A stimulation method suitable for constructing coalbed methane in coal surface mining

Abstract

This invention belongs to the field of coalbed methane extraction technology, specifically relating to a method for enhancing coalbed methane production in structural coalfields. Addressing the shortcomings of poor fracture system connectivity and ineffective conventional hydraulic fracturing in structural coalfield development zones, this invention combines radial borehole technology with electro-pulse technology to enhance coal seam production, achieving high-yield and high-efficiency coalbed methane development in structural coalfields. The method involves: arranging multi-layered, multi-directional radial boreholes in the coal seam section exposed by the vertical wellbore of the coalbed methane surface well, based on the coal seam geological conditions; after well flushing, using electro-pulse technology to impact radial boreholes layer by layer. The multi-layered, multi-directional, long-distance radial boreholes of this invention effectively connect the fracture system around the wellbore, significantly increasing the single-well control area of ​​coalbed methane wells in structural coalfield development zones. The high-energy shock waves generated by the electro-pulse technology further enhance the coal reservoir through fracture creation, unblocking, and desorption promotion effects.

Description

Technical Field

[0001] This invention belongs to the field of coalbed methane extraction technology, specifically relating to a method for enhancing coalbed methane production in structural coalfield development. Background Technology

[0002] Coalbed methane (CBM) primarily exists in an adsorbed state within coal seams. Therefore, its development involves depressurization to desorb the gas, allowing the free gas to migrate through the coal seam fracture system to the wellbore and reach the surface. The core of CBM development is depressurization and desorption, ensuring unobstructed coal seam fracture pathways. Common depressurization measures include drainage, while fracturing the reservoir is used to improve fracture patency.

[0003] Surface development of coalbed methane in fragmented and soft coal seams has always been a challenge in the industry. The reasons for this are directly related to the inherent physical properties of these seams. First, the most significant characteristic of fragmented and soft coal seams is their fractured and loose structure, generally with a strength coefficient (f) less than 0.5. Particularly, granular coal and mylonite, two types of structural coal, can be easily crumbled into powder by hand. Therefore, conventional hydraulic fracturing techniques are ineffective in fragmented and soft coal seams, failing to achieve the formation of a fracture network system. Second, the pore-fracture connectivity in fragmented and soft coal seams is very poor, and they are predominantly micropores, resulting in poor drainage and depressurization / desorption effects. These unique characteristics lead to the poor effectiveness of fracturing and other reservoir stimulation methods in structural coal seams, making it difficult to achieve depressurization / desorption through drainage.

[0004] For many years, the inventors of this invention have been dedicated to solving the problem of stimulation and production enhancement of low-yield coalbed methane wells. In 2018, the inventors published a paper entitled "Application of High-Energy Electric Pulse Technology in Coalbed Methane Wells in the Qinshui Basin," demonstrating that electric pulse technology has adaptive selection and good application prospects for unblocking and increasing production in the near-wellbore zone of coalbed methane wells. However, the application of electric pulse production enhancement technology in coal seams with low coal rock strength and fragmented coal structure has limitations. Due to its relatively weak stimulation effect and small radius of action, electric pulse technology can only be used as a secondary stimulation measure for coal reservoirs and cannot replace conventional fracturing technology as a primary stimulation measure for coal seams.

[0005] This invention integrates radial drilling technology to address the limitations of applying electro-pulse enhancement technology in coal seams with low coal and rock strength and fragmented coal structure. Summary of the Invention

[0006] To address the shortcomings of poor fracture system connectivity and poor performance of conventional hydraulic fracturing in tectonic coal development zones, this invention proposes a method for enhancing coal seam production through a combination of radial hole technology and electrical pulse technology in tectonic coal development zones, thereby achieving high-yield and high-efficiency surface development of coalbed methane in these zones.

[0007] This invention provides the following technical solution: a method for enhancing coalbed methane production in coal seam development, which involves arranging multi-layered and multi-directional radial boreholes in the coal seam section exposed by the vertical wellbore of the coalbed methane surface, based on the geological conditions of the coal seam; after well cleaning, using electric pulse technology to perform impact drilling layer by layer.

[0008] Furthermore, the greater the coal seam thickness, the more perforated layers there are, and the more fragmented the coal body, the greater the spacing between perforated layers.

[0009] Furthermore, for coal seams dominated by fractured coal, a layer of radial boreholes is arranged at 50 cm intervals from the bottom plate upwards; for coal seams dominated by granular coal and mylonite, a layer of radial boreholes is arranged at 70-80 cm intervals from the bottom plate upwards; the direction of the radial boreholes is perpendicular to or oblique to the direction of the primary fractures of the coal seam at an angle of more than 60°.

