HYPERGRAVITY EXPERIMENTAL APPARATUS AND METHOD FOR NATURAL GAS HYDRATE EXPLOITATION BY HYDRAULIC FRACTURING

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of China application serial no. 202411199415.6, filed on Aug. 29, 2024. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification.

BACKGROUND

Technical Field

The disclosure belongs to the field of geotechnical engineering and energy engineering physical modeling technology, relates to a hydrate exploitation simulation experimental apparatus, and particularly relates to a hypergravity experimental apparatus for natural gas hydrate exploitation by hydraulic fracturing.

Description of Related Art

Natural gas hydrates (hereinafter referred to as hydrates) are cage-like crystalline compounds formed by natural gas molecules (mainly methane) and water molecules under high pressure and low temperature conditions. Due to their considerable resource potential and pollution-free combustion, they are recognized as the most promising clean energy to substitute conventional oil and gas in the 21-st century. Marine hydrates are generally found in sediment pores at water depths of kilometers and burial depths of hundreds of meters, with reservoir thicknesses reaching tens of meters. The reservoir deformation, strength, seepage, and stability characteristics are significantly affected by the gravitational field. In recent years, China and Japan have carried out several in-situ production tests of marine hydrates. However, due to poor stability and permeability of marine hydrate reservoirs and insufficient depressurization efficiency, the gas production efficiency is still about one order of magnitude lower than the industrial exploitation standard. The excessively low permeability of in-situ hydrate reservoirs is one of the essential issues limiting the hydrate exploitation efficiency. Hydrates are generally distributed in sediments in cementing, pore filling, lenses/veins, and nodules/chunks morphologies and thus occupy a large amount of sediment pore space. Formation permeability is thereby significantly lowered. Hydraulic fracturing, reinforcement, and modification of the hydrate reservoirs may greatly improve the depressurization efficiency and formation stability during hydrate exploitation, and a breakthrough in the production efficiency of in-situ hydrate exploitation is thereby achieved.

Hydraulic fracturing technology has been widely applied in high-strength, dense, and continuous reservoirs such as coal and shale gas, but extending it to hydrate reservoirs with low strength and stiffness and poor continuity characteristics still requires extensive research. The initiation and development behavior of hydraulic fracturing fractures are significantly affected by the formation stress level. At present, the hydrate reservoir hydraulic fracturing modeling apparatuses are mostly in the form of triaxial experimental apparatuses, which are mainly used to simulate the stress levels of centimeter-scale hydrate-bearing sediment unit bodies at a specific depth in large-scale reservoirs. These apparatuses cannot simulate the geothermal gradient and overall formation stress distribution of large-scale reservoirs, and do not have the capability to simulate hydrate exploitation after fracturing. The aforementioned apparatuses are mainly used to study the fracturability of formations under specific stress conditions, making it difficult to reflect the fracture development and final distribution patterns within large-scale reservoirs. Further, the impact of fracturing fractures on hydrate exploitation productivity is not revealed as well. The currently-available hydrate exploitation modeling apparatuses are constant gravity experimental apparatuses that are not equipped with hydraulic fracturing simulation capabilities.

To realistically model the large-scale hydrate reservoirs' hydraulic fracturing-exploitation process, the essential technologies that need to be urgently addressed in the experimental apparatus include: accurately controlling the temperature, hydraulic pressure environment, gravitational stress gradient, and effective stress levels of the reservoir model on the centrifuge, remotely regulating the optimization of injection fracturing and reinforcing fluids, temperature and flow rate, and remote servo switching the work model of production well between fluid injection and depressurization exploitation.

Therefore, in the state of the art, a solution for realistically simulating the large-scale hydraulic fracturing-exploitation process of hydrate reservoirs is required.

SUMMARY

In order to solve the problems found in the related art, the disclosure aims to provide a hypergravity experimental apparatus for natural gas hydrate exploitation by hydraulic fracturing, so as to fill the gap in experimental modeling of field-scale hydrate reservoir exploitation by hydraulic fracturing-exploitation and its' production efficiency.

The technical solutions adopted by the disclosure are provided as follows.

1. A hypergravity experimental apparatus and method for natural gas hydrate exploitation by hydraulic fracturing is provided.

A high-pressure vessel, a water bath temperature control module, an effective stress control module, a fracturing reinforcement exploitation module, a hydrate preparation module, and a model multi-physical field monitoring module are included. The high-pressure vessel is provided with a hydrate reservoir model inside and is entirely 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 communicates with the inside of the high-pressure vessel and applies stress control to the hydrate reservoir model. The fracturing reinforcement exploitation module communicates with the inside of the high-pressure vessel and conducts fracturing, reinforcement, and exploitation experiments 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. The hydrate preparation module communicates with the hydrate reservoir model inside the high-pressure vessel.

The high-pressure vessel, the effective stress control module, and the fracturing reinforcement exploitation module are all mounted within a centrifuge basket and operate under 1 g to 500 g hypergravity. The hydrate preparation module and the water bath temperature control module are placed in the centrifuge power room and work under 1 g normal gravity.

The high-pressure vessel is provided with a loading plate inside. The loading plate divides a space inside the high-pressure vessel into an upper chamber and a lower chamber, the lower chamber is provided with the hydrate reservoir model inside, and the upper chamber is provided with an axial pressure fluid layer inside. The fracturing reinforcement exploitation module includes a hydrate reservoir hydraulic fracturing module, an injection fluid switching and reservoir reinforcement module, and a hydrate exploitation module. The hydrate reservoir hydraulic fracturing module, the injection fluid switching and reservoir reinforcement module, and the hydrate exploitation module are all communicate with the hydrate reservoir model. The effective stress control module communicates with the axial pressure liquid layer and is connected to the hydrate exploitation module.

The water bath temperature control module includes a water bath jacket, a constant temperature water bath, and a water bath circulation pump. The high-pressure vessel is placed in the water bath jacket with a guide groove. The water bath jacket, the constant temperature water bath, and the water bath circulation pump communicate in series for circulation, with the water bath circulation pump driving antifreeze liquid to circulate within the water bath jacket and the constant temperature water bath. The effective stress control module includes an axial pressure pump, an injection pipeline, a back-pressure pump, and a buffer tank. An input end of the axial pressure pump communicates with a water storage container, and an output end of the axial pressure pump communicates with the axial pressure fluid layer inside the high-pressure vessel through the injection pipeline. An output end of the back-pressure pump communicates with an upper portion of the buffer tank, and a lower end of the buffer tank is connected to a control end of the hydrate exploitation module.

The hydrate reservoir hydraulic fracturing module includes a piston temperature control container, an advection pump, a single-degree-of-freedom loading device, an injection pipeline, and an injection end head. An input end of the advection pump communicates with the water storage container, and an output end of the advection pump communicates with one end of the piston temperature control container. The piston temperature control container is provided with fracturing fluid inside in advance, and the other end of the piston temperature control container communicates with an upper end of the injection pipeline. A lower end of the injection pipeline is inserted into a production well in the hydrate reservoir model and is installed with the specially designed injection end head. A production well perforation corresponding to the injection end head is arranged on a side wall of the production well. An upper portion of the injection pipeline is installed on a loading arm of the single-degree-of-freedom loading device, and the single-degree-of-freedom loading device drives the injection pipeline to move up and down. The injection end 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 formed in a middle of the top plate, the top plate and the bottom plate are fixedly connected by four cylindrical connecting rods, and the top plate and a periphery of the bottom plate are sealed and connected to an inner wall of the production well through the sealing ring. The injection fluid switching and reservoir reinforcement module includes two piston temperature control containers, the advection pump, the single-degree-of-freedom loading device, the injection pipeline, and the injection end head. The injection fluid switching and reservoir reinforcement module shares the advection pump, the single-degree-of-freedom loading device, the injection pipeline, and the injection end head with 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 to the piston temperature control container of the hydrate reservoir hydraulic fracturing module and are pre-filled with a gel breaker and a reinforcing agent.

