Gas hydration solidification outburst prevention method based on soft and hard composite coal seam

Abstract

The invention discloses a gas hydration solidification outburst prevention method based on a soft and hard composite coal seam, and belongs to the technical field of coal and gas outburst disaster prevention and control. Comprising the following steps: conventionally sampling a to-be-mined coal seam, and measuring coal seam characteristics; the method comprises the following steps: dividing a coal seam into a soft coal seam and a hard coal seam, carrying out detailed block division on each soft coal seam, and formulating a block-by-block hydrate solidification scheme; the temperature, the pressure and the hydrate generation state of the injection area are tracked in real time through monitoring equipment, the reaction condition is adjusted to ensure that the hydrate is solidified layer by layer and block by block, after solidification in each area is completed, the solidification radius is determined, and the solidification effect of the coal seam is evaluated. According to the method, block-by-block curing is carried out in the soft coal seam to form the curing area, the design of the coal seam gas hydration curing radius is combined, the overall strength of the coal seam is remarkably improved, and the risk of coal seam gas outburst is reduced.

Description

Technical Field

[0001] The present invention relates to a gas hydration solidification method for preventing coal and gas outbursts, in particular to a gas hydration solidification method for preventing coal and gas outbursts based on a soft-hard composite coal seam, and belongs to the technical field of preventing and controlling coal and gas outburst disasters. Background Art

[0002] Against the backdrop of the current energy transition and growing environmental protection needs, coal, as a vital energy resource, faces significant challenges in improving safety and environmental performance. In particular, gas outbursts in coal mining operations not only severely impact miner safety but also negatively impact coal mining efficiency. Therefore, finding effective technical means to reduce outburst risks and enhance coal strength has become a pressing issue for the coal mining industry.

[0003] In recent years, researchers have begun to focus on the potential of gas hydrates in coal reinforcement. Gas hydrate is a solid substance formed by water and gas at specific temperature and pressure. Its properties make it an ideal reinforcement material. By introducing hydrates into coal seams, micro-cracks in the coal body can be effectively filled, improving its mechanical strength and thus reducing the risk of gas outbursts. In addition, the design method of soft and hard composite coal seams has gradually gained attention. This method makes the entire coal seam structure more stable by alternating soft coal and hard coal seams in the coal seam and taking advantage of the difference in physical properties between the two. This layered design not only helps to enhance the compressive capacity of the coal body, but also improves the safety of the coal seam under dynamic loads.

[0004] At present, in the existing technology, although there are some methods that attempt to enhance the strength of coal bodies through physical or chemical means, most of the methods have problems such as high cost, difficulty in implementation, and insignificant effect. For example, the application announcement number CN211292236U is a coal body preparation device containing gas hydrates and a mechanical property testing system. It is a laboratory small coal sample hydrate generation device, which is a laboratory testing method and cannot be applied to coal seam gas hydration and solidification. For another example, the authorization announcement number CN112031858B is a synthesis device for forming gas hydrates in coal seams. Although it mentions coal seam gas hydration and solidification, it is still lacking in the hydration and solidification process parameters, and the reliability and accuracy of coal seam hydration and solidification cannot be determined; in addition, cooling liquid circulation is used during the drilling process, and the temperature of frictional heat generated during the drilling process cannot be ignored, which brings great obstacles to cooling. Therefore, it is worth considering whether the integrated hydrate device is effective and convenient. It is an innovation in the coalbed gas hydration solidification device. The process does not consider the investigation of the solidification effect, which is a major limitation in the solidification of soft and hard composite coal seams. The soft layer itself is more dangerous and especially requires scientific solidification technology guidance.

[0005] Therefore, combining the technical ideas of gas hydrate formation and soft-hard composite coal seam design will provide a new solution to reduce the risk of coal mine outbursts. Summary of the Invention

[0006] The purpose of the present invention is to solve the problems of high cost, great difficulty in implementation and unclear effect in the existing gas hydration and solidification for preventing gas outbursts, and to provide a gas hydration and solidification method for preventing gas outbursts based on soft and hard composite coal seams. By solidifying block by block in the soft coal seam to form a solidification area, and combining the design of the coal seam gas hydration and solidification radius, the overall strength of the coal seam is significantly improved, the risk of coal seam gas outburst is reduced, and a basis for reliability and accuracy is provided for the subsequent construction of coal seam hydration and solidification.

