A crack fractal characterization method, system and readable storage medium

Abstract

The present application provides a method, system and readable storage medium for fractal characterization of fractures, which includes quantitatively describing the distribution characteristics of the fractal dimension of induced fractures based on the fractal characteristics of fractures in different regions, taking into account the fractal of local fractures and the change of fractal dimension with the fracture position; deriving the seepage parameters of complex fractures based on the distribution characteristics of the fractal dimension of induced fractures, and performing fractal characterization of fractures based on the derived seepage parameters of complex fractures, wherein the seepage parameters of complex fractures include at least one of porosity, permeability and compressibility. The implementation of this method can accurately describe the complexity of fracture structures between different regions and improve the accuracy of analysis.

Description

Technical Field

[0001] The present application relates to the technical field of fracture structure description, and in particular to a fracture fractal characterization method, system and readable storage medium. Background Art

[0002] Conventional oil and gas resources have been gradually exhausted after long-term exploitation. With the advancement of technology, unconventional oil and gas exploitation has gradually become the first highlight of global energy development. Due to the poor distribution stability of unconventional reservoirs, the large number of reservoir interlayers, and the prominent low porosity, low permeability and low pressure characteristics, the oil well production has declined rapidly, the reservoir utilization level is low, and effective development faces many challenges. At present, the hydraulic fractures formed by volume fracturing technology can communicate with natural fractures to form a complex fracture network, increase the permeability and oil leakage area of ​​the reservoir, and thus increase the oil well production and reservoir recovery. However, the complex fracture network structure exacerbates the heterogeneity of the reservoir, and the reservoir flow pattern and mechanism are more complex. The traditional seepage mechanics theoretical model cannot be applied to the study of such reservoirs. After a deep understanding of the complex fracture structure characteristics, a method for describing the fractures of unconventional reservoirs and characterizing the seepage parameters has been established, which is of great significance for the evaluation of the unconventional volume fracturing transformation effect and the efficient development of oil and gas resources. However, for the conventional fractal fracture characterization method, only the fractal characteristics of the overall fracture are currently considered, which cannot accurately describe the complexity of the fracture structure between different regions, and there is a problem of low analysis accuracy. Summary of the invention

[0003] The purpose of the embodiments of the present application is to provide a method, system and readable storage medium for fractal characterization of fractures, which can accurately describe the complexity of fracture structures between different regions and improve the accuracy of analysis.

[0004] The present application also provides a method for characterizing crack fractals, comprising the following steps:

[0005] S1. Based on the fractal characteristics of cracks in different regions, the distribution characteristics of the fractal dimension of induced cracks are quantitatively described, taking into account the fractal of local cracks and the change of fractal dimension with the crack position;

[0006] S2. According to the distribution characteristics of the induced fracture fractal dimension, complex fracture seepage parameters are derived, and fracture fractal characterization is performed based on the derived complex fracture seepage parameters, wherein the complex fracture seepage parameters include at least one of porosity, permeability and compressibility.

[0007] In a second aspect, the embodiment of the present application further provides a fracture fractal characterization system, the system comprising a quantitative description module and a parameter derivation module, wherein:

[0008] The quantitative description module is used to quantitatively describe the distribution characteristics of the fractal dimension of the induced cracks according to the fractal characteristics of the cracks in different regions, taking into account the fractal of the local cracks and the fractal dimension changes with the crack position;

[0009] The parameter derivation module is used to derive complex fracture seepage parameters according to the distribution characteristics of the induced fracture fractal dimension, and to perform fracture fractal characterization according to the derived complex fracture seepage parameters, wherein the complex fracture seepage parameters include at least one of porosity, permeability and compressibility.

[0010] In a third aspect, an embodiment of the present application further provides a readable storage medium, wherein the readable storage medium includes a crack fractal characterization method program, and when the crack fractal characterization method program is executed by a processor, the steps of a crack fractal characterization method as described in any one of the above items are implemented.

[0011] As can be seen from the above, the fracture fractal characterization method, system and readable storage medium provided by the embodiments of the present application can better describe the differences between fractures by quantitatively describing the distribution characteristics of the induced fracture fractal dimension compared with the conventional fractal dimension characterization method. Combining the distribution characteristics of the induced fracture fractal dimension, the derivation of complex fracture seepage parameters can derive a more general distribution of complex fracture seepage parameters, which can further improve the analysis accuracy while accurately describing the complexity of the fracture structure between different regions.

[0012] Other features and advantages of the present application will be described in the following description, and partly become apparent from the description, or be understood by practicing the embodiments of the present application. The purpose and other advantages of the present application can be realized and obtained by the structures specifically pointed out in the written description, claims, and drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings required for use in the embodiments of the present application will be briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present application and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without paying creative work.

