A method for tracing CO2 migration using rare earth elements in CO2-ECBM

Abstract

The present invention discloses a method for tracing CO2 migration using rare earth elements in CO2-ECBM, comprising the following steps: selecting injection wells and extraction wells in a study area for tracing rare earth elements; selecting a certain rare earth element or a combination of several elements to mark CO2 in different injection wells with different rare earth elements; injecting CO2 with different rare earth markers into each injection well, collecting gas samples in the extraction well and testing the rare earth element content to obtain rare earth element characteristics; judging whether CO2 migrates to the extraction well and which corresponding injection well the CO2 originates from based on whether the rare earth marker is detected in the sample and what the rare earth marker is. The present invention uses a method for tracing CO2 migration using rare earth elements in CO2-ECBM, and obtains information on the migration status of CO2 by comparing the rare earth element characteristics of the gas in the injection well and the extraction well. The method has clear ideas, strong applicability, simple operation, safety and environmental protection.

Description

Technical Field

[0001] The invention belongs to the field of mining, and in particular to a method for tracing CO2 migration using rare earth elements in coal seam CO2 geological storage and CH4 enhanced mining (CO2-ECBM). Background Art

[0002] 1.1 Coalbed Methane (CBM)

[0003] Coalbed methane (CBM) is a type of unconventional natural gas produced and stored in coal seams with a methane content of about 90% or even higher. It is called gas in coal mining production, and is called coalbed methane or coalbed methane from the perspective of energy utilization. [1] .

[0004] my country's coalbed methane resources are very large, which is basically equivalent to the total conventional natural gas resources of 33.11 trillion cubic meters in the country. The coalbed methane above the -2000-meter level reaches 32.15 trillion cubic meters. [1] Coalbed methane is not only a high-quality unconventional natural gas resource, but also a harmful gas that seriously threatens coal mine production safety and seriously pollutes the environment. Therefore, from the perspectives of resource development, production safety, and environmental protection, the efficient development and effective utilization of coalbed methane has very important practical significance. [1] .

[0005] 1.2 Enhanced Coal Bed Methane Recovery (ECBM)

[0006] There are many ways to improve coalbed methane recovery rate. The existing methods and technologies mainly include: [2] : Drainage and pressure reduction method, which reduces the formation pressure by drainage to achieve CH4 desorption and thus increase the coalbed methane recovery rate; gas displacement method, which replaces CH4 by injecting different displacement agents (such as water, N2, CO2 gas, etc.) into the coal seam to increase the production of coalbed methane; hydraulic fracturing method, which uses a ground high-pressure pump to inject a large amount of chemical substances mixed with water, sand, gravel, etc. into the reservoir for hydraulic fracturing to release natural gas; multi-branch horizontal well technology, which drills multiple branch wellbores on both sides of a main horizontal wellbore as gas release channels, and drills a vertical well to connect the main horizontal wellbore, which can greatly improve the coalbed methane extraction efficiency; thermal promotion method, which directly injects conventional thermal fluids, or passes direct current to heat the reservoir, or injects oxidants to cause oxidation reactions in the formation to generate heat, destroying the original adsorption balance of the gas and reducing the gas adsorption capacity, thereby increasing the coalbed methane recovery rate.

[0007] As one of the common and effective methods, Enhanced Coal Bed Methane Recovery (ECBM) is to inject CO2 into coal seams, abandoned mines, saline water layers, etc. to form competitive adsorption with CH4 inside, thereby driving CH4 out. [3] This method can not only promote the improvement of coalbed methane (CBM) recovery rate, but also seal CO2 in coal seams, abandoned mines, idle and abandoned sites, etc., which can greatly reduce the greenhouse effect and is a feasible method. [3] .

[0008] 1.3 Coalbed CO2 Geological Storage and CH4 Enhanced Mining (CO2-ECBM)

[0009] The CO2 Geological Storage-Enhanced Coalbed Methane Recovery (CO2-ECBM) technology can replace a large amount of CH4 in coalbeds due to the competitive adsorption advantage of CO2, thereby achieving enhanced coalbed methane production and improving coalbed methane recovery, integrating greenhouse gas emission reduction and new energy development, and has great development prospects. Whether it is reducing carbon emissions or efficiently developing coalbed methane, both are major needs of my country, and coalbed CO2-ECBM has great practical significance [4] .

[0010] CO2-ECBM technology is an emerging technology. Currently, micro-pilot tests are being conducted in many regions around the world, and multi-well pilot test technology is also being gradually developed, but it has not yet been commercialized. [5] CO2-ECBM technology is still in the experimental exploration stage in my country; China has abundant coal resources and widely distributed coal seams, and CO2-ECBM technology has broad application prospects in my country. [5] Therefore, the experimental exploration and scientific research work on geological storage of CO2 and enhanced mining of CH4 in coal seams (CO2-ECBM) should be strengthened.

[0011] 1.4 Carbon dioxide migration

[0012] In the entire CO2-ECBM technology, how to inject CO2 into coal seams to achieve the goal of reducing CO2 emissions and increasing CH4 production without causing catastrophic CO2 leakage requires a lot of theoretical analysis, experimental testing, numerical simulation, technology research and development, and engineering cases. Among them, the migration status of carbon dioxide is one of the important concerns among many links.

[0013] From the perspective of the effectiveness of CO2 displacing CH4, a large amount of CO2 is injected into the coal seam from the injection well for a long time. Has the CO2 injected into the coal seam migrated? Where has it migrated? It may have been stored in the coal seam, migrated to the surrounding rock, migrated to the extraction well, migrated to the coal seam outcrop, migrated to the open fault, etc. The information on the migration of CO2 is very important for evaluating the effectiveness of CO2-ECBM technology.

