A system and method for studying hydraulic fracturing crack propagation using nuclear magnetic resonance

Abstract

The present invention belongs to the field of unconventional natural gas development such as coalbed methane and shale gas, and specifically relates to a system and method for studying hydraulic fracturing crack expansion using nuclear magnetic resonance. A core holder, a hydraulic fracturing conveying device, a gas injection device, a vacuum device and a gas metering device are used to simulate hydraulic fracturing through physical experiments, causing cracks in the core, and then nitrogen and carbon dioxide are injected into the core separately or simultaneously at different proportions and flow rates, and the migration of methane is detected by nuclear magnetic resonance, and the development of cracks in the core is obtained by calculation, thereby realizing the function of simulating the use of nitrogen or carbon dioxide single gas, or nitrogen and carbon dioxide mixed gas in hydraulic fracturing to study the development of cracks and the process of displacing natural gas.

Description

Technical Field

[0001] The present invention belongs to the field of development of unconventional natural gas such as coalbed methane and shale gas, and specifically relates to a system and method for studying hydraulic fracturing crack expansion using nuclear magnetic resonance. Background Art

[0002] Unconventional natural gas such as shale gas and coalbed methane is mainly stored in unconventional natural gas reservoirs in an adsorbed or adsorbed state. Unconventional natural gas reservoirs generally have low permeability and low natural production capacity. After production reaches a certain stage, new production technology is urgently needed to increase natural gas production.

[0003] Currently, injecting carbon dioxide or nitrogen into unconventional natural gas reservoirs to displace natural gas is an emerging production-increasing technology. The principle is to inject carbon dioxide or nitrogen into unconventional natural gas reservoirs, and through the competitive adsorption of carbon dioxide or nitrogen with methane, reduce the effective partial pressure of methane, promote the desorption of methane adsorbed in unconventional natural gas reservoirs, and displace free gas at the same time, thereby increasing unconventional natural gas production and recovery rate.

[0004] Nuclear magnetic resonance is a means of characterizing internal structure through spectral signals. Compared with other characterization methods, nuclear magnetic resonance is more accurate and has more important guiding significance. Using this technology can make research on reservoir pores and fractures more precise and accurate.

[0005] The main technical problems existing in the currently published patents are:

[0006] 1. In current research, there are few experimental systems and methods for mixed gas displacement of natural gas, and there are few studies on how to effectively mix carbon dioxide with nitrogen and methane.

[0007] 2. In current research, there are few experimental systems and methods for real-time research on mixed gas displacement of natural gas. Currently, the displacement effect is mostly calculated by adsorption volume, gas injection flow rate, etc., and there is little research on the mechanism of displacement in cores.

[0008] 3. Currently, there is little research on the detailed characterization of the form, direction, height and length of hydraulic fracturing crack expansion. Summary of the invention

[0009] The present invention proposes a system and method for studying hydraulic fracturing crack expansion using nuclear magnetic resonance to solve the problems in the prior art that carbon monoxide cannot be effectively mixed with nitrogen and methane, there is little research on the mechanism of displacement and replacement in rock cores, and there is little research on the form, direction, and refined characterization of hydraulic fracturing crack expansion height and length.

[0010] To achieve the above object, the present invention proposes the following technical solutions:

[0011] A system for studying hydraulic fracturing crack expansion using nuclear magnetic resonance, comprising a core holder, a hydraulic fracturing conveying device, a gas injection device, a vacuum device and a gas metering device;

[0012] The inlet end of the core holder is connected to a hydraulic fracturing conveying device, a gas injection device and a vacuum pumping device, and the outlet end of the core holder is connected to a gas metering device;

[0013] The core holder is placed in a second temperature-controlled box, the second temperature-controlled box is externally connected to a nuclear magnetic resonance device, and the core holder is connected to a pressure reducing valve via a pressure control device.

[0014] Preferably, the hydraulic fracturing delivery device comprises a storage tank, a first valve, a booster pump and a first flow meter connected in sequence;

[0015] The storage tank contains a mixture of fracturing fluid and proppant and has scales;

[0016] A first pressure gauge and a first thermometer are arranged above the storage tank.

[0017] Preferably, the vacuum pump comprises a ninth valve and a vacuum pump; the vacuum pump is connected to the inlet of the core holder through the ninth valve and the eighth pressure gauge.

