Low-permeability coal seam acid-alkali synergistic penetration enhanced coalbed methane extraction method

Abstract

The present application relates to coal seam permeability enhancement technology in underground coal mine, specifically to a low-permeability coal seam acid-alkali synergistic permeability enhancement and coalbed methane extraction method, comprising the following steps: S100, determining the minerals in the coal sample and performing macroscopic mechanical property characterization experiment on the coal sample; S200, setting the permeability enhancement sequence of the target coal seam according to the test results of step S100; S300, processing the coal sample according to the permeability enhancement sequence determined in step S200, and then performing experiment on the processed coal sample; the experiment includes x-ray diffraction experiment, Fourier infrared spectrum analysis experiment, scanning electron microscope experiment, uniaxial compression experiment and seepage experiment; S400, comparing the experimental results in step S300 with the experimental results in step S100, and determining the optimal compatibility of the acid permeability enhancement liquid and the alkali permeability enhancement liquid respectively; S500, extracting coalbed methane according to the optimal compatibility of the acid permeability enhancement liquid and the alkali permeability enhancement liquid. The present application can greatly improve the coal seam permeability enhancement effect and effectively reduce the damage of the permeability enhancement liquid to the coal reservoir.

Description

Technical Field

[0001] This invention relates to the field of coal seam permeability enhancement technology in underground coal mines, specifically a method for enhancing coalbed methane extraction by acid-base synergistic permeability enhancement in low-permeability coal seams. Background Technology

[0002] my country possesses abundant coalbed methane (CBM) resources with broad development prospects. The reserves of CBM resources at depths shallower than 2000m are approximately 30.1 trillion m³, with 12.5 trillion m³ being recoverable under current technological conditions. However, the "microporosity, strong adsorption, and low permeability" characteristics of Chinese coal seams severely restrict CBM extraction. In recent years, with the continuous increase in mining depth, the characteristics of high ground stress, high gas pressure, high gas content, and low permeability in deep coal seams have become more pronounced, making it difficult to extract the large amounts of CBM resources present in the coal seams. This also poses a serious threat to the safe and efficient production of coal mines. Therefore, efficient extraction of CBM from low-permeability coal seams has always been a technological bottleneck restricting the industrialization of CBM in my country and the prevention and control of coal mine gas disasters.

[0003] Coal reservoirs are typical pore-fracture type reservoirs, containing abundant microstructures such as pores and fractures, forming a unique dual-pore structure. Pores are the primary sites for the accumulation of adsorbed (approximately 90%) and free coalbed methane, and their storage capacity is related to the degree of pore development and pore structure within the coal seam. Fractures are the main channels for coalbed methane migration, and their development characteristics directly affect the permeability of the coal seam. The coalbed methane extraction process is essentially a "desorption-diffusion-seepage" process within the coal structure. A large amount of coalbed methane adsorbed in the pores must first be desorbed and diffused into the fractures before it can seep and migrate to the extraction well through the fracture structure. Therefore, to achieve efficient coalbed methane extraction, it is necessary to both modify the coal reservoir to increase its permeability and desorb the adsorbed coalbed methane to form a free state, thereby increasing the amount of coalbed methane desorbed. Currently, in the field of permeability enhancement technology for low-permeability coal seams, the most widely used methods are to mechanically fracture the coal body to increase the pore structure and thus improve the permeability of the coal seam. When dealing with mineral-rich coal seams, domestic and international scholars have proposed injecting acid into the coal seam using hydraulic fracturing technology. The acid can effectively dissolve the minerals filling the pores and fractures in the coal, further improving the permeability. However, these methods have very limited effect on the gas adsorption properties of the coal, and the low viscosity of the acid leads to significant filtration loss after injection, resulting in waste and potential equipment corrosion during subsequent mining. Furthermore, different coal seams exhibit significant differences in pore structure, mechanical properties, mineral composition and content, and adsorption characteristics. This means that using the same fracturing technology and fracturing fluid to enhance permeability in different coal seams often yields vastly different results. Therefore, it is urgently needed to develop a method for enhancing coalbed methane extraction in low-permeability coal seams through a synergistic acid-base permeability enhancement approach, addressing the current problems of poor permeability enhancement effects, lack of specificity, and low coalbed methane extraction efficiency. Summary of the Invention

[0004] To address the aforementioned issues, a method for enhancing coalbed methane extraction in low-permeability coal seams using a synergistic acid-alkali permeability enhancement approach is proposed. This method involves preparing acidic and alkaline permeability enhancement solutions, and then performing two permeability enhancement processes, either acid-first followed by alkaline or vice versa, depending on the coal seam conditions. This approach, building upon hydraulic fracturing, dissolves the minerals filling the coal pores, further increasing the coal seam permeability and reducing the coal's gas adsorption capacity. Simultaneously, it reduces the filtration loss of the enhancement solution and the corrosion of equipment during subsequent coal mining, achieving clean and efficient coalbed methane extraction.

