Method for predicting seismic fractures in granite reservoir well

Abstract

The invention discloses a method for predicting seismic fractures in a granite reservoir well. The method comprises the following steps: S1, acquiring seismic data in the well; selecting an excitation point location by using an annular fixed well spacing VSP technology, and receiving the excitation point location through a three-component geophone in the well to obtain seismic data in the well; s2, processing seismic data in the well to obtain imaging data; the processing flow comprises time difference correction, first arrival pickup, three-component rotation processing, wave field separation processing and imaging processing; s3, analyzing the first arrival time obtained in the step S2, and predicting the fracture development direction; and S4, performing correlation calculation on the seismic imaging data in the well obtained in the step S2 to form coherent body data so as to predict the spatial distribution characteristics of the crack. According to the method, the development orientation and spatial distribution characteristics of the granite reservoir fracture can be predicted, an effective means is provided for predicting the fracture in the granite reservoir, and the method is suitable for predicting the seismic fracture in the granite reservoir.

Description

Technical Field

[0001] The present invention relates to a method for predicting seismic fractures, specifically a method for predicting in-well seismic fractures in granite reservoirs. Background Art

[0002] As one of the geothermal resource rock masses, the granite reservoir buried deep underground has broad development potential. To efficiently utilize the geothermal resources of the granite reservoir, it is necessary to evaluate the fluid flow and heat transfer channels in the granite reservoir and master the fracture orientation and fracture parameters in the granite reservoir. At present, the fracture prediction method is mainly completed based on surface seismic data. However, due to the weak wave impedance interfaces in most of the granite reservoir, it is difficult for surface seismic to obtain a fidelity effective reflection signal. At the same time, it is also difficult for surface seismic resolution in the deep layer to meet the prediction requirements of small fractures. Summary of the Invention

[0003] To solve the above deficiencies in the prior art, the present invention aims to provide a method for predicting in-well seismic fractures in granite reservoirs to achieve the purpose of predicting the development orientation and spatial distribution of fractures in granite reservoirs.

[0004] To achieve the above object, the technical solution adopted by the present invention is as follows: A method for predicting in-well seismic fractures in granite reservoirs, comprising the following steps: S1. In-well seismic data acquisition Using the annular fixed well-source distance VSP technology, select the excitation point positions, and receive through in-well three-component geophones to obtain in-well seismic data; S2. Process the in-well seismic data to obtain imaging data Process the in-well seismic data obtained in step S1. The processing flow includes time difference correction, first arrival picking, three-component rotation processing, wave field separation processing, and imaging processing, and finally obtain the processed first arrival time and imaging profile; S3. Predict the fracture development direction Analyze the first arrival time obtained in step S2, compare the first arrival times of all shot points with the same well-source distance received by the geophones at the same depth, and the direction with the shorter time is the fracture orientation; S4. Predict the spatial distribution characteristics of fractures Perform relevant calculations on the in-well seismic imaging data obtained in step S2 to form coherent body data, use coherent slices and rose diagrams to predict the development degree of fractures in different directions around the well at different depths, and use the coherent body data to predict the spatial distribution characteristics of fractures.

[0005] As a definition of the present invention: The basis for selecting the excitation points in step S1 has two aspects. One is the distance from the wellhead of the observation well, and the distance is between one-half and two-thirds of the well depth. The other is the azimuth, that is, the interval degree between two adjacent excitation points on the same horizontal plane is 10°, 15° or 20°.

[0006] As another definition of the present invention: The travel-time correction in step S2 includes excitation point elevation static correction and source-receiver distance correction; The first arrival picking means that, by using the method of man-machine interaction magnification, the position of the first arrival to be picked is magnified to a size that can meet the requirements of first arrival picking, ensuring that the time error between the picked first arrival time and the actual time is within 1 ms; The three-component rotation processing means that, based on the energy received by the three-component geophones, the azimuth angle and polarization angle are calculated, and the energy of the borehole seismic three-component geophones is redistributed to obtain the rotated borehole seismic data, which is then completed through Geoeast software; The borehole seismic wavefield separation processing method includes median filtering processing, frequency-wavenumber filtering processing and singular value decomposition filtering; The imaging processing uses single-shot migration imaging, and then all the imaging point gathers are interpolated into a three-dimensional volume to obtain the imaging data.

[0007] As another definition of the present invention: The first arrival time analysis, that is, in the first arrival times picked in step S2, the first arrival times with the same source-receiver distance of each level of geophones are extracted and arranged according to the azimuth corresponding to the excitation points. If it is circular, it means that the fractures are not developed in all directions. If it is elliptical, the short axis direction of the ellipse is the main development direction of the fractures.