[0014] There has been a lot of concern about the safety of CO2 displacement of CH4. For example, if the geological safety limit of the reservoir is exceeded during CO2 displacement and storage, CO2 leakage will occur, causing harm to the regional natural environment and human life. [6] Although CO2 can be safely and long-term stored in underground structural or lithological traps, there are still many uncertainties that may lead to CO2 leakage and environmental damage. Many cases show that the risk of CO2 leakage exists during CO2 displacement and geological storage, and it may cause great environmental damage. [7] The environmental risk of CO2 storage refers to the possibility of sudden or slow leakage of CO2 if the process selection or storage site selection is inappropriate, which may cause a series of environmental problems such as groundwater pollution, soil acidification, and ecological damage. [8] ; etc.; Therefore, obtaining as much information as possible on the CO2 migration status is very important for evaluating the safety of CO2-ECBM technology.

[0015] 1.5 Characteristics of rare earth elements

[0016] Rare earth elements are not one element. Usually, the 15 elements of the lanthanide series and yttrium are called rare earth elements, including yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). In English, they are called Rare Earth Elements, which can be abbreviated as REE. [9] The similarity of the atomic structure and crystal chemical properties of rare earth elements makes rare earth elements a special "element family". They coexist closely in nature. Generally speaking, in various types of rocks and minerals, as long as one of the rare earth elements appears, the others will also exist at the same time. At the same time, due to the similarity of rare earth elements in crystal chemical properties, element alkalinity, element valence (Ce 4+ 、Eu 2+), the ability to form complexes, the strength of ion adsorption, etc., cause the differentiation of rare earth elements in nature; the movement and combination of rare earth elements are a reflection of certain geological and physical and chemical conditions. Conversely, the physical and chemical conditions of geological processes can be explained based on the composition characteristics of rare earth elements in geological bodies. This is the theoretical basis for rare earth elements to be used as geochemical indicators. [9] There are many testing methods used for rare earth element research at home and abroad, mainly including neutron activation method, plasma spectroscopy, isotope dilution mass spectrometry, spark source mass spectrometry, X-ray fluorescence spectroscopy, etc. At present, the application of plasma mass spectrometry is increasing. [9] .

[0017] Rare earth element characteristics [9] , mainly including the standardized distribution pattern of rare earth elements in chondrites and the geochemical parameters of rare earth elements. In the case of rare earth element markers, the focus is on the content of the marked rare earth elements.

[0018] Standardized distribution pattern of rare earth elements in chondrites: The logarithm of the rare earth element content after chondrite standardization is a function of its atomic number, and the distribution pattern of rare earth elements obtained is almost close to a straight line. The standardized diagram of rare earth elements in chondrites eliminates the disadvantage of jagged graphs due to the even number rule when directly plotting the rare earth element content or percentage; the standardized diagram of rare earth elements in chondrites provides a method to intuitively represent the distribution characteristics of rare earth elements, and is therefore widely used in the study of rare earth element geochemistry. [9] .

[0019] Figure 2 This is a standardized distribution pattern diagram of rare earth elements in chondrites.

[0020] Geochemical parameters of rare earth elements: Geochemical parameters of rare earth elements are one of the main means to reflect the characteristics of rare earth elements. There are many geochemical parameters of rare earth elements, and the most commonly used ones are ∑REE, LREE, HREE, LREE / HREE, (La / Yb) N , δEu, δCe, etc. [9] .

[0021] ∑REE: refers to the total content of rare earth elements, usually refers to the sum of the contents of 15 elements from La to Lu, and is also the sum of LREE and HREE.

[0022] LREE: refers to the content of light rare earth elements, usually refers to the sum of the contents of elements from La to Eu.

[0023] HREE: refers to the content of heavy rare earth elements, usually refers to the sum of the contents of each element from Gd to Lu.

[0024] LREE / HREE: refers to the ratio of LREE to HREE. This parameter reflects the degree of differentiation of rare earth elements to a certain extent.

[0025] (La / Yb) N : Refers to the ratio of La and Yb standardized by chondrites, reflecting the degree of differentiation of rare earth elements.

[0026] δEu: Indicates the degree of abnormality of the Eu element; reflected in the standardized distribution pattern diagram of rare earth element chondrites, the curve shows a "peak" or "valley" at Eu. The "peak" is called the Eu positive anomaly and the "valley" is called the Eu negative anomaly.

[0027] δCe: indicates the degree of abnormality of the Ce element; if δCe>1, Ce is positively abnormal; if δCe≈1, Ce is not abnormal; if δCe<1, Ce is negatively abnormal.

[0028] Regarding the background values ​​of various rare earth elements in coal seams, coalbed methane, CH4, and CO2 as injected gas, which are closely related to CO2-ECBM technology: Generally speaking, the background values ​​of rare earth element content in coal seams, CH4, and CO2 are very low. The content of each rare earth element in coal seams is usually in the range of a few tenths, a few tens, or even tens of ppm (parts per million, 1×10 -6 ), Swaine (1990) shows the rare earth element content in coal from all over the world in Table 1 [9] The results of Wang Yunquan (1997) on 89 samples from China are shown in Table 2 [9] The test results of 32 coal samples from Huainan mining area, Huaibei coalfield and Weishan Lake mining area by Zhao Zhigen (2002) are shown in Table 3. [9] The rare earth element content of CH4 and CO2 as raw gases is even lower, at ppb (parts per billion, 1×10 -9 ), or ppt (parts per trillion, 1×10 -12 ), or even be negligible.

