A method and system for evaluating the stability of seafloor carbon dioxide hydrates

Abstract

The application provides a method and system for evaluating the stability of seabed carbon dioxide hydrate. The method comprises: filling the reactor with sediments to form a storage area and a capping layer; filling the reactor with carbon dioxide gas to form carbon dioxide hydrate in the storage area; filling the capping layer with seawater to submerge all sediments in the capping layer; taking liquid samples from different heights in the capping layer respectively, and testing the carbon dioxide concentration in the liquid samples; calculating the dissolution-diffusion degradation rate of the carbon dioxide hydrate based on the carbon dioxide concentration, and evaluating the stability of the seabed carbon dioxide hydrate. The system comprises a reactor, a temperature adjusting device, a carbon dioxide gas tank and a constant pressure seawater tank. The application can simulate the storage of carbon dioxide hydrate in seabed sediments and the dissolution-diffusion degradation process of the carbon dioxide hydrate after decomposition under the seabed undercurrent, so that the simulation process is more realistic and the result is more reliable.

Description

TECHNICAL FIELD

[0001] The application relates to a method and system for evaluating the stability of seabed carbon dioxide hydrate, and belongs to the technical field of carbon dioxide geological storage. BACKGROUND

[0002] In recent decades, the burning of a large amount of fossil energy has led to a sharp increase in the content of carbon dioxide in the atmosphere, and the climate change problem caused thereby has been listed as the primary environmental problem. Carbon dioxide capture and storage technology is an important means to solve the problem.

[0003] At present, a variety of carbon dioxide geological storage technology ideas have been proposed. Mainly including carbon dioxide storage in a trap formation, bound storage in a porous rock pore structure, carbon dioxide storage in the form of dissolution in seawater, mineral form storage of carbon dioxide converted into minerals, and carbon dioxide storage in the form of carbon dioxide hydrate formed in sediments. Therefore, the geological bodies suitable for carbon dioxide storage mainly include basalt layers, deep saltwater layers, oil and gas fields being exploited or depleted, and shallow sea sediments. The main potential risk thereof is that it may increase the acidity of water bodies, affect geochemistry and ecological balance, and large-scale carbon dioxide leakage may occur. Unlike other methods, the carbon dioxide storage method in the form of hydrate exists in a solid state, has the advantages of environmental friendliness, high safety, and low leakage risk, and has attracted widespread attention in recent years.

[0004] Carbon dioxide can spontaneously form carbon dioxide hydrate by combining with water in seabed sediments. After carbon dioxide storage is completed, carbon dioxide hydrate and seawater in the pore of the sediments reach a stable state. Since the temperature and pressure conditions of the seabed are relatively stable, when there is no large-scale tectonic event, the main control factor of the stability of carbon dioxide hydrate is the change in the concentration of carbon dioxide in seawater induced by seabed undercurrent. Therefore, the determination of the dissolution-diffusion degradation rate of carbon dioxide hydrate under different seabed undercurrent flow rates plays an important role in further establishing environmental impact, establishing a method for enhancing hydrate storage, and optimizing injection and storage areas. However, the current research on the stability of carbon dioxide hydrate mainly focuses on the decomposition behavior of hydrate, and cannot reflect the dissolution-diffusion degradation process of carbon dioxide hydrate in actual seabed sediments. Simulating the process needs to establish a constant-pressure water-rich sediment layer condition, prepare a simulation sample including a carbon dioxide hydrate storage area and an overlying sediment layer area, and simulate the complete process of seabed undercurrent. The current research lacks effective experimental simulation means and a method for measuring the degradation rate of carbon dioxide hydrate. SUMMARY

[0005] To solve the above technical problems, the present application aims to provide a method for evaluating the stability of seabed carbon dioxide hydrate. The method can simulate the storage of carbon dioxide hydrate in seabed sediments and the dissolution-diffusion degradation process of carbon dioxide hydrate after decomposition under the seafloor undercurrent, and obtain the dissolution-diffusion degradation rate of carbon dioxide hydrate.

[0006] The present application also aims to provide a system for evaluating the stability of seabed carbon dioxide hydrate. The system is used to implement the above method.

[0007] To achieve the above-mentioned purposes, the first aspect of the present application provides a method for evaluating the stability of seabed carbon dioxide hydrate, comprising the following steps:

[0008] S1: sample preparation

[0009] S101: adjust the temperature in the reaction kettle to the test temperature T 测 ;

[0010] S102: fill the reaction kettle with sediments; the sediments include two layers, the lower layer filling phase is wet sediments, which is used to provide a storage area; the upper layer filling phase is dry sediments, which is used to simulate the cap layer above the storage area;

[0011] S103: fill the reaction kettle with carbon dioxide gas to form carbon dioxide hydrate in the storage area, and after a period of time, the carbon dioxide hydrate is generated;

[0012] S104: fill the cap layer with seawater to at least submerge all the sediments of the cap layer, and the pressure in the reaction kettle reaches the test pressure P 测 , thereby forming a storage area including sediments, seawater and carbon dioxide hydrate in the lower layer of the reaction kettle, and forming a cap layer including sediments and seawater in the upper layer of the reaction kettle;

[0013] S2: test process

[0014] Under the conditions of the test temperature T 测 , constant test pressure P 测 , dynamic environment and / or static environment, liquid samples are taken from different heights in the cap layer at intervals, and the carbon dioxide concentration in the liquid samples is tested;

[0015] S3: sample analysis

[0016] Based on the carbon dioxide concentration in the liquid samples taken from different heights in the cap layer, the dissolution-diffusion degradation rate of carbon dioxide hydrate is calculated, and the stability of seabed carbon dioxide hydrate is evaluated.

[0017] In the above method, preferably, in step S101, the temperature inside the reactor is adjusted to the test temperature T 测 by a temperature adjusting device arranged outside the reactor. More preferably, the temperature adjusting device is a water bath.

[0018] In the above method, preferably, in step S102, the wet deposit is a mixture of the deposit and seawater.

[0019] In the above method, preferably, in step S103, carbon dioxide gas is filled into the reactor to a pressure of 3-4 MPa inside the reactor.

[0020] In the above method, preferably, in step S103, the completion of the formation of carbon dioxide hydrate is determined by the following method: when the pressure inside the reactor decreases by 0.01 MPa or less in 1 h, the formation of carbon dioxide hydrate is completed.

