Liquid nitrogen and static crushing agent coupled coal seam fracturing method

Abstract

The invention discloses a liquid nitrogen and static crushing agent coupled coal seam fracturing method. The method comprises the following steps: preparing frozen static crushing agent particles by using liquid nitrogen; the frozen static crushing agent particles are injected into the drill hole along with liquid nitrogen, the liquid nitrogen drives the coal body to generate cracks and continuously expand and develop due to the fact that the low temperature and the liquid nitrogen pressure are gradually increased, and the frozen static crushing agent particles flow into the cracks generated by the coal seam along with the liquid nitrogen; after liquid nitrogen fracturing is completed, the temperature of the coal seam is gradually recovered along with the influence of the ground temperature, frozen static crushing agent particles start to dissolve, at the moment, the static crushing agent and water start to generate chemical reaction volume expansion, and then the static crushing agent expands in the crack to form a static crushing agent expansion area to conduct secondary expansion fracturing on the crack; meanwhile, the static crushing agent expansion area supports the crack, the crack is prevented from being closed under the action of ground stress, and coal seam fracturing is completed. By means of the method, efficient fracturing of the coal seam can be achieved, cracks can be effectively supported, and therefore the follow-up closing condition is reduced, and finally the mining efficiency of the coal seam gas is effectively improved.

Description

Technical Field

[0001] The invention relates to a coal seam fracturing method coupling liquid nitrogen and a static crushing agent, belonging to the technical field of coal seam gas mining. Background Art

[0002] With the gradual depletion of shallow coal resources, coalbed methane mining has gradually moved to deeper areas. my country's coalbed methane resources generally have the characteristics of low porosity and low permeability. The extremely poor porosity and permeability characteristics hinder the effective seepage of coalbed methane, seriously restricting the rapid development of the coalbed methane industry.

[0003] In order to extract coalbed methane, the coal seam needs to be fractured. At present, hydraulic fracturing is mainly used. However, the fracturing technology has problems such as low efficiency and high cost, which limits the effective extraction of coalbed methane. Liquid nitrogen has a temperature of -196℃ under normal pressure. When it comes into contact with solids, it can quickly reduce the temperature of solids, causing the solids to shrink and form thermal stress along the internal radial direction. At the same time, when liquid nitrogen vaporizes and expands into pure nitrogen at 21℃, it has an expansion rate of 696 times, which can generate huge gas pressure in a limited space. If there is water in the pores inside the solid, it will also cause the water ice phase change to produce a volume expansion of about 9%, which can theoretically generate a frost heave force of up to 207MPa. Therefore, liquid nitrogen has also begun to be used in the coalbed fracturing process, and its permeability enhancement effect is good. Although liquid nitrogen fracturing technology can improve the efficiency and effect of fracturing, it lacks effective fracture support means after fracturing, resulting in the closure of fractures formed by fracturing due to the influence of ground stress, which ultimately leads to low efficiency of coalbed methane extraction.

[0004] Therefore, how to provide a new method that can not only achieve efficient fracturing of coal seams, but also effectively support the formed cracks, thereby reducing their subsequent closure, and ultimately effectively improving the efficiency of coalbed methane extraction, is the research direction required by the present invention. Summary of the invention

[0005] In view of the problems existing in the above-mentioned prior art, the present invention provides a coal seam fracturing method coupled with liquid nitrogen and a static crushing agent, which can not only achieve efficient fracturing of the coal seam, but also effectively support the formed cracks, thereby reducing the subsequent closure situation, and ultimately effectively improving the extraction efficiency of coalbed methane.

[0006] In order to achieve the above-mentioned purpose, the technical solution adopted by the present invention is: a coal seam fracturing method coupled with liquid nitrogen and static crushing agent, the specific steps are:

[0007] Step 1, preparing frozen static breaker particles: pouring a certain amount of liquid nitrogen into a heat preservation tank, installing a sieve plate with multiple sieve holes in the heat preservation tank, and the sieve plate is above the liquid nitrogen surface; preparing a static breaker solution by mixing a static breaker and water in a required ratio, and pouring the static breaker solution into the heat preservation tank, at which time the static breaker solution first flows through the sieve plate, passes through each sieve hole to form static breaker droplets, and the static breaker droplets are poured into the liquid nitrogen and quickly frozen to form frozen static breaker particles;

