Shale volatile oil reservoir phase change experimental device and method based on online nuclear magnetism

Abstract

The invention discloses a shale volatile oil reservoir phase change experimental device and method based on online nuclear magnetism, and belongs to the technical field of shale oil and gas exploitation, the device comprises a nuclear magnetism equipment analyzer, an intermediate container with a piston, a constant-speed constant-pressure pump, a core holder special for nuclear magnetism and an oil-gas separation metering device; two-dimensional nuclear magnetic resonance is used for testing T1-T2 maps of the shale volatile oil reservoir in different states and at different pressure points in the depressurization exploitation process, and the change of oil and gas signals in the T1-T2 maps in the saturated oil state and in the depletion exploitation process under each pressure is compared and analyzed. The oil-gas phase state change characteristics in the nanopores in the depletion exploitation process of the shale volatile oil reservoir are further known, and exploration and development of the shale volatile oil reservoir are effectively guided.

Description

Technical Field

[0001] The invention belongs to the technical field of shale oil and gas exploitation, and in particular relates to an experimental device and method for phase change of shale volatile oil reservoir based on online nuclear magnetic resonance. Background Art

[0002] Shale oil reservoirs are rich in reserves, widely distributed, and have great development potential, but they are difficult to develop and have a low degree of exploitation. The main reason is that the pore structure of shale is complex, with pore sizes ranging from nanometers to micrometers, and even micro-millimeter microcracks. Nanopores are the main component of shale, and the nanoconfinement effect is significant. The deviation phenomena caused include changes in saturation point and increased capillary force, resulting in different physical properties of fluids in the bulk phase and nanopores. PVT (pressure-volume-temperature) technology has been very mature in the study of bulk phase state, but there is no conclusion on the phase state changes in real reservoir nanopores. Shale volatile oil reservoirs have the characteristics of high gas-oil ratio, light oil and heavy gas, high content of intermediate components, and sufficient elastic energy. The phase state changes are complex during the development process. In the depletion development stage, when the reservoir pressure is higher than the bubble point pressure, the production gas-oil ratio is stable and the oil production is relatively stable. When the reservoir pressure is lower than the bubble point pressure, the shale volatile oil reservoir will volatilize a large amount of light hydrocarbon components, generate bubbles, and the production gas-oil ratio increases sharply, and the crude oil shrinks sharply. If the pressure is continued to be reduced after the bubble point appears, the Jamin effect will occur, reducing the degree of oil and gas recovery. In-depth research on the phase change characteristics of shale volatile oil reservoirs during the depletion of microscopic pore structure can effectively guide the exploration and development of shale volatile oil reservoirs.

[0003] The conventional oil and gas phase state test method is mainly PVT testing, which mainly includes equal component expansion and constant volume depletion experiments. At the formation temperature, the fluid sample is injected into the PVT cylinder and pressurized to the saturation pressure to make it in a single phase. The volume is measured by step-by-step pressure reduction above the bubble point pressure, and the PV curve is drawn to obtain the bubble point pressure. This type of test is carried out without porous media conditions and provides characteristic parameters of phase state changes in the bulk phase of the reservoir during the depletion process. When the reservoir rock is micro-nano pores, the flow of shale volatile oil reservoirs in actual reservoirs is affected by the pore storage space, and the nano-confinement effect is significant. The nanoscale limits the freedom of movement of molecules, and the phase state characteristics in porous media are different from the PVT test results. Therefore, the PVT test method is not suitable for studying the phase change characteristics of oil and gas in shale reservoirs with developed micro-nano pores.

[0004] X-ray CT scanning technology can obtain the complete core structure without damage. By testing the attenuation coefficient of the material to X-rays, the composition and density of the material can be determined. The density of oil and gas components varies greatly, and the difference in X-ray attenuation coefficients can be used to distinguish oil and gas. X-ray CT scanning technology can obtain three-dimensional visualization of the phase changes of fluids in the pore space inside the real core. This method has been applied to the study of the phase changes of condensate gas. At present, nano CT scanning technology is expensive and has high technical requirements. The higher the scanning accuracy, the more limited the scanning area is, and it is not universal. The most commonly used micron CT scanning technology has a maximum scanning accuracy of 0.7μm, which cannot identify phase changes in shale nanopores.

