Method for characterizing carbon dioxide flooding efficiency-burying capacity of fractured tight reservoir under micro-nano scale and experimental device for characterizing carbon dioxide flooding efficiency-burying capacity of fractured tight reservoir

Abstract

The invention belongs to the technical field of oil and gas field development, and particularly relates to a method for representing fractured tight reservoir carbon dioxide oil displacement efficiency-burying capacity under the micro-nano scale and an experimental device thereof. According to the characterization method, on the basis of a micro-fluidic chip, the remaining oil is subjected to image processing, color segmentation, morphological analysis and a machine learning algorithm, the occurrence form of crude oil is automatically recognized, the CO2 huff and puff process is analyzed, and the total burying amount of CO2 is calculated. The experimental device comprises a high-temperature and high-pressure visible kettle for placing a micro-fluidic chip, and a CO2 piston container, a crude oil piston container and a fracturing fluid piston container which are sequentially arranged in parallel. According to the characterization method, the remaining oil occurrence analysis, the fracturing fluid displacement influence and the accurate characterization of the CO2 oil displacement-burying process can be completed at the same time under the micro-nano scale, the storage amount of CO2 in the gas phase and the oil phase is accurately calculated, the limitation of a traditional method on the assumption of CO2 solubility uniformity is broken through, and the CO2 storage amount estimation accuracy is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of oil and gas field development, and particularly relates to a method for characterizing the oil displacement efficiency - storage amount of carbon dioxide in a fractured tight oil reservoir at the micro - nano scale and an experimental device therefor. Background Art

[0002] Tight oil reservoirs generally have characteristics such as ultra - low permeability, small pore - throat radius, and developed micro - fractures. The crude oil in such reservoirs is dispersed, and the reservoir seepage capacity is limited, resulting in a low recovery rate of tight oil reservoirs. Therefore, during the development process, it is necessary to use hydraulic fracturing to artificially create fractures to improve the reservoir physical properties and seepage capacity to achieve efficient development. At the same time, CO 2 flooding is also one of the key technologies for improving the recovery rate (EOR) of tight oil. CO 2 has a dissolution and extraction effect, which can not only effectively reduce the viscosity of crude oil, increase the swept volume, but also form a weak acid environment in the reservoir to improve the reservoir permeability. Moreover, CO 2 injection can achieve long - term geological storage.

[0003] However, there is still a lack of accurate characterization methods for the diffusion, dissolution, and migration laws of CO 2 in the oil phase. In particular, the microscopic seepage law of CO 2 in tight reservoirs is not clear. Especially after undergoing fracturing transformation, the oil displacement - storage process of CO 2 is simultaneously affected by the dual characteristics of the fracture network and the matrix. Existing studies are difficult to evaluate the dynamic distribution and storage capacity of CO 2 in the oil phase at the micro - nano scale, resulting in insufficient understanding of the full - cycle seepage law of CO 2 in tight oil reservoirs.

[0004] Although traditional core experiments can study the fluid migration law, it is difficult to accurately characterize the dissolution, diffusion, and swept process of CO 2 at the micro - nano scale. Moreover, the core will show irreversible damage with a pore collapse rate > 20%, resulting in low experimental repeatability and significant data deviation.

[0005] In summary, it is urgent to research and develop a method that can accurately characterize the oil displacement efficiency and storage situation of CO 2 in a fractured tight oil reservoir at the micro - nano scale. Summary of the Invention

[0006] The purpose of the present invention is to provide a method for characterizing the oil displacement efficiency - storage amount of carbon dioxide in a fractured tight oil reservoir at the micro - nano scale. This characterization method can simultaneously complete the analysis of the remaining oil occurrence, the influence of fracturing fluid displacement, and the accurate characterization of the CO 2 oil displacement - storage process at the micro - nano scale, and accurately calculate the CO 2The storage amounts in the gas phase and the oil phase break through the limitations of the uniform solubility assumption of CO in the traditional method, improving the estimation accuracy of the CO storage amount. Visual quantitative characterization of the CO flooding and sequestration process is realized to optimize the EOR plan and improve the development efficiency of tight reservoirs. 2 The storage amounts in the gas phase and the oil phase break through the limitations of the uniform solubility assumption of CO in the traditional method, improving the estimation accuracy of the CO storage amount. Visual quantitative characterization of the CO flooding and sequestration process is realized to optimize the EOR plan and improve the development efficiency of tight reservoirs. 2 The storage amounts in the gas phase and the oil phase break through the limitations of the uniform solubility assumption of CO in the traditional method, improving the estimation accuracy of the CO storage amount. Visual quantitative characterization of the CO flooding and sequestration process is realized to optimize the EOR plan and improve the development efficiency of tight reservoirs. 2 The storage amounts in the gas phase and the oil phase break through the limitations of the uniform solubility assumption of CO in the traditional method, improving the estimation accuracy of the CO storage amount. Visual quantitative characterization of the CO flooding and sequestration process is realized to optimize the EOR plan and improve the development efficiency of tight reservoirs.

[0007] The specific technical solution is as follows: A method for characterizing the CO flooding efficiency - buried amount in a fractured tight reservoir at the micro - nano scale, which is based on a microfluidic chip. Through image processing, color segmentation, morphological analysis and machine learning algorithms for the remaining oil, the occurrence form of crude oil is automatically identified, and the CO 2 huff - puff process is analyzed to calculate the total buried amount of CO. 2 The specific steps are as follows:

[0008] The specific steps are as follows: S1. Microfluidic experiment data acquisition: Based on a microfluidic chip to simulate the CO displacement process in a fractured tight reservoir, and collect microfluidic experiment images. High - resolution optical microscopes or nuclear magnetic resonance (NMR) equipment can be used to obtain microfluidic experiment images and save them in standard formats (such as.tiff,.png). 2 S1. Microfluidic experiment data acquisition: Based on a microfluidic chip to simulate the CO displacement process in a fractured tight reservoir, and collect microfluidic experiment images. High - resolution optical microscopes or nuclear magnetic resonance (NMR) equipment can be used to obtain microfluidic experiment images and save them in standard formats (such as.tiff,.png).

[0009] S2. Image pre - processing: Perform image pre - processing on the collected microfluidic experiment images, including image grayscale conversion, Gaussian filtering for denoising, and Sobel edge detection for enhancement. ImageJ can be used for batch image processing to improve the analysis efficiency.

[0010] Image grayscale conversion: Convert the experimental image to a grayscale image through Image→Type→8 - bit in ImageJ to reduce the computational complexity.

[0011] Gaussian filtering for denoising: Use Gaussian filtering to remove image noise and enhance the contrast of oil - water - pore; sigma (σ)=2.

[0012] Sobel edge detection for enhancement: Use the Sobel operator for edge enhancement to improve the accuracy of oil - water boundary recognition.

[0013] S3. Oil - water region segmentation: First, segment oil and water in the HSB color space; then, generate a binary mask (Mask) to extract the oil - phase region.

[0014] Use the ColorThreshold function to segment oil and water based on the HSB color space: The color hue range is 109 - 204 (corresponding to the cyan - green to purple region in the HSV color ring), the saturation interval is 53 - 255, and the brightness ≤ 255.

[0015] S4. Morphological feature extraction: First, the area, perimeter, and roundness of the oil phase region are statistically analyzed; then, the porosity and wettability change are calculated.

[0016] Use the AnalyzeParticles function to statistically analyze the area, perimeter, and roundness of the oil phase region, where the area is greater than a certain threshold and the roundness ranges from 0.5 to 1.0.

[0017] Porosity is the total area of pores divided by the total area of the chip etching region. The total pore area is calculated by identifying and statistically analyzing the pore regions in the matrix region of the microfluidic chip used. The total area of the chip etching region is obtained by multiplying the image width by the height.

[0018] The wettability change is obtained by measuring the contact angle between crude oil or water and the pore wall surface. When it is greater than 90°, it is oil-wet; when it is less than 90°, it is water-wet.