[0029] Table 1 Rare earth element content in coal worldwide

[0030] Rare Earth Elements Content range (ppm) Rare Earth Elements Content range (ppm) La 0.58～92 G 0.66～7.6 Ce 0.44～183 Tb <0.02～2.1 Pr 1.7～9.9 Dy <0.07～16 Nd 1.3～85 Ho 0.07～11 Pm — Er 0.5～6.9 Sm 0.08～19 Yb 0.046～10 Eu 0.02～3.8 Lu 0.002～30

[0031] Table 2 Rare earth element content in Chinese coal

[0032] element Min.(ppm) Maximum (ppm) Average(ppm) La 0.58 70.90 24.06 Ce 2.76 158.00 44.96 Nd 3.77 67.40 21.18 Sm 0.08 14.30 3.80 Eu 0.02 2.90 0.72 Tb 0.07 2.11 0.54 Yb 0.05 5.97 1.47 Lu 0.01 1.04 0.26

[0033] Table 3 Rare earth element content in coal from some mining areas

[0034] element Min.(ppm) Maximum (ppm) Average(ppm) La 1.80 51.54 22.30 Ce 4.80 99.01 45.62 Nd 2.90 40.49 16.82 Sm 0.84 6.72 2.92 Eu 0.23 1.22 0.64 Tb 0.16 0.75 0.46 Yb 0.44 4.62 1.98 Lu 0.078 0.81 0.34

[0035] 1.6 Rare earth element tracer

[0036] There is a need for CO2 tracing in CO2-ECBM: CO2-ECBM technology is a major demand in my country, whether it is to reduce carbon emissions or to efficiently develop coalbed methane, and coalbed CO2-ECBM has great practical significance; CO2-ECBM technology is an emerging technology and is in the experimental exploration stage in my country. We should strengthen the experimental exploration and scientific research work on coalbed CO2 geological storage and CH4 enhanced mining (CO2-ECBM); from the effectiveness of CO2 displacing CH4, and from the safety of CO2 displacing CH4, it is necessary to understand the CO2 migration situation.

[0037] Rare earth elements have excellent performance: some inorganic salt tracers have large detection errors, large dosages, high costs, and may affect the mineralization of formation water, which limits the use of inorganic salt tracers; the tracer of radioactive elements is also increasingly restricted due to its potential environmental hazards; rare earth elements have excellent performance, a wide variety, no interference with each other, and are non-toxic, non-radioactive, and have extremely strong thermal stability and biological stability. At the same time, they have the advantages of perfect testing technology and high precision.

[10] .

[0038] Rare earth elements have many tracer applications: rare earth elements have stable chemical properties, high degree of homogeneity, and are not easily disturbed by metamorphism. Once "recorded" in a geological body, they are easy to be preserved, so rare earth elements are a good geochemical indicator. [9] Rare earth elements not only serve as geochemical indicators, but also have a wide range of tracer effects in many fields. For example, in soil erosion research, the principle of REE tracer technology is to evenly mix REE tracer compounds with soil and then place them in different terrain parts of the studied area, so that they can be transported and deposited with sediment during the entire soil erosion process. The type and content of REE in the sediment samples are then determined, and the laws of erosion and sediment production such as soil erosion and sediment source distribution are further analyzed.

[11] The rare earth element tracer monitoring method for horizontal well fracturing is to add different rare earth element tracers to different fracturing sections. Due to the differences in fracturing effects and reservoir properties, the output concentration and curve shape of the tracers in each fracturing section will also be different. By detecting the concentration of different tracers in the flowback fluid, the fracturing flowback effect of each section can be determined.

[12] At the same time, there are several patents on rare earth element tracing that can be publicly consulted, such as the method of using rare earth elements to trace the loss of agricultural non-point source phosphorus, using the similar characteristics of rare earth elements and phosphorus in rainfall runoff, applying multi-source material tracing of rare earth elements to the tracing of agricultural non-point source phosphorus loss, and effectively identifying high-risk areas for phosphorus loss in complex terrain basins.

[13] The method of using rare earth elements to trace soil loss in karst areas can quantitatively define the contribution of surface soil loss and underground soil loss in karst desertification areas. It is generally applicable in karst areas, with high accuracy and simple operation.

[14] .

[0039] Based on the recognition that there is a need for CO2 tracing in CO2-ECBM, rare earth elements have excellent properties, and rare earth elements already have many tracing applications, this patent proposes a method for tracing CO2 migration in CO2-ECBM using rare earth elements.

[0040] 1.7 Summary

[0041] Through the above background introduction, we can understand that:

[0042] 1. CO2-ECBM technology integrates greenhouse gas emission reduction and new energy development, and has great development prospects. CO2-ECBM technology is still in the experimental exploration stage in my country.

[0043] 2. In CO2-ECBM technology, CO2 migration is one of the important concerns, both in terms of the effectiveness of CO2 displacing CH4 and the safety of CO2 geological storage.

[0044] 3. Rare earth elements have many excellent properties. They not only serve as geochemical indicators, but also have a wide range of tracer effects in various fields.

[0045] 4. The present invention proposes a method for tracing CO2 migration in CO2-ECBM using rare earth elements.

