CN110230896B - Downhole heat extraction device and downhole heat extraction method - Google Patents

Abstract

The invention provides a downhole heat extraction device and a downhole heat extraction method. The downhole heat extraction device comprises: a water pump, a first sleeve and an extension pipe, both ends of the first sleeve are connected with end caps, and a water inlet of the water pump It is arranged in the first sleeve, one end of the extension pipe is communicated with the first sleeve, and the other end is used to extend downhole. The invention alleviates the technical problems in the prior art that the water pump needs to have a larger lift, larger required power and higher cost when extracting formation water.

Description

Downhole heat extraction device and downhole heat extraction method

technical field

The invention relates to the technical field of geothermal exploitation, in particular to a downhole heat extraction device and a downhole heat extraction method.

Background technique

In the process of mining geothermal heat, it is usually necessary to pump underground hot water to the surface, and a water pump is generally used to pump water to a suitable position below the hydrostatic liquid level. However, in the actual operation process, when the water pump is pumping water at a certain position, it is often encountered that the water temperature at the position is low and it is difficult to obtain enough heat; when this happens, it is usually necessary to increase the depth of the pumping position , extracting formation water from deeper locations above the surface.

To increase the depth of the pumping position, it is necessary to increase the lift of the pump and increase the working power of the pump. Therefore, the cost of mining will be too high and the economic benefit will be reduced.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a downhole heat extraction device and a downhole heat extraction method to alleviate the technical problems in the prior art that when extracting formation water, the water pump needs to have a larger lift, larger required power and higher cost.

Above-mentioned purpose of the present invention can adopt following technical scheme to realize:

The invention provides an underground heat extraction device, comprising: a water pump, a first sleeve and an extension pipe, both ends of the first sleeve are connected with end covers, and the water inlet of the water pump is arranged in the first sleeve In the barrel, one end of the extension pipe is communicated with the first sleeve, and the other end is used to extend downhole.

In a preferred embodiment, the water pump is a submersible pump, and the submersible pump is arranged in the first sleeve; the water outlet of the submersible pump is connected with a water outlet pipe extending outside the first sleeve.

In a preferred embodiment, the extension pipe is made of thermal insulation material.

In a preferred embodiment, a first end cap is connected to the lower end of the first sleeve, the first end cap is provided with a taper pipe threaded hole, and the extension pipe is connected to the taper pipe threaded hole.

In a preferred embodiment, a second end cap is connected to the upper end of the first sleeve; the water outlet pipe includes a first pipe section and a second pipe section, and the lower end of the first pipe section is connected to the water outlet of the water pump , the upper end of the first pipe section is connected to the second end cover; the lower end of the second pipe section is connected to the second end cover and communicated with the first pipe section.

In a preferred embodiment, the downhole heat extraction device comprises a second casing and an exhaust pipe, the second casing is arranged in the first casing, and the water pump is arranged in the second casing The lower part of the second sleeve is closed, and the upper part is provided with a communication hole communicating with the first sleeve; the exhaust pipe communicates with the upper part of the second sleeve.

In a preferred embodiment, the communication hole is provided on the side wall of the second sleeve.

In a preferred embodiment, the side wall of the second sleeve is provided with a plurality of communication holes distributed at intervals around the axis of the second sleeve.

In a preferred embodiment, a third end cap is connected to the upper end of the second sleeve; the water outlet pipe includes a first pipe section and a second pipe section, and the lower end of the first pipe section is connected to the water outlet of the water pump , the upper end of the first pipe section is connected to the third end cover; the lower end of the second pipe section is connected to the third end cover and communicated with the first pipe section, and the upper end of the second pipe section extends to the outside of the first sleeve.

The present invention provides a downhole heat extraction method, which adopts the above-mentioned downhole heat extraction device, including:

Step S100, drilling according to the design depth;

Step S200, running the first casing into the well so that the hydrostatic liquid level is higher than the first casing;

In step S300, the water pump is turned on to exploit the formation water in the well.

