A method for testing lost gas in shale gas content based on a depletion experimental device

Abstract

The application discloses a method for testing lost gas in shale gas content based on a depletion experiment device, and relates to the technical field of oil and gas field development.The depletion experiment device comprises a core holder, a heating jacket, a confining pressure pump, a gas inlet assembly, a gas outlet assembly, a vacuum pumping device, a gas reference container and an arc-shaped radiation slot plate.The application fully utilizes the characteristic that the horizontal permeability of shale is much greater than the vertical permeability, designs a test device fully consistent with the real environment, and can make methane truly saturate the shale core and greatly reduce the time required for saturating the shale core on one hand, and can directly test the lost gas part in the shale gas content on the other hand, thereby improving the test efficiency and the accuracy of the test results, and providing strong evidence support for shale gas reservoir evaluation and reserve calculation.

Description

Technical Field

[0001] The present invention relates to the technical field of oil and gas field development, and in particular to a method for testing lost gas in shale gas content based on a depletion experimental device. Background Art

[0002] Shale gas is natural gas found in shale formations, primarily composed of methane. Shale gas exists in shale primarily in the form of adsorbed and free phases. Shale gas content is a key parameter characterizing shale reservoir quality, and must be determined before shale gas reservoirs can be mined. Gas content is typically measured by shale gas content, which is the volume of shale gas under standard gas conditions per unit mass of shale. There are two existing shale gas content testing technologies:

[0003] One is the on-site staged desorption method, which is to seal the core obtained from underground drilling into a container, heat it and measure the amount of shale gas desorbed from the core, and calculate the loss of shale gas released during the process of lifting the shale core from the bottom of the well to the wellhead based on the rate of shale gas escaping from the core. When the desorbed shale gas in the core has completely escaped, the shale core is crushed and heated again to measure the amount of residual shale gas released. Then, the desorbed gas volume of the shale core is added to the loss gas volume and the participating gas volume. The sum of the obtained gas volumes is divided by the mass of the tested shale core, which is the shale gas content. Most of the gas content measured by this testing technology is the shale gas loss that escapes during the process of lifting the shale core from the bottom of the well to the wellhead. However, this part of the gas content is obtained by indirect calculation rather than direct measurement. The basis for its indirect calculation is to assume that the speed at which the shale gas in the core escapes during the process of lifting from the bottom of the well to the wellhead is equal to the speed at which the shale gas escapes during the shale gas desorption test. If this assumption is inconsistent with the actual shale gas escape in the core from the bottom of the well to the wellhead, the shale gas content obtained will not be the actual shale gas content, and it will be difficult to verify by other methods. In addition, the shale core used to test gas content has been in contact with air since it was taken out of the coring barrel. The desorbed gas volume measured by heating the core includes a large portion of air, which is also counted in the desorbed gas volume of the core. There is no means to exclude this air from the desorbed gas volume of the shale core, resulting in the measured shale gas desorption volume being larger than the actual shale core desorption volume.

[0004] The second method is the continuous desorption method coordinated with the pressure-maintaining coring work, that is, the core obtained by pressure-maintaining coring and the coring are heated to the formation temperature, and the amount of shale gas desorbed from the core is measured until no more gas escapes from the pressure-maintaining cylinder. Then the coring cylinder is opened, the core is taken out and weighed, and the amount of desorbed gas escaping from the pressure-maintaining cylinder is divided by the mass of the tested shale core, which is the shale gas content.

[0005] The method measures the gas content of all the gas in the shale core without air mixing, and the core maintains the formation pressure during coring and lifting, and the natural gas in the shale core does not dissipate, so the measured shale gas content data is more accurate and reliable than the previous shale gas testing method. However, this method also has some defects. First, this method requires the cooperation of pressure-maintaining coring, but pressure-maintaining coring is very expensive, and its popularization and application are affected by economic conditions. Second, in method two, the core obtained by pressure-maintaining coring is heated to the formation temperature together with the core and the natural gas escaping therefrom is measured, but there is a safety hazard of the pipeline being ejected due to unpressurization and causing injury. In addition, although the gas content data measured by this method is accurate, the shale gas content data measured by method two cannot be used to correct the shale gas content data measured by method one, that is, it cannot be used to calculate the loss of shale gas during the lifting of the shale core from the bottom of the well to the wellhead.

[0006] At present, many researchers have also designed many experimental methods to calculate the loss of shale gas during the lifting of the shale core from the bottom of the well to the wellhead. A commonly used experimental method is to place the core in a core holder, saturate the core with methane vertically to the formation pressure, then set the pressure and temperature drop rate according to the length of the core lifting time from the bottom of the well to the wellhead, then gradually desorb the core according to the set pressure drop rate, release the methane gas in the core, and measure the desorbed gas, until no more methane escapes from the core. The measured methane desorption gas is the loss gas part of the shale gas content.

[0007] Other researchers also take other methods to calculate the lost gas part in shale gas content, for example, collecting shale cores, obtaining the time required for the shale cores; placing the shale cores into a desorption tank for desorption experiment, obtaining residual gas content; preprocessing the desorption segment experimental data to conform to uniform distribution, obtaining uniform desorption segment experimental data; determining the parameter value of the fitting segmented estimation model according to the uniform desorption segment experimental data and the objective function of the desorption segment in the fitting segmented estimation model; and determining the lost gas content in the shale core according to the time and the lost gas segment objective function. In addition, the patent document with the publication number CN104863579A also discloses a lost gas amount measuring method and a lost gas amount measuring system, and the process thereof is as follows: firstly, under the same conditions, the same core sample is repeatedly sampled at equal time intervals, the gas content of each sampling sample is determined, and then the lost gas amount of the core sample is solved through a "t" function algorithm according to the change relationship between the gas diffusion rate and the time. However, the foregoing technical solution still has certain defects. First, there is a huge difference between the vertical permeability and the horizontal permeability of the shale core, and the horizontal permeability of the shale core is often one to two orders of magnitude higher than the vertical permeability, which makes it very difficult for the methane gas to completely saturate the shale core in the vertical direction of the shale core, resulting in that the lost gas part value in the shale gas content obtained by using the experimental method is very low, and even lower than the desorption gas amount of the shale core. The lost gas amount calculated by the method is neither reliable nor accurate. Second, the method of calculating the lost gas amount by using uniform distribution or "t" function is actually based on the assumption of shale gas desorption rate to inversely calculate the lost gas amount of the shale core, and lacks direct experimental evidence. The foregoing defects affect the reliability of the lost gas part in the shale gas content obtained by relying on the prior art solution, and further adversely affect the shale gas reservoir evaluation and reserve calculation. SUMMARY

[0008] The present application aims to overcome the above technical problems existing in the prior art, and provides a method for testing the lost gas in shale gas content based on a depletion experiment device. The present application fully utilizes the characteristic that the horizontal permeability of shale is much greater than the vertical permeability, and designs a test device that fully conforms to the real environment. On the one hand, the methane can truly saturate the shale core, and the time required for saturating the shale core is greatly reduced. On the other hand, the lost gas part in the shale gas content can be directly tested, which not only improves the test efficiency, but also improves the accuracy of the test results, thereby providing strong evidence support for the shale gas reservoir evaluation and reserve calculation.

