A method for constructing a methane adsorption model for hydrated muddy silt and an experimental device

Abstract

The present invention discloses a method and experimental device for constructing a methane adsorption model for hydrated muddy silt, comprising the following steps: Step S1: obtaining muddy silt under dry conditions and under wet conditions as multiple sets of dry samples and wet samples, and conducting CO2 adsorption experiments at 273K and liquid nitrogen adsorption experiments at 77K on the dry samples to analyze the pore structure characteristics of the muddy silt; Step S2: conducting methane isothermal adsorption experiments on the multiple sets of dry samples and wet samples using a methane isothermal adsorption experimental device based on the volumetric method to analyze the methane adsorption characteristics of the muddy silt; Step S3: modifying the solid isothermal adsorption model to obtain a methane isothermal adsorption model applicable to muddy silt. The present invention conducts low-pressure CO2 and N2 adsorption and high-pressure methane adsorption experiments to obtain the microscopic pore structure characteristics and methane adsorption characteristics of the muddy silt in the South China Sea, and optimizes the methane adsorption model for the muddy silt.

Description

Technical Field

[0001] The present invention relates to the technical field of exploration of methane adsorption in muddy silt, and in particular to a method for constructing a hydrate muddy silt methane adsorption model and an experimental device. Background Art

[0002] The basic idea of ​​natural gas hydrate extraction is to change the phase equilibrium conditions for the stable existence of hydrates, so that the hydrates decompose in situ to produce natural gas and water, and then transport the natural gas to the ground along pores, natural fractures, artificial fractures and pipes, ultimately achieving efficient hydrate extraction. [4] . Because natural gas hydrate reservoirs contain a large number of micro-nano pores and have a large specific surface area, there may be a certain adsorption effect during the hydrate extraction process. During the hydrate extraction process, due to the decrease in pressure, the adsorbed gas desorbs and becomes free gas, which can supplement the gas production; conversely, the gas may be partially adsorbed in the reservoir pores again during the flow process, which brings difficulties to gas extraction. Therefore, studying the adsorption characteristics of methane gas in hydrate reservoirs is of great significance for estimating the natural gas reserves in hydrate reservoirs, determining the timing of hydrate extraction, and the flow of gas in hydrate reservoirs.

[0003] Unlike shale gas reservoirs, hydrate reservoirs do not contain organic matter, but their mineral composition is similar to that of shale. XRD test results of natural gas hydrate reservoirs in the South my country Sea show that carbonate minerals account for 54.4%, of which the main component is aragonite (46.6%); clay minerals account for about 22%, of which the main component is illite (20.59%); others such as quartz account for 17.2%, and feldspar accounts for about 6.3%.

[10] Although organic matter is considered to be the main contributor to adsorption in shale gas reservoirs, it is also generally recognized that clay minerals participate in the adsorption process. Schettler and Parmoly found a higher adsorption capacity in low TOC shales, which the researchers attributed to the adsorption properties of clay minerals, especially illite. They conducted adsorption tests on pure illite and montmorillonite and proved that pure clay minerals have a methane adsorption capacity comparable to that of organic-rich shales.

[11] Lu et al. also believed that the adsorption of methane on illite is considerable.

[12] However, Ross and Bustin et al. pointed out that due to the hydrophilicity of clay mineral surfaces, clay minerals do not contribute to the gas adsorption capacity when water is present, and water blocks the passage of gas molecules to the adsorption sites.

[13] In addition, since adsorption is a spontaneous exothermic reaction, it is more likely to occur at lower temperatures. The temperature of hydrate reservoirs is much lower than that of shale reservoirs, which makes it possible for hydrate reservoirs to have an adsorption capacity comparable to that of shale reservoirs. However, the existing technology lacks exploration of the methane adsorption characteristics of muddy silt, making it difficult to master a muddy silt methane adsorption model that fits the methane adsorption characteristics of muddy silt. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for constructing a hydrate muddy silt methane adsorption model and an experimental device to solve the technical problem of the lack of exploration of the methane adsorption characteristics of muddy silt in the prior art.

[0005] In order to solve the above technical problems, the present invention specifically provides the following technical solutions:

[0006] A method for constructing a methane adsorption model for hydrated muddy silt comprises the following steps:

[0007] Step S1: Obtain muddy silt under dry conditions and under water conditions as multiple groups of dry samples and water samples, and conduct CO2 adsorption experiments at 273K and liquid nitrogen adsorption experiments at 77K on the dry samples based on the standard static volume method on an ASAP2460 surface and pore size analyzer, respectively, to obtain CO2 adsorption data of the dry samples and N2 adsorption and desorption analysis to determine the pore structure characteristics of the muddy silt;

[0008] Step S2: performing methane isothermal adsorption experiments on multiple groups of dry samples and water-containing samples using a methane isothermal adsorption experimental device based on a volumetric method to obtain methane adsorption data for the dry samples and methane adsorption data for the water-containing samples to analyze the methane adsorption characteristics of the muddy silt;

[0009] Step S3: Modify the solid isothermal adsorption model based on the pore structure characteristics and methane adsorption characteristics of the muddy silt to obtain a methane isothermal adsorption applicable model for the muddy silt.

[0010] As a preferred embodiment of the present invention, the method for obtaining the dry sample and the water sample includes:

[0011] The muddy silt was dried in a constant temperature box at 80°C for 12 hours, and then the dried muddy silt was ground into muddy silt powder with a particle size of less than 80 mesh. The muddy silt powder was divided into multiple groups of dry samples according to their mass;

[0012] Based on the water content formula and the water saturation formula, a conversion formula between water content and water saturation is constructed. The conversion formula is expressed as follows:

[0013]

[0014] Where mc is the moisture content, ρ w is the density of water, ρ b is the apparent density of muddy silt, S w is water saturation;

[0015] Obtain the apparent density ρ of muddy silt b , the density of water ρ w Water saturation S w , calculating the existing value of the water content of the muddy silt using a conversion formula, and estimating the actual value of the water content of the muddy silt based on the existing value of the water content of the muddy silt;

[0016] The amount of water to be added to the silt sand is calculated using a moisture content formula based on the actual value of the moisture content of the silt sand. Water is then added to the silt sand based on the amount of water added to obtain a water-containing sample, thereby restoring the moisture content of the silt sand to that of the natural environment storage state in the laboratory.

