A method for determining the gravity diffusion coefficient of carbon dioxide in natural gas

Abstract

The present invention discloses a method for determining the gravitational diffusion coefficient of carbon dioxide in natural gas, which belongs to the field of gas diffusion technology, including: placing carbon dioxide at the bottom and natural gas at the top, and determining the diffusion coefficient of carbon dioxide from bottom to top into natural gas, which is called the carbon dioxide molecular diffusion coefficient; placing carbon dioxide at the top and natural gas at the bottom, and determining the diffusion coefficient of carbon dioxide from top to bottom into natural gas, which is called the carbon dioxide equivalent diffusion coefficient; calculating the difference between the carbon dioxide equivalent diffusion coefficient and the carbon dioxide molecular diffusion coefficient as the gravitational diffusion coefficient of carbon dioxide in natural gas. The present invention proposes for the first time a simple and easy method for determining the gravitational diffusion coefficient of carbon dioxide in natural gas, quantifies the gravitational diffusion coefficient as an indicator, and provides a general method for practitioners to clarify the degree of influence of gravity on gas diffusion.

Description

Technical Field

[0001] The invention belongs to the technical field of gas diffusion and also relates to the technical field of oil and natural gas exploration and development, and specifically is a method for measuring the gravity diffusion coefficient of carbon dioxide in natural gas. Background Art

[0002] Injecting carbon dioxide into gas reservoirs can effectively improve the recovery rate and achieve carbon sequestration, taking into account both economic benefits and environmental protection. The interphase diffusion of carbon dioxide and natural gas is an important mechanism for improving the recovery rate of gas reservoirs. The diffusion between the two includes molecular diffusion caused by pressure gradient, diffusion caused by concentration gradient, and diffusion caused by gravity. Existing research focuses on molecular diffusion caused by pressure and concentration. In fact, due to the difference in density between carbon dioxide and natural gas, the gravity diffusion of carbon dioxide in natural gas will affect the interphase mass transfer. Therefore, studying the gravity diffusion coefficient of carbon dioxide in natural gas under gas reservoir conditions is of great significance for guiding gas reservoir development and carbon sequestration.

[0003] For the determination of the diffusion coefficient of carbon dioxide in natural gas, some current determination devices place natural gas and carbon dioxide in two containers respectively, which are arranged up and down and connected by a pipeline, and then the valve between the two containers is opened to connect them, and the diffusion coefficient under the influence of gravity is determined by measuring the components of the gas in the two containers at different times. It only generally considers the influence of gravity on the diffusion coefficient. The measured diffusion coefficient is actually a mixed diffusion coefficient that combines complex factors such as concentration gradient, gravity, and pressure gradient, and it is impossible to intuitively understand the extent of the influence of gravity on gas diffusion. Summary of the invention

[0004] In order to solve the above problems, the present invention provides a method for determining the gravitational diffusion coefficient of carbon dioxide in natural gas. The present invention draws on the testing method of molecular diffusion coefficient, utilizes the difference in density between carbon dioxide and natural gas, and measures the molar fraction of carbon dioxide in natural gas under different diffusion times and diffusion distances by changing their relative positions in the vertical direction, calculates the diffusion coefficients respectively, and finally deducts the influence of molecular diffusion by subtraction to obtain the gravitational diffusion coefficient.

[0005] In order to achieve the above object, the solution provided by the present invention is as follows:

[0006] A method for measuring the gravitational diffusion coefficient of carbon dioxide in natural gas comprises the following steps:

[0007] S1. Place carbon dioxide at the bottom and natural gas at the top, and measure the diffusion coefficient of carbon dioxide from bottom to top into natural gas, which is called the diffusion coefficient of carbon dioxide molecules;

[0008] S2. Place carbon dioxide at the top and natural gas at the bottom, and measure the diffusion coefficient of carbon dioxide from top to bottom into the natural gas, which is called the equivalent diffusion coefficient of carbon dioxide;

[0009] S3. Calculate the difference between the equivalent diffusion coefficient of carbon dioxide and the diffusion coefficient of carbon dioxide molecules as the gravity diffusion coefficient of carbon dioxide in natural gas.

