A method for evaluating the effect of secondary fracturing steering based on multi-dimensional fracture characteristics

Abstract

The application discloses a secondary fracturing steering effect evaluation method based on multi-dimensional fracture characteristics, and belongs to the technical field of oil and gas field development and reservoir reconstruction. The method comprises the following steps: obtaining target reservoir geomechanics parameters, reservoir physical property parameters, initial fracture characteristic parameters and fracturing construction parameters, and establishing a secondary fracturing fracture propagation simulation model; based on the model, fracture propagation simulation under different working conditions is carried out, and the spatial coordinates of the fracture unit, the fracture length, the fracture azimuth angle and the effective reconstruction area distribution information are extracted; based on the fracture propagation simulation result, the fracture azimuth circle variance, the three-dimensional fractal dimension and the newly added effective reconstruction volume rate are respectively calculated; the above indexes are dimensionless processed and the index weight is determined, the comprehensive fracture steering evaluation index is calculated, and the secondary fracturing fracture steering effect under different working conditions is determined according to the index. The application realizes quantitative evaluation of the secondary fracturing fracture steering degree under the condition of complex irregular fracture network.

Description

Technical Field

[0001] This application relates to the field of oil and gas field development and reservoir stimulation technology, and in particular to a method for evaluating the effect of secondary fracturing diversion based on multidimensional fracture characteristics. Background Technology

[0002] 1. Necessity of secondary fracturing.

[0003] Hydraulic fracturing is a core technique for the efficient development of tight oil reservoirs. It improves reservoir conductivity by artificially creating fractures, thereby achieving efficient oil and gas production. However, during long-term well production, factors such as reservoir rock compaction, proppant embedding, and formation fluid scouring can cause initial fracturing fractures to close or fail, leading to a decline in reservoir conductivity and a continuous decrease in single-well production. Simultaneously, fracturing fractures exhibit significant geometric anisotropy; pressure diffusion and seepage capacity along the fracture direction are significantly higher than in the vertical direction. Therefore, the pressure propagation and seepage control range around the fracture is typically elliptical. This elliptical seepage zone implies the existence of untreated areas within the fractures, easily leading to residual oil within the fractures. To restore and enhance production capacity, repeated fracturing technology has emerged. By reopening failed fractures or creating new ones, it has become a crucial technical guarantee for stable and increased production in shale oil reservoirs.

[0004] 2. Mechanism of secondary hydraulic fracturing fracture reversal.

[0005] In-situ stress variation is one of the main factors influencing fracture morphology changes after repeated fracturing. Due to the influence of various geological structures, the distribution of in-situ stress in the formation is highly complex. During repeated fracturing, the fracture propagation direction may change, even generating more complex fracture networks. Therefore, fracture redirection is an important consideration during repeated fracturing.

[0006] Before repeated fracturing fractures open, multiple induced stress fields exist. These induced stress fields superimpose in their original directions, altering the directions of the maximum and minimum horizontal principal stresses. Therefore, a stress redirection zone exists near the artificial fracture, potentially changing the propagation direction of the repeated fracturing fracture.

[0007] Studies have shown that when artificial fractures exist in oil and gas reservoirs, the stress field around the fractures can be mainly divided into the stress field caused by far-field stress in the production well and near the fracture, and the stress field caused by changes in pore pressure in the reservoir due to long-term production activities of the oil and gas wells. Assuming the rock is in an elastic state, according to the theory of elasticity and rock fracture criteria, fracture initiation always occurs along the direction perpendicular to the minimum horizontal principal stress. Therefore, the stress field distribution plays a decisive role in the timing of new fracture formation and the direction of new fracture extension in repeatedly fractured wells.

[0008] The stress field induced by the two-dimensional vertical crack is:

[0009] ;

[0010] In the formula: Let be the induced stress in the x-direction, in MPa; y is the induced stress in the y direction, MPa; p is the force exerted on the fracture surface during repeated fracturing, MPa; r, , Let be the distances from any point in space to the center, lower end, and upper end of the crack, respectively, in meters; , , Let be the angles (°) between the lines connecting any point to the center, lower end, and upper end of the crack and the z-axis; C is half the crack height, in meters.

[0011] After the initial artificial fracture forms, an elliptical region develops around the fracture as oil and gas well production progresses. Within this region, pore pressure redistributes, influencing the fracture's orientation. Studies show that during oil and gas production, the extension direction of the initial artificial fracture (direction of maximum horizontal principal stress) changes rapidly, while the horizontal stress component perpendicular to this direction (minimum horizontal stress) changes more slowly, both exhibiting a decreasing trend and related to both time and space variables. Therefore, if the induced stress difference generated during repeated fracturing can be comparable to the initial stress difference in the formation, stress redirection will occur within the elliptical region—that is, the directions of minimum and maximum horizontal stress will interchange. During this process, according to elasticity theory, repeated fracturing can generate new fractures perpendicular to the initial artificial fracture direction, connecting more areas in the formation and increasing the production of older oil and gas wells.

[0012] 3. The necessity of evaluating the deflection effect of secondary fracturing.

[0013] In China, repeated fracturing technology is mainly applied to ultra-low permeability and extra-low permeability reservoirs. Due to the poor physical properties and low permeability of these reservoirs, it is difficult to form effective displacement channels during long-term development. As production time increases, the area near the artificial fractures formed during the initial fracturing is preferentially utilized, with crude oil continuously extracted from the fracture walls. However, due to limited pressure propagation and seepage control, residual oil gradually accumulates in the lateral areas of the fractures. Therefore, repeatedly fracturing to create new fracture propagation paths, expand the original fracture control range, and further utilize the residual oil lateral to the fractures is an important way to improve the development efficiency of older wells.

[0014] However, the effectiveness of secondary fracturing depends not only on the scale of fracturing and the volume of newly created fractures, but more importantly, on whether the newly formed fractures can effectively deflect away from the original fracture-controlled area. If the secondary fracturing fractures continue to extend along the original fracture direction, although they can increase the stimulated volume to some extent, their ability to mobilize lateral remaining oil areas is limited. Only when the fractures effectively deflect and form a new network of branching fractures can the reservoir sweep range be expanded and the degree of mobilization of remaining oil be improved. Therefore, evaluating the deflection effect of secondary fracturing fractures is of great significance for determining the effectiveness of repeated fracturing stimulation.

[0015] 4. Existing technologies and their shortcomings.

[0016] Existing technologies have conducted extensive research on fracture redirection and effectiveness evaluation in secondary fracturing. For example, patent CN114592849A, "Experimental Method for Temporary Plugging and Redirecting Fracturing in Unconventional Reservoirs," discloses a method for simulating and reproducing the temporary plugging and redirecting fracturing scenario. This method obtains characteristic information of the initial fracture, primary fracturing fracture, and secondary fracturing fracture, determines the fracture redirection characteristics, and then optimizes the temporary plugging and redirecting fracturing technology. This method can analyze the fracture morphology changes during the temporary plugging and redirection process through experimental means, but it mainly relies on physical simulation experiments, and its ability to continuously quantify the degree of redirection of complex fracture networks under actual reservoir conditions is limited.

