A method and system for azimuthal seismic inversion of stress-level anisotropic media

Abstract

The present invention discloses a method and system for azimuthal seismic inversion of stress-anisotropic media, relating to the field of seismic reservoir identification. The method comprises: establishing a HTI medium model under linear slip boundary conditions and applying a small uniaxial stress parallel to the axis of symmetry to the HTI medium model; decoupling the stiffness tensor induced by cracks and stress in the horizontal uniaxially stressed HTI medium; establishing a PP-wave AVAZ approximate equation for decoupling the anisotropic parameters induced by cracks and stress; and performing PP-wave AVAZ inversion using the difference in reflection coefficient amplitudes between different azimuths to form an azimuthal seismic inversion scheme for stress-anisotropic media. The method utilizes nonlinear acoustoelasticity theory to construct the effective stiffness tensor for horizontal uniaxial or biaxially stressed HTI media, decouples the anisotropic parameters induced by cracks and stress, estimates unknown model parameters using the AVAZ seismic inversion method, and performs inversion using seismic reflection amplitude data at different azimuths, thereby improving the accuracy of underground fracture location prediction.

Description

Technical Field

[0001] The present invention relates to the field of seismic reservoir identification, and in particular to a method and system for azimuthal seismic inversion of a medium with anisotropic stress levels. Background Art

[0002] Detecting seismic anisotropy from surface seismic data helps characterize the extent of subsurface fracture development and guide the prediction of subsurface fracture locations. However, when an initially anisotropic elastic medium is subjected to stress, the symmetry of the stiffness tensor changes, and the altered medium is not completely equivalent to a stress-free anisotropic medium. Therefore, decoupling the two becomes particularly important when studying the effects of fracture- and stress-induced anisotropy on seismic wave velocity, amplitude, or other physical properties of rocks. Furthermore, estimating the geostress field is crucial for subsurface oil and gas exploration and development. Current geostress estimation methods are typically based on ground reflection seismic data, using empirical approximate equations between seismic velocity and geostress. This approach is often limited by the accuracy or applicability of the empirical model. When the empirical model is inaccurate or unsuitable, previous geostress estimation methods suffer from poor accuracy. Summary of the Invention

[0003] In view of the shortcomings of the existing technology, the purpose of the present invention is to provide a method and system for azimuthal seismic inversion of stress-horizontally anisotropic media. The effective stiffness tensor of horizontal uniaxial or biaxial stress HTI media is constructed using nonlinear acoustoelasticity theory, and the anisotropic parameters induced by cracks and stress are decoupled. The unknown model parameters are estimated using the AVAZ seismic inversion method, and the seismic reflection amplitude data at different azimuth angles are used for inversion, which can improve the accuracy of underground crack location prediction.

[0004] In order to achieve the above object, the present invention is implemented through the following technical solutions:

[0005] In a first aspect, an embodiment of the present invention provides a method for azimuthal seismic inversion in a medium with anisotropic stress levels, comprising:

[0006] An HTI medium model under linear slip boundary conditions is established, and a small uniaxial stress parallel to the symmetry axis is applied to the HTI medium model.

[0007] Decoupling the stiffness tensor induced by cracks and stress in horizontal uniaxially stressed HTI media;

[0008] The PP wave AVAZ approximate equation is established to decouple the crack and stress-induced anisotropy parameters;

[0009] The PP wave AVAZ inversion is performed using the difference in reflection coefficient amplitude between different azimuths to form an azimuthal seismic inversion scheme for stress-anisotropic media.

[0010] As a further implementation method, the nonlinear acoustic elasticity theory is used to represent the effective elastic stiffness matrix under horizontal uniaxial stress; based on the small strain assumption, the effective elastic stiffness tensor in stress anisotropic media is simplified.

[0011] As a further implementation method, the expression of the effective elastic stiffness tensor under horizontal uniaxial stress is adjusted based on the weak anisotropy background assumption and the anisotropy induced by small horizontal uniaxial stress (ignoring the approximate high-order infinitesimal terms related to the anisotropy and stress that appear in the calculation).

