A method for improving the thermal conductivity of a dry hot rock borehole formation

Abstract

The present application relates to the technical field of geothermal resource development, and discloses a method for improving the heat conduction performance of a dry hot rock drilling stratum, comprising the following steps: drilling a U-shaped well in a dry hot rock stratum; injecting a heat-conducting composite glue solution into a target layer of the U-shaped well as a fracturing fluid to perform hydraulic fracturing and form a dense fracture network; injecting the heat-conducting composite glue solution into the dense fracture network; and sealing and curing the well to make the heat-conducting composite glue solution hydrophobic and solidified, the heat-conducting material is tightly extruded, the fracture gaps are filled, and a filled high-heat-conducting modified layer is formed. The present application only uses a U-shaped well to form a closed heat exchange loop, and does not need to construct a branch well; the heat-conducting composite glue solution is used to perform hydraulic fracturing at the bottom of the U-shaped well, a dense fracture network is formed around the wellbore in the dry hot rock, and the high-heat-conducting modified layer is formed by completely filling the fracture gaps with the heat-conducting composite glue solution, so that the effective heat exchange area is expanded from the wellbore wall surface to the stratum scale, the heat transfer thermal resistance is significantly reduced, the cold accumulation problem is solved, and the heat exchange efficiency of a geothermal exploitation well is improved.

Description

Technical Field

[0001] This invention belongs to the field of geothermal resource development technology, and in particular relates to a method for improving the thermal conductivity of dry hot rock strata in boreholes. Background Technology

[0002] The core of developing deep geothermal resources such as hot dry rocks is to extract heat from the formation through heat exchange wells. Conventional AGS geothermal systems use multiple horizontal branch wells drilled underground. These branch wells are connected to the inlet and outlet wells to form a closed loop, and the heat exchange medium circulates directly in the well without the need for fracturing to connect the well body. However, the construction of branch wells is costly, time-consuming, and technically challenging.

[0003] Dry hot rock formations are characterized by dense lithology and limited thermal conductivity. The high thermal resistance between the intact wellbore and the heat exchange medium leads to a cold accumulation effect during long-term operation, severely limiting heat extraction efficiency. Existing technologies can only modify the thermal conductivity of a very small area of ​​the wellbore, resulting in limited improvement in heat exchange area. Although hydraulic fracturing can create a dense fracture network, the fracture walls are still composed of low thermal conductivity rock, which is prone to closure failure under geostress, making long-term stable heat exchange impossible. Summary of the Invention

[0004] In view of this, the purpose of this invention is to provide a method for improving the thermal conductivity of hot dry rock formations in boreholes. This method uses only a U-shaped well to form a closed heat exchange loop, eliminating the need for branch wells. At the bottom of the U-shaped well, a thermally conductive composite adhesive is used as the fracturing fluid for hydraulic fracturing, forming a dense network of fractures in the hot dry rock around the wellbore. The thermally conductive composite adhesive completely fills the fracture gaps to form a highly thermally conductive modified layer, expanding the effective heat exchange area from the wellbore wall to the formation scale, significantly reducing heat transfer resistance, solving the cold accumulation problem, and improving the heat exchange efficiency of geothermal wells.

[0005] The present invention solves the above-mentioned technical problems through the following technical means:

[0006] This invention discloses a method for improving the thermal conductivity of hot dry rock strata in boreholes, comprising the following steps:

[0007] U-shaped well completion: The drilling of a U-shaped well is completed in a dry hot rock formation, and the borehole wall is cleaned. The U-shaped well is a closed heat exchange loop without branches. One end of the U-shaped well is the water inlet well, and the other end is the water outlet well.

[0008] Fracturing: Injecting thermally conductive composite adhesive into the target section of the U-shaped well as a fracturing fluid to perform hydraulic fracturing, forming a dense network of fractures in the dry hot rock formation around the borehole of the U-shaped well;

[0009] Adding thermally conductive composite adhesive: Continuously inject the thermally conductive composite adhesive into the dense crack network so that the thermally conductive composite adhesive completely fills all the gaps in the dense crack network.

