An experimental device and method for permeability evolution using scCO2

Abstract

The application discloses an experimental device and method for permeability evolution by using scCO2. The experimental device comprises a CO2 storage tank, a booster pump, a vacuum pump, a stop valve, a pulse manufacturing unit, a pressure regulating valve, a pressure sensor, a temperature sensor, a confining pressure pump, a confining pressure valve, a measuring chamber, a cleaning tank, a control host, a sewage recovery tank, a polymer solution storage tank, a rock sample, a heating device, a rock sample clamping device and a capillary coil. The experimental method measures the viscosity of scCO2 through the capillary coil, measures the permeability of the rock sample crack by using a differential pressure decay method, observes the process of the stress cycle of scCO2 to the artificial crack to cause the evolution of the permeability of the artificial crack and generates a permeability evolution curve. The application observes the permeability evolution of the artificial crack under the stress cycle, studies the stress sensitivity of the crack, fully considers the influence of the viscosity of scCO2 on the permeability of the rock sample, and through the real-time measurement of the viscosity of scCO2, the permeability measurement of the rock sample is more accurate.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of shale gas development stimulation, in particular to an experimental device and method for permeability evolution using scCO2. BACKGROUND

[0002] CO2 can displace shale gas in shale, thereby improving shale gas recovery, while reducing the content of CO2 in the atmosphere and mitigating the greenhouse effect. When the temperature and pressure of CO2 are greater than 31.10°C and 7.38MPa at the same time, CO2 will reach a supercritical state to become scCO2. scCO2 has diffusion similar to gas and solubility and density similar to liquid, and also has the characteristics of low surface tension, small viscosity, good permeability and flowability. Compared with conventional fracturing fluid, scCO2 has no damage to the reservoir, easy flowback, and has great advantages in the fracturing stimulation of unconventional reservoirs. However, the permeability of scCO2 in the fracture varies greatly with different working conditions, and the fracture has stress sensitivity, and the permeability evolves with the change of stress. As a direct index for predicting the production parameters of shale gas industry, the evolution of permeability under stress has great significance for the evaluation of fracturing effect.

[0003] A rock permeability measuring device is disclosed in patent CN107703039A, which can measure different permeability core segments to improve measurement accuracy, but the device has low automation degree and large human measurement error. A device and method for testing core relative permeability are disclosed in patent CN105784567A, which uses a steady-state method to measure permeability, and multiple experiments are required to obtain the core relative permeability. Patent CN106644871B provides a device and method for evaluating the influence of supercritical carbon dioxide fracturing fluid on oil and gas reservoir permeation, which measures permeability before and after the fracturing fluid enters the core, but the steady-state method has low measurement accuracy for low permeability rocks. Patent CN109813645A provides a radial permeability measurement system and method for low-permeability rock core plunger, which measures permeability by applying pressure to the sample cavity to make high-pressure gas flow radially into the core, causing pressure decay, which has high measurement accuracy but complex internal structure. Patent CN113607620B discloses an experimental device for carbon dioxide directional fracturing and permeability testing, which realizes supercritical carbon dioxide directional fracturing shale experiment under simulated formation conditions, and the permeability measurement uses pulse attenuation method, but the viscosity of supercritical carbon dioxide is not measured in real time, which has errors. Patent CN109975140A discloses an experimental device and method for supercritical carbon dioxide pulse fracturing and permeability testing, which has the same defects as patent CN113607620B except for the fracturing process. The above six patents do not measure the viscosity of the medium for measuring permeability in real time, especially the measuring device using supercritical carbon dioxide as the medium, the viscosity of supercritical carbon dioxide changes greatly with temperature and pressure, and slight changes in experimental conditions will cause significant changes, thereby reducing the accuracy of permeability measurement, and some patents use steady-state method to measure permeability, ignoring the problem that this method is not suitable for low-permeability rocks, so the above methods for measuring permeability have certain limitations.

[0004] Patent CN111340298B discloses a shale reservoir induced heterogeneity dynamic evolution prediction method, which calculates the apparent permeability of different sections of the shale reservoir, obtains the change relationship of the apparent permeability of the shale reservoir at different positions in the matrix with time, and quantitatively characterizes the heterogeneity response and evolution in the matrix by using the change relationship. The patent only proposes a method of using multiple formulas to characterize the evolution layer by layer, without specific experimental process. Patent CN105136646A introduces a permeability evolution experimental device and method considering shale adsorption and fracturing process. In the early stage of fracturing, the transient method is used to measure the permeability when the permeability of the sample is small, and the steady-state method is used to measure the permeability when the permeability of the sample is large. In the above two patents and similar patents, there is no experimental device and method for studying stress cycle induced permeability evolution.

[0005] In summary, there is an urgent need for an experimental device and method for applying stress cycle to artificial fractures by using scCO2 to induce permeability evolution, so as to study the influence law of stress cycle induced permeability evolution of artificial fractures. SUMMARY

