A method for predicting shear wave velocity in marine-continental transitional shale formations

Abstract

The present invention discloses a method for predicting shear wave velocity of marine-continental transitional shale formations, and the steps are as follows: S1, calculating the clay content volume fraction and porosity of marine-continental transitional shale rocks; S2, calculating the core density; S3, calculating the rock pore aspect ratio, and obtaining the optimal pore aspect ratio at a specific logging depth; S4, calculating the shear modulus of saturated fluid rock according to the optimal pore aspect ratio, clay content volume fraction, and porosity; S5, calculating the saturated fluid rock according to the shear modulus and core density of the saturated fluid rock. The method of the present invention obtains parameters such as the mud content and porosity of the marine-continental transitional shale in the section according to the logging data, and calculates the formation shear wave velocity based on the established SCA theory, that is, by establishing a theoretical relationship between rock mineral content, pore aspect ratio, pore fluid and formation shear wave velocity, to achieve logging shear wave velocity prediction.

Description

Technical Field

[0001] The invention relates to the technical field of oil and gas field exploitation, in particular to a method for predicting shear wave velocity of a marine-continental transitional phase shale formation. Background Art

[0002] The combination of shear wave velocity and longitudinal wave velocity can realize the identification of formation lithology, determination of fluid properties, porosity calculation, prediction of fracture distribution, evaluation of rock mechanics parameters, etc., which is of great significance to formation logging evaluation. However, array acoustic logging is required to obtain the shear wave velocity of the formation in the wellbore, which is more expensive than conventional logging and is often only used in key exploration wells. It is difficult to measure the entire well. Therefore, it is important to establish a method for predicting the shear wave velocity of the formation. Conventional well logging shear wave velocity prediction methods are mostly based on the measured longitudinal and shear wave velocities of array acoustic logging, and then establish a regional empirical relationship between the longitudinal and shear wave velocities to achieve shear wave velocity prediction.

[0003] Conventional methods for predicting shear wave velocity from well logging have good application effects in sandstone formations or relatively average marine shale formations. However, marine-continental transitional shale has many lithofacies, rapid lateral changes, and diverse pore shapes, so the application effect of conventional well logging shear wave velocity prediction methods is limited. Summary of the invention

[0004] The purpose of the present invention is to provide a shear wave velocity well logging prediction method for marine-continental transitional shale formations in order to address the problem that existing well logging shear wave velocity prediction methods are not applicable to marine-continental transitional shale.

[0005] The method of the present invention calculates the formation shear wave velocity based on the SCA theory, that is, by establishing the theoretical relationship between rock mineral content, pore P-wave and S-wave, pore fluid and formation shear wave velocity, the well logging shear wave velocity prediction is achieved. This technology can effectively reduce the influence of mineral content and pore morphology changes on the shear wave prediction results, thereby more accurately predicting the well logging shear wave velocity of marine-continental transitional shale.

[0006] Since the clay content of the marine-continental transitional shale is high, the establishment of the marine-continental transitional shale rock physics model must consider the influence of factors such as the mineral composition, mineral content and pore aspect ratio of the study area. The method of the present invention equates the marine-continental transitional shale to a rock mixture composed of rock matrix, pores, fluids, etc., and establishes a rock physics model for the marine-continental transitional shale based on this assumption. It is assumed that the matrix of the rock is mainly composed of quartz and clay, the fluid in the pores is formation water, the pores are composed of elliptical pores and spherical pores, and the pore aspect ratio of the total pore space is represented by α.

[0007] The method for predicting shear wave velocity of shale formations in the marine-continental transition phase provided by the present invention comprises the following specific steps:

[0008] S1. Calculate the clay content volume fraction V of marine-continental transitional shale rocks using regional empirical formula sh and porosity

[0009] In the present invention, according to the characteristics of the selected region, the following empirical formula is used for calculation:

[0010] Clay content volume fraction V sh The calculation formula is as follows:

[0011] V sh =0.42*KTH+28.61 (1)

[0012] Where KTH is the uranium-free gamma logging value, API.