[0010] Curing and Molding: During well sealing and maintenance, the hydrophobic composite adhesive is cured using high downhole temperatures. Under in-situ stress, the thermally conductive materials within the adhesive are tightly compressed, filling cracks and gaps to form a filled, highly thermally conductive modified layer. This expands the effective heat exchange area from the wellbore wall to the formation depth. Simultaneously, the composite adhesive forms a continuous network of highly thermally conductive fractures in the dry, hot rock layer, significantly reducing the thermal resistance of heat transfer from the rock to the heat exchange medium, increasing thermal conductivity by more than 5 times.

[0011] Furthermore, in the U-shaped well construction process, the well diameter of the U-shaped well is ≥150mm, the length of the horizontal section is 500~2000 meters, and the vertical section is smoothly connected to the horizontal section; after well construction, rock powder and mud film are removed to ensure the well wall is clean. One end of the U-shaped well is the water inlet well, and the other end is the water outlet well, with a well diameter ≥150mm. After cleaning, it can meet the requirements for the installation of equipment such as casing, spray gun, and packer.

[0012] Furthermore, before fracturing and creating fractures, a fracturing area marking step is performed. This marking step includes: lowering a high-precision geomagnetic guidance locator from the water intake well and probing the entire vertical and horizontal sections of the U-shaped well. Combining data on the porosity, thermal conductivity, and lithological reservoir properties of the hot dry rock, the fracturing target area (the entire horizontal section + the lower 500-1000m of the vertical section) is marked with a positioning accuracy of ±0.5m. Magnetic positioning marks are then made on the fracturing tubing. The geomagnetic positioning technology used in this invention is currently the core positioning technology for deep horizontal well drilling and has been maturely applied in oil wells and geothermal wells above 7000 meters.

[0013] Furthermore, after marking the fracturing area, cementing is also performed in the complex formation area of ​​the vertical section of the U-shaped well. The purpose is to isolate the complex formation and prevent the loss of the heat exchange medium; the fracturing and stimulated section has no casing, and the heat exchange medium is in direct contact with the rock mass. The thermally conductive composite adhesive can make the fractured dry and hot rock layer hydrophobic and dense, so that the heat exchange medium will not be lost.

[0014] The cementing process includes: using a casing string running-in process, sequentially running the positioning casing with magnetic positioning marks and the high-strength production casing from the intake well until the casing string covers the complex formation area of ​​the vertical section of the U-shaped well; after the casing string is run in, cementing is performed using high-temperature and high-pressure cementing cement, with the cementing pressure controlled at 30~40MPa, and curing for 48~72 hours to ensure a firm bond between the casing and the well wall, without any channeling or leakage. The cementing process must ensure that after cementing, the inner wall of the casing is smooth, without protrusions or blockages, meeting the requirements for the running-in and movement of the spray gun and packer.

[0015] Furthermore, prior to the fracturing and fracture creation step, a fracturing segmentation step is included, in which the target layer is segmented using a recyclable packer to implement segmented fracturing.

[0016] Specifically, the fracturing segmentation step includes: lowering a high-temperature, high-pressure retrievable packer from the inlet well, and using a geomagnetic positioning system to set the packer at the segmentation nodes in the fracturing target area. The segmentation spacing for horizontal segments is 50-100m, and the segmentation spacing for vertical segments is 100-200m. Through a surface hydraulic control system, the packer is hydraulically expanded to achieve sealing and separation of each fracturing segment. The sealing pressure difference is ≥100MPa to ensure no cross-segment fluid flow during subsequent segmented fracturing and to guarantee the independence of each fracturing segment.

[0017] Furthermore, the step of injecting the thermally conductive composite adhesive includes: injecting the prepared thermally conductive composite adhesive from the inlet well into the first segmented fracturing section; starting the fracturing truck and gradually increasing the fracturing pressure from the initial pressure of 50 MPa to 100-140 MPa, with the flow rate controlled at 5-8 m³ / h; continuously injecting the adhesive, and when the downhole pressure peak drops by ≥10 MPa within 2 seconds, it is determined that the dry hot rock formation has formed fractures, immediately stopping the pressure increase, maintaining the current fracturing flow rate, and continuing to inject the adhesive for 30-60 minutes to ensure that the thermally conductive composite adhesive completely fills the fracture gaps; after the first segment fracturing is completed, releasing the packer and moving it to the next segment node, repeating the operation to complete the adhesive hydraulic fracturing and fracture filling of all fracturing sections.

