A method and system for optimizing residual acid flowback time in high-speed water injection oil wells

Abstract

This application relates to the field of petroleum engineering and provides a method and system for optimizing the residual acid flowback time in high-speed water injection oil wells. The method includes: testing the critical concentration of fluoride ions in the acid; obtaining a core sample and placing it in a test pipeline, then injecting acid into the test pipeline; allowing the acid to remain in the test pipeline for a specified duration after injection; obtaining the skin factor of the core sample and the fluoride ion concentration of the acid in the core sample at different test durations; determining the test duration at which the skin factor is minimized and the test duration at which the fluoride ion concentration reaches the critical concentration value; selecting the smaller of these two values ​​as the optimal duration; and performing residual acid flowback at the optimal interval after the well is shut down following acid injection. This reduces damage to the reservoir, improves acidizing efficiency, and overcomes the limitation of existing methods that rely solely on engineers' experience to determine the flowback time.

Description

Technical Field

[0001] This invention belongs to the field of petroleum engineering, and in particular relates to a method and system for optimizing the residual acid backflow time of high-speed water injection oil wells. Background Technology

[0002] As oil production continues, reservoirs in high-speed water injection wells become clogged to varying degrees due to various reasons, leading to a slowdown in water injection rates and reduced oil production efficiency. Conventional techniques include acidizing to unblock these clogs. Acidizing involves injecting acid solutions into the formation to dissolve certain minerals, restoring or increasing near-wellbore permeability in oil and gas wells. However, during matrix acidizing, if the residual acid is flowed back into the well too late after pump shutdown, the retained acid can damage reservoir porosity and structure, causing a resurgence of the improved skin factor and reducing the effectiveness of acidizing. Conversely, if the residual acid is flowed back too early, before it has fully dissolved the reservoir rock and become residual acid, the utilization efficiency of the flowed-back residual acid will be reduced, failing to achieve the desired acidizing effect. Therefore, determining the timing of residual acid flowback is crucial for protecting the reservoir and maximizing acid utilization during matrix acidizing operations.

[0003] In existing technologies, the most common practice is for engineers to determine the return time after acidification based on experience, which often results in the residual acid being returned too early or too late. Summary of the Invention

[0004] The purpose of this application is to provide a method for optimizing the residual acid backflow time of high-speed water injection oil wells, aiming to solve the problem in the prior art that engineers determine the residual acid backflow time after acidification based on experience, which easily leads to the residual acid backflow timing being too early or too late.

[0005] This application embodiment is implemented as follows: a method for optimizing the residual acid flowback time of a high-speed water injection oil well, the method comprising:

[0006] S100: Conduct a rock powder dissolution experiment to test the critical concentration value of fluoride ions in terrine. The critical concentration value is the fluoride ion concentration value corresponding to the maximum dissolution rate of rock powder during the dissolution process of terrine.

[0007] S200: Obtain a core specimen, wherein the core specimen is a columnar structure and has an acid inlet end for soil acid infiltration and an acid outlet end for soil acid outfiltration at both ends.

[0008] S300: Place the core specimen in the test pipeline and disconnect the test pipeline, inject soil acid into the test pipeline, and allow the soil acid to seep through the core specimen from the acid inlet end to the acid outlet end.

[0009] S400: Close the inlet and outlet of the test pipeline, allow the acid to remain in the test pipeline for the test duration, obtain the fluoride ion concentration value at each location in the core specimen, and open the inlet and outlet of the test pipeline. Detect the liquid pressure on the acid inlet side and the acid outlet side of the first pipeline in the core specimen, calculate the pressure difference between the two ends of the core specimen, and calculate the skin factor of the core specimen under the test duration based on the pressure difference.

[0010] S500: Based on different test durations, repeat steps S200 to S400 to obtain the skin factor of the core specimen and the fluoride ion concentration at each location in the core specimen under different test durations;

[0011] S600: Obtain the test duration for achieving the minimum skin factor to obtain the first duration; obtain the test duration for achieving the critical concentration value of fluoride ions in the core specimen to obtain the second duration; take the smaller of the first duration and the second duration as the optimal duration; and perform residual acid backflow after stopping the pump following acid injection in the oil well at intervals of the optimal duration.

