Multiphase seepage experimental simulation system for hydrate decomposition process under low-temperature and high-pressure conditions

Abstract

The invention is suitable for the technical field of natural gas hydrate exploitation, and provides a multiphase seepage experiment simulation system for a hydrate decomposition process under low-temperature and high-pressure conditions. According to the invention, improvement is carried out on the basis of a traditional sedimentary layer seepage experiment system, firstly, an unsteady state water-driven gas process is introduced, so that the professional problem of relative permeability measurement in a hydrate decomposition process is solved; meanwhile, simulation of the hydrate depressurization decomposition process is added, the continuous evolution characteristics of the pore structure of the sedimentary layer in the hydrate depressurization decomposition process are accurately described, and the description deviation of the multi-phase seepage parameter dynamic evolution process in the process is eliminated; through the improvement, a more real permeability evolution model is constructed, and a solid theoretical basis and technical support are provided for a numerical simulation method of hydrate pilot production in China.

Description

Technical Field

[0001] The invention belongs to the technical field of natural gas hydrate exploitation, and in particular relates to a multiphase seepage experimental simulation system for a hydrate decomposition process under low temperature and high pressure conditions. Background Art

[0002] Methane hydrate, as a crystalline substance formed by methane molecules embedded in the lattice of water molecules, under specific low temperature and high pressure conditions, the gas and water form an ice-like solid, and its structure has strong stability. This special structure allows methane hydrate to exist in large quantities in specific environments such as the seabed and permafrost. Methane hydrate exists in the porous media of the reservoir, in which the seepage of multiphase fluid is an extremely complex process. Its complexity is mainly reflected in two aspects: first, the presence of hydrates will change the pore structure and permeability of the porous medium, thereby affecting the flow of the fluid; second, changes in temperature, pressure and other conditions during the seepage process will in turn affect the stability and decomposition process of the hydrate. Therefore, it is necessary to conduct experiments to deeply study the mechanism and laws of methane hydrate seepage.

[0003] In order to deeply study the seepage mechanism and laws of methane hydrate, experimental simulation has become a necessary means. However, this requires that the experimental equipment must be able to simulate the formation and existence conditions of methane hydrate in nature, that is, low temperature and high pressure environment. This places high demands on the design and manufacture of experimental equipment, including containers and piping systems that can withstand high pressure, as well as precise temperature control devices to ensure the stability and accuracy of experimental conditions. In the seepage process of methane hydrate, the flow of multiphase substances such as water, gas, and hydrates is involved. How to accurately measure the flow rate, pressure, saturation and other parameters of each phase is an important challenge facing experimental technology. Traditional measurement methods may not meet the requirements of methane hydrate seepage experiments, and new measurement technologies and instruments need to be developed.

[0004] In the existing seepage experiments of hydrate-containing sediments, there are still difficulties in the characterization of the flow process of gas and water multiphase fluids under low temperature and high pressure environment and the measurement of relative permeability. For example, the conventional steady-state measurement method controls the injection of liquid and gas phase fluids into the core in a certain proportion, and counts the flow rate of the two-phase fluid after it flows stably, and then calculates the relative permeability of the liquid and gas phases. However, in the multiphase seepage experiment of the hydrate decomposition process in the existing system, the gas injection valve and the liquid injection valve cannot be opened at the same time, otherwise the high-pressure fluid will damage the liquid injection and gas injection systems, resulting in the destruction of the experimental instruments. In addition, most of the existing instruments consider the constant volume synthesis of hydrates, and do not consider the study of quantitative decomposition of hydrates. They cannot accurately characterize the continuous evolution characteristics of the pore structure of the sedimentary layer during the decomposition of hydrates under reduced pressure, resulting in deviations in their description of the dynamic evolution process of multiphase seepage parameters.

[0005] In view of the above problems, the present invention proposes a multiphase seepage experimental simulation system for hydrate decomposition process under low temperature and high pressure conditions. Summary of the invention

[0006] The purpose of the present invention is to provide a multiphase seepage experimental simulation system for hydrate decomposition process under low temperature and high pressure conditions, aiming to solve the problems raised in the above background technology.

[0007] The purpose of the present invention is achieved through the following technical solutions: The multiphase seepage experimental simulation system of hydrate decomposition process under low temperature and high pressure conditions includes: The liquid injection system includes a horizontal flow pump for controlling the flow rate of liquid phase water, a liquid flow meter for recording the flow rate of liquid phase water, and a liquid injection valve for controlling the opening and closing of liquid injection; the horizontal flow pump is connected to the liquid injection valve, and the liquid injection valve is connected to the liquid flow meter; The gas injection system comprises a gas cylinder for providing gas, a pressure reducing valve for reducing gas pressure, a flow controller for controlling gas flow, and a gas injection valve for controlling the opening and closing of gas injection; the gas cylinder is connected to the pressure reducing valve, the pressure reducing valve is connected to the flow controller, and the flow controller is connected to the gas injection valve; A core holder and a high-pressure reaction column, wherein the core holder is used to hold the core sample and is placed in the high-pressure reaction column; The closed system includes a confining pressure pump for controlling the confining pressure of the core sample, an axial pressure pump for controlling the axial pressure of the core sample, an axial hydraulic cylinder, a vacuum pump and a vacuum valve for maintaining a vacuum environment, and a pressure reducing valve for reducing the pressure of the core sample; the confining pressure pump is connected to the core holder, the axial hydraulic cylinder is connected to the high-pressure reaction column, and the axial pressure pump is connected to the axial hydraulic cylinder; the vacuum pump is connected to the vacuum valve, and the vacuum valve is connected to the high-pressure reaction column; The back pressure system comprises a hand-operated back pressure pump, a back pressure valve for controlling the back pressure, a gas collection bottle and an exhaust valve; the hand-operated back pressure pump is connected to the exhaust valve, and the exhaust valve is connected to the gas collection bottle; The data metering and acquisition module includes a temperature controller, a buffer tank, a pressure sensor, an axial pressure sensor, a gas-liquid separator, a drying tank, a balance and a timer; the temperature controller is connected to the upper part of the high-pressure reaction column, the pressure sensor is connected to the lower part of the high-pressure reaction column, the pressure sensor is connected to the pressure reducing valve, and the axial pressure sensor is connected to the axial hydraulic cylinder; the gas-liquid separator is connected to the pressure sensor, the buffer tank and the drying tank are both connected to the gas-liquid separator, the back pressure valve is connected to the buffer tank, a pressure gauge is installed on the connecting pipeline between the back pressure valve and the buffer tank, and a balance is installed at the drying tank; The unsteady-state water-displacement gas module is used to measure the dynamic evolution of the liquid and gas phase relative permeabilities of core samples under low temperature and high pressure conditions. The unsteady-state water-displacement gas module is based on the Buckley-Leverett model that describes the front advancement process of one-dimensional two-phase flow. The calculation formula of the liquid and gas phase relative permeabilities is as follows: ; ; ; In the formula, are the relative permeabilities of liquid and gas phases, respectively; is the liquid water saturation at the outlet; are the dynamic viscosity coefficients of liquid and gas phase fluids respectively; L is the length of the core sample; for Change in the amount of water in the internal liquid phase; is the time increment; A is the cross-sectional area of ​​the core sample; k 0 is the intrinsic permeability of the core sample; is the displacement pressure difference; is the corrected water-gas ratio; is the average value of the total amount of gas; for Changes in the amount of methane in the internal gas phase; is the pore volume; The calculation formula of the intrinsic permeability of the core sample is: ; In the formula, is the dynamic viscosity system of deionized water; is the deionized water flow rate per unit time; is the fluid density; is the acceleration due to gravity; is the head difference of the fluid at the inlet and outlet; is the pressure difference between the inlet and outlet; The theoretical value of displacement pressure difference is: ; In the formula, is the interfacial tension between gas and liquid phase fluids; is the effective permeability of the core; is the porosity; The methane hydrate quantitative decompression decomposition module is used to describe the continuous evolution characteristics of the pore structure of the core sample. The specific process is as follows: First, the excess gas method was used to synthesize methane hydrate inside the core sample. The preset synthesis pressure was adjusted to control the saturation of methane hydrate in the core sample to the maximum saturation, and the effective permeability and relative permeability of the core sample were measured. Subsequently, the pressure of the high-pressure reaction column is gradually reduced to decompose the methane hydrate to a set saturation. The effective permeability and relative permeability of the core pore structure corresponding to the saturation after the methane hydrate is decomposed are measured. Continue to quantitatively decompose methane hydrate and measure the multiphase seepage parameters of core samples until methane hydrate is completely decomposed.