[0008] Step 2, drilling a hole in the coal seam: drilling a hole in the coal seam by a drilling rig, and installing a sealing device in the hole to seal the hole after completion; the sealing device is equipped with a nitrogen injection pipe and a ventilation pipe, one end of the nitrogen injection pipe and one end of the ventilation pipe are both in the borehole, and the other end of the nitrogen injection pipe and the other end of the ventilation pipe are both outside the borehole, and are respectively equipped with a nitrogen injection stop valve and a ventilation stop valve;

[0009] Step 3, equipment installation: connect the mixer to the inlet of the high-pressure nitrogen injection pump through a connecting pipe, and connect the outlet of the high-pressure nitrogen injection pump to the other end of the nitrogen injection pipe through a pressure-resistant pipe and a connector; the outlet of the high-pressure nitrogen injection pump is located at a pressure gauge for monitoring the nitrogen injection pressure;

[0010] Step 4: Liquid nitrogen fracturing of coal seams: First, open the nitrogen injection stop valve and the ventilation stop valve, pour liquid nitrogen into the mixer, and start the high-pressure nitrogen injection pump. At this time, the liquid nitrogen in the mixer is pumped by the high-pressure nitrogen injection pump, so that the liquid nitrogen is injected into the borehole through the pressure-resistant pipe and the nitrogen injection pipe; due to the high temperature inside the borehole, the liquid nitrogen entering the borehole continues to gasify and reduce the borehole temperature and produce nitrogen, which is discharged from the borehole through the ventilation pipe. As the borehole temperature continues to drop, liquid nitrogen gradually accumulates in the borehole and gradually fills the borehole; when liquid nitrogen is observed to flow out from the other end of the ventilation pipe, the ventilation stop valve is closed at this time to make A closed space is maintained in the borehole, and the frozen static crushing agent particles prepared in step 1 are added to the mixer at the same time. The frozen static crushing agent particles are injected into the borehole along with the liquid nitrogen. As the liquid nitrogen is continuously pressurized and injected into the borehole, the liquid nitrogen in the borehole freezes the coal body due to the low temperature, causing the coal body to become more brittle. At the same time, the liquid nitrogen pressure in the borehole gradually increases, driving the coal body to generate cracks and continuously expand and develop. The frozen static crushing agent particles flow into the cracks generated in the coal seam along with the liquid nitrogen; the frozen static crushing agent particles act as a proppant in the cracks, promoting the flow of liquid nitrogen in the cracks, and further increasing the crack expansion length;

[0011] Step 5, secondary fracturing with static breaker: After the liquid nitrogen fracturing work is completed, the high-pressure nitrogen injection pump and the nitrogen injection stop valve are turned off, and the frozen static breaker particles are retained in the cracks. As the temperature of the coal seam gradually recovers due to the influence of the ground temperature, the remaining liquid nitrogen in the borehole continues to gasify, causing the temperature in the borehole to continue to rise. When the melting point of the frozen static breaker particles is reached, it begins to dissolve. At this time, the static breaker begins to react chemically with water and expands in volume, and then expands in the crack to form a static breaker expansion zone, which performs secondary expansion fracturing on the cracks, so that secondary fracturing cracks are generated around the cracks. At the same time, the static breaker expansion zone supports the cracks to prevent the cracks from closing due to ground stress, completing the coal seam fracturing process.

[0012] Furthermore, the sieve holes on the sieve plate are evenly distributed, and the aperture of the sieve holes is determined according to the particle size of the frozen static breaker particles to be prepared.

[0013] Furthermore, one end of the ventilation pipe is located near the bottom of the borehole, and one end of the nitrogen injection pipe is located near the sealing device in the borehole. This arrangement can ensure that liquid nitrogen can enter the ventilation pipe only after the borehole is basically filled, and it is convenient to determine the accumulated amount of liquid nitrogen in the borehole by observing whether the ventilation pipe discharges liquid nitrogen.

[0014] Furthermore, the static breaker contains calcium oxide, which reacts chemically with water to generate calcium hydroxide, which expands in volume by about one time. This property is used to realize secondary expansion fracturing of the static breaker.

[0015] Furthermore, the borehole is drilled upwards, which makes it easier to inject liquid nitrogen and judge the injection status.