[0019] S5. Classification and identification of remaining oil occurrence patterns: First, after completing the oil-water region segmentation and morphological feature extraction, combined with geometric features and spatial distribution information, the remaining oil is divided into three typical occurrence types: isolated droplet remaining oil, film-like remaining oil, and capillary-bound remaining oil.

[0020] Then, identify and count the number, average diameter, and area ratio in the total oil phase of the isolated droplet oil. Isolated oil droplets usually show the characteristics of being separately distributed, round in shape, small in area, and discontinuous with other oil phases. The determination criteria are: aspect ratio close to 1, area < threshold, and closed edge.

[0021] Identify and count the oil film thickness and pore wall coverage rate of the film-like oil. This type of oil is distributed on the pore surface, usually in a strip shape, adhering to the wall, and distributed along the pore wall, having a high aspect ratio and low roundness. Calculate the skeleton length ratio for classification (such as aspect ratio greater than 3 and roundness less than 0.5). The film thickness is obtained by measuring the thickness of the oil film, and the pore wall coverage rate is the sum of the areas of the film-like oil in contact with the pores divided by the total inner surface area of all pores.

[0022] Identify and count the spatial distribution and relative content of capillary-bound remaining oil. This type of oil is mainly bound by capillary force, commonly found in blind-end pores or microfracture regions, showing small size, irregular shape, low roundness, high aspect ratio, distributed in closed or narrow pores, and difficult to move with the fluid after displacement. Usually, the screening conditions are area less than the set threshold and roundness less than 0.3.

[0023] S6. CO 2 Analysis of the huff and puff process: a. Data acquisition: Process the obtained microfluidic experiment images using ImageJ or Matlab to identify the CO 2 gas phase region and the position of the oil-gas interface.

[0024] Use ImageJ to count CO 2 The number of pixels in the gas phase region (through the Analyze Particles function of ImageJ), and calculate the CO according to the image resolution 2 The two-dimensional area of the region, and combine with the etching depths of the main channel and the matrix region of the microfluidic chip to convert the two-dimensional area into a three-dimensional volume, and calculate the CO 2 Filling volume.

[0025] Extract the distribution area of CO in the chip by color threshold segmentation method, identify and quantify the gaseous CO 2 and liquid CO 2 separately. This segmentation method is similar to the identification and extraction of the remaining oil morphology mentioned above, and is divided into image preprocessing; threshold segmentation to improve the contrast between the CO 2 region and other regions. Identify the gaseous CO 2 through the phase interface between the gaseous CO 2 and the oil phase of the crude oil, and identify the liquid CO 2 through the different color thresholds between the liquid CO 2 and the crude oil, and then the gaseous CO 2 and the liquid CO 2 occupied areas can be identified. The respective proportions of the gaseous CO 2 and the liquid CO 2 are obtained by dividing the areas they occupy by the total area of the distribution region, which is equal to the volume proportion.

[0026] b. Calculate the CO 2 swept range: 2 First, calculate the change of the oil-water interface after the CO enters: Use the color threshold segmentation algorithm to identify the CO 2 , crude oil and water three-phase regions; use the contour extraction algorithm (Canny edge detection) to extract the oil-water interface; mark the interface position and calculate its total length or the pixel positions it occupies; compare the oil-water interface positions at different times, and calculate the change distance of the interface front position per unit time, so as to identify and quantify the changes in the morphology, position and contact relationship of the oil-water interface over time during the CO 2 displacement process; judge the invasion rate of the CO 2 into the oil phase, reflecting the Jamin effect or the non-steady fingering mechanism.

[0027] Statistical expansion rate of the CO 2 swept area at different time points: Set several time nodes (such as t 1 , t 2 , t 3 ), t 2...), compare the affected area at each moment; divide the area difference by the time difference to obtain the average expansion rate of the affected area of CO 2 and calculate the propagation direction and anisotropic expansion velocity. The expansion rate of the CO 2 affected area at different time points refers to quantifying the 2 evolution law of its affected range over time after CO enters, including the pore volume fraction occupied, the front propagation velocity, etc.

[0028] c. Calculate the oil displacement efficiency of CO 2 huff and puff on the remaining oil: (1) Calculate the saturation of isolated-droplet remaining oil, film-like remaining oil, and capillary-bound oil respectively before and after CO 2 displacement; Among them, the calculation formula for the saturation of various remaining oils before CO 2 displacement is as follows: ; In the formula: S boil,i —— The saturation of 2 type of remaining oil before CO i displacement; A before,oil-i —— The volume of 2 type of remaining oil before CO i displacement; A total —— The total pore volume.

[0029] The calculation formula for the saturation of various remaining oils after CO 2 displacement is as follows: ; In the formula: S aoil,i —— The saturation of 2 type of remaining oil after CO i displacement; A after,oil-i —— The volume of 2 type of remaining oil after CO i displacement; A total —— The total pore volume.

[0030] (2) Calculate the oil displacement efficiency of CO 2 on isolated-droplet remaining oil, film-like remaining oil, and capillary-bound oil respectively: ; Eoil,i —— CO 2 For i class remaining oil saturation; S boil,i —— CO 2 Before displacement, i class remaining oil saturation; S aoil,i —— CO 2 After displacement, i class remaining oil saturation.

[0031] S7. Calculate CO 2 Total buried storage: (1) Calculate the buried storage of CO 2 in the gas phase, and take it as the first part of the total buried storage of CO 2 The calculation formula is as follows: ; In the formula: M ,gas —— CO 2 Buried storage in the gas phase; P —— Experimentally measured CO 2 Gas phase pressure, MPa; V ,gas —— CO identified by microfluidic experiment image 2 Gas phase volume, m 3 ; Z —— CO 2 Compression factor (obtained by looking up the table according to temperature and pressure); R —— Universal gas constant, 8.314 J / (mol·K); T —— Experiment temperature, K.

[0032] (2) Calculate the total dissolved amount of CO 2 in the oil phase. Since the solubility of CO 2 in the oil phase is affected by the oil-gas contact area and gradually decreases with the increase of the oil-gas contact distance, it is necessary to use a non-uniform dissolution model to calculate the storage amount of CO 2 in the oil phase.

[0033] ① Establish a solubility distribution model of CO 2 in the oil phase, and use the unsteady diffusion equation (Fick's second law) to calculate the dissolution and diffusion of CO 2 in the oil phase: ; where: D —— CO 2 diffusion coefficient in oil, m 2 / s; x —— depth of oil phase, m; t —— CO 2 action time, s.

[0034] ② For the final state of the experiment, use the semi - infinite diffusion model to calculate the CO 2 solubility distribution: ; where: C(x,t) —— CO concentration at a distance of x from the interface in the oil phase, mol / m 2 ; 3 ; C 0 —— CO 2 saturation solubility at the oil - gas interface (obtained by looking up the table); erf() —— error function, describing the diffusion behavior; x —— depth of oil phase, m; t —— CO 2 action time, s.

[0035] ③ Calculate the total dissolved amount of CO 2 in the oil phase by integration: ; where: M ,oil —— total dissolved amount of CO 2 in the oil phase; L —— length of the oil - phase region; V unit —— chip volume per unit volume of the oil phase; x —— depth of oil phase, m; t —— CO 2 action time, s; Use Matlab / Python for numerical integration to calculate the total dissolved amount of CO 2 in the oil phase.

[0036] (3) Calculate the total buried amount of CO 2 : ; Wherein: M ,total ——CO 2 Total buried storage amount; M ,gas ——CO 2 Buried storage amount in the gas phase; M ,oil ——CO 2 Total dissolved amount in the oil phase.

[0037] By identifying and counting the saturations of different types of remaining oil before and after CO 2 displacement, the oil displacement efficiency is calculated, and by comparing and analyzing the changes in the remaining oil saturation before and after CO 2 displacement, the contribution of CO 2 huff and puff to oil displacement is quantified. Combining with experimental data, the injection pressure, temperature and huff and puff cycle of CO 2 huff and puff are optimized to improve the ultimate recovery rate.

