CN111520119B - Method for efficiently extracting coal bed gas by staged fracturing of large-spacing thin coal seam group multi-bottom horizontal well - Google Patents
Method for efficiently extracting coal bed gas by staged fracturing of large-spacing thin coal seam group multi-bottom horizontal well Download PDFInfo
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Abstract
A method for efficiently extracting coal bed gas by multi-bottom horizontal well staged fracturing of a large-spacing thin coal seam group comprises the steps of combining factors such as coal mining planning and coal bed gas resources, forming 2-3 target production layer groups in intervals of an enriched coal seam, arranging layers of all branch shafts of the multi-bottom horizontal well according to the target production layer groups, determining a U-shaped or L-shaped multi-bottom horizontal well body structure, drilling, casing well cementing construction, perforating in stages and fracturing multiple layers of all branch shafts of the multi-bottom casing horizontal well, performing post-fracturing pressure drop, blowout, sand flushing and well washing construction to meet the requirement of descending of drainage and mining equipment, installing drainage and mining equipment, performing fine drainage and mining, and constructing a cluster U-shaped or L-shaped multi-bottom casing horizontal well group; the method can overcome the problems of insufficient thickness of the current target producing zone, small hydraulic fracturing modification influence range, poor yield increasing effect and the like, realizes the rapid and efficient extraction of the coal bed gas of the large-spacing thin coal bed group, obviously improves the yield and the recovery ratio of the single-well coal bed gas, meets the standard requirement of mine gas treatment, and solves the problem of extraction, excavation and mining imbalance.
Description
Technical Field
The invention relates to the technical field of high-efficiency extraction of coal bed gas (gas), in particular to a method for efficiently extracting coal bed gas by segmented fracturing of a large-spacing thin coal seam group multi-bottom horizontal well.
Background
In coal-containing areas such as the southwest pilferage and the northeast chalky district in China, the thickness of coal-containing strata is large, coal seam groups develop, and the characteristics that the number of coal seam layers is large, the thickness of the coal seams is small, the distance between adjacent coal seams is large, but the distance span between the bottom coal seam and the bottom coal seam of the coal-containing strata is large and the like are presented. In addition, many coal seams are broken and soft, and have very low permeability. In general, the coal bed gas development in the areas adopts the conventional development mode of ground vertical wells, cluster wells or horizontal wells, the gas production effect is not ideal, and the input-output ratio is very low. The reasons for this mainly include:
(1) the method adopts conventional development modes such as ground vertical wells or cluster wells, single-layer or multi-layer hydraulic fracturing, combined-layer drainage and mining, and the like, wherein firstly, the thickness of a single layer of a coal seam is small, and the accumulated thickness of a used target production layer is insufficient; secondly, because the coal bed is broken and soft and has low permeability, the hydraulic fracturing crack has short extension in the relatively plastic coal bed and limited influence range, and the fracturing modification yield-increasing effect is poor; thirdly, in the conventional drainage and mining process of multi-layer combined mining, due to the fact that the distance span between the upper coal seam and the lower coal seam is large, the upper broken soft low-permeability coal seam is exposed above the dynamic liquid level along with the reduction of the dynamic liquid level, the effective stress is excessively increased, pores in coal tend to be closed due to the loss of fluid flowing pressure support, and the permeability tends to zero, so that the gas production rate of the upper coal seam is sharply reduced, even gas is not produced, the gas production rate of a single well is quickly attenuated, and the effects of multi-layer combined mining, contribution of gas production of each layer and superposition increase of the gas production rate cannot be realized.
(2) By adopting a conventional horizontal well development mode, the single well gas production rate is not high due to the fact that the target producing zone thickness of a single coal seam encountered by drilling is not enough, and the problems that a horizontal well is easy to collapse and bury due to drilling accidents and difficult to form a well when being constructed in a broken soft coal seam, the permeability of the broken soft coal seam is low and the like.
