An oilfield tracer and its method for evaluating the oil production contribution of different sections of a horizontal well.

Abstract

This invention provides an oilfield tracer and a method for evaluating the oil production contribution of different sections of a horizontal well, belonging to the field of oilfield development technology. The oilfield tracer provided by this invention comprises white oil, a nonionic surfactant, sodium 3-allyloxy-2-hydroxypropanesulfonate, and alcohols; wherein the rare earth metal salt forms a complex with a complexing agent and sodium 3-allyloxy-2-hydroxypropanesulfonate; the complexing agent is a mixture of glycolic acid, triethylenetetramine, and sodium dimercaptopropanesulfonate in a mass ratio of 10:(3-5):(1-2). This oilfield tracer is stable under complex formation conditions and can withstand high salinity. In evaluating the oil production contribution of different sections of a horizontal well using this oilfield tracer, different types and amounts of tracers can be selected for different sections, enabling the evaluation of the oil production contribution of each section of the horizontal well and providing a valid basis for subsequent repeated fracturing.

Description

Technical Field

[0001] This invention belongs to the field of oilfield development technology, and particularly relates to an oilfield tracer and a method for evaluating the oil production contribution of different sections of a horizontal well. Background Technology

[0002] Horizontal wells, due to their unique advantage of large contact area with the oil-bearing formation, significantly increase the production of single wells and are now widely used in various oilfields. Horizontal well staged fracturing technology is a key technology for tight oil development, and tracer monitoring technology is widely used to monitor the fracturing effect. With the widespread application of tracer monitoring technology, some problems have gradually emerged, such as in tracer selection: how to effectively and uniformly inject tracers into the formation and distinguish the oil and water production contributions of different stages of the horizontal well. Furthermore, due to the limitations of the reservoir environment, tracers must also meet requirements for good temperature resistance, salinity resistance, chemical stability, physical stability, and biological stability.

[0003] Currently, commonly used tracers in oilfield development include chemical tracers, isotope tracers, and trace element tracers. Although these tracer technologies are widely used in various oilfields, they also have many problems. For example, chemical tracers have low adsorption on the formation surface, low consumption, and many types, making them easy to detect by spectrophotometry. However, due to the large amount used in the field, their cost is high, their adaptability and selectivity are poor, and their accuracy is low. Furthermore, radioactive isotope tracers are radioactive, which is detrimental to personnel and environmental safety, thus limiting their application. Stable isotope tracers, while having no radioactive hazards, require complex and expensive analytical testing methods. Trace element tracers, as the fourth generation of tracer technology, have advantages such as being non-radioactive, non-polluting, corrosion-resistant, having good safety and stability, requiring small amounts, being added directly from the wellhead, being inexpensive, having low cost, high analytical accuracy, and offering a variety of methods. They have become the current development direction of tracer technology.

[0004] However, the presence of clay minerals in the formation exhibits strong adsorption of the active cations in the tracer. Furthermore, the formation water contains abundant anions such as hydroxide, carbonate, and bicarbonate ions. Single-added metal cations in the formation readily react chemically with these anions, leading to significant losses of the metal cations in the tracer. Moreover, the high mineralization of formation water in western regions necessitates further improvement in the mineralization resistance of current trace element tracers.

[0005] Therefore, it is necessary to provide a new type of tracer and a method for evaluating the oil production contribution of each section of a horizontal well, which can distinguish the oil and water production contributions of each section, so as to screen and obtain tracers with a wider range of applications. Summary of the Invention

[0006] In view of the shortcomings of the prior art, the first objective of the present invention is to provide an oilfield tracer that can exist stably under complex formation conditions, can withstand high salinity, and has wide adaptability.

[0007] The second objective of this invention is to provide a method for evaluating the oil production contribution of each section of a horizontal well using the aforementioned oilfield tracer.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0009] On the one hand, an oilfield tracer comprises 50-60 parts white oil, 2-5 parts nonionic surfactant, 1-2.5 parts sodium 3-allyloxy-2-hydroxypropanesulfonate, and 5-10 parts alcohol; wherein,

[0010] The rare earth metal salt forms a complex with the complexing agent and sodium 3-allyloxy-2-hydroxypropanesulfonate;

[0011] The complexing agent is a mixture of glycolic acid, triethylenetetramine and sodium dimercaptopropanesulfonate in a mass ratio of 10:(3-5):(1-2).