[0007] To achieve the above object, the present invention adopts the following technical solution: a gas hydration solidification method for preventing coal outbursts based on a soft-hard composite coal seam, comprising the following steps: Step S1: routine sampling of the coal seam to be mined is performed using geological exploration techniques to determine the coal seam characteristics: Sampling and measurement include measuring the geological structure and lithologic composition of coal seams, measuring the gas content, gas pressure and gas permeability of coal seams, and detecting natural fractures, porosity and their distribution in coal seams. The measurement results can be used to determine the thickness distribution of coal seams and the relationship between the layers and interlayers of coal seams. Step S2: Divide the coal seams into soft coal seams and hard coal seams according to their mechanical properties, and divide each soft coal seam into detailed blocks, and formulate a hydrate solidification plan for each block: S21. Based on the differences in compressive strength and shear strength between soft and hard coal seams, layers with lower strength, greater deformation, and higher gas content are classified as soft coal seams, while layers with higher hardness and greater support are classified as hard coal seams. The locations and thicknesses of the soft and hard coal seams are determined based on the measurement results. S22. Areas with obvious cracks, cavities, or faults within the soft coal seam are divided into key solidification areas, and divided into high-gas and high-pressure blocks, crack-developed blocks, and low-gas and low-pressure blocks based on gas content and gas pressure; S23. Develop a hydrate solidification plan for soft coal seam blocks based on the thickness and layer uniformity of the soft coal seams. In high-gas, high-pressure blocks, water is injected to generate hydrates. Hydrate injection is carried out in blocks with developed fractures in stages and layers, with the injection process gradually transitioning from shallow to deep layers. In low-gas, low-pressure blocks, the gas injection rate is increased to regulate hydrate formation. Step S3: Setting gas hydrate formation conditions in the soft coal seam, and injecting water and gas into the pores of the coal seam through directional drilling to react and produce gas hydrate: Construct short-distance parallel boreholes toward the interior of the coal seam, and construct gas hydrate injection holes vertically connected to the parallel boreholes in each block using hydraulic slitting and hydraulic coal excavation methods. Lateral embedded casings are installed in the parallel boreholes, embedded capillaries are installed in the vertical hydrate injection holes, and coolant injection pipes and water injection pipes are installed in the lateral embedded casings, with the embedded capillaries extending into the interior of the soft coal seam. The initial critical temperature for the formation of gas hydrates is 273.15K, and the pressure is 2.517MPa. A cooling device or low-temperature gas injection is used to control the temperature between 0°C and 10°C, with a fluctuation of no more than ±1°C. A booster pump is used to gradually increase the coal seam pressure to maintain the pressure between 3MPa and 8MPa, with a fluctuation of ±0.1MPa allowed. The injection rate is controlled by a constant-flow water injection pump. The water injection rate in high-gas content areas is controlled at an injection depth of 0.1-0.2L / m per hour, in medium-gas content areas at 0.05-0.1L / m per hour, and in low-gas content areas at 0.03-0.05L / m per hour. In each block, the water injection rate is maintained at a ratio of 1:5-1:9 to the gas flow rate. Step S4: Using monitoring equipment to track the temperature, pressure, and hydrate formation status of the injection area in real time, and adjusting the reaction conditions to ensure that the hydrate solidifies layer by layer and block by block: Control the injection parameters of gas and water according to the set conditions. Reduce the injection rate in low-strength coal seams with high porosity and dense cracks. Increase the pressure and extend the injection time in coal seams with low porosity, high strength and few cracks. In coal seams with medium porosity, medium strength and moderate cracks, follow the standard values ​​for gas and water injection parameters to ensure uniform solidification effect. Step S5: After solidification is completed in each area, the solidification radius is determined, and the solidification effect of the coal seam is evaluated to determine whether further solidification is required: After the gas hydrate solidification is implemented, the effective solidification range is the coal seam gas hydration solidification radius 𝑅 with the injection borehole as the center. When the solidification radius is within 1~2m, it is judged that the solidification meets the standard, and the solidification area with a solidification radius less than 1m is supplemented with solidification; The relationship between the coal seam gas hydration solidification radius 𝑅 is: , Where 𝑘 is the proportional constant, 𝑃 𝑖 is the injection pressure, 𝑄 𝑔 is the gas flow rate, 𝑄 𝑤 is the water injection rate, 𝛼 is an index related to coal seam properties, temperature and hydrate formation conditions, 𝛽 is an index related to gas diffusion capacity and hydrate formation rate, and 𝛾 is an index related to hydrate formation efficiency.

[0008] In step S1, measuring the geological structure of the coal seam includes measuring the inclination and strike of the coal seam, and evaluating the mechanical parameters of the compressive strength and shear strength of the soft and hard coal seams; measuring the lithologic composition of the coal seam includes measuring the proportion of ash and minerals in the coal seam; and natural cracks, porosity and their distribution in the coal seam are detected by acoustic imaging, CT scanning or drilling.

