A downward coal seam drilling gas extraction efficiency improvement method based on micron bubble regulation

Abstract

The application discloses a downward coal seam drilling gas extraction efficiency improving method based on micron bubble regulation, and belongs to the technical field of coal mine gas extraction. In view of the problems of difficult drainage of water in the downward coal seam drilling, remarkable water locking effect, blocked negative pressure conduction of gas extraction and low gas extraction efficiency, the distribution characteristics of pressure decrease along the hole depth of the downward drilling are utilized, a hole bottom gas bubble volume ratio conversion formula is established based on a gas isothermal state equation, and the initial volume ratio of micron bubbles at the water inlet end can be inversely deduced according to a preset hole bottom target gas bubble proportion. The micron bubble water flow perfusion is switched at the end of drilling, the micron bubbles spontaneously expand in the low pressure area at the hole bottom relying on the original pressure gradient of the drilling, the retained water in the hole is driven and drained through extrusion replacement and interface drag reduction effect, and the water locking damage of the coal body is weakened. The application does not need additional drainage engineering, can quantitatively regulate and control the bubble expansion effect, significantly improves the negative pressure conduction capacity of the downward drilling, the gas extraction concentration and the scalar quantity, and is suitable for downward coal seam gas extraction drillings of various inclination angles.

Description

Technical Field

[0001] This invention relates to the fields of underground coal mine gas extraction and coal seam borehole drainage modification technology, specifically to a method for improving the efficiency of downward coal seam borehole gas extraction based on micron-sized bubble control. Background Technology

[0002] In coal mine underground gas control projects, due to limitations in roadway layout, drilling conditions, and production processes, a large number of gas extraction boreholes must be constructed using a downward inclination method. After the downward drilling is completed, construction wastewater and seepage water from the surrounding rock easily accumulate inside the borehole due to gravity. Conventional methods of drainage, blowing, and negative pressure drainage are insufficient to completely empty the water inside the borehole. The accumulated water remains in the lower half of the borehole and within coal seam fissures for extended periods, creating a severe water-locking effect: on the one hand, the accumulated water blocks the micropores of the coal seam, significantly reducing the gas permeability coefficient; on the other hand, the accumulated water obstructs the transmission of negative pressure during extraction, preventing effective communication of negative pressure in the latter half of the borehole, resulting in extraction blind zones. This leads to low gas extraction concentration, rapid attenuation, and low pure flow rate, severely restricting the efficiency of coal seam gas extraction and increasing the difficulty of underground gas control.

[0003] Existing downhole drainage technologies mainly include high-pressure air duct blowing, auxiliary drainage holes for diversion, and negative pressure pumping. These processes suffer from incomplete drainage, complex construction procedures, and high costs. Furthermore, conventional clear water drilling results in high surface tension and strong wettability of the water, leading to significant water lock damage to the coal seam. Currently, the industry lacks a low-cost, high-efficiency, drill-while-drilling-ready drainage modification technology that requires no additional engineering work.

[0004] Microbubbles possess advantages such as large specific surface area, strong interfacial activity, significant pressure expansion characteristics, and good stability in water. In low-pressure gradient pipelines, bubbles can expand autonomously as pressure decreases. Considering the distribution characteristics of high inlet water pressure, low bottom hydrostatic pressure, and decreasing pressure along the borehole, utilizing the pressure expansion characteristics of microbubbles to displace and drain accumulated water is a novel technical approach to solve the problem of residual water in downholes and weaken the water-locking effect. Therefore, there is an urgent need to develop a complete construction method based on controlling the volume ratio of microbubbles according to pressure decrease for directional drainage of residual water in downholes. Summary of the Invention

[0005] To address the technical challenges of existing downburst coal seam boreholes, such as difficulty in draining accumulated water, significant water-locking effects, poor negative pressure conduction, and low gas extraction efficiency, this invention provides a method for improving gas extraction efficiency in downburst coal seam boreholes based on micron-bubble control. This method utilizes the borehole pressure gradient to achieve controllable expansion of micron-bubbles, precisely replacing residual water in the borehole, mitigating water-locking damage to the coal body, opening deep gas seepage channels, and enhancing gas extraction effectiveness.

