A method for evaluating the oil displacement efficiency of shale gas injection and a gas injection experimental device

Abstract

A method for evaluating the oil displacement efficiency of shale gas injection and a gas injection experimental device. It mainly solves the problem that the existing oil displacement efficiency evaluation methods cannot evaluate the oil displacement efficiency in different pore intervals of the core. The method includes the following steps: S1. Calculate the conversion coefficient C between the nuclear magnetic resonance T2 value of the water-saturated core and the pore throat radius; S2. Test the nuclear magnetic resonance T2 spectrum of the shale oil-saturated core; S3. Use the gas injection experimental device to conduct gas injection huff and puff experiments on the shale oil-saturated core, and test the nuclear magnetic resonance T2 spectrum of the core after each gas injection huff and puff; S4. Convert the nuclear magnetic resonance T2 value of the shale oil-saturated core into the pore throat radius by using the conversion coefficient C obtained in step S1, and calculate the gas injection huff and puff oil displacement efficiency in different pore intervals of the core according to the signal amplitude of the nuclear magnetic resonance T2 spectrum of the shale core. This evaluation method can evaluate the oil displacement efficiency of the core in different huff and puff cycles and the oil displacement efficiency in different pore intervals of the core, and at the same time, high-temperature and high-pressure gas injection huff and puff experiments can be carried out.

Description

Technical Field

[0001] The present invention relates to the field of oil and gas field development, and specifically to a method for evaluating the efficiency of shale gas injection for oil displacement and a gas injection experimental device. Background Art

[0002] Shale reservoirs are different from conventional reservoirs, mainly consisting of micro-nano scale pores, with extremely low permeability and porosity. The production of shale oil decreases rapidly, and the primary recovery degree is low. The development practice of shale oil in the United States shows that the initial production of shale oil wells is high, but it decreases to 25% of the initial production within two years, and the primary recovery degree of primary development mostly ranges from 2% to 8%. It has become difficult to establish a displacement system for water flooding of shale oil. Foreign scholars found through carbon dioxide huff and puff experiments on the Eagle Ford Shale that carbon dioxide huff and puff can increase the shale oil recovery rate by 33% - 85%. It can be seen that gas injection huff and puff can effectively develop shale oil.

[0003] Currently, the main methods for evaluating the development effect of shale oil gas injection huff and puff at home and abroad are: evaluating the oil displacement efficiency of the core by measuring the weight change of the core before and after gas injection huff and puff, such as patents CN113777002A, CN113075110A, CN113504351A, etc. However, these methods have the following deficiencies:

[0004] (1) Using traditional measurement methods, the evaluation of the core oil displacement efficiency is only limited to the core itself, and it is impossible to evaluate the oil displacement efficiency in different pore intervals of the core;

[0005] (2) The entire experimental device is not evacuated, which will affect the purity of the injected gas and thus affect the oil displacement effect;

[0006] (3) The experimental device can only withstand relatively low temperatures and pressures, and it is impossible to achieve experimental temperatures and pressures above 100°C and 40 MPa. Summary of the Invention

[0007] In order to overcome the above deficiencies in the prior art, the present invention provides a method for evaluating the efficiency of shale gas injection for oil displacement and a gas injection experimental device. This evaluation method can evaluate the oil displacement efficiency of the core in different huff and puff cycles and the oil displacement efficiency in different pore intervals of the core, and at the same time, it can carry out high-temperature and high-pressure gas injection huff and puff experiments.

[0008] The technical solution of the present invention is: A method for evaluating the efficiency of shale gas injection for oil displacement, characterized by including the following steps:

[0009] S1. Calculate the conversion coefficient C between the nuclear magnetic resonance T2 value of the water-saturated core and the pore throat radius,

[0010] S1.1. Drill a shale core, evacuate and saturate it with simulated formation water, test the nuclear magnetic resonance T2 spectrum, and calculate the nuclear magnetic pore throat distribution frequency curve from the nuclear magnetic resonance T2 spectrum of the water-saturated core;

[0011] S1.2. Dry the core and conduct a high-pressure mercury injection experiment to obtain a mercury injection pore throat distribution frequency curve.

