Method for pre-extracting cross-cut coal uncovering gas of inclined coal seam group through ground horizontal well

Abstract

The invention discloses a method for pre-extracting cross-cross cut coal uncovering gas of a ground horizontal well penetrating through an inclined coal seam group, and belongs to the technical field of coal mine gas treatment and coal bed gas development. A horizontal well with a three-opening well body structure is constructed on the ground, and a horizontal well section drills along the inclined direction of a coal seam group and sequentially penetrates through all inclined coal seams of a coal series, and then production casing pipes are put down for well cementation; integrating geological logging, geophysical logging, injection pressure drop well testing and sampling test results, and evaluating the coal and gas outburst risk of the drilling coal seam; the method comprises the following steps: selecting all coal seams with outburst danger to carry out perforation, layered or combined-layer hydrofracture air permeability transformation, descending a jet pump to the bottom of an inclined shaft section after overflow is released, and carrying out drainage pressure reduction and gas pre-extraction for 1-1.5 years. According to the method, the coal seam gas in the cross-cross-cut coal uncovering area of the inclined coal seam group can be rapidly pre-pumped, the coal and gas outburst risk in the cross-cross-cut coal uncovering process is eliminated, meanwhile, coal seam gas in a coal mine area can be efficiently developed, and remarkable economic, environmental, safe and social benefits are generated.

Description

Technical Field

[0001] The present invention relates to a method for pre-extracting coal gas from a surface horizontal well through a group of inclined coal seams, in particular to a method for utilizing a surface horizontal well to stage fracturing a coal seam with a risk of coal and gas outburst to improve the permeability of the coal seam, drain water from the layer and reduce pressure to promote CH 4 The invention relates to a method for desorbing and quickly pre-extracting coal seam gas in the coal-excavating area through the stone gates of inclined coal seams, which significantly improves the excavation efficiency and safety of coal mines through the stone gates of inclined coal seams, and belongs to the technical field of coal mine gas control and coal seam gas development. Background Art

[0002] As my country's basic energy and important industrial raw material, coal plays a key supporting role in the development of the national economy. However, in the process of mine construction and coal mining, many problems seriously restrict the high-quality development of coal mining enterprises. On the one hand, the well construction cycle is long, and various safety accidents occur frequently, among which coal mine gas problems are particularly prominent. On the other hand, in the key link of stone gate excavation, there are many difficulties. Due to factors such as the large number of coal uncovering, poor accuracy of advanced geological detection, and difficulty in gas pressure relief and permeability increase, the coal uncovering procedure is complicated, the workload of preventing outbursts is huge, and the time cycle is long. At the same time, the high cost of gas control and low excavation efficiency make stone gate coal uncovering a bottleneck project for coal mining deployment. According to statistical analysis of relevant data, most of the super-large outbursts of more than 1,000 tons occur during stone gate coal uncovering, and the average intensity of stone gate coal uncovering is much higher than the outburst intensity of coal tunnels. The prevention and control of gas in stone gate coal uncovering has become the focus and difficulty of disaster prevention and control of high gas outburst mines, and "safe and fast stone gate coal uncovering" has become a core technical problem that needs to be solved to ensure coal mine safety production and mining efficiency.

[0003] The traditional "short exploration and slow excavation" underground stone gate coal-excavation process can no longer meet the demand. When excavating coal from the stone gate, energy is easily accumulated near the coal-excavation area and suddenly released, and continuous outburst energy can be provided in the distance, which makes it very easy for coal and gas outbursts to occur at the stone gate position, and the amount of coal outbursts is large, which is serious. The geological conditions underground in coal mines are complex, and there is a risk of outbursts throughout the entire process of coal excavation. In addition, the geological forecast is not accurate enough, so operations can only be carried out one by one, and cannot be carried out in parallel. The conventional "geophysical exploration-drilling-excavation" serial and repetitive "short exploration and slow excavation" mode, when facing high-gas and large-span coal seams, has complex coal-excavation procedures, long time, many anti-outburst drilling holes, high underground anti-outburst costs, and slow excavation speed. As coal mining extends to deeper areas, the difficulty of geological structure detection increases. This model is increasingly difficult to meet the needs of rapid tunnel excavation. It is urgent to overcome the formation of "long exploration and fast excavation" type rapid stone gate coal-excavation methods and technologies.

[0004] The advanced treatment of pre-draining gas on the ground is an important technical guarantee for realizing "long exploration and fast excavation". At present, the methods for treating coal mine gas disasters are mainly divided into two categories: surface gas extraction and underground gas extraction. A few coal enterprises have carried out experimental work such as surface advanced extraction and combined surface and underground extraction, accumulating certain experience and laying a technical foundation for long exploration and fast excavation of cross-measure headings and working faces. However, there are still problems such as insufficient exploration of the technologies for one-time "series" excellent and fast drilling and completion of horizontal wells in steeply inclined and large-span coal seam groups, and inadequate matching of fracturing for permeability enhancement and drainage and pressure reduction for pre-draining, resulting in poor effect of rapid pre-draining of gas by surface horizontal wells. Under this background, developing a method that uses sectional fracturing of surface horizontal wells to improve the gas permeability of coal seams, combines layer drainage and pressure reduction to promote CH 4 desorption, and rapidly pre-drains the gas in the coal seams in the cross-measure heading area of cross-cutting steeply inclined coal seam groups, so as to significantly improve the tunneling efficiency and safety of cross-measure headings, is of great significance for solving the current problems of safe tunneling of cross-measure headings underground and efficient coal mining in coal mines. Summary of the Invention

[0005] Technical Problem: The object of the present invention is to overcome the problems such as poor accuracy of advanced geological exploration in the cross-measure heading area of cross-cutting steeply inclined coal seam groups, great difficulty in gas pressure relief and permeability enhancement, poor effect and low efficiency of traditional underground cross-measure heading gas pre-drainage and outburst elimination, resulting in complex cross-measure heading procedures, large amount of outburst prevention work and low tunneling efficiency. A method for pre-draining the gas in the cross-measure headings of cross-cutting steeply inclined coal seam groups by surface horizontal wells is provided. By using sectional fracturing of surface horizontal wells in coal seams with coal and gas outburst danger, the gas permeability of coal seams is improved, combined layer drainage and pressure reduction promotes CH 4 desorption and rapid pre-draining of the gas in the coal seams in the cross-measure heading area of cross-cutting steeply inclined coal seam groups, thereby improving the tunneling efficiency and safety of cross-measure headings in coal mines.

