Device and method for measuring high-temperature and high-pressure diffusion coefficient of low-permeability porous medium gas

Abstract

The invention discloses a device and a method for measuring a high-temperature and high-pressure diffusion coefficient of low-permeability porous medium gas, and relates to the technical field of oil exploitation. The device comprises a self-control constant-temperature box, a constant-speed constant-pressure pump, a high-temperature high-pressure piston intermediate container, a high-temperature high-pressure corrosion-resistant gas storage tank, a high-temperature high-pressure sealed reaction kettle, a three-way valve, a one-way valve, a high-temperature high-pressure corrosion-resistant pipeline, a sensor assembly and a monitoring control terminal, the monitoring control terminal at least comprises a computer, and a pressure data processing system is carried in the computer; the sensor assembly at least comprises a high-precision pressure sensor and a temperature sensor. Through the design of the experimental device and the design of the corresponding experimental method, the experimental condition range is widened, the highest test temperature can reach DEG C, meanwhile, the test pressure can reach MPa, and the high-temperature and high-pressure conditions of the device cover most oil reservoir conditions in China, especially the high-temperature and high-pressure fracture-vug type carbonate reservoir, so that the experimental device has the advantages of being simple in structure and convenient to use. And the tested result has more application significance.

Description

Technical Field

[0001] The present invention relates to the technical field of oil extraction, and specifically to a device and method for measuring the high-temperature and high-pressure diffusion coefficient of gas in low-permeability porous media. Background Art

[0002] Diffusion is a phenomenon of mass transfer, during which substances become more evenly distributed in space over time. The rate of diffusion is characterized by the diffusion coefficient, which is a measure of the rate of the diffusion process. In the field of petroleum engineering, the diffusion coefficient is a very important parameter in the development of oil and gas reservoirs: during the drilling process, the diffusion coefficient of the light components in crude oil in the drilling fluid determines the degree of dissolution and separation of gas in it; during the gas injection enhanced oil recovery process, the sweep efficiency of the injected gas in the formation fluid can be assisted by the gas diffusion coefficient; during the geological process of injecting carbon dioxide, the diffusion coefficient of carbon dioxide in formation water determines the capture amount and migration degree of the water body; during the underground hydrogen storage process, the diffusion coefficient of hydrogen in the formation determines the hydrogen storage amount and production rate in the underground space. In addition, during the numerical simulation process, the diffusion effect of gas is an important parameter for predicting the production performance and development effect of fractured reservoirs. The diffusion coefficient is an important parameter for studying the mechanism of enhanced oil recovery by gas injection. The mass transfer rate of gas determines the dissolved amount of the injected gas in crude oil, thereby affecting the optimization of the gas injection volume; during the gas injection flooding process, mass transfer occurs between the gas and the crude oil, thereby changing the properties of the crude oil, such as viscosity reduction, volume expansion, and change in saturation pressure; during the gas injection huff and puff process, the gas sweep efficiency is different at different mass transfer rates, thereby affecting the optimization of the shut-in time. Therefore, determining the diffusion coefficient of the injected gas under actual reservoir conditions is of great significance for oil development.

[0003] According to the definition of the reservoir space, the space where oil and gas accumulate and can percolate is essentially a porous medium in nature, and the skeleton is mostly composed of sandstone and carbonate rocks. The voids between the skeletons form the storage and flow space of the formation fluid. During the process of enhanced oil recovery by gas injection, most of the injected gas interacts with the crude oil and formation water in the pores in the form of molecules through diffusion mass transfer. Therefore, when determining the diffusion coefficient of the injected gas in the porous medium, the influence of the characteristic parameters of the porous medium itself needs to be considered.

[0004] The development of oil and gas fields is gradually focusing on the "two deep and one unconventional" (deep sea, deep earth, unconventional) fields and has made certain progress. Low-permeability and tight porous media are mostly the reservoir spaces of unconventional oil and gas reservoirs such as tight sandstone and tight carbonate rocks. The reservoir pore throats are extremely developed, with a permeability generally lower than 10 mD and a porosity mostly lower than 10%, even reaching ultra-low permeability. In addition, sand-mud filling, or even dense filling caused by chemical precipitation, is also commonly found in deep fractured-vuggy carbonate rock reservoirs, and this type of oil and gas reservoir generally has high temperature and high pressure. Therefore, during the process of gas injection for developing these two types of reservoirs, the physical property characteristics of the low-permeability and tight porous media directly affect the contact mode and flow law between the injected gas and the formation fluid, and further affect the final effect of enhanced oil recovery by the injected gas. Therefore, it is necessary to measure the bulk diffusion coefficient to assist in analyzing the migration law of the injected gas in the low-permeability and tight porous media.

[0005] In recent years, with the application of gas injection enhanced oil recovery technology in unconventional oil and gas reservoirs such as fractured-vuggy reservoirs, tight oil, and shale oil, clarifying the enhanced oil recovery mechanism of the injected gas, optimizing the gas injection parameters, and efficient gas injection development all require accurate calculation of the diffusion coefficient between the injected gas and the formation fluid. In view of the special high temperature and high pressure conditions (the lowest temperature is 100 °C and the lowest pressure is 30 MPa) and low-permeability and tight reservoir characteristics of unconventional oil and gas reservoirs, there are too many deficiencies in the existing methods for measuring the diffusion coefficient of the injected gas in porous media: the PVT chamber cannot measure the diffusion coefficient of the injected gas in low-permeability and tight porous media; the method of measuring the gas diffusion coefficient with a holder is significantly affected by convection, gravity, etc., and the measurement error is large; the experimental condition range of the autoclave method is limited (temperature range: 25 - 90 °C; pressure: 0.1 - 30 MPa), and it cannot accurately measure the diffusion coefficient of the gas in crude oil under the gas injection conditions of unconventional oil and gas reservoirs.