[0012] S1.3. Fit the nuclear magnetic resonance pore throat distribution frequency curve with the mercury injection pore throat distribution frequency curve to obtain the conversion coefficient C between the nuclear magnetic resonance T2 value and the pore throat radius.

[0013] S2. Test the nuclear magnetic resonance T2 spectrum of the core saturated with shale oil.

[0014] S3. Use the gas injection experimental device to conduct a gas injection huff and puff experiment on the core saturated with shale oil, and test the nuclear magnetic resonance T2 spectrum of the core after each gas injection huff and puff.

[0015] S4. Use the conversion coefficient C obtained in step S1 to convert the nuclear magnetic resonance T2 value of the core saturated with shale oil into the pore throat radius, and calculate the oil displacement efficiency of gas injection huff and puff in different pore intervals of the core according to the signal amplitudes of the nuclear magnetic resonance T2 spectra of the dry shale core, the core saturated with oil shale, and the shale core in different huff and puff cycles.

[0016] Furthermore, step S4 includes

[0017] S4.1. Calculate the oil displacement efficiency of the core after the nth huff and puff, and the expression is

[0018]

[0019] In the formula: E n is the oil displacement efficiency of the core after the nth huff and puff, %; S 饱和油 is the total signal amplitude of the nuclear magnetic resonance T2 spectrum of the core saturated with shale oil, dimensionless; S 干样 is the total signal amplitude of the nuclear magnetic resonance T2 spectrum of the dry shale core, dimensionless; S n is the total signal amplitude of the nuclear magnetic resonance T2 spectrum of the core saturated with shale oil after the nth huff and puff, dimensionless.

[0020] S4.2. Calculate the oil displacement efficiency of the core in different pore intervals after the nth huff and puff, and the calculation expression of the oil displacement efficiency is

[0021]

[0022] In the formula: is the oil displacement efficiency of the core in the pore throat radius interval (i, j] μm after the nth huff and puff, %; is the total signal amplitude of the nuclear magnetic resonance T2 spectrum of the core saturated with shale oil in the pore throat radius interval (i, j] μm, dimensionless; is the total signal amplitude of the nuclear magnetic resonance T2 spectrum of the dry shale core in the pore throat radius interval (i, j] μm, dimensionless; is the sum of the nuclear magnetic resonance T2 spectrum signal amplitudes after the nth huff and puff for the core pore throat radius interval (i, j] μm of saturated shale oil, dimensionless; i and j are the pore throat radius values, μm.

[0023] Further, in the step S1, 3 shale cores are drilled, and the conversion coefficients C1, C2, and C3 between the nuclear magnetic resonance T2 value of the water-saturated core and the pore throat radius are calculated respectively, and the average value is taken to obtain the required pore throat radius conversion coefficient C.

[0024] Further, in the step S1.1, after the core is drilled, it is first dried, and the nuclear magnetic resonance T2 spectrum of the dry core is tested as the base signal.

[0025] Further, in the step S2, a core is drilled at the same position of the reservoir as in the step S1.1, dried first and then the nuclear magnetic resonance T2 spectrum of the dry rock sample is tested as the base signal.

[0026] Further, in the step S3, the injected gas is one of carbon dioxide, associated gas or nitrogen.

[0027] An injection gas device used in the shale gas injection enhanced oil recovery efficiency evaluation method described above includes a constant temperature box. A reaction kettle is provided in the constant temperature box, and a core is provided in the reaction kettle. One air hole of the reaction kettle is connected to a second six-way valve through a pipeline. A vacuum pump and a plunger pump are respectively connected to the second six-way valve, and a gas source is connected to the plunger pump; the other air hole of the reaction kettle is connected to a back pressure valve through a pipeline, and a hand pump is connected to the back pressure valve.

[0028] A first pressure sensor and a first six-way valve are connected to the second six-way valve, and a pressure gauge and a vacuum pump are connected to the first six-way valve.

