A progressive gas grading control method for outburst coal seam based on energy stepwise release

Abstract

This invention discloses a progressive gas classification and treatment method for outburst-prone coal seams based on energy cascade release, which proceeds according to the following time-controlled steps: identifying the main and sub-critical layers of the target coal seam, fracturing the critical layers to achieve large-scale pressure relief of the coal seam; constructing low-disturbance cross-layer boreholes from the bottom drainage roadway to the target coal seam for preliminary gas extraction; constructing hydraulic cavity-forming enhanced extraction boreholes near the pre-extraction low-disturbance conventional boreholes; immediately after the hydraulic cavity-forming boreholes are completed and the drilling is withdrawn, performing compressed air sweeping to remove residual coal slurry and water inside and around the borehole walls; and finally, inserting screen pipes into the hydraulic cavity-forming boreholes and connecting them to the main pipeline network for secondary enhanced extraction of residual gas in the coal seam until the gas pressure is less than 0.74 MPa. This invention is based on the technical path of "fracturing and unloading - pre-drainage and energy reduction - cavity creation and efficiency enhancement". It adopts a graded strategy of fracturing key overburden layers to achieve overall pressure relief, pre-drainage of low-disturbance boreholes to create a safety window, and hydraulic cavity creation to enhance extraction. It coordinates risk control and efficiency improvement in a temporal and spatial sequence, and realizes the tiered release of coal body strain energy and gas expansion energy. It has high safety redundancy and is more suitable for efficient gas control and precise outburst elimination projects in deep coal seams.

Description

Technical Field

[0001] This invention belongs to the field of coal mine gas control technology, specifically relating to a progressive gas classification and control method for outburst-prone coal seams based on energy cascade release. Background Technology

[0002] As coal mining progresses deeper, in-situ stress and gas pressure increase significantly, and the structure and geological environment of coal-bearing reservoirs become increasingly complex, posing a dual challenge to the safety and efficiency of single gas extraction technologies. Furthermore, mining disturbances between multiple coal seams will exacerbate the complexity of the stress field, especially under the superimposed stress coupling effect generated during drilling operations, which poses a risk of inducing dynamic disasters such as gas outbursts.

[0003] Conventional gas drainage drilling produces relatively small disturbances, but its limited borehole diameter and effective pressure relief range make it difficult to release stress over a large area of ​​coal seam. Enhanced drainage technologies, such as hydraulic cavity drilling, can significantly expand the permeability enhancement range and improve drainage efficiency. However, the stress concentration effect and "water-locking effect" within the coal seam caused by hydraulic operations cannot be ignored, posing a risk of delayed blowouts. Therefore, the differences in the ability of different drainage technologies to regulate coal strain energy and gas expansion energy mean that a single drainage technology cannot effectively balance construction safety and drainage efficiency.

[0004] In summary, the purpose of this invention is to provide a progressive, staged extraction method that balances both safety and efficiency. Based on the technical path of "fracturing and unloading – pre-extraction and energy reduction – cavity creation and efficiency enhancement," it utilizes the organic combination of time and space to construct boreholes in stages, thereby achieving the tiered release of coal seam strain energy and gas expansion energy, reducing the risk of gas outbursts and improving extraction efficiency. This provides theoretical support and engineering technical guidance for the safe and efficient development of deep, high-gas, low-permeability coal seams. Summary of the Invention

[0005] To address the problems existing in the prior art, this invention provides a progressive gas classification and control method for outburst-prone coal seams based on energy cascade release. Based on the technical path of "fracturing and unloading - pre-drainage and energy reduction - cavity creation and efficiency enhancement", it adopts a classification strategy of fracturing key overburden layers to achieve overall pressure relief, pre-drainage of low-disturbance boreholes to create a safety window, and hydraulic cavity creation to enhance extraction. It coordinates risk control and efficiency improvement in a temporal and spatial sequence to achieve efficient gas control and precise outburst elimination in deep coal seams.

[0006] To achieve the above objectives, the technical solution adopted by this invention is: a progressive gas classification and control method for outburst-prone coal seams based on energy cascade release, comprising the following five steps:

[0007] S1: Overall pressure relief from hydraulic fracturing of the key layer;

[0008] Based on the relevant geological parameters of the target coal seam and its overlying strata, the main and sub-key strata of the coal seam are identified, and a set of high and low-level fracturing boreholes are constructed on both sides and the top of the roadway. After the matching packers are connected to the boreholes and transported to the preset fracturing points, the main and sub-key strata of the overlying strata are simultaneously fracturing in segments, with the fracturing point intervals being 2-5 m.

