Hydraulic fracturing - production hypergravity physical simulation experimental device and method for natural gas hydrate reservoir

Abstract

The present invention discloses a physical simulation experimental device and method for hydrate reservoir hydraulic fracturing - exploitation under hypergravity. A hydrate reservoir model is placed in a high - pressure vessel and is placed in a water bath environment and connected to a water bath temperature control module. An effective stress control module applies stress control, and a fracturing, reinforcement, and exploitation test module conducts fracturing, reinforcement, and exploitation tests. A multi - physical - field monitoring module for the model conducts monitoring, and a hydrate preparation module is connected to the high - pressure vessel; the high - pressure vessel, the effective stress control module, and the fracturing, reinforcement, and exploitation test module are all under hypergravity; the hydrate preparation module and the water bath temperature control module are under normal gravity. The present invention is carried on a geotechnical centrifuge and can regulate the high - pressure, low - temperature, high - stress or formation stress environment of the hydrate reservoir to simulate the in - situ occurrence environment of the hydrate reservoir, realize the simulation of pressure - reduction exploitation of vertical or horizontal wells, and simulate the hydraulic fracturing transformation of large - scale hydrate reservoirs and the gas - production process of long - duration hydrate exploitation at the model scale through hypergravity experiments.

Description

Technical Field

[0001] The present invention belongs to the technical field of geotechnical engineering and physical simulation technology of energy engineering, and relates to a simulation experimental device for hydrate production, in particular to a physical simulation experimental device for hydrate reservoir hydraulic fracturing-production under hypergravity. Background Art

[0002] Natural gas hydrate (hereinafter referred to as hydrate) is a cage-shaped crystalline compound formed by gas molecules (mainly methane) and water molecules in nature under high pressure and low temperature conditions. Because of its large resource potential and pollution-free combustion, it is recognized as the most potential clean energy to replace conventional oil and gas in the 21st century. Marine hydrates are generally stored in the pores of sediments with a water depth of thousands of meters and a burial depth of hundreds of meters. The reservoir thickness reaches dozens of meters, and the characteristics of reservoir deformation, strength, seepage and stability are significantly affected by the gravity field. In recent years, China and Japan have carried out several in-situ production tests of marine hydrates respectively. However, affected by factors such as poor stability and permeability of hydrate reservoirs and insufficient pressure reduction efficiency, the gas production efficiency of production is still about one order of magnitude lower than the industrial production standard. The extremely low permeability of in-situ hydrate reservoirs is one of the key problems restricting the hydrate production efficiency. Hydrates are often distributed in sediments in the forms of cementation, pore filling, vein-like, etc., occupying a large amount of sediment pore space and significantly reducing the formation permeability. Hydraulic fracturing and reinforcement of hydrate reservoirs are expected to greatly improve the pressure drop transfer efficiency and formation stability during hydrate production, and achieve a breakthrough in the production efficiency of in-situ hydrate production.

[0003] Hydraulic fracturing technology has been widely used in high-strength, dense and continuous reservoirs such as coal and shale gas at present. However, a large number of studies still need to be carried out to extend it to hydrate reservoirs with low strength, low stiffness and poor continuity. The initiation and development behavior of hydraulic fracturing cracks are significantly affected by the formation stress level. At present, most of the hydraulic fracturing simulation devices for hydrate reservoirs are triaxial constant gravity experimental devices, which are mainly used to simulate the stress values of hydrate-bearing sediment unit bodies at the centimeter scale at a certain depth of large-scale reservoirs, and cannot simulate the geothermal gradient and overall formation stress distribution of large-scale reservoirs, and do not have the function of simulating hydrate production after fracturing; the above devices are mainly used to study the fracturability of the formation under a certain stress, and it is difficult to reflect the crack development and final distribution law in large-scale reservoirs, nor can it reveal the influence of splitting cracks on the production capacity of hydrate production. Existing hydrate production simulation devices do not have a constant gravity experimental device with a hydraulic fracturing simulation function.

[0004] To truly simulate the hydraulic fracturing-production process of large-scale hydrate reservoirs, the key technologies that the experimental device urgently needs to solve include: accurately controlling the reservoir model temperature, water pressure environment, self-weight stress and effective stress level on the centrifuge, remotely regulating the optimization of the injection of fracturing and reinforcement fluids, temperature and flow rate, and remotely servo-switching the production well between the two modes of fluid injection and pressure reduction production.

[0005] Therefore, the prior art lacks a solution for truly simulating the hydraulic fracturing-production process of large-scale hydrate reservoirs. SUMMARY OF THE INVENTION

[0006] In order to solve the problems existing in the background art, the object of the present invention is to provide a hypergravity physical simulation experimental device for hydraulic fracturing-production of natural gas hydrate reservoirs, so as to fill the blank of experimental simulation of combined efficient production capacity of hydraulic fracturing-production of hydrate reservoirs.

[0007] The technical solution adopted by the present invention is as follows:

[0008] I. A hypergravity physical simulation experimental device for hydraulic fracturing-production of natural gas hydrate reservoirs:

[0009] It includes a high-pressure vessel, a water bath temperature control module, an effective stress control module, a fracturing-reinforcement-production module, a hydrate preparation module, and a model multi-physical field monitoring module; a hydrate reservoir model is arranged inside the high-pressure vessel, the high-pressure vessel is integrally placed in a water bath environment and connected to the water bath temperature control module, so that the hydrate reservoir model is placed in the water bath environment, the effective stress control module is connected to the inside of the high-pressure vessel and applies stress control to the hydrate reservoir model, the fracturing-reinforcement-production module is connected to the inside of the high-pressure vessel and conducts fracturing, reinforcement, and production tests on the hydrate reservoir model, the model multi-physical field monitoring module is installed on the high-pressure vessel to monitor the hydrate reservoir model, and the hydrate preparation module is connected to the hydrate reservoir model inside the high-pressure vessel.

[0010] The high-pressure vessel, the effective stress control module, and the fracturing-reinforcement-production module are all carried in a centrifuge hanging basket and work under a hypergravity of 1 - 500g; the hydrate preparation module and the water bath temperature control module are placed in the centrifuge power chamber and work under a normal gravity of 1g.

[0011] A loading plate is arranged inside the high-pressure vessel, the loading plate divides the space inside the high-pressure vessel into an upper chamber and a lower chamber, a hydrate reservoir model is arranged in the lower chamber, an axial pressure liquid layer is arranged in the upper chamber, and the fracturing-reinforcement-production module includes a hydrate reservoir hydraulic fracturing module, an injection fluid switching and reservoir reinforcement module, and a hydrate production module, and the hydrate reservoir hydraulic fracturing module, the injection fluid switching and reservoir reinforcement module, and the hydrate production module are all connected to the hydrate reservoir model; the effective stress control module is connected to the axial pressure liquid layer and connected to the hydrate production module.

[0012] The water bath temperature control module includes a water bath jacket, a constant temperature water bath tank, and a water bath circulation pump. The high-pressure container is placed in the water bath jacket with a diversion groove. The water bath jacket, the constant temperature water bath tank, and the water bath circulation pump are connected in series and circulated. The antifreeze is driven by the water bath circulation pump to circulate in the water bath jacket and the constant temperature water bath tank. The effective stress control module includes an axial pressure pump, an injection pipeline, a back pressure pump, and a buffer tank. The input end of the axial pressure pump is connected to the water storage container, and the output end of the axial pressure pump is connected to the axial pressure liquid layer in the high-pressure container through the injection pipeline. The output end of the back pressure pump is connected to the upper part of the buffer tank, and the lower end of the buffer tank is connected to the control end of the hydrate exploitation module.

[0013] The hydrate reservoir hydraulic fracturing module includes a piston temperature control container, a peristaltic pump, a single-degree-of-freedom loading device, an injection pipeline, and an injection head. The input end of the peristaltic pump is connected to the water storage container, and the output end of the peristaltic pump is connected to one end of the piston temperature control container. The piston temperature control container is pre-filled with fracturing fluid. The other end of the piston temperature control container is connected to the upper end of the injection pipeline. The lower end of the injection pipeline is inserted into the production well in the hydrate reservoir model. A specially designed injection head is installed at the lower end of the injection pipeline. The side wall of the production well is provided with production well perforations corresponding to the injection head. The upper part of the injection pipeline is installed on the loading arm of the single-degree-of-freedom loading device, and the injection pipeline is driven by the single-degree-of-freedom loading device to move up and down. The injection head includes a top plate, a bottom plate, a cylindrical connecting rod, and a sealing ring. The top plate is fixedly connected to the lower end of the injection pipeline. A through hole communicating with the lower end of the injection pipeline is opened in the middle of the top plate. The top plate and the bottom plate are fixedly connected by four cylindrical connecting rods. The periphery of the top plate and the bottom plate is hermetically connected to the inner wall of the production well through the sealing ring. The injection fluid switching and reservoir reinforcement module includes two piston temperature control containers, a peristaltic pump, a single-degree-of-freedom loading device, an injection pipeline, and an injection head. The peristaltic pump, the single-degree-of-freedom loading device, the injection pipeline, and the injection head of the injection fluid switching and reservoir reinforcement module share the same components as those of the hydrate reservoir hydraulic fracturing module. The two piston temperature control containers of the injection fluid switching and reservoir reinforcement module are connected in parallel with the piston temperature control container of the hydrate reservoir hydraulic fracturing module. The two piston temperature control containers are pre-filled with a gel breaker and a reinforcing agent respectively.

[0014] The main components of the reinforcing agent are calcium oxide, calcium silicate, and calcium sulfate, and the molar ratio of calcium oxide, calcium silicate, and calcium sulfate is 5:2:1.