[0009] In step S5, the method for determining the solidification radius includes: taking cores at different depths and orientations around the solidification borehole, analyzing the presence of hydrates in the core samples, and determining the solidification radius in combination with physical property tests; simultaneously, monitoring the distribution of hydrates by changes in the propagation velocity of acoustic waves, and using acoustic logging technology to obtain the solidification state of the coal seam to infer the solidification radius; and further, using an infrared thermal imager to monitor temperature changes during the solidification process, and combining the temperature distribution diagram to evaluate the solidification radius range.

[0010] In step S5, different curing cycles are set according to the size of the curing radius: 1) the injection point spacing is increased for areas with a larger curing radius, while the injection points are densely arranged in areas with a smaller curing radius; 2) the injection time is shortened for coal seam areas with a larger curing radius, while the injection time is extended for areas with a smaller curing radius.

[0011] In step S5, the diffusion capacity and pore structure of the coal seam are matched by controlling the injection rate and pressure: the injection rate is reduced in the fracture development area, and the injection pressure is increased in the area with low porosity; in coal seams with different pore structures, the hydrate formation efficiency is enhanced by controlling the ratio of gas and water: the gas ratio is reduced in the coal seam area with high porosity, and the gas ratio is increased in the area with low porosity.

[0012] The beneficial effects of the present invention are: 1) In the method of the present invention, solidification is carried out block by block in the soft coal seam to form a solidified area, and gas hydrates are generated by gas-containing coal under certain temperature, moisture and gas pressure to enhance the strength of the coal body. In combination with the block-by-block solidification process of the soft layers of the soft and hard composite coal seams, hydration and solidification of the soft layers of the composite coal seam are achieved. The soft coal seam is regarded as a weak layer prone to outburst, and the hydrate solidification process is implemented as the focus, while the hard coal seam plays the role of structural support and stress dispersion, ensuring the stability of the entire coal body and significantly improving the overall strength of the coal seam, thereby reducing the risk of outburst.

[0013] 2) The method of the present invention adopts a segmented solidification strategy to gradually treat each soft coal layer in the coal seam, and proposes a design for the coal seam gas hydration solidification radius. Starting from the solidification drill hole and centered on the solidification drill hole, a circular area radius that can achieve the ideal solidification effect is formed. The process parameters of the solidified coal seam are obtained through small segment testing. This parameter is crucial for evaluating the effectiveness of the solidification process and the overall stability of the coal seam, and can provide a reliable and accurate basis for the subsequent construction of coal seam hydration solidification.

[0014] 3) The method of the present invention utilizes gas hydrate formation technology. By controlling the temperature, pressure and moisture conditions in the coal seam, the gas in the coal seam is combined with water to form gas hydrate. This hydrate can fill the cracks and pores in the coal seam, forming a solid structure and significantly enhancing the mechanical strength of the coal body. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 The coal seam distribution profile obtained by the method of the present invention is: Figure 2 A structural diagram of a nested piping arrangement for block-by-block curing in the method of the present invention; Figure 3 The structure of the gas hydrate solidification radius in the method of the present invention and a partially enlarged structural diagram; Figure 4 for Figure 2 Pipe connection structure diagram of the nested piping device.

[0016] In the figure, 1-coal seam roof, 2-coal seam floor, 3-upper hard coal seam, 4-soft coal seam, 5-lower hard coal seam, 6-gas hydrate solidification area, 7-gas hydrate molecules, 8-gas hydrate injection hole, 9-coolant injection pipe, 10-water injection pipe, 11-coal seam parallel drilling hole, 12-solidification effect detection hole, 13-embedded capillary tube, 14-side embedded casing. DETAILED DESCRIPTION

[0017] The present invention will be further explained below with reference to the accompanying drawings and specific embodiments.

[0018] Example: Figures 1 to 4 As shown, the present invention provides a method for preventing gas outbursts based on soft and hard composite coal seams by hydration and solidification, comprising the following steps: Step S1: routine sampling of the coal seam to be mined is performed using geological exploration techniques to determine the coal seam characteristics: Sampling and measurement include measuring the geological structure and lithologic composition of coal seams, measuring the gas content, gas pressure and gas permeability of coal seams, and detecting natural cracks, porosity and their distribution in coal seams; determining the thickness distribution of coal seams and the stratigraphic relationship of coal seams in the geological structure through measurement results, and obtaining the multi-layered and inter-layered composition relationship of coal seams.