[0014] Figure 1 A flow chart of a crack fractal characterization method provided in an embodiment of the present application;

[0015] Figure 2 Schematic diagram of crack distribution inside each unit;

[0016] Figure 3It is a schematic diagram comparing a conventional multi-factor fitting model with the fitting model disclosed in this application;

[0017] Figure 4 This is a schematic diagram of the distribution of multi-branch induced fractures in horizontal wells;

[0018] Figure 5 It is a schematic diagram of reservoir matrix unit;

[0019] Figure 6 Schematic diagram of the comparison results between fractal-induced cracks and equivalent discrete cracks;

[0020] Figure 7 A structural schematic diagram of a crack fractal characterization system provided in an embodiment of the present application. DETAILED DESCRIPTION

[0021] The technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all of the embodiments. The components of the embodiments of the present application described and shown in the drawings here can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present application provided in the drawings is not intended to limit the scope of the application claimed for protection, but merely represents the selected embodiments of the present application. Based on the embodiments of the present application, all other embodiments obtained by those skilled in the art without making creative work belong to the scope of protection of the present application.

[0022] It should be noted that similar reference numerals and letters represent similar items in the following drawings, so once an item is defined in one drawing, it does not need to be further defined and explained in subsequent drawings. At the same time, in the description of this application, the terms "first", "second", etc. are only used to distinguish the description and cannot be understood as indicating or implying relative importance.

[0023] Please refer to Figure 1 , Figure 1 This is a flow chart of a crack fractal characterization method in some embodiments of the present application. Taking the method applied to a computer device (the computer device can be a terminal or a server, and the terminal can be, but is not limited to, various personal computers, laptops, smart phones, tablet computers, and portable wearable devices. The server can be an independent server or a server cluster composed of multiple servers) as an example for explanation, the method includes the following steps:

[0024] Step S1, based on the fractal characteristics of cracks in different regions, taking into account the fractal of local cracks and the change of fractal dimension with the position of cracks, quantitatively describe the distribution characteristics of the fractal dimension of induced cracks.

[0025] Step S2, deriving complex fracture seepage parameters according to the distribution characteristics of the induced fracture fractal dimension, and performing fracture fractal characterization according to the derived complex fracture seepage parameters, wherein the complex fracture seepage parameters include at least one of porosity, permeability and compressibility.

[0026] As can be seen from the above, a fracture fractal characterization method provided in the embodiment of the present application can better describe the differences between fractures by quantitatively describing the distribution characteristics of the induced fracture fractal dimension compared with the conventional fractal dimension characterization method. Combining the distribution characteristics of the induced fracture fractal dimension, the derivation of complex fracture seepage parameters can derive a more general distribution of complex fracture seepage parameters, which can further improve the accuracy of analysis while accurately describing the complexity of fracture structures between different regions.

[0027] In one embodiment, in step S1, the distribution characteristics of the induced crack fractal dimension are quantitatively described by the following steps:

[0028] Step S11 , considering the differences in corresponding fracture structures between different regions, the fracture is equally divided into a plurality of fracture units.

[0029] Specifically, considering the differences in fracture structures between different regions, the computer device divides the fracture into n fracture segments, where the fractures between each unit are continuous, such as Figure 2 shown.

[0030] In one embodiment, assuming that the complexity of the fractures in the divided fracture units is consistent, the idea of ​​fractal statistics can be used to obtain the corresponding fractal dimension for each fracture unit.

[0031] Step S12, using the fractal box dimension method to solve the fractal dimension of each crack unit.

[0032] Specifically, when using fractal theory to characterize the structural characteristics of the fracture network, the fractal box dimension method is generally used to quantitatively characterize the complexity and density of the fracture distribution. It should be noted that fractal dimension is known as the fractal theory of nature's geometry and is a new branch of modern mathematics, but its essence is a new worldview and methodology. Fractal dimension reflects the effectiveness of complex shapes occupying space, and it is a measure of the irregularity of complex shapes. It is cross-combined with the chaos theory of dynamic systems and complements each other. It recognizes that the local part of the world may show similarities with the whole in certain aspects (morphology, structure, information, function, time, energy, etc.) under certain conditions or processes. It recognizes that the change of spatial dimension can be either discrete or continuous, thereby expanding the field of vision.

[0033] Step S13, establishing a distance transformation relationship between the induced crack fractal dimension at different positions and the target distance according to the weighted summation result between the weight function of the fractal dimension and the corresponding fractal dimension, wherein the target distance includes the vertical distance from the reference point.