(3) The prior art discloses a method for reinforcing extraction of a horizontal well by staged fracturing of coal bed gas (patent number ZL201410225854. X). when a single-structure soft coal bed is developed for a horizontal well of coal bed gas, the well track of the horizontal well is controlled in a mudstone or sandstone roof which is 0.5m-1.5m above the top boundary of the coal bed, and the purpose of increasing yield is achieved by staged downward directional perforation and staged fracturing in a horizontal well section. For the extraction of the coal bed gas of the large-spacing thin coal bed group, the method only uses a conventional well type horizontal well, has limitation, is not suitable, and mainly shows that the single-layer thickness of the coal bed of the large-spacing thin coal bed group is small, the extraction problem of a single target coal bed can only be solved by adopting one horizontal section of the roof horizontal well to perform downward directional perforating and sectional fracturing in a sectional mode, the extraction problem of the coal bed group cannot be solved, the thickness of the used target coal bed is insufficient, the gas production effect of the single well is not ideal, the input-output ratio is very low, and the economy is not ideal. The prior art also discloses a non-well-cementing casing well completion coal bed gas horizontal well extraction method (patent number CN201710764015.9), which adopts a casing setting mode, an external self-expansion packer mode and a non-well cementing mode to complete the well, an orientation hydraulic sand blasting perforation mode to carry out perforation construction, and a mode of oil pipe injection and oil casing annulus sand adding combined injection to carry out staged fracturing construction on a perforation section. The horizontal well section of the method is constructed in a coal seam, the problem that only a single coal seam can be extracted exists, the method is only suitable for a hard coal seam with a relatively good coal body structure and is not suitable for a broken soft coal seam, and the technical problems that drilling in the broken soft coal seam is easy to collapse and bury and drill, the well is difficult to form, the drilling is not long into the horizontal well section, the casing is difficult to run and the like are solved. Therefore, the two patents are not suitable for extracting coal bed gas of large-distance thin coal bed groups.
Therefore, in view of the defects, through careful research and design, the designer researches and designs a method for extracting coal bed gas efficiently by segmented fracturing of a large-interval thin coal seam group multi-bottom horizontal well by combining experience and achievement of related industries for many years, so as to overcome the defects.
Disclosure of Invention
The invention aims to provide a method for efficiently extracting coal bed gas by multi-bottom horizontal well staged fracturing of a large-spacing thin coal seam group, which can overcome the problems of low single-well gas yield and the like caused by three adverse effects of insufficient thickness of a target producing layer, small hydraulic fracturing modification influence range, poor yield increasing effect, reduction of permeability and high gas yield attenuation due to the fact that a dynamic liquid level of an upper broken soft coal layer is reduced below the broken soft coal layer in the prior extraction of the large-spacing thin coal seam group, and the like, realize the rapid and efficient extraction of the coal bed gas of the large-spacing thin coal seam group, remarkably improve the single-well coal bed gas yield and recovery ratio, meet the standard requirements of mine gas control, and solve the problem of extraction and mining disorder.
In order to achieve the purpose, the invention discloses a method for efficiently extracting coal bed gas by staged fracturing of a large-spacing thin coal seam group multi-bottom horizontal well, which is characterized by comprising the following steps of:
the method comprises the following steps: combining factors such as coal mining planning and coal bed gas resources, and forming 2-3 target production layer groups in the layer sections of the enriched coal bed;
step two: arranging the layer positions of each branch shaft of the multi-bottom horizontal well according to the target layer group;
step three: determining a U-shaped or L-shaped multi-bottom horizontal well body structure, and performing drilling and casing cementing construction;
step four: perforating and sectionally perforating multi-layer fracturing on the branch shafts of the multi-bottom casing horizontal well to form fracturing cracks, so that the branch shafts of the multi-bottom horizontal well are communicated with a plurality of target coal beds in a target pay group;
step five: performing the construction of pressure drop, open flow, sand washing and well washing after fracturing to meet the requirement of lowering drainage and production equipment;
step six: installing a displacement and mining device for fine displacement and mining, adopting layered pressure control and combined layer displacement and mining in a vertical well for a U-shaped multi-bottom casing horizontal well, and adopting combined layer displacement and mining for an L-shaped multi-bottom casing horizontal well;
step seven: and constructing a cluster U-shaped or L-shaped multi-bottom casing horizontal well group.