[0015] The piston temperature control containers are mainly composed of piston containers, pneumatic valves, and semiconductor chips. The piston containers are respectively connected to the peristaltic pump and the injection pipeline, and the semiconductor chips are installed on the piston containers. There is a piston plate inside the piston container, which divides the piston container into upper and lower chambers. The lower chamber is connected to the output end of the peristaltic pump through the pneumatic valve, and the required substance is pre-added to the upper chamber. The upper chamber is connected to the upper end of the injection pipeline.

[0016] The described hydrate production module includes a production pipeline, a solid separation and metering device, a backpressure valve, and a liquid-gas separation, collection and metering module. The input end of the solid separation and metering device is connected to the top of the production well through the production pipeline, the output end of the solid separation and metering device is connected to the inlet of the backpressure valve, and the outlet of the backpressure valve is connected to the liquid-gas separation, collection and metering module.

[0017] After the hydraulic fracturing is completed, the injection pipeline and the injection head are driven by a single-degree-of-freedom loading device to move up to the top of the production well, so that the production pipeline can be connected to the production well at the chemically modified and reinforced area. At this time, the state of the production well is switched from the reservoir modification mode to the hydrate production mode.

[0018] The described multi-physical field monitoring module of the model includes sensors installed on the high-pressure vessel, acoustic emission probes on the inner wall of the high-pressure vessel, and sapphire endoscopes, and a model monitoring and data acquisition module. The sensors include thermocouples, pressure sensors, earth pressure gauges, resistivity probes, strain gauges, etc. The acoustic emission probes are arranged horizontally facing the production well. The sapphire endoscope is vertically inserted inside the hydrate reservoir model and is parallel to the production well. An endoscope camera is provided inside the sapphire endoscope. The sensors, acoustic emission probes, and endoscope cameras are all communicatively connected to the model monitoring and data acquisition module outside the centrifuge basket through sensor signal lines;

[0019] One pneumatic valve connected to the input end of the pneumatic valve / solid separation and metering device in the piston temperature control container of the described injection fluid switching and reservoir reinforcement module and the described hydrate reservoir hydraulic fracturing module is connected to the solenoid valve group. The solenoid valve group is communicatively connected to the external model monitoring and data acquisition module through the solenoid valve group signal line. The model monitoring and data acquisition module remotely controls the solenoid valve group to further control the opening and closing of each pneumatic valve.

[0020] II. A method for simulating the physical process of hydraulic fracturing and production of natural gas hydrate reservoirs by hypergravity, the process of the method is as follows:

[0021] S1. Hydrate formation process

[0022] The hydrate preparation module generates hydrates in the high-pressure vessel to prepare a hydrate reservoir model;

[0023] S2. Effective stress control process

[0024] After the hydrate is prepared, the high-pressure vessel, effective stress control module, and fracturing reinforcement and exploitation module of the device are mounted in the centrifuge basket. The constant temperature water bath, water bath circulation pump, and hydrate preparation module are placed in the centrifuge power chamber. Through the centrifuge rotary joint, the centrifuge water / gas pipeline, and the water / gas interface in the centrifuge basket are connected to the inside of the high-pressure vessel and the water bath jacket of the high-pressure vessel. After the device is mounted, start the centrifuge. After applying an ng ultra-gravity field through the high-speed rotation of the centrifuge to reach the ng ultra-gravity state, the axial pressure liquid in the water storage container is injected into the pipeline by the axial pressure pump and injected into the axial pressure liquid layer in the high-pressure vessel to increase the pressure. The pressure of the axial pressure liquid layer is transmitted to the hydrate reservoir model through the loading plate, and the overburden total stress σ0 applied by the axial pressure pump is converted into the total stress of the hydrate reservoir model;

[0025] At the same time, backpressure is applied to the backpressure valve through the backpressure pump and buffer tank to control the opening and closing and opening degree of the valve core, so that the backpressure valve is connected to the hydrate reservoir model through the production pipeline, and the effective stress is indirectly controlled by controlling the total stress and pore water pressure of the hydrate reservoir model;

[0026] The centrifuge described is an ultra-gravity centrifuge.

[0027] S3. Hydraulic fracturing process

[0028] Before the reservoir is hydraulically fractured, the injection pipeline and injection head are driven by a single-degree-of-freedom loading device to move down to a position below the connection of the production pipeline to the production well and in the production well in the hydrate reservoir model;

[0029] When performing reservoir hydraulic fracturing, only the pneumatic valve corresponding to piston container I is opened, and the metering pump is remotely controlled to draw liquid from the water storage container, and water is injected at a constant flow rate to push the piston plate inside piston container I, so that the fracturing fluid in the upper chamber of piston container I reaches the injection head at a constant flow rate through the injection pipeline, and then flows out of the injection head to the production well and is ejected from the perforations of the production well into the hydrate reservoir model to form hydraulic fracturing cracks, realizing hydraulic fracturing.

[0030] S4. Reservoir reinforcement process

[0031] After the hydraulic fracturing is completed, only the pneumatic valve corresponding to piston container II is opened, and the metering pump is remotely controlled to draw liquid from the water storage container, and water is injected at a constant flow rate to push the piston plate inside piston container II, so that the breaker in the upper chamber of piston container II reaches the injection head at a constant flow rate through the injection pipeline, and then flows out of the injection head to the production well and flows out of the perforations of the production well into the cracks of the already formed hydraulic fracturing cracks;

[0032] Subsequently, switch to only open the pneumatic valve corresponding to the piston container Ⅲ, and remotely control the peristaltic pump to draw liquid from the water storage container, inject water at a constant flow rate to push the piston plate inside the piston container Ⅲ, so that the consolidant in the upper cavity of the piston container Ⅲ reaches the injection end through the injection pipeline at a constant flow rate, and then flows out from the injection end to the production well and out of the perforation of the production well into the fracture of the hydraulic fracturing fracture that has been injected with the gel breaker, forming a chemical modification and reinforcement area;

[0033] After the consolidant is injected, it hydrates under the action of water and salt in the pore water of the hydrate reservoir model to form a continuous porous surface-like structure to support the fracture surface after several hours.

[0034] S5. Hydrate production process

[0035] Drive the injection pipeline and the injection end up to the top of the production well by the single-degree-of-freedom loading device and be located above the connection of the production pipeline to the production well, so that the production pipeline can be connected to the production well in the chemical modification and reinforcement area;

[0036] Then, the methane gas generated by the decomposition of the hydrate in the hydrate reservoir model enters the production pipeline through the production well, and then enters the liquid-gas separation collection and metering module through the pressure control of only the back-pressure valve after separating the solids by the solid separation and metering device to perform liquid-gas separation and metering, completing the experiment; finally, stop the high-speed rotation of the centrifuge and move the device away from the centrifuge basket.

[0037] S6. Monitoring process

[0038] During the whole process of the experiment from S1 - S5, use the model multi-physical field monitoring module to collect data and perform data analysis to obtain the hydraulic fracturing - production situation.

[0039] During the hydraulic fracturing process, the fracture initiation pressure of the hydraulic fracturing is obtained according to the following formula:

[0040]

[0041] Among them, p f represents the fracture initiation pressure of the hydraulic fracturing; μ and v f represent the viscosity and flow rate of the fracturing fluid respectively; g is the acceleration due to gravity; n is the multiple of the acceleration due to gravity; K0 and ψ are the lateral pressure coefficient and the internal friction angle of the sediment respectively; ρ' is the buoyant density of the sediment; z represents the reservoir burial depth, N p is the pore characteristic constant related to the hydrate saturation, c h is the cohesion of the hydrate-bearing sediment; a f is a constant parameter related to the permeability of the hydrate-bearing sediment and the diversion capacity of the splitting fracture.

[0042] During the hydrate formation process, effective stress control process, hydraulic fracturing process, reservoir reinforcement process of injecting gel breaker and reinforcing agent, and subsequent hydrate production process, the temperature, pore pressure, total stress, resistivity, and reservoir deformation are monitored in real time through sensors such as thermocouples, pressure sensors, earth pressure gauges, resistivity probes, and strain gauges respectively;

[0043] During the hydraulic fracturing process, the fracture splitting strength and splitting position of the hydrate reservoir model during hydraulic fracturing are monitored in real time through acoustic emission probes, and the actual fracture propagation process, fracture position, fracture aperture, and particle migration and deformation response of the hydrate reservoir model are observed in real time through the endoscopic camera inside the sapphire endoscope tube.

[0044] In the method, during the hydraulic fracturing process, the acoustic signal data collected by the acoustic emission probe is processed according to the following formula to obtain the rise time t r and the average frequency AF:

[0045] t r =t d / A e

[0046] AF=NF / t f

[0047] wherein, the rise time t r is the ratio between the delay t d between the start time and the maximum amplitude of the acoustic signal and the amplitude A e ; the average frequency AF is the ratio between the number of effective fracture acoustic signals NF and the total duration t f of the hydraulic fracturing;

[0048] After the start of the hydraulic fracturing, based on the rise time t r and the average frequency AF obtained in real time, judgments are made to obtain different stage judgments of the hydraulic fracturing:

[0049] When the average frequency AF increases by more than a preset threshold compared to the value before the hydraulic fracturing and the change rate of the average frequency AF at adjacent times is lower than the preset slope threshold, and at the same time the rise time t r increases by no more than the preset threshold compared to the value before the hydraulic fracturing and the change rate of the rise time t r at adjacent times is lower than the preset slope threshold, it is considered that the current is in the tensile fracture stage;

[0050] When the average frequency AF continuously increases and the change rate of the average frequency AF at adjacent times is higher than the preset slope threshold, and at the same time the rise time t r continuously increases and the change rate of the rise time t r at adjacent times is higher than the preset slope threshold, it is considered that the current is in the main fracture formation and propagation stage.