[0021] In the above method, preferably, step S104 specifically comprises: turning the reactor by 180°, at this time, the storage area is located in the upper layer of the reactor, and the capping layer is located in the lower layer of the reactor; filling seawater into the capping layer to submerge all the deposit in the capping layer, and discharging the gas inside the reactor from the top of the reactor (the top after the reactor is turned by 180°), and stopping the process of saturating the seawater with carbon dioxide when the seawater continuously discharges from the top of the reactor; continuing to fill seawater into the capping layer to reach a test pressure P 测 ; and then turning the reactor by 180° again, at this time, the storage area is located in the lower layer of the reactor, and the capping layer is located in the upper layer of the reactor, thereby forming the storage area comprising the deposit, seawater and carbon dioxide hydrate in the lower layer of the reactor, and forming the capping layer comprising the deposit and seawater in the upper layer of the reactor.

[0022] In the above method, preferably, after step S104 is completed, the storage area does not comprise carbon dioxide gas, and the capping layer does not comprise carbon dioxide hydrate.

[0023] In the present application, the sample preparation method of step S1 is different from the existing preparation method of carbon dioxide hydrate sample, and step S1 of the present application can simulate the carbon dioxide hydrate storage area and the overlying deposit layer area in the same reactor, and can also regulate the environmental temperature and pressure conditions of the simulated deposit layer.

[0024] In the above method, preferably, in step S2, the test temperature T 测 is 3-7℃, and the test pressure P 测 is 6-12 MPa.

[0025] In the above method, preferably, in step S2, the constant test pressure P 测 By connecting to the constant pressure seawater tank or constant pressure pump of the reactor, the real conditions of the seabed are simulated.

[0026] In the above method, preferably, in step S2, the dynamic environment is an environment in which the seawater in the reactor has a flow behavior simulated by constantly injecting seawater into the cap layer; and the static environment is an environment in which the seawater in the reactor is in a static state.

[0027] In the above method, preferably, step S2 specifically comprises: at the test temperature T 测 , the constant test pressure P 测 , the dynamic environment and / or the static environment, every time interval, liquid samples are taken from different heights in the cap layer by using a sampler, the sampler is connected to a flash metering tank, the sampling volume is obtained through the flash metering tank, the water in the liquid sample is removed by flashing in the flash metering tank, the carbon dioxide gas released by flashing enters a water tank containing water, the carbon dioxide gas makes the water in the water tank drain to a drain metering tank, the volume of the carbon dioxide gas released by flashing is obtained by the amount of water in the drain metering tank, and then the carbon dioxide concentration in the liquid sample taken from different heights in the cap layer is calculated. More preferably, the liquid samples taken from different heights in the cap layer at least include liquid samples taken from the lower surface of the cap layer (i.e. the interface of hydrate-seawater) and liquid samples taken from a certain height from the lower surface of the cap layer.

[0028] In the above method, preferably, in step S2, the carbon dioxide concentration in the taken liquid sample is calculated by the following formula:

[0029] C(T 测 ,P 测 ) = C1 + C2 Formula (1)

[0030] In formula (1), C(T 测 , P 测 ) is the carbon dioxide concentration in the taken liquid sample, with the unit of mol / m 3 ; C1 is the carbon dioxide gas concentration released by flashing of the liquid sample, with the unit of mol / m 3 ; and C2 is the carbon dioxide gas concentration in the liquid sample that is not released by flashing, with the unit of mol / m 3 ;

[0031] C1 is calculated by the following formula:

[0032]

[0033] In formula (2), V Q is the sampling volume obtained by the flash metering tank, in m 3 ; n is the molar amount of carbon dioxide gas released by the liquid sample through flashing, in mol;

[0034] n is calculated by the following formula:

[0035]

[0036] In formula (3), P0 is the atmospheric pressure, with a value of 101325 Pa; V g is the volume of carbon dioxide gas released through flashing obtained by the drainage amount in the drainage metering tank, in m 3 ; R is the gas constant, with a value of 8.31441 J / (mol·K); T0 is the room temperature, with a value of 293.15 K; z is the compression factor, with a value of 0.9945 (obtained by the ideal gas state equation under the conditions of room temperature T0 and atmospheric pressure P0);

[0037] C2 is calculated by the following formula:

[0038] C2 = C sc (T0, P0) formula (4)

[0039] In formula (4), C sc (T0, P0) is the solubility of carbon dioxide gas in the seawater used under the conditions of room temperature T0 and atmospheric pressure P0, in mol / m 3 .

[0040] Wherein, the solubility of carbon dioxide gas in the seawater used under the conditions of room temperature T0 and atmospheric pressure P0 can be tested or calculated by the method in the prior art, for example, calculated by using the model of Duan et al. (Zhenhao Duan et al. An improved model for the calculation of CO2 solubility in aqueous solutions containing Na + , K + , Ca 2+ , Mg 2+ , Cl - , and SO4 2- . Marine Chemistry (2006).

[0041] In the above method, preferably, step S2 further comprises: plotting the carbon dioxide concentration in the liquid sample taken at different heights in the cover layer versus the time of taking the liquid sample.

[0042] In the above method, preferably, in step S3, the dissolution-diffusion degradation rate of the carbon dioxide hydrate at a certain height from the lower surface of the capping layer is calculated by the following formula at a certain time of taking the liquid sample:

[0043]

[0044] In formula (5), DR is the dissolution-diffusion degradation rate of the carbon dioxide hydrate at a certain height from the lower surface of the capping layer, in units of cm / year; J is the CO2diffusion flux at a certain height from the lower surface of the capping layer, in units of mol cO2 / m 2 / s; represents the ideal gas volume corresponding to 1 mol of CO2at standard conditions; represents that 1 volume of carbon dioxide hydrate stores 160 volumes of CO2gas at standard conditions, wherein 0.8 is the pore occupancy of the carbon dioxide hydrate;

[0045] J is calculated by the following formula:

[0046]

[0047] In formula (6), is the porosity of the sediment, in units of dimensionless; dC / dx is the CO2concentration gradient; D s is the CO2diffusion factor at a certain height from the lower surface of the capping layer, in units of m 2 / s; the “-” sign indicates that the diffusion direction is opposite to the CO2concentration gradient;

[0048] D S is calculated by the following formula:

[0049]

[0050] In formula (7), k = C x2 / C x1 ; C x2 is the carbon dioxide concentration in the liquid sample taken at the lower surface of the capping layer (i.e., the hydrate-seawater interface); C x1 is the carbon dioxide concentration in the liquid sample taken at a height of x1from the lower surface of the capping layer, in units of mol / m 3 ; d is the distance between x1and the lower surface of the capping layer, in units of m; t is the time of taking the liquid sample, in units of s;

[0051] dC / dx is the difference between C x2 and C x1 (i.e., Cx2 - C x1 ) divided by the distance between x1 and the lower surface of the cap rock (i.e. d). Wherein the carbon dioxide concentration C x2 is greater than C x1 .