[0034] Specifically, for any crack unit k, assuming its fractal dimension is d fk , and the vertical distance between it and the reference point is x, then in the current embodiment, the relationship between the crack fractal dimension and the distance x can be established by the following formula:

[0035]

[0036] Among them, F(d f ,x) is the fractal dimension d f The relationship function with distance x, is the fractal dimension d fk The weight function, r k is the coverage grid side length of crack element k, in m, N(r k ) is the total number of grids corresponding to cracks in the divided grid.

[0037] Step S14, quantitatively describing the distribution characteristics of the induced crack fractal dimension based on the distance transformation relationship obtained in step S13.

[0038] Specifically, based on the relationship between the fractal dimension of induced cracks and the distance at different positions, it is necessary to establish an objective function to quantitatively describe the distribution characteristics of the fractal dimension of induced cracks. Considering that the change relationship between the fractal dimension and the spatial position is volatile, the conventional characterization model cannot accurately fit the fractal dimension change curve. Therefore, in the current embodiment, the computer device is based on the similarity between the water content change characteristics and the fractal dimension change relationship proposed in the present invention. On the basis of the quantitative characterization model of water content change, combined with the fractal dimension distribution characteristics, a quantitative characterization model of the fractal dimension of induced cracks is constructed.

[0039] In one embodiment, in step S12, for each crack unit, a fractal box dimension method is used to solve the crack fractal dimension of the crack unit, including:

[0040] Step S121, using square grids with a side length of r to divide the crack unit into equal intervals, and counting the total number of grids N(r) corresponding to the cracks in the divided grids.

[0041] Specifically, the computer device uses a square grid with a side length of r to equidistantly divide the fracture model corresponding to the fracture unit, and simultaneously counts the number of grids N(r) with fractures in the divided grids.

[0042] Step S122, reducing the side length of the square grid in sequence according to a preset step length, and repeating the execution process of step S121, recording the total number of grids obtained in each iterative process.

[0043] Specifically, the computer device continuously reduces the grid side length r and, according to the execution principle of step S121, repeatedly counts the number of crack grids N(r) when each side length is r.

[0044] Step S123, according to the change relationship between the changed side length of the grid and the corresponding total number of grids in each iteration process, the fracture fractal dimension d of the fracture unit is obtained by solving the following formula: f :

[0045]

[0046] Specifically, the computer device can draw a relationship curve between ln(N(r)) and -ln(r) on a preset coordinate axis, and determine the fracture fractal dimension d of the fracture unit based on the slope of the curve. f , which is expressed by the above formula (1).

[0047] In one embodiment, in step S14, the distribution characteristics of the induced crack fractal dimension are quantitatively described by the following formula:

[0048]

[0049] Among them, d fmax is the peak value of the fractal dimension; x is the distance between the fracture and the wellbore, in m; x m is the crack position corresponding to the peak value of the fractal dimension, in m; G is the decreasing rate of the fractal dimension, and the decreasing rate of the fractal dimension is proportional to the natural logarithm of the crack position.

[0050] However, since the relationship between the fractal dimension and the crack position is relatively complex, and the corresponding relationship on both sides of the peak is not exactly the same, using the constant G for fitting will result in a large error. Therefore, in the current embodiment, the fractal dimension decreasing rate G is corrected, and it is assumed that the fractal dimension decreasing rate G is proportional to the natural logarithm of the crack position, that is:

[0051] G=α i ln(x);

[0052] Among them, α i is the fractal dimension decreasing coefficient, and, when i = 1, α i That is the decreasing coefficient on the left side of the peak. When i = 2, α i is the decreasing coefficient on the right side of the peak value. Therefore, after the above derivation process, the final quantitative characterization model of the induced crack fractal dimension is shown by the above formula (2).

[0053] In one embodiment, for a conventional multi-factor fitting model, such as Figure 3 As shown in (a), the fitting effect is good, and the fitting rate reaches 0.85. However, this method is limited by the fact that when the complexity of the induced crack model increases, the number of fitting functions will also increase, which will lead to an increase in the number of fitting parameters. The biggest disadvantage is that the mathematical model is not fixed and will change with the fitting model. Figure 3 (b) and Figure 3 It can be seen from (c) that when the fractal dimension decrease rate G is a constant, the fitting effect is relatively poor. The fitting error of the model will increase with the increase of the difference in the change trend on both sides of the peak. The final fitting rate of the model is between 0.7-0.8. However, the modified induced crack fractal dimension quantitative characterization model uses different fractal decrease coefficients to effectively solve the impact of the difference in the change rate on both sides of the peak on the fitting error. Figure 3 In (d), the fitting rate between the induced crack fractal dimension quantitative characterization model curve and the fractal dimension is 0.96, and the two have a high degree of fit.