[0006] To achieve the above objectives, the present invention employs the following technical solution:

[0007] A method for improving the efficiency of gas extraction in downward coal seam boreholes based on micron-sized bubble control includes the following steps:

[0008] Step 1: Calculate the bottom water pressure Pd at the bottom of the borehole based on the downhole drilling parameters. The formula for calculating the bottom water pressure is: Pd = P0 + L·sinα·ρg; where P0 is the standard atmospheric pressure, α is the borehole inclination angle, L is the borehole length, ρ is the density of clean water, and g is the acceleration due to gravity.

[0009] Step 2: Calculate the gas expansion factor A based on the drilling rig borehole inlet water pressure Pr and the gas isothermal state equation, A=Pr / Pd; Combine the expansion factor to calculate the bottom bubble volume ratio Rd, the calculation formula is Rd=A·R0 / (A·R0+1-R0), where R0 is the initial volume ratio of micron-sized bubbles at the water inlet end;

[0010] Step 3: Based on the drainage requirements, preset the target bubble ratio at the bottom of the hole, and deduce the initial volume ratio of micron-sized bubbles at the inlet end in reverse.

[0011] Step 4: Select a micron bubble preparation process to prepare a uniform micron bubble water flow that meets the initial volume ratio requirements;

[0012] Step 5: When the coal seam borehole is near the end of the borehole, switch the original ordinary clean water supply of the drilling rig to micron bubble water flow.

[0013] Step 6: Continue to use micron-sized bubble water flow to complete the remaining drilling operations. After the hole is formed, continue to inject micron-sized bubble water flow until the gas content of the water flowing out of the hole is stable and significantly increased. Stop injecting water to complete the drilling drainage and modification operation, and seal the hole for gas extraction.

[0014] Optionally, the downward drilling is a drilling hole constructed by dipping the coal seam downwards underground, and the drilling inclination angle α ranges from 2° to 60°.

[0015] Optionally, in step two, the drilling rig's inlet water pressure Pr is controlled above 1 MPa, and the bottom hole water pressure Pd is less than the inlet water pressure Pr, forming a pressure gradient environment of high-pressure inlet water and low-pressure bottom hole along the drilling process.

[0016] Optionally, in step three, the proportion of target bubbles at the bottom of the hole, Rd, is above 50%.

[0017] Optionally, in step five, the water flow is switched when drilling reaches a position 1 to 3 m from the end of the borehole to ensure the operation time of the micron bubble water flow.

[0018] Optionally, in step six, the continuous injection time after hole formation shall not be less than 15 to 40 minutes, and the criterion for stopping shall be that the gas content of the water flowing out of the hole reaches more than 40% and is maintained for 5 minutes.

[0019] Optionally, the micron-sized bubble water flow expands in volume under the action of the pressure gradient inside the hole, and the bubbles squeeze and replace the water trapped at the bottom of the hole, weakening the water-locking effect in the coal pores and maintaining the gas seepage channel in the deep borehole.

[0020] Compared with the prior art, the present invention has at least the following advantages:

[0021] 1. Water replacement is achieved by relying on the original pressure field of the borehole: Based on the inherent pressure gradient of high pressure at the borehole opening and low pressure at the bottom of the borehole, the invention utilizes the pressure difference to achieve autonomous expansion of micron-sized bubbles. No additional pressurization equipment is required. Water replacement is achieved by relying on the original pressure field of the borehole, resulting in low energy consumption and strong adaptability.

[0022] 2. Establish a quantitative control method: Establish a calculation model for bottom water pressure and a conversion formula for bubble volume ratio to achieve quantitative prediction of the proportion of influent bubbles and the expansion ratio at the bottom of the hole. This avoids problems such as drainage failure due to insufficient bubble ratio and reduced fluid carrying capacity due to excessive bubble ratio, and provides strong controllability.

[0023] 3. Excellent drainage effect: After the micron-sized bubbles expand, they squeeze, disturb, and replace the water that remains at the bottom of the hole, greatly reducing the amount of residual water in the borehole, breaking through the water barrier layer, and allowing the negative pressure of extraction to smoothly penetrate the lower half of the coal body in the borehole, eliminating the blind zone of deep extraction.

[0024] 4. Reduce water-locking damage to coal: Micron-sized bubbles have hydrophobic interface properties, which can reduce the surface tension of coal and rock pores, break capillary water blockage, improve the connectivity of coal pores, and enhance gas permeability.

[0025] 5. Simple construction and low cost: Based on the existing drilling water supply system, there is no need to add drainage holes or pumping equipment. The construction is carried out while drilling, the process is simple, there are no chemical agents added, and it is green and environmentally friendly with no secondary pollution.