[0018] Preferably, the gas injection device comprises three branches:

[0019] The first branch includes a first gas cylinder, and the outlet end of the first gas cylinder is connected to a first pump, a first storage tank and a fourth valve in sequence;

[0020] The second branch includes a second gas cylinder, and the outlet end of the second gas cylinder is connected to a fifth valve and a second storage tank in sequence;

[0021] The third branch includes a third gas cylinder, the outlet end of the third gas cylinder is connected to the second storage tank, and the second storage tank is connected to a constant speed and constant pressure pump;

[0022] The fourth valve is connected to the constant speed and constant pressure pump and then connected to the eighth pressure gauge and the inlet of the core holder.

[0023] Preferably, the outlet end of the first gas cylinder is connected to the first pump via the second valve and the second pressure gauge, the first pump is connected to the first storage tank via the first one-way valve, and the first storage tank is connected to the fourth valve via the second one-way valve and the first flow meter;

[0024] The outlet end of the second gas cylinder is connected to the second storage tank through the fifth valve, the fourth pressure gauge and the second pump; the second storage tank is connected to the constant speed and constant pressure pump through the fifth one-way valve, the third flow meter and the eighth valve in sequence;

[0025] The outlet end of the third gas cylinder is connected to the second storage tank via the sixth valve and the third pump;

[0026] The side of the first pump is connected to the first air compressor, the top of the first storage tank is connected to the third pressure gauge and the second thermometer, and the bottom is connected to the third valve;

[0027] The second pump is connected to the second air compressor on its side;

[0028] A sixth pressure gauge and a third thermometer are connected to the top of the second storage tank, the second storage tank is placed inside the first temperature-controlled box, and the sixth pressure gauge and the third thermometer are located outside the first temperature-controlled box;

[0029] The seventh valve is connected to the right side of the bottom of the second storage tank; a stirring rod is arranged inside the second storage tank, and the stirring rod is sequentially connected to the right motor, the switch and the battery through the circuit; a seventh pressure gauge is arranged on the top of the constant speed and constant pressure pump.

[0030] A fifth pressure gauge is externally connected to the connection passage between the sixth valve and the third pump, and the side surface of the third pump is connected to the third air compressor.

[0031] Preferably, the outside of the second temperature control box is connected to the radio frequency device and the computer in sequence through a circuit.

[0032] Preferably, the gas metering device comprises a filter, a tenth pressure gauge, a fifteenth valve, a pressure reducing valve, a methane gas infrared online analyzer, a carbon dioxide infrared gas online analyzer, a fourth flow meter and an air bag;

[0033] The filter, the tenth pressure gauge, the fifteenth valve, the pressure reducing valve, the methane gas infrared online analyzer, the carbon dioxide infrared gas online analyzer, the fourth flow meter and the air bag are connected in sequence, and the filter is connected to the core holder outlet.

[0034] Preferably, the pressure control device comprises a hand pump, a twelfth valve, and a buffer tank, and the hand pump, the twelfth valve, and the buffer tank are connected in sequence;

[0035] The buffer tank is connected to three branches, the first branch buffer tank is connected to the tenth valve, and the tenth valve is connected to the inlet side of the core holder;

[0036] The second branch buffer tank is connected to the eleventh valve, and the eleventh valve is connected to the core holder;

[0037] The third branch buffer tank is connected to the fourteenth valve and the pressure reducing valve;

[0038] The top of the buffer tank is connected to the ninth pressure gauge, and the side of the buffer tank is connected to the thirteenth valve.

[0039] A method for studying hydraulic fracturing crack expansion using nuclear magnetic resonance comprises the following steps:

[0040] Step 1: cleaning a system for studying hydraulic fracturing crack propagation using nuclear magnetic resonance, and checking the air tightness of the system;

[0041] Step 2: Load the rock sample into the core holder, evacuate the air in the system, and ensure that the system is closed;

[0042] Step 3: Add axial pressure, confining pressure and back pressure;

[0043] Step 4: Allow the core to absorb methane;

[0044] Step 5: preparing a mixed gas and injecting the mixed gas into the core holder;

[0045] Step 6: Repeat step 4;

[0046] Step 7: pressurizing the mixture of fracturing fluid and proppant through a hydraulic fracturing delivery device and injecting it into the core holder until cracks are generated in the core;

[0047] Step 8: Repeat step 5;

[0048] Step 9: Measure the amount of methane adsorbed and saturated in the core;

[0049] The saturation and relaxation curves in the core are detected by radio frequency devices and computers to obtain the displacement and replacement conditions in the core and the fracture development conditions in the core.