Main components of the reinforcing agent are calcium oxide, calcium silicate, and calcium sulfate, with a molar ratio of the calcium oxide, calcium silicate, and calcium sulfate being 5:2:1.

The piston temperature control container is mainly formed by a piston container, a pneumatic valve, and a semiconductor chip. The piston container communicates with the advection pump and the injection pipeline, and the semiconductor chip is installed on the piston container. A piston plate is inside the piston container, and the piston plate divides the piston container into upper and lower chambers. The lower chamber communicates with the output end of the advection pump through the pneumatic valve, a required substance is pre-added in the upper chamber, and the upper chamber communicates with the upper end of the injection pipeline.

The hydrate exploitation module includes a production pipeline, a solid separation meter, a back-pressure valve, and a liquid-gas separation collection and metering module. An input end of the solid separation meter communicates with a top portion of the production well through the production pipeline, an output end of the solid separation meter communicates with an inlet of the back-pressure valve, and an outlet of the back-pressure valve communicates with the liquid-gas separation collection and metering module.

After hydraulic fracturing is completed, the single-degree-of-freedom loading device drives the injection pipeline and the injection end head to move upwards to the top portion of the production well, so that the production pipeline communicates with the production well at the chemically-modified reinforcement zone. Herein, a state of the production well is switched from a reservoir modification mode to a hydrate exploitation mode.

The model multi-physical field monitoring module includes a sensor installed on the high-pressure vessel, an acoustic emission probe and a sapphire endoscope tube on an inner wall of the high-pressure vessel, and a model monitoring and data collection module. The sensor includes a thermocouple, a pressure sensor, an earth pressure gauge, a resistivity probe, and a strain gauge, etc. The acoustic emission probe is horizontally arranged towards the production well. The sapphire endoscope tube is vertically inserted into the hydrate reservoir model and parallel to the production well, and an endoscope camera is installed inside the sapphire endoscope tube. The sensor, the acoustic emission probe, and the endoscope camera are all connected via sensor signal lines to the model monitoring and data collection module outside the centrifuge basket for communication.

The injection fluid switching and reservoir reinforcement module and one pneumatic valve connected to a pneumatic valve/an input end of a solid separation meter in the piston temperature control container of the hydrate reservoir hydraulic fracturing module are both connected to an electromagnetic valve group. The electromagnetic valve group is communicatively connected via an electromagnetic valve group signal line to the external model monitoring and data collection module. The model monitoring and data collection module remotely controls the electromagnetic valve group to further control the opening and closing of each pneumatic valve.

2. A method for natural gas hydrate exploitation by hydraulic fracturing applied to the hypergravity experimental apparatus and filed exploitation is provided, and the method includes the following steps.

S1: Hydrate Formation Process

The hydrate preparation module prepares a hydrate in the high-pressure vessel to prepare the hydrate reservoir model.

S2: Effective Stress Control Process

After the preparation of the hydraulic is completed, the high-pressure vessel, the effective stress control module, and the fracturing reinforcement exploitation module of the apparatus are all mounted within a centrifuge basket. The constant temperature water bath, the water bath circulation pump, and the hydrate preparation module are placed in the centrifuge power room. Through the centrifuge rotary joint, a centrifuge water/gas pipeline, a water/gas interface in the centrifuge basket are connected with an inner portion of the high-pressure vessel and the water bath jacket of the high-pressure vessel. After the apparatus is loaded, the centrifuge is started. Through high-speed rotation of the centrifuge, an ng hypergravity field is applied to reach an ng hypergravity state, and the axial pressure pump injects liquid from the axial pressure liquid injection pipeline in the water storage container to into the axial pressure liquid layer in the high-pressure vessel to increase pressure. The loading plate transmits the pressure of the axial pressure liquid layer to the hydrate reservoir model and converts the overlying total stress σ₀ applied by the axial pressure pump into a total stress of the hydrate reservoir model.

The back-pressure pump and buffer tank apply back-pressure to the back-pressure valve to control opening, closing, and an opening level of a valve core, so that the back-pressure valve communicates with the hydrate reservoir model via the production pipeline, and an effective stress thereof is indirectly controlled by controlling the total stress and a pore water pressure of the hydrate reservoir model.

The centrifuge is a hypergravity centrifuge.

S3: Hydraulic Fracturing Process

Before reservoir hydraulic fracturing is performed, the single-degree-of-freedom loading device drives the injection pipeline and the injection end head to move downward to a position below where the production pipeline communicates with to the production well and within the production well in the hydrate reservoir model.

When the reservoir hydraulic fracturing is performed, only the pneumatic valve corresponding to a piston container I is opened, the advection pump is remotely controlled to draw liquid from the water storage container, and water is injected at a constant flow speed to push the piston plate inside the piston container I, so that fracturing fluid in an upper chamber of the piston container I flows at a constant flow speed through the injection pipeline to reach the injection end head, flows out from the injection end head into the production well, and is ejected out from the production well perforation to enter the hydrate reservoir model to form a hydraulic fracturing fracture and to implement hydraulic fracturing.

S4: Reservoir Reinforcement Process

After hydraulic fracturing is completed, only the pneumatic valve corresponding to a piston container II is opened, the advection pump is remotely controlled to draw liquid from the water storage container, water is injected at a constant flow speed to push the piston plate inside the piston container II, so that the gel breaker in an upper chamber of piston container II flows at a constant flow speed through the injection pipeline to reach the injection end head, flows out from the injection end head into the production well, flows out from the production well perforation to enter a crack of the formed hydraulic fracturing fracture.

Subsequently, only the pneumatic valve corresponding to a piston container III is opened instead, the advection pump is remotely controlled to draw liquid from the water storage container, and water is injected at a constant flow speed to push the piston plate inside the piston container III, so that the reinforcing agent in an upper chamber of the piston container III flows at a constant flow speed through the injection pipeline to reach the injection end head, and flows out from the injection end head into the production well, flows out of the production well perforation to enter the crack of the hydraulic fracturing fracture that is injected with the gel breaker to form a chemically-modified reinforcement zone.

After the reinforcing agent is injected, continuous porous structure supporting fracture surface is formed under the action of water and salt in the pore water of the hydrate reservoir model for several hours of hydration.

S5 Hydrate Exploitation Process

The single-degree-of-freedom loading device drives the injection pipeline and the injection end head to move upwards to the top portion of the production well and above where the production pipeline communicates with the production well, so that the production pipeline communicates with the production well at the chemically-modified reinforcement zone.

Hydrate in the hydrate reservoir model is decomposed to generate methane gas which enters the production pipeline through the production well, so that only the controlled pressure of the back-pressure valve enters the liquid-gas separation collection and metering module for liquid-gas separation and metering after the solid is separated by the solid separation meter to complete the experiment. Finally, the high-speed rotation of the centrifuge is stopped, and the apparatus is removed from the centrifuge basket.

