Underground enhanced heat exchange method for closed heat removal system

Abstract

The invention discloses an underground enhanced heat exchange method for a closed heat removal system, and belongs to the field of geothermal energy development and utilization. A vertical well is drilled according to the depth and thickness of a heat reservoir, a casing pipe is put down and well cementation is carried out, a central heat preservation pipe is arranged in the casing pipe, and an annular space between the casing pipe and the central heat preservation pipe forms a vertical well annulus; casing lateral windowing is carried out on the vertical well section corresponding to the thermal reservoir, a plurality of branch well holes are drilled, or volume fracturing is carried out to form micro cracks; filling a high-thermal-conductivity material in the multilateral well hole / microcrack, transforming the thermal reservoir and supporting the transformed space to form a high-thermal-conductivity area in the thermal reservoir, and plugging the lateral multilateral well hole to complete the transformation of the thermal reservoir; a heat exchange working medium is injected through a high-pressure pump through a vertical well annulus, is pumped out of the central heat preservation pipe after being subjected to closed type sufficient heat exchange with the transformed heat storage layer, and geothermal energy utilization is achieved through the heat exchanger. The defects of a traditional closed heat removal system are remarkably overcome, the heat removal temperature is increased, the heat removal service life is prolonged, and development and utilization of medium-deep geothermal energy are accelerated.

Description

Technical Field

[0001] The present invention relates to the technical field of geothermal energy development and utilization, and more particularly to an underground enhanced heat exchange method for a closed heat extraction system. Background Art

[0002] Geothermal energy is widely distributed, has abundant reserves, is safe, high-quality and has a stable energy supply. It is an important clean and renewable energy under the background of the "dual carbon target" and energy revolution. Closed heat extraction technology is an important means of geothermal energy development and utilization. The heat extraction cycle working fluid used does not exchange with the formation during the heat extraction process, thus avoiding environmental pollution, equipment corrosion and scaling problems that may be caused by open systems, that is, "extracting heat without taking water", which is particularly suitable for the development of geothermal reservoirs that are water-free, difficult to recharge or extremely difficult to recharge.

[0003] In geothermal closed development, a coaxial casing system is often used to achieve heat exchange. It consists of a vertical well drilled into the geothermal formation and a central insulation pipe in a coaxial position in the wellbore. The circulating fluid is injected into the annulus from the ground through a high-pressure pump, extracts heat from the well wall through thermal convection and heat conduction, and then returns to the ground through the central insulation pipe to flow through the heat exchanger for utilization. Since the closed system and the outside world (geothermal formation) only rely on heat conduction to achieve heat exchange, the system's heat extraction capacity and efficiency are limited.

[0004] Therefore, how to propose a closed heat extraction system underground enhanced heat exchange method to improve the heat extraction efficiency is a problem that technical personnel in this field urgently need to solve. Summary of the invention

[0005] In view of this, the present invention provides a method for enhancing downhole heat exchange in a closed heat extraction system to improve the thermal conductivity of the reservoir, thereby enhancing heat exchange and improving the heat extraction performance of the closed heat extraction system.

[0006] In order to achieve the above object, the present invention adopts the following technical solution:

[0007] A closed heat extraction system underground enhanced heat exchange method, comprising:

[0008] Drill a vertical well according to the depth and thickness of the thermal reservoir, run casing and cement the well, set a central insulation pipe inside the casing, and the annular space between the casing and the central insulation pipe constitutes the vertical well annulus;

[0009] Perform lateral casing windowing in the vertical well section corresponding to the thermal reservoir, drill several branch wellbores, or perform volume fracturing to form micro fractures;

[0010] Fill the branch wellbore / micro-cracks with high thermal conductivity materials to transform the thermal reservoir and support the transformed space, forming a high thermal conductivity area in the thermal reservoir, and plugging the lateral branch wellbore to complete the thermal reservoir transformation;

[0011] The hot heat exchange medium is injected through the annulus of the vertical well by a high-pressure pump, and is extracted from the central insulation pipe after sufficient closed heat exchange with the transformed heat reservoir, and the geothermal energy is utilized by using a heat exchanger.

