An integrated method for fracturing, gasification and carbon sequestration of thin interbedded coal seams

Abstract

An integrated method for fracturing, gasification, and carbon sequestration of thin interbedded coal seams involves geological exploration and detailed coal seam assessment. Intermediate horizontal wells are drilled from the surface downward. Top and bottom horizontal wells are planned and drilled in the sandstone formations at the top and bottom of the interbedded coal seams. Multiple fishbone wells are drilled obliquely downward and upward within the top and bottom horizontal wells. Oxygen-enriched water vapor is injected into the intermediate horizontal well through coiled tubing, forcing the oxygen-enriched water vapor into the reservoir under high pressure for fracturing. This triggers underground gasification of the coal seam and simultaneously extracts coalbed methane. Supercritical CO2 fracturing is performed in the top and bottom horizontal wells. Water-based fracturing fluid is injected into the top and bottom horizontal wells. Gasification products and desorbed methane produced by coal seam combustion and gasification are extracted through the intermediate horizontal wells, and the previously injected supercritical carbon dioxide is stored for a long term. This method addresses the limitations of traditional hydraulic fracturing technology and improves reservoir reconstruction.

Description

Technical Field

[0001] The present invention belongs to the technical field of coal and unconventional natural gas development, and in particular relates to an integrated method for fracturing, gasification and carbon sequestration of thin interbedded coal seams. Background Art

[0002] As a clean energy source, the development of thin interbedded coalbed methane faces multiple difficulties. First, the thickness of thin interbedded coal seams is extremely uneven, and coal seams and sandstone formations frequently alternate, resulting in the need to frequently adjust the mining strategy during the mining process. This not only increases the difficulty and cost of mining, but also thinner coal seams mean that the controlled reserves of a single well are limited, requiring more well locations and a denser well network layout, further pushing up mining costs. Secondly, due to the undeveloped microcracks in the coal seams and generally low permeability, it is difficult for coalbed methane to flow naturally to production wells. This requires that effective production-increasing measures must be taken during the mining process to increase the permeability of the coal seams and increase the production of coalbed methane. In addition, the gas pressure of thin interbedded coal seams is usually low, which means that the natural driving energy of coalbed methane is insufficient, making it difficult to maintain a long-term stable gas production. Therefore, specific technical means are needed to supplement energy and maintain or improve the mining efficiency of coalbed methane. The geological structure of thin interbedded coal seams is often complex, with faults, folds, and other geological phenomena, all of which have a significant impact on the migration and enrichment of coalbed methane. Therefore, the impact of geological structure must be fully considered during the mining process, and well locations and mining plans must be scientifically and rationally designed. Furthermore, more efficient production-enhancing measures, such as supercritical CO2 fracturing and in-situ methane explosive fracturing, need to be explored. To overcome these technical difficulties, innovative mining technologies and methods are needed to achieve clean utilization and carbon sequestration of coalbed methane while improving mining efficiency and safety. Summary of the Invention

[0003] In response to some problems existing in the existing technology, the present invention provides an integrated method for fracturing, gasification and carbon sequestration of thin interbedded coal seams. This method has the advantages of high efficiency, environmental protection and sustainability, and can provide a new technical path for the development of coalbed methane in thin interbedded coal seams. It can not only solve the limitations of traditional hydraulic fracturing technology, but also improve the transformation effect of the reservoir, significantly improve the effect and efficiency of coal seam mining, and has important application prospects and promotion value.

[0004] To achieve the above objectives, the present invention provides an integrated method for fracturing, gasification and carbon sequestration of thin interbedded coal seams, comprising the following steps:

[0005] Step 1: Geological exploration and detailed assessment of coal seams;

[0006] S11: Conduct ground geological surveys of the development block, observe coal seam outcrop samples, and record coal seam characteristics. The surface of the development block is mudstone strata, and the part below the mudstone strata is a thin interbedded structure with alternating coal seams and sandstone strata.

[0007] S12: Detect the distribution of underground coal seams, determine the thickness and distribution range of coal seams, as well as the interbedded relationship between coal seams and mudstone strata, and the interbedded relationship between coal seams and sandstone strata. At the same time, obtain the key parameters of the coal seams, fully understand the geological structure of the coal seams, and analyze the impact of the geological structure of the coal seams on the migration and enrichment of coalbed methane;

[0008] S13: Determine the gas content, pressure data and gas composition in the coal seam through logging, sampling and analysis techniques;

[0009] S14: Comprehensively evaluate the coal seam by integrating geological exploration results, well logging data, gas content and pressure data, determine the coal seam development potential, and scientifically and rationally determine the location of subsequent intermediate horizontal well drilling. At the same time, formulate an integrated plan for subsequent drilling, fracturing, gasification and carbon sequestration;

[0010] Step 2: drilling of intermediate horizontal wells;

[0011] S21: Based on the geological exploration data of the coal seam, the thickness, dip, and permeability of the coal seam are comprehensively analyzed. Based on the distribution range of the mudstone strata, sandstone strata, and coal seams, the coal seam with good reservoir properties in the intermediate position is selected as the drilling position for the intermediate horizontal well;

[0012] S22: Based on the selected drilling horizon, the 3D geological model is used to plan the drilling path to ensure that the drilling trajectory can penetrate the target coal seam and that the horizon encountered by the intermediate horizontal well has good reservoir properties;

[0013] S23: Using CBM drilling equipment and process technology, drill an intermediate horizontal well from the ground downward;

[0014] Step 3: Drilling of top horizontal well and bottom horizontal well;

[0015] Plan and drill the top horizontal well in the uppermost sandstone formation of the thin interbedded coal seam, ensuring that the axis of the top horizontal well is parallel to the axis of the middle horizontal well and that the top horizontal well is located 1 / 5 to 1 / 4 below the interior of the upper sandstone formation.

[0016] In the sandstone formation at the bottom of the thin interbedded coal seam, plan and drill a bottom horizontal well. At the same time, ensure that the axis of the bottom horizontal well is parallel to the axis of the middle horizontal well and that the bottom horizontal well is located at the upper 1 / 5 to 1 / 4 position of the lower sandstone formation.