Wherein: in the second step, the arrangement layer of each branch shaft of the multi-bottom horizontal well is used for selecting the rock stratum interlayer between the coal layers for the broken soft coal layer, and the rock stratum interlayer between the coal layers or the coal layer can be selected for the hard coal layer with good coal body structure, so that the fracture can be ensured to be capable of fracturing and penetrating the communication target coal layer.
Wherein: in the third step: constructing a U-shaped multi-bottom casing horizontal well, constructing a vertical well, drilling to 60 meters below a lowest target coal seam, setting a casing for well cementation, then forming a cave at a target layer of a horizontal shaft to facilitate smooth butt joint with the horizontal well, constructing the horizontal shaft at the target layer in sequence from bottom to top, communicating with a vertical well for drainage and mining at the cave forming position, and setting the casing for well cementation and completion.
Wherein: in the third step: constructing an L-shaped multi-bottom casing horizontal well, constructing horizontal mineshafts at a target layer position in sequence from bottom to top, and setting casings to fix and complete the well.
Wherein: and performing clustering perforation in the fourth step, wherein the perforation adopts a circumferential hole distribution mode and combines the well distribution layer of the horizontal well, and a non-uniform hole distribution mode of the upper half position or the lower half position of the horizontal shaft is adopted according to the requirement of the specific upward or downward extending distance of the fracture, so as to be beneficial to the horizontal well fracturing fracture to penetrate through multiple layers.
Wherein: the staged fracturing mode of the horizontal well adopts pumping bridge plug staged fracturing or continuous oil pipe hydraulic sand blasting perforation annular fracturing.
Wherein: the density of the mixture is 1.052 to 1.26g/cm2The ultra-low density proppant realizes upward extension and propping of the fracture.
Wherein: and in the step five, the horizontal well staged fracturing adopts a pumping bridge plug staged fracturing mode, and pressure drop, blowout, horizontal well drilling plug, vertical well sand washing and well washing are carried out after fracturing.
Wherein: and in the step five, the horizontal well staged fracturing adopts an oil pipe dragging staged sand blasting fracturing mode to perform pressure drop, open flow, vertical well sand washing and well washing after fracturing.
Wherein: in the sixth step: when the layered pressure control and combination layer mode is adopted, the double-sleeve multilayer layered pressure control and discharge and mining equipment is selected for the condition of a plurality of coal seam groups and large water yield, and the double-sleeve multilayer layered pressure control and discharge and mining equipment, the double-pump layered pressure control and discharge and mining equipment or the four-channel layered pressure control and discharge and mining equipment is selected for the condition of a double coal seam group and small water yield.
According to the content, the method for efficiently extracting the coal bed gas by the segmented fracturing of the large-spacing thin coal seam group multi-bottom horizontal well has the following effects:
1. the design of many bottoms horizontal well can exploit a plurality of target layer groups simultaneously in a horizontal well, has increased the accumulative total target coal seam number of piles and the accumulative total target coal seam thickness that the in a plurality of layer groups used, can improve the reserves for the coal seam pneumatics greatly, and mining efficiency is high. The U-shaped horizontal well group well type provides conditions for layered pressure control and combined layer drainage and production.
2. The horizontal sections of the multi-bottom horizontal well are designed in rock strata or coal seams between upper coal seams and lower coal seams of each target production stratum group, when casing horizontal well sections in a certain target production stratum group are subjected to subsection perforation fracturing, fracturing transformation of a plurality of target coal seams in the target production stratum group can be simultaneously realized by utilizing the characteristic that hydraulic fracturing cracks are expanded upwards and downwards to penetrate the stratum, and gas production high-conductivity sand crack channels are established between a horizontal shaft and the plurality of target coal seams in the target production stratum group, so that each target coal seam in the target production stratum group can make contributions, and the gas yield of the multi-bottom horizontal well can be greatly improved.