[0005] This invention adopts the following technical solution: a method for enhancing coalbed methane extraction by acid-alkali synergistic permeability enhancement in low-permeability coal seams, comprising the following steps: S100: sampling and sealing the coal seam to be enhanced, measuring the minerals in the coal sample, and simultaneously conducting macroscopic mechanical property characterization experiments on the coal sample; S200: based on the test results of step S100, setting the permeability enhancement sequence for the target coal seam, the permeability enhancement sequence being the order in which acidic and alkaline permeability enhancement solutions are injected into the coal seam, including two cases: acid first then alkaline, and alkaline first then acid; S 300: The coal sample is processed according to the permeability enhancement sequence determined in step S200, and then experiments are conducted on the processed coal sample; the experiments include X-ray diffraction, Fourier transform infrared spectroscopy, electron microscopy, uniaxial compression, and seepage experiments; S400: The experimental results in step S300 are compared with those in step S100 to determine the optimal compatibility of the acidic and alkaline permeability enhancement solutions; S500: Coalbed methane is extracted according to the optimally compatible acidic and alkaline permeability enhancement solutions.

[0006] In some embodiments, the tests on the coal sample in step S100 include industrial analysis, mineral composition determination, coal macromolecular structure determination, pore and fracture structure analysis, and permeability measurement. Industrial analysis determines the ash content in the coal, mineral composition determination can determine the types and relative contents of minerals in the coal, coal macromolecular structure determination determines the types of functional groups in the primary coal sample, pore and fracture structure analysis reflects the distribution of pores and fractures in the primary coal sample, and permeability measurement reflects the air permeability of the pore and fracture structure in the primary coal sample.

[0007] In some embodiments, in step S200, When the minerals in the coal seam are mainly silicon-containing minerals, the permeability enhancement sequence of first alkali and then acid is adopted. First, alkaline permeability enhancement solution is injected to initially dissolve the silicon-containing minerals or convert them into intermediate products that are easily soluble in acid. At the same time, the content of oxygen-containing functional groups in the coal is increased, the gas adsorption of the coal seam is reduced, and gas desorption is promoted. After the initial permeability enhancement is completed, acidic permeability enhancement solution is injected to completely dissolve the minerals in the coal. When the minerals in the coal seam are mainly carbonate minerals, an acid-to-alkali permeability enhancement sequence is adopted. First, acidic permeability enhancement solution is injected to dissolve the minerals in the coal to complete the initial permeability enhancement. Then, alkaline permeability enhancement solution is injected to neutralize the residual acidic permeability enhancement solution, thereby reducing the corrosive impact on equipment during subsequent coal mining. Furthermore, by increasing the content of oxygen-containing functional groups in the coal, the gas adsorption capacity of the coal is reduced, promoting gas desorption and completing the secondary permeability enhancement.

[0008] In some embodiments, in step S300, in the characterization and analysis section of the experimental scheme, after the coal sample is treated with different concentrations of permeability-enhancing liquid set in step S200, the changes in minerals in the coal sample are characterized by X-ray diffraction, the changes in oxygen-containing functional groups in the coal sample are analyzed by Fourier transform infrared spectroscopy, the changes in micropores and cracks in the coal sample are observed directly by electron microscopy, the changes in mechanical properties of the coal sample are analyzed by uniaxial compression, and the changes in gas permeation characteristics of the coal sample are analyzed by permeation.

[0009] In some embodiments, in step S400, by comparing the experimental results of coal samples before and after treatment with different concentrations of permeation-enhancing liquid, the relationship between the concentration of permeation-enhancing liquid and various characteristic parameters of coal samples is determined from the mineral composition and content, porosity, permeability, adsorption and desorption amount, uniaxial compressive strength and functional group changes of coal before and after permeation-enhancing liquid modification, and the optimal permeation-enhancing liquid formulation scheme is selected.

[0010] In some embodiments, the specific process of step S500 is as follows: S501: Drill a horizontal docking well and a vertical drainage well separately. The horizontal docking well is drilled to the roof strata of the coal seam and then horizontally drilled within a range of 0.5m to 1.5m above the coal seam until it is connected to the vertical drainage well, so that the horizontal docking well and the vertical drainage well form a U-shaped drainage well system, and a surface drainage system is established near the vertical drainage well. S502: Directional perforation is performed through a horizontal docking well to the target coal seam below, establishing a fluid channel between the wellbore and the target coal seam; the initial permeability enhancement fluid and proppant are injected into the target coal seam in stages according to the pre-set permeability enhancement sequence to induce fracturing and enhance permeability; pressure holding and well shut-in are performed to allow the permeability enhancement fluid to fully penetrate the target coal seam, dissolve the minerals in the coal seam, and modify its pore and fracture structure; after pressure holding and well shut-in are completed, gas is returned and produced through the surface drainage system; S503: When the gas production drops to 60% of the average gas production, inject a permeability-enhancing fluid into the target coal seam under high pressure according to step S502 to achieve secondary permeability enhancement of the target coal seam; perform pressure-maintaining well shut-in to further dissolve the minerals in the coal seam and modify its pore and fracture structure; after pressure-maintaining well shut-in, perform return drainage and gas production through the surface drainage system.