[0038] In the present invention, for the method for characterizing the carbon dioxide oil displacement efficiency - buried storage amount in a fractured tight oil reservoir at the micro - nano scale, the microfluidic chip used includes a main fluid channel A, a branched fracture region B, two high - permeability regions C of tight oil reservoirs and two low - permeability regions D of tight oil reservoirs.

[0039] The main fluid channel A represents the artificial fractures generated after fracturing of the tight oil reservoir; the branched fracture region B represents the secondary fracture structure formed by the extension of the artificial fractures into the matrix.

[0040] The high - permeability region C of the tight oil reservoir represents a large - pore high - permeability region with a relatively high content of framework minerals such as quartz and feldspar; the mineral composition of this region is simple, the stability and anti - compaction property of the rock are good, its internal pore diameter is relatively large, mainly micron - scale pores, the pore radius is generally between 2 - 12 μm, and the pores with a radius greater than 8 μm account for a relatively high proportion, and the coordination number is about 3 - 4.

[0041] The low - permeability region D of the tight oil reservoir represents a small - pore low - permeability region with a relatively high content of clay minerals such as illite / montmorillonite mixed layer, illite and chlorite. The mineral composition of this region is complex, and the sensitivity and heterogeneity of the rock are stronger. Its internal pore diameter is smaller, nano - scale pores are more developed, the pore radius is mainly distributed between 2 - 8 μm, and the coordination number is about 1 - 2.

[0042] Among them, the two high - permeability regions C of the tight oil reservoir and the two low - permeability regions D of the tight oil reservoir are arranged alternately on the microfluidic chip, forming an arrangement pattern of C - D - C - D.

[0043] Optionally select one of the C-D region combinations as the matrix pore structure region. The main fluid channel A is located outside the high-permeability region C of the tight oil reservoir. The fluid injection end is above the main fluid channel A, and the fluid production end is below it. The fluid injection end and the fluid production end are respectively located on both sides of the matrix pore structure region. The main fluid channel A is connected to the branched fracture region B; the branched fracture region B only penetrates and communicates within the C-D region combination. The remaining two sides of the matrix pore structure region are in an open state, which is used to represent the channels for fluids from other surrounding well groups and fractures to enter during the actual development process.

[0044] The above-mentioned matrix pore structure region contains two types of pore-fracture combinations with different development degrees, which can effectively simulate the influence of fractures on the behavior of reservoir fluids after fracturing in tight oil reservoirs. The fracture morphology and distribution characterized by the microfluidic chip conform to the actual field development.

[0045] Take the other C-D region combination as the matrix region. Three sides of this matrix region are in an open state, which is used to represent the channels for fluids from other surrounding well groups and fractures to enter during the actual development process.

[0046] The etching depth of the main fluid channel A and the branched fracture B is the same, denoted as depth a; the etching depth of the high-permeability region C and the low-permeability region D of the tight oil reservoir is the same, denoted as depth b; the ratio of depth a to depth b is 2-4:1. Such a design enables the microfluidic chip to form a 2.5D space, which can ensure the authenticity of the simulation as much as possible.

[0047] Existing microfluidic chips mostly adopt homogeneous pore networks and cannot accurately characterize the complex heterogeneous structure of tight oil reservoirs. More importantly, the current microfluidic chips do not consider the coupling effect between fractures and the matrix, resulting in a simulation error of up to 30%-40% in oil displacement efficiency. However, the microfluidic chip used in the characterization method of the present invention combines the characteristics of fracturing tight oil reservoirs for CO 2 oil displacement, designs a fracture-matrix coupled flow channel, overcomes the disadvantage of the insufficient adaptability of existing microfluidic chips to complex reservoir environments, and etches a microscopic visualization chip that simulates the internal structure of a tight oil reservoir after actual fracturing, which can characterize the influence of the coupling effect between fractures and the matrix on CO 2 oil displacement and storage. The design and implementation of the microfluidic chip enable three different effects on CO 2 oil displacement and storage to be characterized simultaneously using the same chip, especially the influence of the coupling effect between fractures and the matrix on CO 2 oil displacement and storage. Specifically: a fracture-matrix coupled flow channel is designed in the matrix pore structure region, and this region can characterize the influence of the coupling effect between fractures and the matrix on CO 2The influence on oil displacement and storage; within the matrix region, no artificial fractures are designed to exist, which can characterize the influence of the matrix itself on CO 2 The influence on oil displacement and storage; moreover, by comparing the matrix pore structure region with the matrix region, the influence of fractures on CO 2 The influence on oil displacement and storage can also be characterized.

[0048] Preferably, the length:width of the microfluidic chip is 2:1.

[0049] In the present invention, the method for characterizing the carbon dioxide oil displacement efficiency - storage amount in a fractured tight oil reservoir at the micro-nano scale uses a microfluidic chip fabricated through the following steps: (1) Fabricate a photolithography mask: Obtain the rock structure image of the fractured tight heterogeneous reservoir through a scanning electron microscope, and use the imageJ software to perform 8-bit processing, binary processing, and noise reduction processing on the image.

[0050] (2) Use the PS software to identify the shapes of the rock particles in the processed image to obtain the simplified structure diagrams of rock particles of various sizes.

[0051] (3) Piece together the simplified structure diagrams of rock particles of various sizes into a complete matrix region picture at a certain ratio and orientation, and then convert the designed matrix region picture into a format recognizable by the photolithography mask manufacturing equipment (such as GDSII) to fabricate the photolithography mask.

[0052] (4) Process the complete matrix region picture using the imageJ software, use the AnalyzeParticles function to identify and count the pore regions in the picture, and calculate the total pore area; then calculate the porosity of the matrix region; the overall area of the matrix region is obtained by multiplying the image width by the height; the porosity obtained in this step is exactly the porosity for morphological feature extraction in step S4 of the characterization method of the present invention.

[0053] (5) Etching and bonding to form: Transfer the photolithography mask obtained in step (3) to the substrate for wet etching; bond the cover layer and the substrate through vacuum thermal pressing to form the microfluidic chip. There are many applicable materials for etching the chip substrate, such as quartz, glass, polymethyl methacrylate (PMMA), polydimethylsiloxane (PDMS), and silicon. In the present invention, a heat-resistant and pressure-resistant borosilicate glass material is used as the chip, with a size of 75×75×3.5 mm, which consists of an etching layer and a cover layer with thicknesses of 1.5 mm and 2.0 mm respectively, and the etching layer is the substrate.

[0054] Etching methods include wet etching and dry etching. In this invention, wet etching is selected, and the specific process is as follows: First, put the silicon-based material substrate with a photolithographic mask into the reaction chamber of the etching equipment, evacuate the air, and then introduce a specific etching gas after reducing the pressure. The commonly used wet etching high-frequency hydrofluoric acid etching technology is adopted, that is, etching is carried out through hydrofluoric acid.

[0055] Since the injection end and the outlet end of the chip are extremely easy to be damaged, and in order to facilitate the experimental fluid to enter and exit the chip, and to avoid directly opening holes on the etching layer which may damage the structural integrity of the etching layer itself, holes are selected to be opened on the cover layer with a thickness of 2.0 mm. The diameter of the hole is 2 mm, and the distance between the centers of the two holes is 90 mm. After passing through the cover layer, it is connected to the etching layer, and finally the two glass plates are bonded by vacuum thermal pressing.

[0056] An experimental device for characterizing the carbon dioxide oil displacement efficiency - storage capacity method in tight oil reservoirs under micro-nano scale includes a high-temperature and high-pressure visible kettle for placing a microfluidic chip and a CO 2 piston container, a crude oil piston container, and a fracturing fluid piston container connected in parallel in sequence. The piston containers are all made of heat-resistant and corrosion-resistant materials.

[0057] The inlet end of the CO 2 piston container is connected to the high-pressure injection pump pipeline via the CO 2 piston container inlet end valve, and this high-pressure injection pump is connected to the deionized water storage tank pipeline; the CO 2 outlet end of the piston container is provided with a CO 2 piston container outlet end valve.

