Method for testing lost gas in shale gas content based on depletion experimental device

Abstract

The invention 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-gas field development, the depletion experiment device comprises a rock core holder, a heating jacket, a confining pressure booster pump, a gas inlet assembly, a gas outlet assembly, a vacuumizing device, a gas reference container and an arc radiation trough plate, the loss gas in the shale gas content is tested based on the depletion experiment device, the characteristic that the horizontal permeability of the shale is far greater than the vertical permeability is fully utilized, the testing device completely conforming to the real environment is designed, on one hand, the shale core can be really saturated by methane, the time required for saturating the shale core is greatly shortened, and on the other hand, the loss gas in the shale gas content is greatly reduced; on the other hand, the lost gas part in the shale gas content can be directly tested, the testing efficiency is improved, the accuracy of the testing result is also improved, and therefore powerful evidence support is provided for shale gas reservoir evaluation and reserve calculation.

Description

Technical Field

[0001] The 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 existing in shale layers, and its main component is methane. Shale gas mainly exists in shale in the form of adsorbed phase and free phase. The gas content of shale is a key parameter to characterize the quality of shale reservoirs. Before exploiting shale gas reservoirs, it is necessary to determine the gas content of shale gas reservoirs. The quality of gas content is usually measured by shale gas content, that is, the volume of shale gas under standard gas state contained in unit mass of shale. There are two existing shale gas content testing technologies:

[0003] One is the on-site segmented desorption method, which is to seal the core obtained from underground drilling into a container for heating and measure the amount of shale gas desorbed from the core, and calculate the loss of shale gas 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 continued to be heated to measure the amount of residual shale gas that escapes, and then the desorbed gas volume of the shale core is added to the lost gas volume and the participating gas volume, and the sum of the gas volumes divided by the mass of the tested shale core 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, but this part of the gas content is obtained by indirect calculation rather than direct measurement. The basis for the 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 in the core escapes during the shale gas desorption gas volume 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 real shale gas content and will be difficult to verify by other methods. In addition, the shale core for testing gas content has been in contact with air after being taken out of the coring barrel. The desorbed gas volume measured by heating the core includes a large part of air, which is also included in the desorbed gas volume of the core. There is no means to exclude this part of 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 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. The coring cylinder is then 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 gas content measured by this method is all gas in the shale core, without air mixing, and during the coring and lifting process, the core always maintains the formation pressure, the natural gas in the shale core is not lost, and the measured shale gas content data is more accurate and reliable than the aforementioned shale gas test method 1. However, this method also has some defects. First, this method requires pressure coring work to cooperate, but pressure coring is very expensive, and its promotion and application are affected by economic conditions. Secondly, in method 2, when the core obtained by pressure coring and the coring are heated to the formation temperature and the natural gas escaping therein is measured, there is a safety hazard that the local natural gas in the core is compressed and expanded, forming a high-pressure gas flow, which causes the pipeline to pop out and injure people due to lack of pressure. In addition, although the gas content data measured by this method is accurate, the shale gas content data measured by method 2 cannot be used to correct the shale gas content data measured by method 1, that is, it cannot be used to calculate the loss of shale gas that escapes during the lifting of the shale core from the bottom of the well to the wellhead.

[0006] At present, many scientific researchers have also designed many experimental methods to calculate the amount of shale gas lost during the process of lifting shale cores from the bottom of the well to the wellhead. The commonly used experimental method is to place the core in a core holder, saturate the core with methane to the formation pressure from the vertical direction of the core, and then set the pressure and temperature drop rates according to the length of time it takes to lift the core from the bottom of the well to the wellhead. The core is then gradually desorbed at the set pressure drop rate to release the methane gas in the core, and the desorbed gas volume is measured until no more methane escapes from the core. The measured methane desorption volume is the lost gas volume in the shale gas content.

[0007] Some other researchers have also adopted other methods to calculate the lost gas content in shale gas content, for example, collecting shale cores and obtaining the time required for the shale cores; placing the shale cores in a desorption tank for desorption experiments to obtain residual gas content; pre-processing the desorption section experimental data to conform to uniform distribution to obtain uniform desorption section experimental data; determining the parameter values ​​of the fitting segmented estimation model based on the uniform desorption section experimental data and the objective function of the desorption section in the fitting segmented estimation model; determining the lost gas content in the shale core based on the time and the lost gas section objective function. In addition, the patent document with publication number CN104863579A also discloses a lost gas content determination method and a lost gas content determination system, the process of which is: first, under the same conditions, the same core sample is repeatedly sampled at equal time intervals, and the gas content of each sample is determined, and then the lost gas content of the core sample is solved by the "t" function algorithm based on the relationship between the gas escape rate and time. However, the aforementioned technical solution still has certain defects. First, there is a huge difference between the vertical permeability and the lateral permeability of the shale core. The lateral permeability of the shale core is often one to two orders of magnitude higher than the vertical permeability. This makes it difficult for methane gas to completely saturate the shale core vertically, resulting in a very low value of the lost gas portion of the shale gas content obtained using the experimental method, even lower than the desorbed gas volume of the shale core. The lost gas volume calculated by this method is neither reliable nor accurate. Secondly, the calculation of the lost gas volume by using the uniform distribution or "t" function method is actually based on the assumption of the shale gas desorption rate and the reverse calculation of the lost gas volume of the shale core, which lacks direct experimental evidence. The above-mentioned defects affect the reliability of the lost gas volume in the shale gas content obtained by relying on the existing technical solutions, and thus have an adverse impact on the evaluation of shale gas reservoirs and the calculation of reserves. Summary of the invention

[0008] The purpose of the present invention is to overcome the above-mentioned technical problems existing in the prior art, and to provide a method for testing the lost gas in shale gas content based on a depletion experimental device. The present invention makes full use of the characteristic that the horizontal permeability of shale is much greater than the vertical permeability, and designs a testing device that fully conforms to the real environment. On the one hand, methane can truly saturate the shale core and greatly reduce the time required for saturating the shale core. On the other hand, it can directly test and obtain the lost gas part in the shale gas content, which not only improves the test efficiency, but also improves the accuracy of the test results, thereby providing strong evidence support for shale gas reservoir evaluation and reserve calculation.

