Method for determining CO2 distribution in porous media when CO2 displaces natural gas

Abstract

The present invention discloses a method for determining CO2 distribution in a porous medium when CO2 displaces natural gas, belonging to the field of rock physics and geophysical exploration technology, and comprising the following steps: S1, placing a porous medium core model in a holder, heating and pressurizing it to preset experimental conditions; S2, taking an initial image of the rock by X-CT scanning; S3, displacing CO2 extracted with iodine ethane indicator into the core, and taking a CT image of the rock after CO2 injection by X-CT scanning; S4, characterizing the distribution of CO2 based on the CT images of different CO2 injection amounts. When CO2 displaces natural gas or other gases, the distribution of CO2 can be accurately and quantitatively described by real-time monitoring of the distribution changes of CO2 in the core or simulated formation. This can make a scientific and accurate assessment of the CO2 displacement effect, and plays an important role in studying the laws of migration, storage, diffusion, etc. of CO2 plumes under formation conditions.

Description

Technical Field

[0001] The present invention belongs to the field of rock physics and geophysical exploration technology, and in particular relates to a method for determining CO2 distribution in a porous medium when CO2 displaces natural gas. Background Art

[0002] Carbon capture, utilization, and storage (CCUS) technology focuses on capturing CO2 emitted from industrial sources for subsequent storage or reuse, which holds significant value for mitigating global warming and protecting the environment. Currently, widely used examples of CCUS include CO2 flooding for oil and CO2 flooding for natural gas. These technologies inject CO2 into oil or gas reservoirs to enhance crude oil or natural gas recovery efficiency. The key to this technology is that the injected CO2 interacts with crude oil, natural gas, and other materials to form a miscible front, effectively separating heavier hydrocarbons and dissolving and extracting natural gas, thereby efficiently displacing oil or gas to production wells. This process is governed by the CO2 and formation temperature and pressure, exhibiting gaseous, liquid, and supercritical states. Its transport mechanisms within rock media are complex. Therefore, developing a method to monitor the distribution of CO2 in different states under formation conditions, scientifically evaluate the effectiveness of CO2 flooding, and study its migration and storage patterns within formations is crucial. In particular, there is no effective method for generating high-resolution CO2 plume maps when displacing gases such as natural gas. Summary of the Invention

[0003] In view of the above-mentioned deficiencies in the prior art, the method for determining CO2 distribution in a porous medium by displacing natural gas with CO2 provided by the present invention solves the problem that the prior art is difficult to accurately and quantitatively describe the distribution of CO2 in CO2 fluid displacement experiments.

[0004] In order to achieve the above-mentioned object of the invention, the technical solution adopted by the present invention is: a method for determining CO2 distribution in a porous medium by displacing natural gas with CO2, comprising the following steps:

[0005] S1. Place the porous medium core model in a holder and heat and pressurize it to the preset experimental conditions;

[0006] S2, taking initial images of the rock through X-CT scanning;

[0007] S3, displacing CO2 extracted with iodine ethane indicator into the core, and taking CT images of the rock after CO2 injection through X-CT scanning;

[0008] S4. Characterize the distribution of CO2 based on CT images of different CO2 injection amounts.

[0009] Furthermore, the step S4 includes the following steps:

[0010] S41. Compare the CT images with the initial image at different CO2 injection amounts to obtain a relationship between the total brightness change and the total substance increase of the CT images with different CO2 injection amounts relative to the initial image, and calibrate the absorbance of the extracted ethyl iodide indicator substance based on the relationship and the CO2 injection amount;

[0011] S42. Based on the absorption rate of the iodine ethane indicator substance and the CT images of the rock before and after CO2 injection, the specific CO2 absorption content of each pixel point is calculated to obtain the specific distribution of CO2 at different locations.

[0012] Furthermore, in S41, the relationship between the total brightness change and the total material increase of the CT images of different CO2 injection amounts relative to the initial image is expressed by the following formula:

[0013]

[0014] Where K is the absorption rate of the material, l is the thickness of the material, C is the concentration of the absorbing material, I0 is the initial light intensity, I t is the intensity of light after passing through the substance.

