Testing device and method for water holding capacity parameters and pore water distribution characteristics of methane hydrate-containing sediments

Abstract

The present invention provides a test device and method for water retention parameters and pore water distribution characteristics of methane hydrate-containing sediments, including a pressure chamber, a first constant temperature bath, a low-field nuclear magnetic resonance instrument, a pressure supply system, a gas supply system, a constant-rate and constant-pressure injection pump, and a data acquisition system. In the present invention, nitrogen gas is used to apply air pressure to the specimen in the pressure chamber under high-pressure and low-temperature conditions. The low-field nuclear magnetic resonance instrument is used to reflect the change trend of pore water. The axis translation method is used to control and measure the suction of the methane hydrate-containing specimen. According to the water output of the specimen collected by the data acquisition system, the suction balance is judged and the water content of the specimen under this suction is calculated, and the soil-water characteristic curve of the methane hydrate-containing soil and the pore water distribution state at each suction level are obtained, realizing the research on the water retention parameters of methane hydrate-containing sediments. Its principle conforms to the hydrate formation mode and mining working conditions, and the structure is simple, which can be equipped for most scientific research and design units.

Description

Technical Field

[0001] The present invention relates to the technical field of physical property testing of materials, and particularly to a testing device for water retention parameters and pore water distribution characteristics of methane hydrate-containing sediments. Background Art

[0002] Methane hydrate (commonly known as combustible ice) is a kind of ice-like, non-stoichiometric cage-shaped crystalline compound formed by gas molecules such as methane being inhaled into the voids of cage-shaped water molecule clusters under certain pressure and temperature conditions. It is widely distributed in submarine sediments in the continental margin slope area and permafrost zones, and has characteristics such as high calorific value, large energy, and clean and pollution-free.

[0003] As a new type of clean energy, improper exploitation of methane hydrate will lead to engineering problems such as submarine landslides and ground collapses, and will also release greenhouse gases, resulting in global warming. Since the mechanical properties, gas-water migration process, and occurrence state of methane hydrate sediments are all related to their water retention properties, the water retention parameters of methane hydrate sediments are very important basic parameters in the process of methane hydrate drilling and exploitation. The soil-water characteristic curve is the relationship curve between the water content of the soil and the suction force, which reflects the water retention ability of the soil and is a key function for describing the behavior of unsaturated soil. The soil-water characteristic curve is essentially determined by pore-scale characteristics, including pore shape and size distribution, interconnectivity and spatial variability, fluid and interfacial tension, etc. The hydrates existing in the sediments will largely change the pore-scale characteristics of the soil. In summary, carrying out research on the soil-water characteristic curve of methane hydrate-containing sediments is of great significance for the exploitation of methane hydrates.

[0004] Since methane hydrate sediments generally exist stably in deep-sea sediment areas and permafrost zones on land, it is difficult and costly to obtain test samples. In the prior art, tetrahydrofuran hydrate is usually used instead of methane hydrate for research. In terms of molecular polarity, there are certain differences between tetrahydrofuran and methane, and the testing devices in the prior art cannot truly simulate the seabed situation, resulting in the test results being unable to accurately characterize the soil-water characteristic curve of methane hydrate-containing sediments. Summary of the Invention

[0005] In order to solve the technical problems in the prior art that the testing device for water retention parameters of methane hydrate-containing sediments cannot simulate the seabed environment and the measurement results are inaccurate, the technical solution of the present invention is as follows:

[0006] On the one hand, the present invention provides a testing device for water retention parameters and pore water distribution characteristics of methane hydrate-containing sediments, including a pressure chamber, a first constant temperature bath, a low-field nuclear magnetic resonance instrument, a pressure supply system, a gas supply system, a constant-speed and constant-pressure injection pump, and a data acquisition system.

[0007] The specific structure of this test device is as follows: A specimen is placed inside the pressure chamber. The specimen is carried by a clay plate. The bottom wall of the pressure chamber is externally connected to a drain pipe. The first constant temperature bath is used to control the temperature of the pressure chamber. The low-field nuclear magnetic resonance instrument has a holder, and the pressure chamber is fixed to the low-field nuclear magnetic resonance instrument through the holder. The pressure supply system includes a nitrogen gas cylinder, and the nitrogen gas cylinder is connected to the top wall of the pressure chamber. The gas supply system includes a methane gas cylinder and a gas buffer tank. The methane gas cylinder is connected to the gas buffer tank, and the gas buffer tank is connected to the top wall of the pressure chamber. The constant-speed and constant-pressure injection pump is respectively connected to the side wall and the bottom wall of the pressure chamber. The data acquisition system includes a processor and a pressure sensor one, a pressure sensor two, a pressure sensor three, a temperature sensor one, and a temperature sensor two that are electrically connected to the processor. The pressure sensor one and the temperature sensor one are arranged on the gas buffer tank. The pressure sensor two and the temperature sensor two are arranged on the pressure chamber. The pressure sensor three is arranged on the constant-speed and constant-pressure injection pump.

[0008] The test device for the water retention parameter and pore water distribution characteristics of methane hydrate-containing sediments in the present invention applies air pressure to the specimen in the pressure chamber using nitrogen under high-pressure and low-temperature conditions, uses a low-field nuclear magnetic resonance instrument to reflect the change trend of pore water, controls and measures the suction of the methane hydrate-containing specimen using the axis translation method, judges the suction balance and calculates the water content of the specimen at this suction according to the water output of the specimen collected by the data acquisition system, obtains the soil-water characteristic curve of the methane hydrate-containing soil and the pore water distribution state at each suction level, realizes the research on the water retention parameter of methane hydrate-containing sediments, its principle conforms to the hydrate formation mode and mining working conditions, and the structure is simple, which can be equipped for most scientific research and design units.