[0005] Micro-nanofluidics is a chip with micron or nano-scale pores and channels that uses an image acquisition system to visually detect and operate the fluid state in micro-nano pores and channels. The two-dimensional pore and channel structure of micro-nanofluidics chips is too idealized compared to the three-dimensional porous media pore structure in real reservoirs. The capillary effect, surface tension, and interaction between fluids in the pores are different, and there are certain differences in the phase change law in real shale nanopores.

[0006] In summary, the existing oil and gas phase research methods are not suitable for studying the fluid phase in complex pore structures such as shale volatile oil reservoirs. In order to clarify the phase change characteristics during the development of shale volatile oil reservoirs, the present invention proposes a shale volatile oil reservoir phase change experimental device and method based on online nuclear magnetic resonance. Summary of the invention

[0007] In view of the above problems existing in the prior art, the present invention proposes an experimental device and method for phase change of shale volatile oil reservoir based on online nuclear magnetic resonance, which has a reasonable design, solves the shortcomings of the prior art and has good effects.

[0008] An experimental device for phase change of shale volatile oil reservoir based on online nuclear magnetic resonance, comprising a nuclear magnetic equipment analyzer, three intermediate containers with pistons, three constant speed and constant pressure pumps, a nuclear magnetic core holder and an oil and gas separation metering device; The output end of the first constant speed and constant pressure pump is connected to the input ends of the first intermediate container and the second intermediate container respectively through a high-pressure pipeline via a three-way valve, and the output end of the second constant speed and constant pressure pump is connected to the input end of the third intermediate container through a high-pressure pipeline; the output ends of the first and second intermediate containers are connected to the inlet end of the nuclear magnetic core clamp through a high-pressure pipeline via a first six-way valve, and the output end of the third intermediate container is connected to the upper confining pressure inlet end of the nuclear magnetic core clamp through a high-pressure pipeline via a second six-way valve, and the high-pressure pipelines from the output ends of the three intermediate containers to the nuclear magnetic core clamp are wrapped with electric heating belts.

[0009] Furthermore, each of the three six-way valves is connected to a pressure gauge for displaying the pressure; The outlet end of the nuclear magnetic core holder is connected to the third constant speed and pressure pump and the oil and gas separation metering device through a high-pressure pipeline via a back pressure valve and a third six-way valve, and switches are connected at the inlet and outlet ends of the nuclear magnetic core holder.

[0010] Furthermore, the core is installed inside the nuclear magnetic resonance core holder, and after positioning, it is placed within the magnet coverage range of the nuclear magnetic resonance equipment analyzer, and the heating system uses electric heating to heat the nuclear magnetic resonance core holder.