[0012] As a further implementation method, the linearized PP wave AVAZ reflection coefficient change is expressed as a scattering function using the asymptotic ray theory and the fixed phase method. express:

[0013]

[0014] Where Δ represents the disturbance of the elastic properties of stress-free or stressed rock, Δρ represents the disturbance of density, and η ij Related to the slowness vector and polarization vector of the incident and scattered P-waves; represents the stiffness tensor perturbation in the horizontal uniaxial stress HTI medium, θ represents the incident angle, Indicates the azimuth.

[0015] As a further implementation method, the stiffness tensor perturbation is approximately represented based on the assumptions of weak stress-free background elastic properties, weak stress-free anisotropy of HTI media, and small horizontal uniaxial stress applied to the HTI medium; and the PP-wave AVAZ approximate equations of the decoupled crack and stress anisotropy parameters are obtained based on the stiffness tensor perturbation.

[0016] As a further implementation, the convolution model is used to generate synthetic azimuthal seismic data:

[0017] Δd=Gm,

[0018] Where Δd represents the difference between different azimuth angles, and m represents the unknown model parameters;

[0019]

[0020] W represents the wavelet vector;

[0021] According to the Bayesian inversion framework, synthetic azimuthal seismic data can be solved by combining prior information and likelihood function, that is, the posterior probability distribution function of the unknown model is proportional to the product of the prior information and likelihood function of the unknown model parameters.

[0022] As a further implementation, the objective function Φ(m) is expressed as:

[0023]

[0024] in, represents the regularization coefficient of the unknown model parameters, m i0 represents the low-frequency initial model of the unknown model parameters, represents the variance of the unknown model parameters, represents the variance of the used data, Δd represents the difference between different azimuths, and m represents the unknown model parameters.

[0025] In a second aspect, an embodiment of the present invention further provides an azimuthal seismic inversion system for anisotropic stress media, comprising:

[0026] The medium model building module is configured to: establish an HTI medium model under a linear slip boundary condition, and apply a uniaxial stress parallel to the symmetry axis to the HTI medium model;

[0027] A decoupling module is configured to: decouple the stiffness tensors induced by cracks and stresses in horizontal uniaxially stressed HTI media;

[0028] The approximate equation building module is configured to: establish the PP wave AVAZ approximate equation that decouples crack and stress-induced anisotropy parameters;

[0029] The inversion scheme generation module is configured to: perform PP wave AVAZ inversion using the reflection coefficient amplitude difference between different azimuths to form an azimuthal seismic inversion scheme for stress level anisotropic media.

[0030] In a third aspect, an embodiment of the present invention further provides a computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, implements the steps of the method for azimuthal seismic inversion of anisotropic stress media.

[0031] In a fourth aspect, an embodiment of the present invention further provides a computer device comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein when the processor executes the program, the steps in the method for azimuthal seismic inversion of anisotropic media with respect to stress levels are implemented.

[0032] The beneficial effects of the present invention are as follows:

[0033] (1) The present invention uses nonlinear acoustoelastic theory to describe the P-wave propagation characteristics in stressed HTI media, and studies the weak anisotropic approximate PP-wave azimuthal reflection coefficient at the interface of fractured HTI media under near-horizontal uniaxial stress in stressed media. The decoupled fracture- and stress-induced anisotropic parameters are inverted using azimuthal seismic amplitude difference data, eliminating the influence of the isotropic background elastic properties.