[0046] 5. This invention is funded by the Energy Research Institute Project of Hefei Comprehensive National Science Center: Basic Theoretical Research on Coalbed Methane Development in Low-Permeability Coal Seams with CO2-ECBM (No.: 21KZS218).

[0047] Main references:

[0048] [1] Zhao Zhigen. Development and utilization of coalbed methane in my country. New Energy. 1997, 19(3): 38-41.

[0049] [2]Xu Xin, Liang Meng. Methods and technical progress in improving coalbed methane recovery. China Coalbed Methane. 2016, 13(3): 3-6.

[0050] [3]ECBM_Baidu Encyclopedia (baidu.com) (https: / / baike.baidu.com / item / ECBM / 17581912?fr=aladdin)

[0051] [4] Sang Shuxun. Review of research on effectiveness of geological storage of carbon dioxide and enhanced development of coalbed methane. Coal Geology and Exploration. 2018, 46(5): 1-9.

[0052] [5] Zhang Kun, Sang Shuxun, Liu Shiqi, et al. Feasibility and existing problems of CO2-ECBM technology. China Coalbed Methane. 2016, 13(2): 43-46.

[0053] [6] Li Zhipeng, Bu Lixia. Geological safety limit system in carbon dioxide flooding and storage process. Special Oil and Gas Reservoirs. 2017, 24(2): 141-144.

[0054] [7] Ma Jinfeng, Yang Yang, Cai Bofeng, et al. Environmental monitoring issues and monitoring scope of different types of carbon dioxide geological storage projects. Environmental Engineering. 2018, 36(2): 10-14.

[0055] [8] Ministry of Environmental Protection of the People’s Republic of China. Technical Guidelines for Environmental Risk Assessment of Carbon Dioxide Capture, Utilization and Storage (Trial). Beijing: Ministry of Environmental Protection, 2016.

[0056] [9] Zhao Zhigen. Geochemistry of rare earth elements in coal-bearing rock series. Beijing: Coal Industry Press. 2002.

[0057]

[10] Chen Yan, Huang Jia. Research and exploration of interwell tracer technology in offshore oil fields. Chemical Engineering Design Communications. 2021, 47(7): 11-13.

[0058]

[11] Tang Zhenquan, Liu Gang, Xu Wennian. Rare earth element tracer technology and its application in soil erosion research. Journal of the Chinese Rare Earth Society. 2011, 29(5): 515-522.

[0059]

[12] Liang Jinhui, Wu Chuanwei, Zhang Menglin, et al. Rare earth element tracer monitoring technology for horizontal well fracturing. Chemical Engineering Management. 2021, (23): 185-186.

[0060]

[13] Wang Lingqing, Liang Tao. A method for tracing phosphorus loss from agricultural non-point sources using rare earth elements, CN103063731A.2013.

[0061]

[14] Peng Xudong, Dai Quanhou, Ding Guijie, et al. A method for tracing soil loss in karst areas using rare earth elements, CN108614088B.2021 Summary of the invention

[0062] In order to solve the above technical problems, the present invention provides a method for tracing CO2 migration using rare earth elements, which is a method for tracing CO2 migration using rare earth elements in CO2-ECBM. The present invention uses a method for tracing CO2 migration using rare earth elements in CO2-ECBM, and obtains information on CO2 migration status according to the rare earth marker characteristics by comparing the rare earth element characteristics in the injection well and the extraction well.

[0063] The present invention adopts the following technical means:

[0064] A method for tracing CO2 migration using rare earth elements comprises the following steps:

[0065] (1) Determine the study area, collect and test the rare earth element background values ​​of coal seams, CH4 and CO2 in the study area;

[0066] (2) According to the distribution of well groups in the study area, injection wells and extraction wells were selected for rare earth element tracing;

[0067] (3) Selecting a tracer rare earth element to label CO2 with the rare earth element;

[0068] (4) Inject rare earth-labeled CO2 to maintain normal operation of the extraction well;

[0069] (5) Determine the time to start sampling and the interval between sampling in the extraction well;

[0070] (6) Collecting samples from the extraction well and testing the rare earth element content to obtain the rare earth element characteristics;

[0071] (7) judging whether the injected CO2 has migrated to the extraction well according to whether the rare earth marker is detected in the sample, and judging whether the CO2 originates from the corresponding injection well according to the difference in the characteristics of the rare earth marker; preferably, the ratio of multiple rare earth marker information is used to judge the ratio of CO2 from multiple injection wells to the extraction well;

[0072] (8) Comprehensively analyze the rare earth marker characteristics in the sample to obtain information on the CO2 migration status.

[0073] Furthermore, in step (2), there are m CO2 injection wells in the study area, referred to as injection well 1, injection well 2, ..., injection well m; there are n coalbed methane extraction wells, referred to as extraction well 1, extraction well 2, ..., extraction well n; injection wells and extraction wells are selected to carry out rare earth element tracing.

[0074] Furthermore, in step (3), the tracer rare earth element is selected from one or more of the 15 elements of the lanthanide series and yttrium, and the CO2 is labeled with the rare earth element; the injection well 1 uses rare earth marker 1, the injection well 2 uses rare earth marker 2, ..., and the injection well m uses rare earth marker m.

[0075] Furthermore, in step (4), CO2 is injected to displace CH4, CO2 with rare earth label 1 is injected from injection well 1, CO2 with rare earth label 2 is injected from injection well 2, ..., CO2 with rare earth label m is injected from injection well m; the time when each injection well starts injecting rare earth labeled CO2 is recorded; and the normal operation of the extraction well is maintained.