[0009] To achieve the above object, the technical scheme adopted by the present application is as follows:

[0010] The application discloses a method for testing lost gas in shale gas content based on a depletion experiment device, and the depletion experiment device comprises a core holder, a heating jacket, a confining pressure pump, a gas inlet assembly, a gas outlet assembly, a vacuumizing device, a gas reference container and an arc-shaped radiation groove plate, a core sealing rubber sleeve is fixed in the core holder, the arc-shaped radiation groove plate is arranged in the core sealing rubber sleeve and covers the outer surface of the core and provides a gas seepage channel, the heating jacket is fixed on the core holder, the confining pressure pump is connected to the core holder and used for injecting confining pressure, the gas inlet assembly is connected to the gas inlet end of the core holder and used for inputting gas into the core and measuring indexes of the gas, the gas outlet assembly is connected between the core sealing rubber sleeve and the arc-shaped radiation groove plate at the gas outlet end of the core holder and used for discharging gas and measuring indexes of the gas, and the vacuumizing device and the gas reference container are connected to the gas inlet end of the core holder through the gas inlet assembly.

[0011] The method for testing lost gas in shale gas content comprises the following steps.

[0012] Step 1: obtaining a core, drying the core to constant weight, drilling an axial hole on the core, covering the arc-shaped radiation groove plate on the core, placing the core into the core sealing rubber sleeve of the core holder, sealing the core, connecting the gas inlet of the gas inlet assembly to a gas source, and inserting the gas outlet into the axial hole.

[0013] Step 2: injecting helium into the core through the gas inlet assembly and injecting confining pressure into the sealing property detection device through the confining pressure pump, if the sealing property is good, discharging the helium in the core through the gas outlet assembly and reducing the confining pressure to zero, and then vacuumizing the core holder and the gas reference container to negative pressure through the vacuumizing device.

[0014] Step 3: injecting helium into the core holder through the gas inlet assembly and the gas reference container, calculating the pore volume of the core according to the pressure of the helium, discharging the helium in the core holder and the gas reference container through the gas outlet assembly, and vacuumizing the core holder and the gas reference container to negative pressure through the vacuumizing device.

[0015] Step 4: heating the core through the heating jacket, when the core is heated to the formation temperature of the coring section, saturating the core by alternately injecting confining pressure and methane at each pressure point according to a set pressure gradient through the confining pressure pump and the gas inlet assembly, and measuring the methane inlet amount of the core at each pressure point in the saturation process, when the sum of the methane inlet amounts is greater than the pore volume of the core, determining that the core is completely saturated.

[0016] Step 5: gradually discharging the methane in the core holder at each pressure drop point according to a set pressure drop gradient through the gas outlet assembly, always keeping the confining pressure of the core greater than the methane pressure in the core holder, reducing the methane pressure to the current atmospheric pressure through the gas outlet assembly, and measuring the methane discharge amount, the methane temperature and the methane pressure at each pressure drop point.

[0017] Step 6: Calculate the amount of lost gas of the core according to the methane discharge amount, methane temperature and methane pressure at each pressure drop point.

[0018] In step 1, the obtained core is in a cylindrical structure, the two ends of the core are smooth and flat, and the core is dried to a constant weight at 60℃, the radius of the axial hole is not greater than 1 / 5 of the radius of the core, and the gas outlet of the gas inlet assembly can be inserted.

[0019] In step 3, the calculation method of the pore volume of the core is: after closing the vacuum device, first pump 10MPa of helium into the gas reference container through the gas inlet assembly, then cut off the gas source, and then measure the pressure in the gas reference container when the pressure in the gas reference container reaches stability; Then control the helium in the gas reference container to enter the core holder through the pipeline, and when the pressures in the gas reference container and the core holder are equal and stable, measure the pressure in the core holder, and then calculate the pore volume of the core according to the pressure in the gas reference container and the pressure in the core holder, as follows:

[0020]

[0021] In the formula, V cv is the pore volume of the core, with the unit of ml; P r 1 is the pressure in the gas reference container, with the unit of 10MPa; P c 1 is the pressure in the core holder, with the unit of 10Mpa; Z1 is P r 1 is the methane deviation coefficient under the condition; Z2 is P c 1 is the methane deviation coefficient under the condition; V r is the volume of the gas reference container, with the unit of ml; V e is the volume of the pipeline passed from the gas reference container to the core holder, plus the sum of the dead volume inside the core holder not occupied by the core, with the unit of ml.

[0022] In step 4, the highest pressure of the core saturated with methane is the formation pressure of the core taking section, and the set pressure gradient includes at least 10 pressure points with gradually increasing pressure, and the pressure difference between any two pressure points should not be higher than one tenth of the formation pressure of the shale core taking section.

[0023] In step 4, the specific process of saturating the core is:

[0024] S1: first, operate the confining pressure pump to inject confining pressure higher than the first pressure point set pressure into the core, then control the air inlet assembly to inject methane into the core for saturation, when the methane pressure in the core holder is increased to the pressure of the first pressure point, measure the methane inlet amount into the core at the first pressure point, at the same time stop injecting methane, let the methane saturate in the core, when the measured methane pressure changes less than 3% of the first pressure point pressure within 8h, it is determined that the core is completely saturated at the first pressure point;

[0025] S2: again operate the confining pressure pump to inject confining pressure higher than the second pressure point set pressure into the core, then repeat step S1, complete the complete saturation of the core at the second pressure point, and measure the methane inlet amount into the core at the second pressure point;

[0026] S3: repeat step S2, complete the complete saturation of the core at the remaining pressure points in turn, and measure the methane inlet amount into the core at the remaining pressure points;

[0027] S4: determine whether the core is saturated according to the methane inlet amount into the core at each pressure point, if the sum of all methane inlet amounts is greater than the pore volume of the core, it is determined that the core is completely saturated; if the sum of all methane inlet amounts is less than or equal to the pore volume of the core, it is determined that the core is not completely saturated, then continue to the complete saturation of the core at the last pressure point.

[0028] In step 5, the set pressure drop gradient is the formation pressure minus the current atmospheric pressure divided by the number of hours used by the core from the bottom of the well to the top of the well during coring, the pressure drop gradient includes a plurality of pressure drop points, and the number of pressure drop points is the number of hours used by the core from the bottom of the well to the top of the well during coring.

[0029] In step 5, when discharging methane, first set the temperature drop gradient of the heating jacket according to the temperature drop rate of the core during coring, and then set the exhaust pressure of the exhaust assembly according to the pressure drop process of the core during coring, so that the methane in the core holder is gradually discharged.