[0017] Step 4: Fishbone well drilling;

[0018] In the top horizontal well, use a directional sidetracking tool to drill the first top fishbone well downward at a clockwise angle of 45 to 60 degrees; repeat the above process multiple times to drill the remaining top fishbone wells in the top horizontal well, and ensure that the spacing between adjacent top fishbone wells is between 20 and 30 meters; in the bottom horizontal well, use a directional sidetracking tool to drill the first bottom fishbone well upward at a clockwise angle of 45 to 60 degrees; repeat the above process multiple times to drill the remaining bottom fishbone wells in the bottom horizontal well, and ensure that the spacing between adjacent bottom fishbone wells is between 20 and 30 meters;

[0019] A three-dimensional well pattern is formed in the thin interbedded coal seam structure by using the middle horizontal well, the top horizontal well, the bottom horizontal well, the top fishbone well and the bottom fishbone well;

[0020] Step 5: Fracturing of the intermediate horizontal well;

[0021] S51: Lower the fracturing tool to the predetermined position in the intermediate horizontal well via coiled tubing, install surface equipment and connect it to the fracturing tool, and ensure stable transmission of feedback and control signals during the fracturing process.

[0022] S52: Pure water is converted into water vapor through a heating device on the ground, and a certain proportion of oxygen is mixed in to form oxygen-enriched water vapor;

[0023] S53: Using a surface pump set, oxygen-rich steam is injected into the intermediate horizontal well through coiled tubing. Under high pressure, the oxygen-rich steam is squeezed into the reservoir to perform a fracturing operation. During the fracturing process, the high temperature and high pressure of the oxygen-rich steam induce thermal stress in the reservoir rock, promoting the formation and expansion of cracks, forming a complex large-scale artificial fracture network.

[0024] S54: Continuously pumping in oxygen-rich steam allows the fracture to expand not only within the coal seam where the intermediate horizontal well is located, but also to extend upward and downward into adjacent coal seams, increasing the connectivity of the artificial fracture.

[0025] Step 6: Underground coal seam gasification and coal seam methane extraction;

[0026] S61: During the fracturing process of the intermediate horizontal well, when the oxygen-enriched steam input reaches 300-400 m³, the oxygen-enriched steam injection operation is stopped; then, an ignition rod is dropped to the bottom of the intermediate horizontal well using a dedicated delivery device, and the ignition rod is used to ignite the coal seam, thereby initiating the underground coal seam gasification process;

[0027] S62: After the coal seam is ignited, oxygen-enriched steam is continuously injected into the intermediate horizontal well to maintain the continuous coal seam gasification process. At the same time, the high temperature and high pressure of the oxygen-enriched steam are used to promote further expansion of the fractures. The gas and heat generated by the coal seam gasification are also used to promote the formation and expansion of the fractures.

[0028] While the gasification operation of the middle horizontal well is in progress, the top horizontal well and the bottom horizontal well are used to extract coalbed methane simultaneously. At the same time, the extraction process is used to reduce the pressure in the coal seam, further promoting the in-depth gasification reaction of the middle horizontal well.

[0029] S63: After the gasification process of the middle horizontal well is completed, the gasification agent injection system of the middle horizontal well is closed and the well is shut in to allow the downhole combustion process to fully proceed. Through the combustion process, a combustion void zone is formed around the artificial fracture network in the middle horizontal well. At the same time, the drainage operation of the top horizontal well and the bottom horizontal well is continued until the set drainage time is reached.

[0030] Step 7: Supercritical CO2 fracturing and displacement;

[0031] After the extraction operation is completed, supercritical CO2 fracturing operations are immediately carried out on the top and bottom horizontal wells. During the supercritical CO2 fracturing process, by precisely controlling the injection flow rate of supercritical CO2, the internal cracks of the coal seam are induced to further expand, forming a more complex and more connected three-dimensional fracture network. Simultaneously, the powerful adsorption capacity of supercritical CO2 is used to replace the adsorbed methane in the coal seam with a free state. At the same time, the high diffusivity and low surface tension of supercritical CO2 are used to allow the supercritical CO2 to penetrate into the micropores of the coal seam, further increasing the content of free methane in the coal seam.

[0032] Step 8: Hydraulic fracturing and CO2 storage;

[0033] After the injection of a predetermined amount of supercritical carbon dioxide is completed, water-based fracturing fluid is injected into the top and bottom horizontal wells at a set flow rate. The injected water-based fracturing fluid causes the internal cracks of the coal seam to gradually expand, forming a complex fracture network. At the same time, the water-based fracturing fluid squeezes the supercritical carbon dioxide deep into the reservoir, further displacing the methane gas in the coal seam.

[0034] During the injection of water-based fracturing fluid, the high molecular weight polymer and cross-linking agent in the water-based fracturing fluid gradually form a high-strength and high-toughness plugging layer in the cracks. The formed plugging layer effectively supports the cracks to prevent the cracks from closing after pressure relief. At the same time, the formed plugging layer effectively prevents supercritical carbon dioxide from escaping from the cracks and returning to the wellbore.

[0035] Step 9: Gasification and coalbed methane extraction;

[0036] S91: Gasification products and desorbed methane gas generated by coal seam combustion and gasification are extracted through intermediate horizontal wells and separated and processed by gas separation equipment after being extracted to the surface. During the extraction process, sensors installed in the extraction wells monitor the parameters of the gas returned from the intermediate horizontal wells in real time. When no significant combustible gas is returned from the intermediate horizontal wells for a long period of time and the gas concentration remains stable within the set low concentration range, the intermediate horizontal wells are closed.

[0037] S92: Keep the top and bottom horizontal wells closed at the same time to store the previously injected supercritical carbon dioxide for a long time to achieve carbon emission reduction targets.

[0038] As a preferred embodiment, in step one, geological radar and seismic exploration techniques are used to detect the distribution of underground coal seams.

[0039] Furthermore, in order to ensure the scientific and rationality of subsequent fracturing, gasification and carbon sequestration plans, in step 2 S23, after the intermediate horizontal well drilling operation is completed, the drilling effect is evaluated through logging technology. The evaluation content includes the accuracy of the drilling trajectory and the penetration of the coal seam. Based on the evaluation results, the subsequent fracturing, gasification and carbon sequestration plans are adjusted and optimized.

[0040] Furthermore, in order to ensure the quality of the horizontal well and the subsequent fracturing effect, during the drilling process in step three, the drilling trajectory and formation changes are monitored in real time to ensure the accuracy of the position of the horizontal well and the smoothness of the trajectory. At the same time, the top horizontal well and the bottom horizontal well share a vertical well.