The features and advantages of the present invention are: when using the underground heat extraction device provided by the present invention to exploit formation water, the first casing is lowered into the well, and the extension pipe extends from the first casing to a deeper position in the well. The first casing communicates with the space in the well through the extension pipe; due to the action of the liquid column pressure difference, the formation water in the well enters the first casing through the extension pipe, and the water pump can pump the water in the first casing to the ground.

The formation water needs to enter from the lower end opening of the extension pipe. Therefore, when the pump is pumping water, the formation water in the well located above the lower end opening of the extension pipe needs to flow from top to bottom to the lower end opening of the extension pipe before entering the extension pipe. middle. The process of the formation water flowing from top to bottom can absorb and utilize the heat at the deeper position in the well, and be gradually heated, so that the temperature of the formation water entering the first casing through the extension pipe is higher.

In the downhole heat extraction device provided by the present invention, the lower end of the extension pipe can be lowered into the well deeper than the first casing, and the formation water at the deeper position in the well enters the first casing through the extension pipe; The formation water is pumped to the surface. In this way, the water pump is pumped with a smaller head, and the power is lower, so that the groundwater with a higher temperature can be extracted from a deeper position in the well, which saves costs and has better economic benefits.

Description of drawings

In order to illustrate the technical solutions in the embodiments of the present invention more clearly, the following briefly introduces the accompanying drawings used in the description of the embodiments. Obviously, the accompanying drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can also be obtained from these drawings without creative effort.

FIG. 1 is a schematic structural diagram of a downhole heat extraction device running into a well provided by an embodiment of the present invention;

FIG. 2 is a schematic structural diagram of a downhole heat extraction device running into a well provided by another embodiment of the present invention;

3 is a schematic structural diagram of a second sleeve in the downhole heat extraction device shown in FIG. 2;

4 is a schematic structural diagram of a first end cover in the downhole heat extraction device provided by the present invention;

5 is a schematic structural diagram of a second end cover in the downhole heat extraction device shown in FIG. 1;

6 is a schematic structural diagram of a second end cover in the downhole heat extraction device shown in FIG. 2;

7 is a schematic structural diagram of a third end cover in the downhole heat extraction device shown in FIG. 2;

8 is a schematic structural diagram of a fourth end cover in the downhole heat extraction device shown in FIG. 2;

9 is a schematic structural diagram of an extension pipe in the downhole heat extraction device provided by the present invention;

10 is a schematic structural diagram of the first pipe section in the downhole heat extraction device provided by the present invention;

11 is a schematic structural diagram of the second pipe section in the downhole heat extraction device provided by the present invention;

FIG. 12 is a schematic diagram of the downhole heat extraction method provided by the present invention.

Description of reference numerals: 10, the first sleeve; 20, the extension pipe; 30, the water pump; 31, the water inlet of the water pump; 40, the water outlet pipe; 41, the first pipe section; 42, the second pipe section; 51, the first end Cover; 511, the first taper pipe threaded hole; 52, the second end cap; 521, the second taper pipe threaded hole; 522, the third taper pipe threaded hole; 60, the second sleeve; 61, the communicating hole; 62, Exhaust pipe; 71, third end cover; 711, fourth taper pipe threaded hole; 712, fifth taper pipe threaded hole; 713, sixth taper pipe threaded hole; 72, fourth end cover; 81, hydrostatic liquid level ; 82, the liquid level in the first casing; 83, the liquid level in the second casing; 91, the upper wellbore; 92, the lower wellbore.