[0017] As a preferred embodiment of the present invention, the pore structure characteristics of muddy silt are analyzed based on the CO2 adsorption data and N2 adsorption and desorption data of multiple groups of dry samples, including:

[0018] CO2 adsorption data of multiple groups of dry samples were used to draw CO2 gas isothermal adsorption curves of the dry samples at 273K. Adsorption potential theory was introduced into the analysis of the CO2 gas isothermal adsorption curves at 273K to obtain CO2 low-temperature adsorption pore structure parameters of the multiple groups of dry samples characterized by the Dubinin-Radushkevich equation and the Dubinin-Astakhov equation. Based on the CO2 low-temperature adsorption pore structure parameters, density functional theory was used to draw micropore size distribution curves of the multiple groups of dry samples to obtain the micropore size structure and distribution characteristics of the muddy silt.

[0019] The N2 adsorption and desorption data of multiple groups of dry samples were used to draw the N2 isothermal adsorption and desorption curves of the dry samples at 77K. The BET theory and Gurvich rule were used to characterize the N2 low-temperature adsorption pore structure parameters of the multiple groups of dry samples. The Barrett-Joyner-Halenda method was then used to draw the BJH pore size distribution curves of the multiple groups of dry samples to obtain the medium and large pore size structure and distribution characteristics of the muddy silt.

[0020] The pore structure characteristics of the muddy silt are obtained based on the micropore size structure and distribution characteristics and the medium and large pore size structure and distribution characteristics of the muddy silt.

[0021] As a preferred embodiment of the present invention, the methane adsorption characteristics of muddy silt are analyzed based on the methane adsorption data of multiple groups of dry samples and the methane adsorption data of water-containing samples, including:

[0022] The methane adsorption isotherm curves of dry samples at 17°C were drawn using the methane adsorption data of multiple groups of dry samples. The first methane adsorption characteristic, which characterizes the change of methane adsorption capacity of muddy silt with pressure, was obtained by analyzing the methane adsorption isotherm curves of dry samples at 17°C.

[0023] The methane adsorption isotherm curve of the water-containing sample at 17°C was drawn using the methane adsorption data of the water-containing sample. The methane adsorption isotherm curve of the water-containing sample at 17°C was combined with the methane adsorption isotherm curve of the dry sample at 17°C for analysis to obtain the second methane adsorption characteristic that characterizes the influence of water content on the first methane adsorption characteristic.

[0024] As a preferred embodiment of the present invention, the method for constructing the applicable model for methane isotherm adsorption includes:

[0025] A correlation formula between excess adsorption capacity and absolute adsorption capacity is established, and the correlation formula is:

[0026]

[0027] Where n e is the excess adsorption capacity, n a is the absolute adsorption capacity, ρ g is the gas density, ρ a is the density of the adsorbed phase, V a is the volume of the adsorbed phase;

[0028] The correction factor is set based on the correlation formula, and the calculation formula of the correction factor is:

[0029]

[0030] According to the pore structure characteristics and methane adsorption characteristics, the Langmuir model and the DR micropore filling model are selected as the solid isothermal adsorption model. Based on the correction factor and the Langmuir model and the DR micropore filling model, the model is corrected to obtain the corrected Langmuir model and the DR micropore filling model, so as to realize the correction of the model to be applied in the experimental environment. The expressions of the corrected Langmuir model and the DR micropore filling model are respectively:

[0031]

[0032] Where n0 is the maximum adsorption capacity, D is a constant related to the affinity coefficient of the adsorbate, p0 is the saturated vapor pressure, n a is the absolute adsorption amount, n L is the Langmuir adsorption capacity, p L is the Langmuir pressure;

[0033] The modified Langmuir and DR equations were quadratically corrected by using gas density instead of pressure to achieve the correction of the model to apply to supercritical adsorption. The expressions of the quadratically corrected Langmuir and DR equations are:

[0034]

[0035] Where, ρ L is the Langmuir gas density, k H is Henry's constant;

[0036] The quadratically corrected Langmuir and DR equations were used as applicable models for methane adsorption isotherms. The methane adsorption data of dry samples and the methane adsorption data of water-containing samples were fitted respectively using the quadratically corrected Langmuir and DR equations as applicable models for methane adsorption isotherms. The mean absolute error was used to characterize the fitting effect. The expression for the mean absolute error is:

[0037]

[0038] Where N is the number of adsorption equilibrium points, and are the simulated value and experimental value of the adsorption amount at the i-th adsorption equilibrium point, respectively;

[0039] The fitting effects of the quadratically corrected Langmuir and DR equations as applicable models for methane isotherm adsorption on the methane adsorption data of dry samples and methane adsorption data of water-containing samples were analyzed. The quadratically corrected Langmuir model was used as the applicable model for methane isotherm adsorption under lower pressure conditions, and the quadratically corrected DR equation was used as the applicable model for methane isotherm adsorption under medium and high pressure conditions.

[0040] As a preferred embodiment of the present invention, the experimental process of the methane isothermal adsorption experiment includes:

[0041] Step 1: calibrate the volume of a sample kettle in a methane isothermal adsorption experimental apparatus using helium adsorption, and place a dry sample / water sample into the sample kettle and calibrate the free space volume using helium adsorption. The free space volume is characterized by the difference between the sample kettle volume and the dry sample / water sample volume.

[0042] Step 2: A fixed amount of methane gas is introduced into the methane isothermal adsorption experimental apparatus after volume calibration, and the initial pressure of the methane isothermal adsorption experimental apparatus is recorded. By opening and closing valves D4 and D6 in the methane isothermal adsorption experimental apparatus, the methane gas in the reference kettle in the methane isothermal adsorption experimental apparatus is gradually transferred to the sample kettle. After the pressure stabilizes, the equilibrium pressures in the reference kettle and the sample kettle are recorded respectively.

[0043] Step 3: Change the initial pressure and repeat step 2 multiple times to obtain multiple sets of equilibrium pressures. Then, calculate the methane gas adsorption data of the dry sample / water sample at different equilibrium pressures according to the law of conservation of mass. The calculation formula of the methane gas adsorption data is:

[0044]

[0045] Among them, n e,i is the methane adsorption capacity of the dry sample / water sample at the i-th adsorption equilibrium, n in,i is the initial gas volume in the reference kettle, n ref,i is the amount of gas in the reference kettle at the i-th adsorption equilibrium, n equ,i is the amount of gas in the sample kettle at the i-th adsorption equilibrium, ρ in,i is the reference kettle density at initial pressure, ρ ref,i is the gas density in the reference kettle at the i-th adsorption equilibrium, ρ equ,i is the gas density in the sample kettle at the i-th adsorption equilibrium, M is the molar mass of methane gas, and m is the mass of dry sample / water sample.

[0046] As a preferred embodiment of the present invention, the calibration process of the sample kettle volume and the free space volume includes:

[0047] Step 11: Place stainless steel beads in the sample kettle, set the oil bath temperature to the experimental temperature of 17°C, fill a fixed amount of helium into the reference kettle, and record the initial pressure of the methane isothermal adsorption experimental device after the pressure stabilizes.