[0050] Preferably, in step three, the specific operations are:

[0051] Add axial pressure: open the twelfth valve and the tenth valve, inject water into the core holder to compact the sample in the core holder, and close the tenth valve;

[0052] Add confining pressure: After opening the twelfth and eleventh valves, inject water into the core holder through a hand pump to compact the sample in the core holder, and close the eleventh and twelfth valves;

[0053] Add back pressure: Open valve 14 to increase the pressure of the pressure reducing valve.

[0054] The present invention is beneficial in that:

[0055] A core holder, a hydraulic fracturing conveying device, a gas injection device, a vacuum device and a gas metering device are used to form a system for studying the expansion of hydraulic fracturing cracks using nuclear magnetic resonance. The hydraulic fracturing conveying device is used to simulate hydraulic fracturing to cause cracks in the core. The gas injection device injects mixed gas into the core, and the migration of methane is detected by the nuclear magnetic resonance device. The crack development in the core is calculated by the gas metering device, thereby realizing the function of simulating the use of nitrogen or carbon dioxide single gas, or nitrogen and carbon dioxide mixed gas to study the crack development and the process of displacing natural gas in hydraulic fracturing.

[0056] Based on a system for studying hydraulic fracturing crack extension using nuclear magnetic resonance, a method for studying hydraulic fracturing crack extension using nuclear magnetic resonance is proposed, and real-time data feedback is achieved according to feedback from a radio frequency device and a computer. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] The accompanying drawings constituting a part of the present invention are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the accompanying drawings:

[0058] Figure 1 This is a system structure diagram for studying hydraulic fracturing crack expansion using nuclear magnetic resonance in the present invention.

[0059] Among them, 1 is a storage tank; 2 is a first pressure gauge; 3 is a first thermometer; 4 is a first valve; 5 is a booster pump; 6 is a first flow meter; 7 is a first gas cylinder; 8 is a second valve; 9 is a second pressure gauge; 10 is a first air compressor; 11 is a first pump; 12 is a first non-return valve; 13 is a third pressure gauge; 14 is a second thermometer; 15 is a first storage tank; 16 is a second non-return valve; 17 is a third valve; 18 is a second flow meter; 19 is a fourth valve; 20 is a second gas cylinder; 21 is a fifth valve; 22 is a fourth pressure gauge; 23 is a second air compressor; 24 is a second pump; 25 is a third gas cylinder; 26 is a sixth valve; 27 is a fifth pressure gauge; 28 is a third air compressor; 29 is a third pump; 30 is a sixth pressure gauge; 31 is a third thermometer; 32 is a second storage tank; 33 is The first temperature control box; 34 is an electric motor; 35 is a switch; 36 is a battery; 37 is a third one-way valve; 38 is a seventh valve; 39 is a third flow meter; 40 is an eighth valve; 41 is a seventh pressure gauge; 42 is a constant speed constant pressure pump; 43 is a ninth valve; 44 is a vacuum pump; 45 is an eighth pressure gauge; 46 is a tenth valve; 47 is an eleventh valve; 48 is a second temperature control box; 49 is a core holder; 50 is a hand pump; 51 is a twelfth valve; 52 is a buffer tank; 53 is a ninth pressure gauge; 54 is a thirteenth valve; 55 is a fourteenth valve; 56 is a filter; 57 is a tenth pressure gauge; 58 is a fifteenth valve; 59 is a pressure reducing valve; 60 is a methane infrared analyzer; 61 is a carbon dioxide infrared analyzer; 62 is a fourth flow meter; 63 is an air bag; 64 is a radio frequency device; 65 is a computer. DETAILED DESCRIPTION

[0060] The present invention will be described in detail below with reference to the accompanying drawings and in combination with embodiments. It should be noted that the embodiments and features in the embodiments of the present invention can be combined with each other without conflict.

[0061] The following detailed description is an exemplary description, which is intended to provide further detailed description of the present invention. Unless otherwise specified, all technical terms used in the present invention have the same meaning as those generally understood by those skilled in the art to which the present invention belongs. The terms used in the present invention are only for describing specific embodiments, and are not intended to limit the exemplary embodiments according to the present invention.