[0052] In the above method, preferably, step S3 further comprises: plotting the dissolution-diffusion degradation rate of carbon dioxide hydrate at a certain height from the lower surface of the cap rock versus the time of taking liquid samples. More preferably, step S3 further comprises: calculating the dissolution-diffusion degradation rate of carbon dioxide hydrate at different heights from the lower surface of the cap rock, and plotting the dissolution-diffusion degradation rate of carbon dioxide hydrate at different heights from the lower surface of the cap rock versus the time of taking liquid samples.

[0053] In the above method, preferably, in step S3, the stability of the seafloor carbon dioxide hydrate is evaluated by determining the value of the dissolution-diffusion degradation rate of carbon dioxide hydrate at a certain height from the lower surface of the cap rock when the dissolution-diffusion degradation rate is basically unchanged with the time of taking liquid samples as the limiting value of the dissolution-diffusion degradation rate, and evaluating the stability of the seafloor carbon dioxide hydrate according to the magnitude of the limiting value of the dissolution-diffusion degradation rate. That is, the stability of the carbon dioxide hydrate at a certain height from the lower surface of the cap rock under the conditions of a test temperature T 测 , a constant test pressure P 测 , a dynamic environment and / or a static environment is evaluated.

[0054] The second aspect of the present application provides a system for evaluating the stability of seafloor carbon dioxide hydrate, which is used to implement the above-mentioned method for evaluating the stability of seafloor carbon dioxide hydrate, and the system comprises: a reaction kettle, a temperature adjusting device, a carbon dioxide gas tank and a constant pressure seawater tank;

[0055] Wherein, the reaction kettle is filled with sediments; the sediments comprise two layers, the lower layer is filled with wet sediments to provide a storage area, and the upper layer is filled with dry sediments to simulate the cap rock above the storage area;

[0056] The temperature adjusting device is arranged outside the reaction kettle to adjust the temperature in the reaction kettle to a test temperature T 测 ;

[0057] The carbon dioxide gas tank is connected to the reaction kettle to fill the storage area of the reaction kettle with carbon dioxide gas to form carbon dioxide hydrate in the storage area;

[0058] The constant-pressure seawater tank is connected to the reactor, and is used at least for filling seawater into the capping layer of the reactor to at least submerge all sediments in the capping layer, and to make the pressure in the reactor reach a test pressure P 测 .

[0059] In the system, the constant-pressure seawater tank is further used for constant-speed injection of seawater into the capping layer to simulate the flow behavior of seawater in the reactor, and to simulate a dynamic environment. The constant-pressure seawater tank is further used for maintaining the pressure in the reactor as a constant test pressure P when a dynamic environment is used 测 .

[0060] In the system, preferably, the reactor is a high-pressure resistant reactor.

[0061] In the system, preferably, the temperature adjusting device is a water bath.

[0062] According to the specific embodiment of the present application, preferably, the system further comprises a constant-pressure pump connected to the reactor, and the constant-pressure pump is used for maintaining the pressure in the reactor as a constant test pressure P when a static environment is used 测 .

[0063] According to the specific embodiment of the present application, preferably, the system further comprises a sampler, a flash measurement tank, a water tank and a water measurement tank; the sampler is in communication with the flash measurement tank when in use, the water tank contains water, the flash measurement tank is in communication with the water tank, and the water tank is in communication with the water measurement tank; the sampler is used for taking liquid samples at different heights in the capping layer, the flash measurement tank is used for obtaining a sampling volume and for flash removal of water in the liquid sample, the water tank is used for receiving carbon dioxide gas released by flash and for discharging water in the water tank to the water measurement tank, and the water measurement tank is used for obtaining the volume of carbon dioxide gas released by flash through the amount of water therein.

[0064] The technical solution of the present application has at least the following advantages and beneficial effects:

[0065] The application develops a new type of carbon dioxide hydrate containing sediment layer generation method, which can simulate carbon dioxide hydrate storage area and overlying sediment layer area in the same reaction kettle, and can also regulate the simulated sediment layer environmental temperature and pressure conditions. The application can simulate the real marine conditions, i.e. marine sediments and water-rich state, so that seawater fills the entire sediment layer. After the storage stage, a constant pressure seawater tank is used to simulate the process of free seawater permeation, and different submarine undercurrent flow rates (i.e. dynamic environment) can be simulated. The application uses a sampling and metering method to more accurately determine the CO2 concentration in the sampled sample. The method and system provided by the application can simulate the carbon dioxide hydrate storage process and its dissolution-diffusion degradation process under different submarine storage environments, obtain the dissolution-diffusion degradation rate of carbon dioxide hydrate, and further evaluate the stability of submarine carbon dioxide hydrate. The method and system of the application make the simulation process more realistic and the results more reliable, and make up for the blank of the prior art in the determination of the dissolution-diffusion degradation rate of carbon dioxide hydrate in submarine sediments, which has important significance for establishing environmental impact, establishing enhanced hydrate storage method, and optimizing injection and storage area. BRIEF DESCRIPTION OF DRAWINGS

[0066] Figure 1 The structural schematic diagram of the system for evaluating the stability of submarine carbon dioxide hydrate in Example 1.

[0067] Figure 2 The carbon dioxide concentration evolution graph in the liquid samples taken at three different heights in the cap layer in Example 3 with sampling time.

[0068] Figure 3 The dissolution-diffusion degradation rate evolution graph of carbon dioxide hydrate at different heights from the lower surface of the cap layer in Example 3 with sampling time.

[0069] BRIEF DESCRIPTION OF DRAWINGS

[0070] 1-reaction kettle; 2-temperature adjusting device; 3-carbon dioxide gas tank; 4-constant pressure seawater tank; 5-sampler; 6-flash metering tank; 7-drainage tank; 8-drainage metering tank. DETAILED DESCRIPTION

[0071] In order to have a clearer understanding of the technical features, objectives and beneficial effects of the application, the technical solutions of the application will be described in detail below, but it cannot be understood as limiting the implementable scope of the application.

