A device and method for studying wellbore instability and gas leakage during hydrate mining

Abstract

The present invention belongs to the field of geotechnical engineering and discloses a device and method for studying wellbore instability and gas leakage during hydrate extraction. The device includes a reactor system, a well system, a hydrate generation system, a hydrate extraction system, and a monitoring system. The device realizes real-time monitoring of wellbore instability and gas leakage during hydrate extraction. Compared with traditional research devices, the fluid-solid coupling process during extraction is considered, that is, under the action of hydrate phase change and sediment, the deformation of the sediment reservoir causes squeezing and shear damage to the wellbore and cement sheath, resulting in gas escape and leakage along the deformed wellbore, which will cause peeling of the cement sheath and corrosion of the wellbore, accelerate the instability of the wellbore and cement sheath, and further hinder gas and water production. The device has certain guiding significance for studying the fluid-solid coupling effect of sediments and wellbore in the hydrate extraction process under laboratory conditions.

Description

Technical Field

[0001] The present invention belongs to the field of geotechnical engineering and relates to a device and method for studying wellbore instability and gas leakage during hydrate mining. Background Art

[0002] Marine gas hydrates are ice-like crystalline compounds composed of water and natural gas, found in deep-sea sediments under conditions of low temperature and high pressure. Commercial extraction of gas hydrates is a goal pursued by countries around the world, but the safety issues that arise during extraction are difficult and complex.

[0003] During the drilling process, changes in formation pressure and temperature around the wellbore inevitably lead to the decomposition of hydrates in the formation, causing the formation rock to lose its cementation and skeletal support, leading to wellbore collapse. The water produced by this decomposition increases the water content of the wellbore formation, and the gas increases the pore pressure, weakening the connection between particles and causing wellbore instability. A wellbore instability loading test platform and loading method (CN111024510A) considers the impact of changes in formation temperature and pressure conditions on wellbore stability, but does not consider the mechanical effects of hydrate decomposition on the wellbore. The wellbore is composed of casing and cement sheath. Uncoordinated deformation of the cement sheath and casing leads to failure of the cementing surface, causing the cement sheath to crack or even break, losing its protective function. Under the non-uniform load applied to the wellbore, the casing deforms and damages. External load is the external non-uniform stress imposed by the reservoir on the wellbore during drilling. When the applied external load exceeds the yield stress of the wellbore, it will cause shear and extrusion damage to the wellbore.

[0004] Natural gas hydrate decomposition can also lead to water flow and irregular wellbore expansion, creating microannuli between the wellbore and the cement sheath. When microannuli form in the cement sheath, methane decomposition gas can leak along these annuli, potentially increasing wellbore pressure above formation pressure and causing wellbore instability. While the experimental device for simulating methane gas leakage from natural gas hydrate decomposition (CN105372392A) considers gas leakage caused by hydrate decomposition, it does not account for the impact of hydrate decomposition on the wellbore wall, creating microannuli and leaking along the wellbore wall.

[0005] In view of the above background, the present invention proposes a device for studying wellbore instability and gas leakage during hydrate extraction, which provides support for studying the safe and efficient development mechanism of hydrates under laboratory conditions. Summary of the Invention

[0006] The present invention proposes a device for studying wellbore instability and gas leakage during hydrate extraction, which can monitor the impact of external loads on wellbore stability at a laboratory scale, and study the wellbore instability and gas leakage caused by hydrate decomposition.

[0007] The technical solutions of the present invention are as follows:

[0008] A device for studying wellbore instability and gas leakage during hydrate production, comprising a reactor system 1, a well system 2, a hydrate generation system 3, a hydrate production system 4, and a monitoring system 5;

[0009] The reactor system 1 is mainly composed of a reactor 11 and a constant temperature chamber 12; the reactor 11 is made of a pressure-resistant stainless steel inner liner and a stainless steel outer shell, with a hollow space in the middle of the shell serving as the constant temperature chamber 12; the reactor 11 is a cube with a cube cavity inside; the top of the reactor 11 is a square upper cover with a well hole in the middle of the upper cover; through holes are provided at the four corners of the four side walls of the reactor 11, which are transparent and smooth, and the guide shaft 312 of the confining pressure device passes through the through holes to connect the confining pressure frame 31 and the confining pressure loading plate 318; a threaded hole is provided in the center of each side wall of the reactor 11, which is transparent and has threads inside, and the lead screw 316 of the confining pressure device passes through the threaded hole to connect the confining pressure frame 31 and the confining pressure head 317; the sample wrapped by the rubber membrane 39 is placed in the cube cavity of the reactor;

