A device and method for simulating changes in gas composition in different reservoirs during coal measure gas exchange processes

Abstract

The application discloses a device and method for simulating gas component change in different reservoirs in coal measure gas exchange process, and relates to the technical field of coal measure gas component dynamic change simulation. The device comprises a gas preparation system, a gas exchange simulation system and a gas detection system. The gas preparation system is used for generating coal measure gas. The gas exchange simulation system is used for simulating coal measure gas exchange, and comprises a plurality of parallelly arranged core holders. The core holders are communicated with gas storage tanks. Adjacent core holders are communicated with each other through a gas exchange pipeline for gas exchange between different reservoirs. The core holders are connected with the gas detection system for detecting gas components. Based on the device, the characteristics of different gas enrichment in different reservoirs can be intuitively restored, and the mechanism of the current adjacent reservoirs with large coal measure gas component difference is revealed. A method of gas component test-temperature change-pressure-gas balance-gas component test-gas exchange balance is proposed, which can effectively monitor the change of gas component after coal measure gas exchange. Based on the gas component test results in the core holder, the difference between the adsorption / desorption gas and the gas component concentration in the reservoir in different balance processes can be compared, and the dynamic evolution law of the competitive adsorption dominated coal measure gas component concentration can be quantitatively characterized.

Description

Technical Field

[0001] This invention relates to the field of simulation technology for dynamic changes in coalbed methane composition, specifically to a method and apparatus for simulating changes in gas composition caused by coalbed methane exchange between different reservoirs under in-situ conditions. Background Technology

[0002] Coal-bearing strata refer to a sedimentary system with a genetically related relationship containing coal seams or coal veins. They typically contain shale, sandstone, and limestone. Natural gas found in different reservoirs is called coalbed methane, which can be classified based on reservoir type as coalbed methane, coalbed shale gas, tight sandstone gas, and coalbed limestone gas. Coalbed methane is a high-quality clean energy source; pre-extraction and utilization of coalbed methane can minimize carbon emissions from coal mining activities. Large-scale development and utilization of coalbed methane can reduce the use of high-carbon energy sources and achieve substantial, low-cost CO2 emission reduction. Currently, coalbed methane evaluation focuses primarily on gas content, with less attention paid to gas composition. Coalbed methane is typically dominated by CH4, containing minor components such as CO2 and N2. High concentrations of these minor components directly reduce the quality of the coalbed methane. Furthermore, significant differences in coalbed methane composition have emerged between adjacent reservoirs. For instance, in Turkey's Zonguldak Basin, coalbed methane has an N2 concentration of less than 5%, while adjacent siltstone gas has an N2 concentration greater than 30%. In China's Huoxi Coalfield, coalbed methane has a CH4 concentration of less than 20%, a CO2 concentration of greater than 20%, and an N2 concentration of greater than 60% (no development value), while adjacent limestone gas has a CH4 concentration of greater than 90% (high development value). These phenomena indicate that understanding the variation patterns of gas composition differences between different reservoirs is crucial for the efficient development of coalbed methane.

[0003] In coal-bearing strata, source rocks (generating coal-bearing gas) are mostly coal seams and organic-rich mudstones and shale. Low organic matter content in limestone and siltstone leads to poor hydrocarbon generation capacity. Therefore, coal-bearing gas preserved in strata with poor hydrocarbon generation capacity mostly originates from the influx of gases generated from source rocks. Furthermore, due to differences in pore structure, coal-bearing gas occurrence state, and different gas adsorption orders (CO2>CH4>N2) among different reservoirs, and because formation uplift and deposition cause changes in reservoir temperature and pressure, coal-bearing gas in different reservoirs has been in a dynamic exchange state over a long geological history. This is likely the fundamental reason for the significant differences in coal-bearing gas composition among adjacent reservoirs. Therefore, conducting coal-bearing gas exchange simulations is of great significance for deeply revealing the coal-bearing gas enrichment mechanism and also has practical significance for determining whether strata with abnormally high concentrations of minor components can be used as indicators of coal-bearing gas enrichment in adjacent reservoirs.

[0004] Currently, some technicians are attempting to study changes in coalbed methane composition by simulating external gas injection. For example, Chinese patent CN201420777316.7 discloses a nitrogen injection displacement coalbed methane simulation experimental device, which simulates the changes in coalbed methane composition and flow rate after nitrogen injection, laying the foundation for scientifically analyzing the intrinsic quantitative relationship between nitrogen injection volume and coalbed methane output. However, this simulation experiment differs significantly from the coalbed methane exchange process in different reservoirs: 1) Simple external gas injection fails to effectively simulate the complex process of coalbed methane generation-adsorption-desorption-diffusion in actual formations. For example, the amount of nitrogen injected cannot represent the nitrogen source in actual formations and the amount of nitrogen change under different temperature and pressure conditions; 2) After external gas injection, only the changes in gas composition of the injected layer are considered. The coalbed methane exchange process is a process of differential equilibrium of different gas components in different reservoirs, and the changes in gas composition of adjacent reservoirs also need to be studied; 3) It is difficult for actual formations to have coalbed methane with a single component. Summary of the Invention

[0005] The purpose of this invention is to solve the problems existing in the prior art and to provide an apparatus and method for simulating the changes in different reservoir gas components during the coalbed methane exchange process.

