A method for exploiting dry hot rock based on a closed loop geothermal system

Abstract

This invention discloses a method for exploiting hot dry rock using an open-hole serpentine horizontal well based on a closed-loop geothermal system, belonging to the field of geothermal development. In the target exploitation area, an injection well and a production well are drilled. The injection well is an open-hole serpentine horizontal well in the hot dry rock reservoir. A low-temperature heat transfer medium is injected into the injection well, and a high-temperature heat transfer medium is produced from the production well to achieve the purpose of obtaining heat from the hot dry rock reservoir. This invention reduces the process steps of running casing, enhances feasibility, and lowers exploitation costs. The optimized horizontal well shape increases the heat exchange area between the flowing medium and the hot dry rock reservoir, extends the residence time, and appropriately increases the flow resistance. It ensures uniform flow of the flowing medium, allowing for sufficient heat exchange between the heat transfer medium and the reservoir, resulting in higher heat extraction efficiency.

Description

Technical Field

[0001] This invention belongs to the field of geothermal development, specifically, it relates to a method for mining dry hot rock using a naked-eye serpentine horizontal well based on a closed-loop geothermal system. Background Technology

[0002] Hot dry geothermal resources (hot dry rock) generally refer to high-temperature rock masses buried thousands of meters below the Earth's surface, with temperatures exceeding 180°C, and containing little or no internal fluids. As a widely distributed, abundant, clean, and pollution-free renewable green energy source, hot dry rock resources offer advantages such as uninterrupted power supply and significant development potential. my country's total hot dry rock resources amount to 2.52 × 10²⁵ J (equivalent to 8.56 × 10⁵ billion tons of standard coal), accounting for one-sixth of the world's total. Therefore, the rational development and utilization of hot dry rock resources, and the accelerated breakthrough of technological bottlenecks to achieve efficient and stable development, are of great significance for the transformation of my country's energy structure and the early realization of carbon peaking and carbon neutrality goals.

[0003] Currently, there are two main types of development and utilization of hot dry rock resources. One is Enhanced Geothermal Systems (EGS), which first artificially form geothermal reservoirs and then extract heat energy through injection and production wells by circulating the heat extraction medium. The other is Closed Loop Geothermal Systems (CLGS), which utilize the heat extraction medium to extract heat in a closed loop within a horizontal wellbore, thus avoiding the complex construction of artificial heat reservoirs.

[0004] Conventional EGS (Enhanced Thermal Storage) methods for mining hot dry rock involve fracturing to create fractures for heat exchange. This method not only has drawbacks such as high investment costs and easy loss of the heat transfer medium, but also has a significant environmental impact, potentially triggering geological disasters such as earthquakes. Closed-loop circulation (CLS) technology for mining hot dry rock can avoid the loss of heat transfer medium caused by flow short circuits and dead zones. Horizontal well technology for mining hot dry rock can achieve high production and efficiency by increasing the contact area between the reservoir and the wellbore. Combining closed-loop circulation systems with horizontal well technology can significantly improve the heat recovery rate of a single well and increase energy production. However, existing methods for mining hot dry rock have drawbacks such as poor feasibility and high cost. For example, CN109798091A discloses a closed-loop well and a method for developing hot dry rock, and CN113846968A discloses a side-drilled branch well heat extraction device and its heat extraction method suitable for hot dry rock development. Both involve well operations such as running casing and cementing. Under high temperature conditions, the casing process is complex and difficult to operate. CN208966316U discloses a U-shaped horizontal well, which has a simple structure but a limited heat exchange area. Therefore, it is necessary to find an economical, feasible and efficient method for mining hot dry rock. Summary of the Invention