[0029] The reaction kettle includes a cylinder body. An oil drain hole is provided at the bottom of the cylinder body. The upper part of the cylinder body is connected to an upper plug, and a pressure cap is connected to the upper part of the upper plug. Two air holes are provided on the upper plug, and a pressure pad is provided at the bottom of the upper plug.

[0030] The present invention has the following beneficial effects: Due to the adoption of the above scheme, the present invention combines the gas injection huff and puff experiment technology, the low-field nuclear magnetic resonance technology and the mercury intrusion technology. As long as the conversion coefficient between the nuclear magnetic resonance T2 value and the pore throat radius, the nuclear magnetic resonance T2 spectrum of the shale saturated with shale oil, and the nuclear magnetic resonance T2 spectrum of the shale after gas injection huff and puff are known, the gas injection huff and puff enhanced oil recovery efficiency of the shale core and the gas injection huff and puff enhanced oil recovery efficiency in different pore intervals of the shale can be calculated, providing basic parameters for the high-benefit development of Gulong shale oil. It is mainly used for quantitatively evaluating the gas injection huff and puff enhanced oil recovery efficiency of shale, but it is also applicable to the evaluation of the enhanced oil recovery efficiency of tight sandstone and low-permeability sandstone. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 is the flow chart of the present invention;

[0032] Figure 2 is a schematic structural diagram of the gas injection experimental device in the present invention;

[0033] Figure 3 is a schematic diagram of a high-pressure carbon dioxide-resistant reactor;

[0034] Figure 4 is a fitting curve of the nuclear magnetic pore throat distribution frequency and the mercury injection pore throat distribution frequency;

[0035] Figure 5 is the nuclear magnetic resonance T2 spectrum of shale gas injection huff and puff;

[0036] Figure 6 is the oil displacement efficiency diagram of shale gas injection huff and puff;

[0037] Figure 7 is the oil displacement efficiency diagram of shale gas injection huff and puff in different pore intervals.

[0038] In the figure, 1 - gas source, 2 - plunger pump, 3 - vacuum pump, 4 - first six-way valve, 5 - vacuum pressure gauge, 6 - second six-way valve, 7 - first pressure sensor, 8 - spring pipeline, 9 - constant temperature box, 10 - high-pressure carbon dioxide-resistant reactor, 11 - shale core, 12 - second pressure sensor, 13 - third six-way valve, 14 - hand pump, 15 - back pressure valve, V1 to V10 are all valves, 16 - air hole, 17 - upper plug, 18 - compression cap, 19 - cylinder body, 20 - pressure pad, 21 - oil drain hole. Detailed implementation manners

[0039] The present invention will be further described below with reference to the accompanying drawings:

[0040] As Figures 1 to 7 shown, a method for evaluating the oil displacement efficiency of shale gas injection includes the following steps:

[0041] S1. Calculate the conversion coefficient C between the nuclear magnetic resonance T2 value of the water-saturated core and the pore throat radius.

[0042] S1.1. Drill at least 3 columnar shale cores with a diameter of 2.5 cm and a length of 2.5 cm from the same reservoir. To prevent the shale cores from breaking, wrap the shale cores with heat shrinkable tubes, wash the oil and dry the shale cores wrapped with heat shrinkable tubes, and measure the dry weight of the shale cores.

[0043] Put the dry shale cores into the nuclear magnetic resonance probe coil respectively. Select the CPMG sequence, set the nuclear magnetic resonance measurement parameters, set the waiting time (TW) to 2000 ms, the echo interval (TE) to 0.06 ms, the number of echo (NECH) to 3000, and the number of accumulations (NS) to 24 times, and measure the T2 spectrum of the dry rock sample as the base signal.

[0044] The shale core is evacuated and saturated with simulated formation water, the weight of the saturated shale core is measured, the porosity of the shale core is calculated, and the water-saturated core is placed in the nuclear magnetic resonance probe coil to measure the nuclear magnetic resonance T2 spectrum. The nuclear magnetic parameters are set as above.

[0045] S1.2. Dry the core and conduct a high-pressure mercury injection experiment to obtain the mercury injection pore throat distribution frequency curve.