[0048] Step 22: Open valves D4 and D6 in the methane isothermal adsorption experimental device to connect the reference kettle and the sample kettle, and isothermally expand helium into the sample kettle. After adsorption equilibrium is reached, record the equilibrium pressure.

[0049] Step 33: Change the number of steel balls in the sample kettle and repeat steps 11 and 22 multiple times to obtain three sets of initial pressure and equilibrium pressure values. According to the law of conservation of mass, a first helium mass conservation formula is constructed. The first helium mass conservation formula is:

[0050] ρ ref V ref =ρ equ (V ref +V sam -ΔV);

[0051] The first helium mass conservation formula is converted into a steel ball volume calculation formula, which is:

[0052]

[0053] Where ΔV is the volume of the steel ball, ρ ref , ρ equ are the densities corresponding to the initial pressure and equilibrium pressure, V ref is the reference kettle volume, V sam is the volume of the sample kettle;

[0054] Step 44: Calculate multiple sets of initial density and equilibrium density using temperature, multiple sets of initial pressure and equilibrium pressure values, and plot ΔV versus (ρ equ -ρ ref ) / ρ equ The relationship curve between them is shown in Figure 1, where the slope of the curve is the reference kettle volume and the intercept of the straight line on the y-axis is the sample kettle volume;

[0055] Step 55: Load the dry sample / water sample into the sample kettle, set the oil bath temperature to the experimental temperature of 17°C, inject helium into the methane isothermal adsorption experimental device, record the initial pressure in the reference kettle and the equilibrium pressure after the reference kettle and the sample kettle are connected, and construct a second helium mass conservation formula based on the law of conservation of mass. The second helium mass conservation formula is:

[0056] ρ ref V ref =ρ equ (V ref +V void );

[0057] The second helium mass conservation formula is converted into a free space volume calculation formula, which is:

[0058]

[0059] Among them, V void is the free space volume.

[0060] As a preferred embodiment of the present invention, the present invention provides a methane isothermal adsorption experimental device according to the method for constructing a hydrate muddy silt methane adsorption model, comprising a helium gas supply tank, a methane gas supply tank, a sample kettle, a reference kettle, valve D1, valve D2, valve D3, valve D4, valve D5, valve D6, pipelines, a pressure sensor and an oil bath, wherein:

[0061] The helium gas supply tank and the methane gas supply tank are respectively connected to the A end and the B end of the valve D1 through pipelines, the C end of the valve D1 and the A end of the valve D3 are respectively connected to the A end and the B end of the valve D2 through pipelines, the B end of the valve D3 is connected to the A end of the valve D5 through a pipeline, the C end of the valve D2 and the A end of the valve D4 are respectively connected to the kettle mouth end of the reference kettle through pipelines, the B end of the valve D4 and the B end of the valve D5 are respectively connected to the A end of the valve D6 through pipelines, and the B end of the valve D6 is connected to the kettle mouth end of the sample kettle. Pressure sensors are arranged inside the sample kettle and the reference kettle to measure the internal pressure of the sample kettle and the reference kettle. The sample kettle, the reference kettle, the valve D4, the valve D5, the valve D6 and the connecting pipelines between the sample kettle, the reference kettle, the valve D4, the valve D5 and the valve D6 are placed inside the oil bath to maintain a constant temperature during the methane isothermal adsorption experiment.

[0062] As a preferred solution of the present invention, the valves D4, D5, D6 and the connecting pipelines between the valves D4, D5 and D6 are used to gradually transfer the helium or methane gas in the reference kettle to the sample kettle, so as to achieve the effect of one-time gas injection into the reference kettle and multiple adsorption of dry samples / water samples in the sample kettle.

[0063] As a preferred embodiment of the present invention, the methane adsorption capacity obtained by performing a methane isothermal adsorption experiment using the methane isothermal adsorption experimental device is the excess adsorption capacity.

[0064] Compared with the prior art, the present invention has the following beneficial effects:

[0065] The present invention conducts low-pressure CO2 and N2 adsorption and high-pressure methane adsorption experiments to obtain the microscopic pore structure characteristics and methane adsorption properties of the muddy silt in the South China Sea, preliminarily determine the methane adsorption magnitude and adsorption influencing factors of the muddy silt, and preferably applies a methane adsorption model to the muddy silt to obtain key adsorption parameters, providing data support and theoretical guidance for subsequent research work. BRIEF DESCRIPTION OF THE DRAWINGS

[0066] To more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for the embodiments or the description of the prior art. Obviously, the drawings described below are merely exemplary, and those skilled in the art can derive other implementation drawings based on the provided drawings without inventive effort.

[0067] Figure 1 A flow chart of a method for constructing a hydrate muddy silt methane adsorption model provided in an embodiment of the present invention;

[0068] Figure 2 Provided isothermal adsorption curve of CO2 gas at 273K for dry samples in the embodiment of the present invention;

[0069] Figure 3 A diagram of the CO2 low-temperature adsorption pore structure parameters of a dry sample provided by an embodiment of the present invention;

[0070] Figure 4 A micropore size distribution curve of a dried sample provided in an embodiment of the present invention;

[0071] Figure 5 N2 isothermal adsorption and desorption curves of the dry sample provided in the embodiment of the present invention at 77K;

[0072] Figure 6 A diagram of the pore structure parameters of N2 low-temperature adsorption provided by an embodiment of the present invention;

[0073] Figure 7 BJH pore size distribution curve of the dried sample provided in the embodiment of the present invention;

[0074] Figure 8 This is a graph showing the methane adsorption isotherm of a dry sample at 17°C provided in an embodiment of the present invention;

[0075] Figure 9 Isothermal adsorption curves of the KT4-16 sample provided in an embodiment of the present invention under dry and water conditions;

[0076] Figure 10 The KT4-16 dried sample provided in the embodiment of the present invention is fitted with the quadratically corrected Langmuir and DR models.

[0077] Figure 11 The KT4-17 dried sample provided in the embodiment of the present invention is fitted with the quadratically corrected Langmuir and DR models.

[0078] Figure 12 The KT4-18 dried sample provided in the embodiment of the present invention is fitted with the quadratically corrected Langmuir and DR models.

[0079] Figure 13 This is a diagram showing the fitting results of the KT4-16 aqueous sample using the quadratic modified Langmuir and DR models according to an embodiment of the present invention;

[0080] Figure 14 The results of fitting the KT4-17 dry sample, KT4-17 dry sample, KT4-18 dry sample and KT4-16 water sample using the quadratic corrected Langmuir and DR models provided in the embodiments of the present invention are shown in FIG.

[0081] Figure 15 Schematic diagram of the structure of the methane isothermal adsorption experimental device provided in an embodiment of the present invention.