S6: Monitoring Process

The entire experimental process from S1 to S5 may use the model multi-physical field monitoring module to collect and analyze data to obtain a hydraulic fracturing-exploitation condition.

In the hydraulic fracturing process, fracture initiation pressure strength is obtained according to the following equation:

p f = a f ⁢ v f ⁢ μ + K 0 ⁢ tan 2 ( π / 4 + ψ / 2 ) - 1 tan 2 ( π / 4 + ψ / 2 ) - 1 ⁢ ρ ′ ⁢ ngz + 2 ⁢ N p ⁢ c h ⁢ tan ⁡ ( π / 4 + ψ / 2 ) tan 2 ( π / 4 + ψ / 2 ) - 1 ,

where p_f represents the fracture initiation pressure strength of hydraulic fracturing, μ and v_f represent a viscosity and a flow rate of the fracturing fluid, respectively, g is the gravitational acceleration, n is a multiple of gravitational acceleration, K₀ and ψ represent a sediment lateral pressure coefficient and an internal friction angle, respectively, ρ′ is a sediment buoyant density, z represents a reservoir burial depth, N_p is a pore characteristic constant related to hydrate saturation, c_h is a cohesion of hydrate-bearing sediment, and a_f is a constant parameter related to permeability of hydrate-bearing sediment and the flow conductivity of splitting fracture.

In the hydrate formation process, the effective stress control process, the hydraulic fracturing process, the reservoir reinforcement process by injecting the gel breaker and the reinforcing agent, and the subsequent hydrate exploitation process, real-time monitoring of temperature, pore pressure, total stress, resistivity, and reservoir deformation is performed through sensors including a thermocouple, a pressure sensor, an earth pressure gauge, a resistivity probe, and a strain gauge, respectively.

In the hydraulic fracturing process, real-time monitoring of fracture splitting strength and a splitting location of the hydraulic fracture inside the hydrate reservoir model is performed through the acoustic emission probe, and an actual fracture propagation process, a fracture location, a fracture opening level, and particle migration and deformation response of the hydrate reservoir model are observed in real-time through the endoscope camera inside the sapphire endoscope tube.

In the method, in the hydraulic fracturing process, acoustic signal data collected by the acoustic emission probe is processed according to the following equations to obtain a rise time t_r and an average frequency AF of an acoustic signal:

t r = t d / A e ⁢ and AF = NF / t f ,

where the rise time t_r is a ratio of a delay t_d from a moment the acoustic signal starts to a maximum amplitude thereof to an amplitude A_e, and the average frequency AF is a ratio of the number of effective fracture acoustic signals NF to total duration of hydraulic fracturing t_f.

After the hydraulic fracturing begins, determination is made based on the real-time obtained rise time t_r and the average frequency AF to determine different stages of hydraulic fracturing.

When the average frequency AF increases by greater than a predetermined threshold compared to a value before hydraulic fracturing and a rate of change of the average frequency AF between adjacent moments is lower than a predetermined slope threshold, while the rise time t_r increases by less than a predetermined threshold compared to the value before hydraulic fracturing and the rate of change of the rise time t_r between adjacent moments is lower than the predetermined slope threshold, it is considered to be in a tensile fracture stage.

When the average frequency AF continuously increases and the rate of change of the average frequency AF between adjacent moments is greater than the predetermined slope threshold, while the rise time t_r continuously increases and the rate of change of the rise time t_r between adjacent moments is greater than the predetermined slope threshold, it is considered to be in a main fracture formation and propagation stage.

In the disclosure, through the design of the effective stress control module and the fracturing reinforcement exploitation module working together with the experimental process, both the hydraulic fracturing function and the exploitation function are implemented, and the hydraulic fracturing-exploitation function is thus achieved.

In the disclosure, the high-pressure, low-temperature, and stress environment of the deep-sea hydrate reservoir is superimposed with a hypergravity field, so the in-situ large-scale hydrate reservoir occurrence conditions are accurately restored. Through the developed hydrate reservoir hydraulic fracturing module and the injection fluid switching and reservoir reinforcement module, hydraulic fracturing and reinforcement modification of the hydrate reservoir model under hypergravity conditions are implemented. The developed hydrate reservoir depressurization exploitation module is used to conduct depressurization exploitation on the hydraulically fractured reservoir model. The apparatus is capable of revealing the initiation and development patterns of hydraulic fracturing fractures in a large-scale hydrate reservoir and evaluating the effectiveness of reservoir reinforcement and changes in hydrate exploitation energy efficiency. As such, support for formulating hydrate energy development strategies and research on exploitation process regulation is provided, and the bottleneck problems of low permeability and poor stability in the hydrate energy development process are overcome.

In the disclosure, the hydraulic fracturing exploitation process of deep-sea natural gas hydrate reservoir under in-situ large-scale low-temperature environment, self-weight stress field, and effective reservoir stress is accurately simulated. The key technologies to achieve this function are: precisely controlling the effective stress of deep-sea in-situ hydrate reservoir formations under hypergravity conditions, accurately restoring the true stress level of large-scale reservoirs at the hundred-meter scale, achieving temperature control, uniform-speed injection, and remote switching of different types of fracturing fluids, accurately reproducing in-situ hydraulic fracturing and reservoir modification characteristics, achieving remote control of hydraulic fracturing locations and remote switching between fracturing fluid injection and depressurization exploitation modes for production wells, simulating the in-situ exploitation process, achieving effective monitoring of fracture distribution and morphology during hydraulic fracturing and depressurization exploitation processes, and revealing the hydraulic fracturing response mechanism under original conditions. The aforementioned hypergravity condition refers to the experimental condition where the experimental device is mounted in a centrifuge basket rotating to produce ng hypergravity, which is n times the Earth's gravitational acceleration g.

Beneficial effects provided by the disclosure include the following.

1) The apparatus may be mounted on a geotechnical centrifuge. Through the combined action of the hypergravity field and the effective stress control module of the apparatus, the in-situ large-scale reservoir soil skeleton stress field and the pore water pressure field are reproduced on the model scale. By adjusting the initial temperature and the circulation flow speed of the cooling liquid in the water bath jacket, the in-situ large-scale reservoir geothermal gradient is reproduced. During the experiment, the differences in mechanical response of the solid skeleton and the thermodynamic behavior characteristics of the hydrate at different depths of the reservoir are reproduced.

2) The hydrate reservoir hydraulic fracturing module and the injection fluid switching and reservoir reinforcement module of the apparatus are capable of implementing remote control of switching different types of injection fluids and servo control of flow speed, as well as precise control of injection fluid temperature on the centrifuge. During hydraulic fracturing, the actual hydraulic fracturing points in the production well are autonomously adjusted. After hydraulic fracturing, reservoir chemical modification reinforcement and hydrate depressurization exploitation modeling are conducted. The simulation of in-situ marine hydrate reservoir hydraulic fracturing exploitation process is thus realistic and comprehensive.

3) The model multi-physical field monitoring module of the apparatus, based on being equipped with sensors such as thermocouples, pressure sensors, earth pressure gauges, resistivity probes, and strain gauges, is further equipped with acoustic emission probes that may monitor the initiation and development of reservoir fractures and violent deformation of the reservoir during hydrate exploitation. A sapphire endoscope tube that can observe the fracture expansion process, fracture location, and fracture opening level in real time through an endoscopic camera is also included. Through the above monitoring methods, the development and distribution patterns of hydraulic fractures are accurately revealed, and the particle migration and deformation response of the reservoir during reservoir modification and depressurization exploitation are observed.

