A three-horizontal well group structure and method for exploiting natural gas hydrates

Abstract

The present invention discloses a three-horizontal well group structure and method for exploiting natural gas hydrates, which belongs to the technical field of natural gas hydrate exploitation. The three-horizontal well group structure includes a production well, a heat injection well, and a sand drainage well extending to the natural gas hydrate reservoir. Each well includes a vertical section and a horizontal section located in the natural gas hydrate reservoir. Several insulation pipe groups are evenly spaced along the length direction on the well wall of the horizontal section of each well. Each insulation pipe group consists of an insulation pipe arranged on the production horizontal section, an insulation pipe arranged on the heat injection horizontal section, and an insulation pipe arranged on the sand drainage horizontal section. The insulation pipes in the same insulation pipe group are aligned up and down; openings are provided on the parts of the well wall of the production horizontal section, the heat injection horizontal section, and the sand drainage horizontal section where insulation pipes are not provided. The well group structure and method of the present invention can effectively discharge sand and water while providing the required heat for the decomposition of natural gas hydrates, thereby preventing reservoir collapse.

Description

Technical Field

[0001] The present invention belongs to the technical field of natural gas hydrate exploitation, and in particular relates to a three-horizontal well group structure and method for exploiting natural gas hydrates. Background Art

[0002] Natural gas hydrates (NGHs) are a type of cage-like crystalline substance formed by natural gas and water under specific temperatures and pressures (low temperature and high pressure). They resemble ice and snow and are commonly known as "combustible ice" because they can ignite. Natural gas hydrates are a clean and efficient emerging energy source with vast reserves. The total organic carbon content of NGHs on Earth is approximately twice that of oil, natural gas, and coal combined, with 99% of the total NGH resources located in marine areas. With the increasing research into NGH extraction, commercial production is becoming increasingly feasible.

[0003] Currently, the main theoretical methods for extracting natural gas hydrates include pressure reduction, heat injection, and CO2 displacement. Due to the limitations of the displacement method, such as long reaction cycles, slow reaction rates, and low efficiency, pressure reduction and heat injection are often used in practice. While the pressure reduction method is more economical, the poor permeability, low heat and mass transfer efficiency, and low conductivity of marine hydrate reservoirs make pressure reduction extraction unsustainable and efficient. Furthermore, during pressure reduction, the heat absorbed by hydrate decomposition lowers the reservoir temperature, which can easily lead to channel blockage near the decomposition layer.

[0004] The heat injection method involves injecting a hot fluid (such as steam, hot water, hot brine, or other hot fluid) from the surface into a natural gas hydrate reservoir using a high-pressure pump, raising the temperature of the hydrate reservoir and thereby achieving the purpose of decomposing the natural gas hydrate. The heat injection method can effectively promote hydrate decomposition and has a wide range of applications, but it has disadvantages such as large heat loss, low heat utilization efficiency, a small heating area, and uncontrollable heat transfer direction, resulting in most of the heat being used to heat pore gas and liquid and sediment. Patent publication number CN108005626A discloses a natural gas hydrate production device based on heat pipe technology, comprising a plurality of production wells and a production well extending into the natural gas hydrate reservoir. Each production well includes a vertical production well section and a horizontal production well section located in the natural gas hydrate reservoir. The production well includes a vertical production well section and a horizontal production well section located in the natural gas hydrate reservoir. A gas-liquid separator is connected to the production well head located in the vertical production well section. The horizontal production well section is radially and evenly distributed with a plurality of heat pipes extending into the natural gas hydrate reservoir. This invention uses heat pipes to transfer the heat of hot water in the mining well to the natural gas hydrate reservoir, and most of the heat is used to destroy the hydrate phase equilibrium instead of heating the pore gas liquid and sediment, thereby achieving directional heat transfer, expanding the heating area, and improving heat utilization efficiency. It overcomes the shortcomings of traditional heat injection methods such as small heating area and uncontrollable heat transfer direction. However, this invention uses single-well circulating heat injection, and the development efficiency is poor. In addition, this invention uses a dual-horizontal well group mode. During the mining process, sand particles precipitated from the hydrate reservoir can easily cause wellbore blockage, affecting the mining effect. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a three-horizontal well group structure and method for exploiting natural gas hydrates, which can effectively discharge sand and water and prevent reservoir collapse while providing the required heat for decomposition of natural gas hydrates.