3. The method adopts a cluster multi-bottom horizontal well staged fracturing mode, is suitable for construction in large areas with complex terrain and fluctuation, can save floor area, can reduce construction investment and accelerate construction speed.
4. The method for efficiently extracting the coal bed gas by the segmented fracturing of the multi-bottom horizontal well of the large-interval thin coal bed group is suitable for multiple coal beds (groups) and broken soft coal beds (groups), can quickly reduce the content and pressure of the coal bed gas of the coal beds, remarkably improve the extraction amount and extraction efficiency of the coal bed gas, greatly improve the operation efficiency and reduce the engineering cost, enable efficient economic extraction and disaster reduction of the coal bed gas (gas) in the thin coal bed region to be possible, is an important breakthrough in the technical field of coal bed gas (gas) extraction, and has important significance for improving the extraction technical level and economic benefit of the coal bed gas (gas).
The details of the present invention can be obtained from the following description and the attached drawings.
Drawings
Fig. 1 shows a schematic diagram (one) of a method for efficiently extracting coal bed gas by staged fracturing of a large-spacing thin coal seam group U-shaped multi-bottom horizontal well.
Fig. 2 shows a schematic diagram (II) of the method for efficiently extracting coal bed gas by staged fracturing of the large-spacing thin coal seam group U-shaped multi-bottom horizontal well.
Fig. 3 shows a schematic diagram (III) of the method for efficiently extracting coal bed gas by staged fracturing of the large-spacing thin coal seam group U-shaped multi-bottom horizontal well.
Fig. 4 shows a schematic diagram (one) of the method for efficiently extracting coal bed gas by staged fracturing of the large-spacing thin coal seam group L-shaped multi-bottom horizontal well.
Fig. 5 shows a schematic diagram (II) of the method for efficiently extracting coal bed gas by staged fracturing of the large-spacing thin coal seam group L-shaped multi-bottom horizontal well.
Fig. 6 shows a schematic diagram (III) of the method for efficiently extracting coal bed gas by staged fracturing of the large-spacing thin coal seam group L-shaped multi-bottom horizontal well.
Fig. 7 shows a schematic diagram of the method for efficiently extracting coal bed gas by segmented fracturing of the cluster L-shaped multi-bottom horizontal well group of the large-spacing thin coal seam group.
Reference numerals:
1-a vertical well; 2-multiple bottom horizontal wells; 3-a horizontal wellbore; 4-pressing cracks; 5-crushing the soft thin coal layer; 6-small spacing interlayer; 7-large-spacing interlayer; 8-target production layer group.
Detailed Description
The above features and advantages of the present invention will be more fully described below with reference to the accompanying drawings, in which some, but not all embodiments of the invention are shown.
Referring to fig. 1 to 7, the method for extracting coalbed methane efficiently by staged fracturing of the large-spacing thin coal seam group multi-bottom horizontal well comprises the following steps:
the method comprises the following steps: combining the factors of coal mining planning, coal bed gas resources and the like, 2-3 target production layer groups 8 are formed in the layer sections of the enriched coal bed.
Specifically, the target pay group 8 should be selected according to the relative enrichment development degree of the coal seam, and should include factors such as thickness, number of layers, and spacing.
Step two: and setting the layer arranged by each branch wellbore of the multi-bottom horizontal well according to the condition of the selected target pay group 8.
Specifically, for the arrangement layer of each branch shaft of the multi-bottom horizontal well, a rock stratum interlayer between coal seams should be selected first for the broken soft coal seam, a rock stratum interlayer between coal seams can be selected for the hard coal seam with a good coal body structure, and the coal seam itself can also be selected in consideration of ensuring that a fracture crack can press through a communication target coal seam, as shown in fig. 1, 3 and 5, for the broken soft thin coal seam 5, the branch shaft arrangement should be performed in a small distance interlayer 6 between the broken soft thin coal seams 5. For a hard coal seam with a good coal body structure, the arrangement of branch wellbores in the thin coal seams 5 can be selected, as shown in fig. 2, 4 and 6, and the arrangement of branch wellbores in the small distance separation layers 6 between the thin coal seams 5 can also be selected, as shown in fig. 1, 3 and 5. Step three: and determining a well body structure of the U-shaped or L-shaped multi-bottom horizontal well, and performing well drilling and casing cementing construction.