[0041] Furthermore, in order to ensure the quality of the fishbone wells and to effectively increase the control range of the top horizontal well and the bottom horizontal well through the fishbone wells, in step four, during the drilling process of the top fishbone well, the inclination and azimuth of the top fishbone well are monitored in real time to ensure that the top fishbone well extends according to the predetermined trajectory. At the same time, it is ensured that the top fishbone well passes through at least one group of adjacent coal seams and sandstone formations after passing through the formation encountered by the connected top horizontal well; during the drilling process of the bottom fishbone well, the inclination and azimuth of the bottom fishbone well are monitored in real time to ensure that the bottom fishbone well extends according to the predetermined trajectory. At the same time, it is ensured that the bottom fishbone well passes through at least one group of adjacent coal seams and sandstone formations after passing through the formation encountered by the connected bottom horizontal well.

[0042] As a preferred embodiment, in step 4, the multiple bottom fishbone wells and the multiple top fishbone wells are symmetrically distributed, and the spacing between two adjacent bottom fishbone wells and two adjacent top fishbone wells is the same. At the same time, the lengths of the bottom fishbone wells and the top fishbone wells are the same.

[0043] Furthermore, in order to ensure the fracturing effect and efficiency, in step 5 S52, the volume fraction of oxygen mixed in the water vapor is 40% to 60%; in step 5 S53, during the fracturing process, the flow rate of the oxygen-enriched water vapor is adjusted to 3.0 to 4.5 m³ / min.

[0044] Furthermore, in order to ensure the extraction effect and extraction efficiency, in step 6 S63, the extraction time is set to no less than 5 to 7 hours.

[0045] Furthermore, in order to ensure the fracturing effect and efficiency, in step seven, the injection flow rate of supercritical carbon dioxide is 2 to 3 m³ / min.

[0046] Furthermore, in order to ensure the effectiveness and efficiency of water-based fracturing, in step eight, the injection flow rate of the water-based fracturing fluid is 8 to 10 m³ / min.

[0047] The present invention proposes an innovative integrated method for the development of thin interbedded coal seams and carbon emission reduction, which effectively integrates fracturing, gasification and carbon sequestration technologies, thereby achieving efficient resource development and environmentally friendly protection. Specifically, firstly, through the early geological exploration and coal seam detailed evaluation process, the position of the intermediate horizontal well can be reasonably planned, and at the same time, a subsequent integrated drilling, fracturing, gasification and carbon sequestration plan can be scientifically and reasonably formulated; secondly, the positions of the top and bottom horizontal wells are reasonably planned, achieving effective coverage of deep thin interbedded coal seams, thereby providing great convenience for subsequent gasification, fracturing and other operations, which not only reduces the engineering volume and cost, but also improves the overall efficiency of the mining operation; thirdly, by setting a plurality of top fishbone wells obliquely below the top horizontal well, and at the same time, setting a plurality of bottom fishbone wells obliquely above the bottom horizontal well, the control range of the top horizontal well on the thin interbedded coal seam below can be effectively increased, and at the same time, the control range of the bottom horizontal well on the thin interbedded coal seam above can be increased. The control range of the inter-coal seam can be further expanded, and the middle horizontal well, top horizontal well, bottom horizontal well, top fishbone well and bottom fishbone well can be used to form a three-dimensional well network in the thin inter-coal seam structure; secondly, the use of oxygen-rich steam as the fracturing medium not only reduces the dependence on water resources, but also makes full use of the high temperature and high pressure of oxygen-rich steam to generate thermal stress in the reservoir rock, promote the formation and expansion of cracks, and form a complex large-scale artificial fracture network; thirdly, the gasification process of the middle horizontal well is combined with the synchronous extraction of the top and bottom horizontal wells to form an efficient gasification-extraction cycle, which significantly improves the gasification efficiency and extraction efficiency; thirdly, the use of supercritical CO2 fracturing operations further expands the fracture network inside the coal seam, significantly improving the fluidity of coalbed methane. At the same time, through the displacement effect of supercritical CO2, the adsorbed methane in the coal seam can be effectively converted into a free state, thereby significantly improving the production and quality of coalbed methane. Furthermore, through the injection of water-based fracturing fluid, not only can the internal cracks of the coal seam be further expanded, but supercritical carbon dioxide can also be squeezed into the deep reservoir, further displacing the methane gas in the coal seam. At the same time, the high molecular polymers and cross-linking agents in the water-based fracturing fluid gradually form a high-strength and high-toughness plugging layer in the cracks. In this way, the plugging layer can be used to effectively support the cracks to prevent the cracks from closing after unloading. The plugging layer can also effectively prevent the escape of supercritical carbon dioxide from the cracks. Finally, the use of intermediate horizontal wells to extract gasification products and methane gas can achieve more complete mining of thin interbedded coal seams.

[0048] The present invention combines oxygen-enriched steam fracturing, supercritical CO2 fracturing, and traditional hydraulic fracturing technologies. By combining the high temperature and high pressure of oxygen-enriched steam with the support of coal gangue particles, a complex and stable fracture network can be formed, significantly improving the permeability of the reservoir and generating additional gas production. The combined technology of hydraulic fracturing and supercritical CO2 storage further enhances the sealing of the formation, thereby ensuring the carbon sequestration effect. As a result, the present invention significantly improves the aperture of the fractures and the gasification efficiency, and realizes in-situ fluidized mining of coal resources, reducing the generation of solid waste and the degree of damage to the surface.

[0049] In summary, the integrated fracturing, gasification, and carbon sequestration method for thin interbedded coal seams proposed in this invention is highly efficient, environmentally friendly, and sustainable, providing a new technical approach for coalbed methane development in thin interbedded coal seams. This method not only overcomes the limitations of traditional hydraulic fracturing technology but also effectively improves reservoir reconstruction and significantly enhances the effectiveness and efficiency of coal seam mining. Therefore, it has significant application prospects and promotional value, and this technology is expected to become one of the key technical means for coalbed methane development in the future. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 This is a schematic diagram of horizontal well layout in thin interbedded coal seams in the present invention;

[0051] Figure 2 This is a schematic diagram of the drilling and distribution of fishbone wells in the present invention;

[0052] Figure 3 This is a schematic diagram of fracturing and creating fractures in an intermediate horizontal well in the present invention;

[0053] Figure 4 This is a schematic diagram of coalbed gasification and coalbed methane extraction in the intermediate horizontal well of the present invention;

[0054] Figure 5 It is a schematic diagram of fishbone well fracturing in the present invention.

