Method for determining CO2 distribution of CO2 displacement natural gas in porous medium

Abstract

The invention discloses a method for determining CO2 distribution of CO2 displacement natural gas in a porous medium, and belongs to the technical field of rock physics and geophysical exploration, and the method comprises the following steps: S1, putting a porous medium core model into a holder, and heating and pressurizing to preset experimental conditions; s2, shooting an initial picture of the rock through X-CT scanning; s3, the CO2 extracted with the iodoethane indicator is displaced into the rock core, and a CT picture obtained after CO2 is injected into the rock is shot through X-CT scanning; and S4, representing CO2 distribution according to the CT pictures of different CO2 injection amounts. When CO2 displaces natural gas and other gases, the distribution of CO2 can be accurately and quantitatively described by monitoring the distribution change of CO2 in a rock core or a simulated stratum in real time. Therefore, the CO2 gas driving effect can be scientifically and accurately evaluated, and the method plays an important role in researching the rules of migration, burying, diffusion and the like of the CO2 plume under the formation condition.

Description

Technical Field

[0001] The 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] The core of carbon capture, utilization and storage (CCUS) technology is to capture CO2 emitted from industrial sources and subsequently store or reuse it, which is of great value for mitigating global warming and protecting the environment. Currently, the most widely used CCUS examples are CO2 oil recovery technology and CO2 natural gas recovery technology, which improves the recovery efficiency of crude oil or natural gas by injecting CO2 into oil or gas reservoirs. The key point of the technology is that the injected CO2 reacts with crude oil, natural gas, etc. to form a miscible front, effectively separate heavier hydrocarbons, dissolve and extract natural gas, and thus can efficiently drive oil or gas to production wells. This process is restricted by CO2 and formation temperature and pressure conditions, showing gaseous, liquid and supercritical states, and its transmission mechanism in rock media is complex. Therefore, it is very important to develop a method for monitoring the distribution of CO2 in different states under formation conditions, scientifically evaluate the effect of CO2 displacement, and study the migration and storage laws of CO2 under formation conditions. In particular, there is no effective means to form a high-resolution CO2 plume distribution map when CO2 displaces natural gas and other gases. 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 it is difficult to accurately and quantitatively describe the distribution of CO2 in a CO2 fluid displacement experiment in the prior art.

[0004] In order to achieve the above-mentioned invention object, 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 pictures of rocks through X-CT scanning;

[0007] S3, driving 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] Further: S4 includes the following sub-steps:

[0010] S41, comparing the CT images with different CO2 injection amounts with the initial image, obtaining the 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 calibrating the absorbance of the extracted iodoethane indicator substance according to 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 content of CO2 absorbed at each pixel is calculated to obtain the specific distribution of CO2 at different locations.

[0012] Further: In the 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] In the formula, 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 a substance.

[0015] The beneficial effects of the present invention are as follows: the present invention provides a method for determining the distribution of CO2 in a porous medium when CO2 displaces natural gas. 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 play an important role in studying the migration, storage, diffusion and other laws of CO2 plumes under formation conditions. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0017] The specific implementation modes of the present invention are described below so that those skilled in the art can understand the present invention. However, it should be clear that the present invention is not limited to the scope of the specific implementation modes. For those of ordinary skill in the art, as long as various changes are within the spirit and scope of the present invention as defined and determined by the attached claims, these changes are obvious, and all inventions and creations utilizing the concept 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 comprises 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 pictures of rocks through X-CT scanning;

[0021] S3, driving 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, when the present invention displaces gas with CO2, this solution mainly utilizes the strong penetrability of X-rays and the strong extraction ability of CO2 to make an invention. X-rays have strong penetrability, and through X-rays, the situation inside the core can be directly observed through the titanium alloy or aluminum alloy holder. The molecular mass of CO2 atoms is relatively small, and the attenuation of X-rays is relatively weak. It is difficult to accurately observe the change in the amount of CO2 by directly using X-rays to observe the migration and distribution of CO2 in the core.