[0017] Patent CN120688268A, entitled "A Simulation and Optimization Method and System for the Initiation and Propagation of Hydraulic Fractures in Tight Conglomerate," discloses a method for establishing a numerical model based on a physical model of tight conglomerate and the fluid-structure interaction control equations for hydraulic fracture propagation. This method analyzes the initiation and propagation characteristics of hydraulic fractures in tight conglomerate considering interface features, providing a theoretical basis for optimizing fracturing schemes. While this method can predict the initiation and propagation process of hydraulic fractures, it primarily focuses on fracture propagation paths and changes in geometric parameters, and does not establish a method for evaluating the degree of turning of the complex fracture network formed under the competitive action of multiple fractures during secondary fracturing.

[0018] Patent CN120847850A, entitled "An Evaluation Method for Fracturing Controlled Stimulation Volume," discloses a method that simulates basic fracture parameters, determines fracturing fluid efficiency, and quantitatively evaluates half-fracture length and well control radius. This method establishes the distribution pattern of remaining reserves through a comprehensive geological and engineering evaluation. However, while this method can evaluate the fracturing effect through parameters such as the stimulation volume, it primarily reflects the scale of reservoir stimulation and cannot further identify the formation mechanism of newly stimulated areas. It also struggles to distinguish between fractures extending along their original propagation direction and those that deviate to form complex fracture networks.

[0019] Patent CN106869892B, "A Method for Judging the Initiation of Temporary Plugging in Repeatedly Fractured Wells," discloses a method that analyzes the induced stress field generated by the extension of hydraulic fractures, the induced stress field generated by changes in formation pore fluid pressure, and the performance parameters of the temporary plugging agent. This method establishes a criterion for judging the initiation and direction of temporary plugging fractures on the fracture wall, effectively predicting the initiation location and orientation of temporarily plugged fractures within repeatedly fractured wells. While this method can determine the location and direction of fracture re-initiation under temporary plugging conditions, it primarily focuses on establishing the fracture initiation criterion and does not further consider the overall reversal effect after the formation of a multi-branched, multi-directional fracture network during fracture propagation.

[0020] In summary, existing technologies mainly focus on the analysis of turning mechanisms, simulation of crack propagation, and evaluation of fracturing effects, but still have the following shortcomings: (1) Existing evaluation methods mostly use single parameters such as crack length, modified volume, or turning angle, which are difficult to fully characterize the turning characteristics of complex crack networks; (2) Traditional turning angle evaluation methods are mainly applicable to regular crack models, and are not suitable for irregular, multi-branched crack networks formed by actual fracturing; (3) Existing methods lack a comprehensive evaluation system that simultaneously considers crack direction changes, network complexity, and the newly added effective modification range; (4) Some evaluation methods lack integration with production response, and cannot effectively determine whether the newly modified area formed by crack turning can bring about an actual increase in production capacity.

[0021] Therefore, there is an urgent need to propose a method that can quantitatively evaluate the crack redirection effect of secondary fracturing from multiple dimensions, taking into account the characteristics of complex crack networks. Summary of the Invention

[0022] This application provides a method for evaluating the effect of secondary fracturing reversal based on multidimensional fracture characteristics. It aims to solve the problems in the prior art, such as the single evaluation index, the difficulty in applying it to irregular and complex fracture networks, the lack of a comprehensive evaluation system that simultaneously considers fracture direction changes, network complexity and the newly added effective modification range, and the fact that some methods are not combined with production response.

[0023] This application provides a method for evaluating the secondary fracturing redirection effect based on multidimensional fracture characteristics, the method comprising:

[0024] Obtain the target reservoir's geomechanical parameters, reservoir physical property parameters, initial fracture characteristic parameters, and fracturing operation parameters, and establish a simulation model for secondary fracturing fracture propagation.

[0025] Based on the aforementioned secondary fracturing fracture propagation simulation model, fracture propagation simulations were conducted under different construction methods, construction parameters, and geological parameters. The spatial distribution characteristics of the fracture network formed by secondary fracturing were obtained, and the spatial coordinates, fracture length, fracture azimuth angle, and distribution information of the effective modification area of ​​the fracture unit were extracted.

[0026] Based on the simulation results of fracture propagation, fracture turning evaluation indexes are established from three dimensions: fracture direction characteristics, fracture network structure characteristics, and reservoir stimulation range. The fracture azimuth circle variance, three-dimensional fractal dimension, and newly added effective stimulation volume ratio are calculated respectively.

[0027] The crack direction index, crack complexity index, and modification range index are dimensionless. The index weights are determined according to the contribution of each index to the crack reversal effect. A comprehensive crack reversal evaluation index is calculated, and the secondary fracturing crack reversal effect under different working conditions is determined based on the comprehensive evaluation index.

[0028] Optionally, in the above scheme, calculating the circular variance of the crack orientation specifically includes:

[0029] A circular statistical method is used to treat each crack element as a direction vector. At the same time, a length-weighted method is used to calculate the length-weighted circular variance based on the crack element orientation azimuth and crack length data, which is used to describe the dispersion of crack direction distribution.

[0030] The larger the circular square difference value, the more dispersed the crack orientation distribution and the more obvious the crack turning and multi-directional propagation characteristics.

[0031] Optionally, in the above scheme, calculating the three-dimensional fractal dimension specifically includes:

[0032] Extract the spatial coordinate information of the crack elements to form a three-dimensional discrete point set, and normalize the coordinates in the three directions to eliminate the differences in the geometric scale of the cracks.

[0033] The box-dimensional mesh covering method is adopted to divide the normalized crack space into cubic boxes with different side lengths, count the number of non-empty boxes, fit a straight line in double logarithmic coordinates, and the absolute value of the slope of the obtained straight line is the three-dimensional fractal dimension of the crack, which is used to describe the spatial complexity of the crack network.

[0034] The larger the fractal dimension, the more complex the crack network structure, and the more obvious the crack branching and non-uniform propagation characteristics.

[0035] Optionally, in the above scheme, the newly added effective modified volume ratio is the ratio between the newly added effective modified volume of secondary fracturing and the original effective modified volume, which is used to characterize the range of the newly modified area formed by the deflection of secondary fracturing fractures.

[0036] Optionally, in the above scheme, the method further includes:

[0037] By combining the circular variance with the fracture direction rose diagram, the qualitative and quantitative evaluation of the fracture turning characteristics of secondary hydraulic fracturing is carried out.

[0038] The method for drawing the crack direction rose diagram is as follows: the crack direction angle range of 0°~360° is divided into several statistical intervals at 10° intervals, the cumulative length of crack units in each angle interval is counted, the crack azimuth angle is used as the polar angle, and the cumulative crack length in the corresponding azimuth interval is used as the radial distance to draw it in the polar coordinate system, and the crack direction in the range of 180°~360° is mapped to 0°~180°.

[0039] Optionally, in the above scheme, the index weights are obtained by using the analytic hierarchy process (AHP) to construct a judgment matrix based on the contribution of each index to the crack turning characteristics, and by calculating the weights using the geometric mean method.

[0040] Optionally, the method further includes a verification step in the above scheme:

[0041] A secondary fracturing production simulation model was established, and the production capacity response of different fracture diversion schemes was analyzed. By comparing the daily oil production, cumulative oil production, and production dynamics, the consistency between the comprehensive evaluation index and the actual production increase effect was verified.