[0012] In a further embodiment, the oilfield tracer comprises, by weight, the following raw materials: 12 parts rare earth metal salt, 12 parts complexing agent, 55 parts white oil, 3 parts nonionic surfactant, 1.5 parts sodium 3-allyloxy-2-hydroxypropanesulfonate, and 8 parts alcohol.

[0013] In a further embodiment, the rare earth metal salt is selected from at least one of rare earth nitrates, rare earth sulfates, and rare earth hydrochlorides.

[0014] In a further embodiment, the rare earth metal element in the rare earth metal salt is selected from at least one of 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).

[0015] Preferably, the mass ratio of glycolic acid, triethylenetetramine, and sodium dimercaptopropanesulfonate in the complexing agent is 10:4:1.5.

[0016] In a further embodiment, the nonionic surfactant is selected from at least one of Tween 20, Tween 40, Tween 60, and Tween 80.

[0017] In a further embodiment, the alcohol is at least one selected from ethanol, isopropanol, glycerol, and n-butanol.

[0018] As a further preferred embodiment, the tracer for oilfield use also contains 8-10% by weight of phenylsuccinic anhydride acylated chitosan, which is obtained by modifying chitosan with phenylsuccinic anhydride.

[0019] Preferably, the molecular weight of the chitosan is 5-10 kDa; the mass ratio of the phenyl succinic anhydride to the chitosan is (0.2-0.35):1; more preferably, the mass ratio of the phenyl succinic anhydride to the chitosan is 0.28:1.

[0020] Preferably, the phenylsuccinic anhydride acylated chitosan is prepared by the following method:

[0021] Chitosan was dispersed in acetic acid solution, sodium carbonate was added, and the mixture was stirred for 30 min. Then phenyl succinic anhydride was added and reacted at room temperature for 5-6 h. The pH was adjusted to 9-10, and unreacted chitosan was removed by centrifugation. The remaining product was dialyzed, and the pH was adjusted to 3-4 with concentrated hydrochloric acid to obtain a white flocculent precipitate. The precipitate was centrifuged again and dried to obtain the phenyl succinic anhydride acylated chitosan.

[0022] In the preparation of phenylsuccinic anhydride acylated chitosan, the amount of sodium carbonate added is in a mass ratio of 1:(0.2-0.3) to chitosan; the mass fraction of the acetic acid solution is 1-2%.

[0023] In a further embodiment, the method for preparing the complex includes the following steps:

[0024] (1) Dissolve rare earth metal salts in water to prepare rare earth metal salt solutions;

[0025] (2) Add a complexing agent to the rare earth metal salt solution and mix well. Then add sodium 3-allyloxy-2-hydroxypropanesulfonate to react and obtain a rare earth metal complex.

[0026] Preferably, in step (1), the mass fraction of the rare earth metal salt solution is 8-10%.

[0027] Preferably, in step (2), the reaction time is 0.5 to 1.5 h.

[0028] In a further embodiment, step S2, after adding sodium 3-allyloxy-2-hydroxypropanesulfonate, also includes adding phenylsuccinic anhydride-acylated chitosan.

[0029] In a further embodiment, the method for preparing the oilfield tracer includes the following steps:

[0030] A nonionic surfactant is added to a rare earth metal complex, the mixture is stirred and reacted, then white oil and alcohol are added, and the mixture is heated to 50-60°C and stirred and reacted to obtain an oilfield tracer.

[0031] Preferably, the stirring reaction time is 45-70 min; the continued stirring reaction time is 1-2 h.

[0032] On the other hand, a method for evaluating the oil production contribution of each section of a horizontal well includes evaluation using the aforementioned oilfield tracer.

[0033] The method for evaluating the oil production contribution of each section of a horizontal well includes the following steps:

[0034] (1) Extract background samples from each section of the horizontal well for analysis and select a tracer for that section; select a tracer made from rare earth elements that are not present or have a low content in the sample of that section as the tracer for that section.

[0035] (2) Based on the fracture volume, reservoir oil saturation, and the minimum detection limit of the effective tracer in the fracturing design, the amount of tracer to be used is calculated by magnification of 20 times.

[0036] (3) The tracer is pumped into the fracturing fluid in different well sections to start fracturing;

[0037] (4) Samples were taken and analyzed during the fracturing fluid flowback and production process to determine the concentration of chemical tracers in the samples and to calculate the contribution rate of each section to oil and water production.

