Method for testing replacement of shale methane by carbon dioxide / nitrogen mixed gas adsorption

Abstract

This invention provides a method for the adsorption and replacement of methane in shale using a carbon dioxide / nitrogen mixed gas. The method includes: injecting a first preset amount of methane into a reference cell to obtain first pressure change data; injecting a second preset amount of carbon dioxide / nitrogen mixed gas into the reference cell in stages to obtain second pressure change data; and determining the adsorption amounts of carbon dioxide, nitrogen, and methane by the shale at each pressure point, as well as the desorption amounts of methane in the shale at each pressure point, based on the first and second pressure change data, the volume of the reference cell, and the residual volume of the sample cell. This method fully considers the potential impact of bidirectional gas diffusion on the competitive adsorption of the mixed gas during the adsorption and replacement of methane in shale, providing a solid foundation for improving the recovery efficiency of the adsorbed gas and thus effectively improving shale gas recovery.

Description

Technical Field

[0001] This invention relates to the field of shale gas recovery technology, and more specifically, to a test method for adsorbing and replacing methane in shale gas using a carbon dioxide / nitrogen mixed gas. Background Technology

[0002] Shale gas is an unconventional natural gas, mainly existing in adsorbed and free states on the surface of shale organic matter and clay minerals and in the pore-fracture structure. Adsorbed gas accounts for 20%-85% of the total gas content. Hydraulic fracturing is the main technical means for shale gas extraction, but the current shale gas recovery rate is generally low (<30%), and the low efficiency of adsorbed gas recovery is one of the main reasons for the low shale gas recovery rate.

[0003] To address this engineering challenge, scholars both domestically and internationally have proposed drawing upon shale oil reservoir development methods. This approach leverages the differences in shale's adsorption characteristics for various gases (CO2, N2, CH4). By injecting CO2, N2, and their mixtures into the later stages of shale gas reservoir development, the injected fluids are forced to undergo molecular exchange with residual adsorbed CH4 through competitive adsorption. This process transforms adsorbed CH4 into a free state, improving shale gas recovery. Simultaneously, the CO2 involved in the molecular exchange is captured by the shale reservoir, achieving CO2 geological sequestration.

[0004] After CO2 / N2 is injected into shale gas reservoirs, the molecular exchange process between CO2 / N2 and CH4 is a key factor affecting residual shale gas recovery and CO2 sequestration. However, in actual shale gas reservoirs, the process of CO2 / N2 adsorption and replacement of residual CH4 involves a two-way diffusion process of CO2 / N2 adsorption and CH4 desorption. Previous studies have largely focused on competitive adsorption experiments using CO2 / CH4, N2 / CH4, and CO2 / N2 / CH4 mixtures to reflect the molecular exchange process between CO2, N2, and their mixtures with CH4, lacking consideration of the potential impact of two-way diffusion on the competitive adsorption of mixed gases.

[0005] Therefore, a new technical solution is urgently needed to solve the above-mentioned technical problems. Summary of the Invention

[0006] The summary section introduces a series of simplified concepts, which will be further explained in detail in the detailed description section. The summary section of this invention is not intended to limit the key features and essential technical features of the claimed technical solution, nor is it intended to determine the scope of protection of the claimed technical solution.

[0007] In a first aspect, the present invention proposes a method for the adsorption and replacement of shale methane using a carbon dioxide / nitrogen mixed gas, comprising:

[0008] Place the sample into the sample cell of the test system;

[0009] The free space volume of the experimental system was calibrated using helium gas to determine the volume of the reference cell and the residual volume of the sample cell in the system.

[0010] A first preset amount of methane is injected into the reference cell to obtain the first pressure change data in the reference cell;

[0011] A second preset amount of carbon dioxide / nitrogen mixed gas is injected into the reference pool in stages to obtain second pressure change data in the reference pool.

[0012] Based on the first pressure change data, the second pressure change data, the volume of the reference cell, and the residual volume of the sample cell, the adsorption amounts of carbon dioxide, nitrogen, and methane by the shale at each pressure point, as well as the desorption amounts of methane in the shale at each pressure point, were determined.