[0010] The well system 2 is mainly composed of a wellbore 21, a casing 22, a cement ring 23, an annular visual window 24 and a gas collection device 25. The wellbore 21 passes through the upper cover of the reactor 11 and extends to the lower center of the sample. The wellbore 21 is divided into three parts from top to bottom: the first part is outside the reactor 11, where the wellbore 21 is composed of a casing 22, and a gas monitoring probe 53 is provided on the outer wall of the casing 22. A sealed annular visual window 24 is provided on the outer side of the casing 22. The annular visual window 24 is used to observe the changes in the bubbles leaking along the well in real time and is connected to the gas collection device 25 through a pipeline; the second part is above the center of the sample, where the wellbore 21 is composed of the casing 22 and the cement ring 23 from the inside out; the third part is below the center of the sample, where the wellbore 21 is composed of the casing 22 and the cement ring 23, and the outer wall of the casing is provided with a gas monitoring probe 53 and a strain gauge 55;

[0011] The hydrate generation system 3 is mainly composed of a confining pressure device, an axial pressure device, a constant temperature device and a liquid inlet device;

[0012] The confining pressure device includes a confining pressure frame 31 and a confining pressure loading plate 318; the confining pressure loading plate 318 is a contact plate that applies uniform stress to the sample, and the confining pressure loading plate 318 is tightly fitted to the side wall of the sample; the confining pressure frame 31 applies uniform stress to the confining pressure loading plate 318, which is mainly composed of the upper end face 311 of the confining pressure frame, a guide shaft 312, an automatic adjuster 313, a linear bearing 314, the lower end face 315 of the confining pressure frame, a screw 316, a rectangular confining pressure head 317 and a bracket 319; the confining pressure frame 31 is placed horizontally and is arranged on the outside of the outer wall of the reactor 11; through holes are provided at the corners of the upper end face 311 and the lower end face 315 of the confining pressure frame, the guide shaft 312 passes through the through hole provided with the linear bearing 314, and is fixed to the confining pressure loading plate 318 by bolts, and the guide shaft 3 12 Ensure that the rectangular confining pressure head 317 is perpendicular to the confining pressure loading plate 318 to apply uniform stress to the specimen; the left and right sides of the lower end surface 315 of the confining pressure frame are welded together with the adjacent lower end surfaces of the confining pressure frame through brackets 319 to form an internal force self-balancing frame to prevent excessive torque and bending when stress is applied to the confining pressure loading plate 318; the automatic adjuster 313 is connected to the screw 316 through the lower end surface 315 of the confining pressure frame, and the screw 316 is welded to the center of the rectangular confining pressure head 317, and the rectangular confining pressure head 317 and the confining pressure loading plate 318 are connected by bolts; the confining pressure frame 31 extends and retracts the screw 316 through the automatic adjuster 313 to evenly transmit force to the rectangular confining pressure head 317 to disperse pressure, and the confining pressure head then evenly transmits pressure to the confining pressure loading plate 318;

[0013] The axial pressure device includes a water inlet valve 36, an axial pressure water pump 37, and an axial pressure flexible bladder 38. The axial pressure water pump 37 applies axial stress to the sample by filling the axial pressure flexible bladder 38 with water. The axial pressure water pump 37 is connected to the water inlet valve 36 via a pipeline. The water inlet valve 36 is connected to the axial pressure flexible bladder 38 via a pipeline that passes through the outer wall of the reactor 11. The axial pressure flexible bladder 38 is an annular water bladder that is in direct contact with the sample and is used to isolate the pressurized water and the sample, creating a closed environment for the sample. It wraps around the wellbore 21 in the reactor 11 and is located on the inner side of the reactor cover.

[0014] The constant temperature device includes a constant temperature water bath 34 and a circulation pipeline 32; the constant temperature box 34 is connected to the constant temperature chamber 12 through the circulation pipeline 32, and the coolant circulates in the constant temperature chamber 12;

[0015] The liquid inlet device includes a liquid inlet valve 33, a pore water pump 35 and a porous sintered plate 310; the pore water pump 35 is connected to the liquid inlet valve 33 via a pipeline, and the liquid inlet valve 33 is connected to the porous sintered plate 310 via a pipeline passing through the base of the reactor 11. The porous sintered plate 310 is closely attached to the bottom of the sample, and the porous sintered plate 310 ensures that deionized water enters the sample evenly from bottom to top;

[0016] The hydrate production system 4 mainly consists of a back pressure valve 41, a gas-liquid separator 42, and a decomposition gas cylinder group 43, which are connected in sequence. The back pressure valve 41 is connected to the wellbore 21 via a pipeline. The gas-liquid separator 42 is divided into three parts: the middle part performs gas separation, the lower part performs liquid collection, and the upper part is connected to the decomposition gas cylinder group 43 via a pipeline.

