A method for predicting seismic fractures in a well in a granite reservoir

Abstract

The application discloses a kind of granite reservoir well in seismic crack prediction method, comprising the following steps: S1.well seismic data acquisition;Using annular fixed well source distance VSP technology, select shot point, receive by well three-component geophone, obtain well seismic data;S2.well seismic data processing, obtain imaging data;Processing flow includes time difference correction, first arrival picking, three-component rotation processing, wave field separation processing, imaging processing;S3.the first arrival time obtained in step S2 is analyzed, and the development direction of crack is predicted;S4.correlation calculation is carried out to the well seismic imaging data obtained in step S2, and coherent body data is formed to predict the spatial distribution characteristics of crack.The application can predict the development direction and spatial distribution characteristics of granite reservoir cracks, provide an effective means for solving crack prediction in granite reservoir, and be suitable for predicting seismic cracks in granite reservoir.

Description

Technical Field

[0001] This invention relates to a method for predicting seismic fractures, specifically a method for predicting seismic fractures in granite reservoir wells. Background Technology

[0002] Deeply buried granite reservoirs, as one type of geothermal resource rock mass, have broad development potential. To efficiently utilize the geothermal resources of granite reservoirs, it is necessary to evaluate the flow of fluids and heat transfer channels within the reservoirs, and to understand the fracture orientation and parameters. Currently, fracture prediction methods are mainly based on surface seismic data. However, due to the weak acoustic impedance interfaces within most granite reservoirs, surface seismic data struggles to obtain high-fidelity, effective reflection signals. Furthermore, the resolution of surface seismic data at depth is insufficient to predict small fractures. Summary of the Invention

[0003] To address the shortcomings of existing technologies, this invention aims to provide a method for predicting seismic fractures in granite reservoir wells, in order to predict the development orientation and spatial distribution of fractures in granite reservoirs.

[0004] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A method for predicting seismic fractures in granite reservoir wells includes the following steps: S1. Well-drilled seismic data acquisition Using the ring-shaped fixed well source distance VSP technology, the excitation point is selected, and the well seismic data is obtained by receiving it through a three-component geophone in the well. S2. Processing of seismic data in the well to obtain imaging data. The well seismic data obtained in step S1 is processed. The processing flow includes time difference correction, first arrival picking, three-component rotation processing, wavefield separation processing, and imaging processing, and finally the processed first arrival time and imaging profile are obtained. S3. Predicting the direction of crack development Analyze the first arrival times obtained in step S2, and compare the first arrival times of all shot points at the same well source distance received by the geophone at the same depth. The direction with the shorter time is the direction of the fracture. S4. Predicting the spatial distribution characteristics of cracks The well seismic imaging data obtained in step S2 are used to perform relevant calculations to form coherence data. Coherence slices and rose diagrams are used to predict the degree of development of fractures in different directions around the well at different depths. The spatial distribution characteristics of fractures are predicted using coherence data.

[0005] As a limitation of the present invention: the selection of the excitation point in step S1 is based on two aspects. One is the distance from the wellhead of the observation well, which is between one-half and two-thirds of the well depth. The other is the orientation, that is, the interval between two adjacent excitation points on the same horizontal plane is 10°, 15° or 20°.

[0006] As another limitation of the present invention: the time difference correction in step S2 includes static correction of the excitation point elevation and well-source distance correction; First arrival pickup refers to using human-computer interaction to magnify the first arrival position to be picked up to a size that meets the first arrival pickup requirements, ensuring that the error between the first arrival pickup time and the actual time is within 1ms. Three-component rotation processing refers to calculating the azimuth and polarization angles based on the energy received by the three-component geophones, redistributing the energy of the three-component geophones in the well to obtain rotated well seismic data, which is then processed using Geoeast software. Methods for separating and processing seismic wavefields in wells include median filtering, frequency-wavenumber filtering, and singular value decomposition filtering; Imaging processing uses single-shot offset imaging, and then uses all imaging point gathers to interpolate into a three-dimensional volume to obtain imaging data.

[0007] As another limitation of the present invention: first arrival time analysis, that is, in the first arrival time picked up in step S2, the first arrival time of the same well source distance of each level detector is extracted and arranged according to the orientation corresponding to the excitation point. If it is circular, it means that the fracture has not developed in any direction. If it is elliptical, the direction of the minor axis of the ellipse is the main development direction of the fracture.

