A method and experimental device for characterizing carbon dioxide flooding efficiency - burial volume in tight fractured oil reservoirs at the micro - nano scale

Abstract

The present invention belongs to the technical field of oil and gas field development, and particularly relates to a method and an experimental device for characterizing the carbon dioxide oil displacement efficiency - burial amount in a fractured tight oil reservoir at the micro-nano scale. The characterization method is based on a microfluidic chip to automatically identify the occurrence form of crude oil through image processing, color segmentation, morphological analysis and machine learning algorithms for the remaining oil, and analyze the CO2 huff and puff process to calculate the total burial amount of CO2. The experimental device includes a high-temperature and high-pressure visible autoclave for placing the microfluidic chip, and a CO2 piston container, a crude oil piston container and a fracturing fluid piston container which are arranged in parallel in sequence. The characterization method can simultaneously complete the accurate characterization of the remaining oil occurrence analysis, the influence of fracturing fluid displacement and the CO2 oil displacement - burial process at the micro-nano scale, accurately calculate the storage amounts of CO2 in the gas phase and the oil phase, break through the limitation of the uniform solubility assumption of CO2 by the traditional method, and improve the estimation accuracy of the CO2 storage amount.

Description

Technical Field

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

[0002] Tight oil reservoirs generally have characteristics such as ultra - low permeability, small pore - throat radius, and well - developed micro - fractures. The crude oil in such reservoirs is dispersed, and the seepage capacity of the reservoir is limited, resulting in a low recovery rate of tight oil reservoirs. Therefore, during the development process, it is necessary to use hydraulic fracturing and other methods to artificially create fractures to improve the physical properties of the reservoir and enhance the seepage capacity to achieve efficient development. At the same time, CO2 flooding is also one of the key technologies for improving the recovery rate (EOR) of tight oil. CO2 has a dissolution and extraction effect. It 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, and the injection of CO2 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 CO2 in the oil phase. In particular, the microscopic seepage law of CO2 in tight reservoirs is not clear. Especially after undergoing fracturing transformation, the CO2 oil displacement - storage process 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 CO2 in the oil phase at the micro - nano scale, resulting in insufficient understanding of the full - cycle seepage law of CO2 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 CO2 at the micro - nano scale. Moreover, the core will have 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 CO2 oil displacement efficiency and storage situation 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 carbon dioxide oil displacement efficiency - storage amount 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 CO2 oil displacement - storage process at the micro - nano scale, accurately calculate the storage amounts of CO2 in the gas phase and the oil phase, break through the limitation of the uniform solubility assumption of CO2 in traditional methods, and improve the estimation accuracy of CO2 storage amount. It realizes the visual quantitative characterization of the CO2 oil displacement and storage process to optimize the EOR scheme and improve the development efficiency of tight oil reservoirs.

[0007] The specific technical solutions are as follows:

[0008] A method for characterizing the CO2 oil displacement efficiency - storage volume in a fractured tight oil 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 residual oil, it automatically identifies the occurrence form of crude oil and analyzes the CO2 huff - puff process to calculate the total storage volume of CO2.

[0009] The specific steps are as follows:

[0010] S1. Microfluidic experiment data acquisition: Based on a microfluidic chip to simulate the CO2 displacement process in a fractured tight oil reservoir, 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).

[0011] 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.

[0012] Image grayscale conversion: Convert the experimental image to a grayscale image through Image → Type → 8 - bit in ImageJ to reduce the computational complexity.

[0013] Gaussian filtering for denoising: Use Gaussian filtering to remove image noise and enhance the contrast of oil - water - pores; sigma (σ)=2.

[0014] Sobel edge detection for enhancement: Use the Sobel operator for edge enhancement to improve the accuracy of oil - water boundary recognition.

[0015] 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.

[0016] 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 range is 53 - 255, and the brightness ≤ 255.

[0017] S4. Morphological feature extraction: First, statistically analyze the area, perimeter, and roundness of the oil - phase region; then calculate the porosity and wettability change.

[0018] 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 range is 0.5 - 1.0.

[0019] 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 areas 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.

[0020] The change in wettability is obtained by measuring the contact angle between crude oil or water and the pore wall. When it is greater than 90°, it is oil-wet; when it is less than 90°, it is water-wet.

[0021] S5. Classification and identification of remaining oil occurrence patterns: First, after completing the segmentation of oil-water regions and the extraction of morphological features, combining 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.

[0022] 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 individually distributed, round in shape, small in area, and discontinuous with other oil phases. Judgment criteria: aspect ratio close to 1, area < threshold, and closed edge.

[0023] Identify and count the 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, with a relatively 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.

[0024] 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 channels, 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.

[0025] S6. Analysis of the CO2 huff and puff process:

[0026] a. Data acquisition: Process the obtained microfluidic experiment images using ImageJ or Matlab to identify the CO2 gas phase region and the location of the oil-gas interface.

[0027] Use ImageJ to count the number of pixels in the CO2 gas phase region (through the Analyze Particles function of ImageJ), calculate the two-dimensional area of the CO2 region according to the image resolution, and combine the etching depths of the main channel and matrix region of the microfluidic chip to convert the two-dimensional area into a three-dimensional volume and calculate the CO2 filling volume.

[0028] Extract the distribution area of CO2 in the chip by color threshold segmentation method, and identify and quantify the volume fractions of gaseous CO2 and liquid CO2 respectively. This segmentation method is similar to the aforementioned identification and extraction of remaining oil morphology, and is divided into image preprocessing; threshold segmentation to improve the contrast between the CO2 region and other regions. Identify gaseous CO2 through the phase interface between gaseous CO2 and crude oil, and identify liquid CO2 through the different color thresholds of liquid CO2 and crude oil. Furthermore, the areas occupied by gaseous CO2 and liquid CO2 can be identified. The respective ratios of gaseous CO2 and liquid CO2 are obtained by dividing the area they occupy by the total area of the distribution region, which is equal to the volume fraction.