[0082] The numbers included in the figure are:

[0083] 1- Sample kettle; 2- Reference kettle; 3- Pressure sensor; 4- Methane gas supply tank; 5- Helium gas supply tank; 6- Oil bath. DETAILED DESCRIPTION

[0084] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0085] like Figure 1 As shown, the present invention provides a method for constructing a hydrate muddy silt methane adsorption model, comprising the following steps:

[0086] Step S1: Obtain muddy silt under dry conditions and under water conditions as multiple groups of dry samples and water samples, and conduct CO2 adsorption experiments at 273K and liquid nitrogen adsorption experiments at 77K on the dry samples based on the standard static volume method on an ASAP2460 surface and pore size analyzer, respectively, to obtain CO2 adsorption data of the dry samples and N2 adsorption and desorption analysis to determine the pore structure characteristics of the muddy silt;

[0087] The method for obtaining the dry sample and the water sample comprises:

[0088] The muddy silt was dried in a constant temperature box at 80°C for 12 hours, and then the dried muddy silt was ground into muddy silt powder with a particle size of less than 80 mesh. The muddy silt powder was divided into multiple groups of dry samples according to their mass;

[0089] Based on the water content formula and the water saturation formula, a conversion formula between water content and water saturation is constructed. The conversion formula is expressed as follows:

[0090]

[0091] Where m c is the moisture content, ρ w is the density of water, ρ b is the apparent density of muddy silt, S w is water saturation;

[0092] Obtain the apparent density ρ of muddy silt b , the density of water ρ w , water saturation S w, calculating the existing value of the water content of the muddy silt using a conversion formula, and estimating the actual value of the water content of the muddy silt based on the existing value of the water content of the muddy silt;

[0093] The amount of water to be added to the silt sand is calculated using a moisture content formula based on the actual value of the moisture content of the silt sand. Water is then added to the silt sand based on the amount of water added to obtain a water-containing sample, thereby restoring the moisture content of the silt sand to that of the natural environment storage state in the laboratory.

[0094] Water saturation is the volume of water contained in a certain pore volume and is defined as follows:

[0095]

[0096] In the above formula, S w is the water saturation, V w is the volume of water, V b is the apparent volume of muddy silt, φ is the porosity, m w is the mass of water, ρ w is the density of water, m b is the mass of the water sample, ρ b is the apparent density of muddy silt.

[0097] The water content is the ratio of the mass of water in the water-containing sample to the mass of muddy silt:

[0098]

[0099] Among them, m c is the water content. The muddy silt in this embodiment is taken from the surface seabed mud of the muddy silt reservoir in the South China Sea. The water density ρ w Take 1g / cm 3 , apparent density of muddy silt ρ b Take 2.1g / cm 3 , the porosity φ is taken as 0.35, the hydrate saturation of the South China Sea muddy silt sand reservoir is taken as 70%, then the water saturation in the South China Sea muddy silt sand reservoir is 1-70%=30%, and the final calculated moisture content of the muddy silt sand is 5%. Because a large amount of water is produced by the decomposition of hydrates during the mining process, the actual moisture content of the South China Sea muddy silt sand reservoir should be greater than 5%. In this paper, the moisture content of the water sample is set to 20%.

[0100] Based on the CO2 adsorption data and N2 adsorption and desorption data of multiple sets of dry samples, the pore structure characteristics of muddy silt were analyzed, including:

[0101] CO2 adsorption data of multiple groups of dry samples were used to draw CO2 gas isothermal adsorption curves of the dry samples at 273K. Adsorption potential theory was introduced into the analysis of the CO2 gas isothermal adsorption curves at 273K to obtain CO2 low-temperature adsorption pore structure parameters of the multiple groups of dry samples characterized by the Dubinin-Radushkevich equation and the Dubinin-Astakhov equation. Based on the CO2 low-temperature adsorption pore structure parameters, density functional theory was used to draw micropore size distribution curves of the multiple groups of dry samples to obtain the micropore size structure and distribution characteristics of the muddy silt.

[0102] In this embodiment, three groups of dried samples are set, namely KT4-16 dried sample, KT4-17 dried sample and KT4-18 dried sample. The KT4-16 dried sample, KT4-17 dried sample and KT4-18 dried sample are subjected to CO2 adsorption at 273K based on the standard static volume method on the ASAP2460 surface and pore size analyzer. The CO2 gas isotherm adsorption curves of the KT4-16 dried sample, KT4-17 dried sample and KT4-18 dried sample at 273K are as follows: Figure 2 As shown in the figure, the pore structure parameters of CO2 low temperature adsorption of multiple groups of dry samples characterized by the Dubinin-Radushkevich equation and the Dubinin-Astakhov equation are shown in the figure. Figure 3 As shown, based on the CO2 low temperature adsorption pore structure parameters, the density functional theory was used to draw the micropore size distribution curves of multiple groups of dry samples as shown in Figure 4 As shown in the figure, the pore size distribution curve of muddy silt has more small peaks, which indicates that the distribution of muddy silt is more uneven.

[0103] The N2 adsorption and desorption data of multiple groups of dry samples were used to draw the N2 isothermal adsorption and desorption curves of the dry samples at 77K. The BET theory and Gurvich rule were used to characterize the N2 low-temperature adsorption pore structure parameters of the multiple groups of dry samples. The Barrett-Joyner-Halenda method was then used to draw the BJH pore size distribution curves of the multiple groups of dry samples to obtain the medium and large pore size structure and distribution characteristics of the muddy silt.

[0104] The pore structure characteristics of the muddy silt are obtained based on the micropore size structure and distribution characteristics and the medium and large pore size structure and distribution characteristics of the muddy silt.

[0105] In this example, three groups of dried samples were set, namely KT4-16 dried sample, KT4-17 dried sample and KT4-18 dried sample. Liquid nitrogen adsorption experiment was carried out on the ASAP2460 surface and pore size analyzer at 77K based on the standard static volume method for the KT4-16 dried sample, KT4-17 dried sample and KT4-18 dried sample. The N2 isothermal adsorption and desorption curves of the KT4-16 dried sample, KT4-17 dried sample and KT4-18 dried sample at 77K were obtained as shown in FIG. Figure 5 As shown in Figure 1, the BET specific surface area (BETSSA) was calculated from the N2 adsorption and desorption data in the relative pressure range of 0.05 to 0.25. The pore volume (TPV) of the sample was calculated based on the adsorption of liquid nitrogen at a relative pressure of approximately 1 according to the Gurvich rule. The average pore diameter (APD) was calculated from the known BETSSA and TPV as follows: Figure 6 As shown, the BJH pore size distribution curves of multiple groups of dry samples were drawn using the Barrett-Joyner-Halenda method. Figure 7 As shown, from Figure 7 It can be seen that muddy silt is distributed from micropores, mesopores to macropores, among which the micro, meso and macropores are evenly distributed in muddy silt.

