Method for enhancing coalbed methane recovery ratio by external magnetic field

Abstract

The invention discloses a method for enhancing coalbed methane recovery efficiency by an external magnetic field, which comprises the following steps of: arranging a plurality of electrified coils or magnetic field generators connected with a power supply in a coalbed to generate an alternating magnetic field in the coalbed; cH4 molecules show diamagnetism under the action of a magnetic field, a repulsive effect is generated in a coal seam, and the adsorption effect of CH4 and a coal body is weakened; and on the other hand, as the coal body contains ferromagnetic or paramagnetic minerals, the alternating magnetic field excites the annular eddy current in the coal body through electromagnetic induction so as to generate Joule heat. Therefore, coal matrix expansion and microfracture expansion can be induced by temperature rise of the coal body, the permeability of the coal body is further improved, and more channels are provided for CH4 desorption and flowing; the clean, efficient and environment-friendly coal bed gas exploitation method is provided on the basis of superposition of multiple production increasing mechanisms, and a new technical approach is provided for exploration and development of unconventional oil and gas resources.

Description

Technical Field

[0001] The present invention relates to a method for enhancing the recovery rate of coalbed methane by applying an external magnetic field, and belongs to the field of exploration and development of unconventional oil and gas resources. Background Art

[0003] CH4 is mainly stored in coalbed methane reservoirs in three forms: adsorbed state, free state and dissolved state. Free CH4 exists in the fractures and pores of coal seams, dissolved CH4 is dissolved in coal seam water, and adsorbed CH4 is attached to the surface of coal matrix through physical adsorption, which is the main form of coalbed methane, with a volume ratio of more than 85%. Therefore, effectively promoting the desorption of adsorbed CH4 has become the key to coalbed methane exploitation. However, the geological conditions of most coalbed methane reservoirs in China are relatively complex, mainly showing the characteristics of low porosity and low permeability. Low porosity means limited gas storage space in coal reservoirs, while low permeability results in poor gas flow ability in coal seams. Such "double low" characteristics seriously restrict the desorption, diffusion and flow processes of CH4, and limit the implementation effect of traditional exploitation methods. In addition, the occurrence state of coalbed methane is also affected by many factors such as temperature, pressure, coal rank and moisture content, further increasing the complexity of the exploitation process. At present, hydraulic fracturing technology is the main means of commercial production of coalbed methane. This technology transforms coalbed methane reservoirs by pumping high-pressure fracturing fluid (usually a mixture of water, sand grains and chemical additives). However, during the hydraulic fracturing process, when the fracturing fluid leaks or is not properly treated, it may pollute groundwater, and high-pressure injection of fluid may induce microseismicity. In summary, there is an urgent need to develop efficient and environmentally friendly candidate technologies to promote CH4 desorption and increase the production of coalbed methane. Summary of the Invention

[0004] In view of the "double low" (low porosity and low permeability) problems faced in the existing coalbed methane exploitation, as well as the risks of groundwater pollution and microseismicity that may be caused by traditional hydraulic fracturing methods, the present invention provides a method for enhancing the recovery rate of coalbed methane by applying an external magnetic field. By introducing an external magnetic field into the coal body, using the diamagnetic properties of CH4 molecules and the eddy current heating of the coal body generated by electromagnetic induction, the superposition of multiple production enhancement mechanisms is realized, which is specifically manifested as: the repulsive effect weakens the adsorption, the temperature rise promotes desorption, and the expansion of microfractures improves the reservoir permeability, thereby effectively increasing the recovery rate of coalbed methane.

[0005] The object of the present invention is achieved by the following technical solutions: 1. Magnetic field generation: A plurality of energized coils are arranged inside the coal seam. To ensure that the magnetic field covers the target coal seam, a coil is arranged at an interval of 1-5 m, and is connected to a power supply to form an alternating magnetic field inside the coal seam, or directly generated by a magnetic field generator. The energized coils are made of materials with good electrical conductivity, high temperature resistance and corrosion resistance (selected from copper, aluminum, copper-nickel alloy, aluminum alloy); When using an energized coil to generate a magnetic field, the current intensity and frequency can be adjusted to control the field strength and spatial distribution range of the alternating magnetic field. The current intensity range is 10-1000 A, and the frequency range is 50-1000 Hz. When using a magnetic field generator to generate a magnetic field, the magnetic field frequency is 50-1000 Hz, and the magnetic field intensity is 0.1-5T. In addition, permanent magnets can be used as auxiliary materials in magnetic circuit design. Permanent magnets are set at the coil intervals to assist in strengthening the alternating magnetic field, making the magnetization effect of ferromagnetic or paramagnetic minerals more significant, thereby further enhancing the coal heating effect and CH4 desorption process. Permanent magnets can be made of neodymium iron boron materials, aluminum nickel cobalt materials, and samarium cobalt materials.