[0034] (2) The PP wave AVAZ approximate inversion method of the present invention can reasonably estimate the unknown model parameters under moderate noise conditions; this inversion method has high reliability in inverting decoupled fractures and stress-induced anisotropic parameters, and will have a wider application in estimating anisotropic stress fields in underground wide-azimuth seismic data. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] The accompanying drawings, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0036] Figure 1 is a flow chart according to one or more embodiments of the present invention;

[0037] FIG2(a) and FIG2(b) are synthetic azimuthal seismic datasets according to one or more embodiments of the present invention;

[0038] Figure 3 is a pp-wave AVAZ inversion decoupled fracture and stress-induced anisotropy parameter inversion result based on noise-free synthetic data according to one or more embodiments of the present invention;

[0039] Figure 4 is the inversion result of decoupled crack and stress-induced anisotropy parameters using pp-wave AVAZ inversion based on noise synthetic data according to one or more embodiments of the present invention;

[0040] FIG5(a) is a portion of post-stack seismic data with an azimuth angle of 20° and an incidence angle of 10° according to one or more embodiments of the present invention;

[0041] FIG5( b ) is a portion of post-stack seismic data with an azimuth angle of 20° and an incidence angle of 20° according to one or more embodiments of the present invention;

[0042] FIG5(c) is a portion of post-stack seismic data with an azimuth angle of 20° and an incidence angle of 30° according to one or more embodiments of the present invention;

[0043] FIG6( a ) is a portion of post-stack seismic data with an azimuth angle of 65° and an incidence angle of 10° according to one or more embodiments of the present invention;

[0044] FIG6( b ) is a portion of post-stack seismic data with an azimuth angle of 65° and an incidence angle of 20° according to one or more embodiments of the present invention;

[0045] FIG6( c ) is a portion of post-stack seismic data with an azimuth angle of 65° and an incidence angle of 30° according to one or more embodiments of the present invention;

[0046] FIG. 7( a ) is a portion of post-stack seismic data with an azimuth angle of 110° and an incidence angle of 10° according to one or more embodiments of the present invention;

[0047] FIG7( b ) is a portion of post-stack seismic data with an azimuth angle of 110° and an incidence angle of 20° according to one or more embodiments of the present invention;

[0048] FIG7( c ) is a portion of post-stack seismic data with an azimuth angle of 110° and an incidence angle of 30° according to one or more embodiments of the present invention;

[0049] FIG8( a ) is a portion of post-stack seismic data with an azimuth of 155° and an incidence angle of 10° according to one or more embodiments of the present invention;

[0050] FIG8( b ) is a portion of post-stack seismic data with an azimuth of 155° and an incidence angle of 20° according to one or more embodiments of the present invention;

[0051] FIG8( c ) is a portion of post-stack seismic data with an azimuth of 155° and an incidence angle of 30° according to one or more embodiments of the present invention;

[0052] FIG9( a ) is azimuthal amplitude difference seismic data with azimuth angles of 20° to 110° and an incident angle of 10° according to one or more embodiments of the present invention;

[0053] FIG9( b ) is azimuth amplitude difference seismic data with azimuth angles of 20° to 110° and an incident angle of 20° according to one or more embodiments of the present invention;

[0054] FIG9( c ) is azimuthal amplitude difference seismic data with azimuth angles of 20° to 110° and an incident angle of 30° according to one or more embodiments of the present invention;

[0055] FIG10( a ) is azimuthal amplitude difference seismic data with azimuth angles of 65° to 155° and an incident angle of 10° according to one or more embodiments of the present invention;

[0056] FIG10( b ) is azimuthal amplitude difference seismic data with azimuth angles of 65° to 155° and an incident angle of 20° according to one or more embodiments of the present invention;

[0057] FIG. 10( c ) is azimuthal amplitude difference seismic data with azimuth angles of 65° to 155° and an incident angle of 30° according to one or more embodiments of the present invention;

[0058] FIG11( a ) is a diagram showing the normal fracture weakness inversion result using fracture anisotropy parameters based on PP-wave AVAZ inversion according to one or more embodiments of the present invention;

[0059] FIG11( b ) is a diagram showing the inversion results of tangential fracture weakness using fracture anisotropy parameters based on PP-wave AVAZ inversion according to one or more embodiments of the present invention;

[0060] FIG12( a ) is a result of P-wave stress-induced anisotropy parameters based on PP-wave AVAZ inversion according to one or more embodiments of the present invention;

