Rock anisotropy parameter measuring method

Abstract

The invention relates to the technical field of oil-gas exploration rock physics experiment analysis, in particular to a rock anisotropy parameter measuring method. Comprising the following steps: selecting a typical rock core of a target reservoir, performing physical property parameter measurement, acquiring physical property parameters of the rock core, calculating anisotropic parameters of the rock according to the physical property parameters of the rock core, and performing physical property measurement on the same rock core: a, arranging strain sensor combinations in a plurality of orientations on the surface of the rock core, and acquiring strain of the rock in different orientations; b, a cylindrical elastic medium with the same diameter as the core is arranged at the top end of the core, strain sensor combinations are arranged on the surface of the cylindrical elastic medium in multiple directions, and the strain of the cylindrical elastic medium in different directions is obtained; c, calculating rock longitudinal and transverse wave velocities according to the steps a and b; and d, calculating the anisotropic parameters of the rock according to the step c. Measurement of anisotropic parameters of the seismic frequency band is achieved for the first time, measurement precision is high, and measurement workload is small.

Description

Technical Field

[0001] The invention relates to the technical field of petrophysical experimental analysis of oil and gas exploration, and in particular to a method for measuring rock anisotropy parameters. Background Art

[0002] The different strength values ​​of rocks in different directions are called rock anisotropy. There are two types of rock anisotropy: one is due to the existence of microcracks and their arrangement and distribution in different directions. This anisotropy will change with the stress of the rock and can be called stress anisotropy; the other is caused by the directional arrangement of rock particles. This type of rock anisotropy will not change with the stress of the rock.

[0003] Affecting drilling efficiency: Since different strength properties are exhibited in different bedding structures, the drilling workload will increase when drilling into such rock layers, so the anisotropy of the rock will affect the drilling efficiency.

[0004] Impact on borehole deflection: Due to the anisotropy of rock, the drill rod is subjected to unbalanced force during the drilling process, causing the drill rod to deflect or even bend at a certain angle, which will affect the deflection of the borehole.

[0005] At present, the measurement of rock physical anisotropy parameters is generally carried out under ultrasonic frequency or static conditions. By drilling cores in different orientations and measuring their rock physical parameters, the anisotropic parameters of the rock are then obtained.

[0006] Before the emergence of the quasi-static stress-strain method, the above methods have solved the problem of measuring the anisotropic parameters of rocks in the ultrasonic frequency band and provided relatively accurate anisotropic parameters of rocks. However, due to the multiple measurements of different cores, it is impossible to accurately evaluate the influence of factors such as pressure and saturation on the anisotropic parameters. In addition, the measurement frequency band is the ultrasonic frequency band, and it is also impossible to accurately evaluate the variation law of anisotropic parameters in the seismic frequency band. Based on the quasi-static stress-strain method, this paper designs a combination of strain sensors in different orientations on the same core, and with the help of a low-frequency rock physics measurement system, it realizes the simultaneous measurement of rock physics parameters in the seismic frequency band under different pressures and saturated fluids, and obtains the anisotropic parameters of rocks in the seismic frequency band for the first time. There is no corresponding research literature on this method. Summary of the invention

[0007] The purpose of the present invention is to accurately evaluate the relationship between the anisotropic parameters of the seismic frequency band and the rock heterogeneity, effective pressure, and fluid saturation by measuring the anisotropic parameters of the rock under different effective pressures and different fluid conditions in the seismic frequency band, combined with the rock microscopic pore structure, and to provide an experimental basis for constructing an anisotropic rock physics model of the seismic frequency band, calculating rock mechanical parameters, and studying the propagation law of seismic waves in heterogeneous formations of complex media. Specifically, the present invention is a method for measuring the anisotropic parameters of the rock.

[0008] The technical solution is as follows:

[0009] A method for measuring rock anisotropy parameters, selecting a typical core of a target reservoir, measuring physical property parameters, obtaining core physical property parameters, calculating rock anisotropy parameters based on the core physical property parameters, and measuring physical properties on the same core, including the following steps:

[0010] A combination of strain sensors is set at multiple locations on the core surface to obtain strains at different locations of the rock;

[0011] A cylindrical elastic medium with the same diameter as the core is arranged at the top of the core, and a strain sensor combination is arranged at multiple positions on the surface of the cylindrical elastic medium to obtain strains at different positions of the cylindrical elastic medium;

[0012] Calculate rock P- and S-wave velocities according to steps a and b;

[0013] Calculate rock anisotropy parameters according to step c.