[0006] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0007] A three-horizontal well group structure for exploiting natural gas hydrates, comprising a production well, a heat injection well, and a sand drainage well extending into a natural gas hydrate reservoir, wherein the production well, the heat injection well, and the sand drainage well each comprise a vertical section and a horizontal section located in the natural gas hydrate reservoir, wherein the production horizontal section is adjacent to an overlying stratum, the sand drainage horizontal section is adjacent to an underlying stratum, and the heat injection horizontal section is located between the production horizontal section and the sand drainage horizontal section;

[0008] Several heat-insulating pipe groups are evenly spaced along the length direction on the well walls of the production horizontal section, the heat injection horizontal section, and the sand discharge horizontal section. Each heat-insulating pipe group consists of one heat-insulating pipe arranged on the production horizontal section, one heat-insulating pipe arranged on the heat injection horizontal section, and one heat-insulating pipe arranged on the sand discharge horizontal section. The heat-insulating pipes in the same heat-insulating pipe group are aligned vertically.

[0009] Openings are provided on the parts of the well wall of the production horizontal section, the heat injection horizontal section and the sand discharge horizontal section where no insulation pipes are provided.

[0010] In the above-mentioned three-horizontal well group structure, the openings are spaced apart along the length of the wellbore in the production horizontal section, the heat injection horizontal section, or the sand drainage horizontal section, and are evenly distributed along the circumference of the wellbore. Furthermore, the diameter of the openings is preferably at least twice the radius of the largest sand grain in the natural gas hydrate reservoir.

[0011] The present invention also includes a method for exploiting natural gas hydrates using the above-mentioned three-horizontal well group structure, comprising the following steps:

[0012] 1) Injecting thermal fluid (such as steam, hot water, hot brine or other hot fluid) into the production well, heat injection well and sand drainage well respectively, and performing heat circulation inside the production well, heat injection well and sand drainage well. The thermal fluid diffuses outward through the openings on the production well, heat injection well and sand drainage well. Through continuous thermal circulation, a thermal cavity connected vertically is formed in the same vertical area including the corresponding opening positions on the production horizontal section, heat injection horizontal section and sand drainage horizontal section;

[0013] 2) After forming a hot cavity that connects the upper and lower parts, stop injecting hot fluid into the production well and the sand drainage well, and continue injecting hot fluid into the hot injection well to dissolve the natural gas hydrate. The dissolved natural gas is produced through the production well, while the water and sand particles produced by the dissolution of the natural gas hydrate are discharged through the sand drainage well. The flow rate of the output in the production well is controlled by controlling the temperature and pressure of the hot fluid injected into the hot injection well.

[0014] Compared with the existing technology, the present invention adds a sand drainage well near the underlying rock formation, combines the setting of the insulation pipe group and the opening, and through heat circulation, forms a heat cavity connected up and down in the same vertical area corresponding to the opening position on the horizontal section of the three wells. While providing the required heat for the decomposition of natural gas hydrates, the water and sand particles produced by the dissolution of natural gas hydrates are discharged through the sand drainage well, effectively avoiding blockage of the wellbore. At the same time, the natural gas hydrates above and below the insulation pipe group are reserved as a support body to prevent natural gas leakage caused by reservoir collapse. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 This is an implementation of the three-horizontal well group structure for exploiting natural gas hydrates described in the present invention.

[0016] Figure 2 for Figure 1 A schematic cross-sectional view of the embodiment.