[0054] In one embodiment, the method further comprises:

[0055] Step S15, determining the target fractal dimension d at the distance from the reference point according to the distribution characteristics of the induced crack fractal dimension f (x), and combined with the target fractal dimension d f (x), the transformation relationship between the number of induced cracks and the fractal dimension is constructed by the following formula:

[0056]

[0057] Where n(x) is the number of induced fractures, α is a preset constant related to fracture porosity, and D is a constant related to fracture complexity. In general, D = d fw , is the fractal dimension at the reference point.

[0058] Specifically, the transformation relationship between the number of induced cracks and the fractal dimension can be derived through the following steps:

[0059] (1) It is generally believed that there will be secondary fractures around the hydraulic fractures produced by horizontal well volume fracturing, such as Figure 4 According to fractal theory, the total number of induced cracks should be related to the fractal dimension, that is:

[0060]

[0061] Among them, L x,y is the crack area at a distance x from the reference point, n(Lx,y ) is the number of cracks in the area of L x,y

[0062] (2) Based on Equation (31), the relationship between the crack location distribution and the fractal dimension can be expressed as:

[0063]

[0064] where D is the Euclidean dimension and α is a constant directly related to the crack porosity.

[0065] Since, in the traditional concept of fractal crack description, when the morphology of a crack is strictly determined, its fractal dimension is a fixed value, and there are only three cases for the crack distribution. These include: (1) when d f < D, the number of induced cracks decreases with the increase of distance; (2) when d f = D, the number of induced cracks does not change with distance; (3) when d f > D, the number of induced cracks increases with the increase of distance. Therefore, affected by the reservoir conditions and construction parameters of actual cracks, the number of induced cracks should be in a trend of increasing first and then decreasing, rather than an unchanging rule.

[0066] Therefore, this method can explain the crack law to a certain extent, but it cannot accurately describe the complexity of cracks, resulting in a large calculation error. The reason is that only the fractal of the whole crack is considered, resulting in only the dominant law of cracks being described.

[0067] (3) Based on the above determination, in the current embodiment, considering the fractal of local cracks and the change of the fractal dimension with position, on the basis of Equation (32), the transformation relationship between the number of induced cracks and the fractal dimension is redefined as:

[0068]

[0069] where d f (x) is the fractal dimension of the crack at a distance x from the reference point.

[0070] Step S16, combining the transformation relationship between the number of induced cracks and the fractal dimension, and the distribution of the induced crack locations, calculate the induced crack spacing.

[0071] In one embodiment, in step S16, the induced crack spacing is calculated through the following steps:

[0072] Step S161, combining the number of induced cracks n(x) calculated according to Equation (3), the aperture f of the induced crack a and the spacing f of the induced crack s ​, according to the distribution of induced cracks and the principle of volume conservation:

[0073] f s (x)n(x)+f a (x)n(x)=L x，y ; (4)

[0074] Among them, L x,y is the length of the unit area, f s is the spacing of the induced cracks, in m; L x,y is the length of the unit area, in m; x is the distance from the fracture to the wellbore, in m.

[0075] Step S162, combining formula (3) and formula (4), and inducing the crack aperture f a , and the induced crack spacing f s When the ratio between them is ignored, the induced crack spacing f is calculated by the following formula (5): s (x) and the fracture fractal dimension d f (x) is expressed as follows:

[0076]

[0077] Specifically, by combining formula (3) and formula (4), the relationship between the induced fracture aperture, the induced fracture spacing and the fractal dimension can be obtained, namely:

[0078]

[0079] In one embodiment, in a reservoir that has been hydraulically fractured, the ratio between the induced fracture aperture and the induced fracture spacing can be ignored, so the expression for the induced fracture spacing can be:

[0080]

[0081] Step S163, the induced crack spacing value f at the known induced crack reference point sw In the case of , the following formula (6) is obtained:

[0082]

[0083] Specifically, assuming that the induced fracture spacing value at the induced fracture reference point is known, such as near the wellbore or at the fracture perforation point, the above formula (6) can be obtained.

[0084] Step S164, combining formula (5) and formula (6), and using the obtained formula (7) to calculate the induced crack spacing:

[0085]

[0086] in, f sw is the induced crack spacing at the reference point, in m, d fw is the fractal dimension of the induced crack at the reference point.

[0087] Specifically, by combining formula (52) and formula (6), the expression of the induced crack spacing based on the reference point can be obtained as follows:

[0088]

[0089] In one embodiment, since the parameter of the reference point in formula (71) is a constant, let:

[0090]

[0091] Therefore, after substituting formula (72) into formula (71), the final expression of the induced crack spacing can be obtained (for details, please refer to formula (7)).

