Fracture interpretation method based on fracturing tracer flowback curve

Abstract

The present invention provides a fracture interpretation method based on a fracturing tracer flowback curve, comprising: step 1, establishing a theoretical interpretation of flowback curve characteristics under different fracture conditions; step 2, establishing a mathematical model to characterize double-wing seams / compound seams, and establishing a one-to-one correspondence between the flowback curve and the fracture characteristics; step 3, using the established double-wing seam / compound seam mathematical model and the actual flowback curve to perform inversion fitting analysis to obtain fracture parameters; step 4, establishing a fracture interpretation method based on a fracturing tracer flowback curve, and applying it to fracture analysis of the actual fracturing tracer flowback curve. The fracture interpretation method based on the fracturing tracer flowback curve can obtain fracture morphology and fracture parameters, etc. for tracer flowback concentration curves of different fracturing wells, and has small errors and is easy to apply, which plays an important guiding role in subsequent fracturing work of similar well layers.

Description

Technical Field

[0001] The invention relates to the field of oil and gas exploration and development, and in particular to a fracture interpretation method based on a fracturing tracer flowback curve. Background Art

[0002] Multi-stage fracturing is widely used in the development of tight oil and gas and shale oil and gas. In actual field practice, the detailed description of the fracture network formed after fracturing has become a difficult problem in the development of tight oil reservoirs and even other unconventional oil and gas resources. It determines the accuracy of the evaluation of the effect after fracturing and the prediction of production dynamics.

[0003] In multi-stage fracturing operations, different tracers are usually injected into the fracturing fluid in steps to be injected into different fracturing layers. The tracers enter the cracks along with the fracturing fluid for seepage. After the fracturing construction is completed, the tracer will be returned along with the fracturing fluid and the subsequent well production. The tracer concentration data is sampled regularly on the ground. By classifying and testing the tracer samples contained in the produced fluid and measuring the concentration, the relationship curve of the tracer return concentration over time can be obtained. The return concentration curve is often used to evaluate the return efficiency, cleaning efficiency and liquid production contribution rate of the fracturing fluid. At the same time, since the different characteristic parameters of the post-fracturing fracture will lead to different tracer return concentration curves, the tracer return concentration curve also contains the parameter characteristics of the post-fracturing fracture to a certain extent. By analyzing the tracer flowback curve, the fracture morphology after fracturing, fracture-related parameters and production capacity can be inverted and deduced through corresponding theoretical research and practical field application. It can be used to evaluate various fracturing parameters (physical parameters of the fracturing section, mechanical parameters of the fracturing section) and guide reservoir evaluation, geological design, fracturing design and the design of a reasonable working system.

[0004] Therefore, in order to more accurately describe the fracture morphology formed after fracturing construction, it is necessary to establish a fracture interpretation method based on the fracturing tracer flowback curve in the actual field application of oil reservoirs.

[0005] In the Chinese patent application with application number: CN201610875282.9, a fracture identification method and device are involved, which includes: obtaining fracture data of the study area; determining the development characteristics and distribution law of fractures in the study area based on the fracture data; establishing the electrical identification standard of lithology and dividing various lithologies; extracting fracture characteristic parameters of various lithologies respectively according to the logging curve; establishing a fracture nonlinear identification model according to the fracture characteristic parameters and obtaining the fracture identification results of a single well in the study area according to the model; determining the three-dimensional model of fracture strength in the study area according to the fracture identification results of the single well combined with production dynamic data, fracture development characteristics and fracture distribution law. This patent requires modeling and learning of a large amount of data, and requires the use of a probabilistic neural network algorithm, which is complex to operate. Due to the wide range of given thresholds, the error value between the obtained fracture parameters and the actual fracture is large.

[0006] In the Chinese patent application with application number: CN201910926527.X, a quantitative interpretation method of tracer curves for fracture-cavity reservoirs is involved, which includes the following steps: S1 classification of inter-well storage space in fracture-cavity reservoirs; S2 establishment of an interpretation model for inter-well tracer output curves for fracture-cavity reservoirs; S3 fitting and solving inter-well tracer output curves to obtain parameters that quantitatively describe the inter-well connectivity characteristics. This patent only quantitatively describes the inter-well connectivity characteristic parameters between each production well and water injection well, and does not describe the fracturing fracture parameters of a single well.