[0055] In the figure: 1. Mudstone formation, 2. Sandstone formation, 3. Coal seam, 4. Intermediate horizontal well, 5. Top horizontal well, 6. Bottom horizontal well, 7. Top fishbone well, 8. Bottom fishbone well, 9. Oxygen-enriched steam, 10. Artificial fracture network, 11. Coalbed methane, 12. Combustion zone, 13. Supercritical carbon dioxide, 14. Three-dimensional fracture network. DETAILED DESCRIPTION

[0056] The present invention will be further described below.

[0057] like Figures 1 to 5As shown, the present invention provides an integrated method for fracturing, gasification and carbon sequestration of thin interbedded coal seams, which comprehensively applies geological exploration, horizontal well drilling, fracturing, underground gasification and supercritical CO2 sequestration and other technical means, uses oxygen-enriched water vapor, supercritical CO2 and water-based fracturing fluid as working fluids, utilizes part of the underground coal seam as fuel, and improves the coalbed methane desorption efficiency through the high-temperature products and heating effect generated by the gasification of the intermediate coal seam, thereby achieving production increase and transformation of the thin interbedded coal seams, and ultimately achieving efficient development of coalbed methane and carbon sequestration. Specifically, the method includes the following steps:

[0058] Step 1: Geological exploration and detailed assessment of coal seams 3;

[0059] S11: Before developing thin interbedded coal seams, a ground geological survey is first conducted on the development block. Outcrop rock samples of coal seam 3 are observed and the characteristics of coal seam 3 are recorded. Specifically, the occurrence, color, luster, fracture, and other characteristics of coal seam 3 are recorded. The surface of the development block is mudstone stratum 1, and the portion below the mudstone stratum 1 is a thin interbedded coal seam structure with alternating coal seams 3 and sandstone stratum 2.

[0060] S12: Detect the distribution of the underground coal seam 3, determine the thickness and distribution range of the coal seam 3, as well as the interbedded relationship between the coal seam 3 and the mudstone stratum 1, and the interbedded relationship between the coal seam 3 and the sandstone stratum 2. At the same time, obtain key parameters of the coal seam 3, such as the thickness and number of layers of the coal seam 3, fully understand the geological structure of the coal seam 3, such as faults and folds, and analyze the impact of the geological structure of the coal seam 3 on the migration and enrichment of the coalbed methane 11;

[0061] As a preferred method, geological radar, seismic exploration and other technical means are used to detect the distribution of the underground coal seam 3.

[0062] S13: Determine the gas content, pressure data, and gas composition in the coal seam 3 through well logging, sampling, and analysis techniques;

[0063] S14: Comprehensively evaluate Coal Seam 3 based on geological exploration results, well logging data, gas content, and pressure data to determine its development potential and feasibility. The location of the subsequent intermediate horizontal well 4 is scientifically and rationally determined. At the same time, an integrated plan for subsequent drilling, fracturing, gasification, and carbon sequestration is developed, providing a reliable basis for the design of subsequent drilling, fracturing, gasification, and carbon sequestration plans.

[0064] Step 2: drilling of intermediate horizontal well 4;

[0065] S21: If Figure 1As shown, based on the geological exploration data of coal seam 3, a comprehensive analysis of factors such as the thickness, dip, and permeability of coal seam 3 is conducted. Based on the distribution range of mudstone formation 1, sandstone formation 2, and coal seam 3, a coal seam 3 with good reservoir properties in the middle position is selected as the drilling layer for intermediate horizontal well 4. This layer should have good reservoir properties to facilitate subsequent fracturing and gasification operations.

[0066] S22: Based on the selected drilling layer, the 3D geological model is used to plan the drilling path to ensure that the drilling trajectory can pass through the target coal seam 3 and that the layer encountered by the intermediate horizontal well 4 has good reservoir properties;

[0067] S23: Using CBM 11 drilling equipment and process technology, drill an intermediate horizontal well 4 from the ground downward;

[0068] As a preferred option, after the drilling operation of the intermediate horizontal well 4 is completed, the drilling effect is evaluated by means of logging technology, etc. The evaluation content includes the accuracy of the drilling trajectory and the penetration of the coal seam, and based on the evaluation results, the subsequent fracturing, gasification and carbon storage plans are adjusted and optimized.

[0069] Step 3: Drilling the top horizontal well 5 and the bottom horizontal well 6;

[0070] After the drilling of the intermediate horizontal well 4 is completed, the top horizontal well 5 is planned and drilled in the sandstone formation 2 at the uppermost part of the thin interbedded coal seam. At the same time, it is ensured that the axis of the top horizontal well 5 is parallel to the axis of the intermediate horizontal well 4, and the top horizontal well 5 is located at the lower 1 / 5 to 1 / 4 position of the upper sandstone formation 2 to facilitate subsequent fracturing operations;

[0071] In the sandstone formation 2 at the bottom of the thin interbedded coal seam, a bottom horizontal well 6 is planned and drilled. At the same time, the axis of the bottom horizontal well 6 is ensured to be parallel to the axis of the middle horizontal well 4, and the bottom horizontal well 6 is located at the upper 1 / 5 to 1 / 4 position of the lower sandstone formation 2 to facilitate subsequent fracturing operations.

[0072] As a preferred embodiment, during the drilling process, the drilling trajectory and formation changes are monitored in real time to ensure that the horizontal well is accurately positioned and has a smooth trajectory. At the same time, the top horizontal well 5 and the bottom horizontal well 6 share a vertical well.