Specifically, according to the condition of an extraction area, a U-shaped or L-shaped multi-bottom casing horizontal well can adopt a one-way or two-way design.
Specifically, referring to fig. 1, 2 and 3, a U-shaped multi-bottom casing horizontal well is constructed, a vertical well 1 is constructed firstly, the vertical well can adopt a two-opening well body structure, a drill bit with phi 311.15mm is used for drilling to 20m below the surface of a bedrock for one opening, and a surface casing with phi 244.5mm is used for well cementation; and secondly, drilling by adopting a phi 215.9mm drill bit to reach 60 meters below the lowest target coal seam, putting into a phi 139.7mm production casing for well cementation, and then forming a cave at the target layer of the horizontal shaft, so that the horizontal shaft 2 can be smoothly butted. The horizontal well can adopt a three-opening well body structure, the horizontal well shaft 3 is constructed on a target layer in sequence by using the sequence from bottom to top, the horizontal well shaft is communicated with the drainage and production vertical well 1 at the position of a cave, a casing is put into the horizontal well shaft for well cementation and completion, and in the construction process, the constructed horizontal well shaft and the drainage and production vertical well are respectively sealed and isolated, so that the damage of drilling fluid and the like to a drilled horizontal section in the construction process is avoided.
Specifically, referring to fig. 4, 5 and 6, an L-shaped multi-bottom casing horizontal well is constructed, the horizontal well can adopt a three-opening well body structure, a horizontal well shaft 3 is constructed at a target layer in sequence from bottom to top, casing well cementation and completion are performed, and in the construction process, a packing measure is taken for the constructed horizontal well shaft, so that damage to a drilled horizontal section caused by drilling fluid and the like in the construction process is avoided.
Step four: and respectively perforating and sectionally perforating and fracturing multiple layers of the shaft of each branch of the multi-bottom casing horizontal well to form a fracturing crack 4, so that each branch shaft of the multi-bottom horizontal well is communicated with a plurality of target coal beds in a target pay group.
Specifically, the clustering perforation is preferably performed, the perforation is preferably performed in a circumferential hole distribution mode, and a horizontal well distribution layer is combined, and according to the requirement on the specific upward or downward extending distance of the fracture, a non-uniform hole distribution mode at the upper half position or the lower half position of a horizontal shaft can be adopted, so that the horizontal well fracturing fracture can pass through multiple layers.
The staged fracturing mode of the horizontal well is preferably the staged fracturing of a pumping bridge plug, and the hydraulic sand-blasting perforation annular fracturing of a continuous oil pipe can also be selected.
Adopts variable density proppant, namely ultra-low density proppant (density is 1.052 to 1.26 g/cm)2) Realizing upward extension and support of the crack; the conventional density proppant is adopted to realize downward extension and support of the fracture, so that the horizontal well fracture can penetrate through multiple layers and can be effectively supported.
Under the condition of ensuring the construction safety, the fracturing scale is properly increased, and the fracturing construction discharge capacity is controlled to be 8-10m3The average sand ratio is controlled between 12 percent and 15 percent.
Step five: and performing the construction of pressure drop, open flow, sand washing and well washing after fracturing, and meeting the requirement of putting the drainage and production equipment into the well.
Specifically, a pumping bridge plug staged fracturing mode is adopted for horizontal well staged fracturing, and work such as pressure drop, blowout, horizontal well drilling plug, vertical well sand washing, well washing and the like after fracturing needs to be carried out. For horizontal well staged fracturing, an oil pipe dragging staged sand blasting fracturing mode is adopted, and work such as pressure drop, open flow, vertical well sand washing, well washing and the like after fracturing needs to be carried out.
Step six: installing a drainage and production device, performing fine drainage and production, and adopting layered pressure control and lamination drainage and production in a vertical well for the U-shaped multi-bottom casing horizontal well; and adopting combined-layer drainage and mining for the L-shaped multi-bottom casing horizontal well.