[0026] 6. Highly practical for engineering applications: It is suitable for drilling downward coal seams with different inclination angles and hole depths, and is compatible with conventional underground crawler drilling rigs and coal electric drills, and can be promoted and applied on a large scale. Detailed Implementation

[0027] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, specific embodiments of the present invention are described in detail below. Many specific details are set forth in the following description to provide a thorough understanding of the present invention. However, the present invention can be practiced in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of the present invention. Therefore, the present invention is not limited to the basic embodiments disclosed below.

[0028] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.

[0029] Example 1

[0030] The method for improving the efficiency of gas extraction in downward coal seam boreholes based on micron-bubble control according to embodiments of the present invention includes the following steps:

[0031] Step 1: Calculation of Borehole Water Pressure Parameters: Based on the downward borehole inclination angle α and borehole length L, establish the formula for calculating the hydrostatic pressure at the bottom of the borehole: Pd = P0 + L·sinα·ρg; where: P0 is standard atmospheric pressure (0.1013 MPa), ρ is the density of clean water (1000 kg / m³), and g is the acceleration due to gravity (9.8 m / s²); calculate the actual water pressure in the low-pressure zone at the bottom of the borehole to provide basic parameters for calculating the bubble expansion ratio. The downward borehole refers to a borehole drilled downwards in an underground coal seam, with the borehole inclination angle α ranging from 2° to 60°.

[0032] Step 2: Bubble Volume Ratio Conversion: Based on the gas isothermal equation of state and the drilling rig inlet water pressure Pr, calculate the bubble expansion ratio A = Pr / Pd; establish the conversion formula for the inlet and outlet bubble volume ratio Rd = A·R0 / (A·R0+1-R0), clarifying the quantitative relationship between the initial inlet water volume ratio and the volume ratio after expansion at the bottom of the borehole; where R0 is the initial volume ratio of micron-sized bubbles at the inlet end. The drilling rig inlet water pressure Pr is controlled above 1 MPa, and the bottom hole water pressure Pd is less than the inlet water pressure Pr, creating a pressure gradient environment along the drilling path with high-pressure inlet water and low-pressure bottom hole.

[0033] Step 3: Bubble Parameter Matching Design: Based on the drainage intensity requirements, preset the target bubble ratio Rd at the bottom of the pore. The higher the Rd, the stronger the drainage displacement capacity. The initial volume ratio R0 of micron-sized bubbles at the inlet end is determined by reverse derivation. The target bubble ratio Rd at the bottom of the pore is above 50%.

[0034] Step 4: Micron-sized bubble preparation: Using a multi-stage Venturi shear or high-pressure dissolved gas release preparation process, the operating pressure and gas-liquid ratio of the equipment are controlled to generate micron-sized bubble water flow with uniform particle size and strong stability.

[0035] Step 5, Switching to water flow while drilling: When the coal seam borehole is drilled to a position close to the tail end (1-3 m from the tail end), turn off the original ordinary clean water supply of the drilling rig and switch to micron bubble water flow with a constant volume ratio.

[0036] Step Six: Constant Pressure Injection and Drainage: Continuously use micron-sized bubble water flow to complete the final drilling. After drilling, maintain the injection of micron-sized bubble water flow for 15–40 minutes, observing the water outflow status at the borehole opening. Stop injection when the gas content of the water outflow at the opening significantly increases and becomes relatively stable (the criterion for stopping is that the gas content of the water outflow at the opening reaches more than 40% and is maintained for 5 minutes). This completes the drainage of water accumulated in the borehole, followed by borehole sealing and extraction. Here, the micron-sized bubble water flow expands in volume under the pressure gradient within the borehole. The bubbles squeeze and replace the water retained at the bottom of the borehole, weakening the water-locking effect in the coal seam pores and maintaining the deep gas seepage channels in the borehole.

[0037] Example 2

[0038] This embodiment describes an efficiency improvement operation for a gas extraction borehole at a medium dip angle of 25° and a depth of 80m in a certain mine. The specific steps are as follows:

[0039] S1. Parameter Calculation: Drilling inclination angle α = 25°, borehole length L = 80m, calculated bottom water pressure Pd = 0.1013 + 80 × sin25° × 1000 × 9.8 / 10 6 ≈0.43 MPa;

[0040] S2. Parameter matching: Drilling rig inlet water pressure Pr = 2.0 MPa, bubble expansion ratio A = 4.65; set the bottom hole high bubble ratio Rd = 52%, and calculate the initial inlet water volume ratio R0 = 15.8% in reverse.