[0061] FIG12( b ) is a result of S-wave stress-induced anisotropy parameters based on PP-wave AVAZ inversion according to one or more embodiments of the present invention;

[0062] Figure 13 It is the inversion result of decoupled fracture and stress-induced anisotropy parameters of near-wellbore seismic data based on PP-wave AVAZ inversion according to one or more embodiments of the present invention. DETAILED DESCRIPTION

[0063] Example 1:

[0064] This embodiment provides a method for azimuthal seismic inversion of stress-level anisotropic media. Figure 1 Shown, including:

[0065] An HTI medium model under linear slip boundary conditions was established, and a small uniaxial stress (less than 10 MPa, about a quarter of the uniaxial fracture strength of Mississippian-era Berea sandstone) parallel to the symmetry axis was applied to the HTI medium model.

[0066] Decoupling the stiffness tensor induced by cracks and stress in horizontal uniaxially stressed HTI media;

[0067] The PP wave AVAZ approximate equation is established to decouple the crack and stress-induced anisotropy parameters;

[0068] The PP wave AVAZ inversion is performed using the difference in reflection coefficient amplitude between different azimuths to form an azimuthal seismic inversion scheme for stress-anisotropic media.

[0069] Specifically, the effective elastic stiffness matrix under horizontal uniaxial stress can be expressed using nonlinear acoustoelasticity theory, which includes third-order elastic constants to describe the relationship between elastic properties and ground stress. Based on the small strain assumption, the effective elastic stiffness tensor in stress anisotropic media can be simplified to:

[0070] C ijkl ≈δ ik T jl +A ijkl +A ijklmn E mn ,(1)

[0071] in,

[0072]

[0073]

[0074] Among them, δ ip is the Kronecker symbol, T jl represents the pre-existing principal stress; A ijkl represents the fourth-order tensor in the stress-free state, A ijklmn Represents the sixth-order tensor in the stress-free state, used to describe linear and nonlinear deformation; E mn represents the strain tensor from the stress-free state to the deformed state; ρ represents the density, α i represents the P-wave velocity propagating along the coordinate axis in stressed rock, β ij It represents the S-wave velocity propagating along the coordinate axis in stressed rock.

[0075] The effective elastic stiffness tensor C in stressed HTI media ijkl It is asymmetric (C ijkl ≠C jikl , C ijkl ≠C ijlk , when i=k and j=l, C ijkl ≠C jilk ), which is different from the intrinsic anisotropy and crack-induced HTI media.

[0076] Due to T jl The value is much higher than A ijkl and A ijklmn Much smaller, i.e. |A ijklmn |>>|A ijkl |>>|T jl Ignore T in formula (1) jl , based on the assumption of weak anisotropy background and the assumption of anisotropy induced by small horizontal uniaxial stress, the effective elastic stiffness tensor under horizontal uniaxial stress is It can be further expressed as:

[0077]

[0078] in,

[0079]

[0080]

[0081]

[0082]

[0083] C 44 ≈μ,(9)

[0084] C 66 ≈μ(1-δ T )+μΓ S .(10)

[0085] Where λ represents the first Lame constant of stress-free rock, μ represents the second Lame constant of stress-free rock, M = λ + 2μ represents the P-wave modulus of stress-free rock; δ N represents the dimensionless normal crack weakness of stress-free HTI medium, δ T represents the dimensionless tangential crack weakness of stress-free HTI medium; Γ P Represented by the second-order stiffness tensor A ij (i.e., the fourth-order tensor A represented by Voigt symbols ijk ) and the third-order stiffness tensor A ijkl (i.e., the sixth-order tensor A represented by Voigt symbols ijklmn ) related P-wave stress-induced anisotropy parameter, Γ S Represented by the second-order stiffness tensor A ij (i.e., the fourth-order tensor A represented by Voigt symbols ijk ) and the third-order stiffness tensor A ijkl (i.e., the sixth-order tensor A represented by Voigt symbols ijklmn ) related S-wave stress-induced anisotropy parameters:

[0086]

[0087]

[0088] Where K P =2A 155 / A 33 and K S =A 456 / A 55 T represents two constants related to the ratio of the third-order stiffness tensor to the second-order stiffness tensor, which are used to control the stress-induced P-wave and S-wave anisotropy parameters respectively; 11 Represents the horizontal uniaxial stress applied to the rock.