[0006] To achieve the above-mentioned technical objectives, the present invention is implemented through the following technical solution: an apparatus for simulating the changes in gas composition of different reservoirs during coalbed methane exchange, comprising a gas preparation system, a gas exchange simulation system, and a gas detection system; the gas preparation system, used to generate coalbed methane, includes a gas storage tank and a high-temperature and high-pressure reactor; the gas outlet pipeline of the high-temperature and high-pressure reactor is connected to the gas storage tank for transporting the coalbed methane generated by the reaction to the gas storage tank; one gas outlet path of the gas storage tank is connected to the gas detection system for detecting the initial gas composition; the other gas outlet path is connected to the gas exchange simulation system; the gas exchange simulation system, used to simulate the coalbed methane exchange process, includes several core holders arranged in parallel, the core holders are connected to the gas storage tank through gas inlet branch pipes, adjacent core holders exchange gas between different reservoirs through gas exchange pipelines, and the gas outlet of the core holders is connected to the gas detection system through a gas outlet pipe for detecting gas composition.

[0007] Furthermore, there are no fewer than two core holders. Solenoid valves are installed on both the air inlet branch pipe and the gas exchange pipe. Temperature and pressure sensors are connected to the core holders to monitor changes in temperature and pressure inside the device. The core holders are equipped with a confining pressure pump, a booster pump, and a constant temperature electric heating belt. The confining pressure pump uses a pressure sensor to monitor the pressure status. A solenoid valve is installed on the air outlet pipe of the core holder at one end near the core holder and at the other end near the gas detection system. A high-precision flow meter is installed on the air outlet pipe between the two solenoid valves.

[0008] Furthermore, a dryer and a high-precision flow meter are sequentially installed on the gas outlet pipe of the high-temperature and high-pressure reactor. A solenoid valve is installed on the connection between the high-temperature and high-pressure reactor and the dryer, as well as on the connection between the high-precision flow meter and the gas storage tank. A booster pump and a constant-temperature electric heating belt are installed on the high-temperature and high-pressure reactor to regulate the temperature and pressure conditions inside the reactor. A pressure sensor and a temperature sensor are also installed to monitor the pressure and temperature inside the reactor.

[0009] Furthermore, a booster pump and a constant-temperature electric heating belt are connected to the gas storage tank to regulate the temperature and pressure conditions inside the tank. Pressure sensors and temperature sensors are also connected to the gas storage tank to monitor the pressure and temperature inside the tank.

[0010] Furthermore, the gas preparation system also includes a coal-based gas configuration module, which is equivalent to a mixed gas composed of CH4+CO2+N2+C2H6, excluding components such as argon and helium, which have extremely low concentrations (less than 0.01%) and poor adsorption. This module includes four parallel gas supply cylinders and a mixed gas configuration instrument. The four gas supply cylinders are CH4, CO2, N2, and C2H6 cylinders, respectively. The four gas supply cylinders are connected to the mixed gas configuration instrument via gas supply branch pipes. The mixed gas configuration instrument is connected to a gas storage tank. Each gas supply branch pipe is equipped with a pressure reducing valve and a solenoid valve, and the inlet and outlet gas pipelines of the mixed gas configuration instrument are each equipped with a solenoid valve.

[0011] Furthermore, a vent valve is installed on both the gas storage tank and the core holder for depressurization.

[0012] Furthermore, the gas detection system includes a gas chromatograph and a carrier gas cylinder.

[0013] The method of simulating the compositional changes of coalbed methane during exchange in different reservoirs using the above-mentioned device needs to be carried out in two cases:

[0014] 1. For situations where there is strong gas connectivity within the rock strata and a small pressure differential for coalbed methane exchange, the simulation method is as follows:

[0015] 1) Pretreatment: Connect the instrument and place the reservoir sample in different core holders. Apply confining pressure using a confining pressure pump based on the actual formation conditions. If an in-situ hydrocarbon generation simulation sample is used, place the source rock sample in a high-temperature and high-pressure reactor and close all solenoid valves.

[0016] 2) Initial gas component determination: The in-situ hydrocarbon generation temperature and pressure conditions are set using the booster pump and constant temperature electric heating belt on the high temperature and high pressure reactor. The generated mixed product is dried and then introduced into the gas storage tank. The flow rate change is recorded. After a large amount of gas enters the gas storage tank, the connection between the reactor and the gas storage tank is blocked. The corresponding solenoid valve is opened to allow the gas in the gas storage tank to enter the gas phase detection system. The flow rate change is recorded. When the flow rate required for chromatographic detection is reached, the solenoid valve on the connection between the gas storage tank and the gas detection system is closed. The characteristics of the initial gas components in the gas storage tank are then determined.

[0017] 3) Simulation of coalbed methane exchange: Based on reservoir temperature and pressure conditions, adjust the temperature and pressure of the gas storage tank, and open the corresponding solenoid valves to allow the mixed gas to enter the corresponding core holders. Open the gas exchange pipelines between the corresponding core holders to allow continuous gas exchange between adjacent core holders until equilibrium is reached. Close the solenoid valves on the gas exchange pipelines, and open the solenoid valve on the outlet pipeline of one core holder to allow the gas in the core holder to enter the gas chromatograph. Record the flow rate change. When the flow rate reaches the chromatograph detection requirements, close the solenoid valve on the outlet pipeline and determine the gas components. Perform the same operation to determine the gas components in other core holders.

[0018] 4) Adjust the pressure and temperature of different core holders based on formation conditions, open the solenoid valve on the gas exchange pipeline between adjacent core holders, so that the reservoir in the adjacent core holders reaches a new equilibrium under the new temperature and pressure conditions, repeat the process of testing the gas components in different core holders, and repeat the aforementioned steps of gas balance, gas component testing, and temperature and pressure adjustment.

[0019] 5) Gas component calculation: This equipment tests the concentration of free gas components in the core holder. When the reservoir adsorption capacity is low, coal-bearing gas is mostly present in a free state, and the component concentration in the core holder can be approximated as similar to that in the reservoir. However, for reservoirs with a large adsorption capacity, due to differences in the gas adsorption order, the gas component concentration in the coal seam should be significantly different from that in the free gas in the core holder. Therefore, it is necessary to back-calculate the gas component concentration in the reservoir based on the tested gas component characteristics.

