A method for predicting rock physical shear wave velocity of deep water ultra-shallow natural gas reservoir

Abstract

The present application relates to the geological exploration technical field, more particularly, to a deep water super shallow layer natural gas reservoir rock physics shear wave velocity prediction method, comprising: calculating equivalent bulk modulus, shear modulus and density of a plurality of mineral mixed matrix according to shale content; then calculating skeleton bulk modulus and shear modulus; then calculating saturated rock bulk modulus and shear modulus through a model, and calculating saturated rock density through rock matrix density; distributing longitudinal and transverse velocities of suspended state and bearing state, and calculating equivalent longitudinal and transverse wave velocities; using equivalent longitudinal wave velocity to construct an inversion objective function with longitudinal wave velocity as a known condition; comparing actual longitudinal wave velocity with calculated longitudinal wave velocity; realizing nonlinear inversion of transverse wave velocity according to an algorithm, and obtaining transverse wave velocity of a to-be-inverted work area. The present application simultaneously considers rock physics models of particle suspended state and bearing state, and realizes transverse wave velocity prediction and deep water super shallow layer natural gas reservoir transverse wave velocity direct inversion.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of geological exploration, more particularly, to a deep-water super-shallow natural gas reservoir rock physical shear wave velocity prediction method. BACKGROUND

[0002] Deep-water super-shallow natural gas reservoirs generally have the characteristics of loose particles, high porosity and high permeability, and have important oil and gas exploration and development value. Rock physical shear wave velocity inversion is an important technical means for carrying out seismic data interpretation and oil and gas resource evaluation. Through rock physical modeling, the physical property parameters (porosity, saturation and shale content) can be linked with the seismic P and S wave velocities, so as to realize the prediction of the shear wave velocity by using logging data. Generally speaking, seismic inversion is a nonlinear optimization problem, that is, the relationship between the seismic elastic parameters and the physical property parameters is a complex nonlinear relationship. The inversion itself includes two key aspects: one is to use rock physical modeling method to construct the inversion solving equation; the other is to optimize the construction of the optimization algorithm suitable for solving complex inversion problems.

[0003] In the prior art, the P wave velocity of seabed sediments is calculated based on the equal sphere particle contact model and Gassmann equation, the elastic parameters of marine sediments are predicted based on the particle contact model and equivalent medium theory, and the bulk compliance factor and shear compliance factor are proposed based on the Biot theory of porous medium, so as to realize the prediction of sediment velocity. However, the above research generally considers only a single particle contact relationship, which has limited scope of application and is difficult to ensure the accuracy and rationality of the rock physical shear wave velocity prediction of deep-water super-shallow natural gas reservoirs. SUMMARY

[0004] In order to overcome the problem in the prior art that only a single particle contact relationship is generally considered to realize the prediction of sediment velocity, which has limited scope of application and is difficult to ensure the accuracy and rationality of the rock physical shear wave velocity prediction of deep-water super-shallow natural gas reservoirs, the present application provides a deep-water super-shallow natural gas reservoir rock physical shear wave velocity prediction method.

[0005] To solve the above technical problems, the technical scheme adopted by the present application is as follows: a deep-water super-shallow natural gas reservoir rock physical shear wave velocity prediction method, the method comprising:

[0006] calculating the equivalent bulk modulus of a plurality of mineral mixed matrix, the shear modulus of the matrix and the density of the matrix;

[0007] In the bearing state, according to the equivalent bulk modulus of the matrix and the shear modulus of the matrix, the equivalent bulk modulus of the skeleton composed of close-packed equal sphere particles when the porosity is the critical porosity and the shear modulus of the skeleton are calculated;

[0008] Calculate the shear modulus and bulk modulus of the rock skeleton based on the equivalent bulk modulus of the matrix, the equivalent bulk modulus of the skeleton, and the shear modulus of the skeleton.

[0009] The equivalent bulk modulus of the pore fluid was calculated based on the patchy partially saturated fluid filling model.