[0029] b. Calculate the swept area of CO2:

[0030] First, calculate the change in the oil-water interface after the entry of CO2: Use the color threshold segmentation algorithm to identify the three-phase regions of CO2, 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 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 CO2 displacement process; judge the invasion rate of CO2 into the oil phase to reflect the Jamin effect or the unstable fingering mechanism.

[0031] Statistically calculate the expansion rate of the CO2 swept area at different time points: Set several time nodes (such as t1, t2, t3...), compare the swept areas at each moment; divide the area difference by the time difference to obtain the average expansion rate of the CO2 swept area, and calculate the propagation direction and the anisotropic expansion velocity. The expansion rate of the CO2 swept area at different time points refers to quantifying the evolution law of the swept area over time after the entry of CO2, including the pore volume fraction occupied, the front propagation velocity, etc.

[0032] c. Calculate the oil displacement efficiency of CO2 huff and puff on the remaining oil:

[0033] (1) Calculate the saturations of isolated-droplet remaining oil, film-like remaining oil, and capillary-bound oil respectively before and after CO2 displacement;

[0034] Among them, the calculation formulas for the saturations of various types of remaining oil before CO2 displacement are as follows:

[0035] ;

[0036] In the formula: S boil,i ——Before CO2 displacement, i The saturation of

[0037] Abefore,oil-i —— Before CO2 displacement, i The volume of Class remaining oil;

[0038] A total —— The total pore volume.

[0039] The calculation formulas for the saturation of various types of remaining oil after CO2 displacement are as follows:

[0040] ;

[0041] In the formula: S aoil,i —— After CO2 displacement, i The saturation of Class remaining oil;

[0042] A after,oil-i —— After CO2 displacement, i The volume of Class remaining oil;

[0043] A total —— The total pore volume.

[0044] (2) Calculate the oil displacement efficiency of CO2 for isolated-droplet remaining oil, film-like remaining oil, and capillary-bound oil respectively:

[0045] ;

[0046] E oil,i —— The saturation of Class remaining oil by CO2; i The saturation of Class remaining oil;

[0047] S boil,i —— Before CO2 displacement, i The saturation of Class remaining oil;

[0048] S aoil,i —— After CO2 displacement, i The saturation of Class remaining oil.

[0049] S7. Calculate the total CO2 storage:

[0050] (1) Calculate the CO2 storage in the gas phase and take it as the first part of the total CO2 storage. The calculation formula is as follows:

[0051] ;

[0052] In the formula: M ,gas —— The CO2 storage in the gas phase;

[0053] P —— The experimentally measured CO2 gas-phase pressure, MPa;

[0054] V ,gas —— The CO2 gas-phase volume identified by microfluidic experiment image, m 3 ;

[0055] Z —— The compression factor of CO2 (obtained by looking up the table according to temperature and pressure);

[0056] R —— Universal gas constant, 8.314 J / (mol·K);

[0057] T —— Experimental temperature, K.

[0058] (2) Calculate the total dissolved amount of CO2 in the oil phase. Since the solubility of CO2 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 CO2 in the oil phase.

[0059] ① Establish a solubility distribution model of CO2 in the oil phase and use the unsteady diffusion equation (Fick's second law) to calculate the dissolution and diffusion of CO2 in the oil phase: ;

[0060] Among them: D —— The diffusion coefficient of CO2 in the oil, m 2 / s;

[0061] x —— The depth of the oil phase, m;

[0062] t —— The action time of CO2, s.

[0063] ② For the final state of the experiment, use the semi-infinite diffusion model to calculate the CO2 solubility distribution: ;

[0064] Among them: C(x,t) —— The concentration of CO2 at a distance of x from the interface in the oil phase, mol / m 3 ;

[0065] C 0—— The saturated solubility of CO2 at the oil-gas interface (obtained by looking up the table);

[0066] erf() —— The error function, describing the diffusion behavior;

[0067] x —— Depth of the oil phase, m;

[0068] t —— Action time of CO2, s.

[0069] ③ Calculate the total dissolved amount of CO2 in the oil phase through integration: ;

[0070] Where: M ,oil —— Total dissolved amount of CO2 in the oil phase;

[0071] L —— Length of the oil phase region;

[0072] V unit —— Chip volume per unit volume of the oil phase;

[0073] x —— Depth of the oil phase, m;

[0074] t —— Action time of CO2, s;

[0075] Use Matlab / Python for numerical integration to calculate the total dissolved amount of CO2 in the oil phase.

[0076] (3) Calculate the total buried amount of CO2: ;

[0077] Where: M ,total —— Total buried amount of CO2;

[0078] M ,gas —— Buried amount of CO2 in the gas phase;

[0079] M ,oil —— Total dissolved amount of CO2 in the oil phase.

[0080] By identifying and counting the saturations of different types of remaining oil before and after CO2 flooding, the oil displacement efficiency is calculated, the changes in the remaining oil saturation before and after CO2 flooding are compared and analyzed, and the contribution of CO2 huff and puff to oil displacement is quantified. Combining the experimental data, the injection pressure, temperature and huff and puff cycle of CO2 huff and puff are optimized to improve the ultimate recovery rate.

[0081] In the present invention, for the method of characterizing the carbon dioxide oil displacement efficiency - burial volume in a 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 the tight oil reservoir, and two low - permeability regions D of the tight oil reservoir.

[0082] 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.

[0083] 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 properties of the rock are good, its internal pore size 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.

[0084] 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 size is small, and nano - scale pores are more developed. The pore radius is mainly distributed between 2 - 8 μm, and the coordination number is about 1 - 2.