[0038] In a further embodiment, in step (2), the formula for calculating the amount Q of the tracer is:

[0039] ;

[0040] In the formula, Q represents the amount of tracer used (kg), and A represents the crack sweep area (m²). 2 H – Average fracture thickness; φ – Reservoir porosity, %; So – Reservoir oil saturation, %; MDL – Minimum detection limit, dimensionless.

[0041] In a further scheme, in step (4), the sampling is to sample the wellhead produced fluid once every 12 hours for 30 consecutive days.

[0042] In a further embodiment, in step (4), the analytical tests are performed using a mass spectrometer.

[0043] Compared with the prior art, the present invention has the following beneficial effects:

[0044] 1. The oilfield tracer of the present invention can exist stably under complex formation conditions, can withstand high salinity, and has wide adaptability.

[0045] 2. In the oilfield tracer of the present invention, glycolic acid, triethylenetetramine, and sodium dimercaptopropanesulfonate are used as complexing agents. The complexing agents form complexes with rare earth metal elements. The complexing agents coat the surface of rare earth metals, which can effectively prevent anions in water from directly combining with rare earth metal ions, thereby preventing a decrease in product performance. This helps to improve the dispersion stability of the tracer in complex formations and improve its mineralization resistance. The three complexing agents work together to further improve mineralization resistance. When the ratio of the three complexing agents is within a specific range, the resulting tracer has higher heat resistance and salt resistance.

[0046] 3. The addition of sodium 3-allyloxy-2-hydroxypropanesulfonate to the oilfield tracer of the present invention makes the complex structure formed on the surface of rare earth ions more compact, which helps to improve the tracer's resistance to mineralization and salt.

[0047] 4. The oilfield tracer of the present invention incorporates phenyl succinic anhydride-acylated chitosan, and controls the ratio of phenyl succinic anhydride to chitosan, which helps the tracer to diffuse rapidly in the well, improves the dispersion stability of the tracer, prevents aggregation in formation water, and helps to further improve the tracer's resistance to mineralization, temperature and salt.

[0048] 5. This invention utilizes the oilfield tracer to evaluate the oil production contribution of each section of a horizontal well. By selecting different types and amounts of tracers for different layers, the oil production contribution rate of each section of the horizontal well can be determined, which can effectively guide the optimization design of volumetric fracturing. Detailed Implementation

[0049] The following non-limiting embodiments are intended to enable those skilled in the art to gain a more comprehensive understanding of the present invention, but do not limit the invention in any way. The following description is merely an exemplary illustration of the scope of protection of the present invention, and those skilled in the art can make various changes and modifications to the invention based on the disclosed content, which should also fall within the scope of protection of the present invention.

[0050] The present invention will be further described below by way of specific embodiments. Unless otherwise specified, all chemical reagents used in the embodiments of the present invention are obtained through conventional commercial means.

[0051] Example 1

[0052] An oilfield tracer is prepared by the following method:

[0053] S1. Dissolve 10g of ytterbium chloride in water to prepare a 10% rare earth metal salt solution.

[0054] S2. Add 8g of complexing agent (hydroxyacetic acid, triethylenetetramine and sodium dimercaptopropanesulfonate in a mass ratio of 10:3:1) to a rare earth metal salt solution, mix well, add 1g of sodium 3-allyloxy-2-hydroxypropanesulfonate and react for 0.5h to obtain a rare earth metal complex.

[0055] S3. Add 2g Tween 60 to the rare earth metal complex, stir and react for 45min, then add 50g white oil and 5g isopropanol, heat to 50℃ and continue stirring and reacting for 2h to obtain the oilfield tracer.

[0056] Example 2

[0057] An oilfield tracer is prepared by the following method:

[0058] S1. Dissolve 12g of ytterbium chloride in water to prepare a rare earth metal salt solution with a mass fraction of 10%.

[0059] S2. Add 12g of complexing agent (hydroxyacetic acid, triethylenetetramine and sodium dimercaptopropanesulfonate in a mass ratio of 10:4:1.5) to a rare earth metal salt solution, mix well, add 2.5g of sodium 3-allyloxy-2-hydroxypropanesulfonate and react for 0.5h to obtain a rare earth metal complex.