[0006] Technical Solution: A method for pre-draining the gas in the cross-measure headings of cross-cutting steeply inclined coal seam groups by surface horizontal wells according to the present invention is characterized by including the following steps:

[0007] (a) Construct a surface horizontal well with a "triple-opening" wellbore structure for pre-draining the gas in the cross-measure heading area of cross-cutting steeply inclined coal seam groups on the ground in the coal mining area. For the "first opening" of the surface horizontal well, vertically drill to a depth of 20 m below the bedrock interface and then complete the drilling. Lower the surface casing and cement it in place. Then, use cement slurry in the "first opening" section to return from the bottom to the ground for cementing.

[0008] (b) Start building the inclination during the drilling of the "second opening" of the surface horizontal well, gradually increasing the well inclination of the "second opening" section. When the "second opening" is drilled to the tunneling position of the cross-measure heading for pre-draining the steeply inclined coal seam groups, the well inclination of the surface horizontal well increases to 90°. After carrying out geophysical standard logging of the "second opening" section on the ground, lower the technical casing and cement it in place. Then, use cement slurry in the "second opening" section to return from the bottom to the ground for cementing.

[0009] (c) During the "third spud" of the surface horizontal well, drill one or more horizontal drilling trajectories parallel to the crossheading for pre-drainage of the inclined coal seam group. After the "third spud" of the surface horizontal well penetrates all the coal seams of the inclined coal seam group for pre-drainage, continue to drill horizontally for 20 m in the extension direction of the surface horizontal well section, and the "third spud" section is completed.

[0010] (d) After the completion of the "third spud" section, conduct comprehensive geophysical logging of the "third spud" section on the ground, perform injection / drawdown well testing and in-situ stress testing on the inclined coal seam group for pre-drainage, and provide accurate reservoir parameters for the optimization of the selection and combination of fracturing pre-drained coal seams. Then, run the production casing and cement it with G-class oil well cement. Next, the cement slurry of the "third spud" section returns to 150 - 200 m above the "third spud" horizontal well section.

[0011] (e) Combine the results of surface horizontal well drilling, comprehensive geophysical logging, injection / drawdown well testing, and in-situ stress testing to comprehensively evaluate the coal and gas outburst hazards of all the coal seams of the inclined coal seam group encountered during the drilling of the "third spud" section. Then, based on the evaluation results of coal and gas outburst hazards, select the coal seams that can be used for gas pre-drainage in the crossheading uncovering coal area of the surface horizontal well.

[0012] (f) Based on the coal seams selected in step (e) for gas pre-drainage in the crossheading uncovering coal area, determine the coal seams for single-layer or combined-layer segmented fracturing and permeability improvement of the surface horizontal well according to the horizontal distance between coal seams, coal seam thickness, roof and floor lithology, and coal and rock mechanical characteristics. After determining the fracturing and transformation sections, perforate the coal seam at the position of the farthest fracturing section in the "third spud" horizontal well section.

[0013] (g) After perforation, conduct hydraulic fracturing on the farthest fracturing section of the "third spud" horizontal well section, and use quartz sand as a proppant to support the artificial fractures generated during the fracturing process to improve the permeability of the coal seam near the wellbore of the "third spud" horizontal well section.

[0014] (h) After the fracturing construction of the farthest fracturing section of the "third spud" horizontal well section is completed, carry out "pumping bridge plug setting + perforating co-operation" under the condition of pressure around the wellhead, and conduct open-hole hydraulic fracturing transformation in the fracturing and transformation sections determined in the steps until the bridge plug sealing, coal seam position perforation, and hydraulic fracturing construction of all the fracturing and transformation sections of the "third spud" horizontal well section determined in the steps are completed.

[0015] (i) After the fracturing construction of all the fracturing sections of the surface horizontal well is completed, slowly release the overflowing water to reduce the wellhead pressure until the wellhead pressure drops to 0. Then, remove the wellhead generated during the fracturing process, drill the bridge plug, wash the well, and run the production string into the wellbore of the surface horizontal well. Then, carry out rapid drainage and pressure reduction of the surface horizontal well and rapid gas pre-drainage and outburst elimination of the coal seam in the crossheading uncovering coal area; continuously monitor the water flow.

[0016] (j) During the rapid drainage and pressure reduction of the ground horizontal well and the pre-drainage and outburst elimination of gas, a remote refined drainage control system is used to precisely control the bottom-hole flowing pressure gauge, the casing annulus pressure gauge, the daily water production, and the daily gas production, reducing the risk of production decline due to serious damage to the coal seam permeability. Record the bottom-hole flowing pressure, the casing annulus pressure gauge, the daily gas production, and the daily water production data every day, and use this as the basis for the fitting of the historical data of the combined-layer drainage of the ground horizontal well and the reservoir simulation evaluation of the pre-drainage effect of the coal seam gas in the cross-cut uncovering area and the prediction of the long-term drainage effect;

[0017] (k) As the gas is pre-drained by the ground horizontal well drainage, the liquid level position in the casing annulus of the ground horizontal well gradually drops. When the liquid level in the casing annulus drops to the position of the jet pump slotted screen pipe, the liquid level in the casing annulus further decreases. At this time, gradually reduce the casing annulus pressure and continue to carry out the pre-drainage work of the coal seam gas in the cross-cut uncovering area;

[0018] (l) After the casing annulus pressure gauge of the ground horizontal well shows 0, install a water ring vacuum pump on the ground and start the drive motor to carry out negative pressure drainage on the ground horizontal well, further reducing the pressure of the inclined coal seam group and promoting the desorption and production of the adsorbed gas in the coal seam until the CH 4 purity in the pre-drained gas of the ground horizontal well is lower than 100 m 3 / d;

[0019] (m) When the CH 4 purity in the pre-drained gas of the ground horizontal well is lower than 100 m 3 / d, turn off the drive motor of the water ring vacuum pump, stop the pre-drainage work of the coal seam gas in the cross-cut uncovering area of the ground horizontal well, and transfer to the stage of connecting the boreholes in the underground coal mine to continue pre-draining the gas of the inclined coal seam group in the cross-cut uncovering area.

[0020] In step (a), the terrain of the drilling location of the ground horizontal well is flat, meeting the well site, road, water and electricity conditions for subsequent drilling, completion, fracturing transformation and drainage. And the elevation difference between the ground opening position of the ground horizontal well and the cross-cut elevation of the inclined coal seam group to be constructed should be greater than 300 m; the surface casing run in the "first opening" section of the ground horizontal well is a J55 grade thin-wall casing with a wall thickness of 5 - 6 mm and an inner diameter of not less than 360 mm.