[0058] The inlet end of the crude oil piston container is provided with a crude oil piston container inlet end valve, and the outlet end is provided with a crude oil piston container outlet end valve.

[0059] The inlet end of the fracturing fluid piston container is provided with a fracturing fluid piston container inlet end valve, and the outlet end is provided with a fracturing fluid piston container outlet end valve.

[0060] The CO 2 piston container inlet end valve, the crude oil piston container inlet end valve, and the fracturing fluid piston container inlet end valve are all connected to the drain valve A through pipelines; the drain valve A is connected to the waste liquid bottle through a pipeline.

[0061] The CO 2 piston container outlet end valve, the crude oil piston container outlet end valve, and the fracturing fluid piston container outlet end valve are all connected to one port of a four-way through pipelines; the other three ports of this four-way are respectively connected to a drain valve B, a displacement pump opening valve, and one port of a three-way through pipelines; the drain valve B is connected to the waste liquid bottle through a pipeline.

[0062] The displacement pump opening valve is connected to one end of the inlet displacement pump, and the other end of the inlet displacement pump is connected via a CO 2 cylinder flow control valve to a CO 2 cylinder pipeline.

[0063] Another port of the tee is connected to the vacuum pump pipeline via a vacuum control valve A, and a vacuum pump pressure sensor is provided at the inlet end of the vacuum pump.

[0064] Another port of the tee is connected to an inlet pipeline on one side of the high-temperature and high-pressure visual autoclave via a valve at the inlet end of the visual autoclave, and a pressure sensor at the inlet end of the visual autoclave is provided at this inlet; this inlet is connected to the waste liquid bottle pipeline via a drain valve C.

[0065] The second inlet of the high-temperature and high-pressure visual autoclave is connected to the confining pressure tracking pump via a pipeline, and the confining pressure tracking pump is connected to the confining pressure liquid storage tank pipeline.

[0066] The third inlet of the high-temperature and high-pressure visual autoclave is communicated with an outlet pipeline on the other side of the high-temperature and high-pressure visual autoclave via a high-temperature heating device; another outlet on this side is communicated with the gas-liquid separator pipeline via a valve at the outlet end of the visual autoclave, and a pressure sensor at the outlet end of the visual autoclave is provided on the pipeline at the inlet end of the valve at the outlet end of the visual autoclave; a backpressure pump is connected to the bottom of the gas-liquid separator, and a metering device is connected to the gas-liquid separator.

[0067] The top inlet of the high-temperature and high-pressure visual autoclave is connected to the vacuum pump via a vacuum control valve B.

[0068] A microscope is provided above the visual window of the high-temperature and high-pressure visual autoclave, and the microscope is electrically connected to a computer via a transmission line.

[0069] The beneficial effects of the present invention are as follows: The method for characterizing the carbon dioxide oil displacement efficiency - storage amount in a fractured tight oil reservoir at the micro-nano scale according to the present invention is based on the microfluidic chip described above. Through experimental image analysis, gas-liquid state equations, diffusion models, and law identification, the occurrence types of remaining oil are characterized, and the dissolution and diffusion behaviors of CO 2 under different experimental conditions at the micro-nano scale are quantified, and the sweep efficiency and storage amount of CO 2 are calculated and analyzed. In particular, the storage amounts of CO 2 in the gas phase and oil phase can be accurately calculated, breaking through the limitation of the uniform solubility assumption of CO 2 in the traditional method, improving the estimation accuracy of the CO 2 storage amount, and realizing the visual quantitative characterization of the oil displacement and storage process of CO 2 . It breaks through the limitations of poor visibility and low repeatability in traditional core experiments, and provides a visual and quantitative research method for the efficient development and storage of CO 2 in tight oil reservoirs, and can comprehensively evaluate the displacement effect of fracturing fluid, the occurrence state of remaining oil, and the CO 2The oil displacement and storage mechanism provides a technical basis for optimizing the EOR plan and improving the development efficiency of tight oil reservoirs.

[0070] In addition, an experimental device for the above characterization method can perform the injection of fracturing fluid and CO 2 displacement that simulates the actual formation conditions of tight oil reservoirs, and is used to study the swept area at different times, the distribution of remaining oil, the CO 2 storage distribution, and the microscopic displacement mechanism. Description of the Drawings

[0071] Figure 1 It is a step flowchart of the method for characterizing the carbon dioxide oil displacement efficiency - storage volume in a fractured tight oil reservoir at the micro - nano scale according to the present invention.

[0072] Figure 2 It is a schematic diagram of the experimental device for the method of characterizing the carbon dioxide oil displacement efficiency - storage volume in a fractured tight oil reservoir at the micro - nano scale according to the present invention; Among them, 1 is a high - pressure injection pump, 2 is a deionized water storage tank, 3 is a CO 2 piston container, 4 is a crude oil piston container, 5 is a fracturing fluid piston container, 6 is a waste liquid bottle, 7 is a high - temperature and high - pressure visible autoclave, 8 is the inlet end valve of the visible autoclave, 9 is the outlet end valve of the visible autoclave, 10 is a vacuum pump, 11 is a confining pressure tracking pump, 12 is a back - pressure pump, 13 is an inlet displacement pump, 14 is a CO 2 gas cylinder, 15 is a high - temperature heating device, 16 is a microscope, 17 is a computer, 18 is a microfluidic chip, 19 is a CO 2 inlet end valve of the piston container, 20 is the inlet end valve of the crude oil piston container, 21 is the inlet end valve of the fracturing fluid piston container, 22 is a CO 2 outlet end valve of the piston container, 23 is the outlet end valve of the crude oil piston container, 24 is the outlet end valve of the fracturing fluid piston container, 25 is a drain valve A, 26 is a drain valve B, 27 is a drain valve C, 28 is a displacement pump opening valve, 29 is a CO 2 gas cylinder flow control valve, 30 is a vacuum control valve A, 31 is a vacuum control valve B, 32 is a gas - liquid separator, 33 is a metering device, 34 is a confining pressure liquid storage tank, 35 is a vacuum pump pressure sensor, 36 is the inlet end pressure sensor of the visible autoclave, 37 is the outlet end pressure sensor of the visible autoclave, 38 is a four - way joint, 39 is a three - way joint.

[0073] Figure 3 It is a schematic diagram of the structure of the microfluidic chip used in the method for characterizing the carbon dioxide oil displacement efficiency - storage volume in a fractured tight oil reservoir at the micro - nano scale according to the present invention; Among them, 40 is the main fluid channel A, 41 is the fluid injection end, 42 is the fluid production end, 43 is the branched fracture area B, 44 is the high-permeability area C of the tight oil reservoir, and 45 is the low-permeability area D of the tight oil reservoir.

[0074] Figure 4 This is the distribution map of different types of remaining oil after water injection in the actual microfluidic chip for simulating fracturing of a tight oil reservoir in Example 3 of the present invention.

[0075] Figure 5 This is the histogram of the proportion of different types of remaining oil in Example 3 of the present invention.

[0076] Figure 6 This is the oil and gas distribution characteristic map after injecting CO 2 in the actual microfluidic chip for simulating fracturing of a tight oil reservoir in Example 3 of the present invention. Specific Embodiments

[0077] Next, in conjunction with the accompanying drawings, the technical solutions of the present invention will be described in detail.

[0078] Example 1 As Figure 3 shown, the microfluidic chip used in the method for characterizing the oil displacement efficiency - burial volume of carbon dioxide in a fractured tight oil reservoir at the micro-nano scale according to the present invention includes a main fluid channel A40, a branched fracture area B43, two high-permeability areas C44 of the tight oil reservoir, and two low-permeability areas D45 of the tight oil reservoir.

[0079] The main fluid channel A40 represents the artificial fracture generated after fracturing of the tight oil reservoir; the branched fracture area B43 represents the secondary fracture structure formed by the extension of the artificial fracture into the matrix.