[0017] The monitoring system 5 is mainly composed of a computer 51, a flow monitor 52, a gas monitoring probe 53, a gas flow meter 54, a strain gauge 55, a pressure sensor 56, a high-density conductivity line measuring point device 57, and an insulating coating 58. The monitoring system includes three parts to achieve three functions: one is to monitor changes in reservoir physical properties; the other is to monitor gas leakage; and the third is to monitor changes in wellbore stress.

[0018] The purpose of monitoring reservoir physical property changes is to analyze the influence of gas leakage or wellbore instability on physical property changes, and includes a high-density conductivity line measuring point device 57, an insulating coating 58, and a computer 51. The matrix-type high-density conductivity line measuring point device 57 is located on the inner wall of the confining pressure loading plate 318. The device is coated with an insulating coating 58, and the computer 51 converts the conductivity changes during the experiment into corresponding physical property changes.

[0019] The gas leakage monitoring system is used to monitor gas leakage along the well wall and gas leakage into the cement sheath in real time, and includes gas monitoring probes 53, a gas flow meter 54, a flow monitor 52, and a computer 51. One group of gas monitoring probes 53 is equidistantly arranged on the outer wall of the wellbore 21 outside the top of the reactor 11, and another group is equidistantly arranged on the outer wall of the wellbore casing 22 below the center of the sample. The two groups of gas monitoring probes 53 are connected to the gas flow meter 54 via wires, the gas flow meter 54 is connected to the flow monitor 52 via wires, and the flow monitor is connected to the computer 51 via wires.

[0020] The function of monitoring the stress changes in the wellbore is to monitor the stress changes in the wellbore in real time during hydrate extraction and analyze the stability of the wellbore, and includes a strain gauge 55, a pressure sensor 56 and a computer 51; the strain gauge 55 and the pressure sensor 56 are located on the outer wall of the wellbore casing 22 below the center of the sample, and are used to monitor the strain and stress around the wellbore respectively. All signals are collected by connecting the computer 51 through wires.

[0021] Reactor system 1 is used for hydrate generation and production, and to monitor physical property changes, wellbore instability, and gas leakage. The reactor is connected to all other systems. Well system 2 is the main device used to study wellbore instability and gas leakage. The well system is buried deeper than reactor system 1 and connected to hydrate generation system 3. Hydrate generation system 3 controls ground stress by adjusting the confining pressure frame and axial pressure water pump, and controls hydrate production by adjusting the pore water pump. The hydrate generation system uses gas saturation to uniformly generate hydrates within reactor system 1. Hydrate production system 4 achieves varying degrees of hydrate production by adjusting the backpressure valve. The hydrate production system is connected to well system 2 and uses a simulated depressurization method to produce hydrates within reactor system 1. Monitoring system 5 monitors reservoir physical property changes, gas leakage, and wellbore stability. The monitoring system uses high-density conductivity coils laid within reactor system 1 to determine the gas and water distribution in the reservoir and calculate reservoir physical parameters. The monitoring system uses gas monitoring probes and stress strain gauges laid within well system 2 to monitor gas leakage and wellbore stability.

[0022] The effects and benefits of this invention are as follows: This device enables real-time monitoring of wellbore instability and gas leakage during hydrate production. Compared to conventional research devices, this device considers the fluid-solid coupling process during production. Specifically, under the influence of hydrate phase transitions and sediments, deformation of the sediment reservoir causes compression and shear damage to the wellbore and cement sheath, leading to gas leakage along the deformed wellbore. This can cause debonding of the cement sheath and corrosion of the wellbore, accelerating wellbore and cement sheath instability and further hindering gas and water production. This device provides valuable guidance for laboratory studies of the fluid-solid coupling between sediments and wellbores during hydrate production. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a schematic diagram of a device for studying wellbore instability and gas leakage during hydrate extraction.

[0024] Figure 2 It is a schematic diagram of the structure of a device for studying wellbore instability and gas leakage during hydrate mining.

[0025] Figure 3 It is a cross-sectional view of the reactor of a device for studying wellbore instability and gas leakage during hydrate extraction.

[0026] Figure 4 It is a vertical well profile diagram of a device for studying wellbore instability and gas leakage during hydrate extraction.

[0027] Figure 5 It is a schematic diagram of a single confining frame structure of a device for studying wellbore instability and gas leakage during hydrate extraction.

[0028] Figure 6It is a three-dimensional diagram of the overall confining frame structure of the device for studying wellbore instability and gas leakage during hydrate extraction.

[0029] Figure 7 It is a top view of the overall confining frame structure of the device for studying wellbore instability and gas leakage during hydrate extraction.

[0030] Figure 8 It is a schematic diagram of the high-density conductivity line point position of a device for studying wellbore instability and gas leakage during hydrate extraction.

