A minimum miscible phase pressure measurement system and measurement method based on microfluidics technology

Abstract

The present invention relates to a minimum miscible pressure measurement system and a measurement method thereof based on microfluidic technology, and provides a fast and visual miscible pressure measurement method, wherein the specific steps include: encapsulating a microfluidic chip, assembling a microfluidic device; testing the air tightness of the device, and maintaining the constant temperature of the device system; saturating the crude oil in the channel, and regulating the crude oil sample flow rate to a set value; connecting the injected gas, and adjusting the gas injection pressure and the pipeline back pressure to a set value; adjusting the microscope to a fluorescence mode, and using a high-speed camera to record the fluorescence image; processing the fluorescence image, and determining the minimum miscible pressure according to the change in fluorescence intensity. The method of the present invention can directly observe the disappearance of the phase interface, and directly determine the minimum miscible pressure according to the intensity of the fluorescence signal, and also has the advantages of sufficient single experimental data, accurate experimental results, low sample consumption, short pressure measurement time, wide applicable temperature and pressure conditions, and a variety of applicable oil types.

Description

Technical Field

[0001] The invention relates to a minimum miscible phase pressure measurement system based on microfluidic technology and a measurement method thereof, belonging to the technical field of oil and gas field development and recovery enhancement. Background Art

[0002] CO2 flooding is an efficient and environmentally friendly technology for improving oil recovery. It is divided into immiscible flooding and miscible flooding according to the displacement pressure. Due to the large difference in physical properties between CO2 and crude oil, CO2 immiscible flooding is prone to gravity differentiation, capillary fingering and other phenomena that are not conducive to improving oil recovery. CO2 miscible flooding can effectively solve the above problems. CO2 miscible flooding refers to the displacement process in which CO2 and crude oil are in contact with each other for many times under certain pressure conditions and sufficient interphase mass transfer occurs through evaporation or extraction, resulting in the disappearance of the oil-gas phase interface and achieving miscible displacement. CO2 is easily soluble in crude oil. Under a certain pressure, CO2 and crude oil undergo interphase mass transfer to cause the crude oil to expand, reduce the density and viscosity of the crude oil, reduce the interfacial tension, and improve the fluidity of the crude oil, thereby significantly improving the recovery rate to nearly 100% in theory. The key indicator for judging whether an oil and gas field can be miscible flooded is the minimum miscible pressure. Miscible flooding can only be effectively carried out when the reservoir formation pressure is greater than the minimum miscible pressure of CO2 and crude oil. Minimum miscibility pressure (MMP) refers to the minimum pressure required for the injected gas and crude oil gas-liquid interface to disappear, i.e., to form a miscible phase under certain temperature conditions. The commonly used method for measuring the minimum miscibility pressure is the capillary test method. The capillary method has the following problems: the experimental process is complex and time-consuming, there is little measured data, the sample consumption is large, and the minimum miscibility pressure is indirectly determined by linear fitting based on the recovery factor data points. In addition, pressure measurement methods such as the bubble riser method and the interfacial tension disappearance method also lack a unified measurement standard and cannot directly and quantitatively determine the MMP.

[0003] Microfluidics is a technology for controlling fluid flow in microchannels. It has the advantages of in-situ visualization, low sample consumption, short experimental time, and inherent safety. Microfluidics, also known as "laboratory on a chip" technology, can shrink traditional experimental methods and integrate them on microfluidic chips. There are few reports on mixed-phase pressure measurement methods based on microfluidics.

[0004] The microfluidic pressure measurement experiment simulating the capillary method is limited by the experimental method of statistical recovery of the capillary method. The minimum miscible pressure determined by the recovery-related displacement pressure used as secondary experimental data is not the actual measured value of the experiment. The miscible pressure measured in this way is not directly related to the miscible behavior of oil and gas, and fails to fully utilize the advantages of microfluidic technology visualization to effectively improve the original traditional experimental method.

[0005] The microfluidic pressure measurement experiment simulating the bubble rise instrument method is limited by the experimental design ideas and operation methods of the bubble rise instrument method. It is impossible to construct a stable oil-gas phase interface. The gas phase flows in the flowing oil phase in the form of bubbles and is accompanied by unstable deformation. It is difficult to track and capture the microscopic image of the oil-gas phase interface in real time, and therefore it is impossible to accurately describe the quantitative relationship between displacement pressure and oil-gas miscible behavior. The deformation of the gas phase sample bubble in the two-phase flow is the deformation of the oil-gas phase interface. It is affected by factors such as oil phase flow rate, oil phase composition, gas phase flow rate, microchannel shape and size, and cannot be associated only with displacement pressure. The microfluidic pressure measurement experiment simulating the bubble rise instrument method has great difficulties in experimental design and even experimental operation, especially it is difficult to accurately describe the quantitative relationship between displacement pressure and oil-gas miscible behavior. Summary of the invention

[0006] In view of the deficiencies in the prior art, the present invention provides a minimum miscible pressure measurement system and a measurement method thereof based on microfluidic technology.

[0007] The present invention utilizes the visualization and micro-nano confinement technical features of microfluidic technology, and can obtain the MMP direct criterion of oil-gas miscibility at the micro-nano scale by in-situ real-time observation of the disappearance of the phase interface. In addition, the method of the present invention also has the advantages of short pressure measurement time, small consumption of samples such as oil and gas, and can measure the miscible pressure under micro-nano confinement. The single experimental data is sufficient and does not require mathematical interpolation to supplement the data. The experimental method has good repeatability and accurate results, and is applicable to a wide range of temperature and pressure conditions and a variety of applicable oil types.