[0106] Step S2: performing methane isothermal adsorption experiments on multiple groups of dry samples and water-containing samples using a methane isothermal adsorption experimental device based on a volumetric method to obtain methane adsorption data for the dry samples and methane adsorption data for the water-containing samples to analyze the methane adsorption characteristics of the muddy silt;

[0107] Based on the methane adsorption data of multiple sets of dry samples and water samples, the methane adsorption characteristics of muddy silt were analyzed, including:

[0108] The methane adsorption isotherm curves of dry samples at 17°C were drawn using the methane adsorption data of multiple groups of dry samples. The first methane adsorption characteristic, which characterizes the change of methane adsorption capacity of muddy silt with pressure, was obtained by analyzing the methane adsorption isotherm curves of dry samples at 17°C.

[0109] In this embodiment, a methane isothermal adsorption experiment was conducted on three groups of dry samples, namely, KT4-16 dry sample, KT4-17 dry sample and KT4-18 dry sample, using a methane isothermal adsorption experimental device to obtain isothermal adsorption curves of different muddy silt samples under dry conditions at 17°C. Figure 8As shown in the figure, the adsorption curve of muddy silt generally shows an increasing trend with increasing pressure. When the pressure reaches a certain value, some adsorption curves begin to show a slowing or decreasing growth rate. Generally speaking, the gas adsorption capacity measured under laboratory conditions is the excess adsorption capacity, which can be defined as the difference between the amount of gas in the adsorption phase volume in the adsorption state and the amount of gas in the absence of adsorption (free phase) in this volume. When the pressure is low, the adsorption growth rate is faster, and the gas density in the adsorption phase increases. At the same pressure conditions, the gas density of the free phase is low. Therefore, under low pressure conditions, the adsorption capacity increases rapidly with increasing density. When the pressure increases to a certain value, gas adsorption gradually approaches saturation, the gas density growth rate in the adsorption phase slows down, while the gas density in the free phase continues to increase, resulting in a decrease in the difference between the gas density in the adsorption phase and the gas density in the free phase, which is manifested as a decrease in excess adsorption capacity.

[0110] The methane adsorption isotherm curve of the water-containing sample at 17°C was drawn using the methane adsorption data of the water-containing sample. The methane adsorption isotherm curve of the water-containing sample at 17°C was combined with the methane adsorption isotherm curve of the dry sample at 17°C for analysis to obtain the second methane adsorption characteristic that characterizes the influence of water content on the first methane adsorption characteristic.

[0111] To characterize the methane adsorption of muddy silt under water-containing reservoir conditions, this example prepared a KT4-16 water sample with a water content of 20% and conducted a methane isothermal adsorption experiment at a water content of 20%. The water content value is obtained from step S1. The methane isothermal adsorption experiment was conducted on the water sample. At 17°C, the isothermal adsorption curves of the KT4-16 dry sample and the KT4-16 water sample are as follows: Figure 9 As shown. Figure 9 It can be seen that the effect of water content on methane gas adsorption can be divided into three parts: under low pressure (<2MPa), water content has almost no effect on methane adsorption; under medium pressure (2-10MPa), water content in the sample reduces the methane adsorption amount; under higher pressure (>10MPa), water content in the sample increases the adsorption amount.

[0112] Step S3: Modify the solid isothermal adsorption model based on the pore structure characteristics and methane adsorption characteristics of the muddy silt to obtain a methane isothermal adsorption applicable model for the muddy silt.

[0113] The method for constructing the applicable model for methane isotherm adsorption includes:

[0114] The adsorption capacity measured in the experiment is the excess adsorption capacity, while the absolute adsorption capacity is the most commonly used in practical applications. Therefore, it is necessary to establish a correlation formula between the excess adsorption capacity and the absolute adsorption capacity to convert the experimental environment and practical application. The correlation formula is:

[0115]

[0116] Where n e is the excess adsorption capacity, n a is the absolute adsorption capacity, ρ g is the gas density, ρ a is the density of the adsorbed phase, V a is the volume of the adsorbed phase;

[0117] The correction factor is set based on the correlation formula, and the calculation formula of the correction factor is:

[0118]

[0119] According to the pore structure characteristics and methane adsorption characteristics, the Langmuir model and the DR micropore filling model are selected as the solid isothermal adsorption model. Based on the correction factor and the Langmuir model and the DR micropore filling model, the model is corrected to obtain the corrected Langmuir model and the DR micropore filling model, so as to realize the correction of the model to be applied in the experimental environment. The expressions of the corrected Langmuir model and the DR micropore filling model are respectively:

[0120]

[0121]

[0122] Where n0 is the maximum adsorption capacity, D is a constant related to the affinity coefficient of the adsorbate, p0 is the saturated vapor pressure, n a is the absolute adsorption amount, n L is the Langmuir adsorption capacity, p L is the Langmuir pressure;

[0123] The modified Langmuir and DR equations were quadratically corrected by using gas density instead of pressure to achieve the correction of the model to apply to supercritical adsorption. The expressions of the quadratically corrected Langmuir and DR equations are:

[0124]

[0125] Where, ρ L is the Langmuir gas density, k H is Henry's constant;

[0126] The quadratically corrected Langmuir and DR equations were used as the applicable models for methane isotherm adsorption. The methane adsorption data of dry samples and the methane adsorption data of water-containing samples were fitted respectively, and the simulation results of the modified Langmuir and DR models were obtained as shown in the figure. Figures 10 to 14 As shown, the mean absolute error is used to characterize the fitting effect, and the expression of the mean absolute error is:

[0127]

[0128] Where N is the number of adsorption equilibrium points, and are the simulated value and experimental value of the adsorption amount at the i-th adsorption equilibrium point, respectively;

[0129] The fitting effects of the quadratically corrected Langmuir and DR equations as applicable models for methane isotherm adsorption on the methane adsorption data of dry samples and methane adsorption data of water-containing samples were analyzed. The quadratically corrected Langmuir model was used as the applicable model for methane isotherm adsorption under lower pressure conditions, and the quadratically corrected DR equation was used as the applicable model for methane isotherm adsorption under medium and high pressure conditions.