[0042] Optionally, in the above scheme, the secondary fracturing fracture propagation simulation model is established based on the UFM model, using the artificial and natural fracture network formed by the primary fracturing, as well as the reservoir stress field, pressure field, and oil saturation field state parameters after the primary fracturing production, as the initial conditions for the secondary fracturing simulation.

[0043] In the above scheme, the normalization of the three directional coordinates can be optionally performed by normalizing the X-coordinate, Y-coordinate and depth coordinate of the crack element center respectively. The normalized crack spatial coordinates are used to eliminate the influence of the scale difference of the crack in the length, height and width directions on the box dimension calculation results.

[0044] In the above scheme, optionally, the geomechanical parameters include the maximum horizontal principal stress, the minimum horizontal principal stress, the stress difference, and the reservoir pressure; the fracturing operation parameters include the fracturing flow rate and the perforation parameters.

[0045] The different construction methods include non-temporary plugging, uniform temporary plugging, and dominant crack temporary plugging. The effect of temporary plugging is characterized by adjusting the Discharge Coefficient parameter using the equivalent flow resistance method.

[0046] Compared with the prior art, this application has at least the following beneficial effects:

[0047] This application, based on further analysis and research of existing technical problems, recognizes that existing technologies suffer from limitations such as single evaluation indicators, difficulty in applying them to irregular and complex fracture networks, lack of a comprehensive evaluation system that simultaneously considers fracture direction changes, network complexity, and the newly added effective stimulation range, and the failure of some methods to integrate with production response. By acquiring the target reservoir's geomechanical parameters, reservoir physical property parameters, initial fracture characteristic parameters, and fracturing construction parameters, a secondary fracturing fracture propagation simulation model is established. Simulations of fracture propagation under different construction methods, construction parameters, and geological parameter conditions are conducted to obtain the spatial distribution characteristics of the fracture network and the spatial coordinates, fracture length, fracture azimuth, and effective stimulation area distribution information of fracture units. This provides a data foundation for subsequent multidimensional evaluation, overcoming the shortcomings of physical simulation experiments in continuously quantifying evaluation. Furthermore, by analyzing fracture direction characteristics, fracture network structural characteristics, and reservoir stimulation range... The method calculates the circular variance of fracture orientation, the three-dimensional fractal dimension, and the newly added effective modification volume ratio from three dimensions: the degree of dispersion of fracture direction, the spatial complexity of fracture network, and the newly added modification range. This quantitatively characterizes fracture turning features from three perspectives: the degree of dispersion of fracture direction, the spatial complexity of fracture network, and the newly added modification range. This overcomes the shortcomings of traditional single-parameter methods that cannot comprehensively characterize the turning features of complex fracture networks. At the same time, the introduction of circular variance and fractal dimension makes this method applicable to the evaluation of irregular, multi-branch fracture networks, avoiding the problem of insufficient applicability of traditional turning angle evaluation methods. By dimensionlessly processing the fracture direction index, fracture complexity index, and modification range index, and determining the index weight according to the contribution of each index to the fracture turning effect, a comprehensive fracture turning evaluation index is calculated. This achieves the organic integration and comprehensive quantitative evaluation of multi-dimensional information, providing a unified quantitative basis for the comparative evaluation of secondary fracturing schemes under different construction methods, construction parameters, and geological conditions.

[0048] This application also has at least the following beneficial effects:

[0049] (1) This application breaks through the traditional method of evaluating the crack turning effect based on a single crack angle, crack length or stimulation volume, and establishes a multi-dimensional evaluation system that integrates crack direction characteristics, crack network structure characteristics and reservoir stimulation range characteristics, which can more comprehensively characterize the degree of secondary fracturing crack turning.

[0050] (2) This application is applicable to multi-branch, multi-directional and irregular fracture networks formed in actual fracturing process, avoiding the evaluation deviation caused by the traditional turning angle evaluation method relying on a single fracture direction, and improving the applicability of fracture turning evaluation under complex fracture network conditions.

[0051] (3) This application combines the evaluation of crack morphology characteristics with production response analysis, which can determine whether the new effective transformation area formed by crack turning can achieve production capacity improvement, thus improving the engineering guidance significance of the evaluation results.

[0052] (4) This application can be used to compare and evaluate secondary fracturing schemes under different construction methods, construction parameters and geological conditions, and provide a basis for optimizing repeated fracturing construction. Attached Figure Description

[0053] Figure 1 A flowchart illustrating a method for evaluating the diversion effect of secondary fracturing based on multidimensional fracture characteristics, provided in one embodiment of this application;

[0054] Figure 2 A simulation result diagram of the artificial and natural fracture network formed by a single hydraulic fracturing operation is provided for one embodiment of this application.

[0055] Figure 3 A reservoir stress field distribution diagram after a single fracturing operation is provided for one embodiment of this application;

[0056] Figure 4 A diagram of a secondary fracturing fracture propagation model based on primary fracturing and production conditions, provided as an embodiment of this application;

[0057] Figure 5 An example diagram of a rose diagram provided in one embodiment of this application;

[0058] Figure 6 Scheme 1, a ranking chart of evaluation indices, is provided as an embodiment of this application.

[0059] Figure 7 Comparison of crack morphology in Scheme 1 provided in an embodiment of this application (from left to right: working conditions 1-1 to 1-3);

[0060] Figure 8 A cumulative oil production ranking chart of Scheme 1 provided in an embodiment of this application (from top to bottom, operating conditions 1-2, 1-3, and 1-1 are respectively).

[0061] Figure 9 A crack path rose diagram provided for a stress difference of 12 MPa in one embodiment of this application;

[0062] Figure 10 A crack path rose diagram provided for a stress difference of 9 MPa in one embodiment of this application;

[0063] Figure 11 A crack path rose diagram provided for a stress difference of 6 MPa in one embodiment of this application;

[0064] Figure 12 Scheme 2, a ranking chart of evaluation indexes, is provided as an embodiment of this application.

[0065] Figure 13Comparison of crack morphology in Scheme 2 provided in one embodiment of this application (from left to right: working conditions 2-1 to 2-3);

[0066] Figure 14 A cumulative oil production ranking chart of Scheme 2 provided in an embodiment of this application (from top to bottom, operating conditions 2-2, 2-3, and 2-1 are respectively).

[0067] Figure 15 Scheme 3, a ranking chart of steering evaluation indexes, is provided as an embodiment of this application.

[0068] Figure 16 Comparison of crack morphology in Scheme 3 provided in one embodiment of this application (from left to right: working conditions 3-1 to 3-3);

[0069] Figure 17 The cumulative oil production ranking chart of Scheme 3 provided in one embodiment of this application (from top to bottom, the charts are operating conditions 3-1, 3-3, and 3-2). Detailed Implementation

[0070] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0071] In the description of this application: unless otherwise stated, "a plurality of" means two or more. The terms "first," "second," "third," etc., in this application are intended to distinguish the objects referred to and do not have any special meaning in terms of technical connotation (e.g., they should not be construed as an emphasis on importance or order). Expressions such as "including," "comprising," and "having" also mean "not limited to" (certain units, components, materials, steps, etc.).

[0072] In one embodiment, reference Figure 1 This paper provides a method for evaluating the secondary fracturing redirection effect based on multidimensional fracture characteristics. The method includes:

[0073] Obtain the target reservoir's geomechanical parameters, reservoir physical property parameters, initial fracture characteristic parameters, and fracturing operation parameters, and establish a simulation model for secondary fracturing fracture propagation.