[0080] The high-permeability area C44 of the tight oil reservoir represents a large-pore high-permeability area with a relatively high content of framework minerals such as quartz and feldspar; the mineral composition of this area is simple, the stability and anti-compaction properties of the rock are good, its internal pore diameter is relatively large, mainly micron-scale pores, the pore radius is generally between 2 - 12 μm, and the proportion of pores with a radius greater than 8 μm is relatively high, and the coordination number is about 3 - 4.

[0081] The low-permeability area D45 of the tight oil reservoir represents a small-pore low-permeability area with a relatively high content of clay minerals such as illite / montmorillonite mixed layer, illite, and chlorite. The mineral composition of this area is complex, and the sensitivity and heterogeneity of the rock are stronger. Its internal pore diameter is small, nano-scale pores are more developed, the pore radius is mainly distributed between 2 - 8 μm, and the coordination number is about 1 - 2.

[0082] Among them, two high-permeability areas C44 of the tight oil reservoir and two low-permeability areas D45 of the tight oil reservoir are arranged alternately on the microfluidic chip, forming a C-D-C-D arrangement pattern.

[0083] Optionally select one of the C-D region combinations as the matrix pore structure region. The main fluid channel A40 is located outside the high-permeability region C44 of the tight oil reservoir. Above the main fluid channel A40 is the fluid injection end 41, and below is the fluid production end 42. The fluid injection end 41 and the fluid production end 42 are respectively located on both sides of the matrix pore structure region. The main fluid channel A40 is connected to the branched fracture region B43; the branched fracture region B43 only penetrates and communicates within the C-D region combination. The other two sides of the matrix pore structure region are in an open state, which is used to represent the channels for fluids from other surrounding well groups and fractures to enter during the actual development process.

[0084] Take the other C-D region combination as the matrix region. Three sides of this matrix region are in an open state, which is used to represent the channels for fluids from other surrounding well groups and fractures to enter during the actual development process.

[0085] The length of this microchip is 12000 μm, and the width is 6000 μm. Among them, the etching depth of the main fluid channel A40 and the branched fracture region B43 is 30 μm, and the etching depth of the matrix region composed of the high-permeability region C44 and the low-permeability region D45 of the tight oil reservoir is 10 μm.

[0086] Example 2 The experimental device for characterizing the carbon dioxide flooding efficiency - storage capacity method in a fractured tight oil reservoir at the micro-nano scale includes a high-temperature and high-pressure visual autoclave 7 for placing the microfluidic chip 18, and a CO 2 piston container 3, a crude oil piston container 4, and a fracturing fluid piston container 5 arranged in parallel in sequence. All of the piston containers are made of heat-resistant and corrosion-resistant materials.

[0087] The inlet end of the CO 2 piston container 3 is connected to the high-pressure injection pump 1 through the CO 2 piston container inlet end valve 19, and this high-pressure injection pump 1 is connected to the deionized water storage tank 2; the CO 2 outlet end of the piston container 3 is provided with a CO 2 piston container outlet end valve 22.

[0088] The inlet end of the crude oil piston container 4 is provided with a crude oil piston container inlet end valve 20, and the outlet end of the crude oil piston container 4 is provided with a crude oil piston container outlet end valve 23.

[0089] The inlet end of the fracturing fluid piston container 5 is provided with a fracturing fluid piston container inlet end valve 21, and the outlet end of the fracturing fluid piston container 5 is provided with a fracturing fluid piston container outlet end valve 24.

[0090] The CO2 The inlet valve 19 of the piston container, the inlet valve 20 of the crude oil piston container, and the inlet valve 21 of the fracturing fluid piston container are all connected to the drain valve A25 through pipelines; the drain valve A25 is connected to the waste liquid bottle 6 through a pipeline.

[0091] The described CO 2 The outlet valve 22 of the piston container, the outlet valve 23 of the crude oil piston container, and the outlet valve 24 of the fracturing fluid piston container are all connected to one port of the four-way joint 38 through pipelines; the other three ports of the four-way joint 38 are respectively connected to a drain valve B26, a displacement pump opening valve 28, and one port of a three-way joint 39 through pipelines; the drain valve B26 is connected to the waste liquid bottle 6 through a pipeline.

[0092] The displacement pump opening valve 28 is connected to one end of the inlet displacement pump 13, and the other end of the inlet displacement pump 13 passes through the CO 2 The gas cylinder flow control valve 29 is connected to the CO 2 The gas cylinder 14 through pipelines.

[0093] The other port of the three-way joint 39 is connected to the vacuum pump 10 through the vacuum control valve A30, and a vacuum pump pressure sensor 35 is provided at the inlet end of the vacuum pump 10.

[0094] The other port of the three-way joint 39 is connected to an inlet pipeline on one side of the high-temperature and high-pressure visual autoclave 7 through the visual autoclave inlet valve 8, and a visual autoclave inlet pressure sensor 36 is provided at this inlet; this inlet is connected to the waste liquid bottle 6 through the drain valve C27 through pipelines.

[0095] The second inlet of the high-temperature and high-pressure visual autoclave 7 is connected to the confining pressure tracking pump 11 through pipelines, and the confining pressure tracking pump 11 is connected to the confining pressure liquid storage tank 34 through pipelines.

[0096] The third inlet of the high-temperature and high-pressure visual autoclave 7 is connected to an outlet pipeline on the other side of the high-temperature and high-pressure visual autoclave 7 through the high-temperature heating device 15; the other outlet on this side is connected to the gas-liquid separator 32 through the visual autoclave outlet valve 9, and a visual autoclave outlet pressure sensor 37 is provided on the pipeline at the inlet end of the visual autoclave outlet valve 9; the bottom of the gas-liquid separator 32 is connected to the back pressure pump 12, and the gas-liquid separator 32 is connected to the metering device 33.

[0097] The top inlet of the high-temperature and high-pressure visual autoclave 7 is connected to the vacuum pump 10 through the vacuum control valve B31.

[0098] A microscope 16 is provided above the visual window of the high-temperature and high-pressure visual autoclave 7, and the microscope 16 is electrically connected to the computer 17 through a transmission line.

[0099] Example 3 First, use the experimental device described in Example 2 to simulate the fracturing of a tight oil reservoir with CO2 The process of carbon dioxide enhanced oil recovery and storage, and the specific experimental operation steps are as follows: (1) Clean the experimental device with petroleum ether and toluene to remove the oil-soluble impurities in the device pipeline and each piston container. Then rinse repeatedly with deionized water to ensure that no residual organic matter affects the experimental results. The cleaning waste liquid enters the waste liquid bottle 6.

[0100] (2) Place the microfluidic chip 18 in the annular cavity of the high-temperature and high-pressure visual autoclave 7 for fixation, and then evacuate the reservoir confining pressure cavity of the high-temperature and high-pressure visual autoclave 7 and the microfluidic chip 18 for 12 hours through the vacuum pump 10.

[0101] (3) Open the valve 8 at the inlet end of the visual autoclave to saturate with oil, and then increase the pressure to the set pressure of 25 MPa. During this process, simultaneously increase the confining pressure to the set pressure of 27 MPa with the confining pressure tracking pump 11 as the injection pressure increases, ensuring that the pressure in the reservoir confining pressure cavity of the high-temperature and high-pressure visual autoclave 7 is slightly higher than the pressure inside the microfluidic chip 18. Then turn on the high-temperature heating device 15 to make the temperature of the confining pressure liquid in the reservoir confining pressure cavity of the high-temperature and high-pressure visual autoclave 7 reach the set temperature of 90 °C.

[0102] (4) Place the microscope 16 at the position for observing the microfluidic chip 18, and adjust the focusing position and magnification of the microscope 16 so that the situation in the micro-nano scale channels in the experimental area on the microfluidic chip 18 can be clearly recorded by the high-speed camera.

[0103] (5) Fill the CO 2 , crude oil, and the fracturing fluid used in the experiment into the CO 2 piston container 3, crude oil piston container 4, and fracturing fluid piston container 5 respectively. Inject the fracturing fluid at a speed of 10 μl / min to simulate the fracturing fluid injection process in actual oilfield development. The entire cycle process is used to collect experimental image data in real time through the digital monitoring and imaging system. When the remaining oil in the micro-nano scale channels inside the microfluidic chip 18 no longer changes, stop injecting the fracturing fluid.