[0007] In the Chinese patent application with application number: CN201910081443.0, a method for analyzing the characteristics of the tracer output curve of fracture-cavity reservoirs is involved, and the steps are as follows: 1) Analyze the fracture-cavity characteristics of the target reservoir, establish a conceptual model to characterize the fractures / caves of the reservoir, and analyze the flow characteristics of the injected fluid under fracture and cave conditions; 2) Theoretically analyze the influence of the tracer output curve characteristics of fractures and caves, and establish a one-to-one correspondence between theoretical interpretation and output characteristics; 3) Analyze the influence of different fracture and cave combinations on the tracer output curve characteristics, and establish a one-to-one correspondence between theoretical interpretation and output characteristics; 4) Establish an analytical method for the tracer output curve characteristics under fracture-cavity reservoir conditions, and apply it to the actual output curve analysis. This patent can only qualitatively determine whether the formation fractures and caves are fractures or caves, and cannot quantitatively describe them. Moreover, it only divides natural fractures, and does not analyze artificial fractures.

[0008] To this end, we invented a new fracture interpretation method based on fracturing tracer flowback curve to solve the above technical problems. Summary of the invention

[0009] The object of the present invention is to provide a fracture interpretation method based on a fracturing tracer flowback curve which can obtain fracture morphology and fracture parameters, etc., has small error and is easy to apply.

[0010] The purpose of the present invention can be achieved through the following technical measures: a fracture interpretation method based on a fracturing tracer flowback curve, the fracture interpretation method based on a fracturing tracer flowback curve comprising: step 1, establishing a theoretical interpretation of flowback curve characteristics under different fracture conditions; step 2, establishing a mathematical model to characterize double-wing fractures / compound fractures, and establishing a one-to-one correspondence between the flowback curve and the fracture characteristics; step 3, using the established double-wing fracture / compound fracture mathematical model and the actual flowback curve to perform inversion fitting analysis to obtain fracture parameters; step 4, establishing a fracture interpretation method based on a fracturing tracer flowback curve, and applying it to fracture analysis of actual fracturing tracer flowback curves.

[0011] The purpose of the present invention can also be achieved by the following technical measures:

[0012] In step 1, based on different tight oil reservoir fracturing processes, the differences in fracture patterns formed by different fracturing processes are discussed from the perspective of formation rock fracture conditions. Combined with the microseismic monitoring data after different fracturing operations at the oil field site, the on-site fractures are divided into double-wing fractures and compound fractures. The characteristics of the tracer return flow curve are theoretically analyzed, and the flow characteristics of the injected tracer under the conditions of double-wing fractures and compound fractures are analyzed. A theoretical explanation of the return flow curve characteristics under different fracture conditions is established.

[0013] In step 1, the characteristics of the flowback curve of the hydraulic fractures on site are recognized and analyzed, including the curve shape, the number of peaks, the time to see the tracer, and the time when the peak occurs; the actual conditions of hydrodynamic diffusion, convection, and adsorption of the injected tracer in the fracture are considered, the fluid movement state of the injected tracer is analyzed, and the three-dimensional convection-diffusion seepage continuity equation of the hydraulic tracer is established; the physical model of the double-wing seam / compound seam is used to simulate the flow state of the injected tracer in the fracture, so as to analyze the influence of the hydrodynamic diffusion and retention factor on the tracer flowback curve.

[0014] In step 1, the three-dimensional convection-diffusion seepage continuity equation of the fracturing tracer is established as follows:

[0015]

[0016] Where:

[0017] v x ——The migration speed of the tracer in the x direction, m / s;

[0018] v y ——The migration velocity of the tracer in the y direction, m / s;

[0019] v z ——The migration speed of the tracer in the z direction, m / s;

[0020] u i ——Mass flow rate of tracer, kg / m 2 *s;

[0021] C——concentration of tracer, ppb;

[0022] t——tracer migration time, s.

[0023] In step 2, considering the influence of hydrodynamic diffusion and adsorption, on the basis of the classical tracer seepage equation, a mathematical model of tracer flowback corresponding to two different fractures, double-wing fracture and compound fracture, is established by transforming and solving the equation; the transformation method is Laplace transform, in which the superposition principle is also used.