[0062] Embodiment 1:

[0063] See also Figure 1 As shown, the present invention provides a system for studying hydraulic fracturing crack expansion using nuclear magnetic resonance, comprising: a core holder 49, a hydraulic fracturing conveying device, a gas injection device, a vacuum device, a pressure control device and a gas metering device;

[0064] Among them, the core clamp 49 used to hold the core is placed in the second temperature control box 48, the inlet end of the core clamp 49 is connected to a hydraulic fracturing conveying device and a gas injection device, the inlet of the core clamp 49 is connected to a vacuum pumping device, the outlet of the core clamp 49 is connected to a gas metering device, the outside of the second temperature control box 48 is connected to a nuclear magnetic resonance device, and a pressure control device is connected between the core clamp 49 and the pressure reducing valve 59.

[0065] The core holder 49 is a high-pressure resistant piston container, which contains several temperature probes for monitoring the internal temperature of the holder, so that the experiment is more in line with the actual situation.

[0066] The hydraulic fracturing delivery device comprises a storage tank 1, a first valve 4, a booster pump 5 and a first flow meter 6, and the storage tank 1, the first valve 4, the booster pump 5 and the first flow meter 6 are connected in sequence through pipelines.

[0067] The storage tank 1 contains a mixture of fracturing fluid and proppant and is marked with scales so that volume changes of the fracturing fluid and proppant can be visually detected. A first pressure gauge 2 and a first thermometer 3 are provided above the storage tank 1 to detect the temperature and pressure in the storage tank.

[0068] The gas injection device includes three branches:

[0069] The first branch includes a first gas cylinder 7 filled with hydrocarbon gas, and the outlet end of the first gas cylinder 7 is connected with a second valve 8, a second pressure gauge 9, a first pump 11, a first non-return valve 12, a first storage tank 15, a second non-return valve 16, a first flow meter 18, and a fourth valve 19 in sequence; the first storage tank 15 has the characteristics of high temperature and high pressure resistance, the third pressure gauge 13 and the second thermometer 14 are used to monitor the temperature and pressure of the gas in the first storage tank 15; the first non-return valve 12 prevents the pressurized methane gas from flowing back, and the second non-return valve 16 prevents the gas flowing out of the storage tank from flowing back into the storage tank.

[0070] Among them, the side of the first pump 11 is connected to the first air compressor 10, the top of the first storage tank 15 is connected to the third pressure gauge 13 and the second thermometer 14, and the bottom is connected to the third valve 17; the third valve 17 is the drain valve of the first storage tank 7.

[0071] The second branch includes a second gas cylinder 20 filled with nitrogen, and the outlet end of the second gas cylinder 20 is connected to a fifth valve 21, a fourth pressure gauge 22, a second pump 24 and a second storage tank 32 in sequence;

[0072] Among them, the side of the second pump 24 is connected to the second air compressor 23; the second storage tank 32 is placed inside the first temperature control box 33, the top of the second storage tank 32 is connected with the sixth pressure gauge 30 and the third thermometer 31, the sixth pressure gauge 30 and the third thermometer 31 are located outside the first temperature control box 33, and the bottom right side of the second storage tank 32 is connected with the seventh valve 38; the second storage tank 32 has a stirring rod inside, and the stirring rod is connected to the motor 34, the switch 35, and the battery 36 in sequence through the circuit; the seventh valve 38 is the drain valve of the second storage tank 20.

[0073] The second storage tank 32 has the characteristics of being resistant to high temperature and high pressure. The sixth pressure gauge 30 and the third thermometer 31 are used to monitor the temperature and pressure of the gas in the second storage tank 32 .

[0074] The third branch includes a third gas cylinder 25 filled with carbon dioxide, the outlet end of the third gas cylinder 25 is connected in sequence to a sixth valve 26, a third pump 29 and a second storage tank 32;

[0075] A fifth pressure gauge 27 is externally connected to the connection channel between the sixth valve 26 and the third pump 29 , and the side of the third pump 29 is connected to the third air compressor 28 .

[0076] The second storage tank 32 is connected to the third one-way valve 37, the third flow meter 39, the eighth valve 40, and the constant speed and pressure pump 42 in sequence through pipelines; wherein, a seventh pressure gauge 41 is provided on the top of the constant speed and pressure pump 42. The third one-way valve 37 prevents the gas flowing out of the storage tank from flowing back into the storage tank. The constant speed and pressure pump 42 is used to pump the liquid carbon dioxide in the second storage tank 32 into the core holder 49, and the constant speed and pressure pump 42 has the function of measuring the instantaneous mass flow and cumulative mass of carbon dioxide.

[0077] In the gas injection device, the fourth valve 19 is connected to the constant speed constant pressure pump 42 and then sequentially connected to the eighth pressure gauge 45 and the inlet of the core holder 49. The eighth pressure gauge 45 is used to detect the pressure at the inlet end of the core holder 49.

