Geothermal single well fracturing method

Abstract

The present invention provides a geothermal single well fracturing method, which includes the following steps: a main well drilling step, drilling the main well from the ground to the underground, and fracturing the working section of the main well, while monitoring the situation during fracturing; the main well includes: a connected straight well section and a working section; the straight well section is connected to the ground; a branch well drilling step, according to the monitoring result, drilling a branch well at the straight well section of the main well at a preset angle offset from the working section of the main well, and hydraulically communicating the branch well with the working section of the main well; a delivery pipe lowering step, lowering a delivery pipe into the straight well section of the main well or the branch well, so that the fluid is output by the delivery pipe after passing through the main well and the branch well. The present invention allows the fluid to flow between the branch well and the main well, increasing the heat exchange area, and the branch well deviates from the preset angle from the working section of the main well, so that the path of the fluid flowing between the main well and the branch well is extended, increasing the heat exchange area and heat exchange time of the fluid, and improving the heat exchange efficiency.

Description

Technical Field

[0001] The present invention relates to the technical field of thermal energy development, and in particular to a geothermal single well fracturing method. Background Art

[0002] Traditional single-well heat exchange technology typically involves completing the well with casing and sealing the bottom. An insulated central string is then lowered into the well. Cold fluid is injected into the annular space between the central string and the casing, where it is heated by the formation and then discharged through the central string. While this method is relatively inexpensive, it suffers from the drawbacks of rapid temperature drop and low heat exchange efficiency because heat exchange occurs only within the casing. The fluid's exposure to the high-temperature deep formation is short, and the heat exchange contact area is limited.

[0003] To avoid the shortcomings of traditional single-well heat exchange technology, perforations are typically performed in the upper and lower portions of the formation. Two sets of separators are then used to separate the annulus between the casing and the center string, corresponding to the upper and lower perforated sections. This seals the space between the upper and lower perforated sections in the annulus. This forces the cold fluid to be injected into the formation through the upper perforated section to heat it, then flow through the lower perforated section into the wellbore and out through the center string. However, due to the flow resistance of the porous formation, the fluid flows only within a short distance outside the casing, resulting in a small overall heat exchange area and low heat exchange efficiency. Summary of the Invention

[0004] In view of this, the present invention proposes a geothermal single-well fracturing method, which aims to solve the problem of low heat exchange efficiency caused by small heat exchange area in existing single-well heat exchange technology.

[0005] The present invention proposes a geothermal single-well fracturing method, which includes the following steps: a main well drilling step, in which the main well is drilled from the ground to the underground, and the working section of the main well is fracturing, while monitoring the situation during fracturing; wherein the main well includes: a connected vertical well section and a working section; the vertical well section is connected to the ground; a branch well drilling step, in which, according to the monitoring results, a branch well is drilled at the vertical well section of the main well at a preset angle offset from the working section of the main well, and the branch well is hydraulically connected to the working section of the main well; a delivery pipe lowering step, in which a delivery pipe is lowered into the vertical well section of the main well or the branch well, so that the fluid is output from the delivery pipe after passing through the main well and the branch well.

[0006] Furthermore, in the above-mentioned geothermal single-well fracturing method, in the step of drilling the main well, the direction of the main well is consistent with the direction of the minimum principal stress of the formation or deviates from the direction of the minimum principal stress of the formation by a preset angle.

[0007] Furthermore, in the above-mentioned geothermal single-well fracturing method, in the step of drilling the main well, when the main well is completed in full open hole or partially open hole, a general fracturing method is used to fracture the working section of the main well; when the main well is completed in full casing and cementing, a bridge plug and perforation method is used to perform segmented fracturing on the working section of the main well.

[0008] Furthermore, in the above-mentioned geothermal single-well fracturing method, in the step of drilling the main well, the main well is a horizontal well or a inclined well.

[0009] Furthermore, in the above-mentioned geothermal single-well fracturing method, in the step of drilling a branch well, the branch well is drilled toward the cracks generated when the working section of the main well is fracturing, so that the cracks of the branch well are connected with the cracks of the main well to achieve hydraulic communication; or, the branch well is drilled toward the cracks generated when the working section of the main well is fracturing, and the part of the branch well corresponding to the cracks of the main well is fractured, so that the cracks of the branch well are connected with the cracks of the main well to achieve hydraulic communication.