[0024] In step 2, when establishing the mathematical model of tracer flowback corresponding to two different fractures, double-wing fractures and compound fractures, in order to solve the concentration in the tracer flowback stage, it is assumed that the injection stage is still continuing and the flow direction of the tracer is still away from the wellbore; the injection and flowback stages of the tracer are analogized to the inter-well tracer seepage mode of one injection and one production, thereby obtaining the mathematical model of tracer flowback corresponding to two different fractures, double-wing fractures and compound fractures.

[0025] In step 2, the mathematical model of tracer flowback in the double-wing gap is:

[0026]

[0027] Where:

[0028] C 0 ——initial concentration of tracer, ppb;

[0029] C i ——Flowback concentration of tracer, ppb;

[0030] Q i ——Average injection volume during the injection phase, m 3 / d;

[0031] t i ——time of injection phase, h;

[0032] t f ——time of flowback phase, h;

[0033] R – retention factor;

[0034] α——hydrodynamic diffusion;

[0035] V——the volume of the injected tracer;

[0036] s——crack opening, μm;

[0037] b——equivalent diameter of the crack, μm;

[0038] n is the number of cracks.

[0039] In step 2, the mathematical model of tracer flowback in composite fractures is:

[0040]

[0041] Where:

[0042] Δx——the length of the tracer slug, m;

[0043] x——tracer migration distance, m;

[0044] t′——tracer injection time, s;

[0045] t——tracer migration time, s;

[0046] t peak ——The time when the tracer concentration is the highest during the flowback stage, s;

[0047] v i ——injection phase velocity, m / s;

[0048] C 0 ——initial concentration of tracer, ppb;

[0049] C i ——Flowback concentration of tracer, ppb;

[0050] D——comprehensive diffusion coefficient;

[0051] R – retention factor;

[0052] α——Hydrodynamic diffusion.

[0053] In step 3, the actual curve is fitted and verified with the curves obtained by other fitting methods.

[0054] In step 3, the error sum of squares, determination coefficient, adjusted determination coefficient, and root mean square error are used to characterize the fitting effect and determine the crack morphology.

[0055] In step 3, the double-wing crack / compound crack mathematical model established in step 2 is compared with the normal distribution equation, and the crack parameters are obtained by inversion fitting.

[0056] In step 4, the knowledge gained from steps 1-3 is summarized and concluded, and the tracer flowback curve of the actual fracturing well is analyzed and understood, and finally an analysis method for understanding the tracer flowback curve is formed.

[0057] The fracture interpretation method based on the fracturing tracer flowback curve in the present invention provides a reasonable analysis method for interpreting the tracer flowback curve of the fracturing well. The fracture morphology and fracture parameters can be obtained for the tracer flowback concentration curves of different fracturing wells, and the error is small and the application is convenient, which plays an important guiding role in the subsequent fracturing work of similar well layers. The method accurately describes the fracture morphology and fracture parameters after fracturing through multiple fracturing tracer flowback curves, and establishes a fracture interpretation method based on the fracturing tracer flowback curve.

[0058] The beneficial effects of the present invention are as follows: 1. considering the on-site fracturing technology, the fractures after fracturing are divided into two fracture forms, namely double-wing fractures and compound fractures, and the characteristics of the tracer flowback curves under different fracture forms are studied, including the curve form, the number of peaks, the tracer appearance time, the peak appearance time and the like, and the corresponding relationship between the fracture form and the tracer flowback curve is clarified; 2. a physical model experiment is carried out, and the flow state of the injected tracer in the fracture is simulated by using the physical model of the double-wing fracture / compound fracture, and the influence of the hydrodynamic diffusion and the retention factor on the tracer flowback curve is clarified; 3. on the basis of the classical seepage equation of the tracer, a tracer flowback mathematical model corresponding to the two different fractures, namely the double-wing fracture and the compound fracture, is established through the transformation and solution of the equation, and the abstract tracer flowback curve is concretized and digitized; 4. the established double-wing fracture / compound fracture mathematical model is subjected to inversion fitting analysis with the actual flowback curve to obtain the fracture parameters, so as to achieve the purpose of accurately describing the fracture network, thereby evaluating the fracturing effect and guiding the fracturing design. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] Figure 1 It is a schematic diagram of the tracer flowback curve characteristics under double-wing slot conditions in a specific embodiment of the present invention;

[0060] Figure 2 It is a schematic diagram of the tracer flowback curve characteristics under the condition of composite fracture in a specific embodiment of the present invention;

[0061] Figure 3 It is a flowback curve diagram of a field fracturing tracer in a specific embodiment of the present invention;

[0062] Figure 4 It is a fitting curve diagram of the fracturing tracer flowback curve in a specific embodiment of the present invention;

[0063] Figure 5 The flowchart is a specific embodiment of the fracture interpretation method based on the fracturing tracer flowback curve of the present invention. DETAILED DESCRIPTION

[0064] It should be noted that the following detailed descriptions are exemplary and are intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present invention belongs.