4) The hydrate exploitation module of this apparatus may be equipped with vertical wells and horizontal wells as production wells according to experimental requirements. After hydraulic fracturing and reservoir modification are completed, the production well may be switched from the fixed-point injection mode to the depressurization exploitation mode. The pressure inside the production well may be controlled by the combined servo regulation of the plunger pump and the back-pressure valve, and the simulation of depressurization exploitation of the hydrate reservoir model after hydraulic fracturing and chemical reinforcement is thus implemented.

5) By utilizing the principle of scale and time reduction in hypergravity experiments, through the apparatus, the extreme environment of high pressure, low temperature, and high stress in deep-sea natural gas hydrate reservoirs is accurately reproduced. The multi-field and multi-phase interactions between the scaled-down laboratory model and the full-scale field prototype are strictly similar. The hydraulic fracturing response of large-scale hydrate reservoirs and subsequent long-duration exploitation process may be accurately simulated on a model scale through hypergravity experiments. The development pattern of hydraulic fracturing fractures is revealed, and the effect of formation chemical reinforcement and the extent of exploitation production capacity improvement are evaluated. An innovative research method for the industrialized development of deep-sea natural gas hydrate resources is thus provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an overall structure of an apparatus according to an embodiment of the disclosure.

FIG. 2 is a schematic view of an effective stress control principle according to the disclosure.

FIG. 3A and FIG. 3B are schematic views of a production well structure with a hydraulic fracturing function according to an embodiment of the disclosure.

FIG. 4 is a schematic view of hydraulic fracturing exploitation modeling for a horizontal well according to an embodiment of the disclosure.

FIG. 5 is a test result graph of splitting pressure for hydrate-bearing sediments.

FIG. 6 is a flow chart of a hydrate hydraulic fracturing-depressurization exploitation process.

In the figures:

• • 1: high-pressure vessel, 2: temperature control bottom plate, 3: sintered plate, 4: production well, 5: constant temperature water bath, 6: water bath circulation pump, 7: hydrate preparation module, 8: centrifuge rotary joint, 9: centrifuge water/gas pipeline, 10: centrifuge basket, 11: water/gas interface in 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,
  • 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 head, 28: electromagnetic valve group, 29: production pipeline,
  • 30, back-pressure pump, 31: solid separation meter, 32: buffer tank, 33: back-pressure valve,
  • 34: axial pressure liquid layer, 35: gas exhaust pipeline, 36: liquid-gas separation collection and metering module, 37: hydraulic fracturing fracture, 38: chemically-modified reinforcement zone, 39: acoustic emission probe, 40: sapphire endoscope tube, 41: endoscope camera, 42: sensor signal line, 43: model monitoring and data collection module, 44: electromagnetic valve group signal line,
  • 45: production well perforation, 46: sealing ring, 47: top plate, 48: cylindrical connecting rod, and 49: bottom plate.

DESCRIPTION OF THE EMBODIMENTS

The content of the disclosure is further described in detail in combination with accompanying figures and embodiments.

As shown in FIG. 1, a specific embodiment of the disclosure is provided. A hydrate exploitation modeling experimental apparatus includes a high-pressure vessel 1, a water bath temperature control module, an effective stress control module, a fracturing reinforcement exploitation module, a hydrate preparation module 7, and a model multi-physical field monitoring module.

The high-pressure vessel 1 is provided with a hydrate reservoir model 15 and an axial pressure liquid layer 34 inside and is itself entirely placed in a water bath environment 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 communicates with the inside of the high-pressure vessel 1 and applies stress control to the hydrate reservoir model 15. The fracturing reinforcement exploitation module communicates with the inside of the high-pressure vessel 1 and conducts fracturing, reinforcement, and exploitation 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 the water bath environment thereof. The hydrate preparation module 7 communicates with the hydrate reservoir model 15 inside the high-pressure vessel 1.

The apparatus is mounted on a geotechnical centrifuge to conduct hypergravity experiments. When a hypergravity experiment is conducted, as shown in FIG. 1, the high-pressure vessel 1, the effective stress control module, and the fracturing reinforcement exploitation module are all mounted within a centrifuge basket 10 and operate under 1 g to 500 g hypergravity. The hydrate preparation module and a constant temperature water bath and a water bath circulation pump of the water bath temperature control module are placed in a centrifuge power room and work under 1 g normal gravity.

Specifically, when a hypergravity experiment is conducted, a large-scale in-situ reservoir is reduced to a 1/n scale laboratory model as the hydrate reservoir model 15. Through an n-fold hypergravity field, the seepage, deformation, and other processes between the laboratory model scale and the in-situ large-scale reservoir are strictly similar.

Main parameter similarity ratios between the laboratory model and a field prototype are as follows: gravity acceleration: n, length: 1/n, area: 1/n², volume: 1/n³, temperature gradient: n, pressure gradient: n, stress gradient: n, seepage velocity: n, injection flow speed: 1/n, and seepage duration: 1/n².

The disclosure adopts similar particle gradation, and according to in-situ porosity, hydrate saturation, etc., scales down the large-scale marine hydrate reservoir to 1/n times, with physical parameters such as material density, viscosity, specific heat, enthalpy, and intrinsic permeability of sediments being 1:1 similar to those in-situ.

In the in-situ reservoir hydraulic fracturing process, assuming that the fracturing fluid does not lose pressure due to significant loss along the fault, when the injection flow rate of the fracturing fluid per unit time is equal to the seepage loss flow rate into the sediment pores, the horizontal length L_f and total area A_f of the fracture reach their maximum values. At this point, the fracturing fluid filtration relationships in the prototype and model are shown in the following equations, respectively:

Q f = v fl ⁢ 2 ⁢ A f ⁢ and F Q ⁢ Q f = F v ⁢ v fl ⁢ F A ⁢ 2 ⁢ A f ,

where v_fl is the filtration flow speed of the fracturing fluid, A_f is the fracture area, F represents the similarity ratio (model/prototype) of a specific physical quantity κ, where κ=Q, A, and v, Q represents the flow rate, A represents the area, v represents the flow speed, and Q_f represents the injection flow rate of the fracturing fluid.

When the model scale is reduced to 1/n of the in-situ scale, the length scaling factor F_L is 1/n, the area scaling factor F_A is 1/n², and consequently the volume scaling factor F_v may be obtained as 1/n³. The seepage scaling factor F_k and the viscosity scaling factor F_μ are both 1, and the gravitational acceleration scaling factor F_g is n. By comparing equations (1) and (2), it may be known that the seepage scaling factor F_v between the disclosure and the in-situ condition is n, and the scaling factor F_Q of fracturing fluid injection flow rate per unit time is 1/n. The scaling factors between the model experiment of the disclosure and the prototype are summarized in Table 1.