[0013] Optionally, the method further includes the following steps before the experiment begins:

[0014] Turn on the vacuum pump to clean at least one of the mixing tank, booster pump, reference tank, sample tank, and pipeline.

[0015] Optionally, before calibrating the free-space volume of the experimental system using helium, the method further includes:

[0016] Heat the oil bath to the preset temperature;

[0017] Turn on the vacuum pump to perform vacuum treatment.

[0018] Optionally, the method further includes:

[0019] Open the gas sampling needle valve and use a gas chromatograph to calibrate the gas components in the reference cell;

[0020] Close the gas intake needle valve and record the first pressure of the reference pool;

[0021] Open the first valve to allow the carbon dioxide / nitrogen mixture in the reference cell to enter the sample cell;

[0022] During the adsorption test at the first pressure point, the first valve was closed when equilibrium was reached at the first pressure point, and the equilibrium pressure was recorded.

[0023] Open the gas sampling needle valve again and use a gas chromatograph to calibrate the gas components in the adsorption cell after adsorption equilibrium.

[0024] Close the gas intake needle valve and record the second pressure of the reference pool.

[0025] Optionally, the method further includes:

[0026] Repeat the gas component calibration and pressure recording operations until the injected carbon dioxide / nitrogen mixture reaches the target injection volume. During the injection process, the injection volumes of carbon dioxide, nitrogen, and methane must meet the preset ratio.

[0027] Optionally, a second preset amount of carbon dioxide / nitrogen mixture is injected into the reference pool in stages, including:

[0028] Using a booster pump, a carbon dioxide / nitrogen mixture is injected into the reference tank at a preset injection rate and a preset mixing ratio.

[0029] Optionally, based on the first pressure change data, the second pressure change data, the volume of the reference cell, and the residual volume of the sample cell, the adsorption amounts of carbon dioxide, nitrogen, and methane by the shale at each pressure point, and the desorption amounts of methane in the shale at each pressure point, are determined, including:

[0030] The amount of substance of a gas is calculated using the real gas law, where the amount of substance of the gas is used to represent the amount of adsorption and desorption.

[0031] Optionally, the amount of carbon dioxide, nitrogen, and methane adsorbed by the shale at each pressure point is equal to the amount of gas in the reference cell at the pressure point plus the amount of gas in the sample cell at the pressure point, minus the total amount of gas in the reference cell and sample cell after adsorption equilibrium.

[0032] Optionally, the method further includes:

[0033] Obtain the initial pressure and injection pressure before and after methane injection;

[0034] The initial adsorption capacity of methane by the shale was determined based on the initial pressure and injection pressure.

[0035] Optionally, the amount of methane desorbed in the shale at each pressure point is equal to the initial adsorption amount minus the amount of methane adsorbed by the shale after each pressurization by injecting a carbon dioxide / nitrogen mixture.

[0036] According to the above technical solution, by injecting a certain amount of methane and carbon dioxide / nitrogen mixed gas into the test system, the potential impact of bidirectional diffusion of gas on competitive adsorption of mixed gas during the adsorption and replacement of shale methane by carbon dioxide / nitrogen mixed gas is fully considered, providing a solid foundation for improving the efficiency of adsorbed gas recovery, thereby effectively improving the shale gas recovery rate.

[0037] The carbon dioxide / nitrogen mixed gas adsorption and displacement test method for shale methane of the present invention, other advantages, objectives and features of the present invention will be apparent in part from the following description, and in part will be understood by those skilled in the art through study and practice of the present invention. Attached Figure Description

[0038] Various other advantages and benefits will become apparent to those skilled in the art upon reading the following detailed description of preferred embodiments. The accompanying drawings are for illustrative purposes only and are not intended to limit this specification. Furthermore, the same reference numerals denote the same parts throughout the drawings. In the drawings:

[0039] Figure 1 A schematic flowchart of a method for the adsorption and replacement of shale methane using a carbon dioxide / nitrogen mixed gas according to an embodiment of the present invention is shown; and

[0040] Figure 2 A schematic diagram of a test system according to an embodiment of the present invention is shown. Detailed Implementation

[0041] The terms "first," "second," "third," "fourth," etc. (if present) in the specification, claims, and accompanying drawings of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments described herein can be implemented in a sequence other than that illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus. The technical solutions of the embodiments of this application will now be clearly and completely described in conjunction with the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them.

