Method for synergistic permeability enhancement and extraction of coal mine gas

Abstract

Provided is a method for synergistic permeability enhancement and extraction of coal mine gas. The method includes: S1: drilling a plurality of vertical wells and a plurality of horizontal wells in a coal seam area to form a three-dimensional well network; S2: composite electric field fracturing: applying a high-voltage pulsed electric field for transient impact, and then introducing low-frequency electromagnetic waves for sustained action; S3: extracting geothermal water from an underground abandoned mine by using a high-pressure pump; heating the geothermal water to a set temperature using a heat pump, then mixing the geothermal water with high-pressure nitrogen, and introducing the mixed geothermal water and high-pressure nitrogen into a wellhead of the three-dimensional well network in a pulsed manner; after a set period of time, extracting and returning the geothermal water to the underground abandoned mine; and S4: extracting gas from the wellhead location by using negative pressure adsorption equipment.

Description

CROSS REFERENCE TO RELATED APPLICATION

[0001] This patent application claims the benefit and priority of Chinese Patent Application No. 202510078320.7, filed with the China National Intellectual Property Administration on Jan. 17, 2025, the disclosure of which is incorporated by reference herein in its entirety as part of the present application.TECHNICAL FIELD

[0002] The present disclosure relates to the technical field of coal mine gas extraction, and in particular to a method for synergistic permeability enhancement and extraction of coal mine gas.BACKGROUND

[0003] Coal mine gas, also known as coalbed methane, refers to hydrocarbon gas stored in coal seams, primarily composed of methane, mainly adsorbed on the surface of coal matrix particles, partially free in coal pores, or dissolved in coal seam water. It is an associated mineral resource of coal, belonging to unconventional natural gas, and has emerged internationally in the last decade or two as a clean, high-quality energy source and chemical raw material. When the methane concentration in air reaches 5%-16%, it can explode upon encountering an open flame, leading to coal mine explosion accidents. To address the above problems, gas is extracted before coal mining, thereby reducing the coal mine gas explosion rate by 70% to 85%.

[0004] Existing gas extraction methods mainly include:

[0005] 1. Surface drilling extraction: (1) Vertical and horizontal well drilling: drilling into the coal seam from the surface through a drilling rig to establish a wellbore directly reaching the coal seam. This method is suitable for deep coal seams or situations where direct extraction from inside the mine is difficult. This allows pre-drainage of gas in the coal seam, reducing safety risks during mining.

[0006] (2) Hydraulic fracturing for permeability enhancement: Similar to shale gas development, high-pressure fluid (usually water mixed with proppant) is injected into the coal seam to create fractures and improve permeability, thereby enhancing gas mobility and extraction efficiency.

[0007] The drilling extraction method requires substantial capital investment for establishing and maintaining a surface drilling system, including procurement and installation of drilling equipment, as well as daily operating costs. Furthermore, drilling activities may cause damage to the surface ecological environment, and particular attention needs to be paid to environmental protection during construction in nature reserves or ecologically sensitive areas. Meanwhile, for shallow coal seams or areas with complex geological structures, the application effectiveness of the surface drilling technology may be limited.

[0008] 2. Underground borehole extraction: (1) In-seam boreholes: Boreholes are arranged along the coal seam strike, typically located in preparation areas ahead of the working face, to release gas pressure in advance and prevent outburst accidents.

[0009] (2) Cross-measure boreholes: These boreholes penetrate from non-coal rock layers to reach the target coal seam, forming an interconnected network of channels, enabling the convergence and centralized extraction of originally isolated small areas of high-concentration gas.

[0010] The underground borehole extraction method requires drilling operations inside the mine and carries certain dangers. Potential accidents like gas outbursts and roof falls are difficult to completely avoid. Moreover, due to the narrow underground space and limitations such as ventilation conditions, the drilling speed is relatively slow, leading to a longer overall extraction cycle. Additionally, as mining progresses, original boreholes gradually become ineffective, necessitating continuous re-layout of new boreholes, which increases management difficulty and costs.

[0011] 3. Protective layer mining method: A safer rock layer above or below the coal seam is first mined to serve as a “protective layer” that provides a pressure relief effect to reduce the stress in the adjacent coal seam, promoting the trapped gas to naturally migrate into the goaf, where it is then collected. This method not only effectively controls the amount of gas emission but also improves the working environment and reduces the risk of coal dust explosions.

[0012] When the protective layer mining method is used, a portion of high-quality coal resources is occupied, which to some extent causes resource waste. Furthermore, not all mining areas have suitable conditions for protective layer mining, especially when there is no suitable rock layer above the target coal seam, this scheme cannot be implemented. Although gas emission can be significantly reduced in the short term, in the long run, as mining depth increases, the pressure relief effect gradually weakens, and the pressure for later-stage governance remains considerable.

[0013] 4. Electric pulse-induced fracturing: A strong electric field generated by a high-voltage pulse power source applies transient impact force to the coal body, causing damage to the internal structure of the coal seam within a short time and generating a new system of micro-fractures. These artificially induced fractures provide additional migration paths for gas, significantly increasing the coal seam permeability, which is beneficial for the subsequent extraction process.