[0072] Example 1

[0073] This embodiment provides a system for evaluating the stability of submarine carbon dioxide hydrate, as shown in Figure 1As shown, the system includes: a reactor 1, a temperature regulating device 2, a carbon dioxide gas tank 3 and a constant pressure seawater tank 4;

[0074] The reactor 1 is filled with sediments; the sediments include two layers, the lower filling phase is wet sediments, which are used to provide a storage area; the upper filling phase is dry sediments, which are used to simulate the capping layer above the storage area;

[0075] The temperature regulating device 2 is arranged outside the reactor 1 and is used to regulate the temperature inside the reactor 1 to the test temperature T 测 ;

[0076] The carbon dioxide gas tank 3 is connected to the reactor 1 and is used to fill the sealed area of ​​the reactor 1 with carbon dioxide gas to form carbon dioxide hydrate in the sealed area;

[0077] The constant pressure seawater tank 4 is connected to the reactor 1 and is used to fill the sealing layer of the reactor 1 with seawater until at least all the sediments in the sealing layer are submerged and the pressure in the reactor 1 reaches the test pressure P 测 .

[0078] The constant pressure seawater tank 4 is also used to inject seawater into the capping layer at a constant rate to simulate the flow behavior of seawater in the reactor 1, thereby simulating a dynamic environment. The constant pressure seawater tank 4 is also used to maintain the pressure in the reactor 1 at a constant test pressure P when a dynamic environment is used. 测 .

[0079] The system further includes: a sampler 5, a flash evaporation metering tank 6, a drainage tank 7 and a drainage metering tank 8; when in use, the sampler 5 is connected to the flash evaporation metering tank 6, the drainage tank 7 contains water, the flash evaporation metering tank 6 is connected to the drainage tank 7, and the drainage tank 7 is connected to the drainage metering tank 8; the sampler 5 is used to take liquid samples from different heights in the capping layer, the flash evaporation metering tank 6 is used to obtain the sampling volume and to flash evaporate and remove water in the liquid sample, the drainage tank 7 is used to receive the carbon dioxide gas released by flash evaporation and discharge the water in the drainage tank 7 to the drainage metering tank 8, and the drainage metering tank 8 is used to obtain the volume of carbon dioxide gas released by flash evaporation through the drainage volume therein.

[0080] In this embodiment, the reactor 1 is a high-pressure reactor.

[0081] In this embodiment, the temperature adjustment device 2 is a water bath.

[0082] In this embodiment, the system may further include: a constant pressure pump ( Figure 1 (not shown), which is connected to the reactor 1, and the constant pressure pump is used to maintain the pressure in the reactor 1 at a constant test pressure P test when a static environment is adopted.

[0083] The system of the embodiment also comprises valves, temperature and pressure detecting elements and other components, which can be selected and arranged conventionally by those skilled in the art, and the present application does not specially limit them.

[0084] Embodiment 2

[0085] The embodiment provides a method for evaluating the stability of seabed carbon dioxide hydrate, which is performed by using the system of embodiment 1, and the method comprises the following steps:

[0086] S1: sample preparation

[0087] S101: adjusting the temperature in the reaction kettle 1 to a test temperature T 测 ;

[0088] S102: filling the sediment into the reaction kettle 1; the sediment comprises two layers, the lower layer is a wet sediment as a filling phase for providing a storage area, and the upper layer is a dry sediment as a filling phase for simulating a capping layer above the storage area;

[0089] S103: filling carbon dioxide gas into the reaction kettle 1 to form carbon dioxide hydrate in the storage area, and the carbon dioxide hydrate is generated after a period of time;

[0090] S104: filling seawater into the capping layer to at least submerge all the sediment of the capping layer, and the pressure in the reaction kettle 1 reaches a test pressure P 测 , thereby forming a storage area comprising the sediment, seawater and carbon dioxide hydrate in the lower layer of the reaction kettle 1, and forming a capping layer comprising the sediment and seawater in the upper layer of the reaction kettle 1;

[0091] S2: test process

[0092] Under the conditions of a test temperature T 测 , a constant test pressure P 测 , a dynamic environment and / or a static environment, liquid samples are taken from different heights in the capping layer at intervals, and the carbon dioxide concentration in the liquid samples is tested;

[0093] S3: sample analysis

[0094] Based on the carbon dioxide concentration in the liquid samples taken from different heights in the capping layer, the dissolution-diffusion degradation rate of the carbon dioxide hydrate is calculated, and the stability of the seabed carbon dioxide hydrate is evaluated.

[0095] In the embodiment, the temperature in the reaction kettle 1 is adjusted to a test temperature T 测 in step S101, which is achieved by a temperature adjusting device 2 arranged outside the reaction kettle 1.

[0096] In the present embodiment, in step S102, the wet deposit is a mixture of the deposit and seawater. Preferably, the wet deposit is prepared by mixing the deposit with a temperature of -10°C or below and seawater with a temperature of -5 to 0°C at a mass ratio of (6-7): 1.

[0097] In the present embodiment, in step S102, the wet deposit is a mixture of the deposit and seawater. Preferably, the wet deposit is prepared by mixing the deposit with a temperature of -10°C or below and seawater with a temperature of -5 to 0°C at a mass ratio of (6-7): 1.

[0098] In the present embodiment, in step S103, the carbon dioxide gas is filled into the reactor 1 to a pressure of 3-4 MPa in the reactor 1.

[0099] In the present embodiment, in step S103, the completion of the carbon dioxide hydrate is determined by the following method: when the pressure in the reactor 1 decreases by 0.01 MPa or less in 1 h, the carbon dioxide hydrate is determined to be completed. When the carbon dioxide hydrate is completed, the reactor 1 contains the deposit, seawater, carbon dioxide gas, and carbon dioxide hydrate.

[0100] In the present embodiment, step S104 specifically includes: turning the reactor 1 by 180°, at this time, the storage area is located in the upper layer of the reactor 1, and the capping layer is located in the lower layer of the reactor 1; filling seawater into the capping layer to submerge all the deposit in the capping layer, and discharging the gas in the reactor 1 from the top of the reactor 1 (the top after the reactor 1 is turned by 180°), and stopping the process of saturating the seawater with carbon dioxide when the seawater continuously discharges from the top of the reactor 1; continuing to fill seawater into the capping layer to reach a test pressure P 测 ; then turning the reactor 1 by 180° again, at this time, the storage area is located in the lower layer of the reactor 1, and the capping layer is located in the upper layer of the reactor 1, thereby forming the storage area containing the deposit, seawater, and carbon dioxide hydrate in the lower layer of the reactor 1, and forming the capping layer containing the deposit and seawater in the upper layer of the reactor 1. The temperature of the seawater filled in should be the same as the test temperature T 测 , and the valve of the reactor 1 should be opened to discharge the seawater, and step S104 should be performed above the carbon dioxide hydrate phase equilibrium pressure to avoid the decomposition of the carbon dioxide hydrate.