[0018] Furthermore, the thermally conductive composite adhesive comprises a thermally conductive material and an adhesive liquid. The thermally conductive material comprises one or more of the following: nano-graphite, ultrafine graphene powder, ultrafine aluminum powder, and ultrafine copper powder. The adhesive liquid comprises one of the following: nano-silica sol and aluminum sol. The adhesive liquid is an aerogel material that can be hydrophobically cured under high underground temperatures.

[0019] Furthermore, the adhesive solution includes one of nano-silica sol and aluminum sol. Aluminum sol or silica sol are aerogel materials that can achieve hydrophobic solidification in high-temperature downhole environments. The thermally conductive materials such as graphite within the adhesive solution can be tightly compressed under geostress, further filling cracks and fissures.

[0020] Furthermore, the thermally conductive composite adhesive has a viscosity of 50~100 mPa·s, a compressive strength of ≥140 MPa, and a temperature resistance of ≥100℃ at a temperature of 25℃.

[0021] Furthermore, the thermally conductive composite adhesive comprises nano-graphite and nano-silica in a mass ratio of 2:1 to 5:1, as well as a dispersant and a thickener. The thickener is magnesium aluminum silicate, suitable for high temperature and high pressure, preventing the adhesive from demulsifying and agglomerating under fracturing pressure of 100-140 MPa; the dispersant is oleamide propylamine oxide. This thermally conductive composite adhesive ensures both the fluidity of the adhesive during high-pressure transport and the formation of a uniform liquid film layer (50-200 μm thick) on the fracture wall, preventing rapid loss.

[0022] Furthermore, the curing and molding step includes: after all fracturing sections are constructed, the wellheads of the U-shaped wells are sealed and cured for 72-96 hours. The high temperature downhole is used to hydrophobically solidify the aerogel material in the composite adhesive, while the thermally conductive material in the adhesive is tightly squeezed under the action of geostress to fill the cracks and ensure the density and stability of the high thermal conductivity modified layer.

[0023] In summary, this application has the following beneficial effects:

[0024] 1. This invention adopts a U-shaped well design without branch wells, which only requires drilling one U-shaped well to form a closed heat exchange loop. Compared with the traditional AGS geothermal system which requires the construction of multiple horizontal branch wells, it greatly reduces engineering costs and construction difficulty, and is more suitable for large-scale dry hot rock resource development.

[0025] 2. This invention utilizes thermally conductive composite adhesive hydraulic fracturing to construct a dense fracture network in the dry, hot rock formation surrounding the borehole, expanding the effective heat exchange area from the wellbore wall to the formation scale. Simultaneously, the injected thermally conductive composite adhesive completely fills the fracture gaps, forming a continuous, filled, highly thermally conductive modified layer after curing, significantly reducing the thermal resistance of heat transfer from the rock to the heat exchange medium. The combination of these two methods fundamentally solves the problem of cold accumulation during long-term operation, thereby greatly improving the heat exchange efficiency of geothermal wells.

[0026] 3. The main processes and corresponding equipment involved in this invention, such as drilling, casing installation, cementing, staged fracturing, and continuous injection, are all mature technologies in the fields of oil and gas and geothermal extraction. This invention optimizes the synergy between a specific thermally conductive composite adhesive and a specific construction process, thus the overall solution has strong on-site implementability and controllable modification costs. In addition, the optimized thermally conductive composite adhesive itself has the functions of filling, curing, and stabilizing fractures. It hydrophobically cures under the high temperature downhole, and the internal thermally conductive material can be further compressed and filled under the action of geostress. Therefore, it can ensure the long-term stability of fractures without the need for additional proppant injection, avoiding fracture closure caused by geostress. This simplifies the construction process while ensuring the long-term stable operation of the heat exchange system.