[0074] Based on the aforementioned secondary fracturing fracture propagation simulation model, fracture propagation simulations were conducted under different construction methods, construction parameters, and geological parameters. The spatial distribution characteristics of the fracture network formed by secondary fracturing were obtained, and the spatial coordinates, fracture length, fracture azimuth angle, and distribution information of the effective modification area of ​​the fracture unit were extracted.

[0075] Based on the simulation results of fracture propagation, fracture turning evaluation indexes are established from three dimensions: fracture direction characteristics, fracture network structure characteristics, and reservoir stimulation range. The fracture azimuth circle variance, three-dimensional fractal dimension, and newly added effective stimulation volume ratio are calculated respectively.

[0076] The crack direction index, crack complexity index, and modification range index are dimensionless. The index weights are determined according to the contribution of each index to the crack reversal effect. A comprehensive crack reversal evaluation index is calculated, and the secondary fracturing crack reversal effect under different working conditions is determined based on the comprehensive evaluation index.

[0077] In one embodiment, the secondary fracturing fracture propagation simulation model is established based on the UFM model, using the artificial and natural fracture network formed by primary fracturing, as well as the reservoir stress field, pressure field, and oil saturation field state parameters after primary fracturing as the initial conditions for secondary fracturing simulation.

[0078] 1. Input parameters and calculation process of each indicator.

[0079] In one embodiment, a numerical simulation model of fracture propagation after secondary fracturing is established using a horizontal well in a shale oil reservoir that has already undergone primary fracturing and production as the research object. The basic geological parameters of the reservoir include: minimum horizontal principal stress of 57–69 MPa, maximum horizontal principal stress of 62–74 MPa, reservoir pressure of 50–57 MPa, Poisson's ratio of 0.28, reservoir permeability of 0.22 mD, and porosity of 0.05.

[0080] Based on the above parameters, a horizontal well primary fracturing and production model is established, such as... Figure 2 and Figure 3 As shown. Figure 2 The model simulates the hydraulic fracture network formed after a single fracturing process using the UFM model based on the Petrel Kinetix module. It can be seen that the artificial fractures formed during the fracturing process and the original natural fractures in the reservoir together constitute a complex fracture network. Figure 2 These data reflect both artificially created fractures and natural fractures opened or expanded by fracturing disturbances, characterizing the spatial distribution of fractures formed during the initial fracturing process and providing initial fracture conditions for subsequent secondary fracturing fracture propagation simulations. Because the fracture morphology in different fracturing sections is influenced by factors such as the development level of local natural fractures and reservoir properties, the fracture morphology varies at different locations along the wellbore. Furthermore, due to this… Figure 2 As a two-dimensional projection of a three-dimensional model, the visual height and density of fractures at different locations are also affected by Petrel's observation perspective. Therefore, this figure is mainly used to visually show the spatial distribution of artificial and natural fractures in a horizontal well, and is not suitable for comparing the fracture morphology of different fracturing sections based on visual morphology.

[0081] Figure 3 The reservoir stress field distribution after primary fracturing is used to characterize the state of reservoir stress redistribution during the production process. In addition to the stress field, state parameters such as the reservoir pressure field and oil saturation field after production are also updated simultaneously and used as initial conditions for secondary fracturing simulation. This paper only presents representative stress field distribution results. Based on this, the primary fracturing fractures are added to the initial natural fractures, and the reservoir pressure, stress, and oil saturation state after production are used as initial conditions for secondary fracturing simulation to establish a numerical simulation model for fracture propagation in secondary fracturing.

[0082] To reduce interference between different fracture clusters during multi-stage fracturing, only one fracturing stage is selected as the research object. Based on the primary fracturing fracture network and the post-production reservoir state of this stage, a secondary fracturing fracture propagation simulation model is established, such as... Figure 4 As shown. Figure 4 The secondary fracturing fracture propagation simulation model, established based on the aforementioned fracture network and reservoir condition, is used to simulate the propagation and reversal behavior of new fractures during secondary fracturing. After secondary fracturing, the SRV region is displayed in a hierarchical manner, with color codes from top to bottom representing the fracture itself (region 0) and regions from near to far from the fracture (region 10 represents the region furthest from the fracture, i.e., the unmodified region). The spatial distribution of different color regions can intuitively reflect the propagation direction, range, and overall morphology of the secondary fracturing fractures.

[0083] Based on the results of secondary hydraulic fracturing simulation, information for each fracture element can be extracted, including three-dimensional coordinates (x, y, depth), fracture length, and fracture azimuth. This data serves as the basis for subsequent calculation of multidimensional evaluation indicators.

[0084] In one embodiment, calculating the circular variance of the crack orientation specifically includes: using a circular statistical method, treating each crack element as a direction vector, and using a length-weighted method, calculating the length-weighted circular variance based on the crack element orientation azimuth angle and crack length data, to describe the dispersion of the crack orientation distribution; the larger the circular variance value, the more dispersed the crack orientation distribution, and the more obvious the crack turning and multi-directional propagation characteristics.

[0085] The first manifestation of fracture orientation in secondary fracturing is a change in fracture propagation direction. Since fracture azimuth is circular data, circular statistical methods are used for analysis. In circular statistics, a circular statistic, the circular variance V, can also be estimated. Each fracture unit is considered as a direction vector, and considering the different contributions of different fracture lengths to the overall direction, a length-weighted approach can be used to calculate the circular variance. The circular variance index is a measure of the dispersion of a set of directional data; it is similar to variance in linear statistics but is applicable to angular data. Fisher (1995) defined a direction-related circular variance in cyclic statistics. :

[0086] ;

[0087] In the formula, N is the total number of angles used for analysis, R is the magnitude of the vector sum of all unit vectors associated with each direction, and the variance of the cyclic statistics varies between 0 and 1.

[0088] When the variance When the variance is 0, it indicates that all unit vectors point in the same direction, meaning there is no dispersion in the dataset. This implies that all cracks or crack groups tend to share a common orientation; when the variance is 0... A value close to 1 indicates that the unit vector is uniformly distributed in all possible directions, meaning the dispersion in the dataset is at its maximum. This implies that the distribution of cracks is completely random, with no dominant orientation. Circular variance provides a tool for quantifying dispersion when analyzing crack orientation or other angular data.

[0089] This embodiment uses the fracture element orientation azimuth and fracture length data obtained from UFM simulation to calculate the length of the composite vector of fracture direction using the length-weighted axial circular statistical method, and further defines the length-weighted circular variance (as shown in the following formula) to quantitatively characterize the dispersion of the secondary fracturing fracture network orientation and its multi-directional propagation characteristics.

[0090] ;

[0091] In one embodiment, to more intuitively reflect the dominant direction of fracture propagation and specific turning direction, the circular variance is combined with the fracture direction rose diagram to qualitatively and quantitatively evaluate the turning characteristics of secondary fracturing fractures.