[0042] According to a first aspect of the present invention, the present invention provides a test method for adsorbing and displacing shale methane using a carbon dioxide / nitrogen mixed gas. Figure 1 A schematic flowchart of a carbon dioxide / nitrogen mixed gas adsorption and displacement test method 100 for shale methane is shown according to an embodiment of the present invention. Method 100 may include the following steps.

[0043] Step S110: Place the sample into the sample cell of the test system.

[0044] For example, the sample mass is first weighed, and then the weighed sample is placed into the sample cell of the test system. Figure 2 A schematic diagram of a test system according to an embodiment of the present invention is shown. The test system includes a gas supply system, a gas mixing system, a pressurization system, a temperature control system, an adsorption / desorption system, a gas component calibration system, and a computer control system. Figure 2As shown, the gas supply system includes high-pressure gas cylinders (1 is a CO2 cylinder, 2 is a CH4 cylinder, 3 is an N2 cylinder, and 4 is a He cylinder), a pressure reducing valve (5), and a high-pressure pipeline (6); the gas mixing system includes gas flow meters (7 is a CO2 flow meter, 8 is a CH4 flow meter, and 9 is an N2 flow meter) and a mixing tank (10). The flow control range of the gas flow meters is 0-200 mL / min, and the accuracy is 1% of the full scale. The volume of the mixing tank is 1 L; the pressurization system includes a booster pump and an air pump. The compressor (11) adopts the HGS-HTA60-HG / BK pressurization control system; the temperature control system adopts the oil bath constant temperature chamber (12), with a temperature control range of 0-200℃ and an accuracy of 0.1℃; the adsorption system includes a reference cell (13) and a sample cell (14), with a reference cell volume of 80mL and a sample tank volume of 160mL; the gas component calibration system includes a gas sampling needle valve (15) and a gas chromatograph (16); the computer control system uses a desktop computer (17). V1, V2, V3, V4, V5 and V6 are all high-pressure pipeline connection valves; P1 and P2 are both pressure sensors.

[0045] After the system is installed, check its airtightness to prevent gas leakage from reducing the accuracy of the test results.

[0046] Step S120: Use helium to calibrate the free space volume of the test system to determine the volume of the reference cell and the residual volume of the sample cell in the system.

[0047] Preferably, before step S120, the method may further include: heating the oil bath to a preset temperature; and turning on the vacuum pump to perform vacuum treatment.

[0048] For example, the oil bath can be heated to, say, 90°C, and then a vacuum pump can be turned on to perform a vacuuming process for 240 minutes. After the vacuuming is completed, the vacuum pump can be turned off, and then the temperature of the oil bath can be adjusted to the temperature to be tested. It is understood that the temperature to be tested can be set arbitrarily and reasonably according to actual needs or experience, and is not limited here.

[0049] After the oil bath temperature drops to the test temperature and stabilizes, He gas cylinder 4 can be opened for free space volume calibration. After calibration, the reference cell volume and the residual sample cell volume can be obtained. The specific details of free space volume calibration are not elaborated here; any existing or future automated space volume calibration method can be used to obtain the reference cell volume and the residual sample cell volume.

[0050] Step S130: Inject a first preset amount of methane into the reference cell to obtain the first pressure change data in the reference cell.

[0051] For example, open gas cylinder 2, adjust the pressure reducing valve, and then open valves V1, V2, and V3. According to the test plan, inject a first preset amount of methane gas into the reference cell using a booster pump. This first preset amount can be arbitrarily set according to the specific test plan. After the pressure reaches equilibrium, close gas cylinder 1, the pressure reducing valve, and valve V3, and record the pressure in the reference cell. P 0-1 Open valve V5, at which point the gas in the reference cell enters the sample cell. After adsorption equilibrium is reached, close valve V5 and record the pressure in the reference cell. P 0-2 .