[0027] 4. This invention is specifically designed and selected for the high-temperature and high-pressure environment of deep dry hot rock, exhibiting excellent temperature resistance, pressure resistance, and formation compatibility. Therefore, this method is applicable to the development of deep geothermal resources such as dry hot rock at depths of 3000-9000 meters, formation pressures of 30-90 MPa, and temperatures of 100-300℃, with a clear and wide range of applications. Detailed Implementation

[0028] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0029] First, the equipment used in the following examples are all standardized equipment in the fields of deep underground drilling and formation fracturing. They can be directly assembled on-site and are suitable for 7000-meter deep wells, fracturing pressures of 100~140MPa, and high-temperature conditions of 100~200℃. The specific selection is as follows:

[0030] Equipment type Specific selection / parameter requirements core role Geomagnetic positioning system High-precision geomagnetic guidance positioning instrument for deep wells, with a positioning accuracy of ±0.5m, temperature resistance of 300℃, and pressure resistance of 100MPa. Achieve precise positioning of the fracturing zone in the horizontal / vertical sections of U-shaped wells at depths of 3000-9000 meters, guiding casing installation and fracturing operations. casing system Positioning casing (N80 grade oil casing with magnetic positioning mark) + Production casing (P110 grade high-strength casing, temperature resistance 300℃, pressure resistance 140MPa) Achieving wellbore reinforcement in complex formations, providing operational access for fracturing, and using magnetic positioning markers in conjunction with a geomagnetic system to achieve precise fracturing. packer High-temperature and high-pressure recyclable hydraulic expansion packer, with a temperature resistance of 300℃, a pressure resistance of 140MPa, and a sealing pressure differential of ≥100MPa. The U-shaped well fracturing section (horizontal section + part of vertical section) is divided into segments (each segment length 50~200m, adjustable) to achieve segmented fracturing, avoid cross-segment fluid flow, and ensure fracturing effect. Fracturing truck crew High-pressure, high-displacement fracturing trucks, with a single unit output pressure ≥100MPa, and combined operation output pressure 100~140MPa, displacement 5~15m³ / h. This provides high-pressure power for fracturing, enabling high-pressure fracture creation and delivery of the adhesive. adhesive conveying equipment High-pressure, corrosion-resistant glue pump and sand mixing truck, withstanding temperatures up to 300℃ and pressures up to 140MPa. The mixing truck enables continuous and stable delivery of the thermally conductive composite adhesive, and can also achieve uniform mixing of the adhesive. Pressure / flow monitoring system High-precision downhole pressure sensors and surface flow meters, with monitoring accuracy of ±0.1MPa and ±0.1m³ / h respectively. Real-time monitoring of downhole pressure and fluid flow rate during fracturing, and using a sudden drop in peak pressure to determine formation fracture formation. Adhesive recovery equipment High-pressure resistant filter recovery tank, ultrasonic disperser Excess thermally conductive composite adhesive that did not adhere during fracturing is recovered, filtered, dispersed, and reused to reduce engineering costs (using the existing adhesive recovery process).

[0031] Example 1: This example describes the preparation of a nano-graphite-silica thermally conductive composite adhesive.

[0032] The nano-graphite-silica thermally conductive composite adhesive in the following embodiments adjusts the viscosity, dispersion stability, and high pressure resistance, while retaining the core ratio and thermal conductivity / bonding properties. It is suitable for high-pressure delivery, fracture wall adhesion, and deep penetration requirements in the fracturing process, without changing the original environmentally friendly water-based system properties.

[0033] 1. Raw material ratio: The mass ratio of nano-graphite to nano-silica (solid content) remains 2:1 (which can be adjusted according to the porosity / crack width of the dry hot rock), supplemented with a small amount of high-pressure resistant environmentally friendly dispersant and thickener (the thickener is magnesium aluminum silicate, and the dispersant is oleamidopropylamine oxide).