[0065] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, it indicates the presence of features, steps, operations and / or combinations thereof.

[0066] like Figure 1 As shown, Figure 1 The present invention is a flow chart of a fracture interpretation method based on a fracturing tracer flowback curve.

[0067] Step 101: Theoretically analyze the characteristics of the tracer flowback curve, analyze the flow characteristics of the injected tracer under the conditions of double-wing fractures and compound fractures, and establish a theoretical explanation of the flowback curve characteristics under different fracture conditions.

[0068] Based on different tight oil reservoir fracturing processes, the differences in fracture patterns formed by different fracturing processes are discussed from the perspective of formation rock fracture. Combined with the microseismic monitoring data after different fracturing operations in the oil field, the field fractures are divided into two types: double-wing fractures and compound fractures. The physical model of double-wing fractures / compound fractures is used to simulate the flow state of injected tracers in fractures. The seepage and migration of tracers in fractures are not only affected by adsorption, but also by the combined effects of hydrodynamic diffusion and convection. Therefore, the effects of hydrodynamic diffusion and retention factor on tracer flowback need to be considered.

[0069] The theoretical analysis of the characteristics of the tracer flowback curve and the flow characteristics of the injected tracer under the conditions of double-wing seams and compound seams mainly refers to the recognition and analysis of the flowback curve characteristics of the on-site fracturing fractures (double-wing seams and compound seams), including the curve shape, the number of peaks, the time when the tracer is seen, the time when the peak occurs, etc.; considering the actual conditions such as the hydrodynamic diffusion, convection, and adsorption of the injected tracer in the fracture, the fluid motion state of the injected tracer is analyzed, and the classical seepage equation of the tracer is established; the physical model of the double-wing seam / compound seam is used to simulate the flow state of the injected tracer in the fracture, so as to analyze the influence of the hydrodynamic diffusion and the retention factor on the tracer flowback curve.

[0070] Step 102: Establish a mathematical model to characterize the double-wing fracture / compound fracture, and establish a one-to-one correspondence between the flowback curve and the fracture characteristics.

[0071] The aforementioned establishment of a mathematical conceptual model to characterize the double-wing seam / compound seam and the analysis of the influence of different influencing factors on the characteristics of the tracer flowback curve mainly refers to considering the influence of hydrodynamic diffusion and adsorption, and establishing a mathematical model of tracer flowback corresponding to two different fractures, the double-wing seam and the compound seam, through the transformation and solution of the equation on the basis of the classical seepage equation of the tracer. The main method of transformation is Laplace transformation, in which the superposition principle is also used.

[0072] When establishing the mathematical model of tracer flowback corresponding to the two different fractures, double-wing fractures and compound fractures, in order to solve the concentration in the tracer flowback stage, we assume that the injection stage is still continuing and the flow direction of the tracer is still away from the wellbore. The injection and flowback stages of the tracer can be compared to the inter-well tracer seepage mode of "one injection and one production", thus obtaining the mathematical model of tracer flowback corresponding to the two different fractures, double-wing fractures and compound fractures.

[0073] Step 103: Use engineering software and the established double-wing fracture / compound fracture mathematical model to perform inversion fitting analysis with the actual flowback curve to obtain fracture parameters.

[0074] During the inversion fitting, the Curve Fitting Tool of MATLAB is used to fit the actual curve, and the curves obtained by other fitting methods can be mutually verified. First, the built-in function of the engineering software is used for fitting, and the sum of squared errors (SSE), coefficient of determination (R-square), adjusted coefficient of determination (Adjusted R-square), and root mean square error (RMSE) are used to characterize the fitting effect and determine the crack morphology; then the double-wing seam / compound seam mathematical model established in step (2) is used to compare the model with the normal distribution equation, and the crack parameters are obtained by inversion fitting.