[0006] Therefore, a new solution needs to be proposed for the above problems. Summary of the Invention

[0007] The purpose of the present invention is to provide a device and method for measuring the high temperature and high pressure diffusion coefficient of gas in low-permeability porous media to solve the technical problems raised in the background technology.

[0008] To achieve the above purpose, the present invention provides the following technical solution: A device for measuring the high temperature and high pressure diffusion coefficient of gas in low-permeability porous media includes an automatic constant temperature box (8), a constant speed and constant pressure pump (1), a high temperature and high pressure piston intermediate container (2), a high temperature and high pressure corrosion-resistant gas storage tank (4), a high temperature and high pressure sealed autoclave (5), a three-way valve, a one-way valve (3), a high temperature and high pressure corrosion-resistant pipeline, a sensor assembly (6), and a monitoring and control terminal;

[0009] The monitoring and control terminal at least includes a computer, and a pressure data processing system (7) is installed in the computer;

[0010] The sensor assembly (6) at least includes a high-precision pressure sensor and a temperature sensor;

[0011] Inside the automatic constant-temperature box (8), there are arranged a high-temperature and high-pressure piston intermediate container (2), a high-temperature and high-pressure corrosion-resistant gas storage tank (4), a high-temperature and high-pressure sealed reaction kettle (5), a three-way valve, and a sensor assembly (6);

[0012] The constant-speed and constant-pressure pump (1) is connected to the high-temperature and high-pressure piston intermediate container (2) through a high-temperature and high-pressure corrosion-resistant pipeline. The high-temperature and high-pressure piston intermediate container (2) is connected to the high-temperature and high-pressure corrosion-resistant gas storage tank (4) through the cooperation of a high-temperature and high-pressure corrosion-resistant pipeline, a three-way valve, and a check valve (3). The high-temperature and high-pressure corrosion-resistant gas storage tank (4) is connected to the high-temperature and high-pressure sealed reaction kettle (5) through the cooperation of a high-temperature and high-pressure corrosion-resistant pipeline and a three-way valve. The three-way valve is also connected to the sensor assembly (6), and the sensor assemblies (6) are all connected to the monitoring and control terminal;

[0013] Further, the high-temperature and high-pressure piston intermediate container (2), the high-temperature and high-pressure corrosion-resistant gas storage tank (4), and the high-temperature and high-pressure sealed reaction kettle (5) all adopt a corrosion-resistant reinforced graphite self-sealing ring structure, which not only enhances the overall sealing structure of the device, but also reduces the abnormal pressure change caused by gas leakage during the experiment and increases the measurement accuracy.

[0014] Further, the volume of the high-temperature and high-pressure sealed reaction kettle (5) is at least 200 ml. The high-temperature and high-pressure sealed reaction kettle (5) is filled with a filling medium, and the size of the filling medium is at least a diameter of 5 cm and a length of 10 cm.

[0015] A method for measuring the high-temperature and high-pressure diffusion coefficient of gas in low-permeability porous media at least includes the following steps:

[0016] S1: Prepare experimental materials. The experimental materials at least include experimental gas, experimental oil, experimental temperature, and pressure. The experimental gas at least includes high-purity CO2 and high-purity N2. The purities of the high-purity CO2 and high-purity N2 are both 99.99%. The experimental temperature is at least 120 °C, and the pressure of the experiment is at least 50 MPa;

[0017] S2: Set up the experimental equipment. First, detect the sealing performance of the high-temperature and high-pressure piston intermediate container. Clean the high-temperature and high-pressure piston intermediate container (2), the high-temperature and high-pressure corrosion-resistant gas storage tank (4), the high-temperature and high-pressure sealed reaction kettle (5), and the connected pipelines with petroleum ether and dry them. Then connect the experimental equipment in sequence;

[0018] S3: Open all the valves in the connected experimental equipment, and inject high-purity nitrogen gas at a certain pressure into the connected containers. The pressure is at least 10 MPa. Observe the pressure change at the corresponding pressure measurement points through a computer. When the pressure remains stable within 3 hours, it indicates that there is no problem with the sealing of the intermediate container (2) of the high-temperature and high-pressure piston and the pipeline. Then close the non-essential valves.

[0019] S4: Process the filling medium.

[0020] S5: Inject gas and conduct a diffusion experiment. Use a constant-speed and constant-pressure pump (1) to inject the experimental gas into the intermediate container (2) of the high-temperature and high-pressure piston. The pressure at the injection end of the high-temperature and high-pressure corrosion-resistant gas storage tank (4) is slightly higher than the experimental set pressure. After the pressure stabilizes, quickly open the connecting valve to allow the gas to enter the high-temperature and high-pressure sealed reaction kettle (5) filled with the filling medium.

[0021] S6: Conduct a diffusion experiment. Use high-precision pressure sensors and temperature sensors to record the change of experimental data. The recording time interval varies from 0.5 - 10 min. Process the data. When the pressure change is less than 10 kPa within 3 hours after diffusion for a certain time, it is considered that the diffusion has reached equilibrium, and the diffusion experiment is stopped.

[0022] Further, the S4 at least includes the following steps:

[0023] According to the physical properties of the filling medium, establish the filling medium for the internal filling body of the corresponding fractured-vuggy reservoir. Use a vernier caliper to measure the radius and length of the core, and calculate the apparent volume of the filling medium.

[0024] Establish the liquid saturation of the filling medium under reservoir conditions: Place the pressed filling medium model into an independent core holder, evacuate it, saturate it with the corresponding formation water to calculate the pore volume, measure the core water-phase permeability at a certain flow rate, put it into an oven and raise the temperature to 120 °C, inject experimental oil, measure the volume of the displaced water, establish the initial oil saturation, and age it for 24 hours at a constant temperature of 120 °C.