[0010] Both step S1 and step S2 of the present invention involve measuring the diffusion coefficient of carbon dioxide into natural gas, which is a non-steady-state diffusion process, and its diffusion coefficient is related to distance and time. The measurement process includes the following steps:

[0011] S11, injecting carbon dioxide and natural gas into the PVT cylinder so that the carbon dioxide and natural gas are arranged up and down and the volumes of carbon dioxide and natural gas in the PVT cylinder are equal;

[0012] S12, using the initial interface between natural gas and carbon dioxide as a reference plane for measuring the depth of carbon dioxide intrusion into natural gas, and measuring the distribution of carbon dioxide concentration in natural gas as a function of intrusion depth at a certain moment;

[0013] S13, repeating steps S11 and S12 to determine the distribution of carbon dioxide concentration in natural gas as a function of invasion depth at different times;

[0014] S14. Calculate the diffusion coefficient of carbon dioxide into natural gas based on Fick's second law.

[0015] As a specific embodiment of the present invention, in step S11, injecting carbon dioxide and natural gas into the PVT cylinder includes the following steps:

[0016] S111, respectively loading carbon dioxide and natural gas into two sample dispensers, the sample dispenser and the PVT cylinder are both piston containers; the PVT cylinder is arranged vertically, one end of each of the two sample dispensers is connected to the upper end of the PVT cylinder, the other end is connected to the first pump, and the lower end of the PVT cylinder is connected to the second pump;

[0017] S112, injecting hydraulic liquid into the PVT cylinder through the second pump, pushing the piston in the PVT cylinder to the highest point, thereby exhausting the air in the PVT cylinder out of the system;

[0018] S113, adjusting the two sample dispensers to the same temperature and the same pressure, adjusting the PVT cylinder to the target temperature and maintaining the constant temperature;

[0019] S114, start the first pump to inject hydraulic liquid into a certain sample dispenser, and simultaneously start the second pump to discharge the hydraulic liquid in the PVT cylinder. The flow rates of the first pump and the second pump are equal, so that the gas in the sample dispenser is injected into the PVT cylinder at a constant pressure;

[0020] S115, start the first pump to inject hydraulic liquid into another sample dispenser, and simultaneously start the second pump to discharge the hydraulic liquid in the PVT cylinder. The flow rates of the first pump and the second pump are equal, so that the gas in the sample dispenser is injected into the PVT cylinder at a constant pressure;

[0021] Among them, after the gas in the sample dispenser is injected into the PVT cylinder at a constant pressure, the piston in the piston cylinder will drop, and the height to which the piston in the PVT cylinder drops in step S114 and step S115 is equal.

[0022] The method of the present invention can inject the gas in the sample dispenser into the PVT cylinder at a constant pressure. Compared with the conventional gas filling method, this will not cause gas disturbance, that is, it avoids the serious convection diffusion that occurs when the high-pressure gas enters the low-pressure or vacuum environment, which will aggravate gas movement and thus affect the accuracy of subsequent tests.

[0023] Furthermore, in order to further reduce gas volume fluctuations, in step S113, the temperature of the sample dispenser is the same as the target temperature of the PVT cylinder to reduce gas volume changes caused by temperature changes during transportation and ensure the accuracy of gas flow.

[0024] As a specific implementation of the present invention, in step S12, determining the distribution of carbon dioxide concentration in natural gas at a certain moment as a function of invasion depth includes the following steps:

[0025] S121, using a piston container as a sampler, one end of the sampler is connected to the upper end of the PVT cylinder, and the other end is connected to the third pump;

[0026] S122, injecting hydraulic liquid into the sampler through the third pump, pushing the piston in the sampler to the highest point, thereby exhausting the air in the sampler out of the system;

[0027] S123, start the second pump to inject hydraulic liquid into the PVT cylinder, and simultaneously start the third pump to discharge the hydraulic liquid in the sampler. The flow rates of the second pump and the third pump are equal, so that the gas in the PVT cylinder is injected into the sampler at a constant pressure;

[0028] S124. Repeat steps S121 to S123 to take the sample in the PVT cylinder into different samplers.

[0029] Furthermore, in order to further reduce gas volume fluctuations, step S121 also includes adjusting the PVT cylinder and the sampler to the same temperature to reduce gas volume changes caused by temperature changes and ensure the accuracy of gas flow.

[0030] Compared with the prior art, at least one specific implementation of the present invention has the following advantages:

[0031] The present invention proposes for the first time a simple and easy method for determining the gravitational diffusion coefficient of carbon dioxide in natural gas, quantifies the gravitational diffusion coefficient as an indicator, and provides a universal method for practitioners to clarify the degree of influence of gravity on gas diffusion.