[0017] The numbers in the figure are:

[0018] 1. Sand drainage well; 101. Vertical sand drainage section; 102. Horizontal sand drainage section; 2. Heat injection well; 201. Vertical heat injection section; 202. Horizontal heat injection section; 3. Production well; 301. Vertical production section; 302. Horizontal production section; 4. Overburden; 5. Natural gas hydrate reservoir; 6. Underburden; 7. Insulated pipe; 8. Heat cavity; 9. Opening. DETAILED DESCRIPTION

[0019] The three-horizontal well group structure for exploiting natural gas hydrates described in the present invention includes a production well 3, a heat injection well 2, and a sand drainage well 1 extending into a natural gas hydrate reservoir 5. The production well 3, the heat injection well 2, and the sand drainage well 1 each include a vertical section and a horizontal section located in the natural gas hydrate reservoir 5, wherein the production horizontal section 302 is close to the overburden stratum 4, the sand drainage horizontal section 102 is close to the underburden stratum 6, and the heat injection horizontal section 202 is located between the production horizontal section 302 and the sand drainage horizontal section 102;

[0020] Several insulation pipe groups are evenly spaced along the length direction on the well walls of the production horizontal section 302, the heat injection horizontal section 202, and the sand discharge horizontal section 102. Each insulation pipe group consists of an insulation pipe 7 provided on the production horizontal section 302, an insulation pipe 7 provided on the heat injection horizontal section 202, and an insulation pipe provided on the sand discharge horizontal section 102. The insulation pipes 7 in the same insulation pipe group are aligned vertically.

[0021] Openings 9 are provided on the parts of the wellbore wall of the production horizontal section 302 , the heat injection horizontal section 202 and the sand discharge horizontal section 102 where the heat insulation pipes 7 are not provided.

[0022] In the above three-horizontal well group structure, the arrangement of the production well 3, the heat injection well 2 and the sand drainage well 1, the setting of the insulation pipe 7 and the opening 9 on the well wall of each well, and the completion operation are all completed using existing conventional technologies.

[0023] In the aforementioned three-horizontal well group structure, the distance between two adjacent insulated tube groups is determined as needed, preferably set to 10 to 30 meters. The insulated tubes 7 are preferably of equal length. Furthermore, the insulated tubes 7 are preferably wrapped with insulating material to isolate heat exchange between the wellbore and the natural gas hydrate reservoir 5. This prevents the extraction of natural gas hydrates from the corresponding upper and lower portions of the insulated tubes 7, thereby supporting the natural gas hydrate reservoir 5 and maintaining its stability.

[0024] The vertical alignment of the insulated tubes 7 in the same insulated tube group means that the starting ends of the insulated tubes 7 on the production horizontal section 302, the insulated tubes 7 on the heat injection horizontal section 202, and the insulated tubes 7 on the sand discharge horizontal section 102 in the same insulated tube group are aligned, or the absolute value of the maximum distance between the starting ends of the three in the vertical plane is less than or equal to 2 meters.

[0025] In the above-mentioned three-horizontal well group structure, the openings 9 are spaced apart along the length of the well wall on the well wall of the production horizontal section 302, the heat injection horizontal section 202, or the sand drainage horizontal section 102, and are evenly distributed along the circumference of the well wall. The shape of the openings 9 can be regular shapes such as circular, elliptical, rectangular, or square, or irregular shapes. Furthermore, the diameter of the openings 9 is preferably at least twice the radius of the largest sand grain in the natural gas hydrate reservoir 5. Furthermore, in order to facilitate the formation of a vertically connected heat cavity 8 within the same vertical region corresponding to the positions of the openings 9 on the three horizontal sections of the well, the starting ends of the first opening 9 on the production horizontal section 302, the first opening 9 on the heat injection horizontal section 202, and the first opening 9 on the sand drainage horizontal section 102 in the same vertical region are aligned, or the absolute value of the maximum distance between the starting ends of the three in the vertical plane is less than or equal to 2 meters.