[0009] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0010] A method for testing the lost gas in shale gas content based on a depletion experimental device, characterized in that: the depletion experimental device comprises a core holder, a heating sleeve, a confining pressure boosting pump, an air inlet assembly, an air outlet assembly, a vacuum device, a gas reference container and an arc-shaped radial slot plate, a core sealing rubber sleeve is fixed in the core holder, the arc-shaped radial 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 fixing sleeve is arranged on the core holder, the confining pressure boosting pump is connected to the core holder to inject confining pressure, the air inlet assembly is connected to the air inlet end of the core holder to input gas into the core and measure the gas index, the air outlet assembly is connected between the core sealing rubber sleeve and the arc-shaped radial slot plate at the air outlet end of the core holder to discharge gas and measure the gas index; the vacuum device and the gas reference container are connected to the air inlet end of the core holder through the air inlet assembly;

[0011] The method for testing the gas loss in shale gas content comprises the following steps:

[0012] Step 1: Get the core, dry it to constant weight, open an axial hole on the core, cover the core with an arc-shaped radial slot plate, and then seal it in the core sealing rubber sleeve of the core holder, then connect the air inlet of the air inlet assembly to the air source, and extend the air outlet into the axial hole;

[0013] Step 2: Use the air inlet assembly to inject helium and the confining pressure booster pump to test the sealing of the confining pressure test device. If the sealing is intact, open the air outlet assembly to discharge the helium in the core and reduce the confining pressure to zero. Then use the vacuum device to evacuate the core holder and the gas reference container to negative pressure.

[0014] Step 3: Inject helium into the core holder using the gas inlet assembly and the gas reference container, and calculate the pore volume of the core according to the pressure of the helium; then open the gas outlet assembly to discharge the helium in the core holder and the gas reference container, and use the vacuum device to evacuate the core holder and the gas reference container to negative pressure;

[0015] Step 4: The core is heated by the heating sleeve. When the core is heated to the formation temperature of the coring section, the confining pressure booster pump and the air intake assembly saturate the core by alternately injecting confining pressure and methane at each pressure point according to the set pressure gradient. At the same time, the methane intake volume entering the core at each pressure point is measured during the saturation process. When the sum of the methane intake volumes is greater than the pore volume of the core, the core is determined to be fully saturated.

[0016] Step 5: The gas outlet assembly gradually discharges the methane in the core holder at each pressure drop point according to the set pressure drop gradient, and always keeps the core confining pressure greater than the methane pressure in the core holder. At the same time, the gas outlet assembly is used to reduce the methane pressure to the current atmospheric pressure, and the methane discharge amount, methane temperature and methane pressure at each pressure drop point are measured;

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

[0018] In step 1, the obtained rock core is a cylindrical structure with smooth and flat ends. It is dried to constant weight at 60°C. The radius of the axial hole is no more than 1 / 5 of the radius of the rock core and can be used for the air outlet of the air inlet assembly to extend into.

[0019] In step 3, the pore volume of the core is calculated as follows: after the vacuum device is turned off, 10 MPa of helium is first pumped into the gas reference container through the gas inlet assembly 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; then the helium in the gas reference container is controlled to enter the core holder through the pipeline. When the pressure in the gas reference container and the core holder are equal and stable, the pressure in the core holder is measured, and then the pore volume of the core is calculated based on the pressure in the gas reference container and the pressure in the core holder, as follows:

[0020]

[0021] Where V cv is the pore volume of the core, in ml; P r 1 is the pressure in the gas reference container, in units of 10MPa; P c 1 is the pressure in the core holder, in units of 10Mpa; Z1 is P r 1 Methane deviation coefficient under the conditions; Z2 is P c 1 Methane deviation coefficient under the condition; V r V is the volume of the gas reference container in ml; e It 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.

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

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

[0024] S1: First operate the confining pressure booster pump to inject a confining pressure higher than the set pressure at the first pressure point into the core, then control the air intake assembly to inject methane into the core for saturation. When the methane pressure in the core holder is increased to the pressure at the first pressure point, measure the methane intake into the core at the first pressure point, and stop injecting methane at the same time to allow methane to saturate the core. When the measured methane pressure changes within 8 hours and is less than 3% of the pressure at the first pressure point, the core is determined to be completely saturated at the first pressure point.

[0025] S2: operate the confining pressure boosting pump again to inject a confining pressure higher than the set pressure at 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 measure the methane intake into the core at the second pressure point;

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

[0027] S4: Determine whether the core is saturated based on the amount of methane entering the core at each pressure point. If the sum of all methane intake volumes is greater than the pore volume of the core, the core is determined to be fully saturated; if the sum of all methane intake volumes is less than or equal to the pore volume of the core, the core is determined to be incompletely saturated, and then continue at the last pressure point until the core is fully saturated.

[0028] In step 5, the set pressure drop gradient is the formation pressure minus the current atmospheric pressure divided by the number of hours taken by the core to travel from the bottom of the well to the wellhead during the coring process. The pressure drop gradient includes multiple pressure drop points, and the number of pressure drop points is the number of hours taken by the core to travel from the bottom of the well to the wellhead during the coring process.

