Pressurized test device and method for in-situ mining natural gas hydrates by jets

The present invention discloses a pressurized test device and method for in-situ mining natural gas hydrates by jets, relating to the field of exploitation of marine natural gas hydrates. The device comprises an injection system, a jet breakup system, an annular pressure system, an axial pressure system, a backpressure system, a vacuum system, a simulation system, a collecting and processing system and a metering system, all of which can operate independently by controlling pipe valves on pipelines. The loading of the confining pressure of the device is independent of the loading of the axial pressure, without interference to each other. Meanwhile, the jet breakup process of natural gas hydrate-containing sediments can be observed in real time by a video camera.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority from Chinese Patent Application No. CN 201910052889.0, filed on Jan. 21, 2019. The content of the aforementioned application, including any intervening amendments thereto, is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates to the field of exploitation of marine natural gas hydrates and in particular to a pressurized test device and method for in-situ mining natural gas hydrates by jets.

BACKGROUND OF THE PRESENT INVENTION

Natural gas hydrates (NGH), as a white solid crystalline substance formed by the interaction of small-molecular gases such as light hydrocarbons, carbon dioxide and hydrogen sulfide with water under certain conditions, have advantages of high energy density, formation under certain conditions, great reserves, and vast area in distribution. The total amount of organic carbon in natural gas hydrates worldwide is twice the explored conventional fossil organic carbon reserves. Due to the huge potential of natural gas hydrates, there has been a surge of research on prospection, trial production, and exploration of natural gas hydrates worldwide in recent decades. A long-term research plan for natural gas hydrates has been made in the United States, Japan, Canada, India, and South Korea and other countries. How to safely, efficiently and environmentally mine natural gas hydrate resources has become the advanced subject and focus of countries around the world.

At present, conventional natural gas mining methods include thermal excitation mining, pressure relief mining, chemical reagent injection mining and CO2displacement mining. Compared with conventional oil and gas reservoirs, marine natural gas hydrate reservoirs have characteristics of shallow buried depth, non-diagenesis and low permeability. Therefore, if the marine natural gas hydrates are mined simply by the four mining methods, there are great limitations: the pressure relief mining may cause the secondary formation of natural gas hydrates or the formation of ice, thereby blocking the permeation path and being disadvantageous for long-term mining; the thermal excitation mining has low heat utilization and heating only in a small range can be performed; the chemical reagent injection mining has disadvantages of expensive chemical reagents, slow effect to the natural gas hydrate reservoirs, and environmental pollution; and the CO2displacement mining has a long mining cycle and requires the natural gas hydrate reservoirs to be highly permeable.

Breakup by erosion using water jets is a new method for mining marine natural gas hydrates. For mining by water jets, since the natural gas hydrate reservoirs have a mechanical strength lower than deep-sea oil and gas reservoirs and have a shallow occurrence depth, breakup can be realized without huge energy input to obtain natural gas hydrate particles. In addition, the mining by water jets does not need to decompose natural gas hydrates in the reservoirs through pressure or temperature transfer, without any requirement on the heat transfer and pressure transfer channels, thus only low requirement on permeability. Meanwhile, the mining by water jets is not affected by the change in temperature and pressure conditions, which is caused by the decomposition of natural gas hydrates to generate secondary natural gas hydrates that hinder the reaction. This mining method is more broadly applicable than other methods, and is considered to be promising. Therefore, it is of great significance to study the principles and rules of the breakup of natural gas hydrates by jets.

The marine natural gas hydrate-containing sediments exist in a high-pressure environment. When experiments for breakup of natural gas hydrates by water jets are conducted, in order to simulate the in-situ marine environment, it is of great importance to ensure the confining pressure and axial pressure loading to the natural gas hydrate-containing sediments. Failure to meet the actual high-pressure occurrence conditions for natural gas hydrate-containing sediments during the breakup experiments by jets will lead to inaccurate principles and rules of the breakup of natural gas hydrate-containing sediments by jets.