[0051] The present invention can simultaneously achieve the hydraulic fracturing function and the exploitation function through the design of the effective stress control module and the fracturing reinforcement exploitation module and the cooperation of the experimental process, realizing the hydraulic fracturing-exploitation function.

[0052] The present invention superimposes the high-pressure, low-temperature, and stress environment of the deep-sea gas hydrate reservoir with the hypergravity field to accurately restore the occurrence conditions of the in-situ large-scale gas hydrate reservoir; realizes the hydraulic fracturing and transformation reinforcement of the gas hydrate reservoir model under hypergravity conditions through the developed hydraulic fracturing module for gas hydrate reservoir, the injection fluid switching, and the reservoir reinforcement module; and conducts pressure reduction exploitation on the reservoir model after hydraulic fracturing through the developed pressure reduction exploitation module for gas hydrate reservoir. This device can reveal the initiation and development laws of hydraulic fracturing cracks in the large-scale gas hydrate reservoir, evaluate the reservoir reinforcement effect and the change of gas hydrate exploitation energy efficiency, provide support for the formulation of gas hydrate energy development strategies and the research on the regulation of the exploitation process, and help break through the bottleneck problems of low reservoir permeability and poor stability in the process of gas hydrate energy development.

[0053] The present invention can accurately simulate the process of hydraulic fracturing exploitation of the deep-sea natural gas hydrate reservoir under the in-situ large-scale low-temperature environment, self-weight stress field, and reservoir effective stress. The key technologies to achieve this function are: accurately controlling the effective stress of the deep-sea in-situ gas hydrate reservoir formation under hypergravity conditions and accurately restoring the true stress level of the large-scale reservoir of hundreds of meters; realizing the temperature control, uniform injection, and remote switching of different types of fracturing fluids to accurately reproduce the in-situ hydraulic fracturing and reservoir transformation characteristics; realizing the remote control of the hydraulic fracturing position and the remote switching of the production well between the fracturing fluid injection-pressure reduction exploitation mode to simulate the in-situ exploitation process; and effectively monitoring the crack distribution and crack morphology during the hydraulic fracturing and pressure reduction exploitation processes to reveal the in-situ hydraulic fracturing response mechanism. The hypergravity condition refers to the ng hypergravity experimental condition where the experimental device rotates in the centrifuge basket to generate n times the earth's gravitational acceleration g.

[0054] The beneficial effects of the present invention are:

[0055] 1) This device can be mounted on a geotechnical centrifuge. Through the combined action of the hypergravity field and the device's effective stress control module, the in-situ large-scale reservoir soil skeleton stress field and pore water pressure field can be reproduced at the model scale; the in-situ large-scale reservoir geothermal gradient can be reproduced by adjusting the initial temperature and circulation flow rate of the coolant in the water bath jacket; during the experimental process, the differences in the mechanical responses of the solid phase skeleton at different depths of the reservoir and the thermodynamic behavior characteristics of gas hydrates can be reproduced;

[0056] 2) The hydrate reservoir hydraulic fracturing module and the injection fluid switching and reservoir reinforcement module of this device can achieve remote control of switching different types of injection fluids, servo control of flow rate, and precise control of the temperature of the injection fluid on the centrifuge; during hydraulic fracturing, it can autonomously regulate the actual hydraulic fracturing points in the production well, and after hydraulic fracturing, it can carry out reservoir chemical transformation and reinforcement and simulate the depressurization production of hydrates; the simulation of the in-situ marine hydrate reservoir hydraulic fracturing production process is more real and comprehensive.

[0057] 3) The multi-physical field monitoring module of this device is further equipped with acoustic emission probes that can monitor the initiation, development of reservoir fractures, and severe deformation of the reservoir during hydrate production on the basis of sensors such as thermocouples, pressure sensors, earth pressure gauges, resistivity probes, and strain gauges, and sapphire endoscopes that can observe the fracture propagation process, fracture location, and fracture aperture in real time through an endoscope camera; through the above monitoring means, it can accurately reveal the development and distribution law of hydraulic fracturing fractures, and observe the particle migration and deformation response of the reservoir during reservoir transformation and depressurization production.

[0058] 4) The hydrate production module of this device can assemble production wells in the form of vertical wells and horizontal wells according to experimental requirements. After hydraulic fracturing and reservoir transformation are completed, the production well can be switched from the fixed-point injection mode to the depressurization production mode, and the pressure in the production well can be jointly servo-controlled by a piston pump - backpressure valve to achieve the simulation of depressurization production of the hydrate reservoir model after hydraulic fracturing and chemical reinforcement.

[0059] 5) Utilizing the scale-down and time-scale principles of hypergravity experiments, this device accurately reproduces the extreme environment of high pressure, low temperature, and high stress in deep-sea natural gas hydrate reservoirs. The multi-field and multi-phase interactions between the indoor scale model and the on-site full-scale prototype are strictly similar. It can accurately simulate the hydraulic fracturing response of large-scale hydrate reservoirs and the subsequent long-term production process through hypergravity experiments at the model scale, reveal the development law of hydraulic fracturing fractures, evaluate the effect of formation chemical reinforcement, and the improvement amplitude of production capacity, providing an innovative research method for the industrial development of deep-sea natural gas hydrates. Description of the Drawings

[0060] Figure 1 is the overall structural schematic diagram of the device in the embodiment of the present invention;

[0061] Figure 2 is the schematic diagram of the effective stress control principle of the present invention;

[0062] Figure 3 is the structural schematic diagram of the production well with hydraulic fracturing function in the embodiment of the present invention;

[0063] Figure 4 is the schematic diagram of horizontal well hydraulic fracturing production simulation in the embodiment of the present invention;

[0064] In the figure:

[0065] 1. High-pressure vessel; 2. Temperature-controlled bottom plate; 3. Sintered plate; 4. Production well; 5. Constant-temperature water bath; 6. Water bath circulation pump; 7. Hydrate preparation module;

[0066] 8. Centrifuge rotary joint; 9. Centrifuge water / gas pipeline; 10. Centrifuge basket; 11. Water / gas interface inside the centrifuge basket; 12. Axial pressure pump; 13. Sensor; 14. Water bath jacket; 15. Hydrate reservoir model; 16. Axial pressure liquid injection pipeline; 17. Water storage container;

[0067] 18. Advection pump; 19. Pneumatic valve; 20. Semiconductor chip; 21. Piston container I; 22. Piston container II; 23. Piston container III; 24. Loading plate; 25. Single-degree-of-freedom loading device; 26. Injection pipeline; 27. Injection end; 28. Solenoid valve group; 29. Production pipeline;

[0068] 30. Back-pressure pump; 31. Solid separation meter; 32. Buffer tank; 33. Back-pressure valve;

[0069] 34. Axial pressure liquid layer; 35. Exhaust pipeline; 36. Liquid-gas separation, collection and measurement module; 37. Hydraulic fracturing crack; 38. Chemical modification and reinforcement area; 39. Acoustic emission probe; 40. Sapphire endoscope tube; 41. Endoscopic camera; 42. Sensor signal line; 43. Model monitoring and data acquisition module; 44. Solenoid valve group signal line;

[0070] 45. Perforation of production well; 46. Sealing ring; 47. Roof; 48. Cylindrical connecting rod; 49. Bottom plate. Detailed implementation manners

[0071] The following further describes the content of the present invention in conjunction with the drawings and embodiments.

[0072] As Figure 1 shown, it is a specific embodiment of the present invention. The hydrate production simulation experiment device includes a high-pressure vessel 1, a water bath temperature control module, an effective stress control module, a fracturing reinforcement and production module, a hydrate preparation module 7, and a model multi-physical field monitoring module.

[0073] The high-pressure vessel 1 is internally provided with a hydrate reservoir model 15 and an axial pressure liquid layer 34. The high-pressure vessel 1 as a whole is placed in a water bath environment and connected to a water bath temperature control module, so that the hydrate reservoir model 15 is placed in the water bath environment. The effective stress control module is connected to the inside of the high-pressure vessel 1 and applies stress control to the hydrate reservoir model 15. The fracturing and reinforcement mining module is connected to the inside of the high-pressure vessel 1 and conducts fracturing, reinforcement, and mining experiments on the hydrate reservoir model 15. The model multi-physical field monitoring module is installed on the high-pressure vessel 1 to monitor the hydrate reservoir model 15 and its water bath environment. The hydrate preparation module 7 is connected to the hydrate reservoir model 15 inside the high-pressure vessel 1.

[0074] This device is carried on a geotechnical centrifuge to conduct hypergravity experiments. When conducting hypergravity experiments, as Figure 1 shown, the high-pressure vessel 1, the effective stress control module, and the fracturing and reinforcement mining module are all carried in the centrifuge basket 10 and work under 1 - 500g hypergravity; the hydrate preparation module, the constant temperature water bath of the water bath temperature control module, and the water bath circulation pump are placed in the centrifuge power room and work under 1g normal gravity.

[0075] Specifically, when implementing and conducting hypergravity experiments, a large-scale in-situ reservoir is reduced to a 1 / n scale indoor model as the hydrate reservoir model 15, and through an n-fold hypergravity field, the processes such as seepage and deformation between the indoor model scale and the in-situ large-scale reservoir are strictly similar.