[0041] S3. Bubble preparation: A multi-stage Venturi generator is used at a working pressure of 2.0 MPa to prepare a high volume ratio micron-sized bubble water flow with a bubble size of 20-50 μm and a volume ratio of 15.8%.

[0042] S4. Water flow switching: When drilling reaches the 77 m position (3 m from the tail end), switch the water supply system and introduce micron bubble water flow;

[0043] S5. Injection and drainage: After the hole is closed, continue injection for 20 minutes. The water coming out of the hole is mainly gas. The water output will decrease significantly and the gas content will stabilize. Stop the operation and seal the hole for extraction.

[0044] In this embodiment, the volume ratio of air bubbles at the bottom of the borehole reaches 52%. The high gas content creates a strong squeezing and water-driving effect, increasing the gas extraction concentration by 25% and the extraction purity by 20%. There is no water accumulation obstruction in the lower half of the borehole, the negative pressure conduction is smooth, and the extraction effect is significantly improved.

[0045] Example 3

[0046] This embodiment describes an efficiency improvement operation for a shallow, downward-facing gas extraction borehole with a 5° dip angle and a depth of 40m in a certain mine. The specific steps are as follows:

[0047] S1. Parameter Calculation: Drilling inclination angle α = 5°, borehole length L = 40m, calculated bottom water pressure Pd = 0.1013 + 40 × sin5° × 1000 × 9.8 / 10 6 ≈ 0.135MPa;

[0048] S2. Parameter matching: Set the drilling rig inlet water pressure Pr=1.2MPa, calculate the bubble expansion ratio A=Pr / Pd=1.2 / 0.135≈8.89; set the target bubble ratio at the bottom of the hole Rd=55%, substitute into the volume ratio conversion formula and calculate in reverse, to obtain the initial water inlet volume ratio R0≈12.1%;

[0049] S3. Bubble preparation: Using a high-pressure dissolved gas release process, the dissolved gas pressure is controlled at 1.2 MPa to prepare micron-sized bubble water flow with a bubble size of 30-50 μm and an initial volume ratio of 12.1%;

[0050] S4. Water flow switching: When the borehole reaches a position of 39 m (1 m from the tail end), switch to micron bubble water flow;

[0051] S5. Injection and drainage: Micron-sized bubble water flow is used to complete the remaining 1m of drilling to the final hole. After the hole is formed, injection is continued for 15 minutes. When the gas content of the water coming out of the hole is observed to be stable and significantly increased, injection is stopped, and then the hole is sealed and extracted.

[0052] This embodiment targets shallow boreholes with small inclination angles. By quantitatively matching bubble parameters, it effectively drives out water accumulated in the borehole and shallow fissures. On-site monitoring shows that the gas extraction concentration increased by 18%, the extraction purity increased by 15%, and the water-locking effect was significantly weakened.

[0053] Example 4

[0054] This embodiment describes an efficiency improvement operation for a deep, downward gas extraction borehole with a 50° dip angle and a depth of 120m in a certain mine. The specific steps are as follows:

[0055] S1. Parameter Calculation: Given a borehole inclination angle α = 50° and a borehole length L = 120m, calculate the bottom water pressure Pd = 0.1013 + 120 × sin50° × 1000 × 9.8 / 10 6 ≈ 1.002MPa;

[0056] S2. Parameter matching: Set the drilling rig inlet water pressure Pr=2.5MPa, calculate the bubble expansion ratio A=Pr / Pd=2.5 / 1.002≈2.49; set the target bubble ratio at the bottom of the hole Rd=50%, substitute into the volume ratio conversion formula and calculate in reverse, to obtain the initial water inlet volume ratio R0≈28.7%;

[0057] S3. Bubble preparation: A multi-stage Venturi shear combination process was adopted, and the working pressure was controlled at 2.5 MPa to prepare a high volume ratio micron bubble water flow with a bubble size of 20-40 μm and an initial volume ratio of 28.7%.

[0058] S4. Water flow switching: When the borehole reaches the 118 m position (2 m from the tail end), switch to micron bubble water flow;

[0059] S5. Injection and drainage: Micron-sized bubble water flow was used to complete the remaining 2m of drilling to the final hole. After the hole was formed, injection was continued for 40 minutes. It was observed that the gas content of the water coming out of the hole was stable and without significant fluctuations. Injection was then stopped, and the hole was sealed and the water was extracted.