[0020] 2. For situations where the gas connection between rock formations is weak and the pressure difference for the start-up of coalbed methane exchange is large, the simulation method is as follows:

[0021] 1) Preprocessing: Same as case 1;

[0022] 2) Initial gas component determination: Same as case 1;

[0023] 3) Adsorption equilibrium: Based on the temperature and pressure conditions of the reservoir, the temperature and pressure of the gas storage tank are controlled, and the mixed gas is introduced into the core holder to bring different reservoir samples to adsorption equilibrium.

[0024] 4) Simulation of coalbed methane exchange: Open the solenoid valve on the gas outlet pipe of the core holder to allow gas from the core holder to enter the gas chromatograph. Record the flow rate change. When the flow rate reaches the chromatograph's detection requirements, close the solenoid valve on the gas outlet pipe and determine the gas components. Perform the same operation to determine the gas components in other core holders. Adjust the pressure and temperature of different core holders based on formation conditions so that the reservoirs in different core holders reach a new equilibrium under the new temperature and pressure conditions. Repeat the gas component test in different core holders. Open the gas exchange pipeline between the corresponding core holders to allow gas exchange between adjacent core holders. When the preset pressure difference is reached, close the solenoid valve on the gas exchange pipeline so that the reservoirs in the core holders reach equilibrium again after gas exchange. Repeat the aforementioned gas component test, temperature and pressure adjustment, and gas equilibrium steps.

[0025] 5) Gas component calculation: Same as case 1.

[0026] Furthermore, in step 2) of the simulation under two different conditions, if a source rock sample cannot be collected, a mixed gas of arbitrary composition can be prepared using the device: set different component concentrations or volumes using a mixed gas preparer, open the solenoid valve on the gas supply branch pipe and the four gas supply cylinders to allow the prepared mixed gas to enter the gas storage tank, close the solenoid valve on the gas supply branch pipe and the four gas supply cylinders, open the corresponding solenoid valve to allow the gas in the gas storage tank to enter the gas phase detection system, record the flow rate change, and close the solenoid valve on the connection between the gas storage tank and the gas detection system when the required flow rate for chromatographic detection is reached, and determine the initial gas component characteristics in the gas storage tank.

[0027] The beneficial effects of this invention are as follows:

[0028] 1. The device disclosed in this application for simulating the changes in gas components in different reservoirs during the exchange of coal-bearing gas can simulate the differential changes in different gas components in coal-bearing reservoirs driven by temperature and pressure changes, intuitively restore the characteristics of differential enrichment of different gases in different reservoirs, and reveal the mechanism of the current large differences in coal-bearing gas components in adjacent reservoirs.

[0029] 2. The simulation device disclosed in this application is equipped with a hydrocarbon generation simulation module, which can better reflect the in-situ coalbed methane generation-transport-enrichment-preservation process;

[0030] 3. This application is equipped with a mixed gas preparation device consisting of CH4, CO2, N2 and C2H6 gas cylinders (most of which account for more than 99% of the coal-based gas), which can realize the preparation of coal-based gas in any proportion;

[0031] 4. The simulation device disclosed in this application is equipped with an independent pressure control system for different core holders, which effectively simulates the in-situ fluid pressure difference of different reservoirs;

[0032] 5. This application proposes a test method of gas component testing - temperature and pressure variation - gas balance - gas component testing - gas exchange balance, which effectively monitors the changes in gas components after coal-based gas exchange and realizes continuous detection of changes in coal-based gas components;

[0033] 6. Based on differences in formation conditions and research content, this application proposes two simulation schemes for coalbed methane exchange, which simulate coalbed methane exchange at different start-up intensities by controlling the opening state of the solenoid valve;

[0034] 7. Based on the gas composition test results in the core holder, the differences in the concentration of adsorbed / desorbed gas and gas components in the reservoir during different equilibrium processes are compared to quantitatively characterize the dynamic evolution law of coal-associated gas component concentration dominated by competitive adsorption.

[0035] 8. The gas in the simulation process can be directly input into the chromatograph for gas component testing, realizing continuous gas component testing and avoiding external gas pollution, thus improving the accuracy of test results;

[0036] 9. The number of simulated stratigraphic layers can be adjusted by increasing or decreasing the number of core holders based on actual geological conditions;

[0037] 10. The simulation device can also be directly used for simulations of gas displacement and coalbed methane adsorption-desorption, which are frequently used in current coalbed methane research. Attached Figure Description

[0038] Figure 1 This is a structural diagram of a device for simulating the changes in gas composition in different reservoirs during coalbed methane exchange.

[0039] Among them, 1-CH4 gas cylinder, 2-CO2 gas cylinder, 3-N2 gas cylinder, 4-C2H6 gas cylinder, 5 / 6 / 7 / 8-pressure reducing valve, 9 / 10 / 11 / 12 / 18 / 19 / 23 / 24 / 31 / 33 / 34 / 37 / 39 / 43 / 47 / 50 / 52 / 56 / 60 / 63 / 65-solenoid valve, 13 / 29 / 35 / 41 / 48 / 54 / 61 / 67-pressure sensor, 14 / 30 / 36 / 49 / 62-temperature sensor Sensor, 15-High temperature and high pressure reactor, 16 / 26 / 44 / 57 / 69-Booster pump, 17 / 27 / 45 / 58 / 70-Constant temperature electric heating belt, 20-Dryer, 21-Mixed gas preparation instrument, 22 / 32 / 38 / 51 / 64-High precision flow meter, 25-Gas storage tank, 28 / 46 / 59 / 71-Vent valve, 40 / 53 / 66-Core holder, 42 / 55 / 68-Containing pressure pump, 72-Gas chromatograph, 73-Carrier gas cylinder. Detailed Implementation

[0040] The following embodiments further illustrate the content of the present invention, but should not be construed as limiting the present invention. Any modifications and substitutions made to the methods, steps, or conditions of the present invention without departing from the essence of the invention are within the scope of the present invention.