[0034] 2. Key performance optimization: The viscosity of the adhesive is adjusted to 50~100mPa·s (25℃) to ensure fluidity during high-pressure transportation and to form a uniform liquid film layer (thickness 50~200μm) on the crack wall to avoid rapid loss; it can withstand high pressure ≥140MPa and temperature 100~300℃ without performance degradation.

[0035] 3. Preparation process: Based on the original mechanical stirring + ultrasonic dispersion, high-pressure homogenization treatment (pressure 30MPa) is added to further improve the dispersion stability of the adhesive and ensure that no graphite particles settle during fracturing and transportation. The preparation process is still suitable for on-site engineering construction.

[0036] 4. Core functions: ① As a fracturing working fluid, it participates in formation fracturing; ② It solidifies in situ on the fracture wall to form a dense, highly thermally conductive modified layer, reducing the thermal resistance between the fracture wall and the heat exchange medium; ③ It chemically bonds with the fracture wall of dry hot rock, improving the bonding strength between the thermally conductive layer and the formation, and resisting groundwater corrosion and formation stress disturbance.

[0037] Example 2: This example describes the preparation of a nano-graphite-silica thermally conductive composite adhesive.

[0038] The only difference between this embodiment and Embodiment 1 is that the mass ratio of nano-graphite to nano-silica (solid content) in this embodiment is 3.5:1.

[0039] Example 3: This example describes the preparation of a nano-graphite-silica thermally conductive composite adhesive.

[0040] The only difference between this embodiment and Embodiment 1 is that the mass ratio of nano-graphite to nano-silica (solid content) in this embodiment is 5:1.

[0041] Example 4

[0042] This embodiment is a method for improving the thermal conductivity of borehole walls in hot dry rock, including the following steps:

[0043] Step 1: Well completion of deep dry hot rock geothermal U-shaped well (existing mature technology)

[0044] Using deep diamond core drilling technology, a 7,000-meter deep dry hot rock reservoir U-shaped well was completed. After completion, the well structure was kept intact. The length of the horizontal section was designed according to the reservoir distribution (500 in this example), and the vertical section was smoothly connected to the horizontal section. After completion, the well wall was flushed with high-pressure clean water to remove rock powder and mud film, ensuring the well wall was clean.

[0045] Key requirements: The U-shaped well has an inlet well at one end and an outlet well at the other end, with a well diameter ≥150mm, to meet the requirements for lowering equipment such as casing, spray gun, and packer.

[0046] Step 2: Precise geomagnetic positioning of deep wells to mark fracturing areas.

[0047] A high-precision geomagnetic guidance positioning instrument is lowered into the water intake well and probes the entire length of the U-shaped well from the vertical section to the horizontal section. Combined with the physical property data of the hot dry rock reservoir (porosity, thermal conductivity, lithology), the fracturing target area is accurately marked with a positioning accuracy of ±0.5m. Magnetic positioning marks are made on the positioning casing to ensure that subsequent fracturing operations accurately correspond to the target area.

[0048] Geomagnetic positioning technology is currently the core positioning technology for deep horizontal well drilling, and it has been successfully applied in oil wells and geothermal wells above 7,000 meters.

[0049] Step 3: Production casing running and cementing

[0050] The casing string running process is adopted, in which the positioning casing with magnetic positioning mark and the P110 grade high-strength production casing are run sequentially from the water intake well until the casing string covers the complex formation area of ​​the vertical section of the U-shaped well. After the casing string is run, high temperature and high pressure cementing is used for cementing, the cementing pressure is controlled at 30MPa, and it is cured for 48 hours to ensure that the casing is firmly bonded to the well wall and there are no problems of channeling or leakage.

[0051] Core requirements: After casing cementing, the inner wall should be smooth, without protrusions or blockages, to meet the requirements for the insertion and movement of the spray gun and packer.

[0052] Step 4: The retrievable packer is lowered to achieve segmentation of the fracturing section.

[0053] 1. The high-temperature and high-pressure retrievable packer is lowered from the water intake well and, in conjunction with the geomagnetic positioning system, the packer is set at the segment nodes of the fracturing target area (each segment is 50m long, with a smaller spacing of 50m for horizontal segments and a larger spacing of 100m for vertical segments).