[0009] S2: Low-disturbance borehole pre-pulling energy reduction;

[0010] After step S1 is completed, a low-disturbance cross-seam borehole is drilled into the target coal seam in the bottom drainage roadway. After the construction is completed, a screen pipe is installed and connected to the main pipeline. Based on the dynamic change trend of coal seam gas pressure, preliminary gas drainage is carried out on the coal seam for 20-50 days to reduce the gas pressure gradient around the borehole and weaken the outburst potential in the original coal seam, creating a safe window for subsequent enhanced drainage processes.

[0011] S3: Hydraulic cavity creation for pressure relief and increased permeability;

[0012] After the initial gas pre-drainage is completed, hydraulic cavity drilling is carried out near the low-disturbance conventional borehole where gas pre-drainage was implemented to enhance gas extraction. The cavity drilling section covers the full thickness of the coal seam. Cavitation is stopped when coal production stops and the water from the borehole becomes clear.

[0013] S4: Compressed air sweeping hole for slag discharge;

[0014] After the hydraulic cavity drilling for pressure relief is completed and the drilling is withdrawn, compressed air cleaning operation is immediately carried out to remove residual coal slurry and water inside and around the borehole, break the "water lock effect" and prevent delayed blowout.

[0015] S5: Sealed-hole network enhanced extraction;

[0016] After the compressed air cleaning is completed, a screen pipe is lowered into the hydraulic cavity drilling hole and connected to the main pipeline network to carry out secondary enhanced extraction of residual gas in the coal seam until the gas pressure is less than 0.74 MPa.

[0017] Furthermore, the primary and subcritical layers in step S1 are determined based on the "masonry beam" structural theory and numerical simulation, and the initiation pressure and fracturing range are obtained based on theoretical calculations and field tests, respectively.

[0018] Furthermore, the diameter of the fracturing borehole in step S1 is selected in the range of Ф90-120 mm; wherein the high-level fracturing borehole advances the working face by 70 m and penetrates the main and subcritical layers; the low-level fracturing borehole advances the working face by 70 m and only penetrates the subcritical layer; the interval between fracturing points is 2-5 m.

[0019] Furthermore, the starting condition for step S2 needs to be met: after step S1 stops, the decay rate of the fracturing pressure within 30 minutes is not greater than 0.1 MPa / min, and the correlation coefficient of the decay curve is not less than 0.9.

[0020] Furthermore, the through-layer drilling in step S2 is a conventional compressed air slag discharge drilling, with a hole diameter range of Ф70-100 mm, ensuring the formation of an effective unloading zone while significantly reducing the instantaneous disturbance stress on the coal body compared to strong disturbance processes such as hydraulic cavity creation.

[0021] Furthermore, in step S2, the low-disturbance boreholes are uniformly arranged with a spacing not exceeding twice the effective extraction radius to ensure the formation of a continuous unloading zone in the coal seam, thereby reducing the concentration effect of in-situ stress and gas pressure.

[0022] Furthermore, the starting condition for step S3 needs to be met: after taking step S2, the decrease in residual gas pressure in the coal seam between boreholes compared to the original gas pressure is not less than 25%.

[0023] Furthermore, the hydraulic cavity-creating boreholes in step S3 are obtained by creating and enlarging the coal seam section using conventional boreholes with a diameter of Ф70-100 mm, with an injection pressure of 15-20 MPa; the spacing between the boreholes does not exceed twice the effective extraction radius.

[0024] Furthermore, during the drilling process in steps S1 to S3, a blowout prevention device is installed at the borehole opening to prevent dynamic disasters caused by sudden energy release.

[0025] Furthermore, in steps S2 and S5, the screen tubes are sealed using a "two-plug-one-injection" method, with a sealing pressure of 2.0 MPa and a sealing section length of 12 m.

[0026] Compared with existing technologies, the progressive gas classification and control method for outburst-prone coal seams disclosed in this invention has the following beneficial effects:

[0027] Compared to single gas extraction technology, this invention effectively disperses risks through the synergistic and hierarchical management of multiple technologies, resulting in high safety redundancy and superior management effects. Furthermore, the technical approach fully leverages risk control and efficiency improvement in the temporal and spatial sequence, enabling the tiered release of coal body strain energy and gas expansion energy. The progressive management model has clear objectives and close connections, which can shorten the overall management cycle.

[0028] This invention achieves large-scale pressure relief of the coal seam by implementing key layer fracturing to cut off the stress transmission of the overlying strata; on the basis of controlled risk, it adopts low-disturbance borehole pre-drainage to systematically weaken the overall outburst potential of the coal seam in advance, creating a safe window for highly efficient permeability enhancement measures under strong disturbance; finally, it implements hydraulic cavity creation to enhance drainage, achieving large-scale pressure relief of the coal seam; compressed air sweeping effectively solves the water-locking effect and the risk of delayed blowouts after hydraulic operations, ensuring unobstructed borehole channels and the highest efficiency in the initial stage of drainage.