[0006] The application provides an experimental device for permeability evolution by using scCO2, which is characterized by comprising a first CO2 storage tank, a first booster pump, a vacuum pump, a first stop valve, a pulse manufacturing unit, a second stop valve, a first pressure regulating valve, a first pressure sensor, a temperature sensor, a third stop valve, a second pressure regulating valve, a second pressure sensor, a confining pressure pump, a confining pressure valve, a measuring chamber, a third pressure sensor, a fourth stop valve, a second booster pump, a second CO2 storage tank, a fourth pressure sensor, a fifth stop valve, a third booster pump, a cleaning tank, a fifth pressure sensor, a sixth stop valve, a control host, a sixth pressure sensor, a seventh stop valve, a sewage recovery tank, an eighth stop valve, a fourth booster pump, a polymer solution storage tank, a ninth stop valve, a rock sample, a heating device, a rock sample clamping device and a capillary coil; the measuring chamber comprises the rock sample clamping device, the capillary coil and the heating device at the bottom; the confining pressure pump and the confining pressure valve jointly form a confining pressure loading system and are communicated with the measuring chamber through pipelines; the first CO2 storage tank is connected with the axial left end of the rock sample clamping device through the first booster pump, the pulse manufacturing unit, the second stop valve and the first pressure regulating valve; the first CO2 storage tank is connected with the radial upper end of the rock sample clamping device through the third stop valve and the second pressure regulating valve; the first CO2 storage tank is connected with the left end of the capillary coil through the ninth stop valve; the second CO2 storage tank is connected with the axial right end of the rock sample clamping device through the second booster pump and the fourth stop valve; the second CO2 storage tank is connected with the radial lower end of the rock sample clamping device through the second booster pump and the sixth stop valve; the polymer solution storage tank is connected with the left end of the capillary coil through the fourth booster pump and the eighth stop valve; the sewage recovery tank is connected with the left end of the capillary coil through the seventh stop valve; the cleaning tank is connected with the right end of the capillary coil through the third booster pump and the fifth stop valve; the first pressure sensor is distributed at the axial left end of the rock sample clamping device; the second pressure sensor is distributed at the axial right end of the rock sample clamping device; the third pressure sensor is distributed at the radial upper end of the rock sample clamping device; the fourth pressure sensor is distributed at the radial lower end of the rock sample clamping device; the fifth pressure sensor is distributed at the left end of the capillary coil; the sixth pressure sensor is distributed at the right end of the capillary coil; the pulse manufacturing unit, the temperature sensor, the first pressure sensor, the second pressure sensor, the third pressure sensor, the fourth pressure sensor, the fifth pressure sensor and the sixth pressure sensor are connected with the control host through signal lines respectively.

[0007] Further, the measuring chamber is provided with the heating device, the rock sample clamping device and the capillary coil, and the rock sample is fixed by the rock sample clamping device.

[0008] Further, the medium in the cleaning tank is water or ethanol.

[0009] Further, the confining pressure pump and the confining pressure valve are communicated with the measuring chamber through pipelines and inject oil pressure into the measuring chamber, so that the rock sample in the measuring chamber is in a confining pressure environment.

[0010] Further, the control host is used for controlling the frequency, waveform and amplitude of the pulse produced by the pulse production unit, collecting and recording the data of the pressure sensor and the temperature sensor, and processing, calculating and drawing the evolution curve of the data.

[0011] An experimental method for permeability evolution by using scCO2, comprising the following steps:

[0012] S1. Place the first set of rock samples in the rock sample clamping device in the measuring chamber, and ensure that the experimental device is kept in a sealed state;

[0013] S2. Turn on the control host, set the experimental temperature, and control the confining pressure loading system to load 1 MPa of hydrostatic confining pressure to the measuring chamber;

[0014] S3. Set the experimental pressure P, and control the confining pressure loading system to apply a confining pressure of P to the measuring chamber;

[0015] S4. Control the first stop valve, the second stop valve, the third stop valve and the ninth stop valve to open, and open the vacuum pump to vacuum the pipeline;

[0016] S5. Close the first stop valve, the second stop valve, the third stop valve and the ninth stop valve, open the fourth booster pump and the eighth stop valve, and make the polymer solution flow into the capillary coil in the measuring chamber, record the readings on the sixth pressure sensor and the fourth pressure sensor, and record the pressure difference ΔP1 when the pressure difference between the left end of the capillary coil and the right end of the capillary coil is stable;

[0017] S6. Close the fourth booster pump and the eighth stop valve, open the third booster pump, the fifth stop valve and the seventh stop valve, and clean the capillary coil and the related pipeline, so that the sewage flows into the sewage recovery tank;

[0018] S7. Close the third booster pump, the fifth stop valve and the seventh stop valve, open the first booster pump and the ninth stop valve, make the scCO2 enter the capillary coil, record the readings on the sixth pressure sensor and the fourth pressure sensor, and record the pressure difference ΔP2 when the pressure difference between the left end of the capillary coil and the right end of the capillary coil is stable, and calculate the viscosity of the scCO2;

[0019] S8. Close the ninth stop valve, open the second stop valve, the first pressure regulating valve, the fourth stop valve and the second booster pump, adjust the scCO2 pressure to p1 through the first pressure regulating valve, and record the readings of the first pressure sensor and the third pressure sensor;

[0020] S9. After the pressure of the first pressure sensor and the third pressure sensor is balanced, the first booster pump is closed, the host computer controls the pulse manufacturing unit to apply a pressure pulse of Δp to the axial inlet end of the rock sample clamping device, the pressure data of the first pressure sensor and the third pressure sensor are collected until the pressure of the two ends of the rock sample clamping device is balanced, and the host computer draws the pressure and time data collected by the pressure sensor into an attenuation curve and calculates the permeability of the artificial fracture;

[0021] S10. The first booster pump is turned on, the first pressure regulating valve is adjusted, the scCO2 pressure is respectively increased to p2, p3 and p4, step 9 is repeated, the permeability of the artificial fracture at this time is calculated, the scCO2 pressure is decreased back to p1 and then increased to p2, p3 and p4 again, step 9 is repeated, and the permeability of the artificial fracture at this time is calculated;

[0022] S11. The host computer draws the permeability of the artificial fracture of the first set of rock samples at each pressure condition into an evolution curve during the process of increasing the scCO2 pressure from p1 to p2, p3 and p4, decreasing back to p1 and then increasing to p2, p3 and p4 again;

[0023] S12. All valves and pumps are closed, the heating device and the host computer are turned off, the measuring chamber is opened after the experimental device is cooled to room temperature, the first set of rock samples is replaced with the second set of rock samples, the measuring chamber is closed, and the experimental device is kept in a sealed state;