[0089] Formulas (5)-(10) are the decoupled crack and stress-induced stiffness tensors derived in horizontal uniaxial stress HTI media. The T in formula (4) is ignored here. jl The approximate term related to the asymmetry of the effective elastic stiffness tensor is represented by . Therefore, the fourth-order stress stiffness tensor It can be expressed as a second-order matrix using Voigt notation.

[0090] Using the asymptotic ray theory and the fixed phase method, the linearized PP wave AVAZ reflection coefficient is The change of scattering function To express:

[0091]

[0092] Where θ represents the angle of incidence, represents the azimuth, ρ represents the density, and the scattering function It can be expressed as:

[0093]

[0094] Where Δ represents the disturbance of the elastic properties of stress-free or stressed rock, Δρ represents the disturbance of density, and η ij Related to the slowness vector and polarization vector of the incident and scattered P-waves; is the stiffness tensor perturbation in the horizontal uniaxial stress HTI medium. According to the assumption of weak stress-free background elastic properties, the assumption of weak stress-free HTI medium anisotropy, and the assumption of horizontal uniaxial small stress on HTI medium, the stiffness tensor perturbation is It can be approximated as:

[0095]

[0096] in,

[0097]

[0098]

[0099]

[0100]

[0101] ΔC 44 ≈Δμ,(20)

[0102] ΔC 66 ≈Δμ-μΔδ T +μΔΓ S .(twenty one)

[0103] Substituting equations (16)-(21) into equation (13), we can obtain the PP-wave AVAZ approximate equation for the decoupled crack and stress anisotropy parameters:

[0104]

[0105] in,

[0106]

[0107]

[0108]

[0109]

[0110]

[0111]

[0112]

[0113] In the formula and denote the average values ​​of P-wave and S-wave velocities and the mean density on the reflection interface, respectively; Δα, Δβ and Δρ denote the disturbance values ​​of P-wave velocity, S-wave velocity and P-wave density, respectively; Δδ N and Δδ T represent the disturbance values ​​of normal crack weakness and tangential crack weakness respectively; ΔΓ P and ΔΓ S denote the perturbation values ​​of the anisotropy parameters induced by the introduced P-wave and S-wave stresses, respectively; is the azimuth, which is the observation azimuth and crack normal The difference (i.e. ).

[0114] Using the amplitude difference between different azimuths for PP wave AVAZ inversion can eliminate the influence of the isotropic background on the PP wave azimuth amplitude data and directly obtain the decoupled crack and stress anisotropy parameters. When there are M interfaces, N incident angles, and L azimuths, the PP wave reflection coefficients between different azimuths can be expressed in matrix form:

[0115]

[0116] in,

[0117]

[0118]

[0119]

[0120]

[0121]

[0122]

[0123]

[0124]

[0125]

[0126]

[0127]

[0128]

[0129]

[0130]

[0131] Where t1 and t M are the first and last reflection interfaces respectively, and the symbol T represents the transpose of the matrix.

[0132] Afterwards, the convolution model is used to generate synthetic azimuthal seismic data in the following matrix form:

[0133] Δd=Gm, (35)

[0134] in,

[0135] Δd=[WΔR PP ] MNL×1 ,

[0136]

[0137]

[0138] Where Δd represents the difference between different azimuth angles, W represents the wavelet vector, and m represents the unknown model parameters.