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

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

[0031]

[0032] Where V s total is the gas loss volume of the measured shale core, in ml;

[0033] V0 1 is the methane emission at the first pressure drop point, in ml;

[0034] V0 n is the methane emission at the nth pressure drop point, in ml;

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

[0036] P0 n 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 °C;

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

[0039] The air intake assembly comprises an air intake pressure reducing valve, a constant speed and pressure pump, an air intake flow meter, an air intake pressure sensor, a valve A and an air intake pipeline which are connected in sequence. The air intake of the air intake pressure reducing valve is connected to a gas source, and the air outlet of the air intake pipeline is connected to the air intake end of the core holder and extends into the axial hole of the core. The air intake flow meter and the air intake pressure sensor are used to measure the flow index and pressure index of the input gas respectively, and the vacuum pumping device and the gas reference container are connected in sequence between the air intake flow meter and the air intake pressure sensor.

[0040] The surface of the portion of the air intake pipe extending into the axial hole is evenly provided with 4 rows of holes, and the angle between every two rows of holes is 90 degrees.

[0041] The gas outlet assembly comprises a gas outlet pipeline, 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, the temperature index and the flow index of the output gas respectively. The gas outlet pipeline is connected between the core sealing rubber sleeve at the gas outlet end of the core clamp and the arc-shaped radial groove plate.

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

[0043] The advantages of adopting the present invention are:

[0044] 1. The present invention uses a specific sealing structure and a sealing detection process to prevent the core from contacting air during the methane saturation process, thereby preventing the influence of air on the measurement results and improving the accuracy of measuring the gas loss of shale.

[0045] 2. The core decompression desorption process of the present invention fully complies with the situation of the pressure drop inside the core from the bottom of the well to the wellhead in the real environment, which effectively improves the accuracy of measuring the gas loss of shale. The gas loss of the core is directly measured, and no assumption conversion is required, which greatly improves the reliability and accuracy of the gas loss.

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

[0047] 4. The present invention gradually saturates the core by gradually increasing the pressure at each pressure point according to the set pressure gradient, which enables the core to be effectively saturated at each pressure point. Coupled with the comparison with the core pore volume, the core can be accurately fully saturated, which is conducive to 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, so as to obtain a more accurate loss of gas volume.

[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 is used to provide 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 the time required for saturating the shale core is greatly reduced, 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, thereby avoiding the inaccurate measured shale gas loss 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 of 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 evenly arranges 4 rows of holes on the surface of the portion of the air inlet pipe extending into the axial hole, and the angle between every two rows of holes is 90 degrees, which is not only beneficial to the uniform saturation of the core, but also beneficial to shortening the saturation time.

[0053] 10. The present invention sets the number of arc-shaped radial slot plates to 2-6, and makes the sum of the widths of all the arc-shaped radial slot plates less than the chord length of the core sealing rubber sleeve. The advantage is that a suitable number of arc-shaped radial slot plates can be equipped according to the smoothness of the core surface to ensure that the arc-shaped radial slot plates and the core fit tightly. For example, if the core surface is smooth and has good roundness, 2-3 arc-shaped radial slot plates can be used to ensure that the slot plates and the core fit tightly, and the slot plates and the core are evenly stressed. If the core surface is not smooth and has poor roundness, 3-6 arc-shaped radial slot plates can be used to wrap the core, so that the core will not change the local seepage pressure of the core due to uneven confining pressure, and the core will not break due to uneven stress. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] Figure 1 A schematic diagram of the structure of the present invention. DETAILED DESCRIPTION

[0055] Example 1

[0056] This embodiment discloses a method for testing the lost gas in shale gas content based on a depletion test device, such as Figure 1 As shown, the exhaustion experimental device comprises:

[0057] The 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. A core sealing rubber sleeve is fixed inside the shell of the core holder. The core sealing rubber sleeve is used to give a certain confining pressure to the core in the core holder according to the experimental design.

[0058] The arc-shaped radial 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 arc-shaped radial slot plate is essentially an arc-shaped plate adapted to the outer surface of the core, which can be made of 316SS stainless steel with a thickness of less than 5mm, and is provided with uniform flow channels, which serve as the seepage channels of methane on the surface of the core. In addition, 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 less than the chord length of the core sealing rubber sleeve. It should be noted that when the number of arc-shaped radial groove plates is 2, a single arc-shaped radial groove plate occupies 4 / 5 of the chord length of the core sealing rubber sleeve, when the number of arc-shaped radial groove plates is 3, a single arc-shaped radial groove plate occupies 3 / 10 of the chord length of the core sealing rubber sleeve, when the number of arc-shaped radial groove plates is 4, a single arc-shaped radial groove plate occupies 3 / 16 of the chord length of the core sealing rubber sleeve, and when the number of arc-shaped radial groove plates is 5-6, a single arc-shaped radial groove plate occupies 3 / 20 of the chord length of the core sealing rubber sleeve. In this embodiment, the number of arc-shaped radial groove plates is preferably 4, and each groove plate occupies 3 / 16 of the chord length of the core sealing rubber sleeve.

[0059] The gas inlet plug is used to provide a channel for accommodating the gas inlet pipeline for the methane saturated core and to assist the gas inlet sealing plate to isolate the core, ensuring that the core can be under the set saturation pressure.

[0060] The air inlet end sealing plate covers the cross-sectional end surface of the core at the air inlet end of the core holder, and is used to seal the air inlet end of the core and the end surface of the core sealing rubber sleeve, and ensure that the core can be under the set saturation pressure.