[0021] In step (b), when starting to build the inclination during the "second opening" drilling process of the ground horizontal well, the maximum build-up rate does not exceed 7° / 30 m, and the starting build-up position should be close to the starting drilling position of the "second opening" to reduce the maximum build-up rate during the inclination-increasing drilling and ensure the smooth progress of subsequent drill pipe conveyed logging, well testing, casing running and perforating operations; the technical casing run in the "second opening" section of the ground horizontal well is an N80 grade casing with a wall thickness of 7 - 8 mm and an inner diameter of not less than 220 mm.

[0022] In step (c), when driving 1 crossheading for the inclined coal seam group, the drilling trajectory of the "third opening" horizontal well section is directly below the driving trajectory of the crossheading (21) of the inclined coal seam group, and the distance between the drilling trajectory of the "third opening" horizontal well section and the driving trajectory of the crossheading of the inclined coal seam group between the two intersection points with the same coal seam is 10 - 15 m, which not only reduces the risk of coal and gas outburst during the coal uncovering process after surface gas pre-drainage, but also ensures the integrity and stability of the surrounding coal and rock strata after the crossheading driving; when driving 2 crossheadings in parallel for the inclined coal seam group, the distance between the first crossheading through the inclined coal seam group (21-1) and the second crossheading through the inclined coal seam group is not greater than 40 m, the drilling trajectory of the "third opening" horizontal well section is located between the driving trajectories of the first and the second crossheadings through the inclined coal seam group in parallel, and the distance between the drilling trajectory of the "third opening" horizontal well section and the second crossheading through the inclined coal seam group with a relatively higher elevation: the distance between the drilling trajectory of the "third opening" horizontal well section and the first crossheading through the inclined coal seam group with a relatively lower elevation = 2:1.

[0023] In step (d), for the injection / drop test and in-situ stress test of the pre-drained coal seam group through the inclined coal seam group, the double packer setting method is adopted to ensure the accuracy of the coal seam test results and provide accurate reservoir parameters for the optimization of the selection and combination of the pre-fractured coal seam; the production casing is of N80 or P110 steel grade, with a wall thickness of 8 - 9 mm and an inner diameter of not less than 130 mm to ensure the safety of the subsequent hydraulic fracturing construction and the effect of coal seam permeability improvement.

[0024] In step (f), for the fractured and reformed interval, if the horizontal distance between the adjacent pre-drained coal seams is greater than 40 m, the adjacent coal seams are fractured in layers; when the total horizontal distance between the adjacent coal seams is less than 40 m and the true thickness ratio of the thin coal seam to the thick coal seam is 1:1 - 1:2, two or more coal seams with little difference in true thickness are combined into one fractured interval for combined fracturing. When the total horizontal distance between the adjacent coal seams is less than 40 m and the true thickness ratio of the thin coal seam to the thick coal seam is less than 1:2, if two or more coal seams with a large difference in true thickness are combined into one fractured interval for combined fracturing, then ball plugging and diversion are required during the fracturing construction to ensure the fracturing and permeability enhancement effect of the coal seam with a smaller thickness; perforation is carried out by the drill pipe conveyed perforation method, the perforation position is strictly limited between the top and bottom surfaces of the pre-drained coal seam, the perforation device is a 102-type perforating gun and 127-type perforating charges, the spiral hole layout method is adopted, the perforation density is 16 holes / m, and the perforating fluid is formation water or fresh water.

[0025] In step (g), the hydraulic fracturing adopts the bare casing injection method to ensure that the hydraulic fracturing construction achieves more than 8 m 3High-speed injection with a displacement of / min, the fracturing fluid is an active water fracturing fluid, the quartz sand has a particle size of fine-grained quartz sand of 40 to 70 mesh, and the stage of injecting the sand-carrying fluid adopts a stepped and continuous injection method to ensure the fracture support in the wellbore area of the near-horizontal well and the transformation effect of the coal reservoir permeability.

[0026] In step (i), continuously monitor the slowly released overflow water, record the flow rate of the produced fluid, the cumulative produced fluid volume, and the pressure at the fracturing wellhead. Collect an overflow fluid sample every 2 to 4 hours and conduct a water quality test on the overflow fluid. The water quality test indicators include pH value, TDS value, ORP value, Cl - Mass concentration, temperature, and turbidity are used as the basis for adjusting the overflow rate in sequence; lower a production string into the wellbore of the surface horizontal well. The front part of the production string is successively connected with a plug, a sand settling pipe, a slotted screen pipe, a jet pump, and a tubing from bottom to top, which serve as the power source and channel for lifting the liquid in the horizontal wellbore to the surface; among them: the length of the sand settling pipe is not less than 4 m, the length of the slotted screen pipe is not less than 2 m, the bottom end of the slotted screen pipe is above the completion position of "two-opening", and the vertical distance is 10 to 15 m.

[0027] In step (k), the liquid level in the casing annulus gradually drops to 0.5 to 0.6 MPa before dropping to the upper end of the slotted screen pipe of the jet pump; when the liquid level in the casing annulus drops to the upper end of the slotted screen pipe, the pressure in the casing annulus pressure gauge gradually drops from 0.5 to 0.6 MPa to 0. During this process, the pressure drop rate in the casing annulus is controlled at 5 to 6 kPa / d.

[0028] In step (l), during the process of carrying out negative pressure drainage on the surface horizontal well, when the liquid level in the casing annulus gradually rises and is 5 m higher than the upper end of the slotted screen pipe, start the jet pump drive motor on the ground to intermittently drain water to reduce the liquid level position in the casing annulus, so as to promote the desorption and production of coalbed methane.