[0041] Example 1

[0042] This embodiment discloses a device for simulating the changes in gas composition of different reservoirs during coalbed methane exchange, which mainly includes three parts: a gas preparation system, a gas exchange simulation system, and a gas detection system.

[0043] The gas preparation system includes four gas cylinders, a gas storage tank 25, and a high-temperature and high-pressure reactor 15.

[0044] The four gas cylinders are CH4 cylinder 1, CO2 cylinder 2, N2 cylinder 3 and C2H6 cylinder 4, which are connected in parallel. Each of the four gas cylinders is connected to the inlet pipe of the gas mixing instrument 21 (reference model: Beijing Maize Company MIH1000-T3 gas mixing and proportioning instrument) through a gas supply branch pipe. Pressure reducing valves 5 / 6 / 7 / 8 and solenoid valves 9 / 10 / 11 / 12 are installed in sequence along the gas outflow direction on the gas supply branch pipe. The outlet pipe of the gas mixing instrument 21 is connected to the first inlet on the gas storage tank 25. Solenoid valves 19 / 23 are installed on the inlet and outlet pipes of the gas mixing instrument 21, respectively.

[0045] A booster pump 16 and a constant temperature electric heating belt 17 are connected to the high temperature and high pressure reactor 15. The pressure and temperature inside the reactor are monitored by a pressure sensor 13 and a temperature sensor 14. The outlet of the high temperature and high pressure reactor 15 is connected to the second inlet of the gas storage tank 25. A dryer 20 and a high-precision flow meter 22 are installed in sequence on the connection path between the two. A solenoid valve 18 / 24 is installed on the connection path between the high temperature and high pressure reactor 15 and the dryer 20, and on the connection path between the high-precision flow meter 22 and the gas storage tank 25, respectively.

[0046] A booster pump 26 and a constant temperature electric heating belt 27 are connected to the gas storage tank 25. A pressure sensor 29 and a temperature sensor 30 are connected to the gas storage tank 25 to monitor the pressure and temperature inside the tank. The gas storage tank 25 has three gas outlet passages. The first gas outlet passage is used to connect to the gas detection system, and a solenoid valve 31, a high-precision flow meter 32 and a solenoid valve 33 are arranged in sequence along the gas flow direction in the first gas outlet passage. The second gas outlet passage is used to connect to the gas exchange simulation system. The third gas outlet passage is equipped with a vent valve 28.

[0047] The gas exchange simulation system includes three core holders 40 / 53 / 66 arranged in parallel. Each core holder is connected to the second gas outlet passage of the gas storage tank 25 via an air inlet pipe. Each of the three air inlet pipes is equipped with a solenoid valve 34 / 47 / 60. Adjacent core holders are connected via gas exchange pipelines, each equipped with a solenoid valve 43 / 56. Each core holder is connected to a temperature sensor 36 / 49 / 62 and a pressure sensor 35 / 48 / 61 to monitor temperature and pressure changes within the respective core holder. Each core... The clamp is also connected to a confining pressure pump 42 / 55 / 68, a booster pump 44 / 57 / 69, and a constant temperature electric heating belt 45 / 58 / 70. Pressure sensors 41 / 54 / 67 are connected to the three confining pressure pumps respectively. The air outlet of each core clamp is connected to the gas detection system through an air outlet pipe. Each air outlet pipe is equipped with a solenoid valve 37 / 50 / 63, a high-precision flow meter 38 / 51 / 64, and a solenoid valve 39 / 52 / 65 in sequence along the gas outflow direction. A vent valve 46 / 59 / 71 is provided on the exhaust passage of each of the three core clamps.

[0048] The gas detection system includes a gas chromatograph 72 and a carrier gas cylinder 73. The gas chromatograph 72 is connected to the gas storage tank 25 and the core holder 40 / 53 / 66 for gas detection.

[0049] The method for simulating the compositional changes of coalbed methane during exchange in different reservoirs using the above-mentioned device includes connecting the instrument, setting the initial gas composition, adsorption equilibrium, simulating coalbed methane exchange, and ending the simulation. Although the device description disclosed that it includes three core holders arranged in parallel, which can form three simulated layers, for ease of explanation, the following will describe the specific operating steps using the coalbed methane exchange simulation process between two layers.

[0050] Furthermore, since a very small pressure difference can drive gas exchange when the solenoid valve between the two core holders of the simulation device opens, this does not conform to all actual formation conditions. For example, the presence of tight interlayers between the two rock layers will significantly weaken coal-bearing gas exchange, requiring a larger starting pressure difference. Therefore, to address this issue, this application proposes two types of coal-bearing gas exchange simulation schemes:

[0051] Simulation Scheme 1: Suitable for situations where there is strong gas exchange between rock formations and a small pressure differential for coalbed methane exchange.

[0052] 1. Connect the instruments and place the reservoir samples in the core holders 40 and 53 respectively. Apply confining pressure using confining pressure pumps 42 and 55 based on the actual formation conditions. If using in-situ hydrocarbon generation simulation samples, place the source rock samples in the high-temperature and high-pressure reactor 15 and close all solenoid valves.