[0008] The technical solution of the present invention is:

[0009] A minimum miscible pressure measurement system based on microfluidic technology comprises the microfluidic experimental platform and a microfluidic chip. The microfluidic experimental platform comprises a fluid control device, a chip packaging device and a microscopic observation device. The fluid control device is used to control the flow rate or flow velocity of the oil phase sample and the gas phase sample such as CO2 to be tested in the microfluidic chip respectively, and to adjust the injection pressure of the gas phase sample, that is, to control the displacement pressure in the microfluidic chip; the chip packaging device is used to package the microfluidic chip; the microscopic observation device is used to observe the change of fluorescence signal intensity on the oil phase side in the phase interface area of ​​the microfluidic chip under different displacement pressures, and obtain fluorescence image video recordings. After image processing, the relative fluorescence intensity under different displacement pressures is obtained, and then the minimum miscible pressure is obtained.

[0010] Preferably according to the present invention, the fluid control device includes a gas injection device, an oil phase sample injection device, a vacuum device and a back pressure regulating device, and the main instruments include a high-pressure plunger pump, a fluid injection pump, a syringe, a vacuum pump, a pressure sensor, etc.

[0011] The microfluidic chip is provided with three connection ports connected to the external pipeline, namely the first connection port, the second connection port and the third connection port; the first connection port is connected to the gas injection device, the gas injection device includes a first CO2 cylinder and a first high-pressure plunger pump which are interconnected, the first high-pressure plunger pump is connected to the first connection port, and a first pressure sensor is arranged on the pipeline between the first high-pressure plunger pump and the first connection port, the first high-pressure plunger pump is used to inject CO2 into the microfluidic chip channel at a certain displacement pressure, the first pressure sensor is used to monitor the displacement pressure of CO2, the high-pressure plunger pump has a constant pressure output mode and a constant current output mode, and can accurately control the pressure or flow rate of gas injection.

[0012] The second connection port is connected to the oil phase sample injection device, which includes a fluid injection pump and a syringe that are interconnected. The fluid injection pump is used to inject the oil phase sample into the microfluidic chip channel at a certain flow rate; the fluid injection pump can accurately control the flow rate of the oil phase fluid.

[0013] The third connection port is connected to a vacuum pumping device and a back pressure regulating device, the vacuum pumping device mainly includes a vacuum pump, the vacuum pump is connected to the third connection port, and a second pressure sensor is arranged on the pipeline between the vacuum pump and the third connection port; the gas in the chip channel can be evacuated through the vacuum pump in the vacuum pipeline, which is beneficial to the subsequent injection of oil phase samples; the back pressure regulating device includes a second CO2 gas cylinder and a second high-pressure plunger pump which are interconnected, the second high-pressure plunger pump is connected to the third connection port, and a third pressure sensor is arranged on the connecting pipeline between the back pressure regulating device and the microfluidic chip; the back pressure regulating device cooperates with the flow rate or pressure setting of the fluid injection device, and uses the second high-pressure plunger pump to actively adjust the flow rate and pressure of the microfluidic chip channel.

[0014] According to the preferred embodiment of the present invention, the microfluidic chip is prepared by anodic bonding of a single crystal silicon wafer and quartz glass, and a channel pattern is etched on the single crystal silicon wafer to simulate the real reservoir rock structure. The channel pattern includes a channel or a porous medium area with a micro-nano confined size, and a channel with different widths and depths is designed according to the needs; or a porous medium area with different porosity and permeability is designed according to the needs. The pressure and temperature range that this chip can withstand is relatively large (P≤40MPa, T≤250℃). The design of the channel pattern of the present invention is designed as a single channel with a length-to-diameter ratio of 500:1 with reference to the capillary model of the capillary method, so that the fluid flow in the channel is close to one-dimensional flow. In addition, the innovation of the chip design is to add a circular base particle design in a single channel, so that the oil and gas phases can contact multiple times when flowing in the channel and truly have multi-stage contact mixing, that is, dynamic mixing, so that the minimum mixed phase pressure measured is closer to the real multi-stage contact mixing minimum mixed phase pressure.

[0015] Preferably, according to the present invention, the microscopic observation system includes an LED light source, a microscope, a CCD high-speed camera and a computer. The LED light source is used to provide an excitation light source, the microscope is coupled to the CCD high-speed camera for use, and is connected to the computer to transmit microscopic images in real time. The microscope is arranged above the chip packaging device, and the chip packaging device is adjusted in position by an XYZ three-axis slide table so that the oil-gas phase interface in the microfluidic chip channel is always clearly imaged in the focusing plane of the microscope.

[0016] A method for measuring minimum miscible phase pressure based on microfluidic technology, based on the above-mentioned measurement system, includes the following steps:

[0017] Step (1), encapsulating the microfluidic chip in a chip encapsulation device, connecting pipelines and equipment, building a microfluidic experimental platform, checking the air tightness of the microfluidic experimental platform, and maintaining the system at a set ambient temperature;

[0018] Step (2), using a vacuum pump to evacuate the channel of the microfluidic chip, loading the oil phase sample into a syringe, and using a fluid injection pump to inject the oil phase sample into the channel of the microfluidic chip through the syringe at a certain flow rate, so that the oil phase sample fills the channel;

[0019] Step (3), using the first high-pressure plunger pump to inject the gas phase sample into the microfluidic chip channel in a constant pressure output mode, and at the same time adjusting the second high-pressure plunger pump of the back pressure regulating device to make it the same as the injection pressure of the gas injection device, that is, adjusting the second high-pressure plunger pump to the same pressure as the first high-pressure plunger pump; then using the first high-pressure plunger pump to gradually increase the displacement pressure, and maintain a constant pressure for a certain period of time at each displacement pressure, so that the oil and gas phases reach a mass transfer equilibrium state at this pressure. Due to the high thermal conductivity of silicon, the trace oil phase sample introduced into the microfluidic chip reaches thermal equilibrium conditions within a few minutes; in the boost stage, the output pressure of the back pressure regulating device is correspondingly adjusted to be the same as the injection pressure;