[0075] Step 104: Establish a fracture interpretation method based on the fracturing tracer flowback curve, and apply it to fracture analysis of the actual fracturing tracer flowback curve.

[0076] The knowledge gained from step (1), step (2) and step (3) is summarized and concluded, and the tracer flowback curve of the real fracturing well is analyzed and understood, and finally an analysis method for understanding the tracer flowback curve is formed.

[0077] In a specific embodiment of the present invention, the following steps are included:

[0078] Step (1): According to the field data of a certain oil reservoir, based on different tight oil reservoir fracturing processes, the differences in fracture patterns formed by different fracturing processes are discussed from the perspective of formation rock fracture conditions. Combined with the microseismic monitoring data after different fracturing operations in the oil field, the field fractures are divided into two types: double-wing fractures and compound fractures. In the fracture seepage system dominated by double-wing fractures, in the backflow stage after fracturing, since the main seepage channel of the tracer is a single fracture, the tracer is produced rapidly. In a short period of time, the tracer concentration increases rapidly and decreases rapidly at the same time, showing a single-peak shape. The tracer backflow concentration curve dominated by a single large fracture has a sharper peak, showing a spike-type single-peak curve; when multiple fractures are seeping back at the same time, the tracer backflow result is affected by the joint influence of multiple fractures, so the concentration curve appears as a multi-peak curve, as shown in the attached figure. Figure 1 In a fracture system with many large fractures and micro fractures, after the flowback begins, due to the high conductivity of the large fractures, the tracer in the fractures is produced earlier than the micro fractures, so the tracer concentration increases rapidly at the beginning and then gradually decreases in the later period. However, due to the compensation effect of the tracer production in the micro fractures, the concentration decrease rate is slower than the single peak curve above, the flowback time is longer, and the overall tracer concentration curve presents a single peak parabola, as shown in the attached figure. Figure 2 shown.

[0079] When analyzing the fluid movement state of the injected tracer, it was found that the seepage migration of the tracer in the fracture was not only affected by adsorption, but also by the combined influence of hydrodynamic diffusion and convection. The three-dimensional convection-diffusion seepage continuity equation of the fracturing tracer was established as follows:

[0080]

[0081] Where:

[0082] v x ——The migration speed of the tracer in the x direction, m / s;

[0083] v y ——The migration velocity of the tracer in the y direction, m / s;

[0084] v z ——The migration speed of the tracer in the z direction, m / s;

[0085] u i ——Mass flow rate of tracer, kg / m 2 *s;

[0086] C——concentration of tracer, ppb;

[0087] t——tracer migration time, s.

[0088] Step (2): Considering the influence of hydrodynamic diffusion and adsorption, based on the classical tracer seepage equation, a mathematical model of tracer flowback corresponding to two different fractures, double-wing fracture and compound fracture, is established by transforming and solving the equation.

[0089] a. Mathematical model of tracer flowback in double wing gaps:

[0090]

[0091] Where:

[0092] C 0 ——initial concentration of tracer, ppb;

[0093] C i ——Flowback concentration of tracer, ppb;

[0094] Q i ——Average injection volume during the injection phase, m 3 / d;

[0095] t i ——time of injection phase, h;

[0096] t f ——time of flowback phase, h;

[0097] R – retention factor;

[0098] α——hydrodynamic diffusion;

[0099] V——the volume of the injected tracer;

[0100] s——crack opening, μm;

[0101] b——equivalent diameter of the crack, μm;

[0102] n is the number of cracks.

[0103] b. Mathematical model of tracer flowback in composite fractures:

[0104]

[0105] Where:

[0106] Δx——the length of the tracer slug, m;

[0107] x——tracer migration distance, m;

[0108] t′——tracer injection time, s;

[0109] t——tracer migration time, s;

[0110] tpeak ——The time when the tracer concentration is the highest during the flowback stage, s;

[0111] v i ——injection phase velocity, m / s;

[0112] C 0 ——initial concentration of tracer, ppb;

[0113] C i ——Flowback concentration of tracer, ppb;

[0114] D——comprehensive diffusion coefficient;

[0115] R – retention factor;

[0116] α——Hydrodynamic diffusion.