[0008] Furthermore, the measurement of the relative permeability of the liquid and gas phases was carried out at 1°C and 2.93 MPa, and the specific process was as follows: a. Open the nitrogen cylinder and adjust the pressure reducing valve, open the gas injection valve, adjust the gas inlet pressure, and control the nitrogen to be injected into the core sample at a rate of 20 STml / min until the core sample reaches a saturated state; b. Close the nitrogen cylinder and the gas injection valve, open the horizontal flow pump and the liquid injection valve; increase the water inlet pressure, and gradually adjust the back pressure to form a displacement pressure difference; start the timer and start the process of liquid water displacing gaseous nitrogen at a displacement rate of 3 ml / min; c. Keep the back pressure and water inlet pressure unchanged until no gas is discharged from the core sample, and record the gas and water output in real time.

[0009] Furthermore, the measurement process of the intrinsic permeability of the core sample is as follows: a. Place the core sample in the core holder, place filters on both sides of the core sample, and then install the holder into the high-pressure reaction column; b. Adjust the confining pressure pump and the axial pressure pump so that the core sample can withstand a confining pressure of 2 MPa and an axial pressure of 1 MPa; c. Keep all valves closed, set the back pressure to 0 MPa, and adjust the balance to zero; d. Adjust the horizontal flow pump and injection valve to inject deionized water into the core sample at a rate of 3 ml / min; e. Observe the balance readings in real time until the reading difference stabilizes, and record the inlet and outlet pressures.

[0010] Furthermore, the measurement process of the effective permeability is the same as the measurement process of the intrinsic permeability.

[0011] Furthermore, in the step of continuously and quantitatively decomposing methane hydrate, the decomposition amount of methane hydrate each time is: ; In the formula, are the amounts of methane hydrate substances in the core samples before and after decomposition, respectively; is the duration of blood pressure reduction; is the decomposition kinetic coefficient of methane hydrate; is the reaction surface area; represents the phase equilibrium pressure of methane hydrate at 1°C, which is 2.93 MPa; The low pressure environment required for hydrate decomposition; is the reaction order; represents the change in time t; Indicated in The change in the amount of methane hydrate substance over time; is the differential symbol.

[0012] Furthermore, the multiphase seepage parameters of the core sample include porosity , preset synthesis pressure 、Phase equilibrium pressure of methane hydrate at 1℃ , the pore volume of the core sample occupied by gas , methane hydrate saturation in core samples S H , inherent permeability k 0 , effective penetration ; Gas occupies the pore volume of the core sample V G The calculation formula is: ; In the formula, n The amount of substance that is methane gas; is the methane gas density; is the injected volume of methane gas per unit time; is the ideal gas constant; is the experimental temperature; is the molar mass of methane gas; is the pressure increment of the core sample per unit time; Methane hydrate saturation in core samples S H The calculation formula is: ; In the formula, A is the cross-sectional area of ​​the core sample; L is the length of the core sample; Indicates the low temperature environment required for the synthesis of methane hydrate, which is 1°C.

[0013] Furthermore, the core holder is a cylindrical holder made of stainless steel with an inner diameter of 50 mm. Its normal working pressure is 0~25 MPa and the temperature is -5~25℃.

[0014] Compared with the prior art, the present invention has the following beneficial effects: The present invention has made improvements on the basis of the traditional sedimentary layer seepage experimental system. First, a non-steady-state water-driven gas process is introduced, thereby breaking through the professional difficulty of relative permeability measurement during hydrate decomposition. At the same time, the simulation of the hydrate decomposition process during decompression is added, which accurately depicts the continuous evolution characteristics of the pore structure of the sedimentary layer during the hydrate decomposition process, and eliminates the description deviation of the dynamic evolution process of the multiphase seepage parameters in this process. Through the above improvements, the present invention constructs a more realistic permeability evolution model, which provides a solid theoretical basis and technical support for the numerical simulation method of hydrate trial production in my country. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 It is a schematic diagram of the structure of the present invention.

[0016] Figure 2 It is a methane hydrate phase equilibrium state.

[0017] Figure 3 Comparison of the equilibrium states of methane hydrate and nitrogen hydrate.

[0018] Figure 4 Permeability evolution during hydrate decomposition in core samples; (a) effective permeability, (b) normalized permeability.

[0019] Figure 5 Dynamic evolution of liquid and gas relative permeabilities of core samples with a porosity of 0.38; (a) liquid relative permeability, (b) gas relative permeability.

[0020] Figure 6 Dynamic evolution of liquid and gas relative permeabilities of core samples with a porosity of 0.33; (a) liquid relative permeability, (b) gas relative permeability.

