Method for estimating near-surface Q-factor based on VSP data

Abstract

The application belongs to the field of oil seismic exploration, and discloses a method for estimating near-surface Q factor based on VSP data, which comprises the following steps: carrying out wave field separation processing on the vertical component profile after VSP data preprocessing to obtain downgoing wave field containing downgoing direct wave and downgoing multiple wave information under free interface; determining the position of downgoing multiple wave under free interface and the depth point for estimating near-surface Q factor to obtain the amplitude spectrum of downgoing direct wave and downgoing multiple wave under free interface at the depth point; using spectral ratio method to fit binomial to obtain the near-surface Q factor calculated by the data at the depth point; determining other depth points after the same shot in the well, using the data at multiple depth points to calculate multiple near-surface Q factors, and then averaging the multiple near-surface Q factors to obtain the near-surface Q factor at the well point. Downgoing direct wave and downgoing multiple wave under free interface are obtained by using zero well source distance VSP data, and the near-surface Q factor is calculated.

Description

Technical Field

[0001] This invention belongs to the field of petroleum seismic exploration and relates to a method for estimating near-surface Q factors based on VSP data. Background Technology

[0002] Because near-surface sediments are loose, they strongly attenuate seismic wave energy, especially high-frequency energy, which attenuates rapidly, severely reducing seismic record resolution and imaging quality. Q-compensation processing techniques using the near-surface Q-factor can expand or enhance the high-frequency components of seismic data, broaden the frequency band, eliminate or reduce the absorption and attenuation effect of the near-surface on seismic waves, and improve seismic record resolution. Therefore, the accurate determination of the near-surface Q-factor is an urgent problem to be solved in this field.

[0003] Currently, the near-surface Q-factor is mainly obtained using small refraction and micrologging data. However, these methods have several problems: (1) changes in the source excitation environment during acquisition lead to inconsistencies in the excitation wavelets; (2) the use of different geophones in micrologging and at the surface results in differences in geophone response functions; and (3) near-surface acquisition is subject to numerous interference factors, resulting in poor data quality. Due to these issues, the obtained near-surface Q-factor results are unstable and may exhibit outliers, affecting their application in actual seismic data processing. Summary of the Invention

[0004] The purpose of this invention is to provide a method for estimating the near-surface Q factor based on VSP data. The method combines the downlink direct wave obtained from zero-source-distance VSP data with the downlink multiple wave from the free interface to obtain the near-surface Q factor. By using the same shot excitation and the same in-well geophone data for optimal selection, the influence of wavelet inconsistency, geophone response function differences and low VSP data quality on the accuracy of the calculation is eliminated.

[0005] To achieve the above objectives, the technical solution adopted by this invention is as follows:

[0006] A method for estimating near-surface Q factors based on VSP data includes the following steps:

[0007] S1. Preprocess the zero-source-distance VSP data to obtain the vertical component profile, and then perform wavefield separation processing on the vertical component profile to obtain the down-field containing only down-going direct wave and free-interface down-going multiple wave information.

[0008] S2. Using the micrologging interpretation results near the wellhead, analyze the correspondence between the free interface downflow multiples and the near-surface structure in the downflow wave field, and determine the location of the free interface downflow multiples by combining the micrologging interpretation results.

[0009] S3. Pick up the downlink direct wave, correct and flatten the downlink wave field, and extract the downlink direct wave and the downlink multiple wave at the free interface according to a single cycle. After determining the depth point for estimating the near-surface Q factor, perform spectral analysis on the downlink direct wave and the downlink multiple wave at the free interface for a single cycle extracted at that depth point to obtain the amplitude spectrum data of the downlink direct wave and the downlink multiple wave at the free interface at that depth point.

[0010] S4. Fit the binomial equation using the spectral ratio method to obtain the attenuation slope K, and obtain the near-surface Q factor of the depth point selected in step S3.

[0011] S5. Under the same shot excitation, based on the number of stages and the stage spacing of the geophones in the well when acquiring VSP data at zero well source distance, determine other depth points in the well for the same shot excitation. Repeat steps S3 and S4 to obtain the near-surface Q factor for each other depth point in the well. After calculating the near-surface Q factor for each depth point, calculate the average value of the obtained near-surface Q factors, which is the near-surface Q factor at that well point.

[0012] As a limitation, the preprocessing of zero-well-source-spacing VSP data in step S1 includes: trace editing, component extraction, abnormal amplitude suppression, and random noise attenuation of the zero-well-source-spacing VSP data.