[0117] Step (3): Now take the third tracer flowback curve as an example, as shown in the attached figure. Figure 3 As shown in the figure, first use MATLAB's Curve Fitting Tool to fit this curve. The fitting curve is shown in the figure. Figure 4 As shown. By observing the curve, it can be found that the first one is a parabolic peak, and it is preliminarily speculated that the fracture produced by the fracturing is a high conductivity channel, accompanied by branch fractures, and is a composite fracture; the second one is a parabolic peak, and it is preliminarily speculated that the fracture produced by the fracturing is a high conductivity channel, accompanied by branch fractures, and is a composite fracture; the third one is a normal distribution peak, and it is preliminarily speculated that the fracture produced by the fracturing is a high conductivity channel, which is a double-wing fracture.

[0118] The fitted curve equation is as follows:

[0119] General model Gauss4:

[0120] f(x)=a1*exp(-((x-b1) / c1)^2)+a2*exp(-((x-b2) / c2)^2)+

[0121] a3*exp(-((x-b3) / c3)^2)+a4*exp(-((x-b4) / c4)^2)

[0122] Coefficients(with 95%confidence bounds):

[0123] a1=33.13(20,46.26) b1=841.2(830.8,851.7)

[0124] c1=45.95(23.53,68.37) a2=22.7(16.94,28.46)

[0125] b2=6.089(-21.72,33.9) c2=45.45(11.67,79.24)

[0126] a3=31.59(23.28,39.9) b3=109.8(104.2,115.5)

[0127] c3=25.29(16.56,34.01) a4=15.83(11.29,20.38)

[0128] b4=744.8(630.7,858.8) c4=485(279.3,690.6)

[0129] The sum of squared errors (SSE), coefficient of determination (R-square), adjusted coefficient of determination (Adjusted R-square), and root mean square error (RMSE) are used to characterize the fitting effect. According to the above parameters, it means that the fitting effect is good.

[0130] Goodness of fit:

[0131] SSE:891 R-square:0.8513

[0132] Adjusted R-square:0.8094 RMSE:4.78

[0133] Then, the double-wing seam / compound seam mathematical model established in step (2) is compared with the normal distribution equation, and the fracture parameters are obtained by inversion fitting.

[0134] 1) The first parabola peak is the second part of the Gaussian distribution, which plays a major role, that is,

[0135]

[0136] at this time:

[0137]

[0138]

[0139]

[0140] Known v i =0.19976m / h, t=59.5h=3570min Combining the above equations, we can solve:

[0141] Δx=135.625m

[0142] R=1.9520

[0143] D=0.0978

[0144] That is, the crack length L = 135.625 mm.

[0145] According to the field data, the position of the "assumed flowback production point" at this time is x = 2v i t i / R=25m, the above formula can be solved as follows:

[0146]

[0147] at this time

[0148]

[0149] Given V = 1m 3 , t i =1.1167h=67min t f =59.5h=3570min, Q i =325.8806m 3 / h,Q f =0.4522m 3 / h, the equivalent diameter of the crack b = 0.0018m = 1.80mm, and the crack permeability K can be obtained according to the above formula m =492500mD, fracture conductivity 88650mD*m, fracture opening s=0.0024m.

[0150] 2) The second parabola peak is mainly caused by the third part of the Gaussian distribution, that is,

[0151]

[0152] at this time:

[0153]

[0154]

[0155]

[0156] Known v i =0.5106m / h, t=553.5h=33570min, combining the above equations, we can solve:

[0157] Δx=77.135m

[0158] R=1.9527

[0159] D=0.0859

[0160] That is, the crack length L = 77.135m.

[0161] According to the field data, the position of the "assumed flowback production point" at this time is x = 2v i t i / R=213.6m, the above formula can be solved as follows:

[0162]

[0163] at this time

[0164]

[0165] Given V = 1m 3 , t i =1.1167h=67min=4020s, t f =553.5h, Q i =325.8806m 3 / h,Q f =0.4522m 3 / h, the equivalent diameter of the crack b = 0.0076m = 7.60mm, and the crack permeability K can be obtained according to the above formula m =146500mD, fracture conductivity 111360mD*m, fracture opening s=0.0085m.