SUMMARY OF THE PRESENT INVENTION

An objective of the present invention is to provide a pressurized test device for in-situ mining natural gas hydrates by jets. A pipe valve is arranged in each of systems included in the device and the systems can operate independently, leading to high safety.

Another objective of the present invention is to provide a pressurized test method for in-situ mining natural gas hydrates by jets.

The pressurized test device for in-situ mining natural gas hydrates by jets in the present invention comprises an injection system, a jet breakup system, an annular pressure system, an axial pressure system, a backpressure system, a vacuum system, a simulation system, a collecting and processing system and a metering system;

the injection system, the axial pressure system and the vacuum system are connected to a gas inlet of a three-way valve by a gas intake pipe and two gas outlets of the three-way valve are communicated with the simulation system respectively by a gas injection pipe and an axial pressure pipe, and a pressure sensor I and a pipe valve I are arranged on the gas injection pipe; the injection system is configured to inject, into the simulation system, methane gas that is used for synthesis of natural gas hydrates, and pressurize the methane gas to a pressure desired by synthesis of natural gas hydrates; the injection system comprises a methane gas cylinder, a pressure relief valve, a pipe valve II, a pressure regulating valve I, a booster pump, an air compressor, a cushion container, a gas flow control meter, a check valve and a pipe valve III; the methane gas cylinder is connected to the gas intake pipe by a first pipeline on which the pressure relief valve, the pipe valve II, the pressure regulating valve I, the gas flow control meter, the check valve and the pipe valve III are successively arranged; a gas intake end of the booster pump is connected to the air compressor by a second pipeline on which a pressure regulating valve II is arranged; a gas discharge end of the booster pump is connected to the first pipeline respectively by a third pipeline and a fourth pipeline; a joint of the third pipeline and the first pipeline is located between the pressure relief valve and the pipe valve II, and a pipe valve IV is arranged on the third pipeline; a joint of the fourth pipeline and the first pipeline is located between the pipe valve II and the pressure regulating valve I, and a pipe valve V and a pipe valve VI are arranged on the fourth pipeline; a pressure gauge is arranged on the cushion container; the cushion container is connected to the fourth pipeline by a fifth pipeline; and a joint of the fifth pipeline and the fourth pipeline is located between the pipe valve V and the pipe valve VI;

the jet breakup system is communicated with the simulation system by a jet pipe; the jet breakup system is configured to jet, to the simulation system, a high-pressure water flow that breaks natural gas hydrate-containing sediments already formed in the simulation system; the jet breakup system comprises a jet pump, a jet pipe, a jet nozzle and a lifting mechanism; the jet pump is connected to the jet pipe, and a pipe valve VII is arranged between the jet pump and the jet pipe; the jet pipe penetrates through the top of a visual test cabin of the simulation system and extends into the visual test cabin; the jet pipe is fixed on the lifting mechanism; and the jet nozzle is mounted at a jetting end of the jet pipe;

the annular pressure system is communicated with an annular pressure hole on the visual test cabin; the annular pressure system is configured to provide, to the simulation system, a confining pressure of in-situ submarine natural gas hydrate-containing sediments; the annular pressure system comprises an annular pressure pump and an annular pressure rubber sleeve; the annular pressure pump is communicated with the annular pressure rubber sleeve by a pipeline on which a pressure sensor II and a pipe valve VIII are arranged; the annular pressure rubber sleeve is arranged in the visual test cabin; a sealing strip and a sealing ring are arranged in a gap between the annular pressure rubber sleeve and a front end cover and a visual window;

the axial pressure system is communicated with the simulation system; the axial pressure system is configured to provide, to the simulation system, an axial pressure of in-situ submarine natural gas hydrate-containing sediments; the axial pressure system comprises a constant-flux pump, an axial pressure passage, an axial pressure loading chamber, a loading shaft and a pressure plate; the constant-flux pump is connected to the gas intake pipe by a sixth pipeline on which a pipe valve IX is arranged; the axial pressure passage is communicated with the axial pressure loading chamber arranged in a rear end cover; one end of the loading shaft is arranged in the axial pressure loading chamber, and the other end of the loading shaft runs through the rear end cover and into a visual test cabin body to be connected to the pressure plate; and a sealing strip and a sealing ring are arranged in a gap between the pressure plate and the annular pressure rubber sleeve, a gap between the loading shaft and the annular pressure rubber sleeve, a gap between the loading shaft and the rear end cover, and a gap between the axial pressure loading chamber and the rear end cover;

