An experimental device and method for simulating hydrate secondary formation

Abstract

The application is suitable for the technical field of hydrate simulation mining, and provides an experimental device and method for simulating secondary formation of hydrate, the device comprising: a reaction unit, a gas injection unit, a liquid injection unit, a temperature control unit, a vacuum degree control unit, an outlet control unit, a gas circulation unit and a data monitoring and processing unit; the reaction unit comprises a reaction kettle, a vacuum box and a constant-temperature water bath box, the reaction kettle is arranged in the vacuum box, and the vacuum box and the reaction kettle in the vacuum box are arranged in the constant-temperature water bath box. Through the reaction kettle, the converging flow effect that the seepage radius of hydrate decomposition products gradually decreases in the radial seepage process in the near-well area is simulated, the law of secondary formation of hydrate in the near-well area in the mining process is explored, and a feasible scheme and technical guidance are provided for prevention and treatment of secondary hydrate in the natural gas hydrate mining process.

Description

Technical Field

[0001] This invention belongs to the field of hydrate simulation mining technology, and specifically discloses an experimental device and method for simulating the secondary formation of hydrates. Background Technology

[0002] Natural gas hydrates, with their abundant reserves, clean and pollution-free nature, and high energy density, have become an internationally recognized important potential high-efficiency clean energy alternative to oil and gas.

[0003] During the depressurization extraction of hydrates, the hydrate decomposition products, namely methane and water, generated in the hydrate decomposition zone, experience a continuous decrease in temperature due to the throttling expansion effect during seepage into the production well. This leads to the regeneration of hydrates in the near-wellbore area where the hydrates have already completely decomposed, blocking the gas production channel and affecting gas production efficiency. The spatiotemporal evolution of secondary hydrates involves the coupling of multiple physical fields, including temperature, pressure, and ion concentration fields, as well as gas-liquid seepage and phase transition. Studying its laws is of great significance for the future industrial development of hydrates. However, existing experimental setups for secondary hydrate research are mostly one-dimensional and small-scale, failing to consider the converging flow effect as the fluid seepage area gradually decreases during actual extraction, nor the heat exchange between the reactor and the isothermal medium. These setups cannot simulate the impact of the converging flow effect and the throttling expansion effect on the secondary formation of hydrates in the near-wellbore area under non-convective heat transfer conditions, necessitating improvements. Summary of the Invention

[0004] The purpose of this invention is to provide an experimental apparatus and method for simulating the secondary formation of hydrates, aiming to overcome the shortcomings of existing experimental apparatuses that cannot simulate the effects of convergent flow and throttling expansion on the secondary formation of hydrates in the near-wellbore area under non-convective heat transfer conditions; to explore the laws governing the secondary formation of hydrates during mining, to clarify the mechanism of the spatiotemporal evolution of secondary hydrates in the near-wellbore area, and to provide feasible solutions and technical guidance for the prevention and control of secondary hydrates.

[0005] The present invention is implemented as follows: an experimental apparatus for simulating the secondary formation of hydrates, the experimental apparatus for simulating the secondary formation of hydrates includes: a reaction unit, a gas injection unit, a liquid injection unit, a temperature control unit, a vacuum control unit, an outlet control unit, a gas circulation unit, and a data monitoring and processing unit;

[0006] The reaction unit includes a reaction vessel, a vacuum chamber, and a constant temperature water bath. The reaction vessel is located inside the vacuum chamber, and the vacuum chamber and the reaction vessel inside it are located inside the constant temperature water bath.

[0007] The reactor includes two or more cascaded single-stage reactors; the inner diameter of the two or more cascaded single-stage reactors is determined based on the similarity principle to simulate the converging flow effect in which the seepage radius of hydrate decomposition products gradually decreases during radial seepage in the near-well region.

[0008] The gas injection unit is used to inject gas into the reactor to simulate the gas produced by the decomposition of hydrates;

[0009] The liquid injection unit is used to inject liquid into the reactor to simulate the liquid produced by the decomposition of hydrates;

[0010] The temperature control unit is used to control the temperature inside the constant temperature water bath.

[0011] The vacuum control unit is used to control the vacuum level inside the vacuum chamber;

[0012] The outlet control unit is used to control the outlet pressure of the reactor and to separate and measure the gas and liquid generated in the experiment.

[0013] The gas circulation unit is used to re-transport the gas separated from the outlet control unit to the gas injection unit to achieve gas recycling.

[0014] The data monitoring and processing unit is used to monitor and collect physical parameters inside the reactor during the experiment. These physical parameters include at least temperature, pressure, and resistivity.

[0015] Furthermore, any two adjacent single-stage reactor sections are connected by a variable-diameter flange;

[0016] The outermost end face of the single-stage reactor is sealed by a flange, which is provided with an injection port and an outlet to realize the injection and output of gas and liquid.

[0017] Each of the single-stage reactors is equipped with a high-pressure connector for connecting to the data monitoring and processing unit; the pressure resistance range of the single-stage reactor and the flange is 0-35MPa.

[0018] Furthermore, the constant temperature water bath contains a first medium that covers the top of the vacuum chamber;

[0019] The temperature control unit includes a first booster pump, a low-temperature constant temperature bath, and a second booster pump; the low-temperature constant temperature bath is filled with a cooling medium, and the first medium in the constant temperature water bath circulates in a closed loop consisting of the constant temperature water bath, the first booster pump, the low-temperature constant temperature bath, the second booster pump, and the constant temperature water bath.