[0024] CO2 has a strong extraction ability, especially CO2 under high temperature and high pressure, so it is relatively easy to add volatile oily markers. Using CO2 to extract a small amount of ethyl iodide according to the temperature and pressure conditions of the experiment can greatly increase the attenuation rate of CO2 to X-rays, while having little effect on the characteristics of CO2 itself. The same effect can be achieved by using iodopropane, etc. Therefore, ethyl iodide can be used as an X-ray indicator and added to CO2. X-ray CT can easily show the location of CO2. And the distribution of CO2 is quantitatively analyzed through the change of CT value to obtain a high-resolution distribution map of CO2 in rocks.

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

[0026] S41, comparing the CT images with different CO2 injection amounts with the initial image, obtaining the 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 calibrating the absorbance of the extracted iodoethane indicator substance according to 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 content of CO2 absorbed at each pixel 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] In the formula, 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 a 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, take the logarithm of the ratio of the light intensity after attenuation to that before attenuation of the same material, and the result is linearly related to the thickness of the material.

[0034] In this embodiment, the devices required by the present invention include at least: an X-ray CT set, a set of aluminum alloy clamps, a set of CO2 injection metering pumps, 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, and the porous medium core model is placed in the holder, and 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 ane, etc.) indicator into the core and record the injection volume m1. When CO2 is monitored at the outlet, the injection volume does not need to be recorded.

[0038] Use X-CT to take photos of rocks after CO2 is injected, and obtain CT images of different injected amounts of CO2.

[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 material increase relative to the initial image can be expressed as:

[0040]

[0041] Before CO2 comes out of the core model outlet, the total amount of injected CO2 is equal to the added CO2 content in the model. Dividing the corresponding points of each image, taking the logarithm, and then summing them, we can get ∑(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 content. The sum of the CO2 content is equal to the injected CO2, which can be used to calibrate the different absorption coefficients of indicators with different degrees of extraction.

[0042] By using this absorption coefficient into the pictures before and after CO2 injection, dividing the corresponding points of each picture and taking the logarithm, and then dividing by the obtained coefficient K, the specific CO2 absorption content of each pixel can be calculated. The specific distribution of CO2 at different locations can be obtained. In this way, the CO2 distribution and concentration at different locations can be obtained.

[0043] The beneficial effects of the present invention are as follows: the present invention provides a method for determining the distribution of CO2 in a porous medium when CO2 displaces natural gas. 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 play an important role in studying the migration, storage, diffusion and other laws of CO2 plumes under formation conditions.

[0044] In the description of the present invention, it is necessary to understand that the orientation or positional relationship indicated by the terms "center", "thickness", "upper", "lower", "horizontal", "top", "bottom", "inner", "outer", "radial", etc. is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention. In addition, the terms "first", "second", and "third" are used only for descriptive purposes, and cannot be understood as indicating or implying the relative importance or the number of implicitly specified technical features. Therefore, the features defined by "first", "second", and "third" may explicitly or implicitly include one or more of the 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 pictures of rocks through X-CT scanning; S3, driving CO2 extracted with iodine ethane indicator into the core, and taking CT images of the rock after CO2 injection through X-CT scanning; S4. Characterize the distribution of CO2 based on CT images of different CO2 injection amounts.

2. The method for determining CO2 distribution in porous media by displacing natural gas with CO2 according to claim 1, characterized in that: The S4 comprises the following sub-steps: S41, comparing the CT images with different CO2 injection amounts with the initial image, obtaining the 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 calibrating the absorbance of the extracted iodoethane indicator substance according to the relationship and the CO2 injection amount; 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 content of CO2 absorbed at each pixel is calculated to obtain the specific distribution of CO2 at different locations.

3. The method for determining CO2 distribution in a porous medium by displacing natural gas with CO2 according to claim 2, characterized in that: 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: In the formula, 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 a substance.

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