[0053] II. Initial Gas Component Determination: Adjust the booster pump 16 and the constant temperature electric heating belt 17 to set the in-situ hydrocarbon generation temperature and pressure conditions. Open the solenoid valves 18 and 24 so that the generated mixed product enters the gas storage tank 25 after passing through the dryer 20. Use the high-precision flow meter 22 to record the flow rate change. After a large amount of gas enters the gas storage tank 25, close the solenoid valves 18 and 24 and reduce the temperature and pressure of the high-temperature and high-pressure reactor 15 to stop hydrocarbon generation.

[0054] If source rock samples cannot be collected, a mixed gas of arbitrary composition can be prepared using the device: Using the mixed gas preparer 21, different component concentrations (volumes) are set. Solenoid valves 9, 10, 11, 12, 19, and 23, as well as CH4 cylinder 1, CO2 cylinder 2, N2 cylinder 3, and C2H6 cylinder 4, are opened, allowing the prepared mixed gas to enter the storage tank 25. Solenoid valves 9, 10, 11, 12, 19, and 23, and the four gas supply cylinders, are then closed. Solenoid valves 31 and 33 are opened, allowing the gas in the storage tank 25 to enter the gas chromatograph 72. The flow rate change is recorded using a high-precision flow meter 32. When the required flow rate for chromatographic detection is reached, solenoid valves 31 and 33 are closed, and the initial gas component characteristics in the storage tank 25 are determined.

[0055] III. Simulation of Coal-bearing Gas Exchange: Based on reservoir temperature and pressure conditions, the temperature and pressure of the gas storage tank 25 are adjusted using a booster pump 26 and a constant-temperature electric heating belt 27. Solenoid valves 34 and 47 are opened sequentially to allow the mixed gas to enter the core holders 40 and 53. Solenoid valve 43 is opened to allow continuous gas exchange between the two core holders 40 and 53 until equilibrium is reached (until the pressure change is less than 0.01 MPa within 24 hours; the subsequent equilibrium determination conditions are the same). Solenoid valve 43 is closed, and solenoid valves 37 and 39 are opened to allow the gas in the core holder 40 to enter the gas chromatograph 72. A high-precision flow meter 38 is used to record the flow rate change. When the flow rate reaches the chromatograph's detection requirements, solenoid valves 37 and 39 are closed, and the gas components are measured (the amount of gas required for chromatographic detection is relatively small, having a minimal impact on the pressure inside the holders). Open solenoid valves 50 and 52 to allow gas from the core holder 53 to enter the gas chromatograph 72. Record the flow rate change using a high-precision flow meter 51. Once the flow rate reaches the chromatograph's detection requirements, close solenoid valves 50 and 52 and determine the gas components. Adjust the pressure (by opening vent valves 46 and 59 to reduce pressure, and using booster pumps 44 and 57 to increase pressure) and temperature (using constant-temperature electric heating belts 45 and 58) of different core holders based on formation conditions. Open solenoid valve 43 to allow the reservoir in core holders 40 and 53 to reach a new equilibrium under the new temperature and pressure conditions. Repeat the process of testing the gas components in different core holders. Repeat the aforementioned steps of gas equilibration, gas component testing, and temperature and pressure adjustment.

[0056] IV. Gas Component Calculation: This equipment tests the concentration of free gas components in the core holder. When the reservoir adsorption capacity is low (such as limestone and sandstone), coal-bearing gas is mostly present in a free state, and the component concentration in the core holder can be approximated as similar to that in the reservoir. However, for reservoirs with high adsorption capacity (such as coal seams), due to differences in gas adsorption order, the gas component concentration in the coal seam should be significantly different from that in the free gas in the core holder. Therefore, it is necessary to back-calculate the gas component concentration in the reservoir based on the tested gas component characteristics. This paper uses core holder 40 as an example to illustrate the reservoir calculation method.

[0057] 1) Adsorption process (pressurization)

[0058] Taking CO2 as an example, when the reservoir does not adsorb gas, the initial amount of CO2 n0 = 0. The adsorption equilibrium after pressurization (hereinafter referred to as the second equilibrium) is shown in equation (1).

[0059] (1)

[0060] Where: P1, pressure at the second equilibrium, MPa; V j Core holder, 40 cm in volume 3 ; φ1, CO2 concentration measured at the second equilibrium, %; n1, amount of CO2 in the clamp at the second equilibrium, mol; R, gas constant, 8.314 J / (K·mol); T1, temperature at the second equilibrium, K; Z1, CO2 compressibility coefficient at temperature T1 and pressure P1, dimensionless, obtained from the compressibility coefficient chart.

[0061] △n1 (Equation 2) is the amount of CO2 adsorbed. Based on the gas law, the volume V of adsorbed CO2 can be obtained. a-1 (Equation 3)

[0062] (2)

[0063] (3)

[0064] For other components, repeat the above calculation process and calculate the adsorption volume V of the gas for all components where the amount of adsorbed substance after the second equilibrium is <0. b-1 V c-1 ...V i-1 Where i = a, b, c... is used to refer to the i-th gas component, then the total adsorbed gas volume V xf1 From equation (4) we get

[0065] (4)

[0066] Based on this, the concentration of each gas component in the reservoir after the second equilibrium is obtained (Equation 5).

[0067] (5)

[0068] Taking CO2 as an example again, the third equilibrium (still pressure adsorption) is shown in equation (6).

[0069] (6)

[0070] Where: P2, pressure at the third equilibrium, MPa; φ2, CO2 concentration measured at the third equilibrium, %; n2, amount of CO2 in the clamp at the third equilibrium, mol; T2, temperature at the third equilibrium, K; Z2, CO2 compressibility coefficient at temperature T2 and pressure P2, dimensionless, obtained from the compressibility coefficient chart.