[0014] Furthermore, the orientations in steps a and b include orientations of 0, 90, 180, and 270 degrees.

[0015] Further, the strain sensor combination at the 0 degree azimuth is a vertical strain sensor combination, which is used to measure longitudinal strain;

[0016] The 180-degree azimuth strain sensor combination is a horizontal sensor combination, which is used to measure lateral strain;

[0017] The strain sensor combination at the 90-degree orientation is a positive 45-degree strain sensor combination, which is used to measure the positive 45-degree strain;

[0018] The strain sensor combination at the 270-degree orientation is a negative 45-degree strain sensor combination, which is used to measure negative 45-degree strain.

[0019] Furthermore, the cylindrical elastic medium is aluminum-like.

[0020] Further, the step c comprises:

[0021] c1: Poisson's ratio υ in the longitudinal direction of the rock is calculated from the ratio of the longitudinal strain ε3 of the rock to the transverse strain ε1 of the rock 纵向 ;

[0022] c2: Young's modulus E of aluminum sample al , longitudinal strain of aluminum sample and rock longitudinal strain ε3 to calculate rock longitudinal Young's modulus E 纵向 ;

[0023] c3: Young's modulus E 纵向 and Poisson's ratio 纵向 , calculate the shear modulus u 纵向 and bulk modulus K 纵向 ;

[0024] c4: Calculate the longitudinal P- and S-wave velocities of the rock, and use the same method to calculate the P- and S-wave velocities of the rock in the 45-degree and horizontal directions.

[0025] Furthermore, the Poisson's ratio v of the rock in the longitudinal direction in step c1 is 纵向 The calculation formula is:

[0026]

[0027] Where: ε3 and ε1 are the longitudinal and transverse strains of the rock, respectively, which are measured by strain sensors at corresponding positions.

[0028] Furthermore, in step c2, the rock longitudinal Young's modulus E 纵向 The calculation formula is:

[0029]

[0030] Where: E al , They are Young's modulus and longitudinal strain of the aluminum sample, respectively. The longitudinal strain of the aluminum sample is measured by a strain sensor attached to the aluminum sample.

[0031] Furthermore, in step c3, the shear modulus u 纵向 and bulk modulus K 纵向 The calculation formula is:

[0032]

[0033]

[0034] Furthermore, the calculation formula for the longitudinal P- and S-wave velocities of the rock in step c4 is:

[0035]

[0036]

[0037] The same calculation method is used to calculate the longitudinal and transverse wave velocity Vp of the rock at 45 degrees 45° , Vs 45° ; Horizontal longitudinal and transverse wave velocity Vp 水平 , Vs 水平 .

[0038] Further, the step d comprises:

[0039] (17)

[0041] (18)

[0043]

[0044] Where χ is a parameter indicating the anisotropy strength of qP waves; γ is a parameter indicating the anisotropy strength of qS waves; and δ is a transition parameter connecting longitudinal waves and shear waves.

[0045] The beneficial effects of the present invention are:

[0046] First, the measurement of anisotropic parameters in the seismic frequency band was realized for the first time, laying an experimental foundation for the evaluation of reservoir fluids using anisotropic parameters.

[0047] Second, the measurement accuracy is high. This method achieves the simultaneous measurement of elastic parameters in different directions by coupling strain sensors in different directions on the same core, so it can more objectively evaluate the influence of effective pressure, fluid saturation, and fracture media on anisotropic parameters.