TABLE 1
Scaling factors of hypergravity modeling for Hydraulic Fracturing-Exploitation
in Natural Gas Hydrate Reservoir on Centrifuge

		Scaling factors			Scaling factors
Parameters	Symbol	The model/Field	Parameters	Symbol	The model/Field
Acceleration	a g	n	Specific heat	C.	1
			capacity
Length	L	1/n	Temperature	T	1
Area	A	1/n2	Temperature	∇T	n
			gradient
Volume	V	1/n3	Pressure	p	1
Saturation	S	1	Pressure	∇p	n
			gradient
Viscosity	μ	1	Stress	σ	1
Density	ρ	1	Seepage speed	∇σ	n
Permeability	k	1	Seepage speed	v	n
Conductivity	λ	1	Injection flow	Qf	1/n
coefficient			speed
Enthalpy value	E	1	Time	t	1/n2

The high-pressure vessel may be a cylindrical vessel forged from stainless steel or titanium alloy, and is composed of a vessel body, a vessel top cover, and high-strength bolts. The high-pressure vessel acts as a carrier for the high-pressure, low-temperature, and high-stress physical environment of a deep-sea hydrate reservoir model, and the vessel interior can withstand a certain pressure. The center of the vessel top cover is provided with a production well hole, and the vessel body is equipped with a sensor reserved hole that can also act as a horizontal well opening. The vessel top cover is connected to the vessel body through the high-strength bolts, with a contact area sealed by a sealing ring. The exterior of the high-pressure vessel is equipped with a temperature control bottom plate 2 of the water bath temperature control module and a water bath jacket 14 with a guide plate.

The high-pressure vessel 1 is provided with a loading plate 24 inside. The loading plate 24 divides a space inside the high-pressure vessel 1 into an upper chamber and a lower chamber, the lower chamber is provided with the hydrate reservoir model 15 inside, and the upper chamber is provided with a water-formed axial pressure liquid layer 34 inside. The fracturing reinforcement exploitation module includes a hydrate reservoir hydraulic fracturing module, an injection fluid switching and reservoir reinforcement module, and a hydrate exploitation module. The hydrate reservoir hydraulic fracturing module, the injection fluid switching and reservoir reinforcement module, and the hydrate exploitation module are all installed outside the high-pressure vessel 1 and communicate with the hydrate reservoir model 15. The effective stress control module is installed outside the high-pressure vessel 1 and communicates with the axial pressure liquid layer 34 and is connected to the hydrate exploitation module.

In specific implementation, a sintered plate 3 is first arranged at a bottom portion of the high-pressure vessel 1, and then the hydrate reservoir model 15 is prepared. The sintered plate 3 is made by sintering screens with different pore diameters and may be used to improve the uniformity of water injection during a model preparation stage and prevent sediment particles from clogging injection holes at the bottom portion of the high-pressure vessel 1.

The water bath temperature control module includes a water bath jacket 14, a constant temperature water bath 5, and a water bath circulation pump 6. The high-pressure vessel 1 is placed in the water bath jacket 14 with a guide groove, which contains antifreeze liquid mainly composed of ethylene glycol. The water bath jacket 14, the constant temperature water bath 5, and the water bath circulation pump 6 communicate in series for circulation, with the water bath circulation pump 6 driving antifreeze liquid to circulate within the water bath jacket 14 and the constant temperature water bath 5. The antifreeze liquid is mainly composed of ethylene glycol, with a temperature generally ranging from 3° C. to 10° C. The temperature may be adjusted according to experimental needs by regulating the power of the constant temperature water bath.

The hydrate preparation module 7 is for generating the hydrate reservoir model 15 containing methane gas hydrate and other substances, which may communicate with the high-pressure vessel 1.

The hydrate preparation module 7 further includes a gas exhaust pipeline 35 and a pneumatic valve 19. The high-pressure vessel 1 has a gas exhaust pipeline 35 installed on its side wall. The gas exhaust pipeline 35 communicates with an input end of a solid separation meter 31 through one pneumatic valve 19. An inner end of the gas exhaust pipeline 35 communicates with an upper portion of the hydrate reservoir model 15.

In a specific embodiment of the disclosure, as shown in FIG. 1, the water bath temperature control module consists of the constant temperature water bath 5, the water bath circulation pump 6, the temperature control bottom plate 2, and the water bath jacket 14 with the guide groove. The water bath circulation pump drives a low-temperature cooling liquid to flow through a centrifuge rotary joint 8, a centrifuge pipeline 9, and a centrifuge basket internal interface 11, enter the water bath jacket 14 from a top portion inlet, flow downward along the guide plate, then enter the temperature control bottom plate 2 from the bottom portion, and finally return to the constant temperature water bath 5 through the centrifuge basket internal interface 11, the centrifuge pipeline 9, and the centrifuge rotary joint 8. Temperature control is implemented through continuous circulation of the cooling liquid and its heat exchange with the hydrate reservoir model 15. By adjusting the initial temperature of the cooling liquid in the constant temperature water bath 5 and the flow speed of the water bath circulation pump 6, the temperature of the sediment model 15 may vary with depth, simulating the geothermal gradient.

As shown in FIG. 2, the effective stress control module includes a water storage container 17, an axial pressure pump 12, an injection pipeline 16, a back-pressure pump 30, and a buffer tank 32. An input end of the axial pressure pump 12 communicates with the water storage container 17 containing axial pressure liquid. An output end of the axial pressure pump 12 communicates with the axial pressure liquid layer 34 inside the high-pressure vessel 1 through the injection pipeline 16. The back-pressure pump 30 contains axial pressure liquid, and its output end communicates with an upper portion of the buffer tank 32. The buffer tank 32 is pre-filled with axial pressure liquid, and its lower end is connected to a control end of the back-pressure valve 33 in the hydrate exploitation module.

A relationship among total stress, effective stress, and pore pressure at the top portion of the hydrate reservoir model 15 is shown in the following equation:

σ 0 = σ 0 ′ + p 0 ,

where σ₀,

σ 0 ′ ,

and p₀ represent the total attraction, effective stress, and pore pressure of the hydrate reservoir model 15, respectively.

The axial pressure pump 12 injects a liquid to increase pressure into the axial pressure liquid layer 34 inside the high-pressure vessel 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 back pressure p₀ to the back-pressure valve 33. Herein, if the pore pressure of the hydrate reservoir model 15 inside the high-pressure vessel 1 is higher than p₀, the water in the pores may be produced along a production well 4 and a production pipeline 29 and enter a liquid-gas separation collection and metering module 36 through the back-pressure valve 33, so that the pore pressure of the hydrate reservoir model 15 is eventually reduced to p₀, and effective stress control is thus achieved.

As shown in equations (2) and (3), under the influence of the hypergravity field, the effective stress σ′ and the pore pressure p at depth z of the hydrate reservoir model increase gradually in a depth direction. Under n times gravity acceleration, the amplitudes increase by ngρ′ and ngρ_w respectively, as shown in the following equations:

σ ′ = σ 0 ′ + ng ⁢ ρ ′ ⁢ and p = p 0 + ng ⁢ ρ w ,

where ρ′ and ρ_w represent the submerged density of sediment particles and the density of water, respectively.

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

In the hydrate reservoir hydraulic fracturing module, a prediction model for fracture initiation pressure strength considering the effects of fracturing fluid flow, reservoir effective stress, and hydrate cementation during the hydraulic fracturing process may be expressed by the above equation.

The hydrate reservoir hydraulic fracturing module includes a piston temperature control container, an advection pump 18, the water storage container 17, a single-degree-of-freedom loading device 25, an injection pipeline 26, and a specifically designed injection end head 27. An input end of the advection pump 18 communicates with the water storage container 17, and an output end of the advection pump 18 communicates with a lower end of the piston temperature control container through the pneumatic valve 19. The piston temperature control container is provided with fracturing fluid inside in advance, and an upper end of the piston temperature control container communicates with an upper end of the injection pipeline 26 through a flexible hose. The injection pipeline 26 is vertically arranged, and a lower end of the injection pipeline 26 is inserted into the production well 4 pre-embedded in the hydrate reservoir model 15 and is installed with the specially designed injection end head 27. A production well perforation 45 corresponding to the injection end head 27 is arranged on a side wall of the production well 4.