[0020] Step (4), adjust the microscopic observation device to the fluorescence mode. Crude oil itself has a fluorescence effect and can excite fluorescence signals under the corresponding excitation light of the fluorescence microscope. Use a CCD high-speed camera to record the fluorescence image data in the same observation area. In the fluorescence mode of the microscopic observation device, select an objective lens with a suitable magnification, move the XYZ three-axis slide to adjust the position of the chip, so that the oil-gas phase interface in the channel is clearly imaged in the focusing plane of the microscope, observe the change in the fluorescence signal intensity at a fixed position on the oil phase side of the oil-gas phase interface and take a fluorescence video with a constant exposure time;

[0021] Step (5), inputting the fluorescence video data obtained in step (4) into image processing software for image processing, calculating the relative fluorescence intensity of the oil phase side of the oil-gas phase interface at a fixed position away from the phase interface at each displacement pressure, and then determining the minimum miscibility pressure.

[0022] According to a preferred embodiment of the present invention, in step (5), the fluorescence video data obtained in step (4) is input into an image processing software for image processing, and the relative fluorescence intensity of the oil phase side at the oil-gas phase interface at each displacement pressure at a fixed position away from the phase interface is calculated, thereby determining the minimum miscible pressure. The specific process is as follows:

[0023] (5)-1, select a fluorescence image when the fluorescence intensity is stable from the fluorescence video of each displacement pressure, that is, a fluorescence image when the mass transfer equilibrium state is reached under this pressure; the fluorescence intensity is stable, that is, the average gray value in the area at a fixed position away from the oil phase side of the oil-gas phase interface remains unchanged; then, select the same position on the oil phase side of the oil-gas phase interface from the fluorescence image selected at each displacement pressure again, and then circle a certain area from the position to calculate the average gray value in the area;

[0024] (5)-2, calculate the relative fluorescence intensity of the fluorescence image with stable fluorescence intensity at each displacement pressure, and take the average gray value of the initial fluorescence image of the oil phase as the basis. The calculation formula of the relative fluorescence intensity is:

[0025]

[0026] (5)-3, real-time observation of the decay of the fluorescence signal intensity on the oil phase side at the oil-gas phase interface. The displacement pressure corresponding to the relative fluorescence intensity less than 1% is the minimum miscibility pressure. Under a certain pressure, the oil and gas phases undergo interphase mass transfer, that is, the gas phase dissolves in the oil phase and is extracted into the gas phase along with the oil phase, which causes the fluorescence signal of the oil phase to decay. The definition of the minimum miscibility pressure is based on experimental experience: generally, when the oil-gas phase interface disappears, the relative fluorescence signal intensity on the oil phase side is less than 1%. After the relative fluorescence intensity decays to 1%, it remains basically stable, which proves that the oil and gas phases have reached miscibility, and the fluorescence signal intensity of the oil-gas mixture mixed into one phase remains stable.

[0027] Preferably, the image processing software is ImageJ software, which can quickly process fluorescent images in batches, facilitate grayscale analysis, and can be used to directly calculate the grayscale value of a selected area of ​​an image to obtain quantitative data required for the experiment.

[0028] Preferably, the image processing includes contrast adjustment, linear transformation, etc. The overall fluorescence signal of the fluorescent image is enhanced through image processing, and the contrast of regions with different fluorescence signal intensities is clear, which facilitates further direct calculation of the average gray value of the selected region.

[0029] Preferably according to the present invention, the oil phase sample can be crude oil, a single component alkane, a mixture of alkanes with any components and any proportions, etc.

[0030] Preferably, according to the present invention, the microscopic observation device selects the fluorescence mode during observation, and the fluorescence mode refers to converting the LED light source through a filter into an excitation light of a specific wavelength that is easy to fully excite the fluorescence effect of an oil phase sample such as crude oil.

[0031] Preferably, in step (1), the system sets the ambient temperature by the following method: the chip packaging device is externally connected to a circulating water bath thermostat and a temperature sensor, the circulating water bath thermostat is used to adjust the temperature in the microfluidic chip channel, and the temperature sensor is used to monitor the temperature in the microfluidic chip channel, thereby accurately controlling the temperature in the microfluidic chip channel; all pipelines connected to the chip packaging device are surrounded by heating belts for temperature control;

[0032] Alternatively, the system may be placed in a constant temperature box to keep the external ambient temperature of the entire measuring system constant.

[0033] The beneficial effects of the present invention are:

[0034] 1. The method of the present invention can observe the disappearance of the phase interface in real time in situ, directly correspond the oil-gas miscible behavior to the displacement pressure actually measured in real time, and obtain the direct criterion of MMP by monitoring the decay of the fluorescence signal intensity of the oil phase after sufficient oil-gas mass transfer. At the same time, it can record the oil-gas interphase mass transfer phenomenon in the miscible process; observe the oil-gas interphase mass transfer law and phase equilibrium law.

[0035] 2. The method of the present invention can conveniently measure the minimum miscible pressure of oil and gas in channels or porous media areas with different porosities and permeabilities at various micro-nano confinement scales by changing the channel or porous media pattern design of the microfluidic chip.

[0036] 3. The method of the present invention requires less oil and gas samples for pressure measurement experiments, which is 1 to 2 orders of magnitude less than the capillary tube method, and is energy-saving and environmentally friendly.