[0101] In the present embodiment, after step S104 is completed, the storage area does not contain carbon dioxide gas, and the capping layer does not contain carbon dioxide hydrate.

[0102] In the present embodiment, in step S1, the test temperature T 测 is 3-7°C, and the test pressure P 测 is 6-12 MPa.

[0103] In the present embodiment, in step S2, the constant test pressure P 测 The constant pressure seawater tank 4 or the constant pressure pump connected to the reactor 1 is used to maintain the simulated real conditions of the seabed.

[0104] In the present embodiment, in step S2, the dynamic environment is an environment in which the seawater in the reactor 1 has a flow behavior simulated by injecting seawater into the cap layer at a constant speed; the static environment is an environment in which the seawater in the reactor 1 is in a static state, which can be understood as the speed of the injected seawater being 0. The seawater injected into the cap layer at a constant speed is seawater without dissolved CO2.

[0105] In the present embodiment, step S2 specifically comprises: at the test temperature T 测 , the constant test pressure P 测 , the dynamic environment and / or the static environment, every time interval, liquid samples are taken from different heights in the cap layer by using the sampler 5, the sampler 5 is connected to the flash metering tank 6, the sampling volume is obtained through the flash metering tank 6, the water in the liquid sample is removed by flash in the flash metering tank 6, the carbon dioxide gas released by flash enters the water-containing drainage tank 7, the carbon dioxide gas makes the water in the drainage tank 7 drain to the drainage metering tank 8, the volume of the carbon dioxide gas released by flash is obtained by the amount of drainage in the drainage metering tank 8, and then the carbon dioxide concentration in the liquid sample taken from different heights in the cap layer is calculated. The flash in the flash metering tank 6 is atmospheric flash. The taking of liquid samples from different heights in the cap layer at least includes taking liquid samples from the lower surface of the cap layer (i.e. the interface of hydrate-seawater) and taking liquid samples from a certain height away from the lower surface of the cap layer.

[0106] In the present embodiment, in step S2, the interval time of taking liquid samples from different heights in the cap layer each time can be routinely adjusted by those skilled in the art.

[0107] In the present embodiment, in step S2, the carbon dioxide concentration in the taken liquid sample is calculated by the following formula:

[0108] C(T 测 , P 测 ) = C1 + C2 Formula (1)

[0109] In formula (1), C(T 测 , P 测 ) is the carbon dioxide concentration in the taken liquid sample, with the unit of mol / m 3 ; C1 is the carbon dioxide gas concentration released by flash of the liquid sample, with the unit of mol / m 3; C2 is the concentration of carbon dioxide gas not released by the liquid sample after flashing, in units of mol / m 3 ;

[0110] C1 is calculated by the following formula:

[0111]

[0112] In formula (2), V Q is the sampling volume obtained by the flash metering tank 6, in units of m 3 ; n is the molar amount of carbon dioxide gas released by the liquid sample after flashing, in units of mol:

[0113] n is calculated by the following formula:

[0114]

[0115] In formula (3), P0 is the atmospheric pressure, with a value of 101325 Pa; V g is the volume of carbon dioxide gas released by flashing obtained by the amount of drainage in the drainage metering tank 8, in units of m 3 ; R is the gas constant, with a value of 8.31441 J / (mol·K); T0 is the room temperature, with a value of 293.15 K; z is the compression factor, with a value of 0.9945 (obtained by the ideal gas state equation under the conditions of room temperature T0 and atmospheric pressure P0);

[0116] C2 is calculated by the following formula:

[0117] C2 = C sc (T0, P0) formula (4)

[0118] In formula (4), C sc (T0, P0) is the solubility of carbon dioxide gas in the seawater used under the conditions of room temperature T0 and atmospheric pressure P0, in units of mol / m 3 .

[0119] The solubility of carbon dioxide gas in the seawater used under the conditions of room temperature T0 and atmospheric pressure P0 can be tested or calculated by methods in the prior art, for example, calculated by using the model of Duan et al.

[0120] In this embodiment, step S2 further includes: plotting the evolution of the carbon dioxide concentration in the liquid sample taken at different heights in the cover layer over time.

[0121] In this embodiment, in step S3, the dissolution-diffusion degradation rate of carbon dioxide hydrate at a height from the lower surface of the cover layer at a certain time of taking the liquid sample is calculated by the following formula:

[0122]

[0123] In formula (5), DR is a dissolution-diffusion degradation rate of carbon dioxide hydrate at a certain height from the lower surface of the cap layer, in units of cm / year; J is a CO2diffusion flux at a certain height from the lower surface of the cap layer, in units of mol CO2 / m 2 / s; represents a volume of an ideal gas corresponding to 1 mol of CO2at standard conditions; represents that 1 volume of carbon dioxide hydrate stores 160 volumes of CO2gas at standard conditions, in which 0.8 is a pore occupancy of carbon dioxide hydrate;

[0124] J is calculated by the following formula:

[0125]

[0126] In formula (6), is a porosity of the sediment (which is a property of the sediment), in units of dimensionless; dC / dx is a CO2concentration gradient; D S is a CO2diffusion factor at a certain height from the lower surface of the cap layer, in units of m 2 / s; the "-" sign indicates that the diffusion direction is opposite to the CO2concentration gradient;

[0127] D S is calculated by the following formula:

[0128]

[0129] In formula (7), k = C x2 / C x1 ; C x2 is a carbon dioxide concentration in a liquid sample taken at the lower surface of the cap layer (i.e., the hydrate-seawater interface), C x1 is a carbon dioxide concentration in a liquid sample taken at a height of xl from the lower surface of the cap layer, in units of mol / m 3 ; d is a distance between xl and the lower surface of the cap layer, in units of m; t is a time of taking the liquid sample (the sampling time is calculated from the end time of step S104 as 0), in units of s;

[0130] dC / dx is calculated by the difference (i.e., C x2 -C x1 ) between C x2 and C x1 , divided by the distance (i.e., d) between xl and the lower surface of the cap layer.