[0076] Furthermore, in step (5), the sampling start time and interval time are determined based on the distance between the injection well and the extraction well, the physical properties and permeability of the coal seam, the migration velocity of the injected CO2 and the degree of adsorption of CO2 by the coal seam.

[0077] Furthermore, in step (6), according to the determined sampling start time and sampling time interval, a gas sample is collected from the extraction well; the rare earth element content in the gas sample is tested to obtain the rare earth element characteristics. The rare earth element characteristics mainly include the rare earth element chondrite standardized distribution pattern diagram and the geochemical parameters of the rare earth elements (∑REE, LREE, HREE, LREE / HREE, (La / Yb) N In the case of rare earth labeling for tracing, the rare earth element characteristics are mainly the content of the labeled rare earth elements.

[0078] Furthermore, in step (7), the CO2 migration situation is determined: if no rare earth marker feature is detected in the gas sample, it can be determined that the injected CO2 has not yet migrated to the extraction well; or if the information of rare earth marker 1 is detected, and it does not conflict with the rare earth element background value, it can be determined that the CO2 in injection well 1 has migrated to the extraction well, and according to the distance between the injection well and the extraction well, combined with the geological characteristics of the study area, the information on the CO2 migration situation can be obtained through analysis; or if multiple rare earth marker information is detected, and these marker information corresponds to their respective injection wells, it can be determined that the CO2 in multiple injection wells has migrated to the extraction well; optionally, according to the ratio of these marker information, combined with the geological characteristics of the study area, the ratio of CO2 migrated from multiple injection wells to the extraction well can be determined.

[0079] Furthermore, in step (8), the injected CO2 is rare earth labeled, and then the presence of rare earth labels is detected in the extraction well, and the ratio of various labels is calculated. After analysis and judgment, information on the migration status of CO2 can be finally obtained.

[0080] Among them, in step (1), the background value of rare earth elements: the background value of rare earth elements is concerned in order to clarify that there is a significant difference between the rare earth element characteristics of the sample in the extraction well and the characteristics of the background value, so as to exclude that the rare earth mark detected in the extraction well is caused by the background value. Determine a research range, collect and test the background values ​​of various rare earth elements in the coal seam, coalbed methane CH4, and CO2 as the injected gas within the research range; generally speaking, the background values ​​of the rare earth element content of coal seams, CH4, and CO2 are very low, and the content of each rare earth element in the coal in the coal seam is usually in the range of zero, several, or tens of ppm (parts per million, one millionth, 1×10 -6 ), while the rare earth element content of CH4 and CO2 as raw gases is even lower, at ppb (parts per billion, 1×10 -9 ), or ppt (parts per trillion, 1×10 -12 ), or even negligible. The content of rare earth markers is usually at least 1-2 orders of magnitude, or even 2-3 orders of magnitude, higher than the background value to clearly distinguish the background value, so as to determine whether the rare earth element marker characteristics, rare earth element geochemical parameters, and rare earth element chondrite standardized distribution patterns of the injection well and the extraction well are consistent, and then determine the migration status of CO2.

[0081] Wherein, in step (2), the distribution of well groups in the study area: without loss of generality, there are m CO2 injection wells in the study area, referred to as injection well 1, injection well 2, ..., injection well m; there are n coalbed methane extraction wells, referred to as extraction well 1, extraction well 2, ..., extraction well n. If there are many injection wells and extraction wells, some representative injection wells and extraction wells can be selected for rare earth element tracing according to the distribution of wells, geological characteristics of the study area, and the needs of research work.

[0082] Among them, in step (3), rare earth element labeling: rare earth element is not an element, usually 15 elements of lanthanide and yttrium are called rare earth elements. Select a tracer rare earth element or a combination of several elements to label CO2 with rare earth elements; without loss of generality, injection well 1 uses rare earth label 1, injection well 2 uses rare earth label 2, ..., injection well m uses rare earth label m.

[0083] Among them, in step (4), gas injection and extraction: inject CO2 to displace CH4, inject CO2 with rare earth label 1 from injection well 1, inject CO2 with rare earth label 2 from injection well 2, ..., inject CO2 with rare earth label m from injection well m; record the time when each injection well starts injecting rare earth labeled CO2; maintain the normal operation of the extraction well.

[0084] Among them, in step (5), the sampling time is determined: the professional technicians determine the sampling start time according to the distance between the injection well and the extraction well, the migration (diffusion, penetration, flow) speed of the injected CO2, the adsorption degree of the coal seam to CO2, etc. The farther the distance between the injection well and the extraction well, the longer the migration time of CO2 from the injection well to the extraction well; the slower the migration speed of the injected CO2, the longer the migration time of CO2 from the injection well to the extraction well; the higher the adsorption degree of the coal seam to CO2, the longer the migration time of CO2 from the injection well to the extraction well; etc. At the same time, for an unfamiliar research area, or the more sophisticated the research work, the sampling start time should be as early as possible and the sampling interval should be as short as possible; after a research area has been tested many times and experience is formed, it will be more conducive to determining the sampling time and sampling interval. If CO2 is well sealed in the coal seam, it is difficult for CO2 to migrate from the injection well to the extraction well, and it may not even reach the extraction well. In the early stages of research, or in the stage of accumulating experience, sampling can be started earlier and the sampling time interval can be shortened to avoid the situation where one or more rare earth-labeled CO2 has migrated from the injection well to the extraction well, but no samples are collected, and the CO2 migration information cannot be obtained, thus failing to achieve the purpose of tracing.