[0006] 2. CH4 gas production evaluation: In conjunction with the surface or underground production system, key indicators such as CH4 flow, composition and pressure are monitored online or offline to achieve a comprehensive evaluation of the coalbed methane production increase effect under the action of the magnetic field.

[0007] 3. Used in conjunction with conventional coalbed methane extraction technology: While maintaining low-pressure drainage or conventional well control, an external magnetic field is applied. Magnetic field and eddy current heating can also be further introduced on the basis of the original hydraulic fracturing operation to reduce the amount of fracturing fluid, reduce the risk of groundwater contamination and increase coalbed methane production.

[0008] The technical principles of the present invention are as follows: 1. CH4 antimagnetism promotes desorption CH4 is an isoelectronic molecule with the total number of electrons equal to the total number of atoms. It does not contain lone pairs of electrons. The carbon atom forms four covalent bonds with four hydrogen atoms. All electrons appear in pairs, and there are no unpaired electrons. Therefore, in a magnetic field environment, CH4 will produce an induced magnetic moment opposite to the direction of the magnetic field, exhibiting anti-magnetism.

[0009] Under the action of the magnetic field, the induced magnetic moment of ferromagnetic or paramagnetic minerals (such as magnetite and pyrite) in the coal body is consistent with the direction of the magnetic field. CH4, as a diamagnetic gas molecule, has an induced magnetic moment in the opposite direction to the magnetic moment of the minerals in the coal body, thereby producing a repulsive effect with CH4 at multiple sites in the coal body, microscopically weakening the adsorption of CH4 on the coal matrix surface, accelerating the transformation of adsorbed CH4 to the free state, and prompting CH4 to quickly migrate to the drainage wellbore along the channel with relatively strong magnetic repulsion.

[0010] On the other hand, an alternating magnetic field is generated inside the coal seam by an energized coil or a magnetic field generator, and its direction and intensity change periodically with time. According to Faraday's law of electromagnetic induction, the alternating magnetic field will induce eddy currents (closed circular currents) in the coal body, and its direction is opposite to the direction of change of the alternating magnetic field.

[0011] 2. Coal iron / paramagnetic mineral induced eddy current heating Due to the presence of ferromagnetic or paramagnetic minerals in the coal seam, they can serve as the main carriers of eddy currents under the action of an alternating magnetic field. An alternating magnetic field is generated inside the coal seam by an energized coil or a magnetic field generator, which can induce circular eddy currents in the coal body through electromagnetic induction. When the eddy currents flow in the coal body, since the coal body and its contained minerals have a certain resistance, electrical energy is converted into heat energy to generate Joule heat. As the eddy currents continue to be generated and heat accumulates, the temperature of the coal body gradually rises. The temperature increase of the coal body under the action of eddy currents can not only reduce the desorption potential barrier of CH4, transform it from the adsorbed state to the free state, and enter the pore and fracture spaces of the coal seam; the eddy current effect can also induce thermal expansion of the coal matrix and further expand the microfractures in the coal body, further increasing the coal seam permeability and strengthening the CH4 flow performance, thereby providing more channels for CH4 desorption and flow ( Figure 1 ).

[0012] In addition, the diamagnetism of CH4 molecules has a magnetic difference from the surrounding medium, causing additional repulsive forces at multiple sites around the iron / paramagnetic minerals, accelerating the desorption and migration of CH4 at the microscale.