Detailed ways

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, but not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

Example 1

Please refer to FIG. 1 and FIG. 9. The arrows in FIG. 1 indicate the flow path of formation water. The present invention provides a downhole heat extraction device, including: a water pump 30, a first sleeve 10 and an extension pipe 20, the first sleeve Both ends of 10 are connected with end caps, the water inlet 31 of the water pump is arranged in the first casing 10, one end of the extension pipe 20 is communicated with the first casing 10, and the other end is used to extend downhole. When using the downhole heat extraction device provided by the present invention to extract formation water, the first casing 10 is lowered into the well, and the extension pipe 20 extends from the first casing 10 to a deeper position in the well. The first casing 10 is communicated with the space in the well through the extension pipe 20; due to the action of the liquid column pressure difference, the formation water in the well enters the first casing 10 through the extension pipe 20, and the water pump 30 can pump the water into the first casing 10. water is pumped to the ground.

Since the formation water needs to enter from the lower end opening of the extension pipe 20, when the water pump 30 pumps water, the formation water in the well located above the lower end opening of the extension pipe 20 needs to flow from top to bottom to the lower end opening of the extension pipe 20, into the extension tube 20. During the process of the formation water flowing from top to bottom, the heat at the deeper position in the well can be absorbed and used to be gradually heated, so that the temperature of the formation water entering the first casing 10 through the extension pipe 20 is higher.

In the downhole heat extraction device provided by the present invention, the lower end of the extension pipe 20 can be lowered into the well deeper than the first casing 10, and the formation water in the deeper position in the well enters the first casing 10 through the extension pipe 20; the water pump 30 The formation water in the first sleeve 10 is pumped to the surface. In this way, the water pump is pumped with a smaller head, and the power is lower, so that the groundwater with a higher temperature can be extracted from a deeper position in the well, which saves costs and has better economic benefits.

In order to facilitate pumping water from a deep well, the water pump 30 adopts a submersible pump, and the submersible pump is arranged in the first casing 10 ; When pumping water, the first casing 10 is lowered into the well, and the water inlet 31 of the water pump is located below the hydrostatic liquid level 81 in the well. During the pumping process, the water pump 30 sends the formation water in the first casing 10 to the surface through the water outlet pipe 40, and the formation water in the well enters the first casing 10 through the extension pipe 20 for supplement; The liquid level in the barrel 10 is substantially stable, the liquid level being lower than the hydrostatic liquid level 81 . The position where the first sleeve 10 is lowered must ensure that the water inlet 31 of the water pump is located below the liquid surface 82 in the first sleeve.

The temperature of formation water is affected by the geothermal gradient, which is generally 2 to 4 degrees Celsius per 100 meters. In some geothermal wells, the temperature of the formation water gradually increases from top to bottom; the formation water at a deeper position in the well enters the extension pipe 20 and flows from bottom to up in the extension pipe 20. The inventors found that when the extension pipe When the 20 is made of steel pipes, during the process of the formation water flowing in the extension pipe 20, heat transfer occurs with the outside through the pipe wall of the extension pipe 20, resulting in a decrease in the temperature of the formation water in the pipe. Therefore, the inventor has made further improvements: the extension pipe 20 is made of thermal insulation material; in some embodiments, the extension pipe 20 is made of PE (polyethylene, polyethylene) pipe, and the PE pipe has good thermal insulation performance, which can reduce the amount of formation water in the Heat loss during flow. In other embodiments, the extension pipe 20 adopts a PPR (polypropylene random, random copolymer polypropylene) pipe with better thermal insulation performance, so as to reduce the heat loss of the formation water during the flow.

As another embodiment, the extension pipe 20 is wrapped with a thermal insulation layer to block the heat transfer inside and outside the pipe. Specifically, the extension pipe 20 is made of steel pipe, and the steel pipe is wrapped with a thermal insulation layer. In this way, while the thermal insulation effect is achieved, the extension pipe 20 can also be guaranteed to have high strength, which is convenient for running into a deeper position in the well.

In an embodiment of the present invention, the first sleeve 10 is a steel cylinder, so that the first sleeve 10 has high strength. In order to reduce the heat loss of the formation water in the first casing 10 , an insulating layer is provided outside the first casing 10 .