[0071] If CO2 is adsorbed, then Δn2 (Equation 7) < 0. Therefore, the total amount of CO2 adsorbed in the reservoir at the third equilibrium is the sum of the absolute values ​​of n1 and Δn2, i.e., n1 + |Δn2|.

[0072] (7)

[0073] The total adsorbed CO2 volume V was calculated based on the gas law. a-2 (Equation 8)

[0074] (8)

[0075] For other components, the total adsorbed volume V of different gases was calculated repeatedly. b-2…… V i-2 (If the difference between the amount of unadsorbed component in the holder at the third equilibrium and the amount of that component in the holder at the second equilibrium is 0), then the total gas volume V in the reservoir is... xf2 From equation (9) we get

[0076] (9)

[0077] This leads to the concentration of each gas component in the reservoir after the third equilibrium (Equation 10).

[0078] (10)

[0079] When the pressure is further increased, the reservoir gas composition characteristics at the time of rebalancing can be obtained by repeating the above calculation process using the test results of the equilibrium gas composition before and after the pressure adjustment and the reservoir gas composition results before the pressure adjustment.

[0080] 2) Desorption process (pressure reduction)

[0081] This calculation process requires the use of the gas component test results from two equilibrium tests and the reservoir gas component test results before depressurization (for the first depressurization, the gas component test results before depressurization are taken from the corresponding results of the previous adsorption equilibrium, which is the calculation result of the previous adsorption equilibrium process). Taking component CO2 as an example, according to the gas state equation, the equilibrium before depressurization has equation (11).

[0082] (11)

[0083] Where: P3, pressure at equilibrium before pressure reduction, MPa; φ3, CO2 concentration measured at equilibrium before pressure reduction, %; n3, amount of CO2 in the clamp at equilibrium before pressure reduction, mol; T3, temperature at equilibrium before pressure reduction, K; Z3, CO2 compressibility coefficient at temperature T3 and pressure P3, dimensionless, obtained from the compressibility coefficient chart;

[0084] At this point, the amount of CO2 adsorbed in the reservoir is n3 (Equation 12).

[0085] (12)

[0086] The equilibrium after pressure reduction is given by equation (13).

[0087] (13)

[0088] Where: P4, pressure at equilibrium after depressurization, MPa; φ4, CO2 concentration measured at equilibrium after depressurization, %; n4, amount of CO2 in the clamp at equilibrium after depressurization, mol; T4, temperature at equilibrium after depressurization, K; Z4, CO2 compressibility coefficient at temperature T4 and pressure P4, dimensionless, obtained from the compressibility coefficient chart;

[0089] If CO2 desorption occurs, then Δn3 (Equation 14) > 0, and Δn3 is the amount of CO2 desorbed. Based on the gas law, the volume V of desorbed CO2 can be obtained. a-3 (Equation 15)

[0090] (14)

[0091] (15)

[0092] Then the amount of adsorbed CO2 remaining in the reservoir at this time is n3 - Δn3, corresponding to the volume V. a-syxf-3 Calculated using equation (16):

[0093] (16)

[0094] The calculation process was repeated for all other components (the difference between the amount of the undesorbed component in the holder at equilibrium after depressurization and the amount of that component in the holder at equilibrium before depressurization was 0) to obtain the desorption volume V for each gas. b-3 V c-3 ...V i-3 and remaining volume V b-syxf-3 V c-syxf-3 ...V i-syxf-3 Then the total volume of desorbed gas V jx From equation (17), we can obtain the concentration φ of each desorbed gas component. i-jx (Equation 18)

[0095] (17)

[0096] (18)

[0097] Total volume of remaining gas V syxf and the concentration φ of each component in the residual gas i-syxf We obtain the results from equations (19) and (20) respectively.

[0098] (19)

[0099] (20)

[0100] V. Simulation End: After the coalbed methane exchange simulation under the design conditions is completed, open valves 28, 46, 59, and 71. After the gas pressure drops to atmospheric pressure, unload the confining pressure of the core holder, remove the sample from the reactor, replace it with another sample or change the initial gas composition to conduct a new simulation.

[0101] Simulation Scheme 2: This scheme is suitable for situations where the gas connection between rock strata is weak and the pressure difference at the start of coalbed methane exchange is large. This scheme can also record the pressure difference changes of different layers during the coalbed methane balance process.

[0102] 1. Connect the instruments and place the reservoir samples in the core holders 40 and 53 respectively. Apply confining pressure using confining pressure pumps 42 and 55 based on the actual formation conditions. If using in-situ hydrocarbon generation simulation samples, place the source rock samples in the high-temperature and high-pressure reactor 15 and close all solenoid valves.

[0103] II. Initial Gas Component Determination: Adjust the booster pump 16 and the constant temperature electric heating belt 17 to set the in-situ hydrocarbon generation temperature and pressure conditions. Open the solenoid valves 18 and 24 to allow the generated mixed product to enter the gas storage tank 25 after passing through the dryer 20. Record the flow rate change using the high-precision flow meter 22. After a large amount of gas enters the gas storage tank 25, close the solenoid valves 18 and 24 and reduce the temperature and pressure of the high-temperature and high-pressure reactor 15 to stop hydrocarbon generation. If a source rock sample cannot be collected, a mixed gas of any composition can be prepared using the device: Set different component concentrations (volumes) using the mixed gas preparer 21, open the solenoid valves 9, 10, 11, 12, 19, 23 and the CH4 gas cylinder 1, CO2 gas cylinder 2, N2 gas cylinder 3 and C2H6 gas cylinder 4 to allow the prepared mixed gas to enter the gas storage tank 25, and close the solenoid valves 9, 10, 11, 12, 19, 23 and the four gas supply cylinders.