Specifically, by combining a shaft track, considering the conditions of eccentric wear of a pipe column, water yield and the like, selecting matched drainage and production equipment to perform refined drainage and production; when the layered pressure control and combination layer mode is adopted, the double-sleeve multilayer layered pressure control and drainage and mining equipment can be selected for the condition of a plurality of coal seam groups and large water yield, and for the condition of a double-sleeve multilayer layered pressure control and drainage and mining equipment with small water yield, the double-pump layered pressure control and drainage and mining equipment or the four-channel layered pressure control and drainage and mining equipment can also be selected.
Step seven: and constructing a cluster U-shaped or L-shaped multi-bottom casing horizontal well group (see figure 7).
Specifically, in order to realize the regional efficient extraction of the coal bed gas, a cluster U-shaped or L-shaped multi-bottom casing horizontal well group can be designed and constructed by combining the condition of an extraction region, so that the factory cooperative operation is realized.
Therefore, the invention has the advantages that:
1. the design of many bottoms horizontal well can exploit a plurality of target layer groups simultaneously in a horizontal well, has increased the accumulative total target coal seam number of piles and the accumulative total target coal seam thickness that the in a plurality of layer groups used, can improve the reserves for the coal seam pneumatics greatly, and mining efficiency is high. The U-shaped horizontal well group well type provides conditions for layered pressure control and combined layer drainage and production.
2. The horizontal sections of the multi-bottom horizontal well are designed in rock strata or coal seams between upper coal seams and lower coal seams of each target production stratum group, when casing horizontal well sections in a certain target production stratum group are subjected to subsection perforation fracturing, fracturing transformation of a plurality of target coal seams in the target production stratum group can be simultaneously realized by utilizing the characteristic that hydraulic fracturing cracks are expanded upwards and downwards to penetrate the stratum, and gas production high-conductivity sand crack channels are established between a horizontal shaft and the plurality of target coal seams in the target production stratum group, so that each target coal seam in the target production stratum group can make contributions, and the gas yield of the multi-bottom horizontal well can be greatly improved.
3. The method adopts a cluster multi-bottom horizontal well staged fracturing mode, is suitable for construction in large areas with complex terrain and fluctuation, can save floor area, can reduce construction investment and accelerate construction speed.
4. The method for efficiently extracting the coal bed gas by the segmented fracturing of the multi-bottom horizontal well of the large-interval thin coal bed group is suitable for multiple coal beds (groups) and broken soft coal beds (groups), can quickly reduce the content and pressure of the coal bed gas of the coal beds, remarkably improve the extraction amount and extraction efficiency of the coal bed gas, greatly improve the operation efficiency and reduce the engineering cost, enable efficient economic extraction and disaster reduction of the coal bed gas (gas) in the thin coal bed region to be possible, is an important breakthrough in the technical field of coal bed gas (gas) extraction, and has important significance for improving the extraction technical level and economic benefit of the coal bed gas (gas).
It should be apparent that the foregoing description and illustrations are by way of example only and are not intended to limit the present disclosure, application or uses. While embodiments have been described in the embodiments and depicted in the drawings, the present invention is not limited to the particular examples illustrated by the drawings and described in the embodiments as the best mode presently contemplated for carrying out the teachings of the present invention, and the scope of the present invention will include any embodiments falling within the foregoing description and the appended claims.