[0139] According to the Bayesian inversion framework, the objective function of formula (35) can be solved by combining prior information and likelihood function, that is, the posterior probability distribution function (PDF) P(m|Δd) of the unknown model is proportional to the product of the prior PDF P(m) of the unknown model parameters and the likelihood function P(Δd|m):

[0140] P(m|Δd)∝P(m)P(Δd|m), (37)

[0141] The prior PDFP(m) of the unknown model parameters in the formula is given by Cauchy distribution modeling:

[0142]

[0143] The likelihood function P(Δd|m) can be expressed as a Gaussian distribution with an L2 norm constraint:

[0144]

[0145] in, and represent the unknown model parameters and the variance of the used data, respectively.

[0146] Therefore, the posterior PDFP(m|Δd) of the unknown model parameters can be derived as:

[0147]

[0148] The final objective function Φ(m) can be expressed as:

[0149]

[0150] This embodiment also adds a low-frequency initial model, and further writes the objective function as:

[0151]

[0152] In the formula is the regularization coefficient of the unknown model parameters, m i0 is the low-frequency initial model with unknown model parameters.

[0153] The iteratively reweighted least squares (IRLS) algorithm is used to solve Equation (42) to obtain a reasonable estimate of the unknown model parameter m.

[0154] This example uses nonlinear acoustoelastic theory to describe the P-wave propagation characteristics in stressed HTI media. The weakly anisotropic approximate PP-wave azimuthal reflection coefficient at the interface of a fractured HTI medium under near-horizontal uniaxial stress in the stressed medium is studied. Azimuthal seismic amplitude difference data are used to invert the decoupled anisotropic parameters induced by fractures and stress, eliminating the influence of the isotropic background elastic properties.

[0155] Example 2:

[0156] This example uses a synthetic azimuthal seismic dataset to verify the inversion method of Example 1. Synthetic data is generated by convolving a 35 Hz Ricker wavelet with the PP wave azimuthal reflection coefficient. Figure 2(a) shows a noise-free synthetic azimuthal seismic dataset. Gaussian noise is then added to the noise-free data to generate noisy data. Figure 2(b) shows a synthetic azimuthal seismic dataset with a signal-to-noise ratio (SNR) of 2, for the four azimuths of 20°, 65°, 110°, and 155°. Azimuthal difference data are then used to estimate the decoupled fracture and stress-induced anisotropy parameters. Amplitude difference data is selected between 20° and 110° and between 65° and 155°, respectively, to obtain the maximum azimuthal difference amplitude.

[0157] Figure 3 and Figure 4 The inversion results of the crack and stress anisotropy parameters for noise-free and noisy synthetic data using the PP-wave AVAZ inversion method are shown. In the case of noise-free data, all inverted decoupled crack and stress-induced anisotropy parameters are in good agreement with the true model parameters. However, the accuracy of the decoupled crack and stress anisotropy parameters inverted using noisy data is lower than that in the noise-free case, and the final inverted unknown model parameters can still meet the needs of practical applications. Therefore, the inversion method of Example 1 provides a feasible method for estimating decoupled crack and stress-induced anisotropy parameters in the case of moderate noise.

[0158] In addition, this example uses a real dataset obtained from southwest China to validate the proposed inversion method. The target layer is a shale gas reservoir containing numerous high-angle fractures. It has also undergone multiple tectonic events, and horizontal tectonic stress is one source of seismic anisotropy. A simplified horizontal uniaxial stress (HTI) model is used to process azimuthal PP-wave seismic data. The data is then processed through amplitude preservation, denoising, velocity analysis, and migration. The data is then sorted to generate azimuthal stacked seismic data. Figure 5(a)-Figure 5(c) 、 Figure 6(a)-Figure 6(c) 、 Figure 7(a)-Figure 7(c) 、 Figure 8(a)-Figure 8(c) The data are partially stacked with average azimuths of 20°, 65°, 110°, and 155°, as well as stacked data with three incidence angles of 10°, 20°, and 30°. The processed azimuth seismic data have a good signal-to-noise ratio and can be used well for azimuth seismic inversion to decouple fracture and stress-induced anisotropy parameters. Figures 9(a), 9(b), and 9(c) are azimuth amplitude difference seismic data with azimuths of 20° and 110° and incidence angles of 10°, 20°, and 30°, respectively. Figures 10(a), 10(b), and 10(c) are azimuth amplitude difference seismic data with azimuths of 65° and 155° and incidence angles of 10°, 20°, and 30°, respectively.