[0011] An experimental method for phase change of shale volatile oil reservoir based on online nuclear magnetic resonance, using the experimental device as described above, comprises the following steps: Step 1: Select shale with a porosity greater than 2.5% and a permeability of 0.05-0.5mD, and wash the core with a mixed solvent of dichloromethane and methanol. The volume ratio of dichloromethane to methanol is 93:7. After washing the oil, put the shale into an oven and keep it at 120°C for drying; Step 2: Use a sample preparation instrument to prepare volatile oil at actual reservoir pressure and temperature, use an electric heating jacket to heat the first intermediate container to the formation temperature, and transfer the prepared live oil from the sample preparation cylinder to the first intermediate container at constant temperature and pressure; Step 3: injecting methane gas into a second intermediate container with a piston, and using a first constant speed constant pressure pump to increase the pressure to 15-20 MPa lower than the reservoir pressure; Step 4: Take out the dried core and weigh it, use a polytetrafluoroethylene tube to burn a heat shrink sleeve to wrap the core, put it into a nuclear magnetic resonance core holder, and test the T1-T2 spectrum of the initial dry core sample; Step 5: adding fluorinated liquid into the third intermediate container, and using the second constant speed and constant pressure pump to displace the fluorinated liquid in the third intermediate container to control the confining pressure; Step 6: Use the second constant speed and constant pressure pump to displace the fluorinated liquid in the third intermediate container, and inject it into the confining pressure inlet of the nuclear magnetic core holder at a constant pressure of 3 MPa to establish the initial confining pressure; turn on the heating system of the nuclear magnetic core holder, set the heating temperature to the formation temperature and keep it stable; Step 7: Use the first constant speed and constant pressure pump to displace the methane gas in the second intermediate container and inject it into the core at a constant pressure until the gas is discharged from the outlet to establish the system pressure. At the same time, use the second and third constant speed and constant pressure pumps to control the confining pressure and back pressure, and test the T1-T2 spectrum under the saturated methane state; Step 8: Use the first constant speed and constant pressure pump to displace the live oil in the first intermediate container and inject it into the core at a constant pressure to saturate the core, and simultaneously use the second constant speed and constant pressure pump to increase the confining pressure; when oil is seen at the outlet, use the third constant speed and constant pressure pump to increase the back pressure, and after the oil output at the outlet is stable, close the switches at the outlet and inlet of the nuclear magnetic core holder; let the core stand and age for 24 hours, and test the T1-T2 spectrum under the saturated oil state; Step 9: Turn on the outlet switch of the nuclear magnetic core holder, control the third constant speed and constant pressure pump to perform depletion mining in constant pressure mode, and perform pressure reduction mining with the formation pressure as the starting pressure according to the depletion speed. After each pressure drops to a stable level, test the T1-T2 spectrum under the pressure; Step 10: The bubble point pressure is obtained by comparing and analyzing the changes in the oil phase and gas phase area signals in the T1-T2 spectrum under different depletion pressures. The two-dimensional spectrum during the pressure drop process is compared with the two-dimensional spectrum under the saturated oil state. The changes in the oil phase area signal indicate the oil utilization. The pressure when the gas phase signal first appears in the spectrum is the bubble point pressure. At this pressure, the light component liquid hydrocarbons begin to vaporize to form bubbles.

[0012] Beneficial technical effects brought by the present invention: The present invention provides a shale volatile oil reservoir phase change experimental device and method based on online nuclear magnetic resonance. Based on the online nuclear magnetic resonance experiment, two-dimensional nuclear magnetic resonance is used to test the T1-T2 spectrum of shale volatile oil reservoirs in different states and at different pressure points during depressurization production, and the changes in oil and gas signals in the T1-T2 spectrum under saturated oil state and at various pressures during depletion production are compared and analyzed: the change in oil signal represents the use of oil, the pressure at which the gas signal appears represents the bubble point pressure (under this pressure, oil and gas phase change separation occurs, from single phase to two phases), and the signal amount represents the amount of produced gas. By using this technology, the characteristics of oil and gas phase change in nanopores during depletion production of shale volatile oil reservoirs can be further understood under conditions that are more in line with actual oil field field development, effectively guiding the exploration and development of shale volatile oil reservoirs. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 It is a schematic diagram of the experimental device for phase change of shale volatile oil reservoir based on online nuclear magnetic resonance in the present invention.

[0014] Figure 2 The figure is a flow chart of the experimental method for phase change of shale volatile oil reservoir based on online nuclear magnetic resonance in the present invention.

[0015] Figure 3 This is the T1-T2 spectrum of the initial dry core sample in Example 1; Among them, (a) is the T1 cumulative spectrum, (b) is the T1-T2 image, and (c) is the T2 cumulative spectrum.

[0016] Figure 4This is the T1-T2 spectrum under the saturated methane state tested in Example 1; Among them, (a) is the T1 cumulative spectrum, (b) is the T1-T2 image, and (c) is the T2 cumulative spectrum.

[0017] Figure 5 This is the T1-T2 spectrum of shale in the oil-saturated state in Example 1; Among them, (a) is the T1 cumulative spectrum, (b) is the T1-T2 image, and (c) is the T2 cumulative spectrum.