[0012] In a preferred embodiment of this application, the method for conducting a rock powder dissolution experiment to test the critical concentration value of fluoride ions in soil acid includes:

[0013] Obtain core samples, wash the core samples with oil, and dry them;

[0014] The dried core sample was crushed and dried again to obtain rock powder;

[0015] The rock powder is placed in a prepared soil acid, and the relationship between the fluoride ion concentration of the soil acid and the dissolution rate of the rock powder is obtained. The critical fluoride ion concentration value is the fluoride ion concentration value corresponding to the maximum dissolution rate.

[0016] In a preferred embodiment of this application, the skin factor of the core specimen is calculated based on the following formula:

[0017]

[0018] Where S is the skin factor of the core specimen, Pt is the pressure difference between the two ends of the core specimen, φ is the porosity of the core specimen, K is the permeability of the core specimen, L is the length of the core specimen, A is the cross-sectional area of ​​the core specimen, and μ is the viscosity of the soil acid.

[0019] In a preferred embodiment of this application, the method for obtaining the fluoride ion concentration values ​​at various locations in a core specimen includes:

[0020] Acid samples were taken from multiple points within the core specimen to obtain the first sample group.

[0021] The first sample group was detected using the fluoride ion selective electrode method to obtain the fluoride ion concentration at multiple points in the core specimen;

[0022] Based on the fluoride ion concentration at multiple points in the core specimen, the distribution of fluoride ion concentration in the core specimen is obtained, and the fluoride ion concentration value at each location in the core specimen is obtained.

[0023] In a preferred embodiment of this application, the residual acid backflow time optimization method further includes:

[0024] The method for optimizing residual acid backflow time also includes:

[0025] Obtain the distribution of fluoride ion concentration in the core specimen when the test duration is equal to the optimal duration;

[0026] Based on the distribution of fluoride ion concentration in the core specimen when the test duration is equal to the optimal duration, the distance from the position where the fluoride ion concentration in the core specimen equals the critical concentration value to the acid inlet end is obtained, and the effective radius of the soil acid is obtained.

[0027] In a preferred embodiment of this application, the residual acid backflow time optimization method further includes:

[0028] Displacing medium is injected from the acid outlet end of the core specimen, and the displacing medium is used to penetrate into the core specimen to remove the soil acid in the core specimen.

[0029] In a preferred embodiment of this application, the core specimen is injected with acid at a concentration of 12% HCl + 3% HF and a flow rate of 10 ml / min.

[0030] In a preferred embodiment of this application, the rock powder has a particle size of 100 mesh and is dried at a temperature of 60°C.

[0031] This application also proposes a system for optimizing the residual acid flowback time of high-speed water injection oil wells, the system comprising:

[0032] A clamping device includes an outer tube with a sheath inside. The sheath is used to fit over the outside of a core specimen. A sealed compression chamber is formed between the sheath and the outer tube. The compression chamber is connected to a confining pressure pump. The two ends of the sheath are an acid inlet and an acid outlet, respectively. Pressure gauges are connected to the acid inlet and the acid outlet, respectively, and the two pressure gauges are used to detect the pressure values ​​at the acid inlet and the acid outlet.

[0033] A soil acid container, wherein the outlet of the soil acid container is connected to the acid inlet, and the soil acid container is used to hold soil acid;

[0034] A constant flow pump is connected to the soil acid container, and the constant flow pump is used to drive the soil acid in the soil acid container into the core specimen of the clamp.

[0035] A liquid collection component is connected to the acid outlet, and the liquid collection component is used to recover the residual acid flowing out of the acid outlet.

[0036] In a preferred embodiment of this application, a plurality of sampling tubes are spaced apart along the length of the sheath, one end of each sampling tube is connected to the inner cavity of the sheath, and the other end extends to the outside of the outer tube and is connected to a valve.

[0037] This application provides a method for optimizing the residual acid flowback time in high-speed water injection oil wells. The method involves injecting acid into core samples and testing the changes in skin factor and fluoride ion concentration in the soil acid at different test durations. The method identifies the test duration for achieving the minimum skin factor and the test duration for reaching the critical fluoride ion concentration, selecting the smaller of these two values ​​as the optimal duration. Residual acid flowback is then performed after the well is shut down following the acid injection pump, at an interval of this optimal duration. This ensures a lower skin factor in the reservoir, reducing reservoir damage, and prevents secondary precipitation during soil acid acidification that could re-block reservoir voids, thus improving acidification efficiency. Furthermore, by detecting changes in skin factor and fluoride ion concentration in the core samples at different durations after acid injection, the residual acid flowback time is determined, overcoming the limitation of existing methods that rely solely on engineer experience to determine the flowback time, thereby improving acidification effectiveness. Attached Figure Description

[0038] Figure 1 This is a flowchart illustrating a method for optimizing the residual acid backflow time in a high-speed water injection oil well, as described in this application.