[0085] Among them, in step (6), sampling and testing: according to the determined sampling start time and sampling time interval, a gas sample is collected from the extraction well; according to the existing mature plasma spectrometry and plasma mass spectrometry testing techniques already mentioned in the background technology, the rare earth element content in the gas sample is tested to obtain the rare earth element characteristics; in the case of rare earth labeling for tracing, the rare earth element characteristics are mainly the labeled rare earth element content.

[0086] Wherein, in step (7), the CO2 migration situation is judged: if the characteristics of the rare earth marker are not detected in the gas sample of the extraction well, it can be judged that the injected CO2 has not yet migrated to the extraction well; if the information of rare earth marker 1 is detected, and it does not conflict with the background value of rare earth elements, it can be judged that the CO2 in injection well 1 has migrated to the extraction well. According to the distance between the injection well and the extraction well, combined with the geological characteristics of the study area, more information on the CO2 migration situation can be obtained through analysis; if the information of other rare earth markers is detected, a similar judgment or analysis is made as mentioned above; if the information of multiple rare earth markers is detected, and these marker information corresponds to their respective injection wells, it can be judged that the CO2 in multiple injection wells has migrated to the extraction well. Furthermore, according to the ratio of these marker information, combined with the geological characteristics of the study area, it can be helpful to judge the ratio of CO2 migrated from multiple injection wells to the extraction well.

[0087] Among them, in step (8), the method of tracing CO2 migration: In summary, by rare earth labeling the injected CO2, then detecting the presence of rare earth labels in the extraction well, calculating the ratio of various labels, and after analysis and judgment, the information on the CO2 migration status can finally be obtained.

[0088] Following the principle of moving from simple to complex, from theory to practice: Following the principle of moving from simple to complex, from theory to practice, the tracer method was first proposed, and then the CO2 injection was verified under laboratory conditions. After that, small blocks with relatively simple geological conditions were selected to carry out field tests. Finally, experience was formed for reference in production practice or for promotion and application.

[0089] Extension of the tracer method: The proposed tracer method is based on CO2-ECBM, that is, CO2 is injected into the coal seam. Due to the competitive adsorption advantage of CO2, it can replace CH4 in the coal seam, thereby realizing CO2 storage in the coal seam, enhanced coalbed methane production and improved coalbed methane recovery rate. The ideas and measures of this method are expected to be extended to other aspects for use; for example, a large amount of CO2 is injected into the coal seam from the injection well for a long time. The CO2 injected into the coal seam may migrate to the coal seam outcrop or to the open fault; for example, CO2 oil recovery. The carbon dioxide oil recovery technology is to inject carbon dioxide into the oil layer to displace the crude oil in the formation to the production well, thereby increasing the oil recovery rate of the oil field; for example, monitoring whether CO2 leaks in carbon capture and storage. Carbon capture and storage (CCS) technology refers to the process of separating CO2 from industrial or related emission sources, transporting it to a storage site, and isolating it from the atmosphere for a long time. This technology is considered to be the most economical and feasible method to reduce greenhouse gas emissions and slow global warming on a large scale in the future. This storage not only includes storing CO2 deep underground, but also monitoring whether it leaks.

[0090] The beneficial effects of the invention are as follows: CO2-ECBM technology is an emerging technology, which is still in the experimental exploration stage in my country and has broad application prospects. Among them, the migration of carbon dioxide is one of the important concerns in many links. Rare earth elements have excellent performance, a variety of types, no interference with each other, and are non-toxic and non-radioactive. They have strong thermal stability and biological stability. At the same time, they have the advantages of perfect testing technology and high precision. This patent proposes a method for tracing CO2 migration with rare earth elements in CO2-ECBM, and the method is highly applicable and can be extended to other aspects for use. BRIEF DESCRIPTION OF THE DRAWINGS

[0091] Figure 1 Flowchart of the tracer method;

[0092] Figure 2Standardized distribution pattern of rare earth elements in chondrites;

[0093] Figure 3 Take the injection well and the extraction well as Example 1;

[0094] Figure 4 The main working process of Example 1;

[0095] Figure 5 The migration of tracer CO2 in Example 1;

[0096] Figure 6 Take an open fault as Example 2. DETAILED DESCRIPTION

[0097] The present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments. However, the following embodiments are limited to explaining the present invention, and the protection scope of the present invention should include the entire contents of the claims, and through the description of the following embodiments, those skilled in the art can fully implement the entire contents of the claims of the present invention.

[0098] The following examples are given in order to illustrate the present invention in detail.

[0099] Embodiment 1:

[0100] This embodiment takes m injection wells and n extraction wells as cases. Without loss of generality, three injection wells and one extraction well are selected for illustration. Other injection wells and extraction wells can be described according to this embodiment.

[0101] 1. Background value of rare earth elements: The content of each rare earth element in the coal seams of Huainan and Huaibei mining areas is usually in the range of a few tenths, a few tens, or even tens of ppm (parts per million, 1×10 -6 ), while CH4 and CO2 without rare earth element labeling have even lower rare earth element content, in ppb (parts per billion, 1×10 -9 ), or ppt (parts per trillion, 1×10 -12 ) level, or even negligible.

[0102] 2. Distribution of well groups in the study area: Without loss of generality, there are m CO2 injection wells in the study area, referred to as injection well 1, injection well 2, ..., injection well m; there are n coalbed methane extraction wells, referred to as extraction well 1, extraction well 2, ..., extraction well n. According to the distribution of wells, geological characteristics of the study area, and the convenience of research work, three injection wells and one extraction well are selected as examples to carry out rare earth element tracing.