[0025] Take out the aged core and seal the two end faces of the filling medium with epoxy resin.

[0026] Place the sealed filling model in the center of the high-temperature and high-pressure sealed reaction kettle (5), evacuate the whole system for 1 hour, and turn on the automatic constant-temperature oven (8), set the experimental temperature as the experimental temperature, and the stabilization time is more than 4 hours.

[0027] Further, the determination of the initial diffusion pressure at least includes the following steps:

[0028] First, according to the injection gas state equation, determine the pressure P1 in the buffer bottle before injection, which can be obtained through the gas state equations before and after injection. The gas state equation of the buffer gas cylinder before injection is:

[0029] P 0 V 0 =Z 0 nRT

[0030] The gas state equation after the buffer gas cylinder is connected to the diffusion kettle instantaneously during injection is:

[0031] P 1 (V 0 +V 1 )=Z 1 nRT

[0032] According to the magnitude of the initial pressure P1 required by the experiment, the pressure P0 in the buffer gas cylinder can be calculated.

[0033]

[0034] Among them, P1 is the experimental pressure; P0 is the pressure of the buffer gas cylinder; V0 is the known volume of the buffer gas cylinder; V1 is the volume of the pipeline and the cavity of the reaction kettle, which is known; Z0 and Z1 are the compression factors of the injection gas under two pressure conditions, which are known; R is the ideal gas constant; T is the experimental temperature; n is the amount of substance of the injection gas.

[0035] Furthermore, the data processing in S6 at least includes the following steps:

[0036] Combined with the actual experiment, make an assumption;

[0037] According to Fick's first diffusion law, when the diffusion coefficient is a constant and there is no source or sink term, the change of the concentration of a substance with time in a rectangular coordinate system is only related to the position. Therefore, the differential equation of diffusion is:

[0038]

[0039] In formula (1), C represents the concentration of the gas in the solution of the porous medium, D is the diffusion coefficient, t is the diffusion time, and x, y, z are the radial distances of the injection gas diffusing in the porous medium;

[0040] Since the core in the actual experiment is columnar, the expression of formula (1) converted in cylindrical coordinates is:

[0041]

[0042] The cylindrical core used in the experiment has sealed upper and lower end faces, and end-face diffusion is not considered. Therefore, gas can only enter the interior of the filling medium through lateral radial diffusion. Thus, Equation (2) is simplified to a differential equation for one-dimensional radial diffusion:

[0043]

[0044] According to the assumed conditions, the initial conditions and boundary conditions are:

[0045] C (t,r) = 0, 0 < r < r 0 , t = 0 (4)

[0046] C = C 0 , r = r 0 , t ≥ 0 (5)

[0047] By combining Equation (3) - Equation (5) and solving, the solution obtained is:

[0048]

[0049] Among them, J 0 (ra n ) is the zero-order Bessel function of the first kind, J 1 (Ra n ) is the first-order Bessel function of the first kind, a n is the positive root of J 0 (r 0 a n ) = 0;

[0050] Let M t represent the amount of substance of gas diffusing into the interior of the porous medium within time t, and M ∞ represent the amount of substance of gas diffusing into the core when time approaches infinity. Integrating Equation (6) with respect to the radius r gives the relationship between M t and time t:

[0051]

[0052] When , Equation (7) can be simplified to:

[0053]

[0054] Combining with the real gas state equation ΔPV = ZΔnRT and substituting M t = Δn into Equation (8), we can obtain:

[0055]

[0056] By transforming Equation (9), the diffusion coefficient D of the injected gas in the filling medium can be obtainedeff , as shown in Equation (10):

[0057]

[0058] Therefore, the effective diffusion coefficient D of the gas in the porous medium is obtained through Equation (10) eff , where M ∞ is the amount of substance of the gas entering the core when the time approaches infinity, in mol; Z is the gas compressibility factor for the experimental temperature and pressure, dimensionless; R is the universal gas constant, 8.314 Pa·m 2 ·K -1 ·mol -1 ; D eff is the effective diffusion coefficient of the gas, in m 2 / s; r 0 is the radius of the core cross-section, in m; V is the annulus volume between the core and the diffusion reactor, in m 3 ; t is the diffusion time, in s; k is the slope of the linear part in the regression of the pressure difference against the square root of time.

[0059] Furthermore, the assumptions at least include:

[0060] (1) The solid skeleton of the filling medium is incompressible, and the position of the solid skeleton in the diffusion reactor remains fixed and does not move;

[0061] (2) The diffusion coefficient of the injected gas in all directions within the filling medium is constant;

[0062] (3) The pore medium inside the filling medium is homogeneous;

[0063] (4) The natural convection effect of the gas in the filling medium is ignored. Since the diffusion direction of the injected gas is perpendicular to the opposite side of the filling medium and is a radial diffusion, the convection effect is very weak and can be disregarded;

[0064] (5) The concentration at the gas-liquid interface is always the equilibrium concentration;

[0065] (6) The temperature remains constant during the experiment;

[0066] (7) During the measurement process, the compressibility factor Z of the injected gas is constant;

[0067] (8) There is no chemical reaction between the injected gas, the filling medium skeleton, and the formation fluid;

[0068] (9) The surface energy at the solid-liquid, gas-liquid, and gas-solid interfaces is neglected;

[0069] (10) The slippage effect of the gas is not considered;

[0070] (11) There is no phase change mass transfer and heat transfer between the gas and liquid phases.

[0071] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0072] Through the design of the experimental device and the corresponding experimental method, the present invention broadens the range of experimental conditions. The highest test temperature can reach 150 °C, and the test pressure can reach 50 MPa at the same time. The high-temperature and high-pressure conditions of the device cover most of the reservoir conditions in China, especially the fractured-vuggy carbonate reservoirs with high temperature and high pressure. The results obtained by the test are more applicable.