[0008] As another limitation of the present invention: coherent volume data, that is, using the third generation coherent algorithm to calculate the correlation value between the imaging data in step S2 and the seismic data of adjacent traces, and reflecting the lateral continuity of the strata through the similarity of the seismic waveforms; A rose diagram is a spatial rose diagram formed by combining the ridge or valley morphology index in the curvature morphology index with the minimum curvature orientation to count the number of cracks in different orientations in a given surface element. The length of the petals represents the crack development density, and the direction represents the crack development direction.

[0009] By adopting the above technical solution, the beneficial effects achieved by the present invention compared with the prior art are as follows: This invention involves several steps, including acquiring borehole seismic data, processing the data to obtain imaging data, and predicting fracture development direction and spatial distribution characteristics. Ultimately, coherence data is obtained. Coherence slices and rose diagrams are used to predict the development degree of fractures at different depths and directions around the well, and the spatial distribution characteristics of fractures are predicted using coherence data. Compared to using surface seismic data for fracture prediction in granite reservoirs, borehole seismic data offers a unique observation method that can effectively predict fracture development around the well. Furthermore, with the detector placed in the well, the fidelity and resolution of the received signal are generally higher than those of surface seismic data. Therefore, this invention applies annular fixed-source-distance borehole seismic data to fracture prediction in granite reservoirs. This not only successfully predicts fracture orientation and spatial distribution characteristics but also improves signal fidelity and effectively predicts various small fractures in granite reservoirs.

[0010] In summary, this invention can predict the development orientation and spatial distribution characteristics of fractures in granite reservoirs, providing an effective means for predicting fractures in granite reservoirs, and is applicable to predicting seismic fractures in granite reservoirs. Attached Figure Description

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

[0012] Figure 1 The following is a record of the three components of seismic data in a well according to an embodiment of the present invention; wherein (a) is the record of the vertical component Z in the well, (b) is the record of the horizontal component H1 in the well, and (c) is the record of the horizontal component H2 in the well. Figure 2 The following are gathers of all shot points at the same depth before time difference correction in this embodiment of the invention; where (a) is the gather record of the first circle, (b) is the gather record of the second circle, and (c) is the gather record of the third circle. Figure 3 The following are the gathers of all shot points at the same depth after time difference correction according to an embodiment of the present invention; where (a) is the gather record of the first circle, (b) is the gather record of the second circle, and (c) is the gather record of the third circle. Figure 4 shows the first arrival time difference of the excitation points at different azimuths in the embodiment of the present invention; where (a) is the 0-1000m well section and (b) is the 0-2000m well section; Figure 5 This is a schematic diagram of a coherent slice according to an embodiment of the present invention; Figure 6 This is a schematic diagram of a rose diagram in an embodiment of the present invention. Detailed Implementation

[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 granite reservoir wells described herein is a preferred embodiment and is only used for illustration and explanation of the present invention, and does not constitute a limitation thereof.

[0014] Example: A method for predicting seismic fractures in granite reservoir wells This embodiment presents a method for predicting seismic fractures in granite reservoir wells. The following will be combined with the attached... Figures 1-6 This embodiment will be described in further detail.

[0015] This embodiment includes the following steps: S1. Well-drilled seismic data acquisition The ring-type fixed-source-distance wellbore seismic technique is one of the most effective geophysical methods for fracture prediction. First, an excitation point is selected, and surface P-wave sources are used to generate seismic data. The observation well section then receives the data using a three-component geophone, obtaining the seismic data. The observation well section is then moved to this location. It should be noted that the location of the excitation point varies depending on the specific application. Two factors need to be considered when selecting the excitation point: first, the distance from the wellhead, which should be between one-half and two-thirds of the well depth; and second, the azimuth, meaning that the interval between two adjacent excitation points on the same horizontal plane should be 10°, 15°, or 20°. Each excitation point is manually numbered, for example, 1001, 1002, ..., each number representing a station number. When acquiring data using a three-component geophone, due to limitations in the number of stages, the data needs to be acquired in several segments from shallow to deep, with each segment constituting a single acquisition.

