A method for increasing coal seam gas production by combining water jet cutting with hot flue gas displacement

Abstract

The present invention discloses a method for increasing coalbed gas production by synergizing water jet slotting with hot flue gas displacement. First, a slotted gas injection borehole and an extraction borehole are drilled in a low-permeability coal seam. The slotted gas injection borehole is then slotted with high-pressure hydraulic force to form an annular slot groove. Gas injection and extraction pipes are then installed in the slotted gas injection and extraction boreholes, respectively, and the holes are sealed. Free gas in the slotted gas injection borehole is first extracted, and then hot flue gas is injected into the slotted gas injection borehole. The high temperature and displacement properties of the hot flue gas promote gas desorption and flow within the coal pores and fissures, forming new gas migration channels. Finally, the desorbed gas is extracted a second time in the extraction borehole. The present invention combines water jet cutting with hot flue gas displacement. It uses hydraulic cutting to initially improve the permeability of the coal seam through high-pressure water jets, and hot flue gas injection to promote the formation of new pores, drive the outflow of gas and seal a large amount of CO2 in the coal seam, thereby improving the gas extraction efficiency of difficult-to-extract coal seams.

Description

Technical Field

[0001] The present invention belongs to the technical field of coal mine production, and particularly relates to the extraction of coalbed methane (gas), specifically a method for increasing coalbed methane production by coordinating water jet cutting with hot flue gas displacement. Background Art

[0002] Deep coal seams contain gas characterized by high stress, low permeability, microporosity, and strong adsorption. Mining faces challenges related to low coal seam permeability, difficult gas desorption and flow, and low extraction rates, resulting in a significant amount of gas remaining in the seam. During coal mining, this residual gas is not only released into the excavation area, causing gas overshoots, but can also trigger gas outbursts, seriously threatening mine safety. Furthermore, the gas released into the excavation area is emitted into the atmosphere with mine airflow, contributing to the greenhouse effect (the greenhouse effect of gas is approximately 22 to 28 times that of carbon dioxide). Furthermore, over 20 billion cubic meters of gas are emitted into the atmosphere annually, resulting in a significant waste of gas resources.

[0003] The high-temperature flue gas produced by gas-fired power generation primarily consists of nitrogen, carbon dioxide, and hydrogen. If captured and injected into underground gas extraction boreholes or coalbed methane wells, it can displace and replace gas within the coal seam. Furthermore, the resulting thermal effect promotes gas desorption and flow within the pores and fissures of the coal mass, improving gas extraction efficiency. Research has shown that gas adsorption capacity decreases with increasing temperature. Furthermore, the high temperature of the flue gas can cause the coal mass to expand and deform, inducing secondary pores and fissures, which help form new pathways for gas migration. This provides a new approach to reducing carbon emissions and gas extraction. Summary of the Invention

[0004] The present invention aims to address the aforementioned problems in the prior art by providing a method for increasing coalbed gas production by combining water jet slitting with hot flue gas displacement. This method utilizes hot flue gas from gas-fired power plants and water jet permeability enhancement to extract gas. Specifically, hot flue gas is injected during hydraulic slitting to enhance permeability and displace coalbed gas, modifying the pore and fracture structure of the coal body and promoting gas desorption and flow, thereby improving gas extraction efficiency in difficult-to-extract coalbeds.

[0005] The present invention is achieved through the following technical solutions:

[0006] A method for increasing coalbed gas production by combining water jet cutting with hot flue gas displacement comprises the following steps:

[0007] A. Identify the low permeability coal seam, and then use conventional drilling methods to drill holes in the coal seam to be extracted. The top of the hole passes through the coal seam roof 0.5-1m, and the interval between adjacent holes is 5-10m. The holes are arranged in an alternating manner of slotted gas injection holes and extraction holes.

[0008] B. Use a water jet slotting system to perform slotting operations in slotting gas injection drilling holes. Open the water flow electromagnetic shut-off valve and set the water pump pressure to 80-150MP to start hydraulic slotting operations. Slot the same coal seam location for 5-10 minutes. After completing one slotting operation, withdraw the drill 1-2m and then slot again. Repeat this step according to the actual coal seam thickness.

[0009] The extraction borehole is sealed conventionally and the extraction pipe extending from the exposed end of the borehole is connected to the extraction pump in the extraction system.

[0010] C. After the slitting is completed, the drill is withdrawn to release the accumulated water and solid coal slag in the slit gas injection borehole, and a drilling plugging device is used to seal the hole between the coal seam and the rock layer to reduce subsequent gas leakage. The drilling plugging device reserves the slit gas injection hole and the primary extraction hole, and the gas injection pipe and the extraction pipe are extended from the reserved hole into the drilling plugging device 30-40cm above the drilling hole. The gas injection pipe is directly connected to the hot flue gas injection pump.