[0020] The first medium in the constant temperature water bath is in indirect contact with the cooling medium in the low temperature constant temperature bath to exchange heat and maintain the temperature of the reactor at a preset temperature.

[0021] Furthermore, the gas injection unit includes a gas cylinder, a first regulating valve group, and a gas flow controller. The gas cylinder, the first regulating valve group, the gas flow controller, the vacuum chamber, and the reaction vessel are connected in sequence via gas pipelines. The gas cylinder stores methane gas, which flows sequentially through the first regulating valve group, the gas flow controller, and the vacuum chamber before being injected into the reaction vessel. The first regulating valve group is used to control the gas pressure, and the gas flow controller is used to control the gas flow rate.

[0022] The liquid injection unit includes a cascaded storage tank, a constant pressure and constant flow pump, and a second regulating valve group. The storage tank contains water, and the constant pressure and constant flow pump injects the water from the storage tank into the gas pipeline at a constant flow rate through the second regulating valve group. After mixing with the gas, the water is injected into the reaction vessel.

[0023] Furthermore, the vacuum chamber includes a chamber body, an end cap, support members, and a heat insulation plate. The end cap and the chamber body are sealed using bolts and a plastic sealing gasket. The heat insulation plate is laid on the inner bottom surface of the chamber body, and multiple support members are arranged on the heat insulation plate to fix the reactor and suspend the reactor in the chamber body.

[0024] Furthermore, the vacuum control unit includes a vacuum gauge, a third regulating valve group, and a vacuum pump. The vacuum gauge and the third regulating valve group are mounted on the housing, and the vacuum pump is used to evacuate the vacuum chamber.

[0025] Furthermore, the data monitoring and processing unit includes a temperature sensor, a pressure sensor, a resistivity sensor, a data acquisition module, and a data processing module;

[0026] The temperature and pressure sensors are evenly distributed along the axial direction of the reactor and are arranged opposite each other along the center of the reactor to quantitatively monitor the temperature and pressure changes caused by the throttling expansion effect and the formation of secondary hydrates during the experiment.

[0027] The resistivity sensor includes a pair of resistivity probes. Each pair of resistivity probes is also evenly distributed along the axial direction of the reactor and is arranged opposite each other along the center of the reactor to qualitatively monitor the resistivity changes caused by the formation of secondary hydrates.

[0028] The data acquisition module is used to process the signals collected by the temperature sensor, pressure sensor, and resistivity sensor, and transmit the collected signals to the data processing module.

[0029] Furthermore, the outlet control unit includes a fourth valve assembly and a flow meter; the fourth valve assembly is installed at the end cap of the vacuum chamber and connected to the outlet of the reactor via a high-pressure pipeline, and is used to control the outlet pressure of the reactor to simulate the pressure of the production well during hydrate extraction; the flow meter is used to monitor the gas flow rate at the outlet of the reactor.

[0030] Furthermore, the gas circulation unit includes a gas-liquid separator, a balance measuring instrument, a first gas storage tank, a third booster pump, and a second gas storage tank;

[0031] The gas-liquid separator is used to separate the product output from the outlet of the reactor. The balance measuring instrument is located at the outlet end of the gas-liquid separator and is used to measure the weight of the separated liquid. The first gas storage tank is used to collect the separated gas. The third booster pump pressurizes the collected gas and pumps it to the second gas storage tank to re-deliver the gas in the second gas storage tank to the gas injection unit or the reactor to achieve gas circulation.

[0032] To facilitate the implementation of the experimental apparatus for simulating secondary hydrate formation, the present invention also aims to provide an experimental method for simulating secondary hydrate formation, used in the aforementioned experimental apparatus. The method includes the following steps:

[0033] Preparation before the purging experiment: Prepare sediment samples in situ in each section of the single-stage reactor using a simulated medium mixed with distilled water and a layered compaction method; connect each section of the single-stage reactor and the corresponding data monitoring and processing unit; transfer the reactor to the vacuum chamber, connect and fix the corresponding parts of the reactor to the vacuum chamber, and seal the vacuum chamber; check the airtightness of the reactor and the vacuum chamber to ensure good airtightness before proceeding to the next step of the experiment;

[0034] Pre-purge experiment: The reactor was pre-purged with methane gas. After observing that no more water was coming out of the reactor outlet, the collected water was weighed and the residual water saturation of the sediment sample in the reactor was calculated. The reactor outlet was closed, and then gas was injected for a period of time until the reactor was filled with a certain amount of gas. The gas injection port of the reactor was then closed.

[0035] Initial formation and decomposition of hydrates: Lower the temperature of the constant temperature water bath to the first temperature. After the hydrates have completely formed, open the outlet of the reactor to decompose the hydrates. After the hydrates have completely decomposed, adjust the temperature of the constant temperature water bath to the second temperature. After the second temperature has stabilized, perform vacuum treatment. After the vacuum degree reaches 100%, start the purging experiment.

[0036] Purging experiment: During the purging experiment, the changes in temperature, pressure and resistivity at different locations in the reactor are monitored in real time. The monitoring data are used to comprehensively determine the secondary formation of hydrates and the location of secondary formation. The changes in the saturation of secondary hydrates are analyzed in combination with the changes in flow rate.

[0037] The present invention provides an experimental apparatus for simulating the secondary formation of hydrates. The reactor used is a two-dimensional long tube reactor, which is composed of two or more reactors with different inner diameters spliced ​​together by flanges. Compared with the existing one-dimensional reactor, it has the following advantages: the two-dimensional long tube reactor is set based on the similarity principle and can simulate the influence of convergent flow effect and throttling expansion effect on the temperature field and pressure field of near-wellbore hydrate reservoir.