[0039] Figure 2 This is a flowchart illustrating a method for conducting rock powder dissolution experiments to test the critical concentration of fluoride ions in soil acid, as described in this application.

[0040] Figure 3 This is a flowchart illustrating the method for obtaining fluoride ion concentration values ​​at various locations in a core specimen according to embodiments of this application.

[0041] Figure 4 This is a flowchart illustrating the method for obtaining the effective radius of the soil acid in the embodiments of this application;

[0042] Figure 5 This is a schematic diagram of the structure of a device for optimizing the residual acid backflow time of a high-speed water injection oil well, as described in an embodiment of this application.

[0043] Figure 6 This is a curve showing the change in rock powder dissolution rate over time in an embodiment of this application;

[0044] Figure 7This is a curve showing the change of the skin factor of the core specimen over time in the embodiments of this application. Detailed Implementation

[0045] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0046] The specific implementation of this application will be described in detail below with reference to specific embodiments.

[0047] Understandably, after years of development, oilfields suffer varying degrees of reservoir damage, leading to clogged reservoir pores, insufficient fluid supply from oil wells, and increased injection pressure in water wells. This results in a slower water injection rate, failing to meet the requirements for high-speed water injection in oil wells. This application provides a method for optimizing the residual acid backflow time in high-speed water injection oil wells, which can optimize the residual acid backflow time and improve the unblocking effect.

[0048] like Figure 1 As shown in the embodiment of this application, a method for optimizing the residual acid flowback time of a high-speed water injection oil well is provided. The method includes:

[0049] S100: Conduct a rock powder dissolution experiment to test the critical concentration value of fluoride ions in the tereic acid, wherein the critical concentration value is the concentration value of fluoride ions in the tereic acid when precipitates appear during the dissolution of the rock powder by the tereic acid.

[0050] In some embodiments of this application, the method for conducting rock powder dissolution experiments to test the critical concentration value of fluoride ions in soil acid includes:

[0051] S110: Obtain a core sample, wash the core sample with oil, and dry it;

[0052] S120: The dried core sample is crushed and dried again to obtain rock powder;

[0053] S130: The rock powder is placed in the prepared soil acid, and the relationship between the fluoride ion concentration of the soil acid and the dissolution rate of the rock powder is obtained. The critical concentration value of fluoride ions is the fluoride ion concentration value corresponding to the maximum dissolution rate.

[0054] In this embodiment, pulverizing the core sample can increase the dissolution rate and shorten the experimental time. Drying the rock powder can prevent moisture from interfering with the experiment and improve its accuracy. Specifically, in some embodiments of this application, the core sample is a core, rock fragment, or rock block sample from the reservoir to be modified. The prepared acid solution has a concentration of 12% HCl + 3% HF, is dispensed into multiple beakers, and placed in a water bath. The temperature of the water bath is set to the actual temperature of the reservoir to be modified, for example, 92°C. The rock powder has a particle size of 100 mesh, and is dried at 60°C to facilitate better dissolution. In some embodiments, the dissolution rate of the rock powder can be calculated by comparing the mass of the solid in the beaker after the predetermined experimental time with the initial mass of the rock powder.

[0055] In one embodiment of this application, the results of the rock powder dissolution experiment are as follows: Figure 6 As shown, the rock powder dissolution rate increases continuously with time, but after 3 hours, the dissolution rate stops increasing and instead decreases, indicating an inflection point. This is due to the formation of precipitation from a secondary reaction. The fluoride ion concentration at this point, measured using the fluoride ion selective electrode method, is 0.32%, which is the critical concentration value for fluoride ions.

[0056] S200: Obtain a core specimen, which is a columnar structure, with an acid inlet end for soil acid infiltration and an acid outlet end for soil acid outfiltration at both ends.

[0057] In some embodiments, the basic parameters of the core specimen include the core length L, diameter d, initial permeability K, porosity φ, and initial skin factor S0. Specifically, in some embodiments, the specific values ​​of the basic parameters are shown in Table 1.