[0010] The bulk modulus and shear modulus of the saturated rock are calculated based on the bulk modulus of the matrix, the bulk modulus of the rock skeleton, the shear modulus of the rock skeleton, and the equivalent bulk modulus of the pore fluid. The density of the saturated rock is calculated based on the fluid density and the density of the matrix.

[0011] Calculate the equivalent bulk modulus of suspended sediments in the suspended state;

[0012] The P-wave and S-wave velocities in the suspended state are calculated based on the equivalent bulk modulus of the suspended sediment and the density of the saturated rock. The P-wave and S-wave velocities in the bearing state are calculated based on the bulk modulus of the saturated rock, the shear modulus of the saturated rock, and the density of the saturated rock.

[0013] Based on the P-wave and S-wave velocities of the suspended state and the P-wave and S-wave velocities of the loaded state, a loosening parameter is introduced to allocate the P-wave and S-wave velocities of the suspended state and the P-wave and S-wave velocities of the loaded state, and the equivalent P-wave and S-wave velocities are calculated.

[0014] The inversion objective function is constructed based on the equivalent P-wave and S-wave velocities and the actual logging P-wave velocity.

[0015] Based on the aforementioned inversion objective function, the predicted shear wave velocity value of the reservoir is obtained through inversion calculation.

[0016] Preferably, the equivalent bulk modulus K of the mixed matrix of multiple minerals is calculated using the Voigt-Reuss-Hill average modulus estimation method. m , matrix shear modulus μ m and the density ρ of the matrix m ;

[0017] K m =[(V sh K c +(1-V sh )K q )+(V sh / K c +(1-V sh ) / K q ) -1 ] / 2

[0018] μ m =[(V sh μ c +(1-Vsh )μ q )+(V sh / μ c +(1-V sh ) / μ q ) -1 ] / 2

[0019] ρ m =V sh ρ c +(1-V sh )ρ q

[0020] Where V sh K represents the clay content. c and K q The bulk modulus values, in μ, are the clayey and sandy components of the matrix, respectively. c and μ q ρ represents the shear modulus values ​​of the clayey and sandy components in the matrix, respectively. c and ρ q These are the density values ​​of the clay and sand components in the matrix, respectively.

[0021] Preferably, based on the equivalent bulk modulus K of the matrix m and the shear modulus μ of the matrix m The equivalent bulk modulus K of the skeleton composed of closely packed isomorphous particles at the critical porosity was calculated using the Hertz-Mindlin model. s and the shear modulus μ of the skeleton s ;

[0022]

[0023] Where n is the coordination number; φ c The critical porosity value depends on lithology and sedimentary environment, and is given by the data from the work site; v m For particles with Poisson's ratio and v m =(3K) m -2μ m ) / (6K m +2μ m P is the effective pressure, which depends on the depth.

[0024] Preferably, based on the equivalent bulk modulus K of the matrix m The equivalent bulk modulus K of the skeleton at critical porosity. s Shear modulus μ of the skeleton s The shear modulus μ of the rock skeleton was calculated using the Hashin-Shtrikman model. dry and the bulk modulus K of the rock skeleton dry;

[0025]

[0026] and

[0027] Preferably, the equivalent bulk modulus of the pore fluid is calculated using a patch-partially saturated fluid filling model;

[0028] K f =(K w -K hc (1-S) hc ) e +K hc

[0029] Where K f K is the equivalent bulk modulus of the pore fluid. w K hc S represents the bulk modulus of brine and hydrocarbons (oil and gas), respectively. hc =1-S w denoted as saturation of hydrocarbon fluid, and e is an empirical constant.

[0030] Preferably, the bulk modulus K of the saturated rock is calculated using the Gassmann equation based on the bulk modulus of the matrix, the bulk modulus of the rock skeleton, the shear modulus of the rock skeleton, and the equivalent bulk modulus of the pore fluid. Sat Furthermore, since the fluid has a negligible effect on the shear modulus of saturated rock, the shear modulus μ Sat Approximately equal to the dry rock shear modulus;

[0031]

[0032] μ Sat =μ dry

[0033] The density ρ of the saturated rock is calculated based on the fluid density and the matrix density. sat The fluid density is constant;

[0034] ρ Sat =φρ f +(1-φ)ρ m .