[0005] The purpose of this invention is to address the shortcomings of existing technologies by providing a method for exploiting dry hot rock using an open-hole serpentine horizontal well based on a closed-loop geothermal system. Drilling an open-hole horizontal well reduces the number of steps involved in running casing, enhancing feasibility and lowering extraction costs. The optimized horizontal well design increases the heat exchange area between the flowing medium and the reservoir, extends the residence time, and appropriately increases the flow resistance. This ensures uniform flow of the flowing medium, enabling sufficient heat exchange between the heat exchange medium and the reservoir, resulting in higher heat recovery efficiency.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: a method for mining dry hot rock using an open-hole serpentine horizontal well based on a closed-loop geothermal system, characterized by comprising the following steps, which are performed sequentially:

[0007] Step 1: Determine the target mining area;

[0008] Step 2: In the target mining area, drill a vertical well section from the surface into the hot dry rock reservoir. After drilling to the predetermined depth of the hot dry rock reservoir, drill the first inclination section of the injection well. The wellbore curvature is less than or equal to 20° / 30m. Stop drilling when the inclination angle approaches 90°.

[0009] Step 3: Drill the first horizontal section of the injection well with an inclination azimuth of 90° and a length greater than 300m. Then, drill the second directional drilling section of the injection well from the toe of the first horizontal section with a wellbore curvature of less than or equal to 20° / 30m. Stop directional drilling when the inclination azimuth approaches 0°.

[0010] Step 4: Drill the second horizontal section of the injection well with an inclination azimuth of 0° and a section length greater than 80m. Then, drill the third directional drilling section of the injection well from the toe of the second horizontal section with a wellbore curvature of less than or equal to 20° / 30m. Stop directional drilling when the inclination azimuth approaches -90°.

[0011] Step 5: Drill the third horizontal section of the injection well, with its wellbore direction opposite to that of the first horizontal section of the injection well, and a length greater than 300m. Then, drill the fourth directional drilling section of the injection well from the toe of the second horizontal section of the injection well, with a wellbore curvature less than or equal to 20° / 30m, until the directional drilling stops when the well inclination azimuth angle approaches 0°.

[0012] Step 6: Drill the fourth horizontal section of the injection well, with its wellbore direction being the same as that of the second horizontal section of the injection well, and its length being greater than or equal to 80m. Then drill the fifth directional drilling section of the injection well, with a wellbore curvature less than or equal to 20° / 30m, until the inclination angle approaches 90° and then stop directional drilling.

[0013] Step 7: Drill the fifth horizontal section of the injection well, with its wellbore direction being the same as that of the first horizontal section of the injection well, and its length being greater than 300m;

[0014] Step 8: Repeat steps 3 to 7 until the wellbore length of the injection well in the hot dry rock reservoir is greater than 1500m. Drill the last horizontal section of the injection well, which is greater than or equal to 80m in length. This completes the drilling of the injection well.

[0015] Step 9: Drill a vertical well from the surface as a production well, connecting it to the toe of the last horizontal section of the injection well to form a closed-loop structure system.

[0016] Step 10: Turn on the surface injection pump to inject a low-temperature heat transfer medium of 10°C or higher into the injection well. Extract the high-temperature heat transfer medium after sufficient heat exchange from the production well to complete the self-circulation of the heat transfer medium, thereby developing the dry hot rock resources.

[0017] Furthermore, in step one, the temperature of the hot dry rock reservoir in the target mining area is higher than 180℃ and the thickness is greater than 500m.

[0018] Furthermore, the caprock above the selected hot dry rock reservoir is an unstable formation, therefore casing and cementing are required. In step two, a vertical well section of the injection well is drilled from the top of the caprock in the target mining area down to the top of the hot dry rock reservoir. Casing and cementing are then carried out. After cementing is completed, drilling continues downwards until the drilling depth in the hot dry rock reservoir exceeds 15m, at which point it stops.

[0019] The wellbore diameters of both the injection and production wells are controlled within the range of 0.2m to 0.5m. Too small a diameter leads to difficulties in wellbore setup, high flow velocity of the heat-carrying medium within the well, short heat exchange time, and low heat extraction efficiency. Too large a diameter increases drilling costs, but the improvement in heat extraction effect is not significant.