[0013] Porosity The calculation formula is:

[0014] φ=5.93+0.04*AC-5.54*DEN+0.12*U (2)

[0015] Where AC is the acoustic time difference logging value, μs / ft; DEN is the compensated density logging value, g / cm 3 ; U is the logging value of uranium content, ppm.

[0016] S2. Calculate the core density ρ by using the Reuss average theory. The calculation formula is:

[0017]

[0018] In the formula, is the porosity; ρ m is the skeleton density; ρ f is the fluid density;

[0019] Skeleton density ρ m The calculation formula is:

[0020] ρ m =(1-V sh ')*ρ s +V sh '*ρ g (4)

[0021]

[0022] In the formula, ρ s is the density of quartz, ρ g is the density of clay; V sh Clay content volume fraction.

[0023] S3. Calculate the rock pore aspect ratio to obtain the optimal pore aspect ratio α at a specific logging depth.

[0024] The pore aspect ratio is calculated by iterative method based on the SCA model, Voigt average formula, and Gassmann theory.

[0025] To obtain the pore aspect ratio, the P-wave velocity must be calculated first. The relationship between the P-wave velocity and the equivalent elastic bulk modulus K of the core is as follows:

[0026]

[0027] Where ρ is the core density; K is the bulk modulus of the saturated fluid rock, and μ is the shear modulus of the saturated fluid rock;

[0028] The bulk modulus K and shear modulus μ of saturated fluid rock are obtained by the Gassmann equation. The specific formula is as follows:

[0029]

[0030] In the formula, K * and μ * are the bulk modulus and shear modulus of the dry rock skeleton, K 0 is the bulk modulus of shale solid minerals, is the porosity, K f is the bulk modulus of the fluid component.

[0031] Among them, the bulk modulus K of the fluid in the marine-continental transition phase is f It can be calculated using the Reuss average formula.

[0032] K * and μ * The calculation formula is as follows:

[0033]

[0034]

[0035] Where j represents a mineral phase or pore space; c j represents the volume fraction of the jth phase; K j and μ j are the bulk modulus and shear modulus of the jth phase respectively; P *j and Q *j is the geometric factor of the ellipsoidal inclusion j in the background matrix.

[0036] Since the function relationship is complex and cannot be calculated directly, an iterative method is used. The specific calculation formula is:

[0037]

[0038]

[0039]

[0040]

[0041] T iijj and T ijij is a function of the pore aspect ratio and the properties of the matrix and the enclosing fluid.

[0042]

[0043]

[0044] in:

[0045]

[0046]

[0047]

[0048]

[0049]

[0050] F 6 =1+A[1+fR(f+θ)]+B(1-θ)(3-4R)] (21)

[0051]

[0052]

[0053]

[0054]

[0055]

[0056]

[0057]

[0058]

[0059] Where: v m is the Poisson's ratio of the dry rock skeleton; α is the aspect ratio of the rock pores; K 0 and μ 0 are the shear modulus and bulk modulus of the marine-continental transitional shale matrix.

[0060] K 0 and μ 0 The calculation formula is as follows:

[0061]

[0062] K 0 =V sn *K s +V sh *K sh (31)

[0063] μ 0 =V sn *μ s +V sh *μ sh (32)

[0064] In the formula, K s and μ s The bulk modulus and shear modulus of sandstone are obtained by experimental calculation; K sh and μ sh The bulk modulus and shear modulus of mudstone are obtained by experimental calculation; V sn and V sh are the volume fractions of sandstone and clay, respectively.

[0065] The pore aspect ratio is set to a value range of 0.0001 to 1.0000. First, an initial value of the pore aspect ratio is set, and the pore aspect ratio is changed gradually, with each increase being 0.0001. sh , Porosity The pore aspect ratio is calculated to obtain the predicted P-wave velocity V at different pore aspect ratios at logging depth. p ; Then, according to the P-wave velocity calculated by well logging, find the predicted P-wave velocity that is closest to the P-wave velocity calculated by well logging; the pore aspect ratio corresponding to the predicted P-wave velocity is the optimal pore aspect ratio α at the logging depth.