the backpressure system comprises a gas guide pipe, a backpressure valve, a backpressure pump and a backpressure cushion container; one end of the gas guide pipe is communicated with the jet pipe, and the other end of the gas guide pipe is communicated with the backpressure cushion container; the backpressure valve, a pipe valve X and a pressure sensor III are arranged on the gas guide pipe; the backpressure cushion container is communicated with the backpressure pump; and a pipe valve XI is arranged between the backpressure cushion container and the backpressure pump;

the vacuum system comprises a vacuum meter, a vacuum container and a vacuum pump; one end of the vacuum container is connected to the vacuum pump, and the other end of the vacuum container is connected to the gas intake pipe by a seventh pipeline on which a pipe valve XII, a safety valve and a pressure sensor IV are successively arranged; and the vacuum meter is arranged on the vacuum container;

the collecting and processing system comprises a pressure sensor I, a pressure sensor II, a pressure sensor III, a pressure sensor IV, a temperature sensor and a control terminal; and the pressure sensor I, the pressure sensor II, the pressure sensor III, the pressure sensor IV and the temperature sensor are all communicatively connected to the control terminal;

the metering system comprises a dryer, a three-phase separator and a micro-gas metering device; the three-phase separator is communicated with the gas guide pipe; a gas discharge end on the top of the three-phase separator is communicated with the micro-gas metering device; and the dryer is arranged between the three-phase separator and the micro-gas metering device; and

the simulation system comprises a thermostat, the visual test cabin, an overturning support and a video camera; the overturning support is arranged on the inner top of the thermostat; the visual test cabin is arranged on the overturning support and comprises the visual test cabin body, the front end cover and the rear end cover, the front end cover is fastened to a front end of the visual test cabin body by a sealing valve, and the visual window is arranged on the front end cover; the rear end cover is fastened to a rear end of the visual test cabin body by the sealing valve; the video camera is arranged outside the visual test cabin and faces the visual window.

The jet breakup system is 20 mm away from the visual window.

The loading shaft has a piston stroke of 30 mm.

There are three temperature sensors, all of which are arranged on a sidewall of the visual test cabin.

The present invention further provides a pressurized test method for in-situ mining natural gas hydrates by jets, using the test device described above, comprising following steps:

step 1: before testing, cleaning and naturally drying a visual test cabin, and preparing, cleaning with deionized water and oven drying quartz sandstone or silty mudstone;

step 2: uniformly mixing the quartz sandstone or silty mudstone prepared in the step 1 with brine, putting the mixture wrapped by an annular pressure rubber sleeve in a visual test cabin, putting the visual test cabin in a thermostat in a sealed state, and injecting water into an axial pressure loading chamber by a constant-flux pump until an axial pressure of in-situ submarine natural gas hydrate-containing sediments to be simulated is reached; injecting water into an annular pressure hole on the visual test cabin by an annular pressure pump until a confining pressure of in-situ submarine natural gas hydrate-containing sediments to be simulated is reached; adjusting the position and jetting distance of a jet nozzle, adjusting a jet pump, and setting a jetting velocity desired by a test;

step 3: feeding air into the thermostat to cool the whole visual test cabin in air bath, and feeding methane gas into the visual test cabin by an injection system, the amount of methane gas being determined by the saturation of the natural gas hydrate-containing sediments; setting the temperature of the air bath to be a temperature desired by natural gas hydrates; and obtaining natural gas hydrate sediment samples at the end of synthesis of natural gas hydrates;

step 4: jet breakup: at the end of synthesis of natural gas hydrates, decreasing the temperature of the air bath to below 242K-271K, discharging the residual methane gas and feeding brine to flood the natural gas hydrate sediment samples; adjusting the pump capacity and pumping time of the annular pressure pump and the constant-flux pump to reach real axial pressure and confining pressure conditions of in-situ submarine natural gas hydrate-containing sediments, and setting the temperature of the air bath to be a reaction temperature set for the test; activating the jet breakup system for a jet breakup test, capturing the jet breakup process by a video camera, and recording the temperature according to the temperature sensor;