In an embodiment of the present invention, please refer to FIGS. 1 and 4 , a first end cap 51 is connected to the lower end of the first sleeve 10 , and the first end cap 51 is the end connected to the two ends of the first sleeve 10 . one of the covers. The first end cover 51 is provided with a first taper pipe threaded hole 511 , and as shown in FIG. 4 , the inner diameter of the first taper pipe threaded hole 511 gradually decreases from bottom to top. The extension pipe 20 is connected to the first taper pipe threaded hole 511 . The first end cover 51 can play a blocking role, so that the formation water in the well needs to flow down to the lower end of the extension pipe 20, and can enter the first sleeve 10 only through the extension pipe 20; Assembly of a sleeve 10 . The extension pipe 20 is matched with the first taper pipe threaded hole 511 to ensure good sealing of the connection, so as to reduce leakage at the connection.

In an embodiment of the present invention, please refer to FIG. 1 , FIG. 10 and FIG. 11 , the upper end of the first sleeve 10 is connected with a second end cap 52 , and the second end cap 52 is formed by the two ends of the first sleeve 10 . One of the connected end caps; the water outlet pipe 40 includes a first pipe section 41 and a second pipe section 42, the lower end of the first pipe section 41 is connected to the water outlet of the water pump 30, and the upper end of the first pipe section 41 is connected to the second end cover 52; The lower end of the second pipe section 42 is connected to the second end cover 52 and communicated with the first pipe section 41 . The first pipe section 41 hangs the water pump 30 below the second end cover 52 , and by changing the length of the first pipe section 41 , the position of the water pump 30 in the first sleeve 10 can be adjusted. The upper end of the second pipe section 42 extends to the wellhead to send the formation water pumped by the water pump 30 to the surface. The second pipe section 42 is fixed to the wellhead, and the second pipe section 42 bears the gravity of the first casing 10, the gravity of the water pump 30 and the gravity of the extension pipe 20, and plays the role of installing and fixing the downhole heat extraction device provided by the present invention. Meanwhile, by changing the length of the second pipe section 42, the running depth of the first sleeve 10 can be adjusted.

Preferably, the lower end of the first pipe section 41 is provided with a flange plate matched with the water pump 30 so as to be more firmly connected with the water pump 30 .

Preferably, please refer to FIG. 5 , the inner surface of the second end cover 52 is provided with a second taper pipe threaded hole 521 which is matched with the first pipe section 41 . As shown in FIG. 5 , the inner diameter of the second taper pipe threaded hole 521 is from the bottom It gradually decreases upwards; the outer surface of the second end cover 52 is provided with a third taper pipe threaded hole 522 that cooperates with the second pipe section 42. As shown in FIG. 5, the inner diameter of the third taper pipe threaded hole 522 is from top to bottom. Gradually decrease; the second taper pipe threaded hole 521 communicates with the third taper pipe threaded hole 522 to seal between the first pipe section 41 and the second end cover 52 and between the second pipe section 42 and the second end cover 52 Sex is better.

The second pipe section 42 carries the pulling force generated by the gravity of the water pump 30 . Preferably, the axis of the first pipe section 41, the axis of the second pipe section 42, the axis of the rotating shaft of the water pump 30, the axis of the first sleeve 10 and the axis of the extension pipe 20 are all coincident, so that the downhole heat extraction device provided by the present invention has The force distribution is more balanced, the force of the water pump 30 is improved, and the stability of the overall structure is improved.

In the mining experiment, when the underground heat extraction device provided by the present invention is used, the water temperature rises from 42°C to 63°C under the condition that the running depth of the water pump remains unchanged, and the heating effect is obvious.