[0011] Compared with the prior art, the present invention has the following beneficial effects: 1. Based on actual coal seam conditions, this invention selectively injects acidic and alkaline permeability-enhancing solutions alternately into low-permeability coal seams. This achieves both physical permeability enhancement by creating macroscopic fractures in the coal seam through hydraulic fracturing and chemical permeability enhancement by fully dissolving the mineral components in the coal. Simultaneously, the permeability-enhancing solutions can reduce the adsorption of coalbed methane by coal and increase the desorption of coalbed methane, thereby achieving multi-directional synergistic permeability enhancement and efficient extraction of coalbed methane from low-permeability coal seams.

[0012] 2. This invention modifies the permeability of coal reservoirs through alternating acid-base fracturing, which not only greatly improves the permeability enhancement effect of coal seams, but also effectively reduces the damage of permeation fluid to coal reservoirs, reduces the corrosion of equipment during subsequent coal mining, and is conducive to the clean and efficient mining of coalbed methane and subsequent safe production. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of a method for enhancing coalbed methane extraction from low-permeability coal seams using a combination of acid and alkali synergistic permeability enhancement according to the present invention. Figure 2 This is a flowchart illustrating the implementation of a method for enhancing coalbed methane extraction from low-permeability coal seams using a combination of acid and alkali synergistic permeability enhancement. In the diagram: 1—Target coal seam; 2—Roof strata; 3—Horizontal docking well; 4—Drainage well; 5—Fracturing pipeline; 6—Directional perforation; 7—No. 1 permeability enhancement fluid storage tank; 8—No. 2 permeability enhancement fluid storage tank; 9—Fracturing fracture network; 10—Dock junction; 11—Surface drainage system. Detailed Implementation

[0014] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments; based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. The following description, in conjunction with the appendix... Figure 1 The specific embodiments of the present invention will be further described in detail below.

[0015] This invention adopts the following technical solution: a method for enhancing coalbed methane extraction by acid-base synergistic permeability enhancement in low-permeability coal seams, comprising the following steps: S100: Samples are taken from the coal seam requiring permeability enhancement and sealed for preservation. The minerals in the coal samples are analyzed, and macroscopic mechanical properties of the coal samples are characterized. For the coal seam requiring permeability enhancement (target coal seam 1), samples are taken on-site and sealed for preservation. The samples undergo tests including industrial analysis, mineral composition determination, macromolecular structure determination, pore and fracture structure analysis, and permeability determination.

[0016] In step S100, the coal sample testing includes industrial analysis, mineral composition determination, coal macromolecular structure determination, pore and fracture structure analysis, and permeability testing. Industrial analysis determines the ash content in the coal, mineral composition determination determines the types and relative contents of minerals in the coal, coal macromolecular structure determination determines the types of functional groups in the primary coal sample, pore and fracture structure analysis reflects the distribution of pores and fractures in the primary coal sample, and permeability testing reflects the air permeability of the pore and fracture structure in the primary coal sample.

[0017] S200: Based on the coal sample parameters obtained from step S100, set the permeability enhancement sequence for the target coal seam and preliminarily determine the composition and ratio of the acidic and alkaline permeability enhancement solutions.

[0018] The permeability enhancement sequence in step S200 is the order in which acidic and alkaline permeability enhancement solutions are injected into the coal seam, including two cases: acid first then alkaline and alkaline first then acid.

[0019] The permeability enhancement sequence depends on the composition and content of the coal seam minerals. When the coal seam is dominated by silica-containing minerals, an alkaline-to-acid permeability enhancement sequence is adopted. First, an alkaline permeability enhancement solution is injected to initially dissolve the silica-containing minerals or convert them into acid-soluble intermediate products. At the same time, the content of oxygen-containing functional groups in the coal is increased, reducing the gas adsorption capacity of the coal seam and promoting gas desorption. After the initial permeability enhancement is completed, an acidic permeability enhancement solution is injected to completely dissolve the minerals in the coal. When the coal seam is dominated by carbonate minerals, an acid-to-alkaline permeability enhancement sequence is adopted. First, an acidic permeability enhancement solution is injected to dissolve the minerals in the coal to complete the initial permeability enhancement. Then, an alkaline permeability enhancement solution is injected to neutralize the residual acidic permeability enhancement solution, reducing the corrosive impact on equipment during subsequent coal mining. It also reduces the gas adsorption capacity of the coal by increasing the content of oxygen-containing functional groups in the coal, promoting gas desorption and completing the secondary permeability enhancement.