[0021] Figure 7 Dynamic evolution of liquid and gas relative permeabilities of core samples with a porosity of 0.28; (a) liquid relative permeability, (b) gas relative permeability.

[0022] Figure 8 Dynamic evolution of liquid and gas relative permeabilities of core samples with a porosity of 0.43; (a) liquid relative permeability, (b) gas relative permeability.

[0023] Fig. 9 Dynamic evolution of liquid and gas relative permeabilities of core samples with a porosity of 0.48; (a) liquid relative permeability, (b) gas relative permeability.

[0024] In the figure: a horizontal flow pump 11, a liquid flow meter 12, a gas cylinder 21, a pressure reducing valve 22, a flow controller 23, a core holder 03, a high-pressure reaction column 04, a confining pressure pump 51, an axial pressure pump 52, an axial hydraulic cylinder 53, a vacuum pump 54, a hand-cranked back-pressure pump 61, a gas collecting bottle 62, a temperature controller 71, a buffer tank 72, a pressure sensor 73, an axial pressure sensor 74, a gas-liquid separator 75, a drying tank 76, a balance 77, a timer 78, a filter 08, a gas injection valve V1, a vacuum valve V2, a liquid injection valve V3, a pressure reducing valve V4, a back-pressure valve V5, and an exhaust valve V6. DETAILED DESCRIPTION

[0025] In order to have a clearer understanding of the technical features, purposes and beneficial effects of the present invention, the technical solution of the present invention is now described in detail below, but it should not be understood as limiting the scope of the present invention. The experimental methods described in the following examples are conventional methods unless otherwise specified; the reagents and materials can be obtained from commercial sources unless otherwise specified.

[0026] The specific implementation of the present invention is described in detail below in conjunction with specific embodiments.

[0027] like Figure 1 As shown, a multiphase seepage experimental simulation system for hydrate decomposition process under low temperature and high pressure conditions is provided in one embodiment of the present invention, comprising: The liquid injection system includes a horizontal flow pump 11 for controlling the flow rate of liquid phase water, a liquid flow meter 12 for recording the flow rate of liquid phase water, and a liquid injection valve V3 for controlling the opening and closing of liquid injection; the horizontal flow pump 11 is connected to the liquid injection valve V3, and the liquid injection valve V3 is connected to the liquid flow meter 12; The gas injection system includes a gas cylinder 21 for providing gas, a pressure reducing valve 22 for reducing gas pressure, a flow controller 23 for controlling gas flow, and a gas injection valve V1 for controlling the opening and closing of gas injection; the gas cylinder 21 is connected to the pressure reducing valve 22, the pressure reducing valve 22 is connected to the flow controller 23, and the flow controller 23 is connected to the gas injection valve V1; The core holder 03 and the high-pressure reaction column 04, the core holder 03 is used to hold the core sample and is placed in the high-pressure reaction column 04; the core holder 03 is a cylindrical holder made of stainless steel with an inner diameter of 50 mm, and its normal working pressure is 0~25 MPa and the temperature is -5~25℃.

[0028] The closed system includes a confining pressure pump 51 for controlling the confining pressure of the core sample, an axial pressure pump 52 for controlling the axial pressure of the core sample, an axial hydraulic cylinder 53, a vacuum pump 54 and a vacuum valve V2 for maintaining a vacuum environment, and a pressure reducing valve V4 for reducing the pressure of the core sample; the confining pressure pump 51 is connected to the core holder 03, the axial hydraulic cylinder 53 is connected to the high-pressure reaction column 04, and the axial pressure pump 52 is connected to the axial hydraulic cylinder 53; the vacuum pump 54 is connected to the vacuum valve V2, and the vacuum valve V2 is connected to the high-pressure reaction column 04; The back pressure system includes a hand-operated back pressure pump 61, a back pressure valve V5 for controlling the back pressure, a gas collection bottle 62 and an exhaust valve V6; the hand-operated back pressure pump 61 is connected to the exhaust valve V6, and the exhaust valve V6 is connected to the gas collection bottle 62; The data metering and acquisition module includes a temperature controller 71, a buffer tank 72, a pressure sensor 73, an axial pressure sensor 74, a gas-liquid separator 75, a drying tank 76, a balance 77 and a timer 78; the temperature controller 71 is connected to the upper part of the high-pressure reaction column 04, the pressure sensor 73 is connected to the lower part of the high-pressure reaction column 04, and the pressure sensor 73 is connected to the pressure reducing valve V4, and the axial pressure sensor 74 is connected to the axial hydraulic cylinder 53; the gas-liquid separator 75 is connected to the pressure sensor 73, the buffer tank 72 and the drying tank 76 are both connected to the gas-liquid separator 75, the back pressure valve V5 is connected to the buffer tank 72, a pressure gauge is installed on the connecting pipeline between the back pressure valve V5 and the buffer tank 72, and a balance 77 is installed at the drying tank 76; The non-steady-state water-displacement gas module is used to measure the dynamic evolution of the relative permeability of the liquid and gas phases of the core sample under low temperature and high pressure conditions; The methane hydrate quantitative decomposition module is used to describe the continuous evolution characteristics of the pore structure of core samples.

[0029] The specific operation steps of the experimental process of this system are detailed as follows: 1. Core sample preparation; The main components of the sediments in the study area are: a) Detrital minerals (about 60.29%): including quartz, muscovite, plagioclase, etc., formed by rock weathering and erosion.

[0030] b) Clay minerals (about 19.64%): including illite, montmorillonite, chlorite, etc., which have strong adsorption capacity.

[0031] c) Carbonate minerals (about 16.46%): mainly calcite, which is related to the sedimentary environment of the hydrate test mining area.

[0032] As shown in Table 1, the sedimentary components in the test mining area are mainly terrestrial in origin and are dominated by detrital minerals.

[0033]

[0034] In order to prepare muddy siltstone core samples that can better characterize the lithological characteristics of the test mining area, the core sample composition is divided into two parts according to the mineral composition and particle size distribution data of the sediment: core skeleton (detrital minerals + carbonate minerals, about 80%) and clay minerals (about 20%). The core skeleton composed of detrital minerals and carbonate minerals has a larger particle size, and quartz sand (200 mesh) is selected for characterization. The clay mineral with a smaller particle size is selected for characterization by illite (4000 mesh). Quartz sand and illite are mixed in a certain ratio (4:1), and an appropriate amount of distilled water is added and stirred evenly (the water content is set to 10%). The mineral particles are filled into a cylindrical pressure-resistant mold (diameter 0.05 m, height 0.1 m) matched with an electric demoulding instrument, and the mineral particles are formed with the help of a hydraulic instrument to obtain a muddy siltstone core sample without hydrate.