[0031] In the figure: 1 reactor system; 2 well system; 3 hydrate generation system; 4 hydrate extraction system; 5 monitoring system; 11 reactor; 12 constant temperature chamber; 21 wellbore; 22 casing; 23 cement ring; 24 annular visual window; 25 gas collection device; 31 confining pressure frame; 311 confining pressure frame upper end face; 312 guide shaft; 313 automatic regulator; 314 linear bearing; 315 confining pressure frame lower end face; 316 lead screw; 317 confining pressure head; 318 confining pressure loading Plate; 319 bracket; 32 circulation pipeline; 33 liquid inlet valve; 34 constant temperature water bath; 35 pore water pump; 36 water inlet valve; 37 axial pressure water pump; 38 axial pressure flexible bag; 39 rubber membrane; 310 porous sintered plate; 41 back pressure valve; 42 gas-liquid separator; 43 decomposition gas cylinder group; 51 computer; 52 flow monitor; 53 gas monitoring probe; 54 gas flow meter; 55 strain gauge; 56 pressure sensor; 57 high-density conductivity line point; 58 insulation coating. DETAILED DESCRIPTION

[0032] The specific implementation of the present invention is described in detail below in conjunction with the technical solutions and drawings.

[0033] Figure 1 Shown is a schematic diagram of a device for studying wellbore instability and gas leakage during hydrate extraction. The device is divided into five systems: a reactor system, a well system, a hydrate generation system, a hydrate extraction system, and a monitoring system. The reactor system is where hydrates are generated and pre-buried for extraction. The well system is used to study the impact of hydrate decomposition on the wellbore during hydrate extraction. The hydrate generation system first controls ground stress using an axial pressure pump and a confining pressure frame, and the ambient temperature using a thermostat, and then controls the amount of hydrate generated using a pore water pump. The hydrate extraction system acts on the well system, using a backpressure valve to control the degree of pressure reduction during extraction. The monitoring system acts on the inner wall of the confining pressure loading plate and the outer wall of the casing, monitoring changes in reservoir properties such as porosity, thermal conductivity, and latent heat, as well as monitoring gas leakage and stress strain in the wellbore, and compiling the generated data into a computer.

[0034] Figure 2The figure shows a schematic structural diagram of a device for studying wellbore instability and gas leakage during hydrate extraction. The device is divided into five systems: a reactor system 1, a well system 2, a hydrate generation system 3, a hydrate extraction system 4 and a monitoring system 5; the reactor 11 is an important component of the reactor system 1, and the generation, extraction and monitoring of hydrates are all completed in the reactor 11; the wellbore 21 is an important component of the well system 2, and the wellbore 21 is the main structure for studying gas leakage and wellbore stability; the hydrate generation system 3 is composed of a confining pressure frame 31, a circulation pipeline 32, a liquid inlet valve 33, a constant temperature water bath 34, a pore water pump 35, a water inlet valve 36 and an axial pressure water pump 37; the confining pressure frame 31 is used to apply confining pressure to the sample, the circulation pipeline 32 and the constant temperature water bath 34 are used to control the temperature range of the sample, the liquid inlet valve 33 and the pore water pump 35 are used to control the amount of hydrate generated, and the water inlet valve 36 and the axial pressure water pump 37 are used to apply axial pressure to the sample;

[0035] Figure 3 A cross-sectional view of a reactor used to study wellbore instability and gas leakage during hydrate extraction. Reactor 11 is constructed from a pressure-resistant stainless steel inner shell and a stainless steel outer shell, with the center of the shell left hollow. Reactor 11 is a cube, housing a cube-shaped sample. The remaining cavity, excluding the sample, serves as a constant-temperature chamber 12. An axially compressed flexible bladder 38 is positioned on top of the sample. The annular axially compressed flexible bladder is sheathed around the outside of the wellbore, and vaseline is applied to the contact area between the two. When pressurized, the two automatically adhere tightly together. The entire sample is encapsulated with a rubber membrane 39. A porous sintered plate 310 is located at the bottom of reactor 11.