[0035] Preferably, in the suspended state, the equivalent bulk modulus K of the suspended sediment is calculated using the Wood model. R ,

[0036]

[0037] Where φ is porosity, K f It is the fluid equivalent modulus.

[0038] Preferably, based on the density ρ of the saturated rock sat and the equivalent bulk modulus K of floating sediments R Calculate the P-wave and S-wave velocities in the suspended state. Since the fluid is not shear-resistant, the S-wave velocity is zero.

[0039]

[0040] V s1 =0

[0041] Based on the bulk modulus K of the saturated rock Sat The shear modulus μ of the saturated rock Sat and the density ρ of the saturated rock sat Calculate the P-wave and S-wave velocities in the bearing state:

[0042]

[0043] Preferably, the equivalent longitudinal and transverse wave velocities are calculated based on the longitudinal and transverse wave velocities in the suspended state and the longitudinal and transverse wave velocities in the loaded state.

[0044]

[0045] in The porosity parameter characterizes the porosity of sediments.

[0046] Preferably, the inversion objective function is:

[0047]

[0048] in Let be the actual logging P-wave velocity at the i-th sampling point. The calculated P-wave velocity is given by φ. i Let be the porosity at the i-th sampling point. Let be the loose parameter at the i-th sampling point.

[0049] Compared with existing technologies, the beneficial effects of this invention are: This invention simultaneously considers the rock physics model of both suspended and supported states of particles and achieves shear wave velocity prediction. For the weakly consolidated characteristics of deep-water ultra-shallow sediments, it is equivalent to an equivalent containing both suspended and supported states. By introducing a porosity parameter to allocate the P and S wave velocities of the suspended state and the P and S wave velocities of the supported state, it has reasonable rock physics significance. This invention achieves direct inversion of shear wave velocity in deep-water ultra-shallow natural gas reservoirs. The inversion objective function established by this method contains only two parameters to be inverted, φ. i and It improves computational efficiency and enhances inversion stability. Attached Figure Description

[0050] Figure 1 This is a flowchart of the method for predicting the rock physical shear wave velocity of deep-water ultra-shallow natural gas reservoirs according to the present invention;

[0051] Figure 2 This is a graph showing the inversion results of shear wave velocity calculated according to the method of this invention for a certain exploration area. Detailed Implementation

[0052] The technical solution of the present invention will be further described in detail below through specific embodiments and in conjunction with the accompanying drawings:

[0053] Example 1

[0054] like Figure 1 As shown, a method for predicting shear wave velocity in rock physics of deep-water ultra-shallow natural gas reservoirs is described, the method comprising:

[0055] S1: Calculate the equivalent bulk modulus, shear modulus, and density of the mixed mineral matrix; in the bearing state, based on the equivalent bulk modulus and shear modulus of the matrix, calculate the equivalent bulk modulus and shear modulus of the skeleton composed of closely packed equispherical particles when the porosity is critical; based on the equivalent bulk modulus of the matrix, the equivalent bulk modulus of the skeleton, and the shear modulus of the skeleton, calculate the shear modulus and bulk modulus of the rock skeleton.

[0056] S2: Calculate the equivalent bulk modulus of the pore fluid based on the patchy partially saturated fluid filling model;

[0057] S3: Calculate the bulk modulus and shear modulus of the saturated rock based on the bulk modulus of the matrix, the bulk modulus of the rock skeleton, the shear modulus of the rock skeleton, and the equivalent bulk modulus of the pore fluid; calculate the density of the saturated rock based on the fluid density and the density of the matrix.

[0058] S4: In the suspended state, calculate the equivalent bulk modulus of the suspended sediment;

[0059] S5: Calculate the P- and S-wave velocities of the suspended state based on the equivalent bulk modulus of the suspended sediment and the density of the saturated rock; calculate the P- and S-wave velocities of the bearing state based on the bulk modulus, shear modulus, and density of the saturated rock; calculate the equivalent P- and S-wave velocities by allocating the P- and S-wave velocities of the suspended state and the bearing state using a porosity parameter.