[0018] Among them, 1 is the first constant speed and constant pressure pump; 2 is the three-way valve; 3 is the first intermediate container; 4 is the second intermediate container; 5 is the first pressure gauge; 6 is the first six-way valve; 7 is the second pressure gauge; 8 is the second six-way valve; 9 is the third intermediate container; 10 is the second constant speed and constant pressure pump; 11 is the third pressure gauge; 12 is the third six-way valve; 13 is the third constant speed and constant pressure pump; 14 is the gas flow meter; 15 is the oil and gas separation metering device; 16 is the back pressure valve; 17 is the nuclear magnetic equipment analyzer; 18 is the heating system; 19 is the nuclear magnetic core holder. DETAILED DESCRIPTION

[0019] The specific implementation of the present invention is further described below in conjunction with specific embodiments: The phase change experimental device of shale volatile oil reservoir based on online NMR includes NMR equipment analyzer, three intermediate containers with pistons, three constant speed and pressure pumps, NMR special core holder, and oil and gas separation metering device; like Figure 1 As shown, the output end of the first constant speed and constant pressure pump 1 is connected to the input ends of the first intermediate container 3 and the second intermediate container 4 respectively through a high-pressure pipeline via a three-way valve 2, and the output end of the second constant speed and constant pressure pump 10 is connected to the input end of the third intermediate container 9 through a high-pressure pipeline; the output ends of the first and second intermediate containers are connected to the inlet end of the nuclear magnetic core clamp 19 through a high-pressure pipeline via a first six-way valve 6, and the output end of the third intermediate container 9 is connected to the upper confining pressure inlet end of the nuclear magnetic core clamp 19 through a high-pressure pipeline via a second six-way valve 8, and the high-pressure pipelines from the output ends of the three intermediate containers to the nuclear magnetic core clamp 19 are wrapped with electric heating belts.

[0020] The three six-way valves are respectively connected to a first pressure gauge 5, a second pressure gauge 7 and a third pressure gauge 11 for displaying the pressure; The outlet end of the nuclear magnetic core holder 19 is connected to the third constant speed and pressure pump 13 and the oil and gas separation metering device 15 through a high-pressure pipeline via a back pressure valve 16 and a third six-way valve 12. The oil and gas separation metering device 15 and the gas flow meter 14 are integrated. Switches are connected at the inlet and outlet ends of the nuclear magnetic core holder 19.

[0021] The core holder 19 for nuclear magnetic resonance has a core inside, which is positioned and placed within the magnet coverage of the nuclear magnetic resonance analyzer 17. The heating system 18 heats the core holder for nuclear magnetic resonance by electric heating.

[0022] An experimental method for phase change of shale volatile oil reservoir based on online nuclear magnetic resonance, using the experimental device as described above, such as Figure 2 As shown, the following steps are included: Step 1: Select shale with a porosity greater than 2.5% and a permeability of 0.05-0.5mD, and wash the core with a mixed solvent of dichloromethane and methanol. The volume ratio of dichloromethane to methanol is 93:7. After washing the oil, put the shale into an oven and keep it at 120°C for drying; Step 2: Use a sample preparation instrument to prepare volatile oil at actual reservoir pressure and temperature, use an electric heating jacket to heat the first intermediate container to the formation temperature, and transfer the prepared live oil from the sample preparation cylinder to the first intermediate container at constant temperature and pressure; Step 3: injecting methane gas into a second intermediate container with a piston, and using a first constant speed constant pressure pump to increase the pressure to 15-20 MPa lower than the reservoir pressure; Step 4: Take out the dried core and weigh it, use a polytetrafluoroethylene tube to burn a heat shrink sleeve to wrap the core, put it into a nuclear magnetic resonance core holder, and test the T1-T2 (longitudinal relaxation time-transverse relaxation time) spectrum of the initial dry core sample. The longitudinal relaxation time (T1) characterizes the recovery speed of the longitudinal component of the magnetization vector (energy release), and the transverse relaxation time (T2) characterizes the attenuation speed of the transverse component of the magnetization vector (nuclear magnetic moment dispersion).