[0014] The method of electric pulse-induced fracturing requires professional high-voltage power supply devices and supporting instruments and meters, resulting in high one-time construction costs.

[0015] In summary, the current permeability enhancement methods for coal seam gas extraction are relatively limited in variety and fail to deliver satisfactory permeability enhancement results.SUMMARY

[0016] An objective of the present disclosure is to provide a synergistic permeability enhancement and extraction method for coal mine gas, to solve the above technical problems.

[0017] To achieve the above objectives, the present disclosure provides a method for synergistic permeability enhancement and extraction of coal mine gas, including the following steps:

[0018] S1: drilling a plurality of vertical wells and a plurality of horizontal wells in a coal seam area to form a three-dimensional well network, thereby forming primary fractures;

[0019] S2: composite electric field fracturing: applying a high-voltage pulsed electric field for transient impact, and then introducing low-frequency electromagnetic waves for sustained action to form secondary fractures;

[0020] S3: extracting geothermal water from an underground abandoned mine by using a high-pressure pump; heating the geothermal water to a set temperature using a heat pump, then mixing the geothermal water with high-pressure nitrogen, and introducing the mixed geothermal water and high-pressure nitrogen into a wellhead of the three-dimensional well network in a pulsed manner; after a set period of time, extracting and returning the geothermal water to the underground abandoned mine, thereby forming tertiary fractures; and

[0021] S4: extracting gas from the wellhead by using negative pressure adsorption equipment. Preferably, step S1 comprises the following sub-steps:

[0022] S11: geological exploration and planning: conducting geological exploration of the coal seam area using a seismic wave method, and determining an optimal vertical drilling spacing and depth as well as an optimal horizontal drilling spacing and depth based on exploration results, to ensure that the wells fully cover the coal seam;

[0023] S12: vertical drilling: drilling vertically downward from a ground surface at selected locations by using rotary drilling equipment, to form a plurality of vertical wells, while ensuring that the vertical wells reach a predetermined depth of the coal seam; and

[0024] S13: horizontal drilling: based on the vertical wells, drilling a plurality of horizontal wells laterally from bottom to top along the vertical wells by using horizontal directional drilling technology, thereby forming a three-dimensional well network.

[0025] Preferably, step S11 comprises the following sub-steps:

[0026] S111: preliminary investigation: collecting geological maps, mining history, and coal seam distribution data of the coal seam area, and conducting field reconnaissance to record topographic features of the coal seam area;

[0027] S112: simultaneously installing seismometers and seismic source equipment in the coal seam area, and laying out survey lines based on preliminary investigation results;

[0028] S113: activating a seismic source on the laid survey lines by using the seismic source equipment, and recording underground reflected wave signals through the seismometers;

[0029] S114: after preprocessing the reflected wave signals, generating an underground structure image using an inversion calculation, and determining a thickness D, a dip angle, and a depth H of the coal seam; and

[0030] S115: based on the determined thickness, dip angle, and depth of the coal seam, determining a spacing and a depth of the vertical wells, and a spacing and an extension length of the horizontal wells;

[0031] wherein a formula for calculating the spacing of the vertical wells is as follows:Lv=(2×Dh×tan⁢(α2))2+d2;(1)in the formula, Lv represents a minimum spacing between two adjacent vertical wells, measured in meters; Dh represents an extension length of the horizontal well, measured in meters; α represents an angle of the horizontal well relative to a vertical direction; and d represents a diameter of the vertical well, measured in meters;

[0033] a formula for calculating the depth of the vertical wells is as follows:Dv=H+Δ⁢D;(2)in the formula, Dv represents a final depth of the vertical well, measured in meters; H represents the depth of the coal seam, measured in meters; and ΔD represents a reserved safety margin for protrusion, measured in meters;

[0035] a formula for calculating the spacing of the horizontal wells is as follows:Lh=(Dv-H)2+(2×W×tan⁢(β))2;(3)in the formula, Lh represents a minimum spacing between two adjacent horizontal wells, measured in meters; W represents a lateral distance of the coal seam between two adjacent horizontal wells; and β represents an inclination angle of the horizontal well;

[0037] a formula for calculating the extension length of the horizontal wells is as follows:Dh=Lmin+Δ⁢L;(4)in the formula, Lmin represents a minimum effective extension length, measured in meters; and ΔL represents a safety extension margin, measured in meters;

[0039] a formula for calculating the minimum effective extension length Lmin is as follows:Lmin=(Qk1×A)2+4×D2-2⁢D;(5)in the formula, Q represents an expected gas flow rate for extraction, measured in m3 / d; k1 represents a permeability of the coal seam, measured in Darcy; A represents a cross-sectional area of the horizontal well, measured in m2; and D represents the thickness of the coal seam, measured in meters.

[0041] Preferably, in step S12, a verticality check is performed every 5 to 10 meters of drilling to ensure that a vertical deviation of the drilling is controlled within a set range; and

[0042] in step S13, a trajectory of the horizontal well is guided with the aid of a Measurement While Drilling (MWD) system.