[0015] The present invention provides a method for determining CO2 distribution in porous media during CO2 displacement of natural gas. By monitoring the distribution of CO2 in a core or simulated formation in real time, the method can accurately and quantitatively describe the distribution of CO2. This allows for a scientific and accurate assessment of the CO2 displacement effect and plays an important role in studying the migration, storage, and diffusion of CO2 plumes in formation conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 This is a flow chart of the method for determining CO2 distribution in a porous medium when CO2 displaces natural gas according to the present invention. DETAILED DESCRIPTION

[0017] The specific embodiments of the present invention are described below to facilitate understanding of the present invention by those skilled in the art. However, it should be clear that the present invention is not limited to the scope of the specific embodiments. For those skilled in the art, as long as various changes are within the spirit and scope of the present invention as defined and determined by the appended claims, these changes are obvious, and all inventions and creations utilizing the concepts of the present invention are protected.

[0018] like Figure 1 As shown, in one embodiment of the present invention, a method for determining CO2 distribution in a porous medium by displacing natural gas with CO2 includes the following steps:

[0019] S1. Place the porous medium core model in a holder and heat and pressurize it to the preset experimental conditions;

[0020] S2, taking initial images of the rock through X-CT scanning;

[0021] S3, displacing CO2 extracted with iodine ethane indicator into the core, and taking CT images of the rock after CO2 injection through X-CT scanning;

[0022] S4. Characterize the distribution of CO2 based on CT images of different CO2 injection amounts.

[0023] In this embodiment, the present invention utilizes the CO2 displacement gas by combining the strong penetrating properties of X-rays with the robust CO2 extraction capability. X-rays have strong penetrating properties, allowing them to directly penetrate titanium or aluminum alloy holders and observe the interior of the core. CO2 atoms have a relatively small molecular mass and are less susceptible to X-ray attenuation. Directly using X-rays to observe the migration and distribution of CO2 within the core makes it difficult to accurately observe changes in the CO2 content.

[0024] CO2 has a strong extraction capacity, especially under high temperature and pressure, making it relatively easy to incorporate volatile, oily markers. Using CO2 to extract a small amount of ethyl iodide, depending on the experimental temperature and pressure conditions, can significantly increase the CO2's X-ray attenuation rate while minimally affecting the CO2's inherent properties. Similar effects can be achieved using iodopropane or other agents. Therefore, ethyl iodide can be added to CO2 as an X-ray indicator. This allows X-ray CT to easily identify the CO2's location. Changes in CT values can also be used to quantitatively analyze the CO2 distribution, yielding a high-resolution map of its distribution within the rock.

[0025] The S4 comprises the following sub-steps:

[0026] S41. Compare the CT images with the initial image at different CO2 injection amounts to obtain a relationship between the total brightness change and the total substance increase of the CT images with different CO2 injection amounts relative to the initial image, and calibrate the absorbance of the extracted ethyl iodide indicator substance based on the relationship and the CO2 injection amount;

[0027] S42. Based on the absorption rate of the iodine ethane indicator substance and the CT images of the rock before and after CO2 injection, the specific CO2 absorption content of each pixel point is calculated to obtain the specific distribution of CO2 at different locations.

[0028] In S41, the relationship between the total brightness change and the total material increase of the CT images with different CO2 injection amounts relative to the initial image is expressed by the following formula:

[0029]

[0030] Where K is the absorption rate of the material, l is the thickness of the material, C is the concentration of the absorbing material, I0 is the initial light intensity, I t is the intensity of light after passing through the substance.

[0031] The calculation method of the above relationship uses the attenuation law of X-rays passing through matter, which is described by the Lambert-Beer law. The formula is:

[0032]

[0033] For a point in the image, the logarithm of the ratio of the light intensity of the same material after attenuation to the light intensity before attenuation is taken, and the result is linearly related to the thickness of the material.

[0034] In this embodiment, the present invention requires at least one device: an X-ray CT scanner, an aluminum alloy holder, a CO2 injection metering pump, and an outlet CO2 monitoring sensor. The specific operation process of the method of the present invention is as follows:

[0035] A holder made of aluminum alloy is used to place the porous medium core model into the holder, and the core is heated and pressurized to the experimental conditions.

[0036] Initial photographs of the rock were taken using X-CT scanning.

[0037] Drive CO2 extracted with a certain amount of iodine ethane (iodine propyl, etc.) indicator into the core and record the injection volume m1. When CO2 is monitored at the outlet, the injection volume can be stopped.

[0038] X-CT was used to take photos of rocks after CO2 was injected, and CT images of rocks with different injection amounts of CO2 were obtained.