[0060] S6: Construct the inversion objective function based on the equivalent P-wave and S-wave velocities and the actual logging P-wave velocity;

[0061] S7: Based on the aforementioned inversion objective function, the predicted shear wave velocity value of the reservoir is obtained through inversion calculation.

[0062] It should be noted that the method of the present invention can be performed in the order of steps S1 to S7, but is not limited to the above steps. Since there is no order between steps S2 and S1, step S2 can be performed first and then step S1. Since there is no order between steps S4 and steps S1 to S3, the order of steps S4 is not limited.

[0063] It should be noted that, firstly, a petrophysical model of deep-water ultra-shallow natural gas reservoirs is established, treating the mixed matrix of various minerals as a mixture of muddy and sandy components. The equivalent bulk modulus and shear modulus of the mixed matrix are calculated based on the muddy content. In the bearing state, mineral particles are in contact and compressed with each other, resulting in sediments with relatively high stiffness. The suspended state refers to particles distributed within a fluid, forming an extremely loose mixture of solids and fluids.

[0064] Example 2

[0065] A method for predicting shear wave velocity in rock physics of deep-water ultra-shallow natural gas reservoirs, the method comprising:

[0066] The equivalent bulk modulus K of a multi-mineral mixed matrix was calculated using the Voigt-Reuss-Hill average modulus estimation method. m , matrix shear modulus μ m and the density ρ of the matrix m ;

[0067] K m =[(V sh K c +(1-V sh )K q )+(V sh / K c +(1-V sh ) / K q ) -1 ] / 2

[0068] μ m =[(V sh μ c +(1-V sh )μ q )+(V sh / μ c +(1-V sh ) / μ q ) -1 ] / 2

[0069] ρ m =Vsh ρ c +(1-V sh )ρ q

[0070] Where V sh K represents the clay content. c and K q The bulk modulus values, in μ, are the clayey and sandy components of the matrix, respectively. c and μ q ρ represents the shear modulus values ​​of the clayey and sandy components in the matrix, respectively. c and ρ q These are the density values ​​of the clay and sand components in the matrix, respectively.

[0071] It should be noted that the HS model can also be used to calculate the equivalent bulk modulus K of the matrix. m , matrix shear modulus μ m And the density of the matrix.

[0072] In the supported state, according to the equivalent bulk modulus K of the matrix m and the shear modulus μ of the matrix m The equivalent bulk modulus K of the skeleton composed of closely packed isomorphous particles at the critical porosity was calculated using the Hertz-Mindlin model. s and the shear modulus μ of the skeleton s ;

[0073]

[0074] Where n is the coordination number; φ c The critical porosity value depends on lithology and sedimentary environment, and is given by the data from the work site; v m For particles with Poisson's ratio and v m =(3K) m -2μ m ) / (6K m +2μ m P is the effective pressure, which depends on the depth.

[0075] Based on the equivalent bulk modulus K of the matrix m The equivalent bulk modulus K of the skeleton at critical porosity. s Shear modulus μ of the skeleton s The shear modulus μ of the rock skeleton was calculated using the Hashin-Shtrikman model. dry and the bulk modulus K of the rock skeleton dry ;

[0076]

[0077] and

[0078] The equivalent bulk modulus of pore fluid was calculated using a patch-partially saturated fluid-filled model.

[0079] K f =(K w -K hc (1-S) hc ) e +K hc Where K f K is the equivalent bulk modulus of the pore fluid. w K hc S represents the bulk modulus of brine and hydrocarbons (oil and gas), respectively. hc =1-S w denoted as saturation of hydrocarbon fluid, and e is an empirical constant.

[0080] Based on the bulk modulus of the matrix, the bulk modulus of the rock skeleton, the shear modulus of the rock skeleton, and the equivalent bulk modulus of the pore fluid, the bulk modulus K of the saturated rock is calculated using the Gassmann equation. Sat Furthermore, since the fluid has a negligible effect on the shear modulus of saturated rock, the shear modulus μ Sat Approximately equal to the dry rock shear modulus;

[0081]

[0082] μ Sat =μ dry

[0083] The density ρ of the saturated rock is calculated based on the fluid density and the matrix density. sat The fluid density is constant;

[0084] ρ Sat =φρ f +(1-φ)ρ m .