[0054] 2. By using a ground-based hydraulic control system, the packer is hydraulically expanded to achieve sealing and separation of each fracturing section, with a sealing pressure difference ≥100MPa, ensuring no cross-section fluid flow during subsequent segmented fracturing and guaranteeing the independence of each fracturing segment.

[0055] Recyclable packer segmented fracturing technology has been successfully applied in deep oil and gas fracturing, enabling precise segmentation and sealing of wells up to 7,000 meters deep.

[0056] Step 5: Inject thermally conductive composite adhesive in staged fracturing, and use the sudden pressure drop to determine the formation of fractures.

[0057] This step uses high-pressure fracturing of 100~140MPa. The nano-graphite-silica thermally conductive composite adhesive prepared in Example 1 is injected as the fracturing working fluid to achieve formation fracturing and preliminary modification of the fracture wall. Downhole pressure and flow rate are monitored in real time throughout the process.

[0058] 1. Inject the prepared thermally conductive composite adhesive into the first segmented fracturing section from the intake well, start the fracturing truck, and gradually increase the fracturing pressure from the initial pressure of 50MPa to 100MPa, while controlling the discharge rate at 5m³ / h;

[0059] 2. Continuously inject the adhesive. When the downhole pressure peak suddenly drops by ≥10MPa within 2 seconds, it is determined that the dry hot rock formation has formed fractures (a dense fracture network has initially developed). Immediately stop pressurizing, maintain the current fracturing flow rate, and continue injecting the adhesive for 30 minutes to ensure that the thermally conductive composite adhesive fully penetrates the fracture wall and forms a uniform liquid film layer.

[0060] 3. After the first stage of fracturing is completed, release the packer and move it to the next segment node. Repeat the above operation to complete the gel fracturing of all fracturing segments and the adhesion of the fracturing wall.

[0061] 4. During the fracturing process, the solution is drained through the outlet well, filtered and ultrasonically dispersed by the adhesive recovery equipment, and then stored for reuse (using the original adhesive recovery process).

[0062] Key parameters: The injection volume of the single-stage gel is calculated based on the length of the fracturing section and the formation porosity. The injection volume of a single stage in a 7000-meter deep well is 35m³. The fracturing pressure is strictly controlled between 100 and 140MPa to avoid excessive pressure that could lead to casing rupture and formation collapse.

[0063] Step 6: Curing and maintenance of thermally conductive composite adhesive + overall inspection of the fracturing section

[0064] 1. Curing and maintenance: After all fracturing sections are completed, all equipment such as packers and spray guns are removed. The wellheads of the U-shaped well intake and outlet are sealed. The high temperature of the formation (100~300℃) and the pressure of 30~90MPa are used to promote the curing of the thermally conductive composite adhesive on the fracture wall. The curing time is 72 hours to ensure that the adhesive is fully cured and a dense, highly thermally conductive integrated modified layer is formed on the fracture wall.

[0065] 2. Fracturing Section Inspection: After maintenance, a deep-well sonic logging tool and a thermal conductivity logging tool were used to conduct a full-process inspection of the fracturing section from the water intake well, verifying: ① the connectivity and opening of the dense fracture network (opening ≥ 0.3 mm); ② the thermal conductivity of the heat-transferring rock layer in the fracturing section (more than 5 times higher than the thermal conductivity of the original dry hot rock, consistent with the effect of the original technology modification). The test showed that the thermal conductivity of the target dry hot rock layer increased from 2.5 to 15.1, an increase of approximately 6 times.

[0066] 3. Trial production: After passing the inspection, start the trial production of the U-shaped well, inject the heat exchange medium from the intake well and export it from the outlet well, monitor the heat extraction power and formation temperature changes, and verify the heat exchange effect.

[0067] Example 5

[0068] The difference between this embodiment and embodiment 4 is that, in this embodiment,

[0069] The length of the horizontal section in step 1 is approximately 1000 meters;

[0070] In step 3, the cementing pressure is controlled at 35 MPa, and the curing time is 60 hours.