[0013] Advantages of the present invention: (1) Accelerate CH4 desorption: By leveraging the diamagnetic characteristics of CH4 and the temperature increase effect generated by eddy current heating, the adsorption force of the coal body on CH4 is significantly weakened, promoting the easier transformation of adsorbed CH4 into the free state; (2) Improve the permeability of the coal body: Eddy current heating not only raises the temperature of the coal body but also causes thermal expansion of the coal matrix and expansion of microfractures, increasing the porosity and fracture density and enhancing the permeability of the coal seam, providing more channels for subsequent CH4 flow and drainage; (3) Magnetic-thermal synergy to accelerate CH4 migration at the microscale: Under the condition of magnetic difference between the iron / paramagnetic minerals in the coal seam and CH4 molecules, the magnetic field forms repulsive forces on CH4 at multiple sites in the coal body, combined with the thermal effect to further improve the migration efficiency of CH4 at the microscale, forming an efficient drainage channel; (4) Reduce environmental risks: Achieving production increase through the method of applying an external magnetic field and eddy current heating, without the need to use a large amount of fracturing fluid, significantly reducing the risk of water resource consumption and groundwater pollution; at the same time, it also reduces the potential microseismic hazards caused by high-pressure liquid injection, being more environmentally friendly and safe. Description of the Drawings

[0014] Figure 1 It is a schematic diagram of the principle of electromagnetic induction eddy current heating of iron / paramagnetic minerals in the coal seam; Figure 2 It is a schematic diagram of the layout of multiple coils (a) and the layout assisted by permanent magnets (b) inside the coal seam. Detailed Embodiments

[0015] The present invention will be further described below in conjunction with specific embodiments. Implementations are carried out on the premise of the technical solution of the invention, and detailed implementation manners and specific operation processes are given. However, the protection scope of the present invention is not limited to the following embodiments. Example 1

[0016] For a target block with a coal seam depth of about 600 m, a medium-high coal rank, and a coal seam thickness of about 8 m, which has low porosity and low permeability, a plurality of coils are arranged around the underground wellbore at intervals of 5 m, and the fixing method adopts heat-resistant and corrosion-resistant alloy bolts and a frame structure. After the coils are arranged, they are connected to an adjustable AC power supply through a ground power supply system, and the voltage level can be switched between 380 - 660 V. Before the formal power-on, underground temperature sensors and ground gas production monitoring devices are deployed. The temperature sensors are distributed at least in 3 layers along the wellbore up and down, and 2 - 3 measuring points are arranged in each layer to monitor the temperature rise of the coal body in real time; the gas production monitoring device includes a flowmeter, a pressure gauge, and a component analyzer to monitor the CH4 flow rate, formation pressure, and associated components (such as CO2 and N2, etc.) in real time or periodically ( Figure 2 a).

[0017] During the power-on stage, the AC frequency is selected to be 50 Hz, and the current intensity is about 100 A to ensure the formation of an alternating magnetic field in the annular coil area. According to Faraday's law of electromagnetic induction, this alternating magnetic field will induce a closed eddy current ring in the coal body. When the eddy current flows, Joule heat is generated due to the resistance of the coal body, promoting the gradual increase of the coal body temperature. The measured magnetic field intensity near the test coil is about 1.2 T. After continuous power-on for 12 hours, the temperature of the coal body around the wellbore rises from the original normal temperature to 45 °C; when the power-on continues to 72 hours, the coal body temperature further rises to 80 °C. According to the comparison of the measured data underground, during the power-on heating period, the CH4 concentration in the coal body gradually decreases with time, while the CH4 flow rate detected at the wellhead continues to increase. When this well did not adopt the external magnetic field scheme, the average daily CH4 production was about 1600 m 3 / d; after continuously applying the magnetic field and eddy current heating for 72 hours, the average daily production once increased to 2200 m 3 / d, and during the subsequent one-week drainage process, the production continuously remained above 2000 m 3 / d, which is more than 25% higher than the original stable level. Example 2

[0018] In this embodiment, an enhanced mining experiment with an applied magnetic field was carried out on a coal seam with a well depth of about 1000 m and a high coal rank; the coal seam thickness is about 6 m, the coal seam has a higher organic matter maturity, its CH4 adsorption capacity is strong, but the pore permeability performance is not ideal. In this embodiment, a magnetic field generator is used to generate a magnetic field, a frequency of 100 Hz is selected, and an alternating magnetic field with periodic changes is formed around the wellbore. According to the downhole sensor and numerical simulation results, the average magnetic field intensity at the target coal seam can reach about 2.0 T.