[0166] 3) The third normal distribution peak is mainly caused by the first part of the Gaussian distribution, that is,

[0167]

[0168] at this time:

[0169]

[0170]

[0171]

[0172] Known v i =0.9401m / h, t=1752h, combining the above equations, we can solve:

[0173] Δx=194.525m

[0174] R=1.1379

[0175] D=0.034

[0176] That is, the crack length L = 194.525m.

[0177] According to the field data, the position of the "assumed flowback production point" at this time is x = 2v i t i / R=1682.4m, the above formula can be solved as follows:

[0178]

[0179] at this time

[0180]

[0181] Given V = 1m 3 , t i =1.1167h=67min=4020s, t f =1752h, Q i =325.8806m 3 / h,Q f =0.4522m 3 / h, the equivalent diameter of the crack b = 0.0096m = 9.60mm, and the crack permeability K can be obtained according to the above formula m =263900mD, fracture conductivity 253340mD*m, fracture opening s=0.0108m.

[0182] Step (4): By combining the analysis of the tracer production curve characteristics in the above steps (2) and (3), and applying the tracer return data actually collected on site for analysis, the retention factor, hydrodynamic diffusion, fracture half-length, fracture width, fracture permeability, fracture conductivity and other fracture parameters in other tracer return curves of the well can be calculated.

[0183] Table 1 Crack parameters

[0184]

[0185]

[0186] This is consistent with the data obtained by other reservoir monitoring methods. Therefore, the characteristics of the tight oil reservoir fracturing tracer production curve analyzed and recognized by this method are correct and meaningful.

[0187] The present invention relates to the field of unconventional tight oil reservoir development. Aiming at the problem that the fracture network formed after tight oil reservoir fracturing is difficult to accurately describe, a fracture interpretation method based on fracturing tracer flowback curve is proposed. The method combines the tight oil reservoir volume fracturing technology, fully understands the tight reservoir characteristics, fully considers the tracer flowback curve interpretation principle during the volume fracturing process, and achieves the purpose of accurately describing the fracture network after fracturing, evaluating the fracturing effect, and guiding the fracturing design, which has guiding significance for further realizing large-scale and efficient development of tight oil reservoirs. In the fracturing production process, this method achieves the following effects: ① Considering the field fracturing technology, the fractures after fracturing are divided into two fracture forms, double-wing fractures and compound fractures, and the characteristics of tracer flowback curves under different fracture forms are studied, including curve form, number of peaks, tracer appearance time, peak appearance time and other characteristics, and the corresponding relationship between fracture form and tracer flowback curve is clarified; ② Physical simulation experiments are carried out, and the physical model of double-wing fractures / compound fractures is used to simulate the flow state of injected tracers in fractures, and the influence of hydrodynamic diffusion and retention factor on tracer flowback curve is clarified; ③ Based on the classical seepage equation of tracers, through the transformation and solution of the equation, a mathematical model of tracer flowback corresponding to two different fractures, double-wing fractures and compound fractures, is established, and the abstract tracer flowback curve is concretized and digitized; ④ By inversion fitting analysis of the established double-wing fracture / compound fracture mathematical model and the actual flowback curve, the fracture parameters are obtained, so as to achieve the purpose of accurately describing the fracture network. This method is of great significance for evaluating the fracturing effect, guiding the fracturing design, and improving the degree of tight oil recovery.

[0188] Finally, it should be noted that the above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art can still modify the technical solutions recorded in the aforementioned embodiments or replace some of the technical features therein with equivalents. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

[0189] Except for the technical features described in the specification, all other technical features are known technologies to those skilled in the art.