[0023] Step 5: adding fluorinated liquid into the third intermediate container, and using the second constant speed and constant pressure pump to displace the fluorinated liquid in the third intermediate container to control the confining pressure; Step 6: Use the second constant speed and constant pressure pump to displace the fluorinated liquid in the third intermediate container, and inject it into the confining pressure inlet of the nuclear magnetic core holder at a constant pressure of 3 MPa to establish the initial confining pressure; turn on the heating system of the nuclear magnetic core holder, set the heating temperature to the formation temperature and keep it stable; Step 7: Use the first constant speed and constant pressure pump to displace the methane gas in the second intermediate container and inject it into the core at a constant pressure until the gas is discharged from the outlet to establish the system pressure. At the same time, use the second and third constant speed and constant pressure pumps to control the confining pressure and back pressure, and test the T1-T2 spectrum under the saturated methane state; Step 8: Use the first constant speed and constant pressure pump to displace the live oil in the first intermediate container and inject it into the core at a constant pressure to saturate the core, and simultaneously use the second constant speed and constant pressure pump to increase the confining pressure; when oil is seen at the outlet, use the third constant speed and constant pressure pump to increase the back pressure, and after the oil output at the outlet is stable, close the switches at the outlet and inlet of the nuclear magnetic core holder; let the core stand and age for 24 hours, and test the T1-T2 spectrum under the saturated oil state; Step 9: Turn on the outlet switch of the nuclear magnetic core holder, control the third constant speed and constant pressure pump to perform depletion mining in constant pressure mode, and perform depressurization mining with the formation pressure as the starting pressure according to the depletion speed (for example, 2MPa / 1.5h, with a pressure reduction of 2MPa every 1.5h). After each pressure is reduced to a stable level, test the T1-T2 spectrum at that pressure; Step 10: The bubble point pressure is obtained by comparing and analyzing the changes in the oil phase and gas phase area signals in the T1-T2 spectrum under different depletion pressures. The two-dimensional spectrum in the pressure drop process is compared with the two-dimensional spectrum under the saturated oil state. The changes in the oil phase area signal indicate the oil utilization. The pressure when the gas phase signal first appears in the spectrum is the bubble point pressure. At this pressure, the light component liquid hydrocarbons begin to vaporize and form bubbles. The bubble point pressure refers to the pressure at a certain temperature when an infinitesimal amount of gas in a hydrocarbon system coexists in equilibrium with a large amount of liquid phase.