[0011] D. Open the first extraction throttle valve and the first gas extraction device to extract the free and partially desorbed gas in the coal seam after hydraulic fracturing; use the gas-solid filter in the first gas extraction device to filter the extracted gas, and return it to the coal-fired power plant through the gas transportation pipeline for power generation. At the same time, start the hot flue gas injection pump to the high-pressure gas insulation tank, and shut down the hot flue gas injection pump when the gas tank volume is filled to 70%-80%.

[0012] E. When the gas extraction flow rate drops to 20%-45% of the normal range, close the first extraction throttle valve and the first gas extraction device, open the tank outlet valve, adjust the hot flue gas heating device to keep the output hot flue gas temperature at 400-500°C, turn on the hot flue gas booster pump and adjust the pump pressure range to 5-10MPa to inject hot flue gas into the slotted gas injection borehole for displacement. The CO2 in the hot flue gas can drive and replace the gas that cannot be normally resolved in the coal seam. The high temperature causes the coal body to expand and deform and induce secondary holes and cracks.

[0013] F. After the displacement is completed, close the tank outlet valve and the hot flue gas booster pump, open the second extraction throttle valve and the second gas extraction device, and pass the extracted gas through the gas-solid filter in the second gas extraction device to remove impurities. The separated gas is returned to the coal-fired power plant through the gas transportation pipeline for power generation, and N2 and CO2 are collected in the gas storage tank of the second gas extraction device for gas injection displacement in other coal seams.

[0014] Ultimately, the above steps achieve the effect of increasing gas production.

[0015] As a preferred technical solution, an N2 monitoring meter is provided on the drilling plugging device at the slotted gas injection drilling site.

[0016] As an optimal technical solution, the hot flue gas injection method is cyclic injection, and each slit gas injection drilling adopts the "inject for two days and stop for ten days" cycle until the N2 monitoring meter on the drilling plugging device detects N2 and stops gas injection.

[0017] As a preferred technical solution, all downhole equipment adopts intrinsically safe equipment.

[0018] As a preferred technical solution, the main components of hot flue gas are N2, CO2 and high-temperature water vapor.

[0019] As a preferred technical solution, the high-pressure gas insulation tank is provided with a drain port to reduce the adverse effects of moisture on displacement.

[0020] As an optimal technical solution, a real-time gas concentration detection device is provided on the extraction pipe, which automatically closes the first extraction throttle valve when the gas concentration threshold is reached.

[0021] Compared with traditional coal seam permeability enhancement technology, the present invention combines water jet cutting of coal seams with hot flue gas displacement and gas extraction, which has the following advantages:

[0022] (1) First, hydraulic cutting is performed to fully utilize the water hammer pressure of the water jet to destroy the coal rock, which initially improves the permeability of the coal seam and forms a large number of initial cracks, providing channels for gas migration and increasing more injection space and specific surface area in contact with coal for subsequent hot flue gas injection.

[0023] (2) Injecting hot flue gas from a coal mine gas power plant into the borehole to heat the surrounding coal, activate the coal pores, and promote gas desorption. Under the combined action of pressure drive and multi-media competition, the hot flue gas and desorbed gas migrate to the activated pores, and the heat flow carried by the flue gas continues to conduct into the coal body, promoting the formation of new pores and driving the gas outflow, thereby improving the gas extraction efficiency of difficult-to-extract coal seams.

[0024] (3) The present invention seals a large amount of CO2 in hot flue gas in coal seams and proposes a new method of enhanced permeability and displacement of gas, which solves multiple problems such as efficient development of gas in deep, high-gas and difficult-to-extract coal seams and reduction of greenhouse gas emissions such as hot flue gas in coal mining areas, and brings the construction of green mines to a new level. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments of the present invention. It should be understood that the following drawings only illustrate certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other relevant drawings can be obtained based on these drawings without paying any creative work.

[0026] Figure 1Flowchart of the method of the present invention.

[0027] Figure 2 The system block diagram of the implementation process of the method of the present invention.

[0028] Figure 3 Schematic diagram of the arrangement of the holes drilled in the method of the present invention.

