Active treatment method for multi-source residual coal gas of abandoned mine

Abstract

The invention discloses an active treatment method for multi-source residual coal gas of a waste mine, which comprises the following steps: firstly, hydraulic fracturing pressure relief permeability improvement and overlying strata separation layer grouting reinforcement are cooperated, the hydraulic fracturing pressure relief permeability improvement establishes a netted fracture network in a coal body to realize permeability improvement, and the overlying strata separation layer grouting reinforcement compacts goaf gangue through grouting and reinforces an overlying strata to form a continuous bearing consolidation body; and therefore, a pressure relief and permeability increasing-strengthening and reinforcing integrated structure is formed, a stable structure and a penetrating seepage network are provided for subsequent displacement and gas extraction, and the subsequent gas extraction efficiency is guaranteed. Then low-temperature liquid CO2 is injected into the coal seam through the injection well, gas displacement and desorption are conducted on gas adsorbed in the coal seam under the action of phase change and competitive adsorption, expansion and development are conducted on fractures to enable permeability to be secondarily enhanced, and meanwhile part of CO2 obtained after displacement is retained and sealed in a coal body and a goaf; on the premise that the gas extraction concentration and efficiency are effectively improved, CO2 can be stored, and the dual effects of resource utilization and carbon storage are achieved.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of coal mine gas control, and particularly relates to a kind of abandoned mine multi-source residual coal gas active control method. BACKGROUND

[0002] There are a large number of residual coal pillars, corner coal and in-situ unsampled coal seams in abandoned mines. These coal seams still contain considerable gas resources. Because the coal body has been in a closed environment for a long time after the mine stopped production, the adsorbed gas in the coal seam continues to desorb and diffuse. However, after experiencing mining disturbance, stress redistribution, long-term moisture soaking, and coal body micro-crack fatigue degradation, the internal cracks of the coal body close, the pore structure collapses, resulting in extremely low permeability and significantly reduced gas flowability, making it difficult for gas to form effective seepage channels, resulting in extremely low utilization rate of gas resources.

[0003] The overburden strata of the mine will undergo the processes of caving, fracturing, separation and recompaction after mining, forming a multi-scale spatial structure of goaf, caving zone, fracture zone and high separation. These spaces usually have natural gas collection, enrichment and migration functions, and can form a continuous flow network inside the mine. The desorbed gas in the residual coal body can diffuse upwards and outwards in these low-resistance channels. When the fracture zone develops above the interface of the protective layer, the gas may escape slowly and continuously. This uncontrolled escape not only causes greenhouse gas emissions, but also makes abandoned mines a regional greenhouse gas leakage point.

[0004] The current mainstream methods for abandoned mine gas control mainly include extraction ventilation method, closed extraction method and simple CO2 injection and storage method, but all have obvious limitations: 1. Low efficiency of single extraction method: the gas pressure in abandoned mines is low and the permeability is poor, so it is difficult to form an effective seepage channel through conventional negative pressure extraction; 2. Stress concentration and unstable structure: the residual coal area and coal pillars bear residual ground stress, which can easily form a high stress area and limit gas seepage; 3. Lack of coupling and regulation mechanism: CO2 has strong competitive adsorption capacity and can effectively replace the adsorbed gas on the surface of the coal body, achieving efficient gas extraction; however, the existing CO2 injection displacement process is mainly used for complete reservoirs, which lacks adaptability for complex spatial systems in abandoned mines, easily causing CO2 leakage, and thus reducing the efficiency of gas displacement; 4. Disconnection between environmental control and resource utilization: traditional control focuses on safety and storage, ignoring the reuse of gas resources, resulting in the economic potential of abandoned mines not being fully tapped.

[0005] In summary, how to provide a new gas active control method, through the synergistic effect of multiple methods, to achieve efficient extraction of residual coal gas in abandoned mines while maintaining the stability of the structure of the abandoned mine during the extraction process, thereby realizing the recycling of gas resources in abandoned mines, is the direction of the present application.

[0006] A composite engineering method integrating "hydraulic fracturing-delamination grouting" for permeability enhancement and "CO2 injection-gas extraction" is proposed to achieve efficient recovery of gas resources in abandoned mines. Summary of the Invention

[0007] To address the problems existing in the prior art, this invention provides an active treatment method for gas from multiple sources in abandoned mines. Through the synergistic effects of hydraulic fracturing to enhance permeability and relieve pressure, delamination grouting to strengthen the structure, and CO2 injection for displacement and desorption, this method ensures efficient extraction of gas from abandoned coal mines while maintaining the stability of the abandoned mine structure during the extraction process, thereby achieving the recycling and utilization of gas resources from abandoned mines.