[0030] In step 6, the calculation method of the lost gas amount of the core is:

[0031]

[0032] In the formula, V s total V is the measured lost gas amount of the shale core, unit: ml;

[0033] V0 1 is the methane discharge amount at the first pressure drop point, unit: ml;

[0034] V0 n is the methane discharge amount at the nth pressure drop point, unit: ml;

[0035] P0 1 P0 is the methane pressure at the first pressure drop point, in MPa;

[0036] P0 n P0 is the methane pressure at the nth pressure drop point, in MPa;

[0037] T1 is the methane temperature at the first pressure drop point, in ℃;

[0038] T n T0 is the methane temperature at the nth pressure drop point, in ℃.

[0039] The air inlet assembly comprises, in sequence, an air inlet pressure reducing valve, a constant speed constant pressure pump, an air inlet flow meter, an air inlet pressure sensor, a valve A and an air inlet pipeline, the air inlet of the air inlet pressure reducing valve is connected with a gas source, the air outlet of the air inlet pipeline is connected at the air inlet end of the core holder and extends into the axial hole of the core; the air inlet flow meter and the air inlet pressure sensor are used for measuring the flow index and the pressure index of the input gas respectively, and the vacuumizing device and the gas reference container are connected in sequence between the air inlet flow meter and the air inlet pressure sensor.

[0040] The surface of the air inlet pipeline at the part extending into the axial hole is uniformly provided with 4 rows of eyelets, and the included angle between every two rows of eyelets is 90 degrees.

[0041] The air outlet assembly comprises, in sequence, an air outlet pipeline, a safety valve, a valve B, an air outlet pressure sensor, an air outlet end gas automatic pressure regulating valve, an air outlet temperature sensor, an air outlet pressure reducing valve and a normal pressure gas flow meter, the air outlet pressure sensor, the air outlet temperature sensor and the normal pressure gas flow meter are used for measuring the pressure index, the temperature index and the flow index of the output gas respectively, and the air outlet pipeline is connected between the core sealing rubber sleeve and the arc-shaped radiation groove plate at the air outlet end of the core holder.

[0042] The number of the arc-shaped radiation groove plates is 2-6, and the sum of the widths of all the arc-shaped radiation groove plates is less than the chord length of the core sealing rubber sleeve.

[0043] The advantages of the present application are as follows:

[0044] 1. The present application can prevent the core from contacting air during the saturation of methane, prevent the influence of air on the measurement result and improve the accuracy of measuring the shale lost gas amount through the specific sealing structure and sealing property detection process.

[0045] 2. The core decompression desorption process of the present invention fully complies with the actual environment in which the internal pressure of the core decreases from the bottom of the well to the wellhead, effectively improving the accuracy of measuring the loss of shale gas. The loss of the core gas is directly measured, without the need for assumptions and conversions, which greatly improves the reliability and accuracy of the loss of gas.

[0046] 3. The present invention uses an air intake assembly and an arc-shaped radial slot plate to separate the air intake path of the methane saturating the core from the path of methane discharge (escape) from the core, which is beneficial to ensure effective saturation of the core.

[0047] 4. This method gradually saturates the core by increasing the pressure at each pressure point according to a set pressure gradient. This ensures that the core is effectively saturated at every pressure point. Combined with the comparison of the core's pore volume, it accurately achieves complete and sufficient saturation of the core, thereby improving the accuracy of the test.

[0048] 5. The present invention discharges methane by gradually reducing the pressure according to a set pressure drop gradient, which truly simulates the pressure drop in the shale core during the coring process. On the one hand, it ensures the smooth discharge of methane, and on the other hand, it can accurately measure the methane discharge amount to obtain a more accurate amount of lost gas.

[0049] 6. The core sealing rubber sleeve provided in the present invention does not cover the entire cylindrical surface of the core, and a half-moon shaped radial groove plate attached to the core provides a seepage channel for methane on the core surface, thereby avoiding uneven pressure distribution at different positions of the core.

[0050] 7. The present invention sets a core gas inlet pipeline to allow methane gas to saturate the core from the center of the core to the core surface. Methane flows horizontally in the core, making full use of the characteristic that the horizontal permeability of shale is much greater than the vertical permeability, so that methane can truly saturate the shale core and greatly reduce the time required to saturate the shale core, thereby improving the experimental efficiency. At the same time, when simulating the situation where methane escapes from the core during the core extraction process, the flow pattern of methane in the core is also consistent with the actual formation conditions, avoiding the situation where the measured shale gas loss is inaccurate due to changing the flow pattern of methane in the core.

[0051] 8. The present invention adopts an air intake component and an air outlet component with a specific structure, which is conducive to the accurate control of processes such as air intake, exhaust, sealing detection, and gas index testing in the test method, thereby obtaining more accurate test results.

[0052] 9. The present invention provides four rows of holes evenly distributed on the surface of the portion of the air inlet pipe extending into the axial hole, and the angle between each two rows of holes is 90 degrees, which is not only beneficial to the uniform saturation of the core, but also helps to shorten the saturation time.

[0053] 10、The number of the arc-shaped radiation groove plates is 2-6, and the sum of the widths of all the arc-shaped radiation groove plates is less than the chord length of the core sealing rubber sleeve. The advantage is that the number of the arc-shaped radiation groove plates can be matched according to the smoothness of the core surface to ensure that the arc-shaped radiation groove plates and the core are tightly attached. For example, if the core surface is smooth and round, 2-3 arc-shaped radiation groove plates can ensure that the groove plates and the core are tightly attached and the stress on the core is uniform. If the core surface is not smooth and round, 3-6 arc-shaped radiation groove plates can be used to wrap the core, so that the core will not change the local seepage pressure due to uneven confining pressure, and the core will not break due to uneven stress. BRIEF DESCRIPTION OF DRAWINGS

[0054] Figure 1 Structure diagram of the present application. DETAILED DESCRIPTION

[0055] Example 1

[0056] The present embodiment discloses a method for testing lost gas in shale gas content based on a depletion experiment device, as shown in Figure 1 The depletion experiment device comprises:

[0057] A core holder is used to hold and protect the rock sample and seal the cylindrical surface or end surface, usually the end surface of the fluid inlet and outlet. The core holder has a core sealing rubber sleeve fixed inside the shell, which is used to give the core in the core holder a certain confining pressure according to the experimental design.

[0058] Arc-shaped radiation groove plates are arranged in the core sealing rubber sleeve to cover the outer surface of the core and provide a gas seepage channel. The arc-shaped radiation groove plates are essentially arc-shaped plates that match the outer surface of the core, which can be made of 316SS stainless steel with a thickness of less than 5mm, and have uniform flow channels that serve as the seepage channel for methane on the core surface. In addition, the number of arc-shaped radiation groove plates is 2-6, and the sum of the widths of all the arc-shaped radiation groove plates is less than the chord length of the core sealing rubber sleeve. It should be noted that when the number of arc-shaped radiation groove plates is 2, a single arc-shaped radiation groove plate occupies 4 / 5 of the chord length of the core sealing rubber sleeve, when the number of arc-shaped radiation groove plates is 3, a single arc-shaped radiation groove plate occupies 3 / 10 of the chord length of the core sealing rubber sleeve, when the number of arc-shaped radiation groove plates is 4, a single arc-shaped radiation groove plate occupies 3 / 16 of the chord length of the core sealing rubber sleeve, and when the number of arc-shaped radiation groove plates is 5-6, a single arc-shaped radiation groove plate occupies 3 / 20 of the chord length of the core sealing rubber sleeve. The present embodiment preferably has 4 arc-shaped radiation groove plates, each occupying 3 / 16 of the chord length of the core sealing rubber sleeve.