[0103] Figure 3This is a plan view of an example of injection wells and extraction wells. It shows that there are m injection wells and n extraction wells in the study area. Three injection wells and one extraction well are selected for example. Under laboratory conditions, the distance between the three injection wells is about 1 meter, and one extraction well is located in the middle of the three injection wells.

[0104] 3. Rare earth element labeling: Select the tracer rare earth element to label the CO2 of the three injection wells with rare earth elements; without loss of generality, injection well 1 is labeled with lanthanum (La), called rare earth label 1, injection well 2 is labeled with neodymium (Nd), called rare earth label 2, and injection well 3 is labeled with europium (Eu), called rare earth label 3. The content of each rare earth label is positioned at the 100ppm level.

[0105] 4. Gas injection and extraction: Inject CO2 to displace CH4, inject CO2 with rare earth marker 1 from injection well 1, inject CO2 with rare earth marker 2 from injection well 2, and inject CO2 with rare earth marker 3 from injection well 3; record the time when each injection well starts injecting rare earth-labeled CO2; maintain the normal operation of the extraction wells.

[0106] 5. Determine the sampling time: Under laboratory conditions, Figure 3 As shown, when the distance between injection well 1, injection well 2 and injection well 3 is about 1 meter, the initial sampling start time is set to 10 hours and the sampling interval is 1 hour; after the first experiment is over, the sampling start time and sampling interval can be analyzed and adjusted according to the experimental results; if rare earth-labeled CO2 is not collected in the extraction well within 20 hours and 30 hours, the initial sampling time of the next experiment can be determined after 20 hours and 30 hours, and the sampling interval can be 2 hours and 3 hours; if rare earth-labeled CO2 is collected in the extraction well within 8 hours and 5 hours, the initial sampling time of the next experiment can be determined before 8 hours and 5 hours, and the sampling interval can be 0.8 hours and 0.5 hours; after several experiments under the conditions of this case, a more reasonable sampling start time and sampling interval can be obtained.

[0107] 6. Sampling and testing: Collect gas samples from the extraction wells according to the determined sampling start time and sampling time interval; for all collected samples, test the rare earth element content in the gas samples to obtain the rare earth element characteristics.

[0108] Figure 4 This is the main working process of Example 1. Since CO2 is injected into the injection well and the extraction well is in an extraction state, CO2 may migrate from the injection well to the extraction well. Gas samples are collected from the extraction well and the rare earth element content of the samples is tested.

[0109] 7. Determine the migration of CO2: If the lanthanum (La) marked in Injection Well 1, the neodymium (Nd) marked in Injection Well 2, and the europium (Eu) marked in Injection Well 3 reach 0.1-1ppm in the sample of Pumping Well 1, it can be considered that the rare earth element markers are detected. If the characteristics of rare earth markers 1, 2, and 3 are not detected in the gas sample of Pumping Well 1, it can be determined that the CO2 injected from Injection Well 1, Injection Well 2, and Injection Well 3 has not yet migrated to Pumping Well 1. Without loss of generality, if the information of rare earth element marker 1 is detected, it can be determined that the CO2 in Injection Well 1 has migrated to the Pumping Well 1; without loss of generality, if the sample at the 15th hour does not detect the rare earth marker 1, but the sample at the 16th hour has detected it, it is considered that CO2 migrated to the Pumping Well 1 within 15 to 16 hours. If multiple rare earth marker information is detected, and these marker information corresponds to respective injection wells, it can be determined that CO2 from multiple injection wells has migrated to the extraction well; and based on the order in which the rare earth markers are detected, it can be determined when the CO2 from the injection wells corresponding to each marker migrated to the extraction well 1. Furthermore, based on the ratio of these rare earth marker information, for example, if the samples collected at a certain time point have a test result of 50% rare earth marker 1, 30% rare earth marker 2, and 20% rare earth marker 3, without loss of generality, it can be determined that the ratio of CO2 migrated to the extraction well 1 from the injection well 1, injection well 2, and injection well 3 is 50%:30%:20%.

[0110] 8. Method for tracing CO2 migration: As described in this embodiment, the injected CO2 is labeled with rare earth elements, and then samples are taken in the extraction wells to detect the presence of rare earth element labels, thereby obtaining information on whether the injected CO2 has migrated to the extraction wells, when it migrated to the extraction wells, and the proportion of CO2 from each injection well that has migrated to the extraction wells.

[0111] Figure 5 For tracing the migration of CO2 in Example 1, the migration of CO2 is determined by the presence or absence of rare earth element markers, the time when the rare earth markers are detected, and the ratio of various markers.

[0112] Embodiment 2:

[0113] Example 2 is an extension of the tracing method, which describes the scenario of CO2 injected from an injection well migrating to an open fault. This example takes one injection well and one open fault as a case.

[0114] 1. Rare earth element background value: same as in Example 1.

[0115] 2. The main components of the scenario of Example 2: One injection well and one open fault are selected as examples to carry out rare earth element tracing.

[0116] Figure 6This is a cross-sectional schematic diagram of Example 2 using an open fault, showing that there is one injection well and one open fault in the study area; labeled CO2 is injected from the injection well, and a sampling point is set at the position of the open fault.

[0117] 3. Rare earth element labeling: Select tracer rare earth elements to label the CO2 of one injection well with rare earth elements; without loss of generality, injection well 1 is labeled with lanthanum (La), called rare earth label 1, and the content of rare earth label is positioned at 100 ppm level.