[0016] It should be noted that the excitation point of the Walkaround VSP is centered on the observation well, and the fixed well-source distance and equal angular intervals are arranged around the well. It can be appropriately offset according to the field operation conditions, but it must meet the offset amount specified by the industry standard. In order to reduce the randomness of the data, it is generally advisable to collect 2-3 circles. In this embodiment, 3 circles are collected.

[0017] An observation well section refers to the observation well section that includes the target layer being studied, and the scope of the observation well section should be expanded as much as possible. Generally, it involves collecting data from the entire well section. In this example, the entire well section is collected.

[0018] The obtained wellbore seismic data are three-component wellbore seismic data. The data from different extraction points are organized according to station number, and the data from each station are pieced together. (See [link to documentation]). Figure 1 , Figure 1 The horizontal axis represents the formation depth in meters (m); the vertical axis represents the recording time in millimeters (ms).

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

[0020] The time difference correction includes static excitation point elevation correction and well-source distance correction. Static excitation point elevation correction uses a replacement velocity to uniformly correct the excitation point elevation to the wellhead elevation. Well-source distance correction corrects excitation points whose well-source distance is offset due to field operation conditions to the same well-source distance, ensuring the elimination of first arrival time differences caused by the observation system. See [link to relevant documentation]. Figure 2 , Figure 3 , Figure 2 and Figure 3 The horizontal axis represents the number of channels, in channels; the vertical axis represents the recording time, in milliseconds (ms).

[0021] First arrival pickup: During the first arrival pickup process, a human-computer interaction zoom-in method is used to magnify the first arrival position to be picked up to a size that meets the first arrival pickup requirements, ensuring that the error between the first arrival pickup time and the actual time is within 1ms. The accuracy of the first arrival pickup is crucial for subsequent prediction of crack direction.

[0022] Three-component rotation processing refers to calculating the azimuth and polarization angles based on the energy received by the three-component geophones, redistributing the energy of the three-component geophones in the well to obtain rotated well seismic data, which is then processed using Geoeast software.

[0023] The wavefield separation processing method includes median filtering, linear filtering, frequency-wavenumber (FK) filtering, and singular value decomposition (SVD) filtering. For three-component data, one or more methods are selected and combined according to the data conditions. Then, a vector rotation method is used based on the velocity difference between the P-wave and the converted wave to extract the wavefield of different components to the greatest extent and ensure the fidelity of weak signals inside the granite reservoir. It should be noted that the present invention uses a combination of median filtering, frequency-wavenumber (FK) filtering, and singular value decomposition (SVD) filtering.

[0024] Since the number of seismic shot points in a ring-shaped fixed well source distance is not large, it is difficult to achieve good imaging results in three-dimensional volume imaging. However, the imaging processing of this invention uses single-shot migration imaging, and then uses all imaging point gathers to interpolate into a three-dimensional volume.

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

[0026] First arrival time analysis involves extracting the first arrival times of each detector at the same well source distance from the first arrival times picked up in step S2, and arranging them according to the orientation corresponding to the excitation point. If it is circular, it indicates that the fractures are not well developed in all directions. If it is elliptical, then the direction of the minor axis of the ellipse is the main direction of fracture development.

[0027] In particular, to reduce the influence of the overlying strata of granite on the travel time of seismic waves, when calculating the first arrival time difference, the seismic wave travel time difference at different excitation points in the same depth segment can be calculated starting from a certain depth after the seismic wave enters the granite reservoir. See Figure 4, where the unit is m and the colored bars represent the travel time of the seismic wave. It can be seen that the fracture development direction is mainly north-northeast.

[0028] S4. Predicting the spatial distribution characteristics of cracks The well seismic imaging data obtained in step S2 are used to perform relevant calculations to form coherence data. Coherence slices and rose diagrams are used to predict the degree of development of fractures in different directions around the well at different depths. The spatial distribution characteristics of fractures are predicted using coherence data.

[0029] For coherent volume data, the correlation values ​​between the imaging data and adjacent seismic traces in step S2 are calculated using a robust third-generation coherent algorithm. The lateral continuity of the strata is reflected by the similarity of the seismic waveforms. (See [link to relevant documentation]). Figure 5 , Figure 5 The horizontal axis represents the line number, which is dimensionless; the vertical axis represents the track number, which is dimensionless.