[0009] The present invention has the following advantages: First, it can reproduce the occurrence environment of methane hydrate on the seabed under natural conditions; second, it can simply and accurately obtain the soil-water characteristic curve; third, it overcomes the difficulty of accurately measuring the water retention parameter of methane hydrate-containing sediments in the laboratory, providing important data support for the commercial exploitation and numerical simulation of hydrates; fourth, it can accurately obtain the change of pore water in the specimen.

[0010] In a possible design, the pressure chamber includes a pressure chamber top cover, a pressure chamber outer cylinder, and a pressure chamber base. The pressure chamber top cover and the pressure chamber base are hermetically connected to the two open ends of the pressure chamber outer cylinder.

[0011] In a possible design, a jacket is further arranged outside the pressure chamber. The jacket is externally connected to the first constant temperature bath, and circulating heat-conducting liquid is provided to the jacket through the first constant temperature bath.

[0012] In a possible design, the pressure supply system further includes a pressure regulating valve, a pressure regulating knob, and a back pressure valve. The nitrogen gas cylinder, the pressure regulating valve, and the pressure regulating knob are connected in sequence and then branch out into two pipelines. One pipeline is connected to the top wall of the pressure chamber through the back pressure valve, and the other pipeline is directly connected to the top wall of the pressure chamber.

[0013] In a possible design, the test device for the water holding capacity parameter and pore water distribution characteristics of methane hydrate-containing sediments further includes: a gas-liquid separator, and the gas-liquid separator is connected to the back pressure valve.

[0014] In a possible design, the gas supply system further includes a second constant temperature bath, and the second constant temperature bath is used to control the gas temperature in the gas buffer tank.

[0015] In a possible design, a safety valve is also provided on the gas buffer tank.

[0016] On the other hand, the present invention also provides a test method for the water holding capacity parameter and pore water distribution characteristics of methane hydrate-containing sediments. The test method is based on the test device and includes the following steps:

[0017] Step 1, sample preparation: Set the sample as a cylinder and compact it layer by layer using a jack according to the preset water content and dry density.

[0018] Step 2, sample installation: Embed the pre-saturated clay board together with the sealing ring into the outer cylinder of the pressure chamber, install the pressure chamber base and the pressure chamber top cover, place them in the holder, and place the entire holder in the low-field nuclear magnetic resonance instrument; Open the constant-speed and constant-pressure injection pump to inject water from the bottom wall of the pressure chamber and then flow out through the drain pipe. After discharging the air in the pressure chamber, close the drain pipe and start the first constant temperature bath to maintain the temperature of the pressure chamber stable.

[0019] Step 3, gas injection: Open the methane gas cylinder to inject methane gas into the gas buffer tank. When the pressure increases to the target value, close the methane gas cylinder, record the pressure and temperature of the gas buffer tank in the stable state as the initial test values, and calculate the initial amount of the reaction gas according to the gas state equation; Then open the gas buffer tank to inject gas into the pressure chamber, start the constant-speed and constant-pressure injection pump at the same time, and make the pipeline between the constant-speed and constant-pressure injection pump and the bottom wall of the pressure chamber unblocked, and set the constant-speed and constant-pressure injection pump to the tracking mode.

[0020] Step 4, cool down to synthesize hydrate. After the temperature and pressure in the pressure chamber reach stability, set the temperature of thermostat 1 to provide a low-temperature environment for the formation of hydrate and promote hydrate synthesis. When the pressure value and temperature value of the system composed of the pressure chamber and the gas buffer tank remain unchanged, it can be considered that the hydrate synthesis is completed. Close the gas buffer tank and the constant-rate and constant-pressure injection pump, record the temperature and pressure of the system at this time, calculate the remaining amount of the reaction gas according to the gas state equation, and preliminarily calculate the hydrate saturation degree in combination with the hydrate number.

[0021] Step 5, saturate the specimen. Open the nitrogen gas cylinder, apply a certain pressure to the back-pressure valve, and adjust the pressure regulating valve so that the pressure of the back-pressure valve and the pressure in the pressure chamber maintain a set difference. Keep the pipeline between the back-pressure valve and the top wall of the pressure chamber unobstructed. Under a constant pressure difference, open the constant-rate and constant-pressure injection pump to inject water from the side wall of the pressure chamber. When the injected water volume exceeds 2 times the pore volume of the specimen, it is considered that the specimen is completely saturated.

[0022] Step 6, water retention and nuclear magnetic test. After the specimen containing hydrate is saturated, it is necessary to measure the initial moisture distribution state of the specimen before the test. Open the low-field nuclear magnetic resonance instrument, input the corresponding parameters, scan the specimen in this state, and obtain the transverse relaxation time distribution. Then keep the pipeline between the pressure regulating knob and the top wall of the pressure chamber unobstructed and the pipeline between the constant-rate and constant-pressure injection pump and the bottom wall of the pressure chamber unobstructed. Adjust the pressure regulating knob to make the air pressure higher than the pore water pressure, and conduct the test under a specified pressure difference. Record the amount of water discharged from the soil through the constant-rate and constant-pressure injection pump. When the reading of the constant-rate and constant-pressure injection pump is stable, it is considered that the system reaches the equilibrium state at this level of pressure. Then use the low-field nuclear magnetic resonance instrument to measure the moisture distribution state in this equilibrium state, and then apply the next level of pressure. Repeat the above steps to complete the established pressure sequence.

[0023] Step 7, data arrangement. Calculate the water saturation degree after the balance of each level of suction according to the initial mass of the specimen, the pore volume and the water output. Then, based on the relationship between the suction value and the water saturation degree at each level, establish the soil-water characteristic relationship curve of the sediment containing methane hydrate, and combine the pore water distribution situation obtained at each level to obtain the relationship between the pore water distribution and the soil-water characteristic curve.