[0059] The gas inlet end plug is used to provide a channel for the gas inlet pipeline and to assist the gas inlet end sealing plate in sealing the core, so that the core can be kept at a set saturation pressure.

[0060] The gas inlet end sealing plate is used to cover the core cross-section end surface of the core holder gas inlet end, to seal the end surface of the core and the core sealing rubber sleeve, and to ensure that the core can be kept at a set saturation pressure.

[0061] The gas outlet end sealing plate is used to cover the core cross-section end surface of the core holder gas outlet end, to seal the end surface of the core and the core sealing rubber sleeve, and to ensure that the core can be kept at a set saturation pressure.

[0062] The heating sleeve is fixedly sleeved on the core holder and can heat the shale core in the core holder to the formation temperature of the core taking layer, and a circulating hot water heating sleeve device or a circulating hot air heating sleeve device can be used.

[0063] The confining pressure pump is connected to the core sealing rubber sleeve in the core holder for injecting confining pressure, which acts on the core sealing rubber sleeve to keep the core in the core holder at a certain confining pressure.

[0064] The gas inlet assembly has a gas inlet connected to a gas source and a gas outlet connected to the gas inlet end of the core holder, the gas source includes a helium gas source and a methane gas source, and the gas inlet pipe can include a high-pressure resistant pipeline, a valve, a pump, and a sensor, etc., for inputting the gas in the gas source into the core and measuring the indexes of the gas, and the specific indexes to be measured can include pressure, temperature, and / or flow rate, etc.

[0065] The gas outlet assembly is connected between the core sealing rubber sleeve and the arc-shaped radiation slot plate at the gas outlet end of the core holder, for discharging gas and measuring the indexes of the gas.

[0066] The vacuum pumping device is connected to the gas inlet end of the core holder through the gas inlet assembly, for completely removing the air in the core before the experiment to prevent the air from affecting the experimental results.

[0067] The gas reference container is connected to the gas inlet end of the core holder through the gas inlet assembly and is located between the vacuum pumping device and the core holder, for use as a reference chamber when using helium to measure the pore volume of the shale core.

[0068] It should be noted that the specific structure of the gas inlet assembly and the gas outlet assembly is not limited in this embodiment, and any structure that can transport gas and measure the indexes of the gas can be used.

[0069] The depletion experiment device based on the above structure, and the method for testing the loss gas in the shale gas content includes the following steps:

[0070] Step 1: First, obtain a cylindrical core, and dry the core at 60℃ until the weight is constant. Then, cut the two ends of the core flat and polish them as smooth as possible to ensure good contact with the inlet end seal plate and the outlet end seal plate, so as to effectively seal the gas pressure in the core. Then, drill a vertical hole through the core, which is perpendicular to the cross section of the core and the center of the core. The radius of the hole is not greater than 1 / 5 of the radius of the core, and the hole can accommodate the outlet of the inlet assembly. Then, cover the arc-shaped radiation slot plate on the core, and put it into the core sealing rubber sleeve of the core holder. Seal the inlet end and outlet end of the core holder with the inlet end seal plate and the outlet end seal plate respectively, and seal the inlet end with the inlet end plug to ensure the airtightness of the core. Finally, connect the inlet of the inlet assembly to the helium gas source, and extend the outlet into the center hole. At the same time, connect the outlet assembly.

[0071] Step 2: Use the inlet assembly to inject helium, and use the confining pressure pump to test the airtightness of the confining pressure detection device. If the airtightness is good, open the outlet assembly to discharge the helium in the core and reduce the confining pressure to zero. After the helium in the core is completely dissipated, close all valves connecting the core holder to the outside, open the vacuum pump, and vacuum the core holder and the gas reference container to a negative pressure, which can be-1KPa.

[0072] Further, the specific process of testing the airtightness of the device is as follows: first, close all valves connecting the core in the core holder to the outside, then use the inlet assembly to pump helium into the core at a set experimental pressure, and operate the confining pressure pump to keep the confining pressure of the core greater than 5MPa of the set experimental pressure. In this way, the airtightness of the device is tested.

[0073] Step 3: After the core holder and the gas reference container reach the vacuum degree target, close the vacuum pump, open the inlet assembly, and use the inlet assembly and the gas reference container to inject helium into the core holder. The pore volume of the core is calculated according to the pressure of the helium. Then, open the outlet assembly to discharge the helium in the core holder and the gas reference container, and use the vacuum pump to vacuum the core holder and the gas reference container to a negative pressure, which can be-1KPa.

[0074] Further, the calculation method of the pore volume of the core is as follows: after closing the vacuum pump, first pump 10MPa of helium into the gas reference container through the inlet assembly, then cut off the gas source, and measure the pressure in the gas reference container when the pressure is stable. Then, control the helium in the gas reference container to enter the core holder through the pipeline, and measure the pressure in the core holder when the pressure in the gas reference container and the core holder is equal and stable. Then, calculate the pore volume of the core according to the pressure in the gas reference container and the pressure in the core holder, as follows:

[0075]

[0076] wherein V cv is the pore volume of the core, in ml; P r 1 is the pressure in the gas reference container, in 10 MPa; P c 1 is the pressure in the core holder, in 10 MPa; Z1is P r 1 is the methane deviation factor under the condition; Z2is P c 1 is the methane deviation factor under the condition; V r is the volume of the gas reference container, in ml; V e is the volume of the pipeline from the gas reference container to the core holder, plus the dead volume inside the core holder not occupied by the core, in ml.

[0077] Step 4: Turn on the power of the heating jacket to heat the core by the heating jacket. When the core is heated to the formation temperature of the coring section and remains stable, saturate the core by the confining pressure pump and the gas inlet assembly in an alternating manner of injecting confining pressure and methane at each pressure point according to the set pressure gradient. At the same time, measure the methane inlet amount of each pressure point into the core during the saturation process. When the sum of the methane inlet amounts is greater than the pore volume of the core, it is determined that the core is completely saturated.

[0078] It should be noted that the highest pressure of the core saturated with methane is the formation pressure of the core coring section. The set pressure gradient includes at least 10 pressure points with gradually increasing pressure. The pressure difference between any two pressure points should not be higher than one tenth of the formation pressure of the shale coring section.