[0073] Step 4: Fishbone well drilling;

[0074] like Figure 2As shown, in the top horizontal well 5, a directional side drilling tool is used to drill the first top fishbone well 7 in a clockwise direction of 45 to 60 degrees; the above process is repeated many times to drill the remaining top fishbone wells 7 in the top horizontal well 5 in turn, and ensure that the spacing between adjacent top fishbone wells 7 is between 20 and 30 meters, so as to effectively reduce the stress interference between the fishbone wells in the subsequent fracturing process; as a preferred embodiment, during the drilling process of the top fishbone well 7, the inclination and azimuth of the top fishbone well 7 are monitored in real time to ensure that the top fishbone well 7 extends according to the predetermined trajectory. At the same time, it is ensured that the top fishbone well 7 passes through at least one group of adjacent coal seams 3 and sandstone formations 2 after passing through the formation encountered by the connected top horizontal well 5. In this way, the control range of the top horizontal well 5 can be effectively increased.

[0075] In the bottom horizontal well 6, a directional side drilling tool is used to drill the first bottom fishbone well 8 in a clockwise direction of 45 to 60 degrees; the above process is repeated many times to drill the remaining bottom fishbone wells 8 in the bottom horizontal well 6 in turn, and ensure that the spacing between adjacent bottom fishbone wells 8 is between 20 and 30 meters, so as to effectively reduce the stress interference between the fishbone wells in the subsequent fracturing process; as a preferred embodiment, during the drilling process of the bottom fishbone well 8, the inclination and azimuth of the bottom fishbone well 8 are monitored in real time to ensure that the bottom fishbone well 8 extends according to the predetermined trajectory. At the same time, it is ensured that the bottom fishbone well 8 passes through at least one group of adjacent coal seams 3 and sandstone formations 2 after passing through the formation encountered by the connected bottom horizontal well 6. In this way, the control range of the bottom horizontal well 6 can be effectively increased.

[0076] As a further preference, the multiple bottom fishbone wells 8 and the multiple top fishbone wells 7 are symmetrically distributed, and the spacing between two adjacent bottom fishbone wells 8 and two adjacent top fishbone wells 7 is the same. At the same time, the lengths of the bottom fishbone wells 8 and the top fishbone wells 7 are the same.

[0077] A three-dimensional well pattern is formed in the thin interbedded coal seam structure by using the middle horizontal well 4, the top horizontal well 5, the bottom horizontal well 6, the top fishbone well 7 and the bottom fishbone well 8;

[0078] Step 5: Fracturing the intermediate horizontal well 4;

[0079] Since the control range of the three-dimensional well pattern composed of horizontal wells and fishbone wells is limited, in order to expand the influence range of the three-dimensional well pattern, reservoir transformation is continued on the basis of the existing well pattern. The specific process is as follows:

[0080] S51: Lowering the fracturing tool to a predetermined position in the intermediate horizontal well 4 via the coiled tubing, installing surface equipment and connecting it to the fracturing tool, while ensuring stable transmission of feedback signals and control signals during the fracturing process;

[0081] S52: converting pure water into water vapor on the ground through a heating device, and mixing a certain proportion of oxygen into it to form oxygen-enriched water vapor 9; as a preferred embodiment, the volume fraction of oxygen mixed into the water vapor is 40% to 60%;

[0082] S53: Using a surface pump group, oxygen-rich steam 9 is injected into the intermediate horizontal well 4 through the coiled tubing. Under high pressure, the oxygen-rich steam 9 is squeezed into the reservoir to perform a fracturing operation. Figure 3 As shown, during the fracturing process, the high temperature and high pressure of oxygen-rich steam 9 is used to generate thermal stress in the reservoir rock, promoting the formation and expansion of cracks, forming a complex large-scale artificial fracture network 10;

[0083] As a preferred embodiment, during the fracturing process, the flow rate of the oxygen-enriched steam 9 is adjusted to 3.0-4.5 m³ / min.

[0084] S54: Continuously pumping in oxygen-rich steam 9 allows the fracture to expand not only in the coal seam 3 where the intermediate horizontal well 4 is located, but also to extend upward and downward into adjacent coal seams 3, thereby increasing the connectivity range of the artificial fracture 10;

[0085] Step 6: underground gasification of coal seam 3 and extraction of coal seam methane 11;

[0086] S61: During the fracturing process of the intermediate horizontal well 4, when the input volume of the oxygen-enriched steam 9 reaches 300-400 m³, the injection of the oxygen-enriched steam 9 is stopped; an ignition rod is then dropped to the bottom of the intermediate horizontal well 4 using a dedicated dropping device, and the coal seam 3 is ignited by the ignition rod, thereby initiating an underground gasification process of the coal seam 3;

[0087] S62: If Figure 4 As shown, after the coal seam 3 is ignited, oxygen-rich steam 9 is continuously injected into the intermediate horizontal well 4 to maintain the continuous gasification process of the coal seam 3. At the same time, the high temperature and high pressure of the oxygen-rich steam 9 are used to promote further expansion of the cracks, and the gas and heat generated by the gasification of the coal seam 3 are simultaneously used to promote the formation and expansion of the cracks.

[0088] While the gasification operation of the middle horizontal well 4 is in progress, the top horizontal well 5 and the bottom horizontal well 6 are used to simultaneously extract the coalbed methane 11. At the same time, the extraction process reduces the pressure in the coal seam 3, further promoting the in-depth gasification reaction of the middle horizontal well 4, forming a virtuous cycle.

[0089] S63: When the gasification process of the middle horizontal well 4 is completed, the gasification agent injection system of the middle horizontal well 4 is closed, and the well is shut in to allow the downhole combustion process to proceed fully. Through the combustion process, a combustion zone 12 is formed around the artificial seam network 10 in the middle horizontal well 4; at the same time, the extraction operation of the top horizontal well 5 and the bottom horizontal well 6 is continued, and this extraction process is continued until the set extraction time ends. As a preferred embodiment, the extraction time is set to be no less than 5 to 7 hours.

[0090] Step 7: Supercritical CO2 fracturing and displacement;

[0091] like Figure 5 As shown, after the extraction operation is completed, supercritical CO2 fracturing operation is immediately performed on the top horizontal well 5 and the bottom horizontal well 6. During the supercritical CO2 fracturing process, the injection flow rate of supercritical carbon dioxide 13 is precisely controlled to induce further expansion of the internal cracks of the coal seam 3, forming a more complex and more connected three-dimensional fracture network 14. As a preferred embodiment, the injection flow rate of supercritical carbon dioxide 13 is 2 to 3 m³ / min.