[0085] Among them, two high - permeability regions C of the tight oil reservoir and two low - permeability regions D of the tight oil reservoir are arranged alternately on the microfluidic chip, forming a C - D - C - D arrangement pattern.

[0086] Optionally, one of the C - D region combinations is used 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 this C - D region combination. The other two sides of this 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.

[0087] The above - mentioned matrix pore structure region contains two different degrees of developed pore - fracture combinations, which can effectively simulate the influence of fractures on the behavior of reservoir fluids after fracturing of the tight oil reservoir. The fracture morphology and distribution characterized by the microfluidic chip conform to the actual on - site development.

[0088] The other C - D region combination is used 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.

[0089] 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.

[0090] Most of the existing microfluidic chips 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 the oil displacement efficiency. The microfluidic chip used in the characterization method of the present invention combines the characteristics of CO2 flooding in fractured tight oil reservoirs and is designed with a fracture-matrix coupled flow channel, which overcomes the disadvantage of the insufficient adaptability of the existing microfluidic chips to complex reservoir environments and etches a microscopic visualization chip that simulates the internal structure of the actual fractured tight oil reservoir, and can characterize the influence of the coupling effect between fractures and the matrix on CO2 flooding and sequestration. The design of the microfluidic chip enables the simultaneous characterization of the effects of three different actions on CO2 flooding and sequestration using the same chip, especially the influence of the coupling effect between fractures and the matrix on CO2 flooding and sequestration. Specifically: a fracture-matrix coupled flow channel is designed in the matrix pore structure region, which can characterize the influence of the coupling effect between fractures and the matrix on CO2 flooding and sequestration; no artificial fractures are designed in the matrix region, which can characterize the influence of the matrix itself on CO2 flooding and sequestration; moreover, by comparing the matrix pore structure region with the matrix region, the influence of fractures on CO2 flooding and sequestration can also be characterized.

[0091] Preferably, the length:width of the microfluidic chip is 2:1.

[0092] In the present invention, the method for characterizing the CO2 flooding efficiency-sequestration amount of fractured tight oil reservoirs at the micro-nano scale uses a microfluidic chip fabricated through the following steps:

[0093] (1) Fabricate a photolithography mask: Obtain the rock structure image of the fractured tight heterogeneous reservoir through a scanning electron microscope, and use imageJ software to perform 8-bit processing, binary processing, and denoising processing on the image.

[0094] (2) Use PS software to identify the shapes of the rock particles in the processed image to obtain a simplified structure diagram of rock particles of various sizes.

[0095] (3) The simplified structure diagrams of rock particles of various sizes are spliced into a complete matrix area picture at a certain ratio and orientation, and then the designed matrix area picture is converted into a format recognizable by a photolithography mask manufacturing device (such as GDSII) to fabricate a photolithography mask.

[0096] (4) The complete matrix area picture is processed using ImageJ software. By using the Analyze Particles function, the pore areas in the picture are identified and counted to calculate the total pore area; then the porosity of the matrix area is calculated; the overall area of the matrix area 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 described in the present invention.

[0097] (5) Etching and bonding to form: The photolithography mask obtained in step (3) is transferred onto a substrate for wet etching; the cover layer and the substrate are bonded and formed through vacuum thermal pressing to obtain the microfluidic chip. There are many applicable materials for etching the chip substrate, such as quartz, glass, polymethyl methacrylate (PMMA), polydimethylsiloxane (PDMS), and silicon. The present invention uses a temperature- and pressure-resistant borosilicate glass material as the chip, with dimensions 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.

[0098] The etching methods include wet etching and dry etching. The present invention selects wet etching, and the specific process is as follows: First, the silicon-based material substrate with the photolithography mask is placed into the reaction chamber of the etching equipment, evacuated, and after reducing the pressure, a specific etching gas is introduced. The commonly used wet etching high-frequency hydrofluoric acid etching technology is used, that is, etching is carried out through hydrofluoric acid.

[0099] Since the injection end and the outlet end of the chip are extremely prone to breakage, and in order to facilitate the entry and exit of the experimental fluid into and out of the chip, and to avoid directly opening holes on the etching layer that 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 hole diameter 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 through vacuum thermal pressing.

[0100] An 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 visible autoclave for placing the microfluidic chip, and a CO2 piston container, a crude oil piston container, and a fracturing fluid piston container that are connected in parallel in sequence. All of the piston containers are made of heatable and corrosion-resistant materials.

[0101] 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 this 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.

[0102] The inlet end of the crude oil piston container is provided with a valve at the inlet end of the crude oil piston container, and the outlet end is provided with a valve at the outlet end of the crude oil piston container.

[0103] The inlet end of the fracturing fluid piston container is provided with a valve at the inlet end of the fracturing fluid piston container, and the outlet end is provided with a valve at the outlet end of the fracturing fluid piston container.

[0104] The inlet end valves of the CO2 piston container, the crude oil piston container and the fracturing fluid piston container are all connected to the drain valve A through pipelines; the drain valve A is connected to the waste liquid bottle through a pipeline.

[0105] The outlet end valves of the CO2 piston container, the crude oil piston container and the fracturing fluid piston container 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.

[0106] The displacement pump opening valve is connected to one end of an imported displacement pump, and the other end of the imported displacement pump is connected to the CO2 gas cylinder pipeline through a CO2 gas cylinder flow control valve.

[0107] The other port of the three-way is connected to the vacuum pump pipeline through a vacuum control valve A, and a vacuum pump pressure sensor is provided at the inlet end of the vacuum pump.

[0108] The other port of the three-way is connected to an inlet pipeline on one side of a high-temperature and high-pressure visual autoclave through a visual autoclave inlet end valve, and a visual autoclave inlet end pressure sensor is provided at this inlet; this inlet is connected to the waste liquid bottle pipeline through a drain valve C.