[0058]

[0059] S300: Place the core specimen in the test pipeline and disconnect the test pipeline, inject soil acid into the test pipeline, and allow the soil acid to seep through the core specimen from the acid inlet end to the acid outlet end.

[0060] In this embodiment, the tert-acid penetrates into the core specimen, dissolving the carbonate and silicate minerals within, thereby increasing the permeability of the core specimen. In some embodiments of this application, the core specimen is injected with tert-acid at a concentration of 12% HCl + 3% HF at a flow rate of 10 ml / min.

[0061] S400: Close the inlet and outlet of the test pipeline, allow the acid to remain in the test pipeline for the specified test time, obtain the fluoride ion concentration values ​​at various locations in the core specimen, and open the inlet and outlet of the test pipeline. Detect the liquid pressure on the acid inlet side and the acid outlet side of the first pipeline in the core specimen, calculate the pressure difference between the two ends of the core specimen, and calculate the skin factor of the core specimen under the specified test time based on the pressure difference.

[0062] In this embodiment, the core specimen obstructs the flow of acid within the test pipeline, causing a pressure difference between the two ends of the core specimen. This results in a pressure difference between the acid inlet and outlet ends of the core specimen. After stopping the acid injection and allowing the test time to pass, the outlet of the test pipeline is opened and the pump is started to continue injecting acid into the test pipeline. The pipe pressure at both ends of the core specimen in the test pipeline is measured to obtain the pressure values ​​at the acid inlet and outlet ends. The pressure difference between the acid inlet and outlet ends is obtained by subtracting the pressure values ​​at the acid inlet and outlet and taking the absolute value.

[0063] In some embodiments of this application, the skin factor of the core specimen is calculated based on the following formula:

[0064]

[0065] Where S is the skin factor of the core specimen, and P t φ is the pressure difference between the two ends of the core specimen, φ is the porosity of the core specimen, K is the permeability of the core specimen, L is the length of the core specimen, A is the cross-sectional area of ​​the core specimen, and μ is the viscosity of the soil acid.

[0066] The "skin factor" mentioned in this application is a numerical value used to describe the severity of the skin effect, which refers to changes in formation permeability near the wellbore caused by human factors. A skin factor greater than zero indicates flow resistance or formation damage.

[0067] S500: Based on different test durations, repeat steps S200 to S400 to obtain the skin factor of the core specimen and the fluoride ion concentration of the tereic acid in the core specimen under different test durations.

[0068] For example, in some embodiments, the pressure difference between the acid inlet and outlet ends at different test durations after acid injection is shown in Table 2.

[0069]

[0070] Based on the pressure difference, the skin factor of the core specimen under the test duration can be calculated to obtain... Figure 4The relationship between epidermal growth factor and test time is shown. The minimum epidermal growth factor was obtained when the test time was 3 hours, so in this embodiment, the first test duration was 3 hours.

[0071] S600: Obtain the test duration for achieving the minimum skin factor to obtain the first duration; obtain the test duration for the fluoride ion concentration of the soil acid in the core specimen to reach the critical concentration value to obtain the second duration; take the smaller of the first duration and the second duration as the optimal duration; and perform residual acid backflow after the pump is stopped from acid injection in the oil well at intervals of the optimal duration.

[0072] In this embodiment, core samples are injected with acid, and the changes in skin factor and fluoride ion concentration in the acid are tested at different test durations. The test duration for achieving the minimum skin factor and the test duration for reaching the critical fluoride ion concentration are obtained, and the smaller of the two is selected as the optimal duration. After the well is stopped from acid injection, residual acid is flowed back after the optimal interval. This ensures that the reservoir achieves a low skin factor, reducing damage to the reservoir, and prevents secondary precipitation during acidizing that could re-block reservoir voids, improving the acidizing and unblocking effect, and thus better improving the subsequent water injection rate to meet the requirements of high-speed water injection. In addition, by detecting the changes in skin factor and fluoride ion concentration in the core samples at different durations after acid injection, the residual acid flow-back time is obtained, overcoming the deficiency of existing flow-back time determination based solely on engineer experience, and improving the acidizing effect.