[0019] After 96 hours of continuous power-on on site, temperature sensors installed at different depths around the wellbore show that the coal body temperature rises from the initial 28 °C to about 90 °C. Affected by thermal expansion, the microfractures of the coal body are fully expanded, and the micropore network commonly found in the high-rank coal matrix is also connected with the fractures, significantly improving the seepage capacity. At the same time, the applied magnetic field exerts a repulsive force on CH4 molecules at the microscale, accelerating the desorption of CH4. According to the flow monitoring of the surface drainage system, when this well did not adopt the applied magnetic field scheme before, the daily average output was about 1500 m 3 / d; after nearly 4 days of the magnetic-thermal synergistic effect in this embodiment, the peak daily output reaches 2450 m 3 / d, and the average growth rate exceeds 60%. And for some time after the power-on is stopped, the coal seam still maintains a high output level, indicating that after being affected by thermal expansion and microfracture expansion, the overall coal seam system is "activated" to form a long-term effective reservoir channel. Example 3

[0020] An enhanced mining experiment with an applied magnetic field was carried out on a section with a burial depth of about 1200 m, a medium coal rank, and a coal seam thickness of 10 m. The water saturation of the coal seam in this section is relatively high and the pore permeability performance is average, and clay minerals and ferromagnetic minerals coexist in some areas. In terms of coil material selection, in order to balance conductivity and corrosion resistance, this embodiment uses aluminum alloy wire, arranges multiple coils around the wellbore at an interval of 2 m, and sets neodymium iron boron permanent magnets at the coil intervals to play an auxiliary role in focusing the magnetic field. The ground power-on uses a 660 V AC power supply, and the basic frequency is set to 50 Hz ( Figure 2 b).

[0021] In this embodiment, numerical simulation was carried out in advance. The simulation results show that when a current of 300 A is passed through the spiral coil and the frequency is 50 Hz, the average magnetic field intensity within 10 m around the wellbore is about 2.5 T, and the coal body temperature can be increased from the normal temperature of 26 °C to about 80 - 100 °C within 48 hours, thus causing significant thermal expansion and the release of adsorbed CH4. According to the measured data of the temperature probes installed at various depths in the well, after 48 hours of continuous power-on, the measured average value of the coal body temperature is 95 °C; at this time, according to the data of the surface drainage system, the daily CH4 gas production volume rises from the initial 3700 m 3 / d to 5100 m3 / d. After analyzing the coal core samples, it was found that the porosity of the coal samples in the heated area increased by 10 - 15% compared with that before heating. At this time, CH4 can be more easily desorbed from the coal matrix surface and enter the fracture or pore network, and then flow to the drainage system of the wellbore. In addition, the coal seams with high water content are also promoted to drain and depressurize through the eddy current heating effect, thus preventing the adverse effects of high water blockage on CH4 desorption. Finally, after about 4 days of continuous power-on in this embodiment, the daily average output of CH4 is above 5000 m 3 / d. Example 4

[0022] For a coalbed methane well that has been flooded and has a high pressure (the information of the coalbed methane reservoir where it is located: the burial depth is about 800 m, the coal rank is medium - low metamorphic degree, and the coal seam thickness is 9 m), an external magnetic field enhanced mining test is carried out. There is a flooding phenomenon in the well during the conventional drainage process, and the water saturation of the coal body is large, resulting in the difficulty of effective desorption of CH4. Try to adopt the method of combining the external magnetic field strengthening scheme proposed by the present invention with conventional drainage to relieve flooding and increase production. First, a plurality of coils are arranged at intervals of 1 m around the wellbore, and the wire material is selected as tensile - resistant aluminum alloy. To adapt to the relatively soft and water - rich environment underground, a high - strength protective sleeve is used outside the coil to separate it from the well wall to avoid being affected by well - wall collapse or other formation deformations; secondly, the current intensity is set to be dynamically adjustable between 100 - 500 A through a ground - adjustable power supply, and the frequency range is 30 - 100 Hz. It is tracked and regulated according to the on - site temperature and pressure data to obtain the best coupling effect of heating and drainage depressurization.

[0023] First, carry out conventional drainage for about 48 hours to lower the formation water level, reducing the blockage of the pores and fractures of the coal body by water. Then, keep draining under the condition of continuous power - on to form a synergistic process of "gradual decline of water level - heating of coal body - improvement of CH4 desorption". According to the records of downhole sensors, after the action of the external magnetic field, the temperature of the coal body rises from about 24 °C at room temperature to 85 °C within 48 hours, and can reach 90 °C in local areas, which is sufficient to weaken the physical adsorption force between the coal matrix and CH4, and also plays a certain role in expanding the pore and fracture system. The ground drainage system records that the CH4 production increases from the initial 900 m 3 / d to 1800 m 3 / d, and exceeds 2000 m 3 / d at the peak.