[0046] S1.3. Calculate the nuclear magnetic resonance pore throat distribution frequency curve from the nuclear magnetic resonance T2 spectrum of the water-saturated core in step S1.1.

[0047] The expression of the nuclear magnetic resonance T2 relaxation time is:

[0048]

[0049] In the formula: S is the surface area of the pore; V is the volume of the pore; Fs is the geometric shape factor; ρ is the transverse surface relaxation intensity of the rock, μm / ms; C is the conversion coefficient between the nuclear magnetic resonance T2 value and the pore throat radius, μm / ms.

[0050] In formula (1), a series of C values can be given to obtain the pore throat radius r corresponding to the T2 value. According to the nuclear magnetic resonance T2 spectrum of the water-saturated core, divide the sum of the signal amplitudes in each pore throat distribution interval by the sum of the signal amplitudes of the nuclear magnetic resonance T2 spectrum to obtain the nuclear magnetic resonance pore throat distribution frequency in each pore throat distribution interval. Calculate the pore throat distribution frequencies of 23 pore throat distribution intervals respectively. These 23 pore throat distribution intervals are (-∞, 0.001] μm, (0.001, 0.0016] μm, (0.0016, 0.0025] μm, (0.0025, 0.0040] μm, (0.0040, 0.0063] μm, (0.0063, 0.01] μm, (0.01, 0.016] μm, (0.016, 0.025] μm, (0.025, 0.040] μm, (0.040, 0.063] μm, (0.063, 0.1] μm, (0.1, 0.16] μm, (0.16, 0.25] μm, (0.25, 0.4] μm, (0.4, 0.63] μm, (0.63, 1] μm, (1, 1.6] μm, (1.6, 2.5] μm, (2.5, 4] μm, (4, 6.3] μm, (6.3, 10] μm, (10, 16] μm, (16, +∞) μm.

[0051] Fit the nuclear magnetic resonance pore throat distribution frequency curve with the mercury injection pore throat distribution frequency curve obtained in step S1.2, as shown in Figure 4, the conversion coefficient between the nuclear magnetic resonance T2 value and the pore throat radius is obtained. For 3 or more shale cores, the pore throat radius conversion coefficients C1, C2, C3... are calculated respectively, and the average value is taken to obtain the required pore throat radius conversion coefficient C.

[0052] S2. Test the nuclear magnetic resonance T2 spectrum of the saturated shale oil core,

[0053] S2.1. Drill a columnar core with a diameter of 2.5 cm and a length less than 10 cm at the same position of the reservoir in step S1. Wrap the shale core with heat shrinkable tubing, wash the oil, dry it, test the weight of the dry rock sample, and at the same time test the nuclear magnetic resonance T2 spectrum of the dry rock sample as the base signal. The test parameters of the nuclear magnetic resonance T2 spectrum are the same as those in step S1.

[0054] S2.2. Vacuum the shale core for 5 hours until the vacuum degree reaches -0.1 MPa, then saturate the shale oil at 90 °C under a pressure of 30 MPa for 1 week, test the weight of the saturated shale, calculate the porosity of the shale core, and at the same time test the T2 spectrum of the shale saturated with shale oil. The test parameters of the nuclear magnetic resonance T2 spectrum are the same as those in step S1.

[0055] S3. Use the gas injection experimental device to conduct a gas injection and huff and puff experiment on the core saturated with shale oil, and test the nuclear magnetic resonance T2 spectrum of the core after each gas injection and huff and puff.

[0056] S3.1. Detect the airtightness of the experimental device. Open valves V1, V2, V4, V5, V7, and inject gas into the carbon dioxide-resistant reactor 10 through the plunger pump 2 until the pressure reaches 30 MPa. Observe whether the pressure changes within half an hour. If the pressure drops, the pipeline leaks. Then, drop soapy water at each joint of the experimental device. If soap bubbles appear at the joint, it leaks. Release the pressure of the experimental device until the system pressure is 0, then tighten the joint here and continue to pressurize and detect until the pressure stabilizes at 30 MPa and does not change for half an hour.