[0060] This embodiment addresses the high bottom water pressure conditions in deep boreholes with steep inclination angles. By increasing the initial bubble volume ratio, the water-driving capacity at the bottom of the borehole is ensured. On-site monitoring shows that the water accumulated in the deep borehole is effectively driven out, the negative pressure of extraction is connected throughout the entire borehole section, the gas extraction concentration is increased by 30%, and the extraction purity is increased by 26%, effectively solving the problem of low extraction efficiency in deep boreholes.

[0061] It should be noted that, depending on the implementation needs, the steps described in the embodiments of the present invention can be broken down into more steps, or two or more steps or parts of steps can be combined into new steps, or some steps can be replaced with other construction steps with corresponding functions, so as to achieve the purpose of the embodiments of the present invention.

[0062] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A method for improving the efficiency of gas extraction in downward coal seam boreholes based on micron-sized bubble control, characterized in that, Includes the following steps: Step 1: Calculate the bottom water pressure Pd at the bottom of the borehole based on the downhole drilling parameters. The formula for calculating the bottom water pressure is: Pd = P0 + L·sinα·ρg; where P0 is the standard atmospheric pressure, α is the borehole inclination angle, L is the borehole length, ρ is the density of clean water, and g is the acceleration due to gravity. Step 2: Calculate the gas expansion factor A based on the drilling rig borehole inlet water pressure Pr and the gas isothermal state equation, A=Pr / Pd; Combine the expansion factor to calculate the bottom bubble volume ratio Rd, the calculation formula is Rd=A·R0 / (A·R0+1-R0), where R0 is the initial volume ratio of micron-sized bubbles at the water inlet end; Step 3: Based on the drainage requirements, preset the target bubble ratio at the bottom of the hole, and deduce the initial volume ratio of micron-sized bubbles at the inlet end in reverse. Step 4: Select a micron bubble preparation process to prepare a uniform micron bubble water flow that meets the initial volume ratio requirements; Step 5: When the coal seam borehole is near the end of the borehole, switch the original ordinary clean water supply of the drilling rig to micron bubble water flow. Step 6: Continue to use micron-sized bubble water flow to complete the remaining drilling operations. After the hole is formed, continue to inject micron-sized bubble water flow until the gas content of the water flowing out of the hole is stable and significantly increased. Stop injecting water to complete the drilling drainage and modification operation, and seal the hole for gas extraction.

2. The method for improving the efficiency of gas extraction in downward coal seam boreholes based on micron-sized bubble control according to claim 1, characterized in that, The downward drilling is a drilling hole drilled downwards in the underground coal seam, and the drilling inclination angle α ranges from 2° to 60°.

3. The method for improving the efficiency of gas extraction in downward coal seam boreholes based on micron-sized bubble control according to claim 1, characterized in that, In step two, the drilling rig's water inlet pressure Pr is controlled above 1 MPa, and the bottom hole water pressure Pd is less than the inlet pressure Pr, creating a pressure gradient environment with high-pressure inlet water and low-pressure bottom hole along the drilling process.

4. The method for improving the efficiency of gas extraction in downward coal seam boreholes based on micron-sized bubble control according to claim 1, characterized in that, In step three, the target bubble ratio Rd at the bottom of the hole is above 50%.

5. The method for improving the efficiency of gas extraction in downward coal seam boreholes based on micron-sized bubble control according to claim 1, characterized in that, In step five, the water flow is switched when the drilling reaches a position 1 to 3 m from the end of the borehole to ensure the operation time of the micron bubble water flow.

6. The method for improving the efficiency of gas extraction in downward coal seam boreholes based on micron-sized bubble control according to claim 1, characterized in that, In step six, the continuous injection time after hole formation shall not be less than 15 to 40 minutes, and the criterion for stopping shall be that the gas content of the water flowing out of the hole reaches more than 40% and is maintained for 5 minutes.

7. The method for improving the efficiency of gas extraction in downward coal seam boreholes based on micron-sized bubble control according to claim 1, characterized in that, The micron-sized bubble water flow expands in volume under the pressure gradient inside the borehole. The bubbles squeeze and replace the water trapped at the bottom of the borehole, weakening the water-locking effect in the coal seam pores and maintaining the gas seepage channel in the deep borehole.