[0079] Further, the specific process of saturating the core is as follows:

[0080] S1: First, operate the confining pressure pump to inject confining pressure higher than the set pressure of the first pressure point into the core. The set pressure can be 5 MPa. Specifically, inject confining pressure 5 MPa higher than the first pressure point into the core. Then control the gas inlet assembly to inject methane into the core for saturation. When the methane pressure in the core holder is increased to the pressure of the first pressure point, measure the methane inlet amount into the core at the first pressure point, and at the same time stop injecting methane to allow the methane to saturate in the core. It should be noted that the pressure in the core holder gas inlet assembly will decrease during saturation. Therefore, when the change amplitude of the methane pressure is less than 3% of the pressure of the first pressure point within 8h, it is determined that the core has been completely saturated at the first pressure point.

[0081] S2: When the core is fully saturated with methane at the first pressure point, the confining pressure pump is operated again to inject confining pressure higher than the second pressure point to the core, and the confining pressure is set to 5 MPa, that is, the confining pressure higher than the second pressure point is set to 5 MPa. Then, step S1 is repeated to complete the full saturation of the core at the second pressure point, and the methane intake amount at the second pressure point is measured.

[0082] S3: Step S2 is repeated to complete the full saturation of the core at the remaining pressure points in turn, and the methane intake amount at each of the remaining pressure points is measured.

[0083] S4: Whether the core is saturated is determined according to the methane intake amount at each pressure point. If the sum of all the methane intake amounts is greater than the pore volume of the core, it is determined that the core is fully saturated. If the sum of all the methane intake amounts is less than or equal to the pore volume of the core, it is determined that the core is not fully saturated, and then the core is continuously saturated at the last pressure point until the core is fully saturated.

[0084] Step 5: After the core is fully saturated, the methane in the core holder is gradually discharged at each pressure drop point by the gas outlet assembly at a set pressure drop gradient, and the confining pressure of the core is always kept greater than the methane pressure in the core holder, specifically, the confining pressure of the core is kept greater than the methane pressure in the core holder by 5 MPa, and at the same time, the methane pressure is reduced to the current atmospheric pressure by the gas outlet assembly, and the methane discharge amount, methane temperature and methane pressure at each pressure drop point are measured.

[0085] It should be noted that the set pressure drop gradient is the formation pressure minus the current atmospheric pressure divided by the number of hours from the bottom of the well to the top of the well during coring, and the unit is MPa / h. The pressure drop gradient includes multiple pressure drop points, and the number of pressure drop points is the number of hours from the bottom of the well to the top of the well during coring.

[0086] In addition, when discharging the methane, the temperature drop gradient of the heating sleeve is set according to the temperature drop rate of the core during coring, and the exhaust pressure of the gas outlet assembly is set according to the pressure drop process of the core during coring, so that the methane pressure in the core holder is slowly reduced and gradually discharged.

[0087] Step 6: The lost gas amount of the core is calculated according to the methane discharge amount, methane temperature and methane pressure at each pressure drop point.

[0088] Further, the calculation method of the lost gas amount of the core is as follows:

[0089]

[0090] In the formula, V s total is the measured lost gas amount of the shale core, and the unit is ml;

[0091] V0 1 is the methane discharge amount at the first pressure drop point, and the unit is ml;

[0092] V0 n is the methane discharge amount at the nth pressure drop point, and the unit is ml;

[0093] P0 1 is the methane pressure at the first pressure drop point, and the unit is MPa;

[0094] P0 n is the methane pressure at the nth pressure drop point, and the unit is MPa;

[0095] T1 is the methane temperature at the first pressure drop point, and the unit is ℃;

[0096] T n is the methane temperature at the nth pressure drop point, and the unit is ℃.

[0097] From the above, the embodiment fully utilizes the characteristic that the horizontal permeability of shale is much larger than the vertical permeability, and designs a test device fully consistent with the real environment. On the one hand, the methane can be truly saturated in the shale core, and the time required for saturating the shale core is greatly reduced. On the other hand, the lost gas part in the shale gas content can be directly tested, which not only improves the test efficiency, but also improves the accuracy of the test results.

[0098] Finally, after the core loss gas amount is tested by the above technical scheme, the core holder is opened, the core is taken out from the core holder, and the corresponding experiment is carried out according to the provisions of the desorption gas and residual gas measurement method in SY / T6940-2020 “Shale Gas Content Determination Method”. The desorption gas content and residual gas content can be obtained. The desorption gas content, residual gas content and the loss gas amount obtained by the embodiment are added, and then divided by the mass of the core sample, so that the total gas content of the core can be obtained, and the unit is ml / g; thereby providing strong evidence support for shale gas reservoir evaluation and reserve calculation.

[0099] Embodiment 2

[0100] The embodiment further limits the structure of the gas inlet assembly and the gas outlet assembly on the basis of the embodiment 1, as shown in Figure 1 , the gas inlet assembly and the gas outlet assembly are respectively:

[0101] The air inlet assembly comprises an air inlet pressure reducing valve, a constant speed and pressure pump, an air inlet flow meter, an air inlet pressure sensor, a valve A and an air inlet pipeline, the air inlet pressure reducing valve, the constant speed and pressure pump, the air inlet flow meter, the air inlet pressure sensor and the valve A are sequentially connected through a high-pressure resistant pipeline, the air inlet of the air inlet pressure reducing valve is connected with a gas source, the air inlet of the air inlet pipeline is connected with the valve A, and the air outlet of the air inlet pipeline is connected at the air inlet end of the core holder and extends into the axial hole of the core; in order to facilitate the effective input of the gas, preferably, the surface of the air inlet pipeline extending into the axial hole part is uniformly provided with 4 rows of holes, and the included angle between every two rows of holes is 90 degrees. The air inlet flow meter and the air inlet pressure sensor are respectively used for measuring the flow index and the pressure index of the input gas, and the vacuumizing device and the gas reference container are sequentially connected between the air inlet flow meter and the air inlet pressure sensor.

[0102] The air outlet assembly comprises an air outlet pipeline, a safety valve, a valve B, an air outlet pressure sensor, an air outlet end gas automatic pressure regulating valve, an air outlet temperature sensor, an air outlet pressure reducing valve and a normal pressure gas flow meter which are sequentially connected, the air outlet pressure sensor, the air outlet temperature sensor and the normal pressure gas flow meter are respectively used for measuring the pressure index, the temperature index and the flow index of the output gas, and the air outlet pipeline is connected between the core sealing rubber sleeve and the arc-shaped radiation slot plate of the core holder air outlet end.

[0103] Specifically, the functions of the above components are as follows:

[0104] The constant speed and pressure pump is used for increasing the pressure of the methane gas to the pressure required by the experiment.

[0105] The air inlet flow meter is used for measuring the methane flow entering the core holder, and can measure the methane inlet amount entering the core after the methane saturation process of the core in the holder is completed.

[0106] The air inlet pressure sensor is used for measuring the methane pressure entering the core holder.

[0107] The air outlet pressure sensor is used for measuring the methane pressure flowing out of the core holder.

[0108] The normal pressure gas flow meter is used for measuring the methane gas flow escaping from the core, and can measure the total amount of the methane gas escaping from the core after the methane gas escaping in the core holder is completed and the desorption experiment is ended.