[0073] In some embodiments of this application, the method for detecting the fluoride ion concentration of argyric acid in the core specimen includes:

[0074] S410: Take soil acid samples from multiple points inside the core specimen to obtain the first sample group;

[0075] S420: The first sample group is detected using the fluoride ion selective electrode method to obtain the fluoride ion concentration at multiple points in the core specimen;

[0076] S430: Based on the fluoride ion concentration at multiple points in the core specimen, obtain the distribution of fluoride ion concentration in the core specimen, and obtain the fluoride ion concentration value at each location in the core specimen.

[0077] In this embodiment, by obtaining the seeping soil acid from the core specimen to form a first sample group, and detecting the fluoride ion concentration of the first sample group, the fluoride ion concentration at multiple points in the core specimen can be determined. In some embodiments, the fluoride ion concentration value at any point in the core specimen can be obtained through interpolation, thereby revealing the distribution of fluoride ion concentration in the core specimen. Furthermore, the fluoride ion selective electrode method can conveniently obtain the fluoride ion concentration value. The measurement principle and required measuring instruments involved in the fluoride ion selective electrode method are conventional technologies and will not be elaborated further here.

[0078] In some embodiments, multiple sampling points are provided, such as Figure 5 As shown, five sampling points were set up: sampling point A, sampling point B, sampling point C, sampling point D, and sampling point E. These five sampling points were evenly spaced from the acid inlet to the acid outlet. Multiple sampling points allow for the detection of fluoride ion concentrations at multiple points within the core specimen. For example, in some embodiments, the chloride ion concentrations at each sampling point at different testing times after acid injection are shown in Table 3 below.

[0079]

[0080] As can be seen from the table, 3 hours after acid injection, the fluoride ion concentration at sampling point E was 0.30%, which is less than the critical concentration of 0.32%. Therefore, the second duration can be set to 3 hours. From the above, it can be seen that in this embodiment, the test duration corresponding to the minimum skin factor of the core specimen is 3 hours. Therefore, the optimal duration in this embodiment is 3 hours. In some other embodiments, when the first duration and the second duration are not equal, the smaller of the two values ​​is taken as the optimal duration.

[0081] In some instances of this application, the method for optimizing residual acid backflow time further includes:

[0082] S710: Obtain the distribution of fluoride ion concentration in the core specimen when the test duration is equal to the optimal duration;

[0083] S720: Based on the distribution of fluoride ion concentration in the core specimen when the test duration is equal to the optimal duration, obtain the distance from the position where the fluoride ion concentration in the core specimen is equal to the critical concentration value to the acid inlet end, and obtain the effective radius of the soil acid.

[0084] In this embodiment, based on the distribution of fluoride ion concentration in the core specimen when the test duration is equal to the optimal duration, the distance from the position where the fluoride ion concentration in the core specimen is equal to the critical concentration value to the acid inlet is obtained, and the effective radius of the soil acid is obtained. This facilitates subsequent determination of whether the soil acid or the concentration of soil acid is suitable for the reservoir to be optimized.

[0085] In one example of this application, the method for optimizing the residual acid backflow time further includes: injecting a displacement medium from the acid outlet of the core specimen, the displacement medium being used to penetrate into the core specimen to remove the soil acid in the core specimen.

[0086] In this embodiment, the core specimen is inverted, and the soil acid is displaced from the core specimen, allowing for soil acid recovery and preventing environmental pollution. Specifically, in some embodiments, the displacer can be an inert gas, which permeates into the core specimen and displaces the soil acid.

[0087] like Figure 5 As shown in the embodiments of this application, a system for optimizing the residual acid backflow time of a high-speed water injection oil well is also provided. The system includes: a constant flow pump 100, an acid container 210, a clamp 300, and a liquid collection assembly.

[0088] The clamp 300 includes an outer tube, inside which a leather sleeve is provided. The leather sleeve is used to fit over the outside of the core specimen 10. A sealed compression chamber is formed between the leather sleeve and the outer tube. The compression chamber is connected to a confining pressure pump 500. The two ends of the leather sleeve are an acid inlet and an acid outlet, respectively. Pressure gauges 700 are connected to the acid inlet and the acid outlet, respectively. The two pressure gauges 700 are used to detect the pressure values ​​of the acid inlet and the acid outlet, respectively.

[0089] The outlet of the acid container 210 is connected to the acid inlet, and the acid container 210 is used to hold acid.

[0090] A constant flow pump 100 is connected to the soil acid container 210, and the constant flow pump 100 is used to drive the soil acid in the soil acid container 210 into the core specimen of the clamp 300.