[0037] 4. The method of the present invention has a concise experimental process, is easy to operate, takes a short time to measure pressure, and is highly efficient. The average time for a single experiment is within 30 minutes.

[0038] 5. The single experimental data of the method of the present invention is sufficient and does not require mathematical interpolation to supplement the data. The experimental method has good repeatability and accurate results. The single pressure measurement experimental data is about 10 times that of the capillary method, and the refined pressure range makes the pressure measurement experimental results more accurate.

[0039] 6. The method of the present invention is applicable to a variety of oil phase samples, and the oil phase samples can be prepared according to the needs. The oil phase samples include but are not limited to crude oil, single component alkanes, and alkanes mixtures with any components and any proportions.

[0040] 7. The method of the present invention is applicable to the measurement of minimum miscible pressure under most reservoir environmental temperatures and pressures, and the set temperature and pressure conditions can be easily changed under the tolerance conditions of the experimental device.

[0041] 8. The channel design of the method of the present invention can make the oil and gas phases contact multiple times during the flow, and the multi-level contact miscible pressure, namely the dynamic miscible pressure, can be obtained by tracking and observing the oil and gas phase interface.

[0042] 9. The method of the present invention can be used to measure the static miscible pressure, that is, to construct a closed channel environment and fill it with an oil phase sample, so as to conveniently observe the interface between the flowing gas phase sample and the oil phase.

[0043] 10. The method of the present invention can be used to measure the primary miscible pressure of hydrocarbon-rich gases such as condensate gas and crude oil. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 It is a structural schematic diagram of a minimum miscible phase pressure measurement system based on microfluidic technology in the present invention;

[0045] Figure 2 This is a comparison diagram of the in-situ real-time observation of the oil-gas phase interface region before and after the miscibility occurs in the method of the present invention, where the a end is the gas phase and the b end is the oil phase;

[0046] Figure 3 is a graph showing the relationship between displacement pressure and relative fluorescence intensity for determining the minimum miscible pressure according to fluorescence intensity decay in Example 1;

[0047] Figure 4 This is a curve diagram showing the relationship between displacement pressure and recovery factor for determining the minimum miscible pressure based on the recovery factor in Comparative Example 1 using the capillary tube method.

[0048] 1. First connection port, 2. Second connection port, 3. Third connection port, 4. Fluid injection pump, 5. First CO2 gas cylinder, 6. Second CO2 gas cylinder, 7. First high-pressure plunger pump, 8. Second high-pressure plunger pump, 9. First pressure sensor, 10. Second pressure sensor, 11. Third pressure sensor, 12. Single channel, 13. Vacuum pump, 14. Round base particles. DETAILED DESCRIPTION

[0049] The present invention will be further described below in conjunction with the embodiments and the accompanying drawings, but is not limited thereto.

[0050] Example 1

[0051] A minimum miscible pressure measurement system based on microfluidics technology, such as Figure 1As shown, it includes a microfluidic experimental platform and a microfluidic chip. The microfluidic experimental platform includes a fluid control device, a chip packaging device and a microscopic observation device. The fluid control device is used to control the flow rate or flow velocity of the oil phase sample to be tested and the gas phase sample such as CO2 in the microfluidic chip, and to adjust the injection pressure of the gas phase sample, that is, to control the displacement pressure in the microfluidic chip; the chip packaging device is used to package the microfluidic chip; the microscopic observation device is used to observe the change of the fluorescence signal intensity on the oil phase side in the phase interface area of ​​the microfluidic chip under different displacement pressures, and obtain fluorescence image video recordings. After image processing, the relative fluorescence intensity under different displacement pressures is obtained, and then the minimum miscible pressure is obtained.

[0052] The fluid control device includes a gas injection device, an oil phase sample injection device, a vacuum device and a back pressure regulating device. The main instruments include a high-pressure plunger pump, a fluid injection pump 4, a syringe, a vacuum pump 13, a pressure sensor, etc.;

[0053] like Figure 1 As shown, the microfluidic chip is designed with three connection ports connected to the external pipeline, namely the first connection port 1, the second connection port 2 and the third connection port 3. The first connection port 1 is connected to the gas injection device, and the gas injection device includes a first CO2 gas cylinder 5 and a first high-pressure plunger pump 7 that are interconnected. The first high-pressure plunger pump 7 is connected to the first connection port 1, and a ball valve and a first pressure sensor 9 are provided on the pipeline between the first high-pressure plunger pump 7 and the first connection port 1. The first pressure sensor 9 is used to monitor the CO2 injection pressure. The first high-pressure plunger pump 7 is used to inject CO2 into the microfluidic chip channel at a certain displacement pressure; the high-pressure plunger pump has a constant pressure output mode and a constant current output mode, which can accurately control the pressure or flow rate of gas injection.

[0054] The second connection port 2 is connected to the oil phase sample injection device, which includes a fluid injection pump 4 and a syringe that are interconnected. The fluid injection pump 4 is used to inject the oil phase sample into the microfluidic chip channel at a certain flow rate; the fluid injection pump 4 can accurately control the flow rate of the oil phase fluid. A ball valve is provided on the connection pipeline between the fluid injection pump 4 and the second connection port 2.

[0055] The third connection port 3 is connected to a vacuum device and a back pressure regulating device through a connecting tee. The vacuum device mainly includes a vacuum pump 13, which is connected to the third connection port 3, and a second pressure sensor 10 is arranged on the pipeline between the vacuum pump 13 and the third connection port 3; the vacuum pipeline can evacuate the gas in the chip channel through the vacuum pump 13, which is conducive to the subsequent injection of the oil phase sample; the back pressure regulating device includes a second CO2 gas cylinder 6 and a second high-pressure plunger pump 8 that are interconnected, the second high-pressure plunger pump 8 is connected to the third connection port 3, and a third pressure sensor 11 is arranged on the connecting pipeline between the back pressure regulating device and the microfluidic chip. The back pressure regulating device cooperates with the flow rate or pressure setting of the fluid injection device, and uses the second high-pressure plunger pump 8 to actively adjust the flow rate and pressure of the microfluidic chip channel.