[0092] Circular variance can quantitatively characterize the dispersion of fracture orientation by calculating the dispersion of fracture azimuth angles. However, it can only reflect the overall concentration or dispersion of fracture orientation, and cannot intuitively show the dominant propagation direction and specific turning direction of the fracture. Therefore, combining circular variance with fracture strike rose diagrams can describe fracture turning from both quantitative and qualitative perspectives: the rose diagram is used to analyze the changes in dominant orientation and multi-directional propagation characteristics, while circular variance is used to quantify the degree of dispersion of fracture orientation, thus providing a more comprehensive evaluation of fracture turning behavior during secondary fracturing.

[0093] The method for drawing the crack direction rose diagram is as follows: the crack direction angle range of 0°~360° is divided into several statistical intervals at 10° intervals, the cumulative length of crack units in each angle interval is counted, the crack azimuth angle is used as the polar angle, and the cumulative crack length in the corresponding azimuth interval is used as the radial distance to draw it in the polar coordinate system, and the crack direction in the range of 180°~360° is mapped to 0°~180°.

[0094] Based on fracture network data obtained from UFM simulations, the azimuth and corresponding fracture length of each fracture element were extracted, and a fracture strike rose diagram was constructed using a discrete statistical method. First, the fracture strike angle range of 0° to 360° was divided into several statistical intervals at 10° intervals, and the cumulative length of fracture elements within each interval was calculated. Then, using the fracture azimuth as the polar angle and the cumulative fracture length within the corresponding azimuth interval as the radial distance, the fracture lengths in each direction were plotted in a polar coordinate system to form the fracture strike rose diagram. Since the fracture strike has axial characteristics, i.e., 0° and 180° represent the same propagation direction, the fracture directions within the 180°~360° range were mapped to 0~180° during the statistical process to avoid duplicate counting of the same dominant fracture direction. In the rose diagram, the peak direction represents the dominant fracture propagation azimuth, and the radial length reflects the scale of fracture development in that direction. Figure 5 Here is an example of a crack direction rose diagram drawn using the method described above. The length of the sector in different directions represents the cumulative crack length in that direction, with the longest sector corresponding to the dominant crack propagation direction; from this... Figure 5 The fracture direction rose diagram reveals a clear dominant direction for fracture orientation. The cumulative fracture length is primarily concentrated in the ranges of 0°–45° and 180°–225°, with the dominant direction exhibiting the highest cumulative length, reaching approximately 1000–1100 m, significantly higher than other directions. Conversely, fractures deviating from the dominant direction typically have cumulative lengths of around 200–400 m. This indicates that the overall fractures primarily extend along the dominant direction, accompanied by a degree of branching and reversal, exhibiting a spatial distribution characteristic of a prominent primary direction and dispersed secondary directions. By comparing the changes in the dominant direction and its distribution range in the rose diagram under different operating conditions, a direct evaluation of the reversal characteristics of secondary fracturing fractures can be achieved.

[0095] Secondary fracturing fractures exhibit not only changes in direction but also an increase in fracture branching and network complexity. Fractal theory is used to describe the spatial distribution characteristics of the fractures.

[0096] In one embodiment, calculating the three-dimensional fractal dimension specifically includes: extracting the spatial coordinate information of the crack elements to form a three-dimensional discrete point set; normalizing the coordinates in the three directions to eliminate differences in the geometric scale of the cracks; using a box-dimensional mesh covering method, dividing the normalized crack space into cubic boxes of different side lengths, counting the number of non-empty boxes, fitting a straight line in a double logarithmic coordinate system, and the absolute value of the slope of the obtained straight line is the three-dimensional fractal dimension of the crack, which is used to describe the spatial complexity of the crack network; the larger the fractal dimension, the more complex the crack network structure, and the more obvious the crack branching and non-uniform propagation characteristics.

[0097] In one embodiment, normalizing the three directional coordinates specifically involves normalizing the X-coordinate, Y-coordinate, and depth coordinate of the crack element center, respectively. The normalized crack spatial coordinates are used to eliminate the influence of scale differences in the length, height, and width directions of the crack on the box-dimensional calculation results.

[0098] The irregularity of rock fractures can be statistically considered to possess self-similarity, and some scholars use fractal dimension as an indicator of the complexity of fracture networks. Among these, the box-dimensional mesh covering method is most suitable for determining the fractal dimension of fracture networks, and its principle is shown in the following equation.

[0099] ;

[0100] First, select boxes with different side lengths of δ to cover set A. This requires the corresponding number of boxes with different side lengths. Plot the coordinates of each scatter point in a logarithmic coordinate system. Finally, these coordinates are fitted to a straight line using the least squares method, and the absolute value of the slope of the resulting line is the fractal dimension of the crack. Generally speaking, the larger the fractal dimension, the stronger the self-similarity and the more complex the crack network morphology.

[0101] Spatial coordinate information of crack elements is extracted based on UFM simulation results. For each crack element, its geometric center coordinates are obtained: ,in, X-coordinate of the crack element center; : Y-coordinate of the crack element center; : Depth coordinates of the crack element center.

[0102] Using spatial coordinates as input data for fractal dimension calculation, a three-dimensional discrete point set is ultimately formed: Where N is the number of crack elements. Since the crack network exhibits significant scale differences in length, height, and width, directly using actual coordinates for box counting may lead to a dominant scale in one direction. Therefore, the coordinates in the three directions are normalized separately:

[0103] ;

[0104] After normalization: To eliminate the influence of differences in crack geometry on the calculation results, the normalized crack space is divided into sections with side lengths of... The cubic box uses a progressively smaller size design: For each scale, the following steps are performed: spatial meshing is performed on the crack points; the box number of the crack point is determined; duplicate numbers are removed; and the number of non-empty boxes is obtained. As the box size decreases, the number of boxes required to cover the crack increases, satisfying the fractal scaling relation: Take the logarithm of both sides Therefore, by calculation and By performing a linear fit y=ax+b, the fractal dimension D=a can be obtained, and the correlation coefficient can be calculated simultaneously. Evaluate the reliability of fractal fitting.

[0105] In one embodiment, the newly added effective modified volume ratio is the ratio between the newly added effective modified volume of secondary fracturing and the original effective modified volume, used to characterize the range of the new modified area formed by the deflection of secondary fracturing fractures.

[0106] In one embodiment, the index weights are obtained by using the analytic hierarchy process (AHP) to construct a judgment matrix based on the contribution of each index to the crack turning characteristics, and then by calculating the weights using the geometric mean method.

[0107] To avoid the limitations of evaluating fracture redirection behavior using a single index, a multi-index evaluation system based on fractal dimension, circular variance, and the added SRV (Stimulated Reservoir Volume) is established. Considering that fracture redirection is mainly manifested as the deviation of the fracture propagation direction from the original dominant direction, the analytic hierarchy process (AHP) is adopted to construct a judgment matrix based on the contribution of each index to the fracture redirection characteristics, and the index weights are calculated using the geometric mean method. Finally, a fracture redirection evaluation index is established (as shown in the following formula) to achieve a quantitative evaluation of fracture redirection capability under different fracturing conditions.

[0108] ;

[0109] In the formula, FDI is the crack diversion evaluation index. It is the fractal dimension, and V is the circular variance. It is the newly added SRV rate.

[0110] In one embodiment, the method further includes a verification step: establishing a secondary fracturing production simulation model, performing capacity response analysis on different fracture redirection schemes, and verifying the consistency between the comprehensive evaluation index and the actual production increase effect by comparing the daily oil production, cumulative oil production, and production dynamic change patterns.