[0043] Preferably, step S2 comprises the following sub-steps:

[0044] S21: transient impact with a high-voltage pulsed electric field: installing a high-voltage pulse generator at the wellhead of the three-dimensional well network, connecting the high-voltage pulse generator to electrodes within the wells, and after setting initial high-voltage pulse parameters, starting the high-voltage pulse generator to apply a transient high-voltage electric field to the coal seam, causing cracks to form in the coal seam, thereby laying a foundation for the secondary fractures;

[0045] S22: removing the high-voltage pulse generator and the electrodes; and

[0046] S23: sustained action by low-frequency electromagnetic waves: installing a low-frequency electromagnetic wave transmitter at the wellhead of the three-dimensional well network, connecting the low-frequency electromagnetic wave transmitter to an antenna within the wells, and after setting initial low-frequency electromagnetic wave parameters, starting the low-frequency electromagnetic wave transmitter to apply sustained action to the formed cracks, thereby forming the secondary fractures.

[0047] Preferably, in step S21, the initial high-voltage pulse parameters comprise voltage amplitude V, pulse width Tp, and pulse frequency f, wherein a formula for calculating the voltage amplitude V is as follows:V=k2×R×D;(6)in the formula, k2 represents an empirical coefficient; R represents a resistivity of the coal seam, measured in Ω / m;

[0049] a formula for calculating the pulse width Tp is as follows:Tp=qf;(7)in the formula, q represents a constant term; and f represents a pulse frequency, measured in Hz;

[0051] a formula for calculating the pulse frequency f is as follows:f=kR×C;(8)in the formula, C represents a capacitance value of the high-voltage pulse generator;

[0053] in step S23, the initial low-frequency electromagnetic wave parameters comprise transmission power P, operating frequency fLF, and action time t, wherein a formula for calculating the transmission power P is as follows:P=k×σ×Dh;(9)in the formula, σ represents an electrical conductivity of the coal seam, measured in S / m;

[0055] a formula for calculating the operating frequency fLF is as follows:fL⁢F=k×cλ;(10)in the formula, c represents the speed of light, measured in m / s; λ represents a wavelength, measured in meters, and λ=k′×D, wherein k′ is a proportionality coefficient;

[0057] a formula for calculating the action time t is as follows:t=Dhv;(11)in the formula, v represents a propagation speed of electromagnetic waves in the coal seam, measured in m / s.

[0059] Preferably, in step S21 and step S23, a microseismic monitoring system and electromagnetic wave reflection detection equipment are installed at the wellhead of the three-dimensional well network, wherein the microseismic monitoring system is configured to detect vibrations generated during fracture propagation, and the electromagnetic wave reflection detection equipment is configured to collect electromagnetic wave reflection data; based on vibration data and the electromagnetic wave reflection data, an energy difference ΔE, a time difference ΔT, a current difference 41, and a crack length difference ΔL are extracted, and the initial high-voltage pulse parameters and the initial low-frequency electromagnetic wave parameters are adjusted through cyclic iteration;

[0060] wherein a formula for calculating an adjusted voltage amplitude V′ is as follows:V′=V+kv×Δ⁢E;(12)in the formula, kv represents a voltage adjustment coefficient;

[0062] a formula for calculating an adjusted voltage amplitude pulse widthTp′is as follows:Tp′=Tp+kc×Δ⁢T;(13)in the formula, kt represents a time adjustment coefficient;a formula for calculating an adjusted pulse frequency f′ is as follows:f′=f+kf×Δ⁢L;(14)in the formula, kf represents a frequency adjustment coefficient;a formula for calculating an adjusted transmission power P′ is as follows:P′=P+kP×Δ⁢I;(15)in the formula, kP represents a power adjustment coefficient;a formula for calculating an adjusted operating frequencyfL⁢F′is as follows:fL⁢F′=fL⁢F+kf×Δ⁢L;(16)a formula for calculating an adjusted action time t′ is as follows:t′=t+kc×Δ⁢T.(17)Preferably, step S3 comprises the following sub-steps:S31: extracting geothermal water from the underground abandoned mine by using the high-pressure pump;S32: feeding the extracted geothermal water into the heat pump for heating to a set temperature, and outputting the heated geothermal water to a geothermal water pipeline;S33: connecting the geothermal water pipeline and a high-pressure nitrogen pipeline to the wellhead of the three-dimensional well network via dual channels respectively, and setting an input ratio of the geothermal water to the high-pressure nitrogen;S34: introducing the mixed geothermal water and high-pressure nitrogen into the wellhead of the three-dimensional well network in a pulsed manner; andS35: after the set period of time, extracting the geothermal water from the wells by using a water pump and returning the geothermal water to the underground abandoned mine through pipelines, forming a closed-loop system.

[0076] Preferably, in step S32, the set temperature is 200° C. to 350° C.;

[0077] in step S33, the input ratio of the geothermal water to the high-pressure nitrogen is 3:2.5; and in step S34, a duration of each pulse is 10 s to 15 s, and an interval time is 20 s to 35 s.