[0020] The acidic penetrating agent in step S200 comprises acid, surfactant, corrosion inhibitor, etc., wherein the acid includes weak acids such as acetic acid and citric acid, and strong acids such as hydrochloric acid, nitric acid, and hydrofluoric acid; the surfactant includes cationic, anionic, and amphoteric surfactants. The alkaline penetrating agent comprises alkaline solution and surfactant, wherein the alkaline solution is an alkaline solution such as sodium hydroxide or potassium hydroxide, and the surfactant includes cationic, anionic, and amphoteric surfactants.

[0021] The acidic permeability-enhancing solution in step S200 serves the following purpose: the acidic components dissolve carbonate and silicate minerals in the coal. Specifically, hydrochloric acid, nitric acid, hydrofluoric acid, acetic acid, and citric acid effectively dissolve carbonate minerals such as calcite and dolomite in the coal seam; hydrofluoric acid effectively dissolves silicate minerals such as quartz, kaolinite, illite, and montmorillonite; and acetic acid dissolves silicate minerals such as kaolinite. Furthermore, it increases the number of oxygen-containing functional groups in the coal to a certain extent, reducing the coal's gas adsorption capacity, and is tailored to the mineral occurrence conditions of different coal seams. Furthermore, a single acid type or a combination of two or more acid types can be used for permeation enhancement. For environmental protection and equipment protection, weak acids such as acetic acid and citric acid are preferred. Surfactants reduce the contact angle between the permeation fluid molecules and the coal, enhancing the wettability of the coal. They also reduce filtration loss by increasing the viscosity of the permeation fluid and act as corrosion inhibitors, ensuring sufficient contact and reaction between the permeation fluid molecules and the minerals and coal molecules. Corrosion inhibitors effectively reduce the corrosion of equipment by residual acidic permeation fluid in the coal seam during subsequent underground coal mining. Alkaline permeation fluids function as follows: alkali solutions can dissolve silicate minerals such as kaolinite in the coal, converting silicate minerals such as quartz into acid-soluble intermediates, increasing the content of oxygen-containing functional groups in the coal, reducing the gas adsorption capacity of the coal seam, and promoting gas desorption. If alkaline permeation fluid is used for secondary permeation enhancement, it can also neutralize residual acidic permeation fluid in the coal seam and reduce corrosion of equipment during subsequent mining. Surfactants enhance the wettability of the coal seam and reduce filtration loss.

[0022] The experimental parameters for treating coal samples with the permeation-enhancing liquid in step S200 include coal sample morphology, permeation-enhancing liquid concentration, treatment time, and treatment temperature. Coal sample morphologies include coal powder, broken coal pieces, and coal columns. After coal powder treatment, X-ray diffraction experiments are performed to determine changes in mineral content, Fourier transform infrared spectroscopy is performed to determine changes in the macromolecular structure of the coal, and nitrogen adsorption experiments are performed to determine changes in the microporous structure of the coal sample. For broken coal pieces, scanning electron microscopy is used to observe changes in the microporous structure of the coal sample. Coal columns are used for uniaxial compression experiments and permeability testing experiments to determine the mechanical and permeability properties of the coal sample, respectively.

[0023] The ratio is set with a certain concentration gradient. Based on the mineral content in the test results and the expected effect of removing minerals, the mass fraction of the permeation solution is estimated according to the stoichiometric ratio of minerals and permeation solution components. Several mass fraction values ​​are then taken above and below this value with a certain difference to form a concentration gradient.

[0024] S300: The coal sample is processed according to the permeability enhancement sequence determined in step S200, and then the processed coal sample is subjected to experiments; the experiments include X-ray diffraction experiments, Fourier transform infrared spectroscopy analysis experiments, electron microscopy scanning experiments, uniaxial compression experiments and seepage experiments.

[0025] X-ray diffraction, Fourier transform infrared spectroscopy, and electron microscopy are tests for characterizing the microscopic features of coal samples, while uniaxial compression and seepage tests are tests for characterizing the macroscopic mechanical properties of coal samples.

[0026] In step S300, in the characterization and analysis section of the experimental scheme, after the coal sample is treated with different concentrations of permeability-enhancing liquid set in step S200, the changes in minerals in the coal sample are characterized by X-ray diffraction, the changes in oxygen-containing functional groups in the coal sample are analyzed by Fourier transform infrared spectroscopy, the changes in micropores and cracks in the coal sample are observed directly by electron microscopy, the changes in mechanical properties of the coal sample are analyzed by uniaxial compression, and the changes in gas permeation characteristics of the coal sample are analyzed by permeation.

[0027] S400 compares the experimental results in step S300 with those in step S100 to determine the optimal compatibility of acidic and alkaline permeation-enhancing solutions.