[0066] S4, according to the optimal pore aspect ratio α, clay content volume fraction V sh , Porosity Substituting into formulas (7)-(32) we can calculate the shear modulus μ of the fluid-saturated rock.

[0067] S5. Calculate the shear wave velocity V of the saturated fluid rock based on the shear modulus μ of the saturated fluid rock and the core density ρ. S ; The calculation formula is as follows:

[0068]

[0069] Compared with the prior art, the present invention is beneficial in that:

[0070] (1) The prediction results are more accurate. For the transitional shale formation, the average relative error of the well logging shear wave velocity predicted by the method of the present invention is 12.8%, while the average relative error of the empirical statistical method is 29.6%. Therefore, the method of the present invention significantly improves the prediction accuracy of the shear wave velocity of the transitional shale formation.

[0071] (2) Wide application scope. This method can predict the shear wave velocity of marine-continental transitional shale logging based on conventional logging data, without the need for array acoustic wave and other logging data, and is applicable to most wells.

[0072] (3) Low engineering investment. The cost of array acoustic logging is higher than that of conventional logging. This patent predicts the shear wave velocity of the formation based on conventional logging data, which can effectively reduce engineering costs.

[0073] Other advantages, objectives and features of the present invention will be embodied in part through the following description, and in part will be understood by those skilled in the art through study and practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0074] Figure 1 , Comparison of shear wave velocity prediction effects of Well A linear fitting method and SCA model method.

[0075] Figure 2 , relative error diagram of well logging shear wave velocity and SCA model predicted shear wave velocity (a), and fitting predicted shear wave velocity (b) of well A. DETAILED DESCRIPTION

[0076] The preferred embodiments of the present invention are described below in conjunction with the accompanying drawings. It should be understood that the preferred embodiments described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention.

[0077] The method of the present invention is compared with the conventional shear wave velocity prediction method in Well A to illustrate the calculation effect of the method of the present invention. The results are shown in Figure 1 and Figure 2 . Figure 1 This is a comparison chart of the shear wave velocity prediction effects of Well A linear fitting method and SCA model method. Figure 1The fourth track is a comparison track of the logging shear wave velocity and the predicted shear wave velocity. The change trend of the predicted shear wave velocity curve (thick dashed line) is basically consistent with that of the logging shear wave velocity curve (thick solid line). The inaccurately calculated layers are the expansion section and the coal seam section. Excluding the coal seam section (2261.75-2262.625m, 2273.875-2274.625m and 2285.5-2286.125m) (solid line frame), and the expansion section (2263.125-2266.725m, 2274.625-2277.875m, 2288.125-2289.125 and 2296.125-2298.125) (dashed line frame), the average relative error of the shear wave velocity predicted by the method of the present invention is 12.8% ( Figure 2 a). The average relative error of the shear wave velocity obtained by the fitting method is 29.6%, which is significantly higher than the shear wave velocity predicted by the method of the present invention, indicating that the method of the present invention effectively improves the prediction accuracy of the shear wave velocity of the marine-continental transitional shale formation ( Figure 2 b).

[0078] The above description is only a preferred embodiment of the present invention and does not constitute any form of limitation to the present invention. Although the present invention has been disclosed as a preferred embodiment as above, it is not intended to limit the present invention. Any technician familiar with the profession can make some changes or modifications to equivalent embodiments of equivalent changes using the technical contents disclosed above without departing from the scope of the technical solution of the present invention. However, any simple modification, equivalent change and modification made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention still falls within the scope of the technical solution of the present invention.

