A micro crack prediction method based on OVT gather spectral decomposition

Abstract

The present application belongs to the technical field of geophysical exploration, and specifically discloses a kind of micro crack prediction method based on OVT gather spectrum decomposition, which is: S1, the feasibility of using OVT gather data for prestack fracture density and azimuth fine quantification prediction is determined;S2, spectrum analysis, determine the micro crack sensitive frequency band;S3, convert the seismic data to the frequency domain, then take frequency filtering to get the discrete frequency energy body of the micro crack sensitive frequency band;S4, the discrete frequency energy body of the micro crack sensitive frequency band is divided into multiple azimuth angle ranges with equal intervals for stacking to generate azimuth angle gather data body;S5, according to the azimuth angle gather data body, the fracture density and azimuth are predicted.The present application improves the prediction ability and prediction accuracy of OVT domain seismic data on fracture density and azimuth, making the fracture density and azimuth prediction results using frequency division seismic data more reasonable.The present application is suitable for micro crack prediction.

Description

Technical Field

[0001] This invention belongs to the field of geophysical exploration technology, specifically a method for predicting microfractures based on OVT gather spectrum decomposition. Background Technology

[0002] With the continuous development and utilization of oil and gas resources, the focus of oil and gas exploration and development has gradually shifted from conventional oil and gas reservoirs to unconventional ones, among which the exploration and development of fractured reservoir oil and gas reserves is particularly important. According to incomplete statistics, the oil and gas production of fractured reservoirs already accounts for half of the total oil and gas production, making it one of the key areas for increasing oil reserves and production. Currently, in the global oil and gas exploration and development process, the proportion of fractured oil and gas reservoirs is increasing, especially in carbonate rocks, shale, tight sandstone, and igneous rocks. Therefore, the prediction of fractured reservoirs is crucial.

[0003] With the continuous development and advancement of seismic geophysical prediction technology, fracture prediction has evolved from statistical identification based on lithology and well logging data to post-stack qualitative prediction using geophysical methods, and then to pre-stack quantitative fracture prediction. Currently, microfracture prediction methods include prediction techniques based on ant-body attributes and pre-stack fracture density and azimuth prediction techniques based on OVT domain seismic data. For post-stack fracture prediction techniques, limited by seismic resolution, the seismic sensitivity to microfractures is poor, and information on fracture azimuth and density is lacking. For pre-stack fracture prediction techniques, different frequencies show significant differences in sensitivity to the distribution of fractures and fissures at different scales. Low-frequency information mainly reflects the distribution of large-scale fractures and fissures, while high-frequency information is more sensitive to the distribution of small-scale fractures and fissures. When using full-band data to predict fractures and fissures at different scales, some information mixing occurs, resulting in weak classification and certain limitations. Microfractures have a significant effect on improving carbonate, shale, and igneous reservoirs; encountering fractures is a major factor in high single-well production. Therefore, to achieve large-scale efficient development, it is urgent to conduct targeted research on microfracture prediction technologies to improve fracture prediction accuracy. Summary of the Invention

[0004] The purpose of this invention is to provide a method for predicting microcracks based on OVT gather spectrum decomposition, so as to improve the ability and accuracy of OVT domain seismic data in predicting crack development density and orientation.

[0005] To achieve the above objectives, the present invention employs the following technical methods:

[0006] A method for predicting microcracks based on OVT gather spectral decomposition includes the following steps performed sequentially:

[0007] S1. Load OVT gather data, conduct feasibility assessment and analysis of OVT gather data, and clarify the feasibility of using OVT gather data for fine quantitative prediction of pre-stack fracture development density and orientation.

[0008] S2. Perform spectrum analysis based on OVT gather data to determine the frequency band sensitive to microcracks;

[0009] S3. Convert the seismic data to the frequency domain, and then use frequency filtering to obtain the discrete frequency energy volume of the sensitive frequency band of the microcrack;

[0010] S4. Divide the discrete frequency energy volume of the sensitive frequency band of the microcrack into multiple azimuth ranges at equal intervals and superimpose them to generate azimuth gather data volume.

[0011] S5. Predict crack development density and azimuth based on azimuth gather data.