[0104] (6) Close all the valves of the CO 2 piston container 3, crude oil piston container 4, and fracturing fluid piston container 5, open the valve of the CO 2 gas cylinder 14, and control the displacement speed of CO 2 at 5 μl / min through the inlet displacement pump 13 to conduct the full-cycle experiment of CO 2 enhanced oil recovery and storage. The entire cycle process is used to collect experimental image data in real time through the digital monitoring and imaging system. When the remaining oil in the micro-nano scale channels of the microfluidic chip 18 no longer changes, stop the CO 2 displacement process.

[0105] After the displacement is completed, the experiment ends. Based on the real-time digital data recorded by each sensor and the real-time image data recorded by the camera system, the two are corresponded according to time, and the characteristics of fluid flow in the channel at each moment are analyzed to obtain the experimental results.

[0106] Then, based on the above simulation experiment results, the method for characterizing the carbon dioxide oil displacement efficiency - burial volume in tight fractured oil reservoirs at the micro-nano scale according to the present invention specifically comprises the following steps: S1. Microfluidic experiment data acquisition: Based on the microfluidic chip to simulate the CO 2 displacement process of tight fractured oil reservoirs, a high-resolution optical microscope or nuclear magnetic resonance (NMR) equipment is used to obtain microfluidic experiment images and save them in a standard format (such as.tiff,.png).

[0107] S2. Image preprocessing: The collected microfluidic experiment images are processed in batches using ImageJ.

[0108] Image grayscale conversion: Convert the experimental images to grayscale images through Image→Type→8-bit in ImageJ to reduce the computational complexity. The language operation is: java CopyEdit run("8-bit").

[0109] Gaussian filtering for denoising: Gaussian filtering is used to remove image noise and enhance the contrast of oil-water-pores; sigma (σ)=2. The language operation is: java CopyEdit run("GaussianBlur...","sigma=2").

[0110] Sobel edge detection enhancement: The Sobel operator is used for edge enhancement to improve the accuracy of oil-water boundary recognition. The language operation is: java CopyEdit run("FindEdges").

[0111] S3. Oil-water region segmentation: First, use the ColorThreshold function to segment oil and water based on the HSB color space: the color hue range is 109 - 204 (corresponding to the cyan to purple region in the HSV color ring), the saturation interval is 53 - 255, and the brightness ≤ 255. The language operation is as follows: java CopyEdit min[0]=109; max[0]=204; / / Hue: Color range min[1]=53; max[1]=255; / / Saturation: Saturation range min[2]=0; max[2]=255; / / Brightness: Brightness range.

[0112] Then, generate a binary mask (Mask) to extract the oil phase region. The language operations are as follows: java CopyEdit run("ConverttoMask").

[0113] S4. Morphological feature extraction: First, use the AnalyzeParticles function to count the area, perimeter, and circularity of the oil phase region. Among them, the area is greater than a certain threshold, and the circularity ranges from 0.5 to 1.0. The language operations are as follows: java CopyEdit run("AnalyzeParticles", "size=50-Infinity circularity=0.5-1.0 show=Masks display").

[0114] Then, calculate parameters such as porosity and wettability change.

[0115] Porosity is the total area of pores divided by the total area of the chip etching region. The total pore area is calculated by identifying and counting the pore regions in the matrix region of the microfluidic chip used. The total area of the chip etching region is obtained by multiplying the image width by the height.

[0116] The wettability change is obtained by measuring the contact angle between crude oil or water and the pore wall. An angle greater than 90° indicates oil-wet, and an angle less than 90° indicates water-wet.

[0117] S5. Classification and identification of remaining oil occurrence patterns: First, after completing the oil-water region segmentation and morphological feature extraction, combined with geometric features and spatial distribution information, the remaining oil is divided into three typical occurrence types: isolated droplet-like remaining oil, film-like remaining oil, and capillary-bound oil.

[0118] Then, identify and count the number, average diameter, and area ratio in the total oil phase of the isolated droplet-like oil. Isolated oil droplets usually have the characteristics of being individually distributed, round in shape, small in area, and discontinuous with other oil phases. Judgment criteria: aspect ratio close to 1, area < threshold, and the edge is closed. The language operations are as follows: java CopyEdit if (circularity > 0.8 && size > 10) { label = "IsolatedDropletOil";}

[0119] Identify and count the film thickness and pore wall coverage rate of film-like oil. This type of oil is distributed on the pore surface, usually in the shape of long strips, adhering to the wall, distributed along the pore wall, with a high aspect ratio and low circularity. Calculate the skeleton length ratio for classification (such as aspect ratio greater than 3 and circularity less than 0.5). The film thickness is obtained by measuring the thickness of the oil film, and the pore wall coverage rate is obtained by dividing the total area of the film-like oil in contact with the pores by the inner surface area of all pores. The language operation is as follows: java CopyEdit if (aspect_ratio > 3 && circularity < 0.5) { label = "Film-likeOil";}

[0120] Identify and count the spatial distribution and relative content of capillary-trapped oil. This type of oil is mainly bound by capillary force, commonly found in blind-end pores or microfracture areas, characterized by small size, irregular shape, low circularity, high aspect ratio, distributed in closed or narrow pores, and difficult to move with the fluid after displacement. Usually, the screening conditions are area less than the set threshold and circularity less than 0.3. The language operation is as follows: java CopyEdit if (size < 20 && circularity < 0.3) { label = "Capillary-TrappedOil";}

[0121] S6, CO 2 Analysis of the huff and puff process: a. Data acquisition: Process the obtained microfluidic experiment images using ImageJ or Matlab to identify the CO 2 gas phase region and the position of the oil-gas interface.

[0122] Use ImageJ to count the number of pixels in the CO 2 gas phase region (through the Analyze Particles function of ImageJ), calculate the two-dimensional area of the CO 2 region according to the image resolution, and combine the etching depths of the main channel and the matrix region of the microfluidic chip to convert the two-dimensional area into a three-dimensional volume, and calculate the CO 2 filling volume.

[0123] Extract the CO through color threshold segmentation method 2Distribution area in the chip, identify and quantify gaseous CO 2 and liquid CO 2 by volume fraction. This segmentation method is similar to the aforementioned remaining oil morphology identification and extraction, and is divided into image preprocessing; threshold segmentation to improve the contrast between the CO 2 area and other areas. Identify gaseous CO 2 through the phase interface between gaseous CO 2 and the oil phase of crude oil, and identify liquid CO 2 through the different color thresholds between liquid CO 2 and crude oil. Furthermore, the areas occupied by gaseous CO 2 and liquid CO 2 can be identified. Divide the area occupied by each by the total area of the distribution area to obtain the respective proportions of gaseous CO 2 and liquid CO 2 , which is equal to the volume fraction.

[0124] b. Calculate the CO 2 swept area: First, calculate the change in the oil-water interface after CO 2 enters: Use the color threshold segmentation algorithm to identify the three-phase regions of CO 2 , crude oil, and water; use the contour extraction algorithm (Canny edge detection) to extract the oil-water interface; mark the interface position and calculate its total length or occupied pixel position; compare the oil-water interface positions at different times, and calculate the change distance of the interface front position per unit time to identify and quantify the changes in the morphology, position, and contact relationship of the oil-water interface over time during the CO 2 displacement process; judge the invasion rate of CO 2 into the oil phase, reflecting the Jamin effect or the unstable fingering mechanism. The language operation is as follows: java CopyEdit run("Measure").

[0125] Statistical expansion rate of the CO 2 swept area at different time points: Set several time nodes (e.g., t 1 , t 2 , t 3 ...), compare the swept areas at each moment; divide the area difference by the time difference to obtain the average expansion rate of the CO 2 swept area, and calculate the propagation direction and anisotropic expansion velocity. The expansion rate of the CO 2 swept area at different time points refers to quantifying the evolution law of its swept area over time after CO 2 enters, including the pore volume fraction occupied, the front propagation velocity, etc.