[0035] 2. Intrinsic permeability measurement; For core samples with different initial porosities, their intrinsic permeability is first measured: a) Place the core sample in a core holder 03 of the same specification, and place filter screens 08 on both sides of the sample to prevent fine rock particles from migrating with the fluid and causing pipeline blockage. Then install the core holder 03 into the high-pressure reaction column 04.

[0036] b) Adjust the confining pressure pump 51 and the axial pressure pump 52 so that the core sample is subjected to a confining pressure of about 2 MPa and an axial pressure of 1 MPa to ensure the airtightness of the experimental environment and prevent leakage during the subsequent fluid injection process.

[0037] c) Keep all valves closed, set the back pressure to 0 MPa, and adjust the balance 77 to zero.

[0038] d) adjusting the horizontal flow pump 11 and the injection valve V3 to inject deionized water into the core sample at a rate of 3 ml / min; Determining the appropriate water injection rate is the key to accurately measuring the core permeability. If the injection rate is too low, the flow time of water in the core sample will increase significantly, and the long experimental process will cause the hydrates in the core sample to gradually decompose, and the measured absolute permeability will not match the actual corresponding hydrate saturation; if the injection rate is too high, the water flow will impact the core sample, causing the fragile muddy silt structure to be destroyed. After many theoretical and experimental studies, the injection rate of deionized water was determined to be 3 ml / min.

[0039] e) Observe the readings of the balance 77 in real time until the reading difference becomes stable within a certain period of time. This indicates that the water inlet and outlet rates are equal and a stable flow has been formed in the core sample. Record the pressure at the inlet and outlet , calculate the pressure difference , intrinsic permeability k 0 According to the observation data and Darcy's theorem, it is calculated that: Formula 1: ; In the formula, is the dynamic viscosity of deionized water, ; is the flow rate per unit time, ; is the fluid density, kg / m 3 ; is the acceleration due to gravity, 9.8 m / s 2 ; is the head difference of the fluid at the inlet and outlet, Pa; is the core sample length, 0.1 m; is the cross-sectional area of ​​the core, 0.0019625 m 2 ; is the pressure difference between inlet and outlet, Pa.

[0040] 3. Residual water saturation measurement; a) After the intrinsic permeability measurement is completed, the core sample is in a saturated state, and the horizontal flow pump 11 and the injection valve V3 are immediately closed. The methane cylinder 21 is opened and the pressure reducing valve 22 is adjusted to prevent the gas flow rate from being too fast and damaging the fragile muddy silt sand structure. The axial pressure and confining pressure of the high-pressure reaction column 04 are adjusted in real time according to the inlet pressure (confining pressure ≥ inlet pressure + 1 MPa), which not only ensures the airtightness of the experimental environment and prevents the leakage of the injected fluid, but also avoids the large difference between the confining pressure and the gas pressure, which causes the core sample to be compacted.

[0041] b) Record the balance 77 reading , open the gas injection valve V1, and inject methane gas into the core sample at a stable injection rate with the help of flow controller 23. After many theoretical and experimental explorations, the injection rate of methane gas was determined to be 20 ST ml / min.

[0042] c) Observe the reading of the balance 77 , and record it when its value remains unchanged. At this point, the free water in the core sample has been completely driven out, and the water molecules only exist in the form of bound water. Based on the amount of liquid discharged, calculate the residual water saturation of the core sample: ; In the formula, These are the balance 77 readings before and after methane displaces water, kg; is the density of deionized water, 1000 kg / m 3 ; is the volume of water discharged, m3 ; is the residual water saturation; is the porosity, r is the radius of the core sample, h is the height of the core sample.

[0043] The muddy siltstone core sample contains 20% clay minerals and has a high residual water saturation (above 0.40). When the excess gas method is used to synthesize methane hydrate, the ratio of the amount of synthesized hydrate to the amount of water required is about 1:0.8. Therefore, after the residual water saturation is measured, the bound water in the core sample can meet the needs of synthesizing methane hydrate, and there is no need to restore the wettability of the core again.

[0044] 4. Natural gas hydrate synthesis; Use excess gas method to synthesize methane hydrate: a) All valves are kept closed, and the back pressure valve V5 is adjusted to make the back pressure slightly higher than the preset synthesis pressure when synthesizing methane hydrate to the set saturation to ensure the airtightness of the experimental environment. The back pressure is generally greater than 10 MPa.

[0045] b) Open the gas injection valve V1 and adjust the gas inlet pressure to control the injection rate of methane gas (20 ST ml / min). Observe the pressure increment of the core sample per unit time, and calculate the pore volume occupied by the gas in the core based on the ideal state equation: Formula 4: ; In the formula, is the pore volume occupied by gas in the core, m 3 ; n is the amount of substance of methane gas, mol; is the density of methane gas, 0.7174 kg / m 3 ; is the injected volume of methane gas per unit time (take 1 min), ; is the ideal gas constant, 8.314 J / (K·mol); is the experimental temperature, K; is the molar mass of methane gas, 0.016 kg / mol; is the pressure increment of the core sample per unit time (1 min), Pa.

[0046] c) The pressure inside the core sample reaches the preset synthetic pressure value After that, adjust the injection rate of methane gas to gradually reduce it to 0 ml / min. Close the gas injection valve V1 and the methane gas cylinder 21. Perform a leak test on the high-pressure reaction column 04. If the readings of the pressure sensors 73 at the inlet and outlet are stable within 1 hour, the system is well sealed.

[0047] d) Adjust the temperature controller 71 to set the experimental environment to the low temperature conditions required for hydrate synthesis. To prevent the residual water in the core pores from freezing, the synthesis temperature is set to 1°C (274.15 K). Then the temperature is lowered. At low temperature and high pressure (1°C, pressure range ) environment, methane hydrate begins to synthesize at constant volume.

[0048] e) As hydrates continue to form, the gas pressure in the core sample gradually decreases. After a long enough time, the pressure drops to a certain stable value. And it no longer changes, and methane hydrate with a preset saturation is successfully synthesized in the muddy siltstone core. is the methane hydrate phase equilibrium pressure corresponding to the experimental temperature (1°C), such as Figure 2 As shown, its value is about 2.93 MPa.

[0049] f) Based on the ideal state equation, the accurate hydrate saturation in the muddy siltstone core sample is calculated.