[0056] The chip packaging device is externally connected to a circulating water bath thermostat and a temperature sensor. The circulating water bath thermostat is used to adjust the temperature in the microfluidic chip channel, and the temperature sensor is used to monitor the temperature in the microfluidic chip channel, thereby accurately monitoring the temperature in the microfluidic chip channel.

[0057] The chip packaging device includes a stainless steel plate frame cover, a quartz glass cover, a microfluidic chip, a stainless steel fixture carrier, an XYZ three-axis slide, and O-rings, NPT adapters, ferrules, screws and other pressure-resistant connectors and fasteners from top to bottom. The specific structure of the chip packaging device includes but is not limited to Chinese patent document CN111701631A, a temperature-controlled pressure-resistant fixture device for high-throughput visualization microfluidic chips.

[0058] The microfluidic chip is prepared by anodic bonding of a single crystal silicon wafer and quartz glass. A channel pattern is etched on the single crystal silicon wafer to simulate the real reservoir rock structure. The channel pattern includes channels or porous medium areas of micro-nano confined sizes. By changing the channel pattern design of the microfluidic chip, the minimum miscible pressure of oil and gas can be measured in channels with different porosities and permeabilities at a variety of micro-nano confinement scales. The pressure and temperature range that this type of chip can withstand is relatively high (P≤40MPa, T≤250℃). The design of the channel pattern of the present invention refers to the capillary model of the capillary method, which is designed as a single channel 12 with a length-to-diameter ratio of 500:1, so that the fluid flow in the channel is close to one-dimensional flow. In addition, in this embodiment, if Figure 1 As shown, the channel pattern is a single channel 12, and a circular base particle 14 is arranged in the single channel 12, so that the oil and gas phases can contact each other multiple times when flowing in the channel and truly multi-stage contact miscibility, i.e., dynamic miscibility, so that the minimum miscibility pressure measured is closer to the actual multi-stage contact miscibility minimum miscibility pressure. The circular base particle 14 is obtained by etching a single crystal silicon wafer.

[0059] The porous medium area refers to the microfluidic chip as a physical model to simulate the real reservoir rock environment. There are some interconnected channels designed within a certain area. The interconnected channels can better simulate the pores of the porous media in the reservoir rock, so this area is called the porous medium area.

[0060] The microscopic observation system includes an LED light source, a microscope, a CCD high-speed camera and a computer; the LED light source is used to provide an excitation light source, the microscope is coupled to the CCD high-speed camera and connected to the computer to transmit the microscopic image in real time; the microscope is set above the chip packaging device, and the chip packaging device is adjusted in position by an XYZ three-axis slide table so that the oil-gas phase interface in the microfluidic chip channel is always clearly imaged within the focusing plane of the microscope.

[0061] Example 2

[0062] A method for measuring minimum miscible phase pressure based on microfluidic technology, based on the measurement system provided in Example 1, comprises the following steps:

[0063] Step (1), encapsulating the microfluidic chip in a chip encapsulation device, connecting pipelines and equipment, building a microfluidic experimental platform, checking the air tightness of the microfluidic experimental platform, and maintaining the system at a set ambient temperature;

[0064] In step (1), the system sets the ambient temperature by the following method: the chip packaging device is externally connected to a circulating water bath thermostat and a temperature sensor, the circulating water bath thermostat is used to adjust the temperature in the microfluidic chip channel, and the temperature sensor is used to monitor the temperature in the microfluidic chip channel, thereby accurately controlling the temperature in the microfluidic chip channel; all pipelines connected to the chip packaging device are surrounded by heating belts for temperature control;

[0065] Or place the system in a constant temperature box to keep the external ambient temperature of the entire measurement system constant.

[0066] Step (2), using a vacuum pump to evacuate the channel of the microfluidic chip, loading the oil phase sample into a syringe, and using a fluid injection pump 4 to inject the oil phase sample into the channel of the microfluidic chip through the syringe at a certain flow rate, so that the oil phase sample fills the channel;

[0067] Step (3), using the first high-pressure plunger pump 7 to inject the gas phase sample into the microfluidic chip channel in a constant pressure output mode, and at the same time adjusting the second high-pressure plunger pump 8 of the back pressure regulating device to make it the same as the injection pressure of the gas injection device, that is, adjusting the second high-pressure plunger pump 8 to the same pressure as the first high-pressure plunger pump 7; then using the first high-pressure plunger pump 7 to gradually increase the displacement pressure, and maintain a constant pressure for a certain period of time at each displacement pressure, so that the oil and gas phases reach a mass transfer equilibrium state at this pressure. Due to the high thermal conductivity of silicon, the trace oil phase sample introduced into the microfluidic chip reaches thermal equilibrium conditions within a few minutes; in the boost stage, the output pressure of the back pressure regulating device is adjusted accordingly to make it the same as the injection pressure;

[0068] Step (4), adjust the microscopic observation device to the fluorescence mode. The crude oil itself has a fluorescence effect and can excite a fluorescence signal under the corresponding excitation light of the fluorescence microscope. Use a CCD high-speed camera to record the fluorescence image data in the same observation area, such as Figure 2 The dotted line box area is shown. Figure 2 This is a comparison chart of in-situ real-time observation of the oil-gas interface area before and after the mixing occurs. The a end is the gas phase and the b end is the oil phase. In the fluorescence mode of the microscopic observation device, select an objective lens with a suitable magnification, move the XYZ three-axis slide to adjust the position of the chip, so that the oil-gas interface in the channel is clearly imaged in the focal plane of the microscope, observe the change in the fluorescence signal intensity at a fixed position on the oil phase side of the oil-gas interface and take a fluorescence video with a constant exposure time;