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

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

[0063] The confining pressure boosting pump is connected to the core sealing rubber sleeve in the core holder to inject confining pressure. Its function is to inject a certain pressure into the core sealing rubber sleeve, thereby placing the core in the core holder under a certain confining pressure.

[0064] The air inlet assembly has an air inlet connected to a gas source and an air outlet connected to the air inlet end of the core holder. The gas source includes a helium gas source and a methane gas source. The air inlet pipes may include high-pressure resistant pipelines, valves, pumps and sensors, etc., which are used to input the gas in the gas source into the core and measure the gas indicators. The specific indicators to be measured may include pressure, temperature and / or flow, etc.

[0065] The gas outlet assembly is connected between the core sealing rubber sleeve at the gas outlet end of the core holder and the arc-shaped radial slot plate to exhaust gas and measure gas indicators.

[0066] The vacuum device is connected to the air inlet end of the core holder through the air inlet assembly, and is used to remove 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 pump and the core holder. It is used as a reference chamber when measuring the pore volume of the shale core using helium.

[0068] It should be noted that the present embodiment does not limit the specific structure of the air inlet assembly and the air outlet assembly, as long as the phase structure can realize the transmission of gas and the measurement of gas indicators.

[0069] Based on the exhaustion test device of the above structure, the method for testing the gas loss in shale gas content comprises the following steps:

[0070] Step 1: First obtain a core with a cylindrical structure, and dry the core to constant weight at 60°C. Then cut both ends of the core flat and polish them as smooth as possible so that they can be in good contact with the inlet and outlet sealing plates, thereby effectively isolating the gas pressure in the core. Then open an axial hole perpendicular to the cross section of the core and through the core on the core. The radius of the axial hole is not greater than 1 / 5 of the radius of the core, and the outlet of the inlet assembly can be inserted. Then cover the core with an arc-shaped radial groove plate and put it into the core sealing rubber sleeve of the core holder. Use the inlet sealing plate and the outlet sealing plate to seal the inlet and outlet ends of the core holder, and use the inlet plug to seal to ensure the tightness of the core. Finally, connect the inlet of the inlet assembly to the helium gas source, extend the outlet into the axial hole, and connect the outlet assembly at the same time.

[0071] Step 2: Use the air inlet assembly to inject helium and the confining pressure pump to test the sealing of the confining pressure test device. If the sealing is intact, open the air outlet assembly to discharge the helium in the core and reduce the confining pressure to zero. After the helium in the core has completely escaped, close all valves connecting the core holder to the outside world, open the vacuum device, and evacuate the core holder and the gas reference container to negative pressure, specifically to -1KPa.

[0072] Furthermore, the specific process of testing the sealing of the device is: first close all valves connecting the core in the core holder with the outside world, then use the air intake assembly to pump helium of the set experimental pressure into the core, and at the same time operate the confining pressure booster pump to make the confining pressure of the core always greater than the set experimental pressure of 5MPa, so as to test whether the sealing of the device is intact.

[0073] Step 3: After the core holder and the gas reference container have reached the vacuum target, close the vacuum device, open the air inlet assembly, use the air inlet assembly and the gas reference container to inject helium into the core holder, and calculate the pore volume of the core according to the pressure of the helium; then open the air outlet assembly to discharge the helium in the core holder and the gas reference container, and use the vacuum device to evacuate the core holder and the gas reference container to negative pressure, specifically to -1KPa.

[0074] Furthermore, the calculation method of the pore volume of the core is as follows: after the vacuum device is turned off, 10MPa helium is first pumped into the gas reference container through the gas inlet assembly 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; then the helium in the gas reference container is controlled to enter the core holder through the pipeline. When the pressure in the gas reference container and the core holder are equal and stable, the pressure in the core holder is measured, and then the pore volume of the core is calculated based on the pressure in the gas reference container and the pressure in the core holder, as follows:

[0075]

[0076] Where V cv is the pore volume of the core, in ml; P r 1 is the pressure in the gas reference container, in units of 10MPa; P c 1 is the pressure in the core holder, in units of 10Mpa; Z1 is P r 1 Methane deviation coefficient under the conditions; Z2 is P c 1 Methane deviation coefficient under the condition; V r V is the volume of the gas reference container in ml; e It 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 and heat the core through the heating jacket. When the core is heated to the formation temperature of the coring section and remains stable, the confining pressure booster pump and the air intake assembly saturate the core by alternately injecting confining pressure and methane at each pressure point according to the set pressure gradient. At the same time, the methane intake volume entering the core at each pressure point is measured during the saturation process. When the sum of the methane intake volumes is greater than the pore volume of the core, the core is determined to be completely saturated.

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

[0079] Furthermore, the specific process of saturating the core is:

[0080] S1: First operate the confining pressure booster pump to inject a confining pressure higher than the set pressure at the first pressure point into the core. The set pressure can be 5MPa, and specifically, a confining pressure higher than 5MPa at the first pressure point can be injected into the core. Then control the air intake assembly to inject methane into the core for saturation. When the methane pressure in the core holder is increased to the pressure at the first pressure point, measure the methane intake into the core at the first pressure point, and stop injecting methane at the same time to saturate the core with methane. It should be noted that the pressure at the core holder's air intake center will drop when saturated. Therefore, when the measured methane pressure changes within 8 hours and is less than 3% of the pressure at the first pressure point, it can be determined that the core is fully saturated at the first pressure point.

[0081] S2: When the core is completely saturated with methane at the first pressure point, the confining pressure booster pump is operated again to inject a confining pressure higher than the set pressure at the second pressure point into the core. The set pressure is also 5 MPa, that is, a confining pressure higher than 5 MPa at 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 measure the methane intake into the core at the second pressure point.