[0029] In the figure: 1-thermal power plant, 2-hot flue gas injection pump, 3-high-pressure gas insulation tank, 4-gas transport pump, 5-hot flue gas heating device, 6-hot flue gas booster pump, 7-N2 monitoring meter, 8-injection throttle valve, 9-water tank, 10-water pump, 11-water flow electromagnetic shut-off valve, 12-drilling rig, 13-first gas extraction device, 14-gas storage tank, 15-second gas extraction device, 16-second extraction throttle valve, 17-first extraction throttle valve, 18-drill bit, 19-lateral nozzle, 20-extraction pipe, 21-gas injection pipe, 22-coal seam, 23-extraction borehole, 24-slit gas injection borehole, 25-working face return air tunnel, 26-working face air inlet tunnel, 27-secondary extraction pipeline, 28-working face cut hole, 29-rock layer, 31-tank outlet valve, 32-drain port. DETAILED DESCRIPTION

[0030] The present invention will be further described below with reference to the following examples, but it should not be understood that the scope of the present invention is limited to the following examples. Without departing from the above technical ideas of the present invention, various substitutions and modifications can be made according to common technical knowledge and customary means in the art, and all should be included in the scope of protection of the present invention.

[0031] This embodiment provides a method for increasing coalbed gas production by combining water jet cutting with hot flue gas displacement. The various systems involved in the implementation process of this method are as follows: Figure 2 As shown, it includes coal body system, water jet drilling system, hot flue gas system and gas extraction system.

[0032] The coal body system includes coal seams and rock strata, on which extraction boreholes 23 and slotted gas injection boreholes 24 are opened.

[0033] The water jet drilling system includes a drilling rig 12, a water tank 9, a water pump 10, a water flow electromagnetic shut-off valve 11 and a drill bit 18. The water tank 9 is connected to the water pump 10 through a pipeline, the water pump 10 is connected to the water flow electromagnetic shut-off valve 11 through a pipeline, the water flow electromagnetic shut-off valve 11 is connected to the drilling rig 12 through a pipeline, and the drill bit 18 is connected to the drilling rig 12. Several horizontal nozzles 19 are provided on the drill bit 18.

[0034] The hot flue gas system includes a thermal power plant 1, which is connected to a hot flue gas injection pump 2 through a pipeline. The hot flue gas injection pump 2 is connected to a high-pressure gas insulation tank 3 through a pipeline. The high-pressure gas insulation tank 3 is provided with an exhaust port and a drain port. The exhaust port is provided with an outlet valve and is connected to a hot flue gas heating device 5 through a pipeline. The hot flue gas heating device 5 is connected to a hot flue gas booster pump 6 through a pipeline. The hot flue gas booster pump 6 is connected to an injection pipe 21 through a pipeline with an injection throttle valve 8. The injection pipe 21 passes through the drilling sealing device and extends into the slit gas injection borehole 24. An N2 monitoring meter is provided on the drilling sealing device at the slit gas injection borehole 24.

[0035] The gas extraction system includes a gas storage tank 14, a first gas extraction device 13, and a second gas extraction device 15. The gas storage tank 14 is connected to the first gas extraction device 13 and the second gas extraction device 15 through pipelines; the first gas extraction device 13 is connected to the extraction pipe 20 through a pipeline with a first extraction throttle valve, and the extraction pipe 20 passes through the drilling sealing device and extends into the slit gas injection borehole 24; the second gas extraction device 15 is connected to the secondary extraction pipeline 27 through a pipeline with a second extraction throttle valve 16, and the secondary extraction pipeline 27 extends into the extraction borehole 23; the second gas extraction device 15 is connected to the thermal power plant 1 through a pipeline with a gas transport pump 4.

[0036] The above-mentioned method of increasing coal seam gas production by combining water jet cutting with hot flue gas displacement is as follows: Figure 1 As shown, the specific steps include:

[0037] A. Drilling construction: First determine the specific location of the deep low permeability coal seam, such as Figure 3 As shown, a row of through-layer drill holes are constructed from the working face return air tunnel 25 and the working face air inlet tunnel 26 to the working face cut eye 28. The drill holes are alternately arranged as extraction drill holes 23 and slit gas injection drill holes 24. The distance between adjacent drill holes is 5-10m (10m, 7m, 5m, etc. can be selected during specific implementation), and the top of the through-layer drill hole exceeds the roof coal seam by 0.5-1m (1m, 0.5m, 0.8m, etc. can be selected during specific implementation).