[0024] S13. Steps 2 to 7 are repeated, the ninth stop valve is closed, the third stop valve, the second pressure regulating valve, the sixth stop valve and the second booster pump are turned on, the scCO2 pressure is adjusted to p1 through the second pressure regulating valve, and the readings of the second pressure sensor and the sixth pressure sensor are recorded;

[0025] S14. After the pressure of the second pressure sensor and the sixth pressure sensor is balanced, the first booster pump is closed, the host computer controls the pulse manufacturing unit to apply a pressure pulse of Δp to the radial inlet end of the rock sample clamping device, the pressure data of the second pressure sensor and the fifth pressure sensor are collected until the pressure of the two ends of the rock sample clamping device is balanced, and the host computer draws the pressure and time data collected by the pressure sensor into an attenuation curve and calculates the permeability of the artificial fracture;

[0026] S15. The first booster pump is turned on, the second pressure regulating valve is adjusted, the scCO2 pressure is respectively increased to p2, p3 and p4, step 14 is repeated, the permeability of the artificial fracture at this time is calculated, the scCO2 pressure is decreased back to p1 and then increased to p2, p3 and p4 again, step 14 is repeated, and the permeability of the artificial fracture at this time is calculated;

[0027] S16. The control host computer draws an evolution curve of the permeability of the artificial fracture of the second set of rock samples under each pressure condition in the process of increasing the scCO2 pressure from p1 to p2, p3, p4 and then decreasing back to p1 and then increasing again to p2, p3, p4.

[0028] Further, the viscosity μ of the scCO2 in the S7 step is calculated according to the following formula: , wherein ΔP1 is the stable pressure difference between the left end of the capillary tube and the right end of the capillary tube when the polymer solution passes through the capillary tube; ΔP2 is the stable pressure difference between the left end of the capillary tube and the right end of the capillary tube when the scCO2 passes through the capillary tube; and η is the viscosity of the polymer solution under the experimental temperature and experimental pressure.

[0029] Further, the permeability K of the artificial fracture in the S9, S10, S14 and S15 steps is calculated according to the following formula: , , wherein Pu is the stable pressure at the inlet end of the rock sample clamping device after t time, Pf is the pressure when the pressure at the inlet and outlet ends of the rock sample clamping device is balanced after t time, Δp is the pressure pulse applied to the inlet end of the rock sample clamping device by the pulse generating unit, V u is the volume of the first CO2 tank, V d is the volume of the second CO2 tank, α is the slope of the decay curve in the pressure difference-time semilogarithmic graph when the pressure at the inlet and outlet ends of the rock sample clamping device is stable, t is the time elapsed until the pressure at the inlet and outlet ends of the rock sample clamping device is balanced after the pulse is applied, μ is the viscosity of the scCO2 under the experimental temperature and experimental pressure, β is the gas compressibility coefficient of the scCO2 under the experimental temperature and experimental pressure, L is the equivalent length of the rock sample in the measurement direction, and A is the equivalent cross-sectional area of the rock sample in the measurement direction.

[0030] Further, a pre-step is further included, that is, the capillary tube for viscosity measurement and the permeability measurement device of the artificial fracture are placed in the same measurement chamber to ensure that the viscosity measurement and the permeability measurement are performed under the same experimental temperature and experimental pressure, and the viscosity of the polymer solution and the pressure difference between the left and right ends of the capillary tube are used to characterize the viscosity of the scCO2.

[0031] Further, the experiment includes two rock samples: A1. The artificial fracture of the rock sample sawed at an angle of 30° with the long axis of the rock sample, and a diameter of 1.5mm drill hole is drilled on both ends of the rock sample parallel to the long axis of the core, which is distributed on the side close to the fracture and leads to the fracture, and the two drill holes are connected to the axial left end of the rock sample clamping device and the axial right end of the rock sample clamping device respectively; A2. The artificial fracture of the rock sample sawed at an angle of 30° with the short axis of the rock sample, and a diameter of 1.5mm drill hole is drilled on the upper and lower rock sample parallel to the short axis of the core, which is distributed on the side close to the fracture and leads to the fracture, and the two drill holes are connected to the radial upper end of the rock sample clamping device and the radial lower end of the rock sample clamping device respectively.

[0032] The present application provides an experimental device and method for permeability evolution using scCO2, compared with the prior art, the beneficial effects of the present application are:

[0033] (1) The present application optimizes the scCO2 viscosity measurement method, which uses the viscosity of polymer solution and the pressure difference between the left and right ends of the capillary coil to characterize the scCO2 viscosity, so that the structure of the scCO2 viscosity measurement unit is simpler.

[0034] (2) The present application fully considers the influence of scCO2 viscosity on rock sample permeability, and through real-time measurement of scCO2 viscosity, the permeability measurement of rock sample is more accurate.

[0035] (3) The present application can realize the output of the evolution curve of fracture permeability under stress cycle, so as to more comprehensively study and analyze the influence law of stress cycle on fracture permeability.

[0036] (4) The present application can change the experimental conditions according to the actual engineering situation, including experimental pressure, experimental temperature, fracture shape, stress applied by scCO2 to the fracture and stress change gradient, so as to realize the research on the influence law of stress cycle on fracture permeability under different working conditions.