[0076] The main parameter similarity scales between the indoor model and the on-site prototype are as follows: gravitational acceleration: n; length: 1 / n; area 1 / n 2 ; volume 1 / n 3 ; temperature gradient: n; pressure gradient: n; stress gradient: n; seepage velocity: n; injection flow rate: 1 / n; seepage time: 1 / n 2 .

[0077] The present invention uses similar particle gradations and scales a large-scale marine hydrate reservoir to 1 / n times according to the in-situ porosity, hydrate saturation, etc. The physical parameters such as material density, viscosity, specific heat, enthalpy value, and sediment intrinsic permeability are similar to the in-situ at a ratio of 1:1.

[0078] During the in-situ reservoir hydraulic fracturing process, it is assumed that the fracturing fluid does not lose pressure due to a large amount of loss along the fault, and when the injection flow rate of the fracturing fluid per unit time is equal to the seepage loss flow rate of the fracturing fluid invading the sediment pores, the fracture horizontal length L f and the total area A f reach the maximum value. At this time, the fracturing fluid filtration loss relationships in the prototype and the model are respectively shown by the following formulas:

[0079] Q f = v fl 2A f

[0080] F Q Q f = F v v fl F A 2A f

[0081] where v fl is the filtration flow rate of the fracturing fluid; A f is the fracture area; F κ represents the similarity ratio (model / prototype) of a certain physical quantity κ, κ = Q, A, v, Q represents the flow rate, A represents the area, v represents the flow velocity, and Q f represents the injection flow rate of the fracturing fluid.

[0082] When the model scale is reduced to 1 / n of the in-situ scale, the length similarity ratio F L is 1 / n, and the area similarity ratio F A is 1 / n 2 , and then the volume similarity ratio F v can be obtained as 1 / n 3 ; the seepage similarity ratio F k and the viscosity similarity ratio F μ are both 1; the gravitational acceleration similarity ratio F g is n. Comparing equations (1) and (2), it can be seen that the seepage similarity ratio F v between the present invention and the in-situ is n, and the similarity ratio F Q of the injection flow rate of the fracturing fluid per unit time is 1 / n. The similarity ratios between the model test of the present invention and the prototype are summarized in Table 1.

[0083] Table 1. Similarity of the simulation experimental device for hydrate reservoir hydraulic fracturing production on a centrifuge

[0084]

[0085]

[0086] The high-pressure vessel can be a cylindrical container forged from stainless steel or titanium alloy, consisting of a vessel barrel, a vessel top cover, and high-strength bolts. It is the carrier of the high-pressure, low-temperature, and high-stress physical environment of the deep-sea hydrate reservoir model, and the inside of the vessel can withstand a certain pressure. A production well hole is provided at the center of the vessel top cover, and a sensor reserved hole that can also serve as the wellhead of a horizontal well is provided on the vessel barrel. The vessel top cover and the vessel barrel are connected by high-strength bolts, and the contact part is sealed by a sealing ring. A temperature control bottom plate 2 of the water bath temperature control module and a water bath jacket 14 with a baffle are arranged outside the high-pressure vessel.

[0087] A loading plate 24 is arranged inside the high-pressure vessel 1. The loading plate 24 divides the space inside the high-pressure vessel 1 into an upper cavity and a lower cavity. A hydrate reservoir model 15 is arranged in the lower cavity, and an axial-pressure liquid layer 34 formed by water is arranged in the upper cavity. The fracturing and reinforcement mining module includes a hydrate reservoir hydraulic fracturing module, an injection fluid switching and reservoir reinforcement module, and a hydrate mining module. The hydrate reservoir hydraulic fracturing module, the injection fluid switching and reservoir reinforcement module, and the hydrate mining module are all installed outside the high-pressure vessel 1 and communicated with the hydrate reservoir model 15; the effective stress control module is installed outside the high-pressure vessel 1 and communicated with the axial-pressure liquid layer 34 and connected to the hydrate mining module.

[0088] In specific implementation, a sintered plate 3 is first arranged at the bottom inside the high-pressure vessel 1, and then the hydrate reservoir model 15 is prepared. The sintered plate 3 is fired from sieves with different pore diameters, which can improve the water injection uniformity in the model preparation stage and prevent sediment particles from blocking the injection holes at the bottom of the high-pressure vessel 1.

[0089] The water bath temperature control module includes a water bath jacket 14, a constant temperature water bath tank 5, and a water bath circulation pump 6. The high-pressure vessel 1 is placed in the water bath jacket 14 with a diversion groove filled with antifreeze mainly composed of ethylene glycol. The water bath jacket 14, the constant temperature water bath tank 5, and the water bath circulation pump 6 are connected in series and circulated. The antifreeze is driven by the water bath circulation pump 6 to circulate inside the water bath jacket 14 and the constant temperature water bath tank 5. The main component of the antifreeze is ethylene glycol, and the temperature is generally 3 - 10 °C. The power of the constant temperature water bath tank can be adjusted according to experimental requirements to change the temperature.

[0090] The hydrate preparation module 7 is used to generate a hydrate reservoir model 15 containing substances such as methane hydrate and is connected to and controlled inside the high-pressure vessel 1.

[0091] The hydrate preparation module 7 further includes an exhaust pipeline 35 and a pneumatic valve 19. The exhaust pipeline 35 is installed on the side wall of the high-pressure vessel 1. The exhaust pipeline 35 is communicated with the input end of a solid separation and metering device 31 through a pneumatic valve 19, and the inner end of the exhaust pipeline 35 is communicated with the upper part of the hydrate reservoir model 15.

[0092] In a specific embodiment of the present invention, as Figure 1As shown in the figure, the water bath temperature control module consists of a constant temperature water bath 5, a water bath circulation pump 6, a temperature control bottom plate 2, and a water bath jacket 14 with a diversion groove. Among them, the water bath circulation pump drives the low-temperature coolant through the centrifuge rotary joint 8. The centrifuge pipeline 9 and the water / gas interface 11 in the centrifuge basket enter the water bath jacket from the top inlet of the water bath jacket 14 and flow downward along the diversion plate, and enter the temperature control bottom plate 2 from the bottom. Finally, it returns to the constant temperature water bath 5 through the water / gas interface 11, the centrifuge pipeline 9, and the centrifuge rotary joint 8 in the centrifuge basket again. Temperature control is achieved through the continuous circulation of the coolant and its heat exchange with the hydrate reservoir model 15. By adjusting the initial temperature of the coolant in the constant temperature water bath 5 and the flow rate of the water bath circulation pump 6, the temperature of the sediment model 15 can be changed along the depth, realizing the simulation of the geothermal gradient.

[0093] As Figure 2 shown, the effective stress control module includes a water storage container 17, an axial pressure pump 12, an axial pressure liquid injection pipeline 16, a back pressure pump 30, and a buffer tank 32. The input end of the axial pressure pump 12 is connected to the water storage container 17. There is axial pressure liquid in the water storage container 17. The output end of the axial pressure pump 12 is connected to the axial pressure liquid layer 34 in the high-pressure container 1 through the axial pressure liquid injection pipeline 16. There is axial pressure liquid in the back pressure pump 30. The output end of the back pressure pump 30 is connected to the upper part of the buffer tank 32. The buffer tank 32 is pre-filled with axial pressure liquid. The lower end of the buffer tank 32 is connected to the control end of the back pressure valve 33 in the hydrate production module.

[0094] The relationships among the total stress, effective stress, and pore pressure at the top of the hydrate reservoir model 15 are shown in the following formula:

[0095] σ0 = σ0′ + p0

[0096] Where, σ0, σ ′ ′ and p0 are the total gravitational force, effective stress, and pore pressure of the hydrate reservoir model 15, respectively.

[0097] The axial pressure pump 12 injects liquid to increase the pressure into the axial pressure liquid layer 34 in the high-pressure container 1 through the axial pressure liquid injection pipeline 16, and converts the overlying total stress σ0 applied by the axial pressure pump into the total stress of the hydrate reservoir model 15 through the loading plate 24. The back pressure pump 30 and the buffer tank 32 apply a back pressure p0 to the back pressure valve 33. At this time, if the pore pressure in the hydrate reservoir model 15 in the high-pressure container 1 is higher than p0, the water in the pores will be produced along the production well 4 and the production pipeline 29 and enter the liquid-gas separation collection and metering module 36 through the back pressure valve 33, and finally reduce the pore pressure of the hydrate reservoir model 15 to p0, realizing effective stress control.

[0098] As shown in Eqs. (2) and (3), under the action of a supergravity field, the effective stress σ' and pore pressure p at depth z in the hydrate reservoir model increase with the depth gradient, and the increases under supergravity with n times the gravitational acceleration are ngρ' and ngρ respectively. w . As shown in the following equation:

[0099] σ′ = σ′0 + ngρ′

[0100] p = p0 + ngρ w

[0101] where ρ′ and ρ w represent the buoyant density of sediment particles and the density of water respectively.

[0102] The actual effective stress at different heights of the model is the sum of the effective stress at the top of the model and the self-weight effective stress, which is consistent with the stress distribution of the n-fold scale prototype hydrate reservoir of the model.

[0103] In the hydrate reservoir hydraulic fracturing module, the fracture initiation pressure prediction model considering the effects of fracturing fluid flow, reservoir effective stress, and hydrate cementation during the hydraulic fracturing process is as shown in the above equation.