[0069] Step (5), input the fluorescence video data obtained in step (4) into the image processing software for image processing, calculate the relative fluorescence intensity of the oil phase side of the oil-gas phase interface at each displacement pressure at a fixed position away from the phase interface, and then determine the minimum miscible pressure. The specific process is:

[0070] (5)-1, select the fluorescence image when the fluorescence intensity is stable from the fluorescence video of each displacement pressure, that is, the fluorescence image when the mass transfer equilibrium state is reached under this pressure; the fluorescence intensity is stable, that is, the average gray value in the area at a fixed position away from the oil phase side of the oil-gas phase interface remains unchanged; then, select the same position on the oil phase side of the oil-gas phase interface from the fluorescence image selected at each displacement pressure again, and then circle a certain area from this position to calculate the average gray value in the area;

[0071] (5)-2, calculate the relative fluorescence intensity of the fluorescence image with stable fluorescence intensity at each displacement pressure, and take the average gray value of the initial fluorescence image of the oil phase as the basis. The calculation formula of the relative fluorescence intensity is:

[0072]

[0073] (5)-3, real-time observation of the decay of the fluorescence signal intensity on the oil phase side at the oil-gas phase interface. The displacement pressure corresponding to the relative fluorescence intensity less than 1% is the minimum miscibility pressure. Under a certain pressure, the oil and gas phases undergo interphase mass transfer, that is, the gas phase dissolves in the oil phase and is extracted into the gas phase along with the oil phase, thereby causing the fluorescence signal of the oil phase to decay. The definition of the minimum miscibility pressure is based on experimental experience: generally, when the oil-gas phase interface disappears, the relative fluorescence signal intensity on the oil phase side is less than 1%. After the relative fluorescence intensity decays to 1%, the relative fluorescence intensity remains basically stable, which proves that the oil and gas phases have reached miscibility, and the fluorescence signal intensity of the oil-gas mixture mixed into one phase remains stable.

[0074] Image processing software is ImageJ software. Image processing includes contrast adjustment, linear transformation, etc.

[0075] The microscope observation device selects the fluorescence mode during observation. The fluorescence mode refers to converting the LED light source through a filter into an excitation light of a specific wavelength that is easy to fully excite the fluorescence effect of oil phase samples such as crude oil.

[0076] The microfluidic chip is provided with channels or porous media areas of micro-nano confined size inside, and channels with different widths and depths can be designed according to requirements; or porous media areas with different porosities and permeabilities can be designed according to requirements.

[0077] The porous medium area refers to the microfluidic chip as a physical model to simulate the real reservoir rock environment. There are some interconnected channels designed within a certain area. The interconnected channels can better simulate the pores of the porous media in the reservoir rock, so this area is called the porous medium area.

[0078] In this embodiment, the method of the present invention is used to measure the carbon dioxide and dodecane (C 12 H 26 ) is the minimum miscible pressure at 20°C. The specific implementation steps are as follows:

[0079] (1) Connect the pipeline according to the experimental process, and package it as follows Figure 1 The microfluidic chip with the channel pattern shown in the figure was used to build a microfluidic pressure measurement experimental platform. The CO2 gas line was connected, and 2.5MPa of CO2 was introduced using a high-pressure plunger pump in constant pressure injection mode. The pressure sensor values ​​of each branch pipeline were monitored to check the air tightness of the microfluidic pressure measurement experimental platform.

[0080] (2) The experimental device is placed in a constant temperature box, and the experimental environment temperature is maintained at 20°C by the constant temperature box; the temperature of the pressure measuring chip is precisely controlled to be constant at 20°C by using an external circulating water bath connected to the chip packaging platform, and the chip temperature is monitored in real time by a temperature sensor.

[0081] (3) Figure 1The pressure measuring chip shown has three connection ports, namely the first connection port 1, the second connection port 2 and the third connection port 3. The third connection port 3 is externally connected to a vacuum pipeline system, and the microfluidic chip channel is evacuated by a vacuum pump 13, and the vacuum degree is 78.15 kPa.

[0082] (4) Prepare the oil phase sample and fill the microfluidic chip channel with the oil phase sample. In this embodiment, the dodecane oil phase sample is prepared with 0.01wt% Nile red as a fluorescent agent, and the oil phase sample is injected into the microfluidic chip channel at a flow rate of 0.1mL / min through the fluid injection pump 4.

[0083] (5) Use a high-pressure plunger pump to pass CO2 into the microfluidic chip channel at a constant pressure of 2.5 MPa, and adjust the back pressure controller to the same pressure. Adjust the high-pressure plunger pump to gradually increase the pressure to 5 MPa at intervals of 0.1 MPa. Then gradually increase the pressure at intervals of 0.01 MPa, and maintain a constant pressure for 1 minute at each pressure stage to allow the oil-gas mass transfer to reach a balanced state. The back pressure controller and the high-pressure plunger pump maintain synchronous pressure settings.

[0084] (6) The oil-gas interface region was observed using a fluorescence microscope, and the fluorescence image video of the constant pressure stage at each displacement pressure was recorded in real time using a CCD high-speed camera at a frame rate of 40 fps, i.e., an exposure time of 25 ms.