[0130] Depend on Figures 10 to 14 As shown in the figure, the Langmuir model is more suitable for simulating isothermal adsorption at lower pressures, while the modified DR equation is more suitable for characterizing isothermal adsorption under medium and high pressure conditions. This indicates that in hydrate reservoirs, when the pressure is low, methane gas may tend to adsorb on the surface. As the pressure increases, the surface adsorption sites gradually decrease, and methane gas turns to fill the micropores. The maximum adsorption capacity (corresponding to n in the Langmuir model) is L , the order of n0) in the DR equation is Figure 8 The positions of the adsorption curves are basically the same, showing that the adsorption capacity of the dried sample KT4-17 is the strongest, followed by the dried sample KT4-18, and the adsorption capacity of the dried sample KT4-16 is the weakest. L The maximum adsorption capacity of DR, n0, decreased by 21.88%, and the maximum adsorption capacity of DR, n0, decreased by 13.67%.

[0131] The experimental process of the methane isothermal adsorption experiment includes:

[0132] Step 1: calibrate the volume of a sample kettle in a methane isothermal adsorption experimental apparatus using helium adsorption, and place a dry sample / water sample into the sample kettle and calibrate the free space volume using helium adsorption. The free space volume is characterized by the difference between the sample kettle volume and the dry sample / water sample volume.

[0133] Step 2: A fixed amount of methane gas is introduced into the methane isothermal adsorption experimental apparatus after volume calibration, and the initial pressure of the methane isothermal adsorption experimental apparatus is recorded. By opening and closing valves D4 and D6 in the methane isothermal adsorption experimental apparatus, the methane gas in the reference kettle in the methane isothermal adsorption experimental apparatus is gradually transferred to the sample kettle. After the pressure stabilizes, the equilibrium pressures in the reference kettle and the sample kettle are recorded respectively.

[0134] Step 3: Change the initial pressure and repeat step 2 multiple times to obtain multiple sets of equilibrium pressures. Then, calculate the methane gas adsorption data of the dry sample / water sample at different equilibrium pressures according to the law of conservation of mass. The calculation formula of the methane gas adsorption data is:

[0135]

[0136] Among them, n e,i is the methane adsorption capacity of the dry sample / water sample at the i-th adsorption equilibrium, n in,i is the initial gas volume in the reference kettle, n ref,i is the amount of gas in the reference kettle at the i-th adsorption equilibrium, n equ,i is the amount of gas in the sample kettle at the i-th adsorption equilibrium, ρ in,i is the reference kettle density at initial pressure, ρ ref,i is the gas density in the reference kettle at the i-th adsorption equilibrium, ρ equ,i is the gas density in the sample kettle at the i-th adsorption equilibrium, M is the molar mass of methane gas, and m is the mass of dry sample / water sample.

[0137] The calibration process of the sample kettle volume and the free space volume includes:

[0138] Step 11: Place stainless steel beads in the sample kettle, set the oil bath temperature to the experimental temperature of 17°C, fill a fixed amount of helium into the reference kettle, and record the initial pressure of the methane isothermal adsorption experimental device after the pressure stabilizes.

[0139] Step 22: Open valves D4 and D6 in the methane isothermal adsorption experimental device to connect the reference kettle and the sample kettle, and isothermally expand helium into the sample kettle. After adsorption equilibrium is reached, record the equilibrium pressure.

[0140] Step 33: Change the number of steel balls in the sample kettle and repeat steps 11 and 22 multiple times to obtain three sets of initial pressure and equilibrium pressure values. According to the law of conservation of mass, a first helium mass conservation formula is constructed. The first helium mass conservation formula is:

[0141] ρ ref V ref =ρ equ (V ref +V sam-ΔV);

[0142] The first helium mass conservation formula is converted into a steel ball volume calculation formula, which is:

[0143]

[0144] Where ΔV is the volume of the steel ball, ρ ref , ρ equ are the densities corresponding to the initial pressure and equilibrium pressure, V ref is the reference kettle volume, V sam is the volume of the sample kettle;

[0145] Step 44: Calculate multiple sets of initial density and equilibrium density using temperature, multiple sets of initial pressure and equilibrium pressure values, and plot ΔV versus (ρ equ -ρ ref ) / ρ equ The relationship curve between them is shown in Figure 1, where the slope of the curve is the reference kettle volume and the intercept of the straight line on the y-axis is the sample kettle volume;

[0146] Step 55: Load the dry sample / water sample into the sample kettle, set the oil bath temperature to the experimental temperature of 17°C, inject helium into the methane isothermal adsorption experimental device, record the initial pressure in the reference kettle and the equilibrium pressure after the reference kettle and the sample kettle are connected, and construct a second helium mass conservation formula based on the law of conservation of mass. The second helium mass conservation formula is:

[0147] ρ ref V ref =ρ equ (V ref +V void );

[0148] The second helium mass conservation formula is converted into a free space volume calculation formula, which is:

[0149]

[0150] Among them, V void is the free space volume.

[0151] Based on the above-mentioned method for constructing a methane adsorption model for hydrated muddy silt, the present invention provides a methane isothermal adsorption experimental device, which is characterized by comprising a helium gas supply tank, a methane gas supply tank, a sample kettle, a reference kettle, valve D1, valve D2, valve D3, valve D4, valve D5, valve D6, pipelines, a pressure sensor and an oil bath pot, wherein:

[0152] The helium gas supply tank and the methane gas supply tank are respectively connected to the A end and the B end of the valve D1 through pipelines, the C end of the valve D1 and the A end of the valve D3 are respectively connected to the A end and the B end of the valve D2 through pipelines, the B end of the valve D3 is connected to the A end of the valve D5 through a pipeline, the C end of the valve D2 and the A end of the valve D4 are respectively connected to the kettle mouth end of the reference kettle through pipelines, the B end of the valve D4 and the B end of the valve D5 are respectively connected to the A end of the valve D6 through pipelines, and the B end of the valve D6 is connected to the kettle mouth end of the sample kettle. Pressure sensors are arranged inside the sample kettle and the reference kettle to measure the internal pressure of the sample kettle and the reference kettle. The sample kettle, the reference kettle, the valve D4, the valve D5, the valve D6 and the connecting pipelines between the sample kettle, the reference kettle, the valve D4, the valve D5 and the valve D6 are placed inside the oil bath to maintain a constant temperature during the methane isothermal adsorption experiment.

[0153] The valves D4, D5, D6 and the connecting pipelines between the valves D4, D5 and D6 are used to gradually transfer the helium or methane gas in the reference kettle to the sample kettle, so as to achieve the effect of injecting gas into the reference kettle once and adsorbing the dry sample / water sample in the sample kettle multiple times.

[0154] The methane adsorption amount obtained by performing a methane isothermal adsorption experiment with the methane isothermal adsorption experimental device is the excess adsorption amount.

[0155] The present invention conducts low-pressure CO2 and N2 adsorption and high-pressure methane adsorption experiments to obtain the microscopic pore structure characteristics and methane adsorption properties of the muddy silt in the South China Sea, preliminarily determine the methane adsorption magnitude and adsorption influencing factors of the muddy silt, and preferably applies a methane adsorption model to the muddy silt to obtain key adsorption parameters, providing data support and theoretical guidance for subsequent research work.