[0012] As a limitation, the parameters for feasibility assessment and analysis of OVT gather data in step S1 are specifically the uniformity of the number of coverages within the specified offset range and the degree of azimuth anisotropy of amplitude slices and travel time slices.

[0013] As a limitation: the spectrum analysis based on OVT gather data in step S2 specifically involves comparing the characteristics of OVT gathers in different sensitive frequency bands and different azimuth angles, clarifying the degree to which the amplitude and residual time difference of the spiral gather phase axis of the microcrack in different frequency bands fluctuate with the azimuth angle, and determining the frequency bands sensitive to the microcrack.

[0014] As a limitation: in step S3, the generalized S-transform method is used to convert the seismic data to the frequency domain.

[0015] As a limitation: Step S4 specifically involves dividing the discrete frequency energy volume of the sensitive frequency band of the microcrack into six azimuth ranges at equal intervals of 30° and superimposing them to generate azimuth gather data volume.

[0016] As a limitation: Step S5 specifically involves obtaining the root mean square amplitude anisotropy attribute of each azimuth angle gather data volume, performing ellipse fitting based on the root mean square amplitude anisotropy variation corresponding to each azimuth, determining the crack prediction result based on the fitting result, the flattening of the ellipse of the root mean square amplitude anisotropy intensity attribute indicates the crack development density, and the azimuth of the major axis of the ellipse of the root mean square amplitude anisotropy azimuth attribute indicates the crack development azimuth.

[0017] As a further limitation: the ellipse fitting in step S5 adopts the Rüger approximation equation, i.e. In the formula Where A is the azimuth angle, B is the offset factor related to the offset distance, and C is the amplitude modulation factor related to the offset distance and crack characteristics. This represents the azimuth anisotropy characteristic of the amplitude.

[0018] The beneficial effects achieved by this invention, due to the adoption of the above-described solution, compared with the prior art, are as follows:

[0019] This invention provides a method for predicting microfractures based on OVT gather spectral decomposition. Through OVT gather spectral analysis, seismic data spectral decomposition, azimuth-range stacking, and ellipse fitting to obtain root mean square amplitude anisotropy properties, it predicts microfractures, improving the predictive ability and accuracy of OVT domain seismic data for fracture development density and azimuth. This provides strong support for well location deployment and well trajectory optimization in fractured oil and gas reservoirs. This invention uses pre-stack OVT gathers for spectral analysis to determine the sensitive frequency band for microfractures, making the prediction results of fracture development density and azimuth using frequency-divided seismic data more reasonable. Compared with the method of frequency division after azimuth-range stacking, the spectral decomposition based on generalized S-transform of pre-stack OVT gathers has more obvious anisotropic information, is less affected by stacking, and the seismic data of the sensitive frequency band for microfractures is more accurate and reliable. The frequency-divided seismic data obtained by this method has a wider bandwidth and higher dominant frequency, reflecting microfractures with higher accuracy.

[0020] This invention is applicable to the prediction of microcracks. Attached Figure Description

[0021] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0022] Figure 1 This is a flowchart of a microcrack prediction method based on OVT gather spectrum decomposition according to an embodiment of the present invention;

[0023] Figure 2 This is a schematic diagram illustrating the uniformity of coverage frequency within a 600-3900 meter offset range in the Y-well area of ​​southern Sichuan in an embodiment of the present invention.

[0024] Figure 3 This is a schematic diagram illustrating that the amplitude slices and travel time slices of this invention exhibit obvious orientational anisotropy.

[0025] Figure 4 This is a schematic diagram illustrating the optimization of OVT gather data at different sensitive frequencies and azimuth angles through spectrum analysis in an embodiment of the present invention.

[0026] Figure 5 This is a crack development density prediction diagram according to an embodiment of the present invention;

[0027] Figure 6 This is a crack development orientation prediction diagram according to an embodiment of the present invention;

[0028] Figure 7This is a planar distribution diagram of fracture development in wells Y6 and Y7 according to an embodiment of the present invention (the upper part of the diagram shows the well logging interpretation results, and the lower part shows the prediction results of this embodiment).

[0029] Figure 8 This is a planar distribution map of fracture development density in wells Y6 and Y7 according to an embodiment of the present invention (the upper part of the map is the core statistics, and the lower part is the prediction result of this embodiment). Detailed Implementation

[0030] The present invention will be further described below with reference to the embodiments. However, those skilled in the art should understand that the present invention is not limited to the following embodiments. Any improvements and equivalent changes made based on the specific embodiments of the present invention are within the scope of protection of the claims of the present invention.