[0109] The second inlet of the high-temperature and high-pressure visual autoclave is connected to a confining pressure tracking pump through a pipeline, and the confining pressure tracking pump is connected to the confining pressure liquid storage tank pipeline.

[0110] 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 through a high-temperature heating device; the other outlet on this side is communicated with the gas-liquid separator pipeline through a visual autoclave outlet end valve, and a visual autoclave outlet end pressure sensor is provided on the pipeline at the inlet end of the visual autoclave outlet end valve; a back pressure pump is connected to the bottom of the gas-liquid separator, and a metering device is connected to the gas-liquid separator.

[0111] The top inlet of the high-temperature and high-pressure visual autoclave is connected to the vacuum pump through a vacuum control valve B.

[0112] 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 through a transmission line.

[0113] The beneficial effects of the present invention are as follows: The method for characterizing the CO2 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, it characterizes the occurrence types of remaining oil, quantifies the dissolution and diffusion behavior of CO2 at the micro - nano scale under different experimental conditions, calculates and analyzes the CO2 sweep efficiency and storage amount. In particular, it can accurately calculate the storage amounts of CO2 in the gas phase and oil phase, breaks through the limitation of the uniform solubility assumption of CO2 in traditional methods, improves the estimation accuracy of CO2 storage amount, and realizes the visual and quantitative characterization of the CO2 oil displacement and storage process. It breaks through the limitations of poor visibility and low repeatability in traditional core experiments, provides a visual and quantitative research method for the efficient development and storage of CO2 in tight oil reservoirs, can comprehensively evaluate the displacement effect of fracturing fluid, the occurrence state of remaining oil, and the CO2 oil displacement and storage mechanism, and provides a technical basis for optimizing the EOR scheme and improving the development efficiency of tight oil reservoirs.

[0114] In addition, an experimental device for the above - mentioned characterization method is provided, which can carry out the injection of fracturing fluid and CO2 displacement under conditions simulating the actual formation of a tight oil reservoir, and is used to study the swept area at different times, the distribution of remaining oil, the CO2 storage distribution, and the microscopic displacement mechanism. BRIEF DESCRIPTION OF THE DRAWINGS

[0115] Figure 1 It is a flow chart of the steps of the method for characterizing the CO2 oil displacement efficiency - storage amount in a fractured tight oil reservoir at the micro - nano scale according to the present invention.

[0116] Figure 2 It is a schematic diagram of the experimental device for the method for characterizing the CO2 oil displacement efficiency - storage amount in a fractured tight oil reservoir at the micro - nano scale according to the present invention;

[0117] Among them, 1 is a high-pressure injection pump, 2 is a deionized water storage tank, 3 is a CO2 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 visual autoclave, 8 is a valve at the inlet end of the visual autoclave, 9 is a valve at the outlet end of the visual autoclave, 10 is a vacuum pump, 11 is an confining pressure tracking pump, 12 is a back pressure pump, 13 is an inlet displacement pump, 14 is a CO2 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 valve at the inlet end of the CO2 piston container, 20 is a valve at the inlet end of the crude oil piston container, 21 is a valve at the inlet end of the fracturing fluid piston container, 22 is a valve at the outlet end of the CO2 piston container, 23 is a valve at the outlet end of the crude oil piston container, 24 is a valve at the outlet end 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 CO2 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 pressure sensor of the vacuum pump, 36 is a pressure sensor at the inlet end of the visual autoclave, 37 is a pressure sensor at the outlet end of the visual autoclave, 38 is a four-way joint, 39 is a three-way joint.

[0118] Figure 3 It is a schematic structural diagram of the microfluidic chip adopted in the method for characterizing the CO2 oil displacement efficiency - storage volume in a fractured tight oil reservoir at the micro-nano scale according to the present invention;

[0119] Among them, 40 is the main fluid channel A, 41 is the fluid injection end, 42 is the fluid production end, 43 is the branch 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.

[0120] Figure 4 It is a distribution diagram of different types of remaining oil after water injection in the actual microfluidic chip for simulating a fractured tight oil reservoir in Example 3 of the present invention.

[0121] Figure 5 It is a histogram of the proportion of different types of remaining oil in Example 3 of the present invention.

[0122] Figure 6 It is a diagram of the oil-gas distribution characteristics after CO2 injection in the actual microfluidic chip for simulating a fractured tight oil reservoir in Example 3 of the present invention. Detailed implementation manners

[0123] The technical solutions of the present invention will be described in detail below with reference to the accompanying drawings.

[0124] Example 1

[0125] As Figure 3As shown in the figure, the microfluidic chip used in the method for characterizing the carbon dioxide oil displacement efficiency - burial volume in tight oil reservoirs at the micro - nano scale in the present invention includes a main fluid channel A40, a branched fracture region B43, two high - permeability regions C44 of tight oil reservoirs, and two low - permeability regions D45 of tight oil reservoirs.

[0126] The main fluid channel A40 represents the artificial fracture generated after fracturing of the tight oil reservoir; the branched fracture region B43 represents the secondary fracture structure formed by the extension of the artificial fracture into the matrix.

[0127] The high - permeability regions C44 of tight oil reservoirs represent large - pore high - permeability regions with relatively high contents of framework minerals such as quartz and feldspar; the mineral composition of this region 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 pores with a radius greater than 8 μm account for a relatively high proportion, and the coordination number is about 3 - 4.

[0128] The low - permeability regions D45 of tight oil reservoirs represent small - pore low - permeability regions with relatively high contents 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 small, and nano - scale pores are more developed. The pore radius is mainly distributed between 2 - 8 μm, and the coordination number is about 1 - 2.