[0078] Therefore, the present disclosure adopts the above-mentioned solution and has the following beneficial effects:

[0079] 1. Improved gas extraction efficiency: Through the sequential creation of primary, secondary, and tertiary fractures, the fracture network of the coal seam is greatly increased, and the coal seam permeability is improved, thereby significantly enhancing gas mobility and extraction efficiency.

[0080] 2. Enhanced safety performance: During the action of the high-voltage pulsed electric field and low-frequency electromagnetic waves, the fracture propagation is monitored in real time, and parameters are adjusted promptly to ensure the safety and controllability of the operation.

[0081] 3. Environmental protection: The geothermal water in the underground abandoned mine is heated by a heat pump and then re-injected into the well network, which not only makes full use of geothermal resources but also reduces energy consumption.

[0082] Furthermore, the extracted geothermal water and high-pressure nitrogen after fracturing are returned to the underground abandoned mine, avoiding environmental pollution and reducing resource waste.

[0083] 4. Dual action: The combination of hot water and nitrogen exploits both thermal stress and gas expansion, significantly improving the coal seam fracturing effectiveness.

[0084] 5. Optimized drilling layout: Through detailed geological exploration (seismic wave method, inversion calculation, etc.), the optimal drilling spacing and depth are accurately determined, ensuring complete coverage of the target coal seam and improving the drilling success rate and resource utilization efficiency.

[0085] 6. Intelligent parameter adjustment: Based on the microseismic monitoring system and electromagnetic wave reflection data, the initial high-voltage pulse parameters and low-frequency electromagnetic wave parameters are cyclically and iteratively adjusted, making the fracture propagation process more efficient and stable.

[0086] In summary, the method for synergistic permeability enhancement and extraction of coal mine gas provided by the present disclosure not only significantly improves gas extraction efficiency but also enhances operational safety and environmental protection. Through intelligent parameter adjustment and optimized drilling layout, the goal of efficient, safe, and environmentally friendly gas extraction is achieved, possessing broad application prospects and promotion value.

[0087] The technical solutions of the present disclosure will be further described in detail below with reference to drawings and embodiments.BRIEF DESCRIPTION OF THE DRAWINGS

[0088] FIG. 1 is a flowchart of a method for synergistic permeability enhancement and extraction for coal mine gas according to the present disclosure.DETAILED DESCRIPTION OF THE EMBODIMENTS

[0089] In order to make the objectives, technical solutions, and advantages of the embodiments of the present disclosure clearer, the embodiments of the present disclosure are further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only intended to explain the present disclosure and are not intended to limit the embodiments of the present disclosure. All other embodiments obtained by a person of ordinary skill in the art based on the embodiments of the present disclosure without creative efforts shall fall within the protection scope of the present disclosure. The reference numerals of the embodiments are shown in the accompanying drawings. The same or similar numerals represent the same or similar elements (with the same or similar functions) throughout the specification.

[0090] It needs to be noted that the terms “include”, “comprise”, and their variants mean to cover a non-exclusive inclusion. For example, a process, method, system, product or device that includes a list of steps or units is not necessarily limited to those steps or units which are clearly listed. Instead, they may include other steps or units that are not expressly listed or inherent or that are inherent to such a process, method, system, product, or device.

[0091] The following describes in detail the implementations of the present disclosure with reference to the accompanying drawings.

[0092] As shown in FIG. 1, a method for synergistic permeability enhancement and extraction of coal mine gas includes the following steps:

[0093] S1: Drill a plurality of vertical wells and a plurality of horizontal wells in a coal seam area to form a three-dimensional well network, thereby forming primary fractures.

[0094] Step S1 specifically includes the following sub-steps:

[0095] S11: Geological exploration and planning: conduct geological exploration of the coal seam area using a seismic wave method, and determine an optimal vertical drilling spacing and depth as well as an optimal horizontal drilling spacing and depth based on exploration results, to ensure that the wells cover the coal seam;

[0096] Step S11 specifically includes the following sub-steps:

[0097] S111: Preliminary investigation: collect geological maps, mining history, and coal seam distribution data of the coal seam area, and conduct field reconnaissance to record topographic features of the coal seam area.

[0098] S112: Simultaneously install seismometers and seismic source equipment in the coal seam area, and lay out survey lines based on preliminary investigation results.

[0099] S113: Activate a seismic source on the laid survey lines by using the seismic source equipment, and record underground reflected wave signals through the seismometers.

[0100] S114: After preprocessing the reflected wave signals, generate an underground structure image using inversion calculation, and determine a thickness D, a dip angle, and a depth H of the coal seam.

[0101] S115: Based on the determined thickness, dip angle, and depth of the coal seam, determine a spacing and a depth of the vertical wells, and a spacing and an extension length of the horizontal wells.