[0038] The reactor adopts a segmented design, and the influence of the mining radius on the formation law of secondary hydrates can be studied by changing the number of segments of the reactor in the experiment.

[0039] Each reactor section is equipped with multiple temperature sensors, pressure sensors, and resistivity sensors. The temperature and pressure sensors are evenly distributed along the reactor axis and are set opposite each other along the reactor axis. Similarly, the probes of the resistivity sensors are also evenly distributed along the reactor axis and are set opposite each other along the reactor axis. The formation of secondary hydrates and their spatiotemporal evolution can be determined by the changes in temperature, pressure, and resistivity at different points.

[0040] The reactor was placed in a vacuum chamber that could be evacuated, and the experiment was carried out under the condition that there was no thermal convection between the reactor and the external environment, so as to realistically reproduce the characteristics of the changes in the reservoir temperature field and pressure field during the hydrate extraction process.

[0041] The gas discharged from the reactor can be reused to carry out gas purging experiments over long periods of time. Attached Figure Description

[0042] Figure 1 A structural diagram of an experimental apparatus for simulating the secondary formation of hydrates provided in an embodiment of the present invention;

[0043] Figure 2 A structural diagram of a two-stage reaction vessel provided in an embodiment of the present invention;

[0044] Figure 3 for Figure 2 Enlarged view of a local structure in the image;

[0045] Figure 4 A flowchart of an experimental method for simulating the secondary formation of hydrates provided in an embodiment of the present invention.

[0046] In the attached diagram: 1-Gas cylinder; 2-First shut-off valve; 3-Pressure reducing valve; 4-Second shut-off valve; 5-Gas flow controller; 6-Third shut-off valve; 7-Storage tank; 8-Fourth shut-off valve; 9-Constant pressure and constant flow pump; 10-Fifth shut-off valve; 11-Constant temperature water bath; 12-Vacuum chamber; 13-Reaction vessel; 14-First support component; 15-Second support component; 16-Third support component; 17-Fourth support component; 18-Fifth support component; 19-Heat insulation plate; 20-First booster pump; 21-Low temperature constant temperature bath; 22-Second booster pump; 23-Vacuum gauge; 24-Sixth shut-off valve; 25-Seventh shut-off valve 26-Vacuum pump; 27-Eighth shut-off valve; 28-Back pressure valve; 29-Flow meter; 30-Gas-liquid separator; 31-Balance measuring instrument; 32-First gas storage tank; 33-Ninth shut-off valve; 34-Third booster pump; 35-Second gas storage tank; 36-Tenth shut-off valve; 37-Temperature sensor; 38-Pressure sensor; 39-Resistivity sensor; 40-Data acquisition module; 41-Data processing module; 42-End flange; 43-Temperature sensor connector; 44-Resistivity connector; 45-Pressure sensor connector; 46-Single-stage reactor; 47-Sealing surface; 48-Reducing flange. Detailed Implementation

[0047] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0048] The specific implementation of the present invention will be described in detail below with reference to specific embodiments.

[0049] like Figure 1 The diagram shown is a structural diagram of an experimental apparatus for simulating the secondary formation of hydrates according to an embodiment of the present invention, including: a reaction unit, a gas injection unit, a liquid injection unit, a temperature control unit, a vacuum control unit, an outlet control unit, a gas circulation unit, and a data monitoring and processing unit;

[0050] The reaction unit includes a reaction vessel 13, a vacuum chamber 12, and a constant temperature water bath 11. The reaction vessel 13 is disposed inside the vacuum chamber 12, and the vacuum chamber 12 and the reaction vessel 13 inside it are disposed inside the constant temperature water bath 11.

[0051] The reactor 13 includes two or more cascaded single-stage reactors 46 (see...). Figure 2 The inner diameter reduction of the single-stage reactor 46 in two or more cascaded stages is determined based on the similarity principle to simulate the converging flow effect in which the seepage radius of hydrate decomposition products gradually decreases during radial seepage in the near-well region.

[0052] The gas injection unit is used to inject gas into the reactor 13 to simulate the gas produced by the decomposition of hydrates;

[0053] The liquid injection unit is used to inject liquid into the reactor 13 to simulate the liquid produced by the decomposition of hydrates;

[0054] The temperature control unit is used to control the temperature inside the constant temperature water bath 11;

[0055] The vacuum control unit is used to control the vacuum level inside the vacuum chamber 12;

[0056] The outlet control unit is used to control the outlet pressure of the reactor 13 and to separate and measure the gas and liquid generated in the experiment.

[0057] The gas circulation unit is used to re-transport the gas separated from the outlet control unit to the gas injection unit to achieve gas recycling.

[0058] The data monitoring and processing unit is used to monitor and collect physical parameters inside the reactor 13 during the experiment. The physical parameters include at least temperature, pressure and resistivity.

[0059] In this embodiment, the reactor 13 used to load sediment samples is a two-dimensional long-tube reactor, which is assembled from multiple single-section reactors 46 with different inner diameters by flanges. This can realistically simulate the effects of convergent flow and throttling expansion on the temperature and pressure fields of hydrate reservoirs in the near-wellbore area. At the same time, the reactor 13 adopts a segmented design, and the influence of the extraction radius on the formation law of secondary hydrates can be studied by changing the number of reactor sections in the experiment. Each reactor section is equipped with a data monitoring and processing unit to monitor and process the changes in temperature, pressure, and resistivity at different points. The formation of secondary hydrates can be determined and their spatiotemporal evolution law can be analyzed by the changes in temperature, pressure, and resistivity at different points. The reactor 13 is placed in a vacuum chamber 12 that can be evacuated, and the experiment is carried out under the condition that there is no thermal convection between the reactor 13 and the external environment, so as to realistically reproduce the characteristics of the changes in the temperature and pressure fields of the reservoir during the hydrate extraction process. The gas discharged from the reactor 13 can be reused to carry out gas purging experiments over a long period of time.