[0078] The vacuum device includes a ninth valve 43 and a vacuum pump 44, which are connected to the eighth pressure gauge 45 and the inlet of the core holder 49 in sequence. The vacuum pump 44 is used to vacuum the system and remove the influence of air in the experimental system on the experimental accuracy and experimental effect.

[0079] The second temperature control box 48 is externally connected to the radio frequency device 64 and the computer 65 in sequence through a circuit. The second temperature control box 48 has the characteristic of flexible temperature setting, and is also provided with a magnetic field required for nuclear magnetic resonance.

[0080] The radio frequency device 64 mainly includes a radio frequency transmitting part and a set of magnetic resonance signal receiving system. The transmitting part is equivalent to a radio transmitter. It is a single-sideband transmitting device with precisely adjustable waveform and spectrum. Its peak transmission power is adjustable from hundreds of watts to fifteen kilowatts. The receiving system is used to receive the free induction decay signal reflected by the core. Since this signal is extremely weak, the total gain of the receiving system is required to be very high and the noise must be very low. Generally, spectrometers use a superheterodyne receiving system, and its main gain can be obtained from the intermediate frequency amplifier. Since the intermediate frequency amplifier operates in a different frequency band from the transmitting system, direct interference with the transmission can be avoided. A receiving gate is provided between the preamplifier and the intermediate amplifier, which is actually a radio frequency switch. It is mainly closed at the moment when the transmitting system is working to prevent strong radio frequency transmission signals from entering the receiving system. The FID (free decay signal) signal after intermediate frequency amplification generally has an amplitude of more than 0.5 volts and can be detected. After detection, the signal is also amplified and filtered.

[0081] The computer 65 is used to process and store nuclear magnetic resonance waves. The magnetic resonance signal is converted into digital quantity through the converter and stored in the temporary memory. The image processes the raw data according to the required method to obtain different parameter images of magnetic resonance and store them in the image memory. The computer can calculate T1, T2, and proton density. The reconstructed image is sent to the display to show the characteristics of the core fracture morphology and so on.

[0082] The gas metering device includes a filter 56, a tenth pressure gauge 57, a fifteenth valve 58, a pressure reducing valve 59, a methane gas infrared online analyzer 60, a carbon dioxide infrared online analyzer 61, a fourth flow meter 62 and an air bag 63; wherein the filter 56 is connected to the outlet of the core holder 49, and the filter 56, the tenth pressure gauge 57, the fifteenth valve 58, the pressure reducing valve 59, the methane gas infrared online analyzer 60, the carbon dioxide infrared online analyzer 61, the fourth flow meter 62 and the air bag 63 are connected in sequence. The tenth pressure gauge 57 is used to monitor the pressure at the outlet of the core holder 49. The methane infrared gas analyzer 60 and the carbon dioxide infrared gas analyzer 61 are respectively used for real-time online measurement of the volume percentage of methane and carbon dioxide in the mixed gas discharged from the experimental system. The fourth flow meter 62 is a mass flow meter used to measure the flow of the terminal mixed gas.

[0083] The pressure control device includes a hand pump 50, a twelfth valve 51 and a buffer tank 52. After the hand pump 50, the twelfth valve 51 and the buffer tank 52 are connected in sequence, the buffer tank 52 is divided into three branches. The first branch connects the buffer tank 52 with the tenth valve 46, and the tenth valve 46 is connected to the left side of the core clamp 49 to control the axial pressure; the second branch connects the buffer tank 52 with the eleventh valve 47, and the eleventh valve 47 is connected to the side of the core clamp 49 to control the confining pressure; the third branch buffer tank 52 is connected to the fourteenth valve 55 and the pressure reducing valve 59 in sequence to control the back pressure; the top of the buffer tank is connected to the ninth pressure gauge 53, and the side is connected to the thirteenth valve 54.

[0084] The hand pump 50 pumps water into the buffer tank 52, applies axial pressure to the core holder 49 through the tenth valve 46, and applies confining pressure to the core holder 49 through the eleventh valve 47, so that the rock sample in the core holder 49 is more compacted to simulate the formation porosity. The hand pump 50 pumps water into the buffer tank, applies a certain pressure to the high-speed pressure reducing valve 59 through the fourteenth valve 55, thereby adjusting the outlet pressure required for the experiment.