[0036] Figure 4 The invention discloses a vertical well cross-section diagram of a device for studying wellbore instability and gas leakage during hydrate extraction. The well system 2 mainly consists of a wellbore 21, a casing 22, a cement ring 23, an annular visual window 24 and a gas collecting device 25. The wellbore 21 is placed vertically and is divided into three parts from top to bottom: the first part is outside the reactor 11, where the wellbore 21 is composed of a casing 22, a gas monitoring probe 53 is provided on the outer wall of the casing, and the gas monitoring probe is connected to a gas flow meter 54 through a wire, and a sealed annular visual window 24 is provided on the outer side of the casing, and the annular visual window is connected to the gas collecting device 25 through a pipeline; the second part is located at the upper center of the sample, where the wellbore 21 is composed of a casing 22 and a cement ring 23 from the inside out; the third part is located at the lower center of the sample, where the wellbore 21 is composed of a casing 22 and a cement ring 23, and a gas monitoring probe 53, a strain gauge 55 and a pressure sensor 54 are provided on the outer wall of the casing, the gas monitoring probe is connected to the gas flow meter 54 through a wire, and the strain gauge and the pressure sensor are connected to a computer through a wire;

[0037] Figure 5The figure shows a schematic diagram of the structure of a single confining pressure frame of a device for studying wellbore instability and gas leakage during hydrate extraction; the single confining pressure frame of the device is composed of an upper end face 311 of the confining pressure frame, a guide shaft 312, an automatic adjuster 313, a linear bearing 314, a lower end face 315 of the confining pressure frame, a screw 316 and an axial pressure head 317; support legs are set at the bottom of the upper end face 311 and the lower end face 315 of the confining pressure frame, so that the confining pressure frame does not bear downward force but only bears inward compression force; the side of the confining pressure lower end face 315 is connected by a bracket 319 to form an internal force self-balancing frame, which is used to prevent torque from increasing and bending when axial stress is applied to the confining pressure loading plate 318; the automatic adjuster 313 extends and retracts the screw 316 under the action of the guide shaft 312, the screw is connected to the axial pressure head 317, and the axial pressure head is connected to the confining pressure loading plate 318 through bolts;

[0038] Figure 6 and Figure 7 The figures are a three-dimensional view and a top view of the overall confining frame structure of a device for studying wellbore instability and gas leakage during hydrate extraction. The figures are used to illustrate that the bracket 319 is welded to the lower end surface 315 of the confining frame and to illustrate the structure of the internal force self-balancing frame as a whole.

[0039] Figure 8 This is a schematic diagram of the location of high-density conductivity line points for studying wellbore instability and gas leakage during hydrate extraction. On the inner side of the confining pressure loading plate 318, high-density conductivity line points 57 are laid in a matrix and evenly coated with an insulating coating.

[0040] The specific implementation steps are as follows:

[0041] 1) Prefabricate the closed sediment system and pre-bury the production well process: First, insert the complete rubber membrane 39 with the bottom sealed into the reactor, and then place the wellbore 21 at the center of the reactor 11; fill the porous medium into the rubber membrane 39 and compact it to create a reservoir, so that the reservoir height is higher than the buried depth of the gas monitoring probe 53 and the strain gauge 55; then fill the reactor 11 with a porous medium of another material and compact it until the entire remaining reactor space is filled; then turn the excess rubber membrane 39 on the upper part of the reactor outside the reactor 11, insert the annular axial pressure flexible water bag 38 into the outside of the wellbore 21, close the reactor cover, and connect all pipelines;

[0042] 2) Natural gas hydrate formation process: First, the pore water pump 35 and the liquid inlet valve 33 are opened in sequence, and deionized water saturated with natural gas is injected into the porous sintered plate 310 through the reactor base. The deionized water saturated with natural gas will evenly penetrate into the reservoir through the porous sintered plate 310. Then, the axial pressure water pump 37 and the water inlet valve 36 are opened in sequence to inject water into the axial pressure flexible bag 38, and the axial pressure is controlled within the pressure range specified by the experiment. Then, the confining pressure frame 31 is used to slowly move the confining pressure loading plate 318 to apply uniform force to the sample and control the confining pressure within the pressure range specified by the experiment. The constant temperature water bath 34 is adjusted to steadily reduce the temperature to below the hydrate phase equilibrium temperature and control it within the temperature range specified by the experiment. Finally, when the volume of the pore water pump 35 no longer changes significantly, it indicates that the sample has reached water saturation, and the liquid inlet valve 33 and the pore water pump 35 are closed in sequence.

[0043] 3) Preliminary Monitoring Process: After hydrate formation, turn on the flow monitor 52 and computer 51 to preliminarily record reservoir physical parameters, wellbore stress and strain, and gas leakage. After a period of stabilization, record the above data again and use the physical parameters, stress and strain, and leakage at this time as standard data. If any of the above data changes dramatically during the stabilization period, the experiment should be abandoned and the cause should be investigated.

[0044] 4) Natural gas hydrate extraction process: First, the backpressure valve 41 is adjusted to reduce the pore pressure in the wellbore 21 and the hydrate reservoir to below the hydrate phase equilibrium pressure, prompting the decomposition of the natural gas hydrate to simulate the hydrate extraction process by pressure reduction. The extracted gas-water mixture is separated by the gas-liquid separator 42. The decomposed gas passes through the upper part of the gas-liquid separator and is stored in the decomposition gas cylinder group 43. The decomposed water is stored in the lower part of the gas-liquid separator due to gravity. Finally, when the decomposed gas storage flow rate reaches zero, the extraction is completed and the backpressure valve 41 is closed.

