Well completion structure for strengthening heat extraction performance of middle-deep layer closed geothermal single well

Abstract

The invention relates to a well completion structure for strengthening the heat extraction performance of a middle-deep layer closed geothermal single well, a heat extraction unit comprises a well mouth and a heat preservation inner sleeve coaxially arranged below the well mouth, a well cementation unit comprises a low-temperature stratum, a high-temperature stratum and a water-bearing stratum, and the low-temperature stratum is located below the well mouth and coaxially provided with a large sleeve. A water filter pipe and a small sleeve are sequentially and coaxially installed below the large sleeve from outside to inside, the lower end of the water filter pipe penetrates through a high-temperature stratum and a water-bearing layer and is communicated to the bottom of the geothermal well, the lower end of the small sleeve is sealed through a plugging head, and vertical fins are installed on the outer wall, located on the water-bearing layer, of the small sleeve. According to the well completion structure, under the condition that a high-temperature aquifer exists in a stratum, natural convection can be formed at the position close to a shaft, fluid circulating flow in the stratum is enhanced, a heat transfer mode in a closed system close to a well wall is converted into a convection heat transfer mode from a conventional heat conduction mode, and the heat extraction capacity of a middle-deep layer closed single well is effectively improved; and the well completion process is simple, and the economical efficiency is good.

Description

Technical Field

[0001] The present invention belongs to the technical field of development and utilization of medium-deep geothermal energy resources, and specifically relates to a well formation structure for enhancing the heat extraction performance of a medium-deep closed geothermal single well. Background Art

[0002] Geothermal energy is a renewable energy source with abundant reserves, wide distribution, stability and reliability. The traditional mode of utilizing medium and deep geothermal energy is mainly the development and utilization of hydrothermal geothermal resources combined with extraction and injection. As described in the patent with publication number CN107642914B, due to the limitations of geothermal geological conditions, some sandstone formations are difficult to recharge, resulting in a series of problems such as water level drop and increased pump power consumption after long-term operation of the system. In order to further standardize the scientific management of geothermal resources, governments in many places require that geothermal tail water must be recharged in the same layer and the recharge rate must be no less than 95%. For this reason, a number of geothermal wells that cannot meet the recharge requirements have been closed in the Beijing-Tianjin-Hebei region. How to efficiently and sustainably obtain geothermal energy under different geothermal resource conditions and realize clean heating of buildings in northern my country has become the biggest technical bottleneck in the geothermal industry.

[0003] In the past decade, a medium-deep closed single-well heat extraction system technology that "extracts heat without taking water" has developed rapidly. As described in the patent publication number CN112923592A, the system installs a coaxial insulating inner casing in a closed geothermal wellbore, so that the heat exchange medium flows from the wellhead to the bottom of the well in the annular space between the well wall and the inner casing, absorbing the heat of the formation near the well wall along the way, and then returns to the wellhead from the bottom through the central insulating inner casing to complete the heat exchange process.

[0004] At present, when cementing existing medium-deep closed single wells, high thermal conductivity cement slurry is often injected from the bottom of the well to the wellhead to displace water with low thermal conductivity in the formation near the well wall, which improves the thermal conductivity of the formation to a certain extent. However, multiple actual project results have proved that the technical means of injecting high thermal conductivity cement slurry is not very effective in improving the heat extraction power.

[0005] Some experts and scholars have proposed to use a variety of wellbore structures such as closed connecting wells (CN104154668A), closed branch wells (CN106948795B) and closed horizontal wells (CN108302833A) to improve the heat extraction power of medium-deep closed single wells. The core idea is to increase the heat exchange area of ​​the wellbore in the higher formation temperature section to improve the heat extraction capacity of the system. However, under the condition of limited thermal conductivity of the formation and cement, these methods are difficult to achieve significant results. In addition, the high initial investment and operating costs of the medium-deep closed single well heat extraction system limit the promotion and application of this model.

[0006] In view of the above problems, it is necessary to optimize the design of medium-deep closed single-well heat extraction system according to different geothermal geological conditions to improve the single-well heat extraction power. The present invention proposes a new well formation structure for enhancing the medium-deep closed single-well heat extraction performance. Summary of the invention

[0007] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a well-forming structure for enhancing the heat extraction performance of a medium-deep closed geothermal single well. Under the condition that a high-temperature aquifer exists in the formation, natural convection can be formed near the wellbore, the circulation of fluid in the formation can be enhanced, and the heat transfer mode in the heat storage near the well wall of the closed system can be converted from a conventional heat conduction mode to a convection heat exchange mode, thereby effectively improving the heat extraction capacity of the medium-deep closed single well. The well-forming process is simple and economical.