Embodiment 2

In the deep layers of the formation, the phenomenon of water and gas mixing is more obvious, and more gas is mixed in the formation water. When encountering the same layer of water and gas, the phenomenon of water and gas mixing is more obvious. The mixing of water and air will adversely affect the pump's water supply efficiency; when the impact is serious, the pump may not be able to supply water, the pump will run dry and generate high temperature, and the pump and its motor and cables will be burned out.

Therefore, the inventor has improved the downhole heat extraction device provided by the present invention, so that the downhole heat extraction device can separate water and gas, so as to reduce the adverse effects of water and gas mixing. Please refer to FIG. 2 and FIG. 3. The arrows in FIG. 2 indicate the flow paths of formation water and the liquid and gas therein. The downhole heat extraction device includes a second casing 60, an exhaust pipe 62, and the above-mentioned water pump 30 and the first A sleeve 10, the second sleeve 60 is arranged in the first sleeve 10, and the water pump 30 is arranged in the second sleeve 60; The communication hole 61 ; the exhaust pipe 62 communicates with the upper part of the second sleeve 60 .

The formation water enters the first sleeve 10 from the lower end, and in the space other than the second sleeve 60 , the formation water moves upward from the bottom, and then flows into the second sleeve 60 through the communication hole 61 . Since the density of the gas part in the formation water is lower than that of the liquid part, the formation water is separated at the communication hole 61: the gas part moves upward and is discharged to the outside of the first sleeve 10 through the exhaust pipe 62; the liquid part therein Then it flows downward and enters the water inlet 31 of the water pump. In this way, the gas content in the formation water entering the water pump 30 is lower, the adverse effect of the water-gas mixing phenomenon is reduced, and the water feeding efficiency of the water pump 30 is improved.

In the downhole heat extraction device provided by the present invention, the first sleeve 10 and the second sleeve 60 cooperate with each other, so that the formation water moves upward first, and then turns to move downward. Separate and drain upwards. In order to achieve the above effects, the communication hole 61 may be provided on the upper part of the side wall of the second sleeve 60 , or may be provided on the upper end surface of the second sleeve 60 . Preferably, the communication hole 61 is arranged on the upper part of the side wall of the second sleeve 60. When the formation water flows through the communication hole 61 on the side wall, the formation water as a whole has an initial velocity of upward movement; and the water inlet 31 of the water pump is located in the communication hole Below 61, the liquid part of the formation water turns to move downward under the action of its own gravity; the gas part of the formation water, due to its small gravity, continues to move upward under the action of the initial velocity of upward movement, which is easier to interact with. The liquid part is separated.

Further, the second sleeve 60 is a cylindrical steel cylinder, and the side wall of the second sleeve 60 is provided with a plurality of communicating holes 61 distributed at intervals around the axis of the second sleeve 60 , so that the The formation water flows from the periphery to the center and enters the second sleeve 60 . In an embodiment of the present invention, the side wall of the second sleeve 60 is provided with two communication holes 61 , and the axes of the two communication holes 61 overlap and intersect with the axis of the second sleeve 60 .

2 , 7 , 10 and 11 , the upper end of the second sleeve 60 is connected with a third end cover 71 ; the water outlet pipe 40 includes a first pipe section 41 and a second pipe section 42 , and the lower end of the first pipe section 41 Connected to the water outlet of the water pump 30, the upper end of the first pipe section 41 is connected to the third end cover 71; the lower end of the second pipe section 42 is connected to the third end cover 71 and communicated with the first pipe section 41, the second pipe section 42 The upper end extends to the outside of the first sleeve 10 . The first pipe section 41 hangs the water pump 30 below the third end cover 71 , and by changing the length of the first pipe section 41 , the position of the water pump 30 in the second sleeve 60 can be adjusted.