[0032] The present invention proposes a constant-pressure gas injection method, which can inject the gas in the sample dispenser into the PVT cylinder at a constant pressure. Compared with the current operation of adding high-pressure raw gas into a vacuum container, the method has little disturbance to the gas and will not cause serious convection diffusion in the PVT cylinder due to the filling operation, which is beneficial to improving the accuracy of subsequent tests.

[0033] The present invention proposes a constant pressure sampling method, which reduces the disturbance of the gas in the observation container when sampling from the observation container, thereby avoiding the sampling operation affecting the determination of carbon dioxide concentration, which is beneficial to improving the accuracy of measurement. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 is a schematic diagram of the flow of an experimental device used in an embodiment of the present invention;

[0035] Figure 2 This is the distribution diagram of the mole fraction of carbon dioxide in natural gas with the invasion depth after 8 hours of diffusion;

[0036] Figure 3 The scatter plot of carbon dioxide mole fraction in natural gas at different diffusion times at an intrusion depth of 1 cm;

[0037] Figure 4 is the gravity diffusion coefficient of carbon dioxide in natural gas at different diffusion times at an intrusion depth of 1 cm;

[0038] In the figure, there are a PVT instrument 100 , a sampler 300 , a chromatograph 500 , a PVT cartridge 110 , a first pump 410 , a sample dispenser 210 for storing carbon dioxide, a sample dispenser 220 for storing natural gas, a second pump 420 , a third pump 430 , and a drain valve 610 . DETAILED DESCRIPTION

[0039] The present invention will be further described in detail below in conjunction with embodiments and drawings, but the embodiments of the present invention are not limited thereto.

[0040] The natural gas used in this example is a reservoir natural gas sample compounded under the conditions of original formation temperature of 81°C and original formation pressure of 5MPa according to the report on the composition of natural gas produced from a gas field in the South China Sea (Table 1) and the national standard GB / T 26981-2020 "Analysis Method for Physical Properties of Oil and Gas Reservoir Fluids". The carbon dioxide in this example uses commercial gas with a purity of 99.0%. Before the experiment, the natural gas and carbon dioxide were placed in a sample dispenser and stirred at a constant temperature of 81°C and a constant pressure of 5MPa.

[0041] The experimental device used in this embodiment is as follows Figure 1 As shown, it includes a PVT instrument 100, two samplers, four samplers 300, three pumps and a chromatograph 500. Among them, the PVT instrument 100 includes a PVT cylinder 110, two samplers are used to store natural gas samples and carbon dioxide samples respectively, three pumps are displacement pumps, and the chromatograph 500 is used to analyze gas composition. The PVT cylinder 110, the sampler and the sampler 300 are all piston containers, the PVT cylinder 110 is arranged vertically, the two samplers are connected in parallel, and the two samplers are connected at one end to the upper end of the PVT cylinder 110 and the other end to the first pump 410, and the lower end of the PVT cylinder 110 is connected to the second pump 420; the four samplers 300 are connected in parallel, and the four samplers 300 are connected at one end to the upper end of the PVT cylinder 110 and the other end to the third pump 430.

[0042] Table 1 Molar composition of produced natural gas

[0043]

[0044] The method for determining the gravitational diffusion coefficient of carbon dioxide in natural gas in this embodiment includes the following steps:

[0045] S1. Determining the diffusion coefficient of carbon dioxide molecules, specifically comprising the following steps:

[0046] S11, injecting carbon dioxide and natural gas into the PVT cylinder 110 in sequence, so that the carbon dioxide and natural gas are arranged up and down and the volumes of carbon dioxide and natural gas in the PVT cylinder 110 are equal;

[0047] S111, respectively loading carbon dioxide and natural gas into two sample dispensers;

[0048] S112, open the drain valve 610 at the top of the PVT cylinder 110, start the second pump 420 to inject hydraulic liquid into the PVT cylinder 110, push the piston in the PVT cylinder 110 to the highest point, and exhaust the air in the system; after the air is exhausted, close the drain valve 610; record the initial height of the piston in the PVT cylinder 110 at this time, which is 47 cm;

[0049] S113, adjusting the temperatures of the two sample dispensers and the PVT cylinder 110 to 81°C and 5MPa;

[0050] S114, injecting carbon dioxide into the PVT cylinder 110: start the first pump 410 to inject hydraulic liquid into the sample dispenser 210 storing carbon dioxide, and synchronously start the second pump 420 to discharge the hydraulic liquid in the PVT cylinder 110. The flow rates of the first pump 410 and the second pump 420 are equal, both of which are 0.05 ml / min, so that the gas in the sample dispenser is injected into the PVT cylinder 110 at a constant pressure. When the piston of the PVT cylinder 110 drops from 2 cm to 45 cm, stop the first pump 410 and the second pump 420;

