CN-116449429-B - Method for evaluating crack permeability based on stoneley wave high-low frequency reflection coefficient and attenuation

CN116449429BCN 116449429 BCN116449429 BCN 116449429BCN-116449429-B

Abstract

The invention discloses a method for evaluating crack permeability based on stoneley wave high-low frequency reflection coefficient and attenuation, which comprises the following steps of preprocessing an array acoustic waveform, separating CSG data combination of each depth point to obtain up-down traveling wave CSG combination, respectively combining the up-down traveling wave with the down traveling wave to form up-down traveling wave COG data, separating the up-down traveling wave COG data to obtain direct wave, up-down reflected wave, residual direct wave, respectively calculating the reflection coefficient average value of a high frequency band and a low frequency band, calculating and normalizing the difference between the high frequency band and the low frequency band, carrying out CSG attenuation calculation of each depth point after data preprocessing, calculating and normalizing the difference between the high frequency band and the low frequency band, multiplying the difference between the normalized reflection coefficient and the attenuation, and defining the difference as a crack seepage factor for evaluating the crack permeability. The invention can solve the technical problem of detecting permeability structure in stratum.

Inventors

CUI YUNJIANG
MING JUN
WANG PEICHUN
XIONG LEI
ZHANG RENFENG
WANG RUIHONG
LI RUIJUAN
MA CHAO
QI YI

Assignees

中海石油(中国)有限公司
中海石油（中国）有限公司天津分公司

Dates

Publication Date: 20260505
Application Date: 20221018

Claims (4)