[0073] Secondly, the present invention improves the device and measurement method for the diffusion coefficient of the existing filling medium, including increasing the volume of the diffusion kettle. The volume of the diffusion kettle of the device of the present invention is 200 mL, which reduces the error caused by the unstable flow of the injected gas and increases the measurement accuracy; an injection gas buffer gas cylinder is added to eliminate the contact time with the crude oil during the injection gas pressurization process, and the initial pressure value of the contact between the injection gas and the liquid phase is obtained by using the gas state equation calculation method, reducing the measurement error; at the same time, the size of the filling medium is enlarged. Different from the existing porous medium with a diameter of 2.5 cm and a length of 10 cm, the improved filling medium model has various sizes. In the example, the diameter is 5 cm and the length is 10 cm; it can not only meet the diffusion coefficient of single-phase gas in the filling medium, such as non-hydrocarbon gases such as carbon dioxide, nitrogen, and hydrogen, and hydrocarbon gases such as methane and ethane in C1-C4, but also meet the diffusion coefficient of mixed gases with different proportions in the liquid phase, where the mixed gas can be non-hydrocarbon, hydrocarbon or a mixture of hydrocarbon gas and non-hydrocarbon gas, which is more in line with the actual gas injection conditions and the actual reservoir conditions.

[0074] In addition, the types of saturated fluid phases of the filling medium measured are diverse, which can be formation crude oil, formation water with different salinities, or filling media with different water saturations. BRIEF DESCRIPTION OF THE DRAWINGS

[0075] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for the description of the embodiments will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0076] Figure 1 is a schematic structural diagram of the whole of the present invention;

[0077] Figure 2 is a schematic diagram of the model of the diffusion coefficient of the injection gas in the filling medium of the present invention;

[0078] Figure 3 is a schematic diagram of the parameters of the N2-filling medium pressure depletion fitting curve of the present invention;

[0079] Figure 4 Schematic diagram of the relationship between the diffusion pressure difference of nitrogen and the square root of time in the filling medium of the present invention;

[0080] Figure 5 Schematic diagram of the change of the diffusion pressure difference of nitrogen and carbon dioxide injection gas with different ratios in the filling medium of the present invention over time;

[0081] In the figure: 1. Constant speed and constant pressure pump; 2. High-temperature and high-pressure piston intermediate container; 3. Check valve; 4. High-temperature and high-pressure corrosion-resistant gas storage tank; 5. High-temperature and high-pressure sealed reaction kettle; 6. Detection component; 7. Pressure data processing system; 8. Automatic control constant temperature box Specific implementation manners

[0082] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments.

[0083] Experimental materials:

[0084] Experimental gas: High-purity CO2 with a purity of 99.99%; high-purity N2 with a purity of 99.999%.

[0085] Experimental oil: Crude oil from a certain oilfield formation, with a viscosity of 1.42 mPa·s and a density of 0.642 g / cm3 at a formation temperature of 140 °C.

[0086] Experimental temperature: The experiment was carried out under the condition of a simulated formation temperature of 120 °C, and the pressure of the experiment was 50 MPa.

[0087] Test instruments and procedures

[0088] The experimental flow chart is as Figure 1 shown.

[0089] Specific experimental equipment includes:

[0090] HW-Ⅲ type automatic control constant temperature box, ISCO (100D high-temperature and high-pressure piston intermediate container) constant speed and constant pressure pump, high-temperature and high-pressure piston intermediate container, high-temperature and high-pressure corrosion-resistant gas buffer tank, high-temperature and high-pressure sealed reaction kettle, three-way valve, check valve, high-temperature and high-pressure corrosion-resistant pipeline, high-precision pressure sensor and data acquisition system. Among them, the seals of the three high-temperature and high-pressure containers all adopt corrosion-resistant reinforced graphite self-sealing ring structures, which not only enhance the overall sealing structure of the device, but also reduce the abnormal pressure changes caused by gas leakage during the experiment, and increase the measurement accuracy.

[0091] Experimental steps

[0092] (1) Detect the tightness of the intermediate container:

[0093] ① Clean the intermediate container of the high-temperature and high-pressure piston, the corrosion-resistant gas storage tank at high temperature and high pressure, and the high-temperature and high-pressure sealed reaction kettle, as well as the connecting pipelines, with petroleum ether, and then dry them.

[0094] ② Connect the experimental equipment together according to the experimental flow chart. Open all valves and inject high-purity nitrogen at a certain pressure (usually 10 MPa) into the connected containers. Observe the pressure changes at the corresponding pressure measurement points through the computer display instrument. If the pressure remains stable within 3 hours, it indicates that there is no problem with the sealing performance of the intermediate container and the pipelines.

[0095] (2) Treat the filling medium:

[0096] ① Based on the physical properties of the filling medium, establish the filling medium for the internal filling body of the corresponding fracture-vug oil tank. Measure the radius and length of the core with a vernier caliper and calculate the apparent volume of the filling medium.

[0097] ② Establish the liquid saturation of the filling medium under reservoir conditions: Place the prepared filling medium model into an independent core holder, evacuate it, saturate it with the corresponding formation water to calculate the pore volume, measure the water-phase permeability of the core at a certain flow rate, put it into an oven and raise the temperature to 120 °C, inject the experimental oil, measure the volume of the displaced water, establish the initial oil saturation, and age it for 24 hours at a constant temperature of 120 °C.

[0098] ③ Take out the aged core and seal the two ends of the filling medium with epoxy resin. Place the sealed filling model in the center of the high-temperature and high-pressure diffusion kettle, evacuate the entire system for 1 hour, and turn on the constant temperature oven, set the experimental temperature, and keep it stable for more than 4 hours.