[0008] To achieve the above objectives, the technical solution adopted by this invention is: a method for active control of multi-source residual coal gas in abandoned mines, comprising the following steps: Step 1: Hydraulic fracturing for pressure relief and permeability enhancement. First, identify the residual coal seam with high gas content and the undisturbed coal seam. Construct multiple fracturing holes from the surface to the residual coal seam and the undisturbed coal seam. Perform hydraulic fracturing on the residual coal seam and the undisturbed coal seam through each fracturing hole, thereby forming a network of fracture channels in the residual coal seam and the undisturbed coal seam, achieving pressure relief and permeability enhancement. Furthermore, hydraulic fracturing can weaken the local bearing capacity of the coal pillar, destroy the stress arch structure, redistribute the stress in the coal body, accelerate the desorption of adsorbed gas, and facilitate the migration of gas to the fracture channels. Through this pressure relief and permeability enhancement process, the equivalent permeability of the coal seam can be increased by 2 to 10 times.

[0009] Step 2, Overburden Separation Grouting Reinforcement: Grouting pipes are arranged from the ground to the separation space above the goaf, and grout is injected into the separation space at the set grouting pressure. After the grout solidifies in the separation space, the compaction rate of the gangue in the goaf reaches 70-90%, forming a continuous load-bearing solidified body.

[0010] Step 3, Liquid CO2 Injection Displacement: Select a fracturing borehole as a CO2 injection well in the location where permeability enhancement is required in the residual coal area and undisturbed coal seam. Inject liquid CO2 into the residual coal area and undisturbed coal seam through the injection well. The liquid CO2 undergoes phase change and expansion upon heating, causing the fracture channels to develop and expand. It also displaces the gas desorbed from the coal seam into the fracture channels through competitive adsorption.

[0011] Step 4, CO2 Sequestration and Gas Extraction: Select a fracturing well as a gas extraction well at the location where extraction is required in the residual coal area and undisturbed coal seam, and connect it to the surface gas extraction system for negative pressure extraction. This allows the desorbed gas in the fracture channels of the residual coal area and undisturbed coal seam to be extracted to the surface, and some CO2 gas is adsorbed and stored in the coal seam and goaf fractures.

[0012] Furthermore, in step one, the injection pressure for hydraulic fracturing is 8~20 MPa, the injection flow rate for each fracturing hole is 40~80 L / min, and the fracturing radius for each fracturing hole is 8~25 m. These fracturing parameters ensure the desired pressure relief and permeability enhancement effects are achieved.

[0013] Furthermore, in step two, the delamination space is the void zone formed at the top of the overlying strata under the influence of mining; during grouting, 50-200 kg / m³ is injected into the delamination space at a grouting pressure of 1-3 MPa. 3 Slurry.

[0014] Furthermore, in step one, the spacing between the fracturing holes is 5-12 m, and in step two, the spacing between the grouting pipes is 6-15 m. This ensures that after the sequential construction of steps one and two, the overlap rate between the pressure relief zone and the consolidated body is 40-75%. This method of first depressurizing and increasing permeability, then strengthening and reinforcing, with overlapping construction in some areas, creates a pressure relief-strengthening structure above the goaf, providing a stable structure and a continuous seepage network for subsequent displacement and gas extraction.

[0015] Furthermore, in step two, the compressive strength of the continuously supported consolidated body is 3~10 MPa, and the solidification thickness is 0.5~2.5 m. Meeting these parameters can reduce the probability of goaf collapse by 40~70% and reduce roof subsidence by 30~60%.

[0016] Furthermore, in step three, liquid CO2 is injected at a temperature of -40°C to -20°C and an injection pressure of 2–6 MPa, with an injection rate of 200–600 kg / h. Using these parameters, the coal seam permeability is further increased by 30–120% through the phase change expansion and competitive adsorption of liquid CO2.

[0017] Furthermore, in step four, the extraction negative pressure is -15 to -35 kPa, and the extraction flow rate is 3 to 12 m³ / min. Using these parameters ensures efficient gas extraction.

[0018] Compared with the prior art, the present invention has the following advantages: 1. This invention employs a synergistic approach of hydraulic fracturing for pressure relief and permeability enhancement, combined with overlying rock delamination grouting reinforcement. The former establishes a network of fractures within the coal seam to enhance permeability, while the latter compacts the gangue in the goaf through grouting and reinforces the overlying strata to form a continuous load-bearing solidified body. This constructs a composite space by integrating the undisturbed coal seam, the coal pillar in the goaf, and the gangue filling the goaf. With the network of fracture channels as the core, it achieves interconnected seepage and gas storage synergy between the undisturbed coal seam, coal pillar, and goaf, thereby forming an integrated structure of pressure relief and permeability enhancement-reinforcement. This improves the permeability of the coal seam and the stability of the surrounding rock. This method breaks through the limitations of traditional single-medium approaches, achieving both coal pressure relief and structural integrity enhancement. It provides a stable structure and a continuous seepage network for subsequent displacement and gas extraction, ensuring efficient gas extraction.