[0050] Based on the experimental temperature and the pressure values ​​before and after synthesis, the volume of methane consumed in the hydrate synthesis process can be calculated. : Formula 5: ; In the formula, n is the amount of substance of methane gas, mol; Indicates the low temperature environment required for the synthesis of methane hydrate, which is 1°C.

[0051] Under standard conditions, the decomposition of 1 volume of hydrate can produce about 164 volumes of methane gas. , the volume of the synthesized hydrate can be calculated : Formula 6: ; Hydrate saturation is the ratio of the hydrate volume to the pore volume in the core sample: Formula 7: ; Arrange equations 5-7 to obtain the saturation of methane hydrate in the core sample: Formula 8: ; In the formula, Indicates the low temperature environment required for the synthesis of methane hydrate, which is 1°C; is the cross-sectional area of ​​the core, 0.0019635 m 2 ; is the porosity of the core sample; is the core sample length, 0.1 m; is the methane hydrate phase equilibrium pressure corresponding to the experimental temperature (1°C).

[0052] In the above formula, All are known, All are fixed values. Adjustable preset synthesis pressure , to control the saturation of methane hydrate in the muddy siltstone core samples.

[0053] 5. Effective permeability measurement; After hydrate is synthesized in the core sample to the preset saturation, its effective permeability needs to be measured. The detailed operation process of this link is the same as that of the "intrinsic permeability measurement" link.

[0054] When hydrates exist in the core, the absolute permeability measured is the effective permeability ; When there is no hydrate, the measured absolute permeability is the intrinsic permeability .

[0055] 6. Relative permeability measurement; The non-steady-state water-displacement gas method is introduced to measure the dynamic evolution of the relative permeability of the liquid and gas phases of core samples with different hydrate saturations. This method is based on the Buckley-Leverett model that describes the front advancement process of one-dimensional two-phase flow. It does not consider the capillary negative pressure and assumes that the gas and liquid phase fluids are immiscible and incompressible.

[0056] In the process of liquid water displacing gaseous methane, the distribution of gas and liquid saturation inside the core is a function related to time and distance. During the displacement process, the flow of liquid and gas phase fluids obeys Darcy's theorem. The relative permeability of liquid and gas can be calculated: ; In the formula, are the dynamic viscosity coefficients of liquid and gas phase fluids, respectively. are the flow rates of liquid water and gaseous methane respectively; is the pressure gradient at a distance L from the core inlet.

[0057] According to the material balance theory, the continuity equations of liquid and gas phase fluids are calculated: ; In the formula, is the porosity; are the liquid and gas saturations at a distance L from the core inlet, respectively; is the flow gradient of the liquid phase fluid at a distance L from the core inlet; is the flow gradient of the gas phase fluid at a distance L from the core inlet; t To replace time.

[0058] Total flow rate of liquid and gas phase fluid at a distance L from the core inlet for: Formula 13: ; In the formula, are the flow rates of liquid and gas phase fluids per unit time respectively; Combine equation 9 and equation 10 to solve the split flow of liquid and gas phase fluids: ; In the formula, is the fractional flow rate of the liquid phase fluid; is the fractional flow of the phase fluid; is the absolute permeability obtained by water measurement method; is the effective permeability obtained by gas testing; Therefore, the flow equations of liquid and gas phase fluids are: ; Furthermore, according to the continuity equation, the liquid and gas saturations at the outlet boundary can be obtained: ; In the formula, is the cumulative injection volume, dimensionless; is the average liquid saturation of the core at that moment; are the liquid and gas saturations at the outlet boundary, respectively.

[0059] This system uses liquid water to displace gaseous methane, and the displaced phase fluid in this process is non-wetting phase methane gas. The pore pressure in the core sample will decrease with the decrease of gas content during the displacement process, and the non-wetting phase gaseous methane will continue to expand. Therefore, the gas volume calculated by the Buckley-Leverett model needs to be corrected: ; In the formula, are the relative permeabilities of liquid and gas phases, respectively; is the liquid water saturation at the outlet; are the dynamic viscosity coefficients of liquid and gas phase fluids respectively; L is the length of the core sample; for Change in the amount of liquid water in the interior, m 3 ; is the time increment, s; A is the cross-sectional area of ​​the core sample; k0 is the intrinsic permeability of the core sample; is the displacement pressure difference, MPa; is the corrected water-gas ratio, dimensionless; is the average value of the total amount of gas, m 3 ; for Change of gas phase methane, m 3 ; is the pore volume, m 3 .

[0060] The relative permeability is measured under the condition of methane hydrate phase equilibrium (1°C, 2.93MPa). If methane gas is injected into the high-pressure reaction column 04, new hydrates will be generated in the upper part of the core, which will not only fail to achieve the displacement effect, but also destroy the pore structure formed in the core sample. Therefore, it is necessary to find an alternative gas with multiphase seepage properties similar to methane gas and a more stable structure to complete the relative permeability measurement.

[0061] Table 2 shows the seepage parameters of alternative gases with similar physical properties to methane gas, calculated based on the SUPERTRAPP model of the National Bureau of Standards of the United States.

[0062] Table 2 Permeation parameters of common alternative gases (1°C, 2.93 MPa) Physical properties Methane Ethane Propane Methyl chloride carbon dioxide Nitrogen Molar mass (g / mol) 16.04 30.07 44.09 50.49 44.01 28.01 <![CDATA[Density (kg / m 3 )]]> 22.16 398.36 532.39 963.09 73.75 36.33 <![CDATA[Dynamic viscosity coefficient (10 −5 Pa·s)]]> 1.10 6.05 12.92 24.16 1.46 1.72 <![CDATA[Kinematic viscosity (10 −7 m / s 2 )]]> 4.97 1.52 2.43 2.51 1.98 4.87 Thermal conductivity (W / m·k) 0.037 0.101 0.112 0.128 0.021 0.028 <![CDATA[Thermal diffusivity (10 −7 m 2 / s)]]> 6.69 6.85 0.85 0.84 2.06 6.81 Prandtl number 0.7424 2.2304 2.8688 2.9711 0.9632 0.7102 Depend on Figure 3 It can be seen that in the multiphase seepage experimental environment (1°C, 2.93MPa), nitrogen exists stably in the form of gas and does not form nitrogen hydrates. Based on the above analysis, nitrogen is used to replace methane gas in the process of measuring the relative permeability of liquid and gas phases by the non-steady-state water drive gas method.