[0019] Measuring the geological structure of coal seams involves measuring their dip and strike, determining their overall extension and variations, and providing a key basis for solidification design. Mechanical parameters such as the compressive and shear strengths of soft and hard coal seams are assessed to determine the solidification requirements of different layers. Natural fractures, porosity, and their distribution within the coal mass are detected through acoustic imaging, CT scanning, or drilling, providing a basis for subsequent hydrate filling.

[0020] Measure the gas content of coal seams and analyze the differences in gas content between soft and hard coal seams to determine whether the gas conditions required for gas hydrate formation are met. Measure the gas pressure of coal seams to determine the formation and stability of gas hydrates within the coal mass. Differences in gas pressure between different regions also affect the control parameters during gas hydrate formation. Evaluate the gas permeability of coal seams to understand the diffusion rate of gas within the coal mass and determine the location and intensity of water and gas injection. Understand the lithologic composition of the coal mass and analyze the ash and mineral content in the coal to ensure that the coal seam is suitable for hydrate formation. Different mineral compositions may affect the rate and stability of hydrate formation.

[0021] Step S2: Divide the coal seams into soft coal seams and hard coal seams according to their mechanical properties, and divide each soft coal seam into detailed blocks, and formulate a hydrate solidification plan for each block: S21. Based on the differences in mechanical properties of compressive strength and shear strength between soft coal seams and hard coal seams, the mechanical strength can be obtained through laboratory testing or on-site non-destructive testing. Layers with lower strength, easy deformation, and higher gas content are classified as soft coal seams, while layers with higher hardness and strong support are classified as hard coal seams. The location and thickness of soft coal seams and hard coal seams are determined based on the measurement results, such as Figure 1 Coal seam structure diagram shown.

[0022] S22. Areas with obvious cracks, cavities or faults inside the soft coal seam are divided into key solidification areas, and divided into high-gas and high-pressure blocks, crack-developed blocks, and low-gas and low-pressure blocks according to gas content and gas pressure. Generally, areas with higher gas content and higher pressure are more likely to form gas hydrates and are therefore more suitable for solidification treatment. The gas pressure and content in these areas can be determined through gas extraction tests or drilling inspections.

[0023] S23. Hydrate solidification plans are formulated for blocks in soft coal seams based on the thickness and uniformity of the soft coal seams. In high-gas, high-pressure blocks, where both gas content and pressure are higher, hydrate formation is more likely. The solidification plan involves injecting water to generate hydrates. In such blocks, temperature and pressure must be strictly monitored during solidification to ensure the stability of the generated hydrates and prevent gas leakage. Hydrate injection is carried out in blocks with developed fractures in stages and layers to ensure uniform hydrate formation in the fractures. The injection process is gradually transitioned from shallow to deep layers to seal the fractures step by step. In low-gas, low-pressure blocks, where hydrate formation is difficult, the solidification plan involves increasing the gas injection rate to adjust hydrate formation conditions. When necessary, cooling measures are used to lower the coal temperature to promote hydrate formation and stability.

[0024] Step S3: Setting gas hydrate formation conditions in the soft coal seam, and injecting water and gas into the pores of the coal seam through directional drilling to react and produce gas hydrate: like Figure 2 and Figure 4 As shown, after parameter testing, short parallel boreholes 11 are drilled into the coal seam. Depending on the specific situation, one or two parallel boreholes are sufficient to achieve the desired goal. Once these parallel boreholes reach their designated locations, hydraulic slitting and hydraulic coal excavation methods are used to construct gas hydrate injection holes 8 in each block, vertically connected to the parallel boreholes, to ensure the efficiency and accuracy of the injection process. During this stage, a side embedded casing 14 of appropriate length is inserted. This casing contains a coolant injection pipe 9 and a water injection pipe 10 for subsequent liquid injection. An embedded capillary tube 13 is placed in the vertical hydrate injection hole. Once the embedded capillary tube 13 reaches the vertical hydrate injection hole, it is controlled by an electrical signal to extend, guiding the coolant injection pipe 9 and the water injection pipe 10 into the soft layer. The main purpose of injecting coolant is to reduce the ambient temperature, thereby achieving the low-temperature conditions required for hydrate formation. Simultaneously, injecting water is a key step in ensuring the smooth progress of the hydrate formation process.