[0104] The hydrate reservoir hydraulic fracturing module includes a piston temperature control container, a peristaltic pump 18, a water storage container 17, a single-degree-of-freedom loading device 25, an injection pipeline 26, and a specifically designed injection end 27. The input end of the peristaltic pump 18 is connected to the water storage container 17, the output end of the peristaltic pump 18 is connected to one end of the lower part of the piston temperature control container through a pneumatic valve 19. The piston temperature control container is pre-filled with fracturing fluid. The other end of the upper part of the piston temperature control container is connected to the upper end of the injection pipeline 26 through a hose. The injection pipeline 26 is vertically arranged and its lower end is inserted into the production well 4 embedded in the hydrate reservoir model 15. A specifically designed injection end 27 is installed at the lower end of the injection pipeline 26. Production well perforations 45 corresponding to and cooperating with the injection end 27 are provided on the side wall of the production well 4.

[0105] The upper part of the injection pipeline 26 is installed on the loading arm of the single-degree-of-freedom loading device 25. The single-degree-of-freedom loading device 25 can be installed on the high-pressure container 1, and the injection pipeline 26 is driven by the single-degree-of-freedom loading device 25 to move up and down.

[0106] In specific implementation, several production well perforations 45 are provided, and multiple circles of production well perforations are arranged at intervals along the axial direction of the production well 4. Each circle of production well perforations includes multiple production well perforations 45 arranged at intervals circumferentially.

[0107] As Figure 3As shown in (a) of , the injection end 27 includes a top plate 47, a bottom plate 49, a cylindrical connecting rod 48, and a sealing ring 46. The top plate 47 is fixedly connected to the lower end of the injection pipeline 26. A through hole communicating with the lower end of the injection pipeline 26 is provided in the middle of the top plate 47. The top plate 47 and the bottom plate 49 are fixedly connected by four cylindrical connecting rods 48. The periphery of the top plate 47 and the bottom plate 49 is hermetically connected to the inner wall of the production well 4 through the sealing ring 46. The bottom plate 49 is a complete plate without a through hole.

[0108] The liquid entering the injection pipeline 26 enters between the top plate 47 and the bottom plate 49 through the through hole of the top plate 47 of the injection end 27, and due to the action of the sealing ring 46, it flows out from the perforation 45 of the production well between the top plate 47 and the bottom plate 49 to form a hydraulic fracture.

[0109] The injection end 27 is used to limit the discharge position of the fracturing fluid, realize fixed-point fracturing of the sediment model, is hermetically connected to the inner wall of the production well, and is driven by the single-degree-of-freedom loading device 25 to move up and down in the production well 4 to change the position, thereby changing the hydraulic fracturing point.

[0110] The injection fluid switching and reservoir reinforcement module includes two piston temperature control containers, a peristaltic pump 18, a water storage container 17, a single-degree-of-freedom loading device 25, an injection pipeline 26, and a specially designed injection end 27. The peristaltic pump 18, the water storage container 17, the single-degree-of-freedom loading device 25, the injection pipeline 26, and the injection end 27 of the injection fluid switching and reservoir reinforcement module are shared with those of the hydrate reservoir hydraulic fracturing module. The two piston temperature control containers of the injection fluid switching and reservoir reinforcement module are connected in parallel with the piston temperature control containers of the hydrate reservoir hydraulic fracturing module, that is, connected between the output end of the peristaltic pump 18 and the upper end of the injection pipeline 26. A breaker and a reinforcement agent are respectively preset in the two piston temperature control containers.

[0111] The two piston temperature control containers of the injection fluid switching and reservoir reinforcement module are the same. There is a piston plate in the piston container of the piston temperature control container, and a breaker / reinforcement agent is provided in the upper cavity above the piston plate.

[0112] The main component of the reinforcement agent is a slurry of calcium oxide, calcium silicate, and calcium sulfate prepared in a certain proportion. The molar ratio of calcium oxide, calcium silicate, and calcium sulfate is 5:2:1. Through the innovative design of the raw material components and their proportion relationship of the reinforcement agent, the present invention can prevent the reinforcement agent from blocking the injection pipeline 26 and the injection end 27.

[0113] In specific implementation, the water storage containers 17 of the hydrate reservoir hydraulic fracturing module, the injection fluid switching and reservoir reinforcement module, and the effective stress control module can share the same one.

[0114] The piston temperature-controlled container mainly consists of a piston container, a pneumatic valve 19, and a semiconductor chip 20. The piston container is connected to the output end of the peristaltic pump 18 and the upper end of the injection pipeline 26 respectively. The semiconductor chip 20 is installed on the piston container.

[0115] Inside the piston container is a piston plate, which divides the piston container into upper and lower chambers. The lower chamber is connected to the output end of the peristaltic pump 18 through the pneumatic valve 19. The required substances such as fracturing fluid, gel breaker, and reinforcing agent are pre-added into the upper chamber. The upper chamber is connected to the upper end of the injection pipeline 26.

[0116] The piston container of the piston temperature-controlled container in the hydrate reservoir hydraulic fracturing module is the piston container I 21. The piston containers of the two piston temperature-controlled containers in the injection fluid switching and reservoir reinforcement module are the piston container II 22 and the piston container III 23.

[0117] The fracturing fluid is pre-added into the upper chamber above the piston plate in the piston container I 21. The temperature of the fracturing fluid in the piston container I 21 is controlled by the semiconductor chip 20 of its corresponding piston temperature-controlled container. By changing the direction and value of the semiconductor chip current, heating / cooling of the liquid in the container and precise temperature control can be achieved.

[0118] The gel breaker and the reinforcing agent are pre-added into the piston container II 22 and the piston container III 23 of the two piston temperature-controlled containers in the injection fluid switching and reservoir reinforcement module respectively. The temperatures of the gel breaker and the reinforcing agent in the containers are independently controlled by the semiconductor chip 20. By changing the direction and value of the semiconductor chip current, heating / cooling of the liquid in the container and precise temperature control can be achieved.

[0119] During reservoir transformation, the opening and closing of the pneumatic valve 19 of a total of three piston temperature-controlled containers in the injection fluid switching and reservoir reinforcement module and the hydrate reservoir hydraulic fracturing module are controlled by remotely regulating the solenoid valve group 28, so as to achieve remote switching of the injected liquid.

[0120] The hydrate production module includes a production pipeline 29, a solid separation and metering device 31, a backpressure valve 33, and a liquid-gas separation, collection, and metering module 36. The input end of the solid separation and metering device 31 is connected to the top of the production well 4 through the production pipeline 29. The output end of the solid separation and metering device 31 is connected to the inlet of the backpressure valve 33. The outlet of the backpressure valve 33 is connected to the liquid-gas separation, collection, and metering module 36.

[0121] The solid separation and metering device 31 is used to separate the solids in the incoming fluid and meter them. The liquid-gas separation, collection, and metering module 36 is used to separate the incoming liquid and gas and meter them separately. The backpressure valve 33 is used to control the opening and closing and the opening degree of the production pipeline 29.

[0122] In specific implementation, the hydrate preparation module 7 and the hydrate reservoir model 15, and the water bath jacket 14 and the constant temperature water bath 5 and the water bath circulation pump 6 are respectively connected through the water / gas interface 11 in the centrifuge hanging basket, the centrifuge water / gas pipeline 9 and the centrifuge rotary joint 8.

[0123] The model multi-physical field monitoring module includes the sensor 13 installed on the high-pressure vessel 1, the acoustic emission probe 39 on the inner wall of the high-pressure vessel 1, and the sapphire endoscope tube 40, and the model monitoring and data acquisition module 43. The sensor 13 includes a thermocouple, a pressure sensor, an earth pressure gauge, a resistivity probe, a strain gauge, etc. The acoustic emission probe 39 is horizontally oriented towards the production well 4. The sapphire endoscope tube 40 is vertically inserted inside the hydrate reservoir model 15 and is parallel to the production well 4. An endoscope camera 41 is provided inside the sapphire endoscope tube 40. The sensor 13, the acoustic emission probe 39, and the endoscope camera 41 are all communicatively connected to the model monitoring and data acquisition module 43 outside the centrifuge hanging basket 10 through the sensor signal line 42.

[0124] The pneumatic valve 19 in the piston temperature control container of the injection fluid switching and reservoir reinforcement module and the hydrate reservoir hydraulic fracturing module / one of the pneumatic valves 19 connected to the input end of the solid separation and metering device 31 in the hydrate preparation module are all connected to the solenoid valve group 28. The solenoid valve group 28 is communicatively connected to the external model monitoring and data acquisition module 43 through the solenoid valve group signal line 44. The model monitoring and data acquisition module 43 remotely controls the solenoid valve group 28 to control the opening and closing of each pneumatic valve 19, so as to realize the remote switching of the injection liquid and the remote switching of the exhaust opening and closing.

[0125] The side wall of the high-pressure vessel 1 is provided with sensor ear holes and the like, which can also serve as the wellhead of the production well, for arranging horizontal wells to simulate the hydraulic fracturing and pressure reduction production of the horizontal well in the hydrate reservoir. When using a horizontal well, the contact range between the production well and the reservoir is larger, which is beneficial to improving the reservoir pressure reduction range and pressure reduction efficiency. Since the hydraulic fracturing cracks develop along the direction of the vertical plane of the minor principal stress of the reservoir, using a horizontal well to carry out hydraulic fracturing can make the injection direction of the fracturing fluid consistent with the crack development direction, and it is expected to obtain a larger hydraulic fracturing range and reservoir stimulation effect.

[0126] The process of the hydraulic fracturing - production ultra-gravity physical simulation experiment of the natural gas hydrate reservoir of the present invention is as follows:

[0127] S1. Hydrate formation process

[0128] The methane hydrate is generated in the high-pressure vessel 1 by the hydrate preparation module 7 to obtain the hydrate reservoir model 15.