[0030] A rose diagram, which combines the ridge or valley morphology index of curvature morphology with the minimum curvature orientation, counts the number of cracks in different orientations within a given surface element to form a spatial rose diagram. This diagram is used to characterize crack distribution. The length of the petals represents crack development density, and the direction represents crack development direction. See the diagram for a detailed illustration. Figure 6 , Figure 6 The horizontal axis represents the line number, which is dimensionless; the vertical axis represents the track number, which is dimensionless. Figure 6 Each square grid contains a petal, where length refers to the petal's length and direction refers to the petal's orientation, rotating clockwise with true north as 0°. Figure 6 It is evident that the main crack developed in a north-northeast direction.

[0031] Because well-drilled seismic surveys have a unique observation method, they can effectively predict the development of fractures around the well. Furthermore, since the detectors are placed in the well, the fidelity and resolution of the received signals are generally higher than those of surface seismic surveys. This invention applies well-drilled seismic surveys with a fixed well source distance to the prediction of fractures in granite reservoirs. It has successfully predicted the fracture orientation and spatial distribution characteristics, providing an effective means to solve the problem of fracture prediction in granite reservoirs.

[0032] The above description is merely an optional embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A method for predicting seismic fractures in granite reservoir wells, characterized in that, Includes the following steps: S1. Well-drilled seismic data acquisition Using the ring-shaped fixed well source distance VSP technology, the excitation point is selected, and the well seismic data is obtained by receiving it through a three-component geophone in the well. S2. Processing of seismic data in the well to obtain imaging data. The well seismic data obtained in step S1 is processed. The processing flow includes time difference correction, first arrival picking, three-component rotation processing, wavefield separation processing, and imaging processing, and finally the processed first arrival time and imaging profile are obtained. S3. Predicting the direction of crack development Analyze the first arrival times obtained in step S2, and compare the first arrival times of all shot points at the same well source distance received by the geophone at the same depth. The direction with the shorter time is the direction of the fracture. S4. Predicting the spatial distribution characteristics of cracks The well seismic imaging data obtained in step S2 are used to perform relevant calculations to form coherence data. Coherence slices and rose diagrams are used to predict the degree of development of fractures in different directions around the well at different depths. The spatial distribution characteristics of fractures are predicted using coherence data. The selection of the excitation point in step S1 is based on two aspects: first, the distance from the wellhead of the observation well, which is between one-half and two-thirds of the well depth; second, the azimuth, that is, the interval between two adjacent excitation points on the same horizontal plane is 10°, 15° or 20°. Imaging processing uses single-shot offset imaging, and then uses all imaging point gathers to interpolate into a three-dimensional volume to obtain imaging data; First arrival time analysis involves extracting the first arrival times of each detector at the same well source distance from the first arrival times picked up in step S2, and arranging them according to the orientation corresponding to the excitation point. If they are circular, it means that the fractures have not developed in any direction. If they are elliptical, the direction of the minor axis of the ellipse is the main direction of fracture development.

2. The method for predicting seismic fractures in granite reservoir wells according to claim 1, characterized in that, Step S2 time difference correction includes static correction of the excitation point elevation and well-source distance correction; First arrival pickup refers to using human-computer interaction to magnify the first arrival position to be picked up to a size that meets the first arrival pickup requirements, ensuring that the error between the first arrival pickup time and the actual time is within 1ms. Three-component rotation processing refers to calculating the azimuth and polarization angles based on the energy received by the three-component geophones, redistributing the energy of the three-component geophones in the well to obtain rotated well seismic data, which is then processed using Geoeast software. Methods for separating and processing seismic wavefields in wells include median filtering, frequency-wavenumber filtering, and singular value decomposition filtering.

3. The method for predicting seismic fractures in granite reservoir wells according to claim 1, characterized in that, Coherent volume data refers to the correlation values ​​between the imaging data in step S2 and the seismic data of adjacent traces calculated using the third generation coherent algorithm. The lateral continuity of the strata is reflected by the similarity of the seismic waveforms. A rose diagram is a spatial rose diagram formed by combining the ridge or valley morphology index in the curvature morphology index with the minimum curvature orientation to count the number of cracks in different orientations in a given surface element. The length of the petals represents the crack development density, and the direction represents the crack development direction.

Images