[0104] Open solenoid valves 31 and 33 to allow gas from gas storage tank 25 to enter gas chromatograph 72. Use high-precision flow meter 32 to record flow rate changes. When the flow rate required for chromatographic detection is reached, close solenoid valves 31 and 33 to determine the initial gas component characteristics in gas storage tank 25.

[0105] III. Adsorption Equilibrium: Based on the reservoir temperature and pressure conditions, the temperature and pressure of the gas storage tank 25 are adjusted by using the booster pump 26 and the constant temperature electric heating belt 27, and the solenoid valves 34 and 47 are opened in sequence to allow the mixed gas to enter the core holders 40 and 53 (the initial gas pressure of the two core holders is kept the same), so that the different reservoir samples reach adsorption equilibrium.

[0106] IV. Simulation of Coal-bearing Gas Exchange: Solenoid valves 36 and 39 are opened to allow gas from the core holder 40 to enter the gas chromatograph 72. A high-precision flow meter 38 is used to record flow rate changes. Once the flow rate reaches the chromatograph's detection requirements, solenoid valves 36 and 39 are closed, and the gas components are measured (the amount of gas required for chromatographic detection is relatively small, having minimal impact on the pressure within the holder). Solenoid valves 50 and 52 are opened to allow gas from the core holder 53 to enter the gas chromatograph 72. A high-precision flow meter 51 is used to record flow rate changes. Once the flow rate reaches the chromatograph's detection requirements, solenoid valves 50 and 52 are closed, and the gas components are measured. Based on formation conditions, the pressure (by opening vent valves 46 and 59 to reduce pressure, and using booster pumps 44 and 57 to increase pressure) and temperature (using constant-temperature electric heating belts 45 and 58) of different core holders are adjusted to allow the reservoir in different core holders to reach a new equilibrium under new temperature and pressure conditions. The gas components in different core holders are then repeatedly tested. Open solenoid valve 43 to allow gas exchange between core holder 40 and core 53. Once the preset pressure difference is reached (e.g., 0.1 MPa), close solenoid valve 43 to allow the core holder 40 and the reservoir in core 53 to reach equilibrium again after gas exchange. Repeat the aforementioned gas component testing, temperature and pressure variation, and gas equilibrium steps.

[0107] V. Gas component calculation: Same as Scheme 1.

[0108] VI. Simulation End: After the coalbed methane exchange simulation under the design conditions is completed, open the vent valves 28, 46, 59, and 71. After the gas pressure drops to atmospheric pressure, unload the confining pressure of the core holder, remove the sample from the reactor, replace it with another sample or change the initial gas composition to conduct a new simulation.

[0109] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. However, the above description is merely a specific embodiment of the present invention, and the technical features of the present invention are not limited thereto. Any other embodiments derived by those skilled in the art without departing from the technical solution of the present invention should be covered within the patent scope of the present invention.

Claims

1. An apparatus for simulating the change of gas composition in different reservoirs during coal measure gas exchange, characterized in that, The system comprises a gas preparation system, a gas exchange simulation system and a gas detection system; The gas preparation system is used for generating coalbed gas and comprises a gas storage tank and a high-temperature and high-pressure reactor; The gas outlet pipeline of the high-temperature and high-pressure reactor is communicated with the gas storage tank and used for conveying the generated coalbed gas into the gas storage tank; One gas outlet passage of the gas storage tank is connected with the gas detection system and used for detecting the initial gas component; Another gas outlet passage is connected with the gas exchange simulation system; The gas exchange simulation system is used for simulating the coalbed gas exchange and comprises a plurality of core holders arranged in parallel, the core holders are communicated with the gas storage tank through gas inlet branch pipes, the core holders arranged adjacently exchange gas between different reservoirs through gas exchange pipelines, and the gas outlet ends of the core holders are connected with the gas detection system through gas outlet pipelines for detecting the gas component; The number of the core holders is not less than two, electromagnetic valves are arranged on the gas inlet branch pipes and the gas exchange pipelines, temperature sensors and pressure sensors are connected with the core holders for monitoring the temperature and pressure changes in the core holders, the core holders are provided with confining pressure pumps, pressure increasing pumps and constant temperature electric heating belts, and the confining pressure pumps monitor the pressure conditions by using the pressure sensors; One electromagnetic valve is arranged on the gas outlet pipeline of the high-temperature and high-pressure reactor close to the core holder and close to the gas detection system respectively, and a high-precision flowmeter is arranged between the two electromagnetic valves; A dryer and a high-precision flowmeter are arranged on the connecting passage between the high-temperature and high-pressure reactor and the dryer and on the connecting passage between the high-precision flowmeter and the gas storage tank respectively, and one electromagnetic valve is arranged on each of the connecting passages; 2. The device of claim 1, wherein, The high-temperature and high-pressure reactor is provided with a pressure increasing pump and a constant temperature electric heating belt for adjusting the temperature and pressure conditions in the reactor, and a pressure sensor and a temperature sensor are arranged on the high-temperature and high-pressure reactor for monitoring the pressure and temperature in the reactor.

3. The apparatus of claim 1, wherein the apparatus is configured to simulate the change of gas composition in different reservoirs during coalbed gas exchange. The gas detection system comprises a gas chromatograph and a carrier gas bottle. The gas storage tank is connected with a pressure increasing pump and a constant temperature electric heating belt for adjusting the temperature and pressure conditions in the tank, and a pressure sensor and a temperature sensor are connected with the gas storage tank for monitoring the pressure and temperature in the tank.