[0092] In one embodiment, step S2 further includes:

[0093] Step S21, based on the distribution characteristics of the induced fracture fractal dimension, without considering that the induced fracture diameter changes with distance in the reservoir and the permeability of the fracture at the reference point in the reservoir is known, the permeability is derived by the following formula (8):

[0094]

[0095] Among them, k f (x) is the equivalent permeability coefficient of the reservoir, k fw is the permeability at the reference point, in mD; x is the distance from the fracture to the wellbore, in m; x w is the position of the reference point, in m; d fw is the fractal dimension at the reference point; D is a constant related to the complexity of the crack, generally D = d fw , is the fractal dimension at the reference point; d f (x) is the fractal dimension when the crack position is x, γ w is the equivalent permeability at the benchmark point;

[0096] Specifically, assuming that the fracturing reconstruction area is divided into n sufficiently small matrix and fracture grids in the direction perpendicular to the hydraulic fracture, the grid (i, j) in the fracturing reconstruction reservoir is selected as the representative reference volume unit. The length of the reference unit in the direction perpendicular to the hydraulic fracture (x-axis direction) is dx, as shown in Figure 5Considering that the reference unit body is evenly distributed in the direction parallel to the induced crack, it is a fixed value in the direction parallel to the induced crack, which may contain multiple induced cracks.

[0097] In one embodiment, the above formula (8) can be derived by the following steps:

[0098] (1) According to the law of conservation of fluid flow volume in hydraulic fractures and induced fractures, the fluid flow equation of the induced fracture along the x-axis direction can be expressed as:

[0099]

[0100] Where V(x) is the fluid flow rate of the induced crack in the reference unit, in cm 3 / s;k f is the permeability of hydraulic fracture, unit: 10 -12 m 2 ; μ is the viscosity of the fluid, unit: mpa.s; A r is the mesh flow area of ​​the hydraulic fracture, in cm 2 ; A p is the flow area of ​​the induced crack, unit: cm 2 ; dp is the pressure difference of the reference unit in the x-axis direction, the unit is atm; dx is the flow length of the hydraulic fracture system, the unit is cm; dL is the flow length of the induced fracture system, the unit is cm.

[0101] (2) Assuming that there is a plate with a gap in the induced fracture, the specific expressions of the permeability of the induced fracture that have been given include:

[0102]

[0103] Among them, k fi is the permeability of the induced fracture, unit: 10 -12 m 2 Therefore, based on the combination of formula (81) and formula (82), the equivalent permeability of the fracture in the reservoir can be preliminarily expressed as:

[0104]

[0105] (3) Since the model does not consider the change of induced fracture diameter with distance in the reservoir, that is, f a is always a constant value, and assuming that the permeability of the fracture at the reference point (i.e., the benchmark point) is known, then:

[0106]

[0107] Among them, k fwis the equivalent permeability of the fracture at the reference point, unit: 10 -12 m 2 .

[0108] (4) For any reference unit cell i, the size of the fracture model is the same. By combining formula (83) and formula (84), the relationship between the reservoir equivalent permeability and the position and the fractal coefficient can be obtained (for details, please refer to formula (8)).

[0109] Step S22, based on the distribution characteristics of the induced fracture fractal dimension, without considering the porosity of the matrix and when the porosity of the fracture at the reference point in the reservoir is known, the porosity is derived by the following formula (9):

[0110]

[0111] in, is the equivalent porosity coefficient of the reservoir, is the porosity at the reference point, is the equivalent porosity at the reference point.

[0112] Specifically, considering the influence of induced fractures in the reservoir during fracturing on porosity and compressibility, in the current embodiment, the equivalence principle is used to characterize the distribution of fracture porosity and permeability.

[0113] (1) Assume that the porosity of a single induced fracture in the reservoir is The single reservoir matrix porosity is Then, in the induced fracture system, the reservoir equivalent porosity can be preliminarily expressed as:

[0114]

[0115] in, is the equivalent porosity of the reservoir, n m is the number of grids in the reservoir matrix, n f is the number of fracture grids, and n is the total number of reservoir grids perpendicular to the direction of the induced fractures.

[0116] (2) For a specific reservoir, the fracturing transformation area has roughly the same vertical induced fracture direction, e.g. Figure 4 In the reservoir, since the porosity of the fracture is much greater than the porosity of the matrix, when solving the equivalent porosity of the reservoir, the above formula (91) can be simplified to:

[0117]

[0118] At the same time, considering that the fracture porosity at the reference point in the reservoir is known, formula (92) can be used to obtain:

[0119]

[0120] in, is the equivalent permeability of the reservoir at the benchmark point.

[0121] (3) By combining formula (92) and formula (93), the distribution of equivalent porosity in the reservoir can be obtained (for details, please refer to formula (9)).