[0082] S3: Repeat step S2 to complete the complete saturation of the core at the remaining pressure points in turn, and measure the methane intake into the core at the remaining pressure points at the same time.

[0083] S4: Determine whether the core is saturated based on the amount of methane entering the core at each pressure point. If the sum of all methane intake volumes is greater than the pore volume of the core, the core is determined to be fully saturated; if the sum of all methane intake volumes is less than or equal to the pore volume of the core, the core is determined to be incompletely saturated, and then continue at the last pressure point until the core is fully saturated.

[0084] Step 5: After the core is fully saturated, the gas outlet component gradually discharges the methane in the core holder at each pressure drop point according to the set pressure drop gradient, and always keeps the core confining pressure greater than the methane pressure in the core holder. Specifically, the core confining pressure can be kept greater than the methane pressure in the core holder by 5 MPa. At the same time, the methane pressure is reduced to the current atmospheric pressure by the gas outlet component, and the methane discharge, 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 it takes for the core to travel from the bottom of the well to the wellhead during the coring process, in units of MPa / h. The pressure drop gradient includes multiple pressure drop points, and the number of pressure drop points is the number of hours it takes for the core to travel from the bottom of the well to the wellhead during the coring process.

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

[0087] Step 6: Calculate the gas loss of the core according to the methane emission, methane temperature and methane pressure at each pressure drop point.

[0088] Furthermore, the calculation method of the gas loss of the core is:

[0089]

[0090] Where V s total is the gas loss volume of the measured shale core, in ml;

[0091] V0 1 is the methane emission at the first pressure drop point, in ml;

[0092] V0 n is the methane emission at the nth pressure drop point, in ml;

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

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

[0095] T1 is the methane temperature at the first pressure drop point, in °C;

[0096] T n is the methane temperature at the nth pressure drop point, in °C.

[0097] From the above content, it can be seen that this embodiment makes full use of 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, methane can truly saturate the shale core and greatly reduce the time required to saturate the shale core. On the other hand, it can directly test the lost gas part of the shale gas content, which not only improves the test efficiency, but also improves the accuracy of the test results.

[0098] Finally, after the loss gas volume of the core is obtained through the above-mentioned technical test, the core holder is opened, the core is taken out of the core holder, and the corresponding experiments are carried out in accordance with the provisions of SY / T6940-2020 "Determination of Shale Gas Content" on the measurement method of desorbed gas and residual gas. The desorbed gas content and the residual gas content are obtained. The desorbed gas content, the residual gas content and the loss gas volume obtained by this implementation are added, and then divided by the mass of the core sample to obtain the total gas content of the core in ml / g; thereby providing strong evidence support for shale gas reservoir evaluation and reserve calculation.

[0099] Example 2

[0100] Based on Example 1, this embodiment further defines the structures of the air inlet assembly and the air outlet assembly. Figure 1 As shown, the air inlet component and the air outlet component are:

[0101] The air intake assembly includes an air intake pressure reducing valve, a constant speed and pressure pump, an air intake flow meter, an air intake pressure sensor, a valve A and an air intake pipeline. The air intake pressure reducing valve, the constant speed and pressure pump, the air intake flow meter, the air intake pressure sensor and the valve A are connected in sequence through a high pressure resistant pipeline. The air intake of the air intake pressure reducing valve is connected to the gas source, the air intake of the air intake pipeline is connected to the valve A, and the air outlet of the air intake pipeline is connected to the air intake end of the core holder and extends into the axial hole of the core. In order to facilitate the effective input of gas, it is preferred that 4 rows of holes are evenly arranged on the surface of the part of the air intake pipeline extending into the axial hole, and the angle between each two rows of holes is 90 degrees. The air intake flow meter and the air intake pressure sensor are used to measure the flow index and pressure index of the input gas respectively, and the vacuum device and the gas reference container are connected in sequence between the air intake flow meter and the air intake pressure sensor.

[0102] The gas outlet assembly comprises a gas outlet pipeline, 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, the temperature index and the flow index of the output gas respectively. The gas outlet pipeline is connected between the core sealing rubber sleeve at the gas outlet end of the core clamp and the arc-shaped radial groove plate.

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

[0104] The constant speed and constant pressure pump is used to increase the pressure of methane gas to a pressure that meets the experimental requirements.

[0105] The air inlet flow meter is used to measure the methane flow entering the core holder and can measure the methane intake into the core after the methane saturation process of the core in the holder is completed.

[0106] Inlet pressure sensor to measure the methane pressure entering the core holder.

[0107] Outlet pressure sensor to measure the pressure of methane flowing out of the core holder.

[0108] The atmospheric pressure gas flow meter is used to measure the flow rate of methane gas escaping from the core, and can complete the escape of methane gas in the core in the holder. After the desorption experiment is completed, the total amount of methane gas escaping from the core is measured.

[0109] The air inlet pipeline is used to transport pressurized gas into the core, and the pipeline is equipped with a pressure sensor with data acquisition function.

[0110] Gas outlet temperature sensor to measure the temperature of methane gas escaping from the core holder.

[0111] The outlet pressure reducing valve can reduce the high pressure gas in the core holder to atmospheric pressure, making it convenient for the outlet atmospheric pressure gas flow meter to measure.