[0029] The present invention conducts comparative simulation calculations under the same geological conditions. Compared with pre-drainage using only low-disturbance boreholes (Example A) and pre-drainage using low-disturbance boreholes combined with hydraulic cavity creation for enhanced drainage (Example B), the progressive gas grading management method of the present invention (Example C) can achieve significant advantages in terms of drainage effect and time to reach the target. Attached Figure Description

[0030] To make the objectives, methods, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings. Obviously, the drawings described below are merely some practical examples of this invention. For those skilled in the art, other drawings can be obtained from these drawings without any creative effort. In particular:

[0031] Figure 1 This is a flowchart illustrating the overall steps of a progressive gas classification and control method for outburst-prone coal seams based on energy cascade release, as disclosed in an embodiment of the present invention.

[0032] Figure 2 This is a schematic diagram of the key layer fracturing borehole layout for a progressive gas classification and control method for outburst-prone coal seams based on energy cascade release, as disclosed in an embodiment of the present invention.

[0033] Figure 3 This is a schematic diagram of the layout of the extraction borehole group for a progressive gas classification and control method for outburst-prone coal seams based on energy cascade release, as disclosed in an embodiment of the present invention.

[0034] Figure 4 This is a plan view of the final borehole layout for a progressive gas classification and control method for outburst-prone coal seams based on energy cascade release, as disclosed in an embodiment of the present invention.

[0035] Figure 5 This is a cross-sectional view of the extraction borehole layout for a progressive gas classification and control method for outburst-prone coal seams based on energy cascade release, as disclosed in an embodiment of the present invention.

[0036] Figure 6 This is a comparison chart of the extraction effects of a progressive gas classification and control method for outburst-prone coal seams based on energy cascade release, as disclosed in an embodiment of the present invention.

[0037] 1: Main critical layer; 2: Basic roof; 3: Subcritical layer; 4: Immediate roof; 5: Coal seam; 6: Floor; 7: Fracturing drilling site; 8: Low-level fracturing borehole; 9: High-level fracturing borehole; 10: Fracturing point; 11: Hydraulic cavity-building enhanced extraction borehole; 12: Conventional compressed air slag removal borehole; 13: Transport roadway; 14: Bottom extraction roadway. Detailed Implementation

[0038] The preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings. These preferred embodiments are for illustrative purposes only and are not intended to limit the scope of protection of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0039] Figure 1 A flowchart illustrating the overall steps of an embodiment of the present invention; Figure 2 This is a schematic diagram of the key layer fracturing borehole arrangement provided in an embodiment of the present invention; Figure 3 This is a schematic diagram of the layout of the extraction borehole group provided in an embodiment of the present invention; Figure 4 This is a plan view of the final borehole layout for extraction boreholes provided in an embodiment of the present invention; Figure 5 This is a cross-sectional view of the extraction borehole layout provided in an embodiment of the present invention; Figure 6 This is a comparison diagram of extraction effects provided for an embodiment of the present invention. The present invention discloses a progressive gas classification and control method for outburst-prone coal seams based on energy tiered release, which specifically includes the following five steps.

[0040] S1: Overall pressure relief from hydraulic fracturing of the key layer;

[0041] Based on the relevant geological parameters of the target coal seam (5) and its overlying strata (1-4), the main and sub-key strata (1, 3) of the coal seam (5) were identified, and a set of high and low position fracturing boreholes (8, 9) were constructed on both sides and top of the roadway (13). After the matching packer connected the borehole to the preset fracturing point (10), the main and sub-key strata (1, 3) of the overlying strata were fracturing. The diameter of the fracturing borehole was selected as Ф113 mm.

[0042] Among them, the geological conditions of a mine in Shanxi were analyzed based on the "masonry beam" structure theory, numerical simulation and field test; the strata of the main key layer (1) and the sub-key layer (3) were determined, and the fracturing pressure and fracturing range were calculated. The high-level fracturing borehole (9) was advanced 70 m ahead of the working face, penetrated the main and sub-key layers (1, 3), and controlled the range of 30 m on both sides of the transport roadway (13) and 30 m above the immediate roof (4); the low-level fracturing borehole (8) was advanced 70 m ahead of the working face, only penetrated the sub-key layer (3), and controlled the range of 30 m on both sides of the transport roadway (13) and 16 m above the immediate roof (4); the fracturing point interval was set to 3 m, and the main and sub-key layers (1, 3) were continuously pumped with a water injection pressure of 18 MPa for 20 min to carry out segmented synchronous fracturing of the main and sub-key layers (1, 3) in order to cut off the stress transmission path of the overlying strata (1-4) and realize the overall decompression of the coal seam-overlying rock.