[0024] In a possible design, in Step 4, the calculation process of the generated hydrate saturation degree is as follows:

[0025] First, the initial amount of substance of the gas in the gas buffer tank is:

[0026]

[0027] Open the gas buffer tank to inject gas into the pressure chamber. The total amount of substance of the free methane gas in the gas buffer tank, the soil pores and the pipeline after the reaction is:

[0028]

[0029] Wherein: P is the pressure of methane gas; T is the temperature of the gas; R is the ideal gas constant; V is the volume of free methane gas; Z is the gas compressibility factor; a represents the gas buffer tank; b represents the sample pores; c represents the pipeline; i represents the initial state of the reaction; t represents the end state of the reaction;

[0030] By collecting the temperature and pressure changes during the test process, the gas consumption Δn during the synthesis process is calculated according to the gas state equation g , and then the hydrate saturation is obtained, that is:

[0031]

[0032]

[0033] In a possible design, in step seven, the process of drawing the soil-water characteristic relationship curve is as follows:

[0034] After water injection saturation, the pore volume of the sample is V v , assuming complete saturation, then the pore volume is equal to the sum of the volume of methane hydrate and the volume of pore water, that is:

[0035] 1 = S w + S h

[0036] According to the proposed suction steps, apply suction, and record the cumulative water output volume at each level of suction through a constant-speed and constant-pressure injection pump: V out . After equilibrium, the water saturation in the pores of the sample at each level of suction is:

[0037]

[0038] Finally, draw the soil-water characteristic relationship curve.

[0039] This test method is to place a sample with a certain water content on a clay board in a pressure chamber in a constant-temperature bath one, and place it on a low-field nuclear magnetic resonance instrument. Methane gas is introduced into it to generate hydrates in the soil sample under high pressure and low temperature conditions to fill the soil sample, forming a sediment containing hydrates in the pores; after preparing a sample of sediment containing methane hydrates at a certain temperature and pressure, apply various levels of air pressure to the pressure chamber, scan each level of air pressure, control and measure the suction of the methane hydrate-containing sample by the axis translation method, and the data acquisition system automatically collects and records the water output data of the sample in the pressure chamber, and judges the equilibrium and water content of the sample according to the water output state, and obtains the soil-water characteristic relationship curve under different hydrate saturation conditions and the pore water distribution state under each level of suction. The method is simple and accurate and easy to operate. Description of the Drawings

[0040] Figure 1 It is a schematic diagram of a test device for the water-holding capacity parameter and pore water distribution characteristics of methane hydrate-containing sediments provided by an embodiment of the present invention;

[0041] Figure 2 It is a schematic diagram of a pressure chamber provided by an embodiment of the present invention.

[0042] Reference numerals: 10, pressure chamber; 11, pressure chamber top cover; 12, outer cylinder of pressure chamber; 13, pressure chamber base; 14, jacket; 141, liquid inlet; 142, liquid outlet; 15, specimen; 16, clay plate; 17, drain pipe; 20, constant temperature bath I; 30, low-field nuclear magnetic resonance instrument; 31, holder; 41, nitrogen gas cylinder; 42, pressure regulating valve; 43, pressure regulating knob; 44, back pressure valve; 51, methane gas cylinder; 52, gas buffer tank; 53, constant temperature bath II; 54, safety valve; 60, constant speed and constant pressure injection pump; 71, processor; 72, pressure sensor I; 73, pressure sensor II; 74, pressure sensor III; 75, temperature sensor I; 76, temperature sensor II; 77, computer; 80, valve X; 81, valve I; 82, valve II; 83, valve III; 84, valve IV; 85, valve V; 86, valve VI; 87, valve VII; 88, valve VIII; 89, valve IX; 90, gas-liquid separator. Detailed implementation manners

[0043] Next, the technical solutions in the present invention will be described in conjunction with the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments.

[0044] In the description of the present invention, it should be noted that, unless otherwise clearly defined and limited, the terms "installation", "connection", and "connection" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection, an electrical connection, or a communication with each other; it can be directly connected, or indirectly connected through an intermediate medium, and it can be the internal communication of two components or the interaction relationship between two components. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific situations.

[0045] In the description of the present invention, it should be understood that the orientation or positional relationship indicated by the terms "upper", "lower", "side", "inner", "outer", "top", "bottom", etc. is based on the installed orientation or positional relationship, and is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the present invention.

[0046] It should also be noted that in the embodiments of the present invention, the same reference numerals are used to denote the same components or the same parts. For the same parts in the embodiments of the present invention, only one of the parts or components may be marked with a reference numeral in the figure. It should be understood that the reference numerals are equally applicable to other identical parts or components.

[0047] Hereinafter, the terms "first", "second", etc. are only used for descriptive purposes and cannot be construed as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Thus, the features defined with "first", "second", etc. may explicitly or implicitly include one or more of such features.

[0048] Conducting research on the soil-water characteristic curve of methane hydrate-bearing sediments is of great significance for the exploitation of methane hydrates. In order to avoid the high-pressure and low-temperature conditions under which methane hydrates stably exist, some scholars use tetrahydrofuran hydrates instead of methane hydrates for research. However, in the seabed, methane hydrates are mainly present. From the perspective of molecular polarity, there are certain differences between tetrahydrofuran and methane, and many scholars have some objections to this. In order to truly simulate the seabed conditions, the present application has developed an indoor test device based on nuclear magnetic resonance technology for testing the soil-water characteristic relationship curve and pore water distribution of methane hydrate-bearing sediments. This device can better reproduce the methane hydrate formation environment in the laboratory, simulate the growth habits of hydrates in the natural state, can simply and accurately obtain the soil-water characteristic curve of methane hydrate-bearing sediments, can accurately obtain the pore water distribution in each state, and can be applicable to different suction ranges, reducing the artificial error in the traditional axis translation method test.