[0045] 5) Real-time monitoring process: During hydrate production, the gas monitoring probe 53 and gas flow meter 54 at the top of the wellbore are used to monitor the gas leakage along the wellbore in real time. The gas monitoring probe 53 and gas flow meter 54 at the bottom of the wellbore are used to monitor the gas leakage of the cement sheath microannulus in real time, and the flow signal is transmitted to the flow monitor 52. The annular visual window 24 is used to observe the bubbles leaking along the wellbore in real time. The strain gauge 55 is used to monitor the strain of the wellbore in real time, and the pressure sensor 56 is used to monitor the stress changes around the wellbore. The difference between the stress and strain data at this time and the standard stress and strain data represents the force effect of production on the wellbore. The high-density conductivity line point 57 is used to monitor the conductivity change of the reservoir in real time, and the conductivity signal is transmitted to the computer 51 to be converted into physical property changes.

[0046] 6) Data analysis process: Three sets of experimental data are obtained through computer 51: initial data, standard data, and real-time monitoring data. First, the reasons for the differences between the initial physical property changes, wellbore stress and strain, and leakage data and the standard data are analyzed. If the differences are within an acceptable range, it means that the experimental standards for hydrate formation are met in the reactor. Otherwise, the reasons for the erroneous results need to be discussed. If the physical property parameters fluctuate violently, there may be problems with the laying of the high-density conductivity line points or incomplete application of the insulating coating. If the wellbore stress and strain or leakage fluctuate violently, it is very likely that there is a problem with the sealing of the hydrate formation system, resulting in premature decomposition of hydrates. When hydrate formation meets the experimental standards, the differences between the standard data and the real-time monitoring data are analyzed. The differences at this time respectively represent: physical property changes, wellbore stress and strain, and gas leakage during hydrate production.