[0111] To verify the effectiveness of the established evaluation method for secondary fracturing reversal based on multidimensional fracture characteristics, secondary fracturing schemes were designed under three sets of control factors: different construction methods (temporary plugging method), different geological conditions (stress difference), and different construction parameters (discharge rate). The three-dimensional fractal dimension, length-weighted circular variance, newly added SRV rate, and comprehensive fracture reversal evaluation index based on the spatial distribution of fracture elements were calculated respectively. The results are shown in Table 1.

[0112] In one embodiment, the geomechanical parameters include the maximum horizontal principal stress, the minimum horizontal principal stress, the stress difference, and the reservoir pressure; the fracturing construction parameters include the fracturing discharge rate and the perforation parameters; the different construction methods include non-plugging, uniform plugging, and dominant fracture plugging, wherein the plugging effect is characterized by adjusting the DischargeCoefficient parameter using the equivalent flow resistance method.

[0113] Since current fracturing simulation software struggles to directly describe the migration of temporary plugging agent particles, bridging formation, and the evolution of the temporary plugging zone, the equivalent flow resistance method is employed in the simulation. The temporary plugging effect is characterized by adjusting the DischargeCoefficient parameter in the Kinetix model. Considering the different spatial distribution characteristics of the temporary plugging agent in the fracture system, three fracture control modes are established: no plugging, uniform plugging, and dominant fracture plugging. No plugging characterizes the natural fracture propagation characteristics under no flow control conditions; uniform plugging simulates the situation where the temporary plugging agent is uniformly distributed in all fractures, leading to a reduction in overall flow capacity; and dominant fracture plugging simulates the process where the temporary plugging agent preferentially acts on high-conductivity dominant fractures, promoting fluid redistribution and fracture deflection by increasing the flow resistance of the dominant channels.

[0114] ;

[0115] 2. Analysis and verification of the results of Scheme 1 (different temporary blocking methods).

[0116] First, let's look at Scheme 1 (different temporary plugging methods). Table 1 shows that under different plugging methods, the uniform plugging scheme has the highest comprehensive evaluation index (0.717), higher than the non-plugging scheme (0.671) and the dominant fracture plugging scheme (0.673). Specifically, under the non-plugging condition, the fracture fractal dimension is 1.492, the length-weighted circular variance is 0.153, and the newly added SRV rate is 0.647, resulting in the lowest comprehensive evaluation index. This indicates that without plugging, fracturing fluid preferentially enters the existing dominant fracture, the fracture propagation path is controlled by the original fracture's conductivity, and the degree of new fracture turning is limited. After adopting uniform plugging, the fracture fractal dimension increases to 1.536, the length-weighted circular variance increases to 0.204, and the newly added SRV rate increases to 0.674, bringing the comprehensive evaluation index to 0.717. This demonstrates that uniform plugging can effectively improve the flow distribution between fractures, reduce the concentrated absorption of fracturing fluid by dominant fractures, promote the coordinated propagation of multiple fractures, and increase the complexity and scope of the fracture network. Although the temporary plugging scheme for dominant fractures achieved an additional SRV rate of 0.661, higher than the non-plugging condition, the degree of fracture direction adjustment was limited due to the dominant fractures' strong stress release and propagation advantages. Its fractal dimension and circular variance were 1.502 and 0.148, respectively, resulting in a comprehensive evaluation index of only 0.673. This indicates that simply increasing the degree of temporary plugging of local dominant fractures does not necessarily achieve the best fracture redirection effect; rather, rationally controlling the flow distribution among fractures is a crucial condition for promoting balanced fracture propagation.

[0117] Further analysis was conducted on the crack propagation morphology and production data under different operating conditions of Scheme 1 (condition 1-1 without temporary plugging, condition 1-2 with uniform temporary plugging, and condition 1-3 with temporary plugging of dominant cracks). To ensure the reliability of the evaluation results, the same constant pressure production strategy was adopted for different operating conditions under the same scheme during production simulation. At the same time, due to the strong spatial superposition of the propagation morphology of multiple crack clusters, it is difficult to intuitively represent the difference in the degree of turning of the overall crack network. Therefore, when verifying the crack morphology under different operating conditions of the same scheme, a representative cluster of cracks was selected for comparative analysis.

[0118] Figure 7The figure shows a comparison of the fracture morphology under different operating conditions of secondary fracturing in Scheme 1 (from left to right: Schemes 1-1 to 1-3). As can be seen from the figure, there are significant differences in the fracture propagation morphology under different operating conditions. Under Scheme 1-1, the fractures mainly extend along the original dominant propagation direction, with a relatively concentrated overall fracture distribution and low lateral branching development, indicating that the fractures under this condition are strongly influenced by the original stress environment and existing fractures, resulting in limited deflection ability. In contrast, under Scheme 1-2, the fracture propagation range increases significantly, with more lateral deflection and branching fractures, forming a more complex fracture network structure and exhibiting stronger fracture deflection ability. Under Scheme 1-3, the fractures also exhibit some degree of deflection and propagation, with a higher degree of complexity than under Scheme 1-1. However, compared to Scheme 1-2, its fracture propagation direction still has a strong dominant directionality, and the degree of deflection is slightly lower. Therefore, the fracture deflection effect under the three operating conditions is ranked as follows: Scheme 1-2 > Scheme 1-3 > Scheme 1-1. Figure 6 Among the three working conditions, working condition 1-2 has a higher fractal dimension and circular variance, and the highest comprehensive index, indicating that the spatial complexity and directional dispersion of the cracks are relatively stronger under this condition. The indicators of working conditions 1-1 and 1-3 are relatively similar, but their comprehensive indices are lower than those of working condition 1-2. Overall, working condition 1-2 has a more prominent effect on crack turning and complex propagation, and the comprehensive evaluation results show that working condition 1-2 > working condition 1-3 > working condition 1-1. Figure 8 The results of the cumulative oil production under different temporary plugging methods are as follows: Figure 8 As can be seen from the data, under the same production time conditions, the cumulative oil production ranking for each operating condition is also as follows: Operating Condition 1-2 > Operating Condition 1-3 > Operating Condition 1-1, and the ranking is consistent with... Figure 6 Consistent; Figure 7 and Figure 6 , 8 The consistency between the shown turning evaluation index and the cumulative oil production ranking proves that the comprehensive evaluation index can accurately reflect the turning effect of secondary fracturing fractures.

[0119] 3. Analysis and verification of the results of Scheme 2 (different ground stress differences).

[0120] Secondly, Scheme 2 (different stress differences) shows that as the horizontal stress difference decreases from 12 MPa to 6 MPa, the fractal dimension of the crack increases from 1.492 to 1.539. This indicates that the directional constraint on crack propagation is weakened under lower stress difference conditions, and cracks are more likely to branch and propagate in a complex manner, with the complexity of the crack network gradually increasing.