[0010] Furthermore, in the above-mentioned geothermal single-well fracturing method, in the step of drilling branch wells, the preset angle and distance between the branch well and the working section of the main well, as well as the distance between the branch well and the straight well section of the main well during hydraulic communication, all make the flow capacity of the fluid in each fracture match during hydraulic communication.

[0011] Furthermore, in the above-mentioned geothermal single-well fracturing method, in the step of drilling branch wells, there are at least two branch wells, and the starting point of each branch well is placed above the working section of the main well, and there is a preset angle between the working section of the main well and each branch well, and there is a preset angle between each branch well.

[0012] Furthermore, in the above-mentioned geothermal single-well fracturing method, in the step of drilling a branch well, the branch well is a horizontal well or a inclined well.

[0013] Furthermore, in the above-mentioned geothermal single-well fracturing method, in the step of lowering the delivery pipe, the delivery pipe is lowered into the vertical well section of the main well, the end of the delivery pipe is placed below the position of the branch well corresponding to the vertical well section, and the gap between the outer wall of the end of the delivery pipe and the well wall of the main well is sealed; or, the delivery pipe is lowered into the vertical well section of the main well, the delivery pipe extends into the branch well, and the gap between the outer wall of the delivery pipe and the well wall of the branch well is sealed.

[0014] Furthermore, in the above-mentioned geothermal single-well fracturing method, in the step of lowering the delivery pipe, when there are at least two branch wells, the delivery pipe is lowered into the vertical well section of the main well, and the end of the delivery pipe is placed below the position of the vertical well section corresponding to the branch well at the working section closest to the main well, and the gap between the outer wall of the end of the delivery pipe and the well wall of the main well is sealed.

[0015] In the present invention, by hydraulically connecting the branch wells drilled from the straight well section of the main well with the working section of the main well, the branch wells are connected to the working sections of the main well, so that the fluid can flow between the branch wells and the main well, which greatly increases the heat exchange area. In addition, the deviation between the working sections of the branch wells and the main well from the preset angle can extend the path of the fluid flowing between the main well and the branch wells, increase the heat exchange area of ​​the fluid, extend the heat exchange time, effectively increase the temperature of the fluid after heat exchange, and thus improve the heat exchange efficiency, solving the problem of low heat exchange efficiency caused by the small heat exchange area of ​​the existing single-well heat exchange technology. In addition, only the main well is connected to the ground, and the branch wells are drilled from the straight well section of the main well, which realizes that one wellhead can circulate heat for the main well and the branch wells at the same time, improves the heat exchange efficiency, greatly reduces the drilling cost and the area of ​​use of the well site, and thus reduces the cost of geothermal energy utilization. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Various other advantages and benefits will become apparent to those skilled in the art upon reading the detailed description of the preferred embodiment below. The accompanying drawings are for illustration purposes only and are not to be considered as limiting the present invention. The same reference symbols are used throughout the drawings to represent the same components. In the drawings:

[0017] Figure 1 A flow chart of a geothermal single well fracturing method provided by an embodiment of the present invention;

[0018] Figure 2 A schematic diagram of the arrangement of the main well and branch wells in a geothermal single-well fracturing method according to an embodiment of the present invention;

[0019] Figure 3 A schematic top view of the arrangement of the main well and branch wells in the geothermal single-well fracturing method provided in an embodiment of the present invention;

[0020] Figure 4 A schematic diagram of a structure for lowering a delivery pipe for a geothermal single-well fracturing method provided by an embodiment of the present invention;

[0021] Figure 5 Another structural schematic diagram of the lowering of a delivery pipe in the geothermal single-well fracturing method provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0022] The exemplary embodiments of the present disclosure will be described in more detail below with reference to the accompanying drawings. Although exemplary embodiments of the present disclosure are shown in the accompanying drawings, it should be understood that the present disclosure can be implemented in various forms and should not be limited by the embodiments set forth herein. On the contrary, these embodiments are provided to enable a more thorough understanding of the present disclosure and to fully convey the scope of the present disclosure to those skilled in the art. It should be noted that, unless there is a conflict, the embodiments of the present disclosure and the features described in the embodiments can be combined with each other. The present invention will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0023] See also Figure 1 , Figure 1 Flowchart of a geothermal single well fracturing method according to an embodiment of the present invention. As shown in the figure, the geothermal single well fracturing method includes the following steps:

[0024] In step S1 of drilling a main well, the main well is drilled from the ground to the underground, and the working section of the main well is fractured while monitoring the fracturing situation. The main well includes: a vertical well section and a working section connected to each other; the vertical well section is connected to the ground.

[0025] Specifically, see Figure 2 , one end of the vertical well section 11 ( Figure 2 The upper end shown in FIG) is connected to the ground, and the other end of the vertical well section 11 ( Figure 2 The lower end shown in FIG. 1 is connected to the working section 12, and the working section 12 is drilled underground.

[0026] The main well is either horizontal or inclined. When the main well is horizontal, the vertical section is the vertical section of the horizontal well, and the working section is the deflection section and horizontal section of the horizontal well. When the main well is inclined, the vertical section is the vertical section of the inclined well, and the working section is the inclined section of the inclined well.

[0027] The strike of the main well is aligned with the direction of the minimum principal stress of the formation, or the strike of the main well deviates from the direction of the minimum principal stress of the formation by a preset angle. In specific implementations, this preset angle can be determined based on actual conditions and is not limited in this embodiment. Preferably, the angle of deviation between the strike of the main well and the direction of the minimum principal stress of the formation does not exceed plus or minus 30°.

[0028] In specific implementation, the main well can be completed in whole or in part with open hole, or can be completed with all casing and cementing, and this embodiment does not impose any limitation on this.

[0029] The working section of the main well is hydraulically fractured to create fractures. When the main well is completed entirely or partially open-hole, the working section of the main well is fractured using a generalized fracturing method, creating multiple fractures in the formation corresponding to the working section of the main well. Generalized fracturing involves injecting fracturing fluid directly into the entire main well 1 without using any inter-layer isolation tools. When the pressure exceeds the formation's fracture pressure, fractures will initiate in the formation. When the fracturing flow rate reaches a certain level, such as 3-6 m³ / min, multiple fractures can be generated in the formation.

[0030] When the main well is completed with full casing cementing, a bridge-plug-perforation method is used to perform staged fracturing on the working section of the main well, creating multiple fractures in the formation corresponding to the working section of the main well. Bridge-plug-perforation fracturing is a multi-stage fracturing technology specifically designed for casing cementing completions. Generally, starting from the bottom of the well, perforating and fracturing each section, then inserting a bridge plug to seal the first section of the fracture while perforating the second section, and then fracturing again, and so on, to create multiple fractures in the formation.

[0031] When the working section of the main well is being hydraulically fractured, the monitoring system monitors the crack conditions generated in the working section of the main well, wherein the monitoring system may be a microseismic monitoring system.

[0032] In step S2 of drilling a branch well, according to the monitoring result, a branch well is drilled at a vertical well section of the main well at a preset angle offset from the working section of the main well, and the branch well is hydraulically connected to the working section of the main well.

[0033] Specifically, according to the crack conditions at the working section of the main well detected by the monitoring system, they are analyzed and processed, and the location and wellbore trajectory of the branch well are designed and determined.

[0034] Before drilling a branch well, a bridge plug should be installed on the vertical section of the main well and close to the working section to seal the working section of the main well, facilitate the drilling of the branch well, and avoid affecting the main well during the drilling of the branch well.

[0035] See also Figure 2 , the first end of branch well 2 ( Figure 2 The left end of the branch well 2 (shown as a left end) is disposed in the vertical well section 11 of the main well 1, and the first end of the branch well 2 is disposed above the working section 12 of the main well 1. The first end of the branch well 2 is connected to the vertical well section 11 of the main well 1, and the second end of the branch well 2 is a free end. Specifically, the second end of the branch well 2 can be disposed arbitrarily underground, i.e., the second end of the branch well 2 can be disposed above the working section 12 of the main well 1, can also be disposed below the working section 12 of the main well 1, or can extend in other directions in the formation.