[0012] Preferably, several branch wellbores are drilled, including:

[0013] Determine the number of branch wells and the length of each well according to the thickness of the geothermal reservoir and geological characteristics;

[0014] The geological conditions of the thermal reservoir are determined. If the geological conditions of the thermal reservoir are determined to be good, the branch wellbores are evenly distributed in the vertical direction; if the geological conditions of the thermal reservoir are determined to be special, the wellbore density is reasonably determined according to the reservoir characteristics.

[0015] Preferably, volume fracturing to form microcracks includes:

[0016] The hydraulic jet is used to perform lateral fracturing to form one or more main fractures, the density and length of which are determined according to the reservoir characteristics;

[0017] Volume fracturing connects natural fractures and rock stratification, forming secondary fractures laterally to the main fractures. The secondary fractures continue to branch and extend to form secondary secondary fractures, ultimately forming an ideal fracture network.

[0018] Preferably, for conductive heat storage, high thermal conductivity materials are filled in branch wellbores and microfractures.

[0019] Preferably, for convection-type heat storage, there is no need to fill branch wellbores and microfractures with high thermal conductivity materials.

[0020] Preferably, the high thermal conductivity material includes a high thermal conductivity material containing corundum or having a proppant function.

[0021] Preferably, an insulation section is added between the central pipe and the annulus of the vertical well.

[0022] Through the above technical scheme, it can be known that compared with the prior art, the present invention discloses a closed heat extraction system underground enhanced heat exchange method, according to the depth and thickness of the heat reservoir, a vertical well is drilled, casing is lowered and cemented, a central insulation pipe is arranged inside the casing, and the annular space between the casing and the central insulation pipe constitutes the vertical well annulus; the casing is laterally opened in the vertical well section corresponding to the heat reservoir, and several branch wellbores are drilled, or microcracks are formed by volume fracturing; high thermal conductivity materials are filled in the branch wellbores / microcracks, the heat reservoir is transformed and the transformed space is supported to form a high thermal conductivity area in the heat reservoir, and the lateral branch wellbores are blocked to complete the heat reservoir transformation; the warm heat exchange medium is injected through the vertical well annulus via a high-pressure pump, and is fully exchanged with the transformed heat reservoir in a closed manner and then extracted from the central insulation pipe, and the heat exchanger is used to realize the utilization of geothermal energy. The present invention significantly improves the shortcomings of low efficiency and small heat extraction of the traditional closed heat extraction system, increases the heat extraction temperature, prolongs the heat extraction life, and accelerates the development and utilization of medium and deep geothermal energy. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying creative work.

[0024] Figure 1 A schematic diagram of a filled branch wellbore provided by the present invention;

[0025] Figure 2 Schematic diagram of filling microcracks provided by the present invention. DETAILED DESCRIPTION

[0026] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0027] The embodiment of the present invention discloses a closed heat extraction system underground enhanced heat exchange method, comprising:

[0028] S1. Drill a vertical well according to the depth and thickness of the thermal reservoir, run casing and cement the well, set a central insulation pipe inside the casing, and the annular space between the casing and the central insulation pipe constitutes the vertical well annulus. Add an insulation section between the central pipe and the vertical well annulus to prevent the high-temperature fluid from being cooled by the low-temperature fluid in the annulus during the production process.

[0029] S2. Casing lateral opening is performed in the vertical well section corresponding to the thermal reservoir, and several branch wellbores are drilled (refer to Figure 1 ), or volume fracturing to form microcracks (reference Figure 2 ).

[0030] Specifically, the vertical well section should be drilled to the bottom of the thermal reservoir as much as possible to obtain the maximum amount of heat energy. The more horizontal branch wells there are, the larger the artificial transformation volume will be, and the better the heat conduction effect will be. Therefore, the number of horizontal branch wells and the length of each well should be reasonably determined according to the thickness of the geothermal reservoir and the geological characteristics.