[0025] The formation of gas hydrates requires certain temperature and pressure conditions. The initial critical temperature for gas hydrate formation is 273.15K and the pressure is 2.517 MPa. As the temperature decreases below zero, gas hydrate formation becomes more stable. As the temperature increases above zero, there is a critical range for both temperature and pressure. For example, at 2.803 MPa, gas hydrate formation can occur at 274.15K, but when the temperature rises to 275.15K, the pressure is no longer sufficient for gas hydrate formation. The critical conditions for hydrate formation above zero temperature are as follows: 273.15K, 2.517Mpa; 274.15K, 2.803Mpa; 275.15K, 3.111Mpa; 276.15K, 3.453Mpa; 277.15K, 3.833Mpa. During the gas hydrate solidification process, in order to ensure that the hydrate formation in each block is uniform and stable and solidified layer by layer, key parameters such as water injection volume, gas flow rate, and generation temperature and pressure must be strictly controlled.

[0026] Using cooling devices or low-temperature gas injection, the temperature is controlled between 0°C and 10°C, with fluctuations of no more than ±1°C. Booster pumps are used to gradually increase the coal seam pressure, maintaining it between 3 MPa and 8 MPa, with a tolerance of ±0.1 MPa fluctuation. A constant-flow water injection pump controls the water injection rate, adjusting the actual injection volume based on the water content of the coal seam. To avoid oversaturation, the injection volume is designed based on the needs of each layer. The injection rate in high-gas-content areas is controlled at 0.1 to 0.2 L / m per hour per depth, in medium-gas-content areas at 0.05 to 0.1 L / m per hour, and in low-gas-content areas at 0.03 to 0.05 L / m per hour. Within each block, the water injection rate is maintained at a ratio of 1:5 to 1:9 to the gas flow rate, optimizing hydrate formation efficiency and ensuring uniform solidification.

[0027] Step S4: Using monitoring equipment to track the temperature, pressure, and hydrate formation status of the injection area in real time, and adjusting the reaction conditions to ensure that the hydrate solidifies layer by layer and block by block: Control the injection parameters of gas and water according to the set conditions, reduce the injection rate in low-strength coal seams with high porosity and dense cracks to prevent rapid or uneven solidification; increase the pressure and extend the injection time in coal seams with low porosity, high strength and fewer cracks to increase the diffusion depth and solidification effect; in coal seams with medium porosity, medium strength and moderate cracks, the injection parameters of gas and water are implemented according to standard values ​​to ensure uniform solidification effect.

[0028] Step S5: After solidification is completed in each area, the solidification radius is determined, and the solidification effect of the coal seam is evaluated to determine whether further solidification is required: After the implementation of gas hydrate solidification, the range of effective solidification with the injection borehole as the center is the coal seam gas hydration solidification radius 𝑅. When the solidification radius is within 1~2m, it is judged that the solidification meets the standard, and the solidification area with a solidification radius less than 1m is supplemented with solidification.

[0029] The method for determining the solidification radius includes: taking cores at different depths and orientations around the solidification borehole, analyzing the presence of hydrates in the core samples, and determining the solidification radius in combination with physical property tests; at the same time, monitoring the distribution of hydrates through changes in the propagation velocity of acoustic waves, and using acoustic logging technology to obtain the solidification state of the coal seam to infer the solidification radius; in addition, using an infrared thermal imager to monitor temperature changes during the solidification process, and combining the temperature distribution map to evaluate the solidification radius range.

[0030] The relationship between the coal seam gas hydration solidification radius 𝑅 is: , Where 𝑘 is a proportional constant that depends on the physical properties of the coal seam and the experimental conditions. The parameter 𝑘 is a proportional constant that is usually determined by experimental data. Under known specific coal seam conditions, experiments can be conducted to measure the solidification radius 𝑅 under different injection pressures, gas flow rates, and water injection rates, and then the value of 𝑘 can be determined through regression analysis. Under the same coal seam conditions, changing the injection pressure 𝑃 𝑖 , gas flow 𝑄 𝑔 and water injection volume 𝑄 𝑤 , record the curing radius 𝑅 under different conditions, and use linear regression or nonlinear regression method to substitute the collected data into the above comprehensive model to solve the value of 𝑘.

[0031] 𝑃 𝑖 is the injection pressure, 𝑄 𝑔 is the gas flow rate, 𝑄 𝑤 is the water injection rate, 𝛼 is an index related to coal seam properties, temperature and hydrate formation conditions, 𝛽 is an index related to gas diffusion capacity and hydrate formation rate, and 𝛾 is an index related to hydrate formation efficiency. 𝛼, 𝛽 and 𝛾 reflect the sensitivity of the solidification radius to injection pressure, gas flow rate and water injection rate. Keeping other variables unchanged, gradually changing one variable (such as 𝑃 𝑖 ) and measure the curing radius 𝑅, repeat this process, and test each variable separately; substitute the experimental data into the model, and according to the changing trend of the data, use statistical analysis methods such as the least squares method to fit the corresponding exponents 𝛼, 𝛽 and 𝛾.