[0159] The generated azimuthal amplitude difference data are then used to invert the decoupled fracture and stress-induced anisotropy parameters. Figures 11(a) and 11(b) show the fracture-induced normal weakness parameter and tangential weakness parameter results based on PP-wave AVAZ inversion, respectively. It can be seen that the red squares in the gas-bearing target reservoir located in Well A almost completely coincide with the high fracture weakness values. Figures 12(a) and 12(b) show the P-wave and S-wave stress anisotropy parameter results based on PP-wave AVAZ inversion, respectively. In contrast, the stress-induced anisotropy parameter values ​​of the gas-bearing target reservoir located in Well A are lower, which is conducive to the formation of a full-scale tensile fracturing and fracture network, promoting the storage and production of shale gas.

[0160] In order to further verify the reliability of the proposed inversion method, this embodiment uses Figure 13 The near-wellbore seismic data shown here provide inversion results for fracture and stress anisotropy parameters based on PP-wave AVAZ inversion. The results show that this inversion method has good consistency with real well logging data and can provide reasonable and reliable estimates of decoupled fracture and stress-induced anisotropy parameters for shale gas reservoirs with HII symmetry.

[0161] The PP-wave AVAZ approximate inversion method of this embodiment can reasonably estimate unknown model parameters under moderate noise conditions. The field data set collected in shale gas reservoirs further proves the reliability of the inversion method in inverting decoupled fractures and stress-induced anisotropic parameters. It will have a wider application in estimating anisotropic stress fields in underground wide-azimuth seismic data.

[0162] Example 3:

[0163] This embodiment provides an azimuthal seismic inversion system for stress-anisotropic media, comprising:

[0164] The medium model building module is configured to: establish an HTI medium model under a linear slip boundary condition, and apply a uniaxial stress parallel to the symmetry axis to the HTI medium model;

[0165] A decoupling module is configured to: decouple the stiffness tensors induced by cracks and stresses in horizontal uniaxially stressed HTI media;

[0166] The approximate equation building module is configured to: establish the PP wave AVAZ approximate equation that decouples crack and stress-induced anisotropy parameters;

[0167] The inversion scheme generation module is configured to: perform PP wave AVAZ inversion using the reflection coefficient amplitude difference between different azimuths to form an azimuthal seismic inversion scheme for stress level anisotropic media.

[0168] Example 4:

[0169] This embodiment provides a computer-readable storage medium having a computer program stored thereon. When the program is executed by a processor, the steps of the azimuthal seismic inversion method for anisotropic stress media described in the first embodiment are implemented.

[0170] Embodiment 5:

[0171] This embodiment provides a computer device, including a memory, a processor, and a computer program stored in the memory and executable on the processor. When the processor executes the program, the steps of the azimuthal seismic inversion method for anisotropic stress media described in the first embodiment are implemented.

[0172] The above description is merely a preferred embodiment of the present application and is not intended to limit the present application. Various modifications and variations are possible for those skilled in the art. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present application shall be included within the scope of protection of the present application.