[0122] Step S23, according to the distribution characteristics of the induced crack fractal dimension and the same execution principle as step S22, the compression coefficient is derived by the following formula:

[0123]

[0124] Among them, c f (x) is the equivalent compressibility coefficient of the reservoir, c fw is the compression coefficient at the reference point, c w is the equivalent compression coefficient at the reference point.

[0125] Specifically, the porosity / permeability / compression coefficient distribution of fractal induced fractures can be calculated according to formulas (8)-(10). It can be seen from the formula structure of the porosity / permeability / compression coefficient of fractal induced fractures that these three fracture parameters are directly related to the number of fractures. When the number of fractures is clear, the formula structure of porosity / permeability / compression coefficient has certain similarities, and their distribution laws are roughly the same. Therefore, in the current embodiment, only the permeability distribution of fractal induced fractures can be used as an example to verify the accuracy of the calculation results.

[0126] In one embodiment, the relationship between the number of fractal-induced cracks and the number of actual cracks is as follows: Figure 6 As shown in (a), the fractal-induced fracture permeability distribution and the actual equivalent permeability distribution calculated by formula (8) are shown in Figure 6 (b). Figure 6 The comparison results of fractal induced fractures and equivalent discrete fractures show that both the number of fractal induced fractures and the permeability distribution are highly consistent with the actual values, and the correlation between the two is greater than 0.91. Therefore, it can be shown that the calculation accuracy of the fractal induced fracture number and fractal induced fracture permeability model is good. Figure 6 (a) shows that the number of induced cracks increases first and then decreases, which is consistent with the actual observation results. Different from the conventional crack number characterization calculation model (reference formula (32)) which can only describe three special crack types, the crack number characterization calculation model proposed in this application can effectively characterize the differences in the internal structure of cracks by introducing the relationship between fractal dimension and position, and has stronger applicability.

[0127] Please refer to Figure 7 As shown, the embodiment of the present application further provides a fracture fractal characterization system 700, which includes a quantitative description module 701 and a parameter derivation module 702, wherein:

[0128] The quantitative description module 701 is used to quantitatively describe the distribution characteristics of the fractal dimension of the induced fractures according to the fractal characteristics of fractures in different regions, taking into account the fractal of local fractures and the fractal dimension changes with the fracture position.

[0129] The parameter derivation module 702 is used to derive complex fracture seepage parameters according to the distribution characteristics of the induced fracture fractal dimension, and perform fracture fractal characterization according to the derived complex fracture seepage parameters, wherein the complex fracture seepage parameters include at least one of porosity, permeability and compressibility.

[0130] In one of the embodiments, when each functional module in the system is executed, the method in any optional implementation of the above embodiment can be executed.

[0131] The above-mentioned fracture fractal characterization system can better describe the differences between fractures by quantitatively describing the distribution characteristics of the induced fracture fractal dimension compared with the conventional fractal dimension characterization method. Combining the distribution characteristics of the induced fracture fractal dimension, the derivation of complex fracture seepage parameters can derive a more general distribution of complex fracture seepage parameters, which can further improve the analysis accuracy while accurately describing the complexity of the fracture structure between different regions.

[0132] The embodiment of the present application provides a readable storage medium, and when the computer program is executed by a processor, the method in any optional implementation of the above embodiment is executed. Wherein, the readable storage medium can be implemented by any type of volatile or non-volatile storage device or a combination thereof, such as static random access memory (Static Random Access Memory, referred to as SRAM), electrically erasable programmable read-only memory (Electrically Erasable Programmable Read-Only Memory, referred to as EEPROM), erasable programmable read-only memory (Erasable Programmable Read Only Memory, referred to as EPROM), programmable read-only memory (Programmable Red-Only Memory, referred to as PROM), read-only memory (Read-Only Memory, referred to as ROM), magnetic memory, flash memory, disk or optical disk.

[0133] The above-mentioned readable storage medium can better describe the differences between the internal fractures by quantitatively describing the distribution characteristics of the induced fracture fractal dimension compared with the conventional fractal dimension characterization method. Combining the distribution characteristics of the induced fracture fractal dimension, the derivation of the complex fracture seepage parameters can be carried out to obtain a more general distribution of complex fracture seepage parameters, which can further improve the analysis accuracy while accurately describing the complexity of the fracture structure between different regions.

[0134] In the embodiments provided in the present application, it should be understood that the disclosed devices and methods can be implemented in other ways. The device embodiments described above are merely schematic. For example, the division of the units is only a logical function division. There may be other division methods in actual implementation. For example, multiple units or components can be combined or integrated into another system, or some features can be ignored or not executed. Another point is that the mutual coupling or direct coupling or communication connection shown or discussed can be through some communication interfaces, and the indirect coupling or communication connection of the devices or units can be electrical, mechanical or other forms.