[0102] A formula for calculating the spacing of the vertical wells is as follows:Lv=(2×Dh×tan⁢(α2))2+d2(1)

[0103] In the formula, Lv represents a minimum spacing between two adjacent vertical wells, measured in meters; Dh represents an extension length of the horizontal well, measured in meters; α represents an angle of the horizontal well relative to a vertical direction; and d represents a diameter of the vertical well, measured in meters.

[0104] A formula for calculating the depth of the vertical wells is as follows:Dv=H+Δ⁢D(2)

[0105] In the formula, Dv represents a final depth of the vertical well, measured in meters; H represents the depth of the coal seam, measured in meters; and ΔD represents a reserved safety margin for protrusion, measured in meters.

[0106] A formula for calculating the spacing of the horizontal wells is as follows:Lh=(Dv-H)2+(2×W×tan⁢(β))2(3)

[0107] In the formula, Lh represents a minimum spacing between two adjacent horizontal wells, measured in meters; W represents a lateral distance of the coal seam between two adjacent horizontal wells; and β represents an inclination angle of the horizontal well.

[0108] A formula for calculating the extension length of the horizontal wells is as follows:Dh=Lmin+Δ⁢L(4)

[0109] In the formula, Lmin represents a minimum effective extension length, measured in meters; and ΔL represents a safety extension margin, measured in meters.

[0110] A formula for calculating the minimum effective extension length Lmin is as follows:Lmin=(Qk1×A)2+4×D2-2⁢D(5)

[0111] In the formula, Q represents an expected gas flow rate for extraction, measured in m3 / d; k1 represents a permeability of the coal seam, measured in Darcy; λ represents a cross-sectional area of the horizontal well, measured in m2; and D represents the thickness of the coal seam, measured in meters.

[0112] S12: Vertical drilling: drill vertically downward from a ground surface at selected locations by using rotary drilling equipment, to form a plurality of vertical wells, while ensuring that the vertical wells reach a predetermined depth of the coal seam.

[0113] In step S12, a verticality check is performed every 5 to 10 meters of drilling to ensure that a vertical deviation of the drilling is controlled within a set range.

[0114] S13: Horizontal drilling: based on the vertical wells, drill a plurality of horizontal wells laterally from bottom to top along the vertical wells by using horizontal directional drilling technology, thereby forming a three-dimensional well network.

[0115] In step S13, a trajectory of the horizontal well is guided with the aid of a Measurement While Drilling (MWD) system.

[0116] S2: Composite electric field fracturing: apply a high-voltage pulsed electric field for transient impact, and then introduce low-frequency electromagnetic waves for sustained action to form secondary fractures.

[0117] Step S2 specifically includes the following sub-steps:

[0118] S21: Transient impact with a high-voltage pulsed electric field: install a high-voltage pulse generator at a wellhead of the three-dimensional well network, connect the high-voltage pulse generator to electrodes within the wells, and after setting initial high-voltage pulse parameters, start the high-voltage pulse generator to apply a transient high-voltage electric field to the coal seam, causing cracks to form in the coal seam, thereby laying a foundation for the secondary fractures.

[0119] In step S21, the initial high-voltage pulse parameters include voltage amplitude V, pulse width Tp, and pulse frequency f, where a formula for calculating the voltage amplitude V is as follows:V=k2×R×D(6)

[0120] In the formula, k2 represents an empirical coefficient; R represents a resistivity of the coal seam, measured in Ω / m.

[0121] A formula for calculating the pulse width Tp is as follows:Tp=qf(7)In the formula, q represents a constant term; and f represents a pulse frequency, measured in Hz;

[0123] A formula for calculating the pulse frequency f is as follows:f=kR×C(8)

[0124] In the formula, C represents a capacitance value of the high-voltage pulse generator.

[0125] S22: Remove the high-voltage pulse generator and the electrodes.

[0126] S23: Sustained action by low-frequency electromagnetic waves: install a low-frequency electromagnetic wave transmitter at the wellhead of the three-dimensional well network, connect the low-frequency electromagnetic wave transmitter to an antenna within the wells, and after setting initial low-frequency electromagnetic wave parameters, start the low-frequency electromagnetic wave transmitter to apply sustained action to the formed cracks, thereby forming the secondary fractures.

[0127] In step S23, the initial low-frequency electromagnetic wave parameters comprise transmission power P, operating frequency fLF, and action time t, wherein a formula for calculating the transmission power P is as follows:P=k×σ×Dh(9)

[0128] In the formula, σ represents an electrical conductivity of the coal seam, measured in S / m;

[0129] A formula for calculating the operating frequency fLF is as follows:fL⁢F=k×cλ(10)

[0130] In the formula, c represents the speed of light, measured in m / s; λ represents a wavelength, measured in meters, and λ=k′×D, where k′ is a proportionality coefficient.

[0131] A formula for calculating the action time t is as follows:t=Dhv(11)

[0132] In the formula, v represents a propagation speed of electromagnetic waves in the coal seam, measured in m / s.