step 5: gas metering: as the jetting progresses, discharging the mixture from the gas guide pipe of the visual test cabin into a three-phase separator where gas is separated, and drying the gas by a drying pipe; increasing the temperature of the air bath, decomposing remaining natural gas hydrates in the visual test cabin, and metering the total amount of the decomposed methane gas at the end of decomposition; and

step 6: at the end of the test, taking the visual test cabin out, observing and recording the breakup effect of the natural gas hydrate-containing sediments, and analyzing data.

By the design solution, the present invention can have following beneficial effects. With the use of the pressurized test device and method for in-situ mining natural gas hydrates by jets in the present invention, systems included in the device can operate independently. For safety, a pipe valve is arranged in each of the systems. The axial pressure loading direction is identical to the arrangement direction of the visual window, and the axial pressure loading is performed by dual-seal to avoid leakage. The confining pressure loading direction is perpendicular to the arrangement direction of the visual window, and the confining pressure loading direction is independent of the axial pressure loading, without interference to each other. Meanwhile, the jet breakup process of natural gas hydrate-containing sediments can be observed in real time by a video camera. The real confining pressure and axial pressure conditions and the flooded environment of the marine in-situ natural gas hydrate-containing sediments can be simulated and real and reliable data can be provided, thereby providing theoretical support for the mining of marine natural gas hydrates.

REFERENCE NUMERALS

DETAILED DESCRIPTION OF THE PRESENT INVENTION

To explain the present invention more clearly, the present invention will be further described below with reference to the accompanying drawings by preferred embodiments. It should be understood by those skilled in the art that the content to be specifically described below is illustrative rather than limiting, and is not intended to limit the protection scope of the present invention.

As shown inFIGS. 1 to 6, the pressurized test device for in-situ mining natural gas hydrates by jets in this embodiment comprises an injection system, a jet breakup system, an annular pressure system, an axial pressure system, a backpressure system, a vacuum system, a simulation system, a collecting and processing system and a metering system.

The injection system, the axial pressure system and the vacuum system are connected to a gas inlet of a three-way valve17by a gas intake pipe and two gas outlets of the three-way valve17are communicated with the simulation system respectively by a gas injection pipe34and an axial pressure pipe370, and a pressure sensor I1801and a pipe valve I401are arranged on the gas injection pipe34. The injection system is configured to inject, into the simulation system, methane gas that is used for synthesis of natural gas hydrates, and pressurize the methane gas to a pressure desired by synthesis of natural gas hydrates. The injection system comprises a methane gas cylinder1, a pressure relief valve2, a pipe valve II402, a pressure regulating valve I301, a booster pump5, an air compressor7, a cushion container9, a gas flow meter11, a check valve12and a pipe valve III403. The methane gas cylinder1is connected to the gas intake pipe by a first pipeline on which the pressure relief valve2, the pipe valve II402, the pressure regulating valve I301, the gas flow meter11, the check valve12and the pipe valve III403are successively arranged. A gas intake end of the booster pump5is connected to the air compressor7by a second pipeline on which a pressure regulating valve II302is arranged. A gas discharge end of the booster pump5is connected to the first pipeline respectively by a third pipeline and a fourth pipeline. A joint of the third pipeline and the first pipeline is located between the pressure relief valve2and the pipe valve II404, and a pipe valve IV402is arranged on the third pipeline. A joint of the fourth pipeline and the first pipeline is located between the pipe valve II402and the pressure regulating valve I301, and a pipe valve V406and a pipe valve VI405are arranged on the fourth pipeline. A pressure gauge15is arranged on the cushion container9; the cushion container9is connected to the fourth pipeline by a fifth pipeline. A joint of the fifth pipeline and the fourth pipeline is located between the pipe valve V405and the pipe valve VI406. The methane gas in the methane gas cylinder1has a purity of 99.99% and is used for synthesis of natural gas hydrates. The pressure regulating valve I301is used for regulating the pressure for injection of the methane gas, and the maximum pressure at the inlet is 20 MPa. The check valve12withstands 16 MPa and is configured to prevent the back flow of the methane gas. The methane gas is supplied from the methane gas cylinder2. The output pressure of the methane gas is determined by the relief valve2. At insufficient output pressure, the pressure is regulated by the pressure regulating valve I301, the booster pump5and the cushion container9. When the pressure is well regulated, the methane gas is injected into the simulation system successively by the three-way valve17and the gas injection pipe34.