[0060] In one example of this embodiment, the number of single-stage reactors 46 is five, that is, five single-stage reactors 46 are cascaded and connected sequentially. The change in the inner diameter of each single-stage reactor 46 is determined based on the principle of similarity to simulate the converging flow effect where the seepage radius gradually decreases during the radial seepage of hydrate decomposition products in the near-wellbore area. In one implementation scenario, the inner diameter of the five single-stage reactors 46 decreases sequentially. However, in some special scenarios of this example, the inner diameter of the five single-stage reactors 46 is not limited to a regular increase or decrease; it can also be abrupt, which is preferred. Obviously, the inner diameter of the five single-stage reactors 46, which decreases sequentially, can be mainly determined based on the principle of similarity, so as to provide reliable technical guidance for hydrate mining based on the actual simulated hydrate mining area.

[0061] In one example of this embodiment, the data monitoring and processing unit is used to monitor and collect physical parameters inside the reaction vessel 13 during the experiment. The physical parameters may include temperature, humidity, permeability, pressure, and resistivity. In this example, the physical parameters can mainly be temperature, pressure, and resistivity. In this way, the monitoring requirements can be met, the amount of data processing can be reduced, and the processing efficiency can be improved.

[0062] In one example of this embodiment, the cascaded reactor 13 is placed in a detachable vacuum chamber 12, which is placed in a constant temperature water bath 11. The constant temperature water bath 11 contains a sufficient amount of a first medium, such as water. Before the purging experiment, the temperature of the constant temperature water bath 11 is controlled by a temperature control unit to adjust the temperature of the reactor 13 to the temperature required for the experiment to simulate the in-situ temperature of the reservoir. After the temperature stabilizes, the vacuum chamber 12 is evacuated to simulate the condition of no convection heat transfer in the in-situ reservoir, so as to avoid the influence of heat transfer between the reactor 13 and the water bath on the experimental results. This ensures that the temperature change in the reactor 13 during the experiment can truly reflect the throttling expansion effect and the change in the temperature field caused by the formation of secondary hydrates. In the purging experiment, a gas-liquid two-phase flow with a certain pressure and flow rate was supplied to reactor 13 through gas injection and liquid injection units to simulate the gas-water flow in the near-wellbore area during hydrate extraction. The pressure at the outlet of reactor 13 was controlled to simulate the production pressure during extraction. The fluid flowing out of reactor 13 was separated by the outlet control unit, and the separated gas continued to be transported to the inlet of reactor 13 for circulation to meet the needs of long-term experiments. During the experiment, the temperature, pressure, resistivity inside reactor 13 and the flow rate at the reactor outlet were monitored and recorded in real time to analyze the formation law of secondary hydrates.

[0063] like Figure 2 , Figure 3As shown, in one embodiment, the data monitoring and processing unit includes a temperature sensor 37, a pressure sensor 38, a resistivity sensor 39, a data acquisition module 40, and a data processing module 41;

[0064] The temperature sensor 37 and pressure sensor 38 are evenly distributed along the axial direction of the reactor 13 and are arranged opposite each other along the axis of the reactor 13, in order to quantitatively monitor the temperature and pressure changes caused by the throttling expansion effect and the formation of secondary hydrates during the experiment.

[0065] The resistivity sensor 39 includes a pair of resistivity probes. Each pair of resistivity probes is also evenly distributed along the axial direction of the reactor 13 and is arranged facing each other along the axis of the reactor 13, in order to qualitatively monitor the resistivity changes caused by the formation of secondary hydrates.

[0066] The data acquisition module 40 is used to process the signals collected by the temperature sensor 37, pressure sensor 38, and resistivity sensor 39, and transmit the collected signals to the data processing module 41.

[0067] In this embodiment, each segment of the single-stage reactor 46 is provided with three temperature and pressure monitoring points and two resistivity monitoring points, with the resistivity monitoring points located between adjacent temperature and pressure monitoring points. Temperature sensors 37 and pressure sensors 38 are uniformly distributed along the axial direction of the reactor 13 and are arranged opposite each other along the center of the reactor 13, used to quantitatively monitor the temperature and pressure changes caused by the throttling expansion effect and the formation of secondary hydrates during the experiment. Resistivity sensor 39 includes a pair of resistivity probes, each pair of probes also uniformly distributed along the axial direction of the reactor 13 and arranged opposite each other along the center of the reactor 13, used to qualitatively monitor the resistivity changes caused by the formation of secondary hydrates. Data acquisition module 40 is used to process the acquired signals and transmit them to data processing module 41, which can be a computer. The data acquisition module 40 can use a data acquisition instrument that is matched with the temperature sensor 37, pressure sensor 38, and resistivity sensor 39. It can convert the analog signals of the temperature sensor 37, pressure sensor 38, and resistivity sensor 39 into electrical digital signals, which are then processed by the computer. The computer then uses these electrical digital signals to comprehensively determine the secondary formation of hydrates and the location of the secondary formation, and analyzes the changes in the saturation of secondary hydrates in combination with the changes in flow rate.