[0085] (7) Import the fluorescence image recording of each displacement pressure into the image processing software ImageJ, and select the fluorescence image with stable fluorescence intensity of the oil phase at each displacement pressure. In the fluorescence image corresponding to each selected displacement pressure, select the same position on the oil phase side of the oil-gas interface, circle the plane area of ​​the same area, and calculate the average gray value within the area. Based on the gray value of the initial fluorescence image of the oil phase, set its relative fluorescence intensity to 1, and calculate the relative fluorescence intensity of the selected area at each displacement pressure.

[0086] (8) Figure 3 As shown, a displacement pressure and relative fluorescence intensity curve was drawn, and the pressure corresponding to the relative fluorescence intensity being less than 1% was defined as the minimum miscibility pressure. The minimum miscibility pressure of CO2 and dodecane at 20°C was measured to be 5.621 MPa.

[0087] Comparative Example 1

[0088] This comparative example uses the capillary method to measure the minimum miscible pressure. The traditional minimum miscible pressure determination is carried out in a capillary filled with quartz sand or glass beads, so it is called the capillary method. During the experiment, the injection pressure is continuously changed, and the gas is injected into the sand-filled capillary model. The volume or mass of the produced oil phase sample is weighed, and the recovery rate of the oil phase sample when 1.2 times the pore volume of the injected gas is injected at each pressure is calculated. Based on the recovery rate data calculated at each displacement pressure, a curve of the relationship between the recovery rate and the displacement pressure when 1.2 times the pore volume of the injected gas is injected is drawn, as shown in FIG. Figure 4 As shown, the displacement pressure corresponding to the intersection of the immiscible section and the miscible section fitting curve is the minimum miscible pressure. For specific operation methods, please refer to the document SY / T 6573-2016 "Minimum miscible pressure experimental determination method - capillary method". Shen Pingping [1] et al. used the capillary method to measure the minimum miscibility pressure of a crude oil sample with CO2 at 101.6°C to be 27.45 MPa. [2] et al. used the capillary method to measure the minimum miscibility pressure of a certain crude oil and CO2 at 113.8℃, which was 27.76MPa. [1] Shen Pingping, Huang Lei. Study on the multiphase and multicomponent seepage mechanism of carbon dioxide-crude oil [J]. Acta Petrolei Sinica, 2009, 30(02): 247-251. [2] Wu Junfeng, Liu Baozhong, Liu Daojie, Wang Changquan, Li Yinghui, Liu Guohua. Carbon dioxide miscible fracturing huff-and-puff experiment [J / OL]. Special Oil and Gas Reservoirs: 1-9 [2022-09-20].

[0089] The capillary method has the disadvantages of complicated experimental process, long experimental time, few measured data, large sample consumption, and inability to observe oil and gas miscibility. This method performs linear fitting based on the recovery factor data points, and takes the pressure corresponding to the intersection of the fitting curve as the minimum miscibility pressure. The minimum miscibility pressure obtained is not the actual experimental measurement value. The recovery factor data used in this method is secondary experimental data, which cannot accurately express the corresponding relationship between the phase behavior changes such as oil and gas miscibility and the disappearance of the phase interface and pressure, that is, it is impossible to know whether the oil and gas phases are miscible at the minimum miscibility pressure measured by this method.

[0090] The minimum miscible pressure measurement method provided by the present application has the following advantages over the capillary method: First, the required consumption of samples such as oil and gas is small, which is 1 to 2 orders of magnitude less than the capillary method, and is energy-saving and environmentally friendly. Secondly, the pressure measurement takes a short time and is highly efficient. The average time for a single test is within 30 minutes. Furthermore, the single experimental data is sufficient and does not require mathematical interpolation to supplement the data. The experimental method has good repeatability and accurate results. The single pressure measurement test data is about 10 times that of the capillary method, and the refined pressure range makes the pressure measurement test results more accurate.

Claims

1. A minimum miscible pressure measurement system based on microfluidics technology, characterized in that: The microfluidic experimental platform comprises a microfluidic chip, wherein the microfluidic experimental platform comprises a fluid control device, a chip packaging device and a microscopic observation device, wherein the fluid control device is used to control the flow rate or flow velocity of the oil phase sample and the gas phase sample to be tested in the microfluidic chip respectively, and to adjust the injection pressure of the gas phase sample, that is, to control the displacement pressure in the microfluidic chip; The fluid control device includes a gas injection device, an oil phase sample injection device, a vacuum device and a back pressure regulating device; The microfluidic chip is provided with three connection ports connected to the external pipeline, namely, a first connection port, a second connection port and a third connection port, the first connection port is connected to the gas injection device, the gas injection device includes a first CO2 gas cylinder and a first high-pressure plunger pump which are connected to each other, the first high-pressure plunger pump is connected to the first connection port, and the first high-pressure plunger pump is used to inject CO2 into the microfluidic chip channel at a certain displacement pressure; The second connection port is connected to an oil phase sample injection device, which includes a fluid injection pump and a syringe that are connected to each other, and the fluid injection pump is used to inject the oil phase sample into the microfluidic chip channel at a certain flow rate; The third connection port is connected to a vacuum pump and a back pressure regulating device, the vacuum pump includes a vacuum pump, the vacuum pump is connected to the third connection port, and a second pressure sensor is arranged on the pipeline between the vacuum pump and the third connection port; the back pressure regulating device includes a second CO2 gas cylinder and a second high-pressure plunger pump that are interconnected, and a third pressure sensor is arranged on the connecting pipeline between the back pressure regulating device and the microfluidic chip; the back pressure regulating device cooperates with the flow rate or pressure setting of the fluid injection device, and uses the second high-pressure plunger pump to actively adjust the flow rate and pressure of the microfluidic chip channel; The chip packaging device is used to package the microfluidic chip; the microscopic observation device is used to observe the change in fluorescence signal intensity on the oil phase side of the phase interface area of ​​the microfluidic chip under different displacement pressures, and obtain fluorescence image video. After image processing, the relative fluorescence intensity under different displacement pressures is obtained, and then the minimum miscible pressure is obtained.