The jet breakup system is communicated with the simulation system by a jet pipe31, and the jet breakup system is 20 mm away from the visual window43in order to observe the jet breakup process conveniently. The jet breakup system is configured to jet, to the simulation system, a high-pressure water flow that breaks natural gas hydrate-containing sediments already formed in the simulation system. The jet breakup system comprises a jet pump20, a jet pipe31, a jet nozzle32and a lifting mechanism30. The jet pump20is connected to the jet pipe31, and a pipe valve VII407is arranged between the jet pump20and the jet pipe31. The jet pipe31penetrates through the top of a visual test cabin28of the simulation system and extends into the visual test cabin28. The jet pipe31is fixed on the lifting mechanism30. The jet nozzle32is mounted at a jetting end of the jet pipe31. The jet pump20provides a stable and continuous high-pressure water flow having a maximum pressure of 50 MPa and a velocity of 100 m/s to break the natural gas hydrate-containing sediments. Different jet nozzles32are configured to simulate the influence of different diameters and shapes on the breakup effect of the natural gas hydrate-containing sediments. The lifting mechanism30is configured to adjust the distance between the jet nozzles32and the natural gas hydrate-containing sediments.

The annular pressure system is communicated with an annular pressure hole38on the visual test cabin28. The annular pressure system is configured to provide, to the simulation system, a confining pressure of in-situ submarine natural gas hydrate-containing sediments. The annular pressure system comprises an annular pressure pump27and an annular pressure rubber sleeve33. The annular pressure pump27is communicated with the annular pressure rubber sleeve33by a pipeline on which a pressure sensor II1802and a pipe valve VIII408are arranged. The annular pressure rubber sleeve33is arranged in the visual test cabin28. A sealing strip40and a sealing ring41are arranged in a gap between the annular pressure rubber sleeve33and a front end cover46and a visual window43. Dual-seal is formed by the sealing strip40and the sealing ring41so that the whole annular pressure system is in a sealed state, without any gas leakage. The annular pressure pump27has a maximum operating pressure of 30 MPa. An annular pressure is loaded to the natural gas hydrate-containing sediments by the annular pressure pump27to simulate the real natural gas hydrate-containing sediments, in order to faithfully reflect the situation of the natural gas hydrate-containing sediments having a confining pressure.

The axial pressure system is communicated with the simulation system. The axial pressure system is configured to provide, to the simulation system, an axial pressure of in-situ submarine natural gas hydrate-containing sediments. The axial pressure system comprises a constant-flux pump13, an axial pressure passage37, an axial pressure loading chamber35, a loading shaft39and a pressure plate42. The constant-flux pump13is connected to the gas intake pipe by a sixth pipeline on which a pipe valve IX409is arranged. The axial pressure passage37is communicated with the axial pressure loading chamber35arranged in a rear end cover36. One end of the loading shaft39is arranged in the axial pressure loading chamber35, and the other end of the loading shaft39runs through the rear end cover36and into a visual test cabin body to be connected to the pressure plate42. The loading shaft39has a piston stroke of 30 mm. The axial pressure system and the annular pressure system are independent of each other and can give a pressure separately to faithfully simulate the tri-axial pressure state of the marine natural gas hydrate-containing sediments. A sealing strip40and a sealing ring41are arranged in a gap between the pressure plate42and the annular pressure rubber sleeve33, a gap between the loading shaft39and the annular pressure rubber sleeve33, a gap between the loading shaft39and the rear end cover36, and a gap between the axial pressure loading chamber35and the rear end cover36. Dual-seal is formed by the sealing strip40and the sealing ring41so that the whole axial pressure system is in a sealed state. The constant-flux pump13has a maximum operating pressure of 50 MPa. A pressure is provided by the constant-flux pump13to simulate the axial pressure state of the natural gas hydrate-containing sediments.