[0031] First, the thickness of the geothermal reservoir needs to be understood through geological exploration. Generally speaking, the thicker the reservoir, the more branch wells can be considered. In thicker reservoirs, intervals of 2 to 5 meters are usually used. The thermal conductivity and permeability of different rocks determine the conduction and flow rate of thermal energy. The number of wells can be appropriately increased in reservoirs with good thermal conductivity, rich geothermal resources, and relatively simple geological structures. It depends on the scale and design requirements of the project.

[0032] The length of the branch wellbore is usually between tens of meters and hundreds of meters, depending on the thickness of the geothermal reservoir, geological structure, geothermal resource occurrence, etc. In many projects, the length of the branch wellbore is between 100 meters and 300 meters to ensure sufficient heat exchange.

[0033] For thermal reservoirs with good geological conditions, the branch wellbores can be evenly distributed in the vertical direction. For thermal reservoirs with special geological conditions, the wellbore density should be reasonably determined according to the reservoir characteristics.

[0034] Whether the geological conditions are good or not mainly depends on whether there are special geological structures such as faults, the occurrence of geothermal resources, etc. In areas with complex geological conditions and relatively scarce geothermal resources, drilling is difficult or not economical, and reservoir transformation may cause geological disasters, and the geological conditions of the thermal reservoir are considered to be poor. At this time, due to the complex geological conditions in this part of the area, it is not suitable to drill for reservoir transformation, and the branch wellbore can be adjusted to avoid it, that is, a small wellbore density is more appropriate.

[0035] For thermal reservoirs with special geological conditions, special geological structures should be avoided, and lateral branch wells should not be drilled in areas with special geological conditions. A drilling plan with a higher wellbore density should be adopted. The branch well spacing in other areas still meets the commonly used basic standards.

[0036] Furthermore, for micro-fractures, hydraulic jet fracturing can be first performed laterally (through lateral branch wellbores, hydraulic jet fracturing and other fracturing behaviors) to form one or more main fractures, and then volume fracturing can be used to connect natural fractures and rock stratification to form secondary fractures laterally to the main fractures. The secondary fractures then continue to branch and extend to form secondary secondary fractures, and so on, ultimately forming an ideal fracture network that can be used to enhance heat extraction. Similar to branch wellbores, the density and length of the main fractures should be reasonably determined based on reservoir characteristics.

[0037] In areas with complex geological conditions and not rich geothermal resources, fracturing is not carried out, that is, the density of main fractures in these areas is small. The length of the main fracture is usually affected by many factors, including formation characteristics, the nature of the injected fluid, the injection pressure, geological structure, etc. Generally, the length of the main fracture may be between tens of meters and hundreds of meters. Rock strength, folds, faults, etc. may affect the formation and direction of fractures.

[0038] Since the fractures mainly provide space for accommodating high thermal conductivity materials, and the vertical well section uses coaxial casing for closed heat extraction, there is no direct fluid exchange with the heat reservoir, and the formation of fractures has no mandatory requirements and is relatively flexible.

[0039] S3. Fill the branch wellbore / micro-cracks with high thermal conductivity materials to transform the thermal reservoir and support the transformed space, forming a high thermal conductivity area in the thermal reservoir, and plugging the lateral branch wellbore to complete the thermal reservoir transformation.

[0040] In order to avoid fluid exchange between the reservoir and the wellbore through the drilled wellbore in a closed system, it is necessary to block the lateral branch wellbores so that the fluid circulates only in the wellbore to extract heat, that is, to extract heat but not water.

[0041] After the branch wellbore / micro-crack is formed, it is filled with corundum or other high thermal conductivity materials that can act as proppants to improve the thermal conductivity of the reservoir while supporting the fracture to maintain its shape and avoid safety problems. Finally, the lateral branch wellbore is plugged to complete the reservoir transformation, and the coaxial casing closed circulation of the vertical well section is used to extract heat.

[0042] The modified high thermal conductivity area can quickly replenish the low-temperature area after heat exchange, continuously and effectively provide thermal energy to the closed heat extraction system, thereby improving the overall heat extraction efficiency.