[0129] Among them, two high - permeability regions C44 of tight oil reservoirs and two low - permeability regions D45 of tight oil reservoirs are arranged alternately on the microfluidic chip, forming a C - D - C - D arrangement pattern.

[0130] Optionally, one of the C - D region combinations is used 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 this C - D region combination. The other two sides of this matrix pore structure region are in an open state, which is used to characterize the channels for fluids from other surrounding well groups and fractures to enter during the actual development process.

[0131] The other C - D region combination is used as the matrix region. Three sides of this matrix region are in an open state, which is used to characterize the channels for fluids from other surrounding well groups and fractures to enter during the actual development process.

[0132] The length of the microchip is 12000μm, and the width is 6000μm. The etching depth of the main fluid channel A40 and the branched fracture area B43 is 30μm, and the etching depth of the matrix area composed of the high-permeability area C44 and the low-permeability area D45 of the tight reservoir is 10μm.

[0133] Example 2

[0134] The experimental device for characterizing the carbon dioxide flooding efficiency - storage capacity method in a fractured tight reservoir at the micro-nano scale includes a high-temperature and high-pressure visual autoclave 7 for placing a microfluidic chip 18, and a CO2 piston container 3, a crude oil piston container 4, and a fracturing fluid piston container 5 that are connected in parallel in sequence. All of the piston containers are made of heat-resistant and corrosion-resistant materials.

[0135] The inlet end of the CO2 piston container 3 is connected to the pipeline of the high-pressure injection pump 1 via the CO2 piston container inlet end valve 19, and the high-pressure injection pump 1 is connected to the deionized water storage tank 2 through a pipeline; the outlet end of the CO2 piston container 3 is provided with a CO2 piston container outlet end valve 22.

[0136] 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.

[0137] 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.

[0138] The CO2 piston container inlet end valve 19, the crude oil piston container inlet end valve 20, and the fracturing fluid piston container inlet end valve 21 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.

[0139] The CO2 piston container outlet end valve 22, the crude oil piston container outlet end valve 23, and the fracturing fluid piston container outlet end valve 24 are all connected to one port of a 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.

[0140] One end of the displacement pump opening valve 28 is connected to the inlet displacement pump 13, and the other end of the inlet displacement pump 13 is connected to the CO2 gas cylinder 14 through the CO2 gas cylinder flow control valve 29.

[0141] 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.

[0142] Another port of the three-way joint 39 is connected to an inlet pipeline on one side of the high-temperature and high-pressure visible autoclave 7 via the visible autoclave inlet end valve 8, and a visible autoclave inlet end pressure sensor 36 is provided at this inlet; this inlet is connected to the waste liquid bottle 6 pipeline via the drain valve C27.

[0143] The second inlet of the high-temperature and high-pressure visible autoclave 7 is connected to the confining pressure tracking pump 11 through a pipeline, and the confining pressure tracking pump 11 is in pipeline communication with the confining pressure liquid storage tank 34.

[0144] The third inlet of the high-temperature and high-pressure visible autoclave 7 is communicated with an outlet pipeline on the other side of the high-temperature and high-pressure visible autoclave 7 via the high-temperature heating device 15; the other outlet on this side is communicated with the gas-liquid separator 32 pipeline via the visible autoclave outlet end valve 9, and a visible autoclave outlet end pressure sensor 37 is provided on the inlet end pipeline of the visible autoclave outlet end valve 9; a back pressure pump 12 is connected to the bottom of the gas-liquid separator 32, and the gas-liquid separator 32 is connected with a metering device 33.

[0145] The top inlet of the high-temperature and high-pressure visible autoclave 7 is connected to the vacuum pump 10 via the vacuum control valve B31.

[0146] A microscope 16 is provided above the viewing window of the high-temperature and high-pressure visible autoclave 7, and the microscope 16 is electrically connected to the computer 17 via a transmission line.

[0147] Example 3

[0148] First, use the experimental device described in Example 2 to simulate the process of CO2 carbon dioxide flooding and storage in tight oil reservoirs. The specific experimental operation steps are as follows:

[0149] (1) Use petroleum ether and toluene to clean the experimental device to remove the oil-soluble impurities in the device pipeline and each piston container. Then use deionized water to rinse repeatedly to ensure that there is no residual organic matter affecting the experimental results. The cleaning waste liquid enters the waste liquid bottle 6.

[0150] (2) Place the microfluidic chip 18 in the annular cavity of the high-temperature and high-pressure visible autoclave 7 and fix it, and then use the vacuum pump 10 to evacuate the reservoir confining pressure annular cavity of the high-temperature and high-pressure visible autoclave 7 and the microfluidic chip 18 for 12 hours.

[0151] (3) Open the visible autoclave inlet end valve 8 to saturate the 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 annular cavity of the high-temperature and high-pressure visible 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 annular cavity of the high-temperature and high-pressure visible autoclave 7 reach the set temperature of 90 °C.

[0152] (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 conditions in the micro-nano scale channels in the experimental area on the microfluidic chip 18 can be clearly recorded by the high-speed camera.

[0153] (5) Fill CO2, crude oil, and the fracturing fluid used in the experiment into the CO2 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 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.

[0154] (6) Close all the valves of the CO2 piston container 3, crude oil piston container 4, and fracturing fluid piston container 5, open the valve of the CO2 gas cylinder 14, and control the displacement speed of CO2 at 5 μl / min through the inlet displacement pump 13 to conduct the full-cycle experiment of CO2 enhanced oil recovery and storage. The entire cycle process is used to collect experimental image data 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 CO2 displacement process.

[0155] (7) 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 imaging system, the two are corresponded according to time to analyze the characteristics of fluid flow in the channels at each moment, and the experimental results are obtained.