[0013] As a second limitation, in step S3, when determining the depth point for estimating the near-surface Q factor, the depth point is selected based on the principle that there are few interfering factors, the waveforms of the downlink direct wave and the downlink multiple wave at the free interface are stable, and the quality of the zero-well-source-distance VSP data is good.

[0014] As a third constraint, in step S4, the formula for fitting the binomial using the spectral ratio method is:

[0015]

[0016] Among them, A f0 For the amplitude of the downlink direct wave, A ft Δt is the amplitude of the downlink multiple wave at the free interface, Δt is the time difference between the downlink direct wave and the downlink multiple wave at the free interface, in seconds, and f is the frequency of the selected depth point.

[0017] Taking the natural logarithm of both sides of the formula, we obtain the attenuation slope K:

[0018]

[0019]

[0020] The near-surface Q-factor value at the selected depth point is:

[0021]

[0022] The present invention, by adopting the above-described technical solution, achieves the following technical advancements compared to existing technologies:

[0023] (1) This invention uses zero-source-distance VSP data to obtain downlink direct waves and free-interface downlink multiple waves, and realizes the near-surface Q factor. By using the same shot excitation, the same well geophone and the optimal near-surface depth point, the effects of wavelet inconsistency, geophone response function differences and low VSP data quality on the accuracy of the calculation are eliminated, and the accuracy of the near-surface Q factor value is improved.

[0024] (2) This invention expands the VSP technology in wells, from the existing method of using VSP data to obtain Q factor values ​​in the middle and deep layers to obtaining Q factors near the surface, thus realizing the purpose of using VSP data to obtain Q factor values ​​for the entire well section.

[0025] (3) This invention can be applied in multiple VSP wells, conforms to the characteristics of the surface Q factor value in the region, and provides guidance for the study of the absorption and attenuation law of the formation in the well area.

[0026] This invention belongs to the field of petroleum seismic exploration and is based on VSP data to obtain the near-surface Q factor. Attached Figure Description

[0027] Figure 1 This is a flowchart of a method according to an embodiment of the present invention;

[0028] Figure 2 This is a schematic diagram illustrating the principle of the method in an embodiment of the present invention;

[0029] Figure 3 This is the vertical component profile of the zero-well-source-pitch VSP data in this embodiment of the invention;

[0030] Figure 4 This is a schematic diagram of the downlink wavefield of zero-well-source-distance VSP data in an embodiment of the present invention;

[0031] Figure 5 This is a schematic diagram of downlink multiples at the free interface of zero-well-source-distance VSP data in an embodiment of the present invention.

[0032] Figure 6 This is a schematic diagram of the downlink direct wave extracted from zero-well-source-distance VSP data in an embodiment of the present invention;

[0033] Figure 7 This refers to the free-interface downlinked multiple wavelet sub-wave extracted from zero-well-pitch VSP data in this embodiment of the invention.

[0034] Figure 8 This is the amplitude spectrum of the downlink direct wave extracted from zero-well-source-distance VSP data in this embodiment of the invention;

[0035] Figure 9The amplitude spectrum of the free interface downflow multiple is extracted from the zero-well-source-distance VSP data in this embodiment of the invention.

[0036] Figure 10 This is the binomial fitting curve of the zero-well-source-distance VSP data spectral ratio method in this embodiment of the invention;

[0037] Figure 11 This refers to the multi-point average Q-factor value in the embodiments of the present invention;

[0038] Figure 12 This is the Q-factor curve for the entire well section in this embodiment of the invention. Detailed Implementation

[0039] To better explain and facilitate understanding of the present invention, the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0040] Example: A method for estimating near-surface Q-factors based on VSP data

[0041] This embodiment uses zero-well-source-distance VSP (Vertical Seismic Profile) data from the Songliao area as an example for illustration. Figure 1 and Figure 2 As shown, this embodiment includes the following steps:

[0042] S1. Preprocess the zero-source-distance VSP data to obtain the vertical component profile, and then perform wavefield separation processing on the vertical component profile to obtain the down-field containing only down-going direct wave and free-interface down-going multiple wave information.

[0043] In this step, the preprocessing of zero-well-spacing VSP data includes: trace editing, component extraction, anomalous amplitude, and random noise attenuation. The vertical component profile is obtained as follows: Figure 3 As shown, the obtained downlink wave field is as follows Figure 4 As shown. By Figure 4 It can be seen that after wave field separation of the vertical component profile, there is no interference from other waves, and the effect is good. The downlink direct wave and the downlink multiple wave at the free interface have strong energy and are clearly distinguishable.