[0020] The first, second, third, fourth, fifth, and last horizontal sections of the injection well are all located on the same horizontal plane. To ensure a large heat exchange area and the residence time of the heat transfer medium in the dry hot rock reservoir, and to ensure safe and stable drilling of the horizontal sections, the length of the horizontal sections with a well inclination azimuth of 90° or -90° is controlled within the range of 300m to 500m (i.e., the lengths of the first, third, and fifth horizontal sections of the injection well are all controlled within the range of 300m to 500m).

[0021] To ensure that heat exchange between wells does not interfere with each other and thus reduce heat extraction efficiency, the length of the second horizontal section of a water injection well with a well inclination azimuth of 0° is greater than or equal to 80m.

[0022] Preferably, the injection well has a borehole length of 2000m and a diameter of 0.2m, and low-temperature water at 60℃ is injected into the well at a water injection rate of 0.5kg / s. Over a 20-year operation period, the outlet water temperature can be maintained above 150℃, resulting in high heat extraction efficiency.

[0023] Furthermore, by utilizing ground heat exchange equipment and adopting the dual-working-medium power generation technology commonly used in hot dry rocks, the high-temperature heat transfer medium in this invention can complete geothermal power generation, and then continue to utilize the heat transfer medium (which still has a high temperature) for purposes such as heating, bathing, and aquaculture.

[0024] Through the above design scheme, the present invention can bring the following beneficial effects:

[0025] 1. Hot dry rock reservoirs are characterized by high temperature and hardness, resulting in high fracturing costs and insignificant fracturing effects. The open-hole serpentine horizontal well mining method based on a closed-loop geothermal system proposed in this invention not only avoids reservoir damage caused by fracturing fluid entering the reservoir, but also avoids high fracturing costs, resulting in high economic benefits. At the same time, it avoids fluid loss problems in the fracturing reservoir and geological disasters such as earthquakes induced by fracturing.

[0026] 2. Utilizing open-hole horizontal wells for geothermal extraction from hot dry rock offers several advantages. First, it reduces the need for casing and tubing, which are subjected to significant thermal stress and corrosion at high temperatures, leading to a substantial decrease in strength. The joint strength and sealing performance of the casing and tubing are also negatively impacted. Therefore, open-hole drilling leverages the stability characteristics of hot dry rock, improving the feasibility of extraction systems and reducing costs. Second, the heat transfer medium does not come into contact with the reservoir rock, ensuring the formation is not contaminated and preventing corrosion and scaling of the wellbore and surface facilities. This maintains the stability of the heat capacity, viscosity, and thermal conductivity of the heat transfer medium, making the extraction process more reliable and stable.

[0027] 3. Optimized horizontal well designs can increase the heat exchange area between the flowing medium and the reservoir, extend the residence time, and appropriately increase the flow resistance, ensuring uniform flow of the flowing medium and enabling the heat exchange medium to complete sufficient heat exchange with the reservoir, thereby achieving higher heat recovery efficiency. The closed-loop structure of serpentine horizontal wells with a spatial length greater than 1500m provides conditions for optimizing the performance of the heat transfer medium due to its encircling and enclosed spatial characteristics. Attached Figure Description

[0028] The accompanying drawings are provided to further illustrate the invention and form part of this application. The illustrative embodiments and descriptions of the invention are used to understand the invention and do not constitute an undue limitation thereof. In the drawings:

[0029] Figure 1 This is a schematic diagram of a well configuration for exploiting dry hot rock using a naked-eye serpentine horizontal well based on a closed-loop geothermal system.