[0052] Step S140: A second preset amount of carbon dioxide / nitrogen mixed gas is injected into the reference cell in stages to obtain second pressure change data in the reference cell.

[0053] Open gas cylinders 1 and 3, adjust the pressure reducing valve, and inject the second preset amount of carbon dioxide / nitrogen mixture into the reference cell in stages according to the test plan. The second preset amount can be arbitrarily set according to the specific test plan.

[0054] Preferably, a booster pump can be used to inject a carbon dioxide / nitrogen mixture into the reference tank at a preset injection rate and a preset mixing ratio. The preset mixing ratio can be 0.25:0.75, 0.5:0.5, 0.75:0.25, etc.

[0055] Step S150: Based on the first pressure change data, the second pressure change data, the volume of the reference cell, and the residual volume of the sample cell, determine the amount of carbon dioxide, nitrogen, and methane adsorbed by the shale at each pressure point, and the amount of methane desorbed in the shale at each pressure point.

[0056] For example, the amount of substance of a gas can be calculated using the real gas law, where the amount of substance of the gas is used to represent the amount of adsorption and desorption.

[0057] Specifically, the equation of state for a real gas is: ,in, This indicates the pressure at the current pressure point. Indicates gas volume (ml), The amount of substance (mol) of a gas. This represents the gas compressibility factor (dimensionless). Indicates the gas temperature (K).

[0058] Optionally, the amount of carbon dioxide, nitrogen, and methane adsorbed by the shale at each pressure point is equal to the amount of gas in the reference cell at the pressure point plus the amount of gas in the sample cell at the pressure point, minus the total amount of gas in the reference cell and sample cell after adsorption equilibrium.

[0059] During the adsorption experiment, the amount of adsorption at a single pressure point can be calculated using the following equation: ,in, Indicates the first The amount of sample adsorption after the first injection of gas pressurization. Indicates the first The amount of gas in the reference cell after the second pressurization and before adsorption. Indicates the first The amount of gas in the sample cell after the first pressurization and before adsorption. Indicates the first The total amount of gas in the reference cell and sample cell after pressurization and adsorption equilibrium.

[0060] According to the above technical solution, by injecting a certain amount of methane and carbon dioxide / nitrogen mixed gas into the test system, the potential impact of bidirectional diffusion of gas on competitive adsorption of mixed gas during the adsorption and replacement of shale methane by carbon dioxide / nitrogen mixed gas is fully considered, providing a solid foundation for improving the efficiency of adsorbed gas recovery, thereby effectively improving the shale gas recovery rate.

[0061] Optionally, before the experiment begins, the method may also include:

[0062] Turn on the vacuum pump to clean at least one of the mixing tank, booster pump, reference tank, sample tank, and pipeline.

[0063] For example, valve V4 can be opened to start vacuum pump VP1 to clean the mixing tank, booster pump, reference tank, sample tank, and residual methane gas in the pipeline.

[0064] This effectively reduces the possibility of interfering data in subsequent experiments, ensuring the accuracy of the adsorption and desorption amounts obtained in subsequent calculations to the greatest extent possible.

[0065] Optionally, the method may also include:

[0066] Obtain the initial pressure and injection pressure before and after methane injection.

[0067] It is understandable that the initial pressure before and after methane injection can be obtained. P 0-1 and injection pressure P 0-2 .

[0068] The initial adsorption capacity of methane by the shale was determined based on the initial pressure and injection pressure.

[0069] Specifically, the initial adsorption amount of methane can be calculated using the following formula: .

[0070] Optionally, the amount of methane desorbed in the shale at each pressure point is equal to the initial adsorption amount minus the amount of methane adsorbed by the shale after each pressurization by injecting a carbon dioxide / nitrogen mixture.