The backpressure system comprises a gas guide pipe, a backpressure valve21, a backpressure pump22and a backpressure cushion container10. One end of the gas guide pipe is communicated with the jet pipe31, and the other end of the gas guide pipe is communicated with the backpressure cushion container10. The backpressure valve21, a pipe valve X410and a pressure sensor III1803are arranged on the gas guide pipe. The backpressure cushion container10is communicated with the backpressure pump22. A pipe valve XI411is arranged between the backpressure cushion container10and the backpressure pump22, and the backpressure pump22has an operating pressure of 0 MPa to 50 MPa. Due to high pressure, high discharge rate and great pressure fluctuation in the visual test cabin28, the use of the backpressure pump can ensure steady fluid so that it is convenient to conduct the test.

The vacuum system comprises a vacuum meter14, a vacuum container16and a vacuum pump8. One end of the vacuum container16is connected to the vacuum pump8, and the other end of the vacuum container16is connected to the gas intake pipe by a seventh pipeline on which a pipe valve XII412, a safety valve19and a pressure sensor IV1804are successively arranged. The vacuum meter14is arranged on the vacuum container16. The vacuum pump8has a degree of vacuum of 0.1 Pa. The vacuum container16is configured to store gas pumped from the simulation system. The vacuum meter14is configured to indicate the storage amount of gas. At the end of the synthesis of the natural gas hydrate-containing sediments, the visual test cabin28is brought into vacuum, in order to ensure the accuracy of the test.

The collecting and processing system comprises a pressure sensor I1801, a pressure sensor II1802, a pressure sensor III1803, a pressure sensor IV1804, a temperature sensor29and a control terminal6. The pressure sensor I1801, the pressure sensor II1802, the pressure sensor III1803, the pressure sensor IV1804and the temperature sensor29are all communicatively connected to the control terminal6. The collected pressure and temperature data is transmitted to the control terminal6to be processed. The pressure sensor I1801, the pressure sensor II1802, the pressure sensor III1803and the pressure sensor IV1804can measure a maximum pressure of 25 MPa, at a precision of 0.1%. There are three temperature sensors29, all of which are arranged on a sidewall of the visual test cabin28. The temperature sensor29is configured to measure the temperature in the visual test cabin28during the breakup process of the natural gas hydrate-containing sediments. The temperature sensor29can measure the temperature between −20° C. and 100° C.

The metering system comprises a dryer23, a three-phase separator24and a micro-gas metering device26. The three-phase separator24is communicated with the gas guide pipe. A gas discharge end on the top of the three-phase separator24is communicated with the micro-gas metering device26. The dryer23is arranged between the three-phase separator24and the micro-gas metering device26.

The mixture from the gas guide pipe is separated by the three-phase separator24into gas, liquid and solid. The gas is discharged from the top of the three-phase separator24, dried by the dryer23and then passed to the micro-gas metering device26. The micro-gas metering device26is configured to collect gas produced during the breakup process of the natural gas hydrate-containing sediments by water jets, in order to collect and meter the gas.

The simulation system comprises a thermostat25, the visual test cabin28, an overturning support45and a video camera47. The overturning support45is arranged on the inner top of the thermostat25. The visual test cabin28is arranged on the overturning support45and comprises the visual test cabin body, the front end cover46and the rear end cover36, the front end cover46is fastened to a front end of the visual test cabin body by a sealing valve44, and the visual window43is arranged on the front end cover46. The rear end cover36is fastened to a rear end of the visual test cabin body by the sealing valve44. The thermostat25is configured to keep a constant temperature during the synthesis of natural gas hydrates. The visual test cabin28can withstand a pressure 0 MPa to 50 MPa, and is 3000 mm×3000 mm×400 mm in size. The synthesized natural gas hydrate sediment samples are 100 mm×100 mm×150 mm in size. The visual window43is a visual window made of sapphire, which has high strength, and by which the breakup process of the natural gas hydrate-containing sediments is observed and the moment of breakup is captured by the video camera47arranged in opposite to the visual window43. A sealing strip40and a sealing ring41are arranged at the joint of the axial pressure loading chamber35, the front end cover46and the rear end cover36, in order to seal against the outer wall and avoid gas leakage. By adjusting the overturning base45, it is convenient to place or take out the natural gas hydrate sediment samples.