Claims (5)
1. A method for efficiently extracting coal bed gas by staged fracturing of a large-spacing thin coal seam group multi-bottom horizontal well is characterized by comprising the following steps of:
the method comprises the following steps: combining factors of coal mining planning and coal bed gas resources, forming 2-3 target production layer groups in the intervals of the enriched coal bed, wherein the target production layer groups are selected according to the relative enrichment development degree of the coal bed and comprise factors of thickness, layer number and spacing;
step two: arranging layers arranged in each branch shaft of a multi-bottom horizontal well according to a target layer group, selecting a rock stratum interlayer between coal layers for a broken soft coal layer at the arrangement layers of each branch shaft of the multi-bottom horizontal well, selecting a rock stratum interlayer between the coal layers or the coal layers for a hard coal layer with a good coal body structure so as to ensure that a fracturing crack can penetrate through the communication target coal layer, selecting a small-distance interlayer between the broken soft coal layers for the broken soft thin coal layer to arrange the branch shafts, and selecting a thin coal layer for the hard coal layer with the good coal body structure to arrange the branch shafts in the thin coal layer or arrange the branch shafts in the small-distance interlayer between the thin coal layers;
step three: determining a well body structure of a U-shaped or L-shaped multi-bottom horizontal well, drilling, casing and cementing, constructing the U-shaped multi-bottom horizontal well, constructing a vertical well, drilling to 60 meters below a lowest target coal seam, casing and cementing, then forming a cave at a target layer of the horizontal well, facilitating smooth butt joint with the horizontal well, constructing a horizontal well at the target layer in sequence from bottom to top, communicating with a vertical well for drainage at the position of the cave, and casing and cementing and completing the well, or constructing the L-shaped multi-bottom horizontal well, constructing a horizontal well at the target layer in sequence from bottom to top, and casing and cementing and completing the well;
step four: perforating and sectionally perforating multi-layer fracturing on the branch mineshafts of the multi-bottom casing horizontal well to form fracturing cracks, so that the branch mineshafts of the multi-bottom casing horizontal well are communicated with a plurality of target coal seams in a target production stratum group, and performing clustering perforation, wherein the perforation adopts a circumferential hole distribution mode and is combined with a horizontal well distribution layer, and a non-uniform hole distribution mode of the upper half position or the lower half position of the horizontal mineshaft is adopted according to the requirement on the specific upward or downward extending distance of the fractures, so that the fracturing fractures of the horizontal well can pass through multiple layers;
step five: performing construction of pressure reduction, open flow, sand washing and well washing after fracturing to meet the requirement of lowering drainage and production equipment;
step six: installing a displacement and mining device for fine displacement and mining, adopting layered pressure control and combined layer displacement and mining in a vertical well for a U-shaped multi-bottom casing horizontal well, and adopting combined layer displacement and mining for an L-shaped multi-bottom casing horizontal well;
step seven: and constructing a cluster U-shaped or L-shaped multi-bottom casing horizontal well group.
2. The method for efficiently extracting coal bed gas by staged fracturing of the large-spacing thin coal seam group and multi-bottom horizontal well according to claim 1, is characterized by comprising the following steps: the staged fracturing mode of the horizontal well adopts pumping bridge plug staged fracturing or continuous oil pipe hydraulic sand blasting perforation annular fracturing.
3. The method for efficiently extracting coal bed gas by staged fracturing of the large-spacing thin coal seam group and multi-bottom horizontal well according to claim 1, is characterized by comprising the following steps: and in the step five, the horizontal well staged fracturing adopts a pumping bridge plug staged fracturing mode, and after fracturing, pressure reduction, blowout, horizontal well drilling plug, vertical well sand washing and well washing are carried out.
4. The method for efficiently extracting coal bed gas by staged fracturing of the large-spacing thin coal seam group and multi-bottom horizontal well according to claim 1, is characterized by comprising the following steps: and in the step five, the horizontal well staged fracturing adopts an oil pipe dragging staged sand blasting fracturing mode, and after fracturing, pressure reduction, open flow, vertical well sand washing and well washing are carried out.
5. The method for efficiently extracting coal bed gas by staged fracturing of the large-spacing thin coal seam group and multi-bottom horizontal well according to claim 1, is characterized by comprising the following steps: in the sixth step: when the layered pressure control and combination layer mode is adopted, the double-sleeve multilayer layered pressure control and discharge and mining equipment is selected for the condition of a plurality of coal seam groups and large water yield, and the double-sleeve multilayer layered pressure control and discharge and mining equipment, the double-pump layered pressure control and discharge and mining equipment or the four-channel layered pressure control and discharge and mining equipment is selected for the condition of a double coal seam group and small water yield.
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