[0085] The pressure reducing valve 59 has a heating function to prevent the liquid carbon dioxide flowing out of the core clamp 49 from being blocked by ice due to pressure changes, thereby blocking the pipeline and damaging the instrument. The pressure reducing valve 67 reduces the high-pressure gas flowing out of the core clamp 49 to a normal pressure state to prevent the pressure change of the gas in the pipeline from affecting the metering accuracy of the methane gas infrared gas analyzer 60 and the carbon dioxide gas infrared gas analyzer 61.

[0086] The present invention is an experimental system for displacing and replacing gases after mixing them. Nitrogen and carbon dioxide gases are mixed in different proportions according to different volume flow rates, thereby realizing mixed gas displacing and replacing experiments under arbitrary proportion conditions.

[0087] The present invention can simulate hydraulic fracturing through physical experiments to cause cracks in the core, and then inject nitrogen and carbon dioxide into the core separately or simultaneously at different proportions and flow rates, detect the migration of methane through nuclear magnetic resonance, and obtain the crack development in the core through calculation, thereby realizing the function of simulating the use of single nitrogen or carbon dioxide gas, or a mixture of nitrogen and carbon dioxide gas, to study the crack development and the process of displacing natural gas in hydraulic fracturing.

[0088] Embodiment 2:

[0089] A method for studying hydraulic fracturing crack expansion using nuclear magnetic resonance comprises the following steps:

[0090] Step 1: Check air tightness: After connecting the system, clean the equipment, check the components of the system, close the inlet and outlet valves, and check the air tightness of the device.

[0091] Step 2: Sample loading and vacuuming: Load the rock sample into the core holder 49, open the vacuum pump 44, the ninth valve 43, the fourth valve 19, the eighth valve 40, and the fifteenth valve 58, and exhaust the air in the system, thereby eliminating the interference of air on the experimental effect and preparing for the experiment. After vacuuming, close the vacuum pump 44, the ninth valve 43, the fourth valve 19, the eighth valve 40, and the fifteenth valve 58. The rock sample used can be granular, powder, or columnar core, and the core size can be flexibly designed.

[0092] Step 3: Add axial pressure and confining pressure:

[0093] Adding axial pressure: After opening the twelfth valve 51 and the tenth valve 46, water is injected into the core holder 49 through the hand pump 50 to compact the sample in the core holder 49. After increasing the axial pressure to the required level according to the experimental plan, the tenth valve 46 is closed.

[0094] Adding confining pressure: After opening the twelfth valve 51 and the eleventh valve 47, water is injected into the core holder 49 through the hand pump 50 to compact the sample in the core holder 49. According to the experimental plan, after increasing the confining pressure to the required pressure for the experiment, the eleventh valve 47 and the twelfth valve 51 are closed.

[0095] Step 4: Add back pressure: According to the experimental plan, open the fourteenth valve 55 and increase the pressure of the pressure reducing valve 59 to the pressure required for the experiment.

[0096] Step 5: Adsorbing methane: Open the second valve 8, pressurize the gas into the first storage tank 15 through the first pump 11, and close the second valve 8 after the pressure is increased to an appropriate pressure; the appropriate pressure is 40-50MPa. Open the fourth valve 19, set an appropriate displacement to inject methane into the core holder 49, so that the core is saturated with methane; the displacement depends on the flow rate of the pump, generally between 0-1L / min; open the fifteenth valve 58, when the methane gas pressure reaches the pressure of the pressure reducing valve 59, the gas passes through the methane infrared gas analyzer 60, when the metering flow of the second flowmeter 18 and the fourth flowmeter 62 is equal, and the reading of the methane infrared gas analyzer 60 is 100%, and the readings of the second flowmeter 18 and the fourth flowmeter 62 are the same, indicating that the core has reached a saturated state of methane adsorption, close the fourth valve 19.

[0097] Step 6: Mixed gas displacement: Open the fifth valve 21 to inject the nitrogen in the second gas cylinder 20 into the second storage tank 32. After pressurizing to an appropriate pressure, close the fifth valve 21; the appropriate pressure is 40-50MPa. At the same time, open the sixth valve 26 to pressurize the carbon dioxide gas into the second storage tank 32. After pressurizing to an appropriate pressure, close the sixth valve 26; the appropriate pressure is 40-50MPa. Turn on the battery 36, the switch 35, and the motor 34 to fully mix the gas in the second storage tank 32. Open the eighth valve 40 and inject the mixed gas into the core holder 49 through a constant speed and constant pressure pump. The injection flow ratio of nitrogen and carbon dioxide gas is measured by monitoring the injection flow of the second flowmeter 29 and the constant speed and constant pressure pump 47, and the ratio of nitrogen and carbon dioxide gas is flexibly adjusted according to the purpose of the experiment.