[0118] 4. Gas injection and extraction: Inject CO2 to displace CH4, and inject CO2 with rare earth marker 1 from injection well 1.

[0119] 5. Determine the sampling time: Under laboratory conditions, the initial sampling start time is set to 10 hours and the sampling interval is 1 hour; as in Example 1, after the first experiment is completed, the sampling start time and the sampling interval can be analyzed and adjusted according to the experimental results.

[0120] 6. Sampling and testing: According to the determined sampling start time and sampling time interval, gas samples are collected from the location of the open fault; for all collected samples, the rare earth element content in the gas samples is tested to obtain the rare earth element characteristics.

[0121] 7. Determine the migration of CO2: If the characteristics of rare earth marker 1 are not detected in the gas sample at the open fault location, it can be determined that the CO2 injected from injection well 1 has not migrated to the open fault. If the information of rare earth element marker 1 is detected, it can be determined that the CO2 in injection well 1 has migrated to the open fault; without loss of generality, if the sample at the 15th hour does not detect rare earth marker 1, but the sample at the 16th hour has detected it, it is considered that CO2 migrated to the open fault within 15 to 16 hours.

[0122] 8. Method for tracing CO2 migration: As described in this embodiment, the injected CO2 is labeled with rare earth elements, and then samples are taken at the open fault location to detect the presence of rare earth element labels, thereby determining whether the injected CO2 has migrated to the open fault and when it migrated to the open fault.

[0123] The present invention does not elaborate on some of the common technologies of those skilled in the art. The above-described embodiments are only descriptions of preferred implementations of the present invention. The preferred embodiments do not describe all the details in detail, nor do they limit the invention to the specific implementations described. Without departing from the design spirit of the present invention, various modifications and improvements made by ordinary technicians in this field to the technical solution of the present invention should fall within the scope of protection determined by the claims of the present invention.

Claims

1. A method for tracing CO2 migration using rare earth elements in CO2-ECBM, characterized in that: The following steps are involved: (1) Determine the study area, collect and test the rare earth element background values ​​of coal seams, CH4 and CO2 in the study area; the focus on the rare earth element background value is to clarify that there is a significant difference between the rare earth element characteristics of the samples in the extraction well and the characteristics of the background value, so as to exclude that the rare earth markers detected in the extraction well are caused by the background value; (2) According to the distribution of well groups in the study area, injection wells and extraction wells are selected for rare earth element tracing. There are m CO2 injection wells in the study area, referred to as injection well 1, injection well 2, ..., injection well m; there are n coalbed methane extraction wells, referred to as extraction well 1, extraction well 2, ..., extraction well n; some injection wells and extraction wells are selected for rare earth element tracing. (3) Selecting a tracer rare earth element to label CO2 with the rare earth element; (4) Inject rare earth-labeled CO2 to maintain normal operation of the extraction well; (5) Determine the start time and interval of sampling in the extraction well; (6) Collecting gas samples from the extraction wells according to the determined sampling start time and sampling time interval; testing the rare earth element content in the gas samples to obtain rare earth element characteristics, with an emphasis on the content of labeled rare earth elements, and also including the standardized distribution pattern of rare earth elements in chondrites and geochemical parameters of rare earth elements; (7) Determine whether the injected CO2 has migrated to the extraction well based on whether rare earth markers are detected in the sample, and determine that the CO2 originated from the corresponding injection well based on the difference in the rare earth marker characteristics; determine the CO2 migration situation: if the rare earth marker characteristics are not detected in the gas sample, then it is determined that the injected CO2 has not migrated to the extraction well; or if the information of rare earth marker 1 is detected, then it is determined that the CO2 in injection well 1 has migrated to the extraction well, and the speed and flow direction information of the CO2 migration situation is obtained through analysis based on the distance between the injection well and the extraction well, combined with the geological characteristics and groundwater movement characteristics of the study area; if multiple rare earth marker information is detected, and these marker information corresponds to their respective injection wells, then it is determined that the CO2 in multiple injection wells has migrated to the extraction well; at the same time, based on the ratio of these marker information and the geological characteristics of the study area, the ratio of CO2 migrated from multiple injection wells to the extraction well is determined; (8) Comprehensively analyze the rare earth marker characteristics in the sample to obtain information on the CO2 migration status.

2. The method according to claim 1, characterized in that: In step (3), the tracer rare earth element is selected from one or more of the 15 elements of the lanthanide series and yttrium, and the CO2 is labeled with the rare earth element; the injection well 1 uses rare earth marker 1, the injection well 2 uses rare earth marker 2,..., and the injection well m uses rare earth marker m.

3. The method according to claim 2, characterized in that: In step (4), CO2 is injected to displace CH4, CO2 with rare earth label 1 is injected from injection well 1, CO2 with rare earth label 2 is injected from injection well 2, ..., CO2 with rare earth label m is injected from injection well m; the time when each injection well starts injecting rare earth labeled CO2 is recorded; and the normal operation of the extraction well is maintained.

4. The method according to claim 1, characterized in that: In step (5), the sampling start time and interval time are determined based on the distance between the injection well and the extraction well, the migration velocity of the injected CO2 and the degree of adsorption of CO2 by the coal seam.

5. The method according to claim 1, characterized in that: In step (8), the injected CO2 is rare earth labeled, and then the presence of rare earth labels is detected in the extraction well, and the ratio of various labels is calculated. After analysis and judgment, information on the migration status of CO2 is finally obtained.

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