[0060] S3. Add 3g Tween 60 to the rare earth metal complex, stir and react for 60min, then add 55g white oil and 8g isopropanol, heat to 60℃ and continue stirring and reacting for 2h to obtain the oilfield tracer.

[0061] Example 3

[0062] An oilfield tracer is prepared by the following method:

[0063] S1. Dissolve 15g of ytterbium chloride in water to prepare a rare earth metal salt solution with a mass fraction of 12%.

[0064] S2. Add 15g of complexing agent (hydroxyacetic acid, triethylenetetramine and sodium dimercaptopropanesulfonate in a mass ratio of 10:5:2) to a rare earth metal salt solution, mix well, add 2.5g of sodium 3-allyloxy-2-hydroxypropanesulfonate and react for 1.5h to obtain a rare earth metal complex.

[0065] S3. Add 5g Tween 60 to the rare earth metal complex, stir and react for 70min, then add 60g white oil and 10g isopropanol, heat to 60℃ and continue stirring and reacting for 2h to obtain the oilfield tracer.

[0066] Example 4

[0067] The difference between this embodiment and Embodiment 2 is that the complexing agent is glycolic acid, triethylenetetramine and sodium dimercaptopropanesulfonate in a mass ratio of 10:2:0.5, while the total amount of complexing agent remains unchanged.

[0068] Example 5

[0069] The difference between this embodiment and Embodiment 2 is that the complexing agent is glycolic acid, triethylenetetramine and sodium dimercaptopropanesulfonate in a mass ratio of 10:6:3, while the total amount of complexing agent remains unchanged.

[0070] Example 6

[0071] The difference between this embodiment and Embodiment 2 is that the tracer for oil fields also contains 8% by mass of phenylsuccinic anhydride acylated chitosan (i.e., 0.96g), which is added immediately after sodium 3-allyloxy-2-hydroxypropanesulfonate.

[0072] The phenyl succinic anhydride acylated chitosan was prepared by the following method:

[0073] 10g of chitosan (molecular weight 10kDa) was dispersed in 50g of 1% acetic acid solution, 2g of sodium carbonate was added, and the mixture was stirred for 30min. Then, 2g of phenylsuccinic anhydride was added and the mixture was reacted at room temperature for 5h. The pH was adjusted to 10, centrifuged, and the remaining product was dialyzed. The pH was then adjusted to 4 with concentrated hydrochloric acid to obtain a white flocculent precipitate. The precipitate was centrifuged again and dried to obtain the phenylsuccinic anhydride acylated chitosan.

[0074] Example 7

[0075] The difference between this embodiment and Example 6 is that the amount of phenylsuccinic anhydride acylated chitosan added is 10% of the mass of the rare earth metal salt, and the phenylsuccinic anhydride acylated chitosan is prepared by the following method:

[0076] 10g of chitosan (molecular weight 10kDa) was dispersed in 50g of 1% acetic acid solution, 3g of sodium carbonate was added, and the mixture was stirred for 30min. Then, 3.5g of phenylsuccinic anhydride was added and the mixture was reacted at room temperature for 5h. The pH was adjusted to 10, centrifuged, and the remaining product was dialyzed. The pH was then adjusted to 4 with concentrated hydrochloric acid to obtain a white flocculent precipitate. The precipitate was centrifuged again and dried to obtain the phenylsuccinic anhydride acylated chitosan.

[0077] Example 8

[0078] The difference between this embodiment and Example 6 is that, in the preparation process of phenylsuccinic anhydride acylated chitosan, the amount of phenylsuccinic anhydride used is 1g, that is, the mass ratio of phenylsuccinic anhydride to chitosan is 0.1:1.

[0079] Example 9

[0080] The difference between this embodiment and Example 6 is that in the preparation of phenylsuccinic anhydride acylated chitosan, the amount of phenylsuccinic anhydride used is 4g, that is, the mass ratio of phenylsuccinic anhydride to chitosan is 0.4:1.

[0081] Example 10

[0082] The difference between this embodiment and Example 6 is that the phenyl succinic anhydride acylated chitosan is replaced by an equal amount of maleic anhydride acylated chitosan, which is prepared by the following method:

[0083] 10g of chitosan (molecular weight 10kDa) was dispersed in 50g of water, 2.5g of sodium carbonate was added, and the mixture was stirred for 30min. Then, 2.8g of maleic anhydride was added and the mixture was reacted at room temperature for 5h. The pH was adjusted to 10, centrifuged, and the remaining product was dialyzed. The pH was then adjusted to 4 with concentrated hydrochloric acid to obtain a white flocculent precipitate. The precipitate was centrifuged again and dried to obtain the maleic anhydride acylated chitosan.