[0008] The present invention solves the technical problem by the following technical solutions:

[0009] A well-forming structure for enhancing the heat extraction performance of a medium-deep closed geothermal single well, comprising a cementing unit and a heat extraction unit, wherein the heat extraction unit comprises a wellhead and a coaxially arranged heat-insulating inner casing below the wellhead, wherein the cementing unit comprises a low-temperature formation, a high-temperature formation and an aquifer from top to bottom outside the wellhead, wherein a large casing is coaxially arranged below the wellhead for the low-temperature formation section, wherein a water filter pipe and a small casing are coaxially installed in sequence from outside to inside below the large casing, wherein the lower end of the water filter pipe passes through the high-temperature formation and the aquifer and is connected to the bottom of the geothermal well, wherein the lower end of the small casing passes through the high-temperature formation and the aquifer and is sealed by a plugging head, wherein vertical fins are installed on the outer wall of the small casing located in the aquifer section;

[0010] A gap is provided between the water filter pipe and the small sleeve, and the gap between the water filter pipe and the small sleeve forms a natural convection flow channel for hot water in the aquifer.

[0011] The annular space between the large and small casings and the heat-insulating inner casing forms a circulating heat exchange fluid channel; the circular space inside the heat-insulating inner casing forms an output channel after heat extraction.

[0012] Moreover, there are a plurality of vertical ribs, which are evenly distributed and circumferentially arranged on the outer wall of the small sleeve.

[0013] Moreover, the vertical ribs are triangular or sawtooth shaped.

[0014] Moreover, a cementing cement layer is filled between the low-temperature formation and the large casing, and the cement in the cementing cement layer is high thermal conductivity composite cement.

[0015] The advantages and beneficial effects of the present invention are:

[0016] 1. The well formation structure of the present invention relies on the existing closed heat extraction mode of "extracting heat without extracting water". During cementing, the upper low-temperature formation section adopts a composite cement slurry with certain thermal insulation performance to be injected at the outer edge of the large casing, which reduces the heat dissipation of the heat exchange medium to the low-temperature formation and reduces the heat loss rate of the geothermal heat extraction system.

[0017] 2. In the well-forming structure of the present invention, small casing is used as the geothermal well wall for the lower high-temperature formation section and the aquifer section, and a fluid flow gap is formed near the well wall of the aquifer section. The natural convection is caused in the gap due to the influence of the radial temperature difference between cold and hot, which strengthens the heat exchange process between the high-temperature aquifer section and the circulating working fluid in the geothermal well, effectively improves the heat extraction capacity of medium-deep geothermal single wells, and realizes low-cost and high-efficiency heat extraction.

[0018] 3. The well-forming structure of the present invention does not need to inject cement in the high-temperature formation section and the aquifer section. By installing a water filter pipe to support the upper large casing, it can not only ensure the cementing strength, but also reduce the cost of the lower cementing cement, thereby reducing the initial investment of a new geothermal well.

[0019] 4. The well-forming structure of the present invention only requires the installation of a water filter pipe at the bottom and a large casing at the top during well cementing. There are no other special components and no special design of the geothermal well structure is required. The overall construction difficulty is relatively low and the scope of application is very wide. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a schematic diagram of the well formation structure of the present invention;

[0021] Figure 2 for Figure 1 A magnified view of part A;

[0022] Figure 3 is a top view of the aquifer of the present invention;

[0023] Figure 4 A comparison curve of the temperature difference between the wellhead inlet and outlet of the wellhead forming structure of the present invention and the traditional well forming structure;

[0024] Figure 5 The figure is a comparison curve of the heat extraction power between the well forming structure of the present invention and the traditional well forming structure.

[0025] Description of Reference Numerals

[0026] 1. Wellhead; 2. Low-temperature formation; 3. Large casing; 4. Small casing; 5. Water filter; 6. High-temperature formation; 7. Aquifer; 8. Vertical fins; 9. Blocking head; 10. Insulating inner casing; 11. Natural convection flow channel; 12. Circulating heat exchange fluid channel; 13. Output channel; 14- Cementing cement layer. DETAILED DESCRIPTION

[0027] The present invention is further described in detail below through specific examples. The following examples are only illustrative and not restrictive, and the protection scope of the present invention cannot be limited thereto.

