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

Abstract

The utility model 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 circulation flowing 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 utility model belongs to the technical field of the development and utilization of medium and deep geothermal energy resources, and particularly relates to a well completion structure for enhancing the heat extraction performance of a medium and deep closed geothermal single well. Background Technique

[0002] The traditional medium and deep geothermal energy utilization mode is mainly the development and utilization of hydrothermal geothermal resources with combined extraction and injection. As described in the disclosed patent with the publication number CN107642914B, due to the limitation of geothermal geological conditions, it is difficult to inject water back into some sandstone formations, resulting in a series of problems such as the decline of water level and the increase of pump power consumption after the long-term operation of the system. To further standardize the scientific management of geothermal resources, many local governments have successively introduced some restrictive policies, requiring that geothermal tail water must be reinjected into the same layer and the reinjection rate is not less than 95%. For this reason, a number of geothermal wells that cannot meet the reinjection requirements have been shut down in the Beijing-Tianjin-Hebei region. How to efficiently and sustainably obtain geothermal energy under different geothermal resource conditions and realize the clean heating of buildings in northern China has become the biggest technical bottleneck in the geothermal industry.

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

[0004] At present, when the existing medium and deep closed single wells are cemented, high thermal conductivity cement slurry is often poured from the bottom of the well to the wellhead to displace the water with a lower thermal conductivity in the formation near the well wall, which improves the thermal conductivity of the formation to a certain extent. However, the results of multiple actual projects prove that the technical means of pouring high thermal conductivity cement slurry is not obvious for improving the heat extraction power.

[0005] Some experts and scholars have proposed to use various wellbore structures such as closed connected wells (CN104154668A), closed branched wells (CN106948795B), and closed horizontal wells (CN108302833A) to improve the heat extraction power of medium and deep closed single wells. The core idea is to enhance the heat extraction capacity of the system by increasing the heat exchange area of the wellbore in the higher formation temperature section. However, under the limitation of the thermal conductivity of the formation and the cement for well cementing, these methods are difficult to achieve obvious effects. In addition, the relatively high initial investment and operation cost of the medium and deep closed single well heat extraction system limit the popularization and application of this mode.

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

[0007] The purpose of the present utility model is to overcome the deficiencies of the prior art and provide a well completion structure for enhancing the heat extraction performance of medium-deep closed geothermal single wells. Under the condition of high-temperature aquifers existing in the formation, natural convection can be formed near the wellbore, the fluid circulation in the formation can be strengthened, the heat transfer mode in the heat storage near the wellbore of the closed system can be converted from the conventional heat conduction mode to the convective heat transfer mode, the heat extraction capacity of medium-deep closed single wells can be effectively improved, and the well completion process is simple and the economy is good.

[0008] The present utility model solves its technical problems through the following technical solutions:

[0009] A well completion structure for enhancing the heat extraction performance of medium-deep closed geothermal single wells includes a cementing unit and a heat extraction unit. The heat extraction unit includes a wellhead and a heat-insulating inner casing coaxially arranged below the wellhead. The cementing unit includes a low-temperature formation, a high-temperature formation and an aquifer from top to bottom outside the wellhead. A large casing is coaxially arranged below the wellhead in the low-temperature formation section. A filter pipe and a small casing are coaxially installed from outside to inside below the large casing. The lower end of the filter pipe passes through the high-temperature formation and the aquifer and communicates with the bottom of the geothermal well. The lower end of the small casing passes through the high-temperature formation and the aquifer and is sealed by a plug. Vertical fins are installed on the outer wall of the small casing in the aquifer section.

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

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

[0012] Moreover, the number of the vertical fins is several, and they are evenly distributed circumferentially on the outer wall of the small casing.

[0013] Moreover, the vertical fins are triangular or serrated.

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

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

[0016] 1. The well completion structure of the present utility model relies on the existing closed heat extraction mode of "extracting heat without extracting water". When cementing, a composite cement slurry with certain heat preservation performance is poured on the outer edge of the large casing in the upper low-temperature formation section, reducing the heat dissipation of the heat exchange working medium to the low-temperature formation and lowering the heat loss rate of the geothermal heat extraction system.