Claims

1. A method for predicting shear wave velocity of marine-continental transitional shale formations, characterized in that: Here are the steps: S1. Calculate the clay content volume fraction V of the marine-continental transitional shale rock sh and porosity S2, calculate the core density ρ; S3, calculating the rock pore aspect ratio, and obtaining the optimal pore aspect ratio α at a specific logging depth; First, the pore aspect ratio is set to a value range of 0.0001 to 1.0000. First, an initial value of the pore aspect ratio is set, and the pore aspect ratio is changed gradually, with each increase of 0.0001; according to the clay content volume fraction V sh , Porosity The pore aspect ratio is calculated to obtain the predicted P-wave velocity V at different pore aspect ratios at logging depth. p ; Then, according to the well logging calculated P-wave velocity, find the predicted P-wave velocity closest to the P-wave velocity calculated by well logging; the pore aspect ratio corresponding to the predicted P-wave velocity is the optimal pore aspect ratio α at the well logging depth; Predicted P-wave velocity V p The calculation method is as follows: Where ρ is the core density; K is the bulk modulus of the saturated fluid rock, and μ is the shear modulus of the saturated fluid rock; The calculation formulas for K and μ are: In the formula, K * and μ * are the bulk modulus and shear modulus of the dry rock skeleton, respectively; K0 is the bulk modulus of the shale solid mineral, is the porosity, K f is the bulk modulus of the fluid component; K * and μ * The calculation formula is as follows: Where j represents a mineral phase or pore space; c j represents the volume fraction of the jth phase; K j and μ j are the bulk modulus and shear modulus of the jth phase respectively; P *j and Q *j is the geometric factor of inclusion j in the background matrix; Since the functional relationship between formulas (8) and (9) is complex and cannot be calculated directly, an iterative method is used. The specific calculation formula is: T iijj and T ijij is a function of the pore aspect ratio and the properties of the matrix and the enclosing fluid; in: F6=1+A[1+fR(f+θ)]+B(1-θ)(3-4R)] (21) Where: v m is the Poisson's ratio of the dry rock skeleton; α is the aspect ratio of the rock pores; K0 and μ0 are the shear modulus and bulk modulus of the marine-continental transitional shale matrix; the calculation formula is as follows: K0=V sn *K s +V sh *K sh (31) μ0=V sn *m s +V sh *m sh (32) In the formula, K s and μ s The bulk modulus and shear modulus of sandstone are obtained by experimental calculation; K sh and μ sh The bulk modulus and shear modulus of mudstone are obtained by experimental calculation; V sn and V sh are the volume fractions of sandstone and clay, respectively; S4, according to the optimal pore aspect ratio α, clay content volume fraction V sh , Porosity Substituting into formulas (7)-(32) we can calculate the shear modulus μ of the fluid-saturated rock; S5. Calculate the shear wave velocity V of the saturated fluid rock based on the shear modulus μ of the saturated fluid rock and the core density ρ. S .

2. The method for predicting shear wave velocity of marine-continental transitional shale formation according to claim 1, characterized in that: In step S1, the clay content volume fraction V sh The calculation formula is as follows: IN sh =0.42*KTH+28.61 (1) Where KTH is the uranium-free gamma logging value, API.

3. The method for predicting shear wave velocity of marine-continental transitional shale formation according to claim 1, characterized in that: In step S1, the porosity The calculation formula is: φ=5.93+0.04*AC-5.54*DEN+0.12*U (2) Where AC is the acoustic time difference logging value, μs / ft; DEN is the compensated density logging value, g / cm 3 ; U is the logging value of uranium content, ppm.

4. The method for predicting shear wave velocity of marine-continental transitional shale formations according to claim 1, characterized in that: In step S2, the core density ρ is calculated as: In the formula, is the porosity; ρ m is the skeleton density; ρ f is the fluid density; Skeleton density ρ m The calculation formula is: r m =(1-V sh ,*r s +V sh ,*r g (4) In the formula, ρ s is the density of quartz, ρ g is the density of clay; V sh Clay content volume fraction.

5. The method for predicting shear wave velocity of marine-continental transitional shale formation according to claim 1, characterized in that: In step S5, the shear wave velocity V of the saturated fluid rock S The calculation formula is as follows:

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