[0029] Beneficial effects: Due to the adoption of the above technical solutions, the present invention overcomes the problems of poor accuracy of advanced geological exploration in the area of uncovering coal seams through inclined coal seam groups by crosshead, great difficulty in gas pressure relief and permeability enhancement, poor effect and low efficiency of traditional underground crosshead gas pre-drainage and outburst elimination, resulting in complex procedures for uncovering coal seams through crosshead, large amount of outburst prevention work, and low tunneling efficiency. A method for pre-draining gas in the process of uncovering coal seams through inclined coal seam groups by surface horizontal wells is proposed. Using sectional fracturing of surface horizontal wells to improve the permeability of coal seams with coal and gas outburst hazards, and combined layer drainage and pressure reduction to promote CH 4Desorb and rapidly pre-pump the coal seam gas in the cross-cut coal uncovering area of inclined coal seam groups, significantly improving the tunneling efficiency and safety of cross-cuts through inclined coal seam groups in coal mines. First, use a surface horizontal well (1) to pass through all coal seams of the inclined coal seam group at one time, and simultaneously pre-pump the gas in the coal seams with the risk of coal and gas outburst. Second, through horizontal well drilling, comprehensive geophysical logging, injection / drawdown well testing and in-situ stress testing, conduct advanced geological exploration of the cross-cut coal uncovering area, comprehensively evaluate the risk of coal and gas outburst in the encountered inclined coal seam group, and determine the coal seams for gas pre-pumping accordingly. Third, conduct staged single-layer or combined-layer fracturing transformation to significantly improve the coal seam permeability and the gas extraction efficiency and engineering effect. Finally, successively use jet pump drainage and pressure reduction, reduce the annulus pressure of the casing, and use a water ring vacuum pump for negative pressure gas extraction to sequentially extract the coal seam gas, improving the gas extraction rate. The main advantages compared with the existing technology are as follows: ① The existing technology cannot achieve one-time pre-pumping of gas and outburst elimination in multiple cross-cut areas of cross-cuts through inclined coal seam groups. This technology uses a surface horizontal well to pass through all coal seams of the inclined coal seam group at one time, and can realize sequential fracturing transformation and combined-layer gas pre-pumping in multiple cross-cut areas of cross-cuts through inclined coal seam groups. ② The existing technology cannot carry out large-scale hydraulic fracturing permeability transformation in the cross-cut coal uncovering area, resulting in low gas extraction efficiency, long cycle and insignificant outburst elimination effect. This technology can use surface fracturing equipment to carry out large-scale staged single-layer or combined-layer fracturing transformation, and can effectively support the fracturing cracks with proppants, significantly improving the coal seam permeability and the subsequent gas pre-pumping effect. ③ When the existing technology pre-pumps gas, the pressure reduction of the coal seam is not thorough enough, resulting in difficult desorption and production of a large amount of adsorbed gas. This technology successively uses jet pump drainage and pressure reduction, reduces the annulus pressure of the casing, and uses a water ring vacuum pump for negative pressure gas extraction to sequentially extract the coal seam gas, which can significantly reduce the fluid pressure of the coal seam and promote the desorption and production of gas. ④ The horizontal well drilling, completion, staged fracturing and combined-layer drainage and gas pre-pumping process of this technology is simple, the engineering implementation cost is low, and the economic, environmental and social benefits are good. Brief Description of the Drawings

[0030] Figure 1 It is a schematic diagram of the method for pre-pumping the gas in the cross-cut coal uncovering of the inclined coal seam group by the surface horizontal well of the present invention.

[0031] Figure 2 It is a schematic diagram of the positional relationship between a cross-cut through the inclined coal seam group and the "three-opening" horizontal wellbore of the horizontal well on a cross-section along a certain inclined coal seam of the present invention.

[0032] Figure 3 It is a schematic diagram of the positional relationship between two cross-cuts through the inclined coal seam group and the "three-opening" horizontal wellbore of the horizontal well on a cross-section along a certain inclined coal seam of the present invention.

[0033] In the figure: 1 - surface horizontal well; 2 - inclined coal seam group; 3 - interface between Quaternary system and bedrock; 4 - surface casing; 5 - cement slurry for primary cementing in the open hole section; 6 - technical casing; 7 - cement slurry for secondary cementing in the open hole section; 8 - extension direction of the horizontal well section; 9 - production casing; 10 - cement slurry for tertiary cementing in the open hole section; 11 - most distal fracturing interval in the tertiary horizontal well section; 12 - production string (including tubing, jet pump, slotted screen pipe, sand settling pipe, plug); 13 - annulus pressure between pipe and casing; 14 - annulus liquid level between pipe and casing; 15 - jet pump; 16 - slotted screen pipe; 17 - water ring vacuum pump; 18 - driving motor; 19 - initial deviation position; 20 - initial drilling position for secondary opening; 21 - crosscut; 22 - 1 - first crosscut through the inclined coal seam group; 22 - 2 - second crosscut through the inclined coal seam group; 22 - tertiary horizontal well section; 23 - distance between the intersection point of the drilling trajectory of the tertiary horizontal well section, the construction driving trajectory of the crosscut through the inclined coal seam group and the same coal seam; 24 - distance between the first crosscut through the inclined coal seam group and the second crosscut through the inclined coal seam group; 25 - plug; 26 - sand settling pipe; 27 - tubing; 28 - driving position; 29 - bottom hole flowing pressure gauge. Detailed implementation manners

[0034] The present invention will be further described below in conjunction with the embodiments in the accompanying drawings:

[0035] As Figure 1 shown, the method for pre - draining gas from uncovering coal seams through crosscuts in the surface horizontal well for inclined coal seam groups of the present invention specifically comprises the following steps:

[0036] (a) Construct a surface horizontal well 1 for pre - draining gas from the crosscut 21 for uncovering coal seams in the inclined coal seam group 2 with a "tertiary opening" wellbore structure in the coal mine area. Vertically drill the surface horizontal well 1 in the "primary opening" to a depth of 20 m below the interface 3 between the Quaternary system and the bedrock and then complete the drilling. Cement the surface casing 4, and then return the cement slurry 5 for primary cementing in the "primary opening" section from bottom to top to the surface for cementing. The terrain at the opening position of the surface horizontal well 1 is flat, meeting the well - site, road, water and electricity conditions for subsequent drilling, completion, fracturing transformation and production. Moreover, the elevation difference between the elevation of the surface opening position of the surface horizontal well 1 and the elevation of the crosscut 21 for constructing through the inclined coal seam group 2 should be greater than 300 m. The surface casing 4 run in the "primary opening" section of the horizontal well is a J55 steel grade thin - wall casing with a wall thickness of 5 - 6 mm and an inner diameter of not less than 360 mm;

[0037] (b) During the "second spud" drilling of the surface horizontal well, the well is deflected to gradually increase the well inclination of the "second spud" section. When the "second spud" drilling reaches the construction and tunneling position of the crossheading 21 that penetrates the inclined coal seam group 2, the well inclination of the horizontal well increases to 90°. After conducting geophysical standard logging on the "second spud" section on the ground, the technical casing 6 is run in and cemented. The cement slurry 7 for cementing the "second spud" section returns from bottom to top to the ground for cementing. When the "second spud" section of the horizontal well is deflecting to increase the inclination, the maximum build rate does not exceed 7° / 30m, and the starting deflection position 19 should be as close as possible to the starting drilling position 20 of the "second spud" to reduce the maximum build rate during the inclination-increasing drilling and ensure the smooth progress of subsequent drill pipe conveyed logging, well testing, casing running, and perforating operations. The technical casing 6 run in the "second spud" section of the horizontal well is an N80 grade casing with a wall thickness of 7 - 8mm and an inner diameter of not less than 220mm.