[0017] 2. For the well completion structure of the present utility model, small casings are selected as the geothermal well walls in the lower high-temperature formation section and the aquifer section, forming a fluid flow gap near the well wall in the aquifer section. Affected by the radial temperature difference between hot and cold, natural convection is induced in the gap, strengthening the heat exchange process between the high-temperature aquifer section and the circulating working medium in the geothermal well, effectively improving the heat extraction capacity of a single medium-deep geothermal well, and achieving low-cost and high-efficiency heat extraction.

[0018] 3. The well completion structure of the present utility model does not require pouring cement in the high-temperature formation section and the aquifer section. By installing a 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, reducing the initial investment of the newly built geothermal well.

[0019] 4. For the well completion structure of the present utility model, only a filter pipe is installed at the lower part and a large casing is installed at the upper part during cementing. There are no other special components, and no special design of the geothermal well structure is required. The overall construction difficulty is small, and the applicable range is very wide. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 is a schematic diagram of the well completion structure of the present utility model;

[0021] Figure 2 is Figure 1 an enlarged view of part A of

[0022] Figure 3 is a top view of the aquifer of the present utility model.

[0023] DESCRIPTION OF THE REFERENCE NUMERALS

[0024] 1. Wellhead; 2. Low-temperature formation; 3. Large casing; 4. Small casing; 5. Filter pipe; 6. High-temperature formation; 7. Aquifer; 8. Vertical fins; 9. Plug; 10. Heat-insulating inner casing; 11. Natural convection flow channel; 12. Circulating heat exchange fluid channel; 13. Output channel; 14 - Cementing cement layer. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0025] The present utility model will be further described in detail below through specific embodiments. The following embodiments are only descriptive and not restrictive, and the protection scope of the present utility model cannot be limited thereby.

[0026] As Figures 1 to 3As shown in the figure, a well completion structure for enhancing the heat extraction performance of a deep and 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 a heat-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. A large casing 3 is coaxially arranged below the wellhead 1 in the low-temperature formation 2 section. A cementing cement layer 14 is filled between the low-temperature formation 2 and the large casing 3. The cement of the cementing cement layer 13 is a high-thermal-conductivity composite cement, which reduces the heat dissipation of the heat exchange working medium to the low-temperature formation and reduces the heat loss rate of the geothermal heat extraction system.

[0027] The utility model does not need to pour cement in the high-temperature formation section and the aquifer section. By installing a 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, reducing the initial investment of the newly built geothermal well; only a filter pipe is installed at the lower part during cementing, and a large casing is installed at the upper part, without other special components, and there is no need to specially design the geothermal well structure. The overall construction difficulty is small, and the applicable range is very wide.

[0028] A filter pipe 5 and a small casing 4 are coaxially installed from outside to inside in sequence below the large casing 3. There is a gap between the filter pipe 5 and the small casing 4. The gap between the filter pipe 5 and the small casing 4 forms a natural convection flow channel 11 for geothermal water in the aquifer 7. Affected by the low-temperature circulating working medium at the lower end of the small casing 4, the temperature of the geothermal water near the casing periphery is relatively low, and the temperature of the geothermal water at the far end of the aquifer 7 is relatively high. Natural convection is formed around the casing under the action of the gravity thermosiphon effect;

[0029] The lower end of the filter pipe 5 passes through the high-temperature formation 6 and the aquifer 7 and communicates to the bottom of the geothermal well 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 plug 9. Vertical fins 8 are installed on the outer wall of the small casing 4 in the aquifer 7 section. The number of the vertical fins 8 is several, and they are evenly distributed circumferentially on the outer wall of the small casing 4. The vertical fins 8 are triangular or serrated to increase the disturbance of natural convection in the gap between the filter pipe 5 and the small casing 4.