[0024] Subsequent data shows that the retardation of water flooding on the desorption process is significantly alleviated under the technical solution of the present invention: on the one hand, eddy current heating raises the local temperature, effectively reducing the solubility of water in CH4 and promoting gas-water separation; on the other hand, due to diamagnetism, CH4 forms a repulsive force with paramagnetic and ferromagnetic minerals in the coal body, enabling it to move more smoothly to the surrounding fractures and wellbores, reducing the hindrance caused by the water film or capillary force. The drainage process remains relatively stable, without large-scale formation water inrush or ground subsidence. The results of this embodiment show that waterlogged coal seams can still be enhanced in desorption by applying an external magnetic field. After continuous monitoring for one week, the coal body temperature gradually drops back to the intermediate level of 40-50 °C, but during this process, the coal body drainage volume does not drop sharply and still remains at a relatively high level of over 1500 m 3 / d, indicating that the newly formed or expanded fracture network formed by the magnetic-thermal synergy continues to have a positive effect on the release and migration of CH4. Thus, this embodiment fully demonstrates that in waterlogged coal seams with poor fracturing effects, the technology of the present invention still has the comprehensive advantages of rapid temperature rise, repulsive desorption, and enhanced permeability, and has stronger controllability in terms of environmental and safety risks.

[0025] In summary, the above embodiments respectively perform external magnetic field enhancement for production increase on coal seams under different well depths, different coal ranks, different coal body water content characteristics, and different construction process conditions. The results all show that the present invention can achieve efficient desorption of adsorbed CH4 and wellbore drainage while reducing the dependence on traditional hydraulic fracturing and lowering environmental and safety risks. The key lies in that the external magnetic field induces eddy current heating through electromagnetic induction, raising the temperature of the coal body and causing microfracture expansion; on the other hand, the diamagnetism of CH4 molecules can exacerbate the repulsion between them and paramagnetic or ferromagnetic minerals, thus accelerating desorption and improving flow efficiency at the microscopic level. This magnetic-thermal synergy mechanism is particularly effective for "dual-low" coal seams, can significantly enhance the recovery rate of coalbed methane resources, and takes into account green, safety, and economy, providing a new technical option for unconventional natural gas development. The above examples are only typical cases. In actual applications, the coil structure, current parameters, and drainage plan can be flexibly adjusted according to geological conditions, equipment capabilities, and operation requirements, and can also be combined with some conventional production increase means to play a significant role in a wider range of coalbed methane reservoirs.

Claims

1. A method for enhancing the recovery rate of coalbed methane by applying an external magnetic field, characterized in that: By arranging multiple energized coils or magnetic field generators connected to a power source inside the coal seam, an alternating magnetic field is generated inside the coal seam; the alternating magnetic field induces eddy currents in the coal seam, and Joule heat is generated during the flow of the eddy currents, increasing the temperature of the coal body. The heating effect weakens the adsorption of the coal body to CH4 and induces coal matrix swelling and microcrack expansion. At the same time, the diamagnetic characteristics of CH4 molecules are used to form a repulsive force around ferromagnetic or paramagnetic minerals, accelerating the transformation of CH4 from the adsorbed state to the free state, and then migrating to the collection channel, improving the recovery rate of coalbed methane.

2. The method for enhancing the recovery rate of coalbed methane by an externally applied magnetic field according to claim 1, wherein: The energized coils are made of materials with good electrical conductivity, high temperature resistance and corrosion resistance, and the materials are selected from copper, aluminum, copper-nickel alloy, and aluminum alloy.

3. The method for enhancing the recovery rate of coalbed methane by applying an external magnetic field according to claim 1, wherein: The current intensity of the power source is 10 - 1000 A, and the frequency is 50 - 1000 Hz.

4. The method for enhancing the recovery rate of coalbed methane by applying an external magnetic field according to claim 1, wherein: Different energized coils are connected to the power source in series or parallel to meet the production increase requirements.

5. The method for enhancing coalbed methane recovery by an externally applied magnetic field according to any one of claims 1-4, characterized in that: Permanent magnets are arranged at the intervals of the energized coils inside the coal seam or in the area adjacent to the wellbore. The permanent magnets are used to focus the alternating magnetic field or promote the desorption and flow of CH4, thereby further enhancing the coal body heating effect and the CH4 desorption and exhaust process.

6. The method for enhancing the recovery rate of coalbed methane by an externally applied magnetic field according to claim 1, characterized in that: The external magnetic field strengthening method is used in combination with the conventional coalbed methane extraction technology.

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