[0038] (c) During the "third spud" of the surface horizontal well, it is drilled horizontally parallel to the construction and tunneling trajectory of the crossheading 21 that penetrates the inclined coal seam group 2. When the "third spud" of the surface horizontal well 1 passes through all the coal seams of the inclined coal seam group 2, it continues to be drilled horizontally for 20m in the extension direction 8 of the horizontal well section and then the "third spud" section is completed. When there is 1 crossheading 21 constructed and tunneling through the inclined coal seam group 2, the drilling trajectory of the "third spud" horizontal well section is directly below the construction and tunneling trajectory of the crossheading 21 that penetrates the inclined coal seam group 2, and the distance 23 between the two intersection points of the drilling trajectory of the "third spud" horizontal well section 22, the construction and tunneling trajectory of the crossheading 21 that penetrates the inclined coal seam group 2, and the same coal seam is 10 - 15m, which not only reduces the risk of coal and gas outburst during the coal uncovering process of the crossheading 2 after surface gas pre-drainage but also ensures the integrity and stability of the surrounding coal and rock strata after the crossheading 21 is tunneled. When there are 2 crossheadings 21 constructed and tunneling parallel through the inclined coal seam group 2, the distance 24 between the first crossheading parallel to the inclined coal seam group 21-1 and the second crossheading parallel to the inclined coal seam group 21-2 is not greater than 40m. The drilling trajectory of the "third spud" horizontal well section 22 is located between the construction and tunneling trajectories of the first crossheading parallel to the inclined coal seam group 22-1 and the second crossheading parallel to the inclined coal seam group 22-2. And the distance between the drilling trajectory of the "third spud" horizontal well section 22 and the second crossheading parallel to the inclined coal seam group 21-2 with a relatively higher elevation: the distance between the drilling trajectory of the "third spud" horizontal well section 22 and the first crossheading parallel to the inclined coal seam group 21-1 with a relatively lower elevation = 2:1, as Figure 2 Figure 3 shown;

[0039] (d) After the completion of the "third spud" interval, perform comprehensive geophysical logging of the "third spud" interval on the ground, conduct injection / fall-off test and in-situ stress test on the target coal seam section of the inclined coal seam group 2 penetrated by pre-drainage. After running the production casing 9, inject G-class oil well cement for cementing. The cement slurry for cementing the "third spud" interval returns to 150 - 200 m above the "third spud" horizontal well interval; when conducting injection / fall-off test and in-situ stress test on the target coal seams of the inclined coal seam group 2 penetrated by pre-drainage, the double packer setting method is used to ensure the accuracy of the coal seam test results and provide accurate reservoir parameters for the optimization of coal seam selection and combination for pre-fracture pre-drainage; the production casing 9 is of N80 or P110 steel grade, with a wall thickness of 8 - 9 mm and an inner diameter of not less than 130 mm to ensure the safety of subsequent hydraulic fracturing construction and the effect of coal seam permeability improvement.

[0040] (e) Combine the results of surface horizontal well 1 drilling, comprehensive geophysical logging, injection / fall-off test and in-situ stress test to comprehensively evaluate the coal and gas outburst risks of all coal seams in the inclined coal seam group 2 encountered in the "third spud" interval, and optimize the selection of coal seams for gas pre-drainage in the cross-cut 21 uncovering coal area using surface horizontal well 1 in combination with the evaluation results of coal and gas outburst risks.

[0041] (f) Based on the optimized results of gas pre-drainage coal seams, determine the intervals for single-layer or multi-layer staged fracturing permeability improvement of surface horizontal well 1 according to the horizontal distance between coal seams, coal seam thickness, roof and floor lithology, coal and rock mechanical characteristics, etc., and perform perforation at the position of the middle coal seam in the most distal fracturing interval 11 of the "third spud" horizontal well interval after determining the fracturing intervals; when the horizontal distance between adjacent gas pre-drainage coal seams is greater than 40 m, perform layered fracturing on adjacent coal seams. When the total horizontal distance between adjacent coal seams is less than 40 m and the true thickness ratio of thin coal seam to thick coal seam is 1:1 - 1:2, combine two or more coal seams with similar true thicknesses into one fracturing interval for multi-layer fracturing. When the total horizontal distance between adjacent coal seams is less than 40 m and the true thickness ratio of thin coal seam to thick coal seam is less than 1:2, if two or more coal seams with large differences in true thickness are combined into one fracturing interval for multi-layer fracturing, it is necessary to carry out ball plugging and diversion during the fracturing construction to ensure the fracturing permeability improvement effect of the thinner coal seam; perforation is carried out by drill pipe conveyance perforation method, the perforation position is strictly limited between the top and bottom surfaces of the gas pre-drainage coal seam, the perforation device is 102-type perforating gun and 127-type perforating charge, the spiral hole pattern is adopted, the perforation density is 16 holes / m, and the perforating fluid is formation water or fresh water;

[0042] (g) After perforation, conduct hydraulic fracturing on the most distal fracturing interval 11 of the "third spud" horizontal well interval, and use quartz sand as a proppant to effectively support the artificial fractures generated during the fracturing process to improve the coal seam permeability near the wellbore of the "third spud" horizontal well interval; the hydraulic fracturing adopts the bare casing injection method to ensure that the hydraulic fracturing construction can achieve a fracture length greater than 8 m 3 / min high-speed injection, the fracturing fluid is active water fracturing fluid, the quartz sand proppant particle size is 40-70 mesh fine-grained quartz sand, and the sand-carrying fluid injection stage adopts a step-by-step and continuous injection method to ensure the fracture support in the near-horizontal wellbore zone and the permeability transformation effect of the coal reservoir.