[0030] A small casing is selected as the geothermal well wall in the lower high-temperature formation section and the aquifer section, forming a fluid flow gap near the well wall in the aquifer section. Affected by the radial temperature difference between hot and cold, natural convection is caused in the gap, strengthening the heat exchange process between the high-temperature aquifer section and the circulating working medium in the geothermal well, effectively improving the heat extraction capacity of the deep geothermal single well, and realizing low-cost and high-efficiency heat extraction.

[0031] 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.

[0032] The working process of the utility model is as follows:

[0033] The circulating heat exchange fluid medium that has released heat on the ground flows into the circulating heat exchange fluid channel 12 between the large casing 3, the small casing 4 and the heat-insulating inner casing 10 through the wellhead 1. Since the composite cement with heat-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; at the same time, during the further downward flow, it first absorbs the heat at the high-temperature formation 6, and then fully exchanges heat with the high-temperature geothermal water at 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 transfer in the natural convection flow channel 11, the heat exchange between the circulating fluid medium in the wellbore and the geothermal water in the formation at the aquifer 7 is more sufficient, maximizing the absorption of the heat in the aquifer 7. The circulating heat exchange fluid that has completed the heat absorption process flows upward through the output channel 13 inside the heat-insulating inner casing 10 and out of the wellhead 1 device for the heat release process, and circulates in turn.

[0034] The present invention uses variable-diameter casing cementing. During the cementing process, the conventional cementing method is still used in the upper low-temperature formation section, and the composite cement slurry is used to cement tightly against the outer edge of the large casing; while in the lower high-temperature formation section and the aquifer section, the small casing is selected as the outer tube of the geothermal wellbore. Instead of pouring cement on the outer side of the near-wellbore, a filter pipe is installed to support the upper large casing. This well completion structure combines variable-diameter casing and filter pipe (or open hole) to carry out cementing while ensuring the cementing strength, and thus a natural convection flow channel 11 is formed between the small casing and the filter pipe.

[0035] Since the temperature near the wellbore is relatively low and the temperature at the far end of the aquifer is relatively high, the temperature difference between hot and cold generated 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 upward along the aquifer between the small casing and the filter pipe 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 strengthening the heat exchange process between the high-temperature aquifer and the circulating working medium in the geothermal well.

[0036] 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 strengthen the fluid disturbance in the gap, so as to achieve the purpose of strengthening heat transfer and improving the heat extraction power of the closed single-well system.

[0037] Although embodiments and drawings of the present utility model are disclosed for illustrative purposes, those skilled in the art can understand that: various substitutions, changes, and modifications are possible without departing from the spirit and scope of the present utility model and the appended claims. Therefore, the scope of the present utility model is not limited to the content disclosed in the embodiments and drawings.

Claims

1. A well completion structure for enhancing the heat extraction performance of a medium-deep closed geothermal single well, characterized in that: It includes a cementing unit and a heat extraction unit. The heat extraction unit includes a wellhead (1) and a heat-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 around the wellhead (1). A large casing (3) is coaxially arranged below the wellhead (1) in the low-temperature formation (2) section. A filter pipe (5) and a small casing (4) are coaxially installed from outside to inside below the large casing (3). The lower end of the filter pipe (5) passes through the high-temperature formation (6) and the aquifer (7) and communicates to the bottom of the geothermal well. 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). Vertical fins (8) are installed on the outer wall of the small casing (4) in the aquifer (7) section.

2. The well completion structure for enhancing the heat extraction performance of a deep closed geothermal single well according to claim 1, characterized in that: A gap is provided between the filter pipe (5) and the small casing (4), and the gap between the filter pipe (5) and the small casing (4) forms a natural convection flow channel (11) for geothermal water in the aquifer (7).

3. The well completion structure for enhancing the heat extraction performance of a deep and medium-deep closed geothermal single well according to claim 1, characterized in that: 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.

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

5. The well completion structure for enhancing the heat extraction performance of a deep and medium-deep closed geothermal single well according to claim 1, characterized in that: The vertical fins (8) are triangular or serrated.

6. The well completion structure for enhancing the heat extraction performance of a deep and medium-deep closed geothermal single well according to claim 1, characterized in that: 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 (14) is a high-thermal-conductivity composite cement.

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