lower part in the coal seam are communicated with the injection well, and the heads of the gasification channels at the upper part in the coal seam are communicated with the extraction well.