[0057] S3.2. Vacuum the experimental device. Place the shale core saturated with shale oil in the reactor 10, close valves V2 and V4, open valves V3 and V6, and use the vacuum pump 3 to evacuate to -0.1 MPa for half an hour.

[0058] S3.3. Inject gas for the first time. Open the thermostat 9 and heat the spring pipeline 8 and the reactor 10. After the temperature of the thermostat reaches the set temperature T °C, open valves V2, V9, V10, close valve V3, use the hand pump 14 to adjust the back pressure valve pressure to the set pressure P1 MPa, and then use the plunger pump 2 to inject gas into the reactor 10 at a low speed. When the pressure of the first pressure sensor 7 reaches P2 MPa, the plunger pump 2 changes to a constant pressure injection mode and continues to inject for h hours, where the pressure P1 is greater than P2.

[0059] S3.4. The experimental device is depressurized. The plunger pump 2 is closed, and the hand pump 14 is used to slowly depressurize until the pressure of the second pressure sensor 12 drops to 0.

[0060] S3.5. Take out the core from the reactor 10, wipe the shale oil on the surface of the core clean, place the core in the nuclear magnetic resonance probe coil for T2 spectrum test, and set the test parameters the same as in step S1.

[0061] S3.6. Inject gas in a cycle, repeat steps S3.2 to S3.5, and perform the 2nd to the nth gas injection in sequence. After each huff and puff cycle, perform the nuclear magnetic resonance T2 spectrum test, as shown in Figure 5 .

[0062] S4. Use the conversion coefficient C obtained in step S1 to convert the nuclear magnetic resonance T2 value of the core saturated with shale oil into the pore throat radius, and calculate the oil displacement efficiency of gas injection huff and puff in different pore intervals of the core according to the nuclear magnetic resonance T2 spectrum signal amplitudes of the dry shale core, the core saturated with oil shale, and the shale cores in different huff and puff cycles.

[0063] S4.1. Calculate the oil displacement efficiency of the core after the nth huff and puff. The expression is

[0064]

[0065] In the formula: E n is the oil displacement efficiency of the core after the nth huff and puff, %; S 饱和油 is the total sum of the nuclear magnetic resonance T2 spectrum signal amplitudes of the core saturated with shale oil, dimensionless; S 干样 is the total sum of the nuclear magnetic resonance T2 spectrum signal amplitudes of the dry shale core, dimensionless; S n is the total sum of the nuclear magnetic resonance T2 spectrum signal amplitudes of the core saturated with shale oil after the nth huff and puff, dimensionless. See the oil displacement efficiency diagram in Figure 6 .

[0066] S4.2. Calculate the oil displacement efficiency of the core in different pore throat radius intervals after the nth huff and puff. The calculation expression of the oil displacement efficiency is

[0067]

[0068] In the formula: is the oil displacement efficiency of the core in the pore throat radius interval (i, j] μm after the nth huff and puff, %; is the total sum of the nuclear magnetic resonance T2 spectrum signal amplitudes of the core saturated with shale oil in the pore throat radius interval (i, j] μm, dimensionless; is the total sum of the nuclear magnetic resonance T2 spectrum signal amplitudes of the dry shale core in the pore throat radius interval (i, j] μm, dimensionless; is the total amplitude of the nuclear magnetic resonance T2 spectrum signal after the nth huff and puff for the core pore throat radius range (i, j] μm of saturated shale oil, dimensionless; i and j are the pore throat radius values, μm.

[0069] Calculated is the oil displacement efficiency of the core pore throat radius range (i, j] μm after the nth huff and puff. See specifically Figure 7 .

[0070] An injection gas device used in the above shale gas injection oil displacement efficiency evaluation method, including a constant temperature box 9. Inside the constant temperature box 9, there is a reaction kettle 10 that is resistant to pressure and carbon dioxide. The reaction kettle 10 includes a cylinder body 19. A core 11 is placed inside the cylinder body 19. The upper part of the cylinder body 19 is connected to an upper plug 17. The upper part of the upper plug 17 is connected to a compression cap 18. There are two air holes 16 on the upper plug 17. Gas can be injected into or the vacuum can be pumped out of the reaction kettle through the air holes 16. A pressure pad 20 is provided at the bottom of the upper plug 17. An oil discharge hole 21 is provided at the bottom of the cylinder body 19.