[0068] like Figure 2 As shown, in one embodiment, any two adjacent single-stage reactors 46 are connected by a reducing flange 48;

[0069] The outermost single-stage reactor 46 is sealed by a flange, which is provided with an injection port and an outlet to realize the injection and output of gas and liquid.

[0070] Each of the single-stage reactors 46 is equipped with a high-pressure connector for connecting to the data monitoring and processing unit; the pressure resistance range of the single-stage reactor 46 and the flange (i.e., end flange 42) is 0-35 MPa. The high-pressure connectors can be categorized as temperature sensor connectors 43, resistivity connectors 44, pressure sensor connectors 45, etc.

[0071] The single-stage reactors 46 with different inner diameters can be connected by a reducing flange 48. Specifically, the reducing flange 48 connects the sealing surfaces 47 of adjacent single-stage reactors 46 to form a two-dimensional long tube reactor. The reactor 13 formed by this connection is sealed at both ends by flanges, and the reactor 13 and the flanges can be sealed by O-rings, perfectly achieving the design requirements of detachability, high sealing, and high strength. High-pressure connectors are used to connect the data monitoring and processing unit, including temperature monitoring devices, pressure monitoring devices, and resistivity monitoring devices, such as temperature sensor 37, pressure sensor 38, and resistivity sensor 39.

[0072] like Figure 1 As shown, in one embodiment, the vacuum chamber 12 includes a chamber body, an end cap, support members, and a heat insulation plate 19. The end cap and the chamber body are sealed using bolts and a plastic sealing gasket. The heat insulation plate 19 is laid on the inner bottom surface of the chamber body, and a plurality of support members are arranged on the heat insulation plate 19 to fix the reactor 13 and suspend the reactor 13 in the chamber body.

[0073] In this embodiment, the multiple supporting members may be: a first supporting member 14, a second supporting member 15, a third supporting member 16, a fourth supporting member 17, and a fifth supporting member 18; correspondingly, the single-stage reactor 46 included in the reactor 13 may be provided with five stages; the heat insulation plate 19 is laid on the inner bottom surface of the box, and its upper side is along the injected fluid (such as... Figure 1 The first support member 14, the second support member 15, the third support member 16, the fourth support member 17, and the fifth support member 18 are placed sequentially from left to right to fix the corresponding single-section reactor 46 and suspend it in the air, so as to prevent the reactor 13 from exchanging heat with the external environment through the bottom plate of the vacuum box 12. The end cap is detachably installed on the top of the box and sealed with the box body by bolts and plastic sealing gaskets. The heat insulation plate 19 can prevent the reactor 13 from exchanging heat with the external environment through the first support member 14, the second support member 15, the third support member 16, the fourth support member 17, and the fifth support member 18. Taking advantage of the fact that the entire vacuum box 12 reduces the heat transfer difficulty in the in-situ reservoir, it prevents the reactor 13 from exchanging heat with the constant temperature water bath 11, and more realistically reflects the temperature field changes caused by the throttling expansion effect and the formation of secondary hydrates during the experiment.

[0074] In one example of this embodiment, the vacuum chamber 12 includes a chamber body, end caps, support components, and heat insulation plate 19 that are detachable, which facilitates the installation, replacement, disassembly, and maintenance of the reactor 13; it also facilitates the debugging and monitoring of the data acquisition module 40 therein.

[0075] like Figure 1 As shown in one example of this embodiment, the vacuum control unit includes a vacuum gauge 23, a third regulating valve group, and a vacuum pump 26. The vacuum gauge 23 and the third regulating valve group are installed on the housing, and the vacuum pump 26 is used to evacuate the vacuum chamber 12.

[0076] The third regulating valve group includes a sixth shut-off valve 24 and a seventh shut-off valve 25; the vacuum gauge 23 and the sixth shut-off valve 24 are installed at the end cover of the vacuum chamber 12, the vacuum pump 26 is used to evacuate the vacuum chamber 12, and the seventh shut-off valve 25 is installed on the pipeline between the vacuum pump 26 and the vacuum chamber 12.

[0077] like Figure 1 As shown, in one embodiment, the constant temperature water bath 11 contains a first medium that covers the top of the vacuum chamber 12;

[0078] The temperature control unit includes a first booster pump 20, a low-temperature constant temperature bath 21, and a second booster pump 22; the low-temperature constant temperature bath 21 is filled with a cooling medium, and the first medium in the constant temperature water bath 11 circulates in a closed loop consisting of the constant temperature water bath 11, the first booster pump 20, the low-temperature constant temperature bath 21, the second booster pump 22, and the constant temperature water bath 11.

[0079] The first medium in the constant temperature water bath 11 and the cooling medium in the low temperature constant temperature bath 21 are in indirect contact in the low temperature constant temperature bath 21 to exchange heat and maintain the temperature of the reactor 13 at a preset temperature.

[0080] In one example of this embodiment, the first medium can be water, and the cooling medium can be an aqueous ethylene glycol solution. The water in the constant temperature water bath 11 circulates in a closed loop: constant temperature water bath 11 - first booster pump 20 - low temperature constant temperature bath 21 - second booster pump 22 - constant temperature water bath 11. The water in the constant temperature water bath 11 does not directly contact the aqueous ethylene glycol solution in the low temperature constant temperature bath 21, but rather exchanges heat with the aqueous ethylene glycol solution through the coils in the low temperature constant temperature bath 21. Based on this process, temperature control is achieved, maintaining the initial temperature of the reactor 13 at the preset temperature, i.e., the actual reservoir temperature, thus realizing a simulation experiment that conforms to the principle of similarity.