[0048] Third, the measurement workload is small. The previous technology required drilling cores in different directions, multiple measurements, multiple pressurizations, and multiple saturations, which was time-consuming, labor-intensive, and costly. This method effectively overcomes these problems. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 It is a azimuth diagram;

[0050] Figure 2 It is a schematic diagram of the combined layout of strain sensors when the core surface is unfolded;

[0051] Figure 3 It is a schematic diagram of the aluminum sample setting at the top of the core;

[0052] Figure 4 It is a flow chart of the present invention;

[0053] Figure 5 is the χVS frequency of the sample at different pressures;

[0054] Figure 6 is the γVS frequency of the sample at different pressures;

[0055] Figure 7 is the delta VS frequency of the ** sample at different pressures;

[0056] In the figure: 1. rock core, 2. aluminum sample, 3. strain sensor. DETAILED DESCRIPTION

[0057] In order to make the purpose, technical scheme and advantages of the present invention clearer, the present invention is further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings. It should be understood that these descriptions are only exemplary and are not intended to limit the scope of the present invention. In addition, in the following description, the description of well-known structures and technologies is omitted to avoid unnecessary confusion of the concept of the present invention.

[0058] A method for measuring rock anisotropy parameters in a seismic frequency band comprises the following steps:

[0059] (1) Select target reservoir cores for physical property parameter measurement

[0060] Typical rock cores of the target formation are selected to carry out measurements of parameters such as porosity and density, and finally the porosity, density and other data of the cores are obtained, providing necessary data for the subsequent calculation of rock anisotropy parameters.

[0061] (2) Rock strain sensor combination design

[0062] ①Measurement principle

[0063] The isotropic rock physics equivalent medium model, which is widely used at present, is only applicable to isotropic rocks. However, in mudstone, mud sandstone, and shale, the anisotropy of the elastic parameters of the clay itself and the directional arrangement of the cracks will cause the anisotropy of the rock. In the field of seismic and acoustic logging, since the wavelength is larger than the core scale, anisotropic rocks are often approximated as transversely isotropic rocks (VTI) for the convenience of calculation.

[0064] Hooke's law relates strain and stress via the modulus of elasticity:

[0065] σ j =C ij ε j (1)

[0066] where σ j is the stress tensor element, C ij is the elasticity tensor, ε j is the strain tensor element.

[0067] In the case of transversely isotropic symmetry, the rock has five independent elastic tensors, which can be expressed as:

[0068]

[0069] When using the quasi-static stress-strain method, since the low-frequency measurement equipment can only apply sinusoidal stress in the σ3 direction (axial direction) and there is no direct shear stress, equation (2) can be simplified to:

[0070]

[0071] For transversely isotropic media VTI, the following relationship holds:

[0072]

[0073]

[0074]

[0075]

[0076]

[0077]

[0078] Where V P(90°) is the longitudinal wave velocity of the sample at 90 degrees, V P(0°) is the longitudinal wave velocity of the sample at 0 degrees, V P(45°) is the longitudinal wave velocity of the sample at 45 degrees, V SH(90°) is the shear wave velocity of the sample at 90 degrees, V SH(0°) is the shear wave velocity of the sample at 0 degrees, ρ is the density, ε is the longitudinal wave anisotropy parameter, γ is the shear wave anisotropy parameter, and δ is the comprehensive anisotropy parameter.

[0079] Therefore, we only need to measure the longitudinal wave velocity at 90 degrees, the longitudinal wave velocity at 0 degrees, the longitudinal wave velocity at 45 degrees, the shear wave velocity at 90 degrees, and the shear wave velocity at 0 degrees to obtain the five independent elastic tensors of the rock.

[0080] ② Coupling strain sensors in different orientations

[0081] According to the measurement principle introduced in the previous section, strain sensors with different angles are designed to measure the longitudinal and transverse wave velocities in all directions of the core. Figure 1 The core diagram is shown in Figure 2. Figure 2 This is the unfolded diagram of the core surface. Four groups of strain sensors a, b, c, and d are designed on it. Group a and group c are a combination of vertical and horizontal strain sensors, which are used to measure the longitudinal strain and lateral strain in the rock respectively; group b and group d are a combination of positive and negative 45 degree strain sensors, which are used to measure the positive and negative 45 degree strain in the rock respectively.

[0082] (3) Measurement of longitudinal and transverse wave velocities in all directions in the seismic frequency band

[0083] The longitudinal and transverse wave velocities in the seismic frequency band are measured based on the dynamic stress-strain method. The measuring equipment is a low-frequency measurement system with a measuring frequency of 12-200Hz. It can also change parameters such as sample saturation and pressure.