[0026] The method for exploiting natural gas hydrates using the three-horizontal well group structure described in the present invention comprises the following steps:

[0027] 1) injecting thermal fluid into the production well 3, the heat injection well 2, and the sand drainage well 1, respectively, and performing heat circulation inside the production well 3, the heat injection well 2, and the sand drainage well 1. The thermal fluid diffuses outward through the openings 9 on the production well 3, the heat injection well 2, and the sand drainage well 1. Through continuous thermal circulation, a vertically connected thermal cavity 8 is formed in the same vertical region including the corresponding positions of the openings 9 on the production horizontal section 302, the heat injection horizontal section 202, and the sand drainage horizontal section 102;

[0028] 2) After forming the upper and lower connected thermal cavity 8, stop injecting thermal fluid into the production well 3 and the sand drainage well 1, and continue injecting thermal fluid into the heat injection well 2 to dissolve the natural gas hydrate. The dissolved natural gas is produced through the production well 3, while the water and sand particles produced by the dissolution of the natural gas hydrate are discharged through the sand drainage well 1; the flow rate of the output in the production well 3 is controlled by controlling the temperature and pressure of the thermal fluid injected into the heat injection well 2.

[0029] In the above method, first, based on geological data, a natural gas hydrate reservoir 5 with reservoir thickness, storage characteristics and other relevant parameters that meet the requirements is selected as the target reservoir; then, existing conventional technologies are used to complete the layout of the well group structure and completion operations on the target reservoir.

[0030] In the above method, the thermal fluid can be hot water, hot brine, or other thermal fluid commonly used in the prior art.

[0031] In the above method, heat circulation is simultaneously carried out in the production well 3, the heat injection well 2, and the sand drainage well 1. When pressure linkage occurs simultaneously in the three wells (pressure linkage occurs when the pressure of one well changes, and the other two wells change accordingly), it is considered that a vertically connected heat cavity 8 is formed. Within the same vertical area corresponding to the positions of the openings 9 in the production horizontal section 302, the heat injection horizontal section 202, and the sand drainage horizontal section 102, it refers to a space with a certain horizontal distance and a certain vertical distance formed by connecting directly below and directly below the position of the opening 9 in the production horizontal section 302, directly below and directly below the position of the opening 9 in the heat injection horizontal section 202, and directly below and directly below the position of the opening 9 in the sand drainage horizontal section 102.

[0032] In the above method, after the hot cavity 8 connected vertically is formed, the flow ratio of the sand drainage well 1 to the heat injection well 2 is controlled to be greater than or equal to 1 to prevent gas from escaping from the sand drainage well 1 (after the hot cavity 8 connected vertically is formed, the production well 3 and the sand drainage well 1 stop injecting hot fluid, and the heat injection well 2 continues to inject hot fluid. In order to prevent the hot fluid from accumulating and absorbing heat in the reservoir and avoid energy waste, the hot fluid that has released heat and cooled should be discharged through the sand drainage well 1).

[0033] In the above method, after forming the upper and lower connected thermal cavity 8, by controlling the temperature and pressure of the hot fluid injected into the hot injection well 2, the volume and shape of the thermal cavity 8 after natural gas hydrate extraction can also be controlled.

[0034] In the above method, after the upper and lower connected heat cavity 8 is formed, the temperature of the output of the sand drainage well 1 (including condensed water, dissolved water of natural gas hydrate, sand particles in the reservoir, etc.) is preferably not more than 40°C and not less than 0°C, and is adjusted by controlling the temperature and pressure of the hot fluid injected into the heat injection well 2.

[0035] In the above method, when natural gas or steam is produced in the sand drainage well 1, the injection of hot fluid into the heat injection well 2 is stopped, or the temperature and pressure of the hot fluid injected into the heat injection well 2 are reduced, and the temperature difference between the injected hot fluid and the produced liquid is kept within the required range to prevent the sand drainage well 1 from producing natural gas or steam.