[0071] Specifically, the amount of methane desorbed in shale at each pressure point can be calculated using the following formula: .in, Indicates the first The amount of methane adsorbed by shale after being pressurized by injecting a mixture of carbon dioxide and nitrogen.

[0072] Optionally, step S140, which involves injecting a second preset amount of carbon dioxide / nitrogen mixture into the reference pool in stages, may include the following steps.

[0073] Step S141: Open the gas sampling needle valve and use a gas chromatograph to calibrate the gas components in the reference cell.

[0074] For example, the reference cell can be pressurized according to the predetermined pressurization pressure for the test. After the pressure stabilizes, valve V3 is closed, the gas sampling needle valve is opened, and the gas components of the reference cell are calibrated using a gas chromatograph.

[0075] Step S142: Close the gas needle valve and record the first pressure of the reference pool.

[0076] After the gas calibration is completed, close the gas sampling needle valve 15 and record the first pressure of the reference cell. P 1-1 .

[0077] Step S143: Open the first valve to allow the carbon dioxide / nitrogen mixture in the reference cell to enter the sample cell.

[0078] Open the first valve, namely valve V5, to allow the CO2 / N2 mixed gas in the reference cell to enter the sample cell, and start the first pressure point adsorption test.

[0079] In step S144, during the adsorption test at the first pressure point, the first valve is closed when the first pressure point reaches equilibrium, and the equilibrium pressure is recorded.

[0080] After the first pressure point is reached, close valve V5 and record the equilibrium pressure. P 1-2 .

[0081] Step S145: Open the gas sampling needle valve again and use a gas chromatograph to calibrate the gas components in the adsorption cell after adsorption equilibrium.

[0082] Reopen the gas sampling needle valve and use a gas chromatograph to calibrate the gas components in the adsorption cell after adsorption equilibrium. Specific operating procedures will not be detailed here.

[0083] Step S146: Close the gas needle valve and record the second pressure of the reference pool.

[0084] After the gas calibration is completed, close the gas sampling needle valve and record the second pressure of the reference cell. P 1-3 .

[0085] Optionally, the method may further include: repeatedly performing gas component calibration and pressure recording operations until the injected carbon dioxide / nitrogen mixture reaches the target injection amount, wherein the injection amounts of carbon dioxide, nitrogen and methane are maintained at a preset ratio during the injection process.

[0086] It is understood that the specific implementation method of this step is similar to steps S141-S146 above, and will not be repeated here. The test is stopped when the injected carbon dioxide / nitrogen mixture reaches the target injection volume, and the injection ratio of carbon dioxide, nitrogen, and methane meets a preset ratio, such as 1:3:1, 1:3:2, 1:1:1, 3:1:1, or 3:1:2. It is understood that the pressure can be recorded after the first calibration during this process. P n-1 Record the pressure after balancing. P n-2 Record the pressure after the second calibration. P n-3 .

[0087] Specifically, after the first CO2 / N2 injection, based on the system's adsorption equilibrium pressure... P 1-1 , P 1-2 , P 1-3 The adsorption capacity of shale for CO2, N2, and CH4 can be obtained:

[0088] ,

[0089] ,

[0090] .

[0091] Therefore, the adsorption capacity of the shale sample after the first injection of the carbon dioxide / nitrogen mixed gas can be calculated: Therefore, it can be concluded that in the first... After the injection, the adsorption capacity of shale for carbon dioxide, nitrogen, and methane can be obtained:

[0092] ,

[0093] ,

[0094] .

[0095] Therefore, we can obtain the... Adsorption capacity of shale sample after secondary injection:

[0096] .

[0097] Furthermore, based on the above formula, the adsorption capacities of carbon dioxide, nitrogen, and methane by the shale at each pressure point after carbon dioxide / nitrogen injection are as follows: Optionally, of all the above parameters, V R , V S , P n-1 , P n-2 and P n-3 All of these can be obtained through testing. Z n-1 and Z n-2 It can be calculated using REFPROP software.

[0098] Finally, after the test, the gas pressure in the sample cell, booster pump, and mixing tank is released. Once the pressure in the sample cell has dropped below 1 atm, the sample cell is opened and the test sample is removed.