[0036] In the vertical direction of the formation, there is a preset angle α between the wellbore of the branch well 2 and the working section 12 of the main well 1. Figure 3From the top view, the wellbore of the branch well 2 deviates from the working section 12 of the main well 1 by a preset angle α, and the branch well 2 is not placed directly above the working section 12 of the main well 1. In specific implementation, the preset angle can be determined according to actual conditions, and this embodiment does not impose any restrictions on this. For example, the working section 12 of the main well 1 can be drilled at an angle of 0 to 15° (top view) clockwise along the minimum principal stress of the formation, and the branch well 2 can be drilled at an angle of 0 to 15° (top view) counterclockwise along the minimum principal stress of the formation to increase the heat exchange area and time. The angle of the clockwise or counterclockwise deviation along the minimum principal stress of the formation can also be selected from 5°, 7.5°, 10°, etc., and this embodiment does not impose any restrictions on this.

[0037] Preferably, the preset angle α between the wellbore of the branch well 2 and the working section 12 of the main well 1 is 30°-45°.

[0038] The branch wells can be horizontal or inclined. That is, when the main well is horizontal, the branch wells can be horizontal or inclined. When the main well is inclined, the branch wells can be horizontal or inclined. This embodiment does not impose any restrictions on the form of the main well and branch wells.

[0039] During specific implementation, the diameter of the branch well is smaller than the diameter of the main well.

[0040] See also Figure 2 The wellbore of the branch well 2 is connected to the working section 12 of the main well 1 through the crack 3 to achieve hydraulic communication between the branch well 2 and the working section 12 of the main well 1. In this way, the straight well section 11 of the main well 1, the branch well 2 and the working section 12 of the main well 1 are all connected to each other.

[0041] In step S3 of running the delivery pipe, the delivery pipe is run into the vertical well section of the main well or the branch well, so that the fluid passes through the main well and the branch well and is output through the delivery pipe.

[0042] Specifically, before lowering the delivery pipe, the bridge plug in the main well should be removed to allow the delivery pipe to be smoothly delivered to the main well.

[0043] See also Figure 2 The outer diameter of the delivery pipe 4 is smaller than the diameter of the main well 1 and smaller than the diameter of the branch well 2, making it easier for the delivery pipe 4 to be placed in the vertical section 11 of the main well 1 or the branch well 2. Furthermore, the delivery pipe 4 is first inserted from the vertical section 11 of the main well 1. A gap is created between the outer wall of the delivery pipe 4 and the wall of the vertical section 11 of the main well 1. This gap is used to transport cold fluid. The fluid is transported through this gap to the working section 12 of the main well 1 or the branch well 2, and flows through the working section 12 of the main well 1 and the branch well 2. During the flow, the fluid exchanges heat with the formation, thereby increasing the temperature of the fluid. The heated fluid is then transported to the surface through the delivery pipe 4.

[0044] Preferably, the delivery pipe 4 is made of a heat-insulating material so that the fluid after heat exchange can maintain the temperature after heat exchange. In specific implementation, the material of the delivery pipe 4 can be determined according to actual conditions, and this embodiment does not impose any restrictions on this.

[0045] It can be seen that in this embodiment, by hydraulically connecting the branch wells drilled from the vertical section of the main well with the working section of the main well, the branch wells are connected to the working sections of the main well, so that the fluid can flow between the branch wells and the main well, greatly increasing the heat exchange area. In addition, the deviation between the working sections of the branch wells and the main well from the preset angle can extend the path of the fluid flowing between the main well and the branch wells, increase the heat exchange area of ​​the fluid, extend the heat exchange time, effectively increase the temperature of the fluid after heat exchange, and thus improve the heat exchange efficiency, solving the problem of low heat exchange efficiency caused by the small heat exchange area of ​​the existing single-well heat exchange technology. In addition, only the main well is connected to the ground, and the branch wells are drilled from the vertical section of the main well, realizing that one wellhead can circulate heat from the main well and the branch wells at the same time, thereby improving the heat exchange efficiency, greatly reducing the drilling cost and the area of ​​use of the well site, and thus reducing the cost of geothermal energy utilization.