[0030] The markings in the diagram are as follows: 1-Vertical section of the injection well, 2-First directional section of the injection well, 3-First horizontal section of the injection well, 4-Second directional section of the injection well, 5-Second horizontal section of the injection well, 6-Third directional section of the injection well, 7-Third horizontal section of the injection well, 8-Fourth directional section of the injection well, 9-Fourth horizontal section of the injection well, 10-Fifth directional section of the injection well, 11-Fifth horizontal section of the injection well, 12-Last horizontal section of the injection well, 13-Production well, 14-Caprock, 15-Hot dry rock reservoir. Detailed Implementation

[0031] The following is combined with Figure 1 The present invention will be described in further detail below. However, this should not be construed as limiting the scope of protection of the present invention to the following embodiments; all technologies implemented based on the content of the present invention fall within the scope of the present invention. To avoid obscuring the essence of the present invention, well-known methods, processes, and procedures have not been described in detail.

[0032] A method for exploiting dry hot rock using an open-hole serpentine horizontal well based on a closed-loop geothermal system, comprising the following specific steps:

[0033] Step 1: Select a hot dry rock mass with a temperature higher than 180℃ and a thickness greater than 500m as the target hot dry rock reservoir 15. Drilling is carried out using a diamond-impregnated drill bit larger than 8 inches and downhole power drilling tools. To simplify the process, open-hole drilling is performed in the hot dry rock reservoir 15, without considering casing running and cementing processes. The hot dry rock reservoir 15 directly exchanges heat with the flowing heat transfer medium, eliminating problems such as thermal resistance, overheating, scaling, and corrosion caused by heat transfer. The entire drilling process uses high-temperature resistant drilling fluid circulation drilling.

[0034] Step 2: Drill vertically in the target mining area to the top of the hot dry rock reservoir 15, and run casing and cement the well in the caprock 14; after passing through the caprock 14 and encountering the hot dry rock reservoir 15, continue drilling downwards to the vertical section 1 of the injection well, and drill to a depth greater than 15m.

[0035] Step 3: Use the drilling tool to drill the first section 2 of the injection well, and control the wellbore curvature to be less than or equal to 20° / 30m. The purpose is to reduce the difficulty of drilling. Drilling will stop when the well inclination angle is close to 90°.

[0036] Step 4: Using a geological directional drilling tool, inject the first horizontal section 3 of the well into the hot dry rock reservoir 15, with a well inclination azimuth of 90° and a section length of more than 300m;

[0037] Step 5: Drill the second directional drilling section 4 of the injection well at the toe of the first horizontal section 3 of the injection well. The wellbore curvature is less than or equal to 20° / 30m. Drill until the well inclination azimuth is close to 0° and then stop the directional drilling. The purpose of the wellbore curvature being less than or equal to 20° / 30m is to reduce the difficulty of directional drilling.

[0038] Step 6: Drill the second horizontal section 5 of the injection well, with an inclination azimuth of 0° and a section length greater than 80m;

[0039] Step 7: Drill the third build-up section 6 of the injection well, with a wellbore curvature of less than or equal to 20° / 30m, until the well inclination azimuth angle approaches -90° and then stop the build-up; the purpose of a wellbore curvature of less than or equal to 20° / 30m is to reduce the difficulty of the build-up.

[0040] Step 8: Drill the third horizontal section 7 of the injection well, with the wellbore direction opposite to the first horizontal section 3 of the injection well, and a length greater than 300m;

[0041] Step 9: Drill the fourth build-up section 8 of the injection well at the toe of the third horizontal section 7 of the injection well, with a wellbore curvature of less than or equal to 20° / 30m, until the well inclination azimuth angle approaches 0° and then stop the build-up.

[0042] Step 10: Continue drilling the fourth horizontal section 9 of the injection well, whose wellbore direction is the same as that of the second horizontal section 5 of the injection well, and the section length is greater than or equal to 80m;

[0043] Step 11: Drill the fifth build-up section 10 of the injection well, with a wellbore curvature of less than or equal to 20° / 30m, until the well inclination azimuth angle approaches 90° and then stop the build-up; the purpose of having a wellbore curvature of less than or equal to 20° / 30m is to reduce the difficulty of the build-up.