[0024] Embodiment 1; Shale treatment: Select shales with porosity greater than 2.5% and permeability of 0.05-0.5mD, and use a mixed solvent of dichloromethane and methanol (volume ratio 93:7) to circulate and wash the core until the mixed solvent is transparent, indicating that the oil washing is complete. Place the shale in an oven and dry it at 120°C for 48 hours; Preparation of volatile oil under formation conditions: weigh the volume of the liquid hydrocarbon component of the volatile oil, add it to the sample preparation instrument together with 70 mL of ground degassed crude oil, and then inject the volume of each gaseous hydrocarbon component under the calculated sample preparation conditions into the sample preparation instrument in the order of the sample preparation pressure of each gaseous hydrocarbon component. The specific component volumes are shown in Table 1. After the gaseous hydrocarbon is injected, pressurize it to the reservoir pressure of 35 MPa and increase the temperature to the reservoir temperature of 70°C. Swing and mix thoroughly to keep the pressure stable, and the preparation of volatile oil under actual reservoir conditions is completed; Table 1 Volume of each hydrocarbon component required for the preparation of volatile oil ; Constant temperature and constant pressure sample transfer: The first intermediate container is heated to 70°C using an electric heating jacket, and a high-pressure hand pump is used to perform a constant temperature and constant pressure sample transfer process, i.e., the volatile oil in the sample preparation instrument is transferred to the first intermediate container; Methane gas pressurization: transfer the methane gas at 7MPa into the second intermediate container, and use the first constant speed and constant pressure pump to pressurize the methane gas in the second intermediate container to 20MPa to keep the pressure stable; Dry shale T1-T2 spectrum measurement: Take out the dried core and weigh it, wrap the core with a heat shrink sleeve made of a polytetrafluoroethylene tube, put the core wrapped with the heat shrink sleeve into the nuclear magnetic core holder, turn on the nuclear magnetic instrument to test the initial dry sample core T1-T2 spectrum, such as Figure 3 As shown; Confining pressure setting: add 800 mL of fluorinated liquid into the third intermediate container, use the second constant speed and constant pressure pump to displace the fluorinated liquid in the third intermediate container and inject it into the dedicated fluorinated liquid inlet of the nuclear magnetic core holder at a constant pressure of 3 MPa, set the temperature of the heating system of the nuclear magnetic core holder to 70°C, and keep the temperature stable; Measurement of T1-T2 spectrum of saturated methane shale: Use the first constant speed and constant pressure pump to displace the methane in the second intermediate container at a constant pressure of 20MPa, slowly open the switch on the second intermediate container, and observe the injection pressure at all times according to the pressure gauge on the first six-way valve until it reaches 20MPa. Simultaneously use the second constant speed and constant pressure pump to increase the confining pressure at a constant pressure to ensure that the confining pressure is 3MPa greater than the injection pressure until the confining pressure reaches 23MPa; during the gas injection process, when the gas is discharged from the outlet, use the third constant speed and constant pressure pump to increase the back pressure to 20MPa at a constant pressure; after the system pressure stabilizes, close the switch of the second intermediate container and suspend the first constant speed and constant pressure pump. Test the T1-T2 spectrum under the saturated methane state, such as Figure 4 As shown; Shale saturated with oil: Use the first constant speed and constant pressure pump to displace the volatile oil in the first intermediate container at a constant pressure of 35MPa, slowly open the switch on the first intermediate container, and observe the injection pressure according to the pressure gauge on the first six-way valve until it reaches 35MPa. Simultaneously use the second constant speed and constant pressure pump to increase the confining pressure at a constant pressure to ensure that the confining pressure is 3MPa greater than the injection pressure until the confining pressure reaches 38MPa; During the oil saturation process, continue to use the first constant speed and constant pressure pump to displace the volatile oil in the first intermediate container to saturate the core until the outlet end produces oil stably and evenly, use the third constant speed and constant pressure pump to increase the back pressure to 35MPa at a constant pressure, turn off the switches at the outlet and inlet ends of the nuclear magnetic core holder, pause the first constant speed and constant pressure pump, turn off the switch of the first intermediate container, and let the shale stand for 48 hours of aging; T1-T2 spectrum test of oil-saturated shale: After the core aging is completed, turn on the nuclear magnetic resonance instrument to test the T1-T2 spectrum of the shale in the oil-saturated state, such as Figure 5 As shown; Shale depressurization process: Open the outlet switch of the nuclear magnetic core holder, and use the third constant speed constant pressure pump to reduce the pressure from 35MPa to 10MPa at a depressurization rate of 5MPa / h by 5MPa every hour. After each pressure is reduced to a certain level and stabilized, test the T1-T2 spectrum at that pressure, and obtain the T1-T2 spectrum at 5 pressure points in total; Analysis of phase changes during shale depressurization: The T1-T2 spectrum under different pressures is compared with the T1-T2 spectrum under saturated oil state. The distribution change of oil phase signal in the T1-T2 spectrum represents the utilization of oil. The pressure of the first gas phase signal represents the bubble point pressure, and the signal amount represents the amount of produced gas. At this bubble point pressure, light component liquid hydrocarbons begin to vaporize to form bubbles.

[0025] Of course, the above description is not a limitation of the present invention, and the present invention is not limited to the above examples. Changes, modifications, additions or substitutions made by technicians in this technical field within the essential scope of the present invention should also fall within the protection scope of the present invention.

Claims

1. The phase change experimental device of shale volatile oil reservoir based on online nuclear magnetic resonance is characterized by: It includes a nuclear magnetic equipment analyzer, three intermediate containers with pistons, three constant speed and constant pressure pumps, a nuclear magnetic core holder and an oil and gas separation metering device; The output end of the first constant speed and constant pressure pump is connected to the input ends of the first intermediate container and the second intermediate container respectively through a high-pressure pipeline via a three-way valve, and the output end of the second constant speed and constant pressure pump is connected to the input end of the third intermediate container through a high-pressure pipeline; the output ends of the first and second intermediate containers are connected to the inlet end of the nuclear magnetic core clamp through a high-pressure pipeline via a first six-way valve, and the output end of the third intermediate container is connected to the upper confining pressure inlet end of the nuclear magnetic core clamp through a high-pressure pipeline via a second six-way valve, and the high-pressure pipelines from the output ends of the three intermediate containers to the nuclear magnetic core clamp are wrapped with electric heating belts.