[0071] In step 4, each segment is 120m long. The horizontal segments are spaced 75m apart, while the vertical segments are spaced 150m apart.

[0072] In step 5, the nano-graphite-silica thermally conductive composite adhesive prepared in Example 2 is used. The pressure is increased from the initial pressure of 50MPa to 120MPa, and the flow rate is controlled at 7m³ / h. The current fracturing flow rate is maintained, and the adhesive is injected for another 45 minutes.

[0073] The curing time in step 6 is 84 hours.

[0074] Finally, through testing, the thermal conductivity of the target layer of dry hot rock increased from 2.5 to 18.2, an increase of about 7.3 times.

[0075] Example 6

[0076] The difference between this embodiment and embodiment 4 is that, in this embodiment,

[0077] The length of the horizontal section in step 1 is approximately 2000 meters;

[0078] In step 3, the cementing pressure is controlled at 40 MPa, and the curing time is 72 hours.

[0079] In step 4, each segment is 200m long. The horizontal segments are spaced 100m apart, while the vertical segments are spaced 200m apart.

[0080] In step 5, the nano-graphite-silica thermally conductive composite adhesive prepared in Example 3 is used. The pressure is increased from the initial 50 MPa to 140 MPa, and the flow rate is controlled at 8 m³ / h. The current fracturing flow rate is maintained, and the adhesive is injected for another 60 minutes.

[0081] The curing time in step 6 is 96 hours.

[0082] Finally, through testing, the thermal conductivity of the target layer of dry hot rock increased from 2.5 to 19.3, an increase of about 7.7 times.

[0083] Comparative Example 1

[0084] The depth of the U-shaped well in this embodiment is similar to that in Embodiment 2, both around 7000 meters. However, the U-shaped well in this embodiment does not employ fracturing operations.

[0085] Finally, through testing, the thermal conductivity of the target layer of dry hot rock increased from 2.5 to 4.6, an increase of only about 1.84 times.

[0086] Comparative Example 2

[0087] The depth of the U-shaped well in this embodiment is similar to that in Embodiment 2, both around 7000 meters. However, the U-shaped well in this embodiment is constructed using fracturing, but without continuous injection of thermally conductive composite adhesive.

[0088] Finally, through testing, the thermal conductivity of the target layer of dry hot rock increased from 2.5 to 5.8, an increase of only about 2.32 times.

[0089] Through the comparison of the above embodiments 4 to 6, and comparative embodiments 1 to 2, it can be seen that:

[0090] 1. The complete patented solution (Examples 4-6) achieves a significant improvement in thermal conductivity of more than 6 times, far exceeding the effect of any simplified solution (comparative Examples 1-2) with single or two steps. This verifies the necessity and superiority of the synergistic effect of "fracturing and expansion + adhesive modification" described in this invention.

[0091] 2. The results of the comparison example 1 are far inferior to those of the fracturing method, which proves that fracturing to create fractures and significantly increase the heat exchange area is the physical basis for achieving efficient heat exchange and is indispensable.

[0092] 3. The results of Example 2 were worse than those of Example 2, indicating that fracturing alone cannot effectively improve the thermal conductivity of the crack wall. Continuous injection of the thermally conductive composite adhesive is the core material for reducing the thermal resistance of the crack wall and achieving enhanced thermal conductivity.

[0093] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention. Technical aspects, shapes, and structures not described in detail in this invention are all well-known technologies.

Claims

1. A method for improving the thermal conductivity of hot dry rock strata in boreholes, characterized in that, Includes the following steps: U-shaped well completion: The drilling of a U-shaped well is completed in a dry hot rock formation, and the borehole wall is cleaned. The U-shaped well is a closed heat exchange loop without branches. One end of the U-shaped well is the water inlet well, and the other end is the water outlet well. Fracturing: Injecting thermally conductive composite adhesive into the target section of the U-shaped well as a fracturing fluid to perform hydraulic fracturing, forming a dense network of fractures in the dry hot rock formation around the borehole of the U-shaped well; Adding thermally conductive composite adhesive: Continuously inject the thermally conductive composite adhesive into the dense crack network so that the thermally conductive composite adhesive completely fills all the gaps in the dense crack network. Curing and molding: Well sealing and maintenance, using the high temperature downhole to cure the hydrophobic composite adhesive, the thermally conductive materials in the thermally conductive composite adhesive are tightly squeezed under the action of geostress, filling the cracks and gaps, forming a filled high thermal conductivity modified layer.