[0129] S2. Effective stress control process

[0130] As Figure 2As shown in the figure, the axial compression pump 12 injects the axial compression liquid in the water storage container 17 into the pipeline 16 to inject liquid into the axial compression liquid layer 34 in the high-pressure container 1 for pressure increase. The pressure of the axial compression liquid layer 34 is transmitted to the hydrate reservoir model 15 through the loading plate 24, and the overburden total stress σ0 applied by the axial compression pump 12 is converted into the total stress of the hydrate reservoir model 15;

[0131] At the same time, the back pressure pump 30 and the buffer tank 32 apply back pressure to the back pressure valve 33 to control the opening and closing and the opening degree of the valve core, so that the pressure of the back pressure valve 33 is the back pressure p0, and the back pressure valve 33 is connected to the hydrate reservoir model 15 through the production pipeline 29. At this time, the pore pressure of the hydrate reservoir model 15 is finally reduced to the same as the back pressure p0, realizing effective stress control.

[0132] The above-mentioned stress and pore pressure both refer to the pressure of the liquid.

[0133] S3, Hydraulic fracturing process

[0134] Before the reservoir is hydraulically fractured, the injection pipeline 26 and the injection end 27 are driven by the single-degree-of-freedom loading device 25 to move down to below the connection to the production well 4 in the production pipeline 29 and located in the production well 4 in the hydrate reservoir model 15;

[0135] When the reservoir is hydraulically fractured in the hydrate reservoir hydraulic fracturing module, only the pneumatic valve 19 corresponding to the piston container I 21 is opened, and the pneumatic valves 19 corresponding to the other two piston containers II 22 and piston container III 23 are closed. The remote control peristaltic pump 18 draws liquid from the water storage container 17, and injects water at a constant flow rate to push the piston plate inside the piston container I 21, so that the high-viscosity fracturing fluid in the upper chamber of the piston container I 21 reaches the injection end 27 at a constant flow rate through the injection pipeline 26, and then flows out from the injection end 27 to the production well 4 and is ejected from the production well perforation 45 into the hydrate reservoir model 15 to form hydraulic fracturing fractures 37, realizing hydraulic fracturing.

[0136] The hydraulic fracturing fractures 37 are divided into horizontal transverse main fractures and splitting fractures branching from the main fractures. The fracture initiation pressure prediction model considering the influence of fracturing fluid flow, reservoir effective stress and hydrate cementation effect during the hydraulic fracturing process is shown in the following formula.

[0137]

[0138] Among them, p f represents the pressure of hydraulic fracturing; μ and v f respectively represent the viscosity and flow rate of the fracturing fluid; g is the acceleration due to gravity; n is the multiple of the acceleration due to gravity; K0 and ψ are the sediment lateral pressure coefficient and the internal friction angle respectively; ρ' is the sediment floating density; z represents the reservoir burial depth, N p is the pore characteristic constant related to the hydrate saturation, c his the cohesion of hydrate sediments; a f is a constant parameter related to the permeability of hydrate sediments and the diversion capacity of splitting fissures.

[0139] S4. Reservoir reinforcement process

[0140] After the hydraulic fracturing is completed, only the pneumatic valve 19 corresponding to the piston container II 22 is opened, and the pneumatic valves 19 corresponding to the other two piston containers I 21 and piston container III 23 are closed. The remote control peristaltic pump 18 extracts liquid from the water storage container 17, injects water at a constant flow rate to push the piston plate inside the piston container II 22, so that the breaker in the upper chamber of the piston container II 22 reaches the injection end 27 through the injection pipeline 26 at a constant flow rate, and then flows out from the injection end 27 to the production well 4 and out of the perforation 45 of the production well into the formed hydraulic fracture 37 of the fracture, so as to reduce the viscosity of the fracturing fluid in the fracture and facilitate subsequent reservoir transformation and hydrate production.

[0141] Subsequently, switch to only open the pneumatic valve 19 corresponding to the piston container III 23, and close the pneumatic valves 19 corresponding to the other two piston containers I 21 and piston container II 22. The remote control peristaltic pump 18 extracts liquid from the water storage container 17, injects water at a constant flow rate to push the piston plate inside the piston container III 23, so that the reinforcement agent in the upper chamber of the piston container III 23 reaches the injection end 27 through the injection pipeline 26 at a constant flow rate, and then flows out from the injection end 27 to the production well 4 and out of the perforation 45 of the production well into the fracture of the hydraulic fracture 37 that has been injected with the breaker, realizing the support and reinforcement of the fracture and forming a chemical transformation and reinforcement area 38;

[0142] After the injection of the reinforcement agent, it forms a continuous porous surface-like structure under the action of water and salt in the pore water of the hydrate reservoir model 15 through hydration for several hours to support the fracture surface and play a long-term role in increasing permeability, reinforcement, sand prevention, etc.

[0143] In specific implementation, the injection pipeline 26 and the injection end 27 can be driven by the single-degree-of-freedom loading device 25 to move to different positions of the production well 4 located in the hydrate reservoir model 15, and hydraulic fracturing and reservoir reinforcement tests are carried out on the hydrate reservoir model 15 at different heights.

[0144] The reaction formula of the chemical reinforcement of the reinforcement agent is as follows:

[0145] a1CaO·SiO2 + a2H2O → a3CaO·SiO2·a4H2O + a5Ca(OH)2

[0146] CaSO4 + 2H2O → Ca(OH)2 + H2S + O2

[0147] CaO + H2O → Ca(OH)2

[0148] Among them, a1 to a5 represent the proportions of various substances.

[0149] S5. Hydrate production process

[0150] During the hydrate production process, the single-degree-of-freedom loading device 25 drives the injection pipeline 26 and the injection end 27 to move upward until the production pipeline 29 is connected to the production well 4 above the hydrate reservoir model 15 and not in the production well 4 in the hydrate reservoir model 15, so that the production pipeline 29 can be smoothly connected to the production well 4 at the chemical modification and reinforcement area 38.

[0151] Then, the backpressure pump 30 and the buffer tank 32 cooperate to control the opening of the backpressure valve 33. The hydrate substances in the hydrate reservoir model 15 enter the production pipeline 29 through the production well 4, and then only the water and methane gas flowing through the backpressure valve 33 after the solids are separated by the solid separation and metering device 31 enter the liquid-gas separation, collection and metering module 36 for liquid-gas separation and metering, completing the experiment.

[0152] In a specific embodiment of the present invention, as Figure 1 and Figure 3 (b), in the hydrate production module, after the reservoir reinforcement and transformation are completed, the injection end 27 is lifted to the top of the production well by the single-degree-of-freedom loading device 25. At this time, the production well 4 is connected to the production pipeline 29. The backpressure on the backpressure valve spool is uniformly reduced by the plunger pump for servo suction / drainage to achieve servo control of the pressure in the production well. When the pore pressure of the hydrate reservoir model 15 is reduced below the hydrate phase equilibrium pressure, the hydrate decomposes into methane gas and water, and is quickly produced along the production well 4 through the hydraulic fracture 37. After the pressure in the production well is reduced to the production target value, it remains constant until all the hydrates in the pores of the hydrate reservoir model 15 are completely decomposed. After the experiment, the centrifugal acceleration is gradually reduced to 1g to end the experiment.

[0153] S6. Monitoring process

[0154] Then, the sensors 13, acoustic emission probes 39 and sapphire endoscopes 40 in the model multi-physical field monitoring module are used to collect data and send it to the model monitoring and data acquisition module 43, and the model monitoring and data acquisition module 43 analyzes the data to obtain the hydraulic fracturing-production situation.

[0155] The above-mentioned sediment is the soil skeleton in the hydrate reservoir model 15.

[0156] Sensor measuring points such as thermocouples, pressure sensors, earth pressure gauges, resistivity probes, and strain gauges are embedded at different positions in the reservoir during the preparation of the hydrate reservoir soil skeleton, and are respectively used to monitor the temperature, pore pressure, total stress, resistivity and reservoir deformation during hydrate formation, hydraulic fracturing, reservoir reinforcement and hydrate production.

[0157] During the hydrate formation process, effective stress control process, hydraulic fracturing process, reservoir reinforcement process of injecting gel breaker and reinforcing agent, and subsequent hydrate production process, temperature, pore pressure, total stress, resistivity, and reservoir deformation during hydrate formation, hydraulic fracturing, reservoir reinforcement, and hydrate production are monitored in real time through sensors such as thermocouples, pressure sensors, earth pressure gauges, resistivity probes, and strain gauges respectively.

[0158] During the hydraulic fracturing process, the fracture splitting strength and splitting position of the hydraulic fracturing fractures 37 inside the hydrate reservoir model 15 are monitored in real time through the acoustic emission probe 39, and the actual fracture propagation process, fracture position, fracture aperture, and particle migration and deformation response of the hydrate reservoir model 15 are observed in real time through the endoscope camera 41 inside the sapphire endoscope tube 40.

[0159] In specific implementation, the actual fracture propagation process, fracture position, and fracture aperture observed in real time by the endoscope camera 41 inside the sapphire endoscope tube 40 can be used to calibrate and verify the acoustic monitoring results, and the development and distribution law of internal splitting fractures can be inverted.

[0160] During the hydrate reservoir transformation and pressure reduction production, the change of acoustic emission signals can also be used to assist in judging whether the hydrate reservoir model 15 has undergone severe deformation, and the particle migration and deformation response of the hydrate reservoir model 15 can be observed through the sapphire endoscope tube 40.