[0036] In order to better explain the technical solution of the present invention, the present invention is further described in detail below with reference to the accompanying drawings and embodiments, but the embodiments of the present invention are not limited thereto.

[0037] Figure 1 This is an implementation method of the three-horizontal well group structure for exploiting natural gas hydrates described in the present invention. Figure 1In the figure, a three-horizontal well group structure for exploiting natural gas hydrates includes a production well 3, a heat injection well 2, and a sand drainage well 1. The production well 3 includes a production vertical section 301 and a production horizontal section 302. The heat injection well 2 includes a heat injection vertical section 201 and a heat injection horizontal section 202. The sand drainage well 1 includes a sand drainage vertical section 101 and a sand drainage horizontal section 102. The production horizontal section 302, the heat injection horizontal section 202, and the sand drainage horizontal section 102 are all located in the natural gas hydrate reservoir 5, wherein the production horizontal section 302 is close to the overburden stratum 4, and the sand drainage horizontal section 102 is close to the underburden stratum 6. The depth of the heat injection horizontal section 202 is between the depths of the production horizontal section 302 and the depths of the sand drainage horizontal section 102 (e.g., Figure 2 As shown); a plurality of insulation pipe groups are evenly spaced along the length direction on the well wall of the production horizontal section 302, the heat injection horizontal section 202 and the sand discharge horizontal section 102, each insulation pipe group consists of an insulation pipe 7 provided on the production horizontal section 302, an insulation pipe 7 provided on the heat injection horizontal section 202 and an insulation pipe provided on the sand discharge horizontal section 102, the lengths of the insulation pipes 7 are equal, and the insulation pipes 7 are all wrapped with insulation materials, and the starting ends of the insulation pipes 7 in the same insulation pipe group are aligned; in the production horizontal section 302, the heat injection horizontal section 20 2 and the portion of the well wall of the sand drainage horizontal section 102 where the insulation pipe 7 is not provided are provided with openings 9, which are rectangular and are spaced apart along the length direction of the well wall on the well wall of the production horizontal section 302, the heat injection horizontal section 202 or the sand drainage horizontal section 102, and are evenly distributed along the circumference of the well wall; the absolute value of the maximum distance between the starting ends of the first opening 9 on the production horizontal section 302, the first opening 9 on the heat injection horizontal section 202, and the first opening 9 on the sand drainage horizontal section 102 in the same vertical area is less than or equal to 2 meters.

[0038] The method for exploiting natural gas hydrates using the above-mentioned three-horizontal well group structure comprises the following steps:

[0039] 1) After the well group structure is arranged and the well completion operation is completed, hot fluid is injected into the production well 3, the heat injection well 2, and the sand drainage well 1, respectively. Thermal circulation is performed inside the production well 3, the heat injection well 2, and the sand drainage well 1. The hot fluid diffuses outward through the openings 9 on the production well 3, the heat injection well 2, and the sand drainage well 1, forming a thermal cavity 8 near the openings 9 on the sand drainage horizontal section 102, the heat injection horizontal section 202, and the production horizontal section 302. Through continuous thermal circulation, a vertically connected thermal cavity 8 is formed in the same vertical area including the corresponding positions of the openings 9 on the production horizontal section 302, the heat injection horizontal section 202, and the sand drainage horizontal section 102;

[0040] 2) After the upper and lower connected thermal cavity 8 is formed, the injection of thermal fluid into the production well 3 and the sand drainage well 1 is stopped, and the injection of thermal fluid into the heat injection well 2 is continued to dissolve the natural gas hydrate. After the natural gas hydrate is dissolved, the dissolved natural gas enters the production well 3 through the opening 9 on the production well 3 and is produced by the production well 3, while the water and sand particles produced after the dissolution of the natural gas hydrate enter the sand drainage well 1 through the opening 9 on the sand drainage well 1 under the action of gravity and pressure and are discharged from the sand drainage well 1.