[0081] like Figure 1As shown, in one embodiment, the gas injection unit includes a gas cylinder 1, a first regulating valve group, and a gas flow controller 5. The gas cylinder 1, the first regulating valve group, the gas flow controller 5, the vacuum chamber 12, and the reaction vessel 13 are connected in sequence through a gas pipeline. The gas cylinder 1 stores methane gas, which flows sequentially through the first regulating valve group, the gas flow controller 5, and the vacuum chamber 12 before being injected into the reaction vessel 13. The first regulating valve group is used to control the gas pressure, and the gas flow controller 5 is used to control the gas flow rate.

[0082] The liquid injection unit includes a cascaded storage tank 7, a constant pressure and constant flow pump 9, and a second regulating valve group. The storage tank 7 stores water, and the constant pressure and constant flow pump 9 injects the water in the storage tank 7 into the gas pipeline at a constant flow rate through the second regulating valve group. After mixing with the gas, the water is injected into the reaction vessel 13.

[0083] In one example of this embodiment, the first regulating valve group includes: a first shut-off valve 2, a pressure reducing valve 3, a second shut-off valve 4, and a third shut-off valve 6; the methane gas stored in the gas cylinder 1 flows through the first shut-off valve 2, the pressure reducing valve 3, the second shut-off valve 4, the gas flow controller 5, and the third shut-off valve 6 in sequence through the gas pipeline and is then injected into the reactor 13. The pressure reducing valve 3 is used to control the gas inlet pressure, the gas flow controller 5 is used to control the gas flow rate, the first shut-off valve 2 and the second shut-off valve 4 are located on the gas pipeline, and the third shut-off valve 6 is located on the end cap of the vacuum box 12 and is connected to the gas injection port of the reactor 13 through a high-pressure pipeline.

[0084] In one example of this embodiment, the second regulating valve group includes a fourth shut-off valve 8 and a fifth shut-off valve 10; the storage tank 7 stores water or distilled water, and the distilled water in the storage tank 7 is injected into the gas pipeline at a constant flow rate by the constant pressure constant flow pump 9, and after mixing with the gas, it is injected into the reaction vessel 13. The fourth shut-off valve 8 and the fifth shut-off valve 10 are located at the suction port and injection port of the constant pressure constant flow pump 9, respectively.

[0085] like Figure 1 As shown, in one embodiment, the outlet control unit includes a fourth valve assembly and a flow meter 29; the fourth valve assembly is installed at the end cap of the vacuum chamber 12 and connected to the outlet of the reactor 13 via a high-pressure pipeline, and is used to control the outlet pressure of the reactor 13 to simulate the pressure of the production well during hydrate extraction; the flow meter 29 is used to monitor the gas flow rate at the outlet of the reactor 13.

[0086] The fourth valve group includes an eighth shut-off valve 27 and a back pressure valve 28. The eighth shut-off valve 27 is installed at the end cap of the vacuum chamber 12 and connected to the outlet of the reactor 13 through a high-pressure pipeline. The back pressure valve 28 is used to control the outlet pressure of the reactor 13 to simulate the pressure of the production well during hydrate mining. The flow meter 29 monitors the gas flow at the outlet of the reactor 13 and the flow data can be acquired by the data monitoring and processing unit.

[0087] like Figure 1 As shown, in one embodiment, the gas circulation unit includes a gas-liquid separator 30, a balance measuring device 31, a first gas storage tank 32, a third booster pump 34, and a second gas storage tank 35;

[0088] The gas-liquid separator 30 is used to separate the product output from the outlet of the reactor 13. The balance measuring device 31 is located at the outlet end of the gas-liquid separator 30 and is used to measure the weight of the separated liquid. The first gas storage tank 32 is used to collect the separated gas. The third booster pump 34 pressurizes the collected gas and pumps it to the second gas storage tank 35 to re-transport the gas in the second gas storage tank 35 to the gas injection unit or the reactor 13 to achieve gas circulation.

[0089] like Figure 1 As shown in one example of this embodiment, a ninth shut-off valve 33 is provided between the first gas storage tank 32 and the third booster pump 34, and a tenth shut-off valve 36 is provided between the second gas storage tank 35 and the gas pipeline.

[0090] like Figure 4 As shown, to facilitate the implementation of the experimental apparatus for simulating secondary hydrate formation, the present invention also aims to provide an experimental method for simulating secondary hydrate formation, used in the aforementioned experimental apparatus for simulating secondary hydrate formation. The method includes the following steps S401 to S407:

[0091] S401. Preparation before purging experiment: Prepare sediment samples in situ in each section of the single-stage reactor 46 using a simulated medium mixed with distilled water and a layered compaction method; connect each section of the single-stage reactor 46 and the corresponding data monitoring and processing unit; transfer the reactor 13 to the vacuum chamber 12, connect and fix the corresponding parts of the reactor 13 to the vacuum chamber 12, and seal the vacuum chamber 12; check the airtightness of the reactor 13 and the vacuum chamber 12 to ensure good airtightness before proceeding to the next experiment;

[0092] S403, Pre-purge experiment: Use methane gas to pre-purge reactor 13. After observing that no more water is coming out of the outlet of reactor 13, weigh the collected water and calculate the residual water saturation of the sediment sample in reactor 13. Close the outlet of reactor 13, and then inject gas for a period of time until the reactor is filled with a certain amount of gas. Then close the gas injection port of reactor 13.