[0031] Example: A method for predicting microcracks based on OVT gather spectrum decomposition

[0032] This embodiment applies the microfracture prediction method based on OVT gather spectrum decomposition to fracture prediction in the Silurian Longmaxi Formation shale gas reservoir in southern Sichuan. The flowchart of the microfracture prediction method based on OVT gather spectrum decomposition is as follows: Figure 1 As shown, the steps are performed sequentially:

[0033] S1. Load OVT gather data, conduct feasibility assessment and analysis of the OVT gather data, and clarify the feasibility of using OVT gather data for fine quantitative prediction of pre-stack fracture development density and orientation; in this implementation, the Y well area in the southern Sichuan region was covered 196 times, with a relatively uniform number of coverages within the offset range of 600-3900 meters, such as Figure 2 As shown, based on the data foundation of the OVT pre-stack fracture prediction method, the time delay and reflected wave amplitude of the near-vertical incident P-wave at the well point in the study area are further analyzed, such as... Figure 3 As shown, amplitude slices and travel time slices have obvious azimuth anisotropy (AVA) characteristics. At the same time, in areas with good data quality, changing the number of azimuth stacks results in small changes in the azimuth of the fitted ellipse. This clarifies that the fracture prediction of the Silurian Longmaxi Formation shale gas reservoir in the Y well area of ​​southern Sichuan can be performed by using OVT gather data for fine quantitative prediction of pre-stack fracture development density and azimuth.

[0034] S2. Spectral analysis was performed on OVT gather data from high-quality shale sections of the Longmaxi Formation (1800ms-2100ms) with varying degrees of fracture development. Comparison of OVT gather characteristics across different sensitive frequency bands and azimuth angles revealed that the amplitude and residual time difference of the helical gather phase axis of micro-fractures fluctuated with azimuth angle across different frequency bands. The fluctuation was more pronounced in high-frequency data, indicating higher sensitivity to micro-fractures and reflecting fracture development characteristics. In this embodiment, based on OVT gather spectral analysis, the bandwidth was 3-55Hz, the dominant frequency was 25Hz, and the sensitive frequency bands were concentrated in three segments: 3-15Hz, 15-30Hz, and 30-55Hz. Comparison of OVT gather data from different sensitive frequency bands (dominant frequencies 10Hz, 25Hz, and 45Hz) and azimuth angles showed that the 45Hz frequency information most clearly reflected the micro-fracture response characteristics, thus determining the micro-fracture sensitive frequency band to be 30-55Hz. Figure 4 As shown;

[0035] S3. The generalized S-transform method was used to convert the OVT gather data of different fracture development levels in the high-quality shale section of Longmaxi Formation (1800ms-2100ms) to the frequency domain, and then frequency filtering was used to obtain the discrete frequency energy volume of 45Hz.

[0036] S4. Divide the 45Hz discrete frequency energy volume into six azimuth ranges at equal intervals of 30° and superimpose them to generate azimuth gather data volume.

[0037] S5. Obtain the root mean square amplitude anisotropy properties of the gather data volumes for each azimuth. Based on the changes in root mean square amplitude anisotropy corresponding to each azimuth, perform ellipse fitting using the Rüger approximation equation. Determine the crack prediction results based on the fitting results. The ellipse flattening of the root mean square amplitude anisotropy intensity property indicates the crack development density, and the ellipse major axis azimuth of the root mean square amplitude anisotropy azimuth property indicates the crack development azimuth. When the incident angle is fixed, the Rüger approximation equation is: In the formula Where A is the azimuth angle, B is the offset factor related to the offset distance, and C is the amplitude modulation factor related to the offset distance and crack characteristics. This represents the azimuth anisotropy characteristic of the amplitude.