[0091] The liquid collection component is connected to the acid outlet, and the liquid collection component is used to recover the residual acid flowing out of the acid outlet.

[0092] In this embodiment, the core specimen 10 is held by a clamp 300, and a constant flow pump 100 drives the soil acid in the soil acid container 210 to be injected into the acid inlet of the clamp 300 through a pipeline, allowing the soil acid to permeate into the core specimen 10. Part of the soil acid permeates through the core specimen 10 and flows from the acid outlet of the clamp 300 into the liquid collection assembly for recovery. Thus, an acidification experiment is conducted on the core specimen 10 using this device to determine the optimal residual acid backflow time.

[0093] In this embodiment, the confining pressure pump 500 injects gas or liquid into the extrusion chamber, causing the casing to extrude the core specimen 10 to simulate the pressure in the reservoir. Furthermore, the casing is tightly fitted to the core specimen 10 to prevent gaps between the core specimen 10 and the casing, thus ensuring accurate measurement of the pressure difference between the acid inlet and outlet.

[0094] In one embodiment of this application, a plurality of sampling tubes 310 are spaced apart along the length of the sheath. One end of each sampling tube 310 is connected to the inner cavity of the sheath, and the other end extends to the outside of the outer tube and is connected to a valve.

[0095] In this embodiment, multiple sampling tubes 310 are set up. By obtaining the distance from each sampling tube 310 to the acid inlet and the fluoride ion concentration of the soil acid sample in each sampling tube 310, the distribution of fluoride ion concentration of soil acid in the core specimen can be obtained. Then, the distance from the position in the core specimen where the fluoride ion concentration is equal to the critical concentration value to the acid inlet can be obtained to obtain the effective radius of soil acid, so as to determine whether soil acid or soil acid concentration is suitable for the reservoir to be optimized.

[0096] Specifically, such as Figure 5 As shown, in some embodiments of this application, the clamp 300 is connected to a heating box 600 for heating the clamp 300, and the heating box 600 heats the clamp to simulate the temperature in a real reservoir. The liquid collection assembly also includes a residual acid container 220, which is connected to the acid outlet of the clamp 300 via a pipe. In some embodiments, a gas container 230 is also provided between the constant flow pump 100 and the clamp 300. The constant flow pump 100 drives the inert gas in the gas container 230 into the clamp 300 and into the core specimen 10, squeezing out the soil acid in the core specimen 10. In some embodiments of this application, the inlet ends of the soil acid container 210, the residual acid container 220, and the gas container 230 are connected to different valve ports of a six-way valve 400, and the outlet ends of the three are connected to different valve ports of another six-way valve 400. Thus, by adjusting the working position of the six-way valve 400, different pipelines can be switched for operation. For example, the acid container 210 or the gas container 230 can be connected to the constant flow pump 100 to inject acid or inert gas into the clamp 100.

[0097] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for optimizing the residual acid flowback time in high-speed water injection oil wells, characterized in that, The method includes: S100: Conduct a rock powder dissolution experiment to test the critical concentration value of fluoride ions in terrine. The critical concentration value is the fluoride ion concentration value corresponding to the maximum dissolution rate of rock powder during the dissolution process of terrine. S200: Obtain a core specimen, wherein the core specimen is a columnar structure and has an acid inlet end for soil acid infiltration and an acid outlet end for soil acid outfiltration at both ends. S300: Place the core specimen in the test pipeline and disconnect the test pipeline, inject soil acid into the test pipeline, and allow the soil acid to seep through the core specimen from the acid inlet end to the acid outlet end. S400: Close the inlet and outlet of the test pipeline, allow the acid to remain in the test pipeline for the test duration, obtain the fluoride ion concentration value at each location in the core specimen, and open the inlet and outlet of the test pipeline. Detect the liquid pressure on the acid inlet side and the acid outlet side of the test pipeline, calculate the pressure difference between the two ends of the core specimen, and calculate the skin factor of the core specimen under the test duration based on the pressure difference. S500: Based on different test durations, repeat steps S200 to S400 to obtain the skin factor of the core specimen and the fluoride ion concentration at each location in the core specimen under different test durations; S600: Obtain the test duration for achieving the minimum skin factor to obtain the first duration; obtain the test duration for achieving the critical concentration value of fluoride ions in the core specimen to obtain the second duration; take the smaller of the first duration and the second duration as the optimal duration; and perform residual acid backflow after stopping the pump following acid injection in the oil well at intervals of the optimal duration.