[0046] In the above-mentioned step S2 of drilling a branch well, the branch well is drilled toward the cracks generated when the working section of the main well is fracturing, so that the cracks in the branch well and the main well are connected to achieve hydraulic communication; or, the branch well is drilled toward the cracks generated when the working section of the main well is fracturing, and the part of the branch well corresponding to the cracks in the main well is fracturing, so that the cracks in the branch well and the cracks in the main well are connected to achieve hydraulic communication.

[0047] Specifically, one embodiment of the connection between the working section 12 of the main well 1 and the wellbore of the branch well 2 is: Figure 2 The working section 12 of the main well 1 is hydraulically fractured to produce multiple cracks 3 in the formation, and the wellbore of the branch well 2 is provided with the cracks 3 at the working section 12 of the main well 1. In this way, the wellbore of the branch well 2 and the working section 12 of the main well 1 are connected through the cracks 3, thereby realizing hydraulic communication between the wellbore of the branch well 2 and the working section 12 of the main well 1.

[0048] Another embodiment of connecting the working section 12 of the main well 1 with the wellbore of the branch well 2 is: Figure 2The working section 12 of the main well 1 is hydraulically fractured to create multiple fractures 3 in the formation. When the fractures 3 at the working section 12 of the main well 1 are formed in the wellbore of the branch well 2, the branch well 2 and the working section 12 of the main well 1 cannot be well connected through the fractures at the working section 12 of the main well 1. In this case, hydraulic fracturing is performed at the locations of the wellbore of the branch well 2 corresponding to the fractures in the main well 1 to create multiple fractures. In this way, the fractures in the wellbore of the branch well 2 are connected with the fractures in the working section 12 of the main well 1, thereby achieving communication between the working section 12 of the main well 1 and the wellbore of the branch well 2, that is, achieving hydraulic communication between the working section 12 of the main well 1 and the wellbore of the branch well 2.

[0049] It can be seen that in this embodiment, the two implementation methods of connecting the working section of the main well with the wellbore of the branch well can achieve good hydraulic communication between the working section of the main well and the branch well, facilitate the flow of fluid, thereby better exchanging heat with the formation and improving the heat exchange efficiency.

[0050] In the above-mentioned step S2 of drilling the branch well, the preset angle between the working section of the branch well and the main well, the distance between the working section of the branch well and the main well, and the distance between the branch well and the vertical well section of the main well during hydraulic communication all make the flow capacity of the fluid in each fracture match during hydraulic communication.

[0051] Specifically, the preset angle α between the branch well and the working section of the main well, the distance between the branch well and the working section of the main well, and the distance between the location of the branch well body when connected to the working section of the main well and the vertical section of the main well should all ensure that the flow capacity of fluid in each fracture of the main well working section and the branch well body are basically consistent when the main well working section and the branch well body are connected, avoiding the phenomenon of short-circuiting the flow of fluid in a particular fracture. Among them, the distance between the vertical section of the main well and the working section of the main well during hydraulic connection refers to the distance between the location where the fracture is generated by fracturing in the working section of the main well and the vertical section of the main well.

[0052] It can be seen that in this embodiment, the selection of parameters between the branch well and the main well is based on the premise of ensuring that the flow capacity of the fluid in each fracture matches when the hydraulic communication is carried out. This can effectively ensure that the flow capacity of the fluid in each fracture is basically consistent, ensure the uniform flow of the fluid, avoid the short circuit phenomenon of the fluid flow in a certain fracture, greatly extend the flow path of the fluid, and increase the heat exchange area between the fluid and the formation.

[0053] In the above-mentioned branch well drilling step S2, there are at least two branch wells, and the starting point of each branch well is placed above the working section of the main well. In addition, there is a preset angle between the working section of the main well and each branch well, and there is a preset angle between each branch well.

[0054] Specifically, the first end of each branch well is located within the vertical section of the main well, and is positioned above the working section of the main well. The first end of each branch well is connected to the vertical section of the main well, and the wellbore of each branch well is connected to the working section of the main well. In practice, the wellbores of the branch wells can also be connected by creating fractures through hydraulic fracturing.