[0049] The test device for the water retention parameter and pore water distribution characteristics of methane hydrate-bearing sediments in the present invention fills the gap in the indoor test device for obtaining the water retention parameter of methane hydrate-bearing sediments at home and abroad, overcomes the deficiencies in the above-mentioned prior art. This test device and test method are reasonably designed, easy to use, can make the measurement results of the soil-water characteristic relationship curve of methane hydrates accurate, the measurement method convenient, and can analyze the influencing reasons from the perspective of nuclear magnetic resonance.

[0050] As Figure 1-2 shown, an embodiment of the present invention provides a test device for the water retention parameter and pore water distribution characteristics of methane hydrate-bearing sediments, including a pressure chamber 10, a first constant temperature bath 20, a low-field nuclear magnetic resonance instrument 30, a pressure supply system, a gas supply system, a constant-speed and constant-pressure injection pump 60, and a data acquisition system.

[0051] The specific structure of the test device for the water retention parameters and pore water distribution characteristics of methane hydrate-containing sediments is as follows: A specimen 15 is placed inside a pressure chamber 10. The specimen 15 is supported by a clay plate 16. A drain pipe 17 is externally connected to the bottom wall of the pressure chamber 10. A constant-temperature bath 20 is used to control the temperature of the pressure chamber 10. A low-field nuclear magnetic resonance instrument 30 has a holder 31, and the pressure chamber 10 is fixed to the low-field nuclear magnetic resonance instrument 30 through the holder 31. The pressure supply system includes a nitrogen gas cylinder 41, and the nitrogen gas cylinder 41 is connected to the top wall of the pressure chamber 10. The gas supply system includes a methane gas cylinder 51 and a gas buffer tank 52. The methane gas cylinder 51 is connected to the gas buffer tank 52, and the gas buffer tank 52 is connected to the top wall of the pressure chamber 10. A constant-rate and constant-pressure injection pump 60 is respectively connected to the side wall and the bottom wall of the pressure chamber 10. The data acquisition system includes a processor 71 and a pressure sensor 72, a pressure sensor 73, a pressure sensor 74, a temperature sensor 75, and a temperature sensor 76 that are electrically connected to the processor 71. The pressure sensor 72 and the temperature sensor 75 are arranged on the gas buffer tank 52. The pressure sensor 73 and the temperature sensor 76 are arranged on the pressure chamber 10. The pressure sensor 74 is arranged on the constant-rate and constant-pressure injection pump 60.

[0052] The working principle of this embodiment: The prepared specimen 15 is placed inside the pressure chamber 10. The temperature is controlled using the constant-temperature bath 20 and it is placed on the low-field nuclear magnetic resonance instrument 30. Then, a pressure system composed of the pressure supply system and the constant-rate and constant-pressure injection pump 60 is used for high-pressure treatment, so that the specimen 15 is induced in a high-pressure and low-temperature environment to obtain specimens 15 with different hydrate saturations. Then, the constant-rate and constant-pressure injection pump 60 is used to saturate the specimen 15 and the pipeline successively. After that, the pressure supply system is used to apply various levels of air pressure to the pressure chamber 10. The water output collected by the constant-rate and constant-pressure injection pump 60 is fed back to the data acquisition system and then displayed by a computer 77. According to the displayed water output situation, it is judged whether the specimen 15 reaches equilibrium. After the specimen 15 reaches equilibrium, the low-field nuclear magnetic resonance instrument 30 is used to obtain the transverse relaxation time distribution curve at this level of suction. Finally, based on the collected data, the soil-water characteristic curve of methane hydrate-containing sediments under different hydrate saturation conditions and the pore water distribution state at each level of suction are obtained.

[0053] This embodiment has the following advantages and positive effects: First, it can reproduce the occurrence environment of methane hydrate on the seabed under natural conditions; second, it can simply and accurately obtain the soil-water characteristic curve; third, it overcomes the difficulty of accurately measuring the water retention parameters of methane hydrate-containing sediments in the laboratory, providing important data support for the commercial exploitation and numerical simulation of hydrates; fourth, it can accurately obtain the change situation of pore water in the specimen 15.

[0054] Among them, the nuclear magnetic resonance technology of the above-mentioned low-field nuclear magnetic resonance instrument 30 utilizes the hydrogen nuclei of the aqueous phase media in different occurrence states in unsaturated soil, which generate different transverse relaxation times under the action of different radio frequency magnetic fields. Furthermore, the transverse relaxation time distribution curve of the pore water in the specimen 15 is obtained through echo inversion. On this curve, different transverse relaxation times correspond to the sizes of the pore radii occupied by the pore water, and the area under the curve corresponds to the water content within the range of this pore radius.

[0055] In one embodiment, the pressure chamber 10 includes a pressure chamber top cover 11, a pressure chamber outer cylinder 12, and a pressure chamber base 13. The pressure chamber top cover 11 and the pressure chamber base 13 are hermetically connected to the two open ends of the pressure chamber outer cylinder 12.

[0056] The nitrogen gas cylinder 4l is connected to the top wall of the pressure chamber 10, the gas buffer tank 52 is connected to the top wall of the pressure chamber 10, and the second pressure sensor 73 and the second temperature sensor 76 are arranged on the pressure chamber 10. In this embodiment, the pressure chamber top cover 11 has four openings. Two openings are respectively connected to the nitrogen gas cylinder 41 and the gas buffer tank 52 through pipelines, and the other two openings are respectively used for installing the second pressure sensor 73 and the second temperature sensor 76.