4. The apparatus of claim 1, wherein the apparatus is configured to simulate the change of gas composition in different reservoirs during coalbed gas exchange. The gas preparation system further comprises a coalbed gas configuration module, which comprises four gas supply cylinders arranged in parallel and a mixed gas configuration instrument; 5. A method of simulating the change in composition of coal-derived gas as it exchanges between different reservoirs, characterised in that, The four gas supply cylinders are CH4 cylinder, CO2 cylinder, N2 cylinder and C2H6 cylinder respectively, the four gas supply cylinders are communicated with the mixed gas configuration instrument through gas supply branch pipes, the mixed gas configuration instrument is communicated with the gas storage tank, a pressure reducing valve and an electromagnetic valve are arranged on each of the gas supply branch pipes, and one electromagnetic valve is arranged on the gas inlet and outlet pipelines of the mixed gas configuration instrument. One vent valve is arranged on the gas storage tank and the core holder respectively for pressure reduction. The simulation is carried out based on the device of any one of claims 1-4, and two cases are simulated:

1. For the case that the rock layer gas contact is strong and the coalbed gas exchange starting pressure difference is small, the simulation method is as follows: 1) Pretreatment: connect the instruments, place the reservoir samples in different core holders, load the confining pressure by using the confining pressure pump based on the actual formation conditions, if the in-situ hydrocarbon generation simulation sample is used, place the hydrocarbon source rock sample in the high-temperature and high-pressure reactor, and close all the electromagnetic valves; 2) Initial gas component determination: The high-temperature and high-pressure reaction kettle is used with a booster pump and a constant temperature electric heating belt to set the in-situ hydrocarbon generation temperature and pressure conditions. The generated mixed product is dried and then introduced into a gas storage tank. The flow change is recorded. After a large amount of gas enters the gas storage tank, the connection path between the reaction kettle and the gas storage tank is blocked. The corresponding electromagnetic valve is opened to make the gas in the gas storage tank enter the gas phase detection system. The flow change is recorded. When the required flow for chromatographic detection is reached, the electromagnetic valve on the connection path between the gas storage tank and the gas detection system is closed, and the initial gas component characteristics in the gas storage tank are determined; 3) Coal measure gas exchange simulation: The temperature and pressure of the gas storage tank are adjusted based on the reservoir temperature and pressure conditions. The corresponding electromagnetic valve is opened to make the mixed gas enter the corresponding core holder. The gas exchange pipeline between the corresponding core holders is opened to make the gas in the adjacent core holders exchange continuously to the equilibrium state. The electromagnetic valve on the gas exchange pipeline is closed. The electromagnetic valve on the gas outlet pipeline of one core holder is opened to make the gas in the core holder enter the gas chromatograph. The flow change is recorded. When the flow reaches the chromatograph detection requirement, the electromagnetic valve on the gas outlet pipeline is closed, and the gas component is determined. The same operation is performed to determine the gas component in other core holders; 4) The pressure and temperature of different core holders are adjusted based on the formation conditions. The electromagnetic valve on the gas exchange pipeline between the adjacent core holders is opened to make the reservoir in the adjacent core holders reach a new equilibrium under the new temperature and pressure conditions. The process of testing the gas component in different core holders is repeated. The steps of gas equilibrium, gas component testing, and temperature and pressure adjustment are repeated; 5) Gas component calculation: The device tested is the free gas component concentration in the core holder. When the adsorption capacity of the reservoir is low, coal measure gas is mostly in free state. It is considered that the component concentration in the core holder is similar to that in the reservoir. However, for reservoirs with high adsorption capacity, due to the difference in gas adsorption sequence, there is a significant difference between the gas component concentration in the coal seam and the free gas in the core holder. Therefore, the gas component concentration in the reservoir needs to be calculated based on the tested gas component characteristics; 2. For the case where the rock layer gas contact is weak and the coal measure gas exchange starting pressure difference is large, the simulation method is as follows: 1) Pretreatment: Same as case 1; 2) Initial gas component determination: Same as case 1; 3) Adsorption equilibrium: The temperature and pressure of the gas storage tank are adjusted based on the reservoir temperature and pressure conditions. The mixed gas is introduced into the core holder to make different reservoir samples reach adsorption equilibrium; 4) Coal gas exchange simulation: open the electromagnetic valve on the gas outlet pipe of the core holder, so that the gas in the core holder enters the gas chromatograph, record the flow change, when the flow reaches the chromatograph detection requirements, close the electromagnetic valve on the gas outlet pipe, measure the gas composition; according to the same operation, measure the gas composition in other core holders; adjust the pressure and temperature of different core holders based on the formation conditions, so that the reservoirs in different core holders reach a new equilibrium under the new temperature and pressure conditions, repeat the test of gas composition in different core holders; open the gas exchange pipeline between the corresponding core holders to make the gas in adjacent core holders exchange, when the preset pressure difference is reached, close the electromagnetic valve on the gas exchange pipeline, so that the reservoirs in the core holders reach equilibrium again after gas exchange, repeat the gas composition test, temperature and pressure change, and gas equilibrium step; 5) Gas composition calculation: same as case 1.

6. A method of simulating the change in composition of coal measures gas when exchanged between different reservoirs as claimed in claim 5 wherein, In step 2 of the simulation experiment under the two different conditions, if the source rock sample cannot be collected, a mixed gas with any composition can be used: use the mixed gas configuration instrument to set different component concentrations or volumes, open the electromagnetic valve on the gas supply branch pipe and the four gas supply cylinders, so that the configured mixed gas enters the gas storage tank, close the electromagnetic valve on the gas supply branch pipe and the four gas supply cylinders, open the corresponding electromagnetic valve to make the gas in the gas storage tank enter the gas phase detection system, record the flow change, when the flow reaches the chromatographic detection requirements, close the electromagnetic valve on the gas storage tank and the gas detection system connection channel, measure the initial gas composition characteristics in the gas storage tank.

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