[0043] S4. The warm heat exchange medium is injected through the annulus of the vertical well by a high-pressure pump, and is fully exchanged with the transformed thermal reservoir in a closed manner before being extracted from the central insulation pipe to realize geothermal energy utilization using a heat exchanger.

[0044] For conductive heat storage, the reservoir with high thermal conductivity materials filled in branch wellbores and micro-cracks has better thermal conductivity, so that heat energy can be replenished to the low-temperature area generated by heat exchange in time, improving the performance and life of the closed heat extraction system.

[0045] For convection heat storage, branch wellbores and micro-fractures do not need to be filled with high thermal conductivity materials. At this time, the reservoir has an advantageous seepage area, with stronger conductivity, and the heat flow gathers faster near the wellbore, thereby improving the heat extraction performance of the closed heat extraction system.

[0046] The closed heat extraction system underground enhanced heat exchange method proposed by the present invention is not limited to the vertical well type. According to the idea of ​​the present invention, other technologies can also be used to transform the reservoir.

[0047] In this specification, each embodiment is described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the embodiments can be referred to each other. For the device disclosed in the embodiment, since it corresponds to the method disclosed in the embodiment, the description is relatively simple, and the relevant parts can be referred to the method part.

[0048] The above description of the disclosed embodiments enables one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A closed heat extraction system underground enhanced heat exchange method, characterized in that: include: Drill a vertical well according to the depth and thickness of the thermal reservoir, run casing and cement the well, set a central insulation pipe inside the casing, and the annular space between the casing and the central insulation pipe constitutes the vertical well annulus; Perform lateral casing windowing in the vertical well section corresponding to the thermal reservoir, drill several branch wellbores, or perform volume fracturing to form micro fractures; Fill the branch wellbore / micro-cracks with high thermal conductivity materials to transform the thermal reservoir and support the transformed space, forming a high thermal conductivity area in the thermal reservoir, and plugging the lateral branch wellbore to complete the thermal reservoir transformation; The hot heat exchange medium is injected through the annulus of the vertical well by a high-pressure pump, and is extracted from the central insulation pipe after sufficient closed heat exchange with the transformed heat reservoir, and the geothermal energy is utilized by using a heat exchanger.

2. The closed heat extraction system downhole enhanced heat exchange method according to claim 1, characterized in that: Several lateral wellbores were drilled, including: Determine the number of branch wells and the length of each well according to the thickness of the geothermal reservoir and geological characteristics; The geological conditions of the thermal reservoir are determined. If the geological conditions of the thermal reservoir are determined to be good, the branch wellbores are evenly distributed in the vertical direction; if the geological conditions of the thermal reservoir are determined to be special, the wellbore density is reasonably determined according to the reservoir characteristics.

3. The closed heat extraction system downhole enhanced heat exchange method according to claim 1, characterized in that: Volumetric fracturing forms microcracks, including: The hydraulic jet is used to perform lateral fracturing to form one or more main fractures, the density and length of which are determined according to the reservoir characteristics; Volume fracturing connects natural fractures and rock stratification, forming secondary fractures laterally to the main fractures. The secondary fractures continue to branch and extend to form secondary secondary fractures, ultimately forming an ideal fracture network.

4. The closed heat extraction system downhole enhanced heat exchange method according to claim 1, characterized in that: For conductive heat storage, high thermal conductivity materials are filled in branch wellbores and microfractures.

5. The closed heat extraction system downhole enhanced heat exchange method according to claim 1, characterized in that: For convection-type heat storage, there is no need to fill branch wellbores and microfractures with high thermal conductivity materials.

6. The closed heat extraction system downhole enhanced heat exchange method according to claim 1, characterized in that: The high thermal conductivity material includes a high thermal conductivity material containing corundum or having a proppant function.

7. The closed heat extraction system downhole enhanced heat exchange method according to claim 1, characterized in that: Add an insulation section between the center pipe and the annulus of the vertical well.

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