[0032] Different curing cycles are set according to the size of the curing radius: 1) The spacing between injection points is increased in areas with a larger curing radius, while the injection points are densely arranged in areas with a smaller curing radius; 2) The injection time is shortened in coal seam areas with a larger curing radius to avoid over-curing, while the injection time is extended in areas with a smaller curing radius to ensure uniform curing.

[0033] By controlling the injection rate and pressure to match the diffusion capacity and pore structure of the coal seam: reducing the injection rate in the fracture-developed area and increasing the injection pressure in the area with low porosity; in coal seams with different pore structures, the hydrate formation efficiency is enhanced by controlling the ratio of gas and water: reducing the gas ratio in the coal seam area with high porosity and increasing the gas ratio in the area with low porosity.

[0034] like Figure 4 As shown, a main borehole is drilled in the coal seam, and side boreholes are formed using hydraulic slotting and hydraulic excavation. This, combined with a nested telescopic tube system, effectively controls gas hydrates, promoting solidification and soft stratification. The nested telescopic tube system consists of multiple retractable, embedded thin tube segments (13). These segments nest together when not in use to save space. Each segment can independently expand and contract. Each segment contains two flexible hoses, one for delivering coolant and the other for water. The hoses are highly flexible and can withstand the stretching and bending within the side borehole. The inner tube delivers coolant, controlling the temperature within the side borehole by regulating its flow rate and temperature, promoting solidification of gas hydrates. The outer tube provides a metered water supply to maintain appropriate humidity and provide the moisture required for hydrate formation. Temperature and water volume control: During coolant delivery, the inner tube begins delivering coolant, controlling the temperature within the side borehole and promoting solidification of gas hydrates. During water supply, the outer tube delivers a preset amount of water to maintain appropriate humidity and promote soft stratification. Real-time monitoring: The system monitors the temperature and humidity in the side holes through sensors to ensure that the requirements of curing and soft stratification are met. Feedback adjustment: Based on the monitoring data, the control system can automatically adjust the flow of coolant and water to ensure the desired effect.

[0035] The data recorded above regarding the hydrate solidification time and the gas hydrate solidification area (Area 6) will provide important guidance for subsequent construction. During this process, based on the previously determined hydrate solidification radius, a pipeline with an embedded capillary tube 13 and a side-embedded sleeve 14 is designed, leaving a space every two radii. This design ensures efficient injection of the required coolant and water during the solidification process. Next, based on the total length of the borehole, the required number of hydrate solidification boreholes can be calculated and the corresponding composite pipelines can be fabricated. These pipelines will include a certain number of side-embedded sleeves 14 with embedded capillary tubes 13. After completing the long borehole construction, when the borehole reaches the predetermined position, hydraulic slitting and hydraulic coal cutting are performed to precisely carve out the hydrate injection hole. After this, the drill is withdrawn to allow installation of the fabricated composite pipeline. Upon reaching each designated position, an electrical signal is used to control the expansion of the embedded capillary tube 13 on the side-embedded sleeve 14, thereby driving the coolant injection pipe 9 and the water injection pipe 10 into the constructed hydrate injection hole. This process will sequentially transport multiple embedded capillaries 13, coolant injection pipes 9, and water injection pipes 10 to ensure effective solidification of the hydrate. To ensure the reliability of the solidification effect, a solidification test is performed through the solidification effect detection hole 12 every week.

[0036] Throughout the injection process, the curing effect detection hole 12 is used to test the effect every week to monitor changes in the curing area. By testing the curing effect, problems can be discovered and adjusted in a timely manner to ensure the smooth progress of the project. When the range changes slightly, it is necessary to record the time required at this time and the gas hydrate curing area 6 in detail. Recording this data not only helps to evaluate the current treatment effect, but also provides a reference for subsequent injections. By systematically analyzing this data, we can gain a deeper understanding of the impact of different parameters on the curing effect, thereby providing a scientific basis and guidance for subsequent engineering decisions.

[0037] Solidification Effect Testing: The propagation velocity and reflection intensity of acoustic waves in a coal seam can reflect the density and structural changes of the coal seam. After solidification, the density and acoustic wave propagation velocity should increase due to the incorporation of hydrates. Acoustic detection measures the propagation velocity and reflection characteristics by transmitting and receiving acoustic waves to determine the effectiveness of solidification. Acoustic wave transmitters and receivers are placed in the solidified area, with the frequency set between 20 and 40 kHz. Layer by layer, along the borehole, the acoustic wave propagation velocity, attenuation rate, and reflection intensity are recorded. The acoustic wave propagation velocity in the solidified area should be 1.5 to 2 times that of the unsolidified area. In areas with high gas pressure and high hydrate content, the velocity should exceed 2500 m / s. The acoustic wave attenuation rate in the solidified area should be less than 15%. Excessive attenuation indicates incomplete hydrate solidification or the presence of cracks. In the acoustic detection image, the reflection intensity of the solidified area should be at least 30% higher than that of the surrounding area, indicating uniform hydrate filling and a dense structure.