[0084] Comparative Example 1

[0085] The difference between this comparative example and Example 2 is that sodium dimercaptopropanesulfonate in the complexing agent is replaced with an equal amount of sodium sulfosalicylate.

[0086] Comparative Example 2

[0087] The difference between this comparative example and Example 2 is that the complexing agent is glycolic acid and triethylenetetramine in a mass ratio of 10:6, while the total amount of complexing agent remains unchanged.

[0088] Comparative Example 3

[0089] The difference between this comparative example and Example 2 is that the complexing agent is glycolic acid and sodium dimercaptopropanesulfonate in a mass ratio of 10:3, while the total amount of complexing agent remains unchanged.

[0090] Comparative Example 4

[0091] The difference between this comparative example and Example 2 is that the complexing agent is replaced with an equal amount of glycolic acid.

[0092] Comparative Example 5

[0093] The difference between this comparative example and Example 2 is that sodium 3-allyloxy-2-hydroxypropanesulfonate was not added.

[0094] Comparative Example 6

[0095] The difference between this comparative example and Example 2 is that sodium 3-allyloxy-2-hydroxypropanesulfonate is replaced by an equal amount of sodium allylsulfonate.

[0096] Experimental Example 1

[0097] Mineralization resistance

[0098] The tracers from the examples and comparative examples were prepared into tracer solutions with a concentration of 0.0996 μg / L using simulated formation water with a mineralization of 250,000 mg / L. The solutions were placed at 200°C for 360 days, and the tracer mass concentration was detected by mass spectrometry. The concentration retention rate was calculated using the ratio of the concentration after aging to the initial concentration, which was used to evaluate the mineralization resistance.

[0099] The test results are shown in Table 1.

[0100] Table 1

[0101]

[0102] As shown in the table, the tracers prepared in Examples 1-3 of this invention maintained a concentration of over 90% after 360 days in simulated formation water with a mineralization of 250,000 mg / L, demonstrating good mineralization resistance and dispersion stability. Compared to Example 2, the tracers obtained in Examples 6 and 7 showed higher concentration retention rates after 120 and 360 days of storage, respectively. However, in Examples 8-10, the concentration retention rates after changing the preparation conditions of phenylsuccinic anhydride acylated chitosan were only slightly different from those in Example 2. This indicates that adding specific phenylsuccinic anhydride acylated chitosan to the tracer helps improve its mineralization resistance. In Comparative Examples 1-3, changing the complexing agent component significantly reduced the concentration retention rate, demonstrating that the choice of complexing agent has a significant impact on the mineralization resistance of the tracer; selecting a specific complexing agent can greatly improve the mineralization resistance of the tracer.

[0103] Experimental Example 2

[0104] Salt resistance

[0105] Take a mineralization of 250,000 mg / L, Na + Concentration 50000 mg / L, Ca 2+ Simulated formation water with a concentration of 50,000 mg / L was prepared by adding the tracers from the examples and comparative examples to the simulated formation water to form a mixed solution containing 50,000 mg / L. The solution was shaken at formation temperature for 120 days, and the mass concentration of the tracer was detected by mass spectrometry. The loss rate of tracer concentration was calculated by using the ratio of the change in concentration to the initial concentration, which was used to evaluate salt tolerance.

[0106] The test results are shown in Table 2 below.

[0107] Table 2

[0108]

[0109] As shown in the table, the tracers prepared in the embodiments and comparative examples of this invention exhibited small concentration changes and good stability after being placed in high salt concentrations for 30 days. With the increase in placement time to 90 days, significant differences in the concentration changes of the tracers in the embodiments and comparative examples became apparent. It can be seen that the concentration loss rate of the tracers obtained in the embodiments was <4%, especially in Examples 1-3 and Examples 6 and 7, where the concentration loss rate was below 2%. In contrast, the concentration loss rate of the tracers in the comparative examples was higher than 5.5%, significantly higher than that of Example 2, indicating relatively lower salt tolerance. With the increase in placement time to 120 days, the difference in concentration loss rates between the tracers in the embodiments and comparative examples further increased. The concentration loss rate of the comparative examples increased to over 9%, even reaching over 15%, while the concentration loss rate of Example 2 was 4.25%, significantly lower than that of the comparative examples. This demonstrates that the selection of the complexing agent and the addition of sodium 3-allyloxy-2-hydroxypropanesulfonate contribute to improving the salt tolerance of the tracers. It can also be seen that the concentration loss rate of the tracers in Examples 6 and 7 further decreased after the addition of phenylsuccinic anhydride acylated chitosan, indicating that the addition of phenylsuccinic anhydride acylated chitosan can further improve the salt resistance of the tracers.