Preferably, the third end cover 71 is provided with a fourth tapered pipe threaded hole 711 matched with the first pipe section 41 , a fifth tapered pipe threaded hole 712 matched with the second pipe section 42 , and a sixth tapered pipe matched with the exhaust pipe 62 . Pipe threaded holes 713 for better sealing between the first pipe section 41 and the third end cover 71 , between the second pipe section 42 and the third end cover 71 , and between the exhaust pipe 62 and the third end cover 71 . As shown in FIG. 7 , the inner diameter of the fourth tapered pipe threaded hole 711 gradually decreases from bottom to top, the inner diameter of the fifth tapered pipe threaded hole 712 gradually decreases from top to bottom, and the inner diameter of the sixth tapered pipe threaded hole 713 decreases from top to bottom. It gradually decreases from top to bottom, and the fourth taper pipe threaded hole 711 and the fifth taper pipe threaded hole 712 communicate with each other.

Both the second pipe section 42 and the exhaust pipe 62 pass through the first sleeve 10 . The water pump 30 sends the formation water to the surface through the first pipe section 41 and the second pipe section 42 ; the separated gas is discharged to the outside of the first sleeve 10 through the exhaust pipe 62 . In this embodiment, please refer to FIG. 6 , the second end cover 52 is provided with two through holes for passing through the second pipe section 42 and the exhaust pipe 62 respectively, and the second pipe section 42 and the second end cover 52 and between the exhaust pipe 62 and the second end cover 52 are in a sealing fit.

Referring to FIG. 8 , a fourth end cap 72 is fixedly connected to the lower end of the second sleeve 60 , and the fourth end cap 72 is used to close the lower end of the second sleeve 60 .

In an embodiment of the present invention, the second pipe section 42 and the second end cover 52 are fixedly connected. The second pipe section 42 is fixed to the wellhead, and the second pipe section 42 carries the gravity of the first casing 10, the gravity of the second casing 60, the gravity of the water pump 30 and the gravity of the extension pipe 20, so as to install and fix the downhole heat extraction function of the device. Meanwhile, by changing the length of the second pipe section 42, the running depth of the first sleeve 10 and the second sleeve 60 can be adjusted.

Preferably, the axis of the first pipe section 41, the axis of the second pipe section 42, the axis of the rotating shaft of the water pump 30, the axis of the first sleeve 10, the axis of the second sleeve 60 and the axis of the extension pipe 20 are all coincident, so that the The force distribution of the downhole heat extraction device is more balanced, the force of the water pump 30 is improved, and the stability of the overall structure is improved.

As another embodiment of the present invention, the second end cap 52 and the third end cap 71 are end caps with an integrated structure, and the upper end of the first sleeve 10 and the upper end of the second sleeve 60 are both fixed to the integrated structure. The lower surface of the end cover, and the exhaust pipe 62, the first pipe section 41 and the second pipe section 42 are all connected to the end cover of the one-piece structure.

Further, a first centralizer is disposed between the side wall of the first sleeve 10 and the side wall of the second sleeve 60 to ensure the installation accuracy between the first sleeve 10 and the second sleeve 60 . In order to facilitate the installation of the downhole heat extraction device provided by the present invention, a second centralizer is provided outside the first casing 10. The second centralizer helps to ensure the installation accuracy of the first casing 10 in the well, so that the first casing 10 can be installed in the well. The axis of 10 is arranged in the vertical direction.

In an embodiment of the present invention, a one-way valve is connected to the exhaust pipe 62 to prevent the gas from flowing back into the second sleeve 60 . Further, a pressure gauge is connected to the exhaust pipe 62 for controlling the discharge of gas. The exhaust pipe 62 may extend to the wellhead to deliver the separated gas to the surface.

The overall structure of the downhole heat extracting device provided in this embodiment is relatively simple, easy to use, and low in price, and can also be applied to oil and gas exploitation to perform downhole oil and gas separation.

Embodiment 3

As shown in FIG. 12 , the present invention provides a downhole heat extraction method using the above-mentioned downhole heat extraction device. include:

Step S100, drilling according to the design depth;

Step S200, running the first casing 10 into the well, so that the hydrostatic liquid level 81 is higher than the first casing 10;

In step S300, the water pump 30 is turned on to exploit the formation water in the well.