[0126] c. Calculate CO 2 Displacement efficiency of remaining oil by throughput: (1) Calculate CO separately 2 The respective saturations of isolated-droplet remaining oil, film remaining oil, and capillary-bound oil before and after displacement; Among them, CO 2 The calculation formulas for the saturations of various types of remaining oil before displacement are as follows: ; In the formula: S boil,i —— CO 2 Before displacement, i Saturation of class remaining oil; A before,oil-i —— CO 2 Before displacement, i Volume of class remaining oil; A total —— Total pore volume.

[0127] CO 2 The calculation formulas for the saturations of various types of remaining oil after displacement are as follows: ; In the formula: S aoil,i —— CO 2 After displacement, i Saturation of class remaining oil; A after,oil-i —— CO 2 After displacement, i Volume of class remaining oil; A total —— Total pore volume.

[0128] (2)Calculate CO separately 2 Displacement efficiency of isolated-droplet remaining oil, film remaining oil, and capillary-bound oil by CO: ; E oil,i —— CO 2 For i Saturation of class remaining oil; S boil,i —— CO 2 Before displacement, i Saturation of class remaining oil; Saoil,i ——CO 2 After displacement, i the remaining oil saturation of this type.

[0129] The specific results are shown in Table 1.

[0130] Table 1 CO 2 The oil displacement efficiency of huff and puff on remaining oil .

[0131] S7. Calculate the total buried amount of CO 2 Total buried amount: Record the pressure (P) and temperature (T) during the experiment for subsequent calculation of CO 2 solubility and diffusion behavior.

[0132] (1) Calculate the buried amount of CO 2 in the gas phase and take it as the first part of the total buried amount of CO 2 The calculation formula is as follows: ; In the formula: M ,gas ——The buried amount of CO 2 in the gas phase; P ——The experimentally measured CO 2 gas phase pressure, MPa; V ,gas ——The CO 2 gas phase volume identified by microfluidic experiment image, m 3 ; Z ——The compression factor of CO 2 (obtained by looking up the table according to temperature and pressure); R ——Universal gas constant, 8.314 J / (mol·K); T ——Experimental temperature, K.

[0133] Substitute the data P = 25 MPa, V = 2.5×10 -3 m 3 , Z = 0.85, R = 8.314 J / (mol·K), T = 343.15 K, and calculate to get M ,gas = 54.6 mol.

[0134] (2) Calculate the CO 2The total dissolved amount in the oil phase, due to CO 2 The solubility in the oil phase is affected by the oil-gas contact area and gradually decreases with the increase of the oil-gas contact distance. Therefore, a non-uniform dissolution model needs to be used to calculate the CO 2 storage amount in the oil phase.

[0135] ① Establish a solubility distribution model of CO 2 in the oil phase, and use the unsteady diffusion equation (Fick's second law) to calculate the dissolution and diffusion of CO 2 in the oil phase: ; Where: D ——Diffusion coefficient of CO 2 in oil, m 2 / s; x ——Depth of the oil phase, m; t ——Action time of CO 2 in s.

[0136] ② For the final state of the experiment, use the semi-infinite diffusion model to calculate the CO 2 solubility distribution: ; Where: C(x,t) ——Concentration of CO x at a distance from the interface in the oil phase 2 mol / m 3 ; C 0 ——Saturated solubility of CO 2 at the oil-gas interface (obtained by looking up the table); erf() ——Error function, describing the diffusion behavior; x ——Depth of the oil phase, m; t ——Action time of CO 2 in s.

[0137] The diffusion coefficient (D) of CO 2 is 2.5×10 -9 m 2 / s. The CO 2 concentrations at different distances can be calculated, as shown in Table 2.

[0138] Table 2 CO 2 concentrations at different distances .

[0139] ③ Calculate the total dissolved amount of CO 2 in the oil phase by integration: ; Wherein: M ,oil ——CO 2 Total dissolved amount in the oil phase; L ——Length of the oil phase region; V unit ——Chip volume per unit volume of the oil phase; x ——Depth of the oil phase, m; t ——CO 2 Action time, s; Using Matlab / Python for numerical integration, the total dissolved amount of CO 2 in the oil phase is calculated to be M ,oil = 28.3 mol.

[0140] (3)Calculate the total buried amount of CO 2 : M ,total =M ,gas +M ,oil = 54.6 + 28.3 = 82.9 mol.

[0141] S8, Result visualization and scheme optimization: CO 2 oil displacement has the best effect on mobilizing isolated-droplet residual oil, and the oil displacement efficiency is 66.5%; CO 2 solubility in the oil phase is controlled by diffusion, and the solubility is the highest near the oil-gas contact surface; the total CO 2 buried amount is 82.9 mol, of which 34.1% is dissolved in the oil phase and 65.9% is stored in the gas phase.

Claims

1. A method for characterizing the efficiency and storage of carbon dioxide in fractured tight oil reservoirs at a micro-nano scale, characterized in that: This characterization method is based on a microfluidic chip. It uses image processing, color segmentation, morphological analysis and machine learning algorithms to automatically identify the storage form of crude oil, analyze the CO2 throughput process, and calculate the total CO2 storage volume. The specific steps are as follows: S1. Microfluidic experimental data acquisition: Simulate the CO2 displacement process of fracturing tight oil reservoirs based on microfluidic chips and collect microfluidic experimental images; S2. Image preprocessing: The collected microfluidic experimental images are preprocessed, including image grayscale, Gaussian filter denoising, and Sobel edge detection enhancement; S3, oil-water region segmentation: First, the HSB color space is used to segment the oil and water; then, a binary mask is generated to extract the oil phase region; S4. Morphological feature extraction: First, the area, perimeter, and roundness of the oil phase region are counted; then the porosity and wettability changes are calculated; S5. Classification and identification of residual oil occurrence morphology: First, after completing the oil-water area segmentation and morphological feature extraction, the residual oil is divided into three typical occurrence types: solitary drop residual oil, film residual oil and capillary bound oil, combining geometric features and spatial distribution information; Then, the number of isolated oil droplets, their average diameter, and their area ratio in the total oil phase were identified and counted; Identify and count the film thickness and pore wall coverage of thin-film oil; Identify and count the spatial distribution and relative content of capillary bound oil; S6. Analysis of CO2 throughput process: a. Data acquisition: The acquired microfluidic experimental images are processed using ImageJ or Matlab to identify the CO2 gas phase area and the oil-gas interface position; ImageJ was used to count the number of pixels in the CO2 gas phase region, and the two-dimensional area of ​​the CO2 region was calculated according to the image resolution. The two-dimensional area was converted into a three-dimensional volume based on the etching depth of the main channel and matrix area of ​​the microfluidic chip to calculate the CO2 filling volume. The distribution area of ​​CO2 in the chip is extracted by color threshold segmentation method, and the volume proportions of gaseous CO2 and liquid CO2 are identified and quantified respectively; b. Calculate the CO2 impact range: First, the changes in the oil-water interface after CO2 enters are calculated: the color threshold segmentation algorithm is used to identify the three-phase region of CO2, crude oil and water; the contour extraction algorithm is used to extract the oil-water interface; the interface position is marked and its total length or the pixel position occupied is calculated; the oil-water interface position at different times is compared, and the change distance of the interface front position per unit time is calculated, so as to identify and quantify the changes in the morphology, position and contact relationship of the oil-water interface over time during the CO2 displacement process; Count the expansion rate of the CO2 affected area at different time points: set several time nodes and compare the affected area at each moment; divide the area difference by the time difference to get the average expansion rate of the CO2 affected area; c. Calculate the displacement efficiency of CO2 huff and puff on the remaining oil: (1) Calculate the saturation of solitary droplet residual oil, thin film residual oil, and capillary bound oil before and after CO2 displacement; Among them, the calculation formulas for various types of residual oil saturation before CO2 displacement are as follows: ； Where: S boil,i ——Before CO2 displacement, i Class residual oil saturation; A before,oil-i ——Before CO2 displacement, i The volume of residual oil; A total ——total pore volume; The calculation formulas for various types of residual oil saturation after CO2 displacement are as follows: ； Where: S aoil,i ——After CO2 displacement, i Class residual oil saturation; A after,oil-i ——After CO2 displacement, i The volume of residual oil; A total ——total pore volume; (2) Calculate the oil displacement efficiency of CO2 on solitary drop residual oil, thin film residual oil and capillary bound oil respectively: ； E oil,i ——CO2 i Class residual oil saturation; S boil,i ——Before CO2 displacement, i Class residual oil saturation; S aoil,i ——After CO2 displacement, i Class residual oil saturation; S7. Calculate the total CO2 storage: (1) Calculate the storage amount of CO2 in the gas phase using the following formula: ； Where: M ,gas ——the amount of CO2 stored in the gas phase; P ——CO2 gas phase pressure measured experimentally, MPa; V ,gas ——CO2 gas phase volume identified by microfluidic experimental image, m 3 ; Z ——Compression factor of CO2; R ——Universal gas constant, 8.314 J / (mol·K); T ——Experimental temperature, K; (2) Calculate the total amount of CO2 dissolved in the oil phase: ①Use the non-steady-state diffusion equation to calculate the dissolution and diffusion of CO2 in the oil phase: ; in: D ——Diffusion coefficient of CO2 in oil, m 2 / s; x ——Oil phase depth, m; t ——CO2 action time, s; ②Use the semi-infinite diffusion model to calculate the CO2 solubility distribution: ; in: C(x,t) ——Interface in oil phase x CO2 concentration at mol / m 3 ; C 0——CO2 saturation solubility at the oil-gas interface; erf() ——error function; x ——Oil phase depth, m; t ——CO2 action time, s; ③Calculate the total amount of CO2 dissolved in the oil phase: ; in: M ,oil ——The total amount of CO2 dissolved in the oil phase; L ——the length of the oil phase region; V unit ——Chip volume per unit volume of oil phase; x ——Oil phase depth, m; t ——CO2 action time, s; The total amount of CO2 dissolved in the oil phase was calculated by numerical integration using Matlab / Python; (3) Calculate the total storage volume of CO2: ; in: M ,total ——Total CO2 storage; M ,gas ——the amount of CO2 stored in the gas phase; M ,oil ——The total amount of CO2 dissolved in the oil phase.