Claims

1. A device for studying wellbore instability and gas leakage during hydrate extraction, characterized in that: The device for studying wellbore instability and gas leakage during hydrate production includes a reactor system (1), a well system (2), a hydrate generation system (3), a hydrate production system (4), and a monitoring system (5); The reactor system (1) includes a reactor (11) and a constant temperature chamber (12); the reactor (11) is made of a pressure-resistant stainless steel inner shell and a stainless steel outer shell, and the middle of the shell is left hollow, which is the constant temperature chamber (12); the reactor (11) is a cube, and the interior is a cube cavity; the top of the reactor (11) is a square upper cover, and a well hole is left in the middle of the upper cover; through holes are provided at the four corners of the four side walls of the reactor (11), and the through holes are transparent and smooth, and the guide shaft (312) of the confining pressure device passes through the through holes to connect to the confining pressure loading plate (318); a threaded hole is provided in the center of each side wall of the reactor (11), and the threaded hole is transparent and has a thread inside, and the lead screw (316) of the confining pressure device passes through the threaded hole to connect to the confining pressure head (317); the sample wrapped by the rubber membrane (39) is placed in the cube cavity of the reactor; The well system (2) includes a wellbore (21), an annular visual window (24) and a gas collecting device (25); the wellbore (21) passes through the upper cover of the reactor (11) and extends to the lower part of the center of the sample. The wellbore (21) is divided into three parts from top to bottom: the first part is outside the reactor (11), where the wellbore (21) includes a casing (22), the outer wall of the casing (22) is provided with a gas monitoring probe (53), and a sealed annular visual window (24) is provided on the outer side of the casing (22), the annular visual window (24) is used to observe the change of bubbles leaking along the well in real time, and is connected to the gas collecting device (25) through a pipeline; the second part is above the center of the sample, where the wellbore (21) is composed of the casing (22) and the cement ring (23) from the inside to the outside; the third part is below the center of the sample, where the wellbore (21) includes the casing (22) and the cement ring (23), and the outer wall of the casing is provided with a gas monitoring probe (53) and a strain gauge (55); The hydrate formation system (3) includes a confining pressure device, an axial pressure device, a constant temperature device and a liquid inlet device; The confining pressure device includes a confining pressure frame (31) and a confining pressure loading plate (318); the confining pressure loading plate (318) is a contact plate for applying uniform stress to the sample, and the confining pressure loading plate (318) is tightly fitted to the side wall of the sample; the confining pressure frame (31) applies uniform stress to the confining pressure loading plate (318), and includes an upper end face (311) of the confining pressure frame, a guide shaft (312), an automatic regulator (313), a linear bearing (314), a lower end face (315) of the confining pressure frame, a lead screw (316), a rectangular confining pressure head (317) and a bracket (319); the confining pressure frame (31) is placed horizontally and is arranged on the outer side of the outer wall of the reactor (11); through holes are provided at the corners of the upper end face (311) and the lower end face (315) of the confining pressure frame, and the guide shaft (312) passes through the through hole provided with the linear bearing (314) and is fixed to the confining pressure loading plate (318) by bolts. The guide shaft (312) ensures that the rectangular confining pressure head (317) is perpendicular to the confining pressure loading plate (318) to apply uniform stress to the specimen; the left and right sides of the lower end surface (315) of the confining pressure frame are respectively welded to the adjacent lower end surface of the confining pressure frame through the bracket (319) to form an internal force self-balancing frame to prevent excessive torque and bending when stress is applied to the confining pressure loading plate (318); the automatic regulator (313) passes through the lower end surface (315) of the confining pressure frame to connect the screw (316), the screw (316) is welded to the center of the rectangular confining pressure head (317), and the rectangular confining pressure head (317) and the confining pressure loading plate (318) are connected by bolts; the screw (316) is extended and retracted by the automatic regulator (313) to evenly transmit force to the rectangular confining pressure head (317) and distribute pressure, and the confining pressure head then evenly transmits pressure to the confining pressure loading plate (318); The axial pressure device includes a water inlet valve (36), an axial pressure water pump (37) and an axial pressure flexible bag (38); the axial pressure water pump (37) applies axial stress to the sample by filling water into the axial pressure flexible bag (38); the axial pressure water pump (37) is connected to the water inlet valve (36) through a pipeline, and the water inlet valve (36) is connected to the axial pressure flexible bag (38) through the outer wall of the reactor (11) through a pipeline; the axial pressure flexible bag (38) is an annular water bag, which is in direct contact with the sample and is used to isolate the pressurized water and the sample, creating a closed environment for the sample. The axial pressure flexible bag (38) wraps the wellbore (21) in the reactor (11) and is located on the inner side of the reactor cover; The constant temperature device includes a constant temperature water bath (34) and a circulation pipeline (32); the constant temperature water bath (34) is connected to the constant temperature chamber (12) through the circulation pipeline (32), and a coolant circulates in the constant temperature chamber (12); The liquid inlet device includes a liquid inlet valve (33), a pore water pump (35) and a porous sintered plate (310); the pore water pump (35) is connected to the liquid inlet valve (33) through a pipeline, and the liquid inlet valve (33) is connected to the porous sintered plate (310) through the base of the reactor (11) through the pipeline, and the porous sintered plate (310) is closely attached to the bottom of the sample. The porous sintered plate (310) ensures that deionized water enters the sample evenly from bottom to top; The hydrate production system (4) includes a back pressure valve (41), a gas-liquid separator (42), and a decomposition gas cylinder group (43) connected in sequence; the back pressure valve (41) is connected to the wellbore (21) through a pipeline; the gas-liquid separator (42) is divided into three parts: upper, middle, and lower. The middle part realizes the gas separation function, the lower part realizes the liquid collection function, and the upper part is connected to the decomposition gas cylinder group (43) through a pipeline. The monitoring system (5) includes a computer (51), a flow monitor (52), a gas monitoring probe (53), a gas flow meter (54), a strain gauge (55), a pressure sensor (56), a high-density conductivity line point (57) and an insulating coating (58); the monitoring system has three functions: one is to monitor the change of reservoir physical properties; one is to monitor gas leakage; and the third is to monitor the change of wellbore stress; The function of monitoring reservoir physical property changes is to analyze the influence of gas leakage or wellbore instability on physical property changes, and the corresponding function includes high-density conductivity line points (57) and insulating coating (58); the matrix-type high-density conductivity line points (57) are located on the inner wall of the confining pressure loading plate (318), and the device is coated with insulating coating (58), and the conductivity changes during the experiment are converted into corresponding physical property changes through the computer (51); The function of monitoring gas leakage is to monitor gas leakage along the well wall in real time, and the function corresponds to the gas monitoring probes (53), the gas flow meter (54) and the flow monitor (52); one group of gas monitoring probes (53) is equidistantly arranged on the outer wall of the wellbore (21) outside the top of the reactor (11), and the other group is equidistantly arranged on the outer wall of the wellbore casing (22) below the center of the sample; the two groups of gas monitoring probes (53) are respectively connected to the gas flow meter (54) through a wire, the gas flow meter (54) is connected to the flow monitor (52) through a wire, and the flow monitor is connected to the computer (51) through a wire; The function of monitoring the stress changes of the wellbore is to monitor the stress changes of the wellbore in real time during the hydrate extraction process and analyze the stability of the wellbore. The function corresponds to the strain gauge (55) and the pressure sensor (56); the strain gauge (55) and the pressure sensor (56) are located on the outer wall of the wellbore casing (22) below the center of the sample, and are used to monitor the strain and stress around the wellbore respectively. All signals are collected by connecting the computer (51) through wires.