[0133] In step S21 and step S23, a microseismic monitoring system and electromagnetic wave reflection detection equipment are installed at the wellhead of the three-dimensional well network. The microseismic monitoring system is configured to detect vibrations generated during fracture propagation, and the electromagnetic wave reflection detection equipment is configured to collect electromagnetic wave reflection data. Based on vibration data and the electromagnetic wave reflection data, an energy difference ΔE, a time difference ΔT, a current difference ΔI, and a crack length difference ΔL are extracted, and the initial high-voltage pulse parameters and the initial low-frequency electromagnetic wave parameters are adjusted through cyclic iteration.

[0134] A formula for calculating an adjusted voltage amplitude V′ is as follows:V′=V+kv×Δ⁢E(12)

[0135] In the formula, kv represents a voltage adjustment coefficient.

[0136] A formula for calculating an adjusted voltage amplitude pulse widthTp′is as follows:Tp′=Tp+kt×Δ⁢T(13)In the formula, kt represents a time adjustment coefficient.A formula for calculating an adjusted pulse frequency f′ is as follows:f′=f+kf×Δ⁢L(14)In the formula, kf represents a frequency adjustment coefficient.

[0140] A formula for calculating an adjusted transmission power P′ is as follows:P′=P+kP×Δ⁢I(15)

[0141] In the formula, kP represents a power adjustment coefficient.

[0142] A formula for calculating an adjusted operating frequencyfL⁢F′is as follows:fL⁢F′=fL⁢F+kf×Δ⁢L(16)A formula for calculating an adjusted action time t′ is as follows:t′=t+kt×Δ⁢T(17)S3: Extract geothermal water from an underground abandoned mine by using a high-pressure pump; heat the geothermal water to a set temperature using a heat pump, then mix the geothermal water with high-pressure nitrogen, and introduce the mixed geothermal water and high-pressure nitrogen into a wellhead of the three-dimensional well network in a pulsed manner; after a set period of time, extract and return the geothermal water to the underground abandoned mine, thereby forming tertiary fractures.A principle of geothermal water and nitrogen fracturing is as follows: High-temperature hot water causes the rock in the coal seam to expand, generating thermal stress, which leads to the propagation of micro-fractures and the formation of new fractures, thereby improving the permeability of the coal seam. Simultaneously, the hot water heats the adsorbed gases (such as methane) in the coal seam, accelerating its desorption process and increasing gas pressure, which helps to form new fractures or propagate existing ones.After high-pressure nitrogen enters the coal seam, it expands during the cooling process, further enlarging fractures and preventing the fractures from closing. Meanwhile, nitrogen can drive the gas in the coal seam towards the production well, improving gas mobility and recovery. Additionally, the continuous injection of nitrogen can maintain high formation pressure, prevent pressure drops due to steam condensation, and ensure long-term stable gas production.

[0147] Step S3 specifically includes the following sub-steps:

[0148] S31: Extract geothermal water from the underground abandoned mine by using the high-pressure pump.

[0149] S32: Feed the extracted geothermal water into the heat pump for heating to a set temperature, and output the heated geothermal water to a geothermal water pipeline.

[0150] In step S32, the set temperature is 200° C. to 350° C.

[0151] S33: Connect the geothermal water pipeline and a high-pressure nitrogen pipeline to the wellhead of the three-dimensional well network via dual channels respectively, and set an input ratio of the geothermal water to the high-pressure nitrogen.

[0152] In step S33, the input ratio of the geothermal water to the high-pressure nitrogen is 3:2.5.

[0153] S34: Introduce the mixed geothermal water and high-pressure nitrogen into the wellhead of the three-dimensional well network in a pulsed manner.

[0154] In step S34, a duration of each pulse is 10 s to 15 s, and an interval time is 20 s to 35 s.

[0155] S35: After the set period of time, which is 1 h to 1.5 h in this embodiment, extract the geothermal water from the wells by using a water pump and return the geothermal water to the underground abandoned mine through pipelines, forming a closed-loop system.

[0156] S4: Extract gas from the wellhead by using negative pressure adsorption equipment.

[0157] In step S4, after further filtering with activated carbon, the adsorbed pure gas is stored in a gas storage tank.An Illustrative Embodiment

[0158] The present disclosure was applied to extract gas from a specific coal seam area, and the extraction data shown in Table 1 were recorded.TABLE 1Gas Extraction DataRatio ofGas FlowCoal SeamGeothermalFractureRate PermeabilityTemperatureWater toPropagationTime(m3 / d)(Darcy)(° C.)NitrogenStatusDay 15000.0013003:2.5ObviousDay 26000.00123003:2.5FurtherExtensionDay 37000.00153003:2.5ContinuedExtensionDay 48000.00183003:2.5More ObviousDay 59000.0023003:2.5StableExtension

[0159] As can be seen from Table 1, the method described in the present disclosure greatly improves the extraction rate.

[0160] Finally, it should be noted that the foregoing embodiments are only intended to describe, rather than to limit the technical solutions of the present disclosure. Although the present disclosure is described in detail with reference to the preferred embodiments, a person of ordinary skill in the art should understand that modifications or equivalent replacements may be made to the technical solutions of the present disclosure without departing from the spirit and scope of the technical solutions of the present disclosure.