[0063] a) After the effective permeability measurement is completed, the core sample is in a saturated state, and the horizontal flow pump 11 and the injection valve V3 are immediately closed. The nitrogen cylinder 21 is opened and the pressure reducing valve 22 is adjusted, and the injection valve V1 is opened to adjust the inlet pressure, and the nitrogen is controlled to be injected into the core at an appropriate rate (20 ST ml / min) until the core sample reaches a saturated state; b) Close the nitrogen cylinder 21 and the gas injection valve V1, and open the horizontal flow pump 11 and the liquid injection valve V3. Increase the water inlet pressure and gradually adjust the back pressure to form a displacement pressure difference. Start the timer 78 to start the process of liquid water displacing gaseous nitrogen.

[0064] Determining the appropriate displacement rate is the key to accurately measuring the relative permeability of liquid and gas phases. If the displacement rate is too low, the water flow will find it difficult to overcome the end effect of the core, and it will also trigger a water lock reaction, and it will not be possible to ensure that the methane hydrate synthesized in the core sample can be stored for a long time and stably; if the displacement rate is too high, the high-pressure water flow will impact the core sample, causing the fragile muddy silt structure to be destroyed, and turbulence will also form in the core, which does not meet the assumptions of the Buckley-Leverett model front advancement theory. The displacement rate is determined by the displacement pressure. In the process of measuring relative permeability by the non-steady-state water-gas displacement method, the theoretical value of the displacement pressure is: ; In the formula, is the interfacial tension between gas and liquid phases, which is about 70 dyn / cm, or 0.07 N / m, in the experimental environment (1°C, 2.93 MPa); is the effective permeability; is the porosity.

[0065] After many theoretical and experimental studies, the displacement rate of the non-steady-state water-displacement gas process was determined to be 3 ml / min.

[0066] c) Keep the back pressure and water inlet pressure constant until no more gas is discharged from the core sample. During this process, the gas output is recorded in real time. And water output .

[0067] Substituting the recorded parameters into Equations 21 and 22, the relative permeabilities of the liquid and gas phases can be obtained.

[0068] 7. Decomposition of natural gas hydrate; For core samples with different initial porosities, methane hydrates were synthesized at a constant volume to a specific saturation. At each saturation, the hydrate particles in the core samples were evenly dispersed in the pore space. During the decomposition of hydrates under reduced pressure, the evolution of the pore structure of the sedimentary layer is a process that gradually radiates from the low-pressure boundary to the surrounding areas. Studies that only consider the constant-volume synthesis of hydrates and ignore the quantitative decomposition of hydrates cannot accurately characterize the continuous evolution characteristics of the pore structure of the sedimentary layer during the decomposition of hydrates under reduced pressure, resulting in deviations in the description of the dynamic evolution process of multiphase seepage parameters.

[0069] The quantitative decompression decomposition of methane hydrate is introduced to describe the continuous evolution characteristics of the pore structure of the mud siltstone core sample. First, the excess gas method is used to synthesize methane hydrate inside the core sample, and the preset synthesis pressure is adjusted to control the saturation of methane hydrate in the core sample to the maximum saturation, and the effective permeability and relative permeability of the core sample are measured; then, the pressure of the high-pressure reaction column 04 is gradually reduced to decompose the methane hydrate to the set saturation. The specific operation steps are as follows: Open the pressure reducing valve V4 to make the core pressure lower than , wait for a certain period of time Then close the pressure reducing valve V4. Adjust the back pressure valve V5 to ensure that the core sample pressure is always lower than the confining pressure by 1 MPa. Hydrate decomposition produces methane and water, which will cause the core pressure to increase slightly. When the readings of the pressure sensors 73 at the inlet and outlet no longer change, the quantitative decomposition of hydrates is completed, and a muddy siltstone core sample with a specific hydrate saturation is obtained.

[0070] According to the decomposition kinetics formula, the amount of hydrate decomposition can be roughly controlled: ; In the formula, are the amount of methane hydrate in the core sample before and after decomposition, mol; is the duration of decompression, s; is the decomposition kinetic coefficient of methane hydrate, 3.6×10 4 mol / (m 2 ·Pa·s); is the reaction surface area, m 2 ; represents the phase equilibrium pressure of methane hydrate at 1°C, which is 2.93 MPa; The low pressure environment required for the decomposition of methane hydrate, MPa; is the reaction order; represents the change in time t; Indicated in The change in the amount of methane hydrate substance over time; is the differential symbol.

[0071] The effective permeability and relative permeability of the core pore structure corresponding to the saturation after methane hydrate decomposition are measured. Then steps 5-7 are repeated to continuously quantitatively decompose methane hydrate and measure the multiphase seepage parameters of the core sample (including porosity Preset synthesis pressure Phase equilibrium pressure of methane hydrate at 1°C Gas occupies the pore volume of the core sample V G , methane hydrate saturation in core samples S H , inherent permeability k 0 , effective penetration ); until the methane hydrate is completely decomposed.

[0072] Example 1: Measurement of seepage parameters of muddy silt sediment in Shenhu Sea area of ​​South China Sea; The average porosity of the pilot mining site in the Shenhu Sea Area is 0.38, and the hydrate saturation is 0.21. Based on this, muddy siltstone core samples with different initial porosities are designed as shown in Table 3.

[0073]

[0074] For muddy siltstone cores with different initial porosities (0.28, 0.33, 0.38, 0.43, 0.48), the excess gas method was used to synthesize high-saturation hydrates (0.40) inside the cores, and their absolute permeabilities and relative permeabilities were measured. Afterwards, the hydrates were gradually decomposed to specific saturations (0.30, 0.20, 0.10, 0.00), and the absolute permeabilities and relative permeabilities of the core samples at each specific hydrate saturation stage were measured.

[0075] A1-A5 represent the measurements of a muddy siltstone core with an initial porosity of 0.38 after the synthesis of high saturation hydrate (0.40) inside the core, when it was not decomposed (A1, i.e., 0.40 saturation), decomposed to 0.30 (A2), 0.20 (A3), 0.10 (A4), and completely decomposed (A5, i.e., 0.00 saturation), respectively.

[0076] The naming and meaning of B1-B5, C1-C5, D1-D5 and E1-E5 are similar to A1-A5.

[0077] For core samples with different initial porosities, the key parameters of the hydrate constant volume synthesis and quantitative decomposition processes were sorted out, and the absolute permeability evolution results were obtained as shown in Tables 4 and 5.

[0078]

[0079] Table 5 Results of absolute permeability evolution of core during hydrate quantitative decomposition process

[0080] In Tables 4 and 5, k nor Indicates normalized permeability; in Table 5 V 1 and V 2 Respectively represent the methane gas emission before and after hydrate decomposition.