[0028] like Figures 1 to 3 As shown, a well formation structure for enhancing the heat extraction performance of a medium-deep closed geothermal single well is innovative in that it includes a cementing unit and a heat extraction unit, the heat extraction unit includes a wellhead 1 and an insulating inner casing 10 coaxially arranged below the wellhead 1, the cementing unit includes a low-temperature formation 2, a high-temperature formation 6 and an aquifer 7 from top to bottom outside the wellhead 1, the low-temperature formation 2 section is located below the wellhead 1 and a large casing 3 is coaxially arranged, a cementing cement layer 14 is filled between the low-temperature formation 2 and the large casing 3, and the cement of the cementing cement layer 13 is a high thermal conductivity composite cement, which reduces the heat dissipation of the heat exchange medium to the low-temperature formation and reduces the heat loss rate of the geothermal heat extraction system.

[0029] The present invention does not need to inject cement into the high-temperature formation section and the aquifer section. By installing a water filter pipe to support the upper large casing, the cementing strength can be ensured, the cost of the lower cementing cement can be reduced, and the initial investment in a new geothermal well can be reduced. During cementing, only the water filter pipe is installed at the lower part and the large casing is installed at the upper part. There are no other special components and no special design of the geothermal well structure is required. The overall construction difficulty is relatively small and the scope of application is very wide.

[0030] A water filter pipe 5 and a small casing pipe 4 are coaxially installed in sequence from outside to inside below the large casing pipe 3. A gap is provided between the water filter pipe 5 and the small casing pipe 4. The gap between the water filter pipe 5 and the small casing pipe 4 forms a natural convection flow channel 11 for hot water in the aquifer 7. The lower end of the small casing pipe 4 is affected by the low-temperature circulating medium. The temperature of the geothermal water near the casing pipe is relatively low, while the temperature of the geothermal water at the far end of the aquifer 7 is relatively high. Natural convection is formed around the casing pipe under the gravity thermosiphon effect.

[0031] The lower end of the water filter pipe 5 passes through the high-temperature formation 6 and the aquifer 7 and is connected to the bottom of the geothermal well, so as to support the large casing 3 and fix the high-temperature formation 6 and the aquifer 7. The lower end of the small casing 4 passes through the high-temperature formation 6 and the aquifer 7 and is sealed by a plugging head 9. The small casing 4 is located on the outer wall of the aquifer 7 section and is installed with vertical fins 8. The number of the vertical fins 8 is several and is evenly distributed circumferentially on the outer wall of the small casing 4. The vertical fins 8 are triangular or serrated, so as to increase the disturbance of natural convection in the gap between the water filter pipe 5 and the small casing 4.

[0032] Small casing is used as the geothermal well wall in the lower high-temperature formation section and aquifer section, forming a fluid flow gap near the well wall of the aquifer section. The natural convection is caused in the gap due to the radial temperature difference, which strengthens the heat exchange process between the high-temperature aquifer section and the circulating working fluid in the geothermal well, effectively improving the heat extraction capacity of medium-deep geothermal single wells and achieving low-cost and efficient heat extraction.

[0033] The annular space between the large casing 3 and the small casing 4 and the heat-insulating inner casing 10 forms a circulating heat exchange fluid channel 12; the circular space inside the heat-insulating inner casing 10 forms an output channel 13 after heat extraction.

[0034] The working process of the present invention is:

[0035] The circulating heat exchange fluid medium that has completed heat release on the ground flows through the wellhead 1 into the circulating heat exchange fluid channel 12 between the large casing 3, the small casing 4 and the insulating inner casing 10. Since the composite cement with insulating particles is filled between the low-temperature formation 2 and the large casing 3, the heat dissipation of the circulating heat exchange fluid to the low-temperature formation 2 is greatly reduced during the downward flow process; at the same time, in the further downward flow process, it first absorbs the heat from the high-temperature formation 6, and then fully exchanges heat with the high-temperature geothermal water in the aquifer 7. Since the aquifer 7 exchanges heat in a natural convection manner, and the vertical fins 8 are used to further strengthen the convective heat exchange of natural convection in the natural convection flow channel 11, the heat exchange between the circulating fluid medium in the wellbore and the hot water in the formation in the aquifer 7 is more sufficient, and the heat in the aquifer 7 is absorbed to the maximum extent. The circulating heat exchange fluid that has completed the heat absorption process flows upward out of the wellhead 1 through the output channel 13 inside the insulating inner casing 10 to release heat, and circulates in sequence.