[0038] B. Hydraulic slotting: Use the drilling rig 12 to control the opening of the horizontal nozzle 19, open the water flow electromagnetic shut-off valve 11, set the pressure of the water pump 10 to 80-150MP (80MP, 125MP, 150MP, etc. can be selected in specific implementation), and the high-pressure water jet reaches the horizontal nozzle through the water braid to start hydraulic slotting of the slotting and gas injection drill hole. Slotting is performed for 5-10 minutes (8min, 5min, 10min, etc. can be selected in specific implementation) at the same coal seam position; after completing one slotting operation, close the water pump 10 and the water flow electromagnetic shut-off valve 11, retreat the drill 1-2m (2m, 1m, etc. can be selected in specific implementation) and then perform the above slotting operation again; repeat 4 times according to the thickness of the coal seam, complete the slotting operation, close the water pump 10 and the water flow electromagnetic shut-off valve 11, and retreat the drill.

[0039] C. Sealing operation: After completing the hydraulic seam cutting, drain the accumulated water and solid coal slag in the seam gas injection borehole 24. Then, simultaneously extend the gas injection pipe 21 and the extraction pipe 20 into the seam gas injection borehole 24. The gas injection pipe is provided with an injection throttle valve 8, a hot flue gas booster pump 6, a hot flue gas heating device 5 and a high-pressure gas insulation tank 3 in sequence. At the same time, extend the secondary extraction pipe 27 into the extraction borehole 23. A secondary sealing method is used between 22 to carry out the sealing operation to reduce the amount of gas leakage during subsequent gas injection. When sealing, a reserved hole is reserved for the extraction pipe to be inserted, and the extraction pipe 20 is inserted into the reserved hole 30-40 cm beyond the upper part of the drilling sealing device (40 cm, 30 cm, 35 cm, etc. can be selected during specific implementation). An electronic gas flow meter is installed on the first extraction throttle valve 17, and an N2 monitoring meter 7 is installed on the drilling sealing device of the slit gas injection borehole 24.

[0040] D. Initial gas extraction: Open the first extraction throttle valve 17 and the first gas extraction device 13 to extract the free and partially desorbed gas in the coal seam after hydraulic fracturing; filter the gas using the gas-solid filtration device in the first gas extraction device 13 and store it in the gas storage tank 14. A real-time gas concentration detection device is provided on the extraction pipe 20, which automatically closes the first extraction throttle valve 17 when the gas concentration reaches the threshold value; at the same time, open the hot flue gas injection pump 2 to inject the hot flue gas into the high-pressure gas insulation storage tank 3. When the volume of the gas tank 3 is filled to 70%-80%, close the hot flue gas injection pump 2. After a period of rest, open the drain port 32 to discharge the cooling water to reduce the adverse effects of moisture on displacement.

[0041] E. Inject hot flue gas for displacement: According to the real-time results of the electronic gas flowmeter, when the gas flow drops to 20%-45% of the initial flow, the first extraction throttle valve 17 is automatically closed, and the first gas extraction device 13 is closed at this time; the hot flue gas injection throttle valve 8, the hot flue gas heating device 5 and the tank outlet valve 31 are opened, and the hot flue gas booster pump 6 is set to adjust the pump pressure to 5-10MPa (7MPa, 5MPa, 10MPa, etc. can be selected during specific implementation). The hot flue gas is transported to the slotted gas injection borehole 24 through a high-temperature resistant pipeline and injected into the coal seam for displacement. The main components of the hot flue gas are N2, CO2 and high-temperature water vapor; the slotted gas injection borehole 24 adopts the "inject for 2 days and stop for 10 days" method for gas injection, and the gas injection is stopped when the N2 monitoring meter 7 detects N2.

[0042] F. Secondary extraction: After the displacement is completed, close the hot flue gas injection throttle valve 8, the storage tank outlet valve 31, the hot flue gas booster pump 6 and the hot flue gas heating device 5, and open the second extraction throttle valve 16, the second gas extraction device 15 and the gas transport pump 4. The gas-solid filter in the second gas extraction device 15 removes impurities and irrelevant gases from the extracted gas, and the separated N2 and CO2 are collected in the gas storage tank of the second gas extraction device 15 for gas injection displacement in other coal seams. The separated gas is returned to the thermal power plant 1 through the gas transport pipeline and the gas transport pump 4 for power generation.

[0043] The present invention utilizes the water hammer pressure of the water jet through hydraulic cutting to destroy the coal rock and initially improve the permeability of the coal seam, forming a large number of initial cracks, and initially providing channels for gas migration. The subsequent injection of hot flue gas increases more injection space and the specific surface area in contact with the coal. The high-pressure, high-temperature, multi-element and multi-phase characteristics of the hot flue gas are then utilized to promote the generation of pores to form a more complex fracture network, promote the full decomposition of gas, and improve the extraction efficiency.