[0041] During the entire mining process, the temperature and pressure of the hot fluid injected into the heat injection well 2 are controlled to control the flow rate of the output in the production well 3 and the volume and shape of the hot cavity 8 after the natural gas hydrate is mined. During the mining process, when natural gas or steam is produced in the sand drainage well 1, the injection of hot fluid into the heat injection well 2 is stopped, or the temperature and pressure of the hot fluid injected into the heat injection well 2 are reduced to keep the temperature difference between the injected hot fluid and the produced liquid within the required range, thereby preventing the sand drainage well 1 from producing natural gas or steam. During the mining process, the temperature of the output of the sand drainage well 1 (including condensed water, dissolved water of natural gas hydrate, sand particles in the reservoir, etc.) is preferably not more than 40°C and not less than 0°C, and is adjusted by controlling the temperature and pressure of the hot fluid injected into the heat injection well 2.

Claims

1. A three-horizontal well group structure for exploiting natural gas hydrates, characterized by: The invention comprises a production well (3), a heat injection well (2) and a sand drainage well (1) extending to a natural gas hydrate reservoir (5); the production well (3), the heat injection well (2) and the sand drainage well (1) all comprise a vertical section and a horizontal section located in the natural gas hydrate reservoir (5); the production horizontal section (302) is close to the overburden (4); the sand drainage horizontal section (102) is close to the underburden (6); and the heat injection horizontal section (202) is located between the production horizontal section (302) and the sand drainage horizontal section (102); Several heat-insulating pipe groups are evenly spaced along the length direction on the well walls of the production horizontal section (302), the heat injection horizontal section (202) and the sand discharge horizontal section (102), each heat-insulating pipe group is composed of a heat-insulating pipe (7) arranged on the production horizontal section (302), a heat-insulating pipe (7) arranged on the heat injection horizontal section (202) and a heat-insulating pipe (7) arranged on the sand discharge horizontal section (102), and the heat-insulating pipes (7) in the same heat-insulating pipe group are aligned up and down; the heat-insulating pipes (7) are wrapped with insulation material; Openings (9) are provided on the parts of the well wall of the production horizontal section (302), the heat injection horizontal section (202) and the sand discharge horizontal section (102) where the heat insulation pipe (7) is not provided.

2. The three-horizontal well group structure for exploiting natural gas hydrates according to claim 1 is characterized in that: The openings (9) are distributed at intervals along the length of the well wall in the production horizontal section (302), the heat injection horizontal section (202) or the sand discharge horizontal section (102), and are evenly distributed along the circumference of the well wall.

3. The three-horizontal well group structure for exploiting natural gas hydrates according to claim 1 or 2, characterized in that: The diameter of the opening (9) is more than twice the radius of the largest sand grain in the natural gas hydrate reservoir (5).

4. A method for extracting natural gas hydrates using the three-horizontal well structure according to any one of claims 1 to 3, comprising the following steps: 1) injecting hot fluid into the production well (3), the heat injection well (2) and the sand drainage well (1) respectively, performing heat circulation inside the production well (3), the heat injection well (2) and the sand drainage well (1), and the hot fluid diffuses outward through the openings (9) on the production well (3), the heat injection well (2) and the sand drainage well (1). Through continuous heat circulation, a heat cavity (8) connected vertically is formed in the same vertical area including the corresponding positions of the openings (9) on the production horizontal section (302), the heat injection horizontal section (202) and the sand drainage horizontal section (102); 2) After forming the upper and lower connected thermal cavity (8), stop injecting the thermal fluid into the production well (3) and the sand discharge well (1), and continue injecting the thermal fluid into the heat injection well (2) to dissolve the natural gas hydrate. The dissolved natural gas is produced through the production well (3), and the water and sand particles produced by the dissolution of the natural gas hydrate are discharged from the sand discharge well (1); the flow rate of the output in the production well (3) is controlled by controlling the temperature and pressure of the thermal fluid injected into the heat injection well (2).

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