[0092] Simultaneously, the powerful adsorption capacity of supercritical carbon dioxide 13 is utilized to replace the adsorbed methane in the coal seam 3 with a free state. At the same time, the high diffusivity and low surface tension of supercritical carbon dioxide 13 are utilized to allow the supercritical carbon dioxide 13 to penetrate into the micropores of the coal seam 3, further increasing the content of free methane in the coal seam 3.

[0093] Step 8: Hydraulic fracturing and CO2 storage;

[0094] After the injection of a predetermined amount of supercritical carbon dioxide 13 is completed, a water-based fracturing fluid is injected into the top horizontal well 5 and the bottom horizontal well 6 at a set flow rate. As a preferred embodiment, the injection flow rate of the water-based fracturing fluid is 8 to 10 m³ / min. The injected water-based fracturing fluid is used to gradually expand the internal cracks of the coal seam 3, forming a complex fracture network. At the same time, the water-based fracturing fluid is used to squeeze the supercritical carbon dioxide 13 into the deep reservoir, further displacing the methane gas in the coal seam 3.

[0095] During the injection of the water-based fracturing fluid, the high molecular weight polymer and cross-linking agent in the water-based fracturing fluid gradually form a high-strength and high-toughness plugging layer in the cracks. The formed plugging layer effectively supports the cracks to prevent the cracks from closing after pressure relief. At the same time, the formed plugging layer effectively prevents supercritical carbon dioxide 13 from escaping from the cracks and returning to the wellbore.

[0096] Step 9: extraction of gasified products and coalbed methane 11;

[0097] S91: The gases produced during the gasification process are mainly carbon monoxide, hydrogen and other gasified products. The gasified products and desorbed methane gas produced by the combustion and gasification of the coal seam 3 are extracted through the intermediate horizontal well 4. After being extracted to the surface, they are separated and processed by a gas separation device. During the extraction process, the parameters of the gas returned from the intermediate horizontal well 4, such as gas flow rate, gas composition and other parameters, are monitored in real time by sensors installed in the extraction well. When no significant combustible gas is returned from the intermediate horizontal well 4 for a long time and the gas concentration is stably maintained within the set low concentration range, the intermediate horizontal well 4 is closed.

[0098] S92: Keep the top horizontal well 5 and the bottom horizontal well 6 closed at the same time, and store the previously injected supercritical carbon dioxide 13 for a long time to achieve the carbon emission reduction target.

[0099] The present invention proposes an innovative integrated method for the development of thin interbedded coal seams and carbon emission reduction, which effectively integrates fracturing, gasification and carbon sequestration technologies, thereby achieving efficient resource development and environmentally friendly protection. Specifically, firstly, through the early geological exploration and coal seam detailed evaluation process, the position of the intermediate horizontal well can be reasonably planned, and at the same time, a subsequent integrated drilling, fracturing, gasification and carbon sequestration plan can be scientifically and reasonably formulated; secondly, the positions of the top and bottom horizontal wells are reasonably planned, achieving effective coverage of deep thin interbedded coal seams, thereby providing great convenience for subsequent gasification, fracturing and other operations, which not only reduces the engineering volume and cost, but also improves the overall efficiency of the mining operation; thirdly, by setting a plurality of top fishbone wells obliquely below the top horizontal well, and at the same time, setting a plurality of bottom fishbone wells obliquely above the bottom horizontal well, the control range of the top horizontal well on the thin interbedded coal seam below can be effectively increased, and at the same time, the control range of the bottom horizontal well on the thin interbedded coal seam above can be increased. The control range of the inter-coal seam can be further expanded, and the middle horizontal well, top horizontal well, bottom horizontal well, top fishbone well and bottom fishbone well can be used to form a three-dimensional well network in the thin inter-coal seam structure; secondly, the use of oxygen-rich steam as the fracturing medium not only reduces the dependence on water resources, but also makes full use of the high temperature and high pressure of oxygen-rich steam to generate thermal stress in the reservoir rock, promote the formation and expansion of cracks, and form a complex large-scale artificial fracture network; thirdly, the gasification process of the middle horizontal well is combined with the synchronous extraction of the top and bottom horizontal wells to form an efficient gasification-extraction cycle, which significantly improves the gasification efficiency and extraction efficiency; thirdly, the use of supercritical CO2 fracturing operations further expands the fracture network inside the coal seam, significantly improving the fluidity of coalbed methane. At the same time, through the displacement effect of supercritical CO2, the adsorbed methane in the coal seam can be effectively converted into a free state, thereby significantly improving the production and quality of coalbed methane. Furthermore, through the injection of water-based fracturing fluid, not only can the internal cracks of the coal seam be further expanded, but supercritical carbon dioxide can also be squeezed into the deep reservoir, further displacing the methane gas in the coal seam. At the same time, the high molecular polymers and cross-linking agents in the water-based fracturing fluid gradually form a high-strength and high-toughness plugging layer in the cracks. In this way, the plugging layer can be used to effectively support the cracks to prevent the cracks from closing after unloading. The plugging layer can also effectively prevent the escape of supercritical carbon dioxide from the cracks. Finally, the use of intermediate horizontal wells to extract gasification products and methane gas can achieve more complete mining of thin interbedded coal seams.

[0100] The present invention combines oxygen-enriched steam fracturing, supercritical CO2 fracturing, and traditional hydraulic fracturing technologies. By combining the high temperature and high pressure of oxygen-enriched steam with the support of coal gangue particles, a complex and stable fracture network can be formed, significantly improving the permeability of the reservoir and generating additional gas production. The combined technology of hydraulic fracturing and supercritical CO2 storage further enhances the sealing of the formation, thereby ensuring the carbon sequestration effect. As a result, the present invention significantly improves the aperture of the fractures and the gasification efficiency, and realizes in-situ fluidized mining of coal resources, reducing the generation of solid waste and the degree of damage to the surface.

[0101] In summary, the integrated fracturing, gasification, and carbon sequestration method for thin interbedded coal seams proposed in this invention is highly efficient, environmentally friendly, and sustainable, providing a new technical approach for coalbed methane development in thin interbedded coal seams. This method not only overcomes the limitations of traditional hydraulic fracturing technology but also effectively improves reservoir reconstruction and significantly enhances the effectiveness and efficiency of coal seam mining. Therefore, it has significant application prospects and promotional value, and this technology is expected to become one of the key technical means for coalbed methane development in the future.