[0043] (h) After the fracturing construction of the farthest fractured layer 11 of the "three-opening" horizontal well section is completed, the "pumping bridge plug seat sealing + perforation combined operation" is carried out under the condition of wellhead pressure, and the light casing hydraulic fracturing transformation of the next fracturing layer section is carried out in turn until the bridge plug isolation, coal seam position perforation and hydraulic fracturing construction of all constructed fracturing layers of the ground (11) horizontal well are completed; the bridge plug is a drillable bridge plug, and after the soluble sealing ball is dropped into the wellbore, the physical isolation of the far end fractured layer section can be achieved; after the construction of all fracturing layers is completed, the sealing ball is dissolved and unsealed, and the fracturing fluid of all fracturing layers is discharged simultaneously when the wellhead overflows.

[0044] (i) After the fracturing construction of all the constructed fracturing layers of the ground horizontal well 1 is completed, the overflow is slowly released until the wellhead pressure drops to 0, and then the fracturing wellhead is removed, the bridge plug is drilled, and the well is washed, and the drainage and production pipe string 12 is lowered into the wellbore of the ground horizontal well 1, and then the ground horizontal well 1 is quickly drained and depressurized, and the coal seam gas in the Shimen 21 coal uncovering area is quickly pre-extracted and eliminated; during the overflow release process, the flow rate of the return fluid, the cumulative return fluid volume, and the fracturing wellhead pressure are continuously monitored and recorded, and the overflow fluid sample is collected once every 2 to 4 hours, and the overflow fluid water quality test is carried out. The water quality test indicators include pH value, TDS value, ORP value, Cl - The mass concentration, temperature and turbidity are used as the basis for adjusting the overflow speed in turn; the production and drainage string includes a wire plug 25, a sand settling pipe 26, a slotted screen pipe 16, a jet pump 15 and an oil pipe 27 from bottom to top, and they are connected in turn as the power source and channel for lifting the liquid in the horizontal wellbore to the surface; the length of the sand settling pipe 26 is not less than 4m, the length of the slotted screen pipe 16 is not less than 2m, and the bottom end of the slotted screen pipe 16 is located above the "second opening" completion drilling position, and the vertical distance is 10 to 15m.

[0045] (j) During the rapid drainage and pressure reduction and gas pre-extraction and outburst elimination process of the ground horizontal well 1, a remote refined drainage and production control system is used to finely control the bottom hole flow pressure, casing annulus pressure 13, daily water production, and daily gas production to reduce the risk of production capacity reduction due to serious damage to the permeability of the coal seam. The bottom hole flow pressure, casing annulus pressure, daily gas production, and daily water production data are recorded every day, and used as the basis for fitting the historical data of the combined layer drainage of the ground horizontal well 1 and the reservoir simulation evaluation of the coal seam gas pre-extraction effect in the Shimen 21 coal uncovering area and the prediction of the long-term extraction effect; During the rapid drainage and pressure reduction and gas pre-extraction and outburst elimination process of the horizontal well, the serious damage to the permeability of the coal seam is caused by the large fluctuation of the bottom hole flow pressure during the rapid increase of the gas production rate of the horizontal well. In order to avoid the rapid and large fluctuation of the bottom hole flow pressure, it is necessary to gradually reduce the daily drop in the bottom hole flow pressure and control the speed of increasing the daily gas production during the rapid production increase of the horizontal well, so that the fluctuation amplitude of the bottom hole flow pressure of the horizontal well is less than 150 kPa and the fluctuation frequency is greater than 12h / time.

[0046] (k) With the extension of the pre-extraction time of the gas in the ground horizontal well 1, the position of the casing annulus liquid level 14 in the horizontal wellbore gradually decreases. When the casing annulus liquid level 14 drops to the position of the slotted screen 16 of the jet pump 15, the casing annulus liquid level 14 cannot be further reduced. At this time, the casing annulus pressure is gradually reduced to continue the coal seam gas pre-extraction work in the Shimen 21 coal uncovering area; the casing annulus pressure 13 of the horizontal well should be gradually reduced to 0.5-0.6MPa before the casing annulus liquid level 14 drops to the upper end of the slotted screen 16; when the casing annulus liquid level 14 drops to the upper end of the slotted screen 16, the casing annulus pressure 13 is gradually reduced from 0.5-0.6MPa to 0, and the casing annulus pressure 13 reduction rate is controlled at 5-6kPa / d during this process.

[0047] (l) When the annular space pressure 13 of the horizontal well casing drops to 0, a water ring vacuum pump 17 is installed on the ground 1 and a drive motor 18 is started to perform negative pressure extraction on the ground horizontal well 1, further reducing the coal reservoir pressure and promoting the desorption and production of gas adsorbed in the coal seam until CH4+ in the pre-extracted gas in the ground horizontal well 1 is completely extracted. 4 Pure amount less than 100m 3 / d; During the negative pressure extraction process, when the liquid level 14 in the casing annulus gradually rises and is 5m higher than the upper end of the slotted screen pipe 16, the jet pump 17 is started on the ground to drive the motor 18 to perform intermittent drainage to lower the position of the liquid level 14 in the casing annulus, thereby promoting the desorption and production of coalbed methane.

[0048] (m) When the CH 4 Pure amount less than 100m 3 / d, turn off the drive motor 18 of the water ring vacuum pump 17, stop the coal seam gas pre-extraction work in the surface horizontal well 1 stone gate 21 coal uncovering area, and enter the coal mine underground docking drilling and continue the pre-extraction of gas from the inclined coal seam group 2 in the stone gate 21 coal uncovering area.