[0037] (5) The experimental device for applying stress cycle to artificial fracture by scCO2 to evolve its permeability provided by the present application has simple structure, easy-to-understand method and is easy to implement. BRIEF DESCRIPTION OF DRAWINGS

[0038] Figure 1 The device structure diagram of the present application is shown in the figure;

[0039] Figure 2 The structure diagram of the scCO2 viscosity and fracture permeability measurement chamber of the present application is shown in the figure;

[0040] Figure 3 The pressure difference-time semi-logarithmic graph when the inlet and outlet ends of the rock sample clamping device are stable is shown in the figure;

[0041] Figure 4 permeability evolution curve of the artificial fracture of the present application;

[0042] Figure 5 cutting schematic diagram of the first set of rock samples of the present application;

[0043] Figure 6 cutting schematic diagram of the second set of rock samples of the present application;

[0044] In the figure, 1, first CO2 tank; 2, first booster pump; 3, vacuum pump; 4, first stop valve; 5, pulse manufacturing unit; 6, second stop valve; 7, first pressure regulating valve; 8, first pressure sensor; 9, temperature sensor; 10, third stop valve; 11, second pressure regulating valve; 12, second pressure sensor; 13, confining pressure pump; 14, confining pressure valve; 15, measuring chamber; 16, third pressure sensor; 17, fourth stop valve; 18, second booster pump; 19, second CO2 tank; 20, fourth pressure sensor; 21, fifth stop valve; 22, third booster pump; 23, cleaning tank; 24, fifth pressure sensor; 25, sixth stop valve; 26, control host; 27, sixth pressure sensor; 28, seventh stop valve; 29, sewage recovery tank; 30, eighth stop valve; 31, fourth booster pump; 32, polymer solution storage tank; 33, ninth stop valve; 34, rock sample; 35, heating device; 36, rock sample clamping device; 37, capillary coil; a1, axial left end of the rock sample clamping device; a2, axial right end of the rock sample clamping device; b1, radial upper end of the rock sample clamping device; b2, radial lower end of the rock sample clamping device; c1, left end of the capillary coil; c2, right end of the capillary coil. DETAILED DESCRIPTION

[0045] The implementation method of the present application is described in detail below in combination with the drawings, and the described is only part of the embodiments, not all the embodiments, and the representation and description irrelevant to the present application are omitted in the drawings and description for the purpose of clarity.

[0046] As Figure 1As shown, the present application provides an experimental device for permeability evolution using scCO2, characterized by comprising a first CO2 gas tank 1, a first booster pump 2, a vacuum pump 3, a first stop valve 4, a pulse manufacturing unit 5, a second stop valve 6, a first pressure regulating valve 7, a first pressure sensor 8, a temperature sensor 9, a third stop valve 10, a second pressure regulating valve 11, a second pressure sensor 12, a confining pressure pump 13, a confining pressure valve 14, a measuring chamber 15, a third pressure sensor 16, a fourth stop valve 17, a second booster pump 18, a second CO2 gas tank 19, a fourth pressure sensor 20, a fifth stop valve 21, a third booster pump 22, a cleaning tank 23, a fifth pressure sensor 24, a sixth stop valve 25, a control host 26, a sixth pressure sensor 27, a seventh stop valve 28, a sewage recovery tank 29, an eighth stop valve 30, a fourth booster pump 31, a polymer solution storage tank 32, a ninth stop valve 33, a rock sample 34, a heating device 35, a rock sample clamping device 36, a capillary coil 37; the measuring chamber 15 comprises the rock sample clamping device 36, the capillary coil 37 and the heating device 35 at the bottom.

[0047] Wherein, the confining pressure pump 13 and the confining pressure valve 14 jointly constitute a confining pressure loading system, which is communicated with the measuring chamber 15 through a pipeline; the first CO2 gas tank 1 is connected with the axial left end a1 of the rock sample clamping device through the first booster pump 2, the pulse manufacturing unit 5, the second stop valve 6 and the first pressure regulating valve 7; the first CO2 gas tank 1 is connected with the radial upper end b1 of the rock sample clamping device through the third stop valve 10 and the second pressure regulating valve 11; the first CO2 gas tank 1 is connected with the left end c1 of the capillary coil through the ninth stop valve 33; the second CO2 gas tank 19 is connected with the axial right end a2 of the rock sample clamping device through the second booster pump 18 and the fourth stop valve 17; the second CO2 gas tank 19 is connected with the radial lower end b2 of the rock sample clamping device through the second booster pump 18 and the sixth stop valve 25; the polymer solution storage tank 32 is connected with the left end c1 of the capillary coil through the fourth booster pump 31 and the eighth stop valve 30; the sewage recovery tank 29 is connected with the left end c1 of the capillary coil through the seventh stop valve 28; the cleaning tank 23 is connected with the right end c2 of the capillary coil through the third booster pump 22 and the fifth stop valve 21; the first pressure sensor 8 is distributed at the axial left end a1 of the rock sample clamping device; the second pressure sensor 12 is distributed at the axial right end a2 of the rock sample clamping device; the third pressure sensor 16 is distributed at the radial upper end b1 of the rock sample clamping device; the fourth pressure sensor 20 is distributed at the radial lower end b2 of the rock sample clamping device; the fifth pressure sensor 24 is distributed at the left end c1 of the capillary coil; the sixth pressure sensor 27 is distributed at the right end c2 of the capillary coil; the pulse manufacturing unit 5, the temperature sensor 9, the first pressure sensor 8, the second pressure sensor 12, the third pressure sensor 16, the fourth pressure sensor 20, the fifth pressure sensor 24 and the sixth pressure sensor 27 are respectively connected with the control host 26 through signal lines.

[0048] The measuring chamber 15 is provided with a heating device 35, a rock sample clamping device 36, and a capillary coil 37. The heating device 35 at the bottom of the measuring chamber 15 provides a temperature required for the experiment. The rock sample 34 is fixed by the rock sample clamping device 36. All pipelines of the device are provided with a heating jacket to ensure that the experiment is carried out at the experimental temperature. The cleaning tank 23 contains water or ethanol as a medium for cleaning the capillary coil 37 and pipelines. The sewage recovery tank 29 recovers sewage for cleaning the capillary coil 37 and pipelines. The confining pressure pump 13 and the confining pressure valve 14 are connected to the measuring chamber 15 through pipelines and inject oil pressure into the measuring chamber 15, so that the rock sample 34 in the measuring chamber 15 is in a confining pressure environment required by the experiment. The control host 26 can control the frequency, waveform, and amplitude of the pulses generated by the pulse generating unit 5, collect and record the data of the pressure sensor and the temperature sensor, and process and calculate the data and draw the evolution curve.