[0085] Calculate the equivalent bulk modulus of suspended sediments in the suspended state;

[0086] Based on the equivalent bulk modulus of the suspended sediment and the density of the saturated rock, and by introducing a loosening parameter to allocate the P and S wave velocities of the suspended state and the P and S wave velocities of the bearing state, the P and S wave velocities of the suspended state are calculated, and the P and S wave velocities of the bearing state are calculated based on the bulk modulus of the saturated rock, the shear modulus of the saturated rock, and the density of the saturated rock.

[0087] Based on the P-wave and S-wave velocities in the suspended state and the P-wave and S-wave velocities in the loaded state, calculate the equivalent P-wave and S-wave velocities.

[0088] An inversion objective function is constructed based on the equivalent P-wave and S-wave velocities and the actual logging P-wave velocity; the predicted S-wave velocity value of the reservoir is obtained by inversion calculation based on the inversion objective function.

[0089] Example 3

[0090] A method for predicting shear wave velocity in rock physics of deep-water ultra-shallow natural gas reservoirs, the method comprising:

[0091] The equivalent bulk modulus K of a multi-mineral mixed matrix was calculated using the Voigt-Reuss-Hill average modulus estimation method. m , matrix shear modulus μ m And the density of the matrix: ρ m ;

[0092] K m =[(V sh K c +(1-V sh )K q )+(V sh / K c +(1-V sh ) / K q ) -1 ] / 2

[0093] μ m =[(V sh μ c +(1-V sh )μ q )+(V sh / μ c +(1-V sh ) / μ q ) -1 ] / 2

[0094] ρ m =V sh ρ c +(1-V sh )ρ q

[0095] Where V sh K represents the clay content. c and K q The bulk modulus values, in μ, are the clayey and sandy components of the matrix, respectively. c and μ q ρ represents the shear modulus values ​​of the clayey and sandy components in the matrix, respectively. c and ρ q These are the density values ​​of the clay and sand components in the matrix, respectively.

[0096] In the supported state, according to the equivalent bulk modulus K of the matrix m and the shear modulus μ of the matrix m The equivalent bulk modulus K of the skeleton composed of closely packed isomorphous particles at the critical porosity was calculated using the Hertz-Mindlin model. s and the shear modulus μ of the skeleton s ;

[0097]

[0098] Where n is the coordination number, which can generally be 8 or 9; φ c The critical porosity value depends on lithology and sedimentary environment, and is given by the data from the work site; v m For particles with Poisson's ratio and v m =(3K) m -2μ m ) / (6K m +2μ m P is the effective pressure, which depends on the depth.

[0099] Based on the equivalent bulk modulus K of the matrix m The equivalent bulk modulus K of the skeleton at critical porosity. s Shear modulus μ of the skeleton s The shear modulus μ of the rock skeleton was calculated using the Hashin-Shtrikman model. dry and the bulk modulus K of the rock skeleton dry ;

[0100]

[0101] and

[0102] The equivalent bulk modulus of pore fluid was calculated using a patch-partially saturated fluid-filled model.

[0103] K f =(K w -K hc (1-S) hc ) e +K hc

[0104] Where K f K is the equivalent bulk modulus of the pore fluid. w K hc S represents the bulk modulus of brine and hydrocarbons (oil and gas), respectively. hc =1-S w denoted as saturation of hydrocarbon fluid, and e is an empirical constant.

[0105] Based on the bulk modulus of the matrix, the bulk modulus of the rock skeleton, the shear modulus of the rock skeleton, and the equivalent bulk modulus of the pore fluid, the bulk modulus K of the saturated rock is calculated using the Gassmann equation. Sat Furthermore, since the fluid has a negligible effect on the shear modulus of saturated rock, the shear modulus μ Sat Approximately equal to the dry rock shear modulus;

[0106]

[0107] μ Sat =μ dry

[0108] The density ρ of the saturated rock is calculated based on the fluid density and the matrix density. sat The fluid density is constant;

[0109] ρ Sat =φρ f +(1-φ)ρ m .