Claims

1. A method for synergistic permeability enhancement and extraction of coal mine gas, comprising the following steps:S1: drilling a plurality of vertical wells and a plurality of horizontal wells in a coal seam area to form a three-dimensional well network, thereby forming primary fractures;S2: composite electric field fracturing by applying a high-voltage pulsed electric field for transient impact, and then introducing low-frequency electromagnetic waves for sustained action to form secondary fractures;S3: extracting geothermal water from an underground abandoned mine by using a high-pressure pump; heating the geothermal water to a set temperature using a heat pump, then mixing the geothermal water with high-pressure nitrogen, and introducing the mixed geothermal water and high-pressure nitrogen into a wellhead of the three-dimensional well network in a pulsed manner; after a set period of time, extracting and returning the geothermal water to the underground abandoned mine, thereby forming tertiary fractures; andS4: extracting gas from the wellhead by using negative pressure adsorption equipment.

2. The method for synergistic permeability enhancement and extraction of coal mine gas according to claim 1, wherein the step S1 comprises the following sub-steps:S11: geological exploration and planning: conducting geological exploration of the coal seam area using a seismic wave method, and determining an optimal vertical drilling spacing and depth as well as an optimal horizontal drilling spacing and depth based on exploration results, to ensure that the wells fully cover the coal seam;S12: vertical drilling: drilling vertically downward from a ground surface at selected locations by using rotary drilling equipment, to form a plurality of vertical wells, while ensuring that the vertical wells reach a predetermined depth of the coal seam; andS13: horizontal drilling: based on the vertical wells, drilling a plurality of horizontal wells laterally from bottom to top along the vertical wells by using horizontal directional drilling technology, thereby forming a three-dimensional well network.

3. The method for synergistic permeability enhancement and extraction of coal mine gas according to claim 2, wherein the step S11 comprises the following sub-steps:S111: preliminary investigation: collecting geological maps, mining history, and coal seam distribution data of the coal seam area, and conducting field reconnaissance to record topographic features of the coal seam area;S112: simultaneously installing seismometers and seismic source equipment in the coal seam area, and laying out survey lines based on preliminary investigation results;S113: activating a seismic source on the laid survey lines by using the seismic source equipment, and recording underground reflected wave signals through the seismometers;S114: after preprocessing the reflected wave signals, generating an underground structure image using inversion calculation, and determining a thickness D, a dip angle, and a depth H of the coal seam; andS115: based on the determined thickness, dip angle, and depth of the coal seam, determining a spacing and a depth of the vertical wells, and a spacing and an extension length of the horizontal wells;wherein a formula (1) for calculating the spacing of the vertical wells is as follows:Lv=(2×Dh×tan⁡(α2))2+d2;(1)in the formula (1), Lv represents a minimum spacing between two adjacent vertical wells, measured in meters; Dh represents an extension length of the horizontal well, measured in meters; α represents an angle of the horizontal well relative to a vertical direction; and d represents a diameter of the vertical well, measured in meters;a formula (2) for calculating the depth of the vertical wells is as follows:Dv=H+Δ⁢D;(2)in the formula (2), Dv represents a final depth of the vertical well, measured in meters; H represents the depth of the coal seam, measured in meters; and ΔD represents a reserved safety margin for protrusion, measured in meters;a formula (3) for calculating the spacing of the horizontal wells is as follows:Lh=(Dv-H)2+(2×W×tan⁡(β))2;(3)in the formula (3), Lh represents a minimum spacing between two adjacent horizontal wells, measured in meters; W represents a lateral distance of the coal seam between two adjacent horizontal wells; and β represents an inclination angle of the horizontal well;a formula (4) for calculating the extension length of the horizontal wells is as follows:Dh=Lmin+Δ⁢L;(4)in the formula (4), Lmin represents a minimum effective extension length, measured in meters; and ΔL represents a safety extension margin, measured in meters;a formula (5) for calculating the minimum effective extension length Lmin is as follows:Lmin=(Qk1×A)2+4×D2-2⁢D;(5)in the formula (5), Q represents an expected gas flow rate for extraction, measured in m3 / d; k1 represents a permeability of the coal seam, measured in Darcy; A represents a cross-sectional area of the horizontal well, measured in m2; and D represents the thickness of the coal seam, measured in meters.

4. The method for synergistic permeability enhancement and extraction of coal mine gas according to claim 3, wherein in the step S12, a verticality check is performed every 5 to 10 meters of drilling to ensure that a vertical deviation of the drilling is controlled within a set range; andin the step S13, a trajectory of the horizontal well is guided with the aid of a Measurement While Drilling (MWD) system.