[0110] Experimental Example 3

[0111] Heat resistance

[0112] A tracer was added to simulated formation water with a salinity of 250,000 mg / L to prepare a mixed solution with a concentration of 10 μg / L. The solution was placed at 350 °C for 90 days, and the mass concentration of the tracer was detected by mass spectrometry. The concentration loss rate was calculated to evaluate the heat resistance performance.

[0113] The test results are shown in Table 3 below.

[0114] Table 3

[0115]

[0116] As shown in the table, the tracers prepared in the embodiments and comparative examples of the present invention showed low concentration loss rates after aging at 350°C for 30 days. However, as the aging time increased to 60 days, the concentration loss rates of the tracers in Examples 4 and 5 and the comparative example were significantly higher than that in Example 2, indicating a decrease in heat resistance. With an aging time of 90 days, the concentration loss rate of Example 2 was approximately 5%, while the concentration loss rates of the tracers in Examples 4 and 5 and the comparative example were significantly higher than that in Example 2, indicating significantly poorer heat resistance. Therefore, the tracers provided by the present invention exhibit good heat resistance, and a specific ratio of complexing agent helps to improve the heat resistance of the tracers.

[0117] Test Example 4

[0118] Static adsorption test of tracer

[0119] The tracers from the examples and comparative examples were prepared into a 500 mg / L tracer solution. 300 mL of the tracer solution was mixed with 100 g of crushed rock sample, sealed, and shaken at the formation temperature for 60 days. The tracer concentration was measured and the loss rate was calculated.

[0120] The test results are shown in Table 4 below.

[0121] Table 4

[0122]

[0123] As shown in the table, the concentration loss rate of the mixed solution of tracer and rock sample obtained in Examples 1-3 was less than 5.5% after oscillation at formation temperature for 60 days, while the concentration loss rate of tracer in Comparative Examples 1-4 was 11-16%. It can be seen that selecting a specific complexing agent for the tracer can reduce the amount of tracer adsorbed in the rock formation and improve stability.

[0124] Application Example 1

[0125] A method for evaluating the oil production contribution of different sections of a horizontal well using oilfield tracers includes the following steps:

[0126] (1) The test section of the horizontal well to be tested consists of 5 sections. Background samples of each section of the horizontal well are extracted and analyzed. The tracer prepared by the rare earth element that is not present or has a low content in the sample of that section is selected as the tracer for that section. Three tracers are determined: X1 to X3. X1: the tracer prepared in Example 2 above; X2: the tracer prepared by the method described in Example 2 above using samarium chloride as a rare earth metal salt; X3: the tracer prepared by the method described in Example 2 above using praseodymium chloride as a rare earth metal salt. Among them, X1 is used for sections 1 and 4, X2 is used for sections 2 and 3, and X3 is used for section 3.

[0127] (2) Based on the fracture volume, reservoir oil saturation and the minimum detection limit of the effective tracer in the fracturing design, the amount of tracer is calculated by multiplying by 20 and using the following formula.

[0128] (1)

[0129] In formula (1), Q is the amount of tracer used (kg), and A is the crack sweep area (m²). 2 H – Average fracture thickness; φ – Reservoir porosity, %; So – Reservoir oil saturation, %; MDL – Minimum detection limit, dimensionless.

[0130] (3) The tracer is pumped into the fracturing pre-fracturing fluid in different well sections to start fracturing.

[0131] (4) Samples were taken for analysis and testing during the fracturing fluid flowback and production process. The wellhead produced fluid was sampled every 12 hours for 30 consecutive days. The concentration of chemical tracer in the sample was determined by mass spectrometry. The tracer production curve was obtained by plotting the change of chemical tracer concentration over time. The contribution of each section to oil production was calculated by fitting the measured tracer production concentration value and then based on the total production of the field test.