The pipe valve I401, the pipe valve II402, the pipe valve III403, the pipe valve IV404, the pipe valve V405, the pipe valve VI406, the pipe valve VII407, the pipe valve VIII408, the pipe valve IX409, the pipe valve X410, the pipe valve XI411and the pipe valve XII412are configured to determine whether to communicate their pipelines. The safety valve19is configured to control the safety of the whole system.

The pressurized test method for in-situ mining natural gas hydrates by jets in the present invention comprises following steps:

step 1: before testing, cleaning and naturally drying a visual test cabin28, and preparing, cleaning with deionized water and oven drying quartz sandstone or silty mudstone;

step 2: uniformly mixing the quartz sandstone or silty mudstone with brine to obtain a mixture, filling the mixture into the annular pressure rubber sleeve33by a deionized test shovel, putting the mixture wrapped by the annular pressure rubber sleeve33in a visual test cabin28, tightening a front end cover46of the visual test cabin28to the visual test cabin by a sealing valve44at the end of filling and putting the visual test cabin28in a thermostat25, and injecting water into an axial pressure loading chamber35by a constant-flux pump13until an axial pressure of in-situ submarine natural gas hydrate-containing sediments is reached; injecting water into an annular pressure hole38on the visual test cabin28by an annular pressure pump27until a confining pressure of in-situ submarine natural gas hydrate-containing sediments to be simulated is reached; adjusting the position and jetting distance of a jet nozzle32, selecting the specification and jet diameter of the jet nozzle32, adjusting a jet pump20, and setting a jetting velocity desired by a test;

step 3: feeding air into the thermostat25to cool the whole visual test cabin28in air bath, and turning on a pipe valve I401, a pipe valve II402and a pipe valve III403in the injection system to feed methane gas into the visual test cabin28at 350 mL/min, the amount of methane gas being determined by the saturation of the natural gas hydrate-containing sediments, the feeding usually lasting about 20 min to 30 min; setting the temperature of the air bath to be a temperature near the freezing point, and recording the resulting data; and after about 10 h, obtaining natural gas hydrate sediment samples at the end of synthesis of natural gas hydrates;

step 4: jet breakup: at the end of synthesis of natural gas hydrates, decreasing the temperature of the air bath to below 242K-271K, discharging the residual methane gas by a vacuum pump8immediately when the temperature becomes stable, and quickly feeding the cooled brine to flood the natural gas hydrate sediment samples; adjusting the pump capacity and pumping time of the annular pressure pump27and the constant-flux pump13to reach real axial pressure and confining pressure conditions of in-situ submarine natural gas hydrate-containing sediments, and setting the temperature of the air bath to be a reaction temperature set for the test; activating the jet breakup system for a jet breakup test, capturing the jet breakup process by a video camera47, and recording the temperature according to the temperature sensor29;

step 5: gas metering: as the jetting progresses, discharging the mixture from the gas guide pipe of the visual test cabin28into a three-phase separator24where gas is separated, and drying the gas by a drying pipe23; increasing the temperature of the air bath, decomposing remaining natural gas hydrates in the visual test cabin28, and metering the total amount of the decomposed methane gas at the end of decomposition; and

step 6: at the end of the test, shutting off the jet pump20, the constant-flux pump13, the annular pressure pump27and other instruments; taking the visual test cabin28out, and observing and recording the breakup effect of the natural gas hydrate-containing sediments; taking the natural gas hydrate sediment samples out, cleaning the visual test cabin28, and analyzing data.