[0008] As another definition of the present invention: The coherence volume data, that is, the correlation values between the imaging data in step S2 and the adjacent trace seismic data are calculated using the third-generation coherence algorithm, and the lateral continuity of the formation is reflected by the similarity of the seismic waveforms; The rose diagram, that is, by combining the ridge or valley shape index in the curvature shape index with the minimum curvature azimuth, the number of fractures in different azimuths in a given bin is statistically analyzed to form a spatial rose diagram, which is used to depict the fracture distribution. The length of the petals represents the fracture development density, and the direction represents the fracture development direction.

[0009] Due to the adoption of the above technical solutions, compared with the prior art, the beneficial effects achieved by the present invention are as follows: The present invention obtains coherent body data through steps such as collecting borehole seismic data, processing the borehole seismic data to obtain imaging data, predicting the development direction and spatial distribution characteristics of fractures, etc. The development degree of fractures in different directions around the well at different depths is predicted by using coherent slices and rose diagrams, and the spatial distribution characteristics of fractures are predicted by using coherent body data. Compared with predicting fractures in granite reservoirs by using surface seismic data, borehole seismic has a special observation method, which can effectively predict the fracture development around the well, and the geophones are placed in the well, and the fidelity and resolution of the received signals are generally higher than those of surface seismic. Therefore, the present invention applies annular fixed-source-offset borehole seismic to the fracture prediction of granite reservoirs, which can not only successfully predict the fracture strike and spatial distribution characteristics, but also improve the signal fidelity, and can effectively predict various small fractures in granite reservoirs.

[0010] In summary, the present invention can predict the development orientation and spatial distribution characteristics of fractures in granite reservoirs, provides an effective means for solving the fracture prediction in granite reservoirs, and is applicable to predicting seismic fractures in granite reservoirs. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0012] Figure 1 is the original borehole seismic three-component record of the embodiment of the present invention; where (a) is the downhole vertical component Z record, (b) is the downhole horizontal component H1 record, and (c) is the downhole horizontal component H2 record; Figure 2 is the trace gather of all shot points at the same depth before time difference correction in the embodiment of the present invention; where (a) is the trace gather record of the first circle, (b) is the trace gather record of the second circle, and (c) is the trace gather record of the third circle; Figure 3 is the trace gather of all shot points at the same depth after time difference correction in the embodiment of the present invention; where (a) is the trace gather record of the first circle, (b) is the trace gather record of the second circle, and (c) is the trace gather record of the third circle; Figure 4 is the first arrival time difference diagram of excitation points in different directions in the embodiment of the present invention; where (a) is the well section of 0 - 1000m, and (b) is the well section of 0 - 2000m; Figure 5 is the schematic diagram of the coherent body slice in the embodiment of the present invention; Figure 6 is the schematic diagram of the rose diagram in the embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0013] The preferred embodiments of the present invention will be described below with reference to the accompanying drawings. It should be understood that the method for predicting seismic fractures in a granite reservoir well described herein is a preferred embodiment, which is only used to illustrate and explain the present invention and does not constitute a limitation to the present invention.

[0014] Embodiment A method for predicting seismic fractures in a granite reservoir well This embodiment is a method for predicting seismic fractures in a granite reservoir well. The following will further describe this embodiment in detail with reference to the accompanying drawings. Figures 1 to 6 A further detailed description of this embodiment will be given below.

[0015] This embodiment includes the following steps: S1. Acquisition of borehole seismic data The annular fixed-offset VSP (Vertical Seismic Profiling) technology is one of the most effective geophysical means for fracture prediction. First, select the excitation points, use the surface P-wave source for excitation, and use the borehole three-component geophones in the observation section for reception to obtain the borehole seismic data. The observation section is moved here. It should be noted that in practical applications, the positions of the excitation points selected at different locations are different. Two factors need to be considered when selecting the excitation points. One is the distance from the wellhead of the observation well, and the distance is between one-half and two-thirds of the well depth. The other is the azimuth, that is, the interval angle between two adjacent excitation points on the same horizontal plane is 10°, 15°, or 20°. Each excitation point is numbered artificially, for example, 1001, 1002, …, and each number is a stake number. When using three-component geophones for acquisition, due to the limitation of the series, it needs to be divided into several sections for acquisition from shallow to deep, and each section is called a lift.

[0016] It should be noted that the excitation points of the annular fixed-offset VSP are arranged in a circle around the observation well with a fixed offset and equal angular intervals. It can be appropriately offset according to the on-site operation conditions, but the offset must meet the requirements specified by the industry standards. In order to reduce the randomness of the data, it is generally appropriate to collect 2 - 3 circles. In this embodiment, 3 circles are collected.