[0156] The above embodiments are merely exemplary embodiments of the present application and are not intended to limit the scope of the present application. The scope of protection of the present application is defined by the claims. Those skilled in the art may make various modifications or equivalent substitutions to the present application within the essence and scope of protection of the present application, and such modifications or equivalent substitutions shall also be deemed to fall within the scope of protection of the present application.

Claims

1. A method for constructing a methane adsorption model for hydrated muddy silt, characterized in that: The following steps are involved: Step S1: Obtain muddy silt under dry conditions and under water conditions as multiple groups of dry samples and water samples, and conduct CO2 adsorption experiments at 273K and liquid nitrogen adsorption experiments at 77K on the dry samples based on the standard static volume method on an ASAP2460 surface and pore size analyzer, respectively, to obtain CO2 adsorption data of the dry samples and N2 adsorption and desorption analysis to determine the pore structure characteristics of the muddy silt; Step S2: performing methane isothermal adsorption experiments on multiple groups of dry samples and water-containing samples using a methane isothermal adsorption experimental device based on a volumetric method to obtain methane adsorption data for the dry samples and methane adsorption data for the water-containing samples to analyze the methane adsorption characteristics of the muddy silt; Step S3, modifying the solid isothermal adsorption model according to the pore structure characteristics and methane adsorption characteristics of the muddy silt to obtain an applicable methane isothermal adsorption model for the muddy silt; Methods for constructing models suitable for methane adsorption isotherm include: A correlation formula between excess adsorption capacity and absolute adsorption capacity is established, and the correlation formula is: Where n e is the excess adsorption capacity, n a is the absolute adsorption capacity, ρ g is the gas density, ρ a is the density of the adsorbed phase; The correction factor is set based on the correlation formula, and the calculation formula of the correction factor is: According to the pore structure characteristics and methane adsorption characteristics, the Langmuir model and the DR micropore filling model are selected as the solid isothermal adsorption model. Based on the correction factor and the Langmuir model and the DR micropore filling model, the model is corrected to obtain the corrected Langmuir model and the DR micropore filling model, so as to realize the correction of the model to be applied in the experimental environment. The expressions of the corrected Langmuir model and the DR micropore filling model are respectively: Where n0 is the maximum adsorption capacity, D is a constant related to the affinity coefficient of the adsorbate, p0 is the saturated vapor pressure, n L is the Langmuir adsorption capacity, p L is the Langmuir pressure, p is the equilibrium pressure; The modified Langmuir and DR equations were quadratically corrected by using gas density instead of pressure to achieve the correction of the model to apply to supercritical adsorption. The expressions of the quadratically corrected Langmuir and DR equations are: Where, ρ L is the Langmuir gas density, k H is Henry's constant; The quadratically corrected Langmuir and DR equations were used as applicable models for methane adsorption isotherms. The methane adsorption data of dry samples and the methane adsorption data of water-containing samples were fitted respectively using the quadratically corrected Langmuir and DR equations as applicable models for methane adsorption isotherms. The mean absolute error was used to characterize the fitting effect. The expression for the mean absolute error is: Where N is the number of adsorption equilibrium points, and are the simulated value and experimental value of the adsorption amount at the i-th adsorption equilibrium point, respectively; The fitting effects of the quadratically corrected Langmuir and DR equations as applicable models for methane isotherm adsorption on the methane adsorption data of dry samples and methane adsorption data of water-containing samples were analyzed. The quadratically corrected Langmuir model was used as the applicable model for methane isotherm adsorption under lower pressure conditions, and the quadratically corrected DR equation was used as the applicable model for methane isotherm adsorption under medium and high pressure conditions.

2. The method for constructing a hydrate muddy silt methane adsorption model according to claim 1, characterized in that: The method for obtaining the dry sample and the water sample comprises: The muddy silt was dried in a constant temperature box at 80°C for 12 hours, and then the dried muddy silt was ground into muddy silt powder with a particle size of less than 80 mesh. The muddy silt powder was divided into multiple groups of dry samples according to their mass; Based on the water content formula and the water saturation formula, a conversion formula between water content and water saturation is constructed. The conversion formula is expressed as follows: Where m c is the moisture content, ρ w is the density of water, ρ b is the apparent density of muddy silt, S w is the water saturation, φ is the porosity; Obtain the apparent density ρ of muddy silt b , the density of water ρ w Water saturation S w , calculating the existing value of the water content of the muddy silt using a conversion formula, and estimating the actual value of the water content of the muddy silt based on the existing value of the water content of the muddy silt; The amount of water to be added to the silt sand is calculated using a moisture content formula based on the actual value of the moisture content of the silt sand. Water is then added to the silt sand based on the amount of water added to obtain a water-containing sample, thereby restoring the moisture content of the silt sand to that of the natural environment storage state in the laboratory.

3. The method for constructing a hydrate muddy silt methane adsorption model according to claim 2, characterized in that: Based on the CO2 adsorption data and N2 adsorption and desorption data of multiple sets of dry samples, the pore structure characteristics of muddy silt were analyzed, including: CO2 adsorption data of multiple groups of dry samples were used to draw CO2 gas isothermal adsorption curves of the dry samples at 273K. Adsorption potential theory was introduced into the analysis of the CO2 gas isothermal adsorption curves at 273K to obtain CO2 low-temperature adsorption pore structure parameters of the multiple groups of dry samples characterized by the Dubinin-Radushkevich equation and the Dubinin-Astakhov equation. Based on the CO2 low-temperature adsorption pore structure parameters, density functional theory was used to draw micropore size distribution curves of the multiple groups of dry samples to obtain the micropore size structure and distribution characteristics of the muddy silt. The N2 adsorption and desorption data of multiple groups of dry samples were used to draw the N2 isothermal adsorption and desorption curves of the dry samples at 77K. The BET theory and Gurvich rule were used to characterize the pore structure parameters of N2 low-temperature adsorption of multiple groups of dry samples at 77K. The Barrett–Joyner–Halenda method was then used to draw the BJH pore size distribution curves of multiple groups of dry samples to obtain the medium and large pore size structure and distribution characteristics of the muddy silt. The pore structure characteristics of the muddy silt are obtained based on the micropore size structure and distribution characteristics and the medium and large pore size structure and distribution characteristics of the muddy silt.