[0135] In addition, the units described as separate components may or may not be physically separated, and the components shown as units may or may not be physical units, that is, they may be located in one place or distributed on multiple network units. Some or all of the units may be selected according to actual needs to achieve the purpose of the solution of this embodiment.

[0136] Furthermore, the functional modules in the various embodiments of the present application may be integrated together to form an independent part, or each module may exist separately, or two or more modules may be integrated to form an independent part.

[0137] In this document, relational terms such as first and second, etc. are used merely to distinguish one entity or operation from another entity or operation, but do not necessarily require or imply any such actual relationship or order between these entities or operations.

[0138] The above description is only an embodiment of the present application and is not intended to limit the protection scope of the present application. For those skilled in the art, the present application may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A method for characterizing crack fractals, characterized in that: The following steps are involved: S1. Based on the fractal characteristics of cracks in different regions, the distribution characteristics of the fractal dimension of induced cracks are quantitatively described, taking into account the fractal of local cracks and the change of fractal dimension with the crack position; S2. Derivation of complex fracture seepage parameters according to the distribution characteristics of the induced fracture fractal dimension, and fractal characterization of fractures according to the derived complex fracture seepage parameters, wherein the complex fracture seepage parameters include at least one of porosity, permeability and compressibility; In step S1, the distribution characteristics of the induced crack fractal dimension are quantitatively described by the following steps: S11. Considering the differences in the corresponding crack structures between different regions, the cracks are equally divided into multiple crack units; S12, using the fractal box dimension method to solve the fractal dimension of each crack unit; S13, establishing a distance transformation relationship between the fractal dimension of the induced cracks at different positions and the target distance according to the weighted summation result between the weight function of the fractal dimension and the corresponding fractal dimension, wherein the target distance includes the vertical distance from the reference point; S14, quantitatively describing the distribution characteristics of the induced crack fractal dimension according to the distance transformation relationship obtained in step S13; In step S14, the distribution characteristics of the induced crack fractal dimension are quantitatively described by the following formula: Among them, d fmax is the peak value of the fractal dimension; x is the distance between the fracture and the wellbore, in m; x m is the crack position corresponding to the peak value of the fractal dimension, in m; G is the decreasing rate of the fractal dimension, and the decreasing rate of the fractal dimension is proportional to the natural logarithm of the crack position, that is, G = α i ln(x),α i is the fractal dimension decreasing coefficient; Step S2 also includes: S21. Based on the distribution characteristics of the induced fracture fractal dimension, without considering the change of the induced fracture diameter with distance in the reservoir and the permeability of the fracture at the reference point in the reservoir is known, the permeability is derived by the following formula (8): Among them, k f (x) is the equivalent permeability coefficient of the reservoir, k fw is the permeability at the reference point, in mD; x is the distance from the fracture to the wellbore, in m; x w is the position of the reference point, in m; d fw is the fractal dimension at the reference point; D is a constant related to the complexity of the crack, generally D = d fw , is the fractal dimension at the reference point; d f (x) is the fractal dimension when the crack position is x, γ w is the equivalent permeability at the reference point; S22. Based on the distribution characteristics of the induced fracture fractal dimension, without considering the porosity of the matrix and when the porosity of the fracture at the reference point in the reservoir is known, the porosity is derived by the following formula (9): in, is the equivalent porosity coefficient of the reservoir, is the porosity at the reference point, is the equivalent porosity at the reference point; S23, according to the distribution characteristics of the induced crack fractal dimension, based on the same execution principle as step S22, the compression coefficient is derived by the following formula: Among them, c f (x) is the equivalent compressibility coefficient of the reservoir, c fw is the compression coefficient at the reference point, c w is the equivalent compression coefficient at the reference point.

2. The method according to claim 1, characterized in that In step S12, for each crack unit, the fractal box dimension method is used to solve the crack fractal dimension of the crack unit, including: S121, using a square grid with a side length of r to divide the crack unit into equal intervals, and counting the total number of grids N(r) corresponding to the cracks in the divided grids; S122, reducing the side length of the square grid in sequence according to a preset step length, and recording the total number of grids obtained in each iteration process while repeating the execution process of step S121; S123, according to the change relationship between the changed side length of the grid and the corresponding total number of grids in each iteration process, the fracture fractal dimension d of the fracture unit is obtained by solving the following formula: f : Among them, d f is the fractal dimension of the crack; N(r) is the total number of grids with cracks when the edge length is r; r is the edge length of the covered grid.