[0093] S405. Initial formation and decomposition of hydrate: Lower the temperature of the constant temperature water bath 11 to the first temperature. After the hydrate has completely formed, open the outlet of the reactor 13 to decompose the hydrate. After the hydrate has completely decomposed, adjust the temperature of the constant temperature water bath 11 to the second temperature. After the second temperature stabilizes, perform vacuum treatment. After the vacuum reaches 100%, start the purging experiment. The first temperature and the second temperature can be flexibly set according to actual needs. This embodiment is not limited to this.

[0094] S407, Purging Experiment: During the purging experiment, the changes in temperature, pressure and resistivity at different locations in the reactor 13 are monitored in real time. The monitoring data are used to comprehensively determine the secondary formation of hydrates and the location of secondary formation. The changes in the saturation of secondary hydrates are analyzed in combination with the changes in flow rate.

[0095] In this embodiment, the data monitoring and processing unit includes a data processing module 41 and a data acquisition module 40, as well as a temperature monitoring device, a pressure monitoring device, and a resistivity monitoring device electrically connected to the data acquisition module 40, for example... Figure 2 The reactor 13 includes temperature sensors 37, pressure sensors 38, and resistivity sensors 39. Temperature sensors 37 and pressure sensors 38 are uniformly distributed along the axial direction of the reactor 13 and are positioned opposite each other along the axial center of the reactor 13, used to quantitatively monitor temperature and pressure changes caused by the throttling expansion effect and the formation of secondary hydrates during the experiment. Each resistivity sensor 39 contains a pair of resistivity probes, each pair of probes also being uniformly distributed along the axial direction of the reactor 13 and positioned opposite each other along the axial center of the reactor 13, used to qualitatively monitor resistivity changes caused by the formation of secondary hydrates. By real-time monitoring of temperature, pressure, and resistivity changes at different locations within the reactor 13 using temperature sensors 37, pressure sensors 38, and resistivity sensors 39, the data processing module 41 uses this data to comprehensively determine the secondary formation of hydrates and their location, and analyzes the changes in secondary hydrate saturation in conjunction with changes in flow rate.

[0096] The above embodiments of the present invention provide an experimental apparatus for simulating secondary hydrate formation, and based on this apparatus, an experimental method for simulating secondary hydrate formation is provided. The reactor 13 used in this apparatus is a two-dimensional long-tube reactor, which is assembled from multiple single-section reactors 46 with different inner diameters by flange splicing. It can realistically simulate the effects of convergent flow and throttling expansion on the temperature and pressure fields of near-wellbore hydrate reservoirs. At the same time, the reactor 13 adopts a segmented design, and the effect of the extraction radius on secondary hydrate formation can be studied by changing the number of sections of the reactor 13 in the experiment. The formation of secondary hydrates is influenced by various factors. Each reactor section is equipped with a data monitoring and processing unit to monitor and process changes in temperature, pressure, and resistivity at different points. The formation of secondary hydrates is determined and their spatiotemporal evolution is analyzed by observing these changes. Reactor 13 is placed inside a vacuum chamber 12 that can be evacuated. Experiments are conducted under conditions where there is no thermal convection between reactor 13 and the external environment, realistically reproducing the characteristics of changes in the reservoir temperature and pressure fields during hydrate extraction. The gas discharged from reactor 13 can be reused to conduct long-term gas purging experiments.

[0097] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. An experimental apparatus for simulating the secondary formation of hydrates, the experimental apparatus comprising: The unit comprises a reaction unit, a gas injection unit, a liquid injection unit, a temperature control unit, a vacuum control unit, an outlet control unit, a gas circulation unit, and a data monitoring and processing unit; characterized in that, The reaction unit includes a reaction vessel, a vacuum chamber, and a constant temperature water bath. The reaction vessel is located inside the vacuum chamber, and the vacuum chamber and the reaction vessel inside it are located inside the constant temperature water bath. The reactor includes two or more cascaded single-stage reactors; the inner diameter of the two or more cascaded single-stage reactors is determined based on the similarity principle to simulate the converging flow effect in which the seepage radius of hydrate decomposition products gradually decreases during radial seepage in the near-well region. The gas injection unit is used to inject gas into the reactor to simulate the gas produced by the decomposition of hydrates; The liquid injection unit is used to inject liquid into the reactor to simulate the liquid produced by the decomposition of hydrates; The temperature control unit is used to control the temperature inside the constant temperature water bath. The vacuum control unit is used to control the vacuum level inside the vacuum chamber; The outlet control unit is used to control the outlet pressure of the reactor and to separate and measure the gas and liquid generated in the experiment. The gas circulation unit is used to re-transport the gas separated from the outlet control unit to the gas injection unit to achieve gas recycling. The data monitoring and processing unit is used to monitor and collect physical parameters inside the reactor during the experiment. These physical parameters include at least temperature, pressure, and resistivity.

2. The experimental apparatus for simulating secondary hydrate formation according to claim 1, characterized in that, Any two adjacent single-section reactors are connected by a reducing flange. The outermost end face of the single-stage reactor is sealed by a flange, which is provided with an injection port and an outlet to realize the injection and output of gas and liquid. Each of the single-stage reactors is equipped with a high-pressure connector for connecting to the data monitoring and processing unit; the pressure resistance range of the single-stage reactor and the flange is 0-35MPa.

3. The experimental apparatus for simulating secondary hydrate formation according to claim 1, characterized in that, The constant temperature water bath contains a first medium that covers the top of the vacuum box; The temperature control unit includes a first booster pump, a low-temperature constant temperature bath, and a second booster pump; the low-temperature constant temperature bath is filled with a cooling medium, and the first medium in the constant temperature water bath circulates in a closed loop consisting of the constant temperature water bath, the first booster pump, the low-temperature constant temperature bath, the second booster pump, and the constant temperature water bath. The first medium in the constant temperature water bath is in indirect contact with the cooling medium in the low temperature constant temperature bath to exchange heat and maintain the temperature of the reactor at a preset temperature.