[0057] As mentioned above, the bottom wall of the pressure chamber 10 is externally connected to the drain pipe 17, and the constant-speed and constant-pressure injection pump 60 is respectively connected to the side wall and the bottom wall of the pressure chamber 10. In this embodiment, the pressure chamber base 13 has two openings, which are respectively connected to the drain pipe 17 and the constant-speed and constant-pressure injection pump 60; the pressure chamber outer cylinder 12 has one opening for connecting the constant-speed and constant-pressure injection pump 60.

[0058] In one embodiment, a jacket 14 is further arranged outside the pressure chamber 10. The jacket 14 is externally connected to the first constant temperature bath 20, and the circulating heat-conducting fluid is provided to the inside of the jacket 14 through the first constant temperature bath 20.

[0059] The jacket 14 has a liquid inlet 141 and a liquid outlet 142. The liquid inlet 141 and the liquid outlet 142 are respectively connected to the pipelines of the circulating heat-conducting fluid of the first constant temperature bath 20. The heat-conducting fluid of the first constant temperature bath 20 enters the jacket 14 from the liquid inlet 141 to control the temperature of the pressure chamber 10, and the heat-conducting fluid after heat exchange flows out from the liquid outlet 142 and enters the first constant temperature bath 20.

[0060] Among them, the above-mentioned heat-conducting fluid is fluorinated oil or alkylnaphthalene heat-conducting oil. It should be particularly noted that water cannot be used as the heat-conducting fluid because the water for controlling the temperature will also be detected by the low-field nuclear magnetic resonance instrument 30, thereby affecting the measurement results.

[0061] In one embodiment, the low-field nuclear magnetic resonance instrument 30 has a holder 31, and the pressure chamber 10 is fixed on the low-field nuclear magnetic resonance instrument 30 through the holder 31.

[0062] To facilitate the fixation of the pressure chamber 10 and the jacket 14, a gripper 31 is provided outside the pressure chamber 10 and the jacket 14.

[0063] In one embodiment, the pressure supply system further includes a pressure regulating valve 42, a pressure regulating knob 43, and a back pressure valve 44. The nitrogen gas cylinder 41, the pressure regulating valve 42, and the pressure regulating knob 43 are connected in sequence and then branch into two pipelines. One pipeline is connected to the top wall of the pressure chamber 10 through the back pressure valve 44 to control the pressure in the pipeline and relieve the pressure in a timely manner, and the other pipeline is directly connected to the top wall of the pressure chamber 10 to provide pressure.

[0064] The pressure regulating valve 42 can convert the high-pressure gas in the nitrogen gas cylinder 41 into low-pressure gas and transport it to the pressure chamber 10. The pressure regulating knob 43 can increase or decrease the pressure to control the pressure transported into the pressure chamber 10.

[0065] In one embodiment, the test device for the water holding capacity parameters and pore water distribution characteristics of methane hydrate sediments further includes: a gas-liquid separator 90, and the gas-liquid separator 90 is connected to the back pressure valve 44.

[0066] Since methane is a flammable gas, to ensure safety, it is recovered and centrally processed after the test. In this embodiment, the gas-liquid separator 90 is used to recover methane gas to prevent the methane gas from accumulating in the laboratory and causing an explosion hazard.

[0067] In one embodiment, the gas supply system further includes a second constant temperature bath 53, and the second constant temperature bath 53 is used to control the gas temperature in the gas buffer tank 52. The second constant temperature bath 53 can maintain the temperature of the methane gas in the gas buffer tank 52 stable.

[0068] In one embodiment, the gas buffer tank 52 is further provided with a safety valve 54.

[0069] The safety valve 54 has a pressure threshold. When the air pressure in the gas buffer tank 52 is too high, the safety valve 54 automatically opens to relieve the pressure.

[0070] In one embodiment, a first valve 81 is provided on the connecting pipeline between the back pressure valve 44 and the gas-liquid separator 90; a second valve 82 is provided at the liquid outlet of the gas-liquid separator 90, and a third valve 83 is provided at the gas outlet; a fourth valve 84 is provided on the connecting pipeline between the back pressure valve 44 and the pressure chamber 10; a fifth valve 85 is provided on the connecting pipeline between the pressure regulating knob 43 and the pressure chamber 10; a sixth valve 86 is provided at the outlet of the drain pipe 17; a seventh valve 87 is provided on the connecting pipeline between the methane gas cylinder 51 and the gas buffer tank 52; an eighth valve 88 is provided on the connecting pipeline between the gas buffer tank 52 and the pressure chamber 10; a ninth valve 89 is provided on the connecting pipeline between the constant speed and constant pressure injection pump 60 and the pressure chamber base 13; a tenth valve 80 is provided on the connecting pipeline between the constant speed and constant pressure injection pump 60 and the outer cylinder 12 of the pressure chamber.

[0071] In one embodiment, the first valve 81 to the tenth valve 80 can be high-pressure manual valves or solenoid valves.

[0072] In one embodiment, the data acquisition system further includes a computer 77. The processor 71 and the low-field nuclear magnetic resonance instrument 30 are electrically connected through signal lines. Moreover, the first valve 81 to the tenth valve 80 can also be solenoid valves, which are electrically connected to the processor 71 and controlled by the processor 71 to open or close.

[0073] In one embodiment, a test method for the water-holding capacity parameter and pore water distribution characteristics of methane hydrate-containing sediments is also provided. The test method is based on the above test device and includes the following steps:

[0074] Step 1, preparation of the specimen 15. Set the specimen 15 as a cylinder and compact it layer by layer using a jack according to the preset water content and dry density.