Claims

1. A method for pre-extracting coal gas from a horizontal well on the ground through a rock gate of an inclined coal seam group, characterized in that The following steps are involved: (a) constructing a ground horizontal well (1) with a "three-opening" wellbore structure on the ground in a coal mine area to pre-drain gas from the coal area by penetrating the stone gate (21) of the inclined coal seam group (2), drilling the ground horizontal well (1) vertically to 20 m below the bedrock interface (3) and then completing the drilling, inserting a surface casing (4) for cementing, and then returning to the ground from the bottom to the top with cement slurry (5) in the "first opening" well section for cementing; (b) During the drilling process of the second opening of the horizontal well on the ground, the inclination of the well section of the second opening is gradually increased. When the drilling of the second opening reaches the excavation position (28) of the stone gate (21) for pre-draining the inclined coal seam group (2), the inclination of the horizontal well on the ground is increased to 90 degrees. After the standard geophysical logging of the well section of the second opening is carried out on the ground, the technical casing (6) is lowered to cement the well. After that, cement slurry (7) is used to return to the ground from bottom to top to cement the well in the second opening section; (c) constructing one or more excavation tracks parallel to the stone gate (21) of the pre-drained inclined coal seam group (2) in the ground horizontal well "three-opening" to carry out horizontal drilling. After the ground horizontal well (1) "three-opening" passes through all the coal seams of the pre-drained inclined coal seam group (2), continue to drill horizontally for 20m in the extension direction (8) of the ground horizontal well section, and the "three-opening" well section is completed; (d) After the drilling of the "three-opening" section is completed, the "three-opening" section geophysical comprehensive logging is carried out on the ground, and the injection / pressure drop test and in-situ stress test are carried out on the pre-pumping inclined coal seam group (2) to provide accurate reservoir parameters for the selection and combination optimization of the pre-pumping coal seam by fracturing; then, the production casing (9) is lowered and G-grade oil well cement is injected for cementing, and then the cement slurry (10) of the three-opening section is returned to 150 to 200 meters above the "three-opening" horizontal section; (e) combining the results of surface horizontal well drilling, geophysical comprehensive logging, injection / pressure drop test and in-situ stress test, comprehensively evaluating the coal and gas outburst hazards of all coal seams in the inclined coal seam group (2) encountered during the drilling of the "three-opening" well section, and selecting the coal seams that can be used for gas pre-extraction in the surface horizontal well (1) in the stone gate (21) coal uncovering area based on the coal and gas outburst hazard evaluation results; (f) Based on the coal seam selected in step (e) for gas pre-extraction in the stone gate (21) coal uncovering area, the coal seam for single-layer or multi-layer staged fracturing and permeability transformation in the ground horizontal well (1) is determined according to the horizontal spacing of the coal seams, the thickness of the coal seams, the lithology of the roof and floor plates, and the mechanical characteristics of the coal and rock. After the fracturing and transformation layer sections are determined, perforation is carried out at the coal seam position in the farthest fracturing layer section (11) of the "three-opening" horizontal well section; (g) After perforation is completed, hydraulic fracturing is carried out at the farthest fracturing layer (11) of the "three-opening" horizontal well section, and quartz sand is used as a proppant to support the artificial fractures generated during the fracturing process, so as to improve the permeability of the coal seam near the wellbore of the "three-opening" horizontal well section; (h) After the fracturing construction of the farthest fracturing layer section (11) of the "three-opening" horizontal well section is completed, "pumping bridge plug seat sealing + perforation combined operation" is carried out under the condition of pressure around the wellhead, and bare casing hydraulic fracturing transformation is carried out in the fracturing transformation layer section determined in step (f), until the bridge plug sealing, coal seam position perforation and hydraulic fracturing construction of all fracturing transformation layer sections of the "three-opening" horizontal well section determined in step (f) are completed; (i) After the fracturing construction of all the fracturing layers of the ground horizontal well (1) is completed, the overflowed water flow is slowly released to reduce the wellhead pressure until the wellhead pressure drops to 0, and then the wellhead generated during the fracturing process is removed, and after drilling the bridge plug and washing the well, the drainage and production pipe string (12) is lowered into the wellbore of the ground horizontal well (1), and then the ground horizontal well (1) is quickly drained and depressurized, and the coal seam gas is quickly pre-extracted and eliminated in the coal-uncovering area of ​​the stone gate (21); the water flow is continuously monitored (j) During the rapid drainage and pressure reduction and gas pre-extraction and sudden relief process of the surface horizontal well (1), a remote refined drainage control system is used to finely control the bottom hole flow pressure gauge (29), the casing annulus pressure gauge (13), the daily water production, and the daily gas production, so as to reduce the risk of production capacity reduction due to serious damage to the permeability of the coal seam. The bottom hole flow pressure, the casing annulus pressure gauge (13), the daily gas production, and the daily water production data are recorded every day, and used as the basis for fitting the historical data of the combined layer drainage of the surface horizontal well (1) and the reservoir simulation evaluation of the coal seam gas pre-extraction effect in the Shimen (21) coal uncovering area and the prediction of the long-term extraction effect; (k) As the surface horizontal well (1) is drained and pre-extracted, the position of the casing annulus liquid level (14) in the surface horizontal wellbore gradually decreases. When the casing annulus liquid level (14) drops to the position of the slotted screen pipe (16) of the jet pump (15), the casing annulus liquid level (14) further decreases. At this time, the casing annulus pressure is gradually reduced to continue the coal seam gas pre-extraction work in the stone gate (21) coal uncovering area; (l) When the casing annulus pressure gauge (13) of the surface horizontal well (1) shows 0, a water ring vacuum pump (17) is installed on the ground and a drive motor (18) is started to perform negative pressure extraction on the surface horizontal well (1), thereby further reducing the pressure of the inclined coal seam group (2) and promoting the desorption and production of gas adsorbed in the coal seam, until the pure amount of CH4 in the pre-extracted gas in the surface horizontal well (1) is less than 100m 3 / d; (m) When the pure amount of CH4 in the pre-extracted gas in the surface horizontal well (1) is less than 100m 3 / d, turn off the drive motor (18) of the water ring vacuum pump (17), stop the pre-extraction of coal seam gas in the stone gate (21) coal uncovering area of ​​the ground horizontal well (1), and enter the underground coal mine docking drilling and pre-extraction of gas in the inclined coal seam group (2) in the stone gate (21) coal uncovering area.

2. The method for pre-extracting coal gas from a ground horizontal well through a rock gate of an inclined coal seam group according to claim 1, characterized in that: In step (a), the terrain of the drilling location of the ground horizontal well (1) is flat, meeting the well site, road, water and electricity conditions for subsequent drilling and completion, fracturing and production, and the difference between the elevation of the ground opening position of the ground horizontal well (1) and the elevation of the stone gate (21) of the construction inclined coal seam group (2) should be greater than 300m; the surface casing (4) lowered into the "first opening" section of the ground horizontal well is a J55 steel grade thin-walled casing with a wall thickness of 5 to 6mm and an inner diameter of not less than 360mm.

3. The method for pre-extracting coal gas from a ground horizontal well through a rock gate of an inclined coal seam group according to claim 1, characterized in that: In step (b), when the surface horizontal well "second opening" drilling process starts to build inclination, the maximum build inclination rate does not exceed 7° / 30m, and the starting build inclination position (19) should be close to the "second opening" starting drilling position (20) to reduce the maximum build inclination rate during the increasing inclination drilling, so as to ensure the smooth implementation of subsequent drill pipe transmission logging, well testing, casing and perforating work; the technical casing (6) lowered into the "second opening" well section of the surface horizontal well is an N80 steel grade casing with a wall thickness of 7 to 8 mm and an inner diameter of not less than 220 mm.