[0044] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that it is still possible to modify the technical solutions described in the aforementioned embodiments, or to replace some or all of the technical features therein by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the scope of the technical solutions of the embodiments of the present invention, and they should all be included in the scope of the claims and description of the present invention.

Claims

1. A method for increasing coalbed gas production by combining water jet cutting with hot flue gas displacement, characterized in that: The following steps are involved: A. Identify low-permeability coal seams and then drill holes in the coal seams to be extracted using conventional drilling methods. The top of the holes should penetrate 0.5-1m through the coal seam roof, with 5-10m between adjacent holes. The holes should be arranged in an alternating pattern of slotted gas injection holes and extraction holes. B. Use a water jet slotting system to perform slotting operations in slotting gas injection drilling holes. Open the water flow electromagnetic shut-off valve and set the water pump pressure to 80-150MP to start hydraulic slotting operations. Slot the same coal seam location for 5-10 minutes. After completing one slotting operation, withdraw the drill 1-2m and then slot again. Repeat this step according to the actual coal seam thickness. Perform conventional sealing on the extraction borehole and connect the extraction pipe extending from the exposed end of the borehole to the extraction pump in the extraction system; C. After the slotting is completed, the drill is withdrawn to release the accumulated water and solid coal slag in the slotted gas injection borehole, and a borehole sealing device is used to seal the hole between the coal seam and the rock stratum to reduce subsequent gas leakage. The borehole sealing device reserves slotted gas injection holes and primary extraction holes, and the gas injection pipe and extraction pipe are extended from the reserved holes into the borehole sealing device 30-40 cm beyond the borehole. The gas injection pipe is directly connected to the hot flue gas injection pump; D. Open the first extraction throttle valve and the first gas extraction device to extract the free and partially desorbed gas in the coal seam after hydraulic fracturing; use the gas-solid filter in the first gas extraction device to filter the extracted gas and return it to the coal-fired power plant through the gas transportation pipeline for power generation; at the same time, start the hot flue gas injection pump to the high-pressure gas insulation tank and shut down the hot flue gas injection pump when the gas tank volume is 70%-80% filled; E. When the gas drainage flow rate drops to 20%-45% of the normal range, close the first drainage throttle valve and the first gas drainage device, open the storage tank outlet valve, adjust the hot flue gas heating device to maintain the output hot flue gas temperature at 400-500°C, turn on the hot flue gas booster pump and adjust the pump pressure range to 5-10MPa to inject hot flue gas into the slotted gas injection borehole for displacement. The CO2 in the hot flue gas can drive out and replace the gas that cannot be normally resolved in the coal seam. The high temperature causes the coal body to expand and deform, and induces secondary pores and fissures; F. After the displacement is completed, close the tank outlet valve and the hot flue gas booster pump, open the second extraction throttle valve and the second gas extraction device, and pass the extracted gas through the gas-solid filter in the second gas extraction device to remove impurities. The separated gas is returned to the coal-fired power plant through the gas transportation pipeline for power generation. N2 and CO2 are collected in the gas storage tank of the second gas extraction device for gas injection displacement in other coal seams; Ultimately, the above steps achieve the effect of increasing gas production.

2. The method for increasing coalbed gas production by combining water jet cutting with hot flue gas displacement according to claim 1 is characterized in that: An N2 monitoring meter is provided on the drilling plugging device at the slotted gas injection drilling site.

3. The method for increasing coalbed gas production by combining water jet cutting with hot flue gas displacement according to claim 2 is characterized in that: The hot flue gas injection method is cyclic injection. Each slot gas injection drilling adopts the "inject for two days and stop for ten days" method until the N2 monitoring meter on the drilling plugging device detects N2 and stops gas injection.

4. The method for increasing coalbed gas production by combining water jet cutting with hot flue gas displacement according to claim 1 is characterized in that: All underground equipment is intrinsically safe.

5. The method for increasing coalbed gas production by combining water jet cutting with hot flue gas displacement according to claim 1 is characterized in that: The main components of hot flue gas are N2, CO2 and high-temperature water vapor.

6. The method for increasing coalbed gas production by combining water jet cutting with hot flue gas displacement according to claim 1 is characterized in that: The high-pressure gas insulation tank is equipped with a drain port to reduce the adverse effects of moisture on displacement.

7. The method for increasing coalbed gas production by combining water jet cutting with hot flue gas displacement according to claim 1 is characterized in that: A real-time gas concentration detection device is installed on the extraction pipe, which automatically closes the first extraction throttle valve when the gas concentration reaches the threshold value.

Images