2. The method for improving the thermal conductivity of hot dry rock strata in boreholes according to claim 1, characterized in that, In the U-shaped well construction process, the diameter of the U-shaped well is ≥150mm, the length of the horizontal section is 500~2000 meters, and the vertical section is smoothly connected to the horizontal section; after the well is completed, rock powder and mud film are removed to ensure the well wall is clean.

3. A method for improving the thermal conductivity of hot dry rock strata in boreholes according to claim 1 or 2, characterized in that, Before performing the fracturing and fracture creation, a step of marking the fracturing area is also performed; The step of marking the fracturing area includes: lowering a high-precision geomagnetic guidance locator from the water intake well and probing the entire length of the vertical and horizontal sections of the U-shaped well; combining data on the porosity, thermal conductivity, and lithological reservoir properties of the hot dry rock to mark the fracturing target area with a positioning accuracy of ±0.5m; and making magnetic positioning marks on the fracturing tubing.

4. The method for improving the thermal conductivity of hot dry rock strata in boreholes according to claim 3, characterized in that, After the marked fracturing zone step, cementing is also performed in the complex formation area of ​​the vertical section of the U-shaped well.

5. The method for improving the thermal conductivity of hot dry rock strata in boreholes according to claim 4, characterized in that, Before the fracturing and fracture creation step, there is also a fracturing segmentation step, in which the target layer is segmented using a packer to implement segmented fracturing.

6. The method for improving the thermal conductivity of hot dry rock strata in boreholes according to claim 5, characterized in that, The steps for injecting the thermally conductive composite adhesive include: injecting the prepared thermally conductive composite adhesive from the inlet well into the first segmented fracturing section; starting the fracturing truck and gradually increasing the fracturing pressure from an initial pressure of 50 MPa to 100-140 MPa, with the flow rate controlled at 5-8 m³ / h; continuously injecting the adhesive, and when the downhole pressure peak drops by ≥10 MPa within 2 seconds, it is determined that the dry hot rock formation has fractured, immediately stopping the pressure increase, maintaining the current fracturing flow rate, and continuing to inject the adhesive for 30-60 minutes to ensure that the thermally conductive composite adhesive completely fills the fracture gaps; after the first segment fracturing is completed, releasing the packer and moving it to the next segment node, repeating the operation to complete the adhesive fracturing and fracture filling of all fracturing sections.

7. The method for improving the thermal conductivity of hot dry rock strata in boreholes according to claim 1, characterized in that, The thermally conductive composite adhesive comprises a thermally conductive material and an adhesive. The thermally conductive material includes one or more of the following: nano-graphite, ultrafine graphene powder, ultrafine aluminum powder, and ultrafine copper powder. The adhesive is an aerogel material that can be hydrophobically cured under high underground temperatures.

8. The method for improving the thermal conductivity of hot dry rock strata in boreholes according to claim 7, characterized in that, The adhesive solution includes one of nano-silica sol and aluminum sol.

9. A method for improving the thermal conductivity of hot dry rock strata in boreholes according to claim 8, characterized in that, The thermally conductive composite adhesive has a viscosity of 50~100 mPa·s at 25℃, a compressive strength ≥140MPa, and a temperature resistance ≥100℃.

10. A method for improving the thermal conductivity of hot dry rock strata in boreholes according to any one of claims 1 to 9, characterized in that, The curing and molding steps include: after all fracturing sections are completed, the inlet and outlet wells of the U-shaped well are sealed and cured for 72-96 hours. The high temperature downhole is used to make the hydrophobic material in the composite adhesive liquid solidify. At the same time, the thermally conductive material in the adhesive liquid is tightly squeezed under the action of geostress to fill the crack gaps and ensure the density and stability of the high thermal conductivity modified layer.