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

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

[0114] Step 1: First obtain a core with a cylindrical structure, and dry the core to constant weight at 60°C. Then cut both ends of the core flat and polish them as smooth as possible so that they can be in good contact with the air inlet and outlet sealing plates, thereby effectively isolating the gas pressure in the core. Then open an axial hole on the core that is perpendicular to the cross section of the core and passes through the core. The radius of the axial hole is not greater than 1 / 5 of the radius of the core, and the air outlet of the air inlet assembly can be inserted. Then cover the core with an arc-shaped radial groove plate and put it into the core sealing rubber sleeve of the core holder. Use the air inlet sealing plate and the air outlet sealing plate to seal the air inlet and outlet ends of the core holder, and use the air inlet plug to seal to ensure the tightness of the core. Finally, connect the air inlet pressure reducing valve, constant speed and pressure pump, air inlet flowmeter, air inlet pressure sensor, valve A and air inlet pipe in sequence without leakage, connect the air outlet pipe, safety valve, valve B, air outlet pressure sensor, air outlet gas automatic pressure regulating valve, air outlet temperature sensor, air outlet pressure reducing valve and atmospheric pressure gas flowmeter in sequence without leakage, and extend the part with holes on the air inlet pipe into the axial hole.

[0115] Step 2: Use the air inlet assembly to inject helium and the confining pressure pump to test the sealing of the confining pressure test device. If the sealing is intact, open the air outlet assembly to discharge the helium in the core and reduce the confining pressure to zero. After the helium in the core has completely escaped, close all valves connecting the core holder to the outside world, open the vacuum device, and evacuate the core holder and the gas reference container to negative pressure, specifically to -1KPa.

[0116] Furthermore, the specific process of testing the sealing of the device is: first close all valves connecting the core in the core holder with the outside world, then use a constant speed and constant pressure pump to pump helium of the set experimental pressure into the core, and at the same time operate the confining pressure boosting pump to make the confining pressure of the shale core always greater than the set experimental pressure of 5MPa, so as to test whether the sealing of the device is intact.

[0117] Step 3: After the core holder and the gas reference container have reached the vacuum target, close the vacuum pumping device, open the air inlet assembly, use the air inlet assembly and the gas reference container to inject helium into the core holder, and calculate the pore volume of the core according to the pressure of the helium; then open all valves in the air outlet assembly to discharge the helium in the core holder and the gas reference container, and use the vacuum pumping device to evacuate the core holder and the gas reference container to negative pressure, specifically to -1KPa.

[0118] Furthermore, the calculation method of the pore volume of the core is as follows: after the vacuum device is turned off, 10 MPa of helium is first pumped into the gas reference container by a 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 the air inlet pressure sensor; then the helium in the gas reference container is controlled to enter the core holder through the air inlet pipe. When the pressure in the gas reference container and the core holder is equal and stable, the pressure in the core holder is measured by the air 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] Where V cv is the pore volume of the core, in ml; P r 1 is the pressure in the gas reference container, in units of 10MPa; P c 1 is the pressure in the core holder, in units of 10Mpa; Z1 is P r 1 Methane deviation coefficient under the conditions; Z2 is P c 1 Methane deviation coefficient under the condition; V r V is the volume of the gas reference container in ml; e It 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.

[0121] Step 4: Turn on the power of the heating jacket and heat the core through the heating jacket. When the core is heated to the formation temperature of the coring section and remains stable, the confining pressure booster pump and the air intake assembly saturate the core by alternately injecting confining pressure and methane at each pressure point according to the set pressure gradient. At the same time, during the saturation process, the air inlet flowmeter is used to measure the methane intake into the core at each pressure point. When the sum of the methane intakes is greater than the pore volume of the core, the core is determined to be completely saturated.

[0122] It should be noted that the maximum pressure of the core saturated with methane is the formation pressure of the core sampling section. The set pressure gradient includes at least 10 pressure points with gradually increasing pressures, and the pressure difference between any two pressure points shall not be higher than one tenth of the formation pressure of the shale coring section.

[0123] Furthermore, the specific process of saturating the core is:

[0124] S1: First operate the confining pressure booster pump to inject a confining pressure higher than the set pressure at the first pressure point into the core. The set pressure can be 5MPa. Specifically, a confining pressure higher than 5MPa at the first pressure point can be injected into the core. Then control the air intake assembly to inject methane into the core for saturation. When the methane pressure in the core holder is increased to the pressure at the first pressure point, use the air inlet flowmeter to measure the methane intake into the core at the first pressure point, and stop injecting methane at the same time to saturate the core with methane. It should be noted that the pressure at the core holder air intake center will drop when saturated. Therefore, when the measured methane pressure changes within 8 hours and is less than 3% of the pressure at the first pressure point, it can be determined that the core is fully saturated at the first pressure point.

[0125] S2: When the core is completely saturated with methane at the first pressure point, the confining pressure booster pump is operated again to inject a confining pressure higher than the set pressure at the second pressure point into the core. The set pressure is also 5 MPa, that is, a confining pressure higher than 5 MPa at 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 measure the methane intake into the core at the second pressure point.

[0126] S3: Repeat step S2 to complete the complete saturation of the core at the remaining pressure points in turn, and measure the methane intake into the core at the remaining pressure points at the same time.

[0127] S4: Determine whether the core is saturated based on the amount of methane entering the core at each pressure point. If the sum of all methane intake volumes is greater than the pore volume of the core, the core is determined to be fully saturated; if the sum of all methane intake volumes is less than or equal to the pore volume of the core, the core is determined to be incompletely saturated, and then continue at the last pressure point until the core is fully saturated.