An upper portion of the injection pipeline 26 is installed on a loading arm of the single-degree-of-freedom loading device 25, and the single-degree-of-freedom loading device 25 may be installed on the high-pressure vessel 1 and drives the injection pipeline 26 to move up and down.

In a specific implementation, multiple production well perforations 45 are provided and are arranged in multiple circles in an axial direction of the production well 4 at intervals. Each circle of production well perforations includes multiple production well perforations 45 arranged at intervals in a circumferential direction.

As shown in FIG. 3A, the injection end head 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, and a through hole communicating with the lower end of the injection pipeline 26 is formed in a middle of the top plate 47. The top plate 47 and the bottom plate 49 are fixedly connected by four cylindrical connecting rods 48, and the top plate 47 and a periphery of the bottom plate 49 are sealed and connected to an inner wall of the production well 4 through the sealing ring 46. The bottom plate 49 is an intact plate without any through hole.

The liquid entering the injection pipeline 26 flows through the through hole in the top plate 47 of the injection end head 27 into the space between the top plate 47 and the bottom plate 49, and due to the effect of the sealing ring 46, it flows out from the production well perforation 45 between the top plate 47 and the bottom plate 49 to form hydraulic fracturing.

The injection end head 27 is used to limit the discharge position of the fracturing fluid and implement targeted fracturing of the sediment model, is connected to the inner wall of the production well 4 through a sealed connection, and is driven by the single-degree-of-freedom loading device 25 to move up and down within the production well 4 to change its position, so that the hydraulic fracturing point is changed.

The variation of injection pressure with time during the hydraulic fracturing process and its key points are shown in FIG. 5.

The injection fluid switching and reservoir reinforcement module includes two piston temperature control containers, the advection pump 18, the water storage container 17, the single-degree-of-freedom loading device 25, the injection pipeline 26, and the specially designed injection end head 27. The injection fluid switching and reservoir reinforcement module shares the advection pump 18, the water storage container 17, the single-degree-of-freedom loading device 25, the injection pipeline 26, and the injection end head 27 with 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 to the piston temperature control container of the hydrate reservoir hydraulic fracturing module, that is, connected between the output end of the advection pump 18 and the upper end of the injection pipeline 26. The two piston temperature control containers are pre-filled with a gel breaker and a reinforcing agent.

The two piston temperature control containers of the injection fluid switching and reservoir reinforcement module are also the same, where the piston container 21 inside the piston temperature control container has a piston plate, and the upper chamber above the piston plate may contain gel breaker/reinforcing agent.

Main components of the reinforcing agent are calcium oxide, calcium silicate, and calcium sulfate slurry prepared in a specific proportion, with a molar ratio of the calcium oxide, calcium silicate, and calcium sulfate being 5:2:1. In the disclosure, through the innovatively-designed reinforcing agent raw material components and their proportional relationship, the reinforcing agent may be prevented from clogging the injection pipeline 26 and the injection end head 27.

In specific implementation, the hydrate reservoir hydraulic fracturing module, the injection fluid switching and reservoir reinforcement module, and the effective stress control module may share the same water storage container 17.

The piston temperature control container is mainly formed by one piston container 21, the pneumatic valve 19, and a semiconductor chip 20. The piston container 21 communicates with the output end of the advection pump 18 and the upper end of the injection pipeline 26, and the semiconductor chip 20 is installed on the piston container 21.

A piston plate is inside the piston container 21, and the piston plate divides the piston container 21 into upper and lower chambers. The lower chamber communicates with the output end of the advection pump 18 through the pneumatic valve 19, a required substance such as fracturing fluid/gel breaker/reinforcing agent is pre-added in the upper chamber, and the upper chamber communicates with the upper end of the injection pipeline 26.

A piston container of the piston temperature control container in the hydrate reservoir hydraulic fracturing module is a piston container I21, and piston containers of the two piston temperature control containers in the injection fluid switching and reservoir reinforcement module are piston container II22 and piston container III23.

The upper chamber above the piston plate in the piston container I21 is pre-filled with fracturing fluid, and the temperature of the fracturing fluid in the piston container I21 is controlled by the corresponding semiconductor chip 20 of its own piston temperature control container. By changing the direction and value of the electric current in the semiconductor chip, heating/cooling and precise temperature control of the liquid in the container may be achieved.

In the injection fluid switching and reservoir reinforcement module, gel breaker and reinforcing agent are pre-added into the piston container II 22 and the piston container III 23 of the two piston temperature control containers, 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 electric current in the semiconductor chip, heating/cooling and precise temperature control of the liquid in the container may be achieved.

During reservoir modification, the opening and closing of the pneumatic valves 19 of a total of three piston temperature control containers in the injection fluid switching and reservoir reinforcement module and the hydrate reservoir hydraulic fracturing module are controlled by remotely adjusting and controlling an electromagnetic valve group 28, and remote switching of the injection fluid is thus implemented.

The hydrate exploitation module includes the production pipeline 29, the solid separation meter 31, the back-pressure valve 33, and the liquid-gas separation collection and metering module 36. An input end of the solid separation meter 31 communicates with a top portion of the production well 4 through the production pipeline 29, an output end of the solid separation meter 31 communicates with an inlet of the back-pressure valve 33, and an outlet of the back-pressure valve 33 communicates with the liquid-gas separation collection and metering module 36.

The solid separation meter 31 is used to separate solids from the incoming fluid and measure them, and the liquid-gas separation collection and metering module 36 is used to separate the incoming liquid and gas and measure them separately. The back-pressure valve 33 is used for opening, closing, and controlling the opening degree of the production pipeline 29.

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

The model multi-physical field monitoring module includes a sensor 13 installed on the high-pressure vessel 1, an acoustic emission probe 39 and a sapphire endoscope tube 40 on an inner wall of the high-pressure vessel 1, and a model monitoring and data collection module 43. The sensor 13 includes a thermocouple, a pressure sensor, an earth pressure gauge, a resistivity probe, and a strain gauge, etc. The acoustic emission probe 39 is horizontally arranged towards the production well 4. The sapphire endoscope tube 40 is vertically inserted into the hydrate reservoir model 15 and parallel to the production well 4, and an endoscope camera 41 is installed inside the sapphire endoscope tube 40. The sensor 13, the acoustic emission probe 39, and the endoscope camera 41 are all connected via sensor signal lines 42 to the model monitoring and data collection module 43 outside the centrifuge basket 10 for communication.

The injection fluid switching and reservoir reinforcement module and one pneumatic valve 19 connected to a pneumatic valve 19/an input end of the solid separation meter 31 in the hydrate preparation module in the piston temperature control container of the hydrate reservoir hydraulic fracturing module are both connected to the electromagnetic valve group 28. The electromagnetic valve group 28 is communicatively connected via an electromagnetic valve group signal line 44 to the external model monitoring and data collection module 43. The model monitoring and data collection module 43 remotely controls the electromagnetic valve group 28 to further control the opening and closing of each pneumatic valve 19, so that remote switching of injection fluid and remote switching of exhaust opening and closing are implemented.