[0098] Step 7: Repeat step 5.

[0099] Step 8: Hydraulic fracturing: Open the first valve 4, pressurize the mixture of fracturing fluid and proppant in the storage tank 1 to the pressure required for hydraulic fracturing through the booster pump 5, and inject it into the core holder 49 to generate cracks in the core.

[0100] Step 9: Repeat step 6.

[0101] Step 10: Experimental measurement and testing:

[0102] Metering: The amount of methane adsorbed and saturated in the core is measured by the imported second flow meter 18 and the fourth flow meter 62. The third flow meter 39, the fourth flow meter 62, the methane infrared analyzer 60, the carbon dioxide infrared gas analyzer 61, the fourth flow meter 62, and the constant speed and pressure pump 42 are used for metering, thereby realizing the calculation of the displacement effect of methane.

[0103] Detection: During the experiment, the radio frequency device 64 and the computer 65 are turned on to detect the saturation and relaxation curves in the core in real time, thereby obtaining the displacement and replacement conditions in the core and the fracture development conditions in the core.

[0104] Step 11: Change conditions and continue comparative experiments: According to the experimental purpose, change the experimental temperature, axial pressure and back pressure, gas mixing ratio and injection displacement for different rock samples, and carry out comparative experiments under other experimental conditions in the same way.

[0105] It is known from common technical knowledge that the present invention can be implemented by other embodiments that do not deviate from its spirit or essential features. Therefore, the above disclosed embodiments are only illustrative in all respects and are not exclusive. All changes within the scope of the present invention or within the scope equivalent to the present invention are included in the present invention.

Claims

1. A system for studying hydraulic fracturing crack expansion using nuclear magnetic resonance, characterized in that: It includes a core holder (49), a hydraulic fracturing conveying device, a gas injection device, a vacuum pumping device and a gas metering device; The inlet end of the core holder (49) is connected to a hydraulic fracturing conveying device, a gas injection device and a vacuum pumping device, and the outlet end of the core holder (49) is connected to a gas metering device; The core holder (49) is placed in a second temperature-controlled box (48), the second temperature-controlled box (48) is externally connected to a nuclear magnetic resonance device, and the core holder (49) is connected to a pressure reducing valve (59) via a pressure control device; The vacuum pumping device comprises a ninth valve (43) and a vacuum pump (44); the vacuum pump (44) is connected to the inlet of the core holder (49) through the ninth valve (43) and the eighth pressure gauge (45); The gas injection device includes three branches: The first branch comprises a first gas cylinder (7), the outlet end of the first gas cylinder (7) is connected in sequence to a first pump (11), a first storage tank (15) and a fourth valve (19); The second branch comprises a second gas cylinder (20), the outlet end of the second gas cylinder (20) being connected in sequence to a fifth valve (21) and a second storage tank (32); The third branch includes a third gas cylinder (25), the outlet end of the third gas cylinder (25) is connected to the second storage tank (32), and the second storage tank (32) is connected to a constant speed and constant pressure pump (42); The fourth valve (19) is connected to the constant speed constant pressure pump (42) and then connected to the inlet of the eighth pressure gauge (45) and the core holder (49); The outlet end of the first gas cylinder (7) is connected to the first pump (11) via the second valve (8) and the second pressure gauge (9), the first pump is connected to the first storage tank (15) via the first one-way valve (12), and the first storage tank (15) is connected to the fourth valve (19) via the second one-way valve (16) and the second flow meter (18); The outlet end of the second gas cylinder (20) is connected to the second storage tank (32) via a fifth valve (21), a fourth pressure gauge (22) and a second pump (24); the second storage tank (32) is connected to a constant speed and constant pressure pump (42) via a third one-way valve (37), a third flow meter (39) and an eighth valve (40) in sequence; The outlet end of the third gas cylinder (25) is connected to the second storage tank (32) via a sixth valve (26) and a third pump (29); The side of the first pump (11) is connected to the first air compressor (10), the top of the first storage tank (15) is connected to a third pressure gauge (13) and a second thermometer (14), and the bottom is connected to a third valve (17); The second pump (24) is connected to the second air compressor (23) at its side; The top of the second storage tank (32) is connected to a sixth pressure gauge (30) and a third thermometer (31), the second storage tank (32) is placed inside the first temperature control box (33), and the sixth pressure gauge (30) and the third thermometer (31) are located outside the first temperature control box (33); The right side of the bottom of the second storage tank (32) is connected to a seventh valve (38); the second storage tank (32) has a stirring rod inside, and the stirring rod is sequentially connected to the right motor (34), the switch (35), and the battery (36) through a circuit; the top of the constant speed constant pressure pump (42) is provided with a seventh pressure gauge (41); A fifth pressure gauge (27) is externally connected to the connection passage between the sixth valve (26) and the third pump (29), and the side of the third pump (29) is connected to the third air compressor (28); The gas metering device comprises a filter (56), a tenth pressure gauge (57), a fifteenth valve (58), a pressure reducing valve (59), a methane gas infrared online analyzer (60), a carbon dioxide gas infrared online analyzer (61), a fourth flow meter (62) and a gas bag (63); The filter (56), the tenth pressure gauge (57), the fifteenth valve (58), the pressure reducing valve (59), the methane gas infrared online analyzer (60), the carbon dioxide infrared gas online analyzer (61), the fourth flow meter (62) and the air bag (63) are connected in sequence, and the filter (56) is connected to the outlet of the core holder (49).