[0019] 2. Based on the integrated structure of pressure relief and permeability enhancement, this invention injects low-temperature liquid CO2 into the coal seam through injection wells. Under phase change and competitive adsorption, the liquid CO2 displaces and desorbs adsorbed gas within the coal seam, and expands and develops fractures, thus further enhancing permeability. Simultaneously, some of the displaced CO2 is retained and stored in the coal body and goaf, achieving synergistic operation of CO2 injection displacement and gas extraction. This effectively improves gas extraction concentration and efficiency while also enabling CO2 sequestration, achieving the dual benefits of resource utilization and carbon sequestration. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the overall layout of the present invention.

[0021] In the diagram: 1. Undisturbed coal seam; 2. Coal pillar in goaf; 3. Network of fracture channels; 4. Roof; 5. Overlying strata; 6. Grouting pipe; 7. Gas extraction well; 8. Surface gas extraction system; 9. CO2 injection equipment; 10. CO2 injection well; 11. Floor; 12. Gangue. Detailed Implementation

[0022] The present invention will be further described below.

[0023] like Figure 1 As shown, the present invention includes the following steps: Step 1: Hydraulic fracturing for pressure relief and permeability enhancement. First, identify the residual coal seam with high gas content (i.e., the goaf coal pillar 2) and the undisturbed coal seam 1. Construct multiple fracturing holes from the surface into the residual coal seam and the undisturbed coal seam 1, with a spacing of 5-12 m between the holes. Perform hydraulic fracturing on the residual coal seam and the undisturbed coal seam 1 through each fracturing hole. The injection pressure of the hydraulic fracturing is 8-20 MPa, the injection flow rate of each fracturing hole is 40-80 L / min, and the fracturing radius of each fracturing hole is 8-25 m. This forms a network of fracture channels 3 in the residual coal seam and the undisturbed coal seam 1, achieving pressure relief and permeability enhancement. Furthermore, hydraulic fracturing can weaken the local bearing capacity of the coal pillar, destroy the stress arch structure, redistribute the stress in the coal body, accelerate the desorption of adsorbed gas, and facilitate the migration of gas to the fracture channels. Through this pressure relief and permeability enhancement process, the equivalent permeability of the coal seam is increased by 2-10 times.

[0024] Step 2: Grouting reinforcement of overlying strata delamination: Grouting pipes 6 are arranged from the ground to the delamination space above the goaf, with a spacing of 6-15 m. The delamination space is the void zone formed at the top of the overlying strata 5 under the influence of mining. 50-200 kg / m³ of grout is injected into the delamination space at a grouting pressure of 1-3 MPa. 3 The grout, during its solidification process within the delamination space, continuously applies pressure to compact the gangue in the goaf until solidification achieves a compaction rate of 70-90%, forming a continuous load-bearing consolidated body. Its compressive strength is 3-10 MPa, and the solidified thickness is 0.5-2.5 m. Achieving these parameters reduces the probability of goaf collapse by 40-70% and reduces roof subsidence by 30-60%. After sequential construction in steps one and two, the overlap between the pressure-relief zone and the consolidated body is 40-75%. This method of first depressurizing and increasing permeability, then strengthening and reinforcing, with overlapping construction in some areas, creates a pressure-relief-strengthening structure above the goaf, providing a stable structure and a continuous seepage network for subsequent displacement and extraction. Furthermore, the formed continuous load-bearing consolidated body enhances the overall support capacity of the roof and overlying strata 5, while simultaneously compacting the gangue layer, significantly reducing the porosity of the goaf, and achieving safe restoration of the ecological environment in the mining area.

[0025] Step 3: Liquid CO2 Injection and Displacement: A fracturing borehole is selected as CO2 injection well 10 at the location in the residual coal zone and the undisturbed coal seam requiring permeability enhancement. The CO2 injection equipment 9 injects liquid CO2 into the residual coal zone and the undisturbed coal seam 1 through the injection well. Specific injection parameters are: liquid CO2 at -40℃ to -20℃, injection pressure of 2~6 MPa, and injection rate of 200~600 kg / h. The liquid CO2 undergoes a phase change and expansion upon heating, causing the fracture channels to develop and expand. Furthermore, through competitive adsorption, it displaces the desorbed gas within the coal seam into the fracture channels. This secondary permeability enhancement effect further increases the coal seam permeability by 30~120%.