[0132] Analysis revealed that the middle section of the horizontal well made a significant contribution to oil production. Sections 2 and 3 were the main oil-producing sections, with oil production contribution rates of 38% and 31%, respectively. Sections 1, 4, and 5 of the horizontal well contributed 14%, 12%, and 5% of the oil production, respectively.

[0133] The above description of the embodiments is provided to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the invention should be within the protection scope of the present invention.

Claims

1. An oilfield tracer, characterized in that, Contains: 10g rare earth metal salt, 8g complexing agent, 50g white oil, 2g nonionic surfactant, 1g sodium 3-allyloxy-2-hydroxypropanesulfonate, and 5g alcohols; or, Contains: 12g rare earth metal salt, 12g complexing agent, 55g white oil, 3g nonionic surfactant, 2.5g sodium 3-allyloxy-2-hydroxypropanesulfonate, and 8g alcohols; or, Contains: 15g rare earth metal salt, 15g complexing agent, 60g white oil, 5g nonionic surfactant, 2.5g sodium 3-allyloxy-2-hydroxypropanesulfonate, and 10g alcohol; among which, Rare earth metal salts form complexes with complexing agents and sodium 3-allyloxy-2-hydroxypropanesulfonate; The complexing agent is a mixture of glycolic acid, triethylenetetramine and sodium dimercaptopropanesulfonate in a mass ratio of 10:(3-5):(1-2). The rare earth metal salt is selected from at least one of rare earth nitrates, rare earth sulfates, and rare earth hydrochlorides; the rare earth metal element in the rare earth metal salt is selected from at least one of lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium.

2. The oilfield tracer according to claim 1, characterized in that, The nonionic surfactant is selected from at least one of Tween 20, Tween 40, Tween 60, and Tween 80.

3. The oilfield tracer according to claim 1, characterized in that, The alcohol is at least one of ethanol, isopropanol, glycerol, and n-butanol.

4. The oilfield tracer according to any one of claims 1 to 3, characterized in that, The tracer for oil fields also contains phenyl succinic anhydride-acylated chitosan as a raw material. The phenyl succinic anhydride-acylated chitosan is prepared by modifying chitosan with phenyl succinic anhydride.

5. The oilfield tracer according to any one of claims 1 to 3, characterized in that, The preparation method of the complexing agent includes the following steps: (1) Dissolve rare earth metal salts in water to prepare rare earth metal salt solutions; (2) Add a complexing agent to the rare earth metal salt solution and mix well. Then add sodium 3-allyloxy-2-hydroxypropanesulfonate to react and obtain a rare earth metal complex.

6. The oilfield tracer according to claim 5, characterized in that, In step (2), the addition of sodium 3-allyloxy-2-hydroxypropanesulfonate further includes the addition of phenyl succinic anhydride acylated chitosan; the phenyl succinic anhydride acylated chitosan is prepared by modifying chitosan with phenyl succinic anhydride.

7. A method for evaluating the oil production contribution of each section of a horizontal well, characterized in that, This includes evaluation using the oilfield tracer described in any one of claims 1 to 6.

8. The method for evaluating the oil production contribution of each section of a horizontal well according to claim 7, characterized in that, Includes the following steps: (1) Extract background samples from each section of the horizontal well for analysis, and select an oilfield tracer for that section; the oilfield tracer is the oilfield tracer described in any one of claims 1 to 6; (2) Based on the fracture volume, reservoir oil saturation, and the minimum detection limit of the effective tracer in the fracturing design, the amount of tracer to be used is calculated by magnification of 20 times. (3) The tracer is pumped into the fracturing fluid in different well sections to start fracturing; (4) Samples were taken and analyzed during the fracturing fluid flowback and production process to determine the concentration of chemical tracers in the samples and to calculate the contribution rate of each section to oil and water production.

9. The method for evaluating the oil production contribution of each section of a horizontal well according to claim 8, characterized in that, In step (2), the amount Q of the tracer is calculated using the following formula (I): （I） In the formula, Q represents the amount of tracer used (kg), and A represents the crack sweep area (m²). 2 H – Average fracture thickness; φ – Reservoir porosity, %; So – Reservoir oil saturation, %; MDL – Minimum detection limit, dimensionless.

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