Claims

1. A method for integrating fracturing, gasification and carbon sequestration of thin interbedded coal seams, characterized in that: The following steps are involved: Step 1: Geological exploration and coal seam (3) detailed assessment; S11: Conduct ground geological surveys on the development block, observe outcrop rock samples of the coal seam (3), and record the characteristics of the coal seam (3). The surface of the development block is mudstone strata (1), and the part below the mudstone strata (1) is a thin interbedded structure of coal seams (3) and sandstone strata (2) alternating with each other; S12: Detect the distribution of the underground coal seam (3), determine the thickness and distribution range of the coal seam (3), as well as the interbedded relationship between the coal seam (3) and the mudstone stratum (1), and the interbedded relationship between the coal seam (3) and the sandstone stratum (2). At the same time, obtain the key parameters of the coal seam (3), fully understand the geological structure of the coal seam (3), and analyze the influence of the geological structure of the coal seam (3) on the migration and enrichment of coalbed methane (11); S13: Determine the gas content, pressure data and gas composition in the coal seam (3) through logging, sampling and analysis techniques; S14: Comprehensively evaluate the coal seam (3) by integrating geological exploration results, well logging data, gas content and pressure data, determine the development potential of the coal seam (3), and scientifically and rationally determine the location of subsequent intermediate horizontal wells (4) for drilling. At the same time, formulate an integrated plan for subsequent drilling, fracturing, gasification and carbon sequestration; Step 2: Drilling of the intermediate horizontal well (4); S21: Based on the geological exploration data of the coal seam (3), the thickness, dip angle, and permeability of the coal seam (3) are comprehensively analyzed, and based on the distribution range of the mudstone stratum (1), sandstone stratum (2), and coal seam (3), the coal seam (3) with good reservoir properties in the middle position is selected as the drilling layer of the intermediate horizontal well (4); S22: Based on the selected drilling layer, the three-dimensional geological model is used to plan the drilling path to ensure that the drilling trajectory can pass through the target coal seam (3) and that the layer encountered by the intermediate horizontal well (4) has good reservoir properties; S23: Using coalbed methane (11) drilling equipment and process technology, drill an intermediate horizontal well (4) from the ground downward; Step 3: Drilling the top horizontal well (5) and the bottom horizontal well (6); In the uppermost sandstone formation (2) of the thin interbedded coal seam, a top horizontal well (5) is planned and drilled, while ensuring that the axis of the top horizontal well (5) is parallel to the axis of the middle horizontal well (4), and the top horizontal well (5) is located at a position 1 / 5 to 1 / 4 below the interior of the upper sandstone formation (2); In the sandstone formation (2) at the bottom of the thin interbedded coal seam, a bottom horizontal well (6) is planned and drilled, while ensuring that the axis of the bottom horizontal well (6) is parallel to the axis of the middle horizontal well (4), and the bottom horizontal well (6) is located at a position 1 / 5 to 1 / 4 above the interior of the lower sandstone formation (2); Step 4: Fishbone well drilling; In the top horizontal well (5), a directional sidetracking tool is used to drill a first top fishbone well (7) obliquely downward in a clockwise direction of 45 to 60 degrees; the above process is repeated multiple times to drill the remaining top fishbone wells (7) in the top horizontal well (5), and ensure that the spacing between adjacent top fishbone wells (7) is between 20 and 30 meters; in the bottom horizontal well (6), a directional sidetracking tool is used to drill a first bottom fishbone well (8) obliquely upward in a clockwise direction of 45 to 60 degrees; the above process is repeated multiple times to drill the remaining bottom fishbone wells (8) in the bottom horizontal well (6), and ensure that the spacing between adjacent bottom fishbone wells (8) is between 20 and 30 meters; A three-dimensional well network is formed in the thin interbedded coal seam structure by using the middle horizontal well (4), the top horizontal well (5), the bottom horizontal well (6), the top fishbone well (7) and the bottom fishbone well (8); Step 5: Fracturing of the intermediate horizontal well (4); S51: lowering the fracturing tool to a predetermined position in the intermediate horizontal well (4) through the coiled tubing, installing the surface equipment and connecting it to the fracturing tool, and at the same time, ensuring the stable transmission of feedback signals and control signals during the fracturing process; S52: Pure water is converted into water vapor through a heating device on the ground, and a certain proportion of oxygen is mixed in to form oxygen-enriched water vapor (9); S53: Using a surface pump group, oxygen-rich water vapor (9) is injected into the intermediate horizontal well (4) through a continuous oil pipe. Under high pressure, the oxygen-rich water vapor (9) is squeezed into the reservoir to perform a fracturing operation. During the fracturing process, the high temperature and high pressure of the oxygen-rich water vapor (9) is used to generate thermal stress in the reservoir rock, thereby promoting the formation and expansion of cracks and forming a complex large-scale artificial fracture network (10); S54: Continuously pumping in oxygen-rich steam (9) allows the fracture to not only expand in the coal seam (3) where the intermediate horizontal well (4) is located, but also extend upward and downward into the adjacent coal seams (3), thereby increasing the connectivity range of the artificial fracture (10); Step 6: Underground gasification of coal seam (3) and extraction of coalbed methane (11); S61: During the fracturing process of the intermediate horizontal well (4), when the input amount of the oxygen-enriched water vapor (9) reaches 300-400 m³, the injection operation of the oxygen-enriched water vapor (9) is stopped; then, an ignition rod is dropped into the bottom of the intermediate horizontal well (4) through a special dropping device, and the coal seam (3) is ignited by the ignition rod, thereby initiating an underground gasification process of the coal seam (3); S62: After the coal seam (3) is ignited, oxygen-rich steam (9) is continuously injected into the intermediate horizontal well (4) to maintain the continuous gasification process of the coal seam (3). At the same time, the high temperature and high pressure of the oxygen-rich steam (9) is used to promote further expansion of the cracks, and the gas and heat generated by the gasification of the coal seam (3) are simultaneously used to promote the formation and expansion of the cracks. While the gasification operation of the middle horizontal well (4) is being carried out, the top horizontal well (5) and the bottom horizontal well (6) are used to synchronously carry out the extraction operation of the coalbed methane (11). At the same time, the extraction process is used to reduce the pressure in the coal seam (3), further promoting the in-depth progress of the gasification reaction in the middle horizontal well (4); S63: When the gasification process of the middle horizontal well (4) is completed, the gasification agent injection system of the middle horizontal well (4) is closed, and the well is shut in to allow the downhole combustion process to proceed fully. Through the combustion process, a combustion void zone (12) is formed around the artificial fracture network (10) in the middle horizontal well (4); at the same time, the extraction operation of the top horizontal well (5) and the bottom horizontal well (6) is continued, and this extraction process is continued until the set extraction time is completed; Step 7: Supercritical CO2 fracturing and displacement; After the extraction operation is completed, the top horizontal well (5) and the bottom horizontal well (6) are immediately subjected to supercritical CO2 fracturing operation. During the supercritical CO2 fracturing process, by precisely controlling the injection flow rate of supercritical carbon dioxide (13), the internal cracks of the coal seam (3) are induced to further expand, forming a more complex and more connected three-dimensional crack network (14). Simultaneously, the strong adsorption capacity of supercritical carbon dioxide (13) is utilized to replace the adsorbed methane in the coal seam (3) with a free state. At the same time, the high diffusivity and low surface tension characteristics of supercritical carbon dioxide (13) are utilized to allow the supercritical carbon dioxide (13) to penetrate into the micropores of the coal seam (3), further increasing the content of free methane in the coal seam (3); Step 8: Hydraulic fracturing and CO2 storage; After the injection of a predetermined amount of supercritical carbon dioxide (13) is completed, a water-based fracturing fluid is injected into the top horizontal well (5) and the bottom horizontal well (6) at a set flow rate, and the injected water-based fracturing fluid is used to gradually expand the internal cracks of the coal seam (3), forming a complex fracture network; at the same time, the water-based fracturing fluid is used to squeeze the supercritical carbon dioxide (13) into the deep part of the reservoir, further displacing the methane gas in the coal seam (3); During the injection of the water-based fracturing fluid, the high molecular weight polymer and cross-linking agent in the water-based fracturing fluid gradually form a high-strength and high-toughness plugging layer in the cracks. The formed plugging layer effectively supports the cracks to prevent the cracks from closing after pressure relief. At the same time, the formed plugging layer effectively prevents supercritical carbon dioxide (13) from escaping from the cracks and returning to the wellbore. Step 9: Extraction of gasification products and coalbed methane (11); S91: The gasification products and desorbed methane gas generated by the combustion and gasification of the coal seam (3) are extracted through the intermediate horizontal well (4), and separated and processed by a gas separation device after being extracted to the ground; during the extraction process, the parameters of the gas returned from the intermediate horizontal well (4) are monitored in real time by a sensor installed in the intermediate horizontal well, and the intermediate horizontal well (4) is closed when no significant combustible gas is returned from the intermediate horizontal well (4) for a long time and the gas concentration is stably maintained within a set low concentration range; S92: The top horizontal well (5) and the bottom horizontal well (6) are kept closed at the same time, and the supercritical carbon dioxide (13) injected previously is sealed for a long term.