[0026] Step 4, CO2 Sequestration and Gas Extraction: A fracturing well (7) is selected as the gas extraction well in the remaining coal seam area and the location where extraction is required in the undisturbed coal seam 1. This well is connected to the surface gas extraction system (8) for negative pressure extraction. The extraction negative pressure is -15 to -35 kPa, and the extraction flow rate is 3~12 m³ / min. This allows the desorbed gas in the fracture channels of the remaining coal seam and undisturbed coal seam 1 to be extracted to the surface, and some CO2 gas is adsorbed and stored in the fractures of the coal seam and goaf. After the above treatment according to the present invention, the extracted gas concentration is increased to 50~80%.

[0027] Through the method of the present invention described above, under the synergistic effect of hydraulic fracturing, delamination grouting, and CO2 injection, the undisturbed coal seam 1, the goaf coal pillar 2, the gangue 12, and the delamination space together form a composite space. The effective porosity of this composite space is measured to reach 8-18%, and the equivalent gas storage volume can reach 5000-20000 m³. The overall gas extraction rate is increased by 50-120% compared with the benchmark. At the same time, long-term CO2 sequestration is achieved, ultimately realizing the synergistic utilization of efficient recovery of gas resources and greenhouse gas sequestration in abandoned mines, which has significant economic benefits, environmental benefits, and safety guarantees.

[0028] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for active management of multi-source residual coal gas in abandoned mines, characterized in that, Includes the following steps: Step 1: Hydraulic fracturing to relieve pressure and enhance permeability: First, identify the residual coal seam with high gas content and the undisturbed coal seam. Then, construct multiple fracturing holes from the surface to the residual coal seam and the undisturbed coal seam. Hydraulic fracturing is performed on the residual coal seam and the undisturbed coal seam through each fracturing hole, thereby forming a network of fracture channels in the residual coal seam and the undisturbed coal seam, achieving pressure relief and permeability enhancement in the residual coal seam and the undisturbed coal seam. Step 2, Overburden Separation Grouting Reinforcement: Grouting pipes are arranged from the ground to the separation space above the goaf, and grout is injected into the separation space at the set grouting pressure. After the grout solidifies in the separation space, the compaction rate of the gangue in the goaf reaches 70-90%, forming a continuous load-bearing solidified body. Step 3, Liquid CO2 Injection Displacement: Select a fracturing borehole as a CO2 injection well in the location where permeability enhancement is required in the residual coal area and undisturbed coal seam. Inject liquid CO2 into the residual coal area and undisturbed coal seam through the injection well. The liquid CO2 undergoes phase change and expansion upon heating, causing the fracture channels to develop and expand. It also displaces the gas desorbed from the coal seam into the fracture channels through competitive adsorption. Step 4, CO2 Sequestration and Gas Extraction: Select a fracturing well as a gas extraction well at the location where extraction is required in the residual coal area and undisturbed coal seam, and connect it to the surface gas extraction system for negative pressure extraction. This allows the desorbed gas in the fracture channels of the residual coal area and undisturbed coal seam to be extracted to the surface, and some CO2 gas is adsorbed and stored in the coal seam and goaf fractures.

2. The active treatment method for multi-source residual coal gas in abandoned mines according to claim 1, characterized in that, In step one, the injection pressure of hydraulic fracturing is 8~20 MPa, the injection flow rate of each fracturing hole is 40~80 L / min, and the fracturing radius of each fracturing hole is 8~25 m.

3. The active treatment method for multi-source residual coal gas in abandoned mines according to claim 1, characterized in that, In step two, the delamination space is the void zone formed at the top of the overlying strata under the influence of mining; during grouting, 50-200 kg / m³ is injected into the delamination space at a grouting pressure of 1-3 MPa. 3 Slurry.

4. The active treatment method for multi-source residual coal gas in abandoned mines according to claim 1, characterized in that, In step one, the spacing between the fracturing holes is 5-12 m, and in step two, the spacing between the grouting pipes is 6-15 m. After step one and step two are constructed in sequence, the overlap rate between the pressure relief zone and the solidified body is 40-75%.

5. The active treatment method for multi-source residual coal gas in abandoned mines according to claim 1, characterized in that, In step two, the compressive strength of the continuously supported solidified body is 3~10 MPa, and the solidification thickness is 0.5~2.5 m.

6. The active treatment method for multi-source residual coal gas in abandoned mines according to claim 1, characterized in that, In step three, liquid CO2 is injected at a temperature of -40 ℃ to -20 ℃ and an injection pressure of 2~6 MPa, with an injection rate of 200~600 kg / h.

7. The active treatment method for multi-source residual coal gas in abandoned mines according to claim 1, characterized in that, In step four, the extraction negative pressure is -15 to -35 kPa, and the extraction flow rate is 3 to 12 m³ / min.

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