[0081] It can be seen from Tables 4 and 5 that when the hydrate saturation is 0, each mudstone siltstone core sample corresponds to two absolute permeabilities: the intrinsic permeability before hydrate constant volume synthesis and after hydrate complete decomposition. The value before synthesis is generally higher than the value after complete decomposition (for example, the intrinsic permeability before constant volume synthesis of A is k 0is 11.452; the intrinsic permeability after complete decomposition is k(S H ) In S H =0, which is 11.307; 11.452>11.307), which indicates that the synthesis and decomposition process of hydrate is not a simple aggregation and dispersion of hydrate particles in the pore space of the core.

[0082] The effective permeability of hydrate-bearing sediments with different lithologies, different sites with the same lithology, and even the same lithology and the same site but different burial depths are different, so it is meaningless to directly compare their values. Therefore, the normalized permeability is introduced. To characterize the dynamic evolution of the absolute permeability of the sedimentary layer during the hydrate decomposition process at the pore scale, while maintaining the internal relative relationship of each group of data, making them comparable to each other. The calculation formula is as follows: .

[0083] Figure 4 is the absolute permeability and normalized permeability of the hydrate decomposition process of the muddy siltstone core It can be seen that the absolute permeability dynamic evolution trend of muddy siltstone core samples with different initial porosities is consistent during the hydrate decomposition process, and both the effective permeability and the normalized permeability increase in stages with the decrease of hydrate saturation.

[0084] The siltstone cores with different initial porosities (0.28, 0.33, 0.38, 0.43, 0.48) were monitored at the representative hydrate decomposition stage (S H = 0.4, 0.3, 0.2, 0.1, 0.0) and input the obtained parameters into the relative permeability calculation program to obtain the relative permeability evolution curves of liquid and gas phases in the hydrate decomposition process. Figure 5 − Fig. 9 The dynamic evolution of relative permeability of liquid and gas phases of siltstone core samples with different initial porosities under different hydrate saturations is shown. It can be seen that the relative permeability evolution curves of siltstone core samples with different initial porosities are consistent in shape, all showing a sucking type, with only specific characteristic values ​​being different. Figure 5-Figure 9It can be seen that the pore structure of the core directly determines its seepage capacity. The results of the single-phase seepage experiment show that under the same conditions, the higher the porosity of the core, the greater its inherent permeability. From the evolution curves of liquid and gas phase relative permeability, it can be found that the core with small porosity, that is, low effective permeability, has low gas and liquid phase relative permeabilities. This is because the core with small porosity has a small pore channel diameter for fluid flow, and the capillary force is large, which will cause imbibition, further increase the viscosity of the rock, and thus reduce the actual seepage capacity of gas and liquid phase fluids. The blocking effect between multiphase fluids has a relatively large impact on the wetting phase fluid (water), which restricts the ability of the rock to allow the flow of liquid phase fluids, making the liquid phase relative permeability of the mud siltstone core low. When the liquid phase saturation is lower than 0.60, the liquid phase relative permeability does not exceed 0.04. At this stage, the difference in the evolution of liquid phase relative permeability of cores with different porosities is small. As the liquid saturation increases, the multiphase flow process of the core approaches the single-phase flow of liquid water. The capillary pressure of the core with smaller porosity is greater, which is more likely to cause imbibition, causing more water molecules to be adsorbed on the surface of solid particles. This reduces the continuous flow channel of liquid water on the one hand, and limits the volume of liquid that can flow on the other hand. W From 0.6 to 1.0), the liquid phase relative permeability decreases significantly with the decrease of porosity. Undecomposed hydrate particles can be regarded as special solid phase particles, which will adsorb liquid phase water molecules under the action of capillary pressure. Since hydrate particles are often dispersed in loose sediments, they tend to gather at pore throats and small pore channels. After the adsorption of water molecules in these key seepage sites, the flow of fluids will be severely restricted. Therefore, the liquid phase relative permeability of mud siltstone cores with higher hydrate saturation decreases more with the decrease of porosity. The blocking effect between multiphase fluids has relatively little effect on the non-wetting phase fluid (methane), which makes the gas phase relative permeability of mud siltstone cores higher. When the liquid phase saturation is lower than 0.6, the methane gas in the core pores migrates along a continuous and stable flow channel. As a non-wetting phase fluid, methane gas mostly flows in large pores or the center of pore channels. The gas phase flow channel will be indirectly reduced with the decrease of porosity, and its degree of obstruction is lower than that of the liquid phase flow channel. Therefore, the gas phase relative permeability decreases with the decrease of porosity in this stage, but its decrease is smaller than that of liquid phase permeability. Similarly, the influence of hydrate saturation change on gas phase relative permeability is lower than that on liquid phase relative permeability.

[0085] The above are only preferred embodiments of the present invention. It should be pointed out that, for those skilled in the art, several modifications and improvements can be made without departing from the concept of the present invention. These should also be regarded as the protection scope of the present invention. These will not affect the effect of the implementation of the present invention and the practicality of the patent.