1. The method for evaluating crack permeability based on stoneley wave high-low frequency reflection coefficient and attenuation comprises the following steps of firstly, preprocessing an array acoustic wave waveform by adopting band-pass filtering to eliminate random noise interference; Performing wave field separation based on linear prediction on CSG data combinations of each depth point to obtain an uplink wave CSG combination and a downlink wave CSG combination; Step three, the upstream wave and the downstream wave are respectively combined into upstream COG data and downstream COG data, and the upstream COG data are subjected to wave field separation based on linear prediction to obtain direct waves and upstream reflected waves; Calculating the reflection coefficient average value of the high frequency band and the reflection coefficient average value of the low frequency band by using the direct wave and the downlink reflection wave which are obtained by separation to obtain a high-frequency reflection coefficient and a low-frequency reflection coefficient, and calculating and normalizing the difference between the high-frequency reflection coefficient and the low-frequency reflection coefficient; Step five, after finishing data preprocessing, carrying out high-frequency-band average attenuation and low-frequency-band average attenuation calculation on CSG data of each depth point, calculating and normalizing the difference between the high-frequency-band average attenuation and the low-frequency-band average attenuation; step six, multiplying the normalized difference between the high-frequency reflection coefficient and the low-frequency reflection coefficient and the difference between the average attenuation of the high frequency band and the average attenuation of the low frequency band to define a crack seepage factor for evaluating the crack permeability of the stratum; The second and third steps are specifically as follows: The wave field separation method based on linear prediction is based on the following principle: in a direct wave of a well bore with a dispersive effect, slowness can be expressed as The wellbore direct wave signal of the nth receiver may be expressed as: (2) In the formula, For each vibration mode frequency spectrum, L is the number of vibration modes, z is the distance from the sound source to the receiver, and the direct wave of the shaft is transmitted from the nth receiver to the mth receiver and is expressed as (3) According to the linear prediction theory, the matrix form of the linear equation set for solving various direct wave spectrums is as follows: (4) Wherein the method comprises the steps of D is the receiver spacing, Is the actual data spectrum, and the formula 4 is about equal and expressed as a matrix G . A = W (5) The direct wave type is usually smaller than the number of array receivers, i.e. L < N, so only the least squares solution of a can be calculated, which is obtained by: (6) Wherein the method comprises the steps of The complex conjugate transpose of G is adopted, epsilon is a damping factor, and I is a unit matrix; The wave field separation is divided into two steps by utilizing the linear prediction method, wherein the first step is to separate CSG data combination, and the second step is to separate COG data; for each depth point, arranging data into CSG data combination according to a receiver array, wherein the CSG data combination comprises an uplink wave and a downlink wave, the uplink wave comprises a direct wave from a source to the receiver and a reflected wave reflected from a lower reflector, the downlink wave is a wave reflected from an upper reflector, the time offset of the two uplink waves is identical and is equal to stoneley wave slowness "+s", the time shift of the downlink wave is negative, the wave propagation in the array is regarded as apparent slowness "-s"; After the upstream wave and the downstream wave are separated, respectively taking data on the intermediate receiver at each depth, and combining the data into upstream COG data and downstream COG data, wherein the upstream COG data comprises a direct wave with the slowness of 0 and an upstream reflected wave with the slowness of +2s, and the downstream wave comprises a residual direct wave with the slowness of 0 and a downstream reflected wave with the slowness of +2s, and the two groups of data are respectively separated to obtain the direct wave and the downstream reflected wave required by calculation; the fifth step is specifically as follows: And (3) combining the data of the first receiver obtained in the step (A) into COG data combination, and calculating the stoneley wave attenuation by linear fitting, wherein the principle is as follows: Assuming that the amplitude spectrum of the waveform received by the receiver with depth z satisfies equation 8 Wherein S (w) and R (w) represent the frequency spectra of the sound source and the receiver, respectively; A green's function for controlling the geometric diffusion of sound waves from a source to a receiver, T being the propagation time, Q being the quality factor; the spectral ratio at different depths z1, z2 satisfies equation 9 Where f is the frequency and V is the wave velocity, it is assumed that the geometric diffusion G is only depth dependent and frequency independent, so that Treated as a constant independent of frequency, then at the same frequency the natural logarithm of the spectral ratio can be made Linear fitting with distance (z 2-z 1) between two receivers to obtain linear slope I.e. defined as the attenuation factor; for acoustic logging tools, since there are multiple receivers, a linear relationship exists between any two receivers, satisfying equation 10, Wherein the method comprises the steps of For attenuation coefficients within the receiver array span, numerically can be found by a least squares linear fit method, thereby representing stoneley wave attenuation; The average attenuation of the low frequency band and the average attenuation of the high frequency band are calculated respectively according to the frequency dependence characteristic of stoneley wave attenuation, and when the average attenuation of the low frequency band is obviously higher than the average attenuation of the high frequency band, the attenuation is likely to be caused by the permeability crack.
2. The method for evaluating crack permeability based on stoneley wave high-low frequency reflection coefficient and attenuation according to claim 1, wherein the step one specifically comprises: The noise frequency has obvious difference with the Stoneley frequency, the noise can be filtered by adopting a band-pass filter, and the response function of the windowed band-pass filter is as follows: (1) 。
3. The method for evaluating crack permeability based on stoneley wave high-low frequency reflection coefficient and attenuation according to claim 1, wherein the fourth step is specifically as follows: Calculating the ratio of the frequency spectrum of the downlink reflected wave to the direct wave for each depth point to obtain a reflection coefficient which changes along with the frequency, and respectively calculating the average value of the reflection coefficients in 0-1kHz and 1-2kHz to obtain a low-frequency reflection coefficient and a high-frequency reflection coefficient; Theory of calculating stoneley wave reflection Wherein R is a reflection coefficient, z is a fracture zone thickness, the fracture zone may comprise one or more fractures perpendicular or oblique to the well axis, kz and k0 are stoneley wave numbers of the fracture zone and the formation, respectively; The reflection coefficient of the permeability crack calculated by the theory shows the characteristic of increasing along the direction of low frequency along the change of frequency, and the average value of the reflection coefficients of the low frequency and the high frequency is calculated according to the frequency dependence characteristic of the stoneley wave reflection coefficient, when the reflection coefficient of the low frequency is obviously higher than the reflection coefficient of the high frequency, the reflection is caused by the permeability crack.
4. The method for evaluating crack permeability based on stoneley wave high-low frequency reflection coefficient and attenuation according to claim 1, wherein the sixth step is as follows: Multiplying the normalized difference between the reflection coefficients and the attenuation difference to define a crack seepage factor, which is defined as ( 11) When the fracture seepage factor is high, the fracture seepage capability of the stratum is strong, and conversely, the fracture seepage factor is weak.