[0028] The experimental results of step S300 can characterize the permeability enhancement effect of the permeation-enhancing liquid in a relatively specific and intuitive way. Specifically, X-ray diffraction results can reflect the types and contents of residual minerals in the coal; Fourier transform infrared spectroscopy results can reflect the increased oxygen-containing functional groups such as hydroxyl and carbonyl groups in the coal; electron microscopy can visually demonstrate the degree of damage to the coal's microstructure caused by the permeation-enhancing liquid; uniaxial compression experiments combined with electron microscopy can visually reflect the corrosion effect of the permeation-enhancing liquid on secondary minerals in the coal's pore and fracture structure through the cross-sectional characteristics of the coal sample; seepage experiments can reflect the change in permeability of the coal sample after modification with the permeation-enhancing liquid; and adsorption-desorption experiments are used to characterize the effect of permeation-enhancing liquid treatment on the methane adsorption-desorption characteristics of the coal. By comparing the experimental results of coal samples before and after treatment with different concentrations of permeation-enhancing liquid, the relationship between the permeation-enhancing liquid concentration and various characteristic parameters of the coal sample is determined from the changes in mineral composition and content, porosity, permeability, adsorption-desorption amount, uniaxial compressive strength, and functional groups before and after permeation-enhancing liquid modification. Therefore, the optimal permeation-enhancing liquid formulation scheme can be selected according to actual requirements.

[0029] S500: Coalbed methane extraction is carried out using the optimal combination of acidic and alkaline permeation-enhancing solutions.

[0030] S501: Drill a horizontal docking well and a vertical drainage well separately. The horizontal docking well is drilled to the roof strata of the coal seam. Horizontal drilling is carried out within a range of 0.5m to 1.5m above the coal seam using wellbore trajectory control technology and geological steering technology until it is connected to the vertical drainage well. This makes the horizontal docking well and the vertical drainage well form a U-shaped drainage well system. A surface drainage system is established near the vertical drainage well.

[0031] S502: Directional perforation is performed to the target coal seam via a horizontal docking well to establish a fluid channel between the wellbore and the target coal seam; staged hydraulic fracturing technology is used to inject the initial permeability enhancement fluid and proppant into the target coal seam in stages according to a pre-set permeability enhancement sequence to induce fracturing and enhance permeability; pressure holding and well shut-in are performed to allow the permeability enhancement fluid to fully penetrate the target coal seam, dissolve the minerals in the coal seam, and modify its pore and fracture structure; after pressure holding and well shut-in are completed, gas is returned and produced through the surface drainage system.

[0032] S503: When the gas production in the vertical well decreases significantly, inject a permeability-enhancing fluid into the target coal seam under high pressure according to step S502 to achieve secondary permeability enhancement of the target coal seam; perform pressure-maintaining well shut-in to further dissolve the minerals in the coal seam and modify its pore and fracture structure; after pressure-maintaining well shut-in, return and gas production are carried out through the surface drainage system.

[0033] Example 1 (1) Coal sample parameter determination Coal samples from target coal seam 1 were obtained through core drilling and placed in a sealed coal sample container. The coal samples underwent industrial analysis, mineral composition determination, macromolecular structure determination, pore and fracture structure analysis, and permeability measurement. The analysis determined that the coal seam is a low-permeability coal seam, characterized by high-ash bituminous coal with a medium-to-high degree of metamorphism. The mineral composition is dominated by silica-containing minerals such as quartz, kaolinite, illite, and montmorillonite, with the remaining components being carbonate minerals such as dolomite and calcite.

[0034] (2) Determining the order of translucency enhancement Since the mineral composition of this coal seam is mainly composed of silicon-containing minerals such as quartz, kaolinite, illite, and montmorillonite, the permeability enhancement sequence for this coal seam is set as alkali first, then acid. That is, alkaline permeability enhancement solution is first injected to initially dissolve the silicon-containing minerals or convert them into intermediate products that are easily soluble in acid. At the same time, the content of oxygen-containing functional groups in the coal is increased, the gas adsorption of the coal seam is reduced, and gas desorption is promoted. After the initial permeability enhancement is completed, acidic permeability enhancement solution is injected to completely dissolve the minerals in the coal.

[0035] (3) Compatibility of permeation-enhancing solution Based on the mineral content and relative proportions of each mineral component in the coal seam, the preliminary composition of the permeability-enhancing solution was determined to be sodium hydroxide, acetic acid, sodium dodecyl sulfate, and a corrosion inhibitor. Various parameters were set for the permeability-enhancing solution treatment of coal samples. A mixture of sodium hydroxide, sodium dodecyl sulfate, and water with a certain mass fraction was used as the alkaline permeability-enhancing solution, and a mixture of acetic acid, sodium dodecyl sulfate, corrosion inhibitor, and water with a certain mass fraction was used as the acidic permeability-enhancing solution. The coal samples were treated with the permeability-enhancing solution in the order of alkaline followed by acidic. X-ray diffraction, Fourier transform infrared spectroscopy, electron microscopy, uniaxial compression, and seepage experiments were then performed on each treated coal sample. Based on the analysis results of the laboratory experiments, the optimal compatibility between the acidic and alkaline permeability-enhancing solutions was determined.