[0131] In this embodiment, step S3 further includes: plotting the time evolution of the dissolution-diffusion degradation rate of carbon dioxide hydrate at a certain height from the lower surface of the capping layer as a function of the time when the liquid sample is taken. Step S3 may further include: calculating the dissolution-diffusion degradation rate of carbon dioxide hydrate at different heights from the lower surface of the capping layer, and plotting the time evolution of the dissolution-diffusion degradation rate of carbon dioxide hydrate at different heights from the lower surface of the capping layer as a function of the time when the liquid sample is taken.

[0132] In this embodiment, in step S3, the stability of the seabed carbon dioxide hydrate is evaluated in the following manner: based on the dissolution-diffusion degradation rate of the carbon dioxide hydrate at a certain height from the lower surface of the cap layer and the time evolution of the liquid sample, the value at which the dissolution-diffusion degradation rate does not substantially change with the time of taking the liquid sample is determined as the dissolution-diffusion degradation rate limit value, and according to the magnitude of the dissolution-diffusion degradation rate limit value, the stability of the seabed carbon dioxide hydrate at the test temperature T is evaluated. 测 , constant test pressure P 测 The stability of carbon dioxide hydrate at a certain height from the lower surface of the capping layer under the conditions of dynamic environment and / or static environment.

[0133] Example 3

[0134] This embodiment provides a method for evaluating the stability of seabed carbon dioxide hydrates. The method is performed using the system of embodiment 1, and the steps included in the method are the same as those of embodiment 2.

[0135] The reactor 1 has a total height of 1200 mm. The lower layer of wet sediment is 700 mm high, and the upper layer of dry sediment is 500 mm high. The dry sediment used is 40-mesh quartz sand. The wet sediment is prepared by thoroughly mixing 40-mesh quartz sand at -10°C and seawater at 0°C in a mass ratio of 20:3. The seawater used is 3.5% saline water.

[0136] Test temperature T 测 At 3°C, the test pressure P 测 It is 6MPa.

[0137] In step S103, at the test temperature T 测Under the condition of 3℃, carbon dioxide gas is filled into the reactor 1 until the pressure in the reactor 1 is 3.7MPa, and carbon dioxide hydrate is gradually formed in the storage area. After a period of time, when the pressure in the reactor 1 decreases by less than 0.01MPa in 1h, the formation of carbon dioxide hydrate is completed. At this time, the reactor 1 contains sediments, seawater, carbon dioxide gas and carbon dioxide hydrate.

[0138] In step S104, the reactor 1 is turned 180°, and at this time, the storage area is located at the upper layer of the reactor 1, and the capping layer is located at the lower layer of the reactor 1. Under the condition of the test temperature T 测 , the capping layer is filled with seawater until all the sediments in the capping layer are submerged. The temperature of the seawater should be the same as the test temperature T 测 . At the same time, the valve of the reactor 1 is opened, and the gas in the reactor 1 is discharged from the top of the reactor 1 (the top after the reactor 1 is turned 180°). When the seawater continuously discharges from the top of the reactor 1, the process of saturating the seawater with carbon dioxide is stopped. The capping layer is continuously filled with seawater until the pressure in the reactor 1 reaches the test pressure P 测 , i.e. 6MPa. Then, the reactor 1 is turned 180° again, and at this time, the storage area is located at the lower layer of the reactor 1, and the capping layer is located at the upper layer of the reactor 1. Thus, the storage area containing sediments, seawater and carbon dioxide hydrate is formed in the lower layer of the reactor 1, and the capping layer containing sediments and seawater is formed in the upper layer of the reactor 1. In order to avoid the decomposition of carbon dioxide hydrate, step S104 should be performed above the phase equilibrium pressure of carbon dioxide hydrate.

[0139] In step S2, under the condition of the test temperature T 测 , the constant test pressure P 测 , and the static environment, liquid samples are taken from three different heights in the capping layer at intervals of a period of time by using the sampler 5. The sampler 5 is connected with the flash metering tank 6, and the sampling volume is obtained through the flash metering tank 6. The water in the liquid sample is removed by flash in the flash metering tank 6, and the carbon dioxide gas released by flash enters the water tank 7 containing water. The carbon dioxide gas makes the water in the water tank 7 discharge to the drainage metering tank 8, and the volume of the carbon dioxide gas released by flash is obtained by the amount of the water in the drainage metering tank 8. Thus, the carbon dioxide concentration in the liquid samples taken from the three different heights in the capping layer is calculated. The flash in the flash metering tank 6 is atmospheric flash. The pressure in the reactor 1 is maintained as the constant test pressure P 测 by the constant pressure pump. The three different heights are the lower surface of the capping layer (i.e. the interface of hydrate-seawater), 200mm away from the lower surface of the capping layer, and 400mm away from the lower surface of the capping layer, respectively.

[0140] In step S2, the carbon dioxide concentration in the liquid sample taken at three different heights in the cap layer is calculated by equations (1), (2), (3) and (4), and a plot of its evolution over time of taking the liquid sample is drawn, as shown in Figure 2 .

[0141] In step S3, the dissolution-diffusion degradation rate of carbon dioxide hydrate at different heights from the lower surface of the cap layer is calculated by equations (5), (6) and (7), and a plot of its evolution over time of taking the liquid sample is drawn, as shown in Figure 3 .

[0142] A calculation example is as follows:

[0143] When t = 176 h, the sample volume obtained by the flash metering tank 6 at the lower surface of the cap layer is known to be 1.61 x 10 -6 m 3 , and the drainage volume in the drainage metering tank 8 is 2.1193 x 10 -5 m 3 .

[0144] Then, according to equation (3), the amount of carbon dioxide released by the sample flash is:

[0145]

[0146] According to equation (2), the carbon dioxide gas concentration released by the flash of the liquid sample is:

[0147]

[0148] Based on the solubility of carbon dioxide gas in the seawater employed at room temperature T0, atmospheric pressure P0, the carbon dioxide gas concentration not released by the flash of the liquid sample is 32.9721 mol / m 3 . Then, according to equation (1), the carbon dioxide concentration in the liquid sample taken is:

[0149] C(T 测 , P 测 ) = C1 + C2 = 550.2484 + 32.9721 = 583.2205 mol / m 3

[0150] From the calculation, the carbon dioxide concentration in the liquid sample at the lower surface of the cap layer (i.e., the hydrate-seawater interface) when t = 176 h, at a distance of 200 mm from the lower surface of the cap layer, and at a distance of 400 mm from the lower surface of the cap layer, is 583.2205, 69.5310, 58.1320 mol / m 3 , respectively, as shown in the results Figure 2 .Figure 2 The unit is mol / kg, which needs to be converted.