2. The shale volatile oil reservoir phase change experimental device based on online nuclear magnetic resonance according to claim 1 is characterized in that: The three six-way valves are all connected with pressure gauges to display the pressure; The outlet end of the nuclear magnetic core holder is connected to the third constant speed and pressure pump and the oil and gas separation metering device through a high-pressure pipeline via a back pressure valve and a third six-way valve, and switches are connected at the inlet and outlet ends of the nuclear magnetic core holder.

3. The shale volatile oil reservoir phase change experimental device based on online nuclear magnetic resonance according to claim 2 is characterized in that: The core is installed inside the nuclear magnetic resonance core holder, and after positioning, it is placed within the magnet coverage range of the nuclear magnetic resonance equipment analyzer. The heating system heats the nuclear magnetic resonance core holder by electric heating.

4. An experimental method for phase change of shale volatile oil reservoir based on online nuclear magnetic resonance, using an experimental device as described in any one of claims 1 to 3, characterized in that: The following steps are involved: Step 1: Select shale with a porosity greater than 2.5% and a permeability of 0.05-0.5mD, and wash the core with a mixed solvent of dichloromethane and methanol. The volume ratio of dichloromethane to methanol is 93:

7. After washing the oil, put the shale into an oven and keep it at 120°C for drying; Step 2: Use a sample preparation instrument to prepare volatile oil at actual reservoir pressure and temperature, use an electric heating jacket to heat the first intermediate container to the formation temperature, and transfer the prepared live oil from the sample preparation cylinder to the first intermediate container at constant temperature and pressure; Step 3: injecting methane gas into a second intermediate container with a piston, and using a first constant speed constant pressure pump to increase the pressure to 15-20 MPa lower than the reservoir pressure; Step 4: Take out the dried core and weigh it, use a polytetrafluoroethylene tube to burn a heat shrink sleeve to wrap the core, put it into a nuclear magnetic resonance core holder, and test the T1-T2 spectrum of the initial dry core sample; Step 5: adding fluorinated liquid into the third intermediate container, and using the second constant speed and constant pressure pump to displace the fluorinated liquid in the third intermediate container to control the confining pressure; Step 6: Use the second constant speed and constant pressure pump to displace the fluorinated liquid in the third intermediate container and inject it into the confining pressure inlet of the nuclear magnetic core holder at a constant pressure of 3 MPa to establish the initial confining pressure; Turn on the heating system of the NMR core holder, set the heating temperature to the formation temperature and keep it stable; Step 7: Use the first constant speed and constant pressure pump to displace the methane gas in the second intermediate container and inject it into the core at a constant pressure until the gas is discharged from the outlet to establish the system pressure. At the same time, use the second and third constant speed and constant pressure pumps to control the confining pressure and back pressure, and test the T1-T2 spectrum under the saturated methane state; Step 8: Use the first constant speed and constant pressure pump to displace the live oil in the first intermediate container and inject it into the core at a constant pressure to saturate the core, and simultaneously use the second constant speed and constant pressure pump to increase the confining pressure; when oil is seen at the outlet, use the third constant speed and constant pressure pump to increase the back pressure, and after the oil output at the outlet is stable, close the switches at the outlet and inlet of the nuclear magnetic core holder; let the core stand and age for 24 hours, and test the T1-T2 spectrum under the saturated oil state; Step 9: Turn on the outlet switch of the nuclear magnetic core holder, control the third constant speed and constant pressure pump to perform depletion mining in constant pressure mode, and perform pressure reduction mining with the formation pressure as the starting pressure according to the depletion speed. After each pressure drops to a stable level, test the T1-T2 spectrum under the pressure; Step 10: The bubble point pressure is obtained by comparing and analyzing the changes in the oil phase and gas phase area signals in the T1-T2 spectrum under different depletion pressures. The two-dimensional spectrum during the pressure drop process is compared with the two-dimensional spectrum under the saturated oil state. The changes in the oil phase area signal indicate the oil utilization. The pressure when the gas phase signal first appears in the spectrum is the bubble point pressure. At this pressure, the light component liquid hydrocarbons begin to vaporize to form bubbles.

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