[0010] Furthermore, the radial borehole has a diameter of 5 centimeters and a length of 100 meters.

[0011] Furthermore, radial drilling includes the following steps:

[0012] S1: General cleaning of vertical well shaft;

[0013] S2: Window opening in the casing at the drilling location;

[0014] S3: Coiled tubing radial drilling: After opening the window, high-pressure water is injected into the coiled tubing from the ground. The high-pressure water jet breaks and drills into the coal seam until the designed length is reached and the drilling operation ends.

[0015] S4: Repeat steps S2 and S3 from the bottom of the coal seam upwards to form multiple layers of radial boreholes in the coal seam.

[0016] Furthermore, the electrical pulse impact radial drilling includes the following steps:

[0017] S1: Well cleaning and installation of electrical pulse device;

[0018] S2: Implement electric pulse operation: Use magnetic positioning to determine the operation location, start the operation from the radial borehole of the lowest layer of the coal seam, and operate step by step from bottom to top. The discharge voltage of the electric pulse device is 30 kV and the working frequency is 3-6 times / min.

[0019] S3: Remove the electric pulse device and flush the well: After the electric pulse operation of all radial boreholes is completed, remove the electric pulse device, run the flushing tubing to flush the well and loosen and break up the impurities in the wellbore.

[0020] Furthermore, coal seam geological conditions include coal seam thickness, coal body structure, coal seam dip, geostress direction, main fracture direction, and fracture development degree.

[0021] Compared with the prior art, the advantages of the present invention are:

[0022] The multi-layered, multi-directional, long-distance radial drilling of this invention effectively connects the fracture system around the wellbore, greatly increasing the single-well control area of ​​coalbed methane wells in structural coal development areas. Moreover, the multi-layered radial drilling will produce a certain pressure relief effect in the stress-concentrated structural coal, which can improve reservoir permeability. The high-energy shock waves generated by the electric pulse technology further modify the coal reservoir by creating fractures, unblocking blockages, and promoting desorption.

[0023] The electrohydraulic effect of the electric pulse technology can accelerate the desorption of coalbed methane within the radial hole control range; the energy of the electric pulse is much lower than that of fracturing operations, so there is no risk of roof perforation and no secondary safety hazards to coal mining; the radial hole can be constructed faster in structural coal, and the wellbore collapse during drilling and electric pulse operations has a hole enlargement effect; the whole operation process is safer and less expensive. Attached Figure Description

[0024] Figure 1 This is a diagram showing the arrangement of radial borehole layers.

[0025] Figure 2 This is a diagram showing the azimuth layout of radial boreholes.

[0026] Figure 3 This is a schematic diagram of radial drilling operations.

[0027] Figure 4 This is a schematic diagram of an electrical pulse operation.

[0028] In the diagram: 1-Wellbore; 2-Radial borehole; 2.1-First borehole; 2.2-Second borehole; 2.3-Third borehole; 2.4-Fourth borehole; 2.5-Fifth borehole; 2.6-Sixth borehole; 7-Recovery tank; 8-Radial drilling rig; 9-Coiled tubing; 10-Tubing; 11-Tubing annulus; 12-Diverter; 13-Coal seam; 14-Nozzle; 15-Electrical pulse control rig; 16-Transmission cable; 17-Downhole equipment; 18-Storm accumulator; 19-Shock wave generator; x-Main fracture direction. Detailed Implementation

[0029] To provide a clearer understanding of the technical features, objectives, and effects of the present invention, specific embodiments are now described with reference to the accompanying drawings. The figures are for illustrative purposes only and do not represent the actual proportions or shapes of the products; the same reference numerals indicate parts with the same structure or function but similar structures.

[0030] In this document, terms such as "parallel" and "perpendicular" are not strict mathematical and / or geometric limitations; they may also include errors that are understandable to those skilled in the art and permissible in the manufacture or use of the product. Furthermore, "perpendicular" includes not only the perpendicularity of two objects that are directly in contact in space, but also the perpendicularity of two objects that are not in contact in space.