[0151] Taking the thickness of the cap layer as 200 mm as an example, the carbon dioxide concentration in the liquid sample taken at the lower surface of the cap layer and 200 mm away from the lower surface of the cap layer is used for calculation.

[0152] According to formula (7), the CO2diffusion factor at 200 mm away from the lower surface of the cap layer is calculated:

[0153]

[0154] According to formula (6), the CO2diffusion flux at 200 mm away from the lower surface of the cap layer is calculated, wherein, The value is 0.455059:

[0155]

[0156] According to formula (5), the dissolution-diffusion degradation rate of carbon dioxide hydrate at a height of 200 mm away from the lower surface of the cap layer at t = 176 h is calculated:

[0157]

[0158] Based on the above results, the stability of carbon dioxide hydrate on the seabed is evaluated: under the condition of a test temperature T 测 of 3°C, a constant test pressure P 测 of 6 MPa, and a static environment, when the thickness of the cap layer is 200 mm, the limit value of the dissolution-diffusion degradation rate of carbon dioxide hydrate (i.e., the dissolution-diffusion degradation rate tends to a certain stable value and remains basically unchanged with the change of sampling time) is 43.94 cm / year, and when the thickness of the cap layer is 400 mm, the limit value of the dissolution-diffusion degradation rate of carbon dioxide hydrate (i.e., the dissolution-diffusion degradation rate tends to a certain stable value and remains basically unchanged with the change of sampling time) is 4.87 cm / year, indicating that the greater the thickness of the cap layer, the stronger the stability of carbon dioxide hydrate.

Claims

1. A method for evaluating the stability of seabed carbon dioxide hydrate, comprising the following steps: S1: sample preparation S101 : Adjust the temperature inside the reactor to the test temperature T 测 ; S102: filling the reactor with sediment; the sediment comprises two layers, the lower layer is a wet sediment as the filling phase for providing a storage zone; the upper layer is a dry sediment as the filling phase for simulating a capping layer above the storage zone; S103: filling the reactor with carbon dioxide gas to form carbon dioxide hydrate in the storage zone, and the formation of carbon dioxide hydrate is completed after a period of time; S104: turn the reactor 180°, at this time the storage area is located in the upper layer of the reactor, the capping layer is located in the lower layer of the reactor; fill seawater into the capping layer to submerge all the sediments in the capping layer, the gas in the reactor is discharged from the top of the reactor, when there is continuous seawater discharge from the top of the reactor, the seawater saturated carbon dioxide process stops; continue to fill seawater into the capping layer to make the pressure in the reactor reach the test pressure P 测 ; then turn the reactor 180° again, at this time the storage area is located in the lower layer of the reactor, the capping layer is located in the upper layer of the reactor, thereby forming the storage area including sediments, seawater and carbon dioxide hydrate in the lower layer of the reactor, and forming the capping layer including sediments and seawater in the upper layer of the reactor; After step S104 is completed, the storage zone does not contain carbon dioxide gas, and the capping layer does not contain carbon dioxide hydrate; S2: test process at a test temperature T 测 , at a constant test pressure P 测 , under dynamic and / or static environmental conditions, liquid samples are taken at different heights in the cover layer at intervals of time, and the carbon dioxide concentration is tested in each of them. S3: sample analysis Based on the carbon dioxide concentration in the liquid samples taken from different heights in the capping layer, the dissolution-diffusion degradation rate of carbon dioxide hydrate is calculated, and the stability of seabed carbon dioxide hydrate is evaluated.

2. The method for evaluating the stability of a seafloor carbon dioxide hydrate according to claim 1, wherein, In step S102, the wet sediment is a mixture of sediment and seawater.

3. The method for evaluating the stability of a seafloor carbon dioxide hydrate according to claim 1, wherein, In step S103, carbon dioxide gas is filled into the reactor to a pressure of 3-4 MPa in the reactor.

4. The method for evaluating the stability of a seafloor carbon dioxide hydrate according to claim 1, wherein, In step S103, the completion of carbon dioxide hydrate formation is determined by the following method: when the pressure in the reactor decreases by 0.01 MPa or less in 1 hour, the formation of carbon dioxide hydrate is completed.

5. The method for evaluating the stability of a seafloor carbon dioxide hydrate according to claim 1, wherein, In step S2, the test temperature T 测 is 3-7 ℃, and the test pressure P 测 is 6-12 MPa.

6. The method for evaluating the stability of a seafloor carbon dioxide hydrate according to claim 1, wherein In step S2, the dynamic environment is an environment in which seawater flows in the reactor, and the flow behavior is simulated by continuously injecting seawater into the capping layer; the static environment is an environment in which seawater is in a static state in the reactor.

7. The method for evaluating the stability of a seafloor carbon dioxide hydrate according to claim 1, wherein Step S2 specifically comprises: under the conditions of a test temperature T 测 , a constant test pressure P 测 , a dynamic environment and / or a static environment, every time interval, liquid samples are taken from different heights in the capping layer respectively by using a sampler, the sampler is communicated with a flash metering tank, a sampling volume is obtained through the flash metering tank, water in the liquid sample is removed by flash in the flash metering tank, carbon dioxide gas released by flash enters a water containing drainage tank, the carbon dioxide gas makes water in the drainage tank drain to a drainage metering tank, the volume of carbon dioxide gas released by flash is obtained through the drainage amount in the drainage metering tank, and then the carbon dioxide concentration in the liquid sample taken from different heights in the capping layer is calculated.

8. The method for evaluating the stability of a seafloor carbon dioxide hydrate according to claim 1 or 7, wherein In step S2, the liquid samples taken from different heights in the capping layer include at least liquid samples taken from the lower surface of the capping layer and liquid samples taken from a certain height above the lower surface of the capping layer.