[0075] Step 2, installation of the specimen 15. Embed the pre-saturated clay board 16 together with the sealing ring into the outer cylinder 12 of the pressure chamber, install the pressure chamber base 13 and the pressure chamber top cover 11, place them in the holder 31, and then place the entire holder 31 in the low-field nuclear magnetic resonance instrument 30. Open the constant-speed and constant-pressure injection pump 60 to inject water from the bottom wall of the pressure chamber 10, that is, open the ninth valve 89, and then let it flow out through the drain pipe 17, that is, open the sixth valve 86. After discharging the air in the pressure chamber 10, close the drain pipe 17, that is, close the sixth valve 86, and start the first constant-temperature bath 20 to maintain the temperature of the pressure chamber 10 stable.

[0076] Step 3, gas injection. Open the methane gas cylinder 51 to inject methane gas into the gas buffer tank 52, that is, open the seventh valve 87. When the pressure increases to the target value, close the methane gas cylinder 51, that is, close the seventh valve 87. Record the pressure and temperature of the gas buffer tank 52 in the stable state as the initial test values, and calculate the initial amount of the reaction gas according to the gas state equation. Then open the gas buffer tank 52 to inject gas into the pressure chamber 10, that is, open the eighth valve 88. At the same time, start the constant-speed and constant-pressure injection pump 60, and make the pipeline between the constant-speed and constant-pressure injection pump 60 and the bottom wall of the pressure chamber 10 unblocked, that is, open the ninth valve 89, and set the constant-speed and constant-pressure injection pump 60 to the tracking mode (back pressure tracking air pressure).

[0077] Step 4, cool down to synthesize hydrate. After the temperature and pressure in the pressure chamber 10 reach stability, set the temperature of the constant temperature bath 20 to provide a low-temperature environment for the formation of hydrate and promote hydrate synthesis. When the pressure value and temperature value of the system composed of the pressure chamber 10 and the gas buffer tank 52 remain unchanged, it can be considered that the hydrate synthesis is completed. Then, close the gas buffer tank 52 and the constant speed and constant pressure injection pump 60, that is, close valve eight 88 and valve nine 89, record the temperature and pressure of the system at this time, calculate the remaining amount of the reaction gas according to the gas state equation, and preliminarily calculate the hydrate saturation degree in combination with the hydrate number.

[0078] Step 5, saturate the specimen 15. Open the nitrogen gas cylinder 41, apply a certain pressure to the back pressure valve 44, and adjust the pressure regulating valve 42 to maintain a set difference between the pressure of the back pressure valve 44 and the pressure of the pressure chamber 10. Keep the pipeline between the back pressure valve 44 and the top wall of the pressure chamber 10 unblocked, that is, open valve four 84. Under a constant pressure difference, open the constant speed and constant pressure injection pump 60 to inject water from the side wall of the pressure chamber 10, that is, open valve ten 80. When the injected water volume exceeds twice the pore volume of the specimen 15, it is considered that the specimen 15 is completely saturated, and then close valve four 84 and valve ten 80.

[0079] Step 6, water holding capacity and nuclear magnetic test. After the hydrate-containing specimen 15 is saturated, before the test, it is necessary to measure the initial moisture distribution state of the specimen 15. Open the low-field nuclear magnetic resonance instrument 30 and input the corresponding parameters, scan the specimen 15 in this state to obtain the transverse relaxation time distribution. Then, keep the pipeline between the pressure regulating knob 43 and the top wall of the pressure chamber 10 unblocked, and keep the pipeline between the constant speed and constant pressure injection pump 60 and the bottom wall of the pressure chamber 10 unblocked, that is, open valve five 85 and valve nine 89. Adjust the pressure regulating knob 43 to make the air pressure higher than the pore water pressure, and conduct the test under a specified pressure difference. Record the water volume discharged from the soil through the constant speed and constant pressure injection pump 60. When the reading of the constant speed and constant pressure injection pump 60 is stable, it is considered that the system reaches the equilibrium state at this level of pressure. Then, use the low-field nuclear magnetic resonance instrument 30 to measure the moisture distribution state in this equilibrium state, and then apply the next level of pressure. Repeat the above steps to complete the established pressure sequence.

[0080] Step 7, data sorting. Calculate the water saturation degree after the suction balance at each level according to the initial mass, pore volume and water output of the specimen 15. Then, based on the relationship between the suction value and the water saturation degree at each level, establish the soil-water characteristic relationship curve of the methane hydrate-containing sediment, and combine the pore water distribution situation obtained at each level to obtain the relationship between the pore water distribution and the soil-water characteristic curve.

[0081] In one embodiment, in Step 4, the calculation process of the generated hydrate saturation degree is as follows:

[0082] First, the initial amount of substance of the gas in the gas buffer tank 52 is:

[0083]

[0084] Open the gas buffer tank 52 to inject gas into the pressure chamber 10, that is, open the valve eight 88. The total amount of substance of the free methane gas in the gas buffer tank 52, the soil pores and the pipeline at the end of the reaction is:

[0085]

[0086] In the formula: P is the pressure of the methane gas; T is the temperature of the gas; R is the ideal gas constant; V is the volume of the free methane gas; Z is the gas compressibility factor; a represents the gas buffer tank 52; b represents the pores of the specimen 15 (which can be obtained by calculating the initial parameters of the specimen 15); c represents the pipeline; i represents the initial state of the reaction; t represents the end state of the reaction;

[0087] To simplify the calculation process, this application ignores the change in the volume of the sand sample during preloading, gas injection and hydrate synthesis and the change in pore volume caused by the conversion of water to hydrate, that is, the hydrate synthesis process can be simplified to be carried out under constant volume conditions. The volume of the pipeline can be calculated according to the conservation of the amount of gas substance and combined with the real gas state equation.