[0121] However, the length-weighted circular square error did not show a continuous increasing trend with decreasing stress difference, with values ​​of 0.153, 0.151, and 0.148, respectively. This is because the circular square error is mainly controlled by the direction of long cracks, while under low stress difference conditions, newly added cracks mainly exhibit branching extension near the dominant direction, without significantly altering the dominant crack direction. (Reference) Figure 9 The cracks exhibit a clearly dominant orientation, with their cumulative length concentrated in the ranges of 0°–45° and 180°–225°. The cumulative length along the dominant orientation is approximately 6000 m–7000 m, while the cumulative length of cracks deviating from the dominant orientation is relatively smaller. Overall, the cracks extend concentratedly along the principal stress control direction, with the principal orientation being more prominent and the degree of deflection and dispersion in other directions being weaker. (Reference) Figure 10 The cracks still primarily extend along the 0°–45° and 180°–225° directions, but the cumulative length in the dominant direction has significantly increased, reaching a maximum cumulative length of approximately 10,000 m–11,000 m. Simultaneously, cracks with cumulative lengths of approximately 5,000 m–7,000 m are distributed on both sides of the dominant direction. Compared to the 12 MPa condition, the overall scale of crack expansion has increased, and while maintaining the dominant direction, it exhibits a more pronounced multi-directional distribution characteristic. (Reference) Figure 11 The cracks generally continue to propagate primarily in the vicinity of 0° / 180° and on either side, with the maximum cumulative length in the dominant direction being approximately 9000m–10000m, and the cumulative length in other adjacent directions being approximately 5000m–7000m. Compared to the condition with higher stress difference, the cracks show a more significant length contribution on both sides of the dominant direction, indicating that as the stress difference decreases, the constraint of the principal stress on the crack propagation direction weakens, making the cracks more prone to deflection and branching in non-dominant directions.

[0122] Crack direction rose diagrams under different stress differences (e.g.) Figure 9-11 As shown in the figure, as the horizontal stress difference decreased from 12 MPa to 6 MPa, the dominant fracture propagation direction remained within the range of 10° to 20° under all three working conditions. This indicates that the change in stress difference did not cause a significant deflection of the main fracture propagation direction, and the fracture as a whole was still controlled by the original geostress field. However, as the stress difference decreased, the number of fracture branches near the dominant propagation direction increased, the secondary fracture directions gradually became more diverse, and the fracture orientation distribution range expanded, indicating that low stress difference conditions are conducive to fracture network complexity and multi-directional propagation. Since the length-weighted statistical method is more sensitive to long-scale dominant fractures, the dispersion of fracture orientation did not show a significant increasing trend with the decrease in stress difference. Therefore, the fracture turning process is not only manifested in the deflection of the main fracture direction, but also includes characteristics such as branch propagation near the dominant fracture and expansion of the modification range. A single fracture orientation parameter is insufficient to comprehensively evaluate the effect of secondary fracturing turning, and a comprehensive evaluation index needs to be established.

[0123] Meanwhile, the newly added SRV rate increased from 0.647 under 12 MPa conditions to 0.663 under 9 MPa conditions, and then decreased to 0.643 under 6 MPa conditions. This indicates that reducing the stress difference can promote fracture propagation complexity, but under excessively low stress difference conditions, fractures may tend to propagate uniformly, with limited improvement in the effective modified volume. Considering all three indicators, the comprehensive evaluation index was highest at 9 MPa (0.685), followed by 0.684 at 6 MPa, and lowest at 12 MPa (0.671). The evaluation results show that a lower stress difference is beneficial for fracture redirection, but the fracture redirection effect is influenced by both fracture complexity and effective modified volume; a single geometric parameter is insufficient to comprehensively evaluate the fracture redirection effect of secondary fracturing.

[0124] Further analysis was conducted on crack propagation morphology and production data under different operating conditions of Scheme 2 (Scheme 2-1 with a stress difference of 12 MPa, Scheme 2-2 with a stress difference of 9 MPa, and Scheme 2-3 with a stress difference of 6 MPa). During production simulation, the same constant-pressure production strategy was adopted for different operating conditions under the same scheme to ensure the reliability of the results.

[0125] Figure 13 Figure 13 shows a comparison of secondary fracturing fracture morphology under different operating conditions in Scheme 2 (from left to right: Schemes 2-1 to 2-3). As can be seen from Figure 13, there are significant differences in fracture propagation morphology under different operating conditions. Under Scheme 2-1, the secondary fracturing fractures mainly propagate along the dominant direction of the original fractures, with a relatively concentrated fracture morphology, a small lateral propagation range, and a low fracture network complexity, indicating that fracture turning is strongly restricted under this condition. Under Scheme 2-2, the lateral propagation range of the fractures increases significantly, and the fractures form more branch structures on the basis of the main propagation direction, resulting in a more complex fracture spatial distribution, demonstrating a strong ability to adjust direction and expand the modification range. Under Scheme 2-3, the fractures also show a certain degree of lateral propagation and branching development, with a slightly improved fracture turning effect compared to Scheme 2-1, but its fracture propagation range and complexity are still weaker than those of Scheme 2-2.

[0126] Figure 12 The fractal dimension, circular square error, and newly added SRV rate of the three working conditions showed relatively small overall differences. Among them, the fractal dimension and newly added SRV rate of working conditions 2-2 and 2-3 were slightly higher than those of working condition 2-1, while the change in circular square error was limited, so the comprehensive index only increased slightly. This indicates that under these conditions, the influence of different working conditions on crack turning morphology is relatively limited, and the comprehensive turning effect of the three working conditions is quite similar, but working condition 2-2 is slightly better than working condition 2-3, which is slightly better than working condition 2-1. Figure 14 For the comparison of cumulative oil production under different geostress differences, from Figure 13 As can be seen from the data, under the same production time conditions, the cumulative oil production of each operating condition is ranked as follows: 2-2 > Operating Condition 2-3 > Operating Condition 2-1. This ranking is consistent with... Figure 13 Consistent;

[0127] In summary, the crack redirection effect under different working conditions is ranked as follows: Working Condition 2-2 > Working Condition 2-3 > Working Condition 2-1. Figure 13 and Figure 12 , 14 The consistency between the shown turning evaluation index and the cumulative oil production ranking proves that the comprehensive evaluation index can accurately reflect the turning effect of secondary fracturing fractures.

[0128] 4. Analysis and verification of the results of Scheme 3 (different construction discharge volumes).

[0129] Finally, there is Scheme 3 (different construction discharge rates). The evaluation results under different discharge rate conditions show that 12... The displacement-based scheme has the highest comprehensive evaluation index, at 0.711, which is higher than 18. (0.671) and 24 (0.688). Among them, 12 Under the given conditions, the length-weighted circular variance reached 0.223, significantly higher than other displacement conditions. This indicates that a lower displacement can reduce the flow competition effect during rapid crack propagation, allowing more cracks to open and improving crack orientation adjustment capabilities. Simultaneously, its newly added SRV rate reached 0.658, maintaining a high level, thus achieving the highest comprehensive evaluation result. When the displacement increased to 24... At that time, the fractal dimension of the fracture increased to 1.5083, indicating that high flow rate can enhance the complexity of the fracture network. However, the length-weighted circular variance was only 0.174, and the newly added SRV rate decreased to 0.645, suggesting that excessively high flow rate may lead to further concentration of fracturing fluid into dominant fractures, causing fracture propagation to be mainly driven by length increase, while the effective deflection capacity is only slightly improved. Therefore, reducing the construction flow rate within a certain range is beneficial to improving the uniformity of fracture propagation and enhancing the deflection effect of secondary fracturing fractures.