2. A method for studying wellbore instability and gas leakage during hydrate production using the device according to claim 1, characterized in that: The steps include: 1) Prefabrication of a closed sediment system and pre-buried mining well process: First, insert the rubber membrane (39) into the reactor (11), and then place the wellbore (21) at the center of the reactor (11); fill the porous medium into the rubber membrane (39) and compact it to make a reservoir, so that the reservoir height is higher than the buried depth of the gas monitoring probe (53) and the strain gauge (55); then fill the porous medium of another material into the reactor (11) and compact it until the entire remaining reactor space is filled; then turn the excess rubber membrane (39) on the upper part of the reactor outside the reactor (11), insert the annular axial pressure flexible bag (38) into the outside of the wellbore (21), close the reactor cover, and connect all pipelines; 2) Natural gas hydrate formation process: First, the pore water pump (35) and the liquid inlet valve (33) are opened in sequence, and deionized water saturated with natural gas is injected into the porous sintered plate (310) through the reactor base. The deionized water saturated with natural gas will evenly penetrate into the reservoir through the porous sintered plate (310); then, the axial pressure water pump (37) and the water inlet valve (36) are opened in sequence, and water is injected into the axial pressure flexible bag (38), and the axial pressure is controlled within the pressure range specified by the experiment; then, the confining pressure loading plate (318) is slowly moved using the confining pressure frame (31) to apply uniform force to the sample and control the confining pressure within the pressure range specified by the experiment; the constant temperature water bath (34) is adjusted to steadily reduce the temperature to below the hydrate phase equilibrium temperature and control it within the temperature range specified by the experiment; finally, when the volume of the pore water pump (35) no longer changes significantly, it indicates that the sample has reached water saturation, and the liquid inlet valve (33) and the pore water pump (35) are closed in sequence; 3) Preliminary monitoring process: After hydrate formation, turn on the flow monitor (52) and computer (51) to preliminarily record the reservoir physical parameters, stress and strain of the wellbore, and gas leakage; then let it stand for a while, and after the data stabilizes, record the above data again, and use the physical parameters, stress and strain, and leakage at this time as standard data; if any of the above data changes dramatically during the standing time, abandon this group of experiments and investigate the cause; 4) Natural gas hydrate production process: First, the back pressure valve (41) is adjusted to reduce the pore pressure of the wellbore (21) and the hydrate reservoir to below the hydrate phase equilibrium pressure, thereby promoting the decomposition of the natural gas hydrate to simulate the hydrate decompression production process; the produced gas-water mixture is separated by the gas-liquid separator (42), and the decomposed gas is stored in the decomposition gas cylinder group (43) through the upper part of the gas-liquid separator, and the decomposed water is stored in the lower part of the gas-liquid separator (42) due to gravity; finally, when the decomposition gas storage flow rate is zero, it indicates that the production is completed, and the back pressure valve (41) is closed; 5) Real-time monitoring process: During the hydrate production process, the gas leakage along the well is monitored in real time by using the gas monitoring probe (53) and the gas flow meter (54) at the top of the wellbore, and the gas leakage of the cement ring micro-annulus is monitored in real time by using the gas monitoring probe (53) and the gas flow meter (54) at the bottom of the wellbore, and the flow signal is transmitted to the flow monitor (52); the bubbles leaking along the well are observed in real time by using the annular visual window (24); the strain of the wellbore is monitored in real time by using the strain gauge (55), and the stress change around the wellbore is monitored by the pressure sensor (56); the difference between the stress and strain data at this time and the standard stress and strain data represents the force effect of the production on the wellbore; the conductivity change of the reservoir is monitored in real time by using the high-density conductivity line point (57), and the conductivity signal is transmitted to the computer (51) to be converted into physical property change; 6) Data analysis process: three sets of experimental data are obtained through the computer (51): initial data, standard data, and real-time monitoring data; first, the reasons for the differences between the initial physical property changes, wellbore stress and strain, and leakage data and the standard data are analyzed: if the differences are within an acceptable range, it means that the reactor (11) meets the experimental standards for hydrate formation, otherwise it is necessary to discuss the reasons for the erroneous results; when the hydrate formation meets the experimental standards, the difference between the standard data and the real-time monitoring data is analyzed again. The difference at this time is caused by the physical property changes, wellbore stress and strain, and gas leakage during the hydrate production process.

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