[0051] S115, injecting natural gas into the PVT cylinder 110 in the same manner as S114: injecting hydraulic liquid into the sample dispenser 220 storing natural gas, synchronously starting the second pump 420 to discharge the hydraulic liquid in the PVT cylinder 110, the flow rates of the first pump 410 and the second pump 420 are equal, both 0.05 ml / min, so that the natural gas in the sample dispenser is injected into the PVT cylinder 110 at a constant pressure, and when the piston of the PVT cylinder 110 drops from 2 cm to 43 cm, the first pump 410 and the second pump 420 are stopped;

[0052] S116. Close all valves to allow the system to diffuse;

[0053] S12, using the initial interface between natural gas and carbon dioxide as a reference plane for measuring the depth of carbon dioxide intrusion into natural gas, measuring the distribution of carbon dioxide concentration in natural gas at a certain moment with the intrusion depth. In order to accurately obtain the carbon dioxide concentration at each location, this embodiment divides the initial space of natural gas into four equal parts in the horizontal direction, and performs sampling and detection respectively. The gas sampling includes the following steps:

[0054] S121, adjusting the temperature of the sampler 300 to 81°C to be consistent with the PVT cylinder 110;

[0055] S122, injecting hydraulic liquid into the sampler 300 through the third pump 430, pushing the piston in the sampler 300 to the highest point, thereby exhausting the air in the sampler 300 out of the system;

[0056] S123, start the second pump 420 to inject hydraulic liquid into the PVT cylinder 110, and synchronously start the third pump 430 to discharge the hydraulic liquid in the sampler 300. The flow rates of the second pump 420 and the third pump 430 are equal, both of which are 0.05 ml / min, so that the gas in the PVT cylinder 110 is injected into the sampler 300 at a constant pressure. When the piston of the PVT cylinder 110 rises 0.5 cm, stop the second pump 420 and the third pump 430;

[0057] S124, repeating steps S121 to S123, dividing the sample in the PVT cylinder 110 into four portions and taking them out;

[0058] S125, stirring the samples obtained from each sampler 300 to make them fully mixed, and then measuring the content of carbon dioxide in the gas through the chromatograph 500;

[0059] S13, repeating steps S11 and S12 to determine the distribution of carbon dioxide concentration in natural gas as a function of invasion depth at different times;

[0060] S14. Calculate the diffusion coefficient of carbon dioxide into natural gas based on Fick's second law.

[0061] S2. Determine the equivalent diffusion coefficient of carbon dioxide molecules. The specific operation is similar to that of determining the diffusion coefficient of carbon dioxide molecules. The difference is that, when the raw gas is initially charged into the PVT cylinder 110, natural gas is injected first and then carbon dioxide is injected, so that carbon dioxide is placed on the top and natural gas is placed on the bottom. When sampling to detect the carbon dioxide concentration in the later stage, it is necessary to discharge the upper carbon dioxide before sampling, that is, to move the initial interface between natural gas and carbon dioxide to the top of the PVT cylinder 110 before sampling. Please note that when discharging the carbon dioxide on the top of the PVT cylinder 110, first use the same operation as sampling to take the carbon dioxide into the sampler 300 at a constant pressure, and then discharge the gas in the sampler 300. This can avoid causing a large disturbance to the gas in the PVT cylinder 110.

[0062] S3. Calculate the difference between the equivalent diffusion coefficient of carbon dioxide and the diffusion coefficient of carbon dioxide molecules as the gravity diffusion coefficient of carbon dioxide in natural gas.

[0063] This example measures the distribution of carbon dioxide concentration in natural gas as a function of intrusion depth at 1h, 2h, 4h, and 8h. Figure 2 , Figure 3 and Figure 4 , Figure 2 The distribution of carbon dioxide mole fraction in natural gas with invasion depth after 8 hours of diffusion is shown. Figure 3 This is a scatter plot of the mole fraction of carbon dioxide in natural gas at different diffusion times at an intrusion depth of 1 cm. Figure 4 is the gravity diffusion coefficient of carbon dioxide in natural gas at different diffusion times at an intrusion depth of 1 cm.