[0128] Step 5: After the core is fully saturated, set the outlet pressure drop gradient of the automatic gas pressure regulating valve at the outlet end, so that the outlet assembly gradually discharges the methane in the core holder at each pressure drop point according to the set pressure drop gradient, and always keeps the core confining pressure greater than the methane pressure in the core holder. Specifically, the core confining pressure can be kept greater than the methane pressure in the core holder by 5 MPa. At the same time, the outlet pressure reducing valve in the outlet assembly is used to reduce the methane pressure to the current atmospheric pressure, and the atmospheric pressure gas flowmeter, outlet temperature sensor and outlet pressure sensor are used to measure the methane discharge, methane temperature and methane pressure at each pressure drop point 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 it takes for the core to travel from the bottom of the well to the wellhead during the coring process, in units of MPa / h. The pressure drop gradient includes multiple pressure drop points, and the number of pressure drop points is the number of hours it takes for the core to travel from the bottom of the well to the wellhead during the coring process.

[0130] In addition, when discharging methane, the temperature drop gradient of the heating sleeve is first set according to the rate of decrease of the core temperature during the core extraction process, and then the exhaust pressure of the gas outlet component is set according to the process of pressure reduction 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 gas loss of the core can be calculated based on the methane emission, methane temperature and methane pressure at each pressure drop point.

[0132] Example 3

[0133] This example verifies the method of the present invention as follows:

[0134] Step 1: Select a section of shale pressure coring from Longmaxi Formation Longyi 11 sublayer in Ning X0X-2 well in Changning area. The formation pressure in the coring section is 56.2MPa, the formation temperature is 103.5℃, the core extraction time is 10h, and the core diameter is 105mm. Take a complete section of smooth core, dry the core at 60℃ to constant weight, cut both ends of the full-diameter shale core flat and polish it as smooth as possible, and drill a 1cm diameter round hole in the middle of the core end face with a liquid nitrogen-cooled drilling machine, and clean up the debris during the drilling process. Then cover the core with an arc-shaped radial groove plate and put it into the core sealing rubber sleeve of the core holder to seal it, and then connect the air inlet of the air inlet assembly to the air source, and extend the air outlet into the axial hole.

[0135] Step 2: Test the sealing of the device. If the test results show that the sealing of the device is intact, open the gas outlet assembly to discharge the helium in the core and reduce the surrounding pressure to zero. Then use the vacuum device to evacuate the core holder and the gas reference container to negative pressure.

[0136] Step 3: Use the gas inlet assembly and the gas reference container to inject 5 MPa of helium into the core holder, and calculate the pore volume of the core as 76.1 cm based on the helium pressure. 3 Then, the gas outlet assembly is opened to discharge the helium in the core holder and the gas reference container, and the core holder and the gas reference container are evacuated to negative pressure using a vacuum device.

[0137] Step 4: Heat the core to the formation temperature of 103.5℃ through the heating sleeve. When the core is heated to the formation temperature of the coring section, set the pressure gradient to 20MPa / h, and use the confining pressure booster pump and the air intake assembly to saturate the core by alternately injecting confining pressure and methane at each pressure point according to the set pressure gradient until the gas pressure in the core reaches 56.2MPa. At the same time, measure the methane intake into the core at each pressure point during the saturation process. When the sum of the methane intake is greater than the pore volume of the core, the core is determined to be fully saturated.

[0138] Step 5: According to the core extraction time, the pressure drop gradient is set to 5.61MPa / h, and the pressure drop time is 10h. The gas outlet component gradually discharges the methane in the core holder at each pressure drop point according to the set pressure drop gradient, and always keeps the core confining pressure greater than the methane pressure in the core holder. At the same time, the gas outlet component is used to reduce the methane pressure to the current atmospheric pressure, and the methane discharge, methane temperature and methane pressure at each pressure drop point are measured. The experimental data obtained are as follows:

[0139]

[0140]

[0141] Step 6: According to the methane emission, methane temperature and methane pressure at each pressure drop point, substitute them into the calculation formula for the gas loss of the core, and finally calculate that the gas loss of the core is 6521ml.

[0142]

[0143] Comparative verification: The gas loss result calculated in step 6 was compared with the gas loss data measured during the pressure-maintaining coring process. The difference between the two was only 50 ml, which proved that the gas loss result obtained by the method of the present invention was accurate and reliable.

[0144] The above description is only a specific implementation mode of the present invention. Any feature disclosed in this specification, unless otherwise stated, can be replaced by other alternative features that are equivalent or have similar purposes; all the disclosed features, or all the steps in the methods or processes, except for mutually exclusive features and / or steps, can be combined in any way.

Claims

1. A method for testing lost gas in shale gas content based on a depletion experimental device, characterized in that: The exhaustion experiment device comprises a core holder, a heating sleeve, a confining pressure boosting pump, an air inlet assembly, an air outlet assembly, a vacuum device, a gas reference container and an arc-shaped radial slot plate. A core sealing rubber sleeve is fixed in the core holder, the arc-shaped radial 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 fixing sleeve is arranged on the core holder, the confining pressure boosting pump is connected to the core holder to inject confining pressure, the air inlet assembly is connected to the air inlet end of the core holder to input gas into the core and measure the gas index, the air outlet assembly is connected between the core sealing rubber sleeve and the arc-shaped radial slot plate at the air outlet end of the core holder to discharge gas and measure the gas index; the vacuum device and the gas reference container are connected to the air inlet end of the core holder through the air inlet assembly; The method for testing the gas loss in shale gas content comprises the following steps: Step 1: Get the core, dry it to constant weight, open an axial hole on the core, cover the core with an arc-shaped radial slot plate, and then seal it in the core sealing rubber sleeve of the core holder, then connect the air inlet of the air inlet assembly to the air source, and extend the air outlet into the axial hole; Step 2: Use the air inlet assembly to inject helium and the confining pressure booster pump to test the sealing of the confining pressure test device. If the sealing is intact, open the air outlet assembly to discharge the helium in the core and reduce the confining pressure to zero. Then use the vacuum device to evacuate the core holder and the gas reference container to negative pressure. Step 3: Inject helium into the core holder using the gas inlet assembly and the gas reference container, and calculate the pore volume of the core according to the pressure of the helium; then open the gas outlet assembly to discharge the helium in the core holder and the gas reference container, and use the vacuum device to evacuate the core holder and the gas reference container to negative pressure; Step 4: The core is heated by the heating sleeve. When the core is heated to the formation temperature of the coring section, the confining pressure booster pump and the air intake assembly saturate the core by alternately injecting confining pressure and methane at each pressure point according to the set pressure gradient. At the same time, the methane intake volume entering the core at each pressure point is measured during the saturation process. When the sum of the methane intake volumes is greater than the pore volume of the core, the core is determined to be fully saturated. Step 5: The gas outlet assembly gradually discharges the methane in the core holder at each pressure drop point according to the set pressure drop gradient, and always keeps the core confining pressure greater than the methane pressure in the core holder. At the same time, the gas outlet assembly is used to reduce the methane pressure to the current atmospheric pressure, and the methane discharge amount, methane temperature and methane pressure at each pressure drop point are measured; Step 6: Calculate the gas loss of the core according to the methane emission, methane temperature and methane pressure at each pressure drop point.