[0110] In the suspended state, the equivalent bulk modulus K of the suspended sediments is calculated using the Wood model. R ,

[0111]

[0112] Where φ is porosity, K f It is the fluid equivalent modulus.

[0113] Based on the density ρ of the saturated rock sat and the equivalent bulk modulus K of floating sediments R Calculate the P-wave and S-wave velocities in the suspended state. Since the fluid is not shear-resistant, the S-wave velocity is zero.

[0114]

[0115] V s1 =0

[0116] Based on the bulk modulus K of the saturated rock Sat The shear modulus μ of the saturated rock Sat and the density ρ of the saturated rock sat Calculate the P-wave and S-wave velocities in the bearing state:

[0117]

[0118] Based on the P-wave and S-wave velocities of the suspended state and the loaded state, and by introducing a loosening parameter to allocate the P-wave and S-wave velocities of the suspended state and the loaded state, the equivalent P-wave and S-wave velocities are calculated.

[0119]

[0120] in The porosity parameter characterizes the porosity of sediments.

[0121] Based on the aforementioned inversion objective function, the predicted shear wave velocity value of the reservoir is obtained through inversion calculation. The inversion objective function is:

[0122]

[0123] in Let be the actual logging P-wave velocity at the i-th sampling point. The calculated P-wave velocity is given by φ. i Let be the porosity at the i-th sampling point. Let be the loose parameter at the i-th sampling point.

[0124] Using the simulated annealing algorithm, the predicted shear wave velocity values ​​of the reservoir are obtained by inversion based on the aforementioned objective function. By constructing the objective function and comparing the calculated and actual P-wave velocities, the shear wave velocity inversion calculation is achieved through Monte Carlo sampling statistics within the simulated annealing framework.

[0125] like Figure 2 As shown, the shear wave velocity inversion results calculated by the present invention for a certain exploration area are obtained by the above method. It can be seen that the method can guarantee the accuracy and rationality of the prediction of rock physical shear wave velocity in deep-water ultra-shallow natural gas reservoirs.

[0126] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. Those skilled in the art can make other variations or modifications based on the above description. It is neither necessary nor possible to exhaustively describe all embodiments here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A method for predicting S-wave velocity of rock physics in deepwater ultra- shallow natural gas reservoirs, characterized in that, The method comprises: calculating the equivalent bulk modulus of a plurality of mineral mixed matrix, the shear modulus of the matrix and the density of the matrix; in the bearing state, according to the equivalent bulk modulus of the matrix and the shear modulus of the matrix, the equivalent bulk modulus of the skeleton composed of close-packed isosphere particles when the porosity is critical porosity and the shear modulus of the skeleton are calculated; according to the equivalent bulk modulus of the matrix, the equivalent bulk modulus of the skeleton and the shear modulus of the skeleton, the shear modulus of the rock skeleton and the bulk modulus of the rock skeleton are calculated; calculating the equivalent bulk modulus of the pore fluid according to the patch partial saturation fluid filling model; according to the bulk modulus of the matrix, the bulk modulus of the rock skeleton, the shear modulus of the rock skeleton and the equivalent bulk modulus of the pore fluid, the bulk modulus of the saturated rock and the shear modulus of the saturated rock are calculated, and the density of the saturated rock is calculated according to the fluid density and the density of the matrix; in the suspension state, the equivalent bulk modulus of the suspension state sediment is calculated; according to the equivalent bulk modulus of the suspension state sediment and the density of the saturated rock, the longitudinal and transverse wave velocity of the suspension state is calculated, and the longitudinal and transverse wave velocity of the bearing state is calculated according to the bulk modulus of the saturated rock, the shear modulus of the saturated rock and the density of the saturated rock; according to the longitudinal and transverse wave velocity of the suspension state and the longitudinal and transverse wave velocity of the bearing state, and introducing the loose parameter to distribute the longitudinal and transverse wave velocity of the suspension state and the longitudinal and transverse wave velocity of the bearing state, the equivalent longitudinal and transverse wave velocity is calculated; according to the equivalent longitudinal and transverse wave velocity and the actual logging longitudinal wave velocity, the inversion target function is constructed; according to the inversion target function, the predicted reservoir S-wave velocity value is calculated by inversion calculation; according to the longitudinal and transverse wave velocity of the suspension state and the longitudinal and transverse wave velocity of the bearing state, the equivalent longitudinal and transverse wave velocity is calculated; wherein is a loose parameter, characterizing the loose degree of the deposit; is the longitudinal wave velocity in the suspension state, is the transverse wave velocity in the suspension state, is the longitudinal wave velocity in the bearing state, is the transverse wave velocity in the bearing state.