Claims

1. A method for azimuthal seismic inversion in stress-anisotropic media, characterized in that: include: An HTI medium model under linear slip boundary conditions is established, and a small uniaxial stress parallel to the symmetry axis is applied to the HTI medium model. Decoupling the stiffness tensor induced by cracks and stress in horizontal uniaxially stressed HTI media; The PP wave AVAZ approximate equation is established to decouple the crack and stress-induced anisotropy parameters; The PP wave AVAZ inversion is performed using the difference in reflection coefficient amplitude between different azimuths to form an azimuthal seismic inversion scheme for stress-anisotropic media; the convolution model is used to generate synthetic azimuthal seismic data: in, represents the difference between different azimuths, represents the unknown model parameters; represents the wavelet vector; According to the Bayesian inversion framework, synthetic azimuthal seismic data can be solved by combining prior information and likelihood function, that is, the posterior probability distribution function of the unknown model is proportional to the product of the prior information of the unknown model parameters and the likelihood function; Objective function Expressed as: in, represents the regularization coefficient of the unknown model parameters, represents the low-frequency initial model of the unknown model parameters, represents the variance of the unknown model parameters, Indicates the variance of the used data; Solving the objective function yields reasonable estimates of the unknown model parameters.

2. The azimuthal seismic inversion method for stress-level anisotropic media according to claim 1, characterized in that: The effective elastic stiffness matrix under horizontal uniaxial stress is expressed using nonlinear acoustic elasticity theory. Based on the small strain assumption, the effective elastic stiffness tensor in stress anisotropic media is simplified.

3. The azimuthal seismic inversion method for stress-level anisotropic media according to claim 2, characterized in that: The expression of the effective elastic stiffness tensor under horizontal uniaxial stress is adjusted based on the assumption of weak anisotropy background and the anisotropy induced by small horizontal uniaxial stress.

4. The azimuthal seismic inversion method for stress-level anisotropic media according to claim 1, characterized in that: The linearized PP wave AVAZ reflection coefficient variation is expressed as a scattering function using the asymptotic ray theory and the fixed phase method. express: in, represents the perturbation of the elastic properties of unstressed or stressed rock, represents the density perturbation, Related to the slowness vector and polarization vector of the incident and scattered P-waves; represents the stiffness tensor perturbation in a horizontal uniaxially stressed HTI medium, represents the angle of incidence, Indicates the azimuth.

5. The azimuthal seismic inversion method for stress-level anisotropic media according to claim 4, characterized in that: The stiffness tensor perturbation is approximately expressed based on the assumptions of weak stress-free background elastic properties, weak stress-free anisotropy of HTI media, and small horizontal uniaxial stress applied to HTI media; and the PP wave AVAZ approximate equation of the decoupled crack and stress anisotropy parameters is obtained based on the stiffness tensor perturbation.

6. A azimuthal seismic inversion system for stress-anisotropic media, characterized in that: include: The medium model building module is configured to: establish an HTI medium model under a linear slip boundary condition, and apply a uniaxial stress parallel to the symmetry axis to the HTI medium model; A decoupling module is configured to: decouple the stiffness tensors induced by cracks and stresses in horizontal uniaxially stressed HTI media; The approximate equation building module is configured to: establish the PP wave AVAZ approximate equation that decouples crack and stress-induced anisotropy parameters; The inversion scheme generation module is configured to: perform PP wave AVAZ inversion using the reflection coefficient amplitude difference between different azimuths to form an azimuthal seismic inversion scheme for stress-anisotropic media; and generate synthetic azimuthal seismic data using a convolution model: in, represents the difference between different azimuths, represents the unknown model parameters; represents the wavelet vector; According to the Bayesian inversion framework, synthetic azimuthal seismic data can be solved by combining prior information and likelihood function, that is, the posterior probability distribution function of the unknown model is proportional to the product of the prior information of the unknown model parameters and the likelihood function; Objective function Expressed as: in, represents the regularization coefficient of the unknown model parameters, represents the low-frequency initial model of the unknown model parameters, represents the variance of the unknown model parameters, Indicates the variance of the used data; Solving the objective function yields reasonable estimates of the unknown model parameters.

7. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the program is executed by a processor, the steps of the azimuthal seismic inversion method for stress-level anisotropic media as described in any one of claims 1 to 5 are implemented.

8. A computer device comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein: When the processor executes the program, the steps of the azimuthal seismic inversion method for stress-level anisotropic media are implemented as described in any one of claims 1 to 5.

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