[0099] (3) Inject gas and conduct diffusion experiments:

[0100] ① Use a booster pump to inject the target gas (N2 and CO2 are used in the example) into the intermediate container of the high-temperature and high-pressure piston. The pressure at the injection end of the corrosion-resistant gas storage tank is slightly higher than the experimental pressure set. After the pressure is stable, quickly open the connection valve of the intermediate high-pressure gas cylinder to allow the gas to enter the high-temperature and high-pressure diffusion kettle filled with the filling medium.

[0101] (4) Diffusion experiment: Use pressure sensors and temperature sensors to record the changes in experimental data. The recording time interval varies from 0.5 to 10 minutes. After data processing, if the pressure change is less than 10 kPa within 3 hours after diffusion for a certain period of time, it is considered that the diffusion has reached equilibrium, and the diffusion experiment is stopped.

[0102] (5) Clean the experimental device after the experiment: Clean the experimental equipment with petroleum ether and nitrogen, and conduct the next set of experiments according to the experimental methods in (1) - (4).

[0103] Determination of initial diffusion pressure:

[0104] In the existing measurement of the diffusion coefficient of injected gas by the PVT method, during the gas injection process, the injection pump is used to boost the pressure of the gas in the intermediate container. At the same time, the injected gas contacts the liquid phase. After boosting to the experimental measurement pressure, the injection port is finally closed, and the pressure change during the diffusion process is measured. This method increases the contact time between the injected gas and the crude oil, that is, during the boosting process, the gas contacts the crude oil, and at this time, the injected gas has already started to diffuse into the crude oil. Therefore, the obtained pressure drop curve is inaccurate.

[0105] The present invention adds a gas buffer container to the crude oil, and obtains the initial diffusion pressure through the method of the gas state equation, realizing the accurate measurement of the pressure of the injected gas diffusing upon contact with the crude oil, reducing the contact with the crude oil during the boosting process, and making the measured pressure drop more accurate. The specific method is as follows:

[0106] First, according to the gas state equation of the injected gas, determine the pressure P1 in the buffer bottle before gas injection, which can be obtained through the gas state equations before and after gas injection. The gas state equation of the buffer gas cylinder before gas injection is:

[0107] P 0 V 0 =Z 0 nRT

[0108] The gas state equation after the buffer gas cylinder is connected to the diffusion kettle instantaneously during gas injection is:

[0109] P 1 (V 0 +V 1 )=Z 1 nRT

[0110] According to the size of the initial pressure P 1 required by the experiment, the pressure P 0 in the buffer gas cylinder can be calculated,

[0111]

[0112] wherein, P 1 is the experimental pressure; P 0 is the pressure of the buffer gas cylinder; V 0 is the known volume of the buffer gas cylinder; V 1 is the volume of the pipeline and the cavity of the reaction kettle, which is known; Z 0 and Z 1 are the compression factors of the injected gas under two pressure conditions, which are known; R is the ideal gas constant; T is the experimental temperature; n is the amount of substance of the injected gas.

[0113] Data processing method

[0114] The diffusion process of the injected gas in the filling medium is very complex. When studying this process, it is necessary to simplify this phenomenon to a certain extent. First, it should be determined whether the composition and physical properties of each phase in the model change, and the characteristics and properties of the studied phenomenon should be judged, whether it is steady state or transient. An ideal model should be used to make reasonable assumptions about the problem, so as to simplify the problem.

[0115] Combined with the actual experiment, the following assumptions are made:

[0116] (1) The solid skeleton of the filling medium is incompressible, and the position of the solid skeleton in the diffusion kettle remains fixed and does not move;

[0117] (2) The diffusion coefficient of the injected gas in all directions in the filling medium is a constant;

[0118] (3) The pore medium inside the filling medium is homogeneous;

[0119] (4) The natural convection effect of the gas in the filling medium is ignored. Since the diffusion direction of the injected gas is perpendicular to the opposite side of the filling medium and shows radial diffusion, the convection effect is very weak, so it can be not considered;

[0120] (5) The concentration at the gas-liquid interface is always the equilibrium concentration;

[0121] (6) The temperature remains constant during the experiment;

[0122] (7) During the measurement process, the compressibility factor Z of the injected gas is a constant;

[0123] (8) There is no chemical reaction between the injected gas, the filling medium skeleton and the formation fluid;

[0124] (9) The surface energy of the solid-liquid, gas-liquid and gas-solid interfaces is ignored;

[0125] (10) The slippage effect of the gas is not considered;

[0126] (11) There is no phase change mass transfer and heat transfer between the gas-liquid phases.

[0127] According to Fick's first diffusion law, when the diffusion coefficient is a constant and there is no source and sink term, the change of the concentration of the substance with time in the rectangular coordinate system is only related to the position. Therefore, the differential equation of diffusion is:

[0128]

[0129] In formula (1), C represents the concentration of the gas in the solution of the porous medium, D is the diffusion coefficient, t is the diffusion time, and x, y, z are the radial distances of the injected gas in the diffusion of the porous medium;

[0130] The core in the actual experiment is columnar, so the expression of Equation (1) converted in cylindrical coordinates is as follows:

[0131]

[0132] The columnar core is used in the experiment, and the upper and lower end faces are sealed. Without considering end face diffusion, the gas can only enter the interior of the filling medium through lateral radial diffusion. Therefore, Equation (2) is simplified to a differential equation of one-dimensional radial diffusion:

[0133]

[0134] According to the assumed conditions, the initial conditions and boundary conditions are:

[0135] C (t,r) = 0, 0 < r < r 0 , t = 0 (4)

[0136] C = C 0 , r = r 0 , t ≥ 0 (5)

[0137] By combining Equation (3) - Equation (5) and solving, the obtained solution is:

[0138]

[0139] Among them, J 0 (ra n ) is the zero-order Bessel function of the first kind, J 1 (Ra n ) is the first-order Bessel function of the first kind, a n is the positive root of J 0 (r 0 a n ) = 0;