[0049] The method for applying stress cycles to an artificial fracture by using scCO2 to evolve the permeability of the artificial fracture by using the experimental device described above comprises the following steps:

[0050] S1. Place the first set of rock samples in the rock sample clamping device 36 in the measuring chamber 15, and ensure that the experimental device is kept in a sealed state.

[0051] S2. Turn on the control host 26, set the experimental temperature to 40℃, which can ensure that CO2 becomes scCO2 after entering the rock sample, and control the confining pressure loading system to load a hydrostatic confining pressure of 1MPa to the measuring chamber 15.

[0052] S3. Set the experimental pressure P to 20MPa, and control the confining pressure loading system to apply a confining pressure of P to the measuring chamber 15.

[0053] S4. Open the first stop valve 4, the second stop valve 6, the third stop valve 10, and the ninth stop valve 33, and open the vacuum pump 3 to vacuum the pipelines.

[0054] S5. Close the first stop valve 4, the second stop valve 6, the third stop valve 10, and the ninth stop valve 33, open the fourth booster pump 31 and the eighth stop valve 30, and make the polymer solution flow into the capillary coil 37 in the measuring chamber 15. Record the readings on the sixth pressure sensor 27 and the fourth pressure sensor 20. After the pressure difference between the left end c1 of the capillary coil 37 and the right end c2 of the capillary coil 37 stabilizes, record the pressure difference ΔP1 at this time.

[0055] S6. Close the fourth booster pump 31 and the eighth stop valve 30, and open the third booster pump 22, the fifth stop valve 21, and the seventh stop valve 28 to clean the capillary coil 37 and the related pipelines, and make the sewage flow into the sewage recovery tank 29.

[0056] S7. Close the third booster pump 22, the fifth stop valve 21 and the seventh stop valve 28, open the first booster pump 2, the ninth stop valve 33, make the scCO2 enter the capillary coil 37, record the readings of the sixth pressure sensor 27 and the fourth pressure sensor 20, and when the pressure difference between the left end c1 and the right end c2 of the capillary coil 37 is stable, record the pressure difference ΔP2 at this time, and calculate the viscosity of the scCO2.

[0057] The viscosity μ of the scCO2 in the step S7 is calculated by formula (1):

[0058] (1)

[0059] In formula (1), ΔP1 is the stable pressure difference between the left end c1 and the right end c2 of the capillary coil 37 when the polymer solution passes through the capillary coil 37, and ΔP2 is the stable pressure difference between the left end c1 and the right end c2 of the capillary coil 37 when the scCO2 passes through the capillary coil 37; η is the viscosity of the polymer solution under the experimental temperature and pressure conditions.

[0060] The measured pressure difference ΔP1 before and after the capillary coil 37 is 358.6 Pa, and the pressure difference ΔP2 is 6.5 Pa, and the viscosity η of the polymer solution under the temperature and pressure conditions is 4.325×10 -3 Pa·s, which is substituted into formula (1) to obtain the viscosity of the supercritical carbon dioxide under this condition, which is 7.84×10 -5 Pa·s, and the viscosity of the supercritical carbon dioxide under the temperature and pressure conditions is 7.83×10 -5 Pa·s, which is less than 0.2%.

[0061] S8. Close the ninth stop valve 33, open the second stop valve 6, the first pressure regulating valve 7, the fourth stop valve 17 and the second booster pump 18, adjust the scCO2 pressure to p1 through the first pressure regulating valve 7, and record the readings of the first pressure sensor 8 and the third pressure sensor 16;

[0062] S9. After the pressures of the first pressure sensor 8 and the third pressure sensor 16 are balanced, the first booster pump 2 is closed, the host computer 26 controls the pulse manufacturing unit 5 to apply a pressure pulse of Δp to the axial inlet end a1 of the rock sample clamping device 36, and the pressure data of the first pressure sensor 8 and the third pressure sensor 16 are collected until the pressures of the two ends of the rock sample clamping device 36 are balanced, the host computer 26 draws the pressure and time data collected by the pressure sensor into an attenuation curve, and calculates the permeability of the artificial fracture.

[0063] The permeability K of the fracture in the step S9 is calculated by formula (2) and formula (3):

[0064] (2)

[0065] In formula (2), P u is the stable pressure of the inlet end of the rock sample clamping device 36 after t time, P f is the pressure when the inlet and outlet ends of the rock sample clamping device 36 are balanced after t time, Δp is the pressure pulse applied to the inlet end of the rock sample clamping device 36 by the pulse making unit, V u is the volume of the first CO2 tank 1, V d is the volume of the second CO2 tank 19, and α is the slope of the decay curve in the pressure difference-time semi-logarithmic graph when the inlet and outlet ends of the rock sample clamping device 36 are balanced.

[0066] The control host 26 plots the collected pressure and time data into a pressure difference-time logarithmic graph, as shown in Figure 3 , and obtains the value of α as -7×10 -4 .

[0067] (3)

[0068] In formula (3), K is the permeability of the artificial fracture, μ is the viscosity of scCO2 under the experimental temperature and pressure conditions, β is the gas compressibility coefficient of scCO2 under the experimental temperature and pressure conditions, L is the equivalent length of the rock sample in the measurement direction, and A is the equivalent cross-sectional area of the rock sample in the measurement direction.