[0031] like Figure 1 , Figure 2 The diagram illustrates a method for enhancing coalbed methane production in coal seam development. In the coal seam section exposed by a vertical wellbore, radial drilling technology is used to drill different levels and directions into the coal seam at different depths and along different azimuths, based on factors such as coal seam thickness, dip, stress direction, main fracture direction, and fracture development. The boreholes are approximately 5 cm in diameter and 100 meters long. The thicker the coal seam, the more borehole layers are required, with the specific number determined by the actual conditions of the coal seam. The borehole orientation is arranged as perpendicularly as possible to or at a large angle to the direction of the primary main fractures in the coal seam, maximizing communication with the primary fracture system. After all radial boreholes are drilled according to the design, coal dust and other impurities are flushed through the entire wellbore. High-energy electric pulse technology is then used to impact the radial boreholes layer by layer, completing a combined production enhancement technology using radial drilling and electric pulse techniques. In structurally developed coalfields, the coal seam is fragmented and soft with poor fracture connectivity, resulting in poor production enhancement from hydraulic fracturing. Multi-layered, multi-directional, and long-distance radial drilling effectively connects the fracture system within a 100-meter radius around the wellbore, significantly increasing the single-well control area of ​​coalbed methane wells in these areas. Furthermore, multi-layered radial drilling provides a certain pressure relief effect in stress-concentrated structural coal, improving reservoir permeability. In addition, electro-pulse technology, through the electrohydraulic effect, induces fractures and unblocks in the coal seam, further modifying the pore-fracture system of the coal reservoir.

[0032] In coal seam development zones, the coal body is fragmented and soft with poor fracture connectivity, making conventional hydraulic fracturing techniques largely ineffective. This invention proposes a method for enhancing coal seam production by combining radial drilling and electrical pulse technology. This method mainly involves three steps: after drilling the vertical wellbore of coalbed methane, designing the radial hole layout in the coal seam section, radial drilling with coiled tubing, and electrical pulse enhancement.

[0033] like Figure 1 , Figure 2 As shown: The first step is the design of the radial borehole layout. The layout of the radial boreholes is crucial and significantly affects the effectiveness of the production enhancement and stimulation. This invention requires, based on a thorough understanding of geological data such as coal seam thickness, coal body structure, coal seam dip, geostress direction, main fracture direction, and fracture development degree, the arrangement of multiple layers, multiple directions, and long distances of radial boreholes in the coal seam section exposed by the vertical well shaft.

[0034] The specific design method is as follows:

[0035] 1. Design of the number of borehole layers and the spacing between layers: This is mainly determined by the coal seam thickness and coal body structure. The greater the coal seam thickness, the more borehole layers are needed; the more fragmented the coal body, the larger the spacing between borehole layers. Generally, for coal seams dominated by fractured coal, a layer of boreholes is arranged every 50 cm from the bottom of the coal seam upwards. For coal seams dominated by granular coal and mylonite, the spacing between boreholes can be increased to 70-80 cm. For example, for a 3-meter-thick coal seam whose main coal body structure is fractured coal, 5 layers of boreholes spaced 50 cm apart are sufficient.

[0036] 2. Design of borehole orientation and number of boreholes per layer. This is mainly designed based on factors such as coal seam dip, principal stress direction, main fracture direction, and fracture development. Generally, the borehole orientation should be as parallel as possible to the coal seam dip direction, and the borehole orientation should be as perpendicular or obliquely at a large angle to the direction of the primary main fracture in the coal seam. If fracture development is relatively simple, a two-bore arrangement perpendicular or nearly perpendicular to the main fracture direction is adopted (e.g., ...). Figure 2 If the fracture development is complex or the degree of complexity is unclear, a large-angle oblique four-hole arrangement with a double-sided oblique angle of more than 60° to the main fracture direction is adopted (e.g., the first and second holes in the middle). Figure 1 (Third to sixth boreholes in the middle section) to make it possible to connect with the original fracture system of the coal seam as much as possible.

[0037] 3. Design of borehole diameter and extension length. The diameter of each borehole is about 5 cm. The borehole length is determined according to the coal body structure of the coal seam and the specific drilling difficulty, and is generally about 100 meters. If the coal body structure is poor and drilling is difficult, the borehole length can be appropriately shortened.

[0038] like Figure 3 As shown: Step 2, radial drilling. The specific construction procedure is as follows:

[0039] 1. Wellbore cleaning of vertical shafts. After the vertical shafts for coalbed methane are completed, well cleaning operations are carried out to remove impurities from the shaft.

[0040] 2. Casing windowing at the drilling location. The guide is lowered through the tubing, and the lowering depth of the guide is corrected using magnetic positioning and natural gamma logging to determine the casing windowing depth. A gyro inclinometer is used to determine the windowing orientation, and a milling drill bit is used to open the window on the casing.

[0041] 3. Coiled tubing radial drilling. After the window is opened, the surface radial well workover rig injects high-pressure water into the coiled tubing. The high-pressure water jet is then sprayed through the nozzle to break up and drill into the coal seam until the designed length is reached, thus ending the drilling operation.

[0042] 4. According to the borehole design, repeat steps 2 and 3 from the bottom of the coal seam upwards to form multiple radial boreholes with a certain diameter and length in the coal seam. During the operation, the construction return fluid containing coal powder is returned to the ground through the annulus of the oil jacket for recovery.