[0140] Let M t represent the amount of substance of the gas diffusing into the interior of the porous medium within time t, and M ∞ represent the amount of substance of the gas diffusing into the core when the time approaches infinity. Integrating Equation (6) with respect to the radius r, the relationship between M t and time t is obtained:

[0141]

[0142] When , Equation (7) can be simplified to:

[0143]

[0144] Combining the real gas state equation ΔPV = ZΔnRT and substituting M t = Δn into Equation (8), the following can be obtained:

[0145]

[0146] The diffusion coefficient D of the injected gas in the filling medium can be obtained from transformation formula (9). eff , as shown in formula (10):

[0147]

[0148] Therefore, the effective diffusion coefficient D of the gas in the porous medium is obtained through formula (10). eff , where M ∞ is the amount of substance of the gas entering the core when the time approaches infinity, in mol; Z is the gas compressibility factor for the experimental temperature and pressure, dimensionless; R is the universal gas constant, 8.314 Pa·m 2 ·K -1 ·mol -1 ; D eff is the effective diffusion coefficient of the gas, in m 2 / s; r 0 is the radius of the core cross-section, in m; V is the annulus volume between the core and the diffusion reactor, in m 3 ; t is the diffusion time, in s; k is the slope of the linear part in the regression of the pressure difference against the square root of time.

[0149] Based on the above experimental process, the following embodiments are proposed:

[0150] Example 1: Determination of the diffusion coefficient of N 2 in an oil sample

[0151] The pressure depletion experiments of the gas in the high-temperature and high-pressure diffusion vessel are all completed in the filling medium. The initial pressure is about 50 MPa, the experimental temperature is 120 °C, the saturated liquid phase is the crude oil of a certain oilfield formation, the physical properties of the filling medium are shown in Table 1, and the experimental gas is N 2 .

[0152] Table 1 Physical properties of the filling medium for the diffusion coefficient determination experiment of N 2 in the filling medium

[0153]

[0154] Refer to Figure 3 , under the condition of an initial pressure of 50.0947 MPa, the pressure change of nitrogen diffusion in the filling medium is divided into three stages ( Figure 3As shown in the figure: In the initial stage of diffusion when the pressure drops linearly, since nitrogen comes into contact with the filling medium for the first time, the pressure drops linearly, which is different from the nearly exponential drop in the pure oil phase and the decline amplitude is slightly slower. This is mainly because nitrogen molecules first need to dissolve in the crude oil on the surface of the filling medium and then enter the crude oil in the internal pores of the filling medium. When the injected gas enters the crude oil in the pores of the filling medium, the pressure drop gradually slows down, indicating that nitrogen gradually diffuses into the interior of the filling medium under high pressure. Finally, as nitrogen gradually enters the connected pores in the filling medium and fills them up, that is, the dissolved amount of nitrogen in the crude oil inside the filling medium approaches saturation, the pressure drop gradually becomes flat until it approaches the diffusion equilibrium.

[0155] Relationship between production pressure difference and square root of diffusion time Figure 4 Not only can the diffusion coefficient of nitrogen in the filling medium be solved by linear regression, but it can also assist in analyzing the pressure changes in the above three diffusion stages during the diffusion process. As can be seen from the figure, in the stage of 0 - 0.8, the pressure difference and the square root of time show a curve relationship, corresponding to the stage when the pressure drops linearly in the initial stage of diffusion; in the stage of 0.8 - 2.5, the pressure difference and the square root of time show a linear relationship, corresponding to the stage when the pressure drops nearly exponentially in the middle stage of diffusion; when it exceeds 2.5, it corresponds to the nearly diffusion equilibrium stage in the later stage of diffusion.

[0156] Table 2 Calculation parameters of nitrogen diffusion coefficient in collapsed filling medium

[0157]

[0158] Under the condition of an initial pressure of 50.0947 MPa, the pressure difference of N 2 diffusing in the collapsed filling medium is 775.83 Pa. Combining with the compressibility factor, the amount of nitrogen substance dissolved in the collapsed filling medium at this stage is 3.59×10— 5 mol. Through the linear regression of the relationship between the diffusion pressure difference and the square root of time in the final stage, the diffusion coefficient of nitrogen in the filling medium of saturated Tahe S117 oil sample is 4.4134×10 -10 m 2 / s.

[0159] Example 2: Determination of diffusion coefficients of nitrogen and carbon dioxide with different ratios in the filling medium

[0160] The pressure depletion experiments of gases in the high-temperature and high-pressure diffusion vessel are all carried out in the filling medium. The initial pressure is about 50 MPa, the experimental temperature is 120 °C, the saturated liquid phase is the crude oil of a certain oilfield formation, and the experimental gases are pure N 2 , pure CO 2 and their mixed gas. The specific ratios and physical properties of the filling medium are shown in Table 3.

[0161] Table 3 Physical properties of the filling medium in the experiment for measuring the diffusion coefficient of the injected gas in the filling medium

[0162]

[0163] See Figure 5 , the law of pressure change with time can not only assist in analyzing the diffusion law at different time stages, but also can calculate the dissolved amount of the injected gas in the filling medium through the gas state equation, and analyze the dissolution results of the injected gas in filling media with different physical properties. At the same time, combining the square root of time in Figure 5 , redraw the relationship between the pressure difference and the square root of time, and then perform a linear fit on the data of the approximate horizontal section to obtain the slope k of the straight line. Substitute it into Equation (10) to calculate the diffusion coefficient of CO with different contents in the 2 collapsed breccia filling medium. The dissolution and diffusion coefficient results of the injected gas in the collapsed filling medium are shown in Table 3.