[0069] Under the temperature and pressure conditions, the viscosity μ of scCO2 is 7.84×10 -5 Pa·s, the gas compressibility coefficient β of scCO2 is 0.285, the equivalent length L of the rock sample in the measurement direction is 50.8 mm, the equivalent cross-sectional area A of the rock sample in the measurement direction is 1012.9 mm 2 , V u = 0.5 m 2 , V d = 0.5 m 2 , and α = -7×10 -4 . Substituting the above values into formula (3) obtains the permeability K of the artificial fracture under the temperature and pressure conditions as 0.196 mD.

[0070] S10. Turn on the first booster pump 2, adjust the first pressure regulating valve 7, and increase the scCO2 pressure to p2, p3, and p4, respectively. Repeat step 9 to calculate the permeability of the artificial fracture at this time. Reduce the scCO2 pressure back to p1 and increase it to p2, p3, and p4 again. Repeat step 9 to calculate the permeability of the artificial fracture at this time.

[0071] S11. The control host 26 increases the scCO2 pressure from p1 to p2, p3, p4, and then decreases back to p1 and increases again to p2, p3, p4. The permeability of the artificial fracture of the first set of rock samples under each pressure condition is plotted as an evolution curve, as shown in FIG. 2. The curve in the figure is the pressure change curve, and the discrete points are the measured permeability at the pressure and time. Figure 4

[0072] S12. Close all valves and pumps, turn off the heating device 35 and the control host 26, and open the measuring chamber 15. Replace the first set of rock samples with the second set of rock samples, close the measuring chamber 15, and ensure that the experimental device remains sealed.

[0073] S13. Repeat steps 2 to 7, close the ninth stop valve 33, open the third stop valve 10, the second pressure regulating valve 11, the sixth stop valve 25, and the second booster pump 18. Adjust the scCO2 pressure to 8 MPa through the second pressure regulating valve 11, and record the readings of the second pressure sensor 12 and the sixth pressure sensor 24.

[0074] S14. After the pressure of the second pressure sensor 12 and the sixth pressure sensor 24 balances, close the first booster pump 2, and control the control host 26 to apply a 50 psi pressure pulse to the radial inlet end b1 of the rock sample clamping device 36 through the pulse generating unit 5. Collect the pressure data of the second pressure sensor 12 and the fifth pressure sensor 24 until the pressure of the rock sample clamping device 36 balances at both ends. The computer plots the pressure and time data collected by the pressure sensor as a decay curve and calculates the permeability of the artificial fracture.

[0075] S15. Open the first booster pump 2, adjust the second pressure regulating valve 11, and increase the scCO2 pressure to 10 MPa, 12 MPa, and 14 MPa, respectively. Repeat step 14 and calculate the permeability of the artificial fracture at this time. Decrease the scCO2 pressure back to p1 and increase it again to 10 MPa, 12 MPa, and 14 MPa, respectively. Repeat step 14 and calculate the permeability of the artificial fracture at this time.

[0076] S16. The control host 26 increases the scCO2 pressure from 8 MPa to 10 MPa, 12 MPa, and 14 MPa, and then decreases back to 8 MPa and increases again to 10 MPa, 12 MPa, and 14 MPa. The permeability of the artificial fracture of the second set of rock samples under each pressure condition is plotted as an evolution curve.

[0077] A pre-step is also included: placing the capillary coil used for viscosity measurement and the artificial fracture permeability measurement device in the same measuring chamber to ensure that viscosity measurement and permeability measurement are performed at the same experimental temperature and experimental pressure. The viscosity of the polymer solution is used to characterize the scCO2 viscosity, and the pressure difference between the left and right ends of the capillary coil is used to characterize the scCO2 viscosity.​

[0078] Two kinds of rock samples are included in the experiment: A1. The artificial fracture sawn in the rock sample is at an angle of 30° with the long axis of the rock sample, and two holes with a diameter of 1.5 mm are drilled at the two ends of the rock sample parallel to the long axis of the core, which are distributed on the side close to the fracture and lead to the fracture, and the two holes are connected to the axial left end a1 of the rock sample clamping device and the axial right end a2 of the rock sample clamping device respectively, as shown in Figure 5 A2. The artificial fracture sawn in the rock sample is at an angle of 30° with the short axis of the rock sample, and two holes with a diameter of 1.5 mm are drilled on the upper and lower parts of the rock sample parallel to the short axis of the core, which are distributed on the side close to the fracture and lead to the fracture, and the two holes are connected to the radial upper end b1 of the rock sample clamping device and the radial lower end b2 of the rock sample clamping device respectively, as shown in Figure 6 .

[0079] The above is only the preferred embodiment of the present application, and it should be understood that the present application is not limited to the form disclosed herein, and should not be considered as excluding other embodiments, but can be used in various other combinations, modifications and environments, and can be modified within the scope of the concept described herein by the above teaching or related technical or knowledge. Any modification and change made by the person in the art without departing from the spirit and scope of the present application shall be within the protection scope of the claims of the present application.