4. The experimental apparatus for simulating secondary hydrate formation according to claim 1, characterized in that, The gas injection unit includes a gas cylinder, a first regulating valve group, and a gas flow controller. The gas cylinder, the first regulating valve group, the gas flow controller, the vacuum chamber, and the reaction vessel are connected in sequence through a gas pipeline. The gas cylinder stores methane gas, which flows sequentially through the first regulating valve group, the gas flow controller, and the vacuum chamber before being injected into the reaction vessel. The first regulating valve group is used to control the gas pressure, and the gas flow controller is used to control the gas flow rate. The liquid injection unit includes a cascaded storage tank, a constant pressure and constant flow pump, and a second regulating valve group. The storage tank contains water, and the constant pressure and constant flow pump injects the water from the storage tank into the gas pipeline at a constant flow rate through the second regulating valve group. After mixing with the gas, the water is injected into the reaction vessel.

5. The experimental apparatus for simulating secondary hydrate formation according to claim 1, characterized in that, The vacuum chamber includes a chamber body, end caps, support members, and a heat insulation plate. The end caps are sealed to the chamber body using bolts and plastic gaskets. The heat insulation plate is laid on the inner bottom surface of the chamber body, and multiple support members are installed on the heat insulation plate to fix the reactor and suspend the reactor in the chamber body.

6. The experimental apparatus for simulating secondary hydrate formation according to claim 5, characterized in that, The vacuum control unit includes a vacuum gauge, a third regulating valve group, and a vacuum pump. The vacuum gauge and the third regulating valve group are mounted on the housing, and the vacuum pump is used to evacuate the vacuum chamber.

7. The experimental apparatus for simulating secondary hydrate formation according to claim 1, characterized in that, The data monitoring and processing unit includes a temperature sensor, a pressure sensor, a resistivity sensor, a data acquisition module, and a data processing module. The temperature and pressure sensors are evenly distributed along the axial direction of the reactor and are arranged opposite each other along the center of the reactor to quantitatively monitor the temperature and pressure changes caused by the throttling expansion effect and the formation of secondary hydrates during the experiment. The resistivity sensor includes a pair of resistivity probes. Each pair of resistivity probes is also evenly distributed along the axial direction of the reactor and is arranged opposite each other along the center of the reactor to qualitatively monitor the resistivity changes caused by the formation of secondary hydrates. The data acquisition module is used to process the signals collected by the temperature sensor, pressure sensor, and resistivity sensor, and transmit the collected signals to the data processing module.

8. The experimental apparatus for simulating secondary hydrate formation according to claim 1, characterized in that, The outlet control unit includes a fourth valve assembly and a flow meter; the fourth valve assembly is installed at the end cap of the vacuum chamber and connected to the outlet of the reactor through a high-pressure pipeline, and is used to control the outlet pressure of the reactor to simulate the pressure of the production well during hydrate extraction; the flow meter is used to monitor the gas flow rate at the outlet of the reactor.

9. The experimental apparatus for simulating secondary hydrate formation according to claim 1, characterized in that, The gas circulation unit includes a gas-liquid separator, a balance measuring instrument, a first gas storage tank, a third booster pump, and a second gas storage tank. The gas-liquid separator is used to separate the product output from the outlet of the reactor. The balance measuring instrument is located at the outlet end of the gas-liquid separator and is used to measure the weight of the separated liquid. The first gas storage tank is used to collect the separated gas. The third booster pump pressurizes the collected gas and pumps it to the second gas storage tank to re-deliver the gas in the second gas storage tank to the gas injection unit or the reactor to achieve gas circulation.

10. An experimental method for simulating the secondary formation of hydrates, using the experimental apparatus for simulating the secondary formation of hydrates as described in any one of claims 1-9, characterized in that, The method includes the following steps: Preparation before the purging experiment: Prepare sediment samples in situ in each section of the single-stage reactor using a simulated medium mixed with distilled water and a layered compaction method; connect each section of the single-stage reactor and the corresponding data monitoring and processing unit; transfer the reactor to the vacuum chamber, connect and fix the corresponding parts of the reactor to the vacuum chamber, and seal the vacuum chamber; check the airtightness of the reactor and the vacuum chamber to ensure good airtightness before proceeding to the next step of the experiment; Pre-purge experiment: The reactor was pre-purged with methane gas. After observing that no more water was coming out of the reactor outlet, the collected water was weighed and the residual water saturation of the sediment sample in the reactor was calculated. The reactor outlet was closed, and then gas was injected for a period of time until the reactor was filled with a certain amount of gas. The gas injection port of the reactor was then closed. Initial formation and decomposition of hydrates: Lower the temperature of the constant temperature water bath to the first temperature. After the hydrates have completely formed, open the outlet of the reactor to decompose the hydrates. After the hydrates have completely decomposed, adjust the temperature of the constant temperature water bath to the second temperature. After the second temperature has stabilized, perform vacuum treatment. After the vacuum degree reaches 100%, start the purging experiment. Purging experiment: During the purging experiment, the changes in temperature, pressure and resistivity at different locations in the reactor are monitored in real time. The monitoring data are used to comprehensively determine the secondary formation of hydrates and the location of secondary formation. The changes in the saturation of secondary hydrates are analyzed in combination with the changes in flow rate.

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