[0016] Compared with the prior art, the present invention first uses liquid nitrogen to prepare frozen static breaker particles. In this state, water wraps the static breaker in the form of ice. Due to the absence of water, the static breaker cannot undergo chemical reactions, which facilitates subsequent injection into the borehole. Then, a hole is drilled at the location where the coal seam needs to be fractured and liquid nitrogen is injected. The frozen static breaker particles are injected along with the liquid nitrogen. The liquid nitrogen freezes the coal body around the borehole due to the low temperature, which increases the brittleness of the coal body. At the same time, the liquid nitrogen pressure in the borehole gradually increases, driving the coal body to produce cracks and continuously expand and develop. The frozen static breaker particles flow into the cracks produced in the coal seam along with the liquid nitrogen. It plays the role of proppant, promotes the flow of liquid nitrogen in the cracks, and further increases the length of crack expansion; after the completion of liquid nitrogen fracturing, the frozen static breaker particles are retained in the cracks. As the temperature of the coal seam gradually recovers due to the influence of ground temperature, the frozen static breaker particles begin to dissolve. At this time, the static breaker and water begin to chemically react and expand in volume, and then expand in the crack to form a static breaker expansion zone to perform secondary expansion fracturing on the cracks, so that secondary fracturing cracks are generated around the cracks. At the same time, the static breaker expansion zone supports the cracks, preventing the cracks from closing due to ground stress, and finally completing the coal seam fracturing process. In this way, not only can the coal seam be efficiently fractured, but the formed cracks can also be effectively supported, thereby reducing the situation of their subsequent closure, and ultimately effectively improving the efficiency of coalbed methane extraction. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a schematic diagram of the construction layout of the present invention;

[0018] Figure 2 is a schematic diagram of preparing frozen static breaker particles in the present invention;

[0019] Figure 3 It is a schematic diagram of the secondary expansion fracturing in the present invention.

[0020] In the figure: 1-mixer, 2-connecting pipe, 3-high-pressure nitrogen injection pump, 4-pressure gauge, 5-pressure-resistant pipe, 6-connecting head, 7-nitrogen injection pipe, 8-nitrogen injection stop valve, 9-vent pipe, 10-vent stop valve, 11-sealing device, 12-drilling hole, 13-crack, 14-liquid nitrogen, 15-frozen static crushing agent particles, 16-sieve plate, 17-sieve hole, 18-insulation tank, 19-static crushing agent solution, 20-static crushing agent droplets, 21-static crushing agent expansion zone, 22-coal seam. DETAILED DESCRIPTION

[0021] The present invention will be further described below.

[0022] like Figure 1 As shown, the specific steps of the present invention are:

[0023] Step 1: Prepare frozen static crushing agent particles: Figure 2As shown, a certain amount of liquid nitrogen 14 is poured into an insulation tank 18, and a sieve plate 16 with a plurality of sieve holes 17 is installed in the insulation tank 18, and the sieve plate 16 is above the liquid surface of the liquid nitrogen 14; a static breaker solution 19 is prepared by mixing a static breaker and water in a required ratio, wherein the main component of the static breaker is calcium oxide, which is an existing preparation; and the static breaker solution 19 is poured into the insulation tank 18, at which time the static breaker solution 19 first flows through the sieve plate 16, and forms static breaker droplets 20 through each sieve hole 17, and the static breaker droplets 20 are dropped into the liquid nitrogen 14 and quickly frozen to form frozen static breaker particles 15; the sieve holes 17 on the sieve plate 16 are evenly distributed, and the aperture of the sieve holes 17 is determined according to the particle size of the frozen static breaker particles 15 to be prepared.

[0024] Step 2, drilling a hole in the coal seam: a hole 12 is constructed in the coal seam 22 by a drilling rig, and after completion, a sealing device 11 is installed in the hole 12 to seal the hole 12; the sealing device 11 is equipped with a nitrogen injection pipe 7 and a vent pipe 9, one end of the nitrogen injection pipe 7 and one end of the vent pipe 9 are both in the hole 12, the other end of the nitrogen injection pipe 7 and the other end of the vent pipe 9 are both outside the hole 12, and are respectively equipped with a nitrogen injection stop valve 8 and a vent stop valve 10; the hole 12 is drilled upward. This is convenient for liquid nitrogen injection and judgment of the injection situation; one end of the vent pipe 9 is in the hole 12 near the bottom of the hole, and one end of the nitrogen injection pipe 7 is in the hole near the sealing device 11. This setting can ensure that liquid nitrogen 14 can enter the vent pipe 9 only after it basically fills the hole 12, and it is convenient to determine the accumulated amount of liquid nitrogen in the hole 12 by observing whether the vent pipe 9 discharges liquid nitrogen 14.