Description

Method for evaluating crack permeability based on stoneley wave high-low frequency reflection coefficient and attenuation Technical Field The invention belongs to the field of well geophysics, and particularly relates to a method for evaluating permeability of a crack by utilizing low-frequency and high-frequency reflection coefficients and attenuation differences of stoneley waves. Background Fractures are the passage of oil and gas into the wellbore and are therefore important for the characterization of reservoir fractures. Ultrasonic/acoustic techniques commonly used for fracture characterization are ultrasonic borehole imaging, stoneley wave reflection (e.g., hornby et al, 1989; tang and Cheng, 1993; kostek et al, 1998) and cross dipole anisotropy (e.g., joyce et al, 1998; tang and Patterson, 2001) measurements and analysis. While all of these techniques have proven to be effective in detecting cracks, distinguishing crack responses from other effects remains a difficult task. Stoneley waves are low-frequency boundary waves which propagate along a well wall, have large amplitude, have a wave speed slightly lower than the sound velocity of fluid in the well, are sensitive to the change of stratum properties, and are affected by the change of stratum parameters caused by the heterogeneity of the stratum and are reflected on the change of the reflection coefficient of the stoneley waves. When a borehole stoneley wave encounters a fracture passing through the borehole, a portion of the wave reflects off the fracture due to hydraulic interactions caused by the wave between the borehole and the fracture, and fracture density, opening, etc. all have an effect on stoneley wave reflection. Thus, stoneley wave reflection coefficients are generally used as a means of identifying fractures and formation evaluation. However, current stoneley wave reflection techniques used to characterize fractures have difficulty distinguishing stoneley wave reflections caused by formation boundaries and borehole variations. If stoneley wave reflection information can be further mined, reflection caused by factors such as stratum boundaries, borehole changes and the like can be distinguished, and support and basis are provided for permeability evaluation of cracks. Disclosure of Invention The invention aims to provide a method for evaluating fracture permeability based on stoneley wave high-low frequency reflection coefficient and attenuation, which aims to solve the technical problem of detecting permeability structures in stratum by calculating stoneley wave reflection coefficients and attenuation in different frequency ranges. In order to realize the method, the invention adopts the following treatment scheme: step one, preprocessing the array acoustic waveform by adopting band-pass filtering to eliminate random noise interference. And step two, performing wave field separation based on linear prediction on CSG data combinations of each depth point to obtain an upstream CSG combination and a downstream CSG combination. And thirdly, respectively combining the upstream wave and the downstream wave into upstream wave COG data and downstream wave COG data. And performing wave field separation based on linear prediction on the downstream COG data to obtain a downstream reflected wave and a residual direct wave. And step four, respectively calculating the average value of the reflection coefficients of the high frequency band and the low frequency band by using the direct wave and the downlink reflection wave which are obtained through separation, and calculating and normalizing the difference of the high frequency reflection coefficient and the low frequency reflection coefficient. And step five, after finishing data preprocessing, carrying out high-frequency band and low-frequency band attenuation calculation on CSG of each depth point, calculating the difference between the high-frequency attenuation and the low-frequency attenuation, and normalizing. And step six, multiplying the difference between the normalized reflection coefficients and the difference between the attenuation, and defining the multiplied reflection coefficients as fracture seepage factors for evaluating the fracture permeability of the stratum. The first step is specifically as follows: the noise frequency has obvious difference from the Stoneley frequency, and the band-pass filter can be used for filtering the noise. The windowed bandpass filter response function is: The second and third steps are specifically as follows: The wave field separation method based on linear prediction is based on the following principle: In the wellbore direct wave with the dispersion effect, the slowness may be represented as S l (ω), and the wellbore direct wave signal of the nth receiver may be represented as: where a l (ω) is the spectrum of each mode, L is the number of modes, and z is the distance from the sound source to the receiver. The specific type of wellbore direct w