4. The method for pre-extracting coal gas from a ground horizontal well through a rock gate of an inclined coal seam group according to claim 1, characterized in that: In step (c), when one of the stone gates (21) of the inclined coal seam group (2) is being excavated, the drilling trajectory of the "three-opening" horizontal well section is located directly below the excavation trajectory of the stone gate (21) of the inclined coal seam group (2), and the distance (23) between the drilling trajectory of the "three-opening" horizontal well section (22), the excavation trajectory of the stone gate (21) of the inclined coal seam group (2) and two intersections of the same coal seam is 10 to 15 meters, which not only reduces the danger of coal and gas outburst during the coal uncovering process of the stone gate (21) after ground gas pre-extraction, but also ensures the integrity and stability of the surrounding coal and rock layers after the stone gate (21) is excavated; the horizontal well section (22) of the inclined coal seam group (2) is constructed and excavated. When two tunneling lines are carried out, the distance (24) between the first tunneling inclined coal seam group stone gate (21-1) and the second tunneling inclined coal seam group stone gate (21-2) is not greater than 40m, the drilling trajectory of the "three-opening" horizontal well section (22) is located between the construction tunneling trajectory of the first tunneling inclined coal seam group parallel stone gate (22-1) and the second tunneling inclined coal seam group parallel stone gate (22-2), and the distance between the drilling trajectory of the "three-opening" horizontal well section (22) and the second tunneling inclined coal seam group stone gate (21-2) with a relatively high elevation: the distance between the drilling trajectory of the "three-opening" horizontal well section (22) and the first tunneling inclined coal seam group stone gate (21-1) with a relatively low elevation = 2:

1.

5. The method for pre-extracting coal gas from a ground horizontal well through a rock gate of an inclined coal seam group according to claim 1, characterized in that: In step (d), the injection / pressure drop test and ground stress test of the pre-pumping inclined coal seam group (2) are both carried out by using a double packer sealing method to ensure the accuracy of the coal seam test results and provide accurate reservoir parameters for the optimization and combination optimization of the pre-pumping coal seams by fracturing; the production casing (9) is of N80 or P110 steel grade, with a wall thickness of 8 to 9 mm and an inner diameter of not less than 130 mm, to ensure the safety of subsequent hydraulic fracturing construction and the effect of coal seam permeability transformation.

6. The method for pre-extracting coal gas from a ground horizontal well through a rock gate of an inclined coal seam group according to claim 1, characterized in that: In step (f), if the horizontal distance between adjacent gas pre-extraction coal seams in the fracturing transformation layer section is greater than 40m, the adjacent coal seams are subjected to layered fracturing; When the total horizontal spacing between adjacent coal seams is less than 40m, and the true thickness ratio of thin coal seams to thick coal seams is 1:1-1:2, two or more coal seams with similar true thicknesses are combined into one fracturing section for combined fracturing. When the total horizontal spacing between adjacent coal seams is less than 40m, and the true thickness ratio of thin coal seams to thick coal seams is less than 1:2, if two or more coal seams with large true thickness differences are combined into one fracturing section for combined fracturing, it is necessary to perform ball plugging and steering during the fracturing construction to ensure the fracturing and permeability enhancement effect of the thinner coal seams. The perforation adopts the drill pipe transmission perforation method, and the perforation position is strictly limited to between the top and bottom surfaces of the gas pre-extraction coal seam. The perforating device is a 102-type perforating gun and a 127-type perforating bullet. The spiral perforation method is adopted, the perforation density is 16 holes / m, and the perforating fluid is formation water or clean water.

7. The method for pre-extracting coal gas from a ground horizontal well through a rock gate of an inclined coal seam group according to claim 1, characterized in that: In step (g), the hydraulic fracturing adopts a bare casing injection method to ensure that the hydraulic fracturing construction is greater than 8m 3 / min high-speed injection, the fracturing fluid is active water fracturing fluid, the quartz sand particle size is 40-70 mesh fine-grained quartz sand, and the sand-carrying fluid injection stage adopts a step-by-step and continuous injection method to ensure the fracture support in the near-horizontal wellbore zone and the permeability transformation effect of the coal reservoir.

8. The method for pre-extracting coal gas from a ground horizontal well through a rock gate of an inclined coal seam group according to claim 1, characterized in that: In step (i), the slowly released overflow water flow is continuously monitored, the flow rate of the backflow fluid, the cumulative backflow fluid volume, and the fracturing wellhead pressure are recorded, and a sample of the overflow fluid is collected every 2 to 4 hours to conduct a water quality test of the overflow fluid. The water quality test indicators include pH value, TDS value, ORP value, Cl - The mass concentration, temperature and turbidity are used as the basis for adjusting the overflow speed in sequence; the front part of the drainage and production pipe string (12) lowered into the wellbore of the ground horizontal well (1) is provided with a wire plug (25), a sand settling pipe (26), a slotted screen pipe (16), a jet pump (15) and an oil pipe (27) which are connected in sequence from bottom to top as the power source and channel for lifting the liquid in the wellbore of the horizontal well to the surface; wherein: the length of the sand settling pipe (26) is not less than 4m, the length of the slotted screen pipe (16) is not less than 2m, and the bottom end of the slotted screen pipe (16) is located above the "second opening" completion drilling position, and the vertical distance is 10 to 15m.

9. The method for pre-extracting coal gas from a ground horizontal well through a rock gate of an inclined coal seam group according to claim 1, characterized in that: In step (k), the casing annulus liquid level (14) gradually drops to 0.5-0.6 MPa before it drops to the upper end of the slotted screen tube (16) of the jet pump (15); when the casing annulus liquid level (14) drops to the upper end of the slotted screen tube (16), the pressure in the casing annulus pressure gauge (13) gradually drops from 0.5-0.6 MPa to 0, and the casing annulus pressure drop rate in this process is controlled at 5-6 kPa / d.

10. The method for pre-extracting coal gas from a ground horizontal well through a rock gate of an inclined coal seam group according to claim 1, characterized in that: In step (1), during the negative pressure extraction of the ground horizontal well (1), when the casing annulus liquid level (14) gradually rises and is 5 m higher than the upper end of the slotted screen pipe (16), the ground jet pump (17) is started to drive the motor (18) to perform intermittent drainage to lower the casing annulus liquid level (14) to promote coalbed methane desorption and production.

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