The side wall of the high-pressure vessel 1 is provided with a sensor ear hole that may also serve as a production well wellhead used to arrange a horizontal well and simulate hydraulic fracturing, depressurization, and exploitation of the horizontal well of the hydrate reservoir. When the horizontal well is adopted, the contact range between the production well and the reservoir is greater, so that the depressurization range and the depressurization efficiency of the reservoir are improved. Since hydraulic fracturing fractures develop in a direction perpendicular to a minor principal stress plane of the reservoir, the use of the horizontal well for hydraulic fracturing can make the injection direction of the fracturing fluid consistent with the development direction of the fractures, and a greater hydraulic fracturing range and reservoir modification effect are thus achieved.

As shown in FIG. 6, a physical modeling experimental process for natural gas hydrate exploitation by hydraulic fracturing provided by the disclosure is as follows.

S1: Hydrate Formation Process

The hydrate preparation module 7 generates a methane hydrate in the high-pressure vessel 1 to obtain the hydrate reservoir model 15.

S2: Effective Stress Control Process

As shown in FIG. 2, the axial pressure pump 12 injects liquid from the axial pressure liquid injection pipeline 16 in the water storage container 17 into the axial pressure liquid layer 34 inside the high-pressure vessel 1 to increase pressure. The loading plate 24 transmits the pressure of the axial pressure liquid layer 34 to the hydrate reservoir model 15 and converts the overlying total stress σ₀ applied by the axial pressure pump 12 into a total stress of the hydrate reservoir model 15.

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 opening, closing, and an opening level of a valve core, so that the pressure of the back-pressure valve 33 is allowed to be the backpressure ρ₀, and that the back-pressure valve 33 communicates with the hydrate reservoir model 15 via the production pipeline 29. At this point, the pore pressure of the hydrate reservoir model 15 is finally reduced to the same as the backpressure p₀, and effective stress control is thus achieved.

The above-mentioned stress and pore pressures both refer to the pressure of the liquid.

S3: Hydraulic Fracturing Process

The single-degree-of-freedom loading device 25 drives the injection pipeline 26 and the injection end head 27 to move downward to a position below where the production pipeline 29 communicates with to the production well 4 and within the production well 4 in the hydrate reservoir model 15 before reservoir hydraulic fracturing is performed.

When reservoir hydraulic fracturing is performed in the hydrate reservoir hydraulic fracturing module, only the pneumatic valve 19 corresponding to the piston container I 21 is opened, while the pneumatic valves 19 corresponding to the other two piston container II 22 and piston container III 23 are closed. The advection pump 18 is remotely controlled to draw liquid from the water storage container 17, and water is injected at a constant flow speed 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 flows at a constant flow speed through the injection pipeline 26 to reach the injection end head 27, flows out from the injection end head 27 into the production well 4, and is ejected out from the production well perforation 45 to enter the hydrate reservoir model 15 to form a hydraulic fracturing fracture 37 to implement hydraulic fracturing.

The hydraulic fracturing fracture 37 is divided into a horizontal main fracture and a branch fracture branching from the main fracture. The prediction model for fracture initiation pressure strength considering the effects of fracturing fluid flow, reservoir effective stress, and hydrate cementation during the hydraulic fracturing process may be expressed by the equation as follows.

p f = a f ⁢ v f ⁢ μ + K 0 ⁢ tan 2 ( π / 4 + ψ / 2 ) - 1 tan 2 ( π / 4 + ψ / 2 ) - 1 ⁢ ρ ′ ⁢ ngz + 2 ⁢ N p ⁢ c h ⁢ tan ⁡ ( π / 4 + ψ / 2 ) tan 2 ( π / 4 + ψ / 2 ) - 1 ,

where ρ_f represents the fracture pressure strength of hydraulic fracturing, μ and v_f represent a viscosity and a flow speed of the fracturing fluid, respectively, g is the gravitational acceleration, n is a multiple of gravitational acceleration, K₀ and ψ represent a sediment lateral pressure coefficient and an internal friction angle, respectively, ρ′ is a sediment buoyant density, z represents a reservoir burial depth, N_p is a pore characteristic constant related to hydrate saturation, c_h is a cohesion of hydrate-bearing sediment, and a_f is a constant parameter related to permeability of hydrate-bearing sediment and the flow conductivity of splitting fracture.

S4: Reservoir Reinforcement Process

After hydraulic fracturing is completed, only the pneumatic valve 19 corresponding to a piston container I22 is opened, while the pneumatic valves 19 corresponding to the other two piston containers I21 and the piston container II23 are closed. The advection pump 18 is remotely controlled to draw liquid from the water storage container 17. Water is injected at a constant flow speed to push the piston plate inside the piston container II22, so that the gel breaker in an upper chamber of piston container II22 flows at a constant flow speed through the injection pipeline 26 to reach the injection end head 27, flows out from the injection end head 27 into the production well 4, flows out from the production well perforation 45 to enter a crack of the formed hydraulic fracturing fracture 37. In this way, the viscosity of the fracturing fluid in the fracture is lowered to facilitate subsequent reservoir modification and hydrate exploitation.

Subsequently, only the pneumatic valve 19 corresponding to a piston container II23 is opened instead, while the pneumatic valves 19 corresponding to the other two piston containers I21 and the piston container II22 are closed. The advection pump 18 is remotely controlled to draw liquid from the water storage container 17. Water is injected at a constant flow speed to push the piston plate inside the piston container III23, so that the reinforcing agent in an upper chamber of the piston container II23 flows at a constant flow speed through the injection pipeline 26 to reach the injection end head 27, and flows out from the injection end head 27 into the production well 4, flows out of the production well perforation 45 to enter the crack of the hydraulic fracturing fracture 37 that is injected with the gel breaker, so as to support and reinforce the crack and form a chemically-modified reinforcement zone 38.

After the reinforcing agent is injected, a continuous porous surface structure supporting fracture surface is formed under the action of water and salt in the pore water of the hydrate reservoir model 15 for several hours of hydration, and long-term effects of increasing permeability, reinforcement, and sand control are thus achieved.

In specific implementation, the single-degree-of-freedom loading device 25 may drive the injection pipeline 26 and the injection end head 27 to move to different positions of the production well 4 located in the hydrate reservoir model 15, so as to conduct hydraulic fracturing and reservoir reinforcement experiments on the hydrate reservoir model 15 at different heights.

The chemical reinforcement reaction equations for the reinforcing agent are as follows:

a 1 ⁢ CaO · SiO 2 + a 2 ⁢ H 2 ⁢ O → a 3 ⁢ CaO · SiO 2 · a 4 ⁢ H 2 ⁢ O + a 5 ⁢ Ca ( OH ) 2 CaSO 4 + 2 ⁢ H 2 ⁢ O → Ca ( OH ) 2 + H 2 ⁢ S + O 2 , and CaO + H 2 ⁢ O → Ca ( OH ) 2 ,

where a₁ to a₅ represent the proportions of various substances.

S5: Hydrate Exploitation Process

During the hydrate exploitation process, the single-degree-of-freedom loading device 25 drives the injection pipeline 26 and the injection end head 27 to move upward to a position above where the production pipeline 29 communicates with to the production well 4 and not within the production well 4 in the hydrate reservoir model 15, so that the production pipeline 29 may smoothly communicate with the production well 4 in the chemically-modified reinforcement zone 38.