[0055] In the vertical direction of the formation, a preset angle is established between the working section of the main well and each branch well, and each branch well has a preset angle between them. Specifically, a preset angle is established between the working section of the main well and the first branch well drilled, and a preset angle is established between the first branch well and the second branch well drilled. Similarly, each branch well has a preset angle with the previously drilled branch well. The angle between the working section of the main well and the first branch well, as well as the angle between each branch well, can be the same or different, and this embodiment does not impose any restrictions on this.

[0056] It can be seen that in this embodiment, by drilling multiple branch wells, the wellbores of each branch well can be connected to the working section of the main well, and the wellbores of each branch well can also be connected, which greatly increases the flow path of the fluid, increases the heat exchange area and heat exchange time. At the same time, one wellhead can simultaneously carry out the heat extraction cycle of the main well and multiple branch wells, realizing stable circulation heat extraction in a single well and greatly reducing the cost of geothermal energy utilization.

[0057] In the above-mentioned step S3 of lowering the delivery pipe, the delivery pipe is lowered into the vertical section of the main well, the end of the delivery pipe is placed below the position of the branch well corresponding to the vertical section, and the gap between the outer wall of the end of the delivery pipe and the well wall of the main well is sealed; or, the delivery pipe is lowered into the vertical section of the main well, the delivery pipe extends into the branch well, and the gap between the outer wall of the delivery pipe and the well wall of the branch well is sealed.

[0058] Specifically, there are two implementation methods for running the delivery pipe, one of which is: Figure 4 , the first end of the delivery pipe 4 ( Figure 4 The upper end shown in FIG) is connected to the ground, and the second end of the delivery pipe 4 ( Figure 4 The lower end shown in the figure is placed in the vertical well section 11 of the main well 1, and if the position of the first end of the branch well 2 corresponding to the vertical well section 11 of the main well 1 is recorded as the first position, the second end of the conveying pipe 4 is placed below the first position of the vertical well section 11 of the main well 1.

[0059] A separator 5 is installed in the gap between the outer wall of the second end of the delivery pipe 4 and the wall of the main well 1. The separator 5 seals the space between the second end of the delivery pipe 4 and the main well 1. The delivery pipe 4 is used to transfer the fluid in the gap between the delivery pipe 4 and the main well 1 through the branch well 2 and the working section 12 of the main well 1 for heat exchange before output.

[0060] When collecting heat, see Figure 4 The cold fluid is transported from the gap between the delivery pipe 4 and the main well 1 to the vertical well section 11 of the main well 1. Since the space between the second end of the delivery pipe 4 and the main well 1 is blocked, the fluid cannot enter the working section 12 of the main well 1 and can only enter the branch well 2. Then the fluid enters the working section 12 of the main well 1 through the crack between the wellbore of the branch well 2 and the working section 12 of the main well 1. During this flow process, the fluid exchanges heat with the formation. The fluid heated by heat exchange flows from the working section 12 of the main well 1 to the vertical well section 11. Since the space between the second end of the delivery pipe 4 and the main well 1 is blocked, the fluid heated by heat exchange will flow into the delivery pipe 4 and be transported to the ground through the delivery pipe 4, completing a heat recovery cycle. Repeat the above operation to achieve multiple heat recovery cycles.

[0061] Another way to run the delivery pipe is: Figure 5 , the first end of the delivery pipe 4 ( Figure 5 The upper end shown in FIG) is connected to the ground, and the second end of the delivery pipe 4 ( Figure 5 The lower end of the delivery pipe 4 (shown) is placed in the branch well 2. A separator 5 is installed in the gap between the outer wall of the second end of the delivery pipe 4 and the wall of the branch well 2 to seal the space between the second end of the delivery pipe 4 and the branch well 2. The delivery pipe 4 is used to transfer the fluid in the gap between the delivery pipe 4 and the main well 1 through the working section 12 of the main well 1 and the branch well 2 for heat exchange before output.