[0044] Step 12: Drill the fifth horizontal section 11 of the injection well, with the wellbore direction being the same as the first horizontal section 3 of the injection well, and the length being greater than 300m;

[0045] Step 13: Repeat steps 3 to 12 until the well length in the hot dry rock reservoir 15 is greater than 1500m;

[0046] Step 14: Drill the last horizontal section 12 of the injection well, with a section length of ≥80m;

[0047] Step 15: Drill a vertical well from the surface as production well 13, connecting it with the toe of the last horizontal section 12 of the serpentine injection well to form a closed-loop structure system, and cement the production well 13 located in the caprock 14.

[0048] Step 16: Turn on the surface injection pump and inject a low-temperature heat transfer medium of 10°C or higher into the injection well at an appropriate injection rate. Extract the high-temperature heat transfer medium after sufficient heat exchange from production well 13 to complete the self-circulation flow of the heat transfer medium, thereby developing the dry hot rock resources.

[0049] Step 17: Using ground heat exchange equipment, adopt the dual-working-medium power generation technology commonly used in hot dry rock to enable the high-temperature heat transfer medium to complete geothermal power generation, and then continue to use the heat transfer medium (which still has a high temperature) for purposes such as heating, bathing, and aquaculture.

[0050] In this invention, the heat transfer medium can be water, or supercritical carbon dioxide, nitrogen, etc., which have better heat transfer performance than water. Other fluids with excellent heat transfer properties, such as water mixed with ethanol or ethylene glycol, can also be selected. That is, based on a thorough study of the thermal properties of water and the heat exchange in wellbore flow, the heat transfer medium formulation suitable for this geothermal extraction method is optimized to improve corrosion resistance and heat exchange capacity. Different heat transfer media require different treatment methods. For supercritical carbon dioxide, a condensation mechanism is used to cool the carbon dioxide, and then heat convection and temperature exchange with the high-temperature dry hot rock reservoir 15 causes the carbon dioxide to change from liquid to gas. This gas can then be used for power generation via a steam turbine system. For mixed fluids, because of their sensitive phase change, the phase of the heat transfer medium can be changed to gaseous state by slightly altering the internal pressure of the system, thereby rapidly and efficiently releasing the carried heat energy.

[0051] Drilling stability is crucial in the exploitation of hot dry rock resources, necessitating the use of high-temperature resistant drilling fluids. Hot dry rock reservoirs often exceed 180°C; prolonged exposure to this high temperature environment severely degrades the performance of drilling fluids, impacting borehole stability and rock-carrying capacity. Commonly used high-temperature resistant drilling fluids include SMC, SMP, and sulfonated drilling fluids, which possess strong salt resistance, good compressibility, and excellent anti-collapse and anti-sticking properties.

[0052] The above is a specific embodiment of the present invention. The specific embodiments of the present invention are not limited to this. For those skilled in the art, other similar changes can be made without departing from the concept of the present invention, and these should all be considered within the protection scope of the present invention.