[0088] By collecting the temperature and pressure changes during the test process, the gas consumption Δn during the synthesis process is calculated according to the gas state equation g , and then the hydrate saturation is calculated, that is:

[0089]

[0090]

[0091] In one embodiment, in step seven, the process of drawing the soil-water characteristic relationship curve is as follows:

[0092] After water injection saturation, the pore volume of the specimen 15 is V v , assuming complete saturation, then the pore volume is equal to the sum of the volume of methane hydrate 1 = S w +S h

[0093] That is, the sum of the volume of the hydrate and the volume of the pore water, that is:

[0094] According to the proposed suction step, apply suction, and record the cumulative water output volume at each level of suction through the constant-speed and constant-pressure injection pump 60: V out , after equilibrium, the water saturation in the pores of the specimen 15 at each level of suction is:

[0095]

[0096] Finally, draw the soil-water characteristic relationship curve.

[0097] In this test method, a specimen 15 with a certain moisture content is placed on a clay plate 16 in a pressure chamber 10 that is in a constant temperature bath -20, and it is placed on a low-field nuclear magnetic resonance instrument 30. Methane gas is introduced into it so that hydrates are formed in the soil sample under high pressure and low temperature conditions and filled in the soil sample, forming a sediment with hydrates in the pores; after obtaining a specimen 15 of methane hydrate sediment at a certain temperature and pressure, various levels of air pressure are applied to the pressure chamber 10, and each level of air pressure is scanned. The axis translation method is used to control and measure the suction of the methane hydrate-containing specimen 15. The data acquisition system automatically collects and records the water output data of the specimen 15 in the pressure chamber 10. According to the water output state, the balance and water content of the specimen 15 are judged, and the soil-water characteristic relationship curve under different hydrate saturation conditions and the pore water distribution state under each level of suction are obtained. The method is simple and accurate and easy to operate.

[0098] As described above, the above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention can easily think of changes or substitutions, which should all be covered within the protection scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope of the claims described.

Claims

1. A test device for water retention parameters and pore water distribution characteristics of methane hydrate-containing sediments, characterized in that, Comprising: A pressure chamber (10), inside which a specimen (15) is placed. The specimen (15) is carried by a clay plate (16), and a drain pipe (17) is externally connected to the bottom wall of the pressure chamber (10); A first constant temperature bath (20) for controlling the temperature of the pressure chamber (10); A low-field nuclear magnetic resonance instrument (30) having a holder (31), and the pressure chamber (10) is fixed to the low-field nuclear magnetic resonance instrument (30) through the holder (31); A pressure supply system including a nitrogen gas cylinder (41) connected to the top wall of the pressure chamber (10); A gas supply system including a methane gas cylinder (51) and a gas buffer tank (52). The methane gas cylinder (51) is connected to the gas buffer tank (52), and the gas buffer tank (52) is connected to the top wall of the pressure chamber (10). A valve eight (88) is provided on the pipeline connecting the gas buffer tank (52) and the pressure chamber (10); A constant speed and constant pressure injection pump (60) respectively connected to the side wall and the bottom wall of the pressure chamber (10); A data acquisition system including a processor (71) and a pressure sensor one (72), a pressure sensor two (73), a pressure sensor three (74), a temperature sensor one (75), and a temperature sensor two (76) electrically connected to the processor (71). The pressure sensor one (72) and the temperature sensor one (75) are provided on the gas buffer tank (52), the pressure sensor two (73) and the temperature sensor two (76) are provided on the pressure chamber (10), and the pressure sensor three (74) is provided on the constant speed and constant pressure injection pump (60).

2. The test device for water retention parameters and pore water distribution characteristics of methane hydrate-containing sediments according to claim 1, characterized in that The pressure chamber (10) includes a pressure chamber top cover (11), a pressure chamber outer cylinder (12), and a pressure chamber base (13). The pressure chamber top cover (11) and the pressure chamber base (13) are hermetically connected to the two ends of the pressure chamber outer cylinder (12).

3. The test device for water retention parameters and pore water distribution characteristics of methane hydrate-containing sediments according to claim 2, characterized in that, An outer jacket (14) is further provided outside the pressure chamber (10). The outer jacket (14) is externally connected to the first constant temperature bath (20), and a circulating heat-conducting liquid is provided into the outer jacket (14) through the first constant temperature bath (20).

4. The test device for the water retention parameter and pore water distribution characteristics of methane hydrate-containing sediments according to claim 1, characterized in that The pressure supply system further includes a pressure regulating valve (42), a pressure regulating knob (43), and a back pressure valve (44). The nitrogen gas cylinder (41), the pressure regulating valve (42), and the pressure regulating knob (43) are sequentially connected and then branch out two pipelines. One pipeline is connected to the top wall of the pressure chamber (10) through the back pressure valve (44), and the other pipeline is directly connected to the top wall of the pressure chamber (10).

5. The test device for water retention parameters and pore water distribution characteristics of methane hydrate-containing sediments according to claim 4, characterized in that, Further comprising: A gas-liquid separator (90) connected to the back pressure valve (44).

6. The test device for the water retention parameter and pore water distribution characteristics of methane hydrate-containing sediments according to claim 1, characterized in that The gas supply system further includes a second constant temperature bath (53) for controlling the gas temperature inside the gas buffer tank (52).

7. The test device for the water-holding capacity parameters and pore water distribution characteristics of methane hydrate-containing sediments according to claim 1, characterized in that, The gas buffer tank (52) is also provided with a safety valve (54).