[0044] S2. Using the micrologging interpretation results near the wellhead, analyze the correspondence between the free interface downflow multiples and the near-surface structure in the downflow wave field, and determine the location of the free interface downflow multiples by combining the micrologging interpretation results.

[0045] In this step, the micrologging interpretation results near the wellhead refer to the micrologging interpretation results at a distance relatively close to the wellhead; the free interface downlink multiples of the zero source distance VSP data are the waves that are generated by the seismic source and propagate upwards, then reflect downwards after encountering the free surface (ground and air) and are received by the geophone in the well. After propagating near the surface, the free interface downlink multiples carry near-surface absorption and attenuation information. Therefore, the near-surface absorption and attenuation can be studied by the energy changes of the free interface downlink multiples.

[0046] Downward multiples at free surfaces show a good correlation with the near-surface structure above the seismic source. Combined with micrologging interpretation results, the source strata of the downward multiples at free surfaces can be determined, allowing for the identification and determination of their location. For example... Figure 5 As shown, the micrologging interpretation results reveal three velocity interfaces (including the free interface between the ground and the air) above the excitation source. In the VSP downlink wave field, there are three strong reflection phase axes behind the downlink direct wave (labeled a in the figure). Based on the arrival time of the seismic waves at the geophone, the strong reflection arrow (labeled d in the figure) behind the downlink direct wave is determined to be a downlink multiple wave from the free interface. This wave is the seismic wave that propagates upward, is reflected downward by the free surface, and is received by the geophone in the well.

[0047] S3. Pick up the downlink direct wave, correct and flatten the downlink wave field, and extract the downlink direct wave and the downlink multiple wave at the free interface according to a single cycle. After determining the depth point for estimating the near-surface Q factor, perform spectral analysis on the downlink direct wave and the downlink multiple wave at the free interface for a single cycle extracted at that depth point to obtain the amplitude spectrum data of the downlink direct wave and the downlink multiple wave at the free interface at that depth point.

[0048] When determining the depth point for estimating the near-surface Q-factor, the selection is based on the principles of minimal interference factors, stable waveforms of the downlink direct wave and the free interface downlink multiple wave, and good quality of zero-source-distance VSP data. In this step, a depth of 1200 meters was selected, and spectral analysis was performed on the downlink direct wave and the free interface downlink multiple wave of a single cycle intercepted from the 1200-meter depth point to obtain amplitude spectrum data at the same depth.

[0049] Among them, the downlink direct wave and the downlink multiple waves of the free interface in a single cycle intercepted at this depth point are as follows: Figure 6 and Figure 7 As shown, the amplitude spectrum data of the downlink direct wave and the downlink multiple wave at the free interface at this depth point are as follows: Figure 8 and Figure 9 As shown.

[0050] S4. Fit the binomial equation using the spectral ratio method to obtain the attenuation slope K, and obtain the near-surface Q factor of the depth point selected in step S3.

[0051] In this step, the formula for fitting the binomial using the spectral ratio method is:

[0052]

[0053] Among them, A f0 For the amplitude of the downlink direct wave, A ft Δt is the amplitude of the downlink multiple wave at the free interface, Δt is the time difference between the downlink direct wave and the downlink multiple wave at the free interface, in seconds, and f is the frequency of the selected depth point.

[0054] Taking the natural logarithm of both sides of the formula, we obtain the attenuation slope K:

[0055]

[0056]

[0057] In the formula, by Figure 6 and Figure 7 The time difference Δt between the downlink direct wave and the downlink multiple wave at the free interface can be calculated to be 0.075 s. Figure 8 and Figure 9 It can be seen that A corresponds to different frequencies f. f0 and A ft Values, Δt, A f0 A ft Substituting f into the formula, we get Figure 10 Binomial fitting was performed within the frequency band of 22Hz-30Hz to obtain the formula:

[0058] y = -0.1168x + 2.2176.

[0059] The slope is thus obtained: K = -0.1168;

[0060] The near-surface Q-factor value at the selected depth point is then obtained as follows:

[0061]

[0062] Within the fitting frequency band of the binomial fitting method, the fitting interval is best selected as the symmetrical interval between the main frequency of the fitting frequency band at the selected depth point. This interval concentrates the main energy of the downlink direct wave and the downlink multiple wave amplitude at the free interface, and the obtained Q factor value is the most stable.