2. According to claim 1, a minimum miscible pressure measurement system based on microfluidic technology is characterized in that: The microfluidic chip is prepared by anodic bonding of a single crystal silicon wafer and quartz glass. A channel pattern is etched on the single crystal silicon wafer to simulate the structure of real oil reservoir rock formations. The channel pattern includes channels or porous medium areas with micro-nano confined sizes. Channels with different widths and depths are designed according to needs. Or design porous media areas with different porosity and permeability according to needs.

3. The minimum miscible pressure measurement system based on microfluidic technology according to claim 1, characterized in that: The microscopic observation system includes an LED light source, a microscope, a CCD high-speed camera and a computer. The LED light source is used to provide an excitation light source. The microscope is coupled to the CCD high-speed camera and connected to the computer to transmit the microscopic image in real time. The microscope is set above the chip packaging device, and the chip packaging device is adjusted in position by an XYZ three-axis slide table so that the oil-gas phase interface in the microfluidic chip channel is always imaged within the focusing plane of the microscope.

4. A method for measuring minimum miscible pressure based on microfluidic technology, according to any one of claims 1-3, characterized in that: The steps include: Step (1), encapsulating the microfluidic chip in a chip encapsulation device, connecting pipelines and equipment, building a microfluidic experimental platform, checking the air tightness of the microfluidic experimental platform, and maintaining the system at a set ambient temperature; Step (2), using a vacuum pump to evacuate the channel of the microfluidic chip, and using a fluid injection pump to inject the oil phase sample into the channel of the microfluidic chip through a syringe at a certain flow rate, so that the oil phase sample fills the channel; Step (3), using the first high-pressure plunger pump to inject the gas phase sample into the microfluidic chip channel in a constant pressure output mode, and at the same time adjusting the second high-pressure plunger pump of the back pressure regulating device to make it the same as the injection pressure of the gas injection device, that is, adjusting the second high-pressure plunger pump to the same pressure as the first high-pressure plunger pump; then using the first high-pressure plunger pump to gradually increase the displacement pressure, and maintaining a constant pressure for a certain period of time at each displacement pressure, so that the oil and gas phases reach a mass transfer equilibrium state at this pressure; During the pressure-increasing stage, the output pressure of the back pressure regulating device is adjusted accordingly to be the same as the gas injection pressure; Step (4), adjusting the microscopic observation device to the fluorescence mode, using a CCD high-speed camera to record the fluorescence image data in the same observation area, moving the XYZ three-axis slide table to adjust the position of the chip so that the oil-gas phase interface in the channel is imaged in the focal plane of the microscope, observing the change in the fluorescence signal intensity at a fixed position of the phase interface on the oil phase side containing the oil-gas phase interface and taking a fluorescence video; Step (5), inputting the fluorescence video data obtained in step (4) into image processing software for image processing, calculating the relative fluorescence intensity of the oil phase side of the oil-gas phase interface at a fixed position away from the phase interface at each displacement pressure, and then determining the minimum miscibility pressure.

5. The method for measuring minimum miscible pressure based on microfluidic technology according to claim 4, characterized in that: Step (5), input the fluorescence video data obtained in step (4) into the image processing software for image processing, calculate the relative fluorescence intensity of the oil phase side of the oil-gas phase interface at each displacement pressure at a fixed position away from the phase interface, and then determine the minimum miscible pressure. The specific process is: (5)-1, select the fluorescence image when the fluorescence intensity is stable from the fluorescence video of each displacement pressure. The stable fluorescence intensity means that the average gray value in the area at a fixed position on the oil phase side of the oil-gas phase interface at the distance from the phase interface remains unchanged; Then, the same position on the oil phase side of the oil-gas phase interface is selected again from the fluorescent image selected at each displacement pressure, and a certain area range is circled from the position, and the average gray value in the area range is calculated; (5)-2, calculate the relative fluorescence intensity of the fluorescence image with stable fluorescence intensity at each displacement pressure, and take the average gray value of the initial fluorescence image of the oil phase as the basis. The calculation formula of the relative fluorescence intensity is: ， (5)-3, real-time observation of the decay of the fluorescence signal intensity on the oil phase side at the oil-gas phase interface. The displacement pressure corresponding to the relative fluorescence intensity being less than 1% is the minimum miscible pressure.

6. The method for measuring minimum miscible pressure based on microfluidic technology according to claim 4, characterized in that: The image processing software is ImageJ software.

7. The method for measuring minimum miscible phase pressure based on microfluidic technology according to claim 4, characterized in that: The image processing includes contrast adjustment and linear transformation.

8. The method for measuring minimum miscible pressure based on microfluidic technology according to claim 4, characterized in that: The oil phase sample is crude oil, single-component alkane, or a mixture of alkanes with any components and any proportions.

9. The method for measuring minimum miscible pressure based on microfluidic technology according to claim 4, characterized in that: In step (1), the system sets the ambient temperature by the following method: the chip packaging device is externally connected to a circulating water bath thermostat and a temperature sensor, the circulating water bath thermostat is used to adjust the temperature in the microfluidic chip channel, and the temperature sensor is used to monitor the temperature in the microfluidic chip channel, thereby accurately controlling the temperature in the microfluidic chip channel; all pipelines connected to the chip packaging device are surrounded by heating belts for temperature control; Alternatively, the system may be placed in a constant temperature box to keep the external ambient temperature of the entire measuring system constant.

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