[0062] Specifically, the second end of the delivery pipe 4 can be bent so that it extends into the branch well 2. Alternatively, an insertable tailpiece can be provided at the second end of the delivery pipe 4, which bends and extends into the branch well 2. This embodiment imposes no restrictions on the configuration of the second end of the delivery pipe 4, as long as the second end of the delivery pipe 4 can be placed in the branch well 2.

[0063] When collecting heat, see Figure 5 The cold fluid is transported from the gap between the delivery pipe 4 and the main well 1 to the vertical well section 11 of the main well 1. Since the space between the second end of the delivery pipe 4 and the branch well 2 is blocked, the fluid cannot enter the branch well 2 and can only enter the working section 12 of the main well 1. The fluid then enters the branch well 2 through the cracks between the wellbore of the branch well 2 and the working section 12 of the main well 1. During this flow, the fluid exchanges heat with the formation. Since the space between the second end of the delivery pipe 4 and the branch well 2 is blocked, the fluid heated by heat exchange will flow from the branch well 2 to the delivery pipe 4 and be transported to the ground through the delivery pipe 4, completing a heat recovery cycle. Repeat the above operation to achieve multiple heat recovery cycles.

[0064] It can be seen that in this embodiment, by sealing the gap between the outer wall at the end of the delivery pipe and the wall of the main well or the branch well, the fluid passes through the branch well and the main well and is then output to the ground by the delivery pipe, which greatly extends the working path of the fluid, increases the heat exchange area, and improves the heat exchange efficiency.

[0065] In step S3 of lowering the delivery pipe, when there are at least two branch wells, the delivery pipe is lowered into the vertical well section of the main well, and the end of the delivery pipe is placed below the position of the vertical well section of the branch well corresponding to the working section closest to the main well, and the gap between the outer wall of the end of the delivery pipe and the well wall of the main well is sealed.

[0066] Specifically, the first end of the delivery pipe 4 is connected to the ground, and the second end of the delivery pipe 4 is placed in the vertical well section 11 of the main well 1. Moreover, if the first end of the branch well 2 at the working section 12 closest to the main well 1 corresponds to the position of the vertical well section 11 of the main well 1 as the second position, then the second end of the delivery pipe 4 is placed below the second position of the vertical well section 11 of the main well 1.

[0067] A separator is provided in the gap between the outer wall of the second end of delivery pipe 4 and the wall of main well 1 to seal the space between the second end of delivery pipe 4 and main well 1. Delivery pipe 4 is used to sequentially transfer the fluid in the gap between delivery pipe 4 and main well 1 through each branch well 2 and working section 12 of main well 1 for heat exchange before output.

[0068] During heat extraction, cold fluid is transported from the gap between the delivery pipe 4 and the main well 1 to the vertical well section 11 of the main well 1. Since the space between the second end of the delivery pipe 4 and the main well 1 is blocked, the fluid cannot enter the working section 12 of the main well 1 and can only enter the branch wells 2. Then the fluid enters the working section 12 of the main well 1 through the cracks between the wellbore of the branch well 2 and the working section 12 of the main well 1, or the fluid passes through the cracks between the branch wells 2 and the working section 12 of the main well 1 and finally enters the working section 12 of the main well 1. During this flow process, the fluid exchanges heat with the formation. The fluid heated by heat exchange flows from the working section 12 of the main well 1 to the vertical well section 11. Since the space between the second end of the delivery pipe 4 and the main well 1 is blocked, the fluid heated by heat exchange will flow into the delivery pipe 4 and be transported to the ground through the delivery pipe 4, thus completing a heat extraction cycle. Repeat the above operation to achieve multiple heat extraction cycles.

[0069] It can be seen that in this embodiment, by sealing the gap between the outer wall at the end of the delivery pipe and the wall of the main well, the fluid can enter each branch well, and then flow from the cracks between the branch well and the working section of the main well to the working section of the main well. Since the fluid can flow to the cracks at multiple branch wells, the working path of the fluid can be extended to the greatest extent, the heat exchange area is increased, and the heat exchange efficiency is improved.