[0161] During the hydraulic fracturing process, the acoustic signal data collected by the acoustic emission probe 39 is processed according to the following formula to obtain the rise time t r and the average frequency AF:

[0162] t r =t d / A e

[0163] AF=NF / t f

[0164] wherein, the rise time t r is the ratio between the delay t d between the start time and the maximum amplitude of the acoustic signal and the amplitude A e ; the average frequency AF is the ratio between the number of effective fracture acoustic signals NF and the total duration t f of the hydraulic fracturing;

[0165] After the start of the hydraulic fracturing, based on the rise time t r and the average frequency AF obtained in real time, judgments are made to obtain different stage judgments of the hydraulic fracturing:

[0166] When the average frequency AF increases by more than a preset threshold compared to the value before hydraulic fracturing and the change rate of the average frequency AF at adjacent times is lower than the preset slope threshold, and at the same time the rise time t r increases by no more than the preset threshold compared to the value before hydraulic fracturing and the rise time t r at adjacent times has a change rate lower than the preset slope threshold, it is considered that the current is in the tensile fracture stage;

[0167] When the average frequency AF continuously increases and the change rate of the average frequency AF at adjacent times is higher than the preset slope threshold, and at the same time the rise time t r continuously increases and the rise time t r at adjacent times has a change rate higher than the preset slope threshold, it is considered that the current is in the main fracture formation and propagation stage.

[0168] Thus, through experiments, it is found that a relatively high average frequency AF and a relatively low rise time t can be monitored in the initial stage of tensile fracture formation r , and when the main fracture forms and propagates, the average frequency AF rapidly decreases and the rise time t r significantly increases.

[0169] According to a specific embodiment of the present invention, such as Figure 4 , the sensor ear hole on the side wall of the high-pressure vessel 1 can also serve as the wellhead of the production well for arranging horizontal wells to simulate the pressure reduction production of hydrate reservoirs by horizontal well hydraulic fracturing. When using horizontal wells, the contact range between the production well and the reservoir is larger, which is beneficial to increasing the pressure reduction range and efficiency of the reservoir. Since the hydraulic fracturing cracks develop along the direction of the minor principal stress vertical plane of the reservoir, using horizontal wells for hydraulic fracturing can make the injection direction of the fracturing fluid consistent with the crack development direction, and it is expected to obtain a larger hydraulic fracturing range and reservoir stimulation effect.

[0170] Therefore, the present invention is mounted on a geotechnical centrifuge and can servo-control the high-pressure, low-temperature, high-stress or formation stress environment of the hydrate reservoir to simulate the in-situ occurrence environment of the hydrate reservoir.

[0171] The present invention can precisely control the temperature, type, flow rate and injection position of different injection fluids; after hydraulic fracturing, it can remotely switch the injection fluid to inject a gel breaker and a reinforcing agent, reduce the blocking effect of the high-viscosity fracturing fluid in the cracks and support and reinforce the cracks, and can further carry out the simulation of pressure reduction production of hydrate vertical wells or horizontal wells.

[0172] The present invention can monitor the crack initiation position, crack morphology, crack development process and the particle migration and deformation response of the reservoir during hydrate pressure reduction production through acoustic emission probes and sapphire endoscopes.

[0173] The present invention realizes the simulation of the characteristics of hydraulic fracturing transformation of large-scale hydrate reservoirs and the long-duration gas production process of hydrates through hypergravity experiments at the model scale, reveals the development law of hydraulic fracturing fissures, evaluates the effect of formation chemical reinforcement and the improvement amplitude of production capacity, and provides an innovative research method for the industrial development of deep-sea natural gas hydrate resources.

[0174] The embodiments listed in this specification are only the preferred embodiments of the present invention. Any equivalent technical exchange made under the working principle and idea of the present invention shall be regarded as the protection scope of the present invention.

Claims

1. A natural gas hydrate reservoir hydraulic fracturing-exploitation ultra-gravity physical simulation experimental device, characterized by: The invention comprises a high-pressure container (1), a water bath temperature control module, an effective stress control module, a fracturing reinforcement mining module, a hydrate preparation module (7) and a model multi-physical field monitoring module; the high-pressure container (1) has a built-in hydrate reservoir model (15); the high-pressure container (1) is placed in a water bath environment as a whole and connected to the water bath temperature control module, so that the hydrate reservoir model (15) is placed in the water bath environment; the effective stress control module is connected to the high-pressure container (1) and applies stress control to the hydrate reservoir model (15); the fracturing reinforcement mining module is connected to the high-pressure container (1) and performs fracturing, reinforcement and mining tests on the hydrate reservoir model (15); the model multi-physical field monitoring module is installed on the high-pressure container (1) to monitor the hydrate reservoir model (15); and the hydrate preparation module (7) is connected to the hydrate reservoir model (15) in the high-pressure container (1); The high-pressure container (1), effective stress control module, and fracturing reinforcement mining module are all mounted in a centrifuge basket (10) and operate under 1-500g of supergravity; The high-pressure container (1) is provided with a loading plate (24), the loading plate (24) divides the space in the high-pressure container (1) into an upper chamber and a lower chamber, the lower chamber is provided with a hydrate reservoir model (15), the upper chamber is provided with an axial pressure liquid layer (34), the fracturing reinforcement and mining module comprises a hydrate reservoir hydraulic fracturing module, an injection fluid switching and reservoir reinforcement module and a hydrate mining module, the hydrate reservoir hydraulic fracturing module, the injection fluid switching and reservoir reinforcement module and the hydrate mining module are all connected to the hydrate reservoir model (15); the effective stress control module is connected to the axial pressure liquid layer (34) and is connected to the hydrate mining module; The hydrate reservoir hydraulic fracturing module comprises a piston temperature control container, a horizontal flow pump (18), a single degree of freedom loading device (25), an injection pipeline (26) and an injection end (27); the input end of the horizontal flow pump (18) is connected to the water storage container (17); the output end of the horizontal flow pump (18) is connected to one end of the piston temperature control container; the piston temperature control container is pre-filled with fracturing fluid; the other end of the piston temperature control container is connected to the upper end of the injection pipeline (26); the lower end of the injection pipeline (26) is inserted into the production well (4) in the hydrate reservoir model (15); the lower end of the injection pipeline (26) is installed with the injection end (27); the side wall of the production well (4) is provided with a production well perforation (45) corresponding to the injection end (27); The upper part of the injection pipeline (26) is installed on the loading arm of the single-degree-of-freedom loading device (25), and the injection pipeline (26) is driven by the single-degree-of-freedom loading device (25) to move up and down; the injection end head (27) comprises a top plate (47), a bottom plate (49), a cylindrical connecting rod (48) and a sealing ring (46); the top plate (47) and the lower end of the injection pipeline (26) are fixedly connected; a through hole communicating with the lower end of the injection pipeline (26) is provided in the middle of the top plate (47); the top plate (47) and the bottom plate (49) are fixedly connected via four cylindrical connecting rods (48); the top plate (47) and the bottom plate (49) are sealedly connected to the inner wall of the production well (4) via the sealing ring (46); The injection fluid switching and reservoir reinforcement module comprises two piston temperature control containers, a horizontal flow pump (18), a single degree of freedom loading device (25), an injection pipeline (26) and an injection end head (27); the horizontal flow pump (18), the single degree of freedom loading device (25), the injection pipeline (26) and the injection end head (27) of the injection fluid switching and reservoir reinforcement module are shared with the horizontal flow pump (18), the single degree of freedom loading device (25), the injection pipeline (26) and the injection end head (27) of the hydrate reservoir hydraulic fracturing module; the two piston temperature control containers of the injection fluid switching and reservoir reinforcement module and the piston temperature control container of the hydrate reservoir hydraulic fracturing module are connected in parallel; and a gel breaker and a reinforcement agent are respectively pre-set in the two piston temperature control containers of the injection fluid switching and reservoir reinforcement module.

2. The natural gas hydrate reservoir hydraulic fracturing-exploitation high gravity physical simulation experimental device according to claim 1, characterized in that: The water bath temperature control module comprises a water bath jacket (14), a constant temperature water bath box (5) and a water bath circulation pump (6); the high pressure container (1) is placed in a water bath jacket (14) with a guide groove; the water bath jacket (14), the constant temperature water bath box (5) and the water bath circulation pump (6) are cyclically connected in series; the water bath circulation pump (6) drives the antifreeze to circulate in the water bath jacket (14) and the constant temperature water bath box (5); The effective stress control module comprises an axial pressure pump (12), an axial pressure liquid injection pipeline (16), a back pressure pump (30) and a buffer tank (32); the input end of the axial pressure pump (12) is connected to a water storage container (17); the output end of the axial pressure pump (12) is connected to an axial pressure liquid layer (34) in a high-pressure container (1) via the axial pressure liquid injection pipeline (16); the output end of the back pressure pump (30) is connected to an upper portion of the buffer tank (32); and the lower end of the buffer tank (32) is connected to a control end of a hydrate production module.

3. The natural gas hydrate reservoir hydraulic fracturing-exploitation high gravity physical simulation experimental device according to claim 2, characterized in that: The piston temperature control container is mainly composed of a piston container, a pneumatic valve (19) and a semiconductor chip (20). The piston container is connected to the horizontal flow pump (18) and the injection pipeline (26) respectively, and the semiconductor chip (20) is installed on the piston container. A piston plate is provided inside the piston container, and the piston plate divides the piston container into an upper and lower chamber. The lower chamber is connected to the output end of the horizontal flow pump (18) through the pneumatic valve (19), and the upper chamber is pre-added with the required substance, and the upper chamber is connected to the upper end of the injection pipeline (26). The piston container of the piston temperature control container of the hydrate reservoir hydraulic fracturing module is piston container I (21), and the piston containers of the two piston temperature control containers of the injection fluid switching and reservoir reinforcement module are piston container II (22) and piston container III (23).