[0109] The air inlet pipeline is used for conveying the gas with pressure to the core, and the pipeline is provided with a pressure sensor with data acquisition function.

[0110] The air outlet temperature sensor is used for measuring the temperature of the methane gas escaping from the core holder.

[0111] The outlet pressure reducing valve can reduce the high pressure gas in the core holder to the atmospheric pressure, and facilitate the measurement of the outlet normal pressure gas flow meter.

[0112] The outlet gas automatic pressure regulating valve is located between the outlet pressure sensor and the outlet gas flow meter at the outlet of the core holder, and can automatically reduce the methane gas pressure in the core holder according to the set pressure drop rate. The methane gas from the core holder escapes to the outlet pressure sensor and the outlet gas flow meter through the valve, and then the pressure and flow rate thereof are measured.

[0113] Based on the above defined inlet assembly and outlet assembly, the specific process for testing the lost gas is as follows:

[0114] Step 1: First, obtain a cylindrical core, and dry the core at 60℃ until the weight is constant. Then, cut and polish the two ends of the core as flat and smooth as possible to ensure good contact with the inlet end sealing plate and the outlet end sealing plate, so as to effectively seal the gas pressure in the core. Then, drill an axial hole in the core, which is perpendicular to the cross section of the core and penetrates the core, and the radius of the axial hole is not greater than 1 / 5 of the radius of the core, and the axial hole can be used for the outlet of the inlet assembly. Then, cover the arc-shaped radiation slot plate on the core, and put it into the core sealing rubber sleeve of the core holder, and seal the inlet end and the outlet end of the core holder using the inlet end sealing plate and the outlet end sealing plate respectively, and use the inlet end plug to block, so as to ensure the airtightness of the core. Finally, connect the inlet pressure reducing valve, the constant speed and constant pressure pump, the inlet flow meter, the inlet pressure sensor, valve A and the inlet pipeline in sequence without leakage, connect the outlet pipeline, the safety valve, valve B, the outlet pressure sensor, the outlet gas automatic pressure regulating valve, the outlet temperature sensor, the outlet pressure reducing valve and the normal pressure gas flow meter in sequence without leakage, and extend the part of the inlet pipeline provided with the eyelet into the axial hole,

[0115] Step 2: Use the inlet assembly to inject helium and the confining pressure pump to detect the airtightness of the sealing device. If the airtightness is good, open the outlet assembly to discharge the helium in the core and reduce the confining pressure to zero. After the helium in the core escapes completely, close all valves connecting the core holder to the outside, open the vacuum pump, and vacuumize the core holder and the gas reference container to a negative pressure, which can be-1KPa.

[0116] Further, the specific process for detecting the airtightness of the device is as follows: first, close all valves connecting the core in the core holder to the outside, then use the constant speed and constant pressure pump to pump the helium with the set experimental pressure into the core, and simultaneously operate the confining pressure pump to make the confining pressure of the shale core always greater than the pressure of 5MPa higher than the set experimental pressure, so as to detect whether the airtightness of the device is good.

[0117] Step 3: After the vacuum degree of the core holder and the gas reference container reaches the target, the vacuum device is closed, the gas inlet assembly is opened, the helium is injected into the core holder by the gas inlet assembly and the gas reference container, and the pore volume of the core is calculated according to the pressure of the helium; then all the valves in the gas outlet assembly are opened to discharge the helium in the core holder and the gas reference container, and the core holder and the gas reference container are vacuumized to negative pressure by the vacuum device, specifically to-1KPa.

[0118] Further, the calculation method of the pore volume of the core is as follows: after the vacuum device is closed, 10MPa of helium is pumped into the gas reference container by the constant speed and constant pressure pump, and then the gas source is cut off; after the pressure in the gas reference container reaches stability, the pressure in the gas reference container is measured by using the inlet pressure sensor; then the helium in the gas reference container enters the core holder through the inlet pipeline, and when the pressures in the gas reference container and the core holder are equal and stable, the pressure in the core holder is measured by using the inlet pressure sensor, and then the pore volume of the core is calculated according to the pressure in the gas reference container and the pressure in the core holder, as follows:

[0119]

[0120] In the formula, V cv is the pore volume of the core, with the unit of ml; P r 1 is the pressure in the gas reference container, with the unit of 10MPa; P c 1 is the pressure in the core holder, with the unit of 10Mpa; Z1 is P r 1 the methane deviation factor under the condition; Z2 is P c 1 the methane deviation factor under the condition; V r is the volume of the gas reference container, with the unit of ml; V e is the volume of the pipeline passed from the gas reference container to the core holder, plus the sum of the dead volume in the core holder not occupied by the core, with the unit of ml.

[0121] Step 4: The power supply of the heating jacket is connected, the core is heated by the heating jacket, when the core is heated to the formation temperature of the coring section and remains stable, the confining pressure pump and the gas inlet assembly are used to saturate the core in the form of alternating injection of confining pressure and methane at each pressure point according to the set pressure gradient, and the methane inlet amount of each pressure point into the core is measured by using the inlet flow meter during the saturation process, when the sum of the methane inlet amounts is greater than the pore volume of the core, it is determined that the core is completely saturated.

[0122] It should be noted that the highest pressure of the core saturated with methane is the formation pressure of the core taking section, and the set pressure gradient includes at least 10 pressure points with gradually increased pressure, and the pressure difference between any two pressure points should not be higher than one tenth of the formation pressure of the shale core taking section.

[0123] Further, the specific process of saturating the core is as follows:

[0124] S1: first, operate the confining pressure pump to inject confining pressure higher than the set pressure of the first pressure point into the core, and the set pressure can be 5 MPa, and specifically, the confining pressure 5 MPa higher than the first pressure point can be injected into the core. Then control the gas inlet assembly to inject methane into the core for saturation. When the methane pressure in the core holder is increased to the pressure of the first pressure point, the methane inlet flow meter is used to measure the methane inlet amount of the core at the first pressure point, and at the same time, the injection of methane is stopped to allow the methane to be saturated in the core. It should be noted that the pressure in the gas inlet assembly of the core holder will decrease during saturation, and therefore, when the change amplitude of the methane pressure is less than 3% of the pressure of the first pressure point within 8 hours, it is determined that the core has been completely saturated at the first pressure point.

[0125] S2: after the core is completely saturated with methane at the first pressure point, the confining pressure pump is operated again to inject confining pressure higher than the set pressure of the second pressure point into the core, and the set pressure is also 5 MPa, that is, the confining pressure 5 MPa higher than the second pressure point is also injected into the core. Then repeat step S1 to complete the complete saturation of the core at the second pressure point, and at the same time, the methane inlet amount of the core at the second pressure point is measured.

[0126] S3: repeat step S2 to sequentially complete the complete saturation of the core at the remaining pressure points, and at the same time, the methane inlet amount of the core at the remaining pressure points is measured.