2. The method of predicting S-wave velocities for deep-water ultra- shallow gas reservoirs of claim 1, wherein, The equivalent bulk modulus of a mixed matrix of various minerals is calculated by Voigt-Reuss-Hill average modulus estimation method , shear modulus of the matrix , and density of the matrix ; wherein is the shale content, and are the bulk modulus values of the shale component and the sand component in the matrix component, respectively, and are the shear modulus values of the shale component and the sand component in the matrix component, respectively, and are the density values of the shale component and the sand component in the matrix component, respectively.

3. The method of predicting S-wave velocities for deepwater ultra- shallow gas reservoirs of claim 2, wherein, the equivalent bulk modulus of the matrix and the shear modulus of the matrix the equivalent bulk modulus of the skeleton consisting of close packing of equal sphere particles at the critical porosity is calculated by Hertz-Mindlin model and the shear modulus of the skeleton ; where is the coordination number; is the critical porosity value, depending on lithology and depositional environment, given by the field data; is the particle Poisson's ratio and ; is the effective pressure, depending on depth.

4. The method of predicting S-wave velocities for deepwater ultra- shallow gas reservoirs of claim 3, wherein, equivalent bulk modulus of the matrix equivalent bulk modulus of the matrix at critical porosity shear modulus of the matrix calculating the shear modulus of the rock matrix by the Hashin-Shtrikman model bulk modulus of the rock matrix ; and .

5. The method of predicting S-wave velocities for deepwater ultra- shallow gas reservoirs of claim 4, wherein, the equivalent bulk modulus of the pore fluid is calculated by the patch partial saturation fluid filling model; wherein Kp is the equivalent bulk modulus of the pore fluid, , Ks and Kf are the bulk moduli of the saltwater and hydrocarbon, respectively, S is the hydrocarbon fluid saturation, e is an empirical constant term.

6. The method for predicting the S-wave velocity of deepwater ultra- shallow natural gas reservoirs rock physics of claim 5, wherein, The bulk modulus of the saturated rock is calculated by the Gassmann equation from the matrix bulk modulus, the bulk modulus of the rock skeleton, the shear modulus of the rock skeleton, and the equivalent bulk modulus of the pore fluid , and since the fluid has a weak effect on the shear modulus of the saturated rock, the shear modulus is approximately equal to the shear modulus of the dry rock calculating the density of the saturated rock from the fluid density and the matrix density , the fluid density being constant; 。 7. The method of predicting S-wave velocities for deepwater ultra- shallow gas reservoirs of claim 6, wherein, In the suspension state, the equivalent bulk modulus of the suspension sediments is calculated by the Wood model : wherein, is the porosity, is the equivalent bulk modulus of the pore fluid.

8. The method of predicting S-wave velocities for deepwater ultra- shallow gas reservoirs of claim 7, wherein, Density of the saturated rock and the equivalent bulk modulus of the floating state sediment The P and S velocities of the floating state are calculated. Since the fluid is not shear resistant, the S velocity is zero. the bulk modulus of the saturated rock the shear modulus of the saturated rock and the density of the saturated rock calculate the P and S wave velocities in the loaded state: 。 9. The method of predicting S-wave velocities for deepwater ultra- shallow gas reservoirs of claim 8, wherein, the inversion target function is: wherein is the actual measured P-wave velocity at the i-th sample point, is the calculated P-wave velocity, wherein is the porosity at the i-th sample point, is the looseness parameter at the i-th sample point.

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