2. The method for characterizing the carbon dioxide recovery efficiency-buried storage volume in fractured tight oil reservoirs at micro-nano scale according to claim 1, characterized in that: The microfluidic chip used includes a main fluid channel A, a branch fracture area B, two tight oil reservoir high permeability areas C and two tight oil reservoir low permeability areas D; Among them, two tight oil reservoir high permeability regions C and two tight oil reservoir low permeability regions D are alternately arranged on the microfluidic chip to form a CDCD arrangement pattern; One of the CD area combinations is selected as the matrix pore structure area, the main fluid channel A is located outside the high permeability area C of the tight oil reservoir, and the main fluid channel A is connected to the branch fracture area B; the branch fracture area B only penetrates and connects within the CD area combination; Another CD region is combined as the matrix region; The etching depth of the main fluid channel A and the branch fracture B is the same, recorded as depth a; the etching depth of the high permeability region C of the tight oil reservoir and the low permeability region D of the tight oil reservoir is the same, recorded as depth b; the depth a:depth b is 2-4:

1.

3. The method for characterizing the carbon dioxide recovery efficiency-buried storage volume in fractured tight oil reservoirs at micro-nano scale according to claim 2, characterized in that: The length:width of the microfluidic chip is 2:

1.

4. The method for characterizing the carbon dioxide recovery efficiency-buried storage volume in fractured tight oil reservoirs at micro-nano scale according to claim 2, characterized in that: The microfluidic chip is manufactured by the following steps: (1) Making photolithography masks: The rock structure image of the dense heterogeneous reservoir after fracturing was obtained by scanning electron microscopy, and the image was processed by 8-bit, binarized, and denoised using imageJ software; (2) Using PS software to identify the shapes of rock particles in the processed image, a simplified structural diagram of rock particles of various sizes is obtained; (3) Simplified structural images of rock particles of various sizes are spliced ​​into a complete matrix area image, and then the designed matrix area image is converted into a format that can be recognized by the photolithography mask manufacturing equipment to make a photolithography mask; (4) The complete matrix area image is processed using imageJ software, and the AnalyzeParticles function is used to identify and count the pore areas in the image and calculate the total pore area; then the matrix area porosity is calculated; (5) Etching and bonding: transferring the photolithography mask obtained in step (3) to the substrate and performing wet etching; The cover layer and the substrate are bonded together by vacuum hot pressing to obtain the microfluidic chip.

5. An experimental device for characterizing the efficiency of carbon dioxide recovery in fractured tight oil reservoirs at micro-nano scales - buried storage method, characterized in that: It includes a high-temperature and high-pressure visual kettle for placing a microfluidic chip, and a CO2 piston container, a crude oil piston container and a fracturing fluid piston container which are sequentially arranged in parallel; The inlet end of the CO2 piston container is connected to the high-pressure injection pump pipeline via the CO2 piston container inlet end valve, and the high-pressure injection pump is connected to the deionized water storage tank pipeline; the outlet end of the CO2 piston container is provided with a CO2 piston container outlet end valve; The inlet end of the crude oil piston container is provided with a crude oil piston container inlet end valve, and the outlet end of the crude oil piston container is provided with a crude oil piston container outlet end valve; The inlet end of the fracturing fluid piston container is provided with a fracturing fluid piston container inlet end valve, and the outlet end of the fracturing fluid piston container is provided with a fracturing fluid piston container outlet end valve; The valve at the inlet end of the CO2 piston container, the valve at the inlet end of the crude oil piston container and the valve at the inlet end of the fracturing fluid piston container are all connected to the drain valve A through a pipeline; the drain valve A is connected to the waste liquid bottle through a pipeline; The valve at the outlet of the CO2 piston container, the valve at the outlet of the crude oil piston container and the valve at the outlet of the fracturing fluid piston container are all connected to one port of the four-way through a pipeline; the other three ports of the four-way are respectively connected to a discharge valve B, a displacement pump opening valve and a port of a three-way through a pipeline; the discharge valve B is connected to the waste liquid bottle through a pipeline; The displacement pump opening valve is connected to one end of the imported displacement pump, and the other end of the imported displacement pump is connected to the CO2 cylinder pipeline via the CO2 cylinder flow control valve; The other port of the tee is connected to the vacuum pump pipeline via the vacuum control valve A, and a vacuum pump pressure sensor is provided at the inlet end of the vacuum pump; The other port of the tee is connected to an inlet pipeline on one side of the high-temperature and high-pressure visual kettle via a visual kettle inlet valve, and a visual kettle inlet pressure sensor is provided at the inlet; the inlet is connected to the waste liquid bottle pipeline via a drain valve C; The second inlet of the high-temperature and high-pressure visual kettle is connected to the confining pressure tracking pump through a pipeline, and the confining pressure tracking pump is connected to the confining pressure liquid storage tank pipeline; The third inlet of the high-temperature and high-pressure visual kettle is connected to an outlet pipeline on the other side of the high-temperature and high-pressure visual kettle via a high-temperature heating device; the other outlet on this side is connected to the gas-liquid separator pipeline via a valve at the outlet end of the visual kettle, and a visual kettle outlet pressure sensor is provided on the inlet end pipeline of the valve at the outlet end of the visual kettle; a back pressure pump is connected to the bottom of the gas-liquid separator, and the gas-liquid separator is connected to a metering device; The top inlet of the high-temperature and high-pressure visual kettle is connected to the vacuum pump via the vacuum control valve B; A microscope is arranged above the visual window of the high-temperature and high-pressure visual kettle, and the microscope is electrically connected to the computer via a transmission line.

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