5. The method for synergistic permeability enhancement and extraction of coal mine gas according to claim 4, wherein the step S2 comprises the following sub-steps:S21: transient impact with a high-voltage pulsed electric field: installing a high-voltage pulse generator at the wellhead of the three-dimensional well network, connecting the high-voltage pulse generator to electrodes within the wells, and after setting initial high-voltage pulse parameters, starting the high-voltage pulse generator to apply a transient high-voltage electric field to the coal seam, causing cracks to form in the coal seam, thereby laying a foundation for the secondary fractures;S22: removing the high-voltage pulse generator and the electrodes; andS23: sustaining action by low-frequency electromagnetic waves: installing a low-frequency electromagnetic wave transmitter at the wellhead of the three-dimensional well network, connecting the low-frequency electromagnetic wave transmitter to an antenna within the wells, and after setting initial low-frequency electromagnetic wave parameters, starting the low-frequency electromagnetic wave transmitter to apply sustained action to the formed cracks, thereby forming the secondary fractures.

6. The method for synergistic permeability enhancement and extraction of coal mine gas according to claim 5, wherein in the step S21, the initial high-voltage pulse parameters comprise voltage amplitude V, pulse width Tp, and pulse frequency f, wherein a formula (6) for calculating the voltage amplitude V is as follows:V=k2×R×D;(6)in the formula (6), k2 represents an empirical coefficient; R represents a resistivity of the coal seam, measured in Ω / m;a formula (7) for calculating the pulse width Tp is as follows:Tp=qf;(7)in the formula (7), q represents a constant term; and f represents a pulse frequency, measured in Hz;a formula (8) for calculating the pulse frequency f is as follows:f=kR×C;(8)in the formula (8), C represents a capacitance value of the high-voltage pulse generator;in the step S23, the initial low-frequency electromagnetic wave parameters comprise transmission power P, operating frequency fLF, and action time t, wherein a formula (9) for calculating the transmission power P is as follows:P=k×σ×Dh;(9)in the formula (9), σ represents an electrical conductivity of the coal seam, measured in S / m;a formula (10) for calculating the operating frequency fLF is as follows:fL⁢F=k×cλ;(10)in the formula (10), c represents the speed of light, measured in m / s; λ represents a wavelength, measured in meters, and λ=k′×D, wherein k′ is a proportionality coefficient;a formula (11) for calculating the action time t is as follows:t=Dhv;(11)in the formula (11), v represents a propagation speed of electromagnetic waves in the coal seam, measured in m / s.

7. The method for synergistic permeability enhancement and extraction of coal mine gas according to claim 6, wherein in the step S21 and the step S23, a microseismic monitoring system and electromagnetic wave reflection detection equipment are installed at the wellhead of the three-dimensional well network, wherein the microseismic monitoring system is configured to detect vibrations generated during fracture propagation, and the electromagnetic wave reflection detection equipment is configured to collect electromagnetic wave reflection data; based on vibration data and the electromagnetic wave reflection data, an energy difference ΔE, a time difference ΔT, a current difference ΔI, and a crack length difference ΔL are extracted, and the initial high-voltage pulse parameters and the initial low-frequency electromagnetic wave parameters are adjusted through cyclic iteration;wherein a formula (12) for calculating an adjusted voltage amplitude V′ is as follows:V′=V+kv×Δ⁢E;(12)in the formula (12), kv represents a voltage adjustment coefficient;a formula (13) for calculating an adjusted voltage amplitude pulse widthTp′is as follows:Tp′=Tp+kt×Δ⁢T(13)in the formula (13), kt represents a time adjustment coefficient;a formula (14) for calculating an adjusted pulse frequency f′ is as follows:f′=f+kf×Δ⁢L;(14)in the formula (14), kf represents a frequency adjustment coefficient;a formula (15) for calculating an adjusted transmission power P′ is as follows:P′=P+kP×Δ⁢I;(15)in the formula (15), kP represents a power adjustment coefficient;a formula (16) for calculating an adjusted operating frequencyfL⁢F′is as follows:fL⁢F′=fL⁢F+kf×Δ⁢L;(16)a formula (17) for calculating an adjusted action time t′ is as follows:t′=t+kt×Δ⁢T.(17)8. The method for synergistic permeability enhancement and extraction of coal mine gas according to claim 7, wherein the step S3 comprises the following sub-steps:S31: extracting geothermal water from the underground abandoned mine by using the high-pressure pump;S32: feeding the extracted geothermal water into the heat pump for heating to a set temperature, and outputting the heated geothermal water to a geothermal water pipeline;S33: connecting the geothermal water pipeline and a high-pressure nitrogen pipeline to the wellhead of the three-dimensional well network via dual channels respectively, and setting an input ratio of the geothermal water to the high-pressure nitrogen;S34: introducing the mixed geothermal water and high-pressure nitrogen into the wellhead of the three-dimensional well network in a pulsed manner; andS35: after the set period of time, extracting the geothermal water from the wells by using a water pump and returning the geothermal water to the underground abandoned mine through pipelines, forming a closed-loop system.

9. The method for synergistic permeability enhancement and extraction of coal mine gas according to claim 8, wherein in the step S32, the set temperature is 200° C. to 350° C.;in the step S33, the input ratio of the geothermal water to the high-pressure nitrogen is 3:2.5; andin the step S34, a duration of each pulse is 10 s to 15 s, and an interval time is 20 s to 35 s.

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