[0017] The observation section refers to the range of the observation well section that is as large as possible on the basis of including the target layer section to be studied. Generally, it is collected for the entire well section. In this embodiment, it is collected for the entire well section.

[0018] The obtained borehole seismic data is borehole three-component seismic data. The data of different lifts are sorted according to the stake number so that the data of each stake number are spliced together. Refer to Figure 1 , Figure 1 where the abscissa is the formation depth, with the unit of m; the ordinate is the recording time, with the unit of ms.

[0019] S2. Processing of borehole seismic data to obtain imaging data Process the borehole seismic data obtained in step S1. The processing flow includes time difference correction, first arrival picking, three-component rotation processing, wavefield separation processing, and imaging processing, and finally obtain the processed first arrival time and imaging profile.

[0020] Among them, the time difference correction includes shot point elevation static correction and well-source distance correction; the shot point elevation static correction is to uniformly correct the shot point elevation to the wellhead elevation using the replacement velocity, and the well-source distance correction is to correct the shot points with offset well-source distances caused by field operation conditions to the same well-source distance to ensure the elimination of the first arrival time difference caused by the observation system. See Figure 2 、 Figure 3 , Figure 2 and Figure 3 where the abscissa is the number of traces, with the unit of trace; the ordinate is the recording time, with the unit of ms.

[0021] For first arrival picking, during the first arrival picking process, use the method of man-machine interaction magnification to magnify the position of the first arrival to be picked to a size that can meet the requirements of first arrival picking, ensuring that the time error between the picked first arrival time and the actual time is within 1 ms. The accuracy of first arrival picking is crucial for predicting the trend of fractures in the follow-up.

[0022] The three-component rotation processing means calculating the azimuth angle and polarization angle based on the energy received by the three-component geophone, redistributing the energy of the borehole seismic three-component geophone, obtaining the rotated borehole seismic data, and then completing it through Geoeast software.

[0023] The wavefield separation processing methods include median filtering processing, linear filtering processing, frequency-wavenumber (FK) filtering processing, and singular value decomposition (SVD) filtering processing. For three-component data, after selecting one or several methods in combination according to the data situation, then use the vector rotation method according to the velocity difference between the P-wave and converted wave to extract the wavefield on different components to the greatest extent and ensure the fidelity of weak signals inside the granite reservoir; it should be noted that the combination of median filtering processing, frequency-wavenumber (FK) filtering processing, and singular value decomposition (SVD) filtering processing is used in the present invention.

[0024] Since the number of borehole seismic shot points with a circular fixed well-source distance is not large, it is difficult to obtain a good imaging effect for three-dimensional volume imaging. In the present invention, single-shot migration imaging is used for imaging processing, and then all imaging point gathers are interpolated into a three-dimensional volume.

[0025] S3. Predict the fracture development direction Analyze the first arrival time obtained in step S2, compare the first arrival times of all shot points with the same well-source distance received by the geophones at the same depth, and the direction with the shorter time is the fracture trend direction.

[0026] First arrival time analysis, that is, among the first arrival times picked up in step S2, extract the first arrival times with the same well-source distance for each level of geophones, and arrange them according to the azimuth corresponding to the shot point. If it is circular, it means that fractures are not well developed in all directions. If it is elliptical, the short axis direction of the ellipse is the main fracture development direction.

[0027] Specifically, to reduce the influence of the overlying formation of granite on the travel time of seismic waves, when calculating the first arrival time difference, the travel time difference of seismic waves at the same depth segment for different shot points can be calculated starting from a certain depth after the seismic waves enter the granite reservoir. Refer to Figure 4, where the unit in Figure 4 is m, and the color bar represents the travel time of seismic waves. It can be seen that the fracture development azimuth is mainly in the north-northeast direction.

[0028] S4. Predict the spatial distribution characteristics of fractures Perform relevant calculations on the borehole seismic imaging data obtained in step S2 to form coherent body data, and use coherent slices and rose diagrams to predict the development degree of fractures in different directions around the well at different depths, and use the coherent body data to predict the spatial distribution characteristics of fractures.

[0029] For coherent body data, use the third-generation coherent algorithm with strong anti-noise ability to calculate the correlation values between the imaging data in step S2 and the seismic data of adjacent traces, and reflect the lateral continuity of the formation through the similarity of seismic waveforms. Refer to Figure 5 , Figure 5 where the abscissa is the line number, dimensionless; the ordinate is the trace number, dimensionless.