[0099] The above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit it. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application.

Claims

1. A method for adsorbing and displacing shale methane using a carbon dioxide / nitrogen mixed gas, characterized in that, include: Place the sample into the sample cell of the test system; The experimental system was calibrated in free space using helium gas to determine the volume of the reference cell and the residual volume of the sample cell in the system. A first preset amount of methane is injected into the reference cell to obtain first pressure change data in the reference cell; A second preset amount of carbon dioxide / nitrogen mixed gas is injected into the reference cell in stages to obtain second pressure change data in the reference cell; Based on the first pressure change data, the second pressure change data, the volume of the reference cell, and the residual volume of the sample cell, the adsorption amounts of carbon dioxide, nitrogen, and methane by the shale at each pressure point, and the desorption amounts of methane in the shale at each pressure point, are determined. This determination, based on the first pressure change data, the second pressure change data, the volume of the reference cell, and the residual volume of the sample cell, includes: The amount of substance of a gas is calculated using the real gas law, wherein the amount of substance of the gas is used to represent the amount of adsorption and the amount of desorption. Obtain the initial pressure and injection pressure before and after the methane injection; Based on the initial pressure and the injection pressure, the initial adsorption amount of methane by the shale is determined. The amount of methane desorbed in the shale at each pressure point is equal to the initial adsorption amount minus the amount of methane adsorbed by the shale after each pressurization by injecting the carbon dioxide / nitrogen mixed gas. The amount of carbon dioxide, nitrogen, and methane adsorbed by the shale at each pressure point is equal to the amount of gas in the reference cell at that pressure point plus the amount of gas in the sample cell at that pressure point, minus the total amount of gas in the reference cell and sample cell after adsorption equilibrium.

2. The experimental method for adsorbing and replacing shale methane with a carbon dioxide / nitrogen mixed gas as described in claim 1, characterized in that, Before the experiment begins, the method also includes: Turn on the vacuum pump to clean at least one of the mixing tank, booster pump, reference tank, sample tank, and pipeline.

3. The experimental method for adsorbing and replacing shale methane with a carbon dioxide / nitrogen mixed gas as described in claim 1, characterized in that, Before calibrating the free-space volume of the experimental system using helium, the method further includes: Heat the oil bath to the preset temperature; Turn on the vacuum pump to perform vacuum treatment.

4. The experimental method for adsorbing and replacing shale methane with a carbon dioxide / nitrogen mixed gas as described in claim 1, characterized in that, The step of injecting a second preset amount of carbon dioxide / nitrogen mixed gas into the reference cell in stages includes: Open the gas sampling needle valve and use a gas chromatograph to calibrate the gas components in the reference cell; Close the gas sampling needle valve and record the first pressure of the reference pool; Open the first valve to allow the carbon dioxide / nitrogen mixture in the reference cell to enter the sample cell; During the adsorption test at the first pressure point, the first valve is closed when equilibrium is reached at the first pressure point, and the equilibrium pressure is recorded. Open the gas sampling needle valve again and use the gas chromatograph to calibrate the gas components in the adsorption cell after adsorption equilibrium. Close the gas sampling needle valve and record the second pressure of the reference pool.

5. The experimental method for adsorbing and replacing shale methane with a carbon dioxide / nitrogen mixed gas as described in claim 4, characterized in that, The method further includes: Repeat the gas component calibration and pressure recording operations until the injected carbon dioxide / nitrogen mixture reaches the target injection amount, wherein the injection amounts of carbon dioxide, nitrogen and methane are maintained at a preset ratio during the injection process.

6. The method for adsorbing and replacing shale methane with a carbon dioxide / nitrogen mixed gas as described in any one of claims 1 to 5, characterized in that, The step of injecting a second preset amount of carbon dioxide / nitrogen mixed gas into the reference cell in stages includes: The carbon dioxide / nitrogen mixture is injected into the reference pool using a booster pump at a preset injection rate and a preset mixing ratio.

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