9. The method for evaluating the stability of a seafloor carbon dioxide hydrate according to claim 7, wherein, In step S2, the carbon dioxide concentration in the liquid samples taken is calculated by the following formula: Formula (1) In formula (1), C02, taken in the liquid sample, in mol / m3 3 ; C 1 C02, released by the liquid sample by flashing, in mol / m3 3 ; C 2 C02, not released by the liquid sample by flashing, in mol / m3 3 ; C 1 Calculated by the following formula: Formula (2) in formula (2), is the sampling volume obtained by the flash metering tank, in m 3 ; n is the molar quantity of carbon dioxide gas released by the liquid sample by flashing, in mol; n The calculation is made by the following formula: Formula (3) In formula (3), P 0 is atmospheric pressure, and has a value of 101325 Pa; V g is the volume of carbon dioxide gas released by flashing, obtained by measuring the amount of drainage in the drainage tank, and has a unit of m 3 ; R is a gas constant, and has a value of 8.31441 J / (mol·K); T 0 is room temperature, and has a value of 293.15 K; z is a compression factor, and has a value of 0.9945; C 2 Calculated by the following formula: Formula (4) In formula (4), at room temperature T 0 atmospheric pressure P 0 solubility in the seawater employed under the conditions of carbon dioxide gas at room temperature, in mol / m 3 .

10. The method for evaluating the stability of a seafloor carbon dioxide hydrate according to claim 7, wherein Step S2 further comprises: plotting the carbon dioxide concentration in the liquid samples taken from different heights in the capping layer versus the time of taking liquid samples.

11. The method for evaluating the stability of a seafloor carbon dioxide hydrate according to claim 9, wherein, In step S3, at a certain time of taking liquid samples, the dissolution-diffusion degradation rate of carbon dioxide hydrate at a certain height above the lower surface of the capping layer is calculated by the following formula: Formula (5) In formula (5), DR is the dissolution-diffusion degradation rate of the CO2 hydrate at a certain height from the lower surface of the cap rock, in units of cm / year; J is the CO2 diffusion flux at a certain height from the lower surface of the cap rock, in units of mol CO2 / m 2 / s; represents the ideal gas volume corresponding to 1 mol of CO2 at standard conditions; represents that 1 volume of CO2 hydrate stores 160 volumes of CO2 gas at standard conditions, where 0.8 is the pore occupancy of the CO2 hydrate. J The calculation is made by the following formula: Formula (6) In formula (6), φ porosity of the deposit, dimensionless; dC / dx D is the CO2 diffusion factor at a certain height from the lower surface of the cover layer, in m / s; the "−" sign indicates that the diffusion direction is opposite to the CO2 concentration gradient; D S D is the CO2 diffusion factor at a certain height from the lower surface of the cover layer, in m / s; the "−" sign indicates that the diffusion direction is opposite to the CO2 concentration gradient; 2 D is the CO2 diffusion factor at a certain height from the lower surface of the cover layer, in m / s; the "−" sign indicates that the diffusion direction is opposite to the CO2 concentration gradient; D S Calculated by the following formula: Formula (7) C (7) in formula (7), k = C x2 / C x1 ; C x2 C is the concentration of carbon dioxide in the liquid sample taken at the lower surface of the cover layer, C x1 C is the concentration of carbon dioxide in the liquid sample taken at a height of xi from the lower surface of the cover layer, in mol / m3; 3 ; d xi is the distance between xi and the lower surface of the cover layer, in m; t t is the time at which the liquid sample is taken, in s; dC / dx By C x2 and C x1 calculated by dividing the difference between the values of C at x1 and at the lower surface of the capping layer by the distance between x1 and the lower surface of the capping layer.

12. The method for evaluating the stability of a seafloor carbon dioxide hydrate according to claim 11, wherein Step S3 further comprises: plotting the dissolution-diffusion degradation rate of carbon dioxide hydrate at a certain height above the lower surface of the capping layer versus the time of taking liquid samples.

13. The method for evaluating the stability of a seafloor carbon dioxide hydrate according to claim 11, wherein Step S3 further comprises: calculating the dissolution-diffusion degradation rate of carbon dioxide hydrate at different heights above the lower surface of the capping layer, and plotting the dissolution-diffusion degradation rate of carbon dioxide hydrate at different heights above the lower surface of the capping layer versus the time of taking liquid samples.

14. The method for evaluating the stability of a seafloor carbon dioxide hydrate according to claim 12, wherein, In step S3, the stability of the seabed carbon dioxide hydrate is evaluated by determining, based on the time evolution of the dissolution-diffusion degradation rate of the carbon dioxide hydrate at a certain height from the lower surface of the cap rock, the value of the dissolution-diffusion degradation rate when the dissolution-diffusion degradation rate is substantially constant with respect to the time of taking the liquid sample, as the limiting value of the dissolution-diffusion degradation rate, and evaluating the stability of the seabed carbon dioxide hydrate according to the magnitude of the limiting value of the dissolution-diffusion degradation rate.

15. A system for evaluating the stability of a seafloor carbon dioxide hydrate for implementing the method of evaluating the stability of a seafloor carbon dioxide hydrate according to any one of claims 1 to 14, the system comprising: a reaction kettle, a temperature adjusting device, a carbon dioxide gas tank, and a constant-pressure seawater tank; The reaction kettle is filled with a sediment, and the sediment includes two layers, a lower layer filled with a wet sediment for providing a storage area, and an upper layer filled with a dry sediment for simulating a cap rock above the storage area. The temperature adjusting device is arranged outside the reaction kettle and is used for adjusting the temperature in the reaction kettle to a test temperature T 测 ; The carbon dioxide gas tank is connected to the reaction kettle for charging the storage area of the reaction kettle with carbon dioxide gas to form carbon dioxide hydrate in the storage area. The constant pressure seawater tank is connected to the reactor, at least for filling seawater into the cover layer of the reactor to at least submerge all the sediments of the cover layer, and to make the pressure in the reactor reach the test pressure P 测 .

16. The system for evaluating the stability of a seafloor carbon dioxide hydrate according to claim 15, wherein The system further includes a sampler, a flash evaporation measuring tank, a water discharge tank, and a water discharge measuring tank. The sampler is in communication with the flash evaporation measuring tank in use. The water discharge tank contains water. The flash evaporation measuring tank is in communication with the water discharge tank. The water discharge tank is in communication with the water discharge measuring tank. The sampler is used to take liquid samples from different heights in the cap rock. The flash evaporation measuring tank is used to obtain a sampling volume and to flash evaporate water in the liquid sample. The water discharge tank is used to receive carbon dioxide gas released by flash evaporation and to discharge water in the water discharge tank to the water discharge measuring tank. The water discharge measuring tank is used to obtain the volume of carbon dioxide gas released by flash evaporation through the amount of water therein.

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