2. A system for studying hydraulic fracturing crack expansion using nuclear magnetic resonance as claimed in claim 1, characterized in that: The hydraulic fracturing delivery device comprises a storage tank (1), a first valve (4), a booster pump (5) and a first flow meter (6) which are connected in sequence; The storage tank (1) contains a mixture of fracturing fluid and proppant and has scales; A first pressure gauge (2) and a first thermometer (3) are provided above the storage tank (1).

3. A system for studying hydraulic fracturing crack expansion using nuclear magnetic resonance as claimed in claim 1, characterized in that: The outside of the second temperature control box (48) is connected to a radio frequency device (64) and a computer (65) in sequence through a circuit.

4. A system for studying hydraulic fracturing crack expansion using nuclear magnetic resonance as claimed in claim 1, characterized in that: The pressure control device comprises a hand pump (50), a twelfth valve (51), and a buffer tank (52), and the hand pump (50), the twelfth valve (51), and the buffer tank (52) are connected in sequence; The buffer tank (52) is connected to three branches, the first branch buffer tank (52) is connected to the tenth valve (46), and the tenth valve (46) is connected to the inlet side of the core holder (49); The second branch buffer tank (52) is connected to the eleventh valve (47), and the eleventh valve (47) is connected to the core holder (49); The third branch buffer tank (52) is connected to the fourteenth valve (55) and the pressure reducing valve (59); The top of the buffer tank (52) is connected to a ninth pressure gauge (53), and the side of the buffer tank (52) is connected to a thirteenth valve (54).

5. A method for studying hydraulic fracturing crack extension using nuclear magnetic resonance, based on a system for studying hydraulic fracturing crack extension using nuclear magnetic resonance as claimed in any one of claims 1 to 4, characterized in that: The specific steps include: Step 1: cleaning a system for studying hydraulic fracturing crack propagation using nuclear magnetic resonance, and checking the air tightness of the system; Step 2: Load the rock sample into the core holder (49), evacuate the air in the system, and ensure that the system is closed; Step 3: Add axial pressure, confining pressure and back pressure; Step 4: Allow the core to absorb methane; Step 5: preparing a mixed gas and injecting the mixed gas into the core holder (49); Step 6: Repeat step 4; Step 7: pressurizing the mixture of fracturing fluid and proppant through a hydraulic fracturing delivery device and injecting it into the core holder (49) until cracks are generated in the core; Step 8: Repeat step 5; Step 9: Measure the amount of methane adsorbed and saturated in the core; The saturation and relaxation curves in the core are detected by using a radio frequency device (64) and a computer (65), and the displacement and replacement conditions in the core and the fracture development conditions in the core are obtained.

6. A method for studying hydraulic fracturing crack expansion using nuclear magnetic resonance according to claim 5, characterized in that: In step three, the specific operations are: Applying axial pressure: opening the twelfth valve (51) and the tenth valve (46), injecting water into the core holder (49), compacting the sample in the core holder (49), and closing the tenth valve (46); Adding confining pressure: After opening the twelfth valve (51) and the eleventh valve (47), water is injected into the core holder (49) through the hand pump (50) to compact the sample in the core holder (49), and the eleventh valve (47) and the twelfth valve (51) are closed; Add back pressure: Open the fourteenth valve (55) to increase the pressure of the pressure reducing valve (59).

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