[0084] Sinusoidal wave excitation is used in the experiment, that is, low-frequency and low-amplitude sinusoidal stress is applied to the rock. According to the elastic wave propagation theory, the strain generated in the rock is expressed as the first-order partial derivative of the displacement with respect to the spatial position, that is,

[0085]

[0086] Where: x is position; ω is frequency (Hz); ρ is density (g / cm3); E is Young's modulus (GPa); ε is strain, and F is the initial amplitude of the applied stress (N).

[0087] Since the stress on the rock changes with time and is not easy to obtain, it is necessary to stick a cylindrical elastic medium with the same diameter on the top of the core. Here, aluminum samples are selected, such as Figure 3 The aluminum sample used in the experiment is a completely elastic, homogeneous, isotropic material, and its elastic modulus does not change with temperature (-20-100°C), pressure (0-40Mpa), or frequency. The Young's modulus is known (72GPa), so the elastic modulus of the rock can be calculated from the ratio of the strain of the rock and the strain of the reference aluminum sample (four sets of strain sensors also need to be attached to the aluminum sample).

[0088] The Poisson's ratio of the rock longitudinal direction can be calculated from the ratio of the core longitudinal strain to the core transverse strain. 纵向 ,Right now

[0089]

[0090] Where: ε3 and ε1 are the longitudinal and transverse strains of the core, respectively, which are measured by strain sensors at corresponding positions.

[0091] Longitudinal Young's modulus E in rock 纵向 The calculation formula is

[0092]

[0093] Where: E al , They are Young's modulus (Gpa) and longitudinal strain of aluminum, respectively. The longitudinal strain of aluminum is measured by a strain sensor attached to the aluminum sample.

[0094] From Young's modulus E 纵向 and Poisson's ratio 纵向, the shear modulus u can be calculated 纵向 (Gpa) and bulk modulus K 纵向 (Gpa)

[0095]

[0096]

[0097] The longitudinal P-wave and S-wave velocities of the core are

[0098]

[0099]

[0100] The same calculation method can be used to calculate the longitudinal and transverse wave velocity Vp of the core at 45 degrees 45° , Vs 45° ; Horizontal longitudinal and transverse wave velocity Vp 水平 , Vs 水平 .

[0101] Similarly, by changing parameters such as sample saturation and confining pressure, the longitudinal and transverse wave velocities of the core at different saturations and pressures at 12-200Hz in different directions can be obtained, laying the foundation for the subsequent calculation of anisotropic parameters.

[0102] (3) Calculation of anisotropic parameters

[0103] The elastic matrix gives the equation relationship between stress and strain, but the coefficients of the elastic wave equation determined by it have no clear physical meaning. Thomsen gave a set of rock physical parameters with clearer physical meanings to characterize the elastic properties of VTI media:

[0104]

[0105]

[0106]

[0107] Where χ is a parameter indicating the anisotropy strength of qP waves; γ is a parameter indicating the anisotropy strength of qS waves; and δ is a transition parameter connecting longitudinal waves and shear waves.

[0108] Therefore, by measuring and obtaining five independent elastic tensors, the three parameters χ, γ, and δ that characterize the anisotropy of rocks can be obtained.

[0109] Embodiment 2:

[0110] Taking the core sample of ** oil field ** work area *** as an example, the anisotropic parameters under the seismic frequency band were measured based on the above method. ** The core porosity is 4.24%, the dry core density is 2.31g / cm3, and the density after full saturation is 2.332g / cm3. First, an aluminum block is glued to the top of the core, and then a, b, c, d4 groups of strain sensors are designed on the surface of the core and the aluminum block. With the help of a low-frequency rock physics measurement system, the longitudinal and transverse wave velocities of the sample in different directions under different pressures and saturated conditions are measured, and then the anisotropic parameters of the sample are obtained. Figure 5 , 6 , 7 are the seismic frequency band anisotropy parameters χ, γ, δ of the obtained ** samples under different pressures. As can be seen from the figure, the anisotropy parameters χ, γ, and δ are all sensitive to pressure. With the increase of effective pressure, χ, γ, and δ all show a decreasing trend, indicating that the increase of effective pressure can suppress the anisotropy of the sample. The anisotropy parameters χ, γ, and δ are less sensitive to frequency, and the dispersion effect is not obvious. Directly measuring the anisotropy parameters of the seismic frequency band of the core sample has laid a solid experimental foundation for identifying reservoir fluids using anisotropy parameters.