[0025] Step 3, equipment installation: connect the mixer 1 to the inlet of the high-pressure nitrogen injection pump 3 through the connecting pipe 2, and connect the outlet of the high-pressure nitrogen injection pump 3 to the other end of the nitrogen injection pipe 7 through the pressure-resistant pipe 5 and the connector 6; the outlet of the high-pressure nitrogen injection pump 3 is located at the installation pressure gauge 4 for monitoring the nitrogen injection pressure;

[0026] Step 4, coal seam liquid nitrogen fracturing: first open the nitrogen injection stop valve 8 and the ventilation stop valve 10, pour liquid nitrogen 14 into the mixer 1, start the high-pressure nitrogen injection pump 3, at this time, the liquid nitrogen in the mixer 1 is pumped by the high-pressure nitrogen injection pump 3, so that the liquid nitrogen 14 is injected into the borehole 12 through the pressure-resistant pipe 5 and the nitrogen injection pipe 7; due to the high temperature inside the borehole 12, the liquid nitrogen entering the borehole 12 continues to gasify and reduce the borehole temperature and produce nitrogen, and the nitrogen is discharged from the borehole 12 through the ventilation pipe 10. As the temperature of the borehole 12 continues to decrease, the liquid nitrogen gradually accumulates in the borehole 12, and the liquid nitrogen gradually fills the borehole 12; when liquid nitrogen is observed to flow out from the other end of the ventilation pipe 9, the ventilation stop valve 10 is closed at this time to allow the borehole 1 2, and at the same time, the frozen static crushing agent particles 15 prepared in step 1 are added to the mixer 1, and the frozen static crushing agent particles 15 are injected into the borehole 12 along with the liquid nitrogen 14. As the liquid nitrogen 14 is continuously pressurized and injected into the borehole 12, the liquid nitrogen 14 in the borehole 12 freezes the coal body due to the low temperature, resulting in increased brittleness of the coal body. At the same time, the liquid nitrogen pressure in the borehole gradually increases, driving the coal body to generate cracks 13 and continuously expand and develop. The frozen static crushing agent particles 15 flow into the cracks 13 generated in the coal seam 22 along with the liquid nitrogen 14; the frozen static crushing agent particles 15 act as a proppant in the cracks 13, promoting the flow of the liquid nitrogen 14 in the cracks 13, and further increasing the expansion length of the cracks 13;

[0027] Step 5, static breaker secondary fracturing: After the liquid nitrogen 14 fracturing work is completed, the high-pressure nitrogen injection pump 3 and the nitrogen injection stop valve 8 are closed, and the frozen static breaker particles 15 are retained in the cracks 13. As the temperature of the coal seam 22 gradually recovers due to the influence of the ground temperature, the remaining liquid nitrogen 14 in the borehole 12 continues to gasify, so that the temperature in the borehole 12 continues to rise, and when the melting point of the frozen static breaker particles 15 is reached, they begin to dissolve. Figure 3 As shown, the calcium oxide of the static breaker reacts chemically with water to generate calcium hydroxide, and its volume expands by about 1 times, and then expands in the crack 13 to form a static breaker expansion zone 21 to perform secondary expansion fracturing on the crack, so that secondary fracturing cracks are generated around the crack 13. At the same time, the static breaker expansion zone 21 supports the crack 13 to prevent the crack 13 from closing due to the ground stress, thereby completing the coal seam fracturing process.

[0028] The above-mentioned mixer 1, high-pressure nitrogen injection pump 3, insulation tank 18, sieve plate, pressure gauge 4, sealing device 11 and static crushing agent are all existing equipment or components, which can be purchased on the market. The present invention only utilizes their existing functions.