[0070] To sum up, in this embodiment, the working sections of the branch well and the main well are connected, so that the fluid can flow between the branch well and the main well, greatly increasing the heat exchange area. Moreover, the working sections of the branch well and the main well deviate from the preset angle, which can extend the path of the fluid flowing between the main well and the branch well, increase the heat exchange area of ​​the fluid, extend the heat exchange time, effectively increase the temperature of the fluid after heat exchange, and thus improve the heat exchange efficiency. Moreover, only the main well is connected to the ground, and the branch well is drilled from the straight well section of the main well, realizing that one wellhead can circulate heat for the main well and the branch well at the same time, thereby improving the heat exchange efficiency, greatly reducing the drilling cost and the area of ​​use of the well site, and thus reducing the cost of geothermal energy utilization.

[0071] Obviously, those skilled in the art may make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if such changes and modifications fall within the scope of the claims and their equivalents, the present invention is intended to include such changes and modifications.

Claims

1. A geothermal single well fracturing method, characterized in that: The steps include: The main well drilling step comprises drilling the main well from the ground to the underground, and fracturing the working section of the main well, while monitoring the fracturing situation; wherein the main well comprises: a vertical well section and a working section connected to each other; the vertical well section is connected to the ground; A branch well drilling step, according to the monitoring result, drilling a branch well at a vertical well section of the main well at a preset angle offset from the working section of the main well, and hydraulically connecting the branch well with the working section of the main well; A delivery pipe lowering step, wherein the delivery pipe is lowered into the vertical well section of the main well or the branch well, so that the fluid passes through the main well and the branch well and is output from the delivery pipe; In the vertical direction of the formation, there is a preset angle between the wellbore of the branch well and the working section of the main well; In the step of drilling a branch well, The branch well is drilled toward the cracks generated during the fracturing of the working section of the main well, so that the branch well is connected with the cracks of the main well to achieve hydraulic communication; or, The branch well is drilled toward the cracks generated during the fracturing of the working section of the main well, and the branch well is fractured at the portion corresponding to the cracks of the main well, so that the cracks of the branch well are connected with the cracks of the main well to achieve hydraulic communication; The preset angle and distance between the branch well and the working section of the main well, as well as the distance between the branch well and the straight well section of the main well during hydraulic communication, all enable the flow capacity of the fluid in each fracture to match during hydraulic communication.

2. The geothermal single well fracturing method according to claim 1, characterized in that: In the step of drilling the main well, The direction of the main well is consistent with the direction of the minimum principal stress of the formation or deviates from the direction of the minimum principal stress of the formation by a preset angle.

3. The geothermal single well fracturing method according to claim 1, characterized in that: In the step of drilling the main well, When the main well is completed in full open hole or partially open hole, a general fracturing method is used to fractur e the working section of the main well; When the main well is completed by all casing cementing, the working section of the main well is fractured in stages by using a bridge plug and perforation combination method.

4. The geothermal single well fracturing method according to claim 1, characterized in that: In the step of drilling the main well, The main well is a horizontal well or an inclined well.

5. The geothermal single well fracturing method according to claim 1, characterized in that: In the step of drilling a branch well, There are at least two branch wells, and the starting point of each branch well is placed above the working section of the main well. In addition, there is a preset angle between the working section of the main well and each branch well, and there is a preset angle between each branch well.

6. The geothermal single well fracturing method according to claim 1, characterized in that: In the step of drilling a branch well, The branch well is a horizontal well or a inclined well.

7. The geothermal single well fracturing method according to claim 1, characterized in that: In the step of lowering into the delivery pipe, The delivery pipe is lowered into the vertical well section of the main well, the end of the delivery pipe is placed below the position of the branch well corresponding to the vertical well section, and the gap between the outer wall of the end of the delivery pipe and the well wall of the main well is blocked; or, The delivery pipe is lowered into the vertical well section of the main well, the delivery pipe extends into the branch well, and the gap between the outer wall of the delivery pipe and the well wall of the branch well is sealed.

8. The geothermal single well fracturing method according to claim 5, characterized in that: In the step of lowering into the delivery pipe, When there are at least two branch wells, the delivery pipe is lowered into the vertical well section of the main well, and the end of the delivery pipe is placed below the position of the branch well corresponding to the vertical well section at the working section closest to the main well, and the gap between the outer wall of the end of the delivery pipe and the well wall of the main well is sealed.

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