[0111] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for measuring rock anisotropy parameters, selecting a typical core of a target reservoir, measuring physical property parameters, obtaining core physical property parameters, and calculating rock anisotropy parameters based on the core physical property parameters, characterized in that: Physical property measurements on the same core include the following steps: a. Set up strain sensor combinations at multiple locations on the core surface to obtain strains at different locations of the rock; b. A cylindrical elastic medium with the same diameter as the core is arranged at the top of the core, and a strain sensor combination is arranged at multiple positions on the surface of the cylindrical elastic medium to obtain strains at different positions of the cylindrical elastic medium; c. Calculate the rock longitudinal and transverse wave velocities according to steps a and b; d. Calculate rock anisotropy parameters according to step c.

2. A method for measuring rock anisotropy parameters according to claim 1, characterized in that: The orientations in steps a and b include orientations of 0, 90, 180, and 270 degrees.

3. A method for measuring rock anisotropy parameters according to claim 2, characterized in that: The strain sensor combination at the 0 degree azimuth is a vertical strain sensor combination, which is used to measure longitudinal strain; The 180-degree azimuth strain sensor combination is a horizontal sensor combination, which is used to measure lateral strain; The strain sensor combination at the 90-degree orientation is a positive 45-degree strain sensor combination, which is used to measure the positive 45-degree strain; The strain sensor combination at the 270-degree orientation is a negative 45-degree strain sensor combination, which is used to measure negative 45-degree strain.

4. A method for measuring rock anisotropy parameters according to claim 3, characterized in that: The cylindrical elastic medium is aluminum.

5. A method for measuring rock anisotropy parameters according to claim 4, characterized in that: The step c comprises: c1: Poisson's ratio υ in the longitudinal direction of the rock is calculated from the ratio of the longitudinal strain ε3 of the rock to the transverse strain ε1 of the rock 纵向 ; c2: Young's modulus E of aluminum sample al , longitudinal strain of aluminum sample and rock longitudinal strain ε3 to calculate rock longitudinal Young's modulus E 纵向 ; c3: Young's modulus E 纵向 and Poisson's ratio 纵向 , calculate the shear modulus u 纵向 and bulk modulus K 纵向 ; c4: Calculate the longitudinal P- and S-wave velocities of the rock, and use the same method to calculate the P- and S-wave velocities of the rock in the 45-degree and horizontal directions.

6. A method for measuring rock anisotropy parameters according to claim 5, characterized in that: The Poisson's ratio of the rock in the longitudinal direction in step c1 is 纵向 The calculation formula is: Where: ε3 and ε1 are the longitudinal and transverse strains of the rock, respectively, which are measured by strain sensors at corresponding positions.

7. A method for measuring rock anisotropy parameters according to claim 6, characterized in that: The rock longitudinal Young's modulus E in step c2 纵向 The calculation formula is: Where: E al , They are Young's modulus and longitudinal strain of the aluminum sample, respectively. The longitudinal strain of the aluminum sample is measured by a strain sensor attached to the aluminum sample.

8. A method for measuring rock anisotropy parameters according to claim 7, characterized in that: The shear modulus u in step c3 纵向 and bulk modulus K 纵向 The calculation formula is:

9. A method for measuring rock anisotropy parameters according to claim 5, characterized in that: The calculation formula for the longitudinal P- and S-wave velocities of the rock in step c4 is: The same calculation method is used to calculate the longitudinal and transverse wave velocity Vp of the rock at 45 degrees 45o , Vs 45o ; Horizontal longitudinal and transverse wave velocity Vp 水平 , Vs 水平 .

10. A method for measuring rock anisotropy parameters according to claim 9, characterized in that: The step d comprises: Where χ is a parameter indicating the anisotropy strength of qP waves; γ is a parameter indicating the anisotropy strength of qS waves; and δ is a transition parameter connecting longitudinal waves and shear waves.

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