[0029] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A coal seam fracturing method coupled with liquid nitrogen and static crushing agent, characterized in that: The specific steps are: Step 1, preparing frozen static breaker particles: pouring a certain amount of liquid nitrogen into a heat preservation tank, installing a sieve plate with multiple sieve holes in the heat preservation tank, and the sieve plate is above the liquid nitrogen surface; preparing a static breaker solution by mixing a static breaker and water in a required ratio, and pouring the static breaker solution into the heat preservation tank, at which time the static breaker solution first flows through the sieve plate, passes through each sieve hole to form static breaker droplets, and the static breaker droplets are poured into the liquid nitrogen and quickly frozen to form frozen static breaker particles; Step 2, drilling a hole in the coal seam: drilling a hole in the coal seam by a drilling rig, and installing a sealing device in the hole to seal the hole after completion; the sealing device is equipped with a nitrogen injection pipe and a ventilation pipe, one end of the nitrogen injection pipe and one end of the ventilation pipe are both in the borehole, and the other end of the nitrogen injection pipe and the other end of the ventilation pipe are both outside the borehole, and are respectively equipped with a nitrogen injection stop valve and a ventilation stop valve; Step 3, equipment installation: connect the mixer to the inlet of the high-pressure nitrogen injection pump through a connecting pipe, and connect the outlet of the high-pressure nitrogen injection pump to the other end of the nitrogen injection pipe through a pressure-resistant pipe and a connector; the outlet of the high-pressure nitrogen injection pump is located at a pressure gauge for monitoring the nitrogen injection pressure; Step 4: Liquid nitrogen fracturing of coal seams: First, open the nitrogen injection stop valve and the ventilation stop valve, pour liquid nitrogen into the mixer, and start the high-pressure nitrogen injection pump. At this time, the liquid nitrogen in the mixer is pumped by the high-pressure nitrogen injection pump, so that the liquid nitrogen is injected into the borehole through the pressure-resistant pipe and the nitrogen injection pipe; due to the high temperature inside the borehole, the liquid nitrogen entering the borehole continues to gasify and reduce the borehole temperature and produce nitrogen, which is discharged from the borehole through the ventilation pipe. As the borehole temperature continues to drop, liquid nitrogen gradually accumulates in the borehole and gradually fills the borehole; when liquid nitrogen is observed to flow out from the other end of the ventilation pipe, the ventilation stop valve is closed at this time to make A closed space is maintained in the borehole, and the frozen static crushing agent particles prepared in step 1 are added to the mixer at the same time. The frozen static crushing agent particles are injected into the borehole along with the liquid nitrogen. As the liquid nitrogen is continuously pressurized and injected into the borehole, the liquid nitrogen in the borehole freezes the coal body due to the low temperature, causing the coal body to become more brittle. At the same time, the liquid nitrogen pressure in the borehole gradually increases, driving the coal body to generate cracks and continuously expand and develop. The frozen static crushing agent particles flow into the cracks generated in the coal seam along with the liquid nitrogen; the frozen static crushing agent particles act as a proppant in the cracks, promoting the flow of liquid nitrogen in the cracks, and further increasing the crack expansion length; Step 5, secondary fracturing with static breaker: After the liquid nitrogen fracturing work is completed, the high-pressure nitrogen injection pump and the nitrogen injection stop valve are turned off, and the frozen static breaker particles are retained in the cracks. As the temperature of the coal seam gradually recovers due to the influence of the ground temperature, the remaining liquid nitrogen in the borehole continues to gasify, causing the temperature in the borehole to continue to rise. When the melting point of the frozen static breaker particles is reached, it begins to dissolve. At this time, the static breaker begins to react chemically with water and expands in volume, and then expands in the crack to form a static breaker expansion zone, which performs secondary expansion fracturing on the cracks, so that secondary fracturing cracks are generated around the cracks. At the same time, the static breaker expansion zone supports the cracks to prevent the cracks from closing due to ground stress, completing the coal seam fracturing process.

2. The coal seam fracturing method using liquid nitrogen coupled with a static crushing agent according to claim 1, characterized in that: The sieve holes on the sieve plate are evenly distributed, and the aperture of the sieve holes is determined according to the particle size of the frozen static breaker particles to be prepared.

3. The coal seam fracturing method using liquid nitrogen coupled with a static crushing agent according to claim 1, characterized in that: One end of the ventilation pipe is located in the borehole near the bottom of the hole, and one end of the nitrogen injection pipe is located in the borehole near the sealing device.

4. The coal seam fracturing method using liquid nitrogen coupled with a static crushing agent according to claim 1, characterized in that: The static crushing agent contains calcium oxide.

5. The coal seam fracturing method using liquid nitrogen coupled with a static crushing agent according to claim 1, characterized in that: The drilling hole is an upward drilling hole.

Images