[0164] Table 4 Statistics of the diffusion results of the injected gas in the filling medium

[0165]

[0166] When the gas components of the injected gas are different, the dimensionless pressure, the change law of the pressure drop amplitude during the diffusion of the injected gas in the chemically dense filling medium, and the final diffusion coefficient and solubility are also different (Table 4). According to Equation (10), the diffusion coefficients of the injected gas measured in this experiment in the chemically dense filling medium saturated with S117 thin oil are 0.5106×10 -10 m 2 / s, 1.4098×10 -10 m 2 / s, 1.9238×10 -10 m 2 / s and 2.2333×10 -10 m 2 / s, corresponding to pure N 2 , mixed gas (molar ratio of N 2 to CO 2 is 8:2), mixed gas (molar ratio of N 2 to CO 2 is 3:7) and pure CO 2 .

[0167] For those skilled in the art, it is obvious that the present invention is not limited to the details of the above-described exemplary embodiments, and the present invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the present invention. Therefore, in any regard, the embodiments should be regarded as exemplary and non-limiting. The scope of the present invention is defined by the appended claims rather than the above description. Thus, all changes that fall within the meaning and scope of the equivalent elements of the claims are intended to be embraced within the present invention. Any reference signs in the claims should not be construed as limiting the claims involved.

Claims

1. A device for measuring the high-temperature and high-pressure diffusion coefficient of a low-permeability porous medium gas, characterized in that: It comprises an automatic constant temperature box (8), a constant speed constant pressure pump (1), a high temperature and high pressure piston intermediate container (2), a high temperature and high pressure corrosion resistant gas storage tank (4), a high temperature and high pressure sealed reaction kettle (5), a three-way valve, a one-way valve (3), a high temperature and high pressure corrosion resistant pipeline, a sensor assembly (6) and a monitoring and control terminal; The monitoring and control terminal at least comprises a computer, wherein the computer is equipped with a pressure data processing system (7); The sensor assembly (6) comprises at least a high-precision pressure sensor and a temperature sensor; The inner side of the automatic thermostatic box (8) is provided with a high-temperature and high-pressure piston intermediate container (2), a high-temperature and high-pressure corrosion-resistant gas storage tank (4), a high-temperature and high-pressure sealed reaction kettle (5), a three-way valve and a sensor assembly (6); The constant speed and constant pressure pump (1) is connected to the high temperature and high pressure piston intermediate container (2) via a high temperature and high pressure corrosion resistant pipeline, the high temperature and high pressure piston intermediate container (2) is connected to the high temperature and high pressure corrosion resistant gas storage tank (4) via a high temperature and high pressure corrosion resistant pipeline, a three-way valve and a one-way valve (3), the high temperature and high pressure corrosion resistant gas storage tank (4) is connected to the high temperature and high pressure sealed reactor (5) via a high temperature and high pressure corrosion resistant pipeline and a three-way valve, the three-way valve is also connected to a sensor assembly (6), and the sensor assembly (6) is connected to the monitoring and control terminal.

2. The device for measuring the high temperature and high pressure diffusion coefficient of a low permeability porous medium gas according to claim 1, characterized in that: The high-temperature and high-pressure piston intermediate container (2), the high-temperature and high-pressure corrosion-resistant gas storage tank (4) and the high-temperature and high-pressure sealed reactor (5) all adopt a corrosion-resistant reinforced graphite self-sealing ring structure, which not only enhances the overall sealing structure of the device, but also reduces the abnormal pressure changes caused by gas leakage during the experiment, thereby increasing the accuracy of the measurement.

3. The device for measuring the high temperature and high pressure diffusion coefficient of a low permeability porous medium gas according to claim 2, characterized in that: The volume of the high-temperature and high-pressure sealed reaction kettle (5) is at least 200 ml. The high-temperature and high-pressure sealed reaction kettle (5) is filled with a filling medium. The size of the filling medium is at least 5 cm in diameter and 10 cm in length.

4. A method for measuring the diffusion coefficient of a low-permeability porous medium gas at high temperature and high pressure, used in a device for measuring the diffusion coefficient of a low-permeability porous medium gas at high temperature and high pressure as claimed in any one of claims 1 to 3, characterized in that: At least the following steps are included: S1: prepare experimental materials, which at least include experimental gas, experimental oil, experimental temperature and pressure, the experimental gas at least includes high-purity CO2 and high-purity N2, the purity of the high-purity CO2 and high-purity N2 are both 99.99%, the experimental temperature at least includes 120°C, and the experimental pressure at least includes 50MPa; S2: Build the experimental equipment, first test the sealing performance of the high-temperature and high-pressure piston intermediate container, clean the high-temperature and high-pressure piston intermediate container (2), the high-temperature and high-pressure corrosion-resistant gas storage tank (4), the high-temperature and high-pressure sealed reactor (5) and the connected pipelines with petroleum ether, and dry them, and then connect the experimental equipment in sequence; S3: Open all valves in the connected experimental equipment, inject a certain pressure of high-purity nitrogen into the connected container, the pressure is at least 10 MPa, and observe the pressure change of the corresponding pressure measuring point through a computer. If the pressure remains stable within 3 hours, it means that there is no problem with the sealing of the high-temperature and high-pressure piston intermediate container (2) and the pipeline, and close the unnecessary open valves; S4: Processing of filling medium; S5: Inject gas and conduct diffusion experiment. Use constant speed and constant pressure pump (1) to inject experimental gas into high temperature and high pressure piston intermediate container (2). The experimental pressure of the injection end of high temperature and high pressure corrosion resistant gas storage tank (4) is slightly higher than the experimental set pressure. After the pressure is stable, quickly open the connecting valve to allow the gas to enter the high temperature and high pressure sealed reactor (5) filled with filling medium. S6: Conduct a diffusion experiment and use high-precision pressure sensors and temperature sensors to record changes in experimental data. The recording time interval varies from 0.5 to 10 minutes. Use data processing. When the pressure change is less than 10 kPa after a certain period of diffusion, that is, within 3 hours, it is considered that the diffusion has reached equilibrium and the diffusion experiment is stopped.