[0130] Further analysis of Scheme 3 under different operating conditions (Operating condition 3-1 is for a displacement of 12...) Operating condition 3-2 is for a displacement of 18 Operating condition 3-3 is for a displacement of 24. The crack propagation morphology and production data were analyzed. During production simulation, the same constant-pressure production strategy was used for different operating conditions under the same scheme to ensure the reliability of the results.

[0131] Figure 16The figure shows a comparison of fracture morphology under different operating conditions in Scheme 3 (from left to right: Schemes 3-1 to 3-3). As can be seen from the figure, there are significant differences in fracture propagation morphology under different operating conditions. Under Scheme 1, the secondary fracturing fractures exhibit significant lateral propagation based on the original fractures, with a high degree of fracture branching and a large fracture network coverage, demonstrating strong fracture turning ability and reservoir stimulation effect. Under Scheme 3-2, the overall fracture propagation range is relatively small, mainly extending along the dominant direction, with limited lateral deflection and branch fracture development, indicating that fracture turning is somewhat limited under this condition. Under Scheme 3-3, the fracture propagation morphology is between that of Schemes 3-1 and 3-2. Although some degree of directional adjustment and lateral propagation occurs, the fracture complexity and stimulation range are still lower than those under Scheme 3-1. Therefore, the fracture turning effect under different operating conditions is ranked as follows: Scheme 3-1 > Scheme 3-3 > Scheme 3-2.

[0132] Figure 15 The fractal dimension and the rate of new SRV change little across the three operating conditions. The main difference lies in the circular variance, with condition 3-1 having the highest circular variance, condition 3-2 having the lowest, and condition 3-3 being in the middle. This results in the comprehensive index showing that condition 3-1 has the highest, condition 3-3 is the second highest, and condition 3-2 has the lowest. Figure 17 For the comparison of cumulative oil production under different construction displacements, from Figure 17 As can be seen from the data, under the same production time conditions, the cumulative oil production ranking for each operating condition is also as follows: Operating Condition 3-1 > Operating Condition 3-3 > Operating Condition 3-2, and the ranking is consistent with... Figure 16 Consistent; Figure 16 and Figure 15 , 17 The consistency between the shown turning evaluation index and the cumulative oil production ranking proves that the comprehensive evaluation index can accurately reflect the turning effect of secondary fracturing fractures.

[0133] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

Claims

1. A method for evaluating the diversion effect of secondary fracturing based on multidimensional fracture characteristics, characterized in that, The method includes: Obtain the target reservoir's geomechanical parameters, reservoir physical property parameters, initial fracture characteristic parameters, and fracturing operation parameters, and establish a simulation model for secondary fracturing fracture propagation. Based on the aforementioned secondary fracturing fracture propagation simulation model, fracture propagation simulations were conducted under different construction methods, construction parameters, and geological parameters. The spatial distribution characteristics of the fracture network formed by secondary fracturing were obtained, and the spatial coordinates, fracture length, fracture azimuth angle, and distribution information of the effective modification area of ​​the fracture unit were extracted. Based on the simulation results of fracture propagation, fracture turning evaluation indexes are established from three dimensions: fracture direction characteristics, fracture network structure characteristics, and reservoir stimulation range. The fracture azimuth circle variance, three-dimensional fractal dimension, and newly added effective stimulation volume ratio are calculated respectively. The crack direction index, crack complexity index, and modification range index are dimensionless. The index weights are determined according to the contribution of each index to the crack reversal effect. A comprehensive crack reversal evaluation index is calculated, and the secondary fracturing crack reversal effect under different working conditions is determined based on the comprehensive evaluation index.

2. The method according to claim 1, characterized in that, The calculation of the circular variance of the crack orientation specifically includes: A circular statistical method is used to treat each crack element as a direction vector. At the same time, a length-weighted method is used to calculate the length-weighted circular variance based on the crack element orientation azimuth and crack length data, which is used to describe the dispersion of crack direction distribution. The larger the circular square difference value, the more dispersed the crack orientation distribution and the more obvious the crack turning and multi-directional propagation characteristics.

3. The method according to claim 1, characterized in that, Calculating the three-dimensional fractal dimension specifically includes: Extract the spatial coordinate information of the crack elements to form a three-dimensional discrete point set, and normalize the coordinates in the three directions to eliminate the differences in the geometric scale of the cracks. The box-dimensional mesh covering method is adopted to divide the normalized crack space into cubic boxes with different side lengths, count the number of non-empty boxes, fit a straight line in double logarithmic coordinates, and the absolute value of the slope of the obtained straight line is the three-dimensional fractal dimension of the crack, which is used to describe the spatial complexity of the crack network. The larger the fractal dimension, the more complex the crack network structure, and the more obvious the crack branching and non-uniform propagation characteristics.

4. The method according to claim 1, characterized in that, The newly added effective modified volume rate is the ratio between the newly added effective modified volume of secondary fracturing and the original effective modified volume, and is used to characterize the range of the newly modified area formed by the deflection of secondary fracturing fractures.

5. The method according to claim 1, characterized in that, The method further includes: By combining the circular variance with the fracture direction rose diagram, the qualitative and quantitative evaluation of the fracture turning characteristics of secondary hydraulic fracturing is carried out. The method for drawing the crack direction rose diagram is as follows: the crack direction angle range of 0°~360° is divided into several statistical intervals at 10° intervals, the cumulative length of crack units in each angle interval is counted, the crack azimuth angle is used as the polar angle, and the cumulative crack length in the corresponding azimuth interval is used as the radial distance to draw it in the polar coordinate system, and the crack direction in the range of 180°~360° is mapped to 0°~180°.

6. The method according to claim 1, characterized in that, The weights of the indicators are determined using the analytic hierarchy process (AHP). A judgment matrix is ​​constructed based on the contribution of each indicator to the crack turning characteristics, and the matrix is ​​calculated using the geometric mean method.

7. The method according to claim 1, characterized in that, The method also includes a verification step: A secondary fracturing production simulation model was established, and the production capacity response of different fracture diversion schemes was analyzed. By comparing the daily oil production, cumulative oil production, and production dynamics, the consistency between the comprehensive evaluation index and the actual production increase effect was verified.

8. The method according to claim 1, characterized in that, The secondary fracturing fracture propagation simulation model is based on the UFM model. The artificial and natural fracture network formed by primary fracturing, as well as the reservoir stress field, pressure field, and oil saturation field state parameters after primary fracturing, are used as the initial conditions for the secondary fracturing simulation.

9. The method according to claim 3, characterized in that, The normalization process for the three directional coordinates involves normalizing the X-coordinate, Y-coordinate, and depth coordinate of the crack element center, respectively. The normalized crack spatial coordinates are used to eliminate the influence of scale differences in the length, height, and width directions of the crack on the box-dimensional calculation results.

10. The method according to claim 1, characterized in that, The geomechanical parameters include the maximum horizontal principal stress, the minimum horizontal principal stress, the stress difference, and the reservoir pressure; the fracturing operation parameters include the fracturing flow rate and perforation parameters. The different construction methods include non-temporary plugging, uniform temporary plugging, and dominant crack temporary plugging. The effect of temporary plugging is characterized by adjusting the Discharge Coefficient parameter using the equivalent flow resistance method.

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