Claims

1. A multiphase seepage experimental simulation system for hydrate decomposition process under low temperature and high pressure conditions, characterized by: include: The liquid injection system includes a horizontal flow pump for controlling the flow rate of liquid phase water, a liquid flow meter for recording the flow rate of liquid phase water, and a liquid injection valve for controlling the opening and closing of liquid injection; the horizontal flow pump is connected to the liquid injection valve, and the liquid injection valve is connected to the liquid flow meter; The gas injection system comprises a gas cylinder for providing gas, a pressure reducing valve for reducing gas pressure, a flow controller for controlling gas flow, and a gas injection valve for controlling the opening and closing of gas injection; the gas cylinder is connected to the pressure reducing valve, the pressure reducing valve is connected to the flow controller, and the flow controller is connected to the gas injection valve; A core holder and a high-pressure reaction column, wherein the core holder is used to hold the core sample and is placed in the high-pressure reaction column; The closed system includes a confining pressure pump for controlling the confining pressure of the core sample, an axial pressure pump for controlling the axial pressure of the core sample, an axial hydraulic cylinder, a vacuum pump and a vacuum valve for maintaining a vacuum environment, and a pressure reducing valve for reducing the pressure of the core sample; the confining pressure pump is connected to the core holder, the axial hydraulic cylinder is connected to the high-pressure reaction column, and the axial pressure pump is connected to the axial hydraulic cylinder; the vacuum pump is connected to the vacuum valve, and the vacuum valve is connected to the high-pressure reaction column; The back pressure system includes a hand-operated back pressure pump, a back pressure valve for controlling the back pressure, a gas collection bottle and an exhaust valve; the hand-operated back pressure pump is connected to the exhaust valve, and the exhaust valve is connected to the gas collection bottle; The data metering and acquisition module includes a temperature controller, a buffer tank, a pressure sensor, an axial pressure sensor, a gas-liquid separator, a drying tank, a balance and a timer; the temperature controller is connected to the upper part of the high-pressure reaction column, the pressure sensor is connected to the lower part of the high-pressure reaction column, the pressure sensor is connected to the pressure reducing valve, and the axial pressure sensor is connected to the axial hydraulic cylinder; the gas-liquid separator is connected to the pressure sensor, the buffer tank and the drying tank are both connected to the gas-liquid separator, the back pressure valve is connected to the buffer tank, a pressure gauge is installed on the connecting pipeline between the back pressure valve and the buffer tank, and a balance is installed at the drying tank; The unsteady-state water-displacement gas module is used to measure the dynamic evolution of the liquid and gas phase relative permeabilities of core samples under low temperature and high pressure conditions. The unsteady-state water-displacement gas module is based on the Buckley-Leverett model that describes the front advancement process of one-dimensional two-phase flow. The calculation formula of the liquid and gas phase relative permeabilities is as follows: ； ； ； In the formula, are the relative permeabilities of liquid and gas phases, respectively; is the liquid water saturation at the outlet; are the dynamic viscosity coefficients of liquid and gas phase fluids respectively; L is the length of the core sample; for Change in the amount of water in the internal liquid phase; is the time increment; A is the cross-sectional area of ​​the core sample; k 0 is the intrinsic permeability of the core sample; is the displacement pressure difference; is the corrected water-gas ratio; is the average value of the total amount of gas; for Changes in the amount of methane in the internal gas phase; is the pore volume; The calculation formula of the intrinsic permeability of the core sample is: ； In the formula, is the dynamic viscosity system of deionized water; is the deionized water flow rate per unit time; is the fluid density; is the acceleration due to gravity; is the head difference of the fluid at the inlet and outlet; is the pressure difference between inlet and outlet; The theoretical value of displacement pressure difference is: ； In the formula, is the interfacial tension between gas and liquid phase fluids; is the effective permeability of the core; is the porosity; The methane hydrate quantitative decompression decomposition module is used to describe the continuous evolution characteristics of the pore structure of the core sample. The specific process is as follows: First, the excess gas method was used to synthesize methane hydrate inside the core sample. The preset synthesis pressure was adjusted to control the saturation of methane hydrate in the core sample to the maximum saturation, and the effective permeability and relative permeability of the core sample were measured. Subsequently, the pressure of the high-pressure reaction column is gradually reduced to decompose the methane hydrate to a set saturation. The effective permeability and relative permeability of the core pore structure corresponding to the saturation after the methane hydrate is decomposed are measured. Continue to quantitatively decompose methane hydrate and measure the multiphase seepage parameters of core samples until methane hydrate is completely decomposed.

2. The multiphase seepage experimental simulation system for hydrate decomposition process under low temperature and high pressure conditions according to claim 1 is characterized in that: The measurement of the relative permeability of the liquid and gas phases was carried out at 1°C and 2.93 MPa, and the specific process was as follows: a. Open the nitrogen cylinder and adjust the pressure reducing valve, open the gas injection valve, adjust the gas inlet pressure, and control the nitrogen to be injected into the core sample at a rate of 20 ST ml / min until the core sample reaches a saturated state; b. Close the nitrogen cylinder and the gas injection valve, and open the advection pump and the liquid injection valve; Increase the water inlet pressure and gradually adjust the back pressure to form a displacement pressure difference; Turn on the timer to start the process of liquid water displacing gaseous nitrogen at a displacement rate of 3 ml / min; c. Keep the back pressure and water inlet pressure unchanged until no gas is discharged from the core sample, and record the gas and water output in real time.

3. The multiphase seepage experimental simulation system for hydrate decomposition process under low temperature and high pressure conditions according to claim 1 is characterized in that: The measurement process of the intrinsic permeability of the core sample is as follows: a. Place the core sample in the core holder, place filters on both sides of the core sample, and then install the holder into the high-pressure reaction column; b. Adjust the confining pressure pump and the axial pressure pump so that the core sample can withstand a confining pressure of 2 MPa and an axial pressure of 1 MPa; c. Keep all valves closed, set the back pressure to 0 MPa, and adjust the balance to zero; d. Adjust the horizontal flow pump and injection valve to inject deionized water into the core sample at a rate of 3 ml / min; e. Observe the balance readings in real time until the reading difference stabilizes, and record the inlet and outlet pressures.

4. The multiphase seepage experimental simulation system for hydrate decomposition process under low temperature and high pressure conditions according to claim 3 is characterized in that: The measurement process of the effective permeability is the same as that of the intrinsic permeability.

5. The multiphase seepage experimental simulation system for hydrate decomposition process under low temperature and high pressure conditions according to claim 1 is characterized in that: In the step of continuously and quantitatively decomposing methane hydrate, the decomposition amount of methane hydrate each time is: ； In the formula, are the amounts of methane hydrate substances in the core samples before and after decomposition, respectively; is the duration of blood pressure reduction; is the decomposition kinetic coefficient of methane hydrate; is the reaction surface area; represents the phase equilibrium pressure of methane hydrate at 1°C, which is 2.93 MPa; The low pressure environment required for hydrate decomposition; is the reaction order; represents the change in time t; Indicated in The change in the amount of methane hydrate substance over time; is the differential symbol.

6. The multiphase seepage experimental simulation system for hydrate decomposition process under low temperature and high pressure conditions according to claim 1 is characterized in that: The multiphase seepage parameters of the core sample include porosity , preset synthesis pressure 、Phase equilibrium pressure of methane hydrate at 1℃ , the pore volume of the core sample occupied by gas , methane hydrate saturation in core samples S H , inherent permeability k 0 , effective penetration ; Gas occupies the pore volume of the core sample V G The calculation formula is: ； In the formula, n The amount of substance that is methane gas; is the methane gas density; is the injected volume of methane gas per unit time; is the ideal gas constant; is the experimental temperature; is the molar mass of methane gas; is the pressure increment of the core sample per unit time; Methane hydrate saturation in core samples S H The calculation formula is: ； In the formula, A is the cross-sectional area of ​​the core sample; L is the length of the core sample; Indicates the low temperature environment required for the synthesis of methane hydrate, which is 1°C.

7. The multiphase seepage experimental simulation system for hydrate decomposition process under low temperature and high pressure conditions according to claim 1, characterized in that: The core holder is a cylindrical holder made of stainless steel with an inner diameter of 50 mm. Its normal working pressure is 0~25 MPa and the temperature is -5~25℃.

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