[0071] One air hole 16 of the reaction kettle 10 is connected to a valve V7. A spring pipeline 8 is connected to the valve V7. The spring pipeline 8 is located inside the constant temperature box 9. The other end of the spring pipeline 8 extends out of the constant temperature box 9 and is connected to a second six-way valve 6 through a valve V5. Valves V2, V3, and V4 are respectively connected to the second six-way valve 6. Among them, the valve V2 is connected to an 8-plunger pump 2. The other end of the plunger pump 2 is connected to a gas source 1 through a valve V1. The valve V3 is connected to a first six-way valve 4. A pressure gauge 5 and a valve V6 are respectively connected to the first six-way valve 4. The other end of the valve V6 is connected to a vacuum pump 3. A first pressure sensor 7 is connected to the valve V4.

[0072] The other air hole 16 of the reaction kettle 10 is connected to a valve V8. The other end of the valve V8 is connected to a back pressure valve 15. The back pressure valve 15 is connected to a third six-way valve 13 through a valve V9. A second pressure sensor 12 and a valve V10 are respectively connected to the third six-way valve 13. The other end of the valve V10 is connected to a hand pump 14.

[0073] This injection gas device is used for gas injection huff and puff of the core. Among them, the plunger pump 2 is used to increase the pressure of the system and detect the tightness of the pipeline. The vacuum pump 3 is used to evacuate the pipeline. The spring pipeline 8 and the constant temperature box 9 are used to heat the injected gas. The hand pump 14 and the back pressure valve 15 are used to provide back pressure to the system and also play a protective role. The gas will expand rapidly when heated. Once the outlet pressure is higher than the back pressure, the gas will be discharged from the reaction kettle 10.

Claims

1. A method for evaluating the oil displacement efficiency of shale gas injection, characterized in that It includes the following steps: S1. Calculate the conversion coefficient C between the nuclear magnetic resonance T2 value of the saturated water-rock core and the pore throat radius. S1.

1. Drill a shale core, evacuate and saturate it with simulated formation water, test the nuclear magnetic resonance T2 spectrum, and calculate the nuclear magnetic pore throat distribution frequency curve from the nuclear magnetic resonance T2 spectrum of the saturated water-rock core. S1.

2. Dry the core and conduct a high-pressure mercury injection experiment to obtain the mercury injection pore throat distribution frequency curve. S1.

3. Fit the nuclear magnetic pore throat distribution frequency curve and the mercury injection pore throat distribution frequency curve to obtain the conversion coefficient C between the nuclear magnetic resonance T2 value and the pore throat radius. S2. Test the nuclear magnetic resonance T2 spectrum of the saturated shale oil core. S3. Use the gas injection experimental device to conduct a gas injection huff and puff experiment on the saturated shale oil core, and test the nuclear magnetic resonance T2 spectrum of the core after each gas injection huff and puff. S4. Use the conversion coefficient C obtained in step S1 to convert the nuclear magnetic resonance T2 value of the saturated shale oil core into the pore throat radius, and calculate the oil displacement efficiency of gas injection huff and puff in different pore intervals of the core according to the nuclear magnetic resonance T2 spectrum signal amplitudes of the dry shale core, the saturated oil shale core, and the shale core in different huff and puff cycles. The said step S4 includes: S4.

1. Calculate the oil displacement efficiency after the nth huff and puff of the saturated shale oil core. The expression is: （2） Wherein: is the oil displacement efficiency of the core after the nth huff and puff, %; is the total amplitude of the nuclear magnetic resonance T2 spectrum signal of the core saturated with shale oil, dimensionless; is the total amplitude of the nuclear magnetic resonance T2 spectrum signal of the dry shale core, dimensionless; is the total amplitude of the nuclear magnetic resonance T2 spectrum signal of the core saturated with shale oil after the nth huff and puff, dimensionless; S4.