Claims

1. A fracture interpretation method based on a fracturing tracer flowback curve, characterized in that: The fracture interpretation method based on the fracturing tracer flowback curve includes: Step 1: Establish a theoretical explanation of the characteristics of the flowback curve under different fracture conditions; Step 2, establish a mathematical model to characterize the double-wing fracture / compound fracture, and establish a one-to-one correspondence between the flowback curve and the fracture characteristics; Step 3, using the established double-wing fracture / compound fracture mathematical model and the actual flowback curve to perform inversion fitting analysis to obtain fracture parameters; Step 4, establishing a fracture interpretation method based on the fracturing tracer flowback curve, and applying it to the fracture analysis of the actual fracturing tracer flowback curve; In step 1, based on different tight oil reservoir fracturing processes, the differences in fracture patterns formed by different fracturing processes are discussed from the perspective of formation rock fracture conditions. Combined with the microseismic monitoring data after different fracturing operations in the oil field, the field fractures are divided into two types: double-wing fractures and compound fractures. The characteristics of the tracer flowback curve are theoretically analyzed, and the flow characteristics of the injected tracer under the conditions of double-wing fractures and compound fractures are analyzed. The theoretical explanation of the characteristics of the flowback curve under different fracture conditions is established; In step 1, the characteristics of the flowback curve of the hydraulic fractures on site are recognized and analyzed, including the curve shape, the number of peaks, the time to see the tracer, and the time when the peak appears. The actual conditions of the hydrodynamic diffusion, convection, and adsorption of the injected tracer in the fracture are considered, the fluid movement state of the injected tracer is analyzed, and the three-dimensional convection-diffusion seepage continuity equation of the hydraulic tracer is established. The physical model of the double-wing seam / compound seam is used to simulate the flow state of the injected tracer in the fracture, so as to analyze the influence of the hydrodynamic diffusion and retention factor on the tracer flowback curve. In step 1, the three-dimensional convection-diffusion seepage continuity equation of the fracturing tracer is established as follows: Where: v x ——The migration speed of the tracer in the x direction, m / s; v y ——The migration velocity of the tracer in the y direction, m / s; v z ——The migration speed of the tracer in the z direction, m / s; u i ——Mass flow rate of tracer, kg / m 2 *s; C——concentration of tracer, ppb; t——tracer migration time, s; In step 2, when establishing the tracer flowback mathematical model corresponding to two different fractures, namely, double-wing fractures and compound fractures, in order to solve the concentration in the tracer flowback stage, it is assumed that the injection stage is still ongoing and the flow direction of the tracer is still away from the wellbore; the tracer injection and flowback stages are analogized to the inter-well tracer seepage mode of one injection and one production, thereby obtaining the tracer flowback mathematical model corresponding to two different fractures, namely, double-wing fractures and compound fractures; In step 2, the mathematical model of tracer flowback in the double-wing gap is: Where: C0——initial concentration of tracer, ppb; C i ——Flowback concentration of tracer, ppb; Q i ——Average injection volume during the injection phase, m 3 / d; t i ——time of injection phase, h; t f ——time of flowback phase, h; R – retention factor; α——hydrodynamic diffusion; V——the volume of the injected tracer; s——crack opening, μm; b——equivalent diameter of the crack, μm; n——the number of cracks; In step 2, the mathematical model of tracer flowback in composite fractures is: Where: Δx——the length of the tracer slug, m; x——tracer migration distance, m; t′——tracer injection time, s; t——tracer migration time, s; t peak ——The time when the tracer concentration is the highest during the flowback stage, s; v i ——injection phase velocity, m / s; C0——initial concentration of tracer, ppb; C i ——Flowback concentration of tracer, ppb; D——comprehensive diffusion coefficient; R – retention factor; α——Hydrodynamic diffusion.

2. The fracture interpretation method based on the fracturing tracer flowback curve according to claim 1, characterized in that: In step 2, considering the influence of hydrodynamic diffusion and adsorption, on the basis of the classical tracer seepage equation, a mathematical model of tracer flowback corresponding to two different fractures, double-wing fracture and compound fracture, is established by transforming and solving the equation; the transformation method is Laplace transform, in which the superposition principle is also used.

3. The fracture interpretation method based on the fracturing tracer flowback curve according to claim 1, characterized in that: In step 3, the actual curve is fitted and verified with the curves obtained by other fitting methods.

4. The fracture interpretation method based on the fracturing tracer flowback curve according to claim 3, characterized in that: In step 3, the error sum of squares, determination coefficient, adjusted determination coefficient, and root mean square error are used to characterize the fitting effect and determine the crack morphology.

5. The fracture interpretation method based on the fracturing tracer flowback curve according to claim 4, characterized in that: In step 3, the double-wing crack / compound crack mathematical model established in step 2 is compared with the normal distribution equation, and the crack parameters are obtained by inversion fitting.

6. The fracture interpretation method based on fracturing tracer flowback curve according to claim 1, characterized in that: In step 4, the knowledge gained from steps 1-3 is summarized and concluded, and the tracer flowback curve of the actual fracturing well is analyzed and understood, and finally an analysis method for understanding the tracer flowback curve is formed.

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