[0156] Then, based on the above simulation experiment results, the method for characterizing the CO2 enhanced oil recovery efficiency - storage volume in a fractured tight oil reservoir at the micro-nano scale according to the present invention specifically includes the following steps:

[0157] S1. Microfluidic experiment data acquisition: Based on the microfluidic chip to simulate the CO2 displacement process in a fractured tight oil reservoir, use a high-resolution optical microscope or nuclear magnetic resonance (NMR) equipment to obtain microfluidic experiment images and save them in a standard format (such as.tiff,.png).

[0158] S2. Image preprocessing: Perform batch image processing on the collected microfluidic experiment images using ImageJ.

[0159] 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:

[0160] java

[0161] CopyEdit

[0162] run("8-bit").

[0163] Gaussian filtering for denoising: Gaussian filtering is used to remove image noise and enhance the contrast of oil-water-pore; sigma (σ) = 2. The language operation is as follows:

[0164] java

[0165] CopyEdit

[0166] run("GaussianBlur...","sigma=2").

[0167] 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 as follows:

[0168] java

[0169] CopyEdit

[0170] run("FindEdges").

[0171] S3. Oil-water region segmentation: First, the ColorThreshold function is used 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:

[0172] java

[0173] CopyEdit

[0174] min[0]=109;max[0]=204; / / Hue: Color interval

[0175] min[1]=53;max[1]=255; / / Saturation: Saturation interval

[0176] min[2]=0;max[2]=255; / / Brightness: Brightness interval.

[0177] Then, a binary mask (Mask) is generated to extract the oil phase region. The language operation is as follows:

[0178] java

[0179] CopyEdit

[0180] run("ConverttoMask").

[0181] S4. Morphological feature extraction:

[0182] 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 operation is as follows:

[0183] java

[0184] CopyEdit

[0185] run("AnalyzeParticles","size=50-Infinity circularity=0.5-1.0 show=Masks display").

[0186] Then, calculate parameters such as porosity and wettability change.

[0187] 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.

[0188] The wettability change is obtained by measuring the contact angle between crude oil or water and the pore wall. If it is greater than 90°, it is oil-wet; if it is less than 90°, it is water-wet.

[0189] S5. Classification and identification of remaining oil occurrence forms: First, after completing the segmentation of the oil-water region and the extraction of morphological features, 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.

[0190] 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 have the characteristics of being distributed separately, having a round shape, a small area, and being discontinuous with other oil phases. The determination criteria are as follows: the aspect ratio is close to 1, the area < threshold, and the edge is closed. The language operation is as follows:

[0191] java

[0192] CopyEdit

[0193] if(circularity>0.8&&size>10){label="IsolatedDropletOil"}.

[0194] 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, and distributed along the pore wall. It has a high aspect ratio and a low roundness. Calculate the skeleton length ratio for classification (e.g., 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 calculated 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:

[0195] java

[0196] CopyEdit

[0197] if(aspect_ratio>3&&circularity<0.5){label="Film-likeOil";}。

[0198] Identify and count the spatial distribution and relative content of capillary-trapped oil. This type of oil is mainly trapped by capillary forces and is commonly found in blind-end pores or microfracture areas. It is characterized by small size, irregular shape, low roundness, and high aspect ratio. It is distributed in closed or narrow pores and is difficult to move with the fluid after displacement. Usually, the screening conditions are that the area is less than a set threshold and the roundness is less than 0.3. The language operation is as follows:

[0199] java

[0200] CopyEdit

[0201] if(size<20&&circularity<0.3){label="Capillary-TrappedOil";}。

[0202] S6. Analysis of the CO2 Huff and Puff Process:

[0203] a. Data acquisition: Process the obtained microfluidic experiment images using ImageJ or Matlab to identify the CO2 gas phase region and the location of the oil-gas interface.

[0204] Use ImageJ to count the number of pixels in the CO2 gas phase region (through the Analyze Particles function of ImageJ). Calculate the two-dimensional area of the CO2 region based on 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 CO2 filling volume.

[0205] Extract the distribution area of CO2 in the chip by color threshold segmentation method, identify and quantify the volume fractions of gaseous CO2 and liquid CO2 respectively. This segmentation method is similar to the aforementioned identification and extraction of remaining oil morphology, and is divided into image preprocessing; threshold segmentation to improve the contrast between the CO2 region and other regions. Identify gaseous CO2 through the phase interface between gaseous CO2 and crude oil, and identify liquid CO2 through the different color thresholds of liquid CO2 and crude oil. Then, the areas occupied by gaseous CO2 and liquid CO2 can be identified. Divide the area occupied by each by the total area of the distribution region to obtain the respective proportions of gaseous CO2 and liquid CO2, which is equal to the volume fraction.

[0206] b. Calculate the swept area of CO2:

[0207] First, calculate the change in the oil-water interface after the entry of CO2: Use the color threshold segmentation algorithm to identify the three-phase regions of CO2, 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 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 CO2 displacement process; judge the invasion rate of CO2 into the oil phase to reflect the Jamin effect or the unstable fingering mechanism. The language operations are as follows:

[0208] java

[0209] CopyEdit

[0210] run("Measure").

[0211] Statistically analyze the expansion rate of the CO2-swept area at different time points: Set several time nodes (such as t1, t2, t3...), compare the swept areas at each moment; divide the area difference by the time difference to obtain the average expansion rate of the CO2-swept area, and calculate the propagation direction and anisotropic expansion velocity. The expansion rate of the CO2-swept area at different time points refers to quantifying the evolution law of the swept area over time after the entry of CO2, including the pore volume fraction occupied, the front propagation velocity, etc.