2. Calculate the oil displacement efficiency after the nth huff and puff in different pore intervals of the saturated shale oil core. The calculation expression of the oil displacement efficiency is: （3） In the formula: is the core pore throat radius interval after the nth huff and puff Displacement efficiency, %; is the core pore throat radius interval saturated with shale oil Total amplitude of nuclear magnetic resonance T2 spectrum signal, dimensionless; is the core pore throat radius interval of dry shale core Total amplitude of nuclear magnetic resonance T2 spectrum signal, dimensionless; is the core pore throat radius interval saturated with shale oil Total amplitude of nuclear magnetic resonance T2 spectrum signal after the nth huff and puff, dimensionless; i and j are pore throat radius values, μm.

2. The shale gas injection enhanced oil recovery efficiency evaluation method according to claim 1, wherein: In the said step S1, drill 3 shale cores, calculate the conversion coefficients C1, C2, and C3 between the nuclear magnetic resonance T2 value of the saturated water-rock core and the pore throat radius respectively, and take the average value to obtain the required pore throat radius conversion coefficient C.

3. The shale gas injection enhanced oil recovery efficiency evaluation method according to claim 2, wherein: In the said step S1.1, after drilling the core, first dry the core and test the nuclear magnetic resonance T2 spectrum of the dry core as the base signal.

4. The shale gas injection enhanced oil recovery efficiency evaluation method according to claim 3, wherein: In the said step S1.1, before drying the core, in order to prevent the core from breaking, wrap the core with a heat shrinkable tube, wash the oil from the shale core wrapped with the heat shrinkable tube, and dry it.

5. The shale gas injection enhanced oil recovery efficiency evaluation method according to claim 1, wherein: In the said step S2, drill a core at the same reservoir position as in step S1.1, first dry it and then test the nuclear magnetic resonance T2 spectrum of the dry rock sample as the base signal.

6. The shale gas injection enhanced oil recovery efficiency evaluation method according to claim 5, wherein: The said step S2 also includes evacuating the core for 5 hours until the vacuum degree reaches -0.1 MPa, then saturating it with shale oil at a pressure of 30 MPa under the condition of 90 °C for 1 week, testing the weight of the saturated shale, calculating the porosity of the shale core, and simultaneously testing the T2 spectrum of the saturated shale oil shale.

7. The shale gas injection enhanced oil recovery efficiency evaluation method according to claim 1, characterized in that: In the said step S3, the injected gas is one of carbon dioxide, associated gas or nitrogen.

8. The shale gas injection enhanced oil recovery efficiency evaluation method according to claim 7, wherein: The said step S3 includes: S3.

1. Detect the airtightness of the gas injection experimental device until the pressure remains stable at 30 MPa for half an hour without change. S3.

2. Evacuate the gas injection experimental device, place the shale core saturated with shale oil in the reaction kettle (10), and evacuate it to -0.1 MPa for half an hour. S3.

3. First, inject gas and heat the reactor. After the temperature of the constant temperature box reaches the set temperature of T °C, open valves V2, V9, and V10, close valve V3, and use the hand pump (14) to increase the back pressure valve pressure to the set pressure of P1 MPa. Then, use the plunger pump (2) to inject gas into the reactor (10) at a low speed. When the pressure of the first pressure sensor (7) reaches P2 MPa, the plunger pump (2) switches to the constant pressure injection mode and continues to inject for h hours, where the pressure P1 is greater than P2; S3.

4. Release the pressure of the experimental device. Close the plunger pump 2 and use the hand pump 14 to slowly release the pressure until the pressure of the second pressure sensor (12) drops to 0; S3.

5. Take out the core from the reactor (10), wipe the shale oil on the surface of the core clean, and place the core in the nuclear magnetic resonance probe coil for T2 spectrum testing; S3.

6. Inject gas in a cycle and repeat steps S3.2 to S3.5 to perform the second to the nth gas injection in sequence. After each huff and puff cycle, conduct nuclear magnetic resonance T2 spectrum testing.

Images