4. The method for constructing a hydrate muddy silt methane adsorption model according to claim 3, characterized in that: Based on the methane adsorption data of multiple sets of dry samples and water samples, the methane adsorption characteristics of muddy silt were analyzed, including: The methane adsorption isotherm curves of dry samples at 17°C were drawn using the methane adsorption data of multiple groups of dry samples. The first methane adsorption characteristic, which characterizes the change of methane adsorption capacity of muddy silt with pressure, was obtained by analyzing the methane adsorption isotherm curves of dry samples at 17°C. The methane adsorption isotherm curve of the water-containing sample at 17°C was drawn using the methane adsorption data of the water-containing sample. The methane adsorption isotherm curve of the water-containing sample at 17°C was combined with the methane adsorption isotherm curve of the dry sample at 17°C for analysis to obtain the second methane adsorption characteristic that characterizes the influence of water content on the first methane adsorption characteristic.

5. The method for constructing a hydrate muddy silt methane adsorption model according to claim 3, characterized in that: The experimental process of the methane isothermal adsorption experiment includes: Step 1: calibrate the volume of a sample kettle in a methane isothermal adsorption experimental apparatus using helium adsorption, and place a dry sample / water sample into the sample kettle and calibrate the free space volume using helium adsorption. The free space volume is characterized by the difference between the sample kettle volume and the dry sample / water sample volume. Step 2: A fixed amount of methane gas is introduced into the methane isothermal adsorption experimental apparatus after volume calibration, and the initial pressure of the methane isothermal adsorption experimental apparatus is recorded. By opening and closing valves D4 and D6 in the methane isothermal adsorption experimental apparatus, the methane gas in the reference kettle in the methane isothermal adsorption experimental apparatus is gradually transferred to the sample kettle. After the pressure stabilizes, the equilibrium pressures in the reference kettle and the sample kettle are recorded respectively. Step 3: Change the initial pressure and repeat step 2 multiple times to obtain multiple sets of equilibrium pressures. Then, calculate the methane gas adsorption data of the dry sample / water sample at different equilibrium pressures according to the law of conservation of mass. The calculation formula of the methane gas adsorption data is: Among them, n e,i is the methane adsorption capacity of the dry sample / water sample at the i-th adsorption equilibrium, n in,i is the initial gas volume in the reference kettle, n ref,i is the amount of gas in the reference kettle at the i-th adsorption equilibrium, n equ,i is the amount of gas in the sample kettle at the i-th adsorption equilibrium, ρ in,i is the reference kettle density at initial pressure, ρ ref,i is the gas density in the reference kettle at the i-th adsorption equilibrium, ρ equ,i is the gas density in the sample kettle at the i-th adsorption equilibrium, M is the molar mass of methane gas, m is the mass of dry sample / water sample, i is the stoichiometric constant, V ref is the reference kettle volume, V void is the volume of free space.

6. The method for constructing a hydrate muddy silt methane adsorption model according to claim 3, characterized in that: The calibration process for the sample kettle volume and free space volume includes: Step 11: Place stainless steel beads in the sample kettle, set the oil bath temperature to the experimental temperature of 17°C, fill a fixed amount of helium into the reference kettle, and record the initial pressure of the methane isothermal adsorption experimental device after the pressure stabilizes. Step 22: Open valves D4 and D6 in the methane isothermal adsorption experimental device to connect the reference kettle and the sample kettle, and isothermally expand helium into the sample kettle. After adsorption equilibrium is reached, record the equilibrium pressure. Step 33: Change the number of steel balls in the sample kettle and repeat steps 11 and 22 multiple times to obtain three sets of initial pressure and equilibrium pressure values. According to the law of conservation of mass, a first helium mass conservation formula is constructed. The first helium mass conservation formula is: r ref V ref =ρ equ (V ref +V sam -ΔV); The first helium mass conservation formula is converted into a steel ball volume calculation formula, which is: Where ΔV is the volume of the steel ball, ρ ref , ρ equ are the densities corresponding to the initial pressure and equilibrium pressure, V ref is the reference kettle volume, V sam is the volume of the sample kettle; Step 44: Calculate multiple sets of initial density and equilibrium density using temperature, multiple sets of initial pressure and equilibrium pressure values, and plot ΔV versus (ρ equ -ρ ref ) / ρ equ The relationship curve between them is shown in Figure 1, where the slope of the curve is the reference kettle volume and the intercept of the straight line on the y-axis is the sample kettle volume; Step 55: Load the dry sample / water sample into the sample kettle, set the oil bath temperature to the experimental temperature of 17°C, inject helium into the methane isothermal adsorption experimental device, record the initial pressure in the reference kettle and the equilibrium pressure after the reference kettle and the sample kettle are connected, and construct a second helium mass conservation formula based on the law of conservation of mass. The second helium mass conservation formula is: ρ ref V ref =ρ equ (V ref +V void ); The second helium mass conservation formula is converted into a free space volume calculation formula, which is: Among them, V void is the volume of free space.

7. A methane isothermal adsorption experimental device for constructing a methane adsorption model for hydrated muddy silt according to any one of claims 1 to 6, characterized in that: It includes helium gas supply tank, methane gas supply tank, sample kettle, reference kettle, valve D1, valve D2, valve D3, valve D4, valve D5, valve D6, pipeline, pressure sensor and oil bath pot, among which, The helium gas supply tank and the methane gas supply tank are respectively connected to the A end and the B end of the valve D1 through pipelines, the C end of the valve D1 and the A end of the valve D3 are respectively connected to the A end and the B end of the valve D2 through pipelines, the B end of the valve D3 is connected to the A end of the valve D5 through a pipeline, the C end of the valve D2 and the A end of the valve D4 are respectively connected to the kettle mouth end of the reference kettle through pipelines, the B end of the valve D4 and the B end of the valve D5 are respectively connected to the A end of the valve D6 through pipelines, and the B end of the valve D6 is connected to the kettle mouth end of the sample kettle. Pressure sensors are arranged inside the sample kettle and the reference kettle to measure the internal pressure of the sample kettle and the reference kettle. The sample kettle, the reference kettle, the valve D4, the valve D5, the valve D6 and the connecting pipelines between the sample kettle, the reference kettle, the valve D4, the valve D5 and the valve D6 are placed inside the oil bath to maintain a constant temperature during the methane isothermal adsorption experiment.

8. The methane isothermal adsorption experimental device according to claim 7, characterized in that: The valves D4, D5, D6 and the connecting pipelines between the valves D4, D5 and D6 are used to gradually transfer the helium or methane gas in the reference kettle to the sample kettle, so as to achieve the effect of injecting gas into the reference kettle once and adsorbing the dry sample / water sample in the sample kettle multiple times.

9. The methane isothermal adsorption experimental device according to claim 8, characterized in that: The methane adsorption amount obtained by performing a methane isothermal adsorption experiment with the methane isothermal adsorption experimental device is the excess adsorption amount.

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