3. The method according to claim 1 or 2, characterized in that: The method further comprises: S15, determining the target fractal dimension d at the distance from the reference point according to the distribution characteristics of the induced crack fractal dimension f (x), and combined with the target fractal dimension d f (x), the transformation relationship between the number of induced cracks and the fractal dimension is constructed by the following formula: Where n(x) is the number of induced fractures, α is a preset constant related to fracture porosity, and D is a constant related to fracture complexity. In general, D = d fw , is the fractal dimension at the reference point; S16. Calculate the spacing between induced cracks based on the transformation relationship between the number of induced cracks and the fractal dimension, as well as the distribution of induced crack locations.

4. The method according to claim 3, characterized in that In step S16, the induced crack spacing is calculated by the following steps: S161, combined with the number of induced cracks n(x) and the aperture f of the induced cracks calculated according to formula (3) a and the spacing f of the induced cracks s , according to the distribution of induced cracks and the principle of volume conservation: f s (x)n(x)+f a (x)n(x)=L x，y ; (4) Among them, f a is the aperture of the induced crack, in m; f s is the spacing of the induced cracks, in m; L x,y is the length of the unit area, in m; x is the distance of the fracture from the wellbore, in m; S162, combining formula (3) and formula (4), and inducing the crack aperture f a , and the induced crack spacing f s When the ratio between them is ignored, the induced crack spacing f is calculated by the following formula (5): s (x) and the fracture fractal dimension d f (x) is expressed as follows: S163, the induced crack spacing value f at the known induced crack reference point sw In the case of , the following formula (6) is obtained: S164. Combine formula (5) and formula (6) to calculate the induced crack spacing using the resulting formula (7): in, f sw is the induced crack spacing at the reference point, in m; d fw is the fractal dimension of the induced crack at the reference point.

5. A crack fractal characterization system, characterized in that: The system includes a quantitative description module and a parameter derivation module, wherein: The quantitative description module is used to quantitatively describe the distribution characteristics of the fractal dimension of the induced cracks according to the fractal characteristics of the cracks in different regions, taking into account the fractal of the local cracks and the fractal dimension changes with the crack position; The parameter derivation module is used to derive complex fracture seepage parameters according to the distribution characteristics of the induced fracture fractal dimension, and to perform fracture fractal characterization according to the derived complex fracture seepage parameters, wherein the complex fracture seepage parameters include at least one of porosity, permeability and compressibility; The specific implementation of the quantitative description of the distribution characteristics of the induced crack fractal dimension is: Considering the differences in the corresponding fracture structures between different regions, the fractures are equally divided into multiple fracture units; The fractal box dimension method is used to solve the fractal dimension of each crack unit; According to the weighted summation result between the weight function of the fractal dimension and the corresponding fractal dimension, a distance transformation relationship between the fractal dimension of the induced cracks at different positions and the target distance is established, wherein the target distance includes the vertical distance from the reference point; According to the distance transformation relationship obtained, the distribution characteristics of the fractal dimension of the induced cracks are quantitatively described; The distribution characteristics of the induced crack fractal dimension are quantitatively described by the following formula: Among them, d fmax is the peak value of the fractal dimension; x is the distance between the fracture and the wellbore, in m; x m is the crack position corresponding to the peak value of the fractal dimension, in m; G is the decreasing rate of the fractal dimension, and the decreasing rate of the fractal dimension is proportional to the natural logarithm of the crack position, that is, G = α i ln(x),α i is the fractal dimension decreasing coefficient; The parameter derivation module is specifically used for: According to the distribution characteristics of the induced fracture fractal dimension, without considering the change of the induced fracture diameter with distance in the reservoir and the permeability of the fracture at the reference point in the reservoir is known, the permeability is derived by the following formula (8): Among them, k f (x) is the equivalent permeability coefficient of the reservoir, k fw is the permeability at the reference point, in mD; x is the distance from the fracture to the wellbore, in m; x w is the position of the reference point, in m; d fw is the fractal dimension at the reference point; D is a constant related to the complexity of the crack, generally D = d fw , is the fractal dimension at the reference point; d f (x) is the fractal dimension when the crack position is x, γ w is the equivalent permeability at the reference point; According to the distribution characteristics of the induced fracture fractal dimension, without considering the porosity of the matrix and when the porosity of the fracture at the reference point in the reservoir is known, the porosity is derived by the following formula (9): in, is the equivalent porosity coefficient of the reservoir, is the porosity at the reference point, is the equivalent porosity at the reference point; According to the distribution characteristics of the induced crack fractal dimension, the compression coefficient is derived by the following formula according to the same execution principle as step S22: Among them, c f (x) is the equivalent compressibility coefficient of the reservoir, c fw is the compression coefficient at the reference point, c w is the equivalent compression coefficient at the reference point.

6. A readable storage medium, characterized in that: The readable storage medium includes a crack fractal characterization method program, and when the crack fractal characterization method program is executed by a processor, the steps of the method according to any one of claims 1 to 4 are implemented.

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