[0036] In the alkaline permeation-enhancing solution, sodium hydroxide reacts with quartz to form an acid-soluble intermediate product, dissolves silicate minerals such as kaolinite, increases oxygen-containing functional groups in the coal, reduces the coal seam's adsorption of methane, and promotes methane desorption. Sodium dodecyl sulfate enhances the wettability of the permeation-enhancing solution on the coal, reduces filtration loss, and slightly increases the content of oxygen-containing functional groups in the coal seam. In the acidic permeation-enhancing solution, acetic acid dissolves carbonate minerals in the coal seam and dissolves the acid-soluble intermediate products generated after previous alkali injection, increases the content of oxygen-containing functional groups in the coal, reduces the coal seam's methane adsorption, and promotes methane desorption. Acetic acid is also a weak acid, preventing the problem of a small reaction range in the permeation-enhancing solution due to excessively rapid reaction. Sodium dodecyl sulfate enhances the wettability of the permeation-enhancing solution on the coal, reduces filtration loss, increases the viscosity of the permeation-enhancing solution, and plays a certain role in corrosion inhibition, ensuring sufficient contact and reaction between the permeation-enhancing solution and the coal seam.

[0037] (4) After the permeation-enhancing fluid is prepared, the alkaline permeation-enhancing fluid is loaded into the No. 1 permeation-enhancing fluid storage tank 7, and the acidic permeation-enhancing fluid is loaded into the No. 2 permeation-enhancing fluid storage tank 8. Horizontal docking well 3 and drainage well 4 are drilled respectively. The horizontal docking well 3 is drilled to the top rock layer 2 of the coal seam. Horizontal drilling is carried out in the range of 0.5~1.5m above the coal seam using wellbore trajectory control technology and geological guidance technology until it is connected with the drainage well 4 at the docking point 10 to form a U-shaped drainage well system. A surface drainage system 11 is established near the drainage well.

[0038] (5) Directional perforation is made from the horizontal docking well 3 to the target coal seam 1 below to establish a fluid channel between the wellbore and the target coal seam 1; segmented hydraulic fracturing technology is adopted, and alkaline permeation fluid and proppant in the No. 1 permeation fluid storage tank 7 are injected into the target coal seam 1 through the directional perforation 6 via the fracturing pipeline 5 to induce fracturing and permeability, forming a fracturing fracture network 9; pressure holding and well shut-in are carried out to allow the alkaline permeation fluid to fully penetrate into the target coal seam 1 and modify its pore structure and adsorption; after the pressure holding and well shut-in are completed, the gas is returned and produced through the surface drainage system 11.

[0039] (6) When the gas production in the vertical well is significantly reduced, the segmented hydraulic fracturing technology is used again. The acidic permeability enhancement fluid in the No. 2 permeability enhancement fluid storage tank 8 is injected into the target coal seam 1 through the fracturing pipeline 5 for secondary permeability enhancement. The well is then kept under pressure to allow the acidic permeability enhancement fluid to fully penetrate the target coal seam 1, further dissolving the minerals in the coal seam and modifying its pore structure and adsorption properties. After the well is kept under pressure, the gas is returned and produced again through the surface drainage system.

[0040] Example 2 (1) Coal sample parameter determination Coal samples from target coal seam 1 were obtained through core drilling and placed in a sealed coal sample container. The coal samples underwent industrial analysis, mineral composition determination, macromolecular structure determination, pore and fracture structure analysis, and permeability measurement. The analysis determined that the coal seam is a low-permeability coal seam, with a medium-metamorphic, high-ash bituminous coal composition. The main mineral components are carbonate minerals such as dolomite and calcite, with the remaining ash components consisting of small amounts of silica-containing minerals such as quartz, kaolinite, illite, and montmorillonite.

[0041] (2) Determining the order of translucency enhancement Since the mineral composition of this coal seam is mainly carbonate minerals such as dolomite and calcite, the permeability enhancement sequence for this coal seam is set as acid-first, then alkaline. That is, acidic permeability enhancement solution is first injected to dissolve the minerals in the coal to complete the initial permeability enhancement, and then alkaline permeability enhancement solution is injected to neutralize the residual acidic permeability enhancement solution, thereby reducing the corrosive impact on equipment during subsequent coal mining. Furthermore, by increasing the content of oxygen-containing functional groups in the coal, the gas adsorption of the coal is reduced, and gas desorption is promoted to complete the secondary permeability enhancement.