Claims

1. An experimental apparatus for permeability evolution using scCO2, characterized in that, Includes a first CO2 storage tank (1), a first booster pump (2), a vacuum pump (3), a first shut-off valve (4), a pulse manufacturing unit (5), a second shut-off valve (6), a first pressure regulating valve (7), a first pressure sensor (8), a temperature sensor (9), a third shut-off valve (10), a second pressure regulating valve (11), a second pressure sensor (12), a confining pressure pump (13), a confining pressure valve (14), a measuring chamber (15), a third pressure sensor (16), a fourth shut-off valve (17), a second booster pump (18), a second CO2 storage tank (19), a fourth pressure sensor (20), a fifth shut-off valve (21), a third booster pump (22), a cleaning tank (23), a fifth pressure sensor (24), and a sixth shut-off valve (25). 25), control host (26), sixth pressure sensor (27), seventh shut-off valve (28), sewage recovery tank (29), eighth shut-off valve (30), fourth booster pump (31), polymer solution storage tank (32), ninth shut-off valve (33), rock sample (34), heating device (35), rock sample clamping device (36), capillary coil (37); the measuring chamber (15) includes the rock sample clamping device (36), capillary coil (37) and the heating device (35) at the bottom; the confining pressure pump (13) and the confining pressure valve (14) together constitute the confining pressure loading system, which is connected to the measuring chamber (15) through pipelines; the first CO2 storage tank (1) passes through the first booster pump (2), pulse manufacturing unit (5), and second shut-off valve. Valve (6) and first pressure regulating valve (7) are connected to the axial left end (a1) of the rock sample clamping device; the first CO2 storage tank (1) is connected to the radial upper end (b1) of the rock sample clamping device via the first booster pump (2), pulse manufacturing unit (5), third shut-off valve (10), and second pressure regulating valve (11); the first CO2 storage tank (1) is connected to the left end (c1) of the capillary coil via the first booster pump (2) and ninth shut-off valve (33); the second CO2 storage tank (19) is connected to the axial right end (a2) of the rock sample clamping device via the second booster pump (18) and fourth shut-off valve (17); the second CO2 storage tank (19) is connected to the radial lower end (a2) of the rock sample clamping device via the second booster pump (18) and sixth shut-off valve (25). b2) connected; polymer solution storage tank (32) is connected to the left end (c1) of capillary coil via fourth booster pump (31) and eighth shut-off valve (30); sewage recovery tank (29) is connected to the left end (c1) of capillary coil via seventh shut-off valve (28); cleaning tank (23) is connected to the right end (c2) of capillary coil via third booster pump (22) and fifth shut-off valve (21); first pressure sensor (8) is located at the axial left end (a1) of rock sample clamping device; second pressure sensor (12) is located at the radial upper end (b1) of rock sample clamping device; third pressure sensor (16) is located at the axial right end (a2) of rock sample clamping device; fourth pressure sensor (20) is located at the right end (c2) of capillary coil.The fifth pressure sensor (24) is located at the lower radial end (b2) of the rock sample clamping device; the sixth pressure sensor (27) is located at the left end (c1) of the capillary coil; the pulse generation unit (5), temperature sensor (9), first pressure sensor (8), second pressure sensor (12), third pressure sensor (16), fourth pressure sensor (20), fifth pressure sensor (24), and sixth pressure sensor (27) are all connected to the control host (26) via signal lines.

2. The experimental apparatus for permeability evolution using scCO2 according to claim 1, characterized in that, The rock sample (34) is fixed by the rock sample clamping device (36); heating jackets are installed outside all pipelines of the device.

3. The experimental apparatus for permeability evolution using scCO2 according to claim 1, characterized in that, The medium inside the cleaning tank (23) is water or ethanol.

4. The experimental apparatus for permeability evolution using scCO2 according to claim 1, characterized in that, The confining pressure pump (13) and confining pressure valve (14) are connected to the measuring chamber (15) through pipelines and inject static water into the measuring chamber (15) to confin the pressure, so that the rock sample (34) in the measuring chamber is in a confining pressure environment.

5. The experimental apparatus for permeability evolution using scCO2 according to claim 1, characterized in that, The control host (26) is used to control the frequency, waveform and amplitude of the pulse generated by the pulse generation unit (5), collect and record the data of the pressure sensor and temperature sensor, and perform data processing, calculation and evolution curve plotting.