The back-pressure pump 30 and the buffer tank 32 then work together to control the opening level of the back-pressure valve 33. Hydrate in the hydrate reservoir model 15 enters the production pipeline 29 through the production well 4, so that only water and methane gas flowing through the back-pressure valve 33 enters the liquid-gas separation collection and metering module 36 for liquid-gas separation and metering after the solid is separated by the solid separation meter 31, and the experiment is then completed.

In a specific embodiment of the disclosure, as shown in FIG. 1 and FIG. 3B, in the hydrate exploitation module, after the reservoir reinforcement and modification are completed, the injection end head 27 is raised to the top portion of the production well through the single-degree-of-freedom loading device 25, at which time the production well 4 communicates with the production pipeline 29. The back pressure on the valve core of the back-pressure valve is uniformly reduced through plunger pump servo suction/discharge, and servo control of the pressure in the production well is thus achieved. When the pore pressure of the hydrate reservoir model 15 is reduced below a hydrate phase equilibrium pressure, the hydrate decomposes into methane gas and water and is quickly produced along the production well 4 through the fracturing fracture 37. After the pressure in the production well is reduced to a target exploitation value, it is kept constant until the hydrate in the pores of the hydrate reservoir model 15 is completely decomposed. After the experiment is completed, the centrifugal acceleration is gradually reduced to 1 g to end the experiment.

S6: Monitoring Process

The sensor 13, the acoustic emission probe 39, and the sapphire endoscope tube 40 in the model multi-physical field monitoring module are then used to collect and send data to the model monitoring and data collection module 43. The model monitoring and data collection module 43 then conducts data analysis to obtain a hydraulic fracturing-exploitation condition.

The abovementioned sediment is a soil skeleton in the hydrate reservoir model 15.

Sensors including a thermocouple, a pressure sensor, an earth pressure gauge, a resistivity probe, and a strain gauge are pre-embedded at different positions in the reservoir during the preparation process of the hydrate reservoir soil skeleton, which are respectively used to monitor the temperature, pore pressure, total stress, resistivity, and reservoir deformation during hydrate formation, hydraulic fracturing, reservoir reinforcement, and hydrate exploitation.

In the hydrate formation process, the effective stress control process, the hydraulic fracturing process, the reservoir reinforcement process by injecting the gel breaker and the reinforcing agent, and the subsequent hydrate exploitation process, real-time monitoring of the temperature, pore pressure, total stress, resistivity, and reservoir deformation is performed through sensors including a thermocouple, a pressure sensor, an earth pressure gauge, a resistivity probe, and a strain gauge, respectively during hydrate formation, hydraulic fracturing, reservoir reinforcement, and hydrate exploitation.

In the hydraulic fracturing process, real-time monitoring of fracture splitting strength and a splitting location of the hydraulic fracture 37 inside the hydrate reservoir model 15 is performed through the acoustic emission probe 39, and an actual fracture propagation process, a fracture location, a fracture opening level, 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.

In specific implementation, the actual fracture propagation process, the fracture location, and the fracture opening level observed in real-time by the endoscopic camera 41 inside the sapphire endoscope tube 40 may be used to calibrate and verify an acoustic monitoring result and to invert internal fracture development and distribution patterns.

During the process of hydrate reservoir modification and depressurization exploitation, changes in acoustic emission signals may also be used to assist in determining whether the hydrate reservoir model 15 undergoes severe deformation. The particle migration and deformation response of the hydrate reservoir model 15 may be observed through the sapphire endoscope tube 40.

In the hydraulic fracturing process, acoustic signal data collected by the acoustic emission probe 39 is processed according to the following equations to obtain a rise time t_r and an average frequency AF of an acoustic signal:

t r = t d / A e ⁢ and AF = NF / t f ,

where the rise time t_r is a ratio of a delay t_d from a moment the acoustic signal starts to a maximum amplitude thereof to an amplitude A_e, and the average frequency AF is a ratio of the number of effective fracture acoustic signals NF to total duration of hydraulic fracturing t_f

After the hydraulic fracturing begins, determination is made based on the real-time obtained rise time t_r and the average frequency AF to determine different stages of hydraulic fracturing.

When the average frequency AF increases by greater than a predetermined threshold compared to a value before hydraulic fracturing and a rate of change of the average frequency AF between adjacent moments is lower than a predetermined slope threshold, while the rise time t_r increases by less than a predetermined threshold compared to the value before hydraulic fracturing and the rate of change of the rise time t_r between adjacent moments is lower than the predetermined slope threshold, it is considered to be in a tensile fracture stage.

When the average frequency AF continuously increases and the rate of change of the average frequency AF between adjacent moments is greater than the predetermined slope threshold, while the rise time t_r continuously increases and the rate of change of the rise time t_r between adjacent moments is greater than the predetermined slope threshold, it is considered to be in a main fracture formation and propagation stage.

Through the experiments, it is discovered that in the initial stage of tensile fracture formation, a relatively high average frequency AF and a relatively low rise time t_r may be monitored. When the main fracture forms and propagates, the average frequency AF rapidly decreases, and the rise time t_r significantly increases.

According to a specific embodiment of the disclosure, as shown in FIG. 4, the sensor ear hole on the side wall of the high-pressure vessel 1 may also serve as the production well wellhead used to arrange the horizontal well and simulate hydraulic fracturing, depressurization, and exploitation of the horizontal well of the hydrate reservoir. When the horizontal well is adopted, the contact range between the production well and the reservoir is greater, so that the depressurization range and the depressurization efficiency of the reservoir are improved. Since hydraulic fracturing fractures develop in a direction perpendicular to a minor principal stress plane of the reservoir, the use of the horizontal well for hydraulic fracturing can make the injection direction of the fracturing fluid consistent with the development direction of the fractures, and a greater hydraulic fracturing range and reservoir modification effect are thus achieved.

Accordingly, the disclosure is mounted on a geotechnical centrifuge and is capable of servo-regulating the high-pressure, low-temperature, high-stress, or formation stress environments of the hydrate reservoir to simulate in-situ storage environment of the hydrate reservoir.

Steps S3 to S5 and their cycle may also serve as an in-situ hydrate reservoir exploitation process.

In the disclosure, the temperature, type, flow speed, and injection position of different injection fluids may be accurately controlled. After hydraulic fracturing, fluid may be remotely injected instead to allow the gel breaker and the reinforcing agent to be injected, so that the blocking effect produced by high-viscosity fracturing fluid in fractures may be lowered, the fractures may be supported and reinforced, and simulation of depressurization and exploitation of hydrate vertical wells or horizontal wells may be further carried out.

In the disclosure, the reservoir fracture initiation location, fracture morphology, fracture development process, and reservoir particle migration and deformation response during hydrate depressurization and exploitation may be monitored through the acoustic emission probe and the sapphire endoscope tube.

In the disclosure, the simulation of large-scale hydrate reservoir hydraulic fracturing modification characteristics and long-duration hydrate exploitation gas production process are implemented through hypergravity experiments at the model scale. The development pattern of hydraulic fracturing fractures is revealed, and the effect of formation chemical reinforcement and the extent of exploitation production capacity improvement are evaluated. An innovative research method for the industrialized development of deep-sea natural gas hydrate resources is thus provided.

This specification only lists the preferred embodiments of the disclosure. Any equivalent technical exchanges made under the working principle and concept of the disclosure are considered to be within the protection scope of the disclosure.