8. A test method for the water retention parameter and pore water distribution characteristics of methane hydrate-containing sediments, characterized in that, The test method is based on the test device for the water retention parameter and pore water distribution characteristics of methane hydrate-containing sediments described in any one of claims 1-7, and includes the following steps: Step 1, preparation of the specimen (15). The specimen (15) is set as a cylinder and is formed by compaction in layers using a jack according to the preset moisture content and dry density. Step 2, installation of the specimen (15). The previously saturated clay plate (16) together with the sealing ring is embedded in the outer cylinder (12) of the pressure chamber. The base (13) and the top cover (11) of the pressure chamber are installed, and it is placed in the holder (31), and the entire holder (31) is placed in the low-field nuclear magnetic resonance instrument (30). The constant-speed and constant-pressure injection pump (60) is opened to inject water from the bottom wall of the pressure chamber (10) and then flows out through the drain pipe (17). After the air in the pressure chamber (10) is discharged, the drain pipe (17) is closed, and the constant-temperature bath 1 (20) is started to maintain the temperature of the pressure chamber (10) stable. Step 3, gas injection. The methane gas cylinder (51) is opened to inject methane gas into the gas buffer tank (52). When the pressure increases to the target value, the methane gas cylinder (51) is closed. The pressure and temperature of the gas buffer tank (52) in the stable state are recorded as the initial test values, and the initial amount of the reaction gas is calculated according to the gas state equation. Then the gas buffer tank (52) is opened to inject gas into the pressure chamber (10). At the same time, the constant-speed and constant-pressure injection pump (60) is started, and the pipeline between the constant-speed and constant-pressure injection pump (60) and the bottom wall of the pressure chamber (10) is made unobstructed. The constant-speed and constant-pressure injection pump (60) is set to the tracking mode. Step 4, cooling and synthesizing hydrate. When the temperature and pressure of the pressure chamber (10) both reach stability, the temperature of the constant-temperature bath 1 (20) is set to provide a low-temperature environment for the formation of hydrate to promote hydrate synthesis. When the pressure value and temperature value of the system composed of the pressure chamber (10) and the gas buffer tank (52) remain unchanged, it can be considered that the hydrate synthesis is completed. The gas buffer tank (52) and the constant-speed and constant-pressure injection pump (60) are closed, and the temperature and pressure of the system at this time are recorded. The remaining amount of the reaction gas is calculated according to the gas state equation, and the hydrate saturation can be preliminarily calculated in combination with the hydrate number. Step 5, saturation of the specimen (15). The nitrogen gas cylinder (41) is opened, and a certain pressure is applied to the back pressure valve (44). The pressure regulating valve (42) is adjusted so that the pressure of the back pressure valve (44) and the pressure of the pressure chamber (10) maintain a set difference. The pipeline between the back pressure valve (44) and the top wall of the pressure chamber (10) is made unobstructed. Under a constant pressure difference, the constant-speed and constant-pressure injection pump (60) is opened to inject water from the side wall of the pressure chamber (10). When the injected amount of water exceeds twice the pore volume of the specimen (15), it is considered that the specimen (15) is completely saturated. Step 6: Water retention and NMR tests. After the hydrate-containing specimen (15) is saturated, the initial moisture distribution state of the specimen (15) needs to be measured before the test. Turn on the low-field nuclear magnetic resonance instrument (30) and input the corresponding parameters, scan the specimen (15) in this state to obtain the transverse relaxation time distribution. Then, make the pipeline of the pressure regulating knob (43) communicate with the top wall of the pressure chamber (10), and the pipeline of the constant-speed and constant-pressure injection pump (60) communicate with the bottom wall of the pressure chamber (10). Adjust the pressure regulating knob (43) to make the air pressure higher than the pore water pressure, and conduct the test under the specified pressure difference. Record the water volume discharged from the soil through the constant-speed and constant-pressure injection pump (60). When the reading of the constant-speed and constant-pressure injection pump (60) is stable, it is considered that the system reaches the equilibrium state at this stage of pressure. Then, use the low-field nuclear magnetic resonance instrument (30) to measure the moisture distribution state in this equilibrium state, and then apply the next stage of pressure. Repeat the above steps to complete the established pressure sequence. Step 7: Data arrangement. Calculate the water saturation after the suction balance at each stage according to the initial mass, pore volume, and water output of the specimen (15). Then, based on the relationship between the suction value and water saturation at each stage, establish the soil-water characteristic relationship curve of the methane hydrate-containing sediment, and combine the pore water distribution obtained at each stage to obtain the relationship between the pore water distribution and the soil-water characteristic curve.

9. The test method for the water-holding capacity parameters and pore water distribution characteristics of methane hydrate-containing sediments according to claim 8, characterized in that In Step 4, the calculation process of the generated hydrate saturation is as follows: First, the initial amount of substance of the gas in the gas buffer tank (52) is: Open the gas buffer tank (52) to inject gas into the pressure chamber (10). After the reaction ends, the total amount of substance of the free methane gas in the gas buffer tank (52), soil pores, and pipelines is: In the formula: P is the pressure of methane gas; T is the temperature of the gas; R is the ideal gas constant; V is the volume of free methane gas; Z is the gas compression factor; a represents the gas buffer tank (52); b represents the pores of the specimen (15); c represents the pipeline; i represents the initial state of the reaction; t represents the end state of the reaction; By collecting the temperature and pressure changes during the experiment, the gas consumption Δn during the synthesis process is calculated according to the gas state equation g , and then the hydrate saturation is obtained, that is:

10. The test method for the water holding capacity parameter and pore water distribution characteristics of methane hydrate-containing sediments according to claim 9, characterized in that In Step 7, the process of drawing the soil-water characteristic relationship curve is as follows: After water injection saturation, the pore volume of the specimen (15) is V v , assuming complete saturation, then the pore volume is equal to Jia 1 = S w +S h The sum of the volume of methane hydrate and the volume of pore water, that is: Apply suction according to the planned suction steps, and record the cumulative water output volume at each suction level by loading a constant-speed and constant-pressure injection pump (60): V out , after waiting for equilibrium, the water saturation in the pores of the specimen (15) at each suction level is:[[]] Finally, draw the soil-water characteristic relationship curve.

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