When the mixing of water and gas is relatively obvious, the above-mentioned downhole heat extraction device including the second casing 60 is used to separate the water and gas and ensure the stable operation of the water pump.

As an embodiment of the present invention, in step S100, the wellbore structure is designed into two sections. The upper section wellbore 91 is relatively large, and the bottom of the upper section wellbore 91 is about 250 m away from the hydrostatic liquid level 81 ; the lower section wellbore 92 is relatively small. In step S100, the extension pipe 20 is extended downward to the lower section wellbore 92 in the vertical direction; the upper section wellbore 91 serves as a pump chamber, the first casing 10 is set in the upper section wellbore 91, and the outer section of the first casing 10 The diameter is smaller than the inner diameter of the upper section of the wellbore 91 .

When the above-mentioned downhole heat extraction device including the second casing 60 is used, as shown in FIG. 2 , as an embodiment of the present invention, in step S300, the water pump 30 is controlled to run at an appropriate power, so as to run at the second After the liquid surface 83 in the sleeve is highly stable, the liquid surface 83 in the second sleeve is lower than the upper boundary of the communication hole 61, and a cavity is formed above the liquid surface in the second sleeve 60, so that the gas can move upward Move to separate.

The above are only a few embodiments of the present invention, and those skilled in the art can make various changes or modifications to the embodiments of the present invention according to the contents disclosed in the application documents without departing from the spirit and scope of the present invention.

Claims (8)

1. An underground heat extraction device, characterized in that it comprises: a water pump, a first sleeve and an extension pipe, both ends of the first sleeve are connected with end caps, and the water inlet of the water pump is arranged in the In the first sleeve, one end of the extension pipe is communicated with the first sleeve, and the other end is used to extend downhole; The downhole heat extraction device includes a second casing and an exhaust pipe, the second casing is arranged in the first casing, and the water pump is arranged in the second casing; The lower part of the second sleeve is closed, and the upper part is provided with a communication hole communicating with the first sleeve; The exhaust pipe communicates with the upper portion of the second sleeve. 2 . The underground heat extraction device according to claim 1 , wherein the water pump is a submersible pump, and the water outlet of the submersible pump is connected with a water outlet pipe extending outside the first sleeve. 3 . 3 . The underground heat extraction device according to claim 2 , wherein the extension pipe is made of thermal insulation material. 4 . 4 . The downhole heat extraction device according to claim 2 , wherein the lower end of the first sleeve is connected with a first end cover, the first end cover is provided with a taper pipe threaded hole, and the extension pipe connected to the taper pipe threaded hole. 5 . The downhole heat extraction device according to claim 1 , wherein the communication hole is provided on the side wall of the second sleeve. 6 . 6 . The downhole heat extraction device according to claim 5 , wherein the side wall of the second sleeve is provided with a plurality of communication holes distributed at intervals around the axis of the second sleeve. 7 . 7 . The downhole heat extraction device according to claim 2 , wherein a third end cap is connected to the upper end of the second sleeve; the water outlet pipe comprises a first pipe section and a second pipe section, the first pipe section The lower end of the pipe section is connected to the water outlet of the water pump, and the upper end of the first pipe section is connected to the third end cover; The lower end of the second pipe section is connected to the third end cap and communicated with the first pipe section, and the upper end of the second pipe section extends to the outside of the first sleeve. 8. A method for downhole heat extraction, characterized in that, the downhole heat extraction device according to any one of claims 1-7 is adopted, comprising: Step S100, drilling according to the design depth; Step S200, running the first casing into the well so that the hydrostatic liquid level is higher than the first casing; Step S300: Turn on the water pump to exploit the formation water in the well, so that the liquid level in the second casing is lower than the upper boundary of the communication hole, and in the second casing, the second casing is A cavity is formed above the liquid level in the sleeve.

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