2. The integrated method of fracturing, gasification and carbon sequestration of thin interbedded coal seams according to claim 1, characterized in that: In step 1, geological radar and seismic exploration techniques are used to detect the distribution of underground coal seams (3).

3. The integrated method of fracturing, gasification and carbon sequestration of thin interbedded coal seams according to claim 1 or 2, characterized in that: In step 2 S23, after the drilling operation of the intermediate horizontal well (4) is completed, the drilling effect is evaluated by logging technology. The evaluation content includes the accuracy of the drilling trajectory and the penetration of the coal seam (3). Based on the evaluation results, the subsequent fracturing, gasification and carbon sequestration plans are adjusted and optimized.

4. The method for integrating fracturing, gasification and carbon sequestration of thin interbedded coal seams according to claim 3, characterized in that: During the drilling process in step 3, the drilling trajectory and formation changes are monitored in real time to ensure that the position of the horizontal well is accurate and the trajectory is smooth. At the same time, the top horizontal well (5) and the bottom horizontal well (6) share a vertical well.

5. The integrated method of fracturing, gasification and carbon sequestration of thin interbedded coal seams according to claim 4, characterized in that: In step 4, during the drilling process of the top fishbone well (7), the inclination and azimuth of the top fishbone well (7) are monitored in real time to ensure that the top fishbone well (7) extends according to the predetermined trajectory. At the same time, it is ensured that the top fishbone well (7) passes through at least one group of adjacent coal seams (3) and sandstone formations (2) after passing through the formation encountered by the connected top horizontal well (5); during the drilling process of the bottom fishbone well (8), the inclination and azimuth of the bottom fishbone well (8) are monitored in real time to ensure that the bottom fishbone well (8) extends according to the predetermined trajectory. At the same time, it is ensured that the bottom fishbone well (8) passes through at least one group of adjacent coal seams (3) and sandstone formations (2) after passing through the formation encountered by the connected bottom horizontal well (6).

6. The integrated method of fracturing, gasification and carbon sequestration of thin interbedded coal seams according to claim 5, characterized in that: In step 4, the plurality of bottom fishbone wells (8) and the plurality of top fishbone wells (7) are symmetrically distributed, and the spacing between two adjacent bottom fishbone wells (8) and two adjacent top fishbone wells (7) is the same. At the same time, the lengths of the bottom fishbone wells (8) and the top fishbone wells (7) are the same.

7. The integrated method of fracturing, gasification and carbon sequestration of thin interbedded coal seams according to claim 6, characterized in that: In step 5 S52, the volume fraction of oxygen mixed into the water vapor is 40% to 60%; in step 5 S53, during the fracturing process, the flow rate of the oxygen-enriched water vapor (9) is adjusted to 3.0 to 4.5 m³ / min.

8. The integrated method of fracturing, gasification and carbon sequestration of thin interbedded coal seams according to claim 7, characterized in that: In step 6 S63, the extraction time is set to be no less than 5 hours.

9. The integrated method of fracturing, gasification and carbon sequestration of thin interbedded coal seams according to claim 8, characterized in that: In step seven, the injection flow rate of supercritical carbon dioxide (13) is 2 to 3 m³ / min.

10. The integrated method of fracturing, gasification and carbon sequestration of thin interbedded coal seams according to claim 9, characterized in that: In step eight, the injection flow rate of the water-based fracturing fluid is 8 to 10 m³ / min.

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