[0043] like Figure 4 As shown: Step 3, Electrical Pulse Enhancement Project. After all radial boreholes are drilled according to the design, coal dust and other impurities are flushed through the entire wellbore. High-energy electrical pulse technology is then used to impact the radial boreholes layer by layer, completing the combined radial borehole technology and electrical pulse technology for enhanced production. The specific construction procedures are as follows:

[0044] 1. Well cleaning and installation of electrical pulse devices. Well cleaning ensures unobstructed passage for downhole instruments, and electrical pulse devices are installed into the wellbore for drilling operations.

[0045] 2. Implement electrical pulse operation. Use magnetic positioning to determine the operation location, start the operation from the radial borehole at the bottom of the coal seam, and work step by step from bottom to top. The intensity and frequency of operation at each layer are determined according to the actual situation. Generally, the discharge voltage is 30kV and the working frequency is 3-6 times / min.

[0046] 3. Remove the instruments and flush the well. After all radial drilling electrical pulse operations are completed, remove the downhole instruments, run the flushing string to flush the well, and wash away the loosened and broken impurities in the wellbore. The entire operation is now complete.

[0047] It should be understood that although this specification is described according to various embodiments, not every embodiment contains only one independent technical solution. This way of describing the specification is only for clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other implementation methods that can be understood by those skilled in the art.

[0048] The detailed descriptions listed above are merely specific illustrations of feasible embodiments of the present invention and are not intended to limit the scope of protection of the present invention. All equivalent embodiments or modifications made without departing from the spirit of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for enhancing coalbed methane production in coalfield development, characterized in that: For soft and fractured coal seams with a firmness coefficient f less than 0.5, based on the geological conditions of the coal seam, multi-layered and multi-directional radial boreholes are arranged in the coal seam section exposed by the vertical wellbore of the coalbed methane surface well; after well cleaning, electric pulse technology is used to perform impact radial boreholes layer by layer. For coal seams dominated by fractured coal, a layer of radial boreholes is arranged at 50 cm intervals from the bottom of the coal seam upwards; for coal seams dominated by granular coal and mylonite, a layer of radial boreholes is arranged at 70-80 cm intervals from the bottom of the coal seam upwards. The direction of radial drilling is perpendicular to or obliquely intersecting the direction of the primary main fracture of the coal seam at an angle of more than 60°; if the fracture development is simple, a two-hole arrangement perpendicular or nearly perpendicular to the direction of the primary fracture is adopted; if the fracture development is complex or the degree of complexity is unclear, a four-hole arrangement with a large angle of more than 60° obliquely intersecting the direction of the primary fracture on both sides is adopted. The radial borehole has a diameter of 5 cm and a length of 100 m; Radial drilling uses high-pressure water jets ejected from nozzles at the end of continuous tubing to break up and drill into the coal seam.

2. The method for enhancing coalbed methane production in coalfield development according to claim 1, characterized in that: The greater the thickness of the coal seam, the more perforated layers there are; the more fragmented the coal body, the greater the spacing between perforated layers.

3. A method for enhancing coalbed methane production in structural coalfield development according to claim 1 or 2, characterized in that, The radial drilling process includes the following steps: S1: General cleaning of vertical well shaft; S2: Window opening in the casing at the drilling location; S3: Coiled tubing radial drilling: After opening the window, high-pressure water is injected into the coiled tubing from the ground. The high-pressure water jet breaks and drills into the coal seam until the designed length is reached and the drilling operation ends. S4: Repeat steps S2 and S3 from the bottom of the coal seam upwards to form multiple layers of radial boreholes in the coal seam.

4. A method for enhancing coalbed methane production in structural coalfield development according to claim 1 or 2, characterized in that, The aforementioned electrical pulse impact radial drilling includes the following steps: S1: Well cleaning and installation of electrical pulse device; S2: Implement electric pulse operation: Use magnetic positioning to determine the operation location, start the operation from the radial borehole of the lowest layer of the coal seam, and operate step by step from bottom to top. The discharge voltage of the electric pulse device is 30 kV and the working frequency is 3-6 times / min. S3: Remove the electric pulse device and flush the well: After the electric pulse operation of all radial boreholes is completed, remove the electric pulse device, run the flushing tubing to flush the well and loosen and break up the impurities in the wellbore.

5. The method for enhancing coalbed methane production in coalfield development according to claim 1, characterized in that: The geological conditions of the coal seam include coal seam thickness, coal body structure, coal seam dip, geostress direction, main fracture direction, and fracture development degree.

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