[0030] For the rose diagram, that is, by combining the ridge or valley shape index in the curvature shape index with the minimum curvature azimuth, count the number of fractures in different azimuths in a given bin to form a spatial rose diagram to depict the fracture distribution. The length of the petal represents the fracture development density, and the direction represents the fracture development direction. The specific schematic diagram can be seen in Figure 6 , Figure 6 where the abscissa is the line number, dimensionless; the ordinate is the trace number, dimensionless. Figure 6 There is a petal in each square grid of Figure 6 , where the length refers to the length of the petal, and the direction refers to the direction of the petal, with due north as 0°, and rotating clockwise. It can be seen from

[0031] Due to the special observation method of borehole seismic, it can effectively predict the fracture development around the well, and the geophones are placed in the well, and the signal fidelity and resolution of the received signals are generally higher than those of surface seismic. The present invention applies the annular fixed well-source distance borehole seismic to the fracture prediction of granite reservoirs, and successfully predicts the fracture strike and spatial distribution characteristics, providing an effective means for solving the problem of fracture prediction in granite reservoirs.

[0032] The above are only alternative embodiments of the present application and are not intended to limit the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present application shall be included within the protection scope of the present application.

Claims

1. A method for predicting seismic fractures in a granite reservoir well, characterized in that, it includes the following steps: S1. Acquisition of downhole seismic data Using the annular fixed well-source distance VSP technology, select the excitation points, and receive through the downhole three-component geophones to obtain downhole seismic data; S2. Processing of downhole seismic data to obtain imaging data Process the downhole seismic data obtained in step S1. The processing flow includes time difference correction, first arrival picking, three-component rotation processing, wave field separation processing, and imaging processing. Finally, the processed first arrival time and imaging profile are obtained; S3. Predict the fracture development direction Analyze the first arrival time obtained in step S2, and compare the first arrival times of all shot points with the same well-source distance received by the geophones at the same depth. The direction with the shorter time is the trend of the fracture; S4. Predict the spatial distribution characteristics of fractures Perform relevant calculations on the downhole seismic imaging data obtained in step S2 to form coherent body data. Use coherent slices and rose diagrams to predict the development degree of fractures in different directions around the well at different depths, and use the coherent body data to predict the spatial distribution characteristics of fractures.

2. The method for predicting seismic fractures in a granite reservoir well according to claim 1, characterized in that, There are two aspects in the basis for selecting the excitation points in step S1. One is the distance from the wellhead of the observation well, and the distance is between one-half and two-thirds of the well depth; the other is the azimuth, that is, the interval degrees between two adjacent excitation points on the same horizontal plane are 10°, 15°, or 20°.

3. The method for predicting seismic fractures in a granite reservoir well according to claim 1, characterized in that, The time difference correction in step S2 includes excitation point elevation static correction and well-source distance correction; First arrival picking means that by using the method of man-machine interactive magnification, the position of the first arrival to be picked is magnified to a size that can meet the requirements of first arrival picking, ensuring that the error between the picked first arrival time and the actual time is within 1 ms; The three-component rotation processing means that according to the energy received by the three-component geophones, calculate the azimuth angle and polarization angle, re-distribute the energy of the downhole seismic three-component geophones to obtain the rotated downhole seismic data, and then complete it through Geoeast software; The method for separating the downhole seismic wave field includes median filtering processing, frequency-wave number filtering processing, and singular value decomposition filtering; For imaging processing, use single-shot migration imaging, and then interpolate all imaging point gathers into a three-dimensional volume to obtain imaging data.

4. The method for predicting seismic fractures in a granite reservoir well according to claim 1, characterized in that, Analysis of the first arrival time, that is, in the first arrival time picked in step S2, extract the first arrival times of the same well-source distance of each level of geophones, and arrange them according to the azimuth corresponding to the excitation points. If it is circular, it means that the fractures have not developed in all directions. If it is elliptical, the short axis direction of the ellipse is the main fracture development direction.

5. The method for predicting seismic fractures in a granite reservoir well according to claim 1, characterized in that, Coherent body data, that is, use the third-generation coherent algorithm to calculate the correlation values between the imaging data in step S2 and the adjacent trace seismic data, and reflect the lateral continuity of the formation through the similarity of seismic waveforms; Rose diagram, that is, by combining the ridge or valley form index in the curvature form index with the minimum curvature azimuth, the number of fractures in different azimuths in a given bin is statistically counted to form a spatial rose diagram, which is used to depict the fracture distribution. The length of the petal represents the fracture development density, and the direction represents the fracture development direction.

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