4. The natural gas hydrate reservoir hydraulic fracturing-exploitation high gravity physical simulation experimental device according to claim 3, characterized in that: The hydrate production module comprises a production pipeline (29), a solid separation meter (31), a back pressure valve (33), and a liquid-gas separation collection and metering module (36); the input end of the solid separation meter (31) is connected to the top of the production well (4) via the production pipeline (29); the output end of the solid separation meter (31) is connected to the inlet of the back pressure valve (33); and the outlet of the back pressure valve (33) is connected to the liquid-gas separation collection and metering module (36).

5. The natural gas hydrate reservoir hydraulic fracturing-exploitation high gravity physical simulation experimental device according to claim 4, characterized in that: The model multi-physics field monitoring module comprises a sensor (13) installed on a high-pressure container (1), an acoustic emission probe (39) and a sapphire endoscope (40) on the inner wall of the high-pressure container (1), and a model monitoring and data acquisition module (43); the sensor (13) comprises a thermocouple, a pressure sensor, an earth pressure gauge, a resistivity probe and a strain gauge; the acoustic emission probe (39) is arranged horizontally toward the production well (4); the sapphire endoscope (40) is vertically inserted into the hydrate reservoir model (15) and parallel to the production well (4); an endoscope camera (41) is arranged in the sapphire endoscope (40); the sensor (13), the acoustic emission probe (39) and the endoscope camera (41) are all connected to the model monitoring and data acquisition module (43) outside the centrifuge basket (10) through a sensor signal line (42); The pneumatic valves (19) in the piston temperature control containers of the injection fluid switching and reservoir reinforcement module and the hydrate reservoir hydraulic fracturing module, as well as a pneumatic valve (19) connected to the input end of the solid separation meter (31) in the hydrate production module are all connected to the electromagnetic valve group (28), and the electromagnetic valve group (28) is connected to the external model monitoring and data acquisition module (43) via the electromagnetic valve group signal line (44). The model monitoring and data acquisition module (43) remotely controls the electromagnetic valve group (28) and further controls the opening and closing of each pneumatic valve (19).

6. The method for high-gravity physical simulation experiment of natural gas hydrate reservoir hydraulic fracturing and exploitation applied to any one of the experimental devices described in claim 5, characterized in that: The method process is as follows: S1. Hydrate formation process A hydrate preparation module (7) generates hydrates in a high-pressure container (1) to prepare a hydrate reservoir model (15); S2. Effective stress control process The high-pressure container (1), effective stress control module, and fracturing reinforcement mining module of the device are mounted in a centrifuge basket (10), and then the centrifuge is started. After the centrifuge rotates at high speed to apply an ng hypergravity field to reach an ng hypergravity state, an axial pressure pump (12) injects liquid into the axial pressure liquid layer (34) in the high-pressure container (1) through an axial pressure liquid injection pipeline (16) to increase the pressure, and transmits the pressure of the axial pressure liquid layer (34) to the hydrate reservoir model (15) through a loading plate (24), so as to convert the overlying total stress σ0 applied by the axial pressure pump (12) into the total stress of the hydrate reservoir model (15); At the same time, a back pressure is applied to the back pressure valve (33) through the back pressure pump (30) and the buffer tank (32) to control the opening and closing of the valve core and the opening degree, so that the back pressure valve (33) is connected to the hydrate reservoir model (15) through the production pipeline (29), and the effective stress of the hydrate reservoir model (15) is indirectly controlled by controlling the total stress and pore water pressure of the hydrate reservoir model (15); S3. Hydraulic fracturing process Before hydraulic fracturing of the reservoir, the injection pipeline (26) and the injection end head (27) are driven by the single-degree-of-freedom loading device (25) to move downward to the production pipeline (29) connected to the production well (4) below the production well (4) and located in the hydrate reservoir model (15); When hydraulic fracturing the reservoir is performed, only the pneumatic valve (19) corresponding to the piston container I (21) is opened, and the horizontal flow pump (18) is remotely controlled to extract liquid from the water storage container (17), and water is injected at a constant flow rate to push the piston plate inside the piston container I (21), so that the fracturing fluid in the upper chamber of the piston container I (21) reaches the injection end (27) through the injection pipeline (26) at a constant flow rate, and then flows out from the injection end (27) to the production well (4) and is ejected from the perforation (45) of the production well into the hydrate reservoir model (15) to form a hydraulic fracturing crack (37), thereby achieving hydraulic fracturing; S4. Reservoir reinforcement process After the hydraulic fracturing is completed, only the pneumatic valve (19) corresponding to the piston container II (22) is opened, and the horizontal flow pump (18) is remotely controlled to extract liquid from the water storage container (17), and water is injected at a constant flow rate to push the piston plate inside the piston container II (22), so that the gel breaker in the upper chamber of the piston container II (22) passes through the injection pipeline (26) at a constant flow rate to the injection end (27), and then flows out from the injection end (27) to the production well (4) and flows out from the production well perforation (45) to enter the cracks of the hydraulic fracturing cracks (37) that have been formed; Then, the pneumatic valve (19) corresponding to the piston container III (23) is switched to open only, and the horizontal flow pump (18) is remotely controlled to extract liquid from the water storage container (17), and water is injected at a constant flow rate to push the piston plate inside the piston container III (23), so that the reinforcement agent in the upper chamber of the piston container III (23) passes through the injection pipeline (26) at a constant flow rate to the injection end (27), and then flows out from the injection end (27) to the production well (4) and flows out from the perforation (45) of the production well into the cracks of the hydraulic fracturing cracks (37) that have been injected with the gel breaker, forming a chemically modified reinforcement zone (38); After the reinforcement agent is injected, it hydrates under the action of water and salt in the pore water of the hydrate reservoir model (15) for several hours to form a continuous porous surface structure to support the fracture surface; S5. Hydrate mining process The injection pipeline (26) and the injection end head (27) are driven by the single-degree-of-freedom loading device (25) to move upward to the top of the production well (4) and to be located above the point where the production pipeline (29) is connected to the production well (4), so that the production pipeline (29) can be connected to the production well (4) at the chemical transformation reinforcement area (38); Then, the gas generated by the decomposition of hydrates in the hydrate reservoir model (15) enters the production pipeline (29) through the production well (4), and then after the solids are separated by the solid separation meter (31), only the pressure control of the back pressure valve (33) enters the liquid-gas separation collection and metering module (36) for liquid-gas separation and metering, thus completing the experiment; S6. Monitoring process The model's multi-physics field monitoring module is used to collect data and perform data analysis to obtain the hydraulic fracturing-production situation.

7. The method for high-gravity physical simulation experiment of natural gas hydrate reservoir hydraulic fracturing and exploitation according to claim 6, characterized in that: During the hydraulic fracturing process, the hydraulic fracturing initiation pressure is obtained according to the following formula: Among them, p f represents the hydraulic fracturing initiation pressure; μ and v f represents the viscosity and flow rate of the fracturing fluid respectively; g is the gravitational acceleration; n is the gravitational acceleration multiple; K0 and ψ are the sediment side pressure coefficient and internal friction angle respectively; ρ' is the sediment floating density; z represents the reservoir depth, N p is the pore characteristic constant related to hydrate saturation, c h is the cohesion of hydrate-containing sediments; a f It is a constant parameter related to the permeability of hydrate-bearing sediments and the conductivity of cleavage fractures.

8. The method for high-gravity physical simulation experiment of natural gas hydrate reservoir hydraulic fracturing and exploitation according to claim 6, characterized in that: During the hydrate formation process, effective stress control process, hydraulic fracturing process, reservoir reinforcement process by injecting breakers and reinforcement agents, and subsequent hydrate production process, temperature, pore pressure, total stress, resistivity, and reservoir deformation are monitored in real time through thermocouples, pressure sensors, earth pressure gauges, resistivity probes, and strain gauges. During the hydraulic fracturing process, the fracture splitting strength and fracture position of the hydraulic fracture (37) inside the hydrate reservoir model (15) are monitored in real time by an acoustic emission probe (39), and the actual fracture extension process, fracture position and fracture aperture, as well as the particle migration and deformation response of the hydrate reservoir model (15) are observed in real time by an endoscope camera (41) inside a sapphire endoscope (40).

9. The method for high-gravity physical simulation experiment of natural gas hydrate reservoir hydraulic fracturing and exploitation according to claim 6, characterized in that: Method During the hydraulic fracturing process, the acoustic signal data collected by the acoustic emission probe (39) is processed according to the following formula to obtain the rise time t of the acoustic signal: r And the average frequency AF: t r =t d / A e AF=NF / h f Among them, the rise time t r is the delay t between the start of the acoustic signal and its maximum amplitude d With amplitude A e The average frequency AF is the ratio of the number of effective fracture acoustic signals NF to the total hydraulic fracturing time t f The ratio between After the hydraulic fracturing starts, according to the real-time rise time t r And the average frequency AF is used to judge the different stages of hydraulic fracturing: When the average frequency AF increases by more than the preset threshold value compared with the value before hydraulic fracturing and the change rate of the average frequency AF at adjacent moments is lower than the preset slope threshold value, and the rise time t r Compared with the value before hydraulic fracturing, the increase does not exceed the preset threshold and the rise time t of adjacent moments r If the rate of change is lower than the preset slope threshold, it is considered to be in the tension crack stage; When the average frequency AF increases continuously and the average frequency AF change rate at adjacent moments is higher than the preset slope threshold, and the rise time t r The rise time t of adjacent moments increases continuously r If the rate of change is higher than the preset slope threshold, it is considered that the current stage is the main crack formation and propagation stage.

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