[0127] S4: determine whether the core is saturated according to the methane inlet amount of the core at each pressure point. If the sum of all the methane inlet amounts is greater than the pore volume of the core, it is determined that the core is completely saturated; if the sum of all the methane inlet amounts is less than or equal to the pore volume of the core, it is determined that the core is not completely saturated, and then continue to the complete saturation of the core at the last pressure point.

[0128] Step 5: after the core is completely saturated, set the gas pressure drop gradient of the gas outlet automatic pressure regulating valve, so that the gas outlet assembly gradually discharges the methane in the core holder at the set pressure drop gradient, and at the same time, the confining pressure of the core is kept greater than the methane pressure in the core holder by 5 MPa, and at the same time, the methane pressure is reduced to the current atmospheric pressure by using the gas outlet pressure reducing valve in the gas outlet assembly, and the methane discharge amount, methane temperature and methane pressure at each pressure drop point are measured by using the normal pressure gas flow meter, the gas outlet temperature sensor and the gas outlet pressure sensor, respectively.

[0129] It should be noted that the set pressure drop gradient is the formation pressure minus the current atmospheric pressure divided by the number of hours the core takes from the bottom of the well to the top of the well during coring, and the unit is MPa / h. The pressure drop gradient includes a plurality of pressure drop points, and the number of pressure drop points is the number of hours the core takes from the bottom of the well to the top of the well during coring.

[0130] In addition, when the methane is discharged, the temperature drop gradient of the heating sleeve is first set according to the temperature drop rate of the core during the coring process of the core, and then the exhaust pressure of the exhaust assembly is set according to the pressure drop process of the core during the coring process, so that the methane pressure in the core holder is slowly reduced and gradually discharged.

[0131] Step 6: The loss gas amount of the core can be calculated according to the methane discharge amount, methane temperature and methane pressure at each pressure drop point.

[0132] Example 3

[0133] This example verifies the method described in the application, as follows:

[0134] Step 1: Select a shale core of the first layer of Longyi 11 in the Longmaxi Formation of Well NingX0X-2 in Changning area for pressure-maintained coring, the coring section has a formation pressure of 56.2 MPa and a formation temperature of 103.5℃, the coring time is 10 hours, the core diameter is 105 mm, a complete and smooth-surfaced core is selected, the core is dried at 60℃ to constant weight, then the full-diameter shale core is cut flat at both ends and polished as smooth as possible, a 1cm-diameter hole is drilled in the middle of the core end face using a liquid nitrogen-cooled drill, and the debris generated during drilling is cleaned. Then cover the arc-shaped radiation groove plate on the core, seal it in the core sealing rubber sleeve of the core holder, connect the gas inlet of the gas inlet assembly to the gas source, and extend the gas outlet into the axial hole.

[0135] Step 2: Detect the sealing performance of the device, and the detection result shows that if the device is perfectly sealed, the helium in the core is discharged by opening the gas outlet assembly, and the confining pressure is reduced to zero, and then the core holder and the gas reference container are vacuumed to negative pressure by using the vacuum device.

[0136] Step 3: Inject 5MPa of helium into the core holder by using the gas inlet assembly and the gas reference container, and calculate the pore volume of the core as 76.1cm 3 ; then discharge the helium in the core holder and the gas reference container by opening the gas outlet assembly, and vacuum the core holder and the gas reference container to negative pressure by using the vacuum device.

[0137] Step 4: The core is heated to the formation temperature 103.5℃ by a heating jacket, when the core is heated to the formation temperature of the coring section, the pressure gradient is set to 20 MPa / h, the core is saturated by the confining pressure pump and the gas inlet assembly in the form of alternating injection of confining pressure and methane at each pressure point according to the set pressure gradient until the gas pressure in the core reaches 56.2 MPa. At the same time, the methane gas inlet amount of each pressure point into the core is measured during the saturation process, when the sum of the methane gas inlet amount is greater than the pore volume of the core, it is determined that the core is completely saturated.

[0138] Step 5: According to the core taking time, the pressure drop gradient is set to 5.61 MPa / h, and the pressure drop time is 10 h, the methane in the core holder is gradually discharged by the gas outlet assembly according to the set pressure drop gradient at each pressure drop point, and the confining pressure of the core is always greater than the methane pressure in the core holder, at the same time, the methane pressure is reduced to the current atmospheric pressure by the gas outlet assembly, and the methane discharge amount, methane temperature and methane pressure at each pressure drop point are measured, and the experimental data obtained are as follows:

[0139]

[0140]

[0141] Step 6: According to the methane discharge amount, methane temperature and methane pressure at each pressure drop point, the loss gas amount of the core is calculated by substituting into the loss gas amount calculation formula, and the final calculation result is that the loss gas amount of the core is 6521 ml.

[0142]

[0143] Comparison and verification: The loss gas amount result calculated in step 6 is compared with the loss gas amount data measured in the pressure maintaining coring process, and the difference between the two is only 50 ml, thereby proving that the loss gas amount result obtained by the method of the application is accurate and reliable.

[0144] The above is only a specific embodiment of the present application, any feature disclosed in the specification can be replaced by other equivalent or similar purpose replacement features unless specifically described, all features disclosed, or steps in all methods or processes can be combined in any way except mutually exclusive features and / or steps.

Claims

1. A method for testing gas loss in shale gas content based on a depletion experimental device, characterized in that: The failure experiment device comprises a core holder, a heating sleeve, a confining pressure pump, a gas inlet assembly, a gas outlet assembly, a vacuum device, a gas reference container and an arc-shaped radiation slot plate, a core sealing rubber sleeve is fixed in the core holder, the arc-shaped radiation slot plate is arranged in the core sealing rubber sleeve to cover the outer surface of the core and provide a gas seepage channel, the heating sleeve is fixedly sleeved on the core holder, the confining pressure pump is connected to the core holder to inject confining pressure, the gas inlet assembly is connected to the gas inlet end of the core holder to input gas into the core and measure the indicators of the gas, the gas outlet assembly is connected between the core sealing rubber sleeve and the arc-shaped radiation slot plate at the gas outlet end of the core holder to discharge the gas and measure the indicators of the gas; the vacuum device and the gas reference container are connected to the gas inlet end of the core holder through the gas inlet assembly. The method for testing the lost gas in shale gas content comprises the following steps: Step 1: obtaining a core, drying the core to constant weight, drilling an axial hole on the core, covering the arc-shaped radiation slot plate on the core, then placing the core into the core sealing rubber sleeve of the core holder, sealing, connecting the gas inlet of the gas inlet assembly to a gas source, and inserting the gas outlet into the axial hole; Step 2: injecting helium by the gas inlet assembly and injecting confining pressure by the confining pressure pump to detect the sealing performance of the device, if the sealing performance is good, discharging the helium in the core by the gas outlet assembly and reducing the confining pressure to zero, then vacuumizing the core holder and the gas reference container to negative pressure by the vacuum device; Step 3: injecting helium into the core holder by the gas inlet assembly and the gas reference container, calculating the pore volume of the core according to the pressure of the helium, then discharging the helium in the core holder and the gas reference container by the gas outlet assembly, and vacuumizing the core holder and the gas reference container to negative pressure by the vacuum device; Step 4: heating the core by the heating sleeve, when the core is heated to the formation temperature of the coring section, saturating the core by alternately injecting confining pressure and methane at each pressure point according to the set pressure gradient by the confining pressure pump and the gas inlet assembly, and measuring the methane inlet amount of the core at each pressure point in the saturation process, when the sum of the methane inlet amounts is greater than the pore volume of the core, determining that the core is completely saturated; Step 5: discharging the methane in the core holder by the gas outlet assembly at each pressure drop point according to the set pressure drop gradient, and always keeping the confining pressure of the core greater than the methane pressure in the core holder, at the same time, reducing the methane pressure to the current atmospheric pressure by the gas outlet assembly, and measuring the methane discharge amount, methane temperature and methane pressure at each pressure drop point; Step 6: calculating the lost gas amount of the core according to the methane discharge amount, methane temperature and methane pressure at each pressure drop point.