6. An experimental method for permeability evolution using scCO2, employing the experimental apparatus for permeability evolution using scCO2 as described in any one of claims 1-5, characterized in that, Includes the following steps: S1. Place the first set of rock samples in the rock sample clamping device (36) in the measuring chamber (15) to ensure that the experimental device is kept sealed; S2. Turn on the control host (26), set the experimental temperature, and control the confining pressure loading system to load a static water confining pressure of 1MPa onto the measuring chamber (15); S3. Set the experimental pressure P and control the confining pressure loading system to apply a confining pressure of P to the measuring chamber (15); S4. Control the opening of the first shut-off valve (4), the second shut-off valve (6), the third shut-off valve (10), and the ninth shut-off valve (33), and start the vacuum pump (3) to evacuate the pipeline; S5. Close the first shut-off valve (4), the second shut-off valve (6), the third shut-off valve (10), and the ninth shut-off valve (33), and turn on the fourth booster pump (31) and the eighth shut-off valve (30) to allow the polymer solution to flow into the capillary coil (37) in the measuring chamber (15). Record the readings on the sixth pressure sensor (27) and the fourth pressure sensor (20). After the pressure difference between the left end (c1) and the right end (c2) of the capillary coil (37) stabilizes, record the pressure difference ΔP1 at this time. S6. Close the fourth booster pump (31) and the eighth shut-off valve (30), open the third booster pump (22), the fifth shut-off valve (21) and the seventh shut-off valve (28), clean the capillary coil (37) and related pipelines, and let the sewage flow into the sewage recovery tank (29). S7. Close the third booster pump (22), the fifth shut-off valve (21) and the seventh shut-off valve (28), and open the first booster pump (2) and the ninth shut-off valve (33) to allow scCO2 to enter the capillary coil (37). Record the readings on the sixth pressure sensor (27) and the fourth pressure sensor (20). After the pressure difference between the left end (c1) and the right end (c2) of the capillary coil stabilizes, record the pressure difference ΔP2 at this time and calculate the viscosity of scCO2. S8. Close the ninth shut-off valve (33), open the second shut-off valve (6), the first pressure regulating valve (7), the fourth shut-off valve (17), and the second booster pump (18), adjust the scCO2 pressure to p1 through the first pressure regulating valve (7), and record the readings of the first pressure sensor (8) and the third pressure sensor (16); S9. After the pressure of the first pressure sensor (8) and the third pressure sensor (16) is balanced, the first booster pump (2) is turned off. The host (26) controls the pulse generation unit (5) to apply a pressure pulse Δp to the left end (a1) of the rock sample clamping device, and collects the pressure data of the first pressure sensor (8) and the third pressure sensor (16) until the pressure at both ends of the rock sample clamping device (36) is balanced. The host (26) plots the pressure and time data collected by the pressure sensor as an attenuation curve and calculates the permeability of the artificial fracture. S10. Turn on the first booster pump (2), adjust the first pressure regulating valve (7), raise the scCO2 pressure to p2, p3, p4 respectively, repeat step 9, calculate the permeability of the artificial fracture at this time, lower the scCO2 pressure back to p1 and raise it again to p2, p3, p4, repeat step 9, calculate the permeability of the artificial fracture at this time. S11. The host (26) draws the evolution curve of the permeability of the artificial fractures of the first set of rock samples under each pressure condition during the process of raising the scCO2 pressure from p1 to p2, p3, p4 and then lowering it back to p1 and then raising it again to p2, p3, p4. S12. Close all valves and pumps, turn off the heating device (35) and the control host (26), and after the experimental device cools to room temperature, open the measuring chamber (15), replace the first set of rock samples with the second set of rock samples, close the measuring chamber (15), and ensure that the experimental device remains sealed. S13. Repeat steps 2 to 7, close the ninth shut-off valve (33), open the third shut-off valve (10), the second pressure regulating valve (11), the sixth shut-off valve (25), and the second booster pump (18), adjust the scCO2 pressure to p1 through the second pressure regulating valve (11), and record the readings of the second pressure sensor (12) and the sixth pressure sensor (27); S14. After the pressure of the second pressure sensor (12) and the sixth pressure sensor (27) is balanced, the first booster pump (2) is turned off. The host (26) controls the pulse generation unit (5) to apply a pressure pulse Δp to the upper radial end (b1) of the rock sample clamping device (36). The pressure data of the second pressure sensor (12) and the fifth pressure sensor (24) are collected until the pressure at both ends of the rock sample clamping device (36) is balanced. The host (26) plots the pressure and time data collected by the pressure sensors as an attenuation curve and calculates the permeability of the artificial fracture. S15. Turn on the first booster pump (2), adjust the second pressure regulating valve (11), raise the scCO2 pressure to p2, p3, p4 respectively, repeat step 14, calculate the permeability of the artificial fracture at this time, lower the scCO2 pressure back to p1 and raise it again to p2, p3, p4, repeat step 14, calculate the permeability of the artificial fracture at this time. S16. The host (26) controls the artificial fracture permeability of the second set of rock samples under each pressure condition to plot the evolution curve during the process of raising the scCO2 pressure from p1 to p2, p3, p4 and then lowering it back to p1 and then raising it again to p2, p3, p4.

7. The experimental method for permeability evolution using scCO2 according to claim 6, characterized in that, The formula for calculating the viscosity μ of scCO2 in step S7 is as follows: In the formula, △P1 is the stable pressure difference between the left end (c1) and the right end (c2) of the capillary coil when the polymer solution passes through the capillary coil (37); △P2 is the stable pressure difference between the left end (c1) and the right end (c2) of the capillary coil when scCO2 passes through the capillary coil (37); η is the viscosity of the polymer solution under the experimental temperature and pressure conditions.

8. The experimental method for permeability evolution using scCO2 according to claim 6, characterized in that, The formula for calculating the permeability K of the artificial fractures in steps S9, S10, S14, and S15 is as follows: , In the formula, Pu is the stable pressure at the inlet of the rock sample clamping device (36) after time t, Pf is the pressure at which the inlet and outlet of the rock sample clamping device (36) are balanced after time t, Δp is the pressure pulse applied by the pulse generation unit to the inlet of the rock sample clamping device (36), and V u V is the volume of the first CO2 storage tank (1). d Let α be the volume of the second CO2 storage tank (19), α be the slope of the attenuation curve in the semi-logarithmic diagram of the pressure difference time when the pressure at the inlet and outlet ends of the rock sample clamping device (36) is stable, t be the time elapsed from the application of the pulse to the pressure balance at the inlet and outlet ends of the rock sample clamping device (36), μ be the viscosity of scCO2 under experimental temperature and pressure conditions, β be the gas compressibility coefficient of scCO2 under experimental temperature and pressure conditions, L be the equivalent length of the rock sample in the measurement direction, and A be the equivalent cross-sectional area of ​​the rock sample in the measurement direction.

9. The experimental method for permeability evolution using scCO2 according to claim 6, characterized in that, It also includes a preliminary step: placing the capillary coil used for viscosity measurement and the permeability measurement device of the artificial crack in the same measurement chamber to ensure that the viscosity measurement and permeability measurement are carried out at the same experimental temperature and experimental pressure, and using the viscosity of the polymer solution and the pressure difference between the left and right ends of the capillary coil (37) to characterize the viscosity of scCO2.

10. The experimental method for permeability evolution using scCO2 according to claim 6, characterized in that, The experiment included two types of rock samples: A1. The artificial cracks cut into the rock sample formed a 30° angle with the long axis of the rock sample. Two holes with a diameter of 1.5 mm were drilled at both ends of the rock sample, parallel to the long axis of the rock core. These holes were located on the side of the end face closest to the crack and led to the crack. The two holes were connected to the left axial end (a1) and the right axial end (a2) of the rock sample clamping device, respectively. A2. The artificial cracks cut into the rock sample formed a 30° angle with the short axis of the rock sample. Two holes with a diameter of 1.5 mm were drilled at the top and bottom of the rock sample, parallel to the short axis of the rock core. These holes were located on the side of the rock sample closest to the crack and led to the crack. The two holes were connected to the upper radial end (b1) and the lower radial end (b2) of the rock sample clamping device, respectively.

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