5. The method for measuring the high temperature and high pressure diffusion coefficient of a low permeability porous medium gas according to claim 4, characterized in that: The S4 at least comprises the following steps: According to the physical properties of the filling medium, the filling medium of the corresponding internal filling body of the fracture-cavity reservoir is established, the radius and length of the core are measured with a vernier caliper, and the apparent volume of the filling medium is calculated; Establish the liquid saturation of the filling medium under reservoir conditions: put the pressed filling medium model into an independent core holder, evacuate it, saturate it with the corresponding formation water to calculate the pore volume, measure the water phase permeability of the core at a certain flow rate, put it in an oven and heat it to 120℃, inject experimental oil, measure the volume of water driven out, establish the initial oil saturation, and age it at a constant temperature of 120℃ for 24 hours; The aged core was taken out and the two end faces of the filling medium were sealed with epoxy resin; The sealed filling model is placed in the center of a high-temperature and high-pressure sealed reactor (5), the entire system is evacuated for 1 hour, and the automatic constant temperature box (8) is turned on, the experimental temperature is set to the experimental temperature, and the stabilization time is more than 4 hours.

6. The method for measuring the high temperature and high pressure diffusion coefficient of a low permeability porous medium gas according to claim 4, characterized in that: The determination of the diffusion initial pressure comprises at least the following steps: First, according to the injected gas state equation, determine the pressure P1 in the buffer bottle before gas injection. The gas state equation before and after gas injection can be obtained. The gas state equation of the buffer gas bottle before gas injection is: P0V0=Z0nRT The gas state equation after the buffer gas cylinder is connected to the diffusion reactor at the moment of gas injection is: P1(V0+V1)=Z1nRT According to the initial pressure P1 required by the experiment, the pressure P0 in the buffer gas cylinder can be calculated. Among them, P1 is the experimental pressure; P0 is the pressure of the buffer gas cylinder; V0 is the volume of the buffer gas cylinder, which is known; V1 is the volume of the pipeline and the reactor cavity, which is known; Z0 and Z1 are the compression factors of the injected gas under two pressure conditions, which are known; R is the ideal gas constant; T is the experimental temperature; n is the amount of injected gas substance.

7. The method for measuring the high temperature and high pressure diffusion coefficient of a low permeability porous medium gas according to claim 4, characterized in that: The data processing in S6 at least includes the following steps: Make assumptions based on experimental results; According to Fick's first diffusion law, when the diffusion coefficient is a constant, under the condition of no source or sink, the concentration of a substance in a rectangular coordinate system changes with time only with the position, so the differential equation for diffusion is: In formula (1), C represents the concentration of gas in the porous medium solution, D is the diffusion coefficient, t is the diffusion time, high temperature and high pressure piston intermediate container, y, z are the radial distances of the injected gas diffusion in the porous medium; The core in the actual experiment is cylindrical, so the expression of equation (1) converted in cylindrical coordinates is: The experiment used a cylindrical core with sealed upper and lower end faces. End face diffusion was not considered, so the gas could only enter the filling medium through radial diffusion from the side. Therefore, equation (2) was simplified to a one-dimensional radial diffusion differential equation: According to the assumptions, the initial and boundary conditions are: C (t,r) =0,0<r<r0,t=0 (4) C=C0,r=r0,t≥0 (5) Combining equations (3) and (5), the solution is: Among them, J0(ra n ) is the zero-order first-kind Bessel function, J1(Ra n ) is a first-order Bessel function of the first kind, a n J0(r0a n )=0 positive root; M t Represents the amount of gas diffused into the porous medium within time t, M ∞ It represents the amount of material diffused into the core by gas when time approaches infinity. Integrating equation (6) with respect to radius r, we get M t The relationship between time t is: when When , formula (7) can be simplified as: Combining the real gas state equation ΔPV=ZΔnRT and M t =Δn Substituting into formula (8), we can get: By changing equation (9), we can get the diffusion coefficient D of the injected gas in the filling medium: eff , as shown in formula (10): Therefore, the effective diffusion coefficient D of gas in porous media is obtained by formula (10): eff , where M ∞ is the amount of gas entering the core when time tends to infinity, mol; Z is the gas compressibility factor at the experimental temperature and pressure, dimensionless; R is the universal gas constant, 8.314 Pa·m 2 ·K -1 ·mol -1 ;D eff is the effective diffusion coefficient of the gas, m 2 / s; r0 is the core cross-sectional radius, m; V is the annular volume between the core and the diffusion reactor, m 3 ; t is the diffusion time, s; k is the slope of the straight line in the square root regression of pressure difference and time.

8. The method for measuring the high temperature and high pressure diffusion coefficient of a low permeability porous medium gas according to claim 7, characterized in that: The assumptions include at least: (1) The solid skeleton of the filling medium is incompressible, and the position of the solid skeleton in the diffusion reactor remains fixed and does not move; (2) The diffusion coefficient of the injected gas in all directions in the filling medium is constant; (3) The porous medium inside the filling medium is homogeneous; (4) Ignore the natural convection of gas in the filling medium. Since the diffusion direction of the injected gas is perpendicular to the opposite side of the filling medium and diffuses radially, the convection effect is very weak and can be ignored. (5) The concentration at the gas-liquid interface is always the equilibrium concentration; (6) The temperature remained constant during the experiment; (7) During the measurement process, the compression factor Z of the injected gas is a constant; (8) There is no chemical reaction between the injected gas, the filling medium skeleton and the formation fluid; (9) The surface energy of solid-liquid, gas-liquid and gas-solid interfaces is negligible; (10) The gas slip effect is not considered; (11) Mass and heat transfer without phase change between gas and liquid phases.

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