2. The method for optimizing the residual acid flowback time of a high-speed water injection oil well according to claim 1, characterized in that, A method for conducting rock powder dissolution experiments to test the critical concentration of fluoride ions in soil acid includes: obtaining a core sample, washing the core sample with oil, and drying it. The dried core sample was crushed and dried again to obtain rock powder; The rock powder is placed in a prepared soil acid, and the relationship between the fluoride ion concentration of the soil acid and the dissolution rate of the rock powder is obtained. The critical concentration value of the fluoride ion is the fluoride ion concentration value corresponding to the maximum dissolution rate.

3. The method for optimizing the residual acid flowback time of a high-speed water injection oil well according to claim 1, characterized in that, The skin factor of the core specimen was calculated based on the following formula: ； Where S is the skin factor of the core specimen, and P t φ is the pressure difference between the two ends of the core specimen, φ is the porosity of the core specimen, K is the permeability of the core specimen, L is the length of the core specimen, A is the cross-sectional area of ​​the core specimen, and μ is the viscosity of the soil acid.

4. The method for optimizing the residual acid flowback time of a high-speed water injection oil well according to claim 1, characterized in that, The method for obtaining the fluoride ion concentration values ​​at various locations in the core specimen includes: taking soil acid samples at multiple points in the core specimen to obtain a first sample group; The first sample group was detected using the fluoride ion selective electrode method to obtain the fluoride ion concentration at multiple points in the core specimen; Based on the fluoride ion concentration at multiple points in the core specimen, the distribution of fluoride ion concentration in the core specimen is obtained, and the fluoride ion concentration value at each location in the core specimen is obtained.

5. The method for optimizing the residual acid flowback time of a high-speed water injection oil well according to claim 4, characterized in that, The method for optimizing the residual acid backflow time also includes: obtaining the distribution of fluoride ion concentration in the core specimen when the test duration is equal to the optimal duration; Based on the distribution of fluoride ion concentration in the core specimen when the test duration is equal to the optimal duration, the distance from the position where the fluoride ion concentration in the core specimen equals the critical concentration value to the acid inlet end is obtained, and the effective radius of the soil acid is obtained.

6. The method for optimizing the residual acid flowback time of a high-speed water injection oil well according to claim 1, characterized in that, The method for optimizing the residual acid backflow time further includes: injecting a displacement medium from the acid outlet end of the core specimen, wherein the displacement medium is used to penetrate into the core specimen to remove the soil acid in the core specimen.

7. The method for optimizing the residual acid flowback time of a high-speed water injection oil well according to claim 1, characterized in that, The core specimens were injected with acid at a concentration of 12% HCl + 3% HF and a flow rate of 10 ml / min.

8. The method for optimizing the residual acid flowback time of a high-speed water injection oil well according to claim 2, characterized in that, The rock powder has a particle size of 100 mesh and is dried at a temperature of 60°C.

9. A system for optimizing the residual acid flowback time of a high-speed water injection oil well, the system being used to execute the method for optimizing the residual acid flowback time of a high-speed water injection oil well according to claim 1, characterized in that, The system includes: a clamp, the clamp including an outer tube, an inner sleeve provided in the outer tube, the inner sleeve being used to fit over the outside of the core specimen, a closed compression chamber being formed between the inner sleeve and the outer tube, the compression chamber being connected to a confining pressure pump, the two ends of the inner sleeve being an acid inlet and an acid outlet respectively, the acid inlet and the acid outlet being respectively connected to pressure gauges, the two pressure gauges being used to detect the pressure values ​​of the acid inlet and the acid outlet respectively; A soil acid container, wherein the outlet of the soil acid container is connected to the acid inlet, and the soil acid container is used to hold soil acid; A constant flow pump is connected to the soil acid container, and the constant flow pump is used to drive the soil acid in the soil acid container into the core specimen of the clamp. A liquid collection component is connected to the acid outlet, and the liquid collection component is used to recover the residual acid flowing out of the acid outlet.

10. The system for optimizing residual acid flowback time in high-speed water injection oil wells according to claim 9, characterized in that, Multiple sampling tubes are spaced apart along the length of the sheath. One end of each sampling tube is connected to the inner cavity of the sheath, and the other end extends to the outside of the outer tube and is connected to a valve.

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