[0038] After the soft coal seams are solidified block by block, the hard coal seams continue to provide support. The solidified soft-hard composite coal seams not only possess strong compressive strength but also effectively distribute loads under sudden stresses, preventing localized collapse or outbursts. This block-by-block solidification process ensures that each layer of soft coal is fully reinforced by the hydrates, improving the overall stability of the coal seams while effectively reducing the potential risk of gas outbursts.

[0039] The method of the present invention forms a solidified area by performing block-by-block solidification in the soft coal seam, utilizes gas-containing coal to generate gas hydrate under a certain temperature, moisture and gas pressure to enhance the strength of the coal body, and combines the block-by-block solidification process of the soft layers of the soft and hard composite coal seams to achieve layer-by-layer hydration and solidification of the soft layers of the composite coal seam; the soft coal seam is regarded as a weak layer prone to outburst, and the hydrate solidification process is implemented emphatically, while the hard coal seam plays the role of structural support and stress dispersion, ensuring the stability of the entire coal body and significantly improving the overall strength of the coal seam, thereby reducing the risk of outburst.

[0040] The method of the present invention adopts a segmented solidification strategy to gradually process each soft coal layer in the coal seam, and proposes a design of the coal seam gas hydration solidification radius. Starting from the solidification drill hole and centered on the solidification drill hole, a circular area radius that can achieve an ideal solidification effect is formed, and the process parameters of the solidified coal seam are obtained through small section testing. This parameter is crucial for evaluating the effectiveness of the solidification process and the overall stability of the coal seam, and can provide a reliable and accurate basis for the subsequent construction of coal seam hydration solidification.

[0041] The above description is only used to illustrate the technical solution of the present invention and is not intended to limit the present invention. Other modifications or equivalent substitutions made to the technical solution of the present invention by ordinary technicians in this field should be included in the scope of the claims of the present invention as long as they do not depart from the spirit and scope of the technical solution of the present invention.

Claims

1. A method for preventing gas outbursts by hydration and solidification based on soft and hard composite coal seams, characterized in that: The following steps are involved: Step S1: routinely sample the coal seam to be mined using geological exploration techniques to determine the coal seam characteristics: Sampling and measurement include measuring the geological structure and lithology of coal seams, measuring the gas content, gas pressure and gas permeability of coal seams, and detecting the natural cracks, porosity and distribution in coal seams; determining the thickness distribution of coal seams and the stratigraphic relationship of coal seams in the geological structure through the measurement results, and obtaining the multi-layer and inter-layer composition relationship of coal seams; Step S2: Divide the coal seams into soft coal seams and hard coal seams according to their mechanical properties, divide each soft coal seam into detailed blocks, and formulate a block-by-block hydrate solidification plan: S21. Based on the differences in compressive strength and shear strength between soft coal seams and hard coal seams, the seams with lower strength, easy deformation and higher gas content are classified as soft coal seams, while the seams with higher hardness and strong supporting force are classified as hard coal seams, and the locations and thicknesses of the soft coal seams and hard coal seams are obtained based on the measurement results; S22. The areas with obvious cracks, cavities or faults inside the soft coal seam are divided into key solidification areas, and divided into high-gas and high-pressure blocks, crack-developed blocks, and low-gas and low-pressure blocks according to gas content and gas pressure; S23. Formulate a hydrate solidification plan for the soft coal seam blocks based on the thickness and layer uniformity of the soft coal seam. In the high-gas and high-pressure blocks, water is injected to generate hydrates. In the blocks with developed fractures, hydrate injection is carried out in stages and layers, and the injection process is gradually transitioned from shallow to deep layers. In the low-gas and low-pressure blocks, the gas injection volume is increased to regulate hydrate generation. Step S3: Setting gas hydrate formation conditions in the soft coal seam, and injecting water and gas into the pores of the coal seam through directional drilling to react and produce gas hydrate: A short-distance coal seam parallel borehole (11) is constructed toward the interior of the coal seam, and a gas hydrate injection hole (8) vertically connected to the parallel borehole is constructed in each block by hydraulic cutting and hydraulic coal excavation methods, and a side embedded casing (14) is arranged in the coal seam parallel borehole (11), an embedded capillary (13) is arranged in the vertical hydrate injection hole, a coolant injection pipe (9) and a water injection pipe (10) are arranged in the side embedded casing (14), and the embedded capillary (13) is extended into the interior of the soft coal seam; The initial critical temperature for the formation of gas hydrate is 273.15K, and the pressure is 2.517Mpa. A cooling device or low-temperature gas injection is used to control the temperature at 0℃~10℃, with a fluctuation of no more than ±1℃. A booster pump is used to gradually increase the coal seam pressure to maintain the pressure at 3Mpa~8MPa, with a fluctuation of ±0.1MPa allowed. The injection rate is controlled by a constant flow water injection pump. The injection volume in the high gas content area is controlled at an injection depth of 0.1~0.2L / m per hour, the injection volume in the medium gas content area is controlled at 0.05~0.1L / m per hour, and the injection volume in the low gas content area is controlled at 0.03~0.05L / m per hour. In each block, the ratio of water injection volume to gas flow rate is maintained at 1:5~1:

9. Step S4: Use monitoring equipment to track the temperature, pressure and hydrate formation status of the injection area in real time, and adjust the reaction conditions to ensure that the hydrate solidifies layer by layer and block by block: Control the injection parameters of gas and water according to the set conditions, reduce the injection rate in low-strength coal seams with high porosity and dense cracks, increase the pressure and extend the injection time in coal seams with low porosity, high strength and few cracks, and implement the injection parameters of gas and water according to standard values ​​in coal seams with medium porosity, medium strength and moderate cracks to ensure uniform solidification effect; Step S5: After solidification of each area is completed, the solidification radius is determined, and the solidification effect of the coal seam is evaluated to determine whether further solidification is needed: After the gas hydrate solidification is implemented, the effective solidification range is the coal seam gas hydration solidification radius 𝑅 with the injection borehole as the center. When the solidification radius is within 1~2m, it is judged that the solidification meets the standard, and the solidification area with a solidification radius less than 1m is supplemented with solidification; The relationship between the hydration and solidification radius 𝑅 of coal seam gas is: ， Where 𝑘 is the proportional constant, 𝑃 𝑖 is the injection pressure, 𝑄 𝑔 is the gas flow rate, 𝑄 𝑤 is the water injection volume, 𝛼 is an index related to coal seam properties, temperature and hydrate formation conditions, 𝛽 is an index related to gas diffusion capacity and hydrate formation rate, and 𝛾 is an index related to hydrate formation efficiency.

2. A method for preventing gas outbursts by hydration and solidification based on a soft-hard composite coal seam according to claim 1, characterized in that: In step S1, measuring the geological structure of the coal seam includes measuring the inclination and strike of the coal seam, and evaluating the mechanical parameters of the compressive strength and shear strength of the soft and hard coal seams; measuring the lithological composition of the coal seam includes measuring the proportion of ash and minerals in the coal seam; the natural cracks, porosity and distribution in the coal seam are detected by acoustic imaging, CT scanning or drilling.

3. The method for preventing gas outbursts by hydration and solidification based on soft and hard composite coal seams according to claim 1 is characterized in that: In step S5, the method for determining the solidification radius includes: coring at different depths and orientations around the solidification borehole, analyzing the presence of hydrates in the core samples, and determining the solidification radius in combination with physical property tests; at the same time, monitoring the distribution of hydrates through changes in the propagation velocity of sound waves, and using sonic logging technology to obtain the solidification state of the coal seam to infer the solidification radius; in addition, using an infrared thermal imager to monitor temperature changes during the solidification process, and combining with the temperature distribution diagram to evaluate the solidification radius range.

4. The method for preventing gas outbursts by hydration and solidification based on soft-hard composite coal seams according to claim 1 is characterized in that: In step S5, different curing cycles are set according to the curing radius: 1) the distance between injection points is increased in the area with a larger curing radius, and the injection points are densely arranged in the area with a smaller curing radius; 2) For coal seam areas with larger solidification radius, the injection time is shortened, while for areas with smaller solidification radius, the injection time is extended.

5. The method for preventing gas outbursts by hydration and solidification based on soft-hard composite coal seams according to claim 1 is characterized in that: In step S5, the diffusion capacity and pore structure of the coal seam are matched by controlling the injection rate and pressure: the injection rate is reduced in the fracture development area, and the injection pressure is increased in the low porosity area; in coal seams with different pore structures, the hydrate formation efficiency is enhanced by controlling the ratio of gas and water: the gas proportion is reduced in the coal seam area with high porosity, and the gas proportion is increased in the low porosity area.

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