[0038] The prediction result of this embodiment is as follows: Figure 5 and Figure 6 As shown, Figure 5 This is a crack development density distribution map characterized by the anisotropic intensity properties of the 45Hz discrete frequency energy volume amplitude (10ms from bottom to top of the five-peak group). Figure 5It can be seen that the density of fracture development in the high-quality shale layer at the bottom of the Longmaxi Formation is uneven. The higher the fracture development intensity value, the higher the degree of fracture development. The densely developed fracture area is mainly concentrated between 0.55 and 0.98. The high-value fracture development area is mainly distributed along the fault and micro-tectonic areas. Figure 6 This is a map showing the azimuth distribution of crack development represented by the anisotropic azimuth properties of the 45Hz discrete frequency energy volume amplitude (10ms upward from the bottom of the five-peak group). Figure 6 It can be seen that the high-quality shale at the bottom of the Longmaxi Formation has a multi-directional and zonal orientation, mainly NE-SW trending, with NW-SE trending in some areas. Most of the associated fractures around the faults are consistent with the fault orientation or intersect at a small angle. The orientation of fractures in other locations is more distinctly zonal. This feature is closely related to the different orientations and strengths of tectonic stress in the study area during the Indosinian, Yanshanian, and Himalayan periods.

[0039] The quality control analysis of the fracture development density and orientation prediction results in this embodiment was conducted using well logging, drilling data, and microseismic fracturing monitoring event results. The prediction results in this embodiment are macroscopically consistent with geological laws, and microscopically, they show a high degree of agreement with the fracture development density and orientation predicted by drilled core statistics and well logging interpretation, as shown in Table 1. Figure 7 and Figure 8 As shown, this greatly improves the accuracy of fracture prediction, providing guidance for the development and production of Silurian Longmaxi Formation shale gas reservoirs in southern Sichuan, including well location deployment, well trajectory optimization, fracturing parameter design, and drilling and fracturing risk avoidance. This ensures efficient and safe drilling and improves drilling success and recovery rate.

[0040] Table 1 shows the agreement between the predicted results and the degree of fracture development in the well logging interpretation of this embodiment.

[0041]

Claims

1. A method for predicting microcracks based on OVT gather spectrum decomposition, characterized in that, This includes the following steps performed sequentially: S1. Load OVT gather data, conduct feasibility assessment and analysis of OVT gather data, and clarify the feasibility of using OVT gather data for fine quantitative prediction of pre-stack fracture development density and orientation. S2. Perform spectrum analysis based on OVT gather data to determine the frequency band sensitive to microcracks; S3. Convert the seismic data to the frequency domain, and then use frequency filtering to obtain the discrete frequency energy volume of the sensitive frequency band of the microcrack; S4. Divide the discrete frequency energy volume of the sensitive frequency band of the micro-crack into six azimuth ranges at equal intervals of 30° and superimpose them to generate azimuth gather data volume. S5. Predict crack development density and azimuth based on azimuth gather data. The root mean square amplitude anisotropy attribute of each azimuth angle gather data volume is obtained. Ellipse fitting is performed based on the root mean square amplitude anisotropy change corresponding to each azimuth. The crack prediction result is determined based on the fitting result. The flattening of the root mean square amplitude anisotropy intensity attribute ellipse indicates the crack development density, and the major axis azimuth of the root mean square amplitude anisotropy azimuth attribute ellipse indicates the crack development azimuth. The ellipse fitting uses the Rüger approximation equation, i.e. In the formula Where A is the azimuth angle, B is the offset factor related to the offset distance, and C is the amplitude modulation factor related to the offset distance and crack characteristics. This represents the azimuth anisotropy characteristic of the amplitude.

2. The method for predicting microcracks based on OVT gather spectrum decomposition according to claim 1, characterized in that, The parameters for feasibility assessment and analysis of OVT gather data in step S1 are specifically the uniformity of the number of coverages within the specified offset range and the degree of azimuth anisotropy of amplitude slices and travel time slices.

3. The method for predicting microcracks based on OVT gather spectrum decomposition according to claim 1, characterized in that, In step S2, the spectrum analysis based on OVT gather data specifically involves comparing the characteristics of OVT gathers in different sensitive frequency bands and at different azimuth angles, clarifying the degree to which the amplitude and residual time difference of the helical gather phase axis of the microcrack fluctuate with the azimuth angle at different frequency bands, and determining the frequency bands sensitive to the microcrack.

4. The method for predicting microcracks based on OVT gather spectrum decomposition according to claim 1, characterized in that, In step S3, the seismic data is converted to the frequency domain using the generalized S-transform method.

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