Claims

1. A method for exploiting dry hot rock using an open-hole serpentine horizontal well based on a closed-loop geothermal system, characterized in that, The steps are as follows, and they are performed in sequence: Step 1: Determine the target mining area; Step 2: In the target mining area, drill a vertical well section (1) of the injection well from the surface to the dry hot rock reservoir (15). After drilling to the predetermined depth of the dry hot rock reservoir (15), drill the first directional section (2) of the injection well. The wellbore curvature is less than or equal to 20° / 30m. Stop directional drilling when the well inclination angle is close to 90°. Step 3: Drill the first horizontal section (3) of the injection well with an inclination azimuth of 90° and a section length greater than 300m. Then, drill the second directional section (4) of the injection well from the toe of the first horizontal section (3) with a wellbore curvature of less than or equal to 20° / 30m. Stop directional drilling when the inclination azimuth approaches 0°. Step 4: Drill the second horizontal section (5) of the injection well with an inclination azimuth of 0° and a section length greater than 80m. Then, drill the third directional drilling section (6) of the injection well from the toe of the second horizontal section (5) of the injection well with a wellbore curvature of less than or equal to 20° / 30m. Stop directional drilling when the inclination azimuth approaches negative 90°. Step 5: Drill the third horizontal section (7) of the injection well, with its wellbore direction opposite to that of the first horizontal section (3) of the injection well, and a length greater than 300m. Then, drill the fourth directional drilling section (8) of the injection well from the toe of the third horizontal section (7), with a wellbore curvature less than or equal to 20° / 30m, until the directional drilling stops when the well inclination azimuth angle approaches 0°. Step 6: Drill the fourth horizontal section (9) of the injection well, with its wellbore direction being the same as that of the second horizontal section (5) of the injection well, and its length being greater than or equal to 80m. Then drill the fifth directional drilling section (10) of the injection well, with a wellbore curvature less than or equal to 20° / 30m, until the inclination angle approaches 90° and then stop the directional drilling. Step 7: Drill the fifth horizontal section (11) of the injection well, and its wellbore direction is the same as that of the first horizontal section (3) of the injection well, with a length greater than 300m; Step 8: Repeat steps 3 to 7 until the wellbore length of the injection well in the hot dry rock reservoir (15) is greater than 1500m. Drill the last horizontal section (12) of the injection well, which is greater than or equal to 80m in length. This completes the drilling of the injection well. Step 9: Drill a vertical well from the surface as a production well (13), and connect it with the toe of the last horizontal section (12) of the injection well to form a closed-loop structure system; Step 10: Turn on the surface injection pump and inject a low-temperature heat transfer medium of 10°C or higher into the injection well. Extract the high-temperature heat transfer medium after sufficient heat exchange from the production well (13), thus completing the self-circulation flow of the heat transfer medium and developing dry hot rock resources.

2. The method for mining dry hot rock using an open-hole serpentine horizontal well based on a closed-loop geothermal system according to claim 1, characterized in that: In step one, the temperature of the dry hot rock reservoir (15) in the target mining area is higher than 180℃ and the thickness is greater than 500m.

3. The method for mining dry hot rock using an open-hole serpentine horizontal well based on a closed-loop geothermal system according to claim 1, characterized in that: In step two, the vertical well section (1) of the injection well is drilled from the top of the caprock (14) of the target mining area down to the top of the hot dry rock reservoir (15), and the casing is run and cemented. After cementing is completed, drilling continues downward until the drilling depth of the hot dry rock reservoir (15) is greater than 15m.

4. The method for mining dry hot rock using an open-hole serpentine horizontal well based on a closed-loop geothermal system according to claim 1, characterized in that: The wellbore diameters of both the injection well and the production well (13) are controlled within the range of 0.2m to 0.5m.

5. The method for mining dry hot rock using an open-hole serpentine horizontal well based on a closed-loop geothermal system according to claim 1, characterized in that: In step ten, cold water at 60°C is injected into the injection well at a water injection rate of 0.5 kg / s.

6. The method for mining dry hot rock using an open-hole serpentine horizontal well based on a closed-loop geothermal system according to claim 1, characterized in that: The first horizontal well section (3), the second horizontal well section (5), the third horizontal well section (7), the fourth horizontal well section (9), the fifth horizontal well section (11), and the last horizontal well section (12) of the injection well are all on the same horizontal plane.

7. The method for mining dry hot rock using an open-hole serpentine horizontal well based on a closed-loop geothermal system according to claim 1, characterized in that: The lengths of the first horizontal well section (3), the third horizontal well section (7), and the fifth horizontal well section (11) of the injection well are all controlled within the range of 300m to 500m.

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