[0212] c. Calculate the oil displacement efficiency of CO2 huff and puff on the remaining oil:

[0213] (1) Calculate the respective saturations of droplet-shaped remaining oil, film-shaped remaining oil, and capillary-bound oil before and after CO2 displacement respectively;

[0214] Among them, the calculation formulas for the saturations of various types of remaining oil before CO2 displacement are as follows:

[0215] ;

[0216] Wherein: S boil,i —— Before CO2 displacement, i The saturation of Class I remaining oil;

[0217] A before,oil-i —— Before CO2 displacement, i The volume of Class I remaining oil;

[0218] A total —— The total pore volume.

[0219] The calculation formulas for the saturation of various remaining oils after CO2 displacement are as follows:

[0220] ;

[0221] Wherein: S aoil,i —— After CO2 displacement, i The saturation of Class II remaining oil;

[0222] A after,oil-i —— After CO2 displacement, i The volume of Class II remaining oil;

[0223] A total —— The total pore volume.

[0224] (2) Calculate the oil displacement efficiency of CO2 for isolated-droplet remaining oil, film-like remaining oil, and capillary-bound oil respectively:

[0225] ;

[0226] E oil,i —— CO2 for i The saturation of Class III remaining oil;

[0227] S boil,i —— Before CO2 displacement, i The saturation of Class III remaining oil;

[0228] S aoil,i —— After CO2 displacement, i The saturation of Class III remaining oil.

[0229] The specific results are shown in Table 1.

[0230] Table 1 Oil Displacement Efficiency of CO2 Huff and Puff on Remaining Oil

[0231] 。

[0232] S7. Calculate the total CO2 storage volume:

[0233] Record the pressure (P) and temperature (T) during the experiment for subsequent calculation of CO2 solubility and diffusion behavior.

[0234] (1) Calculate the CO2 storage volume in the gas phase and take it as the first part of the total CO2 storage volume. The calculation formula is as follows:

[0235] ;

[0236] In the formula: M ,gas ——The CO2 storage volume in the gas phase;

[0237] P ——The measured CO2 gas phase pressure, MPa;

[0238] V ,gas ——The CO2 gas phase volume identified by microfluidic experiment image, m 3 ;

[0239] Z ——The compression factor of CO2 (obtained by looking up the table according to temperature and pressure);

[0240] R ——Universal gas constant, 8.314 J / (mol·K);

[0241] T ——The experimental temperature, K.

[0242] 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.

[0243] (2) Calculate the total dissolved amount of CO2 in the oil phase. Since the solubility of CO2 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 CO2 in the oil phase.

[0244] ① Establish a solubility distribution model of CO2 in the oil phase and use the unsteady diffusion equation (Fick's second law) to calculate the dissolution and diffusion of CO2 in the oil phase: ;

[0245] Among them:D —— Diffusion coefficient of CO2 in oil, m 2 / s;

[0246] x —— Depth of oil phase, m;

[0247] t —— Action time of CO2, s.

[0248] ② For the final state of the experiment, the semi-infinite diffusion model is used to calculate the CO2 solubility distribution: ;

[0249] Where: C(x,t) —— CO2 concentration at a distance from the interface in the oil phase x at, mol / m 3 ;

[0250] C 0 —— CO2 saturation solubility at the oil-gas interface (obtained by looking up the table);

[0251] erf() —— Error function, describing the diffusion behavior;

[0252] x —— Depth of oil phase, m;

[0253] t —— Action time of CO2, s.

[0254] The CO2 diffusion coefficient (D) is 2.5×10 -9 m 2 / s. The CO2 concentrations at different distances can be calculated, as shown in Table 2.

[0255] Table 2 CO2 Concentrations at Different Distances

[0256] .

[0257] ③ Calculate the total dissolved amount of CO2 in the oil phase by integration: ;

[0258] Where: M ,oil —— Total dissolved amount of CO2 in the oil phase;

[0259] L —— Length of the oil phase region;

[0260] V unit —— Chip volume per unit volume of the oil phase;

[0261] x —— Depth of oil phase, m;

[0262] t —— CO2 action time, s;

[0263] Use Matlab / Python for numerical integration to calculate the total dissolved amount of CO2 in the oil phase M ,oil = 28.3 mol.

[0264] (3) Calculate the total buried amount of CO2: M ,total =M ,gas +M ,oil = 54.6 + 28.3 = 82.9 mol.

[0265] S8, Result visualization and scheme optimization:

[0266] The mobilization effect of CO2 flooding on isolated droplet-shaped residual oil is the best, with an oil displacement efficiency of 66.5%; the solubility of CO2 in the oil phase is diffusion-controlled, and the solubility is the highest near the oil-gas contact surface; the total CO2 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 carbon dioxide flooding efficiency - storage volume in a fractured tight oil reservoir at the micro - nano scale, characterized in that, This characterization method is based on a microfluidic chip. Through image processing, color segmentation, morphological analysis, and machine learning algorithms for residual oil, it automatically identifies the occurrence forms of crude oil and analyzes the CO2 huff and puff process to calculate the total buried amount of CO2. The specific steps are as follows: S1. Microfluidic experiment data acquisition: Simulate the CO2 displacement process in a fractured tight reservoir based on a microfluidic chip and collect microfluidic experiment images. S2. Image preprocessing: Perform image preprocessing on the collected microfluidic experiment images, including image grayscale conversion, Gaussian filtering for noise reduction, and Sobel edge detection for enhancement. S3. Oil-water region segmentation: First, segment oil and water in the HSB color space; then, generate a binary mask to extract the oil phase region. S4. Morphological feature extraction: First, statistically analyze the area, perimeter, and roundness of the oil phase region; then, calculate the porosity and wettability changes. S5. Classification and identification of the occurrence forms of residual oil: First, after completing the oil-water region segmentation and morphological feature extraction, combine geometric features and spatial distribution information to divide residual oil into three typical occurrence types: isolated droplet residual oil, film-like residual oil, and capillary-bound oil. Then, identify and count the number, average diameter of isolated droplet oil, and its area proportion in the total oil phase. Identify and count the film layer thickness of film-like oil and the pore wall coverage rate. Identify and count the spatial distribution and relative content of capillary-bound oil. S6. Analysis of the CO2 huff and puff process: a. Data acquisition: Process the obtained microfluidic experiment images using ImageJ or Matlab to identify the CO2 gas phase region and the position of the oil-gas interface. Use ImageJ to count the number of pixels in the CO2 gas phase region, calculate the two-dimensional area of the CO2 region according to the image resolution, and combine the etching depths of the main channel and matrix region of the microfluidic chip to convert the two-dimensional area into a three-dimensional volume and calculate the CO2 filling volume. Extract the distribution region of CO2 in the chip through color threshold segmentation, and identify and quantify the volume proportions of gaseous CO2 and liquid CO2 respectively. b. Calculate the CO2 sweep range: First, calculate the change in the oil-water interface after CO2 enters: Use the color threshold segmentation algorithm to identify the CO2, crude oil, and water three-phase regions; use the contour extraction algorithm 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 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. Statistically analyze the expansion rate of the CO2 swept region at different time points: Set several time nodes and compare the swept areas at each moment; divide the area difference by the time difference to obtain the average expansion rate of the CO2 swept region. c. Calculate the oil displacement efficiency of CO2 huff and puff on residual oil: (1) Calculate the saturations of isolated droplet residual oil, film-like residual oil, and capillary-bound oil respectively before and after CO2 displacement. Among them, the calculation formulas for the saturations of various types of residual oil before CO2 displacement are as follows: ； In the formula: S boil,i —— Before CO2 displacement, i The remaining oil saturation of this type; A before,oil-i —— Before CO2 flooding, i the volume of the remaining oil of this type; A total —— total pore volume; The calculation formulas for the remaining oil saturation of various types after CO2 displacement are as follows: ； In the formula: S aoil,i —— After CO2 displacement, i Residual oil saturation of this type; A after,oil-i —— After CO2 flooding, i the volume of the remaining oil of this type; A total —— total pore volume; (2) Calculate the oil displacement efficiency of CO2 for isolated droplet-shaped remaining oil, film-shaped remaining oil, and capillary-bound oil respectively: ； E oil,i —— Displacement efficiency of CO2 for i class of remaining oil; S boil,i ——Before CO2 flooding, i type of remaining oil saturation; S aoil,i —— After CO2 displacement, i similar remaining oil saturation; S7. Calculate the total buried amount of CO2: (1) Calculate the buried amount of CO2 in the gas phase. The formula is as follows: ； In the formula: —— The buried amount of CO2 in the gas phase; P —— CO2 gas phase pressure measured by experiment, MPa; —— CO2 gas phase volume identified by microfluidic experiment 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 dissolved amount of CO2 in the oil phase: ① Use the unsteady-state diffusion equation to calculate the dissolution and diffusion of CO2 in the oil phase: ; Wherein: D ——Diffusion coefficient of CO2 in oil, m 2 / s; x ——Depth of the oil phase, m; t ——CO2 action time, s; ② Use the semi-infinite diffusion model to calculate the CO2 solubility distribution: ; Wherein: C(x,t) —— CO2 concentration at a distance from the interface in the oil phase x m, mol / m 3 ; C 0——CO2 saturation solubility at the oil-gas interface; erf() —— error function; x ——Depth of oil phase, m; t ——CO2 action time, s; ③ Calculate the total dissolved amount of CO2 in the oil phase: ; Wherein: ——Total dissolved amount of CO2 in the oil phase; L —— the length of the oil phase region; V unit —— chip volume per unit volume of the oil phase; x ——Depth of oil phase, m; t ——CO2 action time, s; Use Matlab / Python for numerical integration to calculate the total dissolved amount of CO2 in the oil phase; (3) Calculate the total CO2 sequestration amount: ; Wherein: ——Total CO2 burial amount; —— CO2 storage in the gas phase; ——Total dissolved amount of CO2 in the oil phase.

2. 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 claim 1, wherein The microfluidic chip adopted includes a main fluid channel A, a branched fracture region B, two high-permeability regions C of tight reservoirs, and two low-permeability regions D of tight reservoirs; Among them, the two high-permeability regions C of tight reservoirs and the two low-permeability regions D of tight reservoirs are arranged alternately on the microfluidic chip, forming an arrangement pattern of C-D-C-D; 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 reservoir. The main fluid channel A is connected to the branched fracture region B; the branched fracture region B only penetrates and communicates within this C-D region combination; Take the other C-D region combination as the matrix region; The etching depths of the main fluid channel A and the branched fracture B are the same, denoted as depth a; the etching depths of the high-permeability region C of the tight reservoir and the low-permeability region D of the tight reservoir are the same, denoted as depth b; the ratio of depth a to depth b is 2-4:

1.

3. 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 claim 2, wherein, The length:width of the microfluidic chip is 2:

1.

4. The method for characterizing the carbon dioxide flooding efficiency and storage volume in a fractured tight oil reservoir at the micro-nano scale according to claim 2, wherein The microfluidic chip is 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. Use imageJ software to perform 8-bit processing, binary processing, and denoising processing on the image; (2) Use 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; (3) Piece together the simplified structure diagrams of rock particles of various sizes into a complete matrix region picture, and then convert the designed matrix region picture into a format recognizable by the photolithography mask manufacturing equipment to fabricate the photolithography mask; (4) Process the complete matrix region picture with imageJ software. Use the AnalyzeParticles function to identify and count the pore regions in the picture, calculate the total pore area; then calculate the matrix region porosity; (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 hot pressing to form the microfluidic chip obtained.

Images