[0042] (3) Compatibility of permeation-enhancing solution Based on the mineral content and relative proportions of each mineral component in the coal seam, the composition of the permeability-enhancing solution was initially determined to be acetic acid, hydrofluoric acid, sodium hydroxide, and sodium dodecyl sulfate. A mixture of acetic acid, a small amount of hydrofluoric acid, sodium dodecyl sulfate, and water with a certain mass fraction was used as the acidic permeability-enhancing solution, and a mixture of sodium hydroxide, sodium dodecyl sulfate, and water with a certain mass fraction was used as the alkaline permeability-enhancing solution. The coal samples were treated with the permeability-enhancing solution in the order of acid first and then alkaline. After treatment, X-ray diffraction, Fourier transform infrared spectroscopy, electron microscopy, uniaxial compression, and seepage experiments were performed on each coal sample. Based on the analysis results of the laboratory experiments, the optimal compatibility of the acidic and alkaline permeability-enhancing solutions was determined.

[0043] In the acidic permeation-enhancing solution, hydrofluoric acid dissolves carbonate and silicate minerals, while acetic acid dissolves carbonate and kaolinite minerals in the coal seam, increases the content of oxygen-containing functional groups in the coal, reduces the coal seam's gas adsorption capacity, and promotes gas desorption. Acetic acid, being a weak acid, prevents the problem of a small reaction range in the permeation-enhancing solution due to excessively rapid reactions. Sodium dodecyl sulfate enhances the wettability of the permeation-enhancing solution on the coal, reducing filtration loss. In the alkaline permeation-enhancing solution, sodium hydroxide neutralizes the residual acidic permeation-enhancing solution in the coal seam, further dissolves remaining silicate minerals such as kaolinite, increases the content of oxygen-containing functional groups in the coal, reduces the coal seam's gas adsorption capacity, and promotes gas desorption. Sodium dodecyl sulfate enhances the wettability of the permeation-enhancing solution on the coal, reduces filtration loss, and slightly increases the content of oxygen-containing functional groups in the coal seam.

[0044] (4) After the permeation-enhancing fluid is prepared, the acidic permeation-enhancing fluid is loaded into the No. 1 permeation-enhancing fluid storage tank 7, and the alkaline permeation-enhancing fluid is loaded into the No. 2 permeation-enhancing fluid storage tank 8. Horizontal docking well 3 and drainage vertical well 4 are drilled respectively. The horizontal docking well 3 is drilled to the top rock layer 2 of the coal seam. Horizontal drilling is carried out in the range of 0.5~1.5m above the coal seam using wellbore trajectory control technology and geological steering technology until it is connected with the drainage vertical well 4 at the docking point 10 to form a U-shaped drainage well system. A surface drainage system 11 is established near the drainage vertical well.

[0045] (5) Directional perforation is made from the horizontal docking well 3 to the target coal seam 1 below to establish a fluid channel between the wellbore and the target coal seam 1; segmented hydraulic fracturing technology is adopted, and acidic permeable fluid and proppant in the No. 1 permeable fluid storage tank 7 are injected into the target coal seam 1 through the directional perforation 6 via the fracturing pipeline 5 to induce fracturing and permeability, forming a fracturing fracture network 9; pressure holding and well shut-in are carried out to allow the acidic permeable fluid to fully penetrate into the target coal seam 1 and modify its pore structure and adsorption; after the pressure holding and well shut-in are completed, the gas is returned and produced through the surface drainage system 11.

[0046] (6) When the gas production in the vertical well is significantly reduced, the segmented hydraulic fracturing technology is used again. The alkaline permeation-enhancing liquid in the No. 2 permeation-enhancing liquid storage tank 8 is injected into the target coal seam 1 through the fracturing pipeline 5 for secondary permeation enhancement. Pressure-maintaining well simmering is carried out to allow the alkaline permeation-enhancing liquid to fully penetrate into the target coal seam 1, further dissolving the minerals in the coal seam and modifying its pore structure and adsorption properties. After pressure-maintaining well simmering, gas is returned and produced again through the surface drainage system.