[0039] After CO2 invaded the model, the CT scan results were compared with the initial images. The relationship between the total brightness change and the total mass increase relative to the initial image can be expressed as:

[0040]

[0041] Before CO2 is released from the core model outlet, the total amount of injected CO2 is equal to the added CO2 content in the model. Dividing the corresponding points in each image, taking the logarithm, and summing the results yields ∑(K*l*c), or K∑(l*c), where ∑(l*c) is the thickness multiplied by the concentration at each point, representing the sum of the CO2 content. This sum of CO2 content equals the injected CO2, which can be used to calibrate the absorption coefficients of indicators with varying degrees of extraction.

[0042] By substituting this absorption coefficient into images taken before and after CO2 injection, dividing the corresponding points in each image, taking their logarithms, and then dividing by the coefficient K, we can calculate the specific CO2 absorption content at each pixel. This gives us the specific distribution of CO2 at different locations, and thus the CO2 distribution and concentration at different locations.

[0043] The present invention provides a method for determining CO2 distribution in porous media during CO2 displacement of natural gas. By monitoring the distribution of CO2 in a core or simulated formation in real time, the method can accurately and quantitatively describe the distribution of CO2. This allows for a scientific and accurate assessment of the CO2 displacement effect and plays an important role in studying the migration, storage, and diffusion of CO2 plumes in formation conditions.

[0044] In the description of the present invention, it should be understood that the terms "center", "thickness", "upper", "lower", "horizontal", "top", "bottom", "inner", "outer", "radial", etc., indicating the orientation or positional relationship, are based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operate in a specific orientation, and therefore cannot be understood as limiting the present invention. In addition, the terms "first", "second", and "third" are used for descriptive purposes only and cannot be understood as indicating or implying the relative importance or the number of technical features implicitly specified. Therefore, the features defined by "first", "second", and "third" may explicitly or implicitly include one or more of such features.

Claims

1. A method for determining CO2 distribution in a porous medium by displacing natural gas with CO2, characterized in that: The following steps are involved: S1. Place the porous medium core model in a holder and heat and pressurize it to the preset experimental conditions; S2, taking initial images of the rock through X-CT scanning; S3. Flood the core with CO2 extracted with iodine ethane as an indicator, and record the injection volume. When CO2 is detected at the outlet, do not record the injection volume. Take a CT image of the core after CO2 injection by X-CT scanning to obtain CT images of CO2 at different injection volumes. S4. Characterize the distribution of CO2 based on CT images of different CO2 injection amounts; Wherein, said S4 comprises the following sub-steps: S41. Compare the CT images with the initial image at different CO2 injection amounts to obtain a relationship between the total brightness change and the total substance increase of the CT images with different CO2 injection amounts relative to the initial image, and calibrate the absorbance of the extracted ethyl iodide indicator substance based on the relationship and the CO2 injection amount; S42. Based on the absorption rate of the iodine ethane indicator substance and the CT images before and after the CO2 injection into the core, the specific CO2 content of each pixel is calculated to obtain the specific distribution of CO2 at different locations; In S41, the relationship between the total brightness change and the total material increase of the CT images with different CO2 injection amounts relative to the initial image is expressed by the following formula: Where K is the absorptivity of the substance, l is the thickness of the substance, C is the concentration of the absorbing substance, I0 is the initial light intensity, and It is the light intensity after passing through the substance; Before CO2 comes out of the core model outlet, the total amount of CO2 injected is equal to the increased CO2 content in the model. The initial light intensity before light intensity attenuation at the corresponding point in each image is divided by the light intensity after light intensity attenuation, that is, after passing through the substance, and then the logarithm is taken and the sum is obtained to obtain ∑(K*l*c), that is, K∑(l*c), where ∑(l*c) is the thickness of each point multiplied by the concentration, that is, the sum of the CO2 contents; the sum of the CO2 contents is equal to the injected CO2, which is used to calibrate the different absorption coefficients of indicators with different degrees of extraction. The above absorption coefficient is substituted into the images before and after CO2 injection, and the initial light intensity at the corresponding point in each image is divided by the light intensity after passing through the substance, and then the logarithm is taken, and then divided by the obtained absorption coefficient K. The specific CO2 content of each pixel point can be calculated, and the specific distribution of CO2 at different positions can be obtained, thereby obtaining the CO2 distribution and concentration at different positions.

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