[0063] S5. Under the same shot excitation, based on the number of stages and the stage spacing of the geophone in the well when acquiring the zero well source distance VSP data, determine other depth points in the well for the same shot excitation. Repeat steps S3 and S4 to obtain the near-surface Q factor for other depth points in the well. After calculating the near-surface Q factor for each depth point, calculate the average value of the obtained near-surface Q factors, which is the near-surface Q factor at that well point.

[0064] In this step, the geophone in the well has 10 stages with an interval of 10m. Therefore, the depth points received in the well are 10 depth points between 1200m and 1290m. Ten near-surface Q-factors are calculated using data from each of the 10 depth points, and then the average of these 10 near-surface Q-factors is calculated. Figure 11 As shown, the near-surface Q-factor value of this well is 2.0;

[0065] Finally, by combining the near-surface Q factor at the well point obtained in step S5 with the intermediate-deep Q factor obtained at the well point, the Q factor curve of the entire well section from the surface to the intermediate-deep layer can be obtained.

[0066] In this step, the mid-deep Q factor is obtained using existing methods, namely, by using the VSP downlink direct wave to obtain it through the spectral ratio method.

[0067] like Figure 12 As shown, this is the Q-factor curve for the entire well section from the surface to the middle and deep layers; Figure 12 As can be seen, this embodiment fills the gap in obtaining near-surface Q factors using zero-source-distance VSP data, making VSP technology an effective means to study the absorption and attenuation law of the entire formation, and providing guidance for the study of formation absorption and attenuation law in well areas.

[0068] It should be noted that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art can still modify the technical solutions described in the above embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for estimating near-surface Q-factors based on VSP data, characterized in that, Includes the following steps: S1. Preprocess the zero-source-distance VSP data to obtain the vertical component profile, and then perform wavefield separation processing on the vertical component profile to obtain the down-field containing only down-going direct wave and free-interface down-going multiple wave information. S2. Using the micrologging interpretation results near the wellhead, analyze the correspondence between the free interface downflow multiples and the near-surface structure in the downflow wave field, and determine the location of the free interface downflow multiples by combining the micrologging interpretation results. S3. Pick up the downlink direct wave, correct and flatten the downlink wave field, and extract the downlink direct wave and the downlink multiple wave at the free interface according to a single cycle. After determining the depth point for estimating the near-surface Q factor, perform spectral analysis on the downlink direct wave and the downlink multiple wave at the free interface for a single cycle extracted at that depth point to obtain the amplitude spectrum data of the downlink direct wave and the downlink multiple wave at the free interface at that depth point. S4. Fit the binomial equation using the spectral ratio method to obtain the attenuation slope K, and obtain the near-surface Q factor of the depth point selected in step S3. S5. Under the same shot excitation, based on the number of stages and the stage spacing of the geophones in the well when acquiring VSP data at zero well source distance, determine other depth points in the well for the same shot excitation. Repeat steps S3 and S4 to obtain the near-surface Q factor for each other depth point in the well. After calculating the near-surface Q factor for each depth point, calculate the average value of the obtained near-surface Q factors, which is the near-surface Q factor at that well point.

2. The method for estimating near-surface Q-factors based on VSP data according to claim 1, characterized in that, In step S1, the preprocessing of zero-well-source-spacing VSP data includes: trace editing, component extraction, anomalous amplitude suppression, and random noise attenuation.

3. The method for estimating near-surface Q-factors based on VSP data according to claim 1, characterized in that, In step S3, when determining the depth point for estimating the near-surface Q factor, the depth point is selected based on the principle that there are few interfering factors, the waveforms of the downlink direct wave and the downlink multiple wave at the free interface are stable, and the quality of the zero-well-source-distance VSP data is good.

4. The method for estimating near-surface Q-factors based on VSP data according to claim 1, characterized in that, In step S4, the formula for fitting the binomial using the spectral ratio method is: Among them, A f0 For the amplitude of the downlink direct wave, A ft Δt is the amplitude of the downlink multiple wave at the free interface, Δt is the time difference between the downlink direct wave and the downlink multiple wave at the free interface, in seconds, and f is the frequency of the selected depth point. Taking the natural logarithm of both sides of the formula, we obtain the attenuation slope K: The near-surface Q-factor value at the selected depth point is:

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