Claims

1. A method for enhancing coalbed methane extraction by acid-base synergistic permeability enhancement in low-permeability coal seams, characterized in that: Includes the following steps, S100: Sample and seal the coal seam that needs to be permeable, and determine the minerals in the coal sample. At the same time, conduct macroscopic mechanical property characterization experiments on the coal sample. S2 00: Based on the test results of step S100, set the permeability enhancement sequence of the target coal seam. The permeability enhancement sequence is the order in which acidic permeability enhancement liquid and alkaline permeability enhancement liquid are injected into the coal seam, including two cases: acid first and then alkaline, and alkaline first and then acid. When the minerals in the coal seam are mainly silicon-containing minerals, the permeability enhancement sequence of first alkali and then acid is adopted. First, alkaline permeability enhancement solution is injected to initially dissolve the silicon-containing minerals or convert them into intermediate products that are easily soluble in acid. At the same time, the content of oxygen-containing functional groups in the coal is increased, the gas adsorption of the coal seam is reduced, and gas desorption is promoted. After the initial permeability enhancement is completed, acidic permeability enhancement solution is injected to completely dissolve the minerals in the coal. When the minerals in the coal seam are mainly carbonate minerals, an acid-to-alkali permeability enhancement sequence is adopted. First, acidic permeability enhancement solution is injected to dissolve the minerals in the coal to complete the initial permeability enhancement. Then, alkaline permeability enhancement solution is injected to neutralize the residual acidic permeability enhancement solution, thereby reducing the corrosive impact on equipment during subsequent coal mining. Furthermore, by increasing the content of oxygen-containing functional groups in the coal, the gas adsorption capacity of the coal is reduced, and gas desorption is promoted to complete the secondary permeability enhancement. S300: The coal sample is processed according to the permeability enhancement sequence determined in step S200, and then the processed coal sample is subjected to experiments; the experiments include X-ray diffraction experiments, Fourier transform infrared spectroscopy analysis experiments, electron microscopy scanning experiments, uniaxial compression experiments and seepage experiments; In step S300, in the characterization and analysis section of the experimental design, After the coal samples were treated with permeation-enhancing solutions of different concentrations set in step S200, the changes in minerals in the coal samples were characterized by X-ray diffraction experiments, the changes in oxygen-containing functional groups in the coal samples were analyzed by Fourier transform infrared spectroscopy, the changes in micropores and cracks in the coal samples were observed directly by electron microscopy, the changes in mechanical properties of the coal samples were analyzed by uniaxial compression experiments, and the changes in gas permeation characteristics of the coal samples were analyzed by permeation experiments. S400: Compare the experimental results in step S300 with those in step S100 to determine the optimal compatibility of acidic and alkaline permeation-enhancing solutions respectively. S500: Coalbed methane extraction is carried out using the optimal combination of acidic and alkaline permeation-enhancing solutions.

2. The method for enhancing coalbed methane extraction from low-permeability coal seams using acid-base synergistic permeability enhancement according to claim 1, characterized in that: The determination of minerals in the coal sample in step S100 includes industrial analysis, mineral composition determination, coal sample macromolecular structure determination, pore and fissure structure analysis, and permeability testing. Industrial analysis determines the ash content in the coal, mineral composition determination determines the types and relative contents of minerals in the coal, coal sample macromolecular structure determination determines the types of functional groups in the primary coal sample, pore and fissure structure analysis reflects the distribution of pores and fissures in the primary coal sample, and permeability testing reflects the air permeability of the pore and fissure structure in the primary coal sample.

3. The method for enhancing coalbed methane extraction from low-permeability coal seams using a combination of acid and alkali synergistic permeability enhancement as described in claim 1, characterized in that: In step S400, by comparing the experimental results of coal samples before and after treatment with different concentrations of permeability-enhancing liquid, the relationship between the concentration of permeability-enhancing liquid and various characteristic parameters of coal samples is determined from the mineral composition and content, porosity, permeability, adsorption and desorption amount, uniaxial compressive strength and functional group changes before and after the permeability-enhancing liquid is modified into coal, and the optimal permeability-enhancing liquid formulation scheme is selected.

4. The method for enhancing coalbed methane extraction from low-permeability coal seams using a combination of acid and alkali synergistic permeability enhancement as described in claim 1, characterized in that: The specific process of step S500 is as follows: S501: Drill a horizontal docking well and a vertical drainage well separately. The horizontal docking well is drilled to the roof strata of the coal seam and then horizontally drilled within a range of 0.5m to 1.5m above the coal seam until it is connected to the vertical drainage well, so that the horizontal docking well and the vertical drainage well form a U-shaped drainage well system, and a surface drainage system is established near the vertical drainage well. S502: Directional perforation is performed through a horizontal docking well to the target coal seam below, establishing a fluid channel between the wellbore and the target coal seam; the initial permeability enhancement fluid and proppant are injected into the target coal seam in stages according to the pre-set permeability enhancement sequence to induce fracturing and enhance permeability; pressure holding and well shut-in are performed to allow the permeability enhancement fluid to fully penetrate the target coal seam, dissolve the minerals in the coal seam, and modify its pore and fracture structure; after pressure holding and well shut-in are completed, gas is returned and produced through the surface drainage system; S503: When the gas production drops to 60% of the average gas production, inject a permeability-enhancing fluid into the target coal seam under high pressure according to step S502 to achieve secondary permeability enhancement of the target coal seam; perform pressure-maintaining well shut-in to further dissolve the minerals in the coal seam and modify its pore and fracture structure; after pressure-maintaining well shut-in, perform return drainage and gas production through the surface drainage system.

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