[0064] The above are only preferred specific implementations of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by any technician familiar with the technical field within the technical scope disclosed in the embodiments of the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for determining the gravitational diffusion coefficient of carbon dioxide in natural gas, characterized in that: include: S1. Place carbon dioxide at the bottom and natural gas at the top, and measure the diffusion coefficient of carbon dioxide from bottom to top into natural gas, which is called the diffusion coefficient of carbon dioxide molecules; S2. Place carbon dioxide at the top and natural gas at the bottom, and measure the diffusion coefficient of carbon dioxide from top to bottom into the natural gas, which is called the equivalent diffusion coefficient of carbon dioxide; S3, calculating the difference between the carbon dioxide equivalent diffusion coefficient and the carbon dioxide molecular diffusion coefficient as the gravity diffusion coefficient of carbon dioxide in natural gas; Both step S1 and step S2 involve measuring the diffusion coefficient of carbon dioxide into natural gas, and the measurement process includes the following steps: S11, injecting carbon dioxide and natural gas into the PVT cylinder so that the carbon dioxide and natural gas are arranged up and down and the volumes of carbon dioxide and natural gas in the PVT cylinder are equal; S12, using the initial interface between natural gas and carbon dioxide as a reference plane for measuring the depth of carbon dioxide intrusion into natural gas, and measuring the distribution of carbon dioxide concentration in natural gas as a function of intrusion depth at a certain moment; S13, repeating steps S11 and S12 to determine the distribution of carbon dioxide concentration in natural gas as a function of invasion depth at different times; S14. Calculate the diffusion coefficient of carbon dioxide into natural gas based on Fick's second law.

2. The method for determining the gravitational diffusion coefficient of carbon dioxide in natural gas according to claim 1, characterized in that: In step S11, injecting carbon dioxide and natural gas into the PVT cylinder includes the following steps: S111, respectively loading carbon dioxide and natural gas into two sample dispensers, wherein the sample dispenser and the PVT cylinder are both piston containers; the PVT cylinder is arranged vertically, one end of each of the two sample dispensers is connected to the upper end of the PVT cylinder, and the other end is connected to the first pump, and the lower end of the PVT cylinder is connected to the second pump; S112, injecting hydraulic liquid into the PVT cylinder through the second pump to push the piston in the PVT cylinder to the highest point, thereby exhausting the air in the PVT cylinder out of the system; S113, adjusting the two sample dispensers to the same temperature and the same pressure, and adjusting the PVT cylinder to the target temperature and maintaining the constant temperature; S114, starting the first pump to inject hydraulic liquid into a certain sample dispenser, and synchronously starting the second pump to discharge the hydraulic liquid in the PVT cylinder, the flow rates of the first pump and the second pump are equal, so that the gas in the sample dispenser is injected into the PVT cylinder at a constant pressure; S115, start the first pump to inject hydraulic liquid into another sample dispenser, and simultaneously start the second pump to discharge the hydraulic liquid in the PVT cylinder, the flow rates of the first pump and the second pump are equal, so that the gas in the sample dispenser is injected into the PVT cylinder at a constant pressure; Wherein, after the gas in the sample dispenser is injected into the PVT cylinder at a constant pressure, the piston in the piston cylinder will drop, and the height to which the piston in the PVT cylinder drops in step S114 and step S115 is equal.

3. The method for determining the gravitational diffusion coefficient of carbon dioxide in natural gas according to claim 2, characterized in that: In step S113, the temperature of the sample dispenser is the same as the target temperature of the PVT cylinder.

4. The method for determining the gravitational diffusion coefficient of carbon dioxide in natural gas according to claim 2, characterized in that: In step S12, determining the distribution of carbon dioxide concentration in natural gas at a certain moment as a function of invasion depth includes the following steps: S121, using a piston container as a sampler, one end of the sampler is connected to the upper end of the PVT cylinder, and the other end is connected to the third pump; S122, injecting hydraulic liquid into the sampler through the third pump to push the piston in the sampler to the highest point, thereby exhausting the air in the sampler out of the system; S123, starting the second pump to inject hydraulic liquid into the PVT cylinder, and synchronously starting the third pump to discharge the hydraulic liquid in the sampler, the flow rates of the second pump and the third pump are equal, so that the gas in the PVT cylinder is injected into the sampler at a constant pressure; S124, repeating steps S121 to S123, taking the sample in the PVT cylinder into different samplers.

5. The method for determining the gravitational diffusion coefficient of carbon dioxide in natural gas according to claim 4, characterized in that: Step S121 also includes adjusting the PVT cylinder and the sampler to the same temperature.

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