[0043] Upon inspection, the pressure decay rate within 40-70 minutes after the fracturing of the key overburden layer was approximately 0.07 MPa / min, and the correlation coefficient of the decay curve was 0.92, which met the start conditions of step S2.

[0044] S2: Low-disturbance borehole pre-pulling energy reduction;

[0045] In the bottom drainage roadway (14), a Ф73 mm conventional compressed air slag discharge borehole (12) with low disturbance effect is constructed towards the target coal seam (5). Taking the basic geological data and gas parameters of a mine in Shanxi as an example, combined with numerical simulation, it is found that setting the spacing S2 of the conventional compressed air slag discharge borehole (12) to 4 m is relatively reasonable. Figure 3 The diamond-shaped arrangement shown forms a continuous and uniform unloading zone in the target coal seam (5).

[0046] After the construction was completed, the screen pipe was lowered and the hole was sealed using the "two plugs and one injection" method. The sealing pressure was 2.0 MPa and the sealing section was 12 m long. Finally, the main pipeline was connected to the main pipeline network and the coal seam (5) was initially gas-drained for 20 days with a negative pressure of 16 kPa. This reduced the gas pressure gradient around the borehole and weakened the outburst potential in the original coal seam (5), creating a safe window for the subsequent enhanced gas extraction process.

[0047] Upon inspection, it was found that as the extraction time increased, the extraction efficiency of the conventional borehole group continued to decrease and the pressure reduction rate continued to shrink. After completing 20 days of preliminary gas extraction, the residual gas pressure in the coal seam between the boreholes was reduced by about 30% compared with the original gas pressure, which met the starting conditions of step S3.

[0048] S3: Hydraulic cavity creation for pressure relief and increased permeability;

[0049] After the initial gas pre-drainage is completed, a borehole with a diameter of Ф73 mm is constructed near the low-disturbance conventional compressed air slag discharge borehole (12) that has been pre-drained. Then, the borehole is enlarged in the coal seam section (5) with a water injection pressure of 15 MPa until a hydraulic cavity-building enhanced extraction borehole (11) with a diameter of 0.5 m is formed. The cavity-building section covers the full thickness of the coal seam. When coal production stops and the water in the borehole becomes clear, the cavity-building is stopped.

[0050] Taking the basic geological data and gas parameters of a mine in Shanxi as an example, and combining numerical simulation, it is concluded that setting the spacing S1 of the hydraulic cavity-forming enhanced extraction borehole (11) to 8 m is relatively reasonable. Figure 3 The diamond-shaped arrangement shown forms a continuous and uniform unloading zone in the target coal seam (5).

[0051] S4: Compressed air sweeping hole for slag discharge;

[0052] After the hydraulic drilling for enhanced extraction (11) is completed and the drilling is withdrawn, compressed air is immediately used to clean the hole, remove the residual coal slurry and water inside the hole and around the hole wall, break the "water lock effect", prevent delayed blowout, and ensure the smooth flow of the drilling channel and the highest efficiency in the early stage of extraction.

[0053] S5: Sealed-hole network enhanced extraction;

[0054] After the compressed air cleaning is completed, a screen pipe is lowered into the hydraulic cavity enhanced extraction borehole (11) and the hole is sealed using the "two plugs and one injection" method. The sealing pressure is 2.0 MPa and the sealing section is 12 m long. Finally, the gas is fed into the main pipeline and the residual gas in the coal seam (5) is subjected to secondary enhanced extraction at a negative pressure of 16 KPa until the gas pressure is less than 0.74 MPa.

[0055] During the construction of the above-mentioned hydraulic cavity-building enhanced extraction borehole (11) and conventional compressed air slag discharge borehole (12), anti-blowout devices are installed at the borehole opening to prevent dynamic disasters caused by sudden energy release. The borehole controls coal seam gas within a 15 m range of the bottom plate (6) on both sides of the outline of the transport roadway (13).