2. The method of claim 1, wherein the method is based on a depletion experimental device to test the lost gas in the shale gas content, characterized in that: In step 3, the calculation method of the pore volume of the core is as follows: after the vacuum device is closed, 10 MPa of helium is pumped into the gas reference container through the air inlet assembly, the air source is cut off, and then the pressure in the gas reference container is measured after the pressure in the gas reference container is stable; then the helium in the gas reference container is controlled to enter the core holder through the pipeline, and the pressure in the gas reference container and the core holder is equal and stable, then the pressure in the core holder is measured, and then the pore volume of the core is calculated according to the pressure in the gas reference container and the pressure in the core holder, as follows: wherein V cv is the pore volume of the core in ml; P r 1 is the pressure in the gas reference vessel in 10 MPa; P c 1 is the pressure in the core holder in 10 Mpa; Z1 is P r 1 is the methane deviation factor at the conditions; Z2 is P c 1 is the methane deviation factor at the conditions; V r is the volume of the gas reference vessel in ml; V e is the volume of the tubing from the gas reference vessel to the core holder plus the dead volume inside the core holder not occupied by the core in ml.

3. The method of claim 1, wherein the apparatus is a decay experiment apparatus. In step 4, the highest pressure of the core saturated with methane is the formation pressure of the core taking section, and at least 10 pressure points with gradually increased pressure are included according to the set pressure gradient, and the pressure difference between any two pressure points should not be higher than one tenth of the formation pressure of the shale core taking section.

4. The method of claim 3, wherein the method is based on a depletion experimental device to test the lost gas in the shale gas content. In step 4, the specific process of saturating the core is as follows: S1: first, operate the confining pressure pump to inject confining pressure higher than the set pressure of the first pressure point into the core, then control the air inlet assembly to inject methane into the core for saturation, and when the methane pressure in the core holder is increased to the pressure of the first pressure point, the methane inlet amount into the core at the first pressure point is measured, and at the same time, the injection of methane is stopped to allow the methane to be saturated in the core, and when the change amplitude of the methane pressure is less than 3% of the pressure of the first pressure point within 8 hours, it is determined that the core is completely saturated at the first pressure point; S2: again, operate the confining pressure pump to inject confining pressure higher than the set pressure of the second pressure point into the core, and then repeat step S1 to complete the complete saturation of the core at the second pressure point, and at the same time, the methane inlet amount into the core at the second pressure point is measured; S3: repeat step S2 to complete the complete saturation of the core at the remaining pressure points in turn, and at the same time, the methane inlet amount into the core at the remaining pressure points is measured; S4: according to the methane inlet amount into the core at each pressure point, it is determined whether the core is saturated, if the sum of all the methane inlet amounts is greater than the pore volume of the core, it is determined that the core is completely saturated; if the sum of all the methane inlet amounts is less than or equal to the pore volume of the core, it is determined that the core is not completely saturated, and then the last pressure point is continued to the complete saturation of the core.

5. The method of claim 1, wherein the method is based on a depletion experimental device to test the lost gas in the shale gas content. In step 5, the set pressure drop gradient is the formation pressure minus the current atmospheric pressure divided by the number of hours from the bottom of the well to the top of the well during the core taking process, and a plurality of pressure drop points are included according to the set pressure drop gradient, and the number of pressure drop points is the number of hours from the bottom of the well to the top of the well during the core taking process.

6. The method of claim 1, wherein the method is based on a depletion experimental device to test the lost gas in the shale gas content. In step 6, the calculation method of the lost gas amount of the core is as follows: + wherein V s total Loss gas for the measured shale core, unit: ml; V0 1 V0 is the methane discharge at the first pressure drop point, in ml; V0 n Vn is the methane discharge at the nth pressure drop point, in ml; P0 1 P0is the methane pressure at the first pressure drop point, in MPa; P0 n Pn is the methane pressure at the nth pressure drop point, in MPa; T1 is the temperature of methane at the first pressure drop point, unit: ℃; T n Tn is the temperature of the methane at the nth pressure drop point in °C.

7. A method for testing gas loss in shale gas content based on a depletion experimental device according to any one of claims 1-6, characterized in that: The air inlet assembly comprises an air inlet pressure reducing valve, a constant speed and constant pressure pump, an air inlet flow meter, an air inlet pressure sensor, a valve A and an air inlet pipeline connected in sequence, the air inlet of the air inlet pressure reducing valve is connected with the air source, the air outlet of the air inlet pipeline is connected with the air inlet end of the core holder and extends into the axial hole of the core; the air inlet flow meter and the air inlet pressure sensor are respectively used for measuring the flow index and the pressure index of the input gas, and the vacuum device and the gas reference container are connected between the air inlet flow meter and the air inlet pressure sensor in sequence.

8. The method of claim 7, wherein the method is based on a depletion experimental device to test the lost gas in the shale gas content. The surface of the portion of the air intake pipe extending into the axial hole is evenly provided with four rows of holes, and the angle between each two rows of holes is 90 degrees.

9. A method for testing gas loss in shale gas content based on a depletion experimental device according to any one of claims 1-6, characterized by: The gas outlet assembly includes a gas outlet pipe, a safety valve, a valve B, a gas outlet pressure sensor, a gas outlet automatic pressure regulating valve, a gas outlet temperature sensor, a gas outlet pressure reducing valve and a normal pressure gas flow meter, which are connected in sequence. The gas outlet pressure sensor, the gas outlet temperature sensor and the normal pressure gas flow meter are used to measure the pressure index, temperature index and flow index of the output gas respectively. The gas outlet pipe is connected between the core sealing sleeve at the gas outlet end of the core clamp and the arc-shaped radial groove plate.

10. The method of claim 1, wherein the method is based on a depletion experimental device to test the lost gas in the shale gas content. The number of the arc-shaped radial slot plates is 2-6, and the sum of the widths of all the arc-shaped radial slot plates is smaller than the circumference of the core sealing rubber sleeve.

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