2. A method for testing lost gas in shale gas content based on a depletion experimental device according to claim 1, characterized in that: In step 3, the pore volume of the core is calculated as follows: after the vacuum device is turned off, 10 MPa of helium is first pumped into the gas reference container through the gas inlet assembly 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; then the helium in the gas reference container is controlled to enter the core holder through the pipeline. When the pressure in the gas reference container and the core holder are equal and stable, the pressure in the core holder is measured, and then the pore volume of the core is calculated based on the pressure in the gas reference container and the pressure in the core holder, as follows: Where V cv is the pore volume of the core, in ml; P r 1 is the pressure in the gas reference container, in units of 10MPa; P c 1 is the pressure in the core holder, in units of 10Mpa; Z1 is P r 1 Methane deviation coefficient under the conditions; Z2 is P c 1 Methane deviation coefficient under the condition; V r V is the volume of the gas reference container in ml; e It 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.

3. The method for testing the lost gas in shale gas content based on the depletion experimental device according to claim 1, characterized in that: In step 4, the highest pressure of the core saturated with methane is the formation pressure of the core coring section, and the set pressure gradient includes at least 10 pressure points with gradually increasing pressures, and the pressure difference between any two pressure points shall not be higher than one tenth of the formation pressure of the shale coring section.

4. A method for testing lost gas in shale gas content based on a depletion experimental device according to claim 3, characterized in that: In step 4, the specific process of saturating the core is: S1: First operate the confining pressure booster pump to inject a confining pressure higher than the set pressure at the first pressure point into the core, then control the air intake assembly to inject methane into the core for saturation. When the methane pressure in the core holder is increased to the pressure at the first pressure point, measure the methane intake into the core at the first pressure point, and stop injecting methane at the same time to allow methane to saturate the core. When the measured methane pressure changes within 8 hours and is less than 3% of the pressure at the first pressure point, the core is determined to be completely saturated at the first pressure point. S2: operate the confining pressure boosting pump again to inject a confining pressure higher than the set pressure at 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 measure the methane intake into the core at the second pressure point; S3: repeat step S2 to complete the complete saturation of the core at the remaining pressure points in turn, and measure the methane intake into the core at the remaining pressure points at the same time; S4: Determine whether the core is saturated based on the amount of methane entering the core at each pressure point. If the sum of all methane intake volumes is greater than the pore volume of the core, the core is determined to be fully saturated; if the sum of all methane intake volumes is less than or equal to the pore volume of the core, the core is determined to be incompletely saturated, and then continue at the last pressure point until the core is fully saturated.

5. The method for testing lost gas in shale gas content based on a depletion experimental device according to claim 1, characterized in that: In step 5, the set pressure drop gradient is the formation pressure minus the current atmospheric pressure divided by the number of hours taken by the core to travel from the bottom of the well to the wellhead during the coring process. The pressure drop gradient includes multiple pressure drop points, and the number of pressure drop points is the number of hours taken by the core to travel from the bottom of the well to the wellhead during the coring process.

6. The method for testing the lost gas in shale gas content based on the depletion experimental device according to claim 1, characterized in that: In step 6, the calculation method of the gas loss of the core is: Where V s total is the gas loss volume of the measured shale core, in ml; V0 1 is the methane emission at the first pressure drop point, in ml; V0 n is the methane emission at the nth pressure drop point, in ml; P0 1 is the methane pressure at the first pressure drop point, in MPa; P0 n is the methane pressure at the nth pressure drop point, in MPa; T1 is the methane temperature at the first pressure drop point, in °C; T n is the methane temperature at the nth pressure drop point, in °C.

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

8. The method for testing lost gas in shale gas content based on a depletion experimental device according to claim 7, characterized in that: The surface of the portion of the air intake pipe extending into the axial hole is evenly provided with 4 rows of holes, and the angle between every two rows of holes is 90 degrees.

9. A method for testing lost gas in shale gas content based on a depletion test device according to any one of claims 1 to 6, characterized in that: The gas outlet assembly comprises a gas outlet pipeline, 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, the temperature index and the flow index of the output gas respectively. The gas outlet pipeline is connected between the core sealing rubber sleeve at the gas outlet end of the core clamp and the arc-shaped radial groove plate.

10. The method for testing lost gas in shale gas content based on a depletion experimental device according to claim 1, characterized in that: The number of the arc-shaped radial groove plates is 2-6, and the sum of the widths of all the arc-shaped radial groove plates is smaller than the chord length of the core sealing rubber sleeve.

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