[0056] Comparative simulation results conducted under the same geological conditions show that ( Figure 6 In Example A, only low-disturbance borehole pre-drainage was used, and the gas extraction time to meet the standard was approximately 127 days. In Example B, a dual treatment measure of low-disturbance borehole pre-drainage + hydraulic cavity-forming enhanced extraction was used. After 40 days of low-disturbance borehole pre-drainage, hydraulic cavity-forming enhanced extraction was carried out, and the total time to meet the gas extraction standard was approximately 102 days, which is an improvement compared to Example A. In Example C, which is the progressive gas grading treatment of this invention, after fracturing the key layer, only 20 days of gas pre-drainage was required to reduce the gas pressure to about 70% of the original value. With the synergistic effect of secondary enhanced extraction, the total extraction time to meet the standard was only 75 days, which has significant advantages in terms of extraction effect and time to meet the standard.

[0057] Finally, it should be noted that the above embodiments are only used to illustrate the method of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the method described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding method to deviate from the scope defined by the claims of the present invention.

Claims

1. A progressive gas classification and control method for outburst-prone coal seams based on energy cascade release, characterized in that, The graded treatment method is based on the technical path of "fracturing and unloading - pre-extraction and energy reduction - cavity creation and efficiency enhancement", which synergistically achieves efficient gas control and precise gas outburst elimination in a temporal and spatial sequence, including the following five steps: S1: Overall pressure relief from hydraulic fracturing of the key layer; Based on the relevant geological parameters of the target coal seam and its overlying strata, the main and sub-key strata of the coal seam are identified, and a set of high and low-level fracturing boreholes are constructed on both sides and top of the roadway. After the matching packer is connected to the borehole and transported to the preset fracturing point, the main and sub-key strata of the overlying strata are simultaneously fracturing in segments, with a fracturing point interval of 3 m. S2: Low-disturbance borehole pre-extraction energy reduction; When step S1 is completed, if the decay rate of the fracturing pressure is not greater than 0.1 MPa / min and the correlation coefficient of the decay curve is not less than 0.9 within 30 minutes, a low-disturbance cross-seam borehole is constructed in the bottom drainage roadway towards the target coal seam. After the construction is completed, a screen pipe is installed and connected to the main pipeline network. Based on the dynamic change trend of the coal seam gas pressure, preliminary gas drainage is carried out on the coal seam for 20-50 days to reduce the gas pressure gradient around the borehole and weaken the outburst potential in the original coal seam, creating a safe window for subsequent enhanced drainage technology. S3: Hydraulic cavity creation for pressure relief and increased permeability; In the initial gas pre-drainage process of step S2, when the residual gas pressure of the coal seam between boreholes decreases by no less than 25% compared with the original gas pressure, the starting condition of step S3 is met. Hydraulic cavity drilling is constructed near the low-disturbance conventional borehole where gas pre-drainage is carried out to enhance gas extraction. The cavity section covers the full thickness of the coal seam. Cavitation is stopped when coal production stops and the water from the borehole becomes clear. S4: Compressed air sweeping hole for slag discharge; After the hydraulic cavity depressurization drilling is completed and the drilling is withdrawn, compressed air cleaning operation is immediately carried out to remove residual coal slurry and water in the hole and around the hole wall, break the "water lock effect" and prevent delayed blowout. S5: Sealed network enhanced extraction; After the compressed air cleaning is completed, a screen pipe is lowered into the hydraulic cavity drilling hole and connected to the main pipeline network to carry out secondary enhanced extraction of residual gas in the coal seam until the gas pressure is less than 0.74 MPa.

2. The method for graded gas control in outburst-prone coal seams based on energy cascade release as described in claim 1, characterized in that, The key layer position in step S1 is determined based on the structural theory of "masonry beam" and numerical simulation, and the initiation pressure and the range of influence of fracturing are determined by combining theory and field tests.

3. The progressive gas classification and control method for outburst-prone coal seams based on energy cascade release as described in claim 1, characterized in that, The selected range for the diameter of the fracturing borehole in step S1 is Ф90-120 mm; the high-level fracturing borehole advances 70 m ahead of the working face and penetrates the main and subcritical layers; the low-level fracturing borehole advances 70 m ahead of the working face and only penetrates the subcritical layer.

4. The method for graded gas control in outburst-prone coal seams based on energy cascade release as described in claim 1, characterized in that, The through-layer drilling in step S2 is a conventional compressed air slag discharge drilling hole with a diameter of Ф70-100 mm, and is evenly arranged at a spacing not exceeding twice the effective extraction radius.

5. The method for graded gas control in outburst-prone coal seams based on energy cascade release as described in claim 1, characterized in that, The hydraulic cavity-creating boreholes in step S3 are obtained by creating and enlarging conventional boreholes of Ф70-100 mm in the coal seam section, with a water injection pressure of 15-20 MPa. The cavity-creating boreholes are arranged near the cross-layer boreholes in step S2 and connected to the main pipeline network, with the spacing between the boreholes not exceeding twice the effective extraction radius.

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