[0036] The present invention adopts a variable diameter casing for cementing. During the cementing process, the upper low-temperature formation section still adopts conventional cementing methods, and composite cement slurry is used to cement closely against the outer edge of the large casing; while in the lower high-temperature formation section and the aquifer section, a small casing is selected as the outer pipe of the geothermal well wall. Cement is no longer poured on the outside of the near-well wall, but a water filter pipe is installed to support the upper large casing. While ensuring the strength of cementing, the well formation structure adopts a combination of a variable diameter casing and a water filter pipe (or a naked hole) for cementing, thereby forming a natural convection flow channel 11 between the small casing and the water filter pipe.

[0037] Since the temperature near the well wall is lower and the temperature at the far end of the aquifer is higher, the temperature difference between cold and hot in the radial direction will cause natural convection in the pure fluid gap: the high-temperature geothermal water at the far end of the heat reservoir will flow radially to the gap between the small casing and the filter pipe in the upper part of the aquifer through natural convection, and then flow downward along the casing, and flow from the small casing to the far end of the heat reservoir at the bottom of the aquifer, thereby enhancing the heat exchange process between the high-temperature aquifer and the circulating working fluid in the geothermal well.

[0038] In addition, vertical fins are installed on the outer edge of the small casing to increase the heat exchange area outside the casing and at the same time enhance the fluid disturbance in the gap, so as to achieve the purpose of enhancing heat transfer and increasing the heat extraction power of the closed single well system.

[0039] like Figure 4 , 5The figures are respectively a comparison curve of the temperature difference at the wellhead and the wellhead outlet of the well forming structure of the present invention and the traditional well forming structure, and a comparison curve of the heat extraction power. It can be seen from the figures that the well forming structure of the present invention can effectively improve the heat exchange efficiency.

[0040] Although the embodiments and drawings of the present invention are disclosed for illustrative purposes, those skilled in the art will appreciate that various substitutions, changes and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. Therefore, the scope of the present invention is not limited to the contents disclosed in the embodiments and drawings.

Claims

1. A well formation structure for enhancing the heat extraction performance of a single closed geothermal well in a medium-deep layer, characterized by: It comprises a cementing unit and a heat extraction unit, wherein the heat extraction unit comprises a wellhead (1) and a heat-insulating inner casing (10) coaxially arranged below the wellhead (1), wherein the cementing unit comprises a low-temperature formation (2), a high-temperature formation (6) and an aquifer (7) from top to bottom outside the wellhead (1), wherein a large casing (3) coaxially arranged in the low-temperature formation (2) section below the wellhead (1), wherein a water filter pipe (5) and a small casing (4) are coaxially installed in sequence from outside to inside below the large casing (3), wherein the lower end of the water filter pipe (5) passes through the high-temperature formation (6) and the aquifer (7) and is connected to the bottom of the geothermal well, wherein the lower end of the small casing (4) passes through the high-temperature formation (6) and the aquifer (7) and is sealed by a plugging head (9), and wherein a vertical fin (8) is installed on the outer wall of the small casing (4) section located in the aquifer (7); A gap is provided between the water filter pipe (5) and the small sleeve (4), and the gap between the water filter pipe (5) and the small sleeve (4) forms a natural convection flow channel (11) for hot water in the aquifer (7); The annular space between the large casing (3) and the small casing (4) and the heat-insulating inner casing (10) forms a circulating heat exchange fluid channel (12); and the circular space inside the heat-insulating inner casing (10) forms an output channel (13) after heat extraction.

2. The well-forming structure for enhancing the heat extraction performance of a single closed geothermal well in a medium-deep layer according to claim 1 is characterized in that: The number of the vertical ribs (8) is several and they are evenly distributed and circumferentially arranged on the outer wall of the small sleeve (4).

3. The well formation structure for enhancing the heat extraction performance of a single closed geothermal well in a medium-deep layer according to claim 1 is characterized in that: The vertical ribs (8) are triangular or sawtooth shaped.

4. The well formation structure for enhancing the heat extraction performance of a single closed geothermal well in a medium-deep layer according to claim 1 is characterized in that: A cementing cement layer (